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Evidence for the involvement of quinone rings in reactions of some coals with tetralin
Evidence for the involvement of quinone rings in reactions of some coals with tetralin Kay R. Brower Department of chemistry, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA (Received 5 November 1976)
Model compounds representing a variety of hydrocarbon groups and most of the oxygencontaining functional groups have been tested for reactivity with tetralin at 4OO’C. Only the quinones were capable of oxidizing tetralin to naphthalene. Anthraquinone is completely deoxygenated to anthracene. Evidence for homolysis of carbon-carbon bonds was found only in the case of dibenzyl, and this was not accompanied by oxidation of tetralin. The extractability of subbituminous coals with the non-reducing solvent, diphenyl-diphenyl ether eutectic, is correlated with oxygen content and depth of burial. It is suggested that oxidized coals near the surface contain quinone rings, and that dissolution by tetralin is primarily then due to lowering of the solvent parameter as a consequence of deoxygenation.
It has long been known that 1,2,3,4-tetrahydronaphthalene is unusually effective as a solvent for bituminous coal at temperatures near 400°C and that the solution process is accompanied by partial oxidation of the tetralin to naphthalene. Other easily dehydrogenated compounds such as dihydrophenanthrene are also effective in liquefying coal. The residue which remains after evaporation of the solvent, called solvent-refined coal, contains less oxygen and more hydrogen than the original coal. It is fusible and soluble at moderate temperatures in a wide variety of solvents. The nature of the redox reaction which appears to be necessary for the dissolution of many varieties of coal is still a matter for speculation. The change in solubility properties could be due to removal of any of several associational forces such as covalent bonding, charge-transfer complexation, acid-base reactions, dipole-dipole interaction, or hydrogen bonding. Probably the most widely held opinion at present is that carbon-carbon bonds in the coal structure are homolysed, and the radical fragments then abstract hydrogen atoms from tetralin to produce dihydronaphthalene as an intermediate and naphthalene as a final product 1-S. It is said that the process does not appear to involve hydrogen transfer on a catalyst surface’. The rapidity of the reaction when it is performed at the usual temperature of 400°C should provoke curiosity about the nature of the bond which is presumed to homolyse. Ordinary carbon-carbon bonds in simple organic compounds are highly resistant to homolytic scission at such a moderate temperature. In order to shed light on this question, we have tested the effect of tetralin on a variety of model compounds which include most of the known types of oxygen-containing functional groups. The purpose was to explore the range of reactions as widely as possible and to determine whether the chemical reduction of functional groups might play a role in the liquefaction of coal, either alone or in conjunction with other processes. We have also searched for a correlation between oxygen content and liquefiability of several bituminous coals from (tetralin)
the Rocky Mountain area. Tests were made not only with tetralin, which liquefied all of the coals, but also with diphenyl-diphenyl ether eutectic mixture (Dowtherm) which has no chemical reducing power. Some samples were liquefied as well by Dowtherm as tetralin, whereas others showed partial miscibility ranging downwards to nearly total insolubility. These tests were intended to show whether the need for chemical reduction as a prelude to liquefaction diminishes as the original oxygen content of the coal decreases. EXPERIMENTAL Reaction
of model compounds
with tetrulin
A O-5 g sample of each compound listed in Table I was placed in a 10 mm standard wall Pyrex tube, and a 2-fold molar quantity of tetralin (1,2,3,4-tetrahydronaphthalene) was added. The tube was then sealed leaving space for 3x expansion of the liquid. It was placed in the pressure vessel shown in Figure I without the filter assembly, and 25 ml of tetralin was added. Heat was applied directly by gas burners which were adjusted to give the same temperatures at the top and bottom thermocouple wells. In the course of 20 min the temperature rose to 400°C and the gauge pressure of tetralin vapour was 3 MPa. After 5 min at 400°C the burners were extinguished, and the vessel was rapidly cooled by a fan. None of the sealed tubes burst in these experiments, although rupture was frequent during early attempts to heat them without use of the pressurized tetralin vapour bath. The same conditions of temperature and reaction time were sufficient to cause extensive dehydrogenation of tetralin by all coal samples used in tests described below. Reaction mixtures were analysed by gas chromatography using samples of 30-50 /.d which were large enough to permit positive identification of each fraction by i.r. and mass spectrometry. Known mixtures were used to calibrate the quantitative determination of composition from peak areas.
FUEL, 1977, Vol 56, July
245
involvement Table 7
of quinone
rings
in reactionsof some coals with tetralin:K. R. grower
Fate of model compounds
in tetralin
at 400°C
Substance
Reaction
Acetophenone Adamantane I-Adamantanol P-Adamantanone Anthraquinone Benzaldehyde Benzoic acid
None None None None Anthracene Benzene and carbon monoxidea None in glass tube Benzene and carbon dioxide in SS None None Trace of toluene None None I-Naphthol, 1,4-dihydroxyben2ene.a and naphthalene None None in glass tube. Diphenylacetone in SS None None
Benzyl alcohol p-Cresol Dibenzyl Methyl benzoate 2-Naphthol 1,4-Naphthoquinone 1-Octene Phenylacetic
acid
Thiophenol Xanthone a
products
tor. The cooled extract was extended with ligroin, filtered, and distilled to a temperature of 400°C. A portion of the distillate was analysed by gas chromatography to determine the naphthalene/tetralin ratio. The residual char was weighed and both it and the solvent-refined coal were analysed for ash.
