Catalysts Prepared by Chemical Vapor Deposition - American

Department of Chemical Engineering, Tunghai University, Taichung, Taiwan, ROC. Ti/SiO2 samples with different titanium contents were prepared by chemi...
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Epoxidation of Propylene on Ti/SiO2 Catalysts Prepared by Chemical Vapor Deposition Kuo-Tseng Li* and I-Chun Chen Department of Chemical Engineering, Tunghai University, Taichung, Taiwan, ROC

Ti/SiO2 samples with different titanium contents were prepared by chemical vapor deposition (CVD) of TiCl4 on silica gel in the temperature range 400-1100 °C. The resulting samples were used to catalyze the reaction of propylene with tert-butyl hydroperoxide (TBHP) to propylene oxide (PO). It was found that deposition temperature of TiCl4 had a strong effect on the catalyst selectivity but had small effect on the catalyst activity. With the increase of CVD temperature, the yield of propylene oxide increased rapidly and passed through a maximum at 900 °C, which had PO yields of 92.7-94.4% in the Ti content range 0.89-1.07 wt %. XPS data indicated that Si-OTiCl3, formed from the reaction between TiCl4 and a silanol site or a surface siloxane bridge site, was the active species for catalyzing the epoxidation. The increase of epoxide yield with increasing deposition temperature was explained in terms of the decrease of the remaining silanol groups, which exhibited an undesired Bronsted acid property. ICP and BET measurements indicated that the titanium concentrations on the Ti/SiO2 catalysts were between 1.1 and 1.36 µmol/m2 and suggested that almost all isolated Si-OH groups were converted to Si-O-TiCl3 species at the deposition temperature of 900 °C. The propylene epoxidation rate on the Ti/SiO2 catalyst was determined to be first order in TBHP with an activation energy of 36.4 kJ/mol. Introduction Propylene oxide is an important chemical intermediate. It is produced commercially either by chlorohydrin or organic hydroperoxide processes.1,2 The latter has more ecological and economical benefits than the former and accounts for more than one million tons annual production of propylene oxide worldwide.3-5 The hydroperoxide process involves the reaction of propylene with alkyl hydroperoxide (C3H6 + ROOH f C3H6O + ROH, where ROOH is either tert-butyl hydroperoxide or ethylbenzene hydroperoxide), catalyzed by homogeneous molybdenum compounds or heterogeneous titanium silica (Ti/SiO2) catalyst.4 The commercial Ti/SiO2 catalyst was prepared by impregnating silica with TiCl4 or an organic titanium compound followed by calcination.5-8 The active catalyst contains tetrahedral TiIV chemically bonded to siloxane ligands (tSiO).5,9-11 A particularly effective catalyst was obtained when the impregnated Ti/SiO2 sample was silylated with an organic silylating agent in order to remove residual Bronsted acid due to Si-OH groups.5-7,12 Titanium silicalite-1(TS-1) has been used to catalyze the propylene epoxidation by H2O2,13 but it cannot catalyze propylene epoxidation with alkyl hydroperoxide, such as TBHP, owing to the limited size of the silicalite pore (5.6 × 5.3 Å).3 The chemical vapor deposition (CVD) method for catalyst preparation has been defined as the process of deposition using reaction between surface sites such as OH groups and vapors of metal compounds. CVD is a useful method for the preparation of highly dispersed catalysts.14 Bond and Konig15 used the CVD method (reaction of VOCl3 vapor with hydroxyl groups on the surface of anatase at room temperature) to prepare a monolayer vanadium pentoxide-titanium oxide catalyst * To whom correspondence should be addressed. Telephone: +886-4-23590262. Fax: +886-4-23590009. E-mail: [email protected].

