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Relative reactivity of hydrogen donor solvent in coal liquefaction
Relative reactivity in coal liquefaction Yoshio
Kamiya
and Shinichi
Department of Reaction Chemistry, Bunkyo-ku, Tokyo 173, Japan (Received 6 July 7984)
of hydrogen
donor
solvent
Nagae Faculty
of Engineering,
University
of Tokyo, Hongo,
Hydrogen transfer from donor solvent to coal must involve reactions such as hydrogen donation to free radicals and hydrogenation of aromatic structures. The relative reactivities of five typical hydrogen donor solvents, more reactive than tetralin, were determined using a competing elimination reaction in the liquefaction of a bituminous coal at 400°C and a brown coal at 350°C. 9,lO_Dihydroanthracene, 9,10dihydrophenanthrene and 1,2,3,4-tetrahydroquinoline exhibited outstanding hydrogen donating ability. Further, the relative reactivities of five mild hydrogen donor solvents such as acenaphthene and indan were determined by a similar elimination reaction using a bituminous coal at 450°C. (Keywords: coal; liquefaction;
hydrogen donor solvent)
Although it is established that a hydrogen donor solventlm3 such as tetralin is very effective for coal liquefaction, its important role being to promote liquefaction by donating hydrogen atoms to coal fragments derived by thermal cracking and also by retarding char formation, the effects of reduction by hydrogen donor on coal structure have not been studied thoroughly. The effect of hydrogen donor can be twofold4. First, stabilization of free radicals can occur by donation of hydrogen atoms,,for example dibenzyl is converted to toluene, and benzyl phenyl ether to toluene and phenol. Second, there are reduction and hydrogenation effects. For example, benzophenone and diphenyl methanol are reduced to diphenyl methane, naphthoquinone to naphthalene, and phenanthrene to hydrophenanthrene. So far, the second group reaction has been thought to make a small contribution to the cleavage reaction. However, it was found that hydrogenation of polycyclic aromatic rings results in a very fast C-0 bond breaking reaction for diary1 ethe?. The relative reactivities of hydrogen donors in coal liquefaction have been studied by a number of workers. Previously, the overall effect has been compared of several solvents on five types of coa16. It was concluded that hydrogen donors were most effective for all ranks of coal and three-ring hydroaromatics were much more effective than tetralin. The competing elimination of hydrogen donors in coal liquefaction as reported by Whitehurst and his coworkers’ will give more quantitative results on relative reactivities. This paper reports the relative reactivities of 12 aromatic solvents in coal liquefaction using a competing elimination reaction with brown and bituminous coal. The measured pseudo-first-order rate constants graphically obtained after 15 min reaction represent the overall reactivity of each solvent including hydrogen donation toward free radicals and reduction of aromatic and oxygencontaining structures of coal. 001~V2361/85/081242-04$3.00 0 1985 Butterworth & Co. (Publishers)
1242
FUEL, 1985, Vol64,
Ltd
September
EXPERIMENTAL Coal samples Analytical data for the coals used are shown in Table 1. All coals ( < 246 pm) were dried at 105°C for 24 h under reduced pressure. Coal liquefaction
Tetralin and 1-methylnaphthalene were reagent grade and used after washing and subsequent distillation at 70°C under reduced pressure. Various solvents were reagent grade and used as received. A mixture of tetralin (10 ml), 1-methylnaphthalene (10 ml) and testing solvent (10 ml or 10 g) and coal (3.0 g) were added to 90 ml magnetically stirred (500 rev min-‘) autoclave. After pressurizing with H, (1 MPa) the autoclave was heated to the reaction temperature within 30min and maintained at that temperature during the reaction time. At the completion of a run, the autoclave was cooled by electric fan to room temperature. The liquid products were removed through a glass filter, and the residual solids were extracted with tetrahydrofuran and weighed after drying. The amounts of daf coal liquefied were calculated as a percentage of daf feed coal by subtracting the percentage undissolved. Liquid portions of the samples were subjected to gas chromatographic analysis to determine the composition of solvent.
