Direct production of biodiesel from waste oils with a strong solid base from

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Applied Energy
journal homepage: www.elsevier.com/locate/apenergy
Direct production of biodiesel from waste oils with a strong solid base from
alkalized industrial clay ash
Wen-Jie Conga, Yi-Tong Wangb, Hu Lia, Zhen Fanga,⁎,1, Jie Suna, Hai-Tong Liua, Jie-Teng Liua,
Song Tanga, Lujiang Xua
a Biomass Group, College of Engineering, Nanjing Agricultural University, 40 Dianjiangtai Road, Nanjing, Jiangsu 210031, China
b College of Metallurgy and Energy, North China University of Science and Technology, 21 Bohai Street, Tangshan 063210, China
H I G H L I G H T S
• Direct production of biodiesel at 65 °C
from waste oil by solid base was
achieved.
• Solid base from alkalized SBC ash had
basic sites from Na2SiO3 and
Na2SiAlO4.
• Biodiesel from soybean oil reached>
99% yield with 8 cycles (> 95%).
• Biodiesel yield was 96% from SBC oil
(AV 10) at 65 °C (vs. 120 °C with
H2SO4).
• It was magnetized for separation with
biodiesel yield 99% for 3 cycles
(87%).
G R A P H I C A L A B S T R A C T
Solid base synthesized from SBC ash for biodiesel production from waste oils with 8 cycles and anti-saponifi-
cation. It was further magnetized for easy separation.
A R T I C L E I N F O
Keywords:
Biodiesel
Solid base catalyst
Spent bleaching clay
Waste oils
Transesterification
A B S T R A C T
Biodiesel was directly one-step produced from waste oils without pretreatment catalyzed by a solid base alka-
lized from spent bleaching clay (SBC) ash. Optimized conditions were obtained with 99.1% biodiesel yield from
soybean oil with an orthogonal design. The base catalyst was stable within 8 cycles (> 95% biodiesel yield) and
resistant to saponification (AV = 9.7 mg KOH/g, 96.5% biodiesel yield). The base was characterized with XRD,
EDX-mapping, FT-IR, XRF and TPD, and it had similar strong basicity to Na2SiO3 (0.21 vs. 0.22 mmol/g for
Na2SiO3) with active sites of Na2O and CH3ONa evolved from Na2SiO3 and NaAlSiO4 by reactions of NaOH with
oxides (e.g., SiO2, Al2O3) in SBC ash. Furthermore, the base was magnetized with magnetism of 6.86 emu/g by
carbonizing residual oil in SBC as carbon support and reductant (of Fe2O3 to magnetic Fe3O4 particles). It
catalyzed soybean oil to produce biodiesel with 99.2% yield and blended oil (AV = 5.9) to biodiesel with 91.9%
yield without any saponification. The catalyst was magnetically separated and reused for 3 cycles with 87%
yield. The non-magnetic base could also efficiently catalyze actual SBC oil for the production of biodiesel with
95% yield at AV of 10. This work realized the full use of inorganics in SBC, and its oil for direct biodiesel
https://doi.org/10.1016/j.apenergy.2020.114735
Received 28 October 2019; Received in revised form 18 February 2020; Accepted 23 February 2020
⁎ Corresponding author.
E-mail address: zhenfang@njau.edu.cn (Z. Fang).
1 ORCID iD: https://orcid.org/0000-0002-7391-372X; URL: http://biomass-group.njau.edu.cn/.
Applied Energy 264 (2020) 114735
0306-2619/ © 2020 Elsevier Ltd. All rights reserved.
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production at a low temperature (i.e., 65 vs. 120 °C with sulfuric acid process) without wastes produced and
results can easily find practical applications for waste oils.
1. Introduction
The rapid rise in energy demands due to industrialization and
modernization has increased the consumption of fossil fuels, causing
environmental damage. In the last dozen years, biodiesel [1] and bio-
fuels [2] from the transformation of green plants has attracted con-
siderable attention. It was reported that biofuel could remit greenhouse
effect (carbon-neutral), reduce haze and acid rain with low emissions of
carbon monoxide, unburned hydrocarbons, polycyclic aromatic hy-
drocarbons and particulates [3]. Biodiesel (fatty acid methyl esters;
FAMEs), as an environmentally friendly, renewable and degradable
fuel, can be easily produced by esterification [4] or transesterification
reactions with edible and non-edible oils [5]. Although non-edible oils
(e.g., waste cooking and Jatropha oils) had no competition with food
[6], pretreatment is generally necessary for low-grade waste oils since
high acid value (AV; 0.08–44 mg KOH/g [7]) oils are prone to sapo-
nification with bases [8]. Mahlia et al. [8] reviewed 1660 patents re-
lated to biodiesel production and concluded that the main obstacle to
commercialize the production of biodiesel from waste oils was that high
AV oils were hardly directly transesterified by bases due to soap for-
mation without pretreatment step. In order to ensure one-step conver-
sion of high AV oils to biodiesel, acids were applied instead of base
catalysts [9], aiming to resist saponification and simplify biodiesel
production [10]. Carbonaceous-based acid catalysts [11] derived from
palm kernel shell and bamboo [12], lignin [1], Jatropha hulls and re-
sidue [13], Xanthoceras sorbifolia bunge hulls [14] by pyrolysis and
sulfonation were synthesized for esterification of oils with harsh reac-
tion conditions that may consume more energy and resources [15]. It
was reported that base catalysts for transesterification were 4000 times
faster than that of acids [16] and required low temperatures. As a re-
sult, solid base catalysts were widely reported as catalysts for biodiesel
production that were summarized in Table 1. Xie et al. synthesized
magnetic Fe3O4/MCM-41/ECH/Na2SiO3 (MCM-41: molecular sieve;
ECH: epichlorohydrin) [17] and CaO–SnO2 [18] for soybean biodiesel
production with yields of 99.2% and 89.3%, respectively. Many in-
organic wastes were also used to prepare solid bases. Calciferous
feedstocks like eggshells [19], mollusks shells [20] and animal bones
[21] were calcined as solid alkali catalysts by the decomposition of the
main component of calcium carbonate (CaCO3) to activated CaO [22].
Being rich in potassium oxide (K2O), banana peel ash was served as a
heterogeneous catalyst for the production of biodiesel at room tem-
perature [23]. Calcined ceramics waste [24] and calcined rice husk ash
with Li2CO3 [25] were found to be active for biodiesel production with
yields of 99.3% and 98.8%, respectively. However, the recyclability of
solid base catalysts was typically not satisfied due to the leaching of
active species in methanol and saponification reaction with free fatty
acids (FFAs) [26]. Two-step process was commonly applied for bio-
diesel production from waste oils with high AV by bases. Thus, it is
urgent to find a solid base that can resist saponification for one-step
production of biodiesel from high AV oils.
The use of waste source to produce both catalyst and biodiesel is
able to not only reduce the overall production cost but also enhance the
environmental friendliness. In the food industry, about 2–3 wt% acti-
vated bleaching clay was added for the decolonization of oils and ad-
sorption of impurities in refining process of edible oils, and the waste
residue with adsorbed oil was called spent bleaching clay (SBC) [27].
According to the consumption of edible oil around the world, it was
estimated that nearly 2 million tonnes of SBC were produced every year
[28]. Generally, SBC was disposed directly at landfills involving in high
cost and large land area, the residual oil containing in it could be oxi-
dized, causing possible fire hazards and environmental problems [29].
For these reasons, it has been prohibited dumping in landfills or public
disposal sites in many countries. Recently, researches on SBC waste
have been split into two parts, including solid residuals regeneration
and residual oil (SBC oil) recovery. The main components of SBC were
about 20% organics (e.g., oil), and 80% inorganicsubstances (SiO2,
Al2O3 and other metallic oxides) [30]. SBC solid residuals can be reused
by pyrolysis to absorb pollutants [31] or recycled by calcination for
next batch oil refining process [32]. As for the use of residue oil, in-situ
production of biodiesel from SBC was studied extensively, which needs
solvent extraction of oil, using petroleum ether [33], ethyl methyl ke-
tone [34] and alcohol [35] as co-solvents with homogeneous catalysts
such as NaOH, KOH [36] and H2SO4 [37]. Homogeneous base catalysts
easily formed soap while acidic catalysts required severe conditions
(e.g., higher temperature 120 °C) for transesterification [38]. In order
to make full use of SBC, organic oil can be extracted as biodiesel raw
material or directly be carbonized to char as magnetic particle and
catalyst support. Inorganic substances such as SiO2 and Al2O3, can react
with NaOH for synthesizing base. It was demonstrated that solid base
Na2SiO3 could catalyze high AV oils for biodiesel production [39], and
biodiesel produced from SBC oil was successfully used in a diesel engine
[40].
