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Articles Trace-Element Partitioning between Fractions of Coal Liquids during Column Chromatography and Solvent Separation A. A. Herod,* A. George, C. A. Islas,† I. Suelves,‡ and R. Kandiyoti Department of Chemical Engineering and Chemical Technology, South Kensington Campus, Imperial College London, London SW7 2AZ, United Kingdom Received November 5, 2002
A coal tar pitch, a coal liquefaction extract, and a low-temperature coal tar have been fractionated by molecular mass, using column chromatography, and the fractions have been analyzed for trace-element content. The solvents used for sequential extraction were acetonitrile, pyridine, and 1-methyl-2-pyrrolidinone (NMP). Trace elements were determined by inductively coupled plasma-mass spectroscopy (ICP-MS) after digestion of the liquids and fractions in a microwave bomb, using nitric acid and hydrogen peroxide. The mercury content was determined using a Leco model AMA254 analyzer. The larger portion of the trace elements analyzed have been found to associate preferentially with fractions that have been shown by size exclusion chromatography to contain the largest molecules. Some of the largest-molecular-mass material adhered to the silica that was used for fractionations. Trace-element mass balances for fractions separated by column chromatography were very poor, because of higher concentrations of trace elements in the largest organic molecules that were held onto the silica. One of the samples, the coal tar pitch, was fractionated by solvent solubility, without contact with filtration media. The method led to somewhat less-sharp molecular-mass separations; however, trace-element analyses of these fractions gave much-improved mass balances. Structural data from this work and previous characterizations suggest that, within larger molecules, increasingly large polycyclic aromatic (PCA) ring systems are being held together by a variety of aliphatic and alicyclic bridging structures. In the absence of mineral matter or other solids, it is thought that the high traceelement concentrations represented organic associations with these complex molecules.
Introduction Live plants tend to filter out transition- and alkalimetal elements present in host soil almost completely, despite the fact that more than one-fourth of the elements of the periodic table are required for life.1 The trace metal elements required by biological systems1 include vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, selenium, molybdenum, and tin, with sodium, magnesium, potassium, and calcium as bulk biological elements. The high trace-element contents of coals result from the admixture with soil components and the adventitious ingress of inorganic ions from the aquatic cover during deposition and maturation. The fate and environmental implications of trace elements released during coal combustion and * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Present address: Department of Chemistry, Lehigh University, Bethlehem, PA 18015-3172. ‡ Present address: Instituto de Carboquimica (CSIC), Miguel Luesma Castan 4, 50015, Zaragoza, Spain. (1) Kendrick, M. J.; May, M. T.; Plishka, M. J.; Robinson, K. D. Metals in Biological Systems; Ellis Horwood: Chichester, U.K., 1992.
gasification processes have been studied and reviewed in detail.2-6 However, not all trace elements in coals are associated with mineral matter. Evidence from Mo¨ssbauer spectroscopy suggests that iron found in a coal tar pitch and a coal liquefaction extract fraction were in direct organic association.7-9 The presence of paramagnetic centers within samples is likely10 to disrupt carbon sampling in 13C NMR and organically associated iron ssems to be one of the possible causes for incom(2) Swaine, D. J.; Trace Elements in Coal; Butterworths: Sevenoaks, Kent, U.K., 1990. (3) Davidson, R. M.; Clarke, L. B. Trace Elements in Coal; Report IEAPER/21; IEA Coal Research: London, 1996. (4) Smith, I. M. Trace Elements from Coal Combustion: Emissions; Report IEAR/01; IEA Coal Research: London, 1987. (5) Clarke, L. B.; Sloss, L. L. Trace Elements from Coal: Combustion and Gasification; Report IEACR/49; IEA Coal Research: London, 1992. (6) Sloss, L. L. Mercury Emissions and EffectssThe Role of Coal; Report IEAPER/19; IEA Coal Research: London, 1995. (7) Herod, A. J.; Gibb, T. C.; Herod, A. A.; Zhang, S. F.; Xu, B.; Kandiyoti, R. Fuel 1996, 75, 437. (8) Herod, A. J.; Gibb, T. C.; Herod, A. A.; Shearman, J.; Dubau, C.; Zhang, S.-F.; Kandiyoti, R. J. Planar Chromatogr.sMod. TLC 1996, 9, 361-367. (9) Richaud, R.; Lachas, H.; Lazaro, M.-J.; Clarke, L. J.; Jarvis, K. E.; Herod, A. A.; Gibb, T. C.; Kandiyoti, R. Fuel 2000, 79, 57-67.
