Solid Acid Catalysts Based on Supported Tungsten Oxides

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Topics in Catalysis 6 (1998) 87±99 87

Solid acid catalysts based on supported tungsten oxides

a b a, David G. Barton ,StuartL.Soled and Enrique Iglesia ∗ a Department of Chemical Engineering, University of California at Berkeley, Berkeley, CA 94720, USA E-mail: [email protected] b Corporate Research Laboratory, Exxon Research and Engineering, Route 22 East, Annandale, NJ 08801, USA

Supported WOx clusters are active and stable catalysts for isomerization, dehydration, and cracking reactions. Brùnsted acid sites 6+ 6+ form on WOx clusters when a lower valent element replaces W or when W centers reduce slightly during catalytic reactions. WOx clusters of intermediate size provide the balance between reducibility and accessibility required to maximize the number of surface H+ species in WOx±ZrO2, zirconium tungstate, and oxygen-modi®ed WC catalysts. H2 is involved in the generation and maintenance of Brùnsted acid sites during catalytic reactions on WOx clusters. Keywords: tungsten oxides, solid acids, acid-catalyzed reactions

1. Introduction (WOx±ZrO2) by impregnation with a solution of tungstate anions and subsequent oxidation treatments at high tem- Many research groups continue to explore the catalytic peratures (873±1173 K) show strong acidity. Hino and properties of oxide-based strong solid acids in an attempt to Arata suggested, based on color changes of Hammett in- eliminate environmental concerns caused by the use, regen- dicators (H0 14.52), that acid sites stronger than 100% 6 − eration, and disposal of liquid acids and halide-containing sulfuric acid may exist [4]; however, the use of this tech- solids [1,2]. The challenge of obtaining acid site strength nique to measure acid strength of solids is often unreli- and density comparable to those in sulfuric acid, halides, able [5]. These authors also reported that WOx±ZrO2 cat- or oxyhalides remains a formidable one; the smaller elec- alyzes the isomerization of n-pentane at low temperatures, tronegativity differences in metal-oxygen bonds compared but found high selectivity to cracked products ( 50%) ∼ with metal-halide bonds limit the achievable acid strength. even at low conversions [4]. Butene dimerization studies In oxide-based solid acids, protons balance net negative later con®rmed that acid sites on WOx±ZrO2 give catalytic charges produced by the replacement of a high valent cation rates and selectivities comparable to those on SOx±ZrO2 with one of lower valence or by the attachment of an anion for this reaction [6]. The addition of a metal component to the surface of a neutral oxide support. In all cases, the net (e.g., < 1 wt.% Pt) to WOx±ZrO2 and the presence of H2 negative charge is balanced by protons. Among strong solid in the reactant mixture lead to active, stable, and selective acids, sulfated and tungstated zirconia and supported het- catalysts for n-alkane isomerization reactions [7±12]. eropolyacids show interesting catalytic behavior and have The structural richness of tungsten oxide species leads attracted signi®cant attention. Their mesoporous structures to several atomic arrangements that can exhibit the strong allow more facile transport of reactants and products than acidity required for reactions of alkanes at low tempera- the restricted pore structures in microporous acids and mini- tures. 12-Tungstophosphoric acid (H3PW12O40)andWOx± mize undesired side reactions favored by diffusional restric- ZrO2 appear to contain the strongest acid sites among tions and long intrapellet residence times. tungsten oxide-based materials reported in the literature. Effective solid acids require that metal cations or metal The ability to delocalize a negative charge among several oxide clusters interact with oxide supports to form struc- surface oxygen atoms in heteropolytungstate or isopoly- tures that stabilize the protons responsible for Brùnsted tungstate clusters allows hydrogen atoms in OH surface acidity. These isolated metal-oxygen pairs or metal ox- groups and hydrocarbons interacting with such OH groups ide clusters must resist sintering and decomposition during to retain cationic character, if not as a stable species, at hydrocarbon reactions. The recent intense interest in sul- least during the formation of short-lived intermediates or activated complexes required in acid catalysis. In 12- fated zirconia appears to arise from the ability of zirconia to 3 establish this environment for the sulfate ion. Yet, serious tungstophosphoric acid, the negative charge of the PO4− concerns remain about the long-term stability of zirconia- anion is shared throughout a neutral (WO3)12 shell and is supported sulfate species in reducing and oxidizing envi- balanced by protons accessible to reactants at the surface ronments that are typical of hydrocarbon reactions and of of this shell [13,14]. regeneration treatments [3]. In WOx±ZrO2 catalysts, WOx clusters of intermediate size delocalize a net negative charge caused by the slight Tungsten oxide species dispersed on zirconia supports reduction of W6+ centers in reactant environments con-

∗ To whom correspondence should be addressed. taining H2 or hydrocarbons [9]. This temporary charge

 J.C. Baltzer AG, Science Publishers 88 D.G. Barton et al. / Solid acid catalysts on tungsten oxides imbalance leads to the formation of Brùnsted acid (WO3)m At low temperatures, bulk WO3 catalyzes metathesis re- 6 n + W − O3 n-H centers on the zirconia support. These actions of alkenes via a mechanism that involves coordi- { }{ } 6+ Brùnsted acid sites on WOx±ZrO2 appear to form via a nation of alkenes to W Lewis centers [18]. Such path- mechanism similar to that on heteropolytungstate clusters, ways lead to reaction products that differ signi®cantly from but in this case, the charge imbalance is not permanent; it those formed in oligomerization-cracking reactions typical is reversible and created in-situ by the reducing environ- of Brùnsted acid sites. The rate of propene metathesis reac- ment of the acid-catalyzed reaction. WOx±ZrO2 catalysts tions on WO3 crystallites increases with increasing disper- have an advantage over heteropolytungstates in that they sion of these crystallites, which may be achieved by impreg- retain structural integrity during high temperature oxidative nation of aqueous solutions of ammonium paratungstate on treatments (> 873 K), in contrast with heteropolytungstates high surface area supports (SiO2,TiO2,Al2O3,orZrO2) that decompose to bulk WO3 at much lower temperatures [18]. Highly dispersed WO3 species (WOx) with greater ( 673 K). Brùnsted acid site density can also be achieved by adding ∼In this review paper, we discuss several important ex- another metal oxide that forms mixed-oxide materials such amples of solid acids based on tungsten oxides with em- as WO3±P2O5,WO3±MoO3,ZrW2±8O0.5±3.5, or heteropoly- phasis on the role of support and surface structures on the tungstates. The role of heteroatoms in intimate mixtures or rate and selectivity of acid-catalyzed reactions. We also as an oxide support can be two-fold: to increase the disper- describe some recent unpublished results from our group sion of WOx clusters and to introduce a permanent charge with regard to the structure and isomerization pathways on imbalance that stabilizes protons and carbocationic species WOx±ZrO2 and Pt/WOx±ZrO2 catalysts. on the surface of these clusters.

