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Gas Adsorption and Separation by the AlBased Metal-Organic Framework MIL-160 Daiane Damasceno Borges, Perine Normand, Anastasia Permiakova, Ravichandar Babarao, Nicolas Heymans, Douglas Soares Galvao, Christian Serre, Guy De Weireld, and Guillaume Maurin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08856 • Publication Date (Web): 02 Nov 2017 Downloaded from http://pubs.acs.org on November 5, 2017
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Gas Adsorption and Separation by the Al-based Metal-Organic Framework MIL-160 Daiane DAMASCENO BORGES1,2*, Périne NORMAND3, Anastasia PERMIAKOVA4, Ravichandar BABARAO5, Nicolas HEYMANS3, Douglas S. GALVAO2, Christian SERRE6, Guy DE WEIRELD3**, Guillaume MAURIN1*** 1
Institut Charles Gerhardt Montpellier UMR CNRS 5253, Université Montpellier, 34095 Montpellier cedex 05, France.
2
Applied Physics Department and Center for Computational Engineering and Sciences, University of Campinas - UNICAMP, Campinas-SP 13083-959, Brazil.
3
Service de Thermodynamique, Faculté Polytechnique, Université de Mons, Place du Parc 20, 7000 Mons, Belgium.
4
Institut Lavoisier de Versailles, UMR CNRS 8180, Université de Versailles St Quentin en
Yvelines, Université Paris Saclay, 45 avenue des Etats-Unis 78035 Versailles Cedex. France. 5.
School of Science, RMIT University, Melbourne, 3001, and Commonwealth Scientific and
Industrial Research Organisation (CSIRO) Manufacturing, Clayton, Victoria 3169, Australia. 6
Institut des Matériaux Poreux de Paris, FRE 2000 CNRS, Ecole Normale Supérieure, Ecole
Supérieure de Physique et des Chimie Industrielles de Paris, PSL Research University, 75005 Paris, France.
*
Daiane DAMASCENO BORGES:
[email protected] Guy DE WEIRELD:
[email protected] *** Guillaume MAURIN:
[email protected] **
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ABSTRACT
One of the most promising technologies, with a low energy penalty, for CO2 capture from diverse gas mixtures is based on the adsorption process using adsorbents. Many efforts are still currently deployed to search for water stable porous Metal-Organic Frameworks (MOFs) with high CO2 affinity combined with large CO2 uptake. In this context, we have selected the water stable and easily scalable Al- based MOF MIL-160 showing an ultra-microporosity and potential interacting sites, both features being a priori relevant to favor the selective adsorption of CO2 over other gases including H2, N2, CH4 and CO. Density Functional Theory (DFT) and forcefield based Grand-Canonical Monte Carlo (GCMC) simulations were first coupled to predict the strength of host/guest interactions and the adsorption isotherms for all guests as single components and binary mixtures. This computational approach reveals the promises of this solid for the selective adsorption of CO2 with respect to these other investigated gases, controlled by a combination of thermodynamics and confinement effects. These predicted performances were further supported by real-co-adsorption measurements performed on shaped samples which evidenced that MIL-160(Al) show promising performances for the selective CO2 capture in postand pre-combustion conditions.
