Generation of porosity in a hybrid organic-inorganic xerogel by chemical treatment

Abstract

A hybrid xerogel of 1,4-bis(trimethoxysilylethynyl)benzene was prepared by using a sol-gel process. Chemical treatment of this material was performed under mild conditions in order to remove the organic group using fluoride anion as a catalyst. Experimental procedures were carried out using different solvents, acidities and counter-cations in order to modify the nucleophilic power of F^-. Characterization of the residues was done by elemental analysis, spectroscopy (²⁹Si NMR), thermal analysis, pycnometry, surface area measurement and SAXS. The efficiency of the chemical treatment is related to the solvation of the F^- anion. It was possible to demonstrate that elimination of the organic part occurs at the same time as a reorganization of the silica network. This reorganization is the result of two competitive F^--catalyzed chemical reactions: a redistribution and a polycondensation process known in solution and which take place in the solid. These processes account for the textural characteristics of the final material and may efface the organization of the organic spacers in the hybrid xerogel. These results demonstrate the limitations of silica as a molecular imprinting material and open a new approach to porous silica with adjustable pore size.