Mechanism for the Dynamic Adsorption of CO2 and CH4 in a Flexible

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Mechanism for the Dynamic Adsorption of CO2 and CH4 in a Flexible Linear Chain Coordination Polymer as Determined from In Situ Infrared Spectroscopy Jeffrey T. Culp,*,†,‡ A. L. Goodman,† Danielle Chirdon,†,§ S. G. Sankar,| and Christopher Matranga† National Energy Technology Laboratory, United States Department of Energy, P.O. Box 10940, Pittsburgh, PennsylVania 15236, URS-Washington DiVision, Pittsburgh, PennsylVania 15236, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260, and AdVanced Materials Corporation, Pittsburgh, PennsylVania 15220 ReceiVed: August 25, 2009; ReVised Manuscript ReceiVed: December 23, 2009

Adsorption-desorption cycles for CO2 and CH4 on the one-dimensional coordination polymer, catenabis(dibenzoylmethanato)-(4,4′-bipyridyl)nickel(II), “Ni-DBM-BPY”, showed pronounced step-shape isotherms, where minimal gas adsorption was detected below a threshold pressure and rapid gas uptake was observed above this threshold. Desorption isotherms from the saturated state displayed significant hysteresis from the adsorption isotherm path. Such behavior is rare in one-dimensional coordination polymers that lack a robust framework with permanent porosity. This step-shape adsorption behavior for CO2 was shown by in situ FTIR measurements to be the result of a structural phase transition in the Ni-DBM-BPY host which arises from a change in conformation of the DBM ligands. After the structural transition, the adsorption spectrum of the adsorbed CO2 changed significantly due to an enhanced CO2 interaction with the host. A similar mechanism can be inferred for CH4 from the isotherm shape, but the host structural phase transitions could not be observed directly with CH4 uptake, since the threshold conditions were outside the temperature and pressure limits of the instrument. These reported results highlight the importance of in situ FT-IR measurements for determining gas adsorption mechanisms in flexible porous coordination polymers. Introduction Carbon capture and storage technology is essential for maintaining fossil fuels as a viable energy source. The Department of Energy has taken a leading role in this effort through its Carbon Sequestration Program with an overall goal of developing fossil fuel conversion systems that achieve 90% CO2 capture with 99% storage permanence at less than a 10% increase in energy costs.1 Paramount to meeting this goal is the development of new CO2 capture materials. Recent materials which have attracted interest for CO2 capture are porous coordination polymers, including metal organic frameworks (MOFs) and zeolitic imidazolate frameworks (ZIFs).2-8 One attractive feature of porous coordination polymers and MOFs is the dynamic structural response that some of these materials show in the presence of suitable guests such as CO2, CH4, N2, and H2 or via thermal cycling.9-20 These flexible materials can show interesting host-guest phenomena including crystal-crystal transformations and gated adsorption behaviors due to the structural transitions which occur between the guestfree and guest-loaded states.21-38 Such dynamic host-guest behavior is particularly attractive for applications involving sorbent-based gas separations and selective adsorption such as CO2 amd CH4 separations.39-43 In order to develop materials for these applications, a detailed understanding of the adsorption mechanisms of these flexible hosts is needed. Structural changes involving guest induced crystal-crystal transformations can be followed by X-ray diffraction; however, * Corresponding author. E-mail: [email protected]. Phone: 412-386-5393. Fax: 412-385-4542. † National Energy Technology Laboratory. ‡ URS-Washington Division. § University of Pittsburgh. | Advanced Materials Corporation.

