Radical in Zeolite

Vasenkov, S. and Frei, H. (2000). Time-resolved study of acetyl radical in zeolite NaY by step-scan FT-IR spectroscopy. J. Phys. Chem. A 104, 4327 4332... 

The previous chapters and examples were meant to familiarize the reader with current conventional ESR methodology and remove any barriers to the application of this kind of spectroscopy in electron-transfer studies. Unavoidably, not all aspects of inorganic ESR could be treated properly for instance, both the ESR spectroscopy of bioinorganic systems (e.g. heme species or iron-sulfur clusters) or materials chemistry (e.g. defect centers, radicals in zeolites) are vast and entirely independent research areas. Nevertheless, the molecular species presented here might serve as introductory examples to illustrate the versatility of this particular kind of magnetic resonance. 

The isotropic and anisotropic hyperfine coupling terms in a arise from interactions between electron and nuclear spins, and provide information about the nature of the orbital containing the unpaired electron and the extent to which it overlaps with orbitals on adjacent atoms. The anisotropic term can cause similar difficulties to the g tensor anisotropy in analysing spectra of polycrystalline powders extracting coupling constants from spectra of transition metal ions or radicals in zeolites can be difficult or impossible without computer simulation. 

With recent advancement in the measurement techniques and the data analysis methods ESR spectroscopy is an increasingly important tool in the studies on catalysis and solid surfaces. This chapter focuses on the following five specific subjects relevant to the ESR applications in catalysis and environmental science (a) nitric oxide (NO) adsorbed on zeolites, (b) Cu(I)-NO complexes formed in zeolites, (c) structure and dynamics of organic radicals in zeolites, (d) titanium dioxide (Ti02) semiconductor photo-catalysis, and (e) the superoxide (O2 ) ion radical. 

Cage Effects on Stability and Molecular Dynamics of Amine Radicals in Zeolites... 

X-Ray irradiation of quartz or silica particles induces an electron-trap lattice defect accompanied by a parallel increase in cytotoxicity (Davies, 1968). Aluminosilicate zeolites and clays (Laszlo, 1987) have been shown by electron spin resonance (e.s.r.) studies to involve free-radical intermediates in their catalytic activity. Generation of free radicals in solids may also occur by physical scission of chemical bonds and the consequent formation of dangling bonds , as exemplified by the freshly fractured theory of silicosis (Wright, 1950 Fubini et al., 1991). The entrapment of long-lived metastable free radicals has been shown to occur in the tar of cigarette smoke (Pryor, 1987). 

Supramolecular concepts involved in the size- and shape-selective aspects of the channels and cavities of zeolites are used to control the selectivity of reactions of species produced by photoexcitation of molecules encapsulated within zeolites. The photochemistry of ketones in zeolites has been extensively studied. Photoexcitation of ketones adsorbed on zeolites at room temperature produces radical species by the Norrish type 1 reaction. A geminate (born together) radical pair is initially produced by photolysis of the ketone, and the control of the reaction products of such radicals is determined by the initial supramolecular structure... 

In solution, the Norrish type 1 reaction of ketones results in the non-selective free-radical combination reactions to give products AA, AB and BB in the ratio of 1 2 1, whereas photolysis of ketones in zeolites produces ... 

Cation-radicals, stabilized in zeolites, are excellent one-electron oxidizers for alkenes. In this bimolecular reaction, only those oxidizable alkenes can give rise to cation-radicals, which are able to penetrate into the zeolite channels. From two dienes, 2,4-hexadiene and cyclooctadiene, only the linear one (with the cylindrical width of 0.44 nm) can reach the biphenyl cation-radical or encounter it in the channel (if the cation-radical migrates from its site toward the donor). The eight-membered ring is too large to penetrate into the Na-ZSM-5 channels. The cyclooctadiene can be confined if the cylindrical width is 0.61 nm, however the width of the channels in Na-ZSM-5 is only 0.55 nm. No cyclooctadiene reaction with the confined biphenyl cation-radical was detected despite the fact that, in solution, one-electron exchange between cyclooctadiene and (biphenyl) proceeds readily (Morkin et al. 2003). 

