Water Adsorption in Porous Metal–Organic Frameworks and Related Materials

Water adsorption in porous materials
is important for many applications
such as dehumidification, thermal batteries, and delivery of drinking
water in remote areas. In this study, we have identified three criteria
for achieving high performing porous materials for water adsorption.
These criteria deal with condensation pressure of water in the pores,
uptake capacity, and recyclability and water stability of the material.
In search of an excellently performing porous material, we have studied
and compared the water adsorption properties of 23 materials, 20 of
which are metal–organic frameworks (MOFs). Among the MOFs are
10 zirconium­(IV) MOFs with a subset of these, MOF-801-SC (single crystal
form), −802, −805, −806, −808, −812,
and −841 reported for the first time. MOF-801-P (microcrystalline
powder form) was reported earlier and studied here for its water adsorption
properties. MOF-812 was only made and structurally characterized but
not examined for water adsorption because it is a byproduct of MOF-841
synthesis. All the new zirconium MOFs are made from the Zr6O4(OH)4(−CO2)n secondary building units (n = 6,
8, 10, or 12) and variously shaped carboxyl organic linkers to make
extended porous frameworks. The permanent porosity of all 23 materials
was confirmed and their water adsorption measured to reveal that MOF-801-P
and MOF-841 are the highest performers based on the three criteria
stated above; they are water stable, do not lose capacity after five
adsorption/desorption cycles, and are easily regenerated at room temperature.
An X-ray single-crystal study and a powder neutron diffraction study
reveal the position of the water adsorption sites in MOF-801 and highlight
the importance of the intermolecular interaction between adsorbed
water molecules within the pores.