Insights into the Chemistry of the Homogeneous Thermal Oligomerization of Ethylene to Liquid-Fuel-Range Hydrocarbons

Thermal, noncatalytic conversion of light olefins (C2=-C4=) was originally utilized in the production of motor fuels at several U.S. refineries in the 1920s to 1930s. However, the resulting fuels had relatively low octane number and required harsh operating conditions (T > 450 °C, P > 50 bar), ultimately leading to its succession by solid acid catalytic processes. Despite the early utilization of the thermal reaction, relatively little is known about the reaction products, kinetics, and initiation pathway under liquid-producing conditions. In this study, thermal ethylene oligomerization was investigated near industrial operating conditions, i.e, at temperatures between 300 and 500 °C and ethylene pressures from 1.5 to 43.5 bar. Nonoligomer products such as propylene and/or higher odd carbon products were significant at all reaction temperatures, pressures, and reaction extents. Methane and ethane were minor products (<1% each), even at ethylene conversions as high as 74%. The isomer distributions revealed a preference for linear, terminal C4 and C5. The reaction order was found to be second-order with a temperature-dependent overall activation energy ranging from 39.4 to 58.3 kcal mol-1. Four bimolecular initiation reaction steps for ethylene were calculated using DFT. Of these, simple H-transfer to yield vinyl and ethyl radicals was found to have a free energy activation energy barrier higher (about 10 kcal mol-1) than the other three initiation steps forming either cyclobutane, 1-butene, or tetramethylene. The importance of diradical species in generating free radicals during a two-phase initiation process was proposed. The reaction chemistry for ethylene, which has only strong, vinyl C-H bonds, starkly contrasted with propylene, which possesses weaker allylic C-H bonds and showed a preference for dimeric C6 products over C2-C8 nonoligomers. The resulting C4 and C5 nonoligomers from propylene contained more iso-olefins compared to linear C4 and C5.