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New method for elucidating the structures of coal
New method for elucidating the structures of coal Norman C. Deno, Barbara A. Greigger and Stephen G. Stroud Department of Chemistry, Pennsylvania State University, University Park, Pa. 16802, USA (Received 29 August 1977; revised 26 January 1978)
A mixture of 30% aqueous hydrogen peroxide (H202) and trifluoroacetic acid (TFA) dissolves coals to give colourless solutions. Depending on the nature of the coal, 9-55% of the hydrogen in the original coal is converted to simple aliphatic products. The aliphatic products preserve most of the original aliphatic structure of the coal. This technique of oxidative degradation is a powerful method of elucidating the structure of coal. It is particularly applicable to the aliphatic components of coal structure.
Much has been learned about the chemical structures in coals, but it is clear that there is still much more to be learned. The early infrared studies’ and their interpretations’>* showed that both aliphatic and aromatic hydrogen were present. This has been confirmed by l3C n.m.r. studies3, studies on BFg-phenol extracts4, studies on toluene extracts’, and reactivities towards fluorine6. The major problem was that there was no method for degrading coal into small fragments (MW below 400)3 that did not concomitantly alter or destroy much of the structure. We now introduce a technique of oxidative degradation that destroys the aromatic rings while leaving the aliphatic structure relatively untouched. This technique is so simple and direct that we recommend that it be immediately adopted as a general method for characterizing and cataloging the aliphatic structures in coals, lignites, lignins, etc.
practice to destroy peroxide before any measurements or further handling although explosion hazards largely arise only when concentrating peroxide solutions. The reaction products were analysed by three methods. The simplest was to measure the n.m.r. spectrum of the reaction mixture. These data (Tables 1 and 2) are presented in complete form for two illustrative examples, Illinois no. 6 and Pittsburgh Seam coals. The data in Table 3 are in a more selected form. The reaction mixtures were concentrated at 2 kPa in a rotary evaporator. In general this concentration was conducted at 90°C but when it was desired to determine the amount of propionic acid the concentration was conducted at 50°C. Blank experiments showed that over 90% of the propionic acid was retained if the rotary evaporations were conducted at 50°C. Methanol was added to the residue and the solutions refluxed for 2 h. The
EXPERIMENTAL Table I
The reagent is made by mixing 10 ml of 30% aqueous Hz02 and 8 ml of trifluoroacetic acid (TFA) and adding 8 ml of 96% H2SO4 slowly with cooling. A sample of 0.8 g of -20 U.S. mesh coal is added. There is some rise in temperature. With lignins and lignites, a colourless solution forms on stirring l-3 h. With bituminous coals, a colourless or near colourless solution forms on heating at reflux (8.5”) with stirring for 1-4 h. Completion of the reaction is indicated by the solution becoming colourless or near colourless. A small amount of sediment (3-6%) is left at the completion of the reaction. The runs with H202-TFA were identical except that the H2SO4 was omitted. The model compounds were added to the reagent over a period of 20 min in the belief that this slow addition more closely approached the conditions of dissolving coals. This rate of addition kept the temperature at 50-70°C owing to the exothermicity of the reaction. Faster addition gave somewhat different products. CAUTION: Excess peroxide must be destroyed by adding a small amount of 10% Pt on asbestos and waiting until 02 evolution ceases and a Kl test is negative. It should be the
N.m.r.
spectra of solutions of coal in H202-TFA-HzS04 H observed x 100 Organic H in coal
Band position (6,
porn)
1.88 2.00 2.18= 2.5-2.7 2.786 2.8-3.5 3.52 Total
H observed
c (wt %)C Organic H in coal d Moisture fwt 96) i c d
Illinois no. 6
Pittsburgh
0.7 0.7
_ -
6.1 13.4 3.0 -
0.9 0.5 4.4 1.8 1.1
23.9
8.7
70.8 4.3 17.7
79.6 5.2 1.5
Seam
Acetic acid Succinic acid wt % C in coal (as received) multiplied by the ratio of fwt of c0al)lfw-t of coal - wt of moisture) Wt % H (total) minus wt % H due to moisture
OOl6-2361/78/5708--0455802.00 0 1978 IPC Business Press
FUEL, 1978, Vol 57, August
455
New method
for elucidating
the structures of coal: N. C. Deno, 6. A. Greigger and S. G. Stroud
giving many properties including extensive elemental and petrographic analyses. The Illinois no. 6 Monterey coal, the North Dakota lignite, and the lignin were supplied by Dr Malvina Farcasiu and Dr Duayne Whitehurst of Central Research Division, Mobil Research and Development Corporation, P. 0. Box 1025, Princeton, N. J. We are grateful for their kind cooperation.
