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Catalytic Degradation of Acrylonitrile-Butadiene-Styrene into Fuel Oil 1. The Effect of Iron Oxides on the Distribution of Nitrogen-Containing Compounds Mihai Brebu,† M. Azhar Uddin, Akinori Muto, and Yusaku Sakata* Department of Applied Chemistry, Okayama University, Tsushima Naka, Okayama 700-8530, Japan
Cornelia Vasile “P. Poni” Institute of Macromolecular Chemistry, 41 A Gr. Ghica Voda Alley, Ro 6600 Iasi, Romania Received June 22, 2000. Revised Manuscript Received December 22, 2000
The thermal and catalytic degradation of acrylonitrile-butadiene-styrene copolymer (ABS) was performed at 400 °C in static nitrogen atmosphere, by semibatch operation. γ-Fe2O3, an Fe3O4-C composite, and R-FeOOH were used as catalysts, in two contact modes: either mixed with the polymer or in contact with the volatile degradation products of ABS. All iron oxides decrease the concentration of nitrogen (N) in ABS degradation oil. Reactions in a flow-type reactor with 4-phenylbutyronitrile as model N-containing compound show that R-FeOOH is active at low temperatures (250-300 °C) in converting heavy N-containing compounds into light aliphatic nitriles. XRD analysis proved that during reaction R-FeOOH was transformed into Fe3O4 in several steps, with R-Fe2O3 as intermediary compound.
Introduction The degradation of waste plastic into fuel is a sustainable method for saving valuable petroleum resources.1 Acrylonitrile-butadiene-styrene copolymer (ABS), present in municipal plastic waste in an amount of 3-5 wt %,2 causes serious problems during thermal degradation due to the nitrogen (N) coming from the acrylonitrile units. The N-containing compounds are not desired in the fuel oil because they lead to the corrosion of the engine parts and the formation of very harmful compounds such as HCN or NOx. In our previous papers3,4 we reported that nitrogen (N) is present in the thermal degradation oil of ABS, mainly in compounds such as aliphatic and aromatic nitriles. 4-Phenylbutyronitrile is the main N-containing compound obtained from ABS thermal degradation (ca. 19 wt % in oil).4 Hydrogen cyanide and heterocyclic compounds containing one or two N atoms such as pyridine, pyrimidine, and quinoline were identified in * Author to whom correspondence should be addressed. Fax: +8186-251-8082. E-mail:
[email protected]. † Permanent address: “P. Poni” Institute of Macromolecular Chemistry, Ro 6600 Iasi, Romania. (1) Sakata, Y.; Uddin, M. A.; Muto, A.; Koizumi, K.; Narazaki, M.; Murata, K.; Kaiji, M. Polym. Recycling 1996, 2, 309-315. (2) Kajiyama, R. Proceedings of the 1st International Symposium on Feedstock Recycling of Plastics, Sendai, Japan, 1999; pp 9-12. (3) Brebu, M.; Sakata, Y.; Uddin, M. A.; Muto, A.; Murata, K.; Vasile, C. Proceedings of the 1st International Symposium on Feedstock Recycling of Plastics, Sendai, Japan, 1999; pp 123-126. (4) Brebu, M.; Uddin, M. A.; Muto, A.; Sakata, Y.; Vasile, C. Energy Fuels 2000, 14, 920.
small amounts. Day and co-workers5 reported similar N-containing compounds in the ABS pyrolysis oil obtained by pyrolysis/gas chromatography/mass spectrometry experiments. Valuable substituted aromatics such as toluene, ethylbenzene, styrene, isopropylbenzene, and R methylstyrene represent more than 50 wt % of the degradation oil, making the ABS thermal degradation attractive to obtain hydrocarbons or fuel. With this aim it is necessary to decrease as low as possible the concentration of N-containing compounds in the degradation oil. This paper presents the results obtained from catalytic degradation of ABS in semibatch reactor over different iron oxides. The effect of three catalysts (γFe2O3, an Fe3O4-C composite, and R-FeOOH) was tested, using two contact modes. The catalysts were either mixed with the polymer or laid in the atmosphere of ABS volatile degradation products. Gas chromatography with four different types of detector and XRD analysis were used in order to clarify the changes in the amount or/and distribution of N-containing compounds in the degradation oil, and, respectively, the changes in the structure of the catalysts. Degradation of 4-phenylbutyronitrile as N-containing model compound over R-FeOOH was performed in a flow-type reactor in order to establish the active phase of the catalyst and the temperature range for its highest activity. (5) Day, M.; Cooney, J. D.; Touchette-Barrette, C.; Sheehan, S. E. J. Anal. Appl. Pyrolysis 1999, 52, 199-224.
