Characterization of biochars using advanced solid-state 13C nuclear magnetic resonance spectroscopy

Abstract

In this chapter, we first briefly reviewed the knowledge of biochar chemical structures based on solid-state NMR. Then, the reason why the widely applied 13C cross polarization/magic angle spinning (CP/MAS) technique is inappropriate for biochar characterization was explained. Afterwards, advanced solid-state NMR techniques for the characterization of biochars were introduced. 13C direct polarization/magic angle spinning (DP/MAS) and DP/MAS with recoupled dipolar dephasing to quantify biochars are used to obtain quantitative aromaticity and nonprotonated aromatic fraction. The recoupled 1H13C dipolar dephasing technique is applied to distinguish different aromatic carbons in biochars. Combined with the data from 1H 13C recoupled long-range dipolar dephasing, the information on the fraction of aromatic edge carbons can be used to obtain the structural models of aromatic cluster sizes. Finally, a case study on a slow-pyrolysis biochar produced from switchgrass was demonstrated.