Adsorption of Small Molecules on Cu3 (btc) 2 and Cu3âx Zn x (btc) 2

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Adsorption of Small Molecules on Cu3(btc)2 and Cu3-xZnx(btc)2 Metal-Organic Frameworks (MOF) as Studied by Solid-State NMR Farhana Gul-E-Noor, Matthias Mendt, Dieter Michel, Andreas Pöppl, Harald Krautscheid, Jürgen Haase, and Marko Bertmer J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp400869f • Publication Date (Web): 21 Mar 2013 Downloaded from http://pubs.acs.org on March 23, 2013

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The Journal of Physical Chemistry

Adsorption of Small Molecules on Cu3(btc)2 and Cu3-xZnx(btc)2 Metal-Organic Frameworks (MOF) as Studied by Solid-State NMR

Farhana Gul-E-Noora, Matthias Mendta, Dieter Michela, Andreas Pöppla, Harald Krautscheidb, Jürgen Haasea, Marko Bertmera*

a

Institute of Experimental Physics II, University of Leipzig, Linnéstr. 5, D-04103 Leipzig, Germany

b

Institute of Inorganic Chemistry, University of Leipzig, Johannisallee 29, D-04103 Leipzig, Germany

* corresponding author, phone: +49 341 9732617

FAX: +49 341 9732649

E-mail: [email protected]

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ACS Paragon Plus Environment


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Abstract Static and MAS

13

C NMR techniques are used to investigate the interaction of CO and

CO2 molecules with the host structure of the MOFs Cu3(btc)2 and Cu2.97Zn0.03(btc)2. A defined amount of

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C enriched molecules per copper atom was adsorbed. The

13

C chemical shift

anisotropy and isotropic chemical shift were studied over a temperature range from 10 to 353 K. Already above 30 K an isotropic line for CO is found superimposed to the solid-like spectra belonging to the majority of adsorbed CO molecules. For adsorbed CO2 an isotropic line can be detected above 70 K. This observation reflects differences in the local motion of both molecules. At high temperatures it is found that CO is desorbed more easily from the MOF framework in comparison to CO2. This is in agreement with conclusions derived from desorption measurements on Cu3(btc)2. From the temperature dependence of the chemical shift for adsorbed CO2 molecules (measured by means of de-convolution of the overlapping

13

13

C MAS NMR between 213 and 353 K) and from the

C NMR lines for adsorbed CO molecules (between 180 K

and 323 K), the activation energy for the local motion of the adsorbed molecules was determined as 3.3 kJ/mol and 6.1 kJ/mol, respectively. Additionally, the motion is accompanied by a partial desorption of the adsorbed species.

Keywords: Cu3(btc)2 , Cu3-xZnx(btc)2 , Metal-organic framework, gas, adsorption, SSNMR

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Introduction Metal-organic frameworks (MOFs) are porous inorganic-organic hybrid materials that possess high surface area and pore volume.1-3 These versatile materials found their application in various sectors like gas adsorption4-8 and separation9, heterogeneous catalysis10, luminescencebased sensors2, ion exchange11, magnetism12 and are also promising for drug-release carrier in medical applications13. Among the various MOFs some have open metal sites, which are unsaturated upon activation and are assumed to play a vital role in gas adsorption, storage, and separation.14-19 Cu3(btc)220 is one of the prominent MOFs with coordinatively unsaturated metal sites for which potential applications in gas storage and gas separation have been demonstrated.8-9, 21, 22 Sorption properties of Cu3(btc)2 were studied experimentally and by means of molecular simulations for several gas molecules (N2, CH4, CO, CO2, N2O etc.) including the selectivity for an effective separation process (e. g. CO2 and CH4).21,

23-24

Apart from this, an experimental study of the

interaction of the gas molecules with the host material, the identification of the actual adsorption site as well as the investigation of the dynamics of the adsorbed molecules is lacking that would help to better understand the adsorption behavior with the host structure. Wu and Peterson et al. identified the main adsorption sites of CO2 and D2 in Cu3(btc)2 by neutron diffraction studies and observed that the open metal sites are the most favorable ones.25-26 Besides this, the presence of small (∼ 0.5 nm) and relatively large pores (∼ 0.9 to 1 nm) also play an important role for the interaction with the probe molecules. Prestipino and Bordiga et al. studied the host-guest interaction by infrared spectroscopy.22, 27 Very few studies have been conducted to investigate the adsorption of gas molecules at the MOF material by nuclear magnetic resonance (NMR) spectroscopy

28-32

, although NMR is extensively used to study gas adsorption in zeolites,

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molecular sieves, metal catalysts, activated carbon materials and others.33-44 The advantage of NMR is that it is a unique tool to explore the nature of the adsorbed molecules and their interaction with the host surfaces and it is element-selective.34,38-44 It is well known that the NMR spectra are also influenced by the coupling between the nuclear spin moments and the electron spin ones. In case of the studied MOF Cu3(btc)2 there are so-called paddle wheel units with antiferromagnetically coupled Cu2+ S = ½ electronic spins resulting in an S = 1 excited (at temperatures above ca. 70 K) and S = 0 electronic ground state.45 In this work we also use mixed-metal MOFs Cu3-xZnx(btc)2 characterized in our previous papers where the partial substitution of copper by zinc additionally results in isolated S = ½ spin states.46-47 The latter play an important role in the EPR spectra at low temperatures.46 In order to elucidate the behavior of the adsorbed molecules and the information of molecular dynamics in the MOFs by means of NMR spectroscopy, the information on the electronic structure of the MOFs as derived from EPR spectroscopic data for the MOFs used here and for similar substances48-49 is of great value. In this paper, we report on the adsorption of defined amounts of

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CO and

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CO2 on

Cu3(btc)2 and mixed-metal Cu3-xZnx(btc)2, respectively. A detailed analysis is performed by 13C NMR, using also MAS measurements, to follow the adsorption process and the interaction between the gas molecules and the host structure. From the variable temperature study down to 10 K a detailed understanding of the dynamic behavior of the adsorbed gas molecules is obtained as well as information on the adsorption kinetics is available.

