Structural single and multiple molecular adsorption of CO2 and H2O in zeolitic imidazolate framework (ZIF) crystals

• DFT modelling of ZIF1, ZIF4 and ZIF6 structural and crystal-chemical properties.
• Adsorption energy calculated for CO2 and H2O determine adsorption capacities.
• Competitive adsorption of CO2 and H2O inside these ZIF's structures is shown.
• The textural porosity is analysed and compared with experiment.
• Theoretical CO2 and H2O adsorption isotherms have been calculated.

Theoretical methods are used for modelling the capture and storage of CO2 in Zinc-Imidazolate-Frameworks (ZIFs). Density functional theory (DFT) is employed to establish the structure of three ZIFs with small, medium and large cages to predict the uptake behaviour of adsorbed water and carbon dioxide. Water hydration strength and configuration inside the material are investigated with and without the presence of CO2. CO2 is predominantly located around the inner surface of the cages in the ZIFs type ZIFn (n = 1,4,6) through single and multipoint interactions. Water shows a stronger adsorption than carbon dioxide and can play an important role in solvating the CO2 molecules inside the porous. In the final structures, H2O and CO2 do not react with the surface to produce hydroxyl groups or carbonate-like species, respectively. Both species compete for the same adsorption sites. Their maximum adsorption occurs in systems with really accessible surfaces such us ZIF6.A detailed study of the surface-accessibility of a molecule on a “Connolly Surface” is reported, were ZIFs with extra-large pores plays a major role on the adsorption process. Simulated adsorption isotherms throughout MC simulations were obtained for a single CO2 and H2O in which results give a ranking for the most appropriate process conditions.

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