Directory UMM :Data Elmu:jurnal:O:Organic Geochemistry:Vol31.Issue11.Nov2000:
Organic Geochemistry 31 (2000) 1189±1208
www.elsevier.nl/locate/orggeochem
The role of alkenes produced during hydrous pyrolysis
of 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 of
deuterium (and protium) exchange and the role of water during hydrous pyrolysis, we conducted a series of experiments 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 cracking of the aliphatic kerogen
structure. The terminal n-alkenes rapidly isomerize to internal alkenes via acid-catalyzed isomerization under hydrothermal 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
Hoering (1984) described interesting results concerning
the role of water during laboratory hydrous pyrolysis. He
found that a large amount of deuterium was incorporated
into the n-alkanes generated from hydrous pyrolysis of
Messel shale kerogen in D2O. Messel shale was selected
for the experiments due to its low thermal history, its
* Corresponding author. Tel.: +1-541-737-2155; fax: +1541-737-2064.
E-mail address: [email protected] (B.R.T. Simoneit).
1
Present address: Lawrence Livermore National Laboratory, Livermore, CA 94551, USA.
high organic carbon content, and its having been used in
numerous studies. The shale was powdered and extracted prior to heating. For each experiment the shale was
combined with water or heavy water, sealed under
nitrogen in a stainless steel reaction vessel and heated at
330 C for 72 h. The n-alkanes from the D2O pyrolysis
were isolated and analysed by mass spectrometry to
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 abundances of 4±6 deuterium atoms. There was no trend in
substitution pattern as a function of chain length.
To explain the deuterium substitution patterns in the
pyrolysis experiments, a free radical chain mechanism
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
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R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208
pathway to the multiple deuteration could have occurred
by the free radical migration of the ole®n sites. Similar
radical reactions have been proposed by others
(Monthioux et al., 1985; Comet et al., 1986), but Ross
(1992a,b) has shown that direct hydrogen transfer from
water to organic free radicals is endothermic by 25±30
kcal/mol and, therefore, should not be signi®cant at
hydrous pyrolysis conditions. A re-examination of the
Hoering (1984) deuterium isomer pro®le data by
numerical modelling was performed by Ross (1992a).
He concluded that a more likely explanation for the
deuterium isomer distribution in the n-alkanes generated
in the D2O Messel shale pyrolysis is by simultaneous
deuterium exchange at more than one site. He further
suggested a combination of ionic and radical chemistry
to explain the results (Ross, 1992a), although the details
of the actual chemical mechanisms that result in the
observed preferential deuterium substitution at one end
of the isoprenoid and biomarker molecules could still
not be explained. Lewan (1997) has suggested that
under hydrous pyrolysis conditions water molecules can
react directly with organic free radicals generated by the
thermal breakdown of organic matter.
A re-evaluation of the research in pyrolysis and high
temperature aqueous chemistry of hydrocarbons provides 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 importance of both radical and ionic reaction mechanisms
during the pyrolysis of organic matter. This paper
duplicates the original Hoering (1984) Messel shale pyrolysis experiment and presents results from additional
hydrous pyrolysis experiments which provide evidence
for the chemical pathways by which hydrogen exchange
occurs between water and aliphatic hydrocarbons during
hydrous pyrolysis. Molecular probes were used with the
shale 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 the
quarry at Darmstadt, Germany (Matthes, 1966; van den
Berg et al., 1977; van de Meent et al., 1980). Hydrous
pyrolysis experiments were performed using ultrapure H2O
from 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 was
obtained from Cambridge Isotopes Laboratories. Aliphatic
compounds used in pyrolysis experiments were n-tetradeca-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 ndotriacontane (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 prior
to the pyrolysis studies.
2.2. Hydrous pyrolysis experiments
The pyrolysis experiments were performed in passivated Sno-Trik1 T316 stainless steel high pressure pipes
sealed with end caps with a total volume of 2.0 cm3 (Leif
and Simoneit, 1995a). Deoxygenated H2O or D2O was
prepared by bubbling with argon for 45 min. The reaction vessels were loaded with reactant mixtures, sealed
in a glove bag under an argon atmosphere, and placed
in a preheated air circulating oven set at the reaction
temperature and controlled to within 2 C. Durations
of 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 shale
were carried out at 330 C with 0.4 g dried shale powder
and 0.8 ml of D2O. Messel shale pyrolyses with molecular probes were conducted with 0.4 g dried shale
powder, 8 mg each of n-tetradeca-1,13-diene, n-hexadec1-ene, and n-eicosane directly spiked on the shale, and
0.8 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 alkaline
conditions was also carried out at 350 C with 10 mg of
the n-alkane and 0.8 ml D2O where the pH of the D2O
was adjusted to 11.3 (at 25 C) using NaOD.
These hydrous pyrolysis experiments with pre-extracted,
powdered rock and added model compounds in aqueous
solution (330 or 350 C) may not be directly comparable
with 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 in
contact with an aqueous phase, but with an organic
phase that has dissolved water in it. However, after the
oil is expelled from the rock chips it can proceed to react
in an aqueous environment similar to what is occurring
in these experiments, and similar to the reactions
occurring during aquathermolysis experiments (Siskin et
al., 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 were
extracted with two 1 ml portions of methanol followed
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
( C)
Duration
(h)
Liquid medium
Reactants
330
350
350
330
330
330
330
330
330
330
330
330
330
330
330
330
330
330
330
72
72
72
1
5
10
36
72
1
5
10
36
72
1
5
10
36
72
10
D2O
D2O
D2O
D2O
D2O
D2O
D2O
D2O
H2O
H2O
H2O
H2O
H2O
D2O
D2O
D2O
D2O
D2O
D2O
Messel shale (0.4 g)
n-C32H66 (10 mg)
n-C32H66 (10 mg)
1,13-C14:2, 1-C16:1, and C20 (8 mg each)
1,13-C14:2, 1-C16:1, and C20 (8 mg each)
1,13-C14:2, 1-C16:1, and C20 (8 mg each)
1,13-C14:2, 1-C16:1, and C20 (8 mg each)
1,13-C14:2, 1-C16:1, and C20 (8 mg each)
Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)
Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)
Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)
Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)
Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)
Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)
Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)
Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)
Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)
Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)
Elemental sulfur (0.5 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)
(0.8 ml)
(0.8 ml, pH=7.0 at 25 C)
(0.8 ml, pH=11.3 at 25 C)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
and water from each pyrolysis experiment were combined in a centrifuge tube and the organic fraction
separated and collected. The water was extracted with
two additional portions of methylene chloride and the
methylene chloride fractions were combined. Methylene
chloride was dried with anhydrous sodium sulfate. The
methylene chloride extracts from the Messel shale experiments were passed through an activated copper column
to remove elemental sulfur. The solvent was removed to
near dryness by nitrogen blowdown. The total extract was
made up to 2 ml of methylene chloride and deasphalted in
100 ml of heptane. The asphaltenes were allowed to precipitate overnight and separated from the maltenes by
vacuum ®ltration through a BuÈchner funnel with fritted
disk (porosity : 4±5.5 mm) and washed with heptane. The
deasphalted fractions were concentrated to 2 ml using a
rotary evaporator with water bath set at 30 C and fractionated by column chromatography (30 1 cm) packed
with 3.8 g alumina (fully active) over 3.8 g silica gel (fully
active). The samples were separated into three fractions by
elution with 50 ml heptane (nonpolar fraction, F1), 50 ml
toluene (aromatic fraction, F2) and 25 ml methanol
(polar fraction, F3). Separation of the alkenes from the
alkanes was carried out by argentation silica column
chromatography. The normal alkanes of the Messel shale±
D2O pyrolysis were isolated from the nonpolar fraction
by urea adduction, an additional procedure which was
necessary to get a more reliable determination of the
deuterium incorporation of the n-alkanes. Hydrogenation
of selected samples was achieved by bubbling H2 gas
into 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 molecules
using relative response factors.
