Zeolites experiments

In order to evaluate the role of the outer surface of the zeolites, experiments with sodalite were carried out. The sodalite cage should be too small to accommodate the bulky Diels-Alder transition state and the six-membered ring apertinres of sodalite are too small for the product to move out. The width of the product (3) is about 4.2A. 

Experiments which have been carried out indicate that the results discussed above which have all been obtained on H-ZSM-5 zeolites are actually valid for all protonated zeolites. Experiments have so far been carried out on protonated ZSM-11, erionite, mordenite, faujasite (Y-zeolite), offretite, ferrierite and L-zeolite (ref. 4). Till now, all the the investigated zeolites have given fully concordant results. While they all exhibit the same behaviour as regards activation, their resistance to deactivation and their product spectrum are in many cases dramatically different. Offretite for instance deactivates almost completely after a couple of hours with high activity, and the dominant hydrocarbon, by far, is methane (ref. 4). 

One example of the use of molecular mechanics is in investigating the mechanism of oxygen diffusion in albite, NaAlSisOg, a mineral related to zeolites. Experiments indicate that diffusion of oxygen in albite is faster in the presence of water. Molecular mechanics calculations show that the activation energy for OH- diffusing through the solid is lower than that for diffusion for O2-. 

The coupled derivatives are represented by AIPO4, GaP04, AlAs04, FeP04, and others which yield neutral frameworks with similarities to various silica crystal structures. The formation of an AlP04-type compound in the zeolite synthesis gel appeared to be a reasonable approach to the synthesis of phosphorus-substituted zeolites. Experiments therefore were initiated to attempt the isomorphous substitution of [PO4] in zeolite structures using the low-temperature and pressure hydrothermal gel systems generally employed in zeolite synthesis in the Union Carbide laboratories (4). 

In both Figures the uptake of CO2 (upper curves) and the gas temperature (lower curve), measured immediately below the microbalance connected sorbent sample are shown as functions of time, approaching equilibrium after ca. 5 hours. For the dry zeolite experiment an overshooting of the uptake of... 

Malmberg S, Votsmeier M, Gieshoff J, Soger N, MuBmann L, Schuler A, Drochner A (2007) Dynamic phenomena of SCR-catalysts containing Fe-exchanged zeolites - experiments and computer simulations. Topics in Catalysis (42-43) 33-36... 

Zeolites and Catalytic Cracking. The best-understood metal oxide catalysts are zeoHtes, ie, crystalline aluminosihcates (77—79). The zeoHtes are well understood because they have much more nearly uniform compositions and stmctures than amorphous metal oxides such as siUca and alumina. Here the usage of amorphous refers to results of x-ray diffraction experiments the crystaUites of a metal oxide such as y-Al202 that constitute the microparticles are usually so small that sharp x-ray diffraction patterns are not measured consequendy the soHds are said to be x-ray amorphous or simply amorphous. 

Numerical values for solid diffusivities D,j in adsorbents are sparse and disperse. Moreover, they may be strongly dependent on the adsorbed phase concentration of solute. Hence, locally conducted experiments and interpretation must be used to a great extent. Summaries of available data for surface diffusivities in activated carbon and other adsorbent materials and for micropore diffusivities in zeolites are given in Ruthven, Yang, Suzuki, and Karger and Ruthven (gen. refs.). 

From adsorption isotherms and competitive adsorption experiments (ref. 19) the strength of adsorption of the reaction components on zeolite CaY appeared to be HBr > o-dibromo > bromo- > p-dibromobenzene > Br2. 

Recently, high-quality SOD membranes for water separation have been developed by Khajavi etal. [21, 52]. These zeolite membranes should allow an absolute separation of water from almost any mixture since only very small molecules such as water, hydrogen, helium, and ammonia can theoretically enter through the six-membered window apertures. Water/alcohol separation factors 10 000 have been reported with reasonable water fluxes up to 2.25 kg nr h at 473 K in pervaporation experiments. 

The application of zeolite membranes in microreactors is still in an early stage of development, and suffers sometimes from unexpected problems arising from template removal [70]. However, several application examples of zeolite membranes in microstructured devices have been demonstrated yielding similar advantages as were to be expected from experiences on the macroscale. Because of the high surface to volume ratio of microreactors, the application of zeolite membranes in these systems has great potential. 

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. 

The initial experiments were carried out using zeolite Y modified with racemic... 

In a further set of experiments racemic butan-2-ol was reacted with zeolite Y modified with chiral sulfoxides. In the first set of experiments racemic butan-... 

