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Hysteretic adsorption of CO2 onto a Cu2( pzdc)2(bpy) porous coordination polymer and concomitant framework distortion Karina Riascos-Rodríguez,a Aaron J. Schroeder,b Michael R. Arend,b Paul G. Evansb and Arturo J. Hernández-Maldonado*a The present study focuses on the long-range structural changes that occur in the porous coordination polymer Cu2( pzdc)2(bpy) ( pzdc = pyrazine-2,3-dicarboxylate, bpy = 4,4’-bipyridine), also known as CPL-2, upon adsorption of CO2 at 25 °C and up to 7 atm. The structural data were gathered using in situ diffraction studies. CPL-2 exhibited an unexpected hysteretic adsorption–desorption process. The onset of hysteresis occurs at a pressure where full occupancy of the volume of the CPL-2 galleries is achieved while the framework retains a structure similar to what is observed under ambient conditions. Moreover, the onset occurs at a CO2 partial pressure larger than 2 atm and could be related to a combination of
Received 25th March 2014, Accepted 12th May 2014 DOI: 10.1039/c4dt00878b www.rsc.org/dalton
1.
adsorbate–adsorbent interactions and forces exerted onto the CPL-2 framework. Pore volumes estimated from fits of the Dubinin–Astakhov isotherm model against the CO2 desorption data gathered at 25 and −78.5 °C, respectively, provided further evidence of the aforementioned CPL-2 framework changes. These findings are of relevance to the understanding of adsorption processes in metal organic frameworks or coordination polymers under conditions that are of relevance to gas capture at industrial scale.
Introduction
The development of fuel pre- or post-processing treatment and purification operations that are suitable for the growing demand for energy while mitigating climate change and pollution is an area of significant challenges and opportunities. Most of the emissions of CO2 arise from the combustion of fossil fuels and the processing of cement, with the United States alone producing 4.9 billion tons of CO2 annually.1,2 Post combustion capture of CO2 is usually attained via absorption methods that rely on aqueous mono-, di- or tri-alkanolamine solutions. A similar process is also used to remove CO2 from natural gas sources. The amine solutions are volatile and corrosive, and are also associated with the formation of carbamates and bicarbonates, requiring large energy inputs for regeneration.3 The drawbacks of this and other existing technologies have motivated the search for cost effective alternatives for carbon capture and storage technologies based on adsorption. Several studies have demonstrated that metal organic frameworks (MOFs) have tremendous potential as
a Department of Chemical Engineering, University of Puerto Rico-Mayagüez Campus, Mayagüez, PR 00681-9000, Puerto Rico. E-mail: [email protected]; Fax: +787-834-3655; Tel: +787-832-4040 x3748 b Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA
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highly porous adsorption materials for the CO2 capture.1,3–9 However, whether these porous materials perform separations efficiently under conditions similar to those found in power plant flue and natural gas streams or even biogas upgrading remains a topic of intense study. The problem is complex because there are a great variety of MOF chemical structures and the responses to different external stimuli, including temperature and pressure swings, are likely to vary significantly. The structure of a MOF can be designed prior to synthesis10,11 and modified upon post-synthesis functionalization.12 There are a variety of MOFs that exhibit a combination of strong and weak coordination bindings within their frameworks. It is important to note that the combination of strong and weak bonds is not found in zeolites, which are mainly composed of rigid covalent Si–O bonds.13–15 The responses of MOF materials to internal or external potentials depend on the type of adsorbate, high pressure, temperature, or electric fields.16,17 For instance, changes in the unit cellscale structure of a MOF can be correlated with s-shaped, stepwise, or hysteretic single component adsorption–desorption isotherms.5,18–22 The control of such changes can also lead to new applications related to solid state chemistry.5,23 However, further structural studies of the dynamics of adsorption of CO2 onto MOFs are required. There is a need for further insight into the stability of the MOF once either the guest or the adsorbate is removed (via desorption) or the external force or stimu-
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lus is stopped. Such insight is not available through traditional single-component adsorption equilibrium studies. Potential hindrances to applications, such as mechanical stability and structural changes under moderate to high pressure at ambient temperature, must be identified in order to fulfill not only industrial requirements related to CO2 capture processes, but also related to performance for targeted applications.1,24 Several in situ structural (i.e., crystallographic) studies of the adsorption of CO2 are available in the low (0.1 to 2 atm) pressure regime.7,25 A few studies have covered pressure ranges that exceed even those found in industrial CO2 capture applications, relying on high-pressure treatments under hydrostatic and non-hydrostatic conditions.23,26,27 Interestingly, most of these studies have been motivated by the gate opening pressure phenomenon, which refers to an abrupt increase in adsorption at a threshold pressure and explains the high CO2 selectivity of several MOFs. A series of other MOFs do not exhibit a sudden adsorption induced gate opening phenomenon at mid-pressure ranges, but instead show hysteretic adsorption–desorption loops that vary according to the gas loading. For instance, hysteretic adsorption–desorption phenomena can be found in some porous coordination polymers (PCPs), a subclass of the broader series of MOF materials. PCPs consist of single- or multi-dimensional porous frameworks constructed from transition-metal centers that serve as coordination centers to organic bridging ligands.28–32 These materials exhibit unique adsorption phenomena due to structural changes sometimes induced by interactions with the guest molecules. CPL-2 (Cu2( pzdc)2(bpy) [pzdc = pyrazine-2,3dicarboxylate, bpy = 4,4′-bipyridine]), for example, exhibits remarkable breathing framework behavior in the presence of water and benzene vapors at ambient temperature and pressure.18 The adsorption of gases such as N2, O2, CO2 and Xe on CPL-2 at relative low partial pressures at cryogenic temperatures also leads to distortion of the framework upon uptake of the gas that can be explained by a change in the coordination mode of Cu from pyramidal to square planar.21 Hernández-Maldonado and co-workers used in situ XRD and 3C cross-polarization magic angle spinning nuclear magnetic resonance (CP-MAS NMR) spectroscopy to study how CPL-n materials undergo framework changes depending on the preactivation temperature, something that is also critical for multi-cycle adsorption operations.33–35 Further experimental and computational studies have elucidated the capacity of these materials for CO2 storage and delivery at medium pressures (up to 6 atm) and also showed that the observed hysteretic adsorption property is a function of the ultimate gas pressure.36 The magnitude of the hysteresis depended on the highest CO2 pressures during loading, and was completely absent within the atmospheric pressure range. In addition, the hysteresis was clearly a result of interactions between the CO2 and the framework, but there was no direct structural information. Further structural information is of utmost importance in allowing CPL-2 and similar materials to be employed in multi-cycle operations. Structural studies will also address
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the stability of hysteretic MOFs at the interface between the fields of chemical engineering and surface science. We present here an in situ adsorption and powder X-ray diffraction study of the structural mechanisms of the hysteretic adsorption of CO2 onto CPL-2 at pressures ranging from 0 to 7 atm. The structural changes were observed using a specially developed capillary sample environment permitting X-ray diffraction studies of samples held at elevated CO2 pressure. A comparison of the adsorption/desorption gap found in chemical measurements with the X-ray data shows that the hysteresis can be observed using the changes in the volume of the CPL-2 unit cell.
2. Experimental 2.1.
Reagents and materials
2,3-Pyrazinedicarboxylic acid (H2pzdc, 97% purity), 4,4-bipyridine (bpy, 98% purity), copper(II) perchlorate hexahydrate (Cu(ClO4)2·6H2O, 98% purity), denatured ethanol (95% purity) and methanol (99% purity) from Sigma-Aldrich were used as supplied to synthesize CPL-2. Ultra-high purity grade CO2 and N2 gases obtained from Praxair, Inc. were used during the measurements of the adsorption isotherms. CO2 and N2 gases used in the in situ X-ray diffraction experiments were obtained from AirGas, Inc. 2.2.
CPL-2 synthesis
CPL-2 was synthesized at ambient temperature according to procedures available elsewhere.18,34,35 One mmol of H2pzdc (0.1681 g) and 0.5 mmol of bpy (0.078 g) were dissolved in a solution prepared previously by mixing 1 : 1 NaOH 0.04 M and ethanol. The resulting mixture was dropwise added to a second solution consisting of CuClO4·6H2O (0.37 g) and water, all under continuous agitation. The final blend was stirred for 24 h, filtered under vacuum and repeatedly washed with methanol and deionized water. The remaining methanol was removed by heating the solid overnight at 90 °C in air. The final solid sample was stored in a sealed desiccator until further use. 2.3.
