Ind. Eng. Chem. Res. 1996,34,135-139
135
a-Te&IoO, as an Active Species in the Vapor-Phase Selective Oxidation of Ethyl Lactate to Pyruvate over TeOz-Moos Catalysts Hiromu Hayashi,* Shigeru Sugiyama, Naoki Masaoka, and Naoya Shigemoto Department of Chemical Science and Technology, Faculty of Engineering, University of Tokushima, Minamcosanjima, Tokushima 770, Japan
Binary oxide, TeO2-Moos, converted ethyl lactate selectively to pyruvate in a vapor-phase fixedbed flow system. A synergy in activity was observed for TeOz-MoOs calcined at 500 "C, showing a sharp maximum at a composition of MoOy2Te02, and it is concluded with evidence from powder XRD, IR, DTA-TGA, and SEM/EPMA that a-Te2MoO.r is the active species for the present system. TeOs-MOOS calcined at 400 "C was a mixture of the component oxides and crystallized at 450 "C to give a-Te2MoO.l. Phase transition of the a-form to less active amorphous p-Te2Moo7 was observed by calcination a t 600 "C. An optimized catalyst with a composition of Mo03.2Te02 calcined a t 500 "C in air revealed a high conversion of lactate over 99% with 93% selectivity to pyruvate a t 300 "C.
Introduction Pyruvic acid is the simplest homologue of a-keto acids, which were extensively reviewed (Cooper et al., 19831, covering 19 general methods for their synthesis. Some of the routes are elegantly designed for laboratory procedure in organic synthesis, but the applications of catalytic processes are of more recent vintage (Sugiyama et al., 1991, 1992, 1993; Tsujino et al., 1992; Hayashi et al., 1993a-c). An established laboratory procedure (Howard and Fraser, 1945) for pyruvic acid synthesis is the dehydrative decarboxylation of tartaric acid in the presence of an excess of powdered potassium hydrogen sulfate (KHso4). Although the unique dehydrating agent KHSO4 melts at a low temperature of 197 "C, it was successfully adapted for vapor-phase flow operations as a silica-supported pyrosulfate catalyst (K2S207/Si02)to obtain ethyl pyruvate continuously from the tartrate in a rather good yield of 60% a t 300 "C (Sugiyama et al., 1991, 1992). An alternative approach to pyruvic acid, bypassing the expensive tartrate, has been made by the liquid-phase oxidation of sodium lactate (eq 1,R = Na) on lead-modified palladium-on-carbon and related cataCH3CHCOOR
I
+
'I202
CH3CCOOR
+
OH
II
+ H20
(1)
0
lysts (Tsujino et al., 1992; Hayashi et a1.,1993c, 1994), and of ethyl lactate (eq 1,R = C2Hd on oxide catalysts such as TiOz, SnO2, and SnOz-Moos (Hayashi et al., 1993b). The advantage of running the reaction in liquid phase over the gas-phase fixed-bed operation for the production of fine chemicals is generally accepted in terms of inexpensive plant investment, stability and flexibility for operating conditions, and greater ease for renewal and/or making up of catalyst. However, both ethyl lactate and ethyl pyruvate boil at the same temperature of 155 "C (Dean, 1973) and are similar in chemical nature. Ethyl pyruvate was obtained selectively in the liquid phase, but the conversion of lactate was usually 30-50% (Tsujino et al., 1992). Thus the separation of product pyruvate from the unreacted lactate discourages the practical application of the liquid-phase oxidation
* To whom correspondence
should be addressed.
of lactate. Comparative reaction studies for catalyst screening in the vapor-phase oxidation of ethyl lactate suggested a binary oxide, TeO2-Moos, as a promising candidate catalyst to afford pyruvate with a high selectivity over 90% at 250-300 "C (Sugiyama et al., 1993). A synergy in activity was observed in the present work for TeO2-Moos, and a-TezMo0.r is suggested as an active species for the oxidation of ethyl lactate to pyruvate. An optimized catalyst revealed a high conversion of lactate over 99% with 93% selectivity to pyruvate, providing a favorable route bypassing the separation problem.