2. Early studies of acid catalysts based on tungsten 3. Tungstated alumina catalysts (WOx±Al2O3) oxides In 1937, Bliss and Dodge reported that dispersing WOx Catalysts containing tungsten oxides have been used as species on Al2O3 enhanced the activity of bulk WO3 cata- solid acid catalysts since the turn of the century. They were lysts for the hydration of ethylene [19]. More recent stud- the subject of an extensive review of their catalytic prop- ies have addressed the structure of the WOx species and erties about thirty years ago [15]. In early studies, these its role in establishing the density and strength of acid sites materials were shown to be active catalysts for hydrocrack- for acid-catalyzed reactions of hydrocarbons [20±26]. ing, dehydrogenation, isomerization, reforming, alcohol de- WOx species appear to interact strongly with sites on the hydration, and ole®n oligomerization reactions. Bulk WO3 surface of γ-Al2O3. These surface species inhibit the sinter- was reported to be an active catalyst for several of these ing of Al2O3 crystallites and the structural transformation reactions, but high temperatures were required because of of γ-Al2O3 into α-Al2O3 during high temperature oxida- its low surface area and weak acid sites [15]. The density tion treatments [20]. For example, a pure γ-Al2O3 sample and strength of acid sites and the rate of catalytic reactions shows a BET surface area of only 10 m2/g after oxidation on tungsten oxide catalysts increase with slight reduction in air at 1323 K, but a sample containing 6 wt.% W has a of the W6+ centers, which often occurs during catalytic re- surface area of 50 m2/g. The resulting isolated species or actions in reducing hydrocarbon environments. In addition, small clusters of WOx reduce in H2 at much higher temper- the more effective dispersion of WO3 species on high sur- atures than bulk WO3 or WOx clusters supported on silica face area supports and the introduction of a heteroatom in or zirconia. order to create a permanent charge imbalance in the WO3 WOx surface coverage on Al2O3 supports increases with structure increase the density and the strength of the acid increasing temperature of oxidative pre-treatments which sites. sinter the Al2O3 crystallites. Titration of the Brùnsted acid Sabatier ®rst reported that yellow WO3 crystallites were sites with sterically hindered 2,6-dimethylpyridine shows readily reduced during alcohol reactions above 523 K to a that the Brùnsted acid site density increases with increas- stoichiometry corresponding to WO2.5 [16]. This slightly ing oxidation temperature (table 1) [24]. Lewis acid site reduced blue oxide was more active than WO3 and was density, measured by difference using 3,5-dimethylpyridine found to re-oxidize in air at room temperature [16]. Since molecules that titrate both Lewis and Brùnsted acid sites, this discovery, many studies have con®rmed the enhanced decreases with increasing oxidation temperature (table 1) catalytic activity of reduced bulk WO3 for a variety of reac- [24]. This increase in Brùnsted acidity with increasing tions. The reversible reduction-oxidation process and color WOx surface coverage appears to be related to the con- change of bulk WO3 is similar to that observed for hy- version of isolated tungstate groups into polytungstate clus- drogen tungsten bronzes (HxWO3) prepared by exposing ters, which can delocalize protons among neighboring WOx physical mixtures of Pt metal and bulk WO3 to H2 at room species. The appearance of polytungstate clusters as the temperature [17]. H2 dissociation on Pt metal sites leads to oxidation temperature increases was detected in Raman the slight reduction of WO3 at room temperature by H-atom spectra by the appearance of W±O±W bands with increas- diffusion from Pt sites to WO3 interstices. ing WOx coverage [22]. At WOx coverages (> 3.2W- D.G. Barton et al. / Solid acid catalysts on tungsten oxides 89

Table 1 Lutidine (2,6-dimethylpyridine and 3,5-dimethylpyridine) site titration results at 473 K from [24].

Catalyst Oxidation Brùnsted acid sitesa Lewis acid sitesb temperature (K) (µmol/m2)(µmol/m2)

γ-Al2O3 773 0 1.51 10% WO3±Al2O3 773 0.14 1.36 10% WO3±Al2O3 1223 0.38 1.13 Y-zeolite 773 1.8 0.47

