Energy & Fuels 1987,1, 187-193
187
New Chemical Structural Features of Coal. Reactions of Components of a Texas Lignite Ben M. Benjamin,* Emily C. Douglas, and Edward W. Hagaman Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 Received August 18, 1986. Revised Manuscript Received October 30, 1986
The origin of certain products observed after acid-catalyzed transalkylation of a Texas lignite is discussed. Several homologous series of aliphatic-substituted aromatics are formed from components of the lignite. It is shown that the components which give rise to these compounds are long-chain aliphatic carboxylic acids that react with solvent under the experimental conditions. In the presence of the strong acid, protonation-deprotonation, hydride transfers, and hydrogen migrations occur in the primary products, leading to secondary reactions and ring closures. As a result, series of alkyltoluenes, ditolylalkanes, methylalkyltetralins, and methylalkylnaphthalenesare formed. In separate experiments it is also shown that the carboxylic acids in the lignite undergo decarboxylation reactions during liquefaction and their fragments combine with the donor solvent, in this case, tetralin. The end products of these reactions are series of aliphatic-substituted tetralins and aliphatic-substituted naphthalenes that bear an incidental relationship to the products of transalkylation.
Introduction have demonstrated the utility of the Recent transalkylation reaction for determining certain chemical structural units in coal. According to the currently accepted mechanism of the reaction, aliphatic-aromatic C-C bonds are cleaved in the presence of an acid catalyst, and the aliphatic fragments are captured by a suitable acceptor. Aspects of the mechanism are discussed in the earlier papers. As applied here, the acid catalyst is trifluoromethanesulfonic acid (TFMSA) and the acceptor is toluene, which is added in large excess. The final product mixture resulting from the reaction of TFMSA, coal, and toluene consists, in part, of a solution of numerous toluene derivatives that can be identified by capillary GC-MS. Typical product lists were given in earlier publications.1,2 It was shown that the technique can be used to compare structural features of different coals2as well as structural features of coal derivative^.^ Consider the particular example in which a comparison was made between the number of excisable groups in the pyridine-soluble and -insoluble parts of Ill. No. 6 and Ky. No. 9 coals.5 It was quite clear that the pyridine-soluble parts of both coals contained the larger amounts of pendant alkyl groups and methylene connecting groups. In contrast, the pyridineinsoluble parts contained more methine cross-linking groups. Whereas, quantitatively, structural aspects of the two parts of these coals were distinguishable, the qualitative aspects of the two parts were quite similar. That is, the same kinds of substituent groups appears in both parts of the coals. On the basis of available data, it was concluded that the two parts of the coals differed mainly in numbers of substituents and in the sizes and degree of cross-linking of the molecular clusters. (1)Benjamin, B.M.;Douglas, E. C.; Canonico, D. M. Fuel 1984,63, 888. (2)
Benjamin, B. M.; Douglas, E. C.; Hershberger, P. M.; Gohdes, J. W. Fuel 1985, 64, 1340. (3)Farcasiu, M.;Forbes, T. F.; LaPierre, R. B. Prepr.-Am. Chem. Soc., Diu. Pet. Chem., 1983, 28, 279. (4)Farcasiu, M.;Scott, E. S. Y.; Lapierre, R. B. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1985,30, 427. ( 5 ) Benjamin, B. M.; Douglas, E. C. Fuel, in press.
0887-0624/87/2501-Ol87$01.50/0
Table I. Partial Ultimate Analyses of Texas Lignite Fractions anal., % fraction parent coal py sol py insol hex sol to1 sol TIPS C 51.19 72.26 35.35 80.98 76.19 68.46 H 3.89 8.03 3.38 12.08 9.31 6.69 S 2.45 3.55 45.39 0.16 2.28 4.58 ash 26.39
If data acquired by transalkylation of fractions of Ill. No. 6 and Ky. No. 9 coals seem somewhat pedestrian by virtue of their similarities, the same statement can not be made about current studies with a Texas lignite. Extensive work with this coal has generated some interesting findings. Accordingly, this paper is concerned with structural and compositional aspects of Texas lignite and its pyridine-, toluene-, and hexane-soluble fractions. This paper also describes relevant chemical reactions of carboxylic acid componentsof the Texas lignite under both transalkylation and thermal liquefaction conditions. Experimental Section General Procedures. A sample of a Texas lignite was kindly supplied by Prof. Ralph A. Zingaro, Department of Chemistry, Texas A&M University, College Station, TX. Samples were collected from