Hydrocracking of virgin and waste plastics A detailed review

Prévia do material em texto

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Renewable and Sustainable Energy Reviews
journal homepage: www.elsevier.com/locate/rser
Hydrocracking of virgin and waste plastics: A detailed review
Dureem Munira, Muhammad F. Irfanb, Muhammad R. Usmana,c,⁎
a Institute of Chemical Engineering and Technology, University of the Punjab, New Campus, Lahore 54590, Pakistan
bDepartment of Chemical Engineering, College of Engineering, University of Bahrain, Isa Town 32038, Bahrain
c Department of Petroleum and Chemical Engineering, Sultan Qaboos University, Muscat 123, Sultanate of Oman
A R T I C L E I N F O
Keywords:
Hydrocracking
Waste plastic
Tertiary recycling
Polymer degradation
Bifunctional catalyst
A B S T R A C T
The continuous increase in the demand for plastics is causing severe problems of managing increased waste
plastic generation. The hydrocracking of plastic materials is considered an important method of converting these
wastes into liquid fuels of high quality. Many attempts have been made in the search for a suitable catalyst and
optimum operating conditions required for a successful hydrocracking process. In the present work, a review of
the literature regarding hydrocracking of both virgin and waste plastic materials is carried out. The effects of
various hydrocracking variables such as temperature, hydrogen pressure, reaction time, catalyst presence and
type, and type of feed plastic employed are discussed in detail. A few hydrocracking mechanisms relevant to
plastic degradation reported in the literature are described and an exhaustive database of the experimental work
is compiled.
1. Introduction
The production of plastic materials such as polyethylene (PE),
polypropylene (PP), polystyrene (PS), polyethylene terephthalate
(PET), and polyvinyl chloride (PVC) is increasing each year [1] due to
their consumption in packaging, construction, agriculture, electrical
and electronic appliances, and health care applications [2]. Plastics
have some unique characteristics, such as outstanding versatility, light
weight, firmness, resistance to water, and low cost, which make them
indispensable for modern civilization [3,4]. However, the life span of
most of the plastic items is very short and most of these items have a life
time of less than one month [5,6]. Due to poor biodegradability rates
[6–10], the plastic waste occupies the land for a longer period of time
[10,11] as only 1‒3wt% of the hydrocarbon contents of plastics are
degraded in 100 years [10]. The plastic waste, therefore, reduces the
overall landfill volume and becomes a huge environmental problem.
Landfill sites are decreasing [10,12–16] and the cost of landfilling is
soaring [15–17]. New legislations in some countries require a decrease
in the amount of waste plastics sent to landfill sites [6,10]. In a landfill
dump, plastic material is an environmental burden, a waste of resource,
source of pollutants such as stabilizers, and greenhouse gases such as
methane [10,18,19]. Incineration of this plastic waste to produce en-
ergy results in the emission of particulate matter [18,20,21] and
harmful gases including unburned hydrocarbons, nitrous and sulfurous
oxides, and dioxins and furans [19,22–24] that are highly unacceptable
from the environmental point of view. Incineration and landfilling are
therefore not the preferred routes for plastic waste management and
plastic waste needs to be treated in a more sustainable and en-
vironmentally friendly way.
Recycling is an alternate method to disposal in a landfill or in-
cineration. It not only protects the environment [25,26], but also helps
to convert the potential resource to useful products [26–28]. Among the
various recycling methods, tertiary recycling, in which waste plastic
material is converted to petrochemicals and fuels [15,29,30] is gaining
increased attention worldwide [8,15,18,22,29,31,32]. Tertiary re-
cycling is an economical and environmentally friendly way of recycling
plastic and is recognized as the most promising method among the
various waste plastic management methods [1,4,33,34]. Tertiary re-
cycling is carried out either by a chemical or a thermal recycling
technique [29,35]. Condensation polymers such as polyesters and
polyurethanes are subjected to solvolysis such as glycolysis, methano-
lysis, and hydrolysis [15,18,29,32,36], whereas addition polymers (PE,
PP, PS, and PVC) are subjected to thermal methods such as gasification,
thermal cracking or pyrolysis, catalytic cracking, and hydrocracking
[15,18,32,37,38].
Gasification is a process which is carried out in the presence of re-
duced amount of oxygen, air, or steam, to produce mainly a synthesis
gas, i.e., a mixture of CO and H2. Cracking is a process in which heavy
polymeric molecules are broken down into smaller much lighter mo-
lecules of gaseous and liquid range. Cracking can be carried out in the
presence or absence of hydrogen and with and without the use of a
catalyst. Thermal cracking or pyrolysis is usually carried out at elevated
https://doi.org/10.1016/j.rser.2018.03.034
Received 6 June 2016; Received in revised form 28 February 2018; Accepted 14 March 2018
⁎ Corresponding author.
E-mail address: mrusman.icet@pu.edu.pk (M.R. Usman).
Renewable and Sustainable Energy Reviews 90 (2018) 490–515
1364-0321/ © 2018 Elsevier Ltd. All rights reserved.
T
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temperatures and in the presence of an inert atmosphere such as ni-
trogen. Catalytic cracking essentially requires a catalyst, usually an
acidic catalyst, to carry out the process at a lower temperature and to
improve the yield as well as quality of the product. The process of
cracking in the presence of hydrogen is called as hydrocracking.
The hydrocracking process is employed for the conversion of heavy
(high boiling) plastic molecules to lighter (low boiling) molecules and
occurs in the presence of hydrogen through carbon-carbon bond clea-
vage, together with simultaneous or successive hydrogenation of un-
saturated molecules formed during the process [39–41]. Hydrocracking
is one of the most promising methods used for the conversion of waste
plastics into high quality liquid fuels. It is more advantageous compared
to pyrolysis and catalytic cracking as it delivers a highly saturated li-
quid product [42,43] that is directly used, without subsequent proces-
sing, as a transportation fuel or a fuel oil required for energy produc-
tion. Pyrolysis and catalytic cracking are considered to give unsaturated
hydrocarbons [42], a large amount of coke [42], and a product with
wide molecular weight distribution [44]. The application of hydro-
cracking reactions involves lower process temperatures [45] and re-
duced amounts of olefins [33,43,45–47], aromatics [43,47], and coke
formation in the reaction products. Furthermore, the presence of hy-
drogen results in the removal of heteroatoms such as chlorine, bromine,
and fluorine that may exist in waste plastic [2,5,48].
Hydrocracking of a polymer generally occurs in the presence of a
bifunctional catalyst in a stirred batch autoclave [49], at moderate
temperatures and relatively high hydrogen pressures. Typical hydro-
cracking conditions are 300–450 °C and 2–15MPa cold hydrogen
pressure. Heat energy is required to bring about the reaction contents to
the desired temperature and to crack the long hydrocarbon chains.
From an energy perspective, cracking and hydrogenation are com-
plementary reactions, as cracking is an endothermic reaction, while
hydrogenation is an exothermic reaction [10]. High partial pressure of
hydrogen should be used in order to suppress undesirable coking or
repolymerization [50]. Although non-catalytic hydrocracking (thermal
hydrocracking) can possiblybe realized, however, the presence of a
catalyst is needed to stimulate the hydrogen addition. Any catalyst used
for the hydrocracking of waste plastic must have a cracking function
and a hydrogenation-dehydrogenation function. A typical hydro-
cracking catalyst has an acidic support with metal impregnated over it.
The acidic support is responsible for cracking and isomerization reac-
tions, whereas the hydrogenation-dehydrogenation function is per-
formed by the metal loaded over the catalyst. The acidic support is
usually an amorphous oxide such as silica-alumina, crystalline zeolite
such as HZSM-5, a strong solid acid such as sulfated zirconia, or a
combination of these materials [39]. The metal can be a noble metal
(palladium or platinum) or non-noble metal of group VI-A (mo-
lybdenum or tungsten) and group VIII-A (cobalt or nickel) of the peri-
odic table [39].
A hydrocracking process can be used in the direct liquefaction of plastic
materials or in a two-stage liquefaction process where plastic material is first
pyrolyzed, i.e., thermally cracked [15,38,51–61], catalytically cracked [51],
or hydrocracked [45] and then the resulting product is subjected to hydro-
treatment to improve the quality of the final liquid produced. Although not
cost effective, in a two-stage process, deactivation of the catalyst by coke
deposition and heteroatoms (e.g., N, S, Cl) is decreased [58] and reduced
amounts of olefins are produced [58], allowing the fuel product, after the
second stage, to be directly used in an automobile engine. Moreover, de-
chlorination of PVCmaterials can be carried out in the first stage to avoid the
presence of chlorinated compounds in the final liquid fuel produced. Some
researchers have also adopted co-processing schemes and carried out hy-
drocracking of a mixture of plastic and a co-feed such as coal [25,44,62–75];
biomass [76]; and oil residue, wax, waste lube oil, long chain alkane (e.g., n-
hexadecane), or tetralin [15,21,25,44,65,68–73,77–85]. Co-processing is
useful in increasing the plant capacity to make it more economical.
In the present contribution, the literature study related to the hy-
drocracking of a plastic material by a direct liquefaction process and in
the absence of a co-feed material is reviewed in detail. The effect of
various operating variables is surveyed and a comprehensive database
of the experimental findings is developed. The objective is to critically
review the operating conditions and the catalytic materials used for the
hydrocracking reaction and to give appropriate recommendations for
their use.
2. Survey of direct liquefaction of plastic materials by
hydrocracking
Hydrocracking of virgin and waste plastics using a direct liquefac-
tion process without the addition of a co-feed has been studied by a
wide range of investigators. A number of these researchers studied the
hydrocracking of a single type of plastic material, while others studied a
mixture of individual plastics. The hydrocracking reactions were stu-
died both in the presence and absence of a catalyst and the effects of
various operating parameters, such as temperature, hydrogen pressure,
reaction time, catalyst type, and catalyst loading were also studied. The
hydrocracking reactions were generally carried out in a closed tubing
bomb (shaking type) reactor or batch stirred autoclave, where the feed
and catalyst were initially charged and hydrogen pressure was set in
cold conditions. The products of the reaction were analyzed for gas
yield, oil yield (lower molecular weight liquid fraction obtained by
extraction using a solvent such as n-pentane, n-hexane, or n-heptane),
total liquid yield that included pre-asphaltenes and asphaltenes ob-
tained usually by tetrahydrofuran (THF) extraction, and solid residue.
In a few studies, unreacted polymer and coke content were also mea-
sured. GC-MS and GC-FID were used to identify and quantify the pro-
duct components and simulated distillation was carried out to develop a
relationship between the boiling point temperature and the percent
distilled. The relationship was later used to evaluate the yields of low
boiling and high boiling fractions of the oil obtained. Table 1 shows the
list of investigators along with the reaction conditions, catalysts, and
analysis techniques employed, whereas Table A-1 in Appendix A is an
extensive database of their experimental findings. The tables include
only those studies that involve the direct liquefaction of a plastic ma-
terial by hydrocracking reaction, without the addition of any solvent,
waste oil, heavy oil, coal, or biomass. In the sections to follow, the
ratios and percent values, such as catalyst loadings and yields, are re-
ported by weight and hydrogen pressures are at cold conditions unless
clearly stated.
2.1. Effect of reaction parameters on hydrocracking of plastic materials
2.1.1. Effect of catalyst on plastic hydrocracking
Catalysts play a key role in plastic hydrocracking. As discussed
earlier, a bifunctional catalyst is usually required for this purpose. The
presence of acidic function as well as hydrogenation-dehydrogenation
function is normally required for achieving appreciable conversion,
high yield and superior quality of liquid products, and reduced amount
of coke deposits. A metal supported solid acid such as sulfated zirconia
or zeolite is a good choice for this type of reaction. A suitable catalyst
yields a liquid product that has a lower boiling point with greater iso- to
n-alkane ratio and a lower bromine number (reduced quantity of ole-
fins). Different types of commercially available and development cat-
alysts designed for cracking, hydrocracking, and hydrotreating pro-
cesses are used in the literature, to observe their effectiveness in
hydrocracking of a plastic material.
Venkatesh et al. [42] employed ZrO2/SO4, 0.5%Pt/ZrO2/SO4, 2.0%
Ni/ZrO2/SO4, and 1.0%Pt/γ-Al2O3 catalysts for the hydrocracking of
HDPE (high density polyethylene), PP, and PS. No experiment was
performed in non-catalytic conditions for the comparison. For the hy-
drocracking reaction of HDPE at 375 °C and 8.38MPa H2 pressure
(reaction time of 25min and feed to catalyst ratio of 5:1) over 0.5%Pt/
ZrO2/SO4 and 2.0%Ni/ZrO2/SO4, virtually the same conversion was
obtained on each catalyst. In both cases, high iso- to n-alkane ratios
D. Munir et al. Renewable and Sustainable Energy Reviews 90 (2018) 490–515
491
Ta
bl
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1
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A
na
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lm
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.[
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PE PP
N
o
ca
ta
ly
st
15
cm
3
SS
(b
at
ch
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tu
bi
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bo
m
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ty
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C
on
ve
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io
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F
in
so
lu
bl
es
O
rr
et
al
.[
63
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as
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as
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48
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C
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H
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so
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.[
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0
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8
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on
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S
Si
m
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at
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di
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ill
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V
en
ka
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sh
et
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.[
86
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85
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is
ot
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00
0)
0.
5%
Pt
/Z
rO
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O
3
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W
)
27
cm
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tu
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C
W
(S
g
=
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00
)
5.
0%
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/A
C
C
(S
g
=
66
0)
5.
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3
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2
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0
M
Pa
T
=
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0–
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0
°C
So
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si
du
e:
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an
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in
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es
G
as
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an
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es
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la
ar
[6
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as
te
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as
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(T
ab
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PE
(M
=
12
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00
0,
ρ=
0.
95
9)
N
o
ca
ta
ly
st
Fe
2
O
3
/S
O
4
0.
5%
Pt
/A
l 2
O
3
/S
O
4
A
l 2
O
3
/S
O
4
Zr
O
2
/S
O
4
N
iM
o/
A
l 2
O
3
+
Si
O
2
-A
l 2
O
3
(4
:1
)
H
ZS
M
-5
Si
O
2
-A
l 2
O
3
(2
5%
Si
O
2
–7
5%
A
l 2
O
3
)
H
ig
h
pr
es
su
re
th
er
m
og
ra
vi
m
et
ri
c
an
al
yz
er
(T
G
A
)
F
=
0.
03
5
g
C
=
10
–5
0%
p H
2
=
6.
31
M
Pa
TG
A
G
C
-M
S
Ib
ra
hi
m
et
al
.[
90
]
W
as
te
A
PC
pl
as
ti
c
(T
ab
le
4)
El
em
en
ta
l
S
N
an
os
ca
le
A
l 2
O
3
ES
R
ce
ll
C
=
10
–2
0%
p H
2
=
3.
55
M
Pa
T
=
R
T‒
~
45
0
°C
ES
R
sp
ec
tr
os
co
py
Jo
o
an
d
C
ur
ti
s
[7
0]
PS LD
PE
PE
T
N
iM
o/
A
l 2
O
3
(2
.7
2%
N
i-
13
.1
6%
M
o,
pr
es
ul
fi
de
d)
20
cm
3
SS
(b
at
ch
,t
ub
in
g
bo
m
b
re
ac
to
r)
F
=
3.
5
g
C
=
99
:1
t=
60
m
in
p H
2
,0
=
8.
3
M
Pa
T
=
43
0
°C
C
on
ve
rs
io
n:
TH
F
in
so
lu
bl
es
G
as
O
il:
n-
he
xa
ne
so
lu
bl
es
Li
qu
id
:T
H
F
so
lu
bl
es
Si
m
ul
at
ed
di
st
ill
at
io
n
Fe
ng
et
al
.[
71
]
W
as
te
A
PC
pl
as
ti
c
(T
ab
le
4)
N
o
ca
ta
ly
st
H
ZS
M
-5
(S
iA
l
=
80
/2
0)
Si
O
2
-A
l 2
O
3
(S
iA
l
=
85
/1
5)
50
cm
3
(b
at
ch
,
tu
bi
ng
bo
m
b,
sh
ak
in
g
ty
pe
)
F
=
5.
0
g
F/
C
=
9:
1
t=
60
m
in
p H
2
,0
=
5.
62
M
Pa
T
=
43
0–
46
0
°C
To
ta
l
yi
el
d:
TH
F
in
so
lu
bl
e
so
lid
re
si
du
e
O
il:
n-
pe
nt
an
e
so
lu
bl
es
Li
qu
id
:T
H
F
so
lu
bl
es
Si
m
ul
at
ed
di
st
ill
at
io
n
(c
on
tin
ue
d
on
ne
xt
pa
ge
)
D. Munir et al. Renewable and Sustainable Energy Reviews 90 (2018) 490–515
492
Ta
bl
e
1
(c
on
tin
ue
d)
R
es
ea
rc
he
r/
s
(y
ea
r)
Fe
ed
C
at
al
ys
t
R
ea
ct
or
C
on
di
ti
on
A
na
ly
si
s
Zm
ie
rc
za
k
et
al
.[
20
]
PS
(M
=
28
0,
00
0)
PS
(M
=
~
80
0)
N
o
ca
ta
ly
st
Zr
O
2
/S
O
4
(S
g
=
11
5)
Fe
2
O
3
/S
O
4
(S
g
=
70
)
50
cm
3
(b
at
ch
st
ir
re
d
ty
pe
)
F/
C
=
10
0:
1,
20
:1
p H
2
,0
=
2.
17
–1
0.
4
M
Pa
t=
15
–1
20
m
in
T
=
35
0–
45
0
°C
C
on
ve
rs
io
n:
TH
F+
M
eO
H
in
so
lu
bl
e
re
si
du
e
(c
en
tr
if
ug
ed
)
G
as
:L
iq
ui
d
N
2
G
C
G
C
-M
S
O
ch
oa
et
al
.[
44
]
M
D
PE
(M
=
60
00
)
N
o
ca
ta
ly
st
A
l 2
O
3
(S
g
=
15
0)
75
%
Si
O
2
-2
5%
A
l 2
O
3
(S
g
=
27
7)
25
%
Si
O
2
-7
5%
A
l 2
O
3
(S
g
=
23
8)
50
%
Si
O
2
-5
0%
A
l 2
O
3
(S
g
=
25
0)
Si
O
2
(S
g
=
18
7)
H
Y
ze
ol
it
e
(S
g
=
87
2)
ZC
A
T
(2
5%
H
ZS
M
S,
S g
=
40
)
25
cm
3
SS
(b
at
ch
,
tu
bi
ng
bo
m
b,
sh
ak
in
g
ty
pe
)
F
=
2.
5
g
F/
C
=
9:
1
t=
15
–6
0
m
in
p H
2
,0
=
1.
4‒
5.
5
M
Pa
T
=
42
0
°C
G
as
O
il:
n-
pe
nt
an
e
so
lu
bl
es
C
ok
e
G
C
-F
ID
Si
m
ul
at
ed
di
st
ill
at
io
n
Lu
o
an
d
C
ur
ti
s
[6
8]
PI
P
(M
=
~
80
0,
00
0,
ρ=
0.
91
0)
PS
(M
=
23
9,
70
0,
ρ=
1.
05
0)
LD
PE
(M
=
~
10
0,
00
0,
ρ=
0.
91
5)
H
D
PE
(M
=
12
5,
00
0,
ρ=
0.
95
0)
N
o
ca
ta
ly
st
Fe
-N
ap
ht
he
na
te
+
S
(1
00
0
pp
m
Fe
+
60
00
pp
m
S)
M
o-
N
ap
ht
he
na
te
+
S
(1
00
0
pp
m
M
o
+
60
00
pp
m
S)
M
on
tm
or
ill
on
it
e
C
ar
bo
n
bl
ac
k
Lo
w
A
lu
m
in
a
(S
g
=
40
4)
Su
pe
r
N
ov
a-
D
(S
g
=
26
8)
O
ct
ac
at
(S
g
=
26
1)
O
ct
ac
at
-5
0G
(S
g
=
30
8)
20
cm
3
SS
(m
ic
ro
tu
bu
la
r,
sh
ak
in
g
ty
pe
)
F
=
0.
