Elucidation of Hydrogen Behavior in Coal Using a Tritium Tracer

Energy Fuels , 2001, 15 (5), pp 1129–1138. DOI: 10.1021/ef010005x. Publication Date (Web): August 28, 2001. Copyright © 2001 American Chemical Soci...
0 downloads 0 Views 193KB Size
Download PDF
Energy & Fuels 2001, 15, 1129-1138

1129

Elucidation of Hydrogen Behavior in Coal Using a Tritium Tracer Method: Hydrogen Transfer Reaction of Coal with Tritiated Gaseous Hydrogen in a Flow Reactor I Putu Sutrisna, Atsushi Ishihara, Weihua Qian, and Toshiaki Kabe* Department of Chemical Engineering, Tokyo University of Agriculture and Technology, 2-24-16, Nakacho, Koganei, Tokyo 184-8588, Japan Received January 8, 2001. Revised Manuscript Received June 19, 2001

The hydrogen transfer reactions of three Argonne coals with tritiated gaseous hydrogen were carried out using a flow fixed-bed reaction system in the presence of Pt/Al2O3 catalyst at a temperature range 200-300 °C and under pressure of 1.5-5.0 MPa to trace the behavior of hydrogen in coal. In the reactions under a constant pressure of 5.0 MPa, it was found that the hydrogen exchange between tritiated hydrogen atoms generated from the Pt catalyst and hydrogen in functional groups such as hydroxy groups proceeded at temperatures as low as 200 °C for all coals studied. The results also indicate that hydrogen transfers to decompose ether linkages and reduce carbonyl groups in coal proceeded substantially at temperatures as low as 200 °C for lignite Beulah-zap (ND) and middle rank Illinois No. 6 (IL) coals. The amount of functional groups in the tritiated coal achieved a maximum at 250 °C for all coals examined. Hydrogen exchanges with hydrogen in the benzylic position and the phenoxy ring were observed at a perceptible extent at 300 °C. Substantial decarboxylation of lignite ND coal was also observed at 300 °C. When the pressure dependence was investigated at a constant temperature of 250 °C, it was found that the amount of hydroxy groups in tritiated coal increased with both the decomposition of ether linkages and the reduction of carbonyl groups.

Introduction Coal is a macromolecular solid constructed by fused aromatic clusters bound together by various types of linkages such as ether and alkyl linkages. A variety of functional groups are also present in coal, and of those, oxygen-containing functional groups are the most abundant. The reactions and cleavage of oxygen functional groups are believed to play an important role in coal conversion processes. Studies using coal model compounds have shown that oxygen functional groups can enhance the rate of decomposition of ethers and amines,1 and their role in the retrograde reactions have been investigated.2,3 Significant efforts in studies using coal model compounds have been extended to gain important role of hydrogen transfers responsible for the cleavage of ether and alkyl linkages under various coal conversion conditions.4-12 * Corresponding author. Tel: +81-42-388-7063. Fax: +81-42-3878945. E-mail: [email protected]. (1) King, H.; Stock, M. L. Fuel 1984, 63, 810. (2) Eskay, T. P.; Britt, P. F.; Buchanan, A. C., III. Energy Fuels 1996, 10, 1257. (3) Eskay, T. P.; Britt, P. F.; Buchanan, A. C., III. Energy Fuels 1997, 11, 1278. (4) Camaioni, D. M.; Franz, J. A.; Autrey, T. J. Phys. Chem. 1993, 97, 5791. (5) Autrey, T.; Alborn, E. A.; Franz, J. A.; Camaioni, D. M. Energy Fuels 1995, 9, 420. (6) Savage, P. E. Energy Fuels 1995, 9, 590. (7) McMillen, D. F.; Malhotra, R.; Chang, S.; Ogier, W. C.; Nigenda, E. S.; Fleming, R. H. Fuel 1987, 66, 1611. (8) Wei, X.-Y.; Ogata, E.; Niki, E. Bull. Chem. Soc. Jpn. 1992, 65, 1114.

Although studies using model compounds have increased our understanding of hydrogen induced cleavage of thermodynamically stable linkages in coal structure and provided information on the role of oxygen functional groups, details about the actual hydrogen transfers between coal and hydrogen still need to be elucidated. On the other hand, in coal liquefaction research, the use of isotopic tracers has been valuable in the study of reaction mechanisms and in the determination of reactive sites in coal.13-16 We have also reported that tritium tracer techniques are effective in monitoring the reaction pathway of hydrogen atoms in coal liquefaction and give quantitative information related to the mobility of hydrogen in coal.17-20 Recently, we have demon(9) Wei, X.-Y.; Ogata, E.; Niki, E.; Zong, Z.-M. Energy Fuels 1992, 6, 868. (10) Futamura, S.; Koyanagi, S.; Kamiya, Y. Fuel 1988, 67, 1436. (11) Autrey, T.; Linehan, J. C.; Kaune, L.; Powers, T. R.; McMillan, E. F.; Stearn, C.; Franz, J. A. Energy Fuels 1999, 13, 927. (12) McMillen, Ogier, W. C.; Ross, D. S. J. Org. Chem 1981, 46, 3322. (13) Bockrath, B. C.; Finseth, D. H.; Hough, M. R. Fuel 1992, 71, 767. (14) Cronauer, D. C.; Mcneil, R. I.; Young, D. C.; Ruberto, R. G. Fuel 1982, 61, 610. (15) Collin, P. J.; Wilson, M. A. Fuel 1983, 62, 1243. (16) Skowronski, R. P.; Ratto, J. J.; Goldberg, I. B.; Heredy, L. A. Fuel 1984, 66, 1642. (17) Kabe, T.; Kimura, K.; Kameyama, H.; Ishihara, A.; Yamamoto, K. Energy Fuels 1990, 4, 201. (18) Kabe, T.; Horimatsu, T.; Ishihara, A.; Kameyama, H.; Yamamoto, K. Energy Fuels 1991, 5, 459. (19) Kabe, T.; Ishihara, A.; Daita, Y. Ind. Eng. Chem. Res. 1991, 30, 1775. (20) Ishihara, A.; Takaoka, H.; Nakajima, E.; Imai, Y.; Kabe, T. Energy Fuels 1993, 7, 362.