Experimental Section Apparatus and Procedure for Reaction Studies. The reaction was carried out by a conventional fEedbed flow apparatus described in the previous papers (Sugiyama et al., 1991; Hayashi et al., 1993a) at 300 "Cwith space velocity of 1000-3600 h-l. Ethyl lactate was supplied as toluene solution into the reactor and, unless stated, diluted with O D 2 to adjust the gas-phase composition of 5% ethyl lactate with 30% 0 2 . The reactor effluent was scrubbed by ice-cooled 1-propanol and analyzed by gas chromatography (GC) with a Hitachi 163-FID for organic species and a Yanaco G 2800-TCD for gases. Materials. The catalyst, TeO2-Moos, was prepared by kneading the component oxides with an appropriate small amount of water in an automatic porcelain mortar (Type 101, Ishikawa, Tokyo) for 2h. The resultant paste was spread over a glass plate, dried overnight a t 80 "C, crushed, and calcined at 500 "C in air for 5h. Ethyl lactate was purchased from Wako Pure Chemicals, Osaka, and used as suppled. Ethyl pyruvate for the calibration of GC analysis was obtained from Aldrich Chemical Co., Milwaukee. TeO2 and Moo3 were obtained from Wako, Osaka, and Merck, Darmstadt, respectively. Characterization of Catalysts. Powder X-ray diffraction of catalysts was measured by a MXP system of MAC Science Co., Tokyo. Infrared spectra were recorded for catalyst powder tabletted with KBr by a spectrometer, Model FTIR-3 of Japan SpectroscopicCo., Tokyo. Differential thermal analysis-thermogravimetric analysis (DTA-TGA) was measured for TeO2-Moos, kneaded, and dried a t 80 "C by a thermal analyzer,
0888-5885/95/2634-0135$09.00/#0 1995 American Chemical Society
136 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 Scheme I
100 W
H2O
2
CH~CHCO~CZH 5 -CH~CCO~CZHS
I
OH
A
II
0 CH3CCOzH
I/
+
CzH5OH
CH3CHO
i?
C2H4
l o n I 80
I I I I 50 30 40 20 Oxygen in feed (%)
I 10
0
601
0
1
60
100
-
80
Degenerate
T
Temperature
( "c)
Figure 1. Oxidation of ethyl lactate over TeOz-MOOS. 5% Ethyl lactate, 30%0 2 , SV 3600 h-l. Catalyst, MoOy2Te02 (calcined 500 "C, 5h). 0, ethyl lactate; 0 , ethyl pyruvate; A , acetaldehyde; A, ethanol; 0, ethylene.
Model DT-40 of Shimadzu, Kyoto. SEMIEPMA observation with EDS mode was made by an Electron Probe Microanalyzer of Type JXA-840A of JOEL, Tokyo.
Results and Discussion Reaction Network. The major reaction at present is a simple oxidation of secondary alcohol t o carbonyl component, but the product pyruvic acid in its free-acid form is unstable to decompose. Thus the substrate lactic acid was supplied as ethyl ester to protect the carbonyl moiety. Esterification is also of benefit to vapor-phase flow operation in making acids more volatile. Hydrolysis of ethyl lactate gives free pyruvic acid with further decarboxylation to acetaldehyde. Ethanol, which is another fragment of ester hydrolysis, could be either oxidized to acetaldehyde or dehydrated to ethylene a t higher temperature above 350 "C. The reaction network is summarized in Scheme 1. Optimizing Reaction Conditions. 1. Effect of Temperature. Figure 1 shows the effect of reaction temperature on the oxidation of ethyl lactate to pyruvate over TeOz-MoOs catalyst. High selective formation of ethyl pyruvate was observed up to around 300 "C, and the yield showed an increase with increasing temperature to a maximum over 90% at 315 "C. Skeletal decomposition of ethyl pyruvate gives an equimolar yield of acetaldehyde and ethanol, the latter of which was further oxidized to the former aldehyde at above 350 "C,but the dehydration t o ethylene was minor, as given in Figure 1. 2. Effect of Ethyl Lactate and Oxygen Concentration. The effect of oxygen concentration in feed at a given substrate concentration of 5% ethyl lactate on lactate conversion is given in Figure %(a).The catalyst TeO2-Moos showed zero-order kinetics for oxygen with stable activity at an oxidative condition of a high oxygen concentration above 30% 0 2 , but degenerated during on stream without change in the selectivity to pyruvate at a reductive condition of a low oxygen concentration. In reverse, the effect of ethyl lactate concentration in feed at 30% 0 2 is given in Figure 2(b). A constant value
j C
60
8
2 4 0
9 20 0
0
2
4
6
8
1
0
1
2
Ethyl lactate in feed (%) Figure 2. Effect of oxygen (a)and ethyl lactate (b) concentration in feed on lactate conversion. Catalyst, same as in Figure 1.
