Reduction of Carbon Dioxide in Hydrothermal Cracking of Polymer

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Reduction of Carbon Dioxide in Hydrothermal Cracking of Polymer Wastes Xu Zeng,† Fangming Jin,*,‡ Zhibao Huo,† Takeo Mogi,‡ Atsushi Kishita,‡ and Heiji Enomoto‡ †

State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, People’s Republic of China ‡ Graduate School of Environmental Studies, Tohoku University, Sendai 980-8579, Japan

T

he Earth’s environment is seriously threatened by an increase in atmospheric carbon dioxide (CO2) concentrations. To support sustainable development in the 21st century, controlling the global warming caused by the increasing atmospheric CO2 levels has become a main scientific and technological challenge.1 To prevent catastrophic events related to atmospheric CO2, the emission of CO2 should be reduced as soon as possible. One of the most promising solutions to overcome this challenge is the conversion of CO2 into valuable chemicals, thus contributing to the reduction of the atmospheric CO2 load. This process is regarded as one of the most ideal solutions for the “greenhouse effect” problem. Potential techniques have been studied extensively, including Fischer
Tropsch reactions, photochemical or electrochemical conversions, catalytic hydrogenation, and producing valuable compounds, such as methanol and methane.2
5 In recent decades, hydrothermal reactions have received more attention for the treatment of organic wastes and biomass conversion because of the unique inherent properties of hightemperature water (HTW) that include a high ion product (Kw) and a low dielectric constant, which are favorable for promoting reactions without catalysts.6
9 In our previous studies, it was found that hydrogen can be generated during the hydrothermal cracking of bitumen and polymer wastes, such as polyethylene (PE) and sulfur-containing rubber.10
14 A cheap source of hydrogen has become the primary challenge in the conversion of CO2. Hydrogen is currently produced by reforming hydrocarbons, which is an energy-intensive process. If the hydrogen produced during the hydrothermal cracking of polymer wastes could be directly used to reduce CO2, then an efficient process for CO2 conversion and polymer waste use would be realized. In this study, we examined the possibility of CO2 reduction during the hydrothermal cracking of polymer wastes. PE, as a representative of plastic waste, and ethylene propylene diene monomer (EPDM), as a model for sulfur-containing waste rubber, were used as test materials. The molecular structure of EPDM can be found in the Supporting Information. Experiments were conducted at temperatures varying from 300 to 450 °C in a bomb-type batch reactor with a Hastelloy C-276 inner wall and an inner volume of 42 mL. The reactor was equipped with a highpressure valve to allow for pressure measurements, CO2 input, and gas sampling. The experimental setup has been described in detail elsewhere.15 After reactions, gas and liquid samples were collected and analyzed by gas chromatography
thermal conductivity detector (GC
TCD) and gas chromatography
mass spectroscopy (GC
MS), respectively. A more detailed description of experimental procedures and the gas collection method can be found in the Supporting Information. r 2011 American Chemical Society

First, experiments with PE were conducted with various temperatures (300
450 °C), reaction times, and initial pH values with or without CO2. No significant differences were observed in the amounts of CO2 and products after the reactions with or without CO2 for all conditions. These results suggest that CO2 reduction rarely took place during the hydrothermal cracking of PE under the conditions used. However, interestingly, a significant reduction in the CO2 level was observed when using EPDM. As shown in Figure 1, no formic acid (C1) was detected in the absence of CO2 in the water samples after the reactions, only carboxylic acids with 2
6 carbon atoms (C2
C6) appeared, but a big formic acid peak appeared in the presence of CO2 at both 400 and 450 °C, with a larger peak at 450 °C. Experiments with the initial pH values from 4 to 13 in the absence of CO2 were performed to test whether the formic acid formation was due to the change of the initial pH. The results showed that formic acid was not formed at any of the initial pH values tested. Previous studies have demonstrated CO2 can be converted into formic acid under hydrothermal conditions using Fe as a reductant and Ni as a catalyst.16,17 Thus, the formic acid formed in the presence of CO2 can be attributed to the reduction of CO2 during the hydrothermal cracking of EPDM. The analytical results of gas samples with and without CO2 showed that the concentrations of hydrocarbons with 1
6 carbon atoms were significantly higher in the presence of CO2 than those in the absence of CO2, particularly for methane at 450 °C (see Figure 2). On the basis of previous results, which showed that CO2 can be reduced to CH4 with metals as reductants under hydrothermal conditions,9,18
20 although further studies are needed, these results suggested that the methane also could be attributed to the reduction of CO2. Further, quantitative analyses of CO2 remaining after the reactions demonstrated that the CO2 amount decreased greatly after the reactions relative to the initial amount of CO2. With an increase in the temperature, the amount of CO2 after the reaction decreased significantly. The highest efficiency for CO2 reduction was about 20%, which occurred at 450 °C with a reaction time of 2 h. The efficiency of CO2 reduction was defined as the percentage of CO2 remaining after the reaction relative to the initial amount of CO2. It should be notable that we used some special methods during injection of gasous CO2 and collection of gas after the reaction to decrease the error caused by the solubility of CO2 in water (the detail can be found in the Received: March 25, 2011 Revised: May 16, 2011 Published: May 16, 2011 2749

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Figure 2. Concentration of hydrocarbons with 1
6 carbon atoms in gas samples after reactions at (A) 400 °C and (B) 450 °C. Figure 1. GC
MS chromatograms of water samples after reactions at (A) 400 °C and (B) 450 °C.

