New Insights into the Effect of the Zeolites Framework Topology on the Esterification Reactions: A Comparative Study from Experiments and Theoretical Calculations

Abstract

Understanding the catalytic activity of protonic zeolites in biomass conversion allows design catalysts specific for various industrial applications resulting in a strong economic impact. The aim of the present work is to analyze the influence of the pore topology and density of active sites comparing H-ZSM-5, H-Beta and H-Y zeolites as solid acid catalysts for the esterification of oleic acid. The catalysts were characterized using XRF, N2 isotherms, NH3-TPD and, 27Al MAS-NMR techniques, catalytic tests were conducted in a batch reactor (20 bar N2, 100 ℃ and 600 RPM), theoretical investigations by DFT calculations were also performed. Zeolites with lower Al density (H-Y and H-ZSM-5) showed higher acid strength, however, H-Beta showed higher conversion than H-ZSM-5 because in BEA-type structures the Al are found in places that are more accessible. H-Y was highly effective for this reaction (98% conversion) and exhibiting promising recyclability, which could be related to their high specific area, hydrophobicity effect and symbiosis between Brønsted acid sites and the presence of extra-framework aluminum inside the zeolite cavity. H-ZSM-5 showed a 70% conversion yield due to the high Si/Al ratio and the additional porosity of this de-aluminized material. DFT results showed that the channel system of H-Y and H-Beta zeolite framework favor the stabilization of coadsorbed species within their confined environments, and such effect could minimize the activation barrier in consecutive steps of the reaction. Our results suggest the use of protonic zeolites as strategic catalytic material in the recovery of oleaginous residues to obtain products with higher added value.