Evolution with the Temperature of the Compounds ... - ACS Publications

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Ind. Eng. Chem. Res. 2008, 47, 6896–6903

Evolution with the Temperature of the Compounds Obtained in the Catalytic Pyrolysis of Polyethylene over HUSY A. Marcilla,* M. I. Beltrań, and R. Navarro Dpto. Ingeniería Química, UniVersidad de Alicante, Apartado 99, 03080 Alicante, Spain

The catalytic decomposition processes of low- and high-density polyethylene (LDPE and HDPE) have been studied using HUSY zeolite as catalyst in a batch reactor under dynamic conditions. The evolution of the gaseous and condensed products evolved with respect to the temperature has been analyzed and compared to that obtained under similar conditions in the thermal pyrolysis of LDPE and HDPE. The behavior of the gases generated from both polyethylenes was similar, olefins being the more abundant species. Great changes were observed in the composition of the gases evolved at different temperatures. Isoparaffins and olefins showed two maxima at low and high temperatures, whereas the remaining compounds generated presented only one maximum at high temperatures. Analysis of condensed products revealed some differences between the two polyethylenes at the end of the process. Two maxima, one at low and another at high temperatures, appeared in the catalytic pyrolysis of HDPE, where isoparaffins and aromatics were the most abundant condensed products obtained at each maximum. However in the case of LDPE, n-paraffins were the main products at the very end of the process. These different outcomes could be related to the progressive deactivation of the zeolite. 1. Introduction The thermal pyrolysis of plastic wastes produces a broad distribution of hydrocarbons, from methane to waxy products. This process takes place at high temperatures. The gaseous compounds generated can be burned out to provide the process heat requirements, but the overall yield of valuable gasolinerange hydrocarbons is poor, so that the pyrolysis process as a means for feedstock recycling of the plastic waste stream is rarely practiced on an industrial scale at present.1,2 In contrast, thermal cracking at low temperatures is usually aimed at the production of waxy oil fractions, which may be used in industrial units for steam cracking and in fluid catalytic cracking units.3 An alternative to improve gasoline yield from plastics pyrolysis is to introduce suitable catalysts. High conversions and interesting product distributions are obtained when plastics are cracked over zeolites.4-6 Moreover the catalytic cracking of polymers has proven itself to be a very versatile process, since a variety of products can be obtained depending on the catalyst,7-10 the polymer,11,12 the reactor type,13,14 and the experimental conditions used,15,16 among other variables. Most published studies concentrate on discussing the results obtained from the analysis of the global composition of the products generated. However, studies regarding the evaluation of the composition of the products generated in the degradation process at different conversion levels are scarcely available. This type of study can provide very interesting information regarding the reaction sequences during the course of the degradation process or the deactivation suffered by the catalysts. Hesse et al.,17 using a repetitive-injection gas chromatograph/ mass spectrometer (GC/MS) evolved gas analyzer, studied the volatiles obtained from the catalytic cracking of polyethylene over HY catalyst at 5 min intervals and a heating rate of 2 °C min-1. Paraffins were more abundant than olefins, but the evolution profiles for both compound types had similar trends. Aromatics were also generated, although they were detected at somewhat higher temperatures than paraffins and olefins. In a * To whom correspondence should be addressed. E-mail: [email protected].

similar way, Manos et al.18 studied the evolution of the products generated in the catalytic degradation of HDPE over the USY zeolite. Isobutane and isopentane were the main gaseous products in the presence of USY. These authors observed that since the evolution of products depended on the temperature, gaseous and liquid olefins were formed in smaller amounts and later than paraffins. High reactor temperatures and heating rates favored the formation of slightly greater amounts of olefins and aromatics among the liquid products. Bagri et al.19 studied the catalytic pyrolysis of the gases evolved from polyethylene pyrolysis when they were passed through a bed composed of zeolite Y. It was found that with increasing Y zeolite bed temperatures gaseous isoparaffin yield decreased, whereas gaseous olefins increased. In addition, there was a marked increase in the concentration of aromatics in the derived oils with increasing pyrolysis temperature. From the analysis with time of the evolution of products generated, Lin et al.20 studied the deactivation of the catalysts employed in cracking processes. Lin used GC to analyze the gases obtained in the catalytic degradation of polypropylene over several catalysts in a fluidized bed reactor operating isothermally at ambient pressure. Catalyst deactivation was reflected by a decrease in the amount of isobutane produced (a product of bimolecular reactions) and the relative increase in olefins as butenes and pentenes (products of monomolecular reactions). The larger-pore zeolites (HUSY and HMOR) were rapidly deactivated, in contrast to the more restrictive HZSM5 and nonzeolitic catalysts (SAHA and MCM-41). In a recent study,21 the TG/FTIR technique was applied to analyze the products evolved during the catalytic pyrolysis of polyethylene and to investigate the progressive deactivation suffered by HZSM5 and HUSY zeolites. There, appreciable differences were found between the use of fresh and progressively coked catalyst. When the coked catalyst is used in successive pyrolysis cycles, the gases formed are increasingly those generated in the thermal process. The deactivation process suffered by HZSM5 was more progressive than that suffered by HUSY.

