(414w) Computational Screening of Metal-Organic Frameworks for Energy Applications

The demand for developing energy-efficient methods to separate CO₂ from flue gas and natural gas has been rapidly increasing. Capturing CO₂ from coal-fired power plants is crucial for reducing greenhouse gas emissions. Separating CO₂ from natural gas is also required prior to pipeline delivery because CO₂ reduces the energy content of the natural gas.[1] The most widely used conventional method for CO₂ separation is absorption of CO₂ in aqueous solutions of alkanolamines. However, this method is expensive due to the solvent exchange process for regeneration.[2] Adsorption-based and membrane-based separations methods have been recently shown to be promising for capture of CO₂ from flue gas and natural gas streams. Developing new materials that can efficiently and economically separate CO₂ has significant importance in industrial applications. In this study, we aim to assess the potential of new nanoporous materials, bio-metal organic frameworks (bio-MOFs) for adsorbent-based and membrane-based CO₂ separation. Bio-MOFs have been recently synthesized using biomolecules such as amino acids, nucleobases, sugars as linker molecules and biocompatible metal cations.[4] Bio-MOF-1, composed of adenine as a biomolecular ligand and zinc salts as metal centers, was synthesized as the first member of bio-MOF family  reported to show  high CO₂ adsorption capacity due to the its linker molecule’s high affinity for CO₂.[5]  

The storage capacities of bio-MOFs have been reported to exceed those of many traditional nanoporous materials due to the large pore sizes and high pore volumes of bio-MOFs. For example, the CO₂ adsorption capacity of bio-MOF-11 at 1 atm and 298 K (4.06 mmol/g MOF) was reported to be higher than that of many other widely studied MOFs including IRMOF-1, HKUST, ZIF-69, CUK-1 due to the high interaction energy between CO₂ molecules and bio-MOF-11.[6] Large surface areas, high pore volumes, good thermal and mechanical stabilities of bio-MOFs have attracted the interest of reserachers and studies on bio-MOFs have been recently started. There are currently nine bio-MOFs (1, 11, 12, 13, 14, 100, 101, 102, and 103) that have been synthesized in the literature. In a recent study, bio-MOFs-12, -13 and -14 were reported as promising materials for CO₂/N₂ separation due to strong adenine-CO₂ interactions.[7]  Bio-MOF-101, 102 and 103 were demonstrated to be the most porous MOFs that have been reported up to date.[9] The CO₂ selectivity of these bio-MOFs was evaluated based on the single-component adsorption isotherm data rather than the mixture adsorption data, which is much more significant in industrial applications. There is currently no adsorption and transport data for gas mixtures in the pores of bio-MOFs and this lack of information limits the assessment of gas adsorption and separation performance of bio-MOFs.

In this study, we investigated the potential of bio-MOFs for CO₂ separation from CO₂/CH₄, CO₂/N₂ and CO₂/H₂ mixtures using Grand-canonical Monte Carlo and Equilibrium Molecular Dynamics simulations. We first validated the accuracy of our molecular simulations by comparing simulated gas uptake results with the experimental ones and found a good agreement between the predictions of our simulations and experimental measurements. Motivated from this agreement, adsorption-based and membrane-based separation performances of bio-MOFs were calculated based on the results of molecular simulations and compared with the performances of other nanoporous materials. Our results showed that bio-MOF-11, bio-MOF-12 and IZUMUM show higher CO₂ selectivities and working capacities compared to other bio-MOFs and traditional zeolites such as MFI, ERI, DDR and TON. Bio-MOFs that were studied in this work can be used as promising membranes especially in CO₂/H₂ separations due to their high CO₂ permeabilities and high CO₂/H₂ permeation selectivities. Results of this study will be helpful to gain molecular level understanding of bio-MOFs’ performance in CO₂separation applications and direct the experimental researchers to focus on more promising bio-MOFs for adsorption-based and membrane-based gas separations.

References:

[1] Baker, R. W. Future directions of membrane gas separation technology. Ind. Eng. Chem. Res., 2002, 41(6), 1393.

[2] Arstad, B.; Fjellvåg, H.; Kongshaug, K.; Swang, O.; Blom, R. Amine functionalised metal organic frameworks (MOFs) as adsorbents for carbon dioxide. Adsorption, 2008, 14(6), 755..

[3] Robeson, L. M. Polymer membranes for gas separation. Curr. Opin. Solid State Mater. Sci., 1999, 4, 549.

[4] Imaz, I.; Rubio-Martinez, M.; An, J.; Sole-Font, I.; Rosi, N. L.; Maspoch, D. Metal-biomolecule frameworks (MBIOFS). Chem.Commun., 2011, 47(26), 7287.

[5] An, J.; Geib, S. J.; Rosi, N. L. Cation-triggered drug release from a porous zinc-adeninate metal-organic framework. J. Am. Chem. Soc., 2009, 131(24), 8376.

[6] Keskin, S.; Heest, T. M. V.; Sholl, D. S. Can metal–organic framework materials play a useful role in large-scale carbon dioxide separations? ChemSusChem, 2010, 3, 879.

[7] Li, T.; Chen, D.-L.; Sullivan, J. E.; Kozlowski, M. T.; Johnson, J. K.; Rosi, N. L. Systematic modulation and enhancement of CO₂ : N₂ selectivity and water stability in an isoreticular series of bio-MOF-11 analogues. Chem. Sci., 2013, 4(4), 1746.

[8] An, J.; Farha, O. K.; Hupp, J. T.; Pohl, E.; Yeh, J. I.; Rosi, N. L. Metal-adeninate vertices for the construction of an exceptionally porous metal-organic framework. Nat. Commun., 2012, 3, 604.

[9] Li, T.; Kozlowski, M. T.; Doud, E. A.; Blakely, M. N.; Rosi, N. L. Stepwise ligand exchange for the preparation of a family of mesoporous MOFs. J. Am. Chem. Soc., 2013, 135(32), 11688.

[10] Bohrman, J. A.; Carreon, M. A. Synthesis and CO₂/CH₄ separation performance of Bio-MOF-1 membranes. Chem.Commun., 2012, 48(42), 5130.

[11] Xie, Z.; Li, T.; Rosi, N. L.; Carreon, M. A. Alumina-supported cobalt–adeninate MOF membranes for CO₂/CH₄ separation. J. Mater. Chem. A, 2014, 2(5), 1239.