Functional assessment of the strength of solid acid catalysts

Abstract

Abstract We describe here a rigorous method to estimate the deprotonation energy (DPE) and acid strength for solid Bronsted acids with uncertain structure using rate constants for reactions involving cationic transition states. The approach exploits relations between turnover rates for dehydration and isomerization reactions and DPE values on Keggin polyoxometalates and H-BEA solids with known structures. These relations are used to estimate the strength of acid sites in SO 4 –ZrO 2 (SZr), WO x –ZrO 2 (WZr), and perfluorosulfonic resins (SAR) from their alkanol dehydration and alkane isomerization rate constants. Alkanol dehydration and alkane isomerization proceed via pathways independent of acid identity and are limited by steps involving late transition states. Turnover rates (per accessible acid sites measured by titration during catalysis) are related to the relevant rate constants and are used to estimate DPE values for SZr, WZr, and SAR. Isomerization data estimate DPE values of 1110 kJ mol −1 and 1120 kJ mol −1 for SZr and WZr, respectively, while dehydration rate data lead to slightly higher values (1165 kJ mol −1 and 1185 kJ mol −1 ). The DPE value for SAR was 1154 kJ mol −1 from dehydration reactions, but diffusional constraints during reactions of non-polar alkanes precluded isomerization rate measurements. SZr and SAR contain stronger acid sites than zeolites (1185 kJ mol −1 ), but weaker than those in H 3 PW 12 O 40 and H 4 SiW 12 O 40 (1087 kJ mol −1 and 1105 kJ mol −1 ). Acid sites present in WZr during alkane isomerization are stronger than those present in zeolites, but these become similar in strength in the polar environment prevalent during dehydration catalysis. These effects of reaction media (and treatment protocols) reflect differences in the extent of dehydroxylation of catalytic surfaces. OH groups remaining after dehydroxylation are stronger acid sites because of a concomitant decrease in electron density in the conjugate anion and the formation of Bronsted–Lewis acid conjugate pairs. The method proposed and used here probes acid strength (as DPE) on sites of uncertain structure and within solvating media inherent in their use as catalysts. It can be used for any Bronsted acid or reaction, but requires reactivity-DPE relations for acids of known structure, the mechanistic interpretations of rates, and the measurement of accessible protons during catalysis. The resulting DPE values provide a rigorous benchmark for the structural fidelity of sites proposed for acids with uncertain structure, a method to assess the consequences of the dynamic nature of active sites in acid catalysis, and a connection between theory and experiment previously unavailable.