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Selective adsorption of dihydrogen isotopes on DUT-8 (Ni,Co) monitored by in situ electron paramagnetic resonance

Muhammad Fernadi Lukman a, Matthias Mendt b, Volodymyr Bon c, Stefan Kaskel c and Andreas Pöppl *a
aFelix Bloch Institute for Solid State Physics, Leipzig University, 04103 Leipzig, Germany. E-mail: poeppl@physik.uni-leipzig.de
bSaxonQ GmbH, Emilienstr. 15, 04107 Leipzig, Germany
cChair of Inorganic Chemistry I, Technische Universität Dresden, 01069 Dresden, Germany

Received 19th June 2023 , Accepted 20th July 2023

First published on 20th July 2023


Abstract

In situ continuous wave electron paramagnetic resonance investigation has been proven as a powerful method by employing paramagnetic Ni2+–Co2+ pairs as spin probes to follow the isotope-selective gate opening phenomenon on the DUT-8(Ni0.98 Co0.02) framework. This method is very sensitive to detect the phase transition from the closed pore to the open pore phase in response to D2 adsorption in the framework, while no phase transformation has been observed during H2 gas adsorption. More interestingly, it is also able to sense local structural changes around the spin probe during the desorption of D2 gas. Based on these evidences, the in situ continuous wave electron paramagnetic resonance method can be implemented as an efficient and non-invasive technique for the detection of dihydrogen isotopes.


Flexible metal organic frameworks such as MIL-53(Al) and DUT-8(Ni) are considered as very attractive MOFs due to their switchability behaviour towards external stimuli such as the adsorption of specific guest molecules, pressure or temperature.1–5 Recently, Kim and co-workers6 reported a D2-selective breathing behaviour in MIL-53(Al), which is potentially useful for the purification of isotopic mixtures. In addition, some of us also revealed that a flexible DUT-8(Ni) selectively responds to D2 gas, while in a contrasting way, is irresponsive towards HD and H2 gas.7 The flexible DUT-8(Ni) MOF is a primitive cubic (pcu) network that consists of Ni2 paddle wheels as nodes (see Fig. 1), 2,6-naphthalenedicarboxylate (ndc) as a linker and 1,4-diazabicyclo[2,2,2]octane (dabco) as pillars. A pronounced nuclear quantum effect8–11 is suggested to be responsible for the higher adsorption enthalpy of deuterium-containing dihydrogen isotopes to be adsorbed inside the DUT-8(Ni) framework at cryogenic temperatures. Previously, in situ neutron powder diffraction (NPD) and thermal desorption spectroscopy (TDS) investigations were conducted to study this intriguing effect demonstrating a high adsorption selectivity for D2vs. H2 after exposure to 1[thin space (1/6-em)]:[thin space (1/6-em)]1 gas mixtures.7
image file: d3cc02938g-f1.tif
Fig. 1 (A) Structural representation of mixed-metal Ni2+–Co2+ paddle wheel units. Blue, grey and red spheres indicate nitrogen, carbon and oxygen atoms, respectively. A schematic representation of the (B) op phase and (C) cp phase state in response to dihydrogen isotope adsorption is also provided.

On the other hand, from the point of view of electron paramagnetic resonance (EPR) spectroscopy, the isomorphous substitution of Co2+ towards DUT-8 (Ni) has been investigated and comprises a noticeable EPR pattern for an open pore (op) phase species that can be interpreted as an effective spin S = 1/2 ground state of the antiferromagnetically coupled mixed Ni2+–Co2+ paddle wheel unit interacting with its 59Co nuclear spin (I = 7/2).5,12 A lengthy discussion on other possible assignments of EPR spectra for the op phase state and its rejection arguments has been discussed by Ehrling and co-workers.5 Previously, an in situ CW-EPR approach enabled us to follow the gate opening mechanism for a series of DUT-8(Ni1−xCox) samples in response to their exposure to N2 gas close to its standard boiling point (71 K).12 Even preceding that, the exotic in situ CW-EPR investigation at X-band frequency has been implemented as a versatile method to follow the gate opening phenomenon of several flexible MOFs triggered by the response toward guest molecule stimuli.12–16 In the present work, we implement this in situ CW-EPR technique for the first time to monitor an isotope-selective phase transition of DUT-8(Ni0.98 Co0.02) towards dihydrogen isotopes.

