(612f) Thermodynamics of CO2 Adsorption into Porphyrin-Based Covalent Organic Frameworks

Anthropogenic CO₂ emission is known as a major contributor to global warming, posing a serious environmental threat. ^[1] In response to this thread, various porous adsorbents, including zeolite, activated carbon, metal-organic frameworks (MOFs), and covalent-organic frameworks (COFs), have emerged as promising candidates for CO₂ capture. Nevertheless, limitations such as ineffectiveness in humidity, small pore volume, high regeneration cost, and undesired CO₂ selectivity have curbed the potential application of porous materials. Among different adsorbents, MOFs, and COFs have been shown to be less impacted by these limitations and exhibit higher active surface area, chemical stability, and tunable properties with stronger CO₂ molecule affinity under humid conditions. ^[2]. Here, we examined the CO₂ sorption in the thin film of porphyrin-based covalent organic frameworks (MPOR-COFs), where M = H₂, Cu²⁺, and Zn²⁺, synthesized via vapor-phase oxidative polymerization. The CO₂ solubility was measured using the gravimetric method, and the isosteric heat of adsorption was estimated using point isotherms at four different temperatures (10, 20, 30, 40 °C) and six different pressures (50, 100, 125, 150, 175, and 200 psi). Incorporating porphyrin units in the COFs brings unprecedented functionalities that affect carbon selectivity. The present study found that POR-COF demonstrated improved CO₂ solubility compared with MPOR-COF (ZnPOR-COF and CuPOR-COF). POR-COF showed enhanced CO₂ solubility, averaging 0.88 ± 0.18 moles of CO₂/kg of adsorbent. However, ZnPOR-COF and CuPOR-COF exhibited reduced CO₂ update in comparison with POR-COF, averaging 0.40 ± 0.13 and 0.47 ± 0.21 moles of CO₂/kg of adsorbent, respectively, which we attributed to the reduced active sites within their 2D lattice structure. The estimated -∆H for POR-COF was 8.79 ± 1.02 kJ/mol of CO₂, which is within the physisorption range, requiring less energy for regeneration. Whereas, for CuPOR-COF and ZnPOR-COF, the heat of adsorption is found to be -14.66 kJ/mol and -12.14 kJ/mol, respectively, potentially owing to the altered adsorption pathways of CO₂ atoms in MPOR-COF. The sorption isotherms were developed based on Langmuir and Freundlich adsorption models. The above findings highlight the significance of porphyrin-based COFs in CO₂ sorption, facilitating the design of novel carbon capture materials.

[1] J. A. Rudd, “An Industrial Take on Developing and Deploying Carbon Capture at Scale." Nat. Rev. Chem., pp. 8 (1), 1–2., 2024.

[2] R. Suresh and S. Vijayakumar, “Adsorption of Greenhouse Gases on the Surface of Covalent Organic Framework of Porphyrin–An Ab Initio Study.," Phys. E Low-Dimens. Syst. Nanostructures, pp. 126, 114448, 2021.