Extraction of coals with diphenyl-diphenyl ether eu tectic The procedure was similar to that above except that a hydrogen pressure of 12 to 16 MPa was always used. The char and solvent-refined coal were weighed.
Observation of hydrogenation disequilibrium during coal liquefaction A mixture of 20 g of Navajo coal, 22 ml of diphenyldiphenyl ether eutectic, and 8 ml of tetralin was heated to 400°C under 20 MPa of hydrogen, and the conditions were maintained for 2 h. Analysis following workup as above showed 0.4 mol tetralin/mol naphthalene.
Not isolated
Isolation of anthracene from reaction of anthraquinone with tetralin When the standard proportion of substrate and tetralin was used, the tetralin was almost completely converted to naphthalene, and some unreacted anthraquinone was admixed with the anthracene product as indicated by the mass spectrum. Two recrystallizations from alcohol gave 0.05 g of pure anthracene. The experiment was repeated using 0.8 g of anthraquinone (3.85 mmol) and 2.0 g of tetralin (17.7 mmol). The reaction mixture was distilled to 25O”C, and the distillate was analysed by GC. Naphthalene amounted to 39% by weight which corresponds to 11.8 mm01 of hydrogen. The amount calculated for reduction of anthraquinone to anthracene is 11.6 mmol.
Isolation of I-naphthol from reaction of I,4naph thoquinone with tetralin
Figure 1 Coal extractor. (a) Perforated support plate; (b) asbestos fibre filter pad; (cl retainer; (d) threaded rod for insertion and removal of filter assembly
Reaction of coals with tetralin Samples of eight subbituminous coals were supplied by the New Mexico Bureau of Mines and Mineral Resources. Their characteristics are given in Table 2. A 20 g portion of coal in the form of coarse lumps was placed in the extractor shown in Figure 1 together with 30 ml of tetralin. In some experiments an atmosphere of hydrogen was supplied through the upper tubing connection with pressures up to 30 MPa. When no hydrogen was added the autogenous pressure after heating ranged from 4 to 10 MPa. The extractor was heated as described above to 400°C and after 5 min a valve attached to the lower tubing connection was opened to allow the liquid contents to be forced through the filter into a receiver which is similar in construction to the extrac-
246
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1977, Vol 56, July
A reaction mixture originally containing 0.50 g (3-l 6 mmol) of naphthoquinone and 0.70 g of tetralin was analysed by distillation and GC as above. The distillate contained 30% naphthalene which corresponds to 3.28 mmol of hydrogen. The amount calculated for reduction of naphthoquinone to 1,4-dihydroxynapthalene is 3.16 mmol. The residue from the distillation was dissolved in ether and extracted with 5% sodium hydroxide solution. Acidification of the extract gave a tarry precipitate which was subjected to vacuum sublimation. Approximately 10 mg of crude lnaphthol was deposited on the cold-finger condenser. It was identified by comparison of its i.r. spectrum with that of an authentic sample.
Table 2
Source
Characteristics
H
Navajo 5.6 Carthage 4.4 Raton 5.0 San Juan 5.6 Wattis 7.3 Dutch Creek 5.4 Montana 7.2 Green -
of coal samples (as received)
Ultimate analysis C N 0 54.8 54.6 70.2 63-6 71.8 82.6 49.4 66.2
1.0 1.0 1.6 1.1 1.1 1.9 0.7 -
22.2 12.7 8.2 20.0 10.1 5.5 33.1 17.0
S .-
Ash
Fuel value (MJ/kg)
0.7 5.1 0.5 0.5 0.7 0.6 0.7 -
15.7 22.2 14.5 9.2 9.0 4.0 8.9 10.9
21.9 23.0 32.6 23-7 26-7 34.2 20.2 23.5
K. R. Brewer: Involvement of quinone rings in reactions of some coals with tetralin
tion metal catalysts is a known reaction, but it is surprising that it can occur in a glass tube.
RESULTS AND DISCUSSION Reaction of tetralin with quinones Of all the compounds shown in Table I, only the quinones caused oxidation of tetralin to naphthalene. This is a known reaction for quinones having high oxidation potentials owing to the presence of electron-withdrawing substituents. For example, 2,3-dichloro-5,6-dicyanoquinone6 and o-chloranil’ dehydrogenate tetralin via 1,2-dihydronaphthalene in the course of several hours at 80°C. It is therefore not surprising that quinones with very low oxidation potentials such as naphthoquinone and anthraquinone react at 400°C. It is surprising, however, that the reduction proceeds partially to 1-naphthol in the first case and predominantly to anthracene in the second case. Tritium isotope effects6 support the previously held idea’ that the reaction is a hydride transfer as depicted below: H
Ii.
