Catalytic materials and adsorbents

Prévia do material em texto

35
 
3. General mechanisms in adsorption and heterogeneous catalysis. 
Adsorption and absorption on solids. 
Adsorption is a physico-chemical process due to the interaction between particles and a 
selective adsorbent medium. The solid retains some families of molecules while others are 
not retained. The adsorption process is represented by an equilibrium reaction: 
Agas + S  SA 
where S is the clean adsorbent solid while SA is the solid on the surface of which A is 
adsorbed. The reaction is exothermic (H < 0) because a interaction between A and S is 
established. On the other hand, enthopy is decreased (S < 0) because the A molecules 
are blocked on the S surface. Thus, adsorption is an exothermic equilibrium, favoured at 
low temperature and high pressure. Desorption (regeneration of the solid) will be favoured 
at higher temperature and lower pressure. If adsorption is strong (|H| high), the 
adsorption process will be very efficient, but the desorption (regeneration of the adsorbent) 
will be difficult and/or expensive (high temperature, vacuum). Kinetics aspects in 
adsorption are frequently negligible. This means that adsorption is fast and activation 
energy low. 
Depending on the relationship that develops between the adsorbent and adsorbed 
distinction is made between: 
a) physical adsorption: process that quickly reaches the equilibrium is weakly 
exothermic and 'reversible, which generally allows for easy regeneration of the 
adsorbent. Are made between adsorbate and adsorbent interactions of van der 
Waals or weak hydrogen bonds. During the adsorption molecules adsorbed 
forgiveness in part their translational degrees of freedom, but not vibrational and 
rotational, by binding to the surface; binding to the surface can then be seen as a 
bond that can only be removed during a desorption process. 
In most cases, physical adsorption is related to hydrogen-bonding or to 
establishment of strong Van der Waals interactions.The latter are strongly favoured 
by porosity, in particular in the case of molecular sieve materials. 
b) chemical adsorption: there is formation of a chemical legane, for example of 
coordination via Lewis acid-base interactions or by dissociative adsorption via 
Brønsted acid-base interaction, between solid and molecule. This is a much more 
exothermic than physical adsorption, and regeneration of the adsorbent implies 
more energy. 
c) solid state absorption: the reaction between solid and molecule is deeper, not only 
superficial, and because the change of the chemical nature of both: 
ZnO + H2S  ZnS + H2O 
 
 
Morphological properties of solids, porosity and cavity effects. 
 
In addition to the chemical nature of the solid and its surface is relevant morphology and 
porosity, normally defined by the following parameters: 
a) surface area or specific surface 
b) pore size 
c) the total pore volume. 
d) pore size distribution 
 
 
 
 
 36
From the point of view of porosity distinguishes between different types of materials: 
1.macroporous solid: pore size> 500 Å 
2. mesoporous solids: pore size between 20 Å and 500 Å 
3. microporous solid: pore size <20 Å 
4. solid structural or zeolitic porosity: it is crystalline solids whose structure 
provides for the existence of cavities and channels of dimensions 
"molecular". These materials can be called "molecular sieves". 
1. Porosity is normally a morphological factor, i.e. is generated by the particle size and 
shape. In the case of some particular solids, such as zeolites, silicoaluminophosphates 
(SAPO), metal organic frameworks (MOF), covalent organic frameworks (COF), and some 
other additional structure, molecular-size porosity is a structural property. 
 
Cavity size of zeolites, mesoporous materials and porous MOFs compared with standard 
aluminosilicates and aluminophosphates. Porous materials are selected arbitrarily; pore 
sizes are approximate due to the variety of pore shapes involved. 
The molecular size porosity gives rise to 
1. Molecular sieving effect: only molecules small enough to enther the cavities are 
adsorbed. 
2. Shape selectivity: only molecules small enough to enter the cavities participate to 
catalysis as reactants, intermediates or products 
3. Confinement effects: the size of the cavity favours catalytic phenomena. 
 
 37
 
Industrial solid-state adsorption and absorption processes 
 
N2 production from air Gas VPSA Carbon molecular sieves 
O2 production from air Gas VPSA Zeolites (Na-MOR) 
Air drying Gas TSA Zeolites (LTA)/silica 
Hydrogen purification Gas PSA Zeolites (LTA) 
Natural gas drying Gas TSA Zeolites (LTA) 
VOC abatement from waste gases Gas TSA Activated carbons/silica 
Biogas purification from siloxanes Gas No regen. Activated carbons 
Biogas purification from H2S Gas No regen. Impregnated AC’s / Fe(OH)3 
Biogas upgrading to biomethane Gas PSA Zeolite (13X) 
i-alkanes/n-alkanes separation Gas PSA Zeolite 
i-alkanes/n-alkanes separation Liquid purge Zeolite 
Bioethanol drying Gas purge Zeolite (LTA) 
Bioethanol drying Liquid Zeolite 
Mercury abatement natural gas Gas TSA Ag-zeolite 
Mercury abatement natural gas Gas No regen. Activated carbons 
p-Xylene-m-xylene separation Liquid purge Ba,K-FAU zeolite 
Natural gas desulphurization Gas No regen. ZnO 
Naphthas desuphurization Gas TSA (burning) ZnO 
Oil decoloration Liquid No regen. Clays / activated carbons 
Benzene / aromatics dehydration Gas TSA Zeolite / Clays 
Water treatment from organics Liquid Purge/No reg. Activated carbons, clays 
Water treatment from heavy metals Liquid No reg. Activated carbons / clays 
Water sweetening Liquid Washing Ion exchange resins 
Water treatment from arsenicum Liquid No regen. Clays 
Water deionization Liquid Washing (purge) Ion exchange resins 
U/Pu separation in water Liquid Washing Ion exchange resins 
 
 
 
 
 
 
Catalysis on solids 
As said, a catalyst accelerates a chemical reaction. It does so by forming bonds with the 
reacting molecules (in the case of heterogeneous catalysis, by adsorption), such that they 
are “activated”. They can thus react to produce a particular product, which detaches itself 
from the catalyst (i.e. desorption), and leaves the catalyst unaltered so that it is ready to 
interact with the next set of molecules. In fact, we can describe the catalytic reaction as a 
cyclic (turnover) event in which the catalyst participates and is recovered in its original 
form at the end of the cycle. 
 38
 
 
The catalytic mechanism is denoted as of Eley–Rideal type when adsorbed or surface 
species react with gas phase substrate, or of the Langmuir–Hinshelwood type when 
reaction is fully occurring at the catalyst surface. 
 
Mechanisms of catalysis by solids. 
Families of catalyst functionalities 
1. Acid catalysis: Brønsted type 
2. Acid catalysis: Lewis type (including halogenation catalysis) 
3. Basic catalysis 
4. Partial oxidation catalysis 
5. Total oxidation catalysis 
6. Hydrogenation /dehydrogenation catalysis 
7. Polymerization catalysis (Ziegler-Natta type). 
 
Acid Catalysis 
Definitions of acidity and basicity 
1884, S.A., Arrhenius, Nobel prize for Chemistry in 1903, 
HA  H+ + A- acid: release of a proton 
BOH  B+ + OH- base: release of an hydroxide ion 
 
1923, J.M. Brønsted and T.M. Lowry 
HA + B = A- + HB+ acid-base: exchange of protons 
 acid: release of a proton 
 base: bonds with a proton 
1923, G.N. Lewis 
B: + A = +BA- acid: available empty orbital 
 base: available doublet of electrons 
 
 39
Catalysts and conditions of industrial solid acid-catalyzed reactions. 
Reaction Catalyst Reaction conditions 
Isobutane alkylation by isobutene USY zeolite 90 °C/10 bar Liquid phase 
Light paraffin isomerizationChlorided alumina 150°C Gas phase 
 Sulphated zirconia, tungstated zirconia 200 °C Gas phase 
 H-Mordenite zeolite 250 °C Gas phase 
Light olefin oligomerization Amberlite (sulphonated PS-PDV) 100 °C/40 bar Liquid phase 
 Solid phosphoric acid or zeolites 200 °C/30 bar Gas phase 
Benzene alkylations H-beta- or H-MWW zeolites 200 °C/30-40 bar Liquid phase 
Diethyl-ether and dimethyl-ether syntheses -Al2O3 300 °C Gas phase 
Light olefin isomerization H-Ferrierite zeolite 350 °C Gas phase 
 -Al2O3 (silicated or borated) 450 °C Gas phase 
Xylenes isomerization, transalkylation H-ZSM5 zeolite 450 °C Gas phase 
Catalytic cracking Rare earth containing H-Faujasite zeolites 700°C Gas phase 
 
 40
 
The so called Brønsted acids and also the so called Lewis acids catalyse many reactions 
of industrial relevance, as bases also do. In the Table the gas-phase proton affinity scale 
of some hydrocarbons is reported and compared with that of ammonia, a quite strong true 
base. The proton affinity is the measure of the heat evolved by interaction with H+ in the 
gas phase and is currently used as a measure of the absolute basicity of molecules. 
 
Proton affinities (kJ/mol) of hydrocarbons and of ammonia for comparison 
(from NIST database). 
Ammonia 846.0 n-bases 
Isobutylene 802.1  - bases 
Toluene 784.0 
1,3-butadiene 783,4 
Propylene 751.6 
Benzene 750.4 
Ethylene 680.5 
Isobutane 677,8 – bases 
Propane 625,7 
Ethane 596,3 
Methane 543,5 
 
Typical basic species have electron pairs in non-bonding (n-) orbitals, and are denoted as 
n-bases. Hydrocarbons do not have n-orbitals but show weak basicity using lone pairs of 
full bonding - orbitals (aromatics, acetylenics, polyenes and olefins) and even -orbitals 
(the paraffins), thus being denoted as -bases and -bases. 
The PA basicity data follow the trend: -orbital containing compounds (olefins and 
aromatics) > isoalkanes > n-alkanes > methane. 
 
