Zeolites experimental conditions

Fig. 10. Conversion of ethylene to ethylbenzenes during the alkylation of benzene with ethylene on H-Y, R—Y and R-X zeolites. Experimental conditions 177°C 1 atm C6H6/C2H4 = 12 molar ratio contact time, LHSY 5.4.

It is also important to point out that pure cobalt oxide, alone or finely dispersed in Si02 (i.e. Co-Si02, Co-Si02-l and Co-Si02-2 in Table 1), zeolite HY, fullerene (i.e. C q/C-,0 80/20) is at least as effective as the reduced oxides for the production of nanotubules in our experimental conditions. In fact, the catalysts studied in this work are also active if the hydrogenation step is not performed. This important point, is presently being investigated in our laboratory in order to elucidate the nature of the active catalyst (probably a metal carbide) for the production of nanotubules. 

We have demonstrated that a combined experimental (27A1 3Q MAS NMR) and theoretical (QM-Pot employing the bare framework model) approach represents a powerful tool for the determination of the local geometry of framework A104 tetrahedra, the prediction of27A1 isotropic chemical shifts in hydrated silicon rich zeolites, and the identification of A1 siting in the framework of silicon-rich zeolites. Experimental evidence is provided for the occupation of at least 10 out of 24 distinguishable framework T sites by A1 atoms in silicon-rich ZSM-5. The conclusion is reached that the A1 distribution over the framework T sites is neither random nor controlled by a simple rule, but depends on the conditions of the zeolite synthesis. 

The IR spectra of CO adsorbed on CuK-FER with the same Cu/Al ratio (Cu/Al = 0.09) but different Si/Al ratio (nominal value of Si/Al ratio are 8.6 and 27.5) were taken at the same experimental conditions (Figure IB). Since dual cation sites capable of bridging CO should be more abundant in the FER matrix with a lower Si/Al ratio (i.e., with a higher concentration of Al in the framework and, thus, higher concentration of extraframework cations in the zeolite), the band at 2138 cm"1 should be more prominent for the sample with lower Si/Al ratio. Indeed, it is clearly seen that the band at 2138 cm"1 is more pronounced in the case of the CuK-FER sample with lower Si/Al ratio (Figure IB). 

Catalytic experiments. The runs are performed in a 300mL static reactor (Autoclave Engineers Model AE 300) for 15hrs under an initial 20 bar pressure with a sample weight leading to 0.U mg-atom of metal. Neither the unloaded zeolites nor the molecular clusters are active in CO hydrogenation under our experimental conditions. 

Several experimental conditions must be realized for the application of Eq. (72). The donor and acceptor molecules should enter the channels at about the same rate, so that the assumptions made for the initial state are sufficiently well fulfilled. They should not be able to glide past each other once they are inside the channels. The crystals should be so long that molecules entering from both sides do not reach each other in the middle part of the channels during the time of observation. These conditions can be fulfilled for the donor/acceptor pair Py+/Ox+ in zeolite L. Moreover, different stages of the diffusion can be observed by means of an optical microscope. 

Zeolites are crystalline aluminosilicates with porous, framework structures made up of linked [Si04] and [A104] tetrahedra that form channels and cages of discrete size [24]. The framework structures of zeolites bear a net negative charge, which must be balanced by positively charged species, typically alkali or alkaline earth metal cations these cations maybe exchanged for one another under appropriate experimental conditions. Zeolites are capable of... 

Pd/Cu zeolite Y associations were found to be selective catalysts for oxidation of olefins in the presence of steam at temperatures ranging from 373 to 433K [22-30]. Acetone and acetaldehyde were obtained by propylene and ethylene oxidation, with selectivities of at least 90%. Neither Pd/Y nor Cu/Y showed good activity in these reactions. The conversion of different olefins under the same experimental conditions decreases in the following order [23] ethylene > propylene > 1-butene > cis-2-butene - trans-2-butene. 

