INTRODUCTION
Zeolites are aluminosilicate molecular sieves with pores of molecular dimensions.[1'2] Zeolites can be synthesized with a wide range of pore sizes and topologies and are used in applications such as catalysis, chemical separations, and as adsorbents and ion exchangers. The zeolite chemical composition, framework topology, and pore size can be varied to control reactivity and selectivity. Two common zeolites, ZSM-5, and Y, are shown in Fig. 1 with one molecule of adsorbed cyclohexane to show the relative sizes of the pores in these materials. It can be seen that these materials, by their very nature, have dimensions in the nanometer-size regime.
Although the pore dimensions of these materials are in the nanometer-size range, the zeolite crystals prepared through conventional syntheses are typically on the order of micrometers in diameter. Recently, however, there has been a great deal of interest in the synthesis of nano-crystalline zeolites, i.e., zeolites with discrete, uniform crystals with dimensions of less than 100 nm, and their unique properties relative to conventional micrometer-sized zeolite crystals.[3-16] For some applications, it would be advantageous to employ much smaller nanometer-sized zeolite crystals in the range 10-100 nm. New synthetic methods have been developed that enable the selective and quantitative formation of small crystallites.
The impact of crystal size on zeolite properties can be profound. For example, Bein[17] has studied thin films formed using nanometer-sized zeolites (10-100 nm crystallite size) and has found that ”thin films can adsorb and desorb most vapors within a few seconds to minutes, often at room temperature, while the bulk materials usually require substantial heating (ca. 200-300 °C) to remove adsorbed vapors.” The facile desorption of products would be a distinct advantage in many gas phase heterogeneous processes involving zeolites, so nanotech-nology has the potential to be utilized in many zeolite-based applications. The increased surface area of the small particles also provides these materials with a distinct advantage in heterogeneous catalysis. In addition, these nanometer-sized zeolites can then be assembled into nanostructures, such as thin films, fibers, or mem-branes.[17] These zeolite nanostructures can potentially be used to control chemical reactivity on molecular length scales. The objectives of this paper are twofold. First, the synthesis of nanocrystalline zeolite particles and the self-assembly of these particles into thin films, fibers, and coatings are discussed. Second, potential applications of nanocrystalline zeolite materials in environmental catalysis are described.
NANOCRYSTALLINE ZEOLITES MATERIALS: SYNTHESIS AND SELF ASSEMBLY
The synthesis of nanometer-sized zeolites, such as ZSM-5,[18] silicalite,[19] A,[20] Y,[4,15,21] Beta,[3] L,[13] and TS-1[8] (the titanium containing analog of ZSM-5), has been reported in the literature. Typically, the particle size distribution is controlled by varying the gel composition or the synthesis temperature and time. Recently, other methods of controlling particle size distribution, such as confined space[12] or templating methods,[16] have been introduced. In the confined synthesis method, an inert mesoporous support material, such as carbon black, is used to control the particle size in zeolite synthesis. In the templating method, pitch imprinted with colloidal silica is used as the crystal template for preparing zeolites with crystal sizes ranging from 13 to 90 nm in size.[16] The advantage of these approaches is that a wide range of zeolites can be synthesized by the same general method. In addition, a variety of nanoarchitectures can be constructed using nanocrystalline zeolites as the building blocks. Nanoarchitectures have previously been discussed in the context of multifunctional materials.[23,24] A schematic of various possible zeolite nanoarchitectures is shown in Fig. 2. Some examples of specific syntheses and nanoarchitectures are described in more detail below.
