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MATERIALS AND INTERFACES Structural and Mechanical Properties of Nanostructured Granular Alumina Catalysts G. Buelna,†,‡ Y. S. Lin,*,‡ L. X. Liu,§ and J. D. Litster§ Department of Chemical Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0171, and Department of Chemical Engineering, University of Queensland, St. Lucia, Brisbane, Queensland 4072, Australia
Granular γ-Al2O3 support and 8 wt % CuO/γ-Al2O3 catalyst were synthesized by a sol-gel granulation method. The pore structure, crush strength, hardness, and elasticity of these solgel-derived catalysts were studied and compared with similar commercial catalysts prepared by non-sol-gel methods. Alumina and CuO-coated alumina granular particles prepared by different methods have different macro- and microstructure. The sol-gel-derived granular γ-alumina and CuO-coated γ-alumina granular particles have a structure defined by compact packing of uniform, nanosized γ-alumina crystallites. They are characterized by a more uniform pore size distribution and larger surface area as compared to similar commercial samples with a structure defined by packing of aggregates consisting of nonuniform γ-alumina crystallites. Because of the differences in the macro- and microstructure, the sol-gel-derived granular samples offer higher crush strength and greater hardness than the commercial samples. Introduction Most research on heterogeneous catalysis is focused on the catalytic activity, selectivity, and chemical stability of a catalyst. Although mechanical properties of granular catalysts are also important to many industrial catalytic reaction and adsorption separation processes, they have received relatively less attention.1,2 Important mechanical properties of a granular catalyst include its crush strength and attrition resistance. In a fixed-bed process, poor catalyst strength may result in crushing of catalyst pellets, causing poor distribution of fluid flow and a large pressure drop through the reactor. The catalyst mechanical properties are particularly critical in fluidized- or moving-bed processes. For example, the Department of Energy (DOE) developed a moving-bed copper oxide process for flue gas desulfurization (using a γ-alumina-supported CuO catalyst) during 19701980.3,4 However, the poor mechanical strength of the catalyst available at the time presented a major problem for the practical application of the DOE copper oxide process.3 The catalyst used did not have sufficiently high attrition resistance, which impeded the operation of the moving-bed process and substantially increased the operational costs of the copper oxide process. The mechanical properties of a catalyst are determined by the structure of the catalyst support granule, which, for any given material, is controlled to a large extent by the method of granulation. γ-Al2O3 is one of * Corresponding author. Tel: ++ 1 513-556-2761. Fax: ++1 513-556-3473. E-mail:
[email protected]. † Current address: Monitoring and Environmental Characterization Department, Sandia National Laboratories, P.O. Box 5800 MS 0755, Albuquerque, NM 87185-0755. ‡ University of Cincinnati. § University of Queensland.
the most commonly used catalyst support materials. For applications as a catalyst or adsorbent in moving- and fluidized-bed reactors, the alumina granules are preferred in spherical form because this geometry minimizes attrition. These spherical granular γ-Al2O3 supports are produced by traditional methods such as tumble growth or snowball.5 Most granular alumina catalyst supports of 1-5 mm in diameter available in the market do not offer sufficiently high mechanical strength and attrition resistance for moving- or fluidized-bed applications. Thus, the development of an alumina catalyst support with good mechanical properties is of practical importance for many industrial processes. Recently, Lin and co-workers6-8 developed a sol-gel process to prepare γ-alumina granular support and catalyst. These alumina granules, which could be produced from various precursors9 and in a continuous fashion,10 have a high surface area and excellent mechanical strength. In the present work, we further study the mechanical properties and microstructure of the sol-gel-derived alumina granules and, for comparison, similar alumina granules available from two commercial sources (Alcoa and UOP). This paper reports the relationship between the granule structure and its mechanical properties. Experimental Section Spherical γ-Al2O3 granules of 2-5 mm in diameter were prepared by the sol-gel oil-drop method, as described in our previous publication.10 First, 2 M stable boehmite sols were synthesized from hydrolysis and condensation of aluminum tri-sec-butoxide, ALTSB (Alfa Aesar, Mw ) 246.32, 95% purity), using the Yoldas process.11 A total of 100 mL of 2 M stable boehmite sol
