Structural and catalytic characterization of solid acids based on zirconia modified by tungsten oxide

Journal of Catalysis 181, 57–72 (1999)

Article ID jcat.1998.2269, available online at http://ww w .idealibrar y .com on

Structural and Catalytic Characterization of Solid Acids Based

on Zirconia Modiﬁed by Tungsten Oxide

,1

David G. Barton, Stuart L. Soled,† George D. Meitzner,‡ Gustavo A. Fuentes,§ and Enrique Iglesia

Department of Chemical Engineering, University of California at Berkeley, Berkeley, California 94720; †Corporate Research Laboratory,

Exxon Research and Engineering, Route 22 East, Annandale, New Jersey 08801; ‡Edge Analytical, Inc., 2126 Allen Blvd.,

Middleton, Wisconsin 53562; and §Departamento de Ingenieria de Procesos, Universidad Autonoma

Metropolitana-Iztapalapa, A.P. 55-534, 09340 Mexico, D.F., Mexico

Received February 2, 1998; revised August 28, 1998; accepted August 31, 1998

BACKGROUND

Tungsten oxide species form strong acid sites on ZrO₂supports

Current catalytic processes for alkane isomerization and

alkylation require liquid acids (e.g., H₂SO₄, HF, AlCl₃)

or halogen-promoted metal oxides (e.g., Cl–Al₂O₃), which

pose signiﬁcant containment, corrosion, and environmen-

tal challenges (1). These problems can be circumvented

by the development of strong solid acid catalysts consist-

ing of metal oxides. The development of solid acids that

are stable, regenerable, and active at the low temperatures

required for favorable isomerization thermodynamics re-

mains a formidable challenge.

Recently, metal oxides promoted by sulfur compounds

have been studied as strong solid acid catalysts (2, 3), espe-

cially SO_x- promoted zirconia (4) containing iron or man-

ganese oxides as promoters (5) or noble metals to inhibit

deactivation (6–8). Yet, their low isomerization selectivity

for C₇₊alkanes, their poor stability, and their tendency to

form volatile sulfur compounds during catalysis and regen-

eration limit their applicability in isomerization and alky-

lation processes.

Tungsten oxide-based materials comprise another inter-

esting class of solid acids. The small and highly charged

W⁺⁶cations are found in several solid acids: heteropoly-

oxoanion compounds, such as 12-tungstophosphoric acid

(9), oxygen-modiﬁed tungsten carbides (10), and WO_xdis-

persed on -Al₂O₃(11) and ZrO₂(12–19). This paper fo-

cuses on WO_xdispersed on ZrO₂(WO_x–ZrO₂), ﬁrst re-

ported to be strongly acidic by Hino and Arata (20). These

authors suggested, based on color changes by Hammett in-

dicators, that acid sites stronger than 100% sulfuric acid

and inhibit ZrO₂crystallite sintering and tetragonal to monoclinic

structural transformations. W-L_IX-ray absorption near-edge spec-

tra suggest that the W centers are in a distorted octahedral oxygen

environment, even after dehydration at 673 K, in all WO_x–ZrO₂

samples (2–21 wt.% W) oxidized at 1073 K. Maximum o-xylene iso-

merization turnover rates (per W atom) on WO_x–ZrO₂solids occur

at WO_xsurface densities (10 W nm ²) that exceed the theoretical

monolayer capacity of ZrO₂. Similar turnover rates are obtained on

2

WO_x–ZrO₂samples with similar WO_xsurface densities (W nm

)

over a large range of oxidation temperatures (773–1223 K) and

WO_xconcentrations (5–21 wt.% W). UV–visible spectra suggest an

increase in WO_xdomain size with increasing surface density. High

isomerization turnover rates appear to require the presence of WO_x

domains of intermediate size on ZrO₂surfaces. WO_xdomains of in-

termediate size appear to provide a compromise between reducibil-

ity and accessibility of WO_xcenters. These domains are necessary

to delocalize a temporary charge imbalance that forms Brønsted

acid sites in the presence of H₂and stabilizes carbocation interme-

diates. The presence of H₂during o-xylene isomerization increases

turnover rates and prevents rapid deactivation. Slow D₂/o-xylene

exchange reactions indicate that H atoms from H₂are not frequently

involved in the activation or desorption of xylenes. H₂is required,

however, in order to reverse the occasional desorption of H atoms

duringo-xyleneisomerization reactions. Thesedesorption processes

lead to the destruction of Brønsted acid sites by the formation of

strongly adsorbed unsaturated species in the absence of H₂. After

promotion with Pt (0.3 wt.%), WO_x–ZrO₂solids catalyze n-heptane

isomerization in the presence of H₂at 400–500 K with much higher

selectivity than sulfated oxides or zeolitic acids at similar turnover

rates. On Pt/WO_x–ZrO₂, efﬁcient hydrogen transfer steps prevent

extensive cracking of adsorbed carbocations by limiting their sur-

c

face lifetimes.

1999 Academic Press

may exist (H₀

14.52), but the use of this technique as

a measure of acid strength on solids is generally unreli-

able (21). Butene dimerization studies have conﬁrmed that

acid sites on WO_x–ZrO₂have catalytic activity compara-

ble to those on SO_x–ZrO₂(14). Hino and Arata (12) re-

ported low-temperature catalytic activity for n-pentane and

n-hexane isomerization, but cracking selectivities were very

high (>50% ) even at low conversion. The addition of a

¹To whom correspondence should be addressed. Fax: (510) 642-4778.

E-mail: iglesia@cchem.berkeley.edu.

57

0021-9517/99 $30.00

c

Copyright

1999 by Academic Press

All rights of reproduction in any form reserved.

58

BARTON ET AL.

metal component (<1% wt. Pt) to WO_x–ZrO₂catalysts and (OH)_{4 2x, x<2}) solids with ammonium metatungstate solu-

the presence of H₂resulted in very active and selective cata- tions. High-surface-area ZrO_x(OH)₄was prepared by

2x

lysts for n-alkane isomerization (16–18, 22) and cycloalkane hydrolysis of a 0.5 M zirconyl chloride solution (ZrOCl₂

ring-opening (15) reactions with excellent stability and high 8H₂O, Aldrich, >98 wt.% , Hf <0.5 wt.% , 500 cm³h ¹feed)

selectivity to isomers.

using the controlled addition of NH₄OH (14.8 N) to obtain

Acid-catalyzed alkane isomerization has become in- a constant pH of 10. This pH was chosen based on previous

creasingly important in the synthesis of high-octane blend- studies (25–27), in which the mesoporosity, surface area,

ing components and lubricant basestocks. Currently, iso- and total pore volume of ZrO_x(OH)_{4 2x}were shown to in-

merization of heptane and larger alkanes (C₇₊) is limited crease with increasingprecipitation pH. The precipitate was

by high selectivity to cracking products and by poor yields immediatelyﬁltered and washed repeatedlybyredispersion

of high-octane multibranched alkanes containing quater- in a NH₄OH solution (pH 10) in order to remove residual

nary carbons. Long carbocation surface residence times chloride. After each wash, a couple ofdropsofa 3M AgNO₃

during C₇₊alkane isomerization and cracking pathways solution were added to the ﬁltrate liquid until it showed no

that directly form stable tert-butyl surface carbocations visible signsofAgClformation (<10ppm residualCl ). The

favor cracking reactions on strong acids (23). In practice, ZrO_x(OH)₄

gel was dried at 383 K overnight, crushed,

2x

undesired cracking reactions limit parafﬁn isomerization and sieved to retain particles with 0.12–0.38 mm diameter.

processes to C₄–C₆alkanes. The dried particles were impregnated to the point of in-

Iglesia et al. (24) have shown that high isomerization se- cipient wetness (0.29 cm³H₂O per 1.0 g ZrO_x(OH)₄

)

2x

lectivity (>70% ) in n-heptane reactions can be obtained with ammonium metatungstate ((NH₄)₆H₂W₁₂O₄₀, Strem

on oxygen-modiﬁed tungsten carbide catalysts at high tem- Chemicals, 99.9% ) solutions to give 2–21 wt.% W after ox-

peratures ( 623 K). These catalysts behave as bifunctional idation treatments. The impregnated samples were dried at

metal–acid catalysts with a WC dehydrogenation function 383 K overnight and oxidized in ﬂowing dry air at tempera-

and a WO_xacid function, but they require high tempera- tures between 773 and 1273 K. All samples were oxidized in

tures ( 623 K), which favor hydrogenolysis pathways and shallow quartz boats inside a 25-mm-diameter quartz tube

monobranched isoheptanes. In another study, Iglesia et al. with ﬂowing air (Linde Zero Grade, 3.3 cm³s ¹), dried and

(6) reported n-heptane isomerization results at lower tem- puriﬁed with Drierite and 13X molecular sieve traps. The

peratures (473 K) on a stronger acid (Pt/SO_x–ZrO₂). They samples were heated at a rate of 0.167 K s ¹to the ﬁnal tem-

found predominately isobutane and propane among crack- perature (measured in a quartz thermowell just above the

ing products ( 50% total cracking selectivity at low con- sample bed), held for 3 h, and cooled to room temperature

versions 10% ), consistent with acid-catalyzed cracking in ﬂowing air. After oxidation at 1073 K, a portion of the

of heptyl cations. The addition of hydride transfer cocata- sample containing 12 wt.% W was impregnated to the point

lysts showed that slow hydride transfer to surface carbo- of incipient wetness with a solution of tetra-amine platinum

cations limits isomerization rates and leads to long carbo- hydroxide (Degussa), dried at 383 K overnight, and oxi-

cation surface lifetimes and high cracking selectivity (6). dized for 3 h at 723 K (0.3 wt.% Pt). Nominal tungsten and

These studies along with Hino and Arata’s discovery of the platinum contents (after oxidative treatments) were based

unique properties of WO_x–ZrO₂led us to study the syn- on precursor decomposition weight losses obtained from

thesis and characterization of these catalysts promoted by thermogravimetric analysis.

