Influence of steroid structure on the pyrolytic formation of polycyclic aromatic hydrocarbons

Abstract

It is well known that polycyclic aromatic hydrocarbons (PAHs) are formed from the pyrolysis of biomass, but the precursors and pathways that lead to PAHs are not well characterized. In this investigation, the flash vacuum pyrolysis (FVP) and the atmospheric pressure flow pyrolysis of a series of structurally different plant steroids, including stigmasterol, stigmasteryl acetate, β-sitosterol, stigmasta-3,5-diene, cholesterol, cholesteryl acetate, dihydrocholesterol, and ergosterol was investigated at 700 °C to determine the impact of steroid structure on the formation of three, four, and five ring PAHs. FVP of the steroids revealed that PAHs, such as phenanthrene, anthracene, pyrene, chrysene, benz[a]anthracene and their monomethylated derivatives, were formed by a series of unimolecular reactions, and the PAH yields were dependent upon the steroid structure. PAH yields were most sensitive to the number of double bonds in the steroid B-ring. Ergosterol (two double bonds) produced 13-fold more PAHs than dihydrocholesterol (no double bonds) in FVP experiments and 4-fold more PAHs in the flow pyrolysis experiments. Increasing the temperature from 700 to 800 °C in the FVP experiments only slightly increased the PAH yields for dihydrocholesterol relative to cholesterol, but the yield of small aromatic hydrocarbons, such as benzene and toluene, increased approximately 2-fold. Small increases in PAH yields (10–40%) were found in the FVP experiments if a double bond was placed in the steroid A-ring by 1,2-elimination of an ester (as for cholesteryl acetate), but this trend was not observed in the flow pyrolysis experiments. Insight into the structural origins of the PAHs was gained by FVP of [4- 13C]cholesterol, in which the 13C content of the products were determined by mass spectrometry. The PAH yields from the flow pyrolysis of steroids esterified with saturated and unsaturated long chain fatty acids, i.e. cholesteryl stearate, cholesteryl oleate, and cholesteryl linolenate, were 20–40% lower (per gram of steroid) than the free steroid. The yield of three to five ring PAHs did not correlate with the number of double bonds in the ester chain, but the yield of benzene increased as the number of double bonds in the ester chain increased. In general, this investigation has shown that structural differences in steroids can significantly alter the PAH yields formed from the vacuum pyrolysis or atmospheric pressure pyrolysis of steroids.