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Liquefaction of Japanese bituminous Akabira coal catalyzed by molten salts under D2 atmosphere
FUEL PROCESSING TECHNOLOGY Fuel Processing Technology 43 (1995) 213-225
Liquefaction of Japanese bituminous Akabira coal catalyzed by molten salts under D2 atmosphere Masakatsu
Nomura*,
Takeshi
Muratani,
Yasuhiro
Tajima,
Satoru
Murata
Department of Applied Chemistry, Faculty of Engineering, Osaka University 2-I Yamada-oka. Suita, Osaka, 565 Japan
Received 7 July 1994; accepted in revised form 17 February 1995
Abstract Akabira coal (Japanese bituminous coal) was liquefied in the presence of molten salts like SnCl,/KCl under a hydrogen or deuterium atmosphere (at 400°C for 1 h). Resulting products were separated into gaseous products, hexane soluble (HS), hexane insoluble but benzene soluble (HI/BS), benzene insoluble but pyridine soluble (BI/PS) and residue. Two extracts, HS and HI/B& were separated by column chromatography. Three fractions of the HS extract account for up to 90% of the HS and four fractions of the HI/BS extract up to 80% of HI/BS. The distribution of deuterium atoms in three fractions of HS were examined by measuring their FD/MS and ‘H NMR properties, and comparing these results with corresponding data of three fractions of HS obtained in a H, atmosphere. These results strongly suggest the preferential incorporation of deuterium atom at cc-carbon adjacent to aromatic compounds. Kinetic features of the liquefaction are discussed according to the detailed analysis of three fractions of HS.
1. Introduction Studies on coal liquefaction catalyzed by molten salts have been undertaken by many coal chemists. Zielke and his coworkers [l-3] concentrated much attention on the development of a coal liquefaction process using massive amounts of molten salts with much stress on the use of ZnClz. Berg and Malsam [4] indicated the preferable characteristics of ZnClJKCl melts over ZnCl, for coal liquefaction. Bell and his coworkers [S-S] have published several papers concerning kinetic aspects of molten salt-catalyzed liquefaction of coal. We have published a wide range of papers [9-143 on this topic of coal liquefaction in the presence of molten salts media. The aim of this paper is to carry out coal liquefaction experiments in the presence of massive amounts of molten salts under a deuterium atmosphere and to obtain
* Corresponding author 0378-3820/95/$09,50 0
1995 Elsevier Science B.V. All rights reserved
SSDI 0378-3820(95)00027-5
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M. Nomura et al. /Fuel Processing
Technology 43 (1995) 213-225
a detailed analyses of the degree and the distribution of deuterium uptake of the products so as to derive closer understanding of coal liquefaction chemistry and coal structure.
2. Experimental 2.1, Reagents Akabira bituminous coal (2.9 wt% ash) was pulverized to under 100 mesh and dried at 110°C for 12 h under reduced pressure. Elemental analysis of Akabira coal is as follows: C, 81.0%; H, 5.6%; N, 2.0%; S, 0.5%, 0, 11.0% (difference); H/C 0.83. Chemical reagents employed here were used without further purification. The liquefaction was carried out under 25 kg/cm2 of D2. 2.2. Preparation of coal deposited with SnC12/KCl Both SnC12 and KC1 (3 : 2 molar ratio) were solubilized in methanol, being kept at 50°C for 1 h with stirring. After evaporation of methanol from the resulting solution, the uniform mixture of SnCl,/KCl was dried at 160°C under 2 mmHg for 2 h. Deposition of SnC12/KCl over coal particles was achieved by adding coal particles to the methanol solution of SnC12/KCl, followed by complete removal of methanol by evaporation. The coal with SnCl,/KCl was dried at 100°C for 12 h under 2 mmHg. 2.3. Hydroliquefaction of coal The autoclave (50 ml) charged with 4 g of treated coal particles and deuterium (25 kg/cm2) was agitated by a rocking motion. The heating regime was as follows: At first the autoclave was heated up to 200°C at a rate of 12”C/min. Shaking was started with 43 strokes/min at 2Oo”C,as the autoclave was further heated to 400°C at a rate of 8”C/min, and held at 400°C for 1 h. After the reaction, the contents of the autoclave was cooled as quickly as possible by air flow. Gaseous products were recovered in the gas bag, an aliquot of which was injected into a gas chromatograph for quantitative analysis. The contents of the autoclave were washed out into a beaker with a small portion of hexane followed by a small amount of tetrahydrofuran. After the evaporation of these solvents, resulting solid contents were transferred to a Soxhlet thimble and subjected to hexane extraction. The hexane soluble fraction was termed HS. Subsequent extraction with benzene gave a hexane insoluble and benzene soluble fraction (HI/BS). The pyridine soluble fraction of the thimble residue was washed with diluted HCl, deionized wate r, and then 6N HNO, in order to remove the salts. 2.4. Further separation of HS and HIfBS fractions The HS and HI/BS fractions were separated into 7 fractions (HSl. -HS7, HI/BSl-HI/BS7), according to the improved USBM/API method [15].
M. Nomura et al. /Fuel Processing
2.5. Instrumental
Technology43 (1995) 213-225
215
analyses
In order to get information about molecular weight of HSl-HS3 fractions, their FD/MS spectra were measured by JEOL JMSDX303HF. Pyrolysis GC/MS measurements were carried out for HS, HI/BS and several fractionated fractions of HS using a Curie-point pyrolyzer, Japan Analytical Industry JHP-3 under following conditions (670°C 3 s). ‘HNMR, 2H NMR and 13CNMR measurements of HSl-HS3 were undertaken by Bruker AM600 (‘H and 13C) and JEOL JNMGSX400 (‘H) NMR spectrometers, using CDC13 and CHC13.
