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

Abstract

The microporous aluminium methylphosphonate-β (AlMePO-β), prepared in the presence of 1,4-dioxane with the composition per formula unit of Al₂(PO₃CH₃)₃ 0.25C₄H₈O₂ , 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 α polymorph; under water vapour partial pressures of ca. 25 Torr, the transformation proceeds close to completion at temperatures in excess of 460 °C. Calculation of the lattice energies of the two AlMePO polymorphs shows the α form to be more stable by 12.6 kJ mol^–1 per Al₂(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 β 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 α 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.