of lactate conversion was observed below 5% ethyl lactate, showing first-order kinetics for the lactate. The catalyst degenerated again at a reductive condition of high concentration of 10% lactate. Oxidation State of TeOz-Moos. Ethyl pyruvate was obtained even in the absence of oxygen in feed with a low selectivity of 40-50% at the initial 1h on stream followed by rapid degeneration as reported in the previous letter (Hayashi et al., 1993a). X-ray photoelectron spectra revealed that tellurium in the binary oxide catalyst, TeOz-MoO3, was reduced to the metallic state (Te 3d512 573 eV), and the reduction of Te extended over the bulk phase during 5 h on stream. The rapid degeneration shows that simple dehydrogenation on the metallic Te was minor and the reaction to pyruvate was oxidative, for which oxygen was supplied from the bulk phase of oxide lattice in the absence of oxygen in feed. The surface Te 3d5/2 core electron was observed at 577 eV, showing a higher oxidation state, for both 10%0 2 and 30% 0 2 with 5%ethyl lactate. On exposure of bulk Te to the surface by grinding of the catalyst, the ratio of Te(O)/TeO, substantially increased at 10%0 2 , while it remained unchanged at 30% 0 2 . Thus lattice oxygen was employed to make up for the oxygen requirement a t the reacting surface under the conditions of low oxygen concentration in feed, signifying the results given in Figure 2. The catalyst should be used under oxidative, oxygen-rich conditions. Chemical Composition of Active Catalyst. Catalyst screening in the vapor-phase oxidation of ethyl lactate (Sugiyama et al., 1993) showed a binary oxide, TeO2-Moos, to be an active catalyst to afford pyruvate,
Ind. Eng. Chem. Res., Vol. 34,No. 1, 1995 137 Te2MoO7
M003
TeO,
I
(a) Moo3 2Te02
(b) Te02 2Mo03
Calcined 400'C 5h
Calcined 500% 5h
I-
60 40
EM 0
0.2
0.4
0.6 0.8
Te02 Te02 + Moo3
1.0
(mol/mol)
Effect of catalyst composition on lactate conversion. 306 "C, 5%ethyl lactate, 30%02,-SV 3600 h-l.
0
200
400
500
600
800
Temperature ("c, Figure 4. TGA-DTA of TeO2-Moos. MOO3 + 2Te02 + nH2O; kneaded 2 h, dried overnight at 80 "C, and then heated in air up to 900 "C with 10 "C/min.
where the component single oxide Moo3 showed a moderate activity, but another component TeO2 was less active only to give ethanol at a high temperature of 350-400 "C. Thus the activity of TeO2-MoO3 catalysts of various compositions has been compared at a given same condition to elucidate the optimum catalyst composition. A synergy in activity was observed for the present TeO2-MoO3 system as shown in Figure 3. Each of the binary TeO2-MoO3 catalysts gave a higher conversion of ethyl lactate than that of the component oxides of TeO2 and Moos, showing a sharp maximum at a molar ratio of TeOfloO3 = 2/1. An activity pattern (solid line) calculated assuming the catalyst to be composed of Moo3 and Mo03.2Te02 at the Mo-rich region, and of Mo03.2Te02 and TeO2 at.the Te-rich region, accounts for the observed activity (open circles), suggesting an active species of a chemical composition of Mo03-2Te02. It appears that surface area of TeOa-MoOs of 2 m2/g as reported in the previous paper (Sugiyama et al., 1993) does not account for the synergism in activity. Evidence for a-Tea007 as an Active Species. 1. TGA-DTA. Figure 4 shows TGA-DTA of a mixture of TeO2-MoO3 system, where TeOa and Moo3 powder were kneaded 2 h in an automatic porcelain mortar to a paste in a molar ratio of TeOmoO3= 2/1 with a small amount of water and dried overnight at 80 "C, and an
u
iopm Figure 5. SEM and EPMA for TeOz-Moos.
