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Studies related to the structure and reactivity of coals
Studies related to the structure reactivity of coals
and
18. The chemical structures of products from reactions of a suite of higher rank Australian coals Peter J. Redlich, W. Roy Jackson, and lmants Liepat
Frank
P. Larkins*,
Alan
L. Chaffee?
Department of Chemistry, Monash University, Clayton, Victoria, Australia 3 168 *Department of Chemistry, University of Tasmania, G.P.O. Box 252C, Hobart, Tasmania, Australia 700 1 tCSIR0 Division of Fuel Technology, Private Mail Bag 7, Menai, N.S. W., Australia 2234 (Received 4 November 1988; revised 7 June 1989)
The structure of the oils, asphaltenes and residues obtained by the thermal reactions of a suite of Australian higher rank coals under hydrogen or nitrogen have been studied by chemical and spectroscopic methods. The host-guest model that has been used to describe the structure of Australian brown coals cannot be applied directly to the higher rank coals. Evidence is provided that suggests that a modified version of the model may be of use in describing the structure of some subbituminous coals. The methodology has proved to be useful in the understanding of structural features of coals which are often not rank dependent, e.g. Callide coal (ABL2), a subbituminous coal, has been shown to have characteristics of both very high and also low rank coals. (Keywords: characterization of coal; chemical structure; reactivity) It has been shown in previous
paperslp6 that the structure of low sulphur Victorian brown coals can be explained in terms of a two component (guest-host) model. The relative concentration of the components greatly affects the atomic H/C ratio of the brown coal and its reactivity under liquefaction conditions. The reactivity of higher rank coals is also related to the atomic H/C ratio’,2, and it was thus of interest to see if the model could be extended to these coals. It should be stressed that the host-guest model derived by us is similar to other two-component coal structure models developed previously but important differences exist which have been discussed in a previous paper3. The methodology used was similar to that in our previous investigation of brown coals, i.e. reaction products obtained from liquefaction using hydrogen in the presence of either a tin catalyst or tetralin were compared with products of thermolysis under nitrogen, under conditions of moderate severity’g2. Other strategies have frequently been used to study the structure of higher-rank coals. For example, Dong et ~1.‘~~have converted a large part of a subbituminous coal into low molecular weight compounds by repetitive hydrogenation under relatively mild conditions. Others have chemically modified the coal structure prior to reductive degradation. In a recent study coal was alkylated prior to lithium aluminium hydride reduction and product extractiong. These methods, related methods and methods based on oxidative coal degradation have been summarized”,’ ‘. Each strategy has its limitations, but we believe that reaction at moderate temperatures, between 300 and 405”C, can lead to valuable structural information for relatively reactive coals’ ,2.
0016-2361/89/121549--09S3.00 0 1989 Butterworth & Co. (Publishers) Ltd.
The brown coals described in previous work were all highly reactive, but some of the higher rank coals displayed reduced reactivity. Hence, increased emphasis must be placed on the characterization of the higher molecular weight products (asphaltenes and residues) if the structure of higher rank coals is to be elucidated. Although g.c.-m.s. cannot be used with these fractions, other analyses can still give significant information. Apart from reduced reactivity, the higher rank coals differ in other respects to the brown coals. The brown coals previously investigated do not vary in rank even though they exhibit large changes in H/C ratio (0.82-1.25 CO, free basis) 2,12. In contrast, for many higher rank coals, the changes in the H/C ratio are caused by structural changes in the coal matrix so that variations in the nature of the coal liquefaction products are likely to reflect these fundamental changes in structure. Differences in coals may also to some degree reflect different plant origins and different coalification processes. These questions are addressed below. EXPERIMENTAL The coals and their hydroliquefaction reactions have been described previously1*2. Products were analysed by elemental analysis, non-aqueous titration, solid state ’ 3C n.m.r., ‘H and 13C n.m.r. (solution), FT-i.r. and g.c.-m.s. using instrumentation and methods described previously3-5*‘2. Coals
The higher rank coals used in this study, ABLl to ABLlS ranged in rank from subbituminous to semi-anthracite with carbon contents (wt% dmif)
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Table 1 The H/C ratio, carbon content and oil yields from SnO,/ hydrogen reactions without solvent at 405°C of six higher rank coals Carbon content
Oil yield
Code
Coal”
H/C CO, free
(wt% dmif CO, free)b
ABL2 ABL3 ABM ABL8 ABL12 ABL14
Callide Bulli Wongawilli Leigh Creek Greta Taroom
0.58 0.64 0.70 0.73 0.86 0.95
81.3 88.4 87.0 76.2 83.4 77.4
7.8 5.0 12.4 13.8 23.0 31.5
et al.
The chromatograms of the oils from the medium volatile oxygen rich subbituminous coals (Figure 2) are dominated by phenolic products, especially phenol and cresols (A, B, C). Nevertheless, there is a similar reduction in the relative abundance of the n-alkanes, and an increase in the relative abundance of aromatic hydrocarbons (with increase in carbon content) as outlined above within this pair of samples.
‘See Refs. 1 and 2 for full details of coal characterization b For a discussion of the advantages of expressing elemental and conversion data on a CO, free basis see Refs. 2 and 12
between 90% (ABLl) and 73% (ABL8) and atomic H/C ratios of 0.96 (ABL15) to 0.51 (ABLl). Full details of the analytical data for these coals have been given in a previous paper 12. The coals were reacted in batch autoclaves with hydrogen (6 MPa initial) in the presence of tetralin’*2 at 405” and in addition they were reacted with hydrogen in the presence of SnO, but without solvent (405°C 10 MPa initial H,) to obtain oils free from reaction solvent. Experiments were also conducted at 350°C under N, (6 MPa initial) in the absence of solvent and in the presence of decalin (solvent to coal ratio, 2:l).
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A subsuite of six oils was chosen for more detailed analysis on the basis of their parent coal being representative of a particular rank range. This included two oils derived from oxygen rich subbituminous coals. It should be noted that while the oil yields obtained from reaction of the brown coals at 405°C accounted for 15-60 wt% (daf) of the coal’y2, oil yields from higher rank coals represented only 5-35%. The low yields severely limited the number of techniques that could be used for oil analysis. G.c.-m.s. and ‘H n.m.r. were the most useful with other techniques being used only if sufficient samples were available.
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RESULTS
AND DISCUSSION
d
Oils
The carbon content, H/C ratio and oil yield for the subsuite of six coals used to obtain solvent free oils are summarized in Table 1. G.c.-m.s. The oils from each of the six solventless reactions were examined by g.c.-m.s. The results are best considered by distinguishing the two oxygen rich medium volatile subbituminous coals Callide, ABLZ, and Leigh Creek, ABL8, from the medium-high volatile subbituminous and bituminous coals. Figures la-d clearly show that for the latter four coals there is a reduction in the relative abundance of n-alkanes and a corresponding increase in the relative abundance of mono-, di- and tri-aromatic hydrocarbons with increasing carbon content (see peaks D, E, F). Phenolic compounds are present in significant quantities in all cases, but do not exhibit a distinct trend with rank (see peaks A, B, C).
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Figure 1 The total ion chromatograms of oils derived from reaction of medium to high volatile subbituminous and bituminous coals with stannic oxide (SnO,) and hydrogen at 405°C with no solvent: a, ABL3 (88.4 wt% dmif C); b, ABM (87.0 wt% dmif C); c, ABL12 (83.4 wt% dmif C); d, ABL14 (77.4 wt% dmif C). Symbols: A, phenol; B, cresol; C, alkylcresols; D, naphthalene; E, methylnaphthalenes; F, C,alkylnaphthalenes; G, C,-alkylnaphthalenes; H, C,-alkylnaphthalenes; N, C,-alkylbenzenes; 0, C,-alkylbenzenes; T, phenanthrene; U, C,-phenanthrenes; Y, decalins; Z, phthalates; ip, isoprenoids; integer number, carbon number of particular n-alkane
Studies related to the structure
and reactivity
of coals. 18: P. J. Redlich
et al.
34 shows that not only were the shorter chain n-alkanes
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more abundant in the region C9-C1s in the ABL12 oil, but the second maximum occurred at 2-3 carbon number less than in ABL14 derived oil. The n-alkane distribution of the oils derived from ABL6 and ABL3 coals (Figures 3b and 3~) were similar to each other, and showed a broad maximum in the C14-C17 n-alkane range with long chain alkanes of C,, or larger being insignificant. The trend toward decreasing chain length with increasing rank of coals is well documented’4v’5. This is clearly the result of increased metamorphic activity, which culminates in the fully aromatic structure of anthracite. It should be noted that not only did the carbon chain length decrease, I-
i
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Figure 2
The total ion chromatograms of oils derived from reaction of oxygen rich medium volatile subbituminous coals with stannic oxide (SnO,) and hydrogen at 405°C with no solvent: a, ABL2 (81.3 wt% dmif C); b, ABL8 (76.2 wt% dmif C). For identification of peaks see legend of Figure 2
Closer examination of the chromatograms yielded some interesting structural information although the low yields of oil must be taken into account when discussing the coals with lower H/C values. First, the distributions of phenols, cresols, and higher homologues (peaks A, B, C) were seen to be produced in similar ratios for coals varying in oxygen and carbon content (see Figure I). In addition, non-aqueous titration of these oils (ABL14, 12 and 6) showed similar percentages of phenolic material (2.4-2.7%) even though the parent coals had very different oxygen contents”. These observations suggest that the part of the coal oxygen containing structure that forms oil is similar over this group of coals, even though they show a wide range of atomic H/C ratios and oxygen contents. The distribution of aromatic hydrocarbon homologues and isomers also varied regularly as a function of rank. The most intriguing of these variations was for the anthracene/phenanthrene ratio (not illustrated) which was observed to decrease from about 1 for ABL14 to zero for ABL3. Phenanthrene and its homologues are often considered to have formed largely by aromatization of diterpenoid natural products during coalification’ 3. However, the origin of anthracene and its homologues is more obscure. At present we cannot adequately explain this observation. The relative distribution of n-alkanes was more clearly seen in single ion chromatograms of the oils (Figures 3 and 4). Oils from the subbituminous coals ABL14 (Figure 34 and ABLS (Figure 4b) showed a distributional maximum in the CZ3-CZ5 carbon range similar to that observed in brown coals3. The n-alkane distribution pattern for the oil derived from ABL12 (Figure 3c) showed a bimodal distribution with maxima occurring at C,, and C,,. Comparison of the two oils (Figures 3c,
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ip ir) ‘151E i
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Figure 3 The single ion chromatograms (m/z 71) for the n-alkane distributions of oils given in Figure I: a, ABL3 (81.3 wt% dmif C); b, ABL6 (87.0 wt% dmif C); c, ABL12 (83.4 wt% dmif C); d, ABL14 (77.4 wt% dmif C). For identification of peaks see legend of Figure 1
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Studies related to the structure and reactivity of coals. 78: P. J. Redlich et al.
