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 experi-ments using aliphatic compounds (1,13-tetradecadiene, 1-hexadecene, eicosane and dotriacontane) as probe molecules. These compounds were pyrolyzed in D2O, shale/D2O, and shale/H2O and the products analyzed by GC±MS. In the
absence of powdered shale, the incorporation of deuterium from D2O occurred only in ole®nic compounds via double
bond isomerization. The presence of shale accelerated deuterium incorporation into the ole®ns and resulted in a minor amount of deuterium incorporation in the saturated n-alkanes. The pattern of deuterium substitution of the diene closely matched the deuterium distribution observed in then-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 saturatedn-alkanes with concomitant oxidation of ole®ns to ketones. These results show that under hydrous pyrolysis conditions, kerogen breakdown generatesn-alkanes and terminaln-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 hydro-thermal conditions, a signi®cant pathway of deuterium (and protium) exchange between water and the hydrocarbons. Thesen-alkenes simultaneously undergo reduction ton-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 then-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
high organic carbon content, and its having been used in numerous studies. The shale was powdered and extrac-ted 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 330C for 72 h. Then-alkanes from the D
2O 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 eachn-alkane, with the highest relative abun-dances 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. P I I : S 0 1 4 6 - 6 3 8 0 ( 0 0 ) 0 0 1 1 3 - 3
Organic Geochemistry 31 (2000) 1189±1208
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* Corresponding author. Tel.: 541-737-2155; fax: +1-541-737-2064.
E-mail address:[email protected] (B.R.T. Simoneit).
1 Present address: Lawrence Livermore National
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 then-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 pro-vides some insight into the major reactions that alkanes and alkenes undergo (Wilson et al., 1986; Weres et al., 1988; Kissin, 1987, 1990; Siskin et al., 1990; Leif et al., 1992; Stalker et al., 1994, 1998; Seewald, 1994, 1996; Jackson et al., 1995; Burnham et al., 1997; Lewan, 1997; Seewald et al., 1998). These studies point to the impor-tance of both radical and ionic reaction mechanisms during the pyrolysis of organic matter. This paper duplicates the original Hoering (1984) Messel shale pyr-olysis 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 ton-alkane andn-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 weren -tetra-deca-1,13-diene (Aldrich Chemical Co., purity >97%),
n-hexadec-1-ene (Aldrich Chemical Co., purity >97%),
n-eicosane (Aldrich Chemical Co., purity 99%), andn -dotriacontane (Aldrich Chemical Co., purity >97%). The Messel shale used in the experiments was powdered, exhaustively extracted in a Soxhlet apparatus with methanol/methylene chloride for 72 h, and dried prior to the pyrolysis studies.
2.2. Hydrous pyrolysis experiments
The pyrolysis experiments were performed in passi-vated Sno-Trik1T316 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 reac-tion 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 within2C. 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 330C with 0.4 g dried shale powder and 0.8 ml of D2O. Messel shale pyrolyses with
mole-cular probes were conducted with 0.4 g dried shale powder, 8 mg each ofn-tetradeca-1,13-diene,n -hexadec-1-ene, andn-eicosane directly spiked on the shale, and 0.8 ml of either H2O or D2O. Heavy water pyrolyses of
n-C32H66were done at 350C with 10 mg of then-alkane
and 0.8 ml D2O. Pyrolysis of n-C32H66 under alkaline
conditions was also carried out at 350C with 10 mg of then-alkane and 0.8 ml D2O where the pH of the D2O
was adjusted to 11.3 (at 25C) using NaOD.
These hydrous pyrolysis experiments with pre-extracted, powdered rock and added model compounds in aqueous solution (330 or 350C) 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
and water from each pyrolysis experiment were com-bined 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 experi-ments 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 pre-cipitate overnight and separated from the maltenes by vacuum ®ltration through a BuÈchner funnel with fritted disk (porosity : 4±5.5mm) and washed with heptane. The deasphalted fractions were concentrated to 2 ml using a rotary evaporator with water bath set at 30C and frac-tionated by column chromatography (301 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 then-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 65C, 3C/min to 310C and isothermal for 30 min, with the injector at 290C, detector at 325C, 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 col-umn (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 290C and helium
Table 1
Hydrous pyrolysis experiments performed in 2.0 cm3316 stainless steel reactors
Temperature Duration Liquid medium Reactants
(C) (h)
as the carrier gas. The MS data were processed with an on-line Finnigan-Incos 2300 computer data system. The positional isomers of then-alkanones and then-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 ofm/z196±220 forn-C14H30,
m/z224±242 forn-C16H34andm/z280±292 forn-C20H42.
