Organic Geochemistry 31 (2000) 1189±1208 www.elsevier.nl/locate/orggeochem The role of alkenes produced during hydrous of a shale Roald N. Leif 1, Bernd R.T. Simoneit * Environmental and Geochemistry Group, College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA

Received 24 June 1999; accepted 26 July 2000 (returned to author for revision 2 December 1999)

Abstract

Hydrous pyrolysis experiments conducted on Messel shale with D2O demonstrated that a large amount of deuterium becomes incorporated into the hydrocarbons generated from the shale . In order to understand the pathway of deuterium (and protium) exchange and the role of during hydrous pyrolysis, we conducted a series of experi- ments using aliphatic compounds (1,13-tetradecadiene, 1-hexadecene, eicosane and dotriacontane) as probe molecules.

These compounds were pyrolyzed in D2O, shale/D2O, and shale/H2O and the products analyzed by GC±MS. In the absence of powdered shale, the incorporation of deuterium from D2O occurred only in ole®nic compounds via double bond isomerization. The presence of shale accelerated deuterium incorporation into the ole®ns and resulted in a minor amount of deuterium incorporation in the saturated n-alkanes. The pattern of deuterium substitution of the diene closely matched the deuterium distribution observed in the n-alkanes generated from the shale kerogen in the D2O/ shale pyrolyses. The presence of the shale also resulted in reduction (hydrogenation) of ole®ns to saturated n-alkanes with concomitant oxidation of ole®ns to ketones. These results show that under hydrous pyrolysis conditions, kerogen breakdown generates n-alkanes and terminal n-alkenes by free radical hydrocarbon of the aliphatic kerogen structure. The terminal n-alkenes rapidly isomerize to internal alkenes via acid-catalyzed isomerization under hydro- thermal conditions, a signi®cant pathway of deuterium (and protium) exchange between water and the hydrocarbons. These n-alkenes simultaneously undergo reduction to n-alkanes (major) or oxidation to ketones (minor) via alcohols formed by the hydration of the alkenes. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Hydrous pyrolysis; Molecular probes; Messel shale; Deuterium exchange; Ole®ns; Ketones

1. Introduction high organic carbon content, and its having been used in numerous studies. The shale was powdered and extrac- Hoering (1984) described interesting results concerning ted prior to heating. For each experiment the shale was the role of water during laboratory hydrous pyrolysis. He combined with water or heavy water, sealed under found that a large amount of deuterium was incorporated nitrogen in a stainless steel reaction vessel and heated at  into the n-alkanes generated from hydrous pyrolysis of 330 C for 72 h. The n-alkanes from the D2O pyrolysis Messel shale kerogen in D2O. Messel shale was selected were isolated and analysed by mass spectrometry to for the experiments due to its low thermal history, its determine the extent of deuterium incorporation. The substitution ranged from 0 to at least 14 deuterium atoms for each n-alkane, with the highest relative abun- * Corresponding author. Tel.: +1-541-737-2155; fax: +1- dances of 4±6 deuterium atoms. There was no trend in 541-737-2064. substitution pattern as a function of chain length. E-mail address: [email protected] (B.R.T. Simoneit). To explain the deuterium substitution patterns in the 1 Present address: Lawrence Livermore National Labora- pyrolysis experiments, a free radical chain mechanism tory, Livermore, CA 94551, USA. was suggested. This mechanism proposes that one

0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0146-6380(00)00113-3 1190 R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208 pathway to the multiple deuteration could have occurred compounds used in pyrolysis experiments were n-tetra- by the free radical migration of the ole®n sites. Similar deca-1,13-diene (Aldrich Chemical Co., purity >97%), radical reactions have been proposed by others n-hexadec-1-ene (Aldrich Chemical Co., purity >97%), (Monthioux et al., 1985; Comet et al., 1986), but Ross n-eicosane (Aldrich Chemical Co., purity 99%), and n- (1992a,b) has shown that direct transfer from dotriacontane (Aldrich Chemical Co., purity >97%). water to organic free radicals is endothermic by 25±30 The Messel shale used in the experiments was powdered, kcal/mol and, therefore, should not be signi®cant at exhaustively extracted in a Soxhlet apparatus with hydrous pyrolysis conditions. A re-examination of the methanol/methylene chloride for 72 h, and dried prior Hoering (1984) deuterium isomer pro®le data by to the pyrolysis studies. numerical modelling was performed by Ross (1992a). He concluded that a more likely explanation for the 2.2. Hydrous pyrolysis experiments deuterium isomer distribution in the n-alkanes generated in the D2O Messel shale pyrolysis is by simultaneous The pyrolysis experiments were performed in passi- deuterium exchange at more than one site. He further vated Sno-Trik1 T316 stainless steel high pressure pipes suggested a combination of ionic and radical chemistry sealed with end caps with a total volume of 2.0 cm3 (Leif to explain the results (Ross, 1992a), although the details and Simoneit, 1995a). Deoxygenated H2OorD2O was of the actual chemical mechanisms that result in the prepared by bubbling with argon for 45 min. The reac- observed preferential deuterium substitution at one end tion vessels were loaded with reactant mixtures, sealed of the isoprenoid and biomarker molecules could still in a glove bag under an argon atmosphere, and placed not be explained. Lewan (1997) has suggested that in a preheated air circulating oven set at the reaction under hydrous pyrolysis conditions water molecules can temperature and controlled to within Æ2C. Durations react directly with organic free radicals generated by the of the heating experiments ranged from 1 to 72 h. thermal breakdown of organic matter. Table 1 is a listing of the pyrolysis experiments for A re-evaluation of the research in pyrolysis and high this study. The heavy water pyrolyses of Messel shale temperature aqueous chemistry of hydrocarbons pro- were carried out at 330C with 0.4 g dried shale powder vides some insight into the major reactions that alkanes and 0.8 ml of D2O. Messel shale pyrolyses with mole- and alkenes undergo (Wilson et al., 1986; Weres et al., cular probes were conducted with 0.4 g dried shale 1988; Kissin, 1987, 1990; Siskin et al., 1990; Leif et al., powder, 8 mg each of n-tetradeca-1,13-diene, n-hexadec- 1992; Stalker et al., 1994, 1998; Seewald, 1994, 1996; 1-ene, and n-eicosane directly spiked on the shale, and

