A new processing method converts a primary waste product of the paper industry into chemicals used to make nylon.
A research project at the U.S. Department of Energy Ames Laboratory has developed a two-step process that transforms waste Kraft lignin into building blocks used in the manufacture of nylon.
Lignin, the “glue” that holds together wood fibers, is left over after the cellulose is separated from wood pulp for use in paper production. Worldwide, papermaking generates about 50 million tons (45 metric tonnes) of lignin annually. About 85 percent of this is Kraft lignin, a byproduct of the widely used Kraft pulping process, according to ScienceDirect.
“The traditional way of processing Kraft lignin from paper or bioethanol manufacture is just to burn it,” said Igor Slowing, an Ames Laboratory scientist and the lead investigator on the project. However, lignin has potential as a source for valued chemicals.
LIGNIN, AN ABUNDANT SOURCE
“Lignin is a complex plant-derived polymer found in the cell walls of almost all dry-land plants,” explains a lignin processor’s web site. Lignin binds together cellulose and hemicellulose to give wood its stiffness. After cellulose, lignin is the largest renewable source of carbon.
Lignin accounts for 10 to 35 percent of available plant-matter biomass. Though lignin molecules are large and complex, they have potential as a renewable source for useful small molecules, according to a paper on lignin processing.
Lignin is potentially the largest natural, renewable resource for certain organic (carbon-based) building blocks for chemical processes, according to a paper on the Ames Lab process published in Green Chemistry, a journal from the Royal Chemical Society. Other methods have been developed for processing lignin, but the Ames Lab approach offers a pathway for producing value-added chemicals under milder conditions while using water as the solvent, a more efficient and benign process than those using, for example, solvents or concentrated acids, or high temperatures and pressures.
A TWO-STEP PROCESS
The first step in the newly developed process subjects the lignin to a mild alkali solution in water at relatively low temperature (about 200°C, 392°F) and low pressure (1 bar, about 1 atmosphere) to produce an organic compound called guaiacol.
The second step uses catalysts, under even milder conditions, to convert guaiacol into materials used to make nylon. The result of this process is extracting about 10 percent of the lignin molecule as nylon precursor chemicals.
“This two-step process provides a new option for lignin utilization in the production of high-demand value-added chemicals,” said Slowing. “We envision this process as a low-energy path” that has the additional advantage of leaving the other 90 percent of the Kraft lignin available for downstream processing into other chemical commodities. “Ultimately, you can see this as part of a big refinery where many things can be taken out of the lignin,” he said.
The potential value of the large amount of lignin available inexpensively has stimulated research into methods for processing lignin into value-added small molecules. Lots of researchers are working on lots of lignin projects, Slowing said. In some cases, they use pyrolysis (heating in the absence of oxygen) at temperatures up to 600°C (1112°F). They get useful products, but this kind of process takes a lot of energy and produces a lot of waste. By contrast, the Ames Lab technology is among the lowest-energy-consuming pathways to lignan processing, he said.
Selling lignin waste for conversion to useful products instead of burning it could add a revenue stream for paper producers while reducing their overall waste production and carbon footprint.
PROJECTS MAKING WASTE VALUABLE
This project was part of a program at the Ames Lab for recovering value and energy from waste products, Slowing said. Other projects at the lab include recovering critical materials from electronics waste, recycling electric vehicle batteries and transforming waste plastic into products such as lubricants and wax. “We keep doing this work and we hope that someone in the industry can take advantage of it,” Slowing said.
“We want people to use what we develop so they produce less waste.” The Ames Laboratory is associated with Iowa State University, where the office of technology transfer assists companies in acquiring access to new technologies developed at the lab and in the university.