In Situ Formation of Zeolitic Imidazolate Frameworks on Nanocellulose Revealed by Time-Resolved Synchrotron Small-Angle and Wide-Angle X-ray Scattering

Metal-organic frameworks such as ZIF-8, grown in situ on nanocellulose (NC), have gained significant attention in recent years due to the versatility of the processing route and multifaceted application in the field of environmental remediation and biomedical applications. However, insights into the interactions between NC and MOF precursors and MOF structure evolution during in situ synthesis are limited or nonexistent. We report the kinetics of ZIF-8 formation on a nanocellulose (NC) aqueous suspension and in water at room temperature, monitored in real time after the addition of ZIF-8 precursors. This is the first study revealing the mechanism of ZIF-8 formation in the presence of nanocellulose. A combination of synchrotron-based small-angle (SAXS) and wide-angle X-ray scattering (WAXS) enabled us to compare the time evolution of the radius of gyration obtained from SAXS and the extent of crystallization determined by WAXS. Based on the SAXS data, we propose a new model that accounts for the initial rapid formation of primary particles, which subsequently evolve into medium-range structures before growing into the final product. Scanning electron microscopy images supported this mechanism, showing smaller particles at the beginning of the reaction and confirmed interparticle interactions, showing nanocellulose particles decorating the surface of the final ZIF-8 crystals. We demonstrate that the concentration of the starting metal salt significantly influences the kinetics of the reaction but has little effect on the ZIF-8 particle size. In contrast, increasing the NC concentration led to a reduction in the final ZIF-8 particle size, while having a negligible impact on the reaction rate and affording a minor decrease in surface area and micropore volume. We show that at the lower NC concentration that was studied the ZIF-8 particles were covered by NC, and no reduction in porosity was observed. Moreover, the kinetics of formation was shown to be independent of the NC functional group and morphology under the conditions used in this study.