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Hydrogen transfer from tetralin to coal macerals. Kinetic isotope effects
Fuel Processing Technology, 23 (1989) 39-45
39
Elsevier Science Publishers B.V., Amsterdam-- Printed in The Netherlands
H y d r o g e n Transfer from Tetralin to Coal Macerals. Kinetic Isotope Effects JANUSZ PAJAK Institute of Petroleum and Coal Chemistry, Polish Academy of Sciences, 44-100 Gliwice (Poland)
(Received November 22nd, 1988; accepted February 2nd, 1989)
ABSTRACT The proton/deuterium kinetic isotope effects (KIE) for the reaction of the whole coal and maceral groups with tetralin-~-d4 and tetralin-d12 have been measured.The results for macerals differconsiderablyindicating differencesin reaction mechanisms.These differencesseemto arise from the fact that different chemical functional groups are predominantly involvedin the ratedetermining step of the maceral/tetralin reaction.
INTRODUCTION In a previous paper [ 1 ] we have reported significant differences in reactivity in hydrogen transfer from tetralin to coal macerals. It has also been shown t h a t the coal/tetralin reaction may be considered as a set of parallel maceral/tetralin reactions and t h a t results obtained for a whole coal derive from the contribution of each maceral. To get more information on the mechanism of hydrogen transfer from tetralin to coal macerals, we have undertaken studies on the deuterium kinetic isotope effects ( K I E ) for maceral/tetralin reactions. EXPERIMENTAL Maceral samples were isolated from a Polish bituminous coal from Ziemowit mine as described previously [1 ]. Table 1 presents characteristic data of the whole coal and maceral samples. The whole coal sample was composed of 57 vol. % of vitrinite, 13% of exinite, 23% of inertinite and 7% of mineral matter. The procedures to obtain tetralin-~-d4 and tetralin-d12 were described previously [2 ]. The reaction mixtures were prepared by placing coal or maceral with tetralin (ratio 1 : 1, usually 50 mg: 50 ttl ) into glass tubes t h a t were subsequently sealed.
0378-3820/89/$03.50
© 1989 Elsevier Science Publishers B.V.
40 TABLE 1 Characteristic data of coal a n d maceral samples a Sample
Coal Vitrinite Exinite Inertinite
Constituent Ash
C
H
N
S
8.0 2.6 nd" nd
70.8 73.2 76.1 76.0
4.6 5.2 6.7 3.8
1.0 1.1 1.1 1.0
0.8 0.6 1.1 0.6
aAll parameters are in wt.% on a dry basis. ~nd = not determined.
One reaction mixture contained the proton donor and the other the deuterated derivatives. After being heated for a selected time at 310 ° C, the reaction mixtures were extracted with hexane and the extracts were analyzed by gas chromtography (AI 93 instrument with FID detector). RESULTS AND DISCUSSION
The amount of hydrogen transferred to coal material may be followed by measuring the ratio of naphthalene to tetralin in the reaction product. Figures 1-4 present the course of formation of naphthalene in the reaction of tetralin and its deuterated derivatives with coal, vitrinite, exinite and inertinite. For coal the most convenient way to obtain kinetic isotope effects is to express them as the ratio of rates for protonated and deuterated compounds. Since the relative times required for conversion to a given percentage of naphthalene are the reciprocals of the relative rates, they are equal to the kinetic isotope effects expressed a s rH/rD, where r denotes the rate. The values can be read from the plots and Table 2 shows the evaluated results from Figs. 1-4. The kinetic isotope effects for the reaction of tetralin with the whole coal and with vitrinite do not show significant differences. As shown in a previous paper, hydrogen transfer from tetralin to coal should be treated as the sum of parallel maceral/tetralin reactions [1]. This means that KIE for the coal/ tetralin reaction also depends on maceral composition. However, we also found that the rate of hydrogen transfer, which equals the rate of naphthalene formation, is by far the highest for vitrinite and the lowest for exinite. So, the contribution of the kinetic isotope effect of vitrinite to the apparent KIE of the whole coal will be greatest. When we take into account the maceral composition of our coal sample, the similarities between KIE for vitrinite and whole coal are not surprising. The results for vitrinite show substantial isotope effects for tetralin deuter-
41
I
o tetrolin tetralin-c~-d. 111 o tetrulin-dl z
12[
Z~
10~
9:
&
8t
} 1
2
3
4
time [hours] o tetraLin •', tetrolin-cx-d4 o tetrolin-dl 2 o ~ " "
J
10
g cn
6
Z 4
1
2
3
time [hours] Fig. l. Reaction of coal with tetralin. Fig. 2. Reaction of vitrinite with tetralin. ated at a-position and even a much greater effect for the perdeuterated compounds. Such kinetic isotope effects conform well to the pattern predicted for a simultaneous transfer of two hydrogens, since for such a transfer we should expect tetralin, tetralin-a-d4 and tetralin-d12 to have isotope effects in a proportion of I : X : X 2. Similar results were previously obtained for the reaction of a North American subbituminous coal with tetralin, where the values of the
42
12f 11
o tetrotin ~, tetratin- oc- d,, o tetrolin- d12
lO
oJ c3 ¢-
F
t-i E~ Z
1
2
3
4
time [hours] o tetrolin ', t e t r o l i n - ~ - d ~ o tetrolin-d~ z
12 11 10 .4--"
9 8
~
7
r°Z ~
5
z
4
cJ
6
1
2
time
3
[hours]
F i g . 3.
