The upgrading of natural gas on the pristine and functionalized NUM-3a -X (X = -Cl, –NH2, –F) were studied using Monte Carlo simulations in the grand canonical ensemble (GCMC). The equilibrium structures of the functionalized NUM-3a-X were found and confirmed by the quantum mechanical DFT methods. At first, the adsorptions of the components of the natural gas, CH4, CO2, H2S and N2 as pure gases in the pristine MOF were calculated, utilizing the common force fields for the MOFs and compared with the available experimental data to find out the best performing ones for the simulation of adsorption of the studied gases. Based on the obtained results, NUM-3a -Cl showed the highest uptake for the pure gases. Also, we found that the adsorption of CO2 on NUM-3a -X (X = -Cl, –NH2, –F) is higher than that of other gases. Furthermore, our GCMC simulations revealed that the inclusion of functional groups increases the CO2 selectivity in binary mixtures. In addition, the selectivity of NUM-3a-Cl for CO2 was found to be higher than that the other studied MOFs. The simulated CO2 selectivity were in the order of CO2/N2 > CO2/CH4. Our results indicated that the inclusion of electron withdrawing functional groups can enhance the performance of a MOF for CO2 separation applications. In addition, the isosteric heats of adsorption and Henry’s law coefficients were studied.
