Silylation Efficiency of Chorosilanes, Alkoxysilanes, and Monosilazanes on Periodic Mesoporous Silica

Because of their large surface area and distinct pore architecture, periodic mesoporous silica (PMS) are predisposed to study the reactivity of surface silanol groups. Surface silylation involving the formation of thermodynamically stable O−SiR₃ moieties is a powerful and probably the most prominent postsynthesis functionalization method of oxidic support materials. This study will assess the reactivity/silylation behavior of similarly sized dimethyloctylsilazane (Me₂NSiMe₂(C₈H₁₇)), dimethyloctylchlorosilane (ClSiMe₂(C₈H₁₇)), and octyltriethoxysilane ((EtO)₃Si(C₈H₁₇)), toward periodic mesoporous silica MCM-41 (BET surface a_s = 1060 m²/g, pore volume V_p = 1.14 cm³/g, pore diameter d_p = 4.0 nm, silanol population = 3.47 mmol/g) and SBA-1 (a_s = 1260 m²/g, V_p = 0.73 cm³/g, D_me = 4.5 nm, silanol population = 4.45 mmol/g). A medium-long aliphatic tail (C₈) was chosen to ensure a reliable elemental (carbon) analysis and to study any size-selective behavior of cagelike SBA-1. For channel-like MCM-41, the obtained hybrid materials reveal a superior silylation efficiency (SE) of the monosilazane reagent compared to the chloro- and ethoxysilanes (SE = 81% versus 67% versus 22%, use of excess of silylating reagent), the latter two reagents requiring either elevated temperature (125 °C) or stoichiometric amounts of a base (NEt₃). For cagelike SBA-1, site-specific reactivity of the monosilazane reagent at the outer surface and pore openings was observed, finally resulting in pore blockage. Even under harsher reaction conditions, the chloro- and ethoxysilanes gave a more random surface silylation, however with lower SE values (SE = 76% versus 62% versus 32%; use of excess of silylating reagent). All materials were characterized by FTIR spectroscopy, nitrogen physisorption, elemental analysis, and transmission electron microscopy.