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Energy & Fuels 1998, 12, 856-863
Reactions of Pittsburgh No. 8 Coal with Maleic Anhydride. Evidence for the Existence of Reactive Diene Structures in Coal John W. Larsen,* Deanna Metka Quay, and James E. Roberts Department of Chemistry, Lehigh University, 6 East Packer Avenue, Bethlehem, Pennsylvania 18015 Received October 28, 1997
The reaction between 13C-labeled maleic anhydride and Pittsburgh No. 8 coal has been investigated by NMR. The occurrence of the following reactions has been ruled out: ene reaction, maleic anhydride polymerization, ester formation, radical addition, electron transfer, and physical entrapment. Michael addition to -SH has been demonstrated in model systems, but does not occur with -OH groups and cannot explain the observed 10-20% (wt) incorporation of maleic anhydride. All data are consistent with the occurrence of Diels-Alder reactions. There are not enough reactive dienes in any coal structure model to yield the large maleic anhydride incorporations observed. Either current coal structures are seriously incorrect or this coal is exhibiting strongly enhanced reactivity whose origin is not understood.
Introduction This paper reports the results of a study of the reaction between Pittsburgh No. 8 coal and maleic anhydride. Our reason for doing this work was a contradiction between claims that a Diels-Alder reaction occurred between maleic anhydride and coals and published coal structures. None of the published structures contain a large enough population of reactive dienes to explain the amount of maleic anhydride incorporated if only Diels-Alder reactions were occurring. This contradiction is important for several reasons. It may indicate that accepted coal structures omit a significant population of a reactive functional group (dienes). It may be due to the occurrence of Diels-Alder reactions between maleic anhydride and normally unreactive groups. It may be that claims of a Diels-Alder reaction are incorrect, and some other reaction is occurring. Our goal was to uncover what is happening between Pittsburgh No. 8 coal and maleic anhydride. There are three primary methods for investigating coal structure. They are spectroscopy, degradation followed by product identification, and derivitization reactions. The last of these is often coupled with spectroscopy and has proved particularly powerful if the derivitizing agent is labeled with 13C.1,2 We have used the last of these techniques to study the reaction of maleic anhydride with Pittsburgh No. 8 coal. Maleic anhydride undergoes the Diels-Alder reaction with aromatics containing reactive diene structures, anthracene for example (eq 1). It is known to react with larger acenes,3,4 with naphthalene under forcing condi(1) Liotta, R.; Brons, G. J. J. Am. Chem. Soc. 1981, 103, 1735-1742. (2) Mallya, N.; Stock, L. Fuel 1986, 65, 736-738.
(1)
tions more vigorous than those used in this work,5 and with β-naphthols, the reaction driven in part by the formation of a keto product (1).6
1
We are not the first to have studied this reaction of coals. The first reported study was by Duty.7 This was followed by a series of studies by one of us, a paper by Zher’akova and Kochkan’an, and a study by Nishioka.8-12 (3) Diels, O.; Alder, K. Justus Liebigs Annalen Chemie 1928, 460, 98-122. (4) Biermann, D.; Schmidt, W. J. Am. Chem. Soc. 1980, 102, 31633181. (5) Oku, A.; Ohnishi, Y.; Mashio, F. J. Org. Chem. 1972, 37, 42644269. (6) Wariyar, N. S. Proc. Indian Acad. Sci. 1956, 43A, 231-236. (7) Duty, R. C.; Lin, H. F. Fuel 1980, 59, 546-550. (8) Larsen, J. W.; Lee, D. Fuel 1983, 62, 1351-1354. (9) Quinga, E. M. Y.; Larsen, J. W. Energy Fuels 1987, 1, 300-304. (10) Zher’akova, G.; Kochkan’an, R. Fuel 1990, 69, 898-901. (11) Nishioka, M.; Larsen, J. W. Energy Fuels 1989, 4, 100-106. (12) Nishioka, M. Energy Fuels 1991, 5, 523-525.
S0887-0624(97)00201-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/30/1998
Reactions of Pgh No. 8 Coal with Maleic Anhydride
There is agreement that coals take up between 10% and 20% of their weight in maleic anhydride. The maleic anhydride double bond disappears in the reaction, and all except Nishioka have assigned this to the occurrence of Diels-Alder reactions. Nishioka invoked charge transfer complex formation despite strong theoretical13 evidence that it is impossible and the absence of any precedent for its occurrence. There is a feature of past work which demands a thorough reinvestigation of this reaction. The amount of maleic anhydride incorporated is too large to be consistent with any published coal structure if only Diels-Alder reactions are occurring. Consider the published data for Pittsburgh No. 8 coal. Nishioka’s maleic anhydride incorporation was 14.4% (wt).12 If it all reacted with anthracenes, then 31.3% of the total coal carbon would be in anthracenes and anthracenes would contain 42.9% of the coal’s aromatic carbon. The coal carbon distribution used in this calculation was that measured by Solum et al. using NMR.14 If we assume that due to the methanol extraction used by Nishioka all of the reacted maleic anhydride opened to the half ester, then 17.7% of the coal would be anthracene, 24.2% of the aromatic carbon. The anthracene-maleic anhydride adduct opened to the half ester when refluxed overnight with methanol. See Supporting Information Figures A and B. If naphthalene (unreactive under the conditions used) is unreasonably assigned as the reaction partner, the results are more reasonable: “only” 31.0% of the aromatic carbons would be naphthalene (closed maleic anhydride) or 17.5% if the anhydride opened. These results are closer to the coal structure models, but still nonsense. Either some reactions in addition to the Diels-Alder are occurring or coals contain a much higher population of reactive diene structures than previously thought possible or coal reactivity is greatly and inexplicably enhanced. This paper reports our investigation of this important topic. Our approach was to consider every possible reaction which might occur between maleic anhydride and the functional groups in two modern coal structure models, Shinn’s15 and Nomura’s.16 On the general proposition that possible reactions can be conclusively disproved but rarely conclusively proved, we have carefully considered each of the possibilities. Those reactions remaining possible at the end of this exercise are the best current explanation for the incorporation of maleic anhydride into this coal. The reactions considered include DielsAlder, ene, polymerization, esterification, radical addition, Michael addition, and electron transfer interactions. Each is addressed below in an attempt to rectify the conflict between current coal structures and the amount of maleic anhydride incorporated. Reactions of maleic anhydride with Argonne Premium Pittsburgh No. 8 coal and several model compounds were carried out. The reaction products were studied using a variety of techniques including solid-state NMR, solution NMR, diffuse reflectance Fouier transform IR, gas chroma(13) Foster, R. Organic Charge-transfer Complexes; Academic Press: New York, 1975. (14) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187-193. (15) Shin, J. H. Fuel 1984, 63, 1187-1196. (16) Nomura, M.; Matsubayashi, K.; Ida, T.; Murata, S. Fuel Process. Technol. 1992, 31, 169-179.
