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

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)P₈O₃₂·2.4NC₆H₁₆·0.83H₂O] 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 Al₂O₃·1.0 P₂O₅·50 H₂O 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.