Evidence for the redox nature of zeolite catalysts

Flockhart, B. D.; McLoughlin, Lauren; Pink, Richard

doi:10.1016/0021-9517(72)90232-1

Cited by 38 publications

(5 citation statements)

References 10 publications

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“…There are numerous other examples that demonstrate the donor property of Z n - . For instance, it has long been known that trinitrobenezene , and tetracyanoethylene , immediately form the corresponding anion radicals upon contact with Na + Y, even at room temperature. It has been demonstrated that ET takes place from the framework to photosensitized electron acceptors such as pyrene, − tetracyanobenezene, 1,4-dicyanobenzene, pyromellitic dianhydride, and methyl viologen (MV 2+ ) .…”

Section: Resultsmentioning

confidence: 99%

Long-Lived Charge-Separation by Retarding Reverse Flow of Charge-Balancing Cation and Zeolite-Encapsulated Ru(bpy)₃²⁺as Photosensitized Electron Pump from Zeolite Framework to Externally Placed Viologen

et al. 2002

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K(+)-exchanged, Ru(bpy)(3)(2+)-encapsulating zeolite-Y [K(+)-Ru(bpy)(3)(2+)Y] and N-[3-(dicyclohexylmethyl)oxypropyl-N'-methyl-4,4'-bipyridinium [DCH-MV(2+)] were prepared, and visible light-induced electron transfer from the zeolite-encapsulated Ru(II) complex to the size-excluded viologen was studied in acetonitrile. Addition of a series of crown ethers (CEs) into the heterogeneous solution leads to over a 10-fold increase in the yield of DCH-MV(*)(+), where the yield linearly increases as the formation constant of CE with K(+) [K(f)(K(+))(CE)] increases. The following two sequential events are attributed to be responsible for the above novel phenomenon. First, K(+) ions are liberated from the zeolite to solution during interfacial electron transfer from the photoexcited Ru(II) complexes to DCH-MV(2+). Second, the liberated K(+) ions form strong host-guest complexes with the added CE molecules, which leads to retardation of the reverse flow of the cations, hence the charge-balancing electrons, from the solution to the zeolite. Surprisingly, the yield of DCH-MV(*)(+) reaches more than approximately 50 times the amount of Ru(bpy)(3)(2+) situated in the outermost supercages, despite the absence of electron relay in the zeolite. This is attributed to photosensitized electron pumping from the zeolite framework to viologen by the outermost Ru(bpy)(3)(2+) ions. In support of the above conclusion, Ru(bpy)(3)(3+) does not accumulate in the zeolite host while DCH-MV(*)(+) accumulates in the supernatant solution. Consistent with the above, the independently prepared hexafluorophosphate salt of Ru(bpy)(3)(3+) is reduced to Ru(bpy)(3)(2+) in acetonitrile upon contact with Ru(bpy)(3)(2+)-free M(+)Y (M(+) = Li(+), Na(+), K(+), Rb(+), and Cs(+)), where the yield increases as the donor strength of the framework oxygen increases. Although small, thermal electron transfer also takes place from the zeolite framework to DCH-MV(2+), where the yield increases upon increasing the donor strength of the framework, concentration of DCH-MV(2+), temperature, and K(f)(K(+))(CE) (when K(+)Y is the zeolite host). The photoyield is always higher than the thermal yield by 4-30 times, confirming that the zeolite-encapsulated Ru(bpy)(3)(2+) serves as the photosensitized electron pump.

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Section: Resultsmentioning

confidence: 99%

Long-Lived Charge-Separation by Retarding Reverse Flow of Charge-Balancing Cation and Zeolite-Encapsulated Ru(bpy)₃²⁺as Photosensitized Electron Pump from Zeolite Framework to Externally Placed Viologen

et al. 2002

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“…Zeolites are solid acids approaching the acidity of superacids; their application as industrial catalysts is mainly that of heterogeneous acids . In fact, several early studies of organic radical cations on amorphous silica−alumina were undertaken to elucidate the acid properties of these solids. , It is pertinent to summarize the nature and structure of the Brönsted and Lewis acid centers in zeolites.…”