Energy & Fuels, Vol. 12, No. 5, 1998 857
tography, and mass spectrometry. The numbering of maleic anhydride used here is shown in structure 2.
2
The Diels-Alder Reaction. The most commonly proposed reaction for the interaction between maleic anhydride and coal7-10 is the Diels-Alder reaction, a 4 + 2 cycloaddition between a reactive diene and a dienophile.3 A Diels-Alder reaction requires a reactive diene in the coal. Published coal models15-20 do not contain enough reactive diene structures to account for the 10-20% weight increase reported when maleic anhydride reacts with the coal.7-12 2-Naphthols are also capable of undergoing a Diels-Alder reaction with maleic anhydride in molten maleic anhydride at 250 °C as reported by Wariyar,6 a temperature far in excess of the 110 °C used in this work. The large incorporations observed with coals and the high reaction temperature required for β-naphthols suggest that either (1) the maleic anhydride does not incorporate via a reaction with naphthols or (2) the coal contains more reactive groups than those that appear in current model structures or (3) something in the coal catalyzes the DielsAlder reaction. The Ene Reaction. An ene reaction with phenol (eq 2) and similar structures (eq 3) is also conceivable. It
(2)
(3)
is the reaction of an olefin possessing an allylic hydrogen (ene) with another olefin (enophile). However, ene reactions of maleic anhydride with aromatic structures are unprecedented in the literature, and ene reactions generally require higher activation energies than DielsAlder reactions.21 Polymerization. Polymerization of maleic anhydride in the coal is another possible reason for the increased weight of the solid reaction product. Maleic anhydride can be polymerized in the presence of UV or gamma radiation, in the presence of free radical initiators, with various pyridine bases, and electrochemically.23 However, maleic anhydride homopolymerizes (17) Wiser, W. H., Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1975, 20 (2), 122. (18) Given, P. G. Fuel 1960, 39, 147-153. (19) Heredy, L. A.; Wender, I. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1980, 25 (4), 38-45. (20) Solomon, P. R. New Approaches in Coal Chemistry; ACS Symposium Series 169; American Chemical Society: Washington, DC, 1981. (21) Hoffmann, H. M. R. Angew. Chem. Int. Ed. 1969, 8, 556-577. (22) Larsen, J. W.; Lee, D.; Shawver, S. E. Fuel Proc. Technol. 1986, 12, 51-62.
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with great difficulty due to steric effects. Duty et al. attempted to polymerize maleic anhydride in the presence of a catalytic amount of coal and found no solid polymeric maleic anhydride.7 Zher’akova considered the possibility of maleic anhydride-coal copolymerization and reported that it did not occur.10 Ester Formation from the Anhydride. As an anhydride, maleic anhydride might be expected to form esters with the abundant phenolic hydroxyls in coals. Larsen et al. ruled out ester formation with phenolic hydroxyl groups by hydrolyzing the coal-maleic anhydride adduct using aqueous tetraethylammonium hydroxide at room temperature.8 This procedure is known to cleave ester groups in coal.1 Acidification followed by ether extraction revealed no maleic acid by UV monitoring at 210 nm.9 They also reported that the IR doublet characteristic of the anhydride group was present in the coal-maleic anhydride adducts. Zher’akova et al. treated coal in succinic, phthalic, and acetic anhydride melts and found no reaction, further evidence that the acid anhydride functionality is not reacting with the coal.10 Radical Addition. Maleic anhydride undergoes addition reactions in the presence of free radicals.23,24 It adds rapidly to the benzylic carbons of toluene and ethyl benzene in the presence of radical initiators such as benzoyl peroxide at 100 °C (eq 4).24 It is unlikely
(4)
that the stable free radicals found in the coal could promote this type of addition via a radical chain mechanism, but this needs to be checked. This method of addition has not been previously considered. Michael Addition. Maleic anhydride is known to react with thiol groups at 65 °C in the presence of triethylenediamine via the Michael reaction (eq 5).25
(5)
Reactions with amino and phenolic groups are other possibilities. This method of incorporation has not been previously considered. While reactive sulfur and nitrogen concentrations in coals are low, there is a reasonably high concentration of oxygen, particularly in the form of phenolic hydroxyl. However, we can find no evidence in the literature for the occurrence of the Michael addition of maleic anhydride to phenols. (23) Trivedi, B. C.; Culbertson, B. M. In Maleic Anhydride; Plenum Press: New York, 1982. (24) Bickford, W. G., Fisher, G. S., Dollean, F. G., Swift, C. G. J. Am. Oil Chem. Soc. 1948, 251-254. (25) Zienty, Vineyard, Schleppnik, J. Org. Chem. 1962, 27, 31403146.
