Theoretical insights into alkylation catalysis over polyoxometalates

Acid catalysis is at the heart of many hydrocarbon conversion processes including catalytic cracking, isomerization, oligomerization, and alkylation. The first three of these processes are typically carried out over solid acids. However, the alkylation of isobutane with n-butene is predominantly carried out using homogenous HF and H₂SO₄, which are highly corrosive and lead to waste disposal and catalyst separations issues. Heteropolyacids (HPA) are active for the alkylation of isobutane and n-butene but suffer from rapid catalyst deactivation. HPA are proposed to be superacids, and their acid strength has raised hope that their deactivation during alkylation may be surmountable. A discussion on the relationship between measurements of acid strength and the energetics of hydrocarbon conversion processes over HPA covers the reaction energetics of the individual steps in the alkylation over phosphotungstic acid (H₃PW₁₂O₄₀); reaction energetics of alkene adsorption and oligomerization, alkylation, and hydride transfer steps; and prospects of developing a solid acid catalyst for the alkylation of isobutane and n-butene. This is an abstract of a paper presented at the AIChE Annual Meeting and Fall Showcase (Cincinnati, OH 10/30/2005-11/4/2005).