These experiments clearly showed that it is a-oxygen participation that provides FeZSM-5 zeolites with such a remarkable catalytic performance in the reaction of benzene to phenol oxidation. Equations (1-3) written above are the main stages of the reaction mechanism. 

For this purpose we studied a temperature-programmed interaction of CH with a-oxygen. Experiments were carried out in a static setup with FeZSM-5 zeolite catalyst containing 0.80 wt % Fe203. The setup was equipped with an on-line mass-spectrometer and a microreactor which can be easily isolated from the rest part of the reaction volume. The sample pretreatment procedure was as follows. After heating in dioxygen at 823 K FeZSM-5 cooled down to 523 K. At this temperature, N2O decomposition was performed at 108 Pa to provide the a-oxygen deposition on the surface. After evacuation, the reactor was cooled down to the room temperature, and CH4 was fed into the reaction volume at 108 Pa. 

Fig 3 shows the results of two temperature-programmed experiments. In the first (blank) experiment CH4 reacts with a "bare" FeZSM-5 zeolite, while in the second one it reacts with the zeolite after a-oxygen loading on its surface. Obviously, the bare surface is quite inert towards methane (Fig 3a) after reactor opening a weak CH4 adsorption occurs at room temperature. A slight heating results in a complete recovery of the CH4 pressure. 

A wide variety of NMR methods are being applied to understand solid acids including zeolites and metal halides. Proton NMR is useful for characterizing Brpnsted sites in zeolites. Many nuclei are suitable for the study of probe molecules adsorbed directly or formed in situ as either intermediates or products. Adsorbates on metal halide powders display a rich carbenium ion chemistry. The interpretation of NMR experiments on solid acids has been greatly improved by Ae integration of theoretical chemistry and experiment. 

The reader is referred the recent book by Bell and Pines [2] for a more complete overview of the various methods and objectives in NMR studies of solid acids and other heterogeneous catalysis. In the present contribution we illustrate the application of H, and MAS NMR to two archetypal solid acids, Brpnsted sites in zeolites and solid metal halides such as aluminum chloride and bromide powders which exhibit "Lewis superacidity". An important characteristic of the more recent work is the integration of quantum chemical calculations into the design and interpretation of the NMR experiments. 

Figure 2 compares the results of theory and experiment for the specific case of p-fluoronitrobenzene. Inspection of the calculated structure shows that the proton is still on the zeolite, and the F shifts are more like chloroform solution than superacid solution. Furthermore, when the l F chemical shift was calculated for the theoretical structure, it was found to agree with the experimental result. 

Experiments were carried out using isotopically labelled methanol (97% 0) and ethanol (98% purchased from MSD Isotopes. Anhydrous isobutanol was purchased from Aldrich Chemical Co., Inc. and contained the natural abimdances of orygen isotopes, i.e. 99.8% and 0.2% O. Nafion-H was obtained fi om C. G. Processing, Inc. and Amberlyst resins were provided by Rohm and Haas. The 2SM-5 zeolite was provided by Mobil Research Development Corp. H-Mordenite, montmorillonite K-10, and silica-alumina 980 were obtained firom Norton, Aldrich, and Davison, respectively. y-AIumina was prepared from Catapal-B fi om Vista. 

These experiments demonstrate that the surface-catalyzed 2 reaction is far more effident than either the or C pathway for the dehydrative coupling of alcohols over the solid add catalysts tested. High selectivity to configurationally inverted chiral ethers ensues, espedally in the case of the HZSM-5 catalyst, in which the minor C or C paths were further suppressed by "bottling" of 3-ethoxypentane by the narrow zeolite chaimels. 

Characterization. In-situ diffuse reflectance FTIR (DRIFT) experiments were carried out with undiluted samples of the zeolites in a Spectratech DRIFT cell and a Nicolet Magna 550 spectrometer. Most experiments were carried out in a flow mode, passing 0.84 ml/s of a gas mixture containing inert (He, Ar or N2) and N2O, NO, CO or mixtures of these gases continuously through the cell at atmospheric pressure. Each spectrum was recorded by addition of 256 scans and a resolution of 8 cm. ... 

So, in the latter case the apparent activation energy is increased by the heat of adsorption of CO, amounting to about 40-60 kJ/mol as calculated from the IR experiments. Hence, for both the Co and the Cu samples E is slightly larger than 2 (table 2) while for iron ai is considerably lower. All these values are compatible with values reported in the literature for Fe-zeolites [6,7,10,11] or dilute solid solutions of Co in MgO [31]. The kinetic and IR results with NO indicate that, like CO, it can remove the oxygen from the... 

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