Synthesis and characterization of magnesium aluminum phosphates, and related compounds

Abstract

Magnesium aluminophosphate molecular sieves of AlPO$sb4$-5 structure have been synthesized with varying Mg/(Mg + Al + P) mole ratios using triethylamine as an organic template. The synthesized products were characterized by XRD, SEM, TGA, solid state NMR techniques and other analytical methods. The results indicated that highly crystalline MAPO-5 material can be obtained. With starting reaction gel composition containing higher than 30 mole % of Mg addition, new unknown crystalline phases began to appear and one of them becomes the major phase at 70 mole % of Mg addition. At 80 and 100 mole % of Mg addition, other new crystalline phases were formed. The substitution of Mg into the AlPO$sb4$-5 framework was demonstrated by the variation in the unit cell volume with Mg mole fraction, chemical analysis and $sp{31}$P NMR. The pyridine adsorbed IR spectra of MAPO-5 showed the presence of Lewis and Bronsted acid sites while that of AlPO$sb4$-5 showed only Lewis acid sites. Propene was detected as the main product in the isopropanol decomposition reaction for MAPO-5 samples. The 10 and 20 mole% of nominally Mg substitution samples have higher catalytic activity than AlPO$sb4$-5, presumably due to the presence of Bronsted acid sites created by the Mg$sp{2+}$ isomorphous substitution.