Table
1
Elemental
analyses
of coal samples -_--__
Analysis __~~~_._~~___~__~
Yallourn ~__
Akabira
C (wt”,,, daf) H (wt”,, daf) N (wt”,, daf) S (WV’,, daf) 0 (wt”;,. diff.) Ash (wt%. dry coal)
65.8 5.1 0.5 0.2 28.4 2.5
82.7 5.9 1.9 0.4 9.1 5.7
Reactivity
of H-donor
Relative reactivity of hydrogen donor solvent was determined by competing elimination reaction with tetralin. In coal liquefaction, hydrogen donor will be mdehydrogenated by hydrogen donation to coal fragment free radicals and by reduction of coal structure. In the following equation, A represents a testing donor solvent and T represents tetralin, i.e. standard reactive material. X’ and Y represent a free radical from coal and a reducible coal structure, respectively. X’+A~XH+A(-H)’
(1)
X’+T%XH+T(-H)’
(2)
Y+A%
Y’+A’
(3)
Y+T%
Y’+T
(4)
If the cross hydrogen abstraction reaction T( - H)’ and A proceeds very slowly as compared reaction between T(- H)’ and coal fragments, obtain Equations (5) and (6). -d(A)/dt=(k;X’+k;;
Y)(A)=k,(A)
-d(T)/dt=(k;X’+k;Y)(T)=k,(T) Relative reactivity of A against T as a function time can be expressed as Equation (7). [ - d(A )l(A)drll[
between with the we can (5) (6)
of reaction
- d(T)l(T)drl =(kax’+k~Y)/(k;X’+k;Y)=(k,/k,),
(7) Therefore, by measuring the conversion rate of solvent after appropriate reaction time, [ - d(A)/(A)dt],, we can obtain the ratio of apparent pseudo-first-order rate constant, i.e. the relative reactivity of hydrogen donor against tetralin. The apparent reaction rate was not divided by the number of reactive hydrogen atoms per molecule of solvent because the number of most reactive hydrogen atoms is not clear for hydrogen donors. Some hydrogen donors such as dihydroanthracene were not so stable at 400°C and were converted mainly to anthracene by dehydrogenating reaction. Therefore, concentration of hydrogen donor A after the reaction was corrected by multiplying A&A,A& where A, and AB represent the initial and the thermally treated concentrations of A component. Equation (7) can be corrected as follows.
solvent
in coal liquefaction:
Y. Kamiya
and S. Nagae
coals were thermally treated in a mixed solvent composed of 1-methylnaphthalene, tetralin and a strong hydrogen donor (component A), component A was dominantly dehydrogenated. Typical examples of solvent conversion curve in Figure I show that dihydroanthracene is about 10 times as reactive as tetralin in hydrogen donation to Yallourn coal. The rate of conversion of solvent was determined by the slope of a tangential line at the reaction time of 1.5 min. The reaction during the heating up and cooling down periods corresponded to about 5 min at the reaction temperature. The rate of conversion of l-methylnaphthalene was negligible as compared with that of dihydroanthracene. The effect of temperature on conversion of dihydrophenanthrene in the liquefaction of Yallourn coal was remarkable and the conversion of Yallourn coal to THFsoluble and lighter fractions after 30 min was increased from 80 to 9q< with increasing temperature from 350 to 400°C. At 4OO”C, lOmmo1 of dihydrophenanthrene per 1.0 g of Yallourn coal was consumed. In the case of Akabira coal, the conversion of dihydrophenanthrene was much lower than that of Yallourn coal, indicating that a bituminous coal is less reactive at 350°C and also dehydrogenates less hydrogen donor than a brown coal. The conversion of dihydrophenanthrene increased remarkably with increasing the reaction temperature. Accordingly, the conversion of Akabira coal after 30 min reaction was 6q< at 350°C and 98% at 4oo”C, respectively. At 400°C after 30 min, 7.5 mmol of dihydrophenanthrene was consumed per 1.0 g of Akabira coal. In Figure 2, the conversion of active donor solvent in a mixed solution with tetralin and 1-methylnaphthalene was plotted and compared. Homologous conversion curves were obtained for the live strong hydrogen donating solvents. The relative value of solvent conversion corresponding to the first order rate constant can be written as the following order: DHA>DHP> THQ > THN > OHA. It should be mentioned that a small amount of tetralin, described below, was also consumed in the course of reaction. In order to investigate the behaviour of hydrogen donor in coal liquefaction, the conversion of coal was
[-d(A)l(A)dt][A,/(Ao-A,)] [-d(T)/(T)drl[T,l(To (&x’+k; = (k;X’+k;i RESULTS
AND
- TB)]
Y) Y),
(8)
DISCUSSION
