Solvent effects in the depolymerization of coal

Larry J. Darlage and Maurice

E. Bailey

lnstitu te for Mining and Minerals Research, Department Pikeville, Kentucky 4 1501, USA (Received 21 November 1975)

of Chemistry, Pikeville College,

The depolymerization reaction of coal in the presence of Friedel-Crafts catalysts in a variety of phenolic and non-phenolic solvents has been investigated. Only the phenoiic solvents were found to be useful in this reaction. Some interesting substituent effects were discovered when the isomeric cresols and dihydroxybenzenes were employed as solvents. The influence of temperature and catalyst concentration on the degree of depolymerization has also been examined. With non-phenolic solvents, polymerization is the predominant reaction.

Depolymerization of coal using Friedel-Crafts catalysts was first reported in 1962 by Heredy and Neuworth’. They discovered that the solubility of coal in a variety of organic solvents can be considerably increased by treatment with boron trifluoride in phenol at 1OO’C. The mechanism of this reaction presumably involves cleavage of methylenearomatic bonds in the coal structure and subsequent attachment of the methylene fragments to phenol solvent molecules. The net effect of this process is the production of smaller more readily extractable ‘coal molecules’. A variety of catalysts have since been investigated in the depolymerization reaction with the conclusion that p-toluenesulphonic acid and other related sulphur-containing acids are the most efficient when phenol is used as the solvent at 185”C2*3. Further studies have shown that coal rank is also an important factor, lower-rank coals yielding higher solubiIity435. Increased extraction may also be attained by pre-oxidation of coal followed by a depolymerization step6. The depolymerization reaction offers a possible alternative to the usual methods of liquefying coal by hydrogenation, solvent refining, and carbonization’. However, since the reactivity characteristics of solvents other than phenol have not yet been reported, it was of interest to investigate the effects of a variety of solvents on this reaction. Included in this study also was a coal-derived creosote oil with a total acid fraction of approximately 6%.

Some effervescence usually occurred. This mixture was stirred at 70-100°C for two to three hours, and then was decanted into two 50-ml centrifuge tubes. Centrifugation at 1500 g was continued for 15 min, and the supernatant liquid was decanted into a graduated cylinder. The residue remaining in the tubes was centrifuged for another 15-min period followed by decantation. In most reactions approximately 70 ml of decantate was obtained. The precipitates in the tubes and in the reaction flask were washed into a fritted-glass filter funnel with carbon tetrachloride, or benzene, and then washed with more carbon tetrachloride until a clear filtrate was obtained. The carbon tetrachloride washings were combined with the decantate and distilled at reduced pressure yielding a solid residue of depolymerized (DP) coal which was dried to a constant weight in vacua at 100°C (Fraction A). The residue in the filter funnel was washed with hot water until the washings appeared colourless and was then dried to a constant weight in vacua at 100°C (Fraction B). The phenol, p-cresol, and mixed cresols were laboratory grade and were redistilled before use. All other solvents were reagent grade and were used without further purification. The oxidized coal was obtained by reacting 60 g of coal with 200 ml of 2 N nitric acid at 70°C with vigorous stirring for 24 h6. The reaction mixture was then cooled to room temperature and filtered. The oxidized coal was washed twice with 250-ml portions of distilled water after which the washings were neutral to pH paper. The coal was placed

EXPERIMENTAL

The coal investigated was obtained from the Pond Creek seam in Pike County, Kentucky. It is a high-volatile bituminous coal with proximate and ultimate analyses shown in Table 1. The coal was first ground to -200 mesh (U.S.), dried in vacua at 1OO’Cfor 24 h, and stored in a desiccator. The depolymerization reaction was carried out with 10 g of coal, 100 ml of solvent, and the required amount of catalyst in a 250-ml three-neck flask equipped with mechanical stirrer, condenser, and thermometer. The reaction mixture was heated with stirring for the desired length of time after which the temperature was reduced to approximately 7O”C, and a two-fold excess of sodium carbonate monohydrate (Na$Z03.H20) was added slowly.