Activation of basic molecules by Brønsted (protonic) acids. 
The n-type basic molecules can be activated by protic acids, mostly used in water solution, 
or by Lewis acids, which can be used in both water solutions and in non protic solvents. 
The protonation or the coordination of lone pairs perturbs the nearest bonds inducing, e.g. 
nucleophylic attacks by other reactant species. This is, e.g., the case of rections such as 
etherifications and esterifications, where both Brønsted and Lewis acid catalysts can be 
use to attack the most basic oxygen atoms of the reactants. 
The conversion of weak - and -bases such as hydrocarbons is mostly catalyzed by 
Brønsted acids. Olefins can react with protic acids and can produce the so-called trivalent 
“classical” carbocations (carbenium ions) where the -type orbitals disappear and one of 
carbon atoms rehybridizes from sp2 to sp3, the hydrogen becoming covalently bonded to 
the carbon atom via a -bond. The carbenium ions are more stable and more easily 
formed on tertiary carbon atoms, while their formation on primary carbon atoms is very 
difficult. This is associated to the electron-donating properties of alkyl groups that allow the 
cationic charge to be delocalized, thus stabilizing the cation. 
 41
 
The carbenium ions are intermediates or, better, transition states for several further 
evolution of the reaction such as attack of a nucleophile in the electrophilic addition 
reaction, addition to another olefin in cationic polymerization, alkylation of a aromatic, 
elimination of a proton in a position isomerization reaction, skeletal isomerization, ….. 
The -basicity of aromatic hydrocarbons was also observed long ago and the existence of 
quite stable protonated forms of benzenes and the methyl substituent effects on them was 
determined. Protonation of aromatic rings generates arenium ions whose cationic charge 
is delocalized on the ring and in particular in the ortho and para position with respect to the 
position where the attack of the electrophile (the proton in this case) occurred. These are 
transition states e.g. in electrophilic aromatic substitution reactions. 
H+
H+ HH
+
 
More recently, in 1967, George Olah (Nobel prize for Chemistry in 1994) and Hogeveen et 
al. for the first time observed the protonation of alkanes by superacids, thus suggesting 
that alkanes may behave as -bases. 
C H
H+
C H
H+
C C
H+
C C
H+
 
The basicity scale for -bonds of hydrocarbons is reported to be tert-C-H > C-C > sec-C-H 
> prim-C-H > CH4, although this depends also on the protonating agent and the steric 
hindrance of the hydrocarbons. In fact, protonation at C-C bond may be significantly 
affected by steric hindrance. Protonation of alkanes generates the so called “non-classical” 
pentacoordinated carbonium ions, which contain five-coordinated (or higher) carbon 
atoms. 
 42
The carbocations, which may be stabilized by solvation, are more o less stable species 
and may act as intermediate species or as transition states in the conversion of 
hydrocarbons. In this case the acid is regenerated after the completion of the reaction and 
acts consequently as a catalyst. Many of the hydrocarbon conversion industrial processes 
are acid catalysed and the formation of carbenium and carbonium ions is one of the steps 
in the reaction mechanism, both over liquid and over solid acid catalysts. 
 
Solid Brønsted acids versus liquid Brønsted acids. 
The most convenient liquid Bronsted acids used industrially as catalysts are sulphuric acid 
(concentrated or pure) and almost pure hydrofluoric acid. Another family of strong liquid 
acids are those based on liquefied aluminum trichloride, AlCl3, which has been proposed 
as a catalyst for aromatic alkylation and acylation reactions by C. Friedel and J.M. Crafts 
at the end of the XIX century. AlCl3 melts at 193 °C, producing a typical molecular liquid. 
When additivated with proton donor species, such as water or HCl, or its precursors such 
as alkyl halides, alkyl amine salts, imidazolium halides, pyridinium halides, or 
phosphonium halides, AlCl3 gives rise to the formation of ionic liquids with very strong 
Brønsted superacidity, whose strength evaluated to be similar to that of dry HF. 
These three acids are all environmentally unfriendly: sulphuric acid and Fiedel Crafts acids 
are corrosive, their regeneration is very problematic and their disposal very unsafe. 
Hydrofluoric acid is very volatile (Teb = 19°C) and gives rise to extremely toxic vapours. 
In contrast, most of solid acids are manipulated easily, can be quite easily regenerated 
and can be reused when spent. Additionally, their disposal is not dangerous. 
 
Activation of basic molecules by Lewis acids. 
In agreement with the Lewis definition, not only the proton, H+, but any species having 
empty frontier orbitals are acidic, being able to interact with bases, i.e.m molecules having 
available full frontier orbitals. Thus, also metal cations ionically bonded to weakly basic 
anions are acidic. The most typical strong “Lewis acids” are metal compounds such as Al 
and B trihalides, as well as the halides of other cations such as FeCl3, ZnCl2, TiCl4, SnCl4 , 
SbF5, BiCl3, etc.. Several of these compounds, however, are liquid and volatile and easily 
hydrolysed in mild conditions. They are also non-environmentally friendly materials, giving 
rise to easy evolution of hydrogen halides and corrosion problems, being unsafely 
disposed when spent. 
Being oxygen the most electronegative element besides fluorine, the metal-oxygen bond is 
very ionic. Thus, at the surfaces of metal oxides, ionicity of the M-O bond results in 
coordinatively unsaturated cations and anions, with Lewis acidity of the cations and 
basicity of the anion. The balance between Lewis acidity and basicitydepends on the size 
(the radius, r) and charge (C) of the cation, i.e. in its polarizing power (either C/r or C/r2), 
as well as on its overall coordination. The smaller, the more charged and the less 
coordinated the surface cation, the more is its Lewis acidity. 
Lewis acids coordinate the doublets giving rise to basicity. Int his way they perturb the 
molecule making easier the further reaction, such as e.g. the elimination reaction of 
alcohols givig rise to olefins. 
 43
 
While Lewis acid sites are very reactive with respect to n-bases (true bases), they ability to 
coordinate -type orbitals and -type orbitals is not really clear and, in any case, usually 
the interaction is very weak. 
 
Basic catalysis. 
Both from the Brønsted and from the Lewis definitions, bases are species characterized by 
the ability to bond with acids. Molecules having available electron pairs are both Brønsted 
and Lewis acidic, being at least in principle able to interact both with protons and with 
metallic cationic species. A typical behavior of basic molecules is to cause dissociation of 
Brønsted acids. Strong bases also deprotonate weak acids, such as e.g. allow 
deprotonation of “activated” C-H bonds of organic molecules, i.e. from -postions of 
carbonyl and carboxyl compounds, benzylic positions of alkylaromatics and from allylic 
positions of olefins. They are also nucleophilic, i.e. able to interact with electrophilic carbon 
atoms of organic molecules, and of CO2 as well. 
Basic solids have large application as catalysts and adsorbents in fine chemistry and 
environmental chemistry. As mentioned above, in the case of ionic oxides, the smaller and 
the more charged the surface cation, and the lower its coordination, the stronger its 
polarizing power, i.e. its electron-withdrawing power and, consequently, its Lewis acidity. 
As a result, it bonds more strongly the basic oxide anions, thus decreasing their basicity. 
In contrast, the larger and the less charged the cation, the weaker is its Lewis acidity and, 
consequently, the stronger the basicity of the oxide anions. Thus, alkali and alkali earth 
oxides, whose cations are large and low-charge, are the more basic, the larger and 
heavier is the cation. 
 
Solid vs liquid basic catalysts. 
Typical liquid basic catalysts are soda solutions, potash, lime, sodium and potassium 
carbonate, ammonia and amines. The use of all these solutions has significant drawbacks 
in terms of safety, corrosivity, toxicity, difficult regeration and unsafe disposal. Solid bases 
do not present most of these drawbacks. 
 
Acido-basic catalysis. 
In many cases, acid and base catalysis are synergistic, the ame catalyst displaying both. 
This in particular occurs on the surfaces of metal oxides, where the cation is Lewis acidic 
while the oxide ion is basic. Organic molecules also having both acid and basic 
functionalities interact with both sites and are “activated” by both. 
 Mn+ + H2C=CH2 + H2O
 44
Catalysts and conditions of industrial solid base or acido-base catalyzed reactions. 
Reaction Catalyst Reaction conditions 
CH3CHO + HCHO = CH=CHCHO NaOH-SiO2 300-320 °C 
Butadiene from ethanol MgO-SiO2 370-390 °C 
crotonic condensation of 2-butanone Na2O/SiO2, Cs2O/SiO2 325-400 °C 
acetone to methyl isobutyl ketone (Pt)-MgO-Al2O3 (calcined hydrotalcite) 120-250 °C 
methyl methacrylate from methylpropionate 
+ formaldehye Cs2O/SiO2 320-380°C 
phenyl-1-propanone from benzoic acid 
with propionic acid CaO-Al2O3 440-520 °C 
2,4-dimethyl-3-pentanone from isobutyric acid ThO2 or ZrO2 430 °C 
biodiesel (FAME) by transesterification ZnAl2O4 200-250 °C 
epoxide ring-opening/oxyethylation alcohol 
to polyethoxylates MgO-Al2O3 (calcined hydrotalcite) 4-5 bar, 150-180 °C 
 45
 
 
Al3+ O           C        H
 Adsorption of acetone as its enolate anion over acido-basic couple sites of aluminum 
oxide: 
(H3C)2C=O + Al3+O2-  H2C=(H3C)C-O- + Al3+ -OH 
In the example given here, a mechanism of the adsorption of acetone over a acido-basic 
metal oxide is proposed. This adsorbed form is considered to act as precursor for aldic 
condensation of acetone and other carbony compounds. 
 