the Wilcox formation, Freestone County, Rockdale, TX, on the Alcoa properties, and are reported to be of Deltaic origin. Seam depth varied from 10 to 20 ft. Samples were sealed in plastic containers in the presence of moisture until used. Ultimate analyses were performed by Galbraith Laboratories, Knoxville, TN. Analyses for the materials used are recorded in Table I. Gas chromatographic analyses were obtained by using a Hewlett-Packard Model 5880A GC fitted with a 30-m silicabonded-phase narrow-bore column (DB-1-30N) from J&W Scientific Co. and a flame ionization detector. GC-Mass spectra were obtained on a Hewlett-Packard 5995B instrument fitted with a column similar to that described above. Spectra were interpreted by matching recorded spectra with library spectra and from known cracking patterns obtained from specially prepared compounds. Single ion chromatograms were generated by scanning the total ion chromatogram as follows: m / z 105, for alkyltoluenes; m / z 119, for a-methylalkyltoluenes; m / z 195, for ditolylalkanes; m / z = 131, for alkyltetralins, m / z 145, for alkylmethyltetralins; m / z 141, for alkylnaphthalenes; m / z 155, for alkylmethylnaphthalenes. 0 1987 American Chemical Society
188 Energy & Fuels, Vol. 1, No. 2, 1987 Table 11. Products from the Transalkylation of Fractions of Texas Lignite with Concentrations Reported in Numbers of GrouDs Der 100 Carbon Atoms in the Extract py sol py insol hex sol to1 sol TIPS compound isolated 4 cycles 4 cycles 1 cycle 1 cycle 1 cycle 0.070 0.071 0.11 0.20 0.25 ethyltoluene propyltoluene 0.089 0.035 0.18 0.050 0.062 butyltoluene 0.038 0.024 0.055 0.018 0.027 0.023 0.0049 0.018 0.0092 0.013 pent yltoluene 0.017 a 0.016 b phenyltolylmethane 0.010 ditolylmethane 0.31 0.44 0.33 0.16 0.32 0.032 0.11 0.0030 0.11 1,l-ditolylethane 0.058 0.022 0.017 0.023 0.010 0.015 1,2-ditolylethane b 0.0055 0.0085 1,l-ditolylpropane 0.0055 0.016 0.0082 0.011 a b 0.0075 1,2-ditolylpropane 0.0058 b b b 0.012 1,l-ditolylpropene a 0.013 a b b 1,l-ditolylbutane a a tritolylmethane 0.0012 0.049 0.0052 0.0089 0.037 a 0.0023 0.0079 1,1,2-tritolylethane 0.0020 0.0087 1,1,3-tritolylpropene 0.0054 0.015 a 0.0030 0.0012 0.0064 0.0050 0.0050 naphthalene methylnaphthalene 0.051 0.054 0.12 0.029 0.039 dimethyl0.049 0.032 0.061 0.047 0.035 naphthalene trimethyl0.0056 0.0071 0.017 b b naphthalene methyltolyl0.0065 0.025 0.019 0.0041 0.012 naphthalene "No mass spectral evidence for these species could be found. *These species were detected by mass spectrometry but in low concentrations in mixture with other compounds. Reagents were used as received unless otherwise stated. Their purities were confirmed by GC analysis when appropriate. Trifluoromethanesulfonic acid (TFMSA), Eastman Kodak, Lot No. A14B, was transferred to vials in a dry inert atmosphere. The vials were closed with Mininert caps and valves obtained from Supelco, Inc. Measured amounts of acid were removed from the vials with a syringe as needed. Fractionation of Coal. Raw lignite, as received, was dried in a vacuum a t room temperature. It was ground to pass a 100-mesh screen and dried again in a vacuum (0.03 mm) for 2 days a t room temperature. I t was then extracted with pyridine in a Soxhlet apparatus for 3 days. Most of the pyridine was removed from the extract on a rotary evaporator. The dark residue was washed thoroughly with water and dried as described above. Typically 25 g of coal gave 2.8 g of pyridine extract. Part of the dry extract was further fractionated by first extracting it with hexane. Twelve percent was soluble in hexane. The hexane-insoluble part was then extracted with toluene to give 10% of toluene-soluble material. The pyridine-insoluble residue from the pyridine extraction above was washed with 1 L of 5% T H F in water followed by 1 L of hot distilled water. All extracts were dried in an Abderhalden a t 70 "C before being used in reactions described below. Transalkylation Reactions. The general procedure and reaction conditions were described in detail earlier.2 The same reaction conditions are used here. For example, 4 g of the parent coal was mixed with 20 mL of toluene, and 2 mL of trifluoromethanesulfonic acid was added while stirring. The reaction mixture was stirred and heated under reflux for 48 h. The reaction was quenched with 2.5 g of sodium bicarbonate and 20 mL of water, and a measured amount of triphenylmethane in toluene was added for a GC integration reference. Solid residues were removed by filtration, and the toluene solution of products was collected for GC and GC-MS analysis. The solid residue from the reaction was washed free of bicarbonate with hot water, -200 mL, and soluble organic matter was removed by washing with 200 mL of hexane. Water and hexane washes were repeated. The residue was dried and returned for a second reaction with toluene and TFMSA. A total of four sequential reactions were run. A summary of the analytical results are recorded in Table 11. Pyridine-soluble and pyridine-insoluble parts of Texas lignite were reacted as described above. Solid reaction residues and solutions of products derived from the pyridine soluble part were
