Diels-Alder conversion of biomass-derived furans and ethylene to renewable aromatics over mesoporous titanium phosphate

Metal phosphates such as P-Beta, Zr-P and Sn-P have drawn lots of attention as emerging catalysts for biomass conversion due to its mild acidity which results in high selectivity to the targeted products while minimizing the unwanted side reactions such as coking that are prevalent in reactive biomass-derived compounds. In this study, we have investigated the production of sustainable aromatics such as p-xylene (PX) and toluene (TOL) via Diels-Alder cycloaddition (DAC) of biomass-derived 2,5-dimethylfuran (DMF) and 2-methylfuran (MF) with ethylene over titanium phosphate catalysts. Specifically, the effects of synthesis method (e.g., sol-gel vs hydrothermal), activation method and P/Ti molar ratio on the structural and acid properties of titanium phosphate catalysts were systematically studied to understand the catalytic consequences of the titanium phosphate materials for the DAC of furans. In addition, amphiphilic surfactant was applied to all catalysts for the formation of mesoporous structure. The characterization studies of the prepared catalysts using XRD, BET, FT-IR, XPS, NMR spectroscopy and TPD of ammonia indicated that the degree of Ti-O-P bond formation, catalysts surface area and surface acidity are largely affected by the catalysts synthesis conditions. Overall, the mesoporous titanium phosphate catalyst, synthesized by hydrothermal method at 180 ℃ for 12 h followed by ethanol refluxing at 60 ℃ for 24 h at a molar P/Ti ratio of 1 showed the highest surface area and the highest acid site density which arise from its distinct ordered Ti-O-P bonding structure as evidenced by XPS and NMR. The catalyst demonstrated the remarkably high PX selectivity of > 90 % at the high DMF conversion of > 90 % and the reusability after two successive reaction-regeneration cycles, showing high phosphorus leaching tolerance.