CH4 mixture adsorption

Jan 29, 2019 - Maria Sin , Negar Kavoosi , Marcus Rauche , Julia Pallmann , Silvia Paasch , Irena Senkovska , Stefan Kaskel , and Eike Brunner. Langmu...
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Interface-Rich Materials and Assemblies 13

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In situ C NMR spectroscopy study of CO/CH mixture adsorption by metal-organic frameworks: Does flexibility influence selectivity? Maria Sin, Negar Kavoosi, Marcus Rauche, Julia Pallmann, Silvia Paasch, Irena Senkovska, Stefan Kaskel, and Eike Brunner Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03554 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on February 3, 2019

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In situ 13C NMR spectroscopy study of CO2/CH4 mixture adsorption by metal-organic frameworks: Does flexibility influence selectivity? Maria Sin,a Negar Kavoosi,b Marcus Rauche,a Julia Pallmann,a Silvia Paasch,a Irena Senkovska,b Stefan Kaskel,b and Eike Brunnera* aTU

Dresden, Faculty of Chemistry and Food Chemistry, Chair of Bioanalytical Chemistry, Bergstrasse 66, 01062 Dresden, Germany. Fax: +49-351-46337188; Tel: +49-351-46332631; E-mail: [email protected] bTU

Dresden, Faculty of Chemistry and Food Chemistry, Chair of Inorganic Chemistry I, Bergstrasse 66, 01062 Dresden, Germany

KEYWORDS: Gas-mixture adsorption, NMR spectroscopy, flexible metal-organic frameworks, selective CO2 adsorption.

ABSTRACT: Metal-organic frameworks are promising candidates for selective separation processes such as CO2 removal from methane (natural gas sweetening). Framework flexibility, i.e., the ability of a MOF lattice to change its structure as a function of parameters like pressure, temperature, and type of adsorbed molecules is only observed for some special compounds. The main question of our present work is: Does framework flexibility influence the adsorption selectivity? As a direct quantitative method to monitor the adsorption of both, carbon dioxide and methane, we make use of high-pressure in situ 13C NMR spectroscopy of 13CO2/13CH4 gas mixtures. This method allows to distinguish between the two gases as well as between adsorbed molecules and the interparticle gas phase. Gas mixture adsorption is studied under isothermal conditions. The selectivity factor for CO2 adsorption from CO2/CH4 mixtures is measured as a function of total gas pressure. The flexible material SNU-9 as well as the flexible and the non-flexible variant of DUT-8(Ni) are compared. Maximum selectivity factors for CO2 are observed for the flexible variant of DUT-8(Ni) in its open, large-pore state. In contrast, the rigid variant of DUT-8(Ni) and SNU-9 especially in its intermediate state exhibit lower adsorption selectivity factors. This observation indicates significant influence of the framework elasticity on the adsorption selectivity.

Introduction Increasing industrialization and combustion of fossil fuels has caused a dramatic increase of CO2 emissions which severely impacts our environment, especially the climate. The capture of CO2 and its storage is therefore of high importance. Currently existing techniques for CO2 capture from flue gas include CO2 scrubbing with amines, membrane separation1, as well as absorption and adsorption techniques2. There is a wide range of materials studied with respect to CO2 capture including zeolites3, carbon materials4, porous organic polymers5 and others. Metal-organic frameworks (MOFs)6 are intensively investigated as one of the most promising platforms for separation and gas purification. MOFs are also considered as highly potent adsorbents for industrial separation processes. For example, natural gas sweetening, i.e., CO2 removal from CH4 is an attractive application7–10. Flexible MOFs represent a group of highly potent materials with unique properties. Their outstanding feature is a result of step-wise structural transitions induced by external

stimuli, like temperature, pressure, and presence of guest molecules11. In particular, the selective recognition of guest molecules, i.e. pore opening for only one species is a visionary target for highly selective adsorption and separation processes. Several cases of flexible MOFs have been reported showing different adsorption behavior12 or even guest-selective pore opening in single-component isotherms.13,14 However, a fundamental question in mixture gas adsorption is whether or not and to which extent the pore opening by a certain gas is accompanied by the coadsorption of originally non-pore opening gas molecules. For example, MIL-53(Cr) is a MOF with high selectivity towards CO2 compared to CH4 at 303 K in the narrow-pore (np) phase. However, the transition to the large-pore (lp) state at higher working capacities leads to a decreasing selectivity.10,15 In Zn2(fu-bdc)2(dabco), a representative member of the pillared layer MOF family with substituted terephthalic acid linkers, CO2 adsorption induces gating transitions at 278 K. In contrast, CH4 cannot open the pore system and is, therefore, not adsorbed by the MOF. The

