Energy & Fuels 1998, 12, 975-980
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Observation of Retrogressive Reactions under Liquefaction Conditions Utilizing the Oxidized Coal Completely Dissolved in Solvent at Room Temperature Kazuhiro Mae,*,† Chunshan Song,‡ Makoto Shimada,† and Kouichi Miura† Department of Chemical Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-01, Japan, and Fuel Science Program, Pennsylvania State University, 209 Academic Project Buildings, University Park, Pennsylvania 16802 Received February 24, 1998. Revised Manuscript Received July 1, 1998
We have found that the brown coal oxidized by nitric acid at 70 °C can be completely dissolved in tetrahydrofuran at room temperature. Utilizing this oxidized coal, we performed the temperature-programmed reaction under liquefaction conditions in several types of solvents under nitrogen atmosphere to evaluate the retrogressive reactions. More than one-half of the tetrahydrofuran-soluble fraction was converted into the THF-insoluble fractions at 240 °C. The changes in the solid caused by the retrogressive reaction were examined from the change in the TG curves of insoluble fractions in toluene or tetrahydrofuran. From this analysis, it was found that two types of retrogressive reactions occurred during the liquefaction of the oxidized brown coal: one is the cross-linking reaction of THF-soluble fraction by the decomposition of oxygen functional groups at low temperatures and the other is the structural change, such as polymerization of the tetrahydrofuran-insoluble fractions, at >350 °C. The retrogressive reactions at low temperatures were affected strongly by the physicochemical properties of the solvent, such as solubility and vapor pressure.
Introduction It is well-recognized that retrogressive reactions occur to some extent during liquefaction and pyrolysis and that they have a negative impact on the liquid yield. Tyler et al.,1 Flanklin et al.,2 Joseph et al.,3 and Serio et al.4,5 examined the effect of the cation type on the pyrolysis or liquefaction yields; it was shown that the Ca2+ cation suppressed the liquid yield. Serio et al.4 demonstrated that this was due to the cross-linking reaction caused by the decomposition of oxygen functional groups. On the other hand, the oil yield was successfully increased by the demineralization,6 the solvent swelling,3,7 or alkylation8-11 of coal. These results indicate that the secondary interactions such as †
Kyoto University. Pennsylvania State University. (1) Tayler, R. J.; Shafer, H. N. S. Fuel 1980, 59, 487-494. (2) Frankilin, H. D.; Cosway, R. G.; Peters, W. A.; Howard, J. B. Ind. Eng. Chem. Process Des. Div. 1983, 22, 39-42. (3) Joseph, J. T.; Farrai, T. R. Fuel 1992, 71, 75-80. (4) Serio, M. A.; Solomon, P. R.; Kroo, E.; Bassilakis, R.; Malhotra, R.; MacMillan, D. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1990, 35, 61-69. (5) Serio, M. A.; Solomon, P. R.; Kroo, E.; Bassilakis, R.; Malhotra, R.; MacMillan, Proceedings of the International Conference on Coal Science New Catsle, 1991; pp 656-659. (6) Mochida, I. Fuel 1983, 62, 659-664. (7) Miura, K.; Mae, K.; Asaoka, S.; Yoshimura, T.; Hashimoto, K. Energy Fuels 1991, 5, 340-346. (8) Liotta, R.; Rose, K.; Hippo, E. J. Org. Chem. 1981, 46, 277-283. (9) Chu, C. J.; Cannon, S. A.; Hauge, R. H.; Margrave, J. L. Fuel 1986, 65, 1740-1746. (10) Ofosu-Asante, K.; Stock, L. M.; Zabransky, R. F. Fuel 1989, 68, 567-572. (11) Schlosberg, R. H.; Neavel, R. C.; Maa, P. S.; Gorbaty, M. L. Fuel 1980, 59, 45. ‡
cation linkage and hydrogen bonding, are closely related to the retrogressive reactions. On the other hand, Saini et al.12 and Song et al.13 performed the temperature-programmed liquefaction and concluded that the benzyl-type radicals can lead to the retrogressive reactions and these reactions can be induced by the decomposition of the oxygen functional groups and the condensation of the polyhydroxyl structure. Buchanan et al.14-18 and Manion et al.19 examined, in detail, the cross-linking reaction mechanism under liquefaction conditions using several model compounds. In these studies Manion et al.19 reported that the decarboxylation of hydrogen-activated benzoic acids results in products that are susceptible to subsequent electrophilic coupling reactions and that H-donors inhibit coupling but promote decarboxylation in the presence of electron-transfer agents. Buchanan at al.17,18 examined the pyrolysis of 11 kinds of model compounds in the liquid phase. They concluded that the aromatic (12) Saini, A. K.; Coleman, M. M.; Song, C.; Schobert, H. H. Energy Fuels 1993, 7, 328-330. (13) Song, C.; Schorbert, H. H.; Hatcher, P. G. Energy Fuels 1992, 6, 326-328. (14) Buchanan, A. C.; Britt, P. F.; Skeen, J. T. D. