Titanium metal-organic frameworks for photocatalytic CO2 conversion through a cycloaddition reaction

Abstract

The elevated levels of CO₂ in the atmosphere have been a major concern for environmental scientists. Capturing CO₂ gas and its subsequent conversion to useful organic compounds is one of the avenues that have been extensively studied in the last decade. The photocatalytic cycloaddition of CO₂ is a promising approach for effective CO₂ capture and the production of value-added chemicals such as cyclic carbonates. MOF-901, a titanium-based metal-organic framework with hexagonal layers and imine linkages, was successfully oxidized in this study to MOF-997, incorporating amide linkages using Oxone. Both MOFs displayed remarkable photocatalytic activity in CO₂ cycloaddition under mild conditions, including moderate temperatures and visible light exposure. Particularly noteworthy is MOF-997, exhibiting superior performance with donor-acceptor active sites, achieving a 99.9% yield in catalyzing CO₂ conversion from styrene epoxide to styrene carbonate under solvent conditions.

Fig. 1. Synthesis and characterization of MOF-901 and MOF-997. (a) Structure of MOF-901, (b) chemical transformation of imine in MOF-901 to amide in MOF-997, (c) PXRD patterns of MOF-997 slightly shifted to the right compared to MOF-901; (d) overlay of the FTIR spectra of MOF-901 and MOF-997, showing the appearance of amide vibrations in MOF-997; (e) N₂ sorption isotherms of MOF-901 and MOF-997; the porosity of MOF-997 reduced after oxidation due to the mass increase of amide units; (f) overlay of the CO₂ sorption isotherms of MOF-901 and MOF-997. The hysteresis of MOF-997 isotherms indicating a strong interaction between the MOF and CO₂.

Fig. 2. (a) Overlay of the solid-state ¹³C-CP-MAS NMR spectra of MOF-901 and MOF-997, showing the disappearance of imine linkages in MOF-901 and the appearance of amide linkages in MOF-997; (b) bandgap determination of MOF-901 and MOF-997 through the Tauc plot; (c) UV-Vis DRS spectra of MOF-901 and MOF-997. Both MOFs strongly absorb visible light at 450 nm.

Fig. 3. FE-SEM images (a and b) and EDX patterns (c and d) of MOF-901 (a and c) and MOF-997 (b and d).

Fig. 4. Electronic projected density of states (PDOS, in eV⁻¹) for MOF-901 (imine linkages; a) and MOF-997 (amide linkages; b). MOFs computed at the PBE level. Energies (in eV) relative to the Fermi level. The projection of the bands onto the titanium s, p and d atomic orbitals is shown.

Fig. 5. (a) Calculated isosurface representation (isosurface value ±0.1) of the highest occupied band; (b) an unoccupied level with a large component on Ti(3d) atomic orbitals for the imine MOF. The corresponding bands for the amide MOF are similar (not shown). The gray, red, green, pink, and white spheres represent the C, O, N, Ti, and H atoms, respectively. The square planar pyramidal environment around each Ti ion is evident.

Fig. 6. ¹H-NMR spectra of the styrene carbonate product catalyzed by (a) MOF-901, and (b) MOF-997. Inset: a scheme showing the cycloaddition of CO₂ and styrene oxide to form styrene carbonate.

Fig. 7. (a) Kinetics study of yield variation with increasing MOF concentration, (b) styrene carbonate yield versus styrene oxide concentration for a fixed amount of MOF-997 (10 mg), and (c) kinetics study of epoxide consumption over time.

Fig. 8. The potential mechanism depicting the cycloaddition reaction between CO₂ and epoxides facilitated by MOF-997.