Acid Catalysis by Heteropoly Acids

Applied Catalysis A: General 256 (2003) 19–35

Review

Acid catalysis by heteropoly acids

M.N. TimofeevaInstitute of Catalysis, Novosibirsk 630090, Russia

Received 30 September 2002; received in revised form 23 January 2003; accepted 24 January 2003

Abstract

The achievements in the field of acid catalysis by heteropoly acids having different structures HxPW11LO40 (L = ZrIV ,TiIV , ThIV ), HxZW12O40 (Z = SiIV , PV, BIII , CoIII , GeIV ), H21B3W39O132, H6P2W18O62, H6P2W21O71, H6As2W21O69 arereviewed. The data on the acidity of HPAs are generalized. The rules of homogeneous and heterogeneous acid catalysis byHPAs are discussed.© 2003 Published by Elsevier B.V.

Keywords: Heteropoly acid; Keggin-type structure; Dawson-type structure; Acidity; Hammet acidity function; Acetone dimerization tomesitylene oxide; Acetic acid andn-butyl alcohol esterification;l-Sorbose acetonation

1. Introduction

Heteropolyacids due to their unique physicochem-ical properties are widely used as homogeneous andheterogeneous acid and oxidation catalysts. They arealso of great interest as model systems for studyingfundamental problems of catalysis[1–3]. This is stip-ulated by the possibility of varying their acidity andoxidizing potential as well as by similarity of their cat-alytic effects in solution and in the solid state[1–3].

For catalysis, Keggin-type HPAs (H8−xXxMVI12O40,

H8−x+nXxMVI12−nVV

n O40, where X= SiIV , GeIV , PV,AsV; M = MoVI , WV) are of importance (Fig. 1).The considerable number of studies performed dur-ing past 20–25 years allowed to formulate the se-lection principles of effective catalysts in the seriesof Keggin-structure HPAs. Their significantly higherBrønsted acidity, compared with the acidity of tradi-tional mineral acid catalysts, is of great importance

E-mail address: [email protected] (M.N. Timofeeva).

for catalysis[1–4]. Many new catalytic processes forbasic and fine organic syntheses based on their em-ployment have been developed[1–4]. In the future, thenumber of such processes will undoubtedly increasebecause HPA-based catalysts have higher activity thanknown traditional catalysts. Using HPA-based cata-lysts, it is frequently possible to obtain higher selec-tivity and successfully solve ecological problems.

Nowadays, more than 100 heteropolyacids of dif-ferent compositions and structures are known[5–7].A great body of information concerning their syn-thesis methods and their structure has been obtained,however, only the Keggin-structure HPAs are welldescribed in respect of their physicochemical andcatalytic properties[1–4]. Probably, this situationis due to several reasons, one of which is a sim-ple synthesis procedure of Keggin-structure HPAscompared with HPAs of other structures. Besides, incontrast to HPAs with Keggin-structure, heteropoly-acids of other types are thermally less stable andtherefore not capable of dehydrating at 150–200◦Cand thus can not be employed for reactions, which

0926-860X/$ – see front matter © 2003 Published by Elsevier B.V.doi:10.1016/S0926-860X(03)00386-7

20 M.N. Timofeeva / Applied Catalysis A: General 256 (2003) 19–35

Fig. 1. The structures of heteropoly anions (M= MoVI , WVI ).

are conducted at more than 150◦C. Nevertheless,in recent years more and more attention is drawnto obtaining new quantitative data for acid-catalyticproperties of HPAs having other structures and com-positions, mainly, H5PW11XO40 (X(IV) = TiIV ,ZrIV , ThIV ), H6As2W21O69(H2O), �-H6P2W18O62,�-H6P2Mo18O62, H6P2W21O71(H2O)3 and H21B3W39O132 (Fig. 1), in order to formulate certain selec-tion rules for effective acid catalysts in the series ofHPAs of the above structures. According to a simpleelectrostatic theory, these HPAs would be expectedto be stronger Brønsted acids than the Keggin-typeHPAs.

The aim of this review is to discuss the scope andlimitations of the above mentioned HPAs as the acidcatalysts. The review mainly covers the literaturepublished during the last 10 years, although, for amore comprehensive discussion, some earlier worksare also included. More detailed information aboutthe preparation of HPAs and their crystallographicdata were comprehensively reviewed elsewhere[5–8].

2. Acidic properties of heteropoly acids

2.1. Acidic properties of heteropoly acidsin solutions

HPAs are well-known to be strong Brønsted acidsbut only recently their acidity has been quantitativelycharacterized and compared with the acidity of usualmineral acids.

2.1.1. The dissociation constants of HPAsA great deal of works is dedicated to the study

of HPA acidity in solutions. However, data for thedissociation constants of HPAs are quite limited,mainly because of polyanions are labile in solutions.In aqueous solution, HPAs are completely dissociatedat the first three steps, the consecutive dissociationusually being unnoticeable because of leveling effectof the solvent[8,9]. According to the electrocon-ductivity data, H4PMo11VO40 and H5PMo10V2O40are strong 1-4 and 1-5 electrolytes, respectively, inaqueous solution[10]. Using a potentiometric method

M.N. Timofeeva / Applied Catalysis A: General 256 (2003) 19–35 21

Table 1Dissociation constants of heteropoly acids and H3PO4 in aqueous solution at 25◦C