Experimental Sample preparation for gas adsorption

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Cu3(btc)2 and Cu2.97Zn0.03(btc)2 were used for the adsorption of gas molecules whose synthesis procedure is described elsewhere.9,46 It has been reported previously that magnetic properties of this MOF can be tuned by Zn2+ incorporation and that little doping has no significant influence on the NMR spectra but is important for EPR spectroscopy especially at low temperatures46-47 as already mentioned above. In this paper we apply 13C NMR on adsorbed 13

C enriched (99 %) CO and CO2 (ISOTEC). Prior to gas adsorption, the samples were placed in

special glass tubes and activated (dehydrated) in vacuum at 383 K for 24 h to remove residual water molecules coordinated to the copper sites. Then, a defined amount of CO corresponding to 1, 2 and 5 molecules per copper atom was adsorbed via the gas phase on MOF Cu2.97Zn0.03(btc)2. Then the adsorption was carried out in a pressure controlled vacuum apparatus at 77 K. Afterwards the glass tubes were sealed to prevent further modification of the samples. Following the above procedure, CO2 corresponding to 1 and 2 molecules per copper atom was adsorbed on MOF Cu3(btc)2. NMR measurements were performed immediately after the sealing procedure. The notation of the different samples indicates the number of gas molecules per copper atom, e. g., 2 CO/Cu corresponds to 2 molecules of CO per copper atom.

NMR measurements 13

C NMR experiments for CO and CO2 molecules adsorbed on Cu2.97Zn0.03(btc)2 and

Cu3(btc)2 were carried out at magnetic fields of 7.05 and 9.4 T. To improve the resolution in the 13

C NMR spectra, in some cases MAS (magic-angle spinning) experiments are performed.

13

C

static spectra at variable temperature in the range between 10 and 323 K were carried out at a magnetic field strength of 7.04 T, using a TecMag Apollo spectrometer at a frequency of 75.468 MHz. The experiments were run with a 3 mm NMR sample tube using a cryostat. The

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temperature was controlled by an ITC-503S (Intelligent Temperature Controller) within an accuracy of ± 1 K. Due to the low natural abundance of 13C and the broad lines in the 13C NMR spectra of the MOF carbon atoms47, the contribution to the signal from other components than the gas molecules can be neglected. At room temperature the NMR spectra were recorded using a single pulse with 90° pulse lengths of 7 µs and 5 µs for the

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CO and

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CO2 containing

samples, respectively. The pulse length was shorter at lower temperature (3.5 µs) due to improvement in probe Q-factor. The recycle delays were more than five times the longitudinal relaxation time T1. MAS NMR experiments were carried out using a Bruker AVANCE 400 spectrometer (magnetic field strength 9.4 T) with a

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C resonance frequency of 100.613 MHz. The

experiments were performed using a 4 mm MAS probe at a spinning speed of 4 kHz at room temperature (298 K) unless stated otherwise. The spectra were recorded using the DEPTH sequence50-51 to remove probe background signals. The 90° pulse lengths were 2.5 µs and 3.2 µs for 13CO and 13CO2 adsorbed samples, respectively. The recycle delays were 500 ms and 700 ms for

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CO and

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CO2 adsorbed samples, respectively, which is more than five times the

longitudinal relaxation times T1 as measured with the inversion recovery technique. For the temperature dependent measurements in the range between 213 and 353 K, the temperature calibration was done via the temperature dependence of the isotropic chemical shift of Pb(NO3)2.52 All the spectra were referenced to TMS using adamantane (high-frequency methylene carbon at 38.56 ppm from TMS) or ethanol (methylene carbon at 56.83 ppm from TMS) as secondary references.

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Results 13

C NMR spectra of adsorbed CO (measurements without and with MAS) Figure 1 displays the

