93. Solid Acid Catalysts Based on Supported Tungsten Oxides (Barton,98) TopCatal

8/2/2019 93. Solid Acid Catalysts Based on Supported Tungsten Oxides (Barton,98) TopCatal

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Topics in Catalysis 6 (1998) 8799 87

Solid acid catalysts based on supported tungsten oxidesDavid G. Barton a, Stuart L. Soled b and Enrique Iglesia a,

a Department of Chemical Engineering, University of California at Berkeley, Berkeley, CA 94720, USAE-mail: [email protected]

b Corporate Research Laboratory, Exxon Research and Engineering, Route 22 East, Annandale, NJ 08801, USA

Supported WO x clusters are active and stable catalysts for isomerization, dehydration, and cracking reactions. Brnsted acid sitesform on WO x clusters when a lower valent element replaces W 6+ or when W 6+ centers reduce slightly during catalytic reactions. WO xclusters of intermediate size provide the balance between reducibility and accessibility required to maximize the number of surface H +

species in WO x ZrO 2 , zirconium tungstate, and oxygen-modied WC catalysts. H 2 is involved in the generation and maintenance of Brnsted acid sites during catalytic reactions on WO x clusters.

Keywords: tungsten oxides, solid acids, acid-catalyzed reactions

1. Introduction

Many research groups continue to explore the catalyticproperties of oxide-based strong solid acids in an attempt toeliminate environmental concerns caused by the use, regen-eration, and disposal of liquid acids and halide-containingsolids [1,2]. The challenge of obtaining acid site strengthand density comparable to those in sulfuric acid, halides,or oxyhalides remains a formidable one; the smaller elec-

tronegativity differences in metal-oxygen bonds comparedwith metal-halide bonds limit the achievable acid strength.In oxide-based solid acids, protons balance net negativecharges produced by the replacement of a high valent cationwith one of lower valence or by the attachment of an anionto the surface of a neutral oxide support. In all cases, the netnegative charge is balanced by protons. Among strong solidacids, sulfated and tungstated zirconia and supported het-eropolyacids show interesting catalytic behavior and haveattracted signicant attention. Their mesoporous structuresallow more facile transport of reactants and products thanthe restricted pore structures in microporous acids and mini-

mize undesired side reactions favored by diffusional restric-tions and long intrapellet residence times.Effective solid acids require that metal cations or metal

oxide clusters interact with oxide supports to form struc-tures that stabilize the protons responsible for Brnstedacidity. These isolated metal-oxygen pairs or metal ox-ide clusters must resist sintering and decomposition duringhydrocarbon reactions. The recent intense interest in sul-fated zirconia appears to arise from the ability of zirconia toestablish this environment for the sulfate ion. Yet, seriousconcerns remain about the long-term stability of zirconia-supported sulfate species in reducing and oxidizing envi-ronments that are typical of hydrocarbon reactions and of regeneration treatments [3].

Tungsten oxide species dispersed on zirconia supports To whom correspondence should be addressed.

(WO x ZrO 2) by impregnation with a solution of tungstateanions and subsequent oxidation treatments at high tem-peratures (8731173 K) show strong acidity. Hino andArata suggested, based on color changes of Hammett in-dicators (H 0 14 .52), that acid sites stronger than 100%sulfuric acid may exist [4]; however, the use of this tech-nique to measure acid strength of solids is often unreli-able [5]. These authors also reported that WO x ZrO 2 cat-alyzes the isomerization of n -pentane at low temperatures,

but found high selectivity to cracked products ( 50%)even at low conversions [4]. Butene dimerization studieslater conrmed that acid sites on WO x ZrO 2 give catalyticrates and selectivities comparable to those on SO x ZrO 2for this reaction [6]. The addition of a metal component(e.g., < 1 wt.% Pt) to WO x ZrO 2 and the presence of H 2in the reactant mixture lead to active, stable, and selectivecatalysts for n -alkane isomerization reactions [712].

The structural richness of tungsten oxide species leadsto several atomic arrangements that can exhibit the strongacidity required for reactions of alkanes at low tempera-tures. 12-Tungstophosphoric acid (H 3PW12O40) and WO x ZrO 2 appear to contain the strongest acid sites amongtungsten oxide-based materials reported in the literature.The ability to delocalize a negative charge among severalsurface oxygen atoms in heteropolytungstate or isopoly-tungstate clusters allows hydrogen atoms in OH surfacegroups and hydrocarbons interacting with such OH groupsto retain cationic character, if not as a stable species, atleast during the formation of short-lived intermediates oractivated complexes required in acid catalysis. In 12-tungstophosphoric acid, the negative charge of the PO 34anion is shared throughout a neutral (WO 3)12 shell and isbalanced by protons accessible to reactants at the surfaceof this shell [13,14].

In WO x ZrO 2 catalysts, WO x clusters of intermediatesize delocalize a net negative charge caused by the slightreduction of W 6+ centers in reactant environments con-taining H 2 or hydrocarbons [9]. This temporary charge

J.C. Baltzer AG, Science Publishers


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88 D.G. Barton et al. / Solid acid catalysts on tungsten oxides

imbalance leads to the formation of Brnsted acid (WO 3)m{W6 n O3}{n -H+ } centers on the zirconia support. TheseBrnsted acid sites on WO x ZrO 2 appear to form via a

mechanism similar to that on heteropolytungstate clusters,but in this case, the charge imbalance is not permanent; itis reversible and created in-situ by the reducing environ-ment of the acid-catalyzed reaction. WO x ZrO 2 catalystshave an advantage over heteropolytungstates in that theyretain structural integrity during high temperature oxidativetreatments ( > 873 K), in contrast with heteropolytungstatesthat decompose to bulk WO 3 at much lower temperatures( 673 K).

In this review paper, we discuss several important ex-amples of solid acids based on tungsten oxides with em-phasis on the role of support and surface structures on therate and selectivity of acid-catalyzed reactions. We alsodescribe some recent unpublished results from our groupwith regard to the structure and isomerization pathways onWO x ZrO 2 and Pt/WO x ZrO 2 catalysts.

2. Early studies of acid catalysts based on tungstenoxides

Catalysts containing tungsten oxides have been used assolid acid catalysts since the turn of the century. They werethe subject of an extensive review of their catalytic prop-erties about thirty years ago [15]. In early studies, these

materials were shown to be active catalysts for hydrocrack-ing, dehydrogenation, isomerization, reforming, alcohol de-hydration, and olen oligomerization reactions. Bulk WO 3was reported to be an active catalyst for several of thesereactions, but high temperatures were required because of its low surface area and weak acid sites [15]. The densityand strength of acid sites and the rate of catalytic reactionson tungsten oxide catalysts increase with slight reductionof the W6+ centers, which often occurs during catalytic re-actions in reducing hydrocarbon environments. In addition,the more effective dispersion of WO 3 species on high sur-face area supports and the introduction of a heteroatom inorder to create a permanent charge imbalance in the WO 3structure increase the density and the strength of the acidsites.

Sabatier rst reported that yellow WO 3 crystallites werereadily reduced during alcohol reactions above 523 K to astoichiometry corresponding to WO 2. 5 [16]. This slightlyreduced blue oxide was more active than WO 3 and wasfound to re-oxidize in air at room temperature [16]. Sincethis discovery, many studies have conrmed the enhancedcatalytic activity of reduced bulk WO 3 for a variety of reac-tions. The reversible reduction-oxidation process and colorchange of bulk WO 3 is similar to that observed for hy-drogen tungsten bronzes (H x WO3) prepared by exposing

physical mixtures of Pt metal and bulk WO 3 to H2 at roomtemperature [17]. H 2 dissociation on Pt metal sites leads tothe slight reduction of WO 3 at room temperature by H-atomdiffusion from Pt sites to WO 3 interstices.