This work aims to synthesize biodiesel from waste oils (i.e., SBC oil)
via a one-step process without pretreatment (esterification) step at low
temperatures (≤65 °C) with strong solid bases simply produced by al-
kalization of SBC ash with NaOH. Sodium silicate and other bases were
formed by the reactions of NaOH with SiO2 and other metal oxides in
SBC ash. Magnetic and base particles were homogeneously synthesized
in carbonaceous support by pyrolysis of residual oil contained in SBC
for magnetic separation. Non-magnetic and magnetic bases catalyzed
soybean oil, blending high AV oil and SBC oil to produce biodiesel.
Orthogonal and single-factor designs were used for the optimization
experiments. Various characterizations were applied for the explana-
tions of experimental results. The developed catalytic system showed
great potential applications in biodiesel industry as well as base-cata-
lyzed relevant reaction processes.
Table 1
Comparison of different solid bases for biodiesel production.
Catalysta Waste material Raw oil and acid value (mg KOH/g) Reaction temperature Reaction time (h) Biodiesel yield Cycle
Fe3O4/MCM-41/ECH/Na2SiO3 [17] — Soybean oil, 0.1 70 °C 8 99.2% 5 (83.6%)
CaO–SnO2 [18] — Soybean oil, 0.1 70 °C 6 89.3% 5 (84.2%)
CaO [22] animal bone Palm oil, 0.6 65 °C 4 96.8% 5 (83.7%)
CWL-800-2 [24] ceramics waste Soybean oil 65 °C 2 99.3% 4 (93.3%)
Li4SiO4-800 [25] rice husk Soybean oil 65 °C 3 98.8% 10 (83%)
Na2SiO3/NaAlSiO4-SBC600 (this work) SBC Soybean oil, 0.5 65 °C 3 99.1% 8 (95.9%)
Na2SiO3/NaAlSiO4-SBC600 (this work) SBC SBC oil, 9.7 65 °C 3 95.8%
a MCM-4: molecular sieve; ECH: epichlorohydrin; CWL: calcined ceramics wastes; SBC: spent bleaching clay.
W.-J. Cong, et al. Applied Energy 264 (2020) 114735
2
2. Materials and methods
2.1. Materials
Sodium hydroxide (≥96%) was bought from Aladdin Industrial Inc.
(Shanghai). Sulfuric acid (≥98%) was bought from Shanghai chemical
reagent Co., Ltd. (Shanghai). Oleic acid (about 80% purity with other
fatty acids) was from Shanghai Lingfeng chemical regent Co., Ltd.
Analytical reagents of Iron (III) sulfate hydrate (≥99.7%) with 21–23%
Fe, dichloromethane (≥99.5%), anhydrous methanol (≥99.5%), hy-
drochloric acid (36–38%), potassium hydroxide (≥96%) and sodium
silicate (19.3–22.8 wt% of Na2O) were purchased from Sinopherm
Chemical Reagent Co., Ltd. (Shanghai). Standard heptadecanoic acid
methyl ester (C17:0) was purchased from TCI Development Co., Ltd.
(Shanghai) and other methyl esters [palmitate (C16:0), stearate
(C18:0), oleate (C18:1), linoleate (C18:2) and linolenate (C18:3)]
(≥99.0%) were from Sigma-Aldrich (Shanghai). Deionized water was
from a water purification system (electrical conductivity was 18.2
MΩ•cm, Milli-Q Academic, Merck Millipore, Darmstadt, Germany). SBC
and SBC oil (AV of 42.62 mg KOH/g) were collected from a local
company (Xinhuan Utilization of Regenerated Grease Co., Ltd.,
Taizhou, Jiangsu). Soybean oil (AV of 0.34 mg KOH/g, saponification
value (SV) of 192.9 mg KOH/g, and molecular weight (MW) of 874 g/
mol calculated by the formula [MW = (56.1 × 1000 × 3)/(SV – AV)])
was bought from a Carrefour supermarket (Nanjing). Soybean oil was
blended with oleic acid to achieve different AVs (2.03, 3.95, 5.91, 7.88,
9.79, 18.80 and 20.02 mg KOH/g) with the corresponding MWs
(864.41, 834.86, 805.66, 817.77, 869.54, 898.61 and 900.13 g/mol)
for the study of resistance to saponification. Raw SBC oil was obtained
directly from the local company after mechanical squeeze and pur-
ification. The AV was measured as 42.62 mg KOH/g and SV was
211.67 mg KOH/g by titration according to the national standard of
China (GB/T 5530-2005 and GB/T 5534-2008), respectively.
Phenolphthalein was used as an indicator of titration. For AV
measurement, the mixture of ethanol (25 mL) after neutralized by KOH
(0.1 mol/L) and raw oil (2 g) was boiled, titrated with KOH dropwise
until color changed and kept for 15 s. AV was calculated
(AV = 56.1 × volume of KOH× concentration of KOH / weight of raw
oil). For SV measurement, the mixture of KOH (0.1 mol/L) in 25 mL
ethanol and raw oil (2 g) was sealed in a bottle and heated by oil bath
(65 °C) with magnetic stirring for 1 h. After the reaction, the hot so-
lution with phenolphthalein (20 µL) was titrated by HCl until pink color
disappeared. Blank experiment without oil was done as well. SV was
calculated: SV = (volume of HCl consumed in blank experiment - vo-
lume of HCl consumed by raw oil) × 56.1 × HCl concentration / mass
of raw oil.
2.2. Catalyst preparation
2.2.1. Preparation of SBC ash and SBC char
SBC was dried at 105 °C overnight (WFO-710, EYELA, Tokyo
Rikakikai Co., Ltd.), ground and sieved (< 200 mesh or 74 μm). Ash
content of SBC was measured as 71.55% after calcination at 600 °C for
24 h, referred to the national standard of China (GB 1886.169-2016).
The oil content of SBC was 21.69% measured by extraction with pet-
roleum ether for three times. SBC particles were calcined at 600 °C
under N2 atmosphere in a tube furnace (OTF-1200X, Hefei Kejing
Material Technology Co. Ltd) for 2 h to produce SBC char or under air
atmosphere in a muffle furnace (KSL-1100X, Hefei Kejing Material
Technology Co. Ltd) for 2 h to produce SBC ash for preparation of
catalysts.
2.2.2. Na2SiO3/NaAlSiO4-based catalysts derived from SBC ash
About 8 g SBC ash, 8 g NaOH and 100 mL deionized water were
mixed and reacted in a round bottom flask with reflux (or Teflon re-
actor), heated by an electrical heater at 105 °C for 3 h with magnetic
stirring (600 rpm) to produce base catalyst by the reactions of
[2NaOH + SiO2 → Na2SiO3 + 2H2O, 2NaOH + Al2O3 + 2SiO2 →
2NaAlSiO4 + H2O]. Obtained mixture was dried at 105 °C in an oven
(WFO-710, EYELA, Tokyo Rikakikai Co., Ltd.) overnight, ground (< 80
mesh) and heated to 400–600 °C for 2 h (excluding heating and cooling
time) at a heating rate of 4.83 °C/min under inert nitrogen flowing in
the tube furnace. The obtained catalyst was named as Na2SiO3/
NaAlSiO4-SBCT (Subscript T is calcination temperature: 400/500/
600 °C). Similarly, commercial Na2SiO3 was calcined at 600 °C under N2
flow for 2 h, ground and sieved (< 80 mesh) as a catalyst for biodiesel
production for comparison experiments.
2.2.3. Magnetic Na2SiO3/Fe3O4-based catalysts derived from SBC char
Similarly, about 8 g dried SBC char, 19 g NaOH, 20 g Fe2(SO4)3 and
100 mL deionized water were mixed for the synthesis magnetic catalyst
by the reactions of [2NaOH + SiO2 → Na2SiO3 + H2O,
6NaOH + Fe2(SO4)3 → 3Na2SO4 + 2Fe(OH)3 ↓, 12Fe(OH)3 + C →
4Fe3O4 + 18H2O + CO2 ↑]. After calcination, the magnetic catalyst
was named as Na2SiO3/Fe3O4-SBCT (Subscript T is calcination tem-
perature: 400/500/600 °C).
