1-alkenes represent a large part of the commercial market and since oligomers thereof are important intermediates for specialty chemicals, they warrant further study. Metallocene catalysts are a promising development, since metallocenes, such as Cp2ZrCl2 (Cp = cyclopentadienyl), are easily obtainable. Examples of the oligomerisation of higher 1-alkenes are limited and there is a shortage of in-depth studies on these types of catalysts [3, 8–39]. It is also very apparent that only low MAO loadings lead to the formation of oligomers, and at higher loadings, polymerisation takes place [3, 18–29]. In an effort to increase the knowledge base of the oligomerisation of 1-alkenes a series of Cp2MCl2-catalysts (M = Zr (1), Ti (2), Hf (3) and Nb(4)), more complex Zr-metallocene (5 to 7) and tridentate bis(imino)pyridine iron(II) (8 to 11) catalyst systems was investigated (Figure 4). The specific aim was to determine the oligomerisation activity and selectivity of these catalysts in the presence of MAO as co-catalyst. Several factors can influence the catalyst activity during the conversion of 1-alkenes, namely activation temperature (Ta), activation time (ta), reaction temperature (Tr), co-catalyst concentration, the transition metal (M), and the monomer (mo) concentration. All these factors were investigated for the metallocene catalysts (1 to 4). The choice of ligand and bridging compound of the metallocene catalysts influenced the properties of the polymers obtained during reactions. The three more complex metallocene catalysts, 5 to 7, were investigated for their ability to oligomerise higher 1-alkenes. An important development in 1-alkene polymerisation catalysts was the discovery of the Group 8 iron catalysts, the so-called neutral tridentate bis(imino)pyridine iron(II) catalysts. Four catalysts of this type, 8 to 11, were identified for investigation of their activity with regard to the oligomerisation of higher chain 1-alkenes. Many factors can influence the catalyst activity during the conversion of 1-alkenes; in this study, only the reaction temperature (Tr) and co-catalyst concentration were varied because they were found to be major factors in determining the catalytic activity. The activity of the catalysts and degree of oligomerisation (n) were investigated with regard to 1-heptene and 1-octene.
Investigation of the co-catalyst
The co-catalysts investigated were ethylaluminium dichloride (EADC) and methylaluminoxane (MAO) respectively. Under the influence of the MAO co-catalyst the activity of the catalyst is significantly higher when compared with the influence of EADC (Figure 5).
Activation of the Cp2ZrCl2/MAO system
At lower activation temperatures a significantly lower catalyst activity is observed. At higher temperatures the activity starts to decrease from the optimum observed at 50 °C (Figure 6). In Figure 7 the influence of the activation time on the activity is shown. The influence of the activation time on the activity shows a gradual increase from 0 mins until the optimum is reached at 45 mins, whereafter the activity starts to decrease. The optimum activation temperature of 50 °C and the optimum activation time of 45 mins were used in all the subsequent reactions.
Influence of the monomer concentration
The mo/M molar ratio variation investigated did not drastically affect the C8 conversion (Figure 8). The largest mo/M molar ratio of 400 was used in all the subsequent reactions.
Influence of the co-catalyst concentration on the catalytic activity
It was found that the transition metal has a significant influence on the observed activity (Table 1). All the catalysts showed a marked improvement in conversion as the co-catalyst concentration was increased. The following activity trend with regard to the various 1-alkenes can be reported for Cp2MCl2/MAO when the transition metal is varied: M = Zr (1) > Hf (3) > Ti (2) > Nb (4). In Table 2 the percentage C7 conversion at different MAO/M molar ratios for catalysts 5to 11 is summarised. There is a clear increase in activity with an increase in co-catalyst concentration.
Influence of the co-catalyst concentration on the degree of oligomerisation
The 4/MAO system always yielded only dimers, regardless of the MAO/M molar ratio used. At all MAO/M molar ratios the 1/MAO system yielded trace amounts of trimers, with the major product being dimers. The degree of oligomerisation for the 3/MAO and 4/MAO systems increased when the co-catalyst concentration was increased. It was further found that the degree of oligomerisation decreases as the 1-alkene chain length increases. The highest degree of oligomerisation for catalysts 5 to 11 with 1-heptene and 1-octene was investigated. Catalyst 5yields mainly dimers, but trimers and tetramers also form. For catalyst 6, the highest degree of oligomerisation is trimerisation, but the main product is dimers in all cases. Catalyst 7 was not reactive with 1-heptene, but yielded hexamers and octamers with 1-octene. The product distribution when catalyst 7 was used was spread proportionally between the various oligomers formed. When the MAO/M molar ratio was increased, the highest degree of oligomerisation observed for catalysts 8 to 11 during reactions with 1-heptene and 1-octene was either dimers or trimers.
