Dehydroxylation of zeolite

Lewis acid sites may be formed following dehydroxylation of zeolite surface in H-form. At sufficiently high temperatures two Bronsted acid sites can drive off a water molecule and leave behind a coordinatively unsaturated Al site, as illustrated in Figure 13.16 [32]. Here not only the resulting tri-coordinated Al but also the tri-coordinated positively charged Si can act as a Lewis acid. Furthermore dehydroxylation may be followed by framework dealumination, leading to cationic extra-framework species like AlO AlfOHij that can act as Lewis acids [33-37]. 

The effect of cation movement and framework distortion on the infrared pattern of a Ca-exchanged Y zeolite (Si/Al = 2.5) is shown in Figure 12. Dehydration and complete dehydroxylation of zeolites with similar cation composition and framework topology (e.g., Ca-exchanged... 

The acidic and nonacidic hydroxyl protons, in synthetic zeolites, were analyzed by multinuclear solid-state NMR [96H1]. The formation, accessibihty, and localization of hydroxyl protons, dehydroxylation of zeolites and the geometry and parameters of the local structure of OH groups was studied by multinuclear solid state NMR [96H1]. For the proton magnetic resonance of heulandite, see [70G1]. 

Figure 3.53. IR transmission absorption spectrum of pyridine adsorbed on partly dehydroxylated HY zeolite (Van Bekkum et al, 1991) B = Bronsted acid sites L = Lewis acid sites.

Pyridine sorption studies have shown the presence of both Bronsted and Lewis acid sites in USY zeolites, although to a lesser extent than in the corresponding HY zeolite (51,53). Acidity is maintained even after strong dehydroxylation of USY-B at 820°C. Rehydration of the calcined material did not regenerate significantly Bronsted acid sites, due to irreversible changes in the zeolite framework (51). 

The chemistry and structure of the hydrogen form of zeolite Y have been thoroughly investigated 82) and are not considered further. The structure of the dehydroxylated zeolite proposed by Uytterhoeven, Christ-ner, and Hall 15) remains unchanged. Recently Ward, on the basis of infrared studies, suggested that this form may be amorphous 27). The extreme instability of dehydroxylated zeolite Y to moisture complicates detailed study 19). The elucidation of the detailed nature of this material lies in the future. At present, completely dehydroxylated Y is little understood and presents a challenging void in our knowledge of the nature of ammonium zeolite Y thermal decomposition products. 

The relationship between acid site density and effective acidity may account for the interesting observation of Hopkins that maximum cracking activity of n-hexane was obtained over a partially dehydroxylated hydrogen zeolite Y (45). While the normal hydrogen form would contain a greater overall concentration of acid sites, the partially dehydroxylated form may have a greater overall acid activity because of the increased effective acidity of the remaining sites. 

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). 

A linear dependence with pore diameter is observed. The intermolecular distance, directly dependent on the curvature of the pores, i.e. the average pore diameter, is linearly related to the activation energy for dehydroxylation. Extrapolation to smaller pores suggests activation energies of approximately 100 kJ mol 1 for dehydroxylation of hydroxyl groups in e.g. zeolite channels, if the hydroxyls are of a comparable type. Rees published corresponding activation energies for water desorption in dealuminated Y-zeolites.34... 

Of great interest is the question of the role of trigonal aluminum, which is usually assumed to act as a LAS. Such a center should be quite typical of A1203, where it may appear as a result of surface oxygen vacancy formation. These vacancies may either develop due to dehydroxylation or be of a biographical nature. A similar situation may take place in the case of such mixed oxides as amorphous aluminosilicates. Uytterhoeven, Cristner, and Hall 123) have concluded that trigonal aluminum could also appear as a LAS upon dehydroxylation of H forms of zeolites. Their scheme was criticized, however, by Kiihl 124), who has undertaken X-ray fluorescence studies of the dehydroxylated forms of faujasites and found that the dehydroxylation was accompanied by dealumination of a zeolite framework with formation of extralattice aluminum which could also exhibit the Lewis acidity. 

