Zeolites isotope exchange with

Figure 1.29. Calculated changes in the S 0 values of volcanic rocks (5 0 = +7.0%c) as a result of equilibrium oxygen isotope exchange with waters of different initial compositions. The dotted areas represent the ranges of rocks in the zeolite and the sericite-chlorite zones (Green et al., 1983).

The interaction of metal clusters in zeolites with protons has been studied by isotope exchange with D2 280-283). The presence of metals or metal impurities is essential for this exchange to occur at moderate temperatures absolutely no exchange between D2 and zeolite protons in metal-free HY is detected at room temperature. In the presence of Pt or Pd, however, exchange is fast and includes all protons in the zeolite, which are detectable by their O—H vibration bands in FTIR. 

Since spillover phenomena have been most directly sensed through the use of IR in OH-OD exchange [10] (in addition, in the case of reactions of solids, to phase modification), we used this technique to correlate with the catalytic results. One of the expected results of the action of Hjp is the enhancement of the number of Bronsted sites. FTIR analysis of adsorbed pyridine was then used to determine the relative amounts of the various kinds of acidic sites present. Isotopic exchange (OH-OD) experiments, followed by FTIR measurements, were used to obtain direct evidence of the spillover phenomena. This technique has already been successfully used for this purpose in other systems like Pt mixed or supported on silica, alumina or zeolites [10]. Conner et al. [11] and Roland et al. [12], employed FTIR to follow the deuterium spillover in systems where the source and the acceptor of Hjp were physically distinct phases, separated by a distance of several millimeters. In both cases, a gradient of deuterium concentration as a function of the distance to the source was observed and the zone where deuterium was detected extended with time. If spillover phenomena had not been involved, a gradientless exchange should have been observed. 

This discovery was quite unexpected, since iron oxide has been never reported as an active catalyst in either partial or full oxidation. The studies of two simplest reactions, i.e. O2 isotopic exchange and N2O decomposition, revealed a dramatic change of Fe properties in the ZSM-5 matrix compared to Fe203 [4]. Fe atoms lose their ability to activate O2 but gain remarkably in their ability to activate N2O. It gives rise to a great effect of the oxidant nature in the reaction of benzene oxidation over the FeZSM-5 zeolite (Table 1). Thus, in the presence of N2O benzene conversion is 27% at 623 K, while in the presence of O2 it is only 0.3% at 773 K. And what is more, there is a perfect change of the reaction route. Instead of selective phenol formation with... 

Figure 2. Lithium isotope separation effected during ion exchange with synthetic zeolite (Taylor and Urey 1938). As has been demonstrated repeatedly since that study, in both natural and synthetic experiments, Li is fixed more effectively in the exchanger than Li.

The oxygen atoms in the silicalite framework can be classified into approximately three kinds by the isotope exchange method proposed by Endoh et al. [12]. On the exchange reaction of in the zeolite framework with C 02 in the gas phase at 773 K, the most reactive oxygen atoms were in the terminal SiOH... 

We synthesized nine silicalites which had different concentrations of defect sites in the zeolite framework determined by isotope exchange method. These silicalites were treated with aluminium trichloride vapor under the same reaction conditions 923 K temperature, 1 h time, 11 kPa aluminium trichloride vapor pressure. Figure 1 shows the plots of the amount of aluminium atoms introduced into the framework against the amount of oxygen atoms on the defect sites. A... 

The results of many studies carried out with zeolites, mostly in the 1970s and reviewed in ref. [52], did not meet these expectations. Zeolites cannot compete with conventional catalytic systems both in the selective and complete oxidations. The main reason is that the transition metals introduced into a zeolite matrix lose their ability to activate dioxygen, as was evidenced by isotopic 02 exchange [53, 54]. The... 

Cu isotopes both with nuclear spin I-3/2. The nucle r g-factors of these two isotopes are sufficiently close that no resolution of the two isotopes is typically seen in zeolite matrices. No Jahn-Teller effects have been observed for Cu2+ in zeolites. The spin-lattice relaxation time of cupric ion is sufficiently long that it can be easily observed by GSR at room temperature and below. Thus cupric ion exchanged zeolites have been extensively studied (5,17-26) by ESR, but ESR alone has not typically given unambiguous information about the water coordination of cupric ion or the specific location of cupric ion in the zeolite lattice. This situation can be substantially improved by using electron spin echo modulation spectrometry. The modulation analysis is carried out as described in the previous sections. The number of coordinated deuterated water molecules is determined from deuterium modulation in three pulse electron spin echo spectra. The location in the zeolite lattice is determined partly from aluminum modulation and more quantitatively from cesium modulation. The symmetry of the various copper species is determined from the water coordination number and the characteristics of the ESR spectra. 

The fundamental carbonium ion-type reactions of olefins— including double bond and carbon skeleton isomerization, polymerization, isotopic exchange, and hydrogen transfer—have been reviewed earlier (62). The importance of a thorough understanding of the nature of olefin transformations over zeolite catalysts cannot be underestimated. Probably the most important and frequently recurring pattern is the transfer or redistribution of hydrogen that is observed with olefins over acidic crystalline aluminosilicate catalysts. 

Several techniques have been reported to yield particle size and distribution profiles, but their use has not been generalized yet, either because the technique is not straightforward or limited to specific cases or because the method used to process the spectroscopic data is too involved. This can be exemplified by the work of Dalla Betta and Boudart (188a), who determined by infrared (IR) the particle size of platinum encaged in CaY zeolite. The size was deduced from the fraction of OH groups exchanged with deuterium, and it was assumed that the isotopic exchange at low temperature is rapid only in the immediate vicinity of the particle. This... 

In 1966, Hoinkis and Levi reported that Sr2+ self-diffusion in SrX appeared to occur by two processes—one fast and the other slow (33). In 1967, they (34) reported that Cs+ and Rb+ isotope exchange in zeolite A does not follow the rate law for a simple diffusion process and appears to take place with 2 steps. They plotted their isotope exchange data as 1 — U vs. time, where U is the fractional attainment of equilibrium at any instant in time. In order to ascertain whether or not the rate data fit the simple diffusion equation, they replotted their data as... 

Apparatus. A vacuum circular static unit was used to study the isotopic exchange. The temperature in the zeolite bed was maintained with accuracy of 1°C. Omegatrone was employed to analyze the gas isotopic composition. 

Comparing the exchange rates per acid site for FAU and MFI (Fig. Schoofs found that for the same Al/Si + A1 ratio, the exchange rate was higher for MFI than for FAU-type zeolites in accordance with an important structural influence. A kinetic isotope effect (kn/kD = 1.7) was measured for MFI type zeolite, suggesting a faster exchange between CD4 and... 

Kubelkova, L., Novakova, J., Dolysek, Z., and Ziras, P., The effect of decationation of hydroxylated HNaY zeolites on their interaction with ethylene and propylene. Hydrogen complexes oligomerization and isotope exchange. Coll. Czech. Chem. Commun., 45, 3101, 1980. 

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