Hygroscopic metal-organic framework MIL-160(Al): In-situ time-dependent ATR-FTIR and gravimetric study of mechanism and kinetics of water vapor sorption

Abstract

Metal-organic frameworks (MOFs) are advanced highly porous coordination polymers of high interest to separations, environmental remediation, catalysis, and biomedicine. While many MOFs are unstable in water and aqueous solutions, aluminum MOFs (Al-MOFs) offer an unprecedented stability. First, we synthesize unusual highly hygroscopic Al-MOF MIL-160(Al), purify it and assign FTIR peaks to specific groups as potential water binding sites. Further, we introduce a novel method of in-situ time-dependent ATR-FTIR spectroscopy to detect specific binding sites in MIL-160(Al) and investigate the progress of reaction. Specifically, we combine in-situ time-dependent ATR-FTIR spectroscopy with using water as “spectroscopic probe” to determine binding sites in MIL-160(Al) and their evolution during the reaction. The in-situ time-dependent ATR-FTIR spectra provide evidence of water bonding to: the μ-OH group, the carboxylate anion COO– in 2,5-FDCA2- linker, oxygen atom in the furan ring of the linker, and the C–C and C–H bonds of the furan ring of the linker. Then, we conduct mechanistic and kinetic study of sorption of water vapor on MIL-160(Al) in air using the combination of two complementary in-situ time-dependent methods: the ATR-FTIR spectroscopy and gravimetric analysis. Water vapor sorption on MIL-160(Al) results in the solid-state adsorption complex with up to four water molecules per unit of MIL-160(Al). Chemical kinetics of water sorption on MIL-160(Al) follows a pseudo-first order rate law and it is consistent with dynamics and timescale revealed by in-situ time-dependent ATR-FTIR. The combination of two in-situ time-dependent methods, the ATR-FTIR spectroscopy and gravimetry, forms a new powerful experimental approach to facilely study mechanisms, stoichiometry and chemical kinetics of various solid–gas reactions in the ambient and controlled environments.