Influence of Co-adsorbates on CO₂ induced phase transition in functionalized pillared-layered metal–organic frameworks

Abstract

Most studies on flexible MOFs which suggest good separation properties of the framework are solely based on single component isotherms and conclusions drawn from these. However, many factors are not considered, particularly the change of the pore space after the opening of the framework can have a distinct effect on the adsorption of a second gas present in a real separation problem that does not induce the phase transition of the material. Within this study, we focus on a series of flexible pillared-layered MOFs of the type [Zn₂(fu-bdc)₂(dabco)]_n bearing flexible side chains and analyze the gas adsorption behavior when the material is exposed to mixtures of CO₂ with other adsorptives, including N₂, CH₄, C₂H₆ and C₃H₈, to evaluate the influence of the co-adsorbate on the sorption selectivity and phase transition under these conditions.

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Introduction

Among the porous materials, metal–organic frameworks are a class that has gathered a lot of attention in recent years, owing to their outstanding properties.¹ They are built up by the combination of metal clusters and multitopic organic linkers i.e. aromatic carboxylates or N-donor ligands.² The reasons for the high interest in this class of materials arise from their ultrahigh surface areas, their high degree of tunability and their structural flexibility. By combining appropriate linker molecules and metal clusters, record holding materials have been prepared with unprecedented pore volumes and surface areas.³ The aspect of tunability of material properties leads to new tailor-made materials and ranges from simple linker functionalization to more complex functionalization and tuning principles such as defect engineering,⁴ postsynthetic exchange/incorporation of metals/linkers,⁵ the precise assembly of different building blocks per pore,⁶ or postsynthetic metalation of metal sites by atomic layer deposition.⁷ Structural flexibility describes the possibility of certain frameworks to undergo a reversible phase transition between at least two distinct states.⁸ The transition between these two states can be initiated by different stimuli, including guest ad/desorption,⁹ temperature,¹⁰ light,¹¹ or mechanical pressure.¹² Frameworks that can undergo this kind of transition are denoted as flexible MOFs, 3^rd generation MOFs or soft porous crystals in the literature.^8b,13 The ability to switch depending on an external stimulus between different states makes flexible MOFs amenable for a handful of potential applications, including chemical sensing,¹⁴ as dampers,¹⁵ drug release¹⁶ or gas separation and storage.¹⁷

In our study we are focusing on several issues regarding gas separation problems in flexible MOFs, using as an example functionalized derivatives of the well-known compound [Zn₂(bdc)₂(dabco)]_n (bdc = 1,4-benzenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane).¹⁸ These so-called fu-MOFs feature alkoxy sidechains (fu) pinned to the bdc linker to yield fu-bdc (2,5-functionalized-1,4-benzenedicarboxylate). These fu-MOFs are built up from Zn₂ paddle-wheels which are fourfold coordinated by fu-bdc linkers, thus generating a [Zn₂(fu-bdc)₂]_n 2D sheet (Fig. 1). The paddle wheels are coordinated in the axial position by dabco pillars, connecting in this fashion the adjacent layers and forming a 3D structure. In the parent material with bdc as the linker only small structural changes are observed (i.e. bending of the linker molecules) when the as-synthesized DMF containing material is desolvated. In contrast, for the functionalized derivatives a large pore contraction is found after desolvation.¹⁹ This contraction is mostly initiated by interactions of the solvent-like side chains. The solvated state of the material is also referred to as the large pore (lp) form and the contracted state after solvent removal is termed as the narrow pore (np) form. Interestingly, strong sorption selectivities are initiated by the right choice of fu e.g. for C₂H₂ over C₂H₄ when the material is present in the np form.²⁰ Sorption selectivity can also be observed in related materials bearing the same fu-bdc linkers but with a different topology.²¹ Notably, during CO₂ adsorption a transition from the np to the lp phase is detected, for the functionalized pillared-layered MOFs.²²Fig. 1(e) shows the shape of a CO₂ adsorption isotherm typically observed for these kinds of fu-MOFs (pink dotted line). At low pressures the material takes up a small portion of CO₂. When a certain threshold pressure is surpassed, the pore opens which is indicated by a drastic increase of the CO₂ uptake in the material. The threshold pressure of the phase transition can be modulated by the choice of the fu-bdc linker and can be further tuned by the preparation of mixed linker MOFs.²³ Only very little N₂ uptake is monitored for these materials and the common N₂ isotherm shape is also depicted in Fig. 1(e) (green dotted line). For many flexible and guest responsive MOFs this kind of situation is observed for single component adsorption: pore opening at threshold pressure p for gas A and no pore opening for gas B. After the pore opening by A, a steep increase of the adsorption of A is observed, while the uptake of B remains low over the whole pressure range of the adsorption isotherm. From the data obtained from such single component adsorption experiments, it is usually deduced, that the material can separate a gas mixture consisting of the two components A and B. However, the entire pore surface of the flexible MOF is changed after gas A induced the np → lp phase transition in the material and hence two different scenarios are imaginable (Fig. 1(f)):

