Physisorption data for methyl-hybrid silica gels

Abstract

The porous structure of inorganic silica and methyl-ormosil gels were studied using nitrogen adsorption at 77 K. Organically, modified silica gels were produced from mixtures of tetraethoxysilane (TEOS) and triethoxy(methyl)silane (MTES), under basic and acid sol–gel catalysis. Starting from pure TEOS, mixtures of increasing MTES content have been prepared. Although not new, silica-based surface morphology is still a challenging subject, particularly when related to silica functionalise systems. N2 adsorption isotherms were used, and the validity of BET model has been discussed for ormosil systems. The pore size distribution of inorganic and hybrid sol–gel silica materials were calculated by using the adsorption branch of the N2 isotherm. Compared with inorganic silica gel, the microstructure of the resulting hybrid gel has been modified—sorption capacities and average pore radius have intensely increased. Acid and basic catalyst determine the type of gel microstructure. Further, the presence of the more flexible ≡Si–CH3 bonds is the responsible for the increasing mobility during gelation, and for the presence of not energetically equivalent terminal groups, which will affect the final pore structure by the suppleness capacity. The rigid and regular pore shape assumption is no more valid. For MTES content higher than 2 mol%, the N2 isotherms reached no more the equilibrium. N2 adsorption isotherms are though not very suitable for surface characterisation of hybrid silica gels, over a limit organic moiety concentration.