Hydrophilicity/hydrophobicity modulated synthesis of nano-crystalline and hierarchically structured TS-1 zeolites

Abstract

Nanosized crystals and hierarchically structured materials are considered to be two important options to enhance the mass transportation efficiency in microporous zeolites. Herein, we first present a highly efficient gel-crystallization approach for the synthesis of TS-1 nanocrystals with an exceptionally high mass yield of up to 97.4% and a volume yield of ∼0.36 g ml⁻¹. The resultant TS-1 nanozeolites (un-TS-1) possess uniform and tunable particle sizes from 80 nm to 2.0 μm. Compared to the TS-1 counterparts (ht-TS-1) synthesized by a direct hydrothermal method, the obtained un-TS-1 materials showed comparable or improved catalytic performance for epoxidation of 1-hexene and hydroxylation of phenol with dilute H₂O₂ as the oxidant. More importantly, a surface-hydrophilicity/hydrophobicity-modulated mechanism has been proposed to illustrate the distinct structural evolution pathway of un-TS-1 nanocrystals and single-crystalline ZSM-5 HSZs under identical synthetic conditions: the relatively stronger hydrophobicity of TS-1 due to the equal-valence substitution of Ti for Si, is suggested to be responsible for the formation of nanocrystalline TS-1, in comparison with ZSM-5 HSZs of higher hydrophilicity resulting from the tri-valence Al substitution for Si. The successful syntheses of relatively hydrophilic Al-containing TS-1 and Fe-doped silicalite-1 HSZs offer further support to this mechanism.