Engineered assembly of water-dispersible nanocatalysts enables low-cost and green CO2 capture

The study addresses the urgent need to reduce the high energy consumption of conventional CO2 capture processes, particularly the energy-intensive solvent regeneration step in amine-based absorption-desorption systems. While catalytic solvent regeneration can lower desorption temperatures and enable use of lower-grade heat sources, existing heterogeneous catalysts suffer from low efficiency and operational challenges. Metal-organic frameworks (MOFs) offer tunable structures, abundant metal sites, and high porosity, but their predominantly microporous nature and limited exploration of acid-stable MOF-based superacid catalysts constrain performance. The authors propose engineering water-dispersible, acidic, core-shell nanocatalysts by modulating MOF self-assembly on carboxylate-functionalized Fe3O4 nanoclusters to create hierarchical micro-mesoporosity and defect sites for sulfation, thereby enhancing proton donation and enabling low-temperature, energy-efficient CO2 desorption.

Prior work highlights catalytic solvent regeneration as a promising route to reduce regeneration energy, enabling operation below 100 °C and potential use of low-grade heat (e.g., solar hot water). However, prevalent heterogeneous catalysts (e.g., metal oxides, zeolites) have shown limited efficiency and operational difficulties at scale. MOFs have emerged as catalytic platforms due to high surface area and tunability. Yaghi and co-workers demonstrated strong Brønsted acidity in sulfated Zr-MOF (MOF-808-SO4), where chelating sulfates on unsaturated metal clusters act as proton donors. Nonetheless, the microporosity of conventional MOFs limits mass transport and accessibility of active sites in catalysis, and several acid-stable MOFs remain underexplored as superacid catalysts for CO2 desorption. The study builds on these insights to develop defect-engineered, mesopore-rich MOF shells on magnetic cores for enhanced sulfation and acidity.

• Substrate synthesis: Carboxylated Fe3O4 nanoclusters (Fe3O4-COOH) were synthesized via a citrate-assisted solvothermal method using ferric ammonium citrate and sodium acetate in ethylene glycol (200 °C, 10 h). Citrate groups coordinate to Fe ions to aggregate 2–5 nm Fe3O4 nanoparticles into 100–300 nm spherical clusters, imparting surface acidity, hydrophilicity, and water dispersibility.
• Benchmark Fe3O4 nanoparticles were prepared by co-precipitation for comparison.
• Modulated MOF self-assembly: Seven MOF shells were grown on Fe3O4-COOH to form core–shell materials (Fe3O4@MOF): ZIF-8, ZIF-67, MIL-100(Fe), MOF-Fe(II), HKUST-1, UiO-66, UiO-66-NH2. Protocols involved dispersing Fe3O4-COOH, coordinating metal ions (Zn, Co, Fe2+, Fe3+, Cu2+, Zr4+) to surface carboxylates, then adding organic linkers (2-methylimidazole, H3BTC, H2BDC, H2BDC-NH2, H2BDC-N) under controlled temperatures and solvents (MeOH, EtOH, DMF, water) to form MOF shells. Washing, magnetic separation, and vacuum drying steps yielded core–shell powders.
• Sulfation: Core–shell Fe3O4@MOF materials were dispersed in 0.05 M aqueous H2SO4 (pH ~1.3) at room temperature for 24 h, then thoroughly washed with hot water and vacuum dried (150 °C, 48 h) to obtain Fe3O4@MOF-SO4 with chelating sulfates coordinated to defect metal sites.
• Characterization: Morphology by HIM/SEM/TEM/HAADF-EDX; Raman and FTIR; XPS (survey and high-resolution); nitrogen adsorption–desorption for pore structures; TGA for linker deficiency; zeta potential; elemental analysis. Mesoporosity and missing-linker defects were assessed by sorption and TGA (weight loss 100–350 °C for ΔW comparison). Stability under acidic sulfation was examined (e.g., HKUST-1 vs UiO-66 series).
• CO2 desorption testing: Catalytic solvent regeneration evaluated using 5 M monoethanolamine (MEA) solutions at 88 °C. Catalysts tested at low loadings (0.01–0.1 wt%). Metrics included CO2 desorption profiles, relative heat duty (RH, normalized energy consumption), and cyclic CO2 capacity over multiple absorption–desorption cycles. Comparators included Fe3O4-COOH, Fe3O4@MOFs, Fe3O4@MOF-SO4, and commercial heterogeneous catalysts (Al2O3, V2O5, H-Beta, HZSM-5, SO4 2−/ZrO2/Al2O3, SO4 2−/ZrO2/SBA-15).
• Mechanistic analysis: Proposed Brønsted-acid-facilitated proton transfer pathways for carbamate breakdown. DFT calculations (Gaussian 16, M06-L functional; 6-31G(d,p) for main-group atoms and Stuttgart-Dresden pseudopotential for Zr) modeled a UiO-66 defect site with coordinated HSO4− and H2O, showing protonation of carbamate nitrogen and N–C bond cleavage coupled with bicarbonate formation; water was essential for the reaction pathway.