Pt for n-heptane isomerization reactions (Pt/WO_x–ZrO₂).

This catalyst promoted by Pt couples a strong acid com-

ponent active at low temperatures with clean noble metal

Chemisorption and Physisorption Measurements

particles that increase hydride transfer rates (18). Here, we

examine the structural stabilization of the zirconia support

and the nature ofthe surface WO_xgroups. We also probe the

effects of H₂on n-heptane and o-xylene isomerization, and

use the turnover rates for the “monofunctional” isomeriza-

tion of o-xylene in order to compare WO_x–ZrO₂samples

with a wide range of WO_xsurface densities.

Chemisorption data were collected on a Quantachrome

1C Autosorb apparatus. Samples were dried in a box fur-

nace at 723 K in air immediately before loading. Pt/WO_x–

1

ZrO₂samples were heated in H₂at 0.033 K s to the ﬁ-

nal reduction temperature (either 473 or 823 K) and held

isothermally for 1 h. After evacuation, the samples were

cooled to 313 K, and an 11-point H₂adsorption isotherm

was obtained between 2.7 and 53 kPa. The linear portion

of the isotherm was extrapolated to zero pressure to obtain

the combined (total) uptake. After evacuation at 313 K, the

isotherm wasagain measured;the linear portion wasextrap-

olated to zero pressure to obtain the weakly adsorbed frac-

EXPERIMENTAL

Synthesis of WO_x–ZrO₂Catalytic Solids

WO_x–ZrO₂samples were prepared by incipient wet- tion of the total initial uptake, and the strong chemisorption

ness impregnation of zirconium oxyhydroxide (ZrO_xuptake wasobtained bydifference. After thismeasurement,

ZIRCONIA AND TUNGSTEN OXIDE

59

the catalyst was again reduced at the original reduction tem- Thermal Analysis

perature, and oxygen uptakes were obtained at 313 K. CO

Weight changes during sample oxidation (0.030–0.100 g)

were measured by thermal gravimetry (Mettler TA-2000C).

uptakes at 313 K were measured on a fresh sample pre-

treated as described above. A similar procedure was used

to determine reversible H₂chemisorption uptakes at 523 K

on WO_x–ZrO₂samples (without Pt) reduced at 523 K, ex-

cept the uptakes are reported at 45 kPa H₂, which results

in a better estimate of the amount of reactive surface H

species present during reaction.

Physical surface areas were obtained by N₂physisorption

using a Quantasorb surface area analyzer (Quantachrome

Corporation) and a standard multipoint BET isotherm (28).

Samples previously oxidized at high temperatures (>473 K)

were degassed in ﬂowing N₂for 1 h at 473 K; other samples

were degassed for 3 h at 383 K before N₂physisorption

measurements.

Weight changes were measured at a heating rate of 0.167 K

1

s

in a 20% O₂/He stream. Heat changes as a result of

crystallization of amorphous zirconia were measured with

a differential scanning calorimeter (Dupont 2100, 0.167 K

s

¹), using samples (0.025–0.030 g) sealed in aluminum con-

tainers and -Al₂O₃samples as reference.

Temperature-programmed reduction studies were car-

ried out in a Quantasorb surface area analyzer (Quan-

tachrome Corporation) modiﬁed with electronic mass ﬂow

meters, a programmable furnace, and on-line 13X molecu-

lar sieve traps in order to remove the water formed during

reduction. A 20% H₂/Ar mixture (Matheson UHP, certiﬁed

mixture) ﬂowing at 1.33 cm³s ¹was used to reduce samples

(0.025–0.200 g) held within a 4-mm-I.D. quartz u-tube cell

containing a quartz thermowell directly in contact with the

sample bed. Reduction rates were obtained by thermal con-

ductivity measurements of the H₂concentration in the exit

stream as the sample temperature was raised from room

Powder X-Ray Diffraction Measurements

X-ray powder diffraction (XRD) patterns were recorded

using a Siemens D5000 Diffractometer with CuK radi-

ation and a scanning rate of 0.002 s ¹. These patterns

were used to quantify the relative amounts and crystal-

lite sizes of each ZrO₂phase (monoclinic and tetragonal)

and to detect the presence of bulk WO₃crystallites. The

1

temperature to 1400 K at 0.167 K s and held for 0.5 h.

The output analog signal was calibrated using the complete

reduction of a standard CuO powder (Aldrich, 99.995% )

in order to determine H₂consumption rates.

¯

ZrO₂diffraction features of interest are the (111) and (111)

reﬂections of the monoclinic phase and the (111) reﬂec-

tion of the metastable tetragonal phase. Since these re-

ﬂections overlap even in samples oxidized at high tem-

peratures, they were resolved using a model proposed by

Schreiner and Jenkins (29), which ﬁts both the instrumen-

tal and the wavelength-breadth components to asymmetric

Lorentzians. These pattern-resolved reﬂections, corrected

X-Ray Absorption Spectroscopy

X-ray absorption spectra were obtained using beamlines

4-1 and 4-3 at the Stanford Synchrotron Radiation Labora-

tory (SSRL). Spectra were measured in transmission mode,

with Ar in the ion chamber detectors. Monochromators on

both beamlines were equipped with Si(220) crystals, de-

tuned to 50% of the maximum inlet signal intensity in or-

der to minimize harmonics. Data points were obtained at

0.25 eV increments near the W-L_Iand W-L_IIIedges (12100

for K ₁–K

wavelength breadth and for instrumental

2

broadening, were used to calculate the tetragonal volume

percentage (V_t) and the tetragonal and monoclinic crystal-

lite sizes in these crystallographic directions.

and 10207 eV) and at 0.04 A ¹increments in the extended

˚

The integrated areas under the resolved reﬂections (I_t,

I_m) were used to calculate the intensity ratio (X_t) (Eq. [1])

and then V_tusing an empirical nonlinear relationship

(Eq. [2]) obtained by Toraya et al. (30) from physical mix-

tures of monoclinic and tetragonal ZrO₂. The Scherrer rela-

tionship (Eq. [3]) (31) was used to calculate crystallite sizes

(D_hkl) using the integral line widths (B_hkl) of the monoclinic

and tetragonal pattern-resolved reﬂections and a shape fac-

tor equal to one:

ﬁne structure (EXAFS) region. A Hf impurity ( 0.5 wt.% )

in ZrO₂limited the EXAFS to the range between the W-L_III

(10207 eV) and Hf-L_III(10739 eV) edges.

Allcatalyst sampleswere diluted with SiO₂to 2.2wt.% W,

pressed into wafers, crushed, and sieved to retain particles

with 0.18–0.25 mm diameter. Sieved powders were packed

into quartz capillaries (0.9 mm diameter with 0.1 mm

wall thickness) and supported horizontally in a rectangular

X-raybeam path (0.2mm 6.0mm). Four heat cartridgesin

a copper heat sink supplied heat uniformly to the capillary

cell. Samples were dehydrated and oxidized in air ﬂowing

through the capillary(0.167cm³s ¹, Matheson, Ultra Zero).

In processing the spectra, the background was removed

I_t(111)

X_t=

V_t=

[1]

¯

I_m(111) + I_m(111) + I_t(111)

1.311(1 X_t)

[2] by ﬁtting a straight line to the pre-edge region and a third-

order polynomial spline to the EXAFS region. The spectra

1.311 0.311X_t

S

were normalized to the step height, measured at the edge

energy. The edge energy is deﬁned as the energy at the ﬁrst

D_hkl

=

.

[3]

B_hklcos

60

BARTON ET AL.

inﬂection point for W-L_IIIspectra and as the ﬁrst inﬂec- using matrix techniques that account for natural ¹³C abun-

tion point beyond the pre-edge feature for W-L_Ispectra. dance and ion fragmentation kinetics (35).

The normalized EXAFS was converted to units of A ¹and

˚

1

˚

then Fourier transformed over the range 2–11.5 A (32).

RESULTS AND DISCUSSION

Small changes in the W-L_Inear-edge spectra were quanti-

ﬁed by subtracting a bulk WO₃reference spectrum from the ZrO₂Crystal Structure and Crystallite Size

sample spectra, using previously reported procedures (33).

