Pentanoic acid from γ-valerolactone and formic acid using bifunctional catalysis

Abstract

Pentanoic acid (PA) is an industrially relevant chemical used in several applications, currently manufactured from fossil feedstock. Conversion of γ-valerolactone (GVL), a stable platform chemical from cellulosic biorefineries, into PA is studied here in the presence of aqueous formic acid (FA), as a sustainable and available reducing agent. For this purpose, bifunctional catalyst comprising Pt supported on acidic zeolites were utilized. Pt has a dual role, decomposing FA into hydrogen in the fastest step occuring in the initial stage of the reaction, and hydrogenating pentenoic acids (PEAs) intermediates, which are formed through acid-catalyzed ring opening of GVL, to PA. Since ring-opening is thermodynamically disfavored under hydrothermal conditions at high temperature (543 K), hydrogenation on Pt is rate limiting and thus fast provision of hydrogen is a prerequisite to PA formation from GVL. Strong acidity such as on ZSM-5 is required to catalyze the dehydration/ring-opening step in the reaction cascade from GVL to PA. High surface area of Pt improves GVL conversion rate, whereas no dependency of rate on Brønsted acidity is observed in the applied conditions. Strong interaction of the Pt/ZSM-5 catalyst with FA and its decomposition side-products, e.g. CO, retards the hydrogenation step, and therefore may better be added stepwise. The temperature dependency of this cascade reaction was determined, showing an apparent activation energy for GVL conversion and FA dehydrogenation of 73 kJ mol⁻¹ and 19 kJ mol⁻¹, respectively. Finally, the selective one-pot process of levulinic (LA) instead of GVL, to PA using FA as reducing agent was pioneered successfully.