Porosimetry and high pressure CO2 adsorption
Porosimetry data were gathered using a Micromeritics ASAP 2020 volumetric adsorption unit fitted with turbo molecular drag pumps. Activation of the sample was performed prior to each adsorption analysis using the instrument’s sample thermal degassing module. A sample was loaded (∼100 mg) into a quartz-made container fitted with an isolation valve, which permits connection to a vacuum port and subsequent transport between analysis ports without exposure to ambient humidity and/or gases. The sample was initially evacuated at a rate of 50 mmHg s−1 to a final pressure of 5 mmHg. Further activation was conducted immediately by heating the sample in situ to 100 °C (heating ramp of 10 °C min−1) and soaking at this temperature for several hours.33 After the degassing time elapsed, the sample was allowed to cool to ambient tempera-
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ture while still under vacuum conditions. The sample was then backfilled with helium to allow its transfer to the analysis port while under isolation. Once attached to the analysis port, the sample was evacuated to remove He prior to the adsorption tests. The adsorption tests for this part of the work were performed for CO2 uptake at −78.5 °C and the sample pore volume was estimated by assuming that the adsorbed phase behaved similar to that of a liquid (i.e., nitrogen molecules tightly packed) and employing the Dubinin–Astakhov (DA) and modified Dubinin–Astakhov (MDA) isotherm models, respectively.37,38 The models are given by the following expressions: qDA ¼ q0 exp½ðC lnðP o =PÞÞn n
qMDA ¼ q0 ½β1 exp½ðC lnðP o =PÞÞ þ β2 KP where
ð1Þ ð2Þ
P β1 ¼ 1 exp α Po P β2 ¼ exp α Po RT C¼ βE q1 K¼ Po
qDA or qMDA is the equilibrium adsorption loading amount, q0 is the adsorption loading amount at saturation (complete pore filling), R is the universal gas constant, β is the affinity coefficient of the adsorbate, E is the energy of adsorption, P/Po is the relative pressure, n is the heterogeneity coefficient, α is a fitting parameter and K is Henry’s law constant. The DA and MDA models take into consideration surface heterogeneity and pore filling effects (i.e., interaction of the adsorbate volume with the surface). The MDA model (eqn (2)) contains a second term that allows it to comply with Henry’s law limit.37 Pure component CO2 adsorption and desorption isotherms were acquired at 25 °C at pressures up to 7 atm using a Micromeritics ASAP 2050 extended pressure volumetric adsorption instrument. The CPL-2 sample was loaded into a stainless steel sample holder that was otherwise similar to the sample holder used for porosimetry. The sample pre-activation and adsorption steps were similar to those employed to gather porosimetry data, with the exception that the temperature was maintained at 25 °C. In addition, the instrument equilibration time intervals used were 30, 50 and 100 seconds. Since the instrument data gathering algorithm is limited to the analysis of eleven consecutive pressure data points, those time intervals resulted in at least 330, 550 and 1100 seconds worth of transient data, respectively, for each targeted pressure increment. It should be noted that the ASAP 2050 maximum allowable working gas pressure is approximately 9 atm for CO2. 2.4.
In situ CO2 adsorption and X-ray diffraction
Changes in the structure of CPL-2 upon adsorption of CO2 were studied via in situ powder X-ray diffraction. The data were gathered using a four-circle X-ray laboratory diffractometer
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with Cu K-α radiation from an Ultra-X 18 rotating anode X-ray generator (Rigaku, Inc.) operating at 40 kV and 100 mA. Diffraction patterns were obtained for 2θ diffraction angles between 5 and 30° with a step size of 0.03°. Each diffraction pattern was indexed using the DICVOL91 program.39 A cylindrical polyimide capillary with a diameter of 1 mm was used as a sample holder. One end of the polyimide capillary was sealed using an epoxy adhesive. The capillary was attached to a gas manifold, including pressure measurement and facilities for flushing and pressurization with N2 and CO2 gases. The sample was pre-activated in place to remove water traces and light impurities. The pre-activation process consisted of heating using an external source (i.e., heat gun) while the sample was exposed to flowing N2 gas. The temperature during activation was measured near the capillary using a thin-wire thermocouple and carefully controlled to remain within 100 to 106 °C. Heating to 100 °C was found in porosimetry to preserve the gas adsorption and structural properties of CPL-2.33,34 Upon completion of the pre-activation, the sample was returned to room temperature under a flow of N2. The N2 gas flow was later substituted with CO2, the system was sealed using a gas valve and the CO2 supply was continued until the sample environment reached the target pressure for each X-ray diffraction scan. The pressure soaking stage lasted only a few seconds prior to each X-ray diffraction scan, which is justified given the fast kinetics of the CO2 uptake.33 The time scales necessary to attain both pressure and uptake equilibration were kept constant and were identical.
3. Results and discussion The unit cell of the CPL-2 lattice is composed of a copper node (Cu2+) bonded to three pyrazine-2,3-dicarboxylate ( pzdc) units, one of them chelated, to conform to a two-dimensional neutral layer. Diagrams of the a–b and c–b planes of the CPL-2 lattice are shown in Fig. 1. The neutral layers are coupled through 4,4′-bipyridine (bpy) pillars bonded at each end to Cu2+ to conform to a three-dimensional structure. The pillar and neutral layers can be thought of as mechanically rigid because the rings of the bpy and pzdc, respectively, resist rotation. A key feature of the CPL-2 structure is that straight one-dimensional accessible channels, termed galleries, extend along the a-axis and can facilitate adsorption by allowing the rapid transport of gas molecules through the crystal.18 This configuration, frequently found in several MOFs,40 can lead to adsorbate–adsorbate interactions that are sensitive to even minor changes in the total loading of the guest molecules. MOFs or PCPs that include Cu2+ complexes can exhibit tetrahedral, square planar or octahedral coordination at the metal site. A Jahn–Teller distortion of the Cu2+ complex is required to change the energy of eg orbitals, which otherwise would be degenerate. Such distortions are likely to occur upon rotation or cleavage of the carboxylate portions of the neutral layer.18,31,41–43 An interaction between the electric field gradient of the framework surface and the CO2 permanent
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Fig. 1
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CPL-2 framework structure, galleries and diffraction planes of interest.
quadruple moment could be induced by specific functionalities, but could also be facilitated upon an external stimulus such as pressure.36 Therefore, the simultaneous increase in static pressure and the amount of guest in the pores are both factors probably influencing the adsorption process. For CPL-2, the carboxyl groups located near the Cu2+ nodes should offer stronger sites for interaction with CO2 as compared to the bpy pillars environment. The CPL-2 framework configuration should remain stable at ambient pressure until exposure to chemical conditions that induce a reorganization of the O or N atoms of the carboxylate linked to the Cu2+ center. The reorganization can be induced via the rotation or cleavage of bonds, re-distribution of electrons, or changes in the length of bonding. Structural changes can easily spread along the long-range order of the framework, producing a cooperative cleavage of bonds or shrinkage. In the case of several MOF compounds, the structural reorganization can lead to the appearance of new phases or even to amorphization.19,24,44 CPL-2 exhibits fully reversible adsorption–desorption linear paths within the 1 atm CO2 pressure range, a reflection of weak adsorbate–adsorbent interactions.33 The adsorption/ desorption measurements extending to 7 atm are shown in Fig. 2. At higher CO2 pressures, the observed hysteretic relationship between adsorbate loading and pressure indicates that adsorbate interactions can be expected to lead to structural distortion. For CPL-2, the carboxyl groups located near the copper nodes should offer stronger sites for interaction with CO2 as compared to the bpy pillars environment. If a Jahn–Teller is in effect taking place, it could translate to structural dynamic changes and even unique adsorption mechanisms. The CPL material prepared for this study exhibited the crystal structure previously described in the literature.18,33,34 The CPL-2 lattice has monoclinic symmetry (space group P21/
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Fig. 2 (top) CO2 equilibrium adsorption and desorption amounts on CPL-2 gathered at 25 °C and (bottom) corresponding X-ray diffraction patterns. In the diffraction data, PCO2 = 0 atm refers to a gas environment without CO2 (1 atm N2).