Amorphous ZrO_x(OH)_{4 2x}powders treated in air above

723 K convert to crystalline ZrO₂with tetragonal, mono-

clinic, or cubic structures. At temperatures below 1443 K,

Catalytic Measurements

Kinetic studies were carried out in high-pressure ﬂow and monoclinic ZrO₂is the thermodynamically stable crystal

glass recirculating batch reactors described previously (34). structure for crystallites larger than 10 nm (36). Metastable

Catalyst samples were sieved to retain particles with diame- tetragonal ZrO₂crystallites smaller than 10 nm often form

ters between 0.25 and 0.50 mm. These samples were placed after oxidation at temperatures slightly above the ZrO₂

along with quartz powder of similar particle size into the crystallization temperature (723 K). The fraction of tetrag-

reactors. Before catalytic tests, samples were pretreated at onal ZrO₂crystallites formed is strongly inﬂuenced by the

773 K in ﬂowing air (8.33 cm³s ¹, Matheson, Ultra Zero) nature of the ZrO_x(OH)_{4 2x}precursors and by the methods

for 3 h and then reduced at reaction temperature in ﬂowing used in their synthesis (37). After oxidation at higher tem-

hydrogen (8.33 cm³s ¹, Matheson, UHP) for 1 h. Oxygen peratures (>900 K), ZrO₂crystallites sinter and become

and water impurities in H₂were removed using a Pd/Al₂O₃predominately monoclinic, regardless of precursor or syn-

catalyst (Johnson–Mathey) and a molecular sieve trap held thetic method. Pure tetragonal ZrO₂crystallites, however,

at ambient temperature. Gas chromatography (Hewlett– may be stabilized by impregnation with metal oxides before

Packard GC/MS 5890/5993, 50-m Ultra-2 column for C₇high-temperature oxidative treatments (38).

separations and 30-m DB-Wax column for xylene separa-

WO_xspecies strongly inﬂuence ZrO_x(OH)₄

crystal-

2x

tions) with ﬂame ionization and mass spectrometric detec- lization into ZrO₂. Impregnation of ZrO_x(OH)₄

with

2x

tion was used to measure product distributions. Reaction ammonium metatungstate solutions stabilizes tetragonal

ratesare reported asW atom turnovers, deﬁned asthe moles ZrO₂crystallites after high-temperature oxidative treat-

of reactant converted per W atom per second. Product se- ments (90% tetragonal for 7.9 wt.% W/ZrO₂vs 3% tetrag-

lectivities are reported on a carbon basis as the percentage onal for pure ZrO₂at 1073 K). This stabilization of tetrago-

of the converted reactant that appears as a given product.

nal ZrO₂crystallites is shown in Fig. 1a, which compares the

n-Heptane isomerization kinetic experiments were car- tetragonal ZrO₂content in samples containing 7.9 wt.% W

ried out between 453 and 513 K at total pressures between with pure ZrO₂samples at various oxidation temperatures.

300 and 2500 kPa in a plug-ﬂow reactor. Electronic mass In addition to stabilizing tetragonal ZrO₂crystallites, WO_x

ﬂow controllers were used to control gas ﬂow rates and a liq- surface species inhibit ZrO₂crystallite sintering, leading to

uid syringe pump was used to introduce n-heptane (Fluka, smaller crystallites than in pure ZrO₂(Fig. 1b). In pure

>99.7% ) into a heated mixing zone at 523 K before entering ZrO₂, the tetragonal content decreases markedly after oxi-

the reactor.

dation above 800 K. When WO_xsurface species are present

o-Xylene isomerization kinetic experiments were carried (7.9 wt.% W), this phase transition occurs at higher ox-

out in a glass recirculating batch reactor at 523 K and a total idation temperatures (>1073 K). This phase transition is

pressure of 107 kPa (0.67 kPa o-xylene, 106 kPa H₂or He, not accompanied by a rapid increase in crystallite size; the

0.030 g sample). This reactor has a total volume of 693 cm³, sizes of tetragonal and monoclinic crystallites are similar

a gas recirculation rate of 4.17 cm³s ¹, and it is operated at and much smaller than those in pure ZrO₂. This suggests

less than 2% reactant conversion per pass in order to en- that WO_xspecies strongly interacting with ZrO₂crystallites

sure gradientless operation (34). The reactor is evacuated inhibit sintering even at high oxidation temperatures. It ap-

usingmechanicaland diffusion pumpsisolated from the sys- pears that a critical crystallite size is reached ( 20 nm),

tem by liquid nitrogen traps. o-Xylene (NBS, >99.995% ), above which tetragonal crystallites become unstable and

degassed using several freeze–evacuation–thaw cycles, was transform into monoclinic crystallites of identical size.

admitted as a vapor from liquid held at room temperature.

The tetragonal content in ZrO₂also depends on WO_x

Hydrogen transfer pathways were studied by H–D iso- concentration at each oxidation temperature. For example,

topic exchange reactions in the glass recirculating batch after oxidative treatments at 1073 K, the tetragonal ZrO₂

reactor. Deuterium/n-heptane (343 K, 97 kPa D₂, 3.7 kPa percentage increases and the crystallite size decreases with

n-heptane) and deuterium/o-xylene (523 K, 106 kPa D₂, increasing WO_xconcentration (Fig. 2). A minimum ZrO₂

0.67 kPa o-xylene) exchange rates were measured during crystallite size (16 nm) is reached by samples containing

isomerization reactions. The deuterium content in reactants 7.9 wt.% W, and crystallites retain this size as WO_xcon-

and products was calculated from mass spectrometric data centration increases further. At WO_xconcentrations above

ZIRCONIA AND TUNGSTEN OXIDE

61

FIG. 1. Powder X-ray diffraction results of ZrO₂and WO_x–ZrO₂(7.9 wt.% W) samples showing stabilization of (a) the tetragonal ZrO₂phase and

(b) the ZrO₂crystallite size with WO_xsurface species at various oxidation temperatures.

12 wt.% W, ZrO₂crystallites are purely tetragonal and Surface areas for all samples become quite similar after ox-

monoclinic WO₃crystallites become detectable by X-ray idation above 1173 K, apparently because WO_xspecies are

diffraction. Samples with bulk WO₃crystallites can also be ineffective at stabilizing the ZrO₂surface at these temper-

identiﬁed visually by their light yellow color, instead of the atures.

white color of the rest of the samples.

WO_xspecies are not the only reported textural promot-

ers of ZrO₂. Several other oxides also inhibit ZrO₂crystal-

lite sintering at high oxidation temperatures (38). Strongly

Catalyst Surface Area Measurements

ZrO_x(OH)_{4 2x}powders have a high surface area (289 m²bound surface oxides inhibit intercrystallite sintering by re-

g

¹) after drying at 383 K for 12 h. This surface area ducing the rate of ZrO₂surface diffusion (39). These surface

decreases markedly after oxidative treatments above the oxides may lose their effectiveness at high temperatures be-

ZrO₂crystallization temperature (723 K) (Fig. 3). The sin- cause they diffuse into the bulk, decompose and desorb, or

tering of ZrO₂crystallites is inhibited by impregnating “dewet” and agglomerate to form poorly interacting clus-

metatungstate ions onto ZrO_x(OH)₄

before oxidation. ters as a separate phase. At high oxidation temperatures

2x

1

The surface area ofthe 7.9wt.% W sample isabout 40m²g

(>1173 K), surface WO_xspecies agglomerate into mono-

greater than that in ZrO₂samples after oxidation between clinic WO₃crystallites and become less effective sintering

773 and 1173 K (Fig. 3). For example, after oxidation at inhibitors. SO_xspecies also inhibit the loss of ZrO₂surface

1073 K, the surface area of pure ZrO₂is 6 m²g ¹, while the area after oxidative treatments (3). However, these species

1

surface area of a sample containing 7.9 wt.% W is 51 m²g

.

decompose and desorb as SO₃at relatively low oxidation

FIG. 2. Powder X-ray diffraction results of WO_x–ZrO₂samples showing stabilization of (a) the tetragonal ZrO₂phase and (b) the ZrO₂crystallite

size with various WO_xconcentrations after oxidation at 1073 K.

62

BARTON ET AL.

(0.8 W nm ²) and impregnation with excess metatungstate

ions (>7.9 wt.% W) leads to WO_xspecies that are not ef-

fective at additional stabilization of ZrO₂crystallites. Thus,

it appears that these weakly bound WO_xspecies may form

WO_xclusters below WO_xsurface densities corresponding

to the theoretical monolayer. This idea of a maximum

number of strong binding sites is supported by the results of

Fuentes (42), who found that WO_x–ZrO₂samples prepared

by coprecipitation of ZrO_x(OH)₄

and excess ammo-

2x

nium metatungstate attain maximum W concentration of

8 wt.% W, after washing with a NH₄OH solution of pH 10,

a process that removes the weakly bound WO_xspecies.

ZrO₂crystallite phase and size can be controlled by ad-

justing oxidation temperature and WO_xloading. These ex-

perimental variables are useful in the design of catalysts

with a given WO_xsurface density but higher surface areas.

For example, a sample with 21 wt.% W oxidized at 823 K

(140 m²g ¹) has the same nominal WO_xsurface density

(5.0 W nm ²) as a 7.9 wt.% W sample oxidized at 1073 K

FIG. 3. N₂physisorption BET surface area data of ZrO₂and WO_x–

ZrO₂(7.9 wt.% W) samples at various oxidation temperatures.

temperatures ( 873 K), leading to rapid ZrO₂sintering

above 873 K (40).