Jackson et al., 1995; Burnham et al., 1997; Lewan, 1997; 0.8 ml of either H2OorD2O. Heavy water pyrolyses of  Seewald et al., 1998). These studies point to the impor- n-C32H66 were done at 350 C with 10 mg of the n-alkane tance of both radical and ionic reaction mechanisms and 0.8 ml D2O. Pyrolysis of n-C32H66 under alkaline during the pyrolysis of organic matter. This paper conditions was also carried out at 350C with 10 mg of duplicates the original Hoering (1984) Messel shale pyr- the n-alkane and 0.8 ml D2O where the pH of the D2O olysis experiment and presents results from additional was adjusted to 11.3 (at 25C) using NaOD. hydrous pyrolysis experiments which provide evidence These hydrous pyrolysis experiments with pre-extracted, for the chemical pathways by which hydrogen exchange powdered rock and added model compounds in aqueous occurs between water and aliphatic hydrocarbons during solution (330 or 350C) may not be directly comparable hydrous pyrolysis. Molecular probes were used with the with hydrous pyrolysis of rock chips (i.e. Lewan, 1997), shale to determine their relative reactivities with regard because the pore spaces in rock chips become ®lled with to n-alkane and n-alkene production. water-saturated bitumen during hydrous pyrolysis. Maturing kerogen in rock chips is, therefore, not in contact with an aqueous phase, but with an organic 2. Experimental phase that has dissolved water in it. However, after the oil is expelled from the rock chips it can proceed to react 2.1. Chemicals and samples in an aqueous environment similar to what is occurring in these experiments, and similar to the reactions The Messel shale is Eocene and was sampled from the occurring during aquathermolysis experiments (Siskin et quarry at Darmstadt, Germany (Matthes, 1966; van den al., 1990; Siskin and Katritzky, 1991). Berg et al., 1977; van de Meent et al., 1980). Hydrous pyrolysis experiments were performed using ultrapure H2O 2.3. Extraction and fractionation from Burdick and Jackson and D2O (purity >99.9%) from Cambridge Isotopes Laboratories. Both H2O and The reaction vessels were cooled to room temperature D2O were distilled in glass before use. NaOD (purity upon completion of the heating cycle. The vessels were >99.5%) for pyrolysis under alkaline conditions was extracted with two 1 ml portions of methanol followed obtained from Cambridge Isotopes Laboratories. Aliphatic by ®ve 1 ml portions of methylene chloride. The solvents R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208 1191

Table 1 Hydrous pyrolysis experiments performed in 2.0 cm3 316 stainless steel reactors

Temperature Duration Liquid medium Reactants (C) (h)

330 72 D2O (0.8 ml) Messel shale (0.4 g)  350 72 D2O (0.8 ml, pH=7.0 at 25 C) n-C32H66 (10 mg)  350 72 D2O (0.8 ml, pH=11.3 at 25 C) n-C32H66 (10 mg) 330 1 D2O (0.8 ml) 1,13-C14:2, 1-C16:1, and C20 (8 mg each) 330 5 D2O (0.8 ml) 1,13-C14:2, 1-C16:1, and C20 (8 mg each) 330 10 D2O (0.8 ml) 1,13-C14:2, 1-C16:1, and C20 (8 mg each) 330 36 D2O (0.8 ml) 1,13-C14:2, 1-C16:1, and C20 (8 mg each) 330 72 D2O (0.8 ml) 1,13-C14:2, 1-C16:1, and C20 (8 mg each) 330 1 H2O (0.8 ml) Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each) 330 5 H2O (0.8 ml) Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each) 330 10 H2O (0.8 ml) Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each) 330 36 H2O (0.8 ml) Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each) 330 72 H2O (0.8 ml) Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each) 330 1 D2O (0.8 ml) Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each) 330 5 D2O (0.8 ml) Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each) 330 10 D2O (0.8 ml) Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each) 330 36 D2O (0.8 ml) Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each) 330 72 D2O (0.8 ml) Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each) 330 10 D2O (0.8 ml) Elemental sulfur (0.5 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)

and water from each pyrolysis experiment were com- of selected samples was achieved by bubbling H2 gas bined in a centrifuge tube and the organic fraction into the sample for 30 min in the presence of platinum separated and collected. The water was extracted with (IV) oxide (Adam's catalyst). The internal standard two additional portions of methylene chloride and the method was used to quantitate the probe molecules methylene chloride fractions were combined. Methylene using relative response factors. chloride was dried with anhydrous sodium sulfate. The methylene chloride extracts from the Messel shale experi- 2.4. Gas chromatography ments were passed through an activated copper column to remove elemental sulfur. The solvent was removed to Gas chromatography (GC) of the pyrolysates was near dryness by nitrogen blowdown. The total extract was performed with a Hewlett-Packard 5890A instrument made up to 2 ml of methylene chloride and deasphalted in equipped with a 30 m x 0.25 mm i.d. DB-5 capillary 100 ml of heptane. The asphaltenes were allowed to pre- column (0.25 mm ®lm thickness). The GC oven was cipitate overnight and separated from the maltenes by heated using the following program : isothermal for 2 vacuum ®ltration through a BuÈ chner funnel with fritted min at 65C, 3C/min to 310C and isothermal for 30 disk (porosity : 4±5.5 mm) and washed with heptane. The min, with the injector at 290C, detector at 325C, and deasphalted fractions were concentrated to 2 ml using a helium as the carrier gas. The alcohols in the polar rotary evaporator with water bath set at 30C and frac- fractions were converted to the trimethylsilyl derivatives tionated by column chromatography (30 Â 1 cm) packed with BSTFA prior to analysis. with 3.8 g alumina (fully active) over 3.8 g silica gel (fully active). The samples were separated into three fractions by 2.5. Gas chromatography±mass spectrometry elution with 50 ml heptane (nonpolar fraction, F1), 50 ml toluene (aromatic fraction, F2) and 25 ml methanol Gas chromatography±mass spectrometry (GC±MS) (polar fraction, F3). Separation of the alkenes from the was performed on a Finnigan 9610 gas chromatograph alkanes was carried out by argentation silica column equipped with a 30 mÂ0.25 mm i.d. DB-5 capillary col- chromatography. The normal alkanes of the Messel shale± umn (0.25 mm ®lm thickness) coupled to a Finnigan

