Enhancement of p-xylene selectivity in the reaction between 2,5-dimethylfuran and ethanol over an ammonium fluoride-modified ZSM-5 zeolite

A renewable synthesis of p-xylene (PX) via the Diels-Alder cycloaddition of biomass-derived 2,5-dimethylfuran (2,5-DMF) and ethanol was realized over various catalysts. During the tandem reaction, multiple competitive reactions occur, among which the hydrolysis of 2,5-DMF to 2,5-hexanedione, further dehydrating to 3-methyl-2-cyclopenten-1-one is the most detrimental to the main reaction as concluded from DFT calculations and experimental results. Besides, the product distributions vary with different catalysts. In comparison with HBeta and WOx/ZrO2, the HZSM-5 zeolite showed better performance with higher PX selectivity and yield. To further improve the reaction performance, the HZSM-5 zeolite was modified using NH4F at different times. After the modification for 5 min, denoted as FZ5-5, an optimum catalyst with the 2,5-DMF conversion of 70% and PX selectivity of 79% was obtained. Furthermore, the physicochemical properties of catalysts were comprehensively characterized using XRD, N-2 adsorption-desorption isotherms, ICP, NH3-TPD, pyridine-IR, SEM, TEM and MAS NMR. The results demonstrate that the high catalytic activity is attributed to the hierarchical pore structure and specific acidic features caused by simultaneous desilication and dealumination during NH4F etching. The hierarchical pore structure facilitates the diffusion of reactants, and the sufficient acid sites with moderate strength and B/L ratio of 1.17 can effectively reduce the 2,5-DMF hydrolysis reaction rate and promote the main reaction.