Effects of substituent groups and central metal ion on hydrogen adsorption in zeolitic imidazolate frameworks

Grand canonical Monte Carlo (GCMC) simulations were employed to compute the adsorption of hydrogen gas in zeolitic imidazolate frameworks (ZIFs). This set of ZIFs has the same basic structure, but different functional groups or central metal ions were substituted into the zeolitic framework. The amount of hydrogen adsorption in ZIFs is mainly affected by the effective porosity, but the substituent group may also influence the hydrogen adsorption amount. Under both low pressure and high pressure conditions, the trend in hydrogen adsorption amount is: –NO2>–CN (–Cl)>–CH3, which suggests a role for the electronegativity of the substituted functional group. Generally speaking, the higher the electronegativity, the larger the adsorption amount. However, the –C6H4 group is a special functional group with high π-electron delocalization that can greatly increase the saturated adsorption amount by ZIFs. The effect of the benzene (–C6H4 group) fused to imidazole in the large pores of ZIFs is superior relative to that in the small pores. The central metal ion, Zn(II) or Co(II), plays a negligible role in the ZIFs saturation adsorption capacity. These results may be helpful in designing and synthesizing new ZIFs with higher hydrogen uptake capability.