Formation and thermal decomposition of pyridine intercalates of α- and γ-zirconium phosphates

The intercalation of pyridine into α-ZrP takes place with an accompanying uptake of one more mole of water to form a dihydrate Zr(HPO₄)_1.55(C₅H₅NHPO₄)_0.45 · 2 H₂O which has an interlayer spacing of 10.9 Å and contains two types of water with different thermostabilities. The pyridine intercalate of γ-ZrP with 12.3 Å spacing is formed by the replacement of interlayer water by the gest molecules without any appreciable change in interlayer spacing and its composition is Zr(HPO₄)_1.56 (C₅H₅NHPO₄)_0.44 · 0.7 H₂O. The α-intercalate undergoes dehydration at below 140°C, accompanied by its separation into a pyridine-free phase (ζ-ZrP) and a pyridine enriched one, and this dehydration is immediately followed by the desorption of pyridine, resulting in the overall conversion of the initial phase to ζ-ZrP. The γ-intercalate, on the other hand, releases its water without any phase separation at similar temperatures but its depyridination temperature at 250°C is about 110°C higher than that for the other. A molecular packing model is proposed to explain the interlayer spacings, compositions, and thermal decomposition properties of both intercalates.