Effect of Mo on the Active Sites of VPO Catalysts upon the Selective Oxidation of n-Butane

Abstract

The effect of the addition of Mo to VPO formulations on the physicochemical and catalytic properties of VPO solids was studied using X-ray diffraction (XRD), Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, Laser Raman spectroscopy (LRS), temperature-programmed reduction, and a flow reactor system. The addition of Mo to the oxides increases the activity and selectivity of the VPO catalysts. The promoting effect is a function of both the Mo loading and the way such cation was added to the VPO matrix. The best catalyst was obtained when 1% Mo was impregnated on the VOHPO4·0.5H2O phase. At 400°C 36% of molar yield to maleic anhydride was obtained in this catalyst against 12% of the unpromoted catalysts and only 3% of the solids where Mo was added during the phosphatation step. The impregnated 1% Mo catalyst achieved a molar yield of 50% after 700 h under reaction stream (equilibrated catalysts). (VO)2P2O7 was the only phase detected by XRD and LRS in all the catalysts studied. They showed comparable BET surface areas and crystallinity after 400 h under reaction conditions. A local order distortion of the O3–P–O–P–O3 structure was detected by LRS in the impregnated Mo VPO catalysts. After 400 h on stream, both promoted and unpromoted solids only showed VIV on the surface layer. The main effect on the addition of Mo by impregnation to VPO oxides was enhanced by the very strong Lewis acid sites and the liability of the oxygen of (VO)2P2O7. This suggests that the promoting effect is more electronic in nature than structural. Polymeric MO3 species were detected neither by TPR nor by LRS. All the promoted catalysts presented a surface molybdenum enrichment but whereas the coprecipitated Mo VPO solid only shows surface MoVI, both MoVI and MoIV coexist in the impregnated catalyst. The solid with the highest yield of maleic anhydride also has the highest surface MoIV/MoVI ratio.