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Action of solvents on coal at low temperatures
Action of solvents on coal at low temperatures 1. Low-rank
coals
Bert van Bodegom, J. A. Rob van Veen, Gerard M. M. van Kessel, Mieke W. A. Sinnige-Nijssen and Hans C. M. Stuiver KoninkhjkelSheil- Laboratorium, Amsterdam, (Shell Research B. V.), Postbus 3003, Amsterdam, The Netherlands (Received 23 December 7982; revised 5 February 7983) The dissolution behaviour of brown coals (67-75% C, daf) in pyridine, primary amines and aqueous KOH has been studied. The solubility in the last two solvents greatly depends on temperature, but in the first it is relatively temperature-independent. Pretreatment of the brown coals with aqueous HCI or with sodium ethanolate in ethanol leads to enhanced solubility. It is concluded that ester-bond breaking is necessary before extensive dissolution can take place. The solubility of brown coals in amines and aqueous KOH is found to increase with increasing carboxylic-acid group concentration in the coal. The solubility of Morwell brown coal in n-alkylamines at 180°C increases with increasing length of the alkylchain in the solvent. The class of good solvents for brown coals is restricted to strong bases, because: 1. ester bonds have to be broken, 2. the acidic coal fragments have to be solubilized. Because of their capacity to break ester bonds these are so-called reactive solvents. Complete solvent recovery is impossible in the case of amines. (Keywords:
coal; solvents;
low temperatures)
In the past, solvent extraction was used mainly as a method for: 1. isolating a hypothetical coking principle (i.e., substances then thought to be responsible for the caking properties); 2. producing materials of potential industrial value (waxes, resins); and 3. studying the constitution of coal. The very extensive literature on this subject has been reviewed by Kiebler’, Dryden’, KrGger3, Van Krevelen4, Davidson’ and Wender et aL6. The present work falls into the third category, but the focus is on the basic chemistry of the coal conversion process, and therefore on those aspects which relate to the coal dissolution process itself. No detailed knowledge is sought of the molecular structure of coal, but only of the parameters that determine the dissolution of coal. A few models of the coal-dissolution process exist, but most refer mainly to bituminous coals. Two models (viz those of Dryden’ and Van Krevelen’) are of a more general nature. According to Dryden’ coal consists of units, ‘micelles’, assumed to be rigid and comparatively indestructible, which vary in size over a considerable range. A suitable solvent acts by swelling the micelle network so that the smaller units can be extracted by diffusion through the swollen pores of the matrix; the higher the temperature, the larger the micelles that can be extracted. On the basis of this ‘micellar-sieve extraction mechanism’ one would expect the extent of dissolution to be proportional to the extent of swelling, and this is indeed what Dryden observed’ for coals having a carbon content > 77% (daf). Therefore, a solvent is suitable if it promotes swelling and loosening of the coal structure, and ifit stabilizes the 0016-2361/84/030346-09$3.00 @ 1984 Buttenvorth & Co. (Publishers)
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coal micelles in solution8*9. The specific features of the solvent and the coal necessary for a high extract yield were not obtained by Dryden, but the availability of an unshared pair of electrons on the part of the solvent, and a large internal surface area and a high oxygen content on the part of the coal were considered to be basic factors*- lo. Apart from its vagueness, the model can be criticized for not being able to account for the behaviour of the pyridine/coal system”*’ 2. According to Van Krevelen’ coal should be viewed as a cross-linked polymer. The basis of his extraction theory is Flory’s trifunctional polymerization model (which also has problems 13,14). Coalilication is considered to be the progress of a polymerization reaction, with progressive loss of functional groups, the insoluble fraction being the cross-linked gel and the extractable substances being the ‘free’ monomers and oligomers. The amount of extractable material dissolved by a particular solvent is, to a first approximation, determined by the solubility parameter, 6, of that solvent as compared with 6 (coal). Maximum solubility is achieved when both b-values are equal15. According to this theory, low-rank coals consist almost entirely of non-cross-linked oligomers, and should therefore be quite soluble in suitable solvents; this will be investigated in the present paper. However, the involvement of chemical factors is indicated by the fact that some solvents with ‘appropriate’ b-values do not dissolve coal; e.g., ethanol has a S-value intermediate between those of ethylenediamine and monoethanolamine (uide infiu), but fails to dissolve coal to any appreciable extent (the same is true for polymers’ 6).
Action
of solvents on coal at low temperatures.
The best solvents for low-rank coals are aliphatic primary amines and aqueous alkaline solutions (e.g., KOH) which interact strongly with coal. The exact nature of this interaction, at least in the case of the organic solvents, is obscure3,’ 7*18. The possibility that these solvents are reactive, i.e., that they break chemical bonds in the coal structure, has not been much discussed in the literature as most researchers adhere to some version of the two models described above. It is possible, however, that ester bonds have to be broken before extensive dissolution can take place (cf: refs. 19-22). The chemical aspects of the dissolution process are, therefore, still very poorly understood. To investigate this problem, the dissolution behaviour of a range of coals in three of the most common solvents (aqueous KOH, aliphatic primary amines and pyridine) has been studied in the hope that the results would lead to a more satisfactory model. Results obtained with low-rank coals are reported in the present paper, while those concerning high-rank coals will be presented subsequently. EXPERIMENTAL Some characteristics of the brown coals studied are given in Table 1. Coal samples were received in evacuated geological sample bags from Laura en Vereeniging (Heerlen, the Netherlands) and were impact-milled and sieved immediately after opening the bags. The fraction between 100 and 150 ASTM mesh (75-150 pm) sieves, used throughout in this study, was stored in a glass jar. No special precautions, such as milling, or storing, under nitrogen gas, were taken. The moisture content was checked frequently, and the carboxyl-group content occasionally; neither these nor the solubility changed significantly with time. The surface areas of the coals after drying for 2 h in vacuum at 106°C were determined by the CO2 sorption method at 295 K as described by Dubinin23. The following solvents were used : 1,3_diaminopropane, Fluka puriss, ethylene-diamine (EDA), ‘Baker analysed reagent, monoethanolamine (MEA) and monopropanolamine (MPA), Brocacef >98%, n-alkylamines, C, from Fluka (puriss), C, through C,, Aldrich 99% and Merck ‘zur Synthese’, pyridine (Py), Merck p.A., N-methyl-MEA and N,N-dimethyl-MEA, Fluka pract., and triethyleneglycol dimethyl ether (triglyme), Fulka puriss. Aqueous KOH solutions were made up from Merck p.A. KOH pellets and demineralized water. The organic solvents were not completely stable at higher temperatures (see below), MEA being the most unstable24. However, this
Table 1 Some characteristics
1: B. van Bodegom
et al.
instability appeared to be associated with impurities in the solvent, at least for MEA: after refluxing MEA with a portion of brown coal and distilling it off, it proved much less prone to decomposition. MEA was therefore used here after ‘purification’. The number of carboxylic groups present in the coal was determined by the Na-acetate method, as described by McPhail and Murray24 (Table 1). The exchange of the carboxylic proton for Ca* + was effected in a similar manner. To increase the number of carboxylic groups, coal samples were oxidized in moist air at 175°C for various lengths of time. The solubilities of the coals were determined by a 24-h treatment of = 2 g of ‘as-received coal in 100 ml solvent, either under reflux conditions, or in a stirred autoclave (Autoclave Engineers Inc., Model ABP-300, Hastelloy C; autogeneous pressure). The solution was allowed to cool down to room temperature, and extract and residue were then separated by several cycles of centrifugation (3000 rev min - ‘), removal of extract solution, addition of fresh solvent, and shaking by hand. The separation was taken to be complete when the fresh solvent added remained almost clear after centrifuging. The extract was recovered by flashing off the solvent in a rotary evaporator. Extract and residue were dried in an oven, purged with nitrogen, at 105-125°C. All the samples were analysed for nitrogen, and, on the assumption that excess nitrogen was due to unchanged solvent, their weights corrected for retained solvent. Using this correction procedure, the sum of the extract and residue yields frequently amounted to > 100% (up to z 115%). This indicates that some reaction had taken place between coal and solvent with incorporation of some solvent decomposition product (loss of nitrogen) into the coal. This does not vitiate the point of this Paper, but it does preclude the use of elemental analyses to draw any reliable conclusions about extract and residue compositions. It was also found that the extract tended to retain much more solvent than the residue. The same effect was noted for an oxidized carbon black in octylamine: possibly the extract solution is a ‘structured’ or ‘ordered’ liquid 26-28, lowering the vapour pressure of the solvent. For this reason, the percentage residue corrected for retained solvent as described above, and on a dry basis, is quoted throughout this Paper. For residues of KOH extraction, corrections were based on %K, measured by neutron activation. The influence of time (up to 96 h), and atmosphere (N, versus air) on the extract yield in reflux experiments was found to be very small. For the autoclave experiments the
of the coals used
Coal
Country origin
Yallourn Morwell Wyoming N. Dakota Blackfoot Picardville Neurath SRS 800
Australia Australia USA USA Canada Canada Germany UK
of
ASH % (db)
C % (daf
1 .3 3.7 6.8 1 1 .o 11 .7 1 1 .2 7.6 6.6
67.6 69.7 75.9a 74.3a 73.7 72.1 68.4 80.5
H
1
% (daf)
0 % (daf)
0 in COOH, % (db)
Surface area m* g-l
4.7 4.8 5.1a 4.aa 4.9 4.6 4.7 5.3
26.9 24.4 1 7Ja 19.7a 19.0 21.4 25.6 12.3
5.84 3.73 I .a0 1 .95 1 .22 2.66 4.04 0.16
175 194 168 176 260 221 190 128
a dmmf
FUEL, 1984, Vol 63, March
347
Action
of solvents
on coal at low
temperatures.
et al.
