Solvation and Hydrogenation
of Coal MILTON ORCH N AND H. H. STORCH C e n t r a l E x p e r i m e n t S t a t i o n , B u r e a u .f M i n e s , P i t t s b u r g h , f a . Experiments on the pressure extraction of coal at 400 ' C. with a variety of solvents suggest a correlation between the degree of coal liquefaction and the chemical structure of the solvent. High boiling aromatic solvents, such as naphthalene, liquefy 20 to 30q0 of the coal; a readily dehydrogenated, hydroaromatic solvent, such as Tetralin, liquefies about 50q0 of the coal; hydroaromatic compounds containing a phenolic group, such as o-cyclohexylphenol or 1,2,3,4-tetrahydro-5-hydroxynaphthalene, liquefy more than SOYO of the coal and are at present the best known
solvents for coal. The solvation process under these conditions probably involves a mild hydrogenolysis of carbon to oxygen linkages by the hydrogen available from the hydroaromatic solvent and dissociation of the coal or primary products from the coal due to hydrogen bonding of the fragments with the hydroxylated solvent. The liquefaction of coal in the presence of high pressure hydrogen proceeds equally well with an aromatic or hydroaromatic compound, but best results are achieved with vehicles containing a hydroxyl group attached to an aromatic ring.
T
Experimental mine a t Bruceton, Pa., ground so that 90% passed a 100-mesh screen. The composition of the coal on a n ('as-used" basis is shown in Table I. The petrographic analysis of this coal indicated 63% anthraxylon, 22% translucent attritus, 12% opaque attritus, and 3% fusain ( 2 ) .
HERE are numerous literature and patent references to the solution of coal by pressure extraction ( 3 ) . The most ex-
tensive early work of this character was done by the German chemists Pott and Broche, and the commercial process used in Germany is named after these two men. In this early work, Pott and Broche ( 9 ) found that the ternary mixture of Tetralin, phenol, and naphthalene in a weight ratio of 2 to 1 to 2 was the most satisfactory solvent for bituminous coal. The present work was undertaken to test the idea that the essential features of structure deemed responsible for the peculiar effectiveness of the ternary niixtuie could be combined in one pure compound. As coal solvents with the desired characteristics, o-cyclohexylphenol, I, and 1,2,3,4-tetrahydro-5-hydro~ynaphthalene, 11, were suggested.
OH
vI
I1
The features of structure these two molecules have in common are: a dicyclic hydroaromatic system, one saturated ring capable of being aromatized, and a hydroxyl group attached to an aromatic nucleus. SOLVATION EXPERIMENTS
MATERIALS. The coal used in all these experiments was Pittsburgh-bed bituminous coal from the Bureau of Mines
TABLE I. ANALYSEOF BRUCIDTON COALAND Two PETROGRAPHIC CONSTITUENTS
OF
IT-
Composition, Per Cent hloisture
Volatile matter Fixed carbon
Ash
hnthraxylon
Whole coal 2.1 36.0 54.7 7.2 5.2 75.5 1.5
1.3 38.3 59.4 1.0 5.6 82.1 1.7 0.8 8.8
1.5 9.1
Fusain 0.5 18.5 67.2 13.8 2.5 78.6 0.4 0.7 7.0
0- and p-cyclohexylphenol and o-phenylphenol were obtained from the Dow Chemical Company and were used without further purification. The 1,2,3,4-tetrahydro-5-hydroxynaphthalene was prepared by the hydrogenation of pure 1-naphthol (6). The tetrahydronaphthalene was of good commercial grade and was distilled from sodium through a short fractionating column. The methyl ether of o-cyclohexylphenol was prepared from ocyclohexylphenol by methylation with dimethyl sulfate (1). The carefully purified ether was obtained as a color-ss crystal1i:e ; solid with the following properties: bi"o271.3 ; b~4~270.8 n R 1.5350; daz 1.02966; freezing point 27.0". -4nalysis: calculated for,C,,H,,O, C, 82.05; H, 9.54; found, C, 82.12; H, 9.58. Per cent methoxyl: calculated, 16.3; found, 16.3. Dicyclohexyl was prepared by the high pressure hydrogenation of diphenyl. PROCEDURE. In all solvation experiments, 40 grams of coal and 160 grams of the indicated solvent were used. The experiments were conducted in the Bureau of Rlines 1.2-liter horizontal rotating autoclave ( 2 ) . Reaction times were not correctod for the preheating or cooling time, which was kept nearly constant in all experiments. I n all these experiments, the air in the autoclave was replaced with hydrogen at atmospheric pressure. No catalysts were added. The autoclave