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Applied Catalysis A: General 256 (2003) 3–18

Friedel–Crafts acylation and related reactions catalysedby heteropoly acids

I.V. Kozhevnikov∗

The Leverhulme Centre for Innovative Catalysis, Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, UK

Received 30 September 2002; received in revised form 10 January 2003; accepted 14 January 2003

Abstract

Recent studies on catalysis by heteropoly acids (HPA) for the Friedel–Crafts acylation of arenes and related Fries rear-rangement of aryl esters are reviewed. It is demonstrated that HPA-based solid acids are efficient and environmentally friendlycatalysts for these reactions, usually superior in activity to the conventional acid catalysts such as H2SO4 or zeolites.© 2003 Elsevier B.V. All rights reserved.

Keywords: Heteropoly acid; Heterogeneous catalysis; Friedel–Crafts acylation; Fries rearrangement

1. Introduction

The Friedel–Crafts aromatic acylation (Eq. (1)) andrelated Fries rearrangement of aryl esters, e.g. phenylacetate (Eq. (2); Ac = acetyl), catalysed by strongacids are the most important routes for the synthesisof aromatic ketones that are intermediates in manufac-turing fine and speciality chemicals as well as phar-maceuticals [1,2].

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(2)

∗ Tel.: +44-151-794-2938; fax: +44-151-794-3589. E-mail address: [email protected] (I.V. Kozhevnikov).

These reactions involve acylium ion intermediates

that are generated from an acylating agent or arylester by interaction with an acid catalyst. For theFriedel–Crafts chemistry, present industrial practiceuses acyl chlorides or acid anhydrides as acylatingagents and requires a stoichiometric amount of solu-ble Lewis acids (e.g. AlCl3) or strong mineral acids(e.g. HF or H2SO4) as catalysts, which results insubstantial amount of waste and corrosion problems[2]. The overuse of catalyst is caused by-productinhibition—the formation of strong complexes be-tween the aromatic ketone and the catalyst. In view of

the increasingly strict environmental legislation, theapplication of heterogeneous catalysis has becomeattractive. In the last couple of decades, considerableeffort has been put into developing heterogeneouslycatalysed Friedel–Crafts chemistry using solid-acidcatalysts such as zeolites, clays, Nafion-H, heteropolyacids (HPA), etc. [2], zeolites being the most studiedcatalysts ([2–6] and references therein). Likewise,the environmentally benign aromatic acylation withcarboxylic acids (Eq. (3)) instead of the anhydridesand acyl chlorides, resulting in the formation of water

0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0926-860X(03)00406-X

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4 I.V. Kozhevnikov / Applied Catalysis A: General 256 (2003) 3–18

as the only by-product, has been attempted, mostlywith zeolites as catalysts [7–10]. Despite the numer-ous studies, solid-acid catalysts have hitherto been

proved efficient only for the acylation of activatedarenes (e.g. anisole), whereas non-activated arenesremain beyond the reach of heterogeneous cataly-sis. The acylation of anisole with acetic anhydrideusing a zeolite catalyst has been commercialised byRhodia [2].

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Heteropoly acids are promising solid-acid catalystsfor the Friedel–Crafts reactions [11–15,17–20]. Theyare stronger than many conventional solid acids suchas mixed-oxides, zeolites, etc. The Keggin-type HPAstypically represented by the formula H8−x[XM12O40],where X is the heteroatom (most frequently P5+ orSi4+), x is its oxidation state, and M is the addendaatom (usually W6+ or Mo6+), are the most impor-tant for catalysis [11–16]. They have been widelyused as acid and oxidation catalysts for organic syn-thesis and found several industrial applications (fora recent comprehensive review, see the monograph[15]). The aim of this paper is to review recent stud-ies on catalysis by HPA for the Friedel–Crafts acy-lation and related Fries rearrangement of aryl esters.Use of HPA catalysts for Friedel–Crafts alkylationof aromatic compounds has been reviewed elsewhere[14,15].

2. Heteropoly acid catalysts

2.1. Acid sites

In acid-catalysed reactions, bulk and supported het-eropoly acids as well as heteropoly salts are used ascatalysts. Several types of acid sites are present inthese catalysts [12–15].

1. Proton sites in heteropoly acids (e.g. H3[PW12O40]).2. Proton sites in acidic salts (e.g. Cs2.5H0.5[PW12

O40]).3. Lewis acid sites in salts (metal countercations, e.g.

in LaIII[PMo12O40]).

4. Proton sites generated by dissociation of coordi-nated water:

Ln(H2O)n3+ → Ln(H2O)n(OH)2+ +H+

5. Proton sites generated by reduction of salts:

Pd2[SiW12O40]+ 4{H} → 2Pd0 +H4[SiW12O40]

6. Protons generated by partial hydrolysis of polyan-ions:

[PW12O40]3− + 2H2O → [PW11O39]7−

+{WO3} + 4H+

2.2. Bulk heteropoly acids

Solid heteropoly acids possess purely Brønstedacidity and are stronger than such conventionalsolid acids as SiO2-Al2O3, H3PO4 /SiO2, and HXand HY zeolites [11–16]. The acids H3[PW12O40],H4[SiW12O40], H3[PMo12O40], and H4[SiMo12O40]are readily available and most frequently used asacid catalysts, the first two usually being preferred.These acids have fairly high thermal stabilities, de-composing at 465, 445, 375, and 350 ◦C, respectively

[14,15]. Decomposed molybdenum acids may be re-constructed under exposure to water vapour [12]. Formuch less labile tungsten acids such reconstruction isunlikely.

The acid strength of crystalline heteropoly acids de-creases in the series [11–15]:

H3[PW12O40] > H4[SiW12O40] > H3[PMo12O40]

> H4[SiMo12O40]

Usually relative catalytic activities of heteropoly acidsare consistent with this order both in homogeneousand heterogeneous systems [12–15]. The drawbackto the bulk acids is their low surface area (typically,1–10m2 g−1) and low porosity (<0.1cm3 g−1). Be-cause of this, for Friedel–Crafts reactions, supportedheteropoly acids are usually preferred.

2.3. Supported heteropoly acids

Supported heteropoly acid catalysts have muchgreater number of surface acid sites than the bulk

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I.V. Kozhevnikov/ Applied Catalysis A: General 256 (2003) 3–18 5

acids, hence they are more important for applications[11–16]. The acidity and catalytic activity of supportedheteropoly acids depend on the type of carrier, the HPA

loading, conditions of pretreatment, etc. Acidic orneutral substances such as SiO2, active carbon, acidicion-exchange resin, etc. are all suitable as supports, themost often used being SiO2. 12-Tungstophosphoricacid supported on titania [21] and zirconia [22,23]has been characterised. Basic solids like MgO tend todecompose heteropoly acids [24].

