Understanding the heterogeneous catalytic mechanisms of glycerol carbonate synthesis on oil palm ash surface: A density functional theory approach

Abstract

The use of glycerol as an added-value by-product from the transesterification of triglycerides to produce biodiesel has been the subject of intense research due to its versatility in a wide range of applications. In this work, a theoretical study at the density functional theory (DFT) level to categorize the most probable routes to achieve glycerol conversion was performed. Gibbs free energies were computed to assess the feasibility of the steps involved in experimental glycerol carbonate synthesis from glycerol and dimethyl carbonate. The cyclization from methyl glycerol carbonate anion to glycerol carbonate is reasonable due to the computation of the Gibbs free energy amounting to -310.0 kcal/mol, which indicates an exothermic reaction with the spontaneous rising of glycerol carbonate. The assessment of the molecular dimensions at the optimized geometry of glycerol carbonate showed a critical diameter and length of 5.25 Å and 5.90 Å, respectively; with a volume of 127.7 Å3. Charge population analysis provided insights into the electronic transfer during the reaction and the bonding mechanism resulting in the products. Additionally, the heterogeneous catalysis reaction involved in the glycerol carbonate evolution reaction and its subproducts was studied on a periodic surface model of the oil palm fuel ash catalyst in order to evaluate activation energies and the feasibility of the proposed mechanism. That is, the computations revealed an activation energy barrier of 13.4 kcal/mol (0.58 eV), which also certifies the cyclization feasibility in the formation of glycerol carbonate. The results may contribute in the insilico design of the experimental setup to improve efficiency in the process of transesterification reaction of glycerol and dimethyl carbonate to valuable glycerol carbonate.