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Anal. Chem. 1981, 53, 2332-2336
ACKNOWLEDGMENT We thank Robert Hazlett (NRL) and Donald Potter (USAF) for providing samples and their continuing interest in our work. We also compliment glassblower Andy Mollick of the VPI&SU Department of Chemistry. J.F.H. wishes to thank Bill Overstreet, Sedki Riad, and William Davis of the VPI&SU Department of Electrical Engineering for instruction in radio electronics. The assistance of Denny Sledd of VPI&SU Laboratory Support Services in making test equipment available is greatly appreciated. LITERATURE CITED (1) Brown, R. S.; Hausler, D. W.; Taylor, L. T.; Carter, R. 0. Anal. Chem. 1981, 53, 197-201. (2) Hausler, D. W.; Taylor, L. T. Anal. Chem. 1981, 53, 1227-1231.
(3) Haw, James F.; Glass T. E.; Hausler, D. W.; Motell, Edwln; Dorn, H. C. Anal. Chem. 1980, 52, 1135-1140. (4) Bayer, Ernst; Albert, Klaus; Nieder, Michael; Grom, Edgar; Keller, Toni J . Chromafogr. 1979, 186, 497-507. (5) Hirschfleld, Thomas Anal. Chem. 1980, 52, 197 A-312 A. (6) Hoult, D. I.; Richards, R. E. J . Magn. Reson. 1978, 24, 71-85. (7) Brown, J. K.; Ladner, W. R. FuelI980, 39, 87-96. (8) Clutter, D. R.; Petrakis, Leonidas; Stenger, R. L., Jr.; Jensen, R. K. Anal. Chem. 1972, 44, 1395-1405. (9) Solash, Jeffrey; Hazlett, Robert N.; Hall, J a m s M.; Nowack, Clarence J. Fuel 1978, 57, 521-528.
RECEIVEDfor review June 17,1981. Accepted September 21, 1981. We gratefully acknowledge financial support for this work provided by the Naval Research Laboratory (Washington, DC), the U.S. Air Force (Wright-Patterson Air Force Base, Dayton, OH), and the U.S. Department of Energy.
Analysis of Coal Conversion Recycle Solvents by Liquid Chromatography with Nuclear Magnetic Resonance Detection James F. Haw, T. E. Glass, and H. C. Dorn" Department of Chemistty, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 2406 7
The analyses of two coal conversion recycle process solvents, one of them hydrotreated, via high-performance liquld chromatography (HPLC) with a continuous flow nuclear magnetlc resonance (NMR) detector are reported. The differences between these samples with reference to the role of the solvent in coal liquefaction are dlscussed. The LC-lH NMR technique characterized the hydrocarbons and aromatic ethers wlth a level of certainty not posslble wlth conventional detectors. In addition, qualitative GC-MS analysis of off-line LC fractions was performed on the recycle solvent sample. LC-lH NMR and LC-GC-MS were found to be complementary for volatile samples. Gel permeation chromatography-lH nuclear magnetic resonance (GPC-'H NMR) analysls of the recycle solvent sample was not as successful as LC-" NMR slnce class separation is almost essentlal for this sample. But the Introduction of GPC-lH NMR Is still of Interest slnce GPC Is the most approprlate mode of Separation for many samples.
