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Functional group fractionation and characterization of tars and pitches
Functional group fractionation and characterization of tars and pitches* Use of size exclusion chromatography mobile phase Michael
J. Mulligan,
K. Mark
Thomas
and DMF as the
and Andrew
British Gas plc, London Research Station, Michael (Received 30 March 7987; revised 3 June 7987)
P. Tytko
Road, London
SW6 2AD,
UK
The use of a chromatographic system where the separation is based mainly on a combination ofsize exclusion and adsorption processes, for the characterization of coalderived materials has been investigated. A calibration curve based on typical components found in tars has been established and provides the basis for the separation by molecular size and chemical type. A series of coal-derived materials produced by standard solvent fractionation and chromatographic schemes have been used to verify the basis of the separation process. The technique has been applied to a series of tars and pitches and the results have shown that it provides a rapid qualitative characterization of the nature of such materials in terms of chemical composition and molar mass distribution, and can be used to monitor the effect of process variables. (Keywords:pitch; structural analysis; size exclusion chromatography)
The processing of coal during carbonization, gasification and liquefaction gives rise to coalderived materials which are soluble in commonly used organic solvents. The analyses of these extracts can provide information about the chemical changes occurring during processing and also allow the effect of process variables to be monitored. The extracts contain’ numerous components and have a broad molar mass distribution. They contain functional groups such as phenols and consist of mainly small aromatic rings which may be linked together via bridge structures. Clearly, characterization of these materials represents a formidable challenge. To aid characterization, coal-derived materials are subjected to solvent fractionation schemes. The basis of these fractionation procedures is to use organic solvents of increasing polarity such as n-pentane, benzene and tetrahydrofuran (THF) to give fractions of increasing molar mass and heteroatom content. Three typical fractions are: n-pentane solubles, benzene insolubles and asphaltenes (benzene soluble, n-pentane insoluble)2. The fractions can be separated further by chromatographic methods and the fractions characterized by a variety of spectroscopic and chemical methods to provide details of individual components and average structures3. This approach is very time consuming and hypothetical average molecular structures have limited predictive value in complex mixtures where there are a wide range of molar masses. Coal-derived materials are also processed further to produce high value products with specific properties. An example of this is the manufacture of pitch by the distillation of coke oven tar. Pitch is used4 in a variety of processes, for example, it is mixed with various fillers and
carbonized to give products such as carbon anodes for the
* This paper was presented at the conference ‘Pitch: the science of a future material’, Newcastle upon Tyne, UK, 2426 March 1987. 001~2361/87/111472~09$3.00 0 1987 Butterworth & Co. (Publishers)
1472
FUEL, 1987,
Ltd.
Vol 66, November
aluminium industry, graphite electrodes, briquettes and refractory linings. The chemical composition, physical properties and structure of pitches are not known with any degree of certainty. As a result empirical tests have been developed from which the nature of a coal-derived pitch and its behaviour during carbonization can be predicted. Examples of these tests’ are softening point which should be sufficiently low to allow efficient mixing with filler coke grist, density (a measure of aromaticity), coking value, quinoline insolubles (QI) and toluene insolubles (TI). Clearly, there is a need for improved methods which are capable of characterizing the chemical composition of the pitch in terms of size and chemical type. Chromatography is widely used in the separation, fractionation and characterization of complex mixtures of organic molecules. Size exclusion chromatography (s.e.c.) is a technique in which molecules are separated on the basis of their ability to penetrate a porous gel. Large molecules cannot penetrate the gel and are confined to the mobile phase resulting in them travelling through the column at the same speed as the mobile phase. Smaller molecules enter the stationary solvent trapped in the gel pores and their elution is retarded by the gel to an extent which is dependent on their molecular size and the distribution of pore sizes in the gel. This method provides a separation mainly on the basis of molecular size which corresponds to separation on the basis of molar mass. S.e.c. has been extensively used for the separation, fractionation and characterization of coal6 and petroleum’ derivatives. Most investigations have achieved a separationEv9 or fractionation”,” on the basis of molecular size which allows the determination of molar mass distributions by s.e.c.12-14. In general, investigations have shown’ 5-21 that the determination of molar mass distributions by s.e.c. needs considerable care in the selection of the mobile and stationary phases. A knowledge of the character of the derivatives under
Functional Table 1
Characterization
gro’up
data for the samples
fractionation
and characterization
used in this study
(a) Tar and tar fractions Tar Whole Fraction Fraction
A B
C(%)
H (%I
N (%)
0 (%I
S (%)
Atomic ratio
84.8 80.2 88.9
6.4 6.9 5.9
1.1 1.3 1.2
5.3 11.2 3.2
0.7 0.4 0.8
1.11 0.98 1.26
C/H
(b) Pitches
Pitch
Softening point (“C)
Quinoline insoluble content (wt %I
Coking value (wt %)
CTPl CTP2 CTP3 CEP PPl PP2 PP3 A240
100.5 103.9 105.2 151.0 138.0 120.5 101.2 119.0
0.8 12.3 9.7 0 0 1.2 11.6 0.3
21.0 58.2 55.6 63.0 25.5 54.3 56.3 39.3
Atomic ratio
C/H
1.04 1.74 1.71 1.16 0.74 1.35 1.70
et al.
M. J. Mulligan
distributions unreliable with high concentrations of these derivatives present. The deviations from the normal size exclusion process can be used to characterize coal and petroleum derivatives provided care is taken in the selection of the chromatographic system. When dimethylformamide (DMF) is used as the mobile phase it is possible to enhance the adsorption and partition effects because of the relatively poor solubility of non-polar polynuclear aromatic compounds (PAC) in the solvent and its poor compatibility with the neutral gel (typically, a polystyrene/divinylbenzene copolymer) used as the stationary phase. The objectives of this work were to investigate the suitability of s.e.c. described above for: 1. assessing the extent of functional group separation in addition to molecular size; 2. characterizing the composition of a range of coal-derived materials; and 3. monitoring changes in composition with changes in process variables. EXPERIMENTAL
(c)Condensates” Hydrogenation
temperature
Compound
Molar mass (gmol-‘)
760
800
850
900
Benzene Toluene Naphthalene Acenaphthylene Acenaphthalene/biphenyl Fluorene Phenanthreneianthracene Pyrene/fluoranthane Chrysene/benzanthracenes Benzofluorenes Benzopyrenes/perylene Coronene Indenopyrenes C,-Phenols Carbazole C,-Naphthols Benzocarbazoles Dibenzocarbazoles Quinolines Azapyrenes
78 92 128 152 154 160 178 202 228 216 252 300 276 150 167 200 217 267 129 203
21.74 0.69 17.81 1.90 0.50 7.35 20.51 12.46 1.58 1.65 2.56 0.03 0.57 0.99 2.44 0.68 0.63 0.07 0.90 0.21
37.26 0.61 15.49 1.08 0.36 4.79 11.96 10.86 0.82 0.42 2.02 0.04 0.71 0.74 2.51 0.71 0.24 0.43 0.40 0.44
62.03 0.20 6.83 0.16 0.21 0.69 2.59 11.95 0.35 0.12 1.57 0.22 1.89 0.08 0.61 0.57 0.03 0.01 0.19
43.24 0.40 2.52 0.03 0.61 1.15 2.24 14.94 0.61 1.00 2.99 4.48 13.61 ~~ 0.18 0.06 0.08 ~ 0.17
(“C)
_ ’ Gas chromatography/mass omitted)
of tars and pitches:
spectrometry
data
(minor
components
investigation is also desirable so that a suitable calibration curve can be derived from model compounds and polymer standards 22-25. However, although the separation is mainly on the basis of molecular size; there are adsorption, association and partition effects particularly for compounds with low molar mass. These effects which depend on the solubility of the material in the mobile phase and the compatibility of the mobile phase with the macroporous gel, have been usedI to separate cats-condensed and per&condensed polynuclear aromatic hydrocarbons (PAH). THF is a good solvent for most structures found in coal derivatives and it is compatible with most of the gels used as stationary phases in s.e.c. However, separation can still occur partly on the basis of functionality as well as on molecular size. An example of this is the association of THF with phenols and amines to give anomalously low retention volumes which can be used22-24 as the basis of a fractionation of tars. This makes determination of molar mass
Materials
used
The characterization data for the tars, pitches and condensates used in this study are given in Tables la, b and c respectively. Usually, greater than 80% of the coal tars and pitches were soluble in DMF. The insoluble portion being mainly coal/coke dust particles. The petroleumderived pitches were less soluble. Commercially available PACs and other calibration standards were obtained from a variety of sources and used without further purification. H.p.1.c. grade solvents were used throughout this investigation. Sample preparation
and ancillary
analyses
A range of coal-derived materials including tars, pitches and high-yield coal extracts such as asphaltenes were available for this investigation. Some of these were subject to the usual fractionation procedures and typical ancillary analyses such as elemental, functional group, molar mass and g.c.-m.s. have been determined26. A summary of the fractionations typically performed on coal-derived materials is shown in Figure I. First, the coal extracts were fractionated into benzene insolubles, asphaltenes and n-pentane soluble fractions according to the procedures by Bartle et a1.27. The n-pentane soluble fractions were fractionated further by silica gel adsorption chromatography28 yielding paraffin, aromatic (low
Coal
arOmatiCS
derived
material
mono’s
Polar
poly’s
Hexane (neutral Ether (basic N) Chloroform
fractions
Polar
fractions
/di’s
polars
N)
Benzene Chloroform THF/Ethanol
\ S-heterocycles
Figure 1 Fractionation characterization
schemes
available
for aiding
coal
FUEL, 1987, Vol 66, November
extract
1473
Functional
group fractionation
Table 2 Calibration
of standard
and characterization
compounds
of tars and pitches: M. J. Mulligan
on polystyrene/divinyl
benzene gel using DMF
et al.
