Interconversion between γ-valerolactone and pentenoic acid combined with decarboxylation to form butene over silica/alumina

Reaction kinetic studies were carried out of the reversible interconversion between γ-valerolactone (GVL) and pentenoic acid (PEA) combined with the irreversible decarboxylation of both species to form butene and CO₂ over a SiO₂/Al₂O₃ catalyst at pressures from atmospheric to 36 bar, temperatures from 498 to 648 K, different concentrations of GVL and PEA, as well as in the presence of water. The catalyst exhibited reversible deactivation within the initial 24 h on stream (losing about 50% of the initial catalytic activity), followed by a slower rate of deactivation of roughly 0.4-0.5% per hour on stream. Decarboxylation of γ-valerolactone, producing equimolar quantities of butene and CO₂, may possibly occur by two distinct pathways: a direct route from the lactone and an indirect route from PEA. 1-butene is the primary product of decarboxylation, formed via β-scission of intermediate carbenium ions. The apparent activation barrier for decarboxylation of GVL (175 kJ mol^-1) is higher than for decarboxylation of PEA (142 kJ mol^-1). A simple kinetic model with rate expressions accounting for adsorption and unimolecular surface reactions of GVL and PEA is sufficient to describe the trends measured for the rates of GVL ring opening to PEA, GVL decarboxylation, PEA cyclization to GVL, and PEA decarboxylation at different reaction conditions.