Low-Temperature CO ₂ Adsorption Performance and Diffusion Kinetics of CALF-20 Powder and Its Extrudate via the Zero-Length Column Technique

Calgary Framework-20 (CALF-20) shows exceptional CO2 capture potential through its selective physisorption and stability in humid flue gas. However, its CO2 diffusion mechanism remains poorly understood, hindering optimal industrial deployment. We synthesized CALF-20 powder and extrudates via solvothermal and phase inversion methods, characterizing their CO2 adsorption-diffusion behavior using the zero-length column (ZLC) technique. At 0.1 bar and 311 K, CALF-20 exhibited a CO2 uptake of 2.0 mmol g–1 and a reciprocal of the diffusion time constant, D/R2, of 4.36 × 10–2 s–1. Particle size experiments revealed macropore diffusion as the primary mass transfer resistance and identified a critical transition radius of 0.48 mm that governs the CO2 diffusion mechanism in the extrudates. The theoretical model further elucidated a shift in macropore diffusion from combined Knudsen/molecular to molecular diffusion control upon switching the carrier gas from He to N2. This study provides crucial kinetic insights for the industrial application of CALF-20.