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1994, 39, 22-27. (15) Buchanan, A. C.; Britt, P. F. Proceedings of the International Conference on Coal Science, Essen, 1997; pp 1337-1352. (16) Buchanan, A. C.; Britt, P. F. Proceedings of the International Conference on Coal Science, Essen, 1997; pp 1353-1356. (17) Eskay, T. P.; Britt. P. F.; Buchanan, A. C., III Energy Fuels 1997, 11, 1278-1287. (18) Britt, P. F.; Mungall, W. S.; Buchanan, A. C., III Energy Fuels 1998, 12, 660-661. (19) Manion, J. A.; McMillen, D. F.; Malhotra, R. Energy Fuels 1996, 10, 776-788.
S0887-0624(98)00038-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/22/1998
976 Energy & Fuels, Vol. 12, No. 5, 1998
carboxylic acids condense to form anhydrides at 250350 °C, then the linkage of anhydrides restricts the access of solvent and catalyst, resulting in the enhancement of additional cross-linking. Thus, some retrogressive reactions surely occur under the liquefaction conditions, and their mechanism was fairly well-recognized. However, the liquefaction of coal proceeds in the state-restricted physicochemical factor, such as the secondary interaction of a coal molecule, solubility of coal at the reaction temperature, etc. In addition, little is known on the change in the product by the retrogressive reaction because the original coal structure could not be clarified. To overcome this drawback, Iino et al.20,21 examined the change in the CS2-NMP insoluble fractions through the heat treatment at low temperatures using the coal solubilized by CS2-NMP. Their approach is thought to be very quantitative, but unfortunately, only the amount of the fraction insoluble in CS2-NMP was monitored. If the coal is rendered completely soluble at room temperature in a solvent that is used for the sequential extraction of coal liquefaction products, the retrogressive reaction can be examined in more detail. We have recently found that the brown coal oxidized in nitric acid was completely dissolved in tetrahydrofuran (THF) at room temperature.22,23 We also clarified the average structure of the oxidized coal by subjecting this coal to several spectroscopic analyses.24 In this paper, we describe the results of the temperatureprogrammed reaction of such a dissolved coal under inert atmosphere using various solvents. We also examined the change in the quality of the solventinsoluble product by the retrogressive reaction. The retrogressive reaction is discussed from the viewpoint of the physicochemical state of coal partly dissolved by solvent under the liquefaction condition. Experimental Section Liquid Oxidation of Coal. An Australian brown coal (Morwell, MW) was used as a raw coal. The coal was ground into fine particles of less than 74 µm and dried in vacuo at 110 °C for 24 h before use. The oxidation of the coal was performed as follows: 20 mL of 4 N nitric acid was added to 1 g of coal in a 200 mL flask. The mixture was stirred using a magnetic stirrer for 4 h in a water bath kept at a constant temperature of 70 °C. After that an excess amount of cold water was added to the flask to terminate the oxidation. The oxidized coal was filtered and evacuated for 24 h at 60 °C. The oxidized coal is denoted as NO70.4. Temperature-Programmed Liquefaction of Coal. The liquefaction experiments were performed in 25-mL micro autoclaves with 1 g of coal and 3 or 10 g of solvent in nitrogen gas of 5.6 MPa. The reactor was plunged into the sand bath preheated at 420 °C, then after it had reached a fixed temperature, the reactor was immediately cooled by soaking in a cool water bath. The heating rate of the sample was 100 °C/min. Four kinds of solvents, tetralin (Tet), 1-methylnaphthalene (1MN), tetrahydroquinoline (THQ), and methanol-1(20) Shen, J.-L.; Takanohashi, T.; Iino, M. Energy Fuels 1992, 6, 854-858. (21) Iino, M.; Takanohashi, T.; Shen, J.-L. Proceedings of the International Conference on Coal Science, Essen, 1997; pp 1485-1488. (22) Miura, K.; Mae, K.; Maki, T.; Araki, J. Chem. Lett. 1995, 909910. (23) Mae, K.; Maki, T.; Miura, K. Energy Fuels 1997, 825-831. (24) Mae, K.; Maki, T.; Okutsu, H.; Miura, K. Proceedings of the International Conference on Coal Science, Essen, 1997; pp 195-199.