Table 1

Pond Creek seam coal analyses

Proximate analysis (wt %, as rec’d)

Moisture Ash Volatile matter Fixed carbon

1.1 18.1 33.1 47.7

Ultimate analysis (wt %, daf)

Carbon Hydrogen Nitrogen Chlorine Sulphur Oxygen (diff .I

82.6 5.8 1.7 0.1 1.3 8.5

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Solvent effects in the depolymerizafion of coal: Larry J. Darlage and Maurice E. Bailey

7

The optimum amount of sulphuric acid needed for effective catalysis was determined by a series of reactions in which its concentration was varied. The reactions were carried out at 180°C for 24 h using a coal:phenol ratio of 1:lO. The results are illustrated graphically in Figure 2. The percent of depolymerized coal extracted by phenol was determined by the following equation:

Coal

Catalyst

+ phenol

No,CO;.

Distil

Wash with CC14 and filter

Combine

I\

--J-.Wash with l-l,0 and filter

Figure 1

Typical

depolymerization-reaction

flow chart

in a vacuum oven to dry for 24 h at 100°C and then stored in a desiccator. All ash determinations were performed by the ASTM procedure. Infrared spectra were recorded on a PerkinElmer Model 700 spectrometer using potassium bromide pellets in concentrations of 3-5 mg of sample per 100 mg of . potassium bromide. Pyridine extractions were performed in a Soxhlet apparatus using reagent-grade pyridine. Fractions A and B were mixed thoroughly with filter-aid in a ratio of 2: 1 and were then extracted until a clear solution was obtained. The pyridine was evaporated at reduced pressure yielding a residue which was dried in a vacuum oven at 100°C to a constant weight.

RESULTS AND DISCUSSION The general reaction scheme followed is shown in Figure 1. After completion of the depolymerization reaction, the acid catalyst was neutralized with solid Na$03.H20. The insoluble salts produced were precipitated by centrifugation and removed from the insoluble coal material by washing Fraction B with hot water. This method was found to be more efficient than neutralization of the product ‘with aqueous Na2C03 because inseparable emulsions were avoided. Much of the present work on depolymerization was conducted using concentrated sulphuric acid as the catalyst as it was shown to be one of the most effective. Actually, the acid reacts initially with the solvent to produce a sulphonic acid which then serves as the depolymerization catalyst3. In the case of phenol, the product is p-hydroxybenzenesulphonic acid (1) which we have isolated from a reaction of phenol with sulphuric acid and have characterized by the melting point of its p-toluidinium salt (2).

% extracted = 100 -

lizSO

Figure2

206

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1976, Vol 55, July

wt of daf coal

It is interesting to note the ‘negative’ extractability until approximately O-025 mol of catalyst is added. This indicates that some phenol is adding to the coal structure but that the degree of depolymerization is insufficient to render much of the coal extractable. A significant increase in weight is noted even when no catalyst is present, presumably as the result of entrapment of some solvent molecules in the porous coal structure ‘. However, prolonged heating of Fractions Bin a vacuum oven produced no significant weight changes. A comparison of the infrared spectra (Figure 3) of unreacted coal and Fraction B obtained from an uncatalysed reaction illustrates the great similarity of the two samples. The only major difference is the carbonyl (C=O) absorption band at about 1700 cm-l which is slightly more intense in the spectrum of Fraction B (Figure 3b) possibly because of oxidation under the reaction conditions. The presence of phenol is not as evident in Fraction B as in a typical Fraction A (Figure 3c) which would be expected from the .accepted reaction mechanism’. The graph in Figure 2 reaches a plateau at 0.07 mol H#Oq/mol phenol, which was the quantity chosen for most of the subsequent depolymerization experiments. It was of interest to determine the effect of adding a second 0.07 mol H2SO4 after 12 h of reaction in order to test the sufficiency of catalyst throughout the course of the reaction. If the H2SO4 is depleted in the depolymerization reaction, then one might expect a considerable increase in extraction, indicating further depolymerization upon addition of more catalyst. Only a small increase from 29% (Run 1) to 3% (Run 3) was obtained as shown in Table 2, demonstrating that sufficient catalyst was available. Increasing the length of time allowed for reaction also has very little effect, as shown by Run 4 in which the reaction time was 96 h.