- rr-Te2Mo0,
I I
10
20
30
28
40
50
60
("1
Figure 6. Powder XRD patterns of Te2M00, (a), MoO3.2TeO2 (b), and TeO2 (c).
aliquot of the white mass was heated up to 900 "C with 10 "C/min. TGA does not show any appreciable change in the sample weight until a high temperature of 700 "C. Thus water added in kneading was completely evaporated off at 80 "C, and sharp peaks in DTA
138 Ind. Eng. Chem. Res., Vol. 34,No. 1, 1995
1 1200
I
I
I
I
1000
800
600
400
pm in diameter dispersed in MOOSlike seeds of a watermelon or raisin bread, showing reactions between the two component oxides of TeO2 and Moo3 have not yet occurred. In contrast, Te02.2Mo03 calcined at 500 "C (b) gives traces of solid-phase reaction. Domains composed of both elements Te and Mo, and of single Mo, were observed, but of single Te disappeared. 3. XRD and IR Spectra. Figure 6 shows powder XRD patterns of MoOy2Te02 (a) and TeOy2Mo03 (b) both calcined a t 500 "C in air for 5h, and of TeOz (c) for reference. A cystalline phase of a-Te2MoO.r (solid line) (JCPDS) was observed for Mo03.2Te02 (a) as anticipated by the composition. Arnaud et al. (1976) determined the crystal structure of a-TezMo0.1. The oxide is monoclinic with Z = 4 of space group P21lc, where double chains of distorted molybdenum octahedra along the a-direction are linked by tetrahedral oxotellurium units in a three-dimentional arrangement (Arnaud et al., 1976). Two phases of Moo3 (open circles) and a-Te2Moo7 (filled circles) were detected for the Mo-rich specimen of Te02'2Mo03 (b). Thus domains composed of both Te and Mo observed in SEIWEPMA (Figure 5b) are the a-TezMo0.1phase formed by the solid-phase reaction between the component oxides, where TeO2 phase disappeared t o give a-TezMo0.1,and the remainder of excess component of Moo3 was detected as it is. Other evidence for a-TeaMo0.1phase was given by the IR spectra as shown in Figure 7. The spectrum of Mo03.2TeO~calcined at 500 "C in air for 5 h agreed well with that of the authentic a-TezMoO7 reported in the literature (Baran et al., 1981; Dimitriev et al., 19811, and was obviously different from those of the component oxides. Phase Transition of a-TeNo07 to the Less Active /3-Form. Calcination of Mo03.2Te02 at 600 "C in air gave an orange-yellow transparent glass, which was identified as P-TenMoO7, suggesting the double endotherm a t 528 and 542 "C in DTA (Figure 4) to be melting of a-TezMo0.1followed by the phase transition to the p-form. The Mo03.2Te02 calcined at 600 "C was amorphous as shown in Figure 6a (dashed line), and the IR is compared with the a-form in Figure 7 (dashed line), respectively,both in agreement with the literature (Bart et al., 1975; Dimitriev et al., 1981). The P-TezMo0.1was less active in the oxidation of ethyl lactate, of which conversion was 20% a t 300 "C with a space velocity of 3600 h-l for 5% ethyl lactate and 30% 0 2 , while the a-form showed 93% conversion at the same condition. Illustrative Flow Operation with Recycling Use of an Optimized Catalyst. It is difficult t o separate the product ethyl pyruvate from unreacted ethyl lactate by distillation as both of them boil a t the same temperature of 155 "C (Dean, 1973). Chemical separation such as acid-base extraction is not suitable for separa-
Wave Number / cm-I Figure 7. IR spectra of calcined Mo03.2Te02 and the component oxides.
-
m y 1 Lactate
100y-=-=-=@ Ethyl Pyruvate
l
o
50
.-
.
o Lactate
r
Pyruvate
Acetaldehyde
cLl-L-- 0 2 4 6 Time-on-Stream (h)
0 2 4 6 Time-on-Stream (h)
Figure 8. Unfavorable change ' activity of Moo3 in recycling use. Reaction condition: 300 "C, 5%ethyl lactate, 30% 0 2 , SV 1000 h-1. (a) First run, fresh catalyst; (b) second run, after recalcined at 400 "C.
observed a t 450-550 "C without weight loss suggest crystallization, melting, and/or phase transition. 2. SElWEPMA. Figure 5 shows SEIWEPMA for TeO2-MoO3 catalysts calcined at 400 (a) and 500 "C (b) t o compare the difference in morphology at temperatures below and above 450 "C, at which the first sharp peak was observed in DTA (Figure 4). It appears for MoOr2Te02 calcined at 400 "C (a) that TeOz of 5-10 Ethyl Lactate
3100
-E
Lactate
Pyru vate
Pyruvate
- -
L
50
(b)
50-
50
91
Actaldehyde
s
U E
Lactate
A
oO
2
4 6 8 1 0 1 2 Time-on-Stream (h)
A
.
