Carbon-Based Bifunctional Lewis/Brønsted Acid Catalysts for 5-HMF Production from Cellobiose

Bifunctional carbon-based acid catalysts presenting Brønsted and Lewis acid sites in different proportions were synthesized by grafting mesoporous aluminosilicate patches in coexistence with benzyl sulfonic moieties on the solid's surface. The final bifunctional systems were obtained by development of a methodology allowing the production of a model discontinuous silica layer on the carbon substrate before applying the optimized procedure to the desired alumino-silicate layer. Patches were necessary to expose bare carbon surfaces for sulfonic acid grafting employing a diazonium coupling methodology to ensure robust anchoring. The bifunctional solids and their monofunctional counterparts were fully characterized and their acidic nature, together with the presence of both Brønsted and Lewis acid sites, was assessed by Py-FTIR spectroscopy, NH₃-TPD, solid-state ³¹P MAS NMR, and Boehm back titration. The attainment of a mesoporous discontinuous oxide framework was confirmed by nitrogen physisorption and SEM/TEM observations. Finally, the catalytic performance of both the produced bifunctional systems and their monofunctional counterparts was investigated and compared for the upgrading of cellobiose to 5-hydroxymethylfurfural (5-HMF). A maximal yield of 37% of 5-HMF out of a 96% of cellobiose conversion was attained after 23 h with catalyst I4@BATEOS/SO₃H in a designed solvent mixture (THF:mQ water) at 423 K.