Finding A Path
Researchers are closer to finding a catalyst to develop a cost-efficient biofuel.
Chaitan Narula, a senior scientist in ORNL's Materials Science and Technology Division, at times appears to be consulting a mental roadmap as he evaluates the various chemical routes available to turn biomass into a viable transportation fuel. He notes that some options produce byproducts that have no market and that others are not economical. Still other processing options yield fuels that do not perform well in the fuel distribution system or in existing vehicles.
The starting point for creating fuel from biomass is the output of biomass fermentation: a stream of liquid that is approximately seven to ten percent ethanol mixed with water. While this dilute stream could be distilled to pure ethanol and used as a fuel, the energy required and the compromise in engine performance make the option unattractive. The challenge for Narula's research team is to devise a way to convert the ethanol directly to higher hydrocarbons using a catalytic process. The advantage of producing a hydrocarbon- based biofuel is that higher hydrocarbons (fuels based on more complex arrangements of carbon molecules) can be blended directly into existing supplies of gasoline and diesel fuels and used in existing vehicles. Equally important, they provide vehicles with the same amount of energy as traditional transportation fuels.
Previous attempts to convert ethanol to hydrocarbons have not been chemically straightforward. Narula describes one such process that starts with ethanol (an alcohol), converts the ethanol to ethylene (a hydrocarbon) and then to methanol (an alcohol), resulting in yet another hydrocarbon that can be mixed with diesel fuel. "There is no economic reason to convert a hydrocarbon to an alcohol and back to a hydrocarbon," Narula says. "The energy required for these conversions makes these paths unfavorable."
The goal of Narula's research is a process that would produce such a fuel by passing a stream of dilute ethanol through a catalyst, a process similar to that of exhaust gases moving through the catalytic convertor on a car. When the ethanol encounters the catalyst, a chemical reaction occurs, converting the ethanol to hydrocarbons. The hydrocarbons are separated from the water, and the water is recycled. Narula's team has completed preliminary work using a hydrogen zeolite catalyst, successfully converting a dilute stream of ethanol into ethylene, a simple hydrocarbon. The team is planning similar studies on metal catalysts. Both experimental evidence and computer simulations suggest that this approach could enable them to convert ethanol to higher hydrocarbons.
"The metal we will start with is iron," Narula says. "There is evidence from previous studies that, when iron is added, the zeolite catalyst will interact with ethanol to produce higher hydrocarbons." Among the first challenges for Narula's team will be demonstrating that a dilute stream of ethanol can, in fact, be converted to higher hydrocarbons using the catalyst. If successful, the next step will be to find the catalyst that produces the best biofuel output. "The basic catalyst we are currently working with is just one of hundreds of zeolites," Narula observes. "There are many different metals that might be used in conjunction with the catalyst. Given the volume of potential metals, we want to proceed systematically, "
The use of computer-based theoretical simulations is enabling Narula's team to narrow the field of possible catalysts from thousands of possible configurations to a handful that holds the greatest potential for success. "Our timetable does not have the years required to examine every possible catalyst," Narula says. The team believes that a combination of logic and careful experimentation will point to a pathway that produces a conversion of ethanol to hydrocarbons. Starting from that point, the team is trying to understand how the reaction works and, based on that understanding, improve the catalyst.
Simulations on ORNL's high-performance computers also enable Narula's team to examine the mechanics of how these ethanol-catalyst chemical reactions occur by considering the structure of various zeolite catalysts and the structure of ethanol. This analysis helps them identify potential chemical pathways between ethanol and various higher hydrocarbons. In the case of their iron-zeolite catalyst, the researchers intend to evaluate the possible ways in which the catalyst can interact with ethanol, identify the potential intermediate steps along the way to producing higher hydrocarbons and calculate the most energetically favorable method of converting biomass to energy. Once they have identified a pathway, they will seek experimental confirmation of the intermediate chemical steps suggested by the simulation. The process's final steps will include additional simulations designed to improve the efficiency of the reaction and increase the quality and quantity of the hydrocarbons produced.
Having found a potential pathway from ethanol to hydrocarbon fuel, the next part of their journey has begun. As the process of creating affordable, efficient hydrocarbon biofuels from biomass becomes increasingly viable, Narula says his team is now in search of an ideal catalyst, one that is superior to those that are currently commercially available. "We are developing some new catalysts in response to what we have learned from our simulation work and experimentation. We know it can be done. Our goal now is to improve our understanding of the reaction so we can develop a catalyst to optimize the process."