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The chemical nature of flash pyrolysis tars. An N.M.R. study
Fuel Processing Technology, 7 (1983) 145--159
145
Elsevier Scientific Publishing Company, Amsterdam - - Printed in The Netherlands
THE CHEMICAL NATURE OF FLASH PYROLYSIS TARS. AN N.M.R. STUDY
JOHN
R. K E R S H A W
and B A R R Y
A. K E L L Y
CSIRO Division of Applied Organic Chemistry, P.O. Box 4331, Melbourne, Victoria 3001 (Australia) (Received June 10th, 1982; accepted in revised form October 12th, 1982)
ABSTRACT Flash pyrolysis tars from one brown and two bituminous Australian coals were separated into oils, asphaltenes and pre-asphaltenes. The oils were further separated by chromatography while the asphaltenes were separated into basic and acid/neutral fractions. The pre-asphaltenes were silyalated prior to 'sC- and 'H-n.m.r. studies. The brown coal tar was less aromatic and contained more long alkyl chains than the tars from the bituminous coals. Aliphatic constituents of the oils,which were relatively abundant, consisted mainly of n-alkanes and straight chain 1-alkenes with an average chain length of ca. C,8. The pre-asphaltenes were no more aromatic than the asphaltenes from the same tar but had higher molecular weights.
INTRODUCTION
The CSIRO is currently investigating flash pyrolysis as the first stage of a process for deriving transport fuels and chemical feedstocks from Australian coals. A 20 kg h "1 fluidized-bed pyrolysis unit is being used to evaluate the process [1]. As it is envisaged that the flash pyrolysis tar will be further processed by hydrogenation to a refinery feedstock, it is important that it is important that its structural composition should be investigated as a guide to its suitability for hydroprocessing and so that the variations of composition with pyrolysis conditions and the coal used may be determined. In this paper the chemical nature of three flash pyrolysis tars obtained from three different Australian coals, two high volatile bituminous coals and one brown coal, have been studied mainly by n.m.r, spectroscopy. The tars were separated into oils, asphaltenes and pre-asphaltenes. The oils were further separated into three fractions by elution chromatography and the asphaltenes into basic and acid/neutral fractions.
0378-3820/83/$03.00
© 1983 Elsevier Science Publishers B.V.
146 EXPERIMENTAL
Materials Flash pyrolysis tars were obtained from pyrolysis of Loy Yang, a brown coal from Victoria, Mfllmerran, a bituminous coal from Queensland and Liddell, a bituminous coal from New South Wales in a 20 kg h -~ fluidized-bed unit with a residence time of ca. 0.5 s. The pyrolysis temperature was 570°C for Loy Yang and Millmerran coals and 685°C for Liddell coal. The pyrolysis unit has been described in detail elsewhere [1]. Analyses of the coals are given in Table 1.
TABLE 1 Analysis of Australian coals used (percentage)
Moisture a Ash a Volatile Matter b Cb Hb Nb Sb O b (by difference) H/C atom. ratio
Loy Yang (brown)
Millmerran (bituminous)
Liddell (bituminous)
10.8 0.6 51.7 71.0 4.8 0.6 0.3 23.3
3.0 19.8 55.0 78.5 6.7 1.2 0.7 12.9
3.3 28.1 45.5 80.6 6.0 2.1 0.4 10.9
0.81
1.02
0.89
aair dried basis; bd.a.f.
Fractionation o f the tars The tars were extracted with toluene in a Soxhlet extractor for 72 h under argon. After removing the toluene under reduced pressure, the extract was re-dissolved in a small volume of toluene and a 20 fold excess of pentane added to precipitate the asphaltenes. Pre-asphaltenes were obtained by Soxhlet extraction of the toluene insoluble product with pyridine for 48 h under argon and removal of the pyridine under reduced pressure. The oils (pentane-soluble product) were further separated by adsorption chromatography on silica gel. The column was eluted successively with pentane, toluene and chloroform/methanol (1:1. by volume). The asphaltenes were separated into basic and acid/neutral fractions by passing HC1 gas through toluene solutions of the asphaltenes according to the procedure of Sternberg et al. [2].
147 Silylation of the pre-asphaltenes was carried out by the m e t h o d of Snape and Battle [3]. Methylation of the whole coal tars and the asphaltene from Millmerran tar was carried o u t by the m e t h o d of Liota et al. [4,5].
A nalyses Elemental analyses were carried out by the Australian Microanalytical Service. Phenolic h y d r o x y l was determined enthalpimetrically by the m e t h o d of Vaughan and Swithenbank [6]. Molecular weights were determined by vapour pressure osmometry in chloroform solutions. Natural abundance 13C-n.m.r. spectra were measured at 20 MHz on a Varian CFT-20 spectrometer with complete proton decoupling. Spectra were recorded in deuterochloroform containing Cr(AcAc)3 under conditions which give quantitative data [ 7]. IH-n.m.r. spectra were measured at 60 MHz in deuterochloroform on a Varian T-60 spectrometer. Chemical shifts are reported relative to internal tetramethylsilane. RESULTS AND DISCUSSION The yields, composition and elemental analyses of tars are given in Table 2. The flash pyrolysis tars had a high asphaltene and pre-asphaltene c o n t e n t (see Table 2) probably due to the short residence time in the pyrolysis unit (ca. 0.5 s) and indicating t h a t considerable upgrading will be required if the tars are to be converted to transport fuels. The high heteroatom c o n t e n t of the flash pyrolysis tars will require considerable hydrogen consumption for their removal in the hydroprocessing step. TABLE 2 Yields, composition, elemental analysis and aromaticity of tars
Pyrolysis Temperature (oC) Tar Yielda Oilb Asphaltene b Pre-asphalteneb Cb Hb
Nb Sb Ob (by difference) H/C Atom. ratio fa from ~SC-n.m.r. of methylated tarc
Loy Yang
Millmerran
Liddell
570 22.6 46 24 28 73.8 7.7 0.3 0.3 17.9 1.25 0.51
570 33.0 36 36 24 81.4 7.8 1.5 1.3 8.0 1.15 0.59
685 27.0 36 45 15 81.1 6.1 1.7 0.9 10.2 0.90 0.64
awt.% coal d.a.f.; bwt.% tar; Ccorrected for OMe peak.