Oxidation catalysis: adsorption/reaction/activation of oxygen. 
Upon catalysis, oxygen is supposed to adsorb and be “activated” over the catalyst surface, 
where it reacts with the substrate. A variety of different “adsorbed” or surface oxygen 
species can be obtained on solid surfaces: they are summarized in Table 8. Most of them 
have intermediate redox state from dioxygen (O2, with O oxidation state = 0) and the most 
reduced species, the oxide anion, O2- (oxidation state –II). Thus they are, at least 
potentially, oxidizing species and are electrophilic. They are considered to be possibly 
involved in total oxidations. 
The oxide anions, O2-, instead, are nucleophilic, and are the main players in selective 
oxidations. When they are interacting with reducible cations they act as oxygen-insertion 
sites. In this case, the high valency metal cations act as the oxidizing species, that reduce 
to a lower valency state by inserting oxide species in the reacting substrate. 
Metals and other elements in very high oxidation states can give rise to element-oxygen 
“double bonds” in their oxides, i.e. very short bonds. This is the case of vanadyl (V), niobyl 
(V), molybdenyl (VI), chromyl (VI), wolframyl (VI) groups, characterized by very high M=O 
oxygen stretching frequencies in vibrational spectroscopies (IR and Raman). Cations with 
short M=O double bonds are usually considered to be active in oxygen insertion into the 
substrates, even if it has been determined that the oxygen atom inserted is not that 
involved in the double bond. Among these elements, we can cite Mo6+, Cr6+, W6+, and V5+, 
which in fact are the key elements in important families of oxidation catalysts (molybdates, 
chromates, wolframates and vanadates). 
 46
A summary of possible surface oxygen species. 
 
Formula Oxidation state name properties possible structure 
 
O2- -II oxide coordinatively unsaturated 
OM
MM 
 doubly bonded 
M
O
 
 
O- -I radical anion 
O22- -I peroxide side-on 
O O
M
 
 bridging 
O O
M M 
O2- -0,5 superoxide radical anion, end-on, bent 
M
O O
 
 bridging 
O O
M
M
 
 
O2 0 neutral dioxygen 
O2+ +0,5 dioxygen cation radical cation 
 
 
 47
Catalytic oxidations realized on noble metal catalysts 
Reaction Catalyst Reaction conditions 
CO oxidation to CO2 Au/ZrO2 0 °C 
 0.3%Pt-0.03%Fe/-Al2O3 100 °C 
Vinyl acetate synthesis Pd-Au/SiO2 180 °C 
Ethylene epoxidation Ag/-Al2O3 250 °C/20 bar 
Methanol oxydehydrogenation to formaldehyde Ag/-Al2O3 or Ag gauzes 650 °C 
Ammonia oxidation to NO (nitric acid synthesis) Pt(Rh) gauzes 800-900 °C , 1-12 bar 
Ammonia oxidation to N2 (ASO, ammonia slip oxidation) promoted 0.5 % Pt/-Al2O3 275 °C 
Methane ammoxidation to HCN (Andrussow process) Pt(Rh) gauzes 1100 °C 
Methane partial oxidation to syngas (low contact time CPO) Rh/-Al2O3 1000 °C, * = 5 ms 
Catalytic combustion of methane for energy production 5 % PdO/-Al2O3 650-850 °C 
Catalytic combustion of VOCs 0.3 % Pd/-Al2O3 500 °C 
Gasoline engine aftertreatment Pt-Rh(Pd)/ CeO2-ZrO2-Al2O3 /cordierite 400 °C 
* = contact time. 
 
Catalytic oxidations realized on transition metal oxide catalysts 
Reaction Catalyst Reaction conditions 
SO2 to SO3 V2O5/(Cs,K)2SO4/SiO2 400-500 °C 
o-xylene to phthalic anhydrideV2O5/TiO2 (anatase) 400 °C 
n-butane to maleic anhydride (VO)2P2O7 350 °C 
Methanol oxidation to formaldehyde (Formox) MoO3/Fe2(MoO4)3 300 °C 
Propene oxidation to acrolein Bi-Mo (Fe,Co,Ni) multicomponent oxides 320 °C 
Acrolein oxidation to acrylic acid Mo-W-V-Fe oxides 280 °C 
Propene ammoxidation to acrylonitrile Bi-Mo (Fe,Cr,Ni,Co,Mg) multicomponent oxides 350 °C 
Propane ammoxidation to acrylonitrile U-Sb (Ti,Sn,Al,Fe,W,Te) multicomponent oxides 450 °C 
Catalytic abatement oxygenated VOC’s MnOx/-Al2O3 400-500 °C 
Catalytic abatement chlorinated VOC’s CrOx/-Al2O3 400-500 °C 
Butene oxidative dehydrogenation to butadiene (Zn,Mg)Fe2O4 550 °C 
HCl oxidation to Cl2 RuO2/TiO2 (rutile), RuO2/SnO2 300 °C 
 48
 
The mechanisms involving adsorbed oxygen species can be of the Eley–Rideal type 
(adsorbed or surface oxygen reacting with gas phase substrate) or of the Langmuir–
Hinshelwood type (both species react being adsorbed at the catalyst surface). 
In a mechanism usually attributed to Mars and Van Krevelen, the catalyst plays between 
two oxidation states. In an higher oxidation state the catalyst surface acts as the oxidant 
for the substrate, reducing itself to a lower oxidation state. The mechansimcan imply the 
metal as the low oxidation state and a surface oxide or even a bulk oxide as the higher 
oxidation state, or two different oxides. Oxygen reoxidizes in situ the catalyst surface from 
its lower to its higher oxidation state. The Mars-Van Krevelen mechanism (also denoted as 
“redox” mechanism) actually implies that oxygen is adsorbed and activated in the form of 
oxide anions. 
 
Oxidation catalysts are g enerally thought not only to activate oxygen but also to activate 
the substrate to be oxidized. As for hydrocarbons, the most reactive towards oxidation are 
functionalized molecules such as aromatics, with the activation of both ring positions as 
well as of benzylic positions. Also oxidation of olefins are very relevant industrially, with the 
possibility of oxidation at the C=C double bond or at the allylic position. The activation of 
unsaturated C=C bonds is frequently obtained by -bonding over noble metal centers or 
transition metal centers. Alternatively, they can be activated by electrophilic attacks. 
Activated saturated carbon atoms such as those in benzylic and allylic positions are 
activated by Lewis acid sites, which are able to abstract hydride species, or in a radical 
mode. Cations such as Sb3+, Bi3+, Te4+, i.e. cations having non-bonding electron pairs, are 
generally thought to be active in H abstraction from allylic positions. 
Heteroatom-containing molecules such as oxygenates, nitrogenated and halogenated 
compounds, that are in fact n-bases and more reactive to oxidation, are activated more 
easily. Activation likely involves the previous coordination of the molecules on Lewis acid 
sites using the lone pairs on heteroatoms. 
 
Homogeneous oxidation catalysts. 
Liquid or solvent soluble transition metal complexes have also catalytic oxidation activity. 
However, their application makes more complex the process, due to the need of theirs 
separation from products and solvents. They are used when preformant heterogeneous 
catalysts have not been found. 
 
 49
Hydrogenation ad dehydrogenation catalysis 
Hydrogenation reactions are exothermic equilibrium reactions, thus performed at the 
lowest temperature allowed by the catalyst activity, and at medium to high pressure. 
Deydrogenation reactions are endothermic equilibrium reactions, thus performed at the 
moderately high temperature and at low pressure, usually not far from ambient pressure. 
Catalytic activity in hydrogenation and dehydrogenation is associated to the ability of the 
catalytic material to adsorb hydrogen dissociatively, either heterolytically or homolytically, 
thus converting H2 in atomic active forms. 
Hydrogen adsorbs homolytically dissociatively very fast on almost all relevant metal 
surfaces, being the dissociation of hydrogen only weakly activated or even barrierless. As 
for example, it has been found that when an H2 molecule chemisorbs on a Pt surface, the 
antibonding σ* orbital of H2 is completely filled by electrons from platinum. Thus, non-
activated dissociative adsorption occurs, i.e. the adsorption step is not kinetically hindered. 
Only on group 11 metals (Cu, Ag, Au) hydrogen dissociation is significantly activated and 
may be endothermic. On-top, bridge or hollow sites can be occupied by atomic hydrogen 
species on metal surfaces. The formation of subsurface atomic hydrogen is also possible, 
usually with an endothermic and/or a slightly activated process. Tetrahedral and 
octahedral subsurface sites are occupied in this case. Only in the case of palladium, 
migration of hydrogen into the interior of the bulk is apparently exothermic too, due to a 
very large binding energy (-2.5 eV). 
 
 on top bridging hollow 
Two kinds of dissociation mechanisms are supposed to exist over sulphide and oxide 
surfaces: heterolytic dissociation, producing a hydride species and a sulphydryl or oxydryl 
species, and homolytic “reducting” dissociation, producing two sulphidryl or oxydryl 
species and reduced metal centers. 
 