10.1021/ef000124x CCC: $20.00 © 2001 American Chemical Society Published on Web 03/28/2001
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Figure 1. Schematic diagram of ABS thermal and catalytic degradation at 400 °C by semi-batch operation.
Experimental Section 1. Materials. Acrylonitrile-butadiene-styrene powder copolymer (ABS) containing 19-22% acrylonitrile, 37-39% butadiene, and 30-32% styrene units was obtained from Aldrich Chemical Co. 4-Phenylbutyronitrile (99% purity), obtained also from Aldrich Chemical Co., was used as a model N-containing compound in some degradation experiments. Maghemite (γ-Fe2O3; TR 99701), a magnetite-carbon composite (Fe3O4-C; TR 990517), and goethite (R-FeOOH; PDC02) iron oxide catalysts were cooperatively developed with Toda Kogyo Corporation, Hiroshima, Japan. 2. Degradation Experiments. 2.1. Semibatch Reactor. ABS thermal and catalytic degradation was performed at 400 °C in a glass reactor, under a static atmosphere of nitrogen, using semibatch operation (Figure 1). The following temperature program was used: room temperature f 120 °C (rate ) 3 °C/min; hold 60 min, 20 mL/min N2 flow) f 400 °C (rate ) 3 °C/min; hold 540 min). The zero time for the experiments was taken when the furnace was heated from 120 °C to the final degradation temperature of 400 °C. The catalysts were used either mixed with the polymer at the bottom of the reactor (liquid-phase contact modesLPC) or laid on a stainless steel net fixed at 120 mm from the bottom of the reactor (vapor-phase contact modesVPC).6 A 2 g amount of iron oxides was used for degradation of 10 g ABS polymer. The degradation products were characterized by gas chromatography using different types of detectors: thermal conductivity (GC-TCD; YANACO G180) for gas analysis, flame ionization (GC-FID; YANACO G6800) for global characterization by C-NP-gram of the degradation oil, atomic emission (GCAED; HP G2350A) to determine the distribution of N-containing compounds in oil, and mass spectrometry (GC-MSD; HP 5973) to identify the chemical structure of the compounds in the degradation oil. Details on the experimental setup and the conditions for the chromatographic analysis of the products were presented elsewhere.4 2.2. Flow-Type Reactor. Degradation experiments of 4-phenylbutyronitrile as a model N-containing compound (MC) over 2 g of R-FeOOH catalyst of 1 mm diameter size were performed at 250, 300, and 400 °C in a flow-type reactor (Figure 2). The catalyst was placed in the middle of the reactor, between two layers of glass beads. The reactor was flushed with nitrogen (N2) at a flow rate of 20 mL/min, to evacuate the air. The N2 flow was stopped after the furnace was heated to the final temperature for reaction and the liquid reactant was feed at a rate of about 1 mL/h by bubbling N2 gas with a very low flow rate in a recipent with MC, connected to the reactor by a feeding tube. The total time for the reaction was about 2 h. The upper part of the reactor was fixed out of the furnace so the eventually vaporized MC will condense and return in the reaction zone. Due to the high layer of catalyst (∼3 cm) and the low feeding rate, this reactor ensures a long contact time (6) Sakata, Y.; Uddin, M. A.; Muto, A. J. Anal. Appl. Pyrolysis 1999, 51, 135-155.
