Each year, the U.S. transportation industry consumes between 20 and 50 billion gallons of diesel and jet fuel derived from fossil resources. PNNL researchers are working to help replace fossil-based energy resources with cleaner-burning fuels of the future. Their latest advance is the first known methodology for converting problematic oxygenated compounds from bio-oil production—referred to as cyclic or aromatic compounds—into desirable high-molecular-weight paraffins for hydrocarbon fuels. By turning a hydrocarbon liability into a chemical asset, the PNNL approach could help biofuels gain ground in the transportation fuel market.

Bio-oil production routes, such as pyrolysis and hydrothermal liquefaction, generally create a high number of cyclic compounds, also called ring compounds. These rings, including aromatics such as alkylbenzenes and cycloparaffins such as alkylcyclohexanes, can gum up downstream equipment or processing steps. PNNL’s patented conversion process turns pesky 6 carbon (C6) rings into 5 carbon (C5) rings that are easier to convert into the desirable paraffins.

The process uses two reactors with different catalysts under different operating conditions. In the first reactor, a typical biomass feedstock with C6 rings is exposed to an acid ring-contraction catalyst. The ensuing reaction produces C5 ring-containing compounds. These C5 compounds move to a second reactor with a metal catalyst, such as iridium. The second catalytic reaction “opens” or expands the C5 ring to yield a complex biomaterial, or acyclic paraffin, with the desired high molecular weight.

Previous research attempts to convert C6 rings to C5 rings combined the solid and acid catalytic function in one reactor. This mixture causes multiple reactions simultaneously—called cracking reactions—that yield undesirable compounds and lose carbon in the process. In proof-of-concept tests with methylcyclohexane as a model compound, PNNL’s patented methodology with two reactors minimized carbon loss while yielding 64 percent high-molecular weight materials—twice the yield of the conventional single-step process.

Applicability / Other Development & Availability

Converting troublesome byproducts from bioprocessing into desirable chemicals or high-quality bio-oils increases the likelihood of their use in standard fuel delivery and refining systems. PNNL’s patented two-step process provides a path toward this conversion reality and moves biofuels one step closer to the safety, stability, and energy content required by commercial diesel and jet fuel markets.