Chapter 38
Development of Nanocrystalline Zeolite Materials as Environmental Catalysts H . Alwy, G. Li, V. H . Grassian, and S. C. Larsen Department of Chemistry, University of Iowa, Iowa City, IA 52242
Introduction Nanoscale materials are promising catalysts in the broadly defined field of environmental catalysis. Environmental catalysis refers to the use of catalysts to solve environmental problems, in areas such as waste remediation, emission abatement, and environmentally benign chemical synthesis. Some of the advantages that may be achieved by improved environmental nanocatalyst materials include increased energy efficiency and conversion, reductions in chemical waste and effective waste remediation. The environmental benefits of nanocatalysts include cleaner air and water and, ultimately, a more sustainable future. Zeolites, which are widely used in applications in separations and catalysis, are aluminosilicate molecular sieves with pores of molecular dimensions. The crystal size of zeolites formed during conventional synthesis range in size from 1,000 to 10,000 nm. Recently, the synthesis of nanometer-sized zeolites has been reported by several groups.(1-12) There has been a great deal of interest in nanocrystalline zeolites because of their properties, such as improved mass transfer and the ability to form zeolite films and other nanostructures. The synthesis and characterization of nanocrystalline NaY zeolites and the formation of transparent thin films from the nanocrystalline NaY are reported here.
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Materials and Methods Nanocrystalline NaY was synthesized using clear solutions according to the method reported in the literature.(l,2) Ludox silica sol (30 wt. %, Aldrich) was deionized with a cation exchange resin, Amberlite (IR-120, Mallinckrodt). 19.7 g of A1 (S0 )*18H 0 (Aldrich, 98%) was dissolved in 75 mL of deionized water. 35 mL of 27% NH3 solution (Mallinckrodt) was added to precipitate Al(OH) . After centrifugation, the solid was washed with water and the supernatant was discarded. 54 g of tetramethylammonium hydroxide (TMAOH, 25% aqueous solution, Aldrich) was added to the solid and stirred until a clear solution formed. The clear solution was added to 20 g of deionized Ludox sol. The molar composition of the resulting clear solution was; 2.5 (TMA^O : 0.041Na O : 1.0 A1 0 : 3.4 SiOi : 370 ftO. This clear solution was heated in a Teflon-lined stainless steel autoclave for 7 days at 95°C. The resulting solution was centrifuged for 30 min. at 3400 rpm. The product was washed with distilled water and dried in air. The nanocrystalline NaY product was characterized by powder X-ray diffraction (Siemans D5000) to assess crystallinity and to verify the identity of the zeolite, and by scanning electron microscopy (SEM, Hitachi S-4000), to determine particle size and morphology. Atomic force microscopy (AFM) images were recorded using a Digital Instruments Nanoscope III Scanning Probe Microscope. Si (n =59.62 MHz) and A1 (n =78.21 MHz) solid-state NMR spectra were obtained using a 300 MHz wide bore magnet with a Tecmag spectrometer with a Bruker 7.5 mm magic angle spinning (MAS) probe with a spinning speed of -6 kHz. FT-IR spectra were recorded with a Mattson Galaxy 6000 infrared spectrometer equipped with a narrowband MCT detector. Each spectrum was obtained by averaging 500 scans at an instrument resolution of 4 cm" . 2
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Characterization of Nanocrystalline NaY The XRD pattern (not shown) indicated that NaY zeolite was formed by the hydrothermal synthetic method described in the previous section. SEM images of commercial NaY (Aldrich) and nanocrystalline NaY are shown in Figure 1. For the Aldrich NaY, intergrown crystal agglomerates are observed in the SEM image. For nanocrystalline NaY discrete zeolite crystals are observed with a particle size of 46 ± 8nm. The particle size was obtained by measuring the particle sizes of 50 zeolite crystals in the SEM image of nanocrystalline NaY.
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Figure 1. SEM images ofAldrich NaY (left) and nanocrystalline NaY (right).