methyl esters were isolated as described7. The methyl esters were examined both by CC (gas chromatography) and by n.m.r. Some of the data in Table 3 result from these spectra and a complete n.m.r. on the methyl esters is given in Table 4 for Illinois no. 6 and Pittsburgh Seam coals. The n.m.r. spectra were taken on a 60 megacycle instrument and only hydrogen n.m.r. values have been measured up to now. The symbols m (multiplet), s (singlet), d (doublet), and t (triplet) are used in describing the splitting of the n.m.r. signals. Band positions are expressed in 6 which is the ppm measured downfield from the singlet of 2,2dimethyl-2-silapentane-5sulphonate. In the H202-TFA runs, the 6.6-7.5 S region was obscured by OH absorption. In the H202-TFA-H2S04 runs, the OH absorption was pushed downfield so that the whole O-9 6 region was clear and any aryl or vinyl hydrogens as well as all aliphatic hydrogens were clearly visible. Malonic acid and 1,2ethanediol were added as internal standards so that the absolute amount of hydrogen could be calculated. The Illinois no. 6 coal (Penn State no. PSOC-022) and Pittsburgh Seam coal (Penn State no. PSOC-110) were obtained from the Penn State/DOE Coal Sample Base, assembled under the direction of Dr William Spackman with the support of contracts successively from the Office of Coal Research, the Energy Research and Development Administration and the Department of Energy, currently under contract no. E(49-18)-2030. We are grateful to Dr Spackman and Mr Philip Dolsen for making the samples available. Computer print-outs are available for these coals
RESULTS AND DISCUSSION In view of the fact that the coals dissolved to give colourless or near colourless solutions, it is remarkable that 9-5570 of the original hydrogen (excluding hydrogen in H20) appears in products (Tables 1-3). Equally remarkable is that about three-fourths of this hydrogen is present in five simple aliphatic compounds. These are acetic, propionic, succinic, and glutaric acids and methanol. The n.m.r. analysis of the methyl esters of the products appears in Table 4. The dominance of dimethyl succinare confirms that succinic acid was a major product. Dimethyl oxalate is observed, which was expected since benzene itself gave some oxalic acid as a product’. The small number of
Tab/e 4 N.m.r. spectra (in CDCI,) products from Hz02-TFA-HzS04 two coals
of methyl esters of non-volatile and HZOZ-TFA oxidations of
Relative n.m.r.
Table 2 N.m.r. spectra of solutions of coal in HzOz-TFA
Band position
Ii observed
Band position Illinois no. 6
(6. ppm)
-
1.40 2.10a 2.2-2.4 2.62b 3.0-3.4 3.8-4.1 4.3-4.6 Total
i
Pittsburgh 1.4 2.2
17.8 2.2 6.3
1.5 1.6 -
4.0 2.2
2.0
1 .o H observed
Seam
1.0
9.7
33.5
Acetic acid Succinic acid
Hz02-TFA-H2S04
III. no. 6
(6)
x 100 Organic H in coal
6.82
-
3.B8a 3.83 3.78 3.J2b 3.67 3.45
9.5 10.6 25.4 8.6 5.9
3.37 2.62b
6.2 17.0
1.25m 0.9om Other bands
2.2 15
i
oxalate succinate
Dimethyl Dimethyl
band areas H202-TFA
Pitts. Seam
Ill. no. 6
Pitts. Seam
9.7 5.6 5.8 26.7 9.3
7.4 4.8 4.5 34.5 6.0 -
2.1 5.7 10.4 6.9 28.0 8.2 4.1
-
3.8
8.2
5.6
18.5 2.9 9
15.9 6.4 12
21.5 5.6 2.8
7
Tab/e 3 Yields of H appearing in products from coal plus H202-TFA-H+04 C (wt %, dry basis)
Substrate
Organic H in coala (wt %I
Ash (wt %, dry basis)
(Wt %I yield of H appearing Moisture (wt %I
Acetic acid
Propionic acid
Succinic acid
in: Glutaric acid
N.m.r. (I .88Methanol
3.52
Pittsburgh Seam coal Illinois no. 6 coal Illinois no. 6 coal
79.6
5.2 4.3
6.1 8.4
1.5
0.9
0.4
4.4
0.5
0
70.8
17.7
6.1
0.7
13.4
2.2
0
8.7 23.9
(Monterey) Lignite, North
69.7
4.4
10.8
12.8
3.2
c
10.2
c
0
20.6
Dakota
65.3
2.8
8.4
35.7
4.2
c
6.0
C
16.2
54.9
z
See footnotes
c and din
6jb
Table 1
Since there was no H observed outside this range, this represents the total H observed relative to the total organic H in the sample (moisture is excluded) multiplied by 100 to convert to percent ’ Not determined
466
FUEL,
1978,
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New method for elucidating the structures of coal: N. C. Deno, B. A. Greigger and S. G. Stroud Table 5 Model compounds
that produce
succinic acid on oxidation
with HzOz-TFA-H2S04 Yielda
Substrate 1,2-Diphenylethane H,02-TFA-H,S04 H202-TFA O,lO-Dihydrophenanthrene H,Oz--TFA-HzS04 H#-TFA Acenaphthene Hz02-TFA-HzS04 Hz02-TFA lndan Hz02-TFA-H2S04 H202-TFA 5lndanol H202-TFA-H2S04 Smethoxyindan H202-TFA-H2S04 4,J-Dimethylindan H202-TFA-H2S04
Succinic acid
Acetic acid
Table 7 Model compounds that produce largely products other than acetic or succinic acids on oxidation with HzOz-TFA-HzS04
(%)
Glutaric acid
Malonic acid
N.m.r. Substrate Ethylbenzene
73 61
0 0
0 18
0
0
12
23
0
0 0
0 0
64 60
9 0
0 0
30
0
36
0
b
0
b
0
42
50
60a
that produce
N.m.r.
