Cationic cellulose nanocrystals assisted synthesis of mesoporous Silicalite-1 zeolites with fewer silanol defects

Abstract

Cellulose nanocrystals (CNCs) are biomass-derived materials with tunable surface properties, which can be used as additives for facilitating mesoporous zeolite synthesis. Herein, cationic cellulose nanocrystals (quaternary ammonium-modified, CNC-N) and unmodified cellulose nanocrystals (hydroxyl-terminated, CNC-OH) were employed to assist the synthesis of mesoporous silicalite-1 (S-1) zeolites with the reduced usage of tetrapropylammonium (TPA, TPA/SiO₂ = 0.04). Parametric studies were conducted to obtain the well-crystallised mesoporous S-1 zeolites. Results showed that the CNC-N could more effectively produce hydrophobic S-1 zeolites with fewer internal silanol defects and higher mesoporosity (e.g., mesoporosity, f_meso, was 31, 20 and 22 % for S-1 templated by CNC-N, CNC-OH and without CNCs, respectively), which was expected to favour the adsorption of non-polar volatile organic compounds (VOCs). According to the characterisation data of the materials at different stages of the synthesis, the CNC-N could induce strong interaction with the anionic silicate species (via electrostatic force), resulting in ‘deposition’ of silicate and TPA on CNC-N, which exhibited slow non-classical crystallisation behaviour that led to the formation of intergrown S-1 (explaining the improved mesoporosity) with fewer internal silanol defects (due to the slow crystallisation). The obtained mesoporous S-1 showed improved performance in toluene adsorption compared to other reference zeolites under investigation. Findings of the work demonstrated the potential of cationic CNCs as the additives for pore/silanol defects engineering of zeolitic materials.