the role of alkenes produced during hydrous pyrolysis of a shaledirectory.umm.ac.id/data...

The role of alkenes produced during hydrous pyrolysisof a shale

Roald N. Leif 1, Bernd R.T. Simoneit *

Environmental and Petroleum 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 kerogen. In order to understand the pathway ofdeuterium (and protium) exchange and the role of water 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 theabsence of powdered shale, the incorporation of deuterium from D2O occurred only in ole®nic compounds via doublebond isomerization. The presence of shale accelerated deuterium incorporation into the ole®ns and resulted in a minoramount 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-alkaneswith 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 cracking of the aliphatic kerogenstructure. 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 alcoholsformed 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

Hoering (1984) described interesting results concerningthe role of water during laboratory hydrous pyrolysis. Hefound that a large amount of deuterium was incorporated

into the n-alkanes generated from hydrous pyrolysis ofMessel shale kerogen in D2O. Messel shale was selectedfor the experiments due to its low thermal history, its

high organic carbon content, and its having been used innumerous studies. The shale was powdered and extrac-

ted prior to heating. For each experiment the shale wascombined with water or heavy water, sealed undernitrogen in a stainless steel reaction vessel and heated at

330�C for 72 h. The n-alkanes from the D2O pyrolysiswere isolated and analysed by mass spectrometry todetermine the extent of deuterium incorporation. The

substitution ranged from 0 to at least 14 deuteriumatoms for each n-alkane, with the highest relative abun-dances of 4±6 deuterium atoms. There was no trend insubstitution pattern as a function of chain length.

To explain the deuterium substitution patterns in thepyrolysis experiments, a free radical chain mechanismwas suggested. This mechanism proposes that one

0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.

PI I : S0146-6380(00 )00113-3

Organic Geochemistry 31 (2000) 1189±1208

www.elsevier.nl/locate/orggeochem

* Corresponding author. Tel.: +1-541-737-2155; fax: +1-

541-737-2064.

E-mail address: [email protected] (B.R.T. Simoneit).1 Present address: Lawrence Livermore National Labora-

tory, Livermore, CA 94551, USA.

pathway to the multiple deuteration could have occurredby the free radical migration of the ole®n sites. Similarradical reactions have been proposed by others(Monthioux et al., 1985; Comet et al., 1986), but Ross

(1992a,b) has shown that direct hydrogen transfer fromwater to organic free radicals is endothermic by 25±30kcal/mol and, therefore, should not be signi®cant at

hydrous pyrolysis conditions. A re-examination of theHoering (1984) deuterium isomer pro®le data bynumerical modelling was performed by Ross (1992a).

He concluded that a more likely explanation for thedeuterium isomer distribution in the n-alkanes generatedin the D2O Messel shale pyrolysis is by simultaneous

deuterium exchange at more than one site. He furthersuggested a combination of ionic and radical chemistryto explain the results (Ross, 1992a), although the detailsof the actual chemical mechanisms that result in the

observed preferential deuterium substitution at one endof the isoprenoid and biomarker molecules could stillnot be explained. Lewan (1997) has suggested that

under hydrous pyrolysis conditions water molecules canreact directly with organic free radicals generated by thethermal breakdown of organic matter.

A re-evaluation of the research in pyrolysis and hightemperature aqueous chemistry of hydrocarbons pro-vides some insight into the major reactions that alkanes

and alkenes undergo (Wilson et al., 1986; Weres et al.,1988; Kissin, 1987, 1990; Siskin et al., 1990; Leif et al.,1992; Stalker et al., 1994, 1998; Seewald, 1994, 1996;Jackson et al., 1995; Burnham et al., 1997; Lewan, 1997;

Seewald et al., 1998). These studies point to the impor-tance of both radical and ionic reaction mechanismsduring the pyrolysis of organic matter. This paper

duplicates the original Hoering (1984) Messel shale pyr-olysis experiment and presents results from additionalhydrous pyrolysis experiments which provide evidence

for the chemical pathways by which hydrogen exchangeoccurs between water and aliphatic hydrocarbons duringhydrous pyrolysis. Molecular probes were used with theshale to determine their relative reactivities with regard

to n-alkane and n-alkene production.

2. Experimental

2.1. Chemicals and samples

The Messel shale is Eocene and was sampled from thequarry at Darmstadt, Germany (Matthes, 1966; van den

Berg et al., 1977; van de Meent et al., 1980). Hydrouspyrolysis experiments were performed using ultrapure H2Ofrom Burdick and Jackson and D2O (purity >99.9%)from Cambridge Isotopes Laboratories. Both H2O and

D2O were distilled in glass before use. NaOD (purity>99.5%) for pyrolysis under alkaline conditions wasobtained fromCambridge Isotopes Laboratories. Aliphatic

compounds used in pyrolysis experiments were n-tetra-deca-1,13-diene (Aldrich Chemical Co., purity >97%),n-hexadec-1-ene (Aldrich Chemical Co., purity >97%),n-eicosane (Aldrich Chemical Co., purity 99%), and n-

dotriacontane (Aldrich Chemical Co., purity >97%).The Messel shale used in the experiments was powdered,exhaustively extracted in a Soxhlet apparatus with

methanol/methylene chloride for 72 h, and dried priorto the pyrolysis studies.

