Defect Dominated Hierarchical Ti-Metal-Organic Frameworks via a Linker Competitive Coordination Strategy for Toluene Removal

As relatively new porous materials, metal-organic frameworks (MOFs) have become attractive materials for many applications, especially for gas adsorption and photocatalysis, due to their high function tunability, porosity, and crystallinity. Recently, impressive improvements and achievements have been reported thanks to the numerous effort on improving the performance of MOFs in those two aspects. Representatively, the construction of the hierarchical pore in microporous MOFs can overcome the micropore blockage to improve the diffusion of the large molecules and the accessibility of their active sites. However, most of the methods to control the formation of hierarchically porous MOF have focused on the crystal growth steps, topology structure design, or post-treatment, few approaches regarding the nucleation process have been adapted due to its random fluctuation and difficulty to predict.<br/>Herein, we proposed a novel linker competitive coordination strategy based on the electronegativity difference of two kinds of organic linkers (H2BDC and NH2-H2BDC) to tailor the pore size of the Ti-MOF (MIL-125). The different electronegativity can determine the nucleation rate of those linkers. By simply controlling the feed ratio of these two organic linkers, a series of Ti-MOFs with continuously tunable hierarchical porosity is directly obtained as evidenced by the DFT pore size distribution and N₂ sorption isotherms. Besides, we also demonstrated that the missing-linker defect is created during this process and plays dominated role in the construction of the hierarchical pore.<br/>The demonstration of toluene removal shows that the competitive coordination strategy not only contributes to wide pore size distribution to enhance the adsorption performance of toluene but also endows good charge separation ability to facilitate photocatalytic performance. Finally, the toluene removal efficiency of optimal Ti-MOF with mixed organic linkers of a 1:1 molar ratio is 2.14 times and 1.88 times of the pristine MIL-125 and MIL-125(NH2), respectively.<br/>In conclusion, we constructed a hierarchically porous MIL-125 with tailored pore size via a simple linker competitive coordination strategy and verified the superiority of the hierarchical pore structure by evaluating the adsorption and photocatalytic performance of as-obtained Ti-MOFs for toluene mineralization. Given that this approach is a one-pot method and shows some merits, such as low cost, controllable pore size distribution, without impurity, and avoiding additional post-treatment processes, it can be a new prospect to optimize the porosity of Ti-MOF for various heterogeneous catalysis applications.