Dimerization ethylene

Butene. Commercial production of 1-butene, as well as the manufacture of other linear a-olefins with even carbon atom numbers, is based on the ethylene oligomerization reaction. The reaction can be catalyzed by triethyl aluminum at 180—280°C and 15—30 MPa ( 150 300 atm) pressure (6) or by nickel-based catalysts at 80—120°C and 7—15 MPa pressure (7—9). Another commercially developed method includes ethylene dimerization with the Ziegler dimerization catalysts, (OR) —AIR, where R represents small alkyl groups (10). In addition, several processes are used to manufacture 1-butene from mixed butylene streams in refineries (11) (see BuTYLENEs). 

NiO-TiOz Catalyst Modified with WO3 for Ethylene Dimerization... 

Heterogeneous catalysts for the dimerization and oligomerization of olefins have been known for many years. A considerable number of papers have dealt with the problem of nickel-containing catalysts for ethylene dimerization [1-3]. 

Fig. 5. Time-course of ethylene dimerization over catalysts evacuated at 400 C for 1 h ( ) S-NiO-TiOj/lS-WOj ( ) 25-NiO-...

Fig. 6. Catalytic activities of Ni0-Ti02/15-WO3 for ethylene dimerization as a function of NiO content.

Hartmann, M., A. Piippl et al. (1996). Ethylene dimerization and butene isomerization in nickel-containing MCM-41 and A1MCM-41 mesoporous molecular sieves An electron spin resonance and gas chromatography study. J. Phys. Chem. 100 9906-9910. 

Yet another possibility is illustrated by the propene (or ethylene) dimerization catalyzed by 7r-l,l,3,3-tetraphenylallylnickel bromide (26) activated with ethylaluminum dichloride the isolation of considerable amounts of 1,1,3,3-tetraphenylpropene (27) from the reaction mixture suggests that a hydrogen atom has been transferred from the substrate olefin to the sterically hindered 1,1,3,3-tetraphenylallyl system under formation of 3 [Eq. (7)] (81). The subsequent formation of the HNiY species from 3 can then take place by insertion of a second propene molecule and /3-hydrogen elimination, as discussed above. 

The reaction of ethylene at -20°C and 1 atm with the phosphine-free catalyst prepared from 77-allylnickel chloride and ethylaluminum dichloride in chlorobenzene results in the rapid formation of a mixture of ethylene dimers with lesser amounts of higher oligomers. The dimer fraction consists mainly of 2-butenes and the trimer fraction of 3-methylpentenes and 2-ethyl-1-butene as well as a minor amount of hexene (97). From the composition of the products it can be concluded that the displacement reaction predominates over the insertion reaction when using the phosphine-free catalyst and that the direction of addition of both the H—Ni and C2H5—Ni species is mainly of the Ni — C2 type. 

Butadiene-ethylene dimerization (example 3, Table II) has been shown to proceed via a croty 1-nickel complex formed by protonation (54). It should be observed at this point that it cannot be excluded that linear cooligomerization of butadiene with ethylene to give 1,4,9-decatriene... 

Table II also lists several isomerizations and skeletal rearrangements (examples 4-7) which are related to butadiene-ethylene dimerization. Protonation of phosphorus-containing nickel(O) complexes is sufficient to achieve skeletal rearrangement of 1,4-dienes in a few seconds at room temperature, probably via cyclopropane intermediates (example 6, Table II). For small ring rearrangements see Bishop (69).

The calculations of Pabon and Bauld on the hole-catalyzed ethylene dimerization were among the first ab initio studies on radical cation reactions [46]. 

To illustrate the weak effect of electron correlation on the electronic coupling for hole transfer, we mention HE and CASPT2 results for an ethylene dimer at an intermolecular distance of 3.5 A, 0.571 eV, and 0.555 eV, respectively cf. Ref. 58... 

The essential point that distinguishes between allowed and forbidden reactions is the role of the D+A configuration. If the D+A configuration is allowed by symmetry to mix into the transition state wave-function then the transition state will be stabilized and will take on character associated with that configuration. For the ethylene dimerization, D+A is precluded from mixing with DA due to their opposite symmetries. As was discussed in detail in Section 2 (p. 130), DA cannot mix with D+A" since and n orbitals are orthogonal (106). Thus for ethylene dimerization the concerted process... 

Figure 7.10 An orbital correlation diagram for ethylene dimerization. Left two widely separated ethylene molecules. Center two ethylene molecules close enough for significant interactions to occur. Right cyclobutane electron configurations correspond to the ground state for each stage.

Although the preceding argument is correct as far as it goes, and leads to a satisfying insight into the source of the inhibition of ethylene dimerization, it is possible and desirable to carry it a step further and thus obtain an even better understanding of this kind of problem. As stressed already in Section... 

It is possible to show very generally that for two olefins, having w, and m2 n electrons, coming together to form a cyclic olefin with (m] + m2 - 4)/2 n bonds, as shown below, the reaction will be thermally allowed when ms + m2 = 4n + 2 (e.g., 4 + 2 = 6 in the case of the Diels-Alder reaction). On the contrary, when m, + m2 = 4n (e.g., 2 4- 2 = 4 for the ethylene dimerization) the reaction is thermally forbidden but photochemically allowed. For a discussion of this generalization the article of Woodward and Hoffmann should be consulted. 

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