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C spectra of sample 2 CO/Cu (adsorbed on Cu2.97Zn0.03(btc)2)

recorded in the temperature range from 10 to 323 K. At 10 K the spectrum exhibits a typical anisotropic powder pattern of rigid CO with an isotropic chemical shift of 175 ppm. For the further analysis it is not essential that the anisotropic line shape for measurements at 10 K is slightly distorted in comparison to the measurements at 30 K due to the very long relaxation time T1 and the changes in the Q factor. With increasing temperature a narrow resonance line at 180.4 ppm appears as can be seen already in the 30 K spectrum. This narrow line indicates an isotropic motion of the CO molecules inside the MOF framework. The presence of a reorientation of the CO molecules inside the MOF at low temperature is not surprising since it has also been observed in silicalite.39 The simulation was carried out for all the spectra and is shown for the 30 K spectrum in supplementary Figure S1, which clearly indicates the presence of two components (broad and narrow) corresponding to two different fractions of CO (i. e. rigid and mobile). The simulation was done using the dmfit program.53 When the temperature is raised the fraction of the mobile part of the CO inside the MOF framework increases on the cost of the rigid part. The temperature dependence of the intensity ratio Irigid / Imobile is shown in Supplementary Figure S2. A gradual decrease in the intensity ratio with increasing temperature is observed and at 110 K the rigid part completely disappears. At 180 K another sharp resonance at about 183.5 ppm starts to appear (better visible in the 220 K spectrum) which fairly resembles the shift obtained for CO gas (183.7 ppm) without MOF material, as measured separately. Therefore, this resonance can be directly assigned to free CO outside the metal organic framework. The appearance of gaseous CO above 150 K strengthens our interpretation that below this temperature all CO molecules stay

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inside the MOF framework. The relative intensity of the gaseous CO increases as the temperature increases and at 323 K it is about 36%. As a consequence of the experimental findings for CO (and in the following also for CO2) an essential point in the following discussion will be the elucidation of exchange phenomena between various molecular species in the present systems. As well known the respective time scale is given by the chemical shift differences.54 In case of free CO and CO in the MOF we find ca. δ ≈ 5.85 ppm at 323 K, corresponding to a characteristic time of τ = 1/(2πδ fres) ≈ 3.6 × 10-4 s (NMR measurements at fres = 75.468 MHz). Hence, as can be inferred from the good separation of the two lines for gaseous and confined CO, the chemical exchange between these species is slow on this NMR time scale. Another resonance at 125.5 ppm is observed for all samples and is interpreted as CO2 assuming that some CO2 is formed due to disproportionation at the top of the glass ampule because of the higher temperatures during the sealing procedure. Figure 2 shows the temperature dependence of the (apparent) chemical shift anisotropy (δ║-δ┴) of the adsorbed CO. Different chemical shift anisotropy (CSA) parameters of solid CO are reported at different temperatures, namely -365 ± 20 ppm55, -353 ppm56 and -335 ± 20 ppm55 at 4.2, 20, and 46 K, respectively. In the case of CO adsorbed on Cu2.97Zn0.03(btc)2 a value of about -375 ± 6 ppm is found at 10 K. A decrease in the spectrum width is observed with increasing temperature as discussed later. At low temperature (30 to 100 K) no temperature dependence of the

13

C resonance

position is observed (Figure 3). Larger 13C NMR line shifts occur when the temperature is raised. In the intermediate temperature range (110 to 180 K), the 13C NMR line is shifted to lower ppm values. This shift is related to the increasing population of the S = 1 state for the Cu2+ metal ions

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in the paddle wheels. At still higher temperature (above 180 K) the resonance line is shifted to higher ppm values due to an increasing exchange between the molecules attached to Cu2+ and the remaining ones in the gas phase and/or not bound to copper. Finally, magic angle spinning (MAS) is introduced to better resolve the spectra for samples with 1, 2 and 5 CO/Cu which are compared in Figure 4. The sample containing the lowest loading (1 CO/Cu) shows only one resonance line while for the higher amounts (2 and 5 CO/Cu) the spectra consist of two resonance lines, a narrow one and a relatively broad one. The chemical shift of the narrow line is 183.5 ppm and is assigned to gaseous CO. The broad line at about 181.5 ppm is assigned to mobile CO that stays inside the MOF in physisorbed state (as is discussed below). When the amount of CO is higher the intensity of the lines increases accordingly and the ratio between gaseous and physisorbed CO increases. We do not estimate the corresponding fractions from the 13C MAS NMR spectra because we will not discuss here the complete side band spectra in dependence on the MAS frequency. The resonance at 125.5 ppm is interpreted as CO2 (see above). From all the spectra it is clear that most of the CO molecules are present outside the MOF framework and only a small amount is located inside the MOF at room temperature indicating a weak interaction with the MOF framework. From the three samples, sample 2 CO/Cu was investigated in more detail as shown above.

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C NMR spectra of adsorbed CO2 (measurements without and with MAS) Figure 5 displays the

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C spectra of 2 CO2/Cu adsorbed on Cu3(btc)2 recorded in the

temperature range from 10 to 323 K. Similar to CO, the 10 K spectrum exhibits a typical powder pattern resulting from chemical shift anisotropy for the rigid CO2 with an isotropic chemical shift of 129 ppm. A similar spectrum appears at 30 K. The simulation of the 10 K spectrum is shown

