Molecular simulations were performed to study the adsorption behavior of water in the metal−organic framework Cu−BTC. This is one of the better-known materials of this type that is stable upon water adsorption/desorption. The charge of the framework atoms was fitted to reproduce the available experimental adsorption isotherm. This new set of interaction parameters was used to calculate Henry coefficients as well as the energies, entropies, and enthalpies for the different adsorption sites. Our simulations show that water has a surprisingly large affinity for the metal center in Cu−BTC compared to other that for molecules like carbon dioxide, nitrogen, oxygen, or hydrocarbons. This particular behavior could be further exploited for the separation of water from other compounds.

We use Monte Carlo simulations to study the adsorption and separation of the natural gas components in IRMOF-1 and Cu-BTC metal-organic frameworks. We computed the adsorption isotherms of pure components, binary, and five-component mixtures analyzing the siting of the molecules in the structure for the different loadings. The bulk compositions studied for the mixtures were 50 : 50 and 90 : 10 for CH4-CO2, 90 : 10 for N2-CO2, and 95 : 2.0 : 1.5 : 1.0 : 0.5 for the CH4-C2H6-N2-CO2-C3H8 mixture. We choose this composition because it is similar to an average sample of natural gas. Our simulations show that CO2 is preferentially adsorbed over propane, ethane, methane and N2 in the complete pressure range under study. Longer alkanes are favored over shorter alkanes and the lowest adsorption corresponds to N2. Though IRMOF-1 has a significantly higher adsorption capacity than Cu-BTC, the adsorption selectivity of CO2 over CH4 and N2 is found to be higher in the latter, proving that the separation efficiency is largely affected by the shape, the atomic composition and the type of linkers of the structure.

The adsorption of several quadrupolar and nonpolar gases on the Metal Organic Framework Cu-BTC has been studied by combining experimental measurements and Monte Carlo simulations. Four main adsorption sites for this structure have been identified: site I close to the copper atoms, site I' in the bigger cavities, site II located in the small octahedral cages, and site III at the windows of the four open faces of the octahedral cage. Our simulations identify the octahedral cages (sites II and III) and the big cages (site I') as the preferred positions for adsorption, while site I, near the copper atoms, remains empty over the entire range of pressures analyzed due to its reduced accessibility. The occupation of the different sites for ethane and propane in Cu-BTC proceeds similarly as for methane, and shows small differences for O2 and N2 that can be attributed to the quadrupole moment of these molecules. Site II is filled predominantly for methane (the nonpolar molecule), whereas for N2, the occupation of II and I' can be considered almost equivalent. The molecular sitting for O2 shows an intermediate behavior between those observed for methane and for N2. The differences between simulated and experimental data at elevated temperatures for propane are tentatively attributed to a reversible change in the lattice parameters of Cu-BTC by dehydration and by temperature, blocking the accessibility to site III and reducing that to site I'. Adsorption parameters of the investigated molecules have been determined from the simulations.

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