![]() The researchers observed an unexpected increase in selectivity with increasing pressure drop which was attributed to the polymer’s swelling from CO 2 adsorption. (20) The thickness of the nanosheets ranged from 5 to 25 nm, and the other dimensions were in the microscale. ![]() ![]() reported that 2D CuBDC nanosheets were incorporated into polyimide (PI) matrix for the formation of 30–50 μm thick mixed matrix membranes (MMM) which achieved a separation of CO 2 over CH 4 with a selectivity 8 times higher than the isotropic 3D bulk CuBDC MOF and 30–80% higher than the polymer solely. for the separation of CO 2/H 2 mixtures, and a separation factor ranging from 34 to 235 was observed. (18) Thermoswitchable 2D nanosheets of various thickness were synthesized by Wang et al. synthesized Zn 2(Bim) 3 MOF nanosheets for the separation of CO 2/H 2 mixtures with a separation factor of up to 166. In recent years, there has been an increasing number of publications on MON fabrication and utilization for gas separations. The orders of magnitude difference in one dimension results in their characterization as 2D. (17) One of the dimensions (thickness) of the MONs is within the nanoscale while the other dimensions are in the microscale. (14−16) In contrast with isotropic, 3D bulk MOFs, MONs are anisotropic, free-standing 2D MOF structures. (11−14) There are reports in the literature that their 2D MOF nanosheet (MON) counterparts outperform them by demonstrating higher selectivity and high permeability. (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. They are easily tunable by altering these building blocks, which results in a wide range of topologies and pore openings. (7−9) MOFs are composed of metal nodes connected by organic ligands. (5,6) A class of porous, crystalline nanomaterials named metal–organic frameworks (MOFs) has been studied extensively over the past years. Increasing the value of one would decrease the value of the other, which is described as Robeson’s upper bound. (4) A trade-off between selectivity and permeability exists for polymeric membranes. (1) Polymeric membranes are also commercially used, but they can suffer from drawbacks such as low permeability and selectivity and insufficient chemical and thermal stability. However, they are energy demanding and costly. (3) Current technologies for carbon dioxide removal include the usage of liquid solvents. ![]() Furthermore, CO 2 is known to cause pipe corrosion in industry in the presence of water as it forms acidic solutions. (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. Hence, the importance of a defect-free synthetic method for CuBDC nanosheets is underlined.ĬO 2/CH 4 gas mixtures are present in natural gas and biogas. Only 10% of missing linkers result in nonselective nanosheets. Furthermore, the separation is shown to be sensitive to the presence of missing linker defects in the nanosheets. 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. The MON structure achieves a better selectivity of CO 2 over CH 4 compared to the bulk CuBDC MOF which is due to the mass transfer resistance of the methane molecules on the surface of the nanosheet. The EF-NEMD simulations reveal a pore blocking separation mechanism, in which the CO 2 molecules occupy adsorption sites and significantly restrict the diffusion of CH 4. We report the results of external force nonequilibrium molecular dynamics (EF-NEMD) for pure components and binary mixtures. In this work, we model the CO 2/CH 4 separation in both defect-free and defective 2D CuBDC nanosheets and compare their performance with the bulk CuBDC MOF and experimental data. 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). The separation of CO 2/CH 4 gas mixtures is a key challenge for the energy sector and is essential for the efficient upgrading of natural gas and biogas.
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