Evaluation of alkaline permanganate oxidation method for the characterization of young kerogen

Org. Geochem. Vol. 5, No. 3, pp. 111 119, 1983 Printed in Great Britain. All rights reserved

0146-6380/83 $3.00+0.00 Copyright C~ 1983 Pergamon Press Ltd

Evaluation of alkaline permanganate oxidation method for the characterization of young kerogen TSUTOMU MACHIHARA* and RYOSHI ISHIWATARIt Department of Chemistry, Faculty of Science, Tokyo Metropolitan University; Fukazawa 2-l-l, Setagaya-ku, Tokyo 158, Japan (Received I I February 1983; accepted 12 July 1983) Abstract--Alkaline potassium permanganate oxidation of a young kerogen (lacustrine) and 34 model compounds (saturated and unsaturated fatty acids, hydroxy acid, aliphatic dicarboxylic acids, aliphatic alcohols, normal hydrocarbon, t-carotene, phenolic acids, benzenecarboxylic acids, carbohydrates, amino acids and proteins) were conducted, followed by GC and GC-MS analysis of the degradation products. The stability of the degradation products of kerogen in permanganate solution and the relationship between degradation products and kerogen building blocks were determined. The results showed that aliphatic acids Cn-C~6 monocarboxylic acids and C6-Cl0 ct,co-dicarboxylic acids) were rather susceptible to oxidation compared with benzenecarboxylic acids and the former were degraded into lower molecular weight decarboxylic acids. It was concluded that oxidation at milder conditions (60° C, 1 hr) is appropriate for qualitative and quantitative characterization of the aliphatic structure of young kerogen. It was noteworthy that benzoic acid was produced in a significant amount by oxidation of amino acids (phenylalanine) and proteins, Cjs-isoprenoidal ketone from phytol, and C8 and C9 ~t,og-dicarboxylic acids from unsaturated fatty acids, respectively; furthermore, 2,2-dimethyl succinic and 2,2-dimethyl glutaric acids were produced from /Lcarotene.

INTRODUCTION An alkaline permanganate (KMnO4) oxidation method has been used by many workers to investigate the chemical structure of humic acids and kerogen in soils, sediments and sedimentary rocks. By the oxidation, many kerogens produce aliphatic mono-and dicarboxylic acids composed of polymethylene chains (e.g. C4-Cts, and/or aromatic acids. The molecular distribution of the degradation products reflects the chemical structure of kerogen. It is important to understand the relationships between the chemical structure of kerogens and their degradation products in order to better use these degradation methods for recognition of kerogen building blocks. However, this aspect has been studied insufficiently. Reaction conditions (e.g. reaction temperature and time) in KMnO4 oxidation strongly influence the quality and quantity of degradation products, because the yield and stability of intermediate oxidation products from the kerogen in KMnO4 solution may depend on the oxidation conditions. It is desirable to conduct oxidation under the conditions where the kerogen structure is converted into organic acids which do not suffer further degradation. However, it is difficult to satisfy this condition, since aliphatic acids are more susceptible to oxidation than aromatic acids (Randall et al., 1938).

*Present address: Technology Research Center, Japan National Oil Corporation; 3-5-5, Midorigaoka, Hamuracho, Nishitama-gun, Tokyo, 190-11, Japan. tAuthor to whom correspondence should be addressed.

Reaction temperatures used by many workers for oxidative degradation of kerogens are different. Some authors oxidize kerogen in KMnO4 solution under reflux (Philp and Yang, 1977), and others use 75°C (Young and Yen, 1977), 80°C (Djuricic et al., 1971), 90°C (Ogner, 1973) or 20-100°C (Vitorovic et al., 1980). We have used 60°C for the KMnO4 oxidation study of young kerogens (Machihara and Ishiwatari, 1980, 1981). Under the latter condition, solid particles of young kerogens almost totally disappeared after 1 hr oxidation. In this study, we have examined (1) the stability of representative model compounds in the KMnO4 solution (these compounds have been detected as degradation products of kerogen or are related to the precursors of kerogen), (2) the changes with time in the yield of degradation products by the KMnO4 oxidation of a kerogen from a young lacustrine sediment, and (3) the KMnO4 oxidation methods used in our previous papers (Machihara and Ishiwatari, 1980, 1981). The results of this study may be useful for further studies of fossil kerogens. EXPERIMENTAL