2.4. Gas chromatography
Gas chromatography (GC) of the pyrolysates was
performed with a Hewlett-Packard 5890A instrument
equipped with a 30 m x 0.25 mm i.d. DB-5 capillary
column (0.25 mm ®lm thickness). The GC oven was
heated using the following program : isothermal for 2
min at 65 C, 3 C/min to 310 C and isothermal for 30
min, with the injector at 290 C, detector at 325 C, and
helium as the carrier gas. The alcohols in the polar
fractions were converted to the trimethylsilyl derivatives
with BSTFA prior to analysis.
2.5. Gas chromatography±mass spectrometry
Gas chromatography±mass spectrometry (GC±MS)
was performed on a Finnigan 9610 gas chromatograph
equipped with a 30 m0.25 mm i.d. DB-5 capillary column (0.25 mm ®lm thickness) coupled to a Finnigan
4021 quadrupole mass spectrometer operated at 70 eV
over the mass range 50±650 dalton and a cycle time of
2.0 s. The GC oven temperature was programmed as
described above, with the injector at 290 C and helium
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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 m0.25 mm i.d. DB-1
capillary column (0.25 mm ®lm thickness). The GC oven
temperature was programmed at isothermal for 2 min at
100 C, 5 C/min to 260 C, 10 C/min to 300 C, and isothermal 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.
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, with
the objective of duplicating the results of Hoering
(1984), who reported extensive deuterium incorporation
in the saturated hydrocarbons generated from the kerogen under these conditions. Fig. 1a is a bar graph plotted from the original data of Hoering (1984) showing
the distribution of deuterium substitution in the normal
alkanes generated under these conditions. The graph
was derived by calculating the weighted average of the
distribution patterns for the n-C17 to n-C29 alkanes
using the weighting factor of the abundances of the
individual n-alkanes. A similar bar graph of the weighted
average deuterium distribution over the same n-alkane
range was made from the data of this study and shown
in Fig. 1b. A comparison of these results indicates that
there are subtle dierences between the two distributions. The pattern from this study has a smaller amount
of generated n-alkanes in the D0 to D2 substitution
range. 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 in
agreement with those of Hoering (1984) showing extensive
deuterium incorporation in the n-alkanes generated
from the thermal breakdown of Messel shale kerogen,
with some n-alkanes having incorporated up to 20 deuterium atoms.
Fig. 1. Average distribution of deuterium substitution in nalkanes from C17 to C29 generated from: (a) the D2O pyrolysis
of Messel shale (after Hoering, 1984), and (b) the D2O pyrolysis of Messel shale (this study).
3.2. Hydrous pyrolysis of n-C32H66 in D2O (pH=7)
In order to better understand the factors aecting the
aqueous high temperature organic chemistry of heavy nparans, pyrolysis of n-C32H66 with water only or water
with inorganic additives has been studied (Leif et al.,
1992; Leif, 1993). It was demonstrated that extensive
hydrocarbon cracking, with varying degrees of alkene
formation in the cracking products, occurred at
350 C for 72 h, with the aliphatic fraction consisting
of n-alkanes and n-alkenes. The composition of the
products was modi®ed by pH and reactive species such
as elemental sulfur and iron sul®des.
Two hydrous pyrolysis experiments with n-C32H66
were repeated in D2O to aid in elucidating the pathways
by which water chemically reacts with hydrocarbons
under hydrous pyrolysis conditions. The aliphatic fraction from the D2O pyrolysis of n-C32H66 for 72 h at
350 C is shown in Fig. 2. The top ®gure is the gas
chromatogram after the experiment showing the
unreacted n-C32 H66 (o scale) and the products from
hydrocarbon cracking. These products were found to be
primarily n-alkanes and n-alkenes. The large number of
n-alkene isomers and broad, poorly de®ned peak shapes
in the alkene fraction are evidence that acid-catalyzed
double bond isomerization, with some deuterium incorporation 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
peaks. 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 that
no deuterium incorporation occurred in the alkane but
extensive deuterium incorporation occurred in the ole®n. The deuterium incorporation occurred during acidcatalyzed 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 addition
of NaOD (pH=11.3 at 25 C). Fig. 4 shows the gas
chromatogram 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 is
only a doublet at each carbon number, i.e. an n-alkane
and a terminal ole®n. The alkaline system inhibited
double bond migration to give a product distribution
consisting of n-alkanes and terminal n-alkenes. This is a
product distribution expected from the Rice±Kossiakov
reaction sequence (Kossiakov and Rice, 1943) for the
free radical cracking of n-C32H66. Fig. 5 shows the mass
spectrum of n-C17H36 of the alkane fraction and the
mass spectrum of n-C17H36-iDi from the hydrogenated
alkene fraction. These results indicate that no deuteration occurred under these conditions, neither in the
alkane fraction nor in the alkene fraction. Because
alkaline conditions should inhibit the acid-catalyzed
reactions but not aect the free radical exchange reactions, the above experiments (model compounds and
water at 350 C in the absence of sediment) demonstrate
that no detectable direct deuterium exchange occurs
between D2O and organic aliphatic hydrogen via a
radical pathway, whereas some exchange between ole®nic hydrogen and D2O is attributable to an acidcatalyzed, ionic pathway. These two experiments
demonstrate that the mechanistically simple direct
reactions between alkyl free radical sites and water, as
proposed by Lewan (1997), do not occur to any measurable extent under hydrous pyrolysis conditions and
the exchange must be occurring through alternative
reaction pathways.
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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
major reaction pathways for the hydrous pyrolysis
of n-alkanes. The products from these pyrolysis experiments are the result of primary cracking of n-C32H66 to
form n-alkanes and terminal n-alkenes, followed by secondary acid-catalyzed reactions of these terminal nalkenes to form a suite of internal n-alkenes. The only
pathway for the deuterium exchange between water and
hydrocarbons under these conditions is by an ionic
rather than a free radical mechanism. The extent of
double bond isomerization in the water system indicates
that 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 products resulting from free radical b-scission reactions and
therefore molecular probes representing these classes of
compounds have been selected for this study. These
probes were reacted under hydrous pyrolysis conditions
and the relative reactivities of these compounds were
measured. A pyrolysis time series in D2O at 330 C was
conducted to measure the relative rates of deuterium
incorporation for an alkadiene, an alkene and an
alkane. The patterns of deuterium incorporation for the
three hydrocarbons for each experiment are shown in
Fig. 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 deuterium incorporation was observed in the ole®ns and no
incorporation in the alkane. This is expected considering
the 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 involving Messel shale. The ®rst series in H2O was conducted
to measure the relative rates of alkene isomerization
versus hydrogenation for 1,13-tetradecadiene and 1hexadecene when pyrolyzed in the presence of Messel
shale. The data are shown in Table 2. The gas chromatograms for the aliphatic fractions are shown in Fig. 8
and demonstrate that the rate of acid-catalyzed alkene
isomerization is much faster than the rate of hydrogenation. This is shown in Fig. 9, where percentage isomerization and percentage reduction are plotted as a
function of time.
