Energy & Fuels 1997, 11, 1105-1106
1105
Practical Notes on the Use of N-Methyl-2-pyrrolidinone as a Solvent for Extraction of Coal and Coal-Related Materials C. M. White,* P. C. Rohar, G. A. Veloski, and R. R. Anderson Federal Energy Technology Center, P.O. Box 10940, Pittsburgh, Pennsylvania 15236 Received March 26, 1997. Revised Manuscript Received May 23, 1997 N-Methyl-2-pyrrolidinone (NMP) has been widely used as a solvent for extraction of coal and coal-derived materials partly because of the high extraction yields obtained with it. The literature contains dozens of articles that describe its use alone and in combination with carbon disulfide, tetracyanoethylene, and other solvents. Extractions have been performed using Soxhlet apparatus,1 an Erlenmeyer flask with boiling NMP (batch extraction),2-7 and ultrasonic techniques.8-10 Various extraction temperatures have been employed ranging from the boiling point of the solvent2-7 (202 °C at 1 atm) to room temperature.8-10 The use of reduced pressure Soxhlet extraction has also been reported.1 Some authors report performing the extraction under nitrogen,2-7 while others do not report excluding air.1,8-10 Some reports do not mention excluding air from samples during solvent removal, which is usually conducted at elevated temperatures.1-9 Some authors mention the need to distill NMP under nitrogen5 before use as an extracting solvent, while still others use the commercially available solvent without further purification.9 One study reports that higher extraction yields are obtained with Upper Freeport coal when performed under nitrogen in a glovebox.10 NMP extraction of coal has been performed under widely varying conditions, and it remains unclear what effect oxygen has on these extractions. A number of reports indicate that N-methyl-2-pyrrolidinone is chemically inert during extraction and does not react with coal.7,8,11 Iino et al. report that no significant reaction occurs between coal and NMP when ultrasonic extraction is performed at room temperature in air.8 Cagniant et al. concluded that NMP did not react with three Polish coals or a series of model compounds they extracted using boiling NMP (202 °C) * To whom correspondence should be addressed. (1) Chawla, B.; Davis, B. H. Fuel Process. Technol. 1989, 23, 133148. (2) Renganathan, K.; Zondlo, J. W.; Stiller, A. H.; Phillips, G.; Mintz, E. A. 1987 International Conference on Coal Science; Moulijn, J. A., et al., Eds.; Elsevier Science Publishers B.V.: Amsterdam, 1987; pp 367370. (3) Renganathan, K.; Zondlo, J. W.; Mintz, E. A.; Kneisl, P.; Stiller, A. H. Fuel Process. Technol. 1988, 18, 273-278. (4) Renganathan, K.; Zondlo, J. W. Fuel Sci. Technol. Int. 1993, 11, 677-695. (5) Chervenick, S. W.; Smart, R. B. Fuel 1995, 74, 241-245. (6) Cai, M. F.; Smart, R. B. Proceedings of the Tenth Annual Pittsburgh Coal Conference; S. H. Chiang, Ed.; 1993; pp 1153-1158. (7) Cagniant, D.; Gruber, R.; Lacordaire, C.; Jasienko, S.; Machnikowska, H.; Salbut, P. D.; Bimer, J.; Puttmann, W. Fuel 1990, 69, 902910. (8) Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Fuel 1988, 67, 1639-1647. (9) Seki, H.; Ito, O.; Iino, M. Fuel 1990, 69, 1047-1051. (10) Ishizuka, T.; Takanohashi, T.; Ito, O.; Iino, M. Fuel 1993, 72, 579-580. (11) Seehra, M. S.; Ghosh, B.; Zondlo, J. W.; Mintz, E. A. Fuel Process. Technol. 1988, 18, 279-286.
