Structure and function of metal cations in light alkane reactions catalyzed by modified H-ZSM5

Biscardi, Joseph A.; Iglesia, Enrique

doi:10.1016/s0920-5861(96)00028-4

Cited by 282 publications

(329 citation statements)

References 60 publications

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“…The rate-determining nature of hydrogen removal steps and the role of Zn cations as recombinative desorption sites were first proposed by Mole et al 1 The similar role of Ga and Zn cations, as sites for the recombinative desorption of H 2 , was later confirmed by kinetic and isotopic tracer studies. 6,9 The higher propane conversion and aromatics formation rates and the higher selectivities to dehydrogenated products and to H 2 observed as the Co content increases suggest a similar role of Co 2+ cations in propane reaction pathways. Initial propane dehydrogenation turnover rates (per Al) (total propane turnover rate minus cracking rate) on Zn/H-ZSM5 (Zn/ Al ) 0.20; 5.2 × 10 -3 s -1 ) are higher than on Co/H-ZSM5 (Co/Al ) 0.22; 3.3 × 10 -3 s -1 ) (Table 4).…”

Section: Resultsmentioning

confidence: 97%

“…These effects reflect the ability of Co 2+ sites to desorb H atoms formed in C-H bond activation steps as H 2 , as shown previously for Zn cations. 1,[6][7][8] The effects of contact time on conversion and selectivity are similar on Co/H-ZSM5 samples with a wide range of Co/Al ratios; these trends are shown in Figure 13 for the sample with a Co/Al of 0.17. The selectivities to C 1 -C 2 and propene products show a non-zero intercept suggesting that these products can form via primary cracking reactions of propane.…”

Section: Resultsmentioning

confidence: 99%

“…Aromatics form predominately via acidcatalyzed, cation-assisted secondary reactions involving oligomerization, dehydrogenation, and cracking of alkenes, as previously shown on Zn-and Ga-exchanged H-ZSM5 catalysts. 3,[6][7][8] Initial propane conversion turnover rates and aromatics sitetime yields increased with increasing Co content (Figure 14), SCHEME 2: Formation of Exchanged Hydrated Co 2+ Cations indicating that the rate-limiting step for propane dehydrogenation is catalyzed by Co cations. The effects of Co on the product selectivity are shown in Figure 15 at the same propane conversion (∼10%).…”

Section: Resultsmentioning

confidence: 99%

“…[1][2][3][4][5][6][7][8] Cations increase propane conversion turnover rates and aromatics and H 2 formation rates by catalyzing the recombinative desorption of H 2 , which occurs slowly on H-ZSM5. 8, 9 Hydrogen desorption steps are not quasi-equilibrated during alkane reactions on cation-exchanged H-ZSM5.…”

Section: Introductionmentioning

confidence: 99%

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Structure and Properties of Cobalt-Exchanged H-ZSM5 Catalysts for Dehydrogenation and Dehydrocyclization of Alkanes

Li¹,

Yu²,

Meitzner³

et al. 2001

J. Phys. Chem. B

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101

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Co/H-ZSM5 catalysts with Co/Al ratios of 0.09-0.22 were prepared by aqueous exchange. Turnover rates for propane conversion to propene and to C 6 -C 8 aromatics on these catalysts are about 10-fold higher than on H-ZSM5. The selectivities to propene, aromatics, and H 2 are also higher on Co/H-ZSM5 than on H-ZSM5. The rate of D 2 exchange with OH groups increases with increasing Co/Al ratio, suggesting that Co cations catalyze D 2 dissociative chemisorption steps that limit the rate of isotopic exchange. Co cations also catalyze hydrogen recombinative desorption steps, which limit the rate of propane dehydrogenation and aromatization reactions. The density of residual zeolitic hydroxyls was measured by D 2 -OH isotopic exchange and by changes in the intensity of OH infrared bands as a function of Co content. D 2 -OH and infrared measurements showed that Co 2+ cations replace 1.1-1.3 zeolitic protons, suggesting the predominant presence of Co 2+ -O-Co 2+ dimers, with some Co 2+ monomers, each bridging two next-nearest neighbor Al sites. The location and structure of exchanged Co cations were probed using X-ray absorption spectroscopy (XAS) and temperature-programmed reduction (TPR). No H 2 consumption was detected up to 1273 K during TPR in any of the Co/H-ZSM5 samples, consistent with the absence of CoO x crystallites, which reduce at ∼800 K. In situ near-edge X-ray absorption studies confirmed that Co species remain as divalent cations during exposure to H 2 or C 3 H 8 at 773 K. Near-edge and fine structure analysis detected Co 2+ cations with similar structure in all Co/H-ZSM5 samples (Co/Al < 0.22), and Co coordination changes from octahedral to tetrahedral upon sample dehydration at 773 K in He. Radial structure functions showed weak contributions from the first and second shells around Co. This reflects the nonuniform nature of the distance and orientation in Al-Al nextnearest neighbor sites in ZSM5.

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Section: Resultsmentioning

confidence: 97%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Structure and Properties of Cobalt-Exchanged H-ZSM5 Catalysts for Dehydrogenation and Dehydrocyclization of Alkanes

Li¹,

Yu²,

Meitzner³

et al. 2001

J. Phys. Chem. B

Self Cite

101

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“…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%

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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BTX from Light Hydrocarbons

Tan

Yang

et al. 2023

Industrial Arene Chemistry

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Structure and function of metal cations in light alkane reactions catalyzed by modified H-ZSM5

Cited by 282 publications

References 60 publications

Structure and Properties of Cobalt-Exchanged H-ZSM5 Catalysts for Dehydrogenation and Dehydrocyclization of Alkanes

Structure and Properties of Cobalt-Exchanged H-ZSM5 Catalysts for Dehydrogenation and Dehydrocyclization of Alkanes

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

BTX from Light Hydrocarbons

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