CALF-20 for Post-Combustion Carbon Capture
CALF-20 — formally Calgary Framework 20, molecular formula [Zn2(1,2,4-triazolate)2(oxalate)] — is a zinc-based metal-organic framework (MOF) engineered for post-combustion CO2 capture from industrial flue gas streams. First reported in Science in December 2021 by researchers at the University of Calgary, the University of Alberta, and the University of Ottawa, CALF-20 addresses a core challenge in solid sorbent carbon capture: capturing CO2 selectively and durably in the presence of water vapor, acid gases, and the full complexity of real industrial exhaust.
CALF-20 physisorbs CO2 with high capacity (4.07 mmol per gram at 1.2 bar and 293 K; 2.6 mmol per gram at 0.15 bar under flue gas conditions), a CO2/N2 selectivity of 230 by ideal adsorbed solution theory, and a low heat of adsorption of -39 kJ per mol — meaning significantly less energy is required to regenerate it compared to amine-based liquid sorbents. Its single-step synthesis from commercially available precursors, making large-scale production practical. CALF-20 is being sold and licensed by Existent Sorbents Inc. Existent supplies in 1 g, 10 g, 100 g, and 1 kg amounts, or in custom volumes for buyers.
What Post-Combustion Carbon Capture Targets
Post-combustion carbon capture removes CO2 from exhaust gas after fuel combustion in air. Operators can retrofit these systems onto existing facilities rather than redesigning combustion equipment, which significantly lowers the capital barrier to deployment. Typical industrial sources include natural gas combined-cycle turbines, natural gas and coal-fired boilers, combined heat and power (CHP) systems, diesel engines and generators, cement kilns, lime kilns, refinery fired heaters and furnaces, petrochemical process furnaces, glass furnaces, and waste-to-energy combustion units.
These facilities represent the most concentrated stationary point sources of CO2 emissions globally. Post-combustion capture retrofitted to existing infrastructure provides a near-term, high-impact pathway for carbon capture and storage (CCS) deployment without requiring the replacement of long-lived capital equipment.
Why CALF-20 Fits Post-Combustion Flue Gas
Flue gas from combustion sources is inherently humid. Water vapor is the most common contaminant that degrades the performance of conventional solid sorbents, particularly zeolites and activated carbons. CALF-20 was designed around this challenge. Its hydrophobic ultramicroporous pore geometry — with a pore limiting diameter of approximately 3 angstroms and a largest cavity diameter of approximately 4.3 angstroms — suppresses water hydrogen-bonding networks within the framework. As a result, CO2 selectively physisorbs preferentially over water up to approximately 40 to 47 percent relative humidity, and the presence of CO2 actually suppresses water adsorption rather than competing with it — a behavior confirmed computationally and experimentally across multiple independent research groups.
Published research and industrial validation document CALF-20’s performance under the full range of demanding flue gas conditions:
Multi-year cycling stability: Multiple users and research have confirmed 1,000,000’s (millions) of adsorption-desorption cycles under high temperature vacuum and steam conditions with no loss of CO2 capacity.
Acid gas tolerance: Demonstrated stability against wet acid gases (SOx, NOx) and prolonged direct natural gas flue gas exposure.
Humid VSA performance: Vacuum swing adsorption (VSA) experiments at 13 to 45 percent relative humidity demonstrate CO2 purity of approximately 95 percent and recovery of approximately 71 to 81 percent — essentially matching dry-gas performance, a critical result for practical deployment without upstream drying.
Temperature swing adsorption (TSA): CALF-20 achieves complete regeneration over multiple TSA cycles, with CO2 uptake up to 2.55 mmol per gram in lab-scale fixed-bed reactors.
These attributes collectively make CALF-20 a optimal candidate sorbent for natural gas, coal, cement, diesel, and other industrial post-combustion applications.
Adsorption Process Compatibility: VSA, TSA, and VPSA
CALF-20 has been validated across multiple adsorption regeneration process configurations, each suited to different operating contexts.