Benjamin et al. separated by centrifugation because a gelatinous mass rendered filtration impossible. Some minor losses are experienced in these operations. Only one cycle of transalkylation was run on the hexane solubles, toluene solubles, and toluene-insoluble pyridine solubles. Most of the hexane solubles and toluene solubles remained in solution after the first cycle, rendering further treatment impractical. Model Experiments under Transalkylation Conditions. Stearic acid, CH3(CHz)16COOH,2.0 g, was mixed with 10 mL of toluene and 1mL of TFMSA, and the mixture was heated under reflux for 48 h. When cool, the reaction was quenched by the addition of excess sodium bicarbonate and water. A GC integration reference was added, and the toluene solution was removed and washed free of unreacted starting material with dilute NaHC03 solution. Similar experiments were run with capric acid, CH3(CHz),COOH, and cetyl alcohol, CH3(CHZ),,OH. The toluene solutions of products were analyzed by capillary GC and GC/MS. Liquefaction Procedure. In general, the detailed procedure described by Mudamburi and Given6was used. Dry coal, 2 g, was placed in a tubing bomb reactor with 5.6 g of tetralin. The mixture was heated a t 425 "C for 1.3 h. Contents of the reactor were removed with the aid of ethyl acetate solvent, and the solids were removed by filtration and thoroughly washed with ethyl acetate. On the basis of the weight of the recovered solids, conversion was -50% based on the whole coal. After the ethyl acetate was removed by distillation, excess tetralin and naphthalene were removed at 0.5 mm pressure. T o the residue was added 1 mL of toluene, and the mixture was stirred until it was homogeneous. A large excess of hexane was then added; after 30 min the precipitated asphaltenes were separated by filtration and thoroughly washed with hexane. The weight of the dry asphaltenes was -10% of the starting coal. Most of the hexane was removed from the solubles on a rotary evaporator, and the resulting oils were analyzed by GC and GC-MS. Control experiments with added components were done in a similar manner. The coal, 2 g, was mixed with 100 mg of capric acid and 100 mg of stearic acid. A second control experiment was done using 100 mg of cetylstearate in place of the free acids. The remainder of the procedure is exactly as described above. Similar experiments were run on Ill. No. 6 coal alone and in mixture with capric acid and stearic acid. Model Experiments under Liquefaction Conditions. A mixture of 0.676 g (2.38 "01) of stearic acid, 1.310 g (7.616 mmol) of capric acid, and 4.4 g of tetralin was placed in a tubing bomb, which was closed and heated at 420 "C for 1.8 h. The tubing bomb was cooled and opened, and a GC integration reference was added. Unreacted stearic acid (-1.4 mmol) and capric acid (-4.4 mmol) were recovered with aqueous sodium bicarbonate. The organic solution was concentrated, and products were analyzed by capillary GC and GC-MS. Hydrocarbons, substituted tetralins, and substituted naphthalenes were found in the reaction mixture. CP/MAS 13C NMR. Measurements were made on a modified Varian XL-100-15spectrometer (2.35 T) equipped with an external 19Ffield-frequency lock, Nicolet NMR-80 data system, and a double-tuned single-coilprobe operating at 25.16 and 100.07 MHz for 13C and 'H, respectively. A single contact cross polarization experiment with radio frequency field strengths ycHlc = y H H l H = 48 kHz was used. Powder samples were packed in an Andrews-type rotor (0.27 cm3) machined from HP-grade boron nitride. The rotor base is faced with a ca. 0.10-mm film of epoxy resin to prevent abrasive wear of the BN. The rotor give no background signal. Coal samples were spun at 4.25 f 0.05 kHz. All coal spectra (2K FIDs) were recorded with a 1.0-ms cross-polarization contact time and a 1.0-s repetition rate.
Results and Discussion Transalkylation of the Texas Lignite and Extracts. A general description of the experimental results is given i n t h i s first p a r a g r a p h . S u b s e q u e n t paragraphs c o n t a i n m o r e detailed descriptions of specific segments of the results. Preliminary results were reported i n previous pub~
~
~~~
(6) Mudamburi, 2.; Given, P. H. Org. Geochem. 1985, 8,441.
