Gold Catalysts

A substance containing gold and having a catalytic function is called a gold catalyst. Gold is a transition metal and is located in Group IB of the sixth period. Gold usually appears in its form as an elementary substance in nature. Gold is chemically inert and does not chemically react with most chemicals. It does not react with oxygen at normal temperature, and only the heating and other processes can produce gold oxide. Gold compounds can form trivalent and monovalent gold ions after dissolving in solution. The valence of gold in common gold catalysts is +1 or +3.

Figure 1. Gold catalyst catalyzed Suzuki reaction

Applications:

Because of their rich varieties, good catalytic performance and environmental protection, gold catalysts are widely used in organic synthesis and environmental protection.

Oxidation reaction: The propylene oxide has many important uses in the industry. At present, the production of propylene oxide by the chlorohydrin method is industrially used, but the method has environmental problems. The supported nano-gold catalyst is a green catalyst which can be used for the catalytic oxidation of olefins. In the catalytic process, the catalyst activity is inversely proportional to the nanogold particle size, and the reaction selectivity is very high. Alcohol selective oxidation is an important route for the synthesis of carbonyl compounds or related derivatives, and these carbonyl compounds or carbonyl derivatives are often used as intermediates for pharmaceuticals or pesticides. Nanogold can also be used as a gold catalyst to catalyze the selective oxidation of alcohols. For example, the oxidation of an alcohol to an aldehyde can be catalyzed by a nano-gold catalyst prepared by using a Cu-Mg-Al ternary composite oxide as a support. The nano-gold catalyst can also catalyze the oxidation of the aldehyde to form the corresponding carboxylic acid. The use of a nano-gold catalyst to catalyze the oxidation of an aldehyde has the advantages of a wide solvent system, good catalytic performance, and high yield.

• Reduction reaction: The hydrogenation reduction of unsaturated olefins is widely used in industrial production. For olefins containing multiple unsaturated carbons, the selectivity to the catalyst is high due to structural complexity during reductive hydrogenation. Gold catalysts have their own advantages in selective hydrogenation reduction and have wide applications in the reduction of polyunsaturated olefins. The selective reduction of unsaturated aldehydes is a common reaction for the synthesis of fine chemicals such as pharmaceutical intermediates and perfumes. Generally, the hydrogenation of unsaturated aldehydes is mostly hydrogenation of double bonds to form saturated aldehydes. The gold catalyst has the function of activating CO bonds, and thus can be used for selective reduction of unsaturated aldehydes. In addition, the gold catalyst is also very suitable for the selective reduction reaction of unsaturated aromatic nitro compounds.
• Hydrocarbon activation: The hydrocarbon activation can achieve the construction of C—X (X is C, N, O, S) bonds. The complex formed by an electron donor such as triphenylphosphine and gold has high catalytic performance and can be used for catalyzing hydrocarbon activation. For example, CH₃AuL (L is PPh₃) is used as a catalyst to catalyze the coupling reaction of Grignard reagent, alkyllithium and halogenated hydrocarbon, so that the construction of C-C bond can be realized. The reaction mechanism is that the metal alkylate is first electrophilically substituted with a monovalent gold salt to form an alkyl gold, and then coupled with a halogenated hydrocarbon to form a C-C bond.

Figure 2. Gold catalyst catalyzed C-H activation reaction

• Environmental protection: In the field of environmental protection, gold catalysts are widely used in automobile exhaust gas treatment, organic matter catalytic decomposition and sewage treatment due to their good catalytic performance. For example, the organic wastewater containing phenol and aromatic hydrocarbon compounds produced by the herbicide p-nitrophenol (4-NP) and its derivatives poses a serious environmental and ecological threat. The gold catalyst with high molecular weight polymer has the advantages of high stability, good catalytic performance, convenient recovery and recyclability. This gold catalyst can reduce 4-NP to 4-AP, which is the best way to degrade 4-NP.

References

1. Qi, Caixia; Zheng. (2019). "CO oxidation over gold catalysts supported on CuO/Cu₂O both in O₂-rich and H₂-rich streams: Necessity of copper oxide." Applied Catalysis, B: Environmental 253, 160-169.
2. Brill, Marcel. (2018), "Chiral carbophilic gold lewis acid complexes in enantioselective catalysis." Topics in Organometallic Chemistry 62, 51-90.
3. Megias-Sayago, Cristina. (2018), "Understanding the Role of the Acid Sites in 5-Hydroxymethylfurfural Oxidation to 2,5-Furandicarboxylic Acid Reaction over Gold Catalysts:Surface Investigation on CexZr1-xO₂ Compounds" ACS Catalysis, 8(12), 1115 4-11164.