10.1021/ef020267e CCC: $25.00 © 2003 American Chemical Society Published on Web 05/22/2003
Partitioning between Fractions of Coal Liquids
plete carbon sampling. Mercury has been found in organic association in a coal tar pitch with negligible mineral content.6,11 During the processing of coalderived liquids,12,13 the deposition of numerous heavy metals onto catalysts has been reported to degrade activity over extended periods of usage.14-16 Available data thus enable the conclusion, with reasonable confidence, that many trace elements associated with coals are or may be organically associated. The list of such trace elements is extensive: magnesium, aluminum, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, germanium, arsenic, strontium, yttrium, zirconium, cadmium, tin, antimony, barium, lanthanum, cerium, hafnium, tellurium, lead, bismuth, and thallium. Little is known, however, concerning the structures and sizes of molecules within which trace-element atoms seem to be embedded. The definition of trace element used here is that the metal element should be present at concentrations on the order of mg/kg, represented here as ppm. In general, when complex mixtures are characterized by almost any analytical technique, the more abundant material tends to mask the features of the less abundant fraction. Separation into molecular size fractions thus allows enhancement of the resolution of most analytical tools and serves to identify structural differences between fractions with increasing ranges of molecular masses. In the present study, several coal-derived liquids have been fractionated, in terms of ascending molecular mass (and partly by polarity), before analysis for trace-element content. In the first instance, the samples were separated by column chromatography. This method is capable of achieving levels of separation that are comparable to that of planar chromatography, while handling significant amounts of sample (∼1 g); this is sufficient for structural characterizations by 13C NMR, as well as trace-element analysis by inductively coupled plasmamass spectroscopy (ICP-MS).17,18 However, a small amount of sample cannot be removed from the silica during this procedure. As will be explained below, this was found to distort trace-element mass balances. The trends obtained using this method were then compared with those from sample fractionation that were obtained by straightforward solvent solubility. Although this method does not afford as sharp a molecular-mass separation as that of column or planar chromatography, it avoids the use of solid media onto which the sample (10) Wind, R. A.; Maciel, G. E.; Botto, R. E. Quantitation in 13C NMR Spectroscopy of Carbonaceous Solids; In Magnetic Resonance of Carbonaceous Solids; Botto, R. E., Sanada, Y., Eds.; Advances in Chemistry Series 229; American Chemical Society: Washington, DC, 1993; Chapter 1. (11) Richaud, R.; Lachas, H.; Collot, A.-G.; Mannerings, A. G.; Herod, A. A.; Kandiyoti, R. Fuel 1998, 77, 359-368. (12) Harrison, J. S.; Kimber, G. M.; Gray, M. D. Proceedings of the 1989 International Conference on Coal Science (Tokyo, Japan, October 23-27, 1989); IEA: Tokyo, 1989; pp 655-658. (13) Moore, S. A.; Jones, M. A.; Kimber, G. M. Proceedings of the 1989 International Conference on Coal Science (Tokyo, Japan, October 23-27, 1989); IEA: Tokyo, 1989; pp 663-665. (14) Cloke, M. Fuel 1986, 65, 417. (15) Cloke, M.; Hamilton, S.; Wright, J. P. Fuel 1987, 66, 1685 (16) Cloke, M.; Hamilton, S.; Wright, J. P. In 1987 International Conference on Coal Science; Moulijn, J. T., Nater, K. A., Chermin, H. A. G., Eds.; Elsevier Science Publishers: Amsterdam, 1987; p 235. (17) Islas, C. A.; Suelves, I.; Li, W.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Fuel, in press. (18) Islas, C. Ph.D. Thesis, University of London, U.K., 2001.
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might adhere. Results described below show that solvent separation did allow better trace-element mass balances to be achieved and served to confirm that trends obtained by column chromatography showing most of the trace elements concentrated in the largest-molecular-mass fraction to be valid. Experimental Section Samples. A coal tar pitch, a coal liquefaction extract, and a low-temperature tar (LTT) have been examined. In this order, the samples reflect diminishing intensities of thermal exposure. All three samples were subsamples of large-scale production processes that were considered to produce relatively homogeneous liquid tar materials under the processing conditions. No special procedures were needed to collect the samples. The coal tar pitch is the residue from the distillation of tars collected during pyrolysis in a high-temperature coke oven. The present sample is a “soft” pitch, which contains some light ends (from anthracene oil), such as phenanthrene. It is used as a laboratory standard, because of its homogeneity and relative abundance; its elemental composition is 91.4% carbon, 4.1% hydrogen, 1.3% nitrogen, and 1.8% sulfur. The liquefaction extract was a sample drawn downstream of the “digester” and hot-filtration stages of the Point of Ayr Coal Liquefaction Facility (British Coal). This sample was of particular interest as a coal extract solution in a coal-derived solvent, because it has suffered less thermal degradation than the aforementioned pitch sample. However, the largest-molecular-mass material in the extract is thought to have been removed during the filtration stage. The elemental composition of the sample was 85.9% carbon, 6.8% hydrogen, and 0.8% nitrogen. The LTT sample came from the Coalite process, which is used to produce a smokeless solid fuel; the tar was the feedstock for a distillation stage, followed by solvent extraction. The elemental composition of the tar sample was 82.3% carbon, 7.83% hydrogen, and 0.91% nitrogen. Of the three samples used in the study, this material was expected to show the least thermal degradation. Fractionation Methods. The initial method of fractionation was based on column chromatography using silica. Two separate batches of 10 and 5 g of SIGMA silica gel for column chromatography (15-40 µm diameter; average pore size of 60 Å) were weighed and dried under vacuum for 3 h at 200 °C. One gram of sample was slurried in acetonitrile (4% w/w), ultrasonicated for 30 min, mixed with the 10 g of silica, and ultrasonicated for 30 min and finally vacuum-dried in a rotary evaporator at 45 °C to total dryness. The 5-g portion of fresh silica was placed in a cylindrical tube (20 cm × 3 cm i.d.) with a sintered glass plate bottom. The sample-coated silica was then placed on top of the fresh silica. Acetonitrile, pyridine, and 1-methyl-2-pyrrolidinone (NMP) were successively passed through the column17 as follows: 75 mL of acetonitrile was eluted through the column by gravity, and a second aliquot of 75 mL acetonitrile was eluted under vacuum. Air was drawn through the column to remove residual solvent. Seventy-five milliliters of pyridine was then eluted under gravity; a second aliquot of 75 mL was eluted under vacuum. The column was dried of pyridine by passing air under vacuum. Finally, 150 mL of NMP was eluted under vacuum; NMP is too viscous for elution under gravity. The separated fractions were dried in the rotary evaporator at 90°C, and residual solvent was removed in a vacuum oven at 150 °C. The fractions were weighed and mass balances were calculated. The silica was found to retain residual sample and was observed to become a gray color after completion of the elution sequence. The trace-element mass balances calculated from the trace-element analysis of fractions from column chromatography were poor (see below). The pitch sample was then
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fractionated by solvent solubility, using acetone and pyridine. Approximately 10 g of pitch were crushed in an agate mortar and extracted using acetone at room temperature over a period of several days. The sample bottle was shaken frequently and ultrasonicated. The acetone solution was decanted from the pitch residue and dried. The residue was dried and extracted using pyridine, again over a period of several days, using similar agitation methods. The pyridine solution was decanted from the residue and dried. The insoluble residue and the solvent extracts were vacuum-dried to constant weight. Fraction weights were as follows: acetone solubles, 3.3421 g; pyridine solubles, 3.2042 g; pyridine insolubles, 3.0743 g; total recovered, 9.6206 g. The initial weight of pitch was 9.4087 g, and the recovery was 102%, indicating that some residual solvent remained. Size Exclusion Chromatography (SEC). Size exclusion chromatograpy (SEC) with NMP as the eluent has been shown to provide separation based on size rather than surface effects.18-23 The procedure has been described elsewhere.19 The polystyrene/polydivinylbenzene column (Mixed-D with 5-µmdiameter beads; Polymer Laboratories Ltd., U.K.) was operated at 80 °C, with NMP as the eluent. Detection was by UV absorbance (Perkin-Elmer model LC 290) at 450 nm, using an Applied Biosystems diode array detector that was set at 280, 300, 350, and 370 nm. The amount of injected sample solution was extract > LTT). These chromatograms have been presented in detail elsewhere.17,18,31,32 (30) Islas, C. A.; Suelves, I.; Li, W.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 2002, 47 (2), 643-644. (31) Islas, C. A.; Suelves, I.; Carter, J. F.; Herod, A. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 2000, 14, 1766-1782. (32) Islas, C. A.; Suelves, I.; Carter, J. F.; Li, W.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 2002, 16, 774-784.
Fractions mobile in NMP were found to be richer in material excluded from the porosity of the Mixed-D column used, corresponding to masses of >200 000 u, in terms of the elution times of polystyrene molecular mass standards. Structural Characterization of the Sample Fractions. Trace elements detected at concentrations above the limits of quantification in this study (Tables 2-4) have been recovered in solutions obtained by flowing a sequence of solvents through the silica bed. It seems reasonable to consider these elements, therefore, as being in organic association; in the case of metals, it seems that we must consider the presence of organometallic compounds. Findings outlined below show that most of the traceelement loading of the original samples is found in the heaviest fraction from each of the samples. The structural makeup of their higher mass fractions thus appears relevant to how the organic associations of these elements may be viewed. Figure 3a and b compare UV-fluorescence spectra of the entire coal tar pitch and its fractions that are separated by column chromatography. Evaluation of spectra from complex mixtures must consider the much smaller quantum yields of large-molecular-mass materials, particularly if they contain large polycyclic aromatic (PCA) ring systems. Clearly, in the presence of small molecules with intense fluorescence, the weak fluorescence of the large molecules cannot be observed with clarity. Once again, fractionation has been shown to assist in the acquisition of data that are more specific to larger-molecular-mass fractions. In Figure 3b, the spectra have been presented in peak-normalized mode, to compare peak positions. Within the limits of the discussions in refs 24 and 25, Figure 3b shows greater concentrations of large PCA ring systems in the NMPmobile fraction, compared to the other two spectra. Analogous differences in the UV-fluorescence spectra (not shown) of fractions separated from the Point of Ayr coal liquefaction extract and the LTT were somewhat less pronounced. The identification by 13C NMR of greater aliphatic contents in the largest-molecular-mass fractions was one of the unexpected findings of the characterization work on separated fractions of the coal tar pitch,20 Point of Ayr coal extract,18 and the LTT.32 At first glance, this appeared to contradict the UV-fluorescence spectra, suggesting the presence of larger PCA ring systems in the heavier fractions. Pyrolysis gas chromatographymass spectroscopy (GC-MS) systematically gave rather sparse spectra, showing only a few aliphatic fragments in the two heaviest fractions of the coal tar pitch, which are likely to have evolved from the breakup of aliphatic/ alicyclic bridging structures between PCA ring systems. The coal liquefaction extract and the LTT gave morecomplex aliphatic structures, compared to the thermally more intensely treated pitch. However, no aromatic material could be identified in the mass spectra of any of the heavy fractions. It would appear that the PCA
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Figure 2. Size exclusion chromatograms of the three fractions separated from (a) the coal tar pitch, (b) the Point of Ayr coal digest, and (c) the LTT, by column chromatography. Notation for spectra is as follows: (1) acetonitrile-mobile, (2) pyridinemobile, and (3) NMP-mobile fractions. Detection by UV absorption at 350 nm.