10.1021/ef010005x CCC: $20.00 © 2001 American Chemical Society Published on Web 08/28/2001

1130

Energy & Fuels, Vol. 15, No. 5, 2001

Sutrisna et al.

Table 1. Analyses of Coals Useda ultimate (%, daf) coal

C

H

N

S

O

ND IL POC

72.9 77.7 91.1

4.8 4.0 4.4

1.2 1.4 1.3

0.7 2.4 0.5

20.4 13.6 2.7

a Abbreviations: ND, North Dakota Beulah-Zap; IL, Illinois No.6; POC, Pocahontah No. 3.

Table 2. Functional Group Composition of Coals (wt % dmmf)a,b hydrogen coal

carbon

Hal HOH Har Htotal Har/Htotal

ND 2.02 0.34 1.58 3.94 IL 3.41 0.23 2.07 5.71 POC 1.97 0.06 2.19 4.22

0.40 0.36 0.52

Cal

oxygen OOH Oether carbonyl

13.47 5.50 5.00 22.73 3.75 2.25 13.93 1.00 1.25

13 2 99% pure) were purchased from Kishida Chemicals. Reaction Procedure. The hydrogen transfer reactions were carried out in a fixed bed reactor (reactor volume of 15 mL) that is presented schematically in Figure 1. One gram of (21) Qian, W.; Ishihara, A.; Fujimura, H.; Saito, M.; Godo, M.; Kabe, T. Energy Fuels 1997, 11, 1288. (22) Kabe, T.; Saito, M.; Qian, W.; Ishihara, A. Fuel 2000, 79, 311. (23) (a)Ishihara, A.; Nishigori, D.; Saito, M.; Qian, W.; Kabe, T. Energy Fuels 2000, 14, 706. (b) Cronauer, D. C.; Ruberto, R. G.; Jenkins, R. G.; Davis, A.; Painter, P. C.; Hoover, D. S.; Starsinic, M. E.; Schlyer, D. Fuels 1983, 62, 1124. (24) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Serio, M. A.; Despande, G. V. Energy Fuels 1988, 2, 405.

Figure 1. Diagram of the experimental apparatus. coal was directly mixed with 0.1 g of 3 wt % Pt catalyst then the mixed coal-catalyst was charged into the reactor. The tritiated gaseous hydrogen was passed at a constant flow of 50 mL/min under pressure (1.5-5.0 MPa). The reactions were carried out at temperatures ranging between 200 and 300 °C. The change in radioactivity of the tritiated hydrogen gas exiting the reactor was continuously detected by a radioanalyzer (Aloka RLC-701) at the outlet of the reactor. To determine the amount of tritiated hydrogen transferred into the coals after the reaction, the tritiated coals were combusted by an automatic sample combustion system (Aloka ADS-113R), and the tritiated water produced was dissolved into a monophase scintillator reagent and its radioactivity was measured by a liquid scintillation counter (LSC, Beckman LS 6500). FTIR spectra of vacuum-dried IL coal and tritiated IL coals were recorded on a Jasco FTIR-5300 spectrometer at least 600 scans at a resolution of 4 cm-1. Samples were prepared as KBr pellets, where predried coal or tritiated coal (2.5 mg) was mixed with KBr (250 mg). The KBr was dried under vacuum at 110 °C over one night prior to sample preparation. The exchange reactions of model compounds with gaseous deuterium 99.5% in the presence of coal were carried out in a 15 mL stainless steel vessel. The reactor was charged with about 0.5 g reactant and 0.05 g IL No. 6 coal and after purging with deuterium gas, it was pressurized to an initial pressure of 2.5 MPa. The reaction temperature (400 °C) was reached within 5 min and held for the desired reaction time after which the reactor was quenched in an ice bath. The liquid products were analyzed by gas chromatography with an FID detector (Shimadzu 17 A) using a commercially available column (60 m × 0.25 mm DB-1). The deuterium content in phenol or in toluene after the reaction was determined by NMR spectroscopy at 400 MHz. GC-MS analysis of the liquid product was also performed on a Shimadzu GCMS-QP5000 instrument equipped with a 60 m × 0.25 mm DB-1 capillary column to identify deuterated products by comparison of the mass spectral fragmentation patterns and normalized ion intensities with those of undeuterated samples. The mass spectra of deuterated products obtained from the reaction of benzyl phenyl ether were as follows: m/z (relative intensity) phenol 97(3.4), 96(21.2), 95(68.0), 94(100); toluene 93(7.2), 92(79.1), 91(100); benzene 79(11.6), 78(100.0); p-benzylphenol 186(7.2), 185(40.2), 184(100); o-benzylphenol 186(8.4), 185(41.6), 184(100); diphenylmethane 169(19.2), 168(100). The mass spectra of deuterated products obtained from the reaction of dibenzyl ether were as follows: m/z (relative intensity) toluene 93(13.5), 92(78.6), 91(100); benzene 79(18.9), 78(100); diphenylmethane 169(22.6), 168(100); bibenzyl 183(8.3), 92(21.8), 91(100).

