Energy & Fuels 1987,1, 228-230
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Figure 1. Radical concentration of 12-PAC complexes vs. coal conversion.
Figure 2. Radical concentration of I,-PAH complexes vs. structural parameter of Ci/C,.
were taken as the indexes of the electron-donor ability of the PAC. The log of the radical concentration of the I,-PAC complexes was plotted against the percentage of coal con~ersion'~ (Figure 1). For the eight PAC, including a five-membered ring and heterocyclic aromatic compounds, so far studied, a linear relation was found. The complexes associated with a relatively higher radical concentration have shown higher conversion. The deviation of individual points from the line may originate both in uncertainties of dissolution experiments and in uncertainties of measuring the radical concentration. On the other hand, little correlation was found between ionization potentialg or the highest occupied orbital energy and the extent of coal conversion. From the present knowledge of the thermal reactivity of PAC, molecules of a peri-condensed structure should be desirable for coal liquefaction solvents because, in general, values of maximum free valence as the active site in peri-type compounds are smaller than cata-type polynuclear aromatic hydrocarbons (PAH).'O In order to evaluate the condensed structure of PAH, it is convenient to use the structural parameter Ci/Ca." Ci denotes internal carbon in condensed aromatic sheets and C, is the total number of aromatic carbon atoms per molecule. The ratio can give information on the compactness of molecules. Figure 2 shows the relation between Ci/C, and the radical concentration of 1,-PAH complexes. The points lie close to a straight line. The higher the value of Ci/C, is, the higher the radical concentration of 12-PAC complex becomes. It can be concluded that the iodine doping technique as the monitor of radicals induced by the formation of an 12-PAC complex is useful for evaluation the solvent quality
of PAC in coal dissolution a t high temperature. Registry No. 12, 7553-56-2;biphenyl, 92-52-4; naphthalene, 91-20-3;anthracene, 120-12-7; phenanthrene,85-01-8;fluorene, 86-73-7;fluoranthene,206-44-0; pyrene, 129-00-0;carbazole, 8674-8; biphenyl-iodine complex, 61599-36-8;naphthalene-iodine complex, 29513-36-8; anthracene-iodine complex, 61599-32-4; phenanthrene-iodine complex, 61599-33-5;fluorene-iodine complex, 106519-99-7;fluoranthene-iodine complex, 106520-00-7; pyrene-iodine complex, 54524-64-0;carbazole-iodine complex, 76790-02-8.
(9) Lewis, 1. C.; Edstrom, T. J . Org. Chem. 1963, 28, 2050. (10) Yokono, T.; Miyazawa, K.; Sanada, Y.; Marsh H. Fuel 1979,58, 692. (11) Speight, J. G.Fuel 1970, 49, 76. (12) The eight PAC so far tested are biphenyl (l), naphthalene (2), anthracene (3), phenanthrene (4), fluorene (S),fluoranthene (6), pyrene (7), and carbazole (8), respectively. Numbers (designation) refer to the points in Figures 1 and 2. (13) The data on coal conversion were taken from the report by Davis et al.' In their studies, a coal containing 84% daf carbon was heated to 400 "C for 1 h in the presence of a 3/1 weight ratio of various compounds under an inert atmosphere. Coal conversion was measured by quinoline solubility of the products.
0887-0624/87/2501-0228$01.50/0
Tetsuro Yokono,* Naoki Takahashi, Yuzo Sanada Faculty of Engineering Hokkaido University Sapporo 060, J a p a n Received September 19, 1986 Revised Manuscript Received December 9, 1986
Solid % ' ! NMR Tracer Studies To Probe Coalification
Sir: In an earlier investigation, it was shown that synthetic coal macerals can be prepared from selected biological source materials a t relatively mild temperatures (150-200 O C ) in the presence of montmorillonite clay.' The solid I3C NMR evidence strongly suggested that the gross chemical changes associated with these simulated transformations of lignin parallel those previously identified during the initial stages of natural coal f o r m a t i ~ n . ~I ?now ~ report in this communication the synthesis and characterization (solid NMR and IR spectroscopy; MS) of ['Vllignins and their use to probe the chemistry of coalification. The [13C]ligninswere prepared by enzymatic polymerization4of coniferyl alcohols specifically enriched with the (1) Hayatsu, R.; McBeth, R. L.; Scott, R. G.; Botto, R. E.; Winans, R. E. Org. Geochem. 1984,6, 463-471. (2) Hatcher, P. G.;Breger, I. A.; Earl, W. L. Org. Geochem. 1981,3,
49-55. (3) Hatcher, P.G.;Breaer, I. A.; Szeverensi, N.; Maciel, G. E. 0r.g. Geochem. 1982,4,9-18.. (4) Kirk, T. K.;Connors, W. J.; Blearn, R. D.; Hackett, W. F.; Zeikus, J. G. Proc. Natl. Acad. Sci. U.S.A. 1983, 72, 2515-2519.
