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N Capture Performances of the Hybrid Porous MIL-101(Cr): From Prediction Towards Experimental Validation Renjith S. Pillai, Ji Woong Yoon, Seung-Joon Lee, Young Kyu Hwang, Youn-Sang Bae, Jong-San Chang, and Guillaume Maurin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07029 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 21, 2017

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The Journal of Physical Chemistry

N2 Capture Performances of the Hybrid Porous MIL-101(Cr): from Prediction Towards Experimental Validation Renjith S. Pillai†,#, Ji Woong Yoon‡, #, Seung-Joon Lee≠, Young Kyu Hwang‡, Youn-Sang Bae≠, Jong-San Chang‡,§,*, and Guillaume Maurin†,* †

Institut Charles Gerhardt Montpellier, UMR-5253, Université de Montpellier, CNRS, ENSCM, Place E. Bataillon, 34095 Montpellier cedex 05, France ‡

Research Center for Nanocatalysts, Korea Research Institute of Chemical Technology (KRICT); Daejeon 305-600, South Korea. ≠

Department of Chemical and Biomolecular Engineering, Yonsei University; Seoul 120-749, South Korea.

§

Department of Chemistry, Sungkyunkwan University; Suwon 440-476, South Korea

#

These authors equally contributed to this work

* Corresponding Author: Guillaume Maurin ([email protected]) and Jong-San Chang ([email protected])

ACS Paragon Plus Environment

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ABSTRACT

The purification of nitrogen-containing gas mixtures, natural/shale gas, and dry air calls for economically viable adsorptive separation processes involving an adsorbent with a higher affinity for N2 over hydrocarbons and oxygen. This led to discover a new class of unprecedented N2-selective Metal-Organic Frameworks (MOFs) with coordinatively unsaturated chromium (III) sites, e.g. MIL100(Cr) (MIL: Materials of Institut Lavoisier). Following this preliminary study, here grand canonical Monte Carlo simulations identified MIL-101(Cr), an analogue of MIL-100(Cr), as another N2selective adsorbent from mixtures of both CH4-N2 (natural gas purification) and O2-N2 (air purification). This prediction was further compared to single gas adsorption and breakthrough separation experiments. It was evidenced that only the more energetic coordinatively unsaturated chromium sites released using an activation temperature of 523 K are responsible for the N2-selective behavior of MIL-101(Cr). The separation mechanisms were then elucidated at the molecular-level and this emphasized the central role played by the concentration of coordinatively unsaturated chromium (III) sites in MIL-101(Cr) that can be controlled by the activation temperature of the sample.


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1. Introduction Natural gas, a clean and cheap energy source, is projected to be the next generation fuel for vehicles. 1-3

However, the main drawback in this field is still the purification of this fossil fuel4 since many

methane reservoirs contain unacceptable amounts of impurities such as CO2, N2 and H2O.4-5 This upgrading is nowadays one of the leading examples of industrial importance together with olefin/paraffin separation.1-2 Economically viable advanced adsorptive separation technologies are currently available for the removal of CO2 and H2O from natural gas.1-2, 6-7 However this is far to be the case for the selective capture of N2 from CH4 which has been mostly achieved by means of the complex, costly and energy consuming cryogenic separation technology.8-10 The use of physisorptionbased separation is commonly a more efficient process, however in the specific case of N2/CH4 separation, this process is particularly complex because of a lack of suitable adsorbents. The main reason is that most of the adsorbents prefer CH4 to N2 in adsorptive separation utilizing the equilibrium-adsorption method.1-3, 10 The use of CH4-selective adsorbent implies an additional costly desorption step, using inert gas or applying vacuum, to obtain high purity.2-4 On the other hand, kinetic and/or molecular sieving driven separation of N2 over CH4 has been achieved using the titanium silicate ETS-4 with a relative high N2/CH4 selectivity (205 at 283 K) however at the expenses of a low N2 uptake (4000 m2.g−1, Vpore> ~2.0 cm3.g−1).14-15 This extended version of MIL-100(Cr) equally contains two coordinatively unsaturated Cr sites (CUS-Cr(III)) sites per trimers once the terminal water molecules are removed by heating. This computational study evidenced that this material shows similar N2/CH4 and N2/O2 selectivities than the values shown by its MIL-100(Cr) analogue, however the N2 uptake is significantly increased. These calculations further confirmed that the presence of the CUS-Cr(III) sites governs the separation mechanisms. As a further step, single component adsorption, breakthrough separation and Infra-red experiments have been conducted to confirm the expectations of this material with a special attention paid on the activation condition of this material, a key to a fine control of the separation performances of this class of MOFs. The performances of MIL-101(Cr) have been further compared to that currently reported in the literature for a series of existing adsorbents.


