Unveiled the Source of the Structural Instability of HKUST-1 Powders upon Mechanical Compaction: Definition of a Fully Preserving Tableting Method

Abstract

Metal–organic frameworks (MOFs) are getting closer to finally being used in commercial applications. In order to maximize their packing density, mechanical strength, stability in reactive environments, and many other properties, the compaction of MOF powders is a fundamental step for the application field of research of these extraordinary materials. In particular, HKUST-1 is among the most promising and studied MOF. Contrary to what reported so far in the literature, here we prove that the tableting of HKUST-1 powders without any damage of the lattice is possible and easy to get. For the first time, this kind of investigation has been performed exploiting its peculiar magnetic properties with the aid of electron paramagnetic resonance spectroscopy. Indeed, they have allowed us to explore in detail all the smallest changes induced in the paramagnetic paddle-wheel units by the application of the mechanical pressure on the material. This original approach has permitted us to unveil the main source of structural instability of HKUST-1 during compaction, that is, the water molecules adsorbed by the powdered sample before tableting and finally to establish a proper compaction protocol. Our conclusions are also fully supported by the results obtained with powder X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetric analysis, water sorption isotherms, and surface area estimation with the Brunauer–Emmett–Teller method, which prove that the tablet of HKUST-1 obtained by this new protocol actually preserves the crystal structure and porosity of the pristine powders. A morphological characterization has also been conducted with a combined use of optical and atomic force microscopies.

An Optimised Compaction Process for Zr-Fumarate (MOF-801)

“…This suggests that the water present within the pores might act as a "cushion" and help to evenly dissipate the applied pressure, preventing the collapse of the framework during compression. Similar behaviour was previously reported for MOF-5 [18] and HKUST-1 [19,20]. The durability of the pellets prepared using the…”

Densification-Induced Structure Changes in Basolite MOFs: Effect on Low-Pressure CH4 Adsorption

“…In agreement, McKellar et al reported variations of 2.6% for densification pressures of 3.9 GPa [ 50 ]. Likewise, the non-appearance of new crystalline peaks indicates that the cubic structure is maintained [ 51 , 52 ]. Therefore, the pressure effect on Basolite C300 consists of crystallinity destruction, in agreement with the BET surface area and pore volume reduction with the pressure, but remaining unaltered the cubic structure of unaltered cells.…”

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

“…[4,[11][12][13][14][15][16][17][18][19][20][21] Recently, after many unsuccessful attempts, [4,[11][12][13] it has been found that HKUST-1 (or Cu-BTC), which is considered a reference system for all of the copper paddle wheel MOFs like STAM-17-OEt, is actually very easy to compact if the appropriate protocol is applied. [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.…”

“…[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.…”

“…[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. [8,10] Regarding STAM-17-OEt, these peculiar magnetic properties have been successfully used in previous works to study the hemilability and the other effects induced by hydration in this material, [8] as well as in composite systems involving active carbon, [10] by analyzing the changes in the EPR spectrum.…”