148
The brown coal gave a lower tar yield than either of the bituminous coals but the tar had a higher H/C atomic ratio and lower aromaticity. Suuberg et al. [8] reported a similar finding during their study of the pyrolysis of a Montana lignite and a Pittsburg bituminous coal. The lignite gave a lower tar yield but with a higher H/C atomic ratio for the tar. The oil content of the tars is higher for the brown coal than for the bituminous coals. The pre-asphaltene content is lowest for the Liddell tar which maybe due to the higher reactor temperature used in the pyrolysis of Liddell coal (685°C) compared to Loy Yang and MiUmerran coals (570°C). Oils
The oils (pentane soluble product) were separated by adsorption chromatography on silica gel into three fractions, a pentane eluate, a toluene eluate and a chloroform/methanol eluate. Analytical and n.m.r, data for the oils and their fractions are given in Tables 3--5. The ~3C-n.m.r. spectra of the total otis, toluene and chloroform/methanol eluates were assigned in a similar way to that used by Snape et al. [9] (see Table 4). The integral of the ~ and e carbons of long alkyl chains are also included so that the number of long chains can be estimated. In the ~3C-n.m.r. spectra of the total oil (see Table 4) the alkene carbons are included in the aromatic carbons. The error caused by this will be very small. The n.m.r, data for the oils (Tables 3 and 4) shows that the oil from Loy Yang tar is the least aromatic with more long alkyl chains, as indicated by the intensity of the e CH2 peak of long aliphatic chains at 29.5 ppm, whilst the Liddell oil is the most aromatic with fewer long aliphatic chains. The H/C atomic ratio is in agreement with the n.m.r, data and is highest for the Loy Yang oil and lowest for Liddell oil. The H/C atomic ratio of Loy Yang and Millemerran oils are high and both oils contain a significant number of long alkyl chains as indicated by their ~3C-n.m.r. spectra (see Table 4). The heteroatom content of the oils, especially the oxygen content in Loy Yang oil, is relatively high. The presence of alkenes in the oils is shown by the ~H-n.m.r. spectra of all three oils. Molecular weights of the three oils were similar. The 13C-n.m.r. spectra of the pentane eluates of all three tars indicate that the principal components are n-alkanes and unbranched 1-alkenes (see Table 5). The latter are indicated by peaks at 114 ppm (terminal methylene carbon, = CH2), 139 ppm (the/3 -- CH carbon) and 34 ppm (~ carbon, CH2 next to CH=CH2 group). Peaks at 14, 23, 32, 29 and 29.5 ppm are due to the a,/~,7,~ and e carbons of long aliphatic chains [ 10]. In the case of Millmerran and LiddeU tar, the ratio of n-alkanes to 1-alkenes is about 3:2". However, in Loy *For 1-alkenes the intensity of the a-carbon of long aliphatic chains at 14 p p m should be same as the intensityof the C H = C H 2 peaks at 114 p p m and 139 ppm. F r o m the intensities of the peaks at 139 ppm, 114 p p m and 14 ppm, the relativeamounts of 1-alkenesand n-alkanes can be estimated.
Yield (wt.% t a r )
aincludes phenolic OH.
Aromatic Phenolic OH Olefmic Benzylic Aliphatic
% H from IH-n.m.r.
Mol. w t .
11 3 2 19 65
292
1.47
0.3 12.5
S O (by difference)
H/C a t o m . r a t i o
77.6 9.5 0.1
46
C H N
(wt.%)
Elemental analysis
19 a
31 50
38 47
284
1.35
0.3 14.8
76.2 8.6 0.1
24
15
311
1.32
0.3 2.4
87.6 9.6 0.1
11
14 2 2 27 55
303
1.36
0.5 6.5
82.8 9.4 0.8
36
37 44
19
339
1.14
0.8 5.2
85.6 8.1 0.3
9
Toluene eluate
Total
CHCIs/CHsOH eluate
Total
Toluene eluate
Millmerran
Loy Yang
Yields, e l e m e n t a l analyses, m o l e c u l a r w e i g h t s a n d IH-n.m.r. d a t a for oils
TABLE 3
34 41
25 a
303
1.19
79.8 7.9 1.8 0.6 9.9
15
CHCIJCH3OH eluate
23 3 2 29 43
290
1.19
83.0 8.2 0.6 0.5 7.7
36
Total
Liddell
40 28
32
272
1.01
86.9 7.3 0.4 0.8 4.6
14
Toluene eluate
38 29
33 a
277
1.12
82.2 7.7 1.4 0.6 8.1
15
CHCIs/CHsOH eluate
33.3
27.5--37
18--22.5
29--30
8 and e CH 2 groups of long aliphatic chains
a
.
.
0.41
18
14.1
3.9
2.6
6.8
6.0
0.53
15
7.4
6.8
4.1
8.1
6.8
15.5
5.4
6.0 25.7 18.9 2.7
0.40
17
13.6
5.5
3.4
6.8
8.9
25.8
9.3
3.8 15.2 13.6 7.6
0.47
13
10.6
4.7
2.5
6.2
6.5
27.6
5.3
4.4 22.1 17.1 3.7
0.55
11
7.4
5.8
2.7
8.5
5.4
16.2
6.6
2.3 25.1 23.6 3.8
a s m u n m g a m a j o r i t y o f a l k y l c h a i n s are p r e s e n t as a l k y l s u b s t i t u e n t $ as o p p o s e d t o n - a l k a n e s .
fa
N u m b e r average chain length of alkyl groups a
10--15
15--18
CHs 7 or f u r t h e r from a r o m a t i c ring
--CH s of ethyl groups
a--CH s
22.5--27.5
6.8
37--60
C H 2 adj t o t e r m i n a l C H 3 in a l k y l g r o u p s >C4; C H 2 in tetzalin, i n d a n e a n d in p r o p y l g r o u p s
3.4 20.5 13.3 3.4
148--168 129--148 118--129 100--118
Toluene
Total
CHCls/CHsOH
Total
Toluene
l~lh~er£an
Loy Yang
A r o m a t i c C---O A r o m a t i c C--C Aromatic C--H Aromatic C--H ortho to a r o m a t i c C--O CH a to aromatic ring; Az---CH2--Ar; C H in alkyl groups CH 2 in l o n g chalns~ C H 2 a r o m a t i c ring; CH 2 fl t o a r o m a t i c ring in a l k y l chains
C a r b o n (%)
Chemical shift ( p p m )
Assignment
13C-n.m.r. d a t a for oils
TABLE 4
0.54
12
7.1
6.3
3.4
8.2
6.0
16.0
6.7
9.7 23.1 16.4 4.5
CHCIs/CH3OH
0.57
14
7.2
5.2
3.2
6.8
5.2
16.5
5.6
7.2 27.3 20.1 2.8
Total
Liddell
0.70
10
3.5
4.6
2.8
7.4
3.2
8.1
4.2
4.2 31.7 31.7 2.1
Toluene
0.69
11
3.5
4.7
3.9
7.0
3.1
8.9
3.9
13.2 29.8 20.9 4.7
CHCIJCHsOH
o
151 TABLE 5 Analytical and n.m.r, data for the pentane eluate of the oils
Yield (wt.% of tar) H/C atom. ratio Mol. wt. n-alkanes and 1-alkenes (wt.% of pentane eluate) Average chain length
Loy Yang
Millmerran
Liddell
11 1.81 272 59
12 1.83 279 52
7 1.82 318 55
18
18
18
Yang tar the 13C-n.m.r. spectra indicates that 1-alkenes predominate. The average chain length of the n-alkanes/1-alkenes, indicated by '3C-n.m.r. spectroscopy, was about C18 in all three tars. A more precise estimate was difficult to obtain due to peak overlap when both n-alkenes and 1-alkenes are present. (n-Alkanes are symmetrical whereas 1-alkenes are not.) The pentane eluate also contained branched alkanes, internal alkenes and small amounts of aromatic hydrocarbons. In the region 100--150 ppm of the ~3C-n.m.r.spectra, two strong signals were present at 114 ppm and 139 ppm due to 1-alkanes, while the other peaks in this region may be due to either internal alkenes or aromatic hydrocarbons. 1H-n.m.r. spectra indicated that both internal alkenes and aromatic hydrocarbons were present. The amount (wt.% tar) of pentane eluate is considerably lower for the Liddell tar then for the Loy Yang or Millmerran tars. The higher pyrolysis temperature used to produce Liddell tar would be expected to reduce both the number and length of n-alkanes and alkenes [12]. However, the chain length of the alkanes/alkenes was the same in the Liddell tar as the other two tars and it is probably the higher rank of the Liddell coal that mainly accounts for the lower alkane/alkene content rather than the pyrolysis temperature. Though both Millmerran and Liddell coals are bituminous coals, Millmerran is not typical having an exceptionally high H/C atomic ratio and volatile matter content. The very short residence time in the pyrolysis unit may limit the thermal breakdown of aliphatic chains and, therefore, the pyrolysis temperature may not have a significant effect on the composition of the alkanes. The amount (wt.% tar) of the toluene eluate, the aromatic hydrocarbon fraction of the otis, was highest for Liddell tar. The aromatic fraction of the oil from this tar was more aromatic and contained fewer long aikyl chains, as indicated by the integral of the e (and ~ ) CH2 peak of long alkyl chains in the 13C-n.m.r. spectra, than the aromatic fractions of the otis from the other tars. ~H-n.m.r., i.r. and OH analysis indicate that the oxygen was present as ether oxygen in the toluene eluates. From the analytical and n.m.r, data in Tables 3 and 4 possible representative average structures can be proposed.