3. Results 3. I. Hydroliquefaction
of Akabira coal
Table 1 lists the hydroliquefaction of Akabira coal in the presence of molten salts under H2 or D2 atmospheres. Since D uptake in liquefied products is considered very informative in evaluating coal structure and the liquefaction mechanisms of coal organic materials, liquefaction under a H2 atmosphere was also carried out to provide supportive analytical data. The yields of gaseous products were estimated according to the results of liquefaction under H2 atmosphere. Table 1 indicates that conversion ranges from 89% to 98% were achieved. Under these reaction conditions, gaseous products were as follows; (under H2) methane 26%, ethane 16%, propane 19%, n-butane 9%, i-butane 9%, COZ 21%; (under D2) methane 22%, ethane 17%, propane 17%, n-butane 8%, i-butane 8%, CO2 28%. 3.2. Characterization
of HS and HIjBS
Elemental analyses of HS and HI/BS fractions are listed in Table 2. Due to the incorporation of D atoms into liquefied products, products under D2 atmosphere were not submitted for elemental analysis. Table 2 suggests that the H/C atomic ratio of HS is 1.21 while that of HI/BS is 0.95. These fractions were submitted to Curiepoint pyrolysis. Volatile manner of HS (H2) and HS (D2) were 64% (H2) and 63% (D2) respectively, tar matter and coke being 25% (H,) and 23% (D2), 11% (H,) and 14% (D2) respectively. Since volatile matter is the fraction injected into the gas chromatograph, about $ of the mass of these samples could be analyzed instrumentally.
Table 1 Results of hydroliquefaction of Akabira coal (wt% daf) Atmosphere
Gas
HS
HI/BS
BIjPS
Conversion
HZ I&
13 13
20 23
16 18
49 35
98 89
216
M. Nomura et al. /Fuel Processing
Table 2 Elemental
analysis
of HS and HI/BS fractions
Technology 43 (1995) 213-225
(wt%)
Fraction
C
H
N
S
O(diff.)
H/Ca
HS HI/BS
86.3 85.0
8.7 6.7
0.9 1.3
0.2 1.2
3.9 5.8
1.21 0.95
a Atomic
Table 3 Distribution
ratio
of seven fractions
Atmosphere
of HS and HI/BS
Fractions
(wt%)
1
2
3
4
5
6
7
HS
H2 DZ
34 32
17 19
34 35
4 2
2 1
7 9
HI/BS
HZ D2
2 2
8 8
7 8
20 17
9 11
29 29
_ _ 15 19
However, in the case of the HI/BS fraction, volatiles were only 35% (Hz, Dz), tar being 29% (H,) and 21% (DJ, and coke being 36% (H,) and 44% (Dz). This higher tendency of the HI/BS fraction to form coke during pyrolysis can be understood due to its high contents of heteroatoms like nitrogen, sulfur and oxygen, and its lower H/C value. The lower H/C value suggests the HI/BS fraction is more aromatic, and the degree of condensation of aromatic compounds should be higher than the HS fraction. Table 3 shows the distribution of seven fractions of HS. From this table, HSl to HS3 contains about 90% of the whole sample. The distribution of the HI/BS fraction also indicates that the HI/BS4 to HI/BS7 fractions account for almost 80% of the injected product.
4. Discussion 4. I. Structural
characteristics
4.1. I. Structural features
of Akabira coal
of HSI fraction
The FD/MS spectrum of HSl (H,) is shown in Fig. l(a) along with that of HSl (D2) (Fig. l(b)). Clear peaks appearing at regular interval of m/z 14 are considered to be alkanes of CnH2n+Z while these peaks disappear in the FD/MS of HSl (DJ. These findings suggest that deuterium atoms are incorporated into alkane derivatives. The HSl fraction is evaluated as a saturate fraction by referring to the definition of the
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Technology 43 (1995) 233-225
217
R ;
2E-
0 4
Fig. 1. FD/MS spectra of HSl fraction from the liquefaction product under (a) H, and (b) D,.
improved USBM/API method. However, the H/C ratio of HSl (H,) was found to be 1.45, which suggests contamination with small amounts of aromatic compounds. GC/MS analysis of HSl (Hz) indicated the presence of alkylbenzene, alkylnaphthalene and tetralin derivatives. As for the separation of a saturate fraction from HS according to the improved USBM/API method, our separation was found to be insufficient. We will discuss this point later. Coal derived liquid is very complicated as suggested by the appearance of its FD/MS spectra so the ‘H NMR and ‘H NMR spectra of these two fractions (HSI (H,) and HSl (DJ) were measured. According to the Brown-Ladner assignments [16], the distribution of four protons; H,, (9.00-6.00 ppm), H, (5.00-2.00 ppm), H, (2.00-1.05 ppm), H, (1.05-0.20 ppm) were estimated: HSl (H,) H,, 16%; H, 27%; H, 43%; H, 14%: HSl (Dz) H,, 25%; H, 36%; H, 30%; H, 9%: These values are only
218
kf. Nomura et aLlFuel Processing Technology 43 (1995) 213-225
relative ratios because we could not quantify the exact amount of deuterium incorporated in the HSl fraction. However, the comparison of these values reveals the very interesting features of the resulting HSl fraction: in fact D atoms are incorporated into H,, and H, preferentially. These tendencies are very important to our interpretation of the mechanism of incorporation of deuterium atoms. Complete separation of saturate from HS was made by fractioning 50 ml of pentane elute in place of 250 ml of pentane. The measurement of 2H NMR of the resulting fraction indicates the disappearance of peaks around 9.00-2.00 ppm. Partial regions (15-13.5 ppm) of 13C NMR spectra of HSl (H,) and HSl (D2) are shown in Fig. 2. In Fig. 2(b), we can observe clear triplet due to the presence of -CH2D while this spectra cannot be observed in Fig. 2(a). Another partial region (23.5-22 ppm) of 13C NMR spectra of HSl (H,) and HSl (D2) are indicated in Fig. 3. The presence of -CHD- and -CD2- is strongly suggested by the appearance of peaks around 22.5 ppm in Fig. 3(b). From these considerations, deuterium atoms
a)
I
- CH,CH,CHQ$
-1
I 15
14.5
.I 14
I13.5
Chemical shift @pm) Fig. 2. 13C NMR spectra of HSl fraction from the liquefaction product under (a) H, and (b) D,.
