Operando infrared spectroscopy to probe the importance of framework selection in silicon-doped aluminophosphates, SAPO-5, SAPO-11, SAPO-18, and SAPO-34, for acid catalysed dimethyl ether formation

Abstract

Methanol dehydration chemistry is heavily reliant on solid acid catalysts for the formation of a wide range of hydrocarbons. Whilst olefins are routinely formed on strong Brønsted acid sites, there is a growing interest in dimethyl ether production, due to its potential as a sustainable fuel alternative, which is compatible with current petroleum infrastructure. The effective formation of dimethyl ether over extended time periods typically favours weaker acid sites. Here, two methanol molecules can couple together, reducing the formation of larger aromatic products that facilitate the methanol-to-olefin process, but which can also facilitate catalyst deactivation. In this manuscript, we use operando diffuse reflectance infrared Fourier transform spectroscopy to probe methanol dehydration on a range of microporous silicon-doped aluminophosphates (SAPO-5, SAPO-11, SAPO-18, and SAPO-34), correlating the findings with catalytic data to highlight the key parameters for an effective methanol-to-dimethyl ether catalyst. In doing so, we demonstrate that weaker acid sites play a key role in the production of dimethyl ether by permitting bound methoxy species and unbound methanol molecules to co-exist, triggering dimethyl ether formation.