Titanium Phosphate Grafted on Mesoporous SBA-15 Silica as a Solid Acid Catalyst for the Synthesis of 5-Hydroxymethylfurfural from Glucose

The grafting of titania on SBA-15 followed by its phosphation was presented to prepare a mesoporous Lewis-Brønsted bi-functional solid acid catalyst for the tandem conversion of glucose via fructose to 5-hydroxymethylfurfural (HMF). Titania was dispersed on SBA-15 as an amorphous surface layer containing abundant coordinatively unsaturated tetrahedral Ti ions, which was reactive and readily transformed upon phosphation into a new titanium phosphate phase with the chemical formula identified as Ti2O3(H2PO4)2·2H2O. The ordered mesoporous structure was well maintained after three modification cycles, affording a desirable surface area over 300 m2/g. The SBA-15-supported titanium phosphate layer affords higher overall acidity and Brønsted to Lewis acid ratio, compared with the conventional post-phosphated bulk anatase titania. The tetrahedral Ti ions and protonated phosphate groups on the titanium phosphate layer were closely arranged forming Lewis-Brønsted acid pairs, which largely enhanced the selective tandem catalysis for glucose conversion via fructose to HMF. An optimized HMF yield of 71% was achieved at 160 °C in a water-methyltetrahydrofuran biphasic system over the SBA-15-supported titanium phosphate catalyst. The catalyst exhibited good hydrothermal stability with a rather limited silicon and phosphate leaching, and no distinct pore collapse or performance loss over three sequential reaction runs.