Tungstated Zirconia Catalysts for Liquid-Phase Beckmann

Apr 10, 2009 - E-mail: (N.R.S.) [email protected]; (D.R.B.) [email protected]. Fax: +44 (0) 1484 472182., †. University of Huddersfield. , ‡...
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J. Phys. Chem. C 2009, 113, 7735–7742

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Tungstated Zirconia Catalysts for Liquid-Phase Beckmann Rearrangement of Cyclohexanone Oxime: Structure-Activity Relationship N. R. Shiju,*,† M. AnilKumar,‡ W. F. Hoelderich,‡ and D. R. Brown*,† Materials and Catalysis Research Centre, Department of Chemical and Biological Sciences, UniVersity of Huddersfield, Queensgate, Huddersfield HD1 3DH, U.K., and Chemical Technology and Heterogeneous Catalysis Department, UniVersity of Technology RWTH Aachen, Worringerweg 1, 52074 Aachen, Germany ReceiVed: December 1, 2008; ReVised Manuscript ReceiVed: February 20, 2009

The performance of tungstated zirconia catalysts with different tungsten loadings and calcination temperatures for liquid-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam was studied and the relation of their activity with the structure of tungsten species on zirconia surface was investigated. Cyclohexanone was the major product when zirconia was used as the catalyst while ε-caprolactam was formed in major amounts with tungstated zirconia. The relative amounts of these products depended on the W surface density, and the maximum selectivity to ε-caprolactam was observed at tungsten loadings near that required for the formation of a monolayer. UV-visible absorption edge energies suggested that the surface contains polytungstate species at these loadings. These polytungstate domains create Brønsted acid centers on the surface, as suggested by acidity measurements using NH3 adsorption microcalorimetry and FTIR of adsorbed pyridine, thereby making the tungstated zirconia catalysts active for the formation of ε-caprolactam. 1. Introduction The commercial production of ε-caprolactam, the precursor of Nylon 6, involves the Beckmann rearrangement of cyclohexanone oxime (Scheme 1) using fuming sulfuric acid as both catalyst and reaction medium.1 A large amount of ammonium sulfate is produced during the neutralization of sulfuric acid to release the ε-caprolactam. The development of a Beckmann rearrangement process using solid acids as catalysts replacing concentrated sulfuric acid can lead to a clean process avoiding the problem of corrosion and the formation of ammonium sulfate byproduct. A number of solid acid catalysts such as zeolites, MCM-41, silica, alumina, silica-alumina, and supported metal oxides have been investigated for the vapor-phase rearrangement of cyclohexanone oxime; however, investigations of liquid-phase rearrangement are rather limited.1-35 The vapor-phase reactions require higher temperatures, usually above 573 K, to keep the oxime and products in the vapor phase. This leads to a decrease in the lactam selectivity and a fast deactivation of the catalyst due to coke formation. Hence, it is desirable to conduct the reaction under relatively mild liquid phase conditions, which may also be preferred energetically. In this work, we have investigated tungstated zirconia catalysts for the liquid-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam. Zirconia-supported oxoanions have been examined as solid acid catalysts recently in industrial isomerization and alkylation processes instead of the highly corrosive and pollutant liquid acids.36 Among them, tungstated zirconia possesses some advantages such as higher stability, lower deactivation rates, and easier regeneration. Since the discovery of the ability of tungstated zirconia to catalyze alkane isomerization at low temperatures, there were several reports of the use of these materials for other reactions such as * To whom correspondence should be addressed. E-mail: (N.R.S.) [email protected]; (D.R.B.) [email protected]. Fax: +44 (0) 1484 472182. † University of Huddersfield. ‡ University of Technology RWTH Aachen.