1: 8. van Bodegom
influence of stirring speed was also checked. In most cases no difference was found between stirring speeds of 2000 rev min-’ and 500 rev min - ‘; but in a few cases (e.g., Morwell in heptylamine) a difference was observed, so all the experiments were conducted at 2000 rev min-’ (the maximum rotational velocity of the apparatus was 2500 rev min-‘). Another potentially weak point in the experimental procedure was the separation of extract and residue by centrifugation. However, it is felt that a clear separation is effected by this technique, since: 1. extract yields obtained in Soxhlet extraction equalled those of the reflux experiments in the cases checked (Morwell/MEA, Py); and 2. the extract yield in octylamine did not change on changing the temperature of centrifugation to 12O”C, thus greatly lowering the viscosity of the amine. Apparently, the largest colloidal particles dissolved are still much smaller than the original coal particles. Indeed, electron micrographs of ‘dried’ extracts showed hardly any particles, and those observed were < 1 pm while the sieve fraction used was 75-150 pm. To check on some of the conclusions concerning the interaction between coal and solvent, the Fouriertransform (FT-i.r.) spectra of various samples were recorded. The experimental technique has been described previously29.
8C
Q,
m
P i f 40
20
0
I
/I
I 200
I
100
150 T (“Cl
Figure 7 Plot of percentage residue versus extraction temperature. 0, Morwell brown coal/pentylamine; 0, North Dakota brown coal/hexylamine; A, Wyoming brown coal/hexylamine
RESULTS Temperature
AND DISCUSSION dependence of extract
yield
One of the most striking features of the dissolution of brown coals in amines and aqueous KOH is its strong dependence on temperature. This dependence is more marked for younger coals, and this is perhaps the reason why this effect has apparently gone unnoticed in the literature as brown coals have not been studied very extensively because of their extreme chemical heterogeneity5. In amine solvents the dissolution process reaches an appreciable rate at z 140°C (Figure I), and in aqueous KOH at ~80°C (Figure 2). It is instructive to compare these results with some obtained by Dryden3’, who found that a plot of log (extract yield (in EDA)) versus l/Twas linear over the entire temperature interval studied (room temperature to 117”Q in the present study such plots show pronounced kinks (Figure 3). In view of these results, it would seem that this is not a simple, unhindered extraction of some soluble component of coal, but that a chemical reaction between solvent and coal must occur before extensive dissolution can take place. As to the nature of the supposed chemical reaction, it is proposed that, although the temperatures involved may seem somewhat high, it is the breaking of ester bonds in coal that is the first step in brown-coal dissolution. That the change in solubility with temperature takes place over a relatively broad temperature range could then be ascribed to differences in physical and chemical environment between the various ester groups. It is of course conceivable that there are other reactive bonds in brown coals. For example, in Wender’s picture of a typical brown coa131, there occur a-hydroxy-ether OH (AH-04
linkages, which are easily broken by acids and bases, indeed, this is why cellulose degrades so quickly. Also,
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FUEL, 1984, Vol 63, March
4 80-
y.****.....*.
60-
2 0 % (L
I I I I \ \
$
.. *. .. .. .... ..
T(‘C)
Figure 2 Plot of percentage residue versus temperature of extraction with KOH/HzO (1 /lo wt/wt). 0, Morwell brown coal; A, Wyoming brown coal
P-hydroxy-ketones OH
0
(A-CH2-L
may occur, which can undergo retro-aldol reactions. However, it is felt that, given the relatively large amounts of carboxylic and hydroxyl groups, ester bonds will form the main linkages between different coal fragments. To check whether 140°C is a reasonable temperature
Action
of solvents on coal at low temperatures.
5-
I
I
2.4 I/T,
2.8
32.10
K-’ (~10~)
Plot of log (extract yield) versus reciprocal temperature. 0, Morwell/pentylamine; A, Wyoming/aq. KOH Figure 3
for the initiation of aminolysis of coalLester groups, an i.r. study of the reaction between octylamine and methyl oleate and phenyl benzoate was conducted and the appearance and rate of growth of two peaks at 1655 and 1550 cm-‘, respectively, which are ascribed to the corresponding amine groups formed (RCOOR’ + R”NH,--, RCONHR” + R’OH) were monitored. It was found that at ~90°C (the benzoate is somewhat more reactive than the oleate) the aminolysis began to take place at a considerable rate. This is satisfactory, considering the effect of reagent access on the reactivity of coa132, though perhaps not completely convincing (see section on solvent effects). Problems due to limited accessibility of coal towards some reagents have been experienced in the determinations of COOH groups (see below), and in the esterification of these groups. In amyl alcohol/HCl some of the carboxylic groups could be esterified, but virtually none could when using dimethylformamide dipropylaceta13j (i.r. results). However, in the dissolution process accessibility problems will not be quite as severe, since amines and aqueous KOH swell brown coals strongly. Assuming that ester-bond breaking is the necessary first step in brown-coal dissolution the implications of this
Table 2 The effect of pretreatment
on the solubility
Coal
Solvent
Yallourn Blackfoot Wyoming Morwell Morwell Morwell Yallourn Yallourn
KOH/H20 l/10 KOH/H20 l/IO KOH/H20 1 00 EDA (reflux) EDA (reflux) PY (Soxhlet) EDA (reflux) EDA (reflux)
(reflux) (reflux) (reflux)
1: B. van Bodegom
et al.
hypothesis will be discussed. First, one would expect that the solubility in pyridine would not depend on temperature to any great extent, as pyridine is unable to break ester bonds. This is indeed the case for Morwell brown coal. Second, it should be possible to separate the bondbreaking from the dissolution step. Since ethanol and water are not solvents for coal, breaking of the esterbonds without dissolution can be effected by sodium ethanolate in ethanol, and by aqueous HCl. The results obtained by pretreating brown coals with EtONa or HCl (bond-breaking step), before treating them with a good solvent (dissolution step), are shown in Table 2. It is evident that the ethanolate treatment is very effective in enhancing the solubility of brown coals which supports the suggested hypothesis. The results of the aqueous HCl treatment are less clear. In the case of Morwell brown coal the effect of HCl is twofold: 1. it leaches out the Ca2+ ions bound to some carboxylic groups; and 2. it, supposedly, catalyses the hydrolysis of the ester groups present. The first effect in itself enhances coal solubility. To ascertain the influence of each effect in the present case, Morwell coal was treated with 0.1 N HCl at room temperature, which removes virtually all Ca2+, but hopefully does not affect greatly the ester groups; and with 1 N HCl, under reflux. The latter treatment does lead to a higher solubility than the former, but some doubt remains about the value of this piece of evidence for the hypothesis. Indeed, with Yallourn brown coal, which contains virtually no Ca2 + ions, such an HCl treatment does not result in an enhanced solubility in EDA. Taking into account that this particular coal has the highest functional-group content, it is possible that hydrolysis and re-esterification more or less compensate each other. To prevent ester linkages reforming between coal fragments a large amount of ethanol was added, which led to the expected enhanced solubility. Esterification of the carboxyl groups present as such is not judged to be the cause of this, since a treatment with amyl alcohol/HCl, admittedly not esterifying all (viz. z 75%) the carboxylic acid groups, did not lead to any change in solubility. This argument will only stand, however, if transesterification has not taken place and this is considered likely, in view of the bulk of the alcohol present which would cause insufficient access to reagent. Overall, it is considered justifiable to conclude that an ester-bond breaking step is necessary for extensive dissolution to take place. In this context it is perhaps worth noting that small molecules, which are soluble in solvents such as ethanol and pyridine, can be split off on alkaline hydrolysis34. The
of some brown coals
% Res.
Pretreatment
% Res. after pretreatment
21 92 62 57 57 87 34 34
EtONa/ETOH l/IO (150°C) EtONa/ETOH l/10 (150°C) EtONa/ETOH 1 /lO (150°C) 0.1 N HCI (room temp.) 1 N HCI (reflux) 1 N HCI (reflux) 1 N HCI (reflux) 1 N HCl/50% EtOH (reflux)
5 18 20 28 14 67 30 12
FUEL, 1984, Vol 63, March
349
Action of solvents on coal at low temperatures. I: B. van Bodegom et al.