2.3. Catalyst characterization
The inorganic elements of catalysts were measured by XRF (ZSX
Primus II, Rigaku, Tokyo) and element C was by an element analyzer
(Vario EL III CHONS, Elementar Analysensysteme GmbH, Hanau,
Germany). X-ray diffraction(XRD) diffractometer (Smartlab 9KW,
Rigaku) with Ni-filtered Cu Kα1 radiation (λ = 0.154 nm) was scanned
from 2θ of 5° to 90° to obtain XRD patterns. Fourier transform-infrared
(FT-IR) spectra were obtained in a spectrophotometer (Nicolet IS10,
Thermo Fisher Scientific Inc., Waltham, WA) for finely ground samples
(1%) in KBr pellets at 400–4000 cm−1. The specific surface area, pore
volume and pore size of samples were measured by Bruner Emmett and
Teller (BET) and Barrett-Joyner-Halenda (BJH) methods (Nova 4200e,
Quantachrome Instruments, Boynton Beach, FL) with N2 adsorption.
Temperature programmed desorption (TPD; Chembet PULSAR TPR/
TPD, Quantachrome, Boynton Beach, FL) was used to measure the
surface basicity. In TPD process, about 50 mg catalyst was heated to
400 °C at a heating rate of 5 °C/min under He flowing (85 mL/min),
cooled to room temperature and absorbed by flushing pure CO2
(85 mL/min) for 80 min at 50 °C. For the subsequent adsorption, the
sample was heated to 400 °C at 5 °C /min and kept for another 80 min.
Five different volumes (0.1–0.5 mL) of a standard CO2 gas (100%
purity) were used to calibrate the basicity curve (Fig. S1 in supple-
mentary materials) and background curve was obtained by He flowing
over the same catalyst. Thermogravimetric analysis-differential scan-
ning calorimetry (TGA-DSC; STA459 F3, Netzsch, Selb, Germany)
analysis was conducted from room temperature to 1000 °C at heating
rate of 5 °C /min under nitrogen atmosphere. The morphologies were
examined with scanning electronic microscope (SEM; Regulus 8220,
Hitachi, Tokyo). The magnetism of catalysts was determined by a vi-
brating sample magnetometer (VSM; Squid-VSM, Quantum, San Diego,
CA).
2.4. Biodiesel production and analysis
All the experiments were conducted in a 50 mL glass bottle sealed
by a rubber-aluminum cap in an oil bath (35–75 °C) with magnetic
stirring. Each run was repeated twice, the listed biodiesel yield was
averaged from the two repeated tests with standard deviation (σ) of
0.03–5.16%. Biodiesel was analyzed by gas chromatography (GC; GC-
2010 plus, Shimadzu, Kyoto).
2.4.1. Pretreatment (esterification) of raw SBC oil
Since its high AV, H2SO4 was used for reducing the AV of raw SBC
oil by esterification to test catalyst resistance to saponification from
W.-J. Cong, et al. Applied Energy 264 (2020) 114735
3
FFAs at a lower AV. About 30 g SBC oil with 7.77 g methanol (me-
thanol/oil molar ratio of 8:1) and 1.2 g H2SO4 (4 wt%) were mixed at
65 °C for 3 h reaction with experimental details given in next section.
After pre-esterification, the raw oil was transferred to a separating
funnel, washed with hot water (80 °C) for three times until pH 7, dried
in an oven at 105 °C overnight for biodiesel production. The AV was
measured again and decreased from 42.62 to about 20.12 mg KOH/g.
2.4.2. Biodiesel production
To screen a stable catalyst from the ones calcined at 400–600 °C, 4 g
soybean oil, 1.6 g dehydrated methanol (methanol/ oil molar ratio was
11/1) and 0.32 g catalyst (8 wt%) were reacted at 60 °C in 3 h [41].
After reactions, the catalyst was separated by precipitation standing
overnight at room temperature. A magnet was used for separating
magnetic catalyst. All catalysts were recycled without washing. The
catalysts calcined at 600 °C with the highest biodiesel yields (96.33%
for Na2SiO3/NaAlSiO4-SBC600 and 93.06% for magnetic Na2SiO3/
Fe3O4-SBC600) were chosen for the following optimization experiments.
Three types of oils (about 4 g soybean oil, blended oil, and SBC oil)
were catalyzed by both selected Na2SiO3/NaAlSiO4-SBC600 and mag-
netic Na2SiO3/Fe3O4-SBC600 for biodiesel production. For Na2SiO3/
NaAlSiO4-SBC600, an orthogonal table (4 factors and 3 levels) was de-
signed to optimize soybean oil transesterification under conditions of
methanol to oil molar ratio of 7/1–11/1, catalyst of 4–8 wt%, reaction
temperature of 45–65 °C and time of 1–3 h. While, for the magnetic
Na2SiO3/Fe3O4-SBC600 catalyst, single-factor experiments were con-
ducted to transesterify soybean oil to biodiesel under conditions of
methanol/oil molar ratio of 5/1–13/1, catalyst dosage of 2–10 wt% and
temperature of 35–75 °C for 1–3 h.
2.4.3. Biodiesel analysis
After transesterification reactions, three phases were formed in the
bottle, crude biodiesel at the top, glycerin and superfluous methanol in
the middle, and solid catalyst at the bottom (while, after pre-ester-
ification of SBC oil, methanol and water at the top, SBC oil in the
middle, and H2SO4 and water at the bottom). Solid Na2SiO3/NaAlSiO4-
SBC600 or Na2SiO3/Fe3O4-SBC600 was separated from the liquid pro-
ducts by precipitation for standing overnight or magnet attraction, re-
spectively. The catalyst was named as the recovered catalyst after col-
lected and used directly for recycle experiments without any washing.
The crude biodiesel was filtered (pore size 0.22 μm), and washed by hot
water (80 °C) in a separating funnel to remove by-products and im-
purities from saponification, dried at 105 °C overnight and analyzed by
the GC with a capillary column of Rtx-Wax (30 m ×
Ф0.25 mm × 0.25 μm). Analytical conditions were injector tempera-
ture 260 °C, column temperature 220 °C, detector temperature 280 °C,
carrier gas of nitrogen with a flow rate 1 mL/min and split ratio 40/1.
Heptadecanoic acid methyl ester (HAME, C17:0) was used as internal
standard for quantitative analysis, while biodiesel yield was calculated
by the weights of standard methyl eaters, calibration factors and peak
areas (GC graph given in Fig. S2). The equation is as follow:
∑=
×
×
Biodiesel yield (wt%)
(
Chromatographic peak area of methyl esters
calibration factors
)/
(
Chromatographic peak area of HAME weight of HAME
weight of crude biodiesel sample
) 100%. (1)
Calibration factors for each FAME were determined by the external
reference method [42], in which the mixed standard methyl eaters were
put into a 2 mL sample bottle and three times dilution by di-
chloromethane. For palmitate (C16:0), stearate (C18:0), oleate (C18:1),
linoleate (C18:2) and linolenate (C18:3) to HAME, the calibration
Fig. 1. A general workflow for the synthesis of non-magnetic and magnetic catalysts for biodiesel production.
W.-J. Cong, et al. Applied Energy 264 (2020) 114735
4
factors (each with 5 points, R2 ≥ 0.9999) were 1.006, 0.960, 1.045,
1.043, and 0.985, respectively (Fig. S3a–f and Table S1 were in sup-
plementary materials).
3. Results and discussion
Both non‐magnetic and magnetic catalysts were synthesized from
SBC ash or char for biodiesel production from plant oils (soybean,
blended high AV and SBC oils) with a general workflow for catalyst
synthesis and biodiesel production (Fig. 1). Tables 1 and 2 summarize
the base catalysts reported for biodiesel production and the experi-
mental data during the screening catalysts at different calcination
temperatures, respectively. Optimization of biodiesel yield with non‐-
magnetic and magnetic catalysts (Table 3 and Fig. 2), catalyst cycles
(Fig. 3), XRF and BET characterizations (Tables 4 and 5) were given
respectively. XRD, FT-IR and TGA-DSC curves for both catalysts were
plotted in Figs. 4–6. SEM images, CO2-TPD profiles and EDX-mapping
for Na2SiO3/NaAlSiO4-SBC600 were shown in Figs. 7–9, respectively.