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c, a = 4.71160 Å, b = 27.833 Å, c = 10.8881 Å, β = 96.0102°).18 An ambient pressure diffraction pattern for CPL-2 acquired after the activation step is shown as curve (a) in Fig. 2. The series of X-ray diffraction patterns labeled (a) through (h) in Fig. 2 were gathered following the path established in the CO2 adsorption isotherm also shown in Fig. 2. It is apparent from the shift of the 2θ angles of the X-ray reflections that the CPL-2 undergoes a pressure-responsive structural change without a major loss of crystallinity. The pressure-induced structural change is characterized by a sustained shift of the (0 2 0) reflection to smaller 2θ angles, corresponding to an increase in the (0 2 0) d-spacing. The increased (0 2 0) d-spacing persists after depressurization (rapid CO2 release), which is indicative of a pore expansion. The structural changes can be interpreted in two pressure or equilibration regimes. In the first of these, at low gas loadings (i.e., CO2 partial pressures smaller than 2 atm), the dominant host–guest interactions probably facilitated the displacement of CO2 toward adsorption sites. The displacement in turn results in a stronger interaction of the CO2 with the CPL-2, perhaps at sites in the vicinity of the Cu2+ nodes. We note that this response can occur rapidly throughout the crystal because there is minimal major resistance to diffusion due to the kinetic diameter of CO2 (∼3.3 Å), which is considerably smaller than the CPL-2 pore dimensions. At higher CO2 pressures, an increase in CO2 loadings inside the pores resulted in structural changes only after the adsorbate–adsorbent energy of interaction was equilibrated or the CO2 occupied the available void volume while the framework was in a relaxed state. At larger CO2 loadings an extra physical force produced by static or dynamic pressure could influence the framework stability in advancement to a different crystal configuration, which probably could be evidenced by a further long-range expansion. This second pressure or equilibration regime or domain also involved slow dynamics. Fig. 3 shows
Fig. 3 CO2 adsorption (filled symbols) and desorption (open symbols) amounts on CPL-2 gathered at 25 °C and different equilibration time intervals.
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that the longer the adsorption equilibration time interval, the larger the adsorption of CO2 at pressures above 2 atm. Furthermore, at an equilibration time interval of 100 s and a CO2 partial pressure of 6 atm the adsorption leg of the isotherm shows an inflection, most likely associated with the onset of a plausible gate or structure opening process. Capture of the whole process was not possible due to limitations of the adsorption instrument capability. Fig. 4 shows CO2 equilibrium adsorption–desorption data gathered in CPL-2 at −78.5 °C. At this temperature and a pressure near 1 atm, the CO2 is adsorbed within the pores of a relaxed CPL-2 framework in a tightly packed fashion while its desorption proceeded without any hysteresis. This provides further evidence that the hysteretic adsorption process observed at 25 °C and large CO2 pressures is likely due, for the most part, to the larger pressure or force applied onto the structure. The changes in the crystal structure induced by CO2 adsorption can be pictured geometrically by considering the structure shown in Fig. 1. The neutral layers of CPL-2 are parallel to the ac plane. We conceptually divide these layers in turn into 2 new imaginary layers defined by the copper atom positions. These layers can be also visualized as planes that could experience sliding or expansion as a result of an eventual un-foiling of the non-bonded flexible carboxyl groups coordinated to the copper node along c. The axis that is perpendicular to the pillar ligand and axis a form the ab plane, which can be readily displaced vertically by the elongation of the neutral layer. In order to test the lattice distortion scheme proposed in the previous paragraph we consider the evolution of the entire series of d-spacings in the range of reflections appearing in the X-ray diffraction pattern. Fig. 5 shows that there is a significant difference between the variation of Δd for the (0 2 0) and (0 1 2) planes, which correspond to spacings in the cross sectional view of the framework along a, and for planes (0 2 0)
Fig. 4 CO2 adsorption (filled symbols) and desorption (open symbols) amounts on CPL-2 gathered at −78.5 °C.
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Fig. 5 Changes in d-spacing (Δd = d − d0) for CPL-2 crystallographic planes (0 2 0), (0 1 2) and (1 0 0) as a function of CO2 (left) or N2 (right) gas partial pressure during adsorption (filled symbols) and desorption (open symbols). The dotted and continuous lines were inserted to help visualize the trends and correspond to polynomial fits that have no relationship with models or any physical interpretation. PCO2 = 0 atm refers to a gas environment without CO2 (1 atm N2). d0 therefore refers to d-spacing values estimated for the CPL-2 when PCO2 = 0 atm or PN2 = 1 atm.
and (1 0 0), for which spacings are in the cross sectional view along c. The same behavior was observed for other homologous planes that were also indexed but not discussed or presented here to avoid redundancy. A control experiment was conducted using N2 exposure at ambient temperature. Fig. 5 shows Δd profiles gathered for CPL-2 only under pure N2 at different pressures. Since the CPL-2 material adsorbs negligible amounts of N2 at room temperature,33 there is no systematic variation of the d-spacings with pressure. We thus can exclude experimental artifacts associated with systematic mechanical distortion of the X-ray apparatus at high pressure. Although changes related to the d-spacing strongly indicate that there are underlying structural changes in the framework, the specific data provided only a fraction of the information necessary to understand the symmetry related to those transformations. Fig. 6 shows the evolution of the changes experi-
Fig. 6 Changes in the CPL-2 pore gallery cross sectional area (ΔS = S − S0) along a or c during adsorption (filled symbols) and desorption (open symbols). PCO2 = 0 atm refers to a gas environment without CO2 (1 atm N2). S0 refers to cross sectional area values estimated for the CPL-2 when PCO2 = 0 atm.
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enced by the CPL-2 apparent gallery cross sectional areas (ΔS) that are orthogonal to a and c, respectively. These changes were calculated using the observed values of Δd and the dimensions of a conceptual parallelogram located within a “channel” perpendicular to the ab or bc planes. Fig. 6 shows that there are expansive non-symmetrical structural changes that prevailed during the CO2 adsorption leg and that no significant contraction was observed during most of the gas desorption process. Interestingly, at the desorption step corresponding to 2 atm CO2, the CPL-2 lattice exhibited a remarkable decrease in ΔS along both a and c. Further confirmation will be required to understand whether this observation indeed represents a new physical effect or whether the system follows the simpler hysteretic behavior suggested by the trends of the other lattice parameters upon desorption. A fit of the MDA isotherm model shown in eqn (2) to the adsorption data gathered for CO2 at −78.5 °C (shown in Fig. 4) indicated that the maximum volume available for adsorption for a relaxed CPL-2 structure at 1 atm was 0.213 cm3 g−1. This result and the corresponding MDA model parameters are gathered in Table 1. The pore volume calculation assumed that the adsorbed phase resembles that of a liquid phase (i.e., tightly packed molecules), which is classical and should hold valid given that CO2 solidifies at −78.5 °C and above 1 atm. A fit of the DA (eqn (1)) model against the CO2 desorption leg data gathered at 25 °C (Fig. 3) indicated that the pore volume of the CPL-2 material expanded as a result of the significant increase in CO2 partial pressure (see Table 1). These results match well the unit cell volume expansion depicted in Fig. 7. This is due to a variation in the unit cell parameters b and β calculated for cases where the adsorbent was exposed to larger partial pressures. Lattice b happens to be associated with the length or orientation of the bpy pillars and, therefore, the changes could make physical sense since a pore expansion could only be attained via swing-like movements of the pillars (see Fig. 1). In general, all of the observed CPL-2 structural changes could translate to a cooperative pore expansion to allocate extra CO2 molecules and suggests that the adsorption mechanism at
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Table 1 Dubinin–Astakhov (DA) and modified Dubinin–Astakhov (MDA) isotherm model parameters and pore volume data obtained from CO2 desorption in CPL-2
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DA or MDA isotherm parameters Temperature (°C)
Equil. time interval (s)
qo (mmol g−1)
C (−)
K (mmol g−1 atm−1)
α (−)
n (−)
Std. Dev.a
Vmp (cm3 g−1)
−78.5 25
30 30 50 100
5.586 4.284 5.149 5.299
0.173 0.206 0.207 0.195
1.01 × 10−3 — — —
418.7 — — —
1.130 2.448 2.272 2.831
±0.043 ±0.035 ±0.039 ±0.044
0.213 0.275 0.331 0.340
a Standard deviation calculated based on residuals between the observed and calculated equilibrium loading amounts for the complete pressure range.
Fig. 7 CPL-2 unit cell lattice parameters as a function of CO2 gas partial pressure during adsorption (filled symbols) and desorption (open symbols). PCO2 = 0 atm refers to a gas environment without CO2 (1 atm N2).
large pressure involves also adsorbate–adsorbate interactions rather than the sole surface binding forces.