5‒
2.
0
g
F/
C
=
1.
5‒
49
:1
t=
30
m
in
p H
2
,0
=
5.
5‒
6.
9
M
Pa
T
=
40
0–
44
0
°C
C
on
ve
rs
io
n:
TH
F
in
so
lu
bl
es
G
as
O
il:
n-
he
xa
ne
so
lu
bl
es
Li
qu
id
:T
H
F
so
lu
bl
es
Lu
o
an
d
C
ur
ti
s
[6
9]
H
D
PE
LD
PE
PE
T
PS 50
%
H
D
PE
+
30
%
PE
T
+
20
%
PS
Lo
w
A
lu
m
in
a
(S
g
=
40
4)
Su
pe
r
N
ov
a-
D
(S
g
=
26
8)
H
ZS
M
-5
(S
g
=
~
30
0)
20
cm
3
SS
(m
ic
ro
tu
bu
la
r,
sh
ak
in
g
ty
pe
)
F
=
2.
0
g
C
=
4‒
9:
1
t=
30
m
in
p H
2
,0
=
2.
3‒
8.
6
M
Pa
T
=
40
0–
44
0
°C
C
on
ve
rs
io
n:
TH
F
in
so
lu
bl
es
G
as
O
il:
n-
he
xa
ne
so
lu
bl
es
Li
qu
id
:T
H
F
so
lu
bl
es
Ib
ra
hi
m
an
d
Se
eh
ra
[9
1]
H
D
PE
W
as
te
A
PC
pl
as
ti
c
(T
ab
le
4)
El
em
en
ta
l
S
N
iM
o/
A
l 2
O
3
H
ZS
M
-5
ES
R
ce
ll
C
=
10
%
‒2
0%
p H
2
=
3.
55
M
Pa
T
=
R
T–
~
45
0
°C
ES
R
sp
ec
tr
os
co
py
H
uff
m
an
et
al
.[
95
]
W
as
te
A
PC
pl
as
ti
c
(T
ab
le
4)
W
as
te
D
SD
pl
as
ti
c
(T
ab
le
4)
N
o
ca
ta
ly
st
H
ZS
M
-5
Zr
O
2
/W
O
3
Fe
rr
ih
yd
ri
te
-C
A
(F
er
ri
hy
dr
it
e
tr
ea
te
d
w
it
h
ci
tr
ic
ac
id
)
5%
M
o/
Fe
rr
ih
yd
ri
te
Si
O
2
-A
l 2
O
3
Ti
O
2
-S
iO
2
(T
i/
(T
i
+
Si
)
=
0.
85
)
Ti
O
2
-S
iO
2
(T
i/
(T
i
+
Si
)>
0.
85
)
50
cm
3
(b
at
ch
,
tu
bi
ng
bo
m
b,
sh
ak
in
g
ty
pe
)
F
=
10
g
F/
C
=
24
–9
9:
1
t=
30
–6
0
m
in
p H
2
,0
=
1.
48
‒7
.0
M
Pa
T
=
40
0–
44
5
°C
C
on
ve
rs
io
n:
TH
F
so
lu
bl
es
G
as
:L
iq
ui
d
N
2
O
il:
n-
pe
nt
an
e
so
lu
bl
es
Li
qu
id
:T
H
F
so
lu
bl
es
Si
m
ul
at
ed
di
st
ill
at
io
n
G
C
-M
S
(c
on
tin
ue
d
on
ne
xt
pa
ge
)
D. Munir et al. Renewable and Sustainable Energy Reviews 90 (2018) 490–515
493
Ta
bl
e
1
(c
on
tin
ue
d)
R
es
ea
rc
he
r/
s
(y
ea
r)
Fe
ed
C
at
al
ys
t
R
ea
ct
or
C
on
di
ti
on
A
na
ly
si
s
D
in
g
et
al
.[
88
]
H
D
PE
(M
=
12
5,
00
0)
W
as
te
A
PC
pl
as
ti
c
(T
ab
le
4)
N
o
ca
ta
ly
st
Ti
C
l 3
H
ZS
M
-5
(3
5%
bi
nd
er
;S
i/
A
lm
ol
e
ra
ti
o
=
35
)
27
cm
3
SS
(b
at
ch
,
tu
bi
ng
bo
m
b,
sh
ak
in
g
ty
pe
)
15
0
cm
3
SS
(b
at
ch
st
ir
re
d
au
to
cl
av
e)
F
=
2.
0‒
20
g
F/
C
=
20
–5
0:
1
t=
60
m
in
p H
2
,0
=
7.
0
M
Pa
T
=
40
0–
43
5
°C
So
lid
re
si
du
e
G
as
O
il:
n-
pe
nt
an
e
so
lu
bl
es
Li
qu
id
G
C
-F
ID
G
C
-M
S
D
in
g
et
al
.[
93
]
W
as
te
A
PC
pl
as
ti
c
(T
ab
le
4)
W
as
te
D
SD
pl
as
ti
c
(T
ab
le
4)
N
o
ca
ta
ly
st
K
C
-2
00
1
(p
re
su
lfi
de
d)
K
C
-2
60
0
(p
re
su
lfi
de
d)
2.
6%
N
i/
H
Si
A
l
(p
re
su
lfi
de
d)
W
he
re
,
H
Si
A
l
(4
:1
m
ix
tu
re
of
Si
O
2
-A
l 2
O
3
an
d
H
ZS
M
-5
)
an
d
H
ZS
M
-5
(1
0–
40
%
bi
nd
er
,
po
re
si
ze
=
6.
2
Å
,S
g
>
20
0)
Si
O
2
-A
l 2
O
3
(1
3%
A
l 2
O
3
,
po
re
si
ze
=
65
Å
,S
g
=
47
5)
27
cm
3
(b
at
ch
,
tu
bi
ng
bo
m
b,
sh
ak
in
g
ty
pe
)
F
=
2.
0
g
F/
C
=
20
–4
0%
t=
60
m
in
T
=
37
5–
48
0
°C
p H
2
,0
=
7.
0
M
Pa
C
on
ve
rs
io
n:
n-
pe
nt
an
e
in
so
lu
bl
es
G
as
O
il:
n-
pe
nt
an
e
so
lu
bl
es
G
C
-F
ID
G
C
-M
S
Si
m
ul
at
ed
di
st
ill
at
io
n
D
in
g
et
al
.[
94
]
H
D
PE
(M
=
12
5,
00
0,
ρ=
0.
95
)
W
as
te
A
PC
pl
as
ti
c
(T
ab
le
4)
N
o
ca
ta
ly
st
K
C
-2
60
0
(N
iM
o/
ze
ol
it
e
an
d/
or
N
iM
o/
A
l 2
O
3
,h
yd
ro
cr
ac
ki
ng
ca
ta
ly
st
)
2.
6%
N
i/
H
Si
A
l
(p
re
su
lfi
de
d)
2.
6%
N
i-
7.
0%
M
o/
H
Si
A
l
(p
re
su
lfi
de
d)
Fo
r
H
Si
A
l
se
e
D
in
g
et
al
.[
93
]
27
cm
3
(b
at
ch
,
tu
bi
ng
bo
m
b,
sh
ak
in
g
ty
pe
)
F
=
2.
0
g
C
=
20
–4
0%
t=0‒
60
m
in
p H
2
,0
=
1.
83
‒7
.0
M
Pa
T
=
35
0–
37
5
°C
C
on
ve
rs
io
n:
n-
pe
nt
an
e
in
so
lu
bl
es
G
as
O
il:
n-
pe
nt
an
e
so
lu
bl
es
G
C
-F
ID
G
C
-M
S
Si
m
ul
at
ed
di
st
ill
at
io
n
R
ot
he
nb
er
ge
r
an
d
C
ug
in
i
[7
2]
H
D
PE
(ρ
=
0.
96
,T
m
=
13
5
°C
)
PS
(ρ
=
1.
0,
T m
=
95
°C
)
PE
T
(ρ
=
1.
4,
T m
=
21
5
°C
)
PP
(ρ
=
0.
94
,
T m
=
17
6
°C
)
N
o
ca
ta
ly
st
13
X
(P
or
e
si
ze
=
10
Å
)
A
O
-6
0
(a
ge
d,
N
iM
o/
A
l 2
O
3
)
43
cm
3
SS
C
=
0.
7
g
t=
60
m
in
p H
2
,0
=
7.
0
M
Pa
T
=
43
0–
44
5
°C
C
O
:H
2
m
ix
tu
re
as
1:
1
m
ol
%
C
on
ve
rs
io
n:
TH
F
so
lu
bl
es
G
as
O
il:
n-
he
pt
an
e
so
lu
bl
es
Sh
ab
ta
i
et
al
.[
89
]
H
D
PE
(M
=
12
5,
00
0,
ρ=
0.
95
9,
T m
=
13
0
°C
)
PP
(M
=
25
0,
00
0,
ρ=
0.
90
0,
T m
=
18
9
°C
,i
so
ta
ct
ic
)
PB
D
(M
=
19
7,
00
0,
ρ=
0.
91
0,
T s
=
14
0–
17
0
°C
)
N
o
ca
ta
ly
st
Fe
2
O
3
/
SO
4
Zr
O
2
/
SO
4
A
l 2
O
3
/
SO
4
0.
5%
Pt
/Z
rO
2
/
SO
4
50
cm
3
(b
at
ch
st
ir
re
d
au
to
cl
av
e)
30
0
cm
3
(b
at
ch
st
ir
re
d
au
to
cl
av
e)
F
=
5.
0‒
10
g
F/
C
=
2–
10
0:
1
t=
30
–1
80
m
in
p H
2
,0
=
3.
55
–1
0.
4
M
Pa
T
=
35
0–
46
5
°C
C
on
ve
rs
io
n:
so
lid
re
si
du
e
G
as
:L
iq
ui
d
N
2
Li
qu
id
G
C
-F
ID
G
C
-M
S
Si
m
ul
at
ed
di
st
ill
at
io
n
D
uf
au
d
an
d
Ba
ss
et
[4
6]
PE
(f
or
m
ed
in
-s
it
u)
PE
(M
=
C
18
‒C
50
)
LD
PE
(M
=
12
5,
00
0)
PP
(M
=
25
0,
00
0,
is
ot
ac
ti
c)
Zr
H
su
pp
or
te
d
on
Si
O
2
-A
l 2
O
3
IR
ce
ll
48
2
cm
3
gl
as
s
ve
ss
el
F
=
11
5
m
g
F/
C
=
1.
64
:1
t=
90
0–
37
20
m
in
p H
2
=
0.
04
‒0
.1
01
M
Pa
T
=
15
0
°C
‒1
90
°C
IR
sp
ec
tr
os
co
py
G
C
R
am
do
ss
an
d
Ta
rr
er
[1
25
]
W
as
te
pl
as
ti
c
(T
ab
le
4)
N
o
ca
ta
ly
st
50
cm
3
(b
at
ch
,t
ub
in
g
bo
m
b
re
ac
to
r)
F
=
6.
0
g
t=
5.
0‒
30
m
in
p H
2
,0
=
0.
79
M
Pa
T
=
47
5–
52
5
°C
U
nr
ea
ct
ed
pl
as
ti
c:
TH
F
in
so
lu
bl
es
,
bu
t
de
ca
lin
so
lu
bl
es
@
14
0
°C
G
as
O
il:
n-
he
xa
ne
so
lu
bl
es
C
ok
e:
D
ec
al
in
in
so
lu
bl
es
Si
m
ul
at
ed
di
st
ill
at
io
n
(c
on
tin
ue
d
on
ne
xt
pa
ge
)
D. Munir et al. Renewable and Sustainable Energy Reviews 90 (2018) 490–515
494
Ta
bl
e
1
(c
on
tin
ue
d)
R
es
ea
rc
he
r/
s
(y
ea
r)
Fe
ed
C
at
al
ys
t
R
ea
ct
or
C
on
di
ti
on
A
na
ly
si
s
Jo
o
an
d
C
ur
ti
s
[7
3]
LD
PE
Z-
75
3
(h
yd
ro
cr
ac
ki
ng
ca
ta
ly
st
)
N
iM
o/
A
l 2
O
3
(2
.7
2%
N
i-
13
.1
6%
M
o,
pr
es
ul
fi
de
d)
N
iM
o/
ze
ol
it
e
(<
25
%
M
oO
2
,1
‒1
0%
N
iO
,p
re
su
lfi
de
d)
10
%
Z-
75
3–
90
%
N
iM
o/
A
l 2
O
3
~
20
cm
3
SS
(b
at
ch
,t
ub
in
g
bo
m
b
re
ac
to
r)
F
=
1.
0
g
F/
C
=
99
:1
t=
30
–6
0
m
in
T
=
40
0–
43
0
°C
p H
2
,0
=
8.
3
M
Pa
C
on
ve
rs
io
n:
TH
F
in
so
lu
bl
es
G
as
O
il:
n-
he
xa
ne
so
lu
bl
es
Li
qu
id
:T
H
F
so
lu
bl
es
Si
m
ul
at
ed
di
st
ill
at
io
n
Sh
ah
et
al
.[
51
]
W
as
te
A
PC
pl
as
ti
c
(T
ab
le
4)
W
as
te
D
SD
pl
as
ti
c
(T
ab
le
4)
N
o
ca
ta
ly
st
H
ZS
M
-5
Zr
O
2
/W
O
3
Fe
rr
ih
yd
ri
te
-C
A
(F
er
ri
hy
dr
it
e
tr
ea
te
d
w
it
h
ci
tr
ic
ac
id
,C
A
)
5%
M
o/
Fe
rr
ih
yd
ri
te
Si
O
2
-A
l 2
O
3
Ti
O
2
-S
iO
2
(T
i/
(T
i
+
Si
)
=
0.
85
)
Ti
O
2
-S
iO
2
(T
i/
(T
i
+
Si
)>
0.
85
)
50
cm
3
(b
at
ch
,
tu
bi
ng
bo
m
b,
sh
ak
in
g
ty
pe
)
F
=
10
g
F/
C
=
99
:1
t=
30
–6
0
m
in
p H
2
,0
=
1.
48
M
Pa
T
=
41
5–
45
5
°C
C
on
ve
rs
io
n:
TH
F
so
lu
bl
es
G
as
:L
iq
ui
d
N
2
O
il:
n-
pe
nt
an
e
so
lu
bl
es
Li
qu
id
:T
H
F
so
lu
bl
es
Si
m
ul
at
ed
di
st
ill
at
io
n
G
C
-M
S
W
al
en
dz
ie
w
sk
i
an
d
St
ei
ni
ng
er
[4
5]
W
as
te
PE
N
iW
+
10
%
H
Y
ca
ta
ly
st
10
00
cm
3
(r
ot
at
ed
ba
tc
h
au
to
cl
av
e)
F
=
25
–1
00
g
F/
C
=
99
–3
32
.3
t=
30
–1
80
m
in
p H
2
,0
=
2‒
5
M
Pa
T
=
37
0–
43
0
°C
G
as
Li
qu
id
:b
p
<
36
0
°C
R
es
id
ue
:b
p
>
36
0
°C
G
C
-F
ID
Br
om
in
e
nu
m
be
r
K
ar
ag
öz
et
al
.[
80
]
LD
PE
(M
=
68
,5
00
,
ρ=
0.
91
83
0.
92
2,
no
st
ab
ili
ze
rs
or
fi
lle
rs
)
D
H
C
-8
(S
g
=
10
2.
0,
no
n-
no
bl
e
m
et
al
s/
Si
O
2
-A
l 2
O
3
)
H
ZS
M
-5
(S
g
=
30
0,
Si
O
2
/A
l 2
O
3
=
21
6
M
)
4.
13
%
C
o/
A
C
B
(S
g
=
20
0.
3)
10
0
cm
3
SS
(b
at
ch
,s
ha
ki
ng
ty
pe
au
to
cl
av
e)
F
=
25
g
F/
C
=
10
:1
T
=
43
5
°C
t=
12
0
m
in
p H
2
,0
=
6.
5
M
Pa
U
nr
ea
ct
ed
pl
as
ti
c:
xy
le
ne
so
lu
bl
es
@
13
0
°C
G
as
O
il:
ce
nt
ri
fu
ge
d
W
ax
:T
H
F
so
lu
bl
es
C
ok
e:
xy
le
ne
in
so
lu
bl
es
G
C
-F
ID
A
li
et
al
.[
96
]
PS
(M
=
18
0,
00
0,
ρ=
1.
05
)
PP
(M
=
75
,0
00
,ρ
=
0.
91
8)
LD
PE
(M
=
80
,0
00
,
ρ=
0.
92
‒0
.9
22
)
H
D
PE
(M
=
85
,0
00
,ρ
=
0.
94
8)
PE
T
ZS
M
-5
(S
iO
2
/A
l 2
O
3
=
30
M
,S
g
=
39
0)
D
H
C
-3
2
(N
iW
/S
iO
2
-A
l 2
O
3
,S
g
=
23
9,
v p
=
0.
36
,p
re
su
lfi
de
d)
FC
C
(m
et
al
/z
eo
lit
e-
Y
,A
l
=
34
%
,
S g
=
19
5)
N
iM
o/
γ-
A
l 2
O
3
(S
g
=
21
0,
v p
=
0.
38
,
pr
es
ul
fi
de
d)
25
cm
3
SS
(b
at
ch
st
ir
re
d
re
ac
to
r)
t=
30
–6
0
m
in
p H
2
,0
=
8.
3
M
Pa
T
=
40
0–
43
0
°C
C
on
ve
rs
io
n:
TH
F
in
so
lu
bl
es
G
as
O
il:
n-
he
xa
ne
so
lu
bl
es
Li
qu
id
:T
H
F
so
lu
bl
es
Si
m
ul
at
ed
di
st
ill
at
io
n
H
es
se
an
d
W
hi
te
[3
3]
PE
(M
=
70
0,
T m
=
80
–9
0
°C
)
Pt
-H
ZS
M
-5
(1
.5
%
A
l 2
O
3
fr
am
e)
Pt
-H
Y
(P
or
e
si
ze
=
7.
4
Å
,S
iO
2
/A
l 2
O
3
=
5.
30
)
Pt
/H
M
C
M
-4
1
(1
7%
A
l 2
O
3
,
po
re
si
ze
=
15
–1
50
Å
)
F
+
C
=
~
5‒
15
m
g
F
=
~
10
%
PE
T
=
10
0–
40
0
°C
G
C
-M
S
M
et
ec
an
et
al
.[
47
]
H
D
PE
(M
=
23
4,
00
0,
ρ=
0.
96
4)
LD
PE
(M
=
21
3,
60
0,
ρ=
0.
91
5)
PP
(M
=
56
5,
70
0,
ρ=
0.
92
0)
D
H
C
-8
(n
on
-n
ob
le
m
et
al
s/
Si
O
2
-A
l 2
O
3
)
H
Y
D
R
O
BO
N
(1
8%
N
iO
/A
l 2
O
3
)
D
H
C
-8
+
H
Y
D
R
O
BO
N
(5
0–
50
%
)
10
0
cm
3
(b
at
ch
,s
ha
ki
ng
ty
pe
au
to
cl
av
e)
F
=
10
g
F/
C
=
20
:1
t=
60
m
in
p H
2
,0
=
5.
