Catalytical advantages of Hf-MOFs in benzaldehyde acetalization

Abstract

The potential use of acetals as bioadditives based on sustainable feedstocks in the automotive industry is of great interest. If such feedstock is a residual stream, the process would represent a dual environmental challenge, such as the valorization of glycerol obtained as a by-product of biodiesel to produce acetals. There are just scarce works about this synthetic route due to the challenge in finding efficient catalytic converters for glycerol valorization in a single process. Herein four different Metal-Organic Frameworks (MOFs) are evaluated as heterogeneous catalysts for the acetalization reaction of benzaldehyde with methanol, specifically, two structures, MOF-808 and UiO-66, containing two structural metal ions, zirconium and hafnium. The aim of this work is to investigate the influence of the MOF structure, the metallic active sites and the reaction conditions on the catalytic performance of this acetal production-type reaction. Furthermore, the recyclability of the catalyst is evaluated, and a potential reaction mechanism is suggested. As relevant results, Hf-MOFs showed higher benzaldehyde conversion than Zr-based ones, and significant improvements in reaction conditions were achieved, such as a significant reduction in catalyst loading of 99.6 % using UiO-66-Hf, and an optimization of reagent ratios reducing methanol consumption by 50 % for a 92 % of benzaldehyde conversion. The Brønsted character of UiO-66-Hf seems to enhance the reaction rate more than the metal center accessibility and larger pore volumes offered by MOF-808.