Figure 2. Schematic diagram of the flow-type reaction experiment. between the MC and the catalyst comparable with ABS residence time in the semibatch reactor. GC-MSD was used for the qualitative analysis of the liquid degradation products. XRD analysis (Shimadzu XD 3A diffractometer, Cu KR radiation) was used for the analysis of the crystalline structure of the catalyst. Identification of the structures was made according to an X-ray powder data file.7
Results and Discussions 1. Thermal and Catalytic Degradation of ABS by Semibatch Operation. Thermal and Catalytic Degradation of ABS by Semibatch Operation. 1.1. Degradation Behavior and Global Characterization of Products. The products of ABS degradation were classified in three groups: gas, oil and degradation residue that represents in LPC mode the tar remaining at the bottom of the reactor and in VPC mode the tar plus the oil adsorbed on the catalyst. The amount of the residue was calculated by subtracting the amount of the catalyst from the amount of the remaining materials in the reactor. Figure 3 shows the cumulative volume of the oil product obtained from 10 g ABS thermal and catalytic degradation at 400 °C, over iron oxides, in liquid- and vapor-phase contact modes. The oil started to accumulate in the graduated cylinder after about 80 min from the time zero of the experiment, when the temperature in the furnace reached about 360 °C. The rate of oil accumulation after the temperature in the furnace reaches 400 °C is presented in Figure 4. In Figures 3 and 4 the curves corresponding to liquid evolution by ABS degradation over different iron oxide catalysts are very close to each other and also very similar to the curve of ABS thermal degradation. However vapor-phase catalysis produced less volume of oil compared with the thermal and liquid-phase contact catalytic degradation. Slight differences in the effect of different types of iron oxide catalysts on ABS degradation appeared for LPC mode, R-FeOOH showing higher (7) Smith, J. V.; Berry, L. G.; Post, B.; Weissmann, S. X-ray Powder Data File; American Society for Testing and Materials: Philadelphia, 1967.
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Figure 3. Cumulative volume of liquid products from thermal and catalytic degradation of ABS at 400 °C. Figure 5. Variation of the temperature at the outlet of the reactor (T outlet) during the degradation process.
Figure 4. The rate of oil accumulation during thermal and catalytic degradation of ABS at 400 °C.
volume of oil and higher rate of oil accumulation with respect to γ-Fe2O3 and Fe3O4-C catalysts. The lower liquid volume is obtained by decomposition over Fe3O4-C catalyst, both in liquid- and vapor-phase contact mode. For thermal degradation of ABS at 400 °C the rate of oil accumulation decreases more than three times in the first 30 min and then has a slight increase with a maximum at about 150 min from the time zero of the experiment (Figure 4). During catalytic degradation the rate of oil accumulation decreases then remains approximately constant, especially for the vapor-phase contact mode. From these data it seems that thermal degradation of ABS occurs in different kinetic steps. The rate of oil accumulation is slightly higher in the first moments of catalytic degradation with respect to ABS thermal degradation but after 40 min it becomes lower. That could be due to the decrease of the catalytic effect of the iron oxides during the degradation process. Figure 5 shows the temperature measured at the outlet of the reactor (T outlet, indicated in Figure 1). Figure 6 shows the temperature measured inside the
Figure 6. Variation of the temperature inside the reactor (T inside) during the degradation process.
reactor (T inside, also indicated in Figure 1), at 130 mm from the bottom, close to the layer of catalyst in vaporphase contact mode. In thermal and LPC catalytic degradation of ABS, the temperature at the outlet of the reactor presents a maximum in the first 100 min and then slowly decreases (Figure 5). This is due to the high rate of ABS degradation at the beginning of the reaction, producing a lot of volatile degradation compounds. When the catalysts are used in VPC mode the T outlet shows a minimum value at about 200 min. This is contrary to the case of LPC mode. The T inside is also about 20 °C lower with respect to LPC mode (Figure 6). These results suggest that endothermic chemical reactions occurs that are not compensated by the controlled temperature at the bottom of the reactor like the case of LPC mode. As a result the temperature of the volatile products leaving the reactor decreases, especially in the
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Table 1. Material Balance for Thermal and Catalytic Degradation of 10 g of ABS at 400 °C product yield (wt %) methoda thermal γ-Fe2O3 (LPC) γ-Fe2O3 (VPC) Fe3O4-C (LPC) Fe3O4-C (VPC) R-FeOOH (LPC) R-FeOOH (VPC)
gas (G)b
oil (L)
residue (R)
oil density (g/mL)
N in oil (mg/mL)
7.2
49.5
43.3
0.868
39.9
8.6 6.9 10.0 6.7 10.4 9.5
44.3 41.3 41.8 39.6 48.7 40.7
47.1 51.8 48.2 53.7 40.9 49.8
0.872 0.860 0.871 0.861 0.885 0.865
22.7 29.9 25.2 26.6 25.8 23.8
a LPC: liquid-phase contact mode; VPC: vapor-phase contact mode. b G ) 100 - (L + R).
