Microkinetic Modeling of the Homogeneous Thermal Oligomerization of Ethylene to Liquid Fuel-Range Hydrocarbons

The thermal oligomerization of ethylene is an intriguing reaction for the production of fuel-range products. Often viewed as a detrimental reaction leading to polymeric deposits in pipelines, recent work suggests that it can be exploited to convert ethylene to higher hydrocarbons in the C5-C12 range. Despite the long history surrounding this reaction, quantum chemical simulations have not been fully deployed to provide full mechanistic understanding, and kinetic models to optimize reaction conditions are lacking. In this work, a microkinetic model based on quantum chemical calculations was developed to unravel the primary drivers of initiation and understand the formation of a variety of products of different carbon number, including odd-numbered products. Through flux analysis, the main driver of initiation was identified to be hydrogen abstraction from ethylene by a 1,4-butyl diradical produced from the reaction of two ethylene molecules. As conversion increased, the primary initiation mode switched to hydrogen abstraction by a butene diradical as 1-butene began to be produced in high quantities. Odd-numbered carbon species were seen to originate from the beta-scission of C8 radical species, with the radical position varied due to intramolecular hydrogen shift reactions from terminal radicals formed via radical addition reactions. The significant quantity of linear terminal olefins in the experimental product distribution was identified to originate from the formation of vinyl radicals through hydrogen abstraction reactions involving ethylene that were then propagated via radical addition reactions to ethylene. The insights from this work can aid in the development of intensified reactor systems to valorize ethane streams from shale gas production, converting waste streams directly to usable fuel products. Microkinetic model quantitatively captures conditions under which thermal oligomerization of ethylene leads to broad product distribution of linear alkenes with both even and odd carbon numbers.