Zeolite decationated

The free dimensions of the windows in zeolite A are 4.3A and those in mordenite are 6.7 X 7.0A. Any of the sorbates of Table VIII could migrate therefore into the crystals with minimal values of E, were it not for an influence of the ions (diameter 2.66A). These ions must obstruct the diffusion paths partially in each case. Indeed, ion exchange is one of the most effective ways of changing the molecular sieve characteristics of zeolites. Decationation, as in H-mordenite or H-clinoptilolite, produces maximum clearance of diffusion paths from metallic ions, so... 

Hyperfine interaction has also been used to study adsorption sites on several catalysts. One paramagnetic probe is the same superoxide ion formed from oxygen-16, which has no nuclear magnetic moment. Examination of the spectrum shown in Fig. 5 shows that the adsorbed molecule ion reacts rather strongly with one aluminum atom in a decationated zeolite (S3). The spectrum can be resolved into three sets of six hyperfine lines. Each set of lines represents the hyperfine interaction with WA1 (I = f) along one of the three principal axes. The fairly uniform splitting in the three directions indicates that the impaired electron is mixing with an... 

The NO spectrum has now been studied for the molecule adsorbed on ZnO, ZnS (18), 7-AI2O3, silica-alumina, silica-magnesia (20), an A-type zeolite (97), H-mordenite (98), and a variety of Y-type zeolites including NaY (19), MgY, CaY, BaY, SrY (81), decationated-Y (19), ScY, LaY, and A1HY (99). The nitric oxide molecule has mainly been used as a... 

A similar site has been observed for decationated-Y, A1HY, MgY, CaY and SrY zeolites but not for BaY and NaY zeolites. The number of sites on MgY, CaY, and SrY is about 50 times less than the number observed... 

Zeolites can be over-washed during workup, resulting in decationization, also known as cation hydrolysis [64]. Decationization involves replacement of alkali... 

Measurement of the thermokinetic parameter can be used to provide a more detailed characterization of the acid properties of solid acid catalysts, for example, differentiate reversible and irreversible adsorption processes. For example, Auroux et al. [162] used volumetric, calorimetric, and thermokinetic data of ammonia adsorption to obtain a better definition of the acidity of decationated and boron-modified ZSM5 zeolites (Figure 13.7). 

Not all zeolite catalysts are used in the decationized or acid form it is also quite common to replace the Na" ions with lanthanide ions such as La " or Ce ". These ions now place themselves so that they can best neutralize three separated negative charges on tetrahedral A1 in the framework. The separation of charges causes high electrostatic field gradients in the cavities which are sufficiently large to polarize... 

The infrared stretching frequency of the hydroxyl associated with the Bronsted sites in decationized zeolites, fails in the range 3600 to 3660 cm. As the Si/Al ratio in the framework increases, this frequency tends to decrease. What does this suggest about the acidity of the highly siliceous zeolites ... 

Decationated zeolites. We start by considering decationated zeolites since they do not contain any metal ions extrinsic to the silica-alumina framework. This type of zeolite is obtained by pretreating, above 350°C, a NH4Y zeolite prepared by exchanging the sodium form of a Y-type zeolite with ammonium ions. Ammonia is evolved leaving a decationated or HY zeolite ... 

When NH4Y is activated at 400°C to form the decationated or HY zeolite, two types of V centers are observed after y irradiation in vacuo (274). Both types give a rather broad EPR line with the same g tensor, x 2.045, g2 = 2.005, and g3 = 2.002, but only one exhibits a six-line hyperfine structure (Ax not determined, A2 = 8.0, A3 = 7.5 G) due to an interaction with 27A1 nuclei (I = f) which is superimposed on the broad EPR line. These V centers are attributed to a positive hole (denoted by the symbol ) trapped either on an oxygen adjacent to both an A1 and a Si atom (V,) or on an oxygen adjacent to two Si atoms (V2). 

It seems that other acidic sites are the most efficient for the alkylation of aromatic compounds than for the reverse reaction, the cracking of alkylaromatic compounds [361]. For the forward process, a linear correlation was observed between the activity of decationized Y zeolites and the number of acidic sites corresponding to H0 < + 3.3, whereas for the cracking, the sites corresponding to H0 < —3.0 correlated with the activity. 