Electron Transfer. Nishioka proposed that maleic anhydride is incorporated into the coal via an electron transfer interaction rather than by the formation of covalent bonds.12 This interaction is proposed to occur when an electron from the coal is transferred to the lowest unoccupied molecular orbital (LUMO) of the maleic anhydride. Maleic anhydride is a good electron acceptor, but such a transfer from aromatic groups is unprecedented and contrary to theory.13 Physical Entrapment. Maleic anhydride could be physically entrapped in the coal matrix. This is unlikely due to the extensive extraction procedures utilized and the high solubility of maleic anhydride in the extraction solvents. Experimental Section All chemicals and reagents were purchased from Aldrich and used as received unless otherwise indicated. The maleic anhydride was obtained from Monstano and recrystallized from toluene. 13C-labeled maleic anhydride was purchased from Cambridge Isotope Laboratories and used after dilution with natural abundance maleic anhydride. Argonne Premium Pittsburgh No. 8 coal was obtained from the Argonne National Laboratory.26 All dry manipulations of the coal were performed in a nitrogen-filled drybox. During drying, vacuum ovens were filled with nitrogen prior to opening. All reactions were performed under nitrogen to minimize oxidation. Poly(maleic anhydride) was obtained from Polyscience, Inc. The coal-maleic anhydride adduct was prepared using a procedure adapted from Nishioka et al.11,12 Samples were prepared by weighing 1 g of Argonne Premium Pittsburgh No. 8 coal into a 50 mL three-neck round-bottom flask fitted with thermometer, stirrer bar, and a small distillation head. After the system was purged with nitrogen, 25-30 mL of anhydrous chlorobenzene was added. The coal was dried by distilling off some chlorobenzene to remove water. The mixture was cooled and fitted with a condenser. A known amount of maleic anhydride (between 0.01 mol (1 g) and 0.0025 mol (0.25 g)) was then added, and the stirred slurry was heated at 110 °C for 10 days under nitrogen. The slurry was filtered at room temperature into a 10 × 50 mm2 Whatman Soxhlet thimble and extracted for 3 days with methanol or acetonitrile. The thimble was dried to constant mass at 60 °C under vacuum. Percent incorporation was determined by comparing the mass of the product to the initial mass of coal. Solid-state NMR spectra of the coal samples were obtained using a General Electric GN 300 solid-state NMR spectrometer operating at 75.4 MHz for carbon utilizing a Doty Scientific, Inc. 7 mm or 5 mm probe and a radio frequency feedback control circuit.27 The spectrometer was operated with radio frequency field strengths of 50-63 kHz for carbon and protons while acquiring 1 K of complex data points. A dwell time of 15 µs, a receiver gate time of 15 µs, and a filter delay of 15.75 µs were used. The times for the recycle delays were selected after the proton and carbon spin lattice relaxation times, T1, 0.23-0.27 s for protons and 1.2-9.8 s for carbons, were determined. Relaxation delays of 2 s for cross polarization (CP) experiments and 60 s for Bloch decay experiments were employed. The data were processed using a baseline correction, exponential multiplication equivalent to 100 Hz line broadening, and one zero-fill, followed by Fourier transformation. The spectra were phased using an autophase function followed by a baseline fix and another autophase so that only minor adjustments were required. Samples were examined (26) Vorres, K. S. Energy Fuels 1990, 4, 420-426. (27) McKay, R. A.; Schaefer, J. Private Communication, Washington U.
Reactions of Pgh No. 8 Coal with Maleic Anhydride under magic angle sample spinning (MASS) and high-power proton decoupling. Sapphire rotors with Kel-F endcaps were used to minimize 13C background. Bloch decay (single pulse) experiments were used for most of the coal spectra. A cross polarization double quantum filter (CP-DQF) pulse sequence was also utilized.28-30 The CP-DQF pulse sequence was evaluated using two low molecular weight test samples. A complete 64 step phase cycle gives maximum suppression of artifacts and single quantum coherence; these experiments were run in blocks of 64 acquisitions each and signal averaged. Each set of CP-DQF experiments was paired with a normal CP protocol for reference. Experiments were completed using a 1 ms Hartmann-Hahn contact time and 40-60 s recycle delays (based on the 5-10 s 1H T ’s for the respective samples.) Spectra were processed 1 with a baseline correction, exponential multiplication equivalent to 25 Hz (model samples) or 100-150 Hz (coal) line broadening, one zero-fill, and Fourier transformation. The CPDQF spectra were processed with a baseline fix and a magnitude calculation, yielding the absolute magnitude for all peaks. This was done to simplify interpretation since the formation of the double quantum coherence adds complexity to the spectral phase. The reference CP experiments were also processed as described above. Several model compounds were synthesized using literature preparations. These included anthracene-9,10-endo-succinic anhydride,31 2-naphthol-maleic anhydride adduct,6 thiophenol-maleic anhydride adduct,25 and the ethylbenzene-maleic anhydride adduct.24 Phenyl succinic anhydride was synthesized by refluxing phenyl succinic acid with acetic anhydride overnight. Product structures were confirmed by melting point, NMR, infrared spectroscopy (IR), and gas chromatography-mass spectrometry (GC-MS). Several