the location of light disordered gaseous guests can be difficult to determine for polycrystalline materials. This, in turn, makes correlations between structural transitions and guest loading difficult to determine. Further complications can also arise in the analysis of step-shaped or sigmoidal gas adsorption isotherms which may occur in rigid porous systems when guest-guest interactions become the dominant interaction above a threshold loading.44 Infrared spectroscopy has been proven to be a valuable experimental technique for investigating CO2 guest-host interactions in coals,45 organic polymers,46-49 and metal organic frameworks.5,7,50,51 In this report, we used in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy to investigate the gas adsorption mechanism in flexible porous coordination polymers. When conducted in situ with an IRactive gas in a high pressure cell, the technique allows the positions and shapes of the host adsorption bands and adsorbed gas spectra to be directly correlated with the gas step-shape adsorption isotherm. This information aids in elucidating guestinduced structure changes of these flexible hosts. The material chosen for study is the linear chain coordination polymer catena-bis(dibenzoylmethanato)-(4,4′-bipyridyl)nickel(II), “Ni-DBM-BPY”. A recent report has detailed the highly dynamic behavior of Ni-DBM-BPY in the presence of various solvents.34 The material was shown to adsorb a significant amount of methane and xenon.34 To our knowledge, a detailed study on the gas adsorption behavior of this material has yet to appear. We report herein the CO2 and CH4 isotherms for NiDBM-BPY. Isotherms for both gases show pronounced steps above the adsorption threshold pressure and an accompanying large desorption hysteresis. The reported gas adsorption/ desorption behavior is unusual for a one-dimensional coordination polymer and further demonstrates that extended open

10.1021/jp908202s  2010 American Chemical Society Published on Web 01/15/2010

Hysteretic Gas Adsorption in [Ni(DBM)2Bpy] structures are not always necessary to achieve attractive gas adsorption properties.28,34,52 Experimental Section Synthesis. All chemicals were purchased from Sigma-Aldrich and used as received. The Ni-DBM-BPY compound was prepared by reaction of anhydrous bis(dibenzoylmethanato)nickel(II) (Ni(DBM)2) with 4,4′-bipyridine in dry THF as previously described.34 Sample purity was verified by the determination of the weight percent of residual NiO by thermal gravimetric analysis (TGA) in dry air at 500 °C. Instrumentation. Carbon dioxide adsorption-desorption isotherms were collected on a pressure-composition isotherm measurement system (Advanced Materials Corporation) for pressures up to 20 atm over a temperature range of 0-40 °C. The instrument is designed on the basis of a conventional Sievert apparatus. Isotherms for CH4 at pressures up to 50 atm over a temperature range of -52 to 20 °C were collected on a similar instrument at Advanced Materials Corporation of Pittsburgh, PA. Prior to the measurements, samples (∼800 mg) were degassed under a vacuum at 85 °C overnight. TGA testing was performed on ∼7 mg samples using a Mettler STARe TGA/ DSC Thermogravimetric Analyzer under a dry air purge of 50 mL/min. The samples were ramped to 500 °C at a rate of 10 °C/min. Fourier transform infrared (FTIR) experiments were conducted using two attenuated total reflectance (ATR) cells from Spectra Tech. This assembly has been described previously.45 Two stainless steel cells were connected in tandem via 0.16 cm stainless steel tubing so that each cell experienced the same pressure. The cells can accommodate pressures up to 136 atm. A cylindrical zinc selenide (ZnSe) ATR crystal was placed inside of each cell. The length of the ATR crystal was 2.8 cm, the diameter of the ATR crystal was 0.6 cm, the incidence angle is 45°, and the number of reflections was 11. One cell contained a ZnSe ATR crystal with no sample present and the other cell contained a ZnSe ATR crystal coated with the sample. Teflon o-rings (size 010) were used to make a pressure seal between the stainless steel chambers and ATR crystal. The cells were placed on a linear translator inside of the FTIR spectrometer. The cells could then be moved so that infrared light could probe either the blank ATR crystal or sample ATR crystal. The cells were enclosed in a temperature jacket that allowed a recirculating bath fluid to flow through the external temperature jacket to either heat or cool the cells. Pressure experiments were performed by connecting the ATR cell assembly to a gas handling system consisting of a syringe pump (ISCO model 260D), a port for gas introduction, and a pressure transducer (OmegaDyne Inc. PX01K1-5KGV). The total volume of the system was estimated to be 1 mL. Carbon dioxide (99.998% supercritical grade) and ultrahigh purity nitrogen (UHP) were used as supplied from Butler gas. All ATR-FTIR data were collected with a single beam FTIR spectrometer (Thermo Electron Nexus 4700 FTIR ESP) equipped with a wide-band MCT detector. Unless otherwise noted, 500 scans were collected with an instrument resolution of 2 cm-1 over the spectral range extending from 4000 to 600 cm-1. Ni-DBM-BPY samples for IR analysis were conducted on the powders produced by the synthetic procedure described above after drying at 85 °C under a vacuum overnight to ensure removal of any residual THF guests. FTIR samples of Ni-DBMBPY were prepped by mixing the evacuated powder in methanol to form a slurry. The ATR crystal was then coated with a thin layer of the slurry (≈10 mg). Once the ATR crystal was coated

