首页|In Situ ~(29)Si solid-state NMR study of grafting of organoalkoxysilanes to mesoporous silica nanoparticles
In Situ ~(29)Si solid-state NMR study of grafting of organoalkoxysilanes to mesoporous silica nanoparticles
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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,(≡SiO-)2SiR(-OH).We used in situ ~(29)Si solid-state NMR experiments to monitor the evolution of T~n sites,(≡SiO-)nSiR(-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.
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