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INTRODUCTION The Porous Coordination Polymers (PCPs) or Metal-Organic Frameworks (MOFs) are crystalline porous hybrid materials1–4 that have been revealed as promising candidates for potential applications in diverse societally relevant applications.5–7 These hybrid porous solids are highly tunable in terms of pore size/topology and chemical functionalities that make them attractive for diverse gas separations or purification via a range of mechanisms including thermodynamics/kinetics and molecular sieving.8 More particularly, MOFs constitute an ideal platform to discover efficient adsorbents for CO2 capture from diverse mixtures including H2, CO, N2 and CH4 combining high selectivity and large CO2 uptake.4 In spite of these attractive properties, some of these MOFs have been shown poorly chemically stable9,10 and this can be a drawback for further applications in the field of gas separation that often occurs in the presence of humidity.11–14 Propitiously, amongst the water stable MOFs reported so far, there is a potential to identify promising candidates for a targeted application. In this context, here, we propose to explore the gas adsorption and separation performances of the Al-based MOF MIL-160(Al) which revealed previously promising properties for water adsorption/desorption and related- heat pump/chiller applications15–17. This MOF, a variant of the parent CAU-1018, is built from inorganic aluminum chains linked via five-membered ring 2,5 furan di-carboxylate ligand. The inorganic building unit consists of AlO6 octahedra that form cis - corner-sharing chains with Al covalently bonded to O atoms from four ligands and two hydroxyl groups (µ2-OH) that bridge the Al centers to create the chains. This leads to the formation of helical chains that run along the c-axis. The resulting 3D-framework exhibits square-shaped channels (Figure 1) with pore size ranging from 4 to 6 Å (see the calculated pore size distribution in Figure S1). Such ultramicroporous MOF decorated by potential interacting sites for the guest, e.g. both the µ 2-OH
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groups and the heteroatom of the furan-linker, is a priori a potential candidate for the selective adsorption of certain gas molecules by a combination of host/guest interactions and confinement effect8. This motivated us to computationally scan the adsorption properties of this material for a series of gas molecules including CO2, N2, CH4, H2 and CO using quantum- and force fieldsimulations and to further predict its promises for the selective capture of CO2 over H2, CO, CH4 and N2. This was performed in tandem with a careful analysis of the strength of the host/guest interactions and of the microscopic mechanisms at the origin of the gas adsorption/separation in this material. The so-obtained separation performances were further compared with the data collected using real co-adsorption experiments on shaped-samples, only rarely reported in the literature, which confirmed that this water-stable MOF shows interesting properties for post- and pre-combustion CO2 capture.
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Figure 1: Top and side view of MIL-160(Al) (1 × 1 × 2) structure. The Al, O, C and H atoms are represented by pink, red, gray and white colors, respectively. The isosurface in blue color shows the accessible pore volume that consists of rectangular-shape sinusoidal channels. MATERIALS AND METHODS Molecular simulations The atomic coordinates of MIL-160(Al) were taken from our previous work15 and the subsequent structure was geometry optimized at the Density Functional Theory (DFT) level. These calculations were performed using the Quickstep module of CP2K code within the PerdewBurke-Ernzerhof (PBE) generalized gradient approximation (GGA) functional19 combined with the use of Gaussian basis set and pseudopotential. For Carbon, Oxygen, and Hydrogen, a triple zeta (TZVP-MOLOPT) basis set was considered, while a double zeta (DZVP-MOLOPT) was applied for Aluminum20. The pseudopotentials used for all atoms were derived by Goedecker, Teter and Hutter21. The van der Waals effects interactions were taken into account via the use of semi-empirical dispersion corrections as implemented in the DFT-D3 method22. These calculations included the relaxation of the atomic positions of the MOF framework as well as the cell parameters while the angles between the cell vectors were maintained fixed at the initial values. The resulting cell parameters, 𝒂 = 𝒃 = 𝟐𝟏. 𝟐𝟑Å, 𝒄 = 𝟏𝟎. 𝟔𝟒𝟐Å and 𝜶 = 𝜷 = 𝜸 = 𝟗𝟎° are in very good agreement with that of the structure model experimentally refined in ref. 15 (i.e. X-ray data refinement: 𝒂 = 𝒃 = 𝟐𝟏. 𝟎𝟕𝟔𝟑(𝟑)Å; 𝒄 = 𝟏𝟎. 𝟔𝟑𝟏𝟐(𝟏)). Single point energy calculation followed by a Mulliken23 analysis were further performed using the Dmol3 program in the Material Studio 8.0 (BIOVIA) to extract the partial charges carried by each atom.
