Polyoxometalates as solid acid catalysts for sustainable chemistry

Abstract

Polyoxometalates, also known as heteropolyacids (HPA), are solid acids widely utilised in heterogeneous catalysis. The physicochemical properties of HPAs facilitate surface-type or bulk-type reactions, dependent on substrate polarity. For the latter, the gas-phase dehydration of ethanol was investigated representing an environmentally friendly solution to produce bio-ethylene, a key compound to the chemical industry; while for the former, the solventless liquid-phase isomerisation of α-pinene was studied due to the widespread applications of its derivates. Supported HPAs, exhibiting the Keggin structure, were prepared on commercial low surface area fumed silica and mesoporous high surface area SBA-15 via wet impregnation technique to elucidate structure-activity behaviours in both catalytic systems relative to their bulk equivalents. In addition, different types of HPAs, phosphotungstic acid (HPW), silicotungstic acid (HSiW) and phosphomolybdic acid (HPMo), were studied to observe the role of acid strength on product distribution. The successful synthesis of supported HPAs was confirmed by multiple characterisation techniques, revealing an inverse relationship between loading and dispersion which is further promoted through the use of mesoporous SBA-15 as the support, whereas non-porous fumed silica induced agglomeration which in turn increases the degree of crystalline water present in the HPAs. n-Propylamine TPD evaluated acid strength decreases in the order of HPW>HSiW>HPMo, which is independent of support. Support choice did not influence selectivity in either reaction, but did impact greatly on catalytic activity, imparting significant reaction rate and turnover frequency enhancement. For surface-type reaction, SBA-15 showed to be the optimal support due to greater active sites accessibility, whereas in the case of the bulk-type reaction, the fumed silica based materials, with larger 3D HPAs structures, even at low loadings, showed superior performance. Product distributions were affected through the choice of HPA, with higher selectivity towards camphene, for α-pinene isomerisation, and ethylene, for ethanol dehydration, observed over the HPW catalysts.