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Figure 1. Illustration of the crystal structure of the mesoporous Cr(III) trimesate based-MIL-101(Cr). The framework atoms are represented as follows: Cr, silver; C, grey; O, red; H, white; CH4, green and N2, blue.

2. Materials and Methods 2.1. Modelling The GCMC simulations were carried out at 283 K to predict the adsorption of CH4, N2, and O2 as single and binary mixtures (N2/CH4 and N2/O2 with molar mixture compositions of 20/80 and 79/21, respectively) in MIL-101(Cr) using the CADSS code.16 The simulation box was made of a single (1×1×1) unit cell of the MOF crystal structure previously reported.13 In this structure, for each trimer, two of the three metal(III) octahedra are CUS sites and the third is bonded to F atoms.. This theoretical crystal model considers that all the CUS-Cr(III) sites are characterized by the same local environment and are thus expected to interact with the guest sites in a similar manner. The van der Waals interactions between the guest molecules and all atoms of the MOF framework except the CUS-Cr(III) were treated by the standard 12-6 Lennard-Jones (LJ) potential. The corresponding LJ parameters


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were obtained using the Lorentz-Berthelot combination rules ( 𝜀𝜀𝑖𝑖𝑖𝑖 = �𝜀𝜀𝑖𝑖 𝜀𝜀𝑗𝑗 , 𝜎𝜎𝑖𝑖𝑖𝑖 = (𝜎𝜎𝑖𝑖 + 𝜎𝜎𝑗𝑗 )/2 , where 𝜀𝜀𝑖𝑖 and 𝜎𝜎𝑖𝑖 are the LJ parameters for the atoms of the MOF framework). The (σ,ε) LJ parameters

for the atoms of the MOF were adopted from the Universal force field (UFF)17 (Table S1). For the guest molecules, CH4 was described by the uncharged single LJ model with potential parameters taken from the TraPPE forcefield18 (Table S2). Both N2 and O2 molecules were represented by a three-site charged model with two LJ sites located at the N and O atoms, respectively, while a third site present at its center of mass (COM) only involves electrostatic interactions as previously described by the TraPPE potential model19 for N2 and the three-site model by Razmuz and Hall20 for O2 (Table S2). The interactions between all these guest molecules and the CUS-Cr(III) were described by a Buckingham potential (see SI) with the corresponding parameters taken from our previously DFT studies on MIL-100(Cr)12 (Table S3). In the case of N2 and O2, an electrostatic contribution for the MOF/guest interaction was considered, where the partial charges for the MOF framework were taken from the previous work reported by Lange et al.21 The Ewald summation method16 was used to calculate the electrostatic interactions and the treatment of the short-range interactions was considered with a cut off distance of 12 Å. For comparison, we also considered the case where the short-range interactions were treated by using a Lennard-Jones contribution term between the guest and all atoms (including CUS-Cr(III)) of the MOF with the (σ,ε) LJ parameters taken from the Universal force field (UFF).17 For each state point, 5×107 Monte Carlo steps have been used for both equilibration and production runs and the adsorption enthalpy at low coverage (∆ℎ) for each gas was calculated using the revised

Widom’s test particle insertion method.22 The Radial Distribution Functions (RDF) reported in Supporting Information for all the guest/MOF pairs were obtained by averaging over the 5×107 Monte Carlo production steps. The selectivity (S) for N2 over CH4 and O2 is defined by the following


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expression: S(N2/CH4) = ( x N 2 / xCH 4 ) ( y CH 4 / y N 2 ) , and S(N2/O2) = ( x N 2 / xO 2 ) ( y O 2 / y N 2 ) , where

xCH 4

xN 2

,

and xO2 , are the molar fractions of N2, CH4 and O2 in the adsorbed phase, respectively, while

y N 2 y CH 4 ,

and y O2 are the molar fractions of N2, CH4 and O2 in the bulk phase, respectively. The

calculated selectivities were obtained for both N2/CH4 (molar ratio = 20/80) and N2/O2 (molar ratio = 79/21).