2.2. Hydrous pyrolysis experiments

The pyrolysis experiments were performed in passi-

vated Sno-Trik1 T316 stainless steel high pressure pipessealed with end caps with a total volume of 2.0 cm3 (Leifand Simoneit, 1995a). Deoxygenated H2O or D2O wasprepared by bubbling with argon for 45 min. The reac-

tion vessels were loaded with reactant mixtures, sealedin a glove bag under an argon atmosphere, and placedin a preheated air circulating oven set at the reaction

temperature and controlled to within �2�C. Durationsof the heating experiments ranged from 1 to 72 h.Table 1 is a listing of the pyrolysis experiments for

this study. The heavy water pyrolyses of Messel shalewere carried out at 330�C with 0.4 g dried shale powderand 0.8 ml of D2O. Messel shale pyrolyses with mole-

cular probes were conducted with 0.4 g dried shalepowder, 8 mg each of n-tetradeca-1,13-diene, n-hexadec-1-ene, and n-eicosane directly spiked on the shale, and0.8 ml of either H2O or D2O. Heavy water pyrolyses of

n-C32H66 were done at 350�C with 10 mg of the n-alkane

and 0.8 ml D2O. Pyrolysis of n-C32H66 under alkalineconditions was also carried out at 350�C with 10 mg of

the n-alkane and 0.8 ml D2O where the pH of the D2Owas adjusted to 11.3 (at 25�C) using NaOD.These hydrous pyrolysis experiments with pre-extracted,

powdered rock and added model compounds in aqueoussolution (330 or 350�C) may not be directly comparablewith hydrous pyrolysis of rock chips (i.e. Lewan, 1997),because the pore spaces in rock chips become ®lled with

water-saturated bitumen during hydrous pyrolysis.Maturing kerogen in rock chips is, therefore, not incontact with an aqueous phase, but with an organic

phase that has dissolved water in it. However, after theoil is expelled from the rock chips it can proceed to reactin an aqueous environment similar to what is occurring

in these experiments, and similar to the reactionsoccurring during aquathermolysis experiments (Siskin etal., 1990; Siskin and Katritzky, 1991).

2.3. Extraction and fractionation

The reaction vessels were cooled to room temperature

upon completion of the heating cycle. The vessels wereextracted with two 1 ml portions of methanol followedby ®ve 1 ml portions of methylene chloride. The solvents

1190 R.N. Leif, B.R.T. Simoneit /Organic Geochemistry 31 (2000) 1189±1208

and water from each pyrolysis experiment were com-bined in a centrifuge tube and the organic fractionseparated and collected. The water was extracted with

two additional portions of methylene chloride and themethylene chloride fractions were combined. Methylenechloride was dried with anhydrous sodium sulfate. The

methylene chloride extracts from the Messel shale experi-ments were passed through an activated copper columnto remove elemental sulfur. The solvent was removed to

near dryness by nitrogen blowdown. The total extract wasmade up to 2 ml of methylene chloride and deasphalted in100 ml of heptane. The asphaltenes were allowed to pre-cipitate overnight and separated from the maltenes by

vacuum ®ltration through a BuÈ chner funnel with fritteddisk (porosity : 4±5.5 mm) and washed with heptane. Thedeasphalted fractions were concentrated to 2 ml using a

rotary evaporator with water bath set at 30�C and frac-tionated by column chromatography (30 � 1 cm) packedwith 3.8 g alumina (fully active) over 3.8 g silica gel (fully

active). The samples were separated into three fractions byelution with 50 ml heptane (nonpolar fraction, F1), 50 mltoluene (aromatic fraction, F2) and 25 ml methanol

(polar fraction, F3). Separation of the alkenes from thealkanes was carried out by argentation silica columnchromatography. The normal alkanes of the Messel shale±D2O pyrolysis were isolated from the nonpolar fraction

by urea adduction, an additional procedure which wasnecessary to get a more reliable determination of thedeuterium incorporation of the n-alkanes. Hydrogenation

of selected samples was achieved by bubbling H2 gasinto the sample for 30 min in the presence of platinum(IV) oxide (Adam's catalyst). The internal standard

method was used to quantitate the probe moleculesusing relative response factors.