SOzOH 1

wt of Fract. B (100 - % ash in Fract.B)

(mollmol

phenol 1

Extraction of reaction product as a function acid concentration used in reaction at 180°C

of sulphuric

Larry J. Darlage and Maurice E. Bailey: Solvent effects in the depolymerization

Wavenumber 4000 tOOr

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600

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90 60 70 60 50 40 30 20 10 0 2.5

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10 11 12 13 14 16

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Wavelength (pm)

Figure3 in phenol,

Infraredspectra of: (8) unreacted coal,(b) Fraction 6 of an uncatelysed reaction (c) Fraction A of an H2S04-cetalysed reaction in phenol, and (d) phenol

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Solvent effects in the depolymerization of coal: Larry J. Darlage and Maurice E. Bailey Table 2 Solubilization of Pond Creek seam coal after depolymerization footnote)

in phenolic solvents (24 h unless stated otherwise in

Catalyst Run

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Type

Mol/mol solvent

Phenol Phenol Phenol Phenol Phenol Phenol Phenol Phenol

H2S04 -

0.07 -

H 2so4

0.1 4a 0.07 o-07 0.07 -

27 28 29 30

29

7.5 10.2 7.1 7.76 10-l f 9.0 9.6 9.8

-5 33 26 0 11 3 3

H 2so4

0.07 0.07 o-14 0.07

150 150 180 190 190 100 190 190 100 190 190 190 190 180 150 150 180

6.5 7.5 9.6 10.5 10.1 10.0 6.8 190 96 8.7 9.7 8.6 9.6 6.8 7.7 6.6 5.9

38 22 4 -7 -2 -6 36 -1 2 12 3 13 3 36 17 33 46

BF3

0.07

150

7.7

20

BF3

0.14

150

6-4

38

H2SD4

0.07

180

7.9

24

H$04 HgS04

0.07 0.07

180 180

7.3 7.2

32 33

H2=4

BF$

’ -

H 2=4 -

0.035 mol of each 0.14 0.07 0.07 0.07 -

BF3 H2SD4 -

0.07 0.07 -

RF3 H2=4

0.07 0.07 -

H2=4

0.07 -

BF3 + HgSO4

Phenol Phenol Phenol o-cresol o-cresol o-cresol m-cresol m-cresol

BF3 BF3 NaHS04

m-cresol p-cresol

-

HgS04 BF3 BF3

Added in two equivalent portions 12 h apart Reaction time was 96 h Boron trifluoride etherate (density, O-44 g/ml) 54% meta, 29% para and 17% other phenols 95:5 mixture Reaction time was 6 h

Table 3

Extractability

of depolymerized coal by pyridine

Solvent

Catalyst

Temp. (“Cl

Pyridine extraction yield (wt%of DP coal)

Phenol Phenol Phenol Phenol

None 0.07 mol/mol 8 F3 0.14 mol/mol BF3 0.13 mol/mol BF3

100 100 100 150

13 42 74 87

Depolymerization

Another catalyst investigated in this reaction was sodium hydrogen sulphate monohydrate (NaHS04.H20). This catalyst was of interest because it would eliminate any

208

Temp. (“C)

Solubilization in solvent (wt % of daf coal)

180 180 180 180 180 100 100 100

H2S04

p-cresol Mixed cresol& Mixed cresolsd Phenol/catechole Phenol/catechole Phenollcatechole Phenol/ resorcinole Phenol/ resorcinole Phenol/ resorcinole Phenol/ hydroquinonee Phenol/o-cresole Phenol/m-cresole

26

a b c d e f

Solvent

Amount of (undissolved coal) Fraction B (g)

FUEL, 1976, Vol 55, July

reaction between catalyst and solvent and it should be relatively soluble in phenol under typical reaction conditions. However, its activity was found to be very slight indeed as Table 2 (Run 11) indicates. Similar results were reported for sodium sulphonates3. Boron trifluoride (BF3) was a very effective catalyst at 150°C (Table 2, Run 9) but at lower temperatures its solubilizing power decreased markedly (Run 6). All previously reported depolymerization reactions using BF3 were performed at lOO”C, but we have now found that more complete reaction can be achieved at 150°C which is readily attained with this catalyst. The amount of each product soluble in pyridine is shown in Table 3 in order that comparisons with other depolymerization results can be of BF3 was also found to be maderm6. The concentration by comparing Runs 9 and 10 in critical, as demonstrated Table 2. The higher degree of extraction when a larger

