Ammonia\nCapture within Zirconium Metal–Organic\nFrameworks: Reversible and Irreversible Uptake

Abstract

Ammonia\nuptake by high-capacity and high-porosity sorbents is a\npromising approach to its storage and release, capture and mitigation,\nand chemical separation. Here, we examined the ammonia sorption behavior\nof several versions of an archetypal zirconium-based metal–organic\nframework (MOF) material, NU-1000a meso- and microporous crystalline\ncompound having the empirical formula (1,3,6,8-tetrakis­(<i>p</i>-benzoate)­pyrene)₂ Zr₆(μ₃-O)₄(μ₃-OH)₄(H₂O)₄(OH)₄ with linkers and nodes arranged to satisfy a <b>csq</b> topology. Depending on the thermal treatment protocol\nused prior to sorption measurements, ammonia can physisorb to NU-1000\nvia hydrogen-bonding and London-dispersion interactions and chemisorb\nvia Brønsted acid–base reactions with node-integrated\nproton donors (μ₃-hydroxos) and node-ligated proton\ndonors (terminal hydroxos), via simple coordination at open Zr­(IV)\nsites, 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\nprocesses. The latter are of particular interest for protection and\nmitigation. Notably, the unexpected dissociative adsorption occurs\nonly with nodes that have been fully dehydrated and irreversibly structurally\ndistorted via thermal pre-treatmenta finding that is supported\nby density functional theory calculations. Differentiating and ranking\nthe relative importance of the many modes of adsorption was facilitated,\nin part, by the availability of variants of NU-1000 that replace the\nmajority of terminal aqua and hydroxo ligands with nonstructural formate\nligands, auxiliary ditopic linkers, or both. The study provides insights\ninto the chemical basis for both reversible and irreversible uptake\nof ammonia by Zr-MOFs and related compounds. The unexpectedly rich\nvariety of sorption motifs suggest the criteria for designing or choosing\nMOFs that are optimal for specific ammonia-centric applications.