2.2. Materials MIL-101(Cr) was prepared from hydrothermal reaction of terephthalic acid (166 mg at 1 mmol) with Cr(NO3)3.9H2O (400 mg at 1 mmol), HF (0.2 ml at 1 mmol), and H2O (4.8 ml at 265 mmol) at 493 K for 8 hours.23 The as-synthesized MIL-101(Cr) was further purified by the following two-step processes using hot ethanol and aqueous NH4F solutions. The crystalline MIL-101(Cr) product in the solution was doubly filtered off using two glass filters with a pore size ranging from 40 and 100 μm to remove the free terephthalic acid. A solvothermal treatment was then sequentially performed using ethanol at 353 K for 24 hours. The resulting solid was soaked in 1 M of NH4F solution at 343 K for 24 hours and immediately filtered, washed with hot water. The solid was finally dried overnight at 423 K under air atmosphere.

2.3. Adsorption measurements 2.3.1. Gas adsorption measurements Prior to each gas adsorption measurement, approximately 100 mg of sample was degassed at either 423 K or 523 K under high vacuum (10-4 mbar) for 12 hours using an outgas station of Autosorb-iQ system (Quantachrome Instruments, Boynton Beach, Florida, USA). The adsorption isotherms of N2,


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CH4 and O2 were then measured at three different temperatures (283 K, 293 K, and 303 K) up to 1 bar using the Autosorb-iQ system. A specially designed home-built water circulating system was used to maintain a constant temperature. High purity N2 (99.999%), CH4 (99.95%) and O2 (99.99%) were purchased from Air Korea, Co., Ltd. and they were used as received. The BET area measurements were performed with N2 physisorption isotherms at liquid nitrogen temperature (77 K) after dehydration under vacuum at 423 K for 12 hours using Micromeritics Tristar 3000. The specific surface areas were calculated by the Brunauer-Emmett-Teller (BET) method in the p/p0 range 0.05 - 0.3.24 The isosteric heat of adsorption (- ΔHads) was calculated by using the Clausius-Clapeyron equation25 from the adsorption isotherms at three temperature. The Clausius-Clapeyron equation is expressed as ln P = - ΔHads /R×(1/T)

(1)

where P, R and T mean the pressure, gas constant and temperature, respectively. The - ΔHads can be obtained from the slope of the plot of Eq. (1). Ideal Adsorbed Solution Theory (IAST) selectivity was calculated by a method derived by Myers and Prausnitz26 to predict the multi-component adsorption isotherms from the pure component isotherms. A dual-site Langmuir-Freundlich equation was used for IAST calculations.12

2.3.2. In-situ and Operando IR spectroscopic analysis For in-situ IR analysis, samples were pressed (~102 MPa) into self-supported disks (2 cm2 area, 710 mg cm-2) and placed in a quartz cell equipped with KBr windows.12 A movable quartz sample holder permits the adjustment of the pellet in the infrared beam for spectra acquisition and to displace it into a furnace at the top of the cell for thermal treatments. The resolution of the spectra was 4 cm-1, and 64 scans were accumulated for each spectrum on a Nicolet Nexus spectrometer equipped with an extended KBr beam splitting device and a mercury cadmium telluride (MCT) cryo-detector. In-situ


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IR spectra of adsorbed CO were obtained at 100 K by cooling the sample holder with liquid N2 after activation for 6 hours at 523 K under vacuum (~10-6 Torr). The addition of exactly known increments of CO probe molecules in the cell was possible via a calibrated volume (1.75 cm3) connected to a pressure gauge for the control of the probe pressure. Operando IR measurements for the determination of N2 adsorption on self-supported MIL101(Cr) wafer were performed at 298 K after dehydration under He (10 cm3 min-1) at 523 K for 12 hours using an IR spectrometer (Thermo Scientific Nicolet 6700) equipped with a “sandwich” reactor-cell.12 The sample was placed in the toroidal sample holder (in the centre of the cylindrical body) in the form of a self-supported wafer of 8-9 mg cm-2.

