Gas Adsorption Mechanism and Kinetics of an Elastic Layer

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Gas Adsorption Mechanism and Kinetics of an Elastic LayerStructured Metal−Organic Framework Atsushi Kondo,*,† Natsuko Kojima,‡ Hiroshi Kajiro,§ Hiroshi Noguchi,‡ Yoshiyuki Hattori,∥ Fujio Okino,∥ Kazuyuki Maeda,† Tomonori Ohba,‡ Katsumi Kaneko,⊥ and Hirofumi Kanoh*,‡ †

Department of Applied Chemistry, Tokyo University of Agriculture and Technology, 2-24-16 Koganei, Naka-cho 184-8588, Japan Graduate School of Science, Chiba University, Yayoi, Inage, Chiba 263-8522, Japan § Nippon Steel Corporation, Shintomi, Futtsu, Chiba 293-8511, Japan ∥ Department of Chemistry, Faculty of Textile Science and Technology, Shinshu University, Ueda 386-8567, Japan ⊥ Research Center for Exotic Nanocarbons (JST), Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan ‡

S Supporting Information *

ABSTRACT: The gate adsorption mechanism and kinetics of an elastic layer-structured metal−organic framework (ELM), [Cu(bpy)2(BF4)2]n (ELM-11), that shows typical single-step CO2 gate adsorption/desorption isotherms accompanied with dynamic structural transformation in a wide temperature range were investigated. Adsorption of quite a small amount of CO2 on the external surface of ELM-11 crystals was observed at the pressure just below a gate adsorption pressure and induced a slight structural change in ELM-11. The structural change should start occurring at the outer parts of ELM-11 and transmit to more inner parts with rising pressure. The adsorption provides the stabilization of the framework through the interaction between fluid−solid and fluid−fluid and enables the framework to expand largely along the stacking direction. The CO2 adsorption rate of ELM-11 is almost comparable to that of Zeolite 5A at around ambient temperatures and shows temperature dependence with an anti-Arrhenius trend: higher adsorption rate with lower temperature.

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INTRODUCTION Carbon dioxide is one of the major greenhouse gases responsible for global warming, and development of adsorbent for effective adsorption/separation of the gas is requisite all over the world. Metal−organic frameworks (MOFs) or porous coordination polymer are promising materials for CO 2 separation/storage because of their excellent adsorption properties based on the high surface area and deep potential well.1−5 Some MOFs show structural transformations by external stimuli, and flexibility is one of the most unique properties of MOFs.6−14 For example, a flexible MOF, [Cu(bpy)2(BF4)2]n (bpy = 4,4′-bipyridine), that is a twodimensional (2D) sheet stacked type of MOF, shows a vertical uptake in adsorption isotherms of several kinds of gases known as a “gate phenomenon”.15−18 The gate adsorption occurs through expansion/shrinkage structural transformation of the 2D layers accompanied with a clathrate formation16,19 between the host framework and guest molecules, and therefore the 2D MOF is called an elastic layer-structured MOF (ELM).20,21 We have investigated the gas adsorption properties of ELM families such as [Cu(bpy) 2 (BF 4 ) 2 ] n (ELM-11), 1 5 − 2 1 [Cu(bpy)2(OTf)2]n (ELM-12) (OTf = trifluoromethanesulfonate),22,23 and [Co(bpy) 2(OTf) 2]n (ELM−22).24 Their adsorption properties can be tuned by changing the metal © 2012 American Chemical Society

species and counteranions: For example, a gate adsorption pressure can be tuned by changing metal species24 or treating the precursor crystal with alcohols,25 and choices of different counteranions provide transformation of adsorption profiles (e.g., single-step to multistep isotherms).21 The gate phenomenon is a promising functionality for effective gas separation, and it is important to understand the phenomenon from many viewpoints with understanding of the mechanism. We have reported that the gate adsorption is in thermal equilibrium.16 However, some questions still remain. How about the kinetics of gas adsorption of ELMs? What is the state of ELMs just before gate adsorption? For the practical usage, the understanding of the kinetics of ELMs is inevitable. In this study, CO2, N2, and O2 adsorption characteristics at 273 K, the CO2 adsorption kinetic properties of ELM-11, and their temperature dependence were investigated in comparison with those of Zeolite 5A which has a high ability for CO2 separation. Received: October 25, 2011 Revised: January 17, 2012 Published: January 18, 2012 4157

