Temperature Induced Structural Transformations and Gas Adsorption

Article pubs.acs.org/JPCA

Temperature Induced Structural Transformations and Gas Adsorption in the Zeolitic Imidazolate Framework ZIF-8: A Raman Study Gayatri Kumari,† Kolleboyina Jayaramulu,† Tapas Kumar Maji,†,‡ and Chandrabhas Narayana*,† †

Chemistry and Physics of Materials Unit and ‡New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore, India 560064 S Supporting Information *

ABSTRACT: Here we have used Raman spectroscopy to investigate molecular level changes in the zeolitic imidazolate framework ZIF-8 (a prototypical zeolitelike porous metal organic framework) as a function of temperature. Temperature dependent Raman spectra suggest that at low temperature the softening of the C−H stretching frequencies is due to the decrease in steric hindrance between the methyl groups of methyl imidazole. The larger separation between the methyl groups opens the window for increased nitrogen and methane uptake at temperatures below 153 K. The appearance of Raman bands at 2323 cm−1 and 2904 cm−1 at or below 153 K in ZIF-8 are characteristic signatures of the adsorbed nitrogen and methane gases respectively. Nanoscale ZIF-8 uptakes more molecules than bulk ZIF-8, and as a result we could provide evidence for encaged CO2 at 203 K yielding its Raman mode at 1379 cm−1.

1. INTRODUCTION Zeolitic imidazolate frameworks (ZIFs) have attracted great interest in various applications like purification of gases, catalysis, sensing, and so forth because of their high surface area with large micro pore volume,1−5 and significant chemical inertness toward alkaline water and various organic solvents.6 ZIFs are structurally analogous to zeolites. The metal atoms in ZIFs are connected via the nitrogen atom of imidazolate or their derivatives giving rise to a rigid framework with characteristic well-defined pores and window sizes. The gas adsorption profiles as a result of sequential filling of large pores in the framework has been speculated to be due to various reasons such as polarizability, gas-induced flexibility or reorganization of structure toward various gas shape and size. The pore size tunability can be achieved by changing the functionality on the imidazole linker as demonstrated by Banerjee et al.7 There are recently numerous literature reports on diverse synthesis methods for ZIFs and nano ZIFs.8−12 Decreasing the crystallite size as in the case of nano ZIFs leads to increased surface area and greater accessibility to pores making nanoscale metal-organic frameworks (MOFs) very promising materials for gas adsorption and catalysis. Zn(II) metal ions bridged by a methyl-imidazolate linker is the basic constitutive unit of ZIF-8. It has a sodalite structure and crystallizes in a cubic lattice with space group I43̅m. Among all the ZIFs known so far, prototypical ZIF-8 is one of the most studied materials attributed to its high stability and flexibility. ZIF-8 can sustain pressure up to 1.6 GPa as revealed by pressure dependent IR studies.13 The narrow window sizes of 3.4 Å interconnecting the larger cavities of size 11.4 Å makes ZIF-8 a prospective material for gas storage and separation.14 Recently, ZIF-8 has been employed for separation of lower © XXXX American Chemical Society

homologues of alkanes, linear and branched alkanes, and other hydrocarbons.1,4 Adsorption of small gases like hydrogen, nitrogen, methane, carbon dioxide by ZIF-8 is quite wellknown.14 It is interesting to note that ZIF-8 can also uptake gases with kinetic diameter larger than its window size, like nitrogen (3.6 Å) and methane (3.8 Å).14−16 This unusual gas adsorption behavior has been previously speculated to be due to the swing effect of the imidazolate moiety resulting in an increase of the channel size.17,18 We have employed Raman spectroscopy to get a microscopic picture and thereby deduce the changes happening at the molecular level in ZIF-8. Raman spectroscopy is powerful technique that gives the information about the vibrational modes associated with the molecule or material. Any subtle changes in the structure brought about by host−guest interactions or induced by change in temperature or pressure can be probed by this technique and is reflected as a change in the modes.19 Recently, Raman spectroscopy has been used to characterize MOFs and the interaction of MOFs with different gas molecules.19−22 But there are very few literature reports on Raman study or vibrational spectroscopic studies to understand gas storage properties in ZIFs. X-ray and NMR have been used extensively to study ZIF-8,23 but these techniques do not provide adequate information about the molecular/atomic interactions in the crystal. The limitation of the X-ray diffraction method lies in its inability to determine hydrogen atom position accurately while NMR is limited by its inability to capture movements in molecular moieties or their dynamics. Around this time, we also Received: August 4, 2013 Revised: October 5, 2013

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dx.doi.org/10.1021/jp407792a | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Figure 1. (A) Three dimensional view of ZIF-8. H atoms have been removed for clarity. C, gray; N, royal blue; Zn, teal. Adsorption−desorption profiles of (B) N2 at 77 K, (C) CO2 at 195 K, and (D) CH4 at 195 K. The red curve represents mZIF-8, and the blue curve represents nZIF-8 in (B), (C), and (D).