former solvent was used in most of the pretreatment experiments (cf: Table 2), and, in fact, dark-yellow solutions were obtained in the case of Yallourn coal (first and last lines of Table 2). Also, the amount of ‘treated coal obtained indicates that a small fraction of the initial coal sample remains behind in the ethanol. Following an HCl pretreatment, the extractability of Morwell coal with pyridine was much enhanced (Table 2). Thus, although the coal as a whole is highly functionalized, a small part (<20x) of the hydrolysis product consists of not extensively functionalized molecules. As shown above, the chemical evidence for the existence of ester groups in brown coals is considerable and confirmation by obtaining the characteristic 1730 cm - ’ absorption band in the FT-i.r. spectrum was sought. Two spectra of Morwell coal are shown in Figure 4. The first is of an untreated sample. At 1730 cm-’ only the slightest of shoulders is visible on the main band due to carboxyl groups although it is perfectly reproducible. The second spectrum is of a sample treated with sodium acetate (or, alternatively, with NH,); if it was possible to convert all the COOH groups into Na carboxylates, thus shifting the absorption frequency to 1550 cm-‘, the 1730 cm-’ peak would be clearer. It was apparent that not all the carboxyl groups could be reached by the NaOAc reagent, although the shoulder was more convincing than in the first case. It should be noted, however, that the physical evidence for the presence of ester groups in brown coals remains rather weak. Dryden’ and Lahaye and Decroocq35 reported that secondary and tertiary amines are much less effective solvents than primary ones. This was confirmed as Nmethyl-MEA and N,N-dimethyl-MEA were found to cause far less solubilization of brown coals than MEA itself. Since N-methyl-MEA should in principle be capable of breaking ester bonds, the accepted explanation in terms of steric hindrance appears to apply. This view is supported by the fact that N-methyl-MEA is easily removed from the coal, whereas MEA is not (see below). The solubility of the coal fragments formed upon the breaking of ester bonds was also investigated. TWO aspects will be discussed: 1. the differences in solvent power between different solvents for the same coal, at the same temperature; and 2. the differences in extract yield
between different coals in the same solvent under reflux conditions. Efect of nature of solvent on coal solubility It is worth noting that the pretreated brown coals (Table 2) remain largely insoluble in the most common
solvents such as ethanol, pyridine, toluene, chloroform and even triglyme. They are very soluble, however, in the solvents that are also capable of breaking ester bonds in coal. As these solvents are strong bases and brown coals are highly acidic4*36, it is possible that acid/base interaction is important. In aqueous KOH, the acidic groups of the coal fragments will ionize, and this apparently gives rise to sufftcient solvation energy for dissolution to take place*. Some light is thrown on what happens in amine solvents by the results shown in Figure 5. It is seen that for Morwell brown coal, at 180°C where the ester-bond breaking reaction may be considered to be more or less complete (Figure I), the solubility depends on the alkylchain length of the n-alkylamines used as solvent. This finding may be interpreted as follows: the coal fragments per se are too polar to be soluble in the rather apolar amines ‘s3’ (some solubility parameters? are shown in Table 3). However, the amine interacts with the acidic groups of the coal, leading to salt or amide formation. Accordingly, the solubility of coal fragments bearing a number of alkyl groups should be considered, rather than that of the simple coal fragments. Therefore, the longer the alkyl chain, the more the coal fragment will resemble the solvent, and will dissolve more easily. A similar effect of chain length on solubility has been observed by Hodek and K611ing39for acetylated coal in pyridine, probably for the same reason. The solvent-polarity argument is strengthened by the fact that MEA dissolves Monvell brown coal almost completely at 170°C even though it gives rise to only very short chains on the coal fragments,
P\\_
60
\
\ 0 C3
b
I
I
I
I
I
C4
=5
=6
C,
C6
Figure 5 Plot of percentage residue as a function of alkyl-chain length; Morwellln-alkyl-amines, 180°C
I 1900
I
AI
I
1700
I 1500
I
I 1300
V (cm-‘) 4 Fourier-transform i.r. Spectra of Morwell (as received), and of Morwell exchanged with NaOAc, ---. Courtesy C. -, A. Emeis Figure
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FUEL, 1984, Vol 63, March
* An explanation in temis of the [-potential is considered unlikely, since the KOH solution used is sufficiently concentrated for the diffuse double layer to be virtually absent 7 The solubility-parameter theory will not be discussed here, see, for example, refs. 15 and 37. Roughly speaking, the lower the solubility parameter, the less polar the solvent will be
Action
of solvents on coal at low temperatures.
1: B. van Bodegom
et al.
Table 3 Solubility parameters, S, caI1/2 crne312, of some soIvants38 Solvent
s
Solvent
6
Propylamine Alkylamine (>C,) Ethylenediamine Monoethanolamine
9.0 8.7 12.3 15.5
Pyridine Ethanol Benzene Chloroform
10.7 12.7 9.2 9.3
probably because MEA is far more polar than the alkylamines (cf. Table 3). The response of the brown coal/EDA system to an increase in temperature is somewhat more complex than that of systems with other amine solvents. At first, the solubility in EDA starts to increase with increasing temperature, as expected, but above 140°C the solubility starts to decrease (see Figure 6). This effect is probably related to the fact that an EDA molecule can interact with two acidic sites. At higher temperatures the second NH, group is apparently able to compete effectively with free EDA molecules for the available acidic groups, linking coal fragments together and making them insoluble. Another interesting feature of Figure 6 is that the sharp increase in solubility seems to occur at much lower temperatures than in the case of pentylamine (cc Figure I), and therefore much nearer the reaction temperatures observed in the model experiments. As EDA is a better coal solvent than the lower alkylamines7p37, this might indicate that the curves in Figure I reflect not only the extent of ester-bond breakage, but also the relatively poor solvent properties of hexyl- and, especially, pentylamine. This effect is illustrated by comparing Figures 1 and 7, in which the solubility of Morwell in a variety of solvents is shown. In conclusion, a good solvent for brown coal should be able to rupture the ester bonds which link the coal fragments together, and to dissolve the reaction product(s) of the solvent and the coal fragments.
oh Figure 7
I 50
I
I 150
100
I 200
b.p. ( “C )
Plot of percentage residue versus boiling point of the
solvent used. Extraction under reflux conditions; Morwell brown coal
Dependence
of solubility on coal characteristics
It is to be expected that there is no single property of brown coal that determines its solubility. It appears, however, that the carboxyl-group content plays a very important role. Figures 8 and 9 show the solubilities of brown coals in MEA and aqueous KOH, respectively, as a function of the number of COOH groups per unit CO2 surface area*. The following features should be noted: 1. First, it is clear that, of the untreated brown coals, two do not follow the main correlation. The reason for this is unknown, although it is likely to be due to a higher content of unreactive macerals which were not measured. 2. It is known that, in some brown coals, not all carboxyl groups are free, as some are associated with alkali or alkaline-earth ions4’. These ions are easily exchangeable and, for example, can be washed out by treating the coal with 0.1 N HCl at room temperature. By analysing the washings it was found that Ca’ + was by far the most abundant exchangeable cation in the samples. From Figure 8 it is apparent that removal of these Ca2 + ions enhances the solubility of the coal in MEA+.. The effect is reversible: after treating Morwell brown coal with a Ca(OAc), solution, its solubility is lower. These results cannot be explained by the fact that there is a carboxylate anion in one case, and a free carboxylic acid group in the other, since exchanging Morwell with NaOAc did not have any effect on its solubility in MEA.
100
I 120
I 140 T (“C)
I 160
I 180
figure 6 Plot of percentage residue as a function of extraction temperature; Morwell/EDA
* Assuming the accessibilities of coal to CO* and CH,COONa to be approximately the same, this is considered to be the best measure of carboxyl-group concentration t It is noteworthy that N. Dakota brown coal is affected by the HCI treatment in the same way as other brown coals, but its y0 res. t~ersus COOH-curve is displaced (by 25% res.) with respect to the main correlation
FUEL, 1984, Vol 63, March
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Action of solvents on coal at low temperatures. 1: B. van Bodegom et al.
coal with ZnCl,/methanol, it is advantageous (in terms of subsequent solubility in Py) to wash the product with aqueous HCl rather than water, HCl probably removing bridging Zn2 ’ ions4’. Essentially the same effect accounts for the increased extractability of Wyodak coal by ethanol/benzene after removal of Ca2 + ions43. 3. The observed relation between the solubility of a coal and its free carboxyl-group concentration stimulated a study of the influence of oxidation/decarboxylation. In agreement with the established trend, oxidation led to enhanced solubility, while decarboxylation strongly decreased the solubility of the coal (Figures 8 and 9). With respect to aqueous KOH, this result tallies with the work of Mukherjee et a1.44,who found that humic acids extracted from coal with aqueous KOH did not redissolve after decarboxylation. The trend observed for brown-coal dissolution in MEA and aqueous KOH at 180°C is also apparent in the case of solvents used under reflux conditions, except for pyridine (Figure 10). This probably means that the fraction of coal extracted by the reactive solvents is representative of the whole coal, which is commonly thought to be the case2*45, while Py extracts only a particular component, that is, Py acts as a ‘nonspecific solvent’2*4 towards low-rank coals. The carboxylic groups in coal react with primary amines > = 140°C to form amides4’j as confirmed by i.r. This amide formation may contribute to the dissolution behaviour of brown coals. To investigate this possibility, the solubility in hexylamine of a High Abrasion Furnace (HAF) black was studied after it had been oxidized by refluxing it with 40% HNO, up to a content of 2.3 meq. COOH g-r black (10’ 7 carboxyl groups per m2), over a
Figure 8 Plot of percentage residue after MEA extraction (170°C) versus carboxyl-group concentration. S, SRS 800; B, Blackfoot; W, Wyoming; M, Morwell; N, Neurath; Y, Yallourn; D, North Dakota; P, Picardville. 0, untreated brown coals; 0, after HCI wash or Ca*’ exchange; H, after oxidation/decarboxylation
100 I AS I I A 80 -’ I I I I 60-
\
I \ I \
!i .2 a cc f
F4P
40\ BA \ \ \ 20-
\ Wb, \ \.
O0
I 2
\
\
.N M/\
[COOHI
Y 4 (10’8m~2)
6
8
Figure 9 Plot of percentage residue after KOH extraction (180°C) versus carboxyl-group concentration. A, untreated brown coals, for letters see caption to Figure 8; A, decarboxylated Morwell brown coal B I
0
Rather, the reason for this is probably the fact that Ca2 + ions can act as a cross-link between different coal fragments (X00 - . . . Ca2 + . , . -OOC-bridges), an effect which is well known in cellulose chemistry4r. It also has a parallel in coal chemistry: after treatment of
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FUEL, 1984, Vol 63, March
0
wzp 2
MN I I 4 I COOH I ( lO’*ti’)
Y I I 6
Figure 70 Plot of percentage residue versus carboxyl-group content for various solvents. Extraction under reflux conditions. For letters see caption to Figure 8. HXA= hexylamine
I 8
Action
of solvents
broad temperature range (5&2oo”C). It was found that the amount of black retained in ‘solution’ after 20 minutes’ centrifugation was independent of temperature. It seems, therefore, that amide formation does not lead to a change in dissolution behaviour as compared with simple salt formation, which from i.r. values takes place at lower temperatures. Solvent recovery
An important condition for the applicability of any solubilization process, be it in solvent mining47,48 or in ash remova14g~50,is the almost complete recovery of the solvent. In this respect, the aliphatic primary amines perform very poorly. Some amine is lost by irreversible reactions with carboxyl and carbonyl groups in coal (up to x 2 mm01 amine g - ’ coal). Second, another part of the solvent is held so strongly by the coal that it is virtually irremovable. The solvent comes away only very slowly at temperatures < 200°C and higher temperatures cannot be used, because the coal starts decomposing, and the amine starts to react further, leading to an additional loss of amine, an effect observed by Dryden”. Neither can the solvent be completely removed by steam stripping or solvent exchange, a complication with the latter method being that part of the treated coal dissolves in the more effective exchange solvents (e.g., ethanol*). This strongly adsorbed fraction amounts to ~4 mmol amine g-’ coal, and, as will be shown in a subsequent paper, probably results from salt formation or strong hydrogen bonding, between phenolic OH groups in the coal and the amine solvent. The recovery of aqueous alkaline solutions is not discussed in any detail, but note that in a simple dissolution-preparation process the likely end result is either a coal product containing much water (up to 9073, or one with a large amount of ash (up to 50%). This is dictated by the nature of brown coals; they have ionizable groups and will therefore swell in contact with pure water, just like cellulose gel (or pul~)~~; the swelling can be suppressed by adding salts to the water phase, but then some of these will be incorporated in the coal mass.
