Kinetic Water Stability of an Isostructural Family of Zinc-Based Pillared Metal–Organic Frameworks

Abstract

The rational design of metal-organic frameworks (MOFs) with structural stability in the presence of humid conditions is critical to the commercialization of this class of materials. However, the systematic water stability studies required to develop design criteria for the construction of water-stable MOFs are still scarce. In this work, we show that by varying the functional groups on the 1,4-benzenedicarboxylic acid (BDC) linker of DMOF [Zn(BDC)(DABCO)(0.5)], we can systematically tune the kinetic water stability of this isostructural, pillared family of MOFs. To illustrate this concept, we have performed water adsorption studies on four novel, methyl-functionalized DMOF variations along with a number of already reported functionalized analogues containing polar (fluorine) and nonpolar (methyl) functional groups on the BDC ligand. These results are distinctly different from previous reports where the apparent water stability is improved through the inclusion of functional groups such as -CH(3), -C(2)H(5), and -CF(3) which only serve to prevent significant amounts of water from adsorbing into the pores. In this study, we present the first demonstration of tuning the inherent kinetic stability of MOF structures in the presence of large amounts of adsorbed water. Notably, we demonstrate that while the parent DMOF structure is unstable, the DMOF variation containing the tetramethyl BDC ligand remains fully stable after adsorbing large amounts of water vapor during cyclic water adsorption cycles. These trends cannot be rationalized in terms of hydrophobicity alone; experimental water isotherms show that MOFs containing the same number of methyl groups per unit cell will have different kinetic stabilities and that the precise placements of the methyl groups on the BDC ligand are therefore critically important in determining their stability in the presence of water. We present the water adsorption isotherms, PXRD (powder X-ray diffraction) patterns, and BET surface areas before and after water exposure to illustrate these trends. Furthermore, we shed light on the important distinction between kinetic and thermodynamic stability in MOFs. Molecular simulations are also used to provide insight into the structural characteristics governing these trends in kinetic water stability.

Evaluation of MOFs for air purification and air quality control applications: Ammonia removal from air

“…However, we did not characterize MOFs post-ammonia exposure in this work. Their ammonia capacities increased to $ 20 mmol/g, $ 1 mmol/g, and $4.6 mmol/g respectively under humid conditions possibly due to ammonia chemically reacting with the frameworks of the water unstable ZnBTTB MOF (Karra, 2011) and kinetically stable MOFs DMOF-A and DMOF-TM2 (Jasuja et al, , 2013. Unusual shapes (additional steps, Fig.…”

“…Synthesis and activation of both of these MOFs were carried out as explained in our previous work (Jasuja et al, , 2013.…”

“…We expect that the UiO-66 variants with acidic functional groups such as -COOH and -SO 3 H will have higher interactions with ammonia. We have also considered two waterstable DMOFs (DMOF-A, DMOF-TM2) discovered from our previous systematic water stability studies (Jasuja et al, , 2013. The overall goal is to identify ammonia-selective, water stable MOF materials, that can be run in series with existing filters, providing significant improvements to the capture of high vapor pressure TICs such as ammonia.…”

Compositional control of pore geometry in multivariate metal–organic frameworks: an experimental and computational study

“…29 Studies were therefore undertaken on selected members of series 1-5 to determine if the addition of a second bdc-X ligand affected the stability in air. A similar protocol has been used to assess the structural stability of MOF-177.…”

Preparation of Hydrophobic Metal-Organic Frameworks via Plasma Enhanced Chemical Vapor Deposition of Perfluoroalkanes for the Removal of Ammonia

“…MOF-5 has also been shown to have its hydrostability increased through doping with Ni 2+ ions 11 . Furthermore, 1,4-diazabicyclo[2.2.2]octane containing MOFs (also known as DMOFs) have been used to show the tuning of water stability through incorporation of various pendant groups on the 1,4-benzene dicarboxylate linker 12,13 .…”