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EXPERIMENTAL SECTION

The sample of ELM-11 was synthesized based on the reported procedure.26 The Zeolite 5A was purchased from Tosoh Cooperation. Synchrotron X-ray powder diffraction patterns of ELM-11 were collected at BL02B2 SPring-8 with a large Debye− Scherrer camera at room temperature. The powder sample was put in a glass capillary (diameter 0.5 mm) and degassed at 373−383 K under vacuum for more than 2 h. The wavelength of incident X-ray is 1.001 ± 0.001 Å. Grand canonical Monte Carlo (GCMC) simulations were performed for evaluating the stability change of ELM-11 by molecular adsorption and expansion. The crystal structure of ELM-11 was obtained from the structural analysis of the guestfree form, as reported before.19 The potential parameters of ELM-11 were taken from the universal force field (Supporting Information).27−30 Partial charges were used in bpy, Cu2+ ion, and BF4− ion. A three-centered Lennard-Jones potential model with three partial charges was used as the CO2 potential model.31 A 3 × 3 Cu grid model with bipyridine termination was used for a layer of ELM-11. The model of an ELM-11 crystal was composed of four layers with ab stacking, corresponding to 4.116 nm × 4.176 nm × t nm3 (Figure 2S, Supporting Information). The t is defined as the thickness of the ELM-11, which was simply varied from 2.021 nm (original structure) to 3.434 nm (50% expanded structure). The unit cell size was 10 × 10 × 10 nm3, and the periodic boundary condition was adopted with the Ewald correction for long-range interactions. The temperature was fixed at 273 K, and the GCMC calculation was performed 106 times per each point. Adsorption isotherms were measured by an automatic volumetric apparatus (Autosorb-1, Quantachrome). For all adsorption measurements, the samples were degassed at 373− 383 K under vacuum (P < 10−1 Pa) for 2 h. Adsorption rates were measured by a laboratory designed volumetric apparatus. It was composed of a dosing cell, an adsorption cell, two vacuum pumps, two pressure transducers, a heating light, and two thermoelectric couples. The apparatus was in an air bath at 298 K, and the sample cell was put in an ice−water bath at 273−303 K to maintain constant temperatures. The volumes of the dosing and adsorption cells were calibrated by He gas. The pressure of each cell was measured by a pressure transducer with high accuracy. The changes of pressure and temperature in the dosing and adsorption cells were monitored in real time. A powder sample (about 50 mg) was put in a stainless cell and attached to the volumetric line. The time course of pressure was recorded with the interval of 0.5−4 s.

Figure 1. Adsorption/desorption isotherms of CO2 (blue), N2 (green), and O2 (red) at 273 K on (a) ELM-11 and (b) Zeolite 5A. Solid and open symbols represent adsorption and desorption, respectively.

due to the difference of interaction potentials between ELM-11 frameworks and gas molecules to induce structural transformation. Zeolite 5A shows steep CO2 uptake at below 10 kPa and the following plateau in the higher-pressure region without hysteresis in the isotherm that is a typical type I isotherm on microporous materials. The maximum adsorbed amount of CO2 is about 230 mg/g at 100 kPa. Different from ELM-11, Zeolite 5A also adsorbs supercritical N2 and O2 gases at 273 K, indicating high adsorption ability of Zeolite 5A, although the adsorbed amounts are quite smaller than that of CO2 (N2: 29 mg/g, O2: 13 mg/g at 100 kPa). The molar ratios of adsorbed amounts (n), n(CO2)/n(N2), and n(CO2)/n(O2) on ELM-11 and Zeolite 5A clearly show specific pressure dependencies as shown in Figure 2.