3.6. Hydrous pyrolysis of Messel shale/molecular probes
in D2O
A series of pyrolyses was conducted in D2O to measure the relative rates of deuterium incorporation for
R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208
1,13-tetradecadiene, 1-hexadecene and eicosane when
pyrolyzed in the presence of Messel shale at 330 C. The
amounts of individually spiked compounds were far in
excess of the yield of corresponding n-alkanes generated
from the Messel shale kerogen. The patterns of deuterium incorporation for the three molecular probes in the
®ve experiments are shown in Fig. 10. These striking
results show extensive deuterium incorporation into the
ole®n molecules and some deuterium incorporation is
also observed in the n-alkanes. In previous experiments
without shale no deuterium incorporation was observed
in the saturated alkane (Fig. 7), but when pyrolyzed
with Messel shale 65% of the recovered eicosane had
incorporated at least 1 deuterium atom. The deuterium
incorporation in the saturated n-alkane is interpreted as
being due exclusively to a radical exchange process, but
the rate of deuterium incorporation in the saturated
hydrocarbon is much slower than in either of the ole®n
species where the exchange occurs by both the radical
and acid-catalyzed ionic pathways.
1195
When comparing the histograms of deuterium incorporation in the n-alkanes from the Messel shale kerogen
to those of the probe molecules, we see the closest match
is for the diene, with much less exchange occurring with
either the alkene or the alkane (Fig. 11). Under these
conditions, deuterium exchange in the aliphatic hydrocarbons occurs by a radical mechanism (incorporation
in the alkane) and an ionic mechanism (isomerization of
a double bond, if a double bond is present). Fig. 11
shows 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 during these hydrous pyrolysis experiments. Fig. 12 shows
the GC traces indicating that the generation of the
ketones proceeds through alcohol intermediates which
are present in the polar fractions during the early stages
of the reactions followed by ketones present in the later
stages of the experiments. The 1 and 5 h experiments
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.
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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 experiments. These ketones, oxidation products of the nalkane 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 kerogen. The elution range for the C16 alkanols and alkanones 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 oxidation of the latter with pyrolysis time.
3.7. Hydrous pyrolysis of sulfur/molecular probes in
D2 O
Fig. 6. Simpli®ed schematic model for deuterium incorporation into pyrolysis products of n-C32H66.
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
deuterium substitution patterns for the three probe
molecules are shown in Fig. 14. Although this was only
R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208
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
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
5
10
36
72
12.6
89.9
96.2
97.2
100.0
n.d.
11.6
13.6
50.5
94.0
5.7
70.0
84.8
91.9
100.0
n.d.
22.2
33.0
65.8
96.0
a
n.d.=Not detected.
a 10 h experiment, extensive deuteration occurred, even
in the saturated n-alkane. These results demonstrate the
large 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 sedimentary
organic matter near sill intrusions of the Guaymas Basin
hydrothermal system (Simoneit and Philp, 1982; Simoneit et al., 1986). These ole®ns, generated by the natural
hydrous pyrolysis occurring at Guaymas Basin, are
evidence for the pyrolytic generation of alkene intermediates during high thermal stress hydrothermal conditions (Simoneit and Philp, 1982; Simoneit et al., 1986).
Petroleum from the Guaymas basin hydrothermal system also contains aliphatic ketones which are synthesized under the hydrous pyrolysis conditions and have
been proposed to be derived via oxidation of alcohols
formed from the hydration of the hydrothermally
derived alkenes (Leif and Simoneit, 1995b). The major
chemical reactions and their relative rates leading to the
pyrolysate distributions of aliphatic material under
hydrous pyrolysis conditions have been identi®ed in the
present series of experiments. This current set of experiments con®rms that hydration of the alkenes can result
in the formation of ketones via alcohol intermediates,
and double bond isomerization of generated alkenes
provides one pathway by which hydrogen from water
can be incorporated into the aliphatic pyrolysates.
Lewan (1992) has demonstrated that during hydrous
pyrolysis water not only acts as solvent but also reacts
chemically, resulting in incorporation of water-derived
hydrogen into the organic matter, with water-derived
oxygen producing elevated amounts of carbon dioxide.
It was not clear to what degree water reacted and by
which mechanisms these processes occurred. This study
focuses on determining some of the likely reaction
mechanisms which can occur between water and organic
matter. One pathway, the quenching of free radical sites
by water as proposed by Lewan (1997) does not appear
to be a signi®cant pathway under typical hydrous pyrolysis conditions. This was demonstrated by the lack of
any measurable D-incorporation in the cracking products formed as a result of the b-scission of n-C32H66
under alkaline conditions. Alternative reaction pathways
between water and hydrocarbons have been identi®ed
and are the following: ionic double-bond isomerization
of transient alkene species, alcohol formation by alkene
hydration followed by oxidation to a ketone, and radical
hydrogen atom exchange reactions via species that act
as free radical hydrogen shuttles (i.e. sul®des or H2S).
During hydrous pyrolysis the initial products generated
by the carbon±carbon bond breaking of the aliphatic
components are n-alkanes and terminal n-alkenes. This
is consistent with the report by Seewald et al. (1998)
where alkenes were identi®ed as reactive intermediates
during the hydrous pyrolysis of shales. This breakdown
of the aliphatic hydrocarbon network occurs through a
pathway of radical b-scission reactions and is the wellknown Rice±Kossiakov mechanism. These thermal
cracking reactions of aliphatic hydrocarbons have been
discussed by other researchers (Ford, 1986; Jackson et
al., 1995; Burnham et al., 1997), and n-alkanes and
terminal n-alkenes are the same products that are generated 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 can
undergo secondary acid-catalyzed double bond isomerization 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-deuterium
exchange process of ole®ns under high temperature- dilute
acid conditions used to generate deuterium labelled compounds (Werstiuk and Timmins, 1985). n-Alkenes were
identi®ed in the aliphatic fractions in the Messel shale
H2O hydrous pyrolysis time series. Homologous series
of terminal n-alkenes and n-alkanes were released after 1
h from the kerogen and present in a 1:2 ratio, followed
by alkene isomerization and a decrease in the alkene to
alkane ratio in the 5 and 10 h experiments (Leif and
R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208
Simoneit, unpublished data). n-Alkenes were not detected in either the 36 or 72 h runs but were likely present
at a low, steady-state concentration.
Free alkenes were also detected in the triterpenoid
hydrocarbons released from the kerogen. Hopenes were
the dominant triterpenoids released after 1 hr during the
hydrous pyrolysis of Messel shale in H2O. The mass
spectra of the hopenes indicate the unsaturated bonds
1199
all occurred in the D or E rings of the C29 and C30
hopenes, and in the alkyl side chains of the C31 and
greater hopenes. This is consistent with double bond
formation via breakage of covalent triterpenoid linkages
at this end of the pentacyclic structure that bind these
compounds to the kerogen. The hopenes were not
detected 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).
1200
R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208
Fig. 9. Isomerization and reduction as a function of time during hydrous pyrolysis at 330 C of: (a) 1-hexadecene and (b)
1,13-tetradecadiene.
