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

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 −CH3, −C2H5, and −CF3 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.