system, which showed high selectivity in the oxidation of o-xylene to phthalic anhydride. Recently, Yang et al.16 catalyzed styrene epoxidation using Ti/SiO2 and Si/Ti/ SiO2 catalysts prepared by chemical grafting of TiCl4 on SiO2 in a temperature range of 200-500 °C. After the grafting stage, their catalysts were treated further with H2O/N2 flow at 25 °C for hydrolysis. The maximum epoxide yield obtained by Yang et al. was less than 26%. Wang et al.17 catalyzed cyclohexene epoxidation using Ti/SiO2 prepared by chemical grafting TiCl4 on deboronated silica gel at 500 °C. They obtained a maximum epoxide selectivity of 82.7% at 84% cyclohexene conversion (i.e., the best epoxide yield was less than 70%). Several studies have been carried out to investigate the reaction between TiCl4 vapor and silica. Kinney and Staley studied the reaction of TiCl4 vapor with silica using infrared photoacoustic spectroscopy and proposed that four possible reactions might occur between TiCl4 vapor and silica.18 Haukka et al. prepared Ti/SiO2 samples by deposition of TiCl4 vapor at various temperatures (175-550 °C) with preheated silica samples, and analyzed the resulting samples by chemical etching, XRD, SEM, TEM, and XPS.19 Leboda et al.20 studied the structure of titania/silica gel and Gun’ko et al.21 studied the structure of titania/fumed silica using chemical vapor deposition of TiCl4 with the silica surface and subsequent hydrolysis of residual Ti-Cl bonds. In the literature, there is no information available about the use of chemical vapor deposition (CVD) method to prepare Ti/SiO2 samples at the deposition temperature greater than 600 °C, probably because the dehydroxylation of Si-OH groups occurs at temperature above 600 °C.18-22 However, silanol groups on the surface of silica exist in several different states,23 and the stronger Si-OH bonds (isolated silanol groups) in silica can exist even at the temperature of 940 °C.24 The stronger silanol groups had an appreciably different environment from the weaker silanol groups. TiCl4 can

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also react with strained siloxane bridge (Si-O-Si) sites,18 which were formed by the removal of surface silanol groups. The siloxane bridge sites can exist at the silica surface even at 1250 °C,25 and the Si-O-Si sites formed at the heat treatment temperature above 600 °C are different from those formed at the lower heat treatment temperature.18 Therefore, the catalytic behaviors of titanium compounds bonded to these stronger Si-OH sites or these stronger Si-O-Si sites are worth being investigated for the epoxidation of propylene. This paper deals with the use of CVD method to prepare Ti/SiO2 catalysts in a deposition temperature range 400-1100 °C and deals with the use of the resulting catalysts for catalyzing the epoxidation of propylene with tert-butyl hydroperoxide. From this, we have observed that the yield of propylene oxide reached a maximum value of 94.4% for the catalyst prepared at the deposition temperature of 900 °C. Experimental Section Catalyst Preparation and Characterization. Chemical vapor deposition of TiCl4 on SiO2 was carried out in a 0.007 m I. D. quartz tube packed with 3 g silica gel powder (Strem Chemicals, Newburyport, MA). The powder had mesh sizes of 80-100 and an average pore radius of 11 nm. An electrically heated tube furnace (Lindberg/Blue M) in which the temperature could be held to within 1 °C of the desired value was used to house the CVD reactor. Before the deposition of TiCl4, silica gel powder was dried under 100 mL/min N2 at 400 °C for 1 h. After the drying stage, a gaseous feed of 0.77 vol % TiCl4 (Showa Chemicals, Tokyo, Japan) vapor in nitrogen was introduced into the CVD reactor with a flow rate of 50 mL/min for 2 h. Titanium contents of the resulting Ti/SiO2 samples were determined with an inductively coupled plasmaatomic emission (ICP-AES) spectrometer (Kontron, model S-35) after HF acid digestion of the solid. The specific surface areas of the Ti/SiO2 samples were determined by nitrogen adsorption at the temperature of liquid nitrogen with a Micrometrics BET surface area analyzer (model Gemini). The X-ray photoelectron spectroscopy (XPS) data were acquired on an ESCA 210 spectrometer (Fisons Surface Science) using Mg KR X-ray radiation. Catalytic Property Measurements. Propylene epoxidation experiments were performed in a 300 mL stirred high-pressure batch reactor (Parr 4842). In a typical experiment, 50 mL of benzene, a prescribed amount of tert-butyl hydroperoxide (Fluka, Switzerland) and Ti/SiO2 catalyst (prepared by the CVD method mentioned above) were charged to the batch reactor. Propylene at 8 kg/cm2 was introduced into the reactor at 5 °C and saturated with the reaction mixture (the amount of propylene inside the reactor was 0.4 g mol, based on the calculation from the solubility data of propylene in benzene26 and from the generalized equation of state.27) The reactor was compressed with 11 kg/ cm2 nitrogen and then heated to the desired temperature. Unless specified otherwise, the reaction temperature was 100 °C, the agitator speed was 100 rpm, and the reaction time was 1 h. The propylene oxide concentration in the product solution was analyzed with a HP5890 gas chromatograph using a 60 m DB-Waxeter column, and the concentration of TBHP was determined by iodometric analysis.