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in Supplementary Figure S3, which indicates the presence of rigid CO2 molecules. Further increase of temperature changes the spectrum dramatically, with a narrowing of the anisotropic line shape which completely disappears at 75 K. Upon raising the sample temperature the mobility of the CO2 molecules increases and with that the width of the powder pattern line decreases. The occurrence of a narrow line above 75 K supports our observation. From 90 K till 290 K only one narrow line is observed revealing changes in the linewidth. Figure 6 represents the temperature dependence of the (apparent) chemical shift anisotropy (δ║-δ┴) of the adsorbed CO2. For solid CO2 a chemical shift anisotropy (CSA) of -335 ppm is reported at about 20 K.56 In the case of CO2 adsorbed on Cu3(btc)2, a value of about -342 ± 3 ppm is found at 10 K, which very well resembles the value found in the literature for pure CO2. Similar to CO a decrease in the spectrum width is observed with increasing temperature as discussed later. Below 60 K the 13C chemical shift derived from the NMR spectra is nearly temperature independent. On the other hand, the isotropic chemical shift of the mobile CO2 shows a temperature dependence and the 13C resonance line is shifted to lower ppm values upon raising the temperature from 60 K to 220 K (Figure 7). It is important to note that above 220 K the resonance line is shifted to higher ppm values. Details will be discussed later. Moreover, the static spectra of samples with 1 and 2 CO2/Cu were recorded at room temperature and are compared in Supplementary Figure S4. For both samples only one resonance line at about 126 ppm can be observed. A further line shape analysis shows that both a Lorentzian and Gaussian line shape can be assumed within the limits of experimental error. This is the reason why linewidths are not further discussed in order to derive conclusions about thermal mobility. Apparently, the linewidths reflect a distribution of local magnetic fields. In

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agreement with this suggestion, MAS NMR spectra were recorded which reveal an appreciable line narrowing, i.e., different 13C lines can be distinguished. The 13C MAS spectra for sample 2 CO2/Cu were measured in the temperature range from 213 to 353 K and are shown as a stack plot in Figure 8. In the spectrum at 353 K two resonance lines at 126 ppm and 125.5 ppm can be differentiated with linewidths of 14 and 23 Hz, respectively. The resonance peak at 125.5 ppm is assigned to the gaseous CO2 in agreement with the chemical shift of the free gas. The resonance line at 126 ppm is related to the physisorbed CO2 which stays inside the metal organic framework. With lowering the temperature the 125.5 ppm signal vanishes at about 273 K, showing only one resonance line with decreasing shift. Accordingly, a clear temperature dependence of the chemical shift is observed for the physisorbed CO2 (Figure 8) and at high temperature the resonance is shifted to higher ppm values.

Discussion 13

C NMR spectra at very low temperatures As shown in Figures 1 (for CO in the MOF) and 5 (for CO2), solid like 13C NMR spectra

appear in a wide range of temperatures (10 to 100 K for CO and 10 to 60 K for CO2) indicating the presence of rigid CO and CO2 molecules. First of all it is interesting to consider the chemical shift anisotropy of the

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C NMR tensors derived from the spectra at very low temperatures

(Figures 2 and 6). The observed values for the rigid CO molecules at 10 K to 50 K differ only slightly from the data observed in the literature at various low temperatures.55-56 Also in the case of CO2 the CSA value is very close to the value found in the literature for the rigid molecules.56 The 13C NMR line shape analysis at the lowest temperatures shown in Supplementary Figures S1 and S3 reveals that we may approximately explain the line shape as originating from CSA. Since

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in case of CO the spectrum at 10 K is apparently distorted due to relaxation effects, a reasonable fit is based on the spectrum for CO measured at 30 K. For CO2, at low temperatures the agreement between the experimental spectra with the evaluated CSA line shape for CO2 is nearly perfect (Supplementary Figure S3). However, we cannot exclude a possible disorder of CO2 orientation and position at low temperature which was observed by Wu et al. by means of neutron diffraction studies of CO2 adsorbed in Cu3(btc)2 and MOF-74.25 From our CSA line shape analysis at low temperature it is not possible to unambiguously state the presence of any distortion in the adsorbed CO2. Obviously, the decrease of the apparent CSA values at higher temperatures (above ca. 50 K for CO and ca. 30 K for CO2, see Figures 2 and 6, respectively) is related to an increasing mobility as discussed below. For CO already at 30 K and higher temperatures an additional symmetric line appears with a center at 180.4 ppm, for CO2 this mobile component is not observed below 50 K. If we derive the isotropic chemical shifts from the center of gravity of the spectra we also observe only a small difference in comparison with the values reported in the literature for solid and liquid samples56 and gaseous CO (without MOF) measured in this work. A similar situation is observed in the case of CO2.56 Thus, apart from the peculiarities at very low temperatures, we may conclude that an influence of adsorption on the copper sites, where a certain number of CO/CO2 is present per Cu atom, cannot be inferred unambiguously from the CSA values. Therefore, no clear information about the adsorption and the exact location of the molecules can be derived since the isotropic chemical shift and the CSA values are not different from those in the solid and liquid state and since, moreover, other information in relation to the distances of the molecules from the Cu atoms is not available from the NMR measurements. Additionally, we could not measure MAS spectra at those low temperatures. It is known that in Cu3(btc)2 the

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antiferromagnetically coupled Cu2+ ion pairs are in the S = 0 electronic ground state at low temperature. Hence, the copper ions show no paramagnetic moments and an influence of the adsorption on the Cu sites cannot be inferred from the 13C NMR spectra at low temperatures. The CO adsorption was studied for the MOF Cu2.97Zn0.03(btc)2. For this system a small number (only 1%) of Cu2+ is present in the S = 1/2 spin state. This system has recently been used for electron spin resonance studies.57 Also here no influence on the NMR spectra can be detected which may be easily understood since the number of these paramagnetic ions is very small and at low temperatures there is no exchange between the small number of adsorbed CO molecules sitting near the paramagnetic sites and the remaining majority of adsorbed species. Even a dynamic averaging due to spin diffusion cannot be detected.

