Metal – Organic Frameworks for CO2 and CH4¬ separation in the presence of water

International audienceStorage and separation of gases is one of the most challenging processes when they have to be implemented at industrial scale. Hundreds of potential materials have been tested over the past years, especially for gas storage (hydrogen and methane for fueling applications) and separation of biogas (a mixture of 55 – 75% of methane 25 – 45% of carbon dioxide). Today, MOFs (Metal-Organic Frameworks) [1-2] are considered as the most promising materials for such applications. MOFs are hybrid, nanoporous materials with a very large surface area, in which pore sizes, shapes and adsorption energies can be almost unlimitedly tailored to fit the required adsorption characteristics. A particular category of these materials are the so-called breathing MOFs, in which a reversible structural transformation (variation of pore volume, pore breathing) can be induced by temperature, pressure or interaction with adsorbate molecules. Molecular modeling is an effective way to rapidly obtain microscopic information on the mechanism of adsorption in these materials (in many cases, not accessible using traditional experimental methods) and to shorten the time necessary to implement research results in an industrial process.In the this work we will present simulations of methane and carbon dioxide adsorption in a wide range of MOF structures from CoRE-MOF database [3], and at variable content of humidity. MOFs with pore limiting diffusion (PLD) over 3.3 Å were chosen for the simulations. Due to the humid conditions we chose structures from the middle range of hydrophilicity, kH between 5·10-2 mol/kg·Pa (CuBTC) and 5·10-6 mol/kg·Pa (ZIF-8). Over 1000 structures have been tested using Grand Canonical Monte Carlo method (as implemented in RASPA code) to find the optimal material for the CH4/CO2 separation in conditions of increasing humidity. The initial screening allowed us to select the most promising structures for more detailed investigation. We show that 16 materials exhibit very promising selectivity (defined as adsorbed amount of CO2¬ to amount of CH4) in range from 50 to 440. Furthermore, adsorption of methane tends to be reduced with increasing humidity. Reference[1]S. Li, Y. G. Chung, and R. Q. Snurr, “High-Throughput Screening of Metal–Organic Frameworks for CO 2 Capture in the Presence of Water,” Langmuir, vol. 32, no. 40, pp. 10368–10376, 2016.[2]R. Krishna, “Screening metal–organic frameworks for mixture separations in fixed-bed adsorbers using a combined selectivity/capacity metric,” RSC Adv., vol. 7, no. 57, pp. 35724–35737, 2017.[3]Y. G. Chung et al., “Computation-Ready, Experimental Metal–Organic Frameworks: A Tool To Enable High-Throughput Screening of Nanoporous Crystals,” Chem. Mater., vol. 26, no. 21, pp. 6185–6192, Nov. 2014