H5PMo10V2O40

[9]H6PMo9V3O40

[9]H7PV12O36

[8]H8NbMo12O42

[10]H8CeMo12O42

[10]H8UMo12O42

[10]H3PO4

[10]

pK4 1.16 1.25 3.4 3.24 – – 2.12 (pK1)pK5 2.14 1.62 4.9 3.43 2.12 – 7.20 (pK2)pK6 – 2.00 6.4 3.64 1.98 2.13 11.9 (pK3)pK7 – – 7.9 4.28 2.99 3.02 –pK8 – – – 5.73 4.16 4.31 –

the dissociation constants (starting fromK4) ofH5PMo10V2O40, H6PMo9V3O40, H7PV12O36 weremeasured in aqueous solution[8,9]. In the aboveacids, the first three protons dissociate completely andthe others do stepwise with increasing pH[9]. Thedissociation constants are presented inTable 1. Notethat the value of dissociation constants of HPAs doesnot vary considerably, while the distinctions betweenvalue of dissociation constants of inorganic acids aremore considerable. For comparison, pKi for H3PO4are given inTable 1. One can see that all the studiedHPAs are much stronger than H3PO4. This may beexplained by taking into account a large size and lowsurface density of the polyanion charge as well asby the peculiarity of the HPAs proton structure[9].Nonaqueous and mixed solvents exhibit differentiat-ing effect on the acid dissociation constants of HPAs.Moreover, HPAs are considerably more stable in

Table 2Dissociation constants of heteropoly acids in various solvents at 25◦C

Acid HOAc [12] CH3CN [15] (CH3)2CO [13,15] C2H5OH [14]

pK1 pK1 pK2 pK3 pK1 pK2 pK3 pK1 pK2 pK3

H6P2W21O71(H2O)3 4.66 1.8 5.6 7.6 – – – – – –H6P2W18O62 4.39 1.8 5.7 7.7 – – – – – –H6P2Mo18O62 4.36 2.0 6.0 8.0 – – – – – –H3PMo12O40 4.68 – – – 2.0 3.6 5.3 1.8 3.4 5.3H4SiW12O40 4.87 1.9 5.9 7.9 2.0 3.6 5.3 2.0 4.0 6.3H3PW12O40 4.70 1.7 5.3 7.2 1.6 3.0 4.1 1.6 3.0 4.1H5PW11TiO40 5.32 2.0 6.0 7.9 1.7 3.2 4.2 – – –H5PW11ZrO40 5.45 1.8 5.5 7.5 2.0 3.4 5.2 – – –H3PW11ThO39 5.48 – – – – – – – – –CF3SO3H 4.97 5.5 – – 2.7 – – – – –HNO3 – – – – 3.6 – – 3.6 – –HClO4 4.87 – – – – – – – – –HBr 5.60 – – – – – – – – –H2SO4 7.00 – – – – – – – – –HCl 8.40 – – – 4.0 – – – – –

organic solutions[11]. In the works[12–15] the acid-ity of HPAs in organic solutions was systematicallystudied. InTable 2, the dissociation constants in ace-tonitrile, acetone, ethanol and acetic acid are given inthe form of pKi = −logKi, whereKi is the acid dis-sociation constant. For comparison, the dissociationconstants of some inorganic acids are presented.

First of all it is clearly seen that all the HPAs arestronger than the usual inorganic acids (HCl, H2SO4,HNO3, HBr) and even such strong acids as HClO4 andCF3SO3H. This is of fundamental importance for theHPAs application in acid catalysis. It is noteworthythat sulphuric acid, which is a traditional acid catalyst,ranks 2–5 units of pK below HPA.

The data given inTable 2show that acidic propertiesof HPAs in CH3CN, C2H5OH and (CH3)2CO do notvary significantly depending on the HPAs compositionand structure. Acidity series differ for each solvent.


Thus, in CH3CN, the acidity of W-containing HPAschanges in the order:

H3PW12O40>H5PW11ZrO40>H6P2W21O71(H2O)3

> �-H6P2W18O62, H6As2W21O69(H2O)

> H4SiW12O40 > H5PW11TiO40

At the same time, in acetone the acidity series falls inthe order:

H3PW12O40, H5PW11TiO40

> H4PW11VO40 > H4SiW12O40 > H5PW11ZrO40

� H3PMo12O40 > H4SiMo12O40

The HPA composition has only a slight effect ontheir acidity, nevertheless, this effect can be seen. Theacidity decreases with the reduction of HPA and re-placement of MoVI or WVI atom by VV atom and/orthe replacement of the central PV atom by SiIV . Inall cases, this decrease in acidity must occur withan increase in basicity of the acid (the number ofequivalent protons connected with heteropolyanion).

The more pronounced acidity dependence on thecomposition and structure of HPA is observed in aceticacid solution[12]. For the W-containing HPAs, theacidity changes in the following series in accordancewith the anion structure:

Dawson> H6P2W21O71(H2O)3 > Keggin

This correlates well with acidity of reducing forms ofthe Dawson- and Keggin-type HPAs in solutions[16].For the Keggin-type HPAs, the dissociation constantsin AcOH decrease in the series[12]:

H3PW12O40 > H4SiW12O40 > H5PW11TiO40

> H5PW11ZrO40 ≈ H3PW11ThO39.

The replacement of WVI by MoVI does not influencethe value of the HPA dissociation constants, while thereplacement of WVI by TiIV , ZrIV or ThIV must resultin the decrease of the dissociation constant accordingto the electrostatic theory.

The basicity of the solvent substantially influencesthe acidity as well as the extent of acidic dissoci-ation (Table 2). Thus in the less polar HOAc, allthe HPAs are comparatively weak monobasic acids.In such polar solvents as CH3CN, C2H5OH and

(CH3)2CO, the Keggin-structure HPAs are com-pletely dissociated at the first step and partiallyat the second one. Such acids as H3PW12O40 andH5PW11TiO40 are completely dissociated at the first,second and partially at the third steps in acetone. ForHPAs of other structures data acquisition in somepolar solvents is impossible due to an intricate de-pendence of electroconductivity on acid concentra-tion in solution [15]. However, it can be supposedthat their acidity will be close to the acidity ofH3PW12O40.

2.1.2. The Hammet acidity function H0In solution, the acidity of HPAs is characterized

quite well by the Hammet acidity functionH0 [17,18].It has been established that the series of the HPAstrength in diluted and concentrated aqueous solutionsdo not coincide with each other. The strength of HPAsin diluted solutions ([HPA]< 0.05 mol/l) decreases inthe series[18]:

H21B3W39O132 > H5PW11TiO40 > H5PW11ZrO40

> H6P2W21O71 > H3PW12O40 > H4SiW12O40

At the same time, the acidity series for concentratedsolutions ([HPA] > 0.05 mol/l) decreases in the order[18]:

H21B3W39O132 > H6P2W21O71 > H5PW11TiO40

> H3PW12O40 ≈ H4SiW12O40 > H5PW11ZrO40

This difference is probably due to the fact that acidityof concentrated solutions is determined not only bythe values of the acid dissociation constants (as it takesplace in the case of diluted acids) but also by the“salt effects” which depend on the composition andstructure of the HPAs[17].

