添加Zr、Mo、Zn组份对VPO催化剂性能的影响

Effects of Zr,Mo and Zn Components on the Performances of VPO Catalysts

通过有机相中还原五价钒制得钒磷氧催化剂的前体 ,然后用浸渍的方法引入Zr、Mo、Zn复合组份 ,并借助机械球磨进一步提高样品的比表面积 .利用XRD、XPS、BET及X射线荧光分析等手段对催化剂的理化性质进行表征 .研究结果表明 ,助剂的引入和机械球磨没有明显影响催化剂的主要晶相组成和结构 ,但不同添加组分的组合对催化剂的粒子大小和比表面积有明显影响 ,从而对催化性能产生直接影响 .其中 ,加入Zr +Mo +Zn三组分助剂并结合球磨过程 ,能最大幅度地使顺酐的收率增加 10 %

VPO catalyst precursors were prepared in organic phase and added to with different combinations of Zr, Mo, and Zn additives by impregnation. Then ball milling process was applied to increase the surface areas during the preparation. Techniques of BET surface area determination, X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray fluorescence analysis were used to investigate the effect of combined additives on the bulk and surface structure of the VPO catalysts which were tested for the selective oxidation of n-butane to maleic anhydride. The results revealed that additives inflicted great influences on the catalyst surface areas and the conversions of n-butane accordingly. Combination of Zr+Mo+Zn increased catalyst surface areas and gave the highest conversions in reactions. And after ball milling process, all the additive combinations could improve the catalytic activity per surface area. It was also indicated that all catalysts showed crystalline phases were close to pure(VO) 2P 2O 7 or B phase which were, both active phases in the selective oxidation of n-butane to MA,with some minor peak drifts. But combined additives, especially Zn+Zr combination, could change the fine crytstalline structures, altering the peak shapes and the relative intensities. Additives did not alter the surface vanadium valence state that was close to +4, a kind of necessary vanadium valence in the catalytic reaction. But combined additives could influence the surface phosphorus/vanadium ratios during the ball milling process. The additives showed different distributions on the catalyst surface, with Zr showing the most enrichment and with Zn the least relative content. It was strange that Zn additive had the extremely low relative content on the catalyst surface and the mechanism was still unknown. Catalytic reaction data revealed that conversions of n-butane and selectivities of maleic anhydride were tightly connected with catalyst structures. Generally, surface area and crystalline phase were closely associated with catalytic converaion and selectivity respectively. However, various reaction conditions and different additives could even further perturb the seemingly simple connections. The ball milled catalysts with Zr+Mo+Zn displayed the yield of 46-mol%,which was pretty good at the selected reaction temperatures and gas hour spatial velocities. It was also reflected that ball milling might be an effective method to improve catalyst activity without destructing the active crystalline phase.