MADISON -- Plants absorb carbon dioxide from the air and combine it with
water molecules and sunshine to make carbohydrate or sugar. Variations on this
process provide fuel for all of life on Earth.
Reporting in the June 21 issue of the journal Nature, University of
Wisconsin-Madison chemical and biological engineering Professor James Dumesic
and his research team describe a two-stage process for turning biomass-derived
sugar into 2,5-dimethylfuran (DMF), a liquid transportation fuel with 40 percent
greater energy density than ethanol.
The prospects of diminishing oil reserves and the threat of global warming
caused by releasing otherwise trapped carbon into the atmosphere have
researchers searching for a sustainable, carbon-neutral fuel to reduce global
reliance on fossil fuels. By chemically engineering sugar through a series of
steps involving acid and copper catalysts, salt and butanol as a solvent,
UW-Madison researchers created a path to just such a fuel.
Currently, ethanol is the only renewable liquid fuel produced on a large
scale," says Dumesic. "But ethanol suffers from several limitations. It has
relatively low energy density, evaporates readily, and can become contaminated
by absorption of water from the atmosphere. It also requires an energy-intensive
distillation process to separate the fuel from water."
Not only does dimethylfuran have higher energy content, it also addresses
other ethanol shortcomings. DMF is not soluble in water and therefore cannot
become contaminated by absorbing water from the atmosphere. DMF is stable in
storage and, in the evaporation stage of its production, consumes one-third of
the energy required to evaporate a solution of ethanol produced by fermentation
for biofuel applications.
Dumesic and graduate students Yuriy Román-Leshkov, Christopher J. Barrett and
Zhen Y. Liu developed their new catalytic process for creating DMF by expanding
upon earlier work. As reported in the June 30, 2006, issue of the journal
Science, Dumesic's team improved the process for making an important chemical
intermediate, hydroxymethylfurfural (HMF), from sugar.
Industry uses millions of tons of chemical intermediates, largely sourced
from petroleum or natural gas, as the raw material for many modern plastics,
drugs and fuels.
The team's method for making HMF and converting it to DMF is a balancing act
of chemistry, pressure, temperature and reactor design. Fructose is initially
converted to HMF in water using an acid catalyst in the presence of a
low-boiling-point solvent. The solvent extracts HMF from water and carries it to
a separate location. Although other researchers had previously converted
fructose to HMF, Dumesic's research group made a series of improvements that
raised the HMF output and made the HMF easier to extract. For example, the team
found that adding salt (NaCl) dramatically improves the extraction of HMF from
the reactive water phase and helps suppress the formation of impurities.
In the June 21, 2007, issue of Nature, Dumesic's team describes its process
for converting HMF to DMF over a copper-based catalyst. The conversion removes
two oxygen atoms from the compound lowering the boiling point, the temperature
at which a liquid turns to gas, and making it suitable for use as transportation
fuel. Salt, while improving the production of HMF, presented an obstacle in the
production of DMF. It contributed chloride ions that poisoned the conventional
copper chromite catalyst. The team instead developed a copper-ruthenium catalyst
providing chlorine resistance and superior performance.
Dumesic says more research is required before the technology can be
commercialized. For example, while its environmental health impact has not been
thoroughly tested, the limited information available suggests DMF is similar to
other current fuel components.
"There are some challenges that we need to address," says Dumesic, "but this
work shows that we can produce a liquid transportation fuel from biomass that
has energy density comparable to petrol."