Chapter 37
Nanocrystalline Metal Oxides: A New Family of Mesoporous Inorganic Materials Useful for Destructive Adsorption of Environmental Toxins
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K. J . Klabunde*, G. Medine, A. Bedilo, P. Stoimenov, and D. Heroux Department of Chemistry, Kansas State University, Manhattan, KS 66506 *Corresponding author:
[email protected] Introduction and Background Although nanoscale materials promise to revolutionize many of our industries, including electronics, health care, energy and more, the near term uses are in environmental remediation and green chemistry applications. One reason for this is that nanomaterials present unique properties as adsorbents and catalysts, because: (1) they possess high surface areas with large surface to bulk ratios so that the nanomaterial is used efficiently; (2) nanocrystals have unusual shapes and possess high surface concentrations of reactive edge, corner, and defect sites that impart intrinsically higher surface reactivities; (3) a wide range of Lewis acid/base properties and oxidation/reduction potential can be engineered into the nanomaterials since the periodic table of the elements (and their oxides, sulfides, etc.) becomes a literal playground for their design, and in a sense becomes three-dimensional since nanocrystal size is important as well as chemical potential; and (4) many nanocrystalline materials, especially ionic metal oxides, can be aggregated (pelletized) while still maintaining high surface areas and open pore structures. Therefore, these nanomaterials represent a new family of porous, inorganic sorbent/catalyst materials perhaps as potentially useful as the zeolites and other fascinating and useful materials, for example the MCM-41 silica series, and the ETS-10 titania-silica zeolitic materials. 1
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Method and Materials
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In this short abstract we present several examples where these new nanostructural materials have proven useful in environmental and catalytic applications. Generally, the nanomaterial in a powder form is allowed to contact a gaseous or liquid adsorbate. The rate of disappearance of the adsorbate is followed by GC-MS while the solid is monitored for changes by solid state NMR, IR, or Raman spectroscopies.
Results and Discussion Destructive Adsorption of Chemical Toxins and Toxic Industrial Chemicals Nanoparticles of MgO, CaO, SrO, A1 0 , and intimately mixed oxides MgO-Al 0 , and SrO-Al 0 (all prepared by modified aerogel proceduresMAP) have proven effective in ambient temperature detoxification of chemical warfare agents (organophosphorus nerve agents and mustard), and their simulants (paraoxon, 2-chloroethylethylsulfide, organophosphorus fluorides and others). ' The reactions of these liquid adsorbates with the dry powder nanomaterials are rapid upon contact, and further penetration into the fine powder is governed by material transfer by adsorbate vapor pressure. This material transfer step can be greatly speeded up by the presence of an inert hydrocarbon or hydrofluorocarbon solvents that dissolve the adsorbate and carry it into the nanomaterial pores. Adsorption capacities are high compared with other more common sorbents such as activated carbon or Ambersorb. Chemical warfare agents are chemically dismantled (destructively adsorbed) into non-toxic fragments. For example, at room temperature paraoxon [(EtO) P(0)OC H4N0 ] suffers bond cleavage of all three P-OR bonds as time goes on (as followed by solid state P NMR). Results with intimately mixed oxides show that further enhancement of reactivity for these detoxification reactions can be achieved. Mixed metal oxide systems of AP-MgO-Al 0 and AP-CaO-Al 0 are better at destructively adsorbing paraoxon than AP-MgO, AP-CaO and AP-A1 0 by themselves. APMgO-Al 0 adsorbs all of the (16ul) paraoxon in less than 20 minutes, whereas AP-A1 0 takes 60 minutes and AP-MgO adsorbs 15ul in approximately 2 hours. AP-CaO-Al 0 also performs well, but not as well as AP-MgO-Al 0 . Sulfated mixed metal oxides also show further improved adsorption suggesting that increasing the acidity of the sample enhances adsorption. Acid gases are also efficiently adsorbed at room temperature, and at elevated temperatures the reactions can be driven to solid-gas stoichiometric ratios. For example, a new SrO-Al 0 nanomaterial adsorbs H S at ambient temperatures. Similarly at 100°C, AP-CaO-Al 0 showed good adsorption of 2
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274 H S. The mixed metal oxides out performed AP-SrO at 250°C and 500°C. APCaSr0 gave a molar ratio of 1:1 at 250°C whereas AP-SrO did not perform as well (molar ratio of 1:3). CP-CaSr0 also gave good results at 500°C, again showing enhanced reactivity over AP-SrO at elevated temperatures. It appears that the mixed metal oxide systems are not as susceptible to crystal growth and reduced activity at higher temperatures, thus making them more suitable for high temperature applications. At elevated temperatures, chlorocarbon reactions can be driven to stoichiometric proportions, especially if small amounts of transition metal catalysts are added. Thus, CCU reacts stoichiometrically with nano-CaO if a monolayer of Fe 0 is placed on the CaO: 2
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The catalytic action of Fe 0 (or other transition metal oxides) appears to be due to the transient intermediacy of mobile FeCl that then exchanges CI" for O " deep into the CaO nanocrystal. 2
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Catalysis Using Nanoscale Metal Oxides Examples of nanoscale metal oxides being used as catalysts for dehydrohalogenation, dehydrogenation, and alkylation are also available. NanoMgO is effective for stripping HC1, HBr, or HI from haloalkanes. This process involves first the conversion of the MgO crystallites to a core/shell structure, for example a MgCl coating on remaining MgO, while retaining to an extent the nanostructured form. Then the nanostructured MgCl coating is a strong enough Lewis acid to serve as a true catalyst: 2
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Oxidative dehydrogenation of propane and butane using nano-MgO as a support for vanadium oxide catalysts shows promise. The higher surface areas and unique morphology offered by the MgO serve to enhance catalyst selectivities. 11
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The use of iodine cocatalyst/promoter for dehydrogenation of butane has also shown promise. 12
C H , + h + MO -> C H + MI + H 0 4
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The reaction sequence involves the conversion of a nanomaterial oxide to an iodide followed by conversion back to metal oxide. Here, again, the high surface area of the nano-oxide coupled with its intrinsically higher surface reactivity allows higher activities, selectivities, and lower temperatures. An additional example of the unusual catalytic properties of nano-MgO is found in the alkylation of toluene or xylene by benzyl chloride. The MgO is a precatalyst that serves as a foundation for the formation of a MgCl coating that serves as the true catalyst. In this study it was found that nanocrystal shape was very important, and 13
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hexagonal platelet morphology was more effective than more polyhedral shapes. Apparently the "molecular trafficking" on the surface of the catalyst was more facile when flat surfaces were available.
Biocidal Properties Nanoscale metal oxides also exhibit biocidal properties due to their abrasive nature, alkaline surfaces, oxidizing power (when elemental halogens are preadsorbed), and the fact that their average particle charge (positive) attracts bacteria (which generally carry overall negative charge). In fact, nanoMgO is biocidal by itself for vegetative cells such as e-coli or vegetative bacillus cereus. However, for spores, which possess a tough protective outer shell, preadsorbed halogens are necessary. These MgO*X halogen adducts are of interest in their own right. The adsorbed halogens (Cl , Br , ICI, IBr, I , IC1 ) are even more chemically reactive than the free halogen molecules. Raman and UV-vis spectra suggest that halogen-halogen bonds are weakened upon adsorption, which apparently is the reason for this enhanced reactivity toward organics and bacteria. These examples serve to point out the potential usefulness of nanoscale metal oxides in environmental remediation, catalysis, and fine chemical syntheses by "greener" chemistry. 14
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