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Ind. Eng. Chem. Res. 2007, 46, 4166-4177
Fundamentals and Applications of Structured Ceramic Foam Catalysts Martyn V. Twigg† and James T. Richardson*,‡
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Catalytic Systems DiVision, Johnson Matthey, Royston, Herts SG8 5HE, United Kingdom, and Department of Chemical and Biomolecular Engineering, UniVersity of Houston, Houston, Texas 77204-4004
This paper reviews the use of ceramic foams as structured catalyst supports. They are open-cell ceramic structures that may be fabricated in a variety of shapes from a wide range of materials, and they exhibit very high porosities with good interconnectivity. These characteristics result in a lower pressure drop than that observed with packed beds and high convection in the tortuous megapores, which, in turn, enhances mass and heat transfer. They are easily coated with high-surface-area catalytic components, using well-established techniques. Research in the past decade has produced a large amount of fundamental information that elucidates the desirable properties of ceramic foams. In addition, many applications involving important reactions have appeared in the open and patent literature, especially for catalytic processes that suffer certain limitations, such as those encountered in relieving high pressure drop with low-contact-time reactions at high space velocities or with narrow reactors in heat-transfer-limited systems and in controlling axial and radial temperature profiles in highly exothermic and endothermic reactions. These important contributions are discussed, and the advantages and shortcomings of using ceramic foams as structured catalyst supports to benefit commercial operations are considered. Introduction In their book on modern chemical process technology, Moulijn et al.1 related many technical problems that can be solved with structured catalyst supports replacing conventional packed-bed reactors. Methods for accomplishing this have been detailed in two reviews that were devoted solely to the subject of structured catalysts.2,3 Many types of structures were examined; however, the use of foamed supports is only now gaining momentum in the research literature. This paper reviews the fundamentals and potential applications of ceramic foams as catalyst supports, with an emphasis on comparisons with packed beds of catalyst particles. Cellular structures in natural materials (wood, bone, coral, etc.) have long been known and studied. Significantly, nature has optimized certain mechanical properties, such as stiffness, strength, and mass, in an efficient manner; however, potential engineering applications did not evolve until ∼50 years ago, when manufacturing methods for making synthetic materials were developed. This inspired investigation into the structure and properties of cellular materials, and this was reviewed by Gibson and Ashby.4,5 Many useful practical applications followed, e.g., insulating materials and lightweight structures. For ceramic foams, their high-temperature characteristics led to use as molten metal filters and kiln furniture, which, today, are the largest applications. Later uses include burner enhancers, soot filters for diesel engine exhausts, catalyst supports, and biomedical devices.6-9 Ceramic foams are characterized as “closed-cell” and “opencell”, and, accordingly, the two characterizations have vastly different properties. Closed-cell foams are composed of polyhedralike cells connected via solid faces, i.e., with no interconnectivity between them, whereas open-cell structures have solid edges and open faces, with fluid flow possible from one cell to another. Figure 1 illustrates the open-cell structure with three compo* To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +1 (713) 743-4324. Fax: +1 (713) 743-4323. † Johnson Matthey. ‡ University of Houston.
Figure 1. Open-cell, retriculated ceramic foam cell structure.
nents: struts, which are composed of solid ceramic material; cells, which are approximately spherical voids enclosed by struts; and windows, which are openings connecting the cells to each other. The open structure is sponge-like, with pore densities of typically 10-100 PPI (pores per inch), creating an interconnecting porosity in the range of 75%-90% or even higher. Consequently, they have low resistance to fluid flow and considerable turbulence is generated by the tortuous flow paths. This review is restricted to open-cell ceramic foams, because they have obvious implications for catalysis. Metal foams, which have similar properties and applications but are manufactured differently and have unique features, are also not included, except where structural similarities add insight into foam behavior. The present authors reviewed the state-of-the art for catalytic applications of ceramic foams and emphasized the following advantages:10,11 (1) the ability to preshape ceramic foam “cartridges” to match complicated reactor configurations and simplify reactor loading; (2) a lower pressure drop, relative to packed beds, with savings in energy efficiency and low-contacttime operation; (3) high geometric surface areas that improve mass transport and lead to higher effectiveness factors for processes that otherwise require large pellets; and (4)
10.1021/ie061122o CCC: $37.00 © 2007 American Chemical Society Published on Web 02/24/2007
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Figure 3. Surface areas obtained from multiple loading of γ-Al2O3 on 30-PPI R-Al2O3 foam (from ref 19).