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Static binding energies for CO2, CH4, N2, CO and H2 in MIL-160 were calculated using density functional theory (DFT) as implemented in the software package VASP24. It is wellknown that standard DFT methods based on generalized gradient approximation do not fully account for the long-range dispersion interactions between the framework and the weakly bound gaseous adsorbates. To accurately estimate static binding energies for weakly bound guest molecules with MIL-160 framework, we implemented dispersion corrections using DFT-D3 method.25 Electron exchange and correlation were described using the generalized gradient approximation Perdew, Burke, and Ernzerhof (PBE)19 form and the projector-augmented wave potentials were used to treat core and valence electrons.19 In all cases, we used a plane-wave kinetic energy cutoff of 600 eV and a Gamma-point mesh for sampling the Brillouin zone. The ionic coordinates were relaxed until the Hellman-Feynman ionic forces were less than 0.01 eV/Å. The initial location of the guest molecule in the unit cell of MIL-160 was obtained from the classical simulated annealing technique using classical force field as implemented in sorption module in the Material Studio.26 In the simulated annealing method, the temperature was lowered stepwise, allowing the gas molecule to reach a desirable configuration based on different moves such as rotation, translation and re-positioning with preset probabilities of occurrence. This process of heating and cooling the system was repeated in several heating cycles to find the local minima. Forty heating cycles were performed where the maximum temperature and the final temperature were 105 K and 100 K, respectively. Static binding energies (ΔE) at 0 K were calculated using the following expression; ∆𝐸 = 𝐸𝑀𝑂𝐹+𝑔𝑎𝑠 − 𝐸𝑀𝑂𝐹 − 𝐸𝑔𝑎𝑠
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where Ex refers, respectively, to the total energies of the MOF + gas complex, the MOF alone, and gas molecule. Grand Canonical Monte Carlo (GCMC) simulations were further performed to predict the adsorption of single components, CO2, N2, CH4, H2 and CO and the binary mixtures CO2/H2 (molar gas composition: 50:50), CO2/N2 (molar gas composition: 15:85), CO2/CH4 (molar gas composition: 50:50) and CO2/CO (molar gas composition: 50:50) using the DFT-optimized structural model. The simulation box was made of 16 (2×2×4) unit cells of MIL-160(Al). The interaction between the MIL-160(Al) framework and the guest species (CO2, CO and N2) was modelled using the sum of a 12-6 Lennard-Jones (LJ) contribution and a coulombic term while for CH4 and H2 only LJ contribution was taken into account. The LJ parameters for describing each atom of the framework were taken from the DREIDING27 force field for the organic linker and from the Universal force field (UFF)28 for the inorganic node. Following the strategy we previously validated on the water adsorption in this material15, the LJ contributions from the Al atoms were not considered. The CO2 molecule was represented by the conventional rigid linear triatomic model, with three charged LJ interaction sites (C−O bond length of 1.149 Ǻ) located on each atom as previously derived by Harris and Yung29, The N2 and CO molecules was also described by a three charged sites model taken from the TraPPE forcefield
30
and from the paper
of Straub et al31, respectively. The H2 molecules were modeled with uncharged two-sites LJ32 and CH4 was described by the TraPPE uncharged single LJ interacting site model 33. The LJ crossing parameters for guest/MOFs interactions were obtained using Lorentz−Berthelot mixing rules. The Ewald summation was used for the calculations of the electrostatic interactions while the short-range contributions were computed with a cutoff distance of 14 Å. Gas-phase fugacity values were calculated with the Peng-Robinson equation of state34. These GCMC simulations 7 ACS Paragon Plus Environment
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were performed using CADSS (Complex Adsorption and Diffusion Simulation Suite)35. For each state point, 2×107 Monte Carlo steps were used for both equilibration and production runs and the adsorption enthalpy at low coverage (∆𝑯) for each gas was calculated through configurational-bias Monte Carlo simulations performed in the µVT ensemble using the revised Widom’s test particle insertion method
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. The radial distribution functions (RDF) for different
guest/atoms of MOFs at different pressures were obtained by averaging over the 2×107 Monte Carlo production steps. The selectivity for CO2 over other gases n, where n = CO, N2, H2 and CH4, is quantified calculating the separation factor, 𝜶, defined as 𝜶 = (𝒚𝒄𝒐𝟐 /𝒚𝒏 )/(𝒙𝒄𝒐𝟐 /𝒙𝒏 ), where y is the mole fractions in the adsorbed phase and x is the mole fractions in the gas phase, both at equilibrium.