SCHEME 1: Beckmann Rearrangement of Cyclohexanone Oxime to ε-Caprolactam

isomerization, esterification, transesterification, cyclohexane ring-opening, benzene hydrogenation, alkene oligomerization, aromatic alkylation with alkenes or methanol, aromatic transalkylation, and heteroatom removal.37-53 Some of the previous studies concentrated on the inorganic structures and reaction pathways responsible for acid catalysis on these materials.37-42 It was suggested by Hino and Arata that the formation of so-called “superacid” sites (H0 ) -14.52) makes these materials active catalysts for isomerization of butane and pentane, as well as Friedel-Craft acylations.36 The formation of these acid sites depends strongly on the preparation conditions and calcination temperatures. Higher calcination temperatures are required for the stabilization of a tetragonal phase and for the formation of WOx species on zirconia surface, thereby leading to the creation of acid sites. It has been observed that turnover rates for xylene isomerization and alcohol dehydration were solely a function of the tungsten surface density. Some previous studies of tungstated zirconia catalysts have shown that the formation of surface polymeric tungsten species correlates well with an increase in tungstated zirconia (WZ) catalyst activity.37,38 It was also suggested that neutral WOx precursors can generate Brønsted acid sites during catalytic reactions via reduction of polytungstate structures.37,38 In the present work, we have examined tungstated zirconia catalysts for the liquidphase Beckmann rearrangement as a function of the tungsten loading and calcination temperature with an objective to gain a fundamental understanding of the structure-activity relationship of WZ for Beckmann rearrangement reaction.

10.1021/jp810542t CCC: $40.75  2009 American Chemical Society Published on Web 04/10/2009

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TABLE 1: BET Surface Area, Surface Density and Absorption Edge Energy Values of Tungstated Zirconia Samples

sample ZrOH 5WZ 10WZ 15WZ 25WZ 35WZ 10WZ373b 10WZ773b 10WZ1073b

BET nominal calcination surface surface W absorption content temperature area density, δ edge energy, (K) (m2/g) (W at/nm2)a (wt%) (eV) 0 5 10 15 25 35 10 10 10

973 973 973 973 973 373 773 1073

274.8 45.3 61.4 67.7 50.5 23.1 231.6 100.4 43.2

3.6 5.3 7.3 16.2 49.6 1.4 3.3 7.6

3.57 3.45 3.35 3.10, 2.71 3.03, 2.71 3.99 3.61 3.20, 2.87

a Calculated based on the amount of tungsten and BET surface area. b Calcination temperatures in K are indicated.

Figure 2. X-ray diffraction patterns of 10 wt % WZ catalyst as a function of calcination temperature.

Figure 1. X-ray diffraction patterns of tungstated zirconia catalysts calcined at 973 K and zirconium hydroxide. The tungsten content of the catalysts in wt % is indicated in the figure.

2. Experimental Details Zirconium oxohydroxide was prepared by the controlled addition of aqueous zirconium oxychloride (ZrOCl2 · 8H2O, Sigma-Aldrich, U.K.) solution to an NH4OH (2M)/NH4Cl (2M) buffer solution to keep the pH at a constant value of 10.5. The solid was filtered and thoroughly washed with distilled water to remove the chlorides. Tungstated zirconia catalysts were prepared by the impregnation of zirconium oxohydroxide using an aqueous solution of ammonium metatungstate (SigmaAldrich, U.K.). The impregnated materials were dried at 333 K overnight and calcined under static air for 4 h. The zeolite samples were obtained from Catal International, Sheffield, U.K. The rearrangement of cyclohexanone oxime (Sigma-Aldrich, U.K.) was carried out in the liquid phase at 353-423 K in a 50 mL glass reactor equipped with a condenser and a magnetic stirrer. Tetradecane (Koch-Light Laboratories, U.K.) was added as a GC internal standard. To monitor the reaction, 0.1 mL samples of the reaction mixture were taken periodically and analyzed by gas chromatography (Perkin-Elmer Clarus 500) using a 50 m BP1 capillary column and an FID detector. The N2 adsorption-desorption isotherms were measured at 77 K on a Micromeritics ASAP-2000 after evacuation at 473 K for 5 h. The surface area was calculated by the Branauer-Emmett-