0
M
Pa
T
=
37
5–
45
0
°C
G
as
Li
qu
id
R
es
id
ue
G
C
-F
ID
Si
m
ul
at
ed
di
st
ill
at
io
n
Br
om
in
e
nu
m
be
r
(c
on
tin
ue
d
on
ne
xt
pa
ge
)
D. Munir et al. Renewable and Sustainable Energy Reviews 90 (2018) 490–515
495
Ta
bl
e
1
(c
on
tin
ue
d)
R
es
ea
rc
he
r/
s
(y
ea
r)
Fe
ed
C
at
al
ys
t
R
ea
ct
or
C
on
di
ti
on
A
na
ly
si
s
M
os
io
-M
os
ie
w
sk
i
et
al
.[
43
]
LD
PE
N
o
ca
ta
ly
st
N
iM
o/
A
l 2
O
3
(S
g
=
26
0)
20
00
cm
3
(b
at
ch
st
ir
re
d
re
ac
to
r)
F
=
83
5
g
F/
C
=
49
:1
t=
12
0
m
in
p H
2
,T
=
10
M
Pa
T
=
42
0
°C
G
as
:g
as
m
et
er
Li
qu
id
G
C
-F
ID
A
tm
os
ph
er
ic
di
st
ill
at
io
n
Br
om
in
e
nu
m
be
r
W
ill
ia
m
s
an
d
Sl
an
ey
[1
26
]
H
D
PE
PP PS PE
T
PV
C
W
as
te
D
SD
pl
as
ti
c
(T
ab
le
4)
W
as
te
Fo
st
Pl
us
44
.4
%
H
D
PE
+
21
.2
%
PP
+
13
.3
%
PS
+
12
.2
%
PV
C
+
8.
9%
PE
T
N
o
ca
ta
ly
st
30
0
cm
3
SS
(b
at
ch
st
ir
re
d
re
ac
to
r)
F
=
30
–4
0
g
t=
60
m
in
p H
2
,0
=
1.
0
M
Pa
T
=
50
0
°C
So
lid
re
si
du
e
G
as
Li
qu
id
:d
ic
hl
or
om
et
ha
ne
so
lu
bl
es
G
C
-F
ID
G
C
-M
S
A
ka
h
et
al
.[
12
7]
H
D
PE
40
%
H
D
PE
+
40
%
PP
+
20
%
PS
40
%
H
D
PE
+
40
%
PP
+
15
%
PS
+
5%
PV
C
35
%
H
D
PE
+
30
%
PP
+
10
%
PS
+
25
%
PE
T
H
U
SY
(S
i/
A
l b
u
lk
=
2.
8)
0.
5%
Pt
/U
SY
(S
i/
A
l b
u
lk
=
2.
8,
Si
/A
l F
ra
m
e
=
9.
0)
1.
0%
Pt
/U
SY
(S
i/
A
l b
u
lk
=
2.
8,
Si
/A
l F
ra
m
e
=
9.
0)
30
0
cm
3
SS
(b
at
ch
st
ir
re
d
au
to
cl
av
e)
t=
5.
0
m
in
p H
2
,0
=
1.
5‒
5.
5
M
Pa
T
=
27
0–
40
0
°C
C
on
ve
rs
io
n:
un
re
ac
te
d
pl
as
ti
c
G
as
O
il
C
ok
e
(x
yl
en
e+
el
em
en
t
an
a.
)
G
C
-F
ID
A
ka
h
et
al
.[
48
]
H
D
PE
PP PS 40
%
H
D
PE
+
40
%
PP
+
20
%
PS
H
U
SY
(S
i/
A
l b
u
lk
=
2.
8)
0.
5%
Pt
/U
SY
(S
i/
A
l b
u
lk
=
2.
8,
Si
/A
l F
ra
m
e
=
9.
0)
1.
0%
Pt
/U
SY
(S
i/
A
l b
u
lk
=
2.
8,
Si
/A
l F
ra
m
e
=
9.
0)
30
0
cm
3
SS(b
at
ch
st
ir
re
d
au
to
cl
av
e)
F
=
18
g
F/
C
=
10
:1
t=
5.
0
m
in
p H
2
,0
=
1.
5‒
5.
5
M
Pa
T
=
27
0–
40
0
°C
C
on
ve
rs
io
n:
un
re
ac
te
d
pl
as
ti
c
G
as
O
il
C
ok
e
(X
yl
en
e+
el
em
en
t
an
a.
)
G
C
-F
ID
M
un
ir
an
d
U
sm
an
[1
23
]
40
%
H
D
PE
+
10
%
LD
PE
+
30
%
PP +
20
%
PS
N
o
ca
ta
ly
st
A
l-S
BA
-1
6
(S
iA
l
=
19
.5
,
S g
=
11
0.
0,
po
re
si
ze
=
13
1.
7
Å
)
A
l-S
BA
-1
6
(S
iA
l
=
10
.8
,
S g
=
22
0.
9,
po
re
si
ze
=
93
.9
Å
)
M
Z-
20
(M
ic
ro
-m
es
op
or
ou
s
ze
ol
it
e,
Si
A
l
=
20
.4
,S
g
=
23
9.
5,
po
re
si
ze
=
11
3.
2
Å
)
60
0
cm
3
(b
at
ch
st
ir
re
d
au
to
cl
av
e)
F
=
10
g
F/
C
=
20
:1
t=
50
m
in
p H
2
,0
=
2.
0
M
Pa
T
=
40
0
°C
C
on
ve
rs
io
n:
TH
F
in
so
lu
bl
e
so
lid
re
si
du
e
G
as
O
il:
n-
he
pt
an
e
so
lu
bl
es
Li
qu
id
:T
H
F
so
lu
bl
es
M
un
ir
et
al
.[
12
4]
40
%
H
D
PE
+
10
%
LD
PE
+
30
%
PP +
20
%
PS
N
o
ca
ta
ly
st
A
l-S
BA
-1
5
(S
iA
l
=
11
.3
8,
S g
=
24
5.
8)
A
l-S
BA
-1
6
(S
iA
l
=
10
.8
,
S g
=
22
1.
0,
po
re
si
ze
=
93
.9
Å
)
M
Z-
15
(M
ic
ro
-m
es
op
or
ou
s
ze
ol
it
e,
Si
A
l
=
47
.0
,S
g
=
11
2.
7)
M
Z-
16
(M
ic
ro
-m
es
op
or
ou
s
ze
ol
it
e,
Si
A
l
=
20
.9
,S
g
=
10
14
.0
,p
or
e
si
ze
=
59
.1
Å
)
60
0
cm
3
(b
at
ch
st
ir
re
d
au
to
cl
av
e)
F
=
10
g
F/
C
=
20
:1
t=
60
m
in
p H
2
,0
=
2.
0
M
Pa
T
=
37
5
°C
–4
25
°C
C
on
ve
rs
io
n:
TH
F
in
so
lu
bl
e
so
lid
re
si
du
e
G
as
O
il:
n-
he
pt
an
e
so
lu
bl
es
Li
qu
id
:T
H
F
so
lu
bl
es
A
ll
%
va
lu
es
ar
e
w
ei
gh
t
pe
rc
en
t.
A
PC
=
A
m
er
ic
an
Pl
as
ti
c
C
ou
nc
il,
D
SD
=
D
ua
le
s
Sy
st
em
D
eu
st
ch
la
nd
,
ES
R
=
el
ec
tr
on
sp
in
re
so
na
nc
e,
FI
D
=
fl
am
e
io
ni
za
ti
on
de
te
ct
or
,
G
C
=
ga
s
ch
ro
m
at
og
ra
ph
,
H
D
N
=
n-
he
xa
de
ca
ne
,
H
D
PE
=
hi
gh
de
ns
it
y
po
ly
et
hy
le
ne
,
IR
=
in
fr
ar
ed
,
LD
PE
=
lo
w
de
ns
it
y
po
ly
et
hy
le
ne
,
M
D
PE
=
m
ed
iu
m
de
ns
it
y
po
ly
et
hy
le
ne
,
M
eO
H
=
m
et
hy
l
al
co
ho
l,
M
S
=
m
as
s
sp
ec
tr
om
et
er
,
PB
D
=
po
ly
bu
ta
di
en
e,
PE
=
po
ly
et
hy
le
ne
,
PE
T
=
po
ly
et
hy
le
ne
te
re
ph
th
al
at
e,
PI
P
=
po
ly
is
op
re
ne
,P
P
=
po
ly
pr
op
yl
en
e,
PS
=
po
ly
st
yr
en
e,
PV
C
=
po
ly
vi
ny
l
ch
lo
ri
de
,S
S
=
st
ai
nl
es
s
st
ee
l,
TG
A
=
th
er
m
og
ra
vi
m
et
ri
c
an
al
ys
is
,
TH
F
=
Te
tr
ah
yd
ro
fu
ra
n.
C
=
ca
ta
ly
st
,g
;F
=
fe
ed
,g
;M
=
m
ol
ec
ul
ar
w
ei
gh
t,
g/
m
ol
;p
H
2
=
hy
dr
og
en
pr
es
su
re
,M
Pa
;p
H
2
,0
=
in
it
ia
l(
co
ld
)
hy
dr
og
en
pr
es
su
re
,M
Pa
;p
H
2
,T
=
hy
dr
og
en
pr
es
su
re
in
it
ia
l,
bu
ta
tr
ea
ct
io
n
te
m
pe
ra
tu
re
,M
Pa
;R
T
=
ro
om
te
m
pe
ra
tu
re
,°
C
;S
g
=
su
rf
ac
e
ar
ea
,m
2
/g
;t
=
re
ac
ti
on
ti
m
e,
m
in
;T
=
te
m
pe
ra
tu
re
,°
C
;T
m
=
m
el
ti
ng
te
m
pe
ra
tu
re
,°
C
;T
s
=
so
ft
en
in
g
te
m
pe
ra
tu
re
,°
C
;v
p
=
po
re
vo
lu
m
e,
cm
3
/g
;ρ
=
de
ns
it
y,
g/
cm
3
.
D. Munir et al. Renewable and Sustainable Energy Reviews 90 (2018) 490–515
496
were obtained. However, with the Ni-loaded sulfated zirconia catalyst
the ratio was slightly better, in the C4‒C9 range. It was proposed by the
authors that the higher amounts of isomers obtained with Ni metal were
due to the lower hydrogenation-dehydrogenation activity of Ni metal
giving a lower concentration of hydride ions on the surface of the
catalyst, due to the carbenium ions formed on the catalyst surface un-
dergoing considerable isomerization before being desorbed by hydride
transfer. For both the above catalysts, PP was degraded completely at
325 °C, however, with no metal function, i.e., over ZrO2/SO4 and with
weak acidic function, i.e., over 1.0%Pt/γ-Al2O3 catalyst, the cracking of
PP was not realized. In both cases insignificant conversions were ob-
tained.
These results suggest the use of a bifunctional catalyst, as expected
for a hydrocracking reaction, with strong acidic function and a hydro-
genation-dehydrogenation function for the hydrocracking of plastic
materials, especially for the conversions at lower temperatures. It is
important to mention here that Venkatesh et al. [42] suspected the
stability of sulfated zirconia catalysts, as they observed a significant loss
in sulfur during the hydrocracking reaction; which is responsible, in the
form of SO4‒2 (sulfate ions), for the activity of the catalyst.
In another study by Venkatesh et al. [86] hydrocracking of HDPE
was reported on two different Pt-loaded zirconia supports. Under the
conditions of 375 °C, 8.38MPa H2 pressure, 25min residence time, and
5:1 feed to catalyst ratio, almost the same conversion was obtained over
both 0.5%Pt/ZrO2/SO4 and 0.5%Pt/ZrO2/WO3 catalysts. Higher liquid
yield and lower amount of gases with lower iso- to n-alkane ratios were
obtained with 0.5%Pt/ZrO2/WO3. Venkatesh et al. [86] explained the
decreased amount of gases produced on the basis of less acidic activity
of 0.5%Pt/ZrO2/WO3 catalyst. However, the ratio of iso- to n-alkane in
the reaction product is generally not only a function of support acidity
it also depends on the relative strength of metal and acidic functions
[87]. With PP at 325 °C, 8.38MPa H2 pressure, 60min reaction time,
and 5:1 feed to catalyst ratio, almost 100% conversion was obtained
over both the catalysts, with very high yields of gases. Again, 0.5%Pt/
ZrO2/SO4 catalyst was found to be more selective towards gases. These
high gas yields over the catalysts were possibly caused due to the
presence of a large amount of catalyst in the reaction mixture [88] and
increased residence time. Although 0.5%Pt/ZrO2/SO4 catalyst was
found to be slightly more active, 0.5%Pt/ZrO2/WO3 catalyst was more
stable, as no loss in tungstate ions was observed.
Another group of researchers, Zmierczak et al. [20], also used solid
superacids (sulfated catalysts), namely, Fe2O3/SO4 and ZrO2/SO4, but
for the hydrocracking of PS. Both catalysts showed enhanced conver-
sion over the thermal reaction. It was found that ZrO2/SO4 catalyst was
more effective compared to Fe2O3/SO4, giving higher conversion and
more monocyclic arenes (fully depolymerized products) and fewer in-
completely depolymerized products such as diphenylalkanes and tri-
phenylalkanes. Experiments were also performed with low molecular
weight PS (M = 800 g/mol), where again ZrO2/SO4 catalyst was found
to be slightly better in converting PS to the reaction products.
In the same group as Zmierczak et al. [20], Shabtai et al. [89]
studied the hydrodegradation of HDPE over ZrO2/SO4 and Pt-loaded
ZrO2/SO4 (0.5%Pt/ZrO2/SO4) catalysts. A similar observation to that of
Venkatesh et al. [42] was reported by comparing the hydrocracking
catalyst (0.5%Pt/SO4/ZrO2) and the cracking catalyst (SO4/ZrO2). The
total conversion was increased from 30.0% to 55.1% (350 °C, 3.55MPa
H2 pressure, 60 min reaction time, and 5:1 feed to catalyst ratio) when
the Pt-loaded catalyst was employed. Under the same conditions, for
thermal hydrocracking (in the absence of a catalyst) negligible con-
version of HDPE was obtained. Shabtai et al. [89] also studied the effect
of catalyst (0.5%Pt/SO4/ZrO2) to feed ratio and, under the conditions
described above, found that the total conversion increased from 55.1%
to 64.7% (17.4% increase in total conversion) when the catalyst to feed
ratio was increased from 1:5 to 1:2.
A disposable and cheap catalyst that does not require reactivation
and regeneration should be extremely useful in improving the overall
economics of the plant, especially when waste plastic material has
harmful impurities that can badly damage a catalyst. This was the in-
spiration behind the work of Nakamura and Fujimoto [78] who used
various types of catalysts with and without metal loadings and observed
the effect of presulfidation with H2S (H2 and H2S mixture, mole ratio
4–1) and addition of 0.03% CS2, on hydrocracking of PP. Experiments
withactivated carbons alone (without metal loading) have shown that
the activated carbon made from brown coal (ACC) was more active and
yielded a greater amount of total liquid, as well as oil, in the range of
naphtha, kerosene, and gas oil and slightly less gas compared to the
activated carbon made from wood (ACW). However, both the activated
carbons were found to be more active and selective towards oil and
liquid yields, than the conditions when no catalyst was employed. 5.0%
Fe/ACC and 5.0%Fe/SiO2-Al2O3 catalysts were also used for the hy-
drocracking reactions. It was found that 5.0%Fe/SiO2-Al2O3 produced
more solid and gaseous products and less liquid product as compared to
5.0%Fe/ACC, whereas the former provided an increased oil yield. When
CS2 was added to 5.0%Fe/SiO2-Al2O3 and 5.0%Fe/ACC, in both cases,
the yield of liquid drastically increased, while the corresponding solid
residue was significantly reduced. However, 5.0%Fe/ACC + CS2 cata-
lyst produced a greater amount of oil and considerably less gas than
5.0%Fe/SiO2-Al2O3 +CS2. Compared to CS2, the presence of H2S with
5.0%Fe/ACC was found to be more effective for the liquefaction of PP
and its small amount significantly contributed to the liquefaction. Ex-
periments with CS2 and H2S without using a catalyst also showed
considerable improvement in conversion and oil and liquid yields,
which led the authors to report that use of sulfur compounds does not
only lead to the sulfidation of iron, the compounds also act as catalysts
in the liquefaction of PP. Based on these findings the authors concluded
that, for the hydrocracking of PP, the iron supported activated carbon
catalyst has the potential to be used as an excellent disposable catalyst
for waste plastics liquefaction, due to its excellent activity and se-
lectivity. The comparison between the results of Nakamura and Fuji-
moto [78] and the work of Venkatesh et al. [42] is difficult to carry out
as lower catalyst concentrations (3.0%) were used by Nakamura and
Fujimoto [78] compared to the 16.7% used by Venkatesh et al. [42].
The catalytic cracking or hydrocracking reaction essentially requires
acidic sites, therefore the knowledge of the quantity and type of acidic
sites is essential in the design of a new catalytic system. This concept
was conceptualized by Ochoa et al. [44], who studied the hydro-
cracking of medium density polyethylene (MDPE) on various catalysts,
mainly to observe the effect of acidity characteristics of a catalyst on the
yield and quality of the oil produced. The catalytic materials used by
Ochoa et al. [44], together with their acidity characteristics, are listed
in Table 2. Ochoa et al. [44] found that pure Al2O3 and pure SiO2 did
not improve the conversion over thermal hydrocracking, as SiO2 had
negligible acidic sites, while Al2O3 did not contain strong acid sites. On
the other hand, for the amorphous SiO2-Al2O3 catalysts containing both
the Brønsted and Lewis acid sites, it was observed that an increase in
total acidity (Brønsted + Lewis) of SiO2-Al2O3 catalyst did not affect
the oil yield. However, oil yield was found to be the sole function of
Brønsted acidity. The yield generally increased with an increase in the
Table 2
Properties of catalysts used by Ochoa et al. [44].
Catalyst Surface area
(m2/g)
Total acidity
(mmol/g)
Brönsted/Lewis
ratio
SiO2 187 0.001 0
25%SiO2-75%Al2O3 238 0.5 0.2
50%SiO2-50%Al2O3 250 0.4 0.3
75%SiO2-25%Al2O3 277 0.3 0.7
Al2O3 150 0.3 0
HY 872 ‒ ‒
ZCAT (25%HZSM-5 + Inert) 40 ‒ ‒
D. Munir et al. Renewable and Sustainable Energy Reviews 90 (2018) 490–515
497
concentration of Brønsted acid sites, suggesting that it was not depen-
dent on the concentration of Lewis acid sites. In comparison to oil yield,
the gas yield was found to be relatively less affected, being reported as
less than 1.0% in all the cases. In the absence of a catalyst at 420 °C and
5.5MPa H2 pressure,> 97% linear molecules were obtained, while in
the presence of a SiO2-Al2O3 catalyst a more branched product was
obtained. Although the boiling range (177–585 °C) measured using si-
mulated distillation was similar in both cases, catalytic and non-cata-
lytic, but a lighter product was obtained in the presence of a catalyst.
With no catalyst in the reaction mixture, 50 vol% of the product boiled
off at ~ 407 °C, while 50 vol% of the product boiled off at only ~ 287 °C
when a catalyst was employed. For the three SiO2-Al2O3 catalysts used,
the least acidic catalyst provided the smallest fraction of the lighter
products. Compared to SiO2-Al2O3 catalysts, HZSM-5 (ZCAT) used in
the study was less active, but showed better selectivity for the gasoline
range product (C5‒C12), whereas the HY zeolite catalyst showed both
the poor activity and selectivity characteristics. The coke formed on
75%SiO2–25%Al2O3 catalyst was about 14.0% as compared to ~ 10%
for both 50%SiO2–50%Al2O3 and 25%SiO2-75%Al2O3 catalysts. HZSM-
5 produced the least amount of coke, only 1.5%. This decreased amount
of coke on HZSM-5 may be due to the smaller pore size of HZSM-5,
which is not selective for large size coke molecules. On a similar basis, a
relatively high content of coke is expected on the HY catalyst. Ochoa
et al. [44] also studied different catalyst loadings such as 1%, 2%, 4%,
and 10% to observe the effect of catalyst concentration in the feed
mixture and to workout the optimum catalyst loading. It was found that
for the catalyst loading below 4% the conversion increased with an
increase in catalyst concentration. However, between 4% and 10%
loading the catalytic degradation of MDPE remained at the same level.
