Photochemical Production of Biphenyls from Oxidized Sulfur

Energy & Fuels 2003, 17, 95-100

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Photochemical Production of Biphenyls from Oxidized Sulfur Compounds Obtained by Oxidative Desulfurization of Light Oils Yasuhiro Shiraishi,* Kenya Tachibana, Takayuki Hirai, and Isao Komasawa Department of Chemical Science and Engineering, Graduate School of Engineering Science, and Research Center for Solar Energy Chemistry, Osaka University, Toyonaka 560-8531, Japan Received June 24, 2002

Photochemical production and recovery process of biphenyls from the desulfurization products, obtained by oxidative desulfurization (ODS) of light oils, has been investigated. The S-oxidized dibenzothiophenes (DBT sulfones), when dissolved in 2-propanol, were converted successfully into the corresponding biphenyls by photoirradiation at wavelengths of λ > 280 nm using a highpressure mercury lamp, under conditions of nitrogen atmosphere and at room temperature. The present photochemical process was found to be also effective for the production of biphenyls from the DBT sulfones present in the desulfurization products of low-aromatic-content light oils. The quantitative recovery of biphenyls from the resulting 2-propanol solution was carried out successfully by adsorption using neutral aluminum oxide and subsequent elution with dichloromethane/n-hexane (20/80 v/v) mixture.

Introduction There has been much recent interest in the deep desulfurization of light oils. The oxidative desulfurization (ODS) process has been investigated in previous work,1 based on chemical oxidation of sulfur compounds and subsequent extraction of the resulting sulfones. Dibenzothiophenes (DBTs) and benzothiophenes (BTs) in light oils are S-oxidized by oxidizing agent (H2O2 with acetic acid) to form the corresponding sulfones under relatively mild conditions at 323-343 K and atmospheric pressure. These sulfones are highly polarized compounds, such that they can be removed from the oil by subsequent extraction using an acetonitrile/water azeotropic mixture. By combination of the processes, the sulfur content of light oils is decreased successfully to < 0.05 wt %, which is below the regulatory levels that presently apply for both Japan and Europe. The ODS process is, however, not based on selective extrusion of sulfur atom from the compounds but rather on removal of the oxidized compounds themselves from the oils, thus causing a low-oil-recovery yield inevitably. A subsequent process, consisting of selective elimination of sulfone group from the sulfone molecules and recovery of the resulting aromatic nuclei, is therefore required for further development of the ODS process. Koch et al.2 have reported that DBT sulfone is converted into the corresponding biphenyl by catalytic pyrolysis using Pt/γ-Al2O3 and Pd/γ-Al2O3 catalysts in * Author to whom correspondence should be addressed. Tel.: +816-6850-6271. Fax: +81-6-6850-6273. E-mail: [email protected]. (1) Shiraishi, Y.; Tachibana, K.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 2002, 40, 4362-4375. (2) Koch, T. A.; Krause, K. R.; Manzer, L. E.; Mehdizadeh, M.; Odom, J. M.; Sengupta, S. K. New J. Chem. 1996, 20, 163-173.

the presence of hydrogen-donating solvents, such as Decalin and tetralin, under conditions at 573 K and H2 pressure of 2 MPa. The process achieves the complete conversion of DBT sulfone into biphenyl but requires severe operating conditions. Wallace and Heimlich3 and LaCount and Friedman4 have reported that DBT sulfone is converted into also biphenyl by alkaline decomposition using melting NaOH or KOH under conditions at 573 K and N2 pressure of 6 MPa. The process also requires severe operating conditions and the product contains a large quantity of impurities, such as dibenzofuran and biphenyl-2-ol. A selective elimination process of the sulfone group from the sulfone molecules, able to be operated under moderate conditions and without the use of noble-metal catalysts and hazardous chemicals, is therefore required. Jenks et al.5-7 have found that the DBT sulfone, when dissolved in 2-propanol, is converted quantitatively into the biphenyl by photoirradiation at wavelengths of λ > 300 nm using a Xe lamp. The reaction proceeds effectively even at room temperature. Thus, such a photochemical process, if applied to the production of the biphenyls from the sulfones, present in the desulfurization products obtained following the ODS treatment of light oils, may thus be able to recover the biphenyls under moderate conditions. Furthermore, if the biphenyls recovered are returned to the light oil, the low (3) Wallace, T. J.; Heimlich, B. N. Tetrahedron 1968, 24, 1311-1322. (4) LaCount, R. B.; Friedman, S. J. Org. Chem. 1977, 42, 27512754. (5) Jenks, W. S.; Taylor, L. M.; Guo, Y.; Wan, Z. Tetrahedron Lett. 1994, 35, 7155-7158. (6) Jenks, W.; Lee, W.; Shutters, D. J. Phys. Chem. 1994, 98, 22822289. (7) Gregory, D. D.; Wan, Z.; Jenks, W. S. J. Am. Chem. Soc. 1997, 119, 94-102.

