Benchmark Acetylene Binding Affinity and Separation through Induced Fit in a Flexible Hybrid Ultramicroporous Material

Abstract

Structural changes at the active site of an enzyme induced by binding to a substrate molecule can result in enhanced activity in biological systems. Herein, we report that the new hybrid ultramicroporous material sql‐SIFSIX‐bpe‐Zn exhibits an induced fit binding mechanism when exposed to acetylene, C₂H₂. The resulting phase change affords exceptionally strong C₂H₂ binding that in turn enables highly selective C₂H₂/C₂H₄ and C₂H₂/CO₂ separation demonstrated by dynamic breakthrough experiments. sql‐SIFSIX‐bpe‐Zn was observed to exhibit at least four phases: as‐synthesised (α); activated (β); and C₂H₂ induced phases (β′ and γ). sql‐SIFSIX‐bpe‐Zn‐β exhibited strong affinity for C₂H₂ at ambient conditions as demonstrated by benchmark isosteric heat of adsorption (Q _st) of 67.5 kJ mol⁻¹ validated through in situ pressure gradient differential scanning calorimetry (PG‐DSC). Further, in situ characterisation and DFT calculations provide insight into the mechanism of the C₂H₂ induced fit transformation, binding positions and the nature of host‐guest and guest‐guest interactions.

We introduce the new flexible physisorbent sql‐SIFSIX‐bpe‐Zn and reveal that it exhibits an induced fit structural change that results in extraordinary affinity for C₂H₂ that in turn enables high selectivity for separation of C₂H₂ over C₂H₄ and CO₂.

Introduction

The induced fit mechanism is a well‐known process for the formation of enzyme‐substrate complexes that enhance activity.^[1] In this phenomenon, the enzyme active site undergoes a structural transformation to optimize the binding site for a specific substrate (Scheme 1 a). Induced fit plays an important role in biological processes such as signal transduction, signal amplification and others.^[2] Whereas induced fit is prominent in enzymatic reactions, it is rare and poorly understood in the context of crystalline porous materials, where such behaviour could result in enhanced selectivity to enable challenging gas capture or purification processes.^[3] Metal organic materials (MOMs)^[4] such as metal organic frameworks (MOFs),^[5] porous coordination polymers (PCPs)^[6] and hybrid ultramicroporous materials (HUMs)^[7] are typically sustained by linker ligands and metal‐based nodes and can overcome the limitations of traditional classes of porous materials such as zeolites.^[8] Such porous coordination networks offer a high degree modularity which makes them amenable to design from first principles using crystal engineering strategies.^[9] A small subset of MOFs and HUMs can undergo structural transformation upon exposure to stimuli^[10] such as heat, gas/vapor or light could therefore be relevant for application in storage,^[11] separations,^[12] catalysis,^[13] molecular sensor^[14] and drug release.^[15] There are at least 150 MOFs^[16] that have been shown to exhibit flexibility among the >75 000 reported MOFs.^[17] The origin of flexibility has been well studied and is attributed to various mechanism such as breathing, swelling, switching, shape memory and others.[^10a, ^10b, ¹⁸] To our knowledge, there are only five previous reports of induced fit driven by gas sorption.^[19]

Scheme 1.

Induced fit transformations. a) Enzyme active sites can undergo structural changes to better fit a substrate and enhance activity. b) A stimulus responsive ultramicroporous physisorbent that exhibits a structural transformation induced by a sorbate would be expected to offer enhanced binding energy and in turn enable better separations.

For example, Rosseinsky et al.^[19b] reported a flexible framework, (ZnGGH‐1⋅DMF‐H₂O) (GGH=tripeptide glycine‐glycine‐l‐histidine) which changes conformation to adapt to the shape and size of specific guest thanks to the flexible skeleton of the organic linker. A CPL network, [Cu₂(pzdc)₂(bpy)] (pzdc=pyrazine‐2,3‐dicarboxylate and bpy=4,4‐bipyridine), was found to contract upon heating the as synthesised phase; further contraction occurred with benzene occupying channels. This induced fit is attributed to isomerisation from square pyramidal to square planar geometry.^[19d] Similarly, a flexible chemisorbent, [Zn₃(OH)₂(btca)₂] (H₂btca=benzotriazole‐5‐carboxylic acid), contracted after activation from its as synthesised phase and further contraction occurred following C₂H₂ adsorption with binding affinity of 47.6 kJ mol⁻¹.^[19c] The mechanism of contraction was driven by the binding affinity of C₂H₂ towards unsaturated Zn centres and self‐adaptive shrinkage of the framework. Whereas physisorbents can exhibit C₂H₂ induced structural transformations,^[20] flexible physisorbents that offer induced fit mechanisms for C₂H₂ (Scheme 1 b) have not yet been reported. We consider this to be a desirable objective since high binding energy, low regeneration energy and good separation performance would be anticipated from such sorbents.

