碱金属改性对ZSM-5结构和吸附甲苯特性的影响及机理

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

ZSM-5是一种常用来吸附甲苯的微孔吸附剂,选择三种碱金属Li、Na和K对ZSM-5进行改性,结合表征手段和数学模型的方式研究引入ZSM-5中的碱金属对微孔结构和吸附甲苯的影响.在此实验中,分别从吸附容量、放热能量、扩散阻力和脱附活化能四方面深入探讨碱金属对吸附甲苯的影响规律.基于实验结果得知:碱金属的引入改变了ZSM-5分子筛的微孔结构并呈现出一定的规律.随着离子半径(Li^(+)Na-ZSM-5>K-ZSM-5.静态饱和吸附量呈Li-ZSM-5(0.363 mmol·g^(-1))>Na-ZSM-5(0.360 mmol·g^(-1))>K-ZSM-5(0.325 mmol·g^(-1))排序.恒定浓度波模型很好的描述甲苯在ZSM-5上的吸附扩散行为,空间位阻和静电束缚力分别在高低进气浓度条件下对甲苯在ZSM-5孔道中的扩散占据主导作用,较高进气质量浓度(155 mg·m^(-3))条件下,碱金属改性对扩散阻力影响规律为Li-ZSM-5Na-ZSM-5>K-ZSM-5.结合脱附动力学分析,Na-ZSM-5因具有较大的孔径和适中的吸附强度,表现出更好的再生潜能.本研究从空间位阻和吸附强度两方面系统阐述了碱金属改性对甲苯吸附行为的影响机理,为在复杂的实际环境应用中选择合适的吸附剂提供了一定的参考意义.

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.