Adsorption and Diffusion of Light Gases in ZIF-68 and ZIF-70: A Simulation Study

Abstract

We have computed the adsorption and diffusion of CO2, N2, CH4, and H2 in zeolitic imidazolate framework (ZIF) materials ZIF-68 and ZIF-70 from atomistic simulations. These simulations were performed using geometries obtained from density functional theory (DFT) optimization of the experimental crystal structure of ZIF-68 and on four structures of ZIF-70, one based on the experimental crystal structure and three having different imidazole/nitroimidazole substitution ratios. The framework charges for charge−quadrupole interaction (CQI) terms in our simulations were parametrized with charges obtained from Bader charge decomposition (periodic DFT calculations) and ChelpG charges (cluster DFT calculations). The adsorption and diffusion of the quadrupolar fluids can be dramatically different when using the Bader and ChelpG charges. Agreement between simulations and experiments for the N2 adsorption isotherms in ZIF-68 and 70 is very good when CQI terms are included. In contrast, simulations overpredict the amount of CO2 adsorbed at 298 K compared with experiments.

Recent Advances in Molecular Dynamics Simulations of Gas Diffusion in Metal Organic Frameworks

“…For example, self and corrected diffusivities of CO 2 , CH 4 and H 2 were simulated using equilibrium MD in ZIF-68 and ZIF-70. (Rankin et al, 2009) That study underlined the importance of including chargequadrupole interactions on the diffusivity of CO 2 . Simulation results clearly revealed that addition of charge-quadrupole interaction terms results in almost one order of magnitude drop in the self and transport diffusivities of CO 2 at low loadings.…”

“…If the charge-quadrupolar interactions are not taken into account in MD simulations then the diffusivities can be significantly overestimated. (Rankin et al, 2009) Force field-based classical MD simulations of MOFs typically treat electrostatic interactions between adsorbates and MOF atoms by assigning fixed point charges to each atom. In this context an important role for quantum mechanics (QM) calculations is to assign the point charges that can later be used in force field calculations.…”

Molecular Simulation Studies of Separation of CO2/N2, CO2/CH4, and CH4/N2 by ZIFs

“…[14][15][16] At present, many studies have been carried out on the adsorption and diffusion of pure gases in ZIFs. [8][9][10][11][12][13][14][15][16][17][18][19][20] Experimentally, for example, Yaghi and co-workers measured the adsorption isotherms of CO 2 , CO, CH 4 , N 2 , and O 2 in several ZIFs. [14][15][16] Wu et al 13 studied the H 2 adsorption sites and binding energies in ZIF-8 using a combined experimental and computational method.…”

van der Waals density functional study of CO2binding in zeolitic imidazolate frameworks

“…Experimental measurements have demonstrated a pronounced dependence on ZIF chemical compositions and structural topologies, for the equilibrium adsorptions of H 2 , CO 2 , CH 4 , N 2 , and CO gases and their mixtures 1,[7][8][9][10][11] . Neutron-scattering, spectroscopy measurements, and Monte-Carlo computer simulations [12][13][14] have provided important insights related to these results, by demonstrating that the gas molecules bind primarily in sites localized near the atoms in the imidazolate linkers in ZIF compounds 8,13,[15][16][17][18][19][20][21][22][23][24][25][26][27] . The magnitudes of the binding energies underlying the interactions between the gas species and ZIF linkers can be estimated based on the measured isosteric heat of adsorption, e.g.…”