Oligomerization of Lower Olefins with Solid Acid Catalysts

1 Oligomerization of Lower Olefins with Solid Acid Catalysts 

The activity of solid acid catalysts for polymerization of olefins to products with higher boiling points has long been known. The polymerization proceeds throu the carbenium ion mechanism proton addition to the double bond followed by a carbenium-ion addition to the double bond. 

The composition of a sample of commercial propene-oligomer gasoline (after hydrogenation) produced over solid phosphoric acid is illustrated in Fig. 4.16. The main products are liquid oligomers (di-, tri-, and tetramers) indicative of the high rate of the chain-transfer. The C9-carbons are predominandy doubly branched, consistent with cationic oligomerization. The structure of oligomers does not correspond exacdy to a simple reaction scheme due to skeletal isomerization. Thus, the dimers do not have exclusively the 2-methylpentyl skeleton, which is expected from the addition of the 2-propyl cation to propene. 3-Methylpentane and 2,3-dimethylbutane are also formed. The skeletons of these molecules may result from methyl shift in the 2-methyl — 3-pentyl cation. 

Similarly, the predominant trimers contain 3,5-dimethylheptane and 2,3-dimethylheptane skeletons, which cannot be formed direcdy from propene and a methylpentyl skeleton. 

Each bsu shows percentage of carbon in the liquid hydrocarbons of that carbon number, and the sections of the bar show how much of this material is atraif t chain, how much single branched, etc., as detailed in the key. 

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