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

Introduction/PurposeStorage and separation of gases are 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.MethodsIn this work, we 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. 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.ResultsMOFs with pore limiting diameter (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). The initial screening allowed us to select the most promising structures for more detailed investigation. We show that 20 materials exhibit very promising selectivity (defined as the adsorbed amount of CO2­ to the amount of CH4) in the range from 100 to 900 and total uptake above 0.5 mol/kg.ConclusionsThe results provide clear information about different mechanisms of adsorption for selected structures. Furthermore, the adsorption of methane tends to be reduced with increasing humidity. Materials with a growing level of selectivity for CO2 and CH4 with increasing hydration degree are the most promising from the future application point of view