Abstract:

Zeolite materials have important application value in the field of adsorption separation. The intracrystalline diffusion behavior of adsorbates in zeolite is one of the important bases for evaluating their adsorption separation effect. The zero length column (ZLC) technique was developed to evaluate intracrystalline diffusivity. This method involves placing a minute quantity of adsorbate between sieve plates and applying a high flow rate of carrier gas, while maintaining the relative partial pressure within the Henry region. The traditional ZLC experiments rely on the bulging method to produce organic vapors with varying partial pressures, which is not suitable for organic molecules with low saturated vapor pressures. To address these limitations, an inverse gas chromatography (IGC)-based ZLC apparatus, complemented by a micro-syringe pump was developed for evaluating the intracrystalline diffusion coefficient. In the IGC-ZLC apparatus, the adsorbent is loaded into a specialized column. A micro-syringe pump continuously injects the adsorbent liquid into the inlet port at a set temperature. The liquid then vaporizes and enters the column, with the eluting molecules detected by a flame ionization detector (FID). The intracrystalline diffusion coefficient is derived from the resulting elution curve. n-Octane, an organic molecule with a low saturated vapor pressure was selected as the adsorbate and ZSM-5 was selected as the adsorbent. The accuracy of the self-developed IGC-ZLC methodology across a range of conditions, including varying partial pressures, carrier gas flow rates, injection volumes, and temperatures was validated. Moreover, the hexane, heptane, nonane, isooctane and cyclooctane were also selected as absorbate for comparison. This work presents a promising new approach for determining intracrystalline diffusion coefficients.

Key words: inverse gas chromatography, zero length column, molecular sieves, diffusion, kinetics

摘要：

分子筛材料在吸附分离领域具有重要的应用价值，吸附质在分子筛中的晶内扩散行为是评价其吸附分离效果的重要依据之一。零长柱方法（zero length column，ZLC）可准确评价晶内扩散行为，但经典的ZLC实验利用鼓泡法获得不同分压的有机蒸气，无法适用于饱和蒸气压较低的吸附质，同时起泡器温度的微小变化也会影响蒸气压。鉴于此，基于反气相色谱（inverse gas chromatography，IGC），结合微量注射泵搭建了一套IGC-ZLC装置评价晶内扩散行为，以饱和蒸气压较低的正辛烷作为吸附质分子，ZSM-5作为吸附剂，在不同分压、载气流量、进样体积和温度下验证了改进IGC-ZLC方法的准确性，同时对比了正己烷、正庚烷、正壬烷、异辛烷和环辛烷的扩散行为，为在极端场景下测定晶内扩散系数提供了一种潜在的新方法。

关键词: 反气相色谱, 零长柱, 分子筛, 扩散, 动力学