Table 2. Balance of N in Different Degradation Products N in degradation products (wt % of initial N content of polymera) gases (G) method thermal γ-Fe2O3 (LPC) γ-Fe2O3 (VPC) Fe3O4-C (LPC) Fe3O4-C (VPC) R-FeOOH (LPC) R-FeOOH (VPC) a
NH3
HCN
oil (L) HCN
organic N
residue (R)b
3.25
0.59
1.92
32.32
61.92
14.29 2.52 11.55 2.03 19.96 8.88
1.29 0.26 0.43 0.23 0.68 1.21
0.70 1.25 0.36 0.98 0.45 0.50
16.64 20.31 17.80 17.43 20.90 16.40
67.08 75.66 69.86 79.33 58.01 73.01
665 mg of N in 10 g of polymer. b R ) 100 - (G + L).
first moments of the process, when the catalysts activity is high. The decrease of the T outlet and the constant value of the T inside after 400 min of the process shows that the catalytic reaction is almost finished. In these conditions a small amount of volatile products leaves the reactor and the T inside temperature shows only the effect of the heat transfer inside the reactor, without significant calorific contribution of chemical reactions. From the material balance of 10 g ABS thermal and catalytic degradation at 400 °C (Table 1) one can see that the catalysts decrease the amount of oil and increase the amount of the residue. Catalysis in VPC mode gives lower amount of gases and the oil is obtained in smaller amount, having lower density with respect to the case of catalysis in LPC mode. Among the studied iron oxides, R-FeOOH gives the highest amount of gases and the obtained oil has the highest density, especially when this catalyst is used in LPC mode. The concentration of N in oil decreases from 40 mg/ mL in the case of thermal degradation, to 23-30 mg/ mL when the iron oxides are used (last column in Table 1). The total amount of HCN (sum of columns 3 and 4 in Table 2) also decreases. Comparing with ABS thermal degradation the amount of NH3 in gases (column 2 in Table 2) increases by a factor of 3.5-6 when γ-Fe2O3 and Fe3O4-C catalysts are used in LPC mode and decreases for degradation in VPC mode. For catalytic degradation using R-FeOOH, ammonia is produced in the highest amount in both LPC and VPC modes. However, in VPC mode the amount of ammonia is less than a half of the amount obtained in LPC mode. 1.2. Composition of the Products Obtained from ABS Thermal and Catalytic Degradation. The main compounds from the gaseous products of ABS degradation (Table 3) are methane (40-47 v %) and ethane (21-24 v %). Other hydrocarbons in gases are ethylene (8.3-
Figure 7. C-NP-gram (a) and N-NP-gram (b) of ABS thermal and catalytic degradation oil.
10.6 v %), propylene (6.2-7.8 v %), propane (7.4-8.5 v %), unsaturated C4 (3.6-8 v %), and saturated C4 (2.84.9 v %). The catalysts did not significantly change the global composition of the gaseous products. However, the VPC mode slightly increases the amount of methane and unsaturated C2 and C4, decreasing the amount of saturated hydrocarbons, with respect to LPC mode. The C-NP-gram8 (Figure 7a; C stands for carbon and NP for normal paraffin) and N-NP-gram9 (Figure 7b; N stands for nitrogen) show the distribution of the compounds (wt %) and the concentration of N (mg/mL) in the degradation oil, with the carbon number, equivalent to the boiling point of normal paraffins. A detailed explanation of the meaning and the procedure to obtain the NP-grams were described in a previous paper.4 The pattern of C-NP-grams is typical for the oil obtained from ABS thermal degradation. The iron oxide catalysts did not change the global aspect of the C-NPgrams but only the ratio between the main peaks. The same results were obtained using these catalysts for the catalytic degradation of PVC mixed plastics.10 (8) Murata, K.; Makino, M. Nippon Kagaku Kaishi 1975, 1, 192200. (9) Shiraga, Y.; Uddin, M. A.; Muto, A.; Narazaki, M.; Sakata, Y.; Murata, K. Proceedings of the 1998 International Symposium on Advanced Energy Technology, Sapporo, Japan; 1998; pp 185-192. (10) Uddin, M. A.; Sakata, Y.; Shiraga, Y.; Muto, A.; Murata, K. Ind. Eng. Chem. Res. 1999, 38, 1406-1410.
