Energy & Fuels 1987,1, 360-362
360
a function of heating rate, being depressed with lower heating rates, indicating slow decomposition of this substance near ita melting temperature. 'H NMR (CDC13): 6 1.95 (s, 6H, Me), 6.88-7.70 (16 aromatic H). '% NMR (CDClS): S 20.9 (CH3);55.8 (C-9,9'); 119.0, 124.0, 125.7, 127.0 (aromatic CH); 140.6, 149.7 (aromatic C). The second substance to elute, 9-methyl-9,9'-bifluorene,was recrystdized from benzene/hexane; mp 170.&171.0 OC. 'H N M R
(CDC13): 6 1.98 (s,3 H, Me), 4.67 (s, l H , H-9'), 6.92-7.80 (m, 16 aromatic H). 13C NMR (CDCl,): 6 26.1 (CH,); 53.1 ((2-9); 55.3 (C-9'); 119.2,119.5,122.9,125.3,125.7,126.7,127.1, 127.2 (aromatic CH); 140.1, 141.7, 144.2, 149.9 (aromatic C).
Acknowledgment. We thank Lloyd L. Brown for ob&hg the high-resolution solution 'H and 13CNMR data reported in this paper.
Hydrogen Donor Ability (Da) and Acceptor Ability (Aa) of Coal and Pitch. 1. Coalification, Oxidation, and Carbonization Paths in the Da-Aa Diagram Tetsuro Yokono,* Naoki Takahashi, and Yuzo Sanada Faculty of Engineering, Hokkido University, Sapporo 060, Japan Received December 30, 1986. Revised Manuscript Received March 23, 1987 Characterization of coal and pitch has been carried out by using a hydrogen donor ability-hydrogen acceptor ability (Da-Aa) diagram. With this diagram, an understanding was gained of the reaction pathways of coalification, oxidation, and carbonization. This graphical representation offers unique coalification tracks. The oxidation pathway shows a characteristic trace in the Da-Aa diagram. Aa shows a minimum at the initial stage of the oxidation reaction. Moreover, solvent fractions of pitch can be clearly classified into several groups according to their different chemical compositions in the Da-Aa diagram.
Introduction I t is recognized that during carbonization and liquefaction reactions, transferable hydrogens stabilize reactive free radicals produced thermally or ~atalytically.'-~Accordingly, a quantitative evaluation of transferable hydrogens in coal, pitch, and solvents used in the coal liquefaction process is necessary to understand the chemistry of carbonization and coal liquefaction. The amount of transferable hydrogen is closely related to the hydrogen donor ability (Da) and acceptor ability (Aa) of coal and pitch. The purpose of this paper is to describe the usefulness of the graphic representation of the Da-Aa diagram for understanding the reaction pathways of coalification, oxidation, and carbonization. The effect of chemical compositions of pitch in the Da-Aa diagram is also studied. Experimental Section Method of Measurement of Da and Aa Values. In order to evaluate the Da for coal and pitch, anthracene13was used as a hydrogen acceptor, while their Aa values were assessed with 9,lO-dihydroanthracene (9,10-DHA)I4 as a hydrogen donor. Anthracene or 9,lGDHA and the sample were mixed (weight ratio 1:l) and heat-treated a t 673 K for 5 min, and the resulting specimen was dissolved in CDC13 and examined by 'H NMR. Da was estimated a t 3.9 ppm (I3.&due to the the intensity of the signals from the 9,lO-positions in 9,10-dihydroanthracene, which are produced by the transfer of hydrogen from samples. Aa was evaluated at 8.4 ppm (Z8,4) due to the hydrogen attached to the 9,lO-positions of anthracene derived from 9,lO-DHA. The justification for the experimental conditions16and chemicals used (1) Yokono, T.;Iyama, S.; Sanada, Y.; Shimokawa, S.; Yamada, E.
Fuel 1986,65, 1701.
(2) Miyazawa, K.;Yokono, T.; Sanada, Y. Carbon 1979,17, 223. (3) Yokono, T.;Kohno, T.; Sanada, Y. Fuel 1986,64,411.
for the measurement of Da and Aa has been described in previous papers."7 Samples. Twenty coals, with a wide range of rank from 68% to 92% carbon content (dry ash-free basis) were selected for the coalification study. The elemental analysis of the coals used are shown in Table I. For oxidation and carbonization reactions, three different coals, Oyubari (87.6% C), Akabira (83.0% C), and Taiheiyo (78.0% C), were extensively used. The coal samples were crushed to pass through 100 Tyler mesh and dried before use. The oxidation reaction was caried out by using an electric furnace at 423 K in air. Carbonization was done in a vertical infrared image furnace with a heating rate of 10 K/min to 873 K in a N2 flow. Coal tar pitches A C were used for characterization of chemical compositions. Pitches A-C were fractionated to hexane solubles (HS), free from polar materials (FP-HS) by means of acid and alkali treatments, and pyridine insolubles (PI) respectively, and Da and Aa values of these solvent fractions were measured.
