Metal-organic frameworks (MOFs) are advanced highly porous coordination polymers, which attract significant interest due to unusual properties. Aluminum MOFs (Al-MOFs) feature structural versatility and stability in air, water, organic solvents, corrosive gases and liquids, as well as ultrahigh sorption capacity and selectivity. This translates to research and applications in separation, sensing, purification, removal of toxic gases, catalysis, photocatalysis, biomedicine. Porphyrin Al-MOFs are unusual and quite interesting materials with ultra large nanopores and significant potential for interactions with various molecules of interest including water. The Al-MOF-TCPPH2 (compound 1) with porphyrin linker has been synthesized, purified and activated to obtain Al-MOF compound 2. Their purity is demonstrated by ATR-FTIR spectroscopy, and assignments of specific functional groups in the TCPP linker are provided as potential binding sites for adsorbates. Compound 2 reversibly sorbs and desorbs significant amount of water (up to 50 wt. %) from humidified air and forms the hydrated Al-MOF compound 3. Compound 3 contains as many as up to 25 water molecules bonded to several distinct groups in the framework. In compound 3, water exists as small ice-like nanoclusters, in which water molecules are three-coordinated to other water molecules. The fourth bond is coordination of the surface water molecules d-H via their H atoms to functional groups in the MOF. Namely, water is bonded to the framework via the N-H group, the carboxylate anion, phenyl group of the linker, and the μ-OH group. Compound 3 facilely loses most of its water to air at moderate ambient relative humidity of 45 %. During water loss, small ice-like nanoclusters with d-H waters are transformed to those with four-coordinated waters denoted s-4. Chemical kinetics of water loss (desorption) has been investigated by in-situ time-dependent gravimetric method under ambient conditions. Kinetics of reaction follows the pseudo-first order rate law. The combination of in-situ time-dependent ATR-FTIR spectroscopy and gravimetry forms a new unique experimental approach with significant potential. It can be facilely utilized in studies of mechanisms of heterogeneous reactions, determination of stoichiometry of complexes of sorbents with adsorbate, and studies of kinetics of "solid-gas" reactions under ambient and controlled environments.

Porphyrin aluminum MOF with ultra-high water sorption capacity: Understanding mechanism of bonding and desorption by in-situ time-dependent ATR-FTIR spectroscopy, gravimetry and XRD

ACS Spring 2022

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