152
Such structures corresponding to the aromatic fraction of the oil from Millmerran tar are given in Fig. la. These formalae represent averages of the chemical type and size of the very large number of individual compounds present. The chloroform/methanol eluate (polar fraction) of the oil from Loy Yang brown coal tar was considerably more aliphatic with more long alkyl chains than those fractions from the bituminous coals (see Table 4) and its yield (wt.% tar) was also considerably higher (see Table 3). Whereas the aromaticit.v and carbon distribution of the polar fractions of the oils from Liddell and Millmerran tars were very similar to the aromatic fractions of these oils, the polar fraction from Loy Yang tar was less aromatic and contained more long alkyl chains than the aromatic fraction of this tar. The chloroform/methanol eluate (polar fraction) of the oil from Liddell tar was the most aromatic with fewer long aliphatic chains. The oxygen content was higher for the polar fraction of Loy Yang oil with about a third of the oxygen being phenolic, however the nitrogen content was much lower than in the polar fractions of the oils from the bituminous coal tars. A peak at 179 ppm in the 13C-n.m.r. spectrum of the polar fraction from Loy Yang oil indicates a COOH group, though the integral for this peak was negligible. Possible average structures for the polar fractions of the oil from Millmerran tar are given in Fig. lb. The 13C-n.m.r. spectra of the total oil, and the pentane elute, the toluene eluate and the chloroform/methanol eluate of the oil from Millmerran flash pyrolysis tar are shown in Fig. 2.
Asphaltenes The asphaltene content of the tar was lowest for Loy Yang and hi~hest for LiddeU (see Table 2). The molecular weight of Loy Yang asphaltene was much lower than that of the other two asphaltenes. The asphaltenes were separated into acid/neutral and basic fractions by bubbling hydrogen chloride into a toluene solution of the asphaltene [2]. The base content of the asphaltenes from the bituminous coal tars were much higher than that from the brown coal tar (see Table 6), presumably due to the much lower nitrogen content in the asphaltene of Loy Yang tar. However a significant part of the HC1 precipitate of all three asphaltenes could not be extracted into chloroform from a sodium hydroxide solution. It appears that hydrogen chloride is precipitating non-basic components, an effect that has also been noted [13] for the asphaltene from a supercritical gas extract, but not in the separation of asphaltenes formed in processes where a considerable amount of hydrogenation takes place [2,14]. The asphaltene from Millmerran coal tar was methylated and separated into acid/neutral and basic fractions with hydrogen chloride. The methylated asphaltene had a much lower base content, 27%, compared with 66% for the unmethylated asphaltene which further supports the observation that hydrogen chloride precipitates non-bases as well as bases from toluene solutions of these flash pyrolysis asphaltenes. Martin et al. [ 13 ] have reported a similar
153
,•(CH2)IO
CHz-
CHcH2__CH 2 3 --CH2--CH3
--CH3
O~cHs CH~
[~_.o- cH~,..~ CH /
CHs CH2
CH~
CH3
~"0~ v NH
~3
(CH2). --CH3
o,.
~ C H
"OH
--CH2--CH2--CH3 CH3 I
OH
CH2--CH3 ~
O-CH~-CH3
C'
-
f,- ~-iF~--F- -CH3
0 --CH2~-~
CH3
3
CH3 CH3
OH H3C'~~CH
3
CH3
CH~
Fig. 1. Possible average s t r u c t u r e s f o r f r a c t i o n s o f M i l l m e r r a n flash p y r o l y s i s tar. (a) = t o l u e n e e l u a t e o f oil; ( b ) ffi c h l o r o f o r m / m e t h a n o l e l u a t e o f oil; (c) = acid f r a c t i o n o f a s p h a l t e n e ; ( d ) ffi base f r a c t i o n o f a s p h a l t e n e ; (e) ffi p r e - a s p h a l t e n e .
154
(a)
_/
J
/
(b)
(c)
(d)
i
~-
/
Fig. 2. lSC-n.m.r, spectra for the oil and its fractions o f MiUmerran flash pyrolysis tar. (a) = total oil, (b) = p e n t a n e eluate, (c) ffi t o l u e n e eluate; (d) = c h l o r o f o r m / m e t h a n o l eluate.
change in the yield of bases from methylated and unmethylated asphaltene obtained from a supercriticai gas extract. The HC1 treatment led to chlorine incorporation into the acid/neutral fraction of the asphaltene which has also been observed by other workers [2,14]. The analytical and n.m.r, data for the asphaltenes and their acid/neutral and basic fractions are given in Tables 6 and 7. The aromaticity, the methyl carbon as a fraction of the aliphatic carbon, and the percentage of aromatic hydrogen are lowest for Loy Yang asphaltene and its acid and base fractions and highest for Liddell asphaltene and
31 c 34 35
34 35
284
1.12
6.4
76.0 7.1 0.1 0.3 15.4 1.1
31 c
307
1.13
N.D.
74.3 7.0 0.1 0.3 18.3 N.D.
74
35 37
28
515
N.D.
N.D.
N.D. b N.D. N.D. N.D. N.D. N.D.
26
35 29
36 c
529
0.91
N.D.
80.3 6.1 2.2 0.9 10.5 N.D.
32 35
33 c
414 (428) a
0.99
5.6
79.1 6.5 0.5 0.6 12.1 1.2
36 ( 7 3 ) a
a f o r m e t h y l a t e d a s p h a l t e n e ; b N . D . = n o t d e t e r m i n e d ; Cincludes p h e n o l i c OH.
Aromatic Phenolic OH Benzylic Aliphatic
%H by 1H-n.m.r.
Mol. wt.
H/C A t o m . r a t i o
OH (wt.%)
(wt.%) C H N S O (by difference) C1
Elemental Analysis
Yield (wt.% asphaltene)
Acid/Neutral
Total
Base
Total
Acid/Neutral
Millmerran
Loy Yang
0.86
N.D.
81.0 5.8 1.8 0.8 10.6 N.D.
Total
35 32
33
33 28
39 c
638 (666) a 538
0.95
2.0
79.3 6.3 2.5 0.9 10.6 0.4
64 ( 2 7 ) a
Base
Liddell
32 6 36 26
349
0.92
4.9
81.9 6.3 0.5 0.5 N.D. N.D.
34
Acid/Neutral
) 59
41
593
0.80
1.8
79.3 5.3 2.0 0.6 N.D. N.D.
66
Base
Yields, e l e m e n t a l analyses, m o l e c u l a r w e i g h t s a n d IH-n.m.r. d a t a f o r t h e a s p h a l t e n e s a n d t h e i r a c i d / n e u t r a l a n d basic f r a c t i o n s
TABLE 6
0.33
0.57
7.2 22.3 27.5 43.0
amethylated asphaltene corrected for OMe peak at ca. 55 ppm.