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was mixed with 20 mL of 1 M HNO3, heated at 70-75 °C, and stirred until the mixture became so viscous that it could not be stirred even with the magnetic stirrer at maximum power. This partially gelled sol was transferred to droppers for the generation of sol droplets of about 5 mm in size. The sol droplets fell through a paraffin oil (mineral oil, from Fisher Scientific, 0.7864 g/cm3 density, and 34.6 cSt kinematic viscosity at 40 °C), forming spherical wet gel particles due to the surface tension. The wet-gel particles then fell into an ammonia solution (10% or 15% NH3) placed underneath the paraffin oil. The structure of the wet-gel particles was strengthened after aging in the ammonia solution for 1 h. The particles were then withdrawn with the flow of an ammonia solution and collected with a sieve. Spherical γ-Al2O3 granules were obtained after these wet particles were dried in air at 40 °C for 48 h and calcined in air at 450 °C for 3 h. Spherical CuO-coated γ-Al2O3 catalyst granules (with ∼8 wt % CuO) were prepared by the one-step solsolution mixing method. Stable 2 M boehmite sol was mixed with a Cu(NO3)2 solution at a predetermined concentration according to the required copper oxide loading. After aging for 0.5 h at 70 °C, the copper nitrate doped boehmite sol reached the gelation point and was dropped into the paraffin oil layer. The wet-gel granules then fell into an ammonia solution containing the same Cu(NO3)2 concentration as the granules to avoid leaching of Cu(NO3)2 from the granules into the ammonia solution. After gelation and aging in the ammoniacupric nitrate solution for 1 h, sufficient copper diffused into the particles. The gel particles were then dried in the same way as that described above for the pure γ-Al2O3 granules. The precursor of the active species, Cu(NO3)2, was coated on the surface of the boehmite primary particles and converted to CuO during a calcination step at 550 °C for 6 h. For comparison with our sol-gel-derived catalyst, we also studied two commercially available γ-Al2O3 and CuO/γ-Al2O3 granules, SOX-3 from UOP and H-156 from Alcoa. These UOP and Alcoa CuO/γ-Al2O3 catalysts contained respectively 6.6 and 9.4 wt % of CuO. The commercial samples were sulfated prior to shipping and contained 3 wt % of sulfur. The Pittsburgh Energy Technology Center (PETC, now National Energy Research Center) and the Institute of Gas Technology (IGT) provided the UOP and Alcoa catalysts, respectively. The UOP SOX-3 and Alcoa H-156 CuO/γ-Al2O3 catalysts were selected as the best catalysts respectively for PETC’s moving-bed copper oxide desulfurization process12,13 and the copper oxide bed regenerable application (COBRA) project in Carbondale, IL.6 The preparation methods for these commercial catalysts were not disclosed. It is possible that the commercial γ-Al2O3 granules were prepared by a traditional granulation method and the CuO/γ-Al2O3 granular catalysts by wet impregnation of copper species on the γ-Al2O3 granules. The Brunauer-Emmett-Teller (BET) surface area, pore volume, pore size distribution, and average pore diameter of the samples were measured by nitrogen adsorption porosimetry (Micromeritics ASAP-2000). The BET surface area and pore volume were calculated from the adsorption isotherm. The pore size distribution was obtained from the desorption isotherm. The average pore diameter was calculated from the pore size distribution data. The copper and sulfur contents of the
Table 1. Pore Structure and Crush Strength of Granular γ-Al2O3 and CuO/γ-Al2O3 BET average crush surface pore pore bulk strength CuO area volume diameter density (N/granule) ( sample wt % (m2/g) (cm3/g) (Å) (g/cm3) std dev Alcoa UOP sol-gel
9.4 0 6.7 0 8.0 0
204 300 99.0 184 248 318
0.23 0.43 0.50 0.78 0.45 0.49
46 48 208 170 73 61
0.76 0.65 0.69 0.53 0.83 0.76
36 ( 8 28 ( 11 40 ( 15 33 ( 12 125 ( 19 128 ( 24
