Copper Catalysts

The copper catalyst refers to a general term for elemental copper, copper oxide and copper salt having catalytic properties. Copper is a magenta metal in the fourth cycle and the IB group of the periodic table. Copper is a less active metal that does not combine with oxygen in dry air at room temperature and produces black copper oxide when heated. The common valence states of copper are 0, +1 and +2. Copper is one of the metals used by humans in the early days. It has many excellent properties, such as good electrical and thermal conductivity, good ductility, low cost and wide range of raw materials. Nano-scale copper has more excellent properties than traditional bulk copper materials, such as large specific surface area, strong plasticity, high strength, and low electrical resistance. Copper and its compounds have excellent activity in the catalytic oxidation, hydrogenation and dehydrogenation of organic compounds, so they are widely used in catalyst preparation. For different catalytic reactions, copper-based catalysts have different requirements for the form of the active copper valence state.

Figure 1. Copper elemental catalyzed ethanol oxidation

Applications:

Copper catalysts are widely used in organic catalytic reactions due to their good catalytic performance, low cost and low toxicity.

• Coupling reaction: Copper of different valence states can be used to catalyze the coupling reaction, especially the Ullmann coupling reaction. The copper salt and copper oxide have good catalytic effects on the arylation reaction of nucleophilic reagent. Copper nanoparticles can catalyze the Ullmann coupling reaction in a wide range of applications, and can also catalyze the coupling reaction of nitrogen-containing nucleophiles, phenols, thiols, xanthogenates, selenium ruthenium nucleophiles and the like.
• Atom transfer radical polymerization: Atom transfer radical polymerization (ATRP) can be used to prepare polymers with well-defined structures and relatively narrow molecular mass distribution. Supported copper catalysts Cu-ZSM-5 and Cu-Al₂O₃ can be directly used for the polymerization of reverse atom transfer radicals of methyl methacrylate and styrene.
• Hydrogenation and dehydrogenation reaction: The increase of CO and CO₂ content in the air raises ecological and environmental issues, and hydrogenation of CO and CO₂ to methanol is one of the effective ways to reduce the content of CO and CO₂ in the air. The hydrogenation of CO and CO₂ usually uses metallic copper and its compounds as catalysts. However, in the process of synthesizing methanol, a separate copper catalyst is generally not used, but a supported copper catalyst is used. The active component of the supported copper catalyst is mainly copper, and its carrier is Al₂O₃, and different additives are added. The choice of copper catalyst will vary depending on the starting materials. Hydrogenation of fatty acid esters to fatty alcohols is also an important hydrogenation reaction, generally employing Cu as a catalyst. The CO low-temperature shift reaction is one of the dehydrogenation reactions. The CO low-temperature shift reaction is usually used for the removal of a large amount of CO in the methanol reforming hydrogen production reaction, and the catalysts used in the reaction are all copper-zinc-based catalysts.
• C-H activation: C-H activation is a direct conversion of C-H bond to C-C or C-Het (Het=Heteroatom) bond, which has good atomic economy and high synthesis efficiency. Because copper is low-toxic, inexpensive, and copper is needed by the organism, copper is often used as a catalyst in the C-H activation reaction. During the reaction, the monovalent copper salt catalyst reacts with the ligand and base to produce an active compound that activates the C-H bond. Proton exchange and oxidative addition are then carried out to form a trivalent copper intermediate. The trivalent copper intermediate is reduced to obtain monovalent copper complex, and the catalysts is recycled while the product is obtained.

Figure 2. Copper salt catalys catalyzes C-H activation

• Environmental protection: Catalytic wet oxidation is an advanced oxidation technique that uses hydroxyl radicals as an oxidant to decompose high molecular organic matter in water into CO₂ and H₂O. The Cu-supported activated carbon catalyst prepared by the impregnation method using activated carbon as a carrier can be used for catalytic wet oxidation treatment of landfill leachate. The addition of a copper catalyst can reduce the activation energy of the reaction and make the reaction more likely to occur. Compared to ordinary copper catalysts, nano-copper has a larger specific surface area and more active centers. Therefore, when nano-copper is used to catalyze an organic reaction, it exhibits high catalytic activity. NH₃-SCR (Ammonia Selective Catalytic Reduction) technology is currently the most widely used denitration technology, and nano copper has very good effect as a catalyst for this reaction.

References

1. Yu, Yang; Jin. (2019). "Highly active and stable copper catalysts derived from copper silicate double-shell nanofibers with strong metal-support interactions for the RWGS reaction." Chemical Communications (Cambridge, United Kingdom) 55(29), 4178-4181.
2. Yoshii, Daichi. (2019), "Selective Dehydrogenative Mono- or Diborylation of Styrenes by Supported Copper Catalysts." ACS Catalysis 9(4), 3011-3016.
3. Coelho, Felipe Lange (2019), "New arylselanylpyrazole-copper catalysts: Highly efficient catalytic system for C-Se and C-S coupling reactions." Catalysis Communications 121, 19-26.