Role of Structure and Chemistry in Controlling Separations of CO2

Article pubs.acs.org/JPCC

Role of Structure and Chemistry in Controlling Separations of CO2/ CH4 and CO2/CH4/CO Mixtures over Honeycomb MOFs with Coordinatively Unsaturated Metal Sites Edder J. García,† John P. S. Mowat,‡ Paul A. Wright,‡ Javier Pérez-Pellitero,† Christian Jallut,§ and Gerhard D. Pirngruber*,† †

IFP Energies nouvelles, Rond Point échangeur de Solaize, 69360 Solaize, France EaStCHEM School of Chemistry, University of St. Andrews, Purdie Building, North Haugh, St. Andrews, Fife KY16 9ST, U.K. § Laboratoire d’Automatique et de Génie des Procédés, Université de Lyon, Université Lyon 1, UMR 5007, CNRS-ESCPE, 43, Bd du 11 Novembre 1918, 69622 Villeurbanne cedex, France ‡

S Supporting Information *

ABSTRACT: A systematic study of the effect of coordinatively unsaturated sites (cus) in the separation of CO2/CH4 and CO/ CO2/CH4 mixtures on CPO-27-M (M = Ni, Co, and Zn) and STA-12-Ni metal−organic frameworks was carried out using gravimetric and breakthrough experiments. The separation selectivity and the working capacity of these structures were evaluated as important performance indicators for CO2 separations by PSA. The results demonstrate a remarkable influence of coordinatively unsaturated sites on the selectivity and the working capacity. Particularly, the high affinity of CPO-27-Ni and CPO-27-Co for CO2 leads to a low working capacity for CO2 (because regeneration of the adsorbents is difficult) but a high CO2/CH4 selectivity. With a ternary CO/CO2/CH4 feed mixture, CPO-27-Ni and -Co prefer the adsorption of CO over CO2 due to the strong specific interaction of CO with cus. Surprisingly, STA-12-Ni does not exhibit the same behavior: it is selective for the adsorption of CO2 in a ternary mixture, just like CPO-27-Zn. Among the four MOFs tested in this study, CPO-27-Zn presents the best compromise between the working capacity and the CO2/CH4 and CO2/CO selectivities. The results are discussed in terms of the coordination chemistry of the coordinatively unsaturated metal sites, their acid−base properties, and their accessibility.

1. INTRODUCTION Today most hydrogen produced in refineries is generated by steam reforming of natural gas. The product of this process is a mixture of hydrogen, methane, carbon dioxide, and carbon monoxide, accompanied by other trace impurities, depending on the technology that is used and the nature of the feedstock. Usually, hydrogen is purified using pressure swing adsorption (PSA) processes. The PSA is a cyclic process based on the selective adsorption of the impurities on porous adsorbents. The most used adsorbents are activated carbons, aluminas, silica gels, and zeolites. The impurities (CO2/CH4/CO) are first adsorbed at high pressure and then recovered during a desorption step at low pressure. The low-pressure stream is called “extract”. Presently, this mixture is used as a fuel and is © XXXX American Chemical Society

then released to the atmosphere. This process leads to CO2 emissions. Concerns about global warming provide a strong incentive for integrating CO2 capture into the production of H2. The extract of the existing H2-PSA units is mainly a mixture of CO2, CO, CH4, and a small fraction of H2. The objective is to separate the extract stream in pure CO2 and a mixture of CH4 and CO that can still be used as a fuel. The CO2 separation can, for example, be carried out in a separate PSA process, either upstream of the H2-PSA or downstream of the H2-PSA, using the extract of the H2-PSA as feed. In both cases, Received: September 25, 2012 Revised: November 17, 2012

A

dx.doi.org/10.1021/jp309526k | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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solvent water molecule that can be removed, thus generating a cus. To date, the CPO-27 topology has one of the highest densities of cus per volume unit found in MOFs. Therefore, this material combines a large pore size with a large number of coordinatively unsaturated sites. We can find a large body of high-pressure adsorption data of the CPO-27 frameworks in the literature. Single component adsorption data of interest for the CO2 purification on CPO27-M (where M = Ni, Co, Mg, and Zn) have been previously reported for H2,14,15 CH4,16−18 CO2,16,19 and CO.20 Additionally, single component isotherms for CO2, CH4, and N2 on CPO-27-Ni and CPO-27-Mg as well as CO2/CH4 and CO2/N2 binary mixture isotherms on CPO-27-Ni were reported using thermogravimetry and breakthrough experiments.16 Similarly, the dynamic separation of CO2/CH4 mixtures on CPO-27-Mg has been studied using cyclic breakthrough experiments.21 It was demonstrated that this material presents a similar separation capacity to NaX zeolite but requires less severe reactivation conditions. The metal−CO 2 interaction is an important factor determining the selectivity at low pressure. The CO2 isosteric heats of adsorption on CPO-27 formed by different cus are 47, 41, and 37 kJ/mol for Mg, Ni, and Co, respectively.19 These heats of adsorption are similar to those exhibited by cationic zeolites like NaY (32 kJ/mol),22 NaX (49 kJ/mol),23 and NaZSM-5 (38 kJ/mol).23 Another honeycomb MOF with cus is the metal phosphonate known as the STA-12 (St. Andrews microporous material no. 12).24,25 In this case, the organic ligand is N,N′piperazinebismethylenephosphonate. This framework can be synthesized with metals such as Ni, Mg, Co, Fe, and Mn.26 The dehydrated framework is built from helical chains to form a honeycomb structure with a large elliptical pore of approximately 8−9 Å (see Figure 2). Materials with the same topology but larger pore diameter (18 Å) have been reported.27 Measurements of the equilibrium isotherms and the heat of adsorption of CH4 and CO2 on STA-12-Ni have shown a highly favorable adsorption of CO2.25 It exhibits a heat of adsorption of CO2 (30−35 kJ/mol) higher than other MOFs with similar pore size, like MIL-47.28 The adsorption sites were identified using IR and CO and CO2 as probe molecules. The three crystallographically different nickel cations give rise to different adsorption sites of different strength.25 Comparatively, both CPO-27 and STA-12 present a similar topology and can be synthesized using several metal centers. This feature provides a straightforward way of changing the polar character of the surface, which has a major importance in the adsorption of CO2. CPO-27 and STA-12 MOFs are promising materials for their application in adsorption due to their high concentration of cus per volume unit. The practical application of these materials requires the knowledge of the multicomponent adsorption data. Despite the interest shown lately in the literature about this class of materials, their separation capabilities have not been compared in detail. The goal of this work is to compare these materials in order to understand the influence of different coordinatively unsaturated sites in the CO2 separation by PSA using CO2/CH4 and CO/CO2/CH4 mixtures as feed. The present paper is organized as follows. In the Experimental Section, the material synthesis and physical characterization techniques are first described. Then, the techniques that are used for the adsorption properties measurements are described. In the Results section, equilibrium

