Coal characterization for liquefaction in tetralin and alcohols

Coal characterization tetralin and alcohols P. N. Kuznetsov, E. D. Korniyets,

G. I. Sukhova, N. A. Belskaya

for liquefaction

in

J. Bimer*, P. D. Salbut*, and N. M. lvanchenko

Institute of Chemistry and Chemical Technology, * institute of Organic Chemistry, 44 Kasprzaka, (Received 6 September 7990; revised 5 March

42 K. Marx St, Krasno yarsk 660049, Warszawa 01-224, Poland 1991)

USSR

Kansk-Achinsk lignite hydrogenation in tetralin, isopropanol, ethanol and methanol was studied. Tetralin was the most active solvent. Synergetical effects were observed when the mixture of tetralin and alcohols was used for liquefaction. The variations in liquid product composition were analysed by a mechanistic numerical model incorporating two competing reactions: alkylation and hydrogen donation. The effect of preliminary 0-methylation with (CH,),SO,, reduction with LiAlH, and with K/isopropanol and reductive methylation with K/CH,J on the oxygen functional group composition and liquefaction behaviour were examined for three coals with different rank. Coal pretreatments were shown to cause dramatic effects on the oxygen-containing group distribution. The liquefaction reactivity with methanol in the presence of ZnCl, and NaOH catalysts decreased slightly. The liquefaction behaviour is strongly influenced by coal rank. (Keywords: liquefaction; alcohol; tetralin)

The understanding of the coal liquefaction process comes about through two stages: increasingly sophisticated investigation of the behaviour of coal model systems; and determination of the coal structural components. The liquefaction is conventionally considered to involve the thermal generation of free radicals, followed by the capping of the radical sites by hydrogen donors’. Partially hydrogenated aromatics such as tetralin are very effective solvents because of their hydrogen-donating property. Derbyshire has stressed that hydrogen-donating ability of the solvent is most important for low-rank coals’. Since Bartle et ~1.~, Ross and Blessing4, and Makabe et al.’ pioneered work on coal liquefaction in low boiling solvents including alcohols under supercritical conditions, alcohol solvents have become of interest for obtaining liquid fuel and for the study of coal structure and the mechanism of conversion into liquid product&23. Low-rank coals were shown to be the most suitable for liquefaction in alcohol solvents’4~‘7,21. Alcohol action is considered to be determined by hydrogen-donor ability4~s~‘9~23-2s and by alkylating properties8g16-23. Th e h y d rogen-donor ability of ethanol and isopropanol was demonstrated using the deuterium isotope technique’ 9g24,2s . The quantitative analysis of the alkylation reaction in coal liquefaction in methanol has been reported elsewhere’7-20. Significant benefits were recently observed by using the hydrogen-donor tetralin and alcohols26 together in the liquefaction of low-rank coals. In contrast, the conversion of bituminous coal did not increase after alcohol addition. The beneficial effect was ascribed to the acceleration of coal swelling improving contact between coal constituents and tetralin during liquefaction. The ester and ether bond cleavage is considered to 00162361/91/09103148 0 1991 Butterworth-Heinemann

Ltd

be an important step in the dissolution of low-rank coals’*8,23. Evaluation of solvent effect as a function of coal rank is very important not only for designing the optimum solvent composition but also for studying the difference in the chemical structure of coals. The structurereactivity relationship in coal liquefaction can be elucidated by selective altering of critical constituents of coal and then examining its subsequent liquefaction behaviour. The oxygen-containing groups of coals, in particular, of lignites, have received special attention because the carbon-oxygen functionalities provide an obvious target for specific chemical alteration. In the chemical techniques used for coal treatment, alkylation and reduction are beneficial procedures for selective alteration of coal structure and functional group composition. 0-alkylation according to Liotta2’ under mildly basic conditions, Friedel-Crafts alkylation28-30 under acidic conditions, and reduction and reductive alkylation according to Sternberg and others31-36 are well known and widely used as pretreatment procedures for enhanced coal solubility. The increase in coal solubility is considered to relate to both depolymerization via ether bond cleavage, esterification, etherification and aromatic ring reduction and alkylation. Earlier the effect of alkylation with isopropyl and methyl halides and reductive treatment with metallic alkali on coal liquefaction in hydrogendonor solvents were studied by Schlosberg et ~l.~‘, Nomura et al. 38 Moore et al.39 and Baldwin et aL4’. They reported the liquefaction degree to be higher for alkylated or reduced coals compared to original samples under the conditions used. The aim of this paper was to study the effect of different solvents (tetralin, alcohols and tetralin-alcohol mixtures) on product composition and liquefaction behaviour of

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Vol 70, September

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Coal characterization: P. IV. Kuznetsov

et al.

