Jingdong Mao, Xiaoyan Cao, and Na Chen

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 1 3C 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. 1 3C 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 ‘H-13C dipolar dephasing tech­nique is applied to distinguish different aromatic carbons in biochars. Combined with the data from ‘H-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.

1 Introduction

The conversion of biomass, such as switchgrass and corn stover, into renewable energy products has been widely investigated owing to the concerns over global warming and limited petroleum resources [4, 9 ] . This process can be achieved through the thermochemical route [ 5]. Thermochemical processing of biomass produces significant biochars. If wisely used, biochars can be beneficial resources but otherwise they could be wastes. Biochars have been used as soil conditioners, carbon sequestration agents, and adsorption agents [ 3, 12]. In addition, their

J. Mao(H) • X. Cao • N. Chen

Department of Chemistry and Biochemistry, Old Dominion University, Norfolk, VA 23529, USA e-mail: JMao@odu. edu

J. W. Lee (ed.), Advanced Biofuels and Bioproducts, DOI 10.1007/978-1-4614-3348-4_5, © Springer Science+Business Media New York 2013

combustion can supply process heat [2]. In order to utilize biochars beneficially, the first step is to characterize them.

Various techniques have been employed to characterize biochars. Among them, solid-state NMR is regarded as one of the best choices because (1) it is nondestruc­tive, (2) it can measure insoluble organic matter, and (3) it also provides comprehen­sive structural information [15]. However, solid-state NMR has been underutilized in the characterization of biochars and most seriously it has been applied inappro­priately in many studies, leading to distorted structural information on chars. In this chapter, we first provide a short review about the chemical structure of biochars obtained from solid-state 13C NMR. Next, the reason why the most-widely used 13C cross polarization/magic angle spinning (CP/MAS) technique is inappropriate for biochar characterization is addressed. Then, the quantitative approach for char char­acterization using direct polarization (DP) techniques is introduced. Afterwards, the advanced solid-state NMR protocol for estimating aromatic cluster sizes of biochars is dealt with. To conclude, the characterization of biochars by advanced solid-state NMR techniques is demonstrated using a slow-pyrolysis biochar produced from switchgrass as an example. The major objective of this chapter is to clarify the mis­leading concepts regarding the characterization of biochars using solid-state NMR spectroscopy and to introduce advanced solid-state NMR techniques for biochar characterization.