Synthesis and characterization of the open-framework magnesium aluminophosphate UiO-28

Abstract

The synthesis and crystal structures of a new magnesium aluminophosphate and its high temperature variant are described. The as-synthesized material (UiO-28-as), of composition MgAl3(PO4)4·C4N3H14·H2O, crystallizes in the orthorhombic space group Pbcm (no. 57) with a = 9.2769(8), b = 14.798(1), c = 14.611(1) A and V = 2005.8(3) A3. UiO-28-as has a two-dimensional eight-ring channel system, and is isostructural with a cobalt aluminophosphate (ACP-2) and a cobalt gallophosphate (GCP-2). Aluminium and magnesium are not uniformly distributed over the three non-equivalent metal framework sites. One site contains solely aluminium; one is Al-rich and the third is Mg-rich, leading to an overall Al/Mg ratio of 3. The magnesium-rich metal atom site is partly coordinated by five water molecules. These water molecules can be removed, leading to an anhydrous variant at 175°C (UiO-28-175). UiO-28-175, of composition MgAl3(PO4)4·C4N3H14, crystallizes in the orthorhombic space group Pbcm (no. 57) with a = 9.2186(4), b = 14.8652(4), c = 14.5811(4) A and V = 1998.1(2) A3. This material represents a new tetrahedral zeolite topology. The framework is built from an octameric secondary building unit (SBU) which, by vertex sharing, forms layers that are stacked along [100]. The layers are connected by means of covalent P–O–M bonds into a 3D framework. The same octameric SBUs have been identified in the ZON and AFR topologies, and in the gallium phosphate TMP-GaPO.