Like the H,, values obtained from the ‘H n.m.r. spectra, the f’ values of the oils calculated from the solution i3C n.m.r. spectra decreased with increasing H/C, from 0.70 for ABL3 to 0.61 for ABL12 and 0.50 for ABL14. The average length of the aliphatic chains was estimated by the method of Cooksoni6 and showed a decrease in the average length of hydrocarbons, from 27 for ABL14 to 14 for ABL3, consistent with the trend seen in the m/z=71 (n-alkane) single-ion plots for the oils from high rank coals (Figure 3). 1
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Figure 4 The single ion chromatograms (m/z 71) for the n-alkane distribution of oils given in Figure 2: a, ABL2 (81.3 wt% dmif C); b, ABLS (76.2 wt% dmif C). For identification of peaks see legend of Figure 1
but there was also a significant decrease in total oil yield from 30 to 5% indicating that the n-alkanes are of decreasing significance in the overall structure of the low H/C, high rank coals. The single ion chromatogram of oil derived from ABL2 (Callide) showed compound distributions seen in both low and very high rank coals with two maxima in the n-alkane distributions near C,, and Cz4 (Figure 4~). ABL2 also shows dichotomous behaviour in features of its elemental analysis in that it has a very low H/C ratio (0.58 CO,-free basis), which is characteristic of very high rank coals in this suite, but the oxygen content of a typical subbituminous coal. Thus the average chain length of the hydrocarbons in coal-derived oils does not correlate with the atomic H/C ratio of the coal, but is more strongly correlated with coal rank or carbon content as is further exemplified by comparison of the single ion chromatograms of the oils from two coal pairs of similar H/C but different rank (ABL6, ABL8, ABL2 and ABL3). However, in contrast it is notable that the relative abundance of the isoprenoid alkanes (C,2-C20) is enhanced for oils from lower H/C ratio coals in both subsets. The ratio of isoprenoid hydrocarbons phytane/pristane (ip-C&p-Cig) also increases with H/C within each subset. N.m.r. The trends observed in g.c.-m.s. studies of the oils were supported by ‘H n.m.r. spectra, which showed the general increase in H, (associated with methylene chains) and decrease in H,, and H, with increasing H/C ratio (Figure 5). The n.m.r. data also show the differences between ABL3 and ABL6 oils as suggested by g.c.-m.s. studies; ABL3 has a higher H, content (0.39) than ABL6 (0.30) in agreement with the former oil having a higher concentration of short chain substituted aromatics (Figures la and lb).
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Asphaltenes Asphaltenes derived from reactions of higher rank coals in the presence of tetralin and hydrogen, and with SnO, in the absence of solvent at 405°C were analysed by non-aqueous titration and by n.m.r. techniques. The two sets of asphaltenes differed only in that those derived from SnO,/no solvent reactions consistently showed lower oxygen contents (by w2%), in agreement with the greater yields of Hz0 and CO, in these reactions’. The similarity in the analyses shows that incorporation of tetralin or its products into asphaltenes was not a major problem. The results presented below refer to asphaltenes obtained from reactions in tetralin. Elemental analysis and non-aqueous titration. The elemental composition of asphaltenes from reaction of a subsuite of higher rank coals at 405°C with tetralin is given in Table 2. As noted previously, the most significant feature was the strong correlation between the H/C ratio of the asphaltenes and that of the parent coals’. The carbon contents of the asphaltenes and of the coals were broadly related as seen from the results in Table 2 and Table 1 of Refs. 1 and 12. A similar but reciprocal trend was seen in oxygen contents though actual values may be subject to relatively high error. In the present work, it was initially assumed that the contribution from acidic
0.5
ABL14
-
a
4 ‘I 3 iiw 8
0.6
-
0.3
_
2 a”
cx
4 :: E
0.2-
0.1 -
1
0.5
,
0.6
0.7
0.8
0.9
1.0
Coal H/C ratio (CO* free)
The relationship between the hydrogen distribution in all the coal derived oils, determined by ‘H n.m.r., and atomic H/C ratio of their parent coals: +, H,,; A, HP; 0, H,. NB. Oils were derived from reaction of coals with SnO, and hydrogen in the absence of solvent Figure 5
Studies
related
to the structure
Table 2 Some chemical data for the asphaltenes derived from reactions of higher rank coals at 405°C in tetralin and hydrogen
Coal
_-__ C
Elemental composition (wt% dmif) H
5.56 84.5 ABL2 5.61 87.9 ABL3 83.4 5.80 ABL4 6.27 85.2 ABL5 86.7 6.16 ABL6 5.95 83.8 ABLI 6.05 ABLlO 84.1 6.35 ABL12 86.5 6.34 ABL13 82.6 ABL14 84.2 6.60 6.67 ABL15 83.7 ___-a Solid state 13C n.m.r. For n.a., Not analysed
0
H/C
Phenohc 0 (wt% dmif)
i3C n.m.r. f,”
1.9 3.5 6.9 6.5 3.9 8.2 8.4 5.0 9.0 7.3 6.6
0.79 0.77 0.84 0.88 0.85 0.85 0.86 0.88 0.92 0.94 0.96
na. n.a. 7.9 n.a. 5.0 8.7 6.4 n.a. 6.2 na. 7.4
0.79 0.71 0.72 0.68 0.71 0.71 0.64 0.66 0.64 0.64 0.66
conditions see Refs. 1 and 12
nitrogen was negligible and that the acidic oxygen present in these asphaltenes is phenolic. No evidence for carboxylic oxygen was found in the FT-i.r. spectra of these asphaltene?. The phenolic oxygen content, determined by nonaqueous titration, is also given in Table 2. However, in most cases, the results were higher than the total oxygen content in the asphaltenes, a feature not noted in analysis of brown coal asphaltenes. Since the analysis assumes that only oxygen groups are acidic, a possible source of error in the acidic oxygen figures is that protons bound to nitrogen are also titrated. It has been reported that protons bound to the nitrogen in carbazole can be easily titrated17, and this has been confirmed in our laboratory. The nitrogen content of the asphaltenes and coals (1.5-2.5 wt%) is significantly higher than in lower rank coals (0.5-1.0%) and if acidic nitrogen is titrated this will affect our acidic oxygen figures. ‘H nmr. spectroscopy. The distribution trends of H,,, H, and H, for the asphaltenes are shown as a function of the H/C ratio of their parent coals in Figure 6. The significant features of the trends in hydrogen distribution are : There was a large variation in all parameters in contrast to the case for brown coal asphaltenes4. This is in agreement with the suggestion that the structural characteristics of the higher rank asphaltenes are dependent on their parent coal’. The H, fraction, but not H,, increased with the H/C ratio of the parent coal. In brown coals the methylene chains appeared mainly in the oil fraction3,4, whereas this observation suggests that the n-alkanes in the oil fraction of high-rank coals do not represent the total n-alkane content of these coals. Probably in high-rank coals many of the methylene chains are more firmly bound to other parts of the coal structure than in brown coals, so that they are not cleaved from them under our liquefaction conditions and are contained in the asphaltene fraction. As for the oils, H,, varied inversely with the H/C ratio of the original coal. The distribution of aromatic protons shows changes in structural characteristics. In the spectra of asphaltenes derived from relatively high rank coals such as ABL6 and ABL3, the distribution of the aromatic hydrogen
and reactivity
of coals. 18: P. J. Redlich
et al.