Fig. 1 (a) Schematic illustration of the building blocks of the pillared layered framework [Zn₂(bdc)₂(dabco)]_n. (b) Depiction of the functionalized derivatives 2 and 3 of the type [Zn₂(fu-bdc)₂(dabco)]_n. (c) Scheme of the structural flexibility of 1 after solvation/desolvation with DMF and (d) of 2 and 3 after solvation/desolvation. Carbon atoms are depicted in grey, nitrogen in blue and oxygen in red, respectively. Coordination environment around zinc atoms is represented by blue polyhedra and functional groups by yellow spheres. (e) Illustration of the adsorption isotherm when two gases are introduced to a flexible MOF. (f) Illustration of two scenarios that can occur when a mixture of gas A and gas B is introduced to a flexible MOF, whereas gas A induces the pore opening and gas B does not.

(a) Gas A opens the pore and is adsorbed inside the framework and gas B remains outside.

(b) Gas A opens the pore and is adsorbed inside the framework and gas B is co-adsorbed with gas A in the lp.

The first scenario would be necessary for a good separation of two compounds. The other scenario, i.e. the transport of species B into the framework opened by gas A, is less desirable and would reduce the sorption selectivity.

With our study presented herein, we wish to address these issues mentioned above. In particular, we want to pursue the question of what happens if the material is exposed to two different gases that both initiate a phase transition (i.e. np → lp). [Zn₂(bdc)₂(dabco)]_n (1) as the non-functionalized and essentially rigid reference material and the two functionalized and responsive fu-MOFs [Zn₂(BME-bdc)₂(dabco)]_n (H₂BME-bdc = 2,5-bis(2-methoxyethoxy)-1,4-benzenedicarboxylic acid) (2) and [Zn₂(DiP-bdc)₂(dabco)]_n (H₂DiP-bdc = 2,5-diisopropoxy-1,4-benzenedicarboxylic acid) (3) were employed (Fig. 2). A series of co-adsorption experiments was conducted using CO₂ (gas A) and a selection of gases B such as N₂, CH₄, C₂H₆ and C₃H₈ which feature different pore opening properties. The results gathered here should be transferable to other flexible MOF materials showing promising adsorption isotherms for gas separation applications.

Fig. 2 Illustration of the organic linker molecules used for the synthesis of the MOF materials 1–3.

Experimental section

All materials were bought from commercial suppliers, such as Sigma Aldrich, TCI and Alfa Aesar and used without further purification unless otherwise noted.

Linker synthesis

The functionalized linkers H₂BME-bdc and H₂DiP-bdc used for the preparation of fu-MOFs [Zn₂(fu-bdc)₂(dabco)]_n were prepared via Williamson ether synthesis from dimethyl 2,5-dihydroxy-1,4-benzenedicarboxylate. A detailed description of the synthetic route to obtain the organic precursors and the linker molecules can be found elsewhere.^19b,20

MOF synthesis

Compounds 1–3 were prepared under solvothermal reaction conditions only slightly modified from already reported synthesis methods.^18,19b

[Zn₂(bdc)₂(dabco)]_n (1), [Zn₂(BME-bdc)₂(dabco)]_n (2), [Zn₂(DiP-bdc)₂(dabco)]_n (3)