Kansk-Achinsk lignite and to examine the influence of chemical pretreatments of coals by means of alkylation and reduction on the reactivity in liquefaction with methanol.

EXPERIMENTAL Three low sulphur coals of different rank were selected. Some characteristics of the original coal samples are given in Table 1.

Coal pretreatments The samples of original coals demineralized with HCl and HF according to Ref. 41 were subjected to the following pretreatments: 0-methylation with (CH,),SO,, reduction with LiAlH,, reduction with K/isopropanol, reductive methylation with K/CH,J and 0-methylation with (CH,),SO,, followed by reductive methylation with K/CH,J.

0-methytation

with (CH3)J0,.

Approximately tog of dried coal were dispersed in 100ml of 10% aq. KOH solution, stirred and, in an inert gas atmosphere 20ml of (CH3)2S0, were added in small amounts for 30min. In 2 h the coal was filtered off, washed with water and the procedure repeated. The methylated product was dried in vacua at 80°C. This reaction procedure transforms phenolic and carboxylic coal acid groups to aryl ether and ester groups, respectively. The reduction was perReduction with LiAlH,. formed by consecutive reactions of coal esterification with triethyl orthoformate and reduction of ethyl esters with LiAlH,. Approximately log of dried coal were mixed with 30ml of triethyl orthoformate and placed in a flask equipped with a Vigreux column. The mixture was heated at 16&l 70°C in an argon atmosphere. Three hours after cooling the reaction mixture the coal was filtered off, washed with methanol and dried in vacua at 80°C. Esterified coal was mixed with 100 ml of freshly purified tetrahydrofuran and placed in a flask equipped with a magnetic stirrer, reflux condenser and argon flow system. Then LiAlH, (1.3 g) was introduced in portions. The mixture was heated for 7 h and was left overnight with stirring under an argon stream. This procedure was repeated twice more. Ethyl acetate (15 ml) was introduced into the cold reaction mixture by drops, then 150ml of water and finally 50ml of cont. HCl were added. The reduced coal was dried in vacua at 80°C. The procedure of carboxylic is shown42 to result in the conversion oxygen to hydroxylic oxygen.

Coal reduction with K/isopropanol and reductive alkyl-

Table 1

Coal characteristics Ultimate

analysis

(X daf)

Coal

Coal basin

Ad(%)

C

H

N

o+s

Lignite

Kansk-Achinsk, USSR Janina, Poland Halemba, Poland

6.2

69.8

4.8

0.8

24.6

8.0 7.9

73.1 83.3

4.9 4.8

1.1 0.8

20.3 11.1

Subbituminous Bituminous

1032

FUEL, 1991, Vol 70, September

ation with K/CH,J were performed

initially with metallic potassium in tetrahydrofuran as reported elsewhere43. The reduced coal sample was subjected to protonation with isopropanol or to alkylation with CH,J. The reduction procedure results in cleavage of some ether bonds and to hydrogenation of certain aromatic rings in coa131,38,40. The reaction with CH,J leads to methylation of aromatic rings and oxygen functional groups31,34,36.

