Application Of Supported Ga 2 3 Catalyst In Ethane Dehydrogenation | Posted on:2010-07-14 | Degree:Master | Type:Thesis | Country:China | Candidate:Z H Shen | Full Text:PDF | GTID:2271360275491307 | Subject:Physical chemistry | Abstract/Summary: | PDF Full Text Request | 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. | Keywords/Search Tags: | supported Ga2O3 catalysts, dehydrogenation of ethane, activity, stability, mechanism, Ga2O3/HZSM-5, alkali treatment, effect of CO2, meso-Al2O3 support | PDF Full Text Request | Related items |
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