J. Phys. Chem. C, Vol. 114, No. 5, 2010 2185 with the sample, the cell was sealed and the sample was flushed with UHP nitrogen for at least 24 h and then stored under UHP nitrogen. Desorption of any methanol adsorbed during the sample preparation was verified by the disappearance of the respective IR adsorption bands at 2960, 2844, 1345, and 1033 cm-1.53 The sample was thermally equilibrated in the ATR cell by recirculating a bath containing a 50% ethylene glycol-50% water through the temperature jacket. Carbon dioxide adsorption on Ni-DBM-BPY was investigated at 3 and 30 °C. The sample was exposed to CO2 as a function of increasing pressure from 0 to 25 atm. Two spectra were recorded: one from the cell containing both CO2 and sample and one from the cell containing CO2. By calculating the difference between the two spectra, an absorption spectrum of CO2 adsorbed on the sample was obtained. This spectral subtraction procedure has been explained before.45 Carbon dioxide adsorption was monitored with ATR-FTIR by following the growth of the ν3 antisymmetric stretching mode (near 2333 cm-1) and ν2 bending mode (near 661 cm-1) of adsorbed CO2 versus time and pressure. While equilibrium was reached within minutes, CO2 adsorption was monitored for 1 h after introduction of CO2. Results and Discussion With the growing interest in the recent literature concerning gas adsorption in flexible porous coordination polymers, we chose to investigate Ni-DBM-BPY as a potential dynamic CO2 sorbent. The structure of Ni-DBM-BPY as described by Soldatov is shown in Figure 1.54 This neutral one-dimensional polymer was propagated by a bridging 4,4′-bipyridine ligand bound to the z-axes of the nickel complexes which were bischelated in a planar fashion by two anionic DBM ligands. The packing arrangement of the infinite chains with included N,Ndimethylformamide (dmf) guests is also shown in Figure 1. The dmf guests reside in the residual space created by the inefficient packing of the two polymer chains. The interesting host-guest chemistry of Ni-DBM-BPY and other M(DBM)2(L) compounds has been well-documented in a series of reports by Soldatov et al.34,55-59 The materials are highly flexible sorbents which adopt a range of structural phases in order to accommodate guest molecules of varying size, shape, and polarity. The versatility of M(DBM)2(Ln) compounds is the result of the conformational flexibility of the DBM ligand. As highlighted in Figure 2, the Ni(DMB)2(L) building block can adopt a range of conformations including the planar arrangement observed in Ni(DBM)2,57 a twisted conformation as seen in Ni(DBM)2(bpy),54 and the bowl-like arrangement found in the Ni2(DBM)4(4,4′-bpy)(pyridine)2 dimer.54 While the majority of the reports on the host-guest behavior of M(DBM)2(L) compounds have focused on cases with small solvent molecules as guests, the adsorption of CH4 and Xe has been reported.34 On the basis of the diverse host-guest behavior of Ni-DBMBPY, we chose to further investigate its gas adsorption properties. The CO2 adsorption-desorption versus pressure isotherms collected at 0 and 25 °C are shown in Figure 3. The data are plotted in relation to both gravimetric uptake and a normalized adsorption of CO2 per mol of Ni-DBM-BPY formula unit. Adsorption isotherms at both temperatures showed relatively low CO2 uptake below a certain threshold pressure (Pth). Above the threshold pressure, a rapid rise in uptake was seen followed by a gradual trend toward saturation. The coverage at the threshold condition appears consistent within experimental error. The subsequent desorption isotherms showed a marked hysteresis, with a wider hysteresis loop observed in the higher temperature isotherm.