reactions were attempted including treating 1-naphthol and 2-naphthol with maleic anhydride at 110 °C with and without a coal catalyst6 and substituting phenol for thiophenol in the Zienty procedure.25 No products were observed using thin layer or gas chromatography. The solution NMR spectra were obtained using a Bruker AM500 MHz spectrometer operating at 125.4 MHz for carbon or a General Electric GN-300 NMR spectrometer operating at 75.4 MHz for carbon. Typical running parameters included a dwell time of 17-25 µs, filter delay of 14-24 µs, receiver gate time of 60-100 µs, 32 k or 64 k complex data points, 5002000 acquisitions, and acquisition times of 0.82-1.11 s. Line broadening equivalent to 1.0 Hz was applied before Fourier transformation. The solution NMR spectra were obtained using a Bruker AM 500 NMR spectrometer operating at 125.4 MHz for carbon. Running parameters included a dwell time of 17 µs, filter delay of 23.8 µs, receiver gate time of 100 µs, and acquisition time of 1.114 s. No line broadening was applied to these spectra. Most solution spectra were run using an attached proton test (APT) protocol.32 A single proton NMR spectrum was obtained using a Bruker AX 360 NMR spectrometer. The operating parameters were similar. Several model reactions were followed using a HewlettPackard 5880A gas chromatograph (GC) with a 15 m, 0.25 mm i.d. SPB-20 column, 20% diphenyl:80% dimethylpolysiloxane. The products were confirmed by GC-MS (Hewlett-Packard) using a 12 m, 0.2 mm i.d. HP Ultra I cross-linked methyl silicon gum capillary column. The running conditions for most samples were injector temperature 250 °C, detector temper(28) Bax, A.; Freeman, R.; Kempsell, S. J. Am. Chem. Soc, 1980, 102, 4849-4851. (29) Menger, E. M.; Vega, S.; Griffin, R. G. J. Am. Chem. Soc, 1989, 108, 2215-2218. (30) Freibolin, H. In Basic one- and Two-Dimensional NMR Spectroscopy; VCH Verlagsgesellschaft: Weinheim, 1991. (31) Bachman, W. F., Kloetzel, M. C. J. Am. Chem. Soc. 1938, 60, 481-485. (32) Patt, S. L.; Shoollery, J. N. J. Magn. Reson. 1982, 46, 535.
Energy & Fuels, Vol. 12, No. 5, 1998 859 ature 250 °C, initial temperature 100 °C for 2 min with a 10 °C/min ramp to 270 °C, and a final hold time of 15 min. The GC-MS also utilized a 2 min solvent delay. The sample injection size was 2 µL. Ab initio and semiempirical molecular orbital calculations were completed using Spartan software33 accessed via an IBM RS 6000 model 340 workstation with AIX 325 from the AFS 3.3 file system. Several basis sets were tried. The ab initio calculations used the standard Pople basis sets 6-31G*, 3-21G(*), and 6-311G**.34,35 The semiempirical calculations used PM3,36,37 MNDO,38-40 and AM141,42 algorithms.
Results and Discussion Using our standard procedure, which involves drying the coal by azeotropic removal of water and Soxhlet extraction using Whatman 33 × 94 thimbles, the maleic anhydride incorporation into Pittsburgh No. 8 coal was 17 ( 3% in numerous repetitions. Elemental analyses show no increase in chlorine content when this coal is refluxed in chlorobenzene, demonstrating that solvent is not incorporated. Changing filters or the drying procedure leads to different and often irreproducible results. Duty observed an average of 12.4% incorporation into Illinois No. 6 coal and Zher’akova observed incorporations of 13% to 27% in coals ranging from 75% C to 92% C.20 They ruled out ester formation, polymerization, and adsorption and assigned the uptake to Diels-Alder reactions. Nishioka observed incorporations of 20% and 14.4% with Pittsburgh No. 8 coal.12 Figure 1a is a 13C NMR spectrum, obtained using Bloch decay and MASS with 1H decoupling of a physical mixture of equal quantities of maleic anhydride and Pittsburgh No. 8 coal. The sharp peaks at 167 and 137 ppm are the normal resonances of the maleic anhydride carbonyl and vinyl carbons, respectively. The area under the maleic anhydride peaks is not equal to that of the coal resonances because the maleic anhydride nuclei were not fully relaxed. Figure 1b is the 13C NMR spectrum of the reaction product obtained using Pittsburgh No. 8 coal and a mixture of 50% maleic anhydride 13C labeled (99%) at C 2 and C3 and 50% natural abundance maleic anhydride. Upon reaction, C2,C3 resonance moves from 137 ppm to about 46 ppm. The peak also broadens significantly compared to the resonances from the physical mixture. This significant change in chemical shift is indicative of covalent bonding with loss of the double bond. Both sp2 hybridized carbons have reacted. The spectrum in Figure 1c is of the reaction product between Pittsburgh No. 8 coal and maleic anhydride 50% labeled with 13C in the carbonyl carbons. The carbonyl carbon chemical shift is now 172 ppm, as expected for an intact saturated anhydride, and is shifted downfield from maleic anhydride (167 ppm) due to the conversion of the neighboring sp2 carbons to (33) Spartan Software. Wavefunction, Inc.: Irvine, CA, 1993. (34) Wilson, S. Adv. Chem. Phys. 1987, 67, 439. (35) Davidson, E. R.; Feller, D. Chem. Rev. 1986, 86, 681. (36) Stewart, J. P. J. Comput. Chem. 1989, 10, 209, 221. (37) Stewart, J. P. J. Comput. Chem. 1990, 11, 543. (38) Dewar, M. J. S.; Theil, W. J. Am. Chem. Soc. 1977, 99, 4907. (39) Dewar, M. J. S.; Rzepa, H. S. J. Am. Chem. Soc. 1978, 777, 784. (40) Dewar, M. J. S.; McKee, M. L. J. Am. Chem. Soc. 1977, 99, 5231. (41) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. P. J. J. Am. Chem. Soc. 1985, 107, 3902. (42) Dewar, M. J. S.; Zoebisch, E. G. Theochem. 1988, 49, 1.