Catalysts comprising H3[PW12O40] supportedon various porous silicas have been characterised[12,14,15,25,26]. Silica is relatively inert towards het-eropoly acids, at least above a certain loading level,although some chemical interaction takes place be-tween heteropoly acids and SiO2, as shown by 1H and31P magic-angle spinning (MAS) NMR [27–30]. Thethermal stability of heteropoly acids on SiO2 seemsto be comparable to or slightly lower than that of theparent HPA [12,31]. At low loadings, H3[PW12O40]and H4[SiW12O40] form finely dispersed specieson the SiO2 surface; HPA crystal phase on silica(200–300m2 g−1) is developed at an HPA loadingabove 20 wt.% [32,33]. Various HPA forms have beenobserved on the silica surface by transmission electronmicroscopy (TEM): discrete molecules, clusters 50 Å

in size and large crystallites of 500 Å. Their relativeamounts depend on the HPA loading [33]. As shownby microcalorimetry [34],whenloadingH3[PW12O40](20 wt.%) on SiO2, the proton sites become weakerand less uniform than those in the bulk H3[PW12O40].The acid strength of H3[PW12O40]/SiO2 increaseswith HPA loading [35]. According to the ammoniathermal desorption data [34], the acid strength of supported H3[PW12O40] decreases in the series of carriers: SiO2 > ␣-Al2O3 > activated carbon.

Heteropoly acids supported on certain activated car-

bons have been considered to be promising fixed-bedacid catalysts for liquid-phase reactions, e.g. esteri-fication, because of their high stability towards HPAleaching from the carrier [36,37]. However, as shownby microcalorimetry [38,39], the acid strength of HPA is greatly reduced in such catalysts. As evi-denced by IR and 31P MAS NMR, H3[PW12O40] andH4[SiW12O40] supported on a chemically (H3PO4)activated carbon retain the Keggin structure at theHPA loading >5 wt.% but decompose at lower load-ings [40].

2.4. Intrazeolite heteropoly acids

Incorporation of heteropoly acids into zeolite

pores to obtain shape-selective catalysts has longbeen a challenge. However, conventional zeolitesare not suitable for this because their pores aretoo small to adsorb large (12 Å) HPA molecules.H3[PW12O40] encapsulated into a mesoporous pure-silica molecular sieve MCM-41 (BET surface areaof 1200m2 g−1, uniform pores 32Å in size) hasbeen prepared [41,42] and characterised by nitro-gen physisorption, XRD, Fourier-transform infraredspectroscopy (FTIR), TEM, and 31P MAS NMR[30,41]. The H3[PW12O40]/MCM-41 compositionswith HPA loadings from 10 to 50 wt.% have ∼30 Åuniformly sized mesopores. Heteropoly acid formsfinely dispersed species on the MCM-41 surface.No HPA crystal phase is observed at HPA load-ings as high as 50wt.%. As shown by TEM [30],the H3[PW12O40] species are mainly located insidethe MCM-41 pores rather than on the outer surface.H4[SiW12O40]/MCM-41 has been characterised [43];it is very similar to H3[PW12O40]/MCM-41. Cesiumsalt of H3[PW12O40] supported on MCM-41 meso-porous silica has been studied [44].

The synthesis of 12-molybdophosphoric acid in the

supercages of Y-type zeolite has been reported [45].This “ship-in-the-bottle” catalyst has been used forliquid-phase reactions [46]. The effect of the Si/Al ra-tio and the countercation in zeolite Y on the encapsula-tion of Keggin heteropoly acids has been studied [47].

2.5. Heteropoly salts

The nature of countercation in heteropoly salts iscritical to their acidity, solubility, porosity, and ther-mal stability. Salts with rather small cations resemble

the parent heteropoly acids; they are readily solu-ble in water, nonporous, and possess surface areasunder 10m2 g−1. In contrast, salts with large mono-valent cations, such as NH4

+, K+, Cs+, etc. arewater-insoluble, have a rigid microporous/mesoporousstructure and can be prepared with surface areas over100m2 g−1 [16,48]. The stoichiometric salts preparedby precipitation from aqueous solutions contain resid-ual quantities of protons, which are apparently respon-sible for the catalytic activity of these salts [16,49].As demonstrated by Misono and others [12,50,51],

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the acidic Cs+ salt, Cs2.5H0.5[PW12O40], has strongacid sites, high surface area (100–200m2 g−1) andhydrophobicity and is a very efficient solid-acid cat-

alyst for a variety of organic reactions, especially inliquid-phase. The Cs+ content controls the pore sizeof the CsxH3−x[PW12O40] salts [52]. Various methodsof the preparation of bulk and supported heteropolysalts have been described [53–56]. The preparationof silica-supported Cs+ salts of H3[PW12O40] withthe egg-white morphology has been reported [57].The proton sites in H3[PW12O40] and its cesium saltshave been characterised by 1H, 2H, 31P MAS NMRand inelastic neutron scattering [58] and in situ FTIR[59]. The 129Xe NMR technique has been appliedto characterise the pore structure of NH4

+, K+, andCs+ salts of Keggin heteropoly acids and confirmedthe presence of microporosity in these salts [60].

Certain neutral heteropoly salts can also gain pro-ton sites upon interaction with reaction medium. Twomechanisms of the proton generation in heteropolysalts are distinguished: the dissociation of coordinatedwater (for salts with the cations like Al3+, Zn2+, etc.)and the reduction of the metal cation (e.g. Ag+, Cu2+,and Pd2+) [61,62].

Al3+ +H2O → Al(OH)2+ +H+

Ag+ + 12 H2 → Ag0 +H+

2.6. Sol–gel catalysts

H3[PW12O40]anditsCs+ salt, Cs2.5H0.5[PW12O40],have been included in the silica matrix by means of a sol–gel technique using the hydrolysis of tetraethylorthosilicate to become water-insoluble and easilyseparable microporous solid-acid catalysts [63,64].The catalysts thus obtained have large surface areas

(400–800m2

g−1

) and are thermally more stable thanAmberlyst-15. They catalyse the hydrolysis of ethylacetate in aqueous phase, showing higher turnoverfrequencies than Amberlyst-15 and HZSM-5. TheKeggin-type heteropoly acids included into a sil-ica matrix by sol–gel technique have been tested inFriedel–Crafts alkylations [65]. However, the sol–gelHPA catalysts, because of stronger interactions of their protons with the silica matrix, appear to havea weaker acid strength compared to silica-supportedHPA [66,67].

