The cadmium catalyst refers to an elemental cadmium or cadmium compound which has a catalytic function. Cadmium is a silver-white lustrous metal with toughness and ductility and is in the transition metal region in the periodic table. Cadmium often exists in three valence states: zero valence, positive valence, and divalent valence. The most common and stable cadmium catalysts are positive trivalent. Cadmium slowly oxidizes in humid air and loses its metallic luster. When heated, the cadmium surface forms a brown oxide layer. If heated above the boiling point, cadmium oxide fumes will be produced. At high temperatures, cadmium reacts strongly with halogen to form cadmium halide. Cadmium can also be directly combined with sulfur to form cadmium sulfide. Cadmium halides and cadmium sulfide are commonly used cadmium catalysts. Cadmium can also be combined with a ligand to form a catalyst in the form of a complex.
Figure 1. Cadmium sulfide as a catalyst
Figure 2. Cadmium halide as a catalyst
Compared with other metal catalysts, cadmium catalysts have the advantages of low price and good catalytic effect. Cadmium catalysts have exerted great applications in organic synthesis and environmental protection.
- Organic Synthesis: Dihydropyrimidin and its derivatives have a wide range of biological activities, such as antiviral, antitumor, anticancer, antihypertensive and anti-inflammatory effects. Furthermore, dihydropyrimidinone and its derivatives have excellent pharmacological activities as antagonists of calcium channel blockers, ɑ1a-antagonists and neuropeptide Y. Therefore, dihydropyrimidinone and its derivatives have great value in biology and medicine, and it is very important to synthesize dihydropyrimidinone and its derivatives. The dihydropyrimidinone and its derivatives are obtained by catalytically condensing the aromatic aldehyde, ketone and urea as raw materials. This method is called Biginelli reaction or Biginelli-like reaction. This type of reaction first uses FeCl3, AlCl3, AlBr3 or Co(OAc)2 as a catalyst, but has disadvantages such as poor catalytic effect. With cadmium chloride as a catalyst, the aromatic aldehyde, aromatic ketone and urea can efficiently synthesize dihydropyrimidinone and its derivatives through the "one-pot method". Because cadmium catalysts are cheap and easy to obtain, and with high catalytic efficiency, they have become the main catalysts for such reactions. Cyanohydrins are a very important class of compounds in which the cyano and hydroxyl groups are readily converted into various useful structural units. The cadmium complex formed by the combination of proline and metal cadmium has water solubility and is a cadmium catalyst with good catalytic effect. Cadmium proline can catalyze the cyanide silicidation of aldehydes in the aqueous phase to obtain the corresponding cyanohydrins in high yield.
- Environmental Protection: The photocatalytic material can utilize the photogenerated electron-hole generated by light irradiation to easily, quickly and effectively degrade organic pollutants in water, and is one of the effective ways to control organic sewage. Due to the narrow band gap (2.4 eV) of CdS semiconductor, it can absorb photogenerated electrons and holes by ultraviolet and visible light with wavelength less than 520 nm, which is a good photocatalyst and has great application in catalytic degradation of organic pollutants in water. Nano-sized CdS materials have significant quantum size effects. As the particle size becomes smaller, the energy gap of the CdS material becomes wider, and the oxidation and reduction ability of the produced photogenerated electrons and holes is enhanced. Moreover, as the particle size becomes smaller, the surface area of the CdS increases, which facilitates the contact of the contaminant with the photocatalyst. At the same time, as the particle size becomes smaller, the mobility constant of CdS electrons from the bulk phase to the surface increases rapidly, and the recombination probability of photogenerated electron-hole pairs is greatly reduced, thereby increasing the quantum yield and reactivity of the catalyst. Therefore, the catalytic performance of nano-CdS photocatalysts is much better than that of ordinary CdS semiconductors. However, if the nano-CdS photocatalyst is directly applied to photocatalytic degradation of organic pollutants in water, it is difficult to separate CdS from water, causing the nano-CdS particles to remain in the water and form secondary pollution. Therefore, when using CdS catalyst to catalyze the degradation of organic pollutants in water, a carrier-supported nano-CdS photocatalyst is usually used.
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