Energy & Fuels 1996, 10, 371-377
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Use of 2,2-Dimethoxypropane and 1H NMR To Distinguish and Quantify the External and Internal Sorbed Water in Coals Daniel A. Netzel,* Thomas F. Turner, Francis P. Miknis, Jeffrey C. Wallace, Jr., Clinton H. Butcher, and Jennifer M. Mitzel Western Research Institute, 365 N. 9th Street, Laramie, Wyoming 82070
Robert J. Hurtubise Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071 Received June 16, 1995. Revised Manuscript Received January 2, 1996X
Physisorbed and chemisorbed water in coal can be effectively removed by the use of 2,2-dimethoxypropane (DMP) which reacts with the water to form methanol and acetone. This reaction is rapid and endothermic. An 1H NMR method, based on this reaction, was developed to measure the amount of water in coals of different rank. Integrations of the methyl resonances from acetone and the methylene resonances of cycloheptane, an internal hydrogen standard, were used to determine the number of moles of water reacted. The method was also used to determine the diffusion kinetic parameters and type of diffusion associated with the chemical dehydration of coals. The initial mechanism for the diffusion of the solvent-reactant into the macromolecular structure of coal can be either Fickian or Case II depending on the coal. From the kinetic study, it was found that external and internal sorbed water were removed sequentially. Free or surface sorbed water is nearly instantaneously removed followed by the water in the internal structure of the coal. It was found that low-rank coals have up to 70% of their total water at or near the surface and readily accessible to chemical dehydration, whereas in high-rank coals about 30% of the water is readily accessible. The remaining water is in the internal structure of the coal.
Introduction The moisture content of coals plays an important role in coal utilization. Indeed, coals are bought and sold on the basis of their heating value, ash, and sulfur contents. The heating value is directly affected by the moisture content. In addition, to obtain the correct elemental composition of coal, the elemental values must be corrected for moisture and ash, or mineral matter. The water content and the state of water in coals are also important in enhancing or lessening coal reactivity toward liquefaction.1-3 Therefore, there is a need for developing more accurate methods to determine the total moisture content in coal and to define the sorbed states of water in order to understand the role of water in coal liquefaction. Water in coals exists in the free, physisorbed and chemisorbed states.4 The free state of water is often referred to as surface, bulk, superficial, or freezable moisture. Air-drying will remove most of this type of water.5 The physisorbed state is water hydrogen bonded to the oxygen-containing functional groups on the coal surface and in the internal macromolecular structure * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, February 15, 1996. (1) Song, C.; Saini, A. K.; Schobert, H. H. Energy Fuels 1994, 8, 301. (2) Okuma, O.; Masuda, M; Murakoshi, K; Yanai, S; Matsumura, T. Nenryo Kyokaishi 1990, 69, 259. (3) Netzel, D. A.; Miknis, F. P.; Wallace, Jr., J. C.; Butcher, C. H.; Mitzel, J. M.; Turner, T. F.; Hurtubise, R. J. DOE Final Report. Western Research Institute Report WRI-95-R023, 1995. (4) Allardice, D. J.; Evans, D. G. In Analytical Methods for Coal and Coal Products; Karr, Jr., C., Ed.; Academic Press: New York, 1978; Vol. I, Chapter 7. (5) Riley, J. T. Am. Lab. 1983, August, 17.
0887-0624/96/2510-0371$12.00/0
of coals. The free and hydrogen-bonded water can also exist in the capillaries of varying radii that are found in coal. Water of this type is designated as inherent or pore moisture and can be released when heated to 110 °C.5 Chemisorbed water is water of hydration of carboxylate cations, calcium sulfate, and clays that are found in coals. Temperatures greater than 110 °C are needed to release chemisorbed water of the carboxylate cations.4 In the case of water of hydration in Kaolinite a temperature of 500 °C is needed to release the water.5 In addition to sorbed water in coals, water can be produced along with carbon dioxide from the decomposition reactions of oxygen containing organic functional groups in low rank coals. This thermal decomposition reaction to produce water can commence at temperature well below 100 °C6,7 and extend to 225 °C.5 A number of physical, chemical, and spectroscopic methods have been used to determine the moisture content in coals.8 Most methods reported are rapid and reasonably accurate but do not give total water content. Also, with current methodology, it is not possible to accurately quantify the different states of water in coals. Thermal methods release a continuum of water with no apparent demarcation for the different water types. In addition, thermal methods of removing water can alter the physical structure of coal and may promote undesirable chemical reactions. (6) Allardice, D. J.; Evans, D. G. Fuel 1971, 50, 219. (7) Swann, P. D.; Harris, J. A.; Siemon, S. R.; Evans, D. G. Fuel 1973, 52, 154. (8) Wroblewski, A. E.; Reinartz, K.; Verkade, J. G. Energy Fuels 1991, 5, 786.
