Large-pore molecular sieve MnAPO-50: synthesis, single- crystal structure analysis and thermal stability

Abstract The large-pore molecular sieve MnAPO-50 of pure, greenish yellow crystals up to 100×100×300 μm was synthesized hydrothermally in the presence of di- n -propylamine. The product is thermally stable up to 250°C in air and up to 430°C in vacuum. It transforms into the dense phases Mn 2 P 2 O 7 and AlPO 4 -tridymite at 550°C in air and at 650°C in vacuum. The single-crystal X-ray structure analysis of the as-synthesized MnAPO-50 (trigonal, P3, a =12.746(1) A, c =9.140(1) A) served as direct evidence of the very high degree of incorporation of Mn into the tetrahedral framework. The presence of Mn and Al on both of the two non-phosphorus framework tetrahedral sites was revealed: the T sites on the general position and on the threefold axis were occupied by 86% of Al, 14% of Mn and 3% of Al, 97% of Mn, respectively. Thirty-five per cent of the non-phosphorus tetrahedral framework sites were thus occupied by Mn, which is in reasonable agreement with the formula (Mn 0.186(6) Al 0.309(9) P 0.504(2) )O 2 , calculated on the basis of the elemental analysis.