Ammonia Capture within Zirconium Metal–Organic Frameworks: Reversible and Irreversible Uptake

Ammonia
uptake by high-capacity and high-porosity sorbents is a
promising approach to its storage and release, capture and mitigation,
and chemical separation. Here, we examined the ammonia sorption behavior
of several versions of an archetypal zirconium-based metal–organic
framework (MOF) material, NU-1000a meso- and microporous crystalline
compound having the empirical formula (1,3,6,8-tetrakis­(p-benzoate)­pyrene)2 Zr6(μ3-O)4(μ3-OH)4(H2O)4(OH)4 with linkers and nodes arranged to satisfy a csq topology. Depending on the thermal treatment protocol
used prior to sorption measurements, ammonia can physisorb to NU-1000
via hydrogen-bonding and London-dispersion interactions and chemisorb
via Brønsted acid–base reactions with node-integrated
proton donors (μ3-hydroxos) and node-ligated proton
donors (terminal hydroxos), via simple coordination at open Zr­(IV)
sites, or via dissociative coordination to Zr­(IV) as NH2– and protonation of a node-based μ3-oxo. Ammonia adsorption occurs via both reversible and irreversible
processes. The latter are of particular interest for protection and
mitigation. Notably, the unexpected dissociative adsorption occurs
only with nodes that have been fully dehydrated and irreversibly structurally
distorted via thermal pre-treatmenta finding that is supported
by density functional theory calculations. Differentiating and ranking
the relative importance of the many modes of adsorption was facilitated,
in part, by the availability of variants of NU-1000 that replace the
majority of terminal aqua and hydroxo ligands with nonstructural formate
ligands, auxiliary ditopic linkers, or both. The study provides insights
into the chemical basis for both reversible and irreversible uptake
of ammonia by Zr-MOFs and related compounds. The unexpectedly rich
variety of sorption motifs suggest the criteria for designing or choosing
MOFs that are optimal for specific ammonia-centric applications.