Hierarchically structure-directing effect of multi-ammonium surfactants for the generation of MFI zeolite nanosheets

Multi-quaternary ammonium surfactants can assemble co-operatively with silicate anions to generate MFI zeolite nanosheets, where the ammonium acts as a structure-directing agent for the microporous zeolite, while the surfactant tails assemble into a lamellar micelle. The two-level structure-directing effects have been investigated for various molecules with the formula of tail-N⁺(CH₃)₂-{spacer-N⁺R ₂}_n-1-R*. Here, n is varied over 1-4. The tail is selected among linear alkyl chains (C_mH_2m+1, m = 6-22). The spacer is an alkylene bridge (-C_iH_2i-, i = 3, 6 and 8). R and R* are alkyl groups (C_jH_2j+1, j = 1-6). The result highlights that the surfactant molecule should be equipped with at least 2 suitable ammonium groups for the formation of zeolite structure, in addition to the amphiphilic nature and micelle packing ability. Moreover, the thickness of the nanosheets can be increased and tailored according to the number of the ammonium groups. Detailed consideration of the effects of template molecular features may be useful for the design of hierarchically structure-directing surfactants for nanomorphic zeolites.