Vacuum Swing Adsorption (VSA): Process modeling and experimental validation using 70 grams of structured CALF-20 have demonstrated 95.5 percent CO2 purity at 88.1 percent recovery — meeting US Department of Energy post-combustion capture targets. A four-step VSA cycle with light-product pressurization achieves these targets under both dry and humid flue gas conditions.
Temperature Swing Adsorption (TSA): CALF-20’s low enthalpic regeneration penalty enables efficient steam-based and direct-contact TSA regeneration. Mechanochemically synthesized CALF-20 has shown stable performance across 10 or more TSA cycles in fixed-bed reactors, with physisorption kinetics that fit a pseudo-first-order model.
Vacuum Pressure Swing Adsorption (VPSA) and biogas upgrading: Multiscale techno-economic analysis of the CALF-20 isoreticular series for CO2/CH4 separation in biogas upgrading confirms that CALF-20 shows the most favorable economics in the series, achieving greater than 97 percent purity methane at competitive operating costs.
Industrially Relevant Form Factors
CALF-20 can be integrated into multiple shaped form factors that align with standard gas separation hardware and reduce pressure drop in packed adsorption beds. The original Science publication demonstrates CO2 isotherm retention across a 3,000,000-fold difference in scale — from milligram laboratory samples to multikilogram structured composites.
Pellets: Binder optimization studies confirm that cellulose acetate binder achieves superior mechanical strength while retaining high CO2 uptake, surpassing polysulfone, PVC, and polyvinyl formal binders.
Extrudates: A rapid synthesis route produces kilogram-scale CALF-20 extrudates in over 95 percent less preparation time than solvothermal methods, with a static CO2 capacity of 2.15 mmol per gram under simulated flue gas conditions (313 K, 0.15 bar) and stable working capacity of 1.51 mmol per gram over 50 VPSA cycles — at approximately one-third the material cost of batch solvothermal production. Existent sorbents is supply 100-200 kg amounts of CALF-20 extradites and will be scaling up to >1 metric ton scale.
Granules, beads, and monoliths: Standard porous solid form factors suitable for packed-bed column integration.
Polymer-composite structured sorbents: 20 percent polysulfone composites demonstrated in the original Science publication with retained porosity and CO2 isotherms.
Hollow fiber sorbent architectures: Polymer-integrated MOF fiber formats show pathways toward advanced structured sorbent contactors with low pressure drop and enhanced heat and mass transfer.
Manufacturing scalability is a core advantage. CALF-20 is synthesized in one step from commercially available, low-cost precursors and can be structured into beads, pellets, granules, extrudates, hollow fibre membranes, monoliths and more by Existent Sorbents Inc.
Where CALF-20 Creates Value
Power generation operators can apply CALF-20 to natural gas combined-cycle units, industrial boilers, combined heat and power systems, and other post-combustion sources. Humid flue gas from natural gas combustion is precisely the operating environment CALF-20 was designed for — no upstream drying required at operating humidity below approximately 47 percent relative humidity.
Cement and lime producers face a hard-to-abate emissions challenge because CO2 is released both from fuel combustion and from the calcination of limestone. Independent research and pilot data confirm CALF-20’s stability in cement kiln flue gas, making it a viable solid sorbent for decarbonizing this sector.
Refinery and petrochemical operators can evaluate CALF-20 across fired heaters and process furnaces that generate dilute CO2 post-combustion streams. CALF-20’s tolerance for acid gases and steam aligns with refinery operating conditions.
Biogas upgrading facilities can use CALF-20 in VPSA processes for CO2/CH4 separation, with techno-economic analysis confirming competitive economics among the broader CALF-20 MOF family.
Across all these sectors, CALF-20 offers a practical, retrofit-compatible pathway to post-combustion carbon capture. It is a proven alternative to molecular sieve (zeolite) sorbents — which CALF-20 outperforms in published head-to-head comparative studies against Zeolite 13X and Mg-MOF-74 — and to liquid amine absorption systems, which carry substantially higher regeneration energy penalties, corrosion risks, solvent degradation costs, and plant footprint requirements.