0.30
Methyl (10--22.5 ppm) as fraction of aliphatic carbon
8.6 23.5 29.4 38.4 0.62
148--168 129--148 100--129 10-60
Aromatic C--O Aromatic C--C Aromatic C--H Aliphatic C
0.31
0.61
6.8 24.3 29.7 39.1
0.41
0.71
8.7 26.6 35.3 29.3
0.42
0.71 (0.69) a
10.8 28.8 31.6 28.8
Acid/Neutral
Total
Base
Total
Acid/Neutral
Millmerran
Lo y Yang
Carbon (%)
fa
Chemical shift (ppm)
Assignments
ISC-n.m.r. data for the asphaltene and their acid/neutral and basic fractions
TABLE 7
8.0 31.9 34.7 25.4
Total
0.41
0.44
0.65 0.75 (0.63) a
4.8 25.4 34.9 34.9
Base
Liddell
0.43
0.69
7.3 29.3 32.3 31.0
Acid/Neutral
0.44
0.74
7.4 32.1 35.0 25.5
Base
C.n O~
157 its fractions. The H/C atomic ratio is highest for Loy Yang asphaltene and lowest for Liddell. The asphaltene from Loy Yang tar is higher in oxygen content but lower in nitrogen content than the black coal asphaltenes. The base fractions of the asphaltenes contained about the same amount of oxygen as the acid fractions but much more nitrogen. The oxygen in the bases is presumably mainly ether oxygen but from enthalpimetric measurements [6] of Liddell and Millmerran base fractions there do appear to be some OH groups present. However, the methoxyl peak in the n.m.r, spectrum of basic fraction of methylated MiUmerran asphaltene was very weak while in the acid/neutral fraction it was quite intense indicating that the vast majority of the OH groups {which were methylated) were in the acid/neutral fraction and not in the basic fraction. The molecular weights of all the basic fractions were considerably higher than the corresponding acid/neutral fraction (see Table 6). However, the carbon and hydrogen distribution appears to be similar for both fractions from the same asphaltene (see Tables 6 and 7). Long alkyl chains, indicated by the e CH2 peak at 29.5 ppm were present in all asphaltenes fractions with e peak being more intense in the 13C-n.m.r. spectra of the Loy Yan~ asphaltene. The ~H-n.m.r. spectrum of the acid/neutral fraction from Liddell tar indicated the presence of a phenolic OH at ca. 5.8 ppm. The phenolic OH peak was not as distinct in the spectra of Loy Yang and Millmerran asphaltene acid fractions, being partly obscured by the aromatic region. Presumably some hydrogen bonding still occurs in the acid fractions, which would tend to broaden the OH peak. The presence of the OH peak in the IH-n.m.r. spectra of the acid fractions of asphaltenes was originally reported by Sternberg et al. [2] l~ut was not observed in the acid fractions of other similar asphaltenes [141. Possible average molecular structures of acid and base fractions from Millmerran asphaltene are shown in Fig. l c and d. The structures shown are in good agreement with the data in Tables 6 and 7.
Pre-asphaltenes The pre-asphaltene content of the flash pyrolysis tars is high (see Table 2). For n.m,r, studies the pre-asphaltenes were silytated by the procedure of Snape and Battle [15] to give extracts that were almost completely soluble in chloroform C>90%). The conversion of the hydroxyl groups to their trinethylsilyl derivatives was confirmed by infrared spectroscopy. Details of the spectroscopic and other analytical data obtained for the pre-asphaltenes are given in Table 8. The pre-asphaltenes are no more aromatic, as shown by 13C-n.m.r. spectroscopy, than the asphaltenes from the corresponding tars. Loy Yang preasphaltene is the least aromatic with the highest H/C atomic ratio while Liddell is most aromatic with the lowest H/C atomic ratio. As the pre-asphal.
158 TABLE 8 Analytical and n.m.r, data for pre-asphaltene Loy Yang
Millmerran
Liddell
Elemental Analysis (wt.%) C H N S O (by difference) H/C atom. ratio
72.3 5.8 2.0 0.3 19.6 0.96
79.4 6.1 3.0 0.5 11.0 0.92
75.5 4.7 4.3 0.4 15.1 0.75
From silylated pre-asphaltenes Mol. wt a %OH from ZH
734 9.0
597 5.9
924 6.4
21
30
36
21 36 43 0.57 0.31
33 33 34 0.66 0.39
35 36 29 0.71 0.41
n.m.r.
Aromatic H(% total H) a
ISC-n.m.r. data (%C) Aromatic C--C and C--O Aromatic C--H Aliphatic C fa Methyl as a fraction of aliphatic carbon
acorrected for Si(CH3)3 content.
tenes have a lower H/C atomic ratio than the corresponding asphaltenes but no higher aromaticity, they presumably have more aliphatic carbon in bridging groups or in alicyclic rings. The molecular weights of the pre-asphaltene were considerably higher than the asphaltenes. The average formulae of the pre-asphaltenes are: C44H43NO9 for Loy Yang, with 4 of the oxygen atoms in OH groups and 1 in 15 molecules containing a sulphur atom; C39H37NO4 for Millmerran, with 2 oxygen atoms in OH groups and 1 in 10 molecules containing a sulphur atom; and CssH43N309 for Liddell, with 4 oxygens atoms in OH groups and 1 in 8 molecules containing a sulphur atom. The 13C-n.m.r. of the silylated Loy Yang pre-asphaltenes had peaks at 174 and 181 ppm which were assigned to ester and COOH groups respectively. The integral of these peaks was not significant. The amount of long alkyl chains indicated by the e CH2 peak at 29.5 ppm, is considerably less in the pre-asphaltenes than in either the asphaltene or oil fractions. A possible average molecular structure for Millmerran pre-asphaltene based on the data in Table 8 is given in Fig. le. General
The H/C atomic ratio decreases and the aromaticity increases of the flash
159 pyrolysis tars, and their fractions, as the rank of the coal increases. Thus the Loy Yang brown coal tar is the least aromatic with the highest H/C atomic ratio and contains more long alkyl chains, especially in the oil fraction. The tar from the highest rank coal, Liddell, is the most aromatic with the lowest H/C atomic ratio and the least long alkyl chains. The flash pyrolysis tar from Liddell coal was produced at a higher temperature, which would be expected to increase its aromaticity. However, the n.m.r, data for the pentane eluates of the oils indicate that the higher rank of Liddell coal is probably the main reason for the higher aromaticity of the Liddell tar. Both Millmerran and especially Loy Yang tars have relatively high H/C atomic ratios and contain appreciable amounts of long alkyl chains either attached to aromatic rings or as straight-chain alkanes (or alkenes). These tars should, therefore, be capable of being hydrotreated to reasonable quality diesel and jet fuels. This seems to us less likely for the Liddell tar with its lower H/C atomic ratio and fewer long alkyl chains. The heteroatom content is high for all three tars. Loy Yang tar contains much oxygen which would consume a considerable amount of hydrogen during the hydroprocessing and, therefore, increase the cost of this step. Though the oxygen content of the Millmerran and Liddell tars is considerably lower than the Loy Yang tar, these tars contain significant amounts of nitrogen and sulphur which may pose problems in their hydroprocessing. ACKNOWLEDGEMENTS
We are grateful to Mrs Irina Salivan for molecular weight and OH determinations, to Mrs Andrea Armstrong for recording the ~3C-n.m.r. spectra and to the CSIRO Division of Fossil Fuels for the tar samples. REFERENCES
1 Edwards, J.H. and Smith, I.W., 1980. Fuel, 59: 674. 2 Sternberg, H.W., Raymond, R. and Schweighardt,F.K., 1975. Science, 188: 49. 3 Snape, C.E., and Bartle, K.D., 1979. Fuel, 58: 898. 4 Liotta, R., 1979. Fuel, 58: 724. 5 Liotta, R., Rose, K. and Hippo, E., 1981. J. Org. Chem., 46: 277. 6 Vaughan, G.A. and Swithenbank, J.J.,1970. Analyst, 95: 891. 7 Shoolery, J.N. and Budde, W.L., 1976. Anal. Chem., 48: 1458. 8 Suuberg, E.M., Peters, W.A. and Howard, J.B., 1980. Amer. Chem. Soc. Adv. Chem. Series, 183: 239. 9 Snape, C.E., Ladner, W.R. and Bartle, K.D., 1979. Anal. Chem., 51: 2189. 10 Pugmire, R.J., Grant, D.M., Zilm, K.W., Anderson, L.L., Oblad, A.G. and Wood, R.E., 1977. Fuel, 56: 295. 11 Vahrman, M., 1970. Fuel, 49: 5. 12 Ker~haw, J.R., Barrass, G., DuPreez, I.C. and Gray, D., 1980. Fuel Process. Technol., 3: 131. 13 Martin, T.G., Smith, C.A., 8nape, C.E. and Starkie, H.C., 1981. Fuel, 60: 365. 14 Bockrath, B.C., Delle Donne, C.L. and Schweighardt, F.K., 1978. Fuel, 57: 4. 15 Snape, C.E. and Bartle, K.D., 1979. Fuel, 58: 898.
