UV-VIS and IR Spectroscopic Investigations on the Generation of Alkenyl Carbocations from Propene in Zeolites of Different Acidity

Förster, H.; Kiricsi, Imre; Seebode, Jürgen

doi:10.1016/s0167-2991(09)60620-0

Cited by 9 publications

(4 citation statements)

References 17 publications

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“…46,47 Kiricsi and coworkers investigated the adsorption and oligomerization of propene on H-ZSM-5, attributing IR bands at 1535, 1520, and 1505 cm −1 to the allylic stretch of monoenyl, dienyl, and trienyl cations by attempting to correlate them with bands observed in the UV−vis, respectively. 49,76 However, we cannot reconcile these assignments, since the results presented here suggest the alkadienyl cation exhibits a vibrational band at higher frequencies than the alkenyl cation. The earliest assignment of the 1510 cm −1 band to an alkyl-substituted cyclopentenyl cation came from Stepanov et al, who correlated their NMR data to previous IR spectroscopic observations.…”

Section: Differentiation Of Acyclic and Cyclic Monoenyl Cations And D...contrasting

confidence: 64%

“…In contrast to NMR and UV–vis spectra of monoenyl and polyenyl cations, IR spectra have not been systematically collected and interpreted. In the majority of reports of IR spectra, oligomeric enylic cations were produced from small olefins such as ethene, propene, and butenes; thus, the identity of the species was not known and had to be inferred. ,,,,− The DRIFT spectra measured during MTO conversion contain features consistent with monoenyl cations; however, the contributions of cyclopentenyl or other cations to these spectra remain unresolved. In this section, monoenyl and dienyl cations are created and their spectra discussed.…”

Section: Resultsmentioning

confidence: 99%

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Spectroscopic Signatures Reveal Cyclopentenyl Cation Contributions in Methanol-to-Olefins Catalysis

Hernandez

Jentoft

2020

ACS Catal.

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The conversion of methanol to olefins on zeolites and zeotypes is an industrially important process, yet the mechanistic details remain unresolved. Various cyclic alkenyl and alkadienyl carbenium ions have been proposed as active intermediates for light olefin production, but direct experimental evidence demonstrating the transformation of these hydrocarbon pool species with product formation is lacking. In this contribution, the interpretations of UV–vis and IR spectra are advanced to provide detail on the species present during catalysis; selected individual species are generated inside the catalyst pores, and their reactivity is tracked. The basis for the interpretation is a multitude of reference spectra of acyclic and cyclic dienes, trienes, and corresponding protonated species in the liquid phase (organic medium or sulfuric acid) or sorbed on dealuminated or acidic zeolites, augmented by trends extracted from these spectra. Accordingly, spectroscopic signatures indicate the presence of both polymethylbenzenium ions and alkyl-substituted cyclopentenyl cations during the conversion of methanol on H-ZSM-5 at 300 °C. To elucidate their role, alkyl-substituted cyclopentenyl cations are generated from acyclic polyenes adsorbed on H-ZSM-5 at room temperature. Cyclization and cleavage are monitored via changes in the electronic and vibrational absorption spectra with increasing temperature. The formation of ethene, propene, and butenes from alkyl-substituted cyclopentenyl cations, in the absence of methylbenzenium ions, is unambiguously demonstrated by temperature-programmed reaction (TPReact) spectroscopy coupled with online gas-phase product analysis. In the UV–vis range, the wavelength of maximum absorption, λmax, of the alkyl-substituted cyclopentenyl cation is linearly dependent on the total number of carbons in the cation. A blue shift in the position of λmax of these cyclopentenyl cations during the TPReact indicates a loss of three carbons, which matches the average size of the concomitantly produced olefins.

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Section: Differentiation Of Acyclic and Cyclic Monoenyl Cations And D...contrasting

confidence: 64%

Section: Resultsmentioning

confidence: 99%

Spectroscopic Signatures Reveal Cyclopentenyl Cation Contributions in Methanol-to-Olefins Catalysis

Hernandez

Jentoft

2020

ACS Catal.