Figure 2. Pressure dependence of (a) adsorbed amounts of CO2 over N2 and (b) CO2 over O2 on ELM-11 (black) and Zeolite 5A (red). Solid and open symbols represent adsorption and desorption, respectively.

Adsorption/desorption isotherms of ELM-11 show quite low ratios in the lower-pressure region than 20 kPa due to no CO2 adsorption and high ratios in the higher-pressure region than the gate pressure in both cases of CO2 over N2 and CO2 over O2. In desorption branches, the molar of adsorbed CO2 is 50 times greater than that of N2 and 60 times greater than that of O2 in the pressure range of 40−100 kPa. Adsorption isotherms of Zeolite 5A show quite high molar ratios in the low-pressure region, and steep decreases of the ratios were observed at higher pressure. The curve of n(CO2)/n(N2) from the desorption branch on Zeolite 5A shows a trend similar to that from the adsorption branch, and on the other hand, the curve of n(CO2)/n(O2) from the desorption branch on Zeolite 5A shows low molar ratio in the pressure range of 0−100 kPa because of a slight hysteresis in the O2 adsorption isotherm. Higher molar ratio indicates purer adsorbed gas, and therefore, ELM-11 should be a hopeful material for CO2 gas separation in

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RESULTS AND DISCUSSION Figure 1 shows the adsorption isotherms of CO2, N2, and O2 at 273 K on ELM-11 and Zeolite 5A over the pressure range of 0−100 kPa. As reported before, ELM-11 has quite a low CO2 adsorbed amount below 31 kPa (gate adsorption pressure), and a steep adsorption uptake follows with about 140 mg/g of CO2 at 50 kPa and 273 K.15 Desorption suddenly occurs at lower than the gate adsorption pressure, leading to a rectangular shape of a large hysteresis loop in the CO2 adsorption isotherm. On the other hand, ELM-11 does not show a gate adsorption and has little N2 and O2 adsorption up to 100 kPa at 273 K. The different behaviors of ELM-11 to the small molecules are 4158

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surface area of 22 m2/g. When the external surface of the ELM11 crystals was fully covered by CO2 molecules, the monolayer capacity was about 7.7 mg/g, which is close to the experimental adsorbed amounts at the pressure just below the gate pressures. In recent years, thermodynamic simulation studies have been reported and given useful information for predicting and understanding the gate adsorption mechanism.27,32,33 We performed GCMC simulation of CO2 adsorption at 273 K. We used nanosized models of ELM-11 composed of four square−grid nanosheets (4.116 × 4.176 nm) with different interlayer distances (Figure 2S, Supporting Information). The interlayer distance varied from 0.458 to 0.687 nm which correspond to the interlayer distance of the original model and the 50% expanded model, respectively. Figure 4 shows

the higher-pressure region than the gate pressure. In fact, ELM11 can separate CO2 gas with high purity from a CO2/N2/O2 mixed gas system at around 100 kPa.20 On the other hand, Zeolite 5A is effective in the low-pressure region. The pressure swing adsorption technique is a conventional method for gas separation, and when we use Zeolite 5A for CO2 gas separation, the pressure swing technique should be effective in the lowpressure region (especially under 1 kPa) and, meanwhile, useful at higher CO2 pressure of 20−60 kPa by using ELM-11. In general, porous materials with deep potential well show high adsorption ability. However, recovery of adsorbed molecules from the materials needs large energy by heating and/or evacuation. Therefore, the materials possessing both abilities of high adsorption and easy recovery are required. ELM-11 shows high CO2 adsorption ability and relatively high and moderate working pressure ranges in the pressure swing method that brings an energetic advantage to recover adsorbed molecules in a practical point of view. To investigate the adsorption of ELM-11 at just below the gate adsorption pressure (Point A in Figure 1a), a time course of CO2 gas pressure at around the gate adsorption pressure was measured at several temperatures between 273 and 303 K (Figure 3). The CO2 gate adsorption pressure on ELM-11