samples were determined by inductively coupled plasma (ICP; Thermo Jarrell Ash ICAP 1100) after digesting the granules in nitric acid. The crystalline structure of powdered sorbents and the extent of dispersion of the active species were examined by X-ray diffraction (XRD; Siemens D-50 with radiation of Cu KR1). A universal testing instrument (Instron 4465) was used to measure the crush strength of the individual granular particles, which was taken as the maximum load applied to break the granule. To measure mechanical properties of the granules such as hardness and Young’s modulus, several particles were mounted in an epoxy resin and polished to the widest cross-sectional area with a diamond paste of 6 and 2 µm in size. The polished surfaces of the samples were indented with a nanoindenter (UMIS 2000). The cross section of a granule undergoing the same preparation was observed by an optical microscope (Nikon Optiphot-100). The surface of the sorbents was observed with a scanning electron microscope (SEM; Hitachi S-4000, 20 kV). For this analysis, several granules were mounted into an aluminum holder using silver paste and vacuum-dried. Then the surface of the samples was coated by physical vapor deposition with a 60% gold/ 40% palladium alloy weld and covered by a graphite paste. Results and Discussion The pore structure, bulk density, and crush strength of sol-gel γ-Al2O3 and 8 wt % CuO/γ-Al2O3 granular catalysts are compared in Table 1 to those of the two as-received commercial catalysts, Alcoa and UOP. The γ-Al2O3 and CuO/γ-Al2O3 sol-gel-derived samples exhibit a larger surface area than the commercial samples. The pore volume and pore size of the sol-gel sorbents fall within the wide range of the commercial samples. As shown in Figure 1, the nitrogen adsorption and desorption isotherms of all of the samples are of type IV, with a hysteresis loop, corresponding to mesoporous materials. The surface area and pore volume of the CuOcoated alumina granules are lower than those of the corresponding pure alumina granules, while the bulk density of the former is higher than that of the pure alumina ones. This is due to the higher density and smaller surface area and pore volume of CuO. The crush strength of the CuO-coated UOP and Alcoa granules is higher than that of the pure alumina granules, whereas the CuO coating made no significant difference in the natively stronger sol-gel granules. From the shape of the hysteresis loop, the pore morphology can be estimated. The new classification of hysteresis loops recommended by IUPAC14 consists of types H1, H2, H3, and H4. Types H1, H2, and H3 correspond to types A, E, and B, respectively, from the original de Boer classification.15,16 The shape of the
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Figure 1. Adsorption and desorption isotherms of (a) γ-Al2O3 sorbents and (b) CuO/γ-Al2O3 sorbents. Open symbols: desorption. Solid symbols: adsorption.
hysteresis loops of all of the γ-Al2O3 samples are essentially the same as those obtained from their respective CuO/γ-Al2O3 samples, indicating that the pore morphology of the samples is not altered by the presence of CuO. The type H3 hysteresis loop of the Alcoa samples indicates narrow, slit-shaped pores associated with platelike structures. The type H1 hysteresis loop in the UOP samples denotes cylindrical-shaped pores. The type H1 loop is related to agglomerates of spheroidal particles of uniform size and array, with only a minor tendency to ink-bottle pores. The cylindrical pores of the UOP samples may indicate calcination of the particles at a high temperature (i.e., 850 °C). Through the sintering process, the size of the pores would increase and more or less cylindrical pores would be formed. The type H2 hysteresis loops of the sol-gel-derived samples indicate the presence of both narrow slit-shaped pores17 and ink-bottle pores.18 It has been reported that after drying and calcination, under conditions similar to those of this study, γ-alumina maintains the basic structure of the slit-shaped boehmite. γ-Alumina is mainly composed of slit-shaped pores with a few isolated ink-bottle pores connected by the slit-shaped pores.19,20 The corresponding pore size distributions of the samples are shown in Figure 2. The sol-gel samples have a more uniform pore size distribution, in the range of 2-10 nm, than the commercial samples. The UOP samples have much larger average pore sizes, while the Alcoa samples have the smallest average pore size with the broadest pore size distribution (ranging from 3 up to 80 nm). The divergence between the pore structure of the sol-gel-derived and commercial samples arises from the different preparation methods and possibly from the use of a binder in the commercial UOP samples. For all three samples, coating CuO lowers the surface area and pore volume but does not exhibit a consistent effect on the average pore size. The pore size distribution data show that the CuO-coated Alcoa
Figure 2. Pore size distribution of (a) γ-Al2O3 sorbents and (b) CuO/γ-Al2O3 sorbents.
Figure 3. XRD patterns of sol-gel-derived and two commercial CuO/Al2O3 sorbents.
