Synthesis of diketopiperazine: A kinetic study by means of thermoanalytical methods

The enalapril maleate (EM) decomposition to diketopiperazine was monitored in helium flow by thermogravimetry at different heating rates between 0.2 and 5 °C min−1. The activation energy value was obtained (198 kJ mol−1) from the Kissinger–Akahira–Sunose isoconversional method. Isothermal experiments were simulated (125, 130, 135 and 155 °C) using the model-free method, employing only the activation energy value. The reaction model of the process was studied by means of the master-plot method. The reaction mechanism depends on the temperature; for experiments below 235 °C, the probable mechanism is nucleation. Instead, when the temperature overcomes the EM melting point, the reaction occurs in liquid state and its kinetics is better described by first order reaction. To avoid the complexity of model changes, experimental results are fitted using the model-free method.

► The enalapril maleate decomposition to diketopiperazine was studied.
► We used thermogravimetry at different heating rates.
► The reaction mechanism depends on the temperature.