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Table 3. The Composition (v %) of Gaseous Products Obtained from ABS Thermal and Catalytic Degradation at 400 °C method
CH4
C2H4
C2H6
C3H6
C3H8
C4H8
C4H10
C5-fraction
thermal γ-Fe2O3 (LPC) γ-Fe2O3 (VPC) Fe3O4-C (LPC) R-FeOOH (LPC) R-FeOOH (VPC)
46.88 40.83 44.05 41.70 40.55 41.60
8.34 9.13 10.43 10.61 9.42 10.58
24.04 22.40 22.16 21.00 21.62 20.92
6.22 7.14 6.63 7.34 7.78 7.35
7.82 8.16 7.41 7.64 8.50 7.71
3.58 6.25 6.51 8.01 7.19 8.09
3.11 4.84 2.81 3.68 4.94 3.75
0.01 1.25
Comparing with ABS thermal degradation, the iron oxide catalysts slowly increase the amount of the substituted aromatics (toluene, ethylbenzene, styrene, cumene, and R methylstyrene) at n-C8-n-C10 range in the oil product and decrease the amount of compounds at n-C13 (Figure 7a). Catalytic degradation of ABS also produces small amounts of new hydrocarbons in n-C20n-C24 range that are not present in the oil obtained by thermal degradation. N-NP-grams (Figure 7b) show that iron oxide catalysts decrease the concentration of all N-containing products in ABS degradation oil. The contact mode for catalysis has a very important role for the N distribution in the oil. When the catalysts are mixed with ABS polymer at the bottom of the reactor (LPC mode) the concentration of light aliphatic nitriles from n-C5-nC6 range decreases to about a half of the concentration in thermal degradation oil. The concentration of Ncontaining products from n-C10, that we suppose to be aniline, pyridine, or amino derivatives, is also decreased to less than a half. However, 4-phenylbutyronitrile, the main N-containing compound obtained from ABS thermal degradation (9.9 mg/mL), still has a very high concentration after catalytic degradation in LPC mode (more than 6.2 mg/mL), as shown by the peak at n-C13. When the catalysts are used in VPC mode the concentration of 4-phenylbutyronitrile decreases to 4.43.8 mg/mL, but the concentration of light aliphatic nitriles from n-C5-n-C6 is higher with respect to LPC mode. In VPC mode R-FeOOH gives a similar amount of 4-phenylbutyronitrile and N-containing compounds at n-C10 as Fe3O4-C catalyst, but gives the lowest amount of light aliphatic nitriles at n-C5-n-C6. XRD analysis of the catalysts after degradation shows that the original crystalline structure of all iron oxides is totally converted to that of Fe3O4. In addition a small peak at the angle 2θ of 44.5° appears that may correspond to metallic iron obtained by advanced reduction of iron oxides in the atmosphere of organic radicals produced during ABS degradation. This supplemental peak is not observed in XRD diffractogram of R-FeOOH catalyst used in vapor-phase contact mode. (2) Thermal and Catalytic Degradation of Model N-Containing Compound in Flow-Type Reactor. 4-Phenylbutyronitrile is the main N-containing compound obtained from ABS degradation.4 Its amount in the oil decreases when iron oxide catalysts are used for degradation, especially in VPC mode. This could be due either to adsorption on catalyst surface or to chemical degradation over the catalyst. To elucidate this aspect we performed thermal and catalytic degradation of pure 4-phenylbutyronitrile as a model N-containing compound (MC) in a flow-type reactor. We choose R-FeOOH as catalyst because its effect on the N distribution in ABS degradation oil is the highest and also is different from γ-Fe2O3 and Fe3O4-C and because of the strong
0.02
dependence of its activity on the contact mode (liquidor vapor-phase), as mentioned before. The flow-type reactor was preferred to the semibatch one because of the better homogeneity of the temperature and because the retention time of the reactant inside the reactor can be controlled by changing the feeding rate. Experiments were performed thermally at 250 °C over quartz wool and catalytically at 250, 300, and 400 °C over R-FeOOH. The produced oil was analyzed by GC-MS and the crystalline structure of the catalyst was determined by XRD. Figure 8 presents the GC-MS chromatograms of 4-phenylbutironitrile as MC and of its liquid degradation products obtained in flow-type reactor at different