Disproportionation, olefins

Disproportionation of Olefins. Disproportionation or the metathesis reaction offers an opportunity to convert surplus olefins to other desirable olefins. Phillips Petroleum and Institut Fransais du Petrc le have pioneered this technology for the dimerization of light olefins. The original metathesis reaction of Phillips Petroleum was intended to convert propylene to 2-butene and ethylene (58). The reverse reaction that converts 2-butene in the presence of excess ethylene to propylene has also been demonstrated (59). A commercial unit with a capacity of about 136,000 t/yr of propylene from ethylene via 2-butene has been in operation in the Gulf Coast since 1985 (60,61). In this process, ethylene is first dimerized to 2-butene foUowed by metathesis to yield propylene. Since this is a two-stage process, 2-butene can be produced from the first stage, if needed. In the dimerization step, about 95% purity of 2-butene is achieved at 90% ethylene conversion. 

A second route based on olefin disproportionation was developed by Phillips Petroleum (131). Here isobutylene reacts with propylene to form isoamylenes, which are dehydrogenated to isoprene. 2-Butene can be used in place of propylene since it also yields isoamylene and the coproduct propylene can be recycled. Use of mixed butylenes causes the formation of pentenes, giving piperjlene, which contaminates isoprene. 

The application of these catalysts in the initial state (without any special treatment of the surface organometallic complexes of such cata-lysts) for ethylene polymerization has been described above. The catalysts formed by the reaction of 7r-allyl compounds with Si02 and AUOj were found to be active in the polymerization of butadiene as well (8, 142). The stereospecificity of the supported catalyst differed from that of the initial ir-allyl compounds. n-Allyl complexes of Mo and W supported on silica were found to be active in olefin disproportionation (142a). 

Olefin disproportionation Catalyst system, MoClg/E ALCL/ 44... 

Olefin disproportionation 02/CH3C00H, caused a higher catalyst stability and activity. Catalysts based on Mo or W, especially 45... 

Olefin disproportionation order to have an increased ration of P-olefins. This caused higher catalyst lifetime. Mo or W03 and alumosilicate as a carrier. 47... 

The homogeneous hydrogenation of soybean oil methyl ester Olefin disproportionation... 

It is to be hoped that measurements will be made in the near future which will put more substantial flesh on the skeleton of known bond enthalpy contributions in organo-transition metal compounds, so that a better understanding of the energetics of reactions such as olefin disproportionation (metathesis) and hydroformylation may be achieved. 

Olefin metathesis (olefin disproportionation) is the reaction of two alkenes in which the redistribution of the olelinic bonds takes place with the aid of transition metal catalysts (Scheme 7.7). The reaction proceeds with an intermediate formation of a metallacyclobutene. This may either break down to provide two new olefins, or open up to generate a metal alkylidene species which -by multiple alkene insertion- may lead to formation of alkylidenes with a polymeric moiety [21]. Ring-opening metathesis polymerization (ROMP) is the reaction of cyclic olefins in which backbone-unsaturated polymers are obtained. The driving force of this process is obviously in the relief of the ring strain of the monomers. 

The variety of transformations possible and the availability of both soluble and supported catalysts should make the industrial future of olefin disproportionation a bright one. 

The report that Mo(CO)6 could be activated for olefin disproportionation by thermal expulsion of CO (46) prompted us to use the dicarbonyl derivative of Table III in a similar manner. No activity was achieved by this technique. Photochemical activation of MoC12(NO)2-(PPh3) 2 was also unsuccessful. 

Many organometallic compounds are moisture sensitive, a particularly important class from the catalytic viewpoint being transition metal hydrocarbyl derivatives (Mn+Rn). These compounds have been used as catalysts for or have been postulated as catalytic intermediates in many olefin reactions such as polymerization, oligomerization, and olefin disproportionation (23). Recent work has shown that compounds containing... 

The reactions chosen for study in this work were olefin polymerization and olefin disproportionation. Catalysts for the former reaction are, in the majority of cases, metal derivatives of Groups IVB-VIB whereas compounds of molybdenum, tungsten, and rhenium are recognized as catalysts for olefin disproportionation. 

We report here a study of Zr, Nb, Cr, and Mo hydrocarbyl compounds grafted onto oxide supports as potential olefin polymerization catalysts and oxide-supported Mo and W 7r-allyl derivatives in olefin disproportionation catalyses. The interaction of these compounds with silica and alumina supports has been examined using ESR and IR, analyses to define the catalytic materials that result. Finally, we consider why chemical support of these organometallic compounds confers on them an enhanced catalytic activity. 

The disproportionation activity in the supported species is parallel to the increased activity of ethylene polymerization on supported catalysts. Many of the steps in the reaction may be identical for example, the initial coordination of olefin to the metal center will be common to both systems. Indeed, some of these catalysts are also ethylene polymerization catalysts (see Table IV) although their activities are much less than the corresponding zirconium derivatives. A possible intermediate common to both disproportionation and polymerization could be the hydrocarbyl-olefin species (Structure I). Olefin disproportionation would result if the metal favored /3-hydrogen elimination to give the diolefin intermediate (Structure II) which is thought to be necessary for olefin disproportionation. Thus, the similarity between the mechanism and activation of olefin disproportionation and polymerization is suggested. 

Finally, just as in the case of polymerization, the particular active site may be a unique combination of organometallic compound and support with the added complication that all the sites catalyzing olefin disproportionation may not have the same activity. 

The alkene metathesis reaction arose serendipitously from the exploration of transition-metal-catalysed alkene polymerisation. Due to the complexity of the polymeric products, the metathetic nature of the reaction seems to have been overlooked in early reports. However, in 1964, Banks and Bailey reported on what was described as the olefin disproportionation of acyclic alkenes where exchange was evident due to the monomeric nature of the products [8]. The reaction was actually a combination of isomerisation and metathesis, leading to complex mixtures, but by 1966 Calderon and co-workers had reported on the preparation of a homogeneous W/Al-based catalyst system that effected extraordinarily rapid alkylidene... 

---