maximized between 7.5 and 8 ppm, which is indicative of a higher concentration of protons attached to polyaromatic clusters rather than to single ringsi8. In lower rank coal derived asphaltenes, the maximum of the aromatic distribution coincided with the CHCl, peak at 7.25 ppm, suggesting a higher concentration of single ring compounds”. 13C solid state n.m.r. The f, values of higher rank coal-derived asphaltenes varied over a narrow range, 0.64-0.72, with the exception of the asphaltene derived from ABL2 (Callide) which had a value of 0.79 (Table 2). Asphaltenes from ABLlO to ABL15 had values of 0.65 f 0.01. These are significantly lower than the values for asphaltenes derived from brown coals of similar H/C ratio4. This observation supports the conclusions drawn from ‘H n.m.r. spectra and elemental analysis above that aliphatic methylene chains are more strongly attached to the macromolecular structure in higher rank coals than in brown coals, so that many are found in the asphaltene fraction of high-rank coal products. Further evidence is seen in the i3C n.m.r. aliphatic resonances of four asphaltenes derived from coals in the higher H/C range 0.7-0.95, which are shown in Figure 7 with their respective H, and H, values. For the asphaltene derived from ABL6 coal, the aliphatic peak at 15-20 ppm was larger than the peak at 30 ppm. However as the H/C ratio of the parent coal (and asphaltene) increased, the 30 ppm peak increased until for ABL14 asphaltene this peak was dominant. This trend corresponds to an increase in H, values from 0.20 -+ 0.35 as well as a decrease in H, values, and is indicative of an increase in concentration of long chain aliphatic material. The increase in intensity of the 30 ppm resonance is probably
415
8
A
l*
A AAA A
A A
I 0.5
1
0.6
0.7
0.8
0.9
Coal H/C ratio (CO2 free)
Figure 6 The relationship between the hydrogen distribution in all the coal derived asphaltenes, determined by ‘H n.m.r. and atomic H/C ratio of their parent coals: +, H,,; A, H,; 0, H,. Integer numbers indicate ABL coal code. NB. Asphaltenes were derived from reaction of coals with tetralin under hydrogen at 405°C
FUEL, 1989, Vol 68, December
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Studies related to the structure and reactivity of coals. 18: P. J. Redlich et al.
fa
There were, however, two exceptions. The phenolic oxygen content of the residues from ABL2 and 8 was only 20-30% of the total oxygen content.
= .66
H, = .31 Hg = .35
13C solid state n.m.r. The fraction of aromatic carbon present in the residues was determined by solid state 13C n.m.r. The f, values (Table 3) varied between 0.81 and 0.91 and showed no significant correlation with H/C of the residues or of the parent coals. The residues could be expected to contain even more quaternary carbon atoms than their parent coals and the difficulties of observing such carbon atoms reduces the accuracy of this technique’ 9.
a
f, = .66 H, = .36 Hg = .27
Reactions of two maceral concentrates from Pike’s Gully coal (ABLS and ABLlO)
fa = .64 Ha = .40 Hg = .23
f, = .71 H, = .38 Hg = .20
d
250
/\/
150 PPM
50
0
Figure 7 The solid state r3C n.m.r. spectra and selected 'H n.m.r. data for asphaltenes derived from reaction of medium-high volatile subbituminous and bituminous coals in tetralin under hydrogen at 405°C: a, ABL14; b, ABL12; c, ABLlO; d, ABL6; +, aromatic spinning sidebands
not due to increasing hydroaromatic species because this would also increase H, values, which did not occur.
Two hand-picked maceral concentrates of one coal, Pike’s Gully, were obtained which had very different H/C ratios. The vitrinite rich sample ABLlO had an H/C value of 0.80 and the inertinite rich sample ABL5 had an H/C ratio of 0.67. Maceral distributions” and reactivities? have been summarized previously. It was noted’ that there was a large difference between the asphaltene yields from these two coals, which was not typical of reaction of higher rank coals. It was thus of interest to study the structure of these asphaltenes. Although the elemental composition of the two samples was similar (Table 2), the n.m.r. data were significantly different. The ABLS asphaltene had H,, 0.40 and H, 0.34 whereas ABLlO asphaltene had H,, 0.32 and H, 0.40, even though f, of both samples was the same. Use of the n.m.r. data in the Brown-Ladner equation” gave values of e (the fraction of available aromatic edge atoms occupied by substituents) of 0.36 for ABR5 and 0.47 for ABRlO suggesting a higher degree of ring substitution in the asphaltene from ABRlO. This conclusion is supported by a comparison of the intensity of the aromatic 700-950 cm-’ C-H band frequencies in the FT-i.r. spectrum of the two asphaltenes (Figure 8). The 750 cm-’ band, attributed to groups of five adjacent aromatic hydrogens, was significantly more intense in the ABL5 (inertinite) asphaltene than in the ABLlO asphaltene, which indicates fewer substituted aromatic rings in ABL5 asphaltene. Thus the larger yield of asphaltene from the vitrinite rich sample is associated with more extensive decom-
Reaction residues
Residues derived from liquefaction of higher rank coals contain a higher ash content (20-60% db) than for brown coals primarily because of the high ash content of the parent coals I2 . Thus the problems of analysis were more acute than those encountered previously4. Elemental analysis. The elemental composition
of 11 residues derived from reaction of their parent coals at 405°C with tetralin is given in Table 3. The most significant feature of the results is that there was a strong correlation between the H/C ratio of the residues derived from the higher rank coals after reaction at 405°C with tetralin and the H/C ratio of their parent coals’. Furthermore, the H/C ratios of residues were significantly higher than those from brown coals of the same H/C value. The acidic oxygen content (Table 3) of the residues, determined by non-aqueous titration, showed that in most cases 40-60% of the oxygen was in phenolic form.
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FUEL, 1989, Vol 68, December
Table 3 Some chemical data for the residues derived from reactions of higher rank coals at 405°C in tetralin and hydrogen Elemental composition (wt% dmif) Coal
C
H
N
O+s”
H/C
ABL2 ABL3 ABL4 ABLS ABL6 ABLI ABLlO ABLll ABLl2 ABL14 ABL15
85.2 91.3 83.1 87.5 90.9 83.8 82.3 83.7 84.0 80.4 84.1
4.00 4.84 4.13 4.31 4.99 4.06 4.93 4.99 5.02 5.32 5.96
1.4 1.6 1.8 2.1 2.4 2.1 2.3 2.4 2.2 2.0 2.0
9.4 2.3 11.1 6.1 1.7 10.0 10.5 8.9 8.8 12.3 7.9
0.56 0.64 0.60 0.59 0.66 0.58 0.72 0.72 0.72 0.79 0.85
Phenohc 0 (wt% dmif) 3.4 1.0 na. 3.2 ;::. 4.1 n.a. n.a. n.a. 3.6
’ By difference b Solid state 13C n.m.r. For conditions see Refs. 1 and 12 n.a., Not analysed
fab 0.83 0.87 0.91
0.86 0.81 0.81 0.81 0.85 n.a. n.a. 0.85
Studies related to the structure and reactivity of coals. 18: P. J. Redlich et al.
asphaltene was significantly more aliphatic than that obtained under standard hydrogenation reaction conditions at 405°C (Table 5)*. Reaction of ABLl5 at 375°C led to higher conversion but the overall characteristics of the asphaltene had become more like those obtained at 405°C in tetralin (Table 5). Therefore, the above results showed that 350°C was required to remove a significant portion of the aliphatic material although some degradation of the macromolecular structure at this temperature may have also contributed to the products. A subsuite of six coals, ranging in H/C values from 0.58 to 0.96, was reacted at 350°C under nitrogen both in the presence and absence of decalin (Table 6). In contrast to reactions of brown coals of similar H/C*, reactions in the absence of decalin gave significantly lower conversions, but the reactions were carried out to obtain oil fractions suitable for analysis. Comparison of the conversion data for decalin reactions with those of brown coals of similar H/C showed similar values*. Coals of H/C < 0.75 remained essentially unreactive at 350°C. The products from these reactions were analysed by elemental analysis, g.c.-m.s. and n.m.r. ?
700
8Ob
900
cm
-1
Figure 8 The FT-i.r. spectra (700-950 cm-’ region) of asphaltenes derived from reaction of: a, ABLS; and b, ABLlO coals with tetralin and hydrogen at 405°C
Table 4 Conversion and product yield data from the reactions of coal ABL15 (79.1 wt% dmif C) in decalin and nitrogen at three temperatures’
Oils. The g.c.-m.s. traces of three of the 350°C oils are shown in Figure 9. Aliphatic products clearly predominate in these oils. The oil from ABL14, with the highest H/C value, shows many similarities to brown coal oils3. All the 350°C oils are broadly similar to the oils produced from reaction of the respective coal at 405°C (cf. Figures 1 and 2), except that the relative abundance of phenols
Table 5 Some chemical data for the asphaltenes reactions of ABLlS at various temperatures
derived
from
Atomic H/C ratio
H,”
1.09 0.99 0.96
0.43 0.39 0.35
Yields (wt% dmif) Temperature (“C)
Total conversion
Asphaltene
Oil
320 350 375
6.9 18.7 37.7
2.8 6.4 13.3
3.2 9.6 19.4
’ Reaction conditions: 2:l decalin:coal, 6 MPa initial N, pressure, 1 h
position of the macromolecular coal structure leading to asphaltene with high ring substitution. In contrast, the heavily substituted rings in the inertinite were unreacted and remained in the residue from the reaction. Products from reactions of higher-rank coals at 350
Reactions of brown coals at 320” either in decalin or in the absence of solvent were shown to dislodge the guest structure from the macromolecular host’,*. In contrast, the higher rank coals showed very little reactivity at this temperature. In an attempt to further elucidate the nature of the bonding between the aliphatic material and the ligno-cellulosic-derived polymer in higher rank coals, reactions of some higher rank coals at temperatures intermediate between 320 and 405°C were carried out. Initially, experiments were carried out with ABL15 which was reacted in 2:l decalin coal slurry under 6 MPa N, for 1 h at 350°C and 375°C (Table 4). The conversion results at 350°C were found to be identical to those for a brown coal of similar H/C ratio at 320°C (CO,-free basis)*. Furthermore, as with the brown coals, the
Elemental composition (wt% dmif) Temperature (“C)
C
H
N
35W 375’ 405d
82.8 82.9 83.7
7.55 6.81 6.67
1.6 2.2 2.3
O+S” 8.1 8.1 7.3 ._.__~
a From ‘H n.m.r. b By difference ’ Reaction conditions as for Table 4 d Reaction conditions: 3:l tetralin-coal, pressure, 1 h
6 MPa
initial hydrogen
Table 6 Conversion and product yields from the reactions of higher rank coals at 350°C in nitrogen Yields (wt% dmif)
Coal
Method
Total conversion
Asphaltene
Oil (by diff.)
