MAPO-43 molecular sieve: Synthesis, characterization and thermal stability

Abstract

A small-pore magnesium-containing aluminophosphate molecular sieve of type 43 (MAPO-43) with three-dimensional structure and monoclinic symmetry [unit cell composition: (Al 6.2Mg 1.8)P 8O 32·2.4NC 6H 16·0.83H 2O] was synthesized. MAPO-43 has gismondine topology. The synthesis was performed by hydrothermal crystallization from a gel of composition 2.35 di- n-propylamine·0.30 MgO·0.85 Al 2O 3·1.0 P 2O 5·50 H 2O at 423 K for 240 h. The crystallization product was characterized by XRD, SEM, TG/DTG/DTA, IR and XPS techniques. Effects of thermal treatment on the structural stability and physical properties ( viz. size and morphology of crystals, surface concentrations and binding energies of Mg 2p, Al 2p and P 2p) of MAPO-43 have been investigated. MAPO-43 shows a low thermal stability. The crystal structure collapse starts above 653 K and is complete above 823 K. At the higher thermal treatment temperature of 923 K, transformation of MAPO-43 phase into tridymite is observed. The surface analysis of MAPO-43 by XPS indicates that the concentration of magnesium is higher in the bulk than on the surface, and a modification of the surface composition occurs upon thermal treatment. The XPS analysis indicates that magnesium is tetrahedrally coordinated into the aluminophosphate framework, as the observed binding energy of Mg 2p, which is 50.3 eV, corresponds to that of tetrahedrally coordinated magnesium. Changes in the binding energies of Mg 2p, Al 2p, P 2p and O 1s are observed with the calcination temperature.