Olefin interconversion

As opinions regarding the metathesis reaction pathway have shifted in recent years to favor a nonpairwise carbene-to-metallocyclobutane transformation, increasing attention has been given to the mechanistic significance of cyclopropane olefin interconversions. This interconversion process seems to occur primarily when certain relatively inefficient catalysts are employed, which in itself raises questions. Under ideal conditions, "good metathesis catalysts are remarkably efficient promoters of transalkylidenation, and consequently are well suited for olefin and polymer syntheses. Thus, most early studies focused primarily on applications. When side reactions did occur, they were usually ignored or presumed to be trivial cationic processes. 

MOI [Mobil olefin interconversion] A process for increasing the yield of propylene from steam crackers and fluid catalytic crackers, using a ZSM-type catalyst. Developed in 1998 by Mobil Technology. 

Investigation of n-butane conversion over H-forms of the ferrierite and theta-1 zeolites demonstrated that the isobutene selectivities were similar (and low) for these catalysts. The maximum selectivities (7-8 %) were obtained at low n-butane conversions (5-10 %) and decreased with increasing conversion of n-butane due to olefin interconversion and aromatisation reactions. Isobutene was in equilibrium with the other butene isomers due to the high isomerisation activity of the parent zeolites. The maximum selectivity to butenes, which was observed at low conversions, was around 20 %. This value reflects a moderate contribution of the dehydrogenation steps in n-butane transformation over H-forms of the ferrierite and theta-1 zeolites and indicates an important role of the n-butane protolytic cracking steps over these two catalysts. 

While the first process is likely in the case of iridium, nickel, and cobalt, it should not be so easy on platinum, because of its competition with carbene-olefin isomerization (see Section III, Scheme 29). We believe that the only way of explaining why 1,2-dicarbenes may account for the hydrogenolysis of cyclic hydrocarbons (Scheme 34), but only for a minor part for the hydrocracking of acyclic hydrocarbons, is the competition, for the latter, between carbene-dicarbene formation and carbene-olefin isomerization. Carbene-olefin interconversions are unlikely in the case of cyclic hydrocarbons, since a dicarbene species cannot transform into a 1,1,2,3-tetraadsorbed species (l-carbene-2,3-olefin) and further into a 1,1,3-triadsorbed species without C-C rupturing. 

Olefin metathesis is being used increasingly in the specialty chemicals market. Olefin interconversion can be used to produce isomerically pure symmetrical internal olefins from a-olefins (Eq. 2 R H), and a-olefins can be produced from internal olefins via ethenolysis. Metathesis of olefins bearing heteroatom functional groups is also a very promising application of the metathesis reaction, which enables the synthesis, in only a few reaction steps, of many products that would otherwise be difficult to obtain. 

Three fluidized bed processes are available for license from Mobil Badger (now ExxonMobil-Badger). These are MBR (Mobil benzene reduction), MOG (Mobil olefins to gasoline), and MOI (Mobil olefin interconversion). All three use zeolite catalyst and a dual dense fluidized bed reactor-regenerator system. The MOI process is shown in Fig. 17. The other two processes are conceptually very similar. 

The competitive alkylidene transfer to olefins and alkylidene to olefin interconversion of the electrophilic carbene complexes [CpFe(CO)L(=CHR)] (where L = CO or PH3 R = Me or Et) has been studied theoretically. These transformations are believed to involve cationic olefin complexes of the type [GpEe(CO)L(olefin)] . ... 

A hydrocarbon pool mechanism via alkylation/dealkylation of hydrocarbon scaffolds. Olefins interconversion via methylation, oligomerization, and cracking. 

Olefins interconversion cycle olefin carbon pool ... 

So, the ethylene production does correlate with coke presence, in particular with aromatics formation as far as the diffusion limitations are not significant. However, it seems that the majority of ethylene is not always formed directly from MeOH [115]. The aromatics and other coke species could be the products of the conversion of primary carbenium ions, which are mobile and could equilibrate each other [28]. This may explain the isotopic distribution in products and retained coke molecules and the coexistence of aromatics and carbenium ions [28], In addition to the coproduction of ethylene with aromatics in olefins interconversion cycle, formation of ethylene via alkylation-dealkylation of methyl aromatics with heavy olefins or with the equivalent carbenium ions like ethyP, propyP, and butyP could be an option. The alkyl aromatics with the side chain length of two carbons or longer are not stable in the pore and dealkylates on the acid sites due to too long residence time and steric hindrances. This may lead to formation of ethylene, other olefins, and alkylaromatics with different structure, namely PMBs [129]. In other words, the ethylene is formed via interaction of the carbenium ions like ethyP, propyP, and butyP formed from MeOH or heavy olefins with aromatics and other coke precursors followed by cracking and in a less extent by a direct alkylation of PMBs with methanol. The speculation is based properly on analysis of the prior arts and is not contradictory with the concept of the aromatic cycle for ethylene formation. 

This means that the catalyst with limited capability to transform ethylene to propylene (limited olefins interconversion activity) offers an opportunity to a selective formation of ethylene from methanol in the presence of ethylene. The experiments with MeOH and ethylene cofeeding have been repeated several times and confirmed in [144]. Kaeding and Butter [121] also suggested ethylene as the initial olefin at low methanol conversion over phosphorus-modified ZSM-5. 

The fact that ZSM-22 does not produce a lot of ethylene and propylene is explained by a suppression of both aromatic/coke and olefins interconversion cycles on this catalyst. SAPO-34 favors aromatic/coke cycle and limits the olefins cycle. On the contrary,... 

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