Genesis of Brønsted Acid Sites during Dehydration of 2-Butanol on Tungsten Oxide Catalysts

Abstract

In situ measurements of Brønsted acid sites (by titration with 2,6-di-tert-butyl-pyridine) and reduced centers (by UV–vis spectroscopy) were carried out on WOx–ZrO2 with catalysts at 2.9–32.3 W/nm2 in order to probe how these species form and act as active sites during 2-butanol dehydration. The rate of 2-butanol dehydration (per W-atom) on WOx–ZrO2 reached maximum values at intermediate WOx surface densities, as was also found previously for o-xylene and n-pentane isomerization. Polytungstate domains prevalent at these surface densities balance the accessibility of W6+ centers with their ability to reduce to form Brønsted acid sites via reduction with 2-butanol. In situ UV–vis spectra showed that 2-butanol reduces WOx species and that the highest density of the Brønsted acid sites formed is obtained at WOx surface densities similar to those required for maximum dehydration rates. Brønsted acid site densities were measured during 2-butanol reaction using sterically hindered 2,6-di-tert-butyl-pyridine, which titrates only Brønsted acid sites. 2-Butanol dehydration requires Brønsted acid sites that form during reaction at low concentrations (0.040 sites/W-atom) and reach their highest concentration at intermediate WOx surface densities (6.8–14.8 W/nm2). Pre-edge features in UV–vis spectra are weak for monotungstate species (<4 W/nm2) and their intensity increase is parallel to that observed in 2-butanol dehydration rates, confirming the requirement for active Hδ+ (WO3)nδ− species, formed during reaction and stabilized by polytungstate domains. Turnover rates (per Brønsted acid site) are much higher on WOx–ZrO2 than on Lewis acid sites active for 2-butanol dehydration on γ-Al2O3. 2-Butanol dehydration turnover rates (per Brønsted acid site) on SiO2-supported tungstophosphoric acid (HPW), however, are higher than those on WOx–ZrO2. The mechanism-based rate equation developed for 2-butanol dehydration and the faster secondary butene isomerization found on HPW suggest that acid sites are weaker on HPW than on WOx–ZrO2. A weaker acidity is indeed expected from the stronger conjugate base provided by the smaller WOx domains and the larger H+/W ratio in HPW clusters. Weaker acid sites allow faster product desorption and higher turnover rates from saturated surfaces during 2-butanol dehydration.