Decomposition Process of Carboxylate MOF HKUST-1 Unveiled at the Atomic Scale Level

Abstract

HKUST-1 is a metal–organic framework (MOF) which plays a significant role in both applicative and basic fields of research, thanks to its outstanding properties of adsorption and catalysis but also because it is a reference material for the study of many general properties of MOFs. Its metallic group comprises a pair of Cu2+ ions chelated by four carboxylate bridges, forming a structure known as paddle-wheel unit, which is the heart of the material. However, previous studies have well established that the paddle-wheel is incline to hydrolysis. In fact, the prolonged exposure of the material to moisture promotes the hydrolysis of Cu–O bonds in the paddle-wheels, so breaking the crystalline network. The main objective of the present experimental investigation is the determination of the details of the structural defects induced by this process in the crystal, and it has been successfully pursued by coupling the electron paramagnetic resonance spectroscopy with other more commonly considered techniques, such as X-ray diffraction, surface area estimation, and scanning electron microscopy. Thanks to this original approach we have recognized three stages of the process of decomposition of HKUST-1, and we have unveiled the details of the corresponding equilibrium structures of the paddle-wheels at the atomic scale level

Graphene inclusion controlling conductivity and gas sorption of metal–organic framework

“…The time-resolved development in the EPR spectrum was monitored on powders of HKUST-1 and HKUST-1/graphene while (i) dehydrating the materials at a temperature corresponding to the outgassing temperature during the gas sorption studies (230 C), (ii) exposure to CO 2 and (iii) rehydration at room temperature. Recent literature describes the result of applying EPR to HKUST-1, and several types of copper(II) sites are discussed: [59][60][61][62][63] (1) monomeric copper such as [Cu(H 2 O) n ] 2+ (n ¼ 5 or 6) captured in the cavities. These have spin quantum number S ¼ 1/2; (2) paddle-wheel units consisting of two copper centres bridged by four carboxylate groups.…”

Micro-spectroscopy of HKUST-1 metal–organic framework crystals loaded with tetracyanoquinodimethane: effects of water on host–guest chemistry and electrical conductivity

“…It is known that HKUST-1 crystals have a limited stability with respect to moisture and water, having potential consequences ranging from Cu reduction to pore collapse depending on the exposure time and water vapour pressure. [43][44][45][46][47][48] To ensure that no such effects occur during the hydration experiments in this work, in situ XRD experiments were performed by recording diffractograms during a temperature-programmed experiment with dosing of water-saturated He gas (60 mL min À1 He flow through a bubbler filled with demineralized H 2 O following the temperature program depicted in Fig. S4 (ESI †), at a heating rate of 5 K min À1 ).…”

Probing the Water Stability Limits and Degradation Pathways of Metal–Organic Frameworks

“…However, we note that the decomposition step proposed here can be regarded as an intermediate in the decomposition pathway, resulting in the species shown by Todaro et al. to contain Cu I ions …”