γ-Valerolactone Ring-Opening and Decarboxylation over SiO2/Al2O3 in the Presence of Water

Abstract

γ-Valerolactone (GVL) has been identified as a promising, sustainable platform molecule that can be produced from lignocellulosic biomass. The chemical flexibility of GVL has allowed the development of a variety of processes to prepare renewable fuels and chemicals. In the present work involving a combination of computational and experimental studies, we explore the factors governing the ring-opening of GVL to produce pentenoic acid isomers, as well as their subsequent decarboxylation over acid catalysts or hydrogenation over metal catalysts. The ring-opening of GVL has shown to be a reversible reaction, while both the decarboxylation and hydrogenation reactions are irreversible and kinetically controlled under the conditions studied (temperatures from about 500 to 650 K). The most significant contributor to lactone reactivity toward ring-opening is the size of the ring, with γ- lactones being more stable and less readily opened than δ- and ε-analogues. We have observed that the presence of either a C═C double bond or a lactone (which opens to form a C═C double bond) is necessary for appreciable rates of decarboxylation to occur. Olefinic acids exhibit higher rates of decarboxylation than the corresponding lactones, suggesting that the decarboxylation of alkene acids provides a lower energy pathway to olefin production than the direct decarboxylation of lactones. We observe lower rates of decarboxylation as the chain length of alkene acids increases; however, acrylic acid (3-carbon atoms) does not undergo decarboxylation at the conditions tested. These observations suggest that particular double bond configurations yield the highest rates of decarboxylation. Specifically, we suggest that the formation of a secondary carbenium ion in the β position leads to high reactivity for decarboxylation. Such an intermediate can be formed from 2- or 3-alkene acids which have at least four carbon atoms.