Among commodity gases, high purity C₂H₂ is the starting material for industrially relevant products such as plastics, vinyl compounds, acrylic derivatives and α‐ethyl alcohols.^[20f] Currently, C₂H₂ is produced by the partial combustion of methane or through the cracking of hydrocarbons. However, this process generates impurities such as C₂H₄ and CO₂. The similar quadruple moments and kinetic diameters (Table S1) of these gases makes the process of selective capture of one gas (for example, C₂H₂) from the other gases difficult and energy intensive. In this context, rigid MOFs with high C₂H₂ affinity have been reported with binding driven by open metal sites,^[21] hydrogen bonding,^[22] synergistic effect of open metal sites and electronegative sites^[23] or acidic/basic functional groups.^[24] HUMs^[7a] have also demonstrated high C₂H₂ binding affinity over other gases such as C₂H₄,^[25] CO₂ ^[26] and other hydrocarbon mixtures.^[27] For example, Li et al. achieved benchmark C₂H₂ selectivity over C₂H₄ using SIFSIX‐14‐Cu‐i.^[25] The high performance exhibited by HUMs has been attributed to ultramicropores (<0.7 nm), strong electrostatic interactions with the inorganic “pillars” (SiF₆ ²⁻ or SIFSIX) and pore structure. Overall, the current C₂H₂ benchmark for isosteric heat of adsorption (Q _st), ≈60 kJ mol⁻¹ at low loading, rests with the rigid ultramicroporous MOF known as NKMOF‐1‐Ni.^[28] The binding affinity of a sorbent can provide a guide to its separation performance for a specific gas in a mixture of gases. Flexible physisorbents which exhibits induced fit have the advantage over chemisorbents. This is because flexible framework can recognise only specific guest molecule vs. different multiple guest and then adopt new configuration from their original structure especially those designed with special binding sites (for example, originating from inorganic pillar) and flexible skeleton from ligand. That newly transformed structure induces tight cavity for that specific guest, combining strong interactions lead to extraordinary guest binding affinity with low regeneration energy. Herein we report that the use of an inorganic pillar, SiF₆ ²⁻ and a flexible dipyridyl linker ligand affords a new flexible physisorbent [Zn(SiF₆)(1,2‐bis(4‐pyridyl)ethane)₂]_n, which exhibits induced fit in single crystal to single (SC‐SC) fashion when exposed to C₂H₂ under ambient conditions. In‐situ X‐ray diffraction was used to study the reversibility of the phase transformations and the switching mechanisms driven by C₂H₂ and CO₂ loading at 195 K. As revealed herein, the observed SC‐SC transformations are driven by host‐guest interactions with the inorganic pillar and therefore differ from the previously reported examples of induced fit binding (Table S2).

Results and Discussion

[Zn(SiF₆)(bpe)₂]_n exists as a 3D pcu or 2D sql network when prepared from 1,2‐bis(4‐pyridyl) ethane and ZnSiF₆ ²⁻ using different solvent combinations. Chloroform and methanol resulted in the pcu variant (Figure S1). Single‐crystal X‐ray diffraction (SC‐XRD) revealed that the organic linker adopts the trans‐ conformation and orthorhombic space group Ibam (Table S3). Single crystals were unstable to loss of guest, making it a 1^st generation PCP.^[6] When pure methanol was used, single crystals of the sql variant were harvested. Crystals were observed to be stable when removed from other liquor and SC‐XRD revealed that bpe linkers are in cis‐ conformation and the monoclinic space group C2/c was adopted (Table S3). The ability of bpe to exhibit conformational flexibility is long known.^[29] The sql phase (sql‐SIFSIX‐bpe‐Zn‐α; Figure 1) exhibits 18 % guest‐accessible volume which is occupied by methanol molecules. The void space was calculated with a probe radius of 1.2 Å and a grid spacing of 0.7 Å. Single crystals of the activated or β phase (sql‐SIFSIX‐bpe‐Zn‐β) were obtained by heating the α phase at 353 K under vacuum for 12 h.

