Evaluating Classical Molecular Dynamics Methods for the Validation of Carbon Dioxide Separation Performance in Polymer Membranes
Climate change is mainly due to CO2 emissions occurring in energy production and transportation. A set of technologies are being developed to separate and sequester CO2 from point sources. Among these are liquid and solid adsorbents as well as membranes and hydrates. Polymer membranes show certain advantages with regards to their storage and disposal properties, they allow for passive operation, have high tolerance to SOx and NOx content and can be integrated within an existing power plant steam cycle (post combustion application). Candidate materials for application in polymer separation membranes must fulfill two key requirements: high CO2 permeability and high CO2/N2 selectivity. Many candidate materials exist today or can be computationally designed; however, the time and cost of experimental lab validation is the principal limitation in materials discovery applications. In this contribution, we investigate the potential of Classical Molecular Dynamics (CMD) simulations as a time- and cost-efficient validation option for suited polymer candidates. CMD simulations have been employed successfully for calculating permeation in glassy polymers , however, significant uncertainties with this approach remain. We have reviewed a set of available CMD methodologies and analyzed their strengths and weaknesses in application to the polymer membrane use case. Specifically, we have investigated effects related to number of atoms, membrane thickness and area, size of polymeric chain and membrane composition, respectively, on both CO2 permeability and selectivity. We show quantitative results for representative polymers that lead to the conclusion that Constant Pressure Difference Molecular Dynamics (CPDMD) [2,3] is the most appropriate methodology. References:  H. Frentrup et al. In Silico Determination of Gas Permeabilities by Non-Equilibrium Molecular Dynamics: CO2 and He through PIM-1, Membranes 5, 99-119 (2015).  X. Kong and J. Liu, An Atomistic Simulation Study on POC/PIM Mixed-Matrix Membranes for Gas Separation, J. Phys. Chem. C 123, 15113-15121 (2019).  J. LKiu and J. Jiang, Molecular Design of Microporous Polymer Membranes for the Upgrading pf Natural Gas, J. Phys.Chem. C 123, 6607-6615 (2019).