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 chemical ly 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 lig ands. The obtained materials were characterized using elemental analys is, high-resolution thermogravimetry (TGA), and nitrogen adsorption at 77 K in a wide range of pressures. Surface coverages of bonded ligand s were between 2.5 and 3.0 mu mol/m(2). It was shown that pore diamete rs of the samples studied decreased systematically with the increase i n size of ligands. The modified materials exhibited narrow and monodis perse pore size distributions, indicating that the chemical bonding pr ocedure did not diminish the structural ordering of the MCM-41 support . TGA data showed that the surface affinity to water was strongly depe ndent on the structure and functionality of the bonded species. Nitrog en adsorption data provided additional information about surface prope rties of the materials. A significant decrease in the amount of nitrog en adsorbed at low pressures was observed for the modified samples, es pecially those with long-chain alkyl groups. Low-pressure adsorption d ata 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 thu s demonstrated that the pore size and surface functionality of ordered mesoporous silicas can be engineered by a proper choice of the pore d iameter of the support and the size and structure of bonded ligands. N itrogen adsorption measurements including low-pressure data were shown to be a powerful tool to characterize structural and surface properti es of unmodified and surface-modified novel porous materials.