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Netzel et al.
Table 1. Proximate Analyses of Six Coal Samples proximate analysis (as received) PSU no. (seam name)
location
apparent rank
moisture, wt %
ash, wt %
volatiles, wt %
fixed carbon, wt %
DECS-6 (Blind Canyon)a DECS-2 (Illinois No. 6)a (Eagle Butte)b,c (Black Thunder)b,d DECS-1 (Bottom)a DECS-11 (Beulah)a
Utah Illinois Wyoming (Powder River Basin) Wyoming (Powder River Basin) Texas North Dakota
hvAb hvCb sub sub sub C lignite
4.73 10.43 29.2 27.63 30.00 33.38
5.56 14.47 4.7 4.49 11.07 6.37
42.40 34.16 30.9 31.78 33.18 37.36
47.31 40.93 35.2 36.10 25.75 22.89
a
Data from Penn State Coal Data Base. b Mine name. c Boysen et al.38
Finseth9 and Olsen and Diehl10 reported the use of an 18O isotope dilution technique to measure the total water content in coal. The method is time consuming and does not differentiate the different types of water. Wroblewski et al.8 and Wroblewski and Verkade11 developed a 31P NMR method to measure the total water content as well as to differentiate between surface and pore water. The determination depends upon derivatizing pyridine-extracted moisture with a 31P NMR tagging agent. The reaction of 2,2-dimethoxypropane (DMP) with water to form methanol and acetone is shown in reaction I. OCH3 CH3
C
CH3 + H2O
O H+
2CH3OH + CH3
C
CH3
(I)
OCH3
The reaction is rapid and endothermic and, in the presence of an acid catalyst, this reaction is nearly instantaneous and quantitative. Thermal methods of removing water can alter the physical structure of coal and may promote undesirable chemical reactions. However, this low-temperature method for the removal of water with DMP preserves the structural integrity, reduces retrograde reactions, and reduces thermal degradation. The amount of water measured in a sample is directly related to the number of moles of acetone formed. The reagent has been used in inorganic chemistry to remove water of hydration in organic compounds,12-14 in biological sciences for drying tissues for microscopic examination,15-17 and as a drying agent for the preparation of infrared samples.18 Bredeweg et al.19 used the reaction to reactivate silica columns. The DMP-H2O reaction was used by Critchfield and Bishop20 to determine the water in materials that cannot be analyzed by Karl Fischer reagent. They quantified the acetone formed using the carbonyl infrared absorption band. Gas chromatographic determination of the reactions prod(9) Finseth, D. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1987, 32(4), 260. (10) Olsen, E. S.; Diehl, J. W. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1988, 33(2), 415. (11) Wroblewski, A. E.; Verkade, J. G. Energy Fuels 1992, 6, 331. (12) Wayland, B. B.; Drago, R. S. J. Am. Chem. Soc. 1966, 88, 4597. (13) Donoghue, J. T.; Drago, R. S. Inorg. Chem. 1962, 1, 866. (14) Donoghue, J. T.; Drago, R. S. Inorg. Chem. 1963, 2, 572. (15) Adams, R. F. J. Chromatogr. 1974, 95, 189. (16) Bousquet, W. F., Christian, J. E.; Knevel, A. M.; Spahr, J. L. Anal. Biochem. 1962, 3, 519. (17) Muller, L. L.; Jacks, T. J. J. Histochem. Cytochem. 1975, 23(2), 107. (18) Erley, D. S. Anal. Chem. 1957, 29, 1564. (19) Bredeweg, R. A.; Rothman, L. D.; Pfeiffer, C. D. Anal. Chem. 1979, 51, 2061. (20) Critchfield, F. E.; Bishop, E. T. Anal. Chem. 1961, 33, 1034.