Figure 4. Simulated CO2 adsorption isotherms on the nanosized model of ELM-11 evaluated by (a) total adsorption amount and (b) adsorption amount in the inner space at 273 K.

simulated CO2 adsorption isotherms on the models of ELM11 at 273 K. Adsorbed amounts of CO2 increase with larger expansion percentage. Because of the nanosized models, adsorbed amounts are quite large compared with the experimental results in all cases. Figure 4b shows adsorption isotherms of CO2 in the inner pores of the models. In all cases, adsorbed amounts are smaller than those in Figure 4a, indicating a large amount of adsorption on the external surface. Interestingly, adsorbed amounts in the inner space of the models with 0−10% expansion are quite small, showing almost no effective inner pore space. However, the adsorbed amounts in the inner pores of the models with more than 20% expansion show obvious uptakes that are almost equivalent to the experimental result. In the experiment, 26% expansion of the interlayer distance and CO2 adsorption of 150 mg/g were observed at 273 K and around 100 kPa.18 The energetic effect of adsorbed CO2 was investigated by potential energy calculations. Figure 5 shows potential energy

Figure 3. Time courses of CO2 pressure on ELM-11 at several temperatures below the gate adsorption pressures. Insertion is the time course over a half day at 298 K.

depends on temperature: the higher the measurement temperature, the higher the gate adsorption pressure.20 Then, initial dose pressure was varied from 49 kPa (273 K) to 141 kPa (303 K). The initial dose gas pressures were set to control equilibrium pressures not to overshoot the gate adsorption pressure at each temperature. At all temperatures, constant pressures were soon observed after CO2 gas diffusion, and the pressures were maintained up to over a half day without gate adsorption. This result supports that the gate adsorption is under thermodynamic control in this time scale. Calculated equilibrium pressures without adsorption were slightly higher than the observed constant pressures at all temperatures. Therefore, the adsorbed amounts of CO2 at each temperature can be estimated from the differences of the pressures. At each temperature, the adsorbed amounts were less than 10 mg/g, and the averaged adsorbed amount is about 5 mg/g. Definite temperature dependence of the adsorbed amounts was not detectable. The averaged crystallite size of ELM-11 was estimated as 0.15 μm from a synchrotron XRD pattern by the Scherrer equation, allowing us to evaluate the external

Figure 5. Potential energy profiles of ELM-11 at 273 K under consideration of (a) CO2 molecules adsorbed on the external surface and (b) CO2 molecules adsorbed in the inner space. 4159

