High Octane Computing: Designing Next Gen Energy Technologies

Stephanie McPherson for TEI

Scott Auerbach, Professor of Chemistry, uses computers to search for “green” alternatives to the use of petroleum and coal, the largest contributors to the carbon emissions causing the human component of global warming. “I think we’ve now learned that energy and environment are inseparable, because when you use energy, you impact the environment in some way,” says Auerbach, who is also an Adjunct Professor in Chemical Engineering. Much of Auerbach’s work has focused on alternative fuels and fuel cells.

Auerbach’s computer modeling work in the field of biofuel refinement is funded by a new $2 million grant from the Department of Energy, which also supports George Huber, Curt Conner and Phil Westmoreland, all from the Chemical Engineering Department, and all members of The Institute for Massachusetts Biofuel Research (TIMBR). Their goal is to apply an existing petroleum refinement catalyst, called a zeolite, to the production of biofuels from solid biomass. A catalyst is a molecule or material that promotes a reaction without being consumed by the reaction, ideally steering the reaction towards the most valuable products. Zeolites are nanoporous solid acids that spark reactions with their acidic nature, steering molecules inside the nanopores (like nanometer-sized reactors) to new shapes that mirror the shape of the small pores. “On a per ton basis, zeolites are the most heavily used catalysts, because they are important in the refinement of crude oil into high octane gasoline,” Auerbach says. “These zeolites have been the workhorse of the petrochemical industry for fifty years, and now we’re thinking of [using them for] what’s called the carbohydrate economy.”

By “carbohydrate economy,” Auerbach means efficiently replacing petroleum products such as fuels and plastics used in massive amounts throughout the world, with new products obtained from the sugars in plant biomass. The biggest draw towards this model of energy is its cyclical nature. After oil is refined and burned, the resulting carbon dioxide gas is released into the atmosphere, likely contributing to anthropogenic global warming. If the economics of biofuels can be improved, and the technology widely adopted, the fuel crops grown to replace those harvested would use carbon for their growth. “That’s the fundamental difference between fossil fuels and biofuels,” says Auerbach. “Biofuels have their own carbon sequestration built into the cycle.”

The difficulty lies in the production. Changing crude oil into high octane gas is relatively simple – a liquid into a refined liquid. Changing solid plant matter into liquid transportation fuel is another matter. “The whole challenge in biomass refinement is getting that solid, recalcitrant plant material to succumb to the dark side of the force and become a liquid,” Auerbach says. Fossil fuels are hydrocarbons, lacking in oxygen, which is the reason for their liquid state. The oxygen in plant matter serves to bind the molecules tightly together. The key biofuel production is the controlled removal of this oxygen without making carbonaceous materials like “char” (although “biochar” is favored by some as a natural fertilizer/carbon sequestration system of its own.)

Huber, a member of TIMBR, developed a method of driving water from the plant matter, leaving hydrocarbon behind. Called “catalytic pyrolysis,” the method involves rapid heating of a plant for a split-second, until the temperature reaches around 600 degrees Celsius. The plant itself is vaporized, leaving the carbohydrate molecules to drift through a zeolites' unusually efficient “shape selective” pores. The majority product in Huber’s process is a high-octane fuel called an aromatic, a molecule similar to benzene.

Catalytic pyrolysis has its drawbacks, however. While it effectively converts roughly half the carbon to usable fuel, half is still rendered useless. To make biofuel refinement cost-effective, nearly all the carbon must be available as fuel. The method needs to be adjusted, but improvements require understanding how the method works. Otherwise, the TIMBR team will spend too much time in trial-and-error research. “During the rapid heating, all the biomass gets vaporized, and we don’t know how that actually works,” Auerbach says. “We don’t know exactly the species [of molecule] that are in the gas phase….but the biomass is gone.” On top of this difficulty, the team is unsure how such rapid heating affects the zeolite itself. They do not know which species of molecules fit inside the zeolite’s pores (though they do know that some make their way there), and they need to better understand the reaction chemistry once the molecule is in the pore. “The biomass molecules are kind of bouncing around like popcorn in a popcorn maker, from time to time having a kind of trajectory that puts it right into one of those pores,” Auerbach says “Once it gets inside there, there can be catalytic reactions and we have no idea what those actually look like.”

The TIMBR team is exploring these issues using a number of methods. Spectroscopy, the identification of chemicals according to their signature response when exposed to certain wavelengths of light, and the use of computer modeling, are giving the scientists a better view of what happens inside the zeolite. Conner and Westmoreland are probing the process using different forms of spectroscopy. To get accurate data under rapid heating, Conner uses vibrational spectroscopy, which uses vibrational frequencies to identify molecules, and Westmoreland employs mass spectrometry, which determines the identities of vaporized biomass molecules from their characteristic masses. In contrast to these experiments, Auerbach is focusing on recreating the situation using computational methods. Auerbach’s team uses quantum mechanics to compute the energies of molecules in zeolites; then his team uses “molecular dynamics” to explore how thermal vibrations in nanopores lead to reactions and eventually to biofuels. “My job is to calculate those energy levels for different kinds of molecules and find out, as a function of those, what’s sort of the least energy pathway to get from reactants to various products,” Auerbach says. “By performing these quantum calculations, it’s sort of the most powerful microscope in the world.”

Once the process is understood and made more efficient, it will need to be commercialized. Huber, who specializes in reaction engineering, is determining the most efficient model for a pilot plant. He is considering a batch reactor, a fixed bed continuous flow reactor, and a fluidized continuous flow reactor. Currently, the best choice seems to be the fluidized continuous flow reactor. By allowing the zeolite to flow freely with the biomass, the team can regenerate the zeolite catalyst without disrupting the process or causing too much excess waste. “So the combination of the reaction engineering, the molecular modeling and the spectroscopy is sort of the magic triangle, the trinity, that’s going to get this problem solved,” Auerbach says. And he is optimistic about his team’s progress. “I predict that in five years we will have a pilot plant commercial process,” he says.

In another project, Auerbach is working with the UMass Amherst “Center for Chemical Innovation on Fueling the Future,” funded by the National Science Foundation. This center, funded at the level of $1.5 million, is developing new understanding and new materials for more efficient charge transfer technologies. This has potential applications, e.g., in improving hydrogen fuel cells, a promising alternative because they are more efficient than internal combustion engines, and their only emission is water. In a fuel cell, electrons start at the high energy of the hydrogen molecule, power a device and hence lose energy, then meet up with protons and oxygen to form water. The problem is that electrons are so fast, protons have a hard time keeping up because they are nearly 2000 times more massive. Auerbach is working with the Center for Fueling the Future to find new nano-structured materials, called proton exchange membranes, through which protons can move more quickly. Other applications of charge transfer being considered by the Center for Fueling the Future include new sensors, new solar energy harvesting materials, new light emitting diodes for display technologies.

Auerbach is also working on a project using zeolites as nanofilters. He models the process of exposing a zeolite to a mixture, then selectively heating the system with microwaves to drive off the more polar species in the mixture. This process has a number of practical implications, including separating volatile organic compounds.

All of Auerbach’s work holds the overarching theme of less waste, more efficient “greentech,” using the nexus between computation and nanotechnologies such as zeolites. At the same time, he realizes that going green is not easy. It’s a big task, but one that may require the power of tiny nanotechnologies. “It’s all about controlled energy transfer from one source to another. Using nanotech to exert such control may mean big breakthroughs from tiny things.” We all hope that Auerbach’s research on computers and nanotech for new energy pays off in the fight against global warming.