4.
of fundamental knowledge related to adsorption-structural changes in metal organic framework (MOF) based adsorbents and under conditions that are of relevance to gas capture at industrial scale.
Conclusions
CPL-2 X-ray diffraction data gathered in situ while adsorbing and desorbing CO2 in a 7 atm partial pressure range under room conditions have allowed a direct assessment of the nonsymmetric character of the structural changes experienced by the porous material. The observations correlated with the CO2 hysteretic adsorption–desorption process that has an apparent onset at a gas pressure corresponding to full occupancy of the volume of the CPL-2 galleries while these were in a relaxed state. The threshold corresponds to a CO2 partial pressure larger than 2 atm and changes observed below this point might be related to a combination of adsorbate–adsorbate and adsorbate–adsorbent interactions under equilibrium. Above the threshold pressure or larger CO2 adsorption loadings, the structural changes are plausibly associated with the expansion of the CPL-2 framework due to the transient process related extra physical force produced by the static or dynamic pressure. These findings are of relevance to the development
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Acknowledgements Funding for this work was provided by the National Science Foundation (NSF) Partnership for Research and Education in Materials (PREM) Award DMR-0934115. The authors gratefully acknowledge partial support of this research by the NSF University of Wisconsin Materials Research Science and Engineering Center (DMR-1121288).
References 1 L. Espinal, D. L. Poster, W. Wong-Ng, A. J. Allen and M. L. Green, Environ. Sci. Technol., 2013, 47, 11960–11975. 2 H. Zhai and E. S. Rubin, Energy Fuels, 2013, 27, 4290–4301.
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3 S. D. Kenarsari, D. L. Yang, G. D. Jiang, S. J. Zhang, J. J. Wang, A. G. Russell, Q. Wei and M. H. Fan, RSC Adv., 2013, 3, 22739–22773. 4 K. W. Chapman, D. F. Sava, G. J. Halder, P. J. Chupas and T. M. Nenoff, J. Am. Ceram. Soc., 2011, 133, 18583–18585. 5 J.-R. Li, J. Sculley and H.-C. Zhou, Chem. Rev., 2012, 112, 869–932. 6 J. Liu, P. K. Thallapally, B. P. McGrail, D. R. Brown and J. Liu, Chem. Soc. Rev., 2012, 41, 2308–2322. 7 K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch, Z. R. Herm, T.-H. Bae and J. R. Long, Chem. Rev., 2012, 112, 724–781. 8 S. Chaemchuen, N. A. Kabir, K. Zhou and F. Verpoort, Chem. Soc. Rev., 2013, 42, 9304–9332. 9 Z. Y. Yeo, P. W. Zhu, A. R. Mohamed and S.-P. Chai, CrystEngComm, 2014, 16, 3072–3075. 10 B. J. Sikora, C. E. Wilmer, M. L. Greenfield and R. Q. Snurr, Chem. Sci., 2012, 3, 2217–2223. 11 B. J. Sikora, R. Winnegar, D. M. Proserpio and R. Q. Snurr, Microporous Mesoporous Mater., 2014, 186, 207–213. 12 K. K. Tanabe and S. M. Cohen, Chem. Soc. Rev., 2011, 40, 498–519. 13 D. W. Breck, Zeolite Molecular Sieves, Wiley, New York, 1973. 14 R. Szostak, Molecular Sieves: Principles of Synthesis and Identification, Blackie Academic and Professional, Thomson Science, New York, 1998. 15 R. T. Yang, Adsorbents: Fundamentals and Applications, Wiley, New York, 2003. 16 S. Henke, A. Schneemann, A. Wütscher and R. A. Fischer, J. Am. Chem. Soc., 2012, 134, 9464–9474. 17 C. R. Murdock, B. C. Hughes, Z. Lu and D. M. Jenkins, Coord. Chem. Rev., 2014, 258–259, 119–136. 18 R. Matsuda, R. Kitaura, S. Kitagawa, Y. Kubota, T. C. Kobayashi, S. Horike and M. Takata, J. Am. Chem. Soc., 2004, 126, 14063–14070. 19 F.-X. Coudert, M. Jeffroy, A. H. Fuchs, A. Boutin and C. Mellot-Draznieks, J. Am. Chem. Soc., 2008, 130, 14294– 14302. 20 J. T. Culp, M. R. Smith, E. Bittner and B. Bockrath, J. Am. Chem. Soc., 2008, 130, 12427–12434. 21 R. Matsuda, R. Kitaura, Y. Kubota, T. C. Kobayashi, M. Takata and S. Kitagawa, Microporous Mesoporous Mater., 2010, 129, 296–303. 22 A. J. Hernández-Maldonado, J. Guerrero-Medina and V. C. Arce-González, J. Mater. Chem. A, 2013, 1, 2343–2350. 23 K. J. Gagnon, C. M. Beavers and A. Clearfield, J. Am. Chem. Soc., 2013, 135, 1252–1255. 24 P. L. Llewellyn, G. Maurin, T. Devic, S. Loera-Serna, N. Rosenbach, C. Serre, S. Bourrelly, P. Horcajada,
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25
26 27 28
29 30 31 32 33 34 35
36
37 38 39 40
41
42 43 44
Y. Filinchuk and G. Férey, J. Am. Chem. Soc., 2008, 130, 12808–12814. K. L. Mulfort, O. K. Farha, C. D. Malliakas, M. G. Kanatzidis and J. T. Hupp, Chem. – Eur. J., 2010, 16, 276–281. K. W. Chapman, G. J. Halder and P. J. Chupas, J. Am. Chem. Soc., 2009, 131, 17546–17547. S. H. Lapidus, G. J. Halder, P. J. Chupas and K. W. Chapman, J. Am. Chem. Soc., 2013, 135, 7621–7628. R. Kitaura, S. Kitagawa, Y. Kubota, T. C. Kobayashi, K. Kindo, Y. Mita, A. Matsuo, M. Kobayashi, H.-C. Chang, T. C. Ozawa, M. Suzuki, M. Sakata and M. Takata, Science, 2002, 298, 2358–2361. S. Kitagawa, R. Kitaura and S. Noro, Angew. Chem., Int. Ed., 2004, 43, 2334–2375. S. Horike, S. Shimomura and S. Kitagawa, Nat. Chem., 2009, 1, 695–704. H. Sakamoto, R. Kitaura, R. Matsuda, S. Kitagawa, Y. Kubota and M. Takata, Chem. Lett., 2010, 39, 218–219. S. Horike, D. Umeyama and S. Kitagawa, Acc. Chem. Res., 2013, 46, 2376–2384. O. J. García-Ricard and A. J. Hernández-Maldonado, J. Phys. Chem. C, 2010, 114, 1827–1834. O. J. García-Ricard, R. Fu and A. J. Hernández-Maldonado, J. Phys. Chem. C, 2011, 115, 3595–3601. O. J. García-Ricard, J. C. Silva-Martínez and A. J. Hernández-Maldonado, Dalton Trans., 2012, 41, 8922– 8930. O. J. García-Ricard, P. Meza-Morales, J. C. Silva-Martínez, M. C. Curet-Arana, J. A. Hogan and A. J. HernándezMaldonado, Microporous Mesoporous Mater., 2013, 177, 54–58. A. Kapoor, J. A. Ritter and R. T. Yang, Langmuir, 1989, 5, 1118–1121. D. D. Do, Adsorption Analysis: Equilibria and Kinetics, Imperial College Press, London, 1998. A. Boultif and D. Louër, J. Appl. Crystallogr., 1991, 24, 987– 993. T. Fukushima, S. Horike, Y. Inubushi, K. Nakagawa, Y. Kubota, M. Takata and S. Kitagawa, Angew. Chem., Int. Ed., 2010, 49, 4820–4824. M. Kondo, T. Okubo, A. Asami, S.-i. Noro, T. Yoshitomi, S. Kitagawa, T. Ishii, H. Matsuzaka and K. Seki, Angew. Chem., Int. Ed., 1999, 38, 140–143. R. Matsuda, T. Tsujino, H. Sato, Y. Kubota, K. Morishige, M. Takata and S. Kitagawa, Chem. Sci., 2010, 1, 315–321. J. Duan, M. Higuchi, M. L. Foo, S. Horike, R. Prabhakara and S. Kitagawa, Inorg. Chem., 2013, 52, 8244–8249. J. T. Culp, L. Sui, A. Goodman and D. Luebke, J. Colloid Interface Sci., 2013, 393, 278–285.
