Nanoporous Zirconium Phosphonate Materials with Enhanced Chemical and Thermal Stability for Sorbent Applications

Nanoporous zirconium phosphonate (ZrP) materials are considered to be promising candidates for practical applications such as catalysis and separation, in particular because of their excellent stability, resulting from the strength of the P–O–Zr bond. However, the functionality of ZrP materials is dependent on the availability of free phosphonate groups uncoordinated by zirconium, the presence of which can decrease the stability. The mechanisms by which nanoporous ZrP materials degrade and lose functionality during thermal and chemical treatment are not well understood. Herein, we address this knowledge gap using nanoporous zirconium aminotris(methylenephosphonic acid) (Zr-ATMP) sorbent materials. Thermal treatment up to 150 °C caused collapse of the nanoporous structure of some Zr-ATMP materials without a significant effect on the chemical structure. On the other hand, contact with 5 M nitric acid changed the chemical structure of the Zr-ATMP materials by catalyzing the formation of P–O–Zr bonds and elemental leaching. Enhancement of the thermal and chemical stability of the Zr-ATMP materials was achieved by decreasing the pH of the synthesis and, interestingly, changing the counterion of the hydroxide used to control the pH also impacted the structure and stability of the resulting materials. The most stable Zr-ATMP material was produced at pH 3 using LiOH, but this material demonstrated lower selectivity than other Zr-ATMP materials, which decreases its practicality for separation applications. The Zr-ATMP material synthesized at pH 3 with NaOH showed an optimal balance between the stability and sorption performance. The enhanced chemical and thermal stability of this material drastically improves its applicability for use in harsh environments, such as in the treatment of radioactive wastes.