3. General features of HPA catalysis

Heteropoly acids catalyse a wide variety of reac-

tions in homogeneous or heterogeneous (liquid–solid,gas–solid or liquid–liquid biphasic) systems, offeringstrong options for more efficient and cleaner process-ing compared to conventional mineral acids [11–16].Being stronger acids, heteropoly acids will have sig-nificantly higher catalytic activity than conventionalcatalysts such as mineral acids, mixed-oxides, zeolites,etc. In particular, in organic media, the molar catalyticactivity of heteropoly acid is often 100–1000 timeshigher than that of H2SO4 [12,14,15]. This makes itpossible to carry out the catalytic process at a lowercatalyst concentration and/or at a lower temperature.Further, heteropoly acid catalysis lacks side reactionssuch as sulfonation, chlorination, nitration, etc. whichoccur with mineral acids [14,15]. As stable, relativelynontoxic crystalline substances, heteropoly acids arealso preferable regarding safety and ease of handling.

The relative activity of Keggin heteropoly acidsprimarily depends on their acid strength. Other prop-erties, such as the oxidation potential as well as thethermal and hydrolytic stability are also important.These properties for the most common heteropolyacids are summarised as follows [14,15].

Acid strength PW > SiW ≥ PMo > SiMo

Oxidation potential PMo > SiMo PW > SiW

Thermal stability PW > SiW > PMo > SiMo

Hydrolytic stability SiW > PW > SiMo > PMo

Usually, tungsten heteropoly acids are the catalysts of choice because of their stronger acidity, higher ther-mal stability and lower oxidation potential comparedto molybdenum acids [14,15]. Generally, if the reac-

tion rate is controlled by the catalyst acid strength,H3[PW12O40] shows the highest catalytic activity inthe Keggin series. However, in the case of less de-manding reactions as well as in reactions at highertemperatures in the presence of water, H4[SiW12O40],having lower oxidation potential and higher hydrolyticstability, could be superior to H3[PW12O40].

The major problem, limiting the utility of homoge-neously catalysed processes, is the well-known diffi-culty in catalyst recovery and recycling. As the cost of heteropoly acids is higher than that of mineral acids,

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the recycling of HPA catalysts is the key issue to theirapplication. Only a few homogeneous reactions, suchas the hydration of lower olefins, allow for easy recy-

cling. In some cases, heteropoly acid can be recoveredfrom polar organic solution without neutralisation byprecipitating with a hydrocarbon solvent. Heteropolyacid can also be extracted from an acidified aqueoussolution of its salt with a polar organic solvent [14,15].Separation of products and recovery and recycling of catalysts often becomes much easier when homoge-neously catalysed reactions are performed in biphasicsystems consisting of two immiscible liquid-phases—a catalyst phase and a product/reactant phase—withintense mass transfer between them. Heteropoly acidsdue to their special solubility properties, i.e. high sol-ubility in a variety of polar solvents and insolubilityin nonpolar solvents, are promising catalysts for op-erating under phase-transfer conditions [14,15]. Theamount of heteropoly acid in the product phase mustbe negligible to allow easy catalyst separation.

The important advantage of heterogeneous systemsincluding solid HPA catalysts over homogeneous onesis the easy separation of catalyst from reaction prod-ucts. Furthermore, since heteropoly acids are solubleonly in wet nucleophilic solvents levelling the acidstrength (dehydrated heteropoly acids are scarcely

soluble in dry polar media), their intrinsic strongacidity cannot be fully utilised in homogeneous sys-tems. Hence, for catalysing highly demanding reac-tions, such as the Friedel–Crafts reaction, heteropolyacid must be used as a solid-acid catalyst in a drynon-nucleophilic medium.

Supported heteropoly acids (usually on silica) aregenerally preferred because of their large surface area.HPA loadings vary from about 10 to 50 wt.% or evenhigher. At lower loadings, the acidity of heteropolyacid decreases because of interaction with support;

such catalysts are also quite sensitive to poisoningby impurities that may be present in the support orfeed. For these reasons, the catalysts containing lessthan 10 wt.% HPA usually show poor activities andare rarely used.

Control of water content in heteropoly acid cata-lysts is essential for their efficient performance. Thiscan be achieved by thermal pretreatment of the cata-lysts, typically at 130–200 ◦C. The effect of water maybe attributed to the HPA acid strength and the numberof proton sites as well as to catalyst deactivation. Ex-

cess water causes a decrease in the HPA acid strength,and thus in its activity. Dehydration of the catalyst in-creases the acid strength but decreases the number of

acid sites, which may reduce the overall catalytic ac-tivity. The strong acid sites thus created tend to deac-tivate (coke) faster [14,15].

Like other solid-acid catalysts, the solid heteropolyacids tend to deactivate during organic reactions be-cause of the formation of carbonaceous deposit (coke)on the catalyst surface. Subsequent regeneration of heteropoly acid catalyst is quite difficult. This problemremains to be solved to put heterogeneous heteropolyacid catalysis in practice.

4. Mechanistic principles

Generally, reactions catalysed by heteropoly acidsmay be represented by the conventional mechanismsof Brønsted acid catalysis. In a simple case of singleproton transfer, the mechanism may include the pro-tonation of the substrate followed by the conversionof the ionic intermediate to yield the reaction product[14,15]:

S1 +H+ S1H+(S2)

→P +H+ (4)

In this equation, S1 and S2 are the substrates and P isthe product. In accordance with this mechanism, thecatalytic activity of heteropoly acids, both in homo-geneous and heterogeneous systems, usually parallelstheir acid strength, i.e. H3[PW12O40] > H4[SiW12O40]> H3[PMo12O40] > H4[SiMo12O40] [14,15]. Beingstronger acids and therefore more efficient protondonors, heteropoly acids usually exhibit higher cat-alytic activities than the conventional acid catalysts.Relatively strong oxidants, molybdenum heteropolyacids are frequently deactivated due to their reduction

by the organic reaction medium; it is not uncommonfor them to show lower activities than those expectedfrom their acid strengths [14,15].

For heterogeneous acid catalysis by solid heteropolycompounds, Misono and others [12,13] advanced amechanistic classification which distinguishes twotypes of catalysis, namely (i) surface type and (ii) bulktype (or “pseudoliquid”). The surface type is a conven-tional acid catalysis on the gas–solid or liquid–solid in-terface. This type applies to reactions occurring on thesurface of bulk or supported heteropoly compounds.

