Catalytic role, zeolites zeolite cracking

Dealuminated Y zeolites which have been prepared by hydrothermal and chemical treatments show differences in catalytic performance when tested fresh however, these differences disappear after the zeolites have been steamed. The catalytic behavior of fresh and steamed zeolites is directly related to zeolite structural and chemical characteristics. Such characteristics determine the strength and density of acid sites for catalytic cracking. Dealuminated zeolites were characterized using X-ray diffraction, porosimetry, solid-state NMR and elemental analysis. Hexadecane cracking was used as a probe reaction to determine catalytic properties. Cracking activity was found to be proportional to total aluminum content in the zeolite. Product selectivity was dependent on unit cell size, presence of extraframework alumina and spatial distribution of active sites. The results from this study elucidate the role that zeolite structure plays in determining catalytic performance. 

We can see that zeolites have come a long way since they were first introduced as catalysts for petroleum cracking in 1962. The number of catalytic applications where zeolites are now used has risen explosively over the past 15 years. Chemical processes that were established in the first half of the twentieth century may not be considered acceptable by a more environmentally conscious society. Catalytic processes play an increasingly important role in modern industrial chemistry, as the alternatives often produce large amounts of undesirable, and often hazardous waste. 

The Future Role of Rare Earth Exchanged Zeolites in Catalytic Cracking ... 

Experiments to further demonstrate the critical role of extraframework Al, or another polyvalent cation, have recently been carried out in our laboratory (19.20). A series of faujasite-type zeolites was prepared that had Alf concentrations between 21 and 54 per u.c. At the low end of the range, AHF was used to remove the framework Al, and an H-ZSM-20 zeolite with 42 Alf/u.c. was synthesized. ZSM-20 is an intergrowth of the cubic faujasite structure and the hexagonal variant know as Breck s structure six (BSS) (21). Thus, it is a faujasite-like material. The catalytic activities of these zeolites for hexane cracking are compared in Figure 5 (lower data set) with the activities of zeolites prepared by steaming or by treatment with SiClA (upper data set). The solid lines represent N(0) distributions. The samples without extraframework Al exhibited very modest activity, even though some of them had a favorable N(0) concentration. 

The matrix plays a critical role in the selective cracking of the bottoms fraction when resid-containing feedstock is processed (9). The generic roles of the matrix are to pre-crack large molecules and adsorb Ni and V preferentially to protect the zeolite component of the FCC particle. In an ideal situation, macropores should lead to mesopores and these to the (zeolitic) micropores. The features and catalytic effects of the various levels of porosity are as follows ... 

Through the use of hexadecane cracking alone, we have been unable in this work to elucidate the role of mesoporosity in the catalytic behavior of calcined or steamed zeolites. Steamed AFS and USY zeolites show differences in mesoporosity but exhibit similar catalytic performance. While mesoporosity may affect diffusion in actual FCC catalysts, larger molecules than hexadecane will be required to determine mesopore effects. 

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. 

Fluid Catalytic Cracking (FCC) is one of the most important process in oil refining. The evaluation of the catalysts in the laboratory scale is often carried out in a micro-reactor, the so called micro-activity test [1-3] (MAT). Coke formation plays an important role in the deactivation of FCC catalysts, which can be deactivated either permanently (loss of surface area, zeolite collapse, metals) or temporarily deactivated (coke). 

The role of catalysis in the petroleum industry has been equally revolutionary. Meta I-supported systems (e.g. of Topsoe and Shell) for catalytic reforming, hydrodesulfurization and hydrodenitrification, alkylation catalysts and shape selective systems (e.g. zeolites and pillared clays) for catalytic cracking (FCC) and production of gasoline from methanol (Mobil MTG) all represent significant technical and commercial achievements. 

As well as having roles as supports for metals, silica and alumina are used directly as heterogeneous catalysts. A major application is in the catalytic cracking of heavy petroleum distillates very fine powders of silica and y-alumina possess a huge surface area of 900m g . Large surface areas are a key property of zeolite catalysts (see Section 13.9), the selectivity of which can be tuned by varying the sizes, shapes and Bronsted acidity of their cavities and channels we discuss these properties more fully in Section 26.7. 

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. 

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