temperatures. A main peak appears in all chromatograms at retention time interval of 12.2 to 14 min that corresponds to MC. The 4-phenylbutironitrile contains some impurities (propane, nitrobenzene, benzeneacetonitrile, tributylamine, benzenepropanol, and some substituted aromatics), shown by the peaks in Figure 8a. Thermal degradation of MC at 250 °C over quartz wool gives mainly degradation products of the impurities. This is proved by the apparition of new peaks (e.g., acetone and butanal) and by the disappearance or the decrease for the peaks of the impurities (Figure 8b). R-FeOOH clearly affects the degradation of 4-phenylbutyronitrile even at 250 °C, very close to the boiling point of MC. The chromatogram in Figure 8c shows new peaks corresponding to aliphatic nitriles (acetonitrile, propanenitrile, butane- and isobutane-nitrile, pentenenitrile) and substituted aromatics (toluene, ethylbenzene, styrene). Benzonitrile (compound 4 in Figure 8c) comes from the degradation of benzeneacetonitrile impurity, and dibuthylamine (compound 5 in Figure 8c) is a degradation product of tributylamine impurity. Increasing the temperature for degradation at 300 °C, new degradation compounds are formed, both lighter and especially heavier than MC (Figure 8d). At 400 °C (chromatogram in Figure 8e) the number and the amount of light aromatic derivatives (retention time lower than 12 min) increases but also the amount of heavy hydrocarbons (retention time higher than 14 min). Some compounds with condensed rings were identified so the degradation products have low quality. These results proved that R-FeOOH has a good selectivity for degradation of 4-phenylbutyronitrile into light aliphatic nitriles only at temperatures lower than 300 °C. XRD analysis of the catalyst (Figure 9) shows the changes appearing in the structure of R-FeOOH after reaction with 4-phenylbutyronitrile. After 2 h of reaction at 250 °C the crystalline structure of R-FeOOH is completely transformed into that of R-Fe2O3. All peaks except for the one at the angle 2θ of 35.5° are broad, suggesting that at this stage R-Fe2O3 has not an advanced degree of crystallinity. After degradation at
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Figure 8. GC-MS chromatograms of model compound (MC) and of its liquid degradation products obtained in flow-type reactor at different temperatures.
R-FeOOH f R-Fe2O3 f Fe3O4. A detailed discussion on the changes in the structure and the catalytic activity of R-FeOOH are presented in a subsequent paper.11 Conclusions Maghemite (γ-Fe2O3), the magnetite-carbon composite (Fe3O4-C), and goethite (R-FeOOH) iron oxides are catalytically active for decreasing the concentration of nitrogen in ABS degradation oil. R-FeOOH is selective in converting 4-phenylbutyronitrile into light aliphatic nitriles only at low temperatures of degradation (250300 °C), as proved by reaction in flow-type reactor. During degradation reaction of 4-phenylbutyronitrile as a model N-containing compound over R-FeOOH, the structure of the catalyst is gradually changed, depending on the temperature. The catalyst is totally converted to R-Fe2O3 above 250 °C and to a mixture of R-Fe2O3 and Fe3O4 at 300 °C. After the reaction at 400 °C R-FeOOH is totally converted into Fe3O4. Figure 9. XRD analysis of R-FeOOH fresh and after reaction with model compound at different temperatures.
300 °C the peaks in the XRD diffractogram corresponding to R-Fe2O3 become sharp, proving a high degree of crystallinity, and new peaks corresponding to Fe3O4 appear. When the degradation reaction of 4-phenylbutyronitrile over R-FeOOH was conducted at 400 °C, the catalyst was totally converted to Fe3O4. These changes can be schematically represented as follows:
Acknowledgment. The authors are thankful to Dr. Katsuhide Murata, Dr. Jale Yanik, and Dr. Nakka Lingaiah for their assistance and support in performing and interpreting the analysis. This research was partially supported by Okayama Foundation for Science and Technology. EF000124X (11) Brebu, M.; Uddin, M. A.; Muto, A.; Sakata, Y.; Vasile, C. Energy Fuels 2001, 15, 565. (Submitted as part 2 of this paper.)
