Molecular mass distribution of modified coal extract

Molecular mass distribution coal extract

of modified

I. E. Nosyrev, M. D. Stefanova*, I. B. Rashkovt, V. I. Bessarabov and A. F. Popov

S. P. Marinov”,

The L. /M. Litvinenko Institute of Physical, Organic and Coal Chemistry, Academy of Sciences, Done&k 340114, Ukraine The Institute of Organic Chemistry and i Polymers, Bulgarian Academy Sofia 17 13, Bulgaria (Received 72 February 1992; revised 15 September 1992) l

Ukrainian of Sciences,

The chloroform soluble portion of dimethylacetamide extract from a subbituminous coal was subjected to different types of chemical modification, i.e. reduction, reductive alkylation, non-reductive alkylation, ‘ionic’ oxidation with subsequent alkylation, etc. The products of the reactions were studied by spectral and chromatographic methods. An increase in molecular mass, prevailing over degradation, was registered by

gel permeation chromatography (g.p.c.) analysis. (Keywords:

coal; reduction;

alkylation)

Different approaches to coal modification, with the purpose of solubilization under mild conditions, have been proposed, i.e. reductive alkylation’p4, acidic depolymerization ‘,‘j, ionic hydrogenation7, etc. A procedure for coal chemical modification by ‘ionic’ oxidation has been applied to coal organic matter (COM)‘-lo. After such treatment the solubility of coal in polar solvents, such as tetrahydrofuran, or NJ-dimethylacetamide (DMAA), reaches 90-95 wt%. The first step in the procedure is COM activation by alkaline metal treatment and, subsequently, reaction with oxygen as an electrophile. This approach is attractive as it does not lead to the addition of alkyl radicals, so that the coal elemental composition was only slightly influenced. In this study products of coal ‘ionic’ oxidation and other types of COM modifications have been compared. Dimethylacetamide (DMAA) extract of coal was prepared and considered as a model of the COM ‘mobile’ phase. Its extensive study provides the basis for ideas to explain coal solubility and coal macromolecular structure.

EXPERIMENTAL Sample

Subbituminous coal, Done&k deposits (Ukraina), with the following characteristics was used: Maceral composition: vitrinite, 82 vol.%; liptinite, 6 vol.%; inertinite, 12 vol.%. Proximate analysis: VM, 42 wt%; ash, 13 wt% (dry basis). Ultimate analysis: (daf) C, 84.8 wt%; H, 5.4 wt%; S, 1.1 wt%; N, 1.6 wt%; Odiff., 7.1 wt%. 0016-2361/94/05/067845 ~0 1994 Butterworth-Heinemann

Ltd

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Extraction

The exhaustive DMAA extraction was performed at ambient temperature. The extracted matter was isolated by HCl precipitation (pH 3) and water washed until it lacked Cl; it was then dried at 80°C in a vacuum oven (2 x lo-” Pa). The extraction sequence is illustrated on Figure

1.

Chemicat

modijcation

The reduction’ of the chloroform soluble portion of DMAA extract was carried out by potassium in tetrahydrofuran (THF) media. THF was purified in situ by anthracene-K ‘blue solution’ and directly transferred by lyophilization under deep vacuum in the reaction vessel”. A portion (2.5 g) of the chloroform soluble part of the DMAA extract and 0.5 g potassium were placed in a round-bottomed flask; THF (50ml) was transferred by dry freezing, and the solution was stirred for 10 h under reflux. All the manipulations were carried out in an inert atmosphere. Reduction.

Reductive butylation. This treatment2 followed the previously described reduction. An excess of n-BuI (three-fold to potassium) distilled over copper wire, was added to the reaction mixture. After 24 h the products of reaction were acidified (3% HCl) and transferred to steam distillation4.

After the reduction step, dry oxygen Oxidation by 02. (99.99 vol.%)” was passed through the reaction mixture at O”C, at a rate of 10 1h-‘. Oxygen absorption was measured by manometric installation. Unreacted potassium was quenched by n-BuOH, then

Molecular

points in the concentration range (4-60 g l- ‘) were prepared and the curve extrapolated to infinite dilution. The h?i, was determined with an accuracy of +_5%.

IC 0,a 11 extraction ~bgWl-

mass distribution of coal extract: I. Nosyrev et al.

by

FT-i.r. spectra. Samples were prepared13 in KBr pellets (1 mg 300 mg- ‘) and spectra were recorded by co-adding 400 ‘scans’ at a resolution of 2 cm- ‘.