(50 m²g ¹). We have also detected by X-ray diffraction the

2

incipient formation of WO₃crystallites at about 9 W nm

,

The surface area of WO_x–ZrO₂samples also depends on

WO_xcoverage. At a given oxidation temperature, surface

area increases with WO_xconcentration (Fig. 4). An asymp-

totic value of surface area is reached near 7.9 wt.% W after

oxidation at 873 or 1073 K. The nominal WO_xsurface den-

sities on 7.9 wt.% W samples oxidized at 873 and 1073 K are

2.3 and 5.0 W nm ², respectively. These WO_xsurface den-

sities are below the theoretical monolayer capacity (7.3 W

atoms nm ²(41)), yet further increases in WO_xconcentra-

tion do not lead to higher surface areas. The WO_xconcen-

tration required for maximum surface area (7.9 wt.% W) is

independent of oxidation temperature and corresponds to

submonolayer coverages. In addition, the maximum tetrag-

onal content was also detected at this WO_xconcentration.

These data suggest that a ﬁxed number of strong binding

sitesfor metatungstate ionsexist on ZrO_x(OH)_{4 2x}powders

irrespective of surface area, WO_xconcentration, or oxida-

tion temperature. The important effects of WO_xsurface

density on catalytic activity and W⁶⁺reducibility are dis-

cussed below.

Thermal Analysis

Weight changes during thermal decomposition of

ZrO_x(OH)_{4 2x}and WO_x–ZrO₂(7.9 wt.% W) samples are

shown in Fig. 5. The ZrO_x(OH)₄

sample loses 19% of

2x

its weight during decomposition to ZrO₂and heating to

1073 K. This weight loss and the weight loss during ammo-

nium metatungstate decomposition ((NH₄)₆H₂W₁₂O₄₀

WO₃; 6.5 wt.% ) were used to calculate nominal WO_x

concentrations (wt.% W) for WO_x–ZrO₂samples after

→

FIG. 5. Thermogravimetric weight loss data of (A) ZrO_x(OH)₄

FIG. 4. N₂physisorption BET surface area data of WO_x–ZrO₂sam- and (B) WO_x–ZrO_x(OH)_{4 2x}(7.9 wt.% W) samples heated at 0.167 K s

ples at various WO_xconcentrations after oxidation at 873 and 1073 K.

in air.

2x

1

ZIRCONIA AND TUNGSTEN OXIDE

63

ZrO_xsurface diffusion rates are required to reach exposed

nucleation sites. This inhibition of ZrO_xsurface diffusion

by WO_xspecies also prevents the rapid sintering of ZrO₂

crystallites and loss of surface area as reported above.

Thermal analysis results for the WO_x–ZrO_x(OH)₄

2x

samples heated to higher temperature (1273 K) have no

additional features that could arise from decomposition or

loss of WO_xspecies. This thermal stability of WO_xspecies

differs markedly from the poor stability of SO_xspecies sup-

ported on ZrO₂. Thermal analysis has detected SO_xdecom-

position ( 873 K) to volatile SO₂and SO₃products in air,

H₂S in H₂, and H₂SO₄when H₂O is present (40). Decom-

position to H₂S occurs at even lower temperatures when Pt

crystallites are present. These difﬁculties limit the applica-

bility of SO_x–ZrO₂catalysts, while the inherent stability of

WO_xspecies permits catalyst regeneration and operation

at much higher temperatures.

FIG. 6. Differential scanning calorimetry data showing the inﬂu-

ence of WO_xconcentration on ZrO₂crystallization exotherms for

(A) ZrO_x(OH)_{4 2x}and WO_x–ZrO_x(OH)_{4 2x}samples with (B) 2.2 wt.% W,

(C) 4.5 wt.% W, (D) 7.9 wt.% W, (E) 16 wt.% W, and (F) 21 wt.% W heated

at 0.167 K s ¹in air.

Temperature-Programmed Reduction

Temperature-programmed reduction (TPR) studies of

supported metal oxides have been used extensively to pro-

vide semi-quantitative descriptions of domain sizes and

monolayer capacities for surface oxides (44). Dispersed

metal oxides tend to reduce at higher temperatures than

bulk crystallites because of strong M–O bonds with the ox-

ide surface and a lack of metal oxide nearest neighbors

to share the reduced charge. TPR methods are often use-

ful indicators of the onset of bulk metal oxide crystallite

formation, caused by a progression from isolated species

to two-dimensional structures and three-dimensional clus-

ters. These methods have been applied to MoO₃on many

supports (45) and to WO₃species on Al₂O₃and SiO₂(46).

Temperature-programmed reduction proﬁles are shown

in Fig. 7 for bulk WO₃, WO_x–Al₂O₃, H₃PW₁₂O₄₀on SiO₂,

and for a range ofWO_xconcentrationson ZrO₂. Allsamples

oxidation. Elemental analysis (Galbraith Laboratories,

Inc., Knoxville, TN) of a WO_x–ZrO₂sample oxidized at

1073 K veriﬁed the accuracy of this method (11.9 wt.% W

nominal vs 11.6 wt.% W analyzed) and conﬁrmed that

WO_xspecies were not lost by sublimation during high-

temperature oxidative treatments.

Differential scanning calorimetry (DSC) data (Fig. 6)

contain a single sharp exotherm at 679 K for a pure

ZrO_x(OH)₄

sample as a result of ZrO₂crystalliza-

2x

tion. ZrO_x(OH)₄

samples impregnated with ammo-

2x

nium metatungstate solutions have an additional broader

exotherm that increases in relative area and occurs at in-

creasing temperatures with increasing WO_xconcentration.

The integrated area of these two exotherms remains con-

stant when normalized by the weight of ZrO₂in these

samples ( 120 J g-ZrO₂¹), which indicates that both of

these exotherms are due to ZrO₂crystallization. The rela-

tive area of the ﬁrst exotherm is a measure of the fraction

of ZrO_x(OH)_{4 2x}crystallites that contain no WO_xsurface

species. It is evident that this area is small even for the

WO_x–ZrO_x(OH)_{4 2x}sample with the lowest WO_xconcen-

tration (2.2 wt.% W, Fig. 6B), indicating that incipient wet-

ness impregnation leads to WO_xspecies that are very well

dispersed.

The relative area of the second exotherm increases and

occurs at higher temperatures with increasing WO_xcon-

centration. WO_xspecies on the surface of amorphous zir-

conia crystallites delay the onset of crystallization by bind-

ing to nucleation sites and by limiting the rate of ZrO_x

surface diffusion, both of which are required for crystal-

lization (43). As a result, the crystallization temperature

of ZrO₂increases with increasing WO_xconcentration be-

cause more nucleation sites become blocked and higher

FIG. 7. Temperature-programmed reduction proﬁles of WO_x–ZrO₂

samples (0.025–0.200 g) after oxidation at 1073 K: (A) 2.2 wt.% W,

(B) 0.3% Pt–7.9 wt.% W, (C) 9.5 wt.% W, (D) 11.6 wt.% W, and

(E) 21 wt.% W and W standards, (F) 12-tungstophosphoric acid on SiO₂

(30 wt.% W), (G) bulk WO₃, and (H) 7.9 wt.% W/Al₂O₃heated at 0.167 K

s

¹(1.33 cm³s ¹, 20% H₂in Ar).

64

BARTON ET AL.

reduced to W metal at temperatures below 1323 K, ex-

cept WO_x–Al₂O₃. Pure ZrO₂samples showed no signi-

ﬁcant H₂consumption up to 1323 K. Three overlapping

peaks were detected for the reduction of bulk WO₃sam-

ples (Fig. 7G), which occurs via stepwise reduction: WO₃→

WO_2.9→WO₂→W (46).

The reduction proﬁles for WO_x–ZrO₂samples are very

broad (800 K) and have four overlapping peaks (Figs. 7A–

7E) that occur at similar temperaturesat allWO_xconcentra-

tions. The ﬁrst three peaks correspond to the same three re-

duction steps found in bulk WO₃. A direct comparison with

bulk WO₃, however, is impaired by marked differences in

crystallite size between bulk and supported samples. An ad-

ditional reduction step occurs at high temperature (1200 K)

in WO_x–ZrO₂samples; it corresponds to the reduction

of WO_xspecies strongly bound to the ZrO₂support

(WO_2.9→W). These WO_xspecies reduce at higher tem-

peratures because they contain unreactive oxygen link-

ages to the ZrO₂support. As WO_xconcentration increases,

the relative area of this high-temperature reduction peak

decreases, which is consistent with a decreasing fraction

of strong W–O–Zr bonds and increasing fraction of WO_x

species in domains that reduce at lower temperatures.

