Galen D. Stucky
Departments of Chemistry, Materials
University of California, Santa Barbara
Santa Barbara, CA 93106
High surface area (300-2,000 m2/g) materials technology is directed primarily towards the creation of inexpensive low bulk volume/area media for applications that require rapid and responsive sampling, selective separations, high throughput catalytic processing, enhanced chemical activity (Klabunde et al. 1996 a and b), or 3-D packaging of supported or entrained nanoscopic structured species (Hornyak et al. 1997; Leon et al. 1995; Blake et al. 1996). High surface areas provide a mechanism to achieve detection sensitivities that are in the range of parts per billion on a short time scale or to rapidly sample and chemically process large volumes of reactants. Control of pore or surface structure at the nanostructure level (< 1 Å) makes it possible to modify separation or catalytic process selectivities by several orders of magnitude. Promising technological areas in this category are chiral active surfaces for pharmaceutical separations and enantiamorph synthesis (Cao et al. 1992); ordered magnetic high surface area materials for oxygen separation; and optically transparent (Schacht et al. 1996; Huo, Feng, et al. 1997; Belcher, Zaremba, et al. 1996) high surface area monoliths (films, spheres, fibers) for chemical sensor applications (Dunbar et al. 1996; Dickinson et al. 1996).
A second important materials technology category for high surface area materials is in the nanostructured modification or design of bulk properties, usually by a synthetic route that accesses metastable phases and kinetically stable intermediates. Here, high surface area interfaces are useful in creating the necessary interface properties for processible composites, designing thermal transport in low density packaging, and determining chemical and mechanical integrity.
In both categories, a key challenge is processing the high surface areas into forms or shapes appropriate for the desired applications. Appropriate forms include thin films (Yan et al. 1992; Aksay et al. 1996; Yang and Ozin 1996; Dunbar et al. 1996; Tolbert, Schäffer, et al. 1997), fibers (Dickinson et al. 1996; Narang et al. 1996; Huo, Zhao, et al. 1997), hard spheres for chromatographic and catalytic fluid bed reactions (Huo, Feng, et al. 1997), and macroscale patterned structures for pattern recognition in optical sensor pattern recognition applications. Hierarchical structure design based on a nanoscale molecular assembly approach that incorporates multi-dimensional control of function and properties on a space and or time basis is an exciting goal in this area that has important ramifications for materials technology in general (Sarikaya and Aksay 1994; Belcher, Wu, et al. 1996; Schacht et al. 1996; Belcher, Zaremba, et al. 1996).
The nanostructure control parameters typically required for the first category of the above applications are comparable to those of enzymes in biocatalysis. Key figure-of-merit variables include the following: (1) molecular recognition parameters, (2) transition state lifetimes, (3) sorption and desorption rates, (4) ability to functionalize the surface, (5) chemical and mechanical stability; (6) defined defect structure, and (7) interface chemistry and structure from both a synthesis and composite property perspective. In porous media, dimensional resolution and definition of structure are optimally at a few percent of the diameter of the pore diameter. Fractional void spaces in porous media are typically desired in the range of 0.9 with pore volumes from 0.6 to 2.7 cc/g. Three-dimensional patterning and periodicity give the best surface area/volume dimensionality, optimum useful access space, and nanoscale control of structure and properties.
High surface area materials are always metastable phases in the sense that they are created by interrupting the kinetics of single-phase assembly. Lower free energy states are invariably accessible; they are created through use of interfaces, including supercritical phases, liquid-liquid, liquid-solid, or even solid-solid interfaces. At the nanostructure level the topology of the created surface is defined by a form of molecular imprinting (Davis et al. 1996; Stucky et al. 1996) that can be achieved using either single molecules or organized arrays of molecules. The nature of the high surface area materials that are created then depends on the relative kinetics and thermodynamics of (1) the polymerization of the species that makes up the final surface, (2) the interface interactions of these species with the molecules that are responsible for creating the ultimate high surface area and surface nanostructure, and (3) the intra-molecular interactions of these molecules. Low-cost synthesis and processing are sought; rapid 3-D assembly of the final high surface area structure is desirable.
