Adsorption And Separation Behaviors Of Alkane Mixtures In Metal-Organic Frameworks

In recent years, metal-organic frameworks (MOFs) have attracted many researchers' interest as a new class of functional materials. Owing to their endless variety of structures and functional groups, high accessible surface area and porosity, spatial and finely controlled structure, MOFs are promising materials for gas storage, adsorption and separation, catalysis, etc. The research of adsorption of alkanes in the nanoporous materials is of great importance because of the separation, purification and process of natural gas and petrochemicals. So it is of theoretical significance and application values to find out the adsorption behaviors and mechanisms in these materials. Due to the limitations of traditional methods, it is difficult to obtain the multi-component adsorption data by experiments. Molecular simulation techniques, as a kind of "computer experiments", offer an attractive method to help us explain the adsorption phenomena observed macroscopically in the nanoporous materials.Grand canonical ensemble Monte Carlo (GCMC) and configurational-bias Monte Carlo (CBMC) techniques were combined to simulate the adsorption behaviors of alkane mixtures in three different topology MOFs: Cu-BTC, MIL-53(Al) and IRMOF-9. Furthermore, an effective method of mCT (Computer Tomography for materials) technique was developed to investigate the adsorption sites of alkane mixtures. The main contents and findings are summarized as follows:1. The adsorption behaviors of methane-ethane-propane and n-butane-i-butane mixtures at 298 K in Cu-BTC were simulated. The results demonstrate that for the methane-ethane-propane mixtures propane is preferentially adsorbed, the selectivity of Cu-BTC for longer chain components increases as the mole fraction of shorter chain components in the gas phase increases. As to the n-butane-i-butane mixtures, i-butane is adsorbed preferentially, the selectivity for i-butane is increasing as the mole fraction of n-butane increases in the gas phase. The adsorption competition between the energy effect and the size effect has been found by the analysis of the adsorption sites. It indicates that at higher pressures propane is mainly occupying the main channels due to the size effect while for methane and ethane the energetically favored ethane occupies the tetrahedron-shaped pockets leaving methane locating at the outer sides of the triangular windows. As to the n-butane-i-butane mixtures, the energy effect is preferential to the size effect, therefore, i-butane is mainly occupying the tetrahedral pockets while n-butane is located in the main channels.2. The adsorption behaviors of methane-ethane-propane mixtures in different topology MOFs were simulated. The results show for that both MIL-53(A1) and IRMOF-9 propane is preferentially adsorbed , the selectivity of both MOFs for longer chain components increases as the mole fraction of shorter chain components in the gas phase increases. But the adsorbed amounts in IRMOF-9 are much larger than those in MIL-53(A1), the adsorption selectivity in IRMOF-9 is a little better than that in MIL-53(A1). Through the analysis of the adsorption sites, ethane and propane are mainly occupying near the octahedral AlO₄(OH)₂ units from top to bottom in the rhombic channels of MIL-53(A1), methane is near the octahedral AlO₄(OH)₂ units from left to right. As to IRMOF-9, ethane and propane are mainly occupying the additional apertures and interpenetrated main channels while methane is located in the interpenetrated sites of two Zn₄O clusters. The interpenetrating of IRMOF-9 framework results in larger adsorption and better selectivity.