Covalent bonding structures of eight oil shales and the characteristics of bond cleavage during the pyrolysis process

The rupture of covalent bond is regarded as the chemical essence of oil shale pyrolysis, while the variations in chemical structure of oil shale with different maturity are the primary factor influencing its pyrolysis behavior. That information, however, is scarce in the literature. In this study, the concentrations of covalent bonds in eight oil shale kerogens were calculated, and the variation of concentrations of eight covalent bonds with H/C in kerogens was summarized as well. Besides, the amount of covalent bonds rupture of eight kerogens during the pyrolysis at 300–420 °C were tested and calculated using 9,10-dihydrophenanthrene (DHP) as a probe molecule. The kinetics of bond cleavage reaction was analyzed, and the relationship between bond cleavage reaction and structure of kerogens was also studied. The results show that the concentrations of C_al-C_al and C_ar-C_ar bonds are 31.09–57.32 and 11.36–32.19 mmol/g, respectively. The concentrations of C_al-C_al and C_al-H bonds of kerogen increase linearly with an increase of H/C, while the concentrations of C_ar-C_ar, C_al-C_ar and C_ar-H bonds have a negative linear relationship with H/C. The amount of covalent bonds rupture of kerogens shows an increasing trend with an increase of pyrolysis temperature or time. The rate of bond cleavage decreases exponentially with the reaction time. The covalent bonds cleavage of kerogen basically follows first-order kinetics. The amount of the covalent bonds that can break at 300–420 °C in kerogens are 0.84–2.32, 2.12–4.43, 4.30–7.13, and 7.03–11.40 mmol/g, respectively. The covalent bonds that rupture of kerogens at 300–420 °C are not primarily C_al-C_al bond but weak bonds with bond energy lower than C_al-C_al bond, and the amount of weak bonds range from 4.53 to 10.42 mmol/g. These results extend the understanding on the mechanism of shale oil and chemicals in oil shale pyrolysis as well as that on the geological evolution of oil shale.