Irradiation zeolites

A five-line spectrum attributed to the NH2 radical has been observed by Sorokin and co-workers (105) in y-irradiated zeolites containing am-... 

So far, we have considered only those systems in which the two nuclei are equivalent. This is not the case for OJ on Mo03/Si02 (81, 82) and some y-irradiated zeolites (81, 83a), where the oxygen nuclei are found to be inequivalent. This case is considered in more detail in the next section. 

P.J. Kasai, Electron Spin Resonance Studies of y- and X-ray-irradiated Zeolites. J. Chem. Phys., 1965, 43, 3322-3327. 

Metal atom clusters in the 26-hedra could (as with water molecules) contain many metal atoms. An example is the fourteen atom Agg+8Ag+ cluster in Y-irradiated zeolite Ag-A[25]. Saturation is achieved for mercury sorbed into silver-exchanged faujasites and other zeolites [26]. The Ag+ is reduced to Ag atoms and then at an approximate critical pressure of mercury vapour there is nucleation of mercury clusters which fill all the pore volume as the pressure of Hg vapour increases further. Mercury-zeolite systems are the oily ones in which sorption isotherms have been investigated quantitatively. Hcwever other metal atoms introduced into zeolites (by ion exchange and reduction, or as metal carbonyls and their decomposition) all show, on heating, a strong tendency to form clusters by migration of atoms, which can aggregate both within and outside the crystals. 

The use of a zeolite as an HPA support has been explored in a classical research work by Ozer and Ferry, who reported the use of a NaY zeolite with H2NaPW 12O40, H4SiWi2O40 or H3PM012O40 as photocatalysts for the degradation of 1,2-dichlorobenzene [71]. In this case HPA has not been previously heterogeneized but just added to the irradiated zeolite suspension. Authors attributed an important... 

As with 16-38, these reactions have sometimes been performed with acid catalysts. Ultrasound has been used to promote the reaction, and it has also been done without solvent using microwave irradiation. Another solid-state variation is done on moist LiBr. Zeolites have been used to promote the... 

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). 

To understand a heterogeneous synthesis process, the following preliminary experiments were performed in a homogeneous solution (1) In the absence of zeolite, the sonochemical reduction of [Pd(NH3)4]2+ to Pd° was found to occur, but the rate of [Pd(NH3)4]2+reduction was much slower than that of [PdCL(]2 reduc-tion. (2) It was confirmed that 2-propanol acted as a precursor to form reductants under ultrasonic irradiation. 

Based on the above results, ultrasonic irradiation to ion-exchanged [Pd(NH3)4]2+-zeolite powders was performed in an aqueous solution containing 2-propanol. The reduction of [Pd(NH3)4]2+-zeolite to Pd°-zeolite was confirmed by XPS analyses. However, from XPS depth analyses of the prepared samples, it was suggested that the [Pd(NH3)4]2+ complexes in the zeolite pore were not sufficiently reduced even in the presence of 2-propanol. Presumably, the reductants formed from 2-propanol sonolysis could not easily diffuse into the zeolite nano-pore (size 1.2 nm) and/or reductants undergo recombination reactions and quenching reactions with the walls. In addition, the results of XPS spectral analyses of the sonochemically prepared Pd-zeolite powders indicated that the average size of the Pd clusters on the zeolite surface is roughly estimated to be less than 1 nm and composed of several dozen Pd atoms. 

Fig. 28. The ESR spectra of Or on - -irradiated alkaline earth zeolites. Only the low-field maxima are shown here 110).

Fig. 30. The ESR spectra of 7-irradiated Y-type zeolites that were dehydroxylated at 600° prior to irradiation (a) A1HY, (b) NH,Y.

A ground mixture of iron(III) nitrate and HZSM-5 zeolite, termed zeofen , has also been used both, in dichloromethane solution and in solid state under MW irradiation conditions [101]. It has been suggested that the zeolite aids the reproducibility of the reaction but any other aluminosilicate support would probably be equally effective. Recent studies point out attractive alternatives that do not employ any of the solid supports in such oxidations with nitrate salts [102]. 

Among various solid supports (K10, acidic alumina, zeolite HY, silica gel) silica gel proved to be the best after irradiation for 4 min the reactions led to yields ranging from 62 to 94%. 

Silica gel or zeolite HY have also been used successfully for the synthesis of imidazoles, isoxazoles, and pyrazoles [59], under irradiation, without solvent (Scheme 8.39). 

The dispersion and solid-state ion exchange of ZnCl2 on to the surface of NaY zeolite by use of microwave irradiation [17] and modification of the surface of active carbon as catalyst support by means of microwave induced treatment have also been reported [18]. The ion-exchange reactions of both cationic (montmorillonites) and anionic clays (layered double hydroxides) were greatly accelerated under conditions of microwave heating compared with other techniques currently available [19.]... 

The most successful application of microwave energy in the preparation of heterogeneous solid catalysts has been the microwave synthesis and modification of zeolites [21, 22], For example, cracking catalysts in the form of uniformly sized Y zeolite crystallites were prepared by microwave irradiation in 10 min, whereas 10-50 h were required by conventional heating techniques. Similarly, ZSM-5 was synthesized in 30 min by use of this technique. The rapid internal heating induced by microwaves not only led to a shorter synthesis time, and high crystallinity, but also enhanced substitution and ion exchange [22]. 

Durable changes of the catalytic properties of supported platinum induced by microwave irradiation have been also recorded [29]. A drastic reduction of the time of activation (from 9 h to 10 min) was observed in the activation of NaY zeolite catalyst by microwave dehydration in comparison with conventional thermal activation [30]. The very efficient activation and regeneration of zeolites by microwave heating can be explained by the direct desorption of water molecules from zeolite by the electromagnetic field this process is independent of the temperature of the solid [31]. Interaction between the adsorbed molecules and the microwave field does not result simply in heating of the system. Desorption is much faster than in the conventional thermal process, because transport of water molecules from the inside of the zeolite pores is much faster than the usual diffusion process. 

An efficient method for conversion of a variety of acids into their corresponding amides in the presence of zeolite-HY under the action of microwave irradiation has recently been described [55], Scheme 10.10. 

Methane has also been used as the reducing agent in the catalytic conversion of NO to N2 over Co-ZSM-5 zeolites [75] in the presence of oxygen. The high NO conversions (>70%) were achieved by microwave irradiation at 250-400 °C, whereas under similar conditions thermal runs failed to convert either NO or methane in significant amounts. The high activity and selectivity of the reduction of NO by methane achieved with microwave irradiation was probably because of the activation of methane to form methyl radicals at relatively low reaction temperatures. 

The Frei photooxygenations in Fig. 32 were all conducted by irradiation of the zeolite powder in the absence of solvent. Solvents shield the substrate from the electrostatic field of the cation and reduces the magnitude of the bathochromic shift.122 Kojima and coworkers reported that irradiation of traw.v-stilbenetajNaY in... 

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