Reaction Pathways and Rate-Determining Steps in Reactions of Alkanes on H-ZSM5 and Zn/H-ZSM5 Catalysts

Biscardi, Joseph A.; Iglesia, Enrique

doi:10.1006/jcat.1998.2312

Cited by 195 publications

(219 citation statements)

References 67 publications

(70 reference statements)

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“…This approach assumes that the addition of gas-phase hydrogen does not affect the rate at which C-H bonds are activated; the assumption is supported by previous findings showing that these steps are quasi-equilibrated during propane conversion on both H-ZSM5 and Zn/H-ZSM5. 3 Figures 6-8 show that reaction rates, selectivities, and deactivation rates reflect these surface hydrogen virtual pressures, irrespective of the actual gas-phase H 2 pressure or the catalyst composition. Reaction rates decreased with increasing (H 2 ) v , because H-atoms reverse C-H bond activation steps, shift the equilibrium position of this quasi-equilibrated step, and lead to fewer or less reactive propane-derived reactive intermediates ( Figure 6).…”

Section: Effect Of Gas-phase H 2 On Propane Dehydrogenation Rates Andmentioning

confidence: 98%

“…The formation of Fe nitrides during NH 3 decomposition on Fe reflects the high virtual N 2 pressure caused by this desorption bottleneck. Formation of iron nitride, Fe 4 N, from Fe at 673 K requires very high N 2 pressures (1.0 × 10 5 kPa); yet, an equimolar mixture of NH 3 and H 2 (100 kPa total) led to the rapid formation of iron nitride, as a result of a steadystate chemical potential of adsorbed nitrogen corresponding to that of 100 MPa N 2 in the gas phase. 15,17,18 Auger electron spectroscopic studies on W 19 and Mo 13 foils confirmed that N* surface coverages during steady-state NH 3 decomposition are much higher than those expected from equilibrium with the prevalent N 2 pressure.…”

Section: Kinetic Treatments Of Catalytic Sequencesmentioning

confidence: 99%

“…Previous studies have confirmed the rate-determining nature of hydrogen removal steps and the catalytic role of cations as hydrogen desorption sites during the conversion of alkanes to aromatics on zeolite catalysts. [1][2][3][4][5] The relative rates of C-H bond activation and of various hydrogen disposal pathways (e.g. recombinative hydrogen desorption, hydrogenation and desorption of unsaturated intermediates) determine the hydrogen chemical potential within the pool of adsorbed species prevalent during steady-state catalytic reactions of alkanes.…”

Section: Introductionmentioning

confidence: 99%

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Kinetic Relevance of Hydrogen Desorption Steps and Virtual Pressures on Catalytic Surfaces during Reactions of Light Alkanes

Biscardi

Iglesia

2002

J. Phys. Chem. B

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Hydrogen removal steps limit alkane dehydrogenation reactions on cation-exchanged H-ZSM5 and cause desorption bottlenecks and the formation of hydrogen-rich reaction intermediates and products. For this reaction, the catalytic surface acts for all kinetic purposes as if it were in equilibrium with a H 2 pressure greater than in the prevalent gas phase. The hydrogen chemical potential within adsorbed intermediates is described rigorously by a virtual H 2 pressure, defined as that required to achieve the prevalent surface hydrogen content if adsorption-desorption steps were equilibrated. These virtual pressures can be measured from the deuterium content in products formed from C 3 H 8 /D 2 reactants. H 2 virtual pressures are high during propane reactions on H-ZSM5, because recombinative desorption of hydrogen atoms formed in C-H activation steps is slow. H 2 virtual pressures decrease as Zn cations replace protons in H-ZSM5, because cations catalyze hydrogen adsorption-desorption steps and provide a kinetic path for communication between H 2 in the gas phase and propane-derived reaction intermediates. As a result, the addition of H 2 to C 3 H 8 reactants decreases propane reaction rates and aromatics selectivity on Zn/H-ZSM5, causing it to resemble kinetically H-ZSM5. The hydrogen chemical potential on these catalytic surfaces and the hydrogen content within reactive intermediates reflect the rate at which hydrogen is formed in either C-H bond activation steps or in the dissociative chemisorption of gas-phase H 2 . The reaction pathways for species derived from each of these two H-sources are kinetically indistinguishable. Both sources contribute hydrogen atoms or hydrogen-rich species to the prevalent pool of reactive intermediates, the hydrogen content of which determines the rate and selectivity of all surface chemical reactions.

