Decationized zeolites, adsorption

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 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... 

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. 

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 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. 

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). 

Note that for the most open clino, decationized ZBS-15, the heats of adsorption for CH4 and N2 are the same. As the zeolite is progressively re-cationized, the accessibility of CH4 to the internal crystal surfaces progressively decreases. This is reflected in a lower determination for (12). Finally, in... 

Inspection of Table 11.3 reveals that there are relatively small differences between the corresponding values of H and E0 for NaY and HY. This is to be expected since the adsorbent-adsorbate interactions are essentially non-specific (see Chapter 1). Decationization of zeolite Y thus has a minimal effect on the energetics of adsorption of the paraffins. The molecular shape of the adsorptive is also unimportant. In accordance with the results in Figures 1.S and 1.6, the molar mass (number of carbon atoms) is much more important than the molecular shape. As before, there is a linear relation between E0 and Nc. An exponential increase of kH with Nc is of course consistent with the form of Equation (4.3). 

The relationships should be established between these constants and such parameters as the type of the zeolite lattice, the type and concentration of cations, the degree of decationization, and the structure of the adsorbate molecule. As a result, semiempirical relationships may be obtained which could be used for the practical calculation of the adsorption equilibrium. 

G. V. Tsitsishvili (Academy of Sciences of the Georgian SSR, Tbilisi, USSR) The nev method of description of the adsorption isotherms allows one, on the basis of the theory of volume filling of micropores, to characterize adsorption on the microporous adsorbent containing adsorption sites of different nature. For zeolites, there are approximately two adsorption sites cations and framework. The existence of cations in different positions must be taken into account. It is important to use the Dubinin approach to desorption-adsorption equilibrium on decationized and other forms of zeolites. We have already obtained some good results in this direction. 

The acid-base properties of the decationated HY zeolites have been extensively studied with adsorption microcalorimetry. Tables II and III present a summary of calorimetric studies of the adsorption of ammonia and other probe molecules on HY zeolites with different Si/AI ratios, preparation methods, pretreatments, adsorption temperatures, and sodium contents. The large variety of conditions used in these studies complicates the comparison of the materials. For example, the initial differential heat of ammonia adsorption at... 

Dehydroxylation of decationated mordenite at high temperatures also produces a substantial change in the acidity spectrum as shown in Table VI. Raising the activation temperature of HM zeolites with a Si/AI ratio of about 9.0 from 703 to 1023 K increases the initial differential heat of ammonia adsorption at 573 K from 165 to 180 kJ moP and sharply decreases the concentration of sites near 160-130 kJ mol and the overall acidity (756). IR spectroscopy of molecular hydrogen adsorbed at low temperature showed that mordenite dehydroxylated at 703 K contains only Brpnsted acid sites and nonacidic terminal Si—OH groups, whereas raising the pretreatment temperature decreases the concentration of acidic bridge-type hydroxyl... 

The acid-base properties of decationated ZSM-5 zeolite have been studied in some detail using adsorption microcalorimetry, as shown in Table VIII (169-173). As the calcination temperature for HZSM-5 zeolites was increased from room temperature to 1073 K, a maximum in acidity was observed while the initial differential heat of ammonia adsorption increased continuously. Vedrine et al. (92) also found a maximum in the intensity of the IR hydroxyl bands (169) of HZSM-5 at 673 K. The IR absorption band of pyridine adsorbed on Brpnsted sites followed the same trend as that found for the hydroxyl stretching bands, confirming that above 673 K the Bronsted acidity decreased as the dehydration temperature increased. 

C) results in high frequency xenon chemical shift from NaX.This 5 = f (N ) variation has the classical form for zeolite -supported metals 8 which is high at low N ( strong metal - Xenon interaction ), decreases as N increases, due to rapid site exchange, then a new increase in 5 when the Xe - Xe interactions become sufficiently important. The very low xenon adsorption of this sample shows that the zeolite lost most of its crystallinity. However, it is known [ 11 ] that the decationized type X,lose their crystallinty after reduction at 300°C. The chemical shift observed in this case is due to the adsorption of xenon on the metal silver particles in a defect structure. 

In order to characterize the types of acid sites, Lygin et al. (41) studied the adsorption of thiophene on Ca and decationized Y zeolites heated in air at 550 °C and then in vacuum at 400°C. Thiophene adsorbs on the zeolite with partial decomposition, and strongly-held chemisorbed species are formed. Methyl, methylene, and CH-groups are detected up to 400 °C. Bands indicative of C=S and thiophene rings are observed up to 300°C. The similarity of the spectra of thiophene chemisorbed on alumina and HY zeolites suggest similar adsorption centers on the 2 surfaces, namely Lewis acid sites. 

N-butylamine was also used by Brueva et al. [130] to determine the number and strength of the acid centers of Na,H-Y zeolites with various amounts of Na ions. The differential adsorption heats, measured at 303 K after out-gassing at 753 K, all presented a wide plateau of heats before a sharp decrease. The initial adsorption heat increased from 105 to 155 kJ mol as the degree of decationization increased up to 80%. This increase was not proportional to the amount of removed Na" ions. A strong increase of the adsorption... 

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