Gallium based hollow silica nanospheres for the acid-catalyzed upgrading of glycerol: Enhanced activity disclosed via an in-depth nuclear magnetic resonance approach

Ga-doped hollow silica nanosphere and nanotubes were synthetized using a soft template sol–gel method. The low dimensional morphologies (0D or 1D) were obtained by simply adjusting the stirring speed during the synthesis procedure. The two materials were fully characterized using different techniques such as ssNMR, N2 physisorption, XRD, TEM or ICP-OES. The influence of the calcination temperature on the coordination environment of gallium as well as the accessibility of the gallium active sites was proved via 71Ga ssNMR. The acid features of the solids were studied via a combined approach based on FT-IR of adsorbed ammonia and 31P ssNMR using trimethylphosphine as a probe molecule. The latter technique allows unveiling a higher Brønsted/Lewis acid sites ratio of Ga-nanospheres as compared to Ga-nanotubes, probably as a consequence of the more defective spherical shell. Both nanostructures were tested for the conversion of glycerol to solketal. Ga-nanospheres revealed improved catalytic performance in comparison with the corresponding nanotubes and displayed outstanding activity with respect to other solid catalysts reported in the literature and tested under the same reaction conditions. Moreover, they proved to be stable and reusable in multiple cycles. The E-factor calculated under the best condition was below 1 thus proving the sustainability of the process.