M. Nomura et al. /Fuel Processing Technology 43 (1995) 213-225
219
I -CH,CH&H,CH,
-1
I 23.5
23
I22.5
22
Chemical shift (ppm)
Fig. 3. 13C NMR spectra
of HSl fraction
from the liquefaction
product
under (a) H, and (b) D,.
incorporated in alkane fraction exist at terminal methyl and methylene adjacent to terminal methyl group. Straight chain alkane is known to show the strongest peak at m/z 57 in its mass spectrum. As expected, the GC/MS of HS (H,) fraction shows the mass spectra of many alkane derivatives that have peaks at m/z 57 of C,Hi with a small isotope peak of m/z 58. However, each alkane of HSl (D2) show Ct fragments with m/z 57,58,59 and 60 (minor), this indicating each fragment contains from zero to three deuterium atoms. However, relative intensity of the peak with m/z 60 is negligible, so being omitted. Table 4 lists isotope distributions of alkanes in HSl (DJ. The degree of deuterium incorporation decreases as the number of carbon atoms in alkane increases. This suggests that the shorter the alkane chain, the greater the probability of alkane existing as an alkyl chain or an alkylene bridge of aromatics (refer to Section 4.2).
220
Table 4 Distribution
M. Nomura et al. /Fuel Processing Technology 43 (1995) 213-225
of isotope
in Cl
fragment
of alkanes
(mol ratio)
Alkanes
C,Hs+
&HsD+
GH7D:
C 12 C 13 C 14 C 15 C 16 C 17 C 18 C 19 C 20 C 1, C 22 C 23 C 24 C 25 C 26 C 27 C 28 C 29
65 68 61 70 58 70 73 72 77 72 81 75 79 78 79 78 80 80
22 20 22 20 27 20 19 20 18 21 15 19 17 17 16 17 16 16
13 12 17 10 1.5 10 8 8 5 7 4 6 4 5 5 5 4 4
4.1.2. Structural features of HS2 and HS3 fractions
The FD/MS spectra of the HS2 and HS3 fractions give number-average molecular weights of ca 461 and ca 509, respectively. The presence of distinct peaks appearing at a regular interval of m/z 14 on FD/MS spectra of HS2 (H,) suggests that various kinds of alkylated aromatic compounds are contained. These features are also observed in the FD/MS spectra of HS3 (H,). Fig. 4 shows the mass spectra of specific products obtained from Curie-point pyrolysis of HS3 (H,) and HS3 (Dz). The xylene spectra of HS3 (H,) and HS3 (Dz) are compared in Fig. 4(A). Similar comparison of dimethylphenol is shown in Fig. 4(B). In the case of xylene, its molecular ion from HS3 (Dz) is higher by 4 mass units than that from HS3 (H,), while the (M-CH3)+ fragment ion from HS3 (D2) is higher by 3 mass units than that from HS3 (Hz). Therefore one deuterium atom is believed to be incorporated in a methyl group of xylene from HS3 (D2). Also in the case of dimethylphenol, the molecular ion from HS3 (Dz) is higher by 3 mass units compared with that from HS3 (Hz). On the other hand, the (M-CH3)+ fragment ion is higher by 2 mass units than that from HS3 (H,). Therefore one deuterium atom is surely incorporated in each methyl group of dimethylphenol. The average proton distributions in HS2 (H,) and HS3 (Hz) and deuterium distributions in HS2 (Dz) and HS3 (Dz) were estimated according to their ‘H NMR And ‘H NMR spectra, respectively. These results are listed in Table 5. From Table 5, it is clear that deuterium atoms are preferentially incorporated into aromatic nuclei and aliphatic carbons adjacent to aromatic compounds. These findings were also observed
M. Nomura et al. /Fuel Processing Technology 43 (199s) 213-225
WY,)+
(W+ 1
lgi
21)
(B)
CM)+
CM-CHJ)’
-
221
e
b) *
A-
109
I 100
I 110
I 120
,. 130
m/z
Fig. 4. (A) Mass spectra of xylene from the Curie-point pyrolysis of HS3 fraction from the liquefaction product under (a) H, and (b) D,. (B) Mass spectra of dimethylphenol from the Curie-point pyrolysis of HS3 fraction from the liquefaction product under (a) H, and (b) D,.
