Selective Production of Terminally Unsaturated Methyl Esters from Lactones Over Metal Oxide Catalysts

Abstract

Metal oxide catalysts were studied for their selectivity for the production of a terminally unsaturated methyl ester, methyl 5-hexenoate (M5H), from a 6 carbon, 6-membered ring lactone, δ-hexalactone (DHL). A 15 wt% Cs/SiO2 catalyst had a selectivity of 55% to M5H. This selectivity was the highest of the metal oxide catalysts studied, which were Cs/SiO2, MgO, SrO, CeO2, ZrO2, Ta2O5, MgAl2O4, and a Mg–Zr mixed oxide. The Cs/SiO2 catalyst was utilized for the ring-opening of γ-valerolactone (GVL), a 5 carbon, 5-membered ring lactone. The catalyst was 88% selective to the terminally unsaturated methyl ester, methyl 4-pentenoate (M4P). Weight hourly space velocity studies determined that the unsaturated ester distributions remained constant and no C=C double bond isomerization occurred. Liquid phase transesterification reactions with DHL and methanol and nuclear magnetic resonance spectroscopy confirmed that DHL undergoes ring-opening transesterification to produce an ω-1 hydroxy methyl ester, methyl 5-hydroxyhexanoate (M5HH). Liquid phase transesterification reactions and thermochemistry calculations established that the equilibrium for GVL transesterification with methanol was favored towards the ring-closed lactone instead of the ring-opened hydroxy ester because of the decreased ring strain of GVL compared to DHL. The difference in terminally unsaturated methyl ester selectivity between GVL and DHL manifests from the difference in ring-strain energy. DHL passes through the M5HH intermediate as a result of greater ring strain, while the production of M4P from GVL most likely occurs through a direct, concerted mechanism.