a Number of Brùnsted acid sites on WOx±Al2O3 calculated from 2,6-dimethylpyridine uptake minus the b uptake of Al2O3. Number of Lewis acid sites calculated as the difference between 3,5-dimethylpyridine uptakes and number of Brùnsted acid sites. atoms/nm2) where polytungstate clusters exist, W-L X- Table 3 I n-Heptane isomerization product selectivity and rate on a Pt/WO ±Al O ray absorption near-edge spectra (XANES) resemble the x 2 3 (0.3 wt.% Pt, 8.4 wt.% W) and oxygen-modi®ed WC (O2 chemisorbed at spectrum of the distorted tungsten oxide octahedral sym- 800 K) catalysts [623 K, 4.4 kPa n-heptane, 96 kPa H2] from [26]. metry in bulk WO [23]. At low WO coverages (1.4 3 x Pt/WO ±Al O WC/O-800 K W-atoms/nm2), the spectra resemble the spectrum of the x 2 3 n-Heptane conversion (%) 13.4 12.9 tungsten oxide tetrahedral symmetry in Al2(WO4)3,which 1 suggests that isolated tetrahedral tungstate groups are re- Pt-Turnover rate (s− ) 0.55 0.17 Reaction selectivity (%) sponsible for the enhanced Lewis acidity at low WOx coverage [23]. Hydrogenolysis 47.9 16.3 Isomerization 35.5 79.5 Highest gas-oil cracking rates were reported on WOx± Cyclization 13.5 0.7 Al2O3 catalysts with surface coverages of about 2.4 W- Dehydrogenation 3.1 3.5 2 atoms/nm [24]. This surface density appears to lead to Cracking pattern (%) polytungstate clusters containing a suf®cient number of 1±2 (terminal) 15.7 32.8 W±O±W linkages for effective delocalization of the Hδ+ 2±3 22.0 30.4 3±4 (Mid-molecule) 62.4 36.8 species among neighboring close-packed WOx species, but which retain a high density of surface WOx groups required Product selectivity (%) for a high acid site density [24]. The rate and selectivity for 2-Methylhexane 14.9 37.3 the isomerization of isobutane and 2-methyl-2-pentene and 3-Methylhexane 17.2 37.4 Ethylpentane 1.9 3.7 the thermal desorption spectra of NH3 and lutidine probe Dimethylpentanes 1.5 1.1 molecules show that WOx±Al2O3 catalysts contain mod- Alkylcyclopentanes 7.6 0.4 erately strong Brùnsted acid sites, intermediate in strength Toluene 5.9 0.3 between those on amorphous silica±alumina catalysts and on Y-zeolite [24,25]. WOx±Al2O3 catalysts retain their ac- high ratio of (2,3) to (2,2) isomers among initial reaction tivity for gas-oil cracking after high temperature hydrother- products, consistent with isomerization that occurs predom- mal treatments, whereas cracking rates on silica±alumina inately by methyl-shift rearrangements on acid sites and and zeolite catalysts decrease markedly after similar treat- not by metal-catalyzed cyclic isomerization pathways [26]. ments (table 2) because of signi®cant loss of surface area. Acid-catalyzed β-scission is the predominant reaction of The introduction of a metal function (0.3 wt.% Pt) on n-heptane at 623 K (table 3). Slow hydride transfer reac- WOx±Al2O3 (8.4 wt.% W) leads to bifunctional alkane tions to surface intermediates lead to long surface residence conversion reactions, in which metal sites catalyze alkane times and a to high probability that they undergo β-scission dehydrogenation and acid sites catalyze isomerization and reactions before they desorb as isomers. β-scission reactions of alkenes [26]. Isomerization of 3,3- dimethylpentane at 623 K on these catalysts leads to a 4. Heteropolytungstates Table 2 Effect of steaming on activity for East Texas Light gas-oil cracking [783 K, Many studies have addressed the catalytic properties of 1 303 kPa total, 283 kPa H2O, WHSVoil = 1.4h− ] from [25]. heteropolytungstate clusters with Keggin structures, such as 12-tungstophosphoric acid (H PW O ), and excellent re- Catalyst Gas-oil cracking activity (cm3 gas/g-cat h) 3 12 40 · view articles have described recent advances [13,14]. The Fresh catalyst Catalyst steamed at 1144 K 3 PW12O − anion (i.e., the ªconjugate baseº) contains the for 16 h 40 smallest negative charge among the different heteropolyan- 10% WO ±Al O 0.45 0.41 4 5 3 2 3 ions with stable Keggin structures (SiW12O40−,BW12O40−, Silica±alumina 0.46 0.13 4 GeW12O40−, etc.). As a result, the corresponding acid form (Davison DA-1) shows the strongest acidity and catalyzes reactions such as Zeolite 0.75 0.24 (Davison FLX-5) hydration of ole®ns, dehydration of alcohols, aromatic alkylation, conversion of methanol, oxygenation of alkenes, and 90 D.G. Barton et al. / Solid acid catalysts on tungsten oxides alcoholysis of epoxides [14]. 12-Tungstophosphoric acid to polar reactants, which tend to swell the structures and 3 consists of a central tetrahedral PO4− anion encapsulated diffuse through the enlarged intracrystalline voids. These within a neutral (WO3)12 shell, on the surface of which swollen structures of the acid forms of heteropolyoxoanions three hydrogens with a net positive charge maintain the have been described as ªpseudo-liquidº media by Misono electroneutrality of the cluster. Thus, 12-tungstophosphoric and co-workers [30]. The insoluble acid salts appear to be acid can be thought of as an ªencapsulatedº form of phos- superior catalysts because they contain a higher permanent phoric acid, which avoids the undesired properties of its surface area accessible to both polar and non-polar mole- liquid form. Similarly, the Si and B forms of heteropoly- cules and because their insoluble nature prevents the unde- tungstates become ªencapsulatedº forms of silicic and boric sired dissolution and elution of soluble acid forms during acids, which are much weaker than phosphoric acid in catalytic reactions of polar molecules. This leaching of cat- the liquid phase. Keggin structures corresponding to se- alytic species causes long-term stability and environmental questered sulfuric acid (H2SW12O40) are not available, ap- contamination problems. parently because the size and coordination symmetry of Impregnation of aqueous solutions of heteropolyacids on the central sulfur atom is inconsistent with a stable Keg- inert oxide supports can be used to attain the protonic form gin structure. If stable, H2SW12O40 clusters with Keggin of the heteropolyacid Keggin structure with a small crys- structures would provide stronger acid sites than currently tal size that increases the accessible external surface area available heteropolytungstate clusters. [31±33]. In this manner, the full density of protons in the The large and soft anionic clusters in Keggin structures parent acid can be retained and sites can remain accessible provide the electron delocalization required for charge sep- to non-polar reactants that normally contact only the ex- aration and for the formation of cationic complexes dur- ternal protons in crystalline heteropolyacids. Supports that ing acid-catalyzed reactions. Heteropolyacid salts, which interact weakly with Keggin clusters (e.g., SiO2)arepre- are produced by partial exchange of the protons in the ferred, because strongly interacting supports such as Al2O3 parent acid with alkali or other cations, form solid acids can lead to hydrolysis and decomposition of the clusters at with morphological and solubility properties very differ- low temperatures [29]. The limitations of these methods for ent from those of the parent acid [27]. For example, par- supporting water-insoluble heteropolyacid salts have been tial exchange of 12-tungstophosphoric acid with Cs+ ions recently overcome [29]. These new methods involve the 2 converts the water-soluble low surface area (< 5m/g) synthesis of SiO2 supported heteropolyacid Cs-salts by re- acid form into a water-insoluble precipitate. This porous placing silanol hydrogens (Si±OH) with Cs+ (Si±OCs) at 2 solid reaches surface areas exceeding 100 m /g after about high pH and then impregnating the Cs2O±SiO2 sample with + + 80% of the H species have been replaced by Cs cations an aqueous solution of H3PW12O40. The heteropolyacid + (Cs2.5H0.5W12O40). These acidic Cs salts were reported precipitates as the acid salt upon contact with Cs cations to be much more active per gram of catalyst than the H- on the surface of SiO2 to form highly dispersed anchored form for alkylation of m-xylene and 1,3,5-trimethylbenzene clusters with Cs2.5H0.5PW12O40 stoichiometry. with cyclohexene [28] and to contain slightly stronger acid sites as measured by a 2-methyl-2-pentene model reaction 5. Oxygen-modi®ed tungsten carbides [29] (table 4). Other acid salts with large cations, such Ê as Rb, K, and NH4, also form small ( 100 A) water- High surface area stoichiometric tungsten carbides (WC) insoluble precipitates with high surface areas∼ and excellent can be prepared by direct carburization of WO3 with catalytic properties, even though a large fraction of the hy- CH4/H2 mixtures (1100 K), followed by a high temper- drogens have been replaced by inactive alkali ions [13,27]. ature H2 treatment (973 K, 0.8 h) to remove polymeric H3PW12O40 contains a high density of protons located pre- carbon that can block carbidic surface sites [34]. Alkane dominantly within dense crystallites that are accessible only dehydrogenation and hydrogenolysis turnover rates on tung- Table 4 sten carbides are similar to those reported on supported no- Catalytic activity for alkylation and acidity of the H3PW12O40 and ble metal catalysts [35]. Chemisorption of oxygen on WC Cs2.5H0.5PW12O40 catalysts [28,29]. surfaces inhibits the rate of these reactions and introduces

H3PW12O40 Cs2.5H0.5PW12O40 Brùnsted acid surface sites similar to those present in sup-

