Direct observation of grafting interlayer phosphate in Mg/Al layered double hydroxides

The grafting of interlayer phosphate in synthetic Mg/Al layered double hydroxides with interlayer hydrogen phosphate (LDH-HPO4) has been studied by XRD, TG/DTA, FT-IR, XPS and XANES. The basal spacing of crystalline LDH-HPO4 decreases in two stages with increasing temperature, from 1.06 nm to 0.82 nm at 333 K in the first transition, and to 0.722 nm at 453 K in the second. The first stage occurs due to the loss of interlayer water and rearrangement of the interlayer HPO42−. In the second transition, the interlayer phosphate is grafted to the layer by the formation of direct bonding to metal cations in the layer, accompanied by a change in polytype of the crystalline structure. The grafted phosphate becomes immobilized and cannot be removed by anion-exchange with 1-octanesulfonate. The LDH is amorphous at 743 K but decomposes to Mg3(PO4)2, AlPO4, MgO and MgAl2O4 after heated to 1273 K.