A substance containing tin while performing a catalytic function is called a tin catalyst. Tin is a silvery metal that has a low melting point. It is not oxidized by air and is mainly present in the form of dioxide and various sulfides. The valence of tin can be 0, +2, +4, and +5, and the valence of tin in most tin catalysts can be +2 or +4.
The advantages of Tin catalysts lie in their high catalytic activity, simple preparation, and reusability, so they play a huge role in the field of organic catalysis and environmental protection.
- Baeyer-Villiger oxidation reaction: The Baeyer-Villiger oxidation reaction is an important reaction for the oxidation of cyclic ketones or chain ketones to lactones or esters. Since the Baeyer-Villiger oxidation reaction can control the stereostructure of the product, it is important for the conversion of functional groups and the expansion of the ring in organic synthesis. The catalytic system with tin as the active center is very helpful. When it is used to catalyze the Baeyer-Villiger oxidation reaction, it benefits the reaction with advantages of reduced product separation steps, simplified purification process and reusability. The tin catalysts commonly used in the Baeyer-Villiger oxidation reaction are heterogeneous tin-containing catalysts and homogeneous tin-containing catalysts. The heterogeneous tin-containing catalyst includes a Sn-β zeolite catalyst, a Sn-MCM-41 catalyst, a hydrotalcite-supported tin catalyst, and a carbon nanotube-supported tin catalyst. The homogeneous tin-containing catalyst is Sn[N(SO2C8F17)2]4. For example, a supported tin catalyst prepared by loading stannous chloride on carbon nanotubes can catalyze the Baeyer-Villiger oxidation of 2-adamantanone.
Figure 1. Tin catalyst-catalyzed Baeyer-Villiger oxidation reaction
- Esterification reaction: Using concentrated sulfuric acid to catalyze the esterification reaction of an acid and an alcohol is a traditional method for synthesizing an ester. However, concentrated sulfuric acid as a catalyst has the disadvantages of severe equipment corrosion, complicated post-treatment process, side reactions and low product yield. Replacing concentrated sulfuric acid with tin chloride will not reduce the catalytic activity, and can avoid the disadvantages of concentrated sulfuric acid catalyst. Therefore, tin chloride (tin chloride, stannous chloride) is often used as a catalyst for the esterification reaction, too. For example, SnCI4·5H2O as a catalyst shows a good catalytic effect in the synthesis of isoamyl acetate, n-pentyl acetate, and n-butyl vine acetate, besides, the yield can reach more than 90% in one hour. SnCI2 also can be used to synthesize acetate and the like.
Figure 2. Tin catalyst catalyzed esterification reaction
- Mannich reaction: The Mannich reaction means that a compound containing α active hydrogen is condensed with an aldehyde and an amine to remove a molecule of water under the catalysis of an acid to obtain a Mannich base (or a salt). With many advantages, this action has been increasingly used in the field of organic synthesis. SnCl4 is one of the catalysts for the Mannich reaction. It can catalyze the “one-pot” Mannich reaction of ketone, amine and aldehyde components, and synthesize a series of β-amino ketone derivatives in one step. At the same time, SnCl4 can also catalyze the "one-pot" Mannich reaction of aromatic ketones, amines and aldehydes with different substituent functional groups.
- Environmental protection: Photocatalytic technology is a promising water treatment technology. It has broad application prospects because it can utilize solar energy and has obvious degradation effects on various pollutants. Some tin catalysts have good photocatalytic oxidation properties and therefore play a vital role in environmental protection. For example, β-SnWO4 with a narrow band gap energy and a special band structure has high photocatalytic activity, and can be used as a catalyst for photocatalytic degradation of rhodamine B. Tin dioxide also has good photocatalytic oxidation activity and can be used for the degradation of phenol.
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- Lee, Jong Kwon. (2016), “Direct dehydrogenation of n-butane over platinum-tin catalysts supported on alumina.” Journal of Nanoscience and Nanotechnology 16(10), 10816-10822.
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