Adsorption/Co-adsorption measurements A green and safer optimized synthesis route was used to produce MIL-160(Al) following larger-scale production of this MOF (kilogram scale).17 The powder was then shaped through a wet granulation route, using 10 wt% of silica sol as a binder, to obtain millimetric granules suitable for industrial processes as Pressure Swing Adsorption (PSA). Pure adsorption isotherm measurements were performed at 303.15 K up to 30 bar for both powdered and shaped samples with an automated in-house built apparatus based on manometric technique. This equipment is composed of two cells (the adsorption cell which contains the adsorbent and the pressure cell, each one fitted with a class A Pt 100 temperature probe (with a precision of 0.2 K at 303.15 K), two BARATRONs 627B pressure transmitters (0-1.333 bar for pressure measurement below 1 bar and 0-33.333 bar for above 1 bar) provided by MKS with an accuracy of 0.12% of the reading value, two electro-valves, a network of stainless steel tubes, a filter, a vacuum pump to 8 ACS Paragon Plus Environment
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realize a vacuum level lower than 10-4 kPa and an in-situ heating system located around the adsorption cell. For each of these gases, the isosteric heat of adsorption at low coverage was calculated by a Clausius-Clapeyron treatment based on the adsorption isotherms collected on the powdered sample at three temperatures (20°C, 30°C and 40 ° C). After outgassing 21.81 g of MIL-160(Al) sample at 150 K during 8 h in the adsorption cell, the adsorption isotherms were collected by gas injection in the pressure cell. When the equilibrium is reached, the electro-valve between the two cells is opened. The gas expands in the whole installation and the adsorption occurs in the adsorption cell. Knowing the installation volume before (pressure cell) and after (pressure cell and adsorption cell) as well as temperatures and pressures (a two-zone temperature model is used, one for each cell), the adsorbed mole number is the difference between the mole numbers in gas phase before and after adsorption calculated with an equation of state (for CO2: Span and Wagner37; for CH4: Setzmann and Wagner38 and for N2: Span et al.39). The whole isotherm was obtained by successive steps of gas admission and adsorption. The maximum relative uncertainty on the adsorbed amount is 6.2% for nitrogen, 3.0 % for methane and 2.5% for carbon dioxide. Co-adsorption measurements were carried out at 303.15 K and 1 and 4 bar with an inhouse built volumetric apparatus on the shaped material. The principle remains identical to the pure component adsorption, but a piston is used to modify the volume of the pressure cell and maintained the pressure at a set point value for co-adsorption equilibrium measurements. Gas composition is analyzed by a micro-chromatograph provided by SRA instruments with a TCD detector. For these experiments, the same sample was outgassed in-situ in the same conditions as pure isotherm measurement. The adsorbed quantities are determined by molar balances before and after adsorption. The gas amounts are calculated by the use of a mixture equation of state
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and the p-T-V-y measurements. The relative uncertainties associated with the adsorbed amounts are lower than 2.5 % for CO2, 8% for CH4 and 12.4% for N2. Co-adsorption apparatus and the experimental procedure have been described in detail in a previous paper 41. Thermodynamic model Ideal Adsorbed Solution Theory (IAST)42 is the most employed macroscopic model to evaluate the performance of adsorbents in gas separation. This model is used to select adsorbents and /or to simulate adsorption processes such as PSA and TSA. It allows the prediction of the mixture adsorption behavior from the single component adsorption isotherms. IAST describes the equilibrium between an ideal adsorbed solution and a perfect gas phase as follows: 𝑝𝑦𝑖 = 𝑝𝑖0 (𝜋)𝑥𝑖
(𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑇)