Figure 2. Steps in the fabrication of ceramic foam using the reticulation technique.
enhanced radial convection, which improves heat transfer and allows better reactor stability for highly exothermic reactions. These previous reviews listed emerging applications at that time as evidence for the benefits of improved performance. Now, over a decade following initial interest, we cover advances in improved characterization, modeling, and process applications. In addition, we discuss potential weaknesses that could affect commercial implementation. Fabrication of Ceramic Foam-Supported Catalysts Most commercial ceramic foams are prepared using a reticulation method that was first patented by Schwartzwalder and Somers.12 The process starts (Figure 2) with polymer foam. This foam is usually polyurethane, although other organic polymers are equally effective. Polyurethane is readily available in a wide range of cell sizes that determine the cell density of the resulting ceramic. The pores of the polymer foam are filled with an aqueous slurry of ceramic (e.g., R-Al2O3, ZrO2, etc.) that is composed of 0.1-10 µm diameter particles, together with appropriate amounts of wetting agents, dispersion stabilizers, and viscosity modifiers.13-15 In the most common approach, low-viscosity suspensions are used and excess slurry is removed by blowing air through the foam or by squeezing and kneading. The wet foam is dried and calcined in air at temperatures of 1000-1700 °C. With careful control, the plastic vaporizes and the ceramic particles sinter, forming a ceramic replica or positive image of the plastic. The surface of the struts surrounding the faces or windows comprises the area available for supporting catalysts. Alternatively, the viscosity of the slurry is increased by adding thickening agents16,17 and excess slurry is removed only at the external surface of the structure, so the ceramic slurry remains in the pores. After calcination and removal of the plastic, ceramic replaces the voids in the original organic material, giving a negative image of the plastic. Pore diameters are smaller, bulk densities are higher, and porosities are lower; however, the mechanical strength is much higher than that of material prepared via the other method. Preformed shapes, such as cylinders, rings, rods, or customdesigned configurations, are fabricated by machining the plastic
Figure 4. Backscattered electron image of the ceramic foam strut.
before or after soaking and drying. Dispersion of the catalytic agents onto the foam surface follows conventional techniques,18 such as single or multiple impregnations of salts or adsorption of ionic precursors from solution, followed by heat treatment at moderate temperatures. Measured Brunauer-Emmett-Teller (BET) surface areas of the foams are low (1-2 m2/g) but may be increased by adding a high-area washcoat, such as γ-Al2O3. Typically, a dilute slurry containing the washcoat precursors (such as boehmite, with binders and small amounts of viscosity-modifying agents) is added to the dry foam, which is then drained and the excess