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

Disilazane reagents of type HN((SiRR22)-R-1)(2) carrying organic substituen ts 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 diffe rent 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 th e standard silylating reagent hexamethyldisilazane, HN(SiMe3)(2), is shown to be controlled by the amount of added silylamine and the contact time, re sulting in effective pore size engineering. Calculations from elemental ana lysis revealed that the degree of silylation (silylation efficiency) and he nce 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 in teraction with nonsilylated silanol groups. The formation of various covale ntly linked siloxy functionalities [(OSiRR22)-R-1] is reflected in the chan ge of the pore volume and mean pore diameter as evidenced by nitrogen physi sorption measurements at 77.4 K. A monofunctional surface reaction and the structural integrity of the immobilized (functionalized) silyl groups is qu antitatively 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 silylamine s. The latter silylation procedure provides important mechanistic and kinet ic details of this peculiar surface silylation reaction emphasizing the pre formation of a four-centered "O ... H ... N ... Si" transposition as the ra te-determining step. Fully silylated materials carrying reactive vinyl moie ties were functionalized by hydroboration with BH3(THF) and 9-BBN.