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two-component gas mixture adsorption study shows increasing CH4 uptake in parallel to the CO2 adsorption after the np/lp transition. The authors conclude that CH4 is able to co-adsorb in the lp phase after CO2 induced structure transition16. 13C

solid-state NMR spectroscopy is a well-established technique to study adsorbed CO2 in porous materials such as MOFs.17-32 The adsorption of CH4 on porous media was also studied previously by NMR spectroscopy.33-37 Pulsed field gradient NMR spectroscopy can be used to study diffusion processes of CO2 and CH4 in MOFs.38,39 In the present study, we quantitatively analyze the selectivity of CO2 adsorption from a mixture with CH4 by monitoring the guest species using in situ 13C NMR spectroscopy on two representative flexible compounds. NMR is a highly appropriate technique for such investigations since the two gases are unambiguously distinguishable by a pronounced chemical shift difference. More importantly, the adsorbed phases are discriminated from the gas phases by the line broadening in case of CO2 and by the chemical shift difference as well as line broadening in case of CH4.

That means, in situ NMR spectroscopy directly monitors the adsorbate amount and its changes and not only the change in the gas phase composition - as commonly done in mixture adsorption or breakthrough experiments. This makes in situ NMR spectroscopy unique for mixed gas adsorption studies. Results and discussion Two flexible MOFs of different composition and flexibility modes were chosen for our study:

Figure 1. View on the crystal structures: (a) SNU-9 in the lp phase, (CCDC 726043); (b) SNU-9 in the np phase (CCDC 961806); (c) DUT-8(Ni) in the op phase (CCDC 760964); (d) DUT-8(Ni) in the cp phase (CCDC-1034317). C – in grey, O – in red, N – in blue, Ni – in green, Zn – in light blue.

(i) Zn2(BPnDC)2(bpy), also denoted as SNU-9 (BPnDC = benzophenone 4,4’-dicarboxylic acid, bpy =

