Nanospace Engineering of Triazine−Thiophene-Intertwined Porous-Organic-Polymers <i>via</i> Molecular Expansion in Tweaking CO₂ Capture

Abstract

Post-combustion CO2 capture, storage, and separation have garnered colossal research interest in the energy industry, although realistic implementation of the available porous adsorbents is restricted owing to their cost competitiveness, stability, and scalability issues. The integration of heteroatom functionalities (N, O, or S) at the molecular level into the organic skeleton of porous framework materials endowed them with superior CO2 adsorbents to mitigate greenhouse gases. In this work, we have successfully introduced triazine–thiophene (Tt) groups to the nanoporous organic polymer (POP) skeleton by Friedel–Craft alkylation of Tt (as a monomer) with a series of cross-linking agents including formaldehyde dimethyl acetal, 1,4-bis(bromomethyl)benzene (BMB), and 4,4′-bis(bromomethyl)biphenyl, which contained methylene, bis-methylene benzene, and bis-methylene biphenyl moieties in each linker unit, respectively. The precise skeleton engineering with the variation of organic cross-linking agents at the molecular level leads to the development of Tt-POP-1, Tt-POP-2, and Tt-POP-3, having nanorod-, nanocoral-, and nanocloud-like morphologies, respectively. In particular, at 273 K, Tt-POP- 1, Tt-POP- 2, and Tt-POP-3 exhibited CO2 uptake capacities of about 33.04, 40.06, and 34.12 cm3/g, respectively, up to 1 bar pressure. Interestingly, Tt-POP-2 bearing a BMB linker exhibited enhanced CO2 uptake capacity both at 298 and 273 K in comparison with the other Tt-POP-1 and Tt-POP-3, respectively. An in-depth study of the CO2 adsorption mechanism by density functional theory calculations showed that the benzyl rings of linker units in Tt-POP-2 and Tt-POP-3 play a pivotal role in CO2 uptake. The more polarized interaction of CO2 with the thiophenyl and benzyl rings compared to the N and S atoms in Tt-POP-2 results in enhanced CO2 uptake capacity with respect to the others.