PKU-5: An aluminoborate with novel octahedral framework topology

A new anhydrous aluminoborate Al2B6O15 (PKU-5) has been synthesized in a boric acid flux in a closed system at 350degreesC. PKU-5, which crystallizes in the space group R3 with the lattice constants a=11.43398(9) and c=6.48307(5) Angstrom, consists of Al octahedra and triangularly coordinated boron. The Al octahedron adopts the (10,3)-a network, in which each octahedron shares three edges with the neighboring octahedra forming ten-membered-ring channels. The octahedral backbone in PKU-5 can be considered as a primary octahedral framework topology and, setting out from the structures of the aluminoborates (PKU-1 and PKU-5), we propose construction rules for the octahedral frameworks. There are two types of connections for edge-sharing octahedra in porous frameworks, trans and cis type, by which various microporous octahedral frameworks of different topologies can be constructed. The borate groups share oxygens with the Al octahedral frameworks forming two kinds of three-membered-ring units consisting of two octahedra and one triangle (2Al+B) and one octahedron and two triangles (Al+2B), respectively.