| Selective oxidation of low carbon alkane to olefins is the hot topic in the field of petrochemical industry and it will become one of the key points in the research and development in the new century.As a kind of important organic petrochemical raw materials,propene is widely used.Therefore,it is very promising to carry out oxidative dehydrogenation of propane to propene(ODHP).Currently,the main problem of this reaction is that the product propene is more active than propane and easy to suffer from oxidation.Therefore,how to design the catalysts with both high activity and high selectivity is the goal of the catalytic researchers.In recent years,the research of cerium-based catalysts has been attracted more and more attention.It has been reported that the cerium phosphate catalysts has an excellent performance for oxidative dehydrogenation of isobutane to isobutene,and our preliminary work also found that the cerium phosphate catalysts synthesized via sol-gel route exhibited an excellent performance for oxidative dehydrogenation of propane to propene.But the influence factors and mechanism of the cerium phosphate catalysts does not been studied deeply.Therefore,in this paper,with the cerium phosphate catalysts as research object and the oxidative dehydrogenation of propane to propene as target reaction.We studied the influences of the different preparation methods and pretreatment conditions on the structure and performance of the catalysts.The main results are as follows:(1)A series of the cerium phosphate catalysts were synthesized via sol-gel,coprecipitation,and hydrothermal routes.The effects of preparation methods on the performance of oxidative dehydrogenation of propane to propene were studied.The results show that different preparation methods of the cerium phosphate catalysts-have great influences on the ODHP performance.The catalyst prepared by sol-gel route has the best catalytic performance,the catalytic performances of cerium phosphate catalysts prepared by precipitation and hydrothermal routes are poor and the catalytic performance of commercial cerium phosphate catalysts is the lowest.XRD?BET?Raman?H2-TPR and TEM techniques were used to study the structure and properties of the catalysts.Based the results of characterization and catalytic performance,it is proposed that the content and state of surface cerium oxide species on cerium phosphate catalysts is closely related to the ODHP performance.The more cerium species on the surface are,the easier they are reduced and the better performance of the catalysts are.(2)The cerium phosphate catalysts synthesized via precipitation route was used as the main research object.The state of surface cerium oxide species were modulated by pretreating the cerium phosphate catalysts with different atmospheres(Ni,H2,Air).Then we studied the influences of the catalysts pretreated by different atmospheres on the ODHP performance.It is the first discover that the cerium phosphate catalysts pretreated by H2 has a significant impact on the catalytic performance.When the pretreatment temperature is 700 ?,the pretreatment time is 1 h,the conversion of propane is increased from the original 3.1%to 21.7%,and the yield of propene is also increased from 2.3%to 12.3%at 550?.(3)XRD?TEM?BET?H2-TPR?Py-IR and UV-vis techniques were used to study the structure and properties of the cerium phosphate catalysts pretreated by H2 and reoxidation.The results show that the cerium oxide species on the catalysts pretreated by H2 were reduced,and then been oxidized again in the reaction atmosphere.Among this reduction-reoxidation process,on one hand,the reducibility of the surface cerium oxygen species(?)is improved,on the other hand,some of the inert cerium oxygen species doped in the cerium phosphate catalysts are partially transformed into the highly active cerium oxygen species which are able to be reduced and reoxidized.The remaining is transformed into CePO4.Both of these are enhanced the catalytic performance of the cerium phosphate catalysts. |
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