The second reduction step (WO_2.9→WO₂) is sensitive

to the size of the WO_xdomains. At low WO_xconcentra-

tions, a large fraction of the WO_xdomains bypass this sec-

ond reduction step and instead reduce directly to W metal

(WO_2.9→W) in the third reduction step; apparently there

are not enough WO_xneighbors in these small domains to

form the stable intermediate WO₂species. The relative area

of this second reduction step increases with increasing WO_x

concentration and ultimately approaches that for stoichio-

metric reduction to WO₂as in WO_x–ZrO₂samples con-

taining primarily bulk WO₃crystallites (21 wt.% W, 1073 K

oxidation). Thus, the increase in the area of this second re-

duction step suggests a growth of WO₃-like domains with

W–O–W linkages as WO_xconcentration increases; these

domains reduce at lower temperatures than isolated WO_x

species.

WO_xspecies on ZrO₂reduce at signiﬁcantly lower tem-

peratures than WO_xon Al₂O₃at similar surface densi-

ties. For example, reduction of the 2.2 wt.% W on ZrO₂

(2.7 W nm ²) sample is complete by 1350 K, whereas re-

duction of the 6 wt.% W on Al₂O₃(2.4 W nm ²) sample is

not ﬁnished at 1400 K. At low WO_xsurface densities, the

reduction temperature of these dispersed WO_xspecies is

determined by the strength of the W–O bonds that contain

oxygens shared with the support. The higher reduction tem-

perature of the WO_x–Al₂O₃sample suggests that the W–O–

Al bonds are stronger than the W–O–Zr bonds. At higher

WO_xsurface densities, the WO_xpolyhedra are connected

by W–O–W bonds, and the ﬁrst two reduction steps of these

polytungstate domains on ZrO₂occur at temperatures sim-

SCHEME 1. Generation of Brønsted acid sites in hydrogen by slight

reduction and delocalization of negative charge in WO_xdomains.

heteropolyacids (H₃PW₁₂O₄₀). This suggests that the onset

of reduction of polytungstate domains is not inﬂuenced by

the support when there are enough WO_xneighbors to share

the reduced charge and bridging W–O–W bonds that can

be broken.

The addition of Pt (0.3 wt.% ) to WO_x–ZrO₂(12.7 wt.%

W) strongly affects the ﬁrst reduction step (WO₃→WO_2.9),

which shifts from 700 to 350 K (Fig. 7B). This suggests that

slight reduction can occur during H₂pretreatment and dur-

ing n-alkane reactions at 440–500 K. The ability of WO_x

domains to undergo slight reduction by delocalizing the

net negative charge may be linked with the strong acid-

ity appearing on WO_xdomains on ZrO₂, for which the

domain sizes become large enough to delocalize the elec-

tron charge over several W atoms. These reducible WO_x

domains may act as redox sites required for the forma-

tion of H⁺species from H₂and alkanes on WO_x-based

solid acids (Scheme 1). The accommodation of a proton

by electron transfer and charge delocalization across an ex-

tended W–O network results in electronic structures simi-

lar to those of heteropolyacids (47). This simple mechanism

may also account for the ability of H atoms to desorb car-

bocations as neutral hydrocarbons and to regenerate neu-

tral WO₃domains (Scheme 2) and for the observed effects

of hydrogen on the rate and selectivity of n-heptane and

o-xylene isomerization reactions.

X-Ray Absorption Spectroscopy

W-L_Inear-edge spectra for Na₂WO₄and (NH₄)₆H₂W₁₂O₄₀

standards and for a WO_x–ZrO₂sample (11.7 wt.% W)

before and after high-temperature oxidation (1073 K) are

shown in Fig. 8. The pre-edge feature at 10 eV reﬂects

a 2s → 5d electronic transition that is dipole-forbidden

in centrosymmetric structures, but becomes allowed in

SCHEME 2. Stabilization of carbocations by slight reduction of WO_x

ilar to those required for the reduction of WO_xdomains in domains.

ZIRCONIA AND TUNGSTEN OXIDE

65

FIG. 8. Normalized W-L_Inear-edge spectra of WO_xstandards and

WO_x–ZrO₂samples: (A) dehydrated Na₂WO₄, (B) 11.6 wt.% W after

1073K, (C) 11.6wt.% W after 423K, and (D) (NH₄)₆H₂W₁₂O₄₀. Difference

spectra at right, with equivalent vertical scale, calculated by subtracting

the WO₃spectrum from the sample spectrum. Spectra recorded in situ at

298 K after dehydration at 423 K.

FIG. 9. Normalized W-L_Inear-edge spectra of WO_x–ZrO₂samples

(1073 K oxidation): (A) 2.2 wt.% W, (B) 11.6 wt.% W, (C) 0.3 wt.% Pt–

12.7 wt.% W, (D) 21 wt.% W, and (E) bulk WO₃. Difference spectra at

right, with equivalent vertical scale, calculated by subtracting the WO₃

spectrum from the sample spectra. Spectra recorded in situ at 298 K after

dehydration at 723 K.

tetrahedral or distorted octahedral structures because of

p–d orbital mixing (48). Thus, changes in the intensity of this

pre-edge feature reﬂect changes in the symmetry of the W

absorber (49). This feature has been used to determine the

distribution of W among octahedral and tetrahedral struc-

tures in Al₂O₃- and TiO₂-supported WO_xcatalysts (50, 51).

These studies have concluded that W atoms can be 4-, 5-,

or 6-coordinate depending on the W coverage and the state

containing different WO_xconcentrations and bulk WO₃.

W centers in bulk WO₃crystallites reside in relatively low

symmetry positions. For example, in monoclinic WO₃there

are 24 different W–O distances per unit cell, ranging from

˚

1.715 to 2.215 A (52). Because of this structural disorder,

coordination numbers and distances derived from the W

radial structure functions are poorly deﬁned. Figure 10

shows, however, that WO_xstructures present on the sur-

face of ZrO₂at all WO_xconcentrations have a static dis-

order similar to that in bulk WO₃. W-L_Inear-edge spectra

and W-L_IIIradial distribution functions in all WO_x–ZrO₂

samples resemble those of bulk WO₃; therefore, W atoms

reside in structures with distorted octahedral symmetry re-

sembling that in bulk WO₃for all WO_xconcentrations and

pretreatment conditions. As a result, signiﬁcant differences

in catalytic activity within these samples cannot be simply

of hydration. The spectrum of a ZrO_x(OH)₄

sample

2x

impregnated with an aqueous ammonium metatungstate

solution and dried in situ at 423 K (Fig. 8C) differs from

the spectrum of pure ammonium metatungstate (Fig. 8D),

indicating that a reaction occurs between the aqueous

metatungstate ions and the ZrO_x(OH)₄

surface during

2x

impregnation. After oxidation at 1073 K, which sinters the

ZrO₂crystallites and markedly increases the WO_xsurface

density, the spectrum changes only slightly (Fig. 8B), sug-

gesting that the local structure and symmetry of the WO_x

species does not change signiﬁcantly after oxidative treat-

ments that lead to catalytically active WO_x–ZrO₂samples.

W-L_Inear-edge spectra for bulk WO₃and WO_x–ZrO₂

samples oxidized at 1073 K with various WO_xconcentra-

tions are shown in Fig. 9. All spectra were measured at room

temperature after in situ dehydration in He at 823 K. The

similarity among the three WO_x–ZrO₂spectra and the spec-

trum of bulk WO₃is conﬁrmed by subtraction of the bulk

WO₃spectrum from each of the three WO_x–ZrO₂spectra

(Fig. 9). These difference spectra differ markedly from the

difference spectrum of Na₂WO₄that contains W atoms only

in tetrahedral sites (Fig. 8A). The difference spectra show

that WO_xstructures have a distorted octahedral symmetry

that is insensitive to WO_xconcentration in a concentration

range that strongly inﬂuences catalytic behavior.

FIG. 10. W-L_IIIradial structure functions, not phase or amplitude cor-

rected, of WO_x–ZrO₂samples after oxidation at 1073 K: (A) 2.2 wt.% W,

(B) 11.6 wt.% W, (C) 0.3 wt.% Pt–12.7 wt.% W, (D) 21 wt.% W, and

The W-L_IIIﬁne structure of the samples in Fig. 9 was used

to obtain the radial structure functions shown in Fig. 10.

There are only slight differences among the four samples (E) bulk WO₃. Spectra recorded in situ at 298 K after dehydration at 723 K.

66

BARTON ET AL.

explained by differences in the local symmetry of the WO_x

octahedra, but may be explained instead by differences in

the size of the WO_xextended domain. We discuss how rates

of o-xylene isomerization relate to the WO_xdomain size de-

termined by UV–visible spectroscopy below.

TABLE 1

Irreversible Chemisorption Uptake^aof H₂, O₂, and CO

on Pt/WO_x–ZrO₂(0.3 wt.% Pt, 10 wt.% W)

Reduction

temperature (K)

H/Pt

O/Pt

CO/Pt

Catalytic Isomerization of n-Heptane

473

673

0.63

0.092

1.25

2.04

0.30

0.050

The addition of a metal function (0.3 wt.% Pt) to WO_x–

ZrO₂leads to high n-heptane isomerization turnover rates

and selectivities at low temperatures (18). The isomeriza-

tion selectivity at 473 K and 50% n-heptane conversion

is 85% on Pt/WO_x–ZrO₂. These isomerization selectivi-

ties are much higher than on Pt/SO_x–ZrO₂(<50% ) and

on other mesoporous acid catalysts. Here, we expand our

previously reported results (18) in order to contrast these

resultswith the effectsof H₂on o-xylene isomerization cata-

lyzed by WO_x–ZrO₂samples without a metal function.