Hydrothermal Synthesis of Nanocrystalline Zeolites X, Y, and ZSM-5
Several methods for the preparation of zeolite Y with nanometer-sized crystals have been reported in the liter-ature.[4,15,21] The synthesis of zeolite Y has been described by Schoeman et al.[21] and Castagnola and Dutta.[4] Typically, this synthetic procedure produces zeolite Y crystals that range in size from 70 to 150 nm with a Si/ Al=1.6. The general features of this synthetic procedure for the synthesis of zeolite Y involve the preparation of a tetramethylammonium hydroxide (TMA)-alu-minate solution that is subsequently added to a silica sol to form a homogeneous solution. The solution is then heated at 100°C in a polypropylene bottle with reflux in a poly(ethylene glycol) bath. Recently, Alwy et al.[25] reported the synthesis of nanocrystalline zeolite Y with a smaller average crystal size of 46 ± 8 nm and have formed transparent films from this material (vide infra). Fig. 3 shows the scanning electron microscope (SEM) image of the nanocrystalline zeolite Y prepared by Alwy et al. Several other synthetic methods used to prepare zeolite Y and X nanocrystals have been described in the liter-ature.[5'15'26-29]
Fig. 1 Diagram of zeolites ZSM-5 and Y and their relative pore size dimensions.
Similar hydrothermal syntheses have been reported for the preparation of nanocrystalline ZSM-5 and silicalite-1 (the purely siliceous form of ZSM-5).[10'11,19'30-32] In these studies, ZSM-5 and silicalite were synthesized with crystal sizes in the range of 10-100 nm. One method involves the hydrothermal crystallization from clear solutions under autogenous pressure,[32] and another approach involves using silicalite seed crystals at atmospheric pressure.[11] An SEM image of nanocrystalline silicalite synthesized in our laboratory with an average particle size of 39 nm is shown in Fig. 4.
Fig. 3 Scanning electron microscopic image of the nanocrystalline zeolite Y film.
Templating Methods for the Syntheses of Nanocrystalline Zeolites
Several hydrothermal methods for the synthesis of zeolites Beta and ZSM-5 with nanometer-sized crystals are available in the literature.1-3’14’19’30’33-1 In addition, tem-plating methods have been recently reported that rely on the growth of zeolite crystals in a carbon-based template such as carbon black or colloid imprinted carbon (CIC). The advantages of these templating methods are that the zeolite crystal size is controlled by the size of the template and the carbon-based templates reported so far can be easily removed by calcination in air.
The confined space synthesis method described by Jacobsen et al.[7'12'18] for the synthesis of zeolites A, X, ZSM-5, and Beta has several distinct advantages over conventional hydrothermal syntheses. The confined space method involves crystallization of a zeolite in the mesopores of an inert porous matrix support. The crystal size distribution is controlled by the size of the mesopores of the inert support and results in a narrow distribution of pore sizes. Carbon black is typically used because it is available with different pore sizes and it can be easily removed by pyrolysis of the carbon.[7,12,18] In addition, in the confined space synthesis method, the gel composition and crystallization temperature and time are not as critical in determining the crystal size distributions. Jacobsen et al.[7,12,18] reported the synthesis of ZSM-5 with particle sizes ranging from 22 to 45 nm depending on the carbon matrix used in the synthesis. Similarly, they reported the confined space synthesis of zeolites Beta, Linde Type A (LTA), and Linde Type X (LTX).[7,12,18] The disadvantage of the confined space synthesis method is that the size distribution of the resulting zeolites could be quite large because the carbon black template is disordered.
Fig. 2 Examples of nanoarchitectures that can be formed from nanocrystalline zeolites. The patterned areas represent nanocrystalline zeolite, and the solid shaded areas indicate a support such as polystyrene beads (diameters of ~ 1-200 nm) or optical fiber.
Fig. 4 Scanning electron microscopic image of silicalite with an average particle size of 39 nm.
A templating method using colloid imprinted carbons (CIC)[22] was recently reported by Kim et al.[16] In this method, colloidal silica is used to imprint mesopores in carbon precursor particles (pitch). In this way, pores of different sizes can be tailored for specific applications.1-22-1 Kim et al.[16] prepared CICs with pore diameters of 12, 22, 45, and 85 nm. These CICs were impregnated to incipient wetness with the zeolite precursor solutions for ZSM-5 and then were subjected to hydrothermal synthesis conditions of ~ 48 hr in an autoclave at 180°C. The CIC was subsequently removed by calcination in air at 570°C. ZSM-5 nanocrystals prepared by this method had average particle sizes of 13, 22, 42, and 90 nm as determined by transmission electron microscopy (TEM). One note about the zeolites produced by this method is that the zeolite crystals are intergrown into aggregates because of the nature of the CIC template.