as the mobile phase
Formula
Retention volume (ml)
Molar mass (g mol- ‘)
C7H8 C,,H,, C,,H,, C,,H,, C,,H,, C,,H,, C,,Hzs C,,H,, C,,H,, C,,H,, C,~H,O C,*H,* C,,H,, CXH,,
27.2 28.3 30.2 27.7 30.3 27.8 30.6 34.1 24.4 29.0 28.3 31.0 27.8 38.2
92 142 154 178 202 178 542 252 382 202 166 228 204 300
Phenols/carboxylic acids (0) 1 3,4-Dimethylphenol 2 Resorcinol 3 Y-Phenanthrol 4 9-Hydroxyl fluorene 5 2-Naphthoic acid
C,H,OH C,H,(QH), C,,H@H C, ,H@H C,,H,OOH
19.6 16.8 20.5 19.2 20. I
122 110 194 182 172
Aliphatic hydrocarbons 1 Octane 2 Nonane 3 Decane 4 Undccane 5 Dodecane 6 Tridecane 7 Tetradecane
C,H,, C,H,, C,,H,, C,,H,, C,,HX, C, ~HZB C,,H,,
28.9 29.4 30.9 31.4 32.8 33.3 34.5
104 128 142 156 170 184 198
C,,H,N C,,H,N, C,,H,N C,,H,,N C,,HgN C, ,H,N C, ,H,N C,,H,,NH C,zH,N
20.9 26.3 25.6 25.0 24.4 25.3 18.0 20.3 24.8
167 180 179 157 143 179 179 169 166
C,,H,NQ, CHN 20 12 4
24.3 20.3
223 310
Ketones/quinones (0) 1 9,10-Phenanthrene quinone 2 Acetophenone 3 Benzophenone 4 Fluorenone
C,,H& C,H,Q C,,H,,Q C,,H,Q
19.8 21.4 22.5 24.0
208 120 182 180
Ethers (0) 1 Dibenzofuran 2 Anisole 3 Dimethoxybenzene
C,,HaQ C,H,Q C,H,,Q,
25.3 23.3 22.4
168 108 140
Cr,H,,S
28.8
184
Compound Aromatic hydrocarbons 1 Toluene 2 I-Methylnaphthalene 3 Acenaphthalene 4 Anthracene 5 Pyrene 6 Phenanthrene 7 Rubrene 8 Perylene 9. Quinqephenyl 10 Fluoranthene 11 Fluorene 12 Benzanthracene 13 1-Phenyl naphthalene 14 Coronene
(0)
(0)
Amines, nitrogen heterocycles etc. (W) 1 Carbazole 2 Phenazine 3 Acridine 4 2,7-Dimethyl quinoline 5 Quinalidine 6 Phenanthridine 7 7,8-Benzoquinoline 8 Diphenylamine 9 Azafluorene 10 9-Nitroanthracene 11 3(2-pyridyl)-5,6diphenyl-1,2,4-triazine
Sulphur compounds 1 Dibenzothiophene
a)
hydroxyl) and polar (high hydroxyi) fractions. The aromatic fraction obtained can be further fractionated into mono (1 ring), di- (2 rings) and polyaromatic (3 3 rings) fractions on an alumina column. Asphaltenes can also be fractionated on a silica gel column using procedures similar to that described by Schwager and Yenz9 to yield fractions of increasing polarity. It is also possible to subject the n-pentane fraction to other chromatographic separations. An example is the use of an OPN/Porasil-C column (porous silica with ophthalonitrile groups chemically bonded to the Porasil-C support medium) which is used to separate out a nitrogen rich fraction using the method described by Burchill et
1474
FUEL, 1987,
Vol 66, November
al 3o A heterocyclic
sulphur rich fraction has also been isolated31*32 from an aromatic n-pentane fraction. This method involves a selective oxidation with mchlorobenzoic acid to produce a sulphone and quinone fraction which is then subsequently reduced by lithium aluminium hydride prior to separation using silica gel adsorption chromatography. Size exclusion
chromatography
The system used was a neutral (polystyrene/divinylbenzene) macroporous gel (‘PL’ gel) with a particle size of IOpm and average pore diameter 10nm. The h.p.1.c.
Functional
group
fractionation
and characterization
of tars and pitches:
Ouinones, Ketones, Amines
Phenols w*
4 2.8
M. J. Mulligan
et al.
Aromatic Hydrocarbons *
Ethers, N, S, heterocycles 4
Aliphatics w
l7
11 n
0
01
z
2.3
c
f
.
3A7
2.2
2.1 I
J
A2 2.0 I
I--
Size exclusion zone
Total, exclusion limit 10
15
.y--permeation +::~Adsorptio; CC mtrol -
,imit
20
I
25
30
35
Retention volume t ml
Figure 2 Calibration graph for s.e.c. column with DMF as the mobile phase showing relative positions of phenols/carboxyEc acids (A), ketones/ quinones (0). ethers (O), aromatic hydrocarbons (a), aliphatic hydrocarbons (V), nitrogen-containing species (m) and sulphursontaining species
(A) pump was an Applied Chromatography Systems (ACS) model 300 pump which was used in conjunction with two detectors. The first monitor was a U.V. detector (ACS model 750111) with a filter of wavelength 254nm. The second monitor which was in series was a differential refractive index detector (ERMA ERC-7510). The flow rate (l.Omlmin~ ‘) was determined with an electronic flow meter. Sample injections were made via a Rheodyne valve system with a 20~1 sample loop using solutions with concentrations of 1-IOmgml-’ in DMF.
RESULTS
AND DISCUSSION
Calibration
procedure
Details of the compounds used in the calibration procedure together with their retention volumes, formulae and molar masses are given in Table 2. Figure 2 shows the calibration graph for the ‘PL’ polystyrene/ divinyl benzene gel s.e.c. column with DMF as the mobile phase. This indicates that the normal size exclusion process has been modified with some compounds being eluted at up to 15 ml greater than the total permeation limit (20 ml). The calibration graph shows that the classes of compounds elute in well defined regions which are given below: Phenols Aromatic hydrocarbons Aliphatic hydrocarbons Amines, nitrogen heterocycles Ketones,lquinones Ethers
etc.
16-20 27-38 29-34 2626 2Ck24 22-25
ml ml ml ml ml ml
Non-polar PAH are eluted well after the total permeation limit (20 ml). A reversal of the normal s.e.c. process occurs within this class, the smaller molecules such as toluene and l-methyl naphthalene being eluted before larger, more condensed molecules such as pyrene and perylene, suggesting the occurrence of adsorption effects. The main exceptions to this general trend in the compounds studied are quinquephenyl and rubrene. The former has the structure C,H,(C,H,), . C,H,, which is a linear molecule whereas the latter is C,,H,(C,H,),. Neither molecules are highly condensed. The series of aliphatic hydrocarbons show identical behaviour with the smaller molecules being eluted first. Both aromatic and aliphatic hydrocarbons have poor solubility in DMF and have a much greater affinity for the stationary phase. These combined effects of adsorption and partition account for their elution well beyond the total permeation limit. In contrast, phenols elute at retention volumes in the range 16-20 ml which are similar to those observed when either chloroform or tetrahydrofuran (THF) are used as the mobile phasesz4 indicating that the normal size exclusion process is operating. The retention volumes obtained in THF are slightly lower than in chloroform and this has been ascribed to association effects between the THF and phenols increasing the apparent molecular size. It appears that the more polar molecules which are reasonably soluble in DMF but with no real affinity for the stationary phase, undergo the normal s.e.c. separation process. The results are consistent with separation in the region between 10 and 20 ml on the basis of molecular size. Nonpolar aromatic and aliphatic molecules are adsorbed and
FUEL, 1987, Vol 66, November
1475
Functional
group fractionation
and characterization
THF
Hexane
I
I
10
20 Retention
volume
of tars and pitches: M. J. Mulligan
fraction
fraction
I
I
30
40
/ ml
Figure 3 Elution profiles for an n-pentane fraction and its subsequent fractionation into hexane, ether and THF fractions on a OPN/Porasil-C column
are eluted in the range 27-35 ml after the total permeation limit. Amines, nitrogen heterocycles, ethers and ketones which have intermediate polarity are eluted in the region 2&27ml with the neutral nitrogen compounds being eluted at the lower end (21-22ml) of the range with compounds such as acetophenone and benzophenone. It is apparent that the separation has been achieved on the basis of a combination of both polarity and molecular size which is very useful in the context of the characterization of complex mixtures of organic compounds such as coal derivatives. Coal-derived fractions
The chromatographic profiles obtained by eluting previously characterized coal-derived materials form an important part of the s.e.c. calibration procedure since this will:
et al.