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In this case, the catalytic activity usually depends onthe surface acidity of heteropoly acid, i.e. the reactionrate is parallel to the number and the strength of the

accessible surface acid sites. The bulk type of mecha-nism is largely relevant to reactions of polar substrates(e.g. alcohol, ether, ketone, amine, etc.) on bulk het-eropoly compounds. These substrates are capable of absorbing into the catalyst bulk, forming HPA sol-vates [12]. In this case, solid heteropoly acids behavelike highly concentrated solutions, and all protons,both in the bulk and on the surface of heteropoly acid,are suggested to participate in the catalytic reaction.Unlike polar molecules, nonpolar reactants (e.g. hy-drocarbons) are incapable of absorbing in the bulk of heteropoly acids. They interact only with the surfaceof the catalyst. Water-insoluble heteropoly salts (e.g.Cs2.5H0.5[PW12O40]) scarcely absorb polar moleculesinto the bulk and hence exhibit surface-type catalysistowards both polar and nonpolar molecules [12]. Thepseudoliquid behaviour appears to be important forreactions of polar molecules at relatively low temper-atures, i.e. when the sorption of substrate in the cata-lyst bulk is significant. Surface and bulk catalysis mayoperate with strongly differing selectivities [12,13].

5. Friedel–Crafts acylation and Fries reactioncatalysed by heteropoly acids

5.1. Acylation

The Friedel–Crafts acylation of aromatic com-pounds is the most important route to aromatic ketonesthat are intermediates in various organic syntheses[1]. The reaction occurs by interacting the aromaticcompound with a carboxylic acid derivative (e.g. acidanhydride, acyl chloride, or the acid itself) in the

presence of an acid catalyst. Reaction mechanisminvolves acylium ion intermediates that are generatedfrom the acylating agent by interaction with the acidcatalyst [1]. In the last couple of decades, solid-acidcatalysts such as zeolites, clays, Nafion-H, heteropolyacids, etc. have been explored for aromatic acylationto replace the conventional soluble acids (e.g. AlCl3,HF, etc.), zeolites being the most studied catalysts([2–10] and references therein). Although relativelyactive catalysts, the zeolites (e.g. H-beta) are deacti-vated during the acylation [6–8]. The main deactiva-

tion is deemed to be reversible; this is attributed tothe strong adsorption of the acylation product on thecatalyst, blocking access to the active sites. Another

type of deactivation, which is irreversible, is causedby tar deposition on the catalyst surface (coking).Heteropoly acids are promising solid-acid catalysts

for aromatic acylation. Below is described their usefor these reactions.

5.1.1. Acylation by acyl chlorides and acid

anhydrides

Izumi et al. [11] pioneered the use of heteropolyacids as catalysts for aromatic acylation. Silica-supported acids H4[SiW12O40] and H3[PW12O40]both effectively catalyse the acylation of p-xylene withbenzoyl chloride (Eq. (5); Bz: benzoyl) and remainunchanged on the SiO2 surface after the reaction.

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But in a more polar reaction medium such aschlorobenzene even H4[SiW12O40] leaches from the

silica support and decomposes in the course of reac-tion. Benzoic anhydride can be used as an acylatingagent with the H4[SiW12O40]/SiO2 catalyst, but ben-zoic acid cannot. A weaker solid acid, H4[SiW12O40]/ carbon, can also catalyse the acylation, but less ef-ficiently than H4[SiW12O40]/SiO2 [11]. In contrast,H3[PMo12O40] in the HPA/SiO2 catalyst was found todecompose during the acylation of p-xylene with ben-zoyl chloride [11]. Probably, the real active species isnot the supported H3[PMo12O40], but some solublespecies which might be formed by the interaction

between H3[PMo12O40] and benzoyl chloride.Cs2.5H0.5[PW12O40] shows high efficiency inthe acylation of activated arenes, such as p-xylene,anisole, mesitylene, etc. by acetic and benzoic an-hydrides and acyl chlorides (Table 1). This catalystprovides higher yields of acylated arenes than the par-ent acid H3[PW12O40], the latter being partly solublein the reaction mixture [68].

The recent study [20] has shown that bulk andsilica-supported H3[PW12O40] exhibit a very highactivity in the acylation of anisole (Eq. (6)) with

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Table 1Friedel–Crafts acylation [68]

Substrates Product yielda

Acylating agent Arene Cs2.5H0.5

[PW12O40]H3[PW12O40]

(PhCO)2O p-Xylene 57 3(PhCO)2O Anisole 85 69Ac2O Anisole 89 50n-C7H15COCl Mesitylene 80 44

a Acylating agent/arene/catalyst= 5/100/0.01 mmol, reflux, 2 h.

Table 2Acylation of anisole with acetic anhydride (2 h) [20]

Catalyst(amount, wt.%)a AN/AA(mol mol−1) T (◦

C) Yieldb

(%) p-MOAP

o-MOAP

H3[PW12O40] (0.83) 10 70 67 c

H3[PW12O40] (0.83) 10 90 96 3.850% PW/SiO2 (0.83) 10 90 88 4.040% PW/SiO2 (0.83) 10 90 88 4.040% PW/SiO2 (0.88) 20 110 98d 2.130% PW/SiO2 (0.83) 10 90 82 3.720% PW/SiO2 (0.83) 10 90 79 3.310% PW/SiO2 (0.83) 10 90 78 3.5H4[SiW12O40] (0.83) 10 70 70 c

40% SiW/SiO2 (0.83) 10 70 61 c

H3[PMo12O40] (0.83) 10 70 0 040% PMo/SiO2 (0.83) 10 70 2 0Cs2.5H0.5[PW12O40]

(0.83)10 90 44 1.5

a The amount of catalysts per total reaction mixture: PW = H3[PW12O40]; SiW = H4[SiW12O40]; and PMo = H3[PMo12O40].

b Yield based on acetic anhydride.c The yield of o-MOAP ca. 2–3%.d Yield in 10 min.

acetic anhydride in liquid-phase, yielding up to 98% para- and 2–4% ortho-isomer of methoxyacetophe-none (MOAP) at 70–110 ◦C and an anisole to aceticanhydride molar ratio AN/AA = 10–20 (Table 2).

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Catalyst pretreatment is essential. Fig. 1 shows the ef-fect of catalyst pretreatment on the yield of p-MOAPand on the initial rate of anisole acylation with bulk

H3[PW12O40] and 50% H3[PW12O40]/SiO2. The ac-tivity passes a maximum at a pretreatment temperatureof 150 ◦C. From TGA, the amount of water remainingin the catalyst after pretreatment at 150 ◦C is about3–4H2O molecules per Keggin unit. Excess watercauses a decrease in the HPA acid strength, and thusin its catalytic activity. Dehydration of the catalystincreases the acid strength but decreases the numberof acid sites, which will reduce the catalytic activityunless the reaction is highly demanding for the cat-alyst acid strength. In addition, the very strong acidsites thus created tend to deactivate (coke) faster. Theacylation of anisole appears to be heterogeneouslycatalysed; no contribution of homogeneous catalysisby HPA is observed. The H3[PW12O40] catalyst isreusable, although gradual decline of activity was ob-served due to the coking of the catalyst. It should benoted that H3[PW12O40] is almost a factor of 100 moreactive than the zeolite H-beta (Table 3), which is inagreement with the higher acid strength of HPA [20].