860 Energy & Fuels, Vol. 12, No. 5, 1998
Figure 1. Solid-state 13C NMR spectra for (A) a physical mixture of Pittsburgh No. 8 coal and maleic anhydride and Pittsburgh No. 8 coal reacted with (B) 13C-2, 13C3-labeled and (C) 13C1-, 13C4-labeled maleic anhydride for 10 days in chlorobenzene at 110 °C, then Soxholet extracted with methanol for 3 days and dried. Upon reaction with coal, the C2 and C3 resonances shift substantially (compare A and B), while the carbonyl lines move very little (compare A and C).
sp3. These three NMR spectra rule out most of the conceivable reactions between maleic anhydride and coal but are consistent with the occurrence of DielsAlder reactions. Electron Transfer. If the maleic anhydride was involved in electron transfer interactions as proposed by Nishioka,12 the resonances of both the vinyl and carbonyl carbons would move and the magnitude of the shifts would correlate linearly with the square of the calculated atomic orbital (AO) coefficients determined for the LUMO of maleic anhydride.43 Figure 1c is the NMR spectrum of the reaction run with the carbonyl carbons labeled. The large peak from the carbonyl carbons has only shifted by 5 ppm, while the vinyl carbons have shifted by 91 ppm. Table 1 shows the atomic orbital coefficients and electron densities (AO coefficients squared) for each of the carbons in maleic anhydride calculated using three different semiempirical methods. Atomic orbital coefficients and electron densities were also calculated using three ab initio basis sets. The electron densities calculated by all these methods are consistently about 0.09 and 0.29 for the C1, C4 and C2, C3 carbons, respectively. Using the values calculated for the electron densities of each atom and the observed change in vinyl carbon chemical shift (91 ppm), and assuming that the chemical shift change is proportional to the electron densities, the calculated change in the carbonyl chemical shift is 28 ppm. Even if our estimate is incorrect, a large change in carbonyl chemical shift is expected if electron transfer as proposed by Nishioka is occurring. There is no evidence for such a shift. Electron transfer from the coal to the (43) Mehring, M.; Spengler, J. Phys. Rev. Lett. 1984, 53, 2441-2444.
Larsen et al.
maleic anhydride as proposed by Nishioka is not occurring. Michael Addition. Michael addition reactions of maleic anhydride with either sulfur or oxygen functionalities present in the coal is another possible incorporation route. The occurrence of a Michael addition reaction between thiophenol and maleic anhydride demonstrates that reaction with sulfur could contribute to the maleic anhydride incorporation.25 The two react smoothly in dioxane at 65° using triethylenediamine base. In the product, the former maleic anhydride C2 and C3 carbons have chemical shifts of 45 and 36 ppm (see Supporting Information Figure C). These are roughly consistent with the coal adduct spectrum. The elemental analysis of Pittsburgh No. 8 coal (listed below in Table 2),26 indicates that 2.2 wt % of the coal is sulfur. Only 0.63 wt % is of organic sulfur, much of it being heteroatoms in aromatic rings.44-46 If we assume all the organic sulfur reacts with maleic anhydride, an unlikely assumption since much of it is in nonreactive forms, the incorporation of maleic anhydride into coal would be 2% (wt). If all aliphatic organic sulfur reacts, still an unlikely assumption because the aliphatic thioether and thiol forms are reported together since they cannot be distinguished in the analyses used, the amount of maleic anhydride incorporation would be about 0.7%. Reactions with sulfur nucleophiles occur, but cannot explain the observed 17% maleic anhydride incorporation. There is no evidence for the reaction of maleic anhydride with oxygen functionalities in the coal. The absence of reaction when 1-naphthol, 2-naphthol, or phenol were treated with maleic anhydride at 110 °C or using the Zienty procedure25 (substituting phenol for thiophenol) demonstrates that a Michael reaction or ester formation at hydroxyl sites in the coal is unlikely. Ester Formation. The absence of ester formation reaction between the maleic anhydride and hydroxyl groups has been noted previously by Larsen et al.8,9 and Zher’akova et al.10 The single unchanged carbonyl peak at 172 ppm in Figure 1 also demonstrates that esterification is not occurring. The failure of 1-naphthol, 2-naphthol, and phenol to react with maleic anhydride under the conditions used for the reaction with coal and the absence of any change in the carbonyl carbon chemical shift on reaction with coal confirm that ester formation is not occurring. Radical Additions. Maleic anhydride adds to alkylbenzenes in the presence of small amounts of benzoyl peroxide.24 Maleic anhydride was reacted with coal in the presence of a free radical initiator to determine if maleic anhydride was adding to the coal via a radical reaction. Dibenzyl mercury was selected as the initiator because of its high thermal stability.47 It has a halflife of 6 days at 110 °C. A control experiment was carried out to demonstrate that dibenzyl mercury did initiate the addition of maleic anhydride to benzyl carbons. The Bickford procedure for addition of maleic anhydride to ethyl benzene was carried out substituting dibenzyl mercury for the benzoyl peroxide.24 Formation (44) George, N. H.; Gorbaty, M. L.; Keleman, S. R.; Sansone, M. Energy Fuels 1991, 5, 93-97. (45) Davidson, R. M. Fuel 1994, 73, 988-1005. (46) Calkins, W. H. Fuel 1994, 73, 475-484. (47) Jackson, R. A.; O’Neill, D. W. J. Chem. Soc., Perkin Trans. II 1978, 509-511.