Toluene 1,2_dimethylbenzene 1,3-dimethylbenzene 1 ,Cdimethylbenzene 2-methylnaphthalene
Acetic acid
14
19
1 .oot 1.63m
Butyric
84
73
7
8
acid
Propionic
acid
2.27
Acetic acid
1.28d 2.4-2.9
lsobutyric
acid
5
10
62
37
21
13
Acetic acid 87 55 82 83 83
2.4-2.9
b
2.27s
Acetic acid
17
48
Butyric acid Propionic acid Acetic acid
81 8 6
JO
Butyric acid Propionic acid Acetic acid
85 6 6
71 7 12
60
4-Propylphenol
0
2-Propylphenol
10 12
0
0
Benzyl alcohol
O-8
Benz@ acetate
1.65r-n 2.27s
c Acetic acid
10 90
1.651n 2.27s
c Acetic acid
8 84
60
100
100
100
90
I-Phenylethanol
Substrate
71
86
1.36d
a Calculated on the basis that one mol of substrate formed one mol of product except for 4,J-dimethylindan. In this case, the basis was that 2 mol of acetic form from one mol of 4,Jdimethylindan b The molar ratio of glutaric acid to succinic acid to methanol was 1.12 to 1.17 to 1, The % yield was not determined
Table 6 Model compounds with Hz02--TFA-HzS04
acid
(“/.I
2.27s
1.22r
Isopropylbenzene 27 13
Propionic
Yielda
2.5Jq
0 0
0 23
1.22r
Relative area
2.50t
11 0
27 27
Identification
2.579 Propylbenzene
71
bands
6
-
acetic acid on oxidation
band area (%) Yield of acetic acid 1.6ag6 13 =25 18
17 17
Other
(56)
0 18b 0
68 46c 66C
0 0
66C d
a
This n.m.r. multiplet at 1.656 was a minor product with all methylbenzenes. It has not been positively identified b A singlet at 2.076 c On the basis that one mol of substrate forms two mol of acetic acid d This was not determined
bands in both Tcbles 1 and 4 shows that relatively few products are formed. The ultimate identification of virtually all of these seems feasible. The data in Tables 1-4 can be interpreted on the basis of model studies, Tables 5- 7. In the model studies it was found that addition of H2SO4 (equal to the TFA used) had an enormously simplifying effect on some of the H202TFA oxidations. This effect was most pronounced with models which produced succinic acid, Table 5. The acetic acid, which is such a prominent product in the H202-TFA oxidations, is completely eliminated in H202-TFA-H2S04 oxidations. With the models in Tables 6 and 7, the effect of H2SO4 was small or negligible. However, the products with H2SO4 were always at least as simple as without H2SO4, and the I l2SO4 technique was adopted. The early data using just
3-Phenyl-1-propanol2.0-3.0 4.62t
Butyrolactone
Methoxybenzene (anisole)
4.05s
Methanold
Diphenylmethane
3.72s
Malonic acid
1,3-Diphenylpropane
1 .J-2.7 2.87 3.72 1.86m 2.48m
Glutaric acid Succinic acid Malonic acid Cyclohexene-1.2. dicarboxylic anhydride
88 8 4 100
e e e
5,6,7,8-Tetrahydro2-naphthol
1.86m 2.4&n
Cyclohexene-1.2. dicarboxylic anhydride
100
e
9,10-Dihydroanthracene
3.72s
Malonic acid
2-Phenylbenzoic acid
8.35m
Phthalic anhydride
1,2,3,4-Tetrahydronaphthalene (tetralin)
9
14
Jlf
71 100
36 e
g, h i
On the basis that one mol of substrate gave one mol of product Some type of isopropyl species, possibly isopropylmaleic acid 2 See footnote a in Tab/e 6 This was an equilibrium mixture of 95 parts methyl trifluoroacetate (4.05s) and 5 parts methanol (3.80s) T Not determined A 59% yield of crystalline product was isolated, m.p. 69-72°C (lit. 72”C, R. E. Buckles, Chem. Revs. 1957. 57,641). The n.m.r. spectrum was identical to that reported in the Sadtler indices g Cyclohexylbenzene and fert-butylbenzene were studied, but only in H202-TFA. They gave predominantly cyclohexanecarboxylic acid and 2,2-dimethylpropionic acid respectively. Presumably, these same products would predominate in HzOz-TFA-HzS04 oxidations h Benzoic acid, nitrobenzene, and benzenesulphonic acid were inert to H202-TFA-HzS04 at 85°C
FUEL,
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New method for elucidating the structures of coal: N. C. Deno, B. A. Greigger and S. G. Stroud
H202-TFA without H2SO4 appear in the MS Thesis of S. G. Stroud. The generalization can be made that H202-TFA-H2S04 oxidation of alkyl benzenes leads to degradation of the benzene ring to a carboxyl group and over 70% preservation of the aliphatic structure. This generalization is true for alkyl bridges as well as simple alkyl substituents. This generalization is less valid for H202-TFA oxidations. The data are summarized in Tables 5- 7. Before the work reported herein, much work had been done on the H202-TFA hydroxylation of alkanes and alkyl chains’. The generalizations emerged that all alkanes are hydroxylated. Alkyl chains could be hydroxylated, but only on positions remote from alcohol or carboxyl groups. The deactivating effect of electronegative groups was evident on carbons at positions 5 and 6 relative to the electronegative substituents. Closer hydrogen atoms were inert. Primary and secondary alcohols were inert to further oxidation. A more complex behaviour was shown by branched alkanes and tertiary alcohols. AIthough these generalizations were shown for H202TFA oxidations, it is believed that they apply to the H202TFA-H2S04 oxidations. It was specifically shown that acetic, propionic, butyric, succinic, and glutaric acids were inert to H202-TFA-H2S04. Further, the formation of the products in Tables 5- 7 in high yields attests to their inertness towards H202-TFA-H2S04. A borderline case is 3-methylbutanoic acid which was 30% oxidized in 3 h at 85°C. Attention is called to the fact that minor products such as acetic and propionic acid from propylbenzene do not arise by degradation of the major product, butyric acid. We can now turn to a discussion of how the results affect the picture of coal structure. It is likely that the acetic acid arises largely from methyl groups attached to benzene rings. Since the yield of acetic acid from methyl benzenes was 66-68% (Table 6), the percentage of hydrogen in a coal due to arylmethyl can be estimated by multiplying the values in Tubles 1 and 3 by 1.5. These estimates range from I .4% for Pittsburgh Seam coal to 9.2% for Illinois no. 6. This wide range emphasizes the varied nature of coal structures. These estimates are very reliable