The GC was equipped with a 30 m0.25 mm i.d. DB-1 capillary column (0.25mm ®lm thickness). The GC oven temperature was programmed at isothermal for 2 min at 100C, 5C/min to 260C, 10C/min to 300C, and iso-thermal for 10 min, with an on-column injector, and helium as the carrier gas. The MS data were processed with Hewlett-Packard Chemstation software. The mass intensity data from the GC-MS analyses were corrected for naturally occurring13C by the method of Biemann
(1962) and Yeh and Epstein (1981) to obtain the extent of deuterium incorporation in then-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 330C, with
the objective of duplicating the results of Hoering (1984), who reported extensive deuterium incorporation in the saturated hydrocarbons generated from the kero-gen under these conditions. Fig. 1a is a bar graph plot-ted from the original data of Hoering (1984) showing the distribution of deuterium substitution in the 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 individualn-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 distribu-tions. 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 somen-alkanes having incorporated up to 20 deu-terium atoms.
3.2. Hydrous pyrolysis ofn-C32H66in D2O (pH=7)
In order to better understand the factors aecting the aqueous high temperature organic chemistry of heavyn -parans, pyrolysis ofn-C32H66with 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 350C 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 frac-tion from the D2O pyrolysis of n-C32H66 for 72 h at
350C is shown in Fig. 2. The top ®gure is the gas chromatogram after the experiment showing the unreacted n-C32H66 (o scale) and the products from
hydrocarbon cracking. These products were found to be primarilyn-alkanes andn-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 incor-poration had occurred. Hydrogenation of the alkene
Fig. 1. Average distribution of deuterium substitution in n -alkanes from C17to C29generated from: (a) the D2O pyrolysis
of Messel shale (after Hoering, 1984), and (b) the D2O
fraction collapsed the multiple ole®n peaks into single peaks. Fig. 3 shows the mass spectrum ofn-C17H36of 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 acid-catalyzed isomerization of the double bond.
3.3. Hydrous pyrolysis ofn-C32H66in D2O (pH=11.3)
The pyrolysis ofn-C32H66 in D2O was repeated, but
this time the system was made alkaline by the addition of NaOD (pH=11.3 at 25C). Fig. 4 shows the gas chromatogram of the aliphatic fraction after heating. Shown is the unreacted n-C32H66starting material (o
scale) and the cracking products, but in this case there is only a doublet at each carbon number, i.e. ann-alkane and a terminal ole®n. The alkaline system inhibited double bond migration to give a product distribution consisting ofn-alkanes and terminaln-alkenes. This is a product distribution expected from the Rice±Kossiakov
reaction sequence (Kossiakov and Rice, 1943) for the free radical cracking ofn-C32H66. Fig. 5 shows the mass
spectrum of n-C17H36 of the alkane fraction and the
mass spectrum ofn-C17H36-iDifrom the hydrogenated
alkene fraction. These results indicate that no deutera-tion occurred under these condideutera-tions, 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 reac-tions, the above experiments (model compounds and water at 350C 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
acid-catalyzed, 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 mea-surable extent under hydrous pyrolysis conditions and the exchange must be occurring through alternative reaction pathways.
Fig. 2. Gas chromatograms of the D2O±n-C32H66system: (a) total nonpolar fraction, (b) alkane fraction, (c) alkene fraction, and (d)
alkene fraction after catalytic hydrogenation. Numbers refer to carbon chain lengths ofn-alkanes. (Note, the enhanced concentration ofn-C34H70is a minor impurity in then-C32H66and the elevated C16represents products from favored midchain cleavage.)
Fig. 6 is a simpli®ed schematic showing the major reaction pathways for the hydrous pyrolysis of n-alkanes. The products from these pyrolysis experi-ments are the result of primary cracking ofn-C32H66to
formn-alkanes and terminaln-alkenes, followed by sec-ondary acid-catalyzed reactions of these terminal n -alkenes to form a suite of internaln-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 pro-ducts resulting from free radicalb-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 330C 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 deu-terium 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 invol-ving Messel shale. The ®rst series in H2O was conducted
to measure the relative rates of alkene isomerization versus hydrogenation for 1,13-tetradecadiene and 1-hexadecene when pyrolyzed in the presence of Messel shale. The data are shown in Table 2. The gas chroma-tograms for the aliphatic fractions are shown in Fig. 8 and demonstrate that the rate of acid-catalyzed alkene isomerization is much faster than the rate of hydro-genation. This is shown in Fig. 9, where percentage iso-merization and percentage reduction are plotted as a function of time.