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Culp et al.

Figure 1. The crystal structure of Ni-DBM-BPY as reported by Soldatov et al.54 Left, a view showing an isolated chain of Ni-DBM-BPY. Right, a view showing the packing arrangement of the chains with DMF guests located in the interchain voids. Key: C, gray; H, yellow; O, red; N, orange; Ni, green.

Figure 2. Three examples highlighting the conformational flexibility of the dibenzylmethanate (DBM) ligand. In the square planar complex Ni(DBM)2, the two DBM ligands are coplanar.34 In the DMF-solvated linear chain polymer [Ni(DBM)2(bpy)]n · 2(DMF), the two DBM ligands are twisted relative to one another by 16.5°.54 In the dimer complex, Ni2(DBM)4(4,4′-bipyridine)(pyridine)2, the DBM ligands bend toward one another in a bowl-like fashion.54 Key: C, gray; H, yellow; O, red; Ni, green.

The isotherm behavior for CO2 on Ni-DBM-BPY is suggestive of a dynamic host where a structural phase change is occurring at Pth. However, a recent report by Walton et al. showed how CO2-CO2 interactions within the pore structure of rigid frameworks can also produce sigmoidal isotherms with no corresponding structure change.44 In an effort to better understand the adsorption mechanism of Ni-DBM-BPY and whether the CO2 interactions observed by Walton et al. play a role in this system, low temperature methane adsorption isotherms were also measured. The results at -52 and -36 °C are shown in Figure 4. The adsorption of CH4 follows a similar trend as seen with CO2 with a low initial uptake followed by a rapid rise above Pth and a subsequent large desorption hysteresis. The coverage at Pth also appears consistent and comparable to the normalized coverage for CO2 at the threshold condition. The similarity in isotherm shape between CO2 and CH4 is supportive of a structural phase change occurring in Ni-DBM-BPY at the threshold pressure, since the type of guest-guest interactions that are known to occur with CO2 would not be expected to occur with CH4. Measurements of the adsorption isotherms for CH4 at higher temperatures showed a logarithmic increase in Pth as a function

of temperature (see the Supporting Information). For the isotherms measured at 0 and 20 °C, the threshold pressures were beyond the accessible range of the instrument. As a result, the adsorption-desorption isotherms measured at 20 °C were entirely below the threshold pressure and showed no step shape or hysteresis. The reversibility of the adsorption-desorption behavior below Pth and the consistent coverage at the threshold pressure permitted a thermodynamics analysis of Pth, and a plot of ln(Pth) vs 1/T could be fitted to a straight line. From the slope of the fit, an estimated enthalpy at the threshold condition (∆Hth) of 20 kJ/mol could be extracted using eq 1

∆Hth ) [d(ln(Pth))/d(1/T)]R

(1)

where T is the temperature and R is the gas constant (see the Supporting Information). In a similar fashion, analysis of the CO2 adsorption isotherms measured at several temperatures gave an enthalpy of 26 kJ/mol at the threshold condition (see the Supporting Information). It is interesting to compare the CO2 and CH4 uptakes with those previously reported for Xe and CH4.34 The previous report

Hysteretic Gas Adsorption in [Ni(DBM)2Bpy]

Figure 3. Adsorption (solid symbols)/desorption (open symbols) isotherms for CO2 on Ni-DBM-BPY at 0 °C (diamonds) and 25 °C (circles). The units of “mol/mol Ni” refer to the moles of gas adsorbed per mole of the Ni(DBM)2(Bpy) formula unit.

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Figure 5. ATR-FTIR spectra of Ni-DBM-BPY measured at 30 °C with no CO2 (bottom spectrum) and with 25 atm of CO2 (top spectrum). The indicated bands at 712, 694, and 683 cm-1 are assigned to NiDBM-BPY, and the indicated bands at 2332, 2320(sh), 661, and 645 cm-1 are assigned to adsorbed CO2.

