Kinetic effects in predicting adsorption using the GCMC method - using CO2 adsorption on ZIFs as an example

Using force field parameters developed and validated for zeolitic imidazolate frameworks (ZIFs) and carbon dioxide (CO2) independently from adsorption data, we predicted CO2/ZIFs adsorption isotherms using the Grand Canonic Monte Carlo (GCMC) method. The results are in sharp contradiction: the calculated adsorption data agree well with the experimental data for SOD-type ZIF-8, but are more than 100% higher than the experimental data for GME-type ZIFs. Using non-equilibrium molecular dynamics simulations and potential of mean force (PMF) calculations, we reveal that the discrepancies are due to the kinetic blockage which is significant for ZIF-68 but negligible for ZIF-8. This study demonstrates that a force field developed independently from the adsorption data can be used to predict the adsorptions accurately; and the kinetic factor must be considered if the bottlenecks exist in the adsorption paths due to geometric and energetic features of adsorbate and adsorbent. It could be very misleading if the force field parameters are adjusted by fitting the GCMC simulation data to experimental data without considering the kinetic factors.