The resultant mZIF-8 was characterized through different techniques. Powder X-ray diffraction (PXRD) pattern was recorded on a Bruker D8 Discover instrument using Cu−Kα radiation. The morphology of ZIF-8 were examined with a transmission electron microscope (TEM; JEOL JEM-3010 with an accelerating voltage of 300 KV). The gases (nitrogen, methane, carbon dioxide, and argon) for adsorption studies were obtained from Chemixgases, India, and were 99.99% pure. Adsorption of N2 (77 K), CO2 (195 K), and CH4 (195 K) was carried out using QUANTACHROME AUTOSORB-1C analyzer. Raman spectra were recorded using a custom built Raman spectrometer with a HeNe red laser (632.8 nm) and 1800 lines/mm grating.25 Laser power was 8 mW at the sample. Temperature dependent Raman studies were done using a Linkam THMS 600 heating−cooling stage. Typical accumulation time was 30 s for all the Raman studies. For gas adsorption Raman studies, activated powder ZIF-8 was mounted on the Linkam stage, and heated to 80 °C and then cooled to room temperature (RT) after which the gases were purged. Temperature was monitored through the temperature controller attached to the Linkam stage. The gas pressure was regulated by the regulator fixed at the inlet and was mostly kept constant around 1 atm. The Raman spectra obtained were smoothened using 5 point FFT, and baseline correction was done to remove the background.

came across a report by Zhang et al. on the crystal structure of N2 inside ZIF-8 and N2 gas adsorption as a function of temperature.16 In our studies in this paper, we compare the gas adsorption profiles of N2, CO2, and CH4 in micrometer ZIF-8 (mZIF-8) and nanoscale ZIF-8 (nZIF-8). Second, we investigate for the first time temperature induced molecular level changes in mZIF-8 through Raman spectroscopy and thereby also demonstrate the plausible reason for adsorption of bigger gas molecules (N2, CO2, CH4) through narrow channels.

2. EXPERIMENTAL DETAILS All the reagents and solvents employed were commercially available and used as supplied without further purification. Zn(NO3)2·4H2O and 2-methyl imidazole were purchased from Sigma-Aldrich. Crystals of mZIF-8 were prepared according to a previously reported procedure.6 A mixture of Zn(NO3)2· 4H2O (0.210 g, 0.8 mmol), 2-methyl imidazole (0.060 g, 0.7 mmol), and 18 mL N,N-dimethylformamide (DMF) was put in a 23 mL Teflon bomb, and the whole reaction mixture was stirred for 1 h. The Teflon bomb was then subsequently placed in a steel autoclave and heated at 140 °C under autogenous pressure for 72 h and then cooled to ambient temperature. Light yellow block shaped crystals of mZIF-8 were isolated by decanting the supernatant liquid and washed thoroughly several times with DMF. (Yield 69%). Anal. Calcd. for C11H23ZnN5O4: C, 35.75; H, 6.58; N, 19.74%. Found: C, 36.09; H, 7; N, 19.49%. Nanoscale ZIF-8 was prepared according to the literature reported by Wiebcke et al.24 In a typical synthesis of nZIF-8, 366 mg of Zn(NO3)2·6H2O and 811 mg of 2-methylimidazole were added to two separate beakers containing 25 mL methanol each. Both clear solutions were mixed under stirring conditions. Stirring was stopped once the two components were mixed properly. After 24 h, a gel like solid was recovered by centrifugation and washed with methanol three times.