sample has a smaller amount of the large pores as compared to the pure Alcoa sample. The XRD patterns of the CuO/γ-Al2O3 sorbents are shown in Figure 3. The XRD patterns of all of the γ-Al2O3 samples essentially mirrored samples obtained from their respective CuO/γ-Al2O3 samples. The absence of peaks corresponding to CuO in all of the samples indicates that the active species is coated on the surface of the support in a monolayer or submonolayer form. The patterns for the UOP and sol-gel samples, with peaks at about 2θ ) 37°, 47°, and 67°, are typical of γ-Al2O3. The peaks found in the Alcoa sorbent do not correspond to γ-Al2O3; instead, they are similar to those of the mineral bauxite, an ore of aluminum that consists of aluminum oxide usually mixed with impurities such as hydroxides iron and titanium oxides.21 The differences in the crystalline structure of the materials identified by XRD analysis may explain the differences found in the pore structure and morphology of the samples. The expected theoretical pore volumes of the CuO/γAl2O3 samples according to the CuO loading, calculated
Ind. Eng. Chem. Res., Vol. 42, No. 3, 2003 445 Table 2. Hardness and Elasticity Results from Nanoindentation
sample Alcoa UOP sol-gel
CuO wt %
hardness H (GPa) at 490 mN ( std dev
Young’s modulus E (GPa)
ratio E/H
0 9.4 0 6.7 0 8.0
0.090 ( 0.021 0.207 ( 0.042 0.091 ( 0.025 0.108 ( 0.02 0.497 ( 0.15 0.706 ( 0.22
5.5 6.9 3.4 2.6 7.0 9.8
60.6 32.8 35.8 23.7 14.0 13.8
from the measured pore volume of the pure alumina support and the density of CuO (6.31 g/cm3), were 0.415, 0.769, and 0.475 cm3/g, for the Alcoa, UOP, and solgel CuO/γ-Al2O3 samples, respectively. The measured pore volumes for the CuO-coated Alcoa and UOP samples are much smaller than these theoretical values, indicating that some of the CuO doped in these two commercial samples may be present as small CuO crystallites that block passage of nitrogen to the pores during nitrogen adsorption porosimetry measurements. The measured pore volume of the sol-gel-derived CuO/ alumina is very close to the theoretical value. This suggests that CuO is well dispersed and presents likely as a surface copper aluminate spinel-like phase22,23 on the internal pore surface of the sol-gel-derived alumina. Conventionally, the crush strength correlates with the pore volume or bulk density; i.e., the higher the bulk density, the higher the strength. However, this relation does not hold in the results presented in Table 1. The crush strength of a material can only be correlated to the pore volume under otherwise similar conditions. Because of the uniform structure of the sol-gel-derived granules, with their very narrow pore size distribution, they have a crush strength much higher than those of the commercial sorbents. In addition, the pore volume of the sol-gel-derived samples is only slightly higher than that of Alcoa and lower than that of UOP. Table 2 summarizes the mechanical properties such as hardness and Young’s modulus of pure γ-Al2O3 and the CuO/γ-Al2O3 sorbents described in Table 1 obtained using a nanoindenter. Hardness is defined as the ability of a material to resist surface deformation or abrasion, and it is indicative of the material’s attrition resistance in an abrasive mode. The CuO-coated aluminas were found to be harder than their respective pure aluminas. The sol-gel alumina sample had 5 times the hardness of the commercial aluminas, whereas the sol-gel CuOcoated alumina sample was 3-6 times harder than the CuO-coated commercial sorbents. According to Archard’s wear mechanism,24 the abrasive wear rate is inversely proportional to the hardness of the material, which indicates that the sol-gel samples have a lower attrition rate than their commercial counterparts. The Young’s modulus measures the elasticity of a material. Young’s modulus, an important property ratio (stress over strain), relies only upon the material, not on its size or shape. The Young’s modulus of the sol-gel samples was higher than that of the commercial samples, indicating that the sol-gel-derived samples were more elastic than the commercial samples. On the basis of the conventional fracture mechanics theory, the ratio of E/H represents the rigidity of a material. The higher the E/H ratio, the less rigid the materials. The fracture toughness of the material (resistance to crack propagation) is also proportional to E/H to the power of 1/2. For all of the materials, the CuO/
Figure 4. Optical photographs of cross-sectional areas of CuO/ γ-Al2O3 sorbents: (a) Alcoa; (b) UOP; (c) sol-gel (particle diameter ) 2 mm).
γ-Al2O3 samples had a lower E/H ratio than the γ-Al2O3 samples, indicating a higher rigidity and lower resistance to crack propagation. As shown in Table 2, the sol-gel-derived samples had lower E/H ratios than the commercial samples. This made the sol-gel samples more rigid and less resistant to crack propagation than the commercial ones, if there were preexisting cracks or flaws in the material. However, as discussed later, the sol-gel-derived granules have a tightly packed structure that is less prone to crack formation than the commercial granules. The above results are related to the microstructure of catalysts prepared by different methods and their mechanical properties. These three alumina granules also are different in macrostructure. Figure 4 show optical photographs of the cross-sectional views of these three granules. The Alcoa sample is formed by concentric layers, whereas the UOP and sol-gel sample appear as a uniform structure. The layers displayed in the Alcoa sample may form during the formation of the alumina granules (for example, by the tumble growth method). The synthesis method for the UOP sample was not disclosed. The uniform structure of the sol-gel sample is obviously related to its unique granulation process, which results in a monolithic granule consisting of nanoscale alumina crystallites.
