Models of Molecules
Harold Schobert
Coal research has a proud history at Penn State, beginning in the 1890s with establishment of one of the first formal schools of mining engineering in the United States. A lineage of prominent coal scientists has followed over the years, including Walter Maximilian Fuchs, a refugee from Nazi Germany, who developed one of the first molecular models of coal in 1942. The Coal Research Section was established in 1957, and ten years later director William Spackman initiated a Coal Sample Bank and Database that is now among the largest in existence, with some 1,100 samples of coal catalogued and stored for researchers around the world.
Capitalizing on this historical strength, in October of last year Penn State announced the launch of a major research alliance with Chevron, one of the world’s leading integrated energy companies, to investigate coal conversion technologies.
All of which makes it a little disconcerting to hear Schobert say, as he did in a recent lecture, that we really don’t know what coal is. But it’s true. “Unlike other complicated materials, like plastics, polymers, or biological substances,” he explains, “there is no single unique molecular structure for a given coal, let alone all coals.”
“Coals are as different as the peoples of the world,” is the way Jonathan Mathews puts it. “You can take two very similar coals, at least by their bulk elemental composition, and yet they behave quite differently. It’s the structural differences that really impact how a coal behaves. If all you’re doing is burning it, this doesn’t matter—you’re hitting it with a sledge hammer. But if you’re doing more subtle chemistry then it makes a huge difference.”
Structural representation of a South African intertinite-rich Highveld coal. Carbon atoms are green, oxygen atoms are red, and sulfur atoms yellow.
Courtesy of Daniel van Niekirk / Jonathan Mathews
Like his illustrious predecessor Fuchs, Mathews, an assistant professor of energy & mineral engineering, attacks this complexity with modeling. But while Fuchs’ early hand-drawn construct was limited to 100 carbon atoms, Mathews, with the aid of powerful computers, now builds models that encompass tens of thousands of atoms. When he manipulates these giant clusters in three-dimensional animations on the large desktop display screen in his cluttered office, the effect is something like flying through space.
His models have to be large, Mathews explains, to encompass a realistic diversity of molecular weights: the correct ratio of carbon atoms to hydrogen, nitrogen, oxygen, and sulfur, as well as other, disparate elements. “The problem is that much of our data gives us averages.”
One tool that helps is the transmission electron microscope, which allows perusal of coal samples at near atomic levels. “We look down at the sharp edges of very small particles,” Mathews explains. “Coal particles are glassy, many of them, so they have these nice sharp edges. We look at these fringes and that gives us a distribution of molecular weights.” Recreating this distribution in a model, he says, can yield a better understanding of how a given coal behaves—and how its behavior can be manipulated. “The ideal is that we can be molecular surgeons, and slice up the structure in a manner that we would like.”
Structural representation of a South African Waterburg coal loaded with a pyridine solvent. (Pyridine is blue.)
Courtesy of Daniel van Niekirk / Jonathan Mathews
The right scalpel for the job could well be an ultra-sophisticated contraption known as a femtosecond laser, which uses an extremely fast (one million billionth of a second) laser pulse to observe and break apart individual chemical bonds within molecules. (To put it in context, a femtosecond pulse lasts as long as the time it takes light to make it one-thousandth of the way across the period at the end of this sentence.) Schobert is collaborating with Christien Strydom, a physical chemist at North West University in South Africa, who is beginning to apply femtochemistry to the structure of coal, and its industrial processing.
Gasification, for example, is a crucial step in the conversion of coal into other forms of energy. The core process is relatively simple: Raw coal is reacted with steam at very high temperatures until carbon and water molecules break apart, their atoms realigning to form carbon monoxide and hydrogen. “But there’s lots of things on a molecular level that we don’t understand,” Strydom says. “We do not know where the bonds break, how they break, how they react eventually to give you these small molecules. Coal is a large polymeric structure, and in gasification it breaks up.”
With femtosecond lasers, Strydom says, “One can follow the movement of atoms. So you can literally follow a chemical reaction as it occurs. And if we can understand that better, maybe we can control it better.”