Thermicity of the Decomposition of Oxygen Functional Groups on Cellulose-Derived Chars

Christin Pflieger; Till Eckhard; Gunnar Schmitz; Vanessa Angenent; Maximilian Göckeler; Osvalda Senneca; Rochus Schmid; Francesca Cerciello; Martin Muhler

doi:10.1021/acsomega.2c07429

Thermicity of the Decomposition of Oxygen Functional Groups on Cellulose-Derived Chars

ACS Omega. 2022 Dec 15;7(51):48606-48614. doi: 10.1021/acsomega.2c07429. eCollection 2022 Dec 27.

Authors

Christin Pflieger¹, Till Eckhard¹, Gunnar Schmitz², Vanessa Angenent², Maximilian Göckeler¹, Osvalda Senneca³, Rochus Schmid², Francesca Cerciello¹, Martin Muhler¹

Affiliations

¹ Laboratory of Industrial Chemistry, Ruhr University Bochum, 44801Bochum, Germany.
² Computational Materials Chemistry Group, Ruhr University Bochum, 44801Bochum, Germany.
³ Istituto di Scienze e Tecnologie per l'Energia e la Mobilità Sostenibili, Consiglio Nazionale delle Ricerche, 80125Napoli, Italy.

Abstract

The evolution of oxygen functional groups (OFGs) and the associated thermic effects upon heat treatment up to 800 °C were investigated experimentally as well as by theoretical calculations. A synthetic carbon with a carbonaceous structure close to that of natural chars, yet mineral-free, was derived from cellulose and oxidized by HNO₃ vapor at different temperatures and for varied durations in order to generate char samples with different concentrations and distributions of OFGs. The functionalized samples were subjected to calorimetric temperature-programmed desorption measurements in correlation with an extensive effluent gas analysis, thereby focusing on the specific heat effects of individual OFG evolution. Interpretation of the experimental results was aided by density functional theory (DFT) calculations which allowed one to infer the thermal stability of different OFGs and the reaction energy associated with their evolution upon heating. Results showed that, with increasing temperature, H₂O was released due to the loss of physisorbed water, the decomposition of clusters bound to carboxylic acids, and condensation reactions. The associated heat uptake amounted to about 100 kJ mol^-1. Contrarily, the release of CO₂, attributed to the decomposition of condensed acids, carboxylic acids, anhydrides, and lactones, resulted in a heat release of about 40 kJ mol^-1. The most strongly pronounced thermic effects were detected for the release of CO, comprising highly exothermic effects due to the decomposition of condensed acids and carbonyls/quinones as well as endothermic effects attributed to anhydrides and phenols/ethers. Notably, anhydrides can be formed during the oxidative treatment as well as during heating by condensation of adjacent carboxylic acids. In the latter case, the two-step decomposition is overall highly exothermic, indicating the associated occurrence of pronounced carbon matrix rearrangements. DFT investigations suggest that these rearrangements not only affect the immediate OFG proximity but also involve several carbon sheets.