Schematics of the mechanisms of dissociation of hydrogen oxides and sulphdes: X = O or 
S, M = metal. 
Also adsorption of the substrate to be hydrogenated or dehydrogenated occurs, as 
mentioned above for catalytic oxidation. 
 50
Catalytic hydrogenations and dehydrogenations on metal catalysts. 
Reaction Catalyst Reaction conditions 
Hydrogenation of nitrogen (ammonia synthesis) Fe (K,Si,Al,Ca,Mg) 400 °C, 200 bar, gas phase 
 Ru/Graphite 300 °C, 100 bar, gas phase 
Hydrogenation of CO,CO2 (methanol synthesis) 55 % Cu/ZnO/ZnAl2O4 250 °C, 100 bar, gas phase 
Hydrogenation of CO,CO2 (methanation) 25 % Ni/ -Al2O3 250 °C, 20-50 bar, gas phase 
 0.3 % Ru/-Al2O3 170 °C, 20-50 bar, gas phase 
Hydrogenation of CO,CO2 (to hydrocarbons, 
LT Fischer Tropsch) Co/ -Al2O3 200 °C, 20-30 bar, slurry 
Hydrogenation of CO,CO2 (to hydrocarbons 
and oxygenates, HT Fischer Tropsch) Fe-Fe5C2/SiO2 300 °C, 20-40 bar, slurry 
Hydrogenation of acetylene (C2 cut treatment) 0.03 % Pd, 0.18 % Ag/ -Al2O3 80 °C, 20 bar, gas phase 
Hydrogenation of acetylenics 0.5 % Pd/ -Al2O3 15 °C, 20 bar, liquid phase, fixed bed 
Benzene saturation to cyclohexane Ni Raney 200 °C / 50 bar, slurry 
Hydrogenation of vegetable oils to margarine Ni/SiO2 200 °C / 5 bar, liquid phase 
Light paraffin dehydrogenation to olefins Pt/Li-Al2O3 550°C 
Heavy paraffin dehydrogenation Pt/Li-Al2O3 475 °C, 2,5 bar 
Aromatization Pt/K-L zeolite 500 °C 
Catalytic reforming of gasoline (aromizing) Pt/K-L zeolite 500 °C 
Dehydrogenation of alcohols to carbonyl compounds Cu-ZnO-Al2O3 200-400 °C 
 CuCr2O4 200-400 °C 
 51
Catalytic hydrogenations and dehydrogenations on non-metallic catalysts. 
Reaction Catalyst Reaction conditions 
Hydrodesulphurization gasolines Ni-MoS2 300 °C- 20 bar, gas phase 
Hydrodesulphurization gasoils Ni-Co-MoS2 350 °C- 50 bar, trickle bed 
Hydrodenitrogenation of hydrocarbons NiS-WS2/Al2O3 350 °C- 50 bar, trickle bed 
Hydrocracking heavy oils and residues NiW-USY zeolite 400 °C, 150 bar, slurry 
Deep hydrocracking (bottom of the barrel treatments) MoS2 450 °C, 200 bat, slurry 
Hydro-deoxygenation of vegetable oils to green Diesel CoS-MoS2/Al2O3 250°C, 50 bar, slurry 
Hydrogenation of CO,CO2 (HP methanol synthesis) ZnO-ZnCr2O4 350 °C, 340 bar 
Hydrodeoxygenation of carboxylicacids to aldehydes Cr2O3-ZrO2, CeO2 310-400 °C, 1-30 bar 
Hydrodealkylantion of toluene Cr2O3/Al2O3 600 °C, 50 bar 
Dehydrogenation of light paraffins to olefins K2O-Cr2O3/Al2O3 550 °C 
Dehydrogenation of ethylbenzene to styrene K2O-Fe3O4 600 °C, steam 
Aromatization of light paraffins Ga-ZSM5 zeolite 500 °C 
Aromatization of light olefins Zn-ZSM5 zeolite 500 °C 
 
 52
Polymerization Catalysis 
Catalytic polymerization is applied mainly to produce high density polyethylene (HDPE) 
and isotactic polypropylene (iPP). It is denoted as stereospecific polymerization or Zigler-
Natta type (ZN) polymerization. The most broadly accepted mechanism for stereoregular 
polymerization on ZN catalysts is the so-called monometallic mechanism proposed by 
Cossee and Arlman. Polymerization would occur via two steps. First, coordination of the 
monomer to the active center occurs, followed by the stereospecific migratory insertion of 
the coordinated monomer into the titanium–carbon bond. In migratory insertion step, a 
vacant coordination site is regenerated, which enables further chain propagation. 
 
Cossee-Arlman mechanism of Ziegler-Natta polymerization catalysis. 
 
Most common heterogeneously catalyzed polymerizon ractions 
 
High density polyethylene TiCl3/MgCl2 Liquid phase, 65-90 °C, 10-35 atm 
 Cr2+/SiO2 Gas Phase, 80-100 ° C, 7- 25 atm 
Isotactic polypropylene TiCl3/MgCl2 Liquid phase, 60-80 °C, 5-15 atm 
Zr(C2H4R)2Cl2/SiO2 substituted heterogenized metallocene 
 
 
4. Catalytic materials 
Strength, amount, and distribution of surface sites on the ideal surface of a solid. 
Over a solid, adsorption is frequently a specific activity of the so called “active sites”. In 
heterogeneous catalysis, the adsorption sites are generally also “catalytically active sites”, 
being the sites where the catalytic reaction occurs. The amount of adsorption per weight or 
volume of adsorbent depends on the amount of these sites or on the surface density of 
these sites. On the other hand, several different families of adsorption sites may occur in 
the same solid surface, characterized by different adsorption strength, so their 
“distribution” (density of sites of any site family) must be characterised. 
Also the catalytic activity (reaction rate) depends on the amount of catalytically active sites 
(e.g. of acid sites having the appropriate strength) present on the catalyst as a whole. This 
means that the “density” of active sites (amount of sites per gram of the solid or per unit 
surface area), is an important parameter. On solids, amount and strength of acid or basic 
sites are quite independent parameters, so both of them must be analysed independently 
for a complete characterisation. 
Additionally, both acidic and basic sites can be present in different position (but frequently 
near each other) on the same solid surface, and can work synergistically. This provides 
evidence for the significant complexity of acid-base characterisation of solids. 
 53
Summary of some most relevant families of industrial catalysts and examples of their applications. 
 
Catalyst family reaction Industrial catalyst Catalyst functionality 
Oxide catalysts 
Bulk single oxide Alcohol dehydration to olefins and ethers -Al2O3 Lewis acidic catalyst 
Bulk mixed oxide Aldol Condensation MgO-MgAl2O4 (calcined hydrotalcite) Basic catalyst 
Multicomponent oxide Propane to acrylonitrile V/Mo/Nb/Sb oxides (Amm)oxidation 
Oxide supported on oxide o-xylene to phtahlic anhydride V2O5/TiO2 Selective oxidation 
 Isobutane to isobutene K2O-Cr2O3/Al2O3 Dehydrogenation 
Impregnated melt or liquid SO2 to SO3 K2SO4-V2O5/SiO2 Oxidation 
 Olefin oligomerization H3PO4/SiO2 (“solid phosphoric acid”) Protonic acid catalyst 
Zeolite catalysts 
Protonic zeolite Benzene+ethylene to ethylbenzene H-BEA Protonic acid catalyst 
Cationic zeolite N2O decomposition/reduction Fe-MFI Redox catalysts 
Metal catalysts 
Bulk metals Ammonia synthesis Fe (CaO, K2O, Al2O3, SiO2 promoters) Hydrogenation 
Metal gauzes Ammonia oxidation to NO Pt (Rh stabilizer) Selective oxidation 
Supported metal Acetylene hydrogenation in ethylene Pd/ Al2O3 (Ag promoter) Preferential hydrogenation 
 Car catalytic mufflers Pt-Rh/ Al2O3-CeO2-ZrO2 Combustion + NO red 
 Alcohols to aldehydes Pt/Carbon Liquid phase oxidation 
 Aromatization of paraffins Pt/K-L zeolite 
 Dehydrog./aromatization 
Sulphide catalysts 
Bulk sulphide Bituminous sands to oil fractions MoS2 Deep hydrocracking 
Supported sulphides Gasoline treatment NiS-MoS2/-Al2O3 Hydrodesulphurization 
Halide catalysts 
Bulk halides Fluorination of chloroalkanes CrF3 Lewis acid catalyst 
Supported halides Ethylene and propene polymerizations TiCl3/MgCl2 Stereospecific 
polymerization 
 Aromatic Alklylation BF3/-Al2O3 Lewis acidity 
 Aldol type condensations KF/-Al2O3 Basic catalyst 
 Ethylene to dichoethane CuCl2/-Al2O3 Oxychlorination
54 
 
 
Surface sites on solid catalysts. 
Several catalytic materials are constituted by polycrystalline powders or supported 
nanocrystal particles. Thus, the active phases may be supposed to be constituted by small 
particles, whose structure is that of nanocrystals or nanopolycristals. Thus, the surface of 
nanocrystalline catalysts may be assumed to be formed by: 
1. extended exposed “perfect” crystal surfaces; 
2. edges between exposed planes; 
3. corners formed by the exposed planes; 
4. surface defects (vacancies, kinks). 
It is likely that in most cases the most reactive and active sites are just the most defective. 
In case of amorphous materials the situation may be similar although less well defined. 
 
Catalytic materials can be roughly divided into two families: 
a) Bulk catalytic materials. 
b) Supported catalytic materials. 
 
Supported catalysts are mostly defined those where a pre-synthesized carrier material is 
used to depose on it a thin layer or the “active phase, or nanoparticles or even, some 
times, isolated complexes or clusters or atoms. Supported catalysts are usually intended 
as those the nature of the support influences the morphological properties and, frequently, 
also the chemical properties of the supported phase, thus participating to the generation of 
the catalytic properties of the pverall catalyst. 
However, in the practical industrial work low-surface area ceramic supports (such as 
corundum powders, carborundum powders, cordierite monolyts) are sometimes used to 
support a bulk catalyst. In this case, where big particles or porous thick layers of a “bulk” 
catalyst are physically deposed on ceramic supports, the support plays a determinant role 
in heat transfer and flow-dynamics of the system, without exerting a definite role in the 
chemical - catalytic behaviour of the material. The term “support” is somehow ambiguous. 
Preparation procedures of catalyst powders may differ significantly between supported and 
unsupported bulk catalysts. However, for most sophisticated materials several 
 
55 
 
components can be included in catalyst formulations using techniques typical for both 
supported and bulk materials preparation. 
 
