Insight into the effect of tetrapropylammonium hydroxide on HZSM-5 zeolite and its application in the reaction between 2,5-dimethylfuran and ethanol to p-xylene

Abstract

The microporous HZSM-5 zeolite was modified by tetrapropylammonium hydroxide (TPAOH) with various concentrations, which was further compared with NaOH modification. The influence of alkali treatment on the porosity, acidity and catalytic performance of HZSM-5 zeolite was systematically investigated. N₂-physisorption measurements demonstrated that the hierarchical zeolite was successfully synthesized by alkali treatment. ²⁹Si MAS NMR proved that the demetallization process was highly more controllable for TPAOH than for NaOH. The different demetallization mechanism caused the variation of acidity as evidenced by NH₃-TPD and Py-IR characterization. What's more, the renewable production of p-xylene from 2,5-DMF and ethanol was realized under alkali treatment of ZSM-5 zeolites. Among them, HZSM-5 treated with 0.3 M TPAOH displayed improved catalytic performance with a 2,5-DMF conversion of 95.0% and a PX selectivity of 67.8%, better than the parent and NaOH modified HZSM-5 zeolite, due to its specific porosity and acidity. To explore the impact of hydrophobicity on catalytic activity, the TPZ-0.3 catalyst was subsequently silylated with several silylating agents with different hydrophobic properties. For the surface modified zeolite, the main reaction activity decreased significantly and even exhibited excellent anti-coking, indicating that the water produced during the reaction was favorable for the main reaction. The discoveries obtained from this research pave a way for the construction of catalysts with higher performance in biomass conversion.