Cracking of pentenes to C2-C4 light olefins over zeolites and zeotypes -: Role of topology and acid site strength and concentration

The cracking of 2-methyl-2-butene was canied out as a model reaction Of C-4(+) olefins cracking by using of various zeolite and zeotype catalysts to obtain light olefins, such as ethylene and propylene. Only ZSM-5 and ZSM-11 zeolites with 10-MRP with intersecting channels and acid sites concentration ranging between 0.03 and 0.1 mmol/g exhibit high and stable selectivity to C-2-C-4 olefins at high conversion level. Active and selective catalysts were prepared by three different methods: direct synthesis of H-ZSM-5 with high Si/Al ratio, steaming of HZSM-5 and partial ammonium ion exchange of Na-ZSM-5. Neither H-(AI)ZSM-5 with high concentration of acid sites nor H-(Fe)ZSM-5 are suitable catalysts as they catalyze hydrogen-transfer reactions yielding paraffins and aromatics, showing the importance of low concentration and high strength of acid sites for cracking activity-selectivity. High yield of butenes in the reaction products indicates that pentenes cracking in 10-MRP proceeds via formation of voluminous carbeniurn ion intermediates demanding sufficient space inside of zeolites channel. Therefore, the presence of intersection of 10-MRP is very important. While in ZSM-5 and ZSM-11 zeolites the low amount of active sites mainly situated in the pores intersection is sufficient to achieve high activity-selectivity in the case of mono-dimensional 12-MRP zeolites such as mordenite and ZSM-12 and also with 10-MRP ferrierite much higher total acid concentration is necessary, as vast of the acid sites, situated out of the main channel do not take part in the reaction. AIPO-11 materials with narrow mono-dimensional channel exhibit low conversion with majority of beta-scission cracking mechanism independently on the acid sites concentration. (c) 2005 Elsevier B.V. All rights reserved.