ZIF-67, a Co-substituted ZIF-8 structure, is investigated as a candidate for the industrially highly demanding propylene/propane separation, with the use of computational techniques for the first time. A new force field for the ZIF-67 framework based on density functional theory calculations is reported along with a recently developed force field for ZIF-8. The new force field is validated through comparison with structural data for ZIF-67 from the literature. Molecular dynamics simulations are reported for ZIF-67, showing a dramatic increase of propylene/propane corrected diffusivities ratio when compared to ZIF-8, implying a huge improvement in the separation of the mixture. The sieving mechanism of ZIF frameworks is investigated, and the results yield a dependency of the swelling motion of the gates from the bonding of the metal atom with its surrounding atoms. The presence of Co in the modified framework results in a tighter structure with a smaller oscillation of the gate opening, which leads to a narrower aperture. The results from the simulations and experiments in ZIF-67 place this new structure at the top of the candidates for propylene/propane separation.

ZIF-8 is a strong candidate for propane/propylene separation, which is regarded as one of the most industrially demanding. Molecular simulation of this separation must account for the flexibility of the structure, which enables the adsorption and diffusion of molecules with kinetic diameter larger than the apertures of the pores. Moreover, this simulation requires modeling subtle changes since the strong sieving effect upon the mixture depends on the very small differences between propane and propylene molecular sizes (∼0.2 Å). In this work, a new force-field for the ZIF-8 structure has been developed from DFT calculations in simplified structures. The new parameter set reproduces structural properties in very good agreement with the experimental measurements reported in literature. Molecular dynamics simulations and the Widom test particle insertion method were then employed for the calculation of diffusivities, activation energies and adsorption properties of propane and propylene. The results are in agreement with experiments and demonstrate that the sieving of such a mixture is a kinetic driven separation process.

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The influence of a zeolitic imidazolate framework (ZIF)'s metal identity on its gas separation performance is studied extensively through molecular simulations for a variety of gases. ZIF-8 is used as the original framework for alterations of different metal substitutes of the Zn metal. ZIF-8 consists of cages connected by narrow apertures that exhibit flexibility through "swelling", allowing for relatively large penetrants to diffuse. Replacing the central metal atom in the basic tetrahedral unit of ZIF-8 with Cd, Co or Be results in three different structures with increasing bonding stiffness with their neighboring atoms. The metal modification approach offers a way to control the flexibility and the size of the aperture, which constitutes the main energy barrier of the penetrant's hop-like diffusion between the framework's cages. Newly developed force fields are reported and utilized here; the new frameworks are compared to the original one, in terms of the diffusivity of various gas molecules as a function of their size (from He to n-butane). The correlation of the gas diffusivity with the aperture flexibility-molecular size relation is investigated. The results reveal that the aperture flexibility-molecular size relation governs the diffusivity, which shapes a common trend along all modifications. Furthermore, a new generalized method is employed for the screening of the various modifications for specific gas separations. This method is useful to detect optimum separation performance for the various modifications: CdIF-1 (Cd) for n-butane/iso-butane mixture; ZIF-67 (Co) for propylene/n-propane and ethylene/ethane mixtures; BeIF-1 (Be) for CO/CH, CO/CH and CO/N mixtures.

Recent advances on the recovery of oil and gas from shale and tight reservoirs have put in focus the need for a better understanding of the behavior of fluids under confinement. Confinement effects must be considered when the pore size is on the order of a few nanometers. Pores of such a small scale are abundant in shale and tight reservoirs, justifying the unique properties and characteristics observed in fluids of such reservoirs. Furthermore, the development of techniques for geological carbon reinjection and storage makes the understanding essential of how CO 2 interacts with the reservoir medium and its fluids.In this work, we use molecular dynamics simulations to predict the behavior of n-alkanes and CO 2 mixtures confined by calcite slit nanopores. We observe that CO 2 displaces the hydrocarbons adsorbed on the calcite surface, while the number of calcium sites controls the amount of CO 2 adsorbed on the pore surface. This suggests that the reinjection of CO 2 in tight oil and gas reservoirs may help enhance hydrocarbon recovery. Furthermore, the temperature, pore size, CO 2 fraction, and n-alkane length are shown to be critical factors for the selective adsorption of CO 2 over n-alkanes.

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ZIF-67, a modification of ZIF-8 framework through Zn substitution with Co, is tested for the first time for the separation of ethylene/ethane mixture using molecular simulations. The framework consists of cages connected with narrow apertures, which exhibit flexibility through a swelling motion, allowing for relatively large penetrants to diffuse. ZIF-67 demonstrates an enhanced separation for the specific mixture. Various computational techniques are employed (conventional molecular dynamics and Monte Carlo simulations, umbrella sampling, and Widom particle insertion), and the separation mechanism is investigated in terms of sorption and diffusion, for both ZIF-8 and ZIF-67. The stiffer bonding of Co with the adjacent N atoms results in a tighter structure and an aperture with smaller size and lower swelling amplitude than ZIF-8. The diffusion results show a clear dependency of the kinetic-driven separation on the aperture flexibility of the different frameworks. The diffusivities of different sized molecules (from He to n-butane) are simulated in both ZIF-8 and ZIF-67 frameworks, and the molecular size is correlated with the aperture’s response variations. A generalized method based on these results is developed which helps the understanding of the sieving mechanism as a function of the penetrant size and of the aperture size and flexibility. This approach provides an efficient screening of modifiable frameworks toward more efficient separations.