Table 5 Average distribution of proton and deuterium in HS2 and HS3 fractions Atmosphere
H,,
HZ
H,
H;
HS2
H, D2
30 37
39 44
24 16
7 3
HS3
H, D2
23 32
34 41
30 22
13 5
in the aromatic compounds of HSl fraction. As for the preferential incorporation of deuterium atoms of cc-carbon sites of aromatic compounds, we have obtained additional information; Fig. 5 shows the region attributed to a-methyl groups in 13C NMR spectra of HS2 (Hz) and HS2 (DJ. Peaks observed at 21.3,19.9 and 19.5 ppm are indicative of a-methyl of aromatic compounds like the methyl of 1-methylnaphthalene (21.6), the methyl of toluene (21.4), the methyl of m-xylene (21.3) the methyl of o-xylene (19.7) and the methyl of 1-methylphenanthlene (19.9). However, in the 13C NMR spectra of HS2 (DJ, two broad peaks centered at 21.4 and 19.7 ppm are observed. Such a broadening of a-methyl peaks is considered due to the presence of coupling between 13C and D which is inserted into a-methyl groups.
222
M. Nomura et al. /Fuel Processing Technology 43 (1995) 213-225
b)
I
I
I
22
20
18
Chemical shift (ppm) Fig. 5. 13C NMR spectra of HS2 fraction from the liquefaction product under (a) H, and (b) D,.
4.2. Possible mechanism of deuterium atom uptake of aromatic and aliphatic compounds
We have published several papers on coal liquefaction in the presence of massive amounts of molten salts. Our results indicated molten salts act as a dispersant of
M. Nomura et al. /Fuel Processing
Technology 43 (1995) 213-225
223
intermediate species from coal organic materials and the catalytic material. We proposed that both ionic and radical mechanism contribute to liquefaction in Lewis acid type molten salt like SnC12/KCl and ZnCl,/KCl. Our finding in this study that deuterium atoms are incorporated into aromatic and aliphatic compounds can be explained in this context. Scheme 1 shows the possible ionic reactions for incorporation of D atoms; reaction (a) indicates that D+ attacks the ipso-position of aromatic compounds, resulting in the formation of deuterium substituted aromatics and deuterium substituted methyl group linking aromatic compounds. From our previous results [ 171, we have shown that the methylene bridges connecting two aromatic compounds is very common in coal organic materials. Reaction (b) also explains the way that deuterium substituted aromatic and deuterium substituted aliphatic compounds are formed. Scheme 2 indicates the radical mechanism of incorporation of deuterium atoms. In reaction (a) homolytic fission of 1,Zdiarylethane yields two benzyl radicals. These radicals abstract D atom from Dz dissolved in melts. In reaction (b) alkyl substituted aromatics yields benzyl type radical and alkyl radical. These two radicals abstract D atom in the way similar to reaction (a) in Scheme 2. The alkyl radical undergoes the disproportionation reaction to yield olefin and alkane. Olefin then takes up D2 to form two deuterium atoms containing alkane by addition reaction.
+--D-
Scheme 1. Ionic mechanism of coal liquefaction in the presence of molten salts.
224
M. Nomura et al. /Fuel Processing
Technology 43 (1995) 213-225
b) “,,
“G,,
6
1;. +..*i %,, ’
Scheme 2. Radical
“Q,,
c
,,.”
,.+”
.-
mechanism
-
of coal liquefaction
Dw
in the presence
of molten
salts.
5. Conclpsion
Liquefaction of Akabira bituminous coal was carried out at 400°C for 1 h using massive amounts of SnCl,-KC1 melt in the presence of H2 or Dz (upto 25 kg/cm2). These liquefaction produced a 90% of pyridine soluble material, both hexane soluble (HS) and hexane insoluble benzene soluble (HI/BS) portions of which were each separated chromatographically into 7 portions. Two HSl fractions (H, and D2: saturate) from above HS fraction according to the improved USBM/API method were submitted to measurements of ‘H NMR and ‘H NMR spectra. Their BrownLadner assignments revealed the very interesting features of two HSl fractions in that deuterium atoms are incorporated into H,, and H, preferentially. 13C NMR spectra of these HSl (H, or D2) also suggested the presence of -CH2D, -CHD- and -CD2-: deuterium atoms incorporated in alkane fraction exist at terminal and methylene adjacent to terminal methyl group. The mass spectra of specific products obtained from Curie-point pyrolysis of HS3 (H2) and HS3 (D2) indicated that at least one deuterium atoms is believed to be incorporated in a methyl group of xylene or each methyl group of dimethylphenol. Deuterium incorporation observed in the structural study on HS fraction can be explained in the context that D+ attacks the ipso-position of aromatic compounds, resulting in the formation of deuterium substituted aromatics and deuterium substituted methyl group linking aromatic compounds or in the radical mechanism of incorporation of deuterium atoms.