145
Elsevier Scientific Publishing Company, Amsterdam - - Printed in The Netherlands
THE CHEMICAL NATURE OF FLASH PYROLYSIS TARS. AN N.M.R. STUDY
JOHN
R. K E R S H A W
and B A R R Y
A. K E L L Y
CSIRO Division of Applied Organic Chemistry, P.O. Box 4331, Melbourne, Victoria 3001 (Australia) (Received June 10th, 1982; accepted in revised form October 12th, 1982)
ABSTRACT Flash pyrolysis tars from one brown and two bituminous Australian coals were separated into oils, asphaltenes and pre-asphaltenes. The oils were further separated by chromatography while the asphaltenes were separated into basic and acid/neutral fractions. The pre-asphaltenes were silyalated prior to 'sC- and 'H-n.m.r. studies. The brown coal tar was less aromatic and contained more long alkyl chains than the tars from the bituminous coals. Aliphatic constituents of the oils,which were relatively abundant, consisted mainly of n-alkanes and straight chain 1-alkenes with an average chain length of ca. C,8. The pre-asphaltenes were no more aromatic than the asphaltenes from the same tar but had higher molecular weights.
INTRODUCTION
The CSIRO is currently investigating flash pyrolysis as the first stage of a process for deriving transport fuels and chemical feedstocks from Australian coals. A 20 kg h "1 fluidized-bed pyrolysis unit is being used to evaluate the process [1]. As it is envisaged that the flash pyrolysis tar will be further processed by hydrogenation to a refinery feedstock, it is important that it is important that its structural composition should be investigated as a guide to its suitability for hydroprocessing and so that the variations of composition with pyrolysis conditions and the coal used may be determined. In this paper the chemical nature of three flash pyrolysis tars obtained from three different Australian coals, two high volatile bituminous coals and one brown coal, have been studied mainly by n.m.r, spectroscopy. The tars were separated into oils, asphaltenes and pre-asphaltenes. The oils were further separated into three fractions by elution chromatography and the asphaltenes into basic and acid/neutral fractions.
0378-3820/83/$03.00
© 1983 Elsevier Science Publishers B.V.
146 EXPERIMENTAL
Materials Flash pyrolysis tars were obtained from pyrolysis of Loy Yang, a brown coal from Victoria, Mfllmerran, a bituminous coal from Queensland and Liddell, a bituminous coal from New South Wales in a 20 kg h -~ fluidized-bed unit with a residence time of ca. 0.5 s. The pyrolysis temperature was 570°C for Loy Yang and Millmerran coals and 685°C for Liddell coal. The pyrolysis unit has been described in detail elsewhere [1]. Analyses of the coals are given in Table 1.
TABLE 1 Analysis of Australian coals used (percentage)
Moisture a Ash a Volatile Matter b Cb Hb Nb Sb O b (by difference) H/C atom. ratio
Loy Yang (brown)
Millmerran (bituminous)
Liddell (bituminous)
10.8 0.6 51.7 71.0 4.8 0.6 0.3 23.3
3.0 19.8 55.0 78.5 6.7 1.2 0.7 12.9
3.3 28.1 45.5 80.6 6.0 2.1 0.4 10.9
0.81
1.02
0.89
aair dried basis; bd.a.f.
Fractionation o f the tars The tars were extracted with toluene in a Soxhlet extractor for 72 h under argon. After removing the toluene under reduced pressure, the extract was re-dissolved in a small volume of toluene and a 20 fold excess of pentane added to precipitate the asphaltenes. Pre-asphaltenes were obtained by Soxhlet extraction of the toluene insoluble product with pyridine for 48 h under argon and removal of the pyridine under reduced pressure. The oils (pentane-soluble product) were further separated by adsorption chromatography on silica gel. The column was eluted successively with pentane, toluene and chloroform/methanol (1:1. by volume). The asphaltenes were separated into basic and acid/neutral fractions by passing HC1 gas through toluene solutions of the asphaltenes according to the procedure of Sternberg et al. [2].
147 Silylation of the pre-asphaltenes was carried out by the m e t h o d of Snape and Battle [3]. Methylation of the whole coal tars and the asphaltene from Millmerran tar was carried o u t by the m e t h o d of Liota et al. [4,5].
A nalyses Elemental analyses were carried out by the Australian Microanalytical Service. Phenolic h y d r o x y l was determined enthalpimetrically by the m e t h o d of Vaughan and Swithenbank [6]. Molecular weights were determined by vapour pressure osmometry in chloroform solutions. Natural abundance 13C-n.m.r. spectra were measured at 20 MHz on a Varian CFT-20 spectrometer with complete proton decoupling. Spectra were recorded in deuterochloroform containing Cr(AcAc)3 under conditions which give quantitative data [ 7]. IH-n.m.r. spectra were measured at 60 MHz in deuterochloroform on a Varian T-60 spectrometer. Chemical shifts are reported relative to internal tetramethylsilane. RESULTS AND DISCUSSION The yields, composition and elemental analyses of tars are given in Table 2. The flash pyrolysis tars had a high asphaltene and pre-asphaltene c o n t e n t (see Table 2) probably due to the short residence time in the pyrolysis unit (ca. 0.5 s) and indicating t h a t considerable upgrading will be required if the tars are to be converted to transport fuels. The high heteroatom c o n t e n t of the flash pyrolysis tars will require considerable hydrogen consumption for their removal in the hydroprocessing step. TABLE 2 Yields, composition, elemental analysis and aromaticity of tars
Pyrolysis Temperature (oC) Tar Yielda Oilb Asphaltene b Pre-asphalteneb Cb Hb
Nb Sb Ob (by difference) H/C Atom. ratio fa from ~SC-n.m.r. of methylated tarc
Loy Yang
Millmerran
Liddell
570 22.6 46 24 28 73.8 7.7 0.3 0.3 17.9 1.25 0.51
570 33.0 36 36 24 81.4 7.8 1.5 1.3 8.0 1.15 0.59
685 27.0 36 45 15 81.1 6.1 1.7 0.9 10.2 0.90 0.64
awt.% coal d.a.f.; bwt.% tar; Ccorrected for OMe peak.