as an upper limit. They are somewhat less reliable as an absolute value because some acetic acid could have arisen from (Ymethylbenzyl alcohol structures as shown by the model compound 1-phenylethanol (Table 6). In any event, the above measurement and calculation is far and away the best method yet devised for analysing coals for arylmethyl. The estimate of arylethyl is even more reliable since propionic acid arises solely from arylethyl. Since a 71% yield of propionic acid was obtained from ethylbenzene, to estimate the percentage of arylethyl, the yield of propionic acid in Table 3 should be multiplied by 100/71. These estimates assume that hydroxy, alkoxy, and alkyl substituents on the benzene or aryl rings will not significantly affect the yield of propionic acid from arylethyl. This would seem to be so, judging by the fact that such substituents did not affect the yield of acetic acid from xylenes, the yield of succinic acid from indans with substituents on the aryl ring, and the yield of butyric acid from propylphenols. As for higher simple alkyl groups, the absence of isobutyric acid rules out isopropyl. The reports of isopropyl4 would seem to be an artifact of the Friedel-Crafts reaction conditions. Propyl, butyl, and other higher alkyl groups are ruled out because of the absence of n.m.r. bands in the
458
FUEL, 1978, Vol 57, August
0.8-1.2 region. Perhaps it is appropriate to emphasize at this point that the data apply only to components in amounts over 0.5%. Higher sensitivity will be readily available through CC analysis, but at present emphasis is placed on the major components. Of the five products in Table 3, methanol shows the greatest variation, being absent in the products of oxidation from typical bituminous coals, but a dominant component from the lignite. Lignite is often considered to represent an early stage in the metamorphic process that produces bituminous coals. If this is so, it is remarkable that coalification is so efficient in removing methoxyl completely. This problem may require reconsideration. Since primary and secondary aliphatic alcohols are inert, the absence of ethanol and higher alcohols in the oxidation products is firm evidence that methoxy is the only alkoxy group present. The appearance of glutaric acid arises either from 1,3diarylpropane or indan structures. There is also the possibility that it could arise from tetralin structures substituted on the l-position, and model experiments on this type of structure are in progress. Succinic acid is the dominant product from H202--TFAH2SO4 oxidations of bituminous coals. It is amazing to us that this one compound accounts for 4.4- 13.4% of the total H in the four coals studied. If allowance is made for the fact that the maximum yield of succinic acid was only 73% in ArCH2CHxAr models and 27-42% in indans, it is apparent that structures generating succinic acid could account for much more than 4.4-13.4% of the H in the original coals. The succinic acid can arise from ArCHzCHzAr and indans. It is suspected that it might also arise from -CH2CH$HOHand -CH$H$HRsystems. Model studies are in progress. Two structural types can probably be eliminated. The failure to observe cyclohexene-1,2-dicarboxylic anhydride mitigates against tetralin structures which are unsubstituted on the aliphatic ring. The absence of benzoic acid or any other aryl H mitigates against aryl carboxyl groups in at least the higher-rank original coals. Such groups should have survived as some kind of substituted benzoic acid unless the aryl ring was heavily alkylated or was hydroxylated. There are several clear paths for the future of this work. Foremost is to improve the yields from the 60-70% range to >90%. Next is to pursue model studies on tetralins and indans substituted on the aliphatic (alicyclic) ring. The minor products of oxidation need identification. Finally the technique after optimization can be applied to the cataloging of the vast variety of coals.
ACKNOWLEDGEMENT This work was supported in part by a grant from the National Science Foundation. We are grateful for this support. We owe special thanks to the Fats and Proteins Research Foundation and Dr Werner Boehme, their Executive Director, because the present work was a spin-off from work they supported on hydroxylation of fatty acids. Our gratitude to the Penn State/DOE Coal Sample Base and the Central Research Division of Mobil was acknowledged in the Experimental Section. We owe special thanks to Professor Peter H. Given (College of Mineral Industries, Penn State) for a truly careful review of this paper.
New method for elucidating the structures of coal: N. C. Deno, B. A. Greigger and S. G. Stroud
REFERENCES
5
1 Brown, J. K.J. Chem. Sot. 1955,744;Browq J. K., Ladner, W. R. and Sheppard, N. Fuel 1960,39,79 2 Given, P. H. Fuel 1960,39,147; 1961,40,427 3 VanderHart, D. L. and Retcofsky, H. L. Fuel 1976,55, 202 and references therein 4 Heredy, L. A., Kostzo, A. E. and Neuworth, M. B. Fuel 1965, 44, 125
6 7 8 9
Bartle, K. D., Martin, T. G. and Williams, D. F. Fuel 1975,54, 226 Huston, J., Scott, R. G. and Studier, M. II. Fuel 1976,55, 281 Durham, L. J., McLeod, D. J. and Cason, J. Org. Syn., Coil. Vol IV, 1962, p 635 Deno, N. C., Greigger, B. A., Messer, L. A., Meyer, M. D. and Stroud, S. G. Tetrahedron Lett. 1977, 1703 Deno, N. C., Jedziniak, E. J., Messer, L. A., Meyer, M. D., Stroud, S. G. and Tomezsko, E. S. Tetrahedron 1977, 33, 2503
FUEL, 1978, Vol 57, August
459
A mixture of 30% aqueous hydrogen peroxide (H202) and trifluoroacetic acid (TFA) dissolves coals to give colourless solutions. Depending on the nature of the coal, 9-55% of the hydrogen in the original coal is converted to simple aliphatic products. The aliphatic products preserve most of the original aliphatic structure of the coal. This technique of oxidative degradation is a powerful method of elucidating the structure of coal. It is particularly applicable to the aliphatic components of coal structure.
