Partial Substitution of titanium with tungsten in MIL-167 Metal Organic Framework: Synthesis, Characterization, and Functional Applications

Abstract

Metal–organic frameworks (MOFs) are highly tunable porous materials whose properties can be tailored through the selection of different metal ions and organic linkers. Among them, titanium-based MOFs have attracted significant interest due to their structural stability, high surface area, and potential applications in gas adsorption, separation, and catalysis. In this work, the titanium-based MOF MIL-167 (Ti(DOBDC)·(Et2MeNH)2·nH2O), was constructed from Ti4+ metal nodes and 2,5-dihydroxyterephthalic acid (DOBDC) linkers. This framework is neutralized by counterions, specifically Et2MeNH+, which are essential for charge balance. MIL-167 was modified through partial substitution of Ti with W at varying molar ratios. This modification was motivated by the difference in oxidation states between Ti4+ and W6+, which could potentially influence the structural and physicochemical properties of the framework. Structural characterization using X-ray powder diffraction indicated that the incorporation of W did not significantly alter the crystalline structure of MIL-167. X-ray fluorescence spectroscopy confirmed the presence of tungsten in samples prepared with different Ti precursor substitution levels, although it remains possible that W is located within the pore system rather than being fully incorporated into the framework nodes. Infrared spectroscopy, differential scanning calorimetry, and thermogravimetric analysis showed no substantial differences between the parent and modified MOFs. Nitrogen physisorption measurements revealed negative adsorption behavior, suggesting that the pores of the modified MIL-167 samples were not accessible. The modified MOFs were also evaluated as heterogeneous catalysts for the cycloaddition of CO2 to epoxides to produce cyclic carbonates; medium catalytic activity was observed. Finally, heterojunctions formed by combining MIL-167 with MIL-125-NH2 exhibited different UV–Vis reflectance behavior compared to pristine and W- modified MIL-167, suggesting the need for further investigation into their optical properties and potential photocatalytic applications.