d
Data from Wyoming Analytical Laboratory, Laramie, Wyoming.
ucts was used by several investigators to determine water in various organic and inorganic materials.21-23 Chen and Fritz24 reported the use of triethyl orthoformate instead of 2,2-dimethoxypropane to determine water in a variety of liquids and solid samples. In this paper the methodologies are described for chemically dehydrating coals and for quantifying, using 1H NMR, the total amount of water in coals. The kinetics for the solvent-reactant diffusion in coals were also investigated. Experimental Section Coal Samples. Of the six coals used in this study four were obtained from the Pennsylvania State University, Coal Research Section, and two were obtained from the Powder River Basin in Wyoming. The coals selected for study consisted of a lignite, three subbituminous, and two bituminous coals. Table 1 lists the coals and the proximate analyses. The coals were used as part of a study to determine the effects of different drying procedures on coal liquefaction yields.3 Reagents. Analytical reagent grade anhydrous magnesium sulfate from Mallinckrodt was used to dry the various reagents used in determining the amount of water in coals by the dehydration method. 2,2-Dimethoxypropane (98%) from Aldrich Chemicals was used as received except for drying with anhydrous magnesium sulfate. Gas chromatographic analysis indicated the purity to be 99% with three impurity components of 0.16, 0.45, and 0.38 relative percent. The NMR spectrum of the reagent showed a small acetone peak. Methanol, HPLC grade, from J. T. Baker was dried with anhydrous magnesium sulfate and was used to prepare standard solutions of water in methanol. A 0.2 N solution of methanesulfonic acid in methanol was prepared using methanesulfonic acid (99%) from Aldrich Chemicals. Cycloheptane (>98%), used as the analytical hydrogen standard, was also obtained from Aldrich Chemicals. Gas chromatographic analysis indicated the purity to be 97.63% with three impurity components of 1.27, 0.67, and 0.69 relative percent. All calculations of the hydrogen content were corrected for the percent purity. Type I distilled water was used to make the standard solutions of water in methanol. The reagents deuterochloroform (CDCl3, 99.8 atom %) and tetramethylsilane (TMS) used for liquid state NMR experiments were obtained from Wilmad Glass Co. Coal Preparation. The coals used in this study were premoisturized. A master batch sample of each coal (∼500 g) was prepared by grinding and screening to -20, +100 mesh particle size (except for Wyoming Black Thunder which was prepared with a mesh size of -100). The ground coal was placed in a wide mouth jar and allowed to equilibrate in an oven with a beaker of water at 30 °C for 24 h. The waterequilibrated coal sample was then removed from the oven and (21) Hager, M.; Baker, G. Proc. Montana Acad. Sci. 1962, 22, 3. (22) Martin, J. H.; Knevel, A. M. J. Pharm. Sci. 1965, 54(10), 1464. (23) Dix, K. D.; Sakkinen, P. A.; Fritz, J. S. Anal. Chem. 1989, 61, 1325. (24) Chen, J.; Fritz, J. S. Anal. Chem. 1991, 63, 2016.