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4,4’-bipyridine)40 and (ii) Ni2(2,6-ndc)2(dabco), also denoted as DUT-8(Ni)41 (2,6-ndc: 2,6-naphthalene dicarboxylate, dabco: 1,4-diazabicyclo [2.2.2]octane). SNU-9 is capable of changing its pore structure between the np (Fig. 1b) and lp (Fig. 1a) state during adsorption of CO2 at 195 K, i.e., it belongs to the MOFs of third generation according to the classification of Kitagawa and coworkers42. The transformation of SNU-9 from np to lp proceeds through the formation of an intermediate phase (ip) (Fig. S1, ESI)43. That means, CO2 is accommodated by the pore system of SNU-9 in the np, ip, and lp state. In contrast, CH4 is not able to initiate such transitions at 195 K at least up to a pressure of 2 bar in SNU-9 (Fig. 2, top). However, at high pressures beyond 20 bar and at 298 K, structure transformation occurs for CH4 as well (Fig. S11, ESI). This behavior makes SNU-9 an interesting compound to evaluate the selectivity for the pore opening process from np to lp state in the presence of gas mixtures. DUT-8(Ni) can be synthesized in a flexible and a nonflexible (rigid) form by controlling the particle size via the synthesis conditions.44–46 DUT-8(Ni) is flexible for crystallites larger than 1 m and rigid for particles considerably smaller than 1 m. Under the synthesis conditions applied here, average particle sizes of about 50-100 m (Fig. S6, ESI) and ca. 300 nm (Fig. S7, ESI) are obtained for the flexible and rigid variant, respectively. The structural reasons for the different behavior of the two variants are not yet fully clear. One striking difference between them is the presence of more structural defects in the rigid variant as could be demonstrated by EPR spectroscopy.46 It should be noted that these two isostructural variants are further denoted as DUT-8(Ni)_flex and DUT-8(Ni)_rigid, respectively. DUT8(Ni)_rigid is a microporous compound with the adsorption behavior typical for MOFs of second generation (Fig. 2, middle, and Figs. S9, S10, S12, ESI). Remarkably, the switchable DUT-8(Ni)_flex undergoes a pronounced onestep transition upon adsorption of gases such as CO2 (195 K), N2 (77 K) or n-butane (298 K): It can adopt a closed-pore (cp) state (Fig. 1d) after solvent removal and an open-pore (op) state (Fig. 1c) induced by gas adsorption under suitable conditions (Fig. S2, ESI). That means, DUT8(Ni)_flex is another MOF of third generation. Volumetrically measured single-component adsorption isotherms as well as combined diffraction and gas adsorption experiments reveal that CO2 does not penetrate into the cp structure below the gate pressure (Fig. 2, bottom, and Fig. S13, ESI). However, CO2 induces the pore opening transition in DUT-8(Ni)_flex beyond 0.5 bar pressure at T = 195 K and is then adsorbed in the op state.47 As expected, the total uptake of CO2 is comparable for DUT-8(Ni)_rigid and DUT-8(Ni)_flex in the op state. Remarkably, CH4 does not induce the opening transition neither at 195 K (up to 2 bar) nor at 298 K (up to 200 bar). It can, therefore, not be adsorbed by DUT-8_flex in contrast to DUT-8_rigid, (Fig. 2, middle and bottom) showing CH4 uptake as expected for microporous materials. It is, therefore, tempting to speculate that the flexible variant has higher selectivity than the rigid variant,

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if the selectivity expected from the single gas adsorption experiment could be maintained for gas mixture adsorption.

Moreover, the adsorption characteristics of the chosen materials may be relevant for highly selective CO2 separation from CO2/CH4 mixtures as applied in natural gas sweetening. In these processes, co-adsorption of CH4 after pore opening by CO2 would significantly reduce the selectivity as already discussed above. To investigate the adsorption of CO2 and CH4 from mixtures and to analyze the adsorbed species by in situ NMR spectroscopy, the previously reported home-built in situ high-pressure apparatus48 was modified to allow mixing of two gases in a desired ratio (see Fig. S14, ESI). The apparatus allows performing experiments at variable temperatures and pressures. The switching behavior and selectivity of gas adsorption from binary gas mixtures were investigated using a mixture of fully 13C-enriched CO2 and CH4 gases in the molar ratio of 2 : 1 at 215 K and up to ca. 8 bar total pressure. Fig. S15 (ESI) displays the 13C NMR spectrum of the pure mixture inside the in situ apparatus without MOF (blank measurement). Two narrow gasphase signals occur at chemical shifts of ca. 127 ppm for 13CO 13CH 2 and -9.5 ppm for 4 in agreement with the literature. Chemical shifts of ca. 126 ppm are reported for pure gaseous 13CO2.18,19 Chemical shifts of -10 to -12 ppm are characteristic for pure gaseous 13CH4.33,34,49 The deviation between the chemical shift in pure phase and the gas mixture studied here is decent and can be attributed to interactions between the different gas molecules at the relatively high applied pressure and/or the low measurement temperature. The selectivity factor, S(CO2/CH4), for carbon dioxide adsorption from the mixture is defined as follows: [CO2]ads[CH4]gas

𝑆(CO2/CH4) = [CH ]

4 ads[CO2]gas

Figure 2. CO2 (circles) and CH4 (squares) measured volumetrically single-component physisorption isotherms for SNU-9 (top), DUT-8(Ni)_rigid (middle), and DUT-8(Ni)_flex (bottom) at 195 K. Filled symbols represent the adsorption, open symbols – desorption branch.

Such special adsorption behavior would render the flexible material as ideal candidate for highly selective CO2 separation from CO2/CH4 mixtures. In mixtures, however, the pore opening induced by CO2 may also facilitate CH4 adsorption and consequently reduce the selectivity as already discussed above. The availability of the DUT-8(Ni) system as flexible and rigid variant renders this framework as an ideal candidate to study a key fundamental question in the field of switchable MOFs addressed here: Does framework flexibility (compliance, framework elasticity) affect the adsorption selectivity?