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 experiments. The mass spectra of the triterpenoid hydrocarbons released from the kerogen during the 72 h
Messel shale D2O hydrous pyrolysis run con®rm that
extensive deuterium incorporation occurred, and the
exchange was localized in the D and E rings or the side
chains of the hopane structures (Leif and Simoneit,
unpublished data). These results are consistent with a
combination of double bond isomerization (ionic), followed by reduction of the double bond (free radical) to
produce the observed deuterium substitution patterns.
A homogeneous radical exchange process would produce uniform deuterium incorporation in all rings of the
pentacyclic structure, which would be distinguishable by
the mass spectra.
Hydration of ole®ns to form alcohols has been
observed during water-ole®n reactions conducted at
temperatures 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 hydration
of the pyrolytically derived ole®ns forming alcohols also
occurred readily as the major reaction pathway to oxygenated 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 hydration of terminal ole®ns to form alkan-2-ols, which follows the Markovnikov rule for an ionic mechanism.
Competing with these ionic reactions of the ole®ns are
the rapid free radical hydrogenation reactions that proceed readily towards generation of saturated hydrocarbons. This was observed in pyrolysis reaction
conditions regardless of the presence of water (Burnham
et al., 1997) and demonstrated in the redox-buered
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 reactions 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 conversion from an alcohol to a ketone provides some
reducing power, in the form of a hydrogen transfer,
which may in turn reduce other unsaturated hydrocarbons. Hydrogenation by molecular hydrogen is
probably not a major pathway under these reaction
conditions. The exact mechanism of how the hydrogen
transfers occur during the oxidation of alcohols is
unknown but the reaction most likely proceeds by
mineral catalysis or by a free radical pathway through a
favorable hydrogen shuttle molecule such as H2S or
sul®des. This mechanism is only speculation but these
results demonstrate that ole®ns and alcohols are intermediates, and a portion of the alcohols is oxidized to
ketones, providing further reducing potential for ole®n
hydrogenation. The identi®ed reactions provide a pathway whereby water can react with the aliphatic portion
of the organic matter to result in hydrogen exchange
and possibly also result in a net transfer of waterderived hydrogen into this pool of organic matter. The
relative rates of the reactions depend on the experimental conditions because some of the components in
the shale can make the H-transfer reactions more facile.
R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208
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 molecular probes and D2O media. Thermal destruction of the
aliphatic kerogen network should also produce double
bonds in the kerogen which can undergo isomerization
reactions to result in deuterium incorporation into the
kerogen network. Double bond isomerization can be
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) 1hexadecene spiked on Messel shale, and (d) eicosane spiked on
Messel shale.
accelerated by acidic mineral sites (i.e. clays) and even
acidic sites in kerogen (Schimmelmann et al., 1999).
In addition to the ionic exchange pathway, direct Dincorporation by a radical pathway also occurs, which is
greatly accelerated by the presence of sulfur and H2S. This
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 petroleum 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/D2O pyrolyses,
we propose a combination of ionic and radical exchange
pathways. This is similar to the pathway for the nC32H66 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 compounds 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 branched and isoprenoid alkenes, terminal n-alkenes, and
even a,o-alkadienes from kerogen is consistent with the
®ndings from the structure elucidations of kerogens 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; DjuricÏic et al., 1971; Simoneit and Burlingame, 1973; VitorovicÂ, 1980). Because branching points
are susceptible to oxidation, monocarboxylic acids and
isoprenoid acids are formed from alkyl groups and isoprenoid 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 fragments, but release primarily n-alkanes and terminal nalkenes. 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 proceeds 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
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
Fig. 14. Histograms showing the extent of deuterium incorporation after hydrous pyrolysis of molecular probes with elemental sulfur in D2O for 10 h at 330 C: (a) 1,13-tetradecadiene,
(b) 1-hexadecene, and (c) n-eicosane.
water-derived hydrogen into organic matter (as opposed
to mere exchange) is likely to be relatively small. Most
of the organic oxidation-reduction reactions occur
among the pools of organic carbon, although hydrogen
exchange between water and the organic pools can be
quite extensive, as demonstrated here and by others (i.e.
Hoering, 1984; Schimmelmann et al., 1999). For oxidation-reduction reactions discussed here, the net hydrogen transfer rates among hydrocarbons are favored
relative to the net hydrogen (and oxygen) transfer rates
between water and organic matter, and therefore the
organic redox reactions will dominate. The consequence
of this is that one hydrocarbon pool is reduced (i.e. alkene
hydrogenation) at the expense of another hydrocarbon
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 oxidized 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 Guaymas 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 conditions 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 secondary 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 molecular 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
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 this
medium not only acts as a solvent but also reacts with
the organic matter present. This was observed by the
extensive deuterium incorporation from the D2O medium into the ole®ns generated by free radical reactions
during the hydrous pyrolysis process. The rate for the
ionic aqueous-organic reaction of ole®n isomerization
was greatly accelerated under these reaction conditions.
In addition to isomerization, the double bonds were
hydrogenated by free radical reactions (major reaction
pathway) and oxidized to ketones (minor reaction
pathway) via hydration through alcohol intermediates.
The results from these hydrous pyrolysis reactions can
be applied directly to the Guaymas Basin hydrothermal
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
water-dominated environment and at comparable temperatures, much like the experimental conditions in this
study. Care must be exercised when simulating processes
that occur over geological time by performing experiments
at elevated temperatures and under greatly accelerated
time conditions. To do this, an overall understanding is
needed of the balance of competing ionic and radical
reactions, how the dierent reaction rates vary as a
function of temperature (Weres et al., 1988; Burnham et
al., 1997), and the degree to which the reactions are
aected by aqueous ionic species, mineral surfaces, and
the amount of sulfur-derived radical species.
5. Conclusions
The pyrolysis of Messel shale in D2O generates hydrocarbons with a large content of deuterium. The deuterium
incorporation occurs by double bond isomerization of
intermediate alkenes produced by the pyrolytic breakdown of the aliphatic kerogen network and by free
radical reactions assisted by H2S and sulfur radical species which make hydrogen transfer more facile. The
major portion of the alkenes are hydrogenated to
alkanes, but a minor portion can undergo hydration to
form alcohols which can subsequently undergo oxidation to alkanones. The major observations are:
1. Hydrocarbon cracking yields n-alkanes and terminal n-alkenes.
2. Under hydrothermal conditions, the terminal nalkenes rapidly isomerize to internal alkenes via
acid-catalyzed isomerization.
3. Hydrogen exchange occurs between water and
alkenes during the isomerization reaction.
4. Hydrogenation of the alkenes (under reducing
conditions) forms alkanes.
R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208
5. Hydration of the alkenes forms transient n-alkanols, some of which are oxidized to n-alkanones.
6. Sulfur radical species and H2S accelerate both the
ionic double bond isomerization and free radical
exchange reactions.
The reaction pathways involving double bonds, either
present in the kerogen or in transient intermediate nalkene species generated by the pyrolytic breakdown of
aliphatic kerogen material, help to explain how deuterium
incorporation occurs in the generated alkanes when
Messel shale is pyrolyzed in D2O, and demonstrate how
deuterium can become enriched at one end of a molecule. The intermediate n-alkenes rapidly isomerize and
simultaneously undergo reduction to n-alkanes and oxidation to ketones via alcohols formed by the hydration
of the alkenes.
