An Improved Catalytic Cracking of n -hexane via Methanol Coupling Reaction Over HZSM-5 Zeolite Catalysts
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Catalysis Letters Vol. 106, Nos. 3–4, February 2006 (Ó 2006) DOI: 10.1007/s10562-005-9626-0
An improved catalytic cracking of n-hexane via methanol coupling reaction over HZSM-5 zeolite catalysts Fuxiang Changa,b, Yingxu Weia, Xianbin Liua,b, Yue Qia, Dazhi Zhanga,b, Yanli Hea, and Zhongmin Liua,* a
Natural Gas Utilization and Applied Catalysis Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, P.R. China b Beijing Graduate School of The Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, P.R. China
Received 6 October 2005; accepted 4 November 2005
The coupling transformation of n-hexane and methanol over HZSM-5 has been investigated with a pulse-reaction system. In the temperature range of 400–500 °C, kinetic data was collected and reaction order was determined. Compared with the pure n-hexane cracking, the increased rate constant and the lowered apparent activation energy clearly demonstrate an improvement of n-hexane activation using methanol as co-reactant and an increased contribution of faster bimolecular mechanism to the n-hexane transformation due to methanol introduction. Similarly, the results of coupling transformation performed over HZSM-5 with different Al content further confirm the transition between reaction mechanisms of n-hexane on account of the introduction of methanol. Moreover, the further investigation suggests that the enhancement of n-hexane activation and the change of reaction mechanism are attributed to the presence of intermediate species evolved from methanol. Thus, a proposed reaction pathway of n-hexane activation with methanol as co-reactant was put forward. KEY WORDS: coupling transformation; mechanism; pulse-reaction; activation energy; Si/Al ratio.
1. Introduction Hydrocarbon cracking is one of the most important processes for light olefins production in petrochemical industry. The disadvantage of this route is its high endothermicity, which makes it a very energy consuming process. Some researchers have studied on catalyst development for high-efficient transformation and less energy cost, while some efforts are also put on alternative way, such as, introducing some exothermic conversion processes for energy supply into the endothermic hydrocarbon cracking. Considering the energy balance and target products, exothermic MTO/MTG process is a good option for this coupling system. Lu¨cke and co-workers [1] investigated the coupling transformation of some hydrocarbons with methanol participation. A high olefins yield up to 1000 g kg)1h)1 in the temperature range of 600–700 °C in a nearly thermo-neutral condition was obtained, and the deactivation behavior of different modification HZSM-5 catalyst was also discussed. Gao and co-workers [2] investigated the coupled conversion of methanol and light hydrocarbons over Ga/HZSM-5 catalyst at moderate temperature (99.996%) of an appropriate flow rate through a saturator containing n-hexane at proper temperature. This stream was then mixed with the methanol stream with desi
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