Materials (1) Young kerogen. A freeze-dried sediment sample (17.8g; organic carbon content 7.32%) from Lake Haruna (Ishiwatari et al., 1980) was exhaustively extracted with benzene-methanol (6:4) to remove lipids. The residue was extracted with 0.5 N NaOH solution to remove humic and fulvic acids, The sediment residue was then treated with 4 6 % H F - 6 N HC1 (1:1) solution to decompose inor111

112

TSUTOMU MACHIHARAand RYOSHI ISHIWATARI

Table 1. Recovery of the original compounds after KMnO4 oxidation at 60° or 80°C for various reaction times (~o of the initial weight) Materials

Degradation products appeared on GC (" of the i n i t i a l weight)

Aliphatic dicarboxylic acids i.

Saturated f a t t y acids a.

CH3(CH2)IoCOOH

C4-CI0 (26) max,C7

none

27

b. c. d. e

CH3(CH2)I2COOH CH3(CH2)14COOH CH3(CH2)I6COOH CH3(CH2)I8COOH

C4-C12 (35) max.C7 C4-C9 (6.5) max,C8 none none

none

29

none

70

C6,C7 mono-acids*(17) max.C7 C7-C9 mono-acids*(18) max.C8 none

O 0 0

none

0

2.

none

96

none

96

Unsaturated f a t t y acids a. b.

CH3(CH2)sCH=CH(CH2)7COOH CH3(CH2)7CH=CH(CH2)7CO0H

C4-C9 (51) max.C8 C4-C9 (62) max.C8

c. d.

CH3~Cti2)4(CH-CHCH2)2(CH2)6COOH CH3CH2(CH=CHCH2)3(CH2)6CO0H

C4-C9 (56) max.Cg C2, C4-C9 (47) max. C9

3.

12-OH Stearic acid

4.

Y,-Dicarboxylic acids

CH3(CH2)5CH(OH)CH2(CH2)gCOOH

5.

Other products

Original material recovered (% of the i n i t i a l weight)

C4-C12 (42)max.Clo

C6,C7 mono-acids*(6.6) max. C6

none none none

a.

HOOC(CH2)4COOH

C4 (0.5)

b.

HOOC(CH2)6COOH

C4-C6 (5.8)

c.

HOOC(CHz)sCOOH

C4-C8 (12)

Aliphatic alcohols a. stearil alcohol b. c.

none

C18 mono-acid*(20) C18-isoprenoidal ketone (43) unknown products

0 0 0

71 2 27

phytol cholesterol cholestanol

none none none

6.

Normal eicosane

none

none

100

7.

#-Carotene

br-C 6 (2.1), br-C 7 (3.4)

none

0

8.

Phenolic acids

d.

9.

unknown products

15

0

a. v a n i l i c acid b. Q-hydroxy benzoic acid

C2 (2) none

none none

0 0

c.

C2 (5)

none

0

Benzenecarboxylic acids a. benzoic acid

none

none

93

b. c.

none none

none none

98 94

none none

none

NO

none

ND

C2 (i0)

none

ND

i0.

p-hydroxy benzoic acid

phthalic acid trimesic acid

a. b.

Carbohydrates glucose starch

c.

cellulose

Ii.