In many of the different processes for coal liquefaction ( I ) , coal is mixed with a recycle solvent and heated to 700-900 O F . Depending on the particular process, the heating is done in the presence of high-pressure hydrogen gas or the recycle solvent has been hydrotreated in a separate step. The role of the solvent is crucial. In addition to dissolving the liquefaction products, it acts as a shuttle of molecular hydrogen to the coal. Tetralins and other reduced polyaromatic species are important shuttles and also have important solvent properties. Aromatic ethers, phenols, and aromatic nitrogen-containing compounds are also present. These compounds generally improve the solubility of coal conversion products. Recycle solvent analysis is important since small changes in the solvent can cause major changes in pilot plant performance. Gas chromatography does not give class separation and poorly resolved peaks may be mixtures of widely different chemical classes. In the work of Whitehurst et al. (2), gas chromatography on packed columns gave poor separations of recycle solvents. Biphenyl and diphenyl ether coeluted and 0003-2700/81/0353-2332$01.25/0
methyltetralin overlapped with naphthalene. In further work, class separation via normal-phase liquid chromatography was used to collect fractions for GC analysis. Tetralin derivatives were thus differentiated from naphthalene derivatives. On-line coupling of different chromatographic modes (e.g., LC-LC and LC-GC) is attaining increasing sophistication in the analysis of complex samples. The analysis of solventrefined coal extract via on-line LC-GC has been performed (3). These techniques are aptly named multidimensional chromatography (4) since two parameters are used for separation (e.g., molecular size and volatility). Alternatively, a single chromatographic stage followed by an information-rich detector may be used. GC-MS, long known to the petroleum industry, is powerful provided the sample is volatile and thermally stable. LCJH NMR (5) is another technique for complex mixture analysis. In this technique, resolution is created along two fundamentally different axes, retention volume and chemical shift. The term multidimensional analysis seems to appropriately describe this and other techniques in which a powerful spectroscopic technique is used on-line with a chromatographic stage. In a companion paper (6) we have described the extension of the LCJH NMR technique to a modern 200-MHz NMR system and improved chromatographic separations. In this paper, we present on-line LC-lH NMR analysis of coal-recycle solvents. We also complement this technique with a GC-MS analysis of off-line LC-fractions. EXPERIMENTAL SECTION The recycle solvent sample (92-03-035) had its origin at the SRC-I plant in Wilsonville, AL. This sample is reported to have a hydrogen content of 8.15%. The other sample was prepared by Conoco via hydrotreatment of a sample similar to 92-03-035 over a cobalt-molybdenum catalyst to yield a hydrogen content of 9.67%. This sample is designated 92-26-019. Both samples were received from the Mobil Research and Development Corp., Central Research Division at Princeton, NJ. Both samples were obtained as vacuum distillation cuts. The simulated boiling range of both samples was labeled as 400-800 O F . Samples were stored sealed under dry nitrogen. Both samples were subjected to LC-'H NMR analysis without pretreatment. 0 1981 American Chemical Soclety
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injections were made. The connection between the outlet of the refractive index detector and the bottom of the NMR flow probe was made with a length of stainless steel capillary tubing. A Jeol FX-200 nuclear magnetic resonance spectrometer equipped with an Oxford 4.7-T superconducting solenoid magnet (54 mm bore) was used to obtain 'H spectra at 199.50 MHz. A floppy disk system was used for data storage and each diskette had sufficient data storage for 58 (1024 point) LC-NMR files. Acquisition of additional files could be done by exchanging diskettes without interrupting operation. A transfer time of approximately 3 s created a short dead time between each file. The process solvent sample was further characterized by GCMS analysis of fractions collected from a liquid chromatographic separation. A 10-pL injection was made onto a Whatman Magnum-9 silica gel column and 10 fractions were collected. The solvent was Freon 113. Aromatic hydrocarbons are retained less strongly on this column than on the PAC column. A Varian Aerograph Series 1400 gas chromatograph interfaced to a Varian MAT 112 mass spectrometer via a jet separator was used for the analysis of the latter eluting fractions. The gas chromatography column (10% SP-2100, 6 ft X 2 mm i.d. glass, Supelco) was temperature programmed from 200 to 300 O C at 10 "C/min.