At low temperatures, bulk WO 3 catalyzes metathesis re-actions of alkenes via a mechanism that involves coordi-nation of alkenes to W 6+ Lewis centers [18]. Such path-

ways lead to reaction products that differ signicantly fromthose formed in oligomerization-cracking reactions typicalof Brnsted acid sites. The rate of propene metathesis reac-tions on WO 3 crystallites increases with increasing disper-sion of these crystallites, which may be achieved by impreg-nation of aqueous solutions of ammonium paratungstate onhigh surface area supports (SiO 2, TiO 2, Al2O3 , or ZrO2)[18]. Highly dispersed WO 3 species (WO x ) with greaterBrnsted acid site density can also be achieved by addinganother metal oxide that forms mixed-oxide materials suchas WO 3P2O5, WO3MoO 3, ZrW28 O0. 53 . 5, or heteropoly-tungstates. The role of heteroatoms in intimate mixtures oras an oxide support can be two-fold: to increase the disper-sion of WO x clusters and to introduce a permanent chargeimbalance that stabilizes protons and carbocationic specieson the surface of these clusters.

3. Tungstated alumina catalysts (WO x Al 2O 3)

In 1937, Bliss and Dodge reported that dispersing WO xspecies on Al 2O3 enhanced the activity of bulk WO 3 cata-lysts for the hydration of ethylene [19]. More recent stud-ies have addressed the structure of the WO x species andits role in establishing the density and strength of acid sites

for acid-catalyzed reactions of hydrocarbons [2026].WO x species appear to interact strongly with sites on thesurface of -Al2O3. These surface species inhibit the sinter-ing of Al 2O3 crystallites and the structural transformationof -Al2O3 into -Al2O3 during high temperature oxida-tion treatments [20]. For example, a pure -Al2O3 sampleshows a BET surface area of only 10 m 2 /g after oxidationin air at 1323 K, but a sample containing 6 wt.% W has asurface area of 50 m 2 /g. The resulting isolated species orsmall clusters of WO x reduce in H 2 at much higher temper-atures than bulk WO 3 or WO x clusters supported on silicaor zirconia.

WO x surface coverage on Al 2O3 supports increases withincreasing temperature of oxidative pre-treatments whichsinter the Al 2O3 crystallites. Titration of the Brnsted acidsites with sterically hindered 2,6-dimethylpyridine showsthat the Brnsted acid site density increases with increas-ing oxidation temperature (table 1) [24]. Lewis acid sitedensity, measured by difference using 3,5-dimethylpyridinemolecules that titrate both Lewis and Brnsted acid sites,decreases with increasing oxidation temperature (table 1)[24]. This increase in Brnsted acidity with increasingWO x surface coverage appears to be related to the con-version of isolated tungstate groups into polytungstate clus-ters, which can delocalize protons among neighboring WO x

species. The appearance of polytungstate clusters as theoxidation temperature increases was detected in Ramanspectra by the appearance of WOW bands with increas-ing WO x coverage [22]. At WO x coverages ( > 3.2 W-


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D.G. Barton et al. / Solid acid catalysts on tungsten oxides 89

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

Catalyst Oxidation Brnsted acid sites a Lewis acid sites b

temperature (K) ( mol/m 2 ) ( mol/m 2)

-Al2O3 773 0 1.5110% WO 3Al2O3 773 0.14 1.3610% WO 3Al2O3 1223 0.38 1.13

Y-zeolite 773 1.8 0.47

a Number of Brnsted acid sites on WO x Al2O3 calculated from 2,6-dimethylpyridine uptake minus theuptake of Al 2 O3 . b Number of Lewis acid sites calculated as the difference between 3,5-dimethylpyridineuptakes and number of Brnsted acid sites.

atoms/nm 2) where polytungstate clusters exist, W-L I X-ray absorption near-edge spectra (XANES) resemble thespectrum of the distorted tungsten oxide octahedral sym-metry in bulk WO 3 [23]. At low WO x coverages (1.4W-atoms/nm 2), the spectra resemble the spectrum of thetungsten oxide tetrahedral symmetry in Al 2(WO 4)3, whichsuggests that isolated tetrahedral tungstate groups are re-sponsible for the enhanced Lewis acidity at low WO x cov-erage [23].

Highest gas-oil cracking rates were reported on WO x Al2O3 catalysts with surface coverages of about 2.4 W-atoms/nm 2 [24]. This surface density appears to lead topolytungstate clusters containing a sufcient number of WOW linkages for effective delocalization of the H +

species among neighboring close-packed WO x species, butwhich retain a high density of surface WO x groups requiredfor a high acid site density [24]. The rate and selectivity forthe isomerization of isobutane and 2-methyl-2-pentene andthe thermal desorption spectra of NH 3 and lutidine probemolecules show that WO x Al2O3 catalysts contain mod-erately strong Brnsted acid sites, intermediate in strengthbetween those on amorphous silicaalumina catalysts andon Y-zeolite [24,25]. WO x Al2O3 catalysts retain their ac-tivity for gas-oil cracking after high temperature hydrother-mal treatments, whereas cracking rates on silicaaluminaand zeolite catalysts decrease markedly after similar treat-ments (table 2) because of signicant loss of surface area.

The introduction of a metal function (0.3 wt.% Pt) onWO x Al2O3 (8.4 wt.% W) leads to bifunctional alkane

conversion reactions, in which metal sites catalyze alkanedehydrogenation and acid sites catalyze isomerization and -scission reactions of alkenes [26]. Isomerization of 3,3-dimethylpentane at 623 K on these catalysts leads to a

Table 2Effect of steaming on activity for East Texas Light gas-oil cracking [783 K,

303 kPa total, 283 kPa H 2O, WHSV oil = 1. 4 h 1] from [25].

Catalyst Gas-oil cracking activity (cm 3 gas/g-cat h)

Fresh catalyst Catalyst steamed at 1144 Kfor 16 h

10% WO 3Al2O3 0.45 0.41Silicaalumina 0.46 0.13

(Davison DA-1)Zeolite 0.75 0.24

(Davison FLX-5)

Table 3n -Heptane isomerization product selectivity and rate on a Pt/WO x Al2O3(0.3 wt.% Pt, 8.4 wt.% W) and oxygen-modied WC (O 2 chemisorbed at

800 K) catalysts [623 K, 4.4 kPa n -heptane, 96 kPa H 2] from [26].

Pt/WO x Al2O3 WC/O-800 K

n -Heptane conversion (%) 13.4 12.9Pt-Turnover rate (s 1) 0.55 0.17

Reaction selectivity (%)Hydrogenolysis 47.9 16.3Isomerization 35.5 79.5Cyclization 13.5 0.7Dehydrogenation 3.1 3.5

Cracking pattern (%)12 (terminal) 15.7 32.823 22.0 30.434 (Mid-molecule) 62.4 36.8

Product selectivity (%)2-Methylhexane 14.9 37.3

3-Methylhexane 17.2 37.4Ethylpentane 1.9 3.7Dimethylpentanes 1.5 1.1Alkylcyclopentanes 7.6 0.4Toluene 5.9 0.3

high ratio of (2,3) to (2,2) isomers among initial reactionproducts, consistent with isomerization that occurs predom-inately by methyl-shift rearrangements on acid sites andnot by metal-catalyzed cyclic isomerization pathways [26].Acid-catalyzed -scission is the predominant reaction of n -heptane at 623 K (table 3). Slow hydride transfer reac-tions to surface intermediates lead to long surface residence

times and a to high probability that they undergo -scissionreactions before they desorb as isomers.

