Zeolite dehydrated

Molecular sieves (dehydrated zeolite) purify petroleum products with their strong affinity for polar compounds such as water, carbon dioxide, hydrogen sulfide, and mercaptans. The petroleum product is passed through the sieve until the impurity is sufficiently removed after which the sieve may be regenerated by heating to 400 - bOO F. 

Intermediates were also observed in the synthesis of a neutral cluster, Ir4(CO)i2, from Ir(CO)2(acac) in the cages of zeohte NaY these were characterized by IR and extended X-ray absorption fine structure (EXAFS) spectroscopies, the latter being a technique ideally suited to investigation of small, highly dispersed species present in small amoimts in sohds. The spectra indicated dimeric intermediates, possibly Ir2(CO)8 [ 16], when the reaction was carried out in the near absence of water in the zeohte in contrast, the reaction in the dehydrated zeolite was faster, and no evidence of intermediates was observed [16]. 

Many experimental and, more recently, simulation methods have been put to use to try to localise the cations in faujasite (figure 1) in different situations hydrated or dehydrated zeolites, zeolites saturated with organic molecules, e.g. benzene, toluene, xylene. The four techniques that are described below have been used in more than 90% of all published works to detect and localise extraframework cations in faujasite type zeolites. 

Although its main use is still the identification of crystalline phases, X-ray diffraction is also the most used technique for the determination of the location of extraframework cations. XRD is well suited to perform structural characterisation of dehydrated zeolites since the framework is highly crystallised and the extraframework cations are often heavy elements. 

As a conclusion of this section, it can be said that the method used has to be carefully chosen according to the sample studied and/or the expected results. Conventional XRD may be sufficient to localise a single cation species in a dehydrated zeolite whereas for bicationic zeolites more elaborate techniques like anomalous XRD or MAS and MQMAS NMR may be necessary. If the focus of the study is more on the influence of adsorbed molecules on the distribution of the cations, neutron scattering may be needed to complete the work. Finally, highly dealuminated zeolites may be difficult to study with diffraction techniques, in this case NMR techniques may be the best available option. 

Other dehydrated zeolites have similar IR spectra with somewhat shifted positions of the bands as shown in Figure 4.34 [53], This fact was assigned to different acidities of the respective... 

Feuerstein, M., Hunger, M., Engel-hardt, G., and Amoureux, J.P. (1996) Characterisation of sodium cations in dehydrated zeolite NaX by Na NMR. Solid State Nucl. Magn. Reson., 7,... 

Even in more conventional PSA dehydration zeolites including types A, X and Y have aU been employed in pressure swing drying. Compound beds of alumina and zeoUtes X or Y have been employed for PSA dehydration and CO2 removal for pre-purification of feed to air separation units. 

The two oxygen-activating complexes [Co(L)j [L = salophen, tetra-tert-butylsalo-phen (55)] have been prepared and were also synthesized within dehydrated zeolite NaY using the intrazeolite ligand synthesis method [164]. These encapsulated metal complexes were shown to be capable of oxidizing hydroquinone and so were then used in a triple catalytic system to mediate the palladium-catalyzed aerobic 1,4-diacetoxylation of 1,3-dienes (Figure 5.28) [165]. The catalytic system involved [Pd(OAc)2], hydroquinone and the [Co(salophen)] complex in acetic acid (Co Pd diene hydroquinone LiOAc = 1 2.23 50 8.3 690, acetic acid, 25 °C,... 

Most studies have been carried out on dehydrated zeolites of the HY or NaY type. Structural and spectroscopic studies have shown the occurrence of interaction between the CO ligands of [Mo(CO)6] and cations not belonging to the zeolite... 

Once more, a dehydrated zeolite shows an Al-content lower than that of the hydrated phase. As in zeolite A, this discrepancy could be interpreted as due to dealumination of the framework during dehydration. 

The normal crystalline zeolites contain water molecules which are coordinated to the exchangeable cations. These structures can be dehydrated by heating under vacuum, and in these circumstances, the cations move position at the same time, frequently settling on sites with a lower coordination number. The dehydrated zeolites are extremely good drying agents, absorbing water to get back to the preferred hydrated condition. 

