A deep insight into the structural characteristics of Yilan oil shale kerogen through selective oxidation

Abstract

The structural characteristics of oil shale are important to its application. In this work, a deep insight into the structural characteristics of Yilan oil shale kerogen (YLK) was studied using alkali-oxygen oxidation at different times and ruthenium ion catalyzed oxidation (RICO). The results indicate that YLK is gradually converted to humic acids (HAs), water-soluble acids (WSAs), carboxylic acids (CAs), CO2 and H2O. 13C NMR analyses of YLK, residues and HAs indicate that C–O bonds, especially Car–O bonds of YLK, are easily cleaved. The condensed aromatic rings and long alkyl chains are difficult to be oxidized during the oxidation process. With increasing oxidation time, the molar fraction of aromatic bridgehead carbon (Xb) of YLK and the residues increases from 0.174 to 0.201, and that of HAs increases from 0.225 to 0.267. The result indicates that the aromatic structures of YLK are composed of 1-ring aromatic clusters (26.5%), 2-ring aromatic clusters (67.1%) and 3 or more-ring aromatic clusters (6.4%). Also, the average methylene chain (Cn) of YLK and the residues increases from 2.0 to 5.1, and that of HAs increases from 1.3 to 2.7, indicating that the alkyl chain length is not average but has a distribution. 2.5% methylene carbons exist as long n-alkyl chains in the alkyl side chains groups (SCGs) on the aromatic rings in YLK. The carbon number of SCGs ranges from 5 to 22, and C15 and C17 are predominant. 3.8% methylene carbons exist as alkyl bridges linkages (BLs) connecting aromatic rings in YLK. The carbon number of BLs connecting aromatic rings ranges from 6 to 19, and C8 is predominant. Because of the existence of above long n-alkyl chains, 93.7% methylene carbons in YLK exist as alkyl chains with an average length of 1.84.