Faujasite surface acidity

Fatty acid, MRNi hydrogenation, 32 243-245 Faujasites, 34 160-183 acidic sites, 27 151-154 alkaline and rare earth forms, 27 160-165 amine titration, 27 163 infrared smdies, 27 160-163 surface acidity and catalytic activity, 27 163-165... 

Most of the published information regarding surface acidity and its relation to catalytic activity has involved zeolites of the faujasite structure as found in zeolites X and Y. A smaller number of investigations of mor-denite have been reported. This discussion will concentrate on studies of these two types of zeolites because their acidic and catalytic properties have been most widely investigated, and because they are both of significant industrial importance. 

The nature of the surface acidity is dependent on the temperature of activation of the NH4-faujasite. With a series of samples of NH4—Y zeolite calcined at temperatures in the range of 200° to 800°C, Ward 148) observed that pyridine-exposed samples calcined below 450°C displayed a strong infrared band at 1545 cm-1, corresponding to pyridine bound at Brpnsted (protonic) sites. As the temperature of calcination was increased, the intensity of the 1545-cm 1 band decreased and a band appeared at 1450 cm-1, resulting from pyridine adsorbed at Lewis (dehydroxylated) sites. The Brtfnsted acidity increased with calcination temperature up to about 325°C. It then remained constant to 500°C, after which it declined to about 1/10 of its maximum value (Fig. 19). The Lewis acidity was virtually nil until a calcination temperature of 450°C was reached, after which it increased slowly and then rapidly at calcination temperatures above 550°C. This behavior was considered to be a result of the combination of two adjacent hydroxyl groups followed by loss of water to form tricoordinate aluminum atoms (structure I) as suggested by Uytterhoeven et al. 146). Support for the proposed dehydroxylation mechanism was provided by Ward s observations of the relationship of Brpnsted site concentration with respect to Lewis site concentration over a range of calcination tem-... 

Surface acidity and catalytic activity. Faujasitic zeolites exchanged with multivalent ions demonstrate significant catalytic activity for reactions involving carbonium ion mechanisms, in contrast to the inactivity of the alkali metal ion-exchanged forms. Several possible sources of the observed activity were proposed initially. Rabo et al. (202, 214) suggested that electrostatic fields associated with the multivalent ions were responsible for the catalytic activity. Lewis acid sites were proposed as the seat of catalytic activity by Turkevich et al. (50) and by Boreskovaet al. (222). Br0nsted acid sites formed by hydrolysis of the multivalent metal ions were proposed as the catalytic centers by Venuto et al. (219) and by Plank (220). 

Relatively few studies have focused on influence of the acid/base properties of the support on the chemisorption of reactants on supported metal clusters. A NMR study by Tong et al.23 showed that the stretching frequency of CO chemisorbed on zeolite supported Pt particles correlates with the surface local density of states (LDOS) of the Pt. The LDOS also showed a correlation with the faujasite framework acidity, but an explanation of this correlation is lacking. Several infrared studies on similar supported Pt catalysts show that the mode of CO... 

Oare earth forms of zeolites X and Y type faujasites possess superior catalytic properties for various reactions such as alkylation, isomerization, and cracking (9, 12, 18). Structural studies involving x-ray diffraction and CO chemisorption have been made to locate the positions of the rare earth (11, 14, 16). Hydroxyl groups and their relationship to surface acidity have been studied by infrared spectroscopy, utilizing the adsorption of pyridine and other basic molecules (2, 6, 21, 22, 23). Since much of the previous research has involved measurements on mixed rare earth faujasites, a need existed for a more systematic study of the individual rare earth zeolites, in regard to both structural and catalytic properties. The present investigation deals with the Y, La, Ce, Pr, Sm,... 

This work deals with the synthesis of monooctylamines by ammonia alkylation with octanol-1 in gaseous phase using various catalysts. These microporous materials were prepared by the hydrothermal method. Y-faujasite and ZSM-5 supports were exchanged by lead and uranyl ions at different concentrations in order to increase their surface acidity necessary for reaction mechanism. The obtained results show that the use of these catalysts results in the formation of primary amines. Monooctylamines selectivities of 90 % were obtained in the present work. It was observed that when SAPO-34 is used, the trioctylamine isomer could be formed in the external surface of the catalyst. 

