关键词: 反气相色谱, 零长柱, 分子筛, 扩散, 动力学

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