A team of researchers at the Center for Direct Catalytic Conversion of Biomass to Biofuels (C3Bio) at Purdue University, West Lafayette, Ind., has developed a process to convert lignin into valuable chemical commodities.
“We are able to take lignin, which most biorefineries consider waste to be burned for its heat, and turn it into high-value molecules that have applications in fragrance, flavoring and high-octane jet fuels,” says lead researcher Mahdi Abu-Omar, professor of chemical engineering and associated director of C3Bio. “We can do this while simultaneously producing from the biomass lignin-free cellulose, which is the basis of ethanol and other liquid fuels. We do all of this in a one-step process.”
The process starts with untreated chipped and milled wood from sustainable poplar, eucalyptus or birch trees. A solvent is added to help dissolve and loosen the materials. Then, the material gets heated for several hours in a pressurized reactor, where a catalyst helps break up the lignin molecules to create lignin-free cellulose and a liquid stream that contains two phenols — a class of aromatic hydrocarbon compounds used in perfumes and flavorings.
The liquid stream containing the solvent is easily evaporated and recycled, adds Abu-Omar. The catalyst also can be recycled and reused.
A recent issue of Green Chemistry provides more details.
Key challenges include scaling-up and running the process in a continuous mode as well as working out efficient products separation. The catalyst is expensive, so the team plans to further study efficient ways to recycle it. The researchers also need to demonstrate that pellet form of the catalyst would be suitable for industrial use.
Current work includes scaling-up the reaction and developing a continuous reactor. “We have been investigating some model compounds to learn more about the mechanism of action and which bonds and oxygen atoms are removed first,” notes Abu-Omar. He hopes to have results and progress to report within the next nine months.
“We have demonstrated the continuous mode of operation at hundreds of milligram scale and are moving towards larger scale. In this configuration we can use the catalyst for tens of hours of operation without loss of activity. We are also looking at using transition metal catalyst that is based on earth-abundant metals that are cheap,” he adds.
The researchers also are currently working on direct production of propylbenzene from the methoxypropylpenols produced from lignin. They use another catalyst to convert the two phenol products into high-octane hydrocarbon fuel suitable for use as drop-in gasoline.
“In collaboration with colleagues in chemical engineering, we have developed a hydrodeoxygenation (HDO) catalyst that is highly selective for removing oxygens from methoxyphenols in the vapor phase. In my colleague Professor Ribeiro’s laboratory they are able now to go directly from methoxyphenols to propylbenzene, a high-octane aromatic hydrocarbon,” Abu-Omar explains.
Abu-Omar says the fuel produced has a research octane rating greater than 100, whereas the average gas for cars has an octane rating in the 80s.
Some smaller companies are in conversations with Abu-Omar, and the team has been working with Purdue startup Spero Energy to utilize this reaction chemistry.
“We are looking for potential toll manufacturing to use our process to make dihydroeugenol at kilos scale and partner potentially with technologists in advancing this process towards commercialization to enable the use of lignin for manufacturing high value chemicals,” he adds.