Coal analysis. Both the demineralized and pretreated coals were subjected to ultimate and oxygen-containing functional group analyses. The content of hydroxyl groups was determined by acetylation in pyridine and the carboxylic oxygen by reaction with calcium acetate and titration of the liberated acetic acid. Coal liquefaction Coal reactivity conditions.

in liquefaction

was tested under various

Non-catalytic coal hydrogenation. Methanol, ethanol, isopropanol, tetralin, toluene and a mixture of tetralin with methanol and ethanol were used as solvents in these hydrogenation experiments. Dry coal sample (8 or 15 g, particle size 0.08-0.20 mm) and solvent (20 g) were placed in a 0.25 1 rotating autoclave. After air removal, the autoclave was pressurized to 5 MPa with hydrogen and then heated at 380°C for 1 h. After the reaction, the gaseous products were separated. The autoclave products were extracted using a Soxhlet apparatus with hexane (malthenes) and then with benzene (asphaltenes). The coal conversion was evaluated based on the quantity of insoluble residue. Malthene yield was evaluated as coal conversion. In alcohol experiments, the gas produced was of both lignite and alcohol origin, therefore the malthene yield was evaluated as coal conversion, gas yield for the tetralin experiment and asphaltene yield. Coal hydrogenation with &Cl, catalyst. The dry coal sample was charged with ZnCl, (1 wt% Zn daf) by impregnation from ZnCl,-methanol solution. The ZnCl,charged dried coal (15 g) and methanol (40 ml) were placed in an 0.25 1 rotating autoclave. An initial hydrogen pressure of 5 MPa was used and the reaction was carried out at 380°C for 1 h. Coal solubilization with NaOH/methanot. The experiments using a NaOH/methanol system were carried out in the absence of H, at 350°C for 1 h. Dry coal (log) was mixed with a solution of NaOH in methanol (10 g/210 ml) and the resultant suspension was placed in a 2 1 autoclave. After reaction, the autoclave products were treated with large amounts of water and acidified with HCI. The water-insoluble product was filtered off, washed with water and dried at 80°C in vacua. The yield of soluble product was determined by Soxhlet extraction with hexane and toluene. Analysis of soluble products. After in vacua distillation of solvents, the malthenes produced were subjected to conventional ultimate analysis for C, H, N and S content and to ‘H n.m.r. spectroscopy. The spectra were recorded from CDCl, solution. Gaseous products were analysed using gas chromatography. Additional details of experiments and product analysis have been published previously12-‘4~‘7-‘0.

Coal characterization: P. N. Kuznetsov RESULTS

cc-region. Differences were observed in the narrow 2.1-2.3 ppm region, due to protons of methyl groups attached to aromatic rings. For ethanol products, as well as this signal, an additional resonance in the 2.4-2.5 ppm region related to the a-methylene groups attached to aromatic rings was observed. The spectra peculiarities observed have been reported earlier16, and explained by formation of alkylated aromatics. From Table 3 it is seen that the products, mostly saturated aliphatic structures, were obtained using methanol and ethanol solvents.

AND DISCUSSION

Lignite hydrogenation in tetralin and alcohols Data on lignite hydrogenation in methanol, ethanol, isopropanol and tetralin in the absence of catalyst at 380°C are presented in Tab/e 2. The solvent reactivity is in the following sequence: tetralin > isopropanol > ethanol >methanol. The highest conversion was observed for tetralin indicating that the hydrogen-donating property is important for coal liquefaction. An indication of the hydrogen-donor ability of ethanol and isopropanol has been provided using the deuterium isotope technique ’ 9s24,25. Using perdeuterated and randomly deuterated ethanol and isopropanol the isotope effect of the a-hydrogen was shown, the latter being close to that observed earlier by Scowrouski and Heredy44 and Cronauer et ~1.~~for deuterated tetralin. In order to compare the malthene composition as a function of solvent type the same distillation procedure was used for all malthene products. According to elemental analysis data (Table 3), malthene composition is strongly influenced by solvent type. The H/C ratio in malthenes from methanol and ethanol experiments is higher than that for the tetralin products. ‘H n.m.r. was used to study proton distribution for different structural groups. The following structural groups have been determined46: the signal in the 6.0-8.0 ppm region with a maximum at 7 ppm is assigned to hydrogen atoms in aromatic rings, H,,; the weak absorption in the 4.0-6.0ppm region is due to protons from hydroxyl groups, Ho”; the 1.9-4.0ppm spectrum region is due to protons in aliphatic radicals with carbon atoms in the cr-position to aromatic rings or to oxygen atoms, H,; the 1.9-0.5 ppm region is due to other aliphatic protons of alkyl substituents and paraffin molecules, H,,. The principal differences in ‘H n.m.r. spectra data for malthenes in different solvents were observed in the

