Multitechnique Analysis of the Hydration in Three Different Copper Paddle-Wheel Metal–Organic Frameworks

Abstract

The structural instability in a humid environment of the majority of metal− organic frameworks (MOFs) is a challenging obstacle for their industrial-scale development.Recently, two water-resistant MOFs have been synthetized, STAM-1 and STAM-17-OEt. They both contain copper paddle wheels, like the well-known water-sensitive HKUST-1, but different organic linkers. The crystal lattice of both the MOFs undergoes a phase transition upon interaction with water molecules. Their unusual flexibility allows the controlled breaking of some interpaddle wheel Cu−O interactions in the so-called crumple zones, with a mechanism called hemilability, which is considered to have a crucial role for the stability toward water. In this work, we present a detailed investigation on the different effects of water exposure on the local and long-range structures of HKUST-1, STAM-1, and STAM-17-OEt. Electron paramagnetic resonance (EPR) spectroscopy has allowed us to characterize the different phases occurring during hydration of each MOF. In particular, we have identified and portrayed the moment of the adsorption of the first water molecule on each copper ion and shown that such soft hydration lead to a similar reversible evolution in all of the three MOFs. This aspect unveiled that the bulk water stability of the MOFs studied is unimportant at this early stage, whereas with a higher degree of hydration (more than few hours in our experimental conditions), we observe the three MOFs embarked on different paths, here carefully described. The evolution of HKUST-1 is not reversible because of its well-known tendency to hydrolysis, but, in contrast, we proved the reversibility of the water effects in STAM-1 and STAM-17-OEt even at the atomic scale level. Furthermore, for the first time, we report a Raman characterization of both STAM-1 and STAM-17-OEt, for each phase of the hydration. The data also include X-ray diffraction, nuclear magnetic resonance measurements, and Brunauer−Emmett−Teller surface area calculations of all the samples.

Structure Effects Induced by High Mechanical Compaction of STAM‐17‐OEt MOF Powders

“…In addition, there are two important obstacles to overcome before the employ of different MOFs in a systematic industrial-scale development: the typical limited chemical stability with respect to water and their largescale synthesis in powder form. [3][4][5][6] Surprisingly, the new MOF STAM-17-OEt (Cu(C 10 O 5 H 8 ) • 1.6 H 2 O) combines all these extraordinary properties: it is flexible (or better, hemilabile), [7][8][9] extremely water-stable, [7,8,10] and with this work we show that it is also very resistant to the mechanical pressures necessary for the industrialization of the material, and therefore easy to densify and commercialize even if originally synthesized in powder form. This last aspect is not at all trivial, because in literature many MOFs have been subjected to densification processes with or without the help of chemical binders, but in most cases a significant loss of crystallinity and surface area have been found, compromising the typical performances of the material, especially as adsorbent or catalyst.…”

“…[22] However, HKUST-1 is very sensitive to water and it typically undergoes severe hydrolysis even by limited exposure to air. [8,[22][23][24][25] In this work, we have tested the effectiveness of a similar compaction protocol to the water-proof and hemilabile STAM-17-OEt. This latter Cu-based MOF, like HKUST-1, is formed by a paddle-wheel unit structure composed of two copper ions coordinated by four carboxylate bridges.…”

“…This latter Cu-based MOF, like HKUST-1, is formed by a paddle-wheel unit structure composed of two copper ions coordinated by four carboxylate bridges. [7,8,26] Such carboxylate groups arise from 5-ethoxy isophthalate organic linkers. [7,8] The lattice of this MOF is composed by an infinite 2D framework with kagome-type topology.…”

“…[7,8,26] Such carboxylate groups arise from 5-ethoxy isophthalate organic linkers. [7,8] The lattice of this MOF is composed by an infinite 2D framework with kagome-type topology. Similarly to HKUST-1, a crystallization water molecule occupies the apical coordination site on each of the Cu 2 + ions of the paddle-wheels, establishing hydrogen bonds to the coordinated carboxylate oxygen atoms lying on the 2D sheets above and below.…”

“…[7] Its particular hemilability arises right from the removal (or addition, during hydration processes) of such coordinated water molecules, process that leads to a single-crystal to single-crystal transition. [7][8][9] The two Cu 2 + ions involved in the paddle-wheels confer to STAM-17-OEt (as well as to HKUST-1) peculiar magnetic properties: the two S = 1/2 spins establish an antiferromagnetic coupling characterized by a spin angular momentum with a singlet (S = 0) ground state and a triplet (S = 1) excited state. [22,23,[27][28][29][30] The latter state generates a characteristic Electron Paramagnetic Resonance (EPR) spectrum.…”