Superacid coal chemistry. 2. Model compound studies under conditions of HF:BF3:H2 catalysed mild coal liquefaction

Superacid

coal chemistry

2. Model compound studies under conditions catalysed mild coal liquefaction George

A. Olah and Altaf

of HF:BF,:H,

Husain

Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, Los Angeles, CA 90089, USA (Received 9 May 7983; revised 12 December 1983)

Selected model compounds representing coal structural entities were studied under the conditions of HF-BF,H, catalysed mild coal liquefaction. Bibenzyl and diphenylmethane gave near quantitative conversion at room temperature without added hydrogen. Biphenyl, however, required hydrogen pressure at 150°C and gave a conversion of only 230%. Among the model compounds containing ether linkages, dibenzyl ether and benzyl phenyl ether gave quantitative conversion at room temperature without added hydrogen. Diphenyl ether in contrast was converted (~70% yield) only under hydrogen pressure at 155°C. Sulphur- and nitrogen-containing model compounds were also studied. At 95°C in the absence of hydrogen, benzyl phenyl sulphide and dibenzyl sulphide gave over 95% conversion. On the other hand diphenyl sulphide and diphenyl disulphide required hydrogen pressure at 150°C to give conversions of z95%. Quinoline gave a conversion of 220% under hydrogen pressure at 15uC. The formation of condensation products in these conversion processes could be suppressed by the use of a good hydrogen donor, such as isopentane. (Keywords: coal; chemistry;

model compounds)

The common methods employed for coal liquefaction as well as degradation in structural studies fall into two general categories: free-radical and cationic. Hydroliquefaction’ and pyrolysis2 are the two major freeradical processes. In recent years, acid-catalysed depolymerization has also been used for converting coal into soluble products for structural studies. There are numerous reports on the acid-catalysed depolymerization of coals for structural studies, gasification and liquefaction3-i9. As described in part 1 of this Paper, a mild superacidic catalyst system has been developed for the depolymerization of coal involving the use of hydrogen fluoride and boron trifluoride, under hydrogen pressure or in the presence of a hydrogen donor solvent (or both)“. The effectiveness of the HF-BF,-H, system for the depolymerization of coals has prompted an investigation of the chemistry of the process to discover the mechanism of the conversion of the coal into pyridine- and cyclohexanesoluble products ,under the mild reaction conditions. Information about the chemistry of coal liquefaction and the effect of such processes on the structure of coal is obtained by studying the products of these processes. However, because of the severe conditions used in most processes, the resulting products may not be truly representative of the starting coal and such studies therefore have limited value. Consequently the study of model compounds in delineating both mechanistic and structural aspects in coal chemistry is useful and receiving much attention. A major advantage of these studies is that both starting materials and products are easily characterizable using simple analytical techniques. Thus the 0016-2361/84/101427~5$3.00 @ 1984 Butterworth & Co. (Publishers) Ltd.

effect of treating model compounds under liquefaction conditions can be readily established by analysing the products. In the present work, a number of model compounds have consequently been studied under the conditions of the HF-BF,-H, catalysed coal liquefaction process. EXPERIMENTAL All model compounds used in the present study were commercially available and used without further purification. Reactions were carried out in a 125 ml Parr Monel400 magnetically stirred reactor fitted with a Teflon liner. The reactor was charged with the model compound (1.Og) and liquid HF (5 ml) at 0°C. After closing the reactor, 6.2 MPa of boron trifluoride and the desired amount of hydrogen were pressurized in. The reactor was then heated in an oil bath at the desired temperature. At the end of a given reaction time the reactor was cooled and slowly depressurized. Gas samples were taken for analysis whenever required. After opening the reactor, the reaction mixture was carefully quenched with ice water. The organic product mixture was extracted in ether and analysed on a Varian model 3700 gas chromatograph (glass capillary column, 15.2 m x 0.25 mm; OV-101; 0.2 MPa He; 6018OC). Ethylbenzene was generally used as an internal standard. The yields of products were calculated as % yield =