4. Heteropolytungstates

Many studies have addressed the catalytic properties of heteropolytungstate clusters with Keggin structures, such as12-tungstophosphoric acid (H 3PW12O40), and excellent re-view articles have described recent advances [13,14]. ThePW12O3

40 anion (i.e., the conjugate base) contains thesmallest negative charge among the different heteropolyan-ions with stable Keggin structures (SiW 12O4

40 , BW12O540 ,

GeW 12O440 , etc.). As a result, the corresponding acid form

shows the strongest acidity and catalyzes reactions such ashydration of olens, dehydration of alcohols, aromatic alky-lation, conversion of methanol, oxygenation of alkenes, and


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alcoholysis of epoxides [14]. 12-Tungstophosphoric acidconsists of a central tetrahedral PO 34 anion encapsulatedwithin a neutral (WO 3)12 shell, on the surface of which

three hydrogens with a net positive charge maintain theelectroneutrality of the cluster. Thus, 12-tungstophosphoricacid can be thought of as an encapsulated form of phos-phoric acid, which avoids the undesired properties of itsliquid form. Similarly, the Si and B forms of heteropoly-tungstates become encapsulated forms of silicic and boricacids, which are much weaker than phosphoric acid inthe liquid phase. Keggin structures corresponding to se-questered sulfuric acid (H 2SW12 O40) are not available, ap-parently because the size and coordination symmetry of the central sulfur atom is inconsistent with a stable Keg-gin structure. If stable, H 2SW12O40 clusters with Kegginstructures would provide stronger acid sites than currentlyavailable heteropolytungstate clusters.

The large and soft anionic clusters in Keggin structuresprovide the electron delocalization required for charge sep-aration and for the formation of cationic complexes dur-ing acid-catalyzed reactions. Heteropolyacid salts, whichare produced by partial exchange of the protons in theparent acid with alkali or other cations, form solid acidswith morphological and solubility properties very differ-ent from those of the parent acid [27]. For example, par-tial exchange of 12-tungstophosphoric acid with Cs + ionsconverts the water-soluble low surface area ( < 5 m2 /g)acid form into a water-insoluble precipitate. This porous

solid reaches surface areas exceeding 100 m2

/g after about80% of the H + species have been replaced by Cs + cations(Cs2. 5H0. 5W12O40). These acidic Cs salts were reportedto be much more active per gram of catalyst than the H-form for alkylation of m -xylene and 1,3,5-trimethylbenzenewith cyclohexene [28] and to contain slightly stronger acidsites as measured by a 2-methyl-2-pentene model reaction[29] (table 4). Other acid salts with large cations, suchas Rb, K, and NH 4, also form small ( 100 A ) water-insoluble precipitates with high surface areas and excellentcatalytic properties, even though a large fraction of the hy-drogens have been replaced by inactive alkali ions [13,27].H3PW12O40 contains a high density of protons located pre-dominantly within dense crystallites that are accessible only

Table 4Catalytic activity for alkylation and acidity of the H 3PW12O40 and

Cs2. 5H0. 5PW12O40 catalysts [28,29].

H3PW12 O40 Cs2. 5H0. 5PW12O40

Surface area (m 2 /g) 6 131Acid amount ( mol/g) a 816 161Acidity parameter 0.6 1.3(3MP2/4MP2) b

Activity (mmol/g-cat h)c

m -Xylene 1.4 24.7Trimethylbenzene 26.2 1.4

a Measured by TPD of NH 3 . b 2-Methyl-2-pentene isomerization selec-tivity ratio [523 K, 1 h]. c Initial rates of alkylation with cyclohexene[327 mmol x -xylene, 216 mmol 1,3,5-trimethylbenzene, 10 mmol cyclo-hexene, 373 K].

to polar reactants, which tend to swell the structures anddiffuse through the enlarged intracrystalline voids. Theseswollen structures of the acid forms of heteropolyoxoanionshave been described as pseudo-liquid media by Misonoand co-workers [30]. The insoluble acid salts appear to besuperior catalysts because they contain a higher permanentsurface area accessible to both polar and non-polar mole-cules and because their insoluble nature prevents the unde-sired dissolution and elution of soluble acid forms duringcatalytic reactions of polar molecules. This leaching of cat-alytic species causes long-term stability and environmentalcontamination problems.

Impregnation of aqueous solutions of heteropolyacids oninert oxide supports can be used to attain the protonic formof the heteropolyacid Keggin structure with a small crys-tal size that increases the accessible external surface area

[3133]. In this manner, the full density of protons in theparent acid can be retained and sites can remain accessibleto non-polar reactants that normally contact only the ex-ternal protons in crystalline heteropolyacids. Supports thatinteract weakly with Keggin clusters (e.g., SiO 2) are pre-ferred, because strongly interacting supports such as Al 2O3can lead to hydrolysis and decomposition of the clusters atlow temperatures [29]. The limitations of these methods forsupporting water-insoluble heteropolyacid salts have beenrecently overcome [29]. These new methods involve thesynthesis of SiO 2 supported heteropolyacid Cs-salts by re-placing silanol hydrogens (SiOH) with Cs + (SiOCs) athigh pH and then impregnating the Cs

2OSiO

2sample with

an aqueous solution of H 3PW12 O40 . The heteropolyacidprecipitates as the acid salt upon contact with Cs + cationson the surface of SiO 2 to form highly dispersed anchoredclusters with Cs 2. 5H0. 5PW12 O40 stoichiometry.

5. Oxygen-modied tungsten carbides

High surface area stoichiometric tungsten carbides (WC)can be prepared by direct carburization of WO 3 withCH4 /H2 mixtures (1100 K), followed by a high temper-ature H 2 treatment (973 K, 0.8 h) to remove polymericcarbon that can block carbidic surface sites [34]. Alkanedehydrogenation and hydrogenolysis turnover rates on tung-sten carbides are similar to those reported on supported no-ble metal catalysts [35]. Chemisorption of oxygen on WCsurfaces inhibits the rate of these reactions and introducesBrnsted acid surface sites similar to those present in sup-ported tungsten oxides [34,36]. Such acid sites catalyzehydroisomerization of alkanes only when tungsten carbidesites are also present.