Reactions of Pgh No. 8 Coal with Maleic Anhydride
Energy & Fuels, Vol. 12, No. 5, 1998 861
Table 1. Atomic Orbital Coefficients for the LUMO of Maleic Anhydride Determined Using Different Semiempirical Algorithms
C1C4 C2C4
MNDO atomic orbital coefficients
MNDO electron density
AM1 atomic orbital coefficients
AM1 electron density
PM3 atomic orbital coefficients
PM3 electron density
0.3043 0.5614
0.09264 0.3152
0.3221 0.5476
0.1037 0.2993
0.3092 0.5565
0.09562 0.3097
Table 2. Average Percent Incorporation of Maleic Anhydride (MA) in Coal reactant
dibenzyl mercury + MA
air-exposed coal and MA
dibenzyl mercurya
no reagentsb
17 ( 4c
17 ( 5
18 ( 3
4(3
-0.6 + 0.6
a
Control experimentsno maleic anhydride present. b Control experimentsall reagents omitted, coal was refluxed in chlorobenzene. c Errors determined as standard deviations. Based on 2-4 repetitions.
of the adduct was confirmed using GC-MS. Dibenzyl mercury catalyzed the addition reaction in the same manner as benzoyl peroxide.24 The coal reactions were run in the usual manner with the addition of 0.3-12 wt % dibenzyl mercury and an excess of maleic anhydride (1 g per 1 g of coal) to allow for additional incorporation. If the maleic anhydride was incorporated via a radical addition mechanism, the presence of additional free radicals should result in greater incorporation. Table 2 lists the incorporation of maleic anhydride with and without dibenzyl mercury. No significant mass increase was observed when radical initiators were utilized. This work was repeated using benzoyl peroxide instead of dibenzyl mercury with no increase in incorporation after 1 day and 10 days. This demonstrates either that radical addition is not occurring or that there are a small number of sites available for this reaction. Coal forms peroxides on exposure to the air.48 The presence of surface peroxides could also lead to an increase in maleic anhydride incorporation if a radical addition mechanism was involved. Table 2 also includes data for a coal sample that was exposed to air for a month and then reacted with an excess of maleic anhydride. All of these reactions yielded the same incorporation. As a control, the coal was treated with just the dibenzyl mercury to ascertain whether it caused a significant gain or loss in the coal mass. Table 2 shows the presence of dibenzyl mercury alone causes a slight mass increase. Since the presence of free radicals initiators does not change the incorporation of the maleic anhydride into the coal, radical addition does not appear to be a significant incorporation route unless all the benzylic positions react so that radical initiators could not increase the incorporation. Using Solum’s structure data for Pittsburgh No. 8 coal, it is easy to show that complete reaction of the benzylic structures would yield much greater maleic anhydride uptakes than any yet observed.14 The easiest way to do the calculation is to assume that there is a reactive benzylic carbon on each aromatic carbon bearing an aliphatic carbon. In Pittsburgh No. 8 coal, 17 of 100 carbons bear an alkyl group. If each reacted with one maleic anhydride, the coal mass would double. There is a possibility that only a small fraction of the benzylic positions are reactive, but that is not a reasonable assumption. We conclude that (48) Liotta, R., Brons, G., Isaacs, J. Fuel 1983, 62, 781-791.
radical addition of maleic anhydride to the coal does not occur. These results are troubling. Why do the benzylic positions in coals not add maleic anhydride when both an initiator and maleic anhydride are present? Accessibility can be invoked in a “hand waving” fashion as an explanation, but is not satisfactory because it applies to all reactions, not just radical addition. This is a reaction which should occur as readily with coals as it does with model compounds. It seems not to and this contradiction is a worthy topic for further study. Ene Reaction. Several reactions can be ruled out by literature precedent. An ene reaction involving an aromatic group has no precedent in the literature. This, together with the generalization that ene reactions require higher activation energies than their corresponding Diels-Alder reactions, makes incorporation by an ene reaction unlikely.21 It is important to consider the possibility of an ene reaction because, like the Diels-Alder, it is thermally reversible and thermal reversibility of maleic anhydride addition to coals has been observed both in this laboratory and by Zher’akova.10 In addition to the lack of literature precedent, toluene, phenol, 1-naphthol, and 2-naphthol failed to give ene products when reacted with maleic anhydride under the conditions used for the reactions with coal. Polymerization. Homopolymerization of maleic anhydride was ruled out by Duty on the basis that insoluble polymer was not formed when maleic anhydride dissolved in toluene was heated in the presence of a catalytic amount of coal.7 Zher’akova also specifically ruled out maleic anhydride polymerization.10 Poly(maleic anhydride) is available commercially. It is soluble in several solvents; however, toluene and chlorobenzene are not among them. If it formed in the coal, it would not be extracted and would result in a weight increase. A solid-state Bloch decay NMR spectrum of poly(maleic anhydride) was acquired, and this molecule has a peak at about 46 ppm. (see Supporting Information Figure D). This chemical shift is similar to the 48 ppm shift observed in the coal reaction product and does not rule out this reaction. Nevertheless, it is an unlikely mode of incorporation. Maleic anhydride homopolymerization requires severe reaction conditions. The radicals present in coals are rather unreactive, those in vitrinite not even capable of initiating styrene polymerization.49 When fully swollen with 4-vinylpyridine, the Argonne coals initiate its polymerization.49 In the case of Pittsburgh No. 8 coal, the mass increase due to 4-vinylpyridine polymerization was 9.6%. The vitrinite radicals seemed to be inert, at least their concentration did not change. It is unreasonable to expect Pittsburgh No. 8 coal to be better at polymerizing maleic anhydride than 4-vinylpyridine. All the data are most consistent with no maleic anhydride polymerization. (49) Flowers, R. A., II; Gebhard, L.; Larsen, J. W.; Silbernagel, B. G. Energy Fuels 1992, 6, 455-459.
