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 experi- ments using aliphatic compounds (1,13-tetradecadiene, 1-hexadecene, eicosane and dotriacontane) as probe molecules. These compounds were pyrolyzed in D2O, shale/D2O, and shale/H2O and the products analyzed by GC±MS. In the absence of powdered shale, the incorporation of deuterium from D2O occurred only in ole®nic compounds via double bond isomerization. The presence of shale accelerated deuterium incorporation into the ole®ns and resulted in a minor amount of deuterium incorporation in the saturated n-alkanes. The pattern of deuterium substitution of the diene closely matched the deuterium distribution observed in the n-alkanes generated from the shale kerogen in the D2O/ shale pyrolyses. The presence of the shale also resulted in reduction (hydrogenation) of ole®ns to saturated n-alkanes with concomitant oxidation of ole®ns to ketones. These results show that under hydrous pyrolysis conditions, kerogen breakdown generates n-alkanes and terminal n-alkenes by free radical hydrocarbon 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. These n-alkenes simultaneously undergo reduction to n-alkanes (major) or oxidation to ketones (minor) via alcohols formed by the hydration of the alkenes. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Hydrous pyrolysis; Molecular probes; Messel shale; Deuterium exchange; Ole®ns; Ketones 1. Introduction high organic carbon content, and its having been used in numerous studies. The shale was powdered and extrac- Hoering (1984) described interesting results concerning ted prior to heating. For each experiment the shale was the role of water during laboratory hydrous pyrolysis. He combined with water or heavy water, sealed under found that a large amount of deuterium was incorporated nitrogen in a stainless steel reaction vessel and heated at into the n-alkanes generated from hydrous pyrolysis of 330 C for 72 h. The n-alkanes from the D2O pyrolysis Messel shale kerogen in D2O. Messel shale was selected were isolated and analysed by mass spectrometry to for the experiments due to its low thermal history, its determine the extent of deuterium incorporation. The substitution ranged from 0 to at least 14 deuterium atoms for each n-alkane, with the highest relative abun- * Corresponding author. Tel.: +1-541-737-2155; fax: +1- dances of 4±6 deuterium atoms. There was no trend in 541-737-2064. substitution pattern as a function of chain length. E-mail address: [email protected] (B.R.T. Simoneit). To explain the deuterium substitution patterns in the 1 Present address: Lawrence Livermore National Labora- pyrolysis experiments, a free radical chain mechanism tory, Livermore, CA 94551, USA. was suggested. This mechanism proposes that one 0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0146-6380(00)00113-3 1190 R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208 pathway to the multiple deuteration could have occurred compounds used in pyrolysis experiments were n-tetra- by the free radical migration of the ole®n sites. Similar deca-1,13-diene (Aldrich Chemical Co., purity >97%), radical reactions have been proposed by others n-hexadec-1-ene (Aldrich Chemical Co., purity >97%), (Monthioux et al., 1985; Comet et al., 1986), but Ross n-eicosane (Aldrich Chemical Co., purity 99%), and n- (1992a,b) has shown that direct hydrogen transfer from dotriacontane (Aldrich Chemical Co., purity >97%). water to organic free radicals is endothermic by 25±30 The Messel shale used in the experiments was powdered, kcal/mol and, therefore, should not be signi®cant at exhaustively extracted in a Soxhlet apparatus with hydrous pyrolysis conditions. A re-examination of the methanol/methylene chloride for 72 h, and dried prior Hoering (1984) deuterium isomer pro®le data by to the pyrolysis studies. numerical modelling was performed by Ross (1992a). He concluded that a more likely explanation for the 2.2. Hydrous pyrolysis experiments deuterium isomer distribution in the n-alkanes generated in the D2O Messel shale pyrolysis is by simultaneous The pyrolysis experiments were performed in passi- deuterium exchange at more than one site. He further vated Sno-Trik1 T316 stainless steel high pressure pipes suggested a combination of ionic and radical chemistry sealed