Figure 1.

Schematic representation of the synthesis and structural transformations of sql‐SIFSIX‐bpe‐Zn. a) Structure of the organic linker (1,2‐bis(4‐pyridyl) ethane) and inorganic components (Zn²⁺ and SIFSIX pillar) that formed a 2D net with sql topology that exhibits channels. b) The as‐synthesised phase, α, underwent reversible transformation to the β phase upon removal/insertion of MeOH or CO₂. C₂H₂ sorption induced another phase, β′⊃2C₂H₂, which accommodated 2C₂H₂ molecules. β⊃2C₂H₂ can adsorb two additional C₂H₂ molecules within the layers at 195 K by a switching transformation from β⊃2C₂H₂ to γ⊃4C₂H₂. Upon desorption, reversible transformation from γ⊃4C₂H₂ and β⊃2C₂H₂ to β occurred at 195 K and 298 K, respectively.

The β phase transformed to the triclinic space group P‐1 with a density of 1.351 g cm⁻³ well below that of the α phase (1.523 g cm⁻³). The crystal structures of the β phase revealed that its 2D networks are distorted and the guest‐accessible volume is reduced to 14 % (Figures 2 a and b). The phase transformation from α to β was also studied by in situ variable temperature powder X‐ray diffraction (PXRD) at intervals of 10 °C every 10 min, which revealed that transformation occurred at 333 K (Figure S2). This phase change was also observed in differential scanning calorimetry (DSC) measurements as a small exothermic peak occurred at 333 K (Figure S3). Thermogravimetric analysis (TGA) indicated that both the α and β phases were stable up to 463 K with ≈13 % weight loss for α, which corresponds to 5 MeOH molecules per unit cell that include one additional methanol from the surface of crystals (Figure S4).

Figure 2.

The phases of sql‐SIFSIX‐bpe‐Zn studied by in situ single‐crystal X‐ray diffraction. a) The as‐synthesized α phase exhibits 1D channels occupied by 5 methanol molecules per unit cell as determined by SCXRD and TGA. b) The α phase underwent transformation to the β phase upon heating at 353 K. This transformation was investigated by in situ variable temperature PXRD and in situ SC‐XRD. c) and d) In‐situ SCXRD measurements at 298 and 195 K enabled determination of the C₂H₂ binding sites in the β′ and γ crystal structures, respectively, which revealed a single binding site (site‐I) in β′⊃2C₂H₂ and multiple binding sites in γ⊃4C₂H₂. The respective crystal photomicrographs of SC‐SC transformations are presented in (a), (b) and (c).

To study the porosity of sql‐SIFSIX‐bpe‐Zn, gas sorption isotherms were measured for CO₂ at 195 K, C₂H₂ at 195 K, and N₂ at 77 K. Prior to gas sorption measurements, the α phase was activated at 353 K under vacuum for 12 h. A stepped isotherm was observed for C₂H₂ adsorption at 195 K (Figure S8). A plateau from 0 to 40 cm³ g⁻¹ (2 guests per unit cell) was observed upon increasing the pressure up to p/p ₀=0.01, followed by a sudden increase in uptake to 82 cm³ g⁻¹ (4 guests per unit cell), which is the saturated uptake at p/p ₀=1. In the case of CO₂ adsorption (Figure S9), a slight inflection was observed up to p/p ₀=0.008 in the uptake range from 30 to 65 cm³ g⁻¹. Saturated CO₂ uptake (80 cm³ g⁻¹, 4 CO₂ per unit cell) was the same as that for C₂H₂ at p/p ₀=1. In both isotherms, desorption matches the adsorption profile and the PXRD pattern after desorption indicates that the β phase is recovered (Figure S11). N₂ sorption revealed a typical type‐I isotherm with saturated uptake limited to only 38 cm³ g⁻¹ at p/p ₀=1, which is approximately equal to 2 guests per unit cell (Figure S10).