Figure 3. (a) Synchronous UV-fluorescence spectra of (a) the coal tar pitch and (b) coal tar pitch fractions separated by column chromatography. In Figure 3b, spectrum 1 denotes the fraction mobile in acetonitrile, spectrum 2 represents the fraction mobile in pyridine, and spectrum 3 is the fraction mobile in NMP.
groups were either embedded in structures that charred readily or were too large to pass through the chromatographic column. It appears therefore that, within larger molecules of coal-derived samples, increasingly large PCA ring systems are being held together by a variety of aliphatic and alicyclic bridging structures. We know very little regarding either the size and conformation of the PCAs or the aliphatic/alicyclic materials that, we presume, bind them together.
Trace-Element Analyses. Tables 2-4 present concentrations of the set of trace elements measured in the three samples (in units of ppm or mg/kg of sample) and the atomic masses used to estimate the concentration of each element. No value has been shown for cases where the concentration in solution was below the limit of detection of the instrument, under the conditions on the day of analysis. Mass balances for each element have been calculated using the fraction weights in Table 1. In some cases, particular elements not detected in
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Table 2. ICP-MS Results of Pitch, and Its Column Chromatography Fractions element Na Mg Al Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Sr Y Zr Cd Sn Sb Ba La Ce Hf Tl Pb Bi Th sum [ppm]
atomic mass
amount of pitch [ppm]
acetonitrile mobiles [ppm]
pyridine mobiles [ppm]
NMP mobiles [ppm]
suma [ppm]
mass balance [%]
23 24 27 45 49 51 52 55 57 59 60 65 66 71 73 75 86 89 91 111 118 121 137 139 140 178 205 208 209 232
427 4.6
19.5
15.6
29 0.22 3.9
46 0.26
0.53
651 123 17 0.30 2.7 0.78 2.9 1.8 81.5 0.86 5.1 17.2 707 2.0 8.6 8.5 5.2 0.02 0.2 7.3
95.9 15.7 31.6 .22 1.3 0.17 1.21 0.65 61.3 0.18 1.71 3.24 108 0.46 1.1 2.3 0.87 0.003 0.22 1.18
22.4 340 ?b 88 ?b 81 110 8 20.6 60 143 140 56 8 16 13.6 212 10 ?b 47
0.73
0.5 1.7
0.06 0.57
10 ?b
0.1 0.07 0.06 8.9 0.2 0.03
0.04 0.008 0.03 3.9 0.06 0.003
?b 40 0.2 2 1.2 7
0.25 0.21 1.1 8.3 297 0.3 1.2 2.3 194 5.7 6.9 16.9 0.41 0.03 2.5 2.0 0.61
0.02 11 203 4.7 0.04 1190
0.14 1.1 0.49
1.2 0.74 195 0.01 0.57 1.4 8.2
0.14 1.9 1.4 33 0.40
0.37
2.3 0.35
0.01 0.75 0.13
0.10 0.03 0.06 5.9 0.08 0.01 261
a
110
1654
b
Sum of fractions for comparison with value for the entire sample. The symbol ? indicates a sum of fractions greater than that in the sample.
