The effect of sol-gel promoters on the characteristics of mixed V-Nb oxides and their catalytic properties in propane oxidative dehydrogenation

Hydrolytic sol–gel procedures are implemented for the preparation of mixed Nb–V oxide systems, the main objective being the development of potential catalysts for the oxidative dehydrogenation (ODH) of propane. The effects of various Nb precursors (alkoxide or chloride), different acid promoters and/or complexing agents (HCl, HNO3, oxalic acid and citric acid) and the influence of the Nb/V ratio on the hydrolytic sol–gel preparation are investigated. Several 1:1 Nb/V systems are prepared. It is found that the addition of citric acid as complexing agent stabilises the solution of Nb and V precursors after the addition of water, favouring the direct formation of a gel. A homogeneous and well-interdispersed 1:1 Nb–V system is obtained by this method. In all the other cases, precipitation of a yellow solid occurs, leading after calcination to heterogeneous systems, in which the presence of segregated V2O5 is detected. The hydrolytic preparation of systems with higher amount of Nb, 9:1 Nb/V and 4.5:1 Nb/V, does not require the addition of a complexing agent like citric acid. Sols containing higher amounts of Nb precursor are stable and turn rapidly into gels. In both 9:1 Nb/V and 4.5:1 Nb/V samples, there is no evidence of crystalline phases containing vanadium. Only the presence of crystalline Nb2O5 is detected by XRD. The effects of sol–gel additives and niobium amount on the morphological and catalytic properties are investigated. The 4.5: