Development of TiO2- and MOF-based photocatalysts for organic synthesis

Abstract

In the recent year, the selective photoreaction of organic compounds under visible light has become an alternative green synthetic approach. The photooxidation and photoreduction of aromatic compounds can produce many valuable organic products for chemical industries and pharmacies. For example, benzaldehyde and N-benzylidene benzylamine are produced from benzyl alcohol and benzylamine photooxidation, respectively, while aniline is obtained from nitrobenzene photoreduction. However, the product yield and selectivity under visible light irradiation from these reactions are still low and the limitation of photocatalyst such as fast electron-hole recombination, wide band gap energy, and poor adsorption ability of substrate can be part of this low outcome. Therefore, a modification of photocatalyst is considered as one of worthwhile methods for enhancing the photoactivity for these photoreactions. TiO2 is a promising photocatalyst which has been used in various photocatalytic reactions due to its exceptional chemical and thermal stability, high capacity to generate charge carrier, and low cost. However, TiO2 still provides a low activity in photoreaction specifically under visible light. This is due to its wide band gap energy (3.1 V), leading to the poor photoactivity in the visible light region. To overcome this limitation, doping TiO2 with nitrogen has previously shown that the photoactivity of TiO2 can be developed. Because nitrogen doping can create new band energy above the valence band of TiO2, this will narrow band gap energy to absorb light in visible region. N-doped-TiO2 is synthesized in this research via the simple thermal hydrolysis of TiOSO4 using NH4OH as a nitrogen source and a precipitating agent. The existence of N 1s in TiO2 structure is evidenced by X-ray photoelectron spectra and the narrowing of band gap energy is confirmed by UV-Vis spectra. It is worth noting that N-doped TiO2 exhibits an excellent performance in the photooxidation of benzyl alcohol and benzylamine (>80% yield), which is higher than those of undoped-TiO2. This also suggests that the method used herein for the nitrogen doping could successfully enhance the photocatalytic activity of TiO2 under visible light region.

In addition, the design of photocatalyst toward target photoreaction is also an interesting approach for enhancing the photocatalytic activity. MOF-525 is one of a high visible-light-harvesting material, which is designed by using a high visible light adsorption ligand (porphyrin) as an organic linker to coordinate with zirconium-oxo cluster, forming a framework of cubic shape, which leads to high stability, large surface area and high visible-light absorption ability. In this work, MOF-525 and metalated- MOF-525 are synthesized for benzylamine photooxidation and nitrobenzene photoreduction. The band gap energy at 1.78 eV and 2.33 eV for MOF-525 and 1.69 eV and 2.34 eV for Fe-MOF-525 from UV-Vis spectra confirm the visible light adsorption ability of catalysts. However, only MOF-525 provides a desired imine product with moderate yield (> 60% yield), possibly due to its suitable HOMO-LUMO energy level to the benzylamine substrate. Inhibition of the photoactivity found for Fe-MOF-525 suggests the important role of n-cation radical active site of free-based porphyrin ring. In nitrobenzene photoreduction, the conversion of nitrobenzene can occur over both catalysts. Interestingly, MOF-525 gives aniline and azoxybenzene as main products, but Fe-MOF-525 is selective only to the aniline product, which is possibly due to different active sites available on the catalyst surface.