Elucidation of Hydrogen Behavior in Coal Elucidation of the Hydrogen Behavior in Coal in the Reaction of Coal with Tritiated Gaseous Hydrogen. To elucidate the hydrogen behavior in coal during the reaction, the hydrogen exchange reaction between the tritiated coal and water was performed at 100 °C for 24 h in a batch reactor. The decrease in radioactivity in the tritiated coal from the exchange reaction with water was considered to be related to the amount of tritiated hydrogen incorporated into functional groups such as hydroxy groups.21-23 The reaction was performed according to the procedure described previously.22,23 The content of tritiated hydrogen remaining in the tritiated coal was measured by a similar way described above. To clarify the exchange level of this procedure, the exchange reaction between phenol-d6 and water or benzoic acid and D2O was carried out at 100 °C for 24 h. NMR analysis of the recovered products showed that the ring of phenol-d6 was not protonated, and that the ring of benzoic acid was also not deuterated. This procedure, therefore, could be used as a tool to determine the amount of tritium incorporated into the hydroxy groups or amount of hydroxy groups may exist in the tritiated coals without any effect of the multiple exchanges into the aromatic ring of these groups. The results of this study were accounted by hydrogen transfer ratio (HTR) that represents the ratio of hydrogen transferred into the coal to total hydrogen content in the raw coal shown in Table 1. The method for calculation of HTR is the same as that of HER (hydrogen exchange ratio) found elsewhere.22,23 Total amount of hydrogen transferred into both oxygen functional groups, such as phenolic or carboxyl groups, and carbon network, such as alkyl or aryl sites, in coal (total HTR) was calculated on the basis of total radioactivity of the tritiated coal. The amount of tritium transferred into the functional groups (HTR-OH) was obtained from a decrease in radioactivity of the tritiated coal after the hydrogen exchange reaction with water. And, the amount of tritium transferred into the carbon network (HTR-CH) was calculated on the basis of radioactivity remaining in tritiated coal after the hydrogen exchange reaction with water.

Energy & Fuels, Vol. 15, No. 5, 2001 1131

Figure 2. Change in radioactivity of the recovered [3H]H2 gas with time from the reaction of IL coal under 5.0 MPa in the presence of Pt catalyst.

Results and Discussion Temperature Dependence. The hydrogen transfer reaction of coal with tritiated gaseous hydrogen was studied from 200 to 300 °C in the presence of a Pt catalyst, at a tritiated hydrogen gas pressure at 5.0 Mpa, and a flow rate of 50 mL/min. The Pt catalyst was used to generate tritiated hydrogen atoms in the reaction system.22,23 The gas was introduced at a constant radioactivity (ca. 1500 counts/min) and the change in radioactivity of the recovered gas during the reaction was monitored by the radioanalyzer. Figure 2 shows change with reaction time in ratio of radioactivity of the recovered gas to initial radioactivity of the gas introduced in the reactions using IL coal. The start of reaction in Figure 2 means that reactor was at reaction temperature. For the reaction at 200 °C (Figure 2a), radioactivity of the recovered gas was observed to decrease at short reaction times. When the temperatures were raised to 250 and 300 °C (Figure 2, parts b and c, respectively), a more extensive decrease in the radioactivity was observed at shorter reaction times, and then the radioactivity increased gradually to achieve a value close to the radioactivity of the gas introduced. The result shows that the activity of the gas phase returns to the original value because the system reaches equilibrium. Profiles of the gas radioactivity shown in Figure 2 were delayed about 7 min from the start of the reaction since it takes the gas 7 min to reach the radioanalyzer.

Figure 3. Change in the total hydrogen transfer ratio of coal with time from the reaction of IL coal under 5.0 MPa at 300 °C in the presence of Pt catalyst.

To clarify the extent of hydrogen transfer into the coals, we determined the hydrogen transfer ratio (HTR) derived from the radioactivity of tritium incorporated into coal. Figure 3 shows change in total HTR of IL coal as a function of reaction time at 300 °C. This result shows that extensive hydrogen transfers occurred at a short reaction time, where a HTR of 34.0% was achieved in 5 min. This is consistence with the results from monitoring the tritiated gas (Figure 2c). The HTR at 0

1132

Energy & Fuels, Vol. 15, No. 5, 2001

Sutrisna et al.

Figure 4. Change in distribution of the hydrogen transfer ratios of coal with time from the reaction with IL coal under 5.0 MPa at 300 °C in the presence of Pt catalyst.