1 extractionby jpetroleum ,ether

13C n.m.r. spectra. ’ 3C n.m.r. spectra were recorded’ 4 in CDCl, at 62.89 MHz on a Bruker WM-250 spectrometer. The inversed gated decoupling mode was used under the following conditions: 30 mg of relaxation reagent Cr(AcAc), and 200mg of the sample; pulse duration 10 ~LS(about 30” pulse angle); 0.25 s data acquisition period with the decoupler on; 6 s delay period with the decoupler off; and number of scans about 6000. RESULTS

AND DISCUSSION

The proposed chemical modification of the coal organic matter (COM) can be divided into two stages:

jl

orlrhtlon OqlBens

c

by

i_

Figure

1

Scheme of isolation

and chemical

modification

of extract

the mixture was acidified to pH 4-5. The excess butylation reagent was distilled. The products were water washed, dried and concentrated at reduced pressure. Non-reductive butylation. A part of the products of ‘ionic’ oxidation was treated by BuLi/BuI in THF”. Briefly, the sample (0.8 g) and freshly distilled THF (‘blue solution’) were mixed with tetramethylethylenediamine (TMEDA) (20ml) under a stirring and cooling argon atmosphere (- 30°C liquid N,). After 30 min, 20 ml BuLi solution were added dropwise over a period of 40 min to the reaction mixture. The mixture was left at room temperature for 2 days. Products of modification were isolated as described previously. Characterization of the modification products Determination of molecular mass distribution by gel permeation chromatography (g.p.c.). The g.p.c. study was

performed with THF as the mobile phase (1 ml mini, 45°C) with columns of @Styrage 100, 50 and 2 x 10nm; DRI was employed as a detector. The concentration of the sample applied was 1 mg ml- ‘. The system was calibrated by polystyrene standards. The error of calculation was f 5%. _ Determination of number average molecular mass (El,). M, was determined by vapour phase osmometry (v.p.0.) in THF. The measurements were performed at 45°C and the system was calibrated by benzyl. Three

1 Reduction by K/THF reflux2. The following reactions can take place: (a) hydrogen exchange at the positions of proton donor groups, and protons at ‘acidic’ centres; (b) electron transfer from the metal (K) to unsaturated coal fragments, resulting in radical anions’ 5 and anion formation; (c) ether bond cleavage and splitting of C-C linkagesi6; and (d) formation of potassium-coal adducts, an analogy to graphite intercalation compounds’ 7. 2. Reaction with the electrophile (e.g. alkyl halide, molecular oxygen). An increase of COM solubility after reductive alkylation can be attributed to the rupture of covalent bonds and to the introduction of bulky alkyl radicals preventing association”. In this study, the effect of different reactions was assessed by means of g.p.c. separation of modified extracts. The first stage of treatment, activation of COM by potassium, was similar for all experiments. Then, the reaction was completed by alkyl halide, in one case (B), and by oxygen in another (C). Oxygen introduction enhanced intermolecular interactions because of the increase in the content of functional groups. Absorption of oxygen containing groups was registered in the i.r. spectra(Figure2)at 1644,170Oand 101&1260cm-‘. On the other hand, introduction of Bu radicals in the extract (reductive butylated, sample B) and in sample D (non-reductively butylated) was accompanied by an H/C atomic ratio increase (Table 1) and enhancement of the vibration frequencies for -CH, and -CH, groups, i.e. 1375, 1450 and 2840-2954 cm-’ (Figure 2). Sample D differs from sample B in the following characteristics: 1. the Bu groups were added after previous oxidation; and 2. Bu groups occur predominantly at CHpositions (for pKa in the range of 19-31 12).

‘ionic’ acidic

Comparison of g.p.c. separations of samples C and D enables the influence of the butyl groups on the molecular mass distribution to be followed. Elution diagrams of the samples under study are shown on Figure 3. Results for number average molecular mass (n;;i,) of the samples determined by v.p.0. and g.p.c. are summarized in Table 2. It is clear from the g.p.c. diagrams, that the

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Molecular Table 1

mass distribution

Characteristics

of extract

of coal extract: I. Nosyrev et al. and products

of its modification Elemental

composition

(wt%)

Sample

Index

C

H

N

S

Dd”‘.

