Acetylene storage performance of [Ni(4,4′-bipyridine)₂(NCS)₂]_n, a switching square lattice coordination network

Abstract

We report that the previously reported square lattice coordination network [Ni(4,4′-bipyridine)₂(NCS)₂]_n, sql-1-Ni-NCS, undergoes acetylene induced switching between closed (nonporous) and open (porous) phases. The resulting stepped sorption isotherms exhibit temperature controlled steps, consistent high uptake and benchmark working capacity (185 cm⁻³ g⁻¹ or 189 cm⁻³, 1–3.2 bar, 288 K) for acetylene storage.

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Acetylene (C₂H₂) is an important chemical feedstock and is also used as a fuel for oxyacetylene torches.¹ However, its highly flammable and explosive nature renders its handling to be more challenging than most gases.^2,3 Current C₂H₂ storage technology involves desensitization by dissolving C₂H₂ in acetone pre-dispersed in a porous monolith that completely fills a gas cylinder.^2,3 The solubility of C₂H₂ in acetone reaches 470.4 cm⁻³ g⁻¹ at 15 bar and 288 K.⁴ Nevertheless, charging C₂H₂ into acetone dramatically expands the volume of acetone and results in relatively low volumetric uptake (192.3 cm⁻³).⁴ In addition, discharging C₂H₂ releases acetone vapour,³ precluding its use in production of fine chemicals and electronic materials. Solid sorbents (e.g. porous coordination networks, PCNs^5,6) offer potential to address these handicaps and broaden the utility of C₂H₂ in this “age of gas”.⁷

Switching coordination networks (CNs) can be classified as third generation CNs as they can undergo guest-induced structural transformation(s) between “closed” nonporous and “open” porous phases.⁸ They represent a small but growing subset of flexible metal–organic frameworks (FMOFs) or soft porous crystals (SPCs).^9–11 Their potential utility is related to their sorption isotherms. Whereas rigid CNs typically exhibit Langmuir (type I) isotherms, switching CNs feature stepped or type F-IV sorption isotherms,^8,12 making them distinctive from most metal–organic materials (MOMs^13,14) by enhancing working capacity for gas storage (Fig. 1).^8,12,15

Fig. 1 Schematic illustration of a type F-IV or stepped sorption isotherm. Such isotherms can offer enhanced gas storage performance as, unlike rigid porous materials, working capacity can be 100% of uptake. Black solid line = adsorption branch; red dash line = desorption branch; P_ga = gate adsorption pressure; P_gd = gate desorption pressure, P_st = gas storage pressure; P_de = gas delivery pressure.

With respect to C₂H₂ storage, a key performance parameter is the working capacity at practically relevant gas delivery (P_de) and storage (P_st) pressures. We recently proposed that a pressure range of 1–15 bar might be used to define working capacity as this range is compatible with existing C₂H₂–acetone technology.⁴ Another important but largely understudied parameter is sorption kinetics, which must be sufficiently fast for gas loading and unloading. To date, more than 100 CNs have been investigated with respect to C₂H₂ sorption and most are rigid CNs with high sorption uptake below 1 bar.^16–27 Whereas high uptake can make a sorbent suitable for C₂H₂ sequestration, the type I isotherms typical of rigid sorbents are unlikely to offer a working capacity that is close or equal to their uptake as would be ideal for C₂H₂ storage/delivery.⁴ Conversely, switching CNs with a type F-IV isotherm can exhibit a working capacity that equals saturation uptake (Fig. 1). Furthermore, an appropriate hysteresis gap would enable C₂H₂ to be stored at lower pressure (i.e. between P_gd and P_ga) than its charging pressure (i.e. P_ga and above). A feature of switching CNs is that P_ga and P_gd can be calculated by applying the Clausius–Clapeyron equation,^4,8 thereby providing an opportunity to calculate C₂H₂ working capacity at higher pressures and avoiding experimental explosion risks. To our knowledge, only two switching CNs, the 3D pillar-layered CN MOF-508 and the 2D square lattice (sql) CN sql-1-Cu-BF₄ (ELM-11) have been studied for their C₂H₂ storage properties.^4,19,28 In this contribution, we report that the sql CN [Ni(bpy)₂(NCS)₂], sql-1-Ni-NCS (1 = bpy = 4,4′-bipyridine) exhibits C₂H₂ induced switching and evaluate its C₂H₂ storage performance by means of variable temperature C₂H₂ sorption studies and in situ synchrotron PXRD studies.