[1]

Here, [CO2]ads and [CO2]gas denote the number densities or mole fractions of adsorbed CO2 and the CO2 gas in the inter-particle space. The analogous nomenclature holds for methane. [X]ads and [X]gas can be replaced by the corresponding NMR signal intensities measured for the signals of the adsorbed and free gas (X = CO2, CH4). Calculation of the selectivity factor thus requires the spectroscopic distinction between adsorbed species and the free gas phase in the inter-particle space. This distinction is easily possible for both gases, CO2 and CH4 (cf. Figs. 3 and 4 as well as Figs. S16-S21, ESI). In the case of CO2, adsorption inside the MOF pores mainly results in a pronounced line broadening. This broad line shape easily allows the differentiation from the narrow isotropic gasphase signal despite the fact that the isotropic chemical shift of adsorbed CO2 differs only slightly from the gasphase signal.26,27 Gas adsorption from the above-described CO2/CH4 mixture (2:1) was studied for SNU-9 under isothermal conditions at 215 K under pressure variation up to 7.6 bar total pressure (see Figs. 3 and 4 as well as Figs. S16 and S19, ESI).

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approximately 150 ppm wide, i.e., much narrower than for fully immobilized CO2. This proves that CO2 molecules confined in the np form of SNU-9 are to some extent still mobile resulting in a reduced line width compared to the fully immobilized state.

Figure 3. In situ 13C NMR signals of 13CO2 on SNU-9 measured at 215 K for four different pressures using a 13CO /13CH mixture (2:1). 2 4

In addition, a narrow CO2 gas phase signal at 127 ppm occurs. Note that SNU-9 starts to transform into the ip form at 0.35 bar total pressure as expected (see Fig. S1, ESI) although the np form is still dominating at this pressure. The onset of the phase transition is indicated by the appearance of a signal at 131 ppm with a line width narrower than adsorbed CO2 in the np phase but broader than the gas phase signal. The broad CO2 signal continuously transforms into this significantly narrower signal. This narrower signal has a width of 8.8 ppm and an isotropic chemical shift of 127.3 ppm at 1.8 bar. The line further changes its chemical shift and width at increasing pressure. At 7.6 bar, it is slightly asymmetric, has a line width of 10.7 ppm, and an isotropic chemical shift of 123.5 ppm. At this pressure, the MOF adopts the lp state.

A total pressure of 7.6 bar corresponds to partial pressures of 5.1 bar for CO2 and 2.5 bar for CH4 in the initial mixture. Inspection of the single-component adsorption isotherms measured at 195 K temperature (Fig. 2) shows that SNU-9 should be in the np state up to ca. 0.2 bar CO2 pressure. Beyond that pressure, it passes through an ip state and switches at further increasing pressure into the lp state. In contrast to CO2, pure methane is not able to induce structural transformations up to 2 bar at 195 K. For SNU-9 in its np phase (0.16 bar), the very broad signal of adsorbed CO2 ranging from ca. 75 to 225 ppm resembles the line shape observed for signals dominated by chemical shift anisotropy in powder samples. This effect is caused by the pronounced chemical shift anisotropy of CO2 which is not averaged out to zero by the spatially anisotropic motions of CO2 molecules confined in the pore system of MOF particles.20-31 Note, that many of the cited references report studies of CO2 adsorption on MOFs with open metal sites as preferred adsorption sites. The MOFs studied here do not exhibit open metal sites in any of their structural states. The adsorption sites for adsorbed guest molecules like xenon were previously studied by computational methods for DUT-8(Ni).48 Such sites are for example located close to the Ni-Ni paddle wheels. However, the flexible MOFs studied here are only found in the open pore state beyond the transition pressure and are then practically filled with adsorbed molecules.48 It must therefore be assumed that the guest molecules cannot preferentially occupy just preferred sites. The line shape observed for 13CO2 can, therefore, be considered as a residual chemical shift anisotropy in analogy to other residual anisotropic magnetic interactions observed in anisotropic media or under the influence of external magnetic/electric fields.50–52 Note that solid, i.e., fully immobilized 13CO2 molecules exhibit ca. 335 ppm chemical shift anisotropy.53,54 The signal observed here is