New Chemical Structural Features of Coal l i c a t i o n ~and , ~ ~other ~ results were reported on Texas lignite, with A1Br3.H20 used as catalyst.' Some of the products are similar to those derived from coals of higher rank. In addition to those usual materials, the lignite give numerous new products not found in the other coals investigated. The new products appear in the GC of the reaction mixture as regularly spaced series of peaks superimposed on the ordinary trace. One series of peaks is very prominent and persists at long retention times. The compounds giving rise to these peaks were identified as a homologous series of straight chain alkyl substituted toluenes. A second less prominent homologous series was identified as ditolylalkanes. Accompanying these were weaker sets of peaks representing homologous series of alkylmethylnaphthalenes and alkylmethyltetralins. The same sets of new materials were obtained in higher concentrations by transalkylation of the pyridine extract of the Texas lignite, but they were absent in the pyridineinsoluble residue. The pyridine-insoluble part gave transalkylation results reminiscent of a bituminous coal. Transalkylation results were also obtained on the hexane-soluble fraction, the toluene-soluble fraction, and the toluene-insoluble-pyridine-soluble (TIPS) fraction. The latter three experiments were done in order to determine the distribution of the newer derivatives among the various coal fractions. As expected the hexane-soluble part had the highest concentration, while lesser amounts appeared in the remaining residues. Table I1 contains a summary of yields of the products referred to above as ordinary materials obtained from the various fractions of the lignite. The list consists of the same 20 derivatives characteristic of the several other coals reported earlier2 and is presented for the purpose of making comparisons. The most outstanding difference is noted in relative concentrations of tritolyImethane, which reflects the number of methine cross-links present. According to these data, the number of methines is very low in all of the extracts. In fact, methines are completely absent in the hexane- and toluene-soluble parts. In contrast the pyridine insoluble residue contains a relatively large amount, that is 0.049 methine cross-links per 100 carbon atoms in the starting sample. The amount of ditolylmethane, which reflects methylene connecting groups, is about the same in all fractions except the toluene solubles in which the methylenes are about half as abundant. The reason for the low concentration in this fraction is not apparent. Among the excised aromatics, methylnaphthalene shows the most variation. It is more concentrated in the hexane-soluble fraction. The higher yield probably results from the large numbers of alkylsubstituted methylnaphthalenes that, after formation as discussed later, can undergo alkyl transfer and thus release the unsubstituted methylnaphthalene. The product of transalkylation of the hexane extract gave a GC rich in peaks. The strongest set of peaks was generated by a homologous series of n-alkyltoluenes, R-Tol. Yields are listed in Table 111. The numbers are calculated in terms of the corresponding straight-chain carboxylic acids for reasons that will be made apparent later. R-To1 is present starting with R = ethyl through R = (CH2)&H3, and the higher molecular weight members, C-24 through (2-34,were most abundant with the even-numbered chain lengths predominating. The sum of the concentrations of the R-groups calculated as carboxylic acids is -6.5% of the weight of the hexane-soluble fraction. Therefore the methylene (-CH,-) units in the normal alkyl groups as (7) Roberts, R. H.; Sweeney, K. M. Fuel 1986, 64, 321.
Energy & Fuels, Vol. 1, No. 2, 1987 189 Table 111. Long-chain n -Alkyl Derivatives in Fractions of Texas Lignite, Reported as Concentrations of the Corresponding Carboxylic Acid length of acids in trans trans trans trans carbon chain hex sola hex solb to1 solb TIPSb py solb 4.4 13 C 13 2.0 d 14 C 1.5 5.5 6.8 d 15 C 1.4 1.7 6.8 0.83 16 C 13 1.3 1.8 0.68 17 2.8 1.0 7.8 d 3.5 18 3.1 9.0 0.6 3.8 1.0 19 0.92 1.3 8.2 5.0 1.3 20 1.6 10 0.5 3.2 0.55 21 1.1 8.2 0.6 4.4 0.73 22 2.2 13 0.9 2.9 1.0 23 1.8 1.1 15 0.75 4.3 24 9.3 26 1.8 0.90 4.7 25 5.3 1.4 13 2.3 0.60 26 36 3.8 52 1.1 9.6 27 13 1.8 14 5.6 0.50 28 157 121 12 32 2.3 29 26 20 2.1 0.52 7.2 30 192 12 126 32 2.5 31 31 19 2.6 C 5.7 32 132 7.2 82 1.2 22 33 20 15 1.3 2.5 C 34 79 53 5.6 d 14 total
714
652
64
16
178
Compounds were detected as methyl esters in the derivatized hexane soluble fraction and reported as gram of carboxylic acid X lo4 per gram of extract. *Compounds were detected as alkyltoluenes in the transalkylated extract and reported as grams of carboxylic acid X lo4 per gram of extract. 'Compounds were detected by GC but the peak was too weak to trigger integration. Compounds were identified by GC-MS in peaks containing other materials. a
represented above comprise -6% of the carbon in the hexane-soluble fraction. As seen in Table 111, the toluene-soluble fraction produced about one-tenth of the amount of n-alkyltoluenes as the hexane-solublepart, and the TIPS fraction produced one-fortieth the amount. Much smaller amounts of a-methyl-substituted alkyltoluenes were present. A potential source of the branched chain is found in aliphatic alcohols or the alcoholic components of esters. Model experiments with cetyl alcohol or cetyl stearate gave product mixtures consisting of numerous isomers of branched-chain C-16 alkyltoluenes including the a-methyl substituent. Small amounts of phenanthrene, methylphenanthrene, dimethylphenanthrene, and fluorenes were present. No substituted phenols could be found except for cresol, which was derived from solvent as indicated by several model experiments. Small peaks were present for many substituted methylnaphthalenes. The regular spacing of the peaks in the single-ion scan for m/z 155 demonstrated the nature of the homologous series. Homologues were present for chain lengths from 3 to 24 carbon atoms, but no particular member of the series predominated. The single-ion scans for mlz 145 showed the presence of a homologous series of alkyl-substituted methyltetralins starting at C-10 and ending at C-26. Other substituted methyltetralins and methylindanes were detected. Analysis of the Hexane-Soluble Fraction for Carboxylic Acids. The hexane-soluble part of the Texas lignite comprised 12% of the pyridine extract or 1.3% of the whole coal, and its physical appearance was a tan sticky solid. Only a very small portion was sufficiently volatile to pass through the GC. This part consisted mainly of a group of aliphatic hydrocarbons and traces of oxygencontaining compounds that were not identified. A fresh dry sample of hexane extract was treated with Methyl-8,*
Benjamin et al.