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profiles of ELM-11 at 273 K under consideration of the effects of CO2 adsorbed on the external surface containing fluid−solid and fluid−fluid interactions. As shown in Figure 5a, although CO2 molecules adsorbed on the external surface bring energetic stabilization, the original structure with no expansion is most stable in all cases at several gas pressures. On the contrary, CO2 molecules adsorbed in the inner space of expanded models strongly influence the potential profiles (Figure 5b). When the gas pressure increases, potential energies of some expanded states become lower than that of the original state. This indicates that ELM-11 becomes more stable energetically by adsorption of gas molecules in the inner pore of the expanded state. In addition, it is clear that stabilization energy through the adsorption accompanied with the structural transformation becomes larger with higher pressure. We measured in situ synchrotron XRD patterns of ELM-11 at different CO2 gas pressures around the gate pressure and at 273 K (Figure 3S, Supporting Information). Almost all peaks did not change by 26.7 kPa with keeping a fundamental crystal structure. However, a weak new peak with the d value of 9.3 Å corresponding to the (200) miller face of the gas adsorbing state was observed even at the pressure below the gate adsorption pressure. When the gas pressure increased up to 33.3 kPa that is higher than gate pressure, many new peaks concerning the gas adsorbing state appeared with peaks relating to the original state, and at higher pressure of 40 kPa the structural change is almost completed. These results imply that ELM-11 crystals partially change the structure even at the pressure below the gate pressure, and the structural change gradually but sensitively occurs in the narrow pressure range around the gate pressure. This is consistent with the result of in situ IR measurement.20 On the basis of the experimental and simulated results, we propose one mechanism of the gate adsorption. Although the CO2 adsorbed amount on the external surface is small, the adsorption is crucial for the gate adsorption. Adsorbed molecules on the external surfaces of ELM-11 induce energetic stabilization and slight structural change even at the pressure below the gate adsorption pressure. The structural change should start at the outer parts of ELM-11 and transmit to more inner parts by pressure rising. When the parts of ELM-11 have the interlayer distance wide enough for gas molecules to pass through the pore entrances, gas molecules are suddenly incorporated into the pores by the deep potential of micropores. This potential energy gain of adsorption accelerates further structural transformation for the clathrate formation. Therefore, gate adsorption sensitively occurs in the narrow pressure range. Adsorption kinetic profiles were measured with introduction of CO2 gas with higher pressure (187 kPa) than the gate adsorption pressure. Figure 6 shows the variation of Mt/Me against time on ELM-11 and Zeolite 5A. Here, Mt is a mass uptake at time t, and Me is a mass uptake at equilibrium. Both samples show the convex curves at all temperatures and fast CO2 adsorption within 5 min to reach equilibrium. Initially steeper uptake appears, and then moderate uptake follows in all cases. It is noted that Zeolite 5A adsorbs CO2 gases faster than ELM-11 at 303 K, and on the contrary ELM-11 adsorbs CO2 gases faster than Zeolite 5A at 273 K, implying different temperature dependence in adsorption kinetics of ELM-11 and Zeolite 5A (Figure 4S, Supporting Information). The temperature dependences of CO2 adsorption kinetics of both samples are also shown in Figure 6. In the case of ELM-11, the

Figure 6. Temperature dependence of Mt/Me−time curve of (a) ELM-11 and (b) Zeolite 5A. Green, red, blue, and black are at 303, 298, 293, and 273 K, respectively.

adsorption rate of CO2 decreases with increasing temperature. On the other hand, the CO2 adsorption rate of Zeolite 5A has no definite temperature dependence at the temperature range from 273 to 303 K. The time of Mt/Me = 0.5 (t1/2) is an indicative value for adsorption rate. The times, t1/2, of ELM-11 and Zeolite 5A are summarized in Table 1. Usually the Table 1. Time of Mt/Me = 0.5 (t1/2) of ELM-11 and Zeolite 5A between 273 and 303 K temperature/K

273

293

298

303

t1/2 (ELM-11)/s t1/2 (Zeolite 5A)/s

∼1 2.5

4 2.5

8 2.5

16 2

adsorption rate of gases on carbon materials shows strong dependence on pressure and relatively weak dependence on temperature.34 In the case of Zeolite 5A, the temperature dependence of the adsorption rate is also quite small. One of the major reasons why ELM-11 shows the different tendency of the adsorption rate is the temperature dependence of the gate adsorption pressure. The CO2 gate adsorption pressures of ELM-11 are 31 kPa (273 K), 65 kPa (293 K), 74 kPa (298 K), and 88 kPa (303 K).20 Therefore, ELM-11 starts to adsorb CO2 gas faster at lower temperature. Another possible reason is the temperature dependence of adsorption amount and adsorption speed on the external surface because the adsorbed molecules induce the structural change. The kinetic profiles of CO2 adsorption on ELM-11 could be fitted using a double exponential equation35 Mt /Me = A1(1 − exp( −k1t )) + A2(1 − exp( −k2t ))

where k1 and k2 are kinetic constants and A1 and A2 are the relative contributions of two barriers controlling the overall process, with A1 + A2 = 1. This model is based on the two energetic barriers due to diffusion through the pore windows, which includes the pore opening process, and along the pore cavities. The fitting result of the double exponential model is shown in Figure 7 with a result of the linear driving force model, which provides a satisfactory description of the adsorption kinetics of various gases/vapors on carbon materials.36,37 The fitting results clearly show the good fit of the double exponential model on the experimental data shown in Figure 7 (see also Figure 5S, Supporting Information). The consistently large value of k1 and smaller value of k2 were observed at all temperatures (Table 1S, Supporting Information). The two components can correspond to the diffusion of CO2 molecules along the micropores of open-formed ELM-11 and multiple steps relating to the pore opening. Two 4160

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greater with lowering temperature, leading to the antiArrhenius behavior.