Anisole acylation is first-order in acetic anhydride,the order in catalyst is 0.66, and the apparent activa-tion energy is 41 kJ mol−1 in the temperature range of

70–110 ◦C [20]. The reaction is inhibited by-productbecause of adsorption of p-MOAP on the catalyst sur-face (Fig. 2). Applying the Langmuir–Hinshelwoodkinetic model, the ratio of adsorption coefficients of

p-MOAP and anisole has been found to be 37 at 90 ◦C.The effect of H3[PW12O40] loading on silica upon theinitial rate of anisole acylation is shown in Fig. 3. Inthese experiments, the total amount of H3[PW12O40]

Table 3Acylation of anisole with acetic anhydride: HPA vs. zeolite (90 ◦C,2 h) [20]

Reaction conditions Catalyst

10% H3

[PW12O40]/ SiO2

10% H3

[PW12O40]/ SiO2

H-betaa

[3]

Catalyst amount (wt.%) 0.83 0.83 1.33AN/AA (mol mol−1) 10 6 6Yield of p-MOAP (%)b 78 50 75TON 780 830 61TOF (min−1) 78 1.2

a Si/Al = 12.5.b Yield based on acetic anhydride.

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Fig. 1. Effect of HPA catalyst pretreatment (specified temperature/0.1 Torr, 1.5 h) in the acylation of anisole at AN/AA = 10molmol−1:(a) yield of p-MOAP (50% H3[PW12O40]/SiO2 (0.83 wt.%), 50 ◦C, 2h); (b) initial rate (bulk H3[PW12O40] (0.83 wt.%), 70 ◦C) [20].

is kept constant. The activity of H3[PW12O40] in-creases with the loading, passing a maximum at about50% loading. Such behaviour may be explained asa result of increasing the HPA acid strength, on theone hand, and decreasing the HPA surface area, onthe other, as the loading increases. It should be notedthat the specific catalytic activity (per Keggin unit)

of supported HPA is greater than that of bulk HPA.This demonstrates that the reaction occurs via the

Fig. 2. Inhibition by-product in the acylation of anisole (40% H3[PW12O40]/SiO2 (0.83 wt.%), AN/AA = 10molmol−1, 70 ◦C, 2h): (1)no p-MOAP added; (2) p-MOAP added initially, AA/ p-MOAP = 2molmol−1; (3) p-MOAP added initially, AA/ p-MOAP = 1molmol−1.Yields are based on acetic anhydride [20].

surface-type catalysis in terms of Misono and othersclassification (“bulk versus surface type”) [12,13].

In contrast to anisole, the acylation of toluene withHPA is far less efficient than that with H-beta [20].These results have been explained by the well-knownstrong affinity of bulk HPA towards polar oxygenates,which would lead to the preferential adsorption of

acetic anhydride on HPA, blocking access for tolueneto the catalyst surface. It appears that the hydrophobic

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Fig. 3. Effect of H3[PW12O40] loading in the H3[PW12O40]/SiO2 catalyst on the initial rate of anisole acylation at AN/AA = 20molmol−1

and a constant total amount of H3[PW12O40] (0.010mmol): (a) 70 ◦C; (b) 90 ◦C [20].

zeolites with high Si/Al ratios less strongly differen-tiate the adsorption than the hydrophilic HPA and,therefore, are more suitable catalysts for the acylationof nonpolar aromatics like toluene.

5.1.2. Acylation by acids

The aromatic acylation with carboxylic acids(Eq. (3)) instead of acid anhydrides and acyl chlorides

has attracted interest, because it is an environmentallybenign reaction, resulting in the formation of wateras the only by-product. It has been attempted withzeolites and clays as catalysts [7–10]. Heteropolyacids have proved to be more active catalysts for thisreaction [17,18,69].

Silica-supported 12-tungstophosphoric acid and itsCs+ salts catalyse the acylation of toluene, p-xyleneand m-xylene with crotonic acid (Eq. (7)) [17,18].Some alkylation of aromatic compounds with crotonicacid also takes place. Heteropoly acid is more active

than zeolites HY and H-beta in the acylation.

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Recently, the acylation of toluene and anisole withC2–C12 aliphatic carboxylic acids in liquid-phasecatalysed by Cs2.5H0.5[PW12O40] has been reported[69]. The acylation of toluene is carried out at a mo-lar ratio PhMe/RCOOH = 50 and 110 ◦C (reflux)in the presence of ca. 10wt.% Cs2.5H0.5[PW12O40]in the reaction mixture. The reaction is clearly het-erogeneous, as Cs2.5H0.5[PW12O40] is insoluble; it

stopped when the catalyst was filtered off the react-ing mixture. With acetic, propionic and butyric acids,the yield of acylated products is very low, thoughincreasing in this series, similar to that observed forzeolites [7,8]. This may be due to the preferential ad-sorption of the lower acids on Cs2.5H0.5[PW12O40],blocking access for toluene to the catalyst surface.The higher acids C6–C12 are more reactive in acy-lation, yielding 31–51% aromatic ketones (Table 4).All three possible isomers, ortho, meta and para, areformed, the para-isomers being the major products

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Table 4Acylation of toluene (100 mmol) with carboxylic acids (2.0 mmol) at 110 ◦C (reflux), 48h [69]

Catalyst (g) Acid Yield (%)a Product distribution (%)

para ortho meta

Cs2.5H0.5[PW12O40] (1.0) Hexanoic 31 55 37 8Cs2.5H0.5[PW12O40] (1.0) Octanoic 47 72 22 6Cs2.5H0.5[PW12O40] (1.0) Dodecanoic 51 71 22 7Cs2.5H0.5[PW12O40] (1.0)b Dodecanoic 44 73 21 6H3[PW12O40] (1.0) Dodecanoic 14 60 23 1740% H3[PW12O40]/SiO2 (2.5) Dodecanoic 35 83 11 6

a The yield of aromatic ketones based on carboxylic acid; the yield based on toluene is 50 times lower.b A reuse of the above run. The catalyst was filtered off, washed with CH 2Cl2, dried and rerun.