The H0 values obtained in aqueous acid solutionswith the same proton concentration differ slightly(Fig. 2, Table 3). This indicates that all the HPAs havesimilar acid strength, close to the acidity of HClO4 andCF3SO3H, and even stronger. The equimolar aqueoussolutions of H21B3W39O132 and H6P2W21O71 havehigher acidity than Keggin-structure HPAs, HClO4and CF3SO3H. Probably, that is because they havemore protons in their composition. The differencein the acidity of aqueous solutions of the HPAs


Fig. 2. H0 for aqueous solutions of HPAs at 20◦C [15].

disappears at the same weight concentration of theHPAs (Table 3).

The differences in acidity become more obvious inwater–organic solutions. Thus, in 90% aqueous ace-tone the following acidity series is observed (acidityis based per proton)[18]:

H6P2W21O71 > H21B3W39O132 > H5PW11TiO40,

H5PW11ZrO40 > H3PW12O40, H4SiW12O40

Table 3Hammet acidity functionH0 values of heteropoly acids in different solution

Acid H2O [18] (CH3)2CO (90%)[18] CH3CN (90%) [18] HOAc (85%) [12]

H0a

(0.1 mol/l)H0

b

(0.3 mol/l)H0

c

(288.2 g/l)H0

a

(0.05 mol/l)H0

b

(0.15 mol/l)H0

a

(0.05 mol/l)H0

b

(0.15 mol/l)H0

a

(0.05 mol/l)H0

b

(0.20 mol/l)

H21B3W39O132 −0.52 +0.21 +0.11 +0.21 +1.55 −0.55 +0.35 – –H6P2W21O71 −0.40 +0.15 +0.04 +0.11 +0.38 −0.58 −0.36 −0.27 −0.20H5PW11TiO40 −0.18 +0.01 −0.06 +1.31 +1.77 +0.06 +0.17 +0.19 +0.40H3PW12O40 −0.05 −0.05 −0.05 +2.17 +2.17 +0.14 +0.14 +0.10 −0.17H5PW11ZrO40 −0.07 +0.03 −0.05 +1.43 +2.02 −0.03 +0.20 +0.79 +1.06H4SiW12O40 −0.03 +0.06 −0.03 +1.36 +2.13 +0.46 +0.60 −0.07 −0.04HClO4 +0.81 +0.36 −0.71 +2.32 +1.80 +0.86 +0.66 – –CF3SO3H +0.56 +0.25 −0.83 +2.00 +1.66 +0.77 +0.62 – –H2SO4 – – – – – – – – +0.75

a The molar concentration of acid.b The proton concentration of acid.c The weight concentration of acid.

In 90% acetonitrile[18] and 85% HOAc[12], the acid-ity series are different:

H6P2W21O71 > H3PW12O40, H5PW11ZrO40,H5PW11TiO40 > H21B3W39O132 > H4SiW12O40and H6P2W21O71(H2O)3 > H4SiW12O40 ≈H3PW12O40 > H5PW11TiO40 > H3PW11ThO39 ≈H5PW11ZrO40, respectively. It is obvious that theacidic series in water–organic solvents does notcorrelate with the thermodynamic dissociation con-


stants in nonaqueous organic media. This shows thatin water–organic solutions HPAs become polyelec-trolytes of a level higher than two and essentiallydiffer from other acids[18].

The acid strength decreases when WVI atom is re-placed for MoVI or VV and/or when the central PV

atom is replaced for SiIV . The effect of the central atomhas been demonstrated for acetonitrile solutions of theKeggin-type heteropolytungstates[19,20]. It has beenestablished that in general the acidity increases with adecrease in the negative charge of the heteropolyanionor an increase in the charge of the central atom:

PV > SiIV , GeIV > BIII > CoIII

It has been found[18] that the acidity of water–organicsolutions of Keggin HPAs depends inversely on thebasicity of the organic solvent. The solvent basic-ity decreases in the series: acetone > acetonitrile >water. The acidity of the solutions of H3PW12O40,H4SiW12O40, H5PW11TiO40 and H5PW11ZrO40changes in the reverse order. This does not occur forthe solutions of H6P2W21O71 and H21B3W39O132most likely due to the influence of the anion structureof HPAs.

The effect of water amount on the acidity ofwater–organic solutions has an extreme character,which is a common phenomenon for mixed solutions.The acidity decreases sharply upon addition of wa-ter, then goes through a minimum and increases withfurther increase of water concentration[17]. Suchcharacter of dependence is supposed to be due to theinfluence of the solvent on the activity coefficient ofreagents[14].

2.1.3. The basicity of heteropolyanionsRecently, the acidity of HPAs has been studied

by using various physicochemical methods, includ-ing comparison of the basicity of heteropolyanons.These data were gained from1H NMR study of com-plexation between the anion and the geminal diol,1,1-dihydroxy-2,2,2-trichloroethane (chloral hydrate),in organic medium[21–23].This reaction is assumed

to yield complexes having two hydrogen bonds be-tween bridging (Ob) oxygen atoms of the heteropoly

anion (Fig. 1). The chemical shifts of the hydroxylprotons of chloral hydrate increase in parallel withthe increase in the basicity of the anion or, in otherwords, with the decrease in the acidity of the corre-sponding HPA. The application of this method for theKeggin HPAs resulted in the following acidity series:

H3PW12O40 > H3PMo12O40 > H4SiW12O40

≈ H4GeW12O40>H4SiMo12O40>H4GeW12O40

In general, this order is in agreement with that obtainedusing indicator tests.