10.1021/ef020139q CCC: $25.00 © 2003 American Chemical Society Published on Web 12/03/2002

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Table 1. (a) Quantities, (b) Sulfur Concentration in the Desulfurization Productsa for Light Oils (100 g) Recovered from the Oil Phase, (c) the Quantities of Photoproducts Obtained Following Photoirradiation of the Desulfurization Products (a, 1 g) in 2-Propanol and Subsequent Adsorption Separation, and (d) the Net Quantities of Biphenyls in (c) feedstocks

(a) (g)

(b) (wt %)

(c) (g)

(d) (g)

LGO CLO LCO

12.02 2.53 9.07

2.857 3.067 0.311

0.237 0.195 0.205

0.210 0.102 0.026

a Desulfurization conditions: quantities of feed light oils, 100 g; time, 30 h; temperature, 343 K; [H2O2] ) 1000- and [AcOH] ) 500-fold molar excesses based on initial sulfur concentration of the feed light oils.

recovery yield of light oil by the ODS process will be much improved. In the present work, detailed investigations on the photoconversion of DBT and BT sulfones and their alkyl-substituted derivatives in 2-propanol have been carried out. The production of biphenyls from the desulfurization products, obtained by the ODS treatment of actual light oils, were then studied. The recovery process of the biphenyls formed was also investigated, and the applicability of the process was examined in detail. Experimental Section 1. Materials. Hexane, dichloromethane, and 2-propanol were purchased from Wako Pure Chemical Industry, Ltd., and were used following dehydration using molecular sieves 5A. BT and DBT sulfones and their methyl-substituted derivatives were synthesized according to the standard procedures.1,8,9 Three light oilssstraight-run light gas oil (LGO; sulfur content, 1.38 wt %), commercial light oil (CLO; 0.179 wt %), and light cycle oil (LCO; 0.132 wt %),sused in a previous ODS study,1 were also employed as the feedstocks. The ODS treatment of the light oils and recovery of the desulfurization products were carried out as described previously1 and as follows: Each light oil (100 g) was mixed with a 1000-fold molar excess of H2O2 based on the initial sulfur concentration of the feed light oils and heated to 343 K. Five-hundred-fold molar excess of acetic acid was then added carefully to the above mixture and stirred for 30 h. The resulting mixture was then cooled to room temperature, and viscous precipitates produced (product 1) were recovered by filtration. The oil phase was then separated from the aqueous phase by decantation and washed several times with water (100 mL). Sulfones remaining in the oil phase (2) were extracted with an acetonitrile/water (84/16 v/v) azeotropic mixture (100 mL) and were then taken up with dichloromethane (100 mL) and concentrated by evaporation. Sulfones in the aqueous H2O2 solution (3) were extracted with dichloromethane (100 mL) and then concentrated. The above desulfurization products recovered [(1)-(3)] were then used for the photoirradiation experiments. The relevant quantities and sulfur concentrations of the desulfurization products for the three light oils, recovered from the oil phase (2), are summarized in Table 1, columns a and b. 2. Procedure and Analysis. Photoirradiation experiments were carried out as follows. Each desulfurization product (1 g) for the light oils was dissolved in 2-propanol (20 mL) within a Pyrex glass test tube (25 mL) and each tube stoppered using a rubber septum cap. N2 gas was bubbled through the solutions for 10 min. These were then photoirradiated by a high-pressure (8) Shiraishi, Y.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 1999, 38, 3300-3309. (9) Gilman, H.; Esmay, D. L. J. Am. Chem. Soc. 1952, 74, 20212024.