O_
f-l”
clloqHo+~, 0
OH O_
The tendency of anthraquinone to deoxygenate completely is probably associated with the well known tautomerization of 9-anthranol to anthrone as shown below. Note that two hydride transfers are required to produce 9.anthranol, and the second stage is probably similar to the third. OH
0
H
Behaviour of adarnan tane and its derivatives in te tralin On the basis of degradation by hypochlorite it has been suggested by Chakrabarttys that the structural framework of coal is a polyamantane. We therefore thought it desirable to test the reactivity of adamantane, 1-adamantanol, and 2adamantanone toward tetralin. These compounds were all found to be inert. Other workers’ have reported that adamantane is stable under coal liquefaction conditions and have used this and other evidence to criticize the polyamantane hypothesis. Behaviour of benzoic acid and phenylace tic acid with tetralin in a stainless-steel vessel In an early series of experiments which taught us the importance of suppressing catalysis by using glass tubes, we placed tetralin solutions of benzoic acid and phenylacetic acid directly in the vessel shown in Figure 1. Benzoic acid was almost completely decarboxylated to benzene, and phenylacetic acid was converted in good yield to diphenylacetone. In sealed glass tubes both of these compounds are inert. The condensation reaction of phenylacetic acid probably involves a catalytic amount of metallic salt as shown below:
Behaviour of dibenzyl in tetralin Dibenzyl was chosen as a substrate likely to homolyse at elevated temperature and abstract hydrogen atoms from tetralin according to the accepted view of hydrogen donation from tetralin to coal. Dissociation into benzyl radicals is favoured by resonance stabilization and by the entropy increase associated with scission as opposed to ring opening. Many chemical systems are known to homolyse more easily such as pentaphenylethanes, peroxides, azo compounds, etc., but it is hard to imagine any of them playing an important role in coal structure. When dibenzyl was given the standard treatment of 5 min at 400°C the gas chromatogram indicated 0.5% conversion to toluene. A second test was carried out at 430°C for 90 min, and the yield of toluene was 30% of theoretical. To our great surprise the yield of naphthalene was many times smaller than needed to explain the presence of toluene. We surmise that the dibenzyl disproportionated and that stilbene is the oxidation product. The elution time of stilbene is too great for detection with our gas chromatograph. Behaviour of benzaldehyde in tetralin Benzaldehyde did not oxidize tetralin to naphthalene, but was partially decarbonylated as indicated by the escape of a pressurized gas when the sealed tube was opened and by the appearance of a 17% yield of benzene in the gas chromatogram. Decarbonylation of benzaldehyde by various transi-
+C6H,CH,),C0
MCO, + 2CsH,CH$0,H
-+ CO, •t H,O +
(CsH,CH,CO,),M
OH
Anthracenc
(C6H5CH,C0,),M++
f MCO,
++
Correlation of extractability of coal with oxygen content A number of coals having the characteristics shown in Table2 were extracted with a eutectic mixture of diphenyl and diphenyl ether. Unlike tetralin which liquefies all of the coals extensively, this solvent shows the complete range of possible effects. Typically the coals lose about 25% of their weight as a result of heating without solvent, so that a yield of 75% char in an extraction test indicates no solvent action. At the other extreme there are coals which leave a char consisting mainly of ash. A hydrogen atmosphere was used in order to drive the extract through the filter. In order to show that liquefaction is not due to chemical reaction with the hydrogen, we pressurized two extractions using Dutch Creek and Wattis coal with nitrogen. The results were nearly the same as with hydrogen. The correlation between extractability and oxygen content is evident in the data of Table3. ‘Ihe oxygen content also appears to decrease with depth of burial. It is hard to resist the inference that Tab/e 3 Extractabilitya,oxygencontent, and depth of coal deposits Oxygen Source
(wt %, as received)
Char bvt %I
Depth
Dutch Creek Raton Wattis Carthage Navajo San Juan Green Montana
5.5 8.2 10.1 12.7 22.2 20.0 17.0 33.1
16 13 21 40 70 75 72 80
500 ft 400 ft Surface Surface Surface Surface 60 ft Surface
a
The solvent is diphenyl-diphenyl
FUEL,
ether eutectic
1977, Vol 56, July
247
involvement of quinone rings in reactions of some coals with tetralin: K. R. grower
the high oxygen content of the coals which lie near the surface is due to air oxidation. The deeply buried coals have only 5- 10% of oxygen (as received basis) which is presumably derived from the original biological source, whereas the shallow coals contain up to 33% oxygen. A wide variety of aromatic compounds can be oxidized to quinones by air, and it seems plausible that the oxygen-rich surface coals contain relatively many quinone rings. It has been shown above that quinones can be reduced to phenols or arenes by tetralin at 4OO”C,and we believe that this is the key to its remarkable solvent power for oxygen-rich coaIs. Compounds in which the quinone ring plays a prominent part are characteristically polar in nature and tend to dissolve better in alcohol than in hydrocarbon solvents. We observed during the course of our sealed-tube experiments, for example, that O-8 g of anthraquinone could not be dissolved in 2.0 g of tetralin even when the tube was heated to 200°C which was as high as we dared to go under direct observation. After reaction at 4OO”C,however, the contents could be completely liquefied in a water bath. On these grounds it is suggested that the insolubility of oxygen-rich coals in nonreducing solvents such as Dowtherm results from a mismatch of solvent parameter lo rather than a high molecular weight. The presence of quinone rings could lead to strong intermolecular association of the type known as dipolar complexation”. If these forces are absent in the beginning as in the low-oxygen coals, or abolished by chemical reduction, then a chemically inert solvent should be effective.