148
The brown coal gave a lower tar yield than either of the bituminous coals but the tar had a higher H/C atomic ratio and lower aromaticity. Suuberg et al. [8] reported a similar finding during their study of the pyrolysis of a Montana lignite and a Pittsburg bituminous coal. The lignite gave a lower tar yield but with a higher H/C atomic ratio for the tar. The oil content of the tars is higher for the brown coal than for the bituminous coals. The pre-asphaltene content is lowest for the Liddell tar which maybe due to the higher reactor temperature used in the pyrolysis of Liddell coal (685°C) compared to Loy Yang and MiUmerran coals (570°C). Oils
The oils (pentane soluble product) were separated by adsorption chromatography on silica gel into three fractions, a pentane eluate, a toluene eluate and a chloroform/methanol eluate. Analytical and n.m.r, data for the oils and their fractions are given in Tables 3--5. The ~3C-n.m.r. spectra of the total otis, toluene and chloroform/methanol eluates were assigned in a similar way to that used by Snape et al. [9] (see Table 4). The integral of the ~ and e carbons of long alkyl chains are also included so that the number of long chains can be estimated. In the ~3C-n.m.r. spectra of the total oil (see Table 4) the alkene carbons are included in the aromatic carbons. The error caused by this will be very small. The n.m.r, data for the oils (Tables 3 and 4) shows that the oil from Loy Yang tar is the least aromatic with more long alkyl chains, as indicated by the intensity of the e CH2 peak of long aliphatic chains at 29.5 ppm, whilst the Liddell oil is the most aromatic with fewer long aliphatic chains. The H/C atomic ratio is in agreement with the n.m.r, data and is highest for the Loy Yang oil and lowest for Liddell oil. The H/C atomic ratio of Loy Yang and Millemerran oils are high and both oils contain a significant number of long alkyl chains as indicated by their ~3C-n.m.r. spectra (see Table 4). The heteroatom content of the oils, especially the oxygen content in Loy Yang oil, is relatively high. The presence of alkenes in the oils is shown by the ~H-n.m.r. spectra of all three oils. Molecular weights of the three oils were similar. The 13C-n.m.r. spectra of the pentane eluates of all three tars indicate that the principal components are n-alkanes and unbranched 1-alkenes (see Table 5). The latter are indicated by peaks at 114 ppm (terminal methylene carbon, = CH2), 139 ppm (the/3 -- CH carbon) and 34 ppm (~ carbon, CH2 next to CH=CH2 group). Peaks at 14, 23, 32, 29 and 29.5 ppm are due to the a,/~,7,~ and e carbons of long aliphatic chains [ 10]. In the case of Millmerran and LiddeU tar, the ratio of n-alkanes to 1-alkenes is about 3:2". However, in Loy *For 1-alkenes the intensity of the a-carbon of long aliphatic chains at 14 p p m should be same as the intensityof the C H = C H 2 peaks at 114 p p m and 139 ppm. F r o m the intensities of the peaks at 139 ppm, 114 p p m and 14 ppm, the relativeamounts of 1-alkenesand n-alkanes can be estimated.
Yield (wt.% t a r )
aincludes phenolic OH.
Aromatic Phenolic OH Olefmic Benzylic Aliphatic
% H from IH-n.m.r.
Mol. w t .
11 3 2 19 65
292
1.47
0.3 12.5
S O (by difference)
H/C a t o m . r a t i o
77.6 9.5 0.1
46
C H N
(wt.%)
Elemental analysis
19 a
31 50
38 47
284
1.35
0.3 14.8
76.2 8.6 0.1
24
15
311
1.32
0.3 2.4
87.6 9.6 0.1
11
14 2 2 27 55
303
1.36
0.5 6.5
82.8 9.4 0.8
36
37 44
19
339
1.14
0.8 5.2
85.6 8.1 0.3
9
Toluene eluate
Total
CHCIs/CHsOH eluate
Total
Toluene eluate
Millmerran
Loy Yang
Yields, e l e m e n t a l analyses, m o l e c u l a r w e i g h t s a n d IH-n.m.r. d a t a for oils
TABLE 3
34 41
25 a
303
1.19
79.8 7.9 1.8 0.6 9.9
15
CHCIJCH3OH eluate
23 3 2 29 43
290
1.19
83.0 8.2 0.6 0.5 7.7
36
Total
Liddell
40 28
32
272
1.01
86.9 7.3 0.4 0.8 4.6
14
Toluene eluate
38 29
33 a
277
1.12
82.2 7.7 1.4 0.6 8.1
15
CHCIs/CHsOH eluate
33.3
27.5--37
18--22.5
29--30
8 and e CH 2 groups of long aliphatic chains
a
.
.
0.41
18
14.1
3.9
2.6
6.8
6.0
0.53
15
7.4
6.8
4.1
8.1
6.8
15.5
5.4
6.0 25.7 18.9 2.7
0.40
17
13.6
5.5
3.4
6.8
8.9
25.8
9.3
3.8 15.2 13.6 7.6
0.47
13
10.6
4.7
2.5
6.2
6.5
27.6
5.3
4.4 22.1 17.1 3.7
0.55
11
7.4
5.8
2.7
8.5
5.4
16.2
6.6
2.3 25.1 23.6 3.8
a s m u n m g a m a j o r i t y o f a l k y l c h a i n s are p r e s e n t as a l k y l s u b s t i t u e n t $ as o p p o s e d t o n - a l k a n e s .
fa
N u m b e r average chain length of alkyl groups a
10--15
15--18
CHs 7 or f u r t h e r from a r o m a t i c ring
--CH s of ethyl groups
a--CH s
22.5--27.5
6.8
37--60
C H 2 adj t o t e r m i n a l C H 3 in a l k y l g r o u p s >C4; C H 2 in tetzalin, i n d a n e a n d in p r o p y l g r o u p s
3.4 20.5 13.3 3.4
148--168 129--148 118--129 100--118
Toluene
Total
CHCls/CHsOH
Total
Toluene
l~lh~er£an
Loy Yang
A r o m a t i c C---O A r o m a t i c C--C Aromatic C--H Aromatic C--H ortho to a r o m a t i c C--O CH a to aromatic ring; Az---CH2--Ar; C H in alkyl groups CH 2 in l o n g chalns~ C H 2 a r o m a t i c ring; CH 2 fl t o a r o m a t i c ring in a l k y l chains
C a r b o n (%)
Chemical shift ( p p m )
Assignment
13C-n.m.r. d a t a for oils
TABLE 4
0.54
12
7.1
6.3
3.4
8.2
6.0
16.0
6.7
9.7 23.1 16.4 4.5
CHCIs/CH3OH
0.57
14
7.2
5.2
3.2
6.8
5.2
16.5
5.6
7.2 27.3 20.1 2.8
Total
Liddell
0.70
10
3.5
4.6
2.8
7.4
3.2
8.1
4.2
4.2 31.7 31.7 2.1
Toluene
0.69
11
3.5
4.7
3.9
7.0
3.1
8.9
3.9
13.2 29.8 20.9 4.7
CHCIJCHsOH
o
151 TABLE 5 Analytical and n.m.r, data for the pentane eluate of the oils
Yield (wt.% of tar) H/C atom. ratio Mol. wt. n-alkanes and 1-alkenes (wt.% of pentane eluate) Average chain length
Loy Yang
Millmerran
Liddell
11 1.81 272 59
12 1.83 279 52
7 1.82 318 55
18
18
18
Yang tar the 13C-n.m.r. spectra indicates that 1-alkenes predominate. The average chain length of the n-alkanes/1-alkenes, indicated by '3C-n.m.r. spectroscopy, was about C18 in all three tars. A more precise estimate was difficult to obtain due to peak overlap when both n-alkenes and 1-alkenes are present. (n-Alkanes are symmetrical whereas 1-alkenes are not.) The pentane eluate also contained branched alkanes, internal alkenes and small amounts of aromatic hydrocarbons. In the region 100--150 ppm of the ~3C-n.m.r.spectra, two strong signals were present at 114 ppm and 139 ppm due to 1-alkanes, while the other peaks in this region may be due to either internal alkenes or aromatic hydrocarbons. 1H-n.m.r. spectra indicated that both internal alkenes and aromatic hydrocarbons were present. The amount (wt.% tar) of pentane eluate is considerably lower for the Liddell tar then for the Loy Yang or Millmerran tars. The higher pyrolysis temperature used to produce Liddell tar would be expected to reduce both the number and length of n-alkanes and alkenes [12]. However, the chain length of the alkanes/alkenes was the same in the Liddell tar as the other two tars and it is probably the higher rank of the Liddell coal that mainly accounts for the lower alkane/alkene content rather than the pyrolysis temperature. Though both Millmerran and Liddell coals are bituminous coals, Millmerran is not typical having an exceptionally high H/C atomic ratio and volatile matter content. The very short residence time in the pyrolysis unit may limit the thermal breakdown of aliphatic chains and, therefore, the pyrolysis temperature may not have a significant effect on the composition of the alkanes. The amount (wt.% tar) of the toluene eluate, the aromatic hydrocarbon fraction of the otis, was highest for Liddell tar. The aromatic fraction of the oil from this tar was more aromatic and contained fewer long aikyl chains, as indicated by the integral of the e (and ~ ) CH2 peak of long alkyl chains in the 13C-n.m.r. spectra, than the aromatic fractions of the otis from the other tars. ~H-n.m.r., i.r. and OH analysis indicate that the oxygen was present as ether oxygen in the toluene eluates. From the analytical and n.m.r, data in Tables 3 and 4 possible representative average structures can be proposed.
