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Metal Oxide Surface Density Effects on the Acidity of Tungstated Zirconia

Nikolaos Soultanidis, Chemical and Biomolecular Engineering, Rice University, 6100 Main Street, MS-362, Houston, TX 77005, Alejandro J. Gonzalez, DCG Partnership, 4170A South Main, Pearland, TX 77581, Antonios C. Psarras, Chemical Engineering, Aristotle University, Aristotle University of Thessaloniki 541 24 Thessaloniki - Greece, Thessaloniki, Greece, Israel E. Wachs, Chemical Engineering, Lehigh University, Iacocca Hall, 111 Research drive, Bethlehem, PA 18015-4791, and Michael S. Wong, Chemical and Biomolecular Engineering & Chemistry, Rice University, 6100 Main Street, MS-362, Houston, TX 77251-1892.

The catalytic isomerization of C4 to C8 paraffins to branched isomers by tungstated zirconia (WOx/ZrO2) has been investigated extensively in the last decade. With the demands for cleaner and cheaper energy rising, tungstated zirconia catalysts attracted the interest of scientists. Among their benefits are their stability at higher temperatures and their low deactivation rate during the isomerization of alkanes to high-octane-number branched alkanes. Various studies have demonstrated the effects of preparation method and pretreatment conditions on the activity and selectivity of this reaction, with several structures postulated for the catalytically active site. The metal oxide surface density model has successfully correlated the volcano-shape dependence of activity to metal oxide surface coverage and metal oxide surface structure for acid catalyzed reactions, such as 2-butanol dehydration, o-xylene isomerization and naphthalene alkylation. Insofar as we know, the tungsten surface density effect in WOx/ZrO2 has not been established for n-pentane (nC5) isomerization, a strong acid-demanding reaction.

In this study, two series of WOx/ZrO2 catalysts were synthesized by incipient wetness impregnation of (1) crystalline ZrO2 and (2) amorphous metastable support Zr(OH) 4 using ammonium metatungstate precursor. The nC5 conversion catalytic activity and product selectivity of these materials were studied with respect to surface density between 2 and 11 tungsten atoms per nm2 (W/nm2), calculated from WO3 weight loading and BET surface area values). Maximum activity in nC5 conversion is observed at ~5.2 W/nm2, indicating that a strong acid-demanding reaction like nC5 isomerization follows a volcano-shape dependence similar to less acid demanding reactions. Results further show that, at a given tungsten surface density, Zr(OH) 4 serves as a better ZrO2 source than crystalline ZrO2 for this reaction, in agreement with observations by other research groups. Pyridine FTIR experiments gave a better insight in the acidic properties of this system. With respect to Brønsted acidity, tungstated zirconia has similar concentrations of Brønsted sites with commercial FCC equilibrium catalysts. The deactivation of Brønsted sites by 44% due to coke deposition indicated the direct correlation between catalytic activity and Brønsted acidity.

By establishing the surface density model and understanding its impact on to the acidic properties of tungstated zirconia, we can improve upon WOx/ZrO2 catalysis through rational nanostructure control.