190 Energy & Fuels, Vol. 1, No. 2, 1987
and the GC of the solution showed strong peaks for a homologous series of methyl esters of long-chain aliphatic carboxylic acids. Structural assignments were confirmed by GC-MS. The analytical data, which are recorded in Table 111, show that the volatile components consist principally of aliphatic carboxylic acids starting with a chain length of C-24 and ending with a chain length of C-34. Furthermore, the concentration of the even-numbered carbon chains predominate over the odd-numbered carbon chains. Acids with chain lengths between 13 and 23 were barely detectable as were acids C-35 and (2-36. It is interesting that this distribution of saturated aliphatic acids is exactly that described for fatty acids commonly found in some plant waxes.g The fatty acid content of several coals has been reported.lOJ1 The sum of the acid concentrations represent 7% of the weight of the hexane-soluble fraction. Thus the CH, groups only in the acids account for about 6.5% of the carbon in the hexane-soluble fraction. This value is comparable to that found in the transalkylation work and is consistent with the 13C solid-state NMR experiment (see below). Three peaks in the GC were generated by substituted aromatic dicarboxylic acids. These compounds were not fully characterized because the mass spectra did not show a molecular ion. Other prominent aspects of the GC trace were peaks for aliphatic hydrocarbons with chain lengths from C-23 through C-33. These materials were probably derived from the corresponding acids that contained one more carbon. They are presumed to have undergone decarboxylation during coalification. Solid-state I3C NMR Spectra. The CP/MAS I3C NMR spectra of the Texas lignite and its pyridine- and hexane-soluble fractions are shown in Figure 1. The aromaticity, fa, of the native coal, 0.44, decreases to 0.34 and 0.13 in the pyridine and hexane extracts, respectively. The gross features of the aromatic carbon resonance band are similar in each of these spectra; i.e., each displays partially resolved bands for ketonic carbon (-200 ppm), carboxyl carbon (-170 ppm), and aromatic carbon (110-160 ppm) in approximately the same proportion. Hence it is possible to null the sp2carbon resonance band in difference spectra between the Texas lignite and its various solvent-extracted residues. This suggests that the sp2carbon resonance band can be used as a monitor of the soluble coal matrix contribution to the spectrum and that the compositional change responsible for the variation in fa in the extracts can be isolated in difference spectra by using an aromatic carbon resonance null criterion. The difference spectrum of the pyridine-soluble fraction minus that of the same material subsequently has been extracted with hexane and toluene, the TIPS fraction, consists of a broad band that extends throughout the chemical shift region for saturated hydrocarbons, 10-60 ppm, and a narrower, intense resonance at 30 ppm, which is attributed to the degenerate resonances of internal methylene carbons of n-alkyl moieties, (CH,),. An approximate deconvolution of the spectrum into these components can be made by assigning the area above the inflection point at one-fourth peak height to (CH,),. This partition and the aromaticity change between the TIPSand (8) Methyl-8, dimethylformamide dimethyl acetal in pyridine, is packed by Pierce Chemical Co., Rockford,IL.The reagent quantitatively esterifies carboxylic acids and is intended to be used for gc analysis. (9) Robinson, T. The Organic Constituents of Higher Plants; Burgeas: Minneapolis, MN, 1963; pp 80-81. (10) Snape, C. E.; Stokes, B. J.; Bartle, K. D. Fuel 1981, 60, 903-908. (11) Chaffee, A. L.; Perry, G. J.; Johns, R. B.; George, A. M. In Coal Structure; Gorbaty, M. L., Ouchi, K., Eds.;Advances in Chemistry Series 192; American Chemical Society: Washington, DC, 1981; pp 113-132.
ll
/I
11
*@@
ppm
O
Figure 1. Solid-state CP/MASW NMR spectra: (a) parent Texas lignite; (b) pyridine extract of Texas lignite; (c) hexane extract of Texas lignite.
hexane-soluble extracts may be used to calculate, as an initial estimate, that 24% of the carbon in the latter fraction occurs as (CH,), (f, of the TIPS fraction is used here in lieu of fa of the pyridine insoluble residue of Texas lignite, which was not determined). If the assumption is made that the fraction of observed carbon which derives from the coal matrix is ca. half the actual value and that the (CH,), component is more realistically measured, the concentration of (CH,), in the hexane soluble extract will be overestimated by a factor between 1and 2.12 Hence a final estimate of the amount of (CH,), in this fraction is 12-24 carbon atom %. Calkind3 reports as much as 10-18% (CH,), in some coals. The polymethylene resonance itself provides no clue as to the nature of the chain termination. Normal alkanes, fatty acids/esters, and aryl-alkyl ketones are all possible contributors. Hence the NMR derived value for (CH,), should be higher than the 7% value determined for the fatty acid concentration in the hexane-solublefraction. It might be expected that parallel trends in the carboxyl carbon and (CH,), concentrations between different solubility fractions codd provide supporting evidence for the assignment of some fraction of (CH,), to fatty acids. In this lignite, the carboxyl carbon resonance concentrations are larger than those required by assuming all (CH,), represent fatty acids with average chain lengths of 25. The expected trend is masked by an overall decrease in carboxyl carbon concentration between the pyridine-soluble and hexane-soluble fractions, which reflects the lower solubility of the coal matrix in hexane. ~~~~~~~
~
~
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(12) Hagaman, E. W.; Chambers, R. R., Jr.; Woody, M. C. Anal. Chem.
