(344z) Hydrothermal Ageing Mechanism of SAPO-34 Studied By Kinetic Analysis of Temperature Programmed Desorption Data

SAPO-34 is a zeotype suitable for use in the methanol to olefins (MTO) process. This catalyst is known to deactivate when exposed to high levels of water at elevated temperature during the MTO reaction. Gaining a fundamental understanding of this hydrothermal ageing will allow the development of improved catalyst formulations.

Samples of SAPO-34 were hydrothermally aged at high temperature for varying times. The aged samples were characterized using ammonia temperature programmed desorption (TPD). TPD is a commonly used thermal analysis technique for investigating adsorption sites. These adsorption sites are important features in the performance of a catalyst and can be influenced by formulation and chemical structure.

Multiple overlapped peaks are common in TPD results and deconvolution of these is important for data analysis. Historically, the shapes of these peaks would be estimated before mechanistic modelling was carried out; introducing assumptions which can be a source of error. In the approach used in this study, no a priori assumptions are made about the peak shapes, rather a modified Sestak-Berggren model is used to fit the data directly.

This model is a numerical fit and the estimated exponent parameters (n and m) indicate which mechanisms could be plausible for the system under investigation. These are analysed, and model discrimination carried out, allowing the fitting of kinetic parameters. These parameters allow comparison between samples which have been aged for different times.

The Sestak-Berggren analysis of these data demonstrated that the adsorption energy values for each thermal event did not change significantly through hydrothermal aging, inferring there is no change in the nature or strength of the acid, adsorption sites. The change is in the pre-exponential factor, indicating clearly that it is the number rather than strength of the acid sites that reduces.