346
Energy & Fuels 1990, 4 , 346-351
of storage. Thus for compzative studies it is required to analyze always fresh sample material. The occurrence of high amounts of BTX in higher rank coals is not surprising. I t is well-known that, upon Curie point pyrolysis experiments of coal, benzene, toluene, and xylenes as well as alkylated phenols occur in the pyrolyzates.% It is assumed that lignin is the natural precursor of such pyrolysis products.w1 Since lignin represents one of the best preserved biopolymers, its final degradation products, i.e., benzene and its alkylated derivatives, are generated in coals at higher coalification stages (R, > 1.0%). Therefore, in medium volatile bituminous coals high amounts of BTX aromatics correspond with the occurrence of good carbonization properties. However, comparison of the BTX yield in the Ruhr coal no. 84166 (731.0 pg/g C0,J and the Saar coal no. 84169 (887.9 hg/g Corg)indicates a similar stage of bitumen degradation despite a difference in vitrinite reflectance values of 0.51 % (Table I). This result is a further confirmation that in the case of Saar coals vitrinite reflectance data do not really (38) Nip, M.; de Leeuw, J. W.; Schenck, P. A. Geochim. Cosmochim. Acta 1988,52,637-648. (39) Hatcher, P. G.; Breger, I. A.; Maciel, G. E.; Szeverenyi, N. M. h o c . Int. Conf. Coal. Sci. 1983. 310-313.
(40)Hatcher, P. G.; Spiker, E. C.; Szeverenyi, N. M.; Maciel, G. E. Nature 1983,305,498-501. (41) Hedges, J. I.; Cowie, G. L.; Ertel, J. R.; Barbour, R. J.; Hatcher, P. G. Geochim. Cosmochim. Acta 1985,49, 701-711.
enable the assessment of the technological properties of the coals.22 Saar coals show much better carbonization properties than one would predict from the measured vitrinite reflectance values.
Conclusions Benzene, toluene, xylenes, and further high volatile aromatic hydrocarbons such as biphenyls obviously play an important role in the process of carbonization. The coking potential of coals depends on the amount of these compounds trapped in the macromolecular network. The experimental conditions used for thermodesorption-gas chromatography simulate the initial step of the carbonization process. Thus, the analysis of the bitumen released during thermodesorption at 300 "C enables prediction of the usefulness of a coal as coking coal. Acknowledgment. Support of this study by Prof. Dr.
M. Wolf (RWTH Aachen) and Prof. Dr. D. H. Welte (KFA Julich GmbH) is gratefully acknowledged. Registry No. 7,142-82-5;8,111-65-9;9,111-84-2; 10,124-18-5; 11,1120-21-4;12,112-40-3;13,629-50-5; 14,629-59-4;15,629-62-9; 16,544-76-3;17,629-78-7;18,593-45-3;19,629-92-5;20,112-95-8; 21,629-94-7;22,629-97-0;23,638-67-5; 24,646-31-1;25,629-99-2; 26,630-01-3;27,593-49-7;28,630-02-4;29,630-03-5;30,638-68-6; 31, 630-04-6; 32, 544-85-4; 33, 630-05-7; a, 71-43-2; b, 108-87-2; c, 108-883;d, 106-42-3;e, 95-47-6;f, 91-20-3; g, 91-57-6;h, 90-12-0; i, 92-52-4;j, 1921-70-6;1, 85-01-8; m,638-36-8; n, 832-71-3; 0 , 2531-84-2; p, 883-20-5;r, 832-69-9;m-xylene, 108-38-3.
Ultrasonically Enhanced Coal Extraction: Use of Base-Solvent Swollen Coal Slurries Michael G. Matturro,* Ronald Liotta,* and Robert P. Reynolds Corporate Research Science Labs, Exxon Research and Engineering Company, Route 22 East, Clinton Township, Annandale, New Jersey 08801 Received September 21, 1989. Revised Manuscript Received M a y 14, 1990 Sonication of base-swollen coal leads to base extraction yields on the order of 70 f 5 wt % for three coals: Illinois No. 6 (bituminous), Rawhide (subbituminous),and Big Brown (lignite). These values are 2.5 times greater than the extent of base extraction (-28 wt % ) observed for the corresponding stirred control mixtures. High extraction values were only obtained for sonicated base solutions containing tetra-n-butylammonium hydroxide, pyridine, and methanol. These solutions were also found to swell coal to a greater extent than any other mixture examined. Swelling is suspected to cause elongation of structural units or chains within coal and to render them, via these orientational effects, more susceptible to cleavage by shear forces. The method provides a convenient procedure to isolate larger portions of extractable coal which, from an analytical standpoint, look chemically like the coal itself. This analytical technique renders the coal amenable to high-resolution chemical analysis.