The bimodal chromatogram of the hexane fraction is a clear indication that the sample contains significant quantities of neutral nitrogen compounds such as carbazole in addition to aromatic species. The ether fraction has a chromatographic profile consistent with the presence of mainly basic nitrogen compounds. The nonpolar nature of the fraction can be clearly seen by comparison of the chromatogtam with that for the THF fraction which contains some phenolic constituents but relatively few PAH. The aromatic n-pentane fraction has been selectively oxidized and fractionated to yield a sulphur-rich fraction. The chromatographic profile of this fraction is shown in Figure 4. This selective oxidation produces sulphone and quinone functional groups from the various PAC present3”. The chromatographic profiles of the two fractions are shown in Figure 4. The sulphone and quinone rich fraction elutes in both the polar region and also in the high molar mass region indicating that some PACs have been converted to high molar mass material that is being excluded from the column which has a total exclusion limit at 10ml. Reduction yields a sulphur fraction rich in dibenzothiophene type structures26. The broad elution profiles of this fraction indicates that there is a wide range of material present with the sulphur species in a variety of environments, but it contains significant quantities which elute in the intermediate or neutral region. Figure 5 illustrates the chromatographic profiles obtained after fractionation of the aromatic portion (> 90%) of a typical hydrogen donor solvent used to extract coal. The effectiveness of the separation into mono-, di- and polyaromatic species has been confirmed by ‘H n.m.r. studies 26. The elution profiles indicate a shift to higher retention volumes with increased ring condensation. However, the profiles in the aromatic region indicate that the natures of all three fractions are broadly similar but the degree of condensation is reflected
confirm the presence of chemical species based on type or functionality by comparison with the calibration graph obtained using model compounds; illustrate the general applicability to complex mixtures that contain numerous individual components that vary in molecular size and chemical types; and help further define the regions that depend on both molecular size and type. These applications are illustrated and discussed below. n-Pentane soluble fractions. The elution profiles for a n-pentane soluble fraction and its subsequent fractionation on an OPN/Porasil-C column to yield hexane, ether and THF fractions is shown in Figure 3. A comparison with the calibration graph derived for model compounds shown in Figure 2 indicates the increasing polar nature of the fractions with increase in polarity of the eluent. This chromatographic separation has been used previously to separate out clearly defined neutral nitrogen species in the hexane fraction and basic nitrogen species into the ether fraction3’. The neutral and basic fractions contain predominantly pyrrolic compounds and aza heterocycles respectively, although nitriles and amines may also be present in small quantities in these fractions3’.
1476
FUEL, 1987, Vol 66, November
Sulphone fraction
and
quinone
J
I
I
10
20
I 30
Retention
volume
I ml
I 40
Figure 4 Elution profiles for the oxidized aromatic fraction of a npentane fraction and its subsequent fractionation into sulphone and quinone fraction (and unoxidized PAC) from which the S-rich fraction arises
Functional
group fractionation
and characterization
in the shift to higher relative retention volumes in the aromatic region of the elution profile. It is clear from the profiles that many small polar and neutral molecules are also found in the mono- and di-aromatic fractions produced using this standard fractionation scheme28. Asphaltenes. The analysis of asphaltenes is difficult because of the presence of high molecular weight compounds which are too complex for individual analysis. In such cases average molecular structures are determined from spectroscopic and analytical data3. The first step in such a structural analysis is to calculate the number of different atoms in an average molecule from the n.m.r.-derived hydrogen and carbon distributions, ultimate analysis and molecular weight. This series of structural parameters is used to construct an average molecular structure to represent a statistical average of the very large number of molecules which vary in both size and chemical type. These average structures are usually not unique but provide a visualization of the types
20
I
I
30
40
Retention
volume
i
ml
Figure 5 Elution proliles for the mono-, di-, and fractions derived from a solvent used to extract coal
Figure 6 Elution profiles of the benzene, chloroform fractions of asphaltene A
Table 3
Characterization
Asphaltene
data for asphaltenes
M. J. Mulligan
et al.
of molecules present in the material. A comparison of three asphaltenes which have been fractionated on a silica gel column to give increasingly polar fractions is shown in Figures 6 and 7. Table 3 summarizes information on the yields obtained in the benzene, chloroform and THF/ethanol fractions after separation of these asphatlenes2’j. The results indicate that asphaltene A has been obtained from a more severe process A that extracts much more of the original coal than process B. This aspect is also reflected in the fact that much less of the extract from process A compared with process B is found to be npentane soluble. The subsequent fractionations of asphaltenes A and B on a silica gel column indicate that the former has lost much more of its heteroatom content during processing and thus the benzene fraction constitutes a much greater percentage of the total asphaltene. The elution profiles for asphaltene C clearly indicate the presence of highly adsorbed material (retention volumes > 35 ml) which represents higher molar mass condensed aromatic material. Figure 6 again illustrates the increasingly polar nature of subsequent fractions of asphaltene A but also importantly indicates the presence of numerous small polar and aromatic molecules. Similarly for asphaltene B, the presence of many small polar molecules especially in the THF fraction is an indication of the imprecise nature of the fractionation. This observation is important since the increasing molar mass and heteroatom content observed in fractions with more polar solvents, might in part, be due to the presence of small polar molecules in these fractions. This is not surprising since as the severity of the extraction process increases, further degradation ofthe coal macromolecules occurs leading to a gradual decrease in heteroatom content and a corresponding increase in the aromatic content of the material extracted. The increased amount of high molar mass polar compounds that is excluded
polyaromatic
and THF/ethanol
A, B and C, including o/OExtract” (‘4 n-pentane)
7; d.a.f. coal
of tars and pitches:
Figure 7 Elution profiles of a pitch asphaltene C (THF/ethanol fraction) and asphaltene B (benzene and THF/ethanol fractions)
their fractionation
‘/, Benzene
into more polar solvents ~_
~--
% Chloroform
7; THF/ethanol ..~
25 12
A B C “Figure
in brackets
refers to portion
29 (18) 35 (35)
soluble
52.6 15.9 82.9
4.7 1.9 2.6
42.1 82.2 14.5
in n-pentane
FUEL, 1987, Vol 66, November
1477
Functional
group fractionation
and characterization
Table 4 An illustration of the s.e.c. selectivity in terms of molecular and chemical type available using DMF as the mobile phase Retention volume (ml)
Molar mass range (gmol-I)
10
>looo
15-20
<500
2C27
1500
27-35
<500
>35
size
Region
__
High molar mass polar material (size exclusion limit) Low molar mass polar material (mainly phenols) up to total permeation limit (20 ml) Low molar mass neutral species (nitrogen heterocycles, ethers, ketones and related species) Low molar mass aromatics, for example, anthracene High molar mass non-polar material adsorbed on column typically highly condensed aromatics and long chain aliphatics
<2000
Fraction
of tars and pitches:
A
Fraction
6
n
12
I
14
I
I
I
I
I
I
I
I
I
I
16
18
20
22
24
26
28
30
32
34
Retention
volume
M. J. Mulligan
et al.
Tars, pitches and condensates Figure 8 shows the chromatographic elution profile for a low temperature coal tar and two fractions derived from this tar. The fractions were produced using the s.e.c. column with THF as the mobile phase by the method described previously24. Fraction A consists of mainly phenols and long-chain paraffins while fraction B contains mainly PAC. The results show that the calibration curve derived from the model compounds applies very well to the complex tar mixture which contains mainly molecules with low molar mass. Figure 9 shows the elution profiles of a variety of coalderived pitches. The results show clearly that the chemical composition of the pitches varies considerably. Pitch CTPl contains a significant proportion of polar materials (mainly phenols) in addition to heterocydic and aromatic compounds. There is a large peak corresponding to small condensed aromatics but the trace tails off relatively quickly indicating only small amounts of large condensed PAH. In comparison, the elution profile for pitch CTP2 which has been heat treated to produce mesophase spheres and contains carbon black as well as primary QI, shows that it contains predominantly small PAH with relatively small amounts of polar and heterocyclic aromatic material. Pitch CTP3 has very little polar material but has significant amounts of small aromatic heterocyclics. The maximum at a retention volume of 32ml is on the high side of the PAH region and there is a large tail to the peak indicating substantial quantities of highly condensed higher molar mass material which is
i ml
Figure 8 Elution profiles for the low temperature coal tar and its fractions: a, tar before fractionation; b, after fractionation
from the column for asphaltene B is a reflection of the fact that these species have retained their heteroatom content while in the case of asphaltene A, the greater severity of the processing conditions has led to greater degradation of the coal macromolecule. The calibration of the s.e.c. chromatographic system with DMF as the mobile phase using model compounds and previously characterized coal-derived materials has enabled regions which depend on both size and chemical type to be defined. This is illustrated in Table 4 and provides the basis for characterizing the complex mixtures of organic molecules whose constituents vary widely. The ability of the technique to characterize complex mixtures of organic molecules is general and only limited by the solubility of individual components in DMF.
