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

Abstract

The metal–organic framework Ni₂(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 N₂ has been probed. While the modified framework, Ni₂(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 N₂ at 25 °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.