Geochemical origin of long chain alkyl aromatics in coal: 2. Model reaction of lignin with alcohol

Short Communications

Geochemical origin of long chain reaction of lignin with alcohol

Ji-Zhou

alkyl aromatics

in coal: 2. Model

Dong and Koji Ouchi

Faculty of Engineering, (Received 16 January

Hokkaido University, 1989; revised 3 May

Sapporo 1989)

060, Japan

The reaction of lignin with n-Cl4 alcohol (as a model of hydrolysate of lipid) at 12&18O”C using active clay as catalyst was studied. The n-hexane insoluble product and lignin itself were then hydrogenated under mild conditions. The benzene soluble fractions of reacted product contained n-C,, substituted phenols and C,-C, substituted phenols. The latter have also been found in the hydrogenation product of lignin itself. Therefore alcohol could link to the benzene nuclei of lignin. As long alkyl chain substituted phenols are found abundantly in coal liquids, pyrolysates or extracts and are regarded as the main constituent of coal, it is suggested that the early diagenesis process includes the reaction between lignin and alcohol (or fatty acid) which may be the hydrolysation product of lipids widely occurring in nature. (Keywords: structural properties; aromatic; hydrogenation)

Earlier studies on the origin and formation of coal suggest that organic plant materials were transformed microbially and/or by oxidation to humic materials, then by abiotic thermal processes to lignite, bituminous coal, and anthracitele3. Cellulose was easily degraded to CO, and water in the early diagenetic stage4-10, while lignin usually survived with only slight alteration of its chemical structure (as phenol more defderivatives)’ l-1 ‘. However, initive evidence is needed. Long alkyd chain aromatic compounds have been found abundantly in coal products such as coal extracts, pyrolysates and hydrogenation products’-“. For example, Allan and Larter” reported that long-chain alkyl aromatics Z-6 to Z-18 were present. More recently, Given et a1.23-26, Sugimoto et a1.27, Yokoyama et a1.28 -3’, and other authors3’-37 also found a number of homologous series of long alkyl chain aromatics or hydroxyl aromatics. Although lignin is thought to be an important source material of coal, it is difficult to visualize any lignin-originated macromolecular structure including long chain alkyl aromatics, since lignin is composed of phenyl propanoid units. Although some special compounds with long alkyl chains, such as Vitamin K and the carotenoids, are usually contained in plants, animals or microorganisms in small amounts ‘9.38m39, it is impossible to accept the concept of coal formation from such a small amount of compounds. Such compounds may have been derived from the reaction between lignin and long straight chain alcohols, fatty acids or their esters, which are widely distributed 32m33.40m45 in the form of 0016~2361/X9;1U1354~04$3.00 (? 1989 Hutterworth & Co. (Publishers) 1354

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esters (lipids) in plants, animals and microorganisms, especially in aquatic plants, e.g. algae. Lipids were probably initially hydrolysed into fatty acids and alcohols during the accumulation stage, which then reacted with lignin or lignin debris (e.g. humic acids) in the presence of clay minerals, thereby producing the corresponding alkylated phenol derivatives. Previously46, we gave evidence for this hypothesis using phenol as a model compound for lignin. Such long alkyl chain phenols are then easily transformed to alkyl naphthalenes and phenanthrenes, with increasing reaction time and temperature. Since these compounds are generally found abundantly in coal products, it seems plausible to suggest that a similar reaction could also be involved during the formation of coal from plant materials. In this report, the above results are extended to the reaction between lignin itself and alcohol. EXPERIMENTAL Samples

Lignin was prepared in an autoclave by heating white birch chips (1 kg) in a lOwt% NaOH solution at 170°C for 3 h under nitrogen. After cooling, the supernatant liquid was filtered, acidified with HCI and a small amount of chloroform was added to the acid solution to coagulate lignin. About 200 g of precipitated lignin (about 20 wt%) was Table 1

Analytical

collected, washed with water to a neutral PH and dried at 50°C under vacuum. Chemical reagent grade of n-C,, alcohol and activated clay were obtained commercially, and used without further purification. Reaction

data of the lignin and the reacted C%

products

H% ~~-

Lignin Fraction

1-2

60.9 76.2

6.0 8.7

of lignin with n-C,,

alcohol

Lignin (2g), n-C,, alcohol (2 g) and activated clay (2 g) were placed in a 50 ml autoclave equipped with a magnetic stirrer, and then subjected to the reaction under OSMPa nitrogen atmosphere at 120°C for 160 h and at 160°C for 96h. After cooling, another 1 g of clay was added to the reactor and the reaction was continued further at 180°C for 376 h. The reason for the stepwise reactions is based on the following considerations: Since the oxygen content (mainly as phenolic and methoxylic functions) is high in lignin (Table I ), a polymerization reaction is favoured at higher temperatures It has also been proved in another preliminary experiment that the above reaction is too slow if the temperature is lower than 150°C After reaction, the contents were recovered using benzene/methanol (7:3 v/v) and refluxed exhaustively, yielding 72wt% of a benzene/methanol soluble portion (fraction 1, based on the input lignin and alcohol) and an insoluble portion including the activated clay (fraction 2). The former was then refluxed (Fraction

