A theoretical investigation into gallic acid pyrolysis

Abstract

Thermodynamic and kinetic information on the first two steps of gallic acid pyrolysis, a decarboxylation followed by a dehydrogenation, is obtained based on density functional theory and quantum chemistry. For the kinetics, transition states are identified with the help of the climbing image nudged elastic band method. Both reactions

Investigating the Thermodynamics and Kinetics of Catechin Pyrolysis for Environmentally Friendly Binders

“…Furthermore, this reaction is calculated to be distinctly endothermic, which is another contrast. In total, the minimum energy path of the second reaction greatly resembles the path for the second reaction of gallic acid pyrolysis as described in Kraus et al, 19 which is expected given the fact that both reactions are dehydrogenations and catechol and pyrogallol only differ by the additional OH group of the latter.…”

“…The nature of both transition states is, again, in agreement with the results previously evaluated for gallic acid pyrolysis. 19 Finally, IV shows o -benzoquinone and H 2 as fully disconnected molecular structures.…”

“…The second reaction also featured a rotational transition state, in accordance with a similar reaction found in gallic acid pyrolysis. 19 In terms of enthalpies, the combined reaction was judged endothermic from 0 to 1250 K, yielding Δ r H°D LPNO-CCSD(T) in the range of 32.4 kcal mol −1 to 36.2 kcal mol −1 . Moreover, this combined reaction was predicted to change from endergonic to exergonic between 750 and 1000 K, with the Δ r G°D LPNO-CCSD(T) value at −13.9 kcal mol −1 for a temperature of 1250 K. Thus, an increase of temperature significantly improves the thermodynamic stability of the pyrolysis products compared to the reactants.…”

“…The thermodynamic quantities of interest, namely, the standard enthalpy of reaction Δ r H ° and the standard Gibbs energy of reaction Δ r G °, were determined on the optimized individual reactant and product molecules for the temperatures 0 K, 273 K, 298 K, 500 K, 750 K, 1000 K, and 1250 K, allowing for comparisons to the gallic acid data reported in Kraus et al 19 In order to gather information on kinetics for the aforementioned temperatures, standard Gibbs energies of activation Δ ‡ G ° and reaction rate constants k were calculated too. For the standard Gibbs energies of activation, the optimized highest-energy transition states from the CI-NEB calculations were evaluated with respect to the individual reactant molecules.…”

“…In this work, CI-NEB is applied in combination with Kohn–Sham density functional theory (DFT , ) to yield minimum energy paths and transition states, whereas the final energetics are evaluated using coupled cluster theory with single and double excitations as well as perturbative triple excitations (CCSD(T) − ), along with a variety of other wave function-based ansatzes and DFT methods. A previous study by Kraus et al on gallic acid, the building block of another class of tannins, the gallotannins, has already demonstrated the capability of CI-NEB calculations to gain insight into tannin pyrolysis, as it was able to trace the loss of carbon during this particular pyrolysis reaction, a phenomenon which is generally linked to the cold crushing strength of the resulting filter . To the best of our knowledge, the minimum energy paths corresponding to the two catechin pyrolysis reactions described above have not been investigated so far.…”