These oxygen-modied tungsten carbides are bifunc-tional; they catalyze alkane dehydrogenation on WC sitesand alkene rearrangements on WO x surface species [26].WO x sites on the carbide surfaces also catalyze methanol

dehydration and propylene oligomerization and cracking re-actions [37], but they do not catalyze propylene metathesisreactions that occur on Lewis acid sites present on WO x Al2O3 and WO x SiO 2 catalysts. It appears that Lewis acid


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sites provided by W 6+ centers are converted into Brnstedacid sites on oxygen-modied WC by interactions of WO xwith spill-over hydrogen atoms formed by H 2 or CH dis-

sociation on WC sites [38].Oxygen-modied tungsten carbides catalyze n -heptaneisomerization with high selectivity ( > 70%) at 623 K (ta-ble 3) [26]. Kinetic and isotopic tracer data show that thereaction proceeds via sequential n -heptane dehydrogenationand heptene isomerization steps. At 623 K, n -heptane con-version rates are limited by rearrangements of intermediateheptenes on WO x acid sites, but n -heptane dehydrogena-tion becomes increasingly rate-limiting as reaction temper-ature is increased. Isomer distributions on oxygen-modiedWC are similar to those found on Pt/WO x Al2O3 and theinitial isomers formed from 3,3-dimethylpentane reactionsare consistent with methyl-shift isomerization pathways (ta-ble 3). The WC function catalyzes the rapid exchange of deuterium atoms into unreacted n -heptane and its reactionproducts at 623 K, suggesting that hydrogen adsorption-desorption steps are quasi-equilibrated during n -heptanereactions. The high selectivity towards cleavage of ter-minal CC bonds indicates that smaller alkanes form inhydrogenolysis reactions on WC sites, rather than by theacid-catalyzed -scission cracking pathways that accountfor the low isomerization selectivity on Pt/WO x Al 2O3 cat-alysts. It appears that the more intimate contact betweenH2 dissociation/alkane dehydrogenation sites and Brnstedacid sites in oxygen-modied WC samples leads to shorter

carbocation residence times and to a lower probability of -scission reactions than on Pt/WO x Al2O3. At lowerreaction temperatures, thermodynamics favor the desiredmultibranched alkanes, but the lower equilibrium alkeneconcentrations make bifunctional pathways involving gasphase alkenes very slow. Stronger WO x acid sites such asthose on Pt/WO x ZrO 2 catalysts and a chain mechanismthat avoids the requirement for gas phase olen intermedi-ates lead to signicantly higher alkane hydroisomerizationrates and selectivities at low temperatures. Pt/WO x ZrO 2catalysts for n -heptane hydroisomerization reactions are de-scribed in detail in a later section of this review.

6. Zirconium tungstate catalysts

Zirconium tungstate solids are used as inorganic cation-exchangers, but their catalytic properties have not beenwidely studied. Ethylene hydration rates on amorphous zir-conium tungstate (ZrW 28O0. 53 . 5) powders are similar on avolumetric basis to those reported on supported phosphoricacid catalysts used in commercial practice [39] (table 5).An active catalyst with a W/Zr ratio of 2.0 was preparedby mixing aqueous Na 2WO 4 and ZrOCl 2 solutions at roomtemperature to give a white gel-like precipitate, which was

acidied using HCl before ltering. Filtered precipitateswere oxidized at 723 K for 7 h to produce an amorphouspale-yellow solid with high surface area (100 m 2 /g) [40].These materials catalyze near equilibrium conversion of

Table 5Catalytic activity of HCl washed zirconium tungstate catalysts for vaporphase ethylene hydration [493 K, 101 kPa, H 2O/C2 H4 = 1. 0, SV =

2300 h 1].

Catalyst W/Zr (atom ratio) Ethanol concentration (wt.%)

Zirconium tungstate 1.0 0.60Zirconium tungstate 2.0 0.90Zirconium tungstate 3.0 0.80H3PO4 /SiO2 0.45

ethylene to ethanol at 553 K after an induction period. Ap-parently, the slow partial reduction of the W 6+ centers, aprocess detected from stoichiometric conversion of ethanolto acetaldehyde during the induction period, leads to in-creased Brnsted acidity in these materials.

Procedures for the synthesis of zirconium tungstate are

similar to those used in the synthesis of supported WO x ZrO 2 catalysts. Zirconium tungstates, however, contain sig-nicant amounts of residual chloride ions, very high WO xconcentrations, and they are typically oxidized at temper-atures below ZrO 2 crystallization temperatures. The na-ture of the acid sites on zirconium tungstate has not beencarefully examined, but it appears that it contains Brnstedacid sites similar to those on WO x ZrO 2. In both types of catalysts, active species consist of amorphous polytungstatespecies that become Brnsted acids only after in-situ partialreduction during catalytic reactions. The residual electron-withdrawing chloride ions in zirconium tungstate powders,may increase the acid strength, but can cause corrosion andcontamination problems by eluting as HCl during catalyticreactions.

7. Isopolytungstate clusters suported on zirconia(WO x ZrO 2)

WO x ZrO 2 powders were rst reported to containstrong acid sites by Hino and Arata [4]. They reportedlow temperature n -butane and n -pentane isomerizationactivity on WO x ZrO 2 catalysts prepared by impregnat-ing ZrO x (OH) 4 2x with aqueous ammonium metatungstate(13 wt.% W) and subsequent high temperature oxida-tion (1073 K). n -Pentane isomerization rates decreasedmarkedly when metatungstate solutions were impregnatedon crystalline ZrO 2 supports instead of amorphous ZrO x(OH) 4 2x supports, suggesting that hydrated WO x speciescombine with ZrO x (OH) 4 2x surface sites and that suchsites are not available after crystalline tetragonal or mono-clinic ZrO 2 structures are formed. It appears that the initialformation of dispersed tungstate species occurs via anionexchange or condensation reactions of hydrated aqueoustungstate anions with surface OH species in ZrO x (OH) 4 2x .The resulting high dispersion of WO x species inhibits thecrystallization and sintering of the ZrO 2 support crystallites

and provides a material that can be sintered in a controlledmanner during subsequent oxidation treatments. The sinter-ing of the ZrO 2 support crystallites leads to the formationof WO x clusters of varying size and catalytic properties.


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Figure 1. The effects of oxidation temperature on n -pentane isomerization[0.5 g catalyst, 13 wt.% W, 553 K, helium 10 cm 3 /min, 1 l n -C5 pulse]

(adopted from [4]).

n -Pentane isomerization rates depend strongly on thetemperature at which impregnated samples are treated inair (gure 1) [4]. High isomerization rates occur in a nar-row range of high oxidation temperatures (9001250 K),during which loss of surface occurs and WO x surface den-sity increases. When n -pentane isomerization reactions arecarried out in helium on these catalysts, selectivity to crack-ing products is high ( 50%) and the catalysts deactivaterapidly.

The addition of small concentrations of a metal com-ponent ( < 0.5 wt.% Pt) to WO x ZrO 2 catalysts and thepresence of H 2 in the reactant stream lead to very active,selective, and stable catalysts for n -alkane hydroisomeriza-tion at 473 K [8,9,12]. The metal function increases the rateof hydrogen transfer to isomerized intermediates, causingtheir desorption as isoalkanes before undesired -scissionreactions occur. In addition, metal sites appear to be in-volved in the formation of Brnsted acid sites via spilloverof hydrogen adatoms from metal sites to WO x species dur-ing the initial stages of the reaction. In the absence of H 2and HH dissociation sites, isomerization still occurs butvia the sacricial use of reactants to form surface hydro-gen species. The desorption of the H species as H 2 duringhydrocarbon reactions, however, leaves unsaturated hydro-carbon fragments permanently attached to the surface andleads to fast deactivation and to a limited number of cat-alytic turnovers.

In our studies, WO x ZrO 2 samples were prepared usingpreviously reported synthesis methods [4], but over a widerrange of composition and oxidation temperatures. High

surface area ZrO x (OH) 4 2x was prepared via hydrolysis of ZrOCl 2 solution using slow addition of NH 4OH to raise thepH to a value of 10. After removing residual chloride ionsby washing, the precipitate was dried and impregnated to in-

Figure 2. The effects of oxidation temperature on BET surface area andcrystal modication of the zirconia support (7.9 wt.% W) [12].

cipient wetness with a solution of ammonium metatungstate((NH 4)6H2W12 O40) and then oxidized at high temperatures(8731100 K) in dry air. Pt (0.3 wt.%) was introducedinto some of the oxidized WO x ZrO 2 samples by incipientwetness with a solution of tetraammine platinum hydroxide,followed by decomposition in air at 723 K, and reductionin H2 at 523 K.