(55–73%), as expected. The reaction selectivity isvirtually 100%, no other products are formed. Theyield increases in the series of acids from hexanoicto dodecanoic acid like in the reaction with CeY ze-olite [7,8] and cation-exchanged montmorillonite [9].Cs2.5H0.5[PW12O40] (S BET, 112m2 g−1) is a muchmore efficient catalyst than the bulk H3[PW12O40](S BET, 7 m2 g−1) (Table 4), which may be ex-plained by a greater number of H+ surface sites inCs2.5H0.5[PW12O40]. The Cs+ salt is also more activethan the silica-supported HPA, 40% H3[PW12O40]/ SiO2, which may be the result of the higher hydropho-

bicity of Cs2.5H0.5[PW12O40] [12,13], favouring theadsorption of nonpolar reactants on the catalyst sur-face and making Cs2.5H0.5[PW12O40] more resistanttowards deactivation by co-product water comparedto the more hydrophilic H3[PW12O40]. After the re-action, Cs2.5H0.5[PW12O40] can be easily separatedby filtration and reused. Some catalyst deactivationwas observed, though, which was probably caused bycoking.

The most important advantage of Cs2.5H0.5[PW12O40] catalyst is that it gives much higher productivity

in aromatic ketones than the zeolite and clay catalystsreported so far, which may be attributed to the strongeracidity of Cs2.5H0.5[PW12O40]. Thus, for the acylationof toluene with dodecanoic acid, Cs2.5H0.5[PW12O40]gives a 1.0% yield of ketone based on toluene(Table 4) which is three times that reported for CeY(0.31%) [7] and for Al3+-montmorillonite (0.32%)[9]. (For CeY, a 96% yield based on dodecanoicacid at PhMe/acid = 313 has been obtained (150 ◦C,48h) [7]. For Al3+-montmorillonite, a 60% yield atPhMe/acid = 187 (110 ◦C, 24h) has been reported

[9].) It should be noted, however, that CeY givesa higher selectivity to the para-acylation (94%) [7]than Cs2.5H0.5[PW12O40], which may be the result of shape-selective catalysis by the zeolite.

The acylation of anisole with C2–C12 acids hasbeen carried out under the same conditions as thatof toluene, except a shorter reaction time (5 h) [69].The acylated anisole forms as the major product( para / ortho = 59:1–96:1 and no meta-isomers) to-gether with esterification products—methyl esters of carboxylic acids and phenol (Table 5). No phenylesters form. The selectivity to esters increases from

acetic to dodecanoic acid, reaching 40% for the lat-ter. The acylation of anisole, in contrast to that of toluene, is most efficient with C2–C6 acids, giving a62–65% yield of acylated products and only 2–6%of methyl esters. When the acylation of anisole byacetic, propanoic or butyric acid is carried out in air

Table 5Acylation of anisole (100 mmol) with carboxylic acids (2.0 mmol)catalysed by Cs2.5H0.5[PW12O40] (1.0g) at 110 ◦C, 5 h [69]

Acid Conversion (%) Product distribution (%)a

para ortho Methyl ester

Acetic 65 96 1 3Propionic 53 93 3 4Butyric 66 89 2 9Hexanoic 71 89 2 9Octanoic 45 73 1 26Dodecanoic 42 59 1 40

a Based on carboxylic acid converted; para and ortho are thecorresponding acylated anisoles. Phenol in a molar ratio of 1:4–1:5to acylated anisole is also formed.

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instead of nitrogen, 4-methoxy-4-acylderivatives of diphenyl ether in a molar ratio to acylated anisole of 1:3–1:5 are obtained. Only traces of such products

form in the case of C6–C12 acids. Apparently, theseproducts are formed by the C–O oxidative couplingof phenol and anisole, followed by acylation.

The acylation of anisole with HZSM-5 zeolite(Si/Al = 30) as a catalyst proceeds differently [10].With C2–C3 acids, at 120 ◦C, PhOMe/acid = 4 and20% HZSM-5, the phenyl esters are the main prod-ucts; no methyl esters have been found. At 150 ◦Cand otherwise the same conditions, a 2:1–5:1 mix-ture of acylated anisole and phenyl ester forms atan 87–100% acid conversion. The conversion dropssharply for the acids higher than C3, down to 0.6% forC12, probably because of restricted access into zeolitepores. Thus Cs2.5H0.5[PW12O40] is a more active aswell as more selective catalyst than HZSM-5 for theanisole acylation.

5.2. Fries rearrangement

The Fries rearrangement of aryl esters is animportant route to aromatic hydroxyketones that areintermediates in manufacturing fine and specialitychemicals as well as pharmaceuticals [1,2]. The re-

arrangement of esters, e.g. phenyl acetate (Eq. (2))to yield 2- and 4-hydroxyacetophenones (2HAP and4HAP) and 4-acetoxyacetophenone (4AAP) togetherwith phenol, involves acylium ion intermediates thatare generated from the ester by interaction with an acidcatalyst, typically a soluble Lewis acid (e.g. AlCl3)or mineral acid (e.g. HF or H2SO4). Heterogeneouscatalysis, using solid acids such as zeolites, clays, het-eropoly acids, etc. has attracted considerable interestto replace the conventional soluble acids in these re-actions ([2,4–6] and references therein). Mechanisms

for the formation of products have been discussed ([4]and references therein). 2HAP, 4AAP and phenol areconsidered to be the primary products, 2HAP beingformed by the intramolecular rearrangement of PhOAc(Eq. (8)) and 4AAP and PhOH by the self-acylation(Eq. (9)).

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In contrast, 4HAP appears to be the secondary prod-uct formed by the intermolecular acylation of phenolwith PhOAc (Eq. (10)). Usually, the yield of phenolis greater than that of 4AAP, as part of PhOH resultsfrom the decomposition and/or hydrolysis of PhOActhat also produce ketene, acetic acid and acetic an-hydride. Solvent plays a significant role in Fries re-action, polar solvents favouring the formation of the

para-acylation products (4AAP and 4HAP).Heteropoly acids, especially H3[PW12O40], have

been demonstrated to be promising catalysts forFries rearrangement of aryl esters [67,70]. The HPA-catalysed rearrangement of phenyl acetate (Eq. (2))occurs in liquid-phase at 100–160 ◦C (Table 6). One

of important advantages of HPA, as compared to ze-olites or mineral acids, is that the reaction can becarried out both homogeneously and heterogeneously.The homogeneous process occurs in polar media, forexample, in neat PhOAc or polar organic solvents likenitrobenzene (PhNO2) or o-dichlorobenzene that arecommonly used for Fries reaction. All these mediawill dissolve H3[PW12O40] at elevated temperatures(ca. 100 ◦C). On the other hand, when using nonpolarsolvents, such as higher alkanes (e.g. dodecane) thatwill not dissolve HPA, the reaction proceeds hetero-

geneously over solid HPA catalysts. In the latter case,supported HPA, preferably on silica, is the catalystof choice, as bulk HPA possesses a low surface area.The HPA catalysts are easily separated from the het-erogeneous system by filtration and can be reused,though with reduced activity. From the homogeneoussystems, HPA can be effectively separated withoutits neutralisation by extraction with water and reusedor utilised otherwise. The heterogeneous catalysis inthe PhOAc—dodecane media was clearly proved byfiltering off the catalyst from the reacting system,

