Liquified or heat-treated biomass is generally not considered to be a stable fuel. This instability is imparted by its high oxygen content.
If there existed a method to convert this highly oxygenated biomass into alkanes, i.e., carbon-based molecules possessing carbon atoms linked together only by carbon-carbon single bonds, it would be a useful fuel. Unfortunately, the most common method currently utilized to remove the oxygen from biomass results in contamination.
Furthermore, the process is readily deactivated by carbon deposition and water. A new process that overcomes these limitations is needed.
Yuan Kou (Peking University, China), Johannes Lercher (Technische Universität München, Germany), and coworkers have developed an improved process for deoxygenating biomass for fuel preparation. They have converted the phenolic compounds in biomass to cyclic alkanes and methanol.
Phenols to fuel.
Biomass is comprised of a large number of carbon-based molecules. Consequently, the scientists intially chose a common molecule in biomass, phenol, as their model compound for deoxygenation studies.
Phenol possesses six carbon atoms in a ring, linked together by alternating carbon-carbon single and double bonds, and an alcohol substituent. It is also known as "hospital odor."
The scientists found that they could chemically convert phenol into cyclohexane (six carbon atoms in a ring, linked together by carbon-carbon single bonds, no alcohol substituent). In other words, the final product is deoxygenated.
The process requires acid, roughly 200°C temperatures, and a carbon-supported metal catalyst. This is a relatively mild protocol.
A range of carbon-supported metal catalysts were successful, such as those based on palladium and rhodium. The reaction yield was approxmately 90%, with 1200 units of product generated per unit of catalyst before catalytic deactivation.
More complex molecules, yet chemically similar to phenol, were also successfully deoxygenated under similar conditions, even phenolic polymers such as those commonly observed in biomass. Here, however, 7%-12% methanol (used as a fuel, among other applications) was also typically observed in the product.
Chemical pathway.
Having a successful deoxygenation process at hand, the scientists then explored the chemical pathway leading to the final product. Such knowledge is useful for predicting how other starting materials will chemically react, and possibly provide insight into how to improve the process.
The scientists found that the fastest step in the deoxygentation process is the chemical conversion of the carbon-carbon double bonds into carbon-carbon single bonds (known as hydrogenation). After that, several chemical reactions (a hydrolysis and a dehydration) proceed in sequence to arrive at the final deoxygenated product.
Utility.
The final primary products in this biomass deoxygenation protocol are carbon-based molecules that are insoluble in water. Thus, the fuel products can be readily separated from the acid and biomass precursors, greatly facilitating purification.
Additionally, the process is atom-efficient, i.e., all of the carbon molecules present in the biomass are converted into useful products (no waste). These are both features of a useful protocol for generating fuel from renewable resources.
Note that burning this fuel will pollute the environment, so it should not be considered as environmentally friendly as hydrogen, which generates the "waste" products water and oxygen. However, it may be viewed as a temporary solution to global energy needs, that avoids extracting petroleum from the environment and takes advantage of global energy distribution networks already in place.
for more information:
Zhao, C., Kou, Y., Lemonidou, A. A., Li, X., & Lercher, J. A. (2009). Highly Selective Catalytic Conversion of Phenolic Bio-Oil to Alkanes Angewandte Chemie International Edition, 48 (22), 3987-3990 DOI: 10.1002/anie.200900404