BET surface area and X-ray diffraction measurements

show that WO x surface species inhibit zirconia sinteringand its tetragonal to monoclinic phase transformation dur-ing the oxidative treatments required for the synthesis of active catalysts. ZrO x (OH) 4 2x powders show high surfaceareas (289 m 2 /g) after drying at 383 K. At 1073 K, a typ-ical oxidation temperature leading to WO x ZrO 2 sampleswith high activity, pure ZrO 2 has a very low surface area(4 m2 /g) and consists of large monoclinic crystals. Impreg-nation with near monolayer WO x surface coverages (6 W-atoms/nm 2) results in higher surface areas (51 m 2 /g) andsmall tetragonal ZrO 2 crystallites after oxidation at 1073 K.Three-dimensional growth of bulk WO 3-like species be-comes detectable by X-ray diffraction and UV-visible mea-surements as this saturation coverage is exceeded. Tung-sten surface densities above 10 W-atoms/nm 2 lead to linescorresponding to bulk WO 3 in X-ray diffraction patterns.

The structure and size of ZrO 2 crystallites can be con-trolled by adjusting the oxidation temperature and theW concentration. At a given W concentration (e.g.,7.9 wt.% W), the BET surface area and the tetragonalcontent in ZrO 2 decrease markedly with increasing oxida-tion temperature (gure 2). WO x species inhibit the sin-tering and the transformation to monoclinic ZrO 2 crystal-lites, but as the oxidation temperature increases tetrago-nal ZrO 2 crystallites ultimately agglomerate and transform

into monoclinic crystallites when a critical crystallite di-ameter is reached [41]. At a given oxidation temperature(e.g., 1073 K), surface area and tetragonal content increasewith increasing W-loading up to 7.9 wt.% W (gure 3)


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Figure 3. The effects of W loading on BET surface area and crystalmodication on the zirconia support (1073 K oxidation) [12].

and then remain constant. It appears that ZrO x (OH) 4 2xpowders contain an intrinsic number of strong binding sitesfor metatungstate ions ( 8 wt.% W), which must be oc-cupied in order to inhibit locally crystallite growth andre-crystallization into the thermodynamically stable mon-oclinic phase [12]. Impregnation with metatungstate ionsbeyond this required surface coverage leads to weakly heldtungstate groups, which agglomerate into WO x clusters andbulk WO 3 crystallites without additional stabilization of the

ZrO 2 support.Control of the W concentration and of the temperatureof oxidation treatments can be used in order to design cat-alysts with a specic WO x surface density and a high sur-face area. For example, WO x ZrO 2 samples containingsimilar WO x surface densities, but very different total sur-face areas, were prepared by impregnation and controlledsynthesis procedures. A sample with 22 wt.% W oxidizedat 823 K had a total surface area of 140 m 2 /g, but the samesurface density (5.0 W-atoms/nm 2) as a 7.9 wt.% W sam-ple oxidized at 1073 K and exhibiting a much lower surfacearea (51 m 2 /g).

Acid-catalyzed skeletal isomerization of o-xylene to m -and p-xylene (523 K, 0.67 kPa o-xylene, 106 kPa H 2) wasused in our study in order to examine the acid properties of WO x ZrO 2 catalysts. Xylene isomerization reactions pro-ceed on Brnsted acid sites via protonation-deprotonation of xylenes with intervening intramolecular methyl shifts thatform meta and para isomers before desorption. The selec-tivity to disproportionation products (toluene and trimethyl-benzenes) was very low ( < 5%), suggesting that bimole-cular isomerization pathways [42] are not involved in xy-lene isomerization reactions on WO x ZrO 2 catalysts at ourreaction conditions. WO x ZrO 2 samples with varying W-loadings (221 wt.% W) and treated at several oxidation

temperatures (8231100 K) were examined using o-xyleneisomerization reactions.

o-Xylene isomerization turnover rates (per W-atom) passthrough a maximum value (0.0017 s 1) at intermediate W

Figure 4. Initial o -xylene isomerization turnover rates at several oxida-tion temperatures and W-loadings in gradientless glass recirculating batch

reactor [523 K, 104 kPa H2, 0.66 kPa o -xylene] [12].

concentrations (12 wt.% W) for samples treated in air at1073 K. At each W-loading (5, 12 or 21 wt.% W), isomer-ization turnover rates also pass through a maximum valueas the oxidation temperature increases (gure 4). The ox-idation temperature at which this maximum value occursdecreases with increasing W-loading. These data suggestthat an intermediate WO x surface density may be requiredfor high densities of Brnsted acid sites. When o-xyleneisomerization turnover rates are plotted as a function of WO x surface density for all WO x ZrO 2 catalysts tested,they merge into a single curve with a sharp maximum at

about 10 W-atoms/nm2

(gure 5). This WO x surface den-sity represents about twice the theoretical monolayer cov-erage of WO x octahedra on a ZrO 2 surface. The narrowrange of WO x surface densities over which WO x ZrO 2samples become active catalysts suggests that high isomer-ization turnover rates require the presence of WO x clusterswithin a narrow range of cluster size, which appear to formselectively as a coverage of 10 W-atoms/nm 2 is reached.These clusters appear to be large enough to delocalize anet negative charge caused by the slight reduction of W 6+

with H 2 leading to the formation of W 6 n O3(n -H+ ) cen-

ters on the zirconia surface, but small enough to containthe resulting protons predominantly at crystallite surfacesrather than at inaccessible intracrystallite positions.

The requirement for slight reduction of W 6+ centersin WO x clusters of intermediate size is consistent withthe strong effect of H 2 on the rate of o-xylene isomer-ization on WO x ZrO 2 catalysts. The addition of dihydro-gen (101 kPa) increases steady-state o-xylene isomeriza-tion rates by more than a factor of four on WO x ZrO 2at 523 K, suggesting the activation and involvement of H 2during monofunctional acid-catalyzed reactions on WO x ZrO 2 [12]. The absence of H 2 leads to the irreversibleloss of active sites, apparently as the result of H 2 desorp-tion and the concomitant destruction of Brnsted acid sites,

formed initially by the sacricial use of hydrogen atomsfrom reactants during o-xylene reactions. Desorption of such H-atoms causes the adsorption of the unsaturated co-product of the initial activation reaction to become irre-


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Figure 5. The inuence of WO x surface density on o -xylene isomerizationturnover rates [523 K, 104 kPa H 2 , 0.66 kPa o -xylene] [12]. The verticaldotted line marks the theoretical monolayer capacity of tetragonal ZrO 2 .

versible. Slow exchange of deuterium during reactions of o-xylene/D 2 mixtures shows that H 2 does not participate inhydrogen transfer elementary steps during xylene isomer-

ization, but it is instead involved in the infrequent regenera-tion of acid sites lost in hydrogen desorption side reactionsduring o-xylene isomerization [12]. These Brnsted acidsites consist of W 6 n O3(n -H+ ) centers, which are formedeither by hydrogen migration from HH or CH dissocia-tion sites to neutral WO x clusters.

Previously reported electronic spectra in the UV-visibleenergy range for WO x ZrO 2 samples and other WO x -containing samples [9] are consistent with the growth of WO x clusters as the WO x surface density increases. AllWO x ZrO 2 samples show a maximum absorbance near4.84.9 eV, which corresponds to ligand-to-metal chargetransfer transitions. UV-visible spectra show that isolatedWO x species are not present in any WO x ZrO 2 sample af-ter 1073 K oxidation and that all samples contain signicantconcentrations of polymeric WOW groups. A secondband that appears at low energies (2.9 eV) in the spec-tra of samples with high WO x surface densities ( > 10 W-atoms/nm 2) shows that WO 3 crystallites with different elec-tronic properties can co-exist with amorphous WO x clusters[9].