152
Such structures corresponding to the aromatic fraction of the oil from Millmerran tar are given in Fig. la. These formalae represent averages of the chemical type and size of the very large number of individual compounds present. The chloroform/methanol eluate (polar fraction) of the oil from Loy Yang brown coal tar was considerably more aliphatic with more long alkyl chains than those fractions from the bituminous coals (see Table 4) and its yield (wt.% tar) was also considerably higher (see Table 3). Whereas the aromaticit.v and carbon distribution of the polar fractions of the oils from Liddell and Millmerran tars were very similar to the aromatic fractions of these oils, the polar fraction from Loy Yang tar was less aromatic and contained more long alkyl chains than the aromatic fraction of this tar. The chloroform/methanol eluate (polar fraction) of the oil from Liddell tar was the most aromatic with fewer long aliphatic chains. The oxygen content was higher for the polar fraction of Loy Yang oil with about a third of the oxygen being phenolic, however the nitrogen content was much lower than in the polar fractions of the oils from the bituminous coal tars. A peak at 179 ppm in the 13C-n.m.r. spectrum of the polar fraction from Loy Yang oil indicates a COOH group, though the integral for this peak was negligible. Possible average structures for the polar fractions of the oil from Millmerran tar are given in Fig. lb. The 13C-n.m.r. spectra of the total oil, and the pentane elute, the toluene eluate and the chloroform/methanol eluate of the oil from Millmerran flash pyrolysis tar are shown in Fig. 2.
Asphaltenes The asphaltene content of the tar was lowest for Loy Yang and hi~hest for LiddeU (see Table 2). The molecular weight of Loy Yang asphaltene was much lower than that of the other two asphaltenes. The asphaltenes were separated into acid/neutral and basic fractions by bubbling hydrogen chloride into a toluene solution of the asphaltene [2]. The base content of the asphaltenes from the bituminous coal tars were much higher than that from the brown coal tar (see Table 6), presumably due to the much lower nitrogen content in the asphaltene of Loy Yang tar. However a significant part of the HC1 precipitate of all three asphaltenes could not be extracted into chloroform from a sodium hydroxide solution. It appears that hydrogen chloride is precipitating non-basic components, an effect that has also been noted [13] for the asphaltene from a supercritical gas extract, but not in the separation of asphaltenes formed in processes where a considerable amount of hydrogenation takes place [2,14]. The asphaltene from Millmerran coal tar was methylated and separated into acid/neutral and basic fractions with hydrogen chloride. The methylated asphaltene had a much lower base content, 27%, compared with 66% for the unmethylated asphaltene which further supports the observation that hydrogen chloride precipitates non-bases as well as bases from toluene solutions of these flash pyrolysis asphaltenes. Martin et al. [ 13 ] have reported a similar
153
,•(CH2)IO
CHz-
CHcH2__CH 2 3 --CH2--CH3
--CH3
O~cHs CH~
[~_.o- cH~,..~ CH /
CHs CH2
CH~
CH3
~"0~ v NH
~3
(CH2). --CH3
o,.
~ C H
"OH
--CH2--CH2--CH3 CH3 I
OH
CH2--CH3 ~
O-CH~-CH3
C'
-
f,- ~-iF~--F- -CH3
0 --CH2~-~
CH3
3
CH3 CH3
OH H3C'~~CH
3
CH3
CH~
Fig. 1. Possible average s t r u c t u r e s f o r f r a c t i o n s o f M i l l m e r r a n flash p y r o l y s i s tar. (a) = t o l u e n e e l u a t e o f oil; ( b ) ffi c h l o r o f o r m / m e t h a n o l e l u a t e o f oil; (c) = acid f r a c t i o n o f a s p h a l t e n e ; ( d ) ffi base f r a c t i o n o f a s p h a l t e n e ; (e) ffi p r e - a s p h a l t e n e .
154
(a)
_/
J
/
(b)
(c)
(d)
i
~-
/
Fig. 2. lSC-n.m.r, spectra for the oil and its fractions o f MiUmerran flash pyrolysis tar. (a) = total oil, (b) = p e n t a n e eluate, (c) ffi t o l u e n e eluate; (d) = c h l o r o f o r m / m e t h a n o l eluate.
change in the yield of bases from methylated and unmethylated asphaltene obtained from a supercriticai gas extract. The HC1 treatment led to chlorine incorporation into the acid/neutral fraction of the asphaltene which has also been observed by other workers [2,14]. The analytical and n.m.r, data for the asphaltenes and their acid/neutral and basic fractions are given in Tables 6 and 7. The aromaticity, the methyl carbon as a fraction of the aliphatic carbon, and the percentage of aromatic hydrogen are lowest for Loy Yang asphaltene and its acid and base fractions and highest for Liddell asphaltene and
31 c 34 35
34 35
284
1.12
6.4
76.0 7.1 0.1 0.3 15.4 1.1
31 c
307
1.13
N.D.
74.3 7.0 0.1 0.3 18.3 N.D.
74
35 37
28
515
N.D.
N.D.
N.D. b N.D. N.D. N.D. N.D. N.D.
26
35 29
36 c
529
0.91
N.D.
80.3 6.1 2.2 0.9 10.5 N.D.
32 35
33 c
414 (428) a
0.99
5.6
79.1 6.5 0.5 0.6 12.1 1.2
36 ( 7 3 ) a
a f o r m e t h y l a t e d a s p h a l t e n e ; b N . D . = n o t d e t e r m i n e d ; Cincludes p h e n o l i c OH.
Aromatic Phenolic OH Benzylic Aliphatic
%H by 1H-n.m.r.
Mol. wt.
H/C A t o m . r a t i o
OH (wt.%)
(wt.%) C H N S O (by difference) C1
Elemental Analysis
Yield (wt.% asphaltene)
Acid/Neutral
Total
Base
Total
Acid/Neutral
Millmerran
Loy Yang
0.86
N.D.
81.0 5.8 1.8 0.8 10.6 N.D.
Total
35 32
33
33 28
39 c
638 (666) a 538
0.95
2.0
79.3 6.3 2.5 0.9 10.6 0.4
64 ( 2 7 ) a
Base
Liddell
32 6 36 26
349
0.92
4.9
81.9 6.3 0.5 0.5 N.D. N.D.
34
Acid/Neutral
) 59
41
593
0.80
1.8
79.3 5.3 2.0 0.6 N.D. N.D.
