Effect of pyrolysis temperature on aromatic cluster size of cellulose char by quantitative multi cross-polarization 13C NMR with long range dipolar dephasing

Recent results obtained by Raman spectroscopy as well as Transmission Electron Microscopy highlight the importance of irregular structures within amorphous chars. Results from DFT simulations, reported here, suggest that irregular ring systems contribute strongly to the asymmetry often observed in the aromatic peak near 140 ppm. By combining a recently developed cross-polarization pulse sequence designed for quantitative analysis of even low hydrogen containing materials with long range dipolar dephasing, we examine the structure of a thermoseries of chars produced from cellulose. These results demonstrate the consistent growth of cluster size with pyrolysis temperature, and examine for the first time the distribution of ether groups and defects within the cluster. These results highlight the contribution of defects to the structure and show localization of ether groups toward the periphery of the clusters. Only minor ether contributions were associated carbon sites most distant from hydrogens, suggesting that cluster growth occurs largely by reaction at these sites. The prevalence of oxygenated and defect carbon sites throughout the aromatic clusters highlights the need for additional characterization of these materials as these sites are likely to offer substantially different reactivity and adsorption behavior compared to regular aromatic structures.

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