Hydropyrolysis of the Murchison macromolecular material releases polyaromatic compounds including phenanthrene, carbazole, fluoranthene, pyrene, chrysene, perylene, benzoperylene and coronene units with varying degrees of alklyation

Gilmour, I

Love, G. D.

Sephton, M. A.

Snape, C. E.

Verchovsky, A. B.

Watson, J. S.

Wright, I. P.

English

Open Research Online

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsHydropyrolysis of high molecular weight organic matterin MurchisonConference or Workshop ItemHow to cite:Sephton, M. A.; Love, G. D.; Watson, J. S.; Verchovsky, A. B.; Wright, I. P.; Snape, C. E. and Gilmour, I(2003). Hydropyrolysis of high molecular weight organic matter in Murchison. In: 34th Lunar and Planetary ScienceConference, 17-21 Mar 2003, Houston, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2003/pdf/1619.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukHYDROPYROLYSIS OF HIGH MOLECULAR WEIGHT ORGANIC MATTER IN MURCHISON.M. A. Sephton1, G. D. Love2, J. S. Watson1, A. B. Verchovsky1, I. P. Wright1, C. E. Snape3 and I. Gilmour1,1Planetary and Space Sciences Research Institute, Open University, Milton Keynes, Buckinghamshire, MK7 6AA,UK, (M.A.Sephton@Open.ac.uk). 2Fossil Fuels and Environmental Geochemistry, School of Civil Engineering andGeosciences, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, UK. 3School of Chemical, En-vironmental and Mining Engineering, University of Nottingham, University Park, Nottingham, NG7 2RD, UK. Introduction:  The Earth-based record of pre-biotic chemical evolution has long-since been obliter-ated by geological processing. However, remains ofmaterials involved in the construction of the Earth arepreserved in ancient asteroids, fragments of which arenaturally-delivered to the Earth as meteorites. Carbo-naceous chondrites, such as the Murchison meteorite,are particularly primitive and contain 2 to 5 wt. % car-bon, most of which is present as organic matter [1]. Atleast 70% of the organic matter in Murchison is a sol-vent-insoluble, high molecular weight macromolecularmaterial. As the dominant organic entity, this compo-nent is key to understanding the origin of meteoriticorganic matter. The macromolecular material consistsof an aromatic framework linked and surrounded byshort functional groups [1]. Pyrolysis-based techniques are probably the mostcommon means of analysing the macromolecular ma-terial in meteorites. Analytical pyrolysis involvesthermally decomposing macromolecular organic mat-ter in an inert atmosphere into lower molecular weightfragments that are more amenable to conventional or-ganic analytical techniques. Hydropyrolysis, whichrefers to pyrolysis assisted by high hydrogen gas pres-sures and a dispersed sulphided molybdenum catalyst,has not been previously applied to the study of highmolecular weight meteoritic organic matter. Methods:  Crushed whole meteorite was solvent-extracted and impregnated with an aqueous solution ofammonium dioxydithiomolybdate [(NH4)2MoO2S2] togive a 2 wt% nominal loading of molybdenum. Hy-dropyrolysis runs were performed in an open-system,temperature-programmed reactor configuration, whichhas been described in detail elsewhere [2]. Compound detection and identification was per-formed by gas chromatography-mass spectrometry(GC-MS) using an Agilent Technologies 6890 gaschromatograph interfaced with a 5973 mass selectivedetector. Analyses were by splitless injection onto aBPX5 capillary column (30 m x 0.25 mm x 0.25 µm).Following a 1 min period at 50 °C the gas chromato-graph oven was programmed from 50 °C to 300 °C at5 °C min-1 and then held at 300 °C for 39 min. Carbon and nitrogen abundance and isotopic com-position of samples were determined using an Ele-mental Analyser-Isotope Ratio Mass Spectrometer(PDZ Europa ANCA-SL on a Geo 20-20) as in [3]. Ahydrous pyrolysis (pyrolysis in the presence of water)residue from a previous