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Synthetic Middle-Distillate-Range Hydrocarbons via Catalytic Dimerization of Branched C6–C8 Olefins Derived from Renewable Dimethyl Ether

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journal contribution
posted on 17.12.2015, 09:38 by Mayank Behl, Joshua A. Schaidle, Earl Christensen, Jesse E. Hensley
2,3,3-Trimethyl-1-butene (triptene) and other branched C6–C8 olefins, having structures characteristic of the products from the low-temperature acid-catalyzed homologation of dimethyl ether (DME), were converted to distillate-range hydrocarbons (C10–C20) with high selectivity via dimerization over a commercial ion-exchange acidic resin (Amberlyst-35) under liquid-phase stirred-batch conditions operating at ambient pressure. Triptene conversion and dimer (2,2,3,5,5,6,6-heptamethyl-3-heptene) production were monitored with time at different temperatures (60, 80, and 100 °C). The dimer production rate increased with increasing temperature; however, dimer concentration decreased with increasing temperature due to competing side reactions. Dimerization, as compared to cracking, isomerization, and oligomerization, was the dominant reaction pathway during the first hours of reaction at all temperatures. Dimerization at 100 °C achieved a conversion of 35% and a molar selectivity to the desired dimer of 71% in 2 h. In longer runs (≥16 h), the highest conversion (80%) was achieved at 100 °C whereas the maximum total C10+ production (1.83 g/batch, 52% by weight of the reactant) was achieved at 80 °C. The nucleophilicity and extent of branching of the C6–C8 olefins were found to have a strong effect on dimerization yields. The cloud point, boiling range, carbon-number distribution, and lower heating value of the dimerized product were compared to ASTM specifications for middle-distillate fuels, and the results suggest that approximately 80% of the product has potential as a jet fuel blend stock.