.
r
.
*
Acetaldehyde
.
' Time-on-Stream (h)
0
2
4
6
Time-on-Stream (h)
Figure 9. Stable activity of Mo3.2Te02 in recycling use. Reaction condition: same as in Figure 8. (a) First run, fresh catalyst; (b) second run, after cooled overnight a t room temperature and used again; (c) third run, after recalcined at 500 "C.
Ind. Eng, Chem. Res., Vol. 34, No. 1, 1995 139 tion of the present two carboxylic acids. The policy in Literature Cited the practical application appears to find an operating b a u d , P. Y.; Averbuch-Pouchot, M. T.; Durif, A.; Guidot, J. condition capable of bypassing the separation problem Structure crystalline de l'oxyde mixte de molybdene-tellure: MoTe207. (Crystal structure of mixed oxide of molybdenumemploying a highly active catalyst. tellurium: MoTez07.)Acta Crystallogr. 1976,B32, 1417-1420. Figure 8 illustrates fxed-bed flow operations over Baran, E. J.; Botto, L. L.; Fournier, L. L. Das SchwingungsspekMoo3 for reference. A fresh catalyst of Moo3 showed a trum von a-TezMoO-, und ein Vorschlag zur Struktur der high activity to convert ethyl lactate over 99% as given Telluromolybdate zweiwertiger Kationen. (Vibrational spectrum in Figure 8a, first run, where the selectivity to pyruvate of a-TezMo07 and a proposal for the structure of telluromolybof 78% was not satisfactory, but in any case it is date of divalent cations.) Z . Anorg. AZlg. Chem. 1981,476,214220. practically needless to separate the unreacted lactate. Bart, J. C. J.; Petrini, G.; Giodano, N. The binary oxide systems However, the Moo3 showed a decrease in activity to 62% TeOz-MoOa. Z . Anorg. Allg. Chem. 1975,412,258-270. conversion of lactate at the same condition in recycling Cooper, A. J. L.; Gions, J. Z.; Meister, A. Synthesis and Properties use as in Figure 8b, second run. Although the selectivof a-Keto Acids. Chem. Rev. 1983,83, 321-358. ity of 78% in the first run (a) was improved to 93% in Daicel Chemical Co., Ltd. Preparation of Pyruvic Acid Esters. second run (b), separation of the unreacted lactate of Japanese Pat. 56-19854, May 9, 1981. Dean, J. A., Ed. Lunge's Handbook of Chemistry, 11th ed.; 38% is rather more disappointing. McGraw-Hill: New York, 1973; pp 7-218, 7-222. In contrast, the binary TeOz-MOOS catalyst of a Dimitriev, Y.; Bart, J . C. J.; Dimitrov, V.; Amaudov, M. Structure composition of Mo03.2Te02 calcined at 500 "C in air of Glasses of the TeOz-MOOS System. Z . Anorg. Allg. Chem. showed stable activity over 99% conversion with a high 1981,479,229-240. selectivity to pyruvate of 93% during 12 h on stream at Hayashi, H.; Shigemoto, N.; Sugiyama, S.; Masaoka, N.; Saitoh, 300 "C for fresh catalyst (a) followed by the second run K. X-ray photoelectron spectra for the oxidation state of TeO2MOOScatalyst in the vapor-phase selective oxidation of ethyl of 8 h on stream (b) at the same condition after once lactate to pyruvate. Catal. Lett. 1993a, 19, 273-277. cooled overnight a t room temperature, as shown in Hayashi, H.; Sugiyama, S.; Katayama, Y.; Sakai, K.; Sugino, M.; Figure 9. In the continued recycling use in the third Shigemoto, N. Liquid-phase oxidation of ethyl lactate to pyrurun, Figure 9c, after recalcination at 500 "C, the TeO2vate over suspended oxide catalysts. J. Mol. Catal. 