slurry is removed. This is followed by calcination at moderate temperatures ( methanation > epoxidation) and effectiveness factors (epoxidation > methanation > Fischer-Tropsch). Because the purpose was to demonstrate the feasibility and advantages for the foam-supported catalysts, no attempt was made to incorporate promoters to improve activity/selectivity or to operate under optimum process conditions. (1) Ethylene Epoxidation. Ethylene epoxidation involves two main reactions:
C2H4 + 0.5O2 ) C2H4O
(∆H°r ) -105 kJ/mol)
C2H4 + 3O2 ) 2CO2 + 2H2O
(10)
(∆H°r ) -1334 kJ/mol) (11)
where reaction 11 is undesirable. The commercial catalyst is ∼10 wt % Ag/R-Al2O3 in the form of pellets 3-12 mm in size packed into tubular reactors 2-5 cm in diameter, operating at 220-235 °C. The conversion per pass is limited to ∼10%, because temperature increases in excess of 30-40 °C are detrimental to both catalyst stability and C2H4O selectivity. Spheres of R-Al2O3 without added washcoat were impregnated with a silver nitrate solution, dried at 120 °C, and calcined at 450 °C. The foams were treated in the same manner, except they were rotated at a rate of 12 rpm during drying in a
Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007 4171 Table 5. Ethylene Epoxidation Rate Comparisonsa powder BET surface area (m2/g) bed density (g/cm3, bed)) Ag loading (wt %) Ag crystallite size (nm) rate of reaction (mol g-1 s-1) 0.5 atm C) 2 , 225 °C effectiveness factor rate at 1 atm (mol s-1 cm-3, bed)) a
1.37 9.90 8.9 1.4 × 10-6 1
1.8 mm spheres
foam
1.37 1.06 9.90 8.9 1.3 × 10-6
1.66 0.60 12.1 7.2 1.5 × 10-6
1 1.38 × 10-6
1 0.90 × 10-6
Data taken from ref 19.
Table 6. Comparison of Simulation Results of a Commercial Reactor Tube (Same Conditions as Figure 6) Loaded with Commercial Catalysts and Ceramic Foam Structuresa property
pellet bed
foam bed case 1
foam bed case 2
Ag loading (%) C2H4 inlet (mol/h) C2H4 conversion (%) λe (W/m) Rw (W/m2) maximum center temp (°C) maximum radial gradient (°C) inlet temperature for runaway productivity ((kg of C2H4)/h)
10 489 9.3 3.26 732 269 20 247 1.38
12.5 489 9.3 28.4 336 251 2 264 1.38
22.5 489 14.8 28.4 336 263 5 247 2.21
a
Data taken from ref 58.
Table 7. Carbon Dioxide Methanation Rate Comparisonsa
wt % Ru BET surface area (m2/g) Ru crystallite size (nm) rate of reaction (mol s-1 g-1) 0.2 atm CO2, 225 °C effectiveness factor rate (mol s-1 cm-3, reactor) a
Figure 8. Simulation of an epoxidation commercial reactor with a foam catalyst. Reactor: 4 cm in diameter, 12 m in length; catalyst: 10% Ag/RAl2O3, 5-mm rings. Inlet conditions: 46% C2H4, 4% CO2, 11% O2, and 39% CH4; gas hourly space velocity, GHSV ) 2890; 242 °C; 20 atm (from ref 58).
microwave oven for 1 min to ensure uniform deposition throughout the foam. Composition and structure of the samples were confirmed by X-ray diffraction. Rate measurements were made under differential flow conditions (C8 a
powder
1.8-mm pellets
foam
20 183 8.0 6.21 × 10-3
20 183 8.0 3.93 × 10-3
20 33.0 6.4 9.33 × 10-3
1
0.63 6.13 × 10-3
∼1 5.97 × 10-3
54 41 5
52 47 1
24 71 5
Data taken from ref 19.
