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Summer 2016

Chopping Oxygen: A Sugary Recipe for Biofuels

Creating materials that rip oxygen out of plants to create fuels from renewable sources

Matthew Gilkey

Plants are remarkable at converting sunlight, water, and atmospheric carbon dioxide into sugary glucose through a process called photosynthesis. While glucose acts as a food source for living organisms, it is also one of nature’s only renewable carbon sources. Poplar trees and switchgrass make up just a few sources of biomass-derived glucose from which we can produce many of our modern commodity chemicals and fuels.

Great! We can just replace our petrochemically derived fuels and chemicals with sugar-based ones, right? Unfortunately, it is not that simple. Along with carbon, glucose contains oxygen. In fact, oxygen makes up roughly 50 percent of a glucose molecule. In stark contrast, fossil fuels typically have very low oxygen content (less than 5 percent). Thus, removal of oxygen—or deoxygenation—remains the grand challenge in using biomass to produce usable fuels.

However, with advances in biomass conversion at Energy Frontier Research Centers such as Catalysis Center for Energy Innovation (CCEI) and Center for Direct Conversion of Biomass to Biofuels (C3Bio), this possibility is becoming increasingly feasible.

Clean ways and messy ways to remove oxygen. There are several ways to remove oxygen from carbon chains. One way is to use strong acids to remove oxygen in the form of water, but these acids are corrosive and are undesirable in large-scale chemical plants.

Instead, metal or metal oxide nanoparticles can be used as catalysts to facilitate deoxygenation by speeding up the rate of a desired chemical reaction (carbon-oxygen bond removal) while eliminating undesired side reactions. Combining such catalytic materials with hydrogen creates a recipe to efficiently remove carbon-oxygen bonds and replace them with carbon-hydrogen bonds in a benign and non-corrosive way. However, tuning this chemistry to be selective for deoxygenation at high conversion remains elusive.

Typically, the first step in glucose conversion is to break it down into a platform chemical known as 5-hydroxymethylfurfural, or more simply, HMF. The benefit in doing so is to create a simpler building block that scientists can subsequently convert to a wider range of commodity chemicals. Unfortunately, HMF is still highly oxygenated. Recently, CCEI unraveled key insights into deoxygenation of HMF into 2,5-dimethylfuran (DMF), a high-energy fuel with an octane number higher than that of gasoline.


Biomass-based carbon cycle. Glucose can be produced and isolated from switchgrass, poplar trees, etc. This oxygen-rich molecule can then be upgraded and deoxygenated to form 2,5-dimethylfuran, a valuable fuel. This can be then burned as fuel, emitting carbon dioxide in the process. Carbon dioxide subsequently acts as a reactant, with the addition of sunlight and water, to photosynthetically produce glucose once more. Image courtesy of Matthew Gilkey.

Researchers discovered that to effectively remove carbon-oxygen bonds, multiple types of active sites—the point on a catalyst where a chemical reaction occurs—are required on a catalytic material. Through a combination of experimental and theoretical contributions, CCEI scientists found that ruthenium (labeled “Ru” on the periodic table) exhibits very desirable properties for deoxygenation because of its ability to form metallic phases as well as metal oxide phases. These phases act synergistically to carry out the chemical reaction.


Getting there! Glucose can be converted to 5-hydroxymethylfurfural (HMF) by removing three carbon-oxygen bonds, which can then form 2,5-dimethylfuran (DMF) by removing terminal carbon-oxygen bonds. DMF has a high energy density and can serve as a potential fuel additive.

Researchers at C3Bio discovered a similar theory after employing a bifunctional zinc- and palladium-based catalyst to remove oxygen from other biomass-based oxygenated compounds, where zinc introduces metal oxide sites and palladium introduces metallic sites. This catalyst resembles that of CCEI by combining two types of synergistic active sites.

In essence, metal oxide phases create an electron-deficient metal atom, which allows electron-rich species such as oxygen atoms to bind. This provides highly oxygenated molecules such as HMF an access point onto the catalyst surface. Once bound, a nearby metallic site can then incorporate a bound hydrogen into the bound molecule, simultaneously breaking a carbon-oxygen bond in the molecule and forming a carbon-hydrogen bond in its place. Without both types of sites on the catalyst surface, removal of oxygen from the molecule cannot occur as effectively.


(a) Proposed reaction mechanism for removing oxygen over metal and metal oxide catalysts, adapted from insights from CCEI and C3Bio. (b) Transmission electron microscope images of the platinum-cobalt catalyst, illustrating the highly ordered nanocrystals. The scale bars are in the lower corners. (c) CCEI’s proposed atomic structure of the platinum-cobalt nanoparticle. Adapted with permission from ACS Catalysis. Copyright 2016 American Chemical Society.

