Local order changes of alkali metal cations in microporous poly(heptazine imides) upon CO2 adsorption

The struggle of the modern society against the global warming demands the implementation of more efficient and less carbon-intensive technologies. Therefore, there is a quest to produce new materials that could efficiently capture carbon dioxide (CO2) from gas streams and be easily regenerated. One promising candidate is the poly(heptazine imide) (PHI), a microporous polymer in which the negatively charged polyheptazine network is balanced by the presence of alkali metal cations. Recently, preliminary adsorption studies revealed this material’s ability to adsorb CO2 from gas streams, presenting an adsorption capacity comparable to that of the benchmark adsorbent Zeolite 13X. Despite its outstanding performance in CO2 capture and total recoverability for numerous adsorption-desorption cycles, the adsorption mechanisms and the structural changes which the material undergoes during the CO2 uptake and release have not yet been revealed. Although it bears certain similarities with zeolites, unlike them PHI is poorly crystalline making the application of X-ray diffraction techniques for the elucidation of the adsorption mechanisms challenging. The preliminary in-situ X-ray diffraction studies have shown that the adsorption is most likely taking place in the structural cavity formed by six heptazines. However, this cavity is occupied by the structural alkali metal cations thus affecting the accessibility of the adsorbate into the pore. The ion-exchange of Na+, the cation originally incorporated during the synthesis for K+, Rb+ and Cs+ clearly affects the CO2 adsorption process, however its effect does not show any clear correlation with the cation size. Hence, the goal of the current proposal is to probe the local environment of K+ and Rb+ in the micropore of PHI adsorbent, under CO2 and N2 adsorption-desorption cycles, by XAS techniques. The information obtained from the suggested experiments will render us invaluable information about the structural features of PHIs helping us to design more efficient adsorbents.