0 1987 American Chemical Society
Energy & Fuels, Vol. 1, No. 2, 1987 229
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Figure 1. CP/MAS 13C NMR spectra of 45% enriched [methoxy:l3C]1ignin (top) and [@-13C]lignin(bottom). Structures designated from left to right are dihydrobenzofuran (phenylcoumaran), @-arylether, and @,@-resin01 subunits, where Ar indicates a 4-hydroxy-3-methoxyphenylgroup. The spectra were recorded on a JEOL FX-6OQ NMR spectrometeroperating at a field of 1.4 T; pertinent operating parameten were a spectral width of 15 kHz, a 2-ms contact time, a 2-s pulse repetition time and a 50-kHz proton decoupling field.
13Cisotope a t the @-positionin the propyl side chain and a t the methoxyl carbon. The synthesis of [13C]coniferyl alcohols will be described in detail el~ewhere.~ Elemental analyses of unlabeled synthetic lignin and a softwood lignin sample are as follows: synthetic (unlabeled) - C, 61.7%; H, 5.4%; softwood lignin - C, 65.6%; H, 5.9%. The FTIR and solid-probe MS analyses of the natural and synthetic lignins also document the similarity in their structure. Mass pyrograms of the lignins are characterized by a sequential evolution of the primary products from pyrolysis. Coniferyl alcohol and coniferyl aldehyde (m/z 180 and 178, respectively) are the first peaks to appear followed by peaks due to (4-hydroxy-3methoxypheny1)acyliumion (m/z 151), dihydroxybenzenes and methoxyphenols (m/z 110,124, and 138), and lesser amounts of phenol (m/z 94) and methyl phenols (m/z 108). The evolution of alkylbenzene fragments a t higher temperatures (>400 "C) is primarily due to secondary pyrolysis reactions. The cross-polarization/magicangle spinning (CP/MAS) 13C NMR spectrum of unlabeled, synthetic lignin shows a remarkably close correspondence to that previously reported for a natural lignin ample.^ The CP/MAS 13C produces a NMR of 45% enriched [meth~xy-~~Cllignin (5) A full paper describin the preparation and characterization of [13C]coniferylalcohols and [' Cllignins will be forthcoming.
f
Figure 2. CP/MAS 13C NMR spectra of [@-13C]lignin samples coalified at 200 "C in the presence of montmorillonite clay for 10 (top) and 25 days (bottom). The spectra were recorded on a Bruker CXP-100 NMR spectrometer operating at a field of 2.3 T; pertinent operating parameters were a spectral width of 10 kHz,a 2-ms contact time, a 2-s pulse repetition time and a 67-kHz proton decoupling field.
