Selective Oxidative Coupling of Amines Using Mesoporous MoO_x Catalysts

Abstract

Herein, we report a synthesis method for highly porous molybdenum oxide via molybdenum-oxo cluster formation under acidic conditions providing extraordinary stability. Synthesized materials indicate higher valences of molybdenum as compared to the commercial standards as verified through X-ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy, and ultraviolet–visible spectroscopy. The formation of a 91% orthorhombic molybdenum oxide bulk structure was verified through powder X-ray diffraction analysis. The effect of the hydrogen peroxide solvent system was optimized to obtain pore diameters as big as 17.4 nm and pore volumes as high as 0.168 cm3/g. These materials serve as great catalysts providing excellent yields of imine via amine coupling, having first-order kinetics with a turnover number as good as 27.93 with a slight decrease to 22.04 even after the fourth cycles. Surface hydroxyl species on the catalyst aid in the solid acid catalysis to jump-start the reaction.

Synthesis of Highly Porous Metal Oxide Nanoparticles for Adsorption Applications

“…Commercial magnesium oxides give only a 9 m 2 /g surface area (Table , entry 18), which suggests that in order to get a higher surface area and pore diameter, nitric acid treatment is mandatory. Figures A–D and S3 show the isotherms and pore size distributions of all the synthesized magnesium catalysts. , Glycol-based (except hexanediol and dodecanediol) meso -MgO gives type IV types of mesoporous isotherms along with higher pore size monomodal distributions. Therefore, magnesium oxide precursors, ethylene glycol solvents, nitric acid inducers, and room temperature are the optimized parameters for the synthesis.…”

“…Porous materials which have pore diameters in between 2 and 50 nm are known as mesoporous materials. , Based on pore size, such materials can be divided into two parts: small-pore-diameter (<10 nm) and large-pore-diameter (>10 nm) mesoporous materials . Comparatively, small ones are easy to synthesize and therefore are very common. , On the other hand, larger-pore-diameter materials need a lot of attention and trials for successful synthesis. − In spite of being not so common, large pore materials have many applications such as catalysis, , adsorption, drug delivery, , chromatography, and diagnosis . Therefore, recently, this field has gathered a lot of researchers’ attention. − At the beginning of the history of mesoporous materials, large pore diameters were not paid much attention due to complexity in synthesis and the fact that not much was known about applications. ,, However, with time, applications started to be discovered, which made scientists get more and more interested in this field. ,,,− These materials have many advantages over non-mesoporous metal oxides. ,,,, Due to the high surface area, mesoporous materials possess a higher number of active sites, which makes them selective and unique for several applications. − , Other than factors like size and shape, oxidation state, preparation methods, and the nature of precursors also play a role in applications. , Metal oxides, sulfides, and nitride are the most common materials which have been targeted for the formation of mesoporous nanoparticles.…”

“…Sizes and shapes of the mesostructures depend on many factors. Some of the important ones are metal precursor and metal concentration, surfactant, acidity of the reaction mixture, solvent, and calcination . Although, large-pore mesoporous materials have been synthesized previously, very few are reported as compared to small-pore systems.…”

“…4,5 On the other hand, larger-pore-diameter materials need a lot of attention and trials for successful synthesis. 6−9 In spite of being not so common, large pore materials have many applications such as catalysis, 10,11 adsorption, 12 drug delivery, 13,14 chromatography, 15 and diagnosis. 13 Therefore, recently, this field has gathered a lot of researchers' attention.…”

“…Some of the important ones are metal precursor and metal concentration, surfactant, acidity of the reaction mixture, solvent, and calcination. 11 Although, large-pore mesoporous materials have been synthesized previously, very few are reported as compared to small-pore systems. This area is still devoid of a well-developed general synthesis procedure.…”