min includes the 5 min heat up time to the reaction temperature. Further, the HTR increased significantly with time up to 40 min and then approached a constant value (ca. 51.0%). The distribution of tritiated hydrogen atoms transferred to hydroxy groups (HTR-OH) and to carbon sites such as alkyl and aryl groups (HTR-CH) in the reactions using IL coal was determined by removing the tritium incorporated in the functional groups with water.21-23 The data are shown in Figure 4. To provide information on the extent of changes in the functional groups under the conditions studied, the amount of hydrogen in the functional groups (FG) of raw IL21 and the maximum amount of hydrogen in hydroxy groups (OHmax) which may be present in this coal were also considered. Here, the OHmax was calculated by the ultimate analysis data listed in Table 1, in which oxygen functional groups assuming all groups are reduced to hydroxy. As shown in this Figure, the HTR-OH achieved a high value (16.8%) even at 5 min, and then almost no longer changed with time. Further, the values of the HTR-OH were much higher than the FG of raw IL coal, and were close to the OHmax. Our previous studies in hydrogen exchange reaction of the coals using a tritium pulse method under a low gas pressure has shown that tritiated hydrogen from the gas phase can exchange to hydrogens in hydroxy groups at temperatures 200-300 °C in the presence of the Pt catalyst,22,23 thus an increase in the HTR-OH of the tritiated coals observed in this study led to the idea that more extensive hydrogen transfer reactions to decompose ether groups or reduce carbonyl groups or both proceed in a tritiated hydrogen gas flow system under relative high pressure (5.0 MPa), generating hydroxy groups. This will be clarified by the FTIR analysis discussed below. While the HTR-OH leveled off from the beginning of the reaction, the HTR-CH increased gradually with time up to 40 min (Figure 4). This indicate that hydrogen transfer reactions to all the oxygen functional groups, i.e., exchange reaction to hydroxy groups, decomposition of ether groups, and reduction of carbonyl groups, in which two later reactions should result in incorporation

Figure 5. FTIR spectra of the raw IL and tritiated IL coals. (a), (b), (c), and (d): spectra of the raw IL, the IL coals tritiated at 200, 250, and 300 °C, respectively.

of tritium into aryl or alkyl sites, are completed at the beginning of the reaction time, and then gradual increase in the HTR-CH should result in tritium exchange to the carbon network in the coal. This clarifies the result shown in Figure 2c. To clarify the changes in the oxygen functional groups, FTIR analyses were performed on the vacuumdried raw IL and tritiated IL coals, and these IR spectra between 900 and 2000 cm-1 were compared in Figure 5. Weak broad C-O absorption between 1150 and 1250 cm-1 reduce apparently in all the tritiated coals. This could possible be due to a loss in the ether groups upon the reaction at temperatures from 200 to 300 °C, although it is not easy to distinguish between the presence of these bands in alcohols and ethers. Changes in other oxygen groups at region 1800-1500 cm-1, e.g., bands at 1740, 1700, 1655, and 1560 cm-1 characterized as alkyl esters, carboxylic acids, highly conjugated carbonyls, and COO- carboxylates, respectively,23b also appear. These changes were revealed only after subtraction of the raw coal spectrum from the spectra of the tritiated coals (Figure 6). The correct degree of subtraction is obtained by using the aromatic bands (1590-1610) as a subtraction standard, since the aromatic groups should be relatively unaffected by the reaction at low temperature. At 200 °C difference spectra (Figure 6a), the quinones (1655 cm-1) and ionized carboxylates appear negative, demonstrating a reduction of these groups upon the reaction at 200 °C. At 250 and 300 °C difference spectra (Figure 6b and c) clearly show the reduction of the various oxygen func-

Elucidation of Hydrogen Behavior in Coal

Energy & Fuels, Vol. 15, No. 5, 2001 1133

Figure 6. FTIR difference spectra of the tritiated IL coal. (a), (b), and (c): difference spectra of the tritiated coals at 200, 250, and 300 °C, respectively.

Figure 8. Distribution of the hydrogen transfer ratios of coal from the reaction under 5.0 MPa for 1 h in the presence of Pt catalyst.

Figure 7. Effect of temperature on the total hydrogen transfer of coal from the reaction under 5.0 MPa for 1 h in the presence of Pt catalyst. 2 ND, 9 IL, b POC coals.

tional groups at region 1800-1500 cm-1. The difference spectra in these cases are rather controversial, since signal-to-noise ratio was too low. This may result from the difference in mineral matter content, as addition of the Pt catalyst. From this result, and in parallel with increasing the HTR-OH observed, it leads to a conclusion that in the presence of the Pt catalyst under relative high tritiated hydrogen gas pressure hydrogen transfer to the coal occurs to decompose ether groups and reduce carbonyl groups. Effect of temperature on the total HTR of the three coals studied is shown in Figure 7. Extensive hydrogen transfer was observed with the lignite ND coal and the tritiated hydrogen atoms even at low temperatures (200-250 °C), and the HTR reached ca. 60% at 300 °C. Middle rank IL coal was less reactive than ND coal at these temperatures, but the HTR of this coal increased

significantly at 300 °C. In contrast, high rank POC coal was much less reactive than the two lower rank coals in the temperature range studied, and the HTR of this coal was less than 10% at 300 °C. These results may be related to the amount of functional groups contained in coals. ND coal which has a large amount of oxygen functional groups (see Table 2) was most reactive in hydrogen transfers at low temperature, IL coal which has a large amount of alkyl groups (Table 2) was reactive at elevated temperature over 250 °C, while POC coal which mainly involve fused aromatic rings and is low in oxygen functional groups (Table 2) was less reactive even at 300 °C. The HTR distribution of these tritiated coals is shown in Figure 8, parts a, b, and c, for ND, IL, and POC coals, respectively. The amount of hydrogen in functional groups (FG)21 and the calculated maximum hydroxy groups (OHmax) of each raw coal are also presented in each figure. From these results, in general, the HTROH of all the coals studied were higher than the FG content of the raw coals even at 200 °C, and increased to achieve a maximum at 250 °C. Furthermore, as compared to the FG of raw coal, the HTR-OH of the coals at 250 °C were ca. 1.7, 2.2, and 2.6 times higher than hydroxyl group concentration of the original ND, IL, and POC coals, respectively, and were close to their OHmax. From these results, it can be noted that the exchange reaction across the entire hydroxy groups of the coals occurs even at 200 °C, and that a significant

1134

Energy & Fuels, Vol. 15, No. 5, 2001

Sutrisna et al.