mercury lamp (300 W, Eikohsha Co., Ltd., Osaka) with magnetic stirring. A Pyrex glass filter was used to give light wavelengths rejection of less than 280 nm. The concentrations of the starting sulfone compounds and the products in 2-propanol were determined by reverse-phase HPLC (Shimadzu LC6A, equipped with spectrophotometric detector SPD-6A) and gas chromatography (Shimadzu GC-14B, equipped with FID). Identification of acidic products was carried out by GC-MS (JEOL, JMS DX303HF) using an electron impact (EI) ionization (70 eV) method following methylation using diazomethane, according to the procedure described previously.8 Analysis of the materials, formed by photoirradiation of the desulfurization products for light oils, was carried out by gas chromatography using an atomic emission detector (GC-AED, Hewlett-Packard 6890, equipped with AED G2350A), in accordance with the previously described procedure.1 LUMO potentials for DBT sulfones were calculated using WinMOPAC ver.3.0 software (Fujitsu Ltd.), according to a procedure described previously.1

Results and Discussion 1. Photolysis of DBT Sulfones. Photoconversion of DBT sulfone into the biphenyl in 2-propanol, when photoirradiated by a Xe lamp at wavelengths of λ > 300 nm, has been studied in detail by Jenks et al.5-7 As shown schematically in Figure 1, it is reported that DBT sulfone 1 is photoexcited by photoirradiation to form highly reactive 1,5-biradical 2 intermediate, which is then hydrogenated by 2-propanol and further photodecomposed to form finally biphenyl 7. When the DBT sulfone, dissolved in 2-propanol, was photoirradiated at λ > 280 nm by a high-pressure mercury lamp under air, the concentration of DBT sulfone decreased with time and the biphenyl concentration increased accordingly. However, when almost all of the DBT sulfone disappeared following 60 h of photoirradiation, only 60% biphenyl yield was obtained. GC-AED analysis detected no peaks, containing a sulfur atom, in the resulting 2-propanol solution. To clarify the photoproducts other than biphenyl, the 2-propanol solution following photoirradiation was treated with diazomethane. The sulfurspecific GC-AED chromatogram for the resulting material showed a new peak containing a sulfur atom, which has a molecular ion at m/z 248. The peak compound was identified from its fragmentation pattern to be a biphenyl-2-sulfonic acid methyl ester, thus indicating that the other photoproduct of DBT sulfone 1 is biphenyl-2-sulfonic acid 8. Jenks et al.5 reported that biphenyl 7 is formed via the photodecomposition of intermediately formed biphenyl-2-sulfinic acid 5. When the reaction was carried out under N2 atmosphere, the biphenyl yield was increased up to >92%. Compound 8 is therefore likely to be formed via the photooxidation of the intermediate 5 with an O2 molecule dissolved in 2-propanol, as schematically shown in Figure 1. The results indicate that the complete conversion of DBT sulfone into the biphenyl requires the complete removal of O2 from 2-propanol. Actual desulfurization products for light oils contain a large quantity of alkyl-substituted DBT sulfones, as described in a previous paper.1 Photoreactivities of methyl-substituted DBT sulfones, such as 4-methylDBT sulfone 9 and 4,6-dimethyl-DBT sulfone 10, were therefore also examined. The photoirradiation of these sulfones, dissolved in 2-propanol, gave rise to the corresponding biphenyls, as also the case for nonsub-

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Figure 1. Photolysis mechanism of DBT sulfone in 2-propanol.

