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Title of Talk:
Development of Nanocrystalline Zeolite Materials as Environmental Catalysts: From Environmentally Benign Synthesis to Emission Abatement

Abstract of Talk:
This project involves the development of nanometer-sized zeolites and hierarchical zeolite structures as environmental catalysts. 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. However, for some applications it would be advantageous to employ much smaller nanometer-sized zeolite crystals in the range, 10-100 nm. Specific advantages to be gained by using zeolite nanostructures include facile adsorption and desorption, the ability to form dense films to facilitate separations applications and optical transparency.

A two pronged approach based on the 1) synthesis and characterization and 2) subsequent application of nanocrystalline zeolites as environmental catalysts has been undertaken. The first aspect involved the synthesis and characterization of nanocrystalline silicalite (particle sizes ranging from 20 nm to 1000 nm), nanocrystalline ZSM-5 (particle sizes ranging from 15-200 nm) and nanocrystalline NaY (particle sizes of 25, 50 and 75 nm). The size-dependent properties of the nanocrystalline zeolites were investigated by powder x-ray diffraction (XRD), scanning electron microscopy (SEM), nitrogen adsorption isotherms, and solid state nuclear magnetic resonance. Hierarchical structures such as rectangular fibers and hollow spheres and tubes were formed from these nanocrystalline zeolites. The second aspect of this approach (and the focus of this presentation) is the application of these nanocrystalline zeolites and hierarchical structures for applications related to environmental catalysis. The use of these nanocrystalline zeolites and hierarchical structures has been evaluated for the following potential applications:

  1. adsorption of VOCs, such as toluene from air and water
  2. selective catalytic reduction (SCR) of NO2 with propylene (or urea) investigated by FTIR spectroscopy and solid state NMR
  3. reduction of Cr(VI) to Cr(III) using iron-loaded hollow zeolite structures

In the current phase of the project, reactivity studies are being undertaken to evaluate the activity of the nanocrystalline zeolites as environmental catalysts. One unique characteristic of nanocrystalline zeolites relative to conventional zeolites is that the external surface area is the same order of magnitude as the internal surface area. Applications are being examined in which the external surface of the nanocrystalline zeolite is utilized for the decomposition of reactant molecules and the internal surface is used for further reaction or for storage. Functionalization of the external surface as a method of varying the hydrophobic/hydrophilic properties of the zeolites is also being examined. In addition, the adsorption of volatile organic compounds (VOCs) on the nanocrystalline zeolites will be further investigated in particular in humid environments and for a variety of different VOC’s.

Environmental Applications/Implications:
Environmental catalysis involves the use of catalysts to solve environmental problems, in areas such as emission abatement and environmentally benign synthesis. Many new catalysts and catalytic processes have been developed to meet the challenges posed by environmental concerns. Recently, zeolites have emerged as important materials for applications in environmental catalysis. Zeolites are aluminosilicate molecular sieves with pores of molecular dimensions. Zeolites can be synthesized with a wide range of pore sizes and topologies and are used in applications such as catalysis and chemical separations. The crystal size of zeolites formed during conventional synthesis range in size from 1,000 to 10,000 nm. However, for some applications it would be advantageous to employ much smaller nanometer-sized zeolite crystals in the range, 10-100 nm. Specific advantages to be gained by using zeolite nanostructures include facile adsorption and desorption, the ability to form dense films to facilitate separations applications and optical transparency. Several applications of nanometer-sized zeolites to environmental catalysis are described in the next three sections.

Environmental Remediation: NOx Emissions Abatement
The emission of NOx and N2O from stationary and automotive sources, such as power plants and lean-burn engines, is a major environmental pollution issue. NOx leads to the production of ground level ozone and acid rain and N2O is a greenhouse gas. The catalytic reduction of nitrogen oxides to N2 is an important environmental challenge for scientists and engineers. Recently, the selective catalytic reduction of NOx and N2O by hydrocarbons (SCR-HC) over transition-metal exchanged zeolites, particularly in the presence of oxygen, has attracted much interest for emission abatement applications in stationary sources, such as natural gas fueled power plants. SCR-HC of NOx and N2O shows promise for applications to lean-burn gasoline and diesel engines where noble-metal three-way catalysts are not effective at reducing NOx in the presence of excess oxygen. The SCR activity of nanocrystalline zeolites, such as NaY, has been investigated.

Environmental Remediation: Photocatalytic Decomposition of Organic Contaminants
The next system that we will investigate involves the photocatalytic oxidation (PCO) of volatile organic compounds (VOC’s) and heavy metals, such as chromium(IV). Photocatalysts, such as TiO2, can be used to degrade a wide range of organic compounds found in polluted water and air. TiO2 photocatalysts are active at ambient temperatures and pressures in the presence of UV irradiation and oxygen. Potential applications include purifying enclosed atmospheres, such as those found in spacecrafts, offices, industrial plants, and homes. The major pollutants in these applications are oxygenates and aromatics. TiO2 photocatalysts have been shown to oxidize toluene, trichloroethylene(TCE), methanol/ethanol and a number of other organic compounds. Additionally, iron nanoparticles have been shown to reduce Cr(VI) to Cr(III) in solution using light. The use of nanometer-sized zeolite TiO2 composites and iron-exchanged zeolites will be evaluated for applications in environmental remediation of VOCs and chromium, respectively.

Adsorption of Volatile Organic Compounds (VOCs)
Zeolites are extremely good adsorbents for many applications involving the adsorption of VOCs from polluted water or air. In this last application, the advantages of nanocrystalline zeolites for the adsorption of VOCs from water and air will be exploited. The nanocrystalline zeolites synthesized in our laboratory will be evaluated for the adsorption of a representative VOC, such as toluene. The adsorption properties of commercial and synthesized zeolites for toluene will be compared. In addition, the nanocrystalline zeolites will be chemically modified in order to tailor the hydrophobic/hydrophilic properties for applications in particular chemical environments. For example, nanocrystalline ZSM5 has been functionalized with octamethylsilane such that the hydrophobicity was dramatically increased. The functionalized zeolites will then be evaluated for the adsorption of toluene in aqueous solution.

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