Table 2 Solvent effect on lignite conversion hydrogenation at 380°C

and product

Product

Lignite hl>drogenation in tetralin-alcohol mixed solvents With the objective of changing the process intensity and selectivity a series of experiments using tetralinalcohol mixed solvents were carried out. In Figures I and 2 results indicating that alcohols promote lignite liquefaction are presented. Significant benefits are observed for solvents containing lo-20% methanol or 20-40% ethanol in tetralin. The maximum lignite conversion in the mixed solvent was - 17% greater than in tetralin alone. Similar changes are observed for

50

40

G D x

Solvent

Gas

Malthenes

Asphaltenes

Tetralin Isopropanol Ethanol Methanol

46.4 38.3 36.0 29.7

9.6 21.0 14.3 17.6

24.3 21.1 25.5 18.6

12.5

30

20

yields during

Lignite conversion (% daf)

10

yields (% daf)

’

1

20

I

1

40

100

80

60

Tetralin

Elemental

composition

and proton

1.o Figure 1 Kansk-Achinsk lignite liquefaction and malthene alkylation dependent on the tetralin-methanol solvent composition (%): (A) conversion; (B) malthene yield; (C)degree of alkylation; (D) asphaltene yield. Conditions: 0.25 1 autoclave,lignite (15g)+mixturesolvent (20g) + H, at 5 MPa, 380°C. 1 h

distribution

at 5 MPa,

in malthenes

produced

Composition (% wt) Solvent Tetralin

C 18.12

lsopropanol

from lignite hydrogenation

in different

‘H n.m.r. data H

H/C atom 1.25

H,,

H OH

K

0.23

0.02

0.32

solvents

at 380°C

H al _

Ha,

L

0.43

3.21

0.61

0.43

4.21

0.60

%,

8.26 _

_

0.19

0.01

0.37

0.14

0.03

0.35

0.48

5.86

0.55

0.13

0.03

0.44

0.40

6.50

0.54

Ethanol

78.44

8.52

1.30

Methanol

78.48

8.62

1.32

For conditions

Methanol Solvent composition (wt %)

0.9 1.5

Conditions: 0.25 1autoclave, coal (15g)+solvent (2Og)+H, 38O”C, I h, Soxhlet extraction with hexane and benzene

Table 3

et al.

see Table 2

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1991,

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Coal characterization:

P. N. Kuznetsov

et al.

i C3H70H

-

liquid product

i C(CH312 + HTO

(21

40

30

g s z s

-z -0

30

2 m z

ae 20 20

$ b z

Evidence for the accuracy of the model can be obtained by numerical comparison of ‘H n.m.r. spectra for malthenes produced by lignite hydrogenation in hydrogen-donor alone (tetralin) and in an alcohol-tetralin mixture with the condition that the hydrogenation by hydrogen donor is the same in both cases. The substitution of some aromatic protons AH,, by CH(CH,), groups should result in a decrease in the aromatic proton fraction and an increase in H, and H,, by 1 AH,, and by 6 AH,,, respectively, in isopropylated products. We can write the following expressions:

10

20

40

60

80

1+6AH,, iC,H+H

Solvent composition (wt %)

HP;

Figure 2 Kansk-Achinsk lignite liquefaction and malthene alkylation dependent on tetralin-ethanol solvent composition (%): (A) conversion; (B) malthene yield; (C)degree of alkylation; (D) asphaltene yield. For conditions see Figure 1

asphaltene yield. The malthene yield however did not vary so greatly. In order to determine the role of donating and alkylating properties of mixed solvent, a thorough analysis of malthene composition as a function of solvent composition was carried out using a mechanistic mode117-20. This model has been described for coal liquefaction in methanol in the presence of hydrogen and includes two competing reactions: coal hydrogenation by hydrogen and alkylation by methanol. The method was based on a detailed comparison of product composition obtained during coal hydrogenation in methanol and without methanol. It has been shown that the peculiarities of the composition of liquid products obtained by lignite hydrogenation in methanol were completely and quantitatively described by methanol alkylation of aromatic rings both in the absence and presence of chlorides and hydroxides of transition metal catalysts. In fact, methanol failed to donate hydrogen under the conditions mentioned. In the presence of NaOH as catalyst, product composition differed considerably from the model composition calculated due to the fact that under NaOH action methanol manifested both alkylating and hydrogendonor properties49 ’ *I ’ 9. Using this method’7-20 as a base the variations in composition of liquid products obtained from lignite hydrogenation experiments with an alcohol-tetralin mixture can be reproduced and the role of different alcohols can be studied. By altering the model, it can be assumed that lignite hydroliquefaction in alcohol-tetralin solvent is an additive process incorporating coal hydrogenation by a hydrogen-donor and coal alkylation by alcohol : +