2 ported tungsten oxides [34,36]. Such acid sites catalyze Surface area (m /g) 6 131 hydroisomerization of alkanes only when tungsten carbide Acid amount (µmol/g)a 8±16 161 Acidity parameter 0.6 1.3 sites are also present. (3MP2/4MP2)b These oxygen-modi®ed tungsten carbides are bifunc- Activity (mmol/g-cat h)c tional; they catalyze alkane dehydrogenation on WC sites · m-Xylene 1.4 24.7 and alkene rearrangements on WOx surface species [26]. Trimethylbenzene 26.2 1.4 WOx sites on the carbide surfaces also catalyze methanol a b dehydration and propylene oligomerization and cracking re- Measured by TPD of NH3. 2-Methyl-2-pentene isomerization selectivity ratio [523 K, 1 h]. c Initial rates of alkylation with cyclohexene actions [37], but they do not catalyze propylene metathesis [327 mmol x-xylene, 216 mmol 1,3,5-trimethylbenzene, 10 mmol cyclo- reactions that occur on Lewis acid sites present on WOx± hexene, 373 K]. Al2O3 and WOx±SiO2 catalysts. It appears that Lewis acid D.G. Barton et al. / Solid acid catalysts on tungsten oxides 91 sites provided by W6+ centers are converted into Brùnsted Table 5 Catalytic activity of HCl washed zirconium tungstate catalysts for vapor acid sites on oxygen-modi®ed WC by interactions of WOx phase ethylene hydration [493 K, 101 kPa, H2O/C2H4 = 1.0, SV = 1 with spill-over hydrogen atoms formed by H2 or C±H dis- 2300 h− ]. sociation on WC sites [38]. Catalyst W/Zr (atom ratio) Ethanol concentration (wt.%) Oxygen-modi®ed tungsten carbides catalyze n-heptane isomerization with high selectivity (> 70%) at 623 K (ta- Zirconium tungstate 1.0 0.60 ble 3) [26]. Kinetic and isotopic tracer data show that the Zirconium tungstate 2.0 0.90 Zirconium tungstate 3.0 0.80 reaction proceeds via sequential n-heptane dehydrogenation H3PO4/SiO2 0.45 and heptene isomerization steps. At 623 K, n-heptane conversion rates are limited by rearrangements of intermediate ethylene to ethanol at 553 K after an induction period. Ap- heptenes on WOx acid sites, but n-heptane dehydrogena- 6+ tion becomes increasingly rate-limiting as reaction temper- parently, the slow partial reduction of the W centers, a ature is increased. Isomer distributions on oxygen-modi®ed process detected from stoichiometric conversion of ethanol to acetaldehyde during the induction period, leads to in- WC are similar to those found on Pt/WOx±Al2O3 and the initial isomers formed from 3,3-dimethylpentane reactions creased Brùnsted acidity in these materials. Procedures for the synthesis of zirconium tungstate are are consistent with methyl-shift isomerization pathways (ta- similar to those used in the synthesis of supported WO ± ble 3). The WC function catalyzes the rapid exchange of x ZrO catalysts. Zirconium tungstates, however, contain sig- deuterium atoms into ªunreactedº n-heptane and its reaction 2 ni®cant amounts of residual chloride ions, very high WO products at 623 K, suggesting that hydrogen adsorption- x concentrations, and they are typically oxidized at temper- desorption steps are quasi-equilibrated during n-heptane atures below ZrO crystallization temperatures. The na- reactions. The high selectivity towards cleavage of ter- 2 ture of the acid sites on zirconium tungstate has not been minal C±C bonds indicates that smaller alkanes form in carefully examined, but it appears that it contains Brùnsted hydrogenolysis reactions on WC sites, rather than by the acid sites similar to those on WO ±ZrO . In both types of acid-catalyzed β-scission cracking pathways that account x 2 catalysts, active species consist of amorphous polytungstate for the low isomerization selectivity on Pt/WO ±Al O cat- x 2 3 species that become Brùnsted acids only after in-situ partial alysts. It appears that the more intimate contact between reduction during catalytic reactions. The residual electron- H dissociation/alkane dehydrogenation sites and Brùnsted 2 withdrawing chloride ions in zirconium tungstate powders, acid sites in oxygen-modi®ed WC samples leads to shorter may increase the acid strength, but can cause corrosion and carbocation residence times and to a lower probability of contamination problems by eluting as HCl during catalytic β-scission reactions than on Pt/WO ±Al O .Atlower x 2 3 reactions. reaction temperatures, thermodynamics favor the desired multibranched alkanes, but the lower equilibrium alkene concentrations make bifunctional pathways involving gas 7. Isopolytungstate clusters suported on zirconia phase alkenes very slow. Stronger WOx acidsitessuchas (WOx±ZrO2) those on Pt/WOx±ZrO2 catalysts and a chain mechanism that avoids the requirement for gas phase ole®n intermedi- WOx±ZrO2 powders were ®rst reported to contain ates lead to signi®cantly higher alkane hydroisomerization strong acid sites by Hino and Arata [4]. They reported rates and selectivities at low temperatures. Pt/WOx±ZrO2 low temperature n-butane and n-pentane isomerization catalysts for n-heptane hydroisomerization reactions are de- activity on WOx±ZrO2 catalysts prepared by impregnat- scribed in detail in a later section of this review. ing ZrOx(OH)4 2x with aqueous ammonium metatungstate (13 wt.% W)− and subsequent high temperature oxidation (1073 K). n-Pentane isomerization rates decreased 6. Zirconium tungstate catalysts markedly when metatungstate solutions were impregnated on crystalline ZrO2 supports instead of amorphous ZrOx Zirconium tungstate solids are used as inorganic cation- (OH)4 2x supports, suggesting that hydrated WOx species − exchangers, but their catalytic properties have not been combine with ZrOx(OH)4 2x surface sites and that such widely studied. Ethylene hydration rates on amorphous zir- sites are not available after− crystalline tetragonal or mono- conium tungstate (ZrW2±8O0.5±3.5) powders are similar on a clinic ZrO2 structures are formed. It appears that the initial volumetric basis to those reported on supported phosphoric formation of dispersed tungstate species occurs via anion acid catalysts used in commercial practice [39] (table 5). exchange or condensation reactions of hydrated aqueous An active catalyst with a W/Zr ratio of 2.0 was prepared tungstate anions with surface OH species in ZrOx(OH)4 2x. − by mixing aqueous Na2WO4 and ZrOCl2 solutions at room The resulting high dispersion of WOx species inhibits the temperature to give a white gel-like precipitate, which was crystallization and sintering of the ZrO2 support crystallites acidi®ed using HCl before ®ltering. Filtered precipitates and provides a material that can be sintered in a controlled were oxidized at 723 K for 7 h to produce an amorphous manner during subsequent oxidation treatments. The sinter- 2 pale-yellow solid with high surface area (100 m /g) [40]. ing of the ZrO2 support crystallites leads to the formation These materials catalyze near equilibrium conversion of of WOx clusters of varying size and catalytic properties. 92 D.G. Barton et al. / Solid acid catalysts on tungsten oxides