Figs. 10–12 display hysteresis loops for magnetic catalyst, biodiesel
yield from high AV oils and at different AVs with the catalysts. Cali-
bration curves, GC and other characterizations (SEM, CO2-TPD and
EDX-mapping analyses) for SBC char and Na2SiO3/Fe3O4-SBC600 were
supplemented in Figs. S1–S6.
3.1. Optimization of biodiesel yield with Na2SiO3/NaAlSiO4-SBC600
Na2SiO3/NaAlSiO4-based catalysts calcined at different tempera-
tures (400–600 °C) have different activities for biodiesel production
from soybean oil. Reaction conditions (11/1 methanol/oil molar ratio,
8 wt% catalyst dosage, 60 °C, 3 h) were selected to screen the catalyst
calcined at 400, 500 and 600 °C [43]. In Table 2, biodiesel yield
reached 96.54%and 96.44% with the catalysts calcined at 400 and
500 °C, respectively, but obvious saponification occurred. At calcina-
tion temperature of 600 °C, little saponification was found with 96.33%
biodiesel yield. So, Na2SiO3/NaAlSiO4-SBC600 was chosen as catalyst
for the subsequent experiments.
In Table 3, four factors and three levels (temperature of 45, 55 and
65 °C; reaction time of 1, 2 and 3 h; methanol/oil molar ratio of 7, 9 and
11 and catalyst dosage of 4, 6 and 8 wt%) orthogonal experiments were
designed to optimize biodiesel yield. Parameter K and R are average
and extreme values, respectively. For temperature, K3 > K2 > K1,
65 °C was chosen to be the best temperature. For reaction time,
K3 > K2 > K2, 3 h was the most suitable time for Na2SiO3/NaAlSiO4-
SBC600 to catalyze the transesterification of soybean oil. So are me-
thanol/oil molar ratio of 11 and catalyst dosage of 8 wt%. Moreover,
parameter R took the order of temperature (R = 18.41) > reaction
time (R = 14.19) > methanol/oil molar ratio (R = 13.59) > catalyst
dosage (R = 9.39). In summary, Na2SiO3/NaAlSiO4-SBC600 catalyzes
soybean oil transesterification reaching the maximum biodiesel yield of
99.1% under the best conditions of methanol /oil molar ratio of 11 with
8 wt% catalyst at 65 °C in 3 h.
3.2. Biodiesel production with magnetic Na2SiO3/Fe3O4-SBC600
The above base catalyst was magnetized for better separation from
liquid products after reactions. Under the same reaction conditions as
the above for non-magnetic catalyst, biodiesel yield was 90.95%,
84.64% and 93.06% when Na2SiO3/Fe3O4-SBCT was obtained by cal-
cining at 400, 500 and 600 °C, respectively (Table 2). Meanwhile,
Na2SiO3/Fe3O4-SBCT calcined at 600 °C was more stable since 90.24%
biodiesel yield was achieved with the used catalyst after 1 cycle while
the other two decreased to 60.05% and 71.87%, respectively. There-
fore, the magnetic catalyst calcined at 600 °C (Na2SiO3/Fe3O4-SBC600)
was selected for the optimization of soybean oil to biodiesel using
single-factor experiments (Fig. 2).
In Fig. 2a, methanol/oil molar ratio was set in 5/1–13/1 under the
temperature of 65 °C for 3 h reaction with 8 wt% catalyst. When the
molar ratio increased from 5/1 to 11/1, biodiesel yield rose from
69.34% to 87.39%, and further increased to 99.19% at 13/1. So, me-
thanol/oil molar ratio of 13/1 was selected as the best value for the
following experiments.
In Fig. 2b, catalyst dosage from 2 to 10 wt% was studied at 65 °C,
13/1 methanol/oil molar ratio and 3 h. Biodiesel yield increased from
84.97% to 98.96% at catalyst from 2 to 8 wt%, and dropped to 88.86%
at 10 wt% catalyst that may be caused by poor mixing of liquid re-
actants and catalyst [26]. Catalyst dosage of 8 wt% was selected.
In Fig. 2c, temperature from 35 to 75 °C was applied to optimize
biodiesel yield under the conditions of 13/1 methanol/oil molar ratio,
8 wt% catalyst and 3 h. Biodiesel yield increased from 39.77% to
99.19% as temperature rose from 35 to 65 °C. At 75 °C, the yield de-
clined to 95.85% (<99.19% at 65 °C). So, 65 °C was chosen to be the
best reaction temperature.
In Fig. 2d, temperature of 65 °C, 13/1 methanol/oil molar ratio, and
8 wt% catalyst were fixed to optimize reaction time from 1 to 3 h.
Table 2
Screening catalysts at different calcination temperatures for biodiesel production from soybean oil.
Catalyst Saponification Biodiesel yield (%) from soybean oila
Fresh catalyst Reused catalyst after 1 cycle
Na2SiO3/NaAlSiO4-SBC400 Yes 96.54 ± 0.04 94.08 ± 0.24
Na2SiO3/NaAlSiO4-SBC500 Yes 96.44 ± 0.11 98.32 ± 1.78
Na2SiO3/NaAlSiO4-SBC600 No 96.33 ± 0.12 96.67 ± 0.07
Na2SiO3/Fe3O4-SBC400 No 90.95 ± 4.45 60.05 ± 6.01
Na2SiO3/Fe3O4-SBC500 No 84.64 ± 6.41 71.87 ± 2.93
Na2SiO3/Fe3O4-SBC600 No 93.06 ± 3.89 90.24 ± 4.31
a Methanol/oil molar ratio: 11/1; catalyst dosage: 8 wt%; reaction time: 3 h; reaction temperature: 60 °C.
Table 3
Optimization of biodiesel production from soybean oil with Na2SiO3/NaAlSiO4-
SBC600 according to an orthogonal experimental design L9 (3)4.
No. A
Temperature
(oC)
B
Reaction
time (h)
C
Methanol/
oil molar
ratio
D Catalyst
dosage
(wt%)
Biodiesel yield (%)
1 45 1 7 4 59.55 ± 2.64
2 45 2 9 6 84.23 ± 0.04
3 45 3 11 8 96.73 ± 2.46
4 55 1 9 8 89.69 ± 2.43
5 55 2 11 4 97.56 ± 3.46
6 55 3 7 6 95.02 ± 0.33
7 65 1 11 6 99.02 ± 0.29
8 65 2 7 8 97.96 ± 0.83
9 65 3 9 4 99.09 ± 0.17
K1 80.277 82.753 84.177 85.400
K2 94.090 93.357 91.110 92.863
K3 98.690 96.947 97.770 94.793
R 18.413 14.194 13.593 9.393
W.-J. Cong, et al. Applied Energy 264 (2020) 114735
5
0
20
40
60
80
100
5/1 7/1 9/1 11/1 13/1
B
io
di
es
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 y
ie
ld
 (%
)
Methanol/oil molar ratio
Reaction temperature: 65 oC
Reaction time: 3 h
Catalyst dosage: 8 wt%
0
20
40
60
80
100
1 1.5 2 2.5 3
B
io
di
es
el
 y
ie
ld
 (%
)
Reaction time (h)
Reaction temperature: 65 oC
Methanol/oil molar ratio: 13/1
Catalyst dosage: 8 wt%
0
20
40
60
80
100
35 45 55 65 75
B
io
di
es
el
 y
ie
ld
 (%
)
Reaction temperature (oC)
Catalyst dosage: 8 wt% 
Methanol/oil molar ratio: 13/1 
Reaction time: 3 h
0
20
40
60
80
100
2 4 6 8 10
B
io
di
es
el
 y
ie
ld
 (%
)
Catalyst dosage (wt%)
Reaction temperature: 65 oC 
Methanol/oil molar ratio: 13/1 
Reaction time: 3 h
(a) (b)
(c) (d)
Fig. 2. Single-factor experiments of soybean oil transesterification with magnetic Na2SiO3/Fe3O4-SBC600 catalyst: (a) methanol/oil molar ratio, (b) catalyst dosage,
(c) reaction temperature, and (d) reaction time.
