The Design And Synthesis Of The Model System For Studying The Active Sites On Chromium Based Catalysts

Light alkane dehydrogenation technology has been paid attention by the researchers for many years as a new method to produce olefin.There are two main catalysts used as industrial dehydrogenation catalysts,the Pt-Sn catalysts and Cr/Al2O₃ catalysts.The former has very complex production process moreover high cost of the active component,While the latter is easy to lose activity needed to be regenerated frequently.To get the rational designed Cr-supported catalysts with high performance,it is needed to have a good understanding of the structure and the mechanism of the reaction of the Cr components as the active sites.There are many reasons why we only have a vague comprehension of the structure of the Cr active sites.The most important one is the complexity of the support.Try to study the different distribution of the Cr components using the ordered zeolite pores under the different preparation methods and to find the relationship of the reaction of alkane dehydrogenation with the Cr distribution.NaY zeolite was used as the supporter.The Cr-supported catalysts were prepared by the ion exchanged and impregnated methods.The p H value of the Cr?NO₃?₃ solution is a main factor to affect the crystallinity of the zeolite.The crystallinity of the exchanged product was low under the condition of uncontrolled pH value.When the pH of the solution was controlled,the crystallinty was obviously increased.Moreover,extending the time of maintained pH could improve the crystallinity of the exchanged product.For the different exchanged condition,it is found that the concentration of the Cr?NO₃?₃ and the exchanged times also affected the zeolite crystallinity.It reduced with the increasing of the Cr?NO₃?₃ concentration and the exchanged times.An initial exploration had been taken using the methods of characterization,such as BET?XRD?IR,and so on.The results showed that the single site catalyst can be got in some appropriate conditions,the active components can be dispersed highly in these situations which can make them perform high activity.This might be connected with the concentration of Cr6+.