the original sample, notably aluminum and titanium, were detected at low concentrations in one or more of the fractions. In regard to the element being found in the original sample at measurable concentration levels and not being detected in any fractions (see below), it was concluded that sample material carrying the element had not been removed from the silica by the sequence of solvent washing. It also seems probable that the excessive quantities of sodium found in the fractions came from surface contamination or from contamination during the processing of solutions, although every attempt was made to avoid contamination. We have noted elsewhere9 that liquefaction extracts from a rank series of coals in tetralin gave trace-element distributions that were not related to the oxygen contents of the original coals. This observation tends to suggest that the carboxyl group concentration, which is expected to be the greatest in low-rank coals and possibly the site of attachment for trace-element cations, was not of particular importance in trace-element associations of these extracts. In the present three samples, the oxygen content (by difference) decreased as the intensity of the thermal treatment increased and could be ranked as follows: LTT > Point of Ayr extract > pitch. It might also be expected that the concentration of carboxyl groups in the samples would decrease as the thermal treatment increased. Thus, if oxygen contents correlated with the trace-element content, their concentrations would be expected to be greatest in the LTT. This was not found to be the case, as will be discussed below. Coal Tar Pitch. The results in Table 2 show an increase of trace-element concentrations, increasing as
the molecular masses (size) increase, as indicated by SEC. Total trace-element contents were 261, 110, and 1654 ppm in the acetonitrile, pyridine, and NMP fractions, respectively, compared with 1190 ppm in the initial sample. Iron was an exception to this trend, showing a greater abundance in the most mobile fraction. However, only 20% of the iron in the original sample could be detected, suggesting that much of the iron-containing compounds remained on the silica column. In previous work, Mo¨ssbauer spectra showed signals that were quite distinct from pyritic or inorganic iron; the detected signal was ascribed to iron that was possibly present in porphyrin or proteinlike structures.7-9 The most-abundant trace elements in the original pitch sample were sodium, iron, zinc, and lead. Of these, sodium may have come from surface contamination of the pitch lumps, because the proportion recovered in the fractions was poor, with only 22% recovered. The presence of lead is thought to have been due to contamination at the tar distillation plant; with a mass balance of only 2%, a retention on silica must be presumed. Other elements detected were magnesium, manganese, chromium, nickel, copper, gallium, germanium, arsenic, cadmium, tin, thallium, and bismuth. Aluminum and titanium were not detected in the entire sample but did appear in the fractions at low levels. This is likely to have been caused by the concentration of these elements in the fractions (or by contamination during the fractionation). The results for the acetonitrile- and pyridine-mobile fractions showed a decrease in the total amount of trace elements, in comparison with the total sample. Iron and zinc were the most-abundant elements in the acetoni-
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Table 3. ICP-MS of Point of Ayr (PoA) Liquefaction Extract, and Its Column Chromatography Fractions element Na Mg Al Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Sr Y Zr Cd Sn Sb Ba La Ce Hf Tl Pb Bi Th sum [ppm] a
atomic mass
amount of PoA liquefaction extract [ppm]
acetonitrile mobiles [ppm]
pyridine mobiles [ppm]
23 24 27 45 49 51 52 55 57 59 60 65 66 71 73 75 86 89 91 111 118 121 137 139 140 178 205 208 209 232
12.3 766 373 0.70 23.8 2.6 5.6 83.1 7170 1.4 6 12.5 32.1 0.44
13.5
36.3 62.2
3.1 21 1.1 1.7 0.15 12.5 1.2 2.3 0.06 0.05 6.7 0.33 8540
0.37 1.7 0.02 0.87 1.4 89.7
0.04
0.11 0.35 0.01 1.0 0.01 109
NMP mobiles [ppm] 720 1699
suma [ppm] 42.8 78.1
mass balance [%] 350 10
0.48 10.9 1.9 2.9 2.0
1.6 93 18.1 11.9 21.6
0.39 6.6 1.23 2.1 1.36
0.46 6.4 3.0 53 0.42
2.5 48.9 64 883 1
0.24 4.13 3.82 90.2 0.16
0.41 0.97 0.11 0.62 0.57
1.2 13.3 1.2 1.4 7.5
0.16 0.77 0.07 0.24 0.45
5 4 6.4 14 300
1.8 0.15 0.22 0.1 0.01 0.33 0.06 0.08
2.0 1.4 2.9 0.03 0.05 14.3
0.62 0.15 0.34 0.03 0.007 1.09 0.02 0.054
5 12 15 50 14 16
185
0.7
56 28 47 38 1.6 17 69 30 280 36
16
3611
Sum of fractions for comparison with value for the entire sample.
trile-mobile fraction, and arsenic and lead were the most abundant in the pyridine-mobile fraction. No iron was found in this fraction at a concentration greater than the limit of detection. The NMP-mobile fraction showed a different behavior, with respect to the other fractions with a much higher concentration of trace elements. The most-abundant elements in this sample were sodium, magnesium, iron, and zinc. Comparison of the previous determinations of trace elements in the sample of coal tar pitch9 indicate that the values shown in Table 2 are similar to the previous measurements, particularly for vanadium, cobalt, zinc, gallium, cadmium, tin, antimony, and lead. Differences were found for chromium (factor 4), manganese (factor 4), arsenic (factor 2), and tellurium (factor 2). For chromium, manganese, and arsenic, these differences may have been caused by variable interference from polyatomic ions from the argon plasma of the ICP-MS. For iron, the previous measurement showed the concentration to be less than the limit of quantitation (LOQ), 254 ppm (because of polyatomic interferences in the plasma), which is similar to the present value, whereas for nickel, the previous value was 200 000 u). As with the other fractions, contamination seems likely for sodium (245% of total), magnesium (1030%), nickel (159%), copper (290%), zinc (116%), and barium (213%).