387-394. .~ .~~ (13) Calkins, W. H., Fuel 1984, 63, 1125.
1986.58. ~~
~
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New Chemical Structural Features of Coal
Energy & Fuels, Vol. I , No. 2, 1987 191
Scheme I. Possible Reaction Scheme Leading to Alkyltoluenes
Scheme 11. Possible Reaction Scheme Leading to Alkyltetralins and Alkylnaphthalenes"
,H
-1
2 -
CH
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- OH' ---*
R
-7
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CH3@$HR
CH3@
-8
'It is recognized that hydride shifts, the first step, to a more stable benzylic position are not favored. Details of this process are discussed in ref 15.
-3
-4
R
1'HO
CH 2 R
-5 Model Experiments under Transalkylation Conditions. Aliphatic carboxylic acids and aromatic solvents undergo Friedel-Crafts reactions when treated with trifluoromethanesulfonic acid. Ketones are readily formed. Further reactions take place in the presence of the strong acid catalyst, and the ketones are converted to an impressive assortment of derivatives. The reaction between toluene and stearic acid (C-18 chain) or capric acid (C-10 chain) serves as an example. Under the experimental conditions described, stearic acid gave a surprising variety of products. Only 35% of unreacted acid was recovered. The main product was n-octadecyltoluene, 5.6%. The expected initial reaction product, tolyl heptadecyl ketone, survived in only 2.2% yield. Other products giving the stronger peaks were methylnaphthalene, methyltetradecylnaphthalene, and ditolyloctadecane. Smaller yields of a series of alkyltoluenes, dialkyltoluenes, ditolylalkanes, methylalkylnaphthalenes, methylalkyltetralins, methylalkylindanes, and methyldialkylnaphthalenes were also found. A material balance could not be obtained because some polymer was formed. Formation of these products can be explained through standard carbonium ion chemistry, Scheme I. Ketone 1, R = (CH2)&H3, is strongly protonated by the TFMSA catalyst to give cation 2. Ion 2 can either abstract a hydride from a convenient source or alternatively can undergo nucleophilic attack by solvent toluene. Three steps follow-deprotonation, dehydroxylation, and hydride transfer-resulting in the neutral molecule 1,l-ditolyloctadecane (3). Several isomers of 3 are found in the
reaction mixture totaling about 0.83 mol % yield. Molecule 3 is very reactive and undergoes protonation and bond scission. Hydride transfer to the resulting cation gives octadecyltoluene (5). Many fragmentation reactions occurred. Fragments of the aliphatic chain reacted to produce a homologous series of alkyl toluenes, Tol-C-6 through Tol-C-14, in low yields. Fragmentation reactions in the presence of Friedel-Crafts catalysts have been reported previou~ly.'~ In addition to the alkyltoluenes, low yields of a homologous series of aliphatic-substituted methylnaphthalenes are formed, and these are accompanied by their immediate precursors, a series of aliphatic-substituted methyltetralins. Substituted methylnaphthalenes were detected with chains containing 10-35 carbon atoms, some disubstituted. T h e monosubstituted methylnaphthalene-C-14 compound was the most abundant with 0.70 mol % yield. A possible reaction scheme for the formation of this compound is presented in Scheme 11. Reactions of this type have been reported in the literature.15 Methylnaphthalenes substituted with chains C-10 through C-17 may be derived from hydrocarbon fragments that react with the methylnaphthalene that was formed earlier by alkyl transfer from a substituted molecule. Small yields of methylnaphthalenes substituted with two chains, that is, C-18 + C-11 through C-18 + '2-17, were detected. These molecules are presumed to be formed either by cyclization of disubstituted toluenes or through aliphatic fragments that transfer to a substituted methylnaphthalene. Capric acid, under the same conditions, is not as reactive; 60% of the acid is recovered. The main products are decyltoluene, 2.8 mol % , methylhexylnaphthalene, 0.83 mol %, tolylnonyl ketone, 8 mol %, and 1,l-ditolyldecane, 3.1 mol % . Other products included alkylmethylnaphthalenes, alkylmethyltetralins, dialkylmethylnaphthalenes, substituted methylindanes, and dialkyltoluenes. (14)Roberts, R. M.; Khalaf, A. A.; Greene, R. N. J . Am. Chem. SOC. 1964,86,2846. (15) Sullivan, R. F.; Edgar, C. J.; Langlois, G. E. J. Catal. 1964,3,183.
Benjamin et al.
