High-Temperature Conversion of Olefins to Liquid Hydrocarbons on γ-Al2O3

Abstract

Noncatalytic thermal conversion of light olefins proceeds at industrially relevant rates at temperatures above 450 °C and pressures above 50 bar. The discovery of solid acid oligomerization catalysts permitted the use of milder conditions (<300 °C) and significantly improved the octane rating. However, Brønsted acid catalysts deactivate and must be regenerated frequently. In this study, at reaction temperatures of about 250–450 °C and pressures of 1 to 40 bar, olefins react on γ-alumina to form higher molecular weight products. The rate of propylene is about ten times higher than that of ethylene. The products, however, are not a simple olefin oligomerization distribution, and many nonoligomer products are formed. The primary products undergo secondary reactions, including double bond isomerization and H-transfer, giving moderate selectivities for saturated products. Depending on the conversion, temperature, and pressure, the rate of ethylene conversion on alumina is more than 100 times that of thermal, noncatalytic conversion. The apparent activation energy for ethylene conversion is 55–75 kJ/mol, which is much lower than ∼244 kJ/mol observed for the thermal gas-phase reaction. On alumina, some reactants and products undergo disproportionation reactions. For example, propylene forms equal molar amounts of ethylene and iso-butene even at very low conversions. Lewis acid sites on γ-alumina have previously been proposed as the active site for double bond isomerization and H–D exchange. Thus, it seems likely that Lewis acid sites are also catalytic for olefin oligomerization and disproportionation reactions. With the γ-alumina catalyst, high liquid yields can be achieved with little formation of coke and minimal deactivation for at least several days.