Determination of the aromaticity and the degree of aromatic condensation of a thermosequence of wood charcoal using NMR

Abstract

Quantifying the role of black carbon (BC) in geochemical processes is difficult due to the heterogeneous character of its chemical structure. Chestnut wood charcoal samples produced at heat treatment temperatures (HTT) from 200–1000 °C were analysed using two different solid state 13C NMR techniques. First, aromaticity was determined as the percentage of total signal present in the aromatic region of 13C direct polarisation (DP) spectra. This was found to increase through the low temperature range of 200–400 °C; at higher temperatures, aromaticity was found to be >90%. Second, aromatic condensation was determined through the measurement of the chemical shift of 13C benzene sorbed to the charcoals, which is influenced by the presence of “ring currents” in the aromatic domains of the charcoals. This technique was less sensitive to molecular changes through the lower temperature range, but showed there was a smooth increase in the degree of condensation of the aromatic structures with increasing temperature through the temperature range 400–1000 °C. Ab initio molecular modelling was used to estimate the size of aromatic domains in the charcoals based on the strength of the ring currents detected. These calculations indicated that charcoals produced at temperatures below 500 °C contain aromatic domains no larger than coronene (7 ring). At higher temperatures the size of these domains rapidly increases, with structures larger than a 19 ring symmetrical PAH being predominant in charcoals produced at temperatures above 700 °C. Data from this study were found to be generally consistent with previously published measurements using the benzenepolycarboxylic acid (BPCA) molecular marker method on the same set of samples.