40
60
80
100
1 2 3 4 5 6 7 8
B
io
di
es
el
 y
ie
ld
 (%
)
Catalyst cycle
(a)
Reaction time: 3 h 
Reaction temperature: 65 oC 
Methanol/oil molar ratio:11/1 
Catalyst dosage: 8 wt.% 
(i) 
(ii) 
(iii) 
(iv) 
60
70
80
90
100
1 2 3 4 5 6 7 8
B
io
di
es
el
 y
ie
ld
 (%
)
Catalyst cycle
Reaction time: 3 h 
Reaction temperature: 65 oC 
Methanol/oil molar ratio:11/1 
Catalyst dosage: 3 wt.% 
(b)
(i) 
(ii) 
Fig. 3. Catalyst cycle for soybean biodiesel production (a) 8 wt% catalyst, and (b) 3 wt% catalyst without washing after each cycle (i: Na2SiO3, ii: Na2SiO3/NaAlSiO4-
SBC600, iii: magnetic Na2SiO3/Fe3O4-SBC600 and iv: Na2SiO3/NaAlSiO4-SBC600 synthesized in a Teflon reactor).
Table 4
Elemental compositions of SBC ash and char, Na2SiO3/NaAlSiO4-SBC600, and magnetic Na2SiO3/Fe3O4-SBC600 catalysts.
Catalyst C analyzed by organic elemental analyzer and inorganics by XRF (wt%)
C S Si Na Al Fe K Ca Mg Other elements (e.g., Ti, Cr, Zn et al.)
SBC char 5.82 0.11 27.24 0.26 6.82 2.98 1.36 1.56 2.47 0.72
SBC ash 0.14 0.13 28.23 0.28 7.05 2.44 1.46 1.71 2.56 0.52
Na2SiO3/NaAlSiO4-SBC600 0.94 0.05 18.21 27.25 4.07 2.10 1.04 1.15 1.98 0.54
Na2SiO3/NaAlSiO4-SBC600 from Teflon reactor 0.78 0.05 17.91 26.65 4.33 2.14 1.10 1.20 1.45 0.55
Na2SiO3/NaAlSiO4-SBC600 after 8 cycles 1.94 0.05 24.39 14.51 5.48 2.66 1.36 1.45 1.29 0.60
Na2SiO3/Fe3O4-SBC600 3.38 11.24 3.71 24.13 0.77 11.46 0.26 0.29 0.17 0.15
Na2SiO3/Fe3O4-SBC600 after 5 cycles 2.84 11.89 5.26 21.80 1.13 10.97 0.29 0.27 0.27 0.16
W.-J. Cong, et al. Applied Energy 264 (2020) 114735
6
Table 5
Physicochemical properties of SBC ash, Na2SiO3/NaAlSiO4-SBC600, and magnetic Na2SiO3/Fe3O4-SBC600 catalysts.
Catalysts Specific surface area (m2/g) Pore volume (cm3/g) Pore diameter (nm) Basicity (mmol/g)
SBC ash 86.27 1.58 × 10−1 3.82 0.01
Na2SiO3 0.48 4.78 × 10−3 3.06 0.22
Na2SiO3 after 8 cycles 6.49 4.70 × 10−3 3.06 0.19
Na2SiO3/NaAlSiO4-SBC600 0.02 5.51 × 10−3 3.32 0.21
Na2SiO3/NaAlSiO4-SBC600 after 8 cycles 3.26 1.74 × 10−2 3.78 0.14
Na2SiO3/Fe3O4-SBC600 0.79 3.28 × 10−3 3.38 0.07
Na2SiO3/Fe3O4-SBC600 after 5 cycles 2.56 1.08 × 10−2 3.82 —
5 15 25 35 45 55 65 75 85
In
te
ns
ity
2-Theta (degree)
1 1
1
1
1
1
1
1
2
2
2
1
1
11
1(a)
(b)
(c)
(d)
1
1
1 3
1
1
1 13
4
4
4
2
2
4 4 4
1 Na2SiO3 2 NaAlSiO4
3 Fe3O4 4 Na2SO4 
Fig. 4. XRD patterns (a) Na2SiO3/NaAlSiO4-SBC600, (b) Na2SiO3/NaAlSiO4-SBC600 after 8 cycles, (c) magnetic Na2SiO3/Fe3O4-SBC600, and (d) magnetic Na2SiO3/
Fe3O4-SBC600 after 5 cycles.
400900140019002400290034003900Tr
an
sm
itt
an
ce
 (%
)
Wavenumber (cm-1)
(c)
(b)
(a)
Si=O
1600
Si-O 
792
Si-O-Si
690, 470
Si-O-Si
1127
Si=O
1440
Si-O-Si
880
Fe-O 
620
Si-O 
506
Si-O-Na
960Si-O-H 
3440
Fig. 5. FT-IR spectra (a) SBC ash, (b) magnetic Na2SiO3/Fe3O4-SBC600, and (c) Na2SiO3/NaAlSiO4-SBC600.
W.-J. Cong, et al. Applied Energy 264 (2020) 114735
7
Biodiesel yield increased from 82.26% to 98.96% as time prolonged
from 1 to 3 h. Little increase in biodiesel yield was found from 1 to
2.5 h, but a distinct growth occurred at 3 h. The best value for reaction
time was chosen as 3 h.
In summary, the best reaction conditions with biodiesel yield of
99.19% for soybean oil transesterification with the magnetic Na2SiO3/
Fe3O4-SBC600 catalyst were {methanol to oil molar ratio of 13/1, cat-
alyst dosage of 8 wt%, reaction temperature of 65 °C and reaction time
of 3 h}. Xie et al. [17] also synthesized recyclable magnetic Fe3O4/
MCM-41/ECH/Na2SiO3 and reached 99.2% biodiesel yield due to
higher surface area. As compared with Na2SiO3/NaAlSiO4-SBC600,
Na2SiO3/Fe3O4-SBC600 needs more methanol (13/1 vs. 11/1) for bio-
diesel production to achieve yield over 99%.
In order to figure out catalyst stability of both non-magnetic and
magnetic catalysts as well as Na2SiO3, catalyst cycle experiments were
studied under conditions of {11/1 methanol/ oil molar ratio and 8 wt%
catalyst dosage at 65 °C for 3 h} with the recovered catalyst at different
cycles for the production of biodiesel.
3.3. Catalyst recycle
Recycle experiments for both non‐magnetic and magnetic catalysts
were conducted under the optimized conditions of {65 °C, 3 h, 8 wt%
catalyst and 11/1 methanol/oil molar ratio}, and calcined Na2SiO3 and
Na2SiO3/NaAlSiO4-SBC600 with All recovered catalyst after reaction
and separation was used directly without any washing. In Fig. 3a for
Na2SiO3/NaAlSiO4-SBC600, at the 8th cycle, biodiesel yield decreased
little to 95.93% from 98.13% at the 1st as compared to 99.08% for
92
94
96
98
100
0 100 200 300 400 500 600 700 800 900 1000
W
ei
gh
t l
os
s (
w
t.%
)
Temperature (oC)
(d)
(b)
(c)
(a)(A)
-25
-20
-15
-10
-5
0
0 100 200 300 400 500 600 700 800 900 1000
D
SC
(m
W
/m
g)
Temperature (oC)
(a)
(c)
(b)
(d)
(B)
Fig. 6. Thermal stability (A) TGA and (B) DSC: (a) Na2SiO3/NaAlSiO4-SBC600 (b) magnetic Na2SiO3/Fe3O4-SBC600 (c) SBC char, and (d) magnetic Na2SiO3/Fe3O4-
SBC600 after 5 cycles.
W.-J. Cong, et al. Applied Energy 264 (2020) 114735
8
Na2SiO3 from 99.78%. Since biodiesel yields changed little at the 8th
cycle, less catalyst dosage (3 wt%) was applied for recycling experi-
ments under the same conditions (Fig. 3b). At the 6th cycle, biodiesel
yield for both Na2SiO3/NaAlSiO4-SBC600 and Na2SiO3 was still over
92%. However, at the 7th and 8th cycles, biodiesel yield decreased to
81.51% and 72.38% with Na2SiO3/NaAlSiO4-SBC600, 94.59% and
79.72% with Na2SiO3, respectively. Meanwhile, magnetic Na2SiO3/
Fe3O4-SBC600 (8 wt%) was cycled under the same conditions (Fig. 3a).
Biodiesel yield decreased from 99.19% to 87.66% at the 3rd cycle and
dropped sharply to 63.55% and 66.13% at the 4th and 5th cycles, re-
spectively. These results clearly show that the synthesized non‐-
magnetic catalyst is highly active and stable for biodiesel production
with similar properties to Na2SiO3. Nevertheless, after magnetized, its
catalytic properties declined. The catalysts were further characterized
by many techniques to further know their physiochemical structures
and properties that influence the production of biodiesel.