Much has been learned about the chemical structures in coals, but it is clear that there is still much more to be learned. The early infrared studies’ and their interpretations’>* showed that both aliphatic and aromatic hydrogen were present. This has been confirmed by l3C n.m.r. studies3, studies on BFg-phenol extracts4, studies on toluene extracts’, and reactivities towards fluorine6. The major problem was that there was no method for degrading coal into small fragments (MW below 400)3 that did not concomitantly alter or destroy much of the structure. We now introduce a technique of oxidative degradation that destroys the aromatic rings while leaving the aliphatic structure relatively untouched. This technique is so simple and direct that we recommend that it be immediately adopted as a general method for characterizing and cataloging the aliphatic structures in coals, lignites, lignins, etc.
practice to destroy peroxide before any measurements or further handling although explosion hazards largely arise only when concentrating peroxide solutions. The reaction products were analysed by three methods. The simplest was to measure the n.m.r. spectrum of the reaction mixture. These data (Tables 1 and 2) are presented in complete form for two illustrative examples, Illinois no. 6 and Pittsburgh Seam coals. The data in Table 3 are in a more selected form. The reaction mixtures were concentrated at 2 kPa in a rotary evaporator. In general this concentration was conducted at 90°C but when it was desired to determine the amount of propionic acid the concentration was conducted at 50°C. Blank experiments showed that over 90% of the propionic acid was retained if the rotary evaporations were conducted at 50°C. Methanol was added to the residue and the solutions refluxed for 2 h. The
EXPERIMENTAL Table I
The reagent is made by mixing 10 ml of 30% aqueous Hz02 and 8 ml of trifluoroacetic acid (TFA) and adding 8 ml of 96% H2SO4 slowly with cooling. A sample of 0.8 g of -20 U.S. mesh coal is added. There is some rise in temperature. With lignins and lignites, a colourless solution forms on stirring l-3 h. With bituminous coals, a colourless or near colourless solution forms on heating at reflux (8.5”) with stirring for 1-4 h. Completion of the reaction is indicated by the solution becoming colourless or near colourless. A small amount of sediment (3-6%) is left at the completion of the reaction. The runs with H202-TFA were identical except that the H2SO4 was omitted. The model compounds were added to the reagent over a period of 20 min in the belief that this slow addition more closely approached the conditions of dissolving coals. This rate of addition kept the temperature at 50-70°C owing to the exothermicity of the reaction. Faster addition gave somewhat different products. CAUTION: Excess peroxide must be destroyed by adding a small amount of 10% Pt on asbestos and waiting until 02 evolution ceases and a Kl test is negative. It should be the
N.m.r.
spectra of solutions of coal in H202-TFA-HzS04 H observed x 100 Organic H in coal
Band position (6,
porn)
1.88 2.00 2.18= 2.5-2.7 2.786 2.8-3.5 3.52 Total
H observed
c (wt %)C Organic H in coal d Moisture fwt 96) i c d
Illinois no. 6
Pittsburgh
0.7 0.7
_ -
6.1 13.4 3.0 -
0.9 0.5 4.4 1.8 1.1
23.9
8.7
70.8 4.3 17.7
79.6 5.2 1.5
Seam
Acetic acid Succinic acid wt % C in coal (as received) multiplied by the ratio of fwt of c0al)lfw-t of coal - wt of moisture) Wt % H (total) minus wt % H due to moisture
OOl6-2361/78/5708--0455802.00 0 1978 IPC Business Press
FUEL, 1978, Vol 57, August
455
New method
for elucidating
the structures of coal: N. C. Deno, 6. A. Greigger and S. G. Stroud
giving many properties including extensive elemental and petrographic analyses. The Illinois no. 6 Monterey coal, the North Dakota lignite, and the lignin were supplied by Dr Malvina Farcasiu and Dr Duayne Whitehurst of Central Research Division, Mobil Research and Development Corporation, P. 0. Box 1025, Princeton, N. J. We are grateful for their kind cooperation.
