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. 2019 Jan 22;7(2):2739-2750.
doi: 10.1021/acssuschemeng.8b05832. Epub 2018 Dec 18.

Molecular Simulations of MOF Membranes and Performance Predictions of MOF/Polymer Mixed Matrix Membranes for CO2/CH4 Separations

Cigdem Altintas  1 , Seda Keskin  1
Affiliations

Molecular Simulations of MOF Membranes and Performance Predictions of MOF/Polymer Mixed Matrix Membranes for CO2/CH4 Separations

Cigdem Altintas et al. ACS Sustain Chem Eng. .

Abstract

Efficient separation of CO2 from CO2/CH4 mixtures using membranes has economic, environmental and industrial importance. Membrane technologies are currently dominated by polymers due to their processing abilities and low manufacturing costs. However, polymeric membranes suffer from either low gas permeabilities or low selectivities. Metal organic frameworks (MOFs) are suggested as potential membrane candidates that offer both high selectivity and permeability for CO2/CH4 separation. Experimental testing of every single synthesized MOF material as membranes is not practical due to the availability of thousands of different MOF materials. A multilevel, high-throughput computational screening methodology was used to examine the MOF database for membrane-based CO2/CH4 separation. MOF membranes offering the best combination of CO2 permeability (>106 Barrer) and CO2/CH4 selectivity (>80) were identified by combining grand canonical Monte Carlo and molecular dynamics simulations. Results revealed that the best MOF membranes are located above the Robeson's upper bound indicating that they outperform polymeric membranes for CO2/CH4 separation. The impact of framework flexibility on the membrane properties of the selected top MOFs was studied by comparing the results of rigid and flexible molecular simulations. Relations between structures and performances of MOFs were also investigated to provide atomic-level insights into the design of novel MOFs which will be useful for CO2/CH4 separation processes. We also predicted permeabilities and selectivities of the mixed matrix membranes (MMM) in which the best MOF candidates are incorporated as filler particles into polymers and found that MOF-based MMMs have significantly higher CO2 permeabilities and moderately higher selectivities than pure polymers.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
(a) Self-diffusivity of gases (D0) as a function of Henry’s constants (K0) calculated at infinite dilution. (b) Comparison of K0, D0, and P0 values of gases.
Figure 2
Figure 2
Adsorption, diffusion and membrane selectivities of MOFs calculated for CO2 over CH4 separation at infinite dilution. The diagonal line is given to guide the eye, the dashed line shows the gas preference of the membrane.
Figure 3
Figure 3
Selectivity and permeability of MOF membranes computed at infinite dilution. The black solid line represents the Robeson’s upper bound. MOFs that can exceed the bound are shown with blue and the top 8 MOF membranes are shown with red symbols.
Figure 4
Figure 4
(a) Separation performance of the top 8 MOF membranes computed at infinite dilution (blue), 1 bar (red), and 10 bar (green). Results of flexible simulations for the two MOFs are also shown at 10 bar (green, crossed). (b) Comparison of adsorption (full symbols), diffusion (empty symbols) and membrane (crossed symbols) selectivities computed at infinite dilution and mixture conditions at 1 bar (circles) and 10 bar (stars).
Figure 5
Figure 5
(a) Porosity and LCD of MOFs. Stars represent the top 8 MOF membranes. (b) Gas permeabilities of MOFs as a function of their LCDs.
Figure 6
Figure 6
Predicted separation performances of MOF-based MMMs (open symbols) together with separation performances of polymeric membranes (closed symbols).
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