Relative reactivities of five hydrogen donors, i.e. 9,10dihydroanthracene (DHA), 9,10-dihydrophenanthrene (DHP), 1,2,3,4-tetrahydroquinoline (THQ), 1,2,3,4tetrahydro-6-naphthol (THN) and 1f2 93 74 35 96 97 38-octahydroanthracene (OHA) were determined by the competing reaction with tetralin in the liquefaction of Yallourn and Akabira coals. When Yallourn or Akabira
I
I
I
0
10
20
Time
I
30
(mln)
of solvents in the liquefaction of Yallourn coal Figure 1 Conversion with mixed solvent of 1-methylnaphthalene, tetralin and DHA at 350°C. 0, Tetralin; 0, dihydroanthracene
FUEL, 1985, Vol 64, September
1243
Reactivity
of H-donor
solvent
in coal liquefaction:
Y. Kamiya
plotted against the amount of hydrogen donor consumed, were observed for all solvent systems,‘suggesting that the number of hydrogen atoms transferred to coal from one mole of solvent is roughly independent of solvent. Increased efficiency of hydrogen donor, corresponding to higher conversion of coal, was observed where tetralin was the most effective hydrogen donor in the solution at the temperature of 450°C. The difference between Yallourn and Akabira coals clearly indicates that a brown coal needs much more hydrogen donor to attain the same level of conversion than a bituminous coal. The conversion of solvent after 30 min of reaction and the rate of solvent consumption (-d(A)/(A)dt),,,,, are shown in Table 2, where Yallourn coal was treated by a mixed solvent at 350°C. Relative reactivity of hydrogen donor toward coal can be evaluated from the value of kA/kT. All five solvents are very active compared with tetralin. According to Whitehurst6, dihydrophenanthrene is 40 times more active than tetralin. The order of hydrogen donating activity of A component was as follows: DHA > DHP > THQ > THN > OHA. Dihydroant hracene, dihydrophenanthrene and tetrahydroquinoline are thought to be very active hydrogen donors. As can be seen in Table 2, the conversion of coal increased with increasing the activity of hydrogen donor and increased almost proportionally to the amount of hydrogen donor consumed (mmol). It has previously been shown that strong hydrogen donors such as DHA and THQ could give much higher conversions of stilbene or benzophenone to diphenylmethane than tetralin’. Therefore, a stronger hydrogen donor is able to transfer more hydrogen atoms to coal structures than tetralin and results in a higher conversion of coal after 30 min of reaction. The conversion of solvent and the rate of solvent consumption (- d(A)/(A)dt), 5minin the liquefaction of Akabira coal are shown in Table 3. In this case, the reactivity of hydrogen donor solvent can be estimated from the values of k, and kA/kT. The relative reactivity of
and S. Nagae
Figure 3. Fairly good correlations
Table 2 Reactivity of strong and component A at 350°C __~___~_____
Component
A
Dihydroanthracene Dihydrophenanthrene Tetrahydroquinoline Tetrahydronaphthol Octahydroanthracene
Table 3 The liquefaction ___-____--~-~--~-~.
Component
A
Dihydroanthracene Dihydrophenanthrene Tetrahydroquinoline Tetrahydronaphthol Octahydroanthracene Tetralin” -~
hydrogen
donors
(A) in the liquefaction
10 Time
I 10
Hydrogen
of Yallourn
(mln)
I I 15 20 donor consumed
coal with mixed solvent composed
kA (component
15 72 89 70 61
39.4 38.9 32.0 24.3 21.5
I.6 1.5 1.4 1.0 8.3
(21.9) (21.6) (25.5) (16.4) (11.3)
coal with mixed solvent composed
I
30
I 25 ( mmsol )
I 30
I
Figure 3 Coal conversion vs hydrogen donor consumption. 0, Yallourn (350°C); 0, Yallourn (450”C);n, Akabira (400°C); A, Akabira (450°C)
Conversion of A after 30 min :‘, (mmol)
of Akabira
I
20
Figure 2 Conversion of strong hydrogen donors in the liquefaction of Yallourn coal with mixed solvent composed of I-methylnaphthalene, tetralin and a strong hydrogen donor A at 350°C. x, Octahydroanthracene; 0, tetrahydronaphthol; A, tetrahydroquinoline; A, dihydrophenanthrene; 0, dihydroanthracene
Conversion of coal after 30 min :< (THF-soluble)
of I-methylnaphthalene,
k,=(-d(X)l(X)dr),,,i, A)
x 10-l X lo-2 x lo-* x 1om2 x 10V3
of I-methylnaphthalene,
tetralin
tetralin
(s-l) kT (tetrahn)
kA IkT
1.1 X lo-3 1.2x 10-j 1.2x 10-a 1.6x 1O-3 1.5 x 1o-3
14.5 12.5 11.7 6.3 5.5
and component
A at 400°C ___-
Conversion of coal after 30 min :< (THF-soluble)
Conversion of A after 30 min O,, (mmol)
98 96 93 93 83 19
38.4 36.6 22.5 25.7 20.8 5.9
’ Net tetralin 30 ml
1244
I
I
0
FUEL, 1985, Vol 64, September
(21.3) (20.3) (18.0) )17.3) (1 I .2) (10.4)
k,=(-d(X)/(X)dt),,,,, kA (component 1.39 1.32 1.0 1.1 5.0
x x x x x
lo-* lo-* 10-r Io-z 10-S
A)
(s-l) kr (tetralin)
kAtkr
I.1 x 1o-3 I.1 x 1o-3 1.0x 10-a
12.6 12 10
0.8 x 1O-3
6
_~__