3.6. Hydrous pyrolysis of Messel shale/molecular probes in D2O
A series of pyrolyses was conducted in D2O to
mea-sure the relative rates of deuterium incorporation for
1,13-tetradecadiene, 1-hexadecene and eicosane when pyrolyzed in the presence of Messel shale at 330C. The amounts of individually spiked compounds were far in excess of the yield of correspondingn-alkanes generated from the Messel shale kerogen. The patterns of deuter-ium incorporation for the three molecular probes in the ®ve experiments are shown in Fig. 10. These striking results show extensive deuterium incorporation into the ole®n molecules and some deuterium incorporation is also observed in then-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 saturatedn-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.
When comparing the histograms of deuterium incor-poration in then-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 hydro-carbons 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 dur-ing 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 C14and C16alkanols 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 ofn-alkanes.
ole®n precursors, the 10 h experiment has a mixture of alkanols and alkanones and the 36 and 72 h experiments show a dominance of alkanones. Of interest is the appearance of C20 ketones in the 36 and 72 h
experi-ments. These ketones, oxidation products of the n -alkane probe molecule, formed as a result of a free radical oxidation pathway. The alkanones ranging from C10 to C33+ (Fig. 12d and e) are derived from the
hydrous pyrolysis breakdown of the Messel shale kero-gen. The elution range for the C16alkanols and
alka-nones is shown expanded in Fig. 13 for the 5 and 72 h experiments. The mass spectra of the alkan-i-ols (i=1± 6) are shown with their characteristic fragmentation patterns in Fig. 13b±e. The dominance of the secondary hexadecan-2-ol over the primary hexadecan-1-ol ®ts with the well known acid catalyzed hydration reaction of alkenes to alcohols. The same isomer distribution is observed for the alkanones (i.e. hexadecan-2-one >> hexadecanal) as for the alkanols, con®rming the oxida-tion of the latter with pyrolysis time.
3.7. Hydrous pyrolysis of sulfur/molecular probes in D2O
One 10 h experiment was performed where the three aliphatic probe molecules were combined with 0.50 g elemental sulfur and D2O and reacted at 330C. The
deuterium substitution patterns for the three probe molecules are shown in Fig. 14. Although this was only
Fig. 5. Mass spectra ofn-C17H36from the D2O±n-C32H66±NaOD system: (a) alkane fraction, and (b) hydrogenated alkene fraction.
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 330C.
R.N.
Leif,
B.R.T.
Simoneit
/
Organic
Geochemist
ry
31
(2000)
1189±12
08
a 10 h experiment, extensive deuteration occurred, even in the saturatedn-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; Simo-neit et al., 1986). These ole®ns, generated by the natural hydrous pyrolysis occurring at Guaymas Basin, are evidence for the pyrolytic generation of alkene inter-mediates during high thermal stress hydrothermal con-ditions (Simoneit and Philp, 1982; Simoneit et al., 1986). Petroleum from the Guaymas basin hydrothermal sys-tem also contains aliphatic ketones which are synthe-sized under the hydrous pyrolysis conditions and 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 experi-ments 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 pyr-olysis conditions. This was demonstrated by the lack of any measurable D-incorporation in the cracking pro-ducts formed as a result of the b-scission of n-C32H66
under alkaline conditions. Alternative reaction pathways between water and hydrocarbons have been identi®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 aren-alkanes and terminaln-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 radicalb-scission reactions and is the well-known 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 terminaln-alkenes are the same products that are gen-erated during Curie-point pyrolyses of hydrocarbons and aliphatic-rich materials (van de Meent et al., 1980; Tegelaar et al., 1989a,b). The terminal n-alkenes can undergo secondary acid-catalyzed double bond isomer-ization under hydrothermal conditions (Weres et al., 1988; Siskin et al., 1990) which results in incorporation (exchange) of hydrogen from water into the hydrocarbon skeleton, similar to the acid-catalyzed protium-deuterium exchange process of ole®ns under high temperature- dilute acid conditions used to generate deuterium labelled com-pounds (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 terminaln-alkenes andn-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
Table 2
Data from the pyrolysis of 1,13-tetradecadiene and 1-hexadecene molecular probes with Messel shale in H2O at 330Ca
1,13-Tetradecadiene 1-Hexadecene
Pyrolysis time (h) % Isomerized % Hydrogenated % Isomerized % Hydrogenated
1 12.6 n.d. 5.7 n.d.
