Chromium Catalysts

A chromium catalyst refers to an elemental chromium or chromium compound which has a catalytic function. Chromium is a silvery white shiny metal that is malleable. Chromium belongs to the VIB family in the periodic table. There is no free state of chromium in nature, and it mainly present in the form of FeCr₂O₄. Chromium is chemically inert and highly resistant to corrosion. In the air, even in hot condition, chromium oxidation is slow. Chromium is commonly present in the form of Cr⁶⁺, Cr⁵⁺, Cr³⁺ and Cr²⁺, so the types of chromium catalysts are abundant. Chromium catalysts can generally be classified into organic chromium catalysts and inorganic chromium catalysts. The chromium catalysts can also be classified into supported chromium catalysts and unsupported chromium catalysts depending on whether it has supported substance.

Figure 1. Inorganic chromium catalyst

Applications:

Due to the wide variety, low cost and excellent performance of chromium catalysts, they have been widely used in various organic synthesis reactions.

• Coupling reaction: Hydrogen cyanide (HCN) is an important fine chemical product widely used in pesticides, pharmaceuticals, metallurgy, dyes and other industries. Its downstream products are abundant and they can be used in the production of plastics, plexiglass, disinfectants and so on. The industrial production method of hydrogen cyanide mainly includes the Andrussow method. The reaction conditions of the Andrussow method are atmospheric pressure, temperature above 1000℃, heating and cooling for a long time. The chromium catalyst has a coupling action and can form hydrogen cyanide with high selectivity. Hydrogen cyanide can be synthesized by a one-step reaction of CH₄ and NH₃ at a temperature lower than 400℃ by means of dielectric barrier discharge plasma technology using a cheap and readily available supported chromium catalyst.
• Dehydrogenation reaction: Many processes involve dehydrogenation reactions such as the bio-p-xylene (PX) process. The chromium catalyst can be used in various hydrocarbon dehydrogenation reactions. The addition of auxiliaries can improve the catalytic performance of chromium, for example, alkali metals can be added to improve the catalytic performance of the chromium catalysts.
• Fluorination reaction: The gas phase fluorine-chloride exchange reaction is a key reaction for the selective synthesis of fluorine-containing compounds. Because the reaction is simple, economical and practical, it has been widely used in the production of hydrofluorocarbons. Chromium oxide is the main catalyst for the gas phase synthesis of fluorinated hydrocarbons. The chromium oxide catalysts for gas phase fluorination of fluorinated hydrocarbons are mainly chromium oxide catalysts and supported chromium oxide catalysts.
• Polyolefin reaction: Polyolefin resins are widely used in agriculture, industry, medical care, and daily life. Due to their high catalytic activity and simple preparation process, chromium-based catalysts, as an important industrial polyolefin catalyst, can catalyze the production of a large number of high-density polyolefins. The polymer catalyzed by the chromium-based catalyst has the advantages of wide molecular weight distribution, excellent processing property and long-chain branching. Further more, a supported chromium-based catalyst is mainly used to catalyze the production of a polyolefin.
• Other reactions: The introduction of a rigid five-membered ring-oxazolidinone structure can greatly improve the thermal stability, glass transition temperature, rigidity and dielectric properties of the epoxy resin. Therefore, epoxy resins modified with oxazolidine have been widely used in the fields of coatings, adhesives and composite materials. The Cr in organometallic chromium catalyst has low electronegativity and strong oxophilicity, and can act well with oxygen, which make the organometallic chromium catalyst better able to catalyze the ring opening reaction between isocyanate and epoxy group. The organometallic chromium catalyst has good selectivity for the formation of the oxazolidinone heterocyclic structure. Therefore, it can be used for the preparation of an oxazolidinone-modified epoxy resin.

Figure 2. Organic chromium catalyst

The chromium catalyst can be used as a cocatalyst to prepare 1,2-propanediol (PG). 1,2-Propylene glycol is an important basic chemical raw material for the production of unsaturated polyester resins, functional fluids, foods, pharmaceuticals, liquid detergents, paints and coatings. 1,2-propanediol can be produced by hydrogenolysis of glycerol. The addition of a certain proportion of the chromium catalyst to the copper-based catalyst not only affects the state of the copper component, but also functions to stabilize the copper particles. The construction of copper-chromium catalyst has good selectivity for the preparation of 1,2-propanediol by hydrogenolysis of glycerol.

References

1. Lee, Hoseong. (2019). "Chromium catalysts for ethylenetrimerization/tetramerization functionalized with ortho-fluorinated arylphosphine ligand." Catalysis Communications 121, 15-18.
2. Zeng, Yanning. (2018). "Silsesquioxane-Supported Chromium Catalyst for Insight into Phillips-Type Ethylene Polymerization." Macromolecular Reaction Engineering 12(5), n/a.
3. Morris, Lilliana S. (2015). "Bimetallic Chromium Catalysts with Chain Transfer Agents: A Route to Isotactic Poly(propylene oxide)s with Narrow Dispersities." Angewandte Chemie, International Edition 57(20), 5731-5734.