The neohexene process

3-Dimethylbut-l-ene (neohexene) is an important intermediate in the synthesis of musk perfume. Its production was begun by the Phillips Petroleum Co. in 1969. Demand has steadily grown (Banks 1982). The process is based on the dimer of 

A simplified scheme of the plant is shown in Fig. 17.3. Commercial diisobutene is first fractionated to remove an oxidation inhibitor, which would otherwise poison the dual catalyst system. The fractionated diisobutene along with the ethene stream enters the top of the reactor containing the catalyst. The ethene, consisting of make-up and recycled ethene, is compressed to the required pressure before it enters the reactor. The separation of reactants and products is achieved by stripping and fractionation. The catalyst is reactivated from time to time, using a mixture of air and inert gas to control the temperature of the coke bum-off In this way the conversion can be maintained between 75% and 50%. The final product has a purity of 96%. 

Neohexene is employed to make the class of synthetic musks designated as bicyclic musks. The reaction with / -cymene, followed by acetylation, gives one such compound, 7-acetyl-1,1,3,4,4,6-hexamethyltetralin (1) sequence (3). Bicyclic musks, with their excellent odour and fixative properties, simulate macrocyclic musks, and have excellent stability towards alkali and light, as well as being colourless and inexpensive to produce. 

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It is still unclear how the initiation step in alkene metathesis occurs and how the initial carbene forms. Commercial applications of metathesis include the triolefin process, in which propylene is converted to ethylene and butene, the neohexene process, in which the dimer of isobutylene, Me3CCH=CMe2, is metathesized with ethylene to give Me3CCH=CH2, an intermediate in the manufacture of synthetic musk, and a 1,5-hexadiene synthesis from 1,5-cy-clooctadiene and ethylene. Two other applications, SHOP and ROMP (Shell higher olefins process and ring-opening metathesis polymerization), are discussed in the next section. 

The neohexene process starts with the acid-catalyzed dimerization of isobutene, followed by metathesis with ethylene, to give neohexene, an intermediate in the manufacture of synthetic musk, and regenerate isobuteneJ... 

Above we have mentioned several heterogeneous applications such as the OCT process and SHOP. Neohexene (3,3-dimethyl-1-butene), an important intermediate in the synthesis of fine chemicals, is produced from the dimer of isobutene, which consists of a mixture of 2,4,4-trimethyl-2-pentene and 2,4,4-trimethyl- 1-pentene. Cross-metathesis of the former with ethene yields the desired product. The catalyst is a mixture of W03/Si02 for metathesis and MgO for isomerisation at 370 °C and 30 bar. The isobutene is recycled to the isobutene dimerisation unit [48],... 

A metathesis similar to OCT came on stream about the same time as the SHOP process. Ethenolysis43 of a mixture of 2,4,4-trimethyl-2-pentene and 2,4,4-trimethyl-1-pentene yields 3,3-dimethyl-l-butene (commonly called neohexene) plus isobutylene (equation 11.15). 

In the commercial manipulation of hydrocarbon feedstocks, the metathesis reaction clearly provides a valuable unit process, which may be combined with other processes either in the same reactor using successive layers of the requisite catalysts, or in consecutive reactors. The neohexene and SHOP processes have already been discussed and three other examples are shown in Scheme 17.1. 

The elucidation of the role of double-bond isomerization activity in metathesis process is an example of the helpfulness of the four-center mechanism. As the scheme predicted, in certain applications the elimination, of double-bond isomerization activity (acidic isomerization sites were destroyed by various mild caustic treatments) prevented secondary metathesis reaction resulting in very high selectivity to specific products ( 5). In contrast, in other applications (e.g., linear olefin and neohexene processes) to obtain a high level of productive metathesis, the mechanistic scheme indicated a need for enhanced isomerization activity this was accomplished by addition of a very selective double-bond isomerization catalyst to the scheme ( ),... 

The latest (1980) commercial application of olefin metathesis is Phillips Neohexene Process ( ). Neohexene, an intermediate in the synthesis of a perfume musk, is produced by cross-metathesis of diisobutylene with ethylene (i,e., ethylene cleavage) over a bifunctional (double-bond isomerization/metathesis) catalyst system (Figure 7) ... 