On Pt/WO_x–ZrO₂, n-heptane isomerization turnover

rates depend weakly on H₂gas phase concentration.

Turnover rates on Pt/WO_x–ZrO₂become negative order

in H₂(0 → 0.5) as H₂pressures increase. These data are

consistent with quasi-equilibrated hydrogen transfer steps,

which lead to short surface lifetimes, low steady-state car-

bocation coverages, and desorption of carbocations before

-scission occurs. On Pt/WO_x–ZrO₂, n-heptane isomeriza-

tion occurs on a surface sparsely populated with carboca-

tions and surface isomerization steps are rate-determining

(Scheme 3). In effect, carbocation formation and desorp-

tion by hydrogen transfer are quasi-equilibrated and car-

bocation coverages are proportional to n-heptane pres-

sure, leading to the observed ﬁrst-order dependence on

n-heptane concentration. The negative H₂pressure order

arises from competitive binding of H⁺and carbocations

at WO_xsites, which causes carbocation surface densities

to decrease with increasing H₂pressure. On Pt/SO_x–ZrO₂,

n-heptane isomerization turnover rates have a positive

H₂order (0.9) and they increase when hydrogen transfer

cocatalysts, such as adamantane, are added to n-heptane re-

actants. Slow hydrogen transfer reactions delay carbocation

desorption and lead to long carbocation surface residence

times and low isomer selectivity (6).

a

Room temperature adsorption; irreversibly chemisorbed species are

deﬁned as those that cannot be removed by evacuation at 313 K.

desorption pathways, which prevent extensive cracking of

adsorbed carbocations. This mechanistic conclusion is con-

ﬁrmed by the rapid isotopic exchange between n-C₇H₁₆and

D₂observed on Pt/WO_x–ZrO₂during n-heptane isomeriza-

tion (18). H–D equilibrium between n-C₇H₁₆and D₂occurs

much more rapidly than isomerization on Pt/WO_x–ZrO₂.

These data show that n-heptane adsorbs, exchanges with a

surface hydrogen pool initially consisting of D atoms, and

frequently desorbs before isomerization occurs.

H₂dissociation is required before it can act as a source

of hydrogen atoms. Reduced Pt sites on Pt/WO_x–ZrO₂dis-

sociate and store H atoms required for hydrogen transfer

and carbocation desorption steps. As a result, Pt clusters

on Pt/WO_x–ZrO₂catalysts chemisorb signiﬁcant amounts

of H₂at room temperature and catalyze exchange of H

atoms between D₂and “unreacted” n-heptane.

Pt dispersion measurements after H₂reduction at 473 K

using H₂, O₂, and CO site titrations show that apparent Pt

dispersions measured by hydrogen uptake exceed those ob-

tained from CO chemisorption (Table 1), suggesting that

hydrogen adatoms spillover onto WO_xcenters during H₂

chemisorption at room temperature. O₂uptakes are much

higher than those of H₂or CO, suggesting that WO_xspecies

reduce slightly during H₂treatments at typical isomeriza-

tion reaction temperatures. At higher reduction tempera-

tures, H₂uptakes decrease and O₂uptakes increase; these

phenomena appear to be related to the decoration of Pt

crystallites by reduced WO_x, as also proposed to explain

similar effects on TiO₂supported metal particles (53). Thus,

hydrogen atoms formed at H₂dissociation sites may be-

come involved not only in the desorption of adsorbed in-

termediates, but also in the generation and maintenance

of Brønsted acid sites. The formation of Brønsted acid sites

during H₂treatment has been detected by Ebitani et al. (54)

using infrared measurements of the density of Lewis and

Brønsted acid sites in Pt/SO_x–ZrO₂. In their study, H₂treat-

ments at temperatures typical of catalytic reactions lead to

an increase in the density of Brønsted acid sites and to a

decrease in the density of Lewis acid sites, suggesting that

Lewis acid sites are converted to Brønsted acid sites by

contact with H₂. We suggest that a similar mechanism leads

to the formation of Brønsted acid sites on Pt/WO_x–ZrO₂

Pt/WO_x–ZrO₂catalyzes H₂activation and storage of H

atoms and provides rapid carbocation neutralization and

SCHEME 3. n-Heptane isomerization on Pt/WO_x–ZrO₂.

ZIRCONIA AND TUNGSTEN OXIDE

67

and WO_x–ZrO₂samples in the presence of H₂at reaction

temperatures. As a result, attempts to measure the number

and nature of the acid sites on these materials must be done

under reaction conditions.

Catalytic Isomerization of o-Xylene

Acid-catalyzed skeletal isomerization of o-xylene

(Eq. [4]) was used to probe the acid properties of WO_x–

ZrO₂samples. Xylene isomerization reactions proceed via

intramolecular 1,2 methyl shifts on Brønsted acid sites (55).

Bimolecular pathways leading to disproportionation side

products (toluene and trimethylbenzenes) can also lead to

isomerization of xylenes (56). In the absence of intrapellet

diffusional restrictions and bimolecular pathways, the rel-

ative selectivity to m- and p-xylenes extrapolated to zero

conversion reﬂects benzenium ion surface lifetimes. Short

FIG. 12. Inﬂuence of hydrogen on o-xylene transformations on a

WO_x–ZrO₂(12 wt.% W) sample oxidized at 1073 K (reaction conditions:

surface lifetimes lead to m-xylene, a product formed by sin- 0.67 kPa o-xylene, 106 kPa H₂or He, 523 K). Reactor loaded with 0.0287 g

for (A) and (B) and run in H₂. Reactor reloaded with 0.0293 g for (C), (D),

and (E) and run in He. (B) and (D) were run immediately after (A) and

(C) without further pretreatment. (E) was run after oxidative treatment.

See Table 2 for pretreatment details.

gle methyl shifts; longer surface residence times lead to xy-

lene isomer distributions near thermodynamic equilibrium

(57)

under these reaction conditions. Tetramethylbenzene, a dis-

proportionation product, became detectable only at high

[4]

conversions. At low conversions, typical m- to p-xylene

ratios are about 5.6 (thermodynamic equilibrium is 2.3 at

523 K) and disproportionation to isomerization ratios are

0.18. Thishigh m-xylene selectivityreﬂectsshort benzenium

ion lifetimes. o-Xylene isomerization turnover rates, based

on the total number of W atoms, were used to compare the

relative catalytic activity of the WO_x–ZrO₂samples. Nor-

malization of the isomerization rates by the total number of

W atoms is the most conservative estimation of the “true”

acid site turnover rate and is a convenient choice in the

absence of an accurate measurement of the acid site den-

sity in the presence of H₂under reaction conditions. Initial

o-xylene isomerization rates are reported at 5–10% conver-

sion in order to avoid including any initial transients.

o-Xylene reactive turnovers (per W atom) are shown as

a function of contact time in Fig. 12 for a 12 wt.% W sample

oxidized at 1073 K after various H₂and He pretreatments

listed in Table 2. The data in Fig. 12A correspond to the

o-Xylene isomerization was carried out in a recirculating

batch reactor at 523 K (0.67 kPa o-xylene; 106 kPa H₂). A

typical plot of selectivity as a function of o-xylene conver-

sion on a 12 wt.% W sample oxidized at 1073 K is shown in

Fig. 11. Aromatic hydrogenation, hydrogenolysis, and xy-

lene oligomerization reaction products were not observed

TABLE 2

Pretreament Conditions and Diluent Gas Composition

for o-Xylene Isomerization Turnover Data in Fig. 12

Figure 12 data

Diluent

Drying

Reduction

A

B

C

D

E

H₂

He

823 K, 2 h

None

823 K, 2 h

None

523 K, 2 h

None

523 K, 2 h

None

FIG. 11. Selectivity as a function of o-xylene conversion on a WO_x–

ZrO₂sample (12 wt.% W) oxidized at 1073 K (reaction conditions:

0.67 kPa o-xylene, 106 kPa H₂, 0.0287 g catalyst, 523 K).

823 K, 2 h

523 K, 2 h

68

BARTON ET AL.

activity, but again deactivated rapidly. The initial activity in

He appears to be caused by a sacriﬁcial loss of hydrogen

from neighboring adsorbed species, which forms Brønsted

acid sites and allows xylene turnovers to occur, but which

leaves behind strongly adsorbed unsaturated species. The

irreversible buildup of these carbonaceous species leads to

the rapid deactivation during o-xylene conversions in He.

After high-temperature oxidative treatments these samples

completely recovered their initial activity.