 
 
In fact, in a typical catalytic material, a number of components can be included. They are: 
a) The active phase, supposed to be responsible for the rate determinant catalytic act. 
b) The support, if needed to produce optimal activity of the active phase and optimal 
morphology and surface area, with optimization sometimes also of heat transfer and flow-
dynamics aspects. 
c) Promoters, that can further improve the catalytic activity. 
d) Stabilizers, which stabilize the catalyst from a number of possible deactivation 
phenomena,such as stabilizers from sintering, from phase transformation, from coke 
deposits formation, from active phase volatilization, etc. 
e) Binders, needed for building extrudates and composite particles. 
f) Diffusion favouring matter, i.e. porous materials added to complex catalyst to favour 
reactants diffusion. 
 
Most relevant catalytic materials. 
Oxide-based materials. 
 
Typical metal oxides are ionic materials. They are characterized by surface Lewis acido-
basicity. The smaller and the more charged the cation, the stronger the Lewis acidity and 
the weaker the basicity. The larger the cation size and the lower its charge, the stronger 
the basicity. For semimetallic elements (such as Si, B, Ge,…) the element-oxygen bond is 
nearly covalent. Lewis acidity disappers, basicity decreases very much, while weak 
Brønsted acidity appears. The oxide of transition metals with very high oxidation states (> 
+V) present strong Brønsted and Lewis acidity and very weak basicity. 
 
The Silicas 
Silica, SiO2, forms many different crystalline and amorphous structures. Among them, only 
in stishovite, a mineral found in some meteoric rocks and stable only above 75 kbar of 
pressure, Si atom takes a octahedral coordination. All structures having practical interest 
present tetrahedrally coordinated silicon atom. At ambient pressure, SiO2 has several 
major polymorphs. Those having thermodynamic stability ranges are: low temperature 
trigonal α-quartz up to 570 °C, high temperature hexagonal β-quartz 570-870 °C, 
hexagonal β-tridymite 870- 1470 °C and high temperature cubic β-cristobalite 1470- 1705 
°C. On the other hand, forms stable at high temperature or never, may exist, as 
metastable stables at r.t.. These phases are available as low-surface area refractory 
ceramic powders, sometimes used as inactive components of catalysts. Crystalline 
metastable phases with zeolite-like porous structure have also been developed, usually 
called silicalites. The tetrahedral-based structures of the silica polymorphs are associated 
to quite a covalent Si-O-Si bond network and differ only for the relative arrangements of 
the tetrahedra. 
56 
 
 
Thermodynamic stability ranges of silica polymorphs The structure of -quartz 
 The structures of silicalite-1 and of crystobalite. 
 
The numerous silica forms which are used in the field of adsorption and catalysis are 
metastable materials, most of them being amorphous. In fact, silica is the best known 
glass forming material, i.e. it has very stable amorphous states, that also consist of a 
tetrahedral covalent network structure, although disordered. These amorphous 
statesmostly characterized by very high surface areas and porosity, and low crystal sizes, 
are kinetically very stable: their sintering and crystallization, usually to cristobalite, are fast 
phenomena only at temperatures of the order of > 800 °C, giving rise to loss of surface 
area and porosity. 
Amorphous silica materials are available in several different forms, depending on the 
morphological properties as a result of different preparation processes. 
 
Precipitated silicas. Although many different recipes have been proposed, precipitated 
silicas are commonly produced by partial neutralization of sodium or potassium silicate 
solutions. Sulphuric acid is mostly used, mixed with sodium silicate in water still retaining 
alkaline pH. Reaction is performed under stirring at 50-90 °C. The precipitate is then 
washed, filtered and dried. During precipitation progressive particle growth occurs up to 4-
57 
 
5 nm clusters, that successively agglomerate to form sponge-like aggregates. Tuning 
preparation procedure parameters (choice of agitation, duration of precipitation, the 
addition rate of reactants, their temperature and concentration, and pH of precipitation, as 
well as drying conditions) allows tuning of final particle size and morphology, thus surface 
area and porosity. Precipitates typically have a broad meso/macroporous morphology. 
Very high surface areas may be obtained with these procedures (up to 750 m2/g), with 
pore volume in the 0.4-1,7 cm3/g range and average pore diameter in the 4-35 nm range. 
Typical impurities of these materials are sodium ions (< 0,8 %) with the likely presence of 
iron and aluminium ions at the 500-1000 ppm level. Precipitated silica are commercially 
available such as the Sipernat family from Evonik and the Zeosil-Micropearl materials from 
Rhodia. 
 
Silica gels. Silica gels are usually produced by dissolving sodium or potassium silicate (10-
20 % silica) into an acid, such as sulphuric acid (pH ~ 0.5-2). If the particles are smaller 
than 100 nm they form silica sols, stabile colloidal dispersions of amorphous silicon dioxide 
particles that can be used e.g. as polishing agent at production of silicon surfaces in the 
electronic industry. A gel is formed when the molecular weight of the micelles reaches 
approximately 6 million , thus the hydrosol viscosity reaches the no-pour point. In a second 
step the liquid is removed leaving a glass-like gel which is broken down into granules and 
then washed, aged, and dried., with 6 % volatiles and 22 A average pore diameter. 
Silica gels have pores with a wide range of diameters, typically between 5 Å and 3000 Å, 
and broad distributions. Silica gels synthesized with surface area as high as 800-900 m2/g, 
an average pore size of about 20Å and effective pore volumes of 0,40 cm3/g, are known 
as narrow pore silica gels, while wide pore silica gels are characterized by surface area ~ 
400 m2/g, average pore size of about 110Å and effective pore volumes of 1,20 cm3/g. 
 TEM micrograph of silica gel Grace 
 
Fumed or pyrogenic silicas. Fumed silicas are produced by flame hydrolysis of silicon 
tetrachloride, a process invented in 1946 by H. Klöpfer a chemist at Degussa (now 
Evonik). This process consists in the reaction of SiCl4 in a hydrogen-oxygen flame at high 
temperature, reported top be near 1100 °C (Degussa – Evonik) or 1800 °C (Cabot), 
producing silica and hydrogen chloride. This procedure produces very small non-porous 
58 
 
amorphous primary particles, that tend to agglomerate in linear and branched chain-like 
structures. The surface area of these materials is moderately high (100-400 m2/g) and 
fully external, essentially depending from the particle size that ranges 5-16 nm. The weight 
loss by drying is quite low, 1-2,5 % depending roughly on the surface area, the 
morphology being stable nearly up to 800 °C, when sintering starts. From the point of view 
of the metal content these materials are very pure. In particular they do not contain alkali 
metal impurities. Typical impuritiy of these materials are residual chlorine, and, to a low 
extent, aluminium, titanium and iron. A typical practical characteristic of these materials is 
the very low apparent density (down to 30 g/l) and the volatility of the particles. 
 
Silica aerogels. Siica aerogels, first prepared in the late 1920s by Samuel Kistler, are 
highly transparent materials with very high surface area (>1000 m2/g) and high void 
volume (85-98 %), prepared by supercritical drying of wet silica gels . Supercritical drying 
process can avoid capillary stress and associated drying shrinkage, which are usually 
prerequisite of obtaining aerogel structure. Commercial aerogels may be hydrophobized, 
e.g. as aerogels from CABOT, mainly used for insulation and daylighting, as intermetallic 
dielectric materials and acoustic applications. 
 
Mesoporous silicas. Mesoporous silicas contain somehow ordered structures of well-
defined channels or interconnected cavities with size from few to several nm. The 
preparation of these materials commonly starts from silicon alkoxides hydrolysis performed 
in the presence of appropriate concentrations of detergent molecules acting as templatesor Structure Directing Agents (SDA). With opportune reaction conditions, pores having 
different geometries can be obtained. Many different materials, with different mostly 
mesoporous pore structure, but having sometimes also some microporosity, may be 
obtained by different preparation procedures and SDAs. Surface areas up to 1500 m2/g 
are obtained, with well-defined mesoporosity. Such mesopores can be constituted by 
linear channels or interconnected cages, or even wormhole-like channels with hexagonal 
symmetry. Although sometimes considered like very large pore zeolites, these materials 
are essentially amorphous silicas with non-structural although sometimes ordered 
mesopores. 
 
Silicalites. Silicalites are fully siliceous zeolites. They are prepared with the typical 
preparation techniques of zeolites, using a pure silicon source and structure directing 
agents. Silicalite-1 is largely the better known and most used siliceous zeolite. Its 
crystalline framework is constituted by Si oxide tetrahedral structure, with the typical 
structural microporosity of the MFI structure zeolites. These materials have interesting 
adsorption capacity for organics, associated to the Van der Waals interactions owiththe 
pores walls. When prepared in a defective structure, silanol groups are present in the 
cavities, providing weak acidity. 
 
 
59 
 
 
 
Silica is a largely covalent oxide, whose surface is constituted by Si-O-Si “siloxane 
bridges” and Si-OH “terminal silanol” groups. Indeed the surface chemistry is dominated 
by the reactivity of terminal silanol groups, Si-OH, which are very weakly acidic but able to 
produce significant H-bonding interaction even weak veryunpolar molelues such as 
hydrocarbons. Isolated, geminal, and vicinal hydrogen bonded silanols exist on the porous 
silica surface. 
Silicas are very largely used in the catalysis field as catalyst supports for metals, oxides 
and sulphides. Tey are also largely used as adsorbents (in particular silica gels) for 
removal of water vapour of even organic vapours from gases. Amorphous silicas and 
silicalites are also useful as adsorbents in liquid phase. Silicas find large application as 
adsorbents in gaschromatography. 
 
