Comparison of thermal stability, acidity, catalytic properties and deactivation behaviour of novel aluminophosphate-based molecular sieves of type 36

Abstract

MnAPO-36, ZAPO-36, CoAPO-36, MAPO-36 and MAPSO-36 with similar molar chemical composition have been prepared and characterized. Magnesium-incorporated aluminophosphate of type 36 showed higher thermal stability than the manganese- zinc- or cobalt-incorporated aluminophosphates. In situ IR acidity measurements of the type 36 aluminophasphates show the presence of Brønsted and Lewis acid sites. In the mono-substituted aluminophosphates, the concentration of Brønsted acid sites and the ratio of Brønsted to Lewis acid sites is higher on MAPO-36 than on MnAPO-36, ZAPO-36 and CoAPO-36. Brønsted acid site density is higher on MAPSO-36 (di-substituted aluminophosphate) than on MAPO-36. The results of stepwise thermal desorption and temperature-programmed desorption of pyridine indicate that the number of strong acid sites and acid strength distribution are strongly affected by the nature of the element substituted in the AlPO₄-36 framework. The MnAPO-36, ZAPO-36, CoAPO-36, MAPO-36 and MAPSO-36 possess significant catalytic activity in the cracking of aliphatic hydrocarbons, conversion of o-xylene and also in the reaction of ethanol to form aromatics at 673 K. In the ethanol, pentane, 3-methylpentane, cyclohexane, o-xylene and ethylbenzene conversion reactions, MAPSO-36 exhibited higher catalytic activity than MnAPO-36, ZAPO-36, CoAPO-36 and MAPO-36. The deactivation behaviour of these aluminophosphate molecular sieves in the ethylbenzene conversion reaction was also investigated. Results show that the type of element incorporated also strongly affects the acidic and catalytic properties of the aluminophosphates.