KINETICS AND MECHANISM OF THE THERMAL-DECOMPOSITION OF ESTERS IN SEDIMENTS

The liquid phase thermal decomposition of a long chain alkyl ester, hexadecyl tetradecanoate, has been investigated in the temperature range 310-365-degrees-C. Accurate measurements of the first-order rate constants have been made and the activation energy and frequency factor for the process determined. Treatment of a solvent-extracted brown coal sample at 330-degrees-C in the presence of tetralin for various times yielded product mixtures containing abundant C24, C26 and C28 alkenes and alkanes. The initial formation of alkenes in this process together with their rate of formation are consistent with their formation via the thermal decomposition of alkyl esters bound in the coal matrix. These results suggest that the bound esters undergo thermal decomposition with similar kinetic and mechanistic characteristics to hexadecyl tetradecanoate. The extent to which thermal decomposition of esters occurs in sediments under geological conditions was assessed by examining sediment samples using FT-IR and pyrolysis-GC techniques. Evidence that a similar reaction occurred in these sediments to that which occurred under laboratory heating conditions was provided by the match in depths where ester depletion had occurred and that calculated using the laboratory determined kinetic parameters.