Catalysis olefin cracking

Most of the commercial zeolite catalyzed processes occur either through acid catalysis fluid catalytic cracking (FCC), aromatic alkylation, methanol to olefins (MTO),... 

Section 5.2 aromatic Friedel-Crafts-type alkylations C. Perego and P. Ingallina, Catal. Today 2002, 73, 3 A. Corma, Chem. Rev. 1995, 95, 559 alkane cracking and isomerization Y. Ono, Catal. Today 2003,81, 3 A. Feller and J. A. Lercher, Adv. Catal. 2004, 48, 229 isoparaffin-olefin alkylation A. Corma and A. Martinez, Catal. Rev. Sci. Eng. 1993, 35, 483 acid-base catalysis with metal oxides K. Tanabe and W. F. Floelderich, Appl. Catal. A Gen. 1999, 181, 399. 

Because the process is dominated by acidic catalysis, if the cracking processes are taken to the extreme, the cracking reactions (known as P-scission) result in propylene and branched olefins such as isobutene. These olefins dominate the light gas products. Ethylene is a very minor component and its presence may be due to a small amount of thermal cracking taking place. 

In 1960, Weisz, Frilette, and co-workers first reported molecular-shape selective cracking, alcohol dehydration, and hydration with small pore zeolites (6,7), and a comparison of sodium and calcium X zeolites in cracking of paraffins, olefins, and alkylaromatics (8). In 1961, Rabo and associates (9) presented data on the hydroisomerization of paraffins over various zeolites loaded with small amounts of noble metals. Since then, the field of zeolite catalysis has rapidly expanded,... 

Medium pore aluminophosphate based molecular sieves with the -11, -31 and -41 crystal structures are active and selective catalysts for 1-hexene isomerization, hexane dehydrocyclization and Cg aromatic reactions. With olefin feeds, they promote isomerization with little loss to competing hydride transfer and cracking reactions. With Cg aromatics, they effectively catalyze xylene isomerization and ethylbenzene disproportionation at very low xylene loss. As acid components in bifunctional catalysts, they are selective for paraffin and cycloparaffin isomerization with low cracking activity. In these reactions the medium pore aluminophosphate based sieves are generally less active but significantly more selective than the medium pore zeolites. Similarity with medium pore zeolites is displayed by an outstanding resistance to coke induced deactivation and by a variety of shape selective actions in catalysis. The excellent selectivities observed with medium pore aluminophosphate based sieves is attributed to a unique combination of mild acidity and shape selectivity. Selectivity is also enhanced by the presence of transition metal framework constituents such as cobalt and manganese which may exert a chemical influence on reaction intermediates. 

The enhanced selectivities observed with medium pore silico-and metalloaluminophosphates may, to a large extent be attributed to a unique combination of mild acidity and shape selectivity. The lack of hydride shift and cracking activity in olefin-mediated reactions is suggestive of mild acidity. The observed resistance to coke deactivation and the enhanced selectivity to para-xylene in methylation and isomerization reactions is evidence of shape-selective catalysis. 

Contact catalysis, mechanism of, 2, 251 Contact catalysts, surface area measurements for studying, 1, 65 Cracked gases, polymerization from, of olefins, 8, 219 Cracking catalysts,... 

Not all petrochemical processes are catalytic—the steam cracking of hydrocarbons to lower olefins is a thermal process at 700 to 800°C or more. However, excluding free-radical polymerization processes, this is a rare example, though severe conditions may still be required in some catalysed processes on thermodynamic grounds or to achieve acceptable rates (several mol h per litre of reaction volume). As we shall see in this and the following chapter, the major impact of catalysis is to provide a remarkably wide range of products from a small number of building blocks. 

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