Phenyl siloxane hybrid xerogels: structure and porous texture

Abstract

The aim of this research is to investigate the effect of phenyltriethoxysilane (PhTEOS) and tetraethoxysilane (TEOS) molar ratios as silicon precursors on the structure and porous texture of xerogels. We have prepared phenyl-silane hybrid xerogels from mixtures of PhTEOS and TEOS at pH 10 and 333 K, using ethanol as a solvent. Characterization techniques include 29Si NMR, FTIR, XRD, FE-SEM, HRTEM, TGA-DSC, helium density, and gas adsorption (N2 at 77 K and CO2 at 273 K). In order to assess the contribution of the quadrupolar moment of N2 and CO2 in the adsorption we obtained the adsorption–desorption isotherm of Ar at 87.3 K for the xerogel synthesized from 50% PhTEOS. The morphology of xerogels changed from aggregates of spherical particles for 20% PhTEOS to lamellae for samples obtained with PhTEOS percentages equal or larger that 60%. The incorporation of phenyl groups into the xerogel matrix caused an increase in the spacing bond between silicon atoms and led to an intramolecular reaction and the formation of lamellar domains. Increasing the PhTEOS molar ratio in the mixture of silicon precursors produced hybrid xerogels with lower specific surface area, pore volume and characteristic energy. The similarity between the isotherms of N2 at 77 K and Ar at 87.3 K indicates that the main retention mechanism is physisorption and that the variation in the surface chemistry with the incorporation of phenyl groups doesn’t inhibit the retention of N2.