2.4. Gas chromatography

Gas chromatography (GC) of the pyrolysates was

performed with a Hewlett-Packard 5890A instrumentequipped with a 30 m x 0.25 mm i.d. DB-5 capillarycolumn (0.25 mm ®lm thickness). The GC oven washeated using the following program : isothermal for 2

min at 65�C, 3�C/min to 310�C and isothermal for 30min, with the injector at 290�C, detector at 325�C, andhelium as the carrier gas. The alcohols in the polar

fractions were converted to the trimethylsilyl derivativeswith BSTFA prior to analysis.

2.5. Gas chromatography±mass spectrometry

Gas chromatography±mass spectrometry (GC±MS)

was performed on a Finnigan 9610 gas chromatographequipped with a 30 m�0.25 mm i.d. DB-5 capillary col-umn (0.25 mm ®lm thickness) coupled to a Finnigan4021 quadrupole mass spectrometer operated at 70 eV

over the mass range 50±650 dalton and a cycle time of2.0 s. The GC oven temperature was programmed asdescribed above, with the injector at 290�C and helium

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 1 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 10 D2O (0.8 ml) Elemental sulfur (0.5 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)

R.N. Leif, B.R.T. Simoneit /Organic Geochemistry 31 (2000) 1189±1208 1191

as the carrier gas. The MS data were processed with anon-line Finnigan-Incos 2300 computer data system. Thepositional isomers of the n-alkanones and the n-alkanolswere identi®ed by comparison with authentic standards.

Deuterium incorporation in the probe molecules wasdetermined by monitoring the distribution in theirmolecular ions after hydrogenation of the ole®n probe

molecules to n-alkanes. GC±MS data was acquiredusing a Hewlett-Packard 5890 Series II GC coupled to aHewlett-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 andm/z 280±292 for n-C20H42.The GC was equipped with a 30 m�0.25 mm i.d. DB-1

capillary column (0.25 mm ®lm thickness). The GC oventemperature was programmed at isothermal for 2 min at100�C, 5�C/min to 260�C, 10�C/min to 300�C, and iso-thermal for 10 min, with an on-column injector, and

helium as the carrier gas. The MS data were processedwith Hewlett-Packard Chemstation software. The massintensity 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 extentof deuterium incorporation in the n-alkanes.

3. Results

3.1. Hydrous pyrolysis of Messel shale in D2O

The ®rst experiment in this series was the hydrous

pyrolysis of Messel shale in D2O for 72 h at 330�C, withthe objective of duplicating the results of Hoering(1984), who reported extensive deuterium incorporation

in the saturated hydrocarbons generated from the kero-gen under these conditions. Fig. 1a is a bar graph plot-ted from the original data of Hoering (1984) showing

the distribution of deuterium substitution in the normalalkanes generated under these conditions. The graphwas derived by calculating the weighted average of thedistribution patterns for the n-C17 to n-C29 alkanes

using the weighting factor of the abundances of theindividual n-alkanes. A similar bar graph of the weightedaverage deuterium distribution over the same n-alkane

range was made from the data of this study and shownin Fig. 1b. A comparison of these results indicates thatthere are subtle di�erences between the two distribu-

tions. The pattern from this study has a smaller amountof generated n-alkanes in the D0 to D2 substitutionrange. The Hoering distribution maximizes at isomer

D5 and the distribution for this study maximizes at D6,but the overall patterns are similar and our results are inagreement with those of Hoering (1984) showing extensivedeuterium incorporation in the n-alkanes generated

from the thermal breakdown of Messel shale kerogen,with some n-alkanes having incorporated up to 20 deu-terium atoms.

3.2. Hydrous pyrolysis of n-C32H66 in D2O (pH=7)

In order to better understand the factors a�ecting theaqueous high temperature organic chemistry of heavy n-

para�ns, pyrolysis of n-C32H66 with water only or waterwith inorganic additives has been studied (Leif et al.,1992; Leif, 1993). It was demonstrated that extensive

hydrocarbon cracking, with varying degrees of alkeneformation in the cracking products, occurred at350�C for 72 h, with the aliphatic fraction consistingof n-alkanes and n-alkenes. The composition of the

products was modi®ed by pH and reactive species suchas elemental sulfur and iron sul®des.Two hydrous pyrolysis experiments with n-C32H66

were repeated in D2O to aid in elucidating the pathwaysby which water chemically reacts with hydrocarbonsunder hydrous pyrolysis conditions. The aliphatic frac-

tion from the D2O pyrolysis of n-C32H66 for 72 h at350�C is shown in Fig. 2. The top ®gure is the gaschromatogram after the experiment showing the

unreacted n-C32 H66 (o� scale) and the products fromhydrocarbon cracking. These products were found to beprimarily n-alkanes and n-alkenes. The large number ofn-alkene isomers and broad, poorly de®ned peak shapes

in the alkene fraction are evidence that acid-catalyzeddouble bond isomerization, with some deuterium incor-poration had occurred. Hydrogenation of the alkene

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

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).


fraction collapsed the multiple ole®n peaks into singlepeaks. Fig. 3 shows the mass spectrum of n-C17H36 of the

alkane fraction and the mass spectrum of n-C17H36-iDi

from the hydrogenated alkene fraction. It is clear thatno deuterium incorporation occurred in the alkane butextensive deuterium incorporation occurred in the ole-

®n. The deuterium incorporation occurred during acid-catalyzed isomerization of the double bond.