moles of product xl00 moles of unrecovered starting material

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Vol 63, October

1427

Superacid coal c~e~is?ry: 2.: G. A. U/ah et al. RESULTS AND DISCUSSION The selection of model compounds is generally based on the structural information available on coals. From time to time various ‘structures’ for coal have been suggested, depending upon the extent of knowledge available at the time. These included those suggested by Given21F2’, Hill and Lyon 23, Wiserz4, Pitt” and Heredy and Wender26. As no uniform composition for coals exists, the most widely held view of coal ‘structure’ pictures bituminous coal as containing statistical proportions of groups of fused aromatic and hydroaromatic ring clusters, composed of an average of two to six ring units, connected by various alkylidene, ether, sulphide and diphenyl bridges. The aromatic backbone of these ring clusters is based primarily on phenanthrene as opposed to anthracene structure2 ‘. As mentioned, alkylidene groups play an important role in linking aromatic and hydroaromatic ring clusters in coals. There have been several reports on the study of model compounds containing these linkages under various coal liquefaction conditions2s-36. The model compounds selected for the present study included bibenzyl, diphenylmethane and biphenyl. When bibenzyl was treated with HF-BF, at -78°C in the absence of hydrogen, no conversion was observed and the starting material was recovered unchanged. However, at room temperature bibenzyl gave >95% conversion (Table I). The only product that could be detected by gas chromatography was benzene. No ethylbenzene could be detected. This is presumably due to the fact that the intermediate phenethyl cation immediately deprotonates to give styrene, which polymerizes under the reaction conditions to products which cannot be detected by g-c. (Sheme I). In recent studies of coal model compounds, Scheme

t

‘C6H6

CH,CH:

-H

@

oCH=CH

-

Polymers

-o-

Poutsma 7 and Gilbert3 * have obtained styrene as one of the products from the thermolysis of 1,3-diphenylpropane. It is expected, however, that at higher temperature and hydrogen pressure or in the presence of a hydrogen donor, i.e. under conditions of hydroliquefaction, this side reaction will be depressed or any Table 1 Conversion

of bibenzyl,

diphenylmethane

polymer formed will eventually be cleaved. Indeed, in the presence of isopentane, bibenzyl gives a much higher amount of benzene. Both bibenzyl and polystyrene treated with HF-BF3 at x 150°C in the presence of 5.5 MPa of hydrogen gave benzene and light hydrocarbons (C,C,) as products. Diphenylmethane when treated with the HF-BF, system gave quantitative conversion. The products were benzene, toluene and anthracene (Table I). The probable path for the formation of these products is shown in Scheme 2. The initially formed benzyl cation reacts with Scheme

Anthrocene

diphenylmethane to give 9,10-dihydroanthracene, which aromatizes under the experimenlal conditions to give anthracene. The formation of anthracenes from methylene-bridged aromatics can be an undesirable side reaction in coal liquefaction. However, when a similar reaction was carried out in the presence of a hydrogen donor such as isopentane, no anthracene was formed and the products consisted of benzene, toluene and other alkyl-substituted benzenes. Biphenyl failed to give any conversion at room temperature in the absence of hydrogen. Even with 3.4 MPa of hydrogen and at 9X, most of the starting material was recovered unchanged. These results are not surprising, since, after the initial protonation of biphenyl, cleavage would result in a highly unstable phenyl cation (Scheme 3). However, when the reaction was carried out at 150°C in presence of 5.5 MPa of hydrogen, appreciable conversion (z 30%) was observed, with the formation of benzene together with light hydrocarbons (C,-C,). This is

and biphenyl Time

Conversion

Products

(h)

(%I c

(mol %I d

nil >95 >95 100 100

none Benzenef26) Benzene(45) Benzene(481, toluene(l4) and anthrscenef45) Benzene(48), toluene(25),p-xylene(l0) and several unidentified products none Benzene(46I, CHs, CzHa, @Ha and Cqs

Hydrogen fMPa)b

Temp. (W

Dibenzyl Dibenzyl Dibenzyle Diphenylmethane Diphenylmethanee

-

-78 25 25 25 25

Biphenyl Biphenyl

3.4 5.5

e

5 ml liquid HF and 6.2 MPa of BF3 (at room temperature)

Model compound

a

-

1 .O g model compound,

95 150

b At room temperature c Based on recovered starting material d Products analysed by g.c.; yield based on converted e 5 ml isopentane

1428

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also used

1984,

Vol 63, October

2

1.5 4 4 4 4 4 10

starting material

nit 30

were used in all reactions

Superacid Scheme

coal chemistry:

2.: G. A. O/ah et al.

