ACIDITY AND ACID STRENGTH DISTRIBUTION OF THE CRYSTALLINE MICROPOROUS MAPO-36 MOLECULAR-SIEVE

In situ i.r. acidity measurements of MAPO-36 show the presence of Bronsted and Lewis acid sites. The temperature-programmed desorption spectra of pyridine, obtained by the mass spectrometric technique, over MAPO-36, AlPO4-5, and SAPO-5 reveal the presence of the two types of acid sites. At a comparable concentration of the substituted element (Si or Mg) in the aluminophosphates of types 5 and 36, the concentration of Bronsted acid sites is significantly higher in MAPO-36. The results of temperature-programmed desorption and stepwise thermal desorption of pyridine (using g.c. techniques) indicates that MAPO-36 possesses a very high number of strong acid sites (i.e., 6, 7.5, and 16 times higher than that of MAPO-5, SAPO-5, and AlPO4-5 respectively) above 673 K and fewer weaker acid sites than the other aluminophosphates of type 5. The generation of negative framework charge, hydroxyl structure, Bronsted acidity, and the chemisorption of pyridine above 673 K indicate the substitution of Mg2+ for some of the Al3+ in the aluminophosphate framework of type 36.