Amino acids and proteins a. b. c.

glutamic acid lysine phenylalanine

C3,C4 (0.2) C4,C5 (0.5) none

none

0

none

0

d.

triptophan

C2 (2)

none

ND

e.

casein

C3-C5 (0.3)

benzoic acid (0,7)

ND

f.

albumin

C4 (0.3)

benzoic acid (1.2)

ND

*

Aliphatic monocarboxylic acid

benzoic acid (56)

0

ND: Not determined

ganic materials and then refluxed with 6N HC1 to remove non-humic materials. The resulting residue (kerogen) was extracted with benzene-methanol (6:4), washed with water and freeze-dried. The yield of kerogen concentrate was 678 rag. The elemental composition of the kerogen was: C ~ 63.8, H~o 4.87, N~o 1.78 (ash-free basis); ash 12.8~o. (2) Model compounds. Model compounds used for

the oxidation study are listed in Table 1. These compounds were selected from the compounds which had been actually detected as degradation products of young kerogens or are related to possible constituents or precursors of young kerogens (e.g. aliphatic alcohols, unsaturated fatty acids, #-carotene, phenolic acids, carbohydrates, amino acids and proteins; Abelson and Hare, 1970, 1971; Hoering 1973; Mac-

113

Characterization of young kerogen hihara and Ishiwatari, 1980, 1981; Ishiwatari and Machihara, 1983, Larter and Douglas, 1980). They were purchased from Wako Chemical Industry, Junsei Chemical Co., Whatman Ltd. (England) or P-L Biochemicals Inc. (Milwaukee, Wisconsin).

Potassium permanganate oxidation A powdered kerogen sample (below 200mesh; 8-9 mg on ash-free basis) or a model compound (0.5-1.5 mg) was suspended in l0 cm 3 (3 cm 3 for a model compound) of 2% KMnO4-1% KOH aqueous solution in a sealed 20cm 3 (10cm 3 for a model compound) glass ampoule and allowed to react at 60%C with mechanical shaking using a Taiyo Incubator (Model M-100) for various durations (30 min to 8 hr). At the end of the heating period, the mixture was cooled in an ice-water bath and treated with NaHSO3 to stop the oxidation reaction. The manganese dioxide generated was dissolved by addition of H2SO4 and NaHSO3. The oxidation products were then extracted from the reaction mixture (pH ~ 1) with ethyl acetate (10cm3x 3 or 5cm3× 3). The organic extract was washed with distilled water (5 cm 3 x 1 or 3 cm 3 × l) and evaporated to dryness under reduced pressure. The extract was silylated with Silyl-8 (Pierce Chemical Co.) and analyzed by gas chromatography (Shimadzu GC-5A, flame ionization detector, 2 m x 3 mm i.d. glass column packed with 1% OV-1 in Chromosorb W A W D M C S , 80-100 mesh, programmed from 90-280°C at a rate of 6°C/min, N2-flow = 40 cm3/min). The degradation products were quantified by measuring their peak areas on the gas chromatograms using a Shimadzu Chromatopack C-RIA digital integrator. Normal saturated monocarboxylic acids (C7, C9, Ct0, C~2, C14, Ci6, C18, C20, C22 and C24), normal saturated ~t¢o-dicarboxylic acids (C2, C4, C6, C8, C10, C12) and benzenecarboxylic acids (benzoic, phthalic acid trimesic acids) were used as standards for quantification. The degradation products were identified with a Shimadzu-LKB 9000 gas chromatograph-mass spectrometer. The GC conditions were the same as used for GC analysis. Helium was used as carrier gas. The mass spectrometer was operated at 70 eV, accelerator voltage of 3.5 kV and an ion source temperature of 310°C.

