Molecular insight into adsorption and diffusion of alkane isomer mixtures in metal-organic frameworks

Adsorption and diffusion of alkane isomer mixtures (C(4) and C(5)) are investigated in catenated and noncatenated metal-organic frameworks (IRMOF-13, IRMOF-14, PCN-6, and PCN-6') using molecular simulations. Competitive adsorption between isomers is observed, particularly at high pressures, at which a linear isomer shows a larger extent of adsorption due to configurational entropy. An inflection is found in the isotherm as a consequence of sequential adsorption in multiple favorable sites. Compared with the noncatenated counterparts, IRMOF-13 and PCN-6 have a greater loading at low pressures because of the constricted pores and stronger affinity with adsorbate. However, the reverse is true at high pressures due to the smaller pore volume. Catenated MOFs exhibit larger adsorption selectivity for alkane mixtures than the noncatenated counterparts. Adsorption selectivity in the four MOFs is comparable to that in carbon nanotube and silicalite, though adsorption capacity is lower in the latter. It is found that diffusivity of alkane in MOFs decreases with the degree of branching because a slender isomer diffuses faster. With the presence of constricted pores, diffusivity in catenated MOFs is smaller than that in noncatenated counterparts. In IRMOF-14 and IRMOF-13 diffusivity decreases monotonically, while it initially increases and then decreases in PCN-6'. The diffusion selectivity in catenated IRMOF-13 and PCN-6 is larger than that in noncatenated IRMOF-14 and PCN-6'. This work provides insightful microscopic mechanisms for the adsorption and diffusion of alkane isomers in MOFs and reveals that both adsorption and diffusion selectivities can be enhanced by catenation.