A different strategy for observing the effectiveness of a catalyst was
adopted by Liu and Meuzelaar [67], who investigated various catalysts for
the hydrodegradation of waste plastic (Table 3) and HDPE in a high pres-
sure TG/GC-MS system under 6.31MPa H2 pressure. The TGA results of
their study are shown in Fig. 1. The figure shows that the presence of each
catalyst increases the rate of decomposition reaction of the waste plastic
studied. It is observed that, for the given conditions, the cracking catalysts
such as SiO2-Al2O3 (25%SiO2 +75%Al2O3) and HZSM-5 showed the
highest activity rates. It is also observed that, at 420 °C the activity de-
creased in the order of SiO2-Al2O3, HZSM-5, pre-sulfided NiMo/Al2O3
mixed with SiO2-Al2O3 (4:1 ratio), and superacids. However, a higher cat-
alyst to feed ratio was set for SiO2-Al2O3 and the mixed catalyst (50%
loading), as compared to HZSM-5 and the others (10% loading). Among the
solid superacids, ZrO2/SO4 proved to be the most effective catalyst and
activity decreased in the following order: ZrO2/SO4>Al2O3/SO4>0.5%
Pt/Al2O3/SO4>Fe2O3/SO4>no catalyst. The products of the TGA system
showed that in the absence of a catalyst, more uniformly distributed long
straight chain aliphatics, including alkenes and alkanes, were produced. The
presence of a catalyst such as Al2O3/SO4, 0.5%Pt/Al2O3/SO4, 0.5%Pt/
ZrO2/SO4, 0.5%Pt/SiO2-Al2O3, and pre-sulfided NiMo/Al2O3 mixed with
SiO2-Al2O3 resulted in the formation of more isomers of aliphatic nature and
revealed that a stronger cracking catalyst produced more lighter aliphatics.
These results support, as discussed above, Ochoa et al. [44], who
also observed an isomerized product over a cracking catalyst and
dominant presence of n-alkanes in the case of thermal hydrocracking
and Venkatesh et al. [86] who suggested that a less active catalyst
produced fewer gases. It was also observed that strong cracking cata-
lysts, e.g., SiO2-Al2O3 and HZSM-5, provided a product that not only
contained light aliphatics, but it also had high yields of substituted
aromatics. The presence of substituted aromatics was explained due to
the occurrence of cyclization reactions on acid catalysts and/or due to
the relatively long residence times available inside the pores of the
catalysts. It was also observed that the effect of catalyst presence and
type on the overall degradation was not important at very high tem-peratures and/or very long residence times.
Ibrahim et al. [90] and Ibrahim and Seehra [91] studied the thermal
and catalytic hydrocracking using ESR (electron spin resonance) spec-
troscopy [92], in which free radicals were detected as they formed.
Elemental sulfur (S), nanoscale Al2O3, NiMo/Al2O3, and HZSM-5 were
used to observe the catalytic effects. HDPE and waste APC plastic
(Table 4) were used and hydrogen pressure of 3.55MPa was set in each
ESR experiment. With waste APC plastic, initially at room temperature,
no ESR signal was detected, showing absence of free radicals. In fact, in
the thermal hydrocracking experiments, no hint of ESR signal appeared
earlier than 360 °C (Fig. 2). However, in the presence of 10%S, 10%
S+10%Al2O3, 10%S+10%NiMo/Al2O3, and 10%HZSM-5, depoly-
merization was detected at 280 °C, 280 °C, 240 °C, and 380 °C, respec-
tively. HZSM-5, therefore, did not reduce the depolymerization tem-
perature and it was reported that nanoscale Al2O3 did not take part in
the depolymerization process. The lower temperature with 10%
S+10%Al2O3 was only due to the sulfur presence, as shown in Fig. 2.
These results agree with the findings of Nakamura and Fujimoto [78],
who proposed that a sulfur compound such as CS2 or H2S acts as a
catalyst and its function is not only the sulfidation of a catalyst. With
HDPE in the presence of 10%S, depolymerization resulted at
Table 3
Proximate and ultimate analyses of comingled waste plastic [62] used by Liu
and Meuzelaar [67].
Proximate analysis Ultimate analysis
Fixed carbon 0.45 Carbon 86.3
Volatile matter 99.7 Hydrogen 14.0
Ash 1.8 Nitrogen 0.05
Moisture 0.06 Sulfur 0.22
Oxygen ‒
Table 4
Analyses of waste APC plastic and waste DSD plastic.
Proximate analysis Ultimate analysis
Waste APC Waste DSD Waste APC Waste DSD
Volatile matter 98.8 93.8 C 84.7 79.0
Fixed carbon 0.74 1.08 H 13.7 13.5
Ash 0.45 4.44 N 0.65 0.67
Moisture 0.01 0.16 S 0.01 0.08
Cl 0.03 1.26
O
(by difference)
0.91 5.49
Fig. 1. High pressure TGA results of the decomposition of commingled waste
plastic (shown in Table 3) in the absence and presence of a catalyst [67]. Used
with permission. Copyright 1996, Elsevier B.V.
D. Munir et al. Renewable and Sustainable Energy Reviews 90 (2018) 490–515
498
approximately 270 °C, close to the value obtained for waste plastic
containing mainly HDPE. With HZSM-5 and NiMo/Al2O3, reduced
concentration of free radicals was observed with an increase in tem-
perature, which showed quenching of the free radicals formed by the
hydrogenation action of these catalysts. On the other hand, it was
proposed that sulfur be used to abstract hydrogen from the hydrocarbon
molecules, to form free radicals responsible for cracking and H2S as a
byproduct. Moreover, in the light of Nakamura and Fujimoto's work on
thermal hydrocracking of PP in the presence of H2S [78], it may be said
that the H2S that is produced as a byproduct also acts as a cracking
catalyst.
Luo and Curtis [68] utilized a series of slurry phase hydrogenation
and cracking catalysts, including Mo-naphthenate + S (1000 ppm Mo
and 6000 ppm S), Fe-naphthenate + S (1000 ppm Fe and 6000 ppm S),
Low Alumina, Super Nova-D, Octacat, Octacat-50G, carbon black, and
montmorillonite clay to test their effectiveness in hydrocracking of
various individual plastics. The properties of some of the FCC (fluid
catalytic cracking) catalysts employed are shown in Table 5. For LDPE
at 400 °C, 5.5 MPa H2, and 30min reaction time, the highest conversion
of 57.9% was reported over Low Alumina catalyst (10% loading),
treated with 5000 ppm Mo-napthenate. For the conditions mentioned,
with carbon black (20% loading and 6.9MPa H2), Super Nova-D (5%
loading), Octacat (5% loading), Octacat-50G (5% loading), Fe-naph-
thenate + S (1000 ppm Fe, 6000 ppm S, 6% loading), and Mo-
naphthenate + S (1000 ppm Mo, 6000 ppm S, 6% loading) the con-
versions were 3.9%, 12.4%, 11.3%, 11.1%, 5.1%, and 5.7%, respec-
tively. For no catalyst conditions only 6.6% conversion was resulted.
With HDPE even lower conversions were obtained in the presence of
slurry phase naphthenate catalysts. In the presence of 10% montmor-
illonite clay only 3.6% conversion was obtained under the same con-
ditions as discussed above for LDPE. The effect of a catalyst was also
studied for PIP (polyisoprene) and PS. For PIP and PS very high con-
versions were obtained with both catalytic and thermal hydrocracking
reactions, although the effect of a catalyst on the conversion under the
conditions studied was difficult to investigate. However, with PS the oil
yield was improved by the addition of a catalyst and the oil yield was
the highest with Mo-naphthenate + S, with least gas yield. For carbon
black with LDPE (6.9MPa H2, 30min, and 400 °C) increasing the cat-
alyst loading from 20% to 40% resulted in only a slight increase in the
conversion, from 3.9% to 4.5%. On the other hand, with Octacat-50G
(5.5MPa H2, 30min, and 400 °C), increasing the catalyst loading from
2% to 5% slightly decreased the conversion from 13.3% to 11.1%.
In another study [69] carried out by the same authors, the hydro-
cracking of a mixture of plastics (50%HDPE, 30% PET, and 20%PS) was
studied in the presence of FCC catalysts such as Low Alumina, Super
Nova-D, and HZSM-5. Generally, both the conversion and oil yield
decreased in the order of HZSM-5, Super Nova-D, and Low Alumina.
Gas yields, however, were virtually unaffected by catalyst type at each
hydrogen pressure studied in their work. As an example, at 440 °C,
5.6 MPa H2 pressure, 10% catalyst, and 30min reaction time, the
conversion was 75.1% over HZSM-5, 73.0% over Super Nova-D, and
68.5% over Low Alumina catalyst. Gas yield and hexane solubles were
also the highest with HZSM-5. This enhanced selectivity of HZSM-5
towards lighter products was also reported by Liu and Meuzelaar [67]
and Ochoa et al. [44]. The increased amount of light product on HZSM-
5 may be explained by its high acidity characteristics and its shape
selectivity (small pore size). However, hydrogenation ability of HZSM-
5, as reported by Ibrahim and Seehra [91], may also contribute in in-
creasing the liquefied products.
Feng et al. [71], while studying the hydrocracking of waste APC
plastic, also found at each temperature that HZSM-5 provided better
gasoline (< 200 °C) and kerosene (200–275 °C) yields compared to
SiO2-Al2O3 catalyst and no catalyst conditions. They also found that in
Fig. 2. ESR spectra for hydrocracking of waste APC plastic under 3.55MPa pressure: a) waste plastic alone, b) waste plastic+ 10%S+nanoscale Al2O3 [90]. Used
with permission. Copyright 1996, Elsevier B.V.
Table 5
Characterization of some of the catalysts used by Luo and Curtis [68] and Luo
and Curtis [69].
Analysis Catalyst
Composition (dry basis, wt%) Low
alumina
Super Nova-
D
Octacat Octacat-50G
SiO2 87.0 55.1 69.4 71.1
Al2O3 12.8 41.8 29.5 28.1
Na 0.05 0.16 0.26 0.35
Fe 0.03 0.47 0.42 0.35
Re2O3 0 2.30 0.25 0.07
Surface area (m2/g) 404 268 261 308
App. bulk density (cm3/g) 0.38 0.77 0.75 0.71
D. Munir et al. Renewable and Sustainable Energy Reviews 90 (2018) 490–515
499
each case, thermal and catalytic, total liquid yield was nearly the same
at 445 °C. At the lower temperature of 430 °C, however, for both the
catalysts, liquid yield obtained improved over that of thermal runs.
Both HZSM-5 and SiO2-Al2O3 offered beneficial effects on oil yield
compared to thermal run, but a slightly lower gas yield was obtained
with thermal hydrocracking at 445 °C. At 445 and 460 °C virtually si-
milar total liquid and oil yields were obtained on each of the two cat-
alysts, but the quality of the oil produced was different in each case, as
observed by simulated distillation. At 430 °C, however, total liquid
yield was much higher for SiO2-Al2O3 than under HZSM-5 or no cata-
lytic conditions.
Taking into consideration that a polymerization catalyst might well
act as a reverse catalyst for the depolymerization of a plastic, Ding et al.
[88] tested TiCl3 (polymerization catalyst)and compared it with HZSM-
5, reported to be a highly selective catalyst by previous researchers
[44,67,68,71]. Ding et al. [88] found that with HDPE, improved con-
version was the result over both the catalysts, compared to the thermal
hydrocracking. However, only HZSM-5 provided a slightly improved
conversion with waste APC plastic. With TiCl3, the conversion was
found to be lower than that obtained with thermal hydrocracking.
Clearly, TiCl3 was not able to resist the impurities present in the waste
plastic. Waste plastic material may have nitrogen, sulfur, chlorine, and
other impurities which may be poisonous [94] or affect the efficiency of
a catalyst. For both the plastics, HZSM-5, although having a slightly
lower loading of 4% than TiCl3 (5%), proved to have better activity to
convert the feed material. It did, however, produce large amounts of
gases and reduced amounts of oil compared to TiCl3. TiCl3 was found to
have better selectivity for the oil products. The reason for the larger
amount of gases produced over the HZSM-5 catalyst was probably high
residence time of 60min in addition to its high activity and small pore
size. With both catalysts, the amounts of olefins were decreased to
negligible compared to thermal hydrocracking, while the amounts of
aromatics were increased. To a large extent, HZSM-5 decreased the
amount of n-paraffins compared to thermal, as well as TiCl3 and pro-
duced a better quality of gasoline containing more naphthenes, iso-
paraffins, and aromatics. The oil obtained over TiCl3 was comprised of
mostly n-paraffins and might need reforming in an additional step. It is
important to mention here that Liu and Meuzelaar [67] also reported
the formation of cyclization products over cracking catalysts, including
HZSM-5.
Ding et al. [93] and Ding et al. [94] extended their work on hy-
drocracking of plastic materials and studied additional catalysts for the
hydrocracking of HDPE, waste APC plastic, and waste DSD plastic
(Table 4). Commercial hydrocracking catalysts, i.e., KC-2600 (possibly
NiMo/SiO2-Al2O3) and KC-2001, NiMo/HSiAl (HSiAl is hybrid support
of HZSM-5 and SiO2-Al2O3 in the ratio of 1:4), and Ni/HSiAl were used
for this purpose. In the absence of a catalyst (thermal hydrocracking)
only a small amount of HDPE was converted at 375 °C, 7.0 MPa H2
pressure, and 60min reaction time. On the other hand, under these
conditions, the presence of each catalyst significantly increased the
conversion. With 40% catalyst in the reaction mixture, HDPE was
converted to 90.0%, 99.3%, 99.6%, and 66.0% over KC-2600, NiMo/
HSiAl, Ni/HSiAl, and HSiAl catalyst, respectively. It is clear that the use
of a hydrogenation function has substantially improved the rate of the
reaction. For each catalyst the oil yield was mainly the same, although
higher gas yields were obtained over Ni/HSiAl compared to KC-2600
and NiMo/HSiAl, for which gas yields were almost the same. For waste
APC plastic, again, Ni/HSiAl was found to be the most active catalyst. It
also produced the maximum oil yield and the lowest oil to gas ratio. For
both types of plastic materials, comparing the use of different hydro-
genation metals, it was observed that Ni/HSiAl had better hydro-
isomerization ability, while NiMo/HSiAl showed better hydrogenation
ability. Generally, increasing catalyst loading from 20% to 40% in-
creased the conversion and gas yields, but decreased the oil yield, while
gas to oil yields were increased considerably. With waste APC plastic,
for Ni/HSiAl catalyst, when the catalyst loading was decreased from
20% to 10%, both the oil yield and conversion were increased. Al-
though not expected, a slightly reduced conversion upon increasing the
catalyst loading was also observed by Luo and Curtis [68]. This in-
creased conversion at lower catalyst loading may be explained on the
basis of improper mixing of reaction contents in a tubing bomb reactor.
Akah et al. [48] supported this reasoning and demonstrated that cata-
lyst effectiveness is greatly improved in the presence of the appropriate
kind of agitator. In a stirred tank reactor, they found that for HDPE at
270 °C and 5.5MPa H2 pressure, a turbine agitator (the type was not
specified, but was possibly straight blade) provided nearly half the
conversion to that obtained with an anchor type agitator. They sug-
gested that better mixing of the reaction contents improve the catalyst
effectiveness and results in the reduction of both the reaction time and
temperature.
Ding et al. [94] also observed that hydrocracking reaction over Ni/
HSiAl catalyst (20% catalyst loading) at 7.0MPa H2 pressure had also
removed impurities such as S and N from the waste APC plastic and
therefore Ni/HSiAl hydrocracking catalyst included the function of
hydrotreating as well. The authors also employed used KC-2600, NiMo/
HSiAl, and Ni/HSiAl catalysts after recalcining and resulfiding and
observed that the conversion, oil yield, and gas yield were unaffected
compared to when fresh catalysts were used. For the waste DSD plastic,
the performance of KC-2001, KC-2600, and Ni/HSiAl catalysts was
compared. Compared to thermal hydrocracking where only ~ 13%
conversion was obtained at 375 °C, 66.7%, 30.9%, and 49.9% conver-
sion was obtained over 40% of each KC-2001, KC-2600, and Ni/HSiAl
catalysts, respectively. Increasing the temperature to 400 °C and de-
creasing half the catalyst loading (20%), the conversion over KC-2001
reached to 78.7% compared to only ~ 36% by thermal hydrocracking.
At 400 °C, over KC-2001, the conversion reached 94.0% for 40% cata-
lyst loading. Although KC-2001 showed improved conversions, oil to
gas ratio was relatively poor (< 1.0) on this catalyst. At 400 °C, KC-
2001 (20% loading) resulted in 38.8% oil and 39.9% gas yield (total
conversion of 78.7%), while KC-2600 gave 48.2% oil and only 15.2%
gas yield (total conversion of 63.4%) under the same reaction condi-
tions.
Rothenberger and Cugini [72] studied the hydrocracking of HDPE
using a molecular sieve (13X) catalyst and AO-60 (an aged Ni-Mo/SiO2-
Al2O3) catalyst. When the molecular sieve catalyst was used higher
hydrocracking rates were observed than those obtained in the absence
of a catalyst (thermal hydrocracking). The conversion at 430 °C (60min
and 7.0MPa H2 pressure) was 72.0% and 56.0% with and without the
use of catalyst, respectively. However using NiMo/SiO2-Al2O3 aged
catalyst (AO-60), the conversion of HDPE was only ~ 18%, even lower
than the thermal reaction. The AO-60 catalyst was therefore highly
inefficient compared to the molecular sieve catalyst. This can be ex-
pected from an aged catalyst which may have lost much of its activity
and the impurities deposited on the catalyst may well hinder the rate of
hydrocracking. The authors, however, suggested that the decreased
activity of the aged catalyst was due to the hydrogenation ability of the
NiMo/SiO2-Al2O3 catalyst, which partly quenched the radicals formed
during the cracking reaction. Contrary to that, Liu and Meuzelaar [67]
and Ding et al. [94] found substantial enhancement in rate compared to
thermal reactions when NiMo/SiO2-Al2O3 catalyst was used. Rothen-
berger and Cugini [72] also performed a single run in the presence of
13X with an H2/CO mixture, at a temperature of 430 °C (60min and
7MPa H2 pressure) for the hydrocracking of a mixture of plastics (50%
HDPE, 35%PS, and 15%PP). They found that very high conversion of
97.0% was possible.
Joo and Curtis [73] in the same group as Luo and Curtis [68], dis-
cussed above, also employed a NiMo/zeolite catalyst (presulfided) and
compared the results with Z-753 (possibly NiW/zeolite) for the hy-
drocracking of LDPE. They found that at a lower temperature (400 °C)
virtually similar results were obtained with both catalysts, while at a
higher temperature, 430 °C, the Mo-containing zeolite catalyst was
more active in converting LDPE to reaction products. Higher gas yields
D. Munir et al. Renewable and Sustainable Energy Reviews 90 (2018) 490–515
500
were also obtained withthe Mo-containing catalyst. Moreover, Joo and
Curtis [73] compared the above two zeolite catalysts with NiMo/Al2O3
and a mixture 10%Z-753–90%NiMo/Al2O3. As expected, the two zeo-
lite catalysts showed much better activity than the Al2O3 based catalyst.
As Al2O3 is only a weak acid compared to a zeolite it is therefore ex-
pected that an Al2O3 catalyst will show less cracking activity than a
zeolite catalyst [42,44,90].