D2O pyrolysis were isolated from the nonpolar fraction 4021 quadrupole mass spectrometer operated at 70 eV by urea adduction, an additional procedure which was over the mass range 50±650 dalton and a cycle time of necessary to get a more reliable determination of the 2.0 s. The GC oven temperature was programmed as deuterium incorporation of the n-alkanes. Hydrogenation described above, with the injector at 290C and helium 1192 R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208 as the carrier gas. The MS data were processed with an on-line Finnigan-Incos 2300 computer data system. The positional isomers of the n-alkanones and the n-alkanols were identi®ed by comparison with authentic standards. Deuterium incorporation in the probe molecules was determined by monitoring the distribution in their molecular ions after hydrogenation of the ole®n probe molecules to n-alkanes. GC±MS data was acquired using a Hewlett-Packard 5890 Series II GC coupled to a Hewlett-Packard 5971 series mass selective detector

(MSD) with mass ranges of m/z 196±220 for n-C14H30, m/z 224±242 for n-C16H34 and m/z 280±292 for n-C20H42. The GC was equipped with a 30 mÂ0.25 mm i.d. DB-1 capillary column (0.25 mm ®lm thickness). The GC oven temperature was programmed at isothermal for 2 min at 100C, 5C/min to 260C, 10C/min to 300C, and iso- thermal for 10 min, with an on-column injector, and helium as the carrier gas. The MS data were processed with Hewlett-Packard Chemstation software. The mass intensity data from the GC-MS analyses were corrected for naturally occurring 13C by the method of Biemann (1962) and Yeh and Epstein (1981) to obtain the extent of deuterium incorporation in the n-alkanes.

Fig. 1. Average distribution of deuterium substitution in n-

3. Results alkanes from C17 to C29 generated from: (a) the D2O pyrolysis of Messel shale (after Hoering, 1984), and (b) the D2O py- rolysis of Messel shale (this study). 3.1. Hydrous pyrolysis of Messel shale in D2O

The ®rst experiment in this series was the hydrous 3.2. Hydrous pyrolysis of n-C32H66 in D2O (pH=7)  pyrolysis of Messel shale in D2O for 72 h at 330 C, with the objective of duplicating the results of Hoering In order to better understand the factors a€ecting the (1984), who reported extensive deuterium incorporation aqueous high temperature organic chemistry of heavy n- in the saturated hydrocarbons generated from the kero- parans, pyrolysis of n-C32H66 with water only or water gen under these conditions. Fig. 1a is a bar graph plot- with inorganic additives has been studied (Leif et al., ted from the original data of Hoering (1984) showing 1992; Leif, 1993). It was demonstrated that extensive the distribution of deuterium substitution in the normal hydrocarbon cracking, with varying degrees of alkene alkanes generated under these conditions. The graph formation in the cracking products, occurred at was derived by calculating the weighted average of the 350C for 72 h, with the aliphatic fraction consisting distribution patterns for the n-C17 to n-C29 alkanes of n-alkanes and n-alkenes. The composition of the using the weighting factor of the abundances of the products was modi®ed by pH and reactive species such individual n-alkanes. A similar bar graph of the weighted as elemental sulfur and iron sul®des. average deuterium distribution over the same n-alkane Two hydrous pyrolysis experiments with n-C32H66 range was made from the data of this study and shown were repeated in D2O to aid in elucidating the pathways in Fig. 1b. A comparison of these results indicates that by which water chemically reacts with hydrocarbons there are subtle di€erences between the two distribu- under hydrous pyrolysis conditions. The aliphatic frac- tions. The pattern from this study has a smaller amount tion from the D2O pyrolysis of n-C32H66 for 72 h at of generated n-alkanes in the D0 to D2 substitution 350C is shown in Fig. 2. The top ®gure is the gas range. The Hoering distribution maximizes at isomer chromatogram after the experiment showing the

D5 and the distribution for this study maximizes at D6, unreacted n-C32 H66 (o€ scale) and the products from but the overall patterns are similar and our results are in hydrocarbon cracking. These products were found to be agreement with those of Hoering (1984) showing extensive primarily n-alkanes and n-alkenes. The large number of deuterium incorporation in the n-alkanes generated n-alkene isomers and broad, poorly de®ned peak shapes from the thermal breakdown of Messel shale kerogen, in the alkene fraction are evidence that acid-catalyzed with some n-alkanes having incorporated up to 20 deu- double bond isomerization, with some deuterium incor- terium atoms. poration had occurred. Hydrogenation of the alkene R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208 1193

Fig. 2. Gas chromatograms of the D2O± n-C32H66 system: (a) total nonpolar fraction, (b) alkane fraction, (c) alkene fraction, and (d) alkene fraction after catalytic hydrogenation. Numbers refer to carbon chain lengths of n-alkanes. (Note, the enhanced concentration of n-C34H70 is a minor impurity in the n-C32H66 and the elevated C16 represents products from favored midchain cleavage.) fraction collapsed the multiple ole®n peaks into single reaction sequence (Kossiakov and Rice, 1943) for the peaks. Fig. 3 shows the mass spectrum of n-C17H36 of the free radical cracking of n-C32H66. Fig. 5 shows the mass alkane fraction and the mass spectrum of n-C17H36-iDi spectrum of n-C17H36 of the alkane fraction and the from the hydrogenated alkene fraction. It is clear that mass spectrum of n-C17H36-iDi from the hydrogenated no deuterium incorporation occurred in the alkane but alkene fraction. These results indicate that no deutera- extensive deuterium incorporation occurred in the ole- tion occurred under these conditions, neither in the ®n. The deuterium incorporation occurred during acid- alkane fraction nor in the alkene fraction. Because catalyzed isomerization of the double bond. alkaline conditions should inhibit the acid-catalyzed reactions but not a€ect the free radical exchange reac-