Later, Cattanach, Wu, and Venuto did an elaborate thermogravi-metric study on the calcination of ammonium zeolite Y and the resulting products (19). They found that the hydrogen zeolite reacted with anhydrous ammonia to yield an ammonium zeolite identical in ammonia content with the initial ammonium zeolite. Further, these workers reported that after loss of chemical water ( dehydroxylation according to Uytter-hoeven, Christner, and Hall or decationization according to Rabo, Pickert, Stamires, and Boyle) the sample became amorphous when exposed to moisture. This observation conflicted with the statement of Rabo et al. (16) in which they emphasized the extreme stability of their decationized Y. The data of Cattanach, Wu, and Yenuto prove, beyond any doubt, that they obtained the expected normal hydrogen zeolite Y prior to the loss of chemical water above 450°. Rabo et al., however, did not prove that the material from which they removed chemical water, was in fact, the hydrogen zeolite. They probably prepared, unknown to them at the time, the ultrastable zeolite described below. 

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). 

The influence of both heat treatment of decationized zeolites and the nature of cations on the proton-donating and electron-deficient zeolite properties has been studied (13-16). However, these works do not allow one to follow clearly the mutually dependent changes in proton-donating and... 

To study the interaction of adsorbed molecules with active sites in decationized zeolites we used optical electronic spectroscopy, which was successful (17-19) with silica-alumina catalysts. The results (17-19) were then extrapolated to zeolites 20-21). 

The object of this work was to study the influence of pretreated, decationized NH4-zeolites on adsorbed A,iV-dimethylaniline molecules. Such influence is caused by, proton-donating and electron-deficient active sites in decationized zeolites. Interaction of an aromatic amine molecule (M) with the proton-donating site leads to the formation of the MH+ molecule ion interaction with the electron-deficient site results in the M+ cation radical. Stabilization of these states by adsorption leads to the... 

Our spectroscopic investigations enable us to distinguish three phenomena which produce and accompany molecular transformations in decationized zeolites ... 

Schemes I—III do not differ significantly from those reported in the literature (8,12). First, the electron-deficient centers in the zeolites must arise at the expense of proton-donating sites. Secondly, the nonproton centers formed in decationized zeolites are essentially different from each other. Both facts are confirmed by the results of our investigations on the electronic spectra of decationized zeolites.

TTigh silica zeolites attract great attention since they are characterized by relatively high thermal stability and considerable acid resistance. Physicochemical properties of high silica zeolites, despite a number of investigations, have not been sufficiently studied. The same is true for L- and clinoptilolite zeolite. The data on synthesis, structure, adsorption properties, decationization, dealuminization, adsorption heats, and other properties of the above-mentioned zeolites have been given (1-15). Results of studies of physicochemical properties of L zeolites and of natural and modified clinoptilolite are given here. 

Natural zeolite was enriched with potassium ions and dealuminized. Enrichment of exchangeable ions of L zeolites and of clinoptilolite was done by multiple treatment with 0.52V solutions of the corresponding nitrates. Decationization and dealuminization were done by treating the natural zeolite with solutions of hydrochloric acid 0.25-12.02V. The Si02/Al203 ratio increased from 8.0 to 69.5, and the CaO content decreased from 6.30 to 1.00 wt % (Table II). 

Figure 6. Temperature dependence of the line width 8 Hp of die NMR signal of OH groups in decationated Y zeolites (75% Na+ ions exchanged, pretreatment temperature 800° and 400° C).

Since heating conditions (32) and aluminum extraction leading to a high stability have been avoided, the increased stability of the catalysts seems related to the presence of exchanged lanthanum. A zeolite with high thermostability can be obtained by introducing only 3.7 La3+ ions/unit cell into a decationated zeolite. A higher content of lanthanum does not... 

The main problem now is to justify the process.schematically depicted by Reactions 11 and 12. From pulse NMR experiments, Mestdagh, Stone, and Fripiat (12) have measured the lifetime (r) of a proton on a surface oxygen site in a decationated Y zeolite... 

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