Stoichiometry
of coal reduction
by tetralin
Determination of the naphthalene/tetralin ratio in the volatile fraction of coal extracts made it possible to calculate the weight of coal which reacts with O-5 mol of tetralin (1 mol of hydrogen). This quantity ranged from 120 to 190 g for the San Juan and Navajo coals. This is difficult to reconcile with the average molecular weight of solvent-refined coal if it be assumed that the principal reaction is hydrogenolysis of carbon-carbon bonds. On the other hand it is easy to understand how so much reducing power could be consumed by reduction of quinone rings, particularly if they are deoxygenated as in the case of anthraquinone. The weight of anthraquinone which reacts with 0.5 mol of tetralin is 69.3 g. It would only be necessary to have one mole of anthraquinone moiety in 360-570 g of coal in order to account for all of the hydrogen consumption. In a recent study of the kinetics and mechanism of solvent refining of coal’* it was noted with surprise that the aromatic carbon, aromatic hydrogen, and polycondensed aromatic ring contents all increased in the liquefaction treatment. These findings harmonize completely with the quinone hypothesis since deoxygenation of a bicondensed quinone ring results in the gain of two aromatic hydrogens, two aromatic carbons, and one polycondensed aromatic ring. Hydrogenation of coal
disequilibrium
in solvent liquefaction
Catalysis by the walls of the stainless-steel pressure vessel is implicated in the decomposition of benzoic acid and the self-condensation of phenylacetic acid. We therefore thought it necessary to study the possibility of indirect transfer of hydrogen from tetralin to coal either by the metal walls or by mineral constituents of the coal samples. To this end we extracted Navajo coal under a hydrogen pressure of 20 MPa
248
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1977, Vol 56, July
with a limited amount of tetralin as described in the Experimental Section. The time was extended to 2 h in order to make the test very stringent. The measured ratio of tetralin to naphthalene at the end of this time was O-4. The equilibrium constant expressed in MPam2for the hydrogenation of naphthalene to tetralin is given by the following equation13: log&
= - 15.13 +7000/r
The value of K at 400°C is O-1867 and the calculated ratio of tetralin to naphthalene is 168. The deviation of this ratio from the equilibrium value by a factor of more than 400 is clear evidence that there is no catalytic exchange of hydrogen between tetrahn and naphthalene with diversion of part of the hydrogen to coal. SUMMARY
AND CONCLUSIONS
Chemicalreduction is not necessary for solvent liquefaction of some subbituminous coals. Two out of three such coals used in our study are deeply buried and have a low oxygen content. All of those which required reduction were found at shallow depth and had a high oxygen content. Model compounds having a wide variety of oxygen-containing functional groups were tested under the conditions used for coal extraction, and only the quinones were capable of oxidizing tetralin to naphthalene. It is possible that subbituminous coal deposits near the surface are oxidized by air with partial conversion of aromatic rings to quinone rings. Quinones are known to have a low solubility in hydrocarbon solvents. We suggest that the effect of hydrogen-donor solvents such as tetralin in liquefying oxidized coals is primarily due to a lowering of the solvent parameter.
ACKNOWLEDGEMENT Thiswork was supported in part by a grant from the Energy Research and Development Program of the State of New Mexico. REFERENCES
4 5 6 7 8 9 10 11 12 13
Curran, G. P., Struck, R. T. and Gorin, E. Preprints, ACS, Div. Petroleum Chem. 1966, C-130-148 Curran, G. P., Struck, R. T. and Gorin, E. Ind. Engng Chem. 1967,6,166 Wiser, W. H. and Hill, G. R. Proc, Symposium on Sci. and Tech. of Coal, Ottawa, 1967, Mines Branch, Dept. of Energy, Mines and Resources (Canada), pp 162-167 Wiser, W. H. Fuel 1968,47,475 Neavel, R. C., Fuel 1976,55,237 Van der Jagt, P. J., de Haan, H. K. and van Zanten, B. Tetrahidron 1971,77,3207 Braude, E. A., Brook, A. G. and Linstead, R. P. J. them. Sot. 1954, p 3569 Chakrabartty, S. K. and Berkowitz, N. Fuel 1974,53,240 Aczel, T., Martin, L. G., Maa, P. S. and Schlosberg, R. H. Fuel 1975,54,295 Hildebrand, J. H. and Scott, R. L. The Solubility of Nonelectrolytes, Dover, New York, 1964, Appendix I Hammett, L. P. Physical Organic Chemistry, 2nd edn, McGraw-Hill, 1970, p 257 Farcasiu, M., Mitchell, T. 0. and Whitehurst, D. D. Preprints of the 1976 Coal Chemistry Workshop, Stanford Research Institute, Menlo Park, California 94025 Wilson, T. P., Caflisch, E. G. and Hurley, G. F. J. phys. Chem. 1958,62,1059
Model compounds representing a variety of hydrocarbon groups and most of the oxygencontaining functional groups have been tested for reactivity with tetralin at 4OO’C. Only the quinones were capable of oxidizing tetralin to naphthalene. Anthraquinone is completely deoxygenated to anthracene. Evidence for homolysis of carbon-carbon bonds was found only in the case of dibenzyl, and this was not accompanied by oxidation of tetralin. The extractability of subbituminous coals with the non-reducing solvent, diphenyl-diphenyl ether eutectic, is correlated with oxygen content and depth of burial. It is suggested that oxidized coals near the surface contain quinone rings, and that dissolution by tetralin is primarily then due to lowering of the solvent parameter as a consequence of deoxygenation.
