Introduction Glassy metal–organic frameworks are emerging types of materials that could combine the modular tunability and high porosity of crystalline metal–organic frameworks (MOF) and the high processability of glassy materials. However, it remains difficult to obtain glassy MOF with high porosity. Thus, an alternative design strategy of embedding crystalline MOF nanocrystals into glassy MOF becomes very attractive to achieve materials with high porosity and liquid-like processability. Glassy materials based on titanium alkoxides are suitable host material for this purpose, which are easy to synthesis at large scale under mild conditions. Also,metal–organic polyhedral (MOP), as the molecular analogue for MOF, are also promising material for synthesizing composite materials. In this report, we developed a series of composite materials made of crystalline MOF and discrete MOP embedded in MOF glasses based on titanium-oxo cluster nodes and alkoxide linkers, and studied their porosity, crystallinity and morphology, which would pave the way for developing composite materials based on MOF glasses.
Methods The prototype crystalline MOF, UiO-66, ZIF-8 and HKUST-1 were synthesized through solvothermal condition according to literature reports. The titanium alkoxide glass is made by mixing titanium precursor, titanium isopropoxide and the corresponding alcohol linker in alcohol solvents followed by evaporating the solvent at elevated temperature. To obtain the MOF@MOF glass composite materials, MOF nanocrystals were simply dissolved in the solution mixtures that were used to obtain titanium alkoxide glasses. The MOF@MOF glass composite materials were then washed out with various solvents and activated with super-critical CO2 drying, and then tested for N2 uptake at 77 K to obtain porosity data. The crystallinity of MOF after synthesis were measured by XRD, and the composite materials were studied by SEM. For the composite material using MOP, the hydroxyl functionalized MOP were synthesized in a method similar to literature, and then reacted with titanium precursor to produce the corresponding Ti-MOP material,following by investigation on its surface area.
Results and discussion The UiO-66 and HKUST-1 remain the crystallinity unvaried before and after synthesis. The composite material of UiO-66@Ti-PEG-BTM and HKUST-1@Ti-PEG-BTM show surface area of 375 m2/g and 368 m2/g, respectively.Conversely, ZIF-8 loses its crystallinity in the composite material, which has very low surface area. It is speculated that the ZIF-8 structure is unstable with the coordination competition of titanium alkoxide glass precursors. The Ti-MOP composite material also shows higher porosity than that of the MOP itself, showing that such synthetic strategy is promising to produce porous materials.
Conclusions We developed a series of optically transparent glassy MOF made from titanium-oxo clusters linked with multi-dentate alcohol linkers, which could be modularly designed and synthesized under mild and environmentally friendly conditions. MOF nanocrystals were succeedingly embedded in the glass to prepare porous composite materials. For UiO-66 and HKUST-1, they retained high crystallinity and porosity in the MOF glasses, whereas ZIF-8 lost its crystallinity. We also developed composite materials based on metal-organic polyhedral and glassy MOF, which also showed high porosity. We therefore developed a new type of MOF glass that could be synthesized under mild condition and showed good compatibility with crystalline MOFs, from which a series of glass–crystal composite material with high porosity were successfully produced. We believe this research provide new opportunities for developing porous composite materials based on glassy MOF.