Reaction mechanisms and hydrogen production in the thermal decomposition of simple carboxylic acids in O2/H2O environments
Yang Yu, Reo Kai, Hiroaki Watanabe
Abstract
Thermochemical hydrogen production from carboxylic acids plays a significant role in the utilization of biofuels. We employed ReaxFF molecular dynamics simulations to investigate the pyrolysis mechanisms of formic acid and acetic acid in O 2 /H 2 O environments. There are two reaction channels in formic acid pyrolysis: the dehydration reaction (HCOOH→H 2 O+CO) and the decarboxylation reaction (HCOOH→H 2 +CO 2 ), with the dehydration reaction predominating, where the major pathway is HCOOH→CHO→CO. For the pyrolysis of acetic acid, the primary pathway involves the sequential steps: CH 3 COOH→CH 3 →CH 3 OH→HCHO→CO. The initial major reactions are CH 3 COOH→CH 3 CO+OH, CH 3 CO→CH 3 +CO, and CH 3 +OH→CH 3 OH, followed by successive dehydrogenation reactions of CH 3 OH to form CO. During oxidation of formic acid, as the oxygen content increases, the production of H 2 and CO decreases, while the production of H 2 O and CO 2 increases. The water-catalyzed pyrolysis generates the most H 2 by inhibiting dehydration and enhancing decarboxylation, with elevated temperatures further increasing the yield. H 2 formation occurs through H-abstraction reactions on acids, H 2 O, and intermediate products by H radicals. Constructing reaction kinetic models for these processes. The decomposition activation energies are in good agreement with experimental data reported in the previous literature. The carboxyl group plays a more predominant role than the methyl group in the initial pyrolysis process. • Thermal decomposition mechanisms of formic and acetic acids were clarified. • Effects of oxidation and hydrolysis on formic acid decomposition were explored. • Decomposition reaction kinetic models were proposed, and the validity of the simulations was verified.