CO2 selective adsorbents are needed that will produce a pure CO2 stream during the desorption step. Another application of CO2 separation by PSA is the purification of biogas or natural gas. In this case, the main objective is to separate CO2 and CH4 to produce a highly pure CH4 stream and to minimize losses of CH4 in the extract stream. In order to achieve this goal, adsorbents for CO2 must fulfill two main properties: they must have a high selectivity for adsorption of CO2 and be able to adsorb a large amount of CO2 during each adsorption−desorption cycle. This latter amount is known as working capacity. Both properties are intrinsically linked to the properties of the adsorbent, such as pore volume, density, or polarity. Metal−organic frameworks (MOFs) are porous solids that can be synthesized in a large variety of pore sizes and compositions. These materials are made of a central metal cluster linked by organic ligands. They present high surface areas and pore volumes; these properties are very useful for gas separation, adsorption, and storage. The suitability of MOFs for capture and separation of CO2 has recently been reviewed.1−4 Among the large number of known frameworks, MOFs with coordinatively unsaturated sites (cus) present the highest adsorption capacities and heat of adsorption of CO2.5−7 During the synthesis of these structures, solvent molecules occupy a terminal site of the metal center. A postsynthesis treatment can remove these solvent molecules to generate coordinatively unsaturated sites. These sites are accessible to guest molecules. On the one hand, the electronic deficiency of these sites produces strong interactions with electronic donors like CO2 or CO. Also, the permanent dipole moment present in these atoms induces the polarization of molecules like CH4. One example of MOFs with coordinatively unsaturated sites is the structure known as Cu-BTC (or HKUST-1).8 This material shows an isosteric heat of adsorption of CO2 of 30−35 kJ/ mol9,10 as well as a good CO2/CH4 selectivity.11 A second example of this type of material is the structure called CPO-27 (Coordination Polymer of Oslo No. 27)12 or MOF-74.13 It exhibits a honeycomb topology where its one-dimensional pore system can be formed by different cus like cobalt, nickel, magnesium, or zinc. The organic ligand is 2,5-dihydroxyterephthalic acid (C8H6O6) (Figure 1). The microporous system presents a pore diameter of approximately 11−12 Å. In the assynthesized material, the metal center is coordinated in octahedral fashion by six oxygen atoms; five of these oxygen atoms come from the ligands, and the last one comes from a

Figure 1. (a) View of the CPO-27-Ni pore. (b) Cluster of the CPO27-Ni. Ni = blue, O = red, C = gray, and H = white. B



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Figure 2. (a) View of the STA-12-Ni pore. (b) Unit cell of the dehydrated STA-12-Ni. Ni = light blue. O = red, C = gray, H = white, P = purple, and N = dark blue.

by using a Bruker AXS D4 Endeavor diffractometer with Cu Kα (λ = 1.542 Å) radiation. All the XRD patterns were recorded using a step size of 0.02° and a scan speed of 60 s/step. The N2 physisorption measurements were performed in a Micromeritics ASAP 2000 instrument. To remove the adsorbed guest molecules, all the samples were heated at 503 K under vacuum overnight before the adsorption measurements. The apparent surface area of N2 was calculated by using the Brunauer− Emmett−Teller (BET) model, and the micropore volume was calculated by using the t-plot method. 2.2. Gravimetric Measurements. Single component adsorption isotherms of CO2, CH4, and CO were measured at 303 K by using a Rubotherm magnetic suspension balance. A mass of approximately 1−2 g of adsorbent was activated and outgassed at 503 K. The isotherms were recorded in the pressure range 0−20 bar. After stabilization of the mass, temperature, and pressure, the change in the mass was recorded at a given pressure. Using this methodology, a direct measurement of the reduced mass (Ω) is obtained. To calculate the absolute adsorbed quantity (qabs), the correction of the buoyancy effect was taken into account. The absolute adsorbed quantity was calculated by the following equation:

data obtained for single, binary, and ternary mixtures by using gravimetric technique and breakthrough curves are given. In the Discussion, the results are commented and interpreted by considering chemical properties of the adsorbents. Finally, before concluding, a discussion concerning the application of the studies materials is provided.

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis and Physical Characterization. The CPO-27-Ni material was synthesized by using the method described in Dietzel et al.29 A solution of nickel acetate tetrahydrate (1.5 mmol) in water (250 mL) and a solution of 2,5-dihydroxyterephthalic acid (0.75 mmol) in tetrahydrofuran (250 mL) were prepared and combined in an autoclave with a Teflon-coated vessel of 500 mL. The solution was heated to 383 K for 3 days. A fine powder was obtained, recovered by filtration, and washed with water. The CPO-27(Co and Zn) were prepared according to the method used by Yaghi et al.30 with slight modifications. A solution was prepared by using 2,5-dihydroxyterephthalic acid (13 mmol) and the acetate tetrahydrates of Ni, Co, or Zn (38 mmol) dissolved in 500 mL of dimethylformanide (DMF) and 10 mL of deionized water. The solution was heated in an autoclave with a Tefloncoated vessel of 1 L at 373 K for 20 h. After cooling down, the liquid solution was successively replaced by pure methanol every 2 days, 7 times for Co and 15 times for Zn. This allows a complete solvent exchange of the DMF, which is strongly adsorbed in the pores, by methanol that can be more easily removed by thermal treatment. The solvent exchange was indispensable in order to achieve high surface areas and pore volumes in the case of Co and Zn, but not in the case of Ni. Yet, we will see in the Results section that a solvent exchange would have, in hindsight, been advisible also for CPO-27-Ni because some traces of DMF seem to remain adsorbed on the Ni sites, even when the expected pore volume has been recovered. The STA-12-Ni was synthesized using the method of Groves et al.24 Briefly, N,N′-piperazinebismethylenehosphonic acid and the nickel acetate were mixed with water in a Teflon-coated vessel by using a M:ligand:H2O molar ratio of 0.02:0.01:9. The vessel was closed and heated to 493 K for 4 days. The final solution was filtrated. The fine powder was washed and dried. The samples were characterized by using XRD and N2 adsorption at 77 K. The XRD measurements were performed

qabs = Ω + ρbulk (Vadsorbent + Vcrucible + Vadsorbed phase)