Figure 4. Adsorption (solid symbols)/desorption (open symbols) isotherms for CH4 on Ni-DBM-BPY at -52 °C (diamonds) and -36 °C (circles). The units of “mol/mol Ni” refer to the moles of gas adsorbed per mole of the Ni(DBM)2(Bpy) formula unit.

Figure 6. ATR-FTIR spectra of Ni-DBM-BPY with adsorbed CO2 at 30 °C as a function of CO2 pressure. The shifts in the Ni-DBM-BPY spectra which occur through phase transition and associated isotherm step are highlighted at 712, 694, and 683 cm-1, and the CO2 bands are indicated at 2332, 2320(sh), 661, and 645 cm-1. Orange, blue, red, purple, green, and light blue spectra indicate CO2 pressure near 2, 11, 12, 13, 14, and 25 atm, respectively.

showed a type I isotherm for Xe with no noticeable inflection and a saturation loading of 3.5 mmol of Xe/g of Ni-DBM-BPY (2.3 mol of Xe/mol of Ni-DBM-BPY). A high pressure loading greater than 100 atm for CH4 was reported in the range 0.6-0.8 mol of CH4/mol of Ni-DBM-BPY, albeit under difficult experimental conditions with rather large experimental error.34 Our results show a similar capacity to Xe for CO2 of 1.8 mol/ mol of Ni-DBM-BPY at 0 °C and 18 atm, with the adsorption still showing a slight positive slope. The CH4 uptake at -52 °C and 50 atm is 1.7 mol of CH4/mol of Ni-DBM-BPY with the adsorption still showing a slight positive slope. The uptake for both gases at high pressure is close to the saturation loading of 2.0 mol of gas/mol of Ni-DBM-BPY expected when considering the host’s ability to adsorb two small solvent guests per Ni-DBM-BPY unit, as indicated in Figure 1.34,54 In the previous report, the adsorption of CH4 was shown to induce a slight structural change in Ni-DBM-BPY as evidenced by powder X-ray diffraction measurements, whereas the adsorption of Xe at 298 K gave a typical type I isotherm with no structural change.34 The step-shaped adsorption isotherms in Ni-DBM-BPY have attractive features for high pressure capture of CO2 and for gas

separation applications. It is therefore very desirable to further understand the highly dynamic adsorption behaviors for CO2 and CH4 in Ni-DBM-BPY and other structurally flexible sorbents. In this effort, gas adsorption in the Ni-DBM-BPY material was investigated by using in situ ATR-FTIR spectroscopy. First, infrared spectra were collected of Ni-DBM-BPY without CO2 present (Figure 5, bottom spectrum). Upon exposure of Ni-DBM-BPY to CO2 (25 atm) (Figure 5, top spectrum), positive absorption bands at 2332, 2320, 712, 683, 661, and 645 cm-1 appeared in the spectrum (Figure 5). In addition, the absorption band at 694 cm-1 decreased in intensity. All other absorption bands remained the same or were shifted by less than 0.5 cm-1. After desorption of CO2 in flowing nitrogen, the spectrum of Ni-DBM-BPY in the presence of CO2 converted back to the original spectrum of Ni-DBM-BPY (Figure 5, bottom spectrum), thus indicating that the interaction between CO2 and the Ni-DBM-BPY was reversible. Next, the interaction of CO2 with Ni-DBM-BPY was carefully monitored as a function of increasing pressure by using ATRFTIR spectroscopy (Figure 6). The sample was exposed to a known pressure of CO2 and then sealed off from the CO2 source. Spectra were then recorded until equilibrium was reached, i.e.,