The Aluminas 
All alumina polymorphs are produced from the different hydroxides or oxyhydroxide by 
thermal decomposition. 
Crystal data of aluminum hydroxides and oxy-hydroxides. 
mineral name Formula Space Group. Z 
Bayerite -Al(OH)3 P21/n 8 
Gibbsite -Al(OH)3 P21/n 8 
Nostrandite Al(OH)3 P1 4 
Doyleite Al(OH)3 P1or P1 2 
Diaspore -AlOOH Pbnm 4 
Boehmite -AlOOH P21/c or Cmc21 4 
Tohdite 5Al2 O3 .H2O P63mc, P31c or Cmc21 2 
60 
 
 
The thermodynamically stable phase of alumina is -Al2O3 (corundum) where all Al ions 
are equivalent in octahedral coordination in a hcp oxide array. Corundum powders can be 
produced at low temperature by decomposing diaspore, or at high temperature by 
calcination of any other alumina or aluminum hydroxides. powders are applied in catalysis 
as supports, e.g. of silver catalysts for ethylene oxidation to ethylene oxide, just because 
they have low Lewis acidity, low catalytic activity (so not producing undesired side 
reactions), while being mechanically and thermally very strong. 
The Corundum Structure 
 
Structure of the layers common to Al(OH)3 polymorphs. 
 
61 
 
Four different polymorphs are known of Al hydroxide Al(OH)3 : Bayerite, usually denoted 
as α–Al(OH)3, Gibbsite, usually denoted as –Al(OH)3, and the less common Al(OH)3 
polymorphs Doyleite and Nordstrandite. The four structures are closely related. They are 
constituted by four different stacking sequences of the same kind of layers, constituted by 
Al(OH)6 edge sharing octahedral forming a planar pseudohexagonal pattern. Thus all Al s 
are octahedrally coordinated while hydroxyl-groups are bridging between two Al atoms. At 
both side of the layers, hydroxyl groups stand. The different Al(OH)3 polymorphs are thus 
associated to different geometries of the H-bondings between the layers. 
 
Two polymorphs are known for the Al oxy-hydroxide AlOOH, diaspore, α–AlOOH , and 
boehmite –AlOOH. Boehmite has a layered structure with octahedral Al ions, 
tetracoordinated oxide ions and bridging hydroxyl groups. Zig-zag chains of hydrogen 
bonds, whose exact geometry has not been completely defined, is formed between the 
layers. Tohidte, with formula Al5O7(OH) can also be considered an oxy-hydroxide. 
 
Crystal structures of boehmite (left) and disapore (right). 
62 
 
 
Most of metastable alumina polymorphs have a structure which can be related to that of 
spinel, i.e. cubic MgAl2O4. -Al2O3, which is the most used form of alumina, is mostly 
obtained by decomposition of the boehmite oxyhydroxide -AlOOH (giving medium surface 
area lamellar powders,  100 m2/g) or of a poorly crystallized hydrous oxyhydroxide called 
“pseudoboehmite” at 600-800 K, giving high surface area materials ( 500 m2/g). The 
decomposition is associate to the endothermic effect in the DSC curve reported in the 
figure below. The materials obtained with these precipitation methods are highly 
microporous. -Al2O3 powders with low porosity may be obtained by flame hydrolysis of 
AlCl3, but they show chlorine surface impurities. 
Nanocrystalline boehmites are even industrially available. They may be prepared by 
precipitation starting from soluble Al salts, but in this case they usually contain non 
negligible amounts of alkali ions. Another way to obtain microporous boehmite comes from 
the so-called Ziegler process, industrially denoted as ALFOL process. This process is 
intended to produce linear fatty alcohols starting from trialkyl-aluminum formed by 
oligomerization reaction of ethylene with Al metal. Oxidation of aluminum trialkyls gives 
rise to aluminum trialkoxides that can be hydrolyzed to alcohols and bohemite. A 
modification of this process allows the production of aluminum trialkoxides and hydrogen 
from alcohols and aluminum metal. Thus, after hydrolysis, boehmite is produced while 
alcohols may be recycled. This way produces high purity alumina, with less than 20 ppm 
sodium and potassium, less than 50 ppm calcium and magnesium, less than 100 ppm iron 
and less than 120 ppm silicon. 
 
 
 
63 
 
 The spinel structure 
 Structural relationshos from boehmite and spinel 
 
 HRTEM of lamellar -Al2O3 particles. 
 
-Al2O3 is one of the most used materials in any field of technologies. However, the details 
of its structure are still matter of controversy. It has a cubic structure described to be a 
defective spinel, although it can be tetragonally distorted. Being the stoichiometry of the 
64 
 
“normal” spinel MgAl2O4 (with Al ions virtually in octahedral coordination and Mg ions in 
tetrahedral coordination) the presence of all trivalent cations in -Al2O3 implies the 
presence of vacancies in usually occupied tetrahedral or octahedral coordination sites. 
Calcination at increasing temperatures gives rise to the sequence -Al2O3  -Al2O3 -
Al2O3  -Al2O3. The ratio between tetrahedrally-coordinated and octahedrally 
coordinated aluminum ions increases upon the sequence -  -  -Al2O3. Tetrahedric 
Al3+ is near 25 % in -Al2O3, 30-37 % in -Al2O3 and 50 % (in principle) in -Al2O3. -Al2O3 
is a tetragonal spinel superstructure whose unit cell is constituted by three spinel unit blocs 
with tetragonal deformation, likely with a partial ordering of Al ions into octahedral sites. It 
is formed continuously in the range 800-900 K. -Al2O3 is formed above 900 K with 
simultaneous decrease of the surface area to near 100 m2/g or less. Its monoclinic 
structure, which is the same of -gallia, can be derived from that of a spinel, with 
deformation and some ordering of the defects, with half tetrahedral andhalf octahedral Al 
ions. During the sequence -Al2O3  -Al2O3  -Al2O3  -Al2O3 the lamellar 
morphology of boehmite is mostly retained but with progressive sintering of the lamellae 
and disappearance of the slit shaped pores. The last step to corundum is responsible for 
the exothermic effect observed in the DSC curve above, typical for the polymorph 
transformation from a metastable phase to a thermodynamically stable phase. 
-Al2O3 is also considered to be a spinel-derived structure but is obtained by 
decomposing bayerite Al(OH)3 (evident again in the DSC curve by a endothermic peak). 
Most authors conclude that -Al2O3 corresponds to a defective spinel like -Al2O3 but with 
a different distribution of vacancies, namely with more tetrahedrally coordinated (35 %) 
and less octahedrally coordinated Al ions. This results in stronger acidity of -Al2O3 with 
respect to -Al2O3. Calcination gives rise to the sequence -Al2O3 -Al2O3  -Al2O3. 
Other metastable forms of alumina, denoted as -Al2O3, -Al2O3 and -Al2O3 also exist 
and can be obtained from the hydroxides gibbsite and tohdite, but they seem to have less 
interest in catalysis. 
The catalytic activity of transitional aluminas (-, -, -, -Al2O3) is undoubtedly mostly 
related to the Lewis acidity of a small number of low coordination surface aluminum ions, 
as well as to the high ionicity of the surface Al-O bond. The alumina’s Lewis sites have 
been characterized to be the strongest among binary metal oxides. The density of the 
very strong adsorption sites is actually very low, near 0.1 sites/nm2 . Taking into account 
the bulk density of -Al2O3, it is easy to calculate that at most one site every 50-100 acts as 
a strong Lewis site on -alumina outgassed at 400-550 °C, the large majority being still 
hydroxylated or not highly exposed at the surface. 
Actually, the true particular sites of aluminas for most catalytic reactions are very likely 
anion-cation couples which have very high activity and work synergistically. The basic 
counterpart may be oxide anions or hydroxyl species. Actually, among the pure ionic 
oxides, aluminas is also one of the strongest Brønsted acids. The activity of pure -Al2O3 
as a good catalyst of skeletal n-butylene isomerization to isobutylene has been attributed 
to its medium-strong Brønsted acidity. 
Transition aluminas, mostly denoted as -Al2O3, but actually being frequently a mixture of 
-Al2O3, -Al2O3 and -Al2O3, or of -Al2O3 and -Al2O3, have wide application as the 
65 
 
catalyst for the Claus process, the production of sulphur from H2S and SO2 in the 
refineries. 
Aluminas are used as commercial catalysts of the alkylations of phenol with alcohols, such 
as the synthesis of o-cresol and 2,6-xylenol using methanol at 300-400 °C 42. Aluminas are 
very active in the dehydration of alcohols to olefins and to ethers, such as methanol to 
dimethylether at 250–280 °C and 0.04–0.05 MPa, and have been used in the sixties for 
producing ethylene from dehydration of bioethanol. 
They may be used for the dehydrofluorination of alkylfluorides which are byproducts of the 
HF catalyzed isobutane / butylene alkylation process. Fluoroalkanes react at 170-220°C, 
being converted to olefins. HF is adsorbed on the alumina to form aluminum fluoride, 
regeneration being needed every 6 months. 
However, the main use of aluminas in hydrocarbon conversions is 
1. as an adsorbent (dehydration of gases, adsorption of heavy metals from waters, 
abatemenet of alkyl halides vapours,… 
2. as a support, for metal catalysts, sulphide catalysts 
3. as a catalyst binder 
4. as an additive (e.g. in FCC catalysis). 
It is also the precursor for fluorided and chlorided aluminas, which may be produced in situ 
upon halogenation, as well as for silicated aluminas (see below), borated aluminas and 
other “modified aluminas” produced ex situ by chemical treatments. 
 