Figure 2. The effects of oxidation temperature on BET surface area and crystal modi®cation of the zirconia support (7.9 wt.% W) [12]. Figure 1. The effects of oxidation temperature on n-pentane isomerization 3 [0.5 g catalyst, 13 wt.% W, 553 K, helium 10 cm /min, 1 µl n-C5 pulse] cipient wetness with a solution of ammonium metatungstate (adopted from [4]). ((NH4)6H2W12O40) and then oxidized at high temperatures (873±1100 K) in dry air. Pt (0.3 wt.%) was introduced n -Pentane isomerization rates depend strongly on the into some of the oxidized WOx±ZrO2 samples by incipient temperature at which impregnated samples are treated in wetness with a solution of tetraammine platinum hydroxide, air (®gure 1) [4]. High isomerization rates occur in a nar- followed by decomposition in air at 723 K, and reduction row range of high oxidation temperatures (900±1250 K), in H2 at 523 K. during which loss of surface occurs and WOx surface den- BET surface area and X-ray diffraction measurements sity increases. When n-pentane isomerization reactions are show that WOx surface species inhibit zirconia sintering carried out in helium on these catalysts, selectivity to crack- and its tetragonal to monoclinic phase transformation during products is high ( 50%) and the catalysts deactivate ing the oxidative treatments required for the synthesis of ∼ rapidly. active catalysts. ZrOx(OH)4 2x powders show high surface The addition of small concentrations of a metal com- areas (289 m2/g) after drying− at 383 K. At 1073 K, a typ- ponent (< 0.5 wt.% Pt) to WOx±ZrO2 catalysts and the ical oxidation temperature leading to WOx±ZrO2 samples presence of H2 in the reactant stream lead to very active, with high activity, pure ZrO2 has a very low surface area selective, and stable catalysts for n-alkane hydroisomeriza- (4 m2/g) and consists of large monoclinic crystals. Impreg- tion at 473 K [8,9,12]. The metal function increases the rate nation with near monolayer WOx surface coverages (6 W- of hydrogen transfer to isomerized intermediates, causing atoms/nm2) results in higher surface areas (51 m2/g) and their desorption as isoalkanes before undesired β-scission small tetragonal ZrO2 crystallites after oxidation at 1073 K. reactions occur. In addition, metal sites appear to be in- Three-dimensional growth of bulk WO3-like species be- volved in the formation of Brùnsted acid sites via spillover comes detectable by X-ray diffraction and UV-visible mea- of hydrogen adatoms from metal sites to WOx species dur- surements as this saturation coverage is exceeded. Tung- 2 ing the initial stages of the reaction. In the absence of H2 sten surface densities above 10 W-atoms/nm lead to lines and H±H dissociation sites, isomerization still occurs but corresponding to bulk WO3 in X-ray diffraction patterns. via the sacri®cial use of reactants to form surface hydro- The structure and size of ZrO2 crystallites can be con- gen species. The desorption of the H species as H2 during trolled by adjusting the oxidation temperature and the hydrocarbon reactions, however, leaves unsaturated hydro- W concentration. At a given W concentration (e.g., carbon fragments permanently attached to the surface and 7.9 wt.% W), the BET surface area and the tetragonal leads to fast deactivation and to a limited number of cat- content in ZrO2 decrease markedly with increasing oxida- alytic turnovers. tion temperature (®gure 2). WOx species inhibit the sin- In our studies, WOx±ZrO2 samples were prepared using tering and the transformation to monoclinic ZrO2 crystal- previously reported synthesis methods [4], but over a wider lites, but as the oxidation temperature increases tetrago- range of composition and oxidation temperatures. High nal ZrO2 crystallites ultimately agglomerate and transform surface area ZrOx(OH)4 2x was prepared via hydrolysis of into monoclinic crystallites when a critical crystallite di- − ZrOCl2 solution using slow addition of NH4OH to raise the ameter is reached [41]. At a given oxidation temperature pH to a value of 10. After removing residual chloride ions (e.g., 1073 K), surface area and tetragonal content increase by washing, the precipitate was dried and impregnated to in- with increasing W-loading up to 7.9 wt.% W (®gure 3) D.G. Barton et al. / Solid acid catalysts on tungsten oxides 93

Figure 4. Initial o-xylene isomerization turnover rates at several oxidation temperatures and W-loadings in gradientless glass recirculating batch reactor [523 K, 104 kPa H2,0.66kPao-xylene] [12]. Figure 3. The effects of W loading on BET surface area and crystal modi®cation on the zirconia support (1073 K oxidation) [12]. concentrations (12 wt.% W) for samples treated in air at 1073 K. At each W-loading (5, 12 or 21 wt.% W), isomer- and then remain constant. It appears that ZrOx(OH)4 2x ization turnover rates also pass through a maximum value powders contain an intrinsic number of strong binding sites− as the oxidation temperature increases (®gure 4). The ox- for metatungstate ions ( 8 wt.% W), which must be oc- idation temperature at which this maximum value occurs cupied in order to inhibit∼ locally crystallite growth and decreases with increasing W-loading. These data suggest re-crystallization into the thermodynamically stable mon- that an intermediate WOx surface density may be required oclinic phase [12]. Impregnation with metatungstate ions for high densities of Brùnsted acid sites. When o-xylene beyond this required surface coverage leads to weakly held isomerization turnover rates are plotted as a function of tungstate groups, which agglomerate into WOx clusters and WOx surface density for all WOx±ZrO2 catalysts tested, bulk WO3 crystallites without additional stabilization of the they merge into a single curve with a sharp maximum at 2 ZrO2 support. about 10 W-atoms/nm (®gure 5). This WOx surface den- Control of the W concentration and of the temperature sity represents about twice the theoretical monolayer cov- of oxidation treatments can be used in order to design cat- erage of WOx octahedra on a ZrO2 surface. The narrow alysts with a speci®c WOx surface density and a high sur- range of WOx surface densities over which WOx±ZrO2 face area. For example, WOx±ZrO2 samples containing samples become active catalysts suggests that high isomer- similar WOx surface densities, but very different total sur- ization turnover rates require the presence of WOx clusters face areas, were prepared by impregnation and controlled within a narrow range of cluster size, which appear to form synthesis procedures. A sample with 22 wt.% W oxidized selectively as a coverage of 10 W-atoms/nm2 is reached. at 823 K had a total surface area of 140 m2/g, but the same These clusters appear to be large enough to delocalize a surface density (5.0 W-atoms/nm2) as a 7.9 wt.% W sam- net negative charge caused by the slight reduction of W6+ 6 n + ple oxidized at 1073 K and exhibiting a much lower surface with H2 leading to the formation of W − O3(n-H ) cen- area (51 m2/g). ters on the zirconia surface, but small enough to contain Acid-catalyzed skeletal isomerization of o-xylene to m- the resulting protons predominantly at crystallite surfaces and p-xylene (523 K, 0.67 kPa o-xylene, 106 kPa H2)was rather than at inaccessible intracrystallite positions. used in our study in order to examine the acid properties of The requirement for slight reduction of W6+ centers WOx±ZrO2 catalysts. Xylene isomerization reactions pro- in WOx clusters of intermediate size is consistent with ceed on Brùnsted acid sites via protonation-deprotonation of the strong effect of H2 ontherateofo-xylene isomer- xylenes with intervening intramolecular methyl shifts that ization on WOx±ZrO2 catalysts. The addition of dihydro- form meta and para isomers before desorption. The selec- gen (101 kPa) increases steady-state o-xylene isomeriza- tivity to disproportionation products (toluene and trimethyl- tion rates by more than a factor of four on WOx±ZrO2 benzenes) was very low (< 5%), suggesting that bimole- at 523 K, suggesting the activation and involvement of H2 cular isomerization pathways [42] are not involved in xy- during ªmonofunctionalº acid-catalyzed reactions on WOx± lene isomerization reactions on WOx±ZrO2 catalysts at our ZrO2 [12]. The absence of H2 leads to the irreversible reaction conditions. WOx±ZrO2 samples with varying W- loss of active sites, apparently as the result of H2 desorp- loadings (2±21 wt.% W) and treated at several oxidation tion and the concomitant destruction of Brùnsted acid sites, temperatures (823±1100 K) were examined using o-xylene formed initially by the sacri®cial use of hydrogen atoms isomerization reactions. from reactants during o-xylene reactions. Desorption of o-Xylene isomerization turnover rates (per W-atom) pass such H-atoms causes the adsorption of the unsaturated co- 1 through a maximum value (0.0017 s− ) at intermediate W product of the initial activation reaction to become irre- 94 D.G. Barton et al. / Solid acid catalysts on tungsten oxides

Figure 6. Direct band gap energy from UV-visible spectra of WOx±ZrO2 catalysts (1073 oxidation) overlaid on o-xylene isomerization rate data