Acknowledgements
We thank the National Aeronautics and Space
Administration (Grants NAGW-2833 and NAGW4172) and the donors of the Petroleum Research Fund
administered by the American Chemical Society for
support of this research. We also thank Dr. Arndt
Schimmelmann and Dr. Gordon Love for their excellent
and detailed reviews which greatly improved
www.elsevier.nl/locate/orggeochem
The role of alkenes produced during hydrous pyrolysis
of 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 of
deuterium (and protium) exchange and the role of water during hydrous pyrolysis, we conducted a series of experiments 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 cracking of the aliphatic kerogen
structure. The terminal n-alkenes rapidly isomerize to internal alkenes via acid-catalyzed isomerization under hydrothermal 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
Hoering (1984) described interesting results concerning
the role of water during laboratory hydrous pyrolysis. He
found that a large amount of deuterium was incorporated
into the n-alkanes generated from hydrous pyrolysis of
Messel shale kerogen in D2O. Messel shale was selected
for the experiments due to its low thermal history, its
* Corresponding author. Tel.: +1-541-737-2155; fax: +1541-737-2064.
E-mail address: [email protected] (B.R.T. Simoneit).
1
Present address: Lawrence Livermore National Laboratory, Livermore, CA 94551, USA.
high organic carbon content, and its having been used in
numerous studies. The shale was powdered and extracted prior to heating. For each experiment the shale was
combined with water or heavy water, sealed under
nitrogen in a stainless steel reaction vessel and heated at
330 C for 72 h. The n-alkanes from the D2O pyrolysis
were isolated and analysed by mass spectrometry to
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 abundances of 4±6 deuterium atoms. There was no trend in
substitution pattern as a function of chain length.
To explain the deuterium substitution patterns in the
pyrolysis experiments, a free radical chain mechanism
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
by the free radical migration of the ole®n sites. Similar
radical reactions have been proposed by others
(Monthioux et al., 1985; Comet et al., 1986), but Ross
(1992a,b) has shown that direct hydrogen transfer from
water to organic free radicals is endothermic by 25±30
kcal/mol and, therefore, should not be signi®cant at
hydrous pyrolysis conditions. A re-examination of the
Hoering (1984) deuterium isomer pro®le data by
numerical modelling was performed by Ross (1992a).
He concluded that a more likely explanation for the
deuterium isomer distribution in the n-alkanes generated
in the D2O Messel shale pyrolysis is by simultaneous
deuterium exchange at more than one site. He further
suggested a combination of ionic and radical chemistry
to explain the results (Ross, 1992a), although the details
of the actual chemical mechanisms that result in the
observed preferential deuterium substitution at one end
of the isoprenoid and biomarker molecules could still
not be explained. Lewan (1997) has suggested that
under hydrous pyrolysis conditions water molecules can
react directly with organic free radicals generated by the
thermal breakdown of organic matter.
A re-evaluation of the research in pyrolysis and high
temperature aqueous chemistry of hydrocarbons provides 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 importance of both radical and ionic reaction mechanisms
during the pyrolysis of organic matter. This paper
duplicates the original Hoering (1984) Messel shale pyrolysis experiment and presents results from additional
hydrous pyrolysis experiments which provide evidence
for the chemical pathways by which hydrogen exchange
occurs between water and aliphatic hydrocarbons during
hydrous pyrolysis. Molecular probes were used with the
shale 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 the
quarry at Darmstadt, Germany (Matthes, 1966; van den
Berg et al., 1977; van de Meent et al., 1980). Hydrous
pyrolysis experiments were performed using ultrapure H2O
from 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 was
obtained from Cambridge Isotopes Laboratories. Aliphatic
compounds used in pyrolysis experiments were n-tetradeca-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 ndotriacontane (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 prior
to the pyrolysis studies.
2.2. Hydrous pyrolysis experiments
The pyrolysis experiments were performed in passivated Sno-Trik1 T316 stainless steel high pressure pipes
sealed with end caps with a total volume of 2.0 cm3 (Leif
and Simoneit, 1995a). Deoxygenated H2O or D2O was
prepared by bubbling with argon for 45 min. The reaction vessels were loaded with reactant mixtures, sealed
in a glove bag under an argon atmosphere, and placed
in a preheated air circulating oven set at the reaction
temperature and controlled to within 2 C. Durations
of 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 shale
were carried out at 330 C with 0.4 g dried shale powder
and 0.8 ml of D2O. Messel shale pyrolyses with molecular probes were conducted with 0.4 g dried shale
powder, 8 mg each of n-tetradeca-1,13-diene, n-hexadec1-ene, and n-eicosane directly spiked on the shale, and
0.8 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 alkaline
conditions was also carried out at 350 C with 10 mg of
the n-alkane and 0.8 ml D2O where the pH of the D2O
was adjusted to 11.3 (at 25 C) using NaOD.
These hydrous pyrolysis experiments with pre-extracted,
powdered rock and added model compounds in aqueous
solution (330 or 350 C) may not be directly comparable
with 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 in
contact with an aqueous phase, but with an organic
phase that has dissolved water in it. However, after the
oil is expelled from the rock chips it can proceed to react
in an aqueous environment similar to what is occurring
in these experiments, and similar to the reactions
occurring during aquathermolysis experiments (Siskin et
al., 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 were
extracted with two 1 ml portions of methanol followed
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
( C)
Duration
(h)
Liquid medium
Reactants
330
350
350
330
330
330
330
330
330
330
330
330
330
330
330
330
330
330
330
72
72
72
1
5
10
36
72
1
5
10
36
72
1
5
10
36
72
10
D2O
D2O
D2O
D2O
D2O
D2O
D2O
D2O
H2O
H2O
H2O
H2O
H2O
D2O
D2O
D2O
D2O
D2O
D2O
Messel shale (0.4 g)
n-C32H66 (10 mg)
n-C32H66 (10 mg)
1,13-C14:2, 1-C16:1, and C20 (8 mg each)
1,13-C14:2, 1-C16:1, and C20 (8 mg each)
1,13-C14:2, 1-C16:1, and C20 (8 mg each)
1,13-C14:2, 1-C16:1, and C20 (8 mg each)
1,13-C14:2, 1-C16:1, and C20 (8 mg each)
Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)
Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)
Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)
Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)
Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)
Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)
Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)
Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)
Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)
Messel shale (0.4 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)
Elemental sulfur (0.5 g) and 1,13-C14:2, 1-C16:1, and C20 (8 mg each)
(0.8 ml)
(0.8 ml, pH=7.0 at 25 C)
(0.8 ml, pH=11.3 at 25 C)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
(0.8 ml)
and water from each pyrolysis experiment were combined in a centrifuge tube and the organic fraction
separated and collected. The water was extracted with
two additional portions of methylene chloride and the
methylene chloride fractions were combined. Methylene
chloride was dried with anhydrous sodium sulfate. The
methylene chloride extracts from the Messel shale experiments were passed through an activated copper column
to remove elemental sulfur. The solvent was removed to
near dryness by nitrogen blowdown. The total extract was
made up to 2 ml of methylene chloride and deasphalted in
100 ml of heptane. The asphaltenes were allowed to precipitate overnight and separated from the maltenes by
vacuum ®ltration through a BuÈchner funnel with fritted
disk (porosity : 4±5.5 mm) and washed with heptane. The
deasphalted fractions were concentrated to 2 ml using a
rotary evaporator with water bath set at 30 C and fractionated by column chromatography (30 1 cm) packed
with 3.8 g alumina (fully active) over 3.8 g silica gel (fully
active). The samples were separated into three fractions by
elution with 50 ml heptane (nonpolar fraction, F1), 50 ml
toluene (aromatic fraction, F2) and 25 ml methanol
(polar fraction, F3). Separation of the alkenes from the
alkanes was carried out by argentation silica column
chromatography. The normal alkanes of the Messel shale±
D2O pyrolysis were isolated from the nonpolar fraction
by urea adduction, an additional procedure which was
necessary to get a more reliable determination of the
deuterium incorporation of the n-alkanes. Hydrogenation
of selected samples was achieved by bubbling H2 gas
into 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 molecules
using relative response factors.
