Heterogeneous catalysis has had a major impact on chemicals and fuels production, environmental protection and remediation, and processing of consumer products and advanced materials. A survey of U.S. industries revealed that the annual revenue from chemical and fuel production topped all other industrial sectors at $210 billion (MIT 1990). Bennet et al.'s survey (1988) showed that over 60% of the 63 major products and 90% of the 34 process innovations from 1930-80 involved catalysis, illustrating the critical role of this field in the fuel and chemical industry. The significance of catalytic processes can be further demonstrated by the value of their products, which amounted to $1 trillion in the United States alone in 1989 (Cusumano 1992). Improvement in catalytic activity and selectivity holds the key to developing more efficient catalytic processes. Ability to tailor catalytic materials with desired microstructure and active component dispersion can bring about significant advances in the field of catalysis. This can be accomplished through nanostructure processing of materials.
Nanostructure processing offers new capabilities of manipulating materials microstructure and compositional variation on the nanometer scale. Two classes of nanostructured materials are being developed through multidisciplinary research efforts: nanocrystalline and nonporous materials.
The first class of nanostructured systems, nanocrystalline materials, involves crystallites of 1-10 nm in dimension (Gleiter 1989; Siegel 1991). Due to their fine grain sizes, ultrahigh surface-to-volume ratio can be achieved readily in nanocrystalline materials. The large number of atoms located at the edges and on the surfaces of nanocrystallites provide active sites for catalyzing surface reactions. Nanocrystallites further possess unique hybrid properties characteristic of neither the molecular nor the bulk solid state limits. Nanocrystalline processing offers a practical way of retaining the results of property manipulation on the atomic or molecular level, producing novel materials with unique size-dependent behavior, including quantum confinement effects, superparamagnetism, greater microstructural uniformity for better mechanical reliability, and high ductility and superplasticity for advanced ceramics. Over the past decade, research on nanocrystalline materials has been greatly accelerated by the advances in our ability to manipulate structures on the molecular or atomic level. However, most of the studies have been directed towards the synthesis, characterization, and application of these systems as structural and optical/electronic materials. As catalysts, nanometer-sized active clusters have been examined for a long time, but they are mainly limited to supported metal systems. Direct synthesis and successful stabilization of nanocrystalline metallic and ceramic materials have only recently been investigated in detail for some catalytic applications (Trudeau and Ying 1996). It would be particularly fruitful to exploit the nanocrystalline systems for their size-dependent effects in structure-sensitive reactions, whereby the catalytic activity depends not only on the number of active sites, but also on the crystal structure, interatomic spacing, and crystallite size of the catalytic material.
A recent study on nanocrystalline cerium oxide-based catalysts demonstrated several unique features of nanocrystalline processing (Ying and Tschöpe 1996). By inert gas condensation of Ce clusters followed by controlled post-oxidation, it is possible to achieve highly non-stoichiometric CeO2-x (Tschöpe and Ying 1994). The unusually high oxygen vacancy concentration possessed and retained by these nanocrystallites is associated with active superoxide surface species. Compared to the conventional high surface area CeO2, nanocrystalline CeO2-x enabled catalytic activation at a significantly lower temperature for SO2 reduction and CO oxidation, and demonstrated superior poisoning resistance (Tschöpe et al. 1995). Nanocrystalline processing further allowed ultrahigh dispersion of components, such as that demonstrated in Cu-CeO2-x nanocomposites (Tschöpe, Ying, and Chiang 1995). Unique chemical and electronic synergistic effects can be obtained from such homogeneously dispersed multicomponent systems, useful towards promoting catalytic activity, especially when they are thermally stable under the reaction conditions.
Future research and development efforts in nanocrystalline materials should be directed towards improved powder processing that will minimize grain growth and loss of surface area in heat treatment. To greatly expand the commercial market for such new materials in catalysis, methods for cost-effective large-scale production of nanocrystals have to be developed, with advanced reactor design, without compromising grain size distribution, and with excellent control on particle agglomeration. There is a great deal of opportunity in synthesizing doped and multicomponent nanocrystalline materials to improve substantially the catalytic activity and selectivity of existing catalysts. Such endeavors are particularly fruitful when a fundamental approach is adopted, whereby the design of the catalyst composition and microstructure is targeted towards solving the bottleneck of specific reactions (i.e., the reactant adsorption, surface reaction, or product desorption step in the reaction mechanism scheme). There is value in pursuing research directed towards attaining a better understanding of the structure-property relationships in the area of catalytic materials. In this important arena, nanocrystalline processing will not only serve to improve the performance in critical catalytic processes but also help establish a superior knowledge base concerning the effects of structural manipulation on the generic catalytic characteristics for the major classes of catalytic reactions.
The second class of nanostructured systems, nonporous materials, is characterized by the molecular assembly of structures consisting of nanometer-sized cavities or pores. Conventional porous structures with long-range crystalline order are typically limited to aluminosilicates (e.g., zeolites) and phosphates with pore openings of less than 15 Å. The inflexibility in the composition and pore size of such materials does not address the needs of the variety of catalytic reactions and gas adsorption applications that exist in chemical industries. Ability to process nanoporous materials with greater control in compositional and pore structure variation will open up tremendous opportunities in advancing catalysis and separation technologies.