Assembly of Nanocrystalline Zeolites
Nanometer-sized zeolites can be assembled into zeolite nanostructures, such as fibers or thin films, to optimize their potential for applications in heterogeneous environmental catalysis or perhaps sensor technology. For applications in thermal chemistry, a primary consideration is to increase the surface area of the zeolite and to reduce intracrystalline diffusion effects. For applications in photocatalysis,[34-36] the use of optically transparent zeolites would increase the efficiency of the process. The formation of a transparent film of nanocrystalline zeolite Y relative to commercial zeolite Y has been demonstrated by Alwy et al.[25] Films of the zeolite Y were prepared by sonication of an aqueous mixture of nanocrystalline zeolite Y for several hours. The resulting hydrosol was pipetted onto a Pyrex slide and dried in ambient air. Films of commercial zeolite Y (Aldrich) were prepared using the same method. Digital images of the zeolite Y films are shown in Fig. 5. In each case, the film was prepared using approximately the same mass of zeolite Y. The film prepared from the nanocrystalline zeolite Y hydrosol is much more uniform than the film prepared from the Aldrich zeolite Y 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 zeolite Y film (right) but is much more difficult to see through the Aldrich zeolite Y film (left). To obtain more quantitative information, the percent transmittance (%T) was measured using UV/Vis spectroscopy. The nanocrystalline zeolite Y film had a %T of 70-80 in the 300-700 nm range compared with a %T of 30-40 for the Aldrich Zeolite Y film in the same range.
In considering all the factors to be optimized, zeolite fibers (either coated or hollow)[37-40] are potentially very promising materials. Reactant absorption and product desorption will be facilitated because of the fact that the zeolite layer is very thin.[17] For applications involving separations and catalytic reactions, zeolite fibers could be positioned within a tube and reactant gases, or liquids could be flowed over the zeolite fibers. For these applications, a large surface area is desirable and can be achieved using bundles of small-diameter zeolite fibers. A fiber bundle 10 cm in length and 1 cm in diameter containing individual fibers of 100-mm diameter with a packing fraction of 50% has a surface area 1570 cm2, compared with a surface area of 31 cm2 if the inside of a 10-cm-long, 1-cm diameter glass tube was coated directly. For zeolite systems in which optical access is needed for photoexcitation or detection, the zeolite fibers will enable the efficient propagation of light through a long path length of zeolite.
Fig. 5 Thin films prepared from hydrosols of Aldrich zeolite Y (left) and nanocrystalline zeolite Y (right).
Several methods for the preparation of zeolite coated or hollow fibers have been reported in the literature. Pradhan et al.[39] reported a method for coating optical fibers with presynthesized zeolites via a sol-gel process. Okada et al.[41] have reported the coating of zeolite Na-X on glass fibers by a soft solution process, and Deng and Balkus have used laser ablation to coat optical fibers.[40] Recently, Ke et al.[42] have reported an electrophoretic method for coating carbon fibers. The carbon fibers can then be removed by combustion leaving hollow zeolite fibers. Huang et al.[6] reported the self-assembly of zeolite nanocrystals (silicalite, ZSM-5, Beta, A, and faujasite) into films, membranes, and fibers. The self-assembled zeolite fibers that were produced in that study were transparent and approximately 27-33 mm in diameter and approximately 1.5 cm in length.
Nanocrystalline silicalite, a purely siliceous form of ZSM-5, was used to prepare zeolite fibers. An SEM image of the 35-nm particle size silicalite is shown in Fig. 4. From this image, it can be seen that the zeolite crystals are uniform in size and are closely packed. The nanocrystal-line particles shown in the SEM image in Fig. 4 self-assemble into transparent, rectangular fibers as illustrated in Fig. 6. These films and fibers (Figs. 5 and 6) formed from nanocrystalline zeolites (Figs. 3 and 4) are both examples of zeolite nanoarchitectures.
