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 heat
|Contact: Kurt Pfitzer|