Improved NH₃ Uptake of a Macromolecule-Metal Complex Constructed with Dual Polymeric Ligands and M(II)

MOFs are considered as efficient NH₃ adsorbents for their high capacity but are accompanied by the collapse of MOFs. In this work, macromolecule-metal complexes (MMCs), which could provide metal sites like MOFs, were developed for reversible NH₃ uptake with high capacity with the assistance of the polymeric ligands. Based on the tunable structure of MMCs, the role of the polymeric ligands and metallic center was investigated. Thereinto, MMCs-3 with dual polymeric ligands presented higher NH₃ adsorption capacity and reversibility of adsorbents compared with MMCs containing a single polymeric ligand (MMCs-1 and MMCs-2). Combined with the NH₃ adsorption test, characterization of FT-IR, UV-vis, EPR spectroscopy, NH₃-TPD measurement, and the DFT calculations, it was found that the neutral polymeric ligands PVIm contributed to improve the stability of MMCs-3 under a NH₃ atmosphere for the tough networks of PVIm-M(II), while the polymeric ligands with a carboxylate anion together with M(II) enhanced the NH₃ capacity for the feasible coordination of a carboxylate anion with M(II). The mechanism of NH₃ uptake by PVIm-Co-PVBA was proposed that the NH₃ was fixed through the coordination with Co(II) along with the departure of PVBA and the following hydrogen bonding interaction with PVBA, while the coordination between PVIm and Co(II) was not destroyed. Thus, MMCs-3 with dual polymeric ligands presented a higher NH₃ uptake capacity and stability. Optimally, PVIm-M-PVBA with the metal center of Co(II), Cu(II), and Ni(II) were obtained with a high capacity of 20.8-23.7 NH₃ mmol/g at 25 °C and 1 bar and a high selectivity of NH₃ over CO₂ (54.9-99.9) and N₂ (73.0-187.6) through the breakthrough measurement with a gas mixture of 0.2% NH₃, 2% CO₂, and 99.6% N₂ at 25 °C.