Hydrogen-free catalytic conversion of highly concentrated glycerol solution: Influence of Si/Al molar ratio in Cu/SBA-15 catalysts

Abstract

Catalytic conversion of glycerol to C₃ products using advanced functional materials is an economical and efficient approach to promoting sustainable energy development. Selective glycerol dehydration to hydroxyacetone (HA) is one such alternative and requires an appropriate balance of catalyst texture, structure, and acidity. In this study, different Cu-based catalysts supported on SBA-15 with different Si/Al molar ratios (2, 10, 15 and ∞) were prepared using the “pH-adjusting” method and ammonia evaporation method and they were evaluated for the dehydration of glycerol at high concentration (80 wt%.) in liquid phase to hydroxyacetone under inert N₂ atmosphere. The formation of 1,2-propanediol (1,2-PDO) was also observed even under N₂ inert atmosphere conditions. The catalysts were characterized by ICP-MS, N₂ adsorption-desorption, XRD, HRTEM, H₂-TPR, passivation of N₂O, NH₃-TPD, TPO, FTIR, FTIR-OH, Py-FTIR and XPS. The Al chemical environment and the Si/Al molar ratio of the supports show a remarkable effect on the catalytic performance. The highest catalytic activity was obtained with the SiAl = 15 molar ratio, reaching a conversion of around 35 % at 120 min of reaction and a selectivity of 84 % to hydroxyacetone at 90 min of reaction. While lower Si/Al molar ratios led to a decrease in catalytic activity and an increase in product distribution. The isomorphic incorporation of Al into the SBA-15 promotes the interlinking cooperative between the properties of textural, acidity, structural, and stability of the catalyst. HRTEM and HAADF-STEM images show a well-defined hexagonal arrangement even when the CuO phase is added to the mesoporous materials with high Al loadings. At high heteroatom loadings, the typical hexagonal arrangement of the mesostructure was preserved but a decrease in the long-range order was observed. The presence of mononuclear and oligonuclear Cu^δ+ species, which is related to their coordination environment and their position in the mesoporous network, was dependent on the variation of the Si/Al molar ratio. The prominent catalytic performance at the SiAl = 15 ratio can be attributed to an optimal cooperative effect of the structural defects and electronic promotion produced by the Al species, which favors the conversion of glycerol to HA. While the increase in the selectivity of 1,2-PDO observed with the molar ratio SiAl = 2 is associated with a conjugation between the higher availability of acid sites on the surface that promote the dehydration of glycerol and the domain size of copper species which would be acting as dehydrogenating sites of glycerol, forming H₂ in-situ leading to the conversion of HA into 1,2-PDO.