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Section: Effect Of Gas-phase H 2 On Propane Dehydrogenation Rates Andmentioning

confidence: 98%

Section: Kinetic Treatments Of Catalytic Sequencesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Kinetic Relevance of Hydrogen Desorption Steps and Virtual Pressures on Catalytic Surfaces during Reactions of Light Alkanes

Biscardi

Iglesia

2002

J. Phys. Chem. B

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“…Likewise, gallium modified HZSM-5 catalyst also had a capability of considerably elevating the selectivity towards aromatics [13][14][15][16], however, due to its high price, gallium was not appropriate for commercial application. In addition, zinc modified HZSM-5 catalyst was also widely reported in large numbers of papers [17][18][19][20][21][22][23][24], but the activity of the catalyst was unable to last long.…”

Section: Introductionmentioning

confidence: 99%

Effect of γ-alumina as active matrix added to HZSM-5 catalyst on the aromatization of methanol

Chen

Zhang

et al. 2015

Appl Petrochem Res

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HZSM-5-based catalyst is a recognized catalyst which is particularly selective towards the formations of aromatics in the methanol reaction. However, studies on HZSM-5-based catalyst were mainly focused on the addition of metallic or/and nonmetallic element. Quite few studies have reported the effect of active matrix such as calumina on the aromatization of methanol. In this study, calumina was introduced into HZSM-5-based catalyst for the purpose of investigating the effect of c-alumina in methanol to aromatics reaction. The catalysts were characterized by X-ray diffraction, Temperature-programmed Desorption of NH 3 (NH 3 -TPD), Pyridine adsorption FT-IR diffuse reflection spectroscopy and adsorption-desorption measurements of nitrogen, respectively. Characterizations showed that the introduction of c-alumina increased the amount of mesopores and acid sites in the catalyst. The experimental mainly includes two parts. Firstly, separate reaction performances over the catalyst with/without calumina and c-alumina showed that c-alumina could significantly promote the formations of aromatics. However, c-alumina alone could merely convert methanol to dimethyl ether with a minor quantity of gaseous hydrocarbons. Acid properties showed that the introduction of c-alumina increased the percentage of Lewis acid on catalyst surface and enhanced acid strength, as a result, promoted the production of active intermediates which was essential for aromatic formation. The rise of aromatics selectivity might be caused by the combined effect of acid site density and acid strength. Follow-up work was mainly focused on the effect of the loading amount and loading order of the catalyst with c-alumina. Results indicated that the total aromatic yield increased gradually with the increasing amount of catalyst with c-alumina regardless of the loading order of the catalyst with c-alumina. Gasoline compositions showed that the increased aromatics were at the expense of paraffins, olefins, and naphthenes. Besides, all single aromatic hydrocarbons increased gradually with the increasing amount of catalyst with c-alumina. And the aromatics had a larger variation change when methanol first passed through the catalyst with c-alumina.

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“…The aromatization of various compounds, including alcohols [5][6][7][8][9][10][11][12], alkanes [13][14][15][16][17][18][19][20][21], and alkenes [22], has been reported on various heterogeneous catalysts, such as silica-alumina, zeolites, hydroxyapatite, and heteropolyacids. Our attention was focused on zeolite catalysts based on the following reports.…”

Section: Introductionmentioning

confidence: 99%

Selective Production of Aromatics from 2-Octanol on Zinc Ion-Exchanged MFI Zeolite Catalysts

2015

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Abstract:The aromatization of 2-octanol derived from castor oil as a byproduct in the formation of sebacic acid was investigated on various zeolite catalysts. Zn ion-exchanged MFI (ZSM-5) zeolites with small silica/alumina ratios and zinc contents of 0.5 to 2.0 wt. % were determined to exhibit good and stable activity for the reaction at 623 to 823 K. The yield of aromatics was 62% at 773 K and the space velocity 350 to 1400 h −1. The temperature and contact time dependences of the product distributions indicated the reaction pathways of 2-octanol→dehydration to 2-octene→decomposition to C5 and C3 compounds→further decomposition to small alkanes and alkenes→aromatization with dehydrogenation. Alcohols with carbon numbers of 5 to 8 exhibited similar distributions of products compared to 2-octanol, while corresponding carbonyl compounds demonstrated different reactivity.

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Reaction Pathways and Rate-Determining Steps in Reactions of Alkanes on H-ZSM5 and Zn/H-ZSM5 Catalysts

Cited by 195 publications

References 67 publications

Kinetic Relevance of Hydrogen Desorption Steps and Virtual Pressures on Catalytic Surfaces during Reactions of Light Alkanes

Kinetic Relevance of Hydrogen Desorption Steps and Virtual Pressures on Catalytic Surfaces during Reactions of Light Alkanes

Effect of γ-alumina as active matrix added to HZSM-5 catalyst on the aromatization of methanol

Selective Production of Aromatics from 2-Octanol on Zinc Ion-Exchanged MFI Zeolite Catalysts

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