To gain insight into the mechanism behind the stepped isotherm, in situ coincidence PXRD measurements during C₂H₂ and CO₂ adsorption at 195 K were conducted. A few selected adsorption and desorption points in each sorption profile in which significant phase change occurred are plotted in Figure 3. Diffraction patterns from point 6 to 87 are shown for C₂H₂ at 195 K in Figure 3 a. C₂H₂ sorption revealed that the β phase remains to point 44 with an uptake of 40 cm³ g⁻¹. Further increase in pressure afforded new peaks after the step that we attribute to the structural transformation from β to a new more open phase, γ (sql‐SISIX‐bpe‐Zn‐γ). The fully loaded γ phase remains unchanged during further adsorption (from point 46 to 55) and desorption (from point 78 to 87). Following the desorption process, we observed that γ returned to β as revealed by Figure 3 a. With respect to in situ CO₂ sorption, the diffraction patterns indicated that transformation occurs from β to α in a reversible manner. Selected patterns from point 1 to 77 are plotted in Figure 3 b. At point 12, new PXRD pattern emerged, which remained unchanged to p/p ₀=1 and after desorption. The PXRD pattern at fully loaded point 33 matches the α phase. Finally, after desorption, α returns to β as shown in Figure 3 b. In‐situ PXRD patterns were also measured during N₂ sorption at 77 K (Figure 3 c). The PXRD patterns indicate that N₂ does not induce a phase change during sorption, presumably because of its weaker interactions with the sorbent. This can also be inferred from the uptake reaching 38 cm³ g⁻¹ at p/p ₀=1, which is half of the uptake vs. C₂H₂ and CO₂. To identify guest binding sites upon C₂H₂ sorption in situ gas loading experiments were conducted on single crystals. A suitable single crystal was selected for these studies and activated in situ at 80 °C for 2 hrs under vacuum. C₂H₂ was dosed into the capillary when the temperature had reached 298 K. Cooling to 195 K revealed that transformation from β to γ occurred within 30 mins, as confirmed by a change in unit cell parameters. SCXRD analysis revealed that the γ phase retained the same space group as the β phase and that there are four distinct C₂H₂ binding sites (Table S3 and Figure S15). The occupancy of each C₂H₂ site was refined at 0.5, which we attribute to slow loading kinetics. The calculated PXRD pattern for the γ phase matches the in situ PXRD pattern at adsorption point 55 as revealed by Figure 3 a and Figure S14. The transformation from the β to γ phase resulted in reduced quality data vs. that obtained for the β phase at 298 K (R=16 %, wR2=45 %).

Figure 3.

In‐situ coincidence PXRD. Numbers in adsorption/desorption traces correspond to numbering of the PXRD patterns: a) In‐situ PXRD measured at 195 K during C₂H₂ adsorption/desorption revealed a reversible switching transformation from β to γ. b) In‐situ PXRD experiments conducted during CO₂ adsorption/desorption at 195 K revealed that switching occurred from β to α in reversible manner. c) In‐situ PXRD measurements conducted under N₂ at 77 K revealed that the β phase remains unchanged during the adsorption/desorption.

