Layered aluminophosphates II. Crystal structure and thermal behaviour of the layered aluminophosphate UiO-15 and its high temperature variants

Abstract

The synthesis and crystal structure of a novel layered aluminophosphate and its two high temperature variants are described. All three structures were solved from powder X-ray diffraction data. The as-synthesized material (UiO-15-as), with composition [NH ₃ (CH ₂ ) ₂ NH ₃ ] ²⁺ [Al ₂ (OH) ₂ (PO ₄ ) ₂ (H ₂ O)] ^2– ·H ₂ O, crystallizes in the space group P1 with a=10.37490(13), b=6.60775(8), c=9.90937(11) Å, α=90.762(1), β=115.265(1) and γ=90.162(1)°. In the aluminophosphate layers the five- and six-coordinated aluminium polyhedra form infinite chains along [010] that are crosslinked by phosphate groups. These layers are held together by a complex hydrogen bonding scheme involving the terminal oxygen atoms of the phosphate groups and the nitrogens of the ethylenediammonium ions. A high temperature variant exists around 125 °C (UiO-15-125). This compound, with composition [NH ₃ (CH ₂ ) ₂ NH ₃ ] ²⁺ [Al ₂ (OH) ₂ (PO ₄ ) ₂ ] ^2– , crystallizes in the monoclinic space group P2 ₁ /c with a=10.28899(23), b=6.75080(13), c=9.62527(20) Å and β=116.124(1)°. The transformation to UiO-15-125 involves removal of an interlamellar water molecule and another water molecule which is coordinated to aluminium. The infinite chains of aluminium polyhedra along [010] and the hydrogen bonding scheme of UiO-15-as is maintained in UiO-15-125. Another high temperature variant exists around 225 °C (UiO-15-225). This compound, with composition [NH ₃ (CH ₂ ) ₂ NH ₃ ] ²⁺ [Al ₂ O(PO ₄ ) ₂ ] ^2– , crystallizes in the monoclinic space group P2 ₁ /c with a=9.42781(37), b=6.91370(19), c=9.40823(27) Å and β=113.002(1)°. The transformation to UiO-15-225 involves the release of a water molecule from UiO-15-125 and the formation of tetrahedral Al-O-Al bonding. UiO-15-225 is therefore the first compound among the aluminophosphates violating Loewensteins rule.