Amorphous silica-aluminas (ASAs). 
Commercial materials are available with any composition starting from pure aluminas to 
pure silicas. The silica-rich materials (usually 15%wt Al2O3) resulting from coprecipitation 
or co-gelling of Si and Al compounds are generally fully amorphous and are called 
“amorphous silica-aluminas” (ASAs). They behave as strongly acidic materials and have 
been used for some decades (1930-1960) as catalysts for catalytic cracking processes, 
and still find relevant industrial application. 
On the surface of SA, medium strength Brønsted acid sites together with very strong Lewis 
acid sites can be detected. Lewis sites are certainly due to highly uncoordinated Al ions 
and correspond to the strongest Lewis sites of transitional alumina or perhaps are even 
stronger, due to the induction effect of the covalent silica matrix. This makes SA also a 
very strong catalyst for Lewis acid catalyzed reactions. Al ions near terminal silanols can 
cause a revelant strengthening of the acidity of terminal silanols. 
Amorphous microporous SA, used in the past for fixed and moving bed catalytic cracking 
starting from the fourties, still finds a number of applications as acid catalysts e.g. the 
dehydrochlorination of halided hydrocarbons. Also, SAs are used as supports of sulphide 
catalysts for hydrotreatings and of catalysts for ring opening of polycyclic compounds, 
useful for the improvement of the technical and environmental quality of Diesel fuels. 
Mesoporous SAs containing big pores with size from few to many nm, have been 
developed recently. Although sometimes considered like very large pore zeolites, these 
materials are essentially amorphous SAs with non-structural although sometimes ordered 
mesopores. The surface chemistry of these materials appears to be closely similar to that 
of amorphous microporous SAs. 
66 
 
 
 
Models for Brønsted acidity in silica-alumionas. 
 
The zirconias 
Zirconia presents three polymorphic structures which are thermodynamically stable in 
three different temperature ranges. Monoclinic zirconia (baddeleyite) is the room 
temperature form, tetragonal zirconia is stable above 1200 K while cubic zirconia is stable 
above 2400 K. Tetragonal and cubic zirconia, however, may exist as metastable forms at 
room temperature, mostly if stabilized by dopants such as Yttrium. Frequently, zirconia 
powders, as prepared, are mixed tetragonal and monoclinic. Several characterization 
studies have been performed on pure zirconias and showed it is a typical ionic material, 
characterized by medium Lewis acidity and significant surface basicity. 
Partially Stabilized Zirconia (PSZ) or tetragonal zirconia polycrystal (TZP) consists of 
zirconia stabilized in the tetragonal form by some mol% of MgO, CaO, Y2O3, CeO2 or 
Sc2O3. They are specialty materials for structural ceramic applications, have very high 
strength, hardness and particularly toughness. 
Fully stabilized cubic zirconia powders, mostly yttria -stabilized zirconia (YSZ), with 8 % 
Y2O3, are available commercially also with significant surface area (50 m2/g). These 
materials find relevant industrial application due to their oxide ion conducting properties. 
They are in fact the solid elctrolytes used in Solid Oxide Fuel Cells (SOFC) as well as in 
oxygen sensors devices such as the -sensor used in the electronic system for the 
management of catalytic converters of gasoline-fuelled cars. 
 
67 
 
 
Pure zirconia or zirconia doped with alkali or alkali-earth cations are applied industrially as 
acido-basic catalysts for some alcohol dehydration. 
Zirconia is active in the heterolytic dissocation of hydrogen: in fact it has application for 
hydrodeoxygenation reactions (e.g. carboxylic acids toaldehyde) and for dehydrogenation 
reactions in the fine chemicals field. 
Zirconia finds many actual or potential applications as support, in particular for metal 
catalysts: it hasan activating effect for gold in the case of Au/ZrO2 CO oxidation catalysts, 
working also at 0°C. 
Sulphated and tungstated zirconias find very high acidity and are applied industrially as 
catalysts for paraffin isomerization. 
 
The titanias. 
The most usual crystal phases of TiO2 are anatase and rutile, the former being always 
metastable, the latter being always thermodynamically stable. Also titanias are highly ionic 
oxides with medium-high Lewis acidity, significant basicity and weak Brønsted acidity, if at 
all. Characterization data show that on anatase stronger Lewis acid sites are usually 
detectable than on rutile. 
Both anatase and rutile find most application as pigments and binders for polymeric 
materials. Anatase is usually prepared by precipitation and is largely used in the catalysis 
field, e.g. as the support for vanadia-based selective oxidation catalysts as well as for 
vanadia-tungsta and vanadia-molybdena catalysts for the Selective Catalytic Reduction of 
NOx. Rutile is the support of RuO2 or SnO2 catalyst for HCl oxidation to Cl2. Titania may 
also be used as support of sulphided hydrodesulphurization catalysts. As a catalyst, 
anatase finds relevant application in the Claus process as an alternative to alumina in 
68 
 
particular for the first higher temperature bed where hydrolysis of COS and CS2 also 
occurs. Titania-anatase is also the basic component of most photocatalysts. 
 
 
Titania and zirconia may be combined with silica and alumina, as well as each other, to 
give interesting and useful high-surface area and high stability materials. These materials 
still retain high Lewis acidity associated to the Al3+ , Ti4+ and Zr4+ cations, as well as 
medium-weak Brønsted acidity, associated to silanols and other hydroxy groups. Titania-
aluminas are important materials as catalyst supports, e.g. for hydrodesulphurization 
catalysts. 
 
Ceria 
Cerium dioxide, CeO2 or ceria, become quite recently a very important member of the 
family of catalytic oxides. It presents, when stoichiometric, the cubic fluorite structure with 
coordination eight for cerium ions and tetarahderal coordination for oxide anions. 
According to the big size of the cations, ceria presents a medium Lewis acidity and 
relevant surface basicity. Acually, its importance is mainly due to its slight easily reversible 
reducibility that produces the so called “oxygen storage capacity”, due to its ability to retain 
slight non-stoichiometry: 
CeO2  CeO2-x + x/2 O2 
For this reason it became, as such or mixed with zirconia and alumina, a most important 
support for metals in oxidation reactions. In particular, CeO2-ZrO2-Al2O3 composite oxides 
are applied as supports/washcoats in the preparation of the Pt-Rh-Pd based catalytic 
converters for gasoline-engine cars, because of its buffering ability on oxygen 
concentration. 
Among tetravalent metal oxides, CeO2 has attracted much interest for its catalytic 
functions in the synthesis of organic compounds, which provides evidence of its relevant 
basicity. In fact thoria, zirconia and ceria based materials (such as CeO2-Al2O3) find 
already practical industrial application in some dehydration and ketonization reactions, 
such as for the synthesis of diisopropyl ketone from isobutyric acid. CeO2-ZrO2 mixed 
oxides form a cubic solid solution in the ceria-rich side, which has relevant ability to adsorb 
69 
 
NOx, further increased by other rare earth doping, and find already practical industrial 
application in some ketonization reactions, such as for the synthesis of diisopropyl ketone 
from isobutyric acid. Ceria based materials are able to adsorb hydrogen reductively, thus 
can alsob applied to hydrodeoxygenation reactions. 
 
Tungsta 
Tungsten oxide WO3 has many crystal structures most of which however are distorted 
forms of the ReO3 –type cubic structure. These structures, where hexavalent tungsten is in 
more or less distorted octahedral sixfold coordination, have an highly covalent character, 
associated to the very high charge and very low size of the W6+ cation. This material has 
very strong acidity both of the Lewis and of the Brønsted type. Pure and silica supported 
WO3 have had industrial application as acid catalysts, e.g. for commercial direct hydration 
of ethylene to ethanol in the gas phase. WO3-ZrO2 is an industrial catalyst for light paraffin 
isomerization. 
WO3 (ReO3) idealized structure. 
 
Niobia, niobic acid and niobium phosphate. 
Hydrated niobium pentoxide (niobic acids, Nb2O5 . n H2O)) calcined at moderate 
temperatures of 100-300 °C are reported to show a strong acidic character with many 
potential applications in catalysis, displaying both Lewis and Brønsted acidity. Niobic acid 
is reported to crystallize as niobium oxide at 853 K, so loosing all its water and hydroxide 
species . The products of the combination of niobium oxide and phosphoric acid are 
niobium phosphates and phosphoric acid-treated niobic acid both reported to be materials 
potentially useful in acid catalysis. Both niobic acid and niobium phosphate find application 
as insoluble solid catalysts in water phase and are applied in the industry for some fine 
chemical acid-catalyzed processes, such as the Fructose dehydration reaction. Niobic acid 
and niobium phosphate are patented as alternatives to solid phosphoric acid for ethylene 
hydration to ethyl alcohol in the gas phase at 200 °C. 
 
Vanadia and molybdena based catalysts for partial oxidations. 
Pentavalent vanadium (V5+) and hexavalent molybdenum (Mo6+) have excellent properties 
for selective oxidation, having also very stable tetravalent (V4+) and pentavalent (Mo5+) 
states, respectively. While pure V2O5 and MoO3 have good catalytic properties, their 
supported species, mixed oxides and salts find even better properties. 
70 
 
 
V2O5-MoO3 mixed oxide-based catalysts. 
Catalysts based on molybdenum-vanadium oxides, with the addition of several other 
components, have been developed for selective oxidations and ammoxidations. This 
catalytic system has received much attention in recent years for the activation and the 
selective oxidation, in particular, of paraffins. The many phases obtained in these systems 
are mostly constituted by edge-sharing distorted MO6 octahedra, bonded with different 
crystal geometries. 
Catalysts with compositions of V2MoO8 to V3Mo2Ox (x  12-14), mostly supported on 
alumina or corundum, have been used in the past to produce maleic anhydride by 
selective oxidation of benzene with air, a process which has been abandoned in favour of 
that using n-butane as the feedstock. 
 
