Solid phosphoric acid catalysts: The effect of free acid composition on selectivity and activity for 1-hexene dimerisation

The rate of dimerisation of 1-hexene in a batch reactor at 200 degrees C was used to compare the activity and dimerisation route selectivity of solid phosphoric acid (SPA) at various phosphoric acid strengths and subsequently free acid compositions. The initial reaction rates, calculated from an elementary kinetic rate model, showed an increase in catalyst activity with increased acid strength. This trend becomes more significant at acid strengths where the more condensed pyrophosphoric acid started becoming present in the free acid. At low acid strengths, a reaction pathway where linear hexenes were directly oligomerised dominated the reaction progression. An increase in acid strength favoured a two-step reaction pathway, where linear hexenes were first isomerised to skeletal hexene isomers before dimerisation. This sequential pathway becomes especially dominant where pyrophosphoric acid became present in the free acid. However, analysis of the final product with proton NMR showed no significant difference in the degree of branching with a variation in dimerisation route selectivity as a function of acid strength. (C) 2009 Elsevier B.V. All rights reserved.