1478
FUEL, 1987,
Vol 66, November
I
I 10
I
I
20 Retention
30 volume
, ml
I
I 40
Figure 9 Elution profiles for various coal-derived
50
pitches
Functional
I
I
20
30 Retention
Figure 10
Elution
group fractionation and characterization
volume
profiles for various
I
I
40
50
of tars and pitches:
M. J. Mulligan et al.
are given in Table le. A comparison of Figure 11 and Table lc shows that the high molar mass condensed aromatics are more clearly identified by the s.e.c. technique. The results outlined above clearly show that the chromatographic technique can be used to monitor the chemical composition of a wide range of coal-derived materials and is sufficiently sensitive to detect compositional changes resulting from small changes in process variables. The use of the technique for characterizing coal-derived materials containing a wide molar mass range and chemical type has demonstrated its usefulness. The alternative approach would be to fractionate the materials and analyse them by gas chromatography and mass spectrometry (g.c.-ms.). This is a rather time consuming procedure and the high molar mass material is not readily detected by the technique. G.c.-m.s. gives detailed information on the concentrations of individual small molecules but is not really suitable for mixtures containing significant quantities of high molar mass material.
ml
petroleum-derived
pitches
strongly adsorbed on the column. Similarly, pitch CEP contains predominantly highly condensed PAH molecules with the maximum in the elution profile occurring outside the range of the calibration curve at > 40 ml. A comparison of Figure 9 with Table lb shows that the relationship between the elution profiles and the pitch characterization parameters is very complex. However, the coking value and QI content tend to correlate with the presence of highly condensed PAH. The relationship between the chemical composition of these pitches and their ability to modify the carbonization process and coke properties will be discussed in a future paper. Figure 10 shows the chromatographic elution profiles of four petroleum-derived pitches. The solubility of the pitches in DMF is the main limitation to the characterization with pitch PP3 being only partly soluble. The soluble portions of all four pitches investigated have a small amount of material (MM > 1000) eluted at the size exclusion limit (10ml) and virtually no low molar mass polar material. The relative amounts of neutral material (mainly nitrogen heterocycles, amines, ethers etc.) varied with PP2 and PP3 having the largest amounts. Otherwise the main difference between the pitches is the extent of the tail on the peak due to small aromatic molecules which is due to high molar mass aromatic material. This tail is largest for Ashland A240 pitch. The ability of the technique to monitor the effect of process variables on the chemical composition of a coalderived material is illustrated in Figure 11. This shows the s.e.c. traces of an anthracene heavy oil condensate which has been produced by a hydrogenation process at various temperatures in the range 76s900°C. Clearly the effect of increasing the temperature is mainly a decrease in the amounts of neutral heterocyclic and other compounds in the region 2&26 ml in the elution profiles. At 900°C the concentration of these compounds is very small and the condensate consists almost exclusively of PAH with some high molar mass condensed aromatics eluting above 35 ml. These samples have been analysed by gas chromatography and mass spectrometry and these data
CONCLUSIONS It is apparent that the use of a highly polar mobile phase (DMF) with a neutral macroporous gel, modifies the separation process considerably from one based mainly on molecular size, to include significant contributions from association, partition and adsorption effects. The calibration of the column with model compounds (mainly constituents of tars), and previously characterized fractions of coal-derived materials has shown that it is
900°C
I
I
I
I
I 30
I
10 Retention
Figure 11 Elution profiles temperatures in the range anthracene oil
I
,
I
I
I
50
volume
I ml
for condensates 760-900°C by
FUEL, 1987,
I
produced at various the hydrogenation of
Vol 66, November
1479
Functional
group fractionation
and characterization
of tars and pitches:
possible to define regions of the chromatogram which depend on both size and chemical type. This twin selectivity provides the basis for a rapid and unique characterization of complex materials, such as tars, coal extracts, pitches etc., which contain numerous individual components varying considerably in molar mass and chemical type. The technique can detect small changes in the relative concentrations of groups of species and hence, monitor the effect of changes in process variables. The overall picture produced has increased our knowledge of the effectiveness of the fractionations that have been performed by various procedures. This investigation has shown that the characterization technique has general applicability to a wide range of coal- and petroleumderived materials; the main limitation being the solubility of the constituents in DMF particularly in some petroleum pitches.
ACKNOWLEDGEMENTS The authors would like to thank British Gas plc for permission to publish this paper. The samples of anthracene oil condensates were supplied by Dr G. H. Rhodes and the g.c.-m.s. for the samples by T. Haley. The samples of pitch were provided by Dr J. W. Patrick and Professor H. Marsh.
REFERENCES 1 2
3 4 5
1480
Pullen, J. R. in ‘Solvent Extraction of Coal’, Report ICTIS/TR16, November 1981, IEA Coal Research, London, UK Mima, M. J., Schultz, H. and McKinsky, W. E. ‘Analytical Methods for Coal and Coal Products’ (Ed. C. Karr), Academic Press, 1978, Ch. 19, pp. 557-568 Herod, A. A., Ladner, W. R. and Snape, C. E. Phil. Trans. R. Sot. A300,3 Rand, B. and McEnaney, B. Br. Ceram. J. 1985, 84, 157 Quinoline Insolubles in Coke Oven Tar, and the effect of
FUEL, 1987, Vol 66, November
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
M. J. Mulligan
et al.
Carbonization Conditions, BCRA Special Publication No. 27, 1982 Bartle, K. D., Collin, G., Stadelhoffer, J. W. and Zander, M. J. Chem. Tech. Biotechnol. 1979, 29, 531 Altgelt, K. H. and Gouw, T. H. Adu. Chromatog. 1975, 13, 71 Meiris, R. B. Chem. Ind. (London) 1973, 642 Altgelt. K. H. Makromol. Chem. 1965. 88. 75 Coleman, W. M., Wooton, D. L., Do&H. C. and Taylor, L. T. Anal. Chem. 1977, 49, 533 Wooton, D. L., Coleman, W. M., Taylor, L. T. and Dorn, H. C. Fuel 1978, 57, 17 Mehesch, H., Hodek, W. and Kolling, G., Compendium 77/78 Supplement to Erdol Kohle Erdgas Petrochemie 1977, p. 283 Lewis, I. C. and Petro, B.A. J. Polym. Sci. Poly. Chem. Ed. 1976, 14, 1975 Mayo, F. R. and Kirschen, N. A. Fuel 1978, 57, 405 Hendrickson, J. G. and Moore, J. C. J. Polym. Sci. 1966,4,167 Edstrom, T. and Petro, B. A. J. Polym. Sci. 1968, C21, 171 Cazes, J. and Gaskill, D. R. Sep. Sci. 1969, 2, 421 Lambert, A. Analyt. Chim. Acta. 1976, Bl, 423 Popl, M., Fahnrich, J. and Stejskal, M. J. Chromatogr. Sci. 1976, 14, 537 Mackay, J. F. and Latham, D. R. Anal. Chem. 1975, 45, 1050 Bergman, J. G., Duffy, L. J. and Stevenson, R. B. Anal. Chem. 1971,43, 131 Philip, C. V. and Anthony, R. G. Am. Chem. Sot. Div. Fuel Chem. Prepr. 1979, 24(3), 204 Philip, C. V. and Anthony, R. G. Fuel 1982,61, 357 Evans, N., Haley, T. M., Mulligan, M. J. and Thomas, K. M. Fuel 1986,65, 694 Bartle, K. D., Mills, D. G., Mulligan, M. J., Amaechina, I. 0. and Taylor, N. Anal. Chem. 1986, 58, 2403 Tytko, A. P. PhD Thesis, University of Leeds, 1986 Bartle, K. D., Ladner, W. R., Martin, T. G., Snape, C. E. and Williams, D. F. Fuel 1979, 58, 413 Marsh, M. K., Smith, C. A., Stokes, B. J. and Snape, C. E. J. Chromatogr. 1984, 283, 173 Schwager, I. and Yen, T. F. Fuel 1979,58, 219 Burchill, P., Herod, A. A., Mahon, J. P. and Pritchard, E. J. Chromatogr. 1983, 281, 109 Rultz, J. M., Carden, B. M., Lena, L. J. and Vincent, E. J. Anal. Chem. 1982, 54, 688 Wiley, C. W., Iwao, M., Castle, R. N. and Lee, M. Anal. Chem. 1981, 53, 400 Burchill, P., Herod, A. A. and Pritchard, E. J. Chromatogr. 1982, 242, 51
J. Mulligan,
K. Mark
Thomas
and DMF as the
and Andrew
British Gas plc, London Research Station, Michael (Received 30 March 7987; revised 3 June 7987)
P. Tytko
Road, London
SW6 2AD,
UK
The use of a chromatographic system where the separation is based mainly on a combination ofsize exclusion and adsorption processes, for the characterization of coalderived materials has been investigated. A calibration curve based on typical components found in tars has been established and provides the basis for the separation by molecular size and chemical type. A series of coal-derived materials produced by standard solvent fractionation and chromatographic schemes have been used to verify the basis of the separation process. The technique has been applied to a series of tars and pitches and the results have shown that it provides a rapid qualitative characterization of the nature of such materials in terms of chemical composition and molar mass distribution, and can be used to monitor the effect of process variables. (Keywords:pitch; structural analysis; size exclusion chromatography)
The processing of coal during carbonization, gasification and liquefaction gives rise to coalderived materials which are soluble in commonly used organic solvents. The analyses of these extracts can provide information about the chemical changes occurring during processing and also allow the effect of process variables to be monitored. The extracts contain’ numerous components and have a broad molar mass distribution. They contain functional groups such as phenols and consist of mainly small aromatic rings which may be linked together via bridge structures. Clearly, characterization of these materials represents a formidable challenge. To aid characterization, coal-derived materials are subjected to solvent fractionation schemes. The basis of these fractionation procedures is to use organic solvents of increasing polarity such as n-pentane, benzene and tetrahydrofuran (THF) to give fractions of increasing molar mass and heteroatom content. Three typical fractions are: n-pentane solubles, benzene insolubles and asphaltenes (benzene soluble, n-pentane insoluble)2. The fractions can be separated further by chromatographic methods and the fractions characterized by a variety of spectroscopic and chemical methods to provide details of individual components and average structures3. This approach is very time consuming and hypothetical average molecular structures have limited predictive value in complex mixtures where there are a wide range of molar masses. Coal-derived materials are also processed further to produce high value products with specific properties. An example of this is the manufacture of pitch by the distillation of coke oven tar. Pitch is used4 in a variety of processes, for example, it is mixed with various fillers and
carbonized to give products such as carbon anodes for the
* This paper was presented at the conference ‘Pitch: the science of a future material’, Newcastle upon Tyne, UK, 2426 March 1987. 001~2361/87/111472~09$3.00 0 1987 Butterworth & Co. (Publishers)
1472
FUEL, 1987,
Ltd.