Introduction Characterization of coal structure by high-resolution spectroscopic methods such as solution NMR is hindered by the insolubility of coals. Chemical degradation schemes usually modify the starting structure extensively, and little knowledge of the original covalency can be deduced from product analysis. Mild chemical schemes for coal solubilization and molecular weight reduction are therefore important for structure characterization. In our work, coal is envisioned as a complex, cross-linked macromolecule derived primarily from lignin with physicochemicalproperties similar to ion-exchange resins. Previously, we de0887-0624/90/ 2504-0346$02.50/0
scribed the neutralization and swelling behavior of solvent-extracted bituminous coal and reported that observations were consistent with a simple polyelectrolyte model.' Since coal behaved like a polymer network, we reasoned that known polymer degradation schemes might be applicable. Acoustic waves over a large frequency range are known to induce structural changes in monomeric and polymeric substrates and to generally increase reaction rates. For (1) Matturro, M. G.; Liotta, R.; Isaacs, J. J. J. Org. Chem. 1986,50, 5560.
1990 American Chemical Society
Ultrasonically Enhanced Coal Extraction Table I. Coal Compositional Analysis coaltype % C %H %N % Sorg % MM" Illinois No. 6 68.4 5.2 1.3 3.1 11.2 Rawhide 67.0 4.7 0.8 0.5 5.4 1.2 12.9 63.3 4.9 1.2 Big Brown "Calculated as 7'0 MM - 1.13 ( % ash) + 0.47 ( % pyritic sulfur). example, poly(methacry1ic acid), when partially neutralized and forced to adopt a rodlike conformation, was more effectively degraded by ultrasound than when the polymer was fully protonated and folded upon itself.2 It is under these conditions, where the carbon-carbon bond axes are oriented in one direction, that a shock wave formed by the collapse of an ultrasonically formed cavity can produce shear forces that may lead to bond scission. We believed that control of coal swelling would allow us to orient the networks in coal so that shear forces would be effective as observed in the oriented polymer systems. Most attempts to enhance the extraction of coal with ultrasound have made use of neat solvents such as pyridine, water, quinoline, tetralin, and tetrahydroquinoline. The latter two solvents are considered "activated" since they can act as a source of hydrogen. In HzO, ultrasonic irradiation produced no noticeable change in extractability or compo~ition.~ The other solvents all lead to moderate extraction increases. The data suggest that ultrasonic energy ruptures bonds in coal that are not affected by solvent alone, such as strong hydrogen bonds and other secondary forces (including van der Waals associations) or aliphatic covalent bonds.4 Implied in some reports is the view that coal possesses a "micellar"-type structure5 that can be disentangled by sufficiently high energy input. The disruption of van der Waals associations at several locations would presumably render dislodged fragments extractable into solution. The extracted or dispersed material may, however, demonstrate thixotropic behavior and become again associated when the agitation is removed.6 We have observed dramatic enhancement in coal extractability using acoustic energy (85 W/cm2) at 23 kHz with tetra-n-butylammonium hydroxide (n-Bu,PJOH) in a solvent mixture containing pyridine and methanol. Increases in base extractability as great as 185% were observed for Illinois No. 6 coal relative to unsonicate slurries (27 wt 70extract for base case coal). Similarly, large increases were also measured for the lower ranked coals, Rawhide and Big Brown. The nature of the swelling medium was found to be crucial for the production of high-yield extraction and was primarily influenced by cation size and solvent composition. Experimental Section Materials. Three coals of different rank were used in the studied described Illinois No. 6 (high-volatile C bituminous), Wyoming Rawhide (subbituminous),and Texas Big Brown (lignate). All coals were dried at 110 "C at 2 Torr for 12 h before use (see Table I for elemental analyses). Solvents and bases used were all ACS reagent grade. n-Bu4NOH(1.0 M in methanol)was purchased from Southwestern Analytical Chemical, Inc., Austin, TX. Elemental analyses were obtained from Galbraith Labs., Knoxville, TN. Static Sonication. All ultrasonic irradiations were made with a collimated 23-kHz beam from a titanium amplifyinghorn driven (2) Alexander, P.; Fox, M. J . Polym. Sei. 1954, 12, 533. (3) Berkowitz, N. Can. J . Technol. 1955,33,378. (4) Anderson, L. L.; Shifai, M. Y.;Hill, G. R. Fuel 1974, 53, 32. (5) Cooke, N. E.; Gaikwad, R. P. Can. J . Chem. Eng. 1983, 61, 697. (6) Berkowitz, N. Nature 1949, 163, 809.
