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Separation Technology

By Ryan Garcia

Novel nonostubble separates desirable from undesirable

If you consider that a human hair is about 100,000 nanometers wide, a five-nanometer-wide hole doesn't seem very big. But it gives Daniel Shantz all the room he needs to work his magic.

That "magic" is the science of organic chemistry that occurs inside each microscopic pore of a spongelike membrane that Shantz is working to develop — a membrane with the potential to help alleviate the nation's dependence on foreign oil.

Shantz, a professor in the Artie McFerrin Department of Chemical Engineering at Texas A&M University, works in materials development, with a particular focus on energy. His latest project, a National Science Foundation–funded effort, is aimed at developing materials that can selectively remove valuable components, such as sugars, from the complex mixtures produced from biomass conversion. These compounds can then be blended with conventional fuels or used as alternative fuels after further processing.

Much as for crude oil, current approaches to biomass conversion result in a crude mixture containing many compounds. Many of these compounds (e.g., water and acids) must be removed before the mixture can be rendered usable, Shantz explains. But unlike crude oil, for which refineries exist to achieve this process, separation technology is relatively nonexistent for bio-oils and cellulose hydrolysis mixtures, Shantz says.

"The chemical nature of these mixtures is very different from those primarily found in current refineries and can't really be used 'as is,'" Shantz says. "Current approaches to biomass conversion, such as cellulose hydrolysis or biomass pyrolysis, result in very complex mixtures. The question then becomes, 'How do I get the valuable compounds out in an efficient way and leave the undesirable stuff behind? If there is 20 percent of something of value, can I separate it out effectively so that I can blend it into existing fuel pools?'"

"Think of these membranes as tiny sponges. Within each of the holes in these sponges, we're growing molecule trees, and we can grow different types of trees depending on what we want them to separate."

In pursuit of that question, Shantz is developing a new membrane that can function as a filter of sorts for these biomass mixtures. Typical filters do their job on the basis of size, similar to how a strainer drains water from spaghetti while retaining the pasta, but Shantz's nanoscale filter relies on chemistry to achieve the desired separations.

Using a ceramic membrane that contains many pores, each five nanometers wide, Shantz is inserting branched layers of organic molecules within each microscopic hole. These organic molecules, known as dendrimers, are grown off the surface of the membrane and attached within each pore by chemical bonds, Shantz explains. It is this "nanostubble" that goes to work, absorbing or binding the molecules Shantz wants to separate.

"Think of these membranes as tiny sponges," Shantz says. "Within each of the holes in these sponges, we're growing molecule trees, and we can grow different types of trees depending on what we want them to separate.

"So instead of sieving based on size, we are looking at achieving a solubility-based separation. Think of the old adage 'like attracts like.' In other words, I have a molecule that is much more soluble in this pore. What that effectively does is partition it across the membrane. These molecules absorb in and then the pores become full of them. These desired elements go into the membrane much more readily and will then move through."

It's an innovative approach to what Shantz calls a "horrendous separation problem" that the petrochemical industry faces as they search for ways to make renewables a larger percentage of their feedstocks and do so by using their existing facilities.

"We are trying to design structure and chemistry on short-length scales," Shantz says. "By changing things very deliberately on the nanometer scale with some organic chemistry, we believe we can make membranes that will sort out a group of molecules if we do the chemistry one way, another group of molecules if we do the chemistry another way."

"By changing things very deliberately on the nanometer scale with some organic chemistry, we believe we can make membranes that will sort out a group of molecules if we do the chemistry one way, another group of molecules if we do the chemistry another way."

That flexibility of the chemistry is something Shantz says adds to the potential attractiveness of this approach. In addition to being less energy-intensive than traditional separation techniques such as distillation, a membrane-based separation technique offers several methods for achieving separation. Employing a solubility-based approach, Shantz could design a dendrimer so that specific molecules of a mixture will react with it more strongly than other molecules, solubilizing and eventually passing through the membrane. In contrast, Shantz is also working to design membranes that can bind up certain components of a mixture, such as in metal sequestration, which removes lead from water.

It's all part of a solution that Shantz says he views as a viable, intermediate-term answer to shifting the fuel pool away from petroleum. Although scientists' long-term goal is to develop processes that break down biomass into only the desired compounds, that has not yet happened, Shantz explains. For now, the ability to glean valuable chemical compounds from these complex mixtures represents a significant step toward a productive transitional period in the nation's adoption of alternative energy, he says.

"We are using chemistry to engineer structure and function at the nanometer-length scale, and by doing that we hope to do separations that people can't do," Shantz says. "We can make things that have engineering importance."

Dr. Daniel Shantz
Dr. Daniel Shantz
Associate Professor
Ray Nesbitt Development Professor III
Chemical Engineering
979.845.3492