indicating potentially larger productivity for reactors. Model studies on this system are in progress and will be reported later. (3) Fischer-Tropsch Synthesis. This is the most demanding example, because of the complexity of both the catalyst and the reactions involved. Brown19 simulated commercial Co/γAl2O3 Fischer-Tropsch catalysts by impregnating 1.8-mm γ-Al2O3 spheres with a cobalt nitrate solution to give 20 wt % cobalt after drying at 105 °C and calcining at 350 °C. To match this, the surface area for the foam should be ∼40 m2/g, which requires a washcoat with a high surface area. After experimenting with several alternative approaches, a coprecipitation procedure was adopted that gave the highest surface area at 20 wt % cobalt. XRD showed the presence of Co3O4 and γ-Al2O3. The powder was slurried with water and foam samples were washcoated with repeated cycles to give a washcoat loading of 17 wt %, corresponding to a surface area of 33 m2/g. Rate comparison test results (Table 8) showed the 1.8-mm pellets with an effectiveness factor of 0.63 and the foam successfully reproducing the volume activity of the pellets with an increase in selectivity to higher hydrocarbons. This example demonstrates that complex reaction chemistry may be successfully incorporated into the foam structure. Future studies will examine the effect of improved heat transfer. These examples demonstrate the advantages of ceramic foam supports for different types of exothermic reactions. For endothermic processes, an important example is the production of synthesis gas with steam or carbon dioxide reforming of methane. (4) Methane Reforming. Richardson et al.59 demonstrated the kinetic superiority of ceramic foams for dry reforming of methane,
CH4 + CO2 / 2CO + 2H2
(13)
using 3.2-mm, 0.5-wt % Rh/γ-Al2O3 spheres and 0.5 wt % Rh/ 30-PPI R-Al2O3 foam with γ-Al2O3 washcoat. Rates were measured in a differential reactor and converted to turnover frequencies using rhodium surface areas measured by H2 chemisorption. The results in Figure 9 clearly indicate that the turnover frequencies of rhodium on the foam are ∼10 times larger than those on the pellets, as anticipated from a high effectiveness factor that was possible with the foam. Methane steam reforming is a major industrial process that proceeds via reaction 14:
CH4 + H2O ) CO + 3H2
(14)
The results of a 2-D simulation comparing commercial pellets and equivalent foam supports is shown in Figure 10 for typical industrial conditions.60 The foam catalyst reaches maximum equilibrium conversion within approximately one-half the tube
Figure 9. Comparison of turnover frequencies for 30 PPI R-Al2O3 foam and pellet catalysts (from ref 59).
Figure 10. Simulation of a methane steam reforming reactor tube under industrial conditions (from ref 60). Table 9. Simulation Comparison between a Conventional Packed-Bed Reformer and a Ceramic Foam Supporta property
conventional
foam
tube length for reaction effectiveness factor at outlet average heat flux pressure drop
916 cm 0.05 31.2 kW m2 1.21 atm
439 cm ∼1 67.1 kW m2 0.14 atm
a Data taken from ref 10. Conditions were as follows: tube diameter, 10 cm; CH4 flow, 1500 mol/h; H2O/CH4 ratio ) 3; wall temperature, 800 °C; inlet temperature, 550 °C; pressure, 20 atm; convention catalyst, multihole cylinders; ceramic foam, 30 PPI R-Al2O3.
length of conventional catalysts, and temperature gradients in the front of the bed are considerably reduced. This is a consequence of improved radial heat transfer and, to a lesser extent, a higher effectiveness factor, which provides a faster approach to equilibrium. These results support earlier predictions reported by Twigg and Richardson,10 using a 1-D model (Table 9). Simulations have also been reported by Thurgood et al.61 and Reitzmann et al.62 for methanol-steam reforming and o-xylene oxidation, respectively, with similar results. These experiments and simulations confirm the inherent advantages of ceramic catalyst supports. Other examples from the literature agree with this observation and are summarized in the following section. Other Process Applications Most reported applications of ceramic catalyst supports are concerned primarily with the chemistry that occurs on the strut surfaces rather than the transport properties previously outlined. They fall into the following classifications: short-contact-time reactions, catalytic combustion, fuel processing, solar methane reforming, and miscellaneous processes.
