New process uses ionic liquid to dissolve wood

By Lisa Gibson
Posted June 1, 2009, at 10:48 a.m. CST

A new process for dissolving wood to separate cellulose-rich material and pure lignin uses ionic liquid and mild temperature and pressure conditions.

Researchers Hector Rodriguez and Robin Rogers from the Queen's University, Belfast, UK, School of Chemistry and Chemical Engineering discovered the method, with the help of researchers from the University of Alabama-Tuscaloosa. It's more eco-friendly than the currently used Kraft pulping process, which originated in the 19th century and uses toxic chemicals. Cellulose can be used in biofuels, paper and textiles, and lignin is useful in developing other chemicals mainly obtained from petroleum-based resources. But developing alternative, environmentally friendly ways to separate them from the wood has posed a challenge.

"With the process that we propose, based in direct dissolution in ionic liquid, the use of noxious substances and generation of toxic waste is minimized," Rodriguez told Biomass Magazine. "Moreover, it pursues an integral recovery of the different biopolymers of wood."

Queen's research found that chips of both hardwood and softwood dissolved completely when placed directly in ionic liquid with the application of pressure and heat (100 degrees Celsius or 212 degrees Fahrenheit), Rodriguez said. The addition of a mixture of water and acetone causes the precipitation of a cellulose-enriched recovered wood, and later, the evaporation of acetone leads to the precipitation of lignin, according to Rodriguez. The ionic liquid is then regenerated by distilling the remaining mixture of ionic liquid and water. Distillation is possible because the ionic liquid does not evaporate. Additionally, the ionic liquid is reusable and biodegradable.

"Ionic liquid is much cleaner and effective in taking lignin out of cellulose," Rodriguez said. Research is ongoing and the separation process hasn't been perfected yet, he said. "This is one of the products we're still working on."

Researchers hope to achieve better dissolution under even softer conditions and are also trying to achieve complete separation of the different elements in one single step, according to Rodriguez. They are still looking for alternative ways to process wood and will look at the addition of eco-friendly additives to the ionic liquid system or the use of catalysts.

The project started in 2002 after the discovery that some ionic liquids could dissolve cellulose directly, Rodriguez said. As research progressed, it was discovered that a suit of biomass also could be dissolved and the team has published the work with a specific ionic liquid that possesses better characteristics than others previously used. "We have demonstrated the process on a laboratory scale, but obviously implementing the process in an industrial plant is not the same," he said.

Both teams also are looking at biomass rich in essential oils that can be used in fragrance manufacturing and other processes.