Adsorption, thermogravimetric, and NMR studies of FSM-16 material functionalized with alkylmonochlorosilanes

Two series of alkyl-modified molecular sieves were prepared using the FSM-16 mesoporous silica. The attachment of alkyl groups to the surface, the structure of the bonded layer, and changes in the mesoporous structure of the FSM-16 material upon functionalization were studied by means of high-resolution thermogravimetry, elemental analysis, nitrogen adsorption, and C-13 and Si-29 NMR. Surface coverages of attached ligands were in the range of 0.85-2.60 mmol/g. All modified samples showed a decrease in the pore size, which was in relation to the size of attached ligands. The uniformity of pores upon modification proves that the mesoporous structure of FSM-16 is retained after modification. The attachment of ligands rakes place in a solvent either with a small amount of water or anhydrous indicating a direct reaction between the chlorosilane molecule and silanol groups and/or a reaction autocatalyzed by the small amount of water present on the FSM-16 surface. Nitrogen adsorption isotherms and the adsorption energy distributions calculated from these isotherms revealed the presence of highly ordered alkyl ligands and the existence of the screening effect produced by alkyl groups as a result of weak interactions of nitrogen probe molecules with attached long alkyl chains. Thermal stability of modified samples greatly depends on the conformation of attached ligands. By using appropriate solvent and/or thermal treatment of modified samples, it was possible to achieve higher ordering of attached groups and consequently higher thermal stability of the resulting bonded phases. The studies of silicon spin-lattice relaxation times T-1 support the formation of ligand structures on the surface and show that the surface accessibility greatly decreases with increasing length of attached ligands and with refinement of the ordering of ligands on the surface. Mesoporous FSM-16 material appeared to be a good model for studying functionalization of the silica surface.