Teller (BET) method. Powder X-ray diffraction (XRD) patterns were collected on a Siemens Diffractometer (D5000) operated at 45 kV and 40 mA using Nickel filtrated Cu KR radiation (1.5406 Å). Raman spectra were collected on a Bruker RFS 100/S with the 1064 nm line of Nd-YAC laser. The system used for ammonia adsorption flow calorimetry has been described previously.54,55 It is based on a Setaram 111 DSC with an automated gas flow and switching system with a mass spectrometer (Hiden HPR20) to sample the downstream gas flow. The sample (20-30 mg) was held on a glass frit in a vertical silica sample tube and activated at 423 K under a dried helium flow (5 mL min-1) for five hours. After activation, the sample temperature was maintained at 423 K and 1 mL pulses of the probe gas (1% ammonia in helium) at atmospheric pressure were injected at regular intervals into the carrier gas stream from a gas sampling valve. The ammonia concentration downstream of the sample was monitored continuously by mass spectroscopy. The pulse interval was chosen to ensure that the ammonia concentration in the carrier gas returned to zero to allow the DSC baseline to stabilize. The net amount of ammonia irreversibly adsorbed from each pulse was determined by comparing the MS signal with that recorded through a control experiment with a blank sample tube. The net heat released by each pulse was calculated from the thermal DSC curve. UV-vis spectra were collected on Perkin-Elmer λ 35 spectrometer, using a labsphere reflectance spectroscopy accessory. The Kubelka-Munk function, F(R∞) for infinitely thick samples was used to convert reflectance measurements (Rsample) into equivalent absorption spectra using the reflectance of MgO as a reference (RMgO).

R∞ ) Rsample /RMgO

and

F(R∞) ) (1-R∞)2 /2R∞

For FTIR spectroscopy of pyridine adsorption, the measurements were performed using Nicolet Prote´ge´ 460 equipped with an evacuable furnace cell with KBr windows, containing a sample wafer. Initially, catalyst powder was pressed into a selfsupported wafer, which was loaded into the IR chamber and heated up to 673 K overnight under reduced pressure of 10-3 mbar. After the cell was cooled down to 323 K, the background spectrum was recorded. Spectra were collected as an average of 200 runs with 0.5 cm-1 definition. The pyridine adsorption was carried out slowly where the catalyst was equilibrated with

Tungstated Zirconia Catalysts

Figure 3. Laser Raman spectra of tungstated zirconia catalysts calcined at 973 K. The tungsten content of the catalysts in wt% is indicated in the figure.

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Figure 5. Diffuse reflectance UV-visible spectra of tungstated zirconia catalysts calcined at 973 K. The tungsten content of the catalysts in wt % is indicated in the figure.

TABLE 2: Performance of Tungstated Zirconia Catalysts in Liquid-Phase Beckmann Rearrangement of Cyclohexanone Oximea sample ZrOH 5WZ 10WZ 15WZ 25WZ 35WZ 10WZ373b 10WZ773b 10WZ1073b HYc HZSM-5c

surface density (W at/nm2)

conversion of oxime, %

selectivity to ε-caprolactam,%

0 3.6 5.3 7.3 16.2 49.6 1.4 3.3 7.6

32.4 33.0 49.3 35.9 37.4 46.1 47.9 33.7 57.1 71.2 22.2

24.5 61.4 79.0 79.3 69.4 57.3 42.0 51.7 62.1 75.0 56.8

a Reaction temperature ) 403 K; solvent ) benzonitrile; oxime ) 0.9 mmol; catalyst ) 20 mg. b 10WZ calcined at different temperatures, see Table 1. c SiO2/Al2O3 ) 6 (HY) and 27 (HZSM-5). Catalyst ) 100 mg and oxime ) 1 mmol.

Figure 4. Laser Raman spectra of 10WZ catalyst as a function of calcination temperature.

pyridine vapor at 323 K. After 60 min evacuation, a spectrum was recorded. The sample was then heated stepwise, scanning with IR spectroscopy. 3. Results and Discussion The tungsten content, BET surface area, and tungsten surface density (δ) of WZ samples are given in Table 1. The presence of zirconia and tungsten oxide crystalline phases in WZ samples calcined at 973 K was identified by XRD (Figure 1). The catalyst samples contain tetragonal phase of zirconia, characterized mainly by the peaks at 2θ ∼ 30, 35, 50, and 60° (Figure 1). It is known that WOx species stabilize tetragonal ZrO2 crystallites while monoclinic ZrO2 is the thermodynamically stable crystal structure for crystallites larger than 10 nm at temperatures below 1443 K, without the presence of WO3. Additional peaks are observed for samples with tungsten loading of 25 and 35 wt % (surface density exceeding 7.3 W atoms nm-2) in the 2θ range of 23-25°. These peaks can be assigned to monoclinic WO3 microcrystallites according to the previous studies.41,45 It has been