HPAs of different structural types (Dawson,H6P2W21O71(H2O)3, H6As2W21O69(H2O), H21B3W39O132) were studied in[23]. Since heteropolyanions of different structures differ substantially insize and are potentially capable of coordinating dis-similar numbers of chloral hydrate molecules, theauthors decided that the data presented inFig. 3,where the �δ 1HCH values are divided by thenumber of WVI or MoVI atoms in the anion, aremore illustrative. In this respect, heteropoly anionsare divided into three groups: 1—S2Mo18O62

4−,PW12O40

3−, PMo12O403−; 2—P2W21O71(H2O)6−,

SiW12O404−, As2W21O69(H2O)6−, B3W39O132

21−;3—P2Mo18O62

6−, P2W18O626−. The basicity of het-

eropoly anions increases in the following order:

S2Mo18O624−, PW12O40

3−, PMo12O403−

< P2W21O71(H2O)6−, SiW12O404−,

As2W21O69(H2O)6−, B3W39O13221−

< P2Mo18O626−, P2W18O62

6−

The specific charge of one atom of WVI or MoVI

changes in the same order. Thus, although the surfaceof the HPA anions is inhomogeneous, the foregoingsuggestion generally confirms the correlation foundpreviously for the HPA anions of the Keggin-structure,namely, with a decrease in the anion charge, the an-ion basicity decreases and, hence, the acidity of thecorresponding HPA increases[24].

2.2. Acidic properties of solid heteropolyacids

The strength and the number of acid centers aswell as related properties of heteropolyacids can becontrolled by the structure and composition of het-


Fig. 3. Variations ofδ 1HCH related to the number of the WVI or MoVI atoms in the heteropoly anions[22].

eropolyanions, extent of hydration, type of support,thermal pretreatment, etc.

Solid heteropolyacids, such as HxZM12O40 (Z= PV, SiIV , GeIV , AlIII ; M = MoVI , WVI ), DawsonH6PM18O62 (M = MoVI , WVI ), H6As2W21O69(H2O),H6P2W21O71(H2O)3, are pure Brønsted acids[25]and are stronger than conventional solid acids suchas SiO2–Al2O3. According to the indicator test,

Fig. 4. IR-spectra of pyridine adsorbed on the surfaces of H5PW11ZrO40 (1) and H5PW11TiO40 (2) [30].

H3PW12O40 has a Hammet acidity function less than−8.2 [26] and it has been suggested to be a superacid[27,28]. A superacid is an acid with a strength greaterthan that of 100% H2SO4, i.e. a value ofH0 < −12[29,30]. Thermal desorption of basic molecules alsoreveals the acidic properties.

At the same time, heteropoly acids such asH5PW11TiO40, H3PW11ThO39 and H5PW11ZrO40


Table 4Average PA values for protons of heteropoly acids substituted bythe pyridinium ion[25]

HPA PA (kJ/mol) n(H+)a

Ib II c

H3PW12O40 1070 4.5 3.0�-H6P2Mo18O62 1110 4.4 6.0CF3SO3H 1120 1.1 –H6P2W21O71 1130 5.6 6.1H4SiW12O40 1140 3.2 4.1H3PMo12O40 1140 2.4 3.0H4SiMo12O40 1150 5.4 4.1H3PW6Mo6O40 1150 3.6 3.0H4PW11VO40 1170 5.5 4.1H21B3W39O132 1180 10 15HClO4 1180 1.07 –H5PW11TiO40 1190 4.9 4.0�-H6P2W18O62 1200 6.5 6.0H6As2W21O69 1200 2.4 6.1H5PW11ZrO40 1210 6.1 3.5H-ZSM-5 1200 – –CF3COOH 1350 – –

a Number of H+ ions substituted by PyH+.b IR spectroscopy.c Weight method.

are both Brønsted and Lewis acids[31]. Fig. 4 showsthe IR-spectra of pyridine adsorbed on H5PW11ZrO40and H5PW11TiO40. In the spectra of all the HPAs theabsorption bands at 1540 cm−1 attributed to Brønstedacid sites and the absorption bands at 1450 cm−1

attributed to Lewis acid sites are observed. The in-tensity of this band for H5PW11ZrO40 is ∼2.5 timeshigher than that for H5PW11TiO40. It is likely thatin acidic reactions Lewis sites as well as Brønstedacid sites significantly contribute to the catalyticactivity of H5PW11ZO40 (Z = ZrIV , TiIV , ThIV )[32].

The strengths of heteropolyacids on the “protonaffinity” scale have been determined by IR spec-troscopy of the pyridinium salts[25]. Table 4demon-strates the PA (“proton affinity”) values of severalHPAs. All the HPAs are strong acids like CF3SO3Hand HClO4 and stronger than zeolites and sulphatedalumina. The structure of the HPA anion has almostno effect on the HPA acidity. H3PW12O40 is thestrongest acid, which agrees well with the data ob-tained for the solid HPA and their solutions by othermethods [12,18,31–34]. PA values of the similar

Keggin HPAs decrease in the series:

H3PW12O40 > H4SiW12O40, H3PMo12O40

> H4SiMo12O40, H3PW6Mo6O40

> H4PW11VO40 � H5PW11TiO40

> H5PW11ZrO40

According to this, the acidity of the HPAs decreaseswith an increase in the anion charge. For the KegginHPA, W-HPAs are stronger acids than Mo-HPAs,whereas for the Dawson HPA�-H6P2M18O62 (M= MoVI , WVI ) this dependence is reverse, which isalso confirmed by the method of complex formationwith chloral hydrate[23].

The acidic strength of heteropolyacids has been de-termined more quantitatively by calorimetry of NH3adsorption[32,35]. It has been determined that aftertreatment at 423 K the initial heats of NH3 adsorp-tion are as follows: 196, 185, 164 and 156 kJ/mol forH3PW12O40, H4SiW12O40, H6P2W21O71(H2O)3 andH6P2W18O62, respectively. These values show thatacidity decreases in the series:

H3PW12O40 > H4SiW12O40 > H6P2W21O71(H2O)3

> H6P2W18O62

These data also indicate that the Keggin-type het-eropoly acids are much stronger acids than theDawson-type HPAs. Furthermore, the heats of theNH3 adsorption confirm that HPAs are strongeracids than zeolites or simple metal oxides: 150 and140 kJ/mol for H-ZSM-5 and�-Al2O3, respectively.

