Chemical-structural changes during the thermal treatment of hydrogen-rich vitrinites caused by the presence of terpene-type resin

Abstract

The vitrain from Teruel (North-Eastern Spain) is a good example of a vitrinite whose perhydrous character and anomalous properties are caused by the presence of terpene-type resin. The sample used in this work was isothermally pyrolysed in an open-medium system at temperatures ranging from 250 to 500 °C. A mass balance, a petrographic and geochemical characterisation of the pyrolysates and compositional and structural analyses (GC and NMR) of the generated oils were performed. The results confirm the association of huminite-resinous substances through covalent bonds and also point to the possible incorporation of aliphatic material via covalent oxygen bonds. The side-chains that serve as linkers between the phenolic subunits derived from the lignin contain more labile thermal bonds than those present in other vitrinites not affected by resin saturation. Consequently, this type of vitrinite undergoes a more intense degradation at lower temperatures than non-perhydrous and other hydrogen-rich vitrinites. The vitrinite network itself due to the incorporation of resin-like substances (cycloparaffinic in nature) may serve as a hydrogen donor, enhancing the hydrogenation processes during pyrolysis with respect to the polymerisation and recombination reactions. The higher ability of hydrogen to stabilise the free radicals formed during pyrolysis explains the higher conversion to liquids at lower temperatures than in the case of non-perhydrous vitrinites. At the same time, this ability limits the growth of the aromatic structures. The evolution of the vitrinite modified by the presence of terpene-type resin is, thus, retarded with respect to non-perhydrous vitrinites, although in both cases the trend followed is very close. In contrast the trend of this type of vitrinite differs considerably from that of perhydrous vitrinites which are affected by hydrocarbon impregnation. Only for the most severe treatments (450/500 °C) was some degree of structural uniformity observed in the solid residues

Mapping the chemistry of resinite, funginite and associated vitrinite in coal with micro‐FTIR

“…Although the influence of liptinite and inertinite on the chemistry of adjacent macerals has been addressed in several studies (Hutton & Cook, 1980; Mastalerz et al , 1993a, b; Suárez‐Ruiz et al , 1994; Stankiewicz et al , 1996; Iglesias et al , 2006), no consensus has been reached about underlying mechanisms. Three possible causes have been suggested to explain alterations of vitrinite's chemistry in the proximity of liptinite and inertinite macerals: (1) botanic precursors and the depositional environment; (2) coalification history of particular samples and (3) intermaceral reactions during the early peat‐forming stage or during later coalification.…”

“…Several studies suggested that the associated liptinite (i.e. resinite, alginite) and inertinite macerals are likely to influence the chemical properties of vitrinite (Hutton & Cook, 1980; Mastalerz et al , 1993a, b; Suárez‐Ruiz et al , 1994; Stankiewicz et al , 1996; Iglesias et al , 2006). However, there is still no consensus on the reasons leading to the variation of vitrinite's chemistry.…”