Synthesis, characterization, thermal stability, acidity and catalytic properties of large-pore MAPO-46

Abstract

Large-pore MAPO-46 with three-dimensional pore structure and hexagonal symmetry has been synthesized. MAPO-46 has AFS topology. The effect of the organic template (di-n-propylamine) concentration in the gel on the crystallinity and phase purity of MAPO-46 was studied. The N₂, H₂O and n-hexane sorption capacity of MAPO-46 is higher than MAPO-36 and AIPO₄-5, which is consistent with the presence of 12- and 8-ring pore openings and a large structural void volume in the crystal structure. Morphological investigations, surface and thermal analysis of the material have been carried out. Effects of thermal treatment on the structural stability and N₂ sorption capacity at 78 K of MAPO-46 have also been investigated. MAPO-46 exhibits a lower thermal stability than MAPO-36 and AIPO₄-5. In situ IR spectroscopic investigations of chemisorbed pyridine on MAPO-46 revealed the presence of Brønsted and Lewis acid sites. The site-energy distribution on MAPO-46 was investigated by the temperature-programmed desorption (TPD) of pyridine and the stepwise-programmed desorption (SPD) of the base from 323 to 673 K. MAPO-46 possesses a stronger acidity than MAPO-5, SAPO-5 and AIPO₄-5. MAPO-46 possesses a significant catalytic activity in the cracking of aliphatic hydrocarbons, conversion of o-xylene, disproportionation of toluene and also conversion of ethanol to aromatics at 673 K. In ethanol, o-xylene, toluene and cumene conversion reactions, MAPO-46 exhibited higher catalytic activity than MAPO-5. The acidity and catalytic activity results both indicated that MAPO-46 has a lower acidity and catalytic activity than MAPO-36. The deactivation of MAPO-46 in the cumene cracking reaction was found to be slower than that of MAPO-36 and MAPO-5 in the same reaction.