The zeolites that are useful as molecular sieves do not demonstrate an appreciable change in the basic framework structure on dehydration although the cations move to positions of lower coordination. After dehydration, zeolite A and others are remarkably stable to heating and do not decompose below about 700°C. The cavities in dehydrated zeolite A amount to about 50% of the volume. 

The dehydrated zeolites exchanged with various cations have been of catalytic interest in many reactions, among which cracking (259) and shape-selective catalysis (260) are most important. Other reactions include oxidation, carbonylation, and related reactions (261) as well as other nonacid catalytic reactions (262). 

If the water content is driven off (usually by heating to 350 °C in a vacuum), the dehydrated zeolite becomes an avid absorber of small molecules, especially water. The size of the molecules that can be absorbed is limited by the zeolite pore diameter, which is different for different zeolites (Table 7.1) a given zeolite (e.g., zeolite 3A) can be a highly selective absorber of, say, small amounts of water from dimethyl sulfoxide (DMSO) solvent. For this reason, dehydrated zeolites are often called molecular sieves. 

Al Quadrupole Coupling Constant, C-rjcc. ond Asymmetry Parameter, tj, of Framework Al Atoms in Dehydrated Zeolites Y (faujasite), MOR (mordenite), and ZSM-5... 

The properties of zeolitic water and the behavior of the exchangeable cations can be studied simultaneously by dielectric measurements (5, 6). In X-type zeolites Schirmer et al. (7) interpreted the dielectric relaxation as a jump of cations from sites II to III or from sites II to II. Jansen and Schoonheydt found only relaxations of cations on sites III in the dehydrated zeolites (8) as well as in the hydrated samples (9). Matron et al. (10) found three relaxations, a, (, and 7, in partially hydrated and hydrated NaX. They ascribed them respectively to cations on sites I and II, on sites III, and to water molecules. 

Table II lists the thermodynamic parameters for the conduction process. For the Na+ samples the activation energies are on the average 3.5 kcal lower than those for the conduction process of the corresponding dehydrated zeolites (<8). For K+-zeolites this difference averages 2.1 kcal. NaF69.8 is not included because of experimental difficulties in pellet preparation. The activation entropies are negative for the X-type zeolites and positive for the Y-type. The activation entropies are higher than those of the dehydrated samples (8) except for KF86.5. The effect of AS on E...

Experimental Technique. The IR spectra of dehydrated samples were recorded by UR 10 spectrometer (VEB Carl Zeiss Jena). To obtain spectra for dehydrated zeolites, samples were activated for 10 hours in air at 570°C, cooled to room temperature in the presence of P4O10, and ground with Nujol. The accuracy of the band maximum determination of the D6-ring band was 1.5 cm-1. IR characterization of the zeolites after CO adsorption was done in a cell with NaCl windows as described by Dunken and coworkers (9). The samples were heated at 550° C for 3 hours under vacuum. After cooling under vacuum to room temperature, CO was adsorbed (pco = 450 torr), and the spectra were recorded. 

We have studied the potassium form of zeolite L (batch 385-386), the same zeolite enriched with sodium and cesium ions (NaL and CsL), and a sample of potassium L-zeolite (sample A). Both zeolites were experimental batches. The chemical composition of dehydrated zeolites is given in Table I. 

Fp is in cm3/gram, %s is in grams/gram, and d is in grams/cm3. This has generally been referred to as the Gurvisch rule (1) and is frequently obeyed by many different adsorbates on different types of microporous adsorbents including silica gel and carbon. It also applies to dehydrated zeolites (2). 

Unlike the usual amorphous, microporous adsorbents, it is possible to calculate the theoretical micropore volume of a dehydrated zeolite from the known crystal structure. We have performed these calculations here for several of the better known zeolites including zeolite A, zeolite X, zeolite L, mordenite (Zeolon), (8) zeolite omega, (4) and the zeolite 0 (offretite... 

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