It has long been known that 1,2,3,4-tetrahydronaphthalene is unusually effective as a solvent for bituminous coal at temperatures near 400°C and that the solution process is accompanied by partial oxidation of the tetralin to naphthalene. Other easily dehydrogenated compounds such as dihydrophenanthrene are also effective in liquefying coal. The residue which remains after evaporation of the solvent, called solvent-refined coal, contains less oxygen and more hydrogen than the original coal. It is fusible and soluble at moderate temperatures in a wide variety of solvents. The nature of the redox reaction which appears to be necessary for the dissolution of many varieties of coal is still a matter for speculation. The change in solubility properties could be due to removal of any of several associational forces such as covalent bonding, charge-transfer complexation, acid-base reactions, dipole-dipole interaction, or hydrogen bonding. Probably the most widely held opinion at present is that carbon-carbon bonds in the coal structure are homolysed, and the radical fragments then abstract hydrogen atoms from tetralin to produce dihydronaphthalene as an intermediate and naphthalene as a final product 1-S. It is said that the process does not appear to involve hydrogen transfer on a catalyst surface’. The rapidity of the reaction when it is performed at the usual temperature of 400°C should provoke curiosity about the nature of the bond which is presumed to homolyse. Ordinary carbon-carbon bonds in simple organic compounds are highly resistant to homolytic scission at such a moderate temperature. In order to shed light on this question, we have tested the effect of tetralin on a variety of model compounds which include most of the known types of oxygen-containing functional groups. The purpose was to explore the range of reactions as widely as possible and to determine whether the chemical reduction of functional groups might play a role in the liquefaction of coal, either alone or in conjunction with other processes. We have also searched for a correlation between oxygen content and liquefiability of several bituminous coals from (tetralin)
the Rocky Mountain area. Tests were made not only with tetralin, which liquefied all of the coals, but also with diphenyl-diphenyl ether eutectic mixture (Dowtherm) which has no chemical reducing power. Some samples were liquefied as well by Dowtherm as tetralin, whereas others showed partial miscibility ranging downwards to nearly total insolubility. These tests were intended to show whether the need for chemical reduction as a prelude to liquefaction diminishes as the original oxygen content of the coal decreases. EXPERIMENTAL Reaction
of model compounds
with tetrulin
A O-5 g sample of each compound listed in Table I was placed in a 10 mm standard wall Pyrex tube, and a 2-fold molar quantity of tetralin (1,2,3,4-tetrahydronaphthalene) was added. The tube was then sealed leaving space for 3x expansion of the liquid. It was placed in the pressure vessel shown in Figure I without the filter assembly, and 25 ml of tetralin was added. Heat was applied directly by gas burners which were adjusted to give the same temperatures at the top and bottom thermocouple wells. In the course of 20 min the temperature rose to 400°C and the gauge pressure of tetralin vapour was 3 MPa. After 5 min at 400°C the burners were extinguished, and the vessel was rapidly cooled by a fan. None of the sealed tubes burst in these experiments, although rupture was frequent during early attempts to heat them without use of the pressurized tetralin vapour bath. The same conditions of temperature and reaction time were sufficient to cause extensive dehydrogenation of tetralin by all coal samples used in tests described below. Reaction mixtures were analysed by gas chromatography using samples of 30-50 /.d which were large enough to permit positive identification of each fraction by i.r. and mass spectrometry. Known mixtures were used to calibrate the quantitative determination of composition from peak areas.
FUEL, 1977, Vol 56, July
245
involvement Table 7
of quinone
rings
in reactionsof some coals with tetralin:K. R. grower
Fate of model compounds
in tetralin
at 400°C
Substance
Reaction
Acetophenone Adamantane I-Adamantanol P-Adamantanone Anthraquinone Benzaldehyde Benzoic acid
None None None None Anthracene Benzene and carbon monoxidea None in glass tube Benzene and carbon dioxide in SS None None Trace of toluene None None I-Naphthol, 1,4-dihydroxyben2ene.a and naphthalene None None in glass tube. Diphenylacetone in SS None None
Benzyl alcohol p-Cresol Dibenzyl Methyl benzoate 2-Naphthol 1,4-Naphthoquinone 1-Octene Phenylacetic
acid
Thiophenol Xanthone a
products
tor. The cooled extract was extended with ligroin, filtered, and distilled to a temperature of 400°C. A portion of the distillate was analysed by gas chromatography to determine the naphthalene/tetralin ratio. The residual char was weighed and both it and the solvent-refined coal were analysed for ash.
