Org. Geochem. Vol. 9, No. 5, pp. 255-264, t986 0146-6380/86 $3.00 + 0.00 Printed in Great Britain Pergamon Journals Ltd The synthesis of alkyl aromatic hydrocarbons and its geochemical implications D. RIGBY, T. D. GILBERT and J. W. SMITH CSIRO Divison of Fossil Fuels, P.O. Box 136, North Ryde, N.S.W. 2113, Australia (Received 1 August 1985; accepted 20 May 1986) Abstract--In the present study clay minerals have been shown to catalyse alkylation at 200-270°C of substituted benzenes and naphthalenes by alcohols and esters that are known to occur naturally in sediments. It has also been demonstrated that the formation of C~C and C~O bond linkages under these experimental conditions is accompanied by some dealkylation of alkyl aromatic products. It therefore seems likely that alkyl aromatic compounds may be synthesized during the thermal treatment of coal. Consequently the significance, in relation to coal structure, of the occurrence of such compounds in the products of coal liquefaction, in artificial maturation experiments and in situations where pyrolytic analytical techniques are used requires re-examination. The value of alkyl aromatics as biomarkers for sedimentary maturation processes is questionable when thermal processes have been used in their separation and identification. Keywords: alkyl aromatic compounds, synthesis, clay catalysis, maturation, coal INTRODUCTION The occurrence of alkyl aromatic compounds in petroleum and coals is of continuing interest to geochemists. It has been suggested that these com- pounds are important as biomarkers (Gallegos, 1981a), as parameters for sediment maturity (Radke et al., 1982; Alexander et al., 1985), in determining the aromaticity of coal macerals and kerogen (Allan and Larter, 1983), and in understanding the structure of coal and kerogen (Baset et al., 1980; Larter and Douglas, 1980; Solli et al., 1980; Allan and Larter, 1983). Radke et al. (1982) have suggested that there are two sources of alkyl aromatics in the solvent extracts of coals: either aromatization of non- aromatic biological precursors trapped in the coals or fragmentation of the coal matrix during maturation. Many of the reports of these compounds in coals occur in studies involving the pyrolysis or lique- faction of coal (Vahrman and Watts, 1972; Maters et al., 1977; Larter and Douglas, 1978; Baset et al., 1980: Philp and Saxby, 1980; Solli et al., 1980; Youtcheff et al., 1983; Wood et al., 1984). Baset et al. (1980) have reported that long chain alkyl benzenes (C14-C34) are detected only in the pyrolytic products and not in the solvent extract of a subbituminous coal. Philp et al. (1981) identified a homologous series of alkyl benzenes in the hydrogenation products of alginite (Tasmanite) and correlated the increased production of alkyl benzenes with higher autoclave temperatures. Therefore the possibility that some alkyl aromatic compounds represent artefacts of the thermal treatment or maturation of coal cannot be ignored. In an attempt to resolve this question, a range of long chain aliphatic compounds commonly occurring in immature sediments was heated at 200-270°C with simple aromatic compounds in the presence of clays. The products were analysed by gas chromatography (GC) and combined gas chromatography/mass spectrometry (GC/MS). EXPERIMENTAL Saturated and unsaturated aliphatic hydrocarbons, alcohols, acids and esters (Table 1) of types likely to occur in immature sediments were heated in the presence of a catalyst with simple aromatic com- pounds that represented structures likely to be found in humic materials. The catalysts used were mont- morillonite from Wyoming, U.S.A. and kaolinite No. 1 supplied by Steetley Industries, Australia. The catalytic effect of Morwell brown coal from Victoria (ash content 3.4% dry basis) was also studied. The aromatic (20--40 mg) and aliphatic compounds were mixed in a Pyrex tube (250 × 6 mm) in the mole ratios shown in Table 1 and 10% by weight of the catalyst was added (except in experiment 8, Table 1). The reactants were frozen at -196°C, evacuated to a pressure of 10 -3 mm Hg and sealed by drawing off the glass tube. The Pyrex reaction tubes were placed in individual copper sheaths and heated at 200 or 270°C for times ranging from 1 to 4 weeks. In a series of control experiments each of the reactants was heated with montmorillonite for 4 weeks at 270~C. After heating for the required period, the tubes were cooled to 20°C and the liquid products were extracted (vith dichloromethane and chro- matographed on a HP5710 GC fitted with a SGE 50 m × 0.2 mm i.d. SE-30 WCOT silica column. The 255