CONCLUSIONS From the observed temperature dependence of the solubility of brown coal in various solvents and from the effect of pretreating brown coals with 1 N HCl or 10% sodium ethanolate on their solubility, it is concluded that the soluble brown-coal fragments are bound in the coal mass via ester bonds and that these ester bonds should be broken before extensive dissolution can take place. It is interesting to note that Whitehurst inferred the existence of ester groups in low-rank coals from the products obtained in short-contact-time liquefaction experiments. However, the physical evidence for ester groups remains rather weak. The coal fragments themselves seem to be soluble only in strong bases. This is probably related to the fact that * This is rather remarkable in view of the fact that the coal fragments per se are hardly soluble in ethanol. Perhaps the incorporation of alkyl groups as a consequence of the coal-amine reaction does indeed diminish internal hydrogen bonding, a similar explanation being frequently put forward in relation to the solubility of acetylated and alkylated coals5’. As an example, the pentylamine extract of Morweli brown coal is 75% soluble in ethanol
on coal at low temperatures.
I: B. van Bodegom
et al.
brown coals contain many acidic groups. It was found that the solubility in primary amines and aqueous KOH correlated with the carboxylic group concentration, though it appears that at least one other coal parameter is important in determining brown-coal solubility. Why is a strong acid/base interaction necessary for the solubilization of brown-coal fragments? In the case of aqueous KOH, the reason is probably that the ionization of the acidic groups provides enough solvation energy for solubilization to take place. In the case of primary amines, it may be that the adducts or other entities formed between the acidic groups of the coal and the basic solvent destroy the inter-fragment hydrogen bonding, thus increasing the solubility of the individual fragments. This explanation may also account for the fact that ethanol is not a good solvent for the brown-coal fragments per se, but is for amine-treated brown coal. The amines are not all equally effective, however, when compared at the same temperature: the more polar solvents, EDA and MEA, are best, while the solvent power of n-alkylamines increases with increasing alkylchain length. It is proposed that this effect should be interpreted in terms of a solubility parameter (6) theory: one should, however, compare 6 (solvent) not with 6 (coal), but with the 6 of the reaction product of the coal with the solvent. It appears, then, that solvents which dissolve brown coals to a considerable extent, aqueous alkali hydroxide and primary amines, are not solvents in the strict sense, but reactive solvents in Van Krevelen’s terminology4, combining the ability to break ester bonds with the ability to solubilize the coal fragments thus produced. Since the days of Dryden’, Kriiger3,54, and Van Krevelen7, little work has been carried out on the dissolution of brown coals in amine solvents. Their theories, however, disregarding as they do the importance of bond-breaking reactions, should now be considered to be incomplete. The dissolution behaviour of brown coals in aqueous alkali-hydroxide solutions has remained under study by various workers in Australia22.32,45*46,55,56.The possibility of ester-bond breaking does not figure strongly in their theories, but then, they effect the extractions often at low temperatures (e.g., at room temperature). Thus, the present findings can be considered to be an extension to their work, rather than superseding it. ACKNOWLEDGEMENT Interesting discussions with Dr P. Biloen and Dr P. A. Verbrugge are gratefully acknowledged. The i.r. spectroscopy work was carried out by Dr C. A. Emeis, the electron microscopy work by Mr A. Knoester. REFERENCES 1 2 3 4 5 6
Kiebler, M. K. ‘Chemistry ofCoal Utilization’(Fd. H. H. Lowry), Wiley, 1945, Vol. I, p. 715 Dryden, I. G. C. ‘Chemistry of Coal Utilization’ (Ed. H. H. Lowry), Supplementary Volume, Wiley, 1963, p. 237 Krijger, C. Erdtil und Kohle 1956, 9,44, 516,620, 839 van Krevelen, D. W. ‘Coal’, Elsevier, 1961 Davidson, R. M. ‘Molecular Structure of Coal’, Report No. ICTIS/TR08, IEA Coal Research, 1980 Wender, I., Heredy, L. A., Neuworth, M. B. and Dryden, I. G. C. ‘Chemistry of Coal Utilization’ (Ed. M. A. Elliott), 2nd Suppl. Vol., Wiley, 1981, p. 425
FUEL, 1984, Vol 63, March
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Action 7 8 9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
354
of solvents on coal at low temperatures.
1: B. van Bodegom
van Krevelen, D. W. Fuel 1965,44,229 Dryden, I. G. C. Fuel 1951,30, 145,217 Dryden, I. G. C. Nature 1949,163, 141 Dryden, I. G. C. Fuel 1951,30, 39 Malanowitz, L. Chim. Ind. 1932,28, 1277 Bunte, K., Briickner, H. and Simpson, H. G. Fuel 1933, 12,26 Stockmayer, W. H. J. Chem. Phys. 1943, 11,45 ZilT, R. M. and Stell, G. J. Chem. Phys. 1980, 73, 3492 Barton, A. F. M. Chem. Rev. 1975, 75, 731 Tager, A. A. and Kohnakova, L. K. Pol. Sci. USSR 1981,22,533 Bangham, D. B. and Dryden, I. G. C. Fuel 1950,29,291 Rybicka, S. M. Fuel 1959,38,45 Harris, R. L., Simons, L. H. and Lagowski, J. J. Am. Chem. Sot. Div. Fuel Chem., Preprints 1979, U, 264 Camier, R. J., Siemon, S. R., Battaerd, H. A. J. and Stanmore, B. R. ‘Coal Structure’ Adu. Chem. Ser. 1981, 192, 311 Liotta, R., Rose, K. and Hippo, E. J. Org. Chem. 1981, 46, 277 Brooks, J. D. and Stemhell, S. Fuel 1958, 37, 124 Dubinin, M. M. Chem. Reu. 1960,60,235 Lindsay, D. A. MSc. Thesis, University of California, 1978, LBL McPhail, I. and Murray, J. B. Miscellaneous Report No. MR155, 1969, State Electricity Commission of Victoria Donnet, J. B. and Voet, A. ‘Carbon Black’, Marcel Dekker Inc., 1976 Donnet, J. B. and Riess, G. Peintures-Pigments-timis 1964,40, 426 Rodgers, B. R. Oak Ridge National Laboratory, ORNL-5631, 1980 Bouwman, R. and Freriks, I. L. C. Fuel 1980,59,315 Dryden, I. G. C. Fuel 1952.31, 176 Winder, I. Catal. Rev-Sci. Eng. 1976, 14,97 Larsen. J. W.. Green. T. K.. Choudhurv. P. and Kuemmerle. E. W. ‘Coal Siructure’ Adv. them. Ser. Ib81, 192, 277 Brechbiihler, H., Biichi, H., Hatz, E., Schreiber, J. and Eschenmoser, A. Helv. Chim. Acta 1965,48, 1746 Chaffee, A. L., Perry, G. J., Johns, R. B. and George, A. M. ‘Coal Structure’ Adu. Chem. Ser. 1981, 192, 113
FUEL, 1984, Vol 63, March
et al.
54
Lahaye, Ph. and Decroocq, D. Rev. Inst. Franc. Petrole 1976,31, 99 Whitehurst, D. D., Mitchell, T. 0. and Farcasiu, M. ‘Coal Liquefaction. The Chemistry and Technology of Thermal Processes’, Academic Press, 1980 Hombach, H. P. Fuel 1980,59,465 ‘Polymer Handbook’ (Eds. J. Brandrup and E. H. Immergut), Interscience, 1975 Hodek, W. and KGlling, G. Fuel 1973,52, 220 Durie, R. A. Fuel 1961,40,407 Paist, W. D. ‘Cellulosics’, Reinhold, 1958 Shinn, J. H. and Vermeulen, T. ‘Coal Liquefaction Fundamentals’ Am. Chem. Sot. Symp. Ser. 1980,139,227 Base&Z. H., Pancirov, R. J. and Ashe, T. R. Adu. Org. Geochem. 1979,619 Mukherjee, P. N., Bhowmik, J. N. and Lahiri, A. Fuel 1957,36, 417 Durie, R. A. and Stemhell, S. Austr. J. Appl. Sci. 1958,9, 370 Butler, R. N., Thornton, J. D. and Moynihan, P. J. Chem. Research (S) 1981, 84 Drinkard, G., Prats, M. and O’Brien, S. M. US Patent 4,032,193, 1974 Wise, D. L. and Augenstein, D. C. In situ 1978, 2, 173 Araya, P. E., Badilla-Ohlbaum, R. and Droguett, S. E. Fuel 1981, 60, 1127 Jenkins, R. Patent WO SI/OZSSO,1981 Pullen, J. R. ‘Solvent Extraction of Coal’, Report No. ICTIS/ TR16. IEA Coal Research. London. 1981 Grignon, J. and Scallan, Al M. J. Abpl. Pol. Sci. 1980,25,2829 Whitehurst, D. D. ‘Organic Chemistry of Coal’ Am. Chem. Sot. Symp. Ser. 1978, 71, 1 KrGger, C. and Kaspers, H. H. Erdiil Kohle-Erdgas-Petrochem.
