Hierarchical self-assembly of cationic zirconium metal-organic cages (Zr-MOCs) with charge-balancing chloride ions through relatively weak interactions result into supramolecular frameworks with high porosity.  

However, due to the high porosity and the weak intermolecular interactions between the Zr-MOCs, the resulting supramolecular frameworks are prone to collapse upon removal of the guest molecular, leading to pore blockage and limitation of their potential applicability in areas requiring solid-state porosity.  

In a study published in CCS Chemistry, Prof. YUAN Daqiang and his colleagues from Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences, reported the construction of Zr-MOC based supramolecular frameworks (MSF-1) with high porosity (74.9 %).  

The researchers considered that the tri-nuclear Zr(IV) clusters in Zr-MOC-based supramolecular frameworks could catalyze the in-situ polymerization of the active monomers, and the resulting polymer may afford a highly stable scaffold interlocking and securing the extrinsic porosity of fragile MOC-based supramolecular framework. 

Upon removal of guest molecules, MSF-1 shows a completely collapsed framework. The tri-nuclear Zr(IV) clusters in MSF-1 was used for the first time to catalyze the transformation of isocyanate to urea compounds.  

The researchers proceeded to introduce in-situ formed polyurea by catalytic polymerization of 1, 6-hexamethylene diisocyanate as a mechanical support for MSF-1. The resultant polyurea@MSF-1 showed significant N₂ enhancement.  

Powder X-ray diffraction study indicated that the crystallinity and structural integrity of the material is maintained after gas adsorption study. The presence of polyurea thereby functions as a mechanical support to maintain the framework structure. 

More interestingly, most of the polyurea was formed on the surface as the initially formed polyurea inhibited further penetration of monomers into the pore of supramolecular frameworks owing to confined porosity.  

In this case, the porosity of MSF-1 can be maintained. The polyurea@MSF-1 showed enhanced methane uptake (184 vs 62 cm³) compared to the collapsed MSF-1, and this indicates the importance of framework retention. The high pressure methane uptake of polyurea@MSF-1 is the highest recorded in MOC-based supramolecular frameworks. 

This study provides an in-situ catalytic polymerization strategy for application-oriented investigations, which not only can enhance the stability of supramolecular frameworks but also may be used to boost the stability of other porous materials. Meanwhile, the induced polyuria may offer the frameworks a new functionality as superior catalytic performance besides increasing surface hydrophobicity.