Selective Catalytic Cracking Of Ethylbenzene In C₈ Aromatics Over Nano-HZSM-5 Zeolite

Ethylbenzene in the mixture of C₈ aromatics can be cracked into benzene and ethylene. This is an attractive reaction for the processing of mixied C₈ aromatics. In this study the catalytic cracking of the mixed C₈ aromatics in the presence of hydrogen gas has been studied. The following results are obtained:(1) The reactions of ethylbenzene, para-xylene, meta-xylene and ortho-xylene over nano-HZSM-5 zeolite indicate that, the major transformation paths of the C₈ aromatic isomers are disproportionation, isomerization and dealkylation. Ethylbenzene is more active than other isomers. High temperature favors the dealkylation of C₈ aromatics. The dealkylation of ethylbenzene occurs at a temperature of at least 480°C.(2) The catalytic performance of nano-HZSM-5 and that of micro-HZSM-5 are comparatively studied. Besides, the effect of beta zeolite adulteration on the catalytic performance of nano-HZSM-5 zeolitic catalyst in the cracking of mixed C₈ aromatics is also investigated. Results show that, the catalytic activity and the ethylene selectivity of nano-HZSM-5 zeolite in the dealkylation of ethylbenzene are much higher than that of micro-HZSM-5. The adulteration of beta zeolite in proper amount favors the activity and durability of the resulted catalyst.(3) The effect of zeolitic acidity on the performance of the catalyst was studied by using steaming and sodium ion impregnation modifications. Results indicate that the acidity of the catalyst is a crucial factor to the production of ethylene from ethylbenzene cracking. In the condition of many strong acid sites existing on the surface of the catalyst, ethylene generated from the cracking of ethylbenzene will be transformed into methane, ethane, propane, etc., via the by-reactions of oligomerization, cracking of the oligomers, and hydride-shift, etc. When the strong acid sites of the catalyst are reduced by sodium cations, the cracking of ethylbenzene can yield as high as 70-80 % ethylene selectivity.(4) The reaction paths of ethylbenzene on alumina supported Mo-Ni, nano-HZSM-5 (NaHZSM-5) zeolite supported Mo-Ni, and NaHZSM-5 catalysts were comparatively studied. Results show that, on the alumina supported Mo-Ni catalyst ethylbenzene is consumed via hydrogenolysis giving methane, ethane, benzene and toluene. On HZSM-5 (NaHZSM-5) catalyst, on the other hand, ethylbenzene is transformed via acid-catalyzed cracking giving ethylene and benzene. However, on the nano-HZSM-5 supported Mo-Ni catalyst reactions of both hydrogenolysis and acid-catalyzed cracking take place.