2.3.3. Measurements of breakthrough gas separation The gas separation properties of MIL-101(Cr) were evaluated by breakthrough experiments using binary gas mixtures of N2/CH4 and N2/O2 with selected molar compositions and flow rate (10 cm3 min). The adsorption separation factor, α, between two components i and j is defined as: 𝑞𝑞𝑖𝑖 �𝑃𝑃 α = 𝑞𝑞 𝑖𝑖 𝑗𝑗 �𝑃𝑃 𝑗𝑗

where 𝑞𝑞𝑖𝑖 or 𝑞𝑞𝑗𝑗 : component molar loading of species i or j, mmol g-1

𝑝𝑝𝑖𝑖 or 𝑝𝑝𝑗𝑗 : partial pressure of species i or j at inlet to fixed bed, Pa

Standard gas mixtures of N2/CH4 and N2/O2 with the molar compositions (10/90) and (79/21), respectively, used in this work were supplied by Rigas (South Korea). Breakthrough separation experiments of N2/O2 gas mixture were performed by passing gas mixtures through a column packed with the adsorbent. The adsorbent were pressed (30 bar) to form pellets which were then crushed and


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sieved to attain particle sizes between 250 and 500 μm (35-60 mesh). The adsorbent bed was made stationary by quartz wool. The remaining void column volume was filled with quartz sands. Prior to conducting breakthrough separation experiments, the adsorbent sample (about 0.5 g after activation) was activated under flowing He at 423-573 K for 12 hours. After activation the temperature was reduced to room temperature and the He flow rate adjusted to 10 cm3 min-1. The flow of gas mixture was controlled with mass flow controllers (Brooks). The column was cooled to 293 K in a thermostated chamber by chiller and the mixture of nitrogen/methane or nitrogen/oxygen (10 cm3 min1

) was introduced when the temperature of column was stabilized. The gas flow rate at the bed exit

was measured with a bubble meter. The outlet gases of the breakthrough column were directly analyzed using a mass spectrometer (Pfeiffer-Vacuum OmniStar).

3. Results and Discussion Figure 2a shows that the GCMC-predicted N2 adsorbed amounts in MIL-101(Cr) at 283 K are much higher than for CH4 and O2 in the whole low-pressure range [0-1 bar]. The higher N2-affinity of this material is confirmed by the higher adsorption enthalpy simulated at low coverage ~-40 kJ/mol (Figure 2b) for this guest as compared to CH4 (-17 kJ/mol) and O2 (-13 kJ/mol). Interestingly, when all the atoms of the MOF are described as LJ interacting sites using (σ,ε) LJ parameters taken from the generic UFF force field, the adsorption isotherm for CH4 remains unchanged although the amount of N2 adsorbed considerably decreases resulting in a material that adsorbs more CH4 than N2 (see Figure S1). This trend is confirmed by a lower simulated adsorption enthalpy at low coverage for N2 ~-11 kJ/mol as compared to CH4 (-17 kJ/mol). This observation emphasizes the key role played by the CUS-Cr(III) in the interactions between MIL-101(Cr) and the quadrupolar N2 molecule and the requirement of a specific force field to describe as accurately as possible the resulting CUS-Cr(III)/ N2 interaction.


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(a)

(b)

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-∆ads h / kJ mol-1

2.0 1.5 1.0 0.5 0.0 0.0

50 40

-1

nads / mmol g

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0.2

0.4

0.6

0.8

1.0

30 20 10 0 0.0

0.4

Pressure / bar

0.8

1.2

1.6

2.0

n / mmol g-1

Figure 2. GCMC-simulated single component adsorption isotherms (a) and enthalpies of adsorption (b) for CH4 (squares), N2 (circles), and O2 (triangles) in MIL-101(Cr) at 283 K.