ABL2
A B A A B A B A B A
7.8 6.0 6.5 9.7 8.0 13.4 6.9 21.3 13.7 18.7
0.3 0.2 3.0 0.6 0.5 5.3 0.5 1.9 0.3 6.4
3.5 3.0 3.3 3.8 3.5 6.5 5.0 15.9 7.4 9.6
ABL6 ABL8 ABLl2 ABL14 ABL15
A, Reaction of coal at 350°C in decalin; B, reaction of coal at 350°C with no solvent
FUEL,
1989,
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Studies related to the structure and reactivity of coals. 78: P. J. Redlich et al.
from reactions of brown coals at 320 and 405°C but less pronounced, and again could indicate the remnants of a host-guest structure for these coals.
b
2000
1000
3000
4000
5000
SCAN
NO.
4”
C
Residues. The elemental analyses of the residues from these reactions and the f, values determined from solid state 13C n.m.r. are shown in Table 7. The 350°C residues derived from ABL14 and ABL12 are similar with respect to their H/C values (x 0.80) and f, values (x0.74). The conversions, however, of 21% and 13% respectively, in decalin, were strongly related to the H/C ratios of the parent coals, that is 0.94 and 0.85, respectively. Furthermore, analysis of their respective 350°C oils clearly showed a similarity in that n-alkanes predominated. This type of result is similar to that expected from reaction of brown coals of similar H/C ratio at 320°C again suggesting that these higher rank coals have remnants of a host-guest structure. The 350°C residue of ABL6, H/C 0.70 Wongawilli, a higher rank and lower H/C ratio coal than ABL14 and ABL12, showed no significant difference in elemental composition from its parent coal. In contrast, reaction of ABLS coal (Leigh Creek H/C 0.73 CO,-free basis) produced a 350°C residue which had an H/C ratio (0.60) significantly lower than the residue from ABL6 (Wongawilli H/C 0.70), even though both coals have similar H/C ratios. This feature can be explained by the structure of ABL8 being more like th;; of brown coals, whereas the 350°C residue from high rank ABL6 coal was probably similar to the parent coal.
CONCLUSIONS 1000
2000
3000
4000
5000
SCAN NO.
Figure 9 The total ion chromatograms of oils derived from reactions of coal at 350°C under nitrogen with no solvent: a, ABL2; b, ABL12; c, ABL14. For identitication of peaks see legend of Figure 1
is lower in the former. This is consistent with the evidence obtained in our study of brown coals that phenolic products are predominantly associated with the macromolecular matrix component of the coals. Interestingly, significant levels of C3- and Cdsubstituted naphthalenes were also released at these relatively low reaction temperatures. The compounds are thought to form predominantly by the decomposition of terpenoid or terpenoid-derived compounds present in the coa13*“, although the distribution of isomers differs here from that observed for low temperature brown coal oils (not illustrated). The ‘H n.m.r. spectra of the 350°C oils from ABL12 and 14 showed lower H,, values (0.14,O.lO respectively) and higher H, values (0.52, 0.62 respectively) than the 405°C oils (H,, 0.25,0.19 and H, 0.38,0.48 respectively), confirming the results obtained by g.c.-m.s. were representative of a significant proportion of the oil. Only small amounts of asphaltenes were obtained from the reactions at 350°C and ‘H n.m.r. spectra were recorded for three of them. The 350°C asphaltenes all showed lower H,, and higher HP-values than the corresponding 405°C asphaltenes, e.g. the values for asphaltenes from ABL15 at 350 and 405°C were: H,,, 0.17, 0.27; H,, 0.22, 0.31; H,, 0.50, 0.35; H,, 0.11, 0.07. This trend is similar to that observed in the asphaltenes Asphaltenes.
1556
FUEL, 1989, Vol68,
December
The work described in this and preceding papersiP6,12 has led to the following general conclusions concerning the structural changes associated with increasing rank for a suite of Australian coals. Long aliphatic chains present in subbituminous coals are more strongly bound (via oxygen or carbon bonds) to, or trapped in, the macromolecular structure than for these same species in brown coals. Similar conclusions concerning the structure of a high volatile, subbituminous coal (Millmerran) have been obtained from the analysis of flash-pyrolysis tars 22*23.In brown coals, n-alkanes are formed at least partly by decomposition of very long chain esters (CC,,) and carboxylic acids. No apparent increase in aromatic cluster size is observed from brown coals to subbituminous coals.
Table 7
Some chemical characteristics of the residues derived from reaction of a subsuite of higher rank coals at 350°C in decalin and nitrogen Elemental composition (wt% dmif)
Coal
C
H
N
o+s*
H/C
f.
ABL2 ABL6 ABL8 ABL12 ABL14 ABLl5
80.9 86.6 75.7 83.0 80.9 80.4
3.68 5.08 3.78 5.63 5.34 5.59
1.2 1.9 1.9 2.0 1.3 1.3
14.2 6.4 18.6 9.4 12.5 12.7
0.55 0.70 0.60 0.81 0.79 0.83
0.85 0.83 0.89 0.73 0.75 0.65
’ Solid state r3C n.m.r. For conditions, see Refs. 1 and 12 * By difference
Studies related to the structure
A significant decrease in average chain length of n-alkanes and a higher proportion of aliphatic material in the form of short side chains and/or linkages between aromatic clusters results for bituminous coals relative to subbituminous coals. The total oxygen content of the coal decreases with increased coalification and a corresponding significant change in oxygen functional group concentration. Carboxylic acids and esters, when present, are largely lost relatively early in the coalification sequence, with phenols and non-acidic oxygen being the predominant residual forms in coals of subbituminous or higher rank. Callide (ABL2), although a subbituminous coal, has some structural characteristics more typical of low rank coals (e.g. carboxylic acids, high nonacidic oxygen content and unusually long n-alkanes) as well as high rank coal features (e.g. large aromatic ring clusters and shorter chain C,,-C,, aliphatic material).
11 12 13
and reactivity
of coals. 18: P. J. Redlich
et al.
Redlich, P. J., Jackson, W. R., Larkins, F. P., Chaffee, A. L. and Liepa, I. Fuel 1989,68, 1538 Redlich, P. J., Jackson, W. R. and Larkins. F. P. Fuel 1989,68, 1544 SupaIuknari, S., Larkins, F. P., Redlich, P. and Jackson, W. R. Fuel Process. Techno/. 1988, 18, 147 Lynch, L. J., Sakurovs, R., Webster, D. S. and Redlich, P. J. Fuel 1988,67, 1036 Dong, J. Z., Katoh, T., Itoh, H. and Ouchi, K. Fuel 1988, 67, 284 and references therein Dong, J. Z. and Ouchi, K. Fuel 1988, 67, 541 Shaw, P. M., Brassell, S. C., Assinder, D. J. and Eglinton, G. Fuel 1988,67, 557 ‘Coal Structure’ (Eds. M. L. Gorbaty and K. Ouchi), Advances in Chemistry Series, American Chemical Society, Washington, USA, 1981, 192 ‘Coal Science & Chemistry’, Coal Science and Technology 10 (Ed. A. Volborth), Elsevier Science Publishers, Amsterdam, 1987 Redlich, P. J., Jackson, W. R., Larkins, F. P. and Rash, D. Fuel 1989,68,22.? Chaffee, A. L., Hoover, D. S., Johns, R. B. and Schweighardt, F. K in ‘Biological Markers in the Sedimentary Record’ (Ed. R. B Johns), Elsevier Science Publishers, Amsterdam, 1986,
p. 11 14
ACKNOWLEDGEMENTS The authors would like to thank the following for their advice and/or assistance: D. Rash and M. Marshall (Monash University); D. Cookson and P. Lloyd (BHP Melbourne Research Laboratory); S. Supaluknari (University of Tasmania); D. Brockway (S.E.C.V.). They also thank the Coal Corporation of Victoria for support.