2.4. Gas chromatography
Gas chromatography (GC) of the pyrolysates was
performed with a Hewlett-Packard 5890A instrument
equipped with a 30 m x 0.25 mm i.d. DB-5 capillary
column (0.25 mm ®lm thickness). The GC oven was
heated using the following program : isothermal for 2
min at 65 C, 3 C/min to 310 C and isothermal for 30
min, with the injector at 290 C, detector at 325 C, and
helium as the carrier gas. The alcohols in the polar
fractions were converted to the trimethylsilyl derivatives
with BSTFA prior to analysis.
2.5. Gas chromatography±mass spectrometry
Gas chromatography±mass spectrometry (GC±MS)
was performed on a Finnigan 9610 gas chromatograph
equipped with a 30 m0.25 mm i.d. DB-5 capillary column (0.25 mm ®lm thickness) coupled to a Finnigan
4021 quadrupole mass spectrometer operated at 70 eV
over the mass range 50±650 dalton and a cycle time of
2.0 s. The GC oven temperature was programmed as
described above, with the injector at 290 C 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 m0.25 mm i.d. DB-1
capillary column (0.25 mm ®lm thickness). The GC oven
temperature was programmed at isothermal for 2 min at
100 C, 5 C/min to 260 C, 10 C/min to 300 C, and isothermal 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.
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, with
the objective of duplicating the results of Hoering
(1984), who reported extensive deuterium incorporation
in the saturated hydrocarbons generated from the kerogen under these conditions. Fig. 1a is a bar graph plotted from the original data of Hoering (1984) showing
the distribution of deuterium substitution in the normal
alkanes generated under these conditions. The graph
was derived by calculating the weighted average of the
distribution patterns for the n-C17 to n-C29 alkanes
using the weighting factor of the abundances of the
individual n-alkanes. A similar bar graph of the weighted
average deuterium distribution over the same n-alkane
range was made from the data of this study and shown
in Fig. 1b. A comparison of these results indicates that
there are subtle dierences between the two distributions. The pattern from this study has a smaller amount
of generated n-alkanes in the D0 to D2 substitution
range. 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 in
agreement with those of Hoering (1984) showing extensive
deuterium incorporation in the n-alkanes generated
from the thermal breakdown of Messel shale kerogen,
with some n-alkanes having incorporated up to 20 deuterium atoms.
Fig. 1. Average distribution of deuterium substitution in nalkanes from C17 to C29 generated from: (a) the D2O pyrolysis
of Messel shale (after Hoering, 1984), and (b) the D2O pyrolysis of Messel shale (this study).
3.2. Hydrous pyrolysis of n-C32H66 in D2O (pH=7)
In order to better understand the factors aecting the
aqueous high temperature organic chemistry of heavy nparans, pyrolysis of n-C32H66 with water only or water
with inorganic additives has been studied (Leif et al.,
1992; Leif, 1993). It was demonstrated that extensive
hydrocarbon cracking, with varying degrees of alkene
formation in the cracking products, occurred at
350 C for 72 h, with the aliphatic fraction consisting
of n-alkanes and n-alkenes. The composition of the
products was modi®ed by pH and reactive species such
as elemental sulfur and iron sul®des.
Two hydrous pyrolysis experiments with n-C32H66
were repeated in D2O to aid in elucidating the pathways
by which water chemically reacts with hydrocarbons
under hydrous pyrolysis conditions. The aliphatic fraction from the D2O pyrolysis of n-C32H66 for 72 h at
350 C is shown in Fig. 2. The top ®gure is the gas
chromatogram after the experiment showing the
unreacted n-C32 H66 (o scale) and the products from
hydrocarbon cracking. These products were found to be
primarily n-alkanes and n-alkenes. The large number of
n-alkene isomers and broad, poorly de®ned peak shapes
in the alkene fraction are evidence that acid-catalyzed
double bond isomerization, with some deuterium incorporation 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
peaks. 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 that
no deuterium incorporation occurred in the alkane but
extensive deuterium incorporation occurred in the ole®n. The deuterium incorporation occurred during acidcatalyzed 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 addition
of NaOD (pH=11.3 at 25 C). Fig. 4 shows the gas
chromatogram 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 is
only a doublet at each carbon number, i.e. an n-alkane
and a terminal ole®n. The alkaline system inhibited
double bond migration to give a product distribution
consisting of n-alkanes and terminal n-alkenes. This is a
product distribution expected from the Rice±Kossiakov
reaction sequence (Kossiakov and Rice, 1943) for the
free radical cracking of n-C32H66. Fig. 5 shows the mass
spectrum of n-C17H36 of the alkane fraction and the
mass spectrum of n-C17H36-iDi from the hydrogenated
alkene fraction. These results indicate that no deuteration occurred under these conditions, neither in the
alkane fraction nor in the alkene fraction. Because
alkaline conditions should inhibit the acid-catalyzed
reactions but not aect the free radical exchange reactions, the above experiments (model compounds and
water at 350 C in the absence of sediment) demonstrate
that no detectable direct deuterium exchange occurs
between D2O and organic aliphatic hydrogen via a
radical pathway, whereas some exchange between ole®nic hydrogen and D2O is attributable to an acidcatalyzed, ionic pathway. These two experiments
demonstrate that the mechanistically simple direct
reactions between alkyl free radical sites and water, as
proposed by Lewan (1997), do not occur to any measurable extent under hydrous pyrolysis conditions and
the exchange must be occurring through alternative
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
major reaction pathways for the hydrous pyrolysis
of n-alkanes. The products from these pyrolysis experiments are the result of primary cracking of n-C32H66 to
form n-alkanes and terminal n-alkenes, followed by secondary acid-catalyzed reactions of these terminal nalkenes to form a suite of internal n-alkenes. The only
pathway for the deuterium exchange between water and
hydrocarbons under these conditions is by an ionic
rather than a free radical mechanism. The extent of
double bond isomerization in the water system indicates
that 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 products resulting from free radical b-scission reactions and
therefore molecular probes representing these classes of
compounds have been selected for this study. These
probes were reacted under hydrous pyrolysis conditions
and the relative reactivities of these compounds were
measured. A pyrolysis time series in D2O at 330 C was
conducted to measure the relative rates of deuterium
incorporation for an alkadiene, an alkene and an
alkane. The patterns of deuterium incorporation for the
three hydrocarbons for each experiment are shown in
Fig. 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 deuterium incorporation was observed in the ole®ns and no
incorporation in the alkane. This is expected considering
the 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 involving Messel shale. The ®rst series in H2O was conducted
to measure the relative rates of alkene isomerization
versus hydrogenation for 1,13-tetradecadiene and 1hexadecene when pyrolyzed in the presence of Messel
shale. The data are shown in Table 2. The gas chromatograms for the aliphatic fractions are shown in Fig. 8
and demonstrate that the rate of acid-catalyzed alkene
isomerization is much faster than the rate of hydrogenation. This is shown in Fig. 9, where percentage isomerization and percentage reduction are plotted as a
function of time.