Scheme 4

3

H2

0 ()

1 CH~ + C2He etc

probably due to the hydrogenation of the aromatic ring followed by cleavage under the reaction conditions. These results are consistent with earlier studies3’ on the reduction and subsequent fragmentation of benzene in superacid under hydrogen pressure. From the above results it is clear why coal is quite susceptible to superacidic HF-BF, catalysed depolymerization under moderate temperatures (x 150-170°C) and hydrogen pressure’giving saturated products having a high H/C ratio. Optimum conditions to achieve this goal and the amount of hydrogen necessary have yet to be established. Coals themselves are sufficiently good hydrogen donors and internal hydrogen transfers are possible by proper selection of reaction conditions. This has been demonstrated in some studies of model compounds35,40 as well as coal itself4iV4’. Thus an overall hydrogen balance with added external hydrogen is to be considered. Another important structural feature of coal involves ether linkages, which also are believed to play an important role in the depolymerization of coal. There are many reports in the literature on the study of model ether compounds under coal liquefaction conditions34.40,43-48. Dibenzyl ether and phenyl benzyl ether, when treated under the mild HF-BF, catalysed liquefaction conditions, gave quantitative conversion even at room temperature and without added hydrogen. The products of these reactions were benzene, toluene, p-xylene, anthracene and 2-methylanthracene. Phenol in quantitative yield was also obtained from phenyl benzyl ether (Table 2). The probable mechanism for the formation of these products is shown in Scheme 4. The crucial step is the protonation of the ether, giving an oxonium ion, followed by cleavage yielding the benzyl cation, which carries the reaction further to the observed products. Once again the formation of condensed products can be suppressed by using a suitable hydrogen donor.

Table 2 Conversion

6

I

-C6H50H

-C,H,OH

+,H. 0 CHz

Anthracene

I

CH,

CH, 2-Methylanthracene

p- Xylene

Not unexpectedly, diphenyl ether failed to give any conversion with HF-BF, at 95°C in the presence of 3.4 MPa of hydrogen. Increasing the hydrogen pressure to 6.2 MPa did not result in any improvement. However, when the reaction was carried out at 155°C and 5.5 MPa of hydrogen, more than half the diphenyl ether was converted, giving benzene, phenol and several yet unidentified products. Light hydrocarbons (C,C,) were also identified in the products. This indicates that conversion of diphenyl ether requires ionic hydrogenation of one of the phenyl rings, followed by cleavage. Sulphur bridges are also an important feature in coal structure4’. Removal of sulphur from coal is also desirable in coal liquefaction, especially in processes where hydrogenation catalysts are employed, as sulphur can poison these catalysts and reduce their activity5’. Sulphur-containing by-products of coal liquefaction are also air and water pollutants51. Removal of sulphur from coal generally requires a catalyst”. Extensive studies of

of ethers

Model compounda

Hydrogen (MPa) b

Dibenzyl

Temp. VW

Time fh)

-

25

4

100

Benzyl phenyl ether

-

25

4

100

Diphenyl Diphenyl Diphenyl

3.4 6.2 5.5

95 95 155

4 6 10

a,b,c,d

1 2

t

ether

ether ether ether

See corresponding

footnotes,

Conversion f%) c

nil nil 70

Products (mof%) d Benzene(l0.3). toluenef7,9),p-xylene (7.1), anthracene(67.7) and 2-methylanthracene (5.9) Benzenef5.9). toluene(3.l).p-xylene (3.3). phenol(100). anthracenef34.4) and Z-methylanthracene(2.9) none none Benzeneftraces), phenol(33.6). CH4, CzHe, C&Ha and C4s

Table 1

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1984,

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Superacid coal chemistry: 2.: G. A. Olah et al. Table 3 Conversion

Model compound

of model compounds

a

Hydrogen (MPa)b

containing

sulphur or nitrogen

Temp.