Table 3. Product Distribution of the Hydrogen Transfer Reaction of Coal at Constant Pressure of 5.0 Mpa in the Presence of the Pt Catalyst product distribution (wt %) coal

T (°C)

solid recovery

liquid

gasa

ND

200 250 300 200 250 300 200 250 300

98.9 95.1 93.1 98.5 94.9 90.2 99.4 98.2 98.0

0.9 2.1 3.2 0.7 2.2 7.5 0.1 1.2 1.5

1.0 2.8 3.7 0.8 2.9 2.3 0.5 0.6 0.5

IL POC

a

By difference.

increase in the amount of hydroxy groups observed for the low ranks of ND and IL coals, regarding the result of FTIR analysis shown in Figures 5b and 6a, leads to the occurrence of extensive hydrogen transfer reactions into ether or carbonyl groups or both upon the reaction at 200 °C, generating hydroxy groups. When temperature was raised to 250 °C, the results provide a strong indication to hydrogen transfer reactions to all the carbonyl and ether groups for not only the two low rank coals but also for the high rank POC coal. Furthermore, regarding the result shown in Figure 2, hydrogen transfer reactions to oxygen functional groups may occur at a fast rate. The maximum values of the HTR-OH at 250 °C close to OHmax is also considered from the contribution of hydrogen transfers to the small amounts of nitrogen and sulfur functional groups available in coal. The decreases in the HTR-OH for ND coal at 300 °C are due to the decomposition of thermally labile functional groups such as carboxy groups.26-28 Besides hydrogen transfers to oxygen functional groups, the hydrogen transfers to aryl or alkyl groups were also observed (Figure 8). HTR-CH of ND coal increased significantly as raising temperature from 200 to 300 °C. While, the HTR-CH of IL coal increased slightly up to 250 °C, and then increased significantly over 250 °C. In contrast, the HTR-CH of POC increased only slightly even at 300 °C, compared with the two lower rank coals. The transfers of tritiated hydrogen atoms to alkyl and aryl groups may be possible through the hydrogen exchange with hydrogen on phenoxy rings or hydrogens on benzylic sites,22,23 and as consequences of decomposition of ether-, alkyl-linkages, and labile functional groups as well as reduction of carbonyl groups. Increase in the HTR-CH of IL coal at 300 °C, however, clearly shows contribution of the hydrogen exchange to carbon network since there is no further change in the functional groups, such as decarboxylation which would result in increasing the HTR-CH. Table 3 shows the result of the recovered products after the hydrogen transfer reaction. This result shows that recoveries of the tritiated coal products were more than 95 wt % for all coals with increasing temperature up to 250 °C, and there was no indication for destruction of coal structures at a perceptible extent. However there (25) Choi, C.; Stock, M. L. J. Org. Chem. 1984, 49, 2871. (26) Artok, L.; Schobert, H. H.; Nomura. M.; Erbatur, O.; Kidena, K. Energy Fuels 1998, 12, 1200. (27) Hayashi, J.; Matsuo, Y.; Kusakabe, K.; Morooko, S. Energy Fuels 1995, 9, 284. (28) Solomon, P. R.; Serio, M. A.; Carangelo, R. M.; Gravel, D.; Baudais, B. F.; Vail, G. Energy Fuels 1990, 4, 319.

Figure 9. Effect of tritiated gaseous hydrogen pressure on the total hydrogen transfer ratio of coal from the reaction at 250 °C for 1 h. 2 ND, 9 IL, b POC coals in the presence of Pt catalyst; 4 ND, 0 IL, O POC coals in the absence of catalyst.

may be the destruction of ND and IL coal structures at 300 °C. The destruction of ND coal structure led to the formation of more light gases, which may be attributed to the significant decarboxylation decreasing HTR-OH. Further, the destruction of IL coal structure led to the formation of liquid products, which may be attributed to hydrogenolysis of a small number of weaker alkyl linkages increasing HTR-CH. In contrast, POC coal showed no significant destruction of structure at a perceptible extent under this condition. Pressure Dependence. Effect of hydrogen pressure on the hydrogen transfer reaction of coal with tritiated gaseous hydrogen was studied in the pressure range 1.5-5.0 MPa, at 250 °C, in the presence of the Pt catalyst. Reactions in the absence of the Pt catalyst were also carried out at 5.0 MPa. As shown in Figure 9, ND coal was remarkably reactive to hydrogen transfers even at the lowest pressure studied (1.5 MPa), where the HTR of ND coal was much greater than those of IL and POC coals, and increased gradually with increasing the pressure. IL coal was also more reactive at higher pressure and a significant increase in the total HTR was observed over 3 MPa. In contrast, POC coal was much less reactive over the range pressure studied compared to the two lower rank coals. This result shows that the structure with more oxygen functional groups is more reactive to hydrogen transfer reactions with hydrogen atoms at low pressure. Moreover, in the absence of the Pt catalyst at 5 MPa, hydrogen transfers of all coals studied hardly occurred, and the HTR of ND coal was only 6% and HTR of IL and POC coal were less than 3%. This shows that hydrogen transfers with hydrogen atoms generated from catalyst is essential to incorporate tritiated hydrogen to coal. The HTR distributions of these tritiated coals as a function of pressure are shown in Figure 10, parts a, b, and c for ND, IL, and POC coals, respectively. The HTR-OH of ND coal (Figure 10a) achieved 31.5% even at 1.5 MPa, and then almost no change as the pressure

