A substance containing tungsten while performing a catalytic function is called a tungsten catalyst. Tungsten is a silvery white shiny metal with a high hardness and a high melting point. Tungsten is chemically stable and does not react with air and water at ambient temperatures. When not heated, hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, and aqua regia of any concentration do not react with tungsten. Tungsten is relatively abundant in the earth's crust, so tungsten catalysts are inexpensive and abundant in variety.
Tungsten catalysts play an important role in the fields of organic synthesis and industrial production because of their excellent catalytic performance, simple preparation, pollution-free and reusability.
- Hydration reaction: Heteropolytungstic acid, with a special acidity, is one of the tungsten catalysts. Having a special solubility which is not possessed by a general solid acid, heteropolytungstic acid can be used as a homogeneous catalyst or a heterogeneous catalyst, and has a wide range of uses. Among them, heteropolytungstic acid is often used as a catalyst in industrial hydration reactions. For example, heteropolytungstic acid catalysts are often used in the industry to catalyze the hydration of n-butene to sec-butanol. When the heteropolytungstic acid catalyst is used as an acidic catalyst in a hydration reaction, it generally has the advantages of low dosage, high yield, good chemical stability, reusability, low cost, non-corrosiveness and simple post-treatment.
- Oxidation reaction: Oxidation reaction, an important method for synthesizing chemical raw materials and fine chemical intermediates, has important applications in medicine, pesticide and other industries. Tungsten catalysts have good catalytic oxidation properties and are often used as catalysts for industrial oxidation processes. The tungsten catalyst used for the oxidation reaction mainly includes a heteropoly compound of tungsten, a tungsten oxide, a tungstic acid, a tungsten salt, and the like. The oxidation reaction catalyzed by the tungsten catalyst mainly includes oxidation reaction of hydrocarbons, alcohols, phenols, ketones and epoxidation of olefins. For example, using Na2WO4·2H2O as a catalyst and [CH3N(n-C8H17)3]HSO4 as a phase transfer catalyst, hydrogen peroxide can directly oxidize cyclohexene to prepare adipic acid. In this reaction, the yield of the product adipic acid is high, reaching 90% or even more. In addition, the catalyst Na2WO4·2H2O is stable in structure and can be separated and reused for many times. For example, when a complex formed by sodium tungstate with a nitrogen-containing bidentate organic ligand (o-phenanthroline) or an oxygen-containing bidentate organic ligand (oxalic acid) as a catalyst, the oxidation of cyclohexanol can be catalyzed. The reaction is carried out to give the product cyclohexanone. This reaction has the advantages of short reaction time and high product yield.
Figure 1. Tungsten catalyst-catalyzed oxidation reaction
- Condensation reaction: It is also often applied to tungsten catalysts in some condensation reactions. For example, the heteropolyacid catalyst Ag3PW12O40 can catalyze the catalyzed synthesis of pyrimidinone derivatives by ketones and alanes. Compared with the traditional three-step method, this method is a new environment-friendly catalytic synthesis process. Phosphotungstic acid (H6P2W18O62·24H2O) as a catalyst can catalyze the Pechmann condensation reaction of phenol and β-ketoester to synthesize various coumarin derivatives under solvent-free reaction conditions.
Figure 2. Tungsten catalyst-catalyzed condensation reaction
- Other reactions: In addition to catalyzing the above several main reactions, the tungsten catalyst also has a good catalytic effect on many reactions such as hydrogenation, dehydrogenation, dehydration, polymerization, alkylation, acylation, esterification, hydrodesulfurization and the like. For example, heteropoly acid H3PW12O40 is supported on activated carbon to prepare a supported tungsten catalyst, which can catalyze the esterification of phthalic anhydride, 2-ethylhexanol and 1-octanol. The SO42--/TiO2-WO3 catalyst was prepared by hydrothermal modification method, which can catalyze the dehydration reaction of cyclohexanol to prepare cyclohexene.
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- Maccarrone, Maria Juliana. (2010), “Nanoparticles of tungsten as low-cost monometallic catalyst for selective hydrogenation of 3-hexyne.” Quimica Nova (2016), 39(1), 1-8.
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