External and Internal Sorbed Water in Coals
Figure 1. Expanded 1H NMR spectrum of the solution from the reaction of water with DMP illustrating the use of the curve-fitting software for determining the areas of the acetone and cycloheptane resonances. placed in a moisture chamber at a temperature of 30 °C until aliquots were taken for the different experiments. The “moisture” value (used as the reference moisture content) for the coals was determined from the weight loss of 1 g of coal in a nitrogen atmosphere at 105 °C for 24 h. NMR Instrumentation. 1H NMR spectra were obtained on a JEOL GSX-270 NMR spectrometer. The experimental conditions for recording a 1H spectrum were eight scans, a pulse width of 5.4 µs (45°), an acquisition time of 1.5 s, a pulse delay of 20 s, and 16K time-domain data points. In some experiments the 1H NMR spectra were obtained on a JEOL GS-400 NMR spectrometer. The experimental conditions for recording a 1H spectrum using this spectrometer were eight scans, a pulse width of 9.9 µs, an acquisition time of 1.0 s, a pulse delay of 20 s, and 16K time-domain data points. A linebroadening factor of 2 Hz was used to obtain the transformed spectra. Resonance areas for acetone (2.18 ppm) and cycloheptane (1.55 ppm) were obtained using a Lab Calc curvefitting software routine (Galactic Industries Corp.). A line width at half-height of 0.010 ppm was used for curve-fitting. Twenty iterations of a Lorentzian line shape gave an acceptable agreement between the experimental and fitted resonance. Figure 1 shows the application of the curve-fitting routine to the 1H signals for acetone and cycloheptane. Using a Lorentzian curve-fitting routine increased the precision and accuracy of the measured area by eliminating distortion at the base of the peak and other instrumental artifacts which contribute to the shape of the peak. Moisture Determination. The standard necessary precautions were taken to prevent atmospheric moisture as a contaminant, which in Laramie, because of its dry climate, was not demanding. All moisture determinations were performed using glassware dried at 110 °C. Initially, a nitrogen-filled glovebox and nitrogen-filled glovebags were used to transfer solutions but this was found to be unnecessary. Transfer of solutions to glass-stoppered 25 mL Erlenmeyer flasks, 10 mL glass-stoppered centrifuge tubes, and glass vials was made expeditiously using syringes with small diameter needles. Solutions of DMP were added to the acid solutions of standards and coal to minimize moisture interference. a. Analysis of Water in Methanol. The precision and accuracy of the 1H NMR method to determine water content in a sample were investigated using known amounts of water in methanol. Various amounts of water (0.0, 0.05, 0.10, 0.20, and 0.30 g) were added to 2 mL of 0.2 N methanesulfonic acid (CH3SO3H) in methanol. Methanesulfonic acid, a strong mineral acid, was used to catalyze the reaction. To this solution was added 4 mL of DMP and 0.7 mL of cycloheptane
Energy & Fuels, Vol. 10, No. 2, 1996 373
Figure 2. 1H NMR spectrum of a methanol-water solution after the reaction of water with 2,2-dimethoxypropane (DMP). as the internal hydrogen standard. After a reaction time of 5 min, a 1/2 mL aliquot of the solution was transferred to a vial, diluted with 1/2 mL of CDCl3 containing TMS and then 0.75 mL of the solution was transferred to a 5 mm NMR tube and the 1H NMR spectrum recorded. A typical 1H NMR spectrum of the solution is shown in Figure 2. The amount of water in methanol was calculated using eq 1.
gH2O ) (fcncwcAAMH2O)/(nAAcMc)
(1)
where gH2O is the grams of water, fc fractional purity of cycloheptane, nc number of hydrogens in cycloheptane molecule, wc weight of cycloheptane (grams), AA 1H area of acetone, MH2O molecular weight of water, nA number of hydrogens in acetone molecule, Ac 1H area of cycloheptane, and Mc molecular weight of cycloheptane. b. Moisture Determination in Coals. Moisture determinations were made on coals of various rank using 2,2-dimethoxypropane. One-half gram of coal was weighed into a 10 mL glass-stoppered centrifuge tube followed by 1 mL of 0.2 N CH3SO3H in CH3OH, 0.7 mL of the reference cycloheptane, and 2 mL of DMP. After 16 h, the solution was stirred and centrifuged prior to removing the aliquots. One-half milliliter aliquot was removed using a syringe, placed into a small vial, and diluted with 1/2 mL of CDCl3. This solution was then transferred to an NMR tube and the 1H spectrum recorded. From the measured areas of the methyl hydrogens of acetone and the hydrogens of cycloheptane, the amount of water was calculated using eq 1 and the amount of water in the coal samples was determined using eq 2.
PH2O ) (gH2O/Wcoal) × 100
(2)
where PH2O is the percent of water in coal, gH2O grams of water (eq 1), and Wcoal weight of coal sample (grams). To determine the kinetics for the amount of water removed, the amount of coal and volumes of the reagents used were twice that used to measure the moisture content. Aliquots of the reaction mixture were removed at 0.25, 0.5, 1.0, 2.0, 4.0, 6.0, 8.0, and 24 h, diluted in CDCl3 and the 1H NMR spectra recorded. In some cases, the aliquots prepared for NMR determination had to be refrigerated until the NMR instrument was available. Storage of NMR prepared aliquots would not affect the kinetic results because the DMP-H2O reaction is no longer in progress. Typical storage times were