Figure 1. Influence of TiCl4 deposition temperature on TBHP conversion and propylene oxide yield.

TBHP conversion was defined as the percentage of TBHP in the feed that had reacted. Propylene oxide yield was defined as the moles of propylene oxide formed per mole of TBHP in the feed. Propylene oxide selectivity was defined as the percentage of TBHP reacted to propylene oxide (i.e., propylene oxide selectivity ) propylene oxide yield/TBHP conversion). Preliminary run was carried out to test the inertness of the reactor at 100 °C. No propylene oxide was detected and TBHP conversion was only 2% when there was no catalyst presented in the reactor. The blank run results confirmed that the reactor system did not aid in the reaction of TBHP to propylene oxide, and thermal decomposition of TBHP was negligible. Results and Discussion Influence of TiCl4 Deposition Temperature. The effect of TiCl4 deposition temperature on TBHP conversion and propylene oxide yield was investigated at a TBHP feed of 0.005 g/mol (i.e., propylene/TBHP molar ratio ) 80) and at the catalyst amount of 0.3 g. The experimental results are shown in Figure 1. For the Ti/ SiO2 catalysts with CVD temperature in the range 4001000 °C, TBHP conversion was between 90.6% and 97.4%, which was not sensitive to the change of deposition temperature. The conversion dropped to 87% for the catalyst prepared at the deposition temperature of 1100 °C which had the smallest surface area (the surface area data will be shown in Figure 7). Propylene oxide yield and selectivity were very sensitive to the change of deposition temperature. The yield was only 50% (the epoxide selectivity was 55%) for the catalyst prepared at the deposition temperature of 400 °C. With the increase of CVD temperature, the propylene oxide yield increased rapidly and passed through a maximum, as shown in Figure 1. The maximum propylene oxide yield was 93% and the best epoxide selectivity was 95%, obtained for the Ti/SiO2 catalyst prepared at the deposition temperature of 900 °C. A further increase in the deposition temperature resulted in a decrease of propylene oxide yield and selectivity. Yang et al.16 studied styrene epoxidation on Ti/SiO2 catalyst prepared at

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Figure 2. Influence of heat treatment temperature on TBHP conversion for (a) SiO2 blanks and (b) Ti/SiO2 catalysts.

Figure 3. Effect of titanium content on TBHP conversion and propylene oxide yield for Ti/SiO2 catalysts prepared at 900 °C.

grafting temperatures ranging from 200 to 500 °C. They also found that epoxide selectivity increased with grafting temperature (the best epoxide selectivity they obtained was 81.7% with styrene conversion of 26.5% for the catalyst prepared at a grafting temperature of 500 °C). Under ordinary conditions, the surface of silica is covered by silanol (Si-OH) groups and the hydroxyl group in Si-OH may behave as Bronsted acid site (SiOH + B ) SiO- + HB+).28,29 These Bronsted acid sites can catalyze the decomposition of TBHP (undesired reaction) and decrease the selectivity to epoxide.5 With an increase of deposition temperature, the number of surface silanol groups decreased due to dehydration.23 The decrease of the number of Bronsted acid sites decreased the decomposition rate of TBHP and resulted in better epoxide selectivity for the Ti/SiO2 catalysts prepared at the deposition temperature of 900 °C, as shown in Figure 1. Catalytic tests were also performed on SiO2 alone when it was exposed to the same conditions of CVD (but without the addition of TiCl4 in gas flow) that the Ti/ SiO2 catalyst was. In the presence of these heat-treated SiO2 samples, no propylene oxide was detected from the reaction of propylene with TBHP, which indicated that all propylene oxides in Figure 1 were produced solely by the catalytic action of titanium species on Ti/SiO2 catalysts. Although the heat-treated SiO2 samples did not aid the reaction of TBHP to propylene oxide, they catalyzed the decomposition of TBHP (an undesired reaction). Figure 2 compares the influence of heattreatment temperature on TBHP conversion for SiO2 blanks and for Ti/SiO2 catalysts, which shows that the latter had much higher conversion than the former. For SiO2 blanks, TBHP conversion was around 29% in the range of 400-600 °C, increased steadily to 54% at 900 °C, and then decreased sharply to 23% at 1000 °C. TBHP decomposition at Te 600 °C should be due to the hydroxyl group in Si-OH groups. The increase of TBHP decomposition in the range of 600-900 °C should be due to the strained siloxane bridge (Si-O-Si) sites, formed from the dehydration of surface silanol groups.25 The results of SiO2 blank runs suggested that the strained