Figure 4. In situ 13C NMR signals of 13CH4 on SNU-9 measured at 215 K for four different pressures using a 13CO /13CH mixture (2:1). 2 4

In the characteristic chemical shift region of methane below zero ppm (see Fig. 4), we observe the signal of gaseous methane at -9.4 ± 0.1 ppm at low pressures (0.16 and 0.35 bar in Fig. 4) which shifts slightly to -9.0 ± 0.1 ppm at 7.6 bar pressure. A second signal at higher shift occurs due to adsorbed CH4. At low pressure, e.g., for p = 0.35 bar, it is weak and relatively broad, centered at about –4 ppm. Pressure increase to 1.8 bar results in an increasing amount of adsorbed methane accompanied by a shift of the adsorbed 13CH4 signal to -6.9 ppm. A few ppm shift to more positive values relative to the gas phase 13C NMR signal are typical for adsorbed methane in porous materials such as zeolites and related compounds.33-37 At further increasing pressures, the signal of adsorbed methane decreases in intensity and occurs finally as a shoulder of the strong gas phase signal. Signal decomposition nevertheless allows intensity determination. The described pressure-dependent changes of the spectra parameters during the np-ip-lp transition process (cf. Fig. S1, ESI)43 are accompanied by

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pronounced intensity changes of the signals due to adsorbed CO2 and CH4 as shown in Fig. 5.

molecules at decreasing pressure. The maximum of the selectivity factor in the desorption branch of Fig. 6 is caused by this replacement of CH4 by CO2.

Figure 6. Selectivity factor for CO2 adsorption (filled symbols) and desorption (empty symbols) on SNU-9 determined from the quantitative in situ 13C NMR data shown in Figure 5.

In order to elucidate the question whether or not the flexibility of a metal-organic framework has an influence upon the adsorption selectivity, we have studied the two isostructural MOFs DUT-8(Ni)_flex and DUT-8(Ni)_rigid in isothermal adsorption/desorption experiments at 215 K (see Figs. 7 and 8) in analogy to the above-described experiments on SNU-9.

Figure 5. Signal intensities for adsorbed 13CO2 and 13CH4 measured by in situ 13C NMR spectroscopy for SNU-9 pressurized with a 13CO2/13CH4 mixture (2:1) at 215 K. A full adsorption (filled symbols and desorption (empty symbols) cycle up to 7.6 bar is shown.

Stepwise pressure increase (adsorption branch) results in a steeply increasing amount of adsorbed CO2 during the structural np-ip-lp transition. The uptake levels off at a maximum value reached in the lp state. The absolute amount of methane co-adsorbed with CO2 is relatively low in the np state. It increases in the ip state and decreases after the transition into the lp state thus passing through a maximum in the ip state. The selectivity factor shown in Fig. 6 thus exhibits a minimum in the adsorption branch when the MOF is passing through the ip state. Pressure release (desorption branch) then results in a continuous release of co-adsorbed methane over the whole pressure region, while CO2 remains adsorbed. Pressure release is accompanied by an increasing amount of adsorbed CO2 in the pore system as reflected by the unexpected signal increase during pressure release. Obviously, CO2 replaces previously adsorbed CH4

It should be noted in this context that the nickel atoms in the Ni-Ni paddle wheel behave non-paramagnetic most likely due to an antiferromagnetic coupling. This is concluded from calculations55 as well as the measured magnetic susceptibility corresponding to a volume susceptibility which is of the order of 10-5 only.41 Typically, paramagnetic susceptibilities are two to three orders larger than that. This explains the absence of a significant paramagnetic shift or line broadening for the adsorbed molecules. CO2 cannot penetrate into the pore system of DUT(8)Ni_flex in the cp state. After switching into the op state, the characteristic broad signal21 is observed (see also Fig. S17, ESI). It is narrower than for SNU-9 in its np phase and exhibits the expected characteristic shape for a residual chemical shift anisotropy. The intensity of this signal increases with increasing total pressure up to ca. 4 bar. However, the shape of this signal, i.e., its residual chemical shift anisotropy is practically constant. This indicates that the structural transition does not proceed via an intermediate state in contrast to SNU-9.