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Hysteretic adsorption of CO2 onto a Cu2( pzdc)2(bpy) porous coordination polymer and concomitant framework distortion Karina Riascos-Rodríguez,a Aaron J. Schroeder,b Michael R. Arend,b Paul G. Evansb and Arturo J. Hernández-Maldonado*a The present study focuses on the long-range structural changes that occur in the porous coordination polymer Cu2( pzdc)2(bpy) ( pzdc = pyrazine-2,3-dicarboxylate, bpy = 4,4’-bipyridine), also known as CPL-2, upon adsorption of CO2 at 25 °C and up to 7 atm. The structural data were gathered using in situ diffraction studies. CPL-2 exhibited an unexpected hysteretic adsorption–desorption process. The onset of hysteresis occurs at a pressure where full occupancy of the volume of the CPL-2 galleries is achieved while the framework retains a structure similar to what is observed under ambient conditions. Moreover, the onset occurs at a CO2 partial pressure larger than 2 atm and could be related to a combination of
Received 25th March 2014, Accepted 12th May 2014 DOI: 10.1039/c4dt00878b www.rsc.org/dalton
1.
adsorbate–adsorbent interactions and forces exerted onto the CPL-2 framework. Pore volumes estimated from fits of the Dubinin–Astakhov isotherm model against the CO2 desorption data gathered at 25 and −78.5 °C, respectively, provided further evidence of the aforementioned CPL-2 framework changes. These findings are of relevance to the understanding of adsorption processes in metal organic frameworks or coordination polymers under conditions that are of relevance to gas capture at industrial scale.
Introduction
The development of fuel pre- or post-processing treatment and purification operations that are suitable for the growing demand for energy while mitigating climate change and pollution is an area of significant challenges and opportunities. Most of the emissions of CO2 arise from the combustion of fossil fuels and the processing of cement, with the United States alone producing 4.9 billion tons of CO2 annually.1,2 Post combustion capture of CO2 is usually attained via absorption methods that rely on aqueous mono-, di- or tri-alkanolamine solutions. A similar process is also used to remove CO2 from natural gas sources. The amine solutions are volatile and corrosive, and are also associated with the formation of carbamates and bicarbonates, requiring large energy inputs for regeneration.3 The drawbacks of this and other existing technologies have motivated the search for cost effective alternatives for carbon capture and storage technologies based on adsorption. Several studies have demonstrated that metal organic frameworks (MOFs) have tremendous potential as
a Department of Chemical Engineering, University of Puerto Rico-Mayagüez Campus, Mayagüez, PR 00681-9000, Puerto Rico. E-mail: [email protected]; Fax: +787-834-3655; Tel: +787-832-4040 x3748 b Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA
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highly porous adsorption materials for the CO2 capture.1,3–9 However, whether these porous materials perform separations efficiently under conditions similar to those found in power plant flue and natural gas streams or even biogas upgrading remains a topic of intense study. The problem is complex because there are a great variety of MOF chemical structures and the responses to different external stimuli, including temperature and pressure swings, are likely to vary significantly. The structure of a MOF can be designed prior to synthesis10,11 and modified upon post-synthesis functionalization.12 There are a variety of MOFs that exhibit a combination of strong and weak coordination bindings within their frameworks. It is important to note that the combination of strong and weak bonds is not found in zeolites, which are mainly composed of rigid covalent Si–O bonds.13–15 The responses of MOF materials to internal or external potentials depend on the type of adsorbate, high pressure, temperature, or electric fields.16,17 For instance, changes in the unit cellscale structure of a MOF can be correlated with s-shaped, stepwise, or hysteretic single component adsorption–desorption isotherms.5,18–22 The control of such changes can also lead to new applications related to solid state chemistry.5,23 However, further structural studies of the dynamics of adsorption of CO2 onto MOFs are required. There is a need for further insight into the stability of the MOF once either the guest or the adsorbate is removed (via desorption) or the external force or stimu-
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lus is stopped. Such insight is not available through traditional single-component adsorption equilibrium studies. Potential hindrances to applications, such as mechanical stability and structural changes under moderate to high pressure at ambient temperature, must be identified in order to fulfill not only industrial requirements related to CO2 capture processes, but also related to performance for targeted applications.1,24 Several in situ structural (i.e., crystallographic) studies of the adsorption of CO2 are available in the low (0.1 to 2 atm) pressure regime.7,25 A few studies have covered pressure ranges that exceed even those found in industrial CO2 capture applications, relying on high-pressure treatments under hydrostatic and non-hydrostatic conditions.23,26,27 Interestingly, most of these studies have been motivated by the gate opening pressure phenomenon, which refers to an abrupt increase in adsorption at a threshold pressure and explains the high CO2 selectivity of several MOFs. A series of other MOFs do not exhibit a sudden adsorption induced gate opening phenomenon at mid-pressure ranges, but instead show hysteretic adsorption–desorption loops that vary according to the gas loading. For instance, hysteretic adsorption–desorption phenomena can be found in some porous coordination polymers (PCPs), a subclass of the broader series of MOF materials. PCPs consist of single- or multi-dimensional porous frameworks constructed from transition-metal centers that serve as coordination centers to organic bridging ligands.28–32 These materials exhibit unique adsorption phenomena due to structural changes sometimes induced by interactions with the guest molecules. CPL-2 (Cu2( pzdc)2(bpy) [pzdc = pyrazine-2,3dicarboxylate, bpy = 4,4′-bipyridine]), for example, exhibits remarkable breathing framework behavior in the presence of water and benzene vapors at ambient temperature and pressure.18 The adsorption of gases such as N2, O2, CO2 and Xe on CPL-2 at relative low partial pressures at cryogenic temperatures also leads to distortion of the framework upon uptake of the gas that can be explained by a change in the coordination mode of Cu from pyramidal to square planar.21 Hernández-Maldonado and co-workers used in situ XRD and 3C cross-polarization magic angle spinning nuclear magnetic resonance (CP-MAS NMR) spectroscopy to study how CPL-n materials undergo framework changes depending on the preactivation temperature, something that is also critical for multi-cycle adsorption operations.33–35 Further experimental and computational studies have elucidated the capacity of these materials for CO2 storage and delivery at medium pressures (up to 6 atm) and also showed that the observed hysteretic adsorption property is a function of the ultimate gas pressure.36 The magnitude of the hysteresis depended on the highest CO2 pressures during loading, and was completely absent within the atmospheric pressure range. In addition, the hysteresis was clearly a result of interactions between the CO2 and the framework, but there was no direct structural information. Further structural information is of utmost importance in allowing CPL-2 and similar materials to be employed in multi-cycle operations. Structural studies will also address
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the stability of hysteretic MOFs at the interface between the fields of chemical engineering and surface science. We present here an in situ adsorption and powder X-ray diffraction study of the structural mechanisms of the hysteretic adsorption of CO2 onto CPL-2 at pressures ranging from 0 to 7 atm. The structural changes were observed using a specially developed capillary sample environment permitting X-ray diffraction studies of samples held at elevated CO2 pressure. A comparison of the adsorption/desorption gap found in chemical measurements with the X-ray data shows that the hysteresis can be observed using the changes in the volume of the CPL-2 unit cell.
2. Experimental 2.1.
Reagents and materials
2,3-Pyrazinedicarboxylic acid (H2pzdc, 97% purity), 4,4-bipyridine (bpy, 98% purity), copper(II) perchlorate hexahydrate (Cu(ClO4)2·6H2O, 98% purity), denatured ethanol (95% purity) and methanol (99% purity) from Sigma-Aldrich were used as supplied to synthesize CPL-2. Ultra-high purity grade CO2 and N2 gases obtained from Praxair, Inc. were used during the measurements of the adsorption isotherms. CO2 and N2 gases used in the in situ X-ray diffraction experiments were obtained from AirGas, Inc. 2.2.
CPL-2 synthesis
CPL-2 was synthesized at ambient temperature according to procedures available elsewhere.18,34,35 One mmol of H2pzdc (0.1681 g) and 0.5 mmol of bpy (0.078 g) were dissolved in a solution prepared previously by mixing 1 : 1 NaOH 0.04 M and ethanol. The resulting mixture was dropwise added to a second solution consisting of CuClO4·6H2O (0.37 g) and water, all under continuous agitation. The final blend was stirred for 24 h, filtered under vacuum and repeatedly washed with methanol and deionized water. The remaining methanol was removed by heating the solid overnight at 90 °C in air. The final solid sample was stored in a sealed desiccator until further use. 2.3.
