Engineers have developed a new method for creating high-performance membranes from crystal sieves called zeolites; the method could increase the energy efficiency of chemical separations up to 50 times over conventional methods and enable higher production rates.
Researchers led by chemical engineer Michael Tsapatsis of the University of Minnesota [Choi et al., DOI: 10.1126/science.1176095] reported this discovery in the July 31, 2009, issue of Science. Tsapatsis’s team developed a rapid heating treatment to remove structural defects in zeolite membranes that limit their performance, a problem that has plagued the technology for decades. This discovery could increase the energy efficiency of producing important chemical solvents for example; renewable biofuels such as ethanol and butanol.
Conventional methods of creating zeolite membranes involves growing a film of crystals with small organic ions added to direct the crystal structure and pore size; two zeolite properties that help determine which molecules can pass through the material. Then they slowly heat the zeolite film in a process called calcination to decompose the ions and open the pores. This method for creating zeolite films often leaves cracks at the boundaries between grains of zeolite crystals.
In an effort to minimize the formation of cracks and other defects, the heating rate during calcination must be very gentle, extending the length of the process to well over 40 hours.
Tsapatsis’s team developed a treatment called Rapid Thermal Processing (RTP), a treatment in which zeolite film is heated to 700 degrees Celsius within one minute and kept at that temperature for no more than two minutes. Acting as an annealing method, RTP refines the granular structure of the zeolite crystal film.
When the researchers examined the RTP-treated films they found no evidence of cracks at grain boundaries. Although they found other types of defects, these don’t seem to affect the membrane properties or performance.
In a comparison to conventionally-made zeolite membranes, Tsapatsis said, “We observed a dramatic improvement in the separation performance of the RTP-treated membranes.” A second round of RTP treatment improved separation performance even further to a level on par with current industry separation methods.
The researchers demonstrated the RTP process on relatively thick (several micrometers) zeolite membranes. Tsapatsis and collaborators are now working towards making zeolite membranes 10 to 100 times thinner to allow molecules to pass through more quickly. They hope to eventually implement RTP treatment with its beneficial effects to these membranes as well.