Synthesis and catalytic properties of MAPO-36 molecular sieve

Abstract

The effects of the conditions of hydrothermal synthesis and the composition of the starting gels on MAPO-36 synthesis have been examined. Pure MAPO-36 was obtained from the gel with Mg/P = 0.08–0.25 after hydrothermal synthesis at 373 K for 44 h and then at 423 K for 22 h.

The infrared spectrum of adsorbed pyridine confirmed that MAPO-36 has Brønsted acid sites in conformity with the expected substitution of Mg²⁺ for Al³⁺ in the framework. The strength of the acid sites on MAPO-36, as estimated by temperature-programmed desorption of ammonia, is as high as that of the sites on H-ZSM-5, though the number is smaller.

The order of the catalytic activities for dehydration of ethanol and cracking of butane and hexane is as follows: H-ZSM-5 > MAPO-36 HY. The higher activity of MAPO-36 compared to that of HY may be caused by the greater strength of acid sites, as indicated by the temperature-programmed desorption of ammonia.

The cracking of isobutane and 3-methylpentane is in the decreasing order: MAPO-36 HY > H-ZSM-5. This order is a result not only of the acidic nature of the catalysts, but also of the pore sizes. The lower activity of H-ZSM-5, in spite of the stronger acid sites, can be explained in terms of spatial constraints on the reactions. On the other hand, the high activity of MAPO-36 confirms that this material is a large-pore molecular sieve.