Vol 66, November
aluminium industry, graphite electrodes, briquettes and refractory linings. The chemical composition, physical properties and structure of pitches are not known with any degree of certainty. As a result empirical tests have been developed from which the nature of a coal-derived pitch and its behaviour during carbonization can be predicted. Examples of these tests’ are softening point which should be sufficiently low to allow efficient mixing with filler coke grist, density (a measure of aromaticity), coking value, quinoline insolubles (QI) and toluene insolubles (TI). Clearly, there is a need for improved methods which are capable of characterizing the chemical composition of the pitch in terms of size and chemical type. Chromatography is widely used in the separation, fractionation and characterization of complex mixtures of organic molecules. Size exclusion chromatography (s.e.c.) is a technique in which molecules are separated on the basis of their ability to penetrate a porous gel. Large molecules cannot penetrate the gel and are confined to the mobile phase resulting in them travelling through the column at the same speed as the mobile phase. Smaller molecules enter the stationary solvent trapped in the gel pores and their elution is retarded by the gel to an extent which is dependent on their molecular size and the distribution of pore sizes in the gel. This method provides a separation mainly on the basis of molecular size which corresponds to separation on the basis of molar mass. S.e.c. has been extensively used for the separation, fractionation and characterization of coal6 and petroleum’ derivatives. Most investigations have achieved a separationEv9 or fractionation”,” on the basis of molecular size which allows the determination of molar mass distributions by s.e.c.12-14. In general, investigations have shown’ 5-21 that the determination of molar mass distributions by s.e.c. needs considerable care in the selection of the mobile and stationary phases. A knowledge of the character of the derivatives under
Functional Table 1
Characterization
gro’up
data for the samples
fractionation
and characterization
used in this study
(a) Tar and tar fractions Tar Whole Fraction Fraction
A B
C(%)
H (%I
N (%)
0 (%I
S (%)
Atomic ratio
84.8 80.2 88.9
6.4 6.9 5.9
1.1 1.3 1.2
5.3 11.2 3.2
0.7 0.4 0.8
1.11 0.98 1.26
C/H
(b) Pitches
Pitch
Softening point (“C)
Quinoline insoluble content (wt %I
Coking value (wt %)
CTPl CTP2 CTP3 CEP PPl PP2 PP3 A240
100.5 103.9 105.2 151.0 138.0 120.5 101.2 119.0
0.8 12.3 9.7 0 0 1.2 11.6 0.3
21.0 58.2 55.6 63.0 25.5 54.3 56.3 39.3
Atomic ratio
C/H
1.04 1.74 1.71 1.16 0.74 1.35 1.70
et al.
M. J. Mulligan
distributions unreliable with high concentrations of these derivatives present. The deviations from the normal size exclusion process can be used to characterize coal and petroleum derivatives provided care is taken in the selection of the chromatographic system. When dimethylformamide (DMF) is used as the mobile phase it is possible to enhance the adsorption and partition effects because of the relatively poor solubility of non-polar polynuclear aromatic compounds (PAC) in the solvent and its poor compatibility with the neutral gel (typically, a polystyrene/divinylbenzene copolymer) used as the stationary phase. The objectives of this work were to investigate the suitability of s.e.c. described above for: 1. assessing the extent of functional group separation in addition to molecular size; 2. characterizing the composition of a range of coal-derived materials; and 3. monitoring changes in composition with changes in process variables. EXPERIMENTAL
(c)Condensates” Hydrogenation
temperature
Compound
Molar mass (gmol-‘)
760
800
850
900
Benzene Toluene Naphthalene Acenaphthylene Acenaphthalene/biphenyl Fluorene Phenanthreneianthracene Pyrene/fluoranthane Chrysene/benzanthracenes Benzofluorenes Benzopyrenes/perylene Coronene Indenopyrenes C,-Phenols Carbazole C,-Naphthols Benzocarbazoles Dibenzocarbazoles Quinolines Azapyrenes
78 92 128 152 154 160 178 202 228 216 252 300 276 150 167 200 217 267 129 203
21.74 0.69 17.81 1.90 0.50 7.35 20.51 12.46 1.58 1.65 2.56 0.03 0.57 0.99 2.44 0.68 0.63 0.07 0.90 0.21
37.26 0.61 15.49 1.08 0.36 4.79 11.96 10.86 0.82 0.42 2.02 0.04 0.71 0.74 2.51 0.71 0.24 0.43 0.40 0.44
62.03 0.20 6.83 0.16 0.21 0.69 2.59 11.95 0.35 0.12 1.57 0.22 1.89 0.08 0.61 0.57 0.03 0.01 0.19
43.24 0.40 2.52 0.03 0.61 1.15 2.24 14.94 0.61 1.00 2.99 4.48 13.61 ~~ 0.18 0.06 0.08 ~ 0.17
(“C)
_ ’ Gas chromatography/mass omitted)
of tars and pitches:
spectrometry
data
(minor
components
investigation is also desirable so that a suitable calibration curve can be derived from model compounds and polymer standards 22-25. However, although the separation is mainly on the basis of molecular size; there are adsorption, association and partition effects particularly for compounds with low molar mass. These effects which depend on the solubility of the material in the mobile phase and the compatibility of the mobile phase with the macroporous gel, have been usedI to separate cats-condensed and per&condensed polynuclear aromatic hydrocarbons (PAH). THF is a good solvent for most structures found in coal derivatives and it is compatible with most of the gels used as stationary phases in s.e.c. However, separation can still occur partly on the basis of functionality as well as on molecular size. An example of this is the association of THF with phenols and amines to give anomalously low retention volumes which can be used22-24 as the basis of a fractionation of tars. This makes determination of molar mass
Materials
used
The characterization data for the tars, pitches and condensates used in this study are given in Tables la, b and c respectively. Usually, greater than 80% of the coal tars and pitches were soluble in DMF. The insoluble portion being mainly coal/coke dust particles. The petroleumderived pitches were less soluble. Commercially available PACs and other calibration standards were obtained from a variety of sources and used without further purification. H.p.1.c. grade solvents were used throughout this investigation. Sample preparation
and ancillary
analyses
A range of coal-derived materials including tars, pitches and high-yield coal extracts such as asphaltenes were available for this investigation. Some of these were subject to the usual fractionation procedures and typical ancillary analyses such as elemental, functional group, molar mass and g.c.-m.s. have been determined26. A summary of the fractionations typically performed on coal-derived materials is shown in Figure I. First, the coal extracts were fractionated into benzene insolubles, asphaltenes and n-pentane soluble fractions according to the procedures by Bartle et a1.27. The n-pentane soluble fractions were fractionated further by silica gel adsorption chromatography28 yielding paraffin, aromatic (low
Coal
arOmatiCS
derived
material
mono’s
Polar
poly’s
Hexane (neutral Ether (basic N) Chloroform
fractions
Polar
fractions
/di’s
polars
N)
Benzene Chloroform THF/Ethanol
\ S-heterocycles
Figure 1 Fractionation characterization
schemes
available
for aiding
coal
FUEL, 1987, Vol 66, November
extract
1473
Functional
group fractionation
Table 2 Calibration
of standard
and characterization
compounds
of tars and pitches: M. J. Mulligan
on polystyrene/divinyl
benzene gel using DMF
et al.
