A team of researchers from Purdue University’s Center for Direct Catalytic Conversion of Biomass to Biofuels, or C3Bio, has developed a process that uses a bimetallic Zn/Pd/C catalyst to convert lignin in intact lignocellulosic biomass directly into two methoxyphenol products (phenols are a class of aromatic hydrocarbon compounds used in perfumes and flavorings) leaving behind the carbohydrates as a solid residue.
Lignin-derived methoxyphenols can be further deoxygenated to propylcyclohexane—a cycloalkane. Cycloalkanes are important components of not only traditional vehicle fuels such as gasoline and diesel, but also jet fuels, such as Jet-A/Jet-A1/JP-8.
The leftover carbohydrate residue is hydrolyzed by cellulases to give glucose in 95% yield, which is comparable to lignin-free cellulose.
In addition, the hemicellulose fraction of the biomass is hydrolyzed in the same step of lignin conversion, with easy separation of the resultant xylose. That xylose fraction can be selectively dehydrated to furfural and subsequently to other furfural-based chemicals or fuels in a biorefinery.
Mahdi Abu-Omar, the R.B. Wetherill Professor of Chemistry and Professor of Chemical Engineering and associate director of C3Bio, led the team.
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. 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.—Prof. Abu-Omar
Plant biomass is made up primarily of lignin and cellulose, a long chain of sugar molecules that is the bulk material of plant cell walls. In standard production of ethanol, enzymes are used to break down the biomass and release sugars. Yeast then feast on the sugars and create ethanol.
Lignin acts as a physical barrier that makes it difficult to extract sugars from biomass and acts as a chemical barrier that poisons the enzymes. Many refining processes include harsh pretreatment steps to break down and remove lignin, Abu-Omar said.
Lignin is far more than just a tough barrier preventing us from getting the good stuff out of biomass, and we need to look at the problem differently. While lignin accounts for approximately 25% of the biomass by weight, it accounts for approximately 37% of the carbon in biomass. As a carbon source lignin can be very valuable, we just need a way to tap into it without jeopardizing the sugars we need for biofuels.—Prof. Abu-Omar
The process starts with untreated chipped and milled wood from sustainable poplar, eucalyptus or birch trees. The catalyst is added to initiate and speed the desired chemical reactions, but is not consumed by them and can be recycled and used again. A solvent is added to the mix to help dissolve and loosen up the materials. The mixture is contained in a pressurized reactor and heated for several hours.
The team also developed an additional process that uses another catalyst to convert the two phenol products into the high-octane (RON > 100) hydrocarbon fuel suitable for use as drop-in gasoline.
The processes and resulting products are detailed in a paper published online in the RSC journal Green Chemistry. The US Department of Energy funded the research.
In addition to Abu-Omar, co-authors include Trenton Parsell, a visiting scholar in the Department of Chemistry; chemical engineering graduate students Sara Yohe, John Degenstein, Emre Gencer, and Harshavardhan Choudhari; chemistry graduate students Ian Klein, Tiffany Jarrell, and Matt Hurt; agricultural and biological engineering graduate student Barron Hewetson; Jeong Im Kim, associate research scientist in biochemistry; Basudeb Saha, associate research scientist in chemistry; Richard Meilan, professor of forestry and natural resources; Nathan Mosier, associate professor of agricultural and biological engineering; Fabio Ribeiro, the R. Norris and Eleanor Shreve Professor of Chemical Engineering; W. Nicholas Delgass, the Maxine S. Nichols Emeritus Professor of Chemical Engineering; Clint Chapple, the head and distinguished professor of biochemistry; Hilkka I. Kenttamaa, professor of chemistry; and Rakesh Agrawal, the Winthrop E. Stone Distinguished Professor of Chemical Engineering.
The catalyst is expensive, and the team plans to further study efficient ways to recycle it, along with ways to scale up the entire process, Abu-Omar said.
The US Department of Energy-funded C3Bio center is an Energy Frontier Research Center. It is part of Discovery Park’s Energy Center and the Bindley Bioscience Center at Purdue.
Purdue Research Foundation has filed patent applications and launched a startup company, Spero Energy, which was founded by Abu-Omar to commercialize the process.
Trenton Parsell, Sara Yohe, John Degenstein, Tiffany Jarrell, Ian Klein, Emre Gencer, Barron Hewetson, Matt Hurt, Jeong Im Kim, Harshavardhan Choudhari, Basudeb Saha, Richard Meilan, Nathan Mosier, Fabio Ribeiro, W. Nicholas Delgass, Clint Chapple, Hilkka I. Kenttämaa, Rakesh Agrawal and Mahdi M. Abu-Omar (2015) “A synergistic biorefinery based on catalytic conversion of lignin prior to cellulose starting from lignocellulosic biomass,” Green Chemistry doi: 10.1039/C4GC01911C
Zemin Tian, Yingjia Zhang, Feiyu Yang, Lun Pan, Xue Jiang, and Zuohua Huang (2014) “Comparative Study of Experimental and Modeling Autoignition of Cyclohexane, Ethylcyclohexane, and n-Propylcyclohexane” Energy & Fuels 2014 28 (11), 7159-7167 doi: 10.1021/ef501389f