On the selectivity and self-diffusion of CO2 and H2 in mixed-layer zeolitic-imidazolate frameworks

Zeolitic-imidazolate frameworks (ZIFs) have shown promise in gas separation through membranes. Nevertheless, the potential of mixed-layer ZIFs to be tailored for targeted gas separation remains largely unexplored. This study aims to fill this research gap through a Molecular Dynamics (MD) study by proposing two molecular models for mixed-layer ZIFs and evaluating their effectiveness in H2 and CO2 separation. MD simulations are conducted to validate and assess the diffusion properties of H2 and CO2 within the mixed-layered ZIF models. The results demonstrate that H2 has higher diffusivity than CO2 within the proposed ZIF models. Mixed-layer ZIF-8/ZIF-7 exhibits higher diffusion coefficients for both H2 (4.79 x 10-9 m2/s) and CO2 (8.13 x 10-11 m2/s) compared to pure ZIF-8, attributed to increased pore flexibility from the ZIF-7 layer. However, this enhancement in diffusion comes at the cost of reduced selectivity due to broader pore size distribution. In contrast, mixed-layer ZIF-8(Zn)/ZIF-8(Co) demonstrates a substantial increase in H2 diffusion (5.17 x 10-9 m2/s) and an exceptional selectivity of 310.00 for H2 over CO2, owing to the altered framework flexibility from incorporating different metal ions. The study further explores the effect of different adsorbate molecular models, revealing that the H2_COMPASS and CO2_TRAPPE combination yields the highest H2/CO2 selectivity. Additionally, increased molecular loading enhances diffusion. These findings underscore the critical role of structural modifications and molecular model selection in optimizing ZIF-based materials for gas separation applications. The proposed models and simulation results offer a foundation for future studies and the development of efficient and sustainable gas capture technologies.

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