Thermal decomposition mechanism of piroxicam

Piroxicam (PRX) is a nonsteroidal anti-inflammatory drug. The thermal decomposition process of PRX was investigated with thermogravimetry and differential scanning calorimetry. The gaseous products generated by thermal decomposition were characterized with thermogravimetric analysis coupled with Fourier transform infrared spectroscopy. The residues of the thermal decomposition at various temperatures were identified with infrared spectroscopy. The molecular bond orders were calculated using an ab initio method from the GAMESS program of quantum chemistry. The mechanism of thermal decomposition for PRX was discussed. The results indicated that the thermal decomposition of PRX is a two-stage process with the initial temperature of 198 °C either in nitrogen or air atmospheres. The thermal decompositions of the first stage in two atmospheres are the same process. The main part of the molecule, including sulfamide, amide, benzene ring and pyridine ring, decompose simultaneously and to form gasifiable small molecules and carbonaceous residue in the first stage. The second stage in nitrogen is a slow thermal pyrolysis process of carbonaceous residue. The forepart of the second stage in air is a slow thermal pyrolysis process as like as in nitrogen, and the later period of the second stage is an oxidation (combustion) reaction process of carbonaceous residue. PRX is stable under ambient temperature and air atmosphere, and it can be preserved for long-term storage under ambient temperature and in air atmosphere.