Tailoring Surface and Structural Properties of MCM-41 Silicas by Bonding Organosilanes

Large pore MCM-41 silica with the pore diameter of 5.0 nm was chemically modified by bonding monomeric-type ligands, such as trimethylsilyl, butyldimethylsilyl, and octyldimethylsilyl, as well as polymeric-type 3-aminopropylsilyl, (hexanoyl-3-aminopropyl)silyl, and octylsilyl ligands. The obtained materials were characterized using elemental analysis, high-resolution thermogravimetry (TGA), and nitrogen adsorption at 77 K in a wide range of pressures. Surface coverages of bonded ligands were between 2.5 and 3.0 ímol/m2. It was shown that pore diameters of the samples studied decreased systematically with the increase in size of ligands. The modified materials exhibited narrow and monodisperse pore size distributions, indicating that the chemical bonding procedure did not diminish the structural ordering of the MCM-41 support. TGA data showed that the surface affinity to water was strongly dependent on the structure and functionality of the bonded species. Nitrogen adsorption data provided additional information about surface properties of the materials. A significant decrease in the amount of nitrogen adsorbed at low pressures was observed for the modified samples, especially those with long-chain alkyl groups. Low-pressure adsorption data were used to calculate adsorption energy distributions (AEDs), and peaks on these distributions were assigned to certain groups present on the silica surface or in the structure of bonded phases. It was thus demonstrated that the pore size and surface functionality of ordered mesoporous silicas can be engineered by a proper choice of the pore diameter of the support and the size and structure of bonded ligands. Nitrogen adsorption measurements including low-pressure data were shown to be a powerful tool to characterize structural and surface properties of unmodified and surface-modified novel porous materials. 1