Selective and Efficient Silylation of Mesoporous Silica: A Quantitative Assessment of Synthetic Strategies by Solid-State NMR

Surface silanol groups in mesoporous silica MCM-41 particles were successfully silylated with trimethylsilyl trifluoromethanesulfonate (TMSOTf). Characterization of modified mesoporous silica materials was conducted using X-ray diffraction, infrared spectroscopy, nitrogen absorption, elemental analysis, and solid-state NMR spectroscopy. In particular, extensive use of ¹H, ¹³C, and ²⁹Si solid-state NMR provided unique insights into the silylation process and served as a key guiding tool for the synthetic effort. Treatment of as-synthesized MCM-41 with TMSOTf was found to selectively and efficiently passivate the external surface of particles without assistance of a base, whereas modification by other silylating reagents, including trimethylchlorosilane (TMCS), <i>N</i>,<i>O</i>-bis­(trimethylsilyl)­acetamide (BSA), and triethoxymethylsilane (MeSi­(OEt)₃), yielded lower coverage and/or resulted in partial silylation of the internal surface. The ²⁹Si and ¹H solid-state NMR spectra gave accurate concentrations of silicon sites and densities of trimethylsilyl (TMS) groups on the external and internal surfaces of MCM-41. The ¹H and ¹³C NMR spectra revealed the definite structures and concentrations of all organic species present in the silylated samples. These data highlighted the importance of choosing a proper concentration of the silylating reagent and finding the washing and extraction conditions that result in efficient sequestration of the structure directing agent (surfactant) without detachment of grafted species or production of unwanted surface alkoxy groups