A comprehensive thermal study of the state of tetrapropylammonium (TPA) phase in ZSM-5

Abstract

A thermal investigation of the decomposition of tetrapropylammonium (TPA) has led to a better insight into its state in the zeolite ZSM-5. DTA has shown an exothermic transition at Tc= 753 K. TPA salts decompose normally with endothermic transitions at lower Tc. TPA decomposes to give a proton at the zeolite framework. The affinity of the anionic framework for the resultant proton is so great that this exothermic peak could be assigned to a Brønsted site = SiO-H bond formation. Various thermodynamic parameters consistently reveal that a high population of TPA leads to a disordered structure. In effect, a progressive loss of TPA modifies the structure into a more ordered phase, which finds supporting evidence from X-ray diffraction studies. The enthalpy, entropy and activation energy of the transition show dependence on the TPA content of the zeolite. Apart from the thermal decomposition, gamma-rays have been found to induce another mechanism of decomposition that could lead to lattice defects of Lewis acid nature. © 1992.