Transformation of butanes over ZSM-5 zeolites. Part 2.—Formation of aromatic hydrocarbons over Zn-ZSM-5 and Ga-ZSM-5

Ono, Yoshio; Kanae, Kunihiko

doi:10.1039/ft9918700669

Cited by 90 publications

(55 citation statements)

References 15 publications

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“…In comparison with Ga/HZSM‐5 isobutane reactions, activation energies were significantly lower for Zn/HZSM‐5; 33 ± 3 kJ mol −1 and 47 ± 2 kJ mol −1 for cracking and dehydrogenation respectively. Ono and Kanae 19 have shown that Zn/HZSM‐5 has weaker acid sites than Ga/HZSM‐5, which is an apparent contradiction to the activation energies listed in Table III. However, our results show that overall yields of all products are lower for Zn/HZSM‐5, which is reflected in the significantly lower preexponential factors.…”

Section: Discussionmentioning

confidence: 73%

Catalytic cracking, dehydrogenation, and aromatization of isobutane over Ga/HZSM‐5 and Zn/HZSM‐5 at low pressures

Sun

Brown

2002

Int J of Chemical Kinetics

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Isobutane cracking, dehydrogenation, and aromatization over Ga/HZSM-5 and Zn/HZSM-5 has been investigated in a Knudsen cell reactor and the kinetics of the primary reaction steps for isobutene and propene formation have been accurately determined. Although cracking is the dominant reaction channel, with propene and methane being primary products, methane formation is significantly less than propene formation. This indicates that a proportion of the cracking proceeds via Lewis acid attack at C C bonds, and not just via alkanium ion formation at Bronsted acid sites. This is particularly apparent over Zn/HZSM-5. Intrinsic rate constants for cracking, calculated from the rate of propene formation, areand for dehydrogenation, calculated from the rate of isobutene formation, areLarge preexponential factors for cracking and dehydrogenation over Ga/HZSM-5 indicate that either the coverage of active sites is significantly less than the coverage of exposed sites or the 468SUN AND BROWN intrinsic reaction step involves a large entropy change between reactant and transition state. For Zn/HZSM-5 the small preexponential factors suggest either small entropy changes during activation, perhaps initiated by Lewis acid sites, or a steady-state distribution of active and exposed sites is rapidly reached. Differences in intrinsic activation energies may reflect the ratio of Lewis and Bronsted acid sites on the respective catalyst surfaces. Aromatization is more prolific over Ga/HZSM-5 than over Zn/HZSM-5 under the low-pressure conditions.

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Section: Discussionmentioning

confidence: 73%

Catalytic cracking, dehydrogenation, and aromatization of isobutane over Ga/HZSM‐5 and Zn/HZSM‐5 at low pressures

Sun

Brown

2002

Int J of Chemical Kinetics

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“…Ono and Kanae studied the conversion of isobutane over H-ZSM-5, ZnO/H-ZSM-5 and Ga-containing H-ZSM-5 [15]. The main products observed were lower alkenes and propane, and the yield of aromatics was only 6.6% over H-ZSM-5.…”

Section: Introductionmentioning

confidence: 95%

Aromatization of Isobutene Using H-ZSM-5/Oxide Composite Catalysts

Al-Otaibi

Hutchings

2009

Catal Lett

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Aromatization of isobutene was investigated using zeolite H-ZSM-5 (Si/Al = 30) as well as physical mixtures of H-ZSM-5 with metal oxides (Ga 2 O 3 , ZnO and CuO). The study was aimed at determining the effect on the selectivity to aromatic products by introducing an oxide/zeolite interface. Of the three oxides, ZnO was found to enhance the yield of aromatic products with the highest aromatic selectivity being attained with ZnO/H-ZSM-5 (1:2 weight ratio). The addition of Ga 2 O 3 had very little effect and the addition of CuO was not found to be beneficial.

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“…[1][2][3][4][5][6][7][8][9][10][11][12][13] Thus the so-called Cyclar process, 14,15 where C 3 -C 5 alkanes are dehydrocyclized to yield aromatic hydrocarbons, proceeds over GaZSM-5 zeolites whereas conventional ZSM-5 aluminosilicates yield mainly paraffins. Specific interest in gallium-containing zeolites stems from their high selectivity to aromatics in the catalytic conversion of olefins and paraffins.…”

Section: Introductionmentioning

confidence: 99%

Characterization of Gallosilicate MFI-Type Zeolites by IR Spectroscopy of Adsorbed Probe Molecules

Areán¹,

Palomino²,

Zecchina³

et al. 1996

J. Phys. Chem.

105

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Three MFI-type, H-GaZSM-5 gallosilicates were synthesized with Si/Ga ratios of 25, 50, and 75, respectively. Incorporation of gallium into the zeolite framework was checked by powder X-ray diffraction, which showed an approximately linear variation of d spacings with Si/Ga ratio. Fourier transform IR spectroscopy of CO and N 2 , adsorbed at liquid nitrogen temperature, was used to characterize these gallosilicates after activation in vacuo at temperatures from 673 to 973 K. It was found that both probe molecules formed stable adducts with polarizing centers having either Brönsted or Lewis acid character. On samples activated at a temperature smaller than 700 K Brönsted sites were found to be predominant; apart from regular Si(OH)Ga sites, adsorbed carbon monoxide also revealed the presence of a small fraction of Brönsted acid groups associated with nonregular sites. These could be located either at the mouth of small cavities (presumably associated with defective regions of the zeolite framework where gallium was segregated) or on amorphous regions. On the sample with Si/Ga ) 25 activated at temperatures higher than 700 K, gallium showed a marked tendency to go into either partial or total extraframework positions, thus generating coordinatively unsaturated Ga 3+ centers with strong Lewis acid character. These centers were revealed by formation of CO and N 2 adducts where stretching frequencies of the adsorbed molecules showed a strong hypsochromic shift, ∆ν. Values of ∆ν as high as 82 and 25 cm -1 were found for CO and N 2 , respectively. In the case of dinitrogen, interaction with Lewis acid centers also brings about a pronounced increase of IR band intensity, and this effect can advantageously be used to monitor extraframework species.

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Transformation of butanes over ZSM-5 zeolites. Part 2.—Formation of aromatic hydrocarbons over Zn-ZSM-5 and Ga-ZSM-5

Cited by 90 publications

References 15 publications

Catalytic cracking, dehydrogenation, and aromatization of isobutane over Ga/HZSM‐5 and Zn/HZSM‐5 at low pressures

Catalytic cracking, dehydrogenation, and aromatization of isobutane over Ga/HZSM‐5 and Zn/HZSM‐5 at low pressures

Aromatization of Isobutene Using H-ZSM-5/Oxide Composite Catalysts

Characterization of Gallosilicate MFI-Type Zeolites by IR Spectroscopy of Adsorbed Probe Molecules

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