(1)

Vadsorbent and Vcrucible were calculated using the He isotherm method.31 The contribution of the adsorbed phase volume (Vadsorbed phase) was approximated by the microporous volume of the samples. We preferred reporting the absolute adsorbed amount over the excess adsorbed amount (the difference between the two is the term ρbulkVadsorbed phase) in order to be consistent with the breakthrough measurements, which directly yield the absolute adsorbed amount. 2.3. Breakthrough Curve Measurements. Breakthrough curves of idealized biogas (CO2/CH4 50/50 v/v) and synthesis gas (CO2/CH4/CO 70/15/15 v/v/v) feeds were carried out on CPO-27-M and STA-12-Ni using a homemade apparatus. The flow of the gases was controlled by means of mass flow controllers in the range 1−10 N·L/h. The total flow was kept at 4 N·L/h. The individual flow of the compounds was adjusted to generate a mixture of the desired composition. The gas concentrations at the system outlet were measured using an online mass spectrometer after dilution by a flow of 100 N·L/h of He. The adsorbent was placed as powder in a stainless steel C



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column with an inner diameter of 10 mm and length of 80 mm. The adsorbent mass was ∼2 g. The adsorbents were activated under He flow at 503 K with a heating rate of 1 K/min. After cooling down, the pressure in the column was adjusted by flowing He through the column. At the same time, the CO2/ CH4 or CO2/CH4/CO mixture was pressurized in the bypass line. The pressure drop was less than 0.1 bar. Then, a direct injection of the mixture was carried out using an automatic valve, and the breakthrough curve was recorded via the mass spectrometer. The breakthrough curves of the more weakly adsorbed components CH4 and CO are characterized by a socalled roll-up. Since the roll-up increases the experimental uncertainty on the calculated uptake, a supplementary experiment was carried out. The column was first saturated by flowing pure CO2, and then the mixture was injected again into the column. This second experiment allows measuring the breakthrough curves of CH4 and CO without roll-up. Additionally, we carried out desorption experiments by passing a flow of He (4 N·L/h) through the column. The absolute adsorbed quantity at equilibrium was calculated from the first moment of the breakthrough curve (μ), according to ⎛ ⎞ Ci ⎜ mads ⎟ Q μ − V + col mads ⎜⎝ ρgrain ⎟⎠

qi =

Table 1. Physical Properties of the Adsorbents

a

⎛

∫ ⎜⎜⎝1 −

Fi Fi,0

⎞ ⎟⎟dt ⎠

(2)

(3)

where Fi,0 and Fi are respectively the flows of the component i at the inlet and the outlet of the column. A numerical integration based on the trapezoidal rule was performed in order to calculate μ. The interval of the integration was taken from the time of injection (t = 0), after correction for the dead time, to the time where the Fi/Fi,0 ratio is stable and close to 1. The selectivities (S) were determined from the curves using the equation

Si , j =

qi /qj Xi / X j

micropore vol (t-plot), cm3/g

grain density,a kg/m3

metal centers density,a mol/m3

CPO-27-Co CPO-27-Ni CPO-27-Zn STA-12-Ni

1093 1266 806 439

0.50 0.58 0.38 0.15

1172 1194 1221 1463

7515 7667 7701 7118

Calculated from the crystallography data.12,13,25,33

than the CPO-27 topology, it is not surprising that this structure presents the lowest pore volume and surface area.19 3.2. Single Component Isotherms: Gravimetric Results. Figure 3 shows the absolute isotherms of CO2, CO, and CH4 at 303 K up to a pressure of 20 bar. In the case of the CPO-27 materials, all the isotherms present a type I shape according to the IUPAC classification.34 No hysteresis loops were observed. In the case of the STA-12-Ni the isotherm of CO2 and CH4 have a type I shape while the isotherm of CO presents a type V shape and exhibits a small hysteresis loop. By using a minimization of the residual sum of squares, the singlesite Langmuir, dual-site Langmuir, Toth, and Langmuir− Freundlich models were fitted to the experimental data. The estimated values of the models parameters are compiled in Tables 2, 3, and 4 and in the Supporting Information. In the case of the CO2 and CO adsorption data, the dual-site Langmuir model allows distinguishing the strong adsorption on the coordinatively unsaturated sites and the weak adsorption in the remaining volume of the pore. When the dual-site Langmuir model was applied to the CH4 adsorption data, the estimated solution converged toward a single-site Langmuir model. However, for CH4 the best fit was obtained by the Langmuir−Freundlich equation. The type V isotherm of CO on STA-12-Ni was not fitted. The single component isotherms are similar to those reported previously.16,19,25 In order to take into account the different densities of CPO-27-M and STA-12-Ni, the isotherms in Figure 3 were expressed in moles per volume unit of the adsorbent. This kind of representation is well suited in order to estimate the required size of the adsorption vessels. The adsorbent density was calculated by using crystallographic data (see Table 1). Expressed in molecules per unit cell, the maximum quantity of adsorbed CO2 is 30.1, 29.9, and 23.2 for CPO-27-Co, -Zn, and -Ni, respectively. The lower saturation capacity of CPO-27-Ni is attributed to an incomplete removal of solvent molecules from the cus (CPO-27-Ni was not solventexchanged by methanol, like the two other CPO-27 materials). In the case of CO and CH4 isotherms, the adsorbed amount that is reached at 20 bar is almost the same for all the studied samples. As far as the initial slopes of the isotherms (slopes at zero pressure) are concerned, in the case of CPO-27-Zn and STA12-Ni, they decrease according to the order CO2 > CO > CH4. In the case of CPO-27-Co and -Ni, the order of affinity is CO > CO2 > CH4. The initial slope of the CO isotherms on CPO-27Ni and Co is very steep, indicating a very strong affinity for CO. Furthermore, the adsorption of CO attains saturation at lower pressure than CO2 and CH4. Figure 4 shows the CO coverage (θ) (that is defined as the number of CO molecules per coordinatively unsaturated site) as a function of pressure.

where qi is the absolute adsorbed amount of the component i, Ci is the gas phase concentration, mads is the adsorbent mass, Q is the total volumetric flow, Vcol is the column volume, and ρgrain is the adsorbent grain density. The first moment can be calculated from μ=

MOF

BET surf. area, cm2/g

(4)

where q and X are the adsorbed amounts and molar fractions in the bulk phase of the components i and j, respectively.