3.3. Hydrous pyrolysis of n-C32H66 in D2O (pH=11.3)

The pyrolysis of n-C32H66 in D2O was repeated, but

this time the system was made alkaline by the additionof NaOD (pH=11.3 at 25�C). Fig. 4 shows the gaschromatogram of the aliphatic fraction after heating.

Shown is the unreacted n-C32H66 starting material (o�scale) and the cracking products, but in this case there isonly a doublet at each carbon number, i.e. an n-alkaneand a terminal ole®n. The alkaline system inhibited

double bond migration to give a product distributionconsisting of n-alkanes and terminal n-alkenes. This is aproduct distribution expected from the Rice±Kossiakov

reaction sequence (Kossiakov and Rice, 1943) for thefree radical cracking of n-C32H66. Fig. 5 shows the mass

spectrum of n-C17H36 of the alkane fraction and themass spectrum of n-C17H36-iDi from the hydrogenatedalkene fraction. These results indicate that no deutera-tion occurred under these conditions, neither in the

alkane fraction nor in the alkene fraction. Becausealkaline conditions should inhibit the acid-catalyzedreactions but not a�ect the free radical exchange reac-

tions, the above experiments (model compounds andwater at 350�C in the absence of sediment) demonstratethat no detectable direct deuterium exchange occurs

between D2O and organic aliphatic hydrogen via aradical pathway, whereas some exchange between ole-®nic hydrogen and D2O is attributable to an acid-

catalyzed, ionic pathway. These two experimentsdemonstrate that the mechanistically simple directreactions between alkyl free radical sites and water, asproposed by Lewan (1997), do not occur to any mea-

surable extent under hydrous pyrolysis conditions andthe exchange must be occurring through alternativereaction pathways.

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.)


Fig. 6 is a simpli®ed schematic showing themajor reaction pathways for the hydrous pyrolysis

of n-alkanes. The products from these pyrolysis experi-ments are the result of primary cracking of n-C32H66 toform n-alkanes and terminal n-alkenes, followed by sec-

ondary acid-catalyzed reactions of these terminal n-alkenes to form a suite of internal n-alkenes. The onlypathway for the deuterium exchange between water and

hydrocarbons under these conditions is by an ionicrather than a free radical mechanism. The extent ofdouble bond isomerization in the water system indicatesthat there can be signi®cant proton exchange between

water and hydrocarbons by this pathway.

3.4. Hydrous pyrolysis of molecular probes in D2O

n-Alkanes and terminal ole®ns are the primary pro-ducts resulting from free radical b-scission reactions and

therefore molecular probes representing these classes ofcompounds have been selected for this study. Theseprobes were reacted under hydrous pyrolysis conditions

and the relative reactivities of these compounds weremeasured. A pyrolysis time series in D2O at 330�C wasconducted to measure the relative rates of deuteriumincorporation for an alkadiene, an alkene and an

alkane. The patterns of deuterium incorporation for thethree hydrocarbons for each experiment are shown inFig. 7. It shows the deuterium incorporation histograms

for 1,13-tetradecadiene, 1-hexadecene and eicosane for®ve time periods of 1, 5, 10, 36 and 72 h. Modest deu-

terium incorporation was observed in the ole®ns and noincorporation in the alkane. This is expected consideringthe results from the pyrolyses of n-C32H66 in D2O

described above.

3.5. Hydrous pyrolysis of Messel shale/molecular probes

in H2O

Two time series experiments were conducted invol-ving Messel shale. The ®rst series in H2O was conducted

to measure the relative rates of alkene isomerizationversus hydrogenation for 1,13-tetradecadiene and 1-hexadecene when pyrolyzed in the presence of Messel

shale. The data are shown in Table 2. The gas chroma-tograms for the aliphatic fractions are shown in Fig. 8and demonstrate that the rate of acid-catalyzed alkene

isomerization is much faster than the rate of hydro-genation. This is shown in Fig. 9, where percentage iso-merization and percentage reduction are plotted as a

function of time.

