PORE-SIZE AND SURFACE TEXTURE MODIFICATION OF SILICA VIA TRIALKYLSILYLATION

Although surface modification of silica via silylation is often undertaken to change surface chemistry and, hence, adsorption characteristics, the use of silylation for modifying pore size, size distribution, and surface texture is rarely considered. In principle, by use of silylating agents of the form RnSiCl4-n where n is typically 3, pore size and texture may be varied by changing the R group, n, and/or surface coverage. When the silica is microporous, this approach should be particularly effective for changing the pore size. However, many questions exist concerning the effect of small-scale surface roughness, the fractional surface coverage, and R group size on the effective pore size. Modification of various silicas (Vycor, CPG-75, two-step acid/base catalyzed silica gel) was conducted by trialkylsilylation. The degree of silylation/surface coverage was monitored by thermogravimetric analysis and elemental analysis. Pore structure was studied by nitrogen adsorption and condensation and small angle X-ray scattering. The results show that surface area, surface texture, pore size distribution, and total pore volume are indeed changed in a controlled fashion due to silylation. Generally, the mean pore size is smaller, the pore size distribution is significantly narrower, and the pore surface is smoother after silylation. The magnitudes of those changes are shown as a function of R, coverage, and surface roughness.