3.4. Catalyst characterization
SBC ash contains 28.23 wt% Si and 7.05 wt% Al detected by XRF
(Table 4) that can be used for the synthesis of base Na2SiO3 and
NaAlSiO4. Differently from SBC ash, SBC char with 5.82 wt% C detected
by organic elemental analyzer from residual oil in SBC, indicating that
it can be magnetized by reducing Fe2O3 to magnetic Fe3O4 with carbon.
In Table 4, after 8 cycles for Na2SiO3/NaAlSiO4-SBC600, Na de-
creased from 27.25% to 14.51% but Al increased from 4.07% to 5.48%,
suggesting that Na2SiO3 may be dissolved during the reactions. Because
Si and Al increased from 18.21 to 24.39 wt%, and 4.07 to 5.48 wt%,
showing the relative stability of NaAlSiO4, with biodiesel yield de-
creased slightly from 98.13% to 95.93% under 11/1 methanol/oil
molar ratio with 8 wt% catalyst at 65 °C in 3 h. It was also found that Na
decreased from 24.13 wt% to 21.80 wt% for Na2SiO3/Fe3O4-SBC600
after 5 cycles, due to the leaching of Na2SiO3 in methanol that made
biodiesel yield decreased from 98.96% to 66.13%.
In Table 5, SBC ash had a comparatively large BET specific surface
area of 86.27 m2/g, pore volume of 1.58 × 10−1 cm3/g and pore
diameter of 3.82 nm. After burning, organics were removed by forming
CO2, producing numerous pores in SBC ash with high specific surface
area. Na2SiO3/NaAlSiO4-SBC600 had a very small specific surface area
(0.02 m2/g) and pore volume of 5.51 × 10−3 cm3/g due to their non-
volatility. After 8 cycles, the catalytic activity reduced slightly with
specific surface area increased to 3.26 m2/g, and pore volume to
1.74 × 10−2 due to the leaching of Na2SiO3 as confirmed by XRD
spectra (Fig. 4) with the disappearance of its main peak. The pore
diameter of catalyst increased slightly during cycles. Furthermore,
Na2SiO3/NaAlSiO4-SBC600 catalyst had smaller specific surface area
(0.02 vs 0.79 m2/g) but larger pore volume (5.51 × 10−3 vs.
3.28 × 10−3 cm3/g) than that of magnetic Na2SiO3/Fe3O4-SBC600.
In Fig. 4a (XRD spectra), non-magnetic Na2SiO3/NaAlSiO4-SBC600
catalyst was composed of Na2SiO3 and NaAlSiO4, with plenty of crystal
structures of Na2SiO3 at 16.9°, 25.02°, 29.34°, 34.86°, 37.2°, 45.72°,
48.08°, 49.58°, 52.18°, 60.2°, 64.24°, 65.74°, and 87.36°, and crystal
structures of NaAlSiO4 at 21°, 34.62° and 61.96° (JCPDS: 74-0748, 16-
0818). In Fig. 4b, nearly all peaks for Na2SiO3 crystals for used
Na2SiO3/NaAlSiO4-SBC600 after 8 cycles were decreased significantly
but those for NaAlSiO4 crystals were still strong. In Fig. 4c and d,
magnetic catalyst of Na2SiO3/Fe3O4-SBC600 was composed of Na2SiO3,
NaAlSiO4, Na2SO4 and Fe3O4. Na2SiO3 peak was much weaker than that
for Na2SiO3/NaAlSiO4-SBC600 due to the relatively small concentration
diluted by other components. Such as Na2SO4 and magnetic Fe3O4 as
confirmed by Na2SO4 peaks found at 23.14°, 24.98°, 31.92°, 41.12°, 43°
and 56.92° (6NaOH + Fe2(SO4)3 → 3Na2SO4 + 2Fe(OH)3 ↓) and Fe3O4
peaks at 30.26° and 35.48° (12Fe(OH)3 + C→ 4Fe3O4 + 18H2O+ CO2
↑) with little change after 5 cycles.
In Fig. 5a, IR peaks for SBC were found at 1600 (Si]O stretching),
792 (SieO vibration of quartz impurities), and 690 and 470 cm−1
corresponding to the vibration band of SieOeSi [44]. In Fig. 5c,
Na2SiO3/NaAlSiO4-SBC600 had similar absorption bands to Na2SiO3/
1 m 10 m
(ii)(a) (i)
10 m1 m
(ii)(b) (i)
Fig. 7. SEM images (a) Na2SiO3/NaAlSiO4-SBC600 and (b) magnetic Na2SiO3/NaAlSiO4-SBC600 after 8 cycles (i: large magnification; ii: small magnification).
W.-J. Cong, et al. Applied Energy 264 (2020) 114735
9
Fe3O4-SBC600 at 506, 880 and 1440 cm−1 corresponding to SieO
bending, the stretching vibrations of SieOeSi and Si]O stretching
vibration, respectively [45]. The vibrations of SieOeSi linking struc-
ture illustrated the formation of sodium silicate. In addition, a peak of
stretching vibration for SieOeNa at 960 cm−1 was found for Na2SiO3/
NaAlSiO4-SBC600 [46]. In biodiesel production, the active sites Na2O
and soluble CH3ONa formed from SieO and SieOeNa in both Na2SiO3
and NaAlSiO4 (Fig. 5, IR spectra) would promote the transesterification
reactions [46]. Obvious absorptions at 970–1200 cm−1 were from the
asymmetric stretching bond of SieOeSi and broad peak at 3440 cm−1
coming from SieOeH stretching vibration because of the absorbed
water molecules on its surface. In Fig. 5b, the peak from Na2SiO3/
Fe3O4-SBC600 at 1127 cm−1 was for SieOeSi absorptions [47]. A
characteristic band of FeeO stretching at about 620 cm−1 wasfor
Fe3O4 [48], making Na2SiO3/Fe3O4-SBC600 had certain magnetism
from magnetic Fe3O4 particles.
In Fig. 6 (TGA-DSC), little weight loss was found for the catalysts. In
Fig. 6A, small weight loss at 700 °C for catalysts (except Na2SiO3/
NaAlSiO4-SBC600 because it was obtained at high calcination tem-
perature) was due to the decomposition of biochar. No obvious peaks in
DSC curves (Fig. 6B) suggested that no phase change and chemical
decomposition for exothermic/ endothermic phenomena during
heating of the catalysts.
In Fig. 7 (SEM), the surface morphology of Na2SiO3/NaAlSiO4-
SBC600 was uneven with lumps of matters covered with Na2SiO3 gel.
After several cycles, catalyst particles turned to be fractured, and were
totally different from the smooth surface of the fresh catalyst, because
Na2SiO3 gel on the surface disappeared to expose the internal structure.
The surface morphology of magnetic Na2SiO3/Fe3O4-SBC600 displayed
similar change to the non-magnetic catalyst, but with rough surface of
SBC char covered with Na2SiO3 gel (Fig. S4).
In Fig. 8 (TPD-CO2), SBC ash had basicity of 0.01 mmol/g due to the
0
100
200
300
400
0
0.5
1
1.5
2
2.5
0 500 1000 1500 2000 2500 3000 3500 4000
Te
m
pe
ra
tu
re
 (°
C)
C
O
2
de
so
rp
tio
n 
si
gn
al
 (m
V
)
Time (s)
(a)
(i) Na2SiO3/NaAlSiO4-SBC600: 0.21 mmol/g
(ii) Na2SiO3: 0.22 mmol/g
0
100
200
300
400
0
0.5
1
1.5
2
2.5
0 500 1000 1500 2000 2500 3000 3500 4000
Te
m
pe
ra
tu
re
 (°
C
)
C
O
2
de
so
rp
tio
n 
si
gn
al
 (m
V
)
Time (s)
(i) Na2SiO3/NaAlSiO4-SBC600:
0.14 mmol/g
(ii) Na2SiO3: 0.19 mmol/g
(b)
Fig. 8. CO2-TPD profiles of (a) fresh catalyst and (b) catalyst after 8 cycle: (i) Na2SiO3/NaAlSiO4-SBC600 and (ii) Na2SiO3.
