Thin-film zeolite membranes with tiny, molecule-sized pores are one step closer to replacing the energy-intensive processes now used in industrial separations, a group of academic researchers is reporting.
Writing this week in Science magazine, the group says the membranes' ability to separate molecules in a mixture is significantly improved by subjecting the zeolite to rapid thermal processing (RTP). By heating the membranes from room temperature to 700 degrees Centigrade in one minute, maintaining this temperature for up to two minutes and then quickly cooling it, the researchers say they have been able to eliminate the formation of grain boundary defects that undermine the sieve-like quality of zeolite's uniformly sized nanopores.
The research group, led by Michael Tsapatsis, Amundson Chair Professor of chemical engineering and materials science at the University of Minnesota, says RTP shows promise in achieving greater yield and energy efficiency in zeolite membrane production.
Tsapatsis' group reported its results in an article titled "Grain Boundary Defect Elimination in a Zeolite Membrane by Rapid Thermal Processing." The article was coauthored by Tsapatsis; Jungkyu Choi, formerly of the University of Minnesota and now a postdoctoral fellow at the University of California at Berkeley; Hae-Kwon Jeong, assistant professor of chemical engineering at Texas A&M University; Mark A. Snyder, assistant professor of chemical engineering at Lehigh University in Bethlehem, Pennsylvania; Jared A. Stoeger, a graduate student at the University of Minnesota; and Richard I. Masel, professor of chemical and biomolecular engineering at the University of Illinois at Urbana-Champaign.
Zeolites are crystalline aluminosilicate materials whose compositions and nanoporous structures can be fine-tuned for applications in catalysis, adsorption and ion exchange. They are called "molecular sieves" because their pores, being small and very uniform in size, can sort and separate molecules selectively according to the molecules' size.
As microscopic particles, zeolites are used in a variety of applications, including the creation of pure streams of oxygen and other gases, the catalytic cracking of petroleum into gasoline, water purification and softening, the dewatering of ethanol, and as additives in laundry detergents.
Zeolite membranes are commonly formed by depositing zeolite crystals on a porous surface and inter-growing these crystals into a continuous film. Several challenges have kept zeolite membranes from achieving their full industrial potential. These include high processing costs; scalability, or the ability to make zeolite membranes in large area; and the difficulty in controlling grain boundary defects, or non-selective pathways at the crystal grain interfaces, which cause poor separation performance.
Meanwhile, an estimated 15 percent of the world's energy consumption is used for the industrial separations of molecules and mixtures, often in volatile, energy-hungry distillation towers. By contrast, zeolite membranes with optimal porosity consume much less energy when they perform separations.
When zeolites are made, structure directing agents, or SDAs, direct the formation of the porous crystalline structure. But the SDAs are then trapped inside the zeolite pores in what scientists call a "ship in the bottle" effect. These SDAs block the zeolite pores and must be removed so other molecules can pass through. High-temperature treatment is typically used to remove the pores, but the heat has little effect on the zeolite, which is stable. But the SDAs, being organic, break up and are removed during heating.
This heat processing must be carried out after the formation of zeolite membranes. Scientists have long believed that this must be done slowly to prevent cracks and other grain boundary defects from forming in the thin film. But the gradual heating promotes the formation of flexible grain boundary defects, or interfaces, between zeolite crystals. These defects can grow much larger than the zeolite pores to become "nonselective" pathways that can be permeated by the very molecules the zeolite is designed to separate.
"The molecules that you are trying to separate with your zeolite film," says Snyder, "can now circumvent the highly selective pathways and pass through the grain boundaries. This has been a major issue for the commercial viability of zeolite membranes."
Tsapatsis' group uses a regimen of RTP to remove the SDA molecules from inside the zeolite pores and to promote what they believe could be chemical "gluing" of the zeolite crystal domains. After heating the zeolite to 700 degrees C, the researchers hold it at that temperature for up to 2 minutes and then cool the material rapidly to room temperature.
The researchers believe the rapid rise in temperature may cause bonding between the crystal domains. "This could possibly decrease the flexibility of the grain boundaries so that they no longer open up between the crystals during operation of the membrane," says Snyder, who worked with Tsapatsis as a postdoctoral researcher. "In short, this type of heat treatment may chemically glue the crystal domains together."
A second round of RTP or conventional slow-heating is then necessary, says Snyder, to completely remove the SDA molecules from inside the pores.
Snyder has used an optical microscopy technique called laser scanning confocal microscopy to examine the grain boundaries in conventionally produced zeolite membranes and in zeolite membranes that have undergone RTP. Confocal microscopy is used widely in biology research to take optical slices of transparent, 3-D structures. Snyder and Tsapatsis were among the first teams to use the technique to characterize zeolite membranes by selectively labeling grain boundary defects with fluorescing molecules. The zeolite they study is silicate-1, one of the more than 170 frameworks of zeolite, which has pores measuring .56 nm in diameter.
"Confocal microscopy shows us the nonselective pathways and other defect features within the membrane," says Snyder.
Confocal microscopy enabled Tsapatsis' group to understand why RTP-treated zeolite membranes achieved a better separation performance than conventionally processed membranes. While a high density of grain boundary defects were observed in conventionally treated membranes, very few defect features were identified in the RTP-treated films. This suggests that grain boundaries in the RTP-treated films are either smaller or less flexible.
This apparent decrease in the number, size and flexibility of grain boundaries in the RTP-treated membranes influences the achievable resolution of the molecular separations, the researchers say.
"A challenging separation that is commonly used to test zeolite membranes," says Snyder, "is that of the isomers orthoxylene and paraxylene. "Orthoxylene is slightly larger and should not pass readily through the pores. One would expect a perfect zeolite membrane to allow a permeation rate for paraxylene that is about two orders of magnitude higher than that of orthoxylene."
"A considerable increase in paraxylene/orthoxylene selectivity was observed for RTP-treated membranes," the researchers wrote in Science, "resulting in an attractive combination of paraxylene permeance and mixture separation factor."
The researchers concluded their article by saying, "If its beneficial effects on performance can be demonstrated for other zeolite types, compositions and microstructures, RTP could contribute, in combination with fast one-step deposition methods, to the realization of large-scale production of zeolite membranes."