DTA-GC-MS coupling for the characterization of the volatile products resulting from the decomposition of organic templates occluded in zeolites

Abstract

The volatile products resulting from the thermal degradation, in an inert atmosphere, of tetrapropylammonium cations, occurring as templates in MFI-type zeolites were analysed at first by means of a set-up coupling the DTA and MS techniques. Next, in order to make their identification easier, the various constituents of the gaseous mixture, evolving from the thermoanalyser, were separated through a chromatographie column before they entered the mass spectrometer. These combined techniques (DTA-GC-MS), revealed the presence of significant amounts of propene during the different stages of decomposition. This suggests for the initial decomposition mechanism a Hofmann elimination reaction followed by β-eliminations. In addition to propene and its oligomers, a variety of hydrocarbons (alkanes, alkenes, aromatics) are evolved. Their nature and proportions depend among others on the temperature and on the acidity of the zeolite. Thus, the main decomposition is accompanied by various secondary reactions due to the catalytic effect of these materials. This coupling of techniques appears as an effective tool to elucidate the complicated process of this type of degradation.