The rapid pace of global climate change has added urgency to developing technologies that reduce the carbon footprint of transportation technologies, especially in sectors that are difficult to electrify. In response, researchers collaborating through the Center for Bioenergy Innovation make the case that scientific advances support a hybrid approach using biological and catalytic methods for producing cellulosic biofuel for use in airplanes, ships and long-haul trucks.

As presented in Energy & Environmental Science, this hybrid approach uses microbes to convert cellulosic biomass such as wood and grass into an intermediate, small-molecule product such as ethanol. The ethanol would then be catalytically upgraded into hydrocarbon fuels suitable for heavier vehicles.

The study states that using the combination of biological and catalytic methods “is a promising approach to bridge the current gap between the fuel molecules that biology most readily makes and the fuel molecules that the world would most value producing from biomass.”

“We’re looking at this as taking the best of both worlds: Using biology for what it really does well, which is to make these small molecules, and then using catalysis to do what it does well, which is to make hydrocarbon fuel mixtures rapidly,” said Brian Davison, co-author and chief science officer for CBI, headquartered at the Department of Energy’s Oak Ridge National Laboratory.

Lee Lynd, an engineering professor and adjunct biology professor at Dartmouth College, who co-led the study with Gregg Beckham at the National Renewable Energy Laboratory, said the field has previously focused on a two-step process of pretreating biomass with heat and/or chemicals before biological processing. This paper is one of only a few that have sought to refine a different approach: biologically creating small molecules, then mechanically disrupting the cell walls during fermentation and introducing a catalyst to create hydrocarbon fuels. This approach is more affordable than producing large fuel molecules directly using microorganisms, Lynd said.

Unlike chemical pretreatment approaches that degrade lignin, the tough polymer that gives plants their structure and rigidity, this method preserves something closer to the lignin’s original structure. As a result, lignin residues can be more easily converted to other products, rather than being unfit for anything but burning, Davison said.

“Sometimes it’s important to go back to basics like this paper does,” said Jim Bielenberg, a senior research associate at ExxonMobil Research and Engineering who closely follows biofuel product research. “We’ve taken a run at cellulosic biofuels, and the cost and performance of those technologies didn’t seem to land where it got a lot of traction. Maybe it’s time to really fundamentally rethink the steps.”