Generation of Microporosity in a Hybrid Material. Access to Pillared Amorphous Organosilicate

Abstract

The potentiality of a new route for creating a controlled porosity in an hybrid xerogel was investigated using the selective removal of the rigid 1,4-bis(ethynyl)benzene group in an hybrid xerogel of the general formula {[(O)1.5Si−C⋮C−C6H4−C⋮C−Si(O)1.5]x[(O)1.5Si−C6H4−C6H4−Si(O)1.5]1-x}. Preparation of these xerogels was achieved by HCl-catalyzed sol−gel copolycondensation of precursor A, 1,4-bis(trimethoxysilylethynyl)benzene, mixed with various amount of precursor B, 4,4‘-bis(trimethoxysilyl)biphenyl, in THF ((A:B) = (100:0), (80:20), (60:40), (40:60), (20:80) and (0:100). Elimination of the 1,4-bis(ethynyl)benzene group of A was achieved by a selective and mild chemical treatment that prevents the elimination of the biphenylene group introduced by precursor B. The porosity of the materials before and after chemical treatment was measured by helium pycnometry, water adsorption, and N2 and Ar porosimetry, this porosity mainly varies in relation to the initial (A:B) ratio. A microporous material with high hydrophobic surface (mean pore diameter φ < 10 Å) is obtained when high proportions of B are used. Lowering the proportion of B mainly leads to an increasing proportion of mesopores (mean pore diameter 10 < φ < 200 Å) that results from a percolation of the voids and the reorganization of the silica network. As a side reaction, the chemical treatment promotes an increase in the level of polycondensation. Interestingly, the porosity of the final material is consistent with a random polycondensation of the precursors during the gelation step.