with p, the total pressure of the gas phase, 𝑦𝑖 , the molar fraction of component i in the gas, 𝑥𝑖 , the molar fraction of component i in the adsorbed phase and 𝑝𝑖0 (𝜋), the equilibrium gas phase pressure of the single component i at the temperature and spreading pressure (𝜋) of the mixture, i.e. in the standard state defined by the equality of spreading pressure for each compound to the mixture spreading pressure. The spreading pressure of compound i is calculated from the Gibbs adsorption isotherm:
0
𝑝𝑖 𝜋𝑖0 𝐴 𝑞𝑖 (𝑝𝑖 ) 𝜋𝐴 = ∫ 𝑑𝑝 = 𝑅𝑇 𝑝𝑖 𝑅𝑇 0
where A is the adsorbent surface area and 𝑞𝑖 (𝑝𝑖 ) is the single component gas adsorption amount of compound i at the pressure 𝑝𝑖 . 10 ACS Paragon Plus Environment
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For given values of p and 𝑦𝑖 , both equations are resolved simultaneously to determine 𝑥𝑖 . Then, the total adsorbed quantity is calculated by: 𝑖
1 𝑥𝑖 =∑ 𝑞𝑡 𝑞𝑖 (𝑝𝑖0 ) 𝑁𝐶
Then, we need an expression to represent as accurately as possible the single component adsorption isotherms in the range of 𝑝𝑖0 in order to solve efficiently these equations. In this work, we use the Jensen and Seaton equation43 𝑡 𝐾𝐻 𝑝 𝑞𝑖 = 𝐾𝐻 (1 + ( )) 𝑎 (1 + κ 𝑝 )
−1/𝑡
where 𝐾𝐻 is the Henry’s law constant, κ is the compressibility of adsorbed phase and t is positive empirical constant.
This equation leads a better fit of the experimental data for
microporous solids than the Langmuir or Toth44 equations especially for adsorbent/adsorbate systems with high Henry’s constants where the amount adsorbed increases rapidly at relatively low pressures and then slows down dramatically. It is necessary to mention that this equation can be reduced to the Toth equation when κ = 0 and moreover, when t=1 the Langmuir equation is obtained. RESULTS AND DISCUSSION The simulated single component adsorption isotherms at 303 K for the series of investigated gases are provided in Figure 2a. All show a Type –I isotherm shape consistent with the behavior of a microporous adsorbent. One observes that the saturation capacity increases following the sequence CO2 > CH4 > CO ~N2 > H2. Interestingly the CO2 uptake remains significantly higher
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than for the other gas molecules in the whole range of explored pressure. The predicted adsorption uptakes of CO2 at 303 K and 1 bar (~3.8 mmol.g-1) and 10 bar (~5.8 mmol.g-1) surpass the performances of other ultra-microporous MOFs previously envisaged for CO2 capture, e.g. SIFSIX-3-Zn (2.3 mmol.g-1 at 308 K and 1 bar)45, UiO-66(Zr)-2CO2H (1.0 mmol.g1
at 303 K and 1 bar)46, MIL-53(Al)-NH2 (1.6 mmol.g-1 at 283 K and 1 bar)47 and MIL-91(Ti)
(~3.0 mmol.g-1 at 1 bar/303 K and ~4.5 mmol.g-1 at 10 bar/303K)48.
Figure 2: GCMC simulated single component adsorption isotherms (a) enthalpy profiles (b) of CO2 (square), CH4 (circle), N2 (up triangle), H2 (lozenge) and CO (down triangle) in MIL160(Al) at 303 K. This promising performance was further confirmed by adsorption measurements (Figure 3a), with a very good agreement between the simulated and the experimental data collected on the powdered samples for CO2, CH4 and N2. One observes that the experiments performed using shaped-samples underestimate the adsorption capacity by a factor of ~13%, which is comparable to the amount of binder (10% mass) introduced to the sample for shaping. The experiments further confirmed that the adsorption uptakes for both CH4 and N2 are significantly lower than 12 ACS Paragon Plus Environment