study [4] was analysed forcomparison. The results are expressed using the δ no-tation as follows: δ (‰) = [(Rsample - Rstandard) / Rstandard]x 1000. Where R = 13C/12C for carbon and 15N/14N fornitrogen; standard = PDB for carbon; air for nitrogen.Results and discussion:  Hydropyrolysis of theMurchison macromolecular material successfully re-leases significant amounts of high molecular weightpolyaromatic hydrocarbons (PAH) (Figure 1). Themajority of the carbon in the Murchison hydropyroly-sate appears to be present as three- to seven-ring PAHincluding phenanthrene, carbazole, fluoranthene, py-rene, chrysene, perylene, benzoperylene and coroneneunits with varying degrees of alklyation. The absenceof long-chain alkyl substituents indicates that aliphaticmoieties exist within or around the aromatic networkas hydroaromatic rings and short alkyl substituents orbridging groups. The molecular weight range of thehydropyrolysis products are noticeably different fromthe predominantly one- and two-ring aromatics ob-tained from Murchison using hydrous pyrolysis [4].However, the  hydrous pyrolysis products were ex-tracted by supercritical fluid extraction which allowsthe retention of components as volatile as benzene andtoluene. Compounds this volatile may have been pro-duced by hydropyrolysis and lost during sample col-lection to leave behind the higher molecular weightcomponents in Figure 1. A summed mass chroma-togram of the Murchison hydropyrolysate (Figure 2)reveals the dominant parent PAH distribution in theextract. It is apparent that the higher molecular weightPAH within the pyrolysable macromolecular materialextend up to coronene. Carbazole is a dominant nitro-gen heterocyclic compound in the hydropyrolysate.Although hydropyrolysis liberates substantialamounts of organic matter from the macromolecularmaterial, over 50% of macromolecular carbon remainsunconverted. Hence, the meteoritic organic networkcontains both labile (pyrolysable) and refractory (non-pyrolysable) fractions. Comparisons with experimentalyields from bituminous coals (over 85%) indicates thatthis refractory residue probably consists of a networkdominated by at least five- or six-ring PAH moietiescross-linked together.Lunar and Planetary Science XXXIV (2003) 1619.pdfHYDROPYROLYSIS OF MURCHISON:  M. A. Sephton et al.It has been established that the labile fraction isrelatively enriched, and the refractory fraction rela-tively depleted, in the heavy isotopes of carbon andnitrogen [5]. Hence, the efficiency of a pyrolysismethod can be gauged by establishing how much ofthe labile 13C- and 15N-enrichments have been re-moved from the total macromolecular fraction. Stableisotopic investigations suggests that the hydropyrolysisresidue (δ13C -20.7‰, δ15N -23.5‰) has had its primi-tive isotopic enrichments removed more efficientlythan the residue from hydrous pyrolysis (δ13C -18.8‰,δ15N +4.2‰).References:  [1] Sephton M. A. (2002) Nat. Prod.Rep. 19, 292-311. [2] Love G. D. et al. (1995) Org.Geochem. 23, 981-986. [3] Sephton M. A. et al. (2002)Geology 30, 1119-1122. [4] Sephton M. A. et al.(1998) Geochim. Cosmochim. Acta 62, 1821-1828. [5]Sephton M. A. et al. (2002) Geochim. Cosmochim.Acta. in press. STIC20 30 40 50 60responseC2retention time (min)Figure 1. Total ion chromatogram (TIC) of the hydropyrolysate fromthe Murchison meteorite. (S) elemental sulfur.Figure 2. Summed mass chromatogram representing the main parentalPAH present in the Murchison hydropyrolysate.Nphenanthrenecarbazolepyrenechryseneperylenebenzo[ghi]perylenecoronenem/z16717820222825227630020 30 40 50 60responseretention time (min)Lunar and Planetary Science XXXIV (2003) 1619.pdf

Hydropyrolysis of high molecular weight organic matter in Murchison

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Hydropyrolysis of high molecular weight organic matter in Murchison

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