Study of competitive adsorption of the N2O-CO2-CH4-N2 quaternary mixture on CuBTC

Abstract

For the widespread use of Metal Organic Frameworks as adsorbents for gas separation and purification, it is crucial to understand the multicomponent adsorption mechanism occurring in them. In this paper, we first provide experimentally consistent and comprehensive set of a quaternary mixture (N2O-CO2-CH4-N2) adsorption data on CuBTC. The measurements were performed at 297 K using a home-made recirculating volumetric apparatus. For the first time, we combine existing results from molecular simulations of the single-gas adsorption and the new experimental quaternary mixture data to get insights in competitive adsorption mechanism. For pure-component adsorption, the affinity of CuBTC for the components is in the following decreasing order N2O, CO2, CH4 and N2. A comparison of the components electronic properties shows that the dipole moment and the polarizability play more role in the adsorption on non-polar adsorbent CuBTC than does the quadrupole moment. The competition in the mixed adsorbed phase is proven by the big difference between the adsorbed quantities of all the components in the single-component phase and the mixture. The experimental data unequivocally confirms that the secondary adsorption sites e.g. sites which adsorption is favoured by adsorption on primary sites, play an important in the adsorption of all the components on CuBTC. For instance, the fact that the secondary adsorption sites of some components represent the primary adsorption sites of others is at the origin of the high competition in adsorption on CuBTC. On the other hand, it is shown that screening adsorbent separation performance with only pure-component data is not appropriate, as evidenced by the high difference between selectivities calculated from experimental mixture data and only pure-component isotherms. Moreover, the working selectivity has been proven to be more useful than the equilibrium selectivity, as it encompasses information on optimal feed gas and desorption pressures.