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Table 6Fries rearrangement of phenyl acetate (2 h)a [67,70]

Catalyst (wt.%) Solvent (PhOAc, wt.%) T (◦C) Conversion (%) Selectivity (%)

PhOH 2HAP 4HAP 4AAP

H3[PW12O40] (0.60) PhOAc (100) 150 5.5 49 5.2 5.6 40H3[PW12O40] (3.0) PhOAc (100) 150 19.2 52 5.7 15 28H3[PW12O40] (3.0) PhNO2 (25) 150 45.8 52 12 24 12H3[PW12O40] (0.60) PhNO2 (25) 130 21.0 46 7.8 18 27H3[PW12O40] (0.60) PhNO2 (50) 100 10.5 55 5.1 10 29H2SO4 (1.4) PhNO2 (25) 130 12.8 67 9.4 7.6 16H3[PW12O40] (0.60) Dodecane (25) 130 3.1 69 8.0 0 2340% PW/SiO2 (1.5) Dodecane (25) 130 8.3 62 10 6.0 2240% PW/SiO2 (1.5)b Dodecane (25) 130 6.7 51 11 5.0 3210% PW/SiO2 (6.0) Dodecane (25) 130 11.8 66 8.0 9.6 1640% PW/SiO2 (3.3)c Dodecane (36) 160 18.0 66 11 8.2 14H-beta (1.3)c,d Dodecane (36) 160 9.3 38 32 6.4 24

Cs2.5H0.5[PW12O40] (0.67) PhNO2 (25) 130 8.7 49 6.1 4.4 41a The reaction with H3[PW12O40] is homogeneous in PhOAc and PhNO2 and heterogeneous in dodecane.b Reuse of the above run.c 5 h.d Si/Al = 11 [4].

which completely terminated the reaction. In con-trast, filtration did not affect the reaction course inhomogeneous systems, e.g. PhOAc–PhNO2 [67,70].

Strong inhibition of HPA-catalysed process with re-action products takes place both in homogeneous and

heterogeneous systems like in anisole acylation. Ad-dition of more HPA catalyst allows reaching a higherPhOAc conversion. Some irreversible catalyst deacti-vation is also observed [67,70].

The total selectivity towards the sum of PhOH,2HAP, 4HAP and 4AAP is over 98%. Some aceticacid and acetic anhydride are also formed. The ho-mogeneous reaction is more efficient than the het-erogeneous one because it makes less phenol andmore acetophenones, the selectivity to the more valu-able para-acetophenones, 4AAP and 4HAP, being

also higher. In terms of turnover frequencies, HPAis almost 200 times more active than H2SO4 in ho-mogeneous reaction, as well as more selective toacetophenones. In heterogeneous systems, HPA isalso two orders of magnitude more active than H-betazeolite, which is one of the best zeolite catalysts forthis reaction [6]. However, H-beta shows a higher to-tal selectivity to acetophenones than HPA (Table 6). Itshould be pointed out that HPA in homogeneous sys-tems gives a higher selectivity to para-acetophenones4AAP and 4HAP than H-beta. The efficiency of

solid HPA (at constant loading) increases in the orderH3[PW12O40] < 40% H3[PW12O40]/SiO2 < 10%H3[PW12O40]/SiO2 in which the number of accessi-ble proton sites increases.

The insoluble salt Cs2.5H0.5[PW12O40] is an effi-

cient solid catalyst for the reaction in polar media suchas PhNO2 (Table 6). Although less active per unitweight than the homogeneous H3[PW12O40] or thesolid catalyst H3[PW12O40]/SiO2, it is more selectiveto acetophenones than the parent HPA. The explana-tion of this may be that the less hydrophilic Cs+ saltpossesses stronger proton sites than the partially hy-drated solid H3[PW12O40] or H3[PW12O40]/SiO2 thatcontained 4–6H2O molecules per Keggin unit [70].

In contrast to silica-supported H3[PW12O40], thesol–gel H3[PW12O40] catalysts prepared by the hy-

drolysis of tetraethyl orthosilicate show only a negli-gible activity in the Fries reaction of phenyl acetate,yielding mainly phenol with 92–100% selectivity.This may be explained by a weaker acid strength of the sol–gel catalysts due to strong interaction of theHPA protons with the silica matrix and the presenceof relatively high amount of water in sol–gel cat-alysts [67]. This is in agreement with the fact thatthe gas-phase isomerisation of butene occurs muchslower over the sol–gel H3[PW12O40] than oversilica-impregnated H3[PW12O40] [66].

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A continuous gas-phase catalytic acetylation of phenol with acetic anhydride to phenyl acetatefollowed by simultaneous Fries rearrangement to

yield ortho- and para-hydroxyacetophenones over asilica-supported heteropoly acid has been described[71]. A complete conversion of phenol to phenylacetate is achieved at 140 ◦C. Upon increasing thetemperature to 200 ◦C, the phenyl acetate formed re-arranges into hydroxyacetophenones with 10% yieldand 90% selectivity to the para-isomer.

The rearrangement of phenyl benzoate (PhOBz) oc-curs similarly to that of PhOAc (Eq. (11)), yielding2- and 4-hydroxybenzophenones (2HBP and 4HBP),4-benzoxybenzophenone (4BBP) and phenol togetherwith benzoic acid.

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Table 7 shows examples of homogeneous (withH3[PW12O40]) and heterogeneous (with Cs+ salt) re-

arrangement of PhOBz in PhNO2 solution [67]. Theproduct selectivities and catalyst activities are quitesimilar to those observed for PhOAc. The differenceis that the amount of phenol formed is nearly equal tothat of 4BBP, indicating that the hydrolysis of PhOBzis less significant in this case.

In the case of rearrangement of p-tolyl acetate( p-TolOAc), the acylation in the para-position isno longer possible. Hence the major products are2-hydroxy-5-methylacetophenone (2H5MAP) and

Table 8Fries rearrangement of p-tolyl acetate (130 ◦C, 2h) [67]

Catalyst (wt.%) Solvent (TolOAc, wt.%) Conversion (%) Selectivity (%)

p-Cresol 2H5MAP 3H6MAP

H3[PW12O40] (0.60)a PhNO2 (25) 8.5 46 54 0H3[PW12O40] (0.60)a o-Cl2C6H4 (25) 5.0 51 49 0Cs2.5H0.5[PW12O40] (1.4)b o-Cl2C6H4 (25) 8.8 17 82 1.4

a Homogeneous reaction.b Heterogeneous reaction.