Zn(NO₃)₂·6H₂O (1.33 g; 4.5 mmol), H₂bdc/H₂fu-bdc (4.5 mmol) and dabco (270 mg; 2.4 mmol) were put in a beaker and 100 ml DMF were added. The mixture was sonicated until the reagents were dissolved. Afterwards, the synthesis solution was divided into three equal portions and the portions were put into three individual 50 ml screw top jar. The jars filled with the solutions were then placed in a preheated oven and left for 24 hours at 120 °C. The oven temperature was then reduced to room temperature. A white, microcrystalline precipitate formed in all three cases (off-white in the case of materials 2 and 3). The solvent from the synthesis was replaced by fresh DMF and a stirring bar was added to each vessel. The mixture was vigorously stirred for 30 minutes, in order to assure a good solvent exchange and was afterwards left to settle for 24 hours. The solvent was removed by decantation and exchanged with CHCl₃ and again stirred for 30 minutes and left to settle for 24 hours. This process was repeated a second time, also with CHCl₃. The three identical batches were combined. Now, the material was filtered and washed three times with 15–20 ml CHCl₃, transferred to a Schlenk tube, and dried in vacuo (oil pump) for 24 hours at elevated temperature (120 °C). The purity of the isolated compounds was controlled by measuring ¹H-NMR spectra (after sample digestion) and PXRD patterns (see ESI†). Yield (based on H₂bdc/H₂fu-bdc): 1 – 79% (1.03 g, 1.8 mmol), 2 – 56% (1.09 g, 1.26 mmol), and 3 – 63% (1.15 g, 1.42 mmol).

Adsorption measurements

The single component adsorption isotherms and co-adsorption measurements were performed using a Belsorp VC apparatus (MicrotracBEL Corp.) coupled with an Agilent 490 Micro gas chromatographic system equipped with a thermal conductivity detector. For each measurement approximately 1 g of sample was used. The MOF was placed in the sample cell, which was sealed with a metal O-ring and connected to the instrument. The sample cell is located within the isothermal part of the Belsorp VC and the temperature within this area can be precisely controlled. The lowest stable temperature was at 278 K and hence all measurements were performed at this temperature. Prior to each measurement, the samples were evacuated at elevated temperatures (120 °C) overnight (first measurement on sample) or at least for three hours (each additional measurement). The single component adsorption isotherms were measured volumetrically. For the co-adsorption measurements, a pre-defined gas mixture with distinct partial pressures for each component was introduced into the manifold of the instrument. In the manifold the two gases are mixed for 2200 seconds and dosed onto the sample. The gas phase was then allowed to equilibrate until the pressure variation was minimized. Afterwards, the total uptake was determined volumetrically. Subsequently a small amount of the gas phase remaining in the manifold was removed and the composition was analyzed by GC (gas chromatography). From the composition of the gas phase not adsorbed on the sample, the exact adsorbed portions of each component of the mixture on the sample were calculated. The process of sampling the gas above the sample followed by equilibration (until pressure variation is minimal) was repeated five times, thus measuring five data points with relatively similar total gas pressure and uptake.

Calculation of selectivity coefficients

The co-adsorption measurements provided experimental values for the molar fraction of both components remaining in the gas phase (y_i) above the sample and adsorbed on the sample (x_i) and hence the selectivity coefficients were calculated by the equation:

General methods

Additional data including PXRDs of the as-synthesized and activated MOFs, IR spectra, TG and NMR spectra of compounds 1–3 are given in the ESI.†

Results and discussion

In the following paragraph, all co-adsorption experiments, measurements and data are represented in a similar manner. Three different data sets were acquired in each experiment. The total uptake was determined volumetrically and is always depicted as green diamonds in the figures. The CO₂ uptake (determined by GC as described in the experimental part) is indicated by red diamonds. The co-adsorbate is depicted by a differently colored diamond (N₂, CH₄, C₂H₆ and C₃H₈ are shown in blue, black, turquoise and orange, respectively). Five measurements were undertaken during each co-adsorption experiment with the selected gas mixtures. The black arrows point from the first to the fifth measurement point. In addition, the relevant single component adsorption isotherms are plotted in the same diagrams for better understanding and comparison of the effect of the simultaneous presence of the two components.

