Development of molybdenum-based oxide heterogeneous catalysts for selective organic syntheses

Abstract

Amines, azo and azoxy compounds are important chemicals that have been largely used in manufacture of pigments, polymers and drugs. Because of the harsh conditions and high energy consumption in the conventional synthesis of these compounds, the heterogeneous catalytic was introduced to overcome the problems. Bismuth molybdate, one of Mo-based oxide materials, has high visible light absorption coefficient which can benefit its utilization in organic syntheses under visible light irradiation. In this work, bismuth molybdate was improved by morphology controlled solvothermal to achieve the suitable morphology. The photocatalytic activity in benzylamine oxidation to imine as well as physical and photoelectrochemical properties of the obtained material was investigated to define the structure-activity relationship. The bismuth molybdate photocatalyst can convert up to 96.9% of benzylamine to imine with the yield of 81.2% within 2 h. According to the characterization results from various techniques i.e., BET surface area, solid ESR, and photoelectrochemical techniques, the solvothermal synthesis induces the morphology that exposes higher surface area and more oxygen vacancy than materials from hydrothermal method. These parameters contribute to the excellent performance in photocatalytic oxidation of benzylamine to imine in the solvothermally prepared material. Moreover, the mechanism of this reaction over the surface of bismuth molybdate was proposed to relate with cation intermediate and superoxide radical due to the suitable band energy levels of the material. The evidence from this work confirms that improvement of bismuth molybdate by surface engineering methods can enhance the photocatalytic activity in organic syntheses without adding cocatalysts. Other than oxidation of benzylamine to imine, the reduction of nitrobenzene is one of the interesting reactions as the different main products i.e., aniline, azobenzene, and azoxybenzene can be produced. As bismuth molybdate shows an excellent photocatalytic activity toward the organic syntheses, this material is worth studying for selectively produce each possible product from tuning the reaction conditions. As bismuth molybdate can possess different phases, in this work, the effect of phase to photocatalytic activity of Bi2MoO6 and Bi4MoO9 in nitrobenzene reduction was investigated. The results show that Bi2MoO6 phase has more suitable energy level and better charge transfer ability than Bi4MoO9. was the most efficient visible-light driven photocatalyst for nitrobenzene reduction. Additionally, it can selectively achieve different products with adjustable reaction conditions. The reduction of nitrobenzene with the addition of hydrazine occurs through the direct pathway and provides aniline in the yield of 94.0% within 14 h. To accomplish the condensation products i.e., azo and azoxy compounds, the alkaline environment was introduced to conduct the reaction pathway through condensation route. The azoxybenzene can be obtained in 96.7%yield within 3 h, but the azobenzene requires hydrazine-assisted to reach 99.9% within 8 h. In conclusion, this result can be a model for applying a single photocatalyst in various applications such as the manufacture of pigments, polymers and drugs. Apart from bismuth molybdate, molybdenum trioxide is another Mo-based oxide material that can present in various phase. The difference in structure of each phase can affect the catalytic activity in nitrobenzene reduction. In this work, MoO3 with two structures i.e., hexagonal (h-MoO3) and orthorhombic (α-MoO3) structures were synthesized and tested in the hydrazine-assisted hydrogenation of nitrobenzene to aniline. The α-MoO3 shows the potential in the reaction as of nitrobenzene in conversion of 99% converted to aniline with >99.9% of selectivity within 6 h at 35 ℃. From the XRD and XPS analysis, this α-MoO3 phase can be reduced by the hydrazine and incorporated by N atom to yield an active phase, (NH4)0.23H0.08MoO3. This active phase consists of Mo5+ which can act as adsorption site for hydrazine and elevate the generation of reactive hydrogen species which can be identified as Hδ-. Despite the change in structure of MoO3, the catalyst can maintain its efficiency within 4 cycles of the reaction. Hence, the Thus, the α-MoO3 catalyst can be used as a catalyst for practically hydrogenation of nitrobenzene to aniline at near room temperature.