Table 7Fries rearrangement of phenyl benzoate in 75:25 (wt.%)PhNO2–PhOBz mixture (130 ◦C, 2h) [67]

Catalyst (wt.%) Conversion(%) Selectivity (%)PhOH 2HBP 4HBP 4BBP

H3[PW12O40] (0.60)a 18.6 40 5.2 19 36Cs2.5H0.5[PW12O40]

(0.67)b4.3 48 2.3 2.5 47

a Homogeneous reaction.b Heterogeneous reaction.

p-cresol (Eq. (12)) together with acetic acid and aceticanhydride. A very small amount of the meta-acylation

product 3-hydroxy-6-methylacetophenone (3H6MAP)may also be formed.

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The homogeneous reaction with H3[PW12O40] in

PhNO2 or o-dichlorobenzene gives almost equalamounts of 2H5MAP and p-cresol, no 3H6MAP be-ing formed (Table 8) [67]. The heterogeneous reactionwith the Cs+ salt gives 2H5MAP with a remarkablyhigh selectivity of 82% and only 17% of p-cresol. Alittle of 3H6MAP (1.4%) is also formed. This indi-cates that the hydrolysis p-TolOAc is less significantwith the Cs+ salt than with H3[PW12O40], which isin agreement with the higher hydrophobicity of theCs+ salt.

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6. Deactivation and regeneration of solid

heteropoly acid catalysts

A serious problem with the solid heteropoly acidcatalysts is their deactivation during organic reactionsbecause of the formation of carbonaceous deposit(coke) on the catalyst surface. Conventional regen-eration by burning coke at 500–550 ◦C, which isroutinely used in the case of aluminosilicates andzeolites, is not applicable to heteropoly acids becausetheir thermal stability is not high enough [15]. There-fore, for heteropoly acids to be more widely used forheterogeneous acid catalysis an efficient and reliablemethodology of their regeneration would be bene-ficial. The development of a technique leading to areduction in the temperature of coke removal wouldbe of importance for the regeneration of deactivatedsolid heteropoly acid catalysts.

The coking and regeneration of silica-supportedH3[PW12O40] during propene oligomerisation hasbeen studied [72,73]. Coke formation causes rapiddeactivation of the catalyst. The coked versus freshcatalysts have been characterised by 31P and 13CMAS NMR, XRD, XPS and TGA/TPO to reveal thatthe Keggin structure of the catalysts remains unaf-fected by coke deposition. The Pd doping has been

shown to affect the nature of coke formed, inhibitingthe formation of polynuclear aromatics. Co-feedingwater to the propene flow greatly inhibits coke for-mation. The removal of coke from HPA catalysts hasbeen attempted using solvent extraction, ozone treat-

Fig. 4. TGA/TPO for 20% H3[PW12O40]/SiO2 coked with propene [72,73].

ment and aerobic oxidation. The aerobic burning of coke on the undoped H3[PW12O40]/SiO2 proceeds tocompletion at the temperature of 500–560 ◦C, which

is higher than the temperature of H3[PW12O40] de-composition. Doping the catalyst with Pd significantlydecreases this temperature (Fig. 4) to allow catalystregeneration at temperatures as low as 350 ◦C withoutloss of catalytic activity.

The catalyst comprising H3[PW12O40] supportedon sulphated zirconia doped with iron(III) for the alky-lation of benzene with propene (100–150 ◦C, 4 MPa)has been regenerated under an air flow at 350 ◦C for2 h [74].

It has been claimed that a heteropoly acid catalyst,e.g. a molybdophosphoric acid catalyst, whose activityhad been lowered could be regenerated by dissolvingand/or suspending it in an aqueous medium and thentreating with an inorganic ion-exchange material, e.g.crystalline antimonic acid [75].

Generally, prevention of catalyst deactivation ispreferable to catalyst regeneration because the lat-ter is usually difficult and expensive. In the case of gas-phase oligomerisation of propene catalysed byH4[SiW12O40], the catalyst cokes less rapidly whensupported on silica than in the pure form [76]. Dilut-ing bulk heteropoly acid catalyst with acid-washed

sand (1 part catalyst to 10 parts sand) dramaticallyincreases the product yield and catalyst lifetime inthe gas-phase oligomerisation of propene (230 ◦C,5MPa) [77]. Treatment of the deactivated heteropolyacid catalyst for the gas-phase 1-butene and n-pentane

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isomerisation with water recovers activity, whereastreatment in air or N2O is ineffective. It is suggestedthat water treatment regenerates acid sites by re-

hydrating partially decomposed Keggin units [35].Co-feeding water has been reported to be crucial to thestable activity of the silica-supported H4[SiW12O40]catalyst for the industrial synthesis of ethyl acetatefrom ethylene and acetic acid in vapour phase bythe new BP AvadaTM process [78]. The process pro-duces 220,000 tonnes of ethyl acetate per year in BPschemical complex near Hull, UK, since 2001 [79].

Some of the above recommendations, such as cata-lyst doping with transition metals and controlled addi-tion of water to the HPA catalyst, might prove useful toprolong the lifetime of HPA catalysts in Friedel–Craftsand related reactions.

7. Conclusion

The recent studies reviewed here demonstrate thatHPA-based solid acids, including bulk and supportedheteropoly acids (preferably H3[PW12O40]) as wellas acidic heteropoly salts (e.g. Cs2.5H0.5[PW12O40]),are active and environmentally friendly catalysts forthe Friedel–Crafts acylation of aromatic compounds

and related Fries rearrangement of aryl esters. Thesesolid acids are superior in activity to the conventionalacid catalysts such as H2SO4 or zeolites, which is inline with the stronger acidity of HPA. The HPA cat-alysts can be reused after a simple work-up, albeitwith reduced activity. Similarly to zeolite catalysts, theHPA-catalysed acylations are inhibited by-productsbecause of strong adsorption of the products on thecatalyst surface. Consequently, to achieve higher con-versions a larger amount of the catalyst is needed or aflow technique should be applied. Adsorption of aro-matic substrate and acylating agent on the catalyst,especially preferential adsorption of one of them (e.g.the acylating agent), can affect (inhibit) the activity of HPA catalyst in acylation. The irreversible deactiva-tion (coking) of HPA catalysts in Friedel–Crafts reac-tions is an issue that needs to be addressed.

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