Scanning electron microscopic image of free-standing fibers formed from nanocrystalline silicalite (100-mm scale bar).
NANOCRYSTALLINE ZEOLITE MATERIALS: APPLICATIONS IN ENVIRONMENTAL CATALYSIS
Recently, zeolites have emerged as important materials for applications in environmental catalysis.[43-46] Environmental catalysis involves the use of catalysts to solve environmental problems in areas such as emission abatement and environmentally benign synthesis.[47] Many new catalysts and catalytic processes have been developed to meet the challenges posed by environmental concerns.[48] Besides emissions abatement, the emphasis of environmental catalysis has expanded to include the development of environmentally benign synthetic routes designed to decrease the amount of chemical waste produced.[46]
As already noted, nanocrystalline zeolites are promising catalytic materials which have higher external surface areas and reduced diffusion path lengths relative to conventional micrometer-sized zeolites that may be beneficial for catalytic activity and selectivity. For example, nanocrystalline ZSM-5 exhibits increased selectivity and toluene conversion into cresol and decreased coke formation relative to conventional ZSM-5 materi-als.[49] Potential uses of nanocrystalline zeolites as environmental catalysts are described below.
Environmentally Benign Synthesis: Selective Partial Oxidation Reactions of Hydrocarbons
The partial oxidation of hydrocarbons is significant to chemical industry because the products are used to convert petroleum hydrocarbon feedstocks into chemicals important in the polymer and petrochemical industries. Liquid-phase air oxidations are generally preferred by chemical industry because of the mild reaction conditions. Notable examples[50] of liquid phase air oxidation reactions are listed in Table 1. As can be seen from Table 1, the conversions of the oxidation processes are typically very low to maintain high selectivity. This is necessary because the desired partial oxidation products can easily be further oxidized under typical reaction conditions. A major motivation for the development of new oxidation routes is the desire to achieve high selectivities at high conversions.
One approach to the partial oxidation of hydrocarbons is to eliminate the use of organic solvents through the use of gas-phase reactants and products. A clean, inexpensive oxidant, molecular oxygen can be used in these reactions. Thus these reactions have the potential to be green processes that use no solvents and minimal energy with catalysts (i.e., zeolites) that have been used in industry for many years. However, several crucial issues related to the application of these processes have emerged. These issues involve improving the efficiency of the photooxidation process and improving the diffusion of product and reactant molecules in the zeolites.[34,35]
Table 1 Examples of liquid-phase oxidation reactions
|
Partial |
Conversion |
|
|
|
Reactant |
oxidation product |
(selectivity) |
Application |
|
Cyclohexane |
Cyclohexyl |
12-13% |
Nylon-6 |
|
(C6H12) |
hydroperoxide, cyclohexanol, cyclohexanone c6h11ooh, Q5H11OH, C6H10O |
(>90%) |
|
|
Isobutane |
tert-butyl |
10-20% |
Propylene |
|
[CH(CH3)3] |
hydroperoxide, tert-butyl alcohol (CH3)3COOH, (CH3)3COH |
oxide |
For the application of zeolites as environmentally benign photooxidation catalysts, a major problem is light scattering by the zeolite. The efficient use of light energy in zeolites requires that light be propagated through a long path length of the zeolite material. However, zeolite crystallites strongly scatter visible light because of their small dimension. Thus only a thin photoreactive zone is obtained regardless of the zeolite bed thickness. In previous photooxidation work, it was determined that yield of photooxidation reactions in zeolites strongly depended on the thickness of the zeolite layer. Using zeolite Y with a crystallite size of approximately 1 mm, product formation for the photooxidation reaction of p-xylene (holding irradiation time and loading constant) was monitored as the sample thickness was varied. The results are shown in Fig. 7 and clearly demonstrate that the effective yield drops drastically as the thickness of the zeolite layer increases. This is because the light is not able to penetrate through more than approximately 100 mm of zeolite and, as stated above, is a problem for applications to photooxidation reactions. Therefore nanocrystalline materials should be better materials for these reactions as they can form optically transparent thin films.