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until CO2 and Ni-DBM-BPY absorption bands no longer changed with time. Equilibrium was reached within minutes, but adsorption was monitored for an additional hour at each pressure step. This procedure was repeated by increasing the CO2 pressure in 1 atm increments until 25 atm was reached. The desorption pathway was also mapped from 25 to 1 atm when the CO2 pressure was decreased. Adsorption and desorption measurements were collected at both 3 and 30 °C. As Ni-DBM-BPY was exposed to CO2 as a function of pressure at 30 °C, a weak positive absorption band at 2333 cm-1 with a shoulder at 2320 cm-1 was observed in the spectra (Figure 6). These absorption bands grew slightly in intensity as the CO2 pressure was increased. When 12 atm of CO2 pressure was reached, a large jump in intensity of these absorption bands was observed and the 2333 cm-1 band shifted slightly to 2332 cm-1 (Figure 6, red spectrum). In addition, positive absorption bands at 712, 683, 661, and 645 cm-1 and a decrease in the absorption band at 694 cm-1 were first observed. The intensity of these bands continued to change significantly as the CO2 pressure increased to 14 atm (Figure 5, green spectrum). After 14 atm, subsequent growth began to taper off. Infrared data collected at 3 °C for CO2 exposure to Ni-DBM-BPY as a function of pressure gave similar results as the 30 °C data with the exception that the large jump in intensity was observed at a lower pressure (∼5 atm) instead of 12 atm. As discussed in detail below, this large jump in CO2 adsorption at both 30 and 3 °C indicated that the Ni-DBM-BPY structure has been altered to a structure with a greater CO2 capacity. The absorption bands at 2332, 2320, 661, and 645 cm-1 have been observed before and are indicative of physical adsorbed CO2.45,60-65 Gaseous CO2 is a linear molecule with two infraredactive absorption bands at 2349 cm-1 (ν3 antisymmetric stretching mode) and 667 cm-1 (ν2 bending mode).66 The ν2 CO2 bending mode is doubly degenerate. Upon adsorption of gaseous CO2, the degeneracy of the ν2 bending mode can be lost because of a change in symmetry. The ν2 band can split into two features: an in-plane bend (lower frequency) and an out-of-plane bend (higher frequency). When CO2 physically interacts with a surface, the resulting absorption band frequencies shift slightly from the gas phase values.60 In this study, the observed absorption band at 2332 cm-1 corresponded to the ν3 antisymmetric CO2 stretching mode of physical adsorbed CO2 and the absorption bands 661 and 645 cm-1 corresponded to the ν2 CO2 bending mode. The shoulder at 2320 cm-1 has been observed before and was assigned to the ν3 + ν2 - ν2 combination band.46,47,51,67-73 If one of the oxygen atoms of the CO2 acts as a Lewis base, the ν3 antisymmetric CO2 stretching mode will shift to frequencies higher than its gaseous counterpart, usually between 2379 and 2340 cm-1, and little to no splitting is observed for the ν2 CO2 bending mode.74-77 Dietzel et al. reported CO2 Lewis base interactions with the nickel cation of a Ni2(dhtp) metal organic framework.5 If the carbon atom of the CO2 acts as a Lewis acid, the ν3 antisymmetric CO2 stretching mode will shift to lower frequencies than its gaseous counterpart, usually between 2339 and 2335 cm-1, and moderate ν2 splitting is observed, usually between 7 and 9 cm-1.46,48,49,71,78-82 Serre et al. found Lewis acid interactions between CO2 and oxygen atoms of the MIL53 framework hydroxyl groups.50,51 When electrostatic interactions are dominant between CO2 and aromatic rings with no Lewis base sites, the ν3 antisymmetric CO2 stretching mode also shifts to lower wavenumbers than its gaseous counterpart, usually between 2335 and 2332 cm-1, and the ν2 split broadens and becomes less defined, usually between 14 and 17 cm-1.46,47,80

