Hafnium Catalysts

The hafnium catalyst refers to the hafnium compound which has a catalytic function. Hafnium is a shiny silver-gray transition metal. The chemical properties of hafnium are relatively stable and have good corrosion resistance. It is not easily eroded by common aqueous acid and alkali solutions. At high temperatures, hafnium can also be directly combined with gases such as oxygen and nitrogen to form oxides and nitrides. The hafnium has a small content in the earth's crust and often coexists with zirconium. The valence of the hafnium is usually +4 in the hafnium compound. The common compounds containing hafnium are hafnium oxide(HfO₂), hafnium tetrachloride(HfCl₄) and hafnium hydroxide (H₄HfO₄). These hafnium compounds are also commonly used as hafnium catalysts. Hafnium and some ligands can form hafnium complexes with catalytic functions, and these complexes can also be used as ruthenium catalysts.

Figure 1. Hafnium catalyst catalyzed condensation reaction

Applications:

Compared with other transition metal catalysts, hafnium catalysts are abundant in variety. At the same time, hafnium catalysts have a good and unique catalytic effect in some reactions. Therefore, hafnium catalysts are widely used in organic synthesis.

• Polymerization: The monoolefin polymerization is an important method for industrial production of olefin copolymers. The use of hafnium catalysts in polymerization is numerous. For example, an ethylene bis(indenyl)diamined hafnium (rac-C₂H₄(Ind)₂Hf(NMe₂)₂) can be used as a catalyst to catalyze the random copolymerization of ethylene with 1-octene. In addition, the direct copolymerization of olefins with polar functional group-containing monomers to produce polyolefins has high requirements on the stereoselectivity of the catalysts, and some hafnium complexes have unique catalytic effects in such polymerizations. For example, by using a dimethylpyridinium hafnium complex as a catalyst, the stereospecific copolymerization of propylene with a terminal halogenated-olefin can be achieved, and isotactic propylene with high molecular weight, high stereoregularity, and high polar monomer insertion rate can be obtained.
• Oxidation reaction: Hafnium catalysts can be used as catalysts for certain oxidation reactions. The pseudo-dicarboxylic acid (PTA) is the main raw material for the polyester industry. Industrially, the PTA is typically produced by the oxidation of p-xylene (PX) by liquid phase oxidation in an acetic acid solution. In the acetic acid reaction system, Hf⁴⁺ has a strong Lewis acidity, so that its catalytic activity is also enhanced. Active intermediates such as peroxides or peroxidic free radicals formed during the reaction easily enter the hafnium coordination sphere to participate in coordination. Under the action of metal ion absorption, the active intermediate product after coordination is prone to rapid intramolecular or intermolecular electron transfer reaction, thereby accelerating the entire reaction process. Therefore, by using a hafnium compound (for example, hafnium acetate or hafnium bromide) as a cocatalyst, the reactivity of the main reaction can be increased, and side reactions can be suppressed.
• Friedel-Crafts acylation: Hafnium catalysts have many applications in the Friedel-Crafts acylation. For example, the hafnium triflate is used to catalyze the Friedel-Crafts acylation of benzoyl chloride and an aromatic hydrocarbon. The fluoroalkylsulfonylamino hafnium can catalyze the Friedel-Crafts acylation of an acid anhydride in a two-phase system. In general, the hafnium catalyst for catalyzing the Friedel-Crafts acylation is easy to separate after the end of the reaction and the catalytic activity is not lowered, and can be recycled and reused.

Figure 2. Hafnium catalyst catalyzed Friedel-Crafts acylation

• Other reactions: In addition to the reactions mentioned above, hafnium catalysts can catalyze many other organic reactions. Hafnium chloride can catalyze the direct condensation of a carboxylic acid and an alcohol to obtain an ester compound. Hafnium chloride can catalyze the Diels-Alder reaction of allyl carboxylate and furan. It can also catalyze the benzylation of aromatic hydrocarbons and heterocyclic aromatic hydrocarbons.

References

1. Yang, Bo. (2015). "Hafnium catalysts for direct alkene epoxidation using molecular oxygen as oxidant." RSC Advances 5(16), 12311-12322.
2. Wang, Xiao-Yan. (2014), "Insights into propylene/ω-halo-α-alkenes copolymerization promoted by rac-Et(Ind)2ZrCl2 and (pyridyl-amido) hafniumcatalysts." Journal of Polymer Science, Part A: Polymer Chemistry 52(23), 3421-3428.
3. Hanawa, Hideo; Kii. (2018), "Bis-zirconium and bis-hafnium catalysts for the strong activation of carbonyl substrates. [Erratum to document cited." Tetrahedron Letters (2002), 43(2), 343.