suggested previously that the agglomeration of WOx species leading to WO3 microcrystallites (2θ ) 23.2, 23.7, and 24.3°) on the zirconia surface occurs when the tungsten coverage exceeds that of a monolayer.41,45 At a given tungsten loading, the structure depends on the calcination temperature (Figure 2). The sample with 10% W loading (10WZ) heated at 373 K was amorphous. When the sample 10WZ was calcined at 773 K, tetragonal zirconia was crystallized, while the calcination at 1073 K led to the formation of WO3 crystallites also. These observations were further confirmed by Raman spectroscopy. The Raman spectra taken at ambient temperature for WZ samples calcined at 973 K are given in Figure 3. In the lower-frequency region, the samples with lower tungsten loading display the band at ca. 650 cm-1, which is characteristic of tetragonal zirconia. In addition, a band at ca. 620 cm-1 of monoclinic zirconia is observed for the sample with the lowest tungsten loading. These features related to the zirconia support are in agreement with the XRD data discussed above. The bands corresponding to surface tungsten species are observed at higher Raman shifts. The bands observed at ∼828 and at ∼980 cm-1 can be assigned to W-O-W stretching

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Figure 6. FTIR spectra of adsorbed pyridine on zirconia (A), 5WZr (B), 10WZr (C), and 25WZr (D). The desorption temperatures are indicated in the figures.

modes and WdO vibrations respectively in hydrated interconnecting polyoxotungstate clusters.52 At tungsten loadings g15 wt %, two new bands ∼715 and 807 cm-1, associated with WdO bending and stretching modes respectively in microcrystalline WO3 species41,45 are observed. Thus, the Raman results confirm the formation of WO3 crystallites at tungsten loadings g15 wt %. The formation of WO3 microcrystallites was also observed when the samples were calcined at higher temperatures. Figure 4 shows the Raman spectra of a sample with 10 wt % W loading calcined at different temperatures. The sample calcined at 1073 K exhibits clearly bands corresponding to WO3 crystallites. The broad band in the 900-950 cm-1 region for the sample calcined at 373 K may be assigned to WOx-O-Zr bonds and the shift in this band with higher calcination temperatures indicates growing interaction between the WOx groups leading to the formation and growth of polytungstate domains. These WOx species have been postulated by previous studies on the basis of the local environment around the W

atoms, where the oxygen coordination number was suggested to be 6, from X-ray absorption spectroscopy studies.38 Figure 5 shows the diffuse reflectance UV-vis spectra for the tungstated zirconia samples calcined at 973 K. All of the WZ catalysts display main absorption features at energies ranging from 2.5 to 4.8 eV due to ligand-to-metal charge transfers in tungsten species (O2p f W5d-O2p) existing on the ZrO2 surface.37 The energy required for this transition depends strongly on WOx concentration and oxidation temperature. Table 1 shows the optical absorption edge energy (AEE) values calculated using these spectra. AEE, defined as the minimum energy required to excite an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), allows a rigorous measurement at the incipient absorption region.37 The AEE values were obtained using the model typically applied for indirect-allowed HOMO f LUMO transitions occurring in amorphous nanosized semiconductor domains, which have been successfully used to characterize WOx

Tungstated Zirconia Catalysts

Figure 7. Heat of NH3 adsorption on tungstated zirconia samples measured by microcalorimetry. The tungsten content in wt % are indicated in the figure.

species supported on metallic oxides.37,39 Thus, the AEE values were determined by finding the x-intercept of the straight tangent line at the proximity of the absorption onset in the [F(R∞)hν]1/η versus hν plots, where F(R∞) is the Kubelka-Munk function, R∞ is the reflectance at infinite sample thickness, hν is the incident photon energy, and η ) 2 for indirect-allowed transitions.37 The absorption energy of WO3 microcrystallites and amorphous WOx species can be better approximated by the indirect electron transition formalism, rather than using directallowed transitions (η ) 1/2).56 Previous studies suggested that the absorption edge energy shifts to lower energies with increasing surface density and is sensitive to the domain size of transition metal oxides.57 The absorption edge energy value obtained in this study for Na2WO4, having molecular (isolated) four-coordinate W6+ centers, is 4.56 eV while that of WO3 that contains WOx species in an extended three-dimensional crystalline network of distorted octahedral coordination is 2.7 eV. For ammonium metatungstate

