Selective Gas Adsorption of Alkane/Alkene in a Single-Crystal and Powder Bimetallic Metal–Organic Framework Compound Cu_2.97Zn_0.03(btc)₂ Studied by Electron Paramagnetic Resonance

Abstract

The metal–organic framework HKUST-1 is an attractive material for gas storage and gas separation applications because of its large surface area and high pore volume, and the presence of coordinatively unsaturated (cus) cupric ion sites. We investigated the interaction of alkenes and alkanes (ethene, 1-butene, ethane, and butane) with Cu2+ ions in their Zn-doped variant Cu2.97Zn0.03(btc)2 by continuous-wave electron paramagnetic resonance spectroscopy. Powder and single-crystal experiments revealed a characteristic change of cupric ion coordination from square planar to square pyramidal for the adsorption of ethylene and 1-butene and confirmed the formation of adsorption complexes at the cus sites of the Cu2+ ion in the mixed metal ion Cu–Zn paddle wheel (PW) units. Quantum chemical calculations based on density functional theory established a weak specific interaction between alkene π-orbitals and Cu2+ ions in a side-on coordination geometry. In contrast, no direct interaction between alkanes and the metal ions was found for this porous material. However, adsorption of ethane and n-butane leads to significant strain effects in the HKUST-1 framework which can be indirectly sensed by the cupric ions in the paramagnetic Cu–Zn PW units.

Isolable 1‐Butene Copper(I) Complexes and 1‐Butene/Butane Separation Using Structurally Adaptable Copper Pyrazolates

“…[8,[11][12] Butene-adsorption methods are mainly focused on the use of zeolites [13][14][15] and metal-organic frameworks (ESI Section 1.1). [8,16] These studies have shown success using flexible frameworks for size discrimination [17] and π-coordination, including those involving Cu(I) and Cu-(II) [10,15,[18][19][20][21][22][23][24] for the separation of butene isomers [9,25] and 1,3butadiene from other C 4 hydrocarbons. [26] Non-porous small molecule adsorbents are an emerging class of materials that can overcome the traditional trade-offs between selectivity vs capacity and selectivity vs heat of adsorption.…”

“…Butene‐adsorption methods are mainly focused on the use of zeolites [13–15] and metal‐organic frameworks (ESI Section 1.1) [8,16] . These studies have shown success using flexible frameworks for size discrimination [17] and π‐coordination, including those involving Cu(I) and Cu(II) [10,15,18–24] for the separation of butene isomers [9,25] and 1,3‐butadiene from other C 4 hydrocarbons [26] …”

Adsorption of Olefins at Cupric Ions in Metal–Organic Framework HKUST-1 Explored by Hyperfine Spectroscopy

“…The molecules constituting such mixtures exhibit similar sizes and physical properties, which makes them extremely challenging to separate. Application of porous MOF adsorbents for this problem has exceptional advantages compared to conventional distillation technologies, because it profits from selective specific chemical interactions between the carbon–carbon double bond of olefins with open metal sites in the MOF. − …”

“… , The exposed surface of the pore contains 12 coordinatively unsaturated Cu 2+ sites available for olefin adsorption. At the same time, we have recently demonstrated that each Cu 2+ site can be exploited as a spin probe addressable by continues wave (CW) electron paramagnetic resonance (EPR) spectroscopy and sensitive toward the olefin binding …”

“…For EPR studies, however, it is common to use a mixed-metal form of the HKUST-1, such as Cu 3– x Zn x (btc) 2 , instead of the stoichiometric MOF. , The reason is that the strong Cu–Cu magnetic coupling within each PW unit complicates the analysis of conventional CW EPR spectra and also leads to prohibitively short electron spin relaxation times for pulsed EPR measurements. Substitution of a fraction of Cu 2+ by EPR-silent Zn 2+ ions creates Cu/Zn PW units while preserving the overall three-dimensional structure and the absorption properties of the MOF. , The magnetically isolated Cu 2+ in the Cu/Zn unit is, thus, a perfectly suitable probe for both conventional and pulsed EPR. Therefore, here and in the previous study, HKUST-1 containing 3% Zn 2+ (Cu 2.97 Zn 0.03 (btc) 2 ) is adopted.…”