Design of highly efficient Mo and W-promoted SnO2 solid acids for heterogeneous catalysis: acetalization of bio-glycerol

Abstract

Development of highly promising solid acids is one of the key technologies to meet the essential challenges of economical and environmental concerns. Thus, novel molybdenum and tungsten promoted SnO₂ solid acids (wet-impregnation) and pure SnO₂ (fusion method) were prepared. The synthesized catalysts were systematically analyzed using various techniques, namely, XRD, BET surface area, pore size distribution, XPS, FTIR, FTIR of adsorbed pyridine, Raman, NH₃-TPD, and H₂-TPR. XRD results suggested formation of nanocrystalline SnO₂ solid solutions due to the incorporation of molybdenum and tungsten cations into the SnO₂ lattice. All the materials exhibited smaller crystallite size, remarkable porosity, and high specific surface area. Raman measurements suggested the formation of more oxygen vacancy defects in the doped catalysts, and the TPR results confirmed facile reduction of the doped SnO₂. NH₃-TPD studies revealed the beneficial role of molybdenum and tungsten oxides on the acidic properties of the SnO₂. FTIR studies of adsorbed pyridine showed the existence of a larger number of Brønsted acidic sites compared to Lewis acidic sites in the prepared catalysts. The resulting catalysts are found to be efficient solid acids for acetalization of glycerol with acetone, furfural, and its derivatives under solvent-free and ambient temperature conditions. Particularly, the Mo⁶⁺-doped SnO₂ catalyst exhibited excellent catalytic performance in terms of both glycerol conversion and selectivity of the products. The increased presence of acidic sites and enhanced specific surface area, accompanied by notable redox properties and superior lattice defects are found to be the decisive factors for better catalytic activity of the Mo⁶⁺-doped SnO₂ sample. The investigated SnO₂ solid acids represent a novel class of heterogeneous catalysts useful for the transformation of glycerol to value-added products in an eco-friendly manner.