The mechanism of H₂promotion in xylene isomerization

reactions was probed by replacing H₂with D₂and moni-

toring the rate of exchange between xylenes and D₂during

o-xylene isomerization. The deuterium contents (Fig. 13)

and the deuterium distributions (Fig. 14) in o-xylene,

m-xylene, and p-xylene are identical, indicating that the ad-

sorption of o-xylene that leads to an isomerization turnover

does not lead to additional deuterium exchange. The rate

of deuterium exchange per W atom (2.3 10 ⁴s ¹) is much

FIG. 13. D₂/o-xylene exchange on a WO_x–ZrO₂(12 wt.% W) sample

oxidized at 1073 K (reaction conditions: 0.67 kPa o-xylene, 106 kPa D₂,

523 K).

experiment in Fig. 11, which was run in H₂at 523 K after lower than o-xylene isomerization turnover rates (1.7

treatment in dry air for 2 h at 823 K and in H₂for 2 h at 10 ³s ¹). For example, only 1.2% of the H atoms in xylenes

523 K. Stable isomerization rates were conﬁrmed by ﬂush- have exchanged at 21% o-xylene chemical conversion. The

ing the system with H₂for 1 h at 523 K, reﬁlling the reactor deuterium content in the m-xylene and p-xylene isomers

with o-xylene/H₂, and remeasuring o-xylene isomerization increased with contact time and showed a binomial dis-

rates (Fig. 12B). Minimal deactivation was detected on this tribution, suggesting that the surface hydrogen/deuterium

sample after o-xylene reactions to near equilibrium con- species are not in equilibrium with the D₂in the gas phase.

version for 2 h, as shown by a nearly complete recovery of As a result, H atoms formed by H₂dissociative chemisorp-

catalytic activity ﬂushing the system with H₂. The inﬂuence tion are not frequently involved in the activation or desorp-

of H₂on o-xylene conversions was tested by replacing H₂tion of the xylene reactants. The presence of H₂is required,

with He (Fig. 12C) as an inert diluent. The initial turnover however, in order to reverse the occasional desorption of

rate was lower in He and rapidly decreased as the sam- H atoms as H₂, which can occur during o-xylene isomeriza-

ple deactivated. This signiﬁcant deactivation is shown by tion reactions by the microscopic reverse of the dissociative

the very low rates in Fig. 12D, which was the experiment chemisorption step. These desorption processes lead to the

immediately following ﬂushing the system with He (no ox- disappearance of Brønsted acid sites and to the formation

idation) and reﬁlling with o-xylene/He. After these two ex- of strongly adsorbed unsaturated species when xylene iso-

periments in He, the sample was reoxidized for 2 h at 823 K merization iscarried out in the absence ofH₂. H₂can restore

and treated in H₂for 2 h at 523 K and tested again with He as these Brønsted acid sites by rereduction of WO_xspecies or

an inert diluent (Fig. 12E). The sample recovered its initial by formation of acidic H_xWO₃species.

FIG. 14. D distribution in o-xylene and in the combination m- and p-xylenes compared to a statistical distribution of 2.4% D atoms.

ZIRCONIA AND TUNGSTEN OXIDE

69

FIG. 16. Inﬂuence of oxidation temperature on o-xylene isomeriza-

tion turnover rates on WO_x–ZrO₂samples at three WO_xconcentrations

(5, 12, and 21 wt.% W) (reaction conditions: 0.67 kPa o-xylene, 106 kPa

H₂, 523 K).

FIG. 15. o-Xylene isomerization turnover rates (per W atom) as a

function of WO_xconcentration on WO_x–ZrO₂samples oxidized at 1073 K

(reaction conditions: 0.67 kPa o-xylene, 106 kPa H₂, 523 K).

o-Xylene isomerization rates on WO_x–ZrO₂samples

with various WO_xconcentrations and oxidation tempera-

tures are shown in Figs. 15 and 16. o-Xylene isomerization

turnover rates for samples oxidized at 1073 K reach a maxi-

mum value at a WO_xconcentration of 12 wt.% W (Fig. 15),

which is similar to that reported by Hino and Arata [12] and

later by Santiesteban et al. (58) for pentane isomerization

reactions. Isomerization rates were negligible on the sample

with the lowest WO_xconcentration (2.2 wt.% W). The sam-

ple with the highest WO_xconcentration tested (21 wt.% W)

gave low but measurable rates, even though it contains pre-

dominately large WO₃crystallites. Clearly, isomerization

rates are not proportional to the number of WO_xsites, but

they are strongly affected by the coverage of WO_xon ZrO₂.

WO_xcoverage and isomerization rates also depend

strongly on oxidation treatment temperatures. ZrO₂crys-

tallites grow and surface areas decrease with increasing ox-

idation temperature, which increases the WO_xsurface den-

sity at a given WO_xconcentration. Three freshly prepared

WO_x–ZrO₂samples (5, 12, and 21 wt.% W) that had only

been dried at 423 K were oxidized in the quartz reactor

at increasing temperatures from 800 to 1300 K. o-Xylene

isomerization rates were measured on each sample with-

out removing them from the reactor after each oxidation

treatment. After catalytic measurements, the reactor was

ﬂushed with H₂at 523 K for 1 h, evacuated, and heated

in ﬂowing dry air to the next highest oxidation tempera-

ture for 2 h. Isomerization rates (per W atom) on these

three samples at several oxidation temperatures are shown

in Fig. 16. Each sample reaches maximum isomerization

turnover rates at intermediate oxidation treatment tem-

peratures. The sample with the highest WO_xconcentration

(21 wt.% W) reaches its maximum isomerization rate

(2.1 10 ³s ¹) after oxidation at 973 K. Samples with lower

W content reach maximum rates only after oxidation at

concentrations in WO_x–ZrO₂samples decreases the oxi-

dation temperature required to reach maximum catalytic

activity and leads to samples with higher surface areas. It

appears that a critical WO_xsurface density is required in or-

der to obtain a maximum number of acid sites for o-xylene

isomerization.

Surface areas were measured for these samples at each

oxidation temperature and used to convert WO_xconcen-

trations to nominal WO_xsurface densities (W nm ²). When

isomerization ratesfor allsamplesfrom Fig. 16are replotted

(Fig. 17) as a function of this WO_xsurface density, they

merge into a single curve with a very sharp maximum rate

at about 10 W nm ². It is apparent that the acid site density

depends only on WO_xsurface density and that it is indepen-

dent of the oxidation temperature and WO_xconcentration

FIG. 17. o-Xylene turnover rates replotted from Fig. 16 as a func-

tion of nominal WO_xsurface density on three WO_x–ZrO₂(5, 12, and

21 wt.% W) samples oxidized at various temperatures (800–1300 K). The

higher temperatures (1100–1200 K). Thus, increasing WO_xtheoretical monolayer capacity for ZrO₂(7 W nm ²) is marked.

70

BARTON ET AL.

used to obtain a particular WO_xsurface density. As a re-

sult, a WO_x–ZrO₂sample with the highest isomerization

1

rate per catalyst weight (1.8 10¹⁹

s

g

¹) was found on

a sample with the highest WO_xconcentration (30 wt.% W,

898 K oxidation, 104 m²g ¹) and was limited only by the

solubility of ammonium metatungstate in water.

The critical WO_xsurface density at which maximum

activity is obtained is higher than the theoretical mono-

layer capacity of ZrO₂(7 W nm ²), calculated by assuming

corner sharing WO_xoctahedra with bond distances de-

rived from monoclinic WO₃; therefore, the most active

WO_x–ZrO₂samples must contain a fraction of the WO_x

species in extended three-dimensional networks. At high

WO_xsurface densities (>10 W nm ²), bulk WO₃has been

detected by X-ray diffraction and therefore isomerization

turnover rates normalized to the total number of W

atoms must decrease because a large fraction of W atoms

reside inside the bulk of these crystallites and become

inaccessible to hydrocarbon reactants. At low WO_xsurface

densities (<5 W nm ²), all the WO_xgroups are accessible,

and X-ray absorption results indicate that the W atoms

reside in a distorted octahedral symmetry similar to that of

the most active species. Isomerization rates are, however,

very low, suggesting that it is the growth of polytungstate

species at WO_xsurface densities higher than a monolayer

that is required for the formation of acid sites. This narrow

range of WO_xsurface densities for which WO_x–ZrO₂

samples become catalytically active suggests that high

isomerization turnover rates require the presence of WO_x

domains of intermediate size, which form selectively as

WO_xsurface densities reach intermediate values.

Previous analysis of optical absorption edge energies

from UV–vis diffuse reﬂectance spectra for WO_x–ZrO₂

sampleswith WO_xsurface densitybelow10W nm ²showed

that the number of WO_xnearest neighbors and the WO_x

domain size increase with increasing WO_xsurface density

(18). Optical absorption edge energies were determined by

plotting ( h )²vs h ( , absorbance; h , photon energy),

which assumes that the fundamental optical absorption is

the result of a direct electronic transition. The fundamen-

tal optical absorption energy for bulk WO₃crystallites and

amorphous WO_xspecies, however, has been shown to be

better approximated by plotting ( h )^1/2vs h , which re-

sults in the indirect absorption edge energy (59). Reanalysis

of the absorption spectra based on the indirect transition

formalism gives lower edge energies, but the trends remain

unchanged (Fig. 18). The position of the UV–visible absorp-

tion edge is sensitive to the domain size of transition metal

oxides, which is related to the number of nearest metal ox-

ide neighbors sharing a common oxygen (M–O–M bonds)

(60). An increase in the domain size results in a decrease in

the absorption edge energy. The decrease of the absorption

edge with increasing WO_xsurface density between 4 and

10 W nm ²reﬂects an increase in the WO_xdomain size. It is

FIG. 18. WO_xdomains with intermediate size lead to maximum

o-xylene isomerization turnover rates. WO_xindirect absorption edge en-

ergies determined from absorption data for WO_x–ZrO₂samples in (18)

are plotted over the o-xylene isomerization rate curve from Fig. 17. The

indirect absorption edge of monoclinic WO₃is marked for reference.

evident when the o-xylene isomerization rate curve is plot-

ted over the UV–visible absorption edge energies (Fig. 18)

that increasing WO_xdomain sizes lead to increasing iso-

merization rates at low surface densities (4–10 W nm ²).