In 1992, Mobil researchers reported that a family of aluminosilicates (termed M41S) with pores larger than 20 Å could be synthesized in an ordered packing through a liquid crystal route (Kresge et al. 1992; Beck et al. 1992). Of particular interest is MCM-41, which has hexagonally-packed cylindrical pore channels containing surface areas greater than 1200 m2/g and uniform pore sizes that can be tailored from 20 to 100 Å in diameter, making for attractive heterogeneous catalysts, catalyst supports, and nanocomposite host materials for a wide range of novel applications (Wu and Bein 1994; Bowes et al. 1996; Sayari 1996; Brinker 1996). The ability to synthesize these materials and to control the structural characteristics on a mesoscopic scale stems from the unique synthesis route of using surfactant solutions to template the inorganic precursors into a continuous solid framework. The formation of mesoporous materials with a variety of crystallographically well-defined frameworks (Huo et al. 1996) has been made possible via a generalized "liquid-crystal templating" (LCT) mechanism (Huo et al. 1994; Chen et al. 1993).
To create materials with superior structural and surface chemical properties, much work has been done to expand upon the LCT synthesis to include non-aluminum metal dopants within the silicate MCM-41 framework (Tanev et al. 1994; Corma et al. 1994), and to derive non-siliceous MCM-41-types of mesostructures (Huo et al. 1994; Braun et al. 1996). The ability to introduce compositional variation into the supramolecular templating of nanoporous structures will present exciting opportunities for attaining a great variety of molecular sieves well beyond the current synthetic flexibility of the zeolitic systems. For example, transition metal oxides play an important role as industrial catalysts in petrochemical production, pollution control, and pharmaceutical and fine chemical syntheses, but they are typically available with relatively low surface areas and poorly-defined pore structures. Improved performance can be achieved by developing transition metal oxide materials with the highly desirable microstructural characteristics of MCM-41. A number of non-siliceous M41S-like materials have been reported for the oxides of such transition metals as antimony, iron, molybdenum, tin, and vanadium; however, these mesostructured materials collapsed upon surfactant removal and did not yield mesoporous molecular sieves (Huo et al. 1994; Stein et al. 1995; Janauer et al. 1996; Ulagappan and Rao 1996). These attempts at synthesizing transition metal oxide mesostructures were based on the LCT mechanism involving Coulombic charge interplay between charged surfactant head groups, metal precursor species, and counterions in solution.
To successfully synthesize transition metal oxide mesoporous materials, it is critical to establish a strong interaction between the surfactant head groups and the inorganic precursor prior to micellar self-assembly (Antonelli and Ying 1996c). Based on this idea, our laboratory has created new organometallic compounds that consist of transition metal alkoxides ligated with a variety of surfactants through covalent bonding. This precursor design proves to be ideal for creating transition metal oxides with a hexagonally packed cylindrical mesoporous structure. The resulting family of transition metal oxides, termed TMS or tech molecular sieves, is characterized by uniform pore diameters that can be flexibly altered through varying the surfactant chain lengths and through the addition of swelling agents (Antonelli and Ying 1995, 1996a, and 1996b; Antonelli, Nakahira, and Ying 1996 ). Consequently, we are able to achieve transition metal oxides with surface areas in excess of 600 m2/g and pore sizes tailored in the range of 20-100 Å. These uniformly large pore openings overcome the configurational diffusion limitations of zeolitic systems. The transition metal-based composition further offers the fundamental chemical flexibility of tailoring mixed oxidation states, electronic interaction, surface modification, and thermal stability.
A wide variety of reactions and processes will profit from the dual microstructural and chemical functionalities of the TMS family of molecular sieves. For example, zirconium oxide-based TMS materials (Wong et al. 1997) could serve as acid catalysts for isoparaffin-olefin alkylation that provides for higher octane number reformulated gasoline blends. The ability to tailor chemical composition and pore sizes may offer significant advantages in controlling surface acidity, catalytic activity, product selectivity, and coking behavior in alkylation catalysis. When developed as solid acid catalysts, TMS materials will serve to reduce the usage of hazardous liquid hydrofluoric and sulfuric acid catalysts in commercial alkylation operations.
Important future directions for the development of nanoporous materials include extending the supramolecular templating synthesis beyond the pore diameter range of 20-100 Å to benefit processes (e.g., enzyme catalysis, bioseparation, and biosensing) that require substrates with uniform distribution of ultralarge pores. Systematic bridging of the microporous and mesoporous range, particularly with tailored pore dimensions of 5-15 Å, would be very useful in overcoming the current limitations with existing zeolitic pore structures for selective petrochemical and fine chemical processing. Such pore size tailoring would be especially useful when attained simultaneously with compositional flexibility. Advanced design of catalytically active sites will call for the ability to ultimately control (1) the concentration of different components in mixed oxides while retaining a well-ordered microstructure, (2) the degree and homogeneity of dispersion of different components and dopants, and (3) the stability of the surface dispersion. Addressing these issues will enable us to tailor the surface chemistry of the active species, the coordination chemistry of the dopants and minor components, and the chemical/electronic interaction between the different components, in the context of a desirable porous microstructure.
Research on the synthesis of advanced nanoporous materials would be most fruitful when targeted towards specific catalytic and separation applications of interest. In particular, mesoporous materials offer unique opportunities not only to provide shape and size selectivity for molecules too large to react within the framework of conventional microporous zeolitic structures, but also to serve as stable catalytic supports for fixating bulky organometallic catalytic complexes. The latter constitute novel heterogeneous catalyst systems that have the advantage over homogeneous catalysts of ease of handling and separation in continuous operations, while retaining the high activity and selectivity associated with designed organometallic ligands. Finally, nanoporous materials can be very useful when processed with desired morphology (Aksay et al. 1996; Yang et al. 1996) and crystal size. They may be used as membranes and sensors, for example, operating on the principle of separation via molecular sieving effects. They may also serve as host matrices for quantum dots and wires in novel optical, electronic, and magnetic devices.
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