was then sealed and the reaction conducted a t the pressure due to the solvent vapors and any permanent gases formed, usually about 250 pounds per square inch. I n most cases, the contents of the autoclave after reaction were fairly fluid. The mixture was poured and scraped into a centrifuge bottle. When the autoclave contents were solid, a mixture of 93% Tetralin and 7% cresol was used to help effect the transfer of the contents to a centrifuge bottle. After centrifugation, the supernatant material was reserved for further investigation which will be the subject of a future report. The balance of the material in the autoclave was transferred with benzene to the insoluble residue in the centrifuge bottle. The residue was extracted exhaustively with benzene, dried, powdered, dried, and analyzed for ultimate composition. The per cent liquefaction obtained in the experiment was calculated from the formula:
% liquefaction =
1
RESTJLTS AND DISCUSSION. Table I1 lists the results of solvation experiments and shows that the vehicles can be grouped into three classes: the very good solvents, 1,2,3,4-tetrahydro-5hydroxynaphthalene and o-cyclohexylphenol; the moderately effective solvent, Tetralin; and the less effective solvents, 1385
,
residue (100 - yoash in residue) - [weight ofweight of dry, ash-free coal
loo
INDUSTRIAL AND ENGINEERING CHEMISTRY
i 386 1.6
genate anthracene. It was of considerable interest in the p instance t'o learn the extent of the hydrogen transfer froin ocyclohexylphenol t o coal. St the conclusion of one solvation experiment, the cyclohexylphenol-phenylphenol fraction \vas separated from the reaction mist,ure by distillation a t atiiio.pheric prcssure and the ultraviolet absorption spectrum of this mixture determined. The ultraviolet absorption spect,ra of pui ii o-phenylphenol and o-cyclohexylphenol were also determiiicd (Figure 1). Calculations using the optical densities of the mixture a t 274 and 246 m,u indicated that 9.9% phenylphenol \viis present in the mixture. 4 s a further rheck, calculations \ ~ a e made using optical densities at 274 and 288 nip. These calculat,ions indicated 8.2% o-phenylphenol present in the mixtuw. The fairly good agreement between the two values obtained from different portions of the spect'rum indicated that there was only a minor amount of material othei than the phenols present. The conversion of 9 % o-cyclohexylphenol to o-phenylpheuol corresponded to a hydrogen consumption of 1.4% by the dIy, ash-frec coal. This small consulliption of hydrogen was vc~Iy effective in producing liquefaction of the coal. The small quantity of hydrogen present (0.09 gram) a t 1 atmosphere in experiment BK-160 probably accounts for the greater liquefaction obtained as conipared t'o cxperiment BIC-162, in which IOU0 pounds per square inch of nitrogen pressure were usrd.
- o-phenylphenol
----
0-yclohexylpknol reaclion mixture
14
I
200
225
250
275
300
325
'
WAVELENGTH I N MILLIMICRONS
Figure 1.
Ultraviolet Absorption Spectra
dicyclohexyl, 2-methoxy-l-cyclohexylbenaene, diphenyl, naphthalene, cresol, and o-phenylphenol. This classification of solvents on the basis of their effectiveness can be correlated with the chemical structure of the solvents. The least effective group of solvents can dissolve 20 t o 30% of the coal a t 400" C. Apparently the only requirement foi this action is that the solvent be a high boiling aromatic compound or a hydroaromatic compound that is only slovily dehydrogenated under these conditions. If the solvent functions as a good hydrogen donor, its effectiveness is enhanced. Tetralin is easily dehydrogenated and accordingly gives moderately good liquefaction results. If, in addition to a hydroaromatic ring, a solvent possesses an aromatic hydroxyl group, this solvent is a n extremely effective vehicle for liquefying coal. Such solvents are o-cyclohexylphenol and 1,2,3,4-tetrahydro-5-hydroxynaphthalene. In the group of less effective solvents, the hydroaromatic compounds are generally superior to the completely aromatic compounds. The effectiveness of Tetralin and the other hydroaromatic solvents is almost certainly due to their ability to act as hydrogen donors t o the coal substance. In pule compound studies, it has been shown (6) that Tetralin can readily hydro-
TABLE 11. SOLVATIOS EXI~ISHI~IEXTS .IT 400 ' C. Experiiiient No.
