We are getting to the heart of the fundamentals of biofuels catalysis. The work tells us that the impurity is unavoidable and we need to make sure it does not build up enough to interfere. Although this is a very fundamental issue, it points out for us what types of things we can do to help extend the lifetime of the catalysts we are using to make bio-oil.

—Roger Rousseau, co-author

Pyrolysis—thermal decomposition of large molecules by heating in the absence of oxygen at more than 500 °C—can be used to convert biomass into a more energy dense—and thus more economically transportable—liquid: bio-oil. The bio-oil liquid contains oxygenated hydrocarbon compounds resulting from the thermal breakdown of lignin, cellulose, and hemicellulose.

Collectively, pyrolysis oil comprises a complex mixture of acids, alcohols, aldehydes, esters, ketones, sugars, phenols, furans, and multifunctional compounds such as hydroxyacetaldehyde. The relative amounts of each compound class can vary depending on the biomass feedstock used and the operating conditions employed during pyrolysis.

In addition to not being as energy-dense as current transportation fuels, bio-oils also have stability issues. Research has shown that upgrading bio-oil to fungible hydrocarbon fuels—such as gasoline and diesel—by employing conventional petroleum refining techniques, such as hydrotreating and hydrocracking, is a promising pathway. According, the US DOE has supported a range of research on stabilizing and upgrading bio-oils (e.g., earlier post).

During the process, phenols undergo a variety of reactions and are converted into ketones. Ketones will link up with others like them and form long chains that “gunk up” the catalysts and interfere with important reactions. Researchers at PNNL wanted to discover the molecular details on how phenol converts to ketone. Ultimately, they discovered, it’s not the catalyst’s fault.

While some ideas existed for how this happens, the team used computers to simulate phenol interacting with catalysts and water to see step-by-step what is going on. To explore water’s role in the reaction, they also simulated the same reactions in a vacuum, which puts everything but the solid catalyst in vapor form. They performed these simulations using resources in EMSL, DOE’s Environmental Molecular Sciences Laboratory at PNNL.

In the simulations, the catalyst is essentially a piece of metal, either nickel or platinum. The phenol molecules and water molecules randomly bounce or land on the metal surface where bonds break and reform between atoms within molecules by shifting electrons around. In this way, a phenol might transform into a ketone.

The team found that the presence of water significantly upped the speed with which the final conversion to a ketone happened. In addition, water also affected how the metal catalyst carried its electrons, which in turn affected how well it catalyzed the reaction between phenol and hydrogen atoms that settle on the catalyst’s surface.

I was surprised at the role liquid plays in the reactivity of the metal catalyst. We know a lot about these reactions in the gas phase, but almost nothing in the liquid. The principles we’ve learned can be applied to other catalyst-driven reactions. They will make working in the complex system of real catalysts making real biofuels easier.

—Yeohoon Yoon, co-author

PNNL colleagues at the Bioproducts, Sciences & Engineering Laboratory, a facility located on the Washington State University Tri-Cities campus where PNNL and WSU researchers collaborate, will use this work to guide development of pyrolysis oil transformation into biofuels.

The researchers also presented this work at the American Chemical Society’s Annual Meeting in San Francisco earlier this month.

This work was supported by the Department of Energy Offices of Science and Energy Efficiency and Renewable Energy.

Resources

• Yeohoon Yoon, Roger Rousseau, Robert S. Weber, Donghai Mei, and Johannes A. Lercher (2014) “First-principles Study of Phenol Hydrogenation on Pt and Ni Catalysts in Aqueous Phase,” J. Am. Chem. Soc. doi: 10.1021/ja501592y