Influence of elemental composition on carbonization pyrolysis of sporopollenin and lignite as models of kerogens

Abstract

Two organic substances (sporopollenin and lignite) were heat-treated as models of kerogens and the resulting chars have been studied by electron microscopy, differential thermal analysis, thermogravimetry and IR spectroscopy. Sporopollenin (H/C = 1.56, O/C = 0.34) melts with decomposition at 436°C. Molecular orientation (1000 Å in extent) appears at 460°C after the weight loss rate maximum. It coincides with the aromatic CH maximum, the OH and carbonyl-carboxyl minima and comes before the aliphatic CH minimum and before the maximum spin concentration as well. Experimental conditions variations (quantity, pressure, action of oxygen) noticeably modify molecular orientation extension (from 50 Å to more than 2000 Å). During pyrolysis, lignite (H/C = 1.12, O/C = 0.44) does not melt and shows a small molecular orientation extension which unlike sporopollenin is not sensitive to experimental conditions. This behaviour is due to persistent oxygenated functions, some of them bonding the aromatic layer stacks. Sporopollenin preheat-treated in air (reticulation) appears comparable to lignite, which confirms the major part played by oxygen as far as melting and molecular orientation are concerned.