hydrogen donor

-

liquid product

dehydrogenated

(1)

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1991,

Vol 70, September

AH,,

1+6AH,,

100 Ethanol

Tetralin

1034

H z.-

iCjH70H _ I-4, -

10

_

-

Htp:fr. + 6AHar 1+6AH,,

(5)

where iC,H,OH and tetr. are related to the malthenes produced in coal hydrogenation in isopropanol-containing media and in tetralin alone, respectively. The denominator takes into account the increase in the whole proton number in the products as a result of substitution of one aromatic proton by seven isopropyl protons. So there are three equations and four unknown values (AH,,, HiyH70H, HLc3H70Hand Hj$“OH). To solve these correlations one can make use of the experimental value of HiCJHToH. It becomes possible to calculate AH,, using: ar H F,‘. _ H;~,H,OH AH,, = (6) 1 +

6

~;i$H70H

which is simply a rearrangement of Equation (3). Using the AH,, value, one can easily compute the values of H;C,H,OH and H~&JHTOH. Comparing the experimental and computed H, and H,, values for malthenes produced in isopropanol-containing solvents, evidence of the accuracy of the model used can be obtained. The AH,, x 100%/H:. ratio is the alkylation degree, i.e. the fraction of aromatic protons substituted by iC,H, groups. Knowing the alkylation degree, elemental composition and molecular weight, we can calculate the number of isopropyl groups introduced into the ‘average molecule’. By analogy, the corresponding calculations can be carried out for products obtained in methanoland ethanol-containing solvents. Data for proton distribution in malthenes, obtained by lignite hydrogenation in tetralin and in different alcohol-containing media are presented in Table 4. Some calculated values using the accepted model are also included. It is seen from Table 4 that with increase in alcohol content in the mixture, the fraction of aliphatic hydrogen in malthenes also increases. For methanol and isopropanol products the increase in aliphatic hydrogen is due to H, atoms and for ethanol due to both H, and H/i,. The results of a quantitative treatment using the model described showed that experimental and calculated values of H, and H,, coincide within the error limits (+O.l) for all ethanol-containing solvents and for

Coal characterization: Table 4

P. IV. Kuznetsov

Proton distribution in malthenes produced from lignite hydrogenation dependent on the tetralin-alcohol H,

et al.

solvent composition H6,

Degree of alkylation (%)

Solvent composition (%)

H,,

H OH

Exp.

Calc.

Exp.

Calc.