Energy & Fuels, Vol. 4, No. 4, 1990 347 by a crystal transducer (Lab-line Ultratip Lab-sonic System No. 9100, Lab-line Instruments, Inc.). Total acoustic powder was -150 w (factory calibrated for H20). Sound intensities at the probe tip were -85 W/cm2. The reactions were performed in a 150-mLvessel placed in a water bath cooled by a chilled circulation system. The temperature of the sonicated reaction mixture was generally maintained at 34 f 4 "C. General Procedure. In a typical experiment, the reaction vessel was filled with 75 mL of pyridine, 25 mL of 1.0 M nByNOH in CH,OH, and 5.0 g of raw coal (-20 mesh). The slurry was cooled, stirred magnetically from below, and ultrasonically irradiated (tip immersion 2-3 cm) under air. The temperature in the cell was monitored with an iron-constantan thermocouple probe located within -1 cm of the titanium tip. Thermal equilibration was reached within a few minutes. Irradiationwas maintained for 2-4 h. Stirringcontrols were run under identical conditions but without ultrasound and at temperatures 5-10 O C less than those maintained during irradiation. The small temperature increases does not account for the large increase in extractability. When sonication was discontinued, the slurry was transferred to 250-mL centrifuge tubes with a minimal amount of methanol (2-3 mL). The mixture was then centrifuged for 30 min at 2500 rpm using a Sorvall Instruments RC-3B refrigerated centrifuge at 25 O C (25-cm rotor). The extract solution was decanted off into a 500-mL round-bottom flask and concentrated by use of a rotary evaporator to give a somewhat viscous residue. About 100 mL of 3N HCl was then added carefully to the residue and swirled for several minutes. The slurry was transferred with deionized HzO to a 500-mL medium sintered glass frit and the solids (extract)were washed with HzOuntil the filtrate was free of C1- (determined by addition of AgN03). This required 10-20 L of HzO. Excess water was removed, and the extract was transferred to a recrystallization dish using a small b m h and dried for 12 h at 110 O C at 2 Torr. This afforded -4.4 g of dried extract. The residue from the centrifugation, the base-nonextractable component, was digested in 150 mL of 3N HCl with stirring for 2 h. The slurry was filtered, again using a 500-mL medium sintered glass frit, and washed with 10-20 L of HzO. The solids were dried at 110 "C at 2 Torr for 12 h and gave -1.2 g of material, (total) mass recovery -110% (see Table 11). Thermal controls (unsonicated) were also worked up as described above and gave isolated base extract and nonextract masses of 1.6 and 4.4 g, respectively (total mass recovery 105%,see Table 11).