M. Nomura et al. /Fuel Processing
Technology 43 (199.5) 213-225
225
Reference Cl1 C.W. Zielke, R.T. Struck, J.M. Evans, C.P. Costanza
and E. Gorin, Ind. Eng. Chem. Process Des. Dev., 5 (1966) 158. PI R.T. Struck, W.E. Clark, P.J. Dudt, W.A. Rosenhoover, C.W. Zielke and E. Gorin. Ind. Eng. Chem. Process Des. Dev., 8 (1969) 546. c31C.W. Zielke, E.B. Klunder, J.T. Maskew and R.T. Struck Ind. Eng. Chem. Process Des. Dev., 19 (1980) 85. using a potassium chloride-zinc chloride molten c41L. Berg and J.S. Malsam, 1973. Coal hydrogenation salt mixture as a catalyst. US Patent 3,736,250. c51D.P. Mobley and A.T. Bell, Fuel, 58 (1979) 661. C61N.D. Taylor and A.T. Bell, Fuel, 59 (1980) 499. c71S.S. Salim and A.T. Bell, Fuel, 61 (1982) 745. PI T.J. Frederick and A.T. Bell, J. Catal. 87 (1984) 226. c91T. Ida, M. Nomura, Y. Nakatsuji and S. Kikkawa, Fuel, 58 (1979) 361. Cl01M. Nomura, M. Miyake, H. Sakashita and S. Kikkawa, Fuel, 61 (1982) 18. Cl11M. Nomura, K. Kimura and S. Kikkawa, Fuel, 61 (1982) 1119. Cl21M. Nomura, H. Sakashita, M. Miyake and S. Kikkawa, Fuel, 62 (1983) 73. Cl31M. Nomura, T. Yoshida and Z. Morita, Ind. Eng. Chem. Product Res. Dev., 23 (1984) 215. Cl41M. Nomura, H. Kawakami and Y. Kawakami, in: N.P. Vasilakos (Ed.), Advances in Coal Chemistry, University of Patras, Patras, 1988, pp. 287-304. Fuel, 55 (1976) 16. Cl51H. Sawatzky, A.E. George, G.T. Smiley and D.S. Montgomery, Cl61J.K. Brown and W.R. Ladner, Fuel, 39 (1960) 87. Cl71M. Nomura, S. Yhuzu, T. Fujimoto, T. Ida and M. Miyake, Fuel, 69 (1990) 972.
Liquefaction of Japanese bituminous Akabira coal catalyzed by molten salts under D2 atmosphere Masakatsu
Nomura*,
Takeshi
Muratani,
Yasuhiro
Tajima,
Satoru
Murata
Department of Applied Chemistry, Faculty of Engineering, Osaka University 2-I Yamada-oka. Suita, Osaka, 565 Japan
Received 7 July 1994; accepted in revised form 17 February 1995
Abstract Akabira coal (Japanese bituminous coal) was liquefied in the presence of molten salts like SnCl,/KCl under a hydrogen or deuterium atmosphere (at 400°C for 1 h). Resulting products were separated into gaseous products, hexane soluble (HS), hexane insoluble but benzene soluble (HI/BS), benzene insoluble but pyridine soluble (BI/PS) and residue. Two extracts, HS and HI/B& were separated by column chromatography. Three fractions of the HS extract account for up to 90% of the HS and four fractions of the HI/BS extract up to 80% of HI/BS. The distribution of deuterium atoms in three fractions of HS were examined by measuring their FD/MS and ‘H NMR properties, and comparing these results with corresponding data of three fractions of HS obtained in a H, atmosphere. These results strongly suggest the preferential incorporation of deuterium atom at cc-carbon adjacent to aromatic compounds. Kinetic features of the liquefaction are discussed according to the detailed analysis of three fractions of HS.
1. Introduction Studies on coal liquefaction catalyzed by molten salts have been undertaken by many coal chemists. Zielke and his coworkers [l-3] concentrated much attention on the development of a coal liquefaction process using massive amounts of molten salts with much stress on the use of ZnClz. Berg and Malsam [4] indicated the preferable characteristics of ZnClJKCl melts over ZnCl, for coal liquefaction. Bell and his coworkers [S-S] have published several papers concerning kinetic aspects of molten salt-catalyzed liquefaction of coal. We have published a wide range of papers [9-143 on this topic of coal liquefaction in the presence of molten salts media. The aim of this paper is to carry out coal liquefaction experiments in the presence of massive amounts of molten salts under a deuterium atmosphere and to obtain
* Corresponding author 0378-3820/95/$09,50 0
1995 Elsevier Science B.V. All rights reserved
SSDI 0378-3820(95)00027-5
214
M. Nomura et al. /Fuel Processing
Technology 43 (1995) 213-225
a detailed analyses of the degree and the distribution of deuterium uptake of the products so as to derive closer understanding of coal liquefaction chemistry and coal structure.
2. Experimental 2.1, Reagents Akabira bituminous coal (2.9 wt% ash) was pulverized to under 100 mesh and dried at 110°C for 12 h under reduced pressure. Elemental analysis of Akabira coal is as follows: C, 81.0%; H, 5.6%; N, 2.0%; S, 0.5%, 0, 11.0% (difference); H/C 0.83. Chemical reagents employed here were used without further purification. The liquefaction was carried out under 25 kg/cm2 of D2. 2.2. Preparation of coal deposited with SnC12/KCl Both SnC12 and KC1 (3 : 2 molar ratio) were solubilized in methanol, being kept at 50°C for 1 h with stirring. After evaporation of methanol from the resulting solution, the uniform mixture of SnCl,/KCl was dried at 160°C under 2 mmHg for 2 h. Deposition of SnC12/KCl over coal particles was achieved by adding coal particles to the methanol solution of SnC12/KCl, followed by complete removal of methanol by evaporation. The coal with SnCl,/KCl was dried at 100°C for 12 h under 2 mmHg. 2.3. Hydroliquefaction of coal The autoclave (50 ml) charged with 4 g of treated coal particles and deuterium (25 kg/cm2) was agitated by a rocking motion. The heating regime was as follows: At first the autoclave was heated up to 200°C at a rate of 12”C/min. Shaking was started with 43 strokes/min at 2Oo”C,as the autoclave was further heated to 400°C at a rate of 8”C/min, and held at 400°C for 1 h. After the reaction, the contents of the autoclave was cooled as quickly as possible by air flow. Gaseous products were recovered in the gas bag, an aliquot of which was injected into a gas chromatograph for quantitative analysis. The contents of the autoclave were washed out into a beaker with a small portion of hexane followed by a small amount of tetrahydrofuran. After the evaporation of these solvents, resulting solid contents were transferred to a Soxhlet thimble and subjected to hexane extraction. The hexane soluble fraction was termed HS. Subsequent extraction with benzene gave a hexane insoluble and benzene soluble fraction (HI/BS). The pyridine soluble fraction of the thimble residue was washed with diluted HCl, deionized wate r, and then 6N HNO, in order to remove the salts. 2.4. Further separation of HS and HIfBS fractions The HS and HI/BS fractions were separated into 7 fractions (HSl. -HS7, HI/BSl-HI/BS7), according to the improved USBM/API method [15].