55 56
Camier, R. J. and Siemon, S. R. Fuel 1978, 57, 85 Camier, R. J., Siemon, S. R. and Stanmore, B. R. Fuel 1980,59,
35 36
37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53
1962,15,90
331
coals
Bert van Bodegom, J. A. Rob van Veen, Gerard M. M. van Kessel, Mieke W. A. Sinnige-Nijssen and Hans C. M. Stuiver KoninkhjkelSheil- Laboratorium, Amsterdam, (Shell Research B. V.), Postbus 3003, Amsterdam, The Netherlands (Received 23 December 7982; revised 5 February 7983) The dissolution behaviour of brown coals (67-75% C, daf) in pyridine, primary amines and aqueous KOH has been studied. The solubility in the last two solvents greatly depends on temperature, but in the first it is relatively temperature-independent. Pretreatment of the brown coals with aqueous HCI or with sodium ethanolate in ethanol leads to enhanced solubility. It is concluded that ester-bond breaking is necessary before extensive dissolution can take place. The solubility of brown coals in amines and aqueous KOH is found to increase with increasing carboxylic-acid group concentration in the coal. The solubility of Morwell brown coal in n-alkylamines at 180°C increases with increasing length of the alkylchain in the solvent. The class of good solvents for brown coals is restricted to strong bases, because: 1. ester bonds have to be broken, 2. the acidic coal fragments have to be solubilized. Because of their capacity to break ester bonds these are so-called reactive solvents. Complete solvent recovery is impossible in the case of amines. (Keywords:
coal; solvents;
low temperatures)
In the past, solvent extraction was used mainly as a method for: 1. isolating a hypothetical coking principle (i.e., substances then thought to be responsible for the caking properties); 2. producing materials of potential industrial value (waxes, resins); and 3. studying the constitution of coal. The very extensive literature on this subject has been reviewed by Kiebler’, Dryden’, KrGger3, Van Krevelen4, Davidson’ and Wender et aL6. The present work falls into the third category, but the focus is on the basic chemistry of the coal conversion process, and therefore on those aspects which relate to the coal dissolution process itself. No detailed knowledge is sought of the molecular structure of coal, but only of the parameters that determine the dissolution of coal. A few models of the coal-dissolution process exist, but most refer mainly to bituminous coals. Two models (viz those of Dryden’ and Van Krevelen’) are of a more general nature. According to Dryden’ coal consists of units, ‘micelles’, assumed to be rigid and comparatively indestructible, which vary in size over a considerable range. A suitable solvent acts by swelling the micelle network so that the smaller units can be extracted by diffusion through the swollen pores of the matrix; the higher the temperature, the larger the micelles that can be extracted. On the basis of this ‘micellar-sieve extraction mechanism’ one would expect the extent of dissolution to be proportional to the extent of swelling, and this is indeed what Dryden observed’ for coals having a carbon content > 77% (daf). Therefore, a solvent is suitable if it promotes swelling and loosening of the coal structure, and ifit stabilizes the 0016-2361/84/030346-09$3.00 @ 1984 Buttenvorth & Co. (Publishers)
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coal micelles in solution8*9. The specific features of the solvent and the coal necessary for a high extract yield were not obtained by Dryden, but the availability of an unshared pair of electrons on the part of the solvent, and a large internal surface area and a high oxygen content on the part of the coal were considered to be basic factors*- lo. Apart from its vagueness, the model can be criticized for not being able to account for the behaviour of the pyridine/coal system”*’ 2. According to Van Krevelen’ coal should be viewed as a cross-linked polymer. The basis of his extraction theory is Flory’s trifunctional polymerization model (which also has problems 13,14). Coalilication is considered to be the progress of a polymerization reaction, with progressive loss of functional groups, the insoluble fraction being the cross-linked gel and the extractable substances being the ‘free’ monomers and oligomers. The amount of extractable material dissolved by a particular solvent is, to a first approximation, determined by the solubility parameter, 6, of that solvent as compared with 6 (coal). Maximum solubility is achieved when both b-values are equal15. According to this theory, low-rank coals consist almost entirely of non-cross-linked oligomers, and should therefore be quite soluble in suitable solvents; this will be investigated in the present paper. However, the involvement of chemical factors is indicated by the fact that some solvents with ‘appropriate’ b-values do not dissolve coal; e.g., ethanol has a S-value intermediate between those of ethylenediamine and monoethanolamine (uide infiu), but fails to dissolve coal to any appreciable extent (the same is true for polymers’ 6).
Action
of solvents on coal at low temperatures.
The best solvents for low-rank coals are aliphatic primary amines and aqueous alkaline solutions (e.g., KOH) which interact strongly with coal. The exact nature of this interaction, at least in the case of the organic solvents, is obscure3,’ 7*18. The possibility that these solvents are reactive, i.e., that they break chemical bonds in the coal structure, has not been much discussed in the literature as most researchers adhere to some version of the two models described above. It is possible, however, that ester bonds have to be broken before extensive dissolution can take place (cf: refs. 19-22). The chemical aspects of the dissolution process are, therefore, still very poorly understood. To investigate this problem, the dissolution behaviour of a range of coals in three of the most common solvents (aqueous KOH, aliphatic primary amines and pyridine) has been studied in the hope that the results would lead to a more satisfactory model. Results obtained with low-rank coals are reported in the present paper, while those concerning high-rank coals will be presented subsequently. EXPERIMENTAL Some characteristics of the brown coals studied are given in Table 1. Coal samples were received in evacuated geological sample bags from Laura en Vereeniging (Heerlen, the Netherlands) and were impact-milled and sieved immediately after opening the bags. The fraction between 100 and 150 ASTM mesh (75-150 pm) sieves, used throughout in this study, was stored in a glass jar. No special precautions, such as milling, or storing, under nitrogen gas, were taken. The moisture content was checked frequently, and the carboxyl-group content occasionally; neither these nor the solubility changed significantly with time. The surface areas of the coals after drying for 2 h in vacuum at 106°C were determined by the CO2 sorption method at 295 K as described by Dubinin23. The following solvents were used : 1,3_diaminopropane, Fluka puriss, ethylene-diamine (EDA), ‘Baker analysed reagent, monoethanolamine (MEA) and monopropanolamine (MPA), Brocacef >98%, n-alkylamines, C, from Fluka (puriss), C, through C,, Aldrich 99% and Merck ‘zur Synthese’, pyridine (Py), Merck p.A., N-methyl-MEA and N,N-dimethyl-MEA, Fluka pract., and triethyleneglycol dimethyl ether (triglyme), Fulka puriss. Aqueous KOH solutions were made up from Merck p.A. KOH pellets and demineralized water. The organic solvents were not completely stable at higher temperatures (see below), MEA being the most unstable24. However, this
Table 1 Some characteristics
1: B. van Bodegom
et al.
instability appeared to be associated with impurities in the solvent, at least for MEA: after refluxing MEA with a portion of brown coal and distilling it off, it proved much less prone to decomposition. MEA was therefore used here after ‘purification’. The number of carboxylic groups present in the coal was determined by the Na-acetate method, as described by McPhail and Murray24 (Table 1). The exchange of the carboxylic proton for Ca* + was effected in a similar manner. To increase the number of carboxylic groups, coal samples were oxidized in moist air at 175°C for various lengths of time. The solubilities of the coals were determined by a 24-h treatment of = 2 g of ‘as-received coal in 100 ml solvent, either under reflux conditions, or in a stirred autoclave (Autoclave Engineers Inc., Model ABP-300, Hastelloy C; autogeneous pressure). The solution was allowed to cool down to room temperature, and extract and residue were then separated by several cycles of centrifugation (3000 rev min - ‘), removal of extract solution, addition of fresh solvent, and shaking by hand. The separation was taken to be complete when the fresh solvent added remained almost clear after centrifuging. The extract was recovered by flashing off the solvent in a rotary evaporator. Extract and residue were dried in an oven, purged with nitrogen, at 105-125°C. All the samples were analysed for nitrogen, and, on the assumption that excess nitrogen was due to unchanged solvent, their weights corrected for retained solvent. Using this correction procedure, the sum of the extract and residue yields frequently amounted to > 100% (up to z 115%). This indicates that some reaction had taken place between coal and solvent with incorporation of some solvent decomposition product (loss of nitrogen) into the coal. This does not vitiate the point of this Paper, but it does preclude the use of elemental analyses to draw any reliable conclusions about extract and residue compositions. It was also found that the extract tended to retain much more solvent than the residue. The same effect was noted for an oxidized carbon black in octylamine: possibly the extract solution is a ‘structured’ or ‘ordered’ liquid 26-28, lowering the vapour pressure of the solvent. For this reason, the percentage residue corrected for retained solvent as described above, and on a dry basis, is quoted throughout this Paper. For residues of KOH extraction, corrections were based on %K, measured by neutron activation. The influence of time (up to 96 h), and atmosphere (N, versus air) on the extract yield in reflux experiments was found to be very small. For the autoclave experiments the
of the coals used
Coal
Country origin
Yallourn Morwell Wyoming N. Dakota Blackfoot Picardville Neurath SRS 800
Australia Australia USA USA Canada Canada Germany UK
of
ASH % (db)
C % (daf
1 .3 3.7 6.8 1 1 .o 11 .7 1 1 .2 7.6 6.6
67.6 69.7 75.9a 74.3a 73.7 72.1 68.4 80.5
H
1
% (daf)
0 % (daf)
0 in COOH, % (db)
Surface area m* g-l
4.7 4.8 5.1a 4.aa 4.9 4.6 4.7 5.3
26.9 24.4 1 7Ja 19.7a 19.0 21.4 25.6 12.3
5.84 3.73 I .a0 1 .95 1 .22 2.66 4.04 0.16
175 194 168 176 260 221 190 128
a dmmf
FUEL, 1984, Vol 63, March
347
Action
of solvents
on coal at low
temperatures.
et al.
