Is silicalite-1 inert or reactive towards alkene oligomerization?

Abstract

Brønsted acid zeolites such as ZSM-5 catalyze alkene oligomerization primarily over strongly acidic bridging hydroxyl groups (Si–OH–Al). Turnover rates of alkene oligomerization have been widely interpreted in terms of the quantity of these bridging hydroxyl groups. In this study, we investigate whether weakly acidic hydroxyl groups such as H-bonded silanol groups catalyze alkene oligomerization. The investigation was based on reacting propene over silicalite-1, a siliceous variant of ZSM-5, at 598–648 K and 0.5 MPa of propene pressure. Results show that vicinal silanols and silanol nests in silicalite-1 catalyze propene oligomerization at mild initial turnover rates (0.0017–0.027 mol mol_SiOH⁻¹ s⁻¹). Results also show that by alkaline treating silicalite-1 silanol nests are healed by reinsertion of dissolved Si species, while new mesopores are formed. These newly formed mesopores shorten diffusion paths to vicinal silanols and lead to significantly improved initial propene oligomerization turnover rates (0.25–0.70 mol mol_SiOH⁻¹ s⁻¹).