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Results and Discussion Sonication. A. Base and Solvent Composition. On the basis of previously mentioned polymer degradation studies, and the hypothesis that highly swollen coal should mechanically degrade more easily than unswollen coal, we have systematically examined the effect of base and solvent composition on the depolymerization of coal. Base extraction yield has been used as an indicator of the extent of molecular disruption or molecular weight reduction. This is a reasonable measure of the extent or bond cleavage occurring within a molecular framework (as in a matrix) a t a cross-linking point or along a chain. Bond rupture should lead to smaller extractable fragments. Table I1 shows a compilation of extraction yields, masses, and recoveries for ultrasonically irradiated basesolvent preswollen Illinois No. 6 coal. Initial inspection of the data shows that a significant extractability enhancement was observed only for the sample sonicated with 0.25 M tetra-n-butylammonium hydroxide in a pyridine and methanol mixture (3:l by volume). Solvents or solvent mixtures alone did not produce appreciable extractability enhancements for the mixed or sonicated samples. Methanol gave a negligible extractability ( B' > A'. More detailed information can be obtained from the respective empirical formulas for the organic components on a 100 carbon atom basis. Table IX gives the formulas for the starting Illinois No. 6 coal and the products A'-E' along with the corresponding H/C, O/C, N/C, and S/C ratios. Relative to carbon, the hydrogen, nitrogen, and oxygen contents are observed to increase while the sulfur content decreases somewhat. The increase in H and N may be due to incorporation of n-Bu4N+groups or pyridine. The exhaus-
Table IX. Empirical Formulas, H/C Ratios, and O/C Ratios for Illinois No. 6 Coal and the Base Extracts Shown in Scheme I1 emDirical formula' H/C O K N/C S/C raw coal C100H91,8N1.BS1.7011.80.92 0.12 0.016 0.017 Cl&IlM,sN&1.3013.3 1.04 0.13 0.024 0.013 A' ClooHlla.rNz.sS1.3014.l 1.16 0.14 0.028 0.013 B' C1&I1&42.2S1.5017,0 1.09 0.17 0.022 0.015 C' 0.017 O . O E b C100H109.2N1.7S1.5b.C l.03 D' ClooHlli,lN2,5S1,401,,~ 1.11 0.15 0.025 0.014 E'
For the organic component. Based on an eetimate of organic sulfur. '0is estimated to be -0 by difference.
tive extraction of B' with H 2 0 gave material E' (Scheme 11). Compositional analysis (Tables VI11 and IX) showed a slight decrease in the amount of added N and H, suggesting that incorporation of nitrogen-containing reagent was minimal. It also suggests that this material may be chemically bound. Alternatively, it could be entrapped in collapsed pores. This might occur when acid is added to reprotonate the coal anions. The drastic deswelling process accompanies acidification. Infrared spectra reveal an increase in the intensity of bands in the aliphatic C-H stretching region (2960-2850 cm-') and in the carbonyl C=O stretching region (1700 cm-' could be assigned to aromatic carboxylic acids or alkyl aryl ketones (including five- and six-membered ring quinones)). Empirical formulas on a 100-carbon basis show an increase in oxygen content for extracted coal with increasing exposure to base and ultrasonic energy. This oxygen increase, however, is not caused by an ultrasound-promotedoxidation, since similar compositional and spectroscopic analyses are obtained for control experiments where coal-base slurries are just stirred. Stirring alone in a pyridine/CH30H solution of n-Bu4NOH under an air atmosphere also leads to oxygen incorporation (-3 oxygens per 100 carbons). Related base-promoted oxidations of c0al~9~ are known, and many other examples using lignin'O as the oxidizable substrates have been studied. Mechanistic Considerations. For ultrasound-promoted reactions in solution, it is generally believed that chemical changes (bond cleavages) are produced by the collapse of sonically formed cavities."J2 The initiation of collapse depends on bubble size which, in turn, depends on sound frequency, solution viscosity, the specific heat of the gas in the bubble, and the total external pressure." The phenomenon of bubble collapse can produce very high local temperatures (thousands of degrees kelvin) ,I3 and (8)Lowry, H. H. Chemistry of Coal Utilization; Wiley: New York, 1963;Chapter 6, p 272. (9)Van Krevelen, D.W. Coal; Elsevier: New York, 1981;Chapter 12, p 219. (10)Gould, R. F., Ed. Lignin Structure and Reactions; Advances in Chemistry Series 59; American Chemical Society: Washington,DC, 1966. (11)Carlin, B. Ultrasonics; McGraw-Hik New York, 1960; p 246. (12)Heuter, T.F.;Bolt, R. H. Sonies; Wiley: New York, 1955; p 229. (13)Margulis, M.A. Russ. J. Phys. Chem. (Engl. Transl.) 1967,50, 1.