3.3. Hydrous pyrolysis of n-C32H66 in D2O (pH=11.3) tions, the above experiments (model compounds and water at 350C in the absence of sediment) demonstrate

The pyrolysis of n-C32H66 in D2O was repeated, but that no detectable direct deuterium exchange occurs this time the system was made alkaline by the addition between D2O and organic aliphatic hydrogen via a of NaOD (pH=11.3 at 25C). Fig. 4 shows the gas radical pathway, whereas some exchange between ole- chromatogram of the aliphatic fraction after heating. ®nic hydrogen and D2O is attributable to an acid- Shown is the unreacted n-C32H66 starting material (o€ catalyzed, ionic pathway. These two experiments scale) and the cracking products, but in this case there is demonstrate that the mechanistically simple direct only a doublet at each carbon number, i.e. an n-alkane reactions between alkyl free radical sites and water, as and a terminal ole®n. The alkaline system inhibited proposed by Lewan (1997), do not occur to any mea- double bond migration to give a product distribution surable extent under hydrous pyrolysis conditions and consisting of n-alkanes and terminal n-alkenes. This is a the exchange must be occurring through alternative product distribution expected from the Rice±Kossiakov reaction pathways. 1194 R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208

Fig. 3. Mass spectra of n-C17H36 from the D2O± n-C32H66 system: (a) alkane fraction, and (b) hydrogenated alkene fraction.

Fig. 6 is a simpli®ed schematic showing the for 1,13-tetradecadiene, 1-hexadecene and eicosane for major reaction pathways for the hydrous pyrolysis ®ve time periods of 1, 5, 10, 36 and 72 h. Modest deu- of n-alkanes. The products from these pyrolysis experi- terium incorporation was observed in the ole®ns and no ments are the result of primary cracking of n-C32H66 to incorporation in the alkane. This is expected considering form n-alkanes and terminal n-alkenes, followed by sec- the results from the pyrolyses of n-C32H66 in D2O ondary acid-catalyzed reactions of these terminal n- described above. alkenes to form a suite of internal n-alkenes. The only pathway for the deuterium exchange between water and 3.5. Hydrous pyrolysis of Messel shale/molecular probes hydrocarbons under these conditions is by an ionic in H2O rather than a free radical mechanism. The extent of double bond isomerization in the water system indicates Two time series experiments were conducted invol- that there can be signi®cant proton exchange between ving Messel shale. The ®rst series in H2O was conducted water and hydrocarbons by this pathway. to measure the relative rates of alkene isomerization versus hydrogenation for 1,13-tetradecadiene and 1-

3.4. Hydrous pyrolysis of molecular probes in D2O hexadecene when pyrolyzed in the presence of Messel shale. The data are shown in Table 2. The gas chroma- n-Alkanes and terminal ole®ns are the primary pro- tograms for the aliphatic fractions are shown in Fig. 8 ducts resulting from free radical b-scission reactions and and demonstrate that the rate of acid-catalyzed alkene therefore molecular probes representing these classes of isomerization is much faster than the rate of hydro- compounds have been selected for this study. These genation. This is shown in Fig. 9, where percentage iso- probes were reacted under hydrous pyrolysis conditions merization and percentage reduction are plotted as a and the relative reactivities of these compounds were function of time.  measured. A pyrolysis time series in D2O at 330 C was conducted to measure the relative rates of deuterium 3.6. Hydrous pyrolysis of Messel shale/molecular probes incorporation for an alkadiene, an alkene and an in D2O alkane. The patterns of deuterium incorporation for the three hydrocarbons for each experiment are shown in A series of pyrolyses was conducted in D2O to mea- Fig. 7. It shows the deuterium incorporation histograms sure the relative rates of deuterium incorporation for R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208 1195

1,13-tetradecadiene, 1-hexadecene and eicosane when When comparing the histograms of deuterium incor- pyrolyzed in the presence of Messel shale at 330C. The poration in the n-alkanes from the Messel shale kerogen amounts of individually spiked compounds were far in to those of the probe molecules, we see the closest match excess of the yield of corresponding n-alkanes generated is for the diene, with much less exchange occurring with from the Messel shale kerogen. The patterns of deuter- either the alkene or the alkane (Fig. 11). Under these ium incorporation for the three molecular probes in the conditions, deuterium exchange in the aliphatic hydro- ®ve experiments are shown in Fig. 10. These striking carbons occurs by a radical mechanism (incorporation results show extensive deuterium incorporation into the in the alkane) and an ionic mechanism (isomerization of ole®n molecules and some deuterium incorporation is a double bond, if a double bond is present). Fig. 11 also observed in the n-alkanes. In previous experiments shows a comparison of the histograms for deuterium without shale no deuterium incorporation was observed substitution patterns for the reactions above. in the saturated alkane (Fig. 7), but when pyrolyzed Examination of the polar fractions indicates that with Messel shale 65% of the recovered eicosane had initially alkanols and then alkanones were formed dur- incorporated at least 1 deuterium atom. The deuterium ing these hydrous pyrolysis experiments. Fig. 12 shows incorporation in the saturated n-alkane is interpreted as the GC traces indicating that the generation of the being due exclusively to a radical exchange process, but ketones proceeds through alcohol intermediates which the rate of deuterium incorporation in the saturated are present in the polar fractions during the early stages hydrocarbon is much slower than in either of the ole®n of the reactions followed by ketones present in the later species where the exchange occurs by both the radical stages of the experiments. The 1 and 5 h experiments and acid-catalyzed ionic pathways. produce mainly C14 and C16 alkanols from the respective