siloxane bridge had higher activity than Si-OH group to catalyze the decomposition of TBHP. However, the siloxane bridge site reacted with TiCl4 during the preparation of Ti/SiO2 catalyst,18 and formed a new SiO-Ti species which was very selective for catalyzing the reaction of propylene and TBHP to propylene oxide, as shown in Figure 1. It is interesting to note that the maximum PO yield of Ti/SiO2 catalysts and the maximum TBHP decomposition rate of SiO2 blanks occurred at the same temperature (900 °C). Since the Ti/SiO2 catalyst prepared at the deposition temperature of 900 °C had the best catalytic performances, it was used for the study on the effects of titanium content, propylene/TBHP feed ratio, and for the kinetic study. Effect of Titanium Content. Ti/SiO2 catalysts with various titanium contents were prepared at 900 °C by varying the time (1-3 h) for chemical vapor deposition. The 1-h CVD sample contained 0.1 wt % Ti and the 2-h CVD sample contained 0.89 wt % Ti, which suggested that most of Ti deposition occurred after the dehydration of silica gel because silica dehydration occurred mostly within 1 h of exposing to 900 °C. Figure 3 shows the relationships between catalytic properties and Ti content for the catalysts with 0.51-1.07 wt % Ti. TBHP conversion was not sensitive to the change of Ti content. However, propylene oxide yield and selectivity were very sensitive to the change of Ti content. PO yield increased rapidly in the range of 0.51-0.89 wt % Ti and then leveled off when Ti content > 0.89 wt %. The maximum yield obtained was 94.4% for the catalyst with 1.07 wt % Ti. The increase of PO yield with the increase of Ti content should be due to the decrease of the remaining Si-OH groups (Bronsted acid sites). Effect of Propylene/TBHP Ratio. For this study, the amount of TBHP fed to the reactor was varied between 0.005 and 0.05 g/mol (i.e., the propylene/TBHP molar ratio was in the range 8-80), and the catalyst amount kept constant at 0.3 g. The decrease of propylene/TBHP ratio (i.e., the increase of the amount of TBHP fed to the reactor) decreased the TBHP conversion and the epoxide selectivity dramatically, as shown in Figure 4. The decrease of TBHP conversion should

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Figure 4. Variation of TBHP conversion and propylene oxide yield as a function of initial TBHP/propylene molar ratio.

Figure 6. Effect of catalyst amount on the reaction rate constant at 100 °C.

Figure 5. First-order kinetic plot for the reaction of propylene with TBHP at three different epoxidation temperatures.

Figure 7. Effect of TiCl4 deposition temperature on catalyst surface area.

be due to the decrease of catalyst/TBHP ratio because the reaction rate constant increased linearly with the increase of catalyst amount, as shown in Figure 6. In Figure 4, the decrease of epoxide selectivity with decreasing propylene/TBHP ratio should be due to the increase of the nonproductive decomposition of TBHP. In commercial propylene oxide processes, a high propylene/TBHP ratio is needed to suppress nonproductive decomposition of TBHP.4 Kinetic Study. For the kinetic study, the amount of TBHP fed to the reactor was 0.005 g/mol and the catalyst amount was 0.3 g. Figure 5 shows the firstorder plot for the reaction of propylene with TBHP at three different epoxidation temperatures (X in the figure represents TBHP conversion). Straight lines passing through zero were obtained in Figure 5, therefore, the rate of propylene epoxidation is first-order with respect to the concentration of TBHP under the experimental

