Tetrahedral aluminium was synthesized inside the TUD-1 structure with a low Si/Al ratio. The Al ion in its tetrahedral structure was stabilized by sodium as a counter cation. However, the tetrahedral aluminium species in the TUD-1 structure reverted to a mixture of hexa-, penta- and tetrahedral coordination sites after the exchange of sodium cations with protons. The Al-TUD-1 materials were found to be catalytically active for the acetal protection of benzaldehyde. (C) 2012 Elsevier B.V. All rights reserved.