Analysis of the single component adsorption mechanism confirmed that the higher N2-affinity of MIL101(Cr) using the more specific host/guest force field is due to the strong interactions between this guest molecule and CUS-Cr(III) with the formation of quasi-linear N-N…CUS-Cr(III) adducts (Figures 3) in both small and large cages, the mean characteristic distance being about 2.09 Å (see Radial Distribution Functions in Figure S2). Operando IR spectroscopy under N2 flow at 283 K and 1 bar confirmed such a strong interaction with the band at 2343-2344 cm-1 (Figure 4) corresponding to the stretching modes of N2 adsorbed on the Cr(III) sites, up-shifted compared to the values (18002220 cm-1)27 observed for the coordinated N2 in 3d dinitrogen metal complexes. We further predicted that the N2-uptake about 2.8 mmol/g at 1 bar and 283K is slightly higher compared to the case of MIL100(Cr)12, i.e. 2.4 mmol/g at 1 bar and 283 K, consistent with a higher pore volume of MIL-101(Cr).


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Figure 3. Local views of the configurations extracted from GCMC simulations performed for N2 (a), CH4 (b), and O2 (c) in MIL-101(Cr) at 0.1 bar and 283 K. The interacting distances are reported in Å and the framework atoms and molecules are represented as follows: Cr, Silver; C, grey; O, red; H, white; CH4, green N2, blue and O2, red.

In contrast to this, CH4 is mainly distributed at the center of these cages leading to only weak interactions with the pore wall as confirmed by characteristic guest/MOF distances over 4.1 Å (Figure S3), while only a very few O2 molecules interact with Cr(III) associated with longer guest/MOF distances of about 2.6 Å as compared to N2 (Figure S4). These spatial distributions are fully consistent with the scenario that we previously reported

12

in the case MIL-100(Cr) and this confirms that the

presence of Cr(III) sites is the key feature to incorporate in a MOF to design an efficient N2-selective adsorbent.


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Absorbance (a.u.)

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2355

373K 423K 473K 523K

0.01

2350

2345

2340

2335

2330

Wavenumber (cm-1) Figure 4. Operando IR spectra of self-supported MIL-101(Cr) wafers under flow of N2 at 298 K and 1 bar. The sample wafers were activated from 373 K to 523 K for 12 hours in He flow prior to gas adsorption.

Further GCMC simulations performed for the binary N2/CH4 (20/80) mixture predicted that N2 is selectively adsorbed over CH4 at 283 K in MIL-101(Cr) with a selectivity value of 5-10 within the 010 bar range. This thermodynamic separation is driven by the stronger interaction of N2 with the CUSCr(III) sites, the location of both N2 and CH4 being the same than that evidenced for the single components (Figure 5a). MIL-101(Cr) was further revealed to be highly N2-selective from a N2/O2 mixture corresponding to the condition of dry air (79/21). The N2/O2 selectivity ranges from 2 to 5 at 283 K in the range 0-10 bar. This separation is also driven by the higher affinity of N2 strongly interacting with the CUS-Cr(III) sites (Figure 5b) while only a small fraction of O2 interacts with other CUS-Cr(III) sites. This emphasizes the ability of MIL-101(Cr) to capture N2 from diverse N2containing gas mixtures. One can notice that the simulated separation performances of both N2/CH4 (20/80) and N2/O2 (79/21) mixtures with MIL-101(Cr) is similar to that reported using MIL-100(Cr).12 This emphasizes that MIL-101(Cr) maintains a high selectivity while allowing an increase of the N2 uptake. ACS Paragon Plus Environment

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Figure 5. Local views of the snapshots extracted from GCMC simulations performed for N2/CH4 (a) and N2/O2 (b) mixture with a molar composition 20/80 and 79/21, respectively, at 283 K in MIL101(Cr).The interacting distances are reported in Å and the framework atoms and molecules are represented as follows: Cr, Silver; C, grey; O, red; H, white; CH4, green N2, blue and O2, red.

