Metal organic frameworks (MOFs) are promising nanoporous materials in gas separation applications due to their tunable pore sizes, large surface areas, high porosities, reasonable thermal and mechanical stabilities. We investigated adsorption-based and membrane-based separation performances of various MOFs for CH4/H2, CO2/H2, CO2/CH4 and CO2/N2 mixtures using molecular simulations. Several MOFs such as PCN-9 (PCN:porous coordination networks, type of MOFs) series and PCN-14 were identified as promising candidates for adsorption-based CH4/H2 separations since they exhibit both high adsorption selectivity and working capacity towards CH4. PCN-9-Co, PCN-9-Mn, PCN-14 and PCN-16 were found to be strong adsorbents for CO2 capture from CO2/H2, CO2/CH4 and CO2/N2 mixtures because of their high CO2 working capacities. Several MOF membranes were shown to exceed Robeson’s upper bound established for CO2/CH4 and CO2/N2 separations. We also developed simple models that can predict adsorption, diffusion and permeation selectivities of MOFs for CH4/H2 and CO2/H2 mixtures based on the simple structural properties of materials which can be simply measured or computed such as pore volume, surface area, pore diameter. Model predictions for adsorbent and membrane selectivities were found to be in a good agreement with the direct results of detailed molecular simulations. For example, adsorption selectivity model suggests that in order to obtain a high selectivity, the differences in the interactions between two gases and MOF must be as large as possible whereas the porosity and surface area of MOF must be as small as possible. Supporting this idea, simulations show that two MOFs which exhibit the highest CH4/H2 selectivity, PCN-13 and PCN-131', have the lowest porosity and surface area. The selectivity models that we will present will be very useful for large scale screening of MOFs in gas separation applications to identify the highly promising materials prior to computationally demanding calculations and extensive experimental efforts. These models can be also used to design and develop new MOF materials to achive high performance gas separation.

Computational screening of MOFs for gas separations

249th National Meeting (2015)

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