Direct synthesis and solid-state NMR characterization of cubic mesoporous silica SBA-1 functionalized with phenyl groups

Well-ordered mesoporous silicas SBA-1 (cubic Pm3n symmetry) functionalized with phenyl groups have been synthesized via co-condensation of tetraethoxysilane (TEOS) and phenyltriethoxysilane (PhTES) under acidic conditions. The synthesis parameters such as temperature, type of surfactant, and synthesis composition have been systematically investigated as a function of PhTES contents. The phenyl-containing units are incorporated quantitatively and reach a maximum PhTES loading up to 33 mol % (based on silicon) without a significant degradation of the structural ordering of the Pm3n mesophase. A combination of multinuclear (¹H, ¹³C, ²⁹Si) solid-state NMR and two-dimensional (2D) solid-state NMR correlation techniques such as ¹³C{¹H} and ²⁹Si{¹H} HETCOR (heteronuclear correlation) and ¹H-¹H exchange NMR has been used to establish framework locations of phenyl functional groups that are incorporated in the mesoporous structure and their interactions with the surfactant molecules. 2D ¹³C{¹H} HETCOR NMR experiments reveal that the phenyl moieties are in close spatial proximity to the trimethylammonium headgroups of the cationic surfactant species in the as-synthesized materials, suggesting that there are some specific interactions between them to maintain the surfactant packing parameter (g) smaller than 1/3 necessary for the formation of the cubic mesophase. The detection of couplings between the protons associated with various ²⁹Si species via ²⁹Si{¹H} HETCOR NMR established that the T silicon species due to the phenyl groups incorporated are in closer proximity to the Q ⁴ silicon species than to the Q³ silicon species. This observation also provides direct molecuiar-level evidence for the co-condensation of PhTES and TEOS in the synthesis of mesoporous organosilicas.