Fig. 4. Gas chromatograms of the D2O±n-C32H66±NaOD system: (a) total nonpolar fraction, (b) alkane fraction, (c) alkene fraction, and (d) alkene fraction after catalytic hydrogenation. Numbers refer to chain lengths of n-alkanes. 1196 R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208

Fig. 5. Mass spectra of n-C17H36 from the D2O±n-C32H66±NaOD system: (a) alkane fraction, and (b) hydrogenated alkene fraction.

ole®n precursors, the 10 h experiment has a mixture of alkanols and alkanones and the 36 and 72 h experiments show a dominance of alkanones. Of interest is the

appearance of C20 ketones in the 36 and 72 h experi- ments. These ketones, oxidation products of the n- alkane probe molecule, formed as a result of a free radical oxidation pathway. The alkanones ranging from

C10 to C33+ (Fig. 12d and e) are derived from the hydrous pyrolysis breakdown of the Messel shale kero-

gen. The elution range for the C16 alkanols and alka- nones is shown expanded in Fig. 13 for the 5 and 72 h experiments. The mass spectra of the alkan-i-ols (i=1± 6) are shown with their characteristic fragmentation patterns in Fig. 13b±e. The dominance of the secondary hexadecan-2-ol over the primary hexadecan-1-ol ®ts with the well known acid catalyzed hydration reaction of alkenes to alcohols. The same isomer distribution is observed for the alkanones (i.e. hexadecan-2-one >> hexadecanal) as for the alkanols, con®rming the oxida- tion of the latter with pyrolysis time.

3.7. Hydrous pyrolysis of sulfur/molecular probes in

D2O

One 10 h experiment was performed where the three aliphatic probe molecules were combined with 0.50 g  elemental sulfur and D2O and reacted at 330 C. The Fig. 6. Simpli®ed schematic model for deuterium incorpora- deuterium substitution patterns for the three probe tion into pyrolysis products of n-C32H66. molecules are shown in Fig. 14. Although this was only ..Li,BRT ioet/OgncGohmsr 1(00 1189±1208 (2000) 31 Geochemistry Organic / Simoneit B.R.T. Leif, R.N. 1197  Fig. 7. Histograms showing the extent of deuterium substitution in 1,13-tetradecadiene, 1-hexadecene, and eicosane as a function of time for pyrolysis in D2O at 330 C. 1198 R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208

Table 2  a Data from the pyrolysis of 1,13-tetradecadiene and 1-hexadecene molecular probes with Messel shale in H2O at 330 C 1,13-Tetradecadiene 1-Hexadecene

Pyrolysis time (h) % Isomerized % Hydrogenated % Isomerized % Hydrogenated

1 12.6 n.d. 5.7 n.d. 5 89.9 11.6 70.0 22.2 10 96.2 13.6 84.8 33.0 36 97.2 50.5 91.9 65.8 72 100.0 94.0 100.0 96.0

a n.d.=Not detected. a 10 h experiment, extensive deuteration occurred, even by water as proposed by Lewan (1997) does not appear in the saturated n-alkane. These results demonstrate the to be a signi®cant pathway under typical hydrous pyr- large degree to which sulfur can accelerate both the olysis conditions. This was demonstrated by the lack of ionic and radical exchange processes. any measurable D-incorporation in the cracking pro-

ducts formed as a result of the b-scission of n-C32H66 under alkaline conditions. Alternative reaction pathways 4. Discussion between water and hydrocarbons have been identi®ed and are the following: ionic double-bond isomerization The presence of ole®ns, especially terminal ole®ns, of transient alkene species, alcohol formation by alkene have been found in the bitumen fractions of sedimentary hydration followed by oxidation to a ketone, and radical organic matter near sill intrusions of the Guaymas Basin hydrogen atom exchange reactions via species that act hydrothermal system (Simoneit and Philp, 1982; Simo- as free radical hydrogen shuttles (i.e. sul®des or H2S). neit et al., 1986). These ole®ns, generated by the natural During hydrous pyrolysis the initial products generated hydrous pyrolysis occurring at Guaymas Basin, are by the carbon±carbon bond breaking of the aliphatic evidence for the pyrolytic generation of alkene inter- components are n-alkanes and terminal n-alkenes. This mediates during high thermal stress hydrothermal con- is consistent with the report by Seewald et al. (1998) ditions (Simoneit and Philp, 1982; Simoneit et al., 1986). where alkenes were identi®ed as reactive intermediates Petroleum from the Guaymas basin hydrothermal sys- during the hydrous pyrolysis of shales. This breakdown tem also contains aliphatic ketones which are synthe- of the aliphatic hydrocarbon network occurs through a sized under the hydrous pyrolysis conditions and have pathway of radical b-scission reactions and is the well- been proposed to be derived via oxidation of alcohols known Rice±Kossiakov mechanism. These thermal formed from the hydration of the hydrothermally cracking reactions of aliphatic hydrocarbons have been derived alkenes (Leif and Simoneit, 1995b). The major discussed by other researchers (Ford, 1986; Jackson et chemical reactions and their relative rates leading to the al., 1995; Burnham et al., 1997), and n-alkanes and pyrolysate distributions of aliphatic material under terminal n-alkenes are the same products that are gen- hydrous pyrolysis conditions have been identi®ed in the erated during Curie-point pyrolyses of hydrocarbons present series of experiments. This current set of experi- and aliphatic-rich materials (van de Meent et al., 1980; ments con®rms that hydration of the alkenes can result Tegelaar et al., 1989a,b). The terminal n-alkenes can in the formation of ketones via alcohol intermediates, undergo secondary acid-catalyzed double bond isomer- and double bond isomerization of generated alkenes ization under hydrothermal conditions (Weres et al., provides one pathway by which hydrogen from water 1988; Siskin et al., 1990) which results in incorporation can be incorporated into the aliphatic pyrolysates. (exchange) of hydrogen from water into the hydrocarbon Lewan (1992) has demonstrated that during hydrous skeleton, similar to the acid-catalyzed protium-deuterium pyrolysis water not only acts as solvent but also reacts exchange process of ole®ns under high temperature- dilute chemically, resulting in incorporation of water-derived acid conditions used to generate deuterium labelled com- hydrogen into the organic matter, with water-derived pounds (Werstiuk and Timmins, 1985). n-Alkenes were oxygen producing elevated amounts of carbon dioxide. identi®ed in the aliphatic fractions in the Messel shale