A Note on Intellectual Property
CALF-20 was invented at the University of Calgary by Prof. George K. H. Shimizu and collaborators. The material is protected by patents CA2904546A1, US9782745B2, CN105051269B, BR112015021875B1, ES2856694T3, EP3784824A1, AU2014231699B2, JP6586366B2, KR102057165B1 which have been licensed for specific fields of use to limited parties. Licensing a patent for a particular application does not confer ownership of the material, its synthesis, or its broader applications. The Calgary Framework 20 designation and the underlying science belong to the research team and institution that developed it, with independent research from institutions across Canada, the United States, Europe, and Asia confirming, extending, and building upon the published record. The vast majority and balance of all the rights to CALF-20 were licensed to Existent Sorbents Inc. in early 2026 which was co-founded by George K. H. Shimizu, the inventor of CALF-20.
Research Foundation: Published Literature on CALF-20
The following publications represent the peer-reviewed research base for CALF-20. This list spans the foundational paper, process engineering studies, computational investigations, materials characterization, synthesis advances, form factor development, and application studies published since 2021.
Foundational and Review
Lin, J.-B. et al. A scalable metal-organic framework as a durable physisorbent for carbon dioxide capture. Science 374(6574), 1464-1469 (2021).
https://doi.org/10.1126/science.abi7281
Drweska, J., Roztocki, K. and Janiak, A. M. Advances in chemistry of CALF-20, a metal-organic framework for industrial gas applications. Chemical Communications 61(6), 1032-1047 (2025).
https://doi.org/10.1039/D4CC05744A
Process Engineering: VSA, TSA, and VPSA
Nguyen, T. T. T., Lin, J.-B., Shimizu, G. K. H. and Rajendran, A. Separation of CO2 and N2 on a hydrophobic metal organic framework CALF-20. Chemical Engineering Journal 442, 136263 (2022).
https://doi.org/10.1016/j.cej.2022.136263
Nguyen, T. T. T., Shimizu, G. K. H. and Rajendran, A. CO2/N2 separation by vacuum swing adsorption using a metal-organic framework, CALF-20: Multi-objective optimization and experimental validation. Chemical Engineering Journal 452, 139550 (2023).
https://doi.org/10.1016/j.cej.2022.139550
Nguyen, T. T. T. et al. Competitive CO2/H2O adsorption on CALF-20. Industrial and Engineering Chemistry Research 63(7), 3265-3281 (2024).
https://doi.org/10.1021/acs.iecr.3c04266
Nguyen, T. T. T. and Rajendran, A. Experimental demonstration of humid post-combustion CO2 capture by vacuum swing adsorption using CALF-20. Adsorption (2025).
https://doi.org/10.1007/s10450-025-00600-z
Peh, S. B., Zhao, D., Jiang, J., Farooq, S. and Zhao, D. Direct contact TSA cycle based on a hydrophobic MOF sorbent for post-combustion CO2 capture from wet flue gas. Chemical Engineering Science 301, 120744 (2025).
https://doi.org/10.1016/j.ces.2024.120744
Hastings, J. et al. Steam isotherms, CO2/H2O mixed-gas isotherms, and single-component CO2 and H2O diffusion rates in CALF-20. Industrial and Engineering Chemistry Research 63(26), 11544-11551 (2024).
https://doi.org/10.1021/acs.iecr.4c00373
Raganati, F. et al. CALF-20 obtained by mechanochemical synthesis for temperature swing adsorption CO2 capture: A thermodynamic and kinetic study. Chemical Engineering Journal 506, 159966 (2025).
https://doi.org/10.1016/j.cej.2025.159966
Multiscale, techno-economic evaluation of isoreticular series of CALF-20 for biogas upgrading using pressure/vacuum swing adsorption (PVSA). Molecular Systems Design and Engineering (2025).
https://doi.org/10.1039/d5me00131e
Mixture equilibrium and kinetics of flue gas components in CALF-20 (binary and ternary CO2/N2/H2O data, 25-150 degrees C). Chemical Engineering Science (2025).
https://doi.org/10.1016/j.ces.2025.004865
Adsorption equilibrium and transport of CO2, N2, and H2O in CALF-20. Industrial and Engineering Chemistry Research (2025).