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Short-Contact-Time Reactions. An extensive series of papers have appeared on a special application of ceramic foams.63-99 Unlike the previous applications that enhance the performance of already existing processes, these papers, which are inspired by Schmidt and co-workers, explore the chemistry at the strut surface and point the way to novel processes. As part of a general study on structured supports (foams, honeycomb monoliths, and other structures), a wealth of information has developed on the chemistry of important reactions at short contact times (on the scale of milliseconds), facilitated by lowpressure drop at high flow rates. This is desirable for reactions in which desired intermediates are generated during consecutive reactions. The studies generally involve partial oxidation63-79 and oxidative dehydrogenation80-99 (i.e., reactions with oxygen under rich conditions that are exothermic and produce fuels and olefins, respectively). In the first type of reaction, CH4 reacts with limited O2, as in reaction 15: O2
O2
CH4 98 CO + 2H2 98 CO2 + 2H2O
(15)
Normally, CO and H2 are produced first and are then converted to CO2 and H2O, consuming all the O2. The unconverted CH4 is subsequently reformed via reactions 13 and 14 back to CO and H2. At contact times on the order of milliseconds, Schmidt et al. measured extremely high selectivities for CO and H2, which they believe are the primary products. Although there is some controversy about this observation, they clearly demonstrated the advantage of the ceramic foam catalysts, not only in providing the short contact time but also in reducing the reactor to a fraction of its normal size. This is especially true when the concept is applied to higher hydrocarbons,73-76 alcohols,77 and biofuels,79,99 where the incentive is to develop small-scale reformers for liquid hydrocarbons and renewable feeds for fuel-cell hydrogen production. The objective of oxidative dehydrogenation80-99 is the production of valuable olefin feedstock from alkanes through extremely low-contact reactions, such as that in reaction 16,
1 i-C4H10 + O2 ) i-C4H8 + H2O 2
(16)
while not permitting oxidation of the alkene product. Alkenes from C2-C4 have been formed over ceramic foams at millisecond contact times during autothermal operation with high alkene selectivity (70%) and alkane conversion (70+%), without carbon formation using catalyst beds 10-100 times smaller than those observed in current technologies. This work, in conjunction with reaction modeling, resulted in greater insight into chemical mechanisms for these reactions, including the role of surface-assisted homogeneous reactions. Catalytic Combustion. An early use for ceramic foams was gas-burner enhancement, where the turbulent mixing of air and fuel and improved radiation within the structure gives very uniform combustion characteristics for flames.6,7,9 By adding catalytic components to the foam, combustion occurs at lower temperature, thereby decreasing NOx emissions. Catalytic combustion using a wide range of ceramic foams and catalytic agents have reinforced these concepts.100-107 Two applications are apparent: (1) for process heating up to ∼1000 °C, high radial heat-transfer rates with foams decrease the size of the catalyst beds, and (2) for gas-turbine applications, the adiabatic bed has better mixing than its honeycomb counterpart and there is less possibility of the development of hot spots.107 Thermal
shock and size issues should be the same, because identical materials are used for both foams and monoliths. Fuel Processing. Many of the features previously discussed suggest that ceramic foam catalysts could fill the niche required for producing hydrogen for fuel-cell applications, either for onboard vehicle processing or at hydrogen refueling stations. Their efficiency in reforming a wide range of hydrocarbons has been demonstrated, with enhanced heat transfer allowing a reduction in reactor size, thereby permitting compact units. This has been demonstrated in several studies, using both metal and ceramic foams for alcohol and hydrocarbon reforming, water gas shift conversion, and selective CO oxidation.108-113 Solar Methane Reforming. An interesting application of ceramic foams has been solar methane reforming with CO2.114-118 The concept is to use the endothermic reaction (eq 13) to absorb heat from a concentrated solar flux, producing CO and H2 that can be stored and later used to generate heat via reaction 17:
CO + 3H2 ) CH4 + H2O
(17)