In the same group as Feng et al. [71], mentioned above, Huffman
et al. [95] and Shah et al. [51] compared the catalytic hydrocracking in
a very small amount of catalyst (feed to catalyst ratio as 99:1) with non-
catalytic thermal hydrocracking for the two waste plastic mixtures
(waste APC plastic and waste DSD plastic). For the cleaner mixture
(waste APC plastic) at 445 °C, 60min, and 1.48MPa H2, the oil yields
obtained from the catalytic and thermal runs were generally not dif-
ferent. However, the quality of oil was affected by the addition of even
such a small amount of catalyst. In general, the addition of 1% catalyst
considerably increased the yield of gasoline range products and the
maximum of this occurred with HZSM-5. Gas yields were somewhat
higher for thermal reactions and for the HZSM-5 catalyst. The above
observation for HZSM-5 is in agreement with the view thus far estab-
lished in our discussion. Experiments performed at 435 °C and 1.48MPa
hydrogen pressure revealed that the effectiveness of a catalyst was
dependent on the reaction temperature. At 435 °C and with a shorter
reaction time (30min) the effect of the type of catalyst was more pro-
nounced for both the oil yield and total conversion. For the waste APC
plastic mixture, HZSM-5 was found to be the most active and selective
towards oil yield, with 5%Mo/Ferrihydrite the least active as well as
selective. Apart from HZSM-5 all the other catalysts produced lower
total conversion as well as oil yield than even thermal hydrocracking.
For the dirtier mixture (waste DSD plastic), again the addition of a
catalyst influenced the hydrocracking reaction. The lowest conversion,
as well as the lowest oil yield, was obtained with ZrO2/WO3, while the
highest oil yield was obtained with a TiO2-SiO2 ((Ti/(Ti + Si)) = 0.85)
catalyst. It was concluded by the authors that at high reaction tem-
peratures, the catalyst loading of 1‒5% had a smaller effect on the
quality and quantity of oil produced. Without a catalyst the tempera-
ture of 440–450 °C was found to be appropriate to obtain a high quality
liquid fuel in sufficient quantity. Feng et al. [71] also found that for the
conversion and total liquid and oil yields the addition of a catalyst was
not important at elevated temperatures. This was not the case for the
quality of oil, however. Experiments at lower temperatures and with
higher loadings of catalysts were probably required to have an other-
wise different conclusion. A low value of catalyst loading of only 1% at
elevated temperatures was probably not the way to determine and
compare the effectiveness of different catalysts.
Walendziewski and Steininger [45] used a commercial catalyst
containing NiW+10%HY for the hydrocracking of waste polythene.
They observed insignificant depolymerization at 370 °C and at a low
reaction time (not mentioned) and probably at a catalyst loading of
0.3‒1% (Unfortunately, the conditions of the reactions are not well
documented in the work and there is no thermal hydrocracking reaction
for the comparison with the catalytic hydrocracking). The above ob-
servation may be explained by the fact that the cracking part (HY
support) was only 10% in the commercial catalyst, which significantly
reduced the concentration of the cracking function and thereby de-
creased the cracking ability of the catalyst. Moreover, waste PE might
contain impurities which could significantly affect the performance of
the catalyst. At 410 °C, 75.4% liquid boiling below 360 °C and a large
amount of gas (17.0%) were produced. The liquid product (< 360 °C)
contained 83.0% content boiling< 180 °C and a small amount of
aromatics (9.0%), which can be expected on a cracking catalyst
[67,88,94]. At 400 °C, the gas produced contained a large amount
(86.7%) of saturated light gases such as methane, ethane, propane, and
butane, but only 0.1% of ethylene. This may be due to the hydro-
genation function of NiW which in the presence of hydrogen saturated
any olefins formed during the course of hydrocracking process.
Dufaud and Basset [46] reported the depolymerization of PE and PP
in the presence of hydrogen and an electrophilic Ziegler-Natta catalyst,
i.e., zirconium hydride supported on silica-alumina catalyst and sug-
gested the possibility of reversibility of the Ziegler-Natta polymeriza-
tion. The in-situ formed PE or PP, when heated to 150 °C under
0.040MPa hydrogen pressure, the polyolefin was successfully depoly-
merized and in 15 h completely converted to only methane and ethane.
Experiments were also performed with commercial (C18 to C50) low
molecular weight PE samples and high molecular weight LDPE (M =
125,000 g/mol) and PP (M = 250,000 g/mol). With low molecular
weight PE samples at higher pressure of 0.101MPa H2 and at the same
temperature of 150 °C, it was found that in 62 h all of PE was converted
to light alkanes. The high molecular weight polyolefin samples were
also able to be depolymerized at these very low temperature and low
pressure conditions.
Karagöz et al. [80] used three catalysts, namely, DHC-8 (non-noble
metals loaded on SiO2-Al2O3), HZSM-5, and Co/ACB (activated carbon
made from biomass) to study the hydrocracking of LDPE at 435 °C (10:1
feed to catalyst ratio, 120min, and 6.5MPa H2). On each catalyst under
these conditions, the conversion of plastic was almost completed.
However, a large amount of semi-solid material was obtained with Co/
ACB, which showed it had a decreased ability to crack. For a high
temperature of 435 °C and reaction time of 120min, although every-
thing was expected to be converted, it was found that the highest gas
yield and the lowest liquid yield were obtained on HZSM-5 catalyst,
while the highest liquid yield and the lowest gas yield were achieved
with Co/ACB. The gas yield was 59.6%, 78.5%, and 28.7%, while liquid
yield was 38.7%, 19.1%, and 71.4% on DHC-8, HZSM-5, and Co/ACB,
respectively. The sulfur content in the liquid products resulted over Co/
ACB catalyst was lower than that of HZSM-5 and suggested that the
activated carbon supported metal catalyst was able to capture sulfur
compounds and therefore also acted as a hydrodesulfurization (HDS)
catalyst. It is important to mention here that the regeneration of a used
ACB catalyst may require special procedures, otherwise the carbon
catalyst has to be disposed of, as reported by Nakamura and Fujimoto
et al. [78] for iron supported activated carbon catalysts.
In follow up work to that of Karagöz et al. [80], Metecan et al. [47]
also employed DHC-8 (a commercial hydrocracking catalyst) and
compared it with commercial hydrotreating catalyst (HYDROBON, 18%
NiO/Al2O3) for the hydroliquefaction of HDPE, LDPE, and PP. Gen-
erally, for each plastic, the HYDROBON catalyst resulted in greater li-
quid yield and less gas yield than DHC-8 and thermal hydrocracking.
DHC-8 provided less liquid yield and higher gas yield than even thermal
hydrocracking. DHC-8 was found to be a good catalyst in reducing the
molecular weight of the liquid obtained and provided a lower mole-
cular weight product than both thermal and catalytic hydrocracking
over the HYDROBON catalyst. However, it produced liquid with high
olefinic content, showing high bromine number, higher than even
thermal hydrocracking of PP and in some cases for LDPE. The HYDR-
OBON catalyst resulted in much reduced bromine numbers compared to
DHC-8 and thermal hydrocracking. At a high temperature of 450 °C, all
the properties and product distribution of the obtained liquid were
independent of the effectiveness of the catalyst used; the exception was
bromine number for polyolefins.These findings supported the works of
Huffman et al. [95], Shah et al. [51], Feng et al. [71], and Liu and
Meuzelaar [67] for the waste plastic mixture used in their study. It was
concluded that another hydrogenation step was required when DHC-8
was employed as catalyst for hydroliquefaction of waste plastic,
whereas the naphtha obtained by using HYDROBON from hydro-
cracking of polyolefins was found to be suitable as feedstock naphtha.
The 50–50% mixture of DHC-8 and HYDROBON catalysts was also
studied in order to utilize the beneficial effects of both catalysts. For
HDPE, the liquid and gas yields were similar to that of DHC-8, while for
LDPE and PP the yields were generally in between the yields obtained
independently over the two catalysts.
Ali et al. [96] studied four catalysts, namely, ZSM-5, presulfided
D. Munir et al. Renewable and Sustainable Energy Reviews 90 (2018) 490–515
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NiMo/γ-Al2O3, presulfided DHC-32 (NiW/SiO2-Al2O3), and an FCC
catalyst (metal supported zeolite Y) for the hydrocracking of HDPE,
LDPE, PS, PP, and PET (at 430 °C, 60min reaction time, and 8.3 MPa
H2). The ZSM-5 catalyst was generally the most active and selective
towards oil produced. For the NiMo/γ-Al2O3 catalyst, as Al2O3 support
is considered less acidic compared to amorphous SiO2-Al2O3 and zeo-
lites, one would expect that supported alumina catalyst is not a good
choice for cracking and hydrocracking reactions, as pointed out by
Venkatesh et al. [42] for 1.0%Pt/Al2O3 and Ochoa et al. [44] and
Ibrahim et al. [90] for pure Al2O3. However, the reactions carried out
by Ali et al. [96] are at a relatively high temperature and for a long
period of time, so one can expect high conversion even in the presence
of a less active catalyst. Moreover, as NiMo/γ-Al2O3 was presulfided,
the presence of sulfur may well play a role in showing the cracking
activity [78] for the Al2O3 support.
The above discussion can be extended by looking at the work of
Mosio-Mosiewski et al. [43] who used NiMo/Al2O3 for studying the
hydrocracking of LDPE. The results of Mosio-Mosiewski et al. [43] are
shown in Table 6. It can be clearly seen that overall conversion is vir-
tually the same in both the non-catalytic and catalytic hydrocracking.
As Al2O3 has reduced acidity, it also has less isomerized product and a
considerably reduced ratio of iso-alkanes to n-alkanes. Moreover, the
absence of aromatics also hints at low activity of this type of catalyst
[67,88,94]. However, due to the strong hydrogenation function of
NiMo, as pointed out by Ding et al. [94], negligible amount of alkenes
was found in the reaction products, validated by bromine number of
only 1.5 g Br2/g obtained with catalytic reaction, compared to 68.9 g
Br2/g with non-catalytic reaction. In addition, lower gas yield may also
be an indication of low cracking ability and relatively superior hydro-
genation function of the catalyst.
Hesse and White [33] studied the catalytic hydrocracking of poly-
ethylene by observing temperature-dependent class-specific evolution
profiles obtained from mass spectra. The catalysts employed were bi-
functional hydrocracking catalysts, namely, Pt/HZSM-5, Pt/HY, and Pt/
HMCM-41. As expected, the hydrocracking reactions were found to
accelerate in the presence of a catalyst. It was observed that olefin and
alkyl aromatic yields were significantly decreased when hydrogen was
used in the cracking system, compared to a helium atmosphere. With
both the Pt/HY and Pt/HMCM-41 catalysts, paraffin to olefin ratio was
higher at 98.0, compared to Pt/HZSM-5, when it was only 18.0. With all
the catalysts, the effective activation energy decreased above 240 °C.
The decrease in activation energy was the highest (much greater) with
Pt/HMCM-41 and the lowest with Pt/HZSM-5. It was reported that
catalyst pore size and acidity are the key factors in determining the
amount of volatile components and the activation energies involved.
Following the work of Hesse and White [33], Akah et al. [48] also
studied the hydrocracking reaction over Pt containing zeolite Y catalyst.
They used Pt/USY (hydrocracking catalyst) and HUSY (cracking cata-
lyst) to study the degradation of HDPE. With HUSY, at 310 °C and
5.5MPa H2 pressure, lower conversion, gas yield, and gasoline yield
(higher gas to gasoline ratio greater than unity), and very high coke
contents, greater than three times than that formed over 1.0%Pt/USY,
were obtained. A large amount of coke was expected to be produced in
the absence of a hydrogenation function over a zeolite of large pore size
since aromatics and polyaromatics (coke precursors) can be formed
more easily using a zeolite Y catalyst. At 270 °C, by decreasing Pt
content from 1.5% to 0.5% on USY zeolite, virtually similar conversion,
and gas and liquid yields were obtained with slightly increased content
of coke. This suggests an increased amount of Pt may increase the
stability of the zeolite catalyst by hydrogenation of the coke precursors.
In the conventional catalysts such as microporous zeolites (pore
size< 2.0 nm), the converted products are mainly comprised of gases
and resulted in rather small quantity of liquids, due to the microporous
cavities of the zeolites that hinder the diffusion of larger molecules.
Moreover, they turn out to be insufficient when the reactant molecules
are larger than their pore dimensions, for the same reasons discussed
above. The mesoporous materials (pore size 2 and 50 nm) have ex-
ceptionally large pore dimensions and possess extremely high surface
areas. The pore size of these materials allows large molecules to reach
the active sites present within the pore channels, with reduced diffu-
sional resistance. The converted product consists of an increased
amount of larger liquid molecules and decreased amount of smaller
gaseous molecules. The mesoporous structures generally have weak
hydrothermal stability and reduced or no acidity required for many
industrially important reactions. Therefore, for processing large mole-
cules such as plastic molecules that need to be cracked and hydro-
cracked and to obtain increased liquid product with reduced gaseous
contents, the mesoporous structures need to be improved and the
acidity has to be introduced as desired. Alternatively, micro-mesoprous
composite catalysts are desired where a combination of mesopores and
micropores, together with the stability of microporous zeolites, is
achieved. In these micro-mesoporous composites, the advantage of the
pore structure of zeolites is obtained, along with the presence of desired
mesopores. In the past few years, many researchers have been working
to develop Al-modified mesoporous catalysts [97–104] and micro-me-
soporous catalysts [102,105–122] for the cracking and hydrocracking
of model compounds or heavy crude oil fractions. However, only a few
studies discussed below have been found on the hydrocracking of
plastic materials.
Munir and Usman [123] and Munir et al. [124] studied the hydro-
cracking of a model waste plastic mixture (40%HDPE, 10%LDPE, 20%
PS, and 30%PP) over Al-modified mesoporous silica materials and
micro-mesoporous composite catalysts from zeolite nanoseeds. Two Al-
SBA-16 catalysts with different Si/Al ratio, one Al-SBA-15 catalyst, and
three composite microporous-mesoporous MZ-15, MZ16, and MZ-20
catalysts made with the precursor of SBA-15 and SBA-16, were utilized
for the hydrocracking of the model waste plastic mixture. Under the
conditions of reaction (375–425 °C, 2.0MPa H2 pressure, 20:1 feed to
catalyst ratio, and 50–60min residence time), Al-SBA-15 catalyst was
generally found to be superior to the other catalysts; which was espe-
cially true at 375 °C. At 400 °C and 60min reaction time micro-meso-
porous composite catalysts competed with the performance of the Al-
SBA-15 catalyst. The composite catalyst with the higher mesoporous
content (MZ-16) performed the best among them all. Comparing the
two Al-SBA-16 catalysts (400 °C and 50min), it was observed that the
Table 6
Findings of Mosio-Mosiewskiet al. [43] for thermal and catalytic hydrocracking
of LDPE. T=420 °C, t=120min, H2 pressure at the start of reaction at
420 °C= 10MPa, feed to catalyst ratio = 49:1.
Results Catalytic hydrocracking
(Ni-Mo/Al2O3 catalyst)
Thermal
hydrocracking
Reaction products (wt%)
Gas products 5.9 8.6
Gasoline 10.0 14.5
Fuel oil 26.2 34.2
Vacuum gas oil 29.1 24.2
Vacuum residue 28.9 18.5
Reactions products (wt%)
n-alkane 79.9 39.5
Naphthenes 1.6 17.3
Iso-alkanes 13.1 22.8
Alkenes 0.0 15.1
Aromatics 0.0 0.6
Unidentified 5.4 4.7
Bromine number (units)
Liquid < 1.0 23.4
Gasoline 1.5 68.9
Diesel oil < 1.0 26.2
Vacuum gas oil 2.1 12.3
t= time, min; T=temperature, °C.
D. Munir et al. Renewable and Sustainable Energy Reviews 90 (2018) 490–515
502
one with higher Si/Al ratio, larger pore size, and slightly enhanced pore
volume resulted in an increased amount of oil, with a corresponding
decreased amount of gas yield for virtually the same conversion value.
Similar findings were obtained when two micro-mesoporous catalysts
were compared at 400 °C (60min).
2.1.1.1. Summary. The presence of a catalyst is useful in decreasing the
temperature as well as time required for the hydrocracking reaction.
The catalyst is desired in order to control the gas, oil, and total liquid
yields. Also, the quality of these products including the oil fraction can
be significantly improved by the use of a suitable catalyst. A catalyst
that has a weak acidic function, such as having alumina or silica as a
support, or a weak hydrogenation-dehydrogenation function (metal)
such as Fe loading, or both, is unable to crack the plastic material at low
temperatures. For hydrocracking reaction to occur at low temperatures,
a supported metal catalyst with a strong acidic function as well as a
strong hydrogenation-dehydrogenation function is useful. The strong
acidic function is offered by the support such as sulfated zirconia,
zeolite, or amorphous silica-alumina, while the strong hydrogenation-
dehydrogenation function is provided by a metal such as Pt. However,
due to the loss of sulfate ions during the course of the hydrocracking
reaction, zeolite and amorphous silica-alumina are the only choice as
strong acidic support. Deactivation behavior of Pt containing catalysts
for waste plastic hydrocracking is, unfortunately, not documented. A
strong acidic function increases the yield of gaseous products and
strong hydrogenation-dehydrogenation function may quench the free
radicals and therefore negatively affect the hydrocracking reaction.
Increased cracking ability of a catalyst may easily increase the amounts
of aromatics by cyclization reactions. A metal function, together with
acidic function, may produce a large amount of aromatics by
dehydrocyclization reactions. A strong hydrogenation function gives
rise to the reduced coke formation by hydrogenating the coke
precursors and enables production of greater amounts of useful liquid
product, i.e., oil by quenching the free radicals to hinder the additional
cracking which may cause excessive gas formation. Strong metal
function is also useful in producing a reduced amount of olefins
(lower bromine number) and increased amount of branched products,
by initializing the formation of alkenes, which can be isomerized
on the associated acidic support. At elevated temperatures, the
depolymerization is not a function of catalyst activity, the conversion
and oil yield are not affected in the presence of a catalyst, although the
quality of the oil may be influenced by the type of catalyst used. High
catalyst loading increases the depolymerization, but also increases the
gas yield. Elemental sulfur and sulfur compounds also act as catalyst
and their objective is not only the sulfidation of a catalyst.
A catalyst with both acidic and hydrogenation-dehydrogenation
function is required. For waste plastic hydrocracking the desired cata-
lyst is one which also has hydrotreating ability or which is unaffected
by the presence of impurities in waste plastics. Deactivation of catalysts
such as those which contain Pt needs to be evaluated before they are
used for commercial applications. If a suitable catalyst required to cope
with the impurities present, is not available, thermal hydrocracking of a
waste plastic material may be warranted. A moderate concentration of
an appropriate catalyst in the range of 3–10% is recommended.
2.1.2. Effect of hydrogen pressure
The presence of hydrogen may reduce the formation of free radicals
responsible for the cracking and overcracking of a hydrocarbon mole-
cule. Also, hydrogen may saturate the olefinic intermediates and ole-
finic reaction products, to produce an increased amount of saturated
hydrocarbons and reduced content of coke formed. It may also enhance
the activity of a catalyst during the course of the reaction by renewing
the active sites of a catalyst. Hydrogen pressure is therefore an im-
portant factor in determining the conversion and yield and quality of
the products formed and in maintaining the activity of a catalyst.
Venkatesh et al. [42] studied the hydrocracking of HDPE at 375 °C
in the presence of 0.5%Pt/ZrO2/SO4 catalyst (16.7% catalyst loading
and 25min reaction time) to observe the effect of hydrogen pressure on
the conversion and yields of the products. No significant effect was
observed on the conversion of HDPE. However, lower hydrogen pres-
sure of 5.27MPa yielded more liquid product (79.8%) with corre-
sponding lower gas yield (19.2%), compared to a higher hydrogen
pressure of 8.38MPa that yielded 69.0% of oil and 30.0% of gas.