66
Base
Yields, e l e m e n t a l analyses, m o l e c u l a r w e i g h t s a n d IH-n.m.r. d a t a f o r t h e a s p h a l t e n e s a n d t h e i r a c i d / n e u t r a l a n d basic f r a c t i o n s
TABLE 6
0.33
0.57
7.2 22.3 27.5 43.0
amethylated asphaltene corrected for OMe peak at ca. 55 ppm.
0.30
Methyl (10--22.5 ppm) as fraction of aliphatic carbon
8.6 23.5 29.4 38.4 0.62
148--168 129--148 100--129 10-60
Aromatic C--O Aromatic C--C Aromatic C--H Aliphatic C
0.31
0.61
6.8 24.3 29.7 39.1
0.41
0.71
8.7 26.6 35.3 29.3
0.42
0.71 (0.69) a
10.8 28.8 31.6 28.8
Acid/Neutral
Total
Base
Total
Acid/Neutral
Millmerran
Lo y Yang
Carbon (%)
fa
Chemical shift (ppm)
Assignments
ISC-n.m.r. data for the asphaltene and their acid/neutral and basic fractions
TABLE 7
8.0 31.9 34.7 25.4
Total
0.41
0.44
0.65 0.75 (0.63) a
4.8 25.4 34.9 34.9
Base
Liddell
0.43
0.69
7.3 29.3 32.3 31.0
Acid/Neutral
0.44
0.74
7.4 32.1 35.0 25.5
Base
C.n O~
157 its fractions. The H/C atomic ratio is highest for Loy Yang asphaltene and lowest for Liddell. The asphaltene from Loy Yang tar is higher in oxygen content but lower in nitrogen content than the black coal asphaltenes. The base fractions of the asphaltenes contained about the same amount of oxygen as the acid fractions but much more nitrogen. The oxygen in the bases is presumably mainly ether oxygen but from enthalpimetric measurements [6] of Liddell and Millmerran base fractions there do appear to be some OH groups present. However, the methoxyl peak in the n.m.r, spectrum of basic fraction of methylated MiUmerran asphaltene was very weak while in the acid/neutral fraction it was quite intense indicating that the vast majority of the OH groups {which were methylated) were in the acid/neutral fraction and not in the basic fraction. The molecular weights of all the basic fractions were considerably higher than the corresponding acid/neutral fraction (see Table 6). However, the carbon and hydrogen distribution appears to be similar for both fractions from the same asphaltene (see Tables 6 and 7). Long alkyl chains, indicated by the e CH2 peak at 29.5 ppm were present in all asphaltenes fractions with e peak being more intense in the 13C-n.m.r. spectra of the Loy Yan~ asphaltene. The ~H-n.m.r. spectrum of the acid/neutral fraction from Liddell tar indicated the presence of a phenolic OH at ca. 5.8 ppm. The phenolic OH peak was not as distinct in the spectra of Loy Yang and Millmerran asphaltene acid fractions, being partly obscured by the aromatic region. Presumably some hydrogen bonding still occurs in the acid fractions, which would tend to broaden the OH peak. The presence of the OH peak in the IH-n.m.r. spectra of the acid fractions of asphaltenes was originally reported by Sternberg et al. [2] l~ut was not observed in the acid fractions of other similar asphaltenes [141. Possible average molecular structures of acid and base fractions from Millmerran asphaltene are shown in Fig. l c and d. The structures shown are in good agreement with the data in Tables 6 and 7.
Pre-asphaltenes The pre-asphaltene content of the flash pyrolysis tars is high (see Table 2). For n.m,r, studies the pre-asphaltenes were silytated by the procedure of Snape and Battle [15] to give extracts that were almost completely soluble in chloroform C>90%). The conversion of the hydroxyl groups to their trinethylsilyl derivatives was confirmed by infrared spectroscopy. Details of the spectroscopic and other analytical data obtained for the pre-asphaltenes are given in Table 8. The pre-asphaltenes are no more aromatic, as shown by 13C-n.m.r. spectroscopy, than the asphaltenes from the corresponding tars. Loy Yang preasphaltene is the least aromatic with the highest H/C atomic ratio while Liddell is most aromatic with the lowest H/C atomic ratio. As the pre-asphal.
158 TABLE 8 Analytical and n.m.r, data for pre-asphaltene Loy Yang
Millmerran
Liddell
Elemental Analysis (wt.%) C H N S O (by difference) H/C atom. ratio
72.3 5.8 2.0 0.3 19.6 0.96
79.4 6.1 3.0 0.5 11.0 0.92
75.5 4.7 4.3 0.4 15.1 0.75
From silylated pre-asphaltenes Mol. wt a %OH from ZH
734 9.0
597 5.9
924 6.4
21
30
36
21 36 43 0.57 0.31
33 33 34 0.66 0.39
35 36 29 0.71 0.41
n.m.r.
Aromatic H(% total H) a
ISC-n.m.r. data (%C) Aromatic C--C and C--O Aromatic C--H Aliphatic C fa Methyl as a fraction of aliphatic carbon
acorrected for Si(CH3)3 content.
tenes have a lower H/C atomic ratio than the corresponding asphaltenes but no higher aromaticity, they presumably have more aliphatic carbon in bridging groups or in alicyclic rings. The molecular weights of the pre-asphaltene were considerably higher than the asphaltenes. The average formulae of the pre-asphaltenes are: C44H43NO9 for Loy Yang, with 4 of the oxygen atoms in OH groups and 1 in 15 molecules containing a sulphur atom; C39H37NO4 for Millmerran, with 2 oxygen atoms in OH groups and 1 in 10 molecules containing a sulphur atom; and CssH43N309 for Liddell, with 4 oxygens atoms in OH groups and 1 in 8 molecules containing a sulphur atom. The 13C-n.m.r. of the silylated Loy Yang pre-asphaltenes had peaks at 174 and 181 ppm which were assigned to ester and COOH groups respectively. The integral of these peaks was not significant. The amount of long alkyl chains indicated by the e CH2 peak at 29.5 ppm, is considerably less in the pre-asphaltenes than in either the asphaltene or oil fractions. A possible average molecular structure for Millmerran pre-asphaltene based on the data in Table 8 is given in Fig. le. General
The H/C atomic ratio decreases and the aromaticity increases of the flash
159 pyrolysis tars, and their fractions, as the rank of the coal increases. Thus the Loy Yang brown coal tar is the least aromatic with the highest H/C atomic ratio and contains more long alkyl chains, especially in the oil fraction. The tar from the highest rank coal, Liddell, is the most aromatic with the lowest H/C atomic ratio and the least long alkyl chains. The flash pyrolysis tar from Liddell coal was produced at a higher temperature, which would be expected to increase its aromaticity. However, the n.m.r, data for the pentane eluates of the oils indicate that the higher rank of Liddell coal is probably the main reason for the higher aromaticity of the Liddell tar. Both Millmerran and especially Loy Yang tars have relatively high H/C atomic ratios and contain appreciable amounts of long alkyl chains either attached to aromatic rings or as straight-chain alkanes (or alkenes). These tars should, therefore, be capable of being hydrotreated to reasonable quality diesel and jet fuels. This seems to us less likely for the Liddell tar with its lower H/C atomic ratio and fewer long alkyl chains. The heteroatom content is high for all three tars. Loy Yang tar contains much oxygen which would consume a considerable amount of hydrogen during the hydroprocessing and, therefore, increase the cost of this step. Though the oxygen content of the Millmerran and Liddell tars is considerably lower than the Loy Yang tar, these tars contain significant amounts of nitrogen and sulphur which may pose problems in their hydroprocessing. ACKNOWLEDGEMENTS
We are grateful to Mrs Irina Salivan for molecular weight and OH determinations, to Mrs Andrea Armstrong for recording the ~3C-n.m.r. spectra and to the CSIRO Division of Fossil Fuels for the tar samples. REFERENCES
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