Effect of Lewis and Bronsted acidity on glucose conversion to 5-HMF and lactic acid in aqueous and organic media

Among the different metal-promoted catalysts for the conversion of C6 sugars to platform chemicals, Sn-based catalysts have demonstrated high activity for different types of reactions, such as isomerizatlon, retro-aldol conversion and dehydration of glucose or its isomers. The form of Sn species, as oxides or ions, along with the properties of the support are important factors determining the prevalent reaction pathway and product distribution. In this framework, we have studied the conversion of glucose mainly in biphasic (H2O/DMSO) media, using four types of Sn-based catalysts, i.e. Sn(II) and Sn(IV) as homogeneous catalysts (Cl-salts) or as the respective oxides in heterogeneous systems, Sn(IV) as SnO2 supported on gamma-Al2O3, zeolites (ZSM-5 and Beta) or mesoporous aluminosilicates (Al-MCM-41), and Sn delta+ as ion-exchange cations on the above zeolites. The solid catalysts were characterized by XRD, FT-IR/pyridine, ICP-OES, XRF and N-2 porosimetry. Under the studied conditions, the main products obtained were 5-HMF and lactic acid in both the homogeneous and heterogeneous systems. Based on the results obtained, it was concluded that the Lewis acidity offered by Sn oxides promotes the retro-aldol reaction pathway towards lactic acid, while the Lewis acidity offered by gamma-Al2O3 enhances the synthesis of both 5-HMF and lactic acid. By combining these two functions, the Sn impregnated gamma-Al2O3 catalysts induce further increase of both 5-HMF and lactic acid yields. In case of catalysts that possess Bronsted acidity as well (i.e. Sn ion-exchanged or impregnated zeolites and Al-MCM-41), dehydration reactions are promoted and the product selectivity is shifted towards 5-HMF synthesis. The highest 5-HMF molar yield of 27.5% (at complete glucose conversion) was achieved by Sn20/gamma-Al2O3 accompanied by lactic acid at 16.5% molar yield. For this catalyst the effect of the solvent, catalyst concentration and reaction temperature and time, were further evaluated.