Results and Discussion Classification of Coal and Pitch on the Da-Aa Diagram. The coalification path on the Da-Aa diagram is shown i n Figure 1. Arrows in the figure represent coalification tracks. Da values passed through a maximum at about 87% carbon content of coking coal with increase in coal rank. This result may be explained as follows: lignite coals, such as Yallourn coal, consume hydrogen mainly due to formation of species such as HzO. Therefore, lignites have relatively high Aa values. On the other hand, (4) Iyama, S.;Yokono, T.; Sanada, Y. Carbon 1986, 24, 423. (5) Yokono, T.:Uno, T.:Obara, T.: Sanada, Y. Trans. Iron Steel Inst. Jpn. 1986, 26, 513. (6) Yokono, T.; Marsh, H. Coal Liauefaction Products NMR SDectroscopic Cha&terization and Proce&ei . Wilev: - New York. 1983:Vol. 1, Chapter 7. (7) Yokono, T.; Marsh, H.; Yokono, M. Fuel 1981, 60, 706.
0887-0624/87/2501-0360$01.50/00 1987 American Chemical Society
Energy & Fuels, Vol. 1, No. 4, 1987 361
Da-Aa Diagram for Coal and Pitch
coal no.a 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 a Numbers
name Yallourn Tempoku Wandoan Clutha Taiheiyo Royal Scott Hunter Valley Akabira Oyubari-1 Masco Yubari shinko Oyubari-2 Mulga South Bulli Goonyella Balmer Peak Downs Saraji Slab Fork Hongei
C
Table I. Elemental Analysis of Coals ultimate anal., wt % (daf) H N S 0 (diff)
66.0 72.7 76.6 77.6 77.8 81.2 81.8 83.0 84.6 86.5 87.0 87.1 88.0 88.0 88.6 89.5 89.9 90.8 91.7 93.0
3.9 5.1 6.3 5.1 6.0 5.1 5.3 6.4 6.4 5.2 6.3 6.1 5.6 4.9 5.1 5.5 5.0 5.2 4.6 3.5
0.5 1.6 1.1 1.8 1.1 1.4 1.5 1.8 2.1 1.4 1.8 1.6 1.7 1.5 1.7 1.3 1.9 1.8 1.2 1.2
0.2 0.5 0.4 0.6 0.2 5.7 0.4 0.1 0.3 0.6 0.2 0.3 1.1 0.4 0.5 0.3 0.6 0.6 1.0 0.4
ash
29.4 20.1 15.6 14.9 14.9 6.6 11.0 8.7 6.6 6.3 4.7 4.9 3.6 5.2 4.1 3.4 2.6 1.6 1.5 1.9
rank designations12 lignite -
0.8 4.7 8.7
subbituminous
...
bituminous
10.7 5.1 6.7 5.0 8.3 7.9 3.6 4.8 6.5 9.4 8.1 10.4 8.0 9.5 5.3 10.6
semianthracite anthracite
refer to points in Figure 1.
-0-
I
A
H
Radical
I f A
3 w rn
-df
u.
-'
0.1
1.0
21)
A-&
3.0
Aa (mgHdgcoal)
Maximum Free Valence
Figure 2. Possible reaction paths of transferable hydrogen.
Coelification path h the Da-Aa d i a g m
0.2
Figure 1. Da and Aa shifts during coalification (numbers refer to specific coals-see Table I).
the transferable hydrogen in lignites treated at 673 K for 5 min is considered to be consumed within the systems, which accounts for their low Da values. Bituminous coals have maximum values of Da, for they include many naphthene and less oxygenated functional groups? For anthracites, such as Hongei coal, Da and Aa values reach nearly zero because high-rank coals mainly contain highly condensed aromatic compounds that have few active sites for hydrogen donation or acceptance. It is useful to consider the possible reaction paths of transferable hydrogen within coal, which are shown in Figure 2. Primary, transferable hydrogens are consumed by thermally induced reactive free radicals (path 1)and oxygenated functional groups within coals (path 2). Secondary, mobile hydrogens react with aromatic carbon atoms with higher electron density (path 3). This thinking was confirmed by calculations of electron density and free valence. The former path might dominate in lignite, because lignite has so many oxygenated functional groups that most of the transferable hydrogen is trapped by functional groups and free radicals produced by cleavage of weaker bonds during Da and Aa measurements at 673 K. The deviation of point from the band may originate from the high sulfur content of Royal Scott coal. Hydrogen (8) Yoahida, T.; Nakata, Y.;Yoahida, R.; Ueda, S.; Kanda, N.; Maekawa, Y. Fuel 1982,61,824.
I
=:
I
I
0
Oyubarl c o a l
0
Akablra coal
A
TaIheIyo c o a
0 0.1
a
n
0 1.o
2.0
3.0
4.0
Aa (meHz /e-coal)
Figure 3. Da and Aa shifts after oxidation treatment (oxidized in air at 423 K for time indicated: m, minutes, h, hours; d, days).
might be consumed by sulfur-containing species. Thus, the Da-Aa diagram is found to be a useful graphical representation for understanding the coalification pathways. The effect of oxidation upon fluidity and coking properties has been well demon~trated.~J~ Even very slight (9) Khan, M. R.; Jenkins, R. G. Fuel 1979, 65, 725.
Yokono et al.