Initially, there was no signal observed for DUT-8(Ni0.98 Co0.02) in the evacuated state (ca. 10−4 mbar), which indicates the presence of closed pore (cp) phase at 21.5 K, in agreement with previous results.5,12 The absence of an EPR signal is probably due to short relaxation times of low symmetric Ni2+–Co2+ paddle wheels. The spectral pattern distinctly evolved after increasing the D2 pressure to 250 mbar, producing a nicely resolved species A (see Fig. 2A and Table 1 for the spin Hamiltonian parameters), which is assigned to the Ni2+–Co2+ paddle wheel signal in the op phase state of DUT-8(Ni0.98 Co0.02). The spin Hamiltonian parameters were determined by spectral simulation of the experimental EPR spectra. The intensity of species was obtained by double integration of the corresponding EPR spectra in Fig. 2A and served as a measure for the volume fraction of the op phase in the sample (Fig. 4). The emergence of the EPR spectrum of the Ni2+–Co2+ paddle wheels at 250 mbar as an indication of the op phase is very well in line with the volumetric D2 adsorption data obtained by Bondorf and co-workers,7 where they also found a similar gate opening pressure and shape of the adsorption hysteresis for D2 but with much longer equilibration times per pressure point.


image file: d3cc02938g-f2.tif
Fig. 2 (A) In situ CW-EPR spectra of DUT-8(Ni0.98 Co0.02) while adsorbing D2 gas from p = 0.001 mbar (black) to p = 680 mbar (green) recorded at 21.5 K. The impurity signal marked with a yellow star is tentatively assigned to some low spin Co2+ defect species while the black circle indicates a background signal from the cryostat. (B) The CW-EPR spectra of Ni2+–Co2+ of species A representing the op phase state of DUT-8(Ni0.98 Co0.02) in response to D2 adsorption recorded in situ at p = 680 mbar and T = 21.5 K (green lines) and its simulated spectra (red lines). Two black circles at magnetic fields of 150 and 340 mT indicate again a background signal from the cryostat.
Table 1 Spin Hamiltonian parameters of the Ni2+–Co2+ mixed paddle wheel species during the adsorption and desorption process of D2. Ref. 12 refers to N2 adsorption on the same sample as the reference. The hyperfine coupling is given in MHz
Species g x g y g z A x A y A z Symmetry
A 2.65(4) 2.59(4) 1.79(4) 510(10) 465(10) 417(10) Rhombic
B 2.62(4) 2.62(4) 1.80(4) 468(10) 468(10) 427(10) Axial
12 2.62(4) 2.62(4) 1.39(4) 538(100) 538(100) 410(100) Axial


Interestingly, after desorption to pressures less than 90 mbar, there is a gradual transformation of species A into a new species B with axial symmetry as presented in Fig. 3A (the percentage of its respective species is tabulated in Table SI1, ESI). Its spectral pattern at 0.001 mbar is shown in Fig. 3B. The transformation of the symmetry of the paddle wheel units from lower symmetry into higher symmetry (C4 as principal rotational axis) at low partial pressure of D2 (p < 35 mbar) indicates less distortion for the paddle wheel units during the evacuation of D2 gas. This phenomenon might originate from the adaptive response of the DUT-8(Ni0.98 Co0.02) framework towards a D2 release via a switching of the linker orientation and can subsequently change the symmetry into a more ordered state.5,17


image file: d3cc02938g-f3.tif
Fig. 3 (A) In situ CW EPR spectra of DUT-8(Ni0.98 Co0.02) while desorbing D2 gas from p = 645 mbar (pink) to p = 0.001 mbar (black). The orange shaded area indicates the superposition of rhombic Ni2+–Co2+ species A and axial Ni2+–Co2+ species B. The yellow shaded area indicates a complete transformation to the axial Ni2+–Co2+ species B. In situ CW EPR signal for species B recorded after desorption of D2 at 0.001 mbar and T = 21.5 K (black lines) with the simulated spectrum (red lines). Two black circles at magnetic field of 150 and 340 mT indicate a background signal from the cryostat.

The sensitivity of the EPR experiment allows one to observe such phase transformation in rather shorter timescales as compared to the volumetric D2 and NPD measurements, which were previously published.7 Note that the intensity of the EPR signal (considering double integration of species A which then transformed into species B) was observed to be constant during the desorption stage and did not revert back to the cp phase at least within the observation times of our EPR measurements (Fig. 4).


image file: d3cc02938g-f4.tif
Fig. 4 Plot of EPR signal intensity of Ni2+–Co2+ species for DUT-8(Ni0.98 Co0.02) normalized with the highest EPR intensity during adsorption and desorption stages (blue shaded arrows).

