Pyrolysis involving n-hexadecane, water and minerals: Insight into the mechanisms and isotope fractionation for water-hydrocarbon reaction

To ascertain the effects of minerals and dissolved salts on gas generation from water-hydrocarbon reaction, a series of pyrolysis of n-C-16 with water and different minerals or salt solutions at 330-420 degrees C and 50 MPa were conducted in a gold-tube pyrolysis apparatus. It was shown that both calcite and montmorillonite exhibited evidently catalytic effects on gas generation in pyrolysis of hydrocarbon involving water at elevated temperature. The evident higher D/H ratios of methane in hydrous pyrolysis using deuterated water demonstrate that water provided H or D for hydrocarbon gas generation in water-hydrocarbon reaction. Meanwhile, the presence of two minerals resulted in the distinct distribution of the isomeric index (i-C-4/n-C-4) and carbon isotopic compositions for gas products. In addition, there was a negative correlation between gas yields/isomeric index and NaCl/KCl concentrations in hydrous pyrolysis of n-C-16. Theoretical calculations based on density functional theory (DFT) and transition states (TS) revealed that the activation energies for reactions between alkenes and H+ or water are much lower than those involving water and other organic compounds. The water-hydrocarbon reaction in this study should mainly occur via ionic mechanism, though free radical reaction between alkyl radicals and water also presented. Moreover, it was demonstrated that ionic and free radical mechanisms dominated the water-hydrocarbon reaction with montmorillonite and calcite, respectively. The effects of dissolved salts on water-hydrocarbon reaction can be interpreted by the evolution of H+ concentration with salt concentration in aqueous solutions at elevated temperature. In addition, the differences in carbon isotope fractionation for methane generation from water-hydrocarbon reaction via free radical and ionic mechanisms were addressed. Hydrogen isotope fractionation for methane during water-hydrocarbon reaction or hydrogenation by water was also elucidated by thermodynamic calculations. Finally, a model for the prediction of hydrogen isotopic ratios of methane after hydrogen transfer equilibrium between water and methane is proposed.