with methane that is formed being recycled in a closed system. Most applications envisage a parabolic dish collector with a ceramic foam absorber receiver (reactor) at the focal point. The receiver comprises a thin, but wide, dish-shaped ceramic foam loaded with reforming catalyst (rhodium) located in a metal reactor with a quartz window. This, indeed, is a true test of the pre-shaping feature of ceramic foam supports, as well as operation under cyclic and transient conditions. The ceramic foam catalysts performed well throughout many trials. Miscellaneous Processes. Other processes involving ceramic foam catalysts that have received recent attention include selective methanol oxidation,119 methanol to olefins,120 diesel exhaust gas,121 the destruction of chlorinated hydrocarbons by steam refoming20 and photocatalysis,122 and countercurrent twophase flow reactors.123 Commercial Aspects Ceramic foam-based catalysts have many advantageous properties that can be exploited in industrial applications. In addition to the research work referenced in this paper, there have been plant trials involving relatively small and quite large ceramic foam-based catalysts. In several instances, the catalytic performance was encouragingly good; however, as yet, there seems to be no full-scale use of these catalysts. Here, we explore why foam-based catalysts have not yet replaced traditional catalysts in two important areas: monolithic autocatalyst applications and processes using conventional pellets, granules, or small extruded catalysts. Monolithic Autocatalysts. The development of catalysts for controlling exhaust gas emissions from cars began in earnest in the early 1970s, following the U.S. Clean Air Act of 1970. At first, an oxidation catalyst was used alone, then two-stage reduction/oxidation systems were fitted that, in turn, were replaced by single three-way catalysts (TWCs).124-127 Much later, diesel-powered cars were fitted with oxidation catalysts. Initially, some companies used traditional pellet catalysts in flat, relatively thin reactors, in which conversion was limited by gas bypass. Loose pellets in these reactors suffered attrition problems, because of gas pulsations and vehicle vibrations. Other companies experimented with ceramic and metal foil monolithic honeycombs that were washcoated with a thin layer of high-surface-area catalyst. Finally, extruded cordierite monoliths gained prominence, after extrusion die technology had been mastered.128,129 At this time, ceramic foam had been introduced
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for filtering liquid metal in casting aluminum and its alloys, and other potential applications were sought. By the early 1980s, there was a flurry of activity seeking auto catalyst applications for ceramic foams. Washcoating was used, as with extruded ceramic monoliths, to overcome the low surface area of the ceramic foam. The resulting catalyst had high activity; however, by then, extruded monolithic honeycomb technology was wellestablished, so the introduction of foam-based autocatalysts was hindered by several factors, including the following: (1) Extruded cordierite monoliths were well-established and were difficult to displace. (2) Ceramic foam was brittle and did not seem to have the strength and durability of extruded ceramic monoliths. (3) The importance of providing foam catalyst with an outer solid skin to facilitate incorporation into a metal can was not properly appreciated at an early stage. (4) Foam structures of similar dimensions as an extruded monolith had higher backpressure. (5) The commercial introduction of ceramic foam catalyst was thought to be relatively costly; because the manufacturing process was complex and expensive, new investment was required to manufacture it. Recently, there has been much research on the use of ceramic foam catalyzed-soot filters for diesels.130-133 Again, it is necessary to displace existing well-established products that generally have a lower backpressure, and, more importantly, the filtration efficiency of ceramic foam is less than that of a ceramic wall-flow filter. For ceramic foam catalysts to enter the autocatalyst area, development work on very large cartridge-like units (e.g., 14 cm × 15 cm) that do not crack during thermal cycling is necessary. They should also have a ceramic skin to enable them to be canned in the same way that cordierite monoliths are today. Moreover, it would be necessary to select the most appropriate pore density and catalyst length carefully to achieve satisfactory catalyst internal surface area while not having too high a backpressure. Process Catalysts. Ceramic foam catalysts can have advantages over their traditional counterparts in the process chemical industry, especially in situations that involve strongly exothermic or endothermic reactions, or where an open pore structure helps to control selectivity. To enter mass production, the cost of manufacturing foam catalyst pieces must be comparable to catalyst costs for traditional pellets, larger Raschig rings, and multihole pellets. It seems likely