To assess the separation performance of MIL-101(Cr) from an experimental standpoint, the adsorption isotherms for all the single components N2, CH4 and O2 were first measured at 283 K. As revealed in our previous paper on MIL-100(Cr)12, the quality of the CUS-Cr(III) containing MOF sample depends on both the purification and activation procedures, especially to ensure an optimal removal of both the solvent and chemical reactants used during the synthesis and the water coordinating the CUS-Cr(III) to make these sites potentially accessible to the guest molecules. To that purpose, the adsorption experiments were performed on MIL-101(Cr) samples preliminary activated at different temperatures. Figure 6 clearly confirms that, the N2-adsorption behavior of MIL-101(Cr) strongly depends on the temperature used during the activation. Typically, Figure 6a shows the gas adsorption isotherms collected for MIL-101(Cr) preliminary activated at 423 K. Here, one observes a preferential adsorption of CH4 vs N2, which corresponds to a reverse selectivity predicted by our GCMC simulations. This experimental observation is similar to that reported by Munusamy et al.28 using a MIL-101(Cr) sample activated at 423 K for 12 hours under vacuum and characterized by a BET area of 2741 m2.g-1 far to be optimal as compared to the theoretical value, i.e. 4000 m2.g-1. To make a more accurate assessment of the separation performances of MIL-101(Cr), we prepared a high


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quality MIL-101(Cr) sample associated with a BET area of 4020 m2/g using a synthesis recipe previously reported by some of us.23 Adsorption results on MIL-101(Cr) sample activated at 423 K show any adsorption preference for N2 vs CH4, here the adsorption isotherms being very similar for both guests suggesting equivalent affinities for the two guests. More interestingly when the activation is performed at 523 K, the N2 becomes preferentially adsorbed over CH4 consistent with the predicted trend (Figure 6b). Analysis of the accessibility of the CUS-Cr(III) sites in the MIL-101(Cr) sample evidenced by IR using CO as a probe molecule (Figure 7) shows that this selective-step change behavior is associated with an increase of the accessible CUS-Cr(III) sites that goes from 1.60 mmol.g1

to 1.75 mmol.g-1 when the activation temperature varies from 423 K to 523 K. This implies that the

N2-selective adsorption of MIL-101(Cr) observed at low pressure comes from the more energetic CUS-Cr(III) sites, strongly bounded to remaining reactants including extra-linkers and metal sites, that require the highest temperature treatment to be free to interact with the guest molecules. Indeed, one observes that while the amount of N2-adsorbed at 0.3 bar increases from 0.26 mmol.g-1 (423 K) to 0.40 mmol.g-1 (523 K) consistent with the adsorption of this guest molecules towards the fraction of CUS-Cr(III) sites (0.15 mmol.g-1) that is made accessible in the temperature range 423-523 K. In contrast, the amount of CH4-adsorbed at 0.3 bar remains similar (~0.26 mmol.g-1) for both samples treated at 423 K and 523 K, suggesting that the additional free CUS-Cr(III) sites do not interact with CH4. The same scenario was encountered for MIL-100(Cr), however associated with a higher concentration of CUS-Cr(III) sites released when the activation temperature varies from 423 K to 523 K (from 2.50 mmol.g-1 to 3.55 mmol.g-1) (see Figure 7). This observation is consistent with the fact that the N2-adsorption performance of MIL-101(Cr) at low pressure than that observed for MIL100(Cr), e.g. N2 uptake at 0.3 bar, 283 K: 1.05 mmol g-1 for MIL-100(Cr)12 vs 0.40 mmol g-1 for MIL101(Cr), and this holds true in the whole range of pressure up to 1 bar. The excellent agreement between the simulated adsorption enthalpy for N2 and the experimental isosteric of adsorption, both


15


~-40 kJ.mol-1, emphasizes that in contrast to the use of a generic force field, the specific force field we derived accurately capture the strength of interactions between CUS-Cr(III) and N2 and indeed this is expected to allow a good description of the adsorption isotherm at very low pressure. However, the experimental N2 uptake for MIL-101(Cr) is lower than the simulated value since in the theoretical model considered for the GCMC simulations the whole fraction of CUS-Cr(III) sites, i.e. 2.93 mmol.g1

, are equivalent and free to interact with the guest molecules. The same trend has been already

observed for MIL-100(Cr) although less pronounced since the fraction of CUS-Cr(III) accessible at high temperature treatment was significantly higher as mentioned above.12 The performance of MIL-101(Cr) (0.95 mmol g-1 at 1 bar and 283 K) fits however in the range of the adsorption capacities reported for other commonly used adsorbents including LiX (1.27 mmol g-1 at 283 K)29 and ETS-4 (0.41 mmol g-1 at 295 K)11. (b) 1,0

