Abstract
Membrane-based separation technologies offer a cost-effective alternative to many energy-intensive gas separation processes, such as distillation. Mixed matrix membranes (MMMs) composed of polymers and metal–organic frameworks (MOFs) have attracted a great deal of attention for being promising systems to manufacture durable and highly selective membranes with high gas fluxes and high selectivities. Therefore, understanding gas transport through these MMMs is of significant importance. There has been longstanding speculation that the gas diffusion behavior at the interface formed between the polymer matrix and MOF particles would strongly affect the global performance of the MMMs due to the potential presence of nonselective voids or other defects. To shed more light on this paradigm, we have performed microsecond long concentration gradient-driven molecular dynamics (CGD-MD) simulations that deliver an unprecedented microscopic picture of the transport of H2 and CH4 as single components and as a mixture in all regions of the PIM-1/ZIF-8 membrane, including the polymer/MOF interface. The fluxes of the permeating gases are computed and the impact of the polymer/MOF interface on the H2/CH4 permselectivity of the composite membrane is clearly revealed. Specifically, we show that the poor compatibility between PIM-1 and ZIF-8, which manifests itself by the presence of nonselective void spaces at their interface, results in a decrease of the H2/CH4 permselectivity for the corresponding composite membrane as compared to the performances simulated for PIM-1 and ZIF-8 individually. We demonstrate that CGD-MD simulations based on an accurate atomistic description of the polymer/MOF composite is a powerful tool for characterization and understanding of gas transport and separation mechanisms in MMMs.
Highlights
Membrane technology plays an important role in today’s industrial gas separation processes and has paramount economic importance.[1−4] The efficiency of membrane-based separation technologies reduces the cost and environmental footprint of many industrial processes.[5]
Even though the presence of microscopic sized voids at the interface has been clearly related to the formation of brittle membranes,[29] the speculation that the presence of these voids might lead to a reduction in selectivity has never been confirmed. The absence of these voids is a signature of good polymer/metal−organic frameworks (MOFs) compatibility, but this is not necessarily related to the good performance of the corresponding membrane for a particular application. To address this still open question, here we report concentration gradient-driven molecular dynamics (CGDMD) simulations of H2 and CH4 transport through a realistic atomistic model of the polymer/MOF membrane with a specific focus on gas transport properties through the interfaces as well as along the individual components of the matrix membranes (MMMs)
The experimental permselectivites for both PIM-1 and ZIF-8 vary within a broad range; the simulated ideal permselectivities lie within this range and are in good agreement with several of the reported permselectivity data
Summary
Membrane technology plays an important role in today’s industrial gas separation processes and has paramount economic importance.[1−4] The efficiency of membrane-based separation technologies reduces the cost and environmental footprint of many industrial processes.[5]. Velioglu et al.[26] and Altintas et al.[27] recently reported the separations of H2/ CH4 and CO2/CH4 mixtures in polymer/MOF MMMs by carrying out screening calculations based on molecular simulations These computational studies predicted the separation performance of the polymer/MOF composites based on the individual constituents of the MMMs by assuming ideal polymer/MOF compatibility and did not consider the impact of the interface on the transport properties. The absence of these voids is a signature of good polymer/MOF compatibility, but this is not necessarily related to the good performance of the corresponding membrane for a particular application To address this still open question, here we report concentration gradient-driven molecular dynamics (CGDMD) simulations of H2 and CH4 transport through a realistic atomistic model of the polymer/MOF membrane with a specific focus on gas transport properties through the interfaces as well as along the individual components of the MMM. We demonstrate below that this is the case in the PIM1/ZIF-8 membrane
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