362 Energy & Fuels, Vol. 1, No.4, 1987
, I
I
I
I
I
073K
ul 'c,
0 0.1
E
Y
m
- -
0
2.0
3.0
Aa
0
Aa (mg-Hdg-coal)
Figure 4. Da and Aa shifts according to carbonization tem-
perature.
oxidation reduces coking capacity, which is significant in industry. Da-Aa shifts after oxidation of three different ranks of coal are shown in Figure 3. In this diagram, each kind of coal has a minimum value of Aa. On the other hand, Da values decreased monotonically with increase in oxidation time. A possible mechanism of oxidation reaction proposed by Van Krevelen is as fo1lows:'l
From this scheme, hydrogen atoms are removed and a quinone type structure is produced in the initial stage of oxidation. The second stage of oxidation is characterized by heterocyclic oxygen. Part of the structure is opened up by heterocyclic oxygen and converted to carboxyl and hydroxyl groups in the third stage. In order to understand the existence of the Aa minimum during the oxidation reaction, an experiment using model compounds was performed. 8-Naphthoquinone and cumarin were selected as model compounds of quinone and lactone structures. Aa values evaluated for 8-naphthoquinone and cumarin were 3.6 and 2.8 mg of HP/g of sample, respectively. The Aa of coumarin is lower than that of 8-naphthoquinone. A change in chemical structure from quinone to cumarin in the initial stage of oxidation might explain the decrease in Aa values. Experiments on the effect of heat-treatment temperature (HTT) on coal on the Da-Aa diagram were carried (10) Lowry, H. H., Ed. Chemistry of Coal Utilization;Wiley: New York, 1945; Vol. I, p 356. (11)Van Krevelen, D. W. Coal; Elaevier: Amsterdam, 1981;Vol. 3, p 245. (12) Stach, E. Coal Petrology, 3rd ed.; Gebfider BomtrQer: Stuttgart, FRG,1982, p 45: (13) The 9,lO-positionsof anthracene are very reactive and are more easily hydrogenated than other positions on almost any other aromatic hydrocarbons. Moreover, the chemical shift of hydrogenated products of 9,10-dihydroanthracene (3.9 ppm) can be easily distinguished because this position is located far from aliphatic and aromatic protons. (14) 9,lO-DHA was selected as a hydrogen donor compound, because of ita high donor ability and the unique chemical shift of ita reaction products at the 9,lO-positions of anthracene (8.4 ppm). (16) Attempts have been made to determine what effect such variables as temperature, residence time, heating rate, etc. have. The optimum conditionefound for Da and Aa measurements were 673 K, a 5-min soak, and a heating rate of 10 K/min.
4.0
(mg-&/g-pitch)
Figure 5. Da and Aa shifts with extraction: pitch B; (m) pitch C.
( 0 )pitch
A; (A)
out for three coals, Taiheiyo, Akabira, and Oyubari. Da and Aa shifts according to Carbonization temperature are shown in the schematic diagram of Figure 4. Arrows indicate the increase of carbonization in HTT. For subbituminous, Taiheiyo coal, the Da and Aa values decrease steadily with increase of HTT. On the other hand, for the medium-volatile bituminous coal of Oyubari, the Da value decreases remarkably with increase of HTT, whereas the Aa value does not show significant change up to a HTT of 693 K, and thereafter it decreases with the increase of HTT. The graph of the high-volatile bituminous coal, Akabira, resembles that of subbituminous coal but it sits between medium-volatile bituminous and subbituminous coals. The decrease of Aa by heat treatment may be attributed to the removal of low molecular mass compounds such as HzO, CO, and COz, which are related to oxygenated functional groups.1° The different chemistry by coal rank on the Da-Aa diagram may be due to the different chemical composition, such as naphthene (donor site) or oxygenated functional group (acceptor site) contents. The above results indicate the difference in chemical reactivity for carbonization reaction with coal rank. Solvent fractionation is one of the methods for separating pitch into several groups with similar chemical compositions. Our interest is in finding how to vary Da and Aa values of pitch fractions by solvent fractionation. The Da and Aa values of solvent fractions (HS, FP-HS, PI) and of the original stocks of three different coal tar pitches A-C are plotted in the Da-Aa diagram in Figure 5. It is apparent that same fractions from different pitches occupy similar regions on the Da-Aa diagram. The HS fraction showed the highest Da values and the order of Da values is, PI < original < HS < FP-HS. This result is in good agreement with that obtained from 'H NMR,which is the proportion of obtained naphthene structure in HS is the highest among solvent fractions tested. Also, when polar material is removed from the HS fraction, an increase in Da and a decrease in Aa are expected. This was confirmed in our experiment, as shown in Figure 5. Since PI fraction has a polycondensed structure with fewer active sites, both Da and Aa are lower than those of the original coal tar pitches. It can be concluded that this approach using the Da-Aa diagram gives us another insight into the characterization of coalification, oxidation, and carbonization reactions. The Da-Aa diagram provides the chemistry of the reactions going on at high temperature. Registry No. H2, 1333-74-0; anthracene, 120-12-7; 9,lO-dihydroanthracene, 613-31-0.
