Propene-producing Via Catalytic Cracking Of Butene Over Nano-sized ZSM-5 Zeolite

Propene is one of the fastest growing petrochemicals, driven primarily by the highgrowth rate of polypropene, propane oxide and acrylnitrile. There has been an increasinginterest in recent years in catalytic cracking of mixed C4 alkene to produce propylene andethylene over zeolite catalysts.Among numerous catalysts which are used in such a process, the ZSM-5 zeolite showeddistinctive potential owing to its high activity, special aperture structure, large surface area, and excellent thermal stability. Due to the relatively strong acdity and large crystal size, muchside products and poor reaction stability in the catalytic cracking of olefins to propene andethene are performed on commercialized conventional ZSM-5 zeolites. Much effort has beenperformed on modifications of the conventional ZSM-5 zeolites in the past years, few reportsdeals with nano-sized ZSM-5 zelites in the field of lower alkene catalytic cracking topropylene, however.In this paper, the catalytic performances of ZSM-5 catalysts with different crystal sizesare investigated in a small fixed bed reactor. The results indicated that the H-type nano-sizedZSM-5(noted as H-ZSM-5(DNZ)) catalyst has better reaction stability, lower velocity of cokeformation and larger coke accommodation ability than that of micro-sized ones. The reactionconditions was also studied on H-ZSM-5(DNZ) catalyst, it was found that relatively highertemperature and space velocity is beneficial for the formation of propene and ethene, but ifthese two values too high, the reaction stability tends to decline. The butene content in LPGalso has an important effect on the reaction performance, when it is too high, the reactionstability also tends to be low.The species of the diluent gases (i.e. H₂O, N₂ and H₂) and diluent ratio were alsoinvestigated on a steaming treated nano-sized catalyst, i.e. St-ZSM-5(DNZ) in the reactionsystem. The catalytic performances of the catalysts are all improved when using diluent gases.Among the diluent gases, the best performance is exibited by using H₂, for its obvious effectsin enhancing the yield of propylene, reducing coke formation and improving the reactionstability. The ratio of diluent gas and LPG also has an important effect on the reaction ofbutene catalytic cracking. Realitively higher diluent ratio is profitable for propene synthensisfor its inhibitability to the side reactions, but when the ratio is too high, it may result in lower butene conversion and propene yield. The best weight ratio of water to LPG is 0.8, while thebest molar ratio of H₂ to LPG is 0.25.The modification studies were also performed on H-ZSM-5(DNZ) catalyst. Steamingtemperature has an obvious effect on zeolites acidity, structure and also their catalyticperfomances. Relatively higher steaming treatment temperature can improve the stability ofcatalysts, and also enhance the selectivity and yield of propene and ethene. It is found that600-650℃is the best steaming temperature.Besides steaming treatments, the effect of phosphorus and metals modification onH-ZSM-5(DNZ) was also concerned. The results indicated that P modification bring aboutobvious decrease of the acid amounts on the zeolite and higher propene yield. Themodification of Mo on H-ZSM-5(DNZ) also reduces the acid amounts of the zeolite.Although the yield of propene is enhanced on Mo-ZSM-5(DNZ), the reaction stability is poor.By using Fe modification on another nano-sized high silica zeolite H-ZSM-5(HSiN), not onlythe yield of propene but also the stability of the catalyst can be improved, owing to animproved activity for butane cracking.Finally, the stability of St-ZSM-5(DNZ) with H₂ as diluent gas was examined. Theresults indicated that this catalyst has high propene yield and (propene+ethene) selectivity andalso rather good reaction stability and regeneration performance. At T=500℃, WHSV=3.5h^-1, p=0.1 MPa, n(H₂/LPG)=0.25, during the TOS of 1000 h, the propene yield almostmaintained at about 35ï¼…. The catalyst after regeneration for five times showed almost thesame good catalytic performance as the fresh one in the long time run of 1000 h.