Constructing Amine-Functionalized Hierarchically Porous Porphyrin-Based Metal–Organic Frameworks for Highly Enhanced Direct Air Capture of CO₂

Direct air capture (DAC) of carbon dioxide (CO₂) has emerged as a prominent negative carbon emission technology for mitigating the greenhouse effect. Among various adsorbents, amine-functionalized metal-organic frameworks (MOFs) have shown exciting potential as adsorbents for atmospheric CO₂ capture. However, the intrinsic microporosity of many MOFs restricts amine loading and uniform dispersion, diminishing the CO₂ adsorption efficiency. In this study, we synthesized a series of hierarchically porous porphyrin-based MOF (HP-PMOF) featuring high-density defects and a combination of micro- and meso- and macropores. The HP-PMOF was subsequently functionalized with short-chain alkyl amines (diethylenetriamine (DETA) and tetraethylenepentamine (TEPA)) and polyamines (polyethylenimine (PEI)) to form HP-PMOF-Amine composites. Evaluations of these adsorbents demonstrated a significantly enhanced CO₂ capture performance in DAC applications. Static CO₂ adsorption isotherms revealed that HP-PMOF-DETA achieved the highest CO₂ uptake of 1.40 mmol g^-1, representing increases of 140 and 20 times over unmodified PMOF and HP-PMOF, respectively. Dynamic DAC performance measurements showed that HP-PMOF-DETA maintained 84% regeneration efficiency after ten cycles under 400 ppm of CO₂. Breakthrough tests demonstrated enhanced CO₂ adsorption capacity across varying relative humidity (0-80%) compared with dry conditions. Mechanistic insights from in situ DRIFTS studies and DFT calculations indicated that, under dry conditions, physisorption and chemisorption synergistically occur between CO₂ and amine groups of DETA, forming carbamate or carbamic acid species. Under humid conditions, water facilitated the adsorption of CO₂ by promoting the conversion of ammonium carbamate to bicarbonate. This work underscores the significant potential of amine-functionalized hierarchically porous MOFs for advancing the efficacy of DAC technologies.