Extraction of coals with diphenyl-diphenyl ether eu tectic The procedure was similar to that above except that a hydrogen pressure of 12 to 16 MPa was always used. The char and solvent-refined coal were weighed.
Observation of hydrogenation disequilibrium during coal liquefaction A mixture of 20 g of Navajo coal, 22 ml of diphenyldiphenyl ether eutectic, and 8 ml of tetralin was heated to 400°C under 20 MPa of hydrogen, and the conditions were maintained for 2 h. Analysis following workup as above showed 0.4 mol tetralin/mol naphthalene.
Not isolated
Isolation of anthracene from reaction of anthraquinone with tetralin When the standard proportion of substrate and tetralin was used, the tetralin was almost completely converted to naphthalene, and some unreacted anthraquinone was admixed with the anthracene product as indicated by the mass spectrum. Two recrystallizations from alcohol gave 0.05 g of pure anthracene. The experiment was repeated using 0.8 g of anthraquinone (3.85 mmol) and 2.0 g of tetralin (17.7 mmol). The reaction mixture was distilled to 25O”C, and the distillate was analysed by GC. Naphthalene amounted to 39% by weight which corresponds to 11.8 mm01 of hydrogen. The amount calculated for reduction of anthraquinone to anthracene is 11.6 mmol.
Isolation of I-naphthol from reaction of I,4naph thoquinone with tetralin
Figure 1 Coal extractor. (a) Perforated support plate; (b) asbestos fibre filter pad; (cl retainer; (d) threaded rod for insertion and removal of filter assembly
Reaction of coals with tetralin Samples of eight subbituminous coals were supplied by the New Mexico Bureau of Mines and Mineral Resources. Their characteristics are given in Table 2. A 20 g portion of coal in the form of coarse lumps was placed in the extractor shown in Figure 1 together with 30 ml of tetralin. In some experiments an atmosphere of hydrogen was supplied through the upper tubing connection with pressures up to 30 MPa. When no hydrogen was added the autogenous pressure after heating ranged from 4 to 10 MPa. The extractor was heated as described above to 400°C and after 5 min a valve attached to the lower tubing connection was opened to allow the liquid contents to be forced through the filter into a receiver which is similar in construction to the extrac-
246
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1977, Vol 56, July
A reaction mixture originally containing 0.50 g (3-l 6 mmol) of naphthoquinone and 0.70 g of tetralin was analysed by distillation and GC as above. The distillate contained 30% naphthalene which corresponds to 3.28 mmol of hydrogen. The amount calculated for reduction of naphthoquinone to 1,4-dihydroxynapthalene is 3.16 mmol. The residue from the distillation was dissolved in ether and extracted with 5% sodium hydroxide solution. Acidification of the extract gave a tarry precipitate which was subjected to vacuum sublimation. Approximately 10 mg of crude lnaphthol was deposited on the cold-finger condenser. It was identified by comparison of its i.r. spectrum with that of an authentic sample.
Table 2
Source
Characteristics
H
Navajo 5.6 Carthage 4.4 Raton 5.0 San Juan 5.6 Wattis 7.3 Dutch Creek 5.4 Montana 7.2 Green -
of coal samples (as received)
Ultimate analysis C N 0 54.8 54.6 70.2 63-6 71.8 82.6 49.4 66.2
1.0 1.0 1.6 1.1 1.1 1.9 0.7 -
22.2 12.7 8.2 20.0 10.1 5.5 33.1 17.0
S .-
Ash
Fuel value (MJ/kg)
0.7 5.1 0.5 0.5 0.7 0.6 0.7 -
15.7 22.2 14.5 9.2 9.0 4.0 8.9 10.9
21.9 23.0 32.6 23-7 26-7 34.2 20.2 23.5
K. R. Brewer: Involvement of quinone rings in reactions of some coals with tetralin
tion metal catalysts is a known reaction, but it is surprising that it can occur in a glass tube.
RESULTS AND DISCUSSION Reaction of tetralin with quinones Of all the compounds shown in Table I, only the quinones caused oxidation of tetralin to naphthalene. This is a known reaction for quinones having high oxidation potentials owing to the presence of electron-withdrawing substituents. For example, 2,3-dichloro-5,6-dicyanoquinone6 and o-chloranil’ dehydrogenate tetralin via 1,2-dihydronaphthalene in the course of several hours at 80°C. It is therefore not surprising that quinones with very low oxidation potentials such as naphthoquinone and anthraquinone react at 400°C. It is surprising, however, that the reduction proceeds partially to 1-naphthol in the first case and predominantly to anthracene in the second case. Tritium isotope effects6 support the previously held idea’ that the reaction is a hydride transfer as depicted below: H
Ii.