H/C atomic ratio

Extract

A

76.86

6.31

1.58

1.82

13.43

0.98

B C

81.3 77.1

7.8 7.2

1.2 1.8

1.5 1.4

8.2 12.5

1.16 1.11

D

83.2

8.3

1.5

0.4

6.6

1.19

Products of modification Reductive butylation ‘Ionic’ oxidation ‘Ionic’ oxidation/ non-reductive butylation

Table 2 Number average molecular masses of the modified extracts riik, initial extract; ti:, extract reductively butylated; $lz, extract after ‘ionic’ oxidation; iii:, extract after ‘ionic’ oxidation/non-reductive butylation Method of determination v.p.0. G.p.c. a Measured bCalculated

470 420

560” 530”

400b 375b

610 490

650” 570”

420b 370*

on alkyl free basis

Figure 3 Molecular mass distribution of coal extract and products of its modification: 1, initial extract; 2, products of ‘ionic’ oxidation; 3, products of reductive butylation; and 4, products of ‘ionic’ oxidation/non-reductive butylation

Figure 2 FT-i.r. spectra of extract and the products of chemical modification: a, initial extract; b, products of ‘ionic’ oxidation; c, products of ‘ionic’ oxidation/non-reductive butylation; and d, products of reductive butylation

molecular mass of the oxidized product C did not decrease during the treatment due to bond cleavages, but it even increased. In the case of alkylated extracts (samples B and D) the values for R, are higher, explained by addition of the Bu groups. Possible explanations for the R,, increase in the process of ‘anion’ oxidation of extract (sample C) are: 1. At the reduction step, the anion centres in COM undergo a number of transformations resulting in radical formation with possible dimerization. Similar reactions, dimerization of naphthalene, pyridine, quinoline, etc., in naphthalene-potassium solution, are described in the literature19-*l.

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2. At the step of ‘polyanion’ treatment by electrophile, the following reactions can take place: (a) an increase in the degree of association on the base of oxygen containing groups, in the case of oxidation; (b) formation of the ether bonds during oxidation; this process is similar to ‘coal weathering’ at moderate heating (up to 80°C) in air’* (besides ether bridges, C-C linkages could be created under lack of oxygen23; and (c) dimerization by the mechanism analogous to the Wurtz procedure at the alkylation stage. One possibility to assess the increase in molecular mass, caused by the influence of alkyl radicals, is to calculate their number (n). The 13C n.m.r. spectra of the modified products were quantitatively interpreted according to the procedure described by Lazarov et c11.‘~.This approach enables C-butyl groups to be estimated from the intensity of the signal of C, carbon atoms at G15 ppm, and of 0-Bu groups by the signal of C, carbon atom at

Molecular

mass distribution

of coal extract: I. Nosyrev et al.

methods is an indication of the existence of substantial association in sample C (extract after ‘ionic’ oxidation). The number of Bu groups calculated on the basis of the difference, li;f!$J$ (substitution of ji?z with tit), increased as the imtlal extract, A, was less influenced by association. However, this value, n= 7.0, is still low compared to that determined by the 13C n.m.r. procedure, where n = 8.9. There is, however, a good coincidence for the n values obtained by g.p.c. and n.m.r. techniques by the differences n:- Hf. The Mt value determined by g.p.c. is slightly affected by association. Thus, g.p.c. separation proceeds on the molecular level as the interaction with the separation gel diminishes association to some extent. The v.p.0. determined values for li;i, are higher compared with calculated values (Table 2). The following formula was employed for the calculation of the number of Bu groups added, for example in the case of sample D: nv.P.O. = (li;r:-R;)xcx104 lG;if:x M,” x C%

160

200

120

40

80

-

0

PPm

Figure 4 13C n m .r. spectra of initial and modified extracts: A, initial extract; B, products of reductive butylation; C, products of ‘ionic’ oxidation; and D, products of‘ionic’oxidation/non-reductive butylation

Table

3

(n per 100

Number C)

of butyl

groups

added

per 100 carbon atoms

Sample

v.p.0.

G.p.c.

‘% n.m.r.

