Effect of hydrophobic character of diammonium-capping agents for the synthesis of nanocrystalline MOR zeolite

초록

Multiammonium surfactants are known to act as effective capping agents in the synthesis of zeolite nanoparticles. While the attractive interactions between ammonium head groups and zeolite surfaces play a critical role in capping, the influence of the surfactant tail structures remains poorly understood. In this study, a series of diammonium surfactants with different tail lengths (i.e., CnH2n+1-N+(CH3)(2)-C6H12-N+(CH3)(2)-CmH2m+1, 4 <= n, m <= 22) were systematically tested to investigate the effect of hydrophobic character on the morphology of mordenite (MOR) zeolite nanocrystals. Capping efficiency was assessed using X-ray diffraction, nitrogen adsorption-desorption isotherms, and scanning electron microscopy. The results showed that surfactants with a moderate hydrophilic/hydrophobic balance-quantified by the carbon-to-nitrogen (C/N+) ratio-exhibited the highest capping efficiency, resulting in smaller particle sizes and larger external surface areas. Furthermore, non-Gemini surfactants (i.e., n not equal m) demonstrated significantly enhanced capping efficiency, compared to symmetric Gemini surfactants (i.e., n = m). These findings offer valuable insights into the molecular design of surfactants in the controlled synthesis of zeolite nanoparticles.

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