0
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Flgure 1. A 200-MHz 'H spectrum of recycle solvent (92-03-035, 400-800 O F ) , obtained under conventional (spinning)conditions. A Whatman Magnum-9 silica gel-PAC column (250 mm X 9 mm id.) was used for normal-phase separations. The packing in this column was silica gel derivatized to introduce amino and cyano furictionalities to the surface. The PAC column was activated with chloroform-dl (99.8% D, Aldrich) and then equilibrated with 95% 1,1,2-trbhlorotrifluoroethane(hereafter Freon 113, Miller-Stephanson Chemical Co.) and 5% chloroform-dl (99.8% D, Aldrich). Hexamethyldisiloxane (HMDS) was added to the solvent to make a Concentration of 0.01% by volume. The solvent w a ~ intentionallynot degassed. A guard column was used for all normal-phase work. Gel permeation Chromatography was carried out on a Waters Microstyragel 100 A column (30 mm X 7.8 mm i.d.). Carbon tetrachloride (Fisher Spectroanalyzed) was dried over D20treated molecular sieves and filtered and enough HMDS added to make it 0.01 % by volume. The GPC column was slowly equilibrated to this solvent. The HPLC hardware was housed in a Plexiglass cabinet located approximately 50 cm from the outer edge of the magnet Dewar. Further protection to the magnet was provided by a Plexiglass barrier. The HPLC components were as described in ref 5. For normal-phase work, 25-jtL injections of neat samples were made. GPC samples were prepiired by diluting with an equal volume of solvent. No precipitant was observed but the diluted samples were filtered since no guard column was available. Again, 25-hL
RESULTS AND DISCUSSION The 200-MHz proton spectrum (obtained under conventional spinning conditions) of the recycle solvent sample is in Figure 1. A few peaks are indicative of specific compounds. For example, the well-separated resonance at 3.6 ppm is characteristic of acenaphthene. In a mixture spectrum like this, some resonances will shift due to hydrogen bonding and other interactions. A common use for spectra like this one is determination of the ratio of aliphatic protons to aromatic protons (Hdiphatic/Haromatic). The analogous spectrum of the hydrotreated recycle solvent differs little qualitatively, but the Hdphatic/Hmtic ratio is much higher. In addition, certain peaks (e.g., 3.6 ppm) are absent. Clearly some separation step is needed for the analysis of this sample. The LC-'H NMR profile of the recycle solvent is shown in Figures 2 and 3. For latter eluting chromatographic peaks, alternate files have been deleted to compress the presentation. Normal-phase chromatography is quite effective for class separations. Files 4-14 are lH NMR spectra taken across the aliphatic peak. For linear alkanes, the methyl/methylene integral ratio uniquely determines the
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,Li 18
I I
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o
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Flgure 2. I G 1 H NMR profile for recycle solvent (92-03-035,400-800 are 15 s in width for files 4-16 and 30 s in width for files 17-32.
OF)
I
-TTTYT-
(files 4-32). Alternate files not shown to compress presentation. Files
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P
v
P
m
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p p m Y - ‘ T T T T
Flgure 3. LC-” profile for recycle solvent (files 34-64). Flles are 60 s in width.
average chain length. File 4 shows no sign of a methine shoulder, and the average structure for this file is n-C14H30. By file 6 the average chain length has dropped to n-Cl1HZ4. Given the boiling range of this sample, alkanes much smaller than Clo should not be important. Files 8-14 show a broad, increasingly prominent resonance signal from 1.4 to 1.9 ppm. To some extent this can be rationalized as methine shoulders from branched open chain alkanes. But by file 14, this region accounts for 30% of the total integrated spectrum. This strongly suggests decalin derivatives. Other condensed aliphatic ring systems are possible. Adamantane derivatives are believed to be important groups in solvent-refined coal products. As will be shown later, methyl-substituted tetralins are major components of this sample. It would not be surprising if repeated catalytic hydrogenation produced some methyl decalins. This is significant since hydrogen consumption represents a major economic barrier to synthetic fuel production. Alkanes play no useful role in coal conversion and may retard conversion by lowering solvent quality (2). Compounds containing one aromatic ring elute in files 18-32. In file 24, there are six resolved aliphatic resonances. Peaks at 2.75 and 1.74 ppm are characteristic of tetralins. The resonance signal at 2.25 ppm is suggestive of a methyl group a to an aromatic ring. Signals at 2.53, 1.59, and 0.93 ppm suggest a n-propyl group attached to an aromatic ring. This