Conversion of fructose over mesoporous Sn-KIT-6-PrSO3H catalysts: Impact of the Brønsted/Lewis acid sites molar ratio on the main reaction pathways

The incorporation of tin and/or propyl-sulfonic groups into the KIT -6 mesoporous material through one -pot hydrothermal synthesis produces materials that retain the typical Ia 3 d symmetry of KIT -6, resulting in high surface areas, large pore diameters, and narrow pore size distribution. Sn(4+ )species primarily create Lewis acid sites (LAS), whereas sulfonic groups mainly produce Bronsted acid sites (BAS). Thus, the varying BAS/LAS acid site molar ratio affects the performance of catalysts in fructose dehydration reactions. Catalysts with a BAS/LAS molar ratio <1 favor the conversion of fructose through parallel reactions that generate 5-hydroxymethyl furfural (HMF) and furfural, suppressing at the same time the formation of levulinic acid. On the other hand, catalysts with a BAS/LAS ratio >1 promote the formation of HMF and its subsequent decomposition to levulinic and formic acids, maintaining the parallel production of furfural. The mesoporous structure of catalysts plays an essential role in producing furfural and levulinic acid, allowing for higher HMF yield due to easier diffusion of reagents and products. The Sn-KIT-6-PrSO3H catalyst showed the highest TOF value (1.0415 s(-1)), suggesting a synergistic effect between Lewis and Bronsted acid sites to promote the desired reaction. The proposed reaction model predicts a selectivity towards HMF higher than 50 % when the fructose conversion is 90 %.