Separation of gas mixtures has social, environmental, industrial and economical importance. Traditional gas separation techniques include distillation, absorption, adsorption and membranes [1]. Among these techniques, adsorption-based and membrane-based gas separation methods have received significant attention from academy and industry since they are energy-efficient and environmentally friendly processes. Various materials such as polymers, zeolites, carbon molecular-sieves, metal organic frameworks have been developed and tested as adsorbents and membranes to achieve high-performance gas separations [2]. Porous coordination networks (PCNs), also known as metal organic frameworks (MOFs), offer high potential for gas storage and separation due to their large surface areas, high porosities, tunable pore sizes, good chemical and thermal stabilities [3]. Various PCNs have been synthesized by different research groups and single-component gas uptake capacities of these PCNs are generally determined by experimental methods or GCMC simulations. However, there has not been any experimental study related with the binary gas adsorption and/or diffusion in PCNs. This lack of information limits the assessment of PCNs’ potential in adsorption-based and membrane-based gas separation applications.
We recently used atomic simulations to examine the separation performances of four PCNs, PCN-9-Co, PCN-9-Fe, PCN-9-Mn and PCN-26 for CH4/H2, CO2/H2, CO2/CH4 and CO2/N2 mixtures [4]. Our results showed that these PCNs can outperform traditional zeolites and widely studied MOFs in gas separations, especially in CO2 related separation processes and PCN-26 was identified as a potential membrane material that can exceed the upper bound established for CO2/CH4 and CO2/N2 separations due to its high CO2 permeability and selectivity [4]. Motivated from these initial results, we carried out a large scale molecular simulation study in this work for twenty different PCNs, which represent the largest number of PCN materials studied in a molecular simulation work, to examine their potential both for adsorption-based and membrane-based gas separations. Additionally, we examined the structure-performance relationships of materials and developed simple selectivity models based on structural properties. We showed that these simple models can make accurate predictions for adsorption, diffusion and membrane selectivity of PCNs without performing computationally demanding molecular simulations. Therefore, these selectivity models will be extremely useful to screen very large groups of PCNs and other similar nanoporous materials to identify the most promising materials in gas separation applications prior to extensive experiments and calculations. We specifically focused on separation of CH4/H2, CO2/H2, CO2/CH4 and CO2/N2 mixtures.
In this work, we examined adsorption-based and membrane-based separation performances of twenty PCNs using molecular simulations to identify the most promising adsorbent and membrane candidates for CH4/H2, CO2/H2, CO2/CH4 and CO2/N2 mixtures. Several PCNs such as PCN-9 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 PCNs membranes were shown to exceed Robeson’s upper bound established for CO2/CH4 and CO2/N2 separations. We showed that both adsorption and diffusion favor the same component CO2 over CH4 (N2) in PCN-10, PCN-11, PCN-131' and make these PCNs highly CO2 selective in membrane-based CO2/CH4 (CO2/N2) separations.
In addition to computing adsorption selectivity, diffusion selectivity and membrane selectivity of PCNs using molecular simulations, we also developed approximate models that can predict adsorption, diffusion and permeation selectivities of PCNs for CH4/H2 and CO2/H2 mixtures. These models were mainly based on the structural properties of the 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. Therefore, these selectivity models will be extremely useful to screen very large groups of PCNs and other similar nanoporous materials to identify the most promising materials in gas separation applications prior to extensive experiments and calculations.
References
[1] Li, J. R.; Kuppler, R. J.; Zhou, H. C. Selective Gas Adsorption and Separation in Metal-Organic Frameworks. Chem. Soc. Rev. 2009, 38, 1477.
[2] Li, J. R.; Sculley, J.; Zhou, H. C. Metal-Organic Frameworks for Separations. Chem. Rev. 2011, 112, 869.
[3] Ma, S.; Simmons, J. M.; Sun, D.; Yuan, D.; Zhou, H. C. Porous Metal-Organic Frameworks
Based on an Anthracene Derivative: Syntheses, Structure Analysis, and Hydrogen Sorption Studies. Inorg. Chem. 2009, 48, 5263.
[4] Ozturk, T. N.; Keskin, S. Predicting Gas Separation Performances of Porous Coordination Networks Using Atomistic Simulations. Ind. Eng. Chem. Res. 2013, 52, 17627.