fraction was not submitted for GC-MS, but a later GC-MS fraction showed 1-propyl-2-methylnaphthalene. Catalytic hydrogenation would be expected to give 1-propyl-2methyltetralin (vide infra). Chemical shift arguments based on model compounds support the 1,2 isomer. The Haliphatic/ Haromatic ratio in this file (corrected for the residual chloroform peak) is slightly low, indicating that some less highly substituted tetralins are also beginning to elute in this file. Alkyl-substituted tetralins elute in files 28-32. The average degree of substitution (6) in file 28 is 3.91. This means that an average structure for this file has nearly four alkyl substituents. Two are due to the fused aliphatic ring and the remaining two are accounted for by methyl and other n-alkyl groups on the aromatic ring. Compounds with two aromatic rings elute in files 34-54. There are five distinct aliphatic resonances in file 34. Four of them are consistent with n-butylnaphthalene and the fifth represents the onset of methylnaphthalene elution. Chemical shifts of reference spectra suggest that 2-butylnaphthalene
is the isomer present. The corresponding GC-MS file indicated a mixture of n-butylnaphthalene (principal ions at m / e = 141,142,184, and 115) and 1-propyl-2-methylnaphthalene (principal ions at m / e = 155,184,156, and 153). The compounds coeluted on both the LC and the GC columns. The reference spectrum of 1-n-butylnaphthalene was not available, so definitive assignment of the isomer was not possible via GC-MS. Files 36-38 are dominated by methyl-, ethyl-, and n-propylnaphthalenes. The average degree of substitution for file 38 is 1.83. Therefore, the average structure for this file has two aliphatic substituents. The small signal at 2.66 ppm is indicative of naphthalenes with methyl substituents in the a positions. The much larger signal at 2.48 ppm is due to methyl substitution in the /3 positions. The similarity between files 24-32 and files 34-37 is worth noting since alkyltetralins are hydrogen shuttles, becoming alkylnaphthalenes after transferring hydrogen to free radicals in the reacting coal. File 40 is the most complex spectrum in this series. The prominent signal at 3.4 ppm has been assigned to the aliphatic protons of acenaphthene. Aromatic signals at 7.5 and 7.35 ppm are suggestive of biphenyl. The broad resonance at 2.41 ppm is consistent with methylbiphenyls. Chromatographic retention volume measurements show that biphenyls elute in this fide. Biphenyls were also detected in the GC-MS run. File 42 is predominantly dibenzofuran. This compound was found in the GC-MS of this sample (principal ions at m / e = 168, 139, 169, and 84). Only signals at 7.25 and 6.97 (characteristic of diphenyl ether) are observed in files 45-48. File 50 is the spectrum of fluorene. Chemical shifts and integral ratios as well as the LC-GC-MS study are conclusive (principal ions at m / e = 166, 165, 83, and 82). Methyl-substituted fluorene elutes in file 52. Phenanthrene elutes in files 55-61. Anthracene has a significant multiplet at 8.1 ppm which is absent from these files. The resonance at 8.66 ppm is due to the 9,lO protons of phenanthrene. The integral ratio of this signal to that of the remaining aromatic protons shows that this chromatographic peak is pure phenanthrene. Nothing else was observed to elute in latter files. The LC-’H NMR profile for the hydrotreated recycle solvent sample is given in Figures 4 and 5. This run was made under conditions identical with the previous one, so file by
ANALYTICAL CHEMISTRY, VOL. 53, NO. 14, DECEMBER 1981
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Flgure 4. LC-lH NMR profile for hydrotreated recycle solvent (92-26-019, 400-800
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Conditions are the same as in Figure 2.
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Figure 5. LC-'H NMR profile for hydrotreated recycle solvent (files 34-64).
file comparisons are possible. The alkane fractions are very similar to those in the recycle solvent analysis and merit no further discussion. A significant difference between the two samples is seen in files 18-32. Hydrotreating has greatly increased the quantity of tetralin derivatives, as would be expected. File 28 of Figure 4 contains 4 times the material of file 28 of Figure 2. The great increase in tetralin derivatives is at the expense of naphthalene derivatives. In file 38, @-methylnaphthalenes again dominate the a-methyl isomers, but the total amount of alkylnaphthalenes is greatly reduced. It is apparent that catalytic hydrogenation of alkylnaphthalenes is producing tetralins with the alkyl substituent exclusively on the aromatic ring. A study of hydrogenation of 2-methylnaphthalene on Raney nickel (7) showed that the unsubstituted ring was more
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Conditions are the same as in Figure 3.
