Adsorbate-dependent phase switching in the square lattice topology coordination network [Ni(4,4′-bipyridine)₂(NCS)₂]_n

Abstract

Switching coordination networks (CNs) featuring stepped sorption isotherms that are accompanied by phase changes offer promise for gas storage and separation applications. However, their responsiveness to different adsorbates remains largely understudied. Herein, we report the variable switching behaviour of a previously known square lattice (sql) topology CN, [Ni(4,4′-bipyridine)₂(NCS)₂] (sql-1-Ni-NCS), with respect to nine gaseous adsorbates.

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The increasing use of gases as fuels or chemical feedstocks has resulted in the “age of gas”.¹ However, the dispersive nature of gases means that they tend to have low densities and form mixtures under ambient and typical industrial process conditions.² This creates several challenges for gas storage and/or separation processes with respect to the associated energy footprint and hazards.^3–6 Porous materials such as activated carbons and zeolites were deployed in the 20th century to mitigate such energy penalty,^3,4 even though their gas sorption uptakes are often modest. In the 1990s, a new class of metal–organic materials (i.e., porous coordination polymers/networks, PCPs/PCNs, and metal–organic frameworks, MOFs) were introduced and it was quickly realized that their inherent modularity enables tunable pore size and chemistry.^7–11 The majority of such materials are first or second-generation CPs, i.e. upon sorbate removal they undergo structural collapse (first generation) or possess rigid structures like classical zeolites (second generation).^8,12 Second-generation CPs typically exhibit Langmuir (type I) sorption isotherms, which necessarily reduces working capacity for gas storage including natural gas storage.¹³

Seminal studies pioneered by Kitagawa, Férey and others introduced “third generation” flexible CPs or “soft porous crystals” in the early 2000s.^14–17 Third generation CPs exhibit structural flexibility when exposed to guest molecules and feature stepped sorption isotherms that are yet to be classified by IUPAC.^17–19 A small but growing subset of flexible CPs are the switching coordination networks (CNs) that can undergo extreme structural transition(s) between “closed” nonporous and “open” porous phases. We have classified the resulting stepped isotherms as type F-IV isotherms.^17,18 Such switching CNs can enable higher working capacity and better thermal management than rigid sorbents with type I isotherms.¹³ Furthermore, switching pressure and adsorption enthalpy can be readily calculated by applying the Clausius–Clapeyron equation,¹⁷ allowing for the strength comparison of different adsorbate–adsorbent interactions.

The switching pressure and sorption uptake of switching CNs can be influenced by various factors.¹⁷ The effects of temperature/pressure, metal–ion and linker on switching CNs have been studied.^13,20–24 The adsorbate also plays a key role in triggering the phase transformations that accompanies switching since adsorbate–adsorbent interactions are the main driving force.^25–27 In general, nonpolar gases such as N₂ and CH₄ tend to exhibit relatively weak interactions with CNs, while hydrocarbons with unsaturated bonds and/or more carbon atoms may induce switching by providing stronger host–guest interactions (Tables S1 and S2, ESI†).¹⁷ However, such empirical rules-of-thumb need to be experimentally verified using a systematic approach.

To our knowledge, there are around 70 switching CNs reported in the literature for which N₂ and CO₂ are the dominant adsorbates, usually studied at their boiling point temperatures of 77 K and 195 K, respectively.¹⁷ Only a handful of switching CNs have been examined across a range of adsorbates at multiple conditions (Table S1, ESI†).^{13,20–23,25–28} This situation hinders our understanding of the adsorbate dependence of switching CNs and in turn limits their potential utility. We address this matter herein through a study of the effect of nine adsorbates on the switching parameters associated with a square lattice (sql) topology coordination network.

Switching CNs often feature square or rhombic cavities that exploit the flexible nature of a rhombus from a mechanical perspective as exemplified by sql CNs with general formula [M(L)₂(A)₂]_n (M = divalent metal cation, L = ditopic linker ligand, A = axial counter anion).† Such CNs are modular and highlight the “node and linker” design strategy developed by Robson and Hoskins over 30 years ago.^29,30 The first reported sorption study on switching sql CNs was conducted on [Cu(bpy)₂(BF₄)₂] (bpy = 4,4′-bipyridine), ELM-11.^31,32 It was observed to exhibit single or multi-step type F-IV sorption isotherms via layer expansion (Fig. 1a) when triggered by gases such as N₂, O₂, Ar, CO₂ and C₂H₂.^33–35 Recently, we studied the sorption properties of three previously known sql CNs [M(bpy)₂(NCS)₂] (M = Fe, Co, or Ni),^36–39sql-1-M-NCS, which are isostructural to the ELM family. Their CO₂ sorption isotherms under low or high temperatures/pressures were observed to exhibit single-step type F-IV isotherms and the switching pressures were found to be metal–ion controlled.³⁸ Amongst the three sql-1-M-NCS CNs, sql-1-Ni-NCS was found to be the “softest” switching CN based on the CO₂ gate-adsorption pressure (P_ga).³⁸ It was later found that sql-1-Ni-NCS exhibits even lower P_ga value and higher sorption uptake for C₂H₂ when compared to its CO₂ sorption.³⁹ These studies prompted us to study the effect of a wider range of gaseous adsorbates on sql-1-Ni-NCS to determine their switching pressures and sorption uptakes.

