PNNL study uncovers role of water in forming impurity in bio-oil upgrading; insight into fundamentals of biofuel catalysis
21 August 2014
In working to elucidate the chemistry of hydrodeoxygenation (HDO) for the catalytic upgrading of pyrolytic bio-oil to fuel-grade products, researchers at Pacific Northwest National Laboratory (PNNL) have discovered that water in the conversion process helps form an impurity which, in turn, slows down key chemical reactions. Results of the study, which was reported in the Journal of the American Chemical Society, can help improve processes that produce biofuels from plants.
The study examines the conversion of bio-oil, produced from biomass such as wood chips or grasses, into transportation fuels. Researchers used density functional theory (DFT)-based ab initio molecular dynamics calculations to provide a detailed atomic-level understanding of how the hydrogenation reactions are influenced by the presence of water and also by the nature of the hydrogenating metal. The results of the study apply not only to water but to related liquids in bio-oil such as alcohols and certain acids.
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.