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This leads to the observed change of the real gas ratio in the sample tube, i.e., enrichment of CH4 in the gas phase. During pressure release, the intensity of the gas-phase CH4 signal drops down due to gas phase removal. The intensity of the gas-phase CO2 signal also drops down. In contrast, the adsorbed CO2 molecules remain in the pore system down to a total gas pressure below 0.5 bar as indicated by the presence and almost unchanged intensity of this broad signal. The latter observation is a consequence of the pronounced adsorption/desorption hysteresis as reflected by the NMR derived adsorption isotherm (Fig. 7, bottom).

Figure 7. Top: In situ 13C NMR spectrum of a 13CO2/13CH4 mixture (2:1) on DUT-8(Ni)_flex measured at 215 K and 4.92 bar. Bottom: Signal intensities for adsorbed 13CO2 measured by in situ 13C NMR spectroscopy. DUT-8(Ni)_flex was pressurized with a 13CO2/13CH4 mixture (2:1) and measured at 215 K. A full adsorption/desorption cycle up to 7.8 bar was measured.

The MOF is completely open and saturated with CO2 molecules at ca. 4 bar. The observation that the gate opening transition proceeds over a certain pressure range can be explained by a variable gate opening pressure of different crystallites within the powdered sample. An analogous behavior of DUT-8(Ni)_flex was already observed for other gases like xenon previously.48 This may be due to different crystallite sizes or variations of the defect concentration.44,46 Remarkably, absolutely no signal of adsorbed CH4 is detectable over the entire pressure range in contrast to SNU-9. The corresponding signal should appear between 3 and -8 ppm (see above). That means, CH4 remains in the gas phase and the co-adsorbed amount is below the detection limit also in the op state of DUT-8(Ni)_flex. The minimum selectivity factor can be estimated considering the detection limit for the adsorbed 13CH4 signal as an upper limit for the maximum possible amount of coadsorbed 13CH4 in equation [1]. This estimation yields a selectivity factor safely larger than 200. The real intensity ratio of gaseous CO2 to gaseous CH4 in the inter-particle space steadily decreases (Fig. S20, ESI) although the used initial gas ratio was 2:1. This is caused by the fact that the total pressure is increased by adding certain portions of the original 2:1 mixture to the sample tube, where the carbon dioxide is preferentially adsorbed.

To answer the question, whether the high selectivity of DUT-8(Ni)_flex for CO2 can be attributed to the flexibility or to the general adsorption properties determined by the DUT-8(Ni) structure and composition, the non-flexible form DUT-8(Ni)_rigid44 was investigated under identical conditions as the flexible variant (Fig. 8 and Fig. S18, ESI). In DUT-8(Ni)_rigid, the line is considerably narrower than in the flexible variant – but still much broader than in the gas phase (cf. Figs. S17 and S18, ESI). The latter observation might be explained by two possible effects: (i) The mean crystallite size in DUT-8(Ni)_rigid is much smaller than in DUT-8(Ni)-flex (see above). This can result in a fast interparticle exchange and thus a further line narrowing effect. (ii) The state of adsorbed CO2 could be influenced by the framework flexibility. The tendency of the MOF lattice to close is driven by the lower overall energy of the cp state and must be counterbalanced by the adsorption enthalpy of the molecules in the pore system.48 In other words, adsorbed CO2 molecules in the op state of DUT-8(Ni)_flex have to keep the pore system open in contrast to CO2 adsorbed in DUT-8(Ni)_rigid which is always found in the op state. In any case, the distinction between adsorbed CO2 and free gas is easily possible in a quantitative manner. The same is true for methane. The narrow signal of free CO2 at 127 ppm is superimposed by the broader signal of adsorbed CO2. The signals are, however, clearly distinguishable due to their strongly different linewidths. The narrow signal of CH4 gas in the interparticle space is detected at ca. -9.4 ppm. Furthermore, a signal at -5.6 ppm appears which is attributed to CH4 co-adsorbed with CO2 inside the pores. The intensity of this signal is approximately constant over the entire pressure range in the adsorption branch. That means, the non-flexible DUT8(Ni)_rigid is not fully selective for CO2 adsorption in contrast to its flexible variant. Nevertheless, the calculated adsorption selectivity shows that CO2 adsorption (see Fig. 8, bottom) is favored under the applied conditions as it can be expected from the volumetrically measured adsorption isotherms (Fig. 2). The highest selectivity factor occurs for the lowest pressure of ca. 0.4 bar. CO2 preferentially adsorbs and is effectively removed from the gas phase (cf. also Fig. S21, ESI). With further increasing pressure, however, the gas phase contains increasing amounts of CO2. This gives rise to the decreasing selectivity factor. Remarkably, the selectivity factor for the adsorption is always considerably lower than 200, which was estimated

