In Situ 29Si solid-state NMR study of grafting of organoalkoxysilanes to mesoporous silica nanoparticles

Since the catalytic activity and the stability of silica-bound organometallic complexes are affected by their interactions with hydroxyl groups on the surface, isolated hydroxyls are often created prior to the introduction of catalytic species. Here, we investigate a method to remove the indigenous hydroxyls and create new isolated hydroxyls by grafting organo-trimethoxysilane (R-TMS) to generate a silicon T-2 site, (equivalent to SiO?)(2)SiR(?OH). We used in situ Si-29 solid-state NMR experiments to monitor the evolution of T-n sites, (equivalent to SiO?)(n)SiR(?OH)(3-n) (n = 1, 2, 3). The study indicates that i) the grafting proceeds in a consecutive manner as T-1 -> T-2 -> T-3, and ii) the kinetics depend on the type of functional groups in the silane. However, the rates of T-1 formation and T-2 -> T-3 conversion are also controlled to a significant extent by the entropy loss associated to the initial silane binding and the spatial arrangement of surface hydroxyls, respectively. The grafting of R-TMS with a basic functional group leads to a lower concentration of T-1 sites. The nucleophilicity of the functional group facilitates the grafting process by lowering the enthalpy barrier, while the T-1 formation rate is more influenced by the entropy barrier than the T-1 -> T-2 conversion rate. Thus, the basic functional group promotes the T-1 -> T-2 conversion more than the T-1 formation, resulting in a lower concentration of T-1 sites.