W03/Si02 has a high potential for practical applications in metathesis. The high-operation temperatures make it less susceptible to trace quantities of catalyst poisons such as air and water and to coke formation. Recently, this catalyst has extensively been studied in relation to its industrial use for the metathesis of long-chain olefins, such as oct-l-ene and industrial hept-l-ene [70], The optimum reaction temperature of an 8 wt% W03/Si02 catalyst was found to be 733 K, The optimum temperature for regeneration in a flow of air is 823 K, A W03/Si02 catalyst is presently used in the OCT process and also in Phillips s neohexene process [2],... 

In the Phillips neohexene process147 2,4,4-trimethyl-2-pentene (8) is converted by cleavage with ethylene to neohexene (9) used in the production of a perfume musk. The starting material is commercial diisobutylene. Since it is a mixture of positional isomers (2,4,4-trimethyl-2-pentene and 2,4,4-trimethyl-l-pentene) and the latter (7) participates in degenerative metathesis, effective utilization of the process requires the isomerization of 7 into 8. A bifunctional catalyst system consisting of an isomerization catalyst (MgO) and a heterogeneous metathesis catalyst is employed 131... 

Cross metathesis of ethylene with internal alkenes provides a facile route to terminal alkenes. A number of processes have been described that use this transformation however, the only products, besides neohexene,that appear to be important are the a,o>-dienes that result from metathesis of cyclic alkenes with an excess of eAylene. This family of compounds should find a wide variety of applications. 

This process, now run by the Chevron Phillips Chemical Company, produces over a million pounds of neohexene annually. Neohexene is a raw material used... 

Neohexene is by far the more expensive of the two starting materials, and so the process shown in Scheme 4.45 is unsatisfactory, since it consumes twice as much of this reagent as is desirable. One way of overcoming this problem is to add a sacrificial oxidant that is less expensive than neohexene. For example, /-butyl chloride, under the influence of aluminium chloride, loses its chlorine to produce the /-butyl cation, which can abstract the hydride ion from p-cymene and thus save one molar equivalent of neohexene. 

From a practical point of view, commercial diisobutene is a mixture of 2,4,4-trimethyl pent-2-ene, used in the cross-metathesis reaction, and of 2,4,4-trimethyl pent-l-ene (not modified by the metathesis reaction), in a relatively large amount (20 to 25%). In order to valorize the process, it is necessary to use commercial diisobutene and so to convert the terminal non useful olefin into the internal useful one. This can be done by mixing the metathesis catalyst (W03/Si02) with an isomerization catalyst (typically magnesia) together in the reactor. Typically, with a 1 3 mixture of W03/Si02 and MgO, at 370°C, under a 30 bar pressure and with a ethylene to diisobutene ratio of 2. A conversion of 65% of diisobutene is achieved with a neohexene selectivity around 85%. 

During a typical process cycle, the diisobutene conversion starts at about 75% and gradually declines. When the conversion reaches 50%, the catalyst system is regenerated using an air-inert gas mixture to control the temperature of the coke burn-off. Throughout the process, the selectivity to neohexene remains constant at c.a. 85%. 

Di-t-butylethene, a component also used as an additive for synthetic gasoline, is obtained from isobutene and ethene with a dual isomerization-disproportionation catalyst [20]. The process begins with dimerization in a reactor for conversion of isobutene to 2,4,4-trimethyl-1-pentene this is followed by isomerization-disproportionation in a bifunctional unit (isomerization of 2,4,4-trimethyl-1-pentene to 2,4,4-tri-methyl-2-pentene and conversion of the latter into di-r-butylethene) (Fig. 6). A by-product of the process, 2,3-dimethyl-2-butene, is recirculated to the disproportionation unit to be cleaved with ethene to isobutene, which is reintroduced into the process. When neohexene (3,3-dimethyl-1-butene) is employed as the starting material in this process, the installation consists solely of the disproportionation and fractionation units. 

FIG. 5 Schematic diagram of the process for neohexene synthesis from isobutene (a) dimerization (b) separation (c) disproportionation (d) separation. (From Ref. 19.)... 

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