Accurate nanoscale assembly therefore always requires the ability to control the kinetics of the surface assembly relative to its "molecular imprinting" definition. On the negative side, this has limited the ability to isolate nanostructured, periodic surfaces having a single composition or a monodispersed pore size. Defect control is always tenuous, and there has been only limited success with creation of designed high surface area macroscale morphologies with monodispersed porosity, such as thin films, beads, fibers, or even millimeter-sized single crystals. On the positive side, if kinetic control of the different interface and assembly processes can be achieved, it is a powerful tool towards fine-tuning over a broad range nanoscale composition, surface area, surface morphology, void space, pore volume, and functionality.
The dimensionality of high surface area porous materials is evolving from the microscale (3-15 Å) domain obtained by the single molecule structure direction used in zeolite and molecular sieve synthesis into the mesoscale (15-150 Å) dimensions of the mesoporous materials (Kresge et al. 1992; Beck et al. 1992; Schüth et al. 1993; Huo et al. 1994). However, progress has been slow in generating monodispersed, high surface area, 3-D periodic arrays of pores and cages above the 10 nm region. In addition, bridging the nanoscale-macroscale (millimeter to centimeter) dimensions in porous material synthesis has to this point meant sacrificing short range nanoscale order to get longer range control of the structure. This is an area that should receive and is receiving considerable attention.
Mechanical and chemical stability are limiting factors for high surface area materials in general and for large pore systems in particular. One way to deal with this is by post-treatment. A second approach is through the use of block copolymer synthesis to create large pores and pore walls with thicknesses of 6 nm or greater (Zhao et al. 1998). An important problem for redox catalytic applications is hydrothermal stability of transition metal defects introduced either by functionalization of the support or by incorporation into the surface during synthesis. Defects, in any case, play an important role in both structure and function; while they can be and are introduced for certain applications, their chemistry and structure are poorly understood.
The synthesis and processing of nanoscale-patterned macroscale shapes and forms of 3-D periodic porous materials is one of the more challenging and exciting recent developments in the field. The synthesis space for this is kinetically controlled and vast, but once defined is simple and can lead to straightforward processing parameters. For example, the simultaneous but independent synthetic definition of hierarchical structure on different length scales can be achieved by multiprocess assembly using emulsion hydrodynamics and surfactants, a route that is highly desirable from a commercial processing perspective. An important area that is being addressed is maximizing access to the interior of these micron-millimeter or larger size continuous structures so that high/space flow velocities for sorbents and reactants and products are possible. The synthesis methodology for creating new cubic and hexagonal mesoporous phases with 3-D access (Huo et al. 1995; Huo et al. 1996), along with spatial pore alignment by applied magnetic (Firouzi et al. 1997; Tolbert, Firouzi, et al. 1997) or electric fields for high surface area mesoporous silica has been demonstrated, making it possible to create highly accessible pore configurations. Quantification of the synthesis conditions, mapping out the pore and cage structures, functionalization, defect chemistry, and diffusion processes with these materials all are essential to successful future applications of the materials.
Commercial applications of high surface area materials already are in the multibillion dollar range; primary contributions to this economic basis are air separation, petroleum and petrochemical processing, environmental cleanup, chemical sensing, fine chemical catalysis, packaging, and chemical separation applications. Economic incentives for development and application of high surface area materials are expected to substantially increase in view of increasing environmental stress, their role as important components of chemical sensors, potential applications in biotechnology, and increasing energy and agricultural consumption demands.
Control of the nanostructure and the nanostructure interface in composite phases can be expected to provide breakthroughs in a number of applications: parallel processing where high density image processing coupled with high space velocity or turnover is required; applications involving pattern recognition, biomaterials synthesis, and design; biotechnology applications; development of small lapel-worn toxic agent detectors for military and environmentally precarious environments; and in general, chemical sensing technology where highly selective, sensitive, rapid detection is required. Breakthroughs are less likely in current bulk processing applications, particularly in the historically more mature air separation, petroleum, and petrochemical industries. The cost of reconfiguring present technologies and plants operating at a relatively high efficiency is a formidable barrier. Nevertheless, the magnitude of the economic impact generated by even small improvements in many of these areas is so substantial that nanostructure-directed modification of the present approaches is still being and will continue to be sought.
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