Thermodynamic and kinetic investigations of tannins using quantum chemistry

The minimum energy paths and transition states for the first two pyrolysis reactions of the tannin building blocks gallic acid and (+)-catechin were calculated by combining density functional theory with the climbing-image nudged elastic band method. For both investigated, the combined pyrolysis reaction was found to be endothermic across the full investigated temperature range and exergonic for temperatures of 1000 K and above when evaluated with the quantum chemical 'gold standard' approach CCSD(T). In the case of gallic acid, the dehydrogenation of pyrogallol was identified as the rate-determining pyrolysis step, whereas the catechol split-off was determined to be the rate-determining step of (+)-catechin pyrolysis. Additionally, simulated Raman spectra were able to explain the presence of subtle shoulder peaks in the spectrum of the binder Carbores®P. Another series of spectra assisted the identification of an ellagic acid pyrolysis product.