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Table 5. Results for the Mercury Determination of Tars, and Their Column Chromatography Fractions fraction
sample weight [mg]
amount of Hg in cell [ng]
entiresample acetonitrile sols pyridine sols NMP sols
33.10, 35.60 56.55 63.50, 64.30 50.90, 102.60
0.33, 0.35 1.82 3.18, 3.13 0.92, 2.39
Hg content [ppm]
mass balance [ppm]a
% of totala
0.0101, 0.0098 0.0323 0.0501, 0.0487 0.0181, 0.0233
[0.0346] 0.0084 0.0240, 0.0233 0.0023, 0.0030
[346] 84 236 26
Pitch
Digest entire sample
?b
?b
0.00098 LTT
entire sample acetonitrile sols pyridine sols NMP sols
217.6, 215.2 186.5, 208.0 105.1, 104.8 33.3, 41.2
91.2, 86.5 8.6, 6.5 13.7, 13.8 28.0, 72.7
0.419, 0.402 0.046, 0.031 0.130, 0.132 0.841, 1.766
[0.0777] 0.0384, 0.0259 0.0065, 0.0066 0.0252, 0.0530
[19] 7.8 1.6 9.5
a Values given in square brackets in table indicate the sum of the fractions. b The symbol ? denotes that values for fractions were not determined, because of a very low mercury concentration in the sample.
Figure 4. Size exclusion chromatograms of the three fractions separated from the coal tar pitch by solvent separation: (1) acetonitrile-soluble, (2) pyridine-soluble, and (3) NMP-soluble fractions. Detection by UV absorption at 350 nm.
Several elements, which were below the limit of quantification in the entire sample, were detected in this fraction: aluminum, titanium, vanadium, cobalt, gallium, strontium, yttrium, zirconium, and cadmium. The mass balances calculated for the elements and shown in Table 4 indicate reasonable balances for only a few elements: scandium, chromium, germanium, arsenic, antimony, and thallium. Many of the mass balances indicated contamination, as discussed previously. Mass-balance shortfalls may indicate that the molecules containing the element were not removed from the silica by the most polar solvent, NMP; iron, lanthanum, cerium, hafnium, tellurium, lead, and bismuth are among these elements. Mercury Determinations. Sample weights that were used in the Leco mercury analyzer, and the concentrations of mercury that were measured, are shown in Table 5. The quantities used, up to 200 mg, corresponded to the total quantity of the less-abundant fractions of some samples, particularly for the LTT; for the NMP fraction, it was necessary to use the entire fraction from one fractionation to obtain a reliable result. Duplicate determinations, which are shown in Table 5 by two values, came from duplicate fraction-
ations. For the pitch, the mass balance indicates more mercury in the fractions than in the original sample, which is possibly an indication of contamination during the fractionation. Previous work11 indicated a mercury content of 0.148 ppm for the pitch, more than 10 times the value found in this work and some 4 times greater than the sum of mercury in the fractions. In the present work, the mercury concentration in the fractions did not increase as the solubility decreased, with the lowest concentration being found in the NMP-soluble fraction. Very little mercury was detected in the entire Point of Ayr liquefaction extract sample (0.00098 ppm), which is less than the concentration in the pitch by a factor 10. Accordingly, the fractions were not examined, because the likely distribution pattern into the fractions would have proved impossible to estimate, except in the presence of very large fraction weights. In contrast, a much higher mercury concentration (0.41 ppm) was found in the LTT. However, the mass balance, which is expressed as the sum of mercury concentrations in the fractions, fell far short of the level in the entire sample, with only 19% being accounted for. The tar fractions showed significant differences in the concentration of mercury, with mercury levels increas-
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Table 6. Trace-Element Concentrations in the Pitch, and the Solvent Solubility Fractionsa element and isotope 24Mg 27Al 44Ca 45Sc 47Ti 51V 52Cr 55Mn 56Fe 59Co 60Ni 63Cu 70Zn 75As 77Se 85Rb 88Sr 89Y 95Mo 111Cd 118Sn 121Sb 133Cs 137Ba 139La 140Ce 182W 204Pb 205Tl 232Th 238U
sum
LODb
LOQc
pitch
acetone
pyrid sol
pyrid insol
normalized sum of fractions
3.9 9.1 39 0.76 1.1 0.02 0.47 0.12 14.7 0.017 0.61 1.45 0.18 0.03 0.12 0.04 0.12 0.006 0.019 0.058 0.20 0.013 0.002 0.27 0.024 0.019 0.0025 0.68 0.003 0.002 0.0008
13.0 30.3 132 2.5 3.6 0.06 1.57 0.42 49 0.058 2.03 4.85 0.59 0.09 0.41 0.14 0.39 0.019 0.063 0.19 0.65 0.044 0.006 0.91 0.081 0.064 0.0085 2.27 0.01 0.006 0.0025
7.4 18.0 59 0.4 0 0.2 1.0 7.37 254 0.24 1.26 1.2 8.64 16.6 2.57 0.31 0.69 0.03 0.15 2.23 1.90 0.30 0.039 0.85 0.45 0.05 0.033 197 8.69 0.004 0.010
0 0 78 1.1 0 0.05 3.6 0.35 50 0.06 0 0 1.83 4.0 0.97 0.24 0.25 0 0.62 0.34 0 0.08 0.04 0.51 0.06 0 0.12 5.4 0.16 0 0.004
11.4 12.4 209 1.1 0 0.43 1.6 2.72 181 0.19 0.82 0 3.49 10.0 1.31 0.21 0.98 0.004 1.82 1.72 0 0.09 0.04 1.68 0.09 0.01 0.73 2.8 0.05 0.002 0.003
37.3 198 133 0.8 7.8 0.63 1.5 19.7 633 0.75 3.18 0.97 20.9 40.8 4.70 0.51 2.03 0.13 0.69 7.23 6.95 1.23 0.15 5.52 0.29 0.25 0.13 547 20 0.025 0.018
16.1 69 143 1.1 2.5 0.37 2.3 7.5 286 0.33 1.32 0.32 8.68 18.2 2.34 0.32 1.08 0.04 1.07 3.07 2.27 0.46 0.078 2.56 0.15 0.085 0.33 182 6.63 0.009 0.008
596
148
446
1695
758
a
Trace elements in pitch and fractions (acetone and pyridine sols/insols); values given in in ppm. An entry of 0 indicates a value less than the LOD. b Limit of detection. c Limit of quantitation.