as the mobile phase
Formula
Retention volume (ml)
Molar mass (g mol- ‘)
C7H8 C,,H,, C,,H,, C,,H,, C,,H,, C,,H,, C,,Hzs C,,H,, C,,H,, C,,H,, C,~H,O C,*H,* C,,H,, CXH,,
27.2 28.3 30.2 27.7 30.3 27.8 30.6 34.1 24.4 29.0 28.3 31.0 27.8 38.2
92 142 154 178 202 178 542 252 382 202 166 228 204 300
Phenols/carboxylic acids (0) 1 3,4-Dimethylphenol 2 Resorcinol 3 Y-Phenanthrol 4 9-Hydroxyl fluorene 5 2-Naphthoic acid
C,H,OH C,H,(QH), C,,H@H C, ,H@H C,,H,OOH
19.6 16.8 20.5 19.2 20. I
122 110 194 182 172
Aliphatic hydrocarbons 1 Octane 2 Nonane 3 Decane 4 Undccane 5 Dodecane 6 Tridecane 7 Tetradecane
C,H,, C,H,, C,,H,, C,,H,, C,,HX, C, ~HZB C,,H,,
28.9 29.4 30.9 31.4 32.8 33.3 34.5
104 128 142 156 170 184 198
C,,H,N C,,H,N, C,,H,N C,,H,,N C,,HgN C, ,H,N C, ,H,N C,,H,,NH C,zH,N
20.9 26.3 25.6 25.0 24.4 25.3 18.0 20.3 24.8
167 180 179 157 143 179 179 169 166
C,,H,NQ, CHN 20 12 4
24.3 20.3
223 310
Ketones/quinones (0) 1 9,10-Phenanthrene quinone 2 Acetophenone 3 Benzophenone 4 Fluorenone
C,,H& C,H,Q C,,H,,Q C,,H,Q
19.8 21.4 22.5 24.0
208 120 182 180
Ethers (0) 1 Dibenzofuran 2 Anisole 3 Dimethoxybenzene
C,,HaQ C,H,Q C,H,,Q,
25.3 23.3 22.4
168 108 140
Cr,H,,S
28.8
184
Compound Aromatic hydrocarbons 1 Toluene 2 I-Methylnaphthalene 3 Acenaphthalene 4 Anthracene 5 Pyrene 6 Phenanthrene 7 Rubrene 8 Perylene 9. Quinqephenyl 10 Fluoranthene 11 Fluorene 12 Benzanthracene 13 1-Phenyl naphthalene 14 Coronene
(0)
(0)
Amines, nitrogen heterocycles etc. (W) 1 Carbazole 2 Phenazine 3 Acridine 4 2,7-Dimethyl quinoline 5 Quinalidine 6 Phenanthridine 7 7,8-Benzoquinoline 8 Diphenylamine 9 Azafluorene 10 9-Nitroanthracene 11 3(2-pyridyl)-5,6diphenyl-1,2,4-triazine
Sulphur compounds 1 Dibenzothiophene
a)
hydroxyl) and polar (high hydroxyi) fractions. The aromatic fraction obtained can be further fractionated into mono (1 ring), di- (2 rings) and polyaromatic (3 3 rings) fractions on an alumina column. Asphaltenes can also be fractionated on a silica gel column using procedures similar to that described by Schwager and Yenz9 to yield fractions of increasing polarity. It is also possible to subject the n-pentane fraction to other chromatographic separations. An example is the use of an OPN/Porasil-C column (porous silica with ophthalonitrile groups chemically bonded to the Porasil-C support medium) which is used to separate out a nitrogen rich fraction using the method described by Burchill et
1474
FUEL, 1987,
Vol 66, November
al 3o A heterocyclic
sulphur rich fraction has also been isolated31*32 from an aromatic n-pentane fraction. This method involves a selective oxidation with mchlorobenzoic acid to produce a sulphone and quinone fraction which is then subsequently reduced by lithium aluminium hydride prior to separation using silica gel adsorption chromatography. Size exclusion
chromatography
The system used was a neutral (polystyrene/divinylbenzene) macroporous gel (‘PL’ gel) with a particle size of IOpm and average pore diameter 10nm. The h.p.1.c.
Functional
group
fractionation
and characterization
of tars and pitches:
Ouinones, Ketones, Amines
Phenols w*
4 2.8
M. J. Mulligan
et al.
Aromatic Hydrocarbons *
Ethers, N, S, heterocycles 4
Aliphatics w
l7
11 n
0
01
z
2.3
c
f
.
3A7
2.2
2.1 I
J
A2 2.0 I
I--
Size exclusion zone
Total, exclusion limit 10
15
.y--permeation +::~Adsorptio; CC mtrol -
,imit
20
I
25
30
35
Retention volume t ml
Figure 2 Calibration graph for s.e.c. column with DMF as the mobile phase showing relative positions of phenols/carboxyEc acids (A), ketones/ quinones (0). ethers (O), aromatic hydrocarbons (a), aliphatic hydrocarbons (V), nitrogen-containing species (m) and sulphursontaining species
(A) pump was an Applied Chromatography Systems (ACS) model 300 pump which was used in conjunction with two detectors. The first monitor was a U.V. detector (ACS model 750111) with a filter of wavelength 254nm. The second monitor which was in series was a differential refractive index detector (ERMA ERC-7510). The flow rate (l.Omlmin~ ‘) was determined with an electronic flow meter. Sample injections were made via a Rheodyne valve system with a 20~1 sample loop using solutions with concentrations of 1-IOmgml-’ in DMF.
RESULTS
AND DISCUSSION
Calibration
procedure
Details of the compounds used in the calibration procedure together with their retention volumes, formulae and molar masses are given in Table 2. Figure 2 shows the calibration graph for the ‘PL’ polystyrene/ divinyl benzene gel s.e.c. column with DMF as the mobile phase. This indicates that the normal size exclusion process has been modified with some compounds being eluted at up to 15 ml greater than the total permeation limit (20 ml). The calibration graph shows that the classes of compounds elute in well defined regions which are given below: Phenols Aromatic hydrocarbons Aliphatic hydrocarbons Amines, nitrogen heterocycles Ketones,lquinones Ethers
etc.
16-20 27-38 29-34 2626 2Ck24 22-25
ml ml ml ml ml ml
Non-polar PAH are eluted well after the total permeation limit (20 ml). A reversal of the normal s.e.c. process occurs within this class, the smaller molecules such as toluene and l-methyl naphthalene being eluted before larger, more condensed molecules such as pyrene and perylene, suggesting the occurrence of adsorption effects. The main exceptions to this general trend in the compounds studied are quinquephenyl and rubrene. The former has the structure C,H,(C,H,), . C,H,, which is a linear molecule whereas the latter is C,,H,(C,H,),. Neither molecules are highly condensed. The series of aliphatic hydrocarbons show identical behaviour with the smaller molecules being eluted first. Both aromatic and aliphatic hydrocarbons have poor solubility in DMF and have a much greater affinity for the stationary phase. These combined effects of adsorption and partition account for their elution well beyond the total permeation limit. In contrast, phenols elute at retention volumes in the range 16-20 ml which are similar to those observed when either chloroform or tetrahydrofuran (THF) are used as the mobile phasesz4 indicating that the normal size exclusion process is operating. The retention volumes obtained in THF are slightly lower than in chloroform and this has been ascribed to association effects between the THF and phenols increasing the apparent molecular size. It appears that the more polar molecules which are reasonably soluble in DMF but with no real affinity for the stationary phase, undergo the normal s.e.c. separation process. The results are consistent with separation in the region between 10 and 20 ml on the basis of molecular size. Nonpolar aromatic and aliphatic molecules are adsorbed and
FUEL, 1987, Vol 66, November
1475
Functional
group fractionation
and characterization
THF
Hexane
I
I
10
20 Retention
volume
of tars and pitches: M. J. Mulligan
fraction
fraction
I
I
30
40
/ ml
Figure 3 Elution profiles for an n-pentane fraction and its subsequent fractionation into hexane, ether and THF fractions on a OPN/Porasil-C column
are eluted in the range 27-35 ml after the total permeation limit. Amines, nitrogen heterocycles, ethers and ketones which have intermediate polarity are eluted in the region 2&27ml with the neutral nitrogen compounds being eluted at the lower end (21-22ml) of the range with compounds such as acetophenone and benzophenone. It is apparent that the separation has been achieved on the basis of a combination of both polarity and molecular size which is very useful in the context of the characterization of complex mixtures of organic compounds such as coal derivatives. Coal-derived fractions
The chromatographic profiles obtained by eluting previously characterized coal-derived materials form an important part of the s.e.c. calibration procedure since this will:
et al.