Supporting Information). These results demonstrated that the reduction of CO2 took place during the hydrothermal treatment of EPDM and the reduced products mainly consisted of formic acid and methane. It should be noted that, in general, a catalyst is needed for the reduction of CO2 under hydrothermal conditions. However, interestingly, an obvious reduction of CO2 was observed without the addition of any catalyst in this study. These results suggest that some components involved in the hydrothermal cracking of EPDM may catalyze the reduction of CO2. Investigation into the catalysis mechanism is now in progress. As shown in Figure 3, the GC
MS spectra of oil samples after hydrothermal reactions in the presence and absence of CO2 indicated that the major products were almost identical, primarily straight-chain n-alkanes with more than nine carbon atoms. These results show that CO2 had no significant influence on the cracking of EPDM into fuel oils under hydrothermal conditions. Thus, the reduction of CO2 and the hydrothermal cracking of EPDM can be achieved simultaneously. The mechanism of CO2 reduction during the hydrothermal cracking of EPDM is not yet clear. Our past studies showed that H2S and hydrogen can be produced during the hydrothermal cracking of sulfur-containing rubber and hydrogen may be

produced from water because of the strong reduction of H2S. Our recent study with H2S as a reductant showed that H2S can reduce H2O into H2 under hydrothermal conditions at temperatures greater than 250 °C.21 On the basis of these results, it is suggested that the mechanism of hydrogen production in the presence of CO2 may be similar to that in the absence of CO2 during the hydrothermal cracking of EPDM. That is, the desulfurization of EPDM releases H2S, which then acts as a reductant to reduce water into hydrogen. Hydrogen reduces CO2 into formic acid and methane. To confirm this consumption, an experiment with Na2S 3 9H2O was performed at 300 °C for 120 min. Hydrogen and methane were found to be the major components of the gas samples. The formation of formic acid was clearly observed (see the Supporting Information). Additionally, as shown in Figure 2, the amount of hydrocarbons, the alkanes from the hydrothermal cracking of EPDM, with 2
6 carbon atoms also increased in the presence of CO2. Currently, we cannot explain the mechanism of the increase in hydrocarbons in the presence of CO2, but any potential mechanism should explain that the reaction equilibrium shifts to the right because of the consumption of hydrogen or hydrogen sulfide in the presence of CO2, as shown in eq 1. EPDM þ H2 O f ½hydrocarbons þ H2 S ðor H2 Þ þ CO2  f organics

ð1Þ 2750

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Figure 3. GC
MS chromatograms of oil samples after reactions for 60 min at (A) 430 °C and (B) 400 °C (C denotes alkanes, and numbers show the carbon number of alkanes).

The proposed process has the following benefits. First, water acts as not only an environmentally benign solvent for CO2 conversion but also a hydrogen source. Second, neither high-purity CO2 nor high-purity hydrogen is required, which would require special pumps and storage, because hydrogen is derived from water and reacted with CO2 in situ. Third, no catalyst is required. Finally, the process is expected to have a low energy cost because the input is organic waste.22
24 In summary, we proposed an attractive technology that reduces CO2 into organic compounds, such as formic acid, during the hydrothermal cracking of EPDM, in which this reduction was accompanied by the conversion of EPDM into fuel oils. Although future work is needed to clarify the reaction mechanism and to optimize the conditions for higher conversion of CO2, the process is a significant potential and sustainable method to reuse CO2 as a C1 building block for energy recovery and to eliminate the immediate threat of surplus CO2 in the atmosphere.

’ ASSOCIATED CONTENT

bS

Supporting Information. Experimental section, molecular structure of EPDM, and HPLC chromatogram of the liquid sample for the reduction of CO2 in the presence of H2S at 300 °C for 120 min. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (20777054 and 21077078) and the National High Technology Research and Development Program of China (2009AA05Z405 and 2009AA063903). ’ REFERENCES (1) Tian, G.; He, C.; Chen, Y.; Yuan, H. M.; Liu, Z. W.; Shi, Z.; Feng, S. H. ChemSusChem 2010, 3, 323–324. (2) Kobayashi, T.; Takahashi, H. Energy Fuels 2004, 18, 285–286. (3) Tambach, T. J.; Mathews, J. P.; Bergen, F. V. Energy Fuels 2009, 23, 4845–4847. (4) Wang, J. G.; Liu, C. J.; Zhang, Y. P.; Eliasson, B. Chem. Phys. Lett. 2003, 368 (3/4), 313–318. (5) Inui, T.; Yamamoto, T.; Inoue, M.; Hara, H.; Takeguchi, T.; Kim, J. B. Appl. Catal., A 1999, 186, 395–406. (6) Akiya, N.; Savage, P. E. Chem. Rev. 2002, 102, 2725–2750. (7) Kishida, H.; Jin, F. M.; Yan, X. Y.; Moriya, T.; Enomoto, H. Carbohydr. Res. 2006, 341, 2619–2623. (8) Jin, F. M.; Kishita, A.; Moriya, T.; Enomoto, H. J. Supercrit. Fluids 2001, 19, 251–262. (9) Jin, F. M.; Enomoto, H. Energy Environ. Sci. 2011, 4, 382–397. (10) Jin, F. M.; Ma, C. X.; Mogi, T.; Kishita, A.; Enomoto, H. Prep. Pap.
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