______
Reactivity of H-donor solvent is similar to the order seen with Yallourn coal. For Akabira coal, the conversion of coal in neat tetralin solvent was also noted. For neat tetralin, the conversion of coal as well as the consumption of solvent were lower than those of other solvents, indicating that a stronger hydrogen donor reduces coal structure to higher extent and results in the higher coal conversion. The high reactivity of DHA and DHP in Tables 2 and 3 is attributable to the active benzylic hydrogen sandwiched by two benzene rings and out of aromatic plane. The bond dissociation energy of the benzylic C-H bond of DHA’ was estimated to be 298 kJ mol-’ (71 kcal mol-‘), which is 46 kJ mol- ’ (11 kcal mol - ‘) lower than that of tetralin. The high reactivity of THQ can be ascribed to the active secondary amine structure adjacent to benzene ring. For THN the measured k,/k, value on Yallourn coal was z 5 as shown in Table 2. This value can be compared with the value of 3 reported by Whitehurst and coworkers*. Much higher reactivity of THN than tetralin may be partly due to the favourable reaction of the hydroxyl group toward oxy radicals resulting from the cleavage of the ether structure of coal. When a mixture of tetralin, I-methylnaphthalene and relatively weak hydrogen-donating solvent (called B component) was used as the solvent in coal liquefaction, tetralin was mainly dehydrogenated. As shown in Table 4, kB/kT values changed from 0.1 to 0.4 according to solvents including 1-met hylnaphthalene. Some aromatics with benzylic hydrogen such as fluorene, acenaphthene and methylnaphthalene were less reactive compared with tetralin. Although hydroaromatic compounds9,‘0 tend to undergo various conversion reactions at 450°C these effects do not significantly influence the relative reactivities in Table 4. Whitehurst and coworkers” reported that pyrene could be hydrogenated to an active solvent such as dihydropyrene in the presence of catalyst and high
solvent
Wh coal
(k&r ),vr,rre
Yallourn
Acenaphthene
0.37 0.34
0.34 0.30
0.36 0.32
0.32 0.3 1 0.28 0.10
0.28 0.14 0.20 0.18
0.30 0.23 0.24 0.14
r;e;;y,_ naphthalene Byrene Indan Fluorene
coal
Akabira
Solvent &_---_--_--_~____
Y. Kamiya
and S. Nagae
pressure of hydrogen gas. In this experiment, a small increase in coal conversion and tetralin conversion in Table 4 might be ascribed to the formation of small amount of dihydropyrene formed by the hydrogen donation from tetralin. Recently, the relative reactivity of many hydrogen donors toward the benzyl radical was measured by Bockrath and Bittner” at 170°C and toward ally1 benzyl radical by Franz and his coworkers’ 3 at 160°C. They have shown a much larger difference in the reactivity of hydrogen donors than seen in Tables 2 and 3. The hydrogen transfer reaction of hydrogen donors to coal and its decomposition products involves the hydrogen abstraction by coal fragments, the hydrogenation of aromatic nucleus and the reduction of oxygencontaining structures of coal. Therefore, the difference of relative reactivities according to free radical or coal can be attributed to the reaction temperature and the reactant.
ACKNOWLEDGEMENT This work was supported by a grant from the Sunshine Project Promotion Headquarters, Agency of Industrial Science and Technology.
REFERENCES 1
2 3 4 5 6
Table 4 Relative reactivity of weak hydrogen donors from the ratio of of solvent (ks) to that of tetralin (kT) at 450°C I-d(Xll(X)dr),,,,, _____.____~____---~-~~ -.-.~~--
in coal liquefaction:
7 8 9 10 11 12 13
Whitehurst, D. D., Mitchell, T. 0. and Farcasiu, M. ‘Coal Liquefaction - The Chemistry and Technology of Thermal Processes’, Academic Press, New York, 1980, pp. 274 Shah, Y. T. ‘Reaction Engineering in Direct Coal Liquefaction’, Addison-Wesley, 1981, pp. 24 Elliot, M. A., Ed. ‘Chemistrv of Coal Utilization, Second Suppl.‘, Wiley Interscience, New York, 1981, pp. 1848 Kamiya, Y. and Yao, T. Bull. Chem. Sot. Jpn 1979, 52, 492 Ogata, E., Nomi, T., Goto, K. and Kamiya, Y. Proc. 19th Conf. on Coal Science in Japan, p. 153, Tokyo, Oct. 1982 Kamiya, Y., Yao, T. and Nagae, S. Bull. Chem. Sot. Jpn 1982,55, 3873 Kamiya, Y., Ohta, H., Fukushima, A., Aizawa, M. and Mizuki, T. Proc. Int. Conf. Coal Science, p. 195, Pittsburgh, Aug. 1983 Korcek, S.,Chenier, J. H. B., Howard, J. A. and Ingold, K. V. Can. J. Chem. 1972, SO, 2285 Sundaram, M. S. and Given, P. H. Am. Chem. Sot. Div. Fuel Chem. Prepr. 1983, 28(5), 26 Shah, Y. T. and Cronauer, D. C. Catal. Rec.-Sci. Eng. 1979,20(2), 209 Derbyshire, F. J., Varghese, P. and Whitehurst, D. D. Proc. Inr. Conf Coal Science, p. 356, Dusseldorf, Sept. 1981 Bockrath, B. C. and Bittner, E. Proc. Int. Conf. Coal Science, p. 212, Pittsburgh, Aug. 1983 Franz, J. A., Barrows, R. D. and Camaioni, D. M. Am. Chem. SOC. Div. Fuel Chem. Prepr. 1983, 28(5), 77