M. Nomura et al. /Fuel Processing
2.5. Instrumental
Technology43 (1995) 213-225
215
analyses
In order to get information about molecular weight of HSl-HS3 fractions, their FD/MS spectra were measured by JEOL JMSDX303HF. Pyrolysis GC/MS measurements were carried out for HS, HI/BS and several fractionated fractions of HS using a Curie-point pyrolyzer, Japan Analytical Industry JHP-3 under following conditions (670°C 3 s). ‘HNMR, 2H NMR and 13CNMR measurements of HSl-HS3 were undertaken by Bruker AM600 (‘H and 13C) and JEOL JNMGSX400 (‘H) NMR spectrometers, using CDC13 and CHC13.
3. Results 3. I. Hydroliquefaction
of Akabira coal
Table 1 lists the hydroliquefaction of Akabira coal in the presence of molten salts under H2 or D2 atmospheres. Since D uptake in liquefied products is considered very informative in evaluating coal structure and the liquefaction mechanisms of coal organic materials, liquefaction under a H2 atmosphere was also carried out to provide supportive analytical data. The yields of gaseous products were estimated according to the results of liquefaction under H2 atmosphere. Table 1 indicates that conversion ranges from 89% to 98% were achieved. Under these reaction conditions, gaseous products were as follows; (under H2) methane 26%, ethane 16%, propane 19%, n-butane 9%, i-butane 9%, COZ 21%; (under D2) methane 22%, ethane 17%, propane 17%, n-butane 8%, i-butane 8%, CO2 28%. 3.2. Characterization
of HS and HIjBS
Elemental analyses of HS and HI/BS fractions are listed in Table 2. Due to the incorporation of D atoms into liquefied products, products under D2 atmosphere were not submitted for elemental analysis. Table 2 suggests that the H/C atomic ratio of HS is 1.21 while that of HI/BS is 0.95. These fractions were submitted to Curiepoint pyrolysis. Volatile manner of HS (H2) and HS (D2) were 64% (H2) and 63% (D2) respectively, tar matter and coke being 25% (H,) and 23% (D2), 11% (H,) and 14% (D2) respectively. Since volatile matter is the fraction injected into the gas chromatograph, about $ of the mass of these samples could be analyzed instrumentally.
Table 1 Results of hydroliquefaction of Akabira coal (wt% daf) Atmosphere
Gas
HS
HI/BS
BIjPS
Conversion
HZ I&
13 13
20 23
16 18
49 35
98 89
216
M. Nomura et al. /Fuel Processing
Table 2 Elemental
analysis
of HS and HI/BS fractions
Technology 43 (1995) 213-225
(wt%)
Fraction
C
H
N
S
O(diff.)
H/Ca
HS HI/BS
86.3 85.0
8.7 6.7
0.9 1.3
0.2 1.2
3.9 5.8
1.21 0.95
a Atomic
Table 3 Distribution
ratio
of seven fractions
Atmosphere
of HS and HI/BS
Fractions
(wt%)
1
2
3
4
5
6
7
HS
H2 DZ
34 32
17 19
34 35
4 2
2 1
7 9
HI/BS
HZ D2
2 2
8 8
7 8
20 17
9 11
29 29
_ _ 15 19
However, in the case of the HI/BS fraction, volatiles were only 35% (Hz, Dz), tar being 29% (H,) and 21% (DJ, and coke being 36% (H,) and 44% (Dz). This higher tendency of the HI/BS fraction to form coke during pyrolysis can be understood due to its high contents of heteroatoms like nitrogen, sulfur and oxygen, and its lower H/C value. The lower H/C value suggests the HI/BS fraction is more aromatic, and the degree of condensation of aromatic compounds should be higher than the HS fraction. Table 3 shows the distribution of seven fractions of HS. From this table, HSl to HS3 contains about 90% of the whole sample. The distribution of the HI/BS fraction also indicates that the HI/BS4 to HI/BS7 fractions account for almost 80% of the injected product.
4. Discussion 4. I. Structural
characteristics
4.1. I. Structural features
of Akabira coal
of HSI fraction
The FD/MS spectrum of HSl (H,) is shown in Fig. l(a) along with that of HSl (D2) (Fig. l(b)). Clear peaks appearing at regular interval of m/z 14 are considered to be alkanes of CnH2n+Z while these peaks disappear in the FD/MS of HSl (DJ. These findings suggest that deuterium atoms are incorporated into alkane derivatives. The HSl fraction is evaluated as a saturate fraction by referring to the definition of the
M. Nomura et al. /Fuel Processing
Technology 43 (1995) 233-225
217
R ;
2E-
0 4
Fig. 1. FD/MS spectra of HSl fraction from the liquefaction product under (a) H, and (b) D,.
