New WSU palladium-iron catalyst could improve drop-in biofuels production from pyrolysis oils

17 October 2014

The addition of palladium (Pd) prevents deactivation (addition of oxygen, red spheres) of an iron catalyst in the reaction that removes oxygen from biofuel feedstock. Credit: ACS, Hensley et al.. Click to enlarge.

Washington State University researchers have developed a new palladium-iron hydrodeoxygenation catalyst (Pd/Fe₂O₃) that could lead to making drop-in biofuels cheaply and more efficiently. Their work is described in two papers in the October issue of the journal ACS Catalysis and is featured on the cover.

The first WSU paper (Hong et.al) describes the synthesis of a series of Pd/Fe₂O₃ catalysts and their performance for the hydrodeoxygenation of m-cresol—a phenolic compounds used as a model compound in the HDO research, as it can be derived from pyrolysis of lignin. The second (Hensley et al.) reports on a combined experimental and theoretical approach to understand the potential function of the surface Pd in the reduction of Pd/Fe₂O₃.

One method of producing biofuels is through the pyrolysis of biomass to bio-oils and successive refinery of bio-oils, and this process has been identified as the most cost-effective approach. The key issue in the bio-oil upgrading is to efficiently reduce their high oxygen content (∼20−40 wt %) to less than 2 wt % for fuel applications. The oxygenates in bio-oils mainly exist in the form of acetic acid, furans, ketones, and phenolics, which are corrosive, thermally unstable, and highly viscous. The removal of oxygen in these bio-oils can be achieved in a similar manner with the hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) processes in petroleum industry, i.e., removing oxygen in the form of water in hydrodeoxygenation (HDO) process.

These catalysts need to be fast and efficient. The WSU study shows how a two-metal approach could improve the catalyst and minimize the amount of hydrogen needed, along with the associated costs.

Iron catalysts have been an inexpensive way to remove oxygen from plant-based materials; however, the catalyst can stop working when it interacts with water, which is a necessary part of biofuels production. The iron rusts. Palladium can work in water, but it is not terrific at removing oxygen; and the metal is very expensive.

Led by Voiland Distinguished Professor Yong Wang, who holds a joint appointment at Pacific Northwest National Laboratory (PNNL) as an associate director at the Institute for Integrated Catalysis, the researchers developed a mixture of the two metals, iron along with a tiny amount of palladium, to serve as a catalyst to efficiently and cheaply remove oxygen.

The synergy between the palladium and the iron is incredible. When combined, the catalyst is far better than the metals alone in terms of activity, stability and selectivity.

Mechanism of Pd−Fe Synergy in HDO of m-Cresol. H₂ preferentially adsorbs and dissociates on the Pd attached to the Fe surface, followed by spillover to the metallic Fe sites where the substrate, m-cresol, adsorbs and activates.
The adsorption mode of m-cresol on the iron surface enables the high selectivity toward direct HDO products—i.e., toluene, benzene, and xylene. Meanwhile, Pd is the active site for activating hydrogen and maintains the high hydrogen coverage on the metallic Fe surface.

Once the product forms via surface reaction on Fe, it readily desorbs to complete the catalytic cycle without further reaction. The surface enrichment of active hydrogen also efficiently removes the oxygen on the surface and thus suppresses the reoxidation of the active Fe under the reaction conditions. Credit: ACS, Hong et al. Click to enlarge.

Adding extremely small amounts of palladium to iron helped cover the iron surface of the catalyst with hydrogen, which caused the reaction to speed up and work better. It also prevented water from interrupting the reactions; further, less hydrogen was needed to remove the oxygen.

With biofuels, you need to remove as much oxygen as possible to gain energy density. Of course, in the process, you want to minimize the costs of oxygen removal. In this case, you minimize hydrogen consumption, increase the overall activity and gain high yields of the desired fuel products using much less expensive and more abundant catalyst materials.

The team used advanced techniques—including high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy and extended X-ray absorption fine structure spectroscopy—to understand how atoms on the catalyst’s surface interact with the plant material lignin. Corresponding theoretical calculations were done by a WSU team led by Jean-Sabin McEwen.

The team is now designing catalysts to work under wetter conditions to better approach realistic conditions rather than just using model compounds.

Resources

• Yongchun Hong, He Zhang, Junming Sun, Karim M. Ayman, Alyssa J. R. Hensley, Meng Gu, Mark H. Engelhard, Jean-Sabin McEwen, and Yong Wang (2014) “Synergistic Catalysis between Pd and Fe in Gas Phase Hydrodeoxygenation of m-Cresol” ACS Catalysis 4 (10), 3335-3345 doi: 10.1021/cs500578g

• Alyssa J. R. Hensley, Yongchun Hong, Renqin Zhang, He Zhang, Junming Sun, Yong Wang, and Jean-Sabin McEwen (2014) “Enhanced Fe₂O3 Reducibility via Surface Modification with Pd: Characterizing the Synergy within Pd/Fe Catalysts for Hydrodeoxygenation Reactions” ACS Catalysis 4 (10), 3381-3392 doi: 10.1021/cs500565e