Effect Of Pore Structure And Acidity Of Zeolite On The Cracking Of LPG

The catalytic cracking of liquified petroleum gas (LPG)for propylene production is studied in a small fixed-bed reactor. By using catalyst characterization means such as XRD,XRF,TG,FT-IR and NH₃-TPD the following results are obtained:(1) Zeolite T, Mordinite, ZSM-5 and BETA/ZSM-5, with different pore structure, exhibit different catalytic activity, selectivity and durability in the reractions of C4 LPG cracking. Zeolite T has the highest selectivity towards propylene owing to the strong shape selectivity of its 8-ring small zeolitic pores. It is believed that the 8-ring small pore zeolites will be useful in the production of propylene from the cracking of C4 LPG. However, zeolite T is suffered from the poor catalytic activity and fast deactivation. Comparatively, ZSM-5 zeolite shows very good comprehensive catalytic performance in the cracking of C4 LPG for its unique 10-ring middle pore system. Thus, catalyst based on ZSM-5 zeolite is hopful.(2) By using the sodium ion impregnation to adjust the acidity of a nano-sized HZSM-5 zeolite, we find that, when catalyst has a large amount of strong acid sites, the propylene selectivity of the C4 LPG cracking would be decreased by the side-reactions of H-shift and aromatization. On the other hand, when catalyst has only weak acid sites the formation of C5+ non-aromatics would be the main reason of low propylene selectivity. In the condition of eliminating all the strong acid sites on the surface of nano-HZSM-5, the cracking of C4 LPG occurs only to butanes, which gives the highest propylene selectivity of about 60% (the selectivity to both propylene and ethylene is about 80%).(3) Modifying the nano-HZSM-5 zeolite with steaming, and the imprenation of different metal ions has been studied. It is found that, all the modifications applied enhance the activity stability and the propylene selectivity of the catalyst to some degrees. However, the so-far obtained catalysts are still suffering from low propylene yield and fast deactivition. To develop a desired catalyst for the production of propylene from the cracking of C4 LPG is still a big challenge.