Reaction of exinite with tetralin.
F i g . 4.
Reaction of inertinite with tetralin.
KIE were 1:2.0:3.7 [2]. Vitrinite is the main component of North American and European coals and thus the results of both studies support the view that concerted transfer of a pair of hydrogens takes place in the rate-determining step of the vitrinite/tetralin reaction. Model compound studies showed a similar pattern of kinetic isotope effects for the reaction of tetralin with quinones
[3].
43 TABLE 2 Deuterium kinetic isotope effects for hydrogenation of maceral groups with tetralin Sample
Tetralin
Tetralin-a-d4
Tetralin-dx2
coal vitrinite exinite inertinite
1 1 1 1
2.2 2.0 2.5 1.7
3.3 3.4 2.5 1.8
Substantial H / D kinetic isotope effects for the reaction of tetralin with whole coals were also reported by Franz and Camaioni [4], Cronauer et al. [5] and Skowronski et al. [6], who used either tetralin-a-d4 or tetralin-d12. There is no record of previous investigation of kinetic isotope effects for maceral groups. The results presented in Table 2 show a completely different pattern of KIE for exinite and inertinite from that for vitrinite. At first these macerals seem to react by a mechanism in which a-hydrogens are in transit in the rate-controlling step. However, the KIE for exinite/tetralin reaction is significantly higher than for inertinite and exinite, and differs so much from other maceral groups [7] as to justify a hypothesis of different reaction mechanism. We also found that only in exinite/decalin reaction large amounts of tetralin are formed, whereas in case of vitrinite and inertinite naphthalene strongly predominates in reaction products. In the recent paper it is stated that exinites contain large quantities of long polymethylene chains, which probably bridge aromatic units [7]. Such structures are known to undergo free radical chain decomposition and 1,3-diphenylpropane and 1,4-diphenylbutane are model compound examples [8,9]. This mechanism, applying to compounds having a long (at least a threeatom) bridge connecting aryl groups, is shown in Scheme I. Scheme I R" + ArCH2 CHz CH2 Ar-~ RH + ARCH2CH2 CHAr
(1)
ARCH2 CH2 (~HAr-~ CHz = CH-Ar + Ar(~H2
(2)
Ar(~H2 + ArCHz CH2 CH2 Ar-~ ARCH2CH2 (~HAr + ARCH3
(3)
Step ( 1 ) serves only as the initiating event, while steps (2) and (3) form chain propagation. Substitution of an oxygen atom for a methylene group leads to even more rapid chain reaction, since dibenzyl ether and fl-phenethyl phenyl ether experience radical chain reaction at much lower temperature than 1,3diphenylpropane [ 10 ].
