Methane diffusion mechanism in catenated metal–organic frameworks

Abstract

In this work, molecular dynamics simulations were performed to investigate the gas diffusion mechanism in catenated metal -organic frameworks (MOFs), for which methane was adopted as a probe and two catenated IRMOFs with interwoven structure, IRMOF-11 and IRMOF-13, were considered. This work reveals that the diffusion pathways of methane molecules in catenated MOFs are mainly governed by the strong confinement in catenation regions, leading to a 3D diffusion along the sheets formed by the A-regions (xy-direction) as well as from one A-region to another by crossing a B-region (z-direction). In addition, the present work shows that the effect of catenation on methane diffusivity is very large, much larger than that on hydrogen diffusivity at room temperature, and that both adsorption selectivity and dynamic selectivity of gas mixtures may be enhanced largely in catenated MOFs, indicating that catenation is a good strategy to improve the overall performance of a material as a membrane in separation applications.

Molecular simulation studies of separation of CH4/H2 mixture in metal-organic frameworks with interpenetration and mixed-ligand

“…A slight increase in diffusion selectivities was found with increasing pressure and this can be attributed to the reason that at higher loadings CH 4 reduces the diffusivity of the faster diffusing component, H 2 , in adsorbed mixture, which causes higher diffusion selectivities toward CH 4 . In our previous work (Xue et al, 2009), we found the relative diffusivity of CH 4 and H 2 (based on the self-diffusivity of the single CH 4 and H 2 component) in the interpenetrated IRMOFs is much higher than those in their corresponding non-interpenetrated counterparts and concluded that interpenetration may significantly enhance the dynamic selectivity for CH 4 /H 2 . By examining the true mixture diffusion, however, we found that the diffusion selectivities of CH 4 /H 2 in L5-L2 and L6-L2 are comparable.…”

“…Electrostatic interactions were not included, as previous simulations have shown that the effects of these interactions on the adsorption of hydrogen in MOFs are very small at room temperature (Garberoglio et al, 2005). The above potential models have been successfully used to model adsorption, diffusion, and separation of methane and hydrogen in MOFs Liu et al, 2008aLiu et al, , 2008bSurblé et al, 2006;Xue et al, 2009;Yang and Zhong, 2005). For the MOFs studied here, the site-site LJ potential was used to calculate the interactions between adsorbate molecules and adsorbents.…”

“…An atomistic representation was used for MOFs studied. The dispersive interactions of all of the atoms in MOFs are modeled by the Universal Force Field (UFF) of Rappé et al (1992), which has been successfully employed to depict the adsorption (Babarao and Jiang, 2008a;Garberoglio et al, 2005;Krungleviciute et al, 2007), diffusion (Liu et al, 2008b;Skoulidas, 2004;Skoulidas and Sholl, 2005;Xue et al, 2009), and separation (Babarao et al, 2007;Babarao and Jiang, 2008b;Keskin and Sholl, 2007;Liu et al, 2008a) of several light gases and their mixtures in MOFs. The potential parameters used are listed in Table 2.…”

“…However, to draw a definite conclusion and design these interpenetrated MOFs for specific applications, we do need to know both adsorption and diffusion characteristics of gas mixtures in these MOF pores quantitatively. To the best of our knowledge, there is no experimental study measuring diffusivity of gas mixtures in interpenetrated MOFs to date, while theoretically, in our previous work, only the effect of interpenetration on single H 2 and CH 4 diffusion were investigated (Liu et al, 2008b;Xue et al, 2009). Regarding the separation of gas mixtures based on diffusion selectivities and permeation selectivities, Keskin and Sholl (2009) systematically investigated the separation of CH 4 /H 2 , CO 2 /CH 4 , and CO 2 /H 2 mixtures in eight MOFs, including one interpenetrated MOFs, i.e.…”

NMR studies of benzene mobility in metal-organic framework MOF-5

“…Meanwhile there exist a large number of computer simulations (see for example refs. [12][13][14][15]) and a few experimental techniques, such as interference and infrared microscopy [16][17][18], quasi elastic neutron scattering (QENS) [19][20][21] and pulsed field gradient (PFG) NMR [22,23], which allow one to determine guest diffusion in MOFs under non-equilibrium and equilibrium conditions, respectively.…”