10.1021/ie800520u CCC: $40.75  2008 American Chemical Society Published on Web 08/06/2008

Ind. Eng. Chem. Res., Vol. 47, No. 18, 2008 6897

Figure 1. Scheme of the batch reactor system.

This paper presents the results obtained during the catalytic pyrolysis of low- and high-density polyethylene (LDPE and HDPE, respectively) over HUSY catalyst carried out in a batch reactor under dynamic conditions. The evolution of the gaseous and liquid products with the temperature is described in depth and discernible changes in the yield of some compounds are interpreted as a consequence of the loss in catalyst activity. 2. Experimental Section 2.1. Materials. Two powdered polymers were used in this study: a low-density polyethylene (LDPE 780R from DOW Chemical) with a density of 0.923 g cm-3 and a melt flow index of 20 g/(10 min), and a high-density polyethylene (HDPE HD3560UR from BP) with a density and melt flow index of 0.935 g cm-3 and 6 g/(10 min), respectively. Both polyethylene samples present a maximum particle size of 500 µm. The catalyst used was HUSY, provided by GRACE-Davison. HUSY is a microporous zeolite which presents supercages (12 Å), tetrahedrally connected by 12-membered ring windows of 7.4 Å diameter, and has a particle size of 1 µm. BET surface area (614 m2 g-1) and micropore volume (0.29 cm3 g-1) were determined by N2 adsorption-desorption at 77 K using an Autosorb-6 Quantachrome apparatus. Data obtained from temperature programmed desorption (TPD) of ammonia, using a Netzsch TG 209 thermobalance, showed a single very broad peak centered at 154 °C with a corresponding ammonia desorption of 2.12 mmol NH3 g-1. 2.2. Equipment and Experimental Procedure. The pyrolysis was carried out in a quartz batch reactor (Figure 1), heated by an electric furnace. The body of the reactor was a cylinder of 470.0 mm height and 30.7 mm internal diameter. A crucible containing 600 mg of polymer and 60 mg of HUSY was placed in the middle of the reactor. A heated exit was located at the top of the reactor. A constant stream of nitrogen was fed to the bottom of the reactor at 150 mL min-1 (STP) for 30 min prior to the experiment and was maintained during the process. The heating rate was 5 °C min-1, and the temperature was varied from 30 to 550 °C. The condensed products generated were collected in cooling traps installed at the reactor outlet, while the gases were collected in sampling bags placed after the cooling traps. From the beginning of the degradation process, condensed and gaseous products were

collected at 5 min intervals (corresponding to approximately 25 °C temperature increments). Under these reaction conditions, the polymer was completely degraded and no solid residue apart from the coke deposited on the catalyst was observed after each experiment. The results shown hereafter are compared with those discussed in a previous paper where the thermal pyrolysis of LDPE and HDPE were carried out under the same conditions described here. A more detailed analysis of the thermal process can be found elsewhere.22 2.3. Analysis of the Products Generated. Gases were identified and quantified using standard gaseous hydrocarbons (Scott Specialty Gases) by an Agilent 6890N gas chromatograph (GC) with a flame ionization detector (FID) and a GS-alumina column (30 m × 0.53 mm i.d.). The peaks that could not be identified were determined by a GC (Agilent 6890N) coupled to a MSD 5973N mass spectrometer (MS) with a GS-GasPro column (30 m × 0.32 mm i.d.). The quantity of condensed products (liquids and waxes) was determined by weighing the preweighed cooling traps after the experiments. Liquids and waxes were extracted with n-hexane and analyzed by GC-MS, using a HP-5MS column (30 m × 0.25 mm i.d.). The internal library (Willey 275) was used for identification of the compound type, whereas the carbon number of the compound was determined by standards. The coke content of zeolites was determined by combustion in oxygen atmosphere (30 mL min-1 STP) in a Mettler Toledo thermobalance (TGA/SDTA851e/LF/1600). Dynamic experiments were carried out from 30 to 900 at 10 °C min-1. 3. Results The series of experiments was duplicated in order to check their reproducibility. Mass balances (gases, condensed products, and coke) of LDPE-HUSY and HDPE-HUSY systems, averages of both experiments, were 98.0 and 82.4%, respectively. The condensation of some high molecular weight compounds on reactor walls, as well as chromatographic analysis precision, may explain the deviations observed, especially in the case of HDPE-HUSY. Figure 2 shows the yield per gram of polyethylene of the gaseous and condensed products obtained at regular intervals of 25 °C (time intervals of 5 min) during the catalytic pyrolysis

