Application Of Supported Ga ₂ 3 Catalyst In Ethane Dehydrogenation

Dehydrogenation of ethane over various supported Ga₂O₃ catalysts,as well asβ-Ga₂O₃,was investigated.During the investigation,three key problems have been raised and discussed:1.Stability of catalysts,2.Reaction mechanism and the effect of CO₂,3.The balance of energy（heat）.Firstly,various supports for the Ga₂O₃-based catalysts have been investigated in dehydrogenation of ethane.Bothβ-Ga₂O₃ and Ga₂O₃/TiO₂ have high initial ethylene yield.Their stabilities are,however,very poor.The high ethylene yield overβ-Ga₂O₃ and Ga₂O₃/TiO₂ can not be sustained for more than 5h on-stream.The rapid deactivation ofβ-Ga₂O₃ on-stream is caused by coking on its small surface area.In comparison,Ga₂O₃/HZSM-5,Ga₂O₃/Al₂O₃ and Ga₂O₃/SiO₂ have better stability in dehydrogenation of ethane..In order to further improve the stability and ethylene selectivity,the Ga₂O₃/HZSM-5 catalysts with different Si/Al molar ratio were investigated.The results of NH₃-TPD and FT-IR spectra of adsorbed pyridine revealed that the enhancement of catalyst stability with increasing Si/Al ratio of the support is caused by the decrease of the acidity of the catalysts,resulting in the suppression of the side reactions,such as cycling and oligomerization.The Ga₂O₃/HZSM-5（97） catalyst exhibits both high activity and stability for dehydrogenation of ethane,with an ethane conversion of 15%and ethylene selectivity of 94%without any observable trend of deactivation in 70h.The dehydrogenation reaction over Ga₂O₃/HZSM-5 is suggested to proceed through a heterolytic dissociation reaction pathway.The activities of the Ga₂O₃/HZSM-5 catalysts are enhanced by BrΦnsted acid sites（mainly medium-strong acid sites） on the HZSM5 supports.Additional meso-pores in the Ga₂O₃/HZSM-5 catalyst can be created by the alkali treatment of the support.The amount of medium-strong acid sites can also be decreased by the alkali treatment.The alkali-treated Ga₂O₃/HZSM-5 catalyst which contains meso pores is not easily deactivated by coking,which could be attributed to the better diffusion properties.Compared with the pure dehydrogenation of ethane,the addition of CO₂ and a little O₂ into the reaction atmosphere can improve both the activity and the stability of β-Ga₂O₃ and Ga₂O₃/HZSM-5.For Ga₂O₃/HZSM-5,when the CO₂ concentration is low,the activity is improved mainly via reverse gas-water shift reaction;when the CO₂ concentration is high,the activity is suppressed mainly via the competition adsorption between CO₂ and ethane.The results of XPS and TGA indicated that the improvement of the stability of Ga₂O₃/HZSM-5 by CO₂ could be attributed to the supplement of lattice oxygen and the suppression of coking.The msu-γ-Al₂O₃ meso-pore material is synthesized via a self-assembly method with a triblock poly（ethylene oxide） surfactant.Ga₂O₃/msu-γ-Al₂O₃ catalyst was then prepared by impregnation.Meanwhile,the Ga-Al-msu-γmeso-pore catalyst is prepared via a co-precipitation method.All the Ga₂O₃/Al₂O₃ catalysts exhibit good stabilities on-stream for ethane dehydrogenation.Compared with the conventional Ga₂O₃/Al₂O₃ catalysts,the meso-pore catalysts,including Ga₂O₃/msu-γ-Al₂O₃ and Ga-Al-msu-γ,show better steady-state ethylene yield,which could be attributed to their meso-pore structure and the enhanced surface acid strength.