The bimodal chromatogram of the hexane fraction is a clear indication that the sample contains significant quantities of neutral nitrogen compounds such as carbazole in addition to aromatic species. The ether fraction has a chromatographic profile consistent with the presence of mainly basic nitrogen compounds. The nonpolar nature of the fraction can be clearly seen by comparison of the chromatogtam with that for the THF fraction which contains some phenolic constituents but relatively few PAH. The aromatic n-pentane fraction has been selectively oxidized and fractionated to yield a sulphur-rich fraction. The chromatographic profile of this fraction is shown in Figure 4. This selective oxidation produces sulphone and quinone functional groups from the various PAC present3”. The chromatographic profiles of the two fractions are shown in Figure 4. The sulphone and quinone rich fraction elutes in both the polar region and also in the high molar mass region indicating that some PACs have been converted to high molar mass material that is being excluded from the column which has a total exclusion limit at 10ml. Reduction yields a sulphur fraction rich in dibenzothiophene type structures26. The broad elution profiles of this fraction indicates that there is a wide range of material present with the sulphur species in a variety of environments, but it contains significant quantities which elute in the intermediate or neutral region. Figure 5 illustrates the chromatographic profiles obtained after fractionation of the aromatic portion (> 90%) of a typical hydrogen donor solvent used to extract coal. The effectiveness of the separation into mono-, di- and polyaromatic species has been confirmed by ‘H n.m.r. studies 26. The elution profiles indicate a shift to higher retention volumes with increased ring condensation. However, the profiles in the aromatic region indicate that the natures of all three fractions are broadly similar but the degree of condensation is reflected
confirm the presence of chemical species based on type or functionality by comparison with the calibration graph obtained using model compounds; illustrate the general applicability to complex mixtures that contain numerous individual components that vary in molecular size and chemical types; and help further define the regions that depend on both molecular size and type. These applications are illustrated and discussed below. n-Pentane soluble fractions. The elution profiles for a n-pentane soluble fraction and its subsequent fractionation on an OPN/Porasil-C column to yield hexane, ether and THF fractions is shown in Figure 3. A comparison with the calibration graph derived for model compounds shown in Figure 2 indicates the increasing polar nature of the fractions with increase in polarity of the eluent. This chromatographic separation has been used previously to separate out clearly defined neutral nitrogen species in the hexane fraction and basic nitrogen species into the ether fraction3’. The neutral and basic fractions contain predominantly pyrrolic compounds and aza heterocycles respectively, although nitriles and amines may also be present in small quantities in these fractions3’.
1476
FUEL, 1987, Vol 66, November
Sulphone fraction
and
quinone
J
I
I
10
20
I 30
Retention
volume
I ml
I 40
Figure 4 Elution profiles for the oxidized aromatic fraction of a npentane fraction and its subsequent fractionation into sulphone and quinone fraction (and unoxidized PAC) from which the S-rich fraction arises
Functional
group fractionation
and characterization
in the shift to higher relative retention volumes in the aromatic region of the elution profile. It is clear from the profiles that many small polar and neutral molecules are also found in the mono- and di-aromatic fractions produced using this standard fractionation scheme28. Asphaltenes. The analysis of asphaltenes is difficult because of the presence of high molecular weight compounds which are too complex for individual analysis. In such cases average molecular structures are determined from spectroscopic and analytical data3. The first step in such a structural analysis is to calculate the number of different atoms in an average molecule from the n.m.r.-derived hydrogen and carbon distributions, ultimate analysis and molecular weight. This series of structural parameters is used to construct an average molecular structure to represent a statistical average of the very large number of molecules which vary in both size and chemical type. These average structures are usually not unique but provide a visualization of the types
20
I
I
30
40
Retention
volume
i
ml
Figure 5 Elution proliles for the mono-, di-, and fractions derived from a solvent used to extract coal
Figure 6 Elution profiles of the benzene, chloroform fractions of asphaltene A
Table 3
Characterization
Asphaltene
data for asphaltenes
M. J. Mulligan
et al.
of molecules present in the material. A comparison of three asphaltenes which have been fractionated on a silica gel column to give increasingly polar fractions is shown in Figures 6 and 7. Table 3 summarizes information on the yields obtained in the benzene, chloroform and THF/ethanol fractions after separation of these asphatlenes2’j. The results indicate that asphaltene A has been obtained from a more severe process A that extracts much more of the original coal than process B. This aspect is also reflected in the fact that much less of the extract from process A compared with process B is found to be npentane soluble. The subsequent fractionations of asphaltenes A and B on a silica gel column indicate that the former has lost much more of its heteroatom content during processing and thus the benzene fraction constitutes a much greater percentage of the total asphaltene. The elution profiles for asphaltene C clearly indicate the presence of highly adsorbed material (retention volumes > 35 ml) which represents higher molar mass condensed aromatic material. Figure 6 again illustrates the increasingly polar nature of subsequent fractions of asphaltene A but also importantly indicates the presence of numerous small polar and aromatic molecules. Similarly for asphaltene B, the presence of many small polar molecules especially in the THF fraction is an indication of the imprecise nature of the fractionation. This observation is important since the increasing molar mass and heteroatom content observed in fractions with more polar solvents, might in part, be due to the presence of small polar molecules in these fractions. This is not surprising since as the severity of the extraction process increases, further degradation ofthe coal macromolecules occurs leading to a gradual decrease in heteroatom content and a corresponding increase in the aromatic content of the material extracted. The increased amount of high molar mass polar compounds that is excluded
polyaromatic
and THF/ethanol
A, B and C, including o/OExtract” (‘4 n-pentane)
7; d.a.f. coal
of tars and pitches:
Figure 7 Elution profiles of a pitch asphaltene C (THF/ethanol fraction) and asphaltene B (benzene and THF/ethanol fractions)
their fractionation
‘/, Benzene
into more polar solvents ~_
~--
% Chloroform
7; THF/ethanol ..~
25 12
A B C “Figure
in brackets
refers to portion
29 (18) 35 (35)
soluble
52.6 15.9 82.9
4.7 1.9 2.6
42.1 82.2 14.5
in n-pentane
FUEL, 1987, Vol 66, November
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Functional
group fractionation
and characterization
Table 4 An illustration of the s.e.c. selectivity in terms of molecular and chemical type available using DMF as the mobile phase Retention volume (ml)
Molar mass range (gmol-I)
10
>looo
15-20
<500
2C27
1500
27-35
<500
>35
size
Region
__
High molar mass polar material (size exclusion limit) Low molar mass polar material (mainly phenols) up to total permeation limit (20 ml) Low molar mass neutral species (nitrogen heterocycles, ethers, ketones and related species) Low molar mass aromatics, for example, anthracene High molar mass non-polar material adsorbed on column typically highly condensed aromatics and long chain aliphatics
<2000
Fraction
of tars and pitches:
A
Fraction
6
n
12
I
14
I
I
I
I
I
I
I
I
I
I
16
18
20
22
24
26
28
30
32
34
Retention
volume
M. J. Mulligan
et al.
Tars, pitches and condensates Figure 8 shows the chromatographic elution profile for a low temperature coal tar and two fractions derived from this tar. The fractions were produced using the s.e.c. column with THF as the mobile phase by the method described previously24. Fraction A consists of mainly phenols and long-chain paraffins while fraction B contains mainly PAC. The results show that the calibration curve derived from the model compounds applies very well to the complex tar mixture which contains mainly molecules with low molar mass. Figure 9 shows the elution profiles of a variety of coalderived pitches. The results show clearly that the chemical composition of the pitches varies considerably. Pitch CTPl contains a significant proportion of polar materials (mainly phenols) in addition to heterocydic and aromatic compounds. There is a large peak corresponding to small condensed aromatics but the trace tails off relatively quickly indicating only small amounts of large condensed PAH. In comparison, the elution profile for pitch CTP2 which has been heat treated to produce mesophase spheres and contains carbon black as well as primary QI, shows that it contains predominantly small PAH with relatively small amounts of polar and heterocyclic aromatic material. Pitch CTP3 has very little polar material but has significant amounts of small aromatic heterocyclics. The maximum at a retention volume of 32ml is on the high side of the PAH region and there is a large tail to the peak indicating substantial quantities of highly condensed higher molar mass material which is
i ml
Figure 8 Elution profiles for the low temperature coal tar and its fractions: a, tar before fractionation; b, after fractionation
from the column for asphaltene B is a reflection of the fact that these species have retained their heteroatom content while in the case of asphaltene A, the greater severity of the processing conditions has led to greater degradation of the coal macromolecule. The calibration of the s.e.c. chromatographic system with DMF as the mobile phase using model compounds and previously characterized coal-derived materials has enabled regions which depend on both size and chemical type to be defined. This is illustrated in Table 4 and provides the basis for characterizing the complex mixtures of organic molecules whose constituents vary widely. The ability of the technique to characterize complex mixtures of organic molecules is general and only limited by the solubility of individual components in DMF.