15 16 17 18
19 20 21
REFERENCES 1 2
Redlich, 1383 Redlich, 231
P., Jackson, P. J., Jackson,
W. R. and Larkins, W. R. and Larkins,
F. P. Fuel 1985, 64, F. P. Fuel 1989,68,
22 23
Brooks, J. D. and Smith, J. W. Geochim. er Casmochim. Acta 1967, 31, 2389 Radke, M., Schaefer, R. G., Leythaeuser, D. and Teichmiiller, M. Geochim. et Cosmochim. Acra 1980. 44, 1787 Cookson, D. J., Latten, J. L., Shaw, I. M. and Smith, B. E. Fuel 1985.64, 509 Katz, M. and Glenn, R. A. Anal. Chem. 1952, 24, 1157 Cookson, D. J. and Smith, B. E. in ‘Coal Science and Chemistry’, Coal Science and Technology 10 (Ed. A. Volborth), Elsevier Science Publishers, Amsterdam, 1987, p. 31 Atemany, L. B., Grant, D. M., Pugmire, R. J., AIger, T. D. and Zilm, K. W. J. Am. Chem. Sot. 1983, 105, 2133 Brown, J. K. and Ladner, W. R. Fuel 1960,39, 87 Strachan, M. G., Alexander, R. and Kagi, R. I. Geochim. et Cosmochim. Acta 1988,52, 1255 Koplick, A. J. and Wailes, P. C. Fuel 1983, 62, 1161 Koplick, A. J., Wailes, P. C., Salivin, I., Vit, I. and Galbraith, M. N. Fuel 1984, 63, 1570
FUEL, 1989,
Vol 68, December
1557
and
18. The chemical structures of products from reactions of a suite of higher rank Australian coals Peter J. Redlich, W. Roy Jackson, and lmants Liepat
Frank
P. Larkins*,
Alan
L. Chaffee?
Department of Chemistry, Monash University, Clayton, Victoria, Australia 3 168 *Department of Chemistry, University of Tasmania, G.P.O. Box 252C, Hobart, Tasmania, Australia 700 1 tCSIR0 Division of Fuel Technology, Private Mail Bag 7, Menai, N.S. W., Australia 2234 (Received 4 November 1988; revised 7 June 1989)
The structure of the oils, asphaltenes and residues obtained by the thermal reactions of a suite of Australian higher rank coals under hydrogen or nitrogen have been studied by chemical and spectroscopic methods. The host-guest model that has been used to describe the structure of Australian brown coals cannot be applied directly to the higher rank coals. Evidence is provided that suggests that a modified version of the model may be of use in describing the structure of some subbituminous coals. The methodology has proved to be useful in the understanding of structural features of coals which are often not rank dependent, e.g. Callide coal (ABL2), a subbituminous coal, has been shown to have characteristics of both very high and also low rank coals. (Keywords: characterization of coal; chemical structure; reactivity) It has been shown in previous
paperslp6 that the structure of low sulphur Victorian brown coals can be explained in terms of a two component (guest-host) model. The relative concentration of the components greatly affects the atomic H/C ratio of the brown coal and its reactivity under liquefaction conditions. The reactivity of higher rank coals is also related to the atomic H/C ratio’,2, and it was thus of interest to see if the model could be extended to these coals. It should be stressed that the host-guest model derived by us is similar to other two-component coal structure models developed previously but important differences exist which have been discussed in a previous paper3. The methodology used was similar to that in our previous investigation of brown coals, i.e. reaction products obtained from liquefaction using hydrogen in the presence of either a tin catalyst or tetralin were compared with products of thermolysis under nitrogen, under conditions of moderate severity’g2. Other strategies have frequently been used to study the structure of higher-rank coals. For example, Dong et ~1.‘~~have converted a large part of a subbituminous coal into low molecular weight compounds by repetitive hydrogenation under relatively mild conditions. Others have chemically modified the coal structure prior to reductive degradation. In a recent study coal was alkylated prior to lithium aluminium hydride reduction and product extractiong. These methods, related methods and methods based on oxidative coal degradation have been summarized”,’ ‘. Each strategy has its limitations, but we believe that reaction at moderate temperatures, between 300 and 405”C, can lead to valuable structural information for relatively reactive coals’ ,2.
0016-2361/89/121549--09S3.00 0 1989 Butterworth & Co. (Publishers) Ltd.
The brown coals described in previous work were all highly reactive, but some of the higher rank coals displayed reduced reactivity. Hence, increased emphasis must be placed on the characterization of the higher molecular weight products (asphaltenes and residues) if the structure of higher rank coals is to be elucidated. Although g.c.-m.s. cannot be used with these fractions, other analyses can still give significant information. Apart from reduced reactivity, the higher rank coals differ in other respects to the brown coals. The brown coals previously investigated do not vary in rank even though they exhibit large changes in H/C ratio (0.82-1.25 CO, free basis) 2,12. In contrast, for many higher rank coals, the changes in the H/C ratio are caused by structural changes in the coal matrix so that variations in the nature of the coal liquefaction products are likely to reflect these fundamental changes in structure. Differences in coals may also to some degree reflect different plant origins and different coalification processes. These questions are addressed below. EXPERIMENTAL The coals and their hydroliquefaction reactions have been described previously1*2. Products were analysed by elemental analysis, non-aqueous titration, solid state ’ 3C n.m.r., ‘H and 13C n.m.r. (solution), FT-i.r. and g.c.-m.s. using instrumentation and methods described previously3-5*‘2. Coals
The higher rank coals used in this study, ABLl to ABLlS ranged in rank from subbituminous to semi-anthracite with carbon contents (wt% dmif)
FUEL, 1989, Vol 68, December
1549
Studies related to the structure
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of coals. 78: P. J. Redlich
Table 1 The H/C ratio, carbon content and oil yields from SnO,/ hydrogen reactions without solvent at 405°C of six higher rank coals Carbon content
Oil yield
Code
Coal”
H/C CO, free
(wt% dmif CO, free)b
ABL2 ABL3 ABM ABL8 ABL12 ABL14
Callide Bulli Wongawilli Leigh Creek Greta Taroom
0.58 0.64 0.70 0.73 0.86 0.95
81.3 88.4 87.0 76.2 83.4 77.4
7.8 5.0 12.4 13.8 23.0 31.5
et al.
The chromatograms of the oils from the medium volatile oxygen rich subbituminous coals (Figure 2) are dominated by phenolic products, especially phenol and cresols (A, B, C). Nevertheless, there is a similar reduction in the relative abundance of the n-alkanes, and an increase in the relative abundance of aromatic hydrocarbons (with increase in carbon content) as outlined above within this pair of samples.
‘See Refs. 1 and 2 for full details of coal characterization b For a discussion of the advantages of expressing elemental and conversion data on a CO, free basis see Refs. 2 and 12
between 90% (ABLl) and 73% (ABL8) and atomic H/C ratios of 0.96 (ABL15) to 0.51 (ABLl). Full details of the analytical data for these coals have been given in a previous paper 12. The coals were reacted in batch autoclaves with hydrogen (6 MPa initial) in the presence of tetralin’*2 at 405” and in addition they were reacted with hydrogen in the presence of SnO, but without solvent (405°C 10 MPa initial H,) to obtain oils free from reaction solvent. Experiments were also conducted at 350°C under N, (6 MPa initial) in the absence of solvent and in the presence of decalin (solvent to coal ratio, 2:l).
J 1000
2000
$I
tb
3000
4000
5000
SCAN NO.
I i
Oils
A subsuite of six oils was chosen for more detailed analysis on the basis of their parent coal being representative of a particular rank range. This included two oils derived from oxygen rich subbituminous coals. It should be noted that while the oil yields obtained from reaction of the brown coals at 405°C accounted for 15-60 wt% (daf) of the coal’y2, oil yields from higher rank coals represented only 5-35%. The low yields severely limited the number of techniques that could be used for oil analysis. G.c.-m.s. and ‘H n.m.r. were the most useful with other techniques being used only if sufficient samples were available.
1000
2000
3000
4000
5000
SCAN NO.
RESULTS
AND DISCUSSION
d
Oils
The carbon content, H/C ratio and oil yield for the subsuite of six coals used to obtain solvent free oils are summarized in Table 1. G.c.-m.s. The oils from each of the six solventless reactions were examined by g.c.-m.s. The results are best considered by distinguishing the two oxygen rich medium volatile subbituminous coals Callide, ABLZ, and Leigh Creek, ABL8, from the medium-high volatile subbituminous and bituminous coals. Figures la-d clearly show that for the latter four coals there is a reduction in the relative abundance of n-alkanes and a corresponding increase in the relative abundance of mono-, di- and tri-aromatic hydrocarbons with increasing carbon content (see peaks D, E, F). Phenolic compounds are present in significant quantities in all cases, but do not exhibit a distinct trend with rank (see peaks A, B, C).
1550
FUEL,
1989,
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2ioo
3odo
4doo
5oio
SCAN NO.
Figure 1 The total ion chromatograms of oils derived from reaction of medium to high volatile subbituminous and bituminous coals with stannic oxide (SnO,) and hydrogen at 405°C with no solvent: a, ABL3 (88.4 wt% dmif C); b, ABM (87.0 wt% dmif C); c, ABL12 (83.4 wt% dmif C); d, ABL14 (77.4 wt% dmif C). Symbols: A, phenol; B, cresol; C, alkylcresols; D, naphthalene; E, methylnaphthalenes; F, C,alkylnaphthalenes; G, C,-alkylnaphthalenes; H, C,-alkylnaphthalenes; N, C,-alkylbenzenes; 0, C,-alkylbenzenes; T, phenanthrene; U, C,-phenanthrenes; Y, decalins; Z, phthalates; ip, isoprenoids; integer number, carbon number of particular n-alkane
Studies related to the structure
and reactivity
of coals. 18: P. J. Redlich
et al.
34 shows that not only were the shorter chain n-alkanes
,1000
2OiO
3000
40do
5060
SCAN NO.
t
more abundant in the region C9-C1s in the ABL12 oil, but the second maximum occurred at 2-3 carbon number less than in ABL14 derived oil. The n-alkane distribution of the oils derived from ABL6 and ABL3 coals (Figures 3b and 3~) were similar to each other, and showed a broad maximum in the C14-C17 n-alkane range with long chain alkanes of C,, or larger being insignificant. The trend toward decreasing chain length with increasing rank of coals is well documented’4v’5. This is clearly the result of increased metamorphic activity, which culminates in the fully aromatic structure of anthracite. It should be noted that not only did the carbon chain length decrease, I-
i
\ 1000
2000
,000
4000
5000
a
1
-l-
14
L
i
I
SCAN NO.