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for CO2 (Figure 3b and Figure 3c). The higher affinity of MIL-160(Al) for CO2 is confirmed by the steeper adsorption isotherm as compared to the other gases in the low-pressure domain. This is also shown by the trend of the simulated adsorption enthalpies plotted in Figure 2b which follows the same sequence as the gas uptake: CO2 > CH4 > CO > N2 > H2. One can notice that the simulated adsorption enthalpy for CO2 (from -27 to -32 kJ/mol in the low-pressure domain) is slightly lower than the value previously reported for the other ultra-microporous MOFs that lie between -45 kJ/mol and -44 kJ/mol for SIFSIX-3-Zn45 and MIL-91(Ti)49, respectively. This result suggests that the CO2 molecule does not interact as strongly as in the MOF mentioned above, however the so-obtained value is significantly higher than the adsorption enthalpy obtained for CH4 (-19 kJ/mol), CO (-18 kJ/mol), N2 (-15 kJ/mol) and H2 (- 7 kJ/mol) which makes this material interesting in the perspective of CO2 capture over these four different guest molecules. Further, these simulated values are in good agreement with the experimental isosteric heats of adsorption calculated for CH4 (-18.7 kJ/mol), CO2 (-33 kJ/mol) and N2 (-16.7 kJ/mol). This relatively moderate CO2 adsorption enthalpy would favor an easier regeneration of MIL160(Al) which is a clear advantage for the use of this material in several adsorption/desorption cycles. Moreover, the enthalpy profile for all adsorbates is almost flat with a slightly increasing as gas loading augments due to the increase of the adsorbate-adsorbate interactions. This observation emphasizes that the adsorption enthalpy difference between CO2 and the other gases is maintained constant in the whole range of pressure, suggesting that the separation performances of MIL-160(Al) are potentially equivalent in a wide range of pressure which is a clear cut-advantage as compared to other MOFs containing strong adsorption sites present in a relatively low concentration that induces a significant drop of the selectivity when the adsorption uptake increases49. Indeed, this result strongly suggests that all gas molecules interact with a 13 ACS Paragon Plus Environment
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relatively homogeneous energetic surface. To shed light on the microscopic adsorption mechanism, a careful analysis of the configurations generated by the Monte Carlo simulations was undertaken.
Figure 3: (a) CO2, (b) CH4 and (c) N2 adsorption isotherms at T=303 K. A comparison between GCMC simulated (blue solid square symbol) and experimental (black open triangular and circle symbols for the powdered and shaped samples) data at 303 K. Indeed, we observed that at the initial stage of adsorption, CO2 is preferentially located in the vicinity of the furan linker as shown by the plot of the radial distribution function (RDF)
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between the oxygen atom of CO2 (Oco2) and the oxygen atom of the furan (Of) reported in Figure 4. The characteristic Oco2-Of interacting distance is relatively long (~3.1 Å) that confirms a moderate strength of CO2/MIL-160(Al) interactions as revealed by the adsorption enthalpy value. Figure 4 clearly emphasizes that CO2 do not interact with the µ2-OH groups due mostly to a limited accessibility of this adsorption site due to a local steric hindrance created by the ligands.
Figure 4: Radial distribution functions (RDFs) between the oxygen atoms of CO2 (Oco2) and the oxygen atoms of the furan (Of), carboxyl (Oca) and hydroxyl groups (µ2-OH) calculated from the GCMC simulations performed at 1 bar and 303 K. The insert reports an illustration of the corresponding preferential arrangement of CO2 in the pores. We further evidenced that each Of adsorption site accommodates one single CO2 molecule, all Of sites being saturated at ~5 bar (see the evolution of coordination number in Figure S2). 15 ACS Paragon Plus Environment
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Similar RDF profiles are observed over the whole range of pressure (see Figure S2) and this is consistent with the relatively flat profile of CO2 adsorption enthalpy. At saturation capacity, Figure 5 highlights that the CO2 molecules are aligned parallel to the direction of the channel (see the orientation distribution in Figure S3). This geometry is favored owing to the preferential interactions with the furan ligand and the small pore size of the channel that both contribute to an ordered packing of the CO2 molecules in the pores.
