Studies On V₂O₅/TiO₂ Catalysts For Ammoxidation Of 3-Picoline

Nicotinic acids are important organic intermediates,which are widely used in medicine,feedstuff additive,chemical assistant and so on.At present,the supply of nicotinic acids in the world market is insufficient, and this situation in China is more serious.Therefore,the production of nicotinic acids should be enhanced urgently.Nicotinic acids can be produced by ammoxidation of 3-picoline and then the produced nicotinonitrile is hydrolyzed into nicotinic acids.This process is most commonly used in the production of nicotinic acids.The most crucial step in the above-mentioned process is ammoxidation of 3-picoline,in which the development of a catalyst with high activity and selectivity is of utmost importance.V₂O₅/TiO₂ catalysts are good for ammoxidation of 3-picoline producing nicotinonitrile.In the current study,eight commercial titanium oxides(TiO₂)as supports were used to prepare a series of V₂O₅/TiO₂ catalysts by wet impregnation,and these catalysts were investigated in ammoxidation of 3-picoline.The main results in terms of catalyst characterization and performance are summarized as follows:(1)The 5 wt.%V₂O₅ loaded on TiO₂(Sigma)catalyst,compared to the catalysts with the same amount of V₂O₅ loaded on the other seven commercial TiO₂ supports,shows the best performance,with a selectivity of 90%for nicotinonitrile and a conversion of 93%for 3-picoline at a reaction temperature of 340℃.(2)When an amount of V₂O₅ loaded on TiO₂(Sigma)is 5 wt.%,in which V₂O₅ is highly dispersed on the support,the catalyst shows the best catalytic properties for ammoxidation of 3-picoline converted into 3-cyanopyridine.(3)The crystal type of TiO₂ affects the catalytic properties of V₂O₅/TiO₂ for the ammoxidation reaction,and the anatase type is better than the rutile one.One of possible reasons is that vanadium oxide can be dispersed better on an anatase support than a rutile one.Another reason could be related to the available acidic sites of the catalysts,i.e.a V₂O₅ supported on anatase catalyst is more acidic than that using a rutile TiO₂ support,resulting in a higher activities for the ammoxidation.(4)The H₂-TPR results indicate that the lower temperature(T_max)for the reduction of V₂O₅/TiO₂ catalysts,the higher conversion for 3-picoline.The selectivity for nicotinonitrile can be related to an H/V value in the reduction of the catalysts.When the H/V value is around 1,the selectivity for nicotinonitrile is higher,under which the V⁵⁺/V⁴⁺transformation during ammoxidation is more favorable than the V⁵⁺/V³⁺transformation.(5)The sulfate present in the support has a negative effect on the catalytic ammoxidation of 3-picoline. (6)When nano-sized TiO₂ powders with a high specific surface area are used as supports,the resulted catalysts show a high conversion of 3-picoline even at low temperatures,but a low selectivity for 3-cyanopyridine.A plausible reason is that V₂O₅ could be supported only on the external surface of a nano-sized TiO₂ particle, and the interection between active species and support becomes weaker,resulting in a high oxidation activity for 3-picoline and a low selectivity for 3-cyanopyridine.The current study also indicates that nano-TiO₂ particles with high BET surface area are not suitable to be used as catalyst supports for 3-picoline ammoxidation.