Metathesis and transalkylation in tandem catalysis

Thesis (Ph.D. (Chemistry))--North-West University, Potchefstroom Campus, 2007

Longer chain terminal alkenes, which are in high demand in industry, can be obtained from widely available shorter chain terminal alkenes in a tandem catalysis reaction. Tandem catalysis involves me combination of multiple catalytic reactions in a single reactor without separation steps in between. In this study, longer chain terminal alkenes were obtained by combining metathesis and transalkylation in a tandem experiment and the results were compared to a different type of tandem experiment, involving metathesis and isomenzation. The metathesis reaction was investigated in Chapter 2, where a short chain terminal alkene is used as the starting material, producing a longer chain internal alkene as primary metathesis product. In these reactions, Grubbs 1 catalyst was used and the experiments were carried out in a mini-reactor with different alkenes (1-pentene, 1-ocene and 1-decene) under different reaction conditions. The PMP yield decreased with increasing alkene:Ru ratio and the alkene chain length influence was not very Clear. Reproducible yields were obtained with all three of the alkenes and the yield increased when the reaction was carried out without solvent or when the gas formed during the reaction was allowed to escape (equilibrium shift). The average PMP yield far 1-octene was 45% after 5 hours at 30oC with an alkene:Ru ratio of 1,000 and above 60% for 1-pentene or when 1-octene was used in reactions without solvent or when the equilibrium was shifted to the right. In Chapter 3 the transalkylation reaction was carried out. This transalkylation reaction a based on the Ziegler "Aufbau” (growth) reactiion and transforms an internal alkene into a terminal alkene. In this project the main focus was to obtain terminal octenes, therefore the starting alkene for the transalkylation reaction was an internal octene. The first part of the transalkylation reaction, the displacement, involves the displacement of the alkyl groups of a trialkylaluminum compound by octene. This internal octene is isomerized to 1-octene before displacing the alkyl group, in the presence of an isomerization catalyst. The second part of the reaction is the back-displacement, where the newly formed octyl groups of the trioctylaluminum compound are replaced again with another (terminal) alkene, setting free a terminal octene. A reactor setup was built that was suitable for these experiments and could also be used in the tandem catalysis experiments. After the development of a quantitative analysis method, which consisted of boiling off the excess octene after displacement in combination with GC analysls, the transalkylation reaction was investigated and optimized. The best results were obtained when TIBA was used in combination with 1-decene as a back-displacing alkene. The optimal yields were obtained when the displacement reaction was carried out at 100oC for 3 hours with 1 mol% Ni(acac)2 catalyst, followed by the back-displacement at 25oC for 2 hours, both reactions with an alkene:Al ratio of 5:1. When starting with 4-octene, an 82% yield was obtained. 1,5-Cyclooctadiene was used to stop isomerization after back-displacement, which proved to be a very successful isomenzation-inhibiting agent. The optimized metathesis and tansalkylation reactions were combined in tandem experiments in Chapter 4. An overall yield of 8.6% was obtained after metathesis of 1-pentene with Grubbs 1 as a catalyst, followed by transalkylatlon where TIBA was used as the aluminum compound and 1-decene as the back-displacing alkene, in the presence of the Ni(acac)2 catalyst. The octene product consisted mostly of 1-octene (95%), but some isomerization had taken place. The metathesis-isomerization experiments showed a very low 1-octene yield in the isomerization step, and therefore the overall yield of these experiments was lower than that of the metathesistransalkylation experiments. An overview of the conclusions of the different chapters was given in Chapter 5, where the potential of the projects on a larger scale was discussed. The tandem catalysis experiments in which metathesis and transalkylation were combined proved to be successful in obtaining longer chain terminal alkenes, as was set as the main aim of this study. Although this method was suitable for laboratory scale experiments, more aspects have to be taken into account to determine the feasibility on a larger industrial scale.

Doctoral