O_
f-l”
clloqHo+~, 0
OH O_
The tendency of anthraquinone to deoxygenate completely is probably associated with the well known tautomerization of 9-anthranol to anthrone as shown below. Note that two hydride transfers are required to produce 9.anthranol, and the second stage is probably similar to the third. OH
0
H
Behaviour of adarnan tane and its derivatives in te tralin On the basis of degradation by hypochlorite it has been suggested by Chakrabarttys that the structural framework of coal is a polyamantane. We therefore thought it desirable to test the reactivity of adamantane, 1-adamantanol, and 2adamantanone toward tetralin. These compounds were all found to be inert. Other workers’ have reported that adamantane is stable under coal liquefaction conditions and have used this and other evidence to criticize the polyamantane hypothesis. Behaviour of benzoic acid and phenylace tic acid with tetralin in a stainless-steel vessel In an early series of experiments which taught us the importance of suppressing catalysis by using glass tubes, we placed tetralin solutions of benzoic acid and phenylacetic acid directly in the vessel shown in Figure 1. Benzoic acid was almost completely decarboxylated to benzene, and phenylacetic acid was converted in good yield to diphenylacetone. In sealed glass tubes both of these compounds are inert. The condensation reaction of phenylacetic acid probably involves a catalytic amount of metallic salt as shown below:
Behaviour of dibenzyl in tetralin Dibenzyl was chosen as a substrate likely to homolyse at elevated temperature and abstract hydrogen atoms from tetralin according to the accepted view of hydrogen donation from tetralin to coal. Dissociation into benzyl radicals is favoured by resonance stabilization and by the entropy increase associated with scission as opposed to ring opening. Many chemical systems are known to homolyse more easily such as pentaphenylethanes, peroxides, azo compounds, etc., but it is hard to imagine any of them playing an important role in coal structure. When dibenzyl was given the standard treatment of 5 min at 400°C the gas chromatogram indicated 0.5% conversion to toluene. A second test was carried out at 430°C for 90 min, and the yield of toluene was 30% of theoretical. To our great surprise the yield of naphthalene was many times smaller than needed to explain the presence of toluene. We surmise that the dibenzyl disproportionated and that stilbene is the oxidation product. The elution time of stilbene is too great for detection with our gas chromatograph. Behaviour of benzaldehyde in tetralin Benzaldehyde did not oxidize tetralin to naphthalene, but was partially decarbonylated as indicated by the escape of a pressurized gas when the sealed tube was opened and by the appearance of a 17% yield of benzene in the gas chromatogram. Decarbonylation of benzaldehyde by various transi-
+C6H,CH,),C0
MCO, + 2CsH,CH$0,H
-+ CO, •t H,O +
(CsH,CH,CO,),M
OH
Anthracenc
(C6H5CH,C0,),M++
f MCO,
++
Correlation of extractability of coal with oxygen content A number of coals having the characteristics shown in Table2 were extracted with a eutectic mixture of diphenyl and diphenyl ether. Unlike tetralin which liquefies all of the coals extensively, this solvent shows the complete range of possible effects. Typically the coals lose about 25% of their weight as a result of heating without solvent, so that a yield of 75% char in an extraction test indicates no solvent action. At the other extreme there are coals which leave a char consisting mainly of ash. A hydrogen atmosphere was used in order to drive the extract through the filter. In order to show that liquefaction is not due to chemical reaction with the hydrogen, we pressurized two extractions using Dutch Creek and Wattis coal with nitrogen. The results were nearly the same as with hydrogen. The correlation between extractability and oxygen content is evident in the data of Table3. ‘Ihe oxygen content also appears to decrease with depth of burial. It is hard to resist the inference that Tab/e 3 Extractabilitya,oxygencontent, and depth of coal deposits Oxygen Source
(wt %, as received)
Char bvt %I
Depth
Dutch Creek Raton Wattis Carthage Navajo San Juan Green Montana
5.5 8.2 10.1 12.7 22.2 20.0 17.0 33.1
16 13 21 40 70 75 72 80
500 ft 400 ft Surface Surface Surface Surface 60 ft Surface
a
The solvent is diphenyl-diphenyl
FUEL,
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1977, Vol 56, July
247
involvement of quinone rings in reactions of some coals with tetralin: K. R. grower
the high oxygen content of the coals which lie near the surface is due to air oxidation. The deeply buried coals have only 5- 10% of oxygen (as received basis) which is presumably derived from the original biological source, whereas the shallow coals contain up to 33% oxygen. A wide variety of aromatic compounds can be oxidized to quinones by air, and it seems plausible that the oxygen-rich surface coals contain relatively many quinone rings. It has been shown above that quinones can be reduced to phenols or arenes by tetralin at 4OO”C,and we believe that this is the key to its remarkable solvent power for oxygen-rich coaIs. Compounds in which the quinone ring plays a prominent part are characteristically polar in nature and tend to dissolve better in alcohol than in hydrocarbon solvents. We observed during the course of our sealed-tube experiments, for example, that O-8 g of anthraquinone could not be dissolved in 2.0 g of tetralin even when the tube was heated to 200°C which was as high as we dared to go under direct observation. After reaction at 4OO”C,however, the contents could be completely liquefied in a water bath. On these grounds it is suggested that the insolubility of oxygen-rich coals in nonreducing solvents such as Dowtherm results from a mismatch of solvent parameter lo rather than a high molecular weight. The presence of quinone rings could lead to strong intermolecular association of the type known as dipolar complexation”. If these forces are absent in the beginning as in the low-oxygen coals, or abolished by chemical reduction, then a chemically inert solvent should be effective.
