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 9 tungsten atoms per nm2 (W/nm2), calculated from WO3 weight loading and BET surface area values). Preliminary data show a maximum in nC5 conversion activity at ~6 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. Efforts to determine the metal oxide surface structure responsible for the strongly acidic active sites and to characterize the nature of surface acidity through IR studies are ongoing. By establishing the surface density model and learning its limitations, we can improve upon WOx/ZrO2 acid catalysis through rational nanostructure control.