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as the minimum selectivity factor for DUT-8(Ni)_flex under identical conditions.

(denoted as (7) in Fig. S14, ESI). The desired CO2:CH4 mixing ratio of 2:1 was adjusted by first filling 13CH4 to a certain pressure into the chamber. Afterwards, 13CO2 was added to finally result in a total pressure corresponding to three times the pressure adjusted initially for 13CH4. The pressures were always well below the critical pressure for both gases which allows to consider the gases in the mixing chamber as ideal. That means, the gas pressure is directly proportional to gas concentration/number density of molecules. Quantitative 13C NMR of the pure mixture confirmed that the desired mixing ratio was indeed obtained (cf. Fig. S15, ESI, see below). The single crystal sapphire tube was filled with sample under argon atmosphere in the glovebox. Prior to the experiments, each sample was activated under high vacuum overnight. The measurements where performed at an Avance 300 NMR spectrometer (Bruker, Karlsruhe, Germany) coupled with a BIOSPIN SA BCU-Xtreme unit (Bruker, Karlsruhe, Germany). To reach thermal equilibrium, the sample was equilibrated for 1 h after major temperature changes. The pressure was incremented stepwise by adding the required portion of the initial gas mixture to the sample tube. An equilibration phase of 15 min was performed for each of the smaller pressure increments in isothermal measurement series, respectively. The 13C NMR spectra were recorded at a resonance frequency of 75.47 MHz (13C) under 1Hdecoupling using a 10 mm probe head, a 13C pulse length of 10 µs and relaxation delay of 5 s.

Figure 8. Top: Signal intensities for adsorbed 13CO2 and 13C NMR spectroscopy. DUT4 measured by in situ 8(Ni)_rigid was pressurized with a 13CO2/13CH4 mixture (2:1) and measured at 215 K. A full adsorption/desorption cycle up to 7.8 bar was measured. Bottom: Selectivity factor for CO2 adsorption (filled symbols) and desorption (empty symbols) determined from the quantitative in situ 13C NMR data shown on top. 13CH

During desorption, methane rapidly leaves the pore system of the MOF (see Fig. 8) while the more preferable CO2 gas remains adsorbed. This is also reflected by the NMR-derived desorption isotherm. The adsorption and desorption curves for both, CO2 and CH4, do not coincide as already observed for SNU-9. During pressure release, methane leaves the pore system earlier than carbon dioxide. Methane desorption is accompanied by the adsorption of corresponding amounts of carbon dioxide, i.e., methane is replaced by carbon dioxide during pressure release before CO2 finally also desorbs at low pressure. Experimental 13C

NMR experiments were performed using a homemade in situ high-pressure gas adsorption apparatus (Fig. S14, ESI). It is equipped with a gas mixing chamber

