H⁺−H⁻ Pairs in Partially Oxidized MAX Phases for Bifunctional Catalytic Conversion of Furfurals into Linear Ketones

Abstract

Currently, less favorable C=O hydrogenation and weak concerted acid catalysis cause unsatisfactory catalytic performance in the upgrading of biomass-derived furfurals (i.e., furfural, 5-methyl furfural, and 5-hydroxymethyl furfural) to ketones (i.e., cyclopentanone, 2,5-hexanedione, and 1-hydroxyl-2,5-hexanedione). A series of partially oxidized MAX phase (i.e., Ti₃ AlC₂ , Ti₂ AlC, Ti₃ SiC₂ ) supporting Pd catalysts were fabricated, which showed high catalytic activity; Pd/Ti₃ AlC₂ in particular displayed high performance for conversion of furfurals into targeted ketones. Detailed studies of the catalytic mechanism confirm that in situ hydrogen spillover generates Frustrated Lewis H⁺ -H^- pairs, which not only act as the hydrogenation sites for selective C=O hydrogenation but also provide acid sites for ring opening. The close intimate hydrogenation and acid sites promote bifunctional catalytic reactions, substantially reducing the reported minimum reaction temperature of various furfurals by at least 30-60 °C.