Mae et al. methylnaphthalene mixture (Me/1MN), were used to examine the effect of the solvent type on the liquefaction yield. The liquefaction of oxidized coal in the absence of solvent was also performed. The liquid and solid products were separated into hexanesoluble (HS), hexane-insoluble/toluene-soluble (HI/TS), tolueneinsoluble/tetrahydrofuran-soluble (TI/THFS), and tetrahydrofuran-insoluble (THFI) components by sequential extraction. The amount of each solvent used for extraction was 100 mL. The extraction was performed for 3 h at room temperature under the irradiation of the ultrasonic wave, this was then filtered in a reducing pressure, and the solid residue recovered was dried in vacuo at 100 °C for 8 h. All the gaseous products were collected in a gas bag and then analyzed using a PerkinElmer Auto-system gas chromatograph. The yields of CO and CO2 were determined using a carboxen-1000, 60/80 mesh column (1.7 mm × 4.5 m) with a thermal conductivity detector. The yields of C1-C4 hydrocarbon gases were determined using Chemipak C18 column (3.2 mm × 1.8 m) with a flame ionization detector. Characterization of Oxidized Coal and Liquefaction Products. The structure of the oxidized coal was examined from the ultimate analysis and 13C NMR and FTIR measurements. The oxidized coal was completely dissolved by THF. The THF extracts of the oxidized coal and the hexaneinsoluble/THF-soluble fractions of the liquefaction products were subjected to molecular weight distribution measurement using the GPC technique.23,25 A liquid chromatograph (Shimadzu Co. Ltd., LC-10A model) equipped with a polystyrene gel column (Shimpak 210H, 8.0 mm in diameter and 0.30 m in length) was used for this purpose. The calibration curve was constructed using eight phenol formaldehyde resins of known molecular weights (180, 250, 335, 580, 820, 1410, 2380, and 2860) as standards.25 The phenolic resins were completely soluble in DMF. Furthermore, the number average molecular weight of the oxidized coal was measured using a vapor pressure osmometer to confirm the calibration curve. Pyrolysis of Solid Products Derived from Liquefaction. To examine the reactivity of the solid products derived from the liquefaction, the hexane-insoluble, toluene-insoluble, and THF-insoluble fractions were pyrolyzed using two types of pryrolyzer. One is flash pyrolysis in an inert atmosphere using a Curie-point pyrolyzer (Japan Analytical Ind., JHP2S). About 2 mg of sample wrapped tightly by a ferromagnetic foil was placed in a small quartz reactor (4.0 mm i.d.) and heated to 764 °C at the rate of 3000 K/s by an induction heating coil to be pyrolyzed rapidly. The tar produced was completely trapped by the quartz wool placed just below the foil. Gaseous products were all led to a gas chromatograph equipped with a Porapak Q column to analyze inorganic gases (IOG; H2, CO, CO2, and H2O) and hydrocarbon gases (HCG; C1-C6 gaseous compounds, benzene, toluene, and xylene). To analyze H2O, lines connecting the pyrolyzer and the GC were all heated to 150 °C to prevent the condensation of H2O. The calibration curve of H2O was constructed by pyrolyzing carefully weighed CuSO4‚5H2O. The yields of char and tar were measured from the weight changes of the foil and the reactor. The second method is temperature-programmed pyrolysis using a thermogravimeric analyzer (Metler, TG50). A 5 mg samples was heated in He gas flow from 25 to 800 °C at the rate of 20 °C/min, monitoring the weight change.