The pulse length corresponds to a pulse flip angle of 60°. This pulse length was chosen to ensure a broad excitation bandwidth of the pulse in order to exclude excitation problems since the spectral band width is relatively high for 13C with its huge chemical shift range. The flip angle of 60° would even allow to shorten the relaxation delay to only 4 times T1 for quantitative measurements. T1-values lower than one second are found under our conditions in full agreement with the literature.32,38,56,57 The chosen recycle delay of 5 s is thus larger than 5 times T1 and ensures full relaxation of the spin system for both gases; saturation effects can be excluded. Decoupling was mandatorily necessary because the signal of methane would otherwise be a multiplet (1:4:6:4:1 quintet with 125 Hz 1J1H-13C -coupling constant35). Without decoupling, the signals of adsorbed methane and free gas severely overlap thus preventing the resolution of these two signals. Inverse gated 1H decoupling was applied which minimizes undesired NOE effects for the methane signals in order to avoid significant errors in the quantification procedure. Temperature calibration was performed using the temperature-dependence of the 1H NMR spectrum of methanol as previously described.58 The 13C NMR chemical shift was referenced relative to tetramethylsilane (TMS). Conclusions In summary, we can draw the following conclusions: (i) The selectivity of adsorption from 13CO2/13CH4 gas mixtures is quantitatively studied by high-pressure in situ

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NMR spectroscopy. It allows to selectively monitor the uptake of the different gas components from the mixture as a function of gas pressure and measurement temperature. The adsorption/desorption processes can be followed under thermodynamically controlled, isothermal conditions. The intensities of the signals due to the different adsorbed components can be quantified and selectivity factors are calculated.

AUTHOR INFORMATION

(ii) The structural transitions induced during gas mixture adsorption on flexible MOFs are monitored. It is demonstrated that the structural transition in SNU-9, i.e., the np-ip-lp transition is reflected by the spectral parameters of the adsorbed 13CO2 and 13CH4. The lowest selectivity factor occurs for SNU-9 in the ip state.

The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

(iii) Comparative investigation of two isostructural MOFs, DUT-8(Ni)_flex and DUT-8(Ni)_rigid, in isothermal adsorption/desorption experiments at 215 K revealed that the state of 13CO2 in the flexible MOF is different from the state in the non-flexible compound. The thermal motions and/or interparticle exchange processes in DUT-8(Ni)_rigid lead to a more isotropic line shape as compared to DUT-8(Ni)_flex. It may be that framework elasticity forces the gas molecules into a stress-induced spatially constrained configuration. Moreover, the selectivity factor S(CO2/CH4) of DUT-8(Ni)_flex is much higher than in DUT-8(Ni)_rigid. These observations indicate that framework flexibility can indeed influence the adsorption selectivity from gas mixtures and point towards a highly selective adsorption of the pore-opening gas species from mixtures by flexible MOFs. This would be a unique and promising feature for future applications in gas separation. Note that we have already studied the adsorption behaviour of the considered MOFs for CO2 and CH4 at high pressures and 298 K, a temperature relevant for practical applications (Fig. S11 – S13, ESI). Interestingly, the characteristic behavior observed at 195 K (Fig. 2) is preserved for all three MOFs at 298 K in single adsorption isotherms: DUT-8(Ni)_rigid is able to adsorb both gases, with some higher affinity to CO2 (Fig. S13, ESI), DUT-8(Ni)_flex can be transformed to the op phase exclusively by CO2 at 27.5 bar. The np to lp transition in SNU-9 could be obtained for both gases differing in the transformation pressure: 5.5 bar vs. 55.0 bar for CO2 and CH4, respectively (Fig. S11, ESI). In our view, the fundamental understanding of switchability phenomena in porous framework materials presented here represents a crucial foundation for a widespread applicability of such responsive materials in future applications, e.g., for gas separation.

Corresponding Author * TU Dresden, Chair of Bioanalytical Chemistry, Bergstr. 66, 01062, Dresden, Germany. Fax: +49-351-46337188; Tel: +49-351-46332631; E-mail: [email protected]

Author Contributions

Funding Sources Financial support from the Excellence Initiative by the German Federal and State Governments (Institutional Strategy, measure “Support the Best” and the German Science Foundation (Deutsche Forschungsgemeinschaft, DFG FOR2433) is gratefully acknowledged.

ABBREVIATIONS MOF, metal organic framework; DUT, Dresden University of Technology; SNU, Seoul National University, NMR, nuclear magnetic resonance.

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ASSOCIATED CONTENT Supporting Information. Scheme of the used apparatus, NMR spectra, CO2 and CH4 adsorption desorption isotherms at 298K, This material is available free of charge via the Internet at http://pubs.acs.org.

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