Abstract
The separation of CO2/CH4 gas mixtures is a key challenge for the energy sector and is essential for the efficient upgrading of natural gas and biogas. A new emerging field, that of metal–organic framework nanosheets (MONs), has shown the potential to outperform conventional separation methods and bulk metal–organic frameworks (MOFs). In this work, we model the CO2/CH4 separation in both defect-free and defective 2D CuBDC nanosheets and compare their performance with the bulk CuBDC MOF and experimental data. We report the results of external force nonequilibrium molecular dynamics (EF-NEMD) for pure components and binary mixtures. The EF-NEMD simulations reveal a pore blocking separation mechanism, in which the CO2 molecules occupy adsorption sites and significantly restrict the diffusion of CH4. The MON structure achieves a better selectivity of CO2 over CH4 compared to the bulk CuBDC MOF which is due to the mass transfer resistance of the methane molecules on the surface of the nanosheet. Our results show that it is essential to consider the real mixture in these systems rather than relying solely on pure component data and ideal selectivity. Furthermore, the separation is shown to be sensitive to the presence of missing linker defects in the nanosheets. Only 10% of missing linkers result in nonselective nanosheets. Hence, the importance of a defect-free synthetic method for CuBDC nanosheets is underlined.
Highlights
CO2/CH4 gas mixtures are present in natural gas and biogas.[1,2] Removing carbon dioxide from methane is important as it will result in a more energy dense product, given that methane is rich in calorific energy while carbon dioxide has no heating value
They demonstrate high surface areas, pore volumes, and high gas uptake that make them great candidates for a range of applications, including gas separations.[11−14] There are reports in the literature that their 2D metal−organic frameworks (MOFs) nanosheet (MON) counterparts outperform them by demonstrating higher selectivity and high permeability.[14−16] In contrast with isotropic, 3D bulk MOFs, metal−organic framework nanosheets (MONs) are anisotropic, free-standing 2D MOF structures.[17]
CuBDC bulk MOF for experiments.[20,21]. In this gas separation work, we have of CO2 applied over CH4 in external-force nonequilibrium molecular dynamics for the separation of equimolar CO2/CH4 mixture with defect-free CuBDC nanosheets, defective nanosheets, and bulk CuBDC MOF
Summary
CO2/CH4 gas mixtures are present in natural gas and biogas.[1,2] Removing carbon dioxide from methane is important as it will result in a more energy dense product, given that methane is rich in calorific energy while carbon dioxide has no heating value. Increasing the value of one would decrease the value of the other, which is described as Robeson’s upper bound.[5,6] A class of porous, crystalline nanomaterials named metal−organic frameworks (MOFs) has been studied extensively over the past years.[7−9] MOFs are composed of metal nodes connected by organic ligands They are tunable by altering these building blocks, which results in a wide range of topologies and pore openings.[10] They demonstrate high surface areas, pore volumes, and high gas uptake that make them great candidates for a range of applications, including gas separations.[11−14] There are reports in the literature that their 2D MOF nanosheet (MON) counterparts outperform them by demonstrating higher selectivity and high permeability.[14−16] In contrast with isotropic, 3D bulk MOFs, MONs are anisotropic, free-standing 2D MOF structures.[17] One of the dimensions (thickness) of the MONs is within the nanoscale while the other dimensions are in the microscale. The thickness of the nanosheets ranged from 5 to 25
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