Results and Discussion Average Structure of Oxidized Coal. The coal oxidized by nitric acid for 4 h at 70 °C (NO70.4) was dissolved completely in THF at room temperature. Therefore, we could characterize its structure by spectroscopic measurements. Table 1 shows the ultimate (25) Masuda, K.; Okuma, O.; Kanamochi, M.; Matsumura, T. J. Jpn. Inst. Energy 1993, 72, 943-949.
Observation of Retrogressive Reactions
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Table 1. Properties of Raw Coal and NO70.4 wt %, daf MW raw NO70.4
C
H
N
S + O (diff.)
O/C
H/C
64.0 56.5
4.7 4.6
0.7 1.0
30.6 37.9
0.36 0.50
0.88 0.98
fa MW raw 0.63 NO70.4 0.56 a
MN M0 N nox[kmol/kmol [kg/kmol] [kg/kmol] (MN/M0) of monomer] 1336 (1550)a
156 205
6.7
2.68 4.17
Determined from VPO.
analysis and the average structural parameters determined by Pugmire’s method.26 The raw coal was not dissolved, so the average molecular weight of the macromolecule, MN, and the number of repeating units, N, could not be determined. The molecular weight of the monomer, M0, between raw coal and NO70.4, its value increased by ca. 50 Da through oxidation. The average chemical formula of the monomer was approximately C20H18O7 for MWRaw and C20H20O10 for NO70.4, calculated from the ultimate analysis and the M0 value. This suggests that there are three oxygens and two hydrogens per monomer through the oxidation. The MN, fa, and nox values of NO70.4 were 1336 kg/kmol, 0.56, and 4.17 mol/mol of monomer, respectively. From these results the average structure of NO70.4 was estimated to be composed of seven monomer units, which is two or three aromatic rings attaching four or five oxygen functional groups. Judging from the fact that NO70.4 was completely dissolved in THF and had many oxygen functional groups, it was considered that the macromolecules in NO70.4 were assembled to form a solid state by secondary interactions such as hydrogen bonding. Temperature-Programmed Liquefaction of NO70.4. Figure 1 shows the change in the product distribution under the temperature-programmed liquefaction of NO70.4 at the tetralin-coal ratio of 3 and 10, respectively. In both cases, the yield of THFI dramatically increased with increasing temperature from 100 to 300 °C and reached a maximum at around 300 °C, then decreased. The maximum yield was up to 50% under the tetralin-coal ratio of 3. This clearly shows that significant retrogressive reactions occurred at low temperatures. Comparing Figure 1b with Figure 1a indicates that the formation of THFI was suppressed at a higher tetralin-coal ratio and the CO2 yield decreased while the yield of the HI/TS fraction increased. This suggests that the precursor, which can be HI/TS fraction, is converted into a THFI fraction by a retrogressive reaction. Thus by utilizing coal samples dissolved completely in THF, it was clearly shown that the retrogressive reaction proceeded significantly at low temperature. Figure 2 shows the molecular weight distributions of HI/THFS fractions obtained at each liquefaction temperature. The profiles were almost the same for all the fractions. They had two sharp peaks at ca. 750 and 1400 kg/kmol, which correspond to the main peaks of NO70.4. Since the average molecular weight of the monomer unit was 205, this result shows that the HI/ (26) Solum, M.S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187-199.
Figure 1. Temprature-programmed reaction of the Morwell coal oxidized by nitric acid with tetralin under an inert gas atmosphere.
Figure 2. Molecular weight distribution of the HI/THFI fractions derived from reaction of NO70.4 with tetralin at each temperature.
THFS fraction consists of two uniform structures having four and seven monomer units. Comparing the GPC profile of NO70.4, the retrogressive reactions appear to convert the fractions with more than 1500 of MN into the THFI fractions.
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Figure 3. Effect of tetralin-coal ratio on product distribution.
Figure 4. Comparison of flash pyrolysis yields of HI fractions derived from reaction with and without solvent.