5 89.9 11.6 70.0 22.2
10 96.2 13.6 84.8 33.0
36 97.2 50.5 91.9 65.8
72 100.0 94.0 100.0 96.0
Simoneit, unpublished data).n-Alkenes were not detec-ted 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
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 330C: (a) 1 h, (b) 5 h, (c) 10 h, (d) 36 h, and (e) 72 h. I.S.=internal standard (n-C24D50).
one characteristic of the early stages of oil generation. Preferential deuterium enrichment at one end of the biomarkers, as observed by others (Hoering, 1984; Stalker et al., 1998), was also observed in these experi-ments. The mass spectra of the triterpenoid hydro-carbons released from the kerogen during the 72 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), fol-lowed by reduction of the double bond (free radical) to produce the observed deuterium substitution patterns. A homogeneous radical exchange process would pro-duce 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 250C (An et al., 1997). Although conversion was low, the ole®n hydration occurred readily and equilibrium was rapidly established. During hydrous pyrolysis of Messel shale, the hydration of the pyrolytically derived ole®ns forming alcohols also occurred readily as the major reaction pathway to oxy-genated products during brief contact times (1±5 h, Fig. 12). As observed by An et al. (1997) the addition of water to ole®ns is regioselective as shown by the hydra-tion of terminal ole®ns to form alkan-2-ols, which fol-lows the Markovnikov rule for an ionic mechanism. Competing with these ionic reactions of the ole®ns are the rapid free radical hydrogenation reactions that pro-ceed readily towards generation of saturated hydro-carbons. This was observed in pyrolysis reaction conditions regardless of the presence of water (Burnham et al., 1997) and demonstrated in the redox-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 reac-tions occurring under hydrous pyrolysis condireac-tions, but we have documented that the generated ole®ns react with water. The hydrogen from water ends up in a reduced hydrocarbon fraction (n-alkanes formed by the hydrogenation of the n-alkenes) and the oxygen from water ends up oxidizing a portion of the alkenes. Lewan (1992, 1997) has observed analogous reactions where increased amounts of CO2 during hydrous pyrolysis
experiments are the result of reactions between water and organic matter. The ketones observed in this study represent only partially oxidized carbon, but the con-version from an alcohol to a ketone provides some reducing power, in the form of a hydrogen transfer, which may in turn reduce other unsaturated hydro-carbons. 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 inter-mediates, and a portion of the alcohols is oxidized to ketones, providing further reducing potential for ole®n hydrogenation. The identi®ed reactions provide a path-way 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 water-derived hydrogen into this pool of organic matter. The relative rates of the reactions depend on the experi-mental conditions because some of the components in the shale can make the H-transfer reactions more facile.
Fig. 9. Isomerization and reduction as a function of time dur-ing hydrous pyrolysis at 330C of: (a) 1-hexadecene and (b)
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 330C.
R.N.
Leif,
B.R.T.
Simoneit
/
Organic
Geochemist
ry
31
(2000)
1189±12
08
This was tested by hydrous pyrolysis studies with mole-cular 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
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 D-incorporation 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 petro-leum formation (Lewan, 1998) and also they have been proposed as species responsible for H-exchange between water and organic matter (Ross, 1992a; Schimmelmann et al., 1999). Sulfur and sulfur species have even been shown to be capable of reacting stoichiometrically and also serving as oxidizing agents (Toland et al., 1958; Toland, 1960; 1961). Therefore, to explain the deuterium patterns observed with the Messel shale/D2O pyrolyses,
we propose a combination of ionic and radical exchange pathways. This is similar to the pathway for the n -C32H66 pyrolyses, but here we include exchange with
presumed ole®n groups in the shale kerogen along with radical exchange processes. Fig. 15 is a schematic showing the major reaction pathways of aliphatic com-pounds observed under hydrous pyrolysis conditions.
The results suggest that deuterium incorporation into hydrocarbons can occur during acid-catalyzed double bond isomerization of alkene intermediates by 1,2-shifts of carbocations. The formation of intermediate bran-ched and isoprenoid alkenes, terminal n-alkenes, and evena,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 Burlin-game, 1973; VitorovicÂ, 1980). Because branching points are susceptible to oxidation, monocarboxylic acids and isoprenoid acids are formed from alkyl groups and iso-prenoid groups, respectively, attached to the kerogen matrix at one point.a,o-Dicarboxylic acids are formed as a result of an alkyl ``bridge'' which is attached to the kerogen at two points. Curie-point pyrolysis suggests that a highly aliphatic polymer is present in Messel shale kerogen (Goth et al., 1988). The conditions during hydrous pyrolysis experiments may yield similar frag-ments, but release primarily n-alkanes and terminaln -alkenes. The double bonds, in the pyrolysate and the remaining aliphatic kerogen network, would then undergo acid-catalyzed double bond isomerization prior to hydrogenation of the double bonds. Hydrogen exchange between water and organic matter also pro-ceeds via sulfur-derived radical species and H2S, and
may also be catalyzed by minerals.