Fig. 28. 36-MHz l5N CP/MAS spectra of pyridine-15N on zeolite HY. The experimental conditions were all the same for (a) and (b), except that sample b was extensively dealuminated by increasing the activation temperature to SSITC (400°C for sample a). Both spectra were acquired at 77 K to prevent chemical exchanges on the NMR time scale, (a) The single resonance at —176 ppm as well as its associated sidebands indicates protonation of pyridine by the Brpnsted sites, (b) In addition to the protonated pyridine, four additional resonances at -68, -88, —116, and -140 ppm are also seen, indicating complexation of pyridine with different extraframework Lewis sites.

In the acylation of alkenes with acetyl chloride120 or acetic anhydride120,121 over zeolites (HX, HY, H-mordenite, H-ZSM-5, H-Beta) usually complex product mixtures including isomeric unsaturated ketones, chloroketones, and acetates were obtained. Zeolite HY proved to be the most active catalyst under mild experimental conditions (25-60°C).120,121... 

Cince the catalytic activity of synthetic zeolites was first revealed (1, 2), catalytic properties of zeolites have received increasing attention. The role of zeolites as catalysts, together with their catalytic polyfunctionality, results from specific properties of the individual catalytic reaction and of the individual zeolite. These circumstances as well as the different experimental conditions under which they have been studied make it difficult to generalize on the experimental data from zeolite catalysis. As new data have accumulated, new theories about the nature of the catalytic activity of zeolites have evolved (8-9). The most common theories correlate zeolite catalytic activity with their proton-donating and electron-deficient functions. As proton-donating sites or Bronsted acid sites one considers hydroxyl groups of decationized zeolites these are formed by direct substitution of part of the cations for protons on decomposition of NH4+ cations or as a result of hydrolysis after substitution of alkali cations for rare earth cations. As electron-deficient sites or Lewis acid sites one considers usually three-coordinated aluminum atoms, formed as a result of dehydroxylation of H-zeolites by calcination (8,10-13). 

Irradiations with PrX zeolite, Table VI, showed that above an exposure of 1019 thermal neutrons cm-2, which is the range of interest, the 142Pr yield decreased, the 141Pr target content increased, and the enrichment factor decreased from >60 to 4. The fast neutron exposure was approximately one-third the thermal. In each test 10 mg of PrX was irradiated, allowed to decay for several days, and then eluted with 2 ml of 7M LiCl. Experimental conditions during these irradiations were not well defined. 

Minachev, Eidus et al. (25) found that Ca, Ni, Co, and NdY zeolites were active in the disproportionation of propylene to ethylene and butenes. The process was accompanied by hydrogen rearrangement to form saturated hydrocarbons and condensation products. The selectivity in this reaction depends on the composition of the catalysts, their pretreatment, and the experimental conditions (26). 

Tphe excellent catalytic activity of lanthanum exchanged faujasite zeo-A lites in reactions involving carbonium ions has been reported previously (1—10). Studies deal with isomerization (o-xylene (1), 1-methy 1-2-ethylbenzene (2)), alkylation (ethylene-benzene (3) propylene-benzene (4), propylene-toluene (5)), and cracking reactions (n-butane (5), n-hexane, n-heptane, ethylbenzene (6), cumene (7, 8, 10)). The catalytic activity of LaY zeolites is equivalent to that of HY zeolites (5 7). The stability of activity for LaY was studied after thermal treatment up to 750° C. However, discrepancies arise in the determination of the optimal temperatures of pretreatment. For the same kind of reaction (alkylation), the activity increases (4), remains constant (5), or decreases (3) with increasing temperatures. These results may be attributed to experimental conditions (5) and to differences in the nature of the active sites involved. Other factors, such as the introduction of cations (11) and rehydration treatments (6), may influence the catalytic activity. Water vapor effects are easily... 

