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

The large-pore molecular sieve MnAPO-50 of pure, greenish yellow crystals u p to 100x100x300 mu m was synthesized hydrothermally in the presence of di- n-propylamine. The product is thermally stable up to 250 degrees C in air a nd up to 430 degrees C in vacuum, It transforms into the dense phases Mn2P2 O7 and AlPO4-tridymite at 550 degrees C in air and at 650 degrees C in vacu um. The single-crystal X-ray structure analysis of the as-synthesized MnAPO -50 (trigonal, P (3) over bar, a = 12.746(1) Angstrom, c = 9.140(1) Angstro m) 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 t wo 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 th e non-phosphorus tetrahedral framework sites were thus occupied by Mn, whic h is in reasonable agreement with the formula (Mn0.186(6)Al0.309(9)P0.504(2 ))O-2, calculated on the basis of the elemental analysis. (C) 2000 Elsevier Science B.V. All rights reserved.