FUEL, 1985, Vol 64, September
1245
Kamiya
and Shinichi
Department of Reaction Chemistry, Bunkyo-ku, Tokyo 173, Japan (Received 6 July 7984)
of hydrogen
donor
solvent
Nagae Faculty
of Engineering,
University
of Tokyo, Hongo,
Hydrogen transfer from donor solvent to coal must involve reactions such as hydrogen donation to free radicals and hydrogenation of aromatic structures. The relative reactivities of five typical hydrogen donor solvents, more reactive than tetralin, were determined using a competing elimination reaction in the liquefaction of a bituminous coal at 400°C and a brown coal at 350°C. 9,lO_Dihydroanthracene, 9,10dihydrophenanthrene and 1,2,3,4-tetrahydroquinoline exhibited outstanding hydrogen donating ability. Further, the relative reactivities of five mild hydrogen donor solvents such as acenaphthene and indan were determined by a similar elimination reaction using a bituminous coal at 450°C. (Keywords: coal; liquefaction;
hydrogen donor solvent)
Although it is established that a hydrogen donor solventlm3 such as tetralin is very effective for coal liquefaction, its important role being to promote liquefaction by donating hydrogen atoms to coal fragments derived by thermal cracking and also by retarding char formation, the effects of reduction by hydrogen donor on coal structure have not been studied thoroughly. The effect of hydrogen donor can be twofold4. First, stabilization of free radicals can occur by donation of hydrogen atoms,,for example dibenzyl is converted to toluene, and benzyl phenyl ether to toluene and phenol. Second, there are reduction and hydrogenation effects. For example, benzophenone and diphenyl methanol are reduced to diphenyl methane, naphthoquinone to naphthalene, and phenanthrene to hydrophenanthrene. So far, the second group reaction has been thought to make a small contribution to the cleavage reaction. However, it was found that hydrogenation of polycyclic aromatic rings results in a very fast C-0 bond breaking reaction for diary1 ethe?. The relative reactivities of hydrogen donors in coal liquefaction have been studied by a number of workers. Previously, the overall effect has been compared of several solvents on five types of coa16. It was concluded that hydrogen donors were most effective for all ranks of coal and three-ring hydroaromatics were much more effective than tetralin. The competing elimination of hydrogen donors in coal liquefaction as reported by Whitehurst and his coworkers’ will give more quantitative results on relative reactivities. This paper reports the relative reactivities of 12 aromatic solvents in coal liquefaction using a competing elimination reaction with brown and bituminous coal. The measured pseudo-first-order rate constants graphically obtained after 15 min reaction represent the overall reactivity of each solvent including hydrogen donation toward free radicals and reduction of aromatic and oxygencontaining structures of coal. 001~V2361/85/081242-04$3.00 0 1985 Butterworth & Co. (Publishers)
1242
FUEL, 1985, Vol64,
Ltd
September
EXPERIMENTAL Coal samples Analytical data for the coals used are shown in Table 1. All coals ( < 246 pm) were dried at 105°C for 24 h under reduced pressure. Coal liquefaction
Tetralin and 1-methylnaphthalene were reagent grade and used after washing and subsequent distillation at 70°C under reduced pressure. Various solvents were reagent grade and used as received. A mixture of tetralin (10 ml), 1-methylnaphthalene (10 ml) and testing solvent (10 ml or 10 g) and coal (3.0 g) were added to 90 ml magnetically stirred (500 rev min-‘) autoclave. After pressurizing with H, (1 MPa) the autoclave was heated to the reaction temperature within 30min and maintained at that temperature during the reaction time. At the completion of a run, the autoclave was cooled by electric fan to room temperature. The liquid products were removed through a glass filter, and the residual solids were extracted with tetrahydrofuran and weighed after drying. The amounts of daf coal liquefied were calculated as a percentage of daf feed coal by subtracting the percentage undissolved. Liquid portions of the samples were subjected to gas chromatographic analysis to determine the composition of solvent.