conditions. Figure 6 shows that the reaction rate constant increased linearly with the amount of catalysts, which indicates that the reaction rates were not affected by the external mass transfer. The activation energy for the reaction of propylene with TBHP, obtained from an Arrhenius plot using the data in Figure 5, was 36.4 kJ/ mol. Catalyst Characterization. Figure 7 shows the effect of CVD temperature on the surface areas of the Ti/SiO2 catalysts. When CVD temperature was below 600 °C, catalyst surface area did not have much change (surface area was 256 m2/g for the fresh silica gel without the deposition of TiCl4 and was 237.4 m2/g for the Ti/SiO2 catalyst prepared at 600 °C). However, surface area decreased significantly with the increase of CVD temperature when T > 700 °C and surface area was only 62.5 m2/g for the catalyst prepared at 1100 °C. The decrease of surface area with increasing CVD

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Figure 9. XPS spectrum for the fresh Ti/SiO2 catalyst prepared at 700 °C. Figure 8. Titanium loading on Ti/SiO2 catalyst as a function of TiCl4 deposition temperature (CVD time ) 2 h). Table 1. Effect of CVD Temperature on the Amount of Ti Deposited per Unit Surface Area CVD temp (°C) 400 600 700 800 900 1000 1100 Ti per unit area (µmol/m2) 1.36 1.3 1.2 1.3 1.1 1.1 1.3

temperature should be due to silica gel sintering which occurred at T g 700 °C.30 Two reasons might be responsible for the sintering:30 (1) condensation of OH groups belonging to different primary particles followed by siloxanone-bridge formation (i.e., Si-OH + Si-OH ) Si-O-Si + H2O); (2) diffusion of inner hydroxyls toward the silica surface which may greatly modify the gel structure. After the epoxidation of propylene with TBHP, the surface areas of the used catalysts were similar to those of the fresh catalysts, which indicated that no coke or gel was produced during the epoxidation process. Figure 8 presents the relationships between catalyst Ti content and CVD temperature (CVD time ) 2 h), which indicates that Ti content decreased continuously with the increase of CVD temperature. On the basis of the surface area data in Figure 7 and the titanium content data in Figure 8, the amounts of Ti deposited per unit surface area are presented in Table 1 for Ti/ SiO2 catalysts prepared at different CVD temperatures. The titanium concentration was between 1.1 and 1.36 µmol/m2, which was not sensitive to the change of CVD temperature. Our data in Table 1 were similar to those reported by Haukka et al.,19 who found that the number of bounded titanium was 0.8 Ti atoms/nm2 (i.e., 1.33 µmol/m2) for the reaction of TiCl4 with silica at 550 °C. Figure 9 shows the XPS spectrum of the fresh catalyst prepared at 700 °C, which indicates the existence of Ti, Cl, Si, and O on the catalyst surface. The binding energy of Ti(2P3/2) peak was 459.4 eV and the binding energy of Cl(2P) peak was 202.7 eV. On the basis of the peak areas and the sensitivity factors, the atomic ratio of Cl/ Ti on the Ti/SiO2 catalyst was found to be 3.04. Figure 10 shows the XPS spectrum recorded from the used Ti/SiO2 catalyst prepared at the deposition temperature of 900 °C. The binding energy for the Ti(2P3/2) peak was 460.4 eV, which was the same as that of the fresh catalyst (before epoxidation). The binding energy

Figure 10. XPS spectrum of Ti(2P3/2) for the used Ti/SiO2 catalyst prepared at 900 °C.

