Study On Preparation Of VPO Catalyst For N-Butane To MA And Oxidation Mechanism

The technological route of selective catalytic oxidation of n-butane to maleic anhydride (MA) becomes preponderant process to produce MA because of its lower price of materials, higher theoretical yield and less environmental pollution. However, the concerned oxidation mechanism over VPO catalyst has not been well understood. Therefore, it is of theoretic and practical significance to study the preparation of VPO catalyst and the oxidation mechanism.In this work, the VPO catalyst and the Ce-promoted VPO catalyst were prepared, and the effects of reaction temperature and the concentration of the phosphoric acid on surface structure and catalytic performance of catalysts were investigated by means of determination of average valence state of vanadium, XRD, BET, pore size analysis and activity evaluation. It was indicated that the catalyst precursor prepared after reduction at 135℃consisted of VOHPO4·0.5H2O, which was converted mainly to (VO)2P2O7 and a littleβ-VOPO4 after the activation, showing better catalytic performance. The water in phosphoric acid could affect the structure of catalyst, and the catalyst prepared by 85% phosphoric acid showed better catalytic performance.The effects of the addition of Ce-promoter and the atomic ratio of Ce/V on the structure and performance of catalyst were studied by means of XRD and TPR-MS. It was showed that suitable atomic ratio of Ce/V was 0.06. The addition of Ce could adjust the proper ratio of V4+/V5+, thus increase the content of oxygen species and the catalytic activity to MA. Furthermore, the addition of Ce could prevent from the decomposition loss of phosphate, and thus improve the stability of catalyst. The influence of process condition on the improved catalyst was investigated, and the optimized process condition were determined as reaction temperature 440℃, initial concentration of n-butane 1.7% and space velocity 2500h-1. Under above condition, the C4H10 conversion was 77.7%, the MA selectivity was 67.4%, and the MA mass yield was 88.5wt%.The adsorption behavior of n-butane on the surface of catalysts was investigated by means of TPD-MS. It was showed that there existed only weaker physical adsorption of n-butane on surface. The oxidation mechanism of n-butane to MA was investigated by means of TPSR-MS. It was showed that different kinds of oxygen species could be formed during the reaction, and the lattice oxygen was the main active oxygen source for the selective oxidation to MA, the adsorbed oxygen species and the gaseous oxygen could create the deep oxidation of n-butane.Finally, the oxidation process of n-butane oxidation with the lattice oxygen in catalyst was explored in a micro fixed bed reactor. It was found that the selectivity to MA could be improved by the lattice oxygen oxidation apparently, when the conversion of n-butane was 15-20%, the MA selectivity was as high as 90%. The new idea about integration of n-butane oxidation to MA with the regeneration of lattice oxygen of catalyst in a fixed-bed was suggested.