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Energy & Fuels 1998, 12, 512-523
Structural Evaluation of Zao Zhuang Coal Masakatsu Nomura,* Levent Artok, Satoru Murata, Akira Yamamoto, Hiroshi Hama, Hong Gao, and Koh Kidena Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565, Japan Received August 13, 1997. Revised Manuscript Received February 17, 1998
The structure of Chinese bituminous Zao Zhuang coal was carefully evaluated on the basis of information from NMR, pyrolysis, ruthenium-catalyzed oxidation reaction (RICO), and single coal particle solvent swelling methods. This coal and its SRC liquefaction fractions show good coking property so that the pyrolysis is accompanied by a lot of coke, this being less useful for its structural elucidation than the case of Japanese Akabira coal. The pretreatment of this coal by the ether bond cleavage reaction can increase volatile-comprising molecules that when recognized provide valuable information on molecular constituents of the coal. SPE/MAS 13C NMR and CP-DD (dipolar dephasing)/MAS 13C NMR techniques were used to assess carbon distribution. The RICO reaction offered information regarding aliphatic functionalities and bridge types and also suggested the presence of some types of molecular units. Solvent swelling experiments implied that this coal has, on average, relatively low cross-link density and verified the structural heterogeneity of the coal. By using the data from the analytical techniques given above, a model structure of Zao Zhuang coal consisting of one MS, one PS, and two PI structures was constructed. The proposed structure is in keeping with various aspects of its reactivity.
Introduction Due to the extremely complex structure of coal organic materials, a number of model structures published so far have been based on the concept of average molecular structure.1-5 Spiro constructed space-filling models for some of these proposed models.6 He determined that, except one,3 these models contained spatially or sterically inaccessible moieties; thus, he had to make alterations on these models to satisfy sterical requirements. On the basis of these models he also proposed a mechanism for thermal decomposition and plasticity.6 Shinn proposed a model structure of Illinois No. 6 coal based on the relatively detailed information about its average structural parameters that were provided from liquefaction processes.5 The liquefaction process has been explained at the molecular level by several researchers by referring to the above models; however, it is true for most chemists who engage in the pyrolytic study of coal that these average structures cannot explain the distribution of products obtained from pyrolysis of coal organic materials and their extracts. Through extensive studies on the pyrolysis of coals, their extracts, and coal model compounds we have begun to construct a coal model structure based on pyrolytic data coupled with the data from a nondestructive analytical method such as solid state 13C NMR.7
However, pyrolysis is accompanied by considerable amounts of coke. Due to this formation of a large amount of coke, one cannot evaluate the organic materials of coal by referring only to its pyrogram. In a previous paper,7 by assuming that the volatile fraction from the pyrolysis of coal represents the whole coal, we have constructed a model structure of Akabira coal, of course, at the same time referring to the distribution of different kinds of carbon in coal organic materials deduced from 13C NMR. We are beginning to feel that we have to examine the justification of the above assumption, because we have encountered an example for which we could construct no reasonable model for a Japanese caking coal (Miike coal) according to our proposed method.8,9 In a preliminary study, we have investigated the chemical structure of Chinese bituminous Zao Zhuang coal by means of Curie-point pyrolysis and solid state 13C NMR, having found a kind of disagreement between pyrolytic and NMR data.10 This means that volatile components from the pyrolysis cannot reflect the constituents of the whole coal. Therefore, we applied ether bond cleavage reaction with SiCl4-NaI reagent for the coal to get more volatile products during pyrolysis.11 As for the chemical structure of Zao Zhuang coal, Takanohashi et al.12 applied a liquefaction technique to degrade carbon disulfide-N-methyl-2-pyrrolidinone
(1) Given, P. H. Fuel 1960, 39, 147. (2) Wiser, W. H. NATO ASI Ser. C 1984, 124, 325. (3) Solomon, P. R. New Approaches in Coal Chemistry. Am. Chem. Soc. Symp. Ser. 1981, No. 169 , 61. (4) Heredy, L. A.; Wender, I. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1980, 25 (4), 38. (5) Shinn, J. H. Fuel 1984, 63, 1187. (6) Spiro, C. L. Fuel 1981, 60, 1121.
(7) Nomura, M.; Matsubayashi, K.; Ida, T.; Murata, S. Fuel Process. Technol. 1992, 31, 169. (8) Hama, H.; Matsubayashi, K.; Murata, S.; Nomura, M. J. Jpn. Inst. Energy 1993, 72, 467. (9) Hama, H.; Murata, S.; Nomura, M. J. Jpn. Inst. Energy 1994, 73, 177. (10) Nomura, M.; Murata, S.; Hama, H.; Yamamoto, A. Unpublished results.