W.-J. Cong, et al. Applied Energy 264 (2020) 114735
10
presence of a small amount of metal existing as oxides like Na2O, Al2O3,
CaO and MgO (Table 4; calibration curve: Fig. S1), and magnetic
Na2SiO3/Fe3O4-SBC600 had a value of 0.07 mmol/g (Fig. S5). Non-
magnetic catalyst of Na2SiO3/NaAlSiO4-SBC600 had basicity of 0.21
(with the approximate value to calcined Na2SiO3, 0.22 mmol/g), but
decreased to 0.14, versus 0.19 mmol/g for that of recovered Na2SiO3
after 8 cycles. Meanwhile, the main desorption peak for Na2SiO3/
NaAlSiO4-SBC600 was lower (140 °C) than that (170 °C) for Na2SiO3.
Furthermore, compared to the reported Na2SiO3 loaded on biochars,
the basicity of Na2SiO3/NaAlSiO4-SBC600 was very low (0.21 vs. 3.18
for Na2SiO3@Ni/C derived from bamboo [49], 3.24 for Na2SiO3@Ni/
JRC derived from Jatropha hulls [13] and 0.56 mmol/g for Na2SiO3@
Fe3O4/C derived from glucose [39]). The reason is that Na2SiO3/
NaAlSiO4-SBC600 had extremely low surface area (0.02 vs. 8.24 for
Na2SiO3@Ni/C, 12.2 for Na2SiO3@Ni/JRC). Biodiesel yield is related to
the basicity [50]. For example, biodiesel yield of 98.13% corresponds to
the high basicity of 0.21 mmol/g for fresh Na2SiO3/NaAlSiO4-SBC600 as
compared to 66.13% yield to basicity of< 0.07 for Na2SiO3/Fe3O4-
SBC600 after 5 cycles.
In Fig. 9 (EDX mapping for Na, Al, Si and O), similar concentration
distribution of Na, Si, Al and O in Na2SiO3/NaAlSiO4-SBC600 catalyst
revealed the co-existence of both Na2SiO3 and NaAlSiO4, and changed
little after 8 cycles. But, for SBC char and magnetic Na2SiO3/Fe3O4-
SBC600 catalyst (Fig. S6), Fe content was measured for magnetism ob-
servation and Si content declined distinctly as proved by XRF (declined
from 27.24 to 3.71 wt%) because Na2SiO3 and Fe3O4 were formed and
covered over the surface of the magnetic catalyst.
In Fig. 10, hysteresis loops measurement gave the specific magnetic
2.5 m
(ii) Na (iii) Al
(a)
(iv) Si (v) O
(i) (b)
2.5 m
(iv) Si (v) O
(i)
(ii) Na (iii) Al
Fig. 9. EDX-mapping images of Na2SiO3/NaAlSiO4-SBC600 (a) before and (b) after 8 cycles.
Sa
tu
ra
tio
n 
m
ag
ne
tiz
at
io
n 
(e
m
u/
g)
Magnetic field (Oe)
(b)
(a)
-5000 50000-10000 10000
5
-5
(a) Ms : 6.86 emu/g 
(b) Ms : 9.23 emu/g 
10
-10
Fig. 10. Hysteresis loops of magnetic Na2SiO3/Fe3O4-SBC600 (a) before and (b) after 5 cycles.
W.-J. Cong, et al. Applied Energy 264 (2020) 114735
11
saturation (Ms) for Na2SiO3/Fe3O4-SBC600 of 6.86 and 9.23 emu/g
before and after 5 cycles, respectively, while SBC particles had no
magnetism. The magnetism of cycled catalysts became stronger due to
the removal of active substances to expose magnetic substance.
3.5. Direct production biodiesel from waste oils
Waste cooking oils with different AVs (2.02–20.12) were directly
used to produce biodiesel with the solid bases.
3.5.1. SBC oil with high AV (20 mg KOH/g)
It is vital that bases could resist FFAs in waste oils to form soap for
the direct production of biodiesel. Here, a very high AV SBC oil was
used to test the base stability with AV of 20.12 mg KOH/g, obtained by
sulfuric acid esterification of crude SBC oil with AV of 42.62.
Experiments under the condition of {11/1 methanol/oil molar ratio and
8 wt% catalyst at 65 °C for 3 h} were designed with the three bases
(Na2SiO3/NaAlSiO4-SBC600, magnetic Na2SiO3/Fe3O4-SBC600 and
commercial Na2SiO3), aiming to confirm their stability. In Fig. 11, SBC
oil achieved biodiesel yield of 72.44% with Na2SiO3/NaAlSiO4-SBC600
as compared to a similar yield of 71.79% from the commercial Na2SiO3
but very low value of 22.31% from magnetic Na2SiO3/Fe3O4-SBC600.
The results illustrate that Na2SiO3/NaAlSiO4-SBC600 base has strong
ability to resist saponification at AV 20.12 with moderate biodiesel
yield (72.44%). In order to reach>90% biodiesel yield for commercial
application, low AV oils were used by blending waste oils with high-
quality soybean oil.
(AV 20.12)
0
20
40
60
80
100
B
io
di
es
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 (%
)
Catalyst
SBC Oil
Methanol/oil molar ratio: 11/1
Temperature: 65 °C 
Reaction time: 3 h
Catalyst dosage: 8 wt%
Na2SiO3/Fe3O4-SBC600Na2SiO3Na2SiO3/NaAlSiO4-SBC600
Blended oil 
(AV 20.02)
Fig. 11. Biodiesel yield from high AV oils (AV = 20 mg KOH/g) with different catalysts.
0
20
40
60
80
100
2.03 3.95 5.91 7.88 9.69 18.8
B
io
di
es
el
 y
ie
ld
 (%
)
Acid value (mg KOH/g)
Na2SiO3/NaAlSiO4-SBC600Na2SiO3/Fe3O4-SBC600 SBC biodiesel
Fig. 12. Production of biodiesel from blended soybean oil at different AV (a) Na2SiO3/NaAlSiO4-SBC600 (b) magnetic Na2SiO3/Fe3O4-SBC600, and (c) SBC biodiesel
production with Na2SiO3/NaAlSiO4-SBC600.
W.-J. Cong, et al. Applied Energy 264 (2020) 114735
12
3.5.2. Simulated blended oil with different AVs (2–20)
Soybean oil was blended with FFA (oleic acid) to reach different AV
oils (2.02–20.02). Blended soybean oil with AV of 20.02 was transes-
terified with similar biodiesel yield (to SBC oil) of 71.19% for Na2SiO3/
NaAlSiO4-SBC600, 70.57% for neat Na2SiO3 and only 21.35% for mag-
netic Na2SiO3/Fe3O4-SBC600 (Fig. 11). Furthermore, a different fraction
of oleic acid (1–10 wt% of soybean oil) was blended to simulate various
high AV (2.03, 3.95, 5.91, 7.88, 9.69 and 18.8 mg KOH/g) waste oils.
Non-magnetic Na2SiO3/NaAlSiO4-SBC600 and magnetic Na2SiO3/
Fe3O4-SBC600 bases were tested for biodiesel production from the
blended soybean oil under the optimal conditions of {methanol/oil
molar ratio of 11/1, 8 wt% catalyst dosage, 65 °C and 3 h}. In Fig. 12,
for non-magnetic Na2SiO3/NaAlSiO4-SBC600, biodiesel yield was over
96% when AV was ≤9.69 mg KOH/g. Meanwhile, when AV reached
18.8 mg KOH/g, biodiesel yield was only 74.19%. Furthermore, for
magnetic Na2SiO3/Fe3O4-SBC600, biodiesel yield was 92.71%, 91.64%,
91.87% at AV of 2.03, 3.95 and 5.91 mg KOH/g, respectively. As AV
increased to 7.88 mg KOH/g, biodiesel yield decreased sharply to only
30.58% with obvious saponification. It can be concluded that non-
magnetic Na2SiO3/NaAlSiO4-SBC600 base has better performance. It can
be used to resist saponification in the transesterification of oil at AVof
9.69 as compared to less resistance ability in the previous work [39]
(with AV of 4.8) due to the high stability of NaAlSiO4 in this work.
3.5.3. SBC oil with low AV (10)
Actual SBC oil with different AVs was also used for biodiesel con-
version by Na2SiO3/NaAlSiO4-SBC600 base. Gratifyingly, similar results
were obtained in Fig. 12c that biodiesel yield achieved 95.8% from SBC
oil at AV of 9.71 (with the composition of FAMEs: 10.3% palmitoleate,
4.1% stearate, 29.7% oleate, 50.4% linoleate and 2.8% linolenate in
Fig. S2).