Figure 5. The in¯uence of WOx surface density on o-xylene isomerization from ®gure 4 (adopted from [9]). turnover rates [523 K, 104 kPa H2,0.66kPao-xylene] [12]. The vertical dotted line marks the theoretical monolayer capacity of tetragonal ZrO . 2 2 for the HOMO-LUMO gap in isolated WO4− (aq.) ions. Optical absorption edge energies for WOx±ZrO2 samples versible. Slow exchange of deuterium during reactions of oxidized at 1073 K were obtained from UV-visible spectra o-xylene/D2 mixtures shows that H2 does not participate in using procedures based on direct transitions between va- hydrogen transfer elementary steps during xylene isomer- lence and conduction bands [44]. Direct band gap energies ization, but it is instead involved in the infrequent regenera- decrease monotonically from 4.15 to 3.80 eV as the WOx tion of acid sites lost in hydrogen desorption side reactions surface density increases from 3.1 to 7.5 W-atoms/nm2 and during o-xylene isomerization [12]. These Brùnsted acid then become constant at higher surface densities (> 7.5W- 6 n + 2 sites consist of W − O3(n-H ) centers, which are formed atoms/nm ) (®gure 6) [9]. This monotonic decrease in the either by hydrogen migration from H±H or C±H dissocia- band gap energy suggests that average WOx cluster size tion sites to neutral WOx clusters. increases with increasing WOx surface density in a range Previously reported electronic spectra in the UV-visible where marked increases in o-xylene isomerization rates also energy range for WOx±ZrO2 samples and other WOx- occur. The spectra of samples with high WOx surface den- containing samples [9] are consistent with the growth of sities (> 10 W-atoms/nm2), however, show two distinct WOx clusters as the WOx surface density increases. All direct band gaps (absorption edges). The low-energy ab- WOx±ZrO2 samples show a maximum absorbance near sorption edge re¯ects the presence of WO3 crystallites with 4.8±4.9 eV, which corresponds to ligand-to-metal charge bulk-like electronic properties, which co-exist with WOx transfer transitions. UV-visible spectra show that isolated clusters of intermediate size. The high-energy absorption WOx species are not present in any WOx±ZrO2 sample af- edge position remains constant (3.8 eV) for all WOx±ZrO2 ter 1073 K oxidation and that all samples contain signi®cant catalysts within the range of WOx surface densities lead- concentrations of polymeric ±W±O±W± groups. A second ingtohigho-xylene isomerization turnover rates (6±15 W- band that appears at low energies (2.9 eV) in the spec- atoms/nm2) (®gure 6). Thus, UV-visible spectra suggest tra of samples with high WOx surface densities (> 10 W- that WOx clusters with intermediate size, de®ned as those 2 atoms/nm ) shows that WO3 crystallites with different elec- with absorption edges similar to ammonium metatungstate tronic properties can co-exist with amorphous WOx clusters (3.5 eV), lead to WOx±ZrO2 samples with maximum ac- [9]. tivity. The low-energy absorption edge in UV-visible electronic X-ray absorption near edge spectroscopy at the W-LI spectra is sensitive to cluster size in the 1±10 nm range edge (12100 eV) is sensitive to the local coordination and for many semiconductors, including WO3 [43]. In this symmetry of the W-atom absorbers. The pre-edge feature size range, the large fraction of atoms residing at the sur- at 5 eV re¯ects a 2s 5d electronic transition that is face alters the cluster electronic properties. As a result, not− allowed in octahedral→ (centrosymmetric) structures, but the energy gap between the valence and conduction bands which occurs in tetrahedral or distorted octahedral struc- increases with decreasing cluster size. This energy gap tures because of d±p orbital mixing [45]. Previously, this ranges from 2.8 eV for crystalline bulk WO3 to 6.22 eV pre-edge feature was used to detect the presence of 4- and D.G. Barton et al. / Solid acid catalysts on tungsten oxides 95

Figure 7. W-LI X-ray absorption near-edge spectra after dehydration (He, 673 K) for WOx±ZrO2 (2±21 wt.% W) and Na2WO4 and WO3 reference materials.

6-coordinated W atoms on WOx±Al2O3 [23]. The degree Figure 8. The in¯uence of WOx surface density on the tempera- of W coordination on Al2O3 supports increases with in- ture programmed reduction pro®les of WOx±ZrO2 samples [10 K/min, creasing concentration of WOx species and with the extent 3 80 cm /min of 20% H2/Ar mixture, 0.08±0.02 g] [9]. of hydration of the catalyst samples [23]. Our studies show that W atoms in WOx±ZrO2 samples charge on the conjugate base may be linked to both the reside in distorted octahedral environments similar to those strong acidity that appears near saturation WOx coverage present in bulk WO3 at all WOx concentration and pre- on ZrO2 and to the easier initial reduction of WOx clusters treatment conditions. Figure 7 shows the W-LI absorption as surface coverage and cluster size increase. It appears edges after dehydration (in He at 673 K) for several WOx± that a critical cluster size is required for both reduction and ZrO2 samples with different W concentrations oxidized at Brùnsted acidity, because the appearance of the latter may 1073 K (2±21 wt.% W) and for two standards: Na2WO4 require the incipient reduction of neutral WO3 clusters by (tetrahedral) and WO3 (distorted octahedral) bulk powders. H-atoms. All WOx±ZrO2 samples show strikingly similar near-edge The addition of a metal function (0.3 wt.% Pt) to WOx± + X-ray absorption spectra. Clearly, none of the WOx±ZrO2 ZrO2 leads to the selective hydroisomerization of C7 alka- samples have W-atoms in the tetrahedral symmetry of the nes at 373±473 K, without the extensive cracking ob- Na2WO4 standard, but instead have W-atoms with distorted served on Pt/SOx±ZrO2 and on zeolites promoted by metals octahedral environments, similar to those found in bulk [8,9,48]. At 473 K, n-heptane isomerization turnover rates WO3 crystallites. are similar on Pt/WOx±ZrO2 and Pt/SOx±ZrO2 (based on Temperature-programmed reduction of supported metal W and S-atoms, respectively), but Pt/WOx±ZrO2 is sig- oxides in hydrogen is also sensitive to the cluster size and ni®cantly more selective to heptane isomers (85% at 50% coverage of surface oxides [46]. For example, isolated n-heptane conversion). At higher reaction temperatures, WOx species on Al2O3 reduce at much higher tempera- Pt/WOx±ZrO2 catalysts are signi®cantly more active than tures than bulk WO3 crystallites [21]. Reduction peaks for Pt/SOx±ZrO2 samples, because they are stable against de- WOx±ZrO2 samples are very broad (600 K) and reduction activation by sublimation, decomposition, and sintering and occurs at much lower temperatures than for WOx±Al2O3 thus retain high isomerization selectivities (71% at 523 K samples at similar WOx surface densities. The reduction and 36% conversion) and rates [9]. of WOx clusters of intermediate size on ZrO2 occurs at n-Heptane isomerization kinetic and isotopic tracer data temperatures typical of those required for the reduction of on Pt/WOx±ZrO2 and Pt/SOx±ZrO2 are consistent with iso- heteropolytungstate clusters (®gure 8) [47]. Whereas, the merization reactions that proceed via chain reactions in- reduction of WOx species at low WOx coverage is strongly volving hydrogen transfer from alkanes, H2, or hydride in¯uenced by the ZrO2 support and occurs at higher tem- donors, such as adamantane, to adsorbed isomerized hep- peratures than bulk WO3.ThelargerWOx clusters present tanes [9,48]. This mechanism contrasts that discussed pre- at higher WOx surface densities delocalize the initial net viously for n-heptane isomerization on oxygen-modi®ed negative charge caused by reduction and lead to species WC and Pt/WOx±Al2O3 catalysts, in which alkene inter- that reduce at lower temperatures; these species then act as mediates form on the dehydrogenation function, diffuse to H2 dissociation centers that increase the rate of subsequent and isomerize on the WOx acid-function. At the higher reduction steps. The ability to delocalize the net negative temperature (623 K vs. 473 K) required for signi®cant iso- 96 D.G. Barton et al. / Solid acid catalysts on tungsten oxides

