Generation, Characterization, and Transformations of Unsaturated Carbenium Ions in Zeolites

Kiricsi, Imre; Förster, H.; Tasi, Gyula; Nagy, J.B.

doi:10.1021/cr9600767

Cited by 133 publications

(125 citation statements)

References 251 publications

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“…Figure 4 depicts the induction periods (black) and active periods (red) of catalysts deactivated by the three different methanol purities as measured by on‐line mass spectrometry. Feedstock impurities clearly have an effect on the induction period as previously reported 40, 61, 62, 63, 64. The results summarized in Figure 4 show that methanol feedstock (U), containing very few impurities, has the longest induction and active periods.…”

Section: Resultssupporting

confidence: 75%

“…Highly poly‐methylated benzenes have been determined to be active species in the formation of olefins and have been suggested to absorb light between 390–400 nm 24, 57, 58, 59, 60, 61. More specifically, some characterization studies suggest that the more active, highly methylated benzenes absorb light around 390 nm, whereas the non‐active, less‐methylated naphthalenic species absorb light near 400 nm 17, 20, 49, 61. This spectral shift from 390 to 400 nm is also observed in the data shown in Figure 1 h, indicated by a dotted line.…”

Section: Resultsmentioning

confidence: 99%

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Effect of Feedstock and Catalyst Impurities on the Methanol‐to‐Olefin Reaction over H‐SAPO‐34

2016

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Operando UV/Vis spectroscopy with on‐line mass spectrometry was used to study the effect of different types of impurities on the hydrocarbon pool species and the activity of H‐SAPO‐34 as a methanol‐to‐olefins (MTO) catalyst. Successive reaction cycles with different purity feedstocks were studied, with an intermittent regeneration step. The combined study of two distinct impurity types (i.e., feed and internal impurities) leads to new insights into MTO catalyst activation and deactivation mechanisms. In the presence of low amounts of feed impurities, the induction and active periods of the process are prolonged. Feed impurities are thus beneficial in the formation of the initial hydrocarbon pool, but also aid in the unwanted formation of deactivating coke species by a separate, competing mechanism favoring coke species over olefins. Further, feedstock impurities strongly influence the location of coke deposits, and thus influence the deactivation mechanism, whereas a study of the organic impurities retained after calcination reveals that these species are less relevant for catalyst activity and function as “seeds” for coke formation only.

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Section: Resultssupporting

confidence: 75%

Section: Resultsmentioning

confidence: 99%

Effect of Feedstock and Catalyst Impurities on the Methanol‐to‐Olefin Reaction over H‐SAPO‐34

2016

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“…12 These data suggest that adsorbed species with varying extent of unsaturation and size coexist during reactions of hydrocarbons on zeolitic acids and that their size and hydrogen content influence the nature of their chemical reactions. 12 The hydrogen chemical potential within this pool of reactive adsorbed species determines the degree of saturation in these intermediates and the products they form. High hydrogen chemical potentials tend to lead to saturated adsorbed species and to hydrogen-rich products, while low hydrogen potentials favor more unsaturated intermediates, which desorb as alkenes or aromatics.…”

Section: Introductionmentioning

confidence: 85%

“…10,11 UV-visible and infrared spectroscopy studies have detected the presence of unsaturated alkenyl carbenium ions during interactions of propene with H-ZSM5. 12 These studies have shown that oligomeric carbenium ions with 6-9 carbon atoms act as persistent carbocations, from which smaller molecules can form via -scission pathways. Mono-, di-, and trienyl carbenium ions have also been detected during adsorption of propene on H-Na-Y.…”

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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“…The UV/vis response (not shown) is in very good agreement with monoenyl (300 nm), dienyl (360 nm), and trienyl (460 nm) carbenium ions reported in the literature for various alkene/ zeolite combinations. 22,23 Note that fluorescence cross sections are considerably (up to 10 orders of magnitude) larger than Raman scattering cross sections. Therefore, the absence of bands characteristic for such conjugated systems in the CARS spectra between −1505 and −1580 cm −1 22,24 does not exclude formation of small amounts of such species, below the CARS detection limit of a few millimoles.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

Tracing Catalytic Conversion on Single Zeolite Crystals in 3D with Nonlinear Spectromicroscopy

et al. 2011

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ABSTRACT:The cost-and material-efficient development of nextgeneration catalysts would benefit greatly from a molecular-level understanding of the interaction between reagents and catalysts in chemical conversion processes. Here, we trace the conversion of alkene and glycol in single zeolite catalyst particles with unprecedented chemical and spatial resolution. Combined nonlinear Raman and two-photon fluorescence spectromicroscopies reveal that alkene activation constitutes the first reaction step toward glycol etherification and allow us to determine the activation enthalpy of the resulting carbocation formation. Considerable inhomogeneities in local reactivity are observed for micrometer-sized catalyst particles. Product ether yields observed on the catalyst are ca. 5 times higher than those determined off-line. Our findings are relevant for other heterogeneous catalytic processes and demonstrate the immense potential of novel nonlinear spectromicroscopies for catalysis research.

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Generation, Characterization, and Transformations of Unsaturated Carbenium Ions in Zeolites

Cited by 133 publications

References 251 publications

Effect of Feedstock and Catalyst Impurities on the Methanol‐to‐Olefin Reaction over H‐SAPO‐34

Effect of Feedstock and Catalyst Impurities on the Methanol‐to‐Olefin Reaction over H‐SAPO‐34

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

Tracing Catalytic Conversion on Single Zeolite Crystals in 3D with Nonlinear Spectromicroscopy

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