Vanadium and niobium mixed-oxide catalysts obtained via sol-gel: preparation and catalytic behaviour in oxidative dehydrogenation of propane

摘要

The hydrolytic sol-gel method was adopted in this work to prepare Nb/V mixed oxides systems as potential catalysts for propane ODH. Several 1:1 Nb/V systems were prepared by varying sol-gel promoter and Nb precursor. It was found that the addition of citric acid as sol-gel promoter stabilises the solution of Nb and V precursors after the addition of water, favouring the direct formation of a gel. As a result, a homogeneous and well-interdispersed 1:1 Nb-V system was obtained by this method. In all the other cases, precipitation of a yellow solid occurred, leading after calcination to heterogeneous systems, in which the presence of segregated V_2O_5 was detected. Two systems with higher amount of Nb, 9:1 Nb/V and 4.5:1 Nb/V, were also prepared by hydrolytic sol-gel method. In both cases, the addition of citric acid was not necessary. Sols containing higher amounts of Nb precursor were stable and turned rapidly into gels. In both 9:1 Nb/V and 4.5:1 Nb/V samples, there was no evidence of crystalline phases containing V. Only the presence of crystalline Nb_2O_5 was detected by XRD. In the 4.5:1 Nb/V sample, however, XRD pattern suggested that some diffusion of V~(5+) ions had occurred into the structure of orthorhombic Nb_2O_5 phase, and Raman patterns evidenced the presence of microcrystalline Nb_(18)V_4O_(55). A Nb-V 'synergistic effect' in the ODH of propane seemed to characterise the 4.5:1 Nb/V sample, which showed both higher activity and selectivity to propene than the 1:1 Nb/V samples. The 9:1 Nb/V sample containing the lowest amount of V was found to behave similarly to pure Nb_2O_5, showing rather poor activity and high selectivity to propene, about 50%. Both activity and selectivity were found to increase only slightly with the 1:1 Nb/V (citric acid) system, containing V mainly as Nb-V mixed oxide. The presence of small amount of V-O-V bonds as crystalline V_2O_5 increased significantly the catalyst activity of precipitated 1:1 Nb/V samples.