methyl esters were isolated as described7. The methyl esters were examined both by CC (gas chromatography) and by n.m.r. Some of the data in Table 3 result from these spectra and a complete n.m.r. on the methyl esters is given in Table 4 for Illinois no. 6 and Pittsburgh Seam coals. The n.m.r. spectra were taken on a 60 megacycle instrument and only hydrogen n.m.r. values have been measured up to now. The symbols m (multiplet), s (singlet), d (doublet), and t (triplet) are used in describing the splitting of the n.m.r. signals. Band positions are expressed in 6 which is the ppm measured downfield from the singlet of 2,2dimethyl-2-silapentane-5sulphonate. In the H202-TFA runs, the 6.6-7.5 S region was obscured by OH absorption. In the H202-TFA-H2S04 runs, the OH absorption was pushed downfield so that the whole O-9 6 region was clear and any aryl or vinyl hydrogens as well as all aliphatic hydrogens were clearly visible. Malonic acid and 1,2ethanediol were added as internal standards so that the absolute amount of hydrogen could be calculated. The Illinois no. 6 coal (Penn State no. PSOC-022) and Pittsburgh Seam coal (Penn State no. PSOC-110) were obtained from the Penn State/DOE Coal Sample Base, assembled under the direction of Dr William Spackman with the support of contracts successively from the Office of Coal Research, the Energy Research and Development Administration and the Department of Energy, currently under contract no. E(49-18)-2030. We are grateful to Dr Spackman and Mr Philip Dolsen for making the samples available. Computer print-outs are available for these coals
RESULTS AND DISCUSSION In view of the fact that the coals dissolved to give colourless or near colourless solutions, it is remarkable that 9-5570 of the original hydrogen (excluding hydrogen in H20) appears in products (Tables 1-3). Equally remarkable is that about three-fourths of this hydrogen is present in five simple aliphatic compounds. These are acetic, propionic, succinic, and glutaric acids and methanol. The n.m.r. analysis of the methyl esters of the products appears in Table 4. The dominance of dimethyl succinare confirms that succinic acid was a major product. Dimethyl oxalate is observed, which was expected since benzene itself gave some oxalic acid as a product’. The small number of
Tab/e 4 N.m.r. spectra (in CDCI,) products from Hz02-TFA-HzS04 two coals
of methyl esters of non-volatile and HZOZ-TFA oxidations of
Relative n.m.r.
Table 2 N.m.r. spectra of solutions of coal in HzOz-TFA
Band position
Ii observed
Band position Illinois no. 6
(6. ppm)
-
1.40 2.10a 2.2-2.4 2.62b 3.0-3.4 3.8-4.1 4.3-4.6 Total
i
Pittsburgh 1.4 2.2
17.8 2.2 6.3
1.5 1.6 -
4.0 2.2
2.0
1 .o H observed
Seam
1.0
9.7
33.5
Acetic acid Succinic acid
Hz02-TFA-H2S04
III. no. 6
(6)
x 100 Organic H in coal
6.82
-
3.B8a 3.83 3.78 3.J2b 3.67 3.45
9.5 10.6 25.4 8.6 5.9
3.37 2.62b
6.2 17.0
1.25m 0.9om Other bands
2.2 15
i
oxalate succinate
Dimethyl Dimethyl
band areas H202-TFA
Pitts. Seam
Ill. no. 6
Pitts. Seam
9.7 5.6 5.8 26.7 9.3
7.4 4.8 4.5 34.5 6.0 -
2.1 5.7 10.4 6.9 28.0 8.2 4.1
-
3.8
8.2
5.6
18.5 2.9 9
15.9 6.4 12
21.5 5.6 2.8
7
Tab/e 3 Yields of H appearing in products from coal plus H202-TFA-H+04 C (wt %, dry basis)
Substrate
Organic H in coala (wt %I
Ash (wt %, dry basis)
(Wt %I yield of H appearing Moisture (wt %I
Acetic acid
Propionic acid
Succinic acid
in: Glutaric acid
N.m.r. (I .88Methanol
3.52
Pittsburgh Seam coal Illinois no. 6 coal Illinois no. 6 coal
79.6
5.2 4.3
6.1 8.4
1.5
0.9
0.4
4.4
0.5
0
70.8
17.7
6.1
0.7
13.4
2.2
0
8.7 23.9
(Monterey) Lignite, North
69.7
4.4
10.8
12.8
3.2
c
10.2
c
0
20.6
Dakota
65.3
2.8
8.4
35.7
4.2
c
6.0
C
16.2
54.9
z
See footnotes
c and din
6jb
Table 1
Since there was no H observed outside this range, this represents the total H observed relative to the total organic H in the sample (moisture is excluded) multiplied by 100 to convert to percent ’ Not determined
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New method for elucidating the structures of coal: N. C. Deno, B. A. Greigger and S. G. Stroud Table 5 Model compounds
that produce
succinic acid on oxidation
with HzOz-TFA-H2S04 Yielda
Substrate 1,2-Diphenylethane H,02-TFA-H,S04 H202-TFA O,lO-Dihydrophenanthrene H,Oz--TFA-HzS04 H#-TFA Acenaphthene Hz02-TFA-HzS04 Hz02-TFA lndan Hz02-TFA-H2S04 H202-TFA 5lndanol H202-TFA-H2S04 Smethoxyindan H202-TFA-H2S04 4,J-Dimethylindan H202-TFA-H2S04
Succinic acid
Acetic acid
Table 7 Model compounds that produce largely products other than acetic or succinic acids on oxidation with HzOz-TFA-HzS04
(%)
Glutaric acid
Malonic acid
N.m.r. Substrate Ethylbenzene
73 61
0 0
0 18
0
0
12
23
0
0 0
0 0
64 60
9 0
0 0
30
0
36
0
b
0
b
0
42
50
60a
that produce
N.m.r.