862 Energy & Fuels, Vol. 12, No. 5, 1998
Duty’s results coupled with the absence of effective initiators in the coal are the grounds on which we rule out maleic anhydride homopolymerization. It is worth noting that the concentration of free radicals in Pittsburgh No. 8 coal is about 1 radical per 2000 carbons so that addition of 1 maleic anhydride to each radical would not yield a significant weight increase. Diels-Alder Reaction. The Diels-Alder is the reaction most commonly proposed to explain the reaction of maleic anhydride with coals.7-10 There are not enough anthracenes (whether or not substituted) or other acenes in current coal structures to account for the 15-20% maleic anhydride incorporations observed.7-12,15-20 Theoretically, the number of reaction sites could be increased if 2-naphthol type structures in coal also undergo Diels-Alder reactions. The product of reaction between anthracene and maleic anhydride, anthracene-9,10-endo-succinic anhydride, has a 13C chemical shift at 46 ppm (see Supporting Information Figure A). The Diels-Alder product derived from the reaction between 2-naphthol and maleic anhydride has chemical shifts at 46 and 36 ppm (see Supporting Information Figure E). These are essentially the same chemical shifts as observed for the coal/product. 2-Naphthol undergoes Diels-Alder reactions with maleic anhydride at temperatures (240 °C) significantly higher than those used for the coal reaction.6 The reaction is carried out in molten 2-naphthol. For this reaction to occur under our conditions (110 °C), something in the coal would have to catalyze the reaction. To test this, a reaction between 1 g of 2-naphthol and 0.8 g of maleic anhydride was tried in the presence of 1 g of coal. There was no reaction between the naphthol and maleic anhydride. The 13C NMR data are consistent with the occurrence of Diels-Alder reactions between maleic anhydride and this coal. There are substantial reasons for doubting the occurrence of all of the alternative reactions. We shall therefore explore in more detail what it means for coal chemistry if the this Diels-Alder reaction is actually occurring. But first, work to define more carefully the chemical shift of the coal-maleic anhydride adduct will be described. If the NMR peaks derived form the C2, C3 peaks of the maleic anhydride could be distinguished from those of the coal, the exact chemical shift or shifts of such peak(s) could be identified. As indicated above, the asymmetric addition of maleic anhydride to 2-naphthol or thiols produces two distinct chemical shifts, one at about 46 ppm and one at 36 ppm. The anthracene derivative would have a single peak due to the symmetry of the molecule. A CP-DQF experiment was carried out in an attempt to distinguish between the chemical shifts arising from the 13C-13C spin pairs from resonances due to the coal. The CP-DQF experiment utilizes the homonuclear dipolar coupling of adjacent 13C nuclei. The 13C- , 13C -labeled carbons derived from 2 3 labeled maleic anhydride molecules are observable, and the natural abundance coal resonances are filtered out. In addition, the use of the CP-DQF experiment should ascertain if any unreacted maleic anhydride is physically entrapped within the coal matrix, as a chemical shift at about 137 ppm (the chemical shift of the C2-C3
Larsen et al.
Figure 2. Solid-state NMR spectra of 13C2-, 13C3-labeled maleic anhydride/Pittsburgh No. 8 adduct (A) cross polarization, (B) cross polarization with a 200 µs double quantum filter. The aliphatic carbon resonances in B and C confirm the double bond of maleic anhydride has reacted.