In the whole suite of reactions occurring under hydrous pyrolysis conditions, the net incorporation of
Fig. 11. Comparison of deuterium incorporation for pyrolysis in D2O with Messel shale at 330C for 72 h for: (a) Messel shale
Fig. 12. Gas chromatograms of the polar NSO fractions (as TMS derivatives) obtained by hydrous pyrolysis of 1,13-tetradecadiene, 1-hexadecene and eicosane with Messel shale in H2O at 330C: (a) 1 h, (b) 5 h, (c) 10 h, (d) 36 h, and (e) 72 h. I.S.=internal standard
(n-C24D50).
Fig. 13. GC±MS data for the alkanol to alkanone progression in the C16elution range in the products from 5 and 72 h hydrous
pyrolyses with alkenes and Messel shale: total ion current traces for C16region (a) 5 h experiment (hexadecanols fori=1±6) and (f) 72
h experiment (hexadecanonesi=1±5+); and mass spectra of the C
16alkanols from the 1 h experiment (b) 2-ol (2A), (c) 3-ol (3A), (d)
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 oxida-tion-reduction reactions discussed here, the net hydro-gen 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
pool, which is simultaneously oxidized (i.e. hydrocarbon aromatization). This process occurs during the natural hydrous pyrolysis of sedimentary organic matter in the Guaymas Basin hydrothermal system (e.g. Kawka and Simoneit, 1987; Simoneit, 1993), a site where hot hydrothermal ¯uids pyrolyze immature sedimentary organic matter to produce oil under reaction conditions comparable to these laboratory hydrous pyrolysis experiments. In the Guaymas Basin, a reduced and alkane rich oil fraction is produced at the expense of a more labile and hydrogen poor fraction. The result is an
n-alkane rich oil which is also highly enriched in oxi-dized organic matter, in the form of polycyclic aromatic hydrocarbons. The presence of the graphitic carburized coating on the walls of hydrous pyrolysis vessels is an example of this type of chemistry. The oils from Guay-mas Basin also contain ketones (Leif and Simoneit, 1995a,b), presumably generated by the pathway identi-®ed in this study, but the ketones are in much lower concentrations relative to the abundant, partially-oxidized polycyclic aromatic hydrocarbons.
Therefore, what is most likely a balanced and realistic view of the chemistry under hydrous pyrolysis condi-tions is a complex set of competing reaccondi-tions where extensive hydrogen exchange within the pools of organic matter and between this organic matter and water are major reactions, but a net transfer of water-derived hydrogen into the organic matter is minor and of sec-ondary importance. If it were the other way around then the petroleum industry would have exploited water as a source of hydrogen years ago, because water as a hydrogen source during upgrading would be much more economical than using expensive catalysts and high pressure mole-cular hydrogen. The use of additives or speci®c H-transfer catalysts may result in ®nding novel reaction pathways leading to processes capable of using water as a signi®cant 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
med-ium 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
Fig. 14. Histograms showing the extent of deuterium incor-poration after hydrous pyrolysis of molecular probes with ele-mental sulfur in D2O for 10 h at 330C: (a) 1,13-tetradecadiene,
(b) 1-hexadecene, and (c)n-eicosane.
matter is pyrolyzed by hot, hydrothermal ¯uids in a water-dominated environment and at comparable tem-peratures, 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
hydro-carbons with a large content of deuterium. The deuterium
incorporation occurs by double bond isomerization of intermediate alkenes produced by the pyrolytic break-down of the aliphatic kerogen network and by free radical reactions assisted by H2S and sulfur radical
spe-cies which make hydrogen transfer more facile. The major portion of the alkenes are hydrogenated to alkanes, but a minor portion can undergo hydration to form alcohols which can subsequently undergo oxida-tion to alkanones. The major observaoxida-tions are:
1. Hydrocarbon cracking yieldsn-alkanes and term-inaln-alkenes.
2. Under hydrothermal conditions, the terminal n -alkenes 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.
5. Hydration of the alkenes forms transient n -alka-nols, some of which are oxidized ton-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 n -alkene 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 mole-cule. The intermediate n-alkenes rapidly isomerize and simultaneously undergo reduction ton-alkanes and oxi-dation to ketones via alcohols formed by the hydration of the alkenes.
Acknowledgements
We thank the National Aeronautics and Space Administration (Grants 2833 and NAGW-4172) 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 this manuscript.
Associate EditorÐL. Schwark
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