Support effects on Brønsted acid site densities and alcohol dehydration turnover rates on tungsten oxide domains

The effects of support identity on catalytic 2-butanol dehydration rates, Brønsted acid site density, and reducibility are examined for WOx domains supported on ZrO2, Al2O3, SiO2 (MCM41), and SnO2. On WOx–Al2O3, 2-butanol dehydration rates (per W atom) increased with increasing WOx surface density and reached maximum values at WOx surface densities (9–10 W nm−2) similar to those required for two-dimensional polytungstates, as also found on WOx–ZrO2. UV–visible edge energies showed that WOx domains become larger as WOx surface density increases. Selective titration of Brønsted acid sites by sterically hindered 2,6-di-tert-butylpyridine during 2-butanol dehydration reaction showed that this reaction occurs predominately on Brønsted acid sites for WOx domains on ZrO2, Al2O3, SiO2, and SnO2 supports. Pre-edge features appear in the UV–visible spectra of WOx–Al2O3 samples during 2-butanol dehydration and their intensity increases with WOx surface density in parallel with measured Brønsted acid site densities and dehydration rates (per W atom). These d–d electronic transitions reflect the formation of reduced centers, consisting of acidic image species, using 2-butanol as a stoichiometric reductant. These processes resemble those on WOx–ZrO2, indicating that temporary acid sites generally form from neutral WOx precursors on all supports. Dehydration turnover rates (per Brønsted acid site) were unaffected by the identity of the support, but for a given WOx surface density, the number of reduced centers and the density of Brønsted acid sites, but not their intrinsic reactivity, depend on the identity of the support; both reduced centers and Brønsted acid sites are more abundant on ZrO2-supported than on Al2O3-supported samples, as a result of electronic isolation of WOx domains on the more insulating and unreducible Al2O3 supports. The dehydration regioselectivity on Brønsted acid sites is strongly influenced by support, with more electropositive support cations leading to stronger interactions between α-hydrogens in reactants and lattice oxygens, favoring sterically hindered transition states required for the formation of cis-2-butene.