44
It was postulated that induced radical chain reaction may be involved in coal liquefaction, but it is uncertain how abundant such structures would be in coal [8]. We believe that the results presented here, namely the magnitude of KIE, selectivity of tetralin at c~-position shown by KIE pattern and similar reaction rate for tetralin and decalin seem to provide the evidence for the involvement of a radical chain mechanism in exinite/tetralin reaction. Kinetic isotope effect in hydrogen transfer from tetralin to exinite may arise from bimolecular reaction of free radicals with tetralin, for example ArCH~ +CloH12-~ArCH3 + C10H~I
(4)
Very strong selectivity of the c~-position in abstraction of hydrogen from tetralin by radicals is well known [11,12]. It has also been found that tetralin has very small effect on the rates of decomposition of 1,3-diphenylpropane and 1,4-diphenylbutane [8 ]. In contrast to exinites, inertinites contain few methylene groups, even less than vitrinites, and do not contain long bridge structures [7]. The highest aromaticity factor (fa) and the lowest oxygen content are characteristic of this maceral group. It seems that kinetic isotope effects for inertinite/tetralin reaction may be explained by bimolecular transfer of hydrogen to polycondensed aromatic rings, in which stepwise loss of hydrogen from tetralin starting at the c~-position takes place. We are yet unable to decide whether the reaction intermediate is a radical or an ion, since the KIE pattern is the same in both cases. In summary, deuterium kinetic isotope effects reveal that hydrogen transfer from tetralin to coal maceral groups differs significantly. These differences seem to be the result of the different contents of various reactive structures present (quinonoid functions, polymethylene bridges and polycondensed rings) that react with tetralin via different routes. ACKNOWLEDGEMENTS
The author wishes to thank S. Pusz and J. Komorek for separation of macerals and B. Setkowska for technical assistance. This work was supported by the Polish Academy of Sciences under Contract No. CPBP 01.16.
REFERENCES 1 Pajak, J., 1989. Fuel Processing Technol., 21: 245-252. 2 Brower, K.R. and Pajak, J., 1984. J. Org. Chem., 49: 3970. 3 Pajak, J. and Brower, K.R., 1985. J. Org. Chem., 50: 2210. 4 Franz, J.A. and Camaioni, C.M., 1984. Fuel, 63: 990. 5 Cronauer, D.C., McNeil, R.I., Young, D.C. and Ruberto, R.G., 1982. Fuel, 61: 610. 6 Skowronski, R.P., Ratto, J.J., Goldberg, I.B. and Heredy, L.A., 1984. Fuel, 63: 440.
45 7 8 9 10 11 12
Choi, C., Wang, S.H. and Stock, L.M., 1988. Energy & Fuels, 2: 37. Poustma, M.L. and Dyer, C.W., 1982. J. Org. Chem., 47: 4903. Gilbert, K.E. and Gajewski, J.J., 1982. J. Org. Chem., 47: 4889. Gilbert, K.E., 1984. J. Org. Chem., 49: 6. Conradi, M.S. and Zeldes, H., 1979. J. Phys. Chem., 83: 633. King, H. and Stock, L.M., 1982. Fuel, 61: 257.
39
Elsevier Science Publishers B.V., Amsterdam-- Printed in The Netherlands
H y d r o g e n Transfer from Tetralin to Coal Macerals. Kinetic Isotope Effects JANUSZ PAJAK Institute of Petroleum and Coal Chemistry, Polish Academy of Sciences, 44-100 Gliwice (Poland)
(Received November 22nd, 1988; accepted February 2nd, 1989)
ABSTRACT The proton/deuterium kinetic isotope effects (KIE) for the reaction of the whole coal and maceral groups with tetralin-~-d4 and tetralin-d12 have been measured.The results for macerals differconsiderablyindicating differencesin reaction mechanisms.These differencesseemto arise from the fact that different chemical functional groups are predominantly involvedin the ratedetermining step of the maceral/tetralin reaction.
INTRODUCTION In a previous paper [ 1 ] we have reported significant differences in reactivity in hydrogen transfer from tetralin to coal macerals. It has also been shown t h a t the coal/tetralin reaction may be considered as a set of parallel maceral/tetralin reactions and t h a t results obtained for a whole coal derive from the contribution of each maceral. To get more information on the mechanism of hydrogen transfer from tetralin to coal macerals, we have undertaken studies on the deuterium kinetic isotope effects ( K I E ) for maceral/tetralin reactions. EXPERIMENTAL Maceral samples were isolated from a Polish bituminous coal from Ziemowit mine as described previously [1 ]. Table 1 presents characteristic data of the whole coal and maceral samples. The whole coal sample was composed of 57 vol. % of vitrinite, 13% of exinite, 23% of inertinite and 7% of mineral matter. The procedures to obtain tetralin-~-d4 and tetralin-d12 were described previously [2 ]. The reaction mixtures were prepared by placing coal or maceral with tetralin (ratio 1 : 1, usually 50 mg: 50 ttl ) into glass tubes t h a t were subsequently sealed.
0378-3820/89/$03.50
© 1989 Elsevier Science Publishers B.V.
