Tribology and Lubrication Technology April 2012 : Page 11Figure 2 | The two types of stable, zeolite nanosheets prepared through a solid exfoliation process show potential as catalysts and filters to separate isomers of organic compounds. (Courtesy of the University of Minnesota) ‘The formation of an intermediate nanocomposite with polystyrene works quite well. Unfortunately, this process takes one day, which makes it very tedious and lengthy.’ The resulting nanosheet-polysty-rene nanocomposite was then added to toluene and sonicated in order to dissolve the polymer and suspend the nanosheets. Centrifuging was then done to separate and filter the larger particles. After isolating the zeolite nanosheets, high-temperature heating (typically 500 C) was done to remove any resid-ual organics by combustion. Tsapatsis says, “The formation of an intermediate nanocomposite with polystyrene works quite well. Unfortu-nately, this process takes one day, which makes it very tedious and lengthy.” Future work by the researchers will be done to find a more efficient meth-od. Tsapatsis is hopeful that a better solvent system can be found. The researchers produced nano-sheets of the MWW and MFI structur-al types. They have typical dimensions of 0.5 micron x 0.5 micron x 3 nano-meters. Tsapatsis explains, “These two structure types have different pore siz-es controlling transport across the zeo- lite nanosheet layers. For MWW, the smaller aperture along the thin dimen-sion of the layers has a diameter of 3 angstroms, which means that this structure type can allow only small molecules such as hydrogen to cross. In the case of MFI, the pore has a di-ameter of 5.5 angstroms, which en-ables the zeolite nanosheets to act as a filter to separate organic compounds such as para-, ortho-and metaxylene.” The zeolite nanosheets were ana-lyzed by transmission electron micros-copy, atomic force microscopy and x-ray diffraction. They can be used to coat porous substrates to form highly packed and oriented coatings. Testing was done to separate ortho-xylene from paraxylene. At tempera-tures up to 200 C, the two xylene iso-mers were incorporated into a nitrogen or argon gas stream and passed through a membrane containing the zeolite nanosheets. Tsapatsis says, “We obtained good separation of the xylene isomers. The flux and selectivity were high.” Besides improving the process, fu-ture work will involve evaluating the zeolite nanosheets under higher tem-peratures and pressures that are more reflective of commercial processing conditions. Liquid feeds also need to be used. Tsapatsis says, “We envision deter-mining if these zeolite nanosheets can work with other chemicals such as separating linear versus branched hy-drocarbons.” The high cost of separating sub-stances (estimated to be 15% of the total energy consumed) means there is great potential for the use of zeolite nanosheets. The researchers have filed a provisional patent and are looking for partners to further develop and commercialize this technology. Further information can be found in a recent paper 2 or by contacting Tsa-patsis at tsapatsis@umn.edu. REfEREncES 1. Canter, N. (2011), “Iron-Based Nanocatalyst,” TLT, 67 (4), pp. 12-13. 2. Varoon, K., Zhang, X., Elyassi, B., Brewer, D., Gettel, M., Kumar, S., Lee, J., Maheshwari, S., Mittal, A., Sung, C., Cococcioni, M., Francis, L, McCormick, A., Mkhoyan, K. and Tsapatsis, M. (2011), “Dispersible Exfoliated Zeolite Nanosheets and Their Applica-tion as a Selective Membrane,” Science , 334 (6052), pp. 72-75. Free half-day nanotribology course May 5 at 67th STLE Annual Meeting & Exhibition in St. Louis. Details at www.stle.org. 11 Publication List Using a screen reader? Click Here |