An increased interest in heterogenization of HPA ona support is caused by a superior activity revealed forthese systems compared to that of bulk HPAs[36–38].Moreover, the supported systems are of practical im-portance because the catalytic activity is determinedby the catalyst surface area for some HPA-catalyzedreactions.

Many porous materials were used as supports forHPAs. A variety of methods has been used to char-acterize HPAs supported on SiO2, carbon, Al2O3,MgF2 [37–42]. 31P NMR [43] and FT-IR data onSiO2, SiO2–Al2O3 and Al2O3 impregnated withH3PW12O40 [44] show that the primary Keggin-structure is preserved but some degradation can beobserved with Al2O3 heated above 200◦C.

Kapustin et al. [45] have found that the acidstrength of supported H3PW12O40 decreases accord-


ing to the series SiO2 > Al2O3 > carbon, whichallowed them to assume a significant interaction be-tween HPA and carbon. The nature of the interactionbetween HPA and carbon depends mainly on the con-centration of the impregnating solutions, solvent usedand pH of the solution. Thus, when H3PW12O40 andH4SiW12O40 are supported from concentrated aque-ous solutions on chemically activated carbon, theyretain their Keggin-structure, and the acids are highlydispersed on the support. At the same time, if the HPAcontent is low, H3PW12O40 is partly decomposed[46].

The results of HPA acidity measurement by twodifferent methods (indicator test and the calorimetryof NH3 adsorption) are compared in[47]. The orderof the acid strength of unsupported HPAs measuredby the indicator test is:

H3PW12O40 > H4SiW12O40 > H4GeW12O40,

H6P2W18O62 > H5BW12O40 > H6CoW12O40

The order obtained by the calorimetry of NH3 adsorp-tion, H3PW12O40 > H4SiW12O40 > H6P2W18O62,is consistent with the series above. An interestingfact is that the acid strength of H6P2W18O62 wasalmost unchanged upon supporting on SiO2. Theacidic strength of 20% H6P2W18O62/SiO2 and 20%H3PW12O40/SiO2 became closer. This result seemsto be quite strange because (see below) upon support-ing H6P2W18O62 on SiO2 as well as on the carbon,the strength of acidic centers decreases due to stronginteraction with the supporting groups of the car-rier [48]. It is likely that such an unusual result iscaused by the high temperature of pre-activation ofH6P2W18O62/SiO2 (200◦C under vacuum), whichresults in isomerization and partial destroying ofH6P2W18O62.

3. Acid catalysis by heteropoly acids

In this part of the review the following questionswill be discussed: (a) comparison of catalytic activ-ity of HPAs having various composition and struc-ture; (b) relationship between the HPAs activity andtheir acidity.Table 5gives examples of reactions cat-alyzed by both the Keggin HPAs and HPAs of otherstructures.

3.1. Homogeneous acid-catalyzed reactions

The mechanism of homogeneous catalysis by het-eropoly acids is in principle similar to the mechanismof catalysis by solutions of inorganic acids. However,heteropolyacids are capable of protonating the sub-strate and activating it for subsequent chemical reac-tions more effectively than usual inorganic acids.

According to physicochemical data, we may expectthat acids with different structures and compositionswill differ slightly in the catalytic activity in homo-geneous acid-catalyzed reactions in water. Consider-able distinctions can be observed in organic solvents.The activity series may differ in different solvents orfor different substrates like it was observed for theHPA-catalyzed ether decomposition[24]. Examplesof relationship between the catalytic activity of HPAand their dissociation constants (the Brønsted equa-tion) are well precedented[17,19,20,31,49].

It is well known that in concentrated aqueous solu-tions of heteropolyacids the reaction rate constant isdependent on the Hammett acidity functionH0 [49].Thus the rate constant for hydration of isobutene inaqueous solution in the presence of H3PW12O40 andH4SiW12O40 obey the equation: log(k) = −1.04H0−3.46. The same relation takes place for H2SO4, HCl,HNO3 and HClO4. Given this fact in mind, it was sug-gested that the hydration of isobutylene in the pres-ence of the HPAs and inorganic acids proceeds via thegenerally accepted mechanism.

However, it turned out that not only concentrationbut also the kind of HPA influences the catalytic activ-ity of HPA for isobutene hydration, which decreasesin the series[50]:

H3PW12O40 > H3PMo12O40 > H4SiW12O40

= H4GeW12O40>H4SiMo12O40>H4GeMo12O40

This order correlates with the relative basicity of con-jugated anions. Judging from the relative basicity ofthe conjugated anions, the strength of H3PW12O40is almost comparable with that of HClO4, however,the catalytic activity of the HPA is much higher ow-ing to the nature of the heteropoly anions[50,24].Thus Izumi et. al.[24] observed that H4SiW12O40is more active for the reaction of dibutyl ether withacetic anhydride than H3PW12O40, despite the factthat H4SiW12O40 is a weaker acid compared to

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Table 5Acid-catalyzed reactions in the presence of heteropoly acids

Reaction Catalyst Ref.