The low-energy absorption edge in UV-visible electronicspectra is sensitive to cluster size in the 110 nm rangefor many semiconductors, including WO 3 [43]. In thissize range, the large fraction of atoms residing at the sur-

face alters the cluster electronic properties. As a result,the energy gap between the valence and conduction bandsincreases with decreasing cluster size. This energy gapranges from 2.8 eV for crystalline bulk WO 3 to 6.22 eV

Figure 6. Direct band gap energy from UV-visible spectra of WO x ZrO 2catalysts (1073 oxidation) overlaid on o -xylene isomerization rate data

from gure 4 (adopted from [9]).

for the HOMO-LUMO gap in isolated WO 24 (aq.) ions.Optical absorption edge energies for WO x ZrO 2 samplesoxidized at 1073 K were obtained from UV-visible spectrausing procedures based on direct transitions between va-lence and conduction bands [44]. Direct band gap energies

decrease monotonically from 4.15 to 3.80 eV as the WOx

surface density increases from 3.1 to 7.5 W-atoms/nm 2 andthen become constant at higher surface densities ( > 7.5 W-atoms/nm 2) (gure 6) [9]. This monotonic decrease in theband gap energy suggests that average WO x cluster sizeincreases with increasing WO x surface density in a rangewhere marked increases in o-xylene isomerization rates alsooccur. The spectra of samples with high WO x surface den-sities (> 10 W-atoms/nm 2), however, show two distinctdirect band gaps (absorption edges). The low-energy ab-sorption edge reects the presence of WO 3 crystallites withbulk-like electronic properties, which co-exist with WO xclusters of intermediate size. The high-energy absorptionedge position remains constant (3.8 eV) for all WO x ZrO 2catalysts within the range of WO x surface densities lead-ing to high o-xylene isomerization turnover rates (615 W-atoms/nm 2) (gure 6). Thus, UV-visible spectra suggestthat WO x clusters with intermediate size, dened as thosewith absorption edges similar to ammonium metatungstate(3.5 eV), lead to WO x ZrO 2 samples with maximum ac-tivity.

X-ray absorption near edge spectroscopy at the W-L Iedge (12100 eV) is sensitive to the local coordination andsymmetry of the W-atom absorbers. The pre-edge featureat 5 eV reects a 2s 5d electronic transition that is

not allowed in octahedral (centrosymmetric) structures, butwhich occurs in tetrahedral or distorted octahedral struc-tures because of dp orbital mixing [45]. Previously, thispre-edge feature was used to detect the presence of 4- and


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Figure 7. W-L I X-ray absorption near-edge spectra after dehydration (He,673 K) for WO x ZrO 2 (221 wt.% W) and Na 2 WO4 and WO 3 reference

materials.

6-coordinated W atoms on WO x Al2O3 [23]. The degreeof W coordination on Al 2O3 supports increases with in-creasing concentration of WO x species and with the extentof hydration of the catalyst samples [23].

Our studies show that W atoms in WO x ZrO 2 samplesreside in distorted octahedral environments similar to thosepresent in bulk WO 3 at all WO x concentration and pre-treatment conditions. Figure 7 shows the W-L I absorption

edges after dehydration (in He at 673 K) for several WO x ZrO 2 samples with different W concentrations oxidized at1073 K (221 wt.% W) and for two standards: Na 2WO4(tetrahedral) and WO 3 (distorted octahedral) bulk powders.All WO x ZrO 2 samples show strikingly similar near-edgeX-ray absorption spectra. Clearly, none of the WO x ZrO 2samples have W-atoms in the tetrahedral symmetry of theNa2WO 4 standard, but instead have W-atoms with distortedoctahedral environments, similar to those found in bulk WO 3 crystallites.

Temperature-programmed reduction of supported metaloxides in hydrogen is also sensitive to the cluster size andcoverage of surface oxides [46]. For example, isolatedWO x species on Al 2O3 reduce at much higher tempera-tures than bulk WO 3 crystallites [21]. Reduction peaks forWO x ZrO 2 samples are very broad (600 K) and reductionoccurs at much lower temperatures than for WO x Al2O3samples at similar WO x surface densities. The reductionof WO x clusters of intermediate size on ZrO 2 occurs attemperatures typical of those required for the reduction of heteropolytungstate clusters (gure 8) [47]. Whereas, thereduction of WO x species at low WO x coverage is stronglyinuenced by the ZrO 2 support and occurs at higher tem-peratures than bulk WO 3 . The larger WO x clusters presentat higher WO x surface densities delocalize the initial net

negative charge caused by reduction and lead to speciesthat reduce at lower temperatures; these species then act asH2 dissociation centers that increase the rate of subsequentreduction steps. The ability to delocalize the net negative

Figure 8. The inuence of WO x surface density on the tempera-ture programmed reduction proles of WO x ZrO 2 samples [10 K/min,

80 cm3 /min of 20% H 2 /Ar mixture, 0.080.02 g] [9].

charge on the conjugate base may be linked to both thestrong acidity that appears near saturation WO x coverageon ZrO 2 and to the easier initial reduction of WO x clustersas surface coverage and cluster size increase. It appears

that a critical cluster size is required for both reduction andBrnsted acidity, because the appearance of the latter mayrequire the incipient reduction of neutral WO 3 clusters byH-atoms.

The addition of a metal function (0.3 wt.% Pt) to WO x ZrO 2 leads to the selective hydroisomerization of C

+7 alka-

nes at 373473 K, without the extensive cracking ob-served on Pt/SO x ZrO 2 and on zeolites promoted by metals[8,9,48]. At 473 K, n -heptane isomerization turnover ratesare similar on Pt/WO x ZrO 2 and Pt/SO x ZrO 2 (based onW and S-atoms, respectively), but Pt/WO x ZrO 2 is sig-nicantly more selective to heptane isomers (85% at 50%n -heptane conversion). At higher reaction temperatures,Pt/WO x ZrO 2 catalysts are signicantly more active thanPt/SO x ZrO 2 samples, because they are stable against de-activation by sublimation, decomposition, and sintering andthus retain high isomerization selectivities (71% at 523 Kand 36% conversion) and rates [9].

n -Heptane isomerization kinetic and isotopic tracer dataon Pt/WO x ZrO 2 and Pt/SO x ZrO 2 are consistent with iso-merization reactions that proceed via chain reactions in-volving hydrogen transfer from alkanes, H 2, or hydridedonors, such as adamantane, to adsorbed isomerized hep-tanes [9,48]. This mechanism contrasts that discussed pre-viously for n -heptane isomerization on oxygen-modied

WC and Pt/WO x Al2O3 catalysts, in which alkene inter-mediates form on the dehydrogenation function, diffuse toand isomerize on the WO x acid-function. At the highertemperature (623 K vs. 473 K) required for signicant iso-


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Scheme 1.

Table 6n -Heptane isomerization selectivity, reaction kinetics, and isotopic tracerdata on Pt/WO x ZrO 2 [0.3 wt.% Pt, 12.7 wt.% W, Pt/WO x ZrO 2 oxida-

tion at 723 K after WO x ZrO 2 oxidation at 1073 K].