It was not clear to what degree water reacted and by H2O hydrous pyrolysis time series. Homologous series which mechanisms these processes occurred. This study of terminal n-alkenes and n-alkanes were released after 1 focuses on determining some of the likely reaction h from the kerogen and present in a 1:2 ratio, followed mechanisms which can occur between water and organic by alkene isomerization and a decrease in the alkene to matter. One pathway, the quenching of free radical sites alkane ratio in the 5 and 10 h experiments (Leif and R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208 1199

Simoneit, unpublished data). n-Alkenes were not detec- all occurred in the D or E rings of the C29 and C30 ted in either the 36 or 72 h runs but were likely present hopenes, and in the alkyl side chains of the C31 and at a low, steady-state concentration. greater hopenes. This is consistent with double bond Free alkenes were also detected in the triterpenoid formation via breakage of covalent triterpenoid linkages hydrocarbons released from the kerogen. Hopenes were at this end of the pentacyclic structure that bind these the dominant triterpenoids released after 1 hr during the compounds to the kerogen. The hopenes were not hydrous pyrolysis of Messel shale in H2O. The mass detected after 10 h and the triterpenoid biomarkers showed spectra of the hopenes indicate the unsaturated bonds a progression from a thermally immature distribution to

Fig. 8. Gas chromatograms of the aliphatic fractions from the pyrolyses of 1,13-tetradecadiene, 1-hexadecene and eicosane with H2O  and Messel shale at 330 C: (a) 1 h, (b) 5 h, (c) 10 h, (d) 36 h, and (e) 72 h. I.S.=internal standard (n-C24D50). 1200 R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208

Hydration of ole®ns to form alcohols has been observed during water-ole®n reactions conducted at temperatures from 180 to 250C (An et al., 1997). Although conversion was low, the ole®n hydration occurred readily and equilibrium was rapidly established. During hydrous pyrolysis of Messel shale, the hydration of the pyrolytically derived ole®ns forming alcohols also occurred readily as the major reaction pathway to oxy- genated products during brief contact times (1±5 h, Fig. 12). As observed by An et al. (1997) the addition of water to ole®ns is regioselective as shown by the hydra- tion of terminal ole®ns to form alkan-2-ols, which fol- lows the Markovnikov rule for an ionic mechanism. Competing with these ionic reactions of the ole®ns are the rapid free radical hydrogenation reactions that pro- ceed readily towards generation of saturated hydro- carbons. This was observed in pyrolysis reaction conditions regardless of the presence of water (Burnham et al., 1997) and demonstrated in the redox-bu€ered hydrothermal experiments where the reaction of alkenes with water forms alkanes (Seewald, 1994, 1996). The simultaneous reduction and oxidation reactions observed in this study are obviously not the only reac- tions occurring under hydrous pyrolysis conditions, but we have documented that the generated ole®ns react with water. The hydrogen from water ends up in a reduced hydrocarbon fraction (n-alkanes formed by the hydrogenation of the n-alkenes) and the oxygen from water ends up oxidizing a portion of the alkenes. Lewan (1992, 1997) has observed analogous reactions where

increased amounts of CO2 during hydrous pyrolysis experiments are the result of reactions between water and organic matter. The ketones observed in this study represent only partially oxidized carbon, but the con- version from an alcohol to a ketone provides some Fig. 9. Isomerization and reduction as a function of time dur- ing hydrous pyrolysis at 330C of: (a) 1-hexadecene and (b) reducing power, in the form of a hydrogen transfer, 1,13-tetradecadiene. which may in turn reduce other unsaturated hydro- carbons. Hydrogenation by molecular hydrogen is one characteristic of the early stages of oil generation. probably not a major pathway under these reaction Preferential deuterium enrichment at one end of the conditions. The exact mechanism of how the hydrogen biomarkers, as observed by others (Hoering, 1984; transfers occur during the oxidation of alcohols is Stalker et al., 1998), was also observed in these experi- unknown but the reaction most likely proceeds by ments. The mass spectra of the triterpenoid hydro- mineral catalysis or by a free radical pathway through a carbons released from the kerogen during the 72 h favorable hydrogen shuttle molecule such as H2Sor Messel shale D2O hydrous pyrolysis run con®rm that sul®des. This mechanism is only speculation but these extensive deuterium incorporation occurred, and the results demonstrate that ole®ns and alcohols are inter- exchange was localized in the D and E rings or the side mediates, and a portion of the alcohols is oxidized to chains of the hopane structures (Leif and Simoneit, ketones, providing further reducing potential for ole®n unpublished data). These results are consistent with a hydrogenation. The identi®ed reactions provide a path- combination of double bond isomerization (ionic), fol- way whereby water can react with the aliphatic portion lowed by reduction of the double bond (free radical) to of the organic matter to result in hydrogen exchange produce the observed deuterium substitution patterns. and possibly also result in a net transfer of water- A homogeneous radical exchange process would pro- derived hydrogen into this pool of organic matter. The duce uniform deuterium incorporation in all rings of the relative rates of the reactions depend on the experi- pentacyclic structure, which would be distinguishable by mental conditions because some of the components in the mass spectra. the shale can make the H-transfer reactions more facile. ..Li,BRT ioet/OgncGohmsr 1(00 1189±1208 (2000) 31 Geochemistry Organic / Simoneit B.R.T. Leif, R.N. 1201  Fig. 10. Histograms showing the extent of deuterium substitution in 1,13-tetradecadiene, 1-hexadecene, and eicosane as a function of time for pyrolysis in D2O with Messel shale at 330 C. 1202 R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208

