Abstract
After crystallization during ionothermal syntheses in phospho-nium-containing ionic liquids, the structure of (NH4)3Al2(PO4)3 [tri-ammonium dialuminum tris-(phosphate)] was refined on the basis of powder X-ray diffraction data from a synchrotron source. (NH4)3Al2(PO4)3 is a member of the structural family with formula A 3Al2(PO4)3, where A is a group 1 element, and of which the NH4, K, and Rb forms were previously known. The NH4 form is isostructural with the K form, and was previously solved from single-crystal X-ray data when the material (SIZ-2) crystallized from a choline-containing eutectic mixture [Cooper et al. (2004 ▸). Nature, 430, 1012-1017]. Our independent refinement incorporates NH4 groups and shows that these NH4 groups are hydrogen bonded to framework O atoms present in rings containing 12 T sites in a channel along the c-axis direction. We describe structural details of (NH4)3Al2(PO4)3 and discuss differences with respect to isostructural forms.
Highlights
Following the discovery of the microporous AlPO4-n series of materials (Wilson et al, 1982), many efforts have been directed toward the synthesis of novel phases utilizing traditional hydrothermal (Wilson, 2007; Yu & Xu, 2006) and solvothermal syntheses (Das et al, 2012)
Ionothermal synthesis has been added to the stable of synthetic methods
Ionothermal synthesis is an extension of the solvothermal method of synthesis using an ionic liquid as the solvent where a portion of the organic structure-directing agent from a typical zeolite synthesis is derived from the ionic liquid (Morris, 2009)
Summary
Following the discovery of the microporous AlPO4-n series of materials (Wilson et al, 1982), many efforts have been directed toward the synthesis of novel phases utilizing traditional hydrothermal (Wilson, 2007; Yu & Xu, 2006) and solvothermal syntheses (Das et al, 2012). Ionothermal synthesis is an extension of the solvothermal method of synthesis using an ionic liquid as the solvent (replacing, for example, water or ethylene glycol) where a portion of the organic structure-directing agent from a typical zeolite synthesis is derived from the ionic liquid (Morris, 2009). Excess water often leads to synthesis of dense AlPO4 phases such as the one with a tridymite-type of structure, which we observed as well during syntheses utilizing 85%wt H3PO4. Ammonium is a good structure-directing agent for aluminophosphate frameworks; multiple ammonium aluminum phosphates are known (Byrne et al, 2009; Vaughan et al, 2012). Low-water-content syntheses clearly favor 2:3 compounds as most of the known materials are synthesized from low-water-content preparations
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