Isomerization selectivity (%) a 89n -Heptane reaction order b 0.9H2 reaction order b 0. 50Adamantane reaction order a,c 0.05Deuterium content in n -heptane (%) d 82Deuterium content in methylhexanes (%) d 83

a [473 K, 650 kPa H 2 , 100 kPa n -heptane, 50% n -heptane conversion].b [473 K, 8002900 kPa H 2 , 33200 kPa n -heptane]. c [01.0 mol.%adamantane]. d [343 K, 3.7 kPa n -heptane, 97 kPa D 2 , 5% n -heptaneconversion].

merization rates on WC and WO x Al2O3 catalysts, equi-librium alkene concentrations are much higher and providean effective diffusive pathway for communication betweenthe two types of sites required in bifunctional isomerizationpathways. On Pt/WO x ZrO 2 and Pt/SO x ZrO 2 (at 473 K),alkenes are not detected in the gas phase and hydrogentransfer reactions via surface diffusion of hydrogen providean alternate and more effective pathway for communicationbetween hydrogenation sites and acid sites.

On Pt/WO x ZrO 2, n -heptane adsorption-desorption stepsare quasi-equilibrated and surface isomerization steps limitthe overall rate (scheme 1). As a result, facile hydrogentransfer steps decrease the surface residence time of inter-mediates and lead to desorption of isomers before unde-sired -scission events occur on the acid sites. Isomeriza-tion turnover rates are rst order in n -heptane and zero tonegative order in H 2 (table 6), suggesting that surface iso-merization is rate-limiting and that hydrogen transfer stepsdecrease the coverage of surface intermediates. Hydrogentransfer occurs primarily using hydrogen adatoms formedby H2 dissociation on surface Pt atoms, which spilloveronto WO x centers (scheme 2). These hydrogen transfer

(C7H15 )(W 6+ )n 1(W5

+ )O3n + HPt x (W6+ O3)n + C7H16(g) + Ptx

Scheme 2.

steps are inhibited on Pt/SO x ZrO 2 catalysts by the dif-cult dissociation of H 2 on Pt clusters poisoned by sulfurspecies, which leads to high cracking selectivity.

Addition of hydrogen transfer co-catalysts, such asadamantane, increase n -heptane isomerization rate and se-lectivity on Pt/SO x ZrO 2, but not on Pt/WO x ZrO 2, be-cause hydrogen transfer steps are quasi-equilibrated on thelatter and do not benet from co-catalysts that increasethe rate of these reactions [9]. Mixtures of n -C7H16 D2 quickly reach HD equilibrium and lead to completescrambling and binomial deuterium distribution in unre-acted n -heptane and in all reaction products, conrm-ing the reversible (quasi-equilibrated) nature of n -heptaneadsorption-desorption steps on Pt/WO x ZrO 2 [9].

Near-edge X-ray absorption studies and temperature-programmed reduction studies have detected the reductionof W6+ centers during hydrogen treatment at 473673 K[9,12]. Site titration studies using H 2, O2 , and CO showthat hydrogen uptake exceeds monolayer coverages on sur-face Pt atoms and that O 2 uptake is signicantly higherthan that of H 2 (table 7), suggesting that hydrogen adatomsspillover onto reduced WO x centers during H 2 treatmentsat typical isomerization reaction temperatures. At higherreduction temperatures, H 2 uptakes markedly decrease andO2 uptakes increase; these effects appear to be related todecoration of Pt crystallites by oxygen-decient WO x , asalso proposed to explain similar effects in TiO 2 supportedmetal particles [49]. The spillover of hydrogen adatoms

appears to involve the formation of acidic hydrogens onthe WO x clusters (scheme 3). Hydrogen adatoms formedby homolytic dissociation of H 2 on Pt sites lose electrondensity to Lewis acid WO x clusters, which delocalize thereduced charge and stabilize the protons. Thus, hydrogenadatoms become involved not only in the desorption of ad-sorbed intermediates, but also in the generation and main-tenance of Brnsted acid sites. The formation of Brnstedacid sites during H 2 treatment has been directly observed byinfrared measurements of pyridine adsorption on Pt/SO x ZrO 2 by Ebitani et al. [50]. They report that the densityof Brnsted acid sites increases during contact with H 2 attemperatures typical of those required for their catalyticaction. The density of Lewis acid sites concurrently de-

Table 7Irreversible chemisorption uptake a of H2 , O2 , and CO on Pt/WO x ZrO 2

(0.3 wt.% Pt, 10 wt.% W).

Reduction H/Pt O/Pt CO/Pttemperature (K)

473 0.63 1.25 0.30673 0.092 2.04 0.050

a Room temperature adsorption; irreversibly chemisorbed species are de-ned as those that cannot be removed by evacuation at room temperature.

HH + (W6+ O3)n (H+ )2(W6

+ )n 2(W5+ )2O3n

Scheme 3.


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Table 8Inuence of the preparative method on the structure, activity, and acidity of WO x ZrO 2

catalysts oxidized at 1098 K [51].

Impregnation a Reux-impregnation b Co-precipitation c

W-loading (wt.% W) 15.0 16.9 15.5

Surface area (m 2 /g) 32 62 62

% tetragonal ZrO 2 30 10 1

Bulk WO 3 present? yes yes nodNumber of strong 0.002 0.002 0.004Brnsted acid sites(meq H + /g)ePentane isomerization acid 2.8 3.9site turnover frequency (s 1)

a Prepared by metatungstate impregnation on pH 9 precipitated ZrO x (OH) 4 2x . b Preparedby reuxing ZrO x (OH) 4 2x overnight prior to metatungstate impregnation. c Preparedby coprecipitation of metatungstate and ZrOCl 2 solutions at pH 9. d 2,6-dimethylpyridine

adsorption results. e [483 K, 2 H 2 / n C5 mol ratio, LHSV = 2 cm 3 n C5 /ml-cath, 2400 kPa.]

creased, suggesting that Lewis acid sites are converted toBrnsted acid sites by contact with H 2. As a result, onlytechniques that measure the Brnsted acid site density un-der catalytic conditions provide an accurate value of theBrnsted acid site density available for reaction.

Several recent patents describe the synthesis and cat-alytic activity of WO x ZrO 2 and Pt/WO x ZrO 2 catalystsfor cyclohexane ring-opening and isomerization, benzenehydrogenation, alkene oligomerization, aromatic alkylationwith C +6 alkenes, toluene alkylation with methanol, aro-matic trans-alkylation reactions, selective catalytic reduc-tion of nitrogen oxides, and the hydrodesulfurization andhydrodenitrogenation of crude oils [11]. These authors re-port that maximum activity for pentane isomerization wasfound on WO x ZrO 2 samples (16 wt.% W, 1098 K oxi-dation) prepared by co-precipitation of ZrOCl 2 and ammo-nium metatungstate solutions, as opposed to impregnationof ammonium metatungstate on ZrO x (OH) 4 2x [51]. It ap-pears that a better dispersion of metatungstate ions on amor-phous ZrO x (OH) 4 2x is achieved during co-precipitationleading to higher surface area, lower monoclinic ZrO 2 con-tent, and fewer inaccessible WO x species within WO 3 clus-ters in the oxidized materials (table 8).