Effect of the Solvent-Coal Ratio on the Product Distribution. Since the extent of the retrogressive reactions was different in different amounts of tetralin, the effect of the tetralin-coal ratio on the product yields was examined at both 300 and 420 °C, as shown in Figure 3. The THFI yield rapidly decreased with tetralin-coal ratios between 0 and 3 and then changed little at high tetralin-coal ratios. At that time, CO2 decreased with the increase of tetralin. Especially NO70.4, which was perfectly soluble in THF and converted into 80% of THFI and 20% of CO2 at 300 °C in the absence of solvent. Therefore, the hydrogen donor is effective for suppressing retrogressive reactions but cannot stop such reactions. Change in the Solid Structure through the Retrogressive Reaction. Next, we examined the change in the quality of heavier products caused by the retrogressive reaction. To do so we performed two types of experiments: flash pyrolysis of the HI fraction at 764 °C and temperature-programmed pyrolysis of the TI and THFI fractions under 20 °C/min in He. Figure 4 shows the char and tar yields during the flash pyrolysis of the HI fraction obtained from the
Mae et al.
Figure 5. Comparison of TG profiles among the TI and THFI fractions.
reaction under liquefaction conditions at each temperature. The abscissa of the figure is the pyrolysis temperature, and the yield is given on daf basis. The HI fraction derived from the reaction with tetralin was easily decomposed and produced a large amount of tar. On the other hand, the char yield did not change and little tar was produced from the HI fraction derived from the pyrolysis without solvent. Since the char yield is based on daf coal, little change in the char yield above 240 °C indicates that the HI fraction became inert without tetralin. On the contrary, two-thirds of the HI fraction was decomposed and the tar yield was up to 20% in the presence of tetralin. This again shows that the solvent plays an important role in suppressing the retrogressive reaction. The above results have established that the retrogressive reaction caused a significant change in the structure of the HI fraction. Consequently, we estimated the structural change by comparing the TG curves of the solvent-insoluble products. The method is simple and convenient, measuring the weight change while heating to 800 °C in He at a constant heating rate. Figure 5 shows the TG profiles of the TI and THFI fractions. The abscissa is the pyrolysis temperature, and the number shown in the graph is the reaction temperature. From the upper graph it was found that the TG curve of the TI fraction obtained at 240 °C crossed that of NO70.4, the main decomposition temperature shifted to higher temperature, and the final conversion was smaller than that of NO70.4. This means that the TI fraction at 240 °C is changed to stable compounds. For the TG curves of the TI fraction obtained at >240 °C, the main decomposition temperature did not change and only the fractions pyrolyzed below 400 °C disappeared. Considering the fact that the CO2 yield corresponded to the THFI yield, the change between 100 and 240 °C in the TI fraction was probably related to the cross-linking reaction by the decomposition of the oxygen functional groups. Britt et al. reported that the dehydration of benzene carboxylic
Observation of Retrogressive Reactions
Figure 6. Effect of solvent type on product distribution at 300 °C.
acids occurred at 250-350 °C.18 NO70.4 contains a large amount of the carboxylic groups, which form tight hydrogen bonding.24 It is assumed that the dissociation energies of some O-H bonds in NO70.4 are reduced by strong hydrogen bondings,27 then decarboxylation or dehydration easily occurs at low temperature. On the other hand, the TG curve of the THFI fractions differs from that of the TI fractions. As shown in the lower graph of Figure 5, the TG curves of the THFI fractions produced at 300 and 360 °C were similar to that produced at 240 °C, although their amounts were changed. However, the TG curve of the THFI fraction produced at 420 °C crossed that produced at 240 °C and the main decomposition temperature shifted to higher temperature. This indicates that the THFI fraction changed into a less decomposable structure between 360 and 420 °C. Namely, a different type of retrogressive reaction occurred at these temperatures. From these observation we can say that two types of retrogressive reactions occur during liquefaction of the low rank coal. The first one is the cross-linking reaction between macromolecules in the THF-soluble fractions at low temperatures. The second one is the recombination or depolymerization of the unit structure in the THFI fractions at >360 °C. Effect of Solvent Type on the Retrogressive Reaction. The retrogressive reaction was suppressed in the presence of tetralin. Kamiya et al.28 and Iino et al.21 also reported that the effect of solvents in suppressing the retrogressive reaction might be explained from the hydrogen-donating ability of the solvent. The hydrogen-donor solvent can stabilize some of the radicals that are responsible for retrogressive reactions around 400 °C but cannot effectively donate hydrogen to suppress the retrogressive reactions at low temperature. To clarify the role of the solvent on the retrogressive reaction at low temperature, we performed the reaction of NO70.4 at 300 °C using various solvents. Figure 6 compares the product distribution. The THFI yield was significantly affected by the solvent type. The THFI yield in 1MN was higher than that in Tet. This means that the retrogressive reactions occurred more extensively in 1MN. On the other hand, the THFI yield was decreased by using THQ. This increase was not caused by some of the nitrogen-containing components (27) Miura, K.; Mae, K.; Morozumi, F. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1997, 42, 209-213. (28) Kamiya, Y.; Nagae, S.; Yao, T.; Hirai, H.; Fukushima, A. Fuel 1982, 61, 906-911.