3.6. Hydrous pyrolysis of Messel shale/molecular probesin D2O

A series of pyrolyses was conducted in D2O to mea-sure the relative rates of deuterium incorporation for

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


1,13-tetradecadiene, 1-hexadecene and eicosane whenpyrolyzed in the presence of Messel shale at 330�C. Theamounts of individually spiked compounds were far inexcess of the yield of corresponding n-alkanes generated

from the Messel shale kerogen. The patterns of deuter-ium incorporation for the three molecular probes in the®ve experiments are shown in Fig. 10. These striking

results show extensive deuterium incorporation into theole®n molecules and some deuterium incorporation isalso observed in the n-alkanes. In previous experiments

without shale no deuterium incorporation was observedin the saturated alkane (Fig. 7), but when pyrolyzedwith Messel shale 65% of the recovered eicosane had

incorporated at least 1 deuterium atom. The deuteriumincorporation in the saturated n-alkane is interpreted asbeing due exclusively to a radical exchange process, butthe rate of deuterium incorporation in the saturated

hydrocarbon is much slower than in either of the ole®nspecies where the exchange occurs by both the radicaland acid-catalyzed ionic pathways.

When comparing the histograms of deuterium incor-poration in the n-alkanes from the Messel shale kerogento those of the probe molecules, we see the closest matchis for the diene, with much less exchange occurring with

either the alkene or the alkane (Fig. 11). Under theseconditions, deuterium exchange in the aliphatic hydro-carbons occurs by a radical mechanism (incorporation

in the alkane) and an ionic mechanism (isomerization ofa double bond, if a double bond is present). Fig. 11shows a comparison of the histograms for deuterium

substitution patterns for the reactions above.Examination of the polar fractions indicates that

initially alkanols and then alkanones were formed dur-

ing these hydrous pyrolysis experiments. Fig. 12 showsthe GC traces indicating that the generation of theketones proceeds through alcohol intermediates whichare present in the polar fractions during the early stages

of the reactions followed by ketones present in the laterstages of the experiments. The 1 and 5 h experimentsproduce 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.


ole®n precursors, the 10 h experiment has a mixture ofalkanols and alkanones and the 36 and 72 h experimentsshow 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 fromC10 to C33+ (Fig. 12d and e) are derived from thehydrous 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 hexperiments. 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 secondaryhexadecan-2-ol over the primary hexadecan-1-ol ®tswith the well known acid catalyzed hydration reaction

of alkenes to alcohols. The same isomer distribution isobserved 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 threealiphatic probe molecules were combined with 0.50 g

elemental sulfur and D2O and reacted at 330�C. Thedeuterium substitution patterns for the three probemolecules are shown in Fig. 14. Although this was only

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

Fig. 6. Simpli®ed schematic model for deuterium incorpora-

tion into pyrolysis products of n-C32H66.


Fig.7.Histogramsshowingtheextentofdeuterium

substitutionin

1,13-tetradecadiene,

1-hexadecene,

andeicosaneasafunctionoftimeforpyrolysisin

D2O

at330� C

.


a 10 h experiment, extensive deuteration occurred, evenin the saturated n-alkane. These results demonstrate thelarge degree to which sulfur can accelerate both the

ionic and radical exchange processes.

4. Discussion

The presence of ole®ns, especially terminal ole®ns,

have been found in the bitumen fractions of sedimentaryorganic matter near sill intrusions of the Guaymas Basinhydrothermal system (Simoneit and Philp, 1982; Simo-

neit et al., 1986). These ole®ns, generated by the naturalhydrous pyrolysis occurring at Guaymas Basin, areevidence for the pyrolytic generation of alkene inter-mediates during high thermal stress hydrothermal con-

ditions (Simoneit and Philp, 1982; Simoneit et al., 1986).Petroleum from the Guaymas basin hydrothermal sys-tem also contains aliphatic ketones which are synthe-

sized under the hydrous pyrolysis conditions and havebeen proposed to be derived via oxidation of alcoholsformed from the hydration of the hydrothermally

derived alkenes (Leif and Simoneit, 1995b). The majorchemical reactions and their relative rates leading to thepyrolysate distributions of aliphatic material underhydrous pyrolysis conditions have been identi®ed in the

present series of experiments. This current set of experi-ments con®rms that hydration of the alkenes can resultin the formation of ketones via alcohol intermediates,

and double bond isomerization of generated alkenesprovides one pathway by which hydrogen from watercan be incorporated into the aliphatic pyrolysates.

Lewan (1992) has demonstrated that during hydrouspyrolysis water not only acts as solvent but also reactschemically, resulting in incorporation of water-derived

hydrogen into the organic matter, with water-derivedoxygen producing elevated amounts of carbon dioxide.It was not clear to what degree water reacted and bywhich mechanisms these processes occurred. This study

focuses on determining some of the likely reactionmechanisms which can occur between water and organicmatter. One pathway, the quenching of free radical sites

by water as proposed by Lewan (1997) does not appearto be a signi®cant pathway under typical hydrous pyr-olysis conditions. This was demonstrated by the lack of

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

between water and hydrocarbons have been identi®edand are the following: ionic double-bond isomerizationof transient alkene species, alcohol formation by alkene

hydration followed by oxidation to a ketone, and radicalhydrogen atom exchange reactions via species that actas free radical hydrogen shuttles (i.e. sul®des or H2S).

