CRYSTALLINE MICROPOROUS MAPO-36 MOLECULAR-SIEVE - SYNTHESIS AND CHARACTERIZATION

The synthesis of MAPO-36 molecular sieve was studied using different gel compositions and hydrothermal crystallization conditions. MAPO-36 is a large-pore molecular sieve which has a pore opening of 7.4 by 6.5 Å. Highly crystalline MAPO-36 molecular sieve was obtained by hydrothermal crystallization of the gel (molar composition: 1.8 Pr3N-0.17 MgO-0.92 Al2O3-1.0 P2O5-40 H20) initially at 378 K for 50 h and finally at 423 K for 24 h. Increased concentration of organic template (tripropylamine) and metal (Mg) produced a decrease in the crystallinity of MAPO-36. Co-phase formation was observed in one particular case for decreased metal concentration. The N2-, H2O-, and n-hexane sorption capacity of MAPO-36 is higher than AlPO4-5, which is due to the presence of the annular side-pockets in the crystal structure. SEM investigations of MAPO-36 crystals showed two different types of morphology. Thermal analysis of Pr3N-MAPO-36 has been studied in the presence of inert and oxidizing atmospheres in the temperature range 303-1273 K. The removal of tripropylamine occluded in the channels of the material occurs in a number of steps and is strongly influenced by the gas atmospheres. XPS analysis indicated that the concentration of magnesium is higher on the surface than in the bulk of MAPO-36, and magnesium is tetrahedrally coordinated into the aluminophosphate framework. In situ IR spectroscopic investigation of chemisorbed pyridine on MAPO-36 revealed the presence of Brønsted and Lewis acid sites. Temperature-programmed desorption (TPD) of pyridine suggests that there exists a site energy distribution (which contains Lewis and Brønsted acid sites). By using GC techniques, TPD of pyridine at different initial sorbate loadings and stepwise thermal desorplion (STD) of the base from 323 to 673 K were investigated. TPD of pyridine over MAPO-36 indicated the presence of two types of acid sites (with the majority being strong acid sites) and a significantly stronger acidity than AlPO4-5, SAPO-5, and MAPO-5. © 1993 Academic Press, Inc.