Sugar into fuel

University of Wisconsin-Madison chemical and biological engineering Prof James Dumesic and his research team have developed 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.

Many researchers are 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,’ said Prof 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.

Prof 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. In 2006, 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, Prof 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.

The  process for converting HMF to DMF over a copper-based catalyst 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 said that more research is required before the technology can be commercialised. 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,’ said Prof Dumesic, ‘but this work shows that we can produce a liquid transportation fuel from biomass that has energy density comparable to petrol.’