Toluene 1,2_dimethylbenzene 1,3-dimethylbenzene 1 ,Cdimethylbenzene 2-methylnaphthalene
Acetic acid
14
19
1 .oot 1.63m
Butyric
84
73
7
8
acid
Propionic
acid
2.27
Acetic acid
1.28d 2.4-2.9
lsobutyric
acid
5
10
62
37
21
13
Acetic acid 87 55 82 83 83
2.4-2.9
b
2.27s
Acetic acid
17
48
Butyric acid Propionic acid Acetic acid
81 8 6
JO
Butyric acid Propionic acid Acetic acid
85 6 6
71 7 12
60
4-Propylphenol
0
2-Propylphenol
10 12
0
0
Benzyl alcohol
O-8
Benz@ acetate
1.65r-n 2.27s
c Acetic acid
10 90
1.651n 2.27s
c Acetic acid
8 84
60
100
100
100
90
I-Phenylethanol
Substrate
71
86
1.36d
a Calculated on the basis that one mol of substrate formed one mol of product except for 4,J-dimethylindan. In this case, the basis was that 2 mol of acetic form from one mol of 4,Jdimethylindan b The molar ratio of glutaric acid to succinic acid to methanol was 1.12 to 1.17 to 1, The % yield was not determined
Table 6 Model compounds with Hz02--TFA-HzS04
acid
(“/.I
2.27s
1.22r
Isopropylbenzene 27 13
Propionic
Yielda
2.5Jq
0 0
0 23
1.22r
Relative area
2.50t
11 0
27 27
Identification
2.579 Propylbenzene
71
bands
6
-
acetic acid on oxidation
band area (%) Yield of acetic acid 1.6ag6 13 =25 18
17 17
Other
(56)
0 18b 0
68 46c 66C
0 0
66C d
a
This n.m.r. multiplet at 1.656 was a minor product with all methylbenzenes. It has not been positively identified b A singlet at 2.076 c On the basis that one mol of substrate forms two mol of acetic acid d This was not determined
bands in both Tcbles 1 and 4 shows that relatively few products are formed. The ultimate identification of virtually all of these seems feasible. The data in Tables 1-4 can be interpreted on the basis of model studies, Tables 5- 7. In the model studies it was found that addition of H2SO4 (equal to the TFA used) had an enormously simplifying effect on some of the H202TFA oxidations. This effect was most pronounced with models which produced succinic acid, Table 5. The acetic acid, which is such a prominent product in the H202-TFA oxidations, is completely eliminated in H202-TFA-H2S04 oxidations. With the models in Tables 6 and 7, the effect of H2SO4 was small or negligible. However, the products with H2SO4 were always at least as simple as without H2SO4, and the I l2SO4 technique was adopted. The early data using just
3-Phenyl-1-propanol2.0-3.0 4.62t
Butyrolactone
Methoxybenzene (anisole)
4.05s
Methanold
Diphenylmethane
3.72s
Malonic acid
1,3-Diphenylpropane
1 .J-2.7 2.87 3.72 1.86m 2.48m
Glutaric acid Succinic acid Malonic acid Cyclohexene-1.2. dicarboxylic anhydride
88 8 4 100
e e e
5,6,7,8-Tetrahydro2-naphthol
1.86m 2.4&n
Cyclohexene-1.2. dicarboxylic anhydride
100
e
9,10-Dihydroanthracene
3.72s
Malonic acid
2-Phenylbenzoic acid
8.35m
Phthalic anhydride
1,2,3,4-Tetrahydronaphthalene (tetralin)
9
14
Jlf
71 100
36 e
g, h i
On the basis that one mol of substrate gave one mol of product Some type of isopropyl species, possibly isopropylmaleic acid 2 See footnote a in Tab/e 6 This was an equilibrium mixture of 95 parts methyl trifluoroacetate (4.05s) and 5 parts methanol (3.80s) T Not determined A 59% yield of crystalline product was isolated, m.p. 69-72°C (lit. 72”C, R. E. Buckles, Chem. Revs. 1957. 57,641). The n.m.r. spectrum was identical to that reported in the Sadtler indices g Cyclohexylbenzene and fert-butylbenzene were studied, but only in H202-TFA. They gave predominantly cyclohexanecarboxylic acid and 2,2-dimethylpropionic acid respectively. Presumably, these same products would predominate in HzOz-TFA-HzS04 oxidations h Benzoic acid, nitrobenzene, and benzenesulphonic acid were inert to H202-TFA-HzS04 at 85°C
FUEL,
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457
New method for elucidating the structures of coal: N. C. Deno, B. A. Greigger and S. G. Stroud
H202-TFA without H2SO4 appear in the MS Thesis of S. G. Stroud. The generalization can be made that H202-TFA-H2S04 oxidation of alkyl benzenes leads to degradation of the benzene ring to a carboxyl group and over 70% preservation of the aliphatic structure. This generalization is true for alkyl bridges as well as simple alkyl substituents. This generalization is less valid for H202-TFA oxidations. The data are summarized in Tables 5- 7. Before the work reported herein, much work had been done on the H202-TFA hydroxylation of alkanes and alkyl chains’. The generalizations emerged that all alkanes are hydroxylated. Alkyl chains could be hydroxylated, but only on positions remote from alcohol or carboxyl groups. The deactivating effect of electronegative groups was evident on carbons at positions 5 and 6 relative to the electronegative substituents. Closer hydrogen atoms were inert. Primary and secondary alcohols were inert to further oxidation. A more complex behaviour was shown by branched alkanes and tertiary alcohols. AIthough these generalizations were shown for H202TFA oxidations, it is believed that they apply to the H202TFA-H2S04 oxidations. It was specifically shown that acetic, propionic, butyric, succinic, and glutaric acids were inert to H202-TFA-H2S04. Further, the formation of the products in Tables 5- 7 in high yields attests to their inertness towards H202-TFA-H2S04. A borderline case is 3-methylbutanoic acid which was 30% oxidized in 3 h at 85°C. Attention is called to the fact that minor products such as acetic and propionic acid from propylbenzene do not arise by degradation of the major product, butyric acid. We can now turn to a discussion of how the results affect the picture of coal structure. It is likely that the acetic acid arises largely from methyl groups attached to benzene rings. Since the yield of acetic acid from methyl benzenes was 66-68% (Table 6), the percentage of hydrogen in a coal due to arylmethyl can be