0,8

0,8

nads / mmol g-1

(a) 1,0

0,6 0,4 0,2 0,0 0,0

0,2

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0,8

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Pressure / bar

(c) 40

-∆ads h / kJ mol-1

nads / mmol g-1

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

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30 20 10 0 0.0

0.2

0.4

0.6

n / mmol g-1


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Figure 6. Gas adsorption isotherms for CH4 (squares), N2 (circles) and O2 (triangles) collected at 283 K for MIL-101(Cr) depending on activation temperature at 423 K (a) and 523 K (b). Experimental isosteric heat of adsorption evaluated for the sample preliminary activated at 523 K (c) for N2 (circles), CH4 (squares) and O2 (triangles).

(a) 2.5

(b)

2.0

4 3

1.5 1.0

2

0.5

1

0.0

0

523 Activation temperature (K)

373

423

473

323K 373K 423K 523K

Absorbance (a.u.)

MIL-100(Cr) MIL-101(Cr)

Total NCUS Cr (mmol/g)

NCUS Cr (per trimer)

0.2

2220

2210

2200

2190

2180

2170

Wavenumber (cm-1)

(c) Absorbance (a.u.)

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


373K 423K 473K 523K

0.1

2220

2210

2200

2190

2180

2170

-1

Wavenumber (cm )

Figure 7. (a) Variation of the number of CUS Cr(III) (NCUS Cr) per trimer of octahedral detected by CO adsorption on (b) MIL-101(Cr) and (c) MIL-100(Cr) versus the temperature of activation using the IR stretching frequency of CO bounded to CUS Cr(III).12


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The N2/CH4 (1.8 at 283 K and 1 bar) and N2/O2 (4.7 at 283 K and 1 bar) selectivities obtained from the application of the ideal adsorbed solution theory (IAST)26 to the experimental adsorption isotherms (Figure 8) shows the same trend as the simulated selectivities.

6 5 Selectivity

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4 3 2 1 0 0.0

0.2

0.4

0.6

0.8

1.0

Pressure / bar

Figure 8: IAST-predicted selectivities for N2/CH4 (black) and N2/O2 (red) from their 20/80 and 79/21 molar ratio gas mixture, respectively, in MIL-101(Cr) as a function of bulk pressure at 283 K.

To confirm the separation performances of MIL-101(Cr), typically breakthrough experiments were carried out on both N2/CH4 (10/90) (Figure 9) and N2/O2 mixtures (79/21) (Figure 10) flowed over a packed bed of MIL-101(Cr) granules at 293 K and 1 bar. The adsorption separation factors of both N2/CH4 and N2/O2 from experimental breakthrough curves strongly depend on activation temperature which is directly related to the concentration of accessible CUS-Cr(III) sites. Activation at 523 K ACS Paragon Plus Environment

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rendered it selective for the N2/CH4 (Figure 9) and N2/O2 separation (Figure 10), i.e. . α(N2/CH4) =1.32 and α(N2/O2) =3.10. This is consistent with the experimental trends on adsorption isotherms (Figure 6b) and isosteric heat of adsorption (Figure 6c). Figure 9a shows that methane is first eluted after 40 seconds, whereas the adsorbent retains nitrogen. After 44 seconds, nitrogen starts eluting. Upon saturation of N2 within the bed, the outlet gas stream finally reaches the original inlet feed concentrations. However, activation at 423 K was very ineffective for the separation, i.e. α(N2/CH4) =0.48 (Figure 9b), showing a CH4-selective property as observed by conventional adsorbents.

(a) 1.5

CH4 N2

(b) 1.5

CH4 N2

1.0 C/C0

1.0

C/C0

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


0.5

0.5

α(N2/CH4) = 0.48

α(N2/CH4) = 1.32 0.0

0.0 0

20

40

60 80 Time (sec)

100

120

0

20

40

60 80 Time (sec)

100

120

Figure 9. Experimental breakthrough curves of a N2/CH4 gas mixture (molar composition 10/90) for MIL-101(Cr), activated (a) 523 K for 12 hours and (b) 423 K for 12 hours in He flow. The breakthrough curves were obtained at 293 K with a total gas flow of 10 cm3 min-1 at atmospheric pressure. The adsorption separation factor between N2 and CH4 is denoted as α(N2/CH4).

