An efficient one-pot synthesis of p-xylene from bio-based 2,5-hexanedione and ethanol

A brand-new p-xylene (PX) production route via 2,5-hexanedione (HDO) and ethanol was put forward. And the reaction was successfully catalyzed by organic base - tetrapropylammonium hydroxide modified HZSM-5 zeolite. Under optimum condition, the conversion of 2,5-HDO and selectivity to PX was 89.6 % and 35.2 %, respectively, much higher than the patent HZSM-5 zeolite. For comparison, PX was also produced from 2,5-dimethyfuran (2,5-DMF) and ethanol or 2,5-HDO and ethylene under the same conditions. However, the PX selectivity and yield obtained from 2,5-HDO and ethanol was much poorer than that when 2,5-DMF or ethylene as reactant due to the series competitive dehydration reactions. To investigate the relationship between catalyst properties and performance, the physicochemical properties of catalysts were comprehensively characterized by XRD, N2 adsorption-desorption isotherms, ICP, NH3/CO2-TPD, pyridine-IR, SEM, TEM and MAS NMR. Conclusion can be drawn that the appropriate acidity and hierarchical pore structure in T0.5-HZ catalyst (HZSM-5 modified by 0.5 M tetrapropylammonium hydroxide) were beneficial to high PX selectivity and yield. Among them, the acid site with weak acid site would catalyze 2,5-HDO intramolecular aldol condensation to 3-methylcyclopentenone (MCO), while strong acid site was beneficial for 2,5-DMF hydrolysis to 2,5-HDO. Thus, acid site with moderate strength was advantageous to the conversion of 2,5-HDO to 2,5-DMF. Besides, the enough amount of acid sites and suitable B/L ratio of 0.98 can satisfy the requirements of various dehydration reactions and Diels-Alder reaction between 2,5-DMF and dienophiles, enhancing PX selectivity and yield. Additionally, the hierarchical pore structure enhanced mass transfer efficiency and improved the reaction rate. The promising production for bio-based PX via 2,5-HDO and ethanol lay solid foundation for industrialized production route from lignicellulose.