Ultrasonically Enhanced Coal Extraction Table X. Extraction Yields (wt %I) and Swelling Values of Illinois No. 6 Coal as a Function of Base Cation Size in Pyridine/CH80H extraction yield base Q, mL/g mixing sonication preswollen 23 Me,NOH 4.3 24 29 EtdNOH 5.4 20 n-Pr4NOH 5.9 17 48 50 73 n-BudNOH 6.1 13 n-Hex,NOH 6.8 14 73
Energy & Fuels, Vol. 4, No. 4, 1990 351 relative ranking of the series of bases. A trend is immediately obvious. As the swelling values increase from 4.3 mL/g for the methyl case to 6.8 mL/g for the tetrahexyl case, a corresponding increase in the extraction yield is observed (from 23 to 73 wt % , respectively). These data support the contention that swelling-related conformational changes in the coal are indeed responsible for the increased disruption of the structure.
intense radial shock waves (with peak pressures from 100 atm to thousands of atmosphere^).'^ Microscopically, these high temperatures and pressures are the cause of observed chemical reactions. Cleavage of covalent bonds by sonication, whether induced by localized heating or intense shear forces, is thought to proceed via radical intermediates. This belief has been supported by studies using radical-trapping agents to measure the rate of radical production in decane15 and to increase cleavage rates in functionalized polymem2 In our work air was used as a blanketing gas, where oxygen would act as a scavenger. Molecular oxygen is known to reaction rapidly even with stabilized radicals and can intercept radical pairs. Preliminary experiments conducted under nitrogen, so that rapid radical trapping is less likely, have given lower extraction values (-61 wt ’%) than the corresponding reaction under air (-77 wt %). The compositional data for all extracts are consistent with this view. To explain the observation that high extraction values are only obtained for a highly swollen coal, we proposed that orientational effects caused by swelling and chain elongtion were operative. To further test this idea, Illinois No. 6 coal was swollen to different extents using a series of tetralkylammoniumhydroxide bases and then sonicated. Swelling values and extraction yields for sonicated and mixed samples are shown in Table X. All reactions were run at 0.25 M base in the usual pyridine/methanol solvent system. These coal/base mixtures were not preswolen overnight but were instead sonciated immediately. The ultimate extraction values would have been higher if they had been preswollen; however, the point here is to see the
Conclusion In summary, our studies have shown that significant increases in base extractability of coal are observed upon ultrasonic irradiation of base slurries at 85 W/cm2 and 23 kHz. Extraction values for three coals of different rank, Illinois No. 6 (Bituminous),Rawhide (subbituminous),and Big Brown (lignite), ranged from 65 to 77 wt ’% under the influence of ultrasound, while control experiments (stirring alone) gave substantially lower values (27-29 wt ’%), No rank-dependent trends were observed. High extraction values were only obtained for base solutions containing n-Bu,NOH, pyridine, and methanol. These solutions were also found to swell Illinois No. 6 coal to a greater extent than any other base mixture examined (e.g., KOH/ pyridine/CH,OH). Swelling is suspected to cause elongation of structural units or chains within coal and to render them, via these orientational effects, more susceptible to cleavage by shear forces. Support for this view was provided by comparing extraction yield and base cation size effects. From a structural standpoint, the extraction of 65-75 wt ’% of coal into a base solution, upon exposure to ultrasound, suggests that a significant number of cross-links in the structure have been cleaved. Whether the extracted organic material is dissolved or, in part, suspended as a dispersed colloid, the magnitude of the extraction yield represents a substantial degradation of the structure. Further evidence for molecular weight reduction is found in the overall volatility increase measured for treated samples (base/ultrasound) relative to raw Illinois No. 6 coal (44 versus 30 wt %). Preliminary studies suggest that a free-radical pathway is operative, wherein oxygen may act as a scavenger and facilitate bond cleavage.
(14)Neppiraa, E.A. Phys. Rep. 1980,61, 159. (15) Suslick, K.S.; Gawlenowski, J. J.; Schubert, P.F.;Wang, H.H. J. Phys. Chem. 1983,87, 2299.
Acknowledgment. J. W. Larsen, M. L. Gorbaty, H. L. Huffman, Jr., C. G. Scouten, D. Brenner, and R. de Coveny are all gratefully acknowledged for their contributions.