3. RESULTS AND DISCUSSION The structure of ZIF-8 is shown in Figure 1A. The PXRD patterns of mZIF-8 and nanoscale hexagonal morphology nZIF8 is shown in Supporting Information, Figure S1. The textural properties like surface area and pore volume have been characterized through nitrogen adsorption−desorption isotherms. Figure 1B shows the nitrogen adsorption isotherm at 77 K (1 atm) for nZIF-8 and mZIF-8 with typical type-I behavior for micro porous materials. The Brunauer−Emmett− B



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Teller (BET) surface area was found to be 520 m2/g with pore volume of 0.31 cm3/g for mZIF-8, and 1120 m2/g with significant pore volume of 0.956 cm3/g for nZIF-8. Optical image of mZIF-8 and TEM image of nZIF-8 has been shown in the Supporting Information, Figures S2 and S3, respectively. Figure 1 shows the adsorption profiles of N2, CO2, and CH4 gases in mZIF-8 and nZIF-8. In all the adsorption profiles, it can be seen that the gas uptake by nZIF-8 is more than that of mZIF-8, which has also been observed in other nanoscale MOFs. Adsorption of guest molecules can occur either on the surface or inside the pores. mZIF-8 has less surface area and adsorption is mainly driven by diffusion into the pores. In the nZIF-8, the surface area is more and pores are easily accessible; hence the diffusion barrier is smaller leading to higher gas uptake. Further, a cylindrical channel allows passage of linear N2 and CO2 molecules while it blocks tetrahedral methane resulting in lesser uptake of methane molecules. In the following Raman studies, we will show that the uptake increases as temperature is lowered making it feasible to capture the signal from encaged gas molecules. 3.1. Raman Study of Nitrogen Adsorption in ZIF-8. Figure 2 shows the RT Raman spectra of mZIF-8 which is

Table 1. Raman Band Assignments of mZIF-826−28 frequency (cm−1)

band assignmenta

168 273 686 755 833 950 1021 1146 1180 1187 1311 1384 1458 1499 1508 2915 2931 3110 3131

ν Zn−N ν Zn−N Imdz ring puckering, H oop bend CN oop bend, δ N−H C−H oop bend (C4−C5) C−H oop bend (C2−H) C−H oop bend ν C5−N ν C−N + N−H wag ν C−N ring expansion + N−H wag δ CH3 C−H wag C2N3 + C4N3 + ν C5N1 + N-Hwag ν C4−C5 νsym C−H (methyl) νasym C−H (methyl) ν C−H (ar) ν C−H (ar)

ν: stretching, δ: bending, oop: out of plane, ar: aromatic, sym: symmetric, asym: aymmetric. a

explained in the following discussion (This phase change is not a structural transition but a change in molecular orientation resulting into pore volume expansion!). It is known that the imidazole ring in mZIF-8 shows flexibility and a swing effect with temperature.17,18 Temperature dependent Raman spectra showed that the imidazole ring puckering mode starts softening around 200 K and hardens beyond 150 K as shown in Figure 3a. Interestingly, the

Figure 2. RT Raman spectrum of mZIF-8. Inset shows the atom nomenclature used for the imidazole ring, where N is shown in blue, Zn in purple, C in gray, and H in white.

dominated by intense bands corresponding to methyl group and imidazole ring vibrations. Band assignments have been summarized in the Table 1 based on the refs 26−28. Very strong bands were observed at 168 cm−1, 686 cm−1, 1146 cm−1, and 1458 cm−1 corresponding to Zn−N stretching, imidazole ring puckering, C5−N stretching and methyl bending, respectively. Previous studies assert that N2 inside mZIF-8 leads to a phase change in mZIF-8 at 113 K as observed by the change in unit cell parameters obtained by X-ray crystallography.16 To get an insight into the phase change resulting because of the change in temperature, we have recorded the temperature dependent Raman spectra of mZIF-8 in a nitrogen atmosphere. As temperature is lowered from RT (293 K) to 83 K, most of the Raman modes harden suggesting a decrease in volume. There is no anomaly in full width at half-maximum (fwhm) in this temperature range suggesting that the lattice retains its crystalline phase throughout the temperature range. Up to 83 K, all the Raman modes of mZIF-8 remain intact implying that the phase change seen in X-ray studies is due to volume change and swinging of the methyl imidazole ring as

Figure 3. Temperature dependence of (a) Imidazole ring puckering and (b) Zn−N stretching in mZIF-8. Red dotted line is guide to the eye.

strongest mode of Zn−N stretching frequency also exhibited a similar trend as a function of the temperature (see Figure 3b) but with a relatively small frequency shift of 2 cm−1 only. This behavior could be explained as follows. As temperature is decreased, mZIF-8 tries to achieve a high volume fraction compact form, which would result in a decrease in bond lengths leading to hardening of frequency. Beyond a certain temperature, ZIF-8 would find it difficult to further compress because of the steric hindrance between the methyl groups on imidazole rings facing each other in pairs inside the window. This would force the methyl-imidazoles to swing away from each other thus C



