A substance containing titanium while performing a catalytic function is referred to as a titanium catalyst. Titanium, a silver-white metal, is one of the transition metals. Dense titanium metal is quite stable in nature, but powdered titanium can cause spontaneous combustion in air. Titanium reacts with many elements and compounds at higher temperatures. Titanium belongs to rare metals, but in fact, titanium is not rare, the crust content of which is much higher than many common metals. The main valence of titanium is +4, and the valence of titanium in most titanium catalysts is +4.
Titanium catalysts have the advantages of being environmentally-friendly, and exhibiting excellent catalytic performance in some reactions, and thus play a role in the field of organic catalysis and environmental protection.
- Asymmetric epoxidation: Asymmetric oxidation of thioethers can convert latent thioethers to chiral sulfoxide compounds. Chiral sulfoxide is an important chiral auxiliary and is widely used in asymmetric synthesis, including asymmetric Michael addition reactions, C-C bond formation reactions, carbonyl reduction reactions, Diels-Alder reactions, and free radical addition reaction, etc. The chiral titanium complex catalyst system has a good catalytic effect on the asymmetric oxidation of thioether, and can asymmetrically oxidize a series of thioethers and achieve high enantioselectivity. Commonly used chiral metal titanium catalyst systems include diol-titanium complexes with a chiral center, chiral triolamine-titanium complexes, salen-titanium complexes, and chiral Schiff base-titanium complexes, etc. For example, a complex formed by a chiral oxazoline ligand and Ti(O-i-Pr)4 can catalyze the asymmetric oxidation of p-tolylmethyl sulfide.
Figure 1. Titanium catalyst-catalyzed asymmetric epoxidation
- Reductive coupling reaction: The reductive coupling reaction, also known as the McMurry reaction, can be used to reduce various compounds such as alkenes, alkynes, and heterocycles. The titanium metal obtained by the reduction of TiCl3 or TiCl4 has a unique catalytic effect on the reduction coupling reaction, thus is often used as a catalyst for the reduction coupling reaction. The common metal-titanium-catalyzed reductive coupling reaction mainly occurs in the intermolecular or intramolecular manner of a compound containing a carbonyl group, a nitrile group or a nitro group.
Figure 2. Titanium catalyst-catalyzed reductive coupling reaction
- Esterification reaction: As one of the esterification reaction catalysts, the titanium catalyst is widely used in the esterification reaction. Compared with the antimony catalyst, the titanium catalyst has the advantages of high catalytic activity, low catalyst dosage and short reaction time. while compared with the germanium catalyst, the titanium catalyst is more environment-friendly. Titanium catalysts are commonly used to catalyze the synthesis of various esters and polyesters. For example, polyethylene terephthalate (PET) is a kind of synthetic material with a large output in polyester products, and is applied to many fields such as fibers, films, and containers. Glycol titanium can catalyze the esterification and polycondensation of terephthalic acid (PTA) and ethylene glycol (EG) to form polyethylene terephthalate.
Figure 3. Titanium catalyst-catalyzed esterification reaction
- Environmental protection: Titanium catalysts also play a large role in environmental protection. Dioxins are persistent organic polluants (POPs) with properties that are difficult to decompose, easy to accumulate, and highly toxic. In the selective catalytic reduction (SCR), the use of titanium dioxide as a catalyst can completely decompose the dioxin and generate non-toxic substances to the environment. In addition, a vanadium-titanium catalyst can be prepared using vanadium and titanium. The catalyst can be used as a catalyst for selective catalytic reduction to achieve flue gas denitration.
- Kaur, Navjeet. (2019), “Application of titanium catalysts for the syntheses of heterocycles.” Synthetic Communications 49(15), 1847-1894.
- Gilja, Vanja. (2018), “Influence of titanium dioxide preparation method on photocatalytic degradation of organic dyes.” Croatica Chemica Acta 91(3), 1-12.
- Rohjans, Stefan H. (2010), “Titanium Catalysts with Linked Indenyl-Amido Ligands for Hydroamination and Hydroaminoalkylation Reactions.” Organometallics 37(23), 4350-4357.
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