steps are inhibited on Pt/SOx±ZrO2 catalysts by the dif®- cult dissociation of H2 on Pt clusters poisoned by sulfur species, which leads to high cracking selectivity. Addition of hydrogen transfer co-catalysts, such as adamantane, increase n-heptane isomerization rate and selectivity on Pt/SOx±ZrO2, but not on Pt/WOx±ZrO2,because hydrogen transfer steps are quasi-equilibrated on the latter and do not bene®t from co-catalysts that increase the rate of these reactions [9]. Mixtures of n-C7H16± D2 quickly reach H±D equilibrium and lead to complete scrambling and binomial deuterium distribution in ªunre- actedº n-heptane and in all reaction products, con®rm- ing the reversible (quasi-equilibrated) nature of n-heptane adsorption-desorption steps on Pt/WOx±ZrO2 [9]. Scheme 1. Near-edge X-ray absorption studies and temperature- Table 6 programmed reduction studies have detected the reduction 6+ n-Heptane isomerization selectivity, reaction kinetics, and isotopic tracer of W centers during hydrogen treatment at 473±673 K data on Pt/WOx±ZrO2 [0.3 wt.% Pt, 12.7 wt.% W, Pt/WOx±ZrO2 oxida- [9,12]. Site titration studies using H2,O2, and CO show tion at 723 K after WOx±ZrO2 oxidation at 1073 K]. that hydrogen uptake exceeds monolayer coverages on sur- Isomerization selectivity (%)a 89 face Pt atoms and that O2 uptake is signi®cantly higher b n-Heptane reaction order 0.9 than that of H2 (table 7), suggesting that hydrogen adatoms b H2 reaction order 0.5±0 − spillover onto reduced WOx centers during H2 treatments Adamantane reaction ordera,c 0.05 Deuterium content in n-heptane (%)d 82 at typical isomerization reaction temperatures. At higher Deuterium content in methylhexanes (%)d 83 reduction temperatures, H2 uptakes markedly decrease and a O2 uptakes increase; these effects appear to be related to [473 K, 650 kPa H2, 100 kPa n-heptane, 50% n-heptane conversion]. b c decoration of Pt crystallites by oxygen-de®cient WOx,as [473 K, 800±2900 kPa H2, 33±200 kPa n-heptane]. [0±1.0 mol.% d adamantane]. [343 K, 3.7 kPa n-heptane, 97 kPa D2,5%n-heptane also proposed to explain similar effects in TiO2 supported conversion]. metal particles [49]. The spillover of hydrogen adatoms appears to involve the formation of acidic hydrogens on merization rates on WC and WOx±Al2O3 catalysts, equi- the WOx clusters (scheme 3). Hydrogen adatoms formed librium alkene concentrations are much higher and provide by homolytic dissociation of H2 on Pt sites lose electron an effective diffusive pathway for communication between density to Lewis acid WOx clusters, which delocalize the the two types of sites required in bifunctional isomerization reduced charge and stabilize the protons. Thus, hydrogen pathways. On Pt/WOx±ZrO2 and Pt/SOx±ZrO2 (at 473 K), adatoms become involved not only in the desorption of ad- alkenes are not detected in the gas phase and hydrogen sorbed intermediates, but also in the generation and main- transfer reactions via surface diffusion of hydrogen provide tenance of Brùnsted acid sites. The formation of Brùnsted an alternate and more effective pathway for communication acid sites during H2 treatment has been directly observed by between hydrogenation sites and acid sites. infrared measurements of pyridine adsorption on Pt/SOx± On Pt/WOx±ZrO2, n-heptane adsorption-desorption steps ZrO2 by Ebitani et al. [50]. They report that the density are quasi-equilibrated and surface isomerization steps limit of Brùnsted acid sites increases during contact with H2 at the overall rate (scheme 1). As a result, facile hydrogen temperatures typical of those required for their catalytic transfer steps decrease the surface residence time of inter- action. The density of Lewis acid sites concurrently de- mediates and lead to desorption of isomers before undesired β-scission events occur on the acid sites. Isomeriza- Table 7 a tion turnover rates are ®rst order in n-heptane and zero to Irreversible chemisorption uptake of H2,O2, and CO on Pt/WOx±ZrO2 (0.3 wt.% Pt, 10 wt.% W). negative order in H2 (table 6), suggesting that surface isomerization is rate-limiting and that hydrogen transfer steps Reduction H/Pt O/Pt CO/Pt decrease the coverage of surface intermediates. Hydrogen temperature (K) transfer occurs primarily using hydrogen adatoms formed 473 0.63 1.25 0.30 by H2 dissociation on surface Pt atoms, which spillover 673 0.092 2.04 0.050 onto WOx centers (scheme 2). These hydrogen transfer a Room temperature adsorption; irreversibly chemisorbed species are de- ®ned as those that cannot be removed by evacuation at room temperature.

6+ 5+ (C7H15)(W )n 1(W )O3n + H±Ptx − 6+ 6+ + 6+ 5+ (W O3)n + C7H16(g) + Ptx H±H + (W O3)n (H )2(W )n 2(W )2O3n −→ −

Scheme 2. Scheme 3. D.G. Barton et al. / Solid acid catalysts on tungsten oxides 97

Table 8 In¯uence of the preparative method on the structure, activity, and acidity of WOx±ZrO2 catalysts oxidized at 1098 K [51].

Impregnationa Re¯ux-impregnationb Co-precipitationc

W-loading (wt.% W) 15.0 16.9 15.5 Surface area (m2/g) 32 62 62

% tetragonal ZrO2 30 10 1

Bulk WO3 present? yes yes no dNumber of strong 0.002 0.002 0.004 Brùnsted acid sites (meq H+/g) ePentane isomerization acid 2.8 3.9 1 site turnover frequency (s− )