The restriction for a nucleophile to penetrate and react with the confined cation-radical sometimes leads to unexpected results. Comparing the reactions of thianthrene cation-radicals, Ran-gappa and Shine (2006) refer to the zeolite situation. When thianthrene is absorbed by zeolites, either by thermal evaporation or from solution, thianthrene cation-radical is formed. The adsorbed cation-radical is stable in zeolite for a very long time. If isooctane (2,2,4-trimethylpentane) was used as a solvent, tert-butylthianthrene was formed in high yield. The authors noted it is apparent that the solvent underwent rupture, but the mechanism of the reaction remains unsolved. ... 

In conclusion, we believe that cracking of gas-oil is taking place on zeolites via carbonium, carb mium ions and radicals. In the case of steam dealujninated samples, when more than 5 Al per unit cell (Si/Al<30) are present in the framework of the zeolite, the ionic mechanism is much more important than 1 he radical one. When the framev ork aluminum decreases and the number of defects increases, the radical mechanism becomes operative and eventually dominant when practically no aluminum is present in l he zeolite framework and superacid Brdnsted sites (at 3610 cm ) are not present. 

When generated in zeolites, alkene or arene radical cations react with the parent molecules to form ti-dimer radical cations. For example, 2,3-dimethyl-l-butene and benzene formed 91 + and 92 +, respectively. The confinement and limited diffusion of the radical cation in the zeolite favor an interaction between a radical cation and a neutral parent in the same channel. 

Let us consider how H+M-M+ is formed, using the scheme for thermal genesis of the active sites and the spectra in Figure 1. On adsorption of Y,Y-dimethylaniline in zeolite 3 in the dark, the 540-600-nm band is observed only in the samples heat treated at 350°, 450°, and 550°C—i.e., when H+M-M+ formation is preceded by formation of both primary MH+ and M+ products. The intensity of bands shows that the most favorable conditions for H+M-M+ formation occurred after zeolite pretreatment at 550°C when the excess M+ cation radicals (430-470 nm) were found together with the H+M+-M+ product. When the samples were treated at 350° and 450°C, all M+ cation radicals reacted. 

Additional dividends from NMR will most likely continue to lie in the area of diffusion and kinetics. Newer NMR techniques here are the ultra-slow motion (25) and rotating frame relaxation (26) techniques which allow measurements of very long jump times. Application of these techniques to the exchange region has been reported for water on NaX in this region they offer a means of deducing second moments of the tightly bound species (9, 52). The CIDNP technique should be applicable to the study of radical reactions on surfaces and in zeolites (58). 

Based on steady-state and time-resolved emission studies, Scaiano and coworkers have concluded that silicalite (a pentasil zeolite) provides at least two types of sites for guest molecules [234-236], The triplet states of several arylalkyl ketones and diaryl ketones (benzophenone, xanthone, and benzil) have been used as probes. Phosphorescence from each molecule included in silicalite was observed. With the help of time-resolved diffuse reflectance spectroscopy, it has been possible to show that these triplet decays follow complex kinetics and extend over long periods of time. Experiments with benzophenone and arylalkyl ketones demonstrate that some sites are more easily accessed by the small quencher molecule oxygen. Also, diffuse reflectance studies in Na + -X showed that diphenylmethyl radicals in various sites decay over time periods differing by seven orders of magnitude (t varies between 20/is and 30 min) [237]. 

The same holds for t-BuOOH and Fe phthalocyanines encapsulated in zeolites or adsorbed on carbon black (121, 124). On the other hand, hydroperoxides have been detected as products in the oxidation of ds-pinane by t-BuOOH in the presence of zeolite Y-encapsulated FePc (133). This is irrefutable evidence of trapping of a radical by dissolved O2. Superior hydroperoxide yields are obtained with FePc in zeolite NaY in comparison with the homogeneous reaction, particularly at subambient temperature ... 

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