Transient absorption spectra from valerophenone in Na-ZSM-5 and Cs-ZSM-5 could be assigned to triplet-triplet transitions no spectra which can be attributed to the hydroxy-1,4-biradicals were detected. Furthermore, the decay of the transient signals could not be fit to either a single or double exponential expression, and samples prepared under apparently identical conditions exhibited half-lives that varied by a factor of 2 [292], All of these spectroscopic observations suggest that the valerophenone molecules reside in a distribution of sites within a zeolite and migration among them under the experimental conditions is slow. Thus, the Norrish II photoproduct ratios must be interpreted in terms of not only the relative populations of alkanophenones at each site type, but also the quantum efficiencies of each and the conformational preferences of the intermediate BR in each environment. 

Different attempts to use some solid acids -as zeolites and phosphates-as catalysts in gas-solid reactions, have been already described (refs. 4-6). Nevertheless, the control of the selectivity appears to be difficult because drastic experimental conditions are always required. 

In the same experimental conditions (lh, 160°C), the protonic zeolite NaHY gives rise to quantitative percentages of conversion, being the rate 2/3 = 33 65, while at 130°C, 3h, the conversion is of a 15% and the main product the ketone 3. 

In all the isomerization reactions carried out in heterogeneous conditions, the nature of the products and product ratio depended largely on the type of catalyst employed, and, moreover, in most of the cases no selectivity was found. Papers have recently appeared concerning the transformation of styrene oxide into phenyl acetaldehyde catalyzed by a series of natural silicates and amorphous silica-alumina (ref. 15) and by pentasil type zeolites (ref. 16). It is said that, in both cases, isomerization occurs on the acidic sites (si lands) of the external surface, which act as active centers even under mild experimental conditions. 

In the fall of 1999, a pilot zeolite barrier was installed at the West Valley Demonstration Project (WVDP) in Western New York. The WVDP is an environmental management project being conducted by the U.S. Department of Energy (DOE) with the cooperation of the New York State Energy Research and Development Authority. Details of the installation are reported by Moore et al. (2000). The clinoptilolite material used in the barrier had previously been studied by Cantrell (1996) for a proposed installation at the DOE Hanford Facility, by Fuhrmann et al. (1995) for use at the WVDP, and by Lee et al. (1998) for the 1998 installation at Chalk River, Ontario. The range of previously estimated distribution coefficients (Kd) was from 650 mL/g (Fuhrmann et al.) to 2600 mL/g (Cantrell). The variation across these studies is most likely attributable to differences in the source water and experimental conditions, although the data interpretation... 

A macrolevel cell can be exemplified by the cellular model [231] and [232] when the cell is incorporated into a system of the strongly bonded Pt crystallites applied to a zeolite. The model allows to describe the complex oscillations of the CO oxidation rate. A heavy dependence of the reaction rates to be computed on the way the coupling rules for the neighboring cells are selected is shown. By varying these rules, it is possible to simulate the various experimental conditions. 

Scanning electron microscopy/energy dispersive X-ray (SEM/EDX) analyses have been used to probe the migration of vanadium in model fluid cracking catalysts (FCC). At the experimental conditions used, vanadium can migrate either from a Eu3+-exchanged Y (EuY) zeolite to an AAA-alumina gel or vice versa, depending on the type of vanadium precursor used. 

Under our experimental conditions, complete removal of the carbonaceous residues leads to the appearance of structure defects and/or short-range amorphization of the zeolitic framework, not detectable by X-ray diffraction. The coke produced during ortho-xylene cracking is the most difficult to remove and its elimination leads to pronounced modifications of the structure and of the crystallinity, doubtless because of its greater poly aromatic (or graphitic) character. 

Twenty-eight kinetics of crystallization of different types of zeolites (A (2,12,13,35,36), X (2,6,37), L (38), P (3 0, ZSM-5 (39-41), synthetic mordenite (2,42) and offretite (43)), synthetized by various authors under various experimental conditions, have been analysed by using Equations (1) and (5). 

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