a b Prepared by metatungstate impregnation on pH 9 precipitated ZrOx(OH)4 2x. Prepared − c by re¯uxing ZrOx(OH)4 2x overnight prior to metatungstate impregnation. Prepared − d by coprecipitation of metatungstate and ZrOCl2 solutions at pH 9. 2,6-dimethylpyridine e 3 adsorption results. [483 K, 2 H2/nC5 mol ratio, LHSV = 2cm nC5/ml-cat h, 2400 kPa.] · creased, suggesting that Lewis acid sites are converted to sites may exist that can catalyze methyl-shift reactions, but Brùnsted acid sites by contact with H2. As a result, only only a few of those active sites may be involved in the C± techniques that measure the Brùnsted acid site density un- H bond activation-initiation step. In addition, the density der catalytic conditions provide an accurate value of the of Brùnsted acid sites before (or after) catalytic reaction is Brùnsted acid site density available for reaction. much lower than those present during catalytic reactions of Several recent patents describe the synthesis and cat- hydrocarbons. As a result, the density of Brùnsted acid sites alytic activity of WOx±ZrO2 and Pt/WOx±ZrO2 catalysts measured by amine adsorption will underestimate the den- for cyclohexane ring-opening and isomerization, benzene sity present at reaction conditions, because Brùnsted acid hydrogenation, alkene oligomerization, aromatic alkylation sites cannot survive in the absence of H2 or hydrocarbon + with C6 alkenes, toluene alkylation with methanol, aro- reactants. matic trans-alkylation reactions, selective catalytic reduc- Recent studies of the thermal desorption of alcohols tion of nitrogen oxides, and the hydrodesulfurization and show that acid sites on WOx±ZrO2 catalysts have similar hydrodenitrogenation of crude oils [11]. These authors re- acid strength as H-mordenite [52]. Infrared measurements port that maximum activity for pentane isomerization was of adsorbed pyridine show that WOx±ZrO2 samples retain found on WOx±ZrO2 samples (16 wt.% W, 1098 K oxi- pyridine on their surface to higher temperatures than on dation) prepared by co-precipitation of ZrOCl2 and ammo- WO ±Al O ,ZrO,orAlO samples [53]. This pyridine nium metatungstate solutions, as opposed to impregnation x 2 3 2 2 3 adsorption study also shows qualitatively that WO ±ZrO of ammonium metatungstate on ZrO (OH) [51]. It ap- x 2 x 4 2x samples that contain Pt have a higher density of Brùnsted pears that a better dispersion of metatungstate− ions on amor- acid sites than those without Pt in an inert diluent [10]. phous ZrOx(OH)4 2x is achieved during co-precipitation − On Pt/WO ±ZrO , butane isomerization rates (573 K) leading to higher surface area, lower monoclinic ZrO con- x 2 2 are inhibited by H ( 0.7 reaction order) and are propor- tent, and fewer inaccessible WO species within WO clus- 2 x 3 tional to butane concentration− (1.0 reaction order). Butane ters in the oxidized materials (table 8). Santiesteban and coworkers also report that their most isomerization selectivity is decreased by the competitive metal-catalyzed hydrogenolysis of butane, which is pro- active WOx±ZrO2 catalyst contains few Brùnsted acid sites (4 µmol H+/g-cat) based on the amount of 2,6- motedbyH2 (table 9) [10]. Hydrogenolysis does not occur dimethylpyridine needed to prevent n-pentane isomeriza- on WOx±ZrO2 samples without Pt, but these samples are tion reactions [51]. They conclude that these minority less active and deactivate more rapidly, apparently by car- 13 Brùnsted acid sites are 104 times more active in pentane bon deposition [54]. C isotopic tracer experiments during isomerization reactions than Brùnsted acid sites present butane isomerization reactions on Pt/WOx±ZrO2 show that in β-zeolite. The mechanism for n-pentane activation on after initiation by a slow dehydrogenation step, bimolecular these catalysts involves a C±H bond activation-initiation reactions lead to C8 intermediates, which then undergo β- step leading to ole®ns, which can be used in carbocation scission to form isomer products [55]. This mechanism is chain reactions. It appears that a small amount of 2,6- similar to that proposed earlier for butane isomerization on dimethylpyridine titrates the strongest acid sites that are re- SOx±ZrO2 catalysts [56,57]. Addition of n-propylamine quired to activate pentane and initiate these chain processes. (98 µmol amine/g-cat) completely suppresses butane iso- 2,6-dimethylpyridine may titrate, however, both Lewis and merization activity, but does not in¯uence hydrogenolysis Brùnsted acid sites at the temperature they used to disperse reactions requiring Pt sites [54], suggesting that the number this basic molecule (573 K). Thus, a greater number of acid of acid sites able to activate butane at 573 K on Pt/WOx± 98 D.G. Barton et al. / Solid acid catalysts on tungsten oxides

Table 9 reaction conditions, because the density of these sites dur- Effect of platinum on the isomerization of n-butane on WOx±ZrO2 catalysts [10]. ing catalysis can differ signi®cantly from those before or after reaction. a b Pt/WOx±ZrO2 Pt/WOx±ZrO2 WOx±ZrO2 Hydrogen transfer reactions play a critical role in alkane (acac) (std) isomerization elementary steps and bene®t from the pres- W-loading (wt.% W) 12 12 12 ence of H2 dissociation sites and Brùnsted acid sites during Pt-Loading (wt.% Pt) 1.2 1.2 0 hydroisomerization reactions. The addition of a metal func- Surface area (m2/g) 44 45 36 CO/Pt adsorption 0.31 0.30 tion to supported WOx acid catalysts leads to active and cButane isomerization 0.66 0.25 0.024 stable alkane hydroisomerization catalysts. Bifunctional rate (µmol/g-cat.s) n-heptane isomerization reactions at 623 K on oxygen- Butane hydrogenolysis 0.26 0 0 modi®ed WC and Pt/WOx±Al2O3 catalysts proceed via rate (µmol/g-cat.s) alkene intermediates that provide effective diffusive path- H2 kinetic order 0.8 0.6 − − ways for the kinetic coupling of steps occurring on dehy- n-C4 kinetic order 0.9 1.0 % coke at 1.5 h 0.013 0.066 0.072 drogenation and acid sites. n-Heptane isomerization reactions at lower temperatures require alternative pathways a Pt impregnation with platinum acetylacetonate after high temperature b for coupling these dehydrogenation and acid sites, because oxidation of WOx±ZrO2 (1096 K). Pt impregnation with PtCl6 prior to c high temperature oxidation of WOx±ZrO2 (1096 K). [573 K, 67.5 kPa low equilibrium alkene concentrations minimize the role of H2, 33.8 kPa n-C4, 2.4% maximum conversion]. bifunctional pathways involving gas phase alkenes. Hydro- gen transfer reactions on Pt/WOx±ZrO2 catalysts provide an ZrO2 in the presence of H2 may larger than the number able alternate and more effective pathway for communication to activate pentane at 483 K on WOx±ZrO2 (4 µmol/g-cat). between dehydrogenation and acid sites and lead to high alkane hydroisomerization rates and selectivities. Quasi- equilibrated hydrogen transfer steps decrease the surface 8. Summary residence time of intermediates and lead to desorption of isomers before undesired β-scission events occur on acid Solid acid catalysts based on supported WOx clusters sites. In addition, hydrogen dissociation at low tempera- are active and stable for isomerization, dehydration, and tures leads to higher Brùnsted acid site densities by migra- cracking reactions. Brùnsted acid sites form on WOx tion of hydrogen adatoms from Pt to WOx clusters. clusters when a lower valent element replaces W6+ or when W6+ centers reduce slightly during catalytic reactions. WOx clusters of intermediate size provide the bal- Acknowledgement ance between reducibility and accessibility required to max- + The authors would like to acknowledge ®nancial sup- imize the number of surface H species in WOx±ZrO2,zirconium tungstate, and oxygen-modi®ed WC catalysts. In port for this work from the National Science Foundation (CTS-9510575). We would also like to thank Prof. Gus- WOx±ZrO2 and zirconium tungstate catalysts, these WOx clusters of intermediate size form after oxidative treatments tavo Fuentes (UAM-Iztapalapa, Mexico) and Dr. George Meitzner (Edge Analytical) for helpful insights, techni- that sinter ZrO2 crystallites and increase WOx surface den- cal discussions, and research collaborations related to the sity. In oxygen-modi®ed WC catalysts, these WOx clusters form after oxygen chemisorption of high surface area WC UV-visible and X-ray absorption spectroscopic studies described in this review article. X-ray absorption experiments powders. 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