Porosimetry and high pressure CO2 adsorption
Porosimetry data were gathered using a Micromeritics ASAP 2020 volumetric adsorption unit fitted with turbo molecular drag pumps. Activation of the sample was performed prior to each adsorption analysis using the instrument’s sample thermal degassing module. A sample was loaded (∼100 mg) into a quartz-made container fitted with an isolation valve, which permits connection to a vacuum port and subsequent transport between analysis ports without exposure to ambient humidity and/or gases. The sample was initially evacuated at a rate of 50 mmHg s−1 to a final pressure of 5 mmHg. Further activation was conducted immediately by heating the sample in situ to 100 °C (heating ramp of 10 °C min−1) and soaking at this temperature for several hours.33 After the degassing time elapsed, the sample was allowed to cool to ambient tempera-
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ture while still under vacuum conditions. The sample was then backfilled with helium to allow its transfer to the analysis port while under isolation. Once attached to the analysis port, the sample was evacuated to remove He prior to the adsorption tests. The adsorption tests for this part of the work were performed for CO2 uptake at −78.5 °C and the sample pore volume was estimated by assuming that the adsorbed phase behaved similar to that of a liquid (i.e., nitrogen molecules tightly packed) and employing the Dubinin–Astakhov (DA) and modified Dubinin–Astakhov (MDA) isotherm models, respectively.37,38 The models are given by the following expressions: qDA ¼ q0 exp½ðC lnðP o =PÞÞn n
qMDA ¼ q0 ½β1 exp½ðC lnðP o =PÞÞ þ β2 KP where
ð1Þ ð2Þ
P β1 ¼ 1 exp α Po P β2 ¼ exp α Po RT C¼ βE q1 K¼ Po
qDA or qMDA is the equilibrium adsorption loading amount, q0 is the adsorption loading amount at saturation (complete pore filling), R is the universal gas constant, β is the affinity coefficient of the adsorbate, E is the energy of adsorption, P/Po is the relative pressure, n is the heterogeneity coefficient, α is a fitting parameter and K is Henry’s law constant. The DA and MDA models take into consideration surface heterogeneity and pore filling effects (i.e., interaction of the adsorbate volume with the surface). The MDA model (eqn (2)) contains a second term that allows it to comply with Henry’s law limit.37 Pure component CO2 adsorption and desorption isotherms were acquired at 25 °C at pressures up to 7 atm using a Micromeritics ASAP 2050 extended pressure volumetric adsorption instrument. The CPL-2 sample was loaded into a stainless steel sample holder that was otherwise similar to the sample holder used for porosimetry. The sample pre-activation and adsorption steps were similar to those employed to gather porosimetry data, with the exception that the temperature was maintained at 25 °C. In addition, the instrument equilibration time intervals used were 30, 50 and 100 seconds. Since the instrument data gathering algorithm is limited to the analysis of eleven consecutive pressure data points, those time intervals resulted in at least 330, 550 and 1100 seconds worth of transient data, respectively, for each targeted pressure increment. It should be noted that the ASAP 2050 maximum allowable working gas pressure is approximately 9 atm for CO2. 2.4.
In situ CO2 adsorption and X-ray diffraction
Changes in the structure of CPL-2 upon adsorption of CO2 were studied via in situ powder X-ray diffraction. The data were gathered using a four-circle X-ray laboratory diffractometer
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with Cu K-α radiation from an Ultra-X 18 rotating anode X-ray generator (Rigaku, Inc.) operating at 40 kV and 100 mA. Diffraction patterns were obtained for 2θ diffraction angles between 5 and 30° with a step size of 0.03°. Each diffraction pattern was indexed using the DICVOL91 program.39 A cylindrical polyimide capillary with a diameter of 1 mm was used as a sample holder. One end of the polyimide capillary was sealed using an epoxy adhesive. The capillary was attached to a gas manifold, including pressure measurement and facilities for flushing and pressurization with N2 and CO2 gases. The sample was pre-activated in place to remove water traces and light impurities. The pre-activation process consisted of heating using an external source (i.e., heat gun) while the sample was exposed to flowing N2 gas. The temperature during activation was measured near the capillary using a thin-wire thermocouple and carefully controlled to remain within 100 to 106 °C. Heating to 100 °C was found in porosimetry to preserve the gas adsorption and structural properties of CPL-2.33,34 Upon completion of the pre-activation, the sample was returned to room temperature under a flow of N2. The N2 gas flow was later substituted with CO2, the system was sealed using a gas valve and the CO2 supply was continued until the sample environment reached the target pressure for each X-ray diffraction scan. The pressure soaking stage lasted only a few seconds prior to each X-ray diffraction scan, which is justified given the fast kinetics of the CO2 uptake.33 The time scales necessary to attain both pressure and uptake equilibration were kept constant and were identical.
3. Results and discussion The unit cell of the CPL-2 lattice is composed of a copper node (Cu2+) bonded to three pyrazine-2,3-dicarboxylate ( pzdc) units, one of them chelated, to conform to a two-dimensional neutral layer. Diagrams of the a–b and c–b planes of the CPL-2 lattice are shown in Fig. 1. The neutral layers are coupled through 4,4′-bipyridine (bpy) pillars bonded at each end to Cu2+ to conform to a three-dimensional structure. The pillar and neutral layers can be thought of as mechanically rigid because the rings of the bpy and pzdc, respectively, resist rotation. A key feature of the CPL-2 structure is that straight one-dimensional accessible channels, termed galleries, extend along the a-axis and can facilitate adsorption by allowing the rapid transport of gas molecules through the crystal.18 This configuration, frequently found in several MOFs,40 can lead to adsorbate–adsorbate interactions that are sensitive to even minor changes in the total loading of the guest molecules. MOFs or PCPs that include Cu2+ complexes can exhibit tetrahedral, square planar or octahedral coordination at the metal site. A Jahn–Teller distortion of the Cu2+ complex is required to change the energy of eg orbitals, which otherwise would be degenerate. Such distortions are likely to occur upon rotation or cleavage of the carboxylate portions of the neutral layer.18,31,41–43 An interaction between the electric field gradient of the framework surface and the CO2 permanent
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Fig. 1
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CPL-2 framework structure, galleries and diffraction planes of interest.
quadruple moment could be induced by specific functionalities, but could also be facilitated upon an external stimulus such as pressure.36 Therefore, the simultaneous increase in static pressure and the amount of guest in the pores are both factors probably influencing the adsorption process. For CPL-2, the carboxyl groups located near the Cu2+ nodes should offer stronger sites for interaction with CO2 as compared to the bpy pillars environment. The CPL-2 framework configuration should remain stable at ambient pressure until exposure to chemical conditions that induce a reorganization of the O or N atoms of the carboxylate linked to the Cu2+ center. The reorganization can be induced via the rotation or cleavage of bonds, re-distribution of electrons, or changes in the length of bonding. Structural changes can easily spread along the long-range order of the framework, producing a cooperative cleavage of bonds or shrinkage. In the case of several MOF compounds, the structural reorganization can lead to the appearance of new phases or even to amorphization.19,24,44 CPL-2 exhibits fully reversible adsorption–desorption linear paths within the 1 atm CO2 pressure range, a reflection of weak adsorbate–adsorbent interactions.33 The adsorption/ desorption measurements extending to 7 atm are shown in Fig. 2. At higher CO2 pressures, the observed hysteretic relationship between adsorbate loading and pressure indicates that adsorbate interactions can be expected to lead to structural distortion. For CPL-2, the carboxyl groups located near the copper nodes should offer stronger sites for interaction with CO2 as compared to the bpy pillars environment. If a Jahn–Teller is in effect taking place, it could translate to structural dynamic changes and even unique adsorption mechanisms. The CPL material prepared for this study exhibited the crystal structure previously described in the literature.18,33,34 The CPL-2 lattice has monoclinic symmetry (space group P21/
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Fig. 2 (top) CO2 equilibrium adsorption and desorption amounts on CPL-2 gathered at 25 °C and (bottom) corresponding X-ray diffraction patterns. In the diffraction data, PCO2 = 0 atm refers to a gas environment without CO2 (1 atm N2).