40 TABLE 1 Characteristic data of coal a n d maceral samples a Sample
Coal Vitrinite Exinite Inertinite
Constituent Ash
C
H
N
S
8.0 2.6 nd" nd
70.8 73.2 76.1 76.0
4.6 5.2 6.7 3.8
1.0 1.1 1.1 1.0
0.8 0.6 1.1 0.6
aAll parameters are in wt.% on a dry basis. ~nd = not determined.
One reaction mixture contained the proton donor and the other the deuterated derivatives. After being heated for a selected time at 310 ° C, the reaction mixtures were extracted with hexane and the extracts were analyzed by gas chromtography (AI 93 instrument with FID detector). RESULTS AND DISCUSSION
The amount of hydrogen transferred to coal material may be followed by measuring the ratio of naphthalene to tetralin in the reaction product. Figures 1-4 present the course of formation of naphthalene in the reaction of tetralin and its deuterated derivatives with coal, vitrinite, exinite and inertinite. For coal the most convenient way to obtain kinetic isotope effects is to express them as the ratio of rates for protonated and deuterated compounds. Since the relative times required for conversion to a given percentage of naphthalene are the reciprocals of the relative rates, they are equal to the kinetic isotope effects expressed a s rH/rD, where r denotes the rate. The values can be read from the plots and Table 2 shows the evaluated results from Figs. 1-4. The kinetic isotope effects for the reaction of tetralin with the whole coal and with vitrinite do not show significant differences. As shown in a previous paper, hydrogen transfer from tetralin to coal should be treated as the sum of parallel maceral/tetralin reactions [1]. This means that KIE for the coal/ tetralin reaction also depends on maceral composition. However, we also found that the rate of hydrogen transfer, which equals the rate of naphthalene formation, is by far the highest for vitrinite and the lowest for exinite. So, the contribution of the kinetic isotope effect of vitrinite to the apparent KIE of the whole coal will be greatest. When we take into account the maceral composition of our coal sample, the similarities between KIE for vitrinite and whole coal are not surprising. The results for vitrinite show substantial isotope effects for tetralin deuter-
41
I
o tetrolin tetralin-c~-d. 111 o tetrulin-dl z
12[
Z~
10~
9:
&
8t
} 1
2
3
4
time [hours] o tetraLin •', tetrolin-cx-d4 o tetrolin-dl 2 o ~ " "
J
10
g cn
6
Z 4
1
2
3
time [hours] Fig. l. Reaction of coal with tetralin. Fig. 2. Reaction of vitrinite with tetralin. ated at a-position and even a much greater effect for the perdeuterated compounds. Such kinetic isotope effects conform well to the pattern predicted for a simultaneous transfer of two hydrogens, since for such a transfer we should expect tetralin, tetralin-a-d4 and tetralin-d12 to have isotope effects in a proportion of I : X : X 2. Similar results were previously obtained for the reaction of a North American subbituminous coal with tetralin, where the values of the
42
12f 11
o tetrotin ~, tetratin- oc- d,, o tetrolin- d12
lO
oJ c3 ¢-
F
t-i E~ Z
1
2
3
4
time [hours] o tetrolin ', t e t r o l i n - ~ - d ~ o tetrolin-d~ z
12 11 10 .4--"
9 8
~
7
r°Z ~
5
z
4
cJ
6
1
2
time
3
[hours]
F i g . 3.
Reaction of exinite with tetralin.
F i g . 4.
Reaction of inertinite with tetralin.
KIE were 1:2.0:3.7 [2]. Vitrinite is the main component of North American and European coals and thus the results of both studies support the view that concerted transfer of a pair of hydrogens takes place in the rate-determining step of the vitrinite/tetralin reaction. Model compound studies showed a similar pattern of kinetic isotope effects for the reaction of tetralin with quinones
[3].