During hydrous pyrolysis the initial products generatedby the carbon±carbon bond breaking of the aliphaticcomponents are n-alkanes and terminal n-alkenes. Thisis consistent with the report by Seewald et al. (1998)

where alkenes were identi®ed as reactive intermediatesduring the hydrous pyrolysis of shales. This breakdownof the aliphatic hydrocarbon network occurs through a

pathway of radical b-scission reactions and is the well-known Rice±Kossiakov mechanism. These thermalcracking reactions of aliphatic hydrocarbons have been

discussed by other researchers (Ford, 1986; Jackson etal., 1995; Burnham et al., 1997), and n-alkanes andterminal n-alkenes are the same products that are gen-erated during Curie-point pyrolyses of hydrocarbons

and aliphatic-rich materials (van de Meent et al., 1980;Tegelaar et al., 1989a,b). The terminal n-alkenes canundergo secondary acid-catalyzed double bond isomer-

ization under hydrothermal conditions (Weres et al.,1988; Siskin et al., 1990) which results in incorporation(exchange) of hydrogen from water into the hydrocarbon

skeleton, similar to the acid-catalyzed protium-deuteriumexchange process of ole®ns under high temperature- diluteacid conditions used to generate deuterium labelled com-

pounds (Werstiuk and Timmins, 1985). n-Alkenes wereidenti®ed in the aliphatic fractions in the Messel shaleH2O hydrous pyrolysis time series. Homologous seriesof terminal n-alkenes and n-alkanes were released after 1

h from the kerogen and present in a 1:2 ratio, followedby alkene isomerization and a decrease in the alkene toalkane ratio in the 5 and 10 h experiments (Leif and

Table 2

Data from the pyrolysis of 1,13-tetradecadiene and 1-hexadecene molecular probes with Messel shale in H2O at 330�Ca

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.


Simoneit, unpublished data). n-Alkenes were not detec-ted in either the 36 or 72 h runs but were likely presentat a low, steady-state concentration.Free alkenes were also detected in the triterpenoid

hydrocarbons released from the kerogen. Hopenes werethe dominant triterpenoids released after 1 hr during thehydrous pyrolysis of Messel shale in H2O. The mass

spectra of the hopenes indicate the unsaturated bonds

all occurred in the D or E rings of the C29 and C30

hopenes, and in the alkyl side chains of the C31 andgreater hopenes. This is consistent with double bondformation via breakage of covalent triterpenoid linkages

at this end of the pentacyclic structure that bind thesecompounds to the kerogen. The hopenes were notdetected after 10 h and the triterpenoid biomarkers showed

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).


one characteristic of the early stages of oil generation.Preferential deuterium enrichment at one end of the

biomarkers, as observed by others (Hoering, 1984;Stalker et al., 1998), was also observed in these experi-ments. The mass spectra of the triterpenoid hydro-

carbons released from the kerogen during the 72 hMessel shale D2O hydrous pyrolysis run con®rm thatextensive deuterium incorporation occurred, and the

exchange was localized in the D and E rings or the sidechains of the hopane structures (Leif and Simoneit,unpublished data). These results are consistent with a

combination of double bond isomerization (ionic), fol-lowed by reduction of the double bond (free radical) toproduce the observed deuterium substitution patterns.A homogeneous radical exchange process would pro-

duce uniform deuterium incorporation in all rings of thepentacyclic structure, which would be distinguishable bythe mass spectra.

Hydration of ole®ns to form alcohols has beenobserved during water-ole®n reactions conducted attemperatures from 180 to 250�C (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 hydrationof 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 arethe 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 (Burnhamet al., 1997) and demonstrated in the redox-bu�eredhydrothermal 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, butwe have documented that the generated ole®ns reactwith water. The hydrogen from water ends up in a

reduced hydrocarbon fraction (n-alkanes formed by thehydrogenation of the n-alkenes) and the oxygen fromwater ends up oxidizing a portion of the alkenes. Lewan(1992, 1997) has observed analogous reactions where

increased amounts of CO2 during hydrous pyrolysisexperiments are the result of reactions between waterand organic matter. The ketones observed in this study

represent only partially oxidized carbon, but the con-version from an alcohol to a ketone provides somereducing power, in the form of a hydrogen transfer,

which may in turn reduce other unsaturated hydro-carbons. Hydrogenation by molecular hydrogen isprobably not a major pathway under these reactionconditions. The exact mechanism of how the hydrogen

transfers occur during the oxidation of alcohols isunknown but the reaction most likely proceeds bymineral catalysis or by a free radical pathway through a

favorable hydrogen shuttle molecule such as H2S orsul®des. This mechanism is only speculation but theseresults demonstrate that ole®ns and alcohols are inter-

mediates, and a portion of the alcohols is oxidized toketones, providing further reducing potential for ole®nhydrogenation. The identi®ed reactions provide a path-

way whereby water can react with the aliphatic portionof the organic matter to result in hydrogen exchangeand possibly also result in a net transfer of water-derived hydrogen into this pool of organic matter. The

relative rates of the reactions depend on the experi-mental conditions because some of the components inthe shale can make the H-transfer reactions more facile.