1-2) H/C

0% .~~ 33.1 15.2

o/c ~~

1.19 1.36

0.48 0.20

Short Communications

II

22

I, 10

methyl indanols

II

28o

150 I

I

0 Figure 1

Gas

1

chromatograms

I

15

I

r

1

30

I

TlME(MIN)

I

TEMP.( I

“C ) ,

1

45

I

I

60

of: a, lignin-derived oil A; and b, from the reaction of lignin with C,, alcohol (lignin oil B). For peak identitication

see Table 2

exhaustively with n-hexane, yielding 29wt% of n-hexane soluble fraction (fraction 1-1) and 43wt% of insoluble fraction (fraction l-2). The analysis of fraction 1-l showed that this comprised mainly unreacted n-C,, alcohol and its derivatives such as C,, alkane and alkenes. Therefore, the transformed materials are contained mainly in fraction 2 and fraction l-2, which were then combined (fraction 3) and subjected to mild hydrogenation in the following ways. Hydrogenation procedure of reaction products or origina/ lignin Fraction 3 thus prepared was hydrogenated in 6ml benzene at 340°C under 10 MPa hydrogen for 2 h using Adkin’s catalyst (20 wt%). After cooling to room temperature, the products were filtered and washed with benzene. Thus about 85 wt% of lignin oil based on the starting materials was obtained (lignin oil B).

Similarly, the original lignin (2.Og) together with 2.Og activated clay and Adkin’s catalyst (20wt%) was hydrogenated and extracted as above. About 75wt% lignin oil of starting lignin was thus obtained (lignin oil A). G.c. and g.c.-m.s. analysis Lignin oils A and B were analysed by gas chromatography using a fused silica capillary column (0.25 mm i.d. x 50 m) coated with SE-52. The temperature was programmed to be 3°C min- 1from 80 to 280°C and was held at this temperature. G.c.-m.s. data were obtained using the same type of column as in g.c., and all the spectra were recorded at an electron impact ionization potential of 20eV. Some of the compounds were tentatively identified by comparison with the published spectra. The main products, o- and p-substituted n-C,, phenols, were identified by comparison with authentic ones which were synthesized according

to the methods of Miller4’ and Read4*. Other chemical structures were estimated from mass spectra and mass chromatograms.

RESULTS

AND DISCUSSION

The elemental analysis of the lignin and the reacted products (fraction 1-2) is given in Table 1. It is clearly shown that fraction l-2 has relatively higher carbon and hydrogen values, but lower oxygen content in comparison to those of original lignin. This is probably due to the dehydroxylation, and/or demethoxylation of the lignin during mild catalytic thermai reaction. The high value of hydrogen in fraction l-2 probably arose from the incorporation of the alkyl chains. Figure I shows the gas chromatograms of lignin oils A and B. The numbered

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Short Communications Table 2

Compounds

Peak no.

identified

Possible chemical type

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

106 116 94 108 108 122 122 122 122 122 122 122 122 136 136

16 17 18 19 20 21 22 23 24

136 136 136 136 136 136 136 134 134

. -

in lignin oil

(Figurr

compound

ethylbenzene unknown phenol o-cresol m- and p-cresol 2,6-dimethylphenol o-ethylphenol 2,4_dimethylphenol 2,5-dimethylphenol m-ethylphenol p-ethylphenol 3,5- and 2,3-dimethylphenol 3,5-dimethylphenol 2-isopropylphenol o-n-propyl and 2,4,6trimethylphenol hydroxyl-p-ethyltoluene hydroxyl-m-ethyltoluene C,-phenol p- and m-propylphenol C,-phenol 2,4,StrimethylphenoI C,-phenol 4-indanol 5-indanol

chromatographic peaks were tentatively identified as in Tuble2. As shown in Figure I, two distinctive groups appeared

in lignin oil B. The early eluting compounds (peaks l-27) in lignin oil B nearly correspond to those in lignin oil A, although the content is somewhat different. However, the later eluting compounds (peak No. 28-74) are not contained in lignin oil A. The later eluting compounds in lignin oil A are mainly C,-indanols and dimeric lignols. This result suggests that the early eluting compounds are those derived from the hgnin structure itself, while the later eluting compounds in lignin oil B are characteristic for the reacted products. As shown in Tuble2, the early eluting compounds are mainly C,-C, phenols and C,+Z, indanols. This was confirmed by both the reaction time and mass spectra. The identifications of other peaks such as l&20,22,27 and 28 are tentative, since we have no available authentic standards for comparison. The presence of C,-C3 phenols as major components (peak 14-15, 19, 21-22) especially C,, was probably derived from the lignin structure itself during the mild hydrogenation, since it is known that lignin comprises propanoid-type phenols49ps0. Indanols such as peaks 23325, 27728 could result as second major components from intramolecular cyclizations of propyl groups according to the following equation.

