The a-axis view of the 2 × 2 × 2 system cell of the metal-organic material (MOM), dia-7i-1-Co, showing multiple representations of methane (CH4) sorbed in the small pores of the framework. The system is shown using licorice representation with the exception of the central region of the framework and all CH4 molecules are depicted using space filling van der Waals illustration. Atom coloring corresponds to: C = gray, H = silver, O = red, N = blue, Co = pink; with the exception of the CH4 molecules sorbed in the central region which are colored green for visual clarity.
Langmuir. 2014 Jun 10;30(22):6454-62.
Pham T, Forrest KA, Tudor B, Elsaidi SK, Mohamed MH, McLaughlin K, Cioce CR, Zaworotko MJ, Space B.
Department of Chemistry, University of South Florida , 4202 East Fowler Avenue CHE205, Tampa, Florida 33620-5250, United States.
Grand canonical Monte Carlo (GCMC) simulations of CO2 and CH4 sorption and separation were performed in dia-7i-1-Co, a metal-organic material (MOM) consisting of a 7-fold interpenetrated net of Co(2+) ions coordinated to 4-(2-(4-pyridyl)ethenyl)benzoate linkers. This MOM shows high affinity toward CH4 at low loading due to the presence of narrow, close fitting, one-dimensional hydrophobic channels-this makes the MOM relevant for applications in low-pressure methane storage. The calculated CO2 and CH4 sorption isotherms and isosteric heat of adsorption, Qst, values in dia-7i-1-Co are in good agreement with the corresponding experimental results for all state points considered. The experimental initial Qst value for CH4 in dia-7i-1-Co is currently the highest of reported MOM materials, and this was further validated by the simulations performed herein. The simulations predict relatively constant Qst values for CO2 and CH4 sorption across all loadings in dia-7i-1-Co, consistent with the one type of binding site identified for the respective sorbate molecules in this MOM. Examination of the three-dimensional histogram showing the sites of CO2 and CH4 sorption in dia-7i-1-Co confirmed this finding. Inspection of the modeled structure revealed that the sorbate molecules form a strong interaction with the organic linkers within the constricted hydrophobic channels. Ideal adsorbed solution theory (IAST) calculations and GCMC binary mixture simulations predict that the selectivity of CO2 over CH4 in dia-7i-1-Co is quite low, which is a direct consequence of the MOM’s high affinity toward both CO2 and CH4 as well as the nonspecific mechanism shown here. This study provides theoretical insights into the effects of pore size on CO2 and CH4 sorption in porous MOMs and its effect upon selectivity, including postulating design strategies to distinguish between sorbates of similar size and hydrophobicity.