1: 8. van Bodegom
influence of stirring speed was also checked. In most cases no difference was found between stirring speeds of 2000 rev min-’ and 500 rev min - ‘; but in a few cases (e.g., Morwell in heptylamine) a difference was observed, so all the experiments were conducted at 2000 rev min-’ (the maximum rotational velocity of the apparatus was 2500 rev min-‘). Another potentially weak point in the experimental procedure was the separation of extract and residue by centrifugation. However, it is felt that a clear separation is effected by this technique, since: 1. extract yields obtained in Soxhlet extraction equalled those of the reflux experiments in the cases checked (Morwell/MEA, Py); and 2. the extract yield in octylamine did not change on changing the temperature of centrifugation to 12O”C, thus greatly lowering the viscosity of the amine. Apparently, the largest colloidal particles dissolved are still much smaller than the original coal particles. Indeed, electron micrographs of ‘dried’ extracts showed hardly any particles, and those observed were < 1 pm while the sieve fraction used was 75-150 pm. To check on some of the conclusions concerning the interaction between coal and solvent, the Fouriertransform (FT-i.r.) spectra of various samples were recorded. The experimental technique has been described previously29.
8C
Q,
m
P i f 40
20
0
I
/I
I 200
I
100
150 T (“Cl
Figure 7 Plot of percentage residue versus extraction temperature. 0, Morwell brown coal/pentylamine; 0, North Dakota brown coal/hexylamine; A, Wyoming brown coal/hexylamine
RESULTS Temperature
AND DISCUSSION dependence of extract
yield
One of the most striking features of the dissolution of brown coals in amines and aqueous KOH is its strong dependence on temperature. This dependence is more marked for younger coals, and this is perhaps the reason why this effect has apparently gone unnoticed in the literature as brown coals have not been studied very extensively because of their extreme chemical heterogeneity5. In amine solvents the dissolution process reaches an appreciable rate at z 140°C (Figure I), and in aqueous KOH at ~80°C (Figure 2). It is instructive to compare these results with some obtained by Dryden3’, who found that a plot of log (extract yield (in EDA)) versus l/Twas linear over the entire temperature interval studied (room temperature to 117”Q in the present study such plots show pronounced kinks (Figure 3). In view of these results, it would seem that this is not a simple, unhindered extraction of some soluble component of coal, but that a chemical reaction between solvent and coal must occur before extensive dissolution can take place. As to the nature of the supposed chemical reaction, it is proposed that, although the temperatures involved may seem somewhat high, it is the breaking of ester bonds in coal that is the first step in brown-coal dissolution. That the change in solubility with temperature takes place over a relatively broad temperature range could then be ascribed to differences in physical and chemical environment between the various ester groups. It is of course conceivable that there are other reactive bonds in brown coals. For example, in Wender’s picture of a typical brown coa131, there occur a-hydroxy-ether OH (AH-04
linkages, which are easily broken by acids and bases, indeed, this is why cellulose degrades so quickly. Also,
348
FUEL, 1984, Vol 63, March
4 80-
y.****.....*.
60-
2 0 % (L
I I I I \ \
$
.. *. .. .. .... ..
T(‘C)
Figure 2 Plot of percentage residue versus temperature of extraction with KOH/HzO (1 /lo wt/wt). 0, Morwell brown coal; A, Wyoming brown coal
P-hydroxy-ketones OH
0
(A-CH2-L
may occur, which can undergo retro-aldol reactions. However, it is felt that, given the relatively large amounts of carboxylic and hydroxyl groups, ester bonds will form the main linkages between different coal fragments. To check whether 140°C is a reasonable temperature
Action
of solvents on coal at low temperatures.
5-
I
I
2.4 I/T,
2.8
32.10
K-’ (~10~)
Plot of log (extract yield) versus reciprocal temperature. 0, Morwell/pentylamine; A, Wyoming/aq. KOH Figure 3
for the initiation of aminolysis of coalLester groups, an i.r. study of the reaction between octylamine and methyl oleate and phenyl benzoate was conducted and the appearance and rate of growth of two peaks at 1655 and 1550 cm-‘, respectively, which are ascribed to the corresponding amine groups formed (RCOOR’ + R”NH,--, RCONHR” + R’OH) were monitored. It was found that at ~90°C (the benzoate is somewhat more reactive than the oleate) the aminolysis began to take place at a considerable rate. This is satisfactory, considering the effect of reagent access on the reactivity of coa132, though perhaps not completely convincing (see section on solvent effects). Problems due to limited accessibility of coal towards some reagents have been experienced in the determinations of COOH groups (see below), and in the esterification of these groups. In amyl alcohol/HCl some of the carboxylic groups could be esterified, but virtually none could when using dimethylformamide dipropylaceta13j (i.r. results). However, in the dissolution process accessibility problems will not be quite as severe, since amines and aqueous KOH swell brown coals strongly. Assuming that ester-bond breaking is the necessary first step in brown-coal dissolution the implications of this
Table 2 The effect of pretreatment
on the solubility
Coal
Solvent
Yallourn Blackfoot Wyoming Morwell Morwell Morwell Yallourn Yallourn
KOH/H20 l/10 KOH/H20 l/IO KOH/H20 1 00 EDA (reflux) EDA (reflux) PY (Soxhlet) EDA (reflux) EDA (reflux)
(reflux) (reflux) (reflux)
1: B. van Bodegom
et al.
hypothesis will be discussed. First, one would expect that the solubility in pyridine would not depend on temperature to any great extent, as pyridine is unable to break ester bonds. This is indeed the case for Morwell brown coal. Second, it should be possible to separate the bondbreaking from the dissolution step. Since ethanol and water are not solvents for coal, breaking of the esterbonds without dissolution can be effected by sodium ethanolate in ethanol, and by aqueous HCl. The results obtained by pretreating brown coals with EtONa or HCl (bond-breaking step), before treating them with a good solvent (dissolution step), are shown in Table 2. It is evident that the ethanolate treatment is very effective in enhancing the solubility of brown coals which supports the suggested hypothesis. The results of the aqueous HCl treatment are less clear. In the case of Morwell brown coal the effect of HCl is twofold: 1. it leaches out the Ca2+ ions bound to some carboxylic groups; and 2. it, supposedly, catalyses the hydrolysis of the ester groups present. The first effect in itself enhances coal solubility. To ascertain the influence of each effect in the present case, Morwell coal was treated with 0.1 N HCl at room temperature, which removes virtually all Ca2+, but hopefully does not affect greatly the ester groups; and with 1 N HCl, under reflux. The latter treatment does lead to a higher solubility than the former, but some doubt remains about the value of this piece of evidence for the hypothesis. Indeed, with Yallourn brown coal, which contains virtually no Ca2 + ions, such an HCl treatment does not result in an enhanced solubility in EDA. Taking into account that this particular coal has the highest functional-group content, it is possible that hydrolysis and re-esterification more or less compensate each other. To prevent ester linkages reforming between coal fragments a large amount of ethanol was added, which led to the expected enhanced solubility. Esterification of the carboxyl groups present as such is not judged to be the cause of this, since a treatment with amyl alcohol/HCl, admittedly not esterifying all (viz. z 75%) the carboxylic acid groups, did not lead to any change in solubility. This argument will only stand, however, if transesterification has not taken place and this is considered likely, in view of the bulk of the alcohol present which would cause insufficient access to reagent. Overall, it is considered justifiable to conclude that an ester-bond breaking step is necessary for extensive dissolution to take place. In this context it is perhaps worth noting that small molecules, which are soluble in solvents such as ethanol and pyridine, can be split off on alkaline hydrolysis34. The
of some brown coals
% Res.
Pretreatment
% Res. after pretreatment
21 92 62 57 57 87 34 34
EtONa/ETOH l/IO (150°C) EtONa/ETOH l/10 (150°C) EtONa/ETOH 1 /lO (150°C) 0.1 N HCI (room temp.) 1 N HCI (reflux) 1 N HCI (reflux) 1 N HCI (reflux) 1 N HCl/50% EtOH (reflux)
5 18 20 28 14 67 30 12
FUEL, 1984, Vol 63, March
349
Action of solvents on coal at low temperatures. I: B. van Bodegom et al.