Moreover, a change in hydrogen pressure showed a slightly different
boiling range of the liquid product, where higher hydrogen pressure
favored the production of lighter hydrocarbons. While working on the
hydrocracking of MDPE over SiO2-Al2O3 and HZSM-5, Ochoa et al. [44]
found that to some extent, the hydrogen pressure increased the yield of
oil. They observed nearly 10% increase in pentane solubles when hy-
drogen pressure was increased from 1.4MPa to 5.5 MPa (420 °C,
15min, 9:1 feed to catalyst ratio). However, they observed a negligible
increase in oil yield in thermal hydrocracking (in the absence of a
catalyst) for the same increase in hydrogen pressure.
Unlike Venkatesh et al. [42] and Ochoa et al. [44], Ding et al. [94],
Luo and Curtis [69], and Akah et al. [48] carried out the hydrocracking
reaction on more than two hydrogen pressures. Ding et al. [94] ob-
served that the hydrocracking reaction was greatly influenced initially,
while the effect was decreased with further increase in hydrogen
pressure. Their results for the effect of hydrogen pressure on the con-
version and yields of gas and oil are shown in Fig. 3. It can be observed
that with an increase in hydrogen pressure from 1.83MPa to 5.27MPa,
the conversion of waste APC plastic using Ni/HSiAl catalyst at 375 °C
increased from 84.9% at 1.83MPa to 98.9% at 5.27MPa. Higher hy-
drogen pressure had shown no significant effect on the conversion. The
oil yield tended to follow the path of conversion, while gas yield vir-
tually remained the same. Generally, at higher hydrogen pressure, the
amounts of n-paraffins, isoparaffins, and cycloparaffins were increased,
while the quantities of olefins and aromatics were decreased. The re-
sults led the authors to conclude that hydrogenation reactions of olefins
and aromatics were improved at high hydrogen pressure to produce
more n-paraffins and cycloparaffins.
Contrary to Ding et al. [94], Luo and Curtis [69], for the hydro-
cracking of a plastic mixture (50%HDPE, 30%PET, and 20%PS), ob-
served that with an increase in hydrogen pressure from 2.3 to 8.6MPa,
in general, the overall conversion and the yield of oil initially decreased
and then increased, while the gas yields remained relatively unaffected.
As an example, for the hydrocracking reaction over HZSM-5 at 2.3, 5.6,
and 8.6MPa H2 pressure (440 °C, reaction time of 30min, and feed to
catalyst ratio of 9:1), the conversions were 87.8%, 75.1%, and 82.5%,
respectively,whereas the yields of hexane solubles were 71.1%, 57.9%,
and 62.4% and gas yields were 10.0%, 10.9%, and 11.4%, respectively.
Fig. 3. Effect of hydrogen pressure on hydrocracking of waste APC plastic over
Ni/HSiAl (27 cm3 reactor, 375 °C, 60min, feed/catalyst = 4.0:1.0) [94].
Adapted with permission. Copyright 1997, American Chemical Society.
D. Munir et al. Renewable and Sustainable Energy Reviews 90 (2018) 490–515
503
Akah et al. [48] studied the hydrocracking of HDPE at three different
hydrogen pressures (1.5, 3.5, and 5.5MPa) at 270 °C, using 1.0%Pt/
USY catalyst.
Similar to Ding et al. [94], Akah et al. [48] observed a greater
change in rates of hydrocracking at lower pressures than at higher
hydrogen pressures. With an increase in hydrogen pressure
from 1.5MPa to 3.5 MPa (270 °C, 5.0min, 10:1 catalyst to feed ratio),
a substantial increase in conversion (45.3–90.0%), gas yield
(10.3–29.4%), and gasoline yield (20.0–53.1%) was observed. With
further increase in pressure from 3.5MPa to 5.5MPa, the conversion,
gas and gasoline yields virtually remained constant. Akah et al. [48]
also measured the yield of coke produced and found that the coke
content decreased continuously with the increase in hydrogen pressure.
With an approximate four-fold increase in hydrogen pressure, they
observed that there was almost a three times decrease in coke content.
This suggests that higher hydrogen pressures are required to quench the
coke precursors.
For the hydrocracking of PS, Zmierczak et al. [20] reported the
effect of final hydrogen pressure in the range of 3.55–17.34MPa. The
PS conversion was found to change rather linearly with the final hy-
drogen pressure. Although the conversion varied noticeably, the pro-
duct distribution was not significantly affected.
2.1.2.1. Summary. The effect of hydrogen pressure on hydrocracking of
waste plastic is studied by only a few researchers. The results of their
work have pointed out that hydrogen pressure does have an effect on
the conversion and liquid yield, but this effect is minimized as hydrogen
pressure is increased. However, the quality of the fuel obtained is
improved at higher hydrogen pressures. Increasing hydrogen pressure
may be useful in removing impurities from waste plastics and coke
formation.
It may be concluded, therefore, that a moderate cold hydrogen
pressure of 2.0‒6.0MPa is the optimum for the hydrocracking of waste
plastics.
2.1.3. Effect of reaction time
Reaction time given to a plastic feed during a hydrocracking op-
eration is another vital factor that decides the extent of degradation and
quality and yield of the gas and liquid products. An optimized reaction
time is therefore required to be worked out to have high conversions,
high oil to gas ratios, and superior quality motor fuels.
Venkatesh et al. [42] observed a negligible effect of reaction time
from 20 to 60min on the conversion of PP over 0.5%Pt/ZrO4/SO4
catalyst (325 °C, 8.38MPa H2, 5:1 catalyst to feed ratio). However, they
found the selectivity of the liquid product was greatly influenced by the
reaction time. For the given reaction conditions, the selectivity to
gaseous products was almost 100% at 60min, whereas at 20min the
liquid yield was about 90.0%. These results show that liquid yield
substantially decreases, while the gas yield significantly increases with
reaction time. In another study over the same catalyst, Venkatesh et al.
[86] studied the effect of reaction time on the hydrocracking of HDPE.
Similar results to the above were found at both 25 and 60min reaction
times. In comparison to Venkatesh et al. [42], Shabtai et al. [89] found
that increasing reaction time increased the conversion even beyond
60min, while working on the hydrocracking of PP on ZrO2/SO4 catalyst
(no Pt metal function) at 410 °C, 10.44MPa H2, and feed to catalyst
ratio as 100:1. They observed that initially the conversion increased
rapidly (at low reaction times) and then remained virtually constant,
i.e., the conversion increased from 87.1% at 30min, to 97.8% at
60min, reached 99.9% and 100% at 120min and 180min, respectively.
The obvious reasons for the difference between the findings of
Venkatesh et al. [42] and Shabtai et al. [89] are the low catalyst loading
of only ~ 1% used by Shabtai et al. [89] in contrast to 16.7% by
Venkatesh et al. [42] and the absence of a strong hydrogenation-de-
hydrogenation metal in the work of Shabtai et al. [89]. The importance
of strong metal function together with strong acidic function in
improved hydrocracking has already been discussed in Section 2.1.2.
The increased catalyst loading and strong metal function considerably
reduced the reaction time for Venkatesh et al. [42], although the re-
action temperature was 85.0 °C less in their work. Shabtai et al. [89]
also studied the hydrocracking of HDPE and, similar to the work of
Venkatesh et al. [42,86] for PP and HDPE hydrocracking, found the
conversion remained virtually the same, the gas yield increased, and
the corresponding liquid yield decreased with reaction time. The yield
of gasoline range product (C5–C12), on the other hand, first increased
and then decreased with the increase in reaction time. In the same
group as Shabtai et al. [89], for the hydrocracking of PS over ZrO2/SO4
catalyst (20:1 feed to catalyst ratio, 375 °C, and final H2 pressure of
10.44MPa), Zmierczak et al. [20] also found an increase in conversion
with an increase in reaction time, where the rate of hydrocracking in-
creased more steadily with time compared to the work of Shabtai et al.
[89]. They observed 43.0%, 62.0%, 77.0%, and 96.0% conversions at
15, 30, 60, and 120min, respectively. Moreover, they observed that
generally light gases, benzene, alkylbenzenes, and naphthalenes in-
creased with reaction time, while di- and triphenylalkanes generally
decreased.
Ochoa et al. [44] worked on the catalytic degradation of MDPE at
420 °C at 5.5MPa of H2 pressure in the presence of various silica-alu-
mina catalysts and HZSM-5 catalyst. As shown in Fig. 4, they found that
increasing reaction time increased the yield of oil produced. The yield
increased rapidly at the start and over a few silica-alumina catalysts
reached to nearly 80% in 15min, thereafter increasing only slightly.
The figure suggests that for the hydrocracking reactions in the presence
of SiO2 catalyst and under no catalyst condition (thermal hydro-
cracking), longer residence times are required to obtain the same oil
yield. These results are in disagreement with those obtained by Shabtai
et al. [89], who observed a constant decrease of hexane solubles with
time.
Joo and Curtis [73] and Ali et al. [96] at 430 °C also observed that
yield of hexane solubles increased with reaction time and appeared to
be doubled when reaction time was increased from 30 to 60min. The
findings of Shabtai et al. [89], where oil yield continuously decreased
(beyond 30min), may be explained on the basis that at a very high
temperature of 450 °C, the transition state beyond which the oil yield
continuously decreased might have already been reached. Moreover,
Joo and Curtis [73] and Ali et al. [96] also observed an increase in
conversion and gas yield with time (at 430 °C), as suggested by Shabtai
et al. [89]. At 400 °C, Joo and Curtis [73] observed unusual behavior at
this lower temperature compared to 430 °C. The conversion decreased
with increasing time for each of the catalysts studied, the gas yield
remained constant with time (30–60min), while oil yield deceased.
This behavior looks suspicious and suggests that the results at 30 min
Fig. 4. Effect of reaction time on oil yield for various catalysts employed by
Ochoa et al. [44]. Adapted with permission. Copyright 1996, Elsevier B.V.
D. Munir et al. Renewable and Sustainable Energy Reviews 90 (2018) 490–515
504
may be mistakenly written for those at 60min and vice versa, otherwise
some extraordinary explanation is required.
Ding et al. [94] and Shah et al. [51] studied the effect of reaction
time on the hydrocrackingof waste APC plastic and waste DSD plastic,
respectively. Ding et al. [94] studied the hydrocracking reaction over
Ni/HSiAl catalyst at 375 °C and 7.0MPa H2 with feed to catalyst ratio
fairly high at 4:1. Their results for the effect of time are shown in Fig. 5.
The figure shows that increasing reaction time increases the conversion,
oil yield, and gas yield. The conversion and oil and gas yields initially
increased slowly with time, increased more rapidly between 15 and
30min, and beyond 30min again slowly increased. Virtually complete
conversion was obtained at 60min. As the reaction temperature used by
Ochoa et al. [44] was much higher than that used by Ding et al. [94], at
a lower temperature, the cracking reactions were initially sluggish and
therefore the rate of hydrocracking to oil yield was not as fast as ob-
served by Ochoa et al. [44]. Shah et al. [51] studied the hydrocracking
reaction at 30 and 60min with and without the addition of HZSM-5
catalyst. In both cases, in the temperature range of 415–445 °C the oil
yield was increased, with an increase in reaction time from 30 to
60min. However, at 445–455 °C, the oil yields were similar for each
time. In the temperature range of 415 °C to approximately 430 °C, the
effect of time was more pronounced for the catalytic process than the
thermal process.
Palmer et al. [65] studied the conversion of PP and PE (the type of
PE is not mentioned) in the presence of hydrogen (3.55MPa), but in the
absence of a catalyst. It was observed by the authors that increase or
decrease of PP or PE conversion with time depends upon the reaction
temperature. At a lower temperature of 425 °C, increasing time from
5min to 60min increased the conversion for both PP and PE almost
four-fold. At higher temperatures of 475 and 525 °C, the conversion of
PP decreased with increasing time, while for PE at 475 °C it remained
approximately the same. This unusual observation is in contrast to the
common understanding by which increasing reaction time should in-
crease the conversion of an irreversible (cracking) reaction. However,
this observation may be explained by the fact that the coke formed
during the hydrocracking reaction was not considered when calculating
the conversions. It is expected that a large amount of coke was pro-
duced at these high temperatures, especially in the absence of a hy-
drogenation catalyst, which remained insoluble in THF and increased
the amount of solid residue.
Ramdoss and Tarrer [125] also studied thermal hydrocracking, but
they reported the conversions based on unreacted plastic after re-
moving coke from the solid residue. For the waste plastic mixture
(waste APC plastic) studied, the authors found that at each tempera-
ture, increasing reaction time increased the conversion of waste plastic
material. It was observed that the change in the rate of conversion with
time was initially (in 5min) very high and later remained virtually
constant with time. The gas yield continuously increased, but the heavy
oil yield continuously decreased with reaction time. For light oil, the
yield usually increased to begin with and then decreased with time.
These results are in line with the findings of Shabtai et al. [89]. In-
creasing reaction time also increased the coke production. As an ex-
ample, nearly 8.0% coke was formed at 30min at the highest tem-
perature of 525 °C.
2.1.3.1. Summary. Increasing reaction time increases the plastic
conversion. The gas yield also increases with reaction time. The
conversion of a plastic material, however, reaches to completion
earlier than the maximum of gas yield. For moderate reaction times,
at a relatively high temperature and/or over an active catalyst, the oil
yield increases sharply with time and then virtually remains constant,
while at a relatively low temperature and/or a less active catalyst, oil
yield increases slowly with time and may reach a constant value. Heavy
liquid yield constantly, but slowly decreases with time and eventually
all of the total liquid (oil plus heavy liquid) may end up in the gas phase
at very long residence times. The time at which the maximum for oil
yield occurs depends upon the reaction conditions and the plastic used.
For an active catalyst at moderate temperature conditions, the re-
action time of greater than 60min is not recommended.
2.1.4. Effect of reaction temperature
Hydrocracking temperature is one of the most important factors that
can bring about considerable change in the conversion, product yield,
and quality of the oil produced. A large number of researchers have
studied the effect of temperature over a wide range to observe how this
influences a hydrocracking reaction.
While studying hydrocracking of HDPE in the presence of 0.5%Pt/
ZrO2/SO4 catalyst (16.7% loading) Venkatesh et al. [42] found that the
higher temperature of 375 °C resulted in higher conversion of 99.0%
(25min) as compared to the hydrocracking reaction at 325 °C (con-
version only 25%) under the same hydrogen pressure of 8.38MPa and
even for a longer period of reaction time (60min). Moreover, improved
selectivity regarding liquid products was obtained at the higher tem-
perature, although this might be due to longer residence time at the
lower temperature. The authors, however, suggested that the reason for
higher yields of gases at low temperature was the poor mass transfer
rates at these conditions, which caused additional cracking of any liquid
product formed initially.
Nakamura and Fujimoto [78] studied the hydrocracking of PP at
two levels of temperature, 380 °C and 400 °C. With and without the
addition of a catalyst, the conversion and the total liquid yield in-
creased with the higher temperature. The gas yield also increased for
the catalytic reaction, but remained virtually unaffected for thermal
hydrocracking. It was observed that the liquid yield increased more
rapidly than the gas yield. For the catalytic reaction, the reason for the
lower conversion at the lower temperature was related to the pore
diameter of the activated carbon involved. It was proposed that the PP
molecules or initially cracked fragments were too large and were,
therefore, unable to be cracked due to the small pore size of the acti-
vated carbon catalyst. However, at the higher temperature, increased
thermal cracking resulted in the formation of smaller molecules which
were easier to be catalytically hydrocracked in the pores of the acti-
vated carbon.
In the same group of researchers, Luo and Curtis [68] and Joo and
Curtis [73] also found that increasing the temperature increased the
degradation rate. For the hydrocracking of LDPE over Low Alumina
catalyst, Luo and Curtis [68] found that increasing the temperature
from 400 °C to 440 °C to some extent doubled the conversion, while Joo
and Curtis [73] found that the lower temperature of 400 °C was not as
effective for the conversion of LDPE over NiMo/Al2O3 or the Z-753
catalyst, compared to when the higher temperature of 430 °C was used.
Joo and Curtis [73] also found that the gas and oil yields increased with
Fig. 5. Effect of reaction time on the conversion, oil yield, and gas yield in the
hydrocracking of waste APC plastic as studied by Ding et al. [94]. Adapted with
permission. Copyright 1997, American Chemical Society.
D. Munir et al. Renewable and Sustainable Energy Reviews 90 (2018) 490–515
505
the higher temperature. Also, for the LDPE hydrocracking, Ali et al.
[96] raised the temperature from 400 to 430 °C over ZSM-5 and NiMo/
γ-Al2O3 catalysts and in both cases observed a substantial increase in
the conversion. Again, the gas yield and oil yield were also improved
through the use of both catalysts.
Like Luo and Curtis [68] and Joo and Curtis [73], Ochoa et al. [44]
also observed an increase in the hydrocracking effect with an increase
in temperature. They observed more than a 50% increase in oil yield
over silica-alumina catalysts for the degradation of LDPE used in their
study, when temperature was raised from 380 °C to 420 °C. Moreover,Rothenberger and Cugini [72] and Ding et al. [94] also found an in-
crease in conversion with temperature. Rothenberger and Cugini [72]
found, for the thermal and catalytic hydrocracking of HDPE and a
mixed plastic feed (50%PE, 35%PS, and 15%PET), respectively, an in-
crease in conversion, as expected, when temperature was increased
from 430 to 445 °C. The gas (C1–C4) yield also increased with the rise
in temperature. For the waste APC plastic, Ding et al. [94] also found
that raising the temperature from 350 °C to 375 °C dramatically in-
creased the conversion for both the catalytic and non-catalytic condi-
tions. The oil yield and the corresponding gas yield were also appre-
ciably increased. In an another study, Ding et al. [93] found similar
findings for the hydrocracking of waste DSD plastic over KC-2001 (40%
loading, 60min, and 7.0MPa H2 pressure) and observed the conversion
increased 66.7–94.0% when temperature was increased from 375 °C to
400 °C.
Orr et al. [63], Palmer et al. [65], Zmierczak et al. [20], Feng et al.
[71], Shabtai et al. [89], Ding et al. [88], Ding et al. [93], Ramdoss and
Tarrer [125], Shah et al. [51], Walendziewski and Steininger [45],
Metecan et al. [47], and Akah et al. [48] studied the hydrocracking
reaction at more than two temperatures, i.e., they studied the effect of
hydrocracking temperature on the variation in change in the rate of the
hydrocracking reaction. Orr et al. [63] hydrocracked the waste APC
plastic in the absence of a catalyst at 400–430 °C. They observed that
the conversion rapidly increased from 400 to 430 °C and then virtually
remained constant. The gas yield slowly increased with temperature
from 400 to 430 °C and then increased more rapidly from 430 °C to
480 °C. The liquid yield, on the other hand, first increased from 400 to
430 °C and then decreased rapidly to reach ~ 6.0% at 480 °C.
Palmer et al. [65] also studied the thermal hydrocracking of a few
plastic materials in the temperature range of 425–525 °C. At 5min re-
action time, the conversion of PE increased continuously with the rise
in temperature, while for PP it first increased and then decreased with
the change in temperature. At 60min time, however, the conversion of
both PE and PP decreased with the temperature rise. These observations
may be explained on the basis that, as expected at these high tem-
peratures, the formation of a large amount of coke (that is a part of solid
residue) remained unaccounted for and therefore resulted in such bi-
zarre observations.
In contrast to Palmer et al. [65], Ramdoss and Tarrer [125] sup-
ported the work of Orr et al. [63] to a certain extent and observed that
for the thermal hydrocracking of a waste plastic mixture, increasing the
temperature rapidly increased the conversion (based on unreacted
plastic), which increased greater than 90% at 475 °C and then slowly
increased towards completion at 525 °C (97.0%). For any specific time,
the conversion, gas yield, and coke formation increased with rise in
temperature, whereas total oil yield decreased.
