Strongly enhanced acidity and activity of amorphous silica–alumina by formation of pentacoordinated Al^V species

Tailoring high-performance aluminosilicates plays a key role in the efficient and clean production of high-value chemicals. Recent work reveals that pentacoordinated Al (Al^V) species can significantly enhance the Brønsted acidity of amorphous silica–alumina (ASA), compared with that typically dominated by tetracoordinated Al species. However, the controlled synthesis of Al^V-rich ASAs is challenging. Employing xylene as the solvent in a flame-spray pyrolysis process, we synthesized Al^V-rich ASAs successfully. The high combustion enthalpy of xylene (36.9 kJ/ml) results in a high flame temperature, promoting the formation and distribution of metastable Al^V species in the silica network forming Brønsted acid sites. This provides a promising route for the controlled synthesis of Al^V-rich ASAs with higher Brønsted acidity. As an example, Al^V-rich ASAs are shown to exhibit superior catalytic performance in phenylglyoxal conversion to ethyl mandelate in ethanol compared with that achieved with other acid catalysts, attaining an ethyl mandelate yield of 99.8%.