At higher surface densities (>10 W nm ²), bulk WO₃was

detected by a second electronic transition edge and isomer-

ization rates per W atom decrease as a result of a decrease

in WO_xdispersion.

The unique acid properties of WO_xdomains of inter-

mediate size appear to be related to their ability to form

W⁶ⁿO_x–(n-H⁺) Brønsted acid centers under reducing con-

ditions at low temperatures (Scheme 1 without Pt). Iso-

lated WO_xspecies that exist at low WO_xsurface densities

(<5 W nm ²) cannot delocalize the negative charge re-

quired for the formation of Brønsted acid centers, whereas

polytungstate species that form at intermediate WO_xsur-

face densities reside in an extended network of WO_xspecies

over which an excess negative charge may be delocalized.

The ability of these WO_xspecies to temporarily reduce

and reversibly adsorb hydrogen was measured by hydro-

gen chemisorption uptakes at 523 K. The reversible H₂

uptakes are plotted on the o-xylene isomerization rate

curve (Fig. 19) and show that reversible H₂adsorption oc-

curs at reaction temperatures. Maximum hydrogen uptakes

were recorded on WO_x–ZrO₂samples that show maximum

catalytic activity, suggesting that the formation of these

W⁶ⁿO_x–(n-H⁺) reduced centers is related to the stabiliza-

tion of carbocationic reaction intermediates. WO_xspecies

present at low surface densities are difﬁcult to reduce and

as a result are not able to form these Brønsted acid cen-

ters required for xylene reactions. Extended WO_xdomains

that form at higher WO_xsurface densities can delocalize

the negative charge required in order to stabilize positively

ZIRCONIA AND TUNGSTEN OXIDE

71

tallites and increase the WO_xsurface density. Surface area

and X-ray diffraction measurements show that dispersed

WO_xspecies inhibit ZrO₂crystallite sintering and stabilize

tetragonal ZrO₂crystallites during high-temperature ox-

idative treatments. UV–visible and X-ray absorption spec-

troscopic studies of precursor and oxidized samples sug-

gest that oxyhydroxide surfaces lead to dispersed WO_x

species (<7.9 wt.% W) after initial contact with aqueous

metatungstate solutions and drying at 400 K. X-ray absorp-

tion spectra at the W-L_Iedge show the predominant pres-

ence of distorted octahedral species similar those in bulk

WO₃at all WO_xsurface densities. The growth of WO_xdo-

mains as WO_xsurface density increases was detected by a

decrease in the absorption edge energies measured by UV–

visible spectroscopy. Temperature-programmed reduction

analysis of WO_x–ZrO₂samples show marked changes in

reduction behavior with increasing WO_xsurface density.

As the WO_xsurface density increases, WO_xspecies con-

vert from strongly bound WO_xspecies to two- and three-

dimensional structures, which reduce at lower tempera-

tures.

The addition of a metal function (0.3% wt. Pt) increases

the turnover rate (based on W atoms) and selectivity for n-

heptane isomerization. Quasi-equilibrated hydrogen trans-

fer elementary steps during n-heptane isomerization reac-

tions decrease the surface residence time of intermediates

and lead to desorption of isomers before -scission events

occur on the acid sites. H₂, O₂, and CO chemisorption up-

takes indicate that H atoms spillover onto WO_xspecies

forming W⁶ⁿO_x–(n-H⁺) Brønsted acid centers under re-

action conditions. H atoms formed at H₂dissociation sites

become involved not only in the desorption of adsorbed

intermediates, but also in the generation and maintenance

of Brønsted acid sites.

Maximum o-xylene isomerization turnover rates were

found on WO_xdomains of intermediate size at WO_xsur-

face densities (10 W nm ²) greater than monolayer cov-

erage on ZrO₂. Turnover rates depend only on the WO_x

surface density, and not independently on the oxidation

temperature and WO_xconcentration used to obtain a spe-

ciﬁc density. WO_xdomains of intermediate size appear to

provide a compromise between reducibility and accessibil-

ity of these WO_xdomains. At low coverages, turnover rates

increase with increasing WO_xsurface density, which is re-

lated to the pronounced ability of the polyoxoanion do-

mains to form W⁶ⁿO_x–(n-H⁺) Brønsted acid centers. As

WO_xsurface density is further increased, W-atom turnover

rates decrease because increasing amounts of WO_xspe-

cies reside inside WO₃crystallites. H₂is required during

o-xylene isomerization reactions on WO_x–ZrO₂to prevent

the occasional desorption of H atoms as H₂, which leads to

the destruction of these Brønsted acid sites via the micro-

scopic reverse of the dissociative chemisorption step. These

desorption processes lead to the disappearance of Brønsted

FIG. 19. Reversible H₂uptakes at 523 K and 45 kPa H₂on WO_x–ZrO₂

samples oxidized at 1073 K, reduced in H₂at 523 K for 1 h, and evacuated

at 523 K, plotted over o-xylene isomerization rates from Fig. 17.

charged species. This delocalization of charge among sev-

eral WO_xgroups to form W⁶ⁿO_x–(n-H⁺) acid centers is

similar to that previouslysuggested for the generation ofthe

strong acid sites in heteropolytungstate clusters (47). In 12-

tungstophosphoric acid, the neutral (WO₃)₁₂shell around

the central PO³₄anion delocalizes the negative charge over

the surface of the shell, which is balanced by three acid hy-

drogens with a net positive charge to maintain electroneu-

trality of the cluster.

o-Xylene isomerization turnover rates are strongly inﬂu-

enced by WO_xsurface density. Two independent param-

eters, WO_xconcentration and oxidation temperature, are

available in order to control the WO_xsurface density. Even

though turnover rates depend on WO_xsurface density, they

are not inﬂuenced by the speciﬁc oxidation temperature

or WO_xconcentration required in order to obtain a given

WO_xsurface density. Maximum turnover rates were ob-

tained on WO_xdomains of intermediate size (10 W nm ²).

These WO_xdomains of intermediate size may provide a

compromise between the reducibility and the accessibility

of WO_xspecies in supported domains. H₂is required during

o-xylene isomerization reactionson WO_x–ZrO₂for the gen-

eration and maintenance of W⁶ⁿO_x–(n-H⁺) Brønsted acid

centers. For this reason, measurements of the Brønsted acid

site density must be carried out under reaction conditions in

order to accurately determine the site density present dur-

ing reaction and to calculate true isomerization turnover

rates.

CONCLUSIONS

WO_x–ZrO₂catalysts active for isomerization reactions

at low temperature were prepared by impregnation of

ZrO_x(OH)_{4 2x}with a solution of ammonium metatungstate

followed by oxidation treatments that sinter the ZrO₂crys-

72

BARTON ET AL.

acid sites and to the formation of strongly adsorbed unsat- 23. Pines, H., “The Chemistry of Catalytic Hydrocarbon Conversions.”

Academic Press, New York, 1981.

24. Iglesia, E., Baumgartner, J. E., Ribeiro, F. H., and Boudart, M., J. Catal.

131, 523 (1991).

urated species that deactivate the catalysts.

ACKNOWLEDGMENTS

25. Rijnten, H. T., Ph.D. thesis, Delft University of Technology, 1971.

26. Crucean, E., and B., R., Trans. J. Brit. Ceram. Soc. 78, 58 (1979).

27. Gimblett, F. G. R., Rahman, A. A., and Sing, K. S. W., J. Colloid

The authors acknowledge ﬁnancial support for this work from the

National Science Foundation (CTS-9510575). X-Ray absorption experi-

Interface Sci. 84, 337 (1981).

ments were performed at the Stanford Synchrotron Radiation Laboratory

28. Smith, J. M., “Chemical Engineering Kinetics,” 2nd ed. McGraw–Hill,

(SSRL) and supported, in part, by the U.S. Department of Energy. We also

New York, 1970.

thank Messrs. W. E. Gates and J. E. Baumgartner of Exxon Research and

Engineering Co. for their help with the high-pressure n-heptane isomer-

29. Schreiner, W. N., and Jenkins, R., Adv. X-Ray Anal. 26, 141 (1983).

30. Toraya, H., Yoshimura, M., and Somiya, S., J. Am. Ceram. Soc. 67, C

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2

(1984).

31. Klug, H. P., and Alexander, L. E., “X-Ray Diffraction Procedures.”

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Article Doi

Structural and catalytic characterization of solid acids based on zirconia modified by tungsten oxide

DOI: 10.1006/jcat.1998.2269

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Article abstract of DOI:10.1006/jcat.1998.2269

Full text of DOI:10.1006/jcat.1998.2269

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