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.[43,51-53] 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.[54] Another aspect of these transition metal-exchanged zeolites that has been reported in the literature is the photocatalytic activity for the direct decomposition of NOx and N2O[55-57] and the SCR-HC of NOx.[58] The use of zeolite nanostructures would provide some of the same advantages as have been discussed for the oxidation reactions such as more efficient light absorption, reactant diffusion, and increased surface area. Applications for which these materials might be important include transparent objects, such as windows, that could be coated with transparent zeolite thin films. The zeolite thin film would then be activated for NOx decomposition by sunlight.
Another interesting question is whether the catalytic activity of transition-metal exchanged zeolites changes when nanometer-sized zeolites are used instead of conventional zeolites. For example, Zhang et al.[14] determined that nanometer-sized HZSM-5 has more Br0nsted acid sites on the external surface than conventional HZSM-5. Iron-exchanged zeolites exhibit high activity for the SCR of NOx and N2O with ammonia or hydrocarbons as the reductant.[59-67] In addition, iron-exchanged zeolites are highly resistant to poisoning by water which is a crucial property for potential applications. The catalytic activity of iron-exchanged zeolites is very sensitive to the exchange procedure used to introduce the iron into the zeolite. Therefore it is conceivable that the properties of iron-exchanged nanozeolites could be dramatically affected by the use of nanometer-sized zeolites and have the potential to be more active catalysts.
Fig. 7 Conversion of paraxylene to paratolualdehyde as a function of BaY film thickness.
Environmental Remediation: Photocatalytic Decomposition of Organic Contaminants
Photocatalysts, such as TiO2, can be used to degrade a wide range of organic compounds found in polluted water and air.[68-70] Much of the research in the last decade has focused on aqueous solution photocatalysis for the decontamination and purification of water. However, gas phase heterogeneous photocatalysis can be an effective way to remove undesirable organic contaminants from air.[70-77] 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/etha-nol, and a number of other organic compounds.1-70-1 Recently, several groups have considered the use of zeolite TiO2 composites,[78-80- TS-1 (titanium-substituted ZSM-5),[81- and nanometer-sized TiO2.
Similar to the problems discussed in the selective oxidation of hydrocarbons, light-scattering problems make these zeolite TiO2 composite catalysts less than optimal. Zeolite TiO2 composites prepared using nanometer-sized zeolites may be useful decomposition catalysts. Molecules such as toluene, benzene, and oxygenates, including aldehydes and ketones, are candidates to evaluate the effectiveness of nanometer zeolite TiO2 composites for the removal of these undesirable organic contaminants from air and water.
CONCLUSION
The development of nanometer-sized zeolites for applications in heterogeneous catalysis can potentially lead to solutions of several important environmental problems. These problems span from new methodologies in environmentally benign synthesis to new methodologies in environmental remediation of pollutants. The development of new green methodologies for the catalytic syntheses of industrially important compounds is necessary if the production of hazardous waste is to be reduced. The remediation of harmful pollutants using nanometer-sized zeolite catalysts has important implications in the area of emission abatement. NOx and volatile organic compounds (VOC) emissions are intimately linked to tropo-spheric ozone formation in a complex nonlinear way.[84-
Because zeolites are also used in sensors and as materials in separation processes, there are other environmental benefits to developing zeolite nanostructured materials that go beyond what has been discussed in this paper. For chemical sensor applications, photoactive molecules can be occluded within the structure of the zeolite.[17,85,86] As other molecules enter the pores of the zeolite, they can be expected to influence the electronic structure and therefore the optical spectrum of the occluded chromophores.[17,86] Detection of the change in the optical spectrum using a fiber optic would enable the construction of chemical sensors. Zeolites-based sensors could be developed for detection of specific pollutants or perhaps chemical warfare agents. Zeolite thin films could be used in applications to separate hazardous substances from environmentally benign species. These and other applications could result from the further development of the zeolite nanostructures. The studies described in this paper are just a first step in the synthesis and potential uses of nanocrystalline zeolite materials.