Stoichiometry
of coal reduction
by tetralin
Determination of the naphthalene/tetralin ratio in the volatile fraction of coal extracts made it possible to calculate the weight of coal which reacts with O-5 mol of tetralin (1 mol of hydrogen). This quantity ranged from 120 to 190 g for the San Juan and Navajo coals. This is difficult to reconcile with the average molecular weight of solvent-refined coal if it be assumed that the principal reaction is hydrogenolysis of carbon-carbon bonds. On the other hand it is easy to understand how so much reducing power could be consumed by reduction of quinone rings, particularly if they are deoxygenated as in the case of anthraquinone. The weight of anthraquinone which reacts with 0.5 mol of tetralin is 69.3 g. It would only be necessary to have one mole of anthraquinone moiety in 360-570 g of coal in order to account for all of the hydrogen consumption. In a recent study of the kinetics and mechanism of solvent refining of coal’* it was noted with surprise that the aromatic carbon, aromatic hydrogen, and polycondensed aromatic ring contents all increased in the liquefaction treatment. These findings harmonize completely with the quinone hypothesis since deoxygenation of a bicondensed quinone ring results in the gain of two aromatic hydrogens, two aromatic carbons, and one polycondensed aromatic ring. Hydrogenation of coal
disequilibrium
in solvent liquefaction
Catalysis by the walls of the stainless-steel pressure vessel is implicated in the decomposition of benzoic acid and the self-condensation of phenylacetic acid. We therefore thought it necessary to study the possibility of indirect transfer of hydrogen from tetralin to coal either by the metal walls or by mineral constituents of the coal samples. To this end we extracted Navajo coal under a hydrogen pressure of 20 MPa
248
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1977, Vol 56, July
with a limited amount of tetralin as described in the Experimental Section. The time was extended to 2 h in order to make the test very stringent. The measured ratio of tetralin to naphthalene at the end of this time was O-4. The equilibrium constant expressed in MPam2for the hydrogenation of naphthalene to tetralin is given by the following equation13: log&
= - 15.13 +7000/r
The value of K at 400°C is O-1867 and the calculated ratio of tetralin to naphthalene is 168. The deviation of this ratio from the equilibrium value by a factor of more than 400 is clear evidence that there is no catalytic exchange of hydrogen between tetrahn and naphthalene with diversion of part of the hydrogen to coal. SUMMARY
AND CONCLUSIONS
Chemicalreduction is not necessary for solvent liquefaction of some subbituminous coals. Two out of three such coals used in our study are deeply buried and have a low oxygen content. All of those which required reduction were found at shallow depth and had a high oxygen content. Model compounds having a wide variety of oxygen-containing functional groups were tested under the conditions used for coal extraction, and only the quinones were capable of oxidizing tetralin to naphthalene. It is possible that subbituminous coal deposits near the surface are oxidized by air with partial conversion of aromatic rings to quinone rings. Quinones are known to have a low solubility in hydrocarbon solvents. We suggest that the effect of hydrogen-donor solvents such as tetralin in liquefying oxidized coals is primarily due to a lowering of the solvent parameter.
ACKNOWLEDGEMENT Thiswork was supported in part by a grant from the Energy Research and Development Program of the State of New Mexico. REFERENCES
4 5 6 7 8 9 10 11 12 13
Curran, G. P., Struck, R. T. and Gorin, E. Preprints, ACS, Div. Petroleum Chem. 1966, C-130-148 Curran, G. P., Struck, R. T. and Gorin, E. Ind. Engng Chem. 1967,6,166 Wiser, W. H. and Hill, G. R. Proc, Symposium on Sci. and Tech. of Coal, Ottawa, 1967, Mines Branch, Dept. of Energy, Mines and Resources (Canada), pp 162-167 Wiser, W. H. Fuel 1968,47,475 Neavel, R. C., Fuel 1976,55,237 Van der Jagt, P. J., de Haan, H. K. and van Zanten, B. Tetrahidron 1971,77,3207 Braude, E. A., Brook, A. G. and Linstead, R. P. J. them. Sot. 1954, p 3569 Chakrabartty, S. K. and Berkowitz, N. Fuel 1974,53,240 Aczel, T., Martin, L. G., Maa, P. S. and Schlosberg, R. H. Fuel 1975,54,295 Hildebrand, J. H. and Scott, R. L. The Solubility of Nonelectrolytes, Dover, New York, 1964, Appendix I Hammett, L. P. Physical Organic Chemistry, 2nd edn, McGraw-Hill, 1970, p 257 Farcasiu, M., Mitchell, T. 0. and Whitehurst, D. D. Preprints of the 1976 Coal Chemistry Workshop, Stanford Research Institute, Menlo Park, California 94025 Wilson, T. P., Caflisch, E. G. and Hurley, G. F. J. phys. Chem. 1958,62,1059