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

Abstract

The effects of support identity on catalytic 2-butanol dehydration rates, Brønsted acid site density, and reducibility are examined for WO x domains supported on ZrO 2, Al 2O 3, SiO 2 (MCM41), and SnO 2. On WO x –Al 2O 3, 2-butanol dehydration rates (per W atom) increased with increasing WO x surface density and reached maximum values at WO x surface densities (9–10 W nm −2) similar to those required for two-dimensional polytungstates, as also found on WO x –ZrO 2. UV–visible edge energies showed that WO x domains become larger as WO x 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 WO x domains on ZrO 2, Al 2O 3, SiO 2, and SnO 2 supports. Pre-edge features appear in the UV–visible spectra of WO x –Al 2O 3 samples during 2-butanol dehydration and their intensity increases with WO x 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 H δ + ( WO 3 ) n δ − species, using 2-butanol as a stoichiometric reductant. These processes resemble those on WO x –ZrO 2, indicating that temporary acid sites generally form from neutral WO x precursors on all supports. Dehydration turnover rates (per Brønsted acid site) were unaffected by the identity of the support, but for a given WO x 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 ZrO 2-supported than on Al 2O 3-supported samples, as a result of electronic isolation of WO x domains on the more insulating and unreducible Al 2O 3 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.