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Title: 129Xe NMR studies of pecan shell-based biochar and structure-process correlations

item MAURI, MICHELE - University Of Milan
item FARINA, MATTEO - University Of Milan
item PATRIARCA, GIORGIO - University Of Milan
item SIMONUTTI, ROBERTO - University Of Milan
item Klasson, K Thomas
item Cheng, Huai

Submitted to: International Journal of Polymer Analysis and Characterization
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/17/2014
Publication Date: 2/1/2015
Citation: Mauri, M., Farina, M., Patriarca, G., Simonutti, R., Klasson, K.T., Cheng, H.N. 2015. 129Xe NMR studies of pecan shell-based biochar and structure-process correlations. International Journal of Polymer Analysis and Characterization. 20:119-129.

Interpretive Summary: There is continuing interest in using natural agri-based raw materials for the generation of activated carbon. The resulting biochar contains micropores that have been found to have useful properties. A method that has been used to study many porous materials is 129Xe NMR, but it has not been applied to biochar thus far. In this work, we explored the potential applications of 129Xe NMR to study pecan shell-based biochar samples made through phosphoric acid activation at a high temperature in a partial oxygen atmosphere. In most 129Xe spectra, two signals were found for these samples, corresponding to the non-adsorbed xenon and xenon adsorbed and permeated in micropores. The linear pressure dependence of chemical shifts and the subsequent 129Xe chemical shifts extrapolated to 0 pressure agreed with previously measured pore size. Through T1 and 2D EXSY experiments, the motion and the diffusion of xenon atoms were studied, providing additional information on the accessibility of the pore system. It appears that 129Xe NMR can be a useful method to characterize biochar, and the methodologies described in this work can possibly be applied to other biochar samples made from different raw materials and with different activation methods.

Technical Abstract: Pecan shell-based biochar is being utilized as filtration medium, sequestrant for metallic ions, soil conditioner, and other applications. One process involves the use of phosphoric acid at high temperature in a partial oxygen atmosphere to produce a highly porous carbonaceous material. In this work, we found 129Xe NMR to be an excellent technique to study micropores in biochar. Thus, the 129Xe chemical shift in biochar was found to vary linearly with the xenon pressure; from the data an estimate of about 8-9 Å could be proposed for the average pore diameter in pecan shell-based biochar. Through saturation recovery and 2D NMR exchange experiments, the information on the exchange between free-versus-bound xenon was obtained. Furthermore, correlations of 129Xe NMR data with the carbonization process conditions were made.