Degradation experiments with model compounds First, we conducted the degradation experiment with model compounds in order to obtain information on (1) the stability of young kerogens degradation products in KMnO4 solution, and (2) the relation between degradation products and the possible chemical structure of young kerogens. (1) Stability. We oxidized the following model compounds at 60°C, the same temperature used for our oxidative degradation studies of kerogen (Machihara and Ishiwatari, 1980, 1981), and at 80°C for comparison: (i) 5 normal aliphatic monocarboxylic acids (C12, C14, C16, Ci8 and C20), (ii) 3 normal ~,a~-dicarboxylic acids (C6, C8 and C10) and (iii) 3 benzenecarboxylic acids (benzoic, phthalic and trimesic acids), which are all actually found in the degradation products of young kerogens. Table 2 summarizes the experimental results. The stability of normal monocarboxylic acids decreased with decreasing chain-length when oxidized at 60°C. This tendency for normal monocarboxylic acids was more pronounced when oxidized at 80°C. When oxidized at 60°C for 1 hr, n-C~8 and n-C20 acids were practically undecomposed, whereas about 30% of n-Ci6 acid and 70% of n-C12 and n-C14 acids decomposed. Approximately 20-50% of the normal C6, C8 and Cm dicarboxylic acids decomposed when oxidized at 60°C for 1 hr. The stability of these dicarboxylic acids seems to increase slightly with decreasing chain-length. As anticipated from a previous study (Randall et al., 1938), benzenecarboxylic acids were resistant to KMnO4 oxidation and did not decompose when oxidized at 60°C for 1 hr. Even when oxidized at 80°C for 1 hr, 78-90% of the acids remained undecomposed. These results showed that under the oxidation condition used by our previous

Table 2. Results of alkaline KMnO4 oxidation experiments of model compounds 60°C

The KMnO4 oxidation of kerogen is considered to proceed by two major steps: (1) attack on the oxidizing reagent on carbon-carbon or carbon-oxygen bonds (including ester linkages) of the kerogen matrix, resulting in the production of mainly carboxylic acids, and (2) the further degradation of the generated carboxylic acids into lower molecular weight compounds. In the degradation experiment of a kerogen, however, it is impossible to differentiate these two steps experimentally, because the second step may proceed in parallel with the first step.

30 min

60 min

120 min

n-Cl2

--

27

.

n-C14

48

29

3

n-C|6

85

70

40

2

n-C18

I00

96

91

21

n-C20

97

96

97

85

n-C 6

--

78

76

--

n-C 8

87

73

44

20

n-C10

79

50

31

12

Benzoic

--

93

--

90

Phthalic

--

98

--

80

Trimesic

--

94

--

78

Saturated

RESULTS

fatty

m,m-Dicarboxylic

Not measured

60 min

acids .

.

. 0.3

acids

Benzene c a r b o x y l i c

--

80°C

acids

114

TSUTOMU MACHIHARA a n d RYOSHI ISHIWATARI

f

C,2

f co l H F F

II I

n

12-OH stearic acid

c'4 H 8c o c

f

CI64

n n

n

Glutamic

n

I"1

O

_ C'81

>

Lysine

n

n

Ilrl

n~ C~8:2

F

i"1 n

n

[1

100~50 Ci83 0

acid

n

3

4

5

6

Carbon

7

8

N 9

Casein 3

4

n

5

6

7

8

9

I0

II

12

Carbon number I0

II

12

number

Fig. 1. Distribution pattern of cq~o-dicarboxylicacids obtained from the oxidation of model compounds.

studies (60°C, 1 hr), more than 50% of major degradation products (n-C16, n-C,s monocarboxylic acids, ct,co-dicarboxylic acids and benzenecarboxylic acids) may remain undecomposed. The stability of the compounds tested decreases in the following order: benzenecarboxylic acids > n-C~s, n-C20 monocarboxylic acids >n-Cl6 ct,co-dicarboxylic acids > n-C~2, n-C~4 monocarboxylic acids. (2) Degradation products by the KMn04 oxidation. Thirty-four model compounds were oxidized and their degradation products were examined. Table 1 summarizes the experimental results. The oxidation condition (60°C, 1 hr) was the same as that used for our previous studies (Machihara and Ishiwatari, 1980, 1981), because the results of degradation products can be directly used for the interpretation of KMnO4 oxidation experiments of young kerogens, and may be applicable for the interpretation of oxidation experiments conducted at different conditions from ours. The following features of degradation products can be noted. Normal unsaturated monocarboxylic acids. The 4 unsaturated monocarboxylic acids produced C8 and C9 ~,~o-dicarboxylic acids as major products and C4-C7 ~,o)-dicarboxylic acids as minor products (Fig. 1). In addition to these degradation products, normal monocarboxylic acids were produced from the oxidation of n-A9CI6:I and n-A9CI8:I acids, and oxalic acid (3%) was generated from n-Ag"2'15C~8:3. The generation of n-C8 and n-C9 ~,m-dicarboxylic acids indicates that the carbon-carbon double bond was easily cleaved by oxidation and the product (n-C 9 dicarboxylic acid) was further oxidized into lower

