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Abstract
The chemistry and nature of biochars are still far from being well understood. In the present work, solid-state 2D HETCOR 1H-13C NMR spectroscopy is introduced for an improved characterization of the aromatic network in biochars. To that end, a pyrochar obtained from the pyrolysis of cellulose at 350 °C for 1 h was used as an example. Variation of the contact time during cross polarization from 50 µs, to 200 µs and 1000 µs gave information about the protonation degree of the different C groups and their interactions. We demonstrated that carbohydrates did not survive the used pyrolysis conditions. Therefore, O-alkyl C was assigned to ethers. Phenols were not identified to a higher extent suggesting that furan and benzofuran-type units determine the O-functionality of the aromatic domains. The latter are directly connected to alkyl chains. Those features are expected to affect chemical but also physical properties of the biochar. Based on our results, we developed a new concept describing the nature of the aromatic network in the studied cellulose-based pyrochars. The latter contrasts common views about the chemical nature of biochar, possibly because pyrolysis temperatures > 350 °C are required for achieving advanced condensation of the aromatic domains.
Highlights
The standard CPMAS spectra of the pure cellulose and its biochar were obtained with a Bruker (Rheinstetten, Germany) Avance III HD 400 MHz wide bore spectrometer, using a triple resonance broadband probe and zirconium rotors of 4 mm OD with KEL-F-caps and applying a magic-angle spinning (MAS) speed of the rotor at 14 kHz
The pyrolysis of cellulose at a temperature of 350 ◦ C for 1 h resulted in a weight loss of 64% (Table 1) corresponding to a C-loss of 55% which is in line with observations that thermal degradation of cellulose starts around 300 ◦ C [15]
The pH change is explainable solely by heat-induced chemical alterations, which may have been the removal of OH-groups of the cellulose by dehydrogenation and the formation of functional groups which increase the acidity of the pyrolysate
Summary
One of the non-degradative techniques that allow a more detailed description of the chemical alteration of organic matter during pyrolysis represents solid-state 13 C nuclear magnetic resonance (NMR) spectroscopy. With this technique, secondary reactions during sample preparation or extraction can be avoided. Modern solid-state NMR spectroscopy offers special pulse sequences which allow an improved assignment of signal intensity to C groups and a more detailed description of the nature of the aromatic network in biochars One of those is the two dimensional (2D) 1 H-13 C heteronuclear correlation (HETCOR) NMR experiment in which 13 C and 1 H are correlated through their spin-spin interactions [18]. It accounts for 40% of the total biomass of wood [24] and 19–33% of grass and wheat straw residues [25]
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