for the Ti(2P3/2) peak was 459.4 eV for the Ti/SiO2 catalysts prepared at 700 °C and at 1100 °C. These results suggested that the change of CVD temperature had little effect on the binding energy in the range of 700-1100 °C. However, the results of Haukka et al.19 indicated that the binding energy of Ti(2P3/2) decreased with the increase of TiCl4 reaction temperature in the range of 250-550 °C. The Ti(2P3/2) binding energy of TiCl3 is 459.5 eV,31 which is very close to the binding energies we obtained for the CVD prepared Ti/SiO2 catalysts. Therefore, the XPS data indicated that the active species for catalyzing propylene epoxidation should be Si-O-TiCl3, which was formed from the reaction took place during the deposition of TiCl4 on silica gel. The XPS results reported here are consistent with the literature information because it is known that SiOTiCl3 species can be formed from the reaction of TiCl4 with either a silanol group(Si-OH + TiCl4 f Si-O-TiCl3 +HCl)18,20,21 or a siloxane bridge site (Si-O-Si+ TiCl4 f Si-O-TiCl3 +SiCl).18 Siloxane bridge sites are Lewis acid sites,25 and silanol groups are Bronsted acid sites. Kinney and Staley18 found that siloxane bridges reacted more quickly with TiCl4 than the silanol groups. On highly or totally dehydrated silica, Morrow et al.25 also found

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that BF3 reacted initially with strained siloxane bridge sites and then with the remaining silanol groups. The siloxane species in Morrow’s studies (formed from the dehydroxylation of isolated surface hydroxyl groups at T > 600 °C and had infrared bands at 908 and 888 cm-1.32) is different from that in Kinney’s work (formed from degassing at only 450 °C and had an infrared peak at 980 cm-1) due to the differences in the heat treatment temperature.18 The concentration of surface silanol groups depends on the temperature of heat treatments (about 5 µmol/m2 at 400 °C, 3.2 µmol/m2 at 600 °C, and 1 µmol/m2 at 900 °C23) and is much larger than the concentration of siloxane bridges (Kinney’s siloxane species concentration was 0.66 µmol/m2 at 600 °C; Morrow’s siloxane species concentration was 0.12 µmol/ m2 at 800 °C and 0.24 µmol/m2 at 1200 °C). The Ti/SiO2 catalyst we prepared at 900 °C for 2 h contained 1.1 µmol of Ti/m2 (as shown in Table 1), which indicated that all silanol groups reacted with TiCl4 at the deposition temperature of 900 °C. The absence of silanol groups (i.e., Bronsted acid sites) should be the reason for the superior epoxide selectivity observed for the catalyst prepared at 900 °C. At a deposition temperature below 900 °C, significant amount of silanol groups still existed after chemical deposition of TiCl4. The Bronsted acid characteristics of the remaining silanol groups catalyzed the decomposition of TBHP and decreased the epoxide selectivity. Conclusions A series of Ti/SiO2 catalysts were prepared by the deposition of TiCl4 vapor on silica gel, and were used to catalyze the reaction of propylene with tert-butyl hydroperoxide (TBHP) to propylene oxide. The deposition temperatures of TiCl4 were varied between 400 and 1100 °C, and the resulting samples were characterized with ICP, XPS, and BET. The change of CVD temperature had much stronger effect on the catalyst selectivity than on the catalyst activity. The best catalytic performance was observed for the catalyst prepared at the deposition temperature of 900 °C for 2-3 h, which had a titanium concentration of 1.1-1.3 µmol of Ti/m2. The titanium concentration was approximately the same as the sum of the reported concentrations of silanol group (1 µmol/m2) and siloxane species (∼0.15 µmol/m2) on the silica gel treated at 900 °C (without CVD), which indicated that almost no silanol groups remained on the Ti/SiO2 catalysts prepared at the deposition temperature of 900 °C for 2-3 h. Since silanol groups are Bronsted acid sites which can catalyze the undesired TBHP decomposition, the best epoxide yield observed for the catalyst prepared at the deposition temperature of 900 °C should be due to the absence of silanol groups on that catalyst. X-ray photoelectron spectroscopic study suggested that SiOTiCl3 was responsible for the epoxidation of propylene. Kinetic studies indicated that the propylene epoxidation on the Ti/SiO2 catalyst was first-order in TBHP with an activation energy of 36.4 kJ/mol. Acknowledgment We gratefully acknowledge the National Science Council of the Republic of China for financial support (Grant No. NSC-89-2214-E-029-009). The authors also thank Mr. Hsieh-Chih Tsai for performing the experimental work on SiO2 blanks and titanium content effect.

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Received for review December 5, 2001 Revised manuscript received May 13, 2002 Accepted June 9, 2002 IE010983O