Cracking reactions using zeolites

In studies involving solid acid-catalyzed hydrocarbon cracking reactions using HZSM-5 zeolite, Haag and Dessau" were able to account nearly quantitatively for H2 formed in the protolysis ionization step of the reaction. This is a consequence of the solid acid zeolite catalyst, which is not easily reduced by the hydrogen gas evolved. 

Efficient contacting of the feed and catalyst is critical for achieving the desired cracking reactions. Steam is commonly used to atomize the feed. Smaller oil droplets increase the availability of feed at the reactive acid sites on the catalyst. With high-activity zeolite catalyst, virtually all of the cracking reactions take place in three seconds or less. 

Auroux et al. [251] used adsorption microcalorimetry of different alkanes to investigate Ga and Al substituted MFI zeolites used as catalysts in dehydrogenation and cracking reactions. 

On the other hand, a remarkable difference between catalysis by Y and 13 zeolites has been found for the Claisen-Sohmidt condensation of acetophenone and benzaldehyde (Table 5). When the cross aldolic reaction is carried out in the presence of HY, together with the expected trans and ois chalcones 5, the 3,3-diphenylpropiophenone 6 is also formed, this product being not detected on 13 zeolites. A likely explanation for the absence of 6 using zeolite beta is that the crystalline structure of this zeolite exerte a spatial constraint making difficult the formation of a big size molecule like 6, especially in the smaller channel. Similar effects due steno limitations on 6 catalysis have been found for the formation of multi-branched products during the cracking of alkanes (ref 8). 

Powders possessing relatively high surface area and active sites can be intrinsically catalytically active themselves. Powders of nickel, platinum, palladium, and copper chromites find broad use in various hydrogenation reactions, whereas zeolites and metal oxide powders are used primarily for cracking and isomerization. All of the properties important for supported powdered catalysts such as particle size, resistance to attrition, pore size, and surface area are likewise important for unsupported catalysts. Since no additional catalytic species are added, it is difficult to control active site location however, intuitively it is advantageous to maximize the area of active sites within the matrix. This parameter can be influenced by preparative procedures. 

The catalytic activity of amorphous silica-alumina ([Si—Al]) in reactions via carbonium ions is due to the existence of Bronsted acid sites on their surface. Consequently, amorphous [Si-Al] acid catalysts provide acid sites and transport to the active sites easily. As a result, amorphous [Si-Al] acid catalysts have been widely operated as cracking catalysts. Acid zeolites have been successfully applied as cracking catalysts. However, in some industrial applications of acid catalysts, for example, in the cracking of hydrocarbons of high molecular weight, zeolites are not useful, since... 

The catalysts used for cracking before the 1960s were amorphous [Si-Al] catalysts. The replacement of these catalysts by faujasite zeolites was a big step forward in the oil refining industry, which led to an increase in the production of gasoline [20], The acid catalyst, currently used in FCC units, is generally composed of 5-40 wt % of 1-5 pm crystals of the H-Y zeolite included in a porous particle composed of an active matrix, which in turn is composed of amorphous alumina, silica, or [Si-Al] and a binder. The porous particle allows the diffusion of the reactants and products of the cracking reaction to and from the micropores of the zeolite [10]. 

Neutrality is satisfied by a cation (e.g., M+) which is usually Na+ derived from the salts used in the synthesis. When the cation is exchanged with a proton an acid site is created. This is the key active site for catalytic cracking reactions. The first exchange is with NH4+ which when heat-treated decomposes to NH3 and the H+ is retained on the zeolite. The acid zeolite is designated HZ... 

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. 

The structural features of dealuminated zeolite samples were characterized using X-ray powder diffraction, porosimetry and solid-state NMR measurements. Hexadecane cracking was used as a probe reaction to investigate catalytic properties of pure zeolites. 

The zeolites have been first used as catalyst in the 1960s for alkane cracking reactions in petroleum industry.They replaced favorably previously employed alumina based catalysts because of their better therm and mechanical stability. Moreover, they showed higher selectivity. The selectivity finds its source in the zeolite micropore structure with different... 

Solid acid zeolites are used as catalysts for a large range of reactions on hydrocarbons. They can for instance induce alkylation, transalkylation, isomerization and cracking reactions. Moreover, they are also used to achieve... 

The diffusion, adsorption and chemical steps for the dehydrogenation and cracking reactions of light alkanes catalyzed by zeolites were studied using a combined classical mechanics (MM, MD and MC) / quantum mechanics approach. 

Four types of REY zeolite (Si/Al = 4.8) with different crystal sizes and acidic properties were used. The physical and chemical properties of the fresh zeolites are given in Table 6.4. Polyethylene plastics-derived heavy oil, shown in Table 6.2, was used as the feed oil. The cracking reaction was conducted in a tubular reactor filled with catalyst particles under the following conditions time factor W/F = 0.2-3.0 kg-catkg oil h and reaction temperature = 300-450°C. The lumping of reaction products were gas (carbon number 1-4), gasoline (5-11), heavy oil (above 12), and a carbonaceous residue referred to as coke. The index of the gasoline quality used was the research octane number (RON), which was calculated from Equation 6.1 [31]. 

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