Carbon dioxide adsorption by physisorption and chemisorption interactions in piperazine-grafted Ni2(dobdc) (dobdc=1,4-dioxido-2,5-benzenedicarboxylate)

The metal-organic framework Ni-2(dobdc) (CPO-27-Ni, where dobdc = 1,4-dioxido-2,5-benzenedicarboxylate) has been post-synthetically modified with piperazine (pip) - a known 'accelerator' to improve the kinetics of CO2 uptake in alkanolamine solvents for chemical absorption - and the impact of the modification on the CO2 uptake and selectivity over N-2 has been probed. While the modified framework, Ni-2(dobdc)(pip)(0.5) (pip-CPO-27-Ni), exhibits a lower uptake of CO2 compared with the non-grafted material, the selectivity for CO2 over N-2 at 25 degrees C and at pressures pertinent to post-combustion flue gas capture (0.1-0.15 bar) is enhanced. Mechanistically, the interaction between the CO2 molecules and the free amine sites in pip-CPO-27-Ni occurs via physisorption and chemisorption interactions, in which CO2 binds to the framework with an isosteric heat of adsorption (-Q(st)) of 40.5 kJ mol(-1) at very low coverage (P = 0.033 mbar), followed by binding at a higher heat of adsorption (-Q(st) = 46.2 kJ mol(-1) at P = 3.55 mbar). Pure water adsorption isotherms revealed a two-step mechanism for uptake in CPO-27-Ni, consistent with adsorption into the first and second hydration spheres of Ni2+ followed by subsequent uptake via physisorption into the pores. Additional steric hindrance in pip-CPO-27-Ni results in a single step only. The working capacity over multiple cycles was also investigated using a temperature swing adsorption process which revealed reversible CO2 adsorption and desorption of 10 wt% over 10 cycles.