maleic anhydride carbon-carbon double bond) would arise from unreacted maleic anhydride. The CP-DQF was performed on several coal-maleic anhydride adducts under the conditions described above. A peak was observed at about 48 ppm. (See Figure 2.) This peak had obvious shoulders. No peaks were observed near 137 ppm. This demonstrates conclusively that the maleic anhydride double bond has completely reacted. Because entrapped maleic anhydride might have a long 1H T1, a 60 s recycle delay was employed to ensure that signal from trapped maleic anhydride would be seen if it was present. No peak at 137 ppm was observed. The adduct was also run at 1.5 and 2.0 kHz spin rate to determine if the shoulders were real peaks or rotational sidebands. From these spectra it appeared that the shoulders on the peak at 48 ppm are rotational sidebands. The spin rate could not be increased because of loss of signal due to the averaging of the homonuclear dipolar interaction, so absolute evidence that these were rotational sidebands is lacking. The experiment was also repeated using a natural abundance (non 13C enriched) maleic anhydride coal adduct. No signal was observed. This control experiment confirms the signal observed at 48 ppm is real and is a direct result of the specific 13C enrichment in the precursor maleic anhydride. Unfortunately, due to the presence of rotational sidebands, it is difficult to determine if the peaks present in the CP-DQF spectra are one peak with sidebands or peaks having several distinct chemical shifts. The sidebands could not be checked further because slower spinning produced more sidebands and a very complex spectrum. Faster spinning caused complete loss of the NMR signal as described above. The presence of a single peak would be indicative of a symmetrical Diels-Alder type adduct. The presence of two or more peaks would demonstrate the existence of
Reactions of Pgh No. 8 Coal with Maleic Anhydride
either several reaction products or a asymmetrical product, for example from a Diels-Alder reaction with a β-naphthol type group or an addition product. It is impossible to use these data to discern between the two possibilities. The CP-DQF experiment confirms that maleic anhydride loses its double bond in the course of the reaction. Summary and Conclusions. All reactions of which we could conceive to account for the incorporation of maleic anhydride into coal have been considered. There is no literature precedent for and model compound evidence against the occurrence of the ene, esterification, and polymerization reactions. Electron transfer has been ruled out by NMR evidence indicating that only the C2, C3 carbons of the maleic anhydride are affected by the reaction. Radical addition reactions appear unlikely since addition of free radical initiators did not increase the incorporation values. The remaining reactions are Michael reactions with sulfur or oxygen functionalities or Diels-Alder reactions. Elemental analysis indicates there is not enough sulfur in the coals to account for the amount of incorporation observed. Michael addition of phenols to maleic anhydride does not occur. A Diels-Alder reaction with 2-naphthol or naphthalene groups is unlikely due to the high temperatures required for these reactions. This leaves the Diels-Alder reaction with reactive dienes, for example anthracene or other acenes.9 Current coal structures do not contain enough anthracene or acene structures to account for the incorporation observed. These data constitute a direct challenge to published coal structures or our understanding of coal reactivity. We first considered whether reactive diene containing structures could exist in coals and have been missed by all previous structure studies. We believe the answer is yes. Consider tetracene as an example. We are not proposing that this structure occurs in coals, but are using it as a concrete example to investigate whether molecules such as this, containing highly reactive conjugated diene structures, could have been detected in studies of coal structure. Spectroscopically, the 13C NMR techniques and IR spectra would not uniquely identify its presence. Its occurrence in coals is consistent with published NMR and IR spectra. It is highly reactive and is readily oxidized.50 It would not survive oxidative degradation and be identified as such. Under the vigorous reductive conditions so far used to probe coal structure, it would rapidly form coke.15 It would coke readily on coal pyrolysis. This molecule is so reactive that it would not have been captured by any of the degradation schemes of which we know, but it would react with maleic anhydride. We do not claim that tetracene is present in coals, but use it as an example. Our argument is that reactive diene structures could exist in coals and could have been missed by most degradation schemes and not uniquely identified spectroscopically. This provides one explanation for the large amounts of maleic anhydride incorporated. If this is the case, then all published coal structures are in error. A problem with this argument is the large amount of diene structures required to explain the (50) Hayatsu, R.; Scott, R. G.; Winans, R.E. In Oxidation in Organic Chemistry; Academic Press: New York, 1982; pp 279-354.
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observed levels of reaction with maleic anhydride. It would be missing the elephant in the haystack. The second possibility is that coal structures are more-or-less correct and that groups in coal have strongly enhanced reactivity. There are several factors which might enhance coal reactivity. The easiest to deal with are steric effects. It is known that native coals are strained. Warming Pittsburgh No. 8 coal in refluxing chlorobenzene takes about two weeks to relax the strain.51 It is conceivable that this structural strain distorts groups in the coal raising their energy and therefore their reactivity. There are no estimates of the stored strain energy. Catalysis by mineral matter is conceivable but difficult to envisage in this high vitrinite low mineral matter coal. Diels-Alder reactions are strongly catalyzed by one-electron oxidation of the ene to a radical cation.52 There is no evidence for groups capable of this difficult oxidation in coals. There is the possibility that several reactions are occurring. Could the net incorporation be due to some Diels-Alder and some radical addition and some Michael addition, etc., all reactions involving the maleic anhydride C ) C? Given the broad NMR peaks and the range of product chemical shifts, such a product mixture might not be identifiable by NMR. The evidence against this is strong, but not conclusive. At least some of the reaction of coals with maleic anhydride is thermally reversible. This clearly implicates the Diels-Alder reaction as a significant participant. The evidence presented here rules out other contributors with varying levels of uncertainty as to the extent to which they might contribute. Except for radical addition reactions, a significant contribution from any of the other reactions is highly unlikely. The observed behavior of this coal under radical addition conditions leaves some uncertainty about its occurrence, but the mystery here is why it does not occur. It is not reasonable that only a small portion (almost 1 out of 4) of the benzylic sites should be capable of undergoing a radical addition reaction. The data presented here are a direct challenge to claims that we have progressed beyond a rudimentary understanding of coal structure/reactivity. Occurrence of Diels-Alder reactions is the best explanation for all of the data available on the reaction between maleic anhydride and coals. If this reaction is occurring, there is a major problem: either coal structures are significantly incorrect or coal reactivity is significantly enhanced in a way not yet characterized. A full understanding of the reaction between maleic anhydride and coals will provide significant insights into coal structure and/or reactivity. Acknowledgment. We are grateful to the U.S. Department of Energy for partial support of this work. EF970201X (51) Larsen, J. W.; Flowers, R. A., II; Hall, P. J.; Carlson, G. Energy Fuels 1997, 11, 998-1002. (52) Bauld, W. L.; Bellville, D. J.; Harirchian, B.; Lorenz, K. T.; Pabon, R. A., Jr.; Reynolds, D. W.; Wirth, D. D.; Chiou, H.-S.; Marsh, B. K. Acc. Chem. Res. 1987, 20, 371-378.
