Identification of alkenylbenzenes in pyrolyzates using GC–MS and GC–FTIR techniques: evidence for kerogen aromatic moieties with various binding sites

Pyrolysis products obtained from heating a shale under vacuum have been analyzed for alkenylbenzenes using GC–MS and GC–FTIR. The shale pyrolysate contains a homologous series (C11–C23) of alkenylbenzenes with terminal unsaturations concomitant with lesser amounts of hitherto unknown trans alk-1-enylbenzenes. These latter compounds are unique constituents of kerogen pyrolyzates and are suggested to be derived from alkylbenzene moieties bound to kerogen at the benzylic position. The presence of these unsaturated compounds in pyrolyzates has been interpreted as evidence for site-specific binding of alkylbenzene moieties in kerogen. Formation of alkenylbenzene moieties is suggested to result from diagenetic cyclization/aromatization reactions of fatty acid precursors. Incorporation of structurally different lipid precursors such as saturated, mono- and polyunsaturated fatty acids is suggested to reflect the nature of the different alkenylbenzene products generated upon pyrolysis. The application of ‘compound specificʹ GC–FTIR techniques, provides a new and complementary tool to the study of geochemical samples. Compound specific IR spectra in conjunction with electron impact mass spectra are shown to facilitate the structural identification of hydrocarbons in geochemical samples.