GAM-8: An organic-inorganic hybrid layered aluminium phosphate obtained by the transformation of AlPO4-5 with cyclohexylamine

Abstract

A hybrid layered aluminium phosphate material GAM-8 was synthesized by a structural change using the aluminophosphate AlPO₄-5 having an AFI-type zeolite structure. This synthesis condition is based on the interzeolite conversion of AlPO₄-5. Cyclohexylamine was used as the organic structure-directing agent (OSDA). It was found that a highly crystalline GAM-8 obtained was isostructural to Co(APSO)_N and its related compounds and UHM-5, whose structures are still unknown. The ab-initio crystal structure analysis by powder X-ray diffraction revealed that a 2D framework structure with a space group of C2/c and lattice constants of a = 37.46 Å, b = 5.32 Å, c = 9.70 Å, and β = 103.41°. Two aluminiumphosphate layers was included in a unit cell. A structural composition per unit cell was determined to be |(C₆H₁₁NH₂)₈|·[Al₈P₈O₂₄(OH)₁₆]. The layered framework was composed of one PO₄ tetrahedral site and two AlO₆ octahedral sites. All the AlO₆ octahedra were edge-shared and connected in a zigzag pattern along the c-axis. The ²⁷Al MAS NMR showed a large quadrupolar interaction of ²⁷Al nuclei. The cyclohexylamine molecules were orderly arranged between the layers, forming a bilayer, and its interlayer distance between the terminal oxygen atoms of the aluminium phosphate layers was considerable, ca. 11.7 Å. The NH₂ group of the OSDA was located close to O atoms of the layer surface with the interatomic distance d(N–O) of ca. 2.7–2.9 Å, suggesting the position of cyclohexylamine was stabilized by the hydrogen bonding of O···H–N.