A Microporous Zr-Tetraphosphonate MOF with Permanent Porosity, Robustness, and Thermochemical Heat Storage Capability

Abstract

Combining high-valent metal cations with phosphonate linkers represents a promising strategy to address the limited stability of MOFs, yet permanently porous phosphonate MOFs (P-MOFs) remain rare due to the strong coordination propensity of phosphonate groups. Herein, we report IEF-10, a new microporous zirconium tetraphosphonate framework constructed from ZrO5F octahedral secondary building units and a pyrene tetraphosphonate linker. The material combines three features that are seldom achieved simultaneously in P-MOFs: permanent accessible microporosity (equivalent BET surface area ∼430 m2·g–1), accessible phosphonic acid functionalities (-PO3H), and exceptional thermal (up to 450 °C), mechanical (>3 tons), and chemical stability (pH 1–12, organic solvents). The structure exhibits reversible water adsorption with moderate uptake and experimentally determined capacities of heat storage of ∼400–500 J·g–1, reflecting a favorable balance between host–guest interactions and regeneration requirements. As a proof of concept, IEF-10 was evaluated for thermochemical heat storage under dynamic moist-gas flow conditions. Calorimetric measurements revealed fast heat-release kinetics (3–4 h), and excellent cyclability in terms of heat release (at least 10 cycles, total test duration of 5 weeks). The hydrophobic surface character is an advantage for long-term storage uses as the adsorbent can remain in a dry state between the charging and discharging stages even at low relative humidity levels in the reactor. These results demonstrate how the combination of permanent porosity, accessible phosphonic acid groups, and exceptional framework robustness can enable functional adsorption-driven energy storage while providing new opportunities for the development of porous zirconium P-MOFs.