improved USBM/API method. However, the H/C ratio of HSl (H,) was found to be 1.45, which suggests contamination with small amounts of aromatic compounds. GC/MS analysis of HSl (Hz) indicated the presence of alkylbenzene, alkylnaphthalene and tetralin derivatives. As for the separation of a saturate fraction from HS according to the improved USBM/API method, our separation was found to be insufficient. We will discuss this point later. Coal derived liquid is very complicated as suggested by the appearance of its FD/MS spectra so the ‘H NMR and ‘H NMR spectra of these two fractions (HSI (H,) and HSl (DJ) were measured. According to the Brown-Ladner assignments [16], the distribution of four protons; H,, (9.00-6.00 ppm), H, (5.00-2.00 ppm), H, (2.00-1.05 ppm), H, (1.05-0.20 ppm) were estimated: HSl (H,) H,, 16%; H, 27%; H, 43%; H, 14%: HSl (Dz) H,, 25%; H, 36%; H, 30%; H, 9%: These values are only
218
kf. Nomura et aLlFuel Processing Technology 43 (1995) 213-225
relative ratios because we could not quantify the exact amount of deuterium incorporated in the HSl fraction. However, the comparison of these values reveals the very interesting features of the resulting HSl fraction: in fact D atoms are incorporated into H,, and H, preferentially. These tendencies are very important to our interpretation of the mechanism of incorporation of deuterium atoms. Complete separation of saturate from HS was made by fractioning 50 ml of pentane elute in place of 250 ml of pentane. The measurement of 2H NMR of the resulting fraction indicates the disappearance of peaks around 9.00-2.00 ppm. Partial regions (15-13.5 ppm) of 13C NMR spectra of HSl (H,) and HSl (D2) are shown in Fig. 2. In Fig. 2(b), we can observe clear triplet due to the presence of -CH2D while this spectra cannot be observed in Fig. 2(a). Another partial region (23.5-22 ppm) of 13C NMR spectra of HSl (H,) and HSl (D2) are indicated in Fig. 3. The presence of -CHD- and -CD2- is strongly suggested by the appearance of peaks around 22.5 ppm in Fig. 3(b). From these considerations, deuterium atoms
a)
I
- CH,CH,CHQ$
-1
I 15
14.5
.I 14
I13.5
Chemical shift @pm) Fig. 2. 13C NMR spectra of HSl fraction from the liquefaction product under (a) H, and (b) D,.
M. Nomura et al. /Fuel Processing Technology 43 (1995) 213-225
219
I -CH,CH&H,CH,
-1
I 23.5
23
I22.5
22
Chemical shift (ppm)
Fig. 3. 13C NMR spectra
of HSl fraction
from the liquefaction
product
under (a) H, and (b) D,.
incorporated in alkane fraction exist at terminal methyl and methylene adjacent to terminal methyl group. Straight chain alkane is known to show the strongest peak at m/z 57 in its mass spectrum. As expected, the GC/MS of HS (H,) fraction shows the mass spectra of many alkane derivatives that have peaks at m/z 57 of C,Hi with a small isotope peak of m/z 58. However, each alkane of HSl (D2) show Ct fragments with m/z 57,58,59 and 60 (minor), this indicating each fragment contains from zero to three deuterium atoms. However, relative intensity of the peak with m/z 60 is negligible, so being omitted. Table 4 lists isotope distributions of alkanes in HSl (DJ. The degree of deuterium incorporation decreases as the number of carbon atoms in alkane increases. This suggests that the shorter the alkane chain, the greater the probability of alkane existing as an alkyl chain or an alkylene bridge of aromatics (refer to Section 4.2).
220
Table 4 Distribution
M. Nomura et al. /Fuel Processing Technology 43 (1995) 213-225
of isotope
in Cl
fragment
of alkanes
(mol ratio)
Alkanes
C,Hs+
&HsD+
GH7D:
C 12 C 13 C 14 C 15 C 16 C 17 C 18 C 19 C 20 C 1, C 22 C 23 C 24 C 25 C 26 C 27 C 28 C 29
65 68 61 70 58 70 73 72 77 72 81 75 79 78 79 78 80 80
22 20 22 20 27 20 19 20 18 21 15 19 17 17 16 17 16 16
13 12 17 10 1.5 10 8 8 5 7 4 6 4 5 5 5 4 4
4.1.2. Structural features of HS2 and HS3 fractions
The FD/MS spectra of the HS2 and HS3 fractions give number-average molecular weights of ca 461 and ca 509, respectively. The presence of distinct peaks appearing at a regular interval of m/z 14 on FD/MS spectra of HS2 (H,) suggests that various kinds of alkylated aromatic compounds are contained. These features are also observed in the FD/MS spectra of HS3 (H,). Fig. 4 shows the mass spectra of specific products obtained from Curie-point pyrolysis of HS3 (H,) and HS3 (Dz). The xylene spectra of HS3 (H,) and HS3 (Dz) are compared in Fig. 4(A). Similar comparison of dimethylphenol is shown in Fig. 4(B). In the case of xylene, its molecular ion from HS3 (Dz) is higher by 4 mass units than that from HS3 (H,), while the (M-CH3)+ fragment ion from HS3 (D2) is higher by 3 mass units than that from HS3 (Hz). Therefore one deuterium atom is believed to be incorporated in a methyl group of xylene from HS3 (D2). Also in the case of dimethylphenol, the molecular ion from HS3 (Dz) is higher by 3 mass units compared with that from HS3 (Hz). On the other hand, the (M-CH3)+ fragment ion is higher by 2 mass units than that from HS3 (H,). Therefore one deuterium atom is surely incorporated in each methyl group of dimethylphenol. The average proton distributions in HS2 (H,) and HS3 (Hz) and deuterium distributions in HS2 (Dz) and HS3 (Dz) were estimated according to their ‘H NMR And ‘H NMR spectra, respectively. These results are listed in Table 5. From Table 5, it is clear that deuterium atoms are preferentially incorporated into aromatic nuclei and aliphatic carbons adjacent to aromatic compounds. These findings were also observed
M. Nomura et al. /Fuel Processing Technology 43 (199s) 213-225
WY,)+
(W+ 1
lgi
21)
(B)
CM)+
CM-CHJ)’
-
221
e
b) *
A-
109
I 100
I 110
I 120
,. 130
m/z
Fig. 4. (A) Mass spectra of xylene from the Curie-point pyrolysis of HS3 fraction from the liquefaction product under (a) H, and (b) D,. (B) Mass spectra of dimethylphenol from the Curie-point pyrolysis of HS3 fraction from the liquefaction product under (a) H, and (b) D,.