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CONCLUSION In conclusion, CO2, N2, and O2 adsorption characteristics at 273 K, gate adsorption mechanism, and CO2 adsorption kinetics of an elastic layer-structured metal−organic framework, [Cu(bpy)2(BF4)2]n (ELM-11), were investigated in comparison with Zeolite 5A. ELM-11 showed high molar ratios of adsorbed CO2 over N2 and O2 at the pressure range of 20−60 kPa. Before gate adsorption, ELM-11 adsorbs a quite small amount of CO2, less than 10 mg/g, and no change of adsorbed amount is observed up to a half day at the temperature range from 273 to 303 K, supporting thermal equilibrium of the gate adsorption. The gate adsorption of ELM-11 should be started by a small amount of adsorption on the external surface, inducing a slight structural change even at the pressure below the gate adsorption pressure. The structural change should start at the outer parts of ELM-11 and transmit to more inner parts by pressure rising. When ELM-11 has the interlayer distance to enable gas molecules to pass through the pore entrances, gas molecules are suddenly adsorbed into the pores by the deep potential of the micropore. The kinetics of CO2 adsorption on ELM-11 follows a double exponential equation based on two energetic barriers due to diffusion through the pore windows, which includes pore opening process, and along the pore cavities. The gate CO2 adsorption rate of ELM-11 that is comparable to that of Zeolite 5A shows temperature dependence, and ELM-11 adsorbs CO2 faster at lower temperature with anti-Arrhenius trend: higher adsorption rate with lower temperature.

Figure 7. Fitting results of CO2 adsorption kinetics on ELM-11 at 303 K (black, experimental data; blue, linear driving force model; red, double exponential model). Inset: squared residuals against time.

parameters, k1 and k2, indicate an anti-Arrhenius trend: The values of k1 and k2 increased with decreasing temperature (Figure 8).

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ASSOCIATED CONTENT

S Supporting Information *

Additional characterized data of simulation details, in situ XRD, CO2 adsorption kinetics. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 8. Plots of kinetic rate constants (k1 and k2) versus 1/T.

An Arrhenius behavior is based on an activation process. The activation process has two important factors: activation energy and temperature. Higher temperature and lower activation energy lead to a faster reaction rate. In an ideal reaction, there is a constant value of activation energy, and when the reaction temperature is changed, a kinetic constant varies according to the Arrhenius equation. As a result, plots of logarithmic rate constants versus reciprocal temperatures become linear with a slope relating to activation energy. In the case of ELM-11, the plots show curved lines indicating anti-Arrhenius behavior (Figure 8). If reaction conditions except temperature are the same, the gas adsorption on ELM-11 should also follow Arrhenius behavior. Therefore, the experimental results imply that states of ELM-11 are different from each other at different temperatures by the effect of surface adsorption. The adsorption on the external surface of ELM-11 induces energetic stabilization, and the effect of adsorption becomes stronger with lower temperature. In addition, the adsorption may cause the energetic barrier for the expansion of layers to be small relative to a thermal energy. The force that accelerates the gate adsorption is based on a larger potential energy gain of the clathrate formation relative to the thermal energy. Accordingly, the relative contribution of the potential energy becomes

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.K.); [email protected] (H.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Satoshi Watanabe and Dr. Hideki Tanaka (Department of Chemical Engineering, Kyoto University, Japan) for fruitful discussions. The synchrotron radiation experiments were performed at SPring-8 with the approval of [Japan Synchrotron Radiation Research Institute (JASRI)] as Nanotechnology Support Project of the Ministry of Education, Culture, Sports, Science and Technology (Proposal No. 2010 B1463/BL02B2).

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