Thermal Stability, Acidity, Catalytic Properties, and Deactivation Behavior of SAPO-5 Catalysts: Effect of Silicon Content, Acid Treatment, and Na Exchange

Abstract

Abstract Crystalline microporous SAPO-5 molecular sieves with different silicon content, acid-treated SAPO-5 and Na-exchanged SAPO-5 were investigated far their thermal stability, and acidic and catalytic properties. SAPO-5 materials with increasing Si framework content exhibited lower thermal stability. The effects of the thermal treatment and Na exchange on the N 2 -sorption capacity (at 78 K) of these materials were studied. In situ IR spectroscopic investigations of pyridine chemisorbed on the aluminophosphate catalysts revealed that the concentration of Bronsted and Lewis acid sites are strongly affected by the Si content in the AlPO 4 framework, acid treatment, and Na exchange. The results of temperature programmed desorption (TPD) and stepwise thermal desorption of pyridine suggest that there exists a broad site energy distribution over the aluminophosphate catalysts. The number of strong acid sites on the aluminophosphate catalysts increases with the increasing Si content in the AlPO 4 framework. The acid treatment and Na exchange showed a decrease in the number of strong acid sites on SAPO-5. The TPD of pyridine over SAPO-5, acid-treated SAPO-5, and Na-exchanged SAPO-5 indicated the presence of two types of acid sites. Correlation between the number of strong acid sites (measured in terms of the chemisorption of pyridine at 673 K) and framework charge on the aluminophosphate catalysts has also been obtained. The catalytic activities of SAPO-5 catalysts in the ethanol, n -hexane, isooctane, toluene, and o -xylene conversion reactions were studied. The catalytic activity of the aluminophosphate increases with the Si content in the AlPO 4 framework. The dependence of cumene cracking activity of the aluminophosphate on time-on-stream show that the deactivation of SAPO-5 with higher Si content is faster. A sharp increase in the deactivation rate of SAPO-5 was observed after the Na exchange.