Influence of the transition metal on the reaction rate
The reaction rate is given as the rate whereby the percentage 1-alkene decreases with time. These reaction rates were measured at the different MAO/M molar ratios; however, for the current discussion only the data for the MAO/M molar ratio of 10 is reported (Table 2). In general, the reaction rate increases in the order: M = Zr > Hf > Ti > Nb.
Influence of the reaction temperature on catalyst activity and the highest degree of oligomerisation observed
The reaction temperature was varied from 0 to 140 °C in order to determine the optimum temperature for the conversion of 1-alkenes (Table 4). The optimum activation temperature for catalyst 1 to 11 was found at 50 °C. When the influence of the reaction temperature on the highest degree of oligomerisation observed is studied, the highest degree of oligomerisation observed for catalysts 1 to 4 varies from dimers to heptamers. Generally, when the reaction temperature is varied from 0 to 110 °C, a decrease in the degree of oligomerisation for catalysts 1 to 4 was observed. Catalysts 5 and 6 yield trimers at all temperatures investigated, but the main products are dimers. With 1-heptene, only small percentages of dimers were observed at 80 °C when catalyst 7 was used. The reactions of catalyst 7 with 1-octene showed products that were spread proportionally between the various oligomers formed. Catalysts 8 to 11 yielded mainly dimers, with only small percentages of trimers present in the product mixtures.
Structure elucidation of the oligomers
A typical chromatogram of the oligomers formed from 1-octene using the catalyst systems 1 to 3and 5 to 7 is illustrated in Figure 10. Catalyst systems 4 and 8 to 11 yielded only the dimer 22and consequently its chromatogram is less complex, as illustrated in Figure 11. The oligomer mixture could not be separated successfully, but since 16a was the major product, it was possible to elucidate the structure from NMR and MS analyses. The proposed structure of 16afrom the NMR analyses is 2-hexyl-1-decene (Table 5). The proposed structure for 16b prepared with 7 is 7-methylpentadecane. The resonance signals are summarised in Table 6. NMR and MS analyses on the dimer product 22 formed exclusively by the catalyst systems 4 and 8 to 11 were conducted, and the proposed structure of 22 (Table 7) is 7-hexadecene. It is proposed that the formation of the dimeric products 16 and 22 occurs according to a metal hydride mechanism (Scheme 2) [3, 24, 40, 47, 57]. The catalytic cycle consists of a series of insertion and β-H-elimination steps that will repeat until all the alkene has been consumed, or until the active species decomposes. It is important to note, that formation of the metal-alkyl-species bounded to carbon-2 would inhibit the formation of the observed dimers.
The activity of catalyst systems 1 to 7 decreases when the reaction temperature is raised to 110 °C, but increases with an increase in the MAO concentration. During the temperature increase, the length of the monomer seems to have a small influence on the activity of catalysts 1 to 4, but a larger influence on the degree of oligomerisation. An increase in the MAO concentration does lead to an increase in the degree of oligomerisation observed. The activity of the reactions in the presence of metallocene catalysts with different transition metals differs significantly. Catalyst system 1 shows higher reaction rates as well as catalytic activity when compared with 2, 3 and 4. The activity of 2 and 3 is in the same order, but the degree of oligomerisation is affected significantly by the reaction conditions used. Similarly, the activity of catalysts 5 and 6 is in the same order. Catalyst system 4 yielded dimer products exclusively, but in low yields. The activity of catalyst 7 is also low, but in contrast to the observations made with 4, the degree of oligomerisation is high. At low co-catalyst loadings, the only iron(II) catalyst that showed promising results was 9. The activity of 8, 10 and 11 was very low and these systems were inactive in more than half the cases investigated. An increase in co-catalyst loading did increase the activity of the various catalyst systems. At lower temperatures, a gradual decrease in activity for catalysts 8 to 11 was observed. A gradual increase in the highest degree of oligomerisation by catalysts 8 to 11 was observed with an increase in co-catalyst concentration. The main oligomer products were dimers, regardless of the temperature used during reactions with catalysts 8 to 11. Therefore, of the iron(II) catalysts investigated, only 9 is recommendable for the synthesis of dimeric oligomers. The low concentration of oligomers other than dimers when the catalyst systems 1, 5 and 6 were used at 50 °C does suggest that these are the catalysts with the most potential for the synthesis of dimer oligomers.
Keywords: 1-alkene, bridged metallocenes, metallocene dichloride catalyst, methylaluminoxane, oligomerisation, iron(II)-complex