Breaking the Catalytic Barrier to Biofuels

Whether it's enzymes for degrading cellulose, microbes for fermenting sugars into ethanol or solid particles for the reforming of syngas to ethanol, the development of highly active and selective catalysts for biofuels production will be key to the success of the industry.
By Jessica Ebert | July 08, 2008
Biorefineries of the future will likely take many lessons from the efficient refineries of today, which pump crude oil in and separate it into the components that are ultimately used to make a portfolio of chemicals and fuels including natural gas, propane, gasoline, asphalt, diesel and jet fuel. The key to the plethora of processes that make the petrochemical industry such an efficient, integrated machine is several decades of research and development, much of which has been devoted to the design and modification of the finely-tuned catalysts that possess just the right kind of chemistry for turning "lead into gold."

"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."

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 phasetypically liquidas 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.

"Since we know so much about zeolites, we should be able to take that knowledge and apply it to the biorefinery of the future to make a portfolio of biobased fuels and products," Auerbach says. "The problem is that the zeolites used in the petroleum industry are by their very nature strong acids." This is a stumbling block for the production of biofuels because the basic reaction for creating these fuels takes simple sugars or gases and converts them into longer-chain alcohols or higher hydrocarbons. Strong acids catalyze the cracking of molecules not their lengthening, but this is a challenge that Auerbach and colleagues, including George Huber and Curt Conner, are working to overcome by changing the composition of a zeolite to make it more strongly basic. To do this, the team is restructuring the zeolite by removing oxygen and adding nitrogen. "Nitrogen-substituted zeolites are generally about twice as basic," Auerbach explains.

Whereas Auerbach and colleagues work to adjust known catalysts to make them more efficient in reactions involving biomass feedstocks, other research groups are working to identify and develop new solid catalysts. Brent Shanks, a chemical engineer at Iowa State University, first gains an understanding of the characteristics of a reaction and then designs catalysts around that. He calls this "rational design." His approach is one of bio-inspiration in that it aims to take certain characteristics of enzymes and build them into chemical catalysts. "Enyzmes are beautiful catalysts but they have some issues such as sometimes they're too specific, too selective, and also you can't go to high temperatures with them," he explains. "With chemical catalysts you can go to higher temperatures but they're not nearly as specific as enzymes."

Advancements in materials chemistry over the past decade or so provide new opportunities to structure materials at the nano-scale. "We can now design at the molecular level, chemical catalysts," Shanks says. "The catalytic site of an enzyme is exquisitely defined at the molecular level. The reactant interacts with the catalyst in a very specific way. We don't have that in chemical catalysts so the question is can we build that same level of molecular specificity into chemical catalysts?"

To that end, Shanks' team is trying to build acid/base functionality into a chemical catalyst. This is based on the knowledge that unlike chemical catalysts, whose function is carried out by metals and enzymes, such as those that break the bonds between molecules of glucose, use organic acids and bases to do the chemistry. "We're interested in marrying the two," Shanks explains. "We want to put organic acids and organic bases on metal oxide materials."

In a different approach, the team at PNNL, which Holladay is a part of, uses high-throughput screening to test multiple catalysts at a time and to increase the number of experiments they can do over a given period of time. This method for identifying new catalysts is carried out at PNNL's Combinatorial Catalysis Lab. Initially, robotic equipment is used to form each catalyst to be tested. Solids handling robots weigh and add an appropriate amount of solid support to a small well on a microtiter plate. Each plate holds 96 wells, so up to 96 catalysts can be developed and tested together. Liquids handling robots then add a salt solution of metals, which fill the pore spaces of the support. The liquid is evaporated leaving the metals embedded in the support. Once the catalyst is treated to set the metals in the active state, the plate is moved to a reactor system where the biomass to be tested is applied to each well. The reaction is carried out in a second reactor and then another set of robotic systems draws samples from each well for analysis, Holladay explains.

This schematic shows the basic steps used to create and test catalysts for high-throughput screening at PNNL's Combinatorial Catalysis Lab.
source: PNNL

"We'll take the ones that show activity and do further experimentation on them," he says. It's these experiments that provide a fundamental understanding of how the catalyst works. Using tools such as gas chromatography, high- pressure liquid chromatography and microscopy techniques that weren't available 20 years ago, new catalysts can be discovered and the surface chemistry can be studied to understand and ultimately improve such things as the interactions between the metals and their supports. "It's kind of a balance of both approaches," Holladay explains. "We start with the discovery phase and then move into the fundamental stage with the overall goal being to develop this industry quickly."
Jessica Ebert is an Ethanol Producer Magazine freelance writer. Reach her at