[Zn₂(bdc)₂(dabco)]_n (1)

The first material under study, in order to test the experimental set-up and measurement conditions, was the parent MOF [Zn₂(bdc)₂(dabco)]_n and the respective single component adsorption isotherms for N₂, CO₂, CH₄, C₂H₆ C₃H₈ at 278 K are shown in Fig. 3(a). The measurements were undertaken in the range from 0 to 3200 kPa (C₂H₆ and C₃H₈ adsorptions were measured in a smaller range, due to their lower condensation pressures). It is directly visible that the adsorbed amount of CO₂ on 1 is much higher than for the other gases. The maximum uptakes are for CO₂, 330 cm³(STP) per g (1624 kPa); for N₂, 107 cm³(STP) per g (3100 kPa); for CH₄, 141 cm³(STP) per g (3050 kPa); for C₂H₆, 127 cm³(STP) per g (1047 kPa) and for C₃H₈, 95 cm³(STP) per g (189 kPa). The shapes of the isotherms do not indicate gate opening behaviour, since for all adsorbed gases no steps are visible in the adsorption isotherm. In fact, this is in good accordance with the expectations, since the parent material 1 only undergoes small structural changes upon guest loading. Fig. 1 highlights the slight structural flexibility upon ad/desorption of the solvent DMF inside the pores of 1.

Fig. 3 Depiction of the excess single component and co-adsorption measurements of material 1. N₂, CO₂, CH₄, C₂H₆ and C₃H₈ single component isotherms are blue, red, black, turquoise and orange, respectively. CO₂, N₂ and CH₄ fractions of the co-adsorption experiments are shown in red, blue and black. Total adsorption is shown in green.

Material 1 was chosen as a reference for comparison and will be also used to explain and evaluate the co-adsorption data. Fig. 3(b) shows the co-adsorption of a mixture of N₂ and CO₂. The applied mixture has a total gas pressure of 925 kPa. The partial pressures of the two components amount to 513 and 421 kPa of N₂ and CO₂, respectively. At the applied pressures barely any N₂ from the mixture is adsorbed in the pore (below detection limit). Moreover, 274 cm³(STP) per g CO₂ are taken up, which coincides nicely with the single component adsorption isotherm at the same pressure. During the CO₂/CH₄ measurement, the measured pressure points are not overlaying with the single component gas adsorption isotherms as nicely as for the CO₂/N₂ mixture. During the measurement a total gas pressure of 1324 kPa was applied to the sample, containing 640 kPa CO₂ and 713 kPa CH₄. A total amount of 169.9 cm³(STP) per g of CO₂ is adsorbed which is distinctively less than for the single component adsorption isotherm at the same pressure. From the mixture a total of 27.7 cm³(STP) per g CH₄ are adsorbed.

[Zn₂(BME-bdc)₂(dabco)]_n (2)

Among the flexible fu-MOF materials that have been introduced by Fischer and co-workers over the recent years, [Zn₂(BME-bdc)₂(dabco)]_n has been the most studied material.^10c,19,22 The material undergoes a np → lp phase transition when the desolvated np phase is exposed to suitable guest molecules (Fig. 1). In Fig. 4 the single component gas adsorption isotherms of this material measured at 278 K for CO₂ and N₂ are displayed. From the isotherms it is directly observable, that the material does not take up any N₂ at 278 K. However, the CO₂ adsorption isotherm shows the anticipated phase transition known for this material featuring a considerably wide hysteresis loop.²² In the low pressure region of the isotherm, before the np → lp transition occurs, already a decent amount of CO₂ is adsorbed (10 cm³(STP) per g). After surpassing the threshold pressure at 1700 kPa, a marked increase of the adsorbed volume of CO₂ can be observed. The overall uptake reaches a value of 83 cm³(STP) per g. The desorption of CO₂ starts at pressures below 1367 kPa and the initial plateau observed during the adsorption is reached again.

Fig. 4 Depiction of the excess single component and co-adsorption measurements of material 2. N₂ and CO₂ single component isotherms are blue and red, respectively. CO₂ and N₂, fractions of the co-adsorption experiments are shown in red and blue, respectively. Total adsorption is shown in green.