3. RESULTS 3.1. Physical Characterization. The XRD patterns of all the samples studied are similar to those published in the literature.12,19,32 No other crystalline phase was detected (see the Supporting Information). The pore volumes and BET apparent surface areas calculated by using N2 physisorption at 77 K are listed in Table 1. The values are in the expected range for these materials. The surface areas and pore volumes decrease in the sequence CPO-27-Ni > CPO-27-Co > CPO-27Zn > STA-12-Ni. The rather low porosity of CPO-27-Zn can be attributed to a lower crystallinity of the desolvated sample. Since STA-12 has a smaller pore diameter and higher density D



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Figure 3. Absolute adsorption isotherms of CO2 (diamonds), CO (circles), and CH4 (triangles) on (a) CPO-27-Co, (b) CPO-27-Zn, (c) CPO-27Ni, and (d) STA-12-Ni at 303 K. The line is the best fit for the adsorption models, i.e., Langmuir−Freundlich for CH4 and dual-site Langmuir for CO2 and CO.

Table 2. Best Fitting Parameters of the Dual-Site Langmuir Model for the CO2 Isotherms at 303 K q1sat, q2sat, 1

mmol/g mmol/g b , bar−1 b2, bar−1

CPO-27-Co

CPO-27-Zn

CPO-27-Ni

STA-12-Ni

5.96 6.68 3.40 0.14

6.43 4.58 1.50 0.25

4.77 4.64 5.24 0.16

7.15 0.00 0.43 0.00

Table 3. Best Fitting Parameters of the Dual-Site Langmuir Model for the CO Isotherms at 303 K CPO-27-Co

CPO-27-Zn

CPO-27-Ni

5.17 9.43 20.01 0.01

5.27 1.68 0.35 0.037

3.73 3.26 73.18 0.04

q1sat, mmol/g q2sat, mmol/g b1, bar−1 b2, bar−1

Figure 4. CO coverage per cus at 303 K. The solid line is the best fitting for the Langmuir dual-site model: filled symbols, adsorption; open symbols, desorption.

In the case of CPO-27-Zn, the CO isotherm gradually increases with pressure, whereas the CO isotherms on CPO-27Ni and Co rise very steeply at the beginning, followed by an almost linear slope. For CPO-27-Co, the knee of the isotherm corresponds to a metal coverage of 1. It must be noted that for CPO-27-Ni the coverage of 1 is not quite attained, due to a incomplete removal of the solvent. STA-12-Ni shows the lowest affinity with CO and presents a S-shaped isotherm with a hysteresis loop.

Table 4. Best Fitting Parameters of the Langmuir− Freundlich Model for the CH4 Isotherms at 303 K

qsat, mmol/g b, bar−1 ξ

CPO-27-Co

CPO-27-Zn

CPO-27-Ni

STA-12-Ni

8.81 0.12 1.01

6.93 0.10 1.12

6.26 0.14 1.12

4.96 0.07 1.05 E



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Figure 5. Breakthrough curves of the CO2/CH4 (50/50) mixture on (a) CPO-27-Co, (b) CPO-27-Zn, (c) CPO-27-Ni, and (d) STA-12-Ni at 303 and 5 bar.

In the adsorption of CO2, the Langmuir constant for the first site increases according to the order CPO-27-Ni > CPO-27-Co > CPO-27-Zn > STA-12-Ni. The Langmuir−Freundlich affinity constant (b) for CH4 decreases in the sequence CPO-27-Ni > CPO-27-Co, Zn > STA-12-Ni. 3.3. Binary Mixtures: Breakthrough Curves. Figure 5 shows the breakthrough curves for the CO2/CH4 (50/50) mixture on CPO-27-M (M = Ni, Co, Zn) and STA-12-Ni at 303 K and 5 bar. The dimensionless x axis is the ratio of the injected volume to the column volume. This representation allows a direct comparison when columns of different sizes are used. The breakthrough curves at 1 and 10 bar are shown in the Supporting Information. All the CPO-27 samples show a higher adsorption capacity than STA-12-Ni, which is in good agreement with the pore volume of the two kinds of materials. All the tested adsorbents have a lower affinity for CH4 than for CO2. Therefore, CH4 always breaks first. All curves exhibit a double roll-up that is more or less visible. The second roll-up is less appreciable in the case of CPO-27-Zn. The first roll-up is due to the CH4 desorption by the advancing front of CO2. The second comes from the exothermic adsorption of CO2 that increases the temperature in the column, thereby further enhancing the desorption of CH4. This peak exhibits similar intensities for CPO-27-Ni, -Co and STA-12-Ni, but it becomes less intense in the case of CPO-27-Zn, indicating a lower heat of adsorption of CO2. Furthermore, if we look carefully at the breakthrough curves of CO2 for the whole series, we will note that the curve for CPO-27-Zn is less steep. This observation also indicates a lower affinity between CO2 and CPO-27-Zn.

Figure 6 displays the desorption curves of the CO2/CH4 mixture by a flow of He. The initial peak in the desorption profile (at t < 0) corresponds to the fast evacuation of the gas contained in the dead volume of the system. For CPO-27-Ni and -Co, CO2 desorption is slower than for CPO-27-Zn and STA-12-Ni. In order to evacuate all the CO2 in the sample, one needs a much higher volume of gas in the case of CPO-27-Ni and Co as compared to STA-12-Ni and CPO-27-Zn. This observation is also valid for desorption curves at 1 and 10 bar (see the Supporting Information). The CO2/CH4 selectivities obtained from the breakthrough experiments are listed in Table 5. We observe an increase in the CO2/CH4 selectivity as a function of the pressure for all the samples, but the experimental uncertainties also increase at higher pressure. There is no significant difference in the selectivity between CPO-27-Ni and Co. STA-12-Ni and CPO27-Zn show a significantly lower selectivity at 1 bar (at 1 bar the values are most reliable). There exists an important difference in the selectivity at 1 bar between STA-12-Ni and CPO-27-Ni, although both materials have the same cus. 3.4. Ternary Mixtures: Breakthrough Curves. Figures 7 and 8 show the breakthrough curves for the CO2/CH4/CO (70/15/15) mixture on CPO-27-M (M = Ni, Mg, Zn) and STA-12-Ni at 303 K and 1 and 5 bar, respectively. The breakthrough curves of the ternary mixture at 10 bar are shown in the Supporting Information The breakthrough curves of the ternary mixture also show a higher adsorption capacity of the CPO-27 samples compared to STA-12-Ni, with a lower pore volume. In the case of CPO-27-Zn, the retention increases according to CH4 < CO < CO2, which is coherent with the slopes of the F