• Water-dispersible acidic substrate: Fe3O4-COOH nanoclusters exhibit strong hydrophilicity and aqueous dispersibility from surface carboxylates; XPS indicates surface enrichment of C and O (C ~29.4%, O ~58.9%; Fe ~9.1% at surface) vs Fe-dominant bulk (~87% Fe), confirming surface functionalization.
• Tailored core–shell structures: Successful growth of diverse MOF shells on Fe3O4-COOH with altered zeta potential (from −43.1 mV to −1.7 to −19.5 mV), confirming surface coverage by MOFs. Core–shell materials display hierarchical micro–mesoporosity with increased total pore volumes (~0.02–0.20 cm3/g); Fe3O4@UiO-66 achieved ~0.20 cm3/g. Mesoporosity fraction for modulated UiO-66 shell ~85.4% vs pristine UiO-66 ~11.1%.
• Defect engineering: TGA indicates substantial missing-linker deficiencies; Cu- and Zr-based MOFs showed high linker deficiency (~56.2–91.5%), higher coating weights (~30.1–34.7 wt%), and homogeneous coatings.
• Sulfation and stability: FTIR confirmed sulfate coordination (S–O ~800–950 cm−1; S=O ~1000–1300 cm−1). Sulfur distributed across shells post-sulfation. Acid stability dictated sulfation success: Fe3O4@HKUST-1 partially disassembled at pH ~1.3 (weak S signals), while Fe3O4@UiO-66-SO4 and Fe3O4@UiO-66-NH2-SO4 retained structure and effectively coordinated sulfate due to robust Zr–O bonds.
• CO2 desorption performance:
  • Fe3O4-COOH at 0.1 wt% reduced energy consumption by ~27.3% vs blank MEA at 88 °C; increasing loading to 1 wt% improved desorption rate but reduced energy efficiency per catalyst mass (2.73 to 0.66).
  • Unsulfated core–shells improved kinetics; Fe3O4@HKUST-1 performed best among them but still showed RH ~74.1%, worse than Fe3O4-COOH (RH ~72.7%), attributed to fewer Brønsted acid sites.
  • Sulfated core–shells exhibited superior performance: Fe3O4@UiO-66-SO4 delivered RH 55.3% (i.e., 44.7% reduction in energy consumption) and desorbed 80.9% more CO2 than blank under the same conditions. Desorption rate advantages diminished over time (24–36 min), evidencing kinetic enhancement.
  • At very low loadings (0.01–0.1 wt%), Fe3O4-COOH, Fe3O4@UiO-66, and Fe3O4@UiO-66-SO4 maintained high cyclic capacities; Fe3O4@UiO-66-SO4 increased cyclic capacity from 0.21 (blank) to 0.30, 0.33, and 0.38 mol CO2/mol MEA at 0.01, 0.05, and 0.1 wt%, respectively, outperforming conventional catalysts typically used at ~1.0–1.1 wt%.
  • Compared to commercial heterogeneous catalysts (Al2O3, V2O5, H-Beta, HZSM-5, sulfate–zirconia composites), Fe3O4@UiO-66-SO4 achieved the lowest RH at only 0.1 wt%, indicating ~10-fold higher efficiency at much lower dosage.
• Recyclability: Fe3O4@UiO-66-SO4 showed excellent stability over 10 cycles; RH increased by ~9% in initial four cycles then stabilized, with no significant changes in XPS, XRD, or SEM.
• Mechanism: Superacid sites formed by chelating sulfate and coordinated water on defect Zr nodes (Ho −14.5) facilitate proton transfer to carbamate, accelerating N–C bond cleavage and forming MEA and HCO3−; DFT indicates water is essential for the catalytic pathway.

The engineered Fe3O4@MOF-SO4 nanocatalysts directly address the challenge of high energy demand in CO2 solvent regeneration by enabling efficient, low-temperature desorption. The carboxylate-rich Fe3O4 core mediates MOF self-assembly to induce missing-linker defects and mesoporosity, enhancing accessibility and providing unsaturated metal sites for effective sulfation. The resulting superacid Brønsted sites drastically improve proton donation, accelerating carbamate breakdown at temperatures below the boiling point of water. Performance comparisons demonstrate that sulfated Zr-MOF shells (UiO-66-SO4) outperform both the uncoated acidic core and conventional heterogeneous catalysts at an order-of-magnitude lower dosage, yielding a 44.7% reduction in energy consumption and substantial gains in cyclic capacity. These results validate the hypothesis that defect-engineered, mesoporous MOF shells with chelating sulfates, in a water-dispersible nanoform, can overcome mass transfer and active site accessibility limitations inherent in microporous MOFs and traditional solids, and can leverage nanofluidic behavior for enhanced catalytic regeneration.

The study introduces a versatile strategy to fabricate water-dispersible, defect-engineered MOF-based core–shell nanocatalysts on carboxylated Fe3O4 nanoclusters. The approach converts microporous pristine MOFs into hierarchical micro–mesoporous shells with abundant unsaturated metal sites amenable to sulfation, creating superacid Brønsted sites. Among the materials, Fe3O4@UiO-66-SO4 showed the best catalytic performance, reducing solvent regeneration energy by 44.7% at only 0.1 wt% loading, with excellent recyclability. The findings highlight the potential for industrially practical, low-dosage catalysts to enable catalytic solvent regeneration using lower-grade heat sources (e.g., solar hot water), advancing greener CO2 capture. Future work could extend this platform to other acid-stable MOFs, further optimize defect densities and sulfation, and scale the synthesis and process integration for industrial applications.

• Sulfation success and stability are MOF-dependent: Cu-based HKUST-1 shells partially disassembled under acidic sulfation (pH ~1.3), limiting sulfate incorporation and performance; Zr-based UiO-66 shells were stable and effective. Thus, chemical stability in acid constrains material choice.
• Performance was demonstrated primarily with MEA (5 M) and at specific operating conditions (e.g., 88 °C); broader solvent systems and process conditions were not detailed.
• While recyclability over 10 cycles was strong, longer-term durability and scale-up considerations were not reported in detail.