192 Energy & Fuels, Vol. 1, No. 2, 1987
RETENTION TIME lminl
Figure 2. Single-ion chromatogram for the m / t 131 ion of oils from liquefaction of (a) Texas lignite and (b) Texas lignite mixed with capric and stearic acids. Several peaks marked with an X at retention times 21-23 min are for tetralyl dimers as explained by Given."
From these experiments it is clear that in the coal transalkylation experiments, unknown fractions of the long-chain aliphatic-substituted methylnaphthalenes, methyltetralins, and toluenes were derived from the reaction of fatty acid components with the solvent and were not part of the original coal polymeric network. Coal Liquefaction Experiments. The liquefaction experiments were performed on unfractionated lignite samples and lignite samples to which extra capric acid and stearic acid had been added. Oils fractions were separated from the reaction mixtures for GC-MS analysis. Normal aliphatic hydrocarbons were the major components. The lower molecular weight members of the series were present in extremely small concentrations while the members starting at C-21 and ending at C-33 were more abundant. The odd-numbered carbon chains predominated as would be expected from decarboxylation of the even-numbered carboxylic acids.16 Samples of coal spiked with capric acid and stearic acid gave products rich in C-15, C-16, and C-17 n-alkanes. The C-9 and smaller species were lost in the workup procedure. Single-ion scans for m/z 131 indicated an array of substituted tetralins (Figure 2). The parent lignite (Figure 2a) gave a series of regularly spaced peaks interspersed with other peaks. The lower molecular weight substituents on tetralins, C-2 through C-11, are most abundant. Higher molecular weight substituents were present, and the compounds gave regularly spaced weak peaks. The products from lignite containing added capric and stearic acids showed increased amounts of n-alkyltetralins (Figure 2b). Concentrations of tetralin-(2-7, -C-8, -C-9, -C-16, and -C-17 compounds are particulaarly enhanced, demonstrating that they are derived from the added carboxylic acids. The distribution of n-alkylnaphthalenes is displayed in Figure 3. The nature of the homologous series is similar to that observed by Given et al.6*16918719In this chromatogram, small peaks for normal alkanes are also present because the mass spectral cracking patterns of these compounds include a low-intensity m/z 141 fragment, but their presence does not interfere with interpretation. Yields of the alkylnaphthalenes are low in the oils produced from (16)Mudamburi, Z.;Given, P. H. Org. Geochem. 1986,8, 221. (17)Sundaran, S.;Given, P. H. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1983,28(5)26-39. (18)Wood, K. V.; Cooks, R. G.; Mudamburi, Z.; Given, P.H. Org. Geochem. 1984, 7, 169. (19)Shadle, L.J.; Jones, A. D.; Deno, N. C.; Given, P.H. Fuel 1986, 65,611.
RETENTION TIME lmin)
Figure 3. Single-ion chromatogram for the m/z 141 ion of oils form liquefaction of (a) Texas lignite and (b) Texas lignite mixed with capric and stearic acids.
the parent lignite (Figure 3a) but are higher in the oils produced by the lignite to which capric and stearic acids have been added (Figure 3b). A sample of Ill. No. 6 coal, which contained no aliphatic carboxylic acids, was liquefied by using tetralin solvent. The oils fraction from this experiment contained small amounts of low molecular weight aliphatic substituted, up to (2-9, tetralins and naphthalenes. On the other hand, the same coal to which capric and stearic acids had been added gave oils that contained significant amounts of tetralins and naphthalenes with substituents up to C-17. Comparable results were obtained by performing the liquefaction experiment on capric acid and stearic acid alone. It is apparent that the shorter alkyl chains are part of the coal and the longer alkyl chains result from the added carboxylic acid. Finally, the Texas lignite was liquefied in the presence of tetralin containing 7% excess carbon-13 in the l-position. The product was fractionated as before, and the oils were analyzed by GC-MS. The mass spectra of the alkyltetralins and alkylnaphthalenes in the oils were inspected to determine if the compounds contained the 13C label originally present in the solvent. Intensity ratios of the fragment ions of masses 131/132 (tetralins) and 141/142 (naphthalenes) were compared with those from experiments starting with natural-abundance tetralin. The results for heptadecyltetralin are typical. For example, the intensity of the ion with mass 132 is 26% of the mass 131 ion for the compound obtained in the experiment with I3C-enriched tetralin and 12% for the compound derived from natural-abundance tetralin. These data show that a large fraction of the alkyltetralins and alkylnaphthalenes are solvent derived. Conclusions This paper critically examined the source of reaction products common to both transalkylation and termolysis reactions on a low-rank coal, that is, homologous series of n-alkyl-substituted low molecular weight cyclic hydrocarbons. The experimental evidence shows that in the transalkylation reaction, the long-chainn-alkyl-substituted methyltetralins, methylnaphthalenes, and toluenes are derived from the reaction of the fatty acid components of the coal with the solvent and are not a part of the original coal polymeric network. This conclusion is supported by model experiments with individual aliphatic acids. Similarly, in the thermolysis reactions, the evidence shows that the n-alkyltetralins and n-alkylnaphthalenes are derived from the reactions of the fatty acids components of the coal with the solvent tetralin. Again the evidence is supported by model experiments with aliphatic acids. Further, coal
Energy & Fuels 1987,1,193-198 thermolysis in the presence of 13C-labeledtetralin shows n-alkylnaphthalenes and n-alkyltetralins contain the 13C label and, therefore, are not an inherent part of the coal network that become soluble during thermolysis. In neither reaction do we mean to imply that all of these products are derived from fatty acid/solvent precursors. Calculations from solid-state 13CNMR data show that the amount of methylene (-CH,-) tied up in long chains is greater than that trapped in the coal as fatty acid. In addition, data from Ill. No. 6 coal, which contains no fatty acids, show small amounts of short chain (up to C-9) materials are present. We cannot exclude the possibility that part of the substituted naphthalenes and tetralins are bound to the coal polymer network or are trapped as noncross-linked components of the coal. We dd caution that it is possible to draw erroneous conclusions from the observation of these products without
193
recognition of their primary source in the reactions of coals.