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Figure 6. Desorption curves of the CO2/CH4 (50/50) mixture on (a) CPO-27-Co, (b) CPO-27-Zn, (c) CPO-27-Ni, and (d) STA-12-Ni at 303 and 5 bar.

and CH4 are quickly eluted. Table 6 summarizes the CO2/CH4 and CO2/CO selectivities for the ternary mixtures at different pressures. The most important property shown in Table 6 is the inversion of the CO2/CO selectivity on CPO-27-Ni and Co. CO is preferentially adsorbed over CO2 in the range of measured pressures. The CO2/CO selectivity increases as pressure is increased. Finally, CPO-27-Zn and STA-12-Ni show a CO2/CO selectivity close to that of CO2/CH4

Table 5. CO2/CH4 Selectivity at Different Pressures MOF CPO-27-Ni CPO-27-Co CPO-27-Zn STA-12-Ni

1 bar 15 12 9 6

± ± ± ±

4 2 2 1

5 bar

10 bar

± ± ± ±

40 ± 16 19 ± 3 22 ± 5 >25

19 16 10 23

7 7 3 2.0

isotherms in Figure 3b. In the case of STA-12-Ni, the order of elution is CO < CH4 < CO2 at 1 bar and CO ∼ CH4 < CO2 at 5 bar. The weak retention of CO at low pressure reflects the low initial slope of the CO isotherm on STA-12-Ni, while the coelution of CO and CH4 above 5 bar is a consequence of the fact that the CO and CH4 isotherms are almost superposed at higher pressures. For CPO-27-Ni and -Co, the retention pattern is entirely different. For these samples the order of elution is CH4 < CO2 < CO; i.e., CO is more strongly retained than CO2. Again, this is a consequence of the very steep isotherms of CO on these materials. As the pressure increases, the breakthrough curve of CO on CPO-27-Co becomes less steep because we are moving into the flat part of the CO isotherm. This effect is less visible in the case of CPO-27-Ni. Also, we note that the roll-up peak of CO2, which is due to the displacement of CO2 by more strongly adsorbed CO, disappears at 5 and 10 bar because the CO/CO2 selectivity decreases. Figure 9 shows the desorption curves for the ternary mixture at 1 bar. The desorption curves confirm the behavior difference of CPO-27-Co and Ni vs CPO-27-Zn and STA-12-Ni. In the former group, CO and CO2 desorb slowly, while CH4 desorbs fast. In the latter group, only CO2 desorbs slowly, while CO

4. DISCUSSION 4.1. Influence of Coordinatively Unsaturated Sites. The presence of cus strongly influences the shape of the singlecomponent CO2 isotherms at low pressures.35 CPO-27-Ni exhibits the steepest initial slope followed by CPO-27-Co and Zn. The steepness of the isotherm is associated with the strength of the adsorbate−solid interaction and is coherent with the heats of adsorption previously reported.16,19 The less favorable CO2−Zn interaction leads to a smoother isotherm and a lower exothermicity in the breakthrough curve reflected in a less marked second roll-up. Also, the CO2 breakthrough curves on CPO-27-Zn are less steep: the sharpening effect of a less favorable adsorption isotherm on the breakthrough curve is weaker.36 Additionally, the desorption experiments show that CO2 is evacuated faster from CPO-27-Zn than from CPO-27Ni and -Co. The question arises as to why the adsorbent−CO 2 interaction is weaker in the case of CPO-27-Zn than in the case of CPO-27-Ni and -Co. Two kinds of arguments can explain this trendone based on coordination chemistry and the other based on Lewis acidity. The adsorption of CO2 results G



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Figure 7. Breakthrough curves of the 70/15/15 CO2/CH4/CO mixture on (a) CPO-27-Co, (b) CPO-27-Zn, (c) CPO-27-Ni, and (d) STA-12-Ni at 303 K and 1 bar.

higher solid−CO2 affinities observed for CPO-27-Ni and -Co shown in Table 2 could be explained in terms of the Lewis acidity of the metal center. Ni and Co in CPO-27 are expected to form stronger Lewis acids than Zn, since these metals more readily accept electrons in the d-orbitals. In the case of STA-12-Ni, the picture becomes more complex since the inclusion of other ligands changes the steric and electronic properties of the metal center. Therefore, a direct comparison with the CPO-27 series is less evident. From the single component adsorption data we can deduce that the environment for the adsorption of CO2 on the cus is less favorable than for CPO-27-Ni and -Co. The dehydrated STA12-Ni framework presents three different types of cus. Ni site I and II are coordinated by two oxygen atoms from the phosphonate group, sharing one, the third oxygen comes from a PO group, and the piperazine nitrogen atom is bounded in equatorial position. In Ni site III, the nitrogen atom is axial and is placed in front of the PO group. Ni sites II and I are less accessible because the PO group that is projected toward the pore prevents a sixth ligand to fully coordinate the metal cation. These sites represent around 66% of the coordinatively unsaturated sites in the framework. This effect has been observed by IR-FT using H2 or CO as probe molecules.25 Furthermore, it has been noted that the isosteric heats of adsorption of H2 and CO on STA-12-Ni are significantly lower than on CPO-27-Ni.25 On the basis of these arguments, we hypothesize that the relatively weak interaction of CO2 with cus sites in STA-12-Ni (compared to

in the formation of a metal−CO2 adduct in which the CO2 molecule is linked through an oxygen atom to the metal. This adduct is reversible at room temperature, as it was demonstrated by the desorption curves. At the same time, the coordination of CO2 to the metal leads to a transition of the metal coordination geometry from square-pyramidal to octahedral. The preferred coordination geometry of the metal center depends on the identity of the metal and the ligands. Zn2+ is versatile with respect to its coordination number. It is well-known that Zn2+ forms tetrahedral and square-pyramidal complexes when the ligands are oxygen or nitrogen as is the case of zeolitic imidazolate frameworks. Nevertheless, Zn2+ has also been reported to form a six-coordinate complex in aqueous solution.37 This coordination is found in the case of the hydrated forms of CPO-27-M and STA-12-M. The Zn−O crystallographic distance for the oxygen of the coordinated water molecule (2.17 Å) is larger than those of the equivalent bonds in hydrated CPO-27-Ni (2.08 Å), -Co (2.16 Å), and STA-12-Ni (2.11 Å).12,13,25,29 This indicates that Zn has a lower affinity for a sixth ligand. Moreover, DFT calculations have shown that the CO2−metal binding distance is larger in CPO27-Zn than in CPO-27-Ni and CPO-27-Mg. 38 These calculations have also shown that the metal−CO2 bonds in CPO-27-Zn exhibit a lower covalent character than in the case of CPO-27-Ni. The cus−CO2 interaction can also be regarded from the angle of a Lewis acid−base point of view. The CO2 molecule can be considered a Lewis base (electron donor) while the metal center behaves as a Lewis acid. Therefore, the H



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Figure 8. Breakthrough curves of the CO2/CH4/CO (70/15/15) mixture on (a) CPO-27-Co, (b) CPO-27-Zn, (c) CPO-27-Ni, and (d) STA-12-Ni at 303 K and 5 bar.