Homogeneous reactionsCondensation: 2CH3COCH3 → CH3COCH=C(CH3)2 H6P2W21O71, HxZW12O40 (Z = SiIV , PV), HxPW11LO40

(L = ZrIV , TiIV , ThIV )[18]

Acetonation ofl-sorbose H6P2W21O71, H6P2W18O62, HxZW12O40 (Z = SiIV , PV),HxPW11LO40 (L = ZrIV , TiIV , ThIV )

[55]

Esterification: BuOH+ HOAc → BuOAc+ H2O H21B3W39O132, H6P2W18O62, H6P2W21O71,H6As2W21O69, HxZW12O40 (Z = SiIV , PV),HxPW11LO40 (L = ZrIV , TiIV , ThIV )

[31]

CH3CH2COOH + iso-C4H9OH → iso-C4H9OCOCH3 + H2O H6P2W18O62, HxZW12O40

(Z = SiIV , PV, BIII , CoIII , GeIV )[52]

iso-C4H9OCOC2H5 → iso-C4H9OH + C2H5COOH

iso-C4H9OCOC2H5 +CH3COOH → iso-C4H9OCOCH3 + C2H5COOH H6P2W18O62, HxZW12O40 (Z = SiIV , PV, GeIV ) [52]Higher aliphatic diols→ cyclic ethers HxZM12O40 (Z = SiIV , PV; M = MoVI , WVI ) [74,75]

H6P2W18O62, HxZW12O40 (Z = SiIV , PV, GeIV ) [54]

(CH3)2C=CH2 + CH3OH → (CH3)3COCH3 H6P2W18O62, HxZW12O40 (Z = SiIV , PV) [64]

Biphasic reactionsCondensation: H6P2W21O71, H6P2M18O62 (M = MoVI , WVI )

HxZW12O40 (Z = Si, P), HxPW11LO40

(L = ZrIV , TiIV , ThIV )

[56,57]

HxZM12O40 (Z = SiIV , PV; M = MoVI , WVI ) [70,71]

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Acetoxylation: HxZW12O40 (Z = SiIV , PV) [72]

Alkylation H6P2W21O71, H6P2W18O62, H3PW12O40 [59]

Heterogeneous reactionsAlkylation: iso-C4H10 + iso-C4H8 → fractions C5–C8 H6P2W18O62, H6P2W18O62/SiO2 [60]

H3PW12O40, �-H6P2W18O62, H6P2W21O71(H2O)3,H6P2W18O62/SiO2, H6P2W18O62/carbon

[48]

Condensation: H6P2W21O71, H6P2W18O62, HxZW12O40 (Z = SiIV , PV),HxPW11LO40 (L = ZrIV , TiIV )

[73]

Higher aliphatic diols→ cyclic ethers HxZM12O40 (Z = SiIV , PV; M = MoVI , WVI ) [74,75]2-iso-C4H8 → dimer and trimer of isobutylene H6P2W21O71, H6P2W18O62, H6As2W21O69 H3PW12O40,

HxPW11LO40 (L = ZrIV , TiIV )[65]

Esterification:iso-C4H8 + CH3OH → iso-C4H9OCH3 HxZW12O40 (Z = SiIV , PV, BIII , CoIII ), H6P2W18O62,H6P2W18O62/SiO2

[47,61]

Dehydration: CH3OH → CH4 + H2O HxZW12O40 (Z = SiIV , PV, BIII ), H21B3W39O132 [62]


H3PW12O40. This difference was explained by sta-bilization of the cationic intermediate via the for-mation of an intermediate polyanion complex dueto the softness of polyanion, which falls the order:SiW12O40

4− > PW12O403− [51]. The effect of the

softness becomes significant for reactions in aqueoussolutions, in which the difference in the acid strengthis less pronounced since most HPAs are completelydissociated.

It was shown that W-containing heteropolyacidsof the Keggin-type demonstrate remarkably high cat-alytic activity [52]. Thus for the liquid-phase homo-geneous decomposition of isobutyl propionate, HPAsappeared to be 60–100 times more active than H2SO4and para-toluenesulfonic acid. The catalytic activ-ity for this reaction increases when the central atomcharge increases. This can be attributed to an increasein the acid strength of the solution. It rises with adiminution of the negative charge of the polyanions.For reactions involving basic molecules such as al-cohols (ester exchange of isobutyl propionate withn-propyl alcohol and esterification of propionic acidwith isobutyl alcohol), no significant differences inthe activity among these catalysts, including H2SO4andpara-toluenesulfonic acid, are observed, suggest-ing that these catalysts are equally strong in solu-tion due to the leveling effect of the basic reactant[52].

In the reaction of acetone dimerization a correla-tion between the catalytic activities of HPAs andH0has been found (Fig. 5) [18]. The catalytic activity de-

Fig. 5. A plot of logarithm of the rate constant of acetone dimer-ization vs.H0 ([H+] = 0.15 mol/l) [15].

creased in the series:

H6P2W21O71 > H5PW11TiO40

≥ H5PW11ZrO40 > H4SiW12O40 ≥ H3PW12O40

The activity of CF3SO3H was higher than that of theKeggin-type heteropolyacids but lower than that ofH6P2W21O71. For HPAs and CF3SO3H, theH0 valuedecreased in the similar order.

In the esterification reaction of acetic acid andn-butyl alcohol the catalytic activity of HPAs hasa good correlation with the dissociation constantof heteropolyacids[31]. The catalytic activity oftungsten-containing heteropolyacids depends on thestructural type and decreases in the series:

H21B3W39O132 > H6P2W18O62

≈ H6P2W21O71 > H6As2W21O69 > Keggin-type

For the heteropoly acids of the Keggin-structure, theactivity decreases in the order:

H5PW11ZrO40 > H4SiW12O40

≥ H3PW12O40>H5PW11TiO40 � H3PW11ThO39

The higher catalytic activity of H5PW11ZrO40 maybe explained by taking into account not only Brønstedacidity but also Lewis acidity. Besides, it has been es-tablished that in the presence of H6P2W21O71(H2O)3the reaction rate substantially depends on the orderof adding the reagents. HPA added immediately aftermixing all the reagents (HOAc, H2O, n-BuOH) is ofhigher catalytic activity than HPAs pre-maintainedduring 1–4 h in aqueous acetic acid medium. For HPAsof other structures such dependence has not been ob-served. On the basis of NMR183W data (Fig. 6) andkinetic measurements, it could be supposed that inpure acetic acid two of three water molecules, incorpo-rated into heteropolyanion and located on its surface(Fig. 6), are replaced for HOAc molecules to formHPA: H6P2W21O71(H2O)2(AcOH) (δ 31P−12.5 ppm)and H6P2W21O71(H2O)(AcOH)2 (δ 31P −11.6 ppm).The third water molecule is located inside the HPAanion and is not capable of such exchange. Thesereactions are reversible, since the intensity of signalsat −12.5 and−11.6 ppm decreases with an increaseof water concentration in HOAc. Mono- and diacy-lated derivatives of the H6P2W21O71(H2O)3 HPA canbe considered as 7- and 8-basic heteropolyacids. An