Figure 2. Conversion of (a) DBT sulfones and (b) BT sulfones (closed bars) and biphenyls yield from DBT sulfones (open bars), following photoirradiation in 2-propanol. Reaction time, 2 h for DBT sulfones and 15 min for BT sulfones; [sulfones]initial ) 5 mM. Symbols: 1, DBT sulfone; 9, 4-methyl-DBT sulfone; 10, 4,6-dimethyl-DBT sulfone; 11, BT sulfone; 14, 2-methylBT sulfone; 15, 3-methyl-BT sulfone; 16, 2,3-dimethyl-BT sulfone.

stituted DBT sulfone. Figure 2a shows the conversion of DBT sulfones and biphenyl yield, following 2 h of photoirradiation to the DBT sulfones in 2-propanol. The

methyl-substituted DBT sulfones were found to be photodecomposed rather faster than nonsubstituted DBT sulfone 1 and the biphenyl yields were higher than those for the nonsubstituted one. The present reaction proceeds via the reductive elimination of the sulfone group from the sulfone molecules. The photochemical reduction of the sulfone compounds usually depends on LUMO potential of the compounds.10 The LUMO potentials for the methyl-substituted DBT sulfones, obtained by MO calculations, lie in the order of 4,6dimethyl-DBT sulfone (-9.396 eV) > 4-methyl-DBT sulfone (-9.477 eV) > DBT sulfone (-9.568 eV), which is the same order as the conversion of DBT sulfones and biphenyl yields, as shown in Figure 2a. This suggests that DBT sulfones, having low LUMO potential, are photoconverted more effectively. However, the detailed description on the relationship between the reactivity and the LUMO potentials cannot be done at this moment and further investigation is therefore required. 2. Photolysis of BT Sulfones. The desulfurization products for actual light oils also contain a large quantity of BT sulfone and its alkyl-substituted derivatives.1 The photoreaction behavior of BT sulfone 11 in 2-propanol was therefore then studied under N2 atmosphere. The photodecomposition of BT sulfone was seen to proceed significantly rather faster than that of DBT sulfones and the BT sulfone disappeared within 2 h of photoirradiation. GC-AED analysis of the resulting 2-propanol solution, following 1 h of photoirradiation, showed two new peaks having molecular ions at m/z 208 and 212, both of which contain a sulfur atom. These peak compounds were identified by GC-MS analyses to be 3-acetylbenzothiophene sulfone 12 and its hydrogenated derivative 13. Following an additional 5 h of photoirradiation, only peak compound 13 remained on the chromatogram. From the above findings, as shown in Figure 3a, BT sulfone is photoexcited by photoirradiation and the hydrogen atom on the C3 position of the molecule is substituted by 2-propanol to give rise to compound 12, which is further hydrogenated, by (10) Samat, A.; Vacher, B.; Chanon, M. J. Org. Chem. 1991, 56, 3524-3530.

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Figure 3. Photoreaction pathway for (a) BT sulfone 11 and (b) 2-methy-BT sulfone 14 in 2-propanol.

photoirradiation in the presence of 2-propanol, to form finally the compound 13. The reactivities of methyl-substituted BT sulfones were also examined. As shown in Figure 2b, the conversion of BT sulfones lies in the order BT sulfone 11 > 2-methyl-BT sulfone 14 > 3-methyl-BT sulfone 15 > 2,3dimethyl-BT sulfone 16, and the reactivity was found to decrease with an increasing number of methyl substituents on the molecule. This tendency differs completely from that obtained for DBT sulfones, as shown in Figure 2a. GC-AED and GC-MS analyses of the resulting solution for 2-methyl-BT sulfone 14 demonstrated a new single peak having a molecular ion at m/z 226, which contains a sulfur atom. The peak compound was identified from its fragmentation pattern to be 2-methyl-3-hydroxyethyl-2,3-dihydrobenzothiophene sulfone 17. The formation mechanism of compound 17 may be expressed as in the same pathway for BT sulfone 11, as shown in Figure 3b. The reaction of 3-methyl-BT sulfone 15 and 2,3-dimethyl-BT sulfone 16 produced highly viscous materials. No new peaks were detected by GC-AED analysis for both cases, and both products were inactive for the treatment with diazomethane. For both products, the IR spectrum demonstrated the presence of carbonyl and sulfone groups and the 13C NMR spectrum showed a large number of resonances attributable to carbonyl groups. These results suggest that the photoirradiation of 15 and 16, having alkyl substituents on the C3 position of the molecule, in 2-propanol does not produce the acetylated products but gives rise to the polymerized materials. The above findings demonstrate that the photoirradiation of BT sulfones does not produce sulfur-free aromatic compounds. 3. Production of Biphenyls from the Desulfurization Product. The present process was then applied to the production of biphenyls from the desulfurization products, obtained by the ODS treatment of actual light oils. A 1 g sample of the desulfurization product for LGO recovered from the oil phase [(2); total quantity, 12.0 g] was first photoirradiated in 2-propanol. The product was concentrated by evaporation and then analyzed by GCAED, following dissolution in dichloromethane. Figure 4 shows the variations in the resulting carbon- and sulfur-specific GC-AED chromatograms. As shown in Figure 4b-ii, the sulfur-specific chromatogram for the

