Dehydration mechanism of AlPO4-5: A high-resolution synchrotron X-ray powder diffraction study

We present the results of a study on the thermal stability and dehydration dynamics of the porous alumino-
phosphate AlPO4-5, from room temperature to 850 °C, carried out by in situ synchrotron X-ray powder dif-
fraction experiments. No phase transitions were observed in the investigated T range. Due to the very low
interactions among framework and water molecules, dehydration is already complete at 152 °C. It occurs in two
main T ranges, one between 60 and 80 °C and the second one between 90 and 120 °C. During these two stages,
the cell volume tends to increase to facilitate the water release. Overall, across the full explored T range, the cell
volume increase is only 0.84%, indicating that AlPO4-5 is one of the most rigid zeolite frameworks studied to
date. Specifically, after complete dehydration at 200 °C, AlPO4-5 undergoes a negative thermal expansion as a
consequence of the decrease in both a and c cell parameters. The expansion coefficients for V, a and c cell
parameters, in the 152–850 °C range, are αV = −7.12 (10−6/K), αa = −2.5 (10−6/K), αc = −2.2 (10−6/K),
respectively.