Modification of the acidic properties by introduction of zinc into ZSM-5 zeolites

Rojasová, Edita; Smiešková, Agáta; Hudec, Pavol; Židek, Zdenek

doi:10.1007/bf02475746

Cited by 9 publications

(7 citation statements)

References 4 publications

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“…The observed decrease in the activity of the mechanical mixture in comparison with the pure acid form can be an indirect evidence that the concentration of proton sites in the mechanical mixtures decreases. This can be supposed on the basis of the results reported in our previous work 10 , where it was shown that increasing the ion-exchange degree the conversion of toluene in ion-exchanged Zn-ZSM-5 samples continuously decreases. In addition on the basis of FTIR measurements using into cationic positions of ZSM-5 zeolites causes decrease in the concentration of proton sites in the catalyst and generation of new, relatively strong Lewis acid sites 9 .…”

Section: Resultsmentioning

confidence: 82%

“…By temperature-programmed desorption of ammonia (TPDA) it was also shown that with increasing amount of zinc in cationic positions, the amount of ammonia desorbed above 450°C increases and the n-hexane conversion at 420°C continuously increases with simultaneous increase in the selectivity to aromatics. In addition, it was shown that the toluene conversion continuously decreases with increasing amount of zinc in cationic positions, indicating a decrease in the concentration of Brønsted acid sites in the zeolite 10 .…”

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confidence: 98%

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Interaction of ZnO with ZSM-5 Zeolites

Rojasová

Smiešková

Hudec

et al. 2000

Collect. Czech. Chem. Commun.

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The possibility of introduction of zinc from ZnO into cationic positions of H-and NH 4 -ZSM-5 zeolites by solid-state ion exchange was investigated. It was concluded that during thermal pretreatment of a mechanical mixture of ZnO with acid forms of ZSM-5 zeolite, introduction of Zn species into cationic positions as a consequence of the proposed solid-state ion exchange proceeds, but its degree is limited. It indicates that the extent of interaction of Zn species is probably controlled by the concentration of cationic sites on the external surface of the zeolite.It has been reported that mechanical mixtures of ZnO with acid forms of ZSM-5 zeolites are highly active and selective in aromatization of lower alkanes 1-3 . The role and the state of metallic species in the mechanical mixtures under the reaction conditions have also been the subject of a wide discussion 2-8 .Kanai and Kawata 2 have reported that a mechanical mixture of ZnO and NH 4 -ZSM-5 is a bifunctional catalyst in which ZnO in the n-hexane aromatization catalyzes dehydrogenation of n-hexane into hexene and of the oligomerized products into aromatics.On the other hand, in our previous work 9 it was observed that both mechanical mixtures of ZnO with alumina and sodium form of ZSM-5 were totally inactive in n-hexane conversion at 420°C, indicating that ZnO as an extra-framework phase is not able to activate n-hexane under these conditions. We have only observed an increased selectivity to aromatics on a mechanical mixture ZnO + NH 4 -ZSM-5 as compared with NH 4 -ZSM-5. We have explained this fact by partial migration of Zn from ZnO into cationic positions due to solid-state ion exchange during thermal pretreatment of the mechanical mixture before measurement.

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Section: Resultsmentioning

confidence: 82%

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confidence: 98%

Interaction of ZnO with ZSM-5 Zeolites

Rojasová

Smiešková

Hudec

et al. 2000

Collect. Czech. Chem. Commun.

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“…The experimental evidence suggests that both zinc and gallium species are either directly or indirectly responsible for increasing the rate of dehydrogenation of the alkane reactant 5. One proposal is that hydride abstraction in the vicinity of Ga 6 and Zn 7 sites is the initial rate‐determining step. Others have concluded that Zn and Ga cations are the driving force for dehydrogenation by combining the H‐atoms, which form during the activation of CH at Bronsted acid sites 8.…”

Section: Introductionmentioning

confidence: 99%

“…This occurs by the cyclic reduction and oxidation of Zn and Ga species 9. All other reaction steps occur on Bronsted acid sites within the zeolite micropores, 5 although it has been suggested that zinc may also be involved in oligomerization of the alkenium ions and dehydrogenation of cycloalkenes 7.…”

Section: Introductionmentioning

confidence: 99%

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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“…The reaction products can be accounted for by the following monomolecular cracking for n-hexane: [15] Variation in mol% of species in gas fraction versus (b) Fig.3 Variation of compounds concentration in gas fraction versus temperature for Ce/H-ZSM-5 catalyst in (a) nitrogen or (b) hydrogen carrier gas, % mol…”

Section: Resultsmentioning

confidence: 99%

Metal impregnated catalysts for bioethanol conversion tested by n-hexane cracking

Proscanu

Matei

Ganea³

et al. 2012

Analele Universitatii "Ovidius" Constanta - Seria Chimie

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The catalytic activity of catalysts for bioethanol conversion to hydrocarbons was tested by cracking nhexane in a glass microreactor in the temperature range of 623 to 823K. The microreactor is a tubular one with axial thermocuple measurements in flow, initially for some experiments in nitrogen and for others by hydrogen. Reaction products were collected and analyzed as gaseous samples by Gas Chromatography. The samples of Me-ZSM-5 zeolites (Me=Fe, La, Ce) were prepared by impregnation method. All the prepared samples had a metal content of approximate to 3 % wt. ZSM-5 zeolite was synthesized by using a structure direct agent, namely tetrapropylammonium hydroxide (TPAOH) and hydrothermal crystallisation method under alkaline conditions according to reported procedures.

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Modification of the acidic properties by introduction of zinc into ZSM-5 zeolites

Cited by 9 publications

References 4 publications

Interaction of ZnO with ZSM-5 Zeolites

Interaction of ZnO with ZSM-5 Zeolites

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

Metal impregnated catalysts for bioethanol conversion tested by n-hexane cracking

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