In order to characterize this gate opening and how it is affected by the presence of N₂, co-adsorption experiments were performed using CO₂/N₂ mixtures containing ratios of 986 : 1051 kPa (Fig. 4(a)) and 1469 : 1068 kPa (Fig. 3(b)). The total pressure (1975 and 2468 kPa, respectively) is higher than the characteristic single-component CO₂ phase transition pressure of material 2. However, the partial CO₂ pressure is below the phase transition pressure. A total CO₂ uptake of 15 cm³(STP) per g is measured at the first equilibration step (Fig. 4(a)) (986 kPa partial CO₂ pressure), slightly more than that of the CO₂ single component adsorption isotherm. It should be noted, that the N₂ uptake during co-adsorption is below the detection limit and thus might have an influence on the correct determination of the CO₂ uptake. During the 5 equilibration steps, the CO₂ uptake slightly increases, most likely due to the longer exposure of the material to CO₂ (higher equilibration time). For the second experiment a substantially higher CO₂ content is present in the CO₂/N₂ mixture, however the partial CO₂ pressure is still below the threshold pressure inducing the phase transition. The total CO₂ uptake is only slightly higher compared to the first experiment (18 cm³(STP) per g). As for the first experiment, the CO₂ uptake increases over the course of the five set equilibrium points. The N₂ uptake again is below the detection limit. Unfortunately, it was not possible to increase the pressure of CO₂ to surpass the threshold pressure of the phase transition due to limitations of the applicable pressures of the co-adsorption instrument. Nonetheless some valuable conclusions can be drawn. Material 2 will not undergo a np → lp phase transition for CO₂/N₂ mixtures unless the characteristic CO₂ threshold is surpassed, i.e. the pore cannot be pushed open by a great excess of N₂ in the presence of CO₂, even when the applied partial CO₂ pressure in the mixture is only slightly lower than the threshold pressure.

[Zn₂(DiP-bdc)₂(dabco)]_n (3)

Since material 2 showed some restrictions, due to the relatively high phase transition pressure under the used conditions, we chose the differently functionalized material [Zn₂(DiP-bdc)₂(dabco)]_n (3) because it reveals the lowest np → lp CO₂ phase transition pressure among our library of fu-MOFs,^19b making this material much more amenable for examining the behaviour when exposed to gas mixtures, in particular when using the specific set up of this study. Fig. 5(a) and (b) shows the single component adsorption isotherms for CO₂, N₂, CH₄, C₂H₆ and C₃H₈ recorded at 278 K. Notably, a stepped isotherm with a hysteresis loop, indicating the np → lp phase transition of the activated material is observed for all probe gases except N₂ and CH₄. The uptake of these two gases is very low. Before the np → lp transition occurs, CO₂, C₂H₆, and C₃H₈ uptakes of 6.5, 3.1 and 2.6 cm³(STP) per g were recorded. After exceeding the threshold pressures of the np → lp transition at 306 kPa (CO₂), 115 kPa (C₂H₆) and 11 kPa (C₃H₈) the uptakes were drastically increased, peaking at 92.1 (CO₂), 53.2 (C₂H₆) and 47.2 (C₃H₈) cm³(STP) per g. For N₂ and CH₄ only comparatively low values of 2.4 and 4.6 cm³(STP) per g were found. Fig. 5(c) and (d) shows the co-adsorption experiments on 3 for CO₂/N₂ mixtures. In one case the partial CO₂ pressure is kept below the threshold pressure at 290 kPa (Fig. 5(c)) and in the other case the partial CO₂ pressure is above the np → lp transition pressure at 621 kPa (Fig. 5(d)). The total pressure of the gas mixture was above the threshold pressure in both cases. The first experiment shows that an excess of N₂ (903 kPa) cannot help to induce the np → lp transition (Fig. 5(c)), as was also observed for material 2. When this mixture is applied the framework takes up 8.5 cm³(STP) per g CO₂ and barely any N₂ (amount below the detection limit). Both values are in line with the values found for the single component isotherms. For the second experiment the threshold pressure of the np → lp transition was surpassed by the applied partial CO₂ pressure (621 kPa) (Fig. 5(d)). In comparison to the previous experiment, the CO₂ uptake is significantly increased and completely in line with the uptake of 3, determined from the single component adsorption isotherm. Interestingly, the N₂ uptake is not increased (compared to the single component isotherm), attesting that the material does not take up any N₂, even when being present in the lp. Thus, this material might be feasible for CO₂/N₂ separations. After these experiments we moved on to a different gas mixture, to assess the ability of 3 to separate CO₂ from a more polarizable co-adsorbate as compared to N₂ and chose CH₄. From the single component adsorption we know that CH₄ does not induce a phase transition and the overall adsorbed amount is only slightly higher than for N₂ (Fig. 5(a)).