https://pubmed.ncbi.nlm.nih.gov/40279447/
Computational Studies: Molecular Simulation and Modelling
Khodadadi, M. S. and Ghaemi, A. Predicting flue gas component adsorption in CALF-20: A molecular dynamics study anchored by experimental CO2 capture. Chemical Engineering Journal Advances 24, 100931 (2025).
https://doi.org/10.1016/j.ceja.2025.100931
Ho, C.-H. and Paesani, F. Elucidating the competitive adsorption of H2O and CO2 in CALF-20: New insights for enhanced carbon capture metal-organic frameworks. ACS Applied Materials and Interfaces 15(41), 48287-48295 (2023).
https://doi.org/10.1021/acsami.3c11092
Oktavian, R. et al. Gas adsorption and framework flexibility of CALF-20 explored via experiments and molecular simulations. Nature Communications 15, 3898 (2024).
https://doi.org/10.1038/s41467-024-48136-0
Gopalsamy, K., Fan, D., Naskar, S., Magnin, Y. and Maurin, G. Engineering of an isoreticular series of CALF-20 metal-organic frameworks for CO2 capture. ACS Applied Engineering Materials 2(1), 96-103 (2024).
https://doi.org/10.1021/acsaenm.3c00622
Grand canonical Monte Carlo and molecular dynamics study: Exploring the potential of CALF-20 for selective gas adsorption at low pressure. Molecules (2023).
https://pmc.ncbi.nlm.nih.gov/articles/PMC9921038/
Krishna, R. and van Baten, J. M. Elucidating the failure of the Ideal Adsorbed Solution Theory for CO2/H2O mixture adsorption in CALF-20. Separation and Purification Technology 352, 128269 (2025).
https://doi.org/10.1016/j.seppur.2024.128269
Rajendran, A., Shimizu, G. K. H. and Woo, T. K. The challenge of water competition in physical adsorption of CO2 by porous solids for carbon capture applications: A short perspective. Advanced Materials (2023).
https://doi.org/10.1002/adma.202301730
Materials Characterization and Structure
Chen, Z. et al. Humidity-responsive polymorphism in CALF-20: A resilient MOF physisorbent for CO2 capture. ACS Materials Letters 5(11), 2942-2947 (2023).
https://doi.org/10.1021/acsmaterialslett.3c00930
CO2 and H2O sorption induced bulk-phase changes of CALF-20 captured using in situ laboratory X-ray powder diffraction. Journal of the American Chemical Society (2025).
https://doi.org/10.1021/jacs.5c06866
Drweska, J., Formalik, F., Roztocki, K., Snurr, R. Q., Barbour, L. J. and Janiak, A. M. Unveiling temperature-induced structural phase transformations and CO2 binding sites in CALF-20. Inorganic Chemistry 63(41), 19277-19286 (2024).
https://doi.org/10.1021/acs.inorgchem.4c02952
Additional Applications
Wei, Y. et al. Efficient Xe selective separation from Xe/Kr/N2 mixtures over a microporous CALF-20 framework. RSC Advances 12(28), 18224-18231 (2022).
https://doi.org/10.1039/D2RA02768B
Mandal, S. and Maiti, P. K. Prediction of thermal conductivity in CALF-20 with first-principles accuracy via machine learning interatomic potentials. Communications Materials 6(1) (2025).
https://doi.org/10.1038/s43246-025-00745-y
Background and Context
IPCC Special Report on Carbon Dioxide Capture and Storage, Chapter 3: Capture of CO2. Intergovernmental Panel on Climate Change (2005).
https://www.ipcc.ch/report/srccs/
Ozin, G., Ye, J. and Bauchman, E. CALF-20: A carbon capture success story. Advanced Science News (January 2022).
https://www.advancedsciencenews.com/calf-20-a-carbon-capture-success-story/
Additional Note
CALF-20 and Calgary Framework 20 are designations for the material [Zn2(1,2,4-triazolate)2(oxalate)] first reported in Science (2021) by Lin, Nguyen, Vaidhyanathan and George K. H. Shimizu et al. at the University of Calgary.