Larry J. Dariage and Maurice E. Bailey: Solvent effects in the depolymerization

Table 4

Solubilization

of Pond Creek seam coal after depolymerization

in non-phenolic solvents (24 h)

Catalyst

Solubilization in solvent (wt%of daf coal)

Solvent

Type

Mol/mol solvent

(“C)

Amount of Fraction B (g)

Toluene Toluene Mixed xylenes Mixed xylenes Anisole Anisole Tetralin Tetralin Creosote 0iP Creosote oil Creosote oil

BFZa H2so4 -

BFs

0.13 0.07 0.07 0.07 0.07 0.07

100 100 150 150 150 150 170 170 170 150

10.6 9.5 12.6 10.5 II.7 IO.5 16.1 9.5 18.6 12.1

-9 5 -31 -1 -20 -5 -72 4 -106 -26

-

-

170

11.6

-16

Temp.

kG04

H2.934

kW4

of coal

a Boron trifluoride etherate (density, 0.44 g/ml) f~ No.1 oil; coal-tar distillate, courtesy of Reilly Tar and Chemical Co.

amount of catalyst is initially present may be associated with a significant loss of BF3 at 150°C. In addition, some BF3 is probably involved in the formation of a white refractory solid in the condenser at this temperature. The chemical nature of this material is uncertain and is under current investigation. A variety of other phenolic solvents were examined using BF3 or H2SO4 as catalyst (see Table 2). All reactions performed at 100°C with BF3 catalyst improved the extraction of coal with a particular solvent only slightly. Again, much better results were obtained at 1SO’C. A comparison of Runs 12, 15 and 18 using H2SO4 as catalyst is fascinating in that substituent effects are evident. For example, m-cresol (3) has electron-donating methyl and hydroxyl groups which reinforce each other and direct electrophiles to the 4 and 6-positions, making these two positions extremely reactive. With o-cresol (4) and p-cresol (S), the two sub-

&

&$

&.&

$

OH 3

4

5

OH 6

7

8

stituents direct electrophiles to different, competing ring positions. On this basis one might predict higher reactivity for m-cresol (Run 15) which indeed appears to be the case. Similar results were obtained for depolymerization reactions using as solvent 95 ml of phenol containing 5 g of resorcinol (6) catechol (7) or hydroquinone (8) represented in Table 2 by Runs 25,22 and 28, respectively. Alternatively, the high yield of solubilized coal when m-cresol is used as the depolymerizing solvent may simply be a solubilization phenomenon and not the result of increased depolymerization. Some support is given to this argument by the results of Runs 29 and 30 (Table 2). Using 95:5 mixtures of phenol with the cresols, very similar results were obtained with the ortho as with the meta derivative. These interesting substituent effects are being investigated further. A number of non-phenolic compounds were also examined as possible depolymerization solvents, and the results are