Extract reductively butylated (B) Extract after ‘ionic’ oxidation/ non-reductively butylated (D)

4.2 7.0” 1.6*

5.4 6.7” 3.6’

1.5

a Calculated b Calculated

composition composition

by Mix - Mt and elemental by Mf: - a: and elemental

8.9 of D of D

55-75 ppm (Figure 4). Independently, the values for n determined by v.p.o., n.m.r. and g.p.c. are compared in Table 3. The calculated values are not considered exact as the applied methods suffer disadvantages: problems of association in the v.p.0. measurements; dependence on the choice of calibration standards in g.p.c. separation; and achievement of quantitative 13C n.m.r. spectra. Nevertheless, the calculated numbers of added groups (n) gives a guide to the reaction mechanism. The difference, a:Ii?:, should correspond to the mass introduced by the alkyl radicals. If the association phenomenon exists for sample C, the measured value for ri;iz will be higher. The latter will affect the difference, and lower values of n will be calculated. On the other hand, alkylation modification has destroyed some association interactions. Hence, the difference, ri;if;‘- &i: will not correspond to the real number of Bu groups added and to the data calculated on the basis of g.p.c. analysis (Table 3). The discrepancies in the values determined by the different

where I$’ is the molecular mass determined by v.p.0. for sample D; M,, is the molecular mass of the butyl group; C is the weight of the carbon atom, in g; and C% is the carbon content for sample D, in wt%. Table 3 shows that the number of Bu groups added strongly depends on the type of modification. The discrepancies between the values determined by v.p.0. and g.p.c. for extracts A and B can be accounted for by the relative error of the measurements. Nevertheless, it is considered that this coincidence is sufficient for the purposes of this study. Extremely high values were measured by v.p.0. for the samples involved in oxygen treatment. An association on the basis of oxygen functional groups is demonstrated to a higher extent in the case of ‘ionic’ oxidation of the extract (sample C). The reaction mechanism of the treatment is complicated, as the main reactions are in opposition: bond cleavage, resulting in COM destruction and ether bridge connection of individual species. An increase in molecular mass during ‘ionic’ oxidation was registered, so a preponderance of ether linkages over covalent bond rupture is assumed. Spectral data for the existence of 0-alk groups were available in the 13C n.m.r. spectra of the extract subjected to ‘ionic’ oxidation (Figure 4). Two ether bonds per 100 C atoms were calculated on the basis of the interpretation of the signal at 55-75 ppm in the 13C n.m.r. spectra14. This number corresponds to two ether linkages per 2.5 ‘average’ molecules, calculated by structural analysis on the basis of elemental composition, R, and n.m.r. data. A higher increase in molecular mass should be registered in the elution diagram. On the contrary, from the elution curve of sample C, compared to the chromatographic behaviour of the initial extract, a slight shift to higher masses is shown. Such a pattern of elution can be explained by assuming a preponderance of ‘intramolecular’ ether bond formation, or by preferable participation of lower molecular weight particles in the reaction creating ether bridges. These data provide ground to propose an association on the basis of oxygen containing groups, and formation of ether bonds in the products of ‘ionic’ oxidation. At the next stage, non-reductive alkylation, ether linkages were split by BuLi treatment, so only evidence of C-alkylation was registered in the 13C n.m.r. spectra of sample D (Figure 4).

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Molecular

mass distribution

of coal extract: I. Nosyrev et al.

COM destruction at the first stage of treatment should be mentioned. Unfortunately, it is impossible to distinguish the decrease in molecular mass on account of rearrangement of associated species, and this is provoked by covalent bond splitting. CONCLUSIONS An increase in molecular mass during ‘ionic’ oxidation of coal extract was registered by spectral and chromatographic methods. An association on the basis of oxygen containing functional group and crosslinking by ether bonds is proposed as a possible explanation. At the same time, a portion of the organic matter was degraded resulting in lower molecular weight particles. Results from the chemical modification of the chloroform soluble portion of dimethylacetamide extract can be interpreted on the basis of the coal organic matter. Extracted substances can be considered as a ‘model’ of the coal structure, representing that portion capable of extraction, the so-called ‘mobile’ phase. In the case of coal, other features have to be taken into account, for example: 1. the differences in coal structure due to rank; 2. the chemical reactions proceed in heterogeneous media so that intensive ether and C-C bridge formations can be expected; and 3. the degradation reaction loosens the coal macromolecular network rather than forming low molecular particles. ACKNOWLEDGEMENTS

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The authors express their gratitude to Ing. Maria Goranova from the Bulgarian Academy of Sciences for the v.p.0. measurements.

682

REFERENCES

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