Figure 5: Illustration of the ordered arrangement of the CO2 molecules along the channel of MIL-160(Al) channel at saturation obtained from the GCMC simulations at 30 bar and 303 K. Very similar conclusion can be drawn for the other gas molecules (see Figure S4-S7). Figure 6 which reports the RDF between Of and the characteristic atom for each guest, emphasizes that the molecules are mostly distributed close to the furan ligand with interacting distances above 3.5 Å, consistent with a weaker host/guest interaction as compared to the case of CO2. At saturation, the other four gases, N2, CO and H2 adopt a less-ordered distribution along the direction of the channel. 16 ACS Paragon Plus Environment
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3.0 2.5 2.0
g(r)
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1.5 1.0 0.5 0.0 2
3
4
5
6
7
8
r(Å) Figure 6: Radial distribution function between the characteristic atom of the guest molecules, i.e. Oco2 (black square), Oco (purple down triangle), CCH4 (red circle), NN2 (blue up triangle) and HH2 (green lozenge) and the oxygen atoms of the furan (Of), calculated from the GCMC simulations performed at 30 bar and 303 K.
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Figure 7: DFT-D3 optimized lowest energy locations of CO2, CH4, N2, CO and H2 molecules in a unit cell of MIL-160 and their corresponding binding energies (B.E) are shown below. Color code: Framework Al, pink polygon; C, grey, H, white; and O, red. Guest molecules: CO2 - C, yellow; O, violet; CH4 – C, cyan, H, orange; N2 - N, blue; CO – O, red, C – yellow; H2 – H, green, respectively. To further substantiate the experimental and GCMC findings of selective CO2 adsorption, dispersion-corrected density functional theory (DFT-D3) calculations were performed to assess the differential interaction of adsorbates with the MIL-160 framework. As evident from Figure 7, CO2 interacts more strongly with the framework than CH4, N2, CO and H2 molecules. In particular, the calculated binding energies based on the DFT-D3 method follows 18 ACS Paragon Plus Environment
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the similar trend in the heat of adsorption predicted from the GCMC simulation (Figure 2b). Also, the DFT-D3 optimized location of adsorbates in MIL-160 framework showed reasonable good agreement with the radial distribution function calculated from the GCMC simulations (Figure 6, Figure S4-S7). For instance, the distance between the oxygen atom of furan (O f) in MIL-160 and the oxygen atom of CO2 from DFT-D3 is around 3 Å, again which is in excellent agreement with the predicted RDF from the simulation. As a further step, GCMC simulations were performed to predict the separation performances of MIL-160(Al) at 303 K for four binary mixtures: CO2/CH4 (50:50), CO2/CO (50:50), CO2/N2 (15:85) and CO2/H2 (50:50). Figure 8 reports the corresponding binary mixture adsorption isotherms which confirm that MIL-160(Al) preferably adsorbs CO2 over the four other gases as expected from the single component adsorption behavior. The corresponding simulated selectivity for CO2/H2, CO2/CO, CO2/N2 and CO2/CH4 are 420, 20, 35 and 10 at 1 bar respectively. The co-adsorption mechanisms of CO2/CH4 and CO2/N2 are illustrated in Figures 9 and 10 at different pressures. This emphasizes that similarly to the single component adsorption behavior, CO2 preferentially sits near the vicinity of the more energetic Of adsorption sites while the other guests are expulsed away from them. This scenario holds true in the whole range of pressure (see RDFs plotted in Figure S9 and S10). At saturation, the efficient packing of CO2 also contributes to a preferential adsorption of this guest molecule over the others.
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Figure 8: GCMC simulated co-adsorption isotherms at 303 K for (a) CO2/H2 (50:50), (b) CO2/CH4 (50:50), (c) CO2/N2 (15:85) and (d) CO2/CO (50:50) in MIL-160 (Al).
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Figure 9: Snapshots of the simulated CO2/CH4 co-adsorption at 0.1, 0.4, 1 and 5 bar. CH4 is represented by green beads.