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c, a = 4.71160 Å, b = 27.833 Å, c = 10.8881 Å, β = 96.0102°).18 An ambient pressure diffraction pattern for CPL-2 acquired after the activation step is shown as curve (a) in Fig. 2. The series of X-ray diffraction patterns labeled (a) through (h) in Fig. 2 were gathered following the path established in the CO2 adsorption isotherm also shown in Fig. 2. It is apparent from the shift of the 2θ angles of the X-ray reflections that the CPL-2 undergoes a pressure-responsive structural change without a major loss of crystallinity. The pressure-induced structural change is characterized by a sustained shift of the (0 2 0) reflection to smaller 2θ angles, corresponding to an increase in the (0 2 0) d-spacing. The increased (0 2 0) d-spacing persists after depressurization (rapid CO2 release), which is indicative of a pore expansion. The structural changes can be interpreted in two pressure or equilibration regimes. In the first of these, at low gas loadings (i.e., CO2 partial pressures smaller than 2 atm), the dominant host–guest interactions probably facilitated the displacement of CO2 toward adsorption sites. The displacement in turn results in a stronger interaction of the CO2 with the CPL-2, perhaps at sites in the vicinity of the Cu2+ nodes. We note that this response can occur rapidly throughout the crystal because there is minimal major resistance to diffusion due to the kinetic diameter of CO2 (∼3.3 Å), which is considerably smaller than the CPL-2 pore dimensions. At higher CO2 pressures, an increase in CO2 loadings inside the pores resulted in structural changes only after the adsorbate–adsorbent energy of interaction was equilibrated or the CO2 occupied the available void volume while the framework was in a relaxed state. At larger CO2 loadings an extra physical force produced by static or dynamic pressure could influence the framework stability in advancement to a different crystal configuration, which probably could be evidenced by a further long-range expansion. This second pressure or equilibration regime or domain also involved slow dynamics. Fig. 3 shows
Fig. 3 CO2 adsorption (filled symbols) and desorption (open symbols) amounts on CPL-2 gathered at 25 °C and different equilibration time intervals.
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that the longer the adsorption equilibration time interval, the larger the adsorption of CO2 at pressures above 2 atm. Furthermore, at an equilibration time interval of 100 s and a CO2 partial pressure of 6 atm the adsorption leg of the isotherm shows an inflection, most likely associated with the onset of a plausible gate or structure opening process. Capture of the whole process was not possible due to limitations of the adsorption instrument capability. Fig. 4 shows CO2 equilibrium adsorption–desorption data gathered in CPL-2 at −78.5 °C. At this temperature and a pressure near 1 atm, the CO2 is adsorbed within the pores of a relaxed CPL-2 framework in a tightly packed fashion while its desorption proceeded without any hysteresis. This provides further evidence that the hysteretic adsorption process observed at 25 °C and large CO2 pressures is likely due, for the most part, to the larger pressure or force applied onto the structure. The changes in the crystal structure induced by CO2 adsorption can be pictured geometrically by considering the structure shown in Fig. 1. The neutral layers of CPL-2 are parallel to the ac plane. We conceptually divide these layers in turn into 2 new imaginary layers defined by the copper atom positions. These layers can be also visualized as planes that could experience sliding or expansion as a result of an eventual un-foiling of the non-bonded flexible carboxyl groups coordinated to the copper node along c. The axis that is perpendicular to the pillar ligand and axis a form the ab plane, which can be readily displaced vertically by the elongation of the neutral layer. In order to test the lattice distortion scheme proposed in the previous paragraph we consider the evolution of the entire series of d-spacings in the range of reflections appearing in the X-ray diffraction pattern. Fig. 5 shows that there is a significant difference between the variation of Δd for the (0 2 0) and (0 1 2) planes, which correspond to spacings in the cross sectional view of the framework along a, and for planes (0 2 0)
Fig. 4 CO2 adsorption (filled symbols) and desorption (open symbols) amounts on CPL-2 gathered at −78.5 °C.
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Fig. 5 Changes in d-spacing (Δd = d − d0) for CPL-2 crystallographic planes (0 2 0), (0 1 2) and (1 0 0) as a function of CO2 (left) or N2 (right) gas partial pressure during adsorption (filled symbols) and desorption (open symbols). The dotted and continuous lines were inserted to help visualize the trends and correspond to polynomial fits that have no relationship with models or any physical interpretation. PCO2 = 0 atm refers to a gas environment without CO2 (1 atm N2). d0 therefore refers to d-spacing values estimated for the CPL-2 when PCO2 = 0 atm or PN2 = 1 atm.
and (1 0 0), for which spacings are in the cross sectional view along c. The same behavior was observed for other homologous planes that were also indexed but not discussed or presented here to avoid redundancy. A control experiment was conducted using N2 exposure at ambient temperature. Fig. 5 shows Δd profiles gathered for CPL-2 only under pure N2 at different pressures. Since the CPL-2 material adsorbs negligible amounts of N2 at room temperature,33 there is no systematic variation of the d-spacings with pressure. We thus can exclude experimental artifacts associated with systematic mechanical distortion of the X-ray apparatus at high pressure. Although changes related to the d-spacing strongly indicate that there are underlying structural changes in the framework, the specific data provided only a fraction of the information necessary to understand the symmetry related to those transformations. Fig. 6 shows the evolution of the changes experi-
Fig. 6 Changes in the CPL-2 pore gallery cross sectional area (ΔS = S − S0) along a or c during adsorption (filled symbols) and desorption (open symbols). PCO2 = 0 atm refers to a gas environment without CO2 (1 atm N2). S0 refers to cross sectional area values estimated for the CPL-2 when PCO2 = 0 atm.
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enced by the CPL-2 apparent gallery cross sectional areas (ΔS) that are orthogonal to a and c, respectively. These changes were calculated using the observed values of Δd and the dimensions of a conceptual parallelogram located within a “channel” perpendicular to the ab or bc planes. Fig. 6 shows that there are expansive non-symmetrical structural changes that prevailed during the CO2 adsorption leg and that no significant contraction was observed during most of the gas desorption process. Interestingly, at the desorption step corresponding to 2 atm CO2, the CPL-2 lattice exhibited a remarkable decrease in ΔS along both a and c. Further confirmation will be required to understand whether this observation indeed represents a new physical effect or whether the system follows the simpler hysteretic behavior suggested by the trends of the other lattice parameters upon desorption. A fit of the MDA isotherm model shown in eqn (2) to the adsorption data gathered for CO2 at −78.5 °C (shown in Fig. 4) indicated that the maximum volume available for adsorption for a relaxed CPL-2 structure at 1 atm was 0.213 cm3 g−1. This result and the corresponding MDA model parameters are gathered in Table 1. The pore volume calculation assumed that the adsorbed phase resembles that of a liquid phase (i.e., tightly packed molecules), which is classical and should hold valid given that CO2 solidifies at −78.5 °C and above 1 atm. A fit of the DA (eqn (1)) model against the CO2 desorption leg data gathered at 25 °C (Fig. 3) indicated that the pore volume of the CPL-2 material expanded as a result of the significant increase in CO2 partial pressure (see Table 1). These results match well the unit cell volume expansion depicted in Fig. 7. This is due to a variation in the unit cell parameters b and β calculated for cases where the adsorbent was exposed to larger partial pressures. Lattice b happens to be associated with the length or orientation of the bpy pillars and, therefore, the changes could make physical sense since a pore expansion could only be attained via swing-like movements of the pillars (see Fig. 1). In general, all of the observed CPL-2 structural changes could translate to a cooperative pore expansion to allocate extra CO2 molecules and suggests that the adsorption mechanism at
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Table 1 Dubinin–Astakhov (DA) and modified Dubinin–Astakhov (MDA) isotherm model parameters and pore volume data obtained from CO2 desorption in CPL-2
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DA or MDA isotherm parameters Temperature (°C)
Equil. time interval (s)
qo (mmol g−1)
C (−)
K (mmol g−1 atm−1)
α (−)
n (−)
Std. Dev.a
Vmp (cm3 g−1)
−78.5 25
30 30 50 100
5.586 4.284 5.149 5.299
0.173 0.206 0.207 0.195
1.01 × 10−3 — — —
418.7 — — —
1.130 2.448 2.272 2.831
±0.043 ±0.035 ±0.039 ±0.044
0.213 0.275 0.331 0.340
a Standard deviation calculated based on residuals between the observed and calculated equilibrium loading amounts for the complete pressure range.
Fig. 7 CPL-2 unit cell lattice parameters as a function of CO2 gas partial pressure during adsorption (filled symbols) and desorption (open symbols). PCO2 = 0 atm refers to a gas environment without CO2 (1 atm N2).
large pressure involves also adsorbate–adsorbate interactions rather than the sole surface binding forces.