S0887-0624(97)00144-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/21/1998
Structural Evaluation of Zao Zhuang Coal
(CS2-NMP) mixed solvent extract of Zao Zhuang coal to smaller recognizable fragments in an attempt to construct a unit model structure for this coal. In this paper, we present additional information regarding the chemical constituents of the coal based on the pyrolytic data of chemically transformed fractions by the ether bond cleavage reaction as pointed out above and solventrefined coal (SRC) liquefaction fractions from this coal. We have recently improved the validity of the rutheniumcatalyzed oxidation reaction (RICO) and employed this technique to identify aliphatic substituents, bridge structures, and alicyclic sites.13 CP-DD (dipolar dephasing)/MAS 13C NMR and SPE/MAS 13C NMR analyses of the coal were carried out to estimate the carbon distribution of the coal. A recently proposed microscopic method for the observation of solvent swelling behavior of single coal particles has been chosen as a probe to investigate the macromolecular nature of the coal.14 Experimental Section Samples, Reagents, and Instruments. Zao Zhuang coal, a medium-volatile bituminous coal, posesses the following elemental composition (on a daf basis): C, 86.6%; H, 5.1%; N, 1.3%; S, 1.3%; O, 5.7% (by difference). All of the reagents commercially available were used without further purification. Solvents (methylene chloride and acetonitrile) for the ether bond cleavage and RICO reaction were purified according to conventional distillation methods. The methodology for the synthesis of 2-benzylphenanthrene and 9-benzylanthracene was given elsewhere.15 CP/MAS 13C NMR measurements were conducted with a JEOL GSH20MU (50 MHz) with high-speed MAS (12 kHz). SPE/MAS 13C NMR was recorded on a Chemagnetics CMX-300 (75 MHz) with 10 kHz MAS. Sample size was ∼150 mg. Scan numbers were 1054 for CP and 512 for SPE methods. Spinning sidebands were largely suppressed in CP experiments but could be suppressed to only 100 s of PD, fa was no longer changed (e.g., at 500 s of PD, fa ) 0.82). Consequently, we found that a quantitative spectrum could be obtained with >100 s of PD. (17) Green, T. K.; Kovac, J.; Larsen, J. W. Fuel 1984, 63, 935. (18) Franz, J. A.; Garcia, R.; Linehan, J. C.; Love, G. D.; Snape, C. E. Energy Fuels 1992, 6, 598. (19) Snape, C. E.; Axelson, D. E.; Botto, R. E.; Delpuech, J. J.; Tekely, P.; Gerstein, B. C.; Pruski, M.; Maciel, G. E.; Wilson, M. A. Fuel 1989, 68, 547. (20) Muntean, J. V.; Stock, L. M. Energy Fuels 1991, 5, 765. (21) Fonseca, A.; Zeuthen, P.; Nagy, J. B. Fuel 1995, 74, 1267. (22) Maciel, G. E.; Bronnimann, C. E.; Jurkiewicz, A.; Wind, R. A.; Pan, V. H. Fuel 1991, 70, 925.
Figure 2. Stack plot of DD/NMR of Zao Zhuang coal.
Figure 1 shows the SPE/MAS 13C NMR spectrum of Zao Zhuang coal; the spectrum was separated into 10 Gaussian curves. The dipolar dephasing method was also applied to get more precise information about the distribution of aromatic carbons.23 It should be noted that we employed here the CP-DD/MAS method with a contact time of 2.5 ms. The resulting spectra and the decay of magnetization are shown in Figures 2 and 3. The decay of magnetization was divided into two decays according to the equation
M(τ) ) M0L exp(-τ/TL) + M0G exp[-0.5(τ/TG)2] (5) where M0L ) 53.0, TL ) 124.7, M0G ) 55.1, and TG ) (23) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187.
Structural Evaluation of Zao Zhuang Coal
Energy & Fuels, Vol. 12, No. 3, 1998 515 Table 2. Aliphatic Monocarboxylic Acids Recovered from RICO Products
Figure 3. Decay of carbon magnetization of aromatic carbon: (a) simulated curve according to eq 5; (b) simulated curve for Lorentzian decay; (c) simulated curve for Gaussian decay. Table 1. Distribution of Carbons in Zao Zhuang Coal carbon distributiona Ar-R CH2 CH Ar-NH bridgeAr-O Ar-S head Ar-H RO q-C CH3 SPE/MAS and curve fitting DD method
5
18
4
14
64 24
2
8
8
fa 0.82
40
a
The absolute deviations for the carbon distribution calculations are about (2%.
