Post-Grafting Amination of Alkyl Halide-Functionalized Silica for Applications in Catalysis, Adsorption, and ¹⁵N NMR Spectroscopy

An anhydrous synthesis of aminosilica materials from alkyl halide-functionalized mesoporous SBA-15 silica by post-grafting amination is introduced for applications in CO₂ adsorption, cooperative catalysis, and ¹⁵N solid-state NMR spectroscopy. The synthesis is demonstrated to convert terminal alkyl halide-functionalized silica materials containing Cl, Br, and I to primary alkylamines using anhydrous ammonia in a high-pressure reactor. The benefits of the post-grafting amination procedure include (i) use of anhydrous isotopically labeled ammonia, ¹⁵NH₃, to create aminosilica materials that can be investigated using ¹⁵N solid-state NMR to elucidate potential intermediates and surface species in CO₂ adsorption processes and catalysis, (ii) similar CO₂ uptake in experiments extracting CO₂ from dry simulated air experiments, and (iii) improved activity in acid–base bifunctional catalysis compared to traditional amine-grafted materials. The effects of the type of halide, the initial halide loading, and the total reaction time on the conversion of the halides to primary amines are explored. Physical and chemical characterizations of the materials show that the textural properties of the silica are unaffected by the reaction conditions and that quantitative conversion to primary amines is achieved even at short reaction times and high initial alkyl halide loadings. Additionally, preliminary ¹⁵N solid-state NMR experiments indicate formation of nitrogen-containing species and demonstrate that the synthesis can be used to create materials useful for investigating surface species by NMR spectroscopy. The differences between the materials prepared via post-grafting amination vs traditional aminosilane grafting are attributed to the slightly increased spacing of the amines synthesized by amination because the alkylhalosilanes are initially better spaced on the silica surface after grafting, whereas the aminosilanes likely cluster to a greater extent when grafted on the silica surface. A slight increase in amine spacing allows for more effective amine–silanol interactions in cooperative catalysis without reducing the amine efficiency in CO₂ uptake under the conditions used here.