Culp et al. When both Lewis acid-Lewis base and electrostatic interactions are available as with CO2 adsorption on poly(ethylene terephthalte) (PET), the spectral data are more difficult to interpret.46,47,51 In this study, Ni-DBM-BPY presents multiple sites for CO2 interaction: aromatic rings in the DBM and BPY ligands, oxygen atoms that link Ni and DBM, and Ni atoms. It is unlikely that CO2 interacts with the Ni atoms because the Ni is coordinatively saturated. The ν3 CO2 antisymmetric stretching frequency that we observed is red-shifted by 17 cm-1 relative to its gaseous counterpart, and the ν2 CO2 bending mode observed appeared as two distinct absorption bands at 661 and 645 cm-1. These band shifts suggested that either the carbon atom of the CO2 acted as a Lewis acid or that aromatic ring electrostatic interactions were primarily responsible for CO2 interactions with Ni-DBM-BPY. If more than one type of adsorption mechanism were available for CO2, distinct absorption bands corresponding to each type of adsorption site would be expected.65,68 The fact that only one distinct symmetrical band ν3 antisymmetric stretching absorption band and two distinct ν2 bending mode absorption bands suggested that there was one type of adsorption site for the CO2 with Ni-DBM-BPY under this pressure regime.47 The ν3 band at 2332 cm-1 and broad ν2 split (17 cm-1) that we observed here suggested specific interactions between CO2 and aromatic rings of Ni-DBM-BPY were responsible for the sorption phenomenon. This assignment coincides with X-ray diffraction data for the Ni-DBM-BPY structure with guest dmf where dmf was shown to primarily interact with the DBM ligands.54 As discussed above, several changes occurred in the infrared spectra when Ni-DBM-BPY was exposed to 12 atm of CO2 pressure at 30 °C. The absorption band at 2332 cm-1 greatly increased in intensity and new changes to absorption bands at 712, 683, 661, 645, and 694 cm-1 were observed. The bands at 2332, 2320, 661, and 645 cm-1 have been assigned to the ν3 antisymmetric stretching mode and the ν2 bending mode of physically adsorbed CO2. However, the bands at 712, 683, and 694 cm-1 also changed in intensity as Ni-DBM-BPY was exposed to CO2 pressure. In order to determine whether these three bands marked different interactions between CO2 and NiDBM-BPY or signified a change in the vibrational spectra and structure of the Ni-DBM-BPY crystal, isotopically labeled CO2 experiments were conducted (see the Supporting Information). Labeled CO2 (13C18O2) gas was added to the Ni-DBM-BPY at 10 atm at 30 °C, a pressure and temperature before the NiDBM-BPY structure has been altered. The cell was then cooled to 3 °C, a temperature and pressure where the Ni-DBM-BPY structure was altered and absorption bands at 2332, 2320, 712, 683, 661, and 645 cm-1 were known to change. The labeled spectra were then compared to the unlabeled CO2 (see the Supporting Information). Both the labeled gaseous and adsorbed CO2 absorption bands shifted to lower wavenumbers as expected. The bands at 712, 683, and 694 cm-1 remained unchanged. This indicates that these bands were not due to adsorption of CO2 but could be directly attributed to a change in the Ni-DBM-BPY structure in the presence of CO2. The absorption bands at 712, 694, and 683 cm-1 were examined more carefully by collecting infrared spectra of the two starting materials of Ni-DBM-BPY without CO2 present. Infrared spectra of the precursor materials 4,4′-bipyridine (BPY) and Ni(DBM)2(H2O)2 at 25 °C were compared with Ni-DBMBPY without CO2 and with Ni-DBM-BPY in the presence of CO2 at 25 atm and 30 °C (see the Supporting Information). The bands at 712 and 683 cm-1 are present in the spectra of both Ni(DBM)2(H2O)2 and Ni-DBM-BPY but not in the spectrum