Shiraishi et al.

desulfurization product appears on higher retention time than that for the feed light oil (Figure 4a-ii). This indicates that the desulfurization product contains a large quantity of sulfones, of higher boiling point than that of the unoxidized sulfur compounds in light oil, as described previously.1 When the product was photoirradiated in 2-propanol for 24 h, as shown in Figure 4ci, new peaks appeared on the carbon-specific chromatogram at 10-15 min as compared to that obtained before photoirradiation (Figure 4b-i), whereas no new peaks on the sulfur-specific chromatogram were observed at 10-15 min following photoirradiation (Figure 4b-ii and 4c-ii). Most of the new peaks appeared in Figure 4c-i, having no sulfur atom, were identified by GC-MS analysis to be alkyl-substituted biphenyls produced by the photolysis of DBT sulfones. The results indicate that the present photochemical process is also effective for the production of biphenyls from the desulfurization products obtained by the ODS treatment of the actual light oil. It is necessary to recover the biphenyls from the resulting 2-propanol solution. Following photoirradiation, the 2-propanol was removed by evaporation and the resulting viscous material was then contacted with n-hexane. GC-AED analysis of the resulting hexane solution detects the unreacted sulfones as well as the biphenyls. To recover the biphenyls more selectively, an adsorption method was then employed as follows: The photoproduct obtained was adsorbed onto 20 g of activated neutral aluminum oxide packed in a glass column (i.d., 20 mm; length, 450 mm). This was then eluted by n-hexane/dichloromethane (80/20 v/v, 100 mL) mixture, and the obtained fraction was concentrated by evaporation. By this means, 0.24 g of viscous material was obtained from 1 g of the desulfurization product for LGO. As shown in Figure 4d, the carbon-specific chromatogram for the obtained material appeared at a lower retention time region than that obtained before adsorption (Figure 4c-i), and any peaks containing a sulfur atom were hardly observed on the sulfur-specific chromatogram. As shown in Figure 5b, an IR spectrum for the obtained material demonstrated no absorption band attributable to the sulfone group at 1130 and 1280 cm-1. These results suggest that the present adsorption process can recover the biphenyls selectively without the contamination of the unreacted sulfones. Following the ODS treatment of LGO (100 g), the desulfurization products are also recovered as precipitate [(1); 2.6 g] and from the aqueous H2O2 phase [(3); 2.0 g] as well as from the oil phase [(2); 12.0 g]. The photoirradiation of the former two products produced also the biphenyls, as in the case for (2). However, the quantities of the photoproducts, recovered from 1 g of the desulfurization products, were 0.04 g for the precipitate (1) and 0.01 g for the aqueous phase (3), respectively, which are significantly lower than those recovered from the oil phase [(2); 0.24 g]. When the desulfurization products for other light oils, such as CLO and LCO, recovered from the oil phase following ODS treatment, were photoirradiated in 2-propanol, the production of biphenyls was also observed. The quantities of the photoproduct, obtained by photoirradiation of the desulfurization product (1 g) and subsequent adsorption, as shown in Table 1c, were found to be 0.20

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Figure 4. (i) Carbon- (193 nm) and (ii) sulfur-specific (181 nm) GC-AED chromatograms for (a) feed LGO, (b) the desulfurization products recovered from the oil phase following 30 h of reaction of (a) at 343 K in the presence of 1000- and 500-fold molar excesses of H2O2 and acetic acid based on the initial sulfur concentration of the feed LGO, (c) the products obtained following photoirradiation of (b) in 2-propanol, and (d) the products recovered following adsorption of (c).