6898 Ind. Eng. Chem. Res., Vol. 47, No. 18, 2008

Figure 2. Evolution of the yield of gaseous and condensed products obtained in the thermal and catalytic pyrolysis of LDPE and HDPE with HUSY.

of LDPE and HDPE with HUSY. The results obtained in a previous study of the thermal pyrolysis of LDPE and HDPE have also been included at the bottom of the figure for comparison.22 As can be observed, the incorporation of HUSY in the reaction medium modifies the evolution of the products generated, due to a change in the reaction mechanism. The generation of gases and condensed products for the LDPEHUSY and HDPE-HUSY systems starts at relatively low temperatures (around 200 °C), but the reaction rate is quite low at the beginning, and the process takes place in a wide range of temperatures, even reaching the temperatures exhibited in the thermal process. As expected, the yield of gases is much higher in the catalytic than in the thermal processes. Comparing both catalytic processes, the yield of condensed products is higher than that of gaseous products in the LDPE-HUSY system, whereas for HDPE-HUSY the contrary trend is observed. Different results can be found in the literature depending on the experimental conditions which have been used. The catalytic cracking of LDPE with HUSY under isothermal conditions at 420 °C was carried out in a batch reactor by van Grieken et al.23 The liquid yield was slightly higher than the gas yield obtained. In a similar way, higher liquid yield was produced from the catalytic degradation of HDPE with HY under dynamic conditions up to mild temperatures by Seo et al.24 (450 °C in the same type of reactor) and by Manos et al.25 (360 °C in a semibatch reactor). However, Hernańdez et al.6 obtained a higher proportion of gases in the catalytic flash pyrolysis of HDPE with HUSY in a fluidized bed reactor working under isothermal conditions at higher temperatures (500, 600, 700, and 800 °C). The results of the analysis of the gases obtained in the catalytic pyrolysis of LDPE and HDPE with HUSY are presented in Figure 3 (grouped by compound type) and Figure 4 (grouped by carbon number). In the gases evolved during the catalytic pyrolysis of both polyethylenes, isoparaffins present two maxima, at low (around 275 °C) and high temperatures (around 450 °C), whereas the remaining compounds present only one maximum at high temperatures (Figure 3). Olefins are the

Figure 3. Evolution with the temperature of the yield of gases (grouped by compound type) obtained in the thermal and catalytic pyrolysis of LDPE and HDPE with HUSY.

major products for both catalytic systems at temperatures higher than 400 °C, whereas isoparaffins present the highest yields at temperatures lower than 400 °C. This behavior could not be observed for the thermal processes which have been included at the bottom of both figures for purposes of comparison. In this case a single step is observed for the evolution of products, and 1-olefins are the major products during the whole process. The yield of isoparaffins is very close to zero. With respect to the carbon number distribution (Figure 4), during the catalytic processes products with four, five, six, and seven carbon atoms are obtained right from the beginning of the process, while compounds with one, two, and three carbon atoms only appear at higher temperatures. The products with four, five, and six carbon atoms are the major compounds. The trend for the gases evolved from both polyethylenes is very similar. For the thermal processes shown at the bottom of Figure 4, hydrocarbons with carbon chain lengths of C2 to C6 are mainly detected, all in similar yields. Both polymers behave in a similar way, as was also discussed in a prior study,21 where the TG/FTIR analysis of the gases evolved in the catalytic pyrolysis of polyethylene with HUSY revealed that the IR spectra were independent of the polymer structure (LDPE and HDPE).


