Cracking and isomerization

Metal oxides, sulfides, and hydrides form a transition between acid/base and metal catalysts. They catalyze hydrogenation/dehydro-genation as well as many of the reactions catalyzed by acids, such as cracking and isomerization. Their oxidation activity is related to the possibility of two valence states which allow oxygen to be released and reabsorbed alternately. Common examples are oxides of cobalt, iron, zinc, and chromium and hydrides of precious metals that can release hydrogen readily. Sulfide catalysts are more resistant than metals to the formation of coke deposits and to poisoning by sulfur compounds their main application is in hydrodesulfurization. 

Cracking and isomerization reactions occur readily in acidic chloroaluminate(III) ionic liquids. A remarkable example of this is the reaction of poly(ethene), which is converted into a mixture of gaseous alkanes of formula (C Ff2n+2, where n = 3-5) and cyclic alkanes with a hydrogen to carbon ratio of less than two (Figure 5.1-4, Scheme 5.1-68) [99]. 

Scheme 5.1-70 Cracking and isomerization of fatty acids and fatty acid methyl esters in...

Some other processes are based on a severe hydrotreatment followed by a stage for octane recovery. Octgain from ExxonMobil [57] and ISAL from UOP-Intevep [58], Deep desulfurization is achieved by an increase in severity, causing lost in octane by olefins saturation. In the first case, in a second reactor octane number is recovered by a combination of cracking and isomerization reactions. In the latter case, the catalyst employed during desulfurization possess isomerization capabilities inhibiting an excessive octane lost. Other mentioned functionalities of the catalyst include dealkylation and conversion. 

On sulfided metallic phases the hydrotreatment reactions also takes place. Noble metal catalysts usually include a zeolitic support. They are particularly used for fulfilling two different objectives, in the case of a gasoline oriented HCK their cracking and isomerization activity is the most important (increasing high octane and conversion yield). In a diesel HCK unit, the noble metal catalyst is mainly oriented to aromatic saturation and cetane improvement. However, in this latter case, also sulfided metal catalysts are used, especially NiW. 

Figure 4.17 Cracking and isomerization pathways of paraffins in hydrocracking (M, metallic site A, acidic site).

The demonstrated performance of ZSM-5 in over 35 cracking units is reviewed. The main features of ZSM-5 are its high activity and stability, favorable selectivity, metals tolerance and flexibility, particularly when used as an additive catalyst. ZSM-5 cracks and isomerizes low octane components in the naphtha produced by the faujasite cracking catalyst. As a result and olefins are produced and gasoline compositional changes occur which explain its increased research and motor octanes. A model was developed which predicts ZSM-5 performance in an FCC unit. 

While such data are not included in this report, our work has also shown that in FCC operations the acidic matrix is able to better crack and isomerize more n-paraffins, thus reducing the n-paraffin content in middle distillate and thereby the characteristic high cloud point associated with it, while also raising octane number by removing or isomerizing n-paraffins in the gasoline fraction. 

Paraffin dehydrogenation, isomerization, and dehydrocyclization Paraffin cracking and isomerization Alcohol -> olefin + H20 Paraffin hydrocracking Olefin polymerization,... 

Linear alkanes, which are known to be less reactive, also undergo H-D exchange by the same mechanistic scheme at slower rates at and above 150°C.54,55 This exchange reaction occurs in a very clean way because no side products from cracking and isomerization are observed. The cations that are adsorbed on the surface are prone to deprotonation, but the alkenes that are formed are rapidly reprotonated before substantial oligomerization can take place. 

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. 

Alkane Cracking and Isomerization on Solid Acid Catalysts... 

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. 

Synthetic zeolites have gained importance as industrial catalysts for cracking and isomerization processes, because of their unique pore structures, which allow the shape-selective conversion of hydrocarbons, combined with their surface acidity, which makes them active for acid-catalyzed reactions. Many attempts have been made to introduce redox-active TMI into zeolite structures to create catalytic activity for the selective oxidation and ammoxidation of hydrocarbons as well as for SCR of nitrogen oxides in effluent gases (69-71). In particular, ZSM-5 doped with Fe ions has attracted attention since the surprising discovery of Panov et al. (72) that these materials catalyze the one-step selective oxidation of benzene to phenol... 

Phosphates having these types of open structures can act as shape-selective acid catalysts, for example, for the cracking and isomerization of hydrocarbons. For examples of lamellar materials, see Section 5.3 and see Intercalation Chemistry). Microporous catalysts are described above and in (see Porous Inorganic Materials and Zeolites). Mesoporous AlPO materials have larger pores within a matrix of amorphous A1P04. ... 

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