3.6. Hydrous pyrolysis of Messel shale/molecular probes
in D2O
A series of pyrolyses was conducted in D2O to measure the relative rates of deuterium incorporation for
R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208
1,13-tetradecadiene, 1-hexadecene and eicosane when
pyrolyzed in the presence of Messel shale at 330 C. The
amounts of individually spiked compounds were far in
excess of the yield of corresponding n-alkanes generated
from the Messel shale kerogen. The patterns of deuterium incorporation for the three molecular probes in the
®ve experiments are shown in Fig. 10. These striking
results show extensive deuterium incorporation into the
ole®n molecules and some deuterium incorporation is
also observed in the n-alkanes. In previous experiments
without shale no deuterium incorporation was observed
in the saturated alkane (Fig. 7), but when pyrolyzed
with Messel shale 65% of the recovered eicosane had
incorporated at least 1 deuterium atom. The deuterium
incorporation in the saturated n-alkane is interpreted as
being due exclusively to a radical exchange process, but
the rate of deuterium incorporation in the saturated
hydrocarbon is much slower than in either of the ole®n
species where the exchange occurs by both the radical
and acid-catalyzed ionic pathways.
1195
When comparing the histograms of deuterium incorporation in the n-alkanes from the Messel shale kerogen
to those of the probe molecules, we see the closest match
is for the diene, with much less exchange occurring with
either the alkene or the alkane (Fig. 11). Under these
conditions, deuterium exchange in the aliphatic hydrocarbons occurs by a radical mechanism (incorporation
in the alkane) and an ionic mechanism (isomerization of
a double bond, if a double bond is present). Fig. 11
shows 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 during these hydrous pyrolysis experiments. Fig. 12 shows
the GC traces indicating that the generation of the
ketones proceeds through alcohol intermediates which
are present in the polar fractions during the early stages
of the reactions followed by ketones present in the later
stages of the experiments. The 1 and 5 h experiments
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 experiments. These ketones, oxidation products of the nalkane 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 kerogen. The elution range for the C16 alkanols and alkanones 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 oxidation of the latter with pyrolysis time.
3.7. Hydrous pyrolysis of sulfur/molecular probes in
D2 O
Fig. 6. Simpli®ed schematic model for deuterium incorporation into pyrolysis products of n-C32H66.
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
deuterium substitution patterns for the three probe
molecules are shown in Fig. 14. Although this was only
R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208
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
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
5
10
36
72
12.6
89.9
96.2
97.2
100.0
n.d.
11.6
13.6
50.5
94.0
5.7
70.0
84.8
91.9
100.0
n.d.
22.2
33.0
65.8
96.0
a
n.d.=Not detected.
a 10 h experiment, extensive deuteration occurred, even
in the saturated n-alkane. These results demonstrate the
large 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 sedimentary
organic matter near sill intrusions of the Guaymas Basin
hydrothermal system (Simoneit and Philp, 1982; Simoneit et al., 1986). These ole®ns, generated by the natural
hydrous pyrolysis occurring at Guaymas Basin, are
evidence for the pyrolytic generation of alkene intermediates during high thermal stress hydrothermal conditions (Simoneit and Philp, 1982; Simoneit et al., 1986).
Petroleum from the Guaymas basin hydrothermal system also contains aliphatic ketones which are synthesized under the hydrous pyrolysis conditions and have
been proposed to be derived via oxidation of alcohols
formed from the hydration of the hydrothermally
derived alkenes (Leif and Simoneit, 1995b). The major
chemical reactions and their relative rates leading to the
pyrolysate distributions of aliphatic material under
hydrous pyrolysis conditions have been identi®ed in the
present series of experiments. This current set of experiments con®rms that hydration of the alkenes can result
in the formation of ketones via alcohol intermediates,
and double bond isomerization of generated alkenes
provides one pathway by which hydrogen from water
can be incorporated into the aliphatic pyrolysates.
Lewan (1992) has demonstrated that during hydrous
pyrolysis water not only acts as solvent but also reacts
chemically, resulting in incorporation of water-derived
hydrogen into the organic matter, with water-derived
oxygen producing elevated amounts of carbon dioxide.
It was not clear to what degree water reacted and by
which mechanisms these processes occurred. This study
focuses on determining some of the likely reaction
mechanisms which can occur between water and organic
matter. One pathway, the quenching of free radical sites
by water as proposed by Lewan (1997) does not appear
to be a signi®cant pathway under typical hydrous pyrolysis conditions. This was demonstrated by the lack of
any measurable D-incorporation in the cracking products formed as a result of the b-scission of n-C32H66
under alkaline conditions. Alternative reaction pathways
between water and hydrocarbons have been identi®ed
and are the following: ionic double-bond isomerization
of transient alkene species, alcohol formation by alkene
hydration followed by oxidation to a ketone, and radical
hydrogen atom exchange reactions via species that act
as free radical hydrogen shuttles (i.e. sul®des or H2S).
During hydrous pyrolysis the initial products generated
by the carbon±carbon bond breaking of the aliphatic
components are n-alkanes and terminal n-alkenes. This
is consistent with the report by Seewald et al. (1998)
where alkenes were identi®ed as reactive intermediates
during the hydrous pyrolysis of shales. This breakdown
of the aliphatic hydrocarbon network occurs through a
pathway of radical b-scission reactions and is the wellknown Rice±Kossiakov mechanism. These thermal
cracking reactions of aliphatic hydrocarbons have been
discussed by other researchers (Ford, 1986; Jackson et
al., 1995; Burnham et al., 1997), and n-alkanes and
terminal n-alkenes are the same products that are generated 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 can
undergo secondary acid-catalyzed double bond isomerization 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-deuterium
exchange process of ole®ns under high temperature- dilute
acid conditions used to generate deuterium labelled compounds (Werstiuk and Timmins, 1985). n-Alkenes were
identi®ed in the aliphatic fractions in the Messel shale
H2O hydrous pyrolysis time series. Homologous series
of terminal n-alkenes and n-alkanes were released after 1
h from the kerogen and present in a 1:2 ratio, followed
by alkene isomerization and a decrease in the alkene to
alkane ratio in the 5 and 10 h experiments (Leif and
R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208
Simoneit, unpublished data). n-Alkenes were not detected in either the 36 or 72 h runs but were likely present
at a low, steady-state concentration.
Free alkenes were also detected in the triterpenoid
hydrocarbons released from the kerogen. Hopenes were
the dominant triterpenoids released after 1 hr during the
hydrous pyrolysis of Messel shale in H2O. The mass
spectra of the hopenes indicate the unsaturated bonds
1199
all occurred in the D or E rings of the C29 and C30
hopenes, and in the alkyl side chains of the C31 and
greater hopenes. This is consistent with double bond
formation via breakage of covalent triterpenoid linkages
at this end of the pentacyclic structure that bind these
compounds to the kerogen. The hopenes were not
detected 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).
1200
R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208
Fig. 9. Isomerization and reduction as a function of time during hydrous pyrolysis at 330 C of: (a) 1-hexadecene and (b)
1,13-tetradecadiene.
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 experiments. The mass spectra of the triterpenoid hydrocarbons released from the kerogen during the 72 h
Messel shale D2O hydrous pyrolysis run con®rm that
extensive deuterium incorporation occurred, and the
exchange was localized in the D and E rings or the side
chains of the hopane structures (Leif and Simoneit,
unpublished data). These results are consistent with a
combination of double bond isomerization (ionic), followed by reduction of the double bond (free radical) to
produce the observed deuterium substitution patterns.