Table
1
Elemental
analyses
of coal samples -_--__
Analysis __~~~_._~~___~__~
Yallourn ~__
Akabira
C (wt”,,, daf) H (wt”,, daf) N (wt”,, daf) S (WV’,, daf) 0 (wt”;,. diff.) Ash (wt%. dry coal)
65.8 5.1 0.5 0.2 28.4 2.5
82.7 5.9 1.9 0.4 9.1 5.7
Reactivity
of H-donor
Relative reactivity of hydrogen donor solvent was determined by competing elimination reaction with tetralin. In coal liquefaction, hydrogen donor will be mdehydrogenated by hydrogen donation to coal fragment free radicals and by reduction of coal structure. In the following equation, A represents a testing donor solvent and T represents tetralin, i.e. standard reactive material. X’ and Y represent a free radical from coal and a reducible coal structure, respectively. X’+A~XH+A(-H)’
(1)
X’+T%XH+T(-H)’
(2)
Y+A%
Y’+A’
(3)
Y+T%
Y’+T
(4)
If the cross hydrogen abstraction reaction T( - H)’ and A proceeds very slowly as compared reaction between T(- H)’ and coal fragments, obtain Equations (5) and (6). -d(A)/dt=(k;X’+k;;
Y)(A)=k,(A)
-d(T)/dt=(k;X’+k;Y)(T)=k,(T) Relative reactivity of A against T as a function time can be expressed as Equation (7). [ - d(A )l(A)drll[
between with the we can (5) (6)
of reaction
- d(T)l(T)drl =(kax’+k~Y)/(k;X’+k;Y)=(k,/k,),
(7) Therefore, by measuring the conversion rate of solvent after appropriate reaction time, [ - d(A)/(A)dt],, we can obtain the ratio of apparent pseudo-first-order rate constant, i.e. the relative reactivity of hydrogen donor against tetralin. The apparent reaction rate was not divided by the number of reactive hydrogen atoms per molecule of solvent because the number of most reactive hydrogen atoms is not clear for hydrogen donors. Some hydrogen donors such as dihydroanthracene were not so stable at 400°C and were converted mainly to anthracene by dehydrogenating reaction. Therefore, concentration of hydrogen donor A after the reaction was corrected by multiplying A&A,A& where A, and AB represent the initial and the thermally treated concentrations of A component. Equation (7) can be corrected as follows.
solvent
in coal liquefaction:
Y. Kamiya
and S. Nagae
coals were thermally treated in a mixed solvent composed of 1-methylnaphthalene, tetralin and a strong hydrogen donor (component A), component A was dominantly dehydrogenated. Typical examples of solvent conversion curve in Figure I show that dihydroanthracene is about 10 times as reactive as tetralin in hydrogen donation to Yallourn coal. The rate of conversion of solvent was determined by the slope of a tangential line at the reaction time of 1.5 min. The reaction during the heating up and cooling down periods corresponded to about 5 min at the reaction temperature. The rate of conversion of l-methylnaphthalene was negligible as compared with that of dihydroanthracene. The effect of temperature on conversion of dihydrophenanthrene in the liquefaction of Yallourn coal was remarkable and the conversion of Yallourn coal to THFsoluble and lighter fractions after 30 min was increased from 80 to 9q< with increasing temperature from 350 to 400°C. At 4OO”C, lOmmo1 of dihydrophenanthrene per 1.0 g of Yallourn coal was consumed. In the case of Akabira coal, the conversion of dihydrophenanthrene was much lower than that of Yallourn coal, indicating that a bituminous coal is less reactive at 350°C and also dehydrogenates less hydrogen donor than a brown coal. The conversion of dihydrophenanthrene increased remarkably with increasing the reaction temperature. Accordingly, the conversion of Akabira coal after 30 min reaction was 6q< at 350°C and 98% at 4oo”C, respectively. At 400°C after 30 min, 7.5 mmol of dihydrophenanthrene was consumed per 1.0 g of Akabira coal. In Figure 2, the conversion of active donor solvent in a mixed solution with tetralin and 1-methylnaphthalene was plotted and compared. Homologous conversion curves were obtained for the live strong hydrogen donating solvents. The relative value of solvent conversion corresponding to the first order rate constant can be written as the following order: DHA>DHP> THQ > THN > OHA. It should be mentioned that a small amount of tetralin, described below, was also consumed in the course of reaction. In order to investigate the behaviour of hydrogen donor in coal liquefaction, the conversion of coal was
[-d(A)l(A)dt][A,/(Ao-A,)] [-d(T)/(T)drl[T,l(To (&x’+k; = (k;X’+k;i RESULTS
AND
- TB)]
Y) Y),
(8)
DISCUSSION
Relative reactivities of five hydrogen donors, i.e. 9,10dihydroanthracene (DHA), 9,10-dihydrophenanthrene (DHP), 1,2,3,4-tetrahydroquinoline (THQ), 1,2,3,4tetrahydro-6-naphthol (THN) and 1f2 93 74 35 96 97 38-octahydroanthracene (OHA) were determined by the competing reaction with tetralin in the liquefaction of Yallourn and Akabira coals. When Yallourn or Akabira