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Figure 10: Snapshots of the simulated CO2/N2 co-adsorption at 0.6, 1, 5 and 10 bar. N2 is represented in blue. 22 ACS Paragon Plus Environment
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To validate these GCMC predictions and further confirm the promises of this MOF for CO2 capture, real co-adsorption measurements were performed on shaped samples for the CO2/N2 and CO2/CH4 binary mixtures at different pressures and different gas compositions. Figure 11 reports the corresponding selectivity plotted for both gas mixtures at 1 bar. A selectivity of about 34 was obtained at 1 bar and 303 K for the mixture CO2/N2 = 17/83 which is in excellent agreement with the GCMC simulated value (35) obtained for a similar gas mixture composition. The agreement between co-adsorption measurements and GCMC simulations is also very good for the CO2/CH4 mixture with an experimental value for the mixture CO2/CH4=49/51 of about 8 (vs 10 for the GCMC simulations). Interestingly the application of the Ideal Adsorption Solution Theory (IAST) macroscopic model to the single component adsorption modeled by Jenson-Seaton equation (Table S1 for the parameters of Jenson-Seaton equation and standard deviation factors) data leads to similar selectivity for both mixtures as compared to the real coadsorption measurements (Figure 11). The relative differences between IAST predictions and experimental measurements on shaped materials are similar for CO2/N2 and CO2/CH4. The maximal relative difference (12%) is observed for CO2/CH4 system. We can note that the IAST predictions are always within the interval defined by the experimental uncertainties. Further, Figure 11 shows that the IAST predicted selectivities are very similar for both shaped and powdered samples. As observed, MIL-160(Al) can be treated as a homogeneous energetic surface as suggested by the adsorption enthalpy profile. Although the corresponding separation performances are not as high as those reported for the best adsorbents so far, they lie in a range of values very attractive for water stable MOFs. The CO2/CH4 separation factor is similar to the performances of other MOFs previously envisaged, such as MIL-53(Al)
50
, MIL-125(Ti) and its NH2-derivative
51
. The
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CO2/N2 selectivity is similar to the performance reported experimentally for other MOFs such as Zn4(pydc)4(DMF)2·3DMF 51, UiO-66 (Zr) BTEC 46, Ni/DOBDC 52 and MIL-91(Ti).53
10
40
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(b)
CO2 / N2 Selectivity
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CO2 / CH4 Selectivity
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4
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10
2
0
0 0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
y CO2
y CO2
Figure 11: (a) CO2/CH4 and (b) CO2/N2 selectivity for different gas composition (yCO2). Comparison between experimental measurements (circle) and IAST macroscopic model prediction for shaped (continuous line) and powdered (dashed line) samples.
CONCLUSIONS GCMC and DFT calculations were performed to probe the sorption properties of CO2, CH4, N2 and CO gases in MIL-160(Al) in terms of affinity and gas uptake. The theoretical predictions show that MIL-160(Al) is a promising candidate for CO2 recovery with a relatively high storage capacity. The host/guest strength interactions were found to be relatively moderate which is an advantage for the regeneration of this solid. GCMC simulations further revealed that this solid is promising for the selective capture of CO2 over the other gases through a thermodynamic driven separation mechanism combined with a confinement effect. At high pressure, the channel-like topology of the structure induces a more efficient packing of CO2 over the other guests. These 24 ACS Paragon Plus Environment
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predictions were further supported by real-co-adsorption measurements performed on both shaped and powdered samples which evidenced that this water stable MIL-160(Al) shows a real interest for CO2 capture in post- and pre-combustion conditions. ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. It contains extra graphs that support the discussion made in this manuscript, such as: Pore size distribution (Figures S1); Orientation probability distribution (Figure S3); Radial Distribution Function (Figures S2, S4-S7, S9-S10); N2 adsorbed snapshot (Figures S8).
Acknowledgement D.D.B. thanks the Research Foundation FAPESP, grant # 2015/14703-9 for financial support. R.B. acknowledges the National Computing Infrastructure (NCI), CSIRO Pearcey cluster and the Pawsey supercomputing facilities for the computing support and the Australian Research Council for the DECRA fellowship (DE160100987). .We thank Jong-San Chang from KRICT for the shaped samples who acknowledge the financial support through the R&D Convergence Program (CRC-14-1-KRICT) funded by NST (National Research Council of Science& Technology) of Korea. We also thank Dr. U-H Lee and Mr. A. H. Valekar from KRICT for their contribution to shaping and F. Nouar from IMAP for the synthesis of the material. The research leading to these results has received funding from the European Community Seventh Program (FP7/2007-2013) under grant agreement n° 608490 (project M4CO2). G.M. thanks Institut Universitaire de France for its support.
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