A homogeneous radical exchange process would produce uniform deuterium incorporation in all rings of the
pentacyclic structure, which would be distinguishable by
the mass spectra.
Hydration of ole®ns to form alcohols has been
observed during water-ole®n reactions conducted at
temperatures 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 hydration
of the pyrolytically derived ole®ns forming alcohols also
occurred readily as the major reaction pathway to oxygenated 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 hydration of terminal ole®ns to form alkan-2-ols, which follows the Markovnikov rule for an ionic mechanism.
Competing with these ionic reactions of the ole®ns are
the rapid free radical hydrogenation reactions that proceed readily towards generation of saturated hydrocarbons. This was observed in pyrolysis reaction
conditions regardless of the presence of water (Burnham
et al., 1997) and demonstrated in the redox-buered
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 reactions 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 conversion from an alcohol to a ketone provides some
reducing power, in the form of a hydrogen transfer,
which may in turn reduce other unsaturated hydrocarbons. Hydrogenation by molecular hydrogen is
probably not a major pathway under these reaction
conditions. The exact mechanism of how the hydrogen
transfers occur during the oxidation of alcohols is
unknown but the reaction most likely proceeds by
mineral catalysis or by a free radical pathway through a
favorable hydrogen shuttle molecule such as H2S or
sul®des. This mechanism is only speculation but these
results demonstrate that ole®ns and alcohols are intermediates, and a portion of the alcohols is oxidized to
ketones, providing further reducing potential for ole®n
hydrogenation. The identi®ed reactions provide a pathway whereby water can react with the aliphatic portion
of the organic matter to result in hydrogen exchange
and possibly also result in a net transfer of waterderived hydrogen into this pool of organic matter. The
relative rates of the reactions depend on the experimental conditions because some of the components in
the shale can make the H-transfer reactions more facile.
R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208
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 molecular probes and D2O media. Thermal destruction of the
aliphatic kerogen network should also produce double
bonds in the kerogen which can undergo isomerization
reactions to result in deuterium incorporation into the
kerogen network. Double bond isomerization can be
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) 1hexadecene spiked on Messel shale, and (d) eicosane spiked on
Messel shale.
accelerated by acidic mineral sites (i.e. clays) and even
acidic sites in kerogen (Schimmelmann et al., 1999).
In addition to the ionic exchange pathway, direct Dincorporation by a radical pathway also occurs, which is
greatly accelerated by the presence of sulfur and H2S. This
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 petroleum 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/D2O pyrolyses,
we propose a combination of ionic and radical exchange
pathways. This is similar to the pathway for the nC32H66 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 compounds 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 branched and isoprenoid alkenes, terminal n-alkenes, and
even a,o-alkadienes from kerogen is consistent with the
®ndings from the structure elucidations of kerogens 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; DjuricÏic et al., 1971; Simoneit and Burlingame, 1973; VitorovicÂ, 1980). Because branching points
are susceptible to oxidation, monocarboxylic acids and
isoprenoid acids are formed from alkyl groups and isoprenoid 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 fragments, but release primarily n-alkanes and terminal nalkenes. 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 proceeds 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
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
Fig. 14. Histograms showing the extent of deuterium incorporation after hydrous pyrolysis of molecular probes with elemental sulfur in D2O for 10 h at 330 C: (a) 1,13-tetradecadiene,
(b) 1-hexadecene, and (c) n-eicosane.
water-derived hydrogen into organic matter (as opposed
to mere exchange) is likely to be relatively small. Most
of the organic oxidation-reduction reactions occur
among the pools of organic carbon, although hydrogen
exchange between water and the organic pools can be
quite extensive, as demonstrated here and by others (i.e.
Hoering, 1984; Schimmelmann et al., 1999). For oxidation-reduction reactions discussed here, the net hydrogen transfer rates among hydrocarbons are favored
relative to the net hydrogen (and oxygen) transfer rates
between water and organic matter, and therefore the
organic redox reactions will dominate. The consequence
of this is that one hydrocarbon pool is reduced (i.e. alkene
hydrogenation) at the expense of another hydrocarbon
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 oxidized 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 Guaymas 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 conditions 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 secondary 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 molecular 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
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 this
medium not only acts as a solvent but also reacts with
the organic matter present. This was observed by the
extensive deuterium incorporation from the D2O medium into the ole®ns generated by free radical reactions
during the hydrous pyrolysis process. The rate for the
ionic aqueous-organic reaction of ole®n isomerization
was greatly accelerated under these reaction conditions.
In addition to isomerization, the double bonds were
hydrogenated by free radical reactions (major reaction
pathway) and oxidized to ketones (minor reaction
pathway) via hydration through alcohol intermediates.
The results from these hydrous pyrolysis reactions can
be applied directly to the Guaymas Basin hydrothermal
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
water-dominated environment and at comparable temperatures, much like the experimental conditions in this
study. Care must be exercised when simulating processes
that occur over geological time by performing experiments
at elevated temperatures and under greatly accelerated
time conditions. To do this, an overall understanding is
needed of the balance of competing ionic and radical
reactions, how the dierent reaction rates vary as a
function of temperature (Weres et al., 1988; Burnham et
al., 1997), and the degree to which the reactions are
aected by aqueous ionic species, mineral surfaces, and
the amount of sulfur-derived radical species.
5. Conclusions
The pyrolysis of Messel shale in D2O generates hydrocarbons with a large content of deuterium. The deuterium
incorporation occurs by double bond isomerization of
intermediate alkenes produced by the pyrolytic breakdown of the aliphatic kerogen network and by free
radical reactions assisted by H2S and sulfur radical species which make hydrogen transfer more facile. The
major portion of the alkenes are hydrogenated to
alkanes, but a minor portion can undergo hydration to
form alcohols which can subsequently undergo oxidation to alkanones. The major observations are:
1. Hydrocarbon cracking yields n-alkanes and terminal n-alkenes.
2. Under hydrothermal conditions, the terminal nalkenes rapidly isomerize to internal alkenes via
acid-catalyzed isomerization.
3. Hydrogen exchange occurs between water and
alkenes during the isomerization reaction.
4. Hydrogenation of the alkenes (under reducing
conditions) forms alkanes.
R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208
5. Hydration of the alkenes forms transient n-alkanols, some of which are oxidized to n-alkanones.
6. Sulfur radical species and H2S accelerate both the
ionic double bond isomerization and free radical
exchange reactions.
The reaction pathways involving double bonds, either
present in the kerogen or in transient intermediate nalkene species generated by the pyrolytic breakdown of
aliphatic kerogen material, help to explain how deuterium
incorporation occurs in the generated alkanes when
Messel shale is pyrolyzed in D2O, and demonstrate how
deuterium can become enriched at one end of a molecule. The intermediate n-alkenes rapidly isomerize and
simultaneously undergo reduction to n-alkanes and oxidation to ketones via alcohols formed by the hydration
of the alkenes.
Acknowledgements
We thank the National Aeronautics and Space
Administration (Grants NAGW-2833 and NAGW4172) and the donors of the Petroleum Research Fund
administered by the American Chemical Society for
support of this research. We also thank Dr. Arndt
Schimmelmann and Dr. Gordon Love for their excellent
and detailed reviews which greatly improved