Environmental Health & Safety / Energy Efficiency

New Feedstock Beckons for Transportation Fuels

Biomass-based material provides alkenes via simple route.

A two-step process starting with ϒ-valerolactone (GVL) can efficiently produce liquid alkenes suitable for jet fuel and gasoline, say researchers at the University of Wisconsin, Madison, Wis. The route could spur large-scale production of both GVL and its precursor, biomass-derived levulinic acid, hopes James Dumesic, professor of chemical and biological engineering and lead researcher.

The process addresses two issues that can affect the economic and environmental attraction of biofuels -- external hydrogen requirements and carbon dioxide emissions, the researchers note. Converting GVL to alkenes doesn't require an outside supply of hydrogen. The CO2 produced in the process is at elevated pressure, which eases capture or use to make methanol or polycarbonates.

GVL itself can serve as a blending stock for gasoline and performs comparably to ethanol but suffers from limitations that blunt such widespread direct use, say the researchers. It has high water solubility, can only be used up to a certain level with conventional combustion engines, and provides lower energy density than petroleum-derived fuels. In addition, combustion of GVL releases more CO2 than alkenes would.

Reducing GVL with hydrogen to produce methyltetrahydrofuran can at least partially alleviate such limitations, the researchers note. However, using GVL to make liquid alkenes makes more sense, they contend, because this avoids the need for external hydrogen and provides a way to tailor molecular weight to fuel requirements.

In the process (Figure 1), GVL in an aqueous feed stream is decarboxylated at 36 bar in a fixed-bed reactor using a silica/alumina catalyst to yield equimolar amounts of butene and carbon dioxide. The exiting gas stream goes to a condenser to remove the water and then to an oligomerization reactor containing a solid acid catalyst. There, butene monomers are coupled to form condensable alkenes; process conditions can be tuned to yield alkenes of molecular weights and degrees of branching suitable for jet fuel and gasoline applications, say the researchers. The stream from this reactor goes to a condenser where it's split into liquid alkenes and CO2 gas.

The process has achieved high conversions of GVL and butenes and greater-than-75% yield of C8+ alkenes, they report in a recent paper in Science.

"The hydrocarbons produced from GVL in this new process are chemically equivalent to those used in the present infrastructure," notes David Alonso, a member of the research team. "The product we make is ready for the jet fuel application and can be added to existing hydrocarbon blends, as needed, to meet specs," he says.

Moreover, the process relies on inexpensive, stable and easily regenerable commercial solid acid catalysts, not precious metal ones, says Dumesic. There's room for improvement, though. Yield of high-molecular-weight alkenes would greatly benefit from development of a water-tolerant butene oligomerization catalyst.

The researchers believe that coupling GVL decarboxylation with butene oligomerization in a single continuous system at elevated pressures should be possible; this would reduce overall capital costs and avoid the need for a compressor once operation reaches a steady state.

On an industrial scale, Dumesic foresees the process being part of an integrated biorefinery. It would catalytically convert biomass to levulinic acid, use the acid to make GVL, and the GVL to make alkenes, while using residual biomass to produce steam and electricity. That's a ways off, though. The GVL-to-alkenes process has been demonstrated on a laboratory scale only within the last six months and pilot scale trials are several years away, says Dumesic.

"The bottleneck in having the fuel ready for prime time is the availability of cost-effective GVL," he notes. Work is underway at the school to develop more efficient methods for making GVL from biomass such as wood, corn stover and switchgrass. "Once the GVL is made effectively, this is an excellent way to convert it to jet fuel."

Meanwhile, Biofine Renewables, Waltham, Mass., already is producing levulinic acid from cellulosic materials at a demonstration plant in Gorham, Me., and considering large-scale plants (see: www.ChemicalProcessing.com/articles/2010/016.html).