Fig. 1 (a) Schematic illustration of the switching mechanism of sql CNs triggered by guest sorption; (b and c) crystal structures of sql-1-Ni-NCS.

sql-1-Ni-NCS was initially synthesized hydrothermally,⁴⁰ and we have recently adopted an alternate route by heating its 1D chain CP precursor that can be obtained by water slurry.³⁸sql-1-Ni-NCS is sustained by Ni(II) ions coordinated equatorially to bpy linker ligands with terminal NCS⁻ anions occupying the axial positions. The interlayer distance is 4.5 Å (Fig. 1b) and the effective dimension of the square cavity is 7.5 Å × 7.5 Å (Fig. 1c). The cavity void is blocked by the interdigitated NCS⁻ ligands (Fig. 1b). sql-1-Ni-NCS is therefore non-porous and is thermally stable up to 180 °C,³⁸ which is sufficient for most practical applications. In contrast to its hydrophilic analogue ELM-11, sql-1-Ni-NCS is hydrophobic towards humidity,³⁸ an important consideration given that water vapour degrades the performance of many adsorbents.

While sql-1-Ni-NCS is ‘‘softer’’ than its Fe and Co analogues, its 77 K N₂ and 195 K CH₄ sorption revealed negligible uptakes (Fig. 2). In contrast, its 195 K C₂H₄ and C₂H₆ isotherms revealed switching behaviour but uptake did not reach saturation at 113 kPa (close to the maximum measurable pressure of the sorption instrument). The corresponding P_ga values for C₂H₄ and C₂H₆ were determined to be 86 and 91 kPa, respectively, much higher than those for CO₂ (4.0 kPa) and C₂H₂ (2.9 kPa) at the same temperature.^38,39 In addition, C₃H₄ (propyne) sorption on sql-1-Ni-NCS exhibited switching behaviour (P_ga = 21.5 kPa) at 273 K with a saturation uptake of 138 cm³ g⁻¹, although its C₃H₆ (propylene) and C₃H₈ (propane) sorption uptakes were negligible (Fig. S1, ESI†). This C₃H₄ uptake matches its CO₂ uptake and corresponds to three C₃H₄ molecules per formula unit (sql-1-Ni-NCS·3C₃H₄). Interestingly, the desorption branch of the C₃H₄ sorption isotherm featured two steps, consistent with a new phase with 2/3 of the saturation uptake (ca. 94 cm³ g⁻¹) between 8 and 20 kPa at 273 K. This data suggests that, before transforming to the closed phase, sql-1-Ni-NCS·3C₃H₄ converted to sql-1-Ni-NCS·2C₃H₄ (two C₃H₄ molecules per formula unit) during desorption. Such a phenomenon (i.e., the desorption branch has more steps than the corresponding adsorption branch) is, to the best of our knowledge, reported herein for the first time in switching CNs.

Fig. 2 77 K N₂, 195 K CH₄/CO₂/C₂H₂/C₂H₄/C₂H₆ and 273 K C₃H₄ sorption isotherms for sql-1-Ni-NCS.

To further study the C₃H₄ sorption behaviour on sql-1-Ni-NCS, C₃H₄ sorption isotherms were collected between 263 and 298 K with a 5 K interval (Fig. 3a and Fig. S2, ESI†). The P_ga values were observed to be 12.5, 16.5, 21.5, 28.0, 35.5 and 45.0 kPa at 263, 268, 273, 278, 283 and 288 K, respectively. These temperatures and P_ga values were fitted to the Clausius–Clapeyron equation (Fig. 3b and Fig. S3, ESI†), to calculate the adsorption enthalpy (ΔH, absolute value) of ca. 32.3 kJ mol⁻¹. This ΔH value is higher than those calculated for the corresponding CO₂ (28.4 kJ mol⁻¹) and C₂H₂ (28.5 kJ mol⁻¹) induced phase transformations.^38,39

Fig. 3 (a) The C₃H₄ sorption isotherms of sql-1-Ni-NCS collected at different temperatures; (b) linear fit of gate adsorption pressure (Ln P_ga) and temperature (1000/T) using the Clausius–Clapeyron equation for sql-1-Ni-NCS.