J. Phys. Chem. C, Vol. 113, No. 18, 2009 7739 having WOx domains of intermediate size, an absorption edge energy of 3.2 eV is obtained, intermediate between the values of Na2WO4 and WO3. The absorption edge energy value obtained for 5 wt % WZ (surface density 3.6 W atoms nm-2) in this work is 3.57 V, lower than that of Na2WO4, which indicates the presence of WOx species interacting with each other to some extent rather than isolated WOx species as in Na2WO4. The absorption edge energies of the samples decrease with increasing surface density (Table 1), which indicates an increase in the interaction of WOx species and consequently a growth in domain size. The edge energy values of samples with intermediate surface density are close to that of ammonium tungstate, the model compound characterized by the existence of WOx species in polytungstate domains. For the samples with higher surface densities (>7 W atoms nm-2), a shoulder at the lower energy side of the main absorption edge becomes apparent, giving rise to a second edge energy value that is closer to that of WO3 (Table 1). Hence, at higher surface densities the increase in domain size finally leads to the conversion of a fraction of WOx species to WO3 microcrystallites, which is also confirmed by X-ray diffraction and Raman spectroscopy, as discussed in the previous paragraphs. A similar trend can be observed for samples with the same tungsten loading calcined at different temperatures. For example, the absorption edge energy of 10WZ dried at 373 K was 3.99 eV, indicative of minimal interaction between the WOx species in this sample. With an increase in calcination temperature, the absorption edge energy decreased indicating the growth of WOx domains. For the sample calcined at 1073 K, the edge energies were 3.20 and 2.87 eV, characteristic of significant interactions between WOx species on the surface of this sample. As the coverage of the ZrO2 support by WOx species increases, the dispersed WOx species eventually form W-O-W bridging bonds between neighboring WOx groups, resulting in the formation of two-dimensional polytungstates and three-dimensional WO3 crystallites. The formation of these W-O-W bonds between WOx octahedra leads to larger domains and to a narrowing of the HOMO-LUMO gap. The increasing domain size of WOx species in going from isolated to polymeric structures enhance the electron delocalization leading to a decreasing trend in AEE.

Figure 8. Variation of absorption edge energy (9) and ε-caprolactam selectivity (b) as a function of W surface density. The data include samples with different tungsten contents as well as 10WZ calcined at different temperatures.

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Figure 9. The dependence of ε-caprolactam selectivity extrapolated to zero conversion of oxime, initial heat of adsorption measured by NH3 microcalorimetry and absorption edge energy obtained from UV-visible spectroscopy on W surface density.

The performances of tungstated zirconia catalysts in the liquid-phase Beckmann rearrangement of cyclohexanone oxime are shown in Table 2. The reactions were conducted at 403 K in benzonitrile solvent. For zirconium oxyhydroxide, the major product was cyclohexanone with the selectivity to ε-caprolactam being around 20%. With the addition of tungsten, the selectivity of ε-caprolactam increased, making it the major product for calcined tungstated zirconia samples. The samples with intermediate WOx surface densities showed highest selectivity to ε-caprolactam. The Beckmann rearrangement is commonly believed to be initiated by protonation at the oxime group, hence the formation of ε-caprolactam typically depends on Brønsted acid sites. The high selectivity toward cyclohexanone with zirconium oxyhydroxide can be explained by the presence of Lewis acid sites since those are proposed to be responsible for cyclohexanone formation. The inverse in selectivity on the addition of tungsten shows that Brønsted acid sites are developed on the surface after tungsten incorporation. The formation of Brønsted acid sites was observed by FTIR using pyridine as the probe molecule. As can be observed from Figure 6, the 1445 cm-1 band, characteristic of pyridine coordinatively bound to