This was tested by hydrous pyrolysis studies with mole- accelerated by acidic mineral sites (i.e. clays) and even cular probes and D2O media. Thermal destruction of the acidic sites in kerogen (Schimmelmann et al., 1999). aliphatic kerogen network should also produce double In addition to the ionic exchange pathway, direct D- bonds in the kerogen which can undergo isomerization incorporation by a radical pathway also occurs, which is reactions to result in deuterium incorporation into the greatly accelerated by the presence of sulfur and H2S. This kerogen network. Double bond isomerization can be is shown in Fig. 14 where extensive D-exchange occurred in all of the probe molecules after only 10 h. The in¯uence

of H2S on free radical cracking is well known (Rebick, 1981; Depeyre et al., 1985; Wei et al., 1992; Godo et al., 1997; 1998). Sulfur radicals are important during petro- leum formation (Lewan, 1998) and also they have been proposed as species responsible for H-exchange between water and organic matter (Ross, 1992a; Schimmelmann et al., 1999). Sulfur and sulfur species have even been shown to be capable of reacting stoichiometrically and also serving as oxidizing agents (Toland et al., 1958; Toland, 1960; 1961). Therefore, to explain the deuterium

patterns observed with the Messel shale/D2Opyrolyses, we propose a combination of ionic and radical exchange pathways. This is similar to the pathway for the n-

C32H66 pyrolyses, but here we include exchange with presumed ole®n groups in the shale kerogen along with radical exchange processes. Fig. 15 is a schematic showing the major reaction pathways of aliphatic com- pounds observed under hydrous pyrolysis conditions. The results suggest that deuterium incorporation into hydrocarbons can occur during acid-catalyzed double bond isomerization of alkene intermediates by 1,2-shifts of carbocations. The formation of intermediate bran- ched and isoprenoid alkenes, terminal n-alkenes, and even a,o-alkadienes from kerogen is consistent with the ®ndings from the structure elucidations of by chemical methods. Carboxylic acids, branched carboxylic acids, a,o-dicarboxylic acids, and isoprenoid acids are common products from kerogen oxidations (Burlingame et al., 1969; DjuricÏ ic et al., 1971; Simoneit and Burlin- game, 1973; Vitorovic , 1980). Because branching points are susceptible to oxidation, monocarboxylic acids and isoprenoid acids are formed from alkyl groups and iso- prenoid groups, respectively, attached to the kerogen matrix at one point. a,o-Dicarboxylic acids are formed as a result of an alkyl ``bridge'' which is attached to the kerogen at two points. Curie-point pyrolysis suggests that a highly aliphatic polymer is present in Messel shale kerogen (Goth et al., 1988). The conditions during hydrous pyrolysis experiments may yield similar frag- ments, but release primarily n-alkanes and terminal n- alkenes. The double bonds, in the pyrolysate and the remaining aliphatic kerogen network, would then undergo acid-catalyzed double bond isomerization prior to hydrogenation of the double bonds. Hydrogen exchange between water and organic matter also pro- Fig. 11. Comparison of deuterium incorporation for pyrolysis  ceeds via sulfur-derived radical species and H2S, and in D2O with Messel shale at 330 C for 72 h for: (a) Messel shale n-alkanes, (b) 1,13-tetradecadiene spiked on Messel shale, (c) 1- may also be catalyzed by minerals. hexadecene spiked on Messel shale, and (d) eicosane spiked on In the whole suite of reactions occurring under Messel shale. hydrous pyrolysis conditions, the net incorporation of R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208 1203

Fig. 12. Gas chromatograms of the polar NSO fractions (as TMS derivatives) obtained by hydrous pyrolysis of 1,13-tetradecadiene,  1-hexadecene and eicosane with Messel shale in H2O at 330 C: (a) 1 h, (b) 5 h, (c) 10 h, (d) 36 h, and (e) 72 h. I.S.=internal standard (n-C24D50). 1204 R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208

Fig. 13. GC±MS data for the alkanol to alkanone progression in the C16 elution range in the products from 5 and 72 h hydrous pyrolyses with alkenes and Messel shale: total ion current traces for C16 region (a) 5 h experiment (hexadecanols for i=1±6) and (f) 72 + h experiment (hexadecanones i=1±5 ); and mass spectra of the C16 alkanols from the 1 h experiment (b) 2-ol (2A), (c) 3-ol (3A), (d) 4-ol (4A) and (e) 5-ol (5A) (as the TMS ethers). R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208 1205