Elucidation of Hydrogen Behavior in Coal

Figure 10. Distribution of the hydrogen transfer ratios of coal from the reaction at 250 °C for 1 h.

was increased. The HTR-OH obtained over the pressure range studied was close to the OHmax. As confirmed by FTIR analysis of the tritiated IL coals, increase in the HTR-OH is a result of reducing oxygen functional groups, such as ether or carbonyl groups. For ND lignite coal, therefore, it could be expected that a maximum hydrogen transfer proceeded to decompose ether linkages and reduce carbonyl groups even at 1.5 MPa. In the cases of two higher rank coals, IL and POC (Figure 10, parts b and c, respectively) the values of HTR-OH at 1.5 and 3.0 MPa were very close to the FG in raw coals and then increased continuously at the pressure over 3.0 MPa. Increasing the pressure up to 5.0 MPa resulted in the increase in HTR-OH of these coals close to their OHmax. This also shows that for higher rank coal decomposition of ether linkages as well as reduction of carbonyl groups also occurred under elevated the pressure over 3.0 MPa. Hydrogen transfers to aryl or alkyl positions for all coals, however, increased only slightly with increasing the pressure. Our previous study has pointed out that the total amount of tritium exchange (HER-total) as well as tritium exchange to hydroxy groups (HER-OH) were almost unaffected by the amount of the Pt catalyst between 5 and 25% of coal loaded.22 At the same study, it has also observed that the particle size of the coal only slightly affected the hydrogen exchange ratios. If the exchange reaction is thought to proceed on the catalyst surface (or catalytic reaction), there should be correlation between the amount of tritium exchange and intensities of the contact of coal particle with the

Energy & Fuels, Vol. 15, No. 5, 2001 1135

catalyst, i.e., amount of the catalyst or coal particle size. On the other hand, the exchange reaction via a spillover of intermediate hydrogen atoms from the catalyst should be relatively unaffected by the amount of the catalyst or coal particle size. This has provided our understanding that tritium from the gas phase can transfer to coal molecules without the requirement of direct contact between coal particle and the Pt catalyst, or the important hydrogen transfer mechanism should be a radical process involving intermediate hydrogen atoms generated from the catalyst. Furthermore, the extensive hydrogen transfer reactions observed at higher gas pressure (over 3 MPa) could be interpreted to the presence of higher concentration of intermediate hydrogen atoms inducing the reactions to reduce carbonyl groups and decompose ether groups at 250 °C for the all coals. The concept that free radical processes predominate during hydrogen transfer reactions to coal molecules is quite widely accepted. For example, Choi and Stoct have shown that benzophenone related to the carbonyl groups is reduced in the hydrogen donor molecules or tetralin, and revealed that the first stage of the reaction is a radical process which gives benzhydrol.25 Furthermore, as reported by several authors,1,31-37 in studies using ether model compounds, decomposition of ether model compounds proceeds through a free-radical process that leads to cleavage of C-O bonds. In our results, a spillover of tritium atoms from the catalyst to coal molecules was expected to induce reduction of carbonyl groups and decomposition of C-O bond, generating hydroxy groups. On the other hand, other possible mechanisms especially the role of hydrogen atoms in the interior of the coal particles should also be considered for the evidence of tritium transfer into the coals, although there is no direct evidence for this in our present study. Exchange Reactions of Coal and Model Compounds with Gaseous Deuterium in the Presence of Coal. The exchange reactions of several model compounds with gaseous deuterium were examined to establish the positions in the coal structure which would exchange with hydrogen (tritium) atoms and to gain enlightenment for the mechanisms of tritium transfer to the coals. Unfortunately, reaction of the model compounds using the Pt catalyst should be unavoidable for a catalytic reaction on the catalyst surface, resulting in hydrogenation of the aromatic ring which would hinder the determination of the actual deuterium exchanged to the ring. Since the important tritium transfer to the coals in the presence of the Pt catalyst should be a radical process involving tritiated hydrogen atoms, this may allow us to compare the tritium transfer (29) King, H.; Stock, M. L. Fuel 1982, 61, 1172. (30) King, H.; Stock, M. L. Fuel 1982, 61, 257. (31) Simmons, M. B.; Klein, M. T. Ind. Eng. Chem. Fundam. 1985, 24, 55. (32) Buchanan, A. C., III.; Britt, P. F.; Skeen, J. T.; Struss, J. A.; Elam, C. L. J. Org. Chem. 1998, 63, 9895. (33) Buchanan, A. C., III.; Britt, P. F.; Thomas, K. B.; Biggs, C. A. J. Am. Chem. Soc. 1996, 118, 2182. (34) Cassidy, P. J.; Hertan, P. A.; Jackson, R. W.; Larkins, P. L.; Rash, D. Fuel 1982, 61, 939. (35) Vuori, A. Fuel 1986, 65, 1575. (36) Bredenberg, J. B.; Ceylan, R. Fuel 1983, 62, 342. (37) Suryam, M. M.; Kafafi, S. A.; Stein, S. E. J. Am. Chem. Soc. 1989, 111, 1423.