molecular weight dicarboxylic acids. We want to emphasize that the distribution of ~t,co-dicarboxylic acids is different from that observed for the oxidation of monocarboxylic acids (Fig. 1). 12-hydroxystearic acid. This acid generated C4-C12 ~,co-dicarboxylic acids, n-CT0 and n-CH being more abundant than others (Fig. 1), indicating that carboncarbon bonds at the position of a hydroxyl group is cleaved by oxidation. Aliphatic ~,~o-dicarboxylic acids. Normal C6, C8 and Cm cq~o-dicarboxylic acids produced C4, C4-C6 and C4-C 8 ~,~o-dicarboxylic acids, respectively. These results indicate that ~,co-dicarboxylic acids produced by oxidation of kerogen are further degraded into lower molecular weight ~,~o-dicarboxylic acids. Phytol. Phytol produced only one degradation product, which was identified as a Ct8 isoprenoidal ketone (6, 10, 14-trimethylpentadecan-2-one) by the comparison of its mass spectrum with that in the literature (e.g. Simoneit and Burlingame, 1974). Normal eicosane. This compound remained unoxidized under these conditions and gave no degradation products. fl-Carotene. These compound decomposed completely and produced two major degradation products, giving the mass spectra as shown in Fig. 2. By comparison of mass spectra of authentic compounds and GC coinjection, these products were identified to be 2,2-dimethyl succinic and 2,2-dimethyl glutaric acids, respectively. These two compounds may have been formed by the reaction illustrated in Fig. 3. Phenolic acids. Degradation products of vanillic, o-and p-hydroxybenzoic acids did not appear in the

Characterization of young kerogen Degradation

115

)roduct- I

I00 147

231 (bose)

50

275(M-15)

190 J ,,

0

~5 c

L2

/

,~JJ

2,2-Dimethyl

I

succinic acid

I00 147

231 (base)

_o

(p rr

50

275 (M-15)

Ii,I,

o

190 , 204

,11 ,,ll

I,

I00

,

ii

I

Ii

I

2oo

150

I

I

25o

300

m/z

Degradation product - 2 I00 204 (base)

147 171

50-

289 (M-15) 245

II

o

I]

II

2 , 2 - Dimethyl

E

II lutaric

h

I

I JI

I

acid

ioo 204 ( bose )

147

289 (M - 15)

171

50--

245

II

o

,I

Ii L

ioo

I

150

LI

200

II

250

300

m/z

Fig. 2. Mass spectra of the two degradation products (trimethylsilyl derivatives) obtained from the oxidation of fl-carotene, and those of authentic standards (trimethylsilyl derivatives): 2,2-Dimethyl succinic acid and 2,2-dimethyl glutaric acid.

/

H3C\

/CH 3

H3C X

CH 3 .

--CH 3

\

.

.

.

H2CI

CH 3

,'0

/C.

H3C\ /CH3 H 2~/C'~c00 H H2C" COOH

H2 Cx C~_.~C~CH 3 H2 \

H3C\ /CH3 C H2C / \COOH

0

11211 C~C--OH

+

\COOH

O /

Fig. 3. Oxidation reaction of fl-carotene by alkaline potassium permanganate. ~,< Site of oxidation cleavage.

