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Preparation, Characterization and Catalytic Performance of HPA/Nanocrystalline HZSM-5 Complex Solid Acidic Catalysts

Author: ChenLiDong
Tutor: WangXiangSheng
School: Dalian University of Technology
Course: Industrial Catalysis
Keywords: Heteropoly acids Nanocrystalline HZSM-5 zeolite in situ Ecaging n-Octane Hydroconversion Dehydration of methanol Dimethyl ether
CLC: O643.36
Type: PhD thesis
Year: 2008
Downloads: 697
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Abstract


The catalysis of heteropoly acids （HPA） in solid state has received much attention owing to their unique acidic and redox activities. Nanocrystalline HZSM-5 zeolites （nano HZSM-5） are thought to be an ideal material for the encapsulation of HPA molecules having a Keggin structure since they have the intergranular secondary pore. Nevertheless, the use of HPA/nano-HZSM-5 as catalyst has not been reported.In this paper, PMo and Ni （II） were loaded in secondary pores from inter crystal gap of nano HZSM-5. Ni-PMo/nano HZSM-5 complex solid acidic catalysts were prepared by impregnation technique. PMo has been decomposed a little due to the deeper interaction with basic extra-framework Al-O species, however, the Keggin structure is essentially preserved on the support. Furthermore, PMo is highly dispersed in the nano HZSM-5, as well as Ni. The concentration of Bronsted and Lewis acid sites is obviously augmented and the Bronsted-to-Total ratios are also improved for the NiPMo-ZSM-5 catalyst.The PMo and Ni in situ encaged in the secondary pore of nano HZSM-5 were prepared. The 31P NMR and FT-IR spectra of catalyst showed that the Keggin structure is essentially preserved on the support after in situ encaging. PMo cannot enter the interior pores of the zeolite, but becomes encaged in the secondary pore of nano HZSM-5. The existent value of Ni is corroborated, especially, the spectra of encaged PMo-Ni and the impregnated PMo-Ni catalysts show the formation of Ni2+, the Ni0 signal can not observed at signals.The hydroconversion of n-octane over various catalysts was investigated. It is concluded that conversion of n-octane, the stability of aromatization is improved, and the yield of light iso-alkanes is enhanced due to the introduction of PMo and Ni. The effects of acidity, WSHVand temperature on the activity of n-octane hydroconversion are demonstrated. The improved activity of n-octane hydroconversion over PMo and Ni in situ encaged in the secondary pores of nano HZSM-5 can be attributed to the increase of acidic concentration and the synergistic effect of Bronsted and Lewis acid sites.The decomposable diagrams of the samples show, for the samples treated at 430-450℃, the features can be related to the orthorhombicα-MoO3 and monoclinicβ-MoO3. The influence of Mo source on the existent state of MoO3 was investigated on nano HZSM-5. It can be found that the acid concentration of MoO3 modified nano HZSM-5 decrease. The all modified catalysts exhibit fewer amounts of Bronsted and more Lewis acid sites than nano HZSM-5. The introduction of PMo in the nano HZSM-5 exhibits most reasonable the concentration and Bronsted-to-Total ratios than any other catalyst. In particular, the use of PMo were increases the ability of aromatization and producing light iso-alkanes for nano HZSM-5. When 0.2wt% of phosphorus is added to the catalyst prepared from Mo7O246-, the activity is suppressed. Theβ-MoO3 precursor favors to increasing of aromatization performance over nano HZSM-5, and orthorhombicα-MoO3 precursor favors to improving producing light iso-alkanes.The catalyst DGDME0609 for conversion of methanol to dimethyl ether was exploitured successfully. The reaction conditions were listed as the following: reaction temperature 200℃; WSHV of methanol is 1.6 and pressure 0.8 MPa. The conversion of methanol is higher than （mol.） 87% and the selectivity of dimethyl ether is higher than （mol.） 99.5%. The reaction progress of dehydration of methanol in gase was discussed. The catalyst shows good foreground in industrial application. It is concluded that this reaction accordings to "Acidic mechanism" over DGDME0609 catalyst, and it will provide academic direct for research the conversion of methanol to dimethyl ether in the next work.

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CLC: > Mathematical sciences and chemical > Chemistry > Physical Chemistry ( theoretical chemistry ),chemical physics > Chemical kinetics,catalysis > Catalytic > Catalyst
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