Thermal decomposition and kinetic studies of solid riboflavin using model-free methods

The thermal stability and decomposition kinetics of riboflavin were investigated by nonisothermal thermogravimetric analysis (TGA) experiments in an inert atmosphere. For kinetic analysis, riboflavin was heated from room temperature to 800 °C with five different heating rates (5,10,15,20 and 30°C min_-1). From the thermal decomposition process, it was found that there are two main stages of pyrolysis. In the DTG thermograms, the temperature peaks at maximum weight loss rate changed with varying heating rate. The kinetic parameters of decomposition including apparent activation energy (E_a) and lnA (pre-exponential factor) under an inert atmosphere have been evaluated from the model-free isoconversion methods of Kissinger-Akahira-Sunose (KAS), Flynn-Wall-Ozawa (FWO) and Friedman over the whole temperature domain. It was found that values of E_a and lnA decrease with increasing conversion (α). The reaction order does not have a significant influence on the process because of the high value of the pre-exponential factor. The apparent activation energy distributions with conversion calculated by these methods ranged between 15.18 and 236.55 kJ mol_-1, and varied over a broad range in a complex manner depending on the heating rate.