Fig. 9. Isomerization and reduction as a function of time dur-

ing hydrous pyrolysis at 330�C of: (a) 1-hexadecene and (b)

1,13-tetradecadiene.


Fig.10.Histogram

sshowingtheextentofdeuterium

substitutionin

1,13-tetradecadiene,1-hexad

ecene,an

deicosaneasafunctionoftimeforpyrolysisin

D2OwithMesselshaleat

330� C

.


This was tested by hydrous pyrolysis studies with mole-cular probes and D2O media. Thermal destruction of thealiphatic kerogen network should also produce doublebonds in the kerogen which can undergo isomerization

reactions to result in deuterium incorporation into thekerogen network. Double bond isomerization can be

accelerated by acidic mineral sites (i.e. clays) and evenacidic sites in kerogen (Schimmelmann et al., 1999).In addition to the ionic exchange pathway, direct D-

incorporation by a radical pathway also occurs, which is

greatly accelerated by the presence of sulfur andH2S. Thisis shown in Fig. 14 where extensive D-exchange occurredin 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 beenproposed as species responsible for H-exchange betweenwater and organic matter (Ross, 1992a; Schimmelmann

et al., 1999). Sulfur and sulfur species have even beenshown to be capable of reacting stoichiometrically andalso serving as oxidizing agents (Toland et al., 1958;Toland, 1960; 1961). Therefore, to explain the deuterium

patterns observed with the Messel shale/D2O pyrolyses,we propose a combination of ionic and radical exchangepathways. This is similar to the pathway for the n-

C32H66 pyrolyses, but here we include exchange withpresumed ole®n groups in the shale kerogen along withradical 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 doublebond isomerization of alkene intermediates by 1,2-shiftsof 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 kerogens bychemical methods. Carboxylic acids, branched carboxylic

acids, a,o-dicarboxylic acids, and isoprenoid acids arecommon products from kerogen oxidations (Burlingameet al., 1969; DjuricÏ icÂ et al., 1971; Simoneit and Burlin-

game, 1973; VitorovicÂ , 1980). Because branching pointsare susceptible to oxidation, monocarboxylic acids andisoprenoid acids are formed from alkyl groups and iso-prenoid groups, respectively, attached to the kerogen

matrix at one point. a,o-Dicarboxylic acids are formedas a result of an alkyl ``bridge'' which is attached to thekerogen at two points. Curie-point pyrolysis suggests

that a highly aliphatic polymer is present in Messel shalekerogen (Goth et al., 1988). The conditions duringhydrous pyrolysis experiments may yield similar frag-

ments, but release primarily n-alkanes and terminal n-alkenes. The double bonds, in the pyrolysate and theremaining aliphatic kerogen network, would then

undergo acid-catalyzed double bond isomerization priorto hydrogenation of the double bonds. Hydrogenexchange between water and organic matter also pro-ceeds via sulfur-derived radical species and H2S, and

may also be catalyzed by minerals.In the whole suite of reactions occurring under

hydrous pyrolysis conditions, the net incorporation of

Fig. 11. Comparison of deuterium incorporation for pyrolysis

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-

hexadecene spiked on Messel shale, and (d) eicosane spiked on

Messel shale.


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).


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).


water-derived hydrogen into organic matter (as opposed

to mere exchange) is likely to be relatively small. Mostof the organic oxidation-reduction reactions occuramong the pools of organic carbon, although hydrogen

exchange between water and the organic pools can bequite extensive, as demonstrated here and by others (i.e.Hoering, 1984; Schimmelmann et al., 1999). For oxida-

tion-reduction reactions discussed here, the net hydro-gen transfer rates among hydrocarbons are favoredrelative to the net hydrogen (and oxygen) transfer ratesbetween water and organic matter, and therefore the

organic redox reactions will dominate. The consequenceof this is that one hydrocarbon pool is reduced (i.e. alkenehydrogenation) at the expense of another hydrocarbon

pool, which is simultaneously oxidized (i.e. hydrocarbonaromatization). This process occurs during the naturalhydrous pyrolysis of sedimentary organic matter in theGuaymas Basin hydrothermal system (e.g. Kawka and