Fig. 5 Depiction of the excess single component and co-adsorption measurements of material 3. N₂, CO₂, CH₄, C₂H₆ and C₃H₈ single component isotherms are blue, red, black, turquoise and orange, respectively. CO₂, N₂, CH₄ and C₂H₆ fractions of the co-adsorption experiments are shown in red, blue, black and turquoise, respectively. The total adsorption is shown in green.

Again two experiments were performed. In the first run, the partial CO₂ pressure is slightly below the np → lp transition pressure and during the second experiment the partial CO₂ pressure was above the np → lp transition pressure. The results are depicted in Fig. 5(e) and (f). From the first experiment (Fig. 5(e)) it can be seen, that the adsorbed amount of CO₂ during the co-adsorption experiments is much higher than for the single component adsorption at the same pressure (23.47 cm³(STP) per g compared to 6.5 cm³(STP) per g), while the CH₄ uptake is only slightly increased (6.5 cm³(STP) per g compared to 4.6 cm³(STP) per g). We propose a synergy of the CH₄ and CO₂ co-adsorption that allows for a lower partial CO₂ pressure to trigger the np → lp transition. In the second experiment (Fig. 5(f)) it can be nicely seen that the CO₂ uptake coincides with the CO₂ uptake of the single component experiment at this pressure after the np → lp pore transition. Most interestingly, also the amount of CH₄ adsorbed in the framework is now drastically increased when compared to the first CO₂/CH₄ co-adsorption experiment. We assume that after the CO₂ induced opening CH₄ is able to co-adsorb in the lp phase. This behaviour is substantially different from the findings obtained from the CO₂/N₂ co-adsorption experiments and attributed to the much higher polarizability of the CH₄ molecule compared to N₂ (see ESI† for information on properties of used adsorptives).

In order to see how material 3 behaves when two guests are adsorbed that both initiate a np → lp transition CO₂/C₂H₆ and CO₂/C₃H₈ co-adsorption experiments were conducted (Fig. 5(g)–(i) (C₂H₆) and ESI† (C₃H₈)). Three different measurement conditions were chosen for CO₂/C₂H₆ mixtures. In the first case, the partial pressures of both gases were below the respective threshold pressures of the np → lp transition (306 and 115 kPa for CO₂ and C₂H₆, respectively). In the second and third case the partial pressure of one of the components was below the corresponding phase transition pressure and the other one was set above. The applied mixtures contained CO₂/C₂H₆ ratios of 250 : 89 kPa (Fig. 5(g)), 249 : 271 kPa (Fig. 5(h)) and 546 : 91 kPa (Fig. 5(i)), respectively. In the first measurement (Fig. 5(g)) a marked increase of the adsorption was found for both components when compared to the respective single component isotherms. Values of 24.4 cm³(STP) per g adsorbed CO₂ and 11.5 cm³(STP) per g of C₂H₆ are measured in contrast to 6.1 cm³(STP) per g and 2.8 cm³(STP) per g (single component adsorption). This suggests that strong cooperative effects between the two gases allow the gate opening transition to occur at lower partial pressures of each component. In the experiments depicted in Fig. 5(h) and (i) in both cases very similar behavior is observed. In Fig. 5(h), p(CO₂) and p(C₂H₆) amount to 249 kPa and 271 kPa, respectively. The measurement reveals, that the CO₂ uptake (28.7 cm³(STP) per g) is significantly increased in comparison to the single component adsorption (6.1 cm³(STP) per g). We conclude that the CO₂ is co-adsorbing with the C₂H₆ that opened the pore. A similar explanation can be drawn for the experiment in Fig. 5(i), where p(CO₂) is above (545.7 kPa) and p(C₂H₆) is below (90.7 kPa) the np → lp transition pressure. The CO₂ adsorption at this point nicely coincides with the single component isotherm, while the C₂H₆ adsorption at 90.7 kPa is notably increased from 2.9 cm³(STP) per g to 10.4 cm³(STP) per g, which is in line with the uptake found in the first co-adsorption experiment of these two species when both partial pressures are below the np → lp transition pressure (Fig. 5(g)).

For the CO₂/C₃H₈ mixtures similar results were found as for CO₂/C₂H₆ and a detailed description can be found in the ESI (Fig. S9†). A summary of the results found for material 3 is shown in Table 1.

