Influence and mechanism of alkali-metal modification on ZSM-5 structure and toluene adsorption

The microporous adsorbent ZSM-5 has been extensively applied for toluene adsorption. In this work, ZSM-5 was modified with alkali metals Li, Na, and K for toluene adsorption. The effects of the alkali metals introduced into ZSM-5 on the ZSM-5 microporous structure and toluene adsorption were studied via characterization techniques and mathematical modeling. Moreover, the influence of alkali metals on toluene adsorption was investigated from four aspects: adsorption capacity, exothermic energy, diffusion resistance, and desorption activation energy. The experimental results show that the introduction of alkali metal affect the ZSM-5 microporous structure in different aspects. The pore size, specific surface area, and pore volume of the modified ZSM-5 were of the following order: Li−ZSM-5 > Na−ZSM-5 > K−ZSM-5, corresponding to increasing ionic radius of the metals (Li⁺ < Na⁺ < K⁺). Likewise, the static saturated adsorption capacity was of the order: Li−ZSM-5 (0.363 mmol·g⁻¹) > Na−ZSM-5 (0.360 mmol·g⁻¹）> K−ZSM-5 (0.325 mmol·g⁻¹). The constant concentration wave model could well fit the adsorption and diffusion behaviors of toluene onto ZSM-5. The steric hindrance and electrostatic binding force played a dominant role in toluene diffusion in the ZSM-5 channel at high and low inlet gas concentrations, respectively. At a higher inlet concentration (155 mg·m⁻³), the influence of alkali metal modification on the internal diffusion resistance for the three adsorbents was of the order: Li−ZSM-5< Na−ZSM-5 < K−ZSM-5, whereas at a lower inlet concentration (25 mg·m⁻³), the trend was Li−ZSM-5 > Na−ZSM-5 > K−ZSM-5. The desorption kinetics analysis show that Na−ZSM-5 exhibite a better regeneration potential, due to its large pore size and moderate adsorption strength. In this study, the mechanism of alkali-metal modification of the adsorption behavior toward toluene was systematically investigated from two aspects: steric hindrance and adsorption strength to provide a certain reference for selecting a suitable adsorbent in complex practical environments.