Figure 2
The total ion chromatograms of oils derived from reaction of oxygen rich medium volatile subbituminous coals with stannic oxide (SnO,) and hydrogen at 405°C with no solvent: a, ABL2 (81.3 wt% dmif C); b, ABL8 (76.2 wt% dmif C). For identification of peaks see legend of Figure 2
Closer examination of the chromatograms yielded some interesting structural information although the low yields of oil must be taken into account when discussing the coals with lower H/C values. First, the distributions of phenols, cresols, and higher homologues (peaks A, B, C) were seen to be produced in similar ratios for coals varying in oxygen and carbon content (see Figure I). In addition, non-aqueous titration of these oils (ABL14, 12 and 6) showed similar percentages of phenolic material (2.4-2.7%) even though the parent coals had very different oxygen contents”. These observations suggest that the part of the coal oxygen containing structure that forms oil is similar over this group of coals, even though they show a wide range of atomic H/C ratios and oxygen contents. The distribution of aromatic hydrocarbon homologues and isomers also varied regularly as a function of rank. The most intriguing of these variations was for the anthracene/phenanthrene ratio (not illustrated) which was observed to decrease from about 1 for ABL14 to zero for ABL3. Phenanthrene and its homologues are often considered to have formed largely by aromatization of diterpenoid natural products during coalification’ 3. However, the origin of anthracene and its homologues is more obscure. At present we cannot adequately explain this observation. The relative distribution of n-alkanes was more clearly seen in single ion chromatograms of the oils (Figures 3 and 4). Oils from the subbituminous coals ABL14 (Figure 34 and ABLS (Figure 4b) showed a distributional maximum in the CZ3-CZ5 carbon range similar to that observed in brown coals3. The n-alkane distribution pattern for the oil derived from ABL12 (Figure 3c) showed a bimodal distribution with maxima occurring at C,, and C,,. Comparison of the two oils (Figures 3c,
4000
4 5000
SCAN NO.
1
b
DbO
sobo I
23 / 14
I
ip ir) ‘151E i
’ \’ IL
1000
2000
ir/ ip
3 I 4000
5000
SCAN NO.
d
AN NO.
Figure 3 The single ion chromatograms (m/z 71) for the n-alkane distributions of oils given in Figure I: a, ABL3 (81.3 wt% dmif C); b, ABL6 (87.0 wt% dmif C); c, ABL12 (83.4 wt% dmif C); d, ABL14 (77.4 wt% dmif C). For identification of peaks see legend of Figure 1
FUEL, 1989,
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68, December
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Studies related to the structure and reactivity of coals. 78: P. J. Redlich et al.
Like the H,, values obtained from the ‘H n.m.r. spectra, the f’ values of the oils calculated from the solution i3C n.m.r. spectra decreased with increasing H/C, from 0.70 for ABL3 to 0.61 for ABL12 and 0.50 for ABL14. The average length of the aliphatic chains was estimated by the method of Cooksoni6 and showed a decrease in the average length of hydrocarbons, from 27 for ABL14 to 14 for ABL3, consistent with the trend seen in the m/z=71 (n-alkane) single-ion plots for the oils from high rank coals (Figure 3). 1
5000
SCAN NO.
Jb
1000
2000
3000
4r
II, 5000
SCAN
NO.
Figure 4 The single ion chromatograms (m/z 71) for the n-alkane distribution of oils given in Figure 2: a, ABL2 (81.3 wt% dmif C); b, ABLS (76.2 wt% dmif C). For identification of peaks see legend of Figure 1
but there was also a significant decrease in total oil yield from 30 to 5% indicating that the n-alkanes are of decreasing significance in the overall structure of the low H/C, high rank coals. The single ion chromatogram of oil derived from ABL2 (Callide) showed compound distributions seen in both low and very high rank coals with two maxima in the n-alkane distributions near C,, and Cz4 (Figure 4~). ABL2 also shows dichotomous behaviour in features of its elemental analysis in that it has a very low H/C ratio (0.58 CO,-free basis), which is characteristic of very high rank coals in this suite, but the oxygen content of a typical subbituminous coal. Thus the average chain length of the hydrocarbons in coal-derived oils does not correlate with the atomic H/C ratio of the coal, but is more strongly correlated with coal rank or carbon content as is further exemplified by comparison of the single ion chromatograms of the oils from two coal pairs of similar H/C but different rank (ABL6, ABL8, ABL2 and ABL3). However, in contrast it is notable that the relative abundance of the isoprenoid alkanes (C,2-C20) is enhanced for oils from lower H/C ratio coals in both subsets. The ratio of isoprenoid hydrocarbons phytane/pristane (ip-C&p-Cig) also increases with H/C within each subset. N.m.r. The trends observed in g.c.-m.s. studies of the oils were supported by ‘H n.m.r. spectra, which showed the general increase in H, (associated with methylene chains) and decrease in H,, and H, with increasing H/C ratio (Figure 5). The n.m.r. data also show the differences between ABL3 and ABL6 oils as suggested by g.c.-m.s. studies; ABL3 has a higher H, content (0.39) than ABL6 (0.30) in agreement with the former oil having a higher concentration of short chain substituted aromatics (Figures la and lb).
1552
FUEL,
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Asphaltenes Asphaltenes derived from reactions of higher rank coals in the presence of tetralin and hydrogen, and with SnO, in the absence of solvent at 405°C were analysed by non-aqueous titration and by n.m.r. techniques. The two sets of asphaltenes differed only in that those derived from SnO,/no solvent reactions consistently showed lower oxygen contents (by w2%), in agreement with the greater yields of Hz0 and CO, in these reactions’. The similarity in the analyses shows that incorporation of tetralin or its products into asphaltenes was not a major problem. The results presented below refer to asphaltenes obtained from reactions in tetralin. Elemental analysis and non-aqueous titration. The elemental composition of asphaltenes from reaction of a subsuite of higher rank coals at 405°C with tetralin is given in Table 2. As noted previously, the most significant feature was the strong correlation between the H/C ratio of the asphaltenes and that of the parent coals’. The carbon contents of the asphaltenes and of the coals were broadly related as seen from the results in Table 2 and Table 1 of Refs. 1 and 12. A similar but reciprocal trend was seen in oxygen contents though actual values may be subject to relatively high error. In the present work, it was initially assumed that the contribution from acidic
0.5
ABL14
-
a
4 ‘I 3 iiw 8
0.6
-
0.3
_
2 a”
cx
4 :: E
0.2-
0.1 -
1
0.5
,
0.6
0.7
0.8
0.9
1.0
Coal H/C ratio (CO* free)
The relationship between the hydrogen distribution in all the coal derived oils, determined by ‘H n.m.r., and atomic H/C ratio of their parent coals: +, H,,; A, HP; 0, H,. NB. Oils were derived from reaction of coals with SnO, and hydrogen in the absence of solvent Figure 5
Studies
related
to the structure
Table 2 Some chemical data for the asphaltenes derived from reactions of higher rank coals at 405°C in tetralin and hydrogen
Coal
_-__ C
Elemental composition (wt% dmif) H
5.56 84.5 ABL2 5.61 87.9 ABL3 83.4 5.80 ABL4 6.27 85.2 ABL5 86.7 6.16 ABL6 5.95 83.8 ABLI 6.05 ABLlO 84.1 6.35 ABL12 86.5 6.34 ABL13 82.6 ABL14 84.2 6.60 6.67 ABL15 83.7 ___-a Solid state 13C n.m.r. For n.a., Not analysed
0
H/C
Phenohc 0 (wt% dmif)
i3C n.m.r. f,”
1.9 3.5 6.9 6.5 3.9 8.2 8.4 5.0 9.0 7.3 6.6
0.79 0.77 0.84 0.88 0.85 0.85 0.86 0.88 0.92 0.94 0.96
na. n.a. 7.9 n.a. 5.0 8.7 6.4 n.a. 6.2 na. 7.4
0.79 0.71 0.72 0.68 0.71 0.71 0.64 0.66 0.64 0.64 0.66
conditions see Refs. 1 and 12
nitrogen was negligible and that the acidic oxygen present in these asphaltenes is phenolic. No evidence for carboxylic oxygen was found in the FT-i.r. spectra of these asphaltene?. The phenolic oxygen content, determined by nonaqueous titration, is also given in Table 2. However, in most cases, the results were higher than the total oxygen content in the asphaltenes, a feature not noted in analysis of brown coal asphaltenes. Since the analysis assumes that only oxygen groups are acidic, a possible source of error in the acidic oxygen figures is that protons bound to nitrogen are also titrated. It has been reported that protons bound to the nitrogen in carbazole can be easily titrated17, and this has been confirmed in our laboratory. The nitrogen content of the asphaltenes and coals (1.5-2.5 wt%) is significantly higher than in lower rank coals (0.5-1.0%) and if acidic nitrogen is titrated this will affect our acidic oxygen figures. ‘H nmr. spectroscopy. The distribution trends of H,,, H, and H, for the asphaltenes are shown as a function of the H/C ratio of their parent coals in Figure 6. The significant features of the trends in hydrogen distribution are : There was a large variation in all parameters in contrast to the case for brown coal asphaltenes4. This is in agreement with the suggestion that the structural characteristics of the higher rank asphaltenes are dependent on their parent coal’. The H, fraction, but not H,, increased with the H/C ratio of the parent coal. In brown coals the methylene chains appeared mainly in the oil fraction3,4, whereas this observation suggests that the n-alkanes in the oil fraction of high-rank coals do not represent the total n-alkane content of these coals. Probably in high-rank coals many of the methylene chains are more firmly bound to other parts of the coal structure than in brown coals, so that they are not cleaved from them under our liquefaction conditions and are contained in the asphaltene fraction. As for the oils, H,, varied inversely with the H/C ratio of the original coal. The distribution of aromatic protons shows changes in structural characteristics. In the spectra of asphaltenes derived from relatively high rank coals such as ABL6 and ABL3, the distribution of the aromatic hydrogen
and reactivity
of coals. 18: P. J. Redlich
et al.