The nature of the pore size and chemistry of the β phase prompted us to explore gas sorption of C₂H₂, C₂H₄, CH₄, C₂H₆, and CO₂ at 298 K. C₂H₂ exhibited strong affinity with sharp uptake at low pressures compared to the other four gases (Figure 4 a and S16). The C₂H₂ uptake was 26 cm³ g⁻¹ at p/p ₀=0.01, reached close to saturation at a relatively low pressure (p/p ₀=0.1) with an uptake of 38 cm³ g⁻¹ and was saturated (40 cm³ g⁻¹) at p/p ₀=1. In contrast, the C₂H₄, C₂H₆ and CO₂ sorption isotherms exhibited negligible uptake at p/p ₀=0.01 whereas increasing pressure to p/p ₀=0.1 resulted in uptakes of 11, 5 and 24 cm³ g⁻¹, respectively. The uptake capacity at p/p ₀=1 was found to be 28, 18 and 40 cm³ g⁻¹ for C₂H₄, C₂H₆ and CO₂, respectively. Interestingly, molecular sieving behaviour towards CH₄ was observed, with a saturated uptake of only 3 cm³ g⁻¹ at p/p ₀=1. When fully saturated at p/p ₀=1, the guest occupancy for C₂H₂, C₂H₄, C₂H₆ and CO₂ was observed to be on the order of 2, 1, 1 and 2 molecules per unit cell, respectively. Cycling experiments indicated that C₂H₂ sorption profiles were reproducible with the same uptake over 10 cycles (Figure S17). Sample regeneration was achieved by vacuum treatment at 313 K for 30 mins without additional heat. In order to calculate Q _st, we measured sorption at various temperatures ranging from 273 to 328 K in 5 °C or 10 °C intervals (Figures 4 b, S18, S19). The resulting Q _st calculations on C₂H₂ sorption revealed that β offers a new benchmark for sorbent‐C₂H₂ interaction energy (Figure 4 c). A Q _st value of 67.5 kJ mol⁻¹ was observed at zero loading, this value decreasing to ≈48 kJ mol⁻¹ and then increased with uptake capacity. Conversely, C₂H₄, CH₄, and CO₂ exhibits low affinity with Q _st values of 38.4, 11, and 33.6 kJ mol⁻¹, respectively (Figures 4 c and S23). The observed interaction energy for C₂H₂ exceeds previously reported materials as detailed in Figure 4 d and Table S4, for example, SIFIX‐3‐Ni (Q _st, 36.7 kJ mol⁻¹),^[26] SIFSIX‐2‐Cu‐i (Q _st, 52.7 kJ mol⁻¹),^[30] TIFSIX‐2‐Cu‐i (Q _st, 46.3 kJ mol⁻¹)[²⁶, ³⁰] and UTSA‐300a (Q _st, 57.6 kJ mol⁻¹),^[20e] Ni₃(pzdc)₂ (7Hade)₂ (Q _st, 44.5 kJ mol⁻¹),^[23] Fe(pyz)Ni(CN)₄ (Q _st, 32.8 kJ mol⁻¹),^[31] Co(pyz)Ni(CN)₄ (Q _st, 45–65 kJ mol⁻¹),^[32] ZUL‐100 Q _st, 65.3 kJ mol⁻¹) and ZUL‐200 (Q _st, 57.6 kJ mol⁻¹).^[20b] Other approaches that focus upon chemisorption can exhibit higher energies such as the nano‐trap MOF ATC‐Cu, for which a Q _st of 79.1 kJ mol⁻¹ driven by coordination between two metal centres was reported.^[33] To further verify the energy of C₂H₂ sorption, we conducted in situ pressure gradient differential scanning calorimetry (PG‐DSC) measurements. The enthalpy obtained for C₂H₂ was in good agreement with the Q _st obtained from the Clausius‐Clapeyron equation (Figures 4 c and S24).^[34] To address gas mixture selectivity we conducted ideal adsorbed solution theory (IAST)^[35] calculations. IAST calculations are not always well‐suited for induced structural transformations but the subtle transformations upon C₂H₂ loading did not result in stepped or “S‐shaped” isotherms, instead they appeared as Type‐I isotherms. Unfortunately, selectivity calculations using breakthrough data were infeasible because of co‐adsorption and steps. Calculated IAST selectivity values at 1 bar for both trace and bulk concentrations were as follows: C₂H₂/C₂H₄ (1:1, 62.8 and 1:99, 53.1), C₂H₂/CH₄ (2:1, 2426 and 1:1, 2302) and C₂H₂/CO₂ (1:1, 8.4) (Figure 4 e and Table S4). The IAST selectivity for C₂H₂/C₂H₄ and C₂H₂/CO₂ can be compared to other leading sorbents (Table S4). TIFSIX‐2‐Cu‐i has slightly higher selectivity for 1:99 C₂H₂/C₂H₄ (55) but is lower for 1:1 C₂H₂/C₂H₄ (45) and 1:1 C₂H₂/CO₂, (6.2) at 1:1.[²⁶, ³⁰] Co(pyz)Ni(CN)₄ offers higher selectivity for 1:1 C₂H₂/CO₂ (36.5) but is lower for 1:99 C₂H₂/C₂H₄ (24.2).^[32] SIFSIX‐3‐Ni exhibited relatively low selectivity of 16.5, 17.6, 0.14 for C₂H₂/C₂H₄ (1:99 and 1:1) and C₂H₂/CO₂ (1:1), respectively.^[26] A number of highly selective materials that were studied for one gas mixture only have recently been reported, for example, ATC‐Cu (1:1 C₂H₂/CO₂=53.6),^[33] Ni₃(pzdc)₂ (7Hade)₂ (1:99 C₂H₂/C₂H₄=168)^[23] and Fe(pyz)Ni(CN)₄ (1:1 C₂H₂/CO₂=24).^[31] In order to further investigate gas mixture separations, we conducted dynamic breakthrough experiments on 1:1 mixture upon Helium dilution. These experiments revealed that the breakthrough time for C₂H₂/C₂H₄ was ≈1 h 15 mins whereas that for C₂H₂/CO₂ was ≈1 h 40 mins (Figure 4 f). C₂H₂ uptake (≈20 cm³ g⁻¹) reached half of the total capacity (40 cm³ g⁻¹) from sorption experiments and outlet purity was found to be ≈99 %. To determine the gate opening temperature for C₂H₂, C₂H₄ and CO₂, we conducted isobar experiments with increasing temperature from 195 K to 328 K (Figure S26). In the C₂H₂ isobar, the uptake capacity from 328 to 278 K remained almost constant with a value of 40 cm³ g⁻¹ at p/p ₀=1. A sudden step occurred from 268 to 258 K, when uptake capacity nearly doubled (≈77 cm³ g⁻¹). Decreasing in temperature from 258 to 238 K resulted in C₂H₂ uptake gradually increasing to ≈82 cm³ g⁻¹, a value that was maintained until 195 K. A similar sorption profile was observed for C₂H₄, but with a very low gate opening temperature. No step was observed from 328 to 238 K while a large step from 228 to 218 K with an uptake of 80 cm³ g⁻¹ was seen. This type of switching isobar trend is rare in MOFs.^[36] The CO₂ isobar exhibits a normal trend, showing a gradual increase in uptake from 40 to 82 cm³ g⁻¹ with decrease in temperature from 328 to 195 K.

