Making Olefins from Soybeans

GREEN CHEMISTRY

Vegetable gardens may seem like unlikely places to look for olefins. But according to a new study, the leafy patch in the backyard may be an ideal spot to collect renewable feed materials. Researchers in Minnesota have shown that olefins and olefinic esters, which are central to polymer production, can be produced from vegetable-oil-derived biodiesel using methods that are more environmentally friendly than conventional methods.

Worldwide, some 300 billion lb of olefins are produced annually--mainly from ethane and other light alkanes--through an energy-intensive process known as steam cracking. According to some estimates, nearly one-third of all pollution emitted by chemical plants is attributed to nitrogen oxides and unburned hydrocarbons released in the flames needed to drive the high-temperature process.

A less polluting and less energy-demanding method for making olefins--and one that also shifts the chemical industry's dependence on petroleum to renewable feed sources--would be an improvement over today's processes. The latest work on biodiesel suggests that such a process may be feasible.

At the University of Minnesota, Twin Cities, Lanny D. Schmidt, a professor of chemical engineering and materials science, and graduate student Ramanathan Subramanian have shown that soy-based biodiesel (a mixture of methyl esters derived from vegetable oils) can be oxidized to valuable olefins and olefinic esters efficiently and fairly selectively. The reaction is conducted in an autothermal catalytic reactor, in which heat is supplied by the exothermic oxidation reactions, not by external heaters [Angew. Chem. Int. Ed., 44, 302 (2004)].

To carry out the oxidation process, the Minnesota group uses an automotive fuel injector to spray droplets of biodiesel, which consists of methyl oleate, methyl linoleate, and related compounds, onto the walls of the reactor where the droplets vaporize. A mixture of the organic material and air is then passed over a catalyst that contains a few percent of rhodium and cerium supported on alumina.

By adjusting the ratio of biodiesel to oxygen (C/O) in the feed stream, the team is able to control the oxidation process and reactor conditions, such as catalyst temperature, and thereby tune the product distribution. For example, at a C/O ratio of roughly 1.3, the reaction yields about 25% ethylene and smaller concentrations of propylene, 1-butene, and 1-pentene. In contrast, at a C/O ratio of 0.9, the product stream consists mainly of hydrogen and CO.

The team notes that C₂ to C₅ products consist almost exclusively of olefins, whereas longer chain products include olefins and olefinic esters. The researchers report that at all C/O ratios, the process yields less than 13% CO₂ (an unwanted product). They add that the catalyst remains stable and resists deactivation by carbon buildup even under extreme conditions.