maximized between 7.5 and 8 ppm, which is indicative of a higher concentration of protons attached to polyaromatic clusters rather than to single ringsi8. In lower rank coal derived asphaltenes, the maximum of the aromatic distribution coincided with the CHCl, peak at 7.25 ppm, suggesting a higher concentration of single ring compounds”. 13C solid state n.m.r. The f, values of higher rank coal-derived asphaltenes varied over a narrow range, 0.64-0.72, with the exception of the asphaltene derived from ABL2 (Callide) which had a value of 0.79 (Table 2). Asphaltenes from ABLlO to ABL15 had values of 0.65 f 0.01. These are significantly lower than the values for asphaltenes derived from brown coals of similar H/C ratio4. This observation supports the conclusions drawn from ‘H n.m.r. spectra and elemental analysis above that aliphatic methylene chains are more strongly attached to the macromolecular structure in higher rank coals than in brown coals, so that many are found in the asphaltene fraction of high-rank coal products. Further evidence is seen in the i3C n.m.r. aliphatic resonances of four asphaltenes derived from coals in the higher H/C range 0.7-0.95, which are shown in Figure 7 with their respective H, and H, values. For the asphaltene derived from ABL6 coal, the aliphatic peak at 15-20 ppm was larger than the peak at 30 ppm. However as the H/C ratio of the parent coal (and asphaltene) increased, the 30 ppm peak increased until for ABL14 asphaltene this peak was dominant. This trend corresponds to an increase in H, values from 0.20 -+ 0.35 as well as a decrease in H, values, and is indicative of an increase in concentration of long chain aliphatic material. The increase in intensity of the 30 ppm resonance is probably
415
8
A
l*
A AAA A
A A
I 0.5
1
0.6
0.7
0.8
0.9
Coal H/C ratio (CO2 free)
Figure 6 The relationship between the hydrogen distribution in all the coal derived asphaltenes, determined by ‘H n.m.r. and atomic H/C ratio of their parent coals: +, H,,; A, H,; 0, H,. Integer numbers indicate ABL coal code. NB. Asphaltenes were derived from reaction of coals with tetralin under hydrogen at 405°C
FUEL, 1989, Vol 68, December
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Studies related to the structure and reactivity of coals. 18: P. J. Redlich et al.
fa
There were, however, two exceptions. The phenolic oxygen content of the residues from ABL2 and 8 was only 20-30% of the total oxygen content.
= .66
H, = .31 Hg = .35
13C solid state n.m.r. The fraction of aromatic carbon present in the residues was determined by solid state 13C n.m.r. The f, values (Table 3) varied between 0.81 and 0.91 and showed no significant correlation with H/C of the residues or of the parent coals. The residues could be expected to contain even more quaternary carbon atoms than their parent coals and the difficulties of observing such carbon atoms reduces the accuracy of this technique’ 9.
a
f, = .66 H, = .36 Hg = .27
Reactions of two maceral concentrates from Pike’s Gully coal (ABLS and ABLlO)
fa = .64 Ha = .40 Hg = .23
f, = .71 H, = .38 Hg = .20
d
250
/\/
150 PPM
50
0
Figure 7 The solid state r3C n.m.r. spectra and selected 'H n.m.r. data for asphaltenes derived from reaction of medium-high volatile subbituminous and bituminous coals in tetralin under hydrogen at 405°C: a, ABL14; b, ABL12; c, ABLlO; d, ABL6; +, aromatic spinning sidebands
not due to increasing hydroaromatic species because this would also increase H, values, which did not occur.
Two hand-picked maceral concentrates of one coal, Pike’s Gully, were obtained which had very different H/C ratios. The vitrinite rich sample ABLlO had an H/C value of 0.80 and the inertinite rich sample ABL5 had an H/C ratio of 0.67. Maceral distributions” and reactivities? have been summarized previously. It was noted’ that there was a large difference between the asphaltene yields from these two coals, which was not typical of reaction of higher rank coals. It was thus of interest to study the structure of these asphaltenes. Although the elemental composition of the two samples was similar (Table 2), the n.m.r. data were significantly different. The ABLS asphaltene had H,, 0.40 and H, 0.34 whereas ABLlO asphaltene had H,, 0.32 and H, 0.40, even though f, of both samples was the same. Use of the n.m.r. data in the Brown-Ladner equation” gave values of e (the fraction of available aromatic edge atoms occupied by substituents) of 0.36 for ABR5 and 0.47 for ABRlO suggesting a higher degree of ring substitution in the asphaltene from ABRlO. This conclusion is supported by a comparison of the intensity of the aromatic 700-950 cm-’ C-H band frequencies in the FT-i.r. spectrum of the two asphaltenes (Figure 8). The 750 cm-’ band, attributed to groups of five adjacent aromatic hydrogens, was significantly more intense in the ABL5 (inertinite) asphaltene than in the ABLlO asphaltene, which indicates fewer substituted aromatic rings in ABL5 asphaltene. Thus the larger yield of asphaltene from the vitrinite rich sample is associated with more extensive decom-
Reaction residues
Residues derived from liquefaction of higher rank coals contain a higher ash content (20-60% db) than for brown coals primarily because of the high ash content of the parent coals I2 . Thus the problems of analysis were more acute than those encountered previously4. Elemental analysis. The elemental composition
of 11 residues derived from reaction of their parent coals at 405°C with tetralin is given in Table 3. The most significant feature of the results is that there was a strong correlation between the H/C ratio of the residues derived from the higher rank coals after reaction at 405°C with tetralin and the H/C ratio of their parent coals’. Furthermore, the H/C ratios of residues were significantly higher than those from brown coals of the same H/C value. The acidic oxygen content (Table 3) of the residues, determined by non-aqueous titration, showed that in most cases 40-60% of the oxygen was in phenolic form.
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FUEL, 1989, Vol 68, December
Table 3 Some chemical data for the residues derived from reactions of higher rank coals at 405°C in tetralin and hydrogen Elemental composition (wt% dmif) Coal
C
H
N
O+s”
H/C
ABL2 ABL3 ABL4 ABLS ABL6 ABLI ABLlO ABLll ABLl2 ABL14 ABL15
85.2 91.3 83.1 87.5 90.9 83.8 82.3 83.7 84.0 80.4 84.1
4.00 4.84 4.13 4.31 4.99 4.06 4.93 4.99 5.02 5.32 5.96
1.4 1.6 1.8 2.1 2.4 2.1 2.3 2.4 2.2 2.0 2.0
9.4 2.3 11.1 6.1 1.7 10.0 10.5 8.9 8.8 12.3 7.9
0.56 0.64 0.60 0.59 0.66 0.58 0.72 0.72 0.72 0.79 0.85
Phenohc 0 (wt% dmif) 3.4 1.0 na. 3.2 ;::. 4.1 n.a. n.a. n.a. 3.6
’ By difference b Solid state 13C n.m.r. For conditions see Refs. 1 and 12 n.a., Not analysed
fab 0.83 0.87 0.91
0.86 0.81 0.81 0.81 0.85 n.a. n.a. 0.85
Studies related to the structure and reactivity of coals. 18: P. J. Redlich et al.
asphaltene was significantly more aliphatic than that obtained under standard hydrogenation reaction conditions at 405°C (Table 5)*. Reaction of ABLl5 at 375°C led to higher conversion but the overall characteristics of the asphaltene had become more like those obtained at 405°C in tetralin (Table 5). Therefore, the above results showed that 350°C was required to remove a significant portion of the aliphatic material although some degradation of the macromolecular structure at this temperature may have also contributed to the products. A subsuite of six coals, ranging in H/C values from 0.58 to 0.96, was reacted at 350°C under nitrogen both in the presence and absence of decalin (Table 6). In contrast to reactions of brown coals of similar H/C*, reactions in the absence of decalin gave significantly lower conversions, but the reactions were carried out to obtain oil fractions suitable for analysis. Comparison of the conversion data for decalin reactions with those of brown coals of similar H/C showed similar values*. Coals of H/C < 0.75 remained essentially unreactive at 350°C. The products from these reactions were analysed by elemental analysis, g.c.-m.s. and n.m.r. ?
