Sn- and Mo-Modified Sulfonated Carbons: Properties and Evaluation as Catalysts for Fructose Conversion in Water and DMSO

Sulfonated carbon-based materials produced from residual glycerol from biodiesel were modified with metallic species (Sn and Mo) to modulate their acidic properties. The materials C, CSn3, CMo3, and CSn3Mo3 presented surface areas of 46.8, 29.3, 67.7, and 43.8 m² g^-1, respectively. The presence of Sn and Mo, which impart Lewis acidity to the systems, can be evidenced by ICP-OES and XRD, while the presence of groups acting as Brønsted acids is clearly observed through FTIR. Their application as heterogeneous catalysts for fructose conversion in water or DMSO revealed that varying the Sn content had a minimal effect on the conversion rates. However, since the CSn3 system stood out for presenting slightly better performance despite having the lowest Sn content among the tested materials (conversion of 41.8% after 6 h), it was chosen to be modified with Mo. The incorporation of Mo into the materials significantly improved the conversion rates, reaching 84.9% for CMo3 and 92.1% for CSn3Mo3 after 6 h. This suggests that the nature of the acidic sites present in the materials played a more critical role in the reaction efficiency than their textural properties. For systems modified with Mo, the reaction produced not only 5-HMF but also intermediates of the retro-aldol pathway (lactic acid (AL), pyruvaldehyde (PYR), and glyceraldehyde (GAA), highlighting the importance of Lewis acid sites in the formation of these species. Additionally, organic acids, such as levulinic acid (LEV) and formic acid (AF), were also detected. In the reuse tests, the observed loss of catalytic activity was attributed to a reduction in the number of Brønsted acid sites and the formation of humins during fructose conversion.