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“…Our efforts to exploit the shape-selective properties of zeolites for the photooxidation of hydrocarbons in zeolites have been complicated by prereaction of alkenes to form polymers. The polymerization of alkenes has been extensively studied on the acid form of zeolites, such as HZSM-5. ,,, Prereaction in ZSM-5 and Beta involves polymerization of the parent alkene which most likely occurs on acid sites.…”

Section: Discussionmentioning

confidence: 99%

Photooxidation of 1-Alkenes in Zeolites: A Study of the Factors that Influence Product Selectivity and Formation

1999

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Photochemical oxidation of hydrocarbons with molecular oxygen is potentially an environmentally benign method for the selective oxidation of hydrocarbons. In this study, in situ FT-IR spectroscopy and ex situ NMR spectroscopy were used to investigate the factors that influence product formation and selectivity in the room-temperature photooxidation of 1-alkenes in zeolites. Upon irradiation with broadband visible light, propylene, 1-butene, and 1-pentene loaded in BaY were photooxidized with molecular oxygen. As discussed in the literature, initial excitation of alkene and molecular complexes results in the selective formation of unsaturated aldehydes and ketones, proposed to occur through a hydroperoxide intermediate. In addition, epoxide and alcohol products are formed when the hydroperoxide intermediate reacts with an unreacted parent alkene molecule. Here it is shown that saturated aldehydes and ketones are formed as well through both a thermal ring-opening reaction of the epoxide in BaY and a second photochemical oxidation route involving a dioxetane intermediate. The yield of saturated aldehydes and ketones increased with decreasing wavelength, increasing temperature, and at a given temperature and wavelength, increasing chain length. Photooxidation of propylene in BaX, BaZSM-5, and BaBeta zeolites was also investigated. Photooxidation in BaX is very similar to that of BaY. In zeolites, BaZSM-5 and BaBeta, propylene polymerized upon adsorption. The polymer, polypropylene, also undergoes photooxidation with molecular oxygen to form an oxygenated polymer product. The results of this study show that product formation and selectivity in the photooxidation of 1-alkenes in zeolites depends on several factors. These factors include thermal reactions of the reactant and photoproduct molecules in the zeolite at ambient temperatures. Several reactions of 1-alkenes in cation-exchanged zeolites contribute to the loss of selectivity; they include: epoxide ring opening, double-bond migration, and alkene polymerization. Some of these reactions are proposed to occur at Brønsted acid sites that are present in various amounts in cation-exchanged zeolites.

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“…47 Dwyer and coworkers 48 identified that the band at 1580 cm À1 was assigned to the v asym C-C-C of allyl carbocations during the isomerization of 1-butene in H-ferrierite zeolite at 300 K and the bands at 4154 cm À1 and 4084 cm À1 are the C-H stretching vibration of the allyl carbocations. Fo ¨rster et al 49 assigned the bands at 1535 cm À1 and 1505 cm À1 to the allyl carbocations of propene in zeolites. Bordiga et al 34 also reported a vibrational ring mode at ca.…”

Section: Experimental Methods To Capture Carbocations In Zeolitesmentioning

confidence: 99%

Carbocation chemistry confined in zeolites: spectroscopic and theoretical characterizations

Chen

Liu

et al. 2022

Chem. Soc. Rev.

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UV-VIS and IR Spectroscopic Investigations on the Generation of Alkenyl Carbocations from Propene in Zeolites of Different Acidity

Cited by 9 publications

References 17 publications

Spectroscopic Signatures Reveal Cyclopentenyl Cation Contributions in Methanol-to-Olefins Catalysis

Spectroscopic Signatures Reveal Cyclopentenyl Cation Contributions in Methanol-to-Olefins Catalysis

Photooxidation of 1-Alkenes in Zeolites: A Study of the Factors that Influence Product Selectivity and Formation

Carbocation chemistry confined in zeolites: spectroscopic and theoretical characterizations

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