Cyclopentanones and 2-cyclopenten-1-ones as major products of hydrous pyrolysis of immature organic-rich shales

Cyclopentanone is an important platform chemical for various industrial syntheses. It has been recently reported as a major product in biomass cellulose-derived biofuels but has rarely been reported in geochemical research. We report here the abundant occurrence of cyclopentanone, 2-cyclopenten-1-one and their alkyl derivatives in pyrolysates of organic-rich shales. Closed system pyrolyses at temperatures from 290 °C to 350 °C have been carried out on thermally immature organic-rich shale samples of Ordovician and Devonian age from Canada. GC-MS analyses of the pyrolysates show that cyclopentanones, 2-cyclopenten-1-ones and phenols dominate over hydrocarbons in the low temperature hydrous pyrolysates, and their abundance relative to hydrocarbons decreases with increasing thermal maturation. However, these low molecular weight cyclic ketones are only minor components compared with hydrocarbons in anhydrous pyrolysates, indicating that water has played an important role in the formation of these oxygen-containing compounds. In addition, n-alkan-2-ones in the range of C6-C35 are also detected in the hydrous pyrolysates, but they are at much reduced concentrations compared with the cyclic ketones. The potential precursors in the shale and the likely chemical reaction mechanism for the formation of these cyclic ketones are discussed in detail in the paper.