AlMePO-b: inclusion and thermal removal of structure directing agent and the topotactic reconstructive transformation to its polymorph AlMePO-a

The microporous aluminium methylphosphonate-beta (AlMePO-beta), prepared in the presence of 1,4-dioxane with the composition per formula unit of Al-2(PO3CH3)(3) . 0.25C(4)H(8)O(2), loses dioxane molecules in two steps upon heating in an inert gas. Further heating in the presence of water vapour results in topotactic reconstructive transformation into the closely related a polymorph; under water vapour partial pressures of ca. 25 Torr, the transformation proceeds close to completion at temperatures in excess of 460 degrees C. Calculation of the lattice energies of the two AlMePO polymorphs shows the a form to be more stable by 12.6 kJ mol(-1) per Al-2(PO3CH3)(3) formula unit, and confirms the thermodynamic reason for the transformation. Close comparison of the structures reveals that the main difference lies in the position of Al(4) in the beta structure, and a mechanistic pathway is suggested and illustrated by which the aluminium migrates following Al-O-P bond hydrolysis and subsequent bond remaking to give rise to the alpha polymorph. The branching nature of the propagation step of the migration, which also results in methylphosphonate group rotations, explains how transformation might occur rapidly in a concerted fashion. A role for the water in catalysing the transformation is proposed.