Methanol 10% + tetralin 90%

0.19

0.01

0.41

0.42

0.39

0.37

Methanol 40% + tetralin 60%

0.18

0.02

0.42

0.43

0.38

0.37

Methanol 80% + tetralin 20%

0.16

0.02

0.43

0.46

0.39

0.36

-21

Methanol 100%

0.14

0.03

0.44

0.49

0.39

0.35

-28

Ethanol 10% + tetralin 90%

0.19

0.01

0.38

0.38

0.42

0.41

8

Ethanol 20% + tetralin 80%

0.17

0.03

0.39

0.38

0.41

0.43

14

10 14

Ethanol 40% + tetralin 60%

0.16

0.02

0.40

0.39

0.42

0.43

17

Ethanol 60% + tetralin 40% Ethanol 100%

0.15 0.14

0.03 0.02

0.41 0.41

0.39 0.39

0.43 0.43

0.44 0.45

20 23

Isopropanol

0.19

0.01

0.37

0.34

0.43

0.47

0.22

0.02

0.37

_

0.39

_

100%

Tetralin 100%

Conditions: 0.25 1 autoclave, lignite (15 g)+solvent

(20 g)+ H, at 5 MPa, 38o”C, 1 h

methanol-tetralin mixtures with a methanol content of < 80%. This means that malthene composition is quantitatively described by hydrogen donation (Equation (1)) and alkylation (Equation (2)). It should be noted that the reactions mentioned also describe the composition of products obtained in the experiments with ethanol alone. There is every indication that ethanol manifests both hydrogen-donor and alkylating properties. Moreover, it is important to note that ethanol and tetralin give the same products from coal via Equation (1). The great differences between calculated and experimental H, and H,, values are observed for products obtained in coal hydrogenation in a methanol-tetralin mixture with high methanol ratio and in the medium of methanol alone. This agrees with the conclusion’g,20 that methanol does not possess hydrogen-donor properties under the given conditions. In the case of isopropanol, differences between computed and experimentally measured H, and H,, values exceed the ‘H n.m.r. error and can apparently be related to the thermal instability of the isopropyl substituent or to its rearrangement. With increase in ethanol content from 10 to lOO%, the degree of alkylation increases from 8 to 23%. Taking into account data on elemental analysis (Table 3) and molecular weight (- 300)16,i9 this corresponds to the introduction of 0.5-l .4 ethyl groups into each ‘molecule’. Figures 1 and 2 show that in contrast to the extreme dependence of coal reactivity on solvent composition, the degree of alkylation increases continuously with the increase in alcohol content in the mixture. The principal increases in lignite reactivity and in degree of alkylation are observed in the same region of solvent composition. Pretreatment solubilization

efSect on coal hydrogenation in methanol

-13

and

To further characterize coal alkylation and reduction and to understand their effects on coal properties and liquefaction reactivity we used three coal samples with different rank. The preliminary structure alteration was performed via selective chemical reduction and alkylation procedures. Kansk-Achinsk lignite, Yanina subbituminous and Halemba bituminous coals were used. The coal

samples were demineralized and subjected to O-methylation with (CH,)$04, reduction with LiAlH, and with K/isopropanol and reductive methylation with K/CH,J. Data on elemental composition, the initial quantity of oxygen-containing carboxyl and hydroxyl groups, reduced and alkylated samples as well as the results on conversion during hydrogenation in methanol at 380 and 350°C both in the absence of catalyst and in the presence of ZnCl, and NaOH are summarized in Table 5. The starting coals significantly differed in elemental and functional composition. From lignite to bituminous coal the content of carboxyl and hydroxyl groups decreased from 3.5 to 0.6% and from 6.9 to 2.5%, respectively. Several trends are evident on close inspection of the data. One can see that reduction and alkylation result in considerable changes in coal composition. Thus, the hydrogen content in lignite and bituminous coal increased from 4.8 to 5.5-5.6% and from 5.1 to 5.6-5.9%, respectively. At the same time, the oxygen functional group composition differed dramatically as a result of pretreatment. After reduction of Kansk-Achinsk lignite with LiAlH, the content of carboxyl groups decreased from 3.5 to 1.l % and the content of hydroxyl groups increased from 6.9 to 8.3%. After treatment with resulting from 0-alkylation, a sharp con(CH,),SO,> centration decrease of both carboxyl (to 0.6%) and hydroxyl groups (to 1.3%) was observed. The analogous tendency took place for Yanina subbituminous and Halemba bituminous coals. After methylation all three coals had similar carboxylic and hydroxylic functional group contents. Data in Table 5 show that except for non-catalytic experiments, the conversion of coals after preliminary 0-methylation reduction and reductive methylation decreases slightly (by 336%) compared with starting samples. This tendency is observed for all coals used, and contrasts with results obtained by other authors37-40,46. For example, Nomura et al. 38 observed enhancement of reactivity of coals in liquefaction in a medium of active hydrogen-donor tetralin at high temperature (425450°C) after they were reduced with metallic potassium in tetrahydrofuran or with sodium in ammonia. One of the reasons for the apparent discrepancy of the low lique-

FUEL,

1991,

Vol 70, September

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Coal characterization: Table 5

Reduction

P. N. Kuznetsov

and alkylation

et al.

effect on coal composition

and liquefaction

behaviour

with methanol Oxygen functional group content (Xwt daf)

Ultimate analysis (%wt daf)

Conversion Run

Coal

Pretreatment

C

H

WC

0

COO”

0 OH

W)

1”

Subbit.

