Hybrid frameworks join forces to clean polluted water

Hybrid materials that merge the porosity of metal–organic frameworks (MOFs) with the robustness of covalent organic frameworks (COFs) are redefining what’s possible in environmental catalysis and pollutant capture. Yet, at the heart of their performance lies an elusive feature: the interface between the two frameworks. How do metal clusters and organic networks communicate across this boundary? And how does that connection dictate the reactivity of the composite material?

Researchers from the Universidad Autónoma de Madrid, in collaboration with the University of Cambridge, have addressed these questions in the paper, Deciphering interfacial interactions in a dual-functional MOF@COF composite for organic pollutant removal from water. Using advanced synchrotron tools at Diamond Light Source; specifically X-ray Absorption Spectroscopy (XAS) on B18 and X-ray Pair Distribution Function (XPDF) analysis on I15-1; they uncovered how subtle coordination changes at the interface create a more active and resilient system for removing organic contaminants such as bisphenol A (BPA) from water.

Representation of the (A) MOF-808 structure and (B) TAPB–BTCA-COF structure. Colour scheme: blue = Zr, grey = C, red = O, lilac = N; hydrogen atoms have been omitted for clarity.

MOF-808 is a porous material made from zirconium atoms and organic linkers that assemble into a rigid 3D network - like a molecular sponge - capable of trapping and breaking down pollutants. Several studies showed that adding a metal ion such as Fe can improve its catalytic properties. A challenge that still needs to be overcome is that the industrial implementation of MOF is hard, as MOF are often prepared as microcrystalline powders, which is hard to process.

The team engineered a Fe-MOF-808@TAPB-BTCA-COF composite, a dual-functional hybrid that combines a zirconium-based MOF (MOF-808) with a triazine-linked COF (TAPB-BTCA). The MOF provides highly accessible Zr-O clusters and open metal sites ideal for adsorption and catalysis, while the COF introduces linkers that improve charge transport and chemical stability.

(A) TEM image of Fe-MOF@COF. (B) FE-SEM image of Fe-MOF@COF.

This design aimed to overcome a common trade-off in water treatment materials: achieving both high pollutant uptake and long-term durability. The researchers proposed that linking the two frameworks would create synergistic charge transfer channels at the interface, improving photocatalytic degradation efficiency for persistent pollutants like BPA, an endocrine-disrupting compound widely found in industrial wastewater.

To confirm these interfacial effects, the team turned to Diamond Light Source for atom-level insights. At B18, X-ray Absorption Spectroscopy (XAS) at the Fe and Zr edges revealed subtle shifts in oxidation states and coordination geometry after COF coating. The data showed the formation of Fe-O-Zr linkages at the boundary, confirming a real chemical bridge rather than a mere physical mixture. These new bonds facilitate electron transfer, supporting efficient redox cycles during pollutant degradation.

Meanwhile, XPDF measurements at I15-1 provided complementary information on short- and long-range ordering. The XPDF patterns captured small distortions in the local Zr-O cluster environment, consistent with strong interfacial coupling, but preserved the overall MOF crystalline network. This combination of structure and chemistry data offered a complete picture of how the two frameworks intertwine at the nanoscale—information inaccessible by conventional diffraction alone.

Together, the synchrotron results demonstrated that interfacial bonding is the key driver of enhanced catalytic activity and material stability.

The Fe-MOF-808@TAPB-BTCA-COF hybrid achieved remarkable pollutant-removal performance, efficiently degrading BPA in both batch and continuous-flow tests while maintaining its structure over multiple cycles. The work not only provides a blueprint for designing MOF@COF hybrids with optimised interfaces but also showcases how multi-beamline synchrotron analysis can unravel the hidden chemistry that governs hybrid material performance.

To find out more about the B18 beamline, please contact the Principal Beamline Scientist Diego Gianolio: diego.gianolio@diamond.ac.uk

To find out more about the I15-1 beamline, please contact the Principal Beamline Scientist Dean Keeble: dean.keeble@diamond.ac.uk