Lewis acid sites, has a higher area than the band at 1545 cm-1 (corresponding to the vibration of pyridinium ions) indicating a higher concentration of Lewis acid sites for the sample with 3.6 W nm-2 (5WZ). The spectra obtained for ZrO2 show that mainly Lewis acid sites are present on this sample (Figure 6A). For the sample with 5.3 W nm-2 (10WZ), the Brønsted/Lewis acid site ratio is much higher than that of the sample with 3.6 W nm-2, especially at the desorption temperatures of 423 and 473 K, showing that the surface now contains Brønsted acid sites predominantly. These Brønsted acid sites are fairly strong as much of the pyridine adsorbed at low temperature was retained on the surface after increasing the temperature (Figure 6). The Brønsted/Lewis acid site ratio calculated based on the areas of bands at 1545 and 1445 cm-1 from the spectra obtained at desorption temperature 423 K (closest to the reaction temperature 403 K) are 0.07, 0.75, 4.5, and 1.71 for ZrO2, 5WZr, 10WZr, and 25WZr, respectively. These values show the trend in the formation of Brønsted acid sites with the addition of tungsten. An approximate trend in acid strength was also provided by NH3 adsorption microcalorimetry. All the tungstated zirconia samples exhibit higher heats of NH3 adsorption than

Tungstated Zirconia Catalysts zirconium oxyhydroxide (Figure 7). Samples with the lowest and highest tungsten loading have the lowest heat of adsorption among the tungstated zirconia samples, which is in agreement with the catalytic activity results. The selectivities to ε-caprolactam extrapolated to zero conversion of oxime are shown in Figure 8 together with absorption edge energy values as a function of W surface density. At low surface densities (10 W atoms nm-2), ε-caprolactam selectivity and the Brønsted/Lewis acid site ratio decrease. NH3 adsorption microcalorimetry shows that the acid strength decreases at higher surface densities (Figure 7) and the lower acid strength of WO3 microcrystallites formed at higher surface densities may play a role in lowering the lactam selectivity. The critical WOx surface density at which maximum selectivity for ε-caprolactam was obtained is near around the theoretical monolayer capacity of ZrO2 (7.3 W nm-2), calculated based on the (001) projection of ZrO2 and assuming WO6 octahedra anchored to exposed Zr-OH sites.39 Hence, polytungstate domains of monolayer volume is an ideal condition for the oxime to go through the desired Beckmann rearrangement for the formation of ε-caprolactam, rather than unwanted hydrolysis to produce cyclohexanone. Since ε-caprolactam is formed by Brønsted acid sites, it follows that these domains of polytungstate species possess highest Brønsted acidity. Heat of adsorption values measured by NH3 adsorption microcalorimetry (Figure 7 and 9) also indicate higher acid strengths at intermediate surface densities, supporting this assumption. This observed trend in acidity may be related to the reducibility of the species at different surface densities. It is reasonable to assume that WOx species at low surface densities are not easily reducible owing to strong interaction with the zirconia support while those in extended polytungstate domains are relatively easily reducible. Previous TPR studies observed that the reducibility of polytungstate species is higher than WOx species present at low surface densities, hence suggesting a lower barrier for the formation of Brønsted acid centers at intermediate surface densities.38 This was also supported by the H2 chemisorption experiments reported previously for tungstated zirconia samples, which showed maximum hydrogen uptakes for samples with intermediate loading, suggesting that the formation of W6-nOx-(nH+) reduced centers is much easier for polytungstate species.37 The extended network of WOx species in them can easily delocalize the negative charge required to form the Brønsted acid centers or to stabilize carbocationic reaction intermediates. As we have discussed before, the ease of electron delocalization with increase in size of this network is responsible for a decrease in absorption edge energy, linking the trends in these variables together (Figure 9). Though the growth of the WOx domains leads to higher selectivity for ε-caprolactam, growth beyond monolayer volume resulting in the formation of WO3 crystallites again decreases the selectivity. Hence the decrease in selectivity at higher surface densities is due to the formation of bulklike WO3, which apparently possesses lower Brønsted acidity than polytungstate species. It is also to be noted that the concentration of strong acid sites (those having heats of adsorption greater than 80 kJ mol-1) is lowest for 35WZ, though having the highest amount of tungsten in this sample. Microcalorimetry results show that though the acid site con-

