Surface characterization and functionalization of MCM-41 silicas via silazane silylation

Disilazane reagents of type HN((SiRR22)-R-1)(2) carrying organic substituents R-1.2 = H, Me, Ph, n-Bu, n-Oct, Vin (Vin vinyl) of varying longitudinal and lateral extension are reacted with high quality MCM-41 samples of different pore sizes, i.e., effective pore diameters of 2.8 and 3.8 nm according to the BJH pore size analysis of the desorption branch. The reaction of the standard silylating reagent hexamethyldisilazane, HN(SiMe3)(2), is shown to be controlled by the amount of added silylamine and the contact time, resulting in effective pore size engineering. Calculations from elemental analysis revealed that the degree of silylation (silylation efficiency) and hence the surface hydroxyl consumption depend on the steric bulk/shape of the groups R. The surface coverage varies from 0.74 to 1.85 silyl groups/nm(2). The sterically least demanding tetramethyl-disilazane, HN(SiHMe2)(2), is the most efficient silylating reagent, while silyl groups with bulky phenyl substituents produce the lowest surface coverage featuring considerable interaction with nonsilylated silanol groups. The formation of various covalently linked siloxy functionalities [(OSiRR22)-R-1] is reflected in the change of the pore volume and mean pore diameter as evidenced by nitrogen physisorption measurements at 77.4 K. A monofunctional surface reaction and the structural integrity of the immobilized (functionalized) silyl groups is quantitatively revealed by means of FT IR and H-1/C-13 MAS NMR spectroscopy. Heterobisilylated organic/ inorganic hybrid materials are synthesized both via consecutive and competitive silylation utilizing mixtures of silylamines. The latter silylation procedure provides important mechanistic and kinetic details of this peculiar surface silylation reaction emphasizing the preformation of a four-centered "O ... H ... N ... Si" transposition as the rate-determining step. Fully silylated materials carrying reactive vinyl moieties were functionalized by hydroboration with BH3(THF) and 9-BBN.