1993b, 83, MOOScatalyst reproduced the excellent activity, while 207-217. Moo3 showed a drastic change in activity (Figure 8). Hayashi, H.; Sugiyama, S.; Shigemoto, N.; Miyaura, K.; Tsujino, S.; Kawashiro, K.; Uemura, S. Formation of an intermetallic It appears worthwhile to compare the activities of compound PdsTe with deactivation of Te/Pd/C catalysts for present catalyst TeOz-MoOa with those disclosed in selective oxidation of sodium lactate to pyruvate in aqueous patent literature as an estimate of economical feasibility phase. Catal. Lett. 1993c, 19, 369-373. in the possible industrial applications. An expired Hayashi, H.; Sugiyama, S.; Katayama, Y.; Kawashiro, K.; Shigepatent (Pfizer, 1963) describes 72% yield of ethyl pyrumoto, N. An alloy phase of Pd3Pb and the activity of Pb/Pd/C vate at 250-270 "C over Fe203-Mo03 supported on catalysts in the liquid-phase oxidation of sodium lactate to pyruvate. J. Mol. Catal. 1994, 91, 129-137. sintered A1203 (Norton SA1011 with an extremely low Howard, J. W.; Fraser, W. A. Pyruvic acid. In Organic Synthesis; space velocity of 120 h-l (residence time 30 s) for a feed Gilman, H., Ed.; Wiley: New York, 1945; Collect. Vol. 1, pp composition of 8.9% ethyl lactate and 18.7% 0 2 . An475-476. other more recent patent (Daicel, 1981) claimed 75.9% Joint Committee of Powder Diffraction Standard (JCPDS)29-915; yield of pyruvate with 97.2% conversion of methyl 30-1339. lactate at 310 "C over V ~ O ~ - M O O ~ - P ~ O ~ / for ( X 6% - A ~ ~ OPfizer, ~ C., and Co. Inc. Preparation of Esters. Japanese Pat. 383662, April 18, 1963. lactate and 8.8% 0 2 at space velocity 1060 h-l (residence Sugiyama, S.; Fukunaga, S.; Ito, K.; Ohigashi, S.; Hayashi, H. time 3.4 8). Conditions of the latter patent are similar Catalysts for vapor-phase dehydration of propylene glycol and to those given in Figure 9, where a high conversion of their application to pyruvic acid synthesis. J. Catal. 1991,129, lactate over 99% with 92.2% yield was observed repro12-18. ducibly, signifying the excellent performance of the Sugiyama, S.;Fukunaga, S.;Kawashiro, K.; Hayashi, H. Catalytic present catalysts. conversion of diethyl tartrate into pyruvate over silica-supported
Conclusions Binary oxides, TeOz-MoOs, converted ethyl lactate selectively to pyruvate in a vapor-phase fixed-bed flow system. A synergy in activity was observed, showing a sharp maximum at a chemical composition of Moos. 2TeO2, and it is concluded with evidence of powder XRD, IR, DTA-TGA, and SEM/EPMA that a-Te2Mo0, is the active species for the present system. Both ethyl lactate and ethyl pyruvate boil a t the same temperature of 155 "C,leading to difficulties in separation, but an optimized catalyst revealed reproducible stable activities in the recycling use with a high conversion of ethyl lactate over 99% and 93% selectivityto pyruvate at 300 "C, providing a favorable route bypassing the separation problem.
potassium disulfate. Bull. Chem. SOC.Jpn. 1992, 65, 20832085. Sugiyama, H.; Shigemoto, N.; Masaoka, N.; Suetoh, S.; Kawami, H.; Miyaura, K.; Hayashi, H. Vapor-phase oxidation of ethyl lactate to pyruvate over various oxide catalysts. Bull. Chem. SOC.Jpn. 1993, 66, 1542-1547. Tsujino, T.; Ohigashi, S.; Sugiyama, S.; Kawashiro, K.; Hayashi, H. Oxidation of propylene glycol and lactic acid in aqueous phase catalyzed by lead-modified palladium-on-carbon and related systems. J . Mol. Catal. 1992, 71,25-35. Received for review January 21, 1994 Revised manuscript received September 12, 1994 Accepted September 24, 1994@ IE940037Q Abstract published in Advance ACS Abstracts, December 1, 1994. @