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Figure 5. IR spectra for (a) desulfurization product of LGO obtained from the oil phase and (b) the product obtained following photoirradiation of (a) in 2-propanol and subsequent adsorption separation.

g for CLO and 0.21 g for LCO, which are lower than those obtained for LGO (0.24 g). 4. Recovery Yield and Purity of Biphenyls. GCAED and GC-MS analyses for the photoproducts obtained following adsorption showed a presence of saturated and aromatic hydrocarbons. These are likely to become mixed during the recovery process of the desulfurization products from light oil by extraction separation. However, in the present process, the recovered photoproducts are returned to the light oil, such that the presence of these hydrocarbons in the photoproducts has no significant effect on the quality of the oil. When the photoproducts were analyzed by IR, as shown in Figure 5b, an adsorption band appeared at 1710 cm-1, which is attributable to a carbonyl group. As described previously,1 the ODS treatment of light oils oxidizes aromatic hydrocarbons to give rise to the corresponding carbonyl compounds. This indicates that the carbonyl compounds in the light oil following oxidation have also become mixed into the desulfurization product during the extraction separation and cannot be separated selectively from the biphenyls by a subsequent adsorption process. The intensity of the absorption band for the carbonyl group in the photoproducts for CLO and LCO was seen to be higher than that for LGO, with the order of LCO > CLO > LGO. The LCO and CLO contain larger quantity of aromatics than LGO, as described previously,1 and larger quantities of the carbonyl compounds are contained in the desulfurization product for LCO and CLO following the ODS treatment. The net quantities of the biphenyls, recovered from 1 g of the desulfurization products, were estimated by GCMS and GC-AED analyses to be 0.210 g for LGO, 0.102

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g for CLO, and 0.026 g for LCO, respectively, with the recovery yields of biphenyls from LGO being highest among the three light oils. As shown in Table 1b, the sulfur content in the desulfurization product for LCO is significantly lower than that of LGO. The low sulfone concentration in the desulfurization product for LCO is one of the reasons for the low biphenyl yield for LCO. As shown in Table 1b, although the sulfur concentration of CLO is comparable to that of the LGO, the recovery yield of biphenyls for CLO is lower than that for LGO. The desulfurization product of CLO contains a larger quantity of the carbonyl compounds than that of LGO. The aromatic hydrocarbons and carbonyl compounds absorb the light at wavelengths of λ > 280 nm, which are essential for the photoexcitation of DBT sulfones. The low recovery yield of the biphenyls from CLO results since the high concentration of aromatics and carbonyl compounds suppress the photoexcitation of DBT sulfones. The results indicate that the present photochemical and adsorption process is effective for the production of the biphenyls from the desulfurization products for low-aromatic-content light oils. To achieve higher recovery yield of biphenyls from high-aromaticcontent light oils, a more selective separation process of the sulfones from the aromatic and carbonyl compounds following the ODS process must be developed. Conclusion The photochemical production and recovery process of biphenyls from desulfurization products, obtained by the ODS treatment of light oils, has been investigated, and the following results have been obtained. (1) DBT sulfones, when dissolved in 2-propanol, are photodecomposed by photoirradiation at wavelengths of λ > 280 nm to give rise to the corresponding biphenyls under moderate conditions. The photoreactivities of DBT sulfones depend on the LUMO potentials for the DBT sulfones, as obtained by semiempirical MO calculations. BT sulfones in 2-propanol are photodecomposed quite faster than DBT sulfones. However, the photoirradiation of BT sulfones does not produce sulfur-free aromatic compounds but produce the acetylated or polymerized compounds. (2) Photoirradiation of the desulfurization products, recovered following the ODS treatment of light oils, when dissolved in 2-propanol, produced also biphenyls successfully. These biphenyls are recovered by adsorption using aluminum oxide and subsequent elution with dichloromethane/n-hexane mixture. The higher recovery yield of biphenyls is achieved for the desulfurization product of low-aromatic-content light oil. The photoproduct obtained from high-aromatic-content light oil contains any impurities, such as aromatic hydrocarbons and their oxidized derivatives. Acknowledgment. The authors are grateful for the financial support of Grants-in-Aid for Scientific Research (No. 12555215) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. The authors are also grateful to the Division of Chemical Engineering for the Lend-Lease Laboratory System. EF020139Q