Fig. 6. 31P NMR spectra of 5× 10−3 mol/l H6P2W21O71(H2O)3solutions in acetic acid with different water concentrations: (1)no water added, (2) 2.5 vol.%, (3) 10 vol.%, (4) 50 vol.%, and (5)aqueous heteropoly acid solution[30].

increase in the HPA basicity leads to a decrease intheir acidity [53]. As a result, H6P2W21O71(H2O)3added simultaneously with the other reagents tothe reaction mixture exhibits higher catalytic ac-tivity compared with H7P2W21O71(H2O)2(AcO) orH8P2W21O71(H2O)(AcO)2. The latter complexes ap-pear after preliminary maintaining of HPA in aqueousHOAc.

The catalytic activity series of HPAs having dif-ferent structures differ depending on the reactiontype. Thus, in the reaction of isobutyl propionate de-composition the following activity series is observed[52]:

H3PW12O40 > H4SiW12O40, H4GeW12O40

> H5BW12O40 > H6P2W18O62 > H6CoW12O40

In the reaction of isobutyl propionate etherifica-tion by acetic acid the order is somewhat different[52]:

H3PW12O40, H4SiW12O40

> H4GeW12O40, H6P2W18O62

In the nucleophilic reaction of the addition of alcoholsto cyclohexenone the catalytic activity changes in theseries[54]:

H4SiW12O40 > H3PW12O40 > H3PMo12O40

> H4GeW12O40 > H6P2W18O62

In the reaction of isobutyl propionate esterificationby n-propyl alcohol different Keggin-structured HPAshave almost the same activity, the activity of theDawson-structure HPA (H6P2W18O62) being higher[52].

At the same time, the HPA activity (per weightunit of a catalyst) in the synthesis of diacetonesorbose(DAS) decreases in the series[55]:

H3PW12O40 > H3PMo12O40, H4SiW12O40

> H4SiW12O40 > �-H6P2W18O62

� H6P2Mo18O62 > H5PW11ZrO40

> H5PW11TiO40 > H6P2W21O71

> �-H6P2W18O62

All these suggests that the catalytic action of HPA inacidic-type reactions depends on the HPA acidity, theheteropolyanion structure and the type of reaction (thenature of the reagents).

3.2. Biphasic catalytic systems

Misono and co-workers[1] demonstrated that crys-talline HPAs in many respects behave like solutions.This is due to the fact that these solids have discreteand mobile ionic structures. Water soluble HPAs havea great affinity for molecules of polar substances (al-cohols, ketones, ethers, etc.) These substances canbe absorbed in a large amount in the bulk of HPAsto form solvates. At the same time in contrast topolar molecules, non-polar reagents are not capableof absorbing in the bulk of HPA. By virtue of theseproperties HPAs can be used under the conditionsof biphasic catalysis. Up to now, only few catalyticHPA-based systems are known (Table 5).

Thus, the employment of the concentrated watersolutions of HPAs permitted the polymerization reac-tion with ring opening of tetrahydrofuran (THF) in the


presence of water[56,57]. The polymer is formed inthe HPA phase and then transferred to the THF phase.The reaction rate increases with decreasing H2O/HPAratio, and the molecular weight of the polymer doesthe same. The process is performed continuously. Thepolyoxytetramethylene glycol with molecular weightof 500–2000 and narrow molecular weight distribu-tion is obtained from the THF phase. According to[57], the reaction depends on the HPA structure andthe acidity of HPAs (per catalyst weight) decreases inthe series:

H6P2W21O71 > H3PW12O40 > H4SiW12O40

> H3PMo12O40 > H5PW11TiO40

� H6P2Mo18O62 > H5PW11ZrO40

> H6P2W18O62

Similarly, the polymerization of cyclic formaldehydeacetal, trimer 1,3-dioxolane and 1,3,5-trioxane are cat-alyzed by the Keggin-type HPAs such as H3PW12O40,H4SiW12O40, H4SiMo12O40 and H3PMo12O40 [58].It has been established that in the polymerization of1,3,5-trioxane in the presence of the HPA, the reactionrate (based on the catalyst weight) is 25 times higherthan that found for BF3·OR2.

The hydroquinone can be alkylated with isobuty-lene in the presence of the HPAs (H6P2W21O71,H3PW12O40, H6P2W18O62) under phase-transfer con-ditions in a biphasic system, including toluene andHPA dioxane etherate (HPA-nC4H8O2-mH2O) [59].The yield of 2-tert-butyl-hydroquinone decreases inthe series:

H3PW12O40 > H6P2W21O71 > H6P2W18O62

This order correlates well with the relative basicityof conjugated anions (seeSection 2.2). Since in theseseries the anion basicity increases, the acidity of thecorresponding HPA falls. H3PW12O40 shows higherefficiency than H2SO4 or H3PO4.

3.3. Heterogeneous catalytic systems

In this section, liquid-phase and gas-phase reac-tions catalyzed by bulk and supported HPAs arediscussed. In heterogeneous as well as homogeneousconditions HPAs are more effective than conventionalcatalysts, such as SiO2–Al2O3, zeolites, etc. The

comparison between the HPA acidity and the acidityof other acidic catalysts was carried out by measur-ing their catalytic activity in different reactions. Todate, a few reactions are known, in which the cat-alytic activity of bulk and supported H6P2W18O62,H6P2W21O71 and H21B3W39O13 is comparedwith that of the Keggin-type heteropolyacids(HnXW12O40; X = PV, SiIV , GeIV , BIII , CoIII ) [47,48,60–62].