ing as solubility decreased. The fraction mobile in NMP showed the highest mercury concentration; however, the amount was still very small. Solvent Separation of Pitch. Clear trends could be established by analyzing the fractions separated by column chromatography. Column chromatography allowed relatively sharp separations of larger amounts of sample. However, sample loss on silica, which leads to poor trace-element mass balances, raised inevitable questions. Figure 4 shows that separation by solvent solubility (acetone/pyridine) cannot provide the level of concentration for high-mass material in the pyridineinsoluble fraction seen for column chromatography in Figure 2a. However, no silica is used and the method can be tailored to avoid the use of filter papers and other media that are likely to adsorb individual sample components selectively. Table 6 presents trace-element analyses of the original pitch and its three fractions: (i) acetone solubles, (ii) acetone insolubles/pyridine solubles, and (iii) pyridine insolubles. Also shown are the trace-element concentrations for the “reconstituted pitch”, calculated from the analyses of the fractions using the fraction weights in Table 1. As expected, for most of the elements, where the limits of detection and quantification were lower than the measured values, the mass balances were much improved, compared to those observed for the column chromatography fractions. The normalized sum of the total concentrations of trace elements was within (20% (758 ppm vs 596 ppm for the entire pitch). These data support the proposition that organic components of the pitch associated with the
bulk of the trace elements could well have been lost on the silica of the chromatographic column. Earlier work8 on the Mo¨ssbauer spectroscopy of fractions separated by planar chromatography had detected organic iron in the coal liquefaction extract fraction that was immobile in pyridine, in situ, without extraction of the organic material from the silica. However, the signal was weak and the organic sample was extracted from the silica into NMP, to provide a more concentrated sample. General Discussion. These three samples were fractionated by extraction from silica in an organic solution, and the metals were assumed to be held in organometallic materials or, at least, in organic association. The highest concentrations of trace elements were found in the fractions containing the largest-molecularmass material (soluble in pyridine or NMP). However, the contaminating elements sodium, magnesium, nickel, copper, and zinc were less likely to be present as organometallics and more likely to be present in association with carboxylic acids. Their tendency to preferentially contaminate the less intensely treated LTT would coincide with a greater concentration of oxygen.17,18 However, the trace elements in the pitch and liquefaction extract are both in samples where the oxygen content has been reduced by either high-temperature pyrolysis (pitch) or by some hydrogenation through interaction of coal with the hydrogenated recycle oil to produce the liquefaction extract;12,13 therefore, the influence of carboxylic acid groups is likely to be absent. Thus, organometallic interactions between aromatic groups and metals are more likely to be responsible for their attachment. The trace elements
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found in these samples are not simply those expected from the occurrence of trace elements in biological systems1 but are more diverse. We have noted28 that the trace elements found in extracts of coal in NMP were more diverse than those extracted from wood, which more closely resembled those of biological origin. The results for mercury indicate that contamination might be the most important source of this element in the fractions. The thermal treatment of the materials would indicate that the mercury in the coal from which the coal tar pitch was prepared should have been completely vaporized in the coke oven; however, in the pipework where the volatile material condensed, large hydrocarbon molecules may have recaptured the element. The atmosphere of the coke oven is almost oxygen-free, and, therefore, interactions with hydrocarbons become more important than mercury oxide chemistry. For the liquefaction extract, the mercury content of the coal has been estimated11 as 0.097 ppm; however, the extract itself appeared to contain such a low concentration that the fractions were not examined. The LTT similarly was derived from a coal of unknown mercury content; however, the lower temperature of thermal treatment may lead to a more-complete trapping of mercury that has volatilized from the coal by the cooler volatile hydrocarbons than with the other two samples. The mass balance for this sample was poor, suggesting that some mercury was not recovered from the silica. Results from two trace-element analyses of the pitch that were conducted on different dates can be compared from data in Tables 2 and 6, with some additional data from elsewhere.9 Tables 2 and 6 do not contain analyses of identical suites of elements, but where the same elements have been analyzed, the values agree within a factor 2 for magnesium, scandium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, arsenic, strontium, yttrium, cadmium, tin, tellurium, lead, and thallium. Results were not similar for zinc and may indicate that this element occurred, in part, as an environmental contaminant. Values for antimony differed by slightly more than a factor 2. The results from our first work9 on pitch and organic trace elements showed that values for gallium, germanium, tellurium, and lead agreed within a factor two with those of Table 2. Results for zinc differed by a factor of 4 from that of Table 2, whereas the result for arsenic was lower than those presented in Tables 2 and 6 by slightly more than a factor of 2. These results were obtained by measuring different isotopes in some cases, and their agreement gives some confidence in the trends identified and the basic methodology employed. Trace elements are found in humic acids from soils.33,34 They have been examined by a combination of aqueous SEC with ICP-MS.33 The main concentrations of trace elements were found in association with the early eluting, large-molecular-size peak, although some trace (33) Bhandari, S. A.; Amarasiriwardena, D.; Xing, B. Application of High Performance Size Exclusion Chromatography (HPSEC) with Detection by Inductively Coupled Plasma-Mass Spectrometry (ICPMS) for the Study of Metal Complexation Properties of Soil Derived Humic Acid Molecular Fractions. In Understanding Humic Substances: Advanced Methods, Properties and Applications; Ghabbour, E. A., Davies, G., Eds.; Special Publication 247; Royal Society of Chemistry: Cambridge, U.K., 1999; p 203.
