Kinetic model for the dimerisation of 1-hexene over a solid phosphoric acid catalyst

Batch kinetic experiments at 150 °C, 200 °C and 250 °C showed that the reaction can be modeled with a three step sequential reaction scheme. This involves firstly linear isomerisation of 1-hexene followed by skeletal isomerisation and finally dimerisation and cracking. The first and last steps in the sequence are modeled as reversible reactions. When first order reaction kinetics is assumed for each of the reactions, the model gave a very good representation of the experimental data. In order to test the validity of the series pathway hypothesis, the reaction was repeated with a skeletal hexene isomer – 2,3-dimethyl-2-butene (DMB) – as reactant. Although the rate and equilibrium constants for the third reaction step as obtained from the 1-hexene conversion data gave a good prediction of the DMB conversion at 200 °C and 250 °C, it failed to predict the reaction rate at 150 °C. This suggests that a different reaction pathway – where linear hexene isomers are directly converted to dimer product – becomes more significant at lower temperatures. The relatively high activation energy of the linear to skeletal hexene reaction may be to blame for this observation. However, this needs to be confirmed be further experimental work. The same equilibrium conversions of both 1-hexene and DMB were observed at all three temperatures investigated—suggesting that the equilibrium conversion is independent of the type of hexene isomer in the reaction mixture.

Graphical abstractA sequential reaction pathway is proposed for this reaction. The three reaction steps included in the model are firstly linear isomerisation of 1-hexene followed by skeletal isomerisation and finally dimerisation and cracking (see figure). The model is tested by using a skeletal hexene isomer – 2,3-dimethyl-2-butene (DMB) – as reactant.Figure optionsDownload full-size imageDownload as PowerPoint slide