BK-134 G-68, BK-100, 143, 146
BK-111
BK-113 BK-112,132 BK-156 BK-160 BK-152 BK-129 BK-141 BK-144 BK-140 BK-162 -4nthraxylon Fu-ain
Vehicle
1,2,3,4-Tetrahydro-6~iydroxyna~litlialenc o-Cycloliexylylienol o-Cyciotirxylphenol 0-Cycl o hexylp tienol 'I'ntralin Naphthalene, phenol, Tetvalin (2:l:Z) U.E.P. cresol a-~~ettiox~-l-c)cloliexylhenzene Dicycloliexyl Saphtlialene o-Phenylplienol Diphenyl U.S.P. cresol o-Cyclohexylpiienol o-Cycloliexylplienol
Lique-
Time, Hoiir. 0 .3
faction,
0 .3 1.0 1 .J 0.3 0 . .i
81.66 82.8 82.7 49.40 51 3"
0.3 0 . .i
31.1 30 2
1.0 0.3
27.2 22.2" 19.6ct
0.3
Vol. 40, No. 8
% 85.3E
0.5
19.4Q
0.5 0,5 0.3
19.2d 94.4 4.2
Transfer t o centrifuge bottles made with aid of Tetralin-rrr-ol. L Average of f o u r determinations; 81.6,82.7,81.0,a n d Sl.0Y0. C Average of t w o determinations; 50.2 arid 48.,6%. d In this experiment, 1000 lb,/sq, in. initial nitrogen grrssiire nsecl
3
? 0
~ 90~
eo
10
20
30
20
IO
TETRALIN, PERCENT 40 50 60 70 CRESOL, PERCENT
80
90
60
70
30
50
40
C
I00
Figure 2. Solvation Experiments with V-arious Proportions of Tetralin and Cresol
The effectiveness of a small quantity of hydrogen avaiia,hle from a hydrogen donor like Tetralin is illustrated by anothcr xries of solvation experiments (Figure 2). In this group o f experiments the solvent consisted of a mixture of Tetralin and cresol in varying proport,ions. In each experiment, 160 gra,ms of solvent mixture were heat,ed with 40 grams of coal for 0.5 hour a t 400' C. These results show that the addition of small quantities of cresol to Tet'ralin or t,he addit'ion of small quantit.ies of Tetralin to cresol results in higher liquefact,ion than that- obtained with the pure solvent alone. The addition of a sina.11 amount of Tetralin t o cresol appears t o have a particularly great effect., which can be ascribed t o the influence of the sma,iI but effective quantiiy of hydrogen furnished by the Tetrdin. One inight expect from these results that the mixture oE Tetialiri, naphthalene, and phenol (Table 11, BII-156) would give more than 51% liquefaction. It is probable, however, that wider these conditions the large concentration of naphthalene ahead>present in the solvent inhibits or retards the convcrsion of Tetralin to naphthalene. The effectiveness of a sinall quantity of hydrogen in producing coal liquefaction is probably due t,o the hydrogenolysis of F ~ J I ~ pec,uliarly susceptible linkages in the coal structure. Thermil treatment alone does not sever these bonds to produce a liquid
I ~
INDUSTRIAL A N D ENGINEERING CHEMISTRY
August 1948
1387
product; but if the coal is made to consume about I to 2% hydrogen, a completely liquid product can be obtained. The hydrogenolyzed bonds are very probably carbon-to-oxygen bonds. Previous work ( I O ) in this laboratory has shown the concomitance of oxygen elimination and liquefact,ion during the hydrogenation of coal. Figure 3 shows the relationship between liquefaction and oxygen elimination during solvation. The percentage oxygen eliminated is calculated as follows:
’3 oxygen - eliminated = weight of residue X % oxygen in residue loo - weight of dry, ash-free coal x % oxygen in coal
1
Inasmuch as the percentage of oxygen is always determined from the results of a n ultimate analysis by difference, the burden of all the errors of analysis falls on the oxygen value. The data for Figure 3 include all the solvation experiments reported in this papcr. The curve shows that it is possible t o liquefy about 20% of the coal under solvation conditions without removing any of the oxygen of the coal. The portion of the coal removed as soluble material is thus more hydrocarbon in nature than the original coal. Beyond about 20% liquefaction there appears to be nearly a straight-line relationship between liquefaction and oxygen removal. This behavior suggests that the hydrogenolysis of oxygen bonds and the dissociation of oxygen-containing fragments of the coal are the essential steps during this portion of the liquefaction process. , The effect of an hydroxyl group in the solvent is indeed striking. 1,2,3,4-Tetrahydronaphthalene gives about 50% liquefaction; 1,2,3,4-tetrahydro-5-hydroxynaphthalenegives 85% liquefaction. The methyl ether of cyclohexylphenol (o-methoxycyclohexylbenzene) gives 29% liquefaction; o-cyclohexylphenol gives 82% liquefaction. The effectiveness of the hydroxyl group is probably related t o its hydrogen-bonding properties. The infrared spectrum in the hydroxyl group region of o-cyclohexylphenol is shown in Figure 4. The broadness of the band at the 2.85micron (3500 cm.-l) region is due t o hydrogen bonding (8). The effectiveness of the hydrogen-bonded solvents suggests that one of the forces holding the reactive, unsaturated fragments of the coal structure together is the associative force resulting from hydrogen bonding. A hydroxylated substance would be expected to dissociate a hydrogen-bonded polymer, because the fragments of the polymer could attach themselves t o the solvent by hydrogen bonding. An alternate reason for the effectiveness of a hydrogen-bonded solvent may be that the primary liquefaction products obtained by mild hydrogcnolysis are themselves
Figure 4.