Time

(W

(h)

Conversion (%) c

Benzyl phenyl sulphide

-

25

4

65

Benzyl phenyl sulphide

-

95

4

100

Dibenzyl

sulphide

-

95

4

100

Diphenyl

sulphide

5.5

150

IO

>95

Diphenyl

disulphide

5.5

150

IO

100

5.5

150

9

20

Quinoline aJJ,cd

See corresponding

footnotes,

Benzene(4.3), toluene(traces),p-xylene(l.2) and anthracene(4.9) Benzene(l6.2). toluene(1 .O),p-xylene(l.4), benzenethiol(4.2) and anthracene(ll.0) Benzene(l3.9). toluene(l.9),p-xylene(10.1), anthracene(39.8) and 2-methylanthracene(9.2) Benzene(61.6). benzenethiol(traces), thianthren(8.4), CH4, CzH6, C3Ha and C4s Benzene(l5.5). toluene(l.5), thianthren(20.91, CH4, CzH,, CaHa and C~S Benzene(traces), CH4, CzH(j, C3H8 and C4s

Table 1

sulphur compounds were conducted by Mobley and Bell” to determine the ability of ZnCl, to catalyse the removal of sulphur from coal-related model compounds. Sulphur model compounds have also been studied by others53*54. The sulphur-containing model compounds chosen for the present work were aromatic and aliphatic sulphides and disulphides. Phenyl benzyl sulphide when treated with HF-BF, at room temperature in the absence of hydrogen gave 65% conversion. However, at 95°C a conversion of >95% was achieved. Products of these reactions were benezene, toluene, p-xylene, benzenethiol and anthracene (Table 3). Similarly dibenzyl sulphide gave a conversion of > 95% at 95°C in the absence of hydrogen, with 2-methylanthracene as an additional product. Diphenyl sulphide, however, required 5.5 MPa of hydrogen at 150°C and then gave a conversion of >95%. Benzene, benzenethiol, thianthrene, and light hydrocarbons (Cl-C,) were obtained as products. Similarly, diphenyl disulphide at 150°C in the presence of 5.5 MPa of hydrogen gave benzene, toluene, thianthrene and light hydrocarbons (C,X,) with a conversion of >95%. Nitrogen is also an important constituent of coals. Nitrogen in coals can have serious effects on catalyst activity and catalyst life in conversion processes50*55. Nitrogen-containing products from coal conversion are also air and water pollutants51. Most of the nitrogen in coal is believed to be present in heterocyclic rings, for example in carbazole- and indole-type structures55. The initial nitrogen model compound selected for the present study was quinoline. At 150°C in the presence of 5.5 MPa of hydrogen, quinoline gave 20% conversion, with benzene and light hydrocarbons (C,-C,) as products (Table 3). The nitrogen of the converted quinoline gives NHiBF; in the acidic system, from which NH, and the acid HF-BF, can be thermally recovered. This represents an added advantage in removal of nitrogen from coal during the liquefaction process. CONCLUSIONS It has been shown that HF-BF,-H, is an efficient system for the conversion under mild conditions of a series of model compounds representing major structural feature of coal. Model compounds such as bibenzyl, diphenylmethane and dibenzyl ether cleave at room temperature, while biphenyl, diphenyl ether and diphenyl sulphide, require higher temperatures and hydrogen pressure for cleavage. In terms of coal liquefaction ability of the

1430

Products (mol%) d

FUEL, 1984, Vol 63, October

superacidic system used, it can be concluded that the high pyridine extractibility achieved at lower temperature results from the cleavage of bridges such as are present in bibenzyl, diphenylmethane and dibenzyl ether. On the other hand, the increased cyclohexane extractibility observed at higher temperatures and under hydrogen pressure reflects the hydrogenation and cleavage of the aromatic backbone in coal structures, similar to what is observed in the conversion of model compounds such as biphenyl and diphenyl ether. Studies on model compounds more representative of the complexities of coal structure, such as polycyclic aromatic systems and compounds with mixed functionalities are currently being conducted to provide further insight into coal processes by ionic depolymerization-liquefaction cleavage and hydrogenation reactions. ACKNOWLEDGEMENT Support of this work by the US Department of Energy is gratefully acknowledged. REFERENCES

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1431