I
I
I
0
10
20
Time
I
30
(mln)
of solvents in the liquefaction of Yallourn coal Figure 1 Conversion with mixed solvent of 1-methylnaphthalene, tetralin and DHA at 350°C. 0, Tetralin; 0, dihydroanthracene
FUEL, 1985, Vol 64, September
1243
Reactivity
of H-donor
solvent
in coal liquefaction:
Y. Kamiya
plotted against the amount of hydrogen donor consumed, were observed for all solvent systems,‘suggesting that the number of hydrogen atoms transferred to coal from one mole of solvent is roughly independent of solvent. Increased efficiency of hydrogen donor, corresponding to higher conversion of coal, was observed where tetralin was the most effective hydrogen donor in the solution at the temperature of 450°C. The difference between Yallourn and Akabira coals clearly indicates that a brown coal needs much more hydrogen donor to attain the same level of conversion than a bituminous coal. The conversion of solvent after 30 min of reaction and the rate of solvent consumption (-d(A)/(A)dt),,,,, are shown in Table 2, where Yallourn coal was treated by a mixed solvent at 350°C. Relative reactivity of hydrogen donor toward coal can be evaluated from the value of kA/kT. All five solvents are very active compared with tetralin. According to Whitehurst6, dihydrophenanthrene is 40 times more active than tetralin. The order of hydrogen donating activity of A component was as follows: DHA > DHP > THQ > THN > OHA. Dihydroant hracene, dihydrophenanthrene and tetrahydroquinoline are thought to be very active hydrogen donors. As can be seen in Table 2, the conversion of coal increased with increasing the activity of hydrogen donor and increased almost proportionally to the amount of hydrogen donor consumed (mmol). It has previously been shown that strong hydrogen donors such as DHA and THQ could give much higher conversions of stilbene or benzophenone to diphenylmethane than tetralin’. Therefore, a stronger hydrogen donor is able to transfer more hydrogen atoms to coal structures than tetralin and results in a higher conversion of coal after 30 min of reaction. The conversion of solvent and the rate of solvent consumption (- d(A)/(A)dt), 5minin the liquefaction of Akabira coal are shown in Table 3. In this case, the reactivity of hydrogen donor solvent can be estimated from the values of k, and kA/kT. The relative reactivity of
and S. Nagae
Figure 3. Fairly good correlations
Table 2 Reactivity of strong and component A at 350°C __~___~_____
Component
A
Dihydroanthracene Dihydrophenanthrene Tetrahydroquinoline Tetrahydronaphthol Octahydroanthracene
Table 3 The liquefaction ___-____--~-~--~-~.
Component
A
Dihydroanthracene Dihydrophenanthrene Tetrahydroquinoline Tetrahydronaphthol Octahydroanthracene Tetralin” -~
hydrogen
donors
(A) in the liquefaction
10 Time
I 10
Hydrogen
of Yallourn
(mln)
I I 15 20 donor consumed
coal with mixed solvent composed
kA (component
15 72 89 70 61
39.4 38.9 32.0 24.3 21.5
I.6 1.5 1.4 1.0 8.3
(21.9) (21.6) (25.5) (16.4) (11.3)
coal with mixed solvent composed
I
30
I 25 ( mmsol )
I 30
I
Figure 3 Coal conversion vs hydrogen donor consumption. 0, Yallourn (350°C); 0, Yallourn (450”C);n, Akabira (400°C); A, Akabira (450°C)
Conversion of A after 30 min :‘, (mmol)
of Akabira
I
20
Figure 2 Conversion of strong hydrogen donors in the liquefaction of Yallourn coal with mixed solvent composed of I-methylnaphthalene, tetralin and a strong hydrogen donor A at 350°C. x, Octahydroanthracene; 0, tetrahydronaphthol; A, tetrahydroquinoline; A, dihydrophenanthrene; 0, dihydroanthracene
Conversion of coal after 30 min :< (THF-soluble)
of I-methylnaphthalene,
k,=(-d(X)l(X)dr),,,i, A)
x 10-l X lo-2 x lo-* x 1om2 x 10V3
of I-methylnaphthalene,
tetralin
tetralin
(s-l) kT (tetrahn)
kA IkT
1.1 X lo-3 1.2x 10-j 1.2x 10-a 1.6x 1O-3 1.5 x 1o-3
14.5 12.5 11.7 6.3 5.5
and component
A at 400°C ___-
Conversion of coal after 30 min :< (THF-soluble)
Conversion of A after 30 min O,, (mmol)
98 96 93 93 83 19
38.4 36.6 22.5 25.7 20.8 5.9
’ Net tetralin 30 ml
1244
I
I
0
FUEL, 1985, Vol 64, September
(21.3) (20.3) (18.0) )17.3) (1 I .2) (10.4)
k,=(-d(X)/(X)dt),,,,, kA (component 1.39 1.32 1.0 1.1 5.0
x x x x x
lo-* lo-* 10-r Io-z 10-S
A)
(s-l) kr (tetralin)
kAtkr
I.1 x 1o-3 I.1 x 1o-3 1.0x 10-a
12.6 12 10
0.8 x 1O-3
6
_~__