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opening the channel enabling increased gas uptake, and will relax the strained Zn−N and imidazole resulting in softening of their modes. On further decreasing the temperature beyond 150 K, we observed hardening of these modes due to further compression of mZIF-8 in the presence of guest molecules. Another implication of ring-opening is that the distance between the two methyl groups (C−H·····H−C) facing inside increases thus reducing the steric hindrance leading to the softening of the C−H stretching frequencies from 2931 cm−1 at RT to 2924 cm−1 at 83 K (Figure 4a and Supporting Figure 5. (a) Raman spectra showing adsorbed nitrogen peak at 2323 cm−1 at 123 K. Red line is the Lorentzian fit to the peak. (b) N2 peak intensity as a function of temperature. Red dotted line is the guide to eye.

behavior of adsorbed N2 on ZIF-8, but has not discussed any molecular level changes from Raman spectroscopic studies.16 3.2. Raman Study of Methane Adsorption in ZIF-8. We have carried out Raman investigations of methane adsorption in ZIF-8 to see if we observe a similar trend as in the case of N2. This is the first report of a Raman study of methane adsorption by ZIF-8. There are other works on Raman investigation of methane adsorption on MOFs which include a report by Hamon et al. who have shown coadsorption of CO2/CH4 mixture in MIL-53(Cr) MOF and Siberio-Peŕez et al. who have demonstrated CH4 and N2 adsorption on iso-reticular MOFs.29,30 In-situ Raman studies of methane adsorption on ZIF-8 show the appearance of a new peak at 2904 cm−1 which is assigned to the adsorbed methane stretching mode (as shown in Figure 6a). As observed in the case of N2, methane’s strongest peak does not appear at the RT whereas starts appearing at around 173 K (see Figure 6b and Supporting Information, Figure S5). Since CH4 is a better Raman scatterer than N2, we do see its existence at 173 K. To confirm that the new peak corresponds to the adsorbed methane and is not due to any kind of methyl− methyl interaction at low temperatures, the above experiment was carried out in an inert argon gas atmosphere. In argon gas, low temperature Raman spectra does not show any new peak in the C−H stretching region of the methyl group (see Figure 6a, red curve) asserting the fact that the 2904 cm−1 peak is indeed due to the C−H stretching of adsorbed methane in ZIF-8. The softening in the C−H mode in comparison to that of free CH4 suggests a weakened C−H bond because of its interaction with mZIF-8 framework. The methane peak intensity decreased as temperature was increased from 133 K to RT indicating a

Figure 4. Temperature dependence of (a) C−H stretching and (b) C−H bending modes of the methyl group in mZIF-8. Red dotted line is guide to the eye.

Information, Figure S4a). Usually softening in stretching frequency is accompanied by hardening in bending mode of the group, and we too observed this trend in the C−H bending frequency as shown in Figure 4b (also Supporting Information, Figure S4b). The temperature dependent increased uptake of gas molecules by ZIF 8 described above is depicted in Scheme 1. The temperature dependent Raman spectra of mZIF-8 in nitrogen environment showed the appearance of a sharp peak at 2323 cm−1 (Figure 5a) at 123 K indicating confinement of nitrogen gas in the pores. There were no changes in the Raman spectra of mZIF-8 suggesting physisorption. Lower N−N stretching frequency of 2323 cm−1 (from 2331 cm−1 in molecular nitrogen gas) indicates bond strength decrease because of its interaction with the mZIF-8 framework. Gradual increase in Raman peak intensity of adsorbed N2 with decrease in temperature (as shown in Figure 5b) indicates sequential filling of N2 in the pores. A recent report demonstrates similar Scheme 1

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Figure 6. (a) Raman spectra of adsorbed CH4 in mZIF-8 at 153 K (CH4, black curve), and Raman spectra of ZIF-8 in argon atmosphere (Ar, red curve) at 153 K. The peak at 2904 cm−1 marked with a star corresponds to that of adsorbed methane. (b) Methane peak intensity as a function of temperature. Red dotted line is guide to the eye.

