Aluminium-derivatized silica monodisperse nanospheres by a one-step synthesis-functionalization method and application as acid catalysts in liquid phase

Abstract

Aluminium-derivatized submicron-in-size silica spheres (Al-MSS), with controllable morphology, particle size, mesoporosity, and incorporation of an active phase at the internal surface, have been prepared by a one-step synthesis-functionalization method in view of potential applications in the liquid phase. Four Al-MSS samples were prepared with a molar Si/Al ratio in the initial gel of 44, 22, 11, and 7, and their properties were compared with those of a purely siliceous sample (MSS). Nitrogen adsorption, XRD, SEM and TEM studies revealed homodispersed particles with average diameters ranging between 350 and 520 nm and possessing radial porosity. BET specific surface areas were included between 900 and 1300 m² g⁻¹ and the average pore diameter varied from 3 to 3.4 nm. Surface reactivity and Al accessibility at the material surface were determined with measurements of two-cycle ammonia adsorption and EDX-based calculations. The best compromise between textural parameters, surface properties, and stoichiometric Al incorporation was achieved with Al-MSS-22. This sample was further characterized by ¹²⁹Xe-NMR, ²⁷Al-MAS-NMR, XPS, NH₃-TPD and IR methods to confirm the high accessibility of its internal porous structure and the presence of weak Brønsted and strong Lewis acid sites on its surface. The catalytic behavior of Al-MSS-22 in the liquid phase was tested in esterification reactions of ethanoic acid with alcohols or polyols such as butanol, isoamyl alcohol, ethylene glycol, and glycerol. In the case of alcohols, the remarkable performance of this aluminosilicate was supported by conversion rates close to 100%, allowing the amount of the catalytic material to be significantly decreased compared with other systems already tested in the literature.