Figure 4.

Pure component C₂H₂, C₂H₄, C₂H₆, CH₄, CO₂ sorption isotherms, their interaction energies and separation performance. a) C₂H₂, C₂H₄, CH₄, and CO₂ sorption isotherms for sql‐SIFSIX‐bpe‐Zn measured at 298 K, plotted in log scale. b) C₂H₂ sorption isotherms measured at different temperatures in 10 °C intervals from 273 to 328 K. c) Isosteric heat of adsorption (Q _st) calculated from Clausius‐Clapeyron equation for C₂H₂ (blue), CO₂ (purple), and C₂H₄ (red). d) C₂H₂ Q _st of sql‐SIFSIX‐bpe‐Zn in this work and other benchmark materials. e) IAST selectivity calculated for C₂H₂/C₂H₄, C₂H₂/CH₄, and C₂H₂/CO₂ gas mixtures for various ratios at 298 K and pressures up to 1 bar. f) Breakthrough separation of C₂H₂/CO₂ and C₂H₂/C₂H₄ measured at 1:1 ratio under helium flow at 298 K.

To gain insight into the sorption performance and benchmark interaction energy towards C₂H₂, we conducted in situ gas loading on single crystals, in situ coincidence PXRD and in situ FTIR measurements at 298 K (Figure 5). From in situ gas loading experiments, we obtained the structures of β′⊃2C₂H₂ and β⊃2C₂H₂ at less than p/p ₀=1. In β′, C₂H₂ molecules were found to align between the SIFSIX pillars (Figure 5 c). In the β phase, C₂H₂ molecules were found to align between the SIFSIX pillars (site‐I) and in adjacent cavities (site‐II) (Figure 5 d). In‐situ coincidence PXRD measurements from selected adsorption (1–22) and desorption points (49–65) are plotted in Figure 5 a. Point 1 indicates that the β phase remains unchanged. When the pressure is increased up to point 6, a slight difference in the peak intensity is observed at lower 2θ values, which nearly matches the calculated β′ powder pattern. Further increase in pressure up to point 18 produces a new PXRD pattern corresponding to the β⊃2C₂H₂ phase. After the desorption process, the pure β phase can be recovered as indicated in the calculated β powder pattern (Figure S11). In‐situ FTIR study of C₂H₂ adsorption reveals two stretching bands that indicate two different types of C₂H₂ binding positions in the framework (Figure 5 b). At low pressures (p/p ₀=0.001 at point 4), a band at 3200 cm⁻¹ emerges, which corresponds to C₂H₂ at site‐I. When the pressure is increased to point 20, another stretching band at 3311 cm⁻¹ appears, which we attribute to C₂H₂ adsorbed at site‐II. The two stretching bands are consistent with the binding positions identified in the in situ structures.

