Hydrothermal Synthesis and Characterization of Sodium Manganese Oxo-Phosphate Na2Mn2O(PO4)2·H2O

Abstract

Sodium manganese oxo-phosphate Na2Mn2O(PO4)2·H2O (MPOS-3) has been prepared in single-crystal and polycrystalline forms by hydrothermal methods. The crystal structure was determined from a 15 × 15 × 80 μm crystal with synchrotron radiation. In the structure, zigzag edge-sharing MnO6 octahedral chains are connected by PO4 tetrahedra to form two-dimensional planes. A second PO43- group sticks out of the planes by sharing three apexes with three MnO6 octahedra of a zigzag chain. The fourth apex of this PO43- groups interacts strongly with sodium ions that are located between the two-dimensional planes. The sodium ions are exchangeable. Ion exchanging half of the Na+ with Ca2+ expands the surface area from 17 to 45 m2/g. MPOS-3 loses water at about 220 °C. The dehydrated form is stable up to 550 °C, above which O2 is liberated. The FTIR spectrum shows two distinct −OH stretching vibration bands, one at 3627 cm-1 and another at 3437 cm-1. MPOS-3 also demonstrates catalytic activity in oxidation of 2-propanol by air to produce acetone. Carbon dioxide is the only byproduct. Preliminary results show 27% conversion of 2-propanol at 450 °C with 65% selectivity toward acetone.