It can be confirmed that the non-magnetic Na2SiO3/NaAlSiO4-
SBC600 base synthesized in this work can be used for the transester-
ification of oils with AV < 9.7 in one step conversion, while magnetic
Na2SiO3/Fe3O4-SBC600 was for oil at AV less than 5.9 mg KOH/g.
Fig. 13 summarized biodiesel production from waste oils at different
AVs from 0.5 to 20 in the presence of Na2SiO3/NaAlSiO4-SBC600 base.
At AV of 40, severe saponification occurred with the mixture solidified
even at 65 °C. At AV of 20 (Fig. 13a), biodiesel yield reached 72.4%
from SBC oil, and 71.2% from blended oil. However, some soap was
produced as proved by the whole mixture formed a bulk solid after
cooled to room temperature. Low AV was required by blending waste
oils with soybean oil. In Fig. 13c (AV 2–10), biodiesel yield reached
96.5% at AV of 9.7, which is good enough for commercial production.
The base is much effective to resist FFAs than commercial KOH ap-
plicable for only AV < 1 oils [51] and could be recycled 8 times when
high-quality soybean oil (AV of 0.5) was used. The base catalyzed ac-
tual SBC oil at AV of 10 (9.71) to give 95.8% biodiesel yield (Fig. 13c).
The non-magnetic Na2SiO3/NaAlSiO4-SBC600 base showed excellent
performance in the one-step production of biodiesel from waste oils
with AV less than 10.
3.5.4. Commercial production of biodiesel from waste oils
Currently, the commercial production of biodiesel is a base-cata-
lyzed process with homogeneous NaOH or KOH. However, homo-
geneous base is easily saponified with FFAs. When NaOH was used as
catalyst to transesterify soybean oil, it caused saponification with only
73% biodiesel yield even at AV of 0.34. For low-grade waste oils, pre-
treatment step is required to reduce AV by esterification with homo-
geneous acid (e.g., H2SO4) before the second step of transesterification.
Acid catalysts were proved to be technically feasible with less complex
one-step process as compared to the current commercial base-catalyzed
system [52], but it was not favorable for industrial application to use
homogeneous acid due to hardly recycle and equipment corrosion [53].
Lam et al. [53] suggested using heterogeneous acid catalyst was the
best option to produce biodiesel from oil with high FFAs, however, high
temperatures were required. Even with strong homogeneous sulfuric
acid, Goff et al. [54] reported 120 °C and 20 h were needed for trans-
esterification. Moreover, Teo et al. [43] produced biodiesel directly
from high AV Jatropha oil with CaSO4/Fe2O3-SiO2 core-shell magnetic
nanoparticles but required conditions of 12 wt% catalyst for 4 h at
120 °C, which are much severer than 8 wt% catalyst for 3 h at 65 °C
reported in this work.
In this work, a solid base catalyst was produced by a simple alka-
lization of SBC ash, which can resist saponification from high AV oils at
9.6 mg KOH/g. The base can be directly applied for biodiesel produc-
tion from waste oils at AV less than 10 without pretreatment with>
96% yield at only 65 °C. For higher AV oil, it is suggested to blend it
with soybean oil to reach AV < 10. The scientific contributions are
found that the mixtures of Na2SiO3 and NaAlSiO4 (by reactions of
NaOH with oxides of SiO2 and Al2O3 in SBC ash) can resist saponifi-
cation and effectively catalyze the transesterification of waste oils at
low temperatures due to the active sites of Na2O and CH3ONa. The base
derived from the waste can be produced in large quantities with good
reusability rather than those prepared by chemical synthesis in small
scale generally using expensive and toxic reagents, which is promising
to replace conventional NaOH or KOH and even solid bases because of
mild reaction conditions, less corrosion to the reactor and catalyst re-
cyclability.
3.6. Catalysts synthesized in Teflon reactor
Since catalysts may be contaminated by glass by reacting SiO2 from
the flask with NaOH (2NaOH + SiO2 → Na2SiO3 + 2H2O), Teflon
reactor replaced glass flask as a reactor to synthesize Na2SiO3/
NaAlSiO4-SBC600 to study the effect of glass reactor with NaOH under
the same condition. In Table 4 (XRF), the Na2SiO3/NaAlSiO4-SBC600
from Teflon reactor contained lower Na (26.65 vs. 27.25 wt% for glass
flask) and Si (17.91 vs. 18.21 wt% for glass flask) as compared with that
from flask. In Fig. 3a-iv, the catalyst from Teflon reactor was conducted
recycle experiments under the same conditions of {8 wt% catalyst and
11/1 methanol/oil molar ratio at 65 °C in 3 h} with biodiesel yield from
99.1% to 96.16%. As compared with the yield from 98.96% to 95.93%
from the 1st to the 8th cycle catalyzed by the catalyst from glass re-
actor, the basicity changed little as measured by CO2-TPD (0.21 vs.
0.21 mmol/g). So, reactor contamination was neglected.
FFAs (%) Biodiesel yield (%)
2
Blended 
oil
Oleic acid (100%)
Soybean oil (0.25%)
40 SBC oil (20%)
Waste oils 
AV (mg KOH/g)
10
Blended oil 
(1-5%)
0.5 99.1% 
8 cycles
 10 SBC oil (5%) 95.8% 
96.5%AV = 9.7
20 SBC oil (10%) 
 Blended oil (10%)
(a)
(b)
(c)
72.4% 
71.2%
Fig. 13. Biodiesel production at 65 °C from waste oils by Na2SiO3/NaAlSiO4-
SBC600 base from (a) SBC oil with high AV (20.1 mg KOH/g); (b) blended oil
with different AVs (2–10), and (c) SBC oil with low AV (10).
W.-J. Cong, et al. Applied Energy 264 (2020) 114735
13
4. Conclusions
A solid base was produced by a simple alkalization of spent
bleaching clay (SBC) ash, which can resist saponification from SBC oil
at high acid value (AV) of 9.71 mg KOH/g without pretreatment with
95.8% biodiesel yield at only 65 °C. Furthermore, the base was mag-
netized for easy separation and achieved 91.9% yield from oil at AV of
5.9. Both catalysts were recyclable and achieved nearly 100% biodiesel
yield from soybean oil. For higher AV oil, it is suggested to blend it with
soybean oil to reach AV<10. The base is easily applied for the com-
mercial production of biodiesel at mild reaction conditions.
CRediT authorship contribution statement
Wen-Jie Cong: Conceptualization, Methodology, Investigation,
Writing - original draft. Yi-Tong Wang: Writing - review & editing. Hu
Li: Conceptualization. Zhen Fang: Supervision, Writing - review &
editing. Jie Sun: Formal analysis. Hai-Tong Liu: Investigation. Jie-
Teng Liu: Investigation. Song Tang: Data curation. Lujiang Xu:
Resources.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgments
The authors wish to acknowledge the financial support from the
Natural Science Foundation of China (No. 21878161) and Nanjing
Agricultural University (68Q-0603).
Appendix A. Supplementary material
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.apenergy.2020.114735.
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	Direct production of biodiesel from waste oils with a strong solid base from alkalized industrial clay ash
	Introduction
	Materials and methods
	Materials
	Catalyst preparation
	Preparation of SBC ash and SBC char
	Na2SiO3/NaAlSiO4-based catalysts derived from SBC ash
	Magnetic Na2SiO3/Fe3O4-based catalysts derived from SBC char
	Catalyst characterization
	Biodiesel production and analysis
	Pretreatment (esterification) of raw SBC oil
	Biodiesel production
	Biodiesel analysis
	Results and discussion
	Optimization of biodiesel yield with Na2SiO3/NaAlSiO4-SBC600
	Biodiesel production with magnetic Na2SiO3/Fe3O4-SBC600
	Catalyst recycle
	Catalyst characterization
	Direct production biodiesel from waste oils
	SBC oil with high AV (20 mg KOH/g)
	Simulated blended oil with different AVs (2–20)
	SBC oil with low AV (10)
	Commercial production of biodiesel from waste oils
	Catalysts synthesized in Teflon reactor
	Conclusions
	CRediT authorship contribution statement
	mk:H1_26
	Acknowledgments
	Supplementary material
	References