9.6. From Figure 3 combined with SPE/MAS results (fa ) 0.82), relative contents of tertiary aromatic carbons and quaternary aromatic carbons were calculated to be 40 and 42% of total carbons in coal organic material (COM), respectively. At a delay time of 20 µs, the peak of aromatic carbons, in which tertiary aromatic carbons had almost decayed, could be separated into three peaks: bridgehead and internal carbons (24%), aromatic carbons bearing alkyl groups (14%), and aromatic carbons bearing oxygen (4%). The latter two values agreed roughly with the value obtained by SPE/MAS 13C NMR and the curve fitting method (Table 1). Pugmire et al.23,24 reported that an average ring size could be calculated on the basis of DD/NMR data. Stock and Muntean also employed Pugmire’s method when they constructed a structural model for Argonne Pocahontas No. 3 coal.25 Thus, we tried to calculate the value of the average ring size according to their method. They used the parameter, χb, which could be calculated according to the following equation: χb ) bridgehead carbons/total aromatic carbons. We could estimate the relative amount of bridgehead carbons and total aromatic carbons as 24 and 82% of total carbons in COM, respectively, so χb is calculated to be 0.29. The value of 0.29 corresponds to tricyclic aromatic compounds such as anthracene or phenanthrene, because these compounds have the value of 0.286 () 4/14). In this study, we employed the CP-DD/MAS method; however, with this method it is possible to underestimate nonprotonated aromatic carbon. Some authors recommended the use of the SPE-DD/MAS method instead of the CP-DD/MAS method.26 We plan to analyze the coal also with this method and to discuss the results in the near future. (24) Fletcher, T. H.; Bai, S.; Pugmire, R. J.; Solum, M. S.; Wood, S.; Grant, D. M. Energy Fuels 1993, 7, 734. (25) Stock, L. M.; Muntean, J. V. Energy Fuels 1993, 7, 704. (26) Love, G. D.; Law, R. V.; Snape, C. E. Energy Fuels 1993, 7, 639.
acid
mol/100 mol of C
acid
mol/100 mol of C
CH3COOH CH3CH2COOH (CH3)2CHCOOH
2.5 0.32 0.06
CH3(CH2)2COOH C2H5CH(CH3)COOH CH3(CH2)3COOH
0.11 0.006 0.002
Functional Groups. On the basis of NMR data and analysis for OH groups, oxygenated functionalities should comprise methoxy and hydroxyl groups only. The number of OH groups was determined as 1.4 per 100 carbons. Oxygenated alkyl carbon was estimated to be 2-3% on the basis of the NMR work. Part of this carbon should belong to the methoxy group. With regard to aliphatic substituents, the RICO reaction was chosen as an analytical tool for the determination of these groups; 93.9% of the carbon content of Zao Zhuang coal was converted to CO2 and soluble products by this method. Table 2 reveals the quantities of monocarboxylic acid products recovered from the RICO reaction. The R group of these acids represents alkyl functional groups of aromatic nucleous. However, these acids could also arise from the oxidation of 1,1-diarylalkanes and corresponding 9-alkyl-substituted hydroaromatics:
The fact that formation of acetic acid is the highest in amount relative to the others indicates that R-methyl groups and other aliphatic species which are capable of yielding acetic acid are the most abundant aliphatic sites. Aliphatic side chains with carbon numbers >4 are insignificant for this coal. We have recently determined from model reactions that the efficiency of acetic acid formation from the RICO reaction of methylsubstituted aromatic carbons is ∼60-70%.13 Efficiency is even lower for the alicyclic systems because of the increased reactivity of the R-alkyl carbon to oxidation reaction. Connecting Bridges and Alicyclic Sites. Accurate determination of the bridge structures of coals is very important in order to estimate their reactivity in various conversion processes. First, regarding oxygen-containing bridge structures, part of the RO- group given in Table 1 may be considered as -CH2O- type connecting linkages of diaryl units, although its exact amount is not known, while the other part is assumed to be a methoxy group as indicated above. Thus, if we deduct the sum of the hydroxyl-substituted carbon, which was estimated by hydroxyl analysis, and -OR group, which was estimated from SPE/MAS 13C NMR analysis and assumed to be completely aromatic bonded, from the percentage of the oxygenated aryl carbon, which was estimated from the CP-DD/MAS 13C NMR method, it can be estimated that diaryl ether linkages should not be