116

TSUTOMU MACHIHARA a n d RYOSH1 ISHIWATAR!

gas chromatograms except for a small amount of oxalic acid. This result indicates that they were attacked at the position of hydroxyl groups (Randall et al, 1938). Benzenecarboxylic acids. As described above, these aromatic acids were resistant to oxidation and no degradation products were detected in the gas chromatograms. Carbohydrates. No degradation products appeared in the gas chromatograms for glucose and starch. On the other hand, cellulose gave a significant amount of oxalic acid. Oxalic acid may have been formed continuously from cellulose during the reaction owing to its slow degradation rate in KMnO4 solution, because according to Randall et al. (1938), oxidation rate of carbohydrates in KMnO4 solution at boiling point decreases in the order of glucose, starch and cellulose. The oxalic acid which was formed just before 1 hr reaction time appears to have been saved from further decomposition. Amino acids and proteins. The result for the oxidation of aliphatic amino acids (lysine and glutamic acid) indicates that oxidative cleavage occurred at the position of carbon with the amino group (Table 1 and Fig. 1). Phenylalanine produced a large amount of benzoic acid, which, on molar basis, corresponds to 77% of phenylalanine. Tryptophan was much less stable than phenylalanine in KMnO 4 solution and gave no degradation products other than oxalic acid. Casein and albumin, which consist of about 20 kinds of amino acids, produced a significant amount of benzoic and succinic acids (Table 1 and Fig. 1).

Oxidative degradation of a young kerogen To examine the attack of the oxidizing reagent on a young kerogen and its degradation products, we

678 45

@

+

Retention

time

D,

Fig. 4. Gas chromatogram of trimethylsilyl derivatives of the ethyl acetate-soluble products obtained from the oxidation (60°C, 2 hr) of Lake Haruna kerogen. The carbon numbers of the e,og-dicarboxylic acids are indicated by the arabic numerals and monocarboxylic acids are indicated by the primed arabic numerals. © etc. indicate benzenecarboxylic acids. ,: Internal standard (C20 normal eicosane).

~(E~ror range :

I f e-----._.o

Total

3

eliphatic

acids

0 ~

Monocarboxylic acids ( C 8 - C l 9 )

8 o~ E

&

o

-~--

~ 05 I/FD.....f._.~-Mon°carb°xyJico ~ e

acids (C2o-C28)

li

ok

_

4

~e

+f >-

2

Dicorboxylic

acids

( C 4 - CI4)

O--

~_ I 0

Benzene

|/~''1g'--~ /~_

I I

I 2

I 3 Reaction

carboxylic acids = ~ ~ I, 4 time

I 5

L 6

I 7

L-8

(hr)

Fig. 5. Changes in the yields of degradation products with reaction time.

oxidized a kerogen from Lake Haruna with KMnO 4 at 60°C and examined the degradation products as a function of reaction time. Figure 4 shows a typical products of the kerogen. As reported previously (Machihara and Ishiwatari, 1980, 1981), major degradation products in the gas chromatogram include normal C8-C2+ monocarboxylic acids, normal C4-C~4 ct,~o-dicarboxylic acids and benzene mona-, di- and tricarboxylic acids. Figure 5 shows changes in the yields of degradation products with reaction time. Three important features of these results are: (1) The yield of n-Cs-Ci9 monocarboxylic acids decreased with reaction time. This indicates that these carboxylic acids were probably trapped in the kerogen matrix and were released and degraded by the KMnO+ solution. A fair amount of C8 and C9 monocarboxylic acids may have been also derived by cleavage of carbon-carbon double bonds from unsaturated fatty acids and/or their related structure in the kerogen (see Table 1). If the oxidation of these monocarboxylic acids follows the first order kinetics, their amount in the initial (unoxidized) kerogen is estimated to be 1.3% of the kerogen. (2) The yield of n-C20-C28 monocarboxylic acids and C 4 - C t 4 ct,co-dicarboxylic acids increased with reaction time to 2 hr (for dicarboxylic acids) or 4 hr (for monocarboxylic acids) and then gradually decreased. The increase of these mona- and dicarboxylic acids is interpreted to be due to cleavage of chemical bonds (e.g. C-C, C-O) in the kerogen matrix. Their decrease after 2 or 4 hr may be a result of degradation into lower molecular weight compounds. As expected from the degradation experiment of model compounds, the relative distribution

