Niobium Catalysts

The niobium catalyst refers to niobium compound which has a catalytic function. The niobium is a shiny gray metal that has paramagnetism. Niobium is stable in air at room temperature and is not completely oxidized when heated in oxygen. At high temperature, niobium is directly combined with sulfur, nitrogen and carbon to form corresponding compounds. Niobium can form various compounds with valence from +5 to −1, but the most common valence of niobium is +5. Niobium oxide (Nb₂O₅), chloride (NbCl₅) and acidifier (NbOPO₄. nH₂O) have different catalytic activities and can catalyze a variety of organic reactions.

Applications:

As a new type of catalytic material, niobium catalyst has good catalytic effect in many organic reactions due to its strong acidity, oxidation reduction and hydrophobic properties. Therefore, niobium is widely used in the field of organic synthesis.

• Friedel-Crafts reaction: Friedel-Crafts reaction is an important acid catalyzed reaction, which can be divided into alkylation reaction and acylation reaction. Lewis and proton acids are often used as conventional catalysts for such reactions. Niobium catalyst overcomes the shortcomings of traditional acid catalyst, has acid and high hydrothermal stability and no corrosion to equipment, and meets the requirements of green catalysis. Therefore, it can be used to catalyze Friedel-Crafts reaction. For example, NbCl₅ can catalyze the acylation reaction of alpha-dimethoxybenzene and acetic acid in friedel-crafts under mild reaction conditions, so as to obtain the acylation products with high yield. P₂O₅-WO₃-Nb₂O₅ has a high catalytic activity and conversion rate. It can catalyze the Friedel -crafts alkylation reaction of phenyl ether and benzyl alcohol to obtain high yield products, and the catalyst can be used repeatedly.

Figure 1. Niobium catalyst catalyzes Friedel-Crafts reaction

• Addition reaction: Sakurai reaction, Mannich reaction and Knoevenagel condensation reaction are important methods to form C-C bonds. Niobium compounds as catalysts can catalyze these addition reactions. For example, niobium pentachloride as a niobium catalyst can promote the Sakurai reaction between aldehydes and allyl silane. The reaction time is short and the yield is high. At room temperature, using ethanol as solvent, niobium pentachloride can catalyze the Mannich reaction of acetophenone, aromatic aldehyde and aromatic amine to produce the intermediates of various drugs and natural products like β-aminoketone and β-aminocarbonyl compounds. The reaction products of Knoevenagel condensation reaction are widely used to prepare many natural products, cosmetics, perfumes and medicines. Under the condition of heating and catalyzed by niobium pentachloride, the aldehyde and the active methylene compound undergo the condensation reaction of Knoevenagel to form the e-alkene. This reaction has high yield and mild reaction conditions, and is suitable for the reaction of various aldehydes and active methylene compounds. In addition to the above reactions, niobium catalyst can also catalyze Diels-Alder cycloaddition reaction, silcyano reaction of carbonyl compounds and synthesis of alpha-aminophenate derivatives.

Figure 2. Niobium catalyst catalyzes addition reaction

• Substitution reaction: Under the catalysis of niobium pentachloride, aryl alcohols and some heteratomic compounds can undergo nucleophilic substitution reactions. The reaction generally has the advantages of high yield, short reaction time, mild conditions and few by-products. Diindolemethane can promote the metabolism of estrogen in human body and has remarkable physiological activity, and it is widely used in drug design. Under the catalysis of niobium pentachloride, the electrophilic substitution reaction between indoles and aromatic aldehydes can take place under the condition of 90 ℃ and with no solvent.
• Cyclization reaction: Benzodiazepine is an important compound widely used in pharmacy, industry and synthesis. Under the catalysis of niobium pentachloride, o-phenylenediamine and ketone can generate benzodiazepine by ring formation reaction. Quinoxoline derivatives are important heterocyclic compounds and intermediates in organic synthesis. Niobium pentachloride can catalyze the reaction of o-phenylenediamine with 1, 2-diketone in acetonitrile solution to produce quinoxoline and its derivatives.

References

1. Das, Lalitendu. (2017). "Oxidative depolymerization of lignin using supported niobium catalysts." ChemEngineering 1(2), 17/1-17/13.
2. Xiang, Wendi. (2018), "Catalysts promoted with niobium oxide for air pollution abatement." Catalysts 7(5), 144/1-144/13.
3. Satoh, Yasushi. (2011), "Active Low-Valent Niobium Catalysts from NbCl5 and Hydrosilanes for Selective Intermolecular Cycloadditions." Journal of Organic Chemistry 76(20), 8569-8573.