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Figure 5. SEM surface pictures of CuO/γ-Al2O3 sorbents: (a) Alcoa, (b) UOP, and (c) sol-gel. The size of the white bar is 5 µm.
Figures 5 and 6 show the scanning electron micrographs of the surface of the granular particles of the three CuO/γ-Al2O3 samples. Scattering particles on the surface of the UOP and Alcoa samples are of copper oxide. No such scattering copper oxide particles are observed on the surface of the sol-gel sample. This implies that in the sol-gel sample the CuO is better dispersed in the matrix structure. A continuous network of cracks 100-200 nm wide was observed in the UOP sample. These cracks may have formed in the steps of drying and calcining during the manufacturing of the granules and could be partly responsible for the low crush strength found in the UOP sorbents. From the two-dimensional SEM pictures of higher magnification, we can infer the particle morphology of the materials. The SEM picture of the CuO/γ-Al2O3 granule surface (Figure 6) suggests that the Alcoa granule consists of fused aggregates of 0.4 µm size, whereas the UOP granule seems to be defined by loosely packed agglomerates of 0.1-0.6 µm size. The sol-gel CuO/γ-Al2O3 surface exhibits a morphology between those of the Alcoa and UOP samples. The particles in the sol-gel sample also look well fused. The above results suggest that the structure of the alumina granule particles is defined by packing of the aggregates consisting of many smaller γ-alumina crystallites. The granule particles include primary pores determined by the intercrystralline space and secondary pores by the interaggregate space. The broader pore size distribution observed for the Alcoa and UOP samples indicates nonuniformity of γ-alumina crystallite particles and a significant contribution of the secondary
Figure 6. SEM surface pictures of (a) Alcoa, (b) UOP, (c) sol-gel CuO/γ-Al2O3 sorbents.
pores. The more uniform pore size distribution in the range of 2-5 nm for the sol-gel-derived alumina granules is a result of uniform γ-alumina crystallites and negligible contribution of the secondary pores. In a sol-gel process, nanoscale primary particles of uniform size are formed in the solvent and are connected to each other to form aggregates during gelation. After drying, the capillary force could cause rearrangement of the structure, resulting in close packing of the primary particles of γ-alumina crystallites. After calcination and sintering, these primary particles are bound together to form a very stiff solid network, and a large intraparticle space with uniform nanoscale pores is formed. As a result, the sol-gel-derived granules have a tightly packed structure with pores highly interconnected, as indicated by their high bulk density, narrow pore size distribution, and large surface area. These macro- and microstructures explain why the sol-gel-derived granules have very high crush strength and hardness compared with the commercial samples, even though the pore volume of the sol-gel-derived samples is slightly higher than that of the Alcoa granules. Conclusions Alumina and CuO-coated alumina granular particles prepared by different methods have different macro- and microstructure that, in turn, affect the mechanical properties of the granules. The sol-gel-derived γ-alu-
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mina and CuO-coated γ-alumina granular particles have a structure defined by close packing of uniform γ-alumina crystallites. They are characterized by more uniform pore size distribution and larger surface area as compared to the commercial samples. The structure of similar alumina and CuO-coated alumina granules obtained from the commercial sources is defined by packing of aggregates consisting of nonuniform γ-alumina crystallite particles. Because of the differences in the macro- and microstructure, the sol-gel-derived granular samples offer higher crush strength and greater hardness than the samples commercially available. It is also found that CuO-coated alumina granules exhibit