The highly energetic interaction of cus with CO has been demonstrated previously using IR, UV, XRD, and microcalorimetry.20 The adsorption of CO produces a monocarboxyle complex with a heat of adsorption of ∼58 kJ/mol. The highly favorable interaction between CO and Co and Ni is due to the formation of σ-bonds stabilized through a π backdonation and a strong electrostatic interaction (ion-induced dipole). Experimental measurements have confirmed a contribution of the π-backbonding in the adsorption of CO on CPO-27-Ni20 and Cu-BTC.40 Recently, an important charge transfer from CO to the cus has been determined by DFT calculations.41 On the other hand, the adsorption behavior of CO on CPO-27-Mg has been investigated by means of IR spectroscopy and ab initio periodic DFT calculations.42 It was shown that in the case of CPO-27-Mg the π back-donation can be neglected since the less energetic CO−Mg interaction is explained by σ-donation and electrostatic contributions. A nearly linear metal−adsorbate complex for CO presenting an average distance molecule−metal center of around 2.06 Å was found. Since from an electronic point of view, Mg and Zn are very close, i.e., both have all valence orbitals filled, it is expected that CPO-27-Zn behaves like CPO-27-Mg. This means that CPO-27-Zn cannot form a π-backbond with CO. In the absence of π-backbonding, the adsorption of CO2 is preferred over the adsorption of CO. As a consequence, CPO-27-Zn displays a high CO2/CO selectivity. At low pressure STA-12-Ni has a much lower affinity for CO than CPO-27-Ni and also prefers the adsorption of CO2 over

CPO-27-Ni) is the consequence of a steric hindrance by the ligands that prevents CO2 from fully approaching the Ni sites. The CH4 affinities of the four solids differ much less than the CO2 affinities. As a consequence, the CO2/CH4 selectivity (at 1 bar) decreases in the order CPO-27-Ni > CPO-27-Co > CPO27-Zn > STA-12-Ni; i.e., the selectivity decreases according to the strength of CO2 adsorption. It must be noted that CPO-27-Ni and -Co are significantly more selective than other MOFs with cus, like Cu-BTC (BTC = 1,3,5-benzenetricalboxylate) or MIL-100(Cr).11,39 This is a consequence of the low stability of Ni and Co in a squarepyramidal coordination in CPO-27, which requires a sixth ligand to stabilize its configuration, while in the case of CuBTC the planar conformation, i.e., without a terminal (CO2) ligand, is fairly stable. While the CO2/CH4 selectivity can be explained just by means of the CO2 affinity, the case of the CO2/CO selectivity is more complex because of the ability of CO to form also specific interactions with cus. In CPO-27-Ni and -Co, the cus−CO interactions are even stronger than the cus−CO2 interactions. This leads to the inversion of the CO2/CO selectivity observed by breakthrough experiments. The selectivity toward CO decreases slowly as the pressure is increased as a result of the saturation of the cus. The metal−CO adduct is reversible at 303 K under helium flow, but in the case of CPO-27-Co and -Ni the displacement of the CO molecules requires the injection of larger amounts of helium, confirming the strength of the interaction. I



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Figure 9. Desorption curves of the CO2/CH4/CO (70/15/15) mixture on (a) CPO-27-Co, (b) CPO-27-Zn, (c) CPO-27-Ni, and (d) STA-12-Ni at 303 K and 1 bar.

Table 6. CO2/CH4 and CO2/CO Selectivities for the Ternary Mixture 1 bar MOF CPO-27-Ni CPO-27-Co CPO-27-Zn STA-12-Ni

CO2/CH4 9 7 10 7

± ± ± ±

2 2 4 3

5 bar

10 bar

CO2/CO

CO2/CH4

CO2/CO

CO2/CH4

CO2/CO

± ± ± ±

19 ± 5 18 ± 11 17 ± 9.0 >11

0.45 ± 0.04 0.49 ± 0.02 12 ± 3 >6

>26 19 ± 11 19 ± 11 >8

0.52 ± 0.08 0.70 ± 0.08 >20 >6

0.20 0.23 6 5

0.04 0.04 1 2

coordination, leading to a full coordination of the sixth ligand. At high pressure of CO, a similar transformation might take place leading to stronger CO affinity. 4.2. Applications. Ideal adsorbents for the PSA process should have a high selectivity combined with a large working capacity.44 The working capacity can be defined as the difference between the adsorbed amount of CO2 when the feed mixture is passed through the column at adsorption pressure (adsorption step) and the CO2 amount remaining adsorbed after evacuation at desorption pressure (desorption step).39 The quantity that is adsorbed in the column in the adsorption step is determined from the breakthrough curve of the feed mixture at adsorption pressure. The quantity that remains adsorbed in the column after desorption is estimated from the adsorbed amount at desorption pressure and at the fluid phase composition at the end of the desorption step. As explained in the Introduction, we need to recover a pure CO2 stream for the purpose of CO2 sequestration. We, therefore, assume that the composition of the fluid phase at the end of the

CO (see Table 6). To explain this phenomenon, we can use the same argument as previously: sites I and II in STA-12-Ni are sterically impeded by ligands pointing toward the pore. Therefore, the CO molecule is unable to fully coordinate to these metal sites. Since CO cannot completely overlap with the metal orbitals, it is unable to form a π-backbonds. In the absence of π-backbonding, the adsorption of CO is systematically weaker than the adsorption of CO2. The isotherm of CO on STA-12 presents an S-shape with a hysteresis loop. An S-shaped type V isotherm may be explained by a low solid−molecule in combination with strong intermolecular interactions.43 CO is not known to produce strong intermolecular interactions in the adsorbed phase, and there is no reason why that should be different in STA-12-Ni. A more plausible explanation for the S-shaped isotherm of CO is, therefore, that the STA-12-Ni structure changes at higher partial pressures of CO. A reversible transformation of the STA-12-Ni structure has been observed upon the adsorption of water.23 In the hydrated form, nickel atoms have a octahedral J