pool, which is simultaneously oxidized (i.e. hydrocarbon aromatization). This process occurs during the natural hydrous pyrolysis of sedimentary organic matter in the Guaymas Basin hydrothermal system (e.g. Kawka and Simoneit, 1987; Simoneit, 1993), a site where hot hydrothermal ¯uids pyrolyze immature sedimentary organic matter to produce oil under reaction conditions comparable to these laboratory hydrous pyrolysis experiments. In the Guaymas Basin, a reduced and alkane rich oil fraction is produced at the expense of a more labile and hydrogen poor fraction. The result is an n-alkane rich oil which is also highly enriched in oxi- dized organic matter, in the form of polycyclic aromatic hydrocarbons. The presence of the graphitic carburized coating on the walls of hydrous pyrolysis vessels is an example of this type of chemistry. The oils from Guay- mas Basin also contain ketones (Leif and Simoneit, 1995a,b), presumably generated by the pathway identi- ®ed in this study, but the ketones are in much lower concentrations relative to the abundant, partially-oxidized polycyclic aromatic hydrocarbons. Therefore, what is most likely a balanced and realistic view of the chemistry under hydrous pyrolysis condi- tions is a complex set of competing reactions where extensive hydrogen exchange within the pools of organic matter and between this organic matter and water are major reactions, but a net transfer of water-derived hydrogen into the organic matter is minor and of sec- ondary importance. If it were the other way around then the petroleum industry would have exploited water as a source of hydrogen years ago, because water as a hydrogen source during upgrading would be much more economical than using expensive catalysts and high pressure mole- cular hydrogen. The use of additives or speci®c H-transfer catalysts may result in ®nding novel reaction pathways leading to processes capable of using water as a signi®cant Fig. 14. Histograms showing the extent of deuterium incor- source of hydrogen for petroleum upgrading. poration after hydrous pyrolysis of molecular probes with ele- This study provides a better understanding of the  mental sulfur in D2O for 10 h at 330 C: (a) 1,13-tetradecadiene, signi®cant results originally presented by Hoering (b) 1-hexadecene, and (c) n-eicosane. (1984). Under hydrous pyrolysis conditions, water is a good solvent for organic molecules (e.g. Connolly, 1966; Price, 1976; 1993) and at elevated temperatures this water-derived hydrogen into organic matter (as opposed medium not only acts as a solvent but also reacts with to mere exchange) is likely to be relatively small. Most the organic matter present. This was observed by the of the organic oxidation-reduction reactions occur extensive deuterium incorporation from the D2O med- among the pools of organic carbon, although hydrogen ium into the ole®ns generated by free radical reactions exchange between water and the organic pools can be during the hydrous pyrolysis process. The rate for the quite extensive, as demonstrated here and by others (i.e. ionic aqueous-organic reaction of ole®n isomerization Hoering, 1984; Schimmelmann et al., 1999). For oxida- was greatly accelerated under these reaction conditions. tion-reduction reactions discussed here, the net hydro- In addition to isomerization, the double bonds were gen transfer rates among hydrocarbons are favored hydrogenated by free radical reactions (major reaction relative to the net hydrogen (and oxygen) transfer rates pathway) and oxidized to ketones (minor reaction between water and organic matter, and therefore the pathway) via hydration through alcohol intermediates. organic redox reactions will dominate. The consequence The results from these hydrous pyrolysis reactions can of this is that one hydrocarbon pool is reduced (i.e. alkene be applied directly to the Guaymas Basin hydrothermal hydrogenation) at the expense of another hydrocarbon system, where unconsolidated sedimentary organic 1206 R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208

Fig. 15. Proposed reaction pathway for the hydrothermal cracking and deuterium exchange processes occurring during the hydrous pyrolysis of Messel shale. matter is pyrolyzed by hot, hydrothermal ¯uids in a incorporation occurs by double bond isomerization of water-dominated environment and at comparable tem- intermediate alkenes produced by the pyrolytic break- peratures, much like the experimental conditions in this down of the aliphatic kerogen network and by free study. Care must be exercised when simulating processes radical reactions assisted by H2S and sulfur radical spe- that occur over geological time by performing experiments cies which make hydrogen transfer more facile. The at elevated temperatures and under greatly accelerated major portion of the alkenes are hydrogenated to time conditions. To do this, an overall understanding is alkanes, but a minor portion can undergo hydration to needed of the balance of competing ionic and radical form alcohols which can subsequently undergo oxida- reactions, how the di€erent reaction rates vary as a tion to alkanones. The major observations are: function of temperature (Weres et al., 1988; Burnham et al., 1997), and the degree to which the reactions are 1. Hydrocarbon cracking yields n-alkanes and term- a€ected by aqueous ionic species, mineral surfaces, and inal n-alkenes. the amount of sulfur-derived radical species. 2. Under hydrothermal conditions, the terminal n- alkenes rapidly isomerize to internal alkenes via acid-catalyzed isomerization. 5. Conclusions 3. Hydrogen exchange occurs between water and alkenes during the isomerization reaction.

The pyrolysis of Messel shale in D2O generates hydro- 4. Hydrogenation of the alkenes (under reducing carbons with a large content of deuterium. The deuterium conditions) forms alkanes. R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208 1207

5. Hydration of the alkenes forms transient n-alka- Depeyre, D., Flicoteaux, C., Blouri, B., Ossebi, J.G., 1985. nols, some of which are oxidized to n-alkanones. Pure n-nonane steam cracking and the in¯uence of sulfur compounds. Industrial Engineering Chemistry Process 6. Sulfur radical species and H2S accelerate both the ionic double bond isomerization and free radical Design and Development 24, 920±924. exchange reactions. DjuricÏ ic , M., Murphy, R.C., Vitorovic , D., Biemann, K., 1971. Organic acids obtained by alkaline permanganate oxidation of kerogen from the Green River (Colorado) shale. Geochi- The reaction pathways involving double bonds, either mica et Cosmochimica Acta 35, 1201±1207. present in the kerogen or in transient intermediate n- Ford, T.J., 1986. Liquid-phase thermal decomposition of hex- alkene species generated by the pyrolytic breakdown of adecane: reaction mechanisms. Industrial Engineering aliphatic kerogen material, help to explain how deuterium Chemistry Fundamentals 25, 240±243. incorporation occurs in the generated alkanes when Godo, M., Saito, M., Sasahara, J., Ishihara, A., Kabe, T.,

Messel shale is pyrolyzed in D2O, and demonstrate how 1997. Elucidation of coal liquefaction mechanism using a deuterium can become enriched at one end of a mole- tritium tracer method. E€ect of H2S and H2O on hydrogen cule. The intermediate n-alkenes rapidly isomerize and exchange reaction of tetralin with tritiated molecular hydro- simultaneously undergo reduction to n-alkanes and oxi- gen. Energy and Fuels 11, 470±476. Godo, M., Saito, M., Ishihara, A., Kabe, T., 1998. Elucidation dation to ketones via alcohols formed by the hydration of coal liquefaction mechanisms using a tritium tracer of the alkenes. method. Hydrogen exchange reaction of solvents with tri- tiated molecular hydrogen in the presence and absence of

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