43 TABLE 2 Deuterium kinetic isotope effects for hydrogenation of maceral groups with tetralin Sample
Tetralin
Tetralin-a-d4
Tetralin-dx2
coal vitrinite exinite inertinite
1 1 1 1
2.2 2.0 2.5 1.7
3.3 3.4 2.5 1.8
Substantial H / D kinetic isotope effects for the reaction of tetralin with whole coals were also reported by Franz and Camaioni [4], Cronauer et al. [5] and Skowronski et al. [6], who used either tetralin-a-d4 or tetralin-d12. There is no record of previous investigation of kinetic isotope effects for maceral groups. The results presented in Table 2 show a completely different pattern of KIE for exinite and inertinite from that for vitrinite. At first these macerals seem to react by a mechanism in which a-hydrogens are in transit in the rate-controlling step. However, the KIE for exinite/tetralin reaction is significantly higher than for inertinite and exinite, and differs so much from other maceral groups [7] as to justify a hypothesis of different reaction mechanism. We also found that only in exinite/decalin reaction large amounts of tetralin are formed, whereas in case of vitrinite and inertinite naphthalene strongly predominates in reaction products. In the recent paper it is stated that exinites contain large quantities of long polymethylene chains, which probably bridge aromatic units [7]. Such structures are known to undergo free radical chain decomposition and 1,3-diphenylpropane and 1,4-diphenylbutane are model compound examples [8,9]. This mechanism, applying to compounds having a long (at least a threeatom) bridge connecting aryl groups, is shown in Scheme I. Scheme I R" + ArCH2 CHz CH2 Ar-~ RH + ARCH2CH2 CHAr
(1)
ARCH2 CH2 (~HAr-~ CHz = CH-Ar + Ar(~H2
(2)
Ar(~H2 + ArCHz CH2 CH2 Ar-~ ARCH2CH2 (~HAr + ARCH3
(3)
Step ( 1 ) serves only as the initiating event, while steps (2) and (3) form chain propagation. Substitution of an oxygen atom for a methylene group leads to even more rapid chain reaction, since dibenzyl ether and fl-phenethyl phenyl ether experience radical chain reaction at much lower temperature than 1,3diphenylpropane [ 10 ].
44
It was postulated that induced radical chain reaction may be involved in coal liquefaction, but it is uncertain how abundant such structures would be in coal [8]. We believe that the results presented here, namely the magnitude of KIE, selectivity of tetralin at c~-position shown by KIE pattern and similar reaction rate for tetralin and decalin seem to provide the evidence for the involvement of a radical chain mechanism in exinite/tetralin reaction. Kinetic isotope effect in hydrogen transfer from tetralin to exinite may arise from bimolecular reaction of free radicals with tetralin, for example ArCH~ +CloH12-~ArCH3 + C10H~I
(4)
Very strong selectivity of the c~-position in abstraction of hydrogen from tetralin by radicals is well known [11,12]. It has also been found that tetralin has very small effect on the rates of decomposition of 1,3-diphenylpropane and 1,4-diphenylbutane [8 ]. In contrast to exinites, inertinites contain few methylene groups, even less than vitrinites, and do not contain long bridge structures [7]. The highest aromaticity factor (fa) and the lowest oxygen content are characteristic of this maceral group. It seems that kinetic isotope effects for inertinite/tetralin reaction may be explained by bimolecular transfer of hydrogen to polycondensed aromatic rings, in which stepwise loss of hydrogen from tetralin starting at the c~-position takes place. We are yet unable to decide whether the reaction intermediate is a radical or an ion, since the KIE pattern is the same in both cases. In summary, deuterium kinetic isotope effects reveal that hydrogen transfer from tetralin to coal maceral groups differs significantly. These differences seem to be the result of the different contents of various reactive structures present (quinonoid functions, polymethylene bridges and polycondensed rings) that react with tetralin via different routes. ACKNOWLEDGEMENTS
The author wishes to thank S. Pusz and J. Komorek for separation of macerals and B. Setkowska for technical assistance. This work was supported by the Polish Academy of Sciences under Contract No. CPBP 01.16.
REFERENCES 1 Pajak, J., 1989. Fuel Processing Technol., 21: 245-252. 2 Brower, K.R. and Pajak, J., 1984. J. Org. Chem., 49: 3970. 3 Pajak, J. and Brower, K.R., 1985. J. Org. Chem., 50: 2210. 4 Franz, J.A. and Camaioni, C.M., 1984. Fuel, 63: 990. 5 Cronauer, D.C., McNeil, R.I., Young, D.C. and Ruberto, R.G., 1982. Fuel, 61: 610. 6 Skowronski, R.P., Ratto, J.J., Goldberg, I.B. and Heredy, L.A., 1984. Fuel, 63: 440.
45 7 8 9 10 11 12
Choi, C., Wang, S.H. and Stock, L.M., 1988. Energy & Fuels, 2: 37. Poustma, M.L. and Dyer, C.W., 1982. J. Org. Chem., 47: 4903. Gilbert, K.E. and Gajewski, J.J., 1982. J. Org. Chem., 47: 4889. Gilbert, K.E., 1984. J. Org. Chem., 49: 6. Conradi, M.S. and Zeldes, H., 1979. J. Phys. Chem., 83: 633. King, H. and Stock, L.M., 1982. Fuel, 61: 257.