Table 1 Summary of the results found for material 3

	p(CO2) < ppt^a	p(CO2) > ppt		p(CO2) < ppt	p(CO2) > ppt
a p pt is the phase transition pressure of the respective single component adsorption isotherm.
CO2/N2	No pore opening; only CO2 adsorbed in np	Pore opening; only CO2 adsorption	CO2/CH4	Pore opening before reaching ppt; enhanced CO2 uptake	Pore opening; CH4 co-adsorbs in lp

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	p(CO2) and p(C2H6) < ppt	p(CO2) < ppt; p(C2H6) > ppt	p(CO2) > ppt; p(C2H6) < ppt
CO2/C2H6	Pore opening before reaching ppt; cooperative induction of phase transition (enhanced uptake of both component)	Pore opening; C2H6 transports CO2 into pore (enhanced CO2 uptake)	Pore opening; CO2 transports C2H6 into pore (enhanced C2H6 uptake)

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Calculation of selectivities

For all co-adsorption measurements the selectivity coefficients were also calculated based on the molar fractions adsorbed on the material and left in the gas phase during each co-adsorption measurement. The calculated values for the 1^st equilibration point for material 3 are presented in Table 2 (values for the other materials and other equilibration points are listed in the ESI†). First of all, it needs to be mentioned, that the CO₂/N₂ measurements conducted on all three materials look very promising and a good separation of the two components is directly visible from the co-adsorption experiments shown before. However, the values determined during the N₂ adsorption are below the detection limit and a meaningful evaluation was not possible. However, in particular for material 3 some interesting conclusions can be drawn. In the case of the CO₂/CH₄ co-adsorption experiments a decent selectivity towards CO₂ is calculated, for the case that the CO₂ pressure is below the threshold pressure of the phase transition (5.85). In this case CH₄ helps inducing a phase transition at lower partial pressures as expected. When the CO₂ pressure is increased and the material is present in the lp, CH₄ is co-adsorbing and the selectivity is drastically increased to 11.57 (the increase in CO₂ adsorption is higher than the increase in CH₄ adsorption induced by the observed cooperative effects). From the CO₂/C₂H₆ co-adsorption measurements only very low selectivities are determined. The values towards CO₂ are in the range of 0.75 to 1.02 – the material seems to slightly favour the adsorption of C₂H₆ and a separation of these two gases over 3 does not seem feasible. The CO₂/C₃H₈ co-adsorption measurements show a clear trend towards a favoured adsorption of C₃H₈ in the presence of CO₂ (see ESI Table S1† for data). In particular at low overall pressures, when the C₃H₈ partial pressure is below the threshold pressure of the phase transition determined by the single component isotherm, a good selectivity of C₃H₈ over CO₂ (7.32 and 3.24) results. This result seems a bit odd, as the adsorbed amount of C₃H₈ is very low compared to the adsorbed CO₂. However, the molar ratios of C₃H₈/CO₂ in the gas phase are much lower than for the adsorbed state. Thus C₃H₈ is relatively enriched over CO₂ by adsorption.

Table 2 Selectivity coefficients calculated from the molar fractions of the components of the binary gas mixture in the gas phase and adsorbed on the material^a

Fig.	Material	Gas mixture	p e /kPa	Molar fraction gas phase		Molar fraction adsorbed		Selectivity (CO2)	Selectivity (co-adsorbate)
				CO2	Co-adsorbate	CO2	Co-adsorbate
a Data is from the 1^st equilibrium point. b Equilibration pressure of the gas mixture.
5(c)	3	CO2/N2	1328.59	0.47	0.53	1.00	0.00	—	—
5(d)	3	CO2/N2	1347.56	0.22	0.78	1.18	−0.18	—	—
5(e)	3	CO2/CH4	1323.98	0.48	0.52	0.84	0.16	5.85	0.17
5(f)	3	CO2/CH4	1167.08	0.24	0.76	0.79	0.21	11.57	0.09
5(g)	3	CO2/C2H6	336.52	0.74	0.26	0.68	0.32	0.75	1.33
5(h)	3	CO2/C2H6	511.79	0.48	0.52	0.48	0.52	1.02	0.98
5(i)	3	CO2/C2H6	630.01	0.86	0.14	0.84	0.16	0.87	1.15

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Conclusions

The results leave a lot of room for discussion and further experiments and theoretical modelling. Below we summarize our findings (I–IV) regarding the adsorption of binary gas mixtures on fu-MOFs of the type [Zn₂(fu-bdc)₂(dabco)]_n and suggest some conclusions that might hold true for other flexible MOFs.