Figure 4. Evolution with the temperature of the yield of gases (grouped by carbon number) obtained in the thermal and catalytic pyrolysis of LDPE and HDPE with HUSY.

The condensed products exhibit a slightly different trend than the gaseous compounds. The evolution of the different condensed compounds is presented in Figure 5. As with the analysis of the gases the thermal processes are included at the bottom of the figure. As observed for gases in the catalytic processes with both polyethylenes, isoparaffins are the first compounds generated and they exhibit two maxima which appear at lower temperatures (around 275 °C) and at temperatures higher than 400 °C, respectively. But, as different from what is observed for the gases, aromatics are the major products at temperatures between 325 and 400 °C. Both polyethylene types show different behaviors from 400 °C on. In the case of LDPE, n-paraffins, 1-olefins, and olefins are also formed in important yields, especially n-paraffins which are the major compounds for temperatures above 400 °C. For HDPE, olefins and n-paraffins are also obtained in this temperature range, though they are formed in smaller quantities than aromatics, which continue to be the main compounds. In the case of the thermal pyrolysis a single process occurs where 1-olefins and n-paraffins are the major compounds obtained with both polyethylenes. Figures 6 and 7 present for the catalytic cracking of LDPE and HDPE with HUSY the distribution of the condensed compounds obtained as a function of the carbon number and the temperature range in which they were generated. In these figures only the temperature ranges in which the more significant changes were observed are shown. In the first fractions obtained

Figure 5. Evolution with the temperature of the yield of condensed products (grouped by compound type) obtained in the thermal and catalytic pyrolysis of LDPE and HDPE with HUSY.

(Figure 6), the distribution of the number of carbon atoms progressively increases with the temperature, since in the fraction generated between 442 and 465 °C, the distribution widens up to compounds with 29 carbon atoms and the maximum yield is obtained for compounds with 10 carbon atoms. In the distribution of the products generated between 466 and 493 °C, compounds with up to 36 carbon atoms are obtained. In the last fraction generated between 494 and 520 °C, the intermediate molecular weight products practically disappear, increasing the fraction of the low molecular weight products (around C9). A small contribution of waxes (C25-C38) can also be observed. In the catalytic pyrolysis of HDPE (Figure 7), the trend observed up to the fraction generated between 440 and 465 °C is similar to that shown for LDPE and HUSY (Figure 6), although the yield obtained with HDPE-HUSY is slightly higher than with LDPE-HUSY. The behavior of both systems differs only during the generation of the last two fractions. In the fraction obtained for 466-491 °C important changes with respect to the LDPE-HUSY system (Figure 6) can be appreciated. As can be observed the distribution is slightly narrower and only widens up to the formation of compounds with 27 carbon atoms, and the yield of the condensed products is clearly lower than in the same fractions obtained with LDPE-HUSY.


Figure 6. Evolution with the temperature of the yield of condensed products (grouped by carbon number) obtained in the catalytic pyrolysis of LDPE with HUSY.

Figure 7. Evolution with the temperature of the yield of condensed products (grouped by carbon number) obtained in the catalytic pyrolysis of HDPE with HUSY.

In the last fraction (492-520 °C), the intermediate and high molecular weight compounds have disappeared completely, increasing the amount of lighter compounds (up to C17). As discussed fairly extensively in a previous study22 the condensed products obtained with the thermal cracking of LDPE and HDPE exhibit a much broader carbon number distribution (up to C42) than with the corresponding catalytic systems. In the final fractions generated, the yield of intermediate molecular weight compounds decreases, favoring the formation of low and high molecular weight compounds, thus resembling the behavior described here for the LDPE-HUSY system during generation of the last fractions obtained. Figures 8 and 9, which represent the evolution of the composition (by weight fraction) of the gaseous and condensed products evolved during the thermal (Figure 8) and catalytic

processes (Figure 9) as a function of the temperature, give another perspective. As can be seen, in the thermal pyrolysis of LDPE and HDPE both gaseous and condensed compounds follow similar trends. At the beginning of the process n-paraffins are the major compounds generated, although, as the temperature increases, their weight fraction decreases, increasing 1-olefins which become the most common products from around 440 °C. The weight fraction of olefins remains practically constant with the temperature until the end of the process when it slightly increases. As has already been commented on above, an interesting behavior can be observed in the presence of HUSY zeolite (Figure 9) as the composition of the products generated strongly depends on the temperature. In the case of the gases, the weight fraction of isoparaffins is close to 80% at the beginning of the


Figure 8. Evolution with the temperature of the weight fraction of products (grouped by compound type) obtained in the thermal pyrolysis of LDPE and HDPE.

