(393b) Microkinetic Analysis of γ-Valerolactone Ring Opening and Decarboxylation over Solid Acids

We recently reported substantial variation in apparent turnover frequencies and kinetic parameters during the catalytic decarboxylation of γ-valerolactone over a series of solid acids including MFI, SiO₂-Al₂O₃, γ-Al₂O₃, and supported phosphotungstic acid. In aluminosilicates, Brønsted sites associated with framework aluminum contribute the majority of decarboxylation activity. Relative to bridging hydroxyls, coordinatively unsaturated aluminum sites are substantially less active, and they do not contribute significantly to butene production rates in materials having both framework and extraframework aluminum. Through comparison of aluminosilicates with supported heteropolyacids, decarboxylation barriers were found to scale with the deprotonation energy of Brønsted acid sites; however, the apparent decarboxylation turnover frequency of a Brønsted site did not necessarily correlate with its deprotonation energy. Here, we attempt to resolve observed differences in apparent phenomena and parameters through microkinetic analysis of the elementary reactions governing valerolactone decarboxylation over Brønsted sites.

On acidic catalyts, valerolactone decarboxylation is likely preceded by ring opening, which forms surface bound alkene-acids that can either desorb as stable products or cleave to form 1-butene and carbon dioxide. Analogous to various alkene chemistries, we cast the reaction pathway as proceeding through a series of alkoxide intermediates. The resulting kinetic model is parameterized at an elementary level using insights from well-studied alkene chemistries and reconciled against experimental data. With a minimal set of variable parameters, we demonstrate good agreement between an elementary-level description of valerolactone decarboxylation and measured production rates in experimental reactors. In a clear nod to Jim and his graduate catalysis course, we employ one of his favorite toolkits to provide a useful and practical framework for interpreting macroscopic kinetic observations at an elementary level.