Influence of motion with increasing temperatures In case of CO the shape of the solid like spectra remains fairly the same from 10 K up to about 50 K (Figure 2) which indicates that the adsorbed CO molecules are rigid and an influence of motion is not detectable. When the sample temperature is raised the apparent chemical shift anisotropy changes and a very strong narrowing of the line width in a small temperature range (50 to 100 K) is observed. At 110 K no influence of the anisotropy may be derived from the spectra but still broad lines are measured. Besides the broad line a relatively narrow line is observed for CO and the chemical shift is again similar to solid, liquid, and gaseous CO. The presence of an isotropic line at very low temperature indicates that some CO molecules (only 4% at 30 K) are freely moving and reveal isotropic reorientation in the adsorbed state. With increasing temperature the intensity of this line increases and above 100 K all the CO molecules possess isotropic motion. In the light of this discussion we interpret the changes in the spectra

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between 50 and 100 K by an increasing anisotropic rotational motion of the majority of CO molecules. A similar effect in the anisotropic line shape is observed for CO2 (Figure 6). The averaging process starts here at 30 K and at about 75 K an isotropic line appears. But at lower temperatures no isotropic line is detectable for CO2 in contrast to CO. Above 70 K the excited S = 1 electronic spin state starts to become populated while at lower temperatures the S = 0 ground state is dominating (Supplementary Figure S5) as calculated from the Boltzmann distribution. An increasing influence of the S = 1 spin state on the 13C NMR line of CO and CO2 is observed upon raising the sample temperature above 100 K for CO and 70 K for CO2 which can be clearly followed from the temperature dependence of the chemical shift (Figures 3 and 7). For the CO containing sample this effect is not significant below 100 K due to higher linewidth. From the figures it is evident that for both molecules the isotropic chemical shift gradually changes to lower ppm values in the temperature range between 110 to 180 K for CO and 75 to 220 K for CO2. In this intermediate temperature range the CO and CO2 molecules already reveal an isotropic motion (as discussed above) and at the same time the population of the S = 1 spin state is increasing (Supplementary Figure S5). Therefore, it is assumed that there is always an exchange between those CO/CO2 molecules interacting with Cu2+ and other ones which are adsorbed on other places inside the host structure (as an excess of CO/CO2 per Cu is present) leading to a chemical shift to lower ppm values (up-field). The up-field isotropic chemical shift is related to the negative hyperfine (hf) coupling and depends on temperature.58-59 Based on that we would also expect a negative hf coupling what has been observed in previous experiments for the Cu—Zn—H2O system.47 The EPR study indicates a negative hf coupling, too.57

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An increase of the sample temperature above 180 K (for CO) gradually changes the chemical shift to higher ppm values and at the same time another resonance line is visible which may be assigned to the gas phase CO (see section ‘Results’). This observation supports our interpretation that CO starts to become free and possesses a weak interaction to the Cu2+ and at a certain temperature only a gas phase CO would be present. The clear separation of the two lines (gas phase and adsorbed but mobile CO) indicates a slow exchange rate. On the other hand, for CO2 only one line is observed up to 290 K and we cannot yet observe a line which could be assigned to gaseous CO2. At temperatures higher than 220 K this single line is slightly shifted to higher ppm values. But it is not clear whether this line is superimposed by another signal. In order to better resolve the lines, MAS NMR spectra were recorded above 200 K and a clear temperature dependence is observed (Figure 8). From the MAS spectra it is apparent that a second line (gas phase CO2) appears only at appreciable higher temperatures in contrast to CO, and at 353 K its fraction is only 13 %. This supports the suggestion that CO2 is stronger bound to the MOF than CO. In the further discussion we will not consider the differences in the linewidth for adsorbed CO and CO2 molecules which can be observed in the spectra because it is not possible to separate the various contributions to them in an unambiguous manner. Hence, besides the differences in the mobility of the molecules, the deeper analysis of which would be interesting, other influences would have to be taken into account (exchange dynamics, different relaxation mechanism, and inhomogeneity in the local magnetic fields) which are not accessible.

Exchange dynamics To derive dynamical data from the temperature dependence of the

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C NMR spectra for

adsorbed CO and CO2 molecules, we will start from the following experimental findings: in case

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of CO, two distinct features are observed in the temperature range between 180 and 323 K, viz., the appearance of two different lines attributed to CO molecules in the gas phase (outside the MOF) and to those adsorbed in the MOF as already mentioned. It is important to note that a similar behavior for CO2 with two different lines is only observed at appreciably higher temperatures. This behavior clearly shows that the probability for the desorption of CO2 from the MOF to the gas phase is much smaller for CO2 in comparison to CO. On the other hand, the 13C NMR lines which are to be assigned to the molecules adsorbed in the MOFs, reveal a similar temperature dependent shift (to higher ppm values) in both cases. This temperature dependence reflects the dynamical behavior of the molecules in the MOFs and can be understood in terms of a fast exchange process between molecules attached to the Cu2+ (Padsorbed) and the remaining ones within the MOF not in direct contact to Cu2+. The situation described leads us to the following model for the interpretation of the NMR spectra: The appearance of two lines in the

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13

C

C NMR spectra of CO indicates a slow

exchange (in the NMR time scale characterized by the shift differences) between adsorbed molecules and the gas phase under equilibrium conditions, without change of the total number of molecules, i.e. only redistribution. This allows us to estimate the temperature dependence of the corresponding fractions of the molecules as a dynamic phenomenon. In the second case we observe fast exchange dynamics between the different types of molecules adsorbed within the MOFs. This leads to a temperature-dependent resonance shift suitable for a dynamic analysis. For CO2 only the latter situation is accessible. Hence, by analyzing both the relative fraction (free and adsorbed CO) and the temperature dependent shifts we can estimate the activation energy for the mobility of the adsorbed molecules.