Figure 9. Evolution with the temperature of the weight fraction of products (grouped by compound type) obtained in the catalytic pyrolysis of LDPE and HDPE with HUSY.

process, but it decreases quickly when the temperature is increased. However, the formation of olefins with the temperature follows the opposite trend. The weight fractions of n-paraffins and 1-olefins present a slight increase with the temperature. Both polyethylenes behave in a similar way. For the condensed products and at low temperatures, the weight fraction of isoparaffins is also very high and it decreases during the course of the degradation process. The concentration of aromatics increases from the beginning of the process for both polyethylenes and remains nearly constant from 325 °C to the end for HDPE, while for LDPE it tends to decrease at the very end, in favor of n-paraffins, olefins, and 1-olefins. As is evidenced in this figure, two steps could be differentiated in the course of the catalytic processes.

4. Discussion Analysis of the gaseous and condensed compounds obtained in the catalytic cracking of polyethylene with HUSY shows a strong relation between the temperature and the composition of the products evolved. The degradation starts relatively early (around 200 °C), and compounds as isoparaffins, olefins and aromatics (in the condensed fractions) are mainly generated. But the process takes place for a long time, and a second degradation step appears for the same temperature range as in the thermal pyrolysis, generating the typical compounds of the thermal process (1-olefins and n-paraffins), apart from those already mentioned. The behavior of HUSY could be related to the progressive deactivation process suffered by this zeolite.


According to Lin et al.20,26 the deactivation process suffered by the zeolite HUSY when it acts as catalyst in the pyrolysis of polymers is reflected in the decrease of the amount of isoparaffins produced (product of bimolecular reactions) and the relative increase in olefins (product of monomolecular reactions). Due to the large pore size of HUSY, bulky bimolecular reactions can occur, ultimately leading to the generation of coke and the subsequent deactivation of the catalyst. It should be remarked that the formation of paraffins involves hydride transfer reactions and, hence, it should be strongly affected by the decrease in acid site density,27 which can be caused by the deactivation processes. Marcilla et al.7 observed a similar premature initiation of the decomposition of different polymers over MCM-41. They related this behavior to the acid sites located on the external surface and the pore mouth region and to the high accessibility to these sites. At the beginning of the process, the cracking of the polymeric chains takes place over the more accessible active sites, which are quickly deactivated, resulting in a noticeable effect on the decrease of the activity of the catalyst. Subsequently, the products obtained in the initial reaction step and the remaining polymer can access the active sites located in the inner part of the catalyst pores and so continue the catalytic cracking reactions. In the case of the catalytic pyrolysis in the presence of HUSY zeolite, a still appreciable catalytic effect is observable at the temperature of maximum degradation rate (i.e., the main degradation step) since an important quantity of gases is generated, and an appreciable proportion of isoparaffins is encountered among the different compounds. But the large pore size of HUSY zeolite also favors the formation of coke molecules during the catalytic process and the coke accumulated may hinder the access of polymer chains to active sites, so part of the polymer decomposition process could proceed via the thermal process, and, therefore, similar products to those obtained in the thermal degradation could be generated (1-olefins and n-paraffins). As noted above, the behavior of LDPE and HDPE in the catalytic cracking is very similar up to high temperatures (around 440 °C), when the thermal degradation of both polymers begins. In the case of the LDPE-HUSY system a greater amount of polyethylene goes through the thermal process route, and this behavior suggests that the deactivation suffered by this system is greater than that observed with HDPE.28 In fact, the amount of coke deposited on the HUSY zeolite after the catalytic cracking of LDPE was slightly higher (16.3 wt % catalyst) than in the case of HDPE (15.7 wt % catalyst). On the other hand the results shown in this paper could explain the wide divergence of the data obtained by different authors in relation to the composition of the gases and condensed products formed in the catalytic pyrolysis of polyethylene in the presence of HUSY zeolite. Hesse et al.17 stated that paraffins were the main components of the gases obtained in the polyethylene catalytic cracking with HY under dynamic conditions in the temperature range from 150 to 300 °C, which coincides with results shown here at low temperatures. Under isothermal conditions (290-520 °C) other authors19,20,29,30 observed, for the composition of the gases evolved in the catalytic pyrolysis carried out in both a fluidized bed reactor20,29,30 and a fixed-bed reactor,19 that as the temperature of the process increased, the proportion of olefins formed increased with respect to that of isoparaffins. However, Hernańdez et al.,6 employing a fluidized bed reactor at different reaction temperatures (500-800 °C), and Park et al.,31 employing a fixed-bed reactor at 450 °C, reported aromatics as the major components of the liquid fraction obtained in the catalytic cracking of HDPE

