Heteropolyacid catalysts for Diels-Alder cycloaddition of 2,5-dimethylfuran and ethylene to renewable p-xylene

Abstract

The Diels-Alder cycloaddition of biomass-derived furans and subsequent dehydration are promising routes for the sustainable production of commodity chemicals such as p-xylene (PX). In this paper, we have investigated the catalytic performances of a range of phosphotungstic acid (HPW) and silicotungstic acid (HSiW) catalysts supported on various oxides, i.e., SiO2, Al2O3, TiO2 and ZrO2 and their structure activity correlation in the conversion of 2,5-dimethylfuran (DMF) and ethylene to PX. The characterization studies of the catalysts using XRD, BET, Raman and P-31 MAS-NMR spectroscopy reveal that all of the supported heteropolyacid (HPA) catalysts (except HPW/ZrO2) retain their Keggin structure on the surface of oxide supports. Results from ammonia- and n-propylamine-TPD studies show that all of the supported HPA catalysts possess well-defined Bronsted acid sites with the total acidity decreasing in the following order: HPA/SiO2 > HPA/Al2O3 > HPA/ZrO2 > HPA/TiO2. The conversion of DMF and the initial rate of PX production generally increase with an increase in the total acidity, with HPA/SiO2 being the most active catalyst. The turnover frequency of PX production for HPA/SiO2 is also considerably greater than those for the HPAs supported on Al2O3, ZrO2, and TiO2, which suggests that the higher activity of HPA/SiO2 is at least partly due to the enhanced strength of Bronsted acid sites. Both the silica-supported HSiW and HPW catalysts demonstrate remarkably high PX selectivity (82-85%) at high DMF conversion (91-94%) at 250 degrees C after 6 h reaction. The effects of reaction conditions such as acid loading, reaction temperature, and reaction time have also been investigated with the most active silica-supported HSiW catalysts to optimize the PX yield. (C) 2016 Elsevier B.V. All rights reserved.