J. Phys. Chem. C, Vol. 113, No. 18, 2009 7741 centration (based on the surface coverage of ammonia with a heat of adsorption of g 80 kJ mol-1) increases initially as the surface density increases. Increase in surface density beyond that of monolayer value decreases the acid site concentration (Figure 7). This is apparently due to the bulklike character of WO3 at higher surface densities that prevents the accessibility of probe molecules (as well as reactant molecules) to WOx centers since many WOx species reside inside the crystallites. Hence WOx domains of an intermediate size having polytungstate species provide maximum accessibility to WOx centers with suitable type and strength of acidity, and thereby favor maximum selectivity for the Beckmann rearrangement product, ε-caprolactam. 4. Conclusions Tungstated zirconia catalysts, prepared by impregnation of ZrOx(OH)4-2x with a solution of ammonium metatungstate followed by oxidation treatments, were found to be active for the liquid-phase Beckmann rearrangement of cyclohexanone oxime. WOx species inhibit ZrO2 crystallite sintering and stabilize tetragonal ZrO2 crystallites during high-temperature oxidative treatments. UV-vis absorption edge energies suggest that zirconia surfaces lead to dispersed WOx species at low surface densities. The decrease in absorption edge energies indicated the growth of WOx domains as WOx surface density increases. The selectivities of Beckmann rearrangement and hydrolysis reactions of oxime strongly depend on the WOx surface density. With zirconia alone, cyclohexanone, the hydrolysis product of oxime, was the major product. ε-Caprolactam, the Beckmann rearrangement product was increased with the addition of tungsten, and maximum ε-caprolactam selectivity was observed for WOx domains of intermediate size at WOx surface densities near to monolayer coverage on ZrO2. At low coverages, ε-caprolactam selectivities increase with increasing WOx surface density, which is related to the increased Brønsted acid centers formed by the polyoxoanion domains. The formation of WO3 crystallites at higher surface densities decreases the acidity and selectivity to ε-caprolactam. WOx domains of intermediate size appear to provide the right kind of acid sites with optimum strength as well as maximum accessibility of reactant molecules to WOx species. Acknowledgment. The authors thank Dr. H. M. Williams and Mr. Ibrahim George, University of Huddersfield for supporting microcalorimetry and UV-visible spectroscopy studies. We also thank Mr. Karl Joseph Vaesen, TCHK, RWTH, and Dr. T. Walter, DWI, RWTH, Aachen for various characterization studies. References and Notes (1) Bellussi, G.; Perego, C. CATTECH 2000, 4, 4. (2) Dahlhoff, G.; Niederer, J. P. M.; Hoelderich, W. F. Catal. ReV. 2001, 43, 381, and references therein. (3) Ichihashia, H.; Ishidab, M.; Shigab, A.; Kitamuraa, M.; Suzukia, T.; Suenobuc, K.; Sugitaa, K. Catal. SurV. Asia 2003, 7, 261, and references therein. (4) Regli, L.; Bordiga, S.; Lamberti, C.; Lillerud, K. P.; Zones, S. I.; Zecchina, A. J. Phys. Chem. C 2007, 111, 2992. (5) Ichihashi, H.; Sato, H. Appl. Catal., A 2001, 221, 359. (6) Anilkumar, M.; Ho¨lderich, W. F. J. Catal. 2008, 260, 17. (7) Kim, S.-G.; Kawakami, T.; Ando, T.; Yukawa, Y. Bull. Chem. Soc. Jpn. 1979, 52, 1115. (8) Botella, P.; Corma, A.; Iborra, S.; Monto´n, R.; Rodrı´guez, I.; Costa, V. J. Catal. 2007, 250, 161. (9) Forni, L.; Fornasari, G.; Tosi, C.; Trifiro`, F.; Vaccari, A.; Dumeignil, F.; Grimblot, J. Appl. Catal. 2003, 248, 47. (10) Curtin, T.; McMonagle, J. B.; Hodnett, B. K. Appl. Catal., A 1992, 93, 75.

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