readily reduced and that methyldecalin could be formed a t temperatures above 250 "C. File 42 is very similar to file 40 of the previous analysis with the exception that acenaphthene is completely absent. Dibenzofuran is also completely absent from this sample but diphenyl ether is a t the same level as in the recycle solvent analysis. This difference presumably reflects relative stability to catalytic hydrogenation. 9,10-Dihydrophenanthreneis present in file 50. The amount of phenanthrene is much less than in the recycle solvent sample. The low level of phenanthrene in the hydrotreated sample may be due to an intrinsic difference between samples since it was derived from a recycle solvent slightly different from 92-03-035. Reduction to 9,10-dihydrophenanthreneand ring clevage to dimethylbiphenyl does not account for the difference in phenanthrene
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Flgure 6. GPC-‘H
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between the two samples. Fluorene is absent from the hydrotreated sample. It is not clear whether it was reduced to another aromatic species or if it was reduced to a fused ring aliphatic compound. Pyrene and its reduced forms, believed to be important in hydrogen shuttling, were not observed in either sample. Either pyrene did not elute or its level was too low for detection by LC-’H NMR. Pyrene was observed in the GC-MS analysis of the last LC fraction (principal ions at m / e = 202,101,100, and 203). But slightly different liquid chromatographic conditions were used to collect LC fractions for GC-MS. Pyrene was not injected to determine retention volume because of this compound’s status as a cancer suspect agent. Phenols are known to be present in recycle solvents at high levels (I). The elution of phenols from normal-phase columns requires relatively polar solvents (e.g., 90% Freon 113, 10% acetonitrile-d,). Large residual proton peaks in this solvent mixture motivated the use of gel permeation chromatography as an alternative. Figure 6 is the profile obtained from a GPC separation of the recycle solvent (GPC-lH NMR). CPC separation offers simplicity. It is generally assumed that any compound which is soluble and unreactive to the gel will elute between the totally excluded and totally permeated limits. The elution volume of some aromatic compounds can be increased by adsorption, however. Adsorption plays a much smaller role in GPC than in normal-phase separations. Comparing a GPC-’H NMR profile with a LG’H NMR profile is a good check for noneluted compounds. The absence of class separation complicates interpretation, but resonances between 2.1 and 2.3 ppm (characteristic of methyl substituted phenols) are visible in files 9-15. Phenol speciation if not possible with these data so a normal-phase LC-’H NMR method for highly polar compounds is under development. Another possible method for phenols is to perform GPC (or I,C)-’’F NMR on a derivatized sample. p-Fluorobenzoyl derivatives (8) are resistant to hydrolysis and should be readily chromatographable. GPC-NMR techniques should be applicable to a wide range of samples. CONCLUSION This comparison study of coal-derived recycle solvents shows that LC-’H NMR provides detailed information about
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samples of moderate complexity. A typical analysis requires 30-50 min. Although further work is needed to improve sensitivity and speed quantitation, the NMR detector is less ambiguous than any other on-line LC detector, including FT-IR. LC-’H NMR provides information which is complimentary to GC-MS. Off-line LC-GC-MS is inconvenient and time-consuming but quite useful for volatile samples. The characterization of solids derived from solvent-refined coal (curently of considerable interest in our laboratory) is hampered by the low volatility of these samples. GC-MS will be of very limited utility but the extention of LC-NMR to these samples should be a fruitful area of research. ACKNO W1,EDGMENT We thank Duayne Whitehurst, Les Rudniclr, and George Odoefer of Mobil Research and Development Corp., Princeton, NJ, for supplying samples and technical information. The assistance of Kim Harich of the VPI&SU Department of Biochemistry and Nutrition in obtaining GC-MS data is greatly appreciated. LITERATURE CITED Whitehurst, D. Duayne; Mitcheil, Thomas 0.; Farcasiu, Malvina “Coal Liquefaction: The Chemistry and Technology of Thermal Processes”; Academic Press: New York, 1980; Chapter 10. Whitehurst, D. Duayne; Mitchell, Thomas 0.; Farcasiu, Malvina “Coal Liquefaction: The Chemlstry and Technology of Thermal Processes”; Academic Press: New York, 1980; Chapter 9. Apffel, A,; McNair, H.M.,unpublished work, Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA, 1981. Majors, R. E. J . Chromatogr. Sei. 1980, 18, 571-582. Haw, James F.; Glass, T. E.; Hausler, D. W.; Motell, Edwin; Dorn, H. C. Anal. Chem. 1980, 52, 1135-1140. Haw, James F.; Glass, T. E.; Dorn, H. C. Anal. Chem., preceding paper in this Issue. Kaliberdo, L. M.; Kuznetsova, V. P.; Shergina, N. I. Izv, Sib. Otd. Akad. Nauk SSSR 1958,77-83. Spratl, M. P.; Dorn, H. C., unpublished work, Department of Chemistry, Virginia Polytechnic Institute and State Unlverslty, Blacksburg, VA, 1981.
RECEIVED for review June 17, 3981. Accepted September 21, 1981. We gratefully acknowledge financial support for this work provided by the Naval Research Laboratory (Washington, DC), the 1J.S. Air Force (WrighbPatterson Air Force Base, Dayton, OH), and the U.S. Department of Energy.
