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.