Herod et al.
elements eluted later, with smaller molecules. Metals associated with the early eluting peak included aluminum, copper, zinc, cadmium, lead, manganese, and arsenic, with evidence for iron. These are some of the elements that were found to associate preferentially with the large-molecular-size components of the NMPsoluble fractions, rather than with the smaller-size fractions. Clearly, it would not be reasonable to consider the molecular structures of the present samples to be similar to those of humic acids, particularly in terms of oxygen content. However, the similarity in behavior was noted, in terms of associations of trace elements with the very largest molecules in both coal-derived materials and humic acids. The relevance of these trace-element data lies both in the probable uses of coal-derived liquids, for the production of liquid fuels by catalytic processes, and in terms of the problems likely to arise during analyses. In the first case, the trace elements are liable to poison catalysts but could be reduced in concentration if the coal liquids were fractionated, to remove the largest molecular fraction. In the second case, analytical methods such as solid-state NMR would suffer interference from the presence of paramagnetic elements, giving erroneous results. The modes of attachment of trace elements to the organic molecules and their structures have not been determined, but they are not considered to be through carboxylic acid salts. Conclusions A coal tar pitch, a coal liquefaction extract, and a lowtemperature coal tar have been fractionated by molecular mass, using column chromatography, and the fractions have been analyzed for trace-element content. The larger portion of the trace elements for which the analyses were performed have been found to preferentially associate with fractions that have been shown, by size exclusion chromatography, to contain the largest molecules. The abundance of trace elements was the greatest in the coal liquefaction extract and the least in the lowtemperature tar; the oxygen contents of the three samples do not correlate with the levels of trace elements in the original samples, which is possibly an indication that the method of binding of the metals is not through carboxylic acid sites. The more-effective binding of the trace elements with larger molecules has parallels with the behavior of humic acids and humic substances, which are used to clean trace metals from water supplies. Some of the largest-molecular-mass material seems to adhere to the silica that is used for the fractionations. However, trace-element mass balances for fractions separated by column chromatography were very poor, much worse than mass closures based on the masses of the fractions themselves. This finding may be explained in terms of the higher concentration of trace elements in the largest organic molecules that are held onto the silica. (34) Ruiz-Haas, P.; Amarasiriwardena, D.; Xing, B. Determination of Trace Metals Bound to Soil Humic Acid Species by Size Exclusion Chromatography and Inductively Coupled Plasma-Mass Spectrometry. In Humic Substances: Structures, Properties and Uses. Davies, G., Ghabbour, E. A., Eds.; Special Publication 228; Royal Society of Chemistry: Cambridge, U.K., 1998; p 147.
Partitioning between Fractions of Coal Liquids
Experiments were undertaken to test the proposition that improved mass closures may be obtained in the absence of the silica. One of the samples, the coal tar pitch, was fractionated by solvent solubility, without contact with filtration media. The method led to somewhat less-sharp molecular mass separations; however, trace-element analyses of these fractions lead to much improved mass balances. Structural data from this work and previous characterizations suggest that, within larger molecules, increasingly large polycyclic aromatic ring systems are being held together by a variety of aliphatic and alicyclic bridging structures. In the absence of mineral matter
Energy & Fuels, Vol. 17, No. 4, 2003 873
or other solids, it is thought that the high trace-element concentrations represented organic associations with these complex molecules. Acknowledgment. The authors would like to thank the British Coal Utilization Research Association (BCURA) and the UK Department of Trade and Industry for supporting this work, under Research Projects B44 and B53. The authors would also like to thank the European Union for a Marie Curie fellowship to I.S. and the Mexican Government for a fellowship to C.I. EF020267E