700
8Ob
900
cm
-1
Figure 8 The FT-i.r. spectra (700-950 cm-’ region) of asphaltenes derived from reaction of: a, ABLS; and b, ABLlO coals with tetralin and hydrogen at 405°C
Table 4 Conversion and product yield data from the reactions of coal ABL15 (79.1 wt% dmif C) in decalin and nitrogen at three temperatures’
Oils. The g.c.-m.s. traces of three of the 350°C oils are shown in Figure 9. Aliphatic products clearly predominate in these oils. The oil from ABL14, with the highest H/C value, shows many similarities to brown coal oils3. All the 350°C oils are broadly similar to the oils produced from reaction of the respective coal at 405°C (cf. Figures 1 and 2), except that the relative abundance of phenols
Table 5 Some chemical data for the asphaltenes reactions of ABLlS at various temperatures
derived
from
Atomic H/C ratio
H,”
1.09 0.99 0.96
0.43 0.39 0.35
Yields (wt% dmif) Temperature (“C)
Total conversion
Asphaltene
Oil
320 350 375
6.9 18.7 37.7
2.8 6.4 13.3
3.2 9.6 19.4
’ Reaction conditions: 2:l decalin:coal, 6 MPa initial N, pressure, 1 h
position of the macromolecular coal structure leading to asphaltene with high ring substitution. In contrast, the heavily substituted rings in the inertinite were unreacted and remained in the residue from the reaction. Products from reactions of higher-rank coals at 350
Reactions of brown coals at 320” either in decalin or in the absence of solvent were shown to dislodge the guest structure from the macromolecular host’,*. In contrast, the higher rank coals showed very little reactivity at this temperature. In an attempt to further elucidate the nature of the bonding between the aliphatic material and the ligno-cellulosic-derived polymer in higher rank coals, reactions of some higher rank coals at temperatures intermediate between 320 and 405°C were carried out. Initially, experiments were carried out with ABL15 which was reacted in 2:l decalin coal slurry under 6 MPa N, for 1 h at 350°C and 375°C (Table 4). The conversion results at 350°C were found to be identical to those for a brown coal of similar H/C ratio at 320°C (CO,-free basis)*. Furthermore, as with the brown coals, the
Elemental composition (wt% dmif) Temperature (“C)
C
H
N
35W 375’ 405d
82.8 82.9 83.7
7.55 6.81 6.67
1.6 2.2 2.3
O+S” 8.1 8.1 7.3 ._.__~
a From ‘H n.m.r. b By difference ’ Reaction conditions as for Table 4 d Reaction conditions: 3:l tetralin-coal, pressure, 1 h
6 MPa
initial hydrogen
Table 6 Conversion and product yields from the reactions of higher rank coals at 350°C in nitrogen Yields (wt% dmif)
Coal
Method
Total conversion
Asphaltene
Oil (by diff.)
ABL2
A B A A B A B A B A
7.8 6.0 6.5 9.7 8.0 13.4 6.9 21.3 13.7 18.7
0.3 0.2 3.0 0.6 0.5 5.3 0.5 1.9 0.3 6.4
3.5 3.0 3.3 3.8 3.5 6.5 5.0 15.9 7.4 9.6
ABL6 ABL8 ABLl2 ABL14 ABL15
A, Reaction of coal at 350°C in decalin; B, reaction of coal at 350°C with no solvent
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Studies related to the structure and reactivity of coals. 78: P. J. Redlich et al.
from reactions of brown coals at 320 and 405°C but less pronounced, and again could indicate the remnants of a host-guest structure for these coals.
b
2000
1000
3000
4000
5000
SCAN
NO.
4”
C
Residues. The elemental analyses of the residues from these reactions and the f, values determined from solid state 13C n.m.r. are shown in Table 7. The 350°C residues derived from ABL14 and ABL12 are similar with respect to their H/C values (x 0.80) and f, values (x0.74). The conversions, however, of 21% and 13% respectively, in decalin, were strongly related to the H/C ratios of the parent coals, that is 0.94 and 0.85, respectively. Furthermore, analysis of their respective 350°C oils clearly showed a similarity in that n-alkanes predominated. This type of result is similar to that expected from reaction of brown coals of similar H/C ratio at 320°C again suggesting that these higher rank coals have remnants of a host-guest structure. The 350°C residue of ABL6, H/C 0.70 Wongawilli, a higher rank and lower H/C ratio coal than ABL14 and ABL12, showed no significant difference in elemental composition from its parent coal. In contrast, reaction of ABLS coal (Leigh Creek H/C 0.73 CO,-free basis) produced a 350°C residue which had an H/C ratio (0.60) significantly lower than the residue from ABL6 (Wongawilli H/C 0.70), even though both coals have similar H/C ratios. This feature can be explained by the structure of ABL8 being more like th;; of brown coals, whereas the 350°C residue from high rank ABL6 coal was probably similar to the parent coal.
CONCLUSIONS 1000
2000
3000
4000
5000
SCAN NO.
Figure 9 The total ion chromatograms of oils derived from reactions of coal at 350°C under nitrogen with no solvent: a, ABL2; b, ABL12; c, ABL14. For identitication of peaks see legend of Figure 1
is lower in the former. This is consistent with the evidence obtained in our study of brown coals that phenolic products are predominantly associated with the macromolecular matrix component of the coals. Interestingly, significant levels of C3- and Cdsubstituted naphthalenes were also released at these relatively low reaction temperatures. The compounds are thought to form predominantly by the decomposition of terpenoid or terpenoid-derived compounds present in the coa13*“, although the distribution of isomers differs here from that observed for low temperature brown coal oils (not illustrated). The ‘H n.m.r. spectra of the 350°C oils from ABL12 and 14 showed lower H,, values (0.14,O.lO respectively) and higher H, values (0.52, 0.62 respectively) than the 405°C oils (H,, 0.25,0.19 and H, 0.38,0.48 respectively), confirming the results obtained by g.c.-m.s. were representative of a significant proportion of the oil. Only small amounts of asphaltenes were obtained from the reactions at 350°C and ‘H n.m.r. spectra were recorded for three of them. The 350°C asphaltenes all showed lower H,, and higher HP-values than the corresponding 405°C asphaltenes, e.g. the values for asphaltenes from ABL15 at 350 and 405°C were: H,,, 0.17, 0.27; H,, 0.22, 0.31; H,, 0.50, 0.35; H,, 0.11, 0.07. This trend is similar to that observed in the asphaltenes Asphaltenes.
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FUEL, 1989, Vol68,
December
The work described in this and preceding papersiP6,12 has led to the following general conclusions concerning the structural changes associated with increasing rank for a suite of Australian coals. Long aliphatic chains present in subbituminous coals are more strongly bound (via oxygen or carbon bonds) to, or trapped in, the macromolecular structure than for these same species in brown coals. Similar conclusions concerning the structure of a high volatile, subbituminous coal (Millmerran) have been obtained from the analysis of flash-pyrolysis tars 22*23.In brown coals, n-alkanes are formed at least partly by decomposition of very long chain esters (CC,,) and carboxylic acids. No apparent increase in aromatic cluster size is observed from brown coals to subbituminous coals.
Table 7
Some chemical characteristics of the residues derived from reaction of a subsuite of higher rank coals at 350°C in decalin and nitrogen Elemental composition (wt% dmif)
Coal
C
H
N
o+s*
H/C
f.
ABL2 ABL6 ABL8 ABL12 ABL14 ABLl5
80.9 86.6 75.7 83.0 80.9 80.4
3.68 5.08 3.78 5.63 5.34 5.59
1.2 1.9 1.9 2.0 1.3 1.3
14.2 6.4 18.6 9.4 12.5 12.7
0.55 0.70 0.60 0.81 0.79 0.83
0.85 0.83 0.89 0.73 0.75 0.65
’ Solid state r3C n.m.r. For conditions, see Refs. 1 and 12 * By difference
Studies related to the structure
A significant decrease in average chain length of n-alkanes and a higher proportion of aliphatic material in the form of short side chains and/or linkages between aromatic clusters results for bituminous coals relative to subbituminous coals. The total oxygen content of the coal decreases with increased coalification and a corresponding significant change in oxygen functional group concentration. Carboxylic acids and esters, when present, are largely lost relatively early in the coalification sequence, with phenols and non-acidic oxygen being the predominant residual forms in coals of subbituminous or higher rank. Callide (ABL2), although a subbituminous coal, has some structural characteristics more typical of low rank coals (e.g. carboxylic acids, high nonacidic oxygen content and unusually long n-alkanes) as well as high rank coal features (e.g. large aromatic ring clusters and shorter chain C,,-C,, aliphatic material).
11 12 13
and reactivity
of coals. 18: P. J. Redlich
et al.
Redlich, P. J., Jackson, W. R., Larkins, F. P., Chaffee, A. L. and Liepa, I. Fuel 1989,68, 1538 Redlich, P. J., Jackson, W. R. and Larkins. F. P. Fuel 1989,68, 1544 SupaIuknari, S., Larkins, F. P., Redlich, P. and Jackson, W. R. Fuel Process. Techno/. 1988, 18, 147 Lynch, L. J., Sakurovs, R., Webster, D. S. and Redlich, P. J. Fuel 1988,67, 1036 Dong, J. Z., Katoh, T., Itoh, H. and Ouchi, K. Fuel 1988, 67, 284 and references therein Dong, J. Z. and Ouchi, K. Fuel 1988, 67, 541 Shaw, P. M., Brassell, S. C., Assinder, D. J. and Eglinton, G. Fuel 1988,67, 557 ‘Coal Structure’ (Eds. M. L. Gorbaty and K. Ouchi), Advances in Chemistry Series, American Chemical Society, Washington, USA, 1981, 192 ‘Coal Science & Chemistry’, Coal Science and Technology 10 (Ed. A. Volborth), Elsevier Science Publishers, Amsterdam, 1987 Redlich, P. J., Jackson, W. R., Larkins, F. P. and Rash, D. Fuel 1989,68,22.? Chaffee, A. L., Hoover, D. S., Johns, R. B. and Schweighardt, F. K in ‘Biological Markers in the Sedimentary Record’ (Ed. R. B Johns), Elsevier Science Publishers, Amsterdam, 1986,
p. 11 14
ACKNOWLEDGEMENTS The authors would like to thank the following for their advice and/or assistance: D. Rash and M. Marshall (Monash University); D. Cookson and P. Lloyd (BHP Melbourne Research Laboratory); S. Supaluknari (University of Tasmania); D. Brockway (S.E.C.V.). They also thank the Coal Corporation of Victoria for support.
15 16 17 18
19 20 21
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