Aromatization of propane over MFI-gallosilicates

Bayense, CR Cees; Pol, van der Ajhp Arjan; Hooff, van Jhc Jan

doi:10.1016/0166-9834(91)85030-y

Cited by 132 publications

(28 citation statements)

References 26 publications

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“…One is the direct recombination of hydride and acidic proton, which resembles the mechanism for alkanes activation over metal modified ZSM-5 catalysts. 72,84 Meanwhile, the work of Meriaudeau et al on the activation of propane over Ga-modified H-ZSM-5 indicated that the spillover of the carbenium ions to Brønsted acid sites was a necessary intermediate step to bypass further C−H bond activation over the Ga species. 85 Thus, the second pathway for the dehydrogenation would involve an exchange of the cyclic carbenium ion with the neighboring acidic proton and subsequent desorption of H 2 on the GaO + ions.…”

Section: Cyclicmentioning

confidence: 99%

A Mechanistic Study of Methanol-to-Aromatics Reaction over Ga-Modified ZSM-5 Zeolites: Understanding the Dehydrogenation Process

Gao

et al. 2018

ACS Catal.

121

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The methanol-to-aromatics (MTA) reaction was investigated on Ga-modified ZSM-5 zeolites (Ga/ZSM-5). As revealed by 71Ga and 1H solid-state NMR and FT-IR of pyridine adsorption measurements, cationic Ga species are formed as Lewis acid sites by substitution of Brønsted acid sites on H-ZSM-5. Further experimental studies show that C5- and C6-cycloalkenes are generated during the MTA reaction, which lead to the formation of cyclic carbocations as precursors to aromatics. Isotope exchange experiments demonstrate that the reactivity of the cyclic carbocations on Ga/ZSM-5 is much higher than that on H-ZSM-5 and they play an intermediate role in the formation of aromatics. In addition to the traditional bimolecular hydrogen transfer (HT) reaction, the dehydrogenation of alkenes with the release of H2 (DeaH2 process) was identified to significantly contribute to the formation of aromatics. The transformation of cycloalkenes to aromatics is favored by promotion of the DeaH2 process and competes with the HT route on Ga/ZSM-5, while these cycloalkenes tend to crack back to alkenes, and the dominating HT route results in lower aromatic selectivity on H-ZSM-5. A DeaH2-aromatization route mediated by the cooperation of cationic Ga species and Brønsted acid sites was proposed for the enhanced formation of aromatics on Ga/ZSM-5 zeolite.

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Section: Cyclicmentioning

confidence: 99%

A Mechanistic Study of Methanol-to-Aromatics Reaction over Ga-Modified ZSM-5 Zeolites: Understanding the Dehydrogenation Process

Gao

et al. 2018

ACS Catal.

121

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“…The dehydrogenation of alkanes to aromatics and H 2 in H-ZSM-5 zeolites increases dramatically upon the incorporation of Ga 2 O 3 or ZnO into the zeolite. − However, despite the fact that the promotional effects of Ga and Zn for alkane dehydrocyclization were discovered many years ago, there is still a great deal that is not known about the nature of the metal sites in the zeolite and the roles played by these metals in the reaction. Although it is generally agreed that both acid sites and metal sites are required for the reaction, with the acid sites cleaving C–H bonds and the metal sites aiding in the release of molecular H 2 , exactly how the metal cations perform this task is not well understood.…”

Section: Introductionmentioning

confidence: 99%

Study of Zn and Ga Exchange in H-[Fe]ZSM-5 and H-[B]ZSM-5 Zeolites

Yeh

Gorte

2016

Ind. Eng. Chem. Res.

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The catalytic properties of H-[Fe]ZSM-5 and H-[B]ZSM-5 were explored after addition of Zn or Ga. TPD−TGA of 2-propanamine adsorbed on Znand Ga-exchanged H-[Fe]ZSM-5 showed a decrease in Brønsted-acid site densities and the formation of new dehydrogenation sites, similar to what is observed following exchange in H-[Al]ZSM-5 and in amorphous silica−alumina. Exchanged Zn cations in [Fe]ZSM-5 also exhibited Lewis-acid character, as demonstrated by the appearance of a υ(CN) stretch at 2310 cm −1 upon adsorption of CD 3 CN. By contrast, the sites in H-[B]ZSM-5 were not capable of protonating 2-propanamine, did not form dehydrogenation sites when Zn or Ga were added, and showed no evidence for sites with Lewis-acid character from the FTIR spectroscopy of CD 3 CN. Neither H(Zn)-[Fe]ZSM-5 nor H(Zn)-[B]ZSM-5 catalyzed reactions of n-hexane at 773 K, but the TPD−TGA of adsorbed propene on H(Zn)-[Fe]ZSM-5 showed strong interactions between the Zn and olefins that might be responsible for the dehydrocyclization of light alkanes in Zn-exchanged, Al-containing zeolites.

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“…4,5 Catalytic activity of these materials is known to involve extra-framework gallium oxide species, the chemistry of which is scarcely known. [6][7][8][9] The surface properties of Ga 2 O 3 have not been much studied, although recent reports 10,11 reveal that both the gand the b-forms present a strong Lewis acidity, attributed to cus Ga 3+ in tetrahedral position on their surface. To our knowledge, however, no study has yet been devoted to the a-polymorph.…”

Section: Introductionmentioning

confidence: 99%

Unexpected similarities between the surface chemistry of cubic and hexagonal gallia polymorphs

Lavalley

Daturi

Montouillout

et al. 2003

Phys. Chem. Chem. Phys.

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g-Ga 2 O 3 and a-Ga 2 O 3 samples were prepared as single-phase materials having a high surface area. 71 Ga NMR showed the simultaneous presence of tetracoordinated (Ga IV ) and hexacoordinated (Ga VI ) in g-Ga 2 O 3 , whereas in a-Ga 2 O 3 most of the gallium present was found to be as Ga VI . However, the apparent rate of methanol conversion into dimethyl ether (at 473 K) was nearly the same for both samples, when due account was taken of their respective surface area. The unexpected high activity of a-Ga 2 O 3 was explained from IR spectroscopic results obtained by using acidity probes like CO and pyridine. These spectroscopic probe molecules showed, for both samples, the presence of strong Lewis acid sites related to coordinatively unsaturated (cus) Ga IV ions which, for the case of a-Ga 2 O 3 , strongly suggests surface reconstruction. By contrast, CO 2 adsorption revealed a higher basicity for the a-polymorph, as compared to g-Ga 2 O 3 . This basicity can be directly correlated to a larger amount of surface Ga VI ions which results in a higher degree of ionicity.

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Aromatization of propane over MFI-gallosilicates

Cited by 132 publications

References 26 publications

A Mechanistic Study of Methanol-to-Aromatics Reaction over Ga-Modified ZSM-5 Zeolites: Understanding the Dehydrogenation Process

A Mechanistic Study of Methanol-to-Aromatics Reaction over Ga-Modified ZSM-5 Zeolites: Understanding the Dehydrogenation Process

Study of Zn and Ga Exchange in H-[Fe]ZSM-5 and H-[B]ZSM-5 Zeolites

Unexpected similarities between the surface chemistry of cubic and hexagonal gallia polymorphs

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