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For the first case, the relative fractions Pinside and Poutside of the adsorbed CO and the free molecules outside the MOF, respectively, can be obtained by means of an intensity analysis of the two 13C NMR lines. As usual, normalized quantities,

Pinside + Poutside = 1

(1)

are used. The Arrhenius plot, ln(Poutside) vs. 1/T, of CO is shown in Supplementary Figure S6. An activation energy Ea of 5.3 kJ/mol is obtained from the slope (escape of molecules from the MOF). In the second case we consider the 13C NMR line shift δ(T)measured measured for CO in the range between 220 and 323 K which is assigned to adsorbed CO molecules. To calculate the occupation number of the adsorbed molecules, the averaged shift due to the fast exchange between the molecules attached to the Cu2+ ions and the remaining CO molecules adsorbed in the MOF is considered, viz.,

δ(T)measured = (1 - Padsorbed)δfree, MOF + Padsorbed * δadsorbed

(2)

For the value δfree, MOF the chemical shift of CO in the gas phase is assumed which is 183.5 ppm. According to the measurements performed at lower temperatures, the chemical shift δadsorbed is temperature dependent due to the interaction of CO with Cu2+ (see above). To estimate P(T)adsorbed using the values δ(T)measured, we extrapolate the measured low temperature values for

δadsorbed (shifts to lower ppm values) towards higher temperatures. Employing the fractions P(T)adsorbed and assuming an Arrhenius plot an activation enthalpy can be derived, the amount of which is 6.1 kJ/mol (local motions in the MOFs). The plot of ln(Padsorbed) vs. 1/T is shown in Supplementary Figure S7. It is interesting that both activation enthalpies are of comparable size

17



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Page 18 of 35

(in the limits of experimental errors). Hence, we may conclude that the estimated energies (ca. 5 to 6 kJ/mol) are related to a local motion which is also influenced by the desorption of CO. A different situation is found for CO2. As mentioned, only at higher temperatures a weak additional NMR line is visible which can be attributed to the gaseous CO2. The fraction of the free CO2 cannot be estimated in a direct way from the spectra. Over a wider temperature range, however, we may again estimate an enthalpy of activation by considering the exchange dynamics for the molecules within the MOFs. Similarly as described for CO we may consider the

13

C

NMR line shift δ(T)measured measured for CO2 by means of MAS in the range between 213 to 353 K which are assigned to adsorbed CO2 molecules in the MOF. Like in the case of CO, the temperature dependence of this shift reflects an exchange process between the molecules attached to Cu2+ (Padsorbed) and the remaining ones within the MOF. Using equation 2 and the (extrapolated) values of δ(T)adsorbed according to figure 7, and the value δfree, MOF = 126.75 ppm from extrapolation of the shift of figure 8 to infinite temperature, the fraction Padsorbed versus temperature T can be estimated from the chemical shift δ(T)measured. Finally, the amount of the activation enthalpy of 3.3 kJ/mol is found from the plot of ln(Padsorbed) vs. 1/T as shown in Supplementary Figure S8. This value is smaller than the one observed in case of CO. The adsorption energies derived here are consistent with the results of recent studies of the CO2 dynamics in a metal-organic framework with open metal sites (Mg2+) by Kong et al.32 By means of

13

C NMR spin-lattice relaxation the authors have derived similar values for the activation

enthalpies (between 3.5 kJ/mol and 6 kJ/mol for different temperature ranges and for 0.5 CO2 per Mg2+) which were attributed to local motions of CO2 in the MOFs. We now try to better understand these values in terms of recently reported results from adsorption kinetic measurements. From the adsorption isotherm an adsorption (desorption)

18


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enthalpy of 25.8 kJ/mol (near ambient temperature) for CO2 was reported and for CO of 17.3 kJ/mol.24 Hence, a stronger adsorption in MOFs is found for CO2 in comparison to CO molecules. This is in agreement with the observation that the gas phase CO2 line appears only at much higher temperatures compared to that of CO. The absolute values for the adsorption enthalpies determined by our NMR measurements are, however, much smaller. Since we are directly investigating the mobility of the guest molecules no macroscopic effects, such as entropy gains because of the modification of the chemical or electronic structure by the first adsorbed molecules, contribute to the observed values. In the limit of very low gas loadings - which we did not investigate - one might be able to determine the macroscopic adsorption enthalpies also with NMR. During the process of adsorption, the gas molecules will very quickly loose rotational degrees of freedom and changes in the vibration are possible. The determined activation energies therefore may correspond to the gain of rotational motion upon release of the molecules from the copper site and/or desorption. Additionally, one has to take into account that the molecules in the gas phase are probably not in equilibrium state for our NMR measurement.

