Porphyrin aluminum MOF with ultra-high water sorption capacity: In-situ time-dependent ATR-FTIR spectroscopy and gravimetry to study mechanism of water bonding and desorption

Abstract

Porphyrin-based aluminum metal-organic framework Al-MOF-TCPPH2 compound 1 is synthesized and activated to obtain compound 2. Its purity is demonstrated by ATR-FTIR spectroscopy, and spectral assignments of specific functional groups within the TCPP linker are provided as potential binding sites for adsorbate. Compound 2 reversibly sorbs and desorbs the significant amount of water (up to 50 wt. %) from humidified air and it forms the hydrated compound 3. Compound 3 contains as many as up to 25 water molecules bonded to several distinct groups in structural unit of Al-MOF-TCPPH2. In compound 3 water exists as small ice-like nanoclusters, in which water molecules are three-coordinated to other water molecules and the fourth bond is coordination of the surface water molecules d-H via their H atoms to functional groups in Al-MOF-TCPPH2. Further, water is bonded to the framework via the NH group, the carboxylate anion −COO−, 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 45 %. During water loss, small ice-like nanoclusters with d-H waters are progressively transformed to those with four-coordinated waters denoted s-4. Chemical kinetics of water loss (desorption) by compound 3 has been investigated by in-situ time-dependent gravimetric method under ambient conditions. Kinetics of reaction of water desorption follows the pseudo-first order rate law with rate constant. The in-situ time-dependent ATR-FTIR spectroscopy demonstrates the capability to facilely identify multiple sorption sites of adsorbate to the MOF. Moreover, the combination of in-situ time-dependent ATR-FTIR spectroscopy and gravimetry forms a new unique experimental approach with significant potential. This approach can be facilely and successfully utilized in studies of mechanisms of reactions of sorption and desorption, determination of stoichiometry of complexes of sorbents with adsorbate, and studies of chemical kinetics of various solid-gas reactions under ambient and controlled environments.