sql-1-Ni-NCS was hydrothermally synthesized in 1999 by Zhang et al.^29,30 we have developed an alternate route by heating the 1D chain coordination polymer {[Ni(bpy)(NCS)₂(H₂O)₂]·bpy} obtained by water slurry.³¹ While the crystal structure (Fig. S1, ESI†) and spectroscopic properties of sql-1-Ni-NCS are known for two decades,^29,30 its sorption properties were unstudied until we reported its CO₂ sorption properties at low (≤1 bar, 195 K) and high (≤38 bar, 273–298 K) temperatures/pressures.³¹ Interestingly, it is the “softest” switching CN with respect to P_gavs. its Fe and Co analogues.^31,32 This prompted us to study its C₂H₂ sorption properties since C₂H₂ generally offers stronger sorbent–sorbate interactions than CO₂.^4,8

The 195 K C₂H₂ sorption isotherm of sql-1-Ni-NCS reveals that the P_ga is 2.9 kPa (Fig. 2), below the P_ga for CO₂ (4.0 kPa).³¹ The C₂H₂ uptake plateaus at 185 cm⁻³ g⁻¹, which suggests 4 C₂H₂ molecules per formula unit (sql-1-Ni-NCS·4C₂H₂). This value is 34% higher than the CO₂ saturation uptake (138 cm⁻³ g⁻¹) of sql-1-Ni-NCS·3CO₂.³¹ A second step appeared at ca. 60 kPa but does not plateau before 120 kPa. At temperatures above 205 K, the second step was not observed while the initial plateau retained the same saturation uptake. The BET surface area and total pore volume of sql-1-Ni-NCS were calculated to be 697.3 m² g⁻¹ and 0.41 cm³ g⁻¹, respectively. P_ga/P_gd values were observed to be 2.9/1.3, 4.1/1.8, 6.8/3.0, 10.2/4.4, 14.9/6.3 and 21.3/9.0 kPa at 195, 199, 205, 210, 215 and 220 K, respectively (Fig. 2 and Fig. S2, ESI†). These temperature and P_ga/P_gd values were fitted to the Clausius–Clapeyron equation (Fig. 3a and Fig. S3, ESI†), which was used to calculate formation (Δ_fH) and dissociation (Δ_dH) enthalpies (absolute values) of ca. 28.5 and 27.7 kJ mol⁻¹, respectively. These ΔH values are comparable to those (28.4/28.2 kJ mol⁻¹) calculated for CO₂ induced phase transition.³¹

Fig. 2 C₂H₂ (195–220 K) adsorption isotherms for sql-1-Ni-NCS.

Fig. 3 (a) Linear fit of gate sorption pressure (LnP_gate) and temperature (1000/T) using the Clausius–Clapeyron equation; (b) calculated P_ga, P_gd and the hysteresis gap (P_ga − P_gd) for sql-1-Ni-NCS.

P _ga/P_gd can be calculated at various temperatures once ΔH has been determined.^4,8 We were therefore able to calculate switching pressure vs temperature from 195 to 298 K for sql-1-Ni-NCS (Fig. 3b and Table S1, ESI†). These plots reveal that the C₂H₂ switching pressure and the hysteresis gap between P_ga and P_gd increase at elevated temperature in a manner similar to that of its CO₂ switching pressures.³¹ For example, P_ga/P_gd were calculated to be 4.4/1.7, 8.5/3.2, and 12.7/4.7 bar at 273, 288, and 298 K, respectively. The corresponding hysteresis gaps were found to be 2.7, 5.3 and 8.0 bar at 273, 288 and 298 K, respectively. These data suggest that C₂H₂ can be stored by sql-1-Ni-NCS at lower pressure (e.g., 3.2 bar, 288 K) than the charging pressure (e.g., ≥8.5 bar, 288 K).

The related sql CN sql-1-Cu-BF₄ was reported to exhibit a type F-IV^m isotherm with three complete C₂H₂ sorption steps at 195 K.^4,8 In contrast, sql-1-Ni-NCS exhibited a type F-IV^s isotherm with a single C₂H₂ sorption step. Although the C₂H₂ uptake of sql-1-Cu-BF₄ (245 cm⁻³ g⁻¹ at the 3rd plateau) is higher than that of sql-1-Ni-NCS (185 cm⁻³ g⁻¹ at the 1st plateau), its working capacity is lower between 1–15 bar under ambient temperatures. This is because only the uptake between the second and the third step of sql-1-Cu-BF₄ can be utilised in this pressure range. The working capacity (163 cm⁻³ g⁻¹) of sql-1-Cu-BF₄ is therefore 66.7% of the sorption uptake.⁴ In contrast, the uptake of sql-1-Ni-NCS can be fully exploited (185 cm⁻³ g⁻¹) under the same conditions so its working capacity is 13.5% above that of sql-1-Cu-BF₄. With respect to other parameters, the switching pressures (P_ga/P_gd) and hysteresis gaps are comparable (Fig. S4 and S5, ESI†): P_gd values for sql-1-Ni-NCS are slightly (0.01–0.34 bar) lower than P_gd3 of sql-1-Cu-BF₄ in the range 195–298 K; P_ga values are 0.04–0.40 bar lower between 195–278 K and 0.08–1.31 bar higher between 283–298 K than those (P_ga3) of sql-1-Cu-BF₄. For example, at 288 K, the P_ga/P_gd values are 8.1/3.5 and 8.5/3.2 bar for sql-1-Cu-BF₄ and sql-1-Ni-NCS, respectively.