______
Reactivity of H-donor solvent is similar to the order seen with Yallourn coal. For Akabira coal, the conversion of coal in neat tetralin solvent was also noted. For neat tetralin, the conversion of coal as well as the consumption of solvent were lower than those of other solvents, indicating that a stronger hydrogen donor reduces coal structure to higher extent and results in the higher coal conversion. The high reactivity of DHA and DHP in Tables 2 and 3 is attributable to the active benzylic hydrogen sandwiched by two benzene rings and out of aromatic plane. The bond dissociation energy of the benzylic C-H bond of DHA’ was estimated to be 298 kJ mol-’ (71 kcal mol-‘), which is 46 kJ mol- ’ (11 kcal mol - ‘) lower than that of tetralin. The high reactivity of THQ can be ascribed to the active secondary amine structure adjacent to benzene ring. For THN the measured k,/k, value on Yallourn coal was z 5 as shown in Table 2. This value can be compared with the value of 3 reported by Whitehurst and coworkers*. Much higher reactivity of THN than tetralin may be partly due to the favourable reaction of the hydroxyl group toward oxy radicals resulting from the cleavage of the ether structure of coal. When a mixture of tetralin, I-methylnaphthalene and relatively weak hydrogen-donating solvent (called B component) was used as the solvent in coal liquefaction, tetralin was mainly dehydrogenated. As shown in Table 4, kB/kT values changed from 0.1 to 0.4 according to solvents including 1-met hylnaphthalene. Some aromatics with benzylic hydrogen such as fluorene, acenaphthene and methylnaphthalene were less reactive compared with tetralin. Although hydroaromatic compounds9,‘0 tend to undergo various conversion reactions at 450°C these effects do not significantly influence the relative reactivities in Table 4. Whitehurst and coworkers” reported that pyrene could be hydrogenated to an active solvent such as dihydropyrene in the presence of catalyst and high
solvent
Wh coal
(k&r ),vr,rre
Yallourn
Acenaphthene
0.37 0.34
0.34 0.30
0.36 0.32
0.32 0.3 1 0.28 0.10
0.28 0.14 0.20 0.18
0.30 0.23 0.24 0.14
r;e;;y,_ naphthalene Byrene Indan Fluorene
coal
Akabira
Solvent &_---_--_--_~____
Y. Kamiya
and S. Nagae
pressure of hydrogen gas. In this experiment, a small increase in coal conversion and tetralin conversion in Table 4 might be ascribed to the formation of small amount of dihydropyrene formed by the hydrogen donation from tetralin. Recently, the relative reactivity of many hydrogen donors toward the benzyl radical was measured by Bockrath and Bittner” at 170°C and toward ally1 benzyl radical by Franz and his coworkers’ 3 at 160°C. They have shown a much larger difference in the reactivity of hydrogen donors than seen in Tables 2 and 3. The hydrogen transfer reaction of hydrogen donors to coal and its decomposition products involves the hydrogen abstraction by coal fragments, the hydrogenation of aromatic nucleus and the reduction of oxygencontaining structures of coal. Therefore, the difference of relative reactivities according to free radical or coal can be attributed to the reaction temperature and the reactant.
ACKNOWLEDGEMENT This work was supported by a grant from the Sunshine Project Promotion Headquarters, Agency of Industrial Science and Technology.
REFERENCES 1
2 3 4 5 6
Table 4 Relative reactivity of weak hydrogen donors from the ratio of of solvent (ks) to that of tetralin (kT) at 450°C I-d(Xll(X)dr),,,,, _____.____~____---~-~~ -.-.~~--
in coal liquefaction:
7 8 9 10 11 12 13
Whitehurst, D. D., Mitchell, T. 0. and Farcasiu, M. ‘Coal Liquefaction - The Chemistry and Technology of Thermal Processes’, Academic Press, New York, 1980, pp. 274 Shah, Y. T. ‘Reaction Engineering in Direct Coal Liquefaction’, Addison-Wesley, 1981, pp. 24 Elliot, M. A., Ed. ‘Chemistrv of Coal Utilization, Second Suppl.‘, Wiley Interscience, New York, 1981, pp. 1848 Kamiya, Y. and Yao, T. Bull. Chem. Sot. Jpn 1979, 52, 492 Ogata, E., Nomi, T., Goto, K. and Kamiya, Y. Proc. 19th Conf. on Coal Science in Japan, p. 153, Tokyo, Oct. 1982 Kamiya, Y., Yao, T. and Nagae, S. Bull. Chem. Sot. Jpn 1982,55, 3873 Kamiya, Y., Ohta, H., Fukushima, A., Aizawa, M. and Mizuki, T. Proc. Int. Conf. Coal Science, p. 195, Pittsburgh, Aug. 1983 Korcek, S.,Chenier, J. H. B., Howard, J. A. and Ingold, K. V. Can. J. Chem. 1972, SO, 2285 Sundaram, M. S. and Given, P. H. Am. Chem. Sot. Div. Fuel Chem. Prepr. 1983, 28(5), 26 Shah, Y. T. and Cronauer, D. C. Catal. Rec.-Sci. Eng. 1979,20(2), 209 Derbyshire, F. J., Varghese, P. and Whitehurst, D. D. Proc. Inr. Conf Coal Science, p. 356, Dusseldorf, Sept. 1981 Bockrath, B. C. and Bittner, E. Proc. Int. Conf. Coal Science, p. 212, Pittsburgh, Aug. 1983 Franz, J. A., Barrows, R. D. and Camaioni, D. M. Am. Chem. SOC. Div. Fuel Chem. Prepr. 1983, 28(5), 77
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