over HUSY, which as shown coincides with results presented here at temperatures above 375 °C. Achilias et al.32 reported a similar trend for LDPE wastes in a laboratory fixed-bed reactor with FCC catalyst at 450 °C. Elordi et al.33 also stated that aromatics and isoparaffins were the main components in the liquid fraction obtained in the catalytic cracking carried out in a conical spouted bed reactor at 500 °C. Garforth et al.29 used a fluidized bed reactor operating in the 290-430 °C range and found a comparable trend for the composition of condensed products with respect to the temperature. Similar results were obtained by Bagri et al.19 in a fixed-bed reactor at different catalyst bed temperatures (400-600 °C) and by Manos et al.,18 who worked under dynamic conditions using different heating rates (2 and 100 K min-1) and final temperatures (529-649 K). They reported that higher reactor temperatures and heating rates favored the formation of slightly higher quantities of olefins and aromatics. 5. Conclusions The results obtained in the present study, carried out under dynamic conditions, show that there are no appreciable differences among the gaseous products generated in the HUSY catalytic pyrolysis of LDPE and HDPE. Isoparaffins and olefins exhibit two maxima, one at low and another at high temperatures, whereas 1-olefins and n-paraffins present only one maximum at high temperatures. Isoparaffins and olefins are the major compounds of the first and the second maximum, respectively. Analysis of the condensed products indicates that the behavior of both polyethylenes is very similar up to the end of the degradation process. Isoparaffins are generated first, while aromatics appear at slightly higher temperatures. From 440 °C to the end of decomposition, aromatics are the products obtained most often with HDPE, while with LDPE an important contribution of n-paraffins and 1-olefins (typical compounds obtained in the thermal degradation) appears. Results suggest that part of the polyethylene degradation process goes via the thermal cracking mechanism due to the progressive deactivation suffered by HUSY. The differences observed between the catalytic cracking of LDPE and HDPE at the end of the process could be related to a faster deactivation of HUSY with LDPE. Acknowledgment Financial support for this investigation has been provided by the Spanish “Comisioń de Investigacioń Cientı´fica y Tecnolo´gica” de la Secretarıá de Estado de Educacioń, Universidades, Investigacioń y Desarrollo and the European Community (FEDER refunds; CICYT CTQ2004-02187), by the Generalitat Valenciana (Project ACOMP/2007/094), and by the University of Alicante, Grupo de Procesado y Piro´lisis de Polı´meros (VIGROB099). Literature Cited (1) Kaminsky, W.; Nun˜ez Zorriqueta, I. J. Catalytical and thermal pyrolysis of polyolefins. J. Anal. Appl. Pyrol. 2007, 79, 368. (2) Predel, M.; Kaminsky, W. Pyrolysis of mixed polyolefins in a fluidised-bed reactor and on a pyro-GC/MS to yield aliphatic waxes. Polym. Degrad. Stab. 2000, 70, 373. (3) Aguado, R.; Olazar, M.; San Jose´, M. J.; Gaisań, B.; Bilbao, J. Wax formation in the pyrolysis of polyolefins in a conical spouted bed reactor. Energy Fuels 2002, 16, 1429. (4) Marcilla, A.; Beltrań, M. I.; Navarro, R. Study of the deactivation process of HZSM5 zeolite during polyethylene pyrolysis. Appl. Catal., A 2007, 333, 57.

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ReceiVed for reView April 2, 2008 ReVised manuscript receiVed June 18, 2008 Accepted June 26, 2008 IE800520U

Evolution with the Temperature of the Compounds ... - ACS Publications

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