O
O
O
+ 2 CO2 + 2 H2O+ 4,5 O2 
 
Catalysts based on Mo–V–W-Fe are used for the selective oxidation of acrolein to acrylic 
acid at 260-300 °C, the main industrial process for producing such an important 
intermediate. 
H2C=(H3C)C-CH=O + ½ O2  H2C=(H3C)C-COOH 
Catalysts based on Mo/V/Nb/Te oxides are reported to allow direct oxidation of propane to 
acrylic acid. Complex catalysts based on Mo/V/Nb/Te and Mo/V/Nb/Sb oxides have also 
been developed for propane ammoxidation to acrylonitrile. The active catalysts belonging 
to these systems contain two predominant phases, so-called M1 (orthorhombic) and M2 
(pseudohexagonal). The M1 phase alone is capable of propane conversion, while the 
presence of the M2 phase may improve selectivity. M1 is a layered structure giving rise to 
two sets of channels, one hexagonal the other nearly heptagonal. In these channels, Te4+ 
cationic sites carrying an electron lone pair possibly relevant for the ammoxidation 
reaction, may be located. 
71 
 
 Structure and catalytically active center of Mo7.8V1.2NbTe0.94O29 (M1) phase in [0 0 1] 
projection. 
 
Metal molybdates. 
Metal molybdates are among the most relevant families of mixed oxides applied 
industrially for partialoxidation reactions. They are roughly distinguished into two main 
families: scheelite type, with the presence of monomeric (MoO4)2- units with tetrahedral 
coordination at molybdenum, and wolframite-type, presenting polymeric anions, with 
octahedral coordination at molybdenum. 
Ferric molybdate phase (Fe2(MoO4)3) is the basic composition of catalysts for the Formox 
process, the selective oxidation of methanol to formaldehyde 
CH3OH + ½ O2  CH2O + H2O 
The complex, monoclinic room-temperature crystal structure of Fe2(MoO4)3 consists of an 
open framework of octahedral FeO6 and tetrahedral MoO4 building blocks which are fused 
together by Fe-O-Mo bonds. This monoclinic structure converts into orthorombic -
Fe2(MoO4)3 at higher temperature where the connectivity of polyhedra remains the same. 
Although it is reported that the active phase of the catalysts is Fe2(MoO4)3, industrial 
catalysts always have an excess of MoO3 so that a typical molybdenum to iron atomic ratio 
72 
 
is 2.2:1. The enhanced catalytic performance of bulk iron molybdate catalysts in the 
presence of excess MoO3 is related to the formation of a surface MoOx monolayer on the 
bulk Fe2(MoO4)3 phase. Thus, the catalytic active phase for bulk iron molybdate catalysts 
is the surface MoOx monolayer on the bulk crystalline Fe2(MoO4)3 phase and the only role 
of the excess crystalline MoO3 is to replenish the surface MoOx lost by volatilization during 
methanol oxidation. 
 
 
Ferric molybdate structure Scheelite structure of CaMoO4 
 
Bismuth molybdates The general chemical formula of bismuth molybdates is Bi2O3·nMoO3 
where n=3, 2 or 1, corresponding to the α, β and γ phase, respectively. The relative activity 
and selectivity of these phases are different for each reaction. 
The crystal structure of -bismuth molybdate Bi2MoO6 is composed of layers of octahedral 
[MoO2]2+ and five-coordinated [Bi2O2]2+ linked together by layers of [O]2− (Aurivillius 
structure). -bismuth molybdate, Bi3(MoO4)3 has a structure of tetrahedral coordination 
with cubic crystal system similar to scheelite. 
Catalysts based on bismuth molybdates have been developed for the selective oxidation 
of propene to acrolein and its ammoxidation to acrylonitrile. They are used today also for 
the oxidation of isobutene to methacrolein and the oxidative dehydrogenation of butane to 
butadiene. Most recent evolution of this catalytic system implies the preparation of 
multicomponent molybdates. Industrial catalysts are based on Bi, Fe, Cr, Ni, Co, Mg 
molybdates where two main phases are formed: Bi/Fe/Cr trivalent scheelite-type 
tetrahedral molybdates constitute the active phase while Ni/Co/Fe/Mg bivalent octahedral 
polymolybdates with the wolframite structure act as catalysts of the reoxidation step. The 
catalysts are supported on silica and used in fluid bed reactor. 
73 
 
 
Aurivillius Structures 
 
Aurivillius structure of Bi2MoO6. Structure of polymeric anion in wolframite structures. 
 
Vanadyl phosphates 
Vanadyl pyrophosphate (VO)2P2O7 (VPP), is the main component of industrial catalysts for 
the selective oxidation of n-butane to maleic anhydride (MA), which produces an MA molar 
yield of between 53 and 65 mol% at n-butane conversion of 80–85 mol%. While the bulk 
VPP is always assumed to constitute the core of the active phase, the nature of the 
surface-active layer is a function of the P/V ratio used for the preparation of the catalyst. A 
slight excess of P with respect to the stoichiometric requirement for the VPP formation is 
necessary to aid the formation of the moderately active but selective δ-VOPO4, that is 
formed during reaction on the surface of VPP. On the contrary, in stoichiometric VPP (P/V 
atomic ratio 1.0), the formation of highly active but quite unselective αI-VOPO4 is obtained. 
The P/V atomic ratio in the most efficient catalysts may range from 1.10 to 1.20. 
The catalyst is prepared from an hydrated vanadyl-orthophosphate precursor. The best 
catalysts have a rose-like morphology, coming from the morphology of the precursor. 
74 
 
 Pseudomorphism between primary particles (top) and secondary particles (bottom) 
VOHPO4–0.5H2O precursor (left) and (VO)2P2O7 catalyst (right) corresponding a topotactic 
reaction driven by calcination. By controlling the morphology of the precursor, one can 
control the morphology of the final catalyst 
 
75 
 
Oxide-supported vanadia. 
The synthesis of phthalic anhydride (precursor of phthalate esters largely used as 
lubricants and plasticizers) is performed industrially over vanadia catalysts (4-10 % V2O5 
wt/wt) supported on titania (anatase polymorph) with surface area 6-25 m2/g, alkali ions (K, 
Rb, Cs), Sb and P playing the role of promoters. 
 
 
 
 
Reactions in similar conditions allow the syntheses of aromatic anhydrides and of aromatic 
nitriles by oxidation and ammoxidation of toluenes and xylenes over vanadia-based 
catalysts such as V2O5 /TiO2 or V2O5/Al2O3. 
CH3
X
CH3
X
O2
O2
CHO
CN
H2O
H2O
+ +
+ 3/2 + 3+ NH3
X
X
 
 
Another example of oxides supported on oxide catalysts is that for the SCR of NOx by 
ammnonia. DeNOxing of waste gases from stationary sources can be achieved efficiently 
by using the so-called SCR process, i.e. the Selective Catalytic Reduction using ammonia 
as the reductant: 
4 NH3 + 4 NO + O2  4 N2 + 6 H2O 
Industrial catalysts are constituted by V2O5-WO3/ TiO2 or V2O5-MoO3/ TiO2 monoliths. TiO2 
in the anatase form supports a “monolayer” of V2O5 and WO3 (or MoO3) deposed by 
impregnation. In general, the overall surface area of the catalysts ks 50-100 m2/g, with 
V2O5 virtual contents of 0,5-3 % w/w and MoO3 or WO3 contents of 8-12 % w/w . Typical 
reaction temperature is around 350 °C. 
In all cases it has been found that the best catalysts contain nearly a full “monolayer” of 
vanadium plus tungsten (or molybdenum) oxides over the TiO2-anatase support. The 
amount of vanadium oxide is variable but generally very small (at least in the most recent 
catalyst formulations). Vanadium oxide species are nearly “isolated” and ly between 
polymeric tungsten oxide species. 
 
Heteropolyacids. 
The most common and thermally stable primary structure of heteropolyacids is that of the 
Keggin unit that consists of a central atom (usually P, Si, or Ge) in a tetrahedral 
arrangement of oxygen atoms, surrounded by 12 oxygen octahedra containing mostly 
tungsten or molybdenum.. There are four types of oxygen atoms found in the Keggin unit, 
the central oxygen atoms, two types of bridging oxygen atoms, and terminal oxygen 
atoms. The secondary structure takes the form of the Bravais lattices, with the Keggin 
units located at the lattice positions. Heteropolyacids possess waters of crystallization that 
O
O
O
CH3
CH3
O2 H2O+ + 33
76 
 
bind the Keggin units together in the secondary structure by forming water bridges. 
Tertiary structures can be observed when heavy alkali salts are formed. 
 
Keggin and Wall-Dawson structures. 
 
The acidity of the heteropolyacids is purely Brønsted in nature. Since the Keggin unit 
possesses a net negative charge, charge compensating protons or cations must be 
present for electroneutrality. The acid form of heteropolyacids is generally soluble in water 
and acts as a liquid acid, and as a homogeneous acid catalyst in water solutions, as well 
as in liquid biphasic systems. 
 
Evaluation of acid strength in solution has shown that HPA’s composed of tungsten are 
more acidic than those composed of molybdenum, and the effect of the central atom is not 
as great as that of the addenda atoms. Nevertheless, phosphorus-based heteropolyacids 
are slightly more acidic than silicon-based heteropolyacids. This gives the general order of 
acidity as H3PW12O40 >