Characterization of young kerogen

30min /,60rain /2hr 4hr ~Shr

~

~4 ~3 ~o ~2 E

T~, >C4

/i

Cs

C6

C7

C8

C9

Methylene chain

I CloI, Ih,. ,,,Ci2 Cll

,, Ci3

,,, Ci4

length

Fig. 6. Yields of C4-C14 ~,e)-dicarboxylic acids from L. Haruna kerogen at different reaction times. of C4-C14 ~¢o-dicarboxylic acids changed with reaction time, as shown in Fig. 6. The yields of C4-C6 ct,co-dicarboxylic acids increased with reaction time, while those of C7-C14 dicarboxylic acids may be due to the secondary degradation of the oxidation products with longer methylene chains. (3) The yield of benzenecarboxylic acids (benzoic acid accounted for 57-74% of them) increased continuously with reaction time (Figure 5). Their yield from the 8 hr oxidation was about twice that from the 1 hr oxidation. These compounds are interpreted to have been formed by decomposition of kerogen. In particular, as the degradation experiment of amino acids and proteins indicated, benzoic acid may have come from phenylalanine-derived structure in the kerogen. DISCUSSION AND CONCLUSION

Oxidation condition

The amounts of kerogen degradation products obtained by KMnO4 oxidation at 60°C for 1 hr are not necessarily the maximum amounts produced during the experiment. As summarized in Table 3 and Fig. 5, the yield of C4-CI4 a,co-dicarboxylic acids for the 1 hr oxidation was nearly equal to that for the 2 hr oxidation. However, the yields of C20-C28 monocarboxylic acids and benzenecarboxylic acids for the 1 hr oxidation were less than their maximum yields (1.5 times larger at 4 h r oxidation). On the other hand, Cs-Cm monocarboxylic acids are not stable in

117

KMnO4 solution and degraded into lower molecular weight compounds with increasing duration of reaction. As shown in Fig. 5, the yield of total aliphatic acids (mono- and dicarboxylic acids) reached a maximum for the 1 hr oxidation. Consequently, the oxidation time of 1 hr may be appropriate to estimate the aliphatic character of young kerogen from its oxidation products. More than 50% of aliphatic mono- and dicarboxylic acids remain without further degradation under this oxidation condition (Table 1). The oxidation conditions used by other workers such as oxidation at 90°C for 6-8 hr (Ogner, 1973), under reflux for 4 hr (Philp and Yang, 1977) or 8 hr (Khan and Schnitzer, 1972) seem to be too severe to preserve degradation products of aliphatic nature in KMnO4 solution. The yield of aromatic degradation products for the 1 hr KMnO4 oxidation was about a half of their maximum yield (at 8 hr oxidation: Table 3). This fact indicates that it is likely that the aromatic structure will be underestimated when the 60°C-1 hr oxidation condition is applied for the characterization of young kerogens. In this case, longer than 1 hr oxidation time will be needed to get more quantitative information on the aromatic structure of young kerogens. Further work is necessary to determine the reaction time enough for estimating their aromatic structures. Correlation between degradation products and precursor structure

Normal C2-C~8 ~,c9-dicarboxylic acids, normal C8-C28 monocarboxylic acids and benzenecarboxylic acids are common oxidative degradation products of young kerogens (Philp and Yang, 1977; Machihara and Ishiwatari, 1980, 1981). Although the number of model compounds examined was limited, this study reveals to some extent the correlation between degradation products and their precursor structure in kerogen. Table 4 gives a list of relationships between possible partial structures of young kerogen and degradation products. The following points are noted from the present study: (1) Alcoholic or amino groups in kerogen would be oxidized into carboxylic groups. (2) The chemical bonds of C = C, C-C(OH)-C,

