Study On The Reactivity Of Kerogen In Oil Shale Based On The Kerogen Abstraction And Molecular Structure

Kerogen in oil shale is the important source of oil and gas. And the first step of kerogen pyrolysis is chemical bond homolysis. The Bond dissociation enthalpy can be regarded as an index to indicate how easy or difficult each bond homolytic cleavage is, and it is closely related to the nature of kerogen pyrolysis. Thus, based on the kerogen structural properties to make kerogen molecular model and calculate various kinds of chemical bond dissociation enthalpy are the key issue to understand the retorting and pyrolysis of kerogen.The molecular structural information of kerogen isolated from Longkou and Yaojie oil shale was obtained by advanced testing technology. And we have predicted bond dissociation enthalpies of C-C bond, C-S bond, C-N bond, C-O bond, C-H bond as well as C=C bond and analyzed the impact of radical stability, the varieties and positions of different functional groups. Finally, we analyzed the reaction process of kerogen pyrolysis by comparing the difference of bond dissociation enthalpies.The results obtained from solid-state 13 C NMR, X-ray photoelectron spectroscopy(XPS), Fourier transform infrared(FT-IR) and X-ray diffraction(XRD) techniques reflect that aliphatic carbons(70.33%) compose the carbon skeletal structure of Longkou kerogen.Although Longkou kerogen has a very low aromaticity of 28.45%, the substitutive degree of aromatic ring is relatively high(0.55-0.75). Organic oxygen mainly exists as C-O 、C-OH 、 O=C-O and C=O. The richest organic nitrogen-containing functional group in Longkou kerogen is pyrrole, followed by amine and cyanobenzene. Organic sulfurs are distributed as sulfoxide, aromatic and aliphatic sulfur. The aliphatic carbon and aromatic carbon of Yaojie kerogen is 54.84% and 44.26% respectively, which means Yaojie kerogen has a big unsaturated degree. And Organic oxygen mainly exists as C-O、 C-OH、 O=C-O and C=O. The richest organic nitrogen-containing functional group in Yaojie kerogen is pyrrole, followed by amine and cyanobenzene. Organic sulfurs are distributed as sulfoxide,aromatic and aliphatic sulfur.On the basis of Density Functional Theoretical Calculation, we have calculated bond dissociation enthalpies of 554 small molecular models. The various subtypes of bond dissociation enthalpies changing tendency is that C-S bond < C-N bond < C-O bond< C-C bond < C-H bond < C=C bond. And we noted the BDE(Bond Dissociation Enthalpy) does not only depend on the stability of radical fragments formed, but also rely on the local environment surrounding the studied bond. Therefore, the BDE of smallmolecules could be a good estimation for the BDE of large molecules in kerogen if the local environments are similar.We have analyzed the reaction process of kerogen pyrolysis by comparing the difference of bond dissociation enthalpies. The first step in kerogen retorting and pyrolysis is bond homolytic cleavage. With respect to predicting the kerogen pyrolysis reaction, our work suggests that the pyrolysates of kerogen result mainly from the cleavage of weak bonds, such as the bridge bond connecting the kerogen macromolecular compounds of structural units, C-S bond, C-N bond and C-O bond, etc. In this process, pyrolysis products are mainly H2 O, CO2, H2 S and other light hydrocarbon. Followed by the chemical bond cleavage of moderate strength, this includes the C-C bond with the aromatic compound,C-C bond with the heteroatom-containing group, the cleavage of alkane and the hydrogen-atom-loss reaction from the substituent α-carbon-centered group on the pyrrole with double bonds is favorable. This process will produce large amounts of oil, gas and semicoke. Finally, the bond cleavage with higher energies is mainly the cleavage of C=C bond, the hydrogen-atom-loss reaction on the aromatic compound as well as C-C bond with the heterocyclic compounds. Thus, these reactions always occur at high temperature and produced semicoke and a small amount of gas.