Catalytic role, zeolites isomerization

Catalytic reactions of hydrocarbons over zeolites are reviewed. The historical development of various mechanistic proposals, particularly of the carbonium ion type, is traced. In spite of numerous catalytic, spectroscopic, and structural studies which have been reported concerning the possible roles of Bronsted acid, Lewis acid, and cationic sites, it still is not possible to formulate a comprehensive mechanistic picture. New activity and product data for cumene cracking and isotope redistribution in deuterated benzenes over Ca-and La-exchanged Y zeolites is presented. Cracking of the isomeric hexanes over alkali metal-exchanged Y and L zeolites has been studied. This cracking is clearly radical rather than carbonium-ion in nature but certain distinct differences from thermal cracking are described. 

The isomerization of n-butenes was used as a test reaction to follow the development of catalytic activity in Na-Y zeolite. The acidity was varied by the substitution of some of the Na " (0.3 to 5.7%) with Ca and by creating a Na deficiency (up to 0.94%) both series of catalysts were studied with and without water added as cocatalyst. A pure Na-Y zeolite containing no decationated sites was found cat-alytically inactive for this reaction. In contrast with silica-alumina catalysts, carbonaceous residues did not appear to play a role in the formation of the catalytic sites as long as H2O was used as cocatalyst. 

This chemistry suggests a role for H2O as cocatalyst with the monovalent zeolites if the equilibrium is moved to the right in the presence of a base. In the present work, the effects of small deliberate replacements of Na by Ca ", of cation deficiency, and of H2O on the catalytic properties of Na-Y zeolite were studied. Also investigated was the possibility that carbonaceous residues form the catalytic sites, as was reported for the isomerization of n-butenes over silica-alumina catalysts (3, 8, 9). The isomerization of the n-butenes provided a useful tool for these studies because it follows first-order kinetics (10) and proceeds over Na-Y zeolite via the 5ec-butylcarbonium ion (11, 12). 

The introduction of zeolites into diesel catalyst formulations, starting in the late 1990s, play an important role in decreasing emissions of gas phase HCs. Zeolites are among the most important materials in industrial catalysis. They dominate the markets for catalytic cracking, hydrocracking and xylene isomerization as well as several smaller scale applications. Their uses stem from their strong acidity and their unique pore structure, which leads to size- and shape-selective effects. 

It is important to emphasize that spectroscopic evidence shows that water transforms the Lewis acid sites of sulfated zirconia into Bronsted acid sites [80]. At the same time, water promotes isomerization reactions over sulfated zirconia for a moderate extent of catalyst dehydration. Similarities were reported between the effect of rehydration on the isomerization activity of sulfated zirconia [81] and on that of other oxide catalysts [49] that are consistent with the role of surface donor sites in hydrocarbon isomerization reactions. However, when spectroscopic methods using basic probes were used to compare sulfated zirconia and zeolites in terms of the strength of their acid sites, the results were inconsistent with all catalytic data. These findings illustrate the danger of comparing the acidity of catalyst systems that differ in structure and composition, such as zeolites and sulfated zirconia in these systems the "catalytic" and the "physicochemical" scales for the strength of acid-base interaction may contain significantly different parameters. 

Molecular heats of adsorption play a role in many catalytic reactions. Figure 6.23 illustrates this for an isomerization reaction catalyzed by a solid acid. As explained in Chapter 3, the hydroisomerization of alkanes on a zeolite-supported metal proceeds through a bifunctional reaction mechanism, in which the metal has the function of activating C-H bonds and H2 at a low reaction temperature. The alkane-alkene equilibrium is established by metal catalysis, and the alkene is protonated and isomerized by the acidic protons of the zeolite... 

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