Atomically Detailed Models for Transport of Gas Mixtures in ZIF Membranes and ZIF/Polymer Composite Membranes

Abstract

In this work, we introduced atomic models for transport of single component gases (CH 4 , CO 2 , H 2 , and N 2 ) and binary gas mixtures (H 2 /CO 2 , H 2 /N 2 , H 2 /CH 4 ) in zeolite imidazolate framework (ZIF) membranes and ZIF/polymer composite membranes. The predictions of atomic models were validated by comparing with the available experimental data for a ZIF-90 membrane. Motivated from the good agreement between experimental measurements and predictions of our molecular simulations for single gas and mixture permeances, we extended atomic modeling methods to an unfabricated ZIF membrane, ZIF-65, for predicting its separation performance. Various selectivities of ZIF membranes such as ideal selectivity, mixture selectivity, adsorption selectivity, and diffusion selectivity were computed for a wide range of operating conditions to assess the potential of ZIF membranes in H 2 /CO 2 separations. We then combined atomic simulations with continuum modeling to estimate the performance of ZIF-90/Matrimid and ZIF-90/Ultem composite membranes for gas separations. Our theoretical predictions agreed very well with the experimental measurements for these two composite membranes, and therefore, we assessed the performances of several ZIF/polymer membranes composed of various polymers, ZIF-90 and ZIF-65, for separation of H 2 from CO 2 .

Computational screening of ZIFs for CO2separations

“…In our previous studies, we showed that the results of molecular simulations employing UFF agree well with the experimental measurements of gas permeances and permeabilities through ZIF membranes. [20,21] Spherical Lennard-Jones (LJ) 12 -6 potentials were used to model H 2 and CH 4 molecules. [22,23] The CO 2 molecule was modelled as a rigid linear molecule which is an all-atom LJ potential with atomic charges to approximate the quadrupole moment of the CO 2 .…”

“…More discussion of computing gas diffusivity in ZIFs by EMD can be found in our earlier study. [20] After computing the adsorption and diffusion selectivities, membrane selectivities of the materials were calculated. IMS of component i from component j was …”

Simulation of separation of C2H6from CH4using zeolitic imidazolate frameworks

“…The potential parameters for frameworks were taken from Universal force field (UFF) [51] (see Table 1). This UFF force field has been validated in our previous work [52] and other studies [17,22,53,54] for gas adsorption in different ZIFs. Moreover, IRMOF-1 and two zeolites, i.e.…”

“…Bux et al [21] investigated C 2 H 6 /C 2 H 4 separation in ZIF-8 membranes by experimentation and simulation, and reported that the total membrane selectivity for C 2 H 6 versus C 2 H 4 was around 2.4. Atci and Keskin [22] predicted the selectivity performance of binary gas mixtures (H 2 /CO 2 , H 2 /N 2 and H 2 /CH 4 ) in ZIF-65 and ZIF-90 membranes using atomic simulation. Han et al [23] examined H 2 adsorption in the alkali metaldoped ZIFs and found the H 2 binding energies and uptakes were enhanced significantly.…”

“…Previous study [14] revealed that a number of ZIFs' pore size were too small (, 2.5 Å ) to effectively separate CH 4 and C 2 H 6 , whereas ZIFs with SOD, GME and RHO topologies possessed applicable pore size and interesting structures. [20,22,47] Since ZIF-90 (SOD), ZIF-80 (GME) and ZIF-71 (RHO) were of interesting topologies and applicable pore size for CH 4 and C 2 H 6 separation, in this paper, we investigated the influence of pore properties on binary gas separations. Equilibrium snapshots and density distribution profiles were used to study the preferential adsorption sites in the ZIFs.…”