4.
of fundamental knowledge related to adsorption-structural changes in metal organic framework (MOF) based adsorbents and under conditions that are of relevance to gas capture at industrial scale.
Conclusions
CPL-2 X-ray diffraction data gathered in situ while adsorbing and desorbing CO2 in a 7 atm partial pressure range under room conditions have allowed a direct assessment of the nonsymmetric character of the structural changes experienced by the porous material. The observations correlated with the CO2 hysteretic adsorption–desorption process that has an apparent onset at a gas pressure corresponding to full occupancy of the volume of the CPL-2 galleries while these were in a relaxed state. The threshold corresponds to a CO2 partial pressure larger than 2 atm and changes observed below this point might be related to a combination of adsorbate–adsorbate and adsorbate–adsorbent interactions under equilibrium. Above the threshold pressure or larger CO2 adsorption loadings, the structural changes are plausibly associated with the expansion of the CPL-2 framework due to the transient process related extra physical force produced by the static or dynamic pressure. These findings are of relevance to the development
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Acknowledgements Funding for this work was provided by the National Science Foundation (NSF) Partnership for Research and Education in Materials (PREM) Award DMR-0934115. The authors gratefully acknowledge partial support of this research by the NSF University of Wisconsin Materials Research Science and Engineering Center (DMR-1121288).
References 1 L. Espinal, D. L. Poster, W. Wong-Ng, A. J. Allen and M. L. Green, Environ. Sci. Technol., 2013, 47, 11960–11975. 2 H. Zhai and E. S. Rubin, Energy Fuels, 2013, 27, 4290–4301.
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3 S. D. Kenarsari, D. L. Yang, G. D. Jiang, S. J. Zhang, J. J. Wang, A. G. Russell, Q. Wei and M. H. Fan, RSC Adv., 2013, 3, 22739–22773. 4 K. W. Chapman, D. F. Sava, G. J. Halder, P. J. Chupas and T. M. Nenoff, J. Am. Ceram. Soc., 2011, 133, 18583–18585. 5 J.-R. Li, J. Sculley and H.-C. Zhou, Chem. Rev., 2012, 112, 869–932. 6 J. Liu, P. K. Thallapally, B. P. McGrail, D. R. Brown and J. Liu, Chem. Soc. Rev., 2012, 41, 2308–2322. 7 K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch, Z. R. Herm, T.-H. Bae and J. R. Long, Chem. Rev., 2012, 112, 724–781. 8 S. Chaemchuen, N. A. Kabir, K. Zhou and F. Verpoort, Chem. Soc. Rev., 2013, 42, 9304–9332. 9 Z. Y. Yeo, P. W. Zhu, A. R. Mohamed and S.-P. Chai, CrystEngComm, 2014, 16, 3072–3075. 10 B. J. Sikora, C. E. Wilmer, M. L. Greenfield and R. Q. Snurr, Chem. Sci., 2012, 3, 2217–2223. 11 B. J. Sikora, R. Winnegar, D. M. Proserpio and R. Q. Snurr, Microporous Mesoporous Mater., 2014, 186, 207–213. 12 K. K. Tanabe and S. M. Cohen, Chem. Soc. Rev., 2011, 40, 498–519. 13 D. W. Breck, Zeolite Molecular Sieves, Wiley, New York, 1973. 14 R. Szostak, Molecular Sieves: Principles of Synthesis and Identification, Blackie Academic and Professional, Thomson Science, New York, 1998. 15 R. T. Yang, Adsorbents: Fundamentals and Applications, Wiley, New York, 2003. 16 S. Henke, A. Schneemann, A. Wütscher and R. A. Fischer, J. Am. Chem. Soc., 2012, 134, 9464–9474. 17 C. R. Murdock, B. C. Hughes, Z. Lu and D. M. Jenkins, Coord. Chem. Rev., 2014, 258–259, 119–136. 18 R. Matsuda, R. Kitaura, S. Kitagawa, Y. Kubota, T. C. Kobayashi, S. Horike and M. Takata, J. Am. Chem. Soc., 2004, 126, 14063–14070. 19 F.-X. Coudert, M. Jeffroy, A. H. Fuchs, A. Boutin and C. Mellot-Draznieks, J. Am. Chem. Soc., 2008, 130, 14294– 14302. 20 J. T. Culp, M. R. Smith, E. Bittner and B. Bockrath, J. Am. Chem. Soc., 2008, 130, 12427–12434. 21 R. Matsuda, R. Kitaura, Y. Kubota, T. C. Kobayashi, M. Takata and S. Kitagawa, Microporous Mesoporous Mater., 2010, 129, 296–303. 22 A. J. Hernández-Maldonado, J. Guerrero-Medina and V. C. Arce-González, J. Mater. Chem. A, 2013, 1, 2343–2350. 23 K. J. Gagnon, C. M. Beavers and A. Clearfield, J. Am. Chem. Soc., 2013, 135, 1252–1255. 24 P. L. Llewellyn, G. Maurin, T. Devic, S. Loera-Serna, N. Rosenbach, C. Serre, S. Bourrelly, P. Horcajada,
10884 | Dalton Trans., 2014, 43, 10877–10884
Dalton Transactions
25
26 27 28
29 30 31 32 33 34 35
36
37 38 39 40
41
42 43 44
Y. Filinchuk and G. Férey, J. Am. Chem. Soc., 2008, 130, 12808–12814. K. L. Mulfort, O. K. Farha, C. D. Malliakas, M. G. Kanatzidis and J. T. Hupp, Chem. – Eur. J., 2010, 16, 276–281. K. W. Chapman, G. J. Halder and P. J. Chupas, J. Am. Chem. Soc., 2009, 131, 17546–17547. S. H. Lapidus, G. J. Halder, P. J. Chupas and K. W. Chapman, J. Am. Chem. Soc., 2013, 135, 7621–7628. R. Kitaura, S. Kitagawa, Y. Kubota, T. C. Kobayashi, K. Kindo, Y. Mita, A. Matsuo, M. Kobayashi, H.-C. Chang, T. C. Ozawa, M. Suzuki, M. Sakata and M. Takata, Science, 2002, 298, 2358–2361. S. Kitagawa, R. Kitaura and S. Noro, Angew. Chem., Int. Ed., 2004, 43, 2334–2375. S. Horike, S. Shimomura and S. Kitagawa, Nat. Chem., 2009, 1, 695–704. H. Sakamoto, R. Kitaura, R. Matsuda, S. Kitagawa, Y. Kubota and M. Takata, Chem. Lett., 2010, 39, 218–219. S. Horike, D. Umeyama and S. Kitagawa, Acc. Chem. Res., 2013, 46, 2376–2384. O. J. García-Ricard and A. J. Hernández-Maldonado, J. Phys. Chem. C, 2010, 114, 1827–1834. O. J. García-Ricard, R. Fu and A. J. Hernández-Maldonado, J. Phys. Chem. C, 2011, 115, 3595–3601. O. J. García-Ricard, J. C. Silva-Martínez and A. J. Hernández-Maldonado, Dalton Trans., 2012, 41, 8922– 8930. O. J. García-Ricard, P. Meza-Morales, J. C. Silva-Martínez, M. C. Curet-Arana, J. A. Hogan and A. J. HernándezMaldonado, Microporous Mesoporous Mater., 2013, 177, 54–58. A. Kapoor, J. A. Ritter and R. T. Yang, Langmuir, 1989, 5, 1118–1121. D. D. Do, Adsorption Analysis: Equilibria and Kinetics, Imperial College Press, London, 1998. A. Boultif and D. Louër, J. Appl. Crystallogr., 1991, 24, 987– 993. T. Fukushima, S. Horike, Y. Inubushi, K. Nakagawa, Y. Kubota, M. Takata and S. Kitagawa, Angew. Chem., Int. Ed., 2010, 49, 4820–4824. M. Kondo, T. Okubo, A. Asami, S.-i. Noro, T. Yoshitomi, S. Kitagawa, T. Ishii, H. Matsuzaka and K. Seki, Angew. Chem., Int. Ed., 1999, 38, 140–143. R. Matsuda, T. Tsujino, H. Sato, Y. Kubota, K. Morishige, M. Takata and S. Kitagawa, Chem. Sci., 2010, 1, 315–321. J. Duan, M. Higuchi, M. L. Foo, S. Horike, R. Prabhakara and S. Kitagawa, Inorg. Chem., 2013, 52, 8244–8249. J. T. Culp, L. Sui, A. Goodman and D. Luebke, J. Colloid Interface Sci., 2013, 393, 278–285.
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