Hierarchical structure ZSM-5/SBA-15 composite with improved hydrophobicity for adsorption-desorption behavior of toluene

Hydrophobic zeolites have been considered as one of the most promising adsorbents for capturing volatile organic compounds (VOCs). In this contribution, hierarchically structured ZSM-5/SBA-15 composites with different morphologies including platelet, hexagonal prism, long-hexagonal prism, and long-rod were prepared. All materials were characterized using XRD, SEM, and TEM-EDX, which confirmed that the high dispersion of two zeolites within the composites. Due to the presence of intracrystalline mesopores that could shorten the diffusion pathway of toluene, the composites resulted in increased effective diffusion and higher adsorption capacity. The composites also possess higher hydrophobicity, leading to much better toluene adsorption performance under humid conditions. The water contact angle of ZSM-5 could be increased from 15.6 degrees to 44.9 degrees by forming well dispersed zeolite composites. The Si-29 MAS NMR results confirmed that the composites possess higher amount of Q(4) groups (75.3%-80.1%), which is the intrinsic reason for higher hydrophobicity. Notably, ZSM-5/SBA-15-hexagonal prism resulted in the best adsorption performance and cycling stability when compared with pure ZSM-5, the breakthrough time was increased from 20.8 to 49.4 min under dry conditions and from 5.9 to 27.3 min in the presence of 50% relative humidity. This work represents an important scheme for making hydrophobic zeolite-based VOCs adsorbents with also improved toluene diffusion properties due to the unique hierarchical porous structure.