with end caps with a total volume of 2.0 cm3 (Leif to explain the results (Ross, 1992a), although the details and Simoneit, 1995a). Deoxygenated H2OorD2O was of the actual chemical mechanisms that result in the prepared by bubbling with argon for 45 min. The reac- observed preferential deuterium substitution at one end tion vessels were loaded with reactant mixtures, sealed of the isoprenoid and biomarker molecules could still in a glove bag under an argon atmosphere, and placed not be explained. Lewan (1997) has suggested that in a preheated air circulating oven set at the reaction under hydrous pyrolysis conditions water molecules can temperature and controlled to within Æ2C. Durations react directly with organic free radicals generated by the of the heating experiments ranged from 1 to 72 h. thermal breakdown of organic matter. Table 1 is a listing of the pyrolysis experiments for A re-evaluation of the research in pyrolysis and high this study. The heavy water pyrolyses of Messel shale temperature aqueous chemistry of hydrocarbons pro- were carried out at 330C with 0.4 g dried shale powder vides some insight into the major reactions that alkanes and 0.8 ml of D2O. Messel shale pyrolyses with mole- and alkenes undergo (Wilson et al., 1986; Weres et al., cular probes were conducted with 0.4 g dried shale 1988; Kissin, 1987, 1990; Siskin et al., 1990; Leif et al., powder, 8 mg each of n-tetradeca-1,13-diene, n-hexadec- 1992; Stalker et al., 1994, 1998; Seewald, 1994, 1996; 1-ene, and n-eicosane directly spiked on the shale, and Jackson et al., 1995; Burnham et al., 1997; Lewan, 1997; 0.8 ml of either H2OorD2O. Heavy water pyrolyses of Seewald et al., 1998). These studies point to the impor- n-C32H66 were done at 350 C with 10 mg of the n-alkane tance of both radical and ionic reaction mechanisms and 0.8 ml D2O. Pyrolysis of n-C32H66 under alkaline during the pyrolysis of organic matter. This paper conditions was also carried out at 350C with 10 mg of duplicates the original Hoering (1984) Messel shale pyr- the n-alkane and 0.8 ml D2O where the pH of the D2O olysis experiment and presents results from additional was adjusted to 11.3 (at 25C) using NaOD. hydrous pyrolysis experiments which provide evidence These hydrous pyrolysis experiments with pre-extracted, for the chemical pathways by which hydrogen exchange powdered rock and added model compounds in aqueous occurs between water and aliphatic hydrocarbons during solution (330 or 350C) may not be directly comparable hydrous pyrolysis. Molecular probes were used with the with hydrous pyrolysis of rock chips (i.e. Lewan, 1997), shale to determine their relative reactivities with regard because the pore spaces in rock chips become ®lled with to n-alkane and n-alkene production. water-saturated bitumen during hydrous pyrolysis. Maturing kerogen in rock chips is, therefore, not in contact with an aqueous phase, but with an organic 2. Experimental phase that has dissolved water in it. However, after the oil is expelled from the rock chips it can proceed to react 2.1. Chemicals and samples in an aqueous environment similar to what is occurring in these experiments, and similar to the reactions The Messel shale is Eocene and was sampled from the occurring during aquathermolysis experiments (Siskin et quarry at Darmstadt, Germany (Matthes, 1966; van den al., 1990; Siskin and Katritzky, 1991). Berg et al., 1977; van de Meent et al., 1980). Hydrous pyrolysis experiments were performed using ultrapure H2O 2.3. Extraction and fractionation from Burdick and Jackson and D2O (purity >99.9%) from Cambridge Isotopes Laboratories. Both H2O and The reaction vessels were cooled to room temperature D2O were distilled in glass before use. NaOD (purity upon completion of the heating cycle. The vessels were >99.5%) for pyrolysis under alkaline conditions was extracted with two 1 ml portions of methanol followed obtained from Cambridge Isotopes Laboratories. Aliphatic by ®ve 1 ml portions of methylene chloride. The solvents R.N. Leif, B.R.T. Simoneit / Organic Geochemistry 31 (2000) 1189±1208 1191 Table 1 Hydrous pyrolysis experiments performed in 2.0 cm3 316 stainless steel reactors Temperature Duration Liquid medium Reactants (C) (h) 330 72 D2O (0.8 ml) Messel shale (0.4 g) 350 72 D2O (0.8 ml, pH=7.0