Cracking acid-catalyzed

The chemistry of acid catalyzed reactions is deeply documented and will not be considered here. It is sufficient to say that they are based in carbenium ion reactions, which yield lower molecular weight compounds and branched paraffins (since tertiary carbenium ions are the most stable). The result of the cracking of a paraffin of n-carbon atoms is another paraffin with (n-x) carbon atoms and an olefin of x-carbon atoms. 

Olefins in the C3-320°C range had significant synthetic value, and additional olefins were produced by thermal cracking in some facilities. Acid-catalyzed and thermal olefin oligomerization were important technologies for the upgrading of Fischer-Tropsch products. 

Sie, S.T. (1993) Acid-catalyzed cracking of paraffinic hydrocarbons 2. Evidence for protonated cyclopropane mechanism from catalytic cracking experiments. Ind. Eng. Chem. Res., 32, 397. 

The catalytic cracking of four major classes of hydrocarbons is surveyed in terms of gas composition to provide a basic pattern of mode of decomposition. This pattern is correlated with the acid-catalyzed low temperature reverse reactions of olefin polymerization and aromatic alkylation. The Whitmore carbonium ion mechanism is introduced and supported by thermochemical data, and is then applied to provide a common basis for the primary and secondary reactions encountered in catalytic cracking and for acid-catalyzed polymerization and alkylation reactions. Experimental work on the acidity of the cracking catalyst and the nature of carbonium ions is cited. The formation of liquid products in catalytic cracking is reviewed briefly and the properties of the gasoline are correlated with the over-all reaction mechanics. 

The acid-catalyzed reactions of olefin polymerization and aromatic alkylation by olefins have been very well explained by the carbonium ion mechanism developed by Whitmore (21). This mechanism provides the basis of the ensuing discussion, which is devoted to the application of such concepts (7,17) to catalytic cracking systems and to the provision of much added support in terms of recently developed structural energy relationships among hydrocarbons and new experimental evidence. 

These acid-catalyzed reactions occur at temperatures lower than those used in conventional liquefaction processes. Phenol induces acid-catalyzed solvolytic reactions at temperatures as low as 120°C. The liquefied products contain chemically bound phenolic groups, a portion of which can be recovered by cracking. 

It is noted that Mo/DM is the best performing catalyst with the highest steady state activity and lowest deactivation rate. The deactivation rate is the lowest even under the influence of intense acid-catalyzed side reactions known to produce coke, i.e. oligomerization of styrene and cracking of ethylbenzene. Obviously, the high surface area and high connectivity of the support have played a determining role in the catalytic reaction. The effects they exert can be looked at in two ways ... 

The purpose of the present work is to incorporate aluminum into the framework of SBA-15 during the synthesis in order to create acid sites on the surface of the material directly and to enhance its activity in acid-catalyzed reactions and to study the stability of SBA and AlSBA molecular sieves under various treatments. The influence of these treatments on the pore size, wall thickness and the environment of Al in these materials are investigated in detail. X-ray diffraction (XRD), Electron Microscopy (TEM) and N2 adsorption were used to characterize the structure, the porosity and the stability of these materials. 27Al MAS NMR was used to ascertain the nature and environment of Al, cumene cracking to test the catalytic activity of parent materials and ammonia chemisorption to probe their surface acidity. 

As shown, the ratio was very high on zeolite catalysts, while that on mesoporous silica was as low as those on AMS and quartz chip. The high ratio on zeolites can not be explained by classical mechanism of acid-catalyzed cracking supposing higher stability of tertiary carbenium ion and its cracking by P-scission, because this supposition predicts that the reaction (2) proceeds in preference to the reaction (1). Rather, a-scission of carbocation [12] may rationalize the higher C3/C4 ratio on zeolite catalysts. 

In the area of waste utilization, Fred s program on the heat content, gasification, and carbonization of forest fuel is now recognized as a major step in our understanding of forest fires. The acid-catalyzed pyrolysis of cellulosic waste to afford l,6-anhydro-3,4-dideoxy-)3-D-g/ycero-hex-3-enopyranos-2-ulose ( levoglucosenone ) pointed the way to another method of chemical conversion of cellulosic wastes similar to cat-cracking in the petrochemical industry. 

Fuels. Catalytic Cracking Catalysts are used lo refined a moderately heavy crude oil fraction known as gas oil to gasoline. The net result of the process is a lighter product with a high content of branched-chain and aromatic hydrocarbons, the species responsible for raising gasoline octane levels. The transformations are complex, hut can be considered lo involve ihe following major acid-catalyzed reactions ... 

Fragmentation of alkylbenzenes over silica-alumina occurs exclusively by acid-catalyzed cracking. The reaction selectively cleaves the bond between the phenyl ring and the a-carbon of the side-chain. This occurs more than 100 times more often than the cracking of all the other bonds combined. Cracking rates of secondary alkylbenzenes are about an order of magnitude higher than those of w-alkylbenzenes. 

Acid-catalyzed cracking and platinum-catalyzed hydrogenolysis proceed simultaneously over dual-function catalysts. The distribution of the scission products is determined by the relative strengths of the acidic and metal-type catalytic components. 

The second drawback—that acid colors can be produced by processes other than simple proton addition—is more serious. The case of alumina is an example. Several of the indicators listed in Table I give faint acid colors with samples of pure, activated aluminas that have previously been shown to be inactive for an acid-catalyzed reaction such as cumene cracking (22). However, closer visual examination indicates that the acid... 

Uncomplicated model reactions should be chosen in which the rate-determining step is an acid-catalyzed transformation. Reactants should therefore be selected that do not undergo extensive side reactions and that undergo kinetically simple reactions such as unimolecular cracking or isomerization. 

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