Mechanistic investigation toward a microkinetic model for decarboxylation of gamma valerolactone over silica alumin

gamma-valerolactone (GVL) ring opening- and decarboxylation rates were measured over amorphous silica alumina (SiO2/Al2O 3) catalyst in the gas phase, using a down configuration fixed bed reactor, operating under anhydrous and differential conditions. By varying temperature, space time and GVL partial pressure, measured rates were leveraged to determine kinetic parameters (apparent activation barriers and pre-exponential factors) for the ring opening and decarboxylation step, as well as thermodynamic parameters (enthalpy- and entropy of reaction) for the ring opening step. The experimentally measured parameters were applied for the development of a microkinetic model that quantitatively describes the overall kinetics involved in the decarboxylation of GVL over SiO2/Al2O3. The microkinetic model predicts that the adsorption of GVL onto the surface of the catalyst and the subsequent ring opening step are two main elementary steps driving the kinetics of the process. In addition, the equilibrium constant of the adsorption step and apparent forward rate constant for the ring opening step were predicted as key parameters associated with the decarboxylation event. By comparing model predicted apparent forward rate constants for all relevant elementary steps involved in the mechanism of GVL decarboxylation, the ring opening step was identified as the slowest step; hence it's probably the rate determining step. In previous studies, decarboxylation of GVL over aluminosilicates with comparable apparent activation barriers and deprotonation energies revealed markedly different turn over frequencies; the microkinetic model introduced suggests that the difference in turn over frequencies is an artifact of local structural effects in the vicinity of the acid site. Although the model describes the kinetics involved, testing more aluminosilicates with different pore sizes will provide more information about the relative importance of the surface adsorption and ring opening steps.