Figure 5.

In‐situ coincidence PXRD, FTIR and gas loading experiments at 298 K. Numbers in adsorption/desorption traces correspond to numbering of the PXRD patterns/FTIR spectra. a) In‐situ PXRD during C₂H₂ adsorption/desorption at 298 K provides insight into phase transformations and the gas diffusion mechanisms at various pressures. b) In‐situ FTIR measurements were conducted during C₂H₂ sorption at 298 K to gain insight into C₂H₂ stretching bands at different gas loading pressures. c) In‐situ gas loading structure of β′⊃2C₂H₂ measured at 298 K. C₂H₂ molecules (orange/grey) were found to occupy the region between the SIFSIX pillars. d) In‐situ structure of β⊃2C₂H₂ at 298 K and 1 bar with C₂H₂ occupying site‐I and site‐II. e) Illustration of the environment of C₂H₂ molecules in β⊃2C₂H₂, and f) Illustration of σ⋅⋅⋅π and π⋅⋅⋅π interactions in β⊃2C₂H₂.

To obtain a better understanding of the interaction energy, we analysed the in situ structures obtained at 298 K. In β′⊃2C₂H₂, C₂H₂ molecules at site‐I position form multiple C−H⋅⋅⋅F hydrogen bonding interactions (D _C⋅⋅⋅F=2.199 Å, 2.200 Å, 2.240 Å, 2.675 Å) with the SIFSIX pillars (Figure 5 c). In β⊃2C₂H₂, C₂H₂ guests are also positioned at site‐I with C−H⋅⋅⋅F hydrogen bonding interactions (D _C⋅⋅⋅F=2.716 Å, 2.766 Å, 3.014 Å) (Figure 5 d). The C₂H₂ molecules at site‐II in β⊃2C₂H₂ exhibit multiple interactions with the aromatic ligand (C−C⋅⋅⋅π with D _C⋅⋅⋅C=2.517 Å, 2.615 Å and C−H⋅⋅⋅π with D _C⋅⋅⋅C=3.448 Å, 3.353 Å) (Figure 5 e). There are also intermolecular hydrogen bonding interactions between C₂H₂ molecules (σ⋅⋅⋅π, C−H⋅⋅⋅C with D _C⋅⋅⋅C=3.316 Å, 4.153 Å, 3.636 Å and π⋅⋅⋅π, C−C⋅⋅⋅C with D _C⋅⋅⋅C=3.189 Å, 3.571 Å, 3.724 Å, 4.108 Å) (Figure 5 f). DFT calculations support the experimentally determined structure of β′⊃2C₂H₂ with C₂H₂ molecules forming favourable interactions between two SIFSIX pillars (D _C⋅⋅⋅F=1.998, 2.113, 2.151, and 2.644 Å) (Figure S31). C₂H₄ and CO₂ molecules align with the SIFSIX pillar in a less favourable perpendicular manner. The interaction distances between the SIFSIX pillar and both C₂H₄ molecules (D _C⋅⋅⋅F=2.31, 2.53, 2.71, and 3.42 Å) and CO₂ molecules (C‐O⋅⋅⋅F with D _O⋅⋅⋅F=2.57 Å) are longer than those for C₂H₂ (Figure S32). For the β⊃2C₂H₂, the calculated binding positions of C₂H₂ also support the experimental results (Figure S33). The calculated binding energies of C₂H₂ in β′⊃2C₂H₂ and β⊃2C₂H₂ and the energy of transition from β⊃empty to β′⊃2C₂H₂ and β′⊃2C₂H₂ to β⊃2C₂H₂ were determined to be −48.26 and −32.87, −17.8, and +5.29 kJ mol⁻¹, respectively. These calculations suggest that β′ is more energetically favourable than β by about 5 kJ mol⁻¹ and that the transition from β′ to β is driven by the heat released from adsorption of the C₂H₂ molecules. Further, to understand the mechanism of induced fit for C₂H₂, we analysed and compared crystal structures of β⊃empty, β′⊃2C₂H₂ and β⊃2C₂H₂. In β′⊃2C₂H₂, the alkane chain of the ligand undergoes