Dual-framework ZIF-7/ZIF-67 composites beat surface area rules for CO2 capture

Carbon capture has long been treated as a numbers game: the more square meters of surface a material packs into every gram, the more carbon dioxide it should soak up. A new study in the Journal of Materials Science upends that intuition. Researchers led by Nur Hidayah Azeman of Universiti Kebangsaan Malaysia report that a composite built from two zeolitic imidazolate frameworks, ZIF-7 and ZIF-67, captures more CO2 than either of its parent materials, even though the winning composition has only a moderate surface area. The best performer, a 75:25 blend of ZIF-7 to ZIF-67, adsorbed 2.63 millimoles of CO2 per gram, comfortably beating pristine ZIF-7 at 1.91 and ZIF-67 at just 1.03 millimoles per gram.

The result matters because it suggests that the secret to better carbon capture lies not in maximizing surface area but in engineering the pore environment itself. Zeolitic imidazolate frameworks, or ZIFs, are a family of metal-organic frameworks in which metal ions are bridged by imidazolate ligands to form zeolite-like topologies. ZIF-7, built from zinc ions and benzimidazole, is famous for its narrow, flexible pores that can swing open like a gate when CO2 approaches. ZIF-67, built from cobalt ions and 2-methylimidazole, is rigid and offers a high surface area but relatively weak CO2 affinity. The Malaysian team reasoned that merging the two could combine the gate-opening flexibility of ZIF-7 with the spacious framework of ZIF-67, creating a material whose pore architecture could be tuned simply by adjusting the blend ratio.

To build the dual-framework composites, the researchers used a solvent-assisted inclusion strategy, integrating the flexible, narrow-pore ZIF-7 with the rigid, high-surface-area ZIF-67 across a series of compositions. The approach preserved the structural integrity of both frameworks, which was confirmed by a battery of characterization techniques. X-ray diffraction of the representative 50:50 composite showed the coexistence of both ZIF-7 and ZIF-67 crystal phases without any phase transformation, while Fourier transform infrared spectroscopy across the whole composite series detected the characteristic vibrational fingerprints of both frameworks, providing evidence that neither had degraded. Field emission scanning electron microscopy then revealed the coexistence of the characteristic morphologies of both parent frameworks within the composite, confirming that the integration was physical rather than a chemical transformation into something new.

Nitrogen physisorption analysis, the standard tool for probing the textural properties of porous materials, revealed dramatic composition-dependent differences. Across the composite series, BET surface areas ranged from 73.14 to 1253.26 square meters per gram, accompanied by systematic variations in pore volume and pore size as the ZIF-7 to ZIF-67 ratio changed. These numbers show that the pore environment can be modulated systematically, giving researchers a dial for controlling how much internal space is available and how accessible it is to gas molecules. But when the team measured CO2 adsorption, the correlation with surface area broke down in a surprising way.

The 75:25 composite, which exhibited the highest CO2 adsorption capacity of 2.63 millimoles per gram, did not have the largest surface area in the series. Instead, it possessed only a moderate BET surface area, yet it outperformed both pristine frameworks. This observation suggests that CO2 adsorption is influenced not solely by surface area but also by the combined effects of framework flexibility, pore accessibility, and framework-specific CO2 interactions. In other words, the gate-opening behavior of ZIF-7, which allows CO2 to slip into otherwise narrow pores, appears to work synergistically with the spacious pores of ZIF-67, creating an adsorption environment that neither framework could achieve alone.

The concept of gate-opening in ZIF-7 is well documented in the literature. Previous work has directly visualized CO2 adsorption in ZIF-7, showing that the framework undergoes a structural transition that opens its narrow pores to CO2 molecules. Other studies have explored how crystal size affects this flexibility, from macro to nanoscale. ZIF-67, meanwhile, has been the subject of efforts to improve its CO2 affinity through post-synthetic ligand exchange. The new study builds on this foundation by showing that simply blending the two frameworks, rather than chemically modifying either one, can produce a material with superior adsorption properties.

The correlation among the structural, textural, and adsorption results establishes a composition-dependent structure-property relationship, demonstrating that dual-framework integration can be used to modulate CO2 adsorption. This finding provides fundamental insights into the design of composition-tunable porous materials with tailored CO2 adsorption properties. Rather than searching for a single magic material, researchers can now think about systematically adjusting the ratio of two frameworks to dial in the desired adsorption behavior, a strategy that could be applied to other dual-framework systems beyond ZIF-7 and ZIF-67.

The implications for real-world carbon capture are significant. Post-combustion carbon capture, the process of stripping CO2 from flue gas, remains one of the most energy-intensive steps in decarbonizing industrial processes. Solid adsorbents like ZIFs offer advantages over liquid solvents because they can be regenerated with less energy and do not corrode equipment. But most ZIFs still fall short of the performance needed for commercial deployment, either because their CO2 capacity is too low or because their selectivity over nitrogen and water vapor is inadequate. The dual-framework strategy offers a new lever for improving both capacity and selectivity, since the pore environment can be engineered to favor CO2 over other gases.

The study also highlights the importance of looking beyond surface area as the sole predictor of adsorption performance. In the field of porous materials, BET surface area has long been treated as the gold standard metric, with higher numbers generally assumed to mean better performance. The new results join a growing body of evidence that pore accessibility, framework flexibility, and specific gas-framework interactions can be equally or more important. As researchers continue to design new materials for carbon capture, energy storage, and gas separation, the lesson from ZIF-7/ZIF-67 composites is clear: the pore environment is a system, and engineering it requires thinking about flexibility, accessibility, and chemistry together rather than optimizing any single parameter in isolation.

The research team, which included collaborators from the National Centre for Physics in Pakistan, Universiti Teknologi MARA, and Universiti Tenaga Nasional, acknowledges that the work is at a fundamental stage. The next steps will likely involve testing the composites under realistic flue gas conditions, evaluating their stability over many adsorption-desorption cycles, and exploring whether the dual-framework strategy can be extended to other pairs of frameworks. But the core message is already clear: by blending two frameworks with complementary properties, researchers can create materials whose adsorption behavior exceeds the sum of their parts, opening a new chapter in the design of porous materials for a warming world.

Subject of Research: Tailoring CO2 adsorption through dual-framework integration of ZIF-7/ZIF-67 composites

Article Title: Tailoring CO2 adsorption through dual-framework integration of ZIF-7/ZIF-67 composites

Article References: Azeman, N. H., Ali, N. Z., Hishamuddin, A. H., Nazri, N. A. A., Bakar, M. H. A., Zolkapli, M., Zain, A. R. M., & Bakar, A. A. A. (2026). Tailoring CO2 adsorption through dual-framework integration of ZIF-7/ZIF-67 composites. Journal of Materials Science, 61(42), 32838-32851. https://doi.org/10.1007/s10853-026-13726-4

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13726-4

Keywords: Tailoring, adsorption, dual-framework, integration, ZIF-7, ZIF-67, composites, scientific research