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Abstract:
Pentanoic acid (PA) is considered as an important derivative from the levulinics family. Herein, γ-valerolactone (GVL) is converted to PA in the presence of aqueous formic acid (FA) as an available, sustainable and alternative reducing agent. For this purpose, bifunctional catalyst comprising Pt supported on acidic zeolites were utilized. Pt has a dual role, decomposing FA into hydrogen which occurs in the initial stage of the reaction, and hydrogenation of the intermediate pentenoic acids (PEAs), which are formed through acid-catalyzed ring opening of GVL, to PA. Since the ring opening is thermodynamically disfavored under hydrothermal condition at high reaction temperature (543 K), hydrogenation on Pt is rate limiting and thus fast provision of hydrogen by decomposing FA on Pt is a prerequisite to PA formation from GVL. High surface area of Pt is indeed improving GVL conversion rates, whereas a rate dependency on the Brønsted acid site, as opposed to similar reaction with pure hydrogen instead of formic acid, is not encountered in this study. The acid sites though should be strong such as on ZSM-5, but stability is challenging for long term reactions in hot liquid water. Balancing the molar ratio of GVL to FA is essential for the catalytic system for two reasons. Sufficient amount of hydrogen and thus FA is needed for complete GVL conversion, but a too high FA content sluggishes hydrogenation due to strong interaction of FA and its decomposition side-products, e.g. CO, with the metal surface, overall hampering the conversion of GVL. The temperature dependence 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, thus being strong for GVL conversion and moderate for FA decomposition. Finally, the selective one-pot process of levulinic (LA), instead of GVL, to PA using FA as a reducing agent was pioneered successfully.