(I) The presence of N₂ is not affecting the CO₂ uptake in fu-MOFs. The results found for 2 and 3 show that the phase transition cannot be shifted to lower pressures and N₂ does not assist in the pore expansion process.

(II) N₂ is not adsorbed in the pore after the CO₂ threshold pressure is surpassed and the np → lp phase transition occurred.

(III) More polar guests, with stronger adsorption enthalpies at the adsorption sites and more favourable interactions with the pendant side chains can help inducing a preliminary CO₂ induced phase transition. Additionally, CH₄ can travel into the pore with the adsorbent and the CH₄ uptake is strikingly higher than during single component adsorption when only the np is present.

(IV) Adsorbates that show stepped isotherms (gate opening behaviour) can induce strong cooperative effects with CO₂. In the presence of C₂H₆ and C₃H₈ the CO₂ adsorption is strongly increased below the CO₂ phase transition pressure.

We conclude that a separation of two gases that possess a two-step isotherm over a flexible MOF is highly unlikely. The gas with the lower threshold pressure will open the pore and considerable amounts of the second gas will be adsorbed in the pore as well. Nevertheless, if the concentrations/pressures of both components are wisely chosen, our data suggest that a separation can be achieved under some specific conditions.

Finally it needs to be highlighted that the np → lp phase transition is a phenomenon governed by thermodynamics. The energy needed for the phase transition must be compensated by the adsorption enthalpy of the respective gases at lp. From our recent molecular dynamics simulations and NMR studies we know that the favoured adsorption sites for CO₂ within this class of fu-MOFs are located in the vicinity of the carboxylate groups connecting to the metal-ion.^23a Thus, the interactions of the guest with the side chains need to be taken into account in order to pass through and reach the adsorption site. Furthermore, the attractive side-chain/side-chain and side-chain/linker interactions favour the pore contraction and need to be compensated to initiate the phase transition. In case of co-adsorption, both components (guests) contribute to the over-all energetics of the np → lp gate-opening phenomenon. It is thus quite conceivable that a cooperative gate opening is possible (as we observed) in case of setting the partial pressures of the two components below the individual phase transition pressures (CO₂/C₂H₆ and CO₂/C₃H₈). If a guest is chosen (N₂) exhibiting low uptake and adsorption enthalpies for the np (experimental evidence) and also (presumably) in the lp state of the material, it will neither assist in the pore opening nor will it significantly co-adsorb. In the case of CO₂/CH₄ experiments, we assume that the adsorption enthalpy of CH₄ is increased in the lp as compared to the np state (increased accessibility of favourable adsorption sites, less hindrance by the side chains to reach adsorption sites). We are aiming to understand the effects reported in this study more quantitatively by focusing on a comprehensive thermodynamic analysis of the co-adsorption process in combination with molecular dynamics simulations in the future. Nevertheless, we suggest some exciting potential functions for flexible MOFs from the current results. For example, it is imaginable that the one gas A of a mixture can work as a switching gas. The second gas B can undergo a reaction or cause a signal inside the framework, only if a certain partial pressure of gas A is set. The obvious necessity of fine-tuning the chosen flexible framework could be done by choosing a suitable linker-functionalization as it is offered by our library of fu-MOFs including mixed-component (or multivariate) fu-MOFs (with different metal ions, linkers and functionalized side chains). Such systems may be tailored to operate at different temperature and pressure regimes and feature guest-specific and adjustable gate-opening properties.

Acknowledgements

A. S. gratefully acknowledges the Ruhr University Research School PLUS for a travel grant to conduct experiments at Hokkaido University, Japan. Ruhr University Research School PLUS is funded by Germany's Excellence Initiative [DFG-GSC 98/3]. The authors gratefully acknowledge Michael S. Denny Jr. (University of California, San Diego) for proof checking the manuscript and helpful discussion.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: PXRDs, NMRs, IR spectra, TGs of the compounds, details on the measurement method and additional selectivity coefficients. See DOI: 10.1039/c6ta03266d

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