Figure 10a shows that O2 is first eluted after 3 seconds, whereas the adsorbent retains nitrogen. After 5 seconds, the outlet gas contains O2 almost 100% pure. Upon saturation of N2 within the bed, the outlet gas stream slowly reaches the original inlet feed concentrations. In contrast, activation at 423


19


K was not effective for the separation, i.e. α(N2/O2) =1.08. (Figure 10b), as also confirmed by adsorption isotherms for MIL-101(Cr) activated at 423 K. (a) 1.5

N2 O2

(b) 1.5

N2 O2

1.0

C/C0

1.0

C/C0

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

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α(N2/O2) = 3.10

0.5

α(N2/O2) = 1.08

0.5

0.0

0.0 0

20

40

Time (sec)

60

80

0

20

40

60

80

Time (sec)

Figure 10. Experimental breakthrough curves of a N2/O2 gas mixture (molar composition 79/21) for MIL-101(Cr), activated (a) 523 K for 12 hours and (b) 423 K for 12 hours in He flow. The breakthrough curves were obtained at 293 K with a total gas flow of 10 cm3 min-1 at atmospheric pressure. The adsorption separation factor between N2 and O2 is denoted as α(N2/O2). 4. Conclusions In this work, grand canonical Monte Carlo simulations based on the use of a specific DFT-derived guest/Cr(III) force field has been applied to reveal the separation performances of MOF incorporating Cr(III) as coordinatively unsaturated sites. The simulations predict that N2 is preferentially adsorbed with respect to CH4 and O2 in MIL-101(Cr) at a given temperature (283 K) and pressure (0-1 bar). Further, experiment adsorption isotherms and IAST results confirmed that an activated MIL-101(Cr) sample at 523 K shows N2-selectivity over CH4 and O2 and this is associated with the preferential interactions between N2 and the strongest interacting CUS-Cr(III) sites that are made free only using an activation at high temperature. Additionally, breakthrough analysis evidences that MIL-101(Cr) shows a very high N2/CH4 and N2/O2 selectivities with good N2 uptake and easy regeneration. According to the molecular simulations, the MIL-101(Cr) incorporating a theoretical concentration of CUS-Cr(III) sites (2.93 mmol g-1) is expected to show even better performance in terms of N2 uptake compared to MIL-100(Cr) while maintaining high N2/CH4 selectivity. Nonetheless, MIL-101(Cr) is an another potential candidate of the MOF family with CUS-Cr(III) sites that allows a higher N2


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selectivity over CH4 and O2 for adsorption-based technologies to achieve unprecedented highlyefficient N2 capture from the natural gas.

Dedication The authors dedicate the manuscript to our late great friend Gérard Férey for his outstanding contribution to the success of this fascinating class of hybrid porous solids and in particular for the discovery of the giant pores MIL-100/MIL-101 among others. We thank him for his permanent invaluable support and source of inspiration. Supporting Information Supporting Information (SI) available: GCMC simulations details, FT-IR analysis details, adsorption measurements and breakthrough analysis. This material is available free of charge via the Internet at http:// pubs.acs.org. Acknowledgements The research leading to these results has received funding from the ANR Chesdens. G.M. thanks the Institut Universitaire de France for its support. Korean authors would like to acknowledge the financial support from the R&D Convergence Program (CRC-14-1-KRICT) of MSIP (Ministry of Science, ICT and Future Planning) and NST (National Research Council of Science & Technology) of Republic of Korea. J.-S.C also acknowledges the financial support through the DRC Program (SKM-1503) funded by NST (National Research Council of Science& Technology) of Korea. We thank Dr. Ji Sun Lee and Ms. Su-Kyung Lee for their experimental assistance, Dr. M. Daturi from LCS, ENSICAEN-France for the IR measurements and Dr. C. Serre from IMAP ENS-ESPCI-France for fruitful discussion.


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