A Fast Track To Green Gasoline

The process of converting wood chips, agricultural wastes, and other stores of biomass into transportation fuels has captured the imaginations of many scientists. One of these researchers is chemical engineer George W. Huber, whose group at the University of Massachusetts, Amherst, is reporting a selective catalytic pyrolysis process that for the first time permits direct conversion of cellulose into compounds that can be used to make gasoline (ChemSusChem, DOI: 10.1002/cssc.200800018).

A big holdup in biomass conversion is the "recalcitrance" of the plant material—that is, the inability to quickly and economically transform the complex matrix of carbohydrates in plant cell walls directly into useful chemicals and transportation fuels. Investigators are pursuing several physical, chemical, and biological approaches to overcome the recalcitrance problem, including the pyrolysis technique developed by Huber and graduate students Torren R. Carlson and Tushar P. Vispute.

Pyrolysis is a well-established method that involves heating solid organic material, including agricultural and industrial wastes, at high temperature in the absence of oxygen. The process decomposes the material into an array of liquid hydrocarbons.

The UMass researchers sorted out the necessary reaction conditions to control the pyrolysis of powdered cellulose and other biomass-derived carbohydrates mixed with fine particles of the zeolite catalyst ZSM5. The cellulose first decomposes into volatile oxygenated organics, which subsequently enter the zeolite's pores and selectively undergo a series of decarbonylation, dehydration, oligomerization, and other reactions, Huber says. The process takes less than two minutes to complete at 600 °C in a specialized reactor and generates a well-defined set of aromatic compounds, including naphthalene, ethylbenzene, toluene, and benzene; by-products include coke, H₂O, CO, and CO₂.

The process has a few caveats, however. For example, the researchers start with purified cellulose, not raw biomass. In addition, environmental and health regulations require that U.S. gasoline blends contain less than 25% aromatics, including less than 1% benzene.

Huber says that using raw biomass should yield results similar to pure cellulose once the process is optimized. He acknowledges that the gasoline limit on aromatics restricts the usefulness of the aromatic products. But he says the aromatics can be blended with alkanes and other components to make gasoline, or the aromatics can be hydrogenated to produce alkanes. "If we combine the hydrogenation step with our process, then we could in principle make a complete gasoline," Huber notes.

The new pyrolysis method is a concise way to treat a great deal of biomass in a short time with effectively a zero net carbon footprint, comments John R. Regalbuto, director of the National Science Foundation's catalysis and biocatalysis program, which supports Huber's work. "Huber's process for the direct conversion of cellulose to gasoline aromatics is at the leading edge of the new green gasoline alternative energy paradigm that NSF, along with other federal agencies, is helping to promote," Regalbuto says.