shown in Table 4. In all catalysed reactions Fraction B weighed more than the initial charge of coal (lO$ used in the reaction, so that the extractabilities have negative values. Apparently polymerization, rather than depolymerization, occurred in these reactions. Condensation reactions of certain aromatic compounds in the presence of FriedelCrafts catalysts are well documented’>” For example, naphthalene readily forms binaphthyl under these reaction conditions”. This type of condensation reaction may be largely responsible for the poor results obtained when creosote oil is used as solvent (Table 4). Creosote oil is a fraction of coaltar distillate which contains a variety of neutral, acidic, and basic compounds. Naphthalene is a major constituent, and the phenols comprise approximately 6% of this oil”. Thus, although some depolymerization of coal may occur as a result of phenolic components in creosote oil, an overwhelming amount of condensation and polymerization must be responsible for the large quantity of insoluble material obtained. Depolymerization of oxidized coal” in phenol with H2SO4 as catalyst resulted in solubilization of 46% of the coal as shown in Table 5. This is a considerable improvement over untreated coal, whose extraction under similar conditions was 29% (Table 2). Even better results were obtained with BF3 in phenol at 150°C. After one 24-h period, more than one-half of the original charge dissolved. When the insoluble portion from this run (Run 3) was reacted for a second 24-h period with fresh solvent and catalyst, an additional 26% was converted to phenol-soluble material. It is of interest to note that decreasing the coal/solvent ratio from 1 :lO to I:20 had no effect on the extractability of the final product (Runs 3 and 4). Finally, as in all previous cases, the use of a non-phenolic solvent, such as tetralin, gave very poor results when reacted with oxidized coal. In summary, the depolymerization reaction of coal works well for most phenolic solvents when catalysed by H2SO4 at 180-200°C or BF3 at 15O’C. Non-phenolic solvents are not useful in this reaction, presumably owing to a lack of affinity for coal and/or a greater propensity for polymerization. Certain substituted phenols, such as m-cresol and resorci-

FUEL, 1976, Vol 55, July

209

Solvent effects in the depolymerization of coal: Larry J. Darlage and Maurice E. Bailey Table 5

Solubilization of oxidized coal after depolymerization

(24 h)

Run

Solvent

Type

Mol/mol solvent

Temp. (“C)

Amount of Fraction B (g)

Solubilization in solvent (wt % of daf coal)

1 2 3 4 5 6 7

Phenol Phenol Phenol Phenola Phenolb Tetral in Tetralin

H2S04

0.07 o-14 0.14 0.14 007 -

180 180 150 150 150 170 170

5.9 10.7 4.8 4.5 1.8 13.9 9.6

46 -5 56 56 26 -41 3

Catalyst

a b

HF3 HF3 HP3 H2S04

Coal:phenol ratio = 1:20 Reaction of 2.5 g of Fraction

-

B from Run 3

r-101, show much more depolymerizing ability than their isomeric compounds which may be explained. by electronic substituent effects on the benzene ring. Pre-oxidized coal is more readily depolymerized in phenol than untreated coal. Considerable further research is needed on the depolymerizing ability of coal-derived solvents such as creosote oil, but our preliminary results are unfavourable because of the high degree of polymerization.

ACKNOWLEDGEMENTS This work was funded by the Institute for Mining and Minerals Research, Lexington, Kentucky. The authors wish to thank Coal Lab, Inc. (Pikeville) for the coal samples and analyses and Reilly Tar and Chemical Co. (Indianapolis) for the coal-tar distillate sample. We also greatly appreciate the laboratory assistance of Cole Anderson, Clarence Awanda, Jerry King, Charlene Mullins, and Vi&i Wray.

210

FUEL, 1976, Vol 55, July

REFERENCES 1 3 4 5 6 7 8 9 10

11

Heredy, L. A. and Neuworth, M. B. Fuel 1962,41, 221 Ouchi, K., Imuta, K. and Yamashita, Y. Fuel 1965,44,29 Ouchi, K., Imuta, K. and Yamashita, Y. Fuel 1973,52, 156 Heredy, L. A., Kostyo, A. E. and Neuworth, M. B. Fuel 1965, 44,125 Ouchi, K., Imuta, K. and Yamashita, Y. Fuel 1965,44,205 Dar&e, L. J., Weidner, J. P. and Block, S. S. Fuel 1974, 53, 54 Goldman, G. K. ‘Liquid FueLr from Coal’, Chemical Process Review No.57, Noyes Data Corp., New Jersey, 1972 Dryden, I. G. C. ‘Btemistry of Coal Utilization’, Suppl. Vol. (Ed. H. H. Lowry), Wiley, New York, 1963, pp 248-249 Scott, C. L. and Steedman, W. Fuel 1972,51,10 Olah, G. A. ‘Friedel-Crafts and Related Reactions’ Vol.1, Interscience Publishers, New York, 1963, p 34 McNeil, D. ‘Kirk-Othmer Encyclopedia of Chemical Technology’, Vol.19 (Ed. A. Standen), Wiley, New York, 1969, pp 653-682