Santiesteban and coworkers also report that their mostactive WO x ZrO 2 catalyst contains few Brnsted acidsites (4 mol H + /g-cat) based on the amount of 2,6-dimethylpyridine needed to prevent n -pentane isomeriza-tion reactions [51]. They conclude that these minorityBrnsted acid sites are 10 4 times more active in pentaneisomerization reactions than Brnsted acid sites presentin -zeolite. The mechanism for n -pentane activation onthese catalysts involves a CH bond activation-initiationstep leading to olens, which can be used in carbocationchain reactions. It appears that a small amount of 2,6-dimethylpyridine titrates the strongest acid sites that are re-

quired to activate pentane and initiate these chain processes.2,6-dimethylpyridine may titrate, however, both Lewis andBrnsted acid sites at the temperature they used to dispersethis basic molecule (573 K). Thus, a greater number of acid

sites may exist that can catalyze methyl-shift reactions, butonly a few of those active sites may be involved in the CH bond activation-initiation step. In addition, the densityof Brnsted acid sites before (or after) catalytic reaction ismuch lower than those present during catalytic reactions of hydrocarbons. As a result, the density of Brnsted acid sitesmeasured by amine adsorption will underestimate the den-sity present at reaction conditions, because Brnsted acidsites cannot survive in the absence of H 2 or hydrocarbonreactants.

Recent studies of the thermal desorption of alcoholsshow that acid sites on WO x ZrO 2 catalysts have similaracid strength as H-mordenite [52]. Infrared measurementsof adsorbed pyridine show that WO x ZrO 2 samples retainpyridine on their surface to higher temperatures than onWO x Al2O3 , ZrO2, or Al2O3 samples [53]. This pyridineadsorption study also shows qualitatively that WO x ZrO 2samples that contain Pt have a higher density of Brnstedacid sites than those without Pt in an inert diluent [10].

On Pt/WO x ZrO 2, butane isomerization rates (573 K)are inhibited by H 2 ( 0.7 reaction order) and are propor-tional to butane concentration (1.0 reaction order). Butane

isomerization selectivity is decreased by the competitivemetal-catalyzed hydrogenolysis of butane, which is pro-moted by H 2 (table 9) [10]. Hydrogenolysis does not occuron WO x ZrO 2 samples without Pt, but these samples areless active and deactivate more rapidly, apparently by car-bon deposition [54]. 13C isotopic tracer experiments duringbutane isomerization reactions on Pt/WO x ZrO 2 show thatafter initiation by a slow dehydrogenation step, bimolecularreactions lead to C 8 intermediates, which then undergo -scission to form isomer products [55]. This mechanism issimilar to that proposed earlier for butane isomerization onSOx ZrO 2 catalysts [56,57]. Addition of n -propylamine

(98 mol amine/g-cat) completely suppresses butane iso-merization activity, but does not inuence hydrogenolysisreactions requiring Pt sites [54], suggesting that the numberof acid sites able to activate butane at 573 K on Pt/WO x


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Table 9Effect of platinum on the isomerization of n -butane on WO x ZrO 2 cata-

lysts [10].

aPt/WO x ZrO 2 bPt/WO x ZrO 2 WO x ZrO 2(acac) (std)

W-loading (wt.% W) 12 12 12Pt-Loading (wt.% Pt) 1.2 1.2 0Surface area (m 2 /g) 44 45 36CO/Pt adsorption 0.31 0.30cButane isomerization 0.66 0.25 0.024rate ( mol/g-cat.s)Butane hydrogenolysis 0.26 0 0rate ( mol/g-cat.s)H2 kinetic order 0. 8 0. 6n -C4 kinetic order 0.9 1.0% coke at 1.5 h 0.013 0.066 0.072

a Pt impregnation with platinum acetylacetonate after high temperatureoxidation of WO x ZrO 2 (1096 K). b Pt impregnation with PtCl 6 prior tohigh temperature oxidation of WO x ZrO 2 (1096 K). c [573 K, 67.5 kPaH2 , 33.8 kPa n -C4 , 2.4% maximum conversion].

ZrO 2 in the presence of H 2 may larger than the number ableto activate pentane at 483 K on WO x ZrO 2 (4 mol/g-cat).

8. Summary

Solid acid catalysts based on supported WO x clustersare active and stable for isomerization, dehydration, andcracking reactions. Brnsted acid sites form on WO x

clusters when a lower valent element replaces W6+

orwhen W 6+ centers reduce slightly during catalytic reac-tions. WO x clusters of intermediate size provide the bal-ance between reducibility and accessibility required to max-imize the number of surface H + species in WO x ZrO 2, zir-conium tungstate, and oxygen-modied WC catalysts. InWO x ZrO 2 and zirconium tungstate catalysts, these WO xclusters of intermediate size form after oxidative treatmentsthat sinter ZrO 2 crystallites and increase WO x surface den-sity. In oxygen-modied WC catalysts, these WO x clustersform after oxygen chemisorption of high surface area WCpowders. Distorted octahedral WO x clusters of intermedi-ate size on ZrO 2 were detected by X-ray absorption andUV-visible spectroscopies and differ markedly in electronicstructure, reduction behavior, and isomerization rate andselectivity from tungstate groups supported on Al 2O3 andfrom bulk WO 3 catalysts.

The unique acid properties on WO x ZrO 2 catalysts ap-pear to be related to their ability to form W 6 n O3(n -H+ ) centers under reducing conditions at low temperatures.These centers can stabilize carbocationic reactive interme-diates by delocalizing the corresponding negative chargeamong several oxygen atoms. This delocalization of chargein supported WO x clusters is similar to the permanent delo-calization of negative charge throughout the (WO 3)12 shell

in 12-tungstophosphoric acid. Hydrogen is involved in thegeneration and maintenance of Brnsted acid sites duringcatalytic reactions on WO x clusters. As a result, Brnstedacid site density measurements must be carried out under

reaction conditions, because the density of these sites dur-ing catalysis can differ signicantly from those before orafter reaction.

Hydrogen transfer reactions play a critical role in alkaneisomerization elementary steps and benet from the pres-ence of H 2 dissociation sites and Brnsted acid sites duringhydroisomerization reactions. The addition of a metal func-tion to supported WO x acid catalysts leads to active andstable alkane hydroisomerization catalysts. Bifunctionaln -heptane isomerization reactions at 623 K on oxygen-modied WC and Pt/WO x Al2O3 catalysts proceed viaalkene intermediates that provide effective diffusive path-ways for the kinetic coupling of steps occurring on dehy-drogenation and acid sites. n -Heptane isomerization re-actions at lower temperatures require alternative pathwaysfor coupling these dehydrogenation and acid sites, becauselow equilibrium alkene concentrations minimize the role of bifunctional pathways involving gas phase alkenes. Hydro-gen transfer reactions on Pt/WO x ZrO 2 catalysts provide analternate and more effective pathway for communicationbetween dehydrogenation and acid sites and lead to highalkane hydroisomerization rates and selectivities. Quasi-equilibrated hydrogen transfer steps decrease the surfaceresidence time of intermediates and lead to desorption of isomers before undesired -scission events occur on acidsites. In addition, hydrogen dissociation at low tempera-tures leads to higher Brnsted acid site densities by migra-tion of hydrogen adatoms from Pt to WO x clusters.

Acknowledgement

The authors would like to acknowledge nancial sup-port for this work from the National Science Foundation(CTS-9510575). We would also like to thank Prof. Gus-tavo Fuentes (UAM-Iztapalapa, Mexico) and Dr. GeorgeMeitzner (Edge Analytical) for helpful insights, techni-cal discussions, and research collaborations related to theUV-visible and X-ray absorption spectroscopic studies de-scribed in this review article. X-ray absorption experimentswere performed at Stanford Synchrotron Radiation Labo-

ratory (SSRL) supported, in part, by US Department of Energy.

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