spectrum, shown in Figure 1 (top), in w@ch the signal from the 13C-labeledmethoxyls is considerably intensified compared to the signals from carbons with the natural I3C abundance. Thus the spectrum consists of a single, relatively sharp resonance centered at 55 ppm. The CP/MAS spectrum of 45% enriched [PJ3C]lignin is also shown in Figwe 1 (bottom). Because its NMR spectrum is consickerably less complex than the unlabeled sample, it is possible to assign the major ,&carbon linkages on the basis of the 13C chemical shifts of model compounds.6 Prominent resonances can be assigned to P-aryl ether (85 ppm), P,&resinol(54 ppm) and dihydrofuran (phenylcoumaran) structures (45 ppm). The presence of coniferyl alcohol end groups is indicated by the intense resonance centered a t 128 ppm. Some details of the transformations of the [13C]lignins at the molecular level can be obtained from solid 13CNMR and MS studies of the coalified materials. Pyrolysis mass spectra for the coalified products (synthetic vitrinites) show a significantly higher proportion of phenol, methylphenol-, and alkylbenzene fragments while dihydroxybenzene structures are present in much lesser amounts. Small amounts of naphthalenes, indans and tetralins also are observed. The shift of the total ion evolution curve to higher temperature is compatible with a higher degree of cross-linking of the coalified materials. In addition, MS analysis reveals that [13C]methanol and [ 13C]dimethyl ether are formed during the coalification of [methoxy13C]lignin. This result strongly supports an ionic reaction mechanism for demethoxylation via aromatic C-0 bond (6) Kringstad, K. P.; Morck, R. Holzforschung 1983, 37, 237-244.
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Energy & Fuels 1987,1, 230-232
scission to form methanol. Methanol subsequently reacts under the acidic conditions to produce dimethyl ether. The CP/MAS spectra shown in Figure 2 for [p-'3C]lignin coalified for increasing periods of time show a progressive increase in the number of aromatic carbons at the expense of aliphatic ether structures. Increased amounts of aliphatic hydrocarbon structures are also observed. The observation that 0-aryl ether groups are cleaved more rapidly than P,@-resinolor dihydrofuran structures is compatible with an ionic reaction mechanism. At later stages of coalification (Figure 2 (bottom)) secondary hydrofuran structures are formed (65 ppm). Changes in the aromatic region are also consistent with the formation of more highly substituted aromatic and polycyclic aromatic species (130-145 ppm), including the formation of benzofuran structures (152 and 110-115 ppm). The catalytic role of smectite clays has been postulated for many natural processes, including petroleum-forming reactions, chemical transformations leading to the formation of humic acids in soils, transformations of steroid biomarkers, and, most recently, reactions related to the chemical evolution of coal^.'^^-'^ In a majority of these studies, a number of the specific catalytic performances of the clays have been evaluated and the advantage of their Brernsted acidity has been realized. The present observations suggest that coalification of lignin with clay is initiated by the heterolytic bond cleavage of labile 0-aryl ether groups forming carbonium ions in the side-chain residues. These intermediate carbocations can then react in one of the following ways: (a) proton abstraction a t neighboring carbons to produce olefinic structures; (b) internal cyclization leading to the formation of hydroaromatic systems, including dihydrofuran structures; (c) intermolecular alkylation of aromatics in nearby polymer chains to produce a more highly cross-linked macromolecule with stable carbon-carbon (rather than carbon-oxygen) linkages. Subsequent hydrogen transfer reactions between side-chain olefinic structures and hydroaromatics produce a macromolecule having aliphatic hydrocarbon linkages intertwined with aromatic structures. The net result of the overall process is the elimination of oxygen with subsequent formation of stable carbon-carbon bonds. These initial experiments demonstrate the feasibility and power of labeling techniques to explore the fundamental chemical processes leading to coalification. Similar coalification studies of lignin employing other isotopic labels (13C, 2H, and 15N)are currently in progress.
Acknowledgment. I am grateful to Robert Scott, who performed the mass spectral analyses, and to Toni Engeklemeier (Argonne Analytical Chemistry Laboratory), who carried out the mass spectral gas analyses. I also am indebted to Dr. G. Joseph Ray and Claude T. Price of Amoco Research Center, Naperville, IL, for performing solid NMR analyses of the synthetic [13C]lignins. This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, US. Department of Energy, under Contract No. W-31-109ENG-38. (7) Theng, B. K. G. The Chemistry of Clay-Organic Reactions; Wiley: New York, 1974; pp 261-291. (8) Fripiat, J. J.; Cruz-Cuplido, M. I. Annu. Rev. Earth Planet. Sci. 1974, 2, 239-256. (9) Johns, W. D. Annu. Reu. Earth Planet. Sci. 1979, 7, 183-198. (10) Corma, A.; Wojciechowski, B. W. Catal. Rev.-,%. Eng. 1985,27, 29-150. (11) Sieskind, 0.;Joly, G.; Albrecht, P. Geochim. Cosmochim. Acta 1979,43, 1675-1679.
