Insights into the functional group transformation of a chinese brown coal during slow pyrolysis by combining various experiments

The effects of heat treatment on the structural transformation of a Chinese brown coal were studied using Fourier transform infrared spectroscopy (FT-IR), high resolution solid-state nuclear magnetic resonance (SSNMR), and Raman spectroscopic techniques. The wide applicabilities of 13C dipolar-decoupling magic-angle-spinning (DDMAS), 1H MAS, 1H-1H homonulcear double-quantum (DQ) MAS and 1H-13C cross-polarization heteronuclear correlation (CP-HETCOR) SSNMR approaches that directly probed the proton and carbon spatial interactions within functional groups of Shengli (SL) brown coal char were tentatively demonstrated. Two distinct transformation intervals, as a function of pyrolysis temperature, were clarified in the SL brown coal char. The initial decomposition (at <300 °C) of coal, with slight change in either functional groups or microstructures, may be caused by the loss of adsorbed gaseous materials and partial scission of weak bonds (such as methylene and ether bridges). Moreover, the progressive dehydrogenation and splitting off of side-chains at 400-700 °C resulted in the drastically cracking of aliphatic groups. However, a significant amount of oxygen-containing functional groups were also found, such as heterocyclic, phenolic hydroxyl groups and crystalline hydrates. The thermal treatment was a crystallite-perfecting process, during which the fraction of the amorphous carbon in the coal chars, determined by Raman spectroscopy, was gradually decreased with increase in the heat-treatment temperature; however, the carbon crystallite size did not show any apparent change.