Zeolites hold great promise in the field of catalysis, ion-exchange, and adsorption/separation because of their high surface area, uniform microporous structure and tunable composition of crystalline frameworks. However, suffering from the sole microporous system of pure zeolites, catalyst deactivation and consequently unfavorable regeneration are often encountered in their practical applications. Therefore, nanosized crystals and hierarchically structured materials are considered to be two major options to enhance the mass transportation in them. Recently, based on the dry-gel conversion method, we first prepared single-crystalline hierarchically structured ZSM-5 zeolites (HSZs) in the absence of additional mesoporous agents. As expected, compared with the conventional microporous ZSM-5 and amorphous mesoporous Al-MCM-41 counterparts, the synthesized materials exhibited significantly enhanced activity and stability and prolonged lifetime in model reactions, e.g. alkylbenzene cracking, <i>o</i>-xylene isomerization, and aldol condensation between benzaldehyde and ethyl alcohol. Importantly, a kinetics controlled nucleation-growth mechanism is suggested for the elucidation of ZSM-5 HSZs formation. However, under the identical synthesis conditions, for TS-1 zeolites, only nanocrystals but not HSZs were obtained. To address the distinct structural evolution pathway between ZSM-5 HSZs and TS-1 nanocrystals, a surface-hydrophilicity/hydrophobicity-modulated mechanism has been proposed. It is suggested that the relatively stronger hydrophobicity of TS-1 due to the equal-valence substitution of Ti for framework Si should be responsible for the formation of nanocrystals, in comparison with ZSM-5 HSZs of higher hydrophilicity resulting from the tri-valence Al substitution for framework Si. Also, the successful synthesis of relatively hydrophilic Al-containing TS-1 and Fe-doped silicalite-1 HSZs offer further support to this mechanism.

Hydrophilicity/hydrophobicity modulating zeolite synthesis: nanocrystals or hierarchically structured materials

254th National Meeting (2017)

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