Zeolite decationized

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 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 ... 

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.

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... 

Bosa5ek et al. (168) used wide-line 27A1 NMR measurements of stationary samples to measure the EFG at the nuclear site in decationated zeolites. In zeolite Na-Y they measured a line half-width of Sv1/2 = 61 kHz (for vL = 16 MHz) which led, via theoretical considerations (173) to vD = 840 kHz the calculated field gradient was 2.9 V/A2. In hydrated samples this gradient was partially averaged by random reorientation of water molecules, giving <5v,/2 = 5.7 kHz and vQ = 256 kHz. 

These results are consistent with the generalized reaction scheme, initially presented by Rabo et al., in which both NH +- and H30+-exchanged zeolites decompose to produce an Br-form that upon further heating becomes a decationized zeolite. This behavior is similar to that reported earlier for calcined ion-exchanged synthetic mordenites, where two distinct sources of acidity were found in the NH4+-form but not in the H30+-form. 

Figure 5. Reaction scheme for the preparation of decationized zeolite ZSM-5.

Up to now, infrared spectroscopy has been used mainly to determine the types of hydroxyl groups and the acidity of zeolites (39). The frequencies of the vertical and horizontal vibrations (with respect to the cavity wall) of H2O molecules adsorbed in zeolite A were determined by measurements in the far infrared ( 220 and —75 cm" ) (37). These values are in agreement with a simple theoretical model. A number of ultraviolet and ESR studies are reviewed (33). The difference has been established between the specific molecular interaction of aromatic molecules on zeolites cationized with alkali cations and the more complex interactions involving charge transfer in CaX and deca-tionized X and Y zeolites. These more complex interactions with CaX zeolites containing protonized vacancies and with decationized zeolites are similar. These phenomena are related to the interactions of molecules with acidic centers in zeolites which are stronger, as compared with the molecular adsorption. 

Figure 1 shows the representation of the experimental isotherm (B. G. Aristov, V. Bosacek, A. V. Kiselev, Trans. Faraday Soc. 1967 63, 2057) of xenon adsorption on partly decationized zeolite LiX-1 (the composition of this zeolite is given on p. 185) with the aid of the virial equation in the exponential form with a different number of coefficients in the series i = 1 (Henry constant), i = 2 (second virial coefficient of adsorbate in the adsorbent molecular field), i = 3, and i = 4 (coefficients determined at fixed values of the first and the second coefficients which are found by the method indicated for the adsorption of ethane, see Figure 4 on p. 41). In this case, the isotherm has an inflection point. The figure shows the role of each of these four constants in the description of this isotherm (as was also shown on Figure 3a, p. 41, for the adsorption of ethane on the same zeolite sample). The first two of these constants—Henry constant (the first virial constant) and second virial coefficient of adsorbate-adsorbate interaction in the field of the adsorbent —have definite physical meanings. 

A study was made of the ultraviolet spectra of benzene, alkyl-, amino-, and nitro-derivatives of benzene, diphenyl-amine, triphenylmethane, triphenylcarbinol, and anthra-quinone adsorbed on zeolites with alkali exchange cations, on Ca- and Cu-zeolites, and on decationized zeolites. The spectra of molecules adsorbed on zeolites totally cationized with alkali cations show only absorption bands caused by molecular adsorption. The spectra of aniline, pyridine, triphenylcarbinol, and anthraquinone adsorbed on decationized zeolite and Ca-zeolite are characterized by absorption of the corresponding compounds in the ionized state. The absorption bands of ionized benzene and cumene molecules appear only after uv-excitation of the adsorbed molecules. The concentration of carbonium ions produced during adsorption of triphenylcarbinol on Ca-zeolite and on the decationized zeolite depends on the degree of dehydroxyla-tion of the zeolite. 

Adsorption on Decationized Zeolites. There is great similarity in the spectral characteristics of adsorption on Ca-zeolites and on decationized zeolites. The adsorption of such molecules as benzene (Figure 4), and cumene (10) on these zeolites is characterized only by molecular adsorption. 

The spectra of aniline (8), diphenylamine, triphenylamine (9, 15), triphenylcarbinol (Figure 2) and anthraquinone (Figure 3) adsorbed either on decationized zeolite or on Ca-zeolite show the presence of new absorption bands caused by ionization of these compounds on the zeolite. 

The observed decrease in the concentration of carbonium ions produced during adsorption of triphenylcarbinol on dehydroxylated Ca-zeolite and on decationized zeolite (Figures 2 and 3) indicates that OH-groups played the determining role in the appearance of proton-donor properties of these zeolites. 

Thus interactions involving charge transfer between acidic centers on the surface of Ca-zeolites and decationized zeolites and the adsorbed molecules depend on the nature and the state of the molecule. These interactions become more pronounced after ultraviolet excitation of the molecules. 

The simple idea involved in the electrostatic theory was very effective in predicting the catalytic activity of various cation-exchanged X and Y catalysts. However, it failed to explain quantitatively the difference in activity between cation-exchanged X and Y, alkaline earth cation X being less active than expected. It did not explain the cause for the similar behavior between cation-exchanged and decationized zeolites, and it did not offer satisfactory chemical evidence for the suggested reaction mechanism. 

Figure 2. Effect of pretreatment temperature on catalytic cracking of 2,3-di-methylbutane, using a decationated zeolite and a palladium-decationated zeolite...

Characterisation of acid sites in decationated zeolites Study of NH3 sorption by frequency-response technique and FTIR spectroscopy... 

The close similarity of CTC of naphthalene adsorbed on the decationated zeolite, porous silica and alumina allows us to make an assumption that the acid sites of Bronsted and Lewis type related to the coordinatively unsaturated silicon and aluminium ions are the real electron-accepting sites. 

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