P _ga can be calculated at given temperatures once ΔH has been determined.^36,38,39 We therefore plotted the P_gavs. temperature from 253 to 298 K for C₃H₄ and compared it with those for CO₂ and C₂H₂ (Fig. 4 and Table S3, ESI†). The plot reveals that the C₃H₄ switching pressure increases at elevated temperature in a manner similar to that of its CO₂ and C₂H₂ counterparts.^38,39 However, the P_ga values for C₃H₄ are much lower than those for CO₂ and C₂H₂. For example, the P_ga value for C₃H₄ at 298 K is only 0.71 bar, while it reaches 12.66 and 18.49 bar for C₂H₂ and CO₂, respectively. Such a large difference in P_ga values suggests that adsorbate–adsorbent interactions strongly affect the switching pressure as reflected in their ΔH values. With respect to the sorption uptake, it is also affected by the adsorbate. For instance, the saturation uptakes of CO₂ and C₃H₄ are both 138 cm³ g⁻¹ (3 mol mol⁻¹), while C₂H₂ uptake reaches 185 cm³ g⁻¹ (4 mol mol⁻¹) at the first plateau, a 33.3% increase in capacity. The adsorbate impact upon switching easiness for the studied gases can be ordered as follows: C₃H₄ > C₂H₂ > CO₂ > C₂H₄ > C₂H₆ > CH₄ ≥ N₂ (Fig. S4, ESI†). This order generally agrees with that reported for other switching CNs (Table S2, ESI†) and follows the trend of the boiling point and vaporisation enthalpy of the gases (Table S4, ESI†).

Fig. 4 Comparison of CO₂, C₂H₂, and C₃H₄ gate-adsorption pressures for sql-1-Ni-NCS.

Structural analysis of previously reported sql-1-M-NCS·xG (Tables S5, S6 and Fig. S5, S6, ESI†) reveals that phase switching involves guest intercalation and/or inclusion phenomena and enables volume expansion of 23.5–114.9%, which can be classified into five distinct phase categories, A–E (Table S6 and Fig. S5, ESI†). The stoichiometric ratio (x) of G: M was found to be 2, 3 or 4. The identical stoichiometric ratio (x = 3) of C₃H₄ and CO₂ and their similar shapes prompted us to conduct molecular simulations to predict the location of C₃H₄ molecules by assuming that the crystal structure of sql-1-Ni-NCS·3C₃H₄ is isostructural to the previously reported structure of sql-1-Ni-NCS·3CO₂.^36,38 The resulting calculations of these “category A” phases indicate that C₃H₄ molecules occupy interlayer voids and internetwork cavities (Figs S6b, ESI†) with C–H⋯π and π⋯π host–guest interactions (Fig. S7, ESI†). Future studies will focus upon in situ PXRD experiments to verify the nature of these structural transformations.

In summary, we herein present the sorption properties of nine gases (N₂, CH₄, CO₂, C₂H₂, C₂H₄, C₂H₆, C₃H₄, C₃H₆, and C₃H₈) of a prototypal switching sql CN, sql-1-Ni-NCS. C₃H₄ sorption was studied at eight temperatures and compared with the previously reported CO₂ and C₂H₂ sorption properties. C₃H₄ induced switching had not been previously reported for CNs, although it has been studied in a soft organic cage.⁴¹ Our results indicate that both the switching pressure and sorption uptake are strongly influenced by the adsorbate. The switching thresholds for each adsorbate are generally compatible with the rules-of-thumb abovementioned. It should be noted that, whereas CH₄, C₃H₆ and C₃H₈ did not trigger switching of sql-1-Ni-NCS at 195 or 273 K and 1 bar, this does not mean that they cannot do so at lower temperatures and/or higher pressures. Overall, the primary message from this study is that nonporous structures (as determined by their crystal structures and/or 77 K N₂ sorption data) should not be discarded as candidates for sorption-based applications as first suggested by Barrer's studies on molecular compounds.⁴² Future studies will explore the sorption behaviour of sql-1-Ni-NCS and related switching adsorbent layered materials (SALMAs) for other adsorbates including the effect of pressure.⁴³

M. J. Z. gratefully acknowledges the support of the Irish Research Council (IRCLA/2019/167) and Science Foundation Ireland (16/IA/4624).

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, sorption isotherms, etc. See DOI: https://doi.org/10.1039/d2cc06549e

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