better mechanical properties than the uncoated alumina granules. Acknowledgment The authors acknowledge C. Ponce de Leo´n and N. Mukherjee for their assistance in performing the ICP and SEM analysis. This work was supported by the Ohio Coal Development Office of the Ohio Department of Development through Grant OCRC/97-A-2.7. Literature Cited (1) Andrew, S. P. S. Theory and Practice of the Formulation of Heterogeneous Catalysts. Chem. Eng. Sci. 1981, 36, 1431. (2) Li, Y. D.; Chang, L. Optimizing the Mechanical Strength of Fe-based Commercial High-temperature Water-gas Shift Catalyst in a Reduction Process. Ind. Eng. Chem. Res. 1996, 35, 4050. (3) Yeh, J. T.; Demski, R. J.; Strakey, J. P.; Joubert, J. I. Combined SO2/NOx Removal from Flue Gas. Detailed Discussion of a New Regenerative Fluidized-Bed Process Developed by the Pittsburgh Energy Technology Center. Environ. Prog. 1985, 4, 223. (4) Yeh, J. T.; Drummond, C. J.; Joubert, J. I. Process Simulation of the Fluidized-Bed Copper-Oxide Process Sulfation Reaction. Environ. Prog. 1987, 6, 44. (5) Pietsch, W. Successfully Use Agglomeration for Size Enlargement. Chem. Eng. Prog. 1996, 92, 29. (6) Deng, S. G.; Lin, Y. S. Granulation of Sol-Gel-Derived Nanostructured Alumina. AIChE J. 1997, 43, 505. (7) Wang, Z. M.; Lin, Y. S. Sol-Gel Synthesis of Pure and Copper Oxide Coated Mesoporous Alumina Granular Particles. J. Catal. 1998, 174, 43. (8) Wang, Z. M.; Lin, Y. S. Sol-gel-derived Alumina-Supported Copper Oxide Sorbent for Flue Gas Desulfurization. Ind. Eng. Chem. Res. 1998, 37, 4675.
(9) Buelna, G.; Lin, Y. S. Sol-Gel-Derived Mesoporous γ-Alumina Granules. Microporous Mesoporous Mater. 1999, 30, 359. (10) Buelna, G.; Lin, Y. S. Preparation of Spherical Alumina and Copper Oxide Coated Alumina Sorbents by Improved SolGel Granulation Process. Microporous Mesoporous Mater. 2001, 42, 67. (11) Chang, C. H.; Gopalan, R.; Lin, Y. S. A Comparative Study on Thermal and Hydrothermal Stability of Alumina, Titania and Zirconia Membranes. J. Membr. Sci. 1994, 91, 27. (12) Hedges, S. W.; Yeh, J. T. Kinetics of Sulfur-dioxide Uptake on Supported Cerium Oxide Sorbents. Environ. Prog. 1992, 11, 98. (13) Yeh, T. Private communications, 1997. (14) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Reporting Physisorption Data for Gas Solid Systems with Special Reference to the Determination of Surface-Area and Porosity (Recommendations 1984). Pure Appl. Chem. 1985, 57, 603. (15) Broekhoff, J. C. P. In Studies in Surface Science and Catalysis 3. Preparation of Catalysts II; Delmon, B., Grange, P., Jacobs, P., Poncelet, G., Eds.; Elsevier Scientific: New York, 1979; p 663. (16) Gregg, S. G.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: San Diego, CA, 1982. (17) Lippens, B. C. Structure and Texture of Aluminas. Ph.D. Thesis, Delft, University of Technology, The Netherlands, 1961. (18) de Boer, J. H. In Structure and Properties of Porous Materials; Everett, D. H., Stone, F. S., Eds.; Butterworth: London, 1958. (19) Leenaars, A. F. M. Preparation, Structure and Separation Characteristics of Ceramic Alumina Membranes. Ph.D. Thesis, University of Twente, Enschede, The Netherlands, 1984. (20) Duisterwinkel, A. E. Clean Coal Combustion with In-situ Impregnated Sol-gel Sorbents. Ph.D. Thesis, Delft University of Technology, Delft, The Netherlands, 1991. (21) la Lande, W. A.; McCarter, W. S. W.; Sanborn, J. B. Bauxite as a Drying Adsorbent. Ind. Eng. Chem. 1944, 36, 99. (22) Centi, G.; Hodnett, B. K.; Jaeger, P.; Macken, C.; Marella, M.; Tomaselli, M.; Paparatto, G.; Peratoner, S. Development of Copper-on-alumina Catalytic Materials for the Cleanup of Flue Gas and the Disposal of Diluted Ammonium Sulfate Solutions. J. Mater. Res. 1995, 10, 553-561. (23) Friedman, R. M.; Freeman, J. J.; Lytle, F. W. Characterization of Cu/Al2O3 Catalysts. J. Catal. 1978, 55, 10-28. (24) Hutchings, I. M. Tribology: Friction and Wear of Engineering Materials; CRC Press: Boca Raton, FL, 1992.
Received for review April 8, 2002 Revised manuscript received November 18, 2002 Accepted November 29, 2002 IE020259L