1136

Energy & Fuels, Vol. 15, No. 5, 2001

Scheme 1. Possible Hydrogen Exchange Reaction between Hydrogen in the Benzylic Position of Toluene with Gaseous Deuterium, in the Presence of Coal

Scheme 2. Possible Hydrogen Exchange Reaction between Hydrogen in Hydroxy and Aromatic Ring Positions of Phenol with Gaseous Deuterium, in the Presence of Coal

reaction with thermal deuterium exchange of the model compounds with coal as a radical initiator. In the exchange reaction with phenol or toluene at 400 °C for 6 h under initial deuterium pressure 2.5 MPa, NMR analysis showed that the recovered phenol was deuterated, not only in hydroxy position but also in ortho and para positions with about the same deuterium content, 32.2% and 38%, respectively, while the recovered toluene was deuterated in the benzylic with deuterium content of 32.8% which is the same deuteration level with the phenoxy ring. Possible hydrogen exchange reaction to the hydrogen in benzylic and phenoxy ring can be illustrated in Schemes 1 and 2, respectively. King and Stock have also shown, in the study of the exchange reaction of phenol with tetralind12 and naphthalene-d8, that ortho and pa-ra positions were exchangeable, and concluded that the exchange reaction to the aromatic hydrogen of phenol proceeded by a free-radical process and not by an electrophilic substitution reaction.29 At the same time, they have also

Sutrisna et al.

reported that the hydrogen exchange between hydrocarbons with benzylic hydrogen atoms and tetralin-d12 occurred selectively at the benzylic position of diphenylpropane or diphenylbutane.30 The reactivity of the benzylic position to hydrogen exchange has been explained by relatively lower C-H bond energy compared with those of in aromatic C-H or secondary aliphatic C-H (i.e., 463 kJ mol-1 for aromatic C-H, 342 kJ mol-1 for the benzylic C-H, and 396 kJ mol-1 for the secondary C-H).30 The result above can be noted that hydrogens in benzylic, ortho and para positions of phenoxy as well are exchangeable, and that the exchange of those positions with gas phase of deuterium in the presence of IL coal is approximately the same rate. Thus the increase in HTR-CH of IL coal from 250 to 300 °C (Figure 8) which is abundance in alkyl groups (Table 2) has presented the hydrogen exchange to the hydrogen in benzylic position. A clear evidence for this exchange has also presented by the result of IL coal as a function of time in Figure 4. While increase in HTR-CH of ND coal at 300 °C which has a large amount of phenoxy groups can be recognized from the contribution of hydrogen exchange with hydrogens on the phenoxy ring. On the other hand, substantial decarboxylation of ND coal was observed at this temperature, thus the significant increase in the HTR-CH of this coal can also be considered from contribution of the decarboxylation process. The hydrogen exchange to the carbon network obtained from high rank POC coal, however, occurred to the much lower extent than those of the two lower ranks even at elevated temperature 300 °C. This leads to ideas, as reported by the result of King and Stock,30 that the presence of free radical initiators from labile linkages or reducible molecules from oxygen functional groups or certain aromatic molecules in coal can enhance the rate of the hydrogen exchange to benzylic position. The reaction of benzyl phenyl ether (BPE) in the presence of IL coal under initial deuterium pressure 2.5 MPa at 400 °C for 1 h yielded phenol, toluene, benzylphenol, and diphenylmethane as the dominant products with minor amounts of benzophenone and bibenzyl, shown in Scheme 3. The products typically accounted for >95% of the products, with small amount of unidentified complex products. Formation of a significant amount of benzylphenol is well-known as a result of the ring recombination of phenoxy radical with benzyl radical. Benzophenone, as reported by Buchanan et al.,32 was generated from a radical rearrangement of an intermediate radical, PhOCH•Ph. A significant amount of diphenylmethane observed may be produced from a second reaction of initial labile product of benzhydrol or benzophenone,25 in which these compounds are generated from a radical rearrangement of a PhOCH•Ph.32 Mass spectral analysis of the liquid product (Scheme 3) indicated that deuterium can incorporate to hydroxy in products 1 and 3 (probability of seeing single D is 61.2 and 27.2%, respectively) and aromatic ring of phenoxy groups (product 1), benzylic position and benzene (products 2, 3, 5, and 4; probability of seeing D is 2, 8.4, 5.6, and 5.2%, respectively). The reaction of dibenzyl ether (DBE) under the same condition above yielded products of toluene, benzaldehyde, benzene,

Elucidation of Hydrogen Behavior in Coal Scheme 3. Product Distribution and Identified Deuterium Positions of the Products from the Reaction between BPE and Gaseous Deuterium, in the Presence of IL Coal

Energy & Fuels, Vol. 15, No. 5, 2001 1137 Scheme 5. Possible Reaction Scheme for Decomposition of Ether Linkages in Coal Correspond to Benzyl Phenyl Ethera

a R‚ is a radical generated from labile linkage or reducible molecules in coal.

Scheme 4. Product Distribution and Identified Deuterium Positions of the Products from the Reaction between DBE and Gaseous Deuterium, in the Presence of IL Coal

bibenzyl, and benzyl alcohol (> 92% of the products, shown in Scheme 4) with some unidentified complex products. The thermolysis of dibenzyl ether has been reported via C-O bond fragmentation to produce toluene and benzaldehyde, in which the latter primary product is capable of secondary reaction to benzyl alcohol and benzene.31 Mass spectral analysis (Scheme 4) showed that deuteration occurred singly at benzylic position of the products 1, 4, 6, and 7 (probability of seeing D is 7.9, 12.5, 1.5, and 5.5%, respectively). The findings of this study, including observations to different reactivity of the three coals to decompose ether groups and the product distribution obtained from the reactions of ether model compounds need to be elucidated. Hence, the following discussion is concerned with the facile reactions of the coal molecules which assist the C-O bond scission. The results indicate that structure molecules with high concentration of hydroxy and carbonyl groups in the two low rank ND and IL coals accelerate the decomposition of ether groups at temperature as low as 200 °C at 5.0 MPa (Figure 8), and that the reactivity of the structure molecules in