distortion (∡_C‐C‐C=113° to 123°), the rotation angle of SIFSIX pillar largely affected (∡_M‐Si‐F=82.63° to 97.36°) and metal to F of SIFSIX bond distances becomes longer (D _Zn⋅⋅⋅F=1.98 to 2.19 Å and D _Si⋅⋅⋅F=1.55 to 1.93 Å) (Figures S27 and S29). In β⊃empty, the alkane chain (∡_C‐C‐C=116° to 118°), rotation angle of SIFISX pillar (∡_M‐Si‐F=88.31° to 91.68°) and metal to F atom of SIFSIX bond distances (D _Zn⋅⋅⋅F=2.07 to 2.09 Å and D _Si⋅⋅⋅F=1.72 to 1.74 Å) were observed to be slightly distorted (Figures S27 and S28), similar to β⊃2C₂H₂. For example, angle of alkane chain (∡_C‐C‐C=112° to 116°), angle of metal to SIFSIX (∡_M‐Si‐F=87.01° to 92.98°) and bond distances of metal to F atom (D _Zn⋅⋅⋅F=2.09 Å and D _Si⋅⋅⋅F=1.72 Å) (Figures S27 and S30). The pore cavity of F to F interactions increased in β′⊃2C₂H₂ (D _F⋅⋅⋅F=7.02 to 7.24 Å) than β⊃empty (D _F⋅⋅⋅F=6.87 to 6.75 Å) and β⊃2C₂H₂ (D _F⋅⋅⋅F=6.84 to 6.96 Å) (Figures S28 to S30). This analysis reveals that pore cavity in β′ structure is more suitable to accommodate C₂H₂ and energetically favourable than β. Therefore, we conclude that the following primary factors contribute to the induced fit mechanism in sql‐SIFSIX‐bpe‐Zn: i) degree of freedom in the structure and electronic structure of the system; ii) multiple hydrogen bonding interactions between C₂H₂ molecules and SIFSIX pillars providing tighter binding site; iii) rotational motion of SIFSIX pillar and organic ligand that can undergo contortion.

Conclusion

In summary, herein we report a new 2D flexible ultramicroporous physisorbent, sql‐SIFSIX‐bpe‐Zn, that exhibits induced fit specifically for C₂H₂ under ambient conditions and undergoes switching from open to more open phases induced by C₂H₂ and CO₂ adsorption at cryogenic temperatures. The induced fit transformation results in benchmark C₂H₂ binding affinity thanks to a tight binding site that enables multiple sorbent‐sorbate interactions. High C₂H₂ selectivity and strong separation performance was found for C₂H₄ and CO₂ binary gas mixtures. In‐situ characterization using PXRD, SC‐XRD, synchrotron diffraction, FTIR, and PG‐DSC measurements were used to elucidate the mechanism of induced fit and identify sorbate binding sites. The detailed in situ characterization results are supported by modelling studies provides insight that could be used to enable a new and general approach to the design of the next‐generation porous physisorbents: flexible ultramicroporous sorbents that exhibit induced fit for of a specific gas molecule, resulting in a selectivity hitherto unseen in rigid porous physisorbents.

Conflict of interest

The authors declare no conflict of interest.

Supporting information

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Supporting Information

M. Shivanna, K.-i. Otake, B.-Q. Song, L. M. van Wyk, Q.-Y. Yang, N. Kumar, W. K. Feldmann, T. Pham, S. Suepaul, B. Space, L. J. Barbour, S. Kitagawa, M. J. Zaworotko, Angew. Chem. Int. Ed. 2021, 60, 20383.