Section: Iib1 Redox Reactivity and Aciditymentioning

confidence: 99%

“…Upon standing of the sample, the aminyl radicals formed (stable) diphenylnitroxyl radicals ( 62 • ). On H-Y zeolite, 60 formed the corresponding radical cation, 60 •+ ; it was characterized on Na-Y by a weak, broad ESR spectrum ( g = 2.002, fwhh = 14 G; 9 × 10 15 spins g -1 ) …”

Section: Va8 Radical Cations Of Nitrogen-containing Substratesmentioning

confidence: 99%

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Generation and Reactions of Organic Radical Cations in Zeolites

2002

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“…The same principle can be extended to zeolite which is a class of three-dimensionally interconnected inorganic polymers consisting of silicon, aluminum, and oxygen atoms, along with the exchangeable charge-balancing cations, which exist within the voids created by the negatively charged framework. It is therefore reasonable to expect that either the framework or the countercation, individually or in combination, would exert a donor − or an acceptor − property toward the incorporated guest molecules depending on their relative electron richness …”

Section: Introductionmentioning

confidence: 99%

Iodine as a Visible Probe for the Evaluation of Zeolite Donor Strength

et al. 1996

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The visible absorption band of iodine adsorbed on zeolite blue-shifted with increasing the electropositivity of the countercation and the aluminum content in the framework. This phenomenon was attributed to the increase in the donor strength of the zeolite framework based on the analogous spectral shift of iodine in solution. In support of this, a negative linear correlation was observed between the measured visible bands of iodine adsorbed on various zeolites and their calculated Sanderson's intermediate electronegativities. However, the simultaneous change in the electrostatic field strength within the zeolite pores, as a result of the change in the number and the size of the cation, has to be taken into account in order to interpret the overall spectral shifts more precisely. The iodine band sensitively blue-shifted with decreasing the moisture content in the framework but red-shifted with the loss of NH3 from NH4 +-exchanged zeolites. The visible band of iodine also progressively red-shifted with increasing the adsorbed amount, presumably due to the nature of iodine to deplete electron density from the framework. The framework structure also affected the spectral shift of the visible iodine band. The overall results established that iodine can be used as a novel molecular probe for the quantitative evaluation of the zeolite donor strength.

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Evidence for the redox nature of zeolite catalysts

Cited by 38 publications

References 10 publications

Long-Lived Charge-Separation by Retarding Reverse Flow of Charge-Balancing Cation and Zeolite-Encapsulated Ru(bpy)₃²⁺as Photosensitized Electron Pump from Zeolite Framework to Externally Placed Viologen

Long-Lived Charge-Separation by Retarding Reverse Flow of Charge-Balancing Cation and Zeolite-Encapsulated Ru(bpy)₃²⁺as Photosensitized Electron Pump from Zeolite Framework to Externally Placed Viologen

Generation and Reactions of Organic Radical Cations in Zeolites

Iodine as a Visible Probe for the Evaluation of Zeolite Donor Strength

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Evidence for the redox nature of zeolite catalysts

Cited by 38 publications

References 10 publications

Long-Lived Charge-Separation by Retarding Reverse Flow of Charge-Balancing Cation and Zeolite-Encapsulated Ru(bpy)32+as Photosensitized Electron Pump from Zeolite Framework to Externally Placed Viologen

Long-Lived Charge-Separation by Retarding Reverse Flow of Charge-Balancing Cation and Zeolite-Encapsulated Ru(bpy)32+as Photosensitized Electron Pump from Zeolite Framework to Externally Placed Viologen

Generation and Reactions of Organic Radical Cations in Zeolites

Iodine as a Visible Probe for the Evaluation of Zeolite Donor Strength

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Product

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Long-Lived Charge-Separation by Retarding Reverse Flow of Charge-Balancing Cation and Zeolite-Encapsulated Ru(bpy)₃²⁺as Photosensitized Electron Pump from Zeolite Framework to Externally Placed Viologen

Long-Lived Charge-Separation by Retarding Reverse Flow of Charge-Balancing Cation and Zeolite-Encapsulated Ru(bpy)₃²⁺as Photosensitized Electron Pump from Zeolite Framework to Externally Placed Viologen