Acknowledgment. We thank the Division of Chemical Sciences/Office of Basic Energy Sciences, U.S.Department of Energy, who sponsored this research under Contract DE-AC05840R21400 with the Martin Marietta Energy Systems, Inc. Registry No. H3C(CH2)&02H, 57-11-4; H3C(CH2)&O2H, 334-48-5;H3C(CH2),50H,36653-82-4;C6H,CH3,108-88-3;tetralin, 119-64-2.
Supplementary Material Available: Table 4, intensities of
P
+ 1 peaks in the mass spectra of substituted tetralins and
naphthalenes from liquefaction of Texas lignite; Figure 4, single ion chromatogram for the mjz 131 ion of oils from liquefaction of (a) Illinois No. 6 coal, (b) Illinois No. 6 coal mixed with capric and stearic acids, and (c) capric and stearic acids alone (2 pages). Ordering information is given on any current masthead page.
Hydrogen-Transfer-PromotedBond Scission Initiated by Coal Fragments Donald F. McMillen,* Ripudaman Malhotra, Georgina P. Hum,? and Sou-Jen Chang* Department of Chemical Kinetics, Chemical Physics Laboratory, SRI International, Menlo Park, California 94025 Received September 12, 1986. Revised Manuscript Received November 21, 1986
We report evidence to support a new mechanism for coal liquefaction in which strong linkages (i.e., nonthermolyzable linkages such as diarylmethanes, other alkylaromatics, and diary1 ethers) are cleaved a t 400 "C as a result of hydrogen transfer from solvent-derived cyclohexadienyl radicals in a direct bimolecular step (radical hydrogen transfer, RHT). Evidence has recently been presented for the operation of this pathway in model compounds. Addition of coal samples to these model compounds markedly accelerated the cleavage of the strong central bonds. Further, this cleavage exhibits the high selectivity typical of the RHT process, suggesting that radical hydrogen transfer, if anything, is more relevant to actual liquefaction than indicated by previous pure model compound studies. The relative liquefaction efficienciesof various donor solvents is shown to be much more readily rationalized by this mechanism than by the traditional bond.scission/radical-capping liquefaction mechanism.
Introduction It has been repeatedly observed' that at liquefaction temperatures coals are very effective promoters of hydrogen-exchange reactions between solvents and coal and among solvent structures themselves. This observation is consistent with the commonly held coal liquefaction mechanism: homolysis of weak linkages in the coal structure produces radicals that can abstract hydrogen (or deuterium) from solvent structures, producing solvent radicals that can,in turn, abstract hydrogen (or deuterium) from coal structures. Hydrogen exchange would then be a natural result of the spontaneous bond scission reactions generally considered to be responsible for coal liquefaction.
* To whom correspondence should be addressed.
Chemistry Laboratory, SRI International. *Postdoctoral Research Associate. Present address: Rohm and Haas Associates, Philadelphia, PA.
0887-0624/87/2501-0193$01.50/0
We wish to report recent results that suggest the opposite: coal liquefaction is a result of hydrogen-exchange reactions, and furthermore, a substantial portion of these transfers do not involve free hydrogen atoms. We have recently suggested2that a significant portion of the molecular weight decrease that occurs during donor-solvent coal liquefaction results from a previously undocumented bimolecular transfer of hydrogen from cyclohexadienyl-type radicals (of either coal or solvent origin) to the ipso positions of linkages to aromatic systems (1) See, for example: (a) King, H.-H.;Stock, L. M. Fuel 1982,61,257. (b) Heredy, L. A.; Fugassi, P. In Coal Science; Gould, R. F., Ed.; Advances in Chemistry 55, American Chemical Society: Washington, DC, 1966; p 448. (2) (a) McMillen, D. F.; Chang, S.-J.; Nigenda, S. E.; Malhotra, R., American Chemical Society, Division of Fuel Chemistry, Preprints; 1985, 30(4),414. (b) McMillen, D. F.; Malhotra, R.; Chang, S.-J.; Nigenda, S. E.; Ogier, W. C.; Fleming, R. H., submitted for publication in F u d .
0 1987 American Chemical Society