lignite ND coal which is has much more hydroxy and carbonyl groups results in decomposition of these groups at low pressure 1.5 MPa at 250 °C (Figure 10). Moreover, the results also indicate that decomposition of ether groups in higher rank IL and POC coals is proceeded only at high concentration of tritiated hydrogen atoms, over 3.0 MPa. These results, hereby, suggest that, in addition to the hydrogen transfer via a spillover of hydrogen atoms from catalyst, there is a strong correlation between the presences of hydroxy or carbonyl groups and easiness of hydrogen transfers to the C-O bond cleavage reactions. In the studies of the decomposition of the anisole model compound, the effect of intramolecular hydrogen bonding available in the o-hydroxy substituent has accelerated the C-O bond cleavage reaction of anisoles,35,36 and resulted in a large bond weakening measured.37 King and Stock1 have shown that the addition of compounds which have oxygen functional groups such as 9,10-anthraquinone, phenol, and benzoic acid accelerates the C-O bond cleavage reactions of benzyl phenyl ether and dibenzyl ether. They have also reported that reducible compounds enhance hydrogen atom transfer reactions by formation of radical molecule at the first stage reaction.30 Therefore, the reactivity of lignite ND coal to decomposition of ether groups can be attributed to the effect of the presences of hydroxy groups and reducible carbonyl groups or other coal molecules. Possible hydrogen transfer pathways for ether linkages in coal correspond to BPE and DBE can be illustrated in Schemes and 6, respectively. The labile BPE may decompose by homolysis of the weak central C-O bond under additional hydrogen-bonding effects to yield benzyl and phenoxyl radicals (eq 8) which may be sequenced by competitive reaction pathways as follows: stabilization of radicals by reaction with tritiated hydrogen atoms available (eq 9), or recombination of these two radicals to form benzylphenol (eq 10). Further, the hydrogen atoms or the radical initiators available may abstract hydrogen from benzylic position of BPE to produce an intermediate radical, CoalPhOCH•Coal (eq 11), followed by radical rearrangement of this radical

1138

Energy & Fuels, Vol. 15, No. 5, 2001

Scheme 6. Possible Reaction Scheme for Decomposition of Ether Linkages in Coal Correspond to Dibenzyl Ether in Coal

Sutrisna et al.

available. This, as reported by Buchanan et al.,32,33 may show the importance of the retrograde reaction pathways involving decomposition of ether linkages in coal via both radical rearrangement of a very reactive phenoxy radical with incipient benzylic radical at the phenoxy ring (eq 10) and 1,2 phenyl shift rearrangement (eq 12). Conclusions

producing an alcohol (eq 12), as reported by Buchanan et al.,32 and this has been confirmed from the reaction BPE model compound. Whereas, the decomposition of ether linkage corresponds to DBE (Scheme 6) has been confirmed to yield toluene and benzaldehyde as the dominant products, and has been reported to proceed by a benzylic hydrogen abstraction, followed by β-scission accounting for its higher C-O bond energy and the formation of benzaldehyde as the dominant product (eqs 14, 15).1,31 It can also be expected that the hydrogen bonding-interaction with hydroxy groups and the presences of free-radical initiators from labile linkages or reducible molecules, as well as hydrogen atoms generated from catalyst can promote the decomposition of stronger ether linkages, not only linkage corresponds to DBE but also linkage corresponds to phenethyl phenyl ether, at much lower temperature than expected for thermolysis. Further, the product corresponding to benzaldehyde is considered to form hydroxy groups (eq 17), accounting for the increase in HTR-OH to achieve maximum hydroxy content which may exist. As shown in Figure 10, parts b and c, for IL and POC coals, the pressure only slightly affected the change in transferring tritiated hydrogen atoms to carbon sites, compared to the increase in contents of hydroxy groups as a result of hydrogenolysis of ether linkages when radicals formed is considered to be stabilized by tritiated hydrogen atom

The hydrogen transfer reaction between coal and tritiated gaseous hydrogen using a fixed-bed flow reactor has been studied. In general, under the relatively mild conditions studied, the results have provided insight into the hydrogen mobility in coal and the changes in coal structure induced by the hydrogen transfers, especially structure related to oxygen-containing groups. The results might indicate that the hydrogen transfer pathway via a spillover of hydrogen atoms from catalyst to coal molecule should be an important pathway to induce hydrogen transfers to (a) hydrogen exchange with hydrogen in functional groups, (b) decomposition of ether linkages, (c) reduction of carbonyl groups, and (d) hydrogen exchange with hydrogen in the benzylic position and the phenoxy ring. These first three processes above occurred at a fast rate reaction at temperature over 200 °C, 5 MPa. It was considered that the presences of the hydroxy groups and radical initiators from labile linkages or reducible molecules such as oxygen functional groups in low-rank coal can enhance hydrogen transfers to decomposition of ether linkages, and that these radical initiators also enhance hydrogen exchange with hydrogen in the benzylic position at 300 °C and 5.0 MPa. The decarboxylation process was also observed for the reaction of lignite ND coal at elevated temperature 300 °C. Acknowledgment. This study was supported by the Research for the Future Project of the Japan Society for the Promotion of Science (the 148 Committee on Coal Utilization Technology) under Contract JSPS-RFTF96R14801. EF010005X