Crystal Structures of Related Novel Aluminophosphate Frameworks: Aipo₄-21(py), Aipo₄-EN3 (en) and a Structural Model for Alpo₄-25

AIPO₄-EN3 is a new aluminophosphate framework synthesised hydrothermally at 200°C using ethylenediamine (en) as a templating agent. The en molecule is incorporated into the structure (P2₁2₁2₁, a = 10.292(2), b = 13.636(2), c = 17.344(3)Å) positioned at the intersection of two 8-ring channels in the (001) plane at about z = 1/4 and 3/4. C-N bonds are oriented along channels in the [100] and [010] directions. The presence of 5-coordinate aluminium polyhedra and 8-ring channels relates the new framework to that of AIPO₄-21 [P2₁/n, a = 8.668(1), b = 17.558(2), c = 9.186(2)Å, β = 107.75(1)°] which can be synthesised using pyrrolidine as a template. In both structures AIV is separated, with pIV, into corrugated sheets of 3-, 4-, 5- and 8-rings, which are linked via chains containing Al and P tetrahedra. In the case of AIPO₄-EN these chains are a zig-zag while they are crankshaft-shaped for AIPO₄-21. Removal of the protonated oxygen forming the edge between 3- and 5- rings suggests a structural model for AIPO₄-25 obtained by calcination of AIPO₄-21 at 500°C. It has essentially the same framework (a = 8.45, b = 18.1, c = 15.24Å) as AIPO₄-21 but with Al and P strictly alternating in 4-,6-, and 8- rings.