Infrared Spectra i n -OH
Bond Region
hydroxylated and hydrogen-bonded and that an excess of a similar solvent serves to keep these fragments dissociated. The relatively poor results obtained with cresol and o-phenylphenol as compared with the excellent results with cyclohexylphenol support this latter view, Both reasons probably account for the effectiveness of hydroxylated solvents. The effect of concentration of the hydroxylated solvent in a mixture used for the solution of coal is shown in Table 111. In these experiments a mixture of various concentrations of ocyclohexylphenol in the methyl ether of o-cyclohexylphenol was used as the vehicle. The results show t h a t a large excess of the hydroxylated solvent must be present to obtain good liquefaction of coal. . It was of considerable interest to study the behavior of pure petrographic constituents of coal during solvation. Samples of pure anthraxylon and pure fusain were hand-picked from Bruceton coal. The chemical analyses of these constituents is shown i n Table I, and the results obtained by treating them with ocyclohexylphenol are shown at the bottom of Table 11. The ease of liquefaction of anthraxylon and the diffidulty expeiienced with fusain correspond to the results obtained with these constituents during hydrogenation (9). The solutions of coal i n cyclohexylphenol are stable. In one experiment such a solution was kept i n air at 45” for 19 days. During this time, virtually no material precipitated. The word “solution” is used i n the sense that no material can be separated from solutions by ordinary centrifugation or filtration. Microscopic examination of these solutions at 400 magnification showed them to be substantially free of solid particles. It is conceivable that these “solutions” are colloidal dispersions (4). HYDROGENATION EXPERIMENTS
I n the first part of this paper, the suggestion was made that the effectiveness of certain solvents i s due i n part t o their ability t o act as hydrogen donors to the coal substance. Experiments were therefore conducted t o show the effect of the presence of highpressure hydrogen gas in the system with some of the same sol-
20
TABLE 111. SOLVATION EXPERIMENTS [ A t 400’ C. (0.5 hour) using various ooncentrations of o-cyclohexyl-
10
- __- - - - - - --
0
20 30 40 SO 60 TO LIQUEFACTION ( D R Y , ASH-FREE BASIS) PERCENT
IO
80
SO
Figure 3. Relation between Oxygen Elimination and Liquefaction during Solvation of Coal
phenol ( R = O H ) i n 2-methoxy-1-cyclohexylbenzene (R=O=CHa) Liquefaction, Vehicle % 100% R - O H 81.6 3870 R-OH. 62% R-OCHa 00.2 32.6 8% R-OH: 92% R-OCHI 100% R-OCHa 30.2
1
INDUSTRIAL AND ENGINEERING CHEMISTRY
1388
TABLE IT'.
Hrrmooi.;sa,rIos
EXPERIMEXTS
[ A t 400° C. a n d 1000 lb. 'sq. in. initial hydrogen pressure (abolit 2300 Ib./sq. in. a trraction temperature) using 100 grains of coal a n d 100 grainh of vehicle for 1 tioiir] Moiitiirr- a n d
1\1-34;BR-125 Tetralin
G-69 G-74
o-Cycloliexylylienol o-Cyclohexylglieno1 o-Cyclohexylphenol o-Cvclohexvluhenol p-C)clohe~.iphenol Coal hvdroeenation tar acids: Coal liydroo.enation,f Petroleum ether extract of cox1 hydrogenation oils C.R.P. cresol U.S.P. cresol U.S.P. crcsol 1.2.3.4-Tetralirdro-3-
2.61,
r.55 0.5a 0.55 0 53
82.8" 80.0 85.0 90.7 78.9 88.4 90 8
10 1 .O
0.55 0.53
84 7 85. 1
3.0 2.4
1.0 0
0.33' 0 0 0.55
91 7 80.8 82.3 83.3
3 1 2.2 2.6 2.G
0 0.5.3 0 0.5: 0.35 0.5; 0.5.;
7 5 . .i 80.8 80.6 78.1 80 4 $11 87. i
2 0
1 1.0
0.65 0
1, U
0 ,11
1.0 0 1 0 1 0
2.7 2.1 2.3 2.3 2 2 2.5
~
G-82 c-77 G-71 BK-114 RK-I 42
RK-103 B I