estimated by multiplying the values in Tubles 1 and 3 by 1.5. These estimates range from I .4% for Pittsburgh Seam coal to 9.2% for Illinois no. 6. This wide range emphasizes the varied nature of coal structures. These estimates are very reliable as an upper limit. They are somewhat less reliable as an absolute value because some acetic acid could have arisen from (Ymethylbenzyl alcohol structures as shown by the model compound 1-phenylethanol (Table 6). In any event, the above measurement and calculation is far and away the best method yet devised for analysing coals for arylmethyl. The estimate of arylethyl is even more reliable since propionic acid arises solely from arylethyl. Since a 71% yield of propionic acid was obtained from ethylbenzene, to estimate the percentage of arylethyl, the yield of propionic acid in Table 3 should be multiplied by 100/71. These estimates assume that hydroxy, alkoxy, and alkyl substituents on the benzene or aryl rings will not significantly affect the yield of propionic acid from arylethyl. This would seem to be so, judging by the fact that such substituents did not affect the yield of acetic acid from xylenes, the yield of succinic acid from indans with substituents on the aryl ring, and the yield of butyric acid from propylphenols. As for higher simple alkyl groups, the absence of isobutyric acid rules out isopropyl. The reports of isopropyl4 would seem to be an artifact of the Friedel-Crafts reaction conditions. Propyl, butyl, and other higher alkyl groups are ruled out because of the absence of n.m.r. bands in the
458
FUEL, 1978, Vol 57, August
0.8-1.2 region. Perhaps it is appropriate to emphasize at this point that the data apply only to components in amounts over 0.5%. Higher sensitivity will be readily available through CC analysis, but at present emphasis is placed on the major components. Of the five products in Table 3, methanol shows the greatest variation, being absent in the products of oxidation from typical bituminous coals, but a dominant component from the lignite. Lignite is often considered to represent an early stage in the metamorphic process that produces bituminous coals. If this is so, it is remarkable that coalification is so efficient in removing methoxyl completely. This problem may require reconsideration. Since primary and secondary aliphatic alcohols are inert, the absence of ethanol and higher alcohols in the oxidation products is firm evidence that methoxy is the only alkoxy group present. The appearance of glutaric acid arises either from 1,3diarylpropane or indan structures. There is also the possibility that it could arise from tetralin structures substituted on the l-position, and model experiments on this type of structure are in progress. Succinic acid is the dominant product from H202--TFAH2SO4 oxidations of bituminous coals. It is amazing to us that this one compound accounts for 4.4- 13.4% of the total H in the four coals studied. If allowance is made for the fact that the maximum yield of succinic acid was only 73% in ArCH2CHxAr models and 27-42% in indans, it is apparent that structures generating succinic acid could account for much more than 4.4-13.4% of the H in the original coals. The succinic acid can arise from ArCHzCHzAr and indans. It is suspected that it might also arise from -CH2CH$HOHand -CH$H$HRsystems. Model studies are in progress. Two structural types can probably be eliminated. The failure to observe cyclohexene-1,2-dicarboxylic anhydride mitigates against tetralin structures which are unsubstituted on the aliphatic ring. The absence of benzoic acid or any other aryl H mitigates against aryl carboxyl groups in at least the higher-rank original coals. Such groups should have survived as some kind of substituted benzoic acid unless the aryl ring was heavily alkylated or was hydroxylated. There are several clear paths for the future of this work. Foremost is to improve the yields from the 60-70% range to >90%. Next is to pursue model studies on tetralins and indans substituted on the aliphatic (alicyclic) ring. The minor products of oxidation need identification. Finally the technique after optimization can be applied to the cataloging of the vast variety of coals.
ACKNOWLEDGEMENT This work was supported in part by a grant from the National Science Foundation. We are grateful for this support. We owe special thanks to the Fats and Proteins Research Foundation and Dr Werner Boehme, their Executive Director, because the present work was a spin-off from work they supported on hydroxylation of fatty acids. Our gratitude to the Penn State/DOE Coal Sample Base and the Central Research Division of Mobil was acknowledged in the Experimental Section. We owe special thanks to Professor Peter H. Given (College of Mineral Industries, Penn State) for a truly careful review of this paper.
New method for elucidating the structures of coal: N. C. Deno, B. A. Greigger and S. G. Stroud
REFERENCES
5
1 Brown, J. K.J. Chem. Sot. 1955,744;Browq J. K., Ladner, W. R. and Sheppard, N. Fuel 1960,39,79 2 Given, P. H. Fuel 1960,39,147; 1961,40,427 3 VanderHart, D. L. and Retcofsky, H. L. Fuel 1976,55, 202 and references therein 4 Heredy, L. A., Kostzo, A. E. and Neuworth, M. B. Fuel 1965, 44, 125
6 7 8 9
Bartle, K. D., Martin, T. G. and Williams, D. F. Fuel 1975,54, 226 Huston, J., Scott, R. G. and Studier, M. II. Fuel 1976,55, 281 Durham, L. J., McLeod, D. J. and Cason, J. Org. Syn., Coil. Vol IV, 1962, p 635 Deno, N. C., Greigger, B. A., Messer, L. A., Meyer, M. D. and Stroud, S. G. Tetrahedron Lett. 1977, 1703 Deno, N. C., Jedziniak, E. J., Messer, L. A., Meyer, M. D., Stroud, S. G. and Tomezsko, E. S. Tetrahedron 1977, 33, 2503
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