To gain insight into the nature of the phase transformation induced by C₂H₂, in situ synchrotron PXRD experiments were conducted. As shown in Fig. 4a, the phase transformation was complete within 8 min under 0.5 bar C₂H₂ at 195 K. Such sorption kinetics are adequate for practical utility and comparable with the CO₂ sorption kinetics.³¹ From a structural perspective, synchrotron PXRD refinement revealed that the unit-cell parameters of sql-1-Ni-NCS·4C₂H₂ differ from those of sql-1-Ni-NCS·3CO₂ (Fig. 4b, Fig. S6 and S7, ESI†). For instance, sql-1-Ni-NCS·3CO₂ retained the same space group, C2/c, as the closed phase of sql-1-Ni-NCS, while sql-1-Ni-NCS·4C₂H₂ adopted space group P2₁/n as did the m-xylene loaded phase (sql-1-Co-NCS·4MX) previously reported by us.³³ The Z value is 4 in the closed phase and 2 in the C₂H₂-loaded phase. The normalized unit-cell volume changes from 2264.9 ų in the closed phase to 3181.8 (i.e., 1590.9 × 2) ų in the C₂H₂-loaded phase, correspond to a 40.5% increase in unit cell volume. Attempts to solve the crystal structure of sql-1-Ni-NCS·4C₂H₂ were unsuccessful, but MX molecules reside in both interlayer spaces and square cavities in sql-1-Co-NCS·4MX (Fig. S8, ESI†).

Fig. 4 (a) In situ synchrotron PXRD patterns for sql-1-Ni-NCS under 0.5 bar C₂H₂ at 195 K; (b) structural parameters of sql-1-Ni-NCS (closed), sql-1-Ni-NCS·3CO_2,sql-1-Ni-NCS·4C₂H₂ and sql-1-Co-NCS·4MX.

Volumetric working capacity for gas storage is also a key performance parameter since container volume is necessarily limited. The network density (excluding C₂H₂) of sql-1-Ni-NCS·4C₂H₂ was calculated to be 1.02 g cm⁻¹ (Fig. 4b), and therefore the volumetric working capacity of C₂H₂ is ca. 189 cm⁻³. This value is higher than sql-1-Cu-BF₄ (174 cm⁻³) and MOF-508 (106 cm⁻³).^4,28 When compared to the industrial adsorbent acetone, which has a volumetric working capacity (170 cm⁻³) between 1–15 bar at 288 K,⁴sql-1-Ni-NCS outperforms it by 11.2% at a safer working pressure range (1–3.2 bar, 288 K).

In summary, we herein report a switching transformation in a 2D sql CN, sql-1-Ni-NCS, induced through exposure to C₂H₂. The C₂H₂ switching pressure, P_ga/P_gd, was controlled by temperature with retention of the saturation uptake. The type F-IV isotherm exhibited by sql-1-Ni-NCS enabled working capacity to be 100% of uptake capacity and the relatively high density resulted in benchmark volumetric working capacity at practically relevant conditions. When combined with other features such as fast sorption kinetics, hydrophobicity, and ease of scale-up,³¹sql-1-Ni-NCS is a promising candidate for enhancing the working capacity of gas storage and highlights the general potential that layered CNs offer for high working capacity. This is perhaps counterintuitive since such CNs are nonporous in their closed phases. Further studies to explore the storage potential of sql-1-Ni-NCS and related switching adsorbent layered materials (SALMAs) for other gases and vapours are in progress.

M. J. Z. gratefully acknowledges the support of the Irish Research Council (IRCLA/2019/167) and Science Foundation Ireland (16/IA/4624). Z. C. and X.-H. B. acknowledge the National Science Foundation of China (NSFC) (21531005) and the Programme of Introducing Talents of Discipline to Universities (B18030). We thank Dr Claire Murray and Dr Chiu C. Tang at the Diamond Light Source, UK, for providing access to the synchrotron X-ray diffraction beamline i11 (EE20500). S.-Q. W. would also like to thank his colleagues, Mr Daniel O’Hearn, Dr Andrey Bezrukov and Dr Soumya Mukherjee, for their assistance at Diamond Light Source.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, desorption isotherms, PXRD patterns, etc. See DOI: 10.1039/d1cc06638b

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