Lifting deoxygenation catalyts to the next level and beyond. The general design strategy that arose from CCEI and C3Bio has most recently led to the design of new, more stable, and more efficient materials for deoxygenation. This past month, CCEI developed a highly active catalyst by alloying platinum and cobalt into highly structured metal nanocrystals (see figure). The catalyst converted HMF to DMF with 98 percent yield! What’s more, the stability of the catalyst is unrivaled, lasting longer than 10 hours, which is a remarkably long lifetime compared to many conventional catalysts.

Interest in biomass will continue for the foreseeable future, and optimal strategies for transforming it into fuels or useful chemicals by oxygen removal are non-trivial, but nonetheless possible. By developing theories and designing efficient catalysts such as those at CCEI and C3Bio, processing of biomass will become more and more viable, contributing to the creation of a more carbon-neutral society.

Acknowledgments

Klein et al. This work was supported by the Center for Direct Catalytic Conversion of Biomass to Biofuels, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

Luo et al. This work was supported by the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, and by useful discussions with Ayman Karim.

Gilkey et al. This work was supported by the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

More Information

Klein I, C Marchum, H Kenttämaa, and MM Abu-Omar. 2016. “Mechanistic Investigation of the Zn/Pd/C Catalyzed Cleavage and Hydrodeoxygenation of Lignin.” Green Chemistry 18:2399-2405. DOI: 10.1039/C5GC01325A

Luo J, H Yun, AV Mironenko, K Goulas, JD Lee, M Monai, C Wang, V Vorotnikov, CB Murray, DG Vlachos, P Fornasiero, and RJ Gorte. 2016. “Mechanisms for High Selectivity in the Hydrodeoxygenation of 5-Hydroxymethylfurfural over PtCo Nanocrystals.” ACS Catalysis 6(7):4095-4104. DOI: 10.1021/acscatal.6b00750

Gilkey MJ, P Panagiotopoulou, AV Mironenko, GR Jenness, DG Vlachos, and B Xu. 2015. “Mechanistic Insights into Metal Lewis Acid-Mediated Catalytic Transfer Hydrogenation of Furfural to 2-Methylfuran.” ACS Catalysis 5(7):3988-3994. DOI: 10.1021/acscatal.5b00586

About the author(s):

  • Matthew Gilkey is a Ph.D. candidate in chemical and biomolecular engineering at the University of Delaware and holds a B.S. in chemical engineering from the University of California, Santa Barbara. He is a graduate researcher in the Catalysis Center for Energy Innovation (CCEI) working under the advisement of Bingjun Xu and Dionisios Vlachos, director of CCEI. His research is centered around designing, developing, and understanding catalysts for upgrading of biomass-derived chemicals to fuels and polymer precursors.

Two to Make the Job Go Right

Scientists combine catalysts, creating long-lasting, biofuel-producing material

Platinum and cobalt could work together to remove oxygen from biomass, one day leading us to a more energy-efficient route to biofuels. Adapted by Cortland Johnson at Pacific Northwest National Lab with permission from ACS Catalysis. Copyright 2016 American Chemical Society.

When it comes to making biofuels, oxygen is a problem atom. The sugars produced by poplar trees, switchgrasses, and other biofuel crops contain a lot of oxygen—significantly more than current petrochemical feedstocks. Turning the sugars into fuels means ripping out the oxygen. Certain designer materials, called catalysts, remove oxygen, but they don’t last long. Recently, scientists combined two key oxygen-removal actions on one catalyst. The result? A more efficient catalyst that lasts longer than many of today’s catalysts. The team showed how to combine the two actions, adding new information to the design guides for biofuel production and creating a more carbon-neutral environment. Two Energy Frontier Research Centers—Catalysis Center for Energy Innovation (CCEI), led by the University of Delaware, and the Center for Direct Catalytic Conversion of Biomass to Biofuels (C3Bio), led by Purdue University—are actively involved in this work.

More Information

Klein I, C Marchum, H Kenttämaa, and MM Abu-Omar. 2016. “Mechanistic Investigation of the Zn/Pd/C Catalyzed Cleavage and Hydrodeoxygenation of Lignin.” Green Chemistry 18:2399-2405. DOI: 10.1039/C5GC01325A

Luo J, H Yun, AV Mironenko, K Goulas, JD Lee, M Monai, C Wang, V Vorotnikov, CB Murray, DG Vlachos, P Fornasiero, and RJ Gorte. 2016. “Mechanisms for High Selectivity in the Hydrodeoxygenation of 5-Hydroxymethylfurfural over PtCo Nanocrystals.” ACS Catalysis 6(7):4095-4104. DOI: 10.1021/acscatal.6b00750

Gilkey MJ, P Panagiotopoulou, AV Mironenko, GR Jenness, DG Vlachos, and B Xu. 2015. “Mechanistic Insights into Metal Lewis Acid-Mediated Catalytic Transfer Hydrogenation of Furfural to 2-Methylfuran.” ACS Catalysis 5(7):3988-3994. DOI: 10.1021/acscatal.5b00586

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.