former solvent was used in most of the pretreatment experiments (cf: Table 2), and, in fact, dark-yellow solutions were obtained in the case of Yallourn coal (first and last lines of Table 2). Also, the amount of ‘treated coal obtained indicates that a small fraction of the initial coal sample remains behind in the ethanol. Following an HCl pretreatment, the extractability of Morwell coal with pyridine was much enhanced (Table 2). Thus, although the coal as a whole is highly functionalized, a small part (<20x) of the hydrolysis product consists of not extensively functionalized molecules. As shown above, the chemical evidence for the existence of ester groups in brown coals is considerable and confirmation by obtaining the characteristic 1730 cm - ’ absorption band in the FT-i.r. spectrum was sought. Two spectra of Morwell coal are shown in Figure 4. The first is of an untreated sample. At 1730 cm-’ only the slightest of shoulders is visible on the main band due to carboxyl groups although it is perfectly reproducible. The second spectrum is of a sample treated with sodium acetate (or, alternatively, with NH,); if it was possible to convert all the COOH groups into Na carboxylates, thus shifting the absorption frequency to 1550 cm-‘, the 1730 cm-’ peak would be clearer. It was apparent that not all the carboxyl groups could be reached by the NaOAc reagent, although the shoulder was more convincing than in the first case. It should be noted, however, that the physical evidence for the presence of ester groups in brown coals remains rather weak. Dryden’ and Lahaye and Decroocq35 reported that secondary and tertiary amines are much less effective solvents than primary ones. This was confirmed as Nmethyl-MEA and N,N-dimethyl-MEA were found to cause far less solubilization of brown coals than MEA itself. Since N-methyl-MEA should in principle be capable of breaking ester bonds, the accepted explanation in terms of steric hindrance appears to apply. This view is supported by the fact that N-methyl-MEA is easily removed from the coal, whereas MEA is not (see below). The solubility of the coal fragments formed upon the breaking of ester bonds was also investigated. TWO aspects will be discussed: 1. the differences in solvent power between different solvents for the same coal, at the same temperature; and 2. the differences in extract yield
between different coals in the same solvent under reflux conditions. Efect of nature of solvent on coal solubility It is worth noting that the pretreated brown coals (Table 2) remain largely insoluble in the most common
solvents such as ethanol, pyridine, toluene, chloroform and even triglyme. They are very soluble, however, in the solvents that are also capable of breaking ester bonds in coal. As these solvents are strong bases and brown coals are highly acidic4*36, it is possible that acid/base interaction is important. In aqueous KOH, the acidic groups of the coal fragments will ionize, and this apparently gives rise to sufftcient solvation energy for dissolution to take place*. Some light is thrown on what happens in amine solvents by the results shown in Figure 5. It is seen that for Morwell brown coal, at 180°C where the ester-bond breaking reaction may be considered to be more or less complete (Figure I), the solubility depends on the alkylchain length of the n-alkylamines used as solvent. This finding may be interpreted as follows: the coal fragments per se are too polar to be soluble in the rather apolar amines ‘s3’ (some solubility parameters? are shown in Table 3). However, the amine interacts with the acidic groups of the coal, leading to salt or amide formation. Accordingly, the solubility of coal fragments bearing a number of alkyl groups should be considered, rather than that of the simple coal fragments. Therefore, the longer the alkyl chain, the more the coal fragment will resemble the solvent, and will dissolve more easily. A similar effect of chain length on solubility has been observed by Hodek and K611ing39for acetylated coal in pyridine, probably for the same reason. The solvent-polarity argument is strengthened by the fact that MEA dissolves Monvell brown coal almost completely at 170°C even though it gives rise to only very short chains on the coal fragments,
P\\_
60
\
\ 0 C3
b
I
I
I
I
I
C4
=5
=6
C,
C6
Figure 5 Plot of percentage residue as a function of alkyl-chain length; Morwellln-alkyl-amines, 180°C
I 1900
I
AI
I
1700
I 1500
I
I 1300
V (cm-‘) 4 Fourier-transform i.r. Spectra of Morwell (as received), and of Morwell exchanged with NaOAc, ---. Courtesy C. -, A. Emeis Figure
350
FUEL, 1984, Vol 63, March
* An explanation in temis of the [-potential is considered unlikely, since the KOH solution used is sufficiently concentrated for the diffuse double layer to be virtually absent 7 The solubility-parameter theory will not be discussed here, see, for example, refs. 15 and 37. Roughly speaking, the lower the solubility parameter, the less polar the solvent will be
Action
of solvents on coal at low temperatures.
1: B. van Bodegom
et al.
Table 3 Solubility parameters, S, caI1/2 crne312, of some soIvants38 Solvent
s
Solvent
6
Propylamine Alkylamine (>C,) Ethylenediamine Monoethanolamine
9.0 8.7 12.3 15.5
Pyridine Ethanol Benzene Chloroform
10.7 12.7 9.2 9.3
probably because MEA is far more polar than the alkylamines (cf. Table 3). The response of the brown coal/EDA system to an increase in temperature is somewhat more complex than that of systems with other amine solvents. At first, the solubility in EDA starts to increase with increasing temperature, as expected, but above 140°C the solubility starts to decrease (see Figure 6). This effect is probably related to the fact that an EDA molecule can interact with two acidic sites. At higher temperatures the second NH, group is apparently able to compete effectively with free EDA molecules for the available acidic groups, linking coal fragments together and making them insoluble. Another interesting feature of Figure 6 is that the sharp increase in solubility seems to occur at much lower temperatures than in the case of pentylamine (cc Figure I), and therefore much nearer the reaction temperatures observed in the model experiments. As EDA is a better coal solvent than the lower alkylamines7p37, this might indicate that the curves in Figure I reflect not only the extent of ester-bond breakage, but also the relatively poor solvent properties of hexyl- and, especially, pentylamine. This effect is illustrated by comparing Figures 1 and 7, in which the solubility of Morwell in a variety of solvents is shown. In conclusion, a good solvent for brown coal should be able to rupture the ester bonds which link the coal fragments together, and to dissolve the reaction product(s) of the solvent and the coal fragments.
oh Figure 7
I 50
I
I 150
100
I 200
b.p. ( “C )
Plot of percentage residue versus boiling point of the
solvent used. Extraction under reflux conditions; Morwell brown coal
Dependence
of solubility on coal characteristics
It is to be expected that there is no single property of brown coal that determines its solubility. It appears, however, that the carboxyl-group content plays a very important role. Figures 8 and 9 show the solubilities of brown coals in MEA and aqueous KOH, respectively, as a function of the number of COOH groups per unit CO2 surface area*. The following features should be noted: 1. First, it is clear that, of the untreated brown coals, two do not follow the main correlation. The reason for this is unknown, although it is likely to be due to a higher content of unreactive macerals which were not measured. 2. It is known that, in some brown coals, not all carboxyl groups are free, as some are associated with alkali or alkaline-earth ions4’. These ions are easily exchangeable and, for example, can be washed out by treating the coal with 0.1 N HCl at room temperature. By analysing the washings it was found that Ca’ + was by far the most abundant exchangeable cation in the samples. From Figure 8 it is apparent that removal of these Ca2 + ions enhances the solubility of the coal in MEA+.. The effect is reversible: after treating Morwell brown coal with a Ca(OAc), solution, its solubility is lower. These results cannot be explained by the fact that there is a carboxylate anion in one case, and a free carboxylic acid group in the other, since exchanging Morwell with NaOAc did not have any effect on its solubility in MEA.
100
I 120
I 140 T (“C)
I 160
I 180
figure 6 Plot of percentage residue as a function of extraction temperature; Morwell/EDA
* Assuming the accessibilities of coal to CO* and CH,COONa to be approximately the same, this is considered to be the best measure of carboxyl-group concentration t It is noteworthy that N. Dakota brown coal is affected by the HCI treatment in the same way as other brown coals, but its y0 res. t~ersus COOH-curve is displaced (by 25% res.) with respect to the main correlation
FUEL, 1984, Vol 63, March
351
Action of solvents on coal at low temperatures. 1: B. van Bodegom et al.
coal with ZnCl,/methanol, it is advantageous (in terms of subsequent solubility in Py) to wash the product with aqueous HCl rather than water, HCl probably removing bridging Zn2 ’ ions4’. Essentially the same effect accounts for the increased extractability of Wyodak coal by ethanol/benzene after removal of Ca2 + ions43. 3. The observed relation between the solubility of a coal and its free carboxyl-group concentration stimulated a study of the influence of oxidation/decarboxylation. In agreement with the established trend, oxidation led to enhanced solubility, while decarboxylation strongly decreased the solubility of the coal (Figures 8 and 9). With respect to aqueous KOH, this result tallies with the work of Mukherjee et a1.44,who found that humic acids extracted from coal with aqueous KOH did not redissolve after decarboxylation. The trend observed for brown-coal dissolution in MEA and aqueous KOH at 180°C is also apparent in the case of solvents used under reflux conditions, except for pyridine (Figure 10). This probably means that the fraction of coal extracted by the reactive solvents is representative of the whole coal, which is commonly thought to be the case2*45, while Py extracts only a particular component, that is, Py acts as a ‘nonspecific solvent’2*4 towards low-rank coals. The carboxylic groups in coal react with primary amines > = 140°C to form amides4’j as confirmed by i.r. This amide formation may contribute to the dissolution behaviour of brown coals. To investigate this possibility, the solubility in hexylamine of a High Abrasion Furnace (HAF) black was studied after it had been oxidized by refluxing it with 40% HNO, up to a content of 2.3 meq. COOH g-r black (10’ 7 carboxyl groups per m2), over a
Figure 8 Plot of percentage residue after MEA extraction (170°C) versus carboxyl-group concentration. S, SRS 800; B, Blackfoot; W, Wyoming; M, Morwell; N, Neurath; Y, Yallourn; D, North Dakota; P, Picardville. 0, untreated brown coals; 0, after HCI wash or Ca*’ exchange; H, after oxidation/decarboxylation
100 I AS I I A 80 -’ I I I I 60-
\
I \ I \
!i .2 a cc f
F4P
40\ BA \ \ \ 20-
\ Wb, \ \.
O0
I 2
\
\
.N M/\
[COOHI
Y 4 (10’8m~2)
6
8
Figure 9 Plot of percentage residue after KOH extraction (180°C) versus carboxyl-group concentration. A, untreated brown coals, for letters see caption to Figure 8; A, decarboxylated Morwell brown coal B I
0
Rather, the reason for this is probably the fact that Ca2 + ions can act as a cross-link between different coal fragments (X00 - . . . Ca2 + . , . -OOC-bridges), an effect which is well known in cellulose chemistry4r. It also has a parallel in coal chemistry: after treatment of
352
FUEL, 1984, Vol 63, March
0
wzp 2
MN I I 4 I COOH I ( lO’*ti’)
Y I I 6
Figure 70 Plot of percentage residue versus carboxyl-group content for various solvents. Extraction under reflux conditions. For letters see caption to Figure 8. HXA= hexylamine
I 8
Action
of solvents
broad temperature range (5&2oo”C). It was found that the amount of black retained in ‘solution’ after 20 minutes’ centrifugation was independent of temperature. It seems, therefore, that amide formation does not lead to a change in dissolution behaviour as compared with simple salt formation, which from i.r. values takes place at lower temperatures. Solvent recovery
An important condition for the applicability of any solubilization process, be it in solvent mining47,48 or in ash remova14g~50,is the almost complete recovery of the solvent. In this respect, the aliphatic primary amines perform very poorly. Some amine is lost by irreversible reactions with carboxyl and carbonyl groups in coal (up to x 2 mm01 amine g - ’ coal). Second, another part of the solvent is held so strongly by the coal that it is virtually irremovable. The solvent comes away only very slowly at temperatures < 200°C and higher temperatures cannot be used, because the coal starts decomposing, and the amine starts to react further, leading to an additional loss of amine, an effect observed by Dryden”. Neither can the solvent be completely removed by steam stripping or solvent exchange, a complication with the latter method being that part of the treated coal dissolves in the more effective exchange solvents (e.g., ethanol*). This strongly adsorbed fraction amounts to ~4 mmol amine g-’ coal, and, as will be shown in a subsequent paper, probably results from salt formation or strong hydrogen bonding, between phenolic OH groups in the coal and the amine solvent. The recovery of aqueous alkaline solutions is not discussed in any detail, but note that in a simple dissolution-preparation process the likely end result is either a coal product containing much water (up to 9073, or one with a large amount of ash (up to 50%). This is dictated by the nature of brown coals; they have ionizable groups and will therefore swell in contact with pure water, just like cellulose gel (or pul~)~~; the swelling can be suppressed by adding salts to the water phase, but then some of these will be incorporated in the coal mass.
