High population and dispersion of pentacoordinated Al species on the surface of flame-made amorphous silica-alumina.

Pentacoordinated Al (Al) species in silica-alumina are promising to promote the formation of acid sites or act as surface defects for tailoring single-atom catalysts. However, pentahedral coordination (Al) is rarely observed in conventionally prepared silica-alumina. Here, we show that high population and dispersion of Al species on the surface of amorphous silica-alumina (ASA) can be achieved by means of flame spray pyrolysis. High resolution TEM/EDX, high magnetic-field NMR and DFT calculations are employed to characterize the structure of as-prepared ASAs. Solid-state Al multi-quantum MAS NMR experiments show that most of the Al species are formed independently from the alumina phase and are accessible for guest molecules on the surface. Upon water adsorption, these Al species are transformed to Al species, structurally similar to surface Al species, as confirmed by DFT calculations. The outstanding catalytic activity of as-synthesized ASA is demonstrated using the in situ H/D exchange reaction with deuterated benzene as an example. The Al-rich ASA provides a much lower activation energy (∼30 kJ/mol) than that reported for zeolite H-ZSM-5 (∼60 kJ/mol). The superior catalytic performance is attributed to the high Al content promoting the surface active sites in ASA. The knowledge gained on the synthesis of Al-rich ASAs and the nature of aluminum coordination in these materials could pave the way to more efficient silica-alumina based catalysts.