Zmierczak et al. [20], over a Fe2O3/SO4 catalyst, observed a pro-
found effect of temperature on the conversion of PS in the low tem-
perature range (350–400 °C), whereas a negligible effect on the con-
version was observed at higher temperatures (425–450 °C); an
observation similar to that made by Orr et al. [63] and Ramdoss and
Tarrer et al. [125] for thermal hydrocracking of waste APC plastic.
Although the effect of temperature was diminished at higher tempera-
tures, the quality of the product was continuously changed. The con-
centration of benzene plus alkylbenzenes continuously increased, while
the yield of diphenylalkanes and triphenylalkanes plus others
continuously decreased and almost reached to zero at 450 °C.
Feng et al. [71] also employed the same plastic material as used by
Orr et al. [63] and observed the effect of temperature at three levels.
Increasing the temperature tended to sharply increase the total liquid
yield as well as oil yield when it was increased from 430 to 445 °C.
When the temperature was increased from 445 to 460 °C there was no
effect on the total liquid and oil yields, apart from thermal runs in
which oil yield was slightly higher at 460 °C compared to 445 °C. Ga-
soline yield (BP<200 °C) generally increased with increasing tem-
perature, apart from when using a SiO2-Al2O3 catalyst, in which case it
decreased slightly from going 445–460 °C. Again, the effect was not
significant beyond 445 °C (except for thermal) and it was suggested that
445 °C should be the maximum temperature of operation.
In their study of HDPE, LDPE, and PS hydrocracking over various
catalysts, Shabtai et al. [89] agreed with those of Zmierczak et al. [20]
that initially at lower temperatures the conversion increased more ra-
pidly, but the effect of temperature was diminished at higher tem-
peratures. As an example of HDPE, the conversion increased from
82.5% at 420 °C to 98.9% at 435 °C and then reached 100% at 450 °C
and 465 °C. Increasing the temperature increased the gas yield, but
decreased the oil yield for each of the plastics used. However, gasoline
range product obtained from HDPE and PP first increased and then
decreased with an increase in temperature. This shows that the tem-
peratures used by Shabtai et al. [89] were high enough for the catalyst
employed, in that the oil fraction had started to convert to coke and
gaseous components. Orr et al. [63] had also observed the same out-
comes for high temperature thermal hydrocracking. A clue can also be
obtained from the work of Palmer et al. [65] for high temperature non-
catalytic hydrocracking. For the gasoline yield the results supported the
work of Feng et al. [71].
Ding et al. [88] investigated the effect of temperature on HDPE and
waste APC plastic material at three levels of temperature, 400, 420, and
430 °C, both in the presence and absence of a catalyst. In each case,
raising the temperature increased the conversion and the gas and oil
yields were also increased with the rise in temperature. In contrast to
the previous works of Zmierczak et al. [20] and Shabtai et al. [89], with
the increase in temperature, the conversion tended at first to increase
slowly and then increased more rapidly. The gas and oil yields followed
the same pattern. This initially reduced rate of hydrocracking compared
to the work of Zmierczak et al. [20] and Shabtai et al. [89] may be due
to the decreased lower temperatures used for HDPE, which is difficult to
degrade at lower temperatures.
In other work by Ding et al. [93], as discussed before, a study was
carried out on the effect of temperature on thermal hydrocracking of
waste DSD plastic. Increasing temperature from 375 °C, the conversion
increased rapidly upto 450 °C and then rather slowly till 480 °C. This is
in line with Zmierczak et al. [20] and Shabtai et al. [89]. Gas yield
increased slowly in the temperature range of 375–430 °C and then in-
creased more rapidly afterwards. On the other hand, oil yield increased
with the temperature rise till 430 °C and followed the same pattern as
the conversion. It then increased slightly, before sharply decreasing to
~ 46% at 480 °C. The decrease in the oil or liquid yield at high tem-
perature (or on a very active catalyst or both), depending upon the
plastic feed, has been reported by other researchers [63,89,125]. The
quality of the oil yield was also found to be dependent on temperature.
At 430 °C and 450 °C a similar type of oil was produced. At a higher
temperature (480 °C), however, a much lighter liquid was obtained. The
temperature of 450 °C was suggested as the optimum for thermal hy-
drocracking. Shah et al. [51] also used the waste DSD plastic and found
that oil yield increased with increasing temperature for both catalytic
and non-catalytic processes. For higher reaction time (60min) the oil
yield increased from 415 °C to 435 °C, beyond which it stayed at the
same level for both the catalytic and non-catalytic processes. However,
forthe lower reaction time (30min) the effect of temperature on in-
crease in oil yield appeared for a much higher temperature (445 °C),
after which the yield remained constant.
D. Munir et al. Renewable and Sustainable Energy Reviews 90 (2018) 490–515
506
Walendziewski and Steininger [45] observed that with an increase
in hydrocracking temperature of the waste polyethylene from 370 to
410 °C, the conversion to gas and liquid products increased progres-
sively, after which it remained almost constant, showing that increasing
temperature beyond ~ 415 °C did not provide any beneficial effects.
The gas yield continuously increased with temperature, but the liquid
yield (BP<360 °C) first increased from 370 °C to 390 °C and then de-
creased with temperature. It was found that the optimum temperature
for hydrocracking was 390 °C, which resulted in the attainment of
conversion higher than 90% and the highest liquid to gas ratio.
Metecan et al. [47], for a part of their experimental runs with HDPE,
LDPE, and PP, noticed that the conversion first increased and then
decreased with temperature, while for PP in some cases it first de-
creased and then increased with temperature. Also, in a few cases it
either increased continuously or decreased continuously with tem-
perature. This shows every possibility. The authors also observed that
increasing temperature increased the gas yield, but decreased the liquid
yield both in the presence and absence of a catalyst, apart from PP
thermal hydrocracking, in which the gas yield first increased and then
decreased with temperature. The quality of the products was also af-
fected by temperature. Increasing the temperature generally produced a
lighter liquid and increased the yield of the naphtha range product.
Molecular weight of the liquids obtained decreased with temperature,
while the bromine number remained nearly the same, except for PP, in
which case it first increased and then decreased with the increase in
temperature. Moreover, methane and ethane yields also tended to in-
crease with higher temperatures.
Akah et al. [48] studied the effect of temperature on HDPE hydro-
cracking over 1.0%Pt/USY catalyst at 270, 310, and 350 °C. Increasing
temperature from 270 to 350 °C resulted in a general increase in both
the gas yield and conversion, while the gasoline yield and coke for-
mation decreased with increasing temperature. The gas composition
(mainly C4 and negligible C1 and C2), however, remained unaltered
with the change in temperature.
Munir and Usman [123] and Munir et al. [124] observed the effect
of temperature on the hydrocracking reaction using Al-modified SBA
catalysts and micro-mesoporous composite catalysts. They found that at
lower reaction temperatures of 375 °C and 400 °C, as expected, the
performance of all the catalysts towards conversion was generally
better than the thermal run. At 375 °C, Al-SBA-15 gave the highest
conversion and liquid yield with the least yield of gaseous product
compared to the other catalysts and the thermal run. On the other hand,
at 400 °C the catalytic reactions were most favored over the MZ-16 (a
micro-mesoprous catalyst with Si/Al ratio of 21, made with a precursor
of SBA-16) with respect to both activity and selectivity of the reaction.
This suggests the zeolite nanoseeds introduction into the mesos-
tructures resulted in increased activity and selectivity of composite MZ-
16, compared to its mesoporous Al-SBA-16 at the temperature of
400 °C. It was also observed that the zeolite building block introduction
into the mesostructures generally resulted in increased gaseous yield of
the composite catalysts, compared to their mesoporous counterparts.
When the reaction temperature of 425 °C was studied, the thermal run
was found to be the most active and produced a high oil yield. This
might occur due to the increased formation of coke, which may result
over an active catalyst, especially one with large pores and at relatively
high temperature of 425 °C. As the conversion was based on THF in-
soluble solid residue that included coke, the apparent conversion, as
discussed above, might decrease with an increase in temperature. It was
observed that the effect of the presence and type of catalyst generally
decreased with an increase in temperature.
2.1.4.1. Summary. In general, increasing the temperature results in an
increase in conversion. A decrease in apparent conversion with
temperature may be due to an increased amount of coke formed at
higher temperatures. Degradation of a plastic material occurs rapidly
with initial increase in temperature and then slowly decreases at high
temperatures to reach to completion. However, with relatively less
active catalysts, the rate of conversion may be lower initially, then it
will rise with the increase in temperature, and slows down again near
the completion. The yield of gases continuously increases with
temperature, and may increase slowly at the start and then more
rapidly at high temperatures. Oil and total liquid yields increase with
the temperature rise, but decrease at higher temperatures as they are
converted to coke and lighter products. Total liquid yield starts
disappearing at lower temperatures compared to oil yield.
For an active catalyst and a moderate reaction time a temperature
ranging between 370–400 °C is recommended.
2.1.5. Effect of the use of different types of plastics
The quality and quantity of the oil obtained depend on the type of
plastic used and the conditions in which the plastic is treated. In the
literature various types of plastic materials are employed by a variety of
researchers to compare the effect of hydrocracking variables on the
conversion, oil and gas yields, and quality of oil produced. Both virgin
and waste plastic materials are used for this purpose.
Venkatesh et al. [42] compared the hydrocracking reaction of
HDPE, PP, and PS over two metal supported sulfated zirconia catalysts.
They observed that PP and PS were completely converted at a tem-
perature much lower than that required for the complete conversion of
HDPE. It was found that at 325 °C (8.38MPa H2, 60min, and 16.7%
catalyst) over 0.5%Pt/ZrO2/SO4 catalyst only 25% of HDPE was con-
verted whereas PP was completely degraded under these conditions. PS
was completely converted even at a lower temperature of 300 °C over
the same catalyst. However, a comparison of PP and PS at 300 °C under
the same reaction conditions was not provided by the authors. For
nearly complete conversions of both PP (325 °C, 8.38MPa H2, and
20min) and HDPE (375 °C, 8.38MPa H2, and 25min), for both catalysts
employed, the lower gas and the higher liquid yields were obtained
when PP was hydrocracked. Moreover, the yield of C4‒C12 products
and the ratio of branched to normal alkane products were also higher
with PP compared to HDPE [86]. The concentration of gasoline range
products in a liquid product, however, was much higher with HDPE.
Venkatesh et al. [86] reported that the reason for the increased con-
version of PP was the branching present in PP at alternate carbon
atoms. Venkatesh et al. [42] also observed that the products obtained
from HDPE and PP were generally gasoline range branched alkanes,
while PS produced benzene, alkylated aromatics, and bicyclic com-
pounds over metal-loaded ZrO2/SO4 catalyst. At a high temperature of
440 °C (30min, 5.6MPa H2, catalyst loading of 20%), using Low Alu-
mina catalyst (FCC catalyst), Luo and Curtis [69], observed that PS was
more difficult to be hydrocracked than that of HDPE. They observed
that for HDPE, PS, and PET, the conversions were 93.3%, 92.3%, and
67.7%, respectively, whereas, hexane solubles, i.e., oil yields were
80.0%, 63.2%, and 34.1% and gas yields were 11.1%, 2.9%, and 25.4%,
respectively. Comparing the work of Venkatesh et al. [42], an obvious
contradiction for PS and HDPE hydrocracking can be noticed. This
disagreement may be due to very high hydrocracking temperatures
involved in the latter study, which resulted in a high percentage of
unaccountedcoke formed, as conversions were based on THF in-
solubles.
In a different study, at a lower temperature of 400 °C (30min and
5.5MPa H2), Luo and Curtis [68] compared the non-catalytic and cat-
alytic hydrocracking of PIP, PS, LDPE, and HDPE. For both the catalytic
and non-catalytic hydrocracking, as expected, HDPE was the most dif-
ficult polymer to be hydrocracked. These results are, unfortunately, not
consistent with their previous work [69], but support the work of
Venkatesh et al. [42]. The conversions obtained were 98.9%, 98.2%,
6.6%, and 3.0% for PIP, PS, LDPE, and HDPE, respectively, under non-
catalytic conditions. Virtually similar conversions to those of non-cat-
alytic hydrocracking were obtained with the catalysts used in their
study.
In the same group as Luo and Curtis [68,69], Joo and Curtis [70]
D. Munir et al. Renewable and Sustainable Energy Reviews 90 (2018) 490–515
507
studied the hydrocracking of PS, LDPE, and PET in the presence of
presulfided NiMo/Al2O3 catalyst (430 °C, 8.3MPa H2, 60min, and 1%
catalyst concentration). Under the conditions of reactions, the order of
conversion was 99.1%, 95.7%, and 65.3% for PS, PET, and LDPE, re-
spectively. These results are again in disagreement with what Luo and
Curtis [69] found in their study and suggest that PET is also less diffi-
cult to be hydrocracked than LDPE (or HDPE). However, similar to Luo
and Curtis [68] and to the work of Venkatesh et al. [42], PS was ob-
served to be the easiest to be hydrocracked, while LDPE was found to be
the most difficult to be converted. PS produced the highest oil yield
(95.3%) and the least gas yield (2.7%), whilst PET gave very high gas
yield of 25.9% compared to oil yield of only 57.6%. PS also produced
the lightest oil fraction, while LDPE provided the heaviest of oils.
Shabtai et al. [89] studied the hydrocracking of HDPE, PP, and
polybutadiene (PBD) in the absence and presence of a catalyst (ZrO2/
SO4). Similar to Venkatesh et al. [42] who also used ZrO2/SO4 (metal-
loaded) catalyst, it was found that HDPE was more difficult to be hy-
drocracked than PP. Moreover, in agreement with the results obtained
by Venkatesh et al. [42], for virtually the same conversion (98.9% for
HDPE at 435 °C and 99.8% for PP at 400 °C), PP produced less gas yield
and higher liquid yield, as well as higher amount of gasoline range
(C5–C12) products. For the same conditions of operation, PBD was
98.5% converted at 415 °C, while PP was converted to 97.8% at 410 °C;
with PBD less gas yield and less liquid yield was obtained under these
conditions. The hydrocracking of HDPE at 450 °C in the presence of
Fe2O3/SO4 gave normal and branched paraffins, 5- and 6-ring non-
substituted ring and mono-, di-, and tri-alkyl substituted cycloparaffins.
Small amounts of olefins and cycloolefins were also present. With PP at
410 °C, branched paraffins, branched olefins, and polymethyl-sub-
stituted naphthenes, while with PBD at 415 °C, paraffins and alkyl-
substituted naphthenes, benzene, alkylbenzene, tetralins and indanes,
and bicyclic arenes were produced.
Ali et al. [96] used five polymeric materials and four different cat-
alysts to study the hydrocracking reaction. For each catalyst, PS was
found to be the most easily degradable plastic and completely con-
verted under the conditions studied, whereas HDPE and LDPE were the
most difficult. The order of ease of conversion for each catalyst was
PS>PET>PP>LDPE & HDPE. These results support the findings of
Venkatesh et al. [42], Luo and Curtis [68], Joo and Curtis [70], and
Shabtai et al. [89]. Generally HDPE provided the highest gas yield and
PS the highest liquid yield. The amount of liquid yield obtained was in
the following order: PS> PP & PET>LDPE >HDPE, while the gas
yield followed the trend as HDPE > LDPE & PS & PP & PET. These
findings also support the work of Venkatesh et al. [42], Joo and Curtis
[70], and Shabtai et al. [89].
Metecan et al. [47] observed that at 375 °C, both HDPE and LDPE
produced waxy products with no liquid yield. On the other hand, PP
was easily cracked at 375 °C and more than 86.0% of liquid yield was
obtained under both thermal and catalytic reactions. Compared to PP,
the naphtha range product obtained over both PEs contained more n-
alkanes and less iso-alkanes. These results supported the works of
Venkatesh et al. [42] and Shabtai et al. [89]. However, an additional
observation was that for PP compared to PEs, temperature had little
effect on the gas composition and the liquid product from PP provided
higher olefinic content and higher bromine numbers.
Williams and Slaney [126] studied various individual plastics such
as HDPE, PP, PS, PVC, and PET without adding a catalyst to the reaction
system. Under the conditions of reaction (1.0 MPa H2 pressure, 60 min,
and 500 °C), HDPE and PP were completely degraded to oil and gas
products (surprisingly no residual solid, i.e., coke). PVC and PET were
difficult to convert to liquid or gas products. The residue left (including
any coke formed) was 0%, 0%, 22.0%, 52.0%, and 41.0% for HDPE, PP,
PS, PVC, and PET, respectively. These observations are in contrast to
those made by the other researchers, as discussed above. However,
again due to a very high temperature of 500 °C and long reaction time,
the coke formed for PS and PET wrongly presented the conversion
values, as the conversions were based on DCM solubles which did not
dissolve the coke formed. The liquid yields were 95.0%, 95.0%, 77.0%,
2.0%, and 27.0% and gas yields were 5.0%, 5.0%, 2.0%, 38.0%, and
32.0% for HDPE, PP, PS, PVC, and PET, respectively. Very high liquid/
gas ratios were obtained with PE, PP, and PS, while PVC and PET
produced very large amounts of gases. Very high gas yields with PET
were also observed by Joo and Curtis [70].
Akah et al. [48] studied hydrocracking of HDPE, PP, and PS at a low
temperature of 310 °C, hydrogen pressure of 5.5 MPa, 5min reaction
time, and over 1.0% Pt/USY catalyst. Close to complete conversion of
each HDPE, PP, and PS was obtained. It was also observed that high
yields of C6 hydrocarbons were obtained with PP. HDPE yielded rela-
tively higher amounts of C4 and C5, whereas PS yielded mostly C3, C4,
C7, and C9. Their results also supported much of the previous studies.
In addition to the above researches a few investigations on mixture
of virgin or waste plastics were also carried out. Rothenberger and
Cugini [72] compared the hydrocracking reaction of HDPE and a plastic
mixture (50%HDPE, 35%PS, and 15%PET) at 430 °C over an AO-60
catalyst. It was found that adding 50%PS plus PET to HDPE, the con-
version was increased from 17.9% to 59.0%. This result was expected as
it was observed by many other researchers that PS and PET are easier to
degrade than HDPE.
Liu and Meuzelaar [67] compared the hydrocracking of HDPE and a
waste plastic mixture (Table 3) over SiO2-Al2O3 and HZSM-5 catalysts
in a high pressure TG analyzer. The decomposition decreased in both
cases and similar TG profiles were obtained for both plastics with each
of the catalysts. However, thermal degradation of HDPE was found to
occur faster than that of waste plastic mixture. The presence of sub-
stituted aromatics in the HDPE products suggested that the aromatics
present in the cracked products of the waste plastic mixture can come
from the cracking of HDPE and do not necessarily require polystyrene
as a component.
Ding et al. [88] also used HDPE and a waste plastic mixture (waste
APC plastic) to study the hydrocracking reaction. Similar to observa-
tions by Liu and Meuzelaar [67], it was found that without a catalyst,
waste plastic was better converted than HDPE, but with catalysts the
opposite was true. As suggested by Ding et al. [88] the presence of
heteroatoms and trace metals that are harmful for the catalysts might
be the reason for this observation. At 435 °C where appreciable con-
versions of both plastics were obtained there was little difference in
conversions, oil yields, and gas yields obtained for each plastic. Underthese conditions, HDPE, in the absence of a catalyst, yielded more
olefins and fewer aromatics. HDPE provided more aromatics and fewer
n-paraffins in the presence of HZSM-5, whereas a reduced amount of
aromatics and increased amount of n-paraffins were produced in the
presence of TiCl3.