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I)

Peak no.

m,l:

type

25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49

148 168 148 148 198 198 212 226 226 240 250 262 276 270 272 268 268 274 290 304 290 290 304 304 304

C,-indanol dibenzofuran C,-indanol C,-indanol unknown isomer of peak 29 unknown unknown isomer of peak 32 unknown unknown unknown unknown unknown C,,-tetralin C,,-naphthalene isomer of peak 40 unknown CIA-CL-phenol 7 C,,-CL-phenol o-n-C, ,-phenol m- and p-n-C 1,-phenol isomer of peak 44 isomer of peak 44 isomer of peak 44

Possible chemical

compound

These were probably produced partially during the mild hydrogenation, and partially during the thermal reaction of lignin with alcohol. The latter seems to be more acceptable, because it was found that indanols were more abundant in lignin oil B than in lignin oil A (see Figure 1). In addition to this, dibenzofuran was present largely in lignin oil B and could probably be the polymerization product of phenols during the catalytic thermal reactions. From the mass chromatograms, small amounts of dihydroxy or methoxy benzenes with C,2&,, were also detected. substituted compounds Thus n-C,, could be produced readily from the mild thermal catalytic reaction of lignin and n-C,, alcohol, as in the reaction between phenol and fatty acid or alcohol in the previous paper46. Although tetralin, naphthalene or phenanthrene nuclei could not be detected in this reaction product, they were produced under severe conditions, for example in the reaction between phenols and fatty acids or alcohols. Long chain phenols similar to those identified above have been found abundantly in the liquefaction products of subbituminous coal. This seems to suggest that the early diagenesis process probably includes reactions between lignin (sometimes lignin modified by microorganic activity or oxidation) and long alkyl-chain alcohols and/or acids that were hydrolysis products from lipids. As suggested previously46, these long alkyl-chain phenols further undergo reactions such as aromatizations, derearrangements or cyclialkylations, zations, when coalification enters its later stages such as catagenesis and metagenesis. In fact, some important

Possible chemical

Peak 110.

50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 14 _. _~~

compound

111 :

type

302 302 304 304 304 318 332 332 316 318 306 334 320 330 344 344 334 344 334 334 344 344 344 344 350

unknown unknown C,,-C,-phenol isomer of peak 52 isomer of peak 52 C,,-C,-phenol C,,-C,-phenol isomer of peak 56 C,,-indanol unknown C,,-dihydroxylphenol unknown C,,-C,-dihydroxylphenol C,,-C,-indanol C,,-C,-indanol isomer of peak 64 C,,-C,-dihydroxylphenol unknown isomer of peak 66? isomer of peak 66? unknown unknown unknown unknown unknown

results have recently been published52~s4, showing that long straight-chain alcohols, fatty acids and esters are abundantly present in peat or brown coal, but only

in small quantities in subbituminous or bituminous coals. No alcohols, fatty acids or esters have as yet been found in anthracite.

CONCLUSIONS A catalytic thermal reaction between lignin and n-C,, alcohol was studied under mild conditions corresponding to extended geological duration of time. The reacted products were then hydrogenated mildly to obtain materials for analysis. The results demonstrated that several C,, alkylated phenols, indanols or dihydroxybenzenes were readily formed. As these compounds are found abundantly in coal liquefaction products, it is suggested that long alkyl chain aromatics in coal were produced from reactions between lignin and alcohol, and/or fatty acids, as demonstrated above, because lignin and lipids materials are abundantly distributed in plants, animals and microorganisms, which are thought to be the source materials of coal. The results also suggest that the diagenesis process is not a simple transformation of lignin or lipid, but include some chemical reactions between them.

ACKNOWLEDGEMENTS The authors are grateful for the financial support of the Sunshine Project promoted by MIT1 and a grant from Ministry of Education (No. 63430016). They also thank Professor J. Hayashi (Department of Applied Chemistry, Faculty of

Short Communications Engineering,

Hokkaido

for his of the lignin

University)

kind advice on preparation sample.

18

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