Simoneit, 1987; Simoneit, 1993), a site where hothydrothermal ¯uids pyrolyze immature sedimentaryorganic matter to produce oil under reaction conditions

comparable to these laboratory hydrous pyrolysisexperiments. In the Guaymas Basin, a reduced andalkane rich oil fraction is produced at the expense of a

more labile and hydrogen poor fraction. The result is ann-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 carburizedcoating on the walls of hydrous pyrolysis vessels is anexample 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 lowerconcentrations 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 whereextensive hydrogen exchange within the pools of organicmatter and between this organic matter and water are

major reactions, but a net transfer of water-derivedhydrogen into the organic matter is minor and of sec-ondary importance. If it were the other way around thenthe petroleum industry would have exploited water as a

source of hydrogen years ago, because water as a hydrogensource during upgrading would be much more economicalthan using expensive catalysts and high pressure mole-

cular hydrogen. The use of additives or speci®c H-transfercatalysts may result in ®nding novel reaction pathwaysleading to processes capable of using water as a signi®cant

source of hydrogen for petroleum upgrading.This study provides a better understanding of the

signi®cant results originally presented by Hoering(1984). Under hydrous pyrolysis conditions, water is a

good solvent for organic molecules (e.g. Connolly, 1966;Price, 1976; 1993) and at elevated temperatures thismedium not only acts as a solvent but also reacts with

the organic matter present. This was observed by theextensive deuterium incorporation from the D2O med-ium into the ole®ns generated by free radical reactions

during the hydrous pyrolysis process. The rate for theionic aqueous-organic reaction of ole®n isomerizationwas greatly accelerated under these reaction conditions.

In addition to isomerization, the double bonds werehydrogenated by free radical reactions (major reactionpathway) and oxidized to ketones (minor reactionpathway) via hydration through alcohol intermediates.

The results from these hydrous pyrolysis reactions canbe applied directly to the Guaymas Basin hydrothermalsystem, where unconsolidated sedimentary organic

Fig. 14. Histograms showing the extent of deuterium incor-

poration after hydrous pyrolysis of molecular probes with ele-

mental sulfur in D2O for 10 h at 330�C: (a) 1,13-tetradecadiene,(b) 1-hexadecene, and (c) n-eicosane.


matter is pyrolyzed by hot, hydrothermal ¯uids in awater-dominated environment and at comparable tem-peratures, much like the experimental conditions in this

study. Care must be exercised when simulating processesthat occur over geological time by performing experimentsat elevated temperatures and under greatly accelerated

time conditions. To do this, an overall understanding isneeded of the balance of competing ionic and radicalreactions, how the di�erent reaction rates vary as a

function of temperature (Weres et al., 1988; Burnham etal., 1997), and the degree to which the reactions area�ected by aqueous ionic species, mineral surfaces, and

the amount of sulfur-derived radical species.

5. Conclusions

The pyrolysis of Messel shale in D2O generates hydro-carbons with a large content of deuterium. The deuterium

incorporation occurs by double bond isomerization ofintermediate alkenes produced by the pyrolytic break-down of the aliphatic kerogen network and by free

radical reactions assisted by H2S and sulfur radical spe-cies which make hydrogen transfer more facile. Themajor portion of the alkenes are hydrogenated to

alkanes, but a minor portion can undergo hydration toform alcohols which can subsequently undergo oxida-tion to alkanones. The major observations are:

1. Hydrocarbon cracking yields n-alkanes and term-inal n-alkenes.

2. Under hydrothermal conditions, the terminal n-alkenes rapidly isomerize to internal alkenes viaacid-catalyzed isomerization.

3. Hydrogen exchange occurs between water and

alkenes during the isomerization reaction.4. Hydrogenation of the alkenes (under reducing

conditions) forms alkanes.

Fig. 15. Proposed reaction pathway for the hydrothermal cracking and deuterium exchange processes occurring during the hydrous

pyrolysis of Messel shale.


5. Hydration of the alkenes forms transient n-alka-nols, some of which are oxidized to n-alkanones.

6. Sulfur radical species and H2S accelerate both theionic double bond isomerization and free radical

exchange reactions.

The reaction pathways involving double bonds, either

present in the kerogen or in transient intermediate n-alkene species generated by the pyrolytic breakdown ofaliphatic kerogen material, help to explain how deuterium

incorporation occurs in the generated alkanes whenMessel shale is pyrolyzed in D2O, and demonstrate howdeuterium can become enriched at one end of a mole-

cule. The intermediate n-alkenes rapidly isomerize andsimultaneously undergo reduction to n-alkanes and oxi-dation to ketones via alcohols formed by the hydrationof the alkenes.

Acknowledgements

We thank the National Aeronautics and SpaceAdministration (Grants NAGW-2833 and NAGW-

4172) and the donors of the Petroleum Research Fundadministered by the American Chemical Society forsupport of this research. We also thank Dr. Arndt

Schimmelmann and Dr. Gordon Love for their excellentand detailed reviews which greatly improved thismanuscript.

Associate EditorÐL. Schwark

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