In another study, Ding et al. [94] studied the same comparison using
different catalysts and again found that waste APC plastic was easier to
be converted in thermal hydrocracking, as they had found in their
previous study, giving roughly three times the conversion and the liquid
yield at 375 °C. Moreover, in contrast to their previous work for each of
the catalyst used, waste APC plastic provided better conversion and
liquid yields, although the product distribution was similar for both
types of plastics.
Huffman et al. [95] and Shah et al. [51] compared two types of
waste plastics, one washed and cleaner (waste APC plastic) and the
other unwashed and dirtier (waste DSD plastic). At 435 °C, on average,
for each catalyst, waste DSD plastic was easier to crack and produced
more pentane solubles than the waste APC plastic mixture. In contrast,
at 445 °C, the waste APC plastic cracked more easily and produced more
total liquid product as well as pentane solubles. However, generally at
445 °C, waste DSD plastic produced more light liquid (gasoline range
product, IBP-200 °C), while waste APC plastic provided more heavy
fraction (≥ 275 °C). The quality of the oil produced by the type of
catalyst used was observed to be affected only for the waste APC plastic.
Similar to Huffman et al. [95] and Shah et al. [51], Ding et al.
D. Munir et al. Renewable and Sustainable Energy Reviews 90 (2018) 490–515
508
[93,94] also found that it was easier to hydrocrack waste APC plastic
under the conditions studied in their work. In the presence of a catalyst
such as NiHSiAl (40% loading) and KC-2600 (40% loading) greater
than 90.0% conversion was obtained. Equal to or greater than 42.0% oil
yield was obtained with waste APC plastic at 375 °C, compared to only
30.9% and 49.9% conversion and 20.7% and 35.1% oil yield over KC-
2600 and Ni/SiAl, respectively for the waste DSD plastic. At 375 °C,
however, a slightly increased conversion in thermal hydrocracking was
obtained with DSD. Compared to their previous work with waste APC
plastic, thermal hydrocracking was found more effective for waste DSD
plastic at each temperature studied.
Williams and Slaney [126] studied the thermal hydrocracking of a
simulated waste mixture and compared its results with two actual waste
plastic mixtures, i.e., waste DSD plastic and waste Fost Plus plastic. The
two waste plastic mixtures yielded 48.2% and 70.6% liquid, 2.2% and
4.9% gas, and 35.1% and 17.8% residue, respectively. The simulated
waste mixture contained HDPE (44%), PP (21.2%), PS (13.3%), PVC
(12.2%), and PET (8.9%). Based on the results with individual plastics,
as discussed above, they calculated the liquid yield as 75.1%, gas yield
as 11.1%, and residue as 12.9%. However, the experimental results
showed that the liquid yield, gas yield, and residue were 55.8%, 4.4%,
and 22.7%, respectively. These findings show the possible interactions
present among the individual plastics when they are hydrocracked in a
mixture and to some extent discourages the possibility, as suggested by
Akah et al. [127], that the product distribution can be controlled by
adjusting the composition of a feed mixture.
2.1.5.1. Summary. In summary, it may be said that the order of ease of
degradation in the presence of hydrogen for various individual plastic
materials is PIP>PS>PET>PP≈ PBD>LDPE>HDPE. Comparing
PP and HDPE, lower gas and higher liquid yields are obtained with PP.
The yield of gasoline range products is also higher and the
concentration of branched products is also higher in PP than HDPE.
Comparing hydrocracking of PS, PET, PP, LDPE, and HDPE, for
identical conversions, PS gives the highest liquid yield, while HDPE
and LDPE give the lowest, but give the highest gas yield and PS gives
the lowest gas yield. The oil fractions of HDPE and LDPE usually
produce n-alkanes and branched alkanes with fewer aromatics and
naphthenes. PP gives higher amounts of isoalkanes, PBD produces high
concentration of aromatics and cycloparaffins, and PS produces
predominantly alkyl benzenes and phenyl alkanes. Significant
interaction between individual plastics is observed when a mixture of
these individual plastic materials is hydrocracked. By adjusting the
composition of a plastic mixture the desired quality of the oil produced
may be obtained. A waste plastic mixture may significantly affect the
performance of a catalyst due to the impurities present in the waste
mixture.
3. Mechanisms and kinetics involved in hydrocracking of plastics
The hydrocracking of a plastic material may be explained by the two
most common mechanistic pathways, one that requires the formation of
free radicals and the other that involves the formation of carbocationic
ions (carbocations). In the radical mechanism which may occur in
thermal hydrocracking or hydrocracking over a solid that contains
weak or no acid sites, e.g., activated carbon, alumina, and TiCl3, the
intermediate free radicals are formed by a thermolytic process and/or
by hydrogen abstraction by the support [78,80]. A free radical is un-
stable and undergoes a cracking (α- or β-scission) reaction to produce
an olefin and another free radical. Termination of a free radical may
occur through radical coupling (recombination), hydrogen abstraction,
or disproportionation reaction [20,88,128]. On an acidic catalyst such
as HZSM-5, a carbenium ion may be formed by hydrogen abstraction
caused by the acid sites of the catalyst. The carbenium ion may undergo
further acid catalyzed reactions, such as isomerization and cracking at
the β-position (β-scission) to produce an olefin and a smaller carbenium
ion. The olefin produced may be hydrogenated in the presence of a high
hydrogen partial pressure before appearing in the product slate. How-
ever, the presence of Brønsted sites may bring about proton transfer,
which leads to the formation of a less likely [129] pentacoordinated
carbonium ion which can undergo β-scission to produce smaller par-
affinic molecules and carbenium ions [130].
On a hydrocracking catalyst that contains both hydrogenation-de-
hydrogenation and acid sites, the hydrocarbon is dehydrogenated on a
metal site to produce an olefin. The olefin is then moved to an acid site
and protonated and isomerized. The protonated branched olefinic
species undergoes β-scission (acid catalyzed reaction) and the cracked
unsaturated species diffuses to a hydrogenation site and therefore a
saturated hydrocarbon is the result. In the presence of catalytic cracking
or hydrocracking catalysts, other acid catalyzed reactions such as de-
hydrocyclization reactions can also occur. Hydrocracking reaction may
also be possible on a catalyst containing a hydrogenolysis metal such as
Rh, Ir, and Os. supported on a non-functional support, where breaking
of C–C linkages may occur together with the hydrogenation of the
product species [41]. It is important to mention here that even in the
presence of an acid (cracking) catalyst or a hydrocracking catalyst, the
formation of free radicals formed by thermal energy addition may not
be ruled out, but may be required to start the catalytic reaction [43,67],
i.e., initially large polymer molecules are broken down into smaller
fragments [78] which are further catalyzed to produce cracked pro-
ducts.
When comparing the role of Pt and Ni loaded over sulfated zirconia,
Venkatesh et al. [86] found that at 375 °C, there was more of an in-
crease in isomerization with Ni than with Pt. It was reasoned by Ven-
katesh et al. [86] that metal function dissociates molecular hydrogen
into atomic form to give hydride ions. The intermediate carbenium ions
formed by the acidic sites can possibly react with a hydride ion and
therefore will be desorbed to the gas phase as products.The higher the
number of hydride ions, the lesser will be isomerization, as inter-
mediate carbocations live for a shorter time. As Ni is less active than Pt,
it produces a reduced number of hydride ions and therefore offers
greater residence time to carbocations to undergo isomerization.
For activated carbon alone (no metal function) Nakamura and
Fujimoto [78] suggested that cracking ability of an activated carbon
catalyst means its ability to abstract hydrogen, especially tertiary hy-
drogen (PP), from a hydrocarbon species. A large hydrocarbon mole-
cule may initially be formed by thermal cracking and then interacts
with the catalyst. Hydrogen is abstracted on the catalyst surface, giving
rise to a hydrocarbon radical which further undergoes β-scission to
form an olefin and another hydrocarbon radical, as shown in Fig. 6
below. For a metal supported activated carbon, Nakamura and Fuji-
moto [78] reported, and were supported by Karagöz et al. [80], that the
metal function dissociates hydrogen molecules into hydrogen atoms.
The hydrogen may quench the free radicals formed by hydrogen ab-
straction or the thermolytic process to avoid overcracking, or saturate
olefins to produce a product with reduced olefins and lighter compo-
nents [80]. The authors suggest that the present mechanism is different
from the usual hydrocracking mechanism which occurs on hydro-
cracking catalysts. It is a radical decomposition mechanism that does
not involve carbenium ion intermediate formed on an acidic support,
but occurs through hydrocarbon radical formation.
In the absence of a catalyst while comparing the cracking of HDPE
Fig. 6. Mechanism of hydrocarbon cracking on an activated carbon [78].
D. Munir et al. Renewable and Sustainable Energy Reviews 90 (2018) 490–515
509
in the presence of nitrogen and hydrogen, Ding et al. [88] suggested
hydrogen as a chain transfer agent, which may undergo chain transfer
reaction and saturate the radicals formed by thermal decomposition
and therefore may prevent further bond scission. Rothenberger and
Cugini [72] also suggested the same mechanism for HDPE cracking in
the presence and absence of a catalyst. This simple mechanism is shown
in Fig. 7.
Based on the high gas yield obtained over HZSM-5 for HDPE
cracking, Ding et al. [88] suggested that hydrocracking of HDPE in-
volves carbocations. In contrast, over TiCl3, it was suggested that HDPE
hydrocracking followed free radical mechanism with recombination
and disproportionation as the chain termination steps [131]. Metecan
et al. [47] studied two commercial catalysts, a hydrotreating catalyst
(HYDROBON), which might have a stronger hydrogenation function,
and a hydrocracking catalyst (DHC-8) which might have a stronger
acidic function (SiO2-Al2O3 compared to Al2O3). For the hydrocracking
of HDPE, two different mechanisms were suggested for the two cata-
lysts. A carbenium ion mechanism, as discussed above, was suggested
for DHC-8 (stronger acidic function), while a radical mechanism was
reported for the HYDROBON catalyst, where hydrogen may be dis-
sociated by the hydrogenation metal and can therefore quench free
radicals and saturate olefins, as discussed above. Mosio-Mosiewski et al.
[43] supported the free radical mechanism on a metal supported alu-
mina support, while studying the hydrocracking of LDPE. It was
reasoned that alumina has only weak acid sites, which means that it
does not give rise to a carbenium ion mechanism.
For the hydrocracking of HDPE over metal supported catalysts, Ding
et al. [94] presented the carbenium ion mechanism and stated that in
the absence of a metal function the rate of formation of carbenium ion
was lower on the hybrid support, as the intermediate olefins were not
formed, which would ultimately decrease the HDPE conversion. How-
ever, the results showed the statement only true for the commercial
hydrocracking catalyst, KC-2600 (possibly NiMo/SiO2-Al2O3). The high
gas yield obtained in the study was explained on the basis of a penta-
coordinated carbonium ion mechanism. However, it was also suggested
that disproportionate reactions can occur. In a disproportionation re-
action alkylation of an olefinic intermediate and an adsorbed carbe-
nium ion occurs, the cracking of which results in a higher carbon
number species and a lower carbon number species than the reactants.
Hesse and White [33] also suggested disproportionation occurred at
low temperature more often than β-scission reactions.
Shabtai et al. [89] reported the carbenium ion mechanisms for both
polyethylene and polypropylene hydrocracking. Their mechanisms are
shown in Figs. 8 and 9. For PP, in the presence of strong acid sites
tertiary carbenium ions are formed. The carbenium ions formed are
cracked at the beta position to generate olefins and lower molecular
weight carbenium ions. These smaller carbenium ions undergo a me-
tathesis reaction with molecular hydrogen to produce a lower mole-
cular weight paraffin hydrocarbon. Additional cracking reactions can
further reduce the length of the hydrocarbons. The olefin formed in the
cracking process is readily protonated and undergoes a metathesis re-
action with hydrogen to produce the corresponding paraffin product.
For HDPE, the hydrocarbon molecule is protonated and cleaved to
produce an alkene and a smaller carbenium ion. The hydride and alkide
shifts produce branching in the hydrocarbon molecule resulting in
lower molecular weight branched paraffin.
Dufaud and Basset [46] proposed a reaction mechanism for poly-
ethylene hydrogenolysis by an electrophilic zirconium hydride on a
Fig. 7. Comparison of the presence of hydrogen and nitrogen for the non-cat-
alytic cracking reaction.
Fig. 8. Mechanism of PP hydrocracking described by Shabtai et al. [89]. Used with permission. Copyright 1997, American Chemical Society.
D. Munir et al. Renewable and Sustainable Energy Reviews 90 (2018) 490–515
510
Fig. 9. Mechanism of the formation of branching in HDPE described by Shabtai et al. [89]. Used with permission. Copyright 1997, American Chemical Society.
Fig. 10. Mechanism of polythene hydrogenolysis proposed by Dufaud and Basset [46].
D. Munir et al. Renewable and Sustainable Energy Reviews 90 (2018) 490–515
511
silica-alumina Ziegler-Natta polymerization catalyst. The mechanism is
outlined below [46,132] and schematically shown in Fig. 10.
• Activation of C–H bond in polymer chain
• The chain is linked to the center of Zr
• β-alkyl transfer to produce an alkyl-olefin complex
• Hydrogenolysis of the complex to produce a hydrido-olefin complex
and a smaller paraffin molecule
• Olefin insertion results in the formation of an alkyl complex
• Hydrogenolysis of the complex leads to the separation of catalyst
and the formation of another saturated hydrocarbon molecule
Only a single study on the kinetics of hydrocracking of plastics has
been found in the literature. Ramdoss and Tarrer [125] studied kinetics
of waste APC plastic mixture at various temperatures and reaction
times. Based on the mechanism proposed (shown in Fig. 11), the fol-
lowing five differential equations Eqs. (1)–(5) were developed, as-
suming each of the reactions was first order with respect to mole
fraction of the corresponding species.
= − − −
dP
dt
k P k P k P1 2 3 (1)
= − − −
dH
dt
k P k H k H k H1 4 6 7 (2)
= + − −
dL
dt
k P k H k L k L2 4 5 8 (3)
= + +
dG
dt
k P k L k H3 5 6 (4)
= +
dC
dt
k H k L7 8 (5)
Assuming Arrhenius temperature dependency of the rate constants,
the values of the pre-exponential factors and activation energies of each
of the reactions was worked out by simultaneously solving the differ-
ential equations and doing regression of the data. Table 7 shows the
results of the kinetic modeling. It is shown in the table that the highest
activation energies such as 121.7 kJ/mol and 113.5 kJ/mol are ob-
tained when light and heavy liquids are converted to gaseous products,
respectively, while the lowest activation energy of 11.8 kJ/mol results
when heavy liquid is convertedto light liquid.
A few of the researchers studied the effect of reaction time on the
hydrocracking reaction, but they did not kinetically model their find-
ings. In the present study, the experimental data of Ding et al. [94] is
used to carry out a simplified kinetic modeling. The weight percent
conversion of plastic material was fitted with an appropriate mathe-
matical function and the derivative of the function obtained at a given
time was used to find the rate of the reaction at that time. The rate and
fractional conversion were related by the following simple power law
equation Eq. (6) for an irreversible reaction. By fitting the rate and
conversion data, the overall rate constant and order of the reaction
were calculated. The results of the kinetic modeling are shown in
Table 8.
− = − =
dX
dt
r kX( )A A An (6)
where, (−rA) is the rate of reaction of plastic (A) degradation in s−1, k
is rate constant in s−1·n−1, XA is conversion in wt%, and n is order of
reaction.
3.1. Summary
Depending on the type of catalyst used, both free radical and car-
benium ion mechanisms are found to occur in the hydrocracking of
plastic materials. On a hydrocracking catalyst a carbenium ion me-
chanism is reported, while a catalyst with weak acidic function would
follow a free radical mechanism. A pentacoordinated carbonium ion
mechanism and disproportionation reactions are also expected to occur
over a hydrocracking catalyst. Kinetic analysis for thermal hydro-
cracking has shown that the highest activation energies are involved
when light and heavy liquids obtained during hydrocracking are con-
verted to gaseous products whereas the lowest activation energy results
when heavy liquid is changed to light liquid.
4. Conclusions
A review of the literature regarding hydrocracking of plastic mate-
rials using the direct liquefaction method and in the absence of a co-
feed has been carried out in detail. The effects of various hydrocracking
variables such as temperature, hydrogen pressure, reaction time, cata-
lyst presence and type, and type of feed plastic employed have been
studied. The hydrocracking mechanisms related to plastic degradation
are discussed and a database of the experimental work has been com-
piled.
It is found that different plastics behave differently to hydrocracking
Fig. 11. Reaction pathways for hydrocracking of waste plastic mixture pro-
posed by Ramdoss and Tarrer [125]. Adapted with permission. Copyright 1996,
Elsevier Ltd.
Table 7
Results of kinetic modeling carried out by Ramdoss and Tarrer [125].
Reaction Pre-exponential factor (min−1) Activation energy (kJ/mol)
1 4612 62.9
2 30 34.7
3 70,792 86.3
4 0.4 11.8
5 4.8×106 121.7
6 3.5×105 113.5
7 1× 105 42.6
8 1803 81.1
Table 8
Results of kinetic modeling for the experimental data of Ding et al. [94].
N Function k n
Mathematical form Constants (s−1 n−1)
5 = + + +X a bt ct dtexp( )2 3 R2
= 0.963
a =2.593517117 4.883 ‒0.3499
b =0.097409994
c =−0.00161467
d =9.08914×10−6
N is number of data points, X is conversion (wt%), and t is time (min). Feed =
waste APC plastic, catalyst = Ni/HSiAl (4:1 mixture of SiO2-Al2O3 and HZSM-
5), feed/catalyst = 1.0/0.2, temperature =375 °C, cold hydrogen pressure
= 7.0MPa.
D. Munir et al. Renewable and Sustainable Energy Reviews 90 (2018) 490–515
512
conditions and a desired quality of oil may be produced by selecting a
particular composition of the plastic feed. Increasing temperature
usually increases the conversion of a plastic material, but more gases
and coke may result at excessively high temperatures. In the presence of
a catalyst, temperature higher than 400 °C is not recommended.
Beneficial effects of pressure are available only at moderate pressures;
very high pressure might not give added benefits when compared to the
cost associated with a high pressure process. Initial cold hydrogen
pressure of 20–60 bar may be suitable for the hydrocracking work.
Increasing reaction time favors the hydrocracking reaction, an ex-
cessively long residence time decreases the yield of liquids and in-
creases the gas production. At typical temperature and pressure con-
ditions with a suitable catalyst, a reaction time of more than 60min is
not advised. A bifunctional catalyst with both the hydrogenation-de-
hydrogenation and cracking ability is the most suitable for the hydro-
cracking of plastic materials. However, additional work is required by
future researchers on the new catalytic materials such as microporous-
mesoporous and aluminum modified mesoporous catalysts, to reduce
the severity of operating conditions and to enhance the quality of the
product. Moreover, detailed deactivation study of the appropriate cat-
alysts is also recommended.
Acknowledgment
The authors acknowledge the funding provided by the Higher
Education Commission of Pakistan, Research Grant No. 20-2308/
NRPU/R&D/HEC/12470 to carry out this work.
Appendix A. Supplementary material
Supplementary data associated with this article can be found in the
online version at https://doi.org/10.1016/j.rser.2018.03.034.
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	Hydrocracking of virgin and waste plastics: A detailed review
	Introduction
	Survey of direct liquefaction of plastic materials by hydrocracking
	Effect of reaction parameters on hydrocracking of plastic materials
	Effect of catalyst on plastic hydrocracking
	Summary
	Effect of hydrogen pressure
	Summary
	Effect of reaction time
	Summary
	Effect of reaction temperature
	Summary
	Effect of the use of different types of plastics
	Summary
	Mechanisms and kinetics involved in hydrocracking of plastics
	Summary
	Conclusions
	Acknowledgment
	Supplementary material
	References