Owing to the possibility of tuning (1) their acidic and basic properties, (2) their surface hydrophilicity, and (3) their adsorption and shape-selectivity properties, catalytic activity of zeolites was investigated in the production of HMF from carbohydrates. Whatever the hexose used as starting material, acidic pillared montmorillonites and faujasite were poorly selective towards HMF, yielding levu-linic and formic acids as the main products [81-83]. 

Therefore, it seems that tridireotional zeolites HY and HB are the most promi sing ones for liquid phase reactions. In the case of HB zeolites, two items deserve special comments. Firstly, the yield (82%) of ethyl phenylacetate for the equimolar esterification of phenylacetic acid and ethanol in the presence of the fl-10 sample is substantially higher than that of the equilibrium (69%) at the same temperature and solvent (Table 3). Analogous results have been already observed with dealuminated acidic Y faujasites and can be due to zeolite water adsorption and/or to the hydrophobicity of the in surfaces (ref. 2). The hydrophobic character of high silica... 

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 effect of intrazeolite protons on pyrrole polymerization in faujasite with ferric ions was examined, in order to distinguish the relative influence of acidity and the one-electron oxidant. If water was co-adsorbed with pyrrole, the authors could prepare materials with conductivities vaiying over a wide range. It is not clear to what extent the conductivity is due to surface-adsorbed polypyrrole, because similar simthetic methods also produced pol)mier coatings on amorphous aluminosilicates. 

The stabilizing effect of the aluminosilicate layer of DAY-Saim and DAY-T can be explained by the elimination of the terminal silanol groups and the blocking of the energy-rich Si-O-Si bonds at the crystal surface, where the water molecules attack the framework. In this case, the question of stability of high-silica faujasites is transferred to the question of stability of the aluminosilicate structures and, accordingly, transferred from the alkaline to the acid mechanism of decomposition which is rate-controlling. 

An aluminosilicate layer at the crystal surface generated subsequently by an alumination of DAY-S zeolite or directly by the steaming of NaY in order to get DAY-T zeolite stabilizes the high-silica faujasites. The decomposition follows the acid hydrolysis of usual aluminosilicates. Consequently, in this case the kinetic model described in ref [5] is valid at least for the surface layer. 

The modification of zeolites mainly relies on secondary synthesis methods. The aim of modification is to reprocess the zeolites using suitable techniques to improve the properties and functions such as (1) acidity, (2) thermal and hydrothermal stability, (3) catalytic performance such as redox catalytic and coordination catalytic properties, etc., (4) channel structures, (5) surface properties and microporous frameworks and charge-balancing ions. Modification is also called secondary synthesis and can lead to new properties that cannot be achieved through direct synthesis. Let us consider the case of faujasite (FAU), the main component of the cracking catalyst, and its catalytic performance (represented by the catalytic activity K/K Std for n-hexane cracking). From Table 6.1 it is seen that the secondary synthesis affects the catalytic performance to a considerable degree. 

It is possible that the initial step in the carbonylation reactions may involve formation of a surface complex of CO with the acid zeolite, which is subsequently attacked by propylene. Indeed, it is known that CO is weakly and reversibly adsorbed on faujasite zeolites containing a variety of cations (48,85). However, the reaction scheme shown in... 

Discussions[9,10,l 1] are available on the concept of polarizabilities in zeolite science and acid catalysis. Barrer and Sutherland[l 1] postulated that the heat of adsorption of hydr ocarbons is dependent on the polarization of molecules. Later on the concept of average curvature of the zeolite surfaces was introduced to predict the heats of adsorptions [12]. However, the role of Si/Al ratio was ignored in this treatment which is equally important to consider while predicting heats of adsorption [13]. Hence, it concerns rightly to examine the basic process of adsorption when any hydrocarbon species comes in the vicinity of acidic sites of zeolite surfaces [14,15]. Besides inter-, and intramolecular forces operative in the process of adsorption, the contribution of periodic minimal surfaces has been discussed by Blum et al [16]. Thomasson et al.[17] have shown that silicon and oxygen atoms in zeolites like faujasites reside on the surface parallel to the periodic minimal surface, the so called "D-surface". A mathematical treatment by these workers gave average curvature, Kav, of a zeolite surface as ... 

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