Table 3. Comparison of the amount of degradation products obtained by KMnO4 oxidation of Lake Haruna kerogen Amount of degradation products (mgC/g-kerogen

C)

Ratio of max. y i e l d to the y i e l d at 60"C, 1 hr

60°C, I hr

Max. y i e l d (oxidation condition)

C8-C19

7.00

9.49 (60°C, 0.5 hr)

1.36

C20-C28

3.05

4.63 (60°C, 4 hr)

1.52

Monocarboxylic acids

~,~-Dicarboxylic acids C4-C14 Benzenecarboxylic acids (mono, di and t r i )

33.3 0.473

35.0

(60°C, 2 hr)

1.05

0.962(60°C, 8 hr)

2.03

TSUTOMUMACHIHARAand RYOSHI ISHIWATARI

118

Table 4. Relationship between possible structure of young kerogen and degradation products Partial structure

Primary degradation products

Secondary degradation products

M~C=CI(CH2)n-CH3

HOOC-(CH2)m-CH 3

(m~n)

HOOC-(CH2)k-COOH (k~m-2)

2a, 2b, la, Ib

M.vC-C-(CH2)n-COOH

HOOC-(CH2)m-COOH(re
HOOC-(CH2)k-COOH (k~m-2)

2a 2d, 4a4c

M~C(OH)-(CH2)n-COOH

HOOC-(CH2)m-COOH (m~n)

HOOC-(CH2)k-COOH

3, 4a~c

M~C-(CH2)n-CH20H

M~C-(CH2)n-COOH

M,~phytol

Cls-isoprenoidal ketone

5b

M~3-carotene

2,2-dimethyl succinic acid 2,2-dimethyl glutaric acid

7

OH M~C-<~OCH 3

HOOC-COOH

8

5a,

HOOC-(CH2)n-COOH (probable)

M~C-<~-(COOH)n

(k~m-2)

Tested compound No. (indicated in Table I)

HOOC-(CH2)m-C@OH (m~n-2)

~>-(COOH)n+I

4a 4c

9, lib,

M~carbohydrates

HOOC-COOH

10

M~amino acids

HOOC-(CH2)n-COOH(n~3) benzoic acid (for phenylalanin)

lla-llc

M~proteins

benzoic acid HOOC-(CH2)n-COOH

11e, l l f

M: kerogen matrix

Fv: unknown linkage

(n ~3) # Randall et al (1938)

C - C ( N H 2 ) - C and probably points of branching are susceptible to oxidation and would produce monocarboxylic acids or dicarboxylic acids, depending on type of terminal groups, which would in turn be degraded into dicarboxylic acids with shorter methylene chains; (3) If carbohydrates, proteins or their reaction products (e.g. melanoidins) are present in or linked with kerogen matrix, they would generate short chain decarboxylic acids, and benzoic acid in the case of proteins. Carbohydrates, proteins or their reaction products are known to be present in young kerogens ( Y a m a m o t o and Ishiwatari, 1981); (4) One can not get information of the presence of phenolic aromatic structures in kerogen because of the lability of such structures to the oxidizing reagent; (5) The structure derived from phytol or carotenoid pigments (e.g./3-carotene), so far as a major part of their structure is preserved, would produce degradation products which are characteristic for the precursors: 6,10,14-trimethylpentadecan-2-one for phytol-derived structure, 2,2-dimethyl succinic and 2,2-dimethyl glutaric acids for /3-carotene-derived structure. Acknowledgements--This work was partly supported by the

Ministry of Education, Science and Culture, Japan (grant No. 56470034) and the Toyota Foundation (grant Nos 78-1-069 and 79-1-161). REFERENCES

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