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in the product because the nitrogen content of the HI obtained from the reaction in THQ was almost similar with those obtained from other conditions. It was assumed that the retrogressive reactions were suppressed in THQ. At 300 °C, the hydrogen transfer from solvent to coal does not occur effectively so the difference is due in part to the other properties of the solvent rather than the chemical property. The solubility potential of coal increases in the order THQ, Tet, and 1MN. Therefore, the degree of coal solubilization at 300 °C may affect the progress of the retrogressive reaction. Namely, NO70.4 was fairly dissolved in THQ, some hydrogen bonding was released, and the frequency of the macromolecules contacting each other was decreased. This state of the coal macromolecule is assumed to diminish the retrogressive reactions. This speculation was supported by the fact that the retrogressive reaction is affected by the solvent-coal ratio (Figures 3 and 4) and that the flash pyrolysis of NO70.4 completely solubilized by solvent brought about the drastic decrease in the char yield of 9%.29 To examine whether the dispersion of NO70.4 molecules solubilized in the solvent affect the retrogressive reaction or not, we performed the liquefaction using a methanol and 1MN mixture which completely dissolves NO70.4 at room temperature. The product distribution was almost equal to that using only 1MN, as shown in Figure 6. This was probably because the methanol evaporated above 100 °C and the solvent in the liquid phase at 300 °C was only 1MN. We have not fully examined the effect of the dispersion of the coal molecule in solvent, but we can say that the solvent should be selected based not only on the hydrogen-donating ability but also on its physicochemical properties, such as the solubility and the vapor pressure. Conclusion We have developed an oxidation method to dissolve the low-rank coal in THF at room temperature. The oxidized brown coal, which was completely soluble in THF, was prepared by the liquid-phase oxidation with nitric acid. The retrogressive reaction under liquefaction conditions was examined by monitoring the change in the THF-soluble fraction using this prepared coal, and the following conclusions were obtained. (1) Two types of retrogressive reactions occurred under liquefaction conditions: cross-linking by decomposition of the oxygen functional groups at low temperature and further cross-linking at temperatures more than 360 °C. (2) The change in the solid caused by the retrogressive reaction was successfully characterized by the pyrolysis methods of the TI and THI fractions. (3) The degree of the retrogressive reactions at low temperature was strongly affected by solvent type. The results suggest that the retrogressive reaction at low temperature may be suppressed by selecting the proper solvent based on its physicochemical properties, such as the solubility parameter and vapor pressure, in addition to the hydrogen-donating ability. (4) The HI-THFS fraction obtained from the liquefaction of the oxidized coal was uniform in structure judging from the GPC profile. This (29) Miura, K.; Mae, K.; Maki, T.; Okutsu, H. Proceedings of the International Conference on Coal Science, Essen, 1997; pp 585-589.
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Mae et al.
suggests that it is possible that the oxidized coal in the liquid phase may be converted into chemicals in high selectivity.
We give sincere thanks to Dr. Kondam M. Reddy and Dr. Masahide Sasaki for their assistance and useful advise in performing the experiment.
Acknowledgment. The authors are grateful to Prof. Harold H. Schobert for his encouragement and support.
EF980038L
