Interest in Molecular Sieve Films Grows

With as much as 3% of U.S. energy being used in distillation processes and the federal government taking the matter increasingly seriously (www.ChemicalProcessing.com/articles/2009/159.html), it’s no wonder that interest in development of molecular sieve films is ever-expanding.

Already polycrystalline zeolite films are used as membranes for alcohol dehydration and are considered among other emerging technologies for various high-resolution molecular separations. They also have been implemented in membrane reactors and used for a range of advanced applications including sensors, corrosion protection coatings, low-k dielectrics and hosts for supramolecular organization of guest molecules.

Unfortunately, grain boundary defects  have hampered their potential for energy-efficient membrane separations . Induced by heat treatment during film manufacture, these defects compromise the membranes’ selectivity, creating non-selective transport pathways for permeating molecules.

Successful approaches to minimizing the effects of grain boundaries and cracks on membrane performance rely on microstructural optimization during film growth, including control of grain orientation and formation of nanocomposites with zeolite crystals embedded in support pores, as well as post-synthesis repair techniques. But the complexity of these processes hinders cost-effective and reliable scale-up of membrane production.

However, this problem appears to have been cracked by a team of researchers led by Michael Tsapatsis of the Department of Chemical Engineering and Materials Science, University of Minnesota, Minn. (see the July 31 issue of Science, 325, 590 2009).

Their new methodology involves a rapid heat-treatment step that enables preparation of silicalite-1 membranes with high separation performance for aromatic and linear versus branched hydrocarbon isomers and holds promise for realizing high-throughput and scalable production of these zeolite membranes with improved energy efficiency. The researchers claim that this could boost energy efficiency of chemical separations up to 50 times over conventional methods while also ensuring higher production rates.

“Using membranes rather than energy-intensive processes such as distillation and crystallization could have a major impact on industry,” says Rosemarie Wesson, National Science Foundation program officer, adding that this discovery could increase energy efficiency of producing important chemical solvents such as xylene and renewable biofuels such as ethanol and butanol.

Researchers create zeolite membranes by growing a film of crystals with small organic ions added to direct crystal structure and pore size — the two zeolite properties that help determine which molecules can pass through the material. Then the zeolite film is slowly heated — or calcined — to remove ions and open transmembrane pores.
However, Tsapatsis explains, “This method for creating zeolite films often leaves cracks at the boundaries between grains of zeolite crystals. These defects have prevented zeolite films from being used effectively as membranes, because molecules of unwelcome chemicals that are rejected by the zeolite pores can still penetrate through the membrane defects.”

Although such defects can be corrected, the repair process is difficult and expensive. That’s why zeolite membranes have so far found use only in specialized, small-scale applications such as removal of water from alcohols or other solvents.

So to minimize formation of cracks and other defects, the heating rate during calcination is very gentle, and the process can take as long as 40 hours. Typically a material is heated up at 1°C per minute to a temperature between 400°C and 500°C, where it’s held for several hours before being allowed to slowly cool. Because conventional calcination is time-consuming and energy-intensive, it has been difficult and expensive to produce zeolite membranes on a large scale.

At the heart of the Minnesota team’s new method is a new treatment called Rapid Thermal Processing (RTP). Here, zeolite film is heated to 700°C 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 zeolite crystal film.

When researchers examined 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 membrane properties or performance.

We observed a dramatic improvement in the separation performance of the RTP-treated membranes [compared to conventionally made zeolite membranes],” Tsapatsis says,  A second round of RTP treatment improved separation performance even further, to a level on par with current industry separation methods.

Researchers demonstrated the RTP process on relatively thick (several micrometers) zeolite membranes. Tsapatsis and collaborators are now working towards making zeolite membranes 10–100 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.