Table 5 Average distribution of proton and deuterium in HS2 and HS3 fractions Atmosphere
H,,
HZ
H,
H;
HS2
H, D2
30 37
39 44
24 16
7 3
HS3
H, D2
23 32
34 41
30 22
13 5
in the aromatic compounds of HSl fraction. As for the preferential incorporation of deuterium atoms of cc-carbon sites of aromatic compounds, we have obtained additional information; Fig. 5 shows the region attributed to a-methyl groups in 13C NMR spectra of HS2 (Hz) and HS2 (DJ. Peaks observed at 21.3,19.9 and 19.5 ppm are indicative of a-methyl of aromatic compounds like the methyl of 1-methylnaphthalene (21.6), the methyl of toluene (21.4), the methyl of m-xylene (21.3) the methyl of o-xylene (19.7) and the methyl of 1-methylphenanthlene (19.9). However, in the 13C NMR spectra of HS2 (DJ, two broad peaks centered at 21.4 and 19.7 ppm are observed. Such a broadening of a-methyl peaks is considered due to the presence of coupling between 13C and D which is inserted into a-methyl groups.
222
M. Nomura et al. /Fuel Processing Technology 43 (1995) 213-225
b)
I
I
I
22
20
18
Chemical shift (ppm) Fig. 5. 13C NMR spectra of HS2 fraction from the liquefaction product under (a) H, and (b) D,.
4.2. Possible mechanism of deuterium atom uptake of aromatic and aliphatic compounds
We have published several papers on coal liquefaction in the presence of massive amounts of molten salts. Our results indicated molten salts act as a dispersant of
M. Nomura et al. /Fuel Processing
Technology 43 (1995) 213-225
223
intermediate species from coal organic materials and the catalytic material. We proposed that both ionic and radical mechanism contribute to liquefaction in Lewis acid type molten salt like SnC12/KCl and ZnCl,/KCl. Our finding in this study that deuterium atoms are incorporated into aromatic and aliphatic compounds can be explained in this context. Scheme 1 shows the possible ionic reactions for incorporation of D atoms; reaction (a) indicates that D+ attacks the ipso-position of aromatic compounds, resulting in the formation of deuterium substituted aromatics and deuterium substituted methyl group linking aromatic compounds. From our previous results [ 171, we have shown that the methylene bridges connecting two aromatic compounds is very common in coal organic materials. Reaction (b) also explains the way that deuterium substituted aromatic and deuterium substituted aliphatic compounds are formed. Scheme 2 indicates the radical mechanism of incorporation of deuterium atoms. In reaction (a) homolytic fission of 1,Zdiarylethane yields two benzyl radicals. These radicals abstract D atom from Dz dissolved in melts. In reaction (b) alkyl substituted aromatics yields benzyl type radical and alkyl radical. These two radicals abstract D atom in the way similar to reaction (a) in Scheme 2. The alkyl radical undergoes the disproportionation reaction to yield olefin and alkane. Olefin then takes up D2 to form two deuterium atoms containing alkane by addition reaction.
+--D-
Scheme 1. Ionic mechanism of coal liquefaction in the presence of molten salts.
224
M. Nomura et al. /Fuel Processing
Technology 43 (1995) 213-225
b) “,,
“G,,
6
1;. +..*i %,, ’
Scheme 2. Radical
“Q,,
c
,,.”
,.+”
.-
mechanism
-
of coal liquefaction
Dw
in the presence
of molten
salts.
5. Conclpsion
Liquefaction of Akabira bituminous coal was carried out at 400°C for 1 h using massive amounts of SnCl,-KC1 melt in the presence of H2 or Dz (upto 25 kg/cm2). These liquefaction produced a 90% of pyridine soluble material, both hexane soluble (HS) and hexane insoluble benzene soluble (HI/BS) portions of which were each separated chromatographically into 7 portions. Two HSl fractions (H, and D2: saturate) from above HS fraction according to the improved USBM/API method were submitted to measurements of ‘H NMR and ‘H NMR spectra. Their BrownLadner assignments revealed the very interesting features of two HSl fractions in that deuterium atoms are incorporated into H,, and H, preferentially. 13C NMR spectra of these HSl (H, or D2) also suggested the presence of -CH2D, -CHD- and -CD2-: deuterium atoms incorporated in alkane fraction exist at terminal and methylene adjacent to terminal methyl group. The mass spectra of specific products obtained from Curie-point pyrolysis of HS3 (H2) and HS3 (D2) indicated that at least one deuterium atoms is believed to be incorporated in a methyl group of xylene or each methyl group of dimethylphenol. Deuterium incorporation observed in the structural study on HS fraction can be explained in the context that D+ attacks the ipso-position of aromatic compounds, resulting in the formation of deuterium substituted aromatics and deuterium substituted methyl group linking aromatic compounds or in the radical mechanism of incorporation of deuterium atoms.
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Technology 43 (199.5) 213-225
225
Reference Cl1 C.W. Zielke, R.T. Struck, J.M. Evans, C.P. Costanza
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