In Situ and Ex Situ 2D Infrared/Fluorescence Correlation Monitoring of Surface Functionality and Electron Density of Biochars

Carboxyl, hydroxyl, and other oxygen-containing functional groups play key roles in the interfacial reactions of soil surfaces including biochar (solid-phase slow pyrolysis product) soil amendment. Intensity and directionality in both real (synchronous) and imaginary (asynchronous) coordinates of 2D infrared correlation spectra were confirmed by the time courses of pyrolysis reaction (temperature × wavenumber × absorbance; 10 °C min^-1, 1 h residence time at 500 °C) utilizing high-density (74 total spectra) in situ diffuse reflectance Fourier transform (DRIFTs) monitoring. Similar primary trends were observed for four different lignocellulosic biomass feedstocks: cottonseed hull, cotton ginning waste, flax shive, and pecan shell. In the OH stretch region (3100-3750 cm^-1), free OH was most sensitive to pyrolysis temperature and reacted before H-bonded OH indicating the evaporation of water, followed by the cleavage of interchain H-bonds. Aromatic CH (R=CH_n) was the primary CH functionality (within 2700-3100 cm^-1) impacted by the pyrolysis temperature perturbation and formed as the aliphatic CH_x was removed. Of C=O/C=C groups, electron-deficient C=O (1740 cm^-1) was most sensitive to pyrolysis, reacted synchronously (in the same direction) with the aromatic C=C (1510 cm^-1), and was formed after the most electron-rich C=O (1620 cm^-1). This electron-density trend in the C=O/C=C (1400-1800 cm^-1) region of infrared coincided with the formation of aromatic extractable carbon before aliphatic structures in 2D fluorescence emission-emission correlation spectra using 340 nm excitation wavelength. Results could be used to drive biomass pyrolysis toward desirable solid- (carboxyl-enriched biochar) and liquid-phase (less hydrophilic bio-oil) products.