Conclusions 13

C NMR spectroscopy is successfully used for a detailed investigation of the adsorption

process of CO and CO2 in Cu2.97Zn0.03(btc)2 and Cu3(btc)2, respectively. The nature of interaction and the dynamic behavior of the molecules are followed by variable temperature

13

C NMR

measurements. For CO already at lower temperature (30 to 100 K) species can be detected showing an isotropic mobility, whereas for CO2 only solid like spectra typical for an anisotropic motion are observed in the temperature range below ca. 60 K. In both cases the solid-like spectra disappear gradually when the sample temperature is raised and finally a complete isotropic

19



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Page 20 of 35

motion is found in the adsorbed state. In the intermediate temperature range both molecules show temperature dependent chemical shifts due to the interaction with Cu2+ in the paddle wheel unit being in the S = 1 electronic spin state. Above 180 K, a high mobility of the CO molecules is observed and molecules staying outside the MOF framework can be detected. A different picture is observed for CO2 even though a high mobility of the molecules is observed. Only a relatively small fraction of gas phase CO2 appears at higher temperature compared to CO which goes along with the observation that adsorption of CO2 is favored over CO. Finally the estimated activation energies for exchange processes within the MOF framework are used to characterize the local motion for CO2 and for CO this local motion is combined with desorption of CO to the outside of the MOF. In an upcoming publication the temperature dependence of the spin-lattice relaxation times of CO and CO2 molecules adsorbed in these MOFs will be discussed.

Acknowledgments The authors thank the German research foundation (DFG) for financial support within the priority program 'porous metal-organic frameworks (MOFs)' (SPP1362).

Supporting Information Available: Static spectrum of CO with simulation at 30 K, temperature dependence of the intensity ratio for the rigid and mobile signal for CO, static spectrum of CO2 with simulation at 30 K, 13C static spectra of 1 and 2 CO2/Cu, temperature dependence of the population of different electronic state, and Arrhenius plots for the calculation of activation energies are available free of charge via the Internet at http://pubs.acs.org.

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[26] Peterson, V. K.; Liu, Y.; Brown, C. M.; Kepert, C. J. Neutron Powder Diffraction Study of D2 Sorption in Cu3(1,3,5-benzenetricarboxylate)2. J. Am. Chem. Soc. 2006, 128, 15578-15579. [27] Bordiga, S.; Regli, L.; Bonino, F.; Groppo, E.; Lamberti, C.; Xiao, B.; Wheatley, P. S.; Morris, R. E.; Zecchina, A. Adsorption Properties of HKUST-1 Toward Hydrogen and Other Small Molecules Monitored by IR. Phys. Chem. Chem. Phys. 2007, 9, 2676-2685. [28] Klein, N.; Herzog, C.; Sabo, M.; Senkovska, I. Getzschmann, J.; Paasch, S.; Lohe, M. R.; Brunner, E.; Kaskel, S. Monitoring Adsorption-Induced Switching by 129Xe NMR Spectroscopy in a New Metal–Organic Framework Ni2(2,6-ndc)2(dabco). Phys. Chem. Chem. Phys. 2010, 12, 11778-11784. [29] Hoffmann, H. C.; Assfour, B.; Epperlein, F.; Klein, N.; Paasch, S.; Senkovska, I.; Kaskel, S.; Seifert, G.; Brunner, E. High-Pressure in Situ

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13

C NMR Investigation of Carbon

Monoxide Adsorbed in Zeolites. Chem. Phys. Lett. 1981, 84, 30-32.

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[34] Michael, A.; Meiler, W.; Michel, D.; Pfeifer, H.; Hoppach, D.; Delmau, J.

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C Nuclear

Magnetic Resonance Investigations of Carbon Monoxide in Decationated Zeolites of Type Y. J. Chem. Soc. Faraday Trans.1986, 82, 3053-3067. [35] Emesh, I. T. A.; Gay, I. D. Adsorption and 13C N.m.r. Studies of Ethene, Ethane and Carbon Monoxide on Zn- and Cd-Exchanged A-Zeolites. J. Chem. Tech. Biotechnol. 1985, 35A, 115120. [36] Sprang, T.; Boddenberg, B. Coadsorption of Xenon and Carbon Monoxide in CadmiumExchanged Zeolite Y Studied with

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Investigations on Carbon Monoxide Hydrogen Bonded to Brønsted Acid Sites in H-Y Zeolites. Chem. Phys. Lett. 1994, 228, 501-505. [40] Gay, I. D. Observation of Bridging CO on Rh-SiO2 by

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C MAS NMR. J. Phys. Chem.

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[51] Cory, D. G.; Ritchey, W. M. Suppression of Signals from the Probe in Bloch Decay Spectra. J. Magn. Reson. 1988, 80, 128-132. [52] van Gorkom, L. C. M.; Hook, J. M.; Logan, M. B.; Hanna, J. V.; Wasylishen, R. E. SolidState Lead-207 NMR of Lead(II) Nitrate: Localized Heating Effects at High Magic Angle Spinning Speeds. Magn. Reson. Chem. 1995, 33, 791-795. [53] Massiot, D.; Fayon, F.; Capron, M.; King, I.; Le Calvé, S.; Alonso, B.; Durand, J.-O.; Bujoli, B.; Gan, Z.; Hoatson, G. Modelling One- and Two-Dimensional Solid-State NMR Spectra. Magn. Reson. Chem., 2002, 40, 70-76. [54] Levitt, M. H. Spin Dynamics Basics of Nuclear Magnetic Resonance, John Wiley & Sons, Ltd: England, 2001. [55] Gibson, A. A. V.; Scott, T. A.; Fukushima, E. Anisotropy of the Chemical Shift Tensor for Solid Carbon Monoxide. J. Magn. Reson. 1977, 27, 29-33. [56] Beeler, A. J.; Orendt, A. M.; Grant, D. M.; Cutts, P. W.; Michl, J.; Zilm, K. W.; Downing, J. W.; Facelli, J. C.; Schindler, M. S.; Kutzelnigg, W. Low-Temperature

