Tribology and Lubrication Technology April 2012 : Page 10tech beat More effective zeolite nanocatalysts and membranes based on layers These structures were prepared using a solid-state exfoliation process with a polymer. catalysts Play an ImPoRtant Role In facIlItatIng the processing of base oils, synthetic lubricant basestocks and additives in reducing emissions. Much attention has been paid to devel-oping nanocatalysts that show the po-tential to provide even better perfor-mance because of their smaller sizes and larger surface areas. In a previous TLT article, a nano-catalyst was prepared by impregnating iron nanoparticles into porous carbon microspheres. 1 Most of the nanoparti-cles have diameters less than 20 nano-meters. The iron-based nanocatalyst very effectively reduced aqueous hexavalent chromium. Future work will determine if it can be used in mul-tipollutant applications such as re-moving NO x from automotive emis-sion streams. Zeolites are a well-known catalyst that have been used as adsorbents and catalysts for several decades. One of their leading uses is as a catalyst in re-fi nery applications such as hydro-cracking. But preparation of suitable zeolite nanocatalysts is diffi cult. Michael Tsa-patsis, professor of chemical engineer-ing and materials science at the Uni-versity of Minnesota in Minneapolis, says, “We have been interested in de-veloping zeolite nanocatalysts that can act as thin fi lms to be used in such ap-plications as fi ltration. The problem is that repeated attempts to develop such nanomaterials led to either destruc-tion of the crystal structure initially formed or aggregation of the layers.” According to Tsapatsis, the ideal structure contains thin, two-dimen-sional planes that are anisotropic. He adds, “A thin layer can help both as a fi lter and as a catalyst. In the former case, diffusion time through a fi lter can be reduced. For the latter, the thin layer can expedite the movement of molecules, thereby speeding up a reac-tion.” Ultimately, Tsapatsis is looking for the sweet spot where the layer struc-ture can be preserved at the nanoscale without causing the crystalline struc-ture to be destroyed. A process has now been developed to achieve this goal. TRIB OL OG Y & L UBRIC A TION TE CHNOL OG Y ‘A thin layer can help both as a fi lter and as a catalyst. In the former case, diffusion time through a fi lter can be reduced. For the latter, the thin layer can expedite the movement of molecules.’ nAnOSHEETS Tsapatsis and his associates have de-veloped a procedure for preparing sta-ble zeolite nanocatalysts. The zeolite is present in the form of crystal-type nanosheets, as shown in Figure 2. Tsapatsis says, “We prepared the nanosheets using a solid-state exfolia-tion process with a polymer. Our pref-erence would have been to use a sol-vent, but the method simply did not work because the nanosheet structures were destroyed.” Instead, the researchers treated the zeolite precursor with polystyrene, ex-hibiting a weight-average molecular weight of 45,000 in a melt blend. This procedure was conducted under a ni-trogen atmosphere in a co-rotating, twin screw extruder. Temperatures be-tween 150 C and 170 C were used in this step of the process. WWW .S TLE. OR G KEY cOncEPTS • Stable zeolite nanosheets were prepared by using a solid-state exfoliation process involving the production of a polystyrene nanocomposite. • Two nanosheet structure types known as MWW and MFI were produced that have typical dimensions of 0.5 micron x 0.5 micron x 3 nanometers. • The zeolite nanosheets demonstrated the ability to act as a fi lter in separating orthoxylene from paraxylene. 10 • APRIL 2 012 Publication List Using a screen reader? Click Here |