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under a nitrogen atmosphere.7 Seehra et al. studied the NMP extraction of coal at 202 °C and 1 atm.11 After examination of the coals, extracts, and residues by EPR, they concluded that there was “no significant reaction of NMP with free radicals” in the coal, making this a simple extraction process “without breaking significantly any new bonds”.11 Alternatively, Cai and Smart concluded that NMP is reactive with coal and is responsible for breaking carbon oxygen bonds when used at its boiling point under nitrogen.11 Cai and Smart “conclude that during NMP-coal extraction, the oxygen-containing bridges are disrupted and the polymer is broken into multiring clusters with hydrocarbon chains”.6 During routine use of N-methyl-2-pyrrolidinone in our laboratories as a solvent for Soxhlet extraction of unburnt carbon isolated from fly ash, we noticed that the solvent quickly darkened during use in blank extractions unless oxygen was strictly purged from the system. Even when blank extractions were performed under nitrogen, the solvent slowly turned brown during use. The NMP used was HPLC grade obtained from Sigma-Aldrich12 and had a stated purity of 99+%. When 150 mL of NMP from a freshly opened bottle was distilled under nitrogen before use as an extracting solvent, 2.1 mL of the solvent distilled between 151 and 180 °C. The boiling point of NMP was 191 °C in our laboratory that day. When NMP was taken from a previously opened bottle that sat on a lab shelf for several months, our experience was that much more material was collected between the initial boiling point and the boiling point of NMP, and the initial boiling point was lower than 151 °C. NMP is hygroscopic, and these observations could be the consequence of a progressive incorporation of water over time. Air must be strictly excluded from the system even during solvent removal steps if heat is used. When bulk NMP was removed from the extract by evaporation on a warm Corning stirrer/hot plate (Model PC-351) in the air in a hood, crystals collected on the sides of the glass evaporating dish. The hot plate was placed at the lowest possible setting that resulted in a temperature of 85 °C measured at the center of the heating surface. The pressure was about 1 atm. The solvent was evaporated over a period of 86 h. The infrared spectrum of crystals collected from the side of the evaporating dish was identical with that of N-methylsuccinimide. When dissolved in methylene chloride and analyzed by gas chromatography using a 30 m × 0.32 mm fused silica open tubular column coated with a 0.25 µm film of HP-5 (12) Use of brand names facilitates understanding and does not necessarily imply endorsement by the USDOE.
This article not subject to U.S. Copyright.
Published 1997 by the American Chemical Society
1106 Energy & Fuels, Vol. 11, No. 5, 1997
(5% phenylmethylpolysiloxane), the retention time of the compound was identical with that of N-methylsuccinimide. The mixed melting point of the collected crystals and authentic N-methylsuccinimide was 6772 °C. Literature values of the melting point of Nmethylsuccinimide range from 66 to 70 °C.13 N-Methylsuccinimide has been reported in the Russian literature to form during the oxidation of NMP.14 Although the reaction of NMP with oxygen to form N-methylsuccinimide has been reported by others, it is not widely known. In addition to the autoxidation of NMP, a metal-catalyzed oxidation by Co(II) is also known.15,16 Coal contains significant amounts of various metals, some of which could catalyze the oxidation of NMP. (13) Beilsteins Handbuch Der Organischen Chemie; Heteroclische Reihe: Berlin, 1918; Bd. 21, S 373. (14) Stepanova, V. N.; Kolesnikova, E. V.; Bochkareva, T. P.; Rastegaeva, N. P. Khim. Promst. (Moscow) 1991, 2, 125-127; CA 115: 116770y. (15) Drago, R. S.; Riley, R. J. Am. Chem Soc. 1990, 112, 215-218. (16) Patton, D. E.; Drago, R. S. J. Chem. Soc., Perkin Trans. 1993, 1, 1611-1615.
Communications
We suspect that a hydroperoxide may be an intermediate in the reaction of NMP with oxygen to yield N-methylsuccinimide and that the hydroperoxide could initiate the oxidation of other substrates.15 For research applications, we recommend using a high-purity NMP and distilling it under nitrogen before use. Whenever NMP is exposed to heat, the NMP should be kept under a nitrogen atmosphere to minimize autooxidation. In any commercial extraction process, recovery of the solvent is an important economic and environmental issue. It may be possible to increase the recovery of NMP by preventing oxidation to N-methylsuccinimide. Additionally, the N-methylsuccinimide that is formed may be more difficult to recover from the coal residue and coal extracts than NMP because N-methylsuccinimide (boiling point 235 °C) has a lower vapor pressure than NMP, causing the weight percent of material extracted to be erroneously high. EF970050Y
