Improving isosteric heat of CO2 adsorption by introducing nitro moieties into jungle-gym-type porous coordination polymers

Abstract

Four types of novel jungle-gym-type porous coordination polymers, [Zn2(bdc)2(dabco)]n (Zn, bdc = 1,4-benzenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane), [Zn2(bdc-NO2)2(dabco)]n (Zn-NO2, bdc-NO2 = nitro-terephthalate), [Cu2(bdc)2(dabco)]n (Cu) and [Cu2(bdc-NO2)2(dabco)]n (Cu-NO2), were synthesised and characterised using gas adsorption measurements to estimate the affinity between CO2 and nitro moieties. N2 adsorption measurements at 77 K for four compounds show type-I isotherms, indicating the permanent porosities, where the BET surface areas are 1417 m2 g−1 (Zn), 1076 m2 g−1 (Zn-NO2), 1688 m2 g−1 (Cu), and 759 m2 g−1 (Cu-NO2). The adsorption capacity of CO2 at 195 K obeys the amount of surface area, whereas the isosteric heat of CO2 adsorption in both Zn-NO2 and Cu-NO2 are higher than the original compounds. The enthalpies at zero coverage of CO2 adsorption are 19 kJ mol−1 (Zn), 27 kJ mol−1 (Zn-NO2), 17 kJ mol−1 (Cu), and 27 kJ mol−1 (Cu-NO2), which may be attributed to the quadruple interaction between CO2 and nitro moieties.