1478
FUEL, 1987,
Vol 66, November
I
I 10
I
I
20 Retention
30 volume
, ml
I
I 40
Figure 9 Elution profiles for various coal-derived
50
pitches
Functional
I
I
20
30 Retention
Figure 10
Elution
group fractionation and characterization
volume
profiles for various
I
I
40
50
of tars and pitches:
M. J. Mulligan et al.
are given in Table le. A comparison of Figure 11 and Table lc shows that the high molar mass condensed aromatics are more clearly identified by the s.e.c. technique. The results outlined above clearly show that the chromatographic technique can be used to monitor the chemical composition of a wide range of coal-derived materials and is sufficiently sensitive to detect compositional changes resulting from small changes in process variables. The use of the technique for characterizing coal-derived materials containing a wide molar mass range and chemical type has demonstrated its usefulness. The alternative approach would be to fractionate the materials and analyse them by gas chromatography and mass spectrometry (g.c.-ms.). This is a rather time consuming procedure and the high molar mass material is not readily detected by the technique. G.c.-m.s. gives detailed information on the concentrations of individual small molecules but is not really suitable for mixtures containing significant quantities of high molar mass material.
ml
petroleum-derived
pitches
strongly adsorbed on the column. Similarly, pitch CEP contains predominantly highly condensed PAH molecules with the maximum in the elution profile occurring outside the range of the calibration curve at > 40 ml. A comparison of Figure 9 with Table lb shows that the relationship between the elution profiles and the pitch characterization parameters is very complex. However, the coking value and QI content tend to correlate with the presence of highly condensed PAH. The relationship between the chemical composition of these pitches and their ability to modify the carbonization process and coke properties will be discussed in a future paper. Figure 10 shows the chromatographic elution profiles of four petroleum-derived pitches. The solubility of the pitches in DMF is the main limitation to the characterization with pitch PP3 being only partly soluble. The soluble portions of all four pitches investigated have a small amount of material (MM > 1000) eluted at the size exclusion limit (10ml) and virtually no low molar mass polar material. The relative amounts of neutral material (mainly nitrogen heterocycles, amines, ethers etc.) varied with PP2 and PP3 having the largest amounts. Otherwise the main difference between the pitches is the extent of the tail on the peak due to small aromatic molecules which is due to high molar mass aromatic material. This tail is largest for Ashland A240 pitch. The ability of the technique to monitor the effect of process variables on the chemical composition of a coalderived material is illustrated in Figure 11. This shows the s.e.c. traces of an anthracene heavy oil condensate which has been produced by a hydrogenation process at various temperatures in the range 76s900°C. Clearly the effect of increasing the temperature is mainly a decrease in the amounts of neutral heterocyclic and other compounds in the region 2&26 ml in the elution profiles. At 900°C the concentration of these compounds is very small and the condensate consists almost exclusively of PAH with some high molar mass condensed aromatics eluting above 35 ml. These samples have been analysed by gas chromatography and mass spectrometry and these data
CONCLUSIONS It is apparent that the use of a highly polar mobile phase (DMF) with a neutral macroporous gel, modifies the separation process considerably from one based mainly on molecular size, to include significant contributions from association, partition and adsorption effects. The calibration of the column with model compounds (mainly constituents of tars), and previously characterized fractions of coal-derived materials has shown that it is
900°C
I
I
I
I
I 30
I
10 Retention
Figure 11 Elution profiles temperatures in the range anthracene oil
I
,
I
I
I
50
volume
I ml
for condensates 760-900°C by
FUEL, 1987,
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produced at various the hydrogenation of
Vol 66, November
1479
Functional
group fractionation
and characterization
of tars and pitches:
possible to define regions of the chromatogram which depend on both size and chemical type. This twin selectivity provides the basis for a rapid and unique characterization of complex materials, such as tars, coal extracts, pitches etc., which contain numerous individual components varying considerably in molar mass and chemical type. The technique can detect small changes in the relative concentrations of groups of species and hence, monitor the effect of changes in process variables. The overall picture produced has increased our knowledge of the effectiveness of the fractionations that have been performed by various procedures. This investigation has shown that the characterization technique has general applicability to a wide range of coal- and petroleumderived materials; the main limitation being the solubility of the constituents in DMF particularly in some petroleum pitches.
ACKNOWLEDGEMENTS The authors would like to thank British Gas plc for permission to publish this paper. The samples of anthracene oil condensates were supplied by Dr G. H. Rhodes and the g.c.-m.s. for the samples by T. Haley. The samples of pitch were provided by Dr J. W. Patrick and Professor H. Marsh.
REFERENCES 1 2
3 4 5
1480
Pullen, J. R. in ‘Solvent Extraction of Coal’, Report ICTIS/TR16, November 1981, IEA Coal Research, London, UK Mima, M. J., Schultz, H. and McKinsky, W. E. ‘Analytical Methods for Coal and Coal Products’ (Ed. C. Karr), Academic Press, 1978, Ch. 19, pp. 557-568 Herod, A. A., Ladner, W. R. and Snape, C. E. Phil. Trans. R. Sot. A300,3 Rand, B. and McEnaney, B. Br. Ceram. J. 1985, 84, 157 Quinoline Insolubles in Coke Oven Tar, and the effect of
FUEL, 1987, Vol 66, November
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et al.
Carbonization Conditions, BCRA Special Publication No. 27, 1982 Bartle, K. D., Collin, G., Stadelhoffer, J. W. and Zander, M. J. Chem. Tech. Biotechnol. 1979, 29, 531 Altgelt, K. H. and Gouw, T. H. Adu. Chromatog. 1975, 13, 71 Meiris, R. B. Chem. Ind. (London) 1973, 642 Altgelt. K. H. Makromol. Chem. 1965. 88. 75 Coleman, W. M., Wooton, D. L., Do&H. C. and Taylor, L. T. Anal. Chem. 1977, 49, 533 Wooton, D. L., Coleman, W. M., Taylor, L. T. and Dorn, H. C. Fuel 1978, 57, 17 Mehesch, H., Hodek, W. and Kolling, G., Compendium 77/78 Supplement to Erdol Kohle Erdgas Petrochemie 1977, p. 283 Lewis, I. C. and Petro, B.A. J. Polym. Sci. Poly. Chem. Ed. 1976, 14, 1975 Mayo, F. R. and Kirschen, N. A. Fuel 1978, 57, 405 Hendrickson, J. G. and Moore, J. C. J. Polym. Sci. 1966,4,167 Edstrom, T. and Petro, B. A. J. Polym. Sci. 1968, C21, 171 Cazes, J. and Gaskill, D. R. Sep. Sci. 1969, 2, 421 Lambert, A. Analyt. Chim. Acta. 1976, Bl, 423 Popl, M., Fahnrich, J. and Stejskal, M. J. Chromatogr. Sci. 1976, 14, 537 Mackay, J. F. and Latham, D. R. Anal. Chem. 1975, 45, 1050 Bergman, J. G., Duffy, L. J. and Stevenson, R. B. Anal. Chem. 1971,43, 131 Philip, C. V. and Anthony, R. G. Am. Chem. Sot. Div. Fuel Chem. Prepr. 1979, 24(3), 204 Philip, C. V. and Anthony, R. G. Fuel 1982,61, 357 Evans, N., Haley, T. M., Mulligan, M. J. and Thomas, K. M. Fuel 1986,65, 694 Bartle, K. D., Mills, D. G., Mulligan, M. J., Amaechina, I. 0. and Taylor, N. Anal. Chem. 1986, 58, 2403 Tytko, A. P. PhD Thesis, University of Leeds, 1986 Bartle, K. D., Ladner, W. R., Martin, T. G., Snape, C. E. and Williams, D. F. Fuel 1979, 58, 413 Marsh, M. K., Smith, C. A., Stokes, B. J. and Snape, C. E. J. Chromatogr. 1984, 283, 173 Schwager, I. and Yen, T. F. Fuel 1979,58, 219 Burchill, P., Herod, A. A., Mahon, J. P. and Pritchard, E. J. Chromatogr. 1983, 281, 109 Rultz, J. M., Carden, B. M., Lena, L. J. and Vincent, E. J. Anal. Chem. 1982, 54, 688 Wiley, C. W., Iwao, M., Castle, R. N. and Lee, M. Anal. Chem. 1981, 53, 400 Burchill, P., Herod, A. A. and Pritchard, E. J. Chromatogr. 1982, 242, 51