Desorption studies of D2 with variability of time shown in Fig. SI3 (ESI) indicated that even up to 33 minutes during pumping off at 0.001 mbar and 21.5 K, the op phase (e.g. Species B) is still observed with relatively constant EPR intensities. This supports the in situ NPD finding that the transition from op phase to cp phase would only be triggered after 24 h evacuation of D2 at similar temperature (ca. 23.3 K).7 One possible explanation might be that D2 molecules are kinetically trapped in the pores since the dimension of the cp phase (2.366 nm × 0.695 nm) structure may accommodate the residual D2 gas that was trapped during transition from the op to cp phase state. Pollock and co-workers18 previously observed this trapping mechanism during the desorption of D2 gas on another variant of flexible MOF material, MIL-53(Al), even at higher temperature (77 K). Moreover, this molecular clamp appeared to be functional until 120 °C when probed using neutron scattering measurements. We also attempted to increase the temperature while desorbing the D2 gas under vacuum at 0.001 mbar, which resulted in a pore closing mechanism being triggered at 30 K (Fig. SI3, ESI). The EPR spectra at that temperature suffered from intensity reduction and line broadening before vanishing at T > 35 K, leaving only the background signal. This result is in agreement with previous TDS results that observe the largest D2 desorption peak between 25–30 K, which implies that the desorption temperature plays an important role in the gate closing mechanism in the DUT-8 (Ni) materials.

In the case of H2 adsorption and desorption on DUT-8(Ni0.98 Co0.02) at 19.5 K, which are provided in Fig. SI1 and SI2 (ESI), the EPR intensities are constant and show only a low-spin Co2+ impurity signal together with some background signals from the cryostat (Fe3+ and Cu2+ species around 150 and 325 mT, respectively). This result signifies that the op phase with its characteristic EPR signal of the Ni2+–Co2+ paddle wheel signal is not present within the pressure range of 0 ≤ p ≤ 929.2 mbar during the H2 adsorption experiment. However, it cannot be ruled out that a trace amount of H2 gas is adsorbed in the cp phase without triggering a pore transition, simply because the energy barrier for the pore transformation is too high for H2 at 19.5 K and pressures up to 1000 mbar.7 The selective opening of DUT-8(Ni0.98 Co0.02) towards D2 owing to the so-called “sizable” chemical affinity quantum sieving (CAQS) effect originates from a slightly lower ZPE (zero point energy) that allows D2 to bind preferentially to various adsorption sites, such as accessible paddle wheel units or linker sites.7,10,19 We should note that the definition of the CAQS effect is not merely constrained to the adsorption of gases in the framework that poses open binding sites. Savchenko and co-workers20 reported that the selectivity of D2 over H2 is mainly caused by different adsorption enthalpies, which in principle, is rooted fundamentally to the small difference in ZPE of adsorbed dihydrogen isotopes on the funnel-like, metal and linker sites in the MOF MFU-4l (at 50 K) where none of them can be considered as open sites in a straightforward definition.

In conclusion, we have demonstrated that the in situ CW-EPR technique at X-band frequencies can be implemented as an efficient technique to monitor the selective-adsorption of dihydrogen isotopes on DUT-8(Ni0.98 Co0.02) MOF, which further confirms the isotope-selective phase transition in response to D2 adsorption. Moreover, as an outlook, the utilisation of in situ pulsed-EPR techniques is in progress to locate the D2 position with respect to the spin probe within the framework of DUT-8(Ni0.98 Co0.02) and estimate how this gas interacts around the accessible paddle wheel units where our EPR active probe resides. This detailed information would provide more clarity in terms of understanding the nature of adsorption sites of the flexible MOF materials.

Muhammad Fernadi Lukman: conceptualization, methodology, investigation, data curation, writing – original draft, and writing – review & editing. Matthias Mendt: investigation, writing – review & editing. Volodymyr Bon: writing – review & editing. Stefan Kaskel: review and supervision. Andreas Pöppl: conceptualization, review, project administration, funding acquisition and supervision.

The authors would like to acknowledge the DFG funding of GRK 2721: Hydrogen Isotopes 1, 2, 3 H with project number 443871192 and FOR 2433 with project number 279409724.

Conflicts of interest

There are no conflicts to declare.

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Footnote

Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cc02938g

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