Self-Etherification of 5-Hydroxymethylfurfural to 5,5′(Oxy-bis(methylene))bis-2-furfural over Hierarchically Micromesoporous ZSM-5: The Role of Brønsted- and Lewis-Acid Sites

The 5,5′(oxy-bis(methylene))bis-2-furfural (OBMF) formation from the 5-hydroxymethylfurfural (HMF) self-etherification over zeolites is an important reaction for the synthesis of value-added chemicals from biomass resources. However, there is still a lack of knowledge about the reaction mechanism and the roles of different acid sites that impedes the exploitation of zeolite catalysts for efficient OBMF production. In the present study, the ZSM-5 (HPZ) catalysts of four hierarchically micromesopores were successfully synthesized in different Si/Al ratios by adjusting the amount of Al in the solvent evaporation with the assistance of a dry gel conversion method. The kinetics of OBMF formation from HMF self-etherification was investigated. The apparent activation energy and the conversion reaction order were calculated to be 104.6 kJ/mol and 0.65, respectively. The intrinsic activity comparison of four catalysts and density functional theory (DFT) calculations showed that HMF adsorption and etherification benefit from Brønsted-acid sites. Besides, the DFT results suggested that the etherification proceeds via a concerted step, instead of the sequential pathways, thus leading to the high selectivity of OBMF on hierarchically micromesoporous ZSM-5 (HPZ) catalysts.