0887-0624/87/2501-0230$01.50/0
Registry No. Lignin, 9005-53-2;lignin-P-l3C, 106542-89-6; coniferyl alcohol, 458-35-5. Robert E. Botto Chemistry Division Argonne National Laboratory Argonne, Illinois 60439 Received November 10, 1986 Revised Manuscript Received December 29, 1986
Hydrogen Bonds from a Subbituminous Coal to Sorbed Solvents. An Infrared Study Sir: Hydrogen bonding between coals and solvents has long been thought to be a dominant factor in coal-solvent interactions.lI2 Despite this, there exists no study of coal-solvent hydrogen bonding using the most direct spectroscopic probe available: changes in the hydroxyl 0-H stretching frequency on hydrogen bond f ~ r m a t i o n . ~ Here we report early results from the study by diffuse reflectance Fourier transform infrared spectroscopy4 (DRIFT) of changes in the 0-H stretching frequency of coals when hydrogen bonds are formed to basic solvents. The frequency shifts are easy to measure if deuteriated solvents are used and there is a correlation between the observed shifts and the basicity of the solvents as measured by their pK in water. A recently published5 structure for the macromolecular network of bituminous coals included a very prominent role for coal-coal hydrogen bonds, together with the claim that these bonds were disrupted by basic solvents but not by nonpolar solvents. Since these claims were based only on thermodynamic data, study of the suggested hydrogen bonds by a technique capable of directly detecting their existence and disruption is necessary. Our results support the earlier claims. There is one recent study of hydrogen bonding between a coal and pyridine that did not include studies of the hydroxyl 0-H stretch.6 This is the most common spectroscopic probe of hydrogen bonds, and there exists an enormous literature, which is fortunately well re~iewed.~-'OIR studies of the 0-H stretch in coals have been used to gain information about coal-coal hydrogen bond~.'l-'~ Figure 1 contains the DRIFT spectrum of Rawhide (1)van Krevelen, D. W. Coal; Elsevier: New York, 1981. (2) Dryden, I. G. C. In Chemistry of Coal Utilization; Lowry, H. H., Ed.; Wiley: New York, 1963. (3) Hydrogen bonding in coals has been reviewed Stenberg, V. I.; Baltisberger, R. J.; Patal, K. M.; Raman, K.; Woolsey, N. F. Coal Sci. 1983,2, 125-171. (4) Studies of coal using DRIFT spectroscopy have previously been reported and the advantages of this technique outlined Fuller, M. P.; Griffiths, P. R. Anal. Chem. 1978, 50, 1906-1910. Fuller, M. P.; Hamadeh, I. M.; Griffiths, P. R.; Lowenhaupt, D. E. Fuel 1982, 61, 529-536. Fuller, E. L., Jr.; Smyrl, N. R. Fuel 1985, 64, 1143-1150. (5) Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985, 50, 4729-4735. (6) Gethner, J. S. Appl. Spectrosc. 1985, 39, 765-777. (7) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; W. H. Freeman: San Francisco, CA, 1960. (8) Joesten, M. D.; Schaad, L. J. Hydrogen Bonding; Marcel Dekker: New York, 1974. (9) Schuster, P.; Zundel, G.; Sandorfy, C. The Hydrogen Bond; North-Holland: New York, 1976; Vol. I1 and 111. (10) Hadzi, D. Hydrogen Bonding; Pergamon Press: New York, 1959. (11)Likhtenshtein, V. I.; Popov, V. K.; Rus'yanova, N. D. Solid Fuel Chem. 1980,14, 14-18. (12) Solomon, P. R.; Carangelo, R. M. Fuel 1982, 61, 663-669. (13) Gethner, J. S. Fuel 1982, 61, 1273-1276. (14) Painter, P.; Bartges, B.; Plasczynski, D.; Plasczynski, T.; Lichtus, A.; Coleman, M. P r e p . Pap.-Am. Chem. Soc., Fuel Diu. 1986. 31(1). 65-69.
0 1987 American Chemical Society