Hysteretic Gas Adsorption in [Ni(DBM)2Bpy] of neat BPY. Thus, it is likely that the DBM ligands and not the BPY ligand were altered upon CO2 adsorption. While it is difficult to definitively assign the origin of bands in this fingerprint region of the infrared, aromatic ring in-plane and out-of-plane bending and ring puckering are located at frequencies between 690 and 900 cm-1. By correlating the changes in the host Ni-DBM-BPY spectra which occur in the region typically assigned to aromatic ring bending and puckering and with the width of the adsorbed CO2 ν2 band split, the mechanism of the Ni-DBM-BPY structural phase transition can be determined as arising from a reorganization of the DBM ligand conformation in order to allow an enhanced ring interaction with the adsorbed CO2. Again, this is consistent with X-ray diffraction data of the guest dmf with Ni-DBM-BPY where dmf was shown to primarily interact with the DBM ligands.54 In order to map out the Ni-DBM-BPY structure change and CO2 adsorption further, adsorption isotherms were constructed using the infrared data. The isotherm shape can be derived by plotting the integrated peak area versus pressure. The peak areas of the ν3 CO2 antisymmetric stretching mode at 2333 cm-1, the ν2 CO2 bending mode at 645 cm-1, and the Ni-DBM-absorption band at 712 cm-1 were measured as a function of CO2 pressure (see Figure 6). These peak areas were plotted as a function of pressure and are shown in Figure 7. Peak areas at 30 °C are shown as solid circles for adsorption and open circles for desorption. The blue, red, purple, and green closed circles correlate to the blue, red, purple, and green spectra shown in Figure 6. Peak areas at 3 °C are shown as solid diamonds for adsorption and open diamonds for desorption. For both 3 and 30 °C isotherms, the ν3 CO2 peak area increased gradually until a threshold pressure was reached. Then, the Ni-DBM-BPY absorption band at 712 cm-1 and ν2 CO2 bands at 661 and 645 cm-1 appeared. At this point, the DBM ligands underwent a conformational change and adsorption greatly increased. This CO2 step increase in adsorption along with simultaneous changes in the Ni-DBM-BPY spectral absorption bands signifies an enhanced interaction with the host following the structural transition. Desorption was also measured. A large hysteresis loop is seen at both temperatures. The CO2 adsorption/desorption mechanism is shown by in situ ATR-FTIR measurements to result from a conformation change of the DBM ligands which interrupts the interchain packing and opens additional pore volume for the subsequent adsorption of guests. Attempts were made to examine the Ni-DBM-BPY structure transition with CH4 using ATR-FTIR spectroscopy. Ni-DBMBPY was exposed to CH4 at 22 atm, the highest pressure that could be attained leak free, and at 1 °C, the lowest temperature feasible using this ATR cell. As expected from the CH4 isotherm data collected at 0 and 20 °C (see the Supporting Information), no significant CH4 adsorption or structural changes were detected at this temperature and pressure. However, taking into account the resemblance in isotherm shape and similar hysteretic behavior in the adsorption-desorption isotherms for CO2 and CH4 at low temperature, it is probable that the adsorption mechanism for CH4 is similar to that of CO2 and involves a host structural transition above the threshold pressure. An investigation into the adsorption behavior of other gases is currently underway in an effort to better understand the thermodynamic requirements for the structural transitions in dynamic host materials. Conclusions The linear chain coordination polymer Ni-DBM-BPY is a highly dynamic sorbent which shows unusual adsorption/

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Figure 7. Adsorption (solid symbols) and desorption (open symbols) isotherms for CO2 on Ni-DBM-BPY as determined from the integrated area of ATR-FTIR spectra at 30 °C (circles) and 3 °C (diamonds), respectively. Color coded solid circles correlate with the color coded spectra shown in Figure 6.

desorption cycles with CO2 and CH4. Both gases show low gradual uptakes below their respective threshold pressures, above which a rapid rise in uptake ensues. Subsequent desorption isotherms from the loaded state for both gases display hysteretic behaviors typically associated with flexible coordination polymers. A detailed correlation can be established between the structural phase transition in the material and the sorption of CO2 by monitoring in situ the adsorption of the gas via ATRFTIR. A spectral transition in the host spectrum occurs simultaneously with a sharp increase in both the intensity and ν2 band splitting of the adsorbed CO2 species. By associating the changes in the host spectra which occur in the region typically assigned to aromatic ring bending and puckering modes with the width of the adsorbed CO2 ν2 band split, the mechanism of the Ni-DBM-BPY structural phase transition can be determined as arising from a reorganization of the DBM ligand conformation in order to allow an enhanced ring interaction with the adsorbed CO2. While the structural phase change pertaining

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