that an automated robotic process will be needed to have an efficient production process. Ceramic foam catalysts can be quite brittle, and breakage during manufacture and loading into a reactor could cause potential problems; however, this could be overcome by having a continuous outer protective ceramic skin (for example, on a cylinder-shaped foam catalyst that has a skin either on the outside perimeter or on the top and bottom faces). Finally, appropriate reactors must be used to take full advantage of the special properties of the ceramic foam, and new reactor designs may well be required. For example, with highly exothermic or endothermic reactions, heat and mass transport into and out of a foam catalyst is better than its conventional counterpart; however, in a practical situation, using a foam catalyst may just result in different limitations restricting the rate of the overall process, so the full benefit of the foam catalyst is not realized. An example might be a strongly endothermic process using thick-walled multitubular reactors. Replacing traditional catalysts with foam catalysts shifts the limitation to heat transfer through the reactor wall, rather than
within the tubes to the catalyst surface; therefore, only a proportion of the ceramic foam catalyst benefit is realized. Complications are possible when long lengths of preformed ceramic foam are used. There could be a practical problem with charging and discharging reactor tubes that are never made perfectly straight, and, in use, this can become extenuated. For instance, bowed and distorted tubes are common in reactors such as steam reformers. This may not be as serious a problem in exothermic processes, where temperatures are generally much lower than those in a steam reformer; however, it would be better to have new reactor designs without inherent difficulties. Conclusions Recently, there have been a considerable number of studies on the fundamentals of fluid transport in ceramic foam catalyst supports. This has led to a greater understanding of pressure drop and heat transfer in ceramic foam beds and has resulted in correlations that, although still empirical, are useful in designing reactors to take advantage of the unique properties of the foam. A large amount of research on applying ceramic foam supports to important reactions has also been reported. The most noteworthy reports fall into two categories: those that benefit from a low-pressure drop at high flow rates, so that consecutive reactions can be run at short contact times to optimize yields of valuable intermediates, and those that are highly endothermic or exothermic and are normally limited by heat transfer. Techniques for successfully loading active components onto ceramic struts have been demonstrated, and simulation studies have verified the considerable advantages that could be gained over conventional processes. One unique property of the foam is transparency, i.e., the ability for high-intensity light to penetrate deep into the structure through radiation from one strut to another. This has been exploited in two applications: the absorption of concentrated solar radiation to drive endothermic reactions that essentially store the solar energy, and use of photocatalytic reactions at lower temperatures to destroy contaminants such as chlorinated hydrocarbons in water. Apart from small applications, such as on-board fuel processors for fuel-cell vehicles, these research achievements have not been applied to replace traditional catalysts in large-scale commercial operations, partly because of resistance to untried innovations and partly because foams may not overcome the advantages of monoliths (e.g., lowest pressure drop) for certain applications. Clearly, additional development work is needed. Ceramic foam catalysts must be manufactured economically, using new production techniques, and must be robust and durable to withstand the rigors of real world use. In addition, special features must be devised to make the foams compatible with process reactors. However, it is also possible to exploit foam advantages by designing new types of reactors or use reactors that operate on different principles. Acknowledgment We are grateful to S. Brown, H. Gadali, and W. Quon for providing unpublished results. Literature Cited (1) Moulijn, J. A.; Makkee, Ir. M.; van Dieper, A. E. Chemical Process Technology; Wiley: New York, 2001. (2) Cybulski, A., Moulijn, J. A., Eds. Structured Catalysts and Reactions; Marcel Dekker: New York, 1998. (3) Cybulski, A., Moulijn, J. A., Eds. Structured Catalysts and Reactions, Second Edition; CRC/Taylor and Francis: Boca Raton, FL, 2006.
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ReceiVed for reView August 24, 2006 ReVised manuscript receiVed January 8, 2007 Accepted January 9, 2007 IE061122O
