“In the world of fuels, the lead is either crude oil or biomass of some sort,” explains Scott Auerbach, a professor of chemistry and chemical engineering at the University of Massachusetts, Amherst. “The gold is either high-octane gasoline, diesel, ethanol, butanol, biodiesel, [or some other biofuel].” Although the paradigm for the production of this symbolic gold is shifting from millennia-old oil pumped from the ground, to biomass harvested from the surface of the Earth, the catalysts for making the transition to biobased refineries are still being developed and optimized.
“There’s a very rich history of catalyst research and understanding as it applies to petroleum and petrochemicals,” says John Holladay, a senior research scientist in the Chemical and Biological Processes Development Group at Pacific Northwest National Laboratory. “Those catalysts are very effective for the feedstocks they use, unfortunately, they’re not for biomass feedstocks. We don’t want to take another 80 years to get to the same point.”
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To the biofuel industry’s benefit, scientists are now armed with modern tools that speed the identification and development of new catalysts suited for biomass conversion as well as the modification of existing catalysts.
These two images show the structure of cellulose and a picture of a zeolite with a glucose molecule in its pore.
Catalyst Basics
Catalysts by definition are facilitators of chemical reactions. Their chemical composition doesn’t change during the reaction so they’re not considered to be direct participants in the reaction. However, they do allow the reaction to proceed under milder conditions. Catalysts typically don’t impact the yield of a reaction or how much of the reactant is converted to product. Most commonly, catalysts change the mechanism of a given reaction and impact both reaction rates and selectivities. By speeding up the formation of certain products and slowing down the formation of others, catalysts effectively steer a reaction to a subset of possible products. In the refinement of biomass-to-fuel, catalysts can steer reactions to the most valuable biofuels and bioproducts thereby minimizing costs associated with product separation and feedstock recycling. “This is the real magic and promise of catalysis,” Auerbach says.
Catalysts come in two main forms: there are biocatalysts, also known as enzymes, which can come from a single cell such as a microbe or from an entire organism such as a plant.
In addition, there are chemical catalysts, which are not associated with living organisms and often are produced synthetically.
Chemical catalysts can be divided into two types: homogeneous catalysts, which are in the same phase—typically liquid—as the reactants in the reaction they catalyze, and heterogeneous catalysts, which are generally solids and out-of-phase with the reaction reactants.
Christopher Jones, a chemical engineer at Georgia Institute of Technology, works from a number of different angles when it comes to biofuels research. The common thread to his team’s projects, however, is that they all focus on lignocellulosic feedstocks, mainly pine and switchgrass, as opposed to edible starches. One of their ongoing projects is gathering data on the behavior of mineral acids such as sulfuric acid in the pretreatment of biomass. “It’s not a particularly interesting or sexy catalytic process,” Jones says. “Mineral acids have been used for a number of years to break down biomass but there are only small, isolated studies in the literature.” Jones’ team is taking a single biomass and systematically studying the effect of certain types of acids and reaction temperatures to gain a greater understanding of how these catalysts act.
This fall Jones will be working on a new project to develop and improve the heterogeneous catalysts used for transforming syngas into cellulosic ethanol.
Superior Solid Catalysts
Heterogenous catalysts typically consist of tiny particles of precious metals such as platinum which are embedded in some kind of support such as silica or alumina. In addition to identifying the right metal and the right amount of that metal to catalyze a particular reaction, optimizing a solid catalyst also involves fine-tuning its support so that the latter is stable, and is porous to allow for the best possible flow of reactants and products, Auerbach explains. One of the most important classes of solid catalysts in oil refining are the zeolites. These solids are naturally occurring but can also be made in the laboratory. They are crystalline aluminosilicates that act as molecule-sized reactors, playing the role of supports and catalysts at the same time.
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