CONCLUSIONS From the observed temperature dependence of the solubility of brown coal in various solvents and from the effect of pretreating brown coals with 1 N HCl or 10% sodium ethanolate on their solubility, it is concluded that the soluble brown-coal fragments are bound in the coal mass via ester bonds and that these ester bonds should be broken before extensive dissolution can take place. It is interesting to note that Whitehurst inferred the existence of ester groups in low-rank coals from the products obtained in short-contact-time liquefaction experiments. However, the physical evidence for ester groups remains rather weak. The coal fragments themselves seem to be soluble only in strong bases. This is probably related to the fact that * This is rather remarkable in view of the fact that the coal fragments per se are hardly soluble in ethanol. Perhaps the incorporation of alkyl groups as a consequence of the coal-amine reaction does indeed diminish internal hydrogen bonding, a similar explanation being frequently put forward in relation to the solubility of acetylated and alkylated coals5’. As an example, the pentylamine extract of Morweli brown coal is 75% soluble in ethanol
on coal at low temperatures.
I: B. van Bodegom
et al.
brown coals contain many acidic groups. It was found that the solubility in primary amines and aqueous KOH correlated with the carboxylic group concentration, though it appears that at least one other coal parameter is important in determining brown-coal solubility. Why is a strong acid/base interaction necessary for the solubilization of brown-coal fragments? In the case of aqueous KOH, the reason is probably that the ionization of the acidic groups provides enough solvation energy for solubilization to take place. In the case of primary amines, it may be that the adducts or other entities formed between the acidic groups of the coal and the basic solvent destroy the inter-fragment hydrogen bonding, thus increasing the solubility of the individual fragments. This explanation may also account for the fact that ethanol is not a good solvent for the brown-coal fragments per se, but is for amine-treated brown coal. The amines are not all equally effective, however, when compared at the same temperature: the more polar solvents, EDA and MEA, are best, while the solvent power of n-alkylamines increases with increasing alkylchain length. It is proposed that this effect should be interpreted in terms of a solubility parameter (6) theory: one should, however, compare 6 (solvent) not with 6 (coal), but with the 6 of the reaction product of the coal with the solvent. It appears, then, that solvents which dissolve brown coals to a considerable extent, aqueous alkali hydroxide and primary amines, are not solvents in the strict sense, but reactive solvents in Van Krevelen’s terminology4, combining the ability to break ester bonds with the ability to solubilize the coal fragments thus produced. Since the days of Dryden’, Kriiger3,54, and Van Krevelen7, little work has been carried out on the dissolution of brown coals in amine solvents. Their theories, however, disregarding as they do the importance of bond-breaking reactions, should now be considered to be incomplete. The dissolution behaviour of brown coals in aqueous alkali-hydroxide solutions has remained under study by various workers in Australia22.32,45*46,55,56.The possibility of ester-bond breaking does not figure strongly in their theories, but then, they effect the extractions often at low temperatures (e.g., at room temperature). Thus, the present findings can be considered to be an extension to their work, rather than superseding it. ACKNOWLEDGEMENT Interesting discussions with Dr P. Biloen and Dr P. A. Verbrugge are gratefully acknowledged. The i.r. spectroscopy work was carried out by Dr C. A. Emeis, the electron microscopy work by Mr A. Knoester. REFERENCES 1 2 3 4 5 6
Kiebler, M. K. ‘Chemistry ofCoal Utilization’(Fd. H. H. Lowry), Wiley, 1945, Vol. I, p. 715 Dryden, I. G. C. ‘Chemistry of Coal Utilization’ (Ed. H. H. Lowry), Supplementary Volume, Wiley, 1963, p. 237 Krijger, C. Erdtil und Kohle 1956, 9,44, 516,620, 839 van Krevelen, D. W. ‘Coal’, Elsevier, 1961 Davidson, R. M. ‘Molecular Structure of Coal’, Report No. ICTIS/TR08, IEA Coal Research, 1980 Wender, I., Heredy, L. A., Neuworth, M. B. and Dryden, I. G. C. ‘Chemistry of Coal Utilization’ (Ed. M. A. Elliott), 2nd Suppl. Vol., Wiley, 1981, p. 425
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Action 7 8 9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
354
of solvents on coal at low temperatures.
1: B. van Bodegom
van Krevelen, D. W. Fuel 1965,44,229 Dryden, I. G. C. Fuel 1951,30, 145,217 Dryden, I. G. C. Nature 1949,163, 141 Dryden, I. G. C. Fuel 1951,30, 39 Malanowitz, L. Chim. Ind. 1932,28, 1277 Bunte, K., Briickner, H. and Simpson, H. G. Fuel 1933, 12,26 Stockmayer, W. H. J. Chem. Phys. 1943, 11,45 ZilT, R. M. and Stell, G. J. Chem. Phys. 1980, 73, 3492 Barton, A. F. M. Chem. Rev. 1975, 75, 731 Tager, A. A. and Kohnakova, L. K. Pol. Sci. USSR 1981,22,533 Bangham, D. B. and Dryden, I. G. C. Fuel 1950,29,291 Rybicka, S. M. Fuel 1959,38,45 Harris, R. L., Simons, L. H. and Lagowski, J. J. Am. Chem. Sot. Div. Fuel Chem., Preprints 1979, U, 264 Camier, R. J., Siemon, S. R., Battaerd, H. A. J. and Stanmore, B. R. ‘Coal Structure’ Adu. Chem. Ser. 1981, 192, 311 Liotta, R., Rose, K. and Hippo, E. J. Org. Chem. 1981, 46, 277 Brooks, J. D. and Stemhell, S. Fuel 1958, 37, 124 Dubinin, M. M. Chem. Reu. 1960,60,235 Lindsay, D. A. MSc. Thesis, University of California, 1978, LBL McPhail, I. and Murray, J. B. Miscellaneous Report No. MR155, 1969, State Electricity Commission of Victoria Donnet, J. B. and Voet, A. ‘Carbon Black’, Marcel Dekker Inc., 1976 Donnet, J. B. and Riess, G. Peintures-Pigments-timis 1964,40, 426 Rodgers, B. R. Oak Ridge National Laboratory, ORNL-5631, 1980 Bouwman, R. and Freriks, I. L. C. Fuel 1980,59,315 Dryden, I. G. C. Fuel 1952.31, 176 Winder, I. Catal. Rev-Sci. Eng. 1976, 14,97 Larsen. J. W.. Green. T. K.. Choudhurv. P. and Kuemmerle. E. W. ‘Coal Siructure’ Adv. them. Ser. Ib81, 192, 277 Brechbiihler, H., Biichi, H., Hatz, E., Schreiber, J. and Eschenmoser, A. Helv. Chim. Acta 1965,48, 1746 Chaffee, A. L., Perry, G. J., Johns, R. B. and George, A. M. ‘Coal Structure’ Adu. Chem. Ser. 1981, 192, 113
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Lahaye, Ph. and Decroocq, D. Rev. Inst. Franc. Petrole 1976,31, 99 Whitehurst, D. D., Mitchell, T. 0. and Farcasiu, M. ‘Coal Liquefaction. The Chemistry and Technology of Thermal Processes’, Academic Press, 1980 Hombach, H. P. Fuel 1980,59,465 ‘Polymer Handbook’ (Eds. J. Brandrup and E. H. Immergut), Interscience, 1975 Hodek, W. and KGlling, G. Fuel 1973,52, 220 Durie, R. A. Fuel 1961,40,407 Paist, W. D. ‘Cellulosics’, Reinhold, 1958 Shinn, J. H. and Vermeulen, T. ‘Coal Liquefaction Fundamentals’ Am. Chem. Sot. Symp. Ser. 1980,139,227 Base&Z. H., Pancirov, R. J. and Ashe, T. R. Adu. Org. Geochem. 1979,619 Mukherjee, P. N., Bhowmik, J. N. and Lahiri, A. Fuel 1957,36, 417 Durie, R. A. and Stemhell, S. Austr. J. Appl. Sci. 1958,9, 370 Butler, R. N., Thornton, J. D. and Moynihan, P. J. Chem. Research (S) 1981, 84 Drinkard, G., Prats, M. and O’Brien, S. M. US Patent 4,032,193, 1974 Wise, D. L. and Augenstein, D. C. In situ 1978, 2, 173 Araya, P. E., Badilla-Ohlbaum, R. and Droguett, S. E. Fuel 1981, 60, 1127 Jenkins, R. Patent WO SI/OZSSO,1981 Pullen, J. R. ‘Solvent Extraction of Coal’, Report No. ICTIS/ TR16. IEA Coal Research. London. 1981 Grignon, J. and Scallan, Al M. J. Abpl. Pol. Sci. 1980,25,2829 Whitehurst, D. D. ‘Organic Chemistry of Coal’ Am. Chem. Sot. Symp. Ser. 1978, 71, 1 KrGger, C. and Kaspers, H. H. Erdiil Kohle-Erdgas-Petrochem.
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35 36
37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53
1962,15,90
331