Figure 7. Fermi resonance (ν− and ν+) modes of adsorbed CO2 in nZIF-8 at 203 K (red curve). Absence of CO2 modes in mZIF-8 at 203 K (black curve). Inset shows the deconvoluted peaks at 1379 cm−1 (CO2) and 1383 cm−1 (δ CH3). The arrow pointing at the red curve (at 1383 cm−1) shows the position of methyl bending mode.

sequential release of methane guest molecules from the ZIF-8 host pores (see Figure 6b). Again, on methane adsorption there is no apparent change in the Raman spectra of ZIF-8 indicating physisorption of the guest molecule. The most intriguing feature is that no guest molecular peak appears at RT while they emerge at low temperatures. There are two possibilities that could explain the above observation. First, at low temperatures, more gas molecules are captured inside the pore than those at RT because of reduced kinetic energy enabling closer packing. Second, at lower temperatures ring-opening through the swinging of methyl-imidazole ring would allow more guest molecules into the pore. It is obvious that both reasons play a significant role in the present scenario, but looking at the Raman spectra we believe that below 150 K swinging of the methyl-imidazole ring is the prime reason for increased adsorption of the gas. Previous neutron diffraction experiments31 and simulation studies17 have predicted two interaction sites in ZIF-8 for methane adsorption at atmospheric pressure. As reported previously, site I refers to the position above C4C5 bond in the plane perpendicular to that of the imidazole ring and is known as “IM” site whereas site II refers to the center of the window formed by the 6 ZnN4 tetrahedron with three methyl groups pointing inside.17 Further loading happens at site III present at the center of the “nanocage” (term coined by previous authors) formed by site I and site II.31,17 Earlier reports have suggested that higher loading of methane molecules at low temperature gives rise to a “ZIF-8HP” structure accompanied by the change in pore shape.17 Site IV is localized in the middle of the 4-ring pore at the center of the cube face as reported previously.17 From the intensity versus temperature plot shown in Figure 6b, it is understood that the methane gas loading occurs sequentially as the intensity does not remain constant but increases exponentially with decreasing temperature. 3.3. Raman Study of Carbon Dioxide Adsorption in ZIF-8. Raman studies of CO2 gas adsorption in mZIF-8 were also conducted but surprisingly, there is no Raman signature of adsorbed CO2 gas in mZIF-8 at RT or at low temperatures (Figure 7, black curve). It should be noted that in our previous studies, we could show Raman signatures of adsorbed CO2 in Zn(Pyz)SiF6 at ambient temperature and pressure.19 Confined CO2 shows two peaks around 1276 cm−1 and 1380 cm−1, but no such signature was observed in the present study with mZIF-8.19 Adsorption studies showed that the nanoscale MOF

shows higher gas uptake than the bulk samples (Figure 1D). So we conducted the Raman measurements of nZIF-8 in a CO2 environment at RT and 203 K (experiments were done above 195 K to avoid direct condensation of CO2), and plotted in Figure 7 (red curve and inset figure). The low frequency Fermi resonance mode (ν−) of CO2 is clearly observed at the frequencies 1270 and 1276 cm−1. This splitting in ν− CO2 could be possibly due to different interaction energies at the surface of nZIF-8 and in the pores. Further, to observe the ν+ mode of CO2, the mode around 1380 cm−1 was deconvoluted as it coincides with the methyl bending modes. On deconvolution, the ν+ CO2 mode was observed at 1379 cm−1. The downshifted Fermi resonance modes are indicative of adsorption of CO2 in nZIF-8.

4. CONCLUSIONS In summary, we have examined molecular level changes in ZIF8 as a function of temperature in the presence of guest molecules through Raman spectroscopy and subsequently addressed the issue of adsorption of bigger gas molecules through narrow channels. ZIF-8 framework flexibility is due to the swinging of the methyl-imidazole ring and weakening of the Zn−N bond which is reflected as softening in its Raman stretching frequency. We have shown that beyond 150 K swinging of methyl-imidazole leads to opening of the channel which favors nitrogen and methane gas uptake at low temperatures as was also shown by the appearance of their Raman modes at 2323 cm−1 and 2904 cm−1 respectively. Though carbon dioxide has similar kinetic diameter (as nitrogen and methane), lower uptake is a result of non accessibility of larger pores at 195 K in mZIF-8. Appearance of adsorbed CO2 modes at 1379 cm−1 and 1270 cm−1 in nZIF-8 can be attributed to the larger adsorption of CO2 molecules because of larger surface area of nZIF-8 and readily accessible pores.

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ASSOCIATED CONTENT

S Supporting Information *

Details of XRD pattern of ZIF-8, optical images, TEM images, and Raman spectra of ZIF-8 in different experimental E



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conditions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91 80 22082810. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Arpan Hazra for BET adsorption measurements. G.K. thanks Balasubramanian Sundaram and Srinu Bhadram for useful discussions. K.J. thanks UGC (SRF) Government of India for a fellowship.

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