Magnetic-Induced Swing Adsorption Using Fe₃O₄/SBA-15-PEI for Rapid and Energy-Efficient Direct Air Capture

Direct air capture (DAC) represents a critical negative emission technology for mitigating rising atmospheric CO₂ levels. However, conventional DAC systems relying on temperature swing adsorption (TSA) often suffer from slow heating/cooling rates and high energy consumption. In this work, we developed Fe₃O₄/SBA-15-PEI, synthesized via co-precipitation and impregnation, and applied for the first time in magnetic-induced swing adsorption (MISA) under DAC conditions. The integration of Fe₃O₄ nanoparticles into the SBA-15 mesostructure endowed the adsorbent with magnetic responsiveness while improving its CO₂ uptake from 1.21 to 1.53 mmol/g (30 °C, 500 ppm CO₂/Ar). The CO₂ adsorption capacity exhibited a strong dependence on both humidity and humidification strategy, reaching 2.05 mmol/g at RH = 30% when dry CO₂ was introduced after prehumidification, and 3.2 mmol/g at RH = 90% when humid CO₂ was directly introduced. Under the MISA process, Fe₃O₄/SBA-15-PEI achieved rapid heating/cooling rates of 12.5/22.7 °C/min under dry conditions and 5.0/11.6 °C/min under humid conditions, which were 328%/483% (dry) and 138%/218% (humid) faster than those under TSA, respectively. Fe₃O₄/SBA-15-PEI also demonstrated enhanced CO₂ desorption performance, achieving desorption capacities of 1.25 mmol/g under dry and 3.24 mmol/g under humid conditions, with corresponding efficiencies of 92.6 and 99.9%, respectively. Additionally, diffusion modeling revealed a favorable time constant (D/r²) of 1.84 × 10^-3 min^-1. The adsorbent also maintained a CO₂ uptake of ∼1.46 mmol/g over 10 cycles of humid DAC tests, indicating good kinetic and stability. Overall, these findings demonstrate the potential of MISA-regulated adsorbents for energy-efficient, rapid-cycle DAC systems and offer insights into the design of smart, responsive adsorbents.