The samples were further characterized by solid state NMR and FTIR spectroscopy. A1 and Si MAS NMR experiments were performed on the nanocrystalline NaY and the Aldrich NaY for comparison. NMR signals from tetrahedral A l and Si atoms were identified in the A1 and Si NMR spectra, respectively. From Si MAS NMR spectra, the Si/Al ratio for nanocrystalline and Aldrich NaY were determined to be 1.7 and 2.4, respectively. By comparing the —Al MAS NMR spectra (Figure 2) of the nanocrystalline NaY with commercial NaY (Aldrich), an increase in linewidth was observed as the NaY particle size decreases. This line-broadening has been observed before with other zeolites and has been attributed to increased site heterogeneity and increased crystal strain as the particle size decreases.(7 5) A similar line broadening effect was observed in the Si MAS NMR spectra. 27
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Figure 2. AIMAS NMR spectra ofA) NaY (Aldrich) andB) nanocrystalline NaY (uncalcined).
280 Preparation of Transparent High Quality NaY Thin Films The inherent problem with zeolites as a medium for photooxidation reactions is that zeolites are typically opaque and scatter light. The result is light penetration through the zeolite is small and much of the sample is not exposed to light. For any applications involving intra-cavity photochemistry (e.g. photooxidation)(i4-/7) or zeolite-based optical sensors(78-20), transparent zeolite films would be desirable. Films of NaY were prepared by sonication of an aqueous mixture of nancrystalline NaY for several hours. The resulting hydrosol was pipeted onto a pyrex slide and dried in ambient air. Films of commercial NaY (Aldrich) were prepared using the same method. Digital images of the NaY films are shown in Figure 3. In each case, the film was prepared using approximately the same mass of NaY zeolite. The film prepared from the nanocrystalline NaY hydrosol is much more uniform than the film prepared from the Aldrich NaY hydrosol. The increased transparency of the films can be observed visually. The " Y " printed on the paper behind the film can be clearly seen through the nanocrystalline NaY film (right) but is much more difficult to see through the Aldrich NaY film (left). To obtain more quantitative information, the percent transmittance was measured using UV/Vis spectroscopy. The nanocrystalline NaY film had a percent transmittance of 7080% in the 300-700 nm range compared to a precent transmittance of 30-40% for the Aldrich NaY film in the same range.
Figure 3. Films prepared from hydrosols of Aldrich NaY (left) and nanocrystalline NaY (right).
Images of the nanocrystalline NaY film were obtained using atomic force microscopy (AFM) are shown in Figure 4. The A F M images of the zeolite films show that the film surface is continuous and smooth on the hundreds of nanometer length scale. Attempts to obtain A F M images of the commercial Aldrich NaY films were unsuccessful due to the extreme roughness of the film.
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Spectroscopic Investigations of N 0 Adsorbed on Nanocrystalline NaY 2
FTIR spectroscopy was also used to characterize the nanocrystalline NaY and to monitor the adsorption, desorption and reaction of various molecules in the zeolite. The FTIR spectra of the adsorption of NO2 at progressively increasing partial pressures of N 0 on nanocrystalline NaY are shown in Figure 5. Spectral features observed can be identified as NO* (2000-2180 cm" spectral region) and N0 " (1300-1600 cm" spectral region). These species are proposed to form through a cooperative effect whereby two adsorbed N 0 molecules in close proximity to one another autoionize according to the reaction: 2
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Further FTIR studies of the nanocrystalline NaY are in progress to examine the reactivity of the nanocrystalline NaY for NO* decomposition reactions. In conclusion, nanocrystalline NaY was successfully synthesized with a 46±8 ran particle size. The NaY was extensively characterized using a variety of techniques including XRD, SEM, A F M , UV/Vis, solid state MAS NMR and FTIR spectroscopy. Transparent, thin films were prepared from the nanocrystalline NaY. Future studies will focus on applications of the nanocrystalline NaY as environmental catalysts.
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Figure 5. FTIR spectra of NOj adsorption on nanocrystalline NaY as a function of increasing N0 pressure (100-1000 mTorrfrombottom to top 2
Acknowledgements Dr. D. Stec and C. Jones are acknowledged for assistance with N M R experiments and B. J. Kruegger is acknowledged for assistance with A F M experiments. The research described in this article has been funded by the Environmental Protection Agency through EPA grant no: R82960001 to SCL and VHG.
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