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conditions. In the case of the application for biogas separation (CO2/CH4 50/50 mixture), we observe a trade-off between selectivity and working capacity. On the one hand, adsorbents with a strong heat of adsorption of CO2, as CPO-27-Ni and -Co, show high selectivity, but their working capacities are poor since these adsorbents attain saturation at low pressure. These materials are more suited for CO2 separations in a low range of partial pressures, in particular for postcombustion CO2 capture.45−48 STA-12-Ni has a low working capacity due to its small pore volume. On the other hand, adsorbents with lower heat of adsorption, as Cu-BTC and MIL47-V, have lower selectivities, but their working capacities are high due to their less steep CO2 isotherms. CPO-27-Zn, MIL-47-V, and MIL100-Cr have similar adsorptive properties. Under typical PSA conditions an increase of the working capacity for a constant selectivity produces a better performance of the unit.44 Therefore, among the MOFs with a working capacity higher than 1 mmol/g compared in this work, the most promising adsorbent for biogas separation is Cu-BTC. For the capture of CO2 from synthesis gas (CO2/CH4/CO 70/15/15 mixture), the inverse CO2/CO selectivity observed for CPO-27-Ni and -Co will prevent the application of these materials for the selective adsorption of CO2 from syngas because the extract always contains CO. To the best of our knowledge, this is the first time that an inverse CO/CO2 selectivity is reported for a MOF. These structures are interesting for other applications, for example, when the objective is the selective adsorption of CO. CPO-27-Zn is a very CO2-selective adsorbent, but its working capacity is lower that of Cu-BTC. In PSA applications where the purity of CO2 is very important, that is the case in the capture and storage of CO2, CPO-27-Zn is more suitable. On the other hand, the higher working capacity of Cu-BTC will lead to smaller adsorption vessels. Simulations of a full PSA cycle, combined with an economic analysis, will be required to choose the best adsorbent.

desorption step is pure CO2. According to Figure 6, this is a realistic assumption. Hence, the amount that remains adsorbed in the column after desorption was estimated from a breakthrough curve of pure CO2 at 1 bar (the desorption pressure). The working capacity is then given by the difference of the adsorbed amounts at adsorption pressure and composition (feed composition) and at desorption pressure and composition (pure CO2). This “equilibrium” working capacity is an approximate indicator of the quantity that can be adsorbed in each cycle of the PSA process. The higher the working capacity, the lower is the amount of adsorbent required for performing the separation. The selectivity of the adsorbent is related to the purity of the CO2 stream. By plotting the selectivity vs the working capacity, we can have a picture of the performance of the adsorbent under a given set of conditions. Figure 10 shows such a plot of working capacity against the

5. CONCLUSIONS Adsorption isotherms and breakthrough curves were measured for CO2, CH4, and CO on a series of MOFs presenting coordinatively unsaturated sites. The experimental data showed a clear influence of the nature of these sites on the adsorption properties. The CO2 and CO affinity by cus plays a main role defining both the shape of the isotherm and selectivity. Therefore, these properties can be tuned by changing the nature of the metal. The high affinity of CO2 on CPO-27-Ni and -Co can be explained by the fact that cus in these materials are more electronically deficient (stronger Lewis acid) than in the case of CPO-27-Zn. The adsorption of CO is also controlled by specific interactions with the cus. In CPO-27Ni and -Co, the adsorption of CO on the cus can be classified as “pseudo”-chemisorption, producing a highly exothermic adsorption and yielding a carbonyl complex that is more stable than the equivalent CO2 adduct. This high affinity is produced by a strong σ-bond stabilized through a π back-donation and a strong electrostatic interaction.20 CPO-27-Ni and STA-12-Ni have very different adsorption properties. The poor accessibility of the cus in STA-12-Ni hinders CO2 and CO to fully coordinate to these metal sites. Concerning the separation of CO2/CH4 mixtures in a medium pressure range (for example, the purification of biogas), we have shown that in MOFs with cus the CO2/ CH4 selectivity is enhanced by a strong CO2 affinity while the

Figure 10. Selectivity at 5 bar vs working capacity for (a) CO2/CH4 50/50 mixture and (b) CO2/CH4/CO 70/15/15.

selectivity. The adsorption pressure in Figure 10 is 5 bar; the feed composition is either CO2/CH4 50/50, which is representative of biogas separation, or CO2/CH4/CO = 70/ 15/15, which roughly reproduces the partial pressures encountered in CO2 capture from synthesis gas. The desorption pressure is 1 bar. For comparison, the procedure outlined above was also applied to some other MOFs, whose adsorption data have been previously reported,39 i.e., MIL-100Cr, MIL-47-V, and Cu-btc. The performance of an adsorbent depends on the feed composition and is a function of the pressure used during the adsorption and desorption steps.39 Therefore, the classification of adsorbents according to their working capacity and selectivity may change when changing the experimental K