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Resonance in Solids. 3 Linear and Pseudolinear Molecules. J. Am. Chem. Soc. 1984, 106, 76727676. [57] Jee, B.; Heine, T.; Hartmann, M.; Pöppl, A. The Adsorption of 13CO2 and 13CO at the Cu-Zn Paddle Wheel Units in the Metal Organic Framework Cu2.97Zn0.03(btc)2 Studied by Continuous Wave and Pulsed Electron Paramagnetic Resonance Spectroscopy. Submitted to J. Phys. Chem.C. [58] La Mar, G. N.; Jr. Horrocks, W. D.; Holm, R. H. NMR of Paramagnetic Molecules Principles and Applications, Academic Press: New York and London, 1973. [59] Bertini, I.; Luchinat, C. The Hyperfine Shift. Coordin. Chem. Rev. 1996, 150, 29-75.

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Page 28 of 35

Figure Captions Figure 1: 13C static spectra of 2 CO/Cu recorded in the temperature range from 10 to 323 K.

Figure 2: Temperature dependence of the chemical shift anisotropy of 2 CO/Cu recorded in the temperature range from 10 to 100 K (open symbols) together with the anisotropy of the bulk 13

CO (solid symbol) according to ref. 56.

Figure 3: Temperature dependence of the isotropic chemical shift of the mobile phase of

13

CO

(2 CO/Cu) in the temperature range from 30 to 323 K.

Figure 4:

13

C MAS NMR spectra of 1, 2 and 5 CO/Cu recorded at 298 K. For a better visual

inspection, the spectrum for 1 CO/Cu is also shown enlarged in the inset. Figure 5: 13C static spectra of 2 CO2/Cu recorded in the temperature range from 10 to 323 K. Figure 6: Temperature dependence of the chemical shift anisotropy of 2 CO2/Cu recorded in the temperature range from 10 to 60 K (open symbols) together with the anisotropy of the bulk 13

CO2 (solid symbol) as from ref. 56.

Figure 7: Temperature dependence of the chemical shift of the mobile phase of

13

CO2 (2

CO2/Cu) in the temperature range from 75 to 323 K. The inset shows the shift dependence in the full temperature range from 10 to 323 K. Figure 8: 13C MAS spectra of 2 CO2/Cu recorded in the temperature range from 213 to 353 K.

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Figures

CO (adsorbed, mobile)

CO (gas) CO (adsorbed, mobile) CO2

110 K

323 K

310 K

100 K

300 K

90 K 290 K

75 K 260 K

60 K 220 K

50 K 180 K

40 K 150 K

30 K 125 K

110 K

10 K

800

600

400

200

0

-200

-400

250

δ/ppm

200

150

100

δ/ppm

Figure 1 29



-400

∆δ CSA (ppm)

-350

-300

-250

-200

0

20

40

60

80

100

T (K)

Figure 2

180

δ iso (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 35

176

172

168

increasing population of S = 1 state 0

50

100

150

200

250

300

350

T (K)

Figure 3

30


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CO (gas) CO2 CO (gas)

CO (adsorbed, mobile) 210

200

190

180

170

160

δ/ppm

5 CO/Cu 2 CO/Cu 1 CO/Cu 220

200

180

160

140

120

δ/ppm

Figure 4

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

CO2 (adsorbed, mobile)

Page 32 of 35

CO2 (adsorbed, mobile)

323 K 300 K

75 K

290 K

60 K 260 K 220 K

50 K

180 K 150 K

40 K 125 K 110 K

30 K 100 K

90 K

10 K 75 K

600

400

200

0

δ/ppm

-200

-400

180

160

140

120

100

80

δ/ppm

Figure 5

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-350

∆δ CSA (ppm)

-300

-250

-200

-150 10

20

30

40

50

60

T (K)

Figure 6

128 δ iso (ppm)

129 128 127 126

δ iso (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


127

125 0

50 100 150 200 250 300 350 T (K)

126

125

increasing population of S = 1 state 50

100

150

200

250

300

350

T (K)

Figure 7 33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 35

CO2 (adsorbed, mobile) CO2 (gas) 353 K

333 K

313 K

298 K

273 K

253 K

233 K

213 K

127

126

125

δ/ppm

Figure 8

34


Page 35 of 35

Table of contents graphic

CO CO CO CO

CO Slow exchange CO

CO CO

323 K 310 K

13C

CO

NMR lines

CO (g as) CO (adso rbed, mo bile)

300 K

separation of

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


290 K 260 K 220 K 250

200

150

100

δ/pp m

35


Adsorption of Small Molecules on Cu3 (btc) 2 and Cu3âx Zn x (btc) 2

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