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. 

The recent studies on the relationship between activation temperature and carbonium ion type catalytic activity of both decationized and cation exchanged zeolites show that at arid above the temperature required for the removal of all observable hydroxyls with vibrational frequencies between 3700-3500 cm" the activity sharply declines. The lowest concentration of acidic lattice hydroxyl required for carbonium ion activity seems to depend on the reaction involved. For example, dehydroxylation of La-exchanged Y to a level at which hydroxyl content was unobservable by currently-used infrared techniques led to total loss of activity to crack n-butane, but only partial loss of activity to crack cumene (vide infra) and to alkylate toluene with propylene (74). The activity and hydroxyl content lost on dehydroxylation can be restored upon subsequent treatment with water (11). Furthermore, alkali metal zeolites, which have little or no carbonium ion type activity can be made to show strong activity by the addition of a proton source, such as alkyl chlorides (51, 58). The similarity of the products obtained with the... 

It is unfortunate that in the published catalytic studies with heat-treated NH4-exchanged Y the catalyst substrate seems to be insufficiently characterized, and the assignment of structural details, which are most important for catalytic properties, are based on assumptions, usually drawn from comparisons with similar zeolite samples. Furthermore, the catalytic properties of zeolites which are more fully characterized usually are not reported in the literature. Consequently, comparison between catalysts prepared by different investigators is very difficult and risky since the catalytically important details depend on several chemical and structural characteristics, all of which usually are not described. As an example of this problem, the dehydroxylation of heat-treated NH4" -exchanged Y depends on the temperature, atmosphere, and time employed on activation, as well as on the Si/Al ratio, the degree of NH4" exchange 16), and the residual cations 108). 

Furthermore, results obtained after heating at 550°C are in good agreement v ith the postulated dehydroxylation of La-H-Y zeolite under heating (3). According to this idea, increasing the temperature beyond 300°—400° C eliminates one proton from the surface then the slope... 

The observed formation and dehydroxylation of Bronsted sites in REY zeolite can be summarized as follows between 200° and 300°C each RE ion dissociates one water molecule with the formation of one Bronsted site in a-cages, followed by a dehydroxylation between 300° and 400°C. This is accompanied by a new dissociation of water molecules, giving rise to Bronsted site in a-cages for each RE ion and to the formation of OH groups in / -cages. 

It seemed of some interest to test the ability of a series of REY zeolites to ionize polynuclear aromatics since the oxidizing properties of zeolites were pointed out (8, 16), but the nature of the electron acceptor site is still under discussion. Hall et al. (5), studying dehydroxylated HY zeolites, presumed it to be molecular oxygen trapped in an anion vacancy, while Hirschler (7) asserted that the protons may be the oxidative centers. In a previous work, as stated by Turkevich et al. (16), we concluded that the active sites are Lewis centers, while the chemisorbed oxygen increases their electron affinity (27). In a recent work, Richardson (14) related spin concentration to the electron affinity of the cation, presuming that the electron transfer took place from the anthracene to the cupric ion, but he could not observe any variation of the Cu peak intensities. 

The acidic properties of some hydroxyl groups have been demonstrated by the existence of a 1540 cm" band in all the spectra of adsorbed pyridine on solids activated at low temperature. Further dehydrated samples behave either as Bronsted or Lewis acid solids the latter are created by dehydroxylation of the zeolite, which occurs in two different ways First... 

XPS of chemisorbed nitrogen containing basic molecules such as pyridine and ammonia have been employed to monitor the strength of acid sites in H and cationic forms of zeolites. One interesting problem is to follow the changes in concentration of the Brbnsted and Lewis acidic centers with the temperature of thermal treatment. Two mechanisms have been proposed for the thermal degradation. Dehydroxylation occurs through a process first described by Uytterhoven et al. [35]... 

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... 

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