Hydrogen transfer, during olefin

Also shown in Tables III through V is the effect of hydrotreating on hydrogen transfer during catalytic cracking. This effect is shown by the ratio of saturated to olefinic C. In catalytic cracking, there are two mechanisms for the formation of saturates. These are the primary cracking reaction, such as... 

Olefins resulting from n-heptane cracking (step 1) are transformed through various reactions (oligomerization, cyclization, hydrogen transfer etc...) into soluble coke molecules sterically blocked in the cavities or at channel intersections (step 2), The same reactions transform soluble coke molecules into non soluble molecules (step 3) that overflow onto the outer surface of the zeolite crystallites. Non soluble coke molecules could also overflow in the mesopores created during zeolite... 

Metal catalysis of olefin and arene hydrogenation is critically dependent on the reactive hydridometal intermediates. However, little is known about the mechanism of hydrogen transfer and hydrometallation and the reactive intermediates involved. The observation of transient charge-transfer absorption during hydrogenation and hydrometallation of olefins with tungsten or molybdenum hydrides [185], opens up the question of a possible electron-transfer mechanism in which the overall hydrogen-atom transfer is the result of a two-step electron-transfer-proton-transfer (ET-PT) process (Eq. 58). 

As already pointed out, light olefins are absent in the liquid-phase the carbonium ions, formed during the reactions, because of their high concentration in the reaction medium and of the high concentration of the reactant are involved in the reaction pathway as external hydrogen transfer promoters, mainly in disproportionation thus, their interaction with the catalyst surface to form olefins, which negatively fect the catalytic activity, is unfavoured and the catalyst stability is widely enhanced. 

This was attributed to the increase in acidity due to the completely isolated framework Al. On the other hand, the hydrogen transfer reactions, which are believed to be responsible for olefin saturation and, consequently, for the parallel decrease in the RON observed, have been related to the density of acid sites. These reasonable assumptions can not fully explain, however, the product distribution observed during the cracking of gas-oil on a series of Y zeolites dealuminated at different levels and by different procedures. This is due to the presence, besides the framework-associated Bronsted sites, of Bronsted and Lewis sites which are associated with extraframework aluminium (EFAL) and which can catalyze carbonium ion as well as radical cracking reactions. 

Cracking. Bond breakage during cracking may be by B-scission of a tri-coordinated carbenium ion (for carbon numbers much greater than 6) or via a penta-coordinated carbonium ion in either case, a shorter-chain olefin is formed. Subsequent hydrogen transfer (HT) reactions may form the shorter-chain paraffin. Isomerization, aromatization, disproportionation and dealkylation are other... 

The discovery of conjunct polymerization, whereby hydrogen transfer occurs between molecules of olefins, leading to the formation of saturated hydrocarbons and polyenes, became the forerunner of the discovery of alkylation and isomerization of paraffins. During World War II these processes were used for the manufacture of high octane aviation gasoline to be used in pursuit planes, and presently products from the alkylation reaction assumed new importance as a major component of unleaded gasoline (Figure 4). 

Mechanism 4 is of importance whenever propane, n-butane, and n-pentane are obtained during alkylation using propylene, n-butenes, and n-pentenes, respectively, as olefin feeds. Mechanism 4 is sometimes referred to as hydrogen transfer or self-alkylation of isobutane. The overall reaction is often depicted as follows (1) ... 

During CCT, there are occasions where organometallic compounds form a Co—C bond that is stable at room temperatures. Investigation of the mechanism of this reaction with deuterated substrates proved that hydrogen transfer from cobalt porphyrin to a double bond (Scheme 8.3) occurred stereospecifically through cis-addition. Cis-addition supposes a concerted reaction mechanism. Stereospecific cis-addition hardly could be observed if reaction of LCo-H addition to a double bond proceeds by a three-center mechanism that requires formation of intermediate radical from the olefin ... 

Coke deposition [207] is known to be the major cause of deactivation during the MTO reaction. On the other hand, the coke species play a role of the active species and a role of an activity regulator to moderate the hydrogen transfer activity of the zeolite and to block the most acidic sites. So, in the same time, the coke content of the catalyst is a factor governing the selectivity and activity of the catalyst [208]. Coke has both a promoting effect and a deactivating effect for the formation of olefins [209]. 

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