For many industrial processes, the typical way to separate gases, liquids, or ions is with heat, using slight differences in boiling points to purify mixtures. These thermal processes account for roughly 10 percent of the energy use in the United States.
MIT chemical engineer Zachary Smith wants to reduce costs and carbon footprints by replacing these energy-intensive processes with highly efficient filters that can separate gases, liquids, and ions at room temperature.
In his lab at MIT, Smith is designing membranes with tiny pores that can filter tiny molecules based on their size. These membranes could be useful for purifying biogas, capturing carbon dioxide from power plant emissions, or generating hydrogen fuel.
"We're taking materials that have unique capabilities for separating molecules and ions with precision, and applying them to applications where the current processes are not efficient, and where there's an enormous carbon footprint," says Smith, an associate professor of chemical engineering.
Smith and several former students have founded a company called Osmoses that is working toward developing these materials for large-scale use in gas purification. Removing the need for high temperatures in these widespread industrial processes could have a significant impact on energy consumption, potentially reducing it by as much as 90 percent.
"I would love to see a world where we could eliminate thermal separations, and where heat is no longer a problem in creating the things that we need and producing the energy that we need," Smith says.
Hooked on research
As a high school student, Smith was drawn to engineering but didn't have many engineering role models. Both of his parents were physicians, and they always encouraged him to work hard in school.
"I grew up without knowing many engineers, and certainly no chemical engineers. But I knew that I really liked seeing how the world worked. I was always fascinated by chemistry and seeing how mathematics helped to explain this area of science," recalls Smith, who grew up near Harrisburg, Pennsylvania. "Chemical engineering seemed to have all those things built into it, but I really had no idea what it was."
At Penn State University, Smith worked with a professor named Henry "Hank" Foley on a research project designing carbon-based materials to create a "molecular sieve" for gas separation. Through a time-consuming and iterative layering process, he created a sieve that could purify oxygen and nitrogen from air.
"I kept adding more and more coatings of a special material that I could subsequently carbonize, and eventually I started to get selectivity. In the end, I had made a membrane that could sieve molecules that only differed by 0.18 angstrom in size," he says. "I got hooked on research at that point, and that's what led me to do more things in the area of membranes."
After graduating from college in 2008, Smith pursued graduate studies in chemical engineering at the University of Texas at Austin. There, he continued developing membranes for gas separation, this time using a different class of materials - polymers. By controlling polymer structure, he was able to create films with pores that filter out specific molecules, such as carbon dioxide or other gases.
"Polymers are a type of material that you can actually form into big devices that can integrate into world-class chemical plants. So, it was exciting to see that there was a scalable class of materials that could have a real impact on addressing questions related to CO2 and other energy-efficient separations," Smith says.
After finishing his PhD, he decided he wanted to learn more chemistry, which led him to a postdoctoral fellowship at the University of California at Berkeley.
"I wanted to learn how to make my own molecules and materials. I wanted to run my own reactions and do it in a more systematic way," he says.
At Berkeley, he learned how make compounds called metal-organic frameworks (MOFs) - cage-like molecules that have potential applications in gas separation and many other fields. He also realized that while he enjoyed chemistry, he was definitely a chemical engineer at heart.
"I learned a ton when I was there, but I also learned a lot about myself," he says. "As much as I love chemistry, work with chemists, and advise chemists in my own group, I'm definitely a chemical engineer, really focused on the process and application."
Solving global problems
While interviewing for faculty jobs, Smith found himself drawn to MIT because of the mindset of the people he met.
"I began to realize not only how talented the faculty and the students were, but the way they thought was very different than other places I had been," he says. "It wasn't just about doing something that would move their field a little bit forward. They were actually creating new fields. There was something inspirational about the type of people that ended up at MIT who wanted to solve global problems."
In his lab at MIT, Smith is now tackling some of those global problems, including water purification, critical element recovery, renewable energy, battery development, and carbon sequestration.
In a close collaboration with Yan Xia, a professor at Stanford University, Smith recently developed gas separation membranes that incorporate a novel type of polymer known as " ladder polymers ," which are currently being scaled for deployment at his startup. Historically, using polymers for gas separation has been limited by a tradeoff between permeability and selectivity - that is, membranes that permit a faster flow of gases through the membrane tend to be less selective, allowing impurities to get through.
Using ladder polymers, which consist of double strands connected by rung-like bonds, the researchers were able to create gas separation membranes that are both highly permeable and very selective. The boost in permeability - a 100- to 1,000-fold improvement over earlier materials - could enable membranes to replace some of the high-energy techniques now used to separate gases, Smith says.
"This allows you to envision large-scale industrial problems solved with miniaturized devices," he says. "If you can really shrink down the system, then the solutions we're developing in the lab could easily be applied to big industries like the chemicals industry."
These developments and others have been part of a number of advancements made by collaborators, students, postdocs, and researchers who are part of Smith's team.
"I have a great research team of talented and hard-working students and postdocs, and I get to teach on topics that have been instrumental in my own professional career," Smith says. "MIT has been a playground to explore and learn new things. I am excited for what my team will discover next, and grateful for an opportunity to help solve many important global problems."