Demineralization

1.2

75.2

5.0

0.80

2.7

4.8

13.5

2”

Subbit.

Reduction

1.1

76.3

5.2

0.82

0.8

5.6

16.4

0.5

0.6

15.3

with LiAIH,

3”

Subbit.

0-methylation

1.4

74.5

5.0

0.81

4b

Subbit.

Demineralization

1.2

73.4

4.9

0.80

3.1

4.1

38.1

56

Subbit.

Reduction

1.1

75.0

5.3

0.85

0.8

5.9

32.2

@

Subbit.

0-methylation

1.4

74.5

5.0

0.81

0.5

1.1

31.5

7s

Subbit.

Reduction with LiAIH,, 0-methylatton wtth (CH3)*S0,

1.4

74.7

5.8

0.93

0.4

1.4

32.1

8’

Lignite

Original

6.2

69.8

4.8

0.82

3.5

6.9

41.9

2.8

73.2

5.6

0.92

1.1

8.3

38.5

2.2

71.7

5.5

0.92

0.6

1.3

35.5

with (CH,),SO,

with LiAIH, with (CH,),SO,

9’

Lignite

Reduction

10’

Lignite

0-methylation

11’

Subbit.

0-methylation with (CH,),SO,, reductive methylation with K/CH,J

1.4

75.5

5.4

0.86

0.4

1.2

28.7

12’

Bituminous

Demineralization

1.8

83.5

5.1

0.73

0.6

2.5

18.2

13

Bituminous

Reduction

with K/isopropanol

1.4

82.9

5.6

0.81

0.2

3.5

16.0

14’

Bituminous

Reductive

alkylation

1.7

83.1

5.9

0.85

0.1

0.8

15.9

“Conditions: bConditions: ‘Conditions:

with LiAIH, with (CH,),SO,

with K/CH,J

0.25 1 autoclave, coal (8 g) + methanol (20 ml) + H, at 5 MPa, 380°C 1 h, Soxhlet extraction with benzene 2.0 I autoclave, coal (lOg)+methanol (210ml)+NaOH (log), 35O”C, 1 h, Soxhlet extraction with toluene 0.25 I autoclave, coal (15g) with ZnCl,(0.31 g)+methanol (40ml)+H, at 5 MPa, 380°C 1 h, Soxhlet extraction

faction reactivity of our preheated coals can be related to their thermally sensitive nature and to the poor hydrogen-donor property of methanol as solvent. These circumstances are probably favourable for crosslinking during the initial reaction stages. In other words, events occurring during the initial stages of liquefaction can render subsequent processing much more difficult if they are not properly controlled. In the presence of an active hydrogen-donor solvent, the tendency for crosslinking reactions to take place is effectively countered and the selectivity of the reaction products towards liquids is enhanced. Coal reactivity in liquefaction to a great extent depends on coal rank. At the transition from Halemba bituminous to Janina subbituminous coals and to Kansk-Achinsk lignites, the degree of conversion increases from 16-18 to 29% and 36-42%, respectively, i.e. coal reactivity increases with decreasing coal rank. Reactivity of coals is influenced by experimental conditions. Thus, the conversion of different Janina coal samples, evaluated by benzene-insoluble residue, at 380°C without a catalyst is 13-16%, with ZnCl, catalyst is 44%, and with NaOH catalyst at a lower temperature of 350°C is 32-38%. The elemental composition of soluble products from different coal samples depended weakly on pretreatment conditions. In ‘H n.m.r. spectra of methanol malthenes the intensity of the signal in the 2.1-2.3ppm region, corresponding to methyl groups attached to aromatic rings, increased from bituminous to subbituminous coals and to lignite, indicating increase in the degree of alkylation. This dependency of malthene alkylation degree on coal rank has been observed previously”. Besides the main signal at 2.1-2.3 ppm a weak signal at 3.774.0 ppm was also observed, which can be linked47,48 to protons of methoxyl groups. In Figure 3 ‘H n.m.r.