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(10) Wang, Q. M.; Shen, D. M.; Bulow, M.; Lau, M. L.; Deng, S. G.; Fitch, F. R.; Lemcoff, N. O.; Semanscin, J. Microporous Mesoporous Mater. 2002, 55, 217−230. (11) Hamon, L.; Jolimaitre, E.; Pirngruber, G. D. Ind. Eng. Chem. Res. 2010, 49, 7497−7503. (12) Dietzel, P. D. C.; Morita, Y.; Blom, R.; Fjelag, H. Angew. Chem. 2005, 117, 6512−6516. (13) Rosi, N. L.; Kim, J.; Eddaoudi, M.; Chen, B. L.; O’Keeffe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2005, 127, 1504−1518. (14) Dietzel, P. D. C.; Georgiev, P. A.; Eckert, J.; Blom, R.; Strassle, T.; Unruh, T. Chem. Commun. 2010, 46, 4962−4964. (15) Zhou, W.; Wu, H.; Yildirim, T. J. Am. Chem. Soc. 2008, 130, 15268−15269. (16) Dietzel, P. D. C.; Besikiotis, V.; Blom, R. J. Mater. Chem. 2009, 19, 7362−7370. (17) Tagliabue, M.; Rizzo, C.; Millini, R.; Dietzel, P.; Blom, R.; Zanardi, S. J. Porous Mater. 2011, 18, 289−296. (18) Wu, H.; Zhou, W.; Yildirim, T. J. Am. Chem. Soc. 2009, 131, 4995−5000. (19) Caskey, S. R.; Wong-Foy, A. G.; Matzger, A. J. J. Am. Chem. Soc. 2008, 130, 10870−10871. (20) Chavan, S.; Vitillo, J. G.; Groppo, E.; Bonino, F.; Lamberti, C.; Dietzel, P. D. C.; Bordiga, S. J. Phys. Chem. C 2009, 113, 3292−3299. (21) Britt, D.; Furukawa, H.; Wang, B.; Glover, T. G.; Yaghi, O. M. Proc. Natl. Acad. Sci. U. S. A 2009, 106, 20637−20640. (22) Maurin, G.; Llewellyn, P. L.; Bell, R. G. J. Phys. Chem. B 2005, 109, 16084−16091. (23) Dunne, J. A.; Mariwals, R.; Rao, M.; Sircar, S.; Gorte, R. J.; Myers, A. L. Langmuir 1996, 12, 5888−5895. (24) Groves, J. A.; Miller, S. R.; Warrender, S. J.; Mellot-Draznieks, C.; Lightfoot, P.; Wright, P. A. Chem. Commun. 2006, 3305−3307. (25) Miller, S. R.; Pearce, G. M.; Wright, P. A.; Bonino, F.; Chavan, S.; Bordiga, S.; Margiolaki, I.; Guillou, N.; Férey, G.; Bourrelly, S.; Llewellyn, P. L. J. Am. Chem. Soc. 2008, 130, 15967−15981. (26) Wharmby, M. T.; Pearce, G. M.; Mowat, J. P. S.; Griffin, J. M.; Ashbrook, S. E.; Wright, P. A.; Schilling, L. H.; Lieb, A.; Stock, N.; Chavan, S.; Bordiga, S.; Garcia, E.; Pirngruber, G. D.; Vreeke, M.; Gora, L. Microporous Mesoporous Mater. 2012, 157, 3−17. (27) Wharmby, M. T.; Mowat, J. P. S.; Thompson, S. P.; Wright, P. A. J. Am. Chem. Soc. 2011, 133, 1266−1269. (28) Bourrelly, S.; Llewellyn, P. L.; Serre, C.; Millange, F.; Loiseau, T.; Ferey, G. J. Am. Chem. Soc. 2005, 127, 13519−13521. (29) Dietzel, P. D. C.; Panella, B.; Hirscher, M.; Blom, R.; Fjellvag, H. Chem. Commun. 2006, 959−961. (30) Rowsell, J. L. C.; Yaghi, O. M. J. Am. Chem. Soc. 2006, 128, 1304−1315. (31) Keller, J.; Staudt, R. Gas Adsorption Equilibria. Experimental Methods and Adsorption Isotherms; Springer: Berlin, 2005. (32) Groves, J. A.; Miller, S. R.; Warrender, S. J.; Mellot-Draznieks, C.; Lightfoot, P.; Wright, P. A. Chem. Commun. 2006, 3305−3307. (33) Dietzel, P. D. C.; Johnsen, R. E.; Blom, R.; Fjellvag, H. Chem. Eur. J. 2008, 14, 2389−2397. (34) Sing, K. S. W. Pure Appl. Chem. 1982, 54, 2201−2218. (35) Dietzel, P. D. C.; Johnsen, R. E.; Fjellvag, H.; Bordiga, S.; Groppo, E.; Chavan, S.; Blom, R. Chem. Commun. 2008, 5125−5127. (36) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; John Wiley & Sons: New York, 1984. (37) Marcus, Y. Chem. Rev. 1988, 88, 1475−1498. (38) Valenzano, L.; Civalleri, B.; Sillar, K.; Sauer, J. J. Phys. Chem. C 2011, 115, 21777−21784. (39) Pirngruber, G. D.; Hamon, L.; Bourrelly, S.; Llewellyn, P. L.; Lenoir, E.; Guillerm, V.; Serre, C.; Devic, T. ChemSusChem 2012, 5, 762−776. (40) Rubes, M.; Grajciar, L.; Bludsky, O.; Wiersum, A. D.; Llewellyn, P. L.; Nachtigall, P. ChemPhysChem 2012, 13, 488−495. (41) Zhou, C.; Cao, L.; Wei, S.; Zhang, Q.; Chen, L. Comput. Theor. Chem. 2011, 976, 153−160. (42) Valenzano, L.; Civalleri, B.; Chavan, S.; Palomino, G. T.; Arean, C.; Bordiga, S. J. Phys. Chem. C 2010, 114, 11185−11191.

working capacity follows the opposite trend. Thus, adsorbents with strong CO2 interaction have a high selectivity but low working capacity and vice versa. Among all the MOFs with cus compared in this work, Cu-BTC seems to be the more suitable for the CO2 separation from biogas. For the CO2 separation from synthesis gas, i.e. mixtures of CO2 with CH4 and CO, we have demonstrated that CPO-27Ni and CPO-27-Co have an inverse CO2/CO selectivity; i.e., they prefer the adsorption of CO over CO2. This illustrates that frameworks with high CO-cus affinity are not suitable for CO2 separation from synthesis gas. As far as CPO-27-Zn is concerned, it is a promising adsorbent for this application because its mild CO2 affinity combined with a weak CO interaction produces a high working capacity and selectivity.

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ASSOCIATED CONTENT

S Supporting Information *

XRD patterns, parameters obtained in the fitting of several adsorption models, breakthrough curves of the 50/50 CO2/ CH4 mixture at 1 bar 10 bar, desorption curves of the 50/50 CO2/CH4 mixture at 1 and 10 bar, breakthrough curve of the 75/15/15 CO2/CH4/CO mixture at 10 bar, and desorption curve of the 75/15/15 CO2/CH4/CO mixture at 5 and 10 bar. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax (+33) 437 702 066; e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Jean-Pierre Courcy for carrying out part of the gravimetric measurements. Dr. Céline Chizallet is acknowledged for fruitful discussions on the coordination chemistry of MOFs.

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ABBREVIATIONS cus, coordinatively unsaturated site; PSA, pressure swing adsorption. REFERENCES

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Role of Structure and Chemistry in Controlling Separations of CO2

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