Title
Kinetics and mechanism of pyrolysis of larch azotannins
Authors
OLGA YU. FETISOVA, NADEZHDA M. MIKOVA, NATALYA YU. VASILYEVA, SVETLANA A. NOVIKOVA and ALEKSANDR S. KAZACHENKO

Received June 15, 2025
Published Volume 60 Issue 1-2 January-February
Keywords tannins, azotannins, thermal decomposition, model-free and model-based methods, activation energy, mechanism of thermal decomposition

Abstract
N-modified compounds were obtained on the basis of previously isolated tannins from larch (T). The estimated composition of the compounds obtained was analyzed by FTIR spectroscopy and elemental analysis. The thermal degradation of larch tannin and its nitrated compounds, such as nitrile azotannin (ATN) and cationic azotannin (CAT), has been studied using thermogravimetry at three heating rates of 5, 10 and 20°/min. Thermal stability of N-modified tannins depends on the nature of the introduced azogroups. The introduction of the azo group – N=N – into the aromatic structure of tannin increases its thermal stability. On the contrary, the presence of a quaternary ammonium group makes the structure of tannin decrease its thermal stability. The main stages, kinetics, and proposed mechanism of pyrolysis during the period of intensive release of volatile substances were determined. Kinetic analysis was performed using both model-free and model-based methods. The results of the study indicate the inaccuracy of model-free methods, in contrast to model-based methods, in relation to the description of the kinetics of thermal decomposition of tannin. The model based method of Coats-Redfern revealed that within the temperature range of 200-350 °C, the thermal decomposition of the initial tannin is constrained by a chemical reaction, likely of the second order (E = 118 kJ/mol). The thermal decomposition of the ATN sample at the second stage of thermolysis (260-340 °C) also takes place under conditions of a chemical reaction, most likely of the first order (E = 53 kJ/mol). For the CAT sample, it is challenging to select a decomposition model unambiguously within the temperature range of 280-340 °C. Models of chemical interaction, diffusion, and nucleus growth show similar high regression coefficients (R2 > 0.99).