Thus, the study of the isopropyl alcohol dehydrationreaction CH3C(OH)HCH3 → CH3CH=CHCH2 +H2O, at 150◦C showed that the HPAs are strongerthan the solid acids widely used in catalysis, such asH3PO4/SiO2 and even zeolites[50,63]. In accordancewith their composition, the HPA acidity changes inthe series[63]:

H3PW12O40 > H4SiW12O40 � H5PW10V2O40

> H3PMo12O40 > H5PW10V2O40

> H2SiMo12O40

It is interesting that the acidity series of solid HPAsagrees with those of acids in solution.

However, for reactions where small polar moleculesare employed, such relation seems to be an excep-tion rather than a rule, since beside the acidity, thecatalytic properties of HPAs may be considerably af-fected by the substrate absorption in the bulk of HPAwith the formation of so called “pseudoliquid phase”.It was demonstrated for reaction of the addition ofmethanol to isobutylene in the presence of HPA. Inthis reaction, the HPA acidity decreases in the series[47,61]:

H6P2W18O62 > H4SiW12O40 > H4GeW12O40

> H3PW12O40 > H5BW12O40, H6CoW12O40

The high activity of acids with a low acidity(H6P2W18O62, H4SiW12O40) was explained by theinfluence of the rate of the substrate absorption inthe bulk of the HPA. This series differs from theactivity series for the esterification reactions in theliquid-phase[64]. The activity (per unit weight) ofthe bulk H6P2W18O62 is at least 13 times higher thanthat of H3PW12O40 [60]. At the same time, the activ-ity (per HPA weight) of H6P2W18O62 is ∼2.7 timeshigher than that of H3PW12O40 in the homogeneous


liquid-phase synthesis of methyl-tert-butyl ether(MTBE) [64].

The reaction of 2,6-di-tert-butyl-4-methylphenol(DMBP) with toluene, which behaves as an accep-tor of tert-butyl, has been studied using bulk andsupported�-H6P2W18O62, H6As2W21O69 and theKeggin-type HPAs, H3PW12O40, H5PW11TiO40,and H5PW11ZrO40 [48]. It has been found that�-H6P2W18O62 and H6As2W21O69 are much moreactive than H3PW12O40. The activity of HPAs con-siderably depends on the anion structure and compo-sition. For the bulk W-containing HPAs the activity(per number of protons) decreases in the followingseries[48]:

H6P2W18O62 > H6As2W21O69 > Keggin.

For the Keggin HPAs, the activity falls in the series:

H3PW12O40 > H5PW11TiO40 > H5PW11ZrO40.

Lewis centers in H5PW11TiO40 and H5PW11ZrO40do not influence much the rate of reaction oftrans-alkylation, as it takes place in homogeneousreactions of acetone dimerization[18] and n-butylalcohol etherification of by acetic acid[31].

In the reaction of isobutylene oligomerizationin decane solution it was demonstrated[65] thatH3PW12O40 is less active than the Keggin-structure

Fig. 7. Dependence of catalytic activity on HPA loading in reaction of 2,6-di-tert-butyl-4-methylphenol with toluene:kb represents theactivity based on the total amount of H3PW12O40 protons (1—H3PW12O40/CFC; 2—H3PW12O40/SiO2; 3—H6P2W18O69/CFC) [48].

HPAs having Lewis centers as well as HPAs of otherstructures:

H6P2W18O62 > H6P2W21O71 > H5PW11TiO40

> H5PW11ZrO40 > H3PW12O40

It seems that in this reaction, Lewis sites along withBrønsted sites significantly contribute to the cat-alytic activity of H5PW11ZrO40. Interestingly, theZrIV -substituted HPA shows higher catalytic activitythan TiIV -substituted HPA.

The support has a great effect on the catalytic activ-ity of supported HPAs. The activity increases with anincrease in the amount of supported HPAs. This oc-curs for many reactions catalyzed by the Keggin-typeHPAs [66–69].

Thus, it was shown that in the reaction of2,6-di-tert-butyl-4-methylphenol with toluene thatacts as atert-butyl acceptor the catalytic activity ofHPAs (H6P2W18O62, H3PW12O40) supported on SiO2and filamentous carbon is very close[48]. The activ-ity of the surface protons of supported HPA increasesmonotonically with increasing acid strength, which inturn rises with the increase in the total HPA loadingand reaches a maximum for the bulk HPA (Fig. 7).

In the gas-phase synthesis of MTBE[47,61], thecatalytic activity of supported Dawson-type acidH6P2W18O62 is compared with that of the Keggin-type


heteropolyacids (HnXW12O40; X = PV, SiIV , GeIV ,BIII , CoIII ). It has been also found that the activity ofH6P2W18O62 is considerably enhanced by supportingon SiO2. However, the activity (per unit weight) ofbulk H6P2W18O62 is lower than that of supportedH6P2W18O62.

Thus, the available data demonstrate that both theacidity of proton sites and the amount of proton sitesare among key factors which determine the catalyticactivity in acidic reactions.

4. Conclusion

The selected examples reviewed show the broadscope of potentially promising applications of HPAsas acid catalysts in various organic reactions. Due totheir unique physicochemical properties, HPAs can beprofitably used in homogeneous, biphasic and hetero-geneous systems. In many cases HPA-based catalystshave higher activity than known traditional catalysts.

The examples given allow to conclude that thecatalytic effect of HPAs in acidic-type reactions de-pends mainly on three factors, namely, the acidity,heteropolyanion structure and type of reaction (natureof reagents). The analysis of investigations carried outfor HPAs of different structures revealed that HPAsare strong acids and that their acidity can be con-trolled by the change of the heteropolyanion charge.The catalytic activity is more dependent on the HPAstructure rather than its composition. Prediction ofcatalytic activity is possible only for HPAs havingsimilar structure.

Acknowledgements

The author thanks Dr. O.A. Kholdeeva for her helpin preparing this manuscript. The author would like tothank Prof. I.V. Kozhevnikov and Dr. G.M. Maksimovfor valuable discussion and comments.

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Acid Catalysis by Heteropoly Acids