1036

FUEL, 1991, Vol 70, September

with hexane

spectra of malthenes obtained from Janina coals during liquefaction in methanol and in toluene are presented. From Figure 3 it is seen that in toluene products the signal from methoxyl protons was observed for malthenes from 0-methylated coal. On liquefaction of different coal samples in methanol in significant quantities gaseous products were formed (with methane constituting > 5&60%). Methanol replacement by toluene did not lead to essential changes in coal conversion but significantly reduced gas yieldmethane in particular (Table 6). In toluene, the highest methane yield was observed for the 0-methylated sample, indicating its origin was from methoxyl groups49. From ‘H n.m.r. data of liquids and the composition of gases produced, it appears that methanol in coal liquefaction as well as CH,J in preliminary O-methylation lead to the formation of methoxylated compounds. The latter are thermally unstable under the liquefaction conditions49,50 and undergo decomposition to methyl aromatics and methane. CONCLUSIONS Kansk-Achinsk lignite hydrogenation in tetralin, isopropanol, ethanol and methanol was studied. Tetralin was the most active solvent. Synergistic effects were observed when the mixture of tetralin-alcohols was used, indicating an interaction between coal and solvent during liquefaction. The variations in liquid product composition were analysed by a mechanistic numerical model incorporating two competitive reactions-alkylation and hydrogen donation. The lignite liquefaction in ethanol alone and in ethanol-tetralin mixture was shown to be the additive process of these reactions. The principal reason for the synergistic effect on lignite conversion seems to be related to the swelling of the coal structure

Coal characterization: Table 6

Effect of subbituminous

coal pretreatment

on the yield of gaseous products

from coal hydrogenation

with methanol

Gas composition Liquefaction solvent

Yield (% daf)

co

Demineralized

Methanol

24.6

Reduced

Methanol

19.1

Methanol Toluene

Coal

with LiAIH,

O-methylated

with (CH,)2S0,

Demineralized Reduced

with LiAIH,

0-methylated

with (CH,),SO,

The experiments

were carried

P. N. Kuznetsov

et al.

and with toluene at 380°C

(%wt)

CO,

CH,

C,H,

13.7

33.0

49.3

0.5

3.5

2.1

29.3

60.1

0.4

7.8

20.7

1.3

26.7

63.0

1.6

6.2

69.9

18.7

0.6

4.6

Toluene

7.4

17.8

43.8

32.1

0.5

5.8

Toluene

10.8

9.1

44.5

43.5

0.3

2.6

out according

C,H,

8.9

to runs 1-3 in Table 5

solubilization in the medium of methanol and in the presence of both ZnCl, and NaOH catalysts. The liquefaction behaviour was strongly influenced by coal rank indicating2~‘4~‘7~21 that low rank coals, such as lignites, liquefy more readily compared with high-rank coals. The results obtained provide further details235 ‘ss2 of coal reactivity in liquefaction. This study will be extended by examining the liquefaction behaviour of pretreated coals in tetralin, by analysing product composition in detail and by discussing the nature of the effects observed.

ACKNOWLEDGEMENTS The authors wish to thank Dr G. Holopova the ‘H n.m.r. spectra.

for providing

REFERENCES 1 2 3 4 5 6 7 8 9 10 11 8

4

6

2

0

ppm Figure 3 ‘H n.m.r. spectra ofmalthenes produced from demineralized (B, E), reduced (C, F), 0-methylated (A, D) Janina coal during hydrogenation in methanol (D-F) and toluene (A-C) solvents. Conditions: 0.25 I autoclave, coal (8 g) + methanol (20 ml) for toluene (40 ml) + H, at SMPa, lh

12 13 14 I5 16

improving the penetration of solvent and the contact between coal constituents with tetralinz5. Preliminary 0-methylation with (CH,),SO,, reduction with LiAlH, and with K/isopropanol and reductive methylation with K/CH,J resulted in considerable changes in coal composition and oxygen-containing group distribution. As a rule preliminary reduction, 0-alkylation and reductive alkylation treatments lead to a slight decrease in coal reactivity in liquefaction and

17 18 19 20 21 22 23

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