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Structural Stability of Metal Organic Framework MOF-177 Dipendu Saha and Shuguang Deng* Department of Chemical Engineering, New Mexico State University, P.O. Box 30001, MSC 3805, Las Cruces, New Mexico 88003, U.S.A.
ABSTRACT Metal organic framework MOF-177 is one of the most promising crystalline porous adsorbents for hydrogen adsorption. The effects of oxygen and water on the structural stability of MOF-177 are investigated by X-ray diffraction (XRD) and thermogravimetric analysis (TGA). A MOF-177 sample is exposed to ambient air for 5 weeks and monitored for its structural changes by XRD. The crystal structure of MOF-177 gradually changes from hexagonal to orthogonal, and then to monoclinic in the 5-week period. The crystal structure of MOF-177 is completely destroyed after it is immersed in water. The weight loss of MOF-177 is negligible at temperatures below 330 °C in the presence of oxygen. However, MOF177 totally converts to zinc oxide at 420 °C. SECTION Nanoparticles and Nanostructures
with XRD. Another fresh MOF-177 sample was analyzed with thermogravimetric analysis (TGA) under an oxygen flow and evacuated conditions. The heat-treated samples in TGA analysis were then characterized for their phase structure with XRD. Figure 1a shows the typical XRD pattern of a MOF-177 sample after an extended long period of evacuation of guest molecules (chloroform) from the as-synthesized sample. The main peaks at 4.7°, 6.2°, 10.1°, and 13.5° are well identified, as shown in the zoom-in view in Figure 1b. In fact, the XRD patterns of MOF-177, presented by different researchers, differ with each other to some extent. For example, in the work by Li and Yang,15 the largest peak appeared at around 5°, unlike the pattern generated by Rowsell's dissertation16 (around 6°) or by our work (4.7°). There are also existing differences in the locations of the smaller peaks. Most probably, the different reaction (crystallization) conditions caused these differences in the peaks. XRD data were also collected on the partially evacuated chloroform exchanged MOF-177 samples and plotted in Figure 1c. It can be seen from Figure 1c that the MOF-177 sample without evacuation of guest molecules does not show any characteristic peaks of MOF-177, and it looks like an amorphous material. This is basically because the guest molecules occupying the internal pores of MOF-177 shift the atomic orientation in crystal planes.13,17 Table 1 summarizes the crystallographic properties of the MOF-177 samples investigated in this work. For the fresh MOF-177 sample with complete evacuation, the optimized cell structure can be indexed as a hexagonal type with P63 (No. 173) space group. This is different from the previously reported trigonal crystal structure with a space group P31c (163).14 The unit cell size is also smaller than that of previously reported results.12
H
ydrogen fuel cell-powered vehicles are promising members of future transportation systems. However, the lack of economic, efficient, and safe on-board hydrogen storage systems has become the main bottleneck of this technology. Metal organic frameworks (MOFs) are a new class of porous crystalline materials with low density and very high specific surface area. The frameworks are built by linking metal ions of Zn, Mn, Cu, or Cr with various organic linker molecules through strong chemical bonds.1-11 MOF-177, a framework consisting of a [Zn4O6]6þ cluster and linker 1,3,5benzenetribenzoate (BTB) ligands, has shown an excess hydrogen adsorption capacity of 7.5 wt % at 77 K and 70 bar8 and an absolute adsorption capacity of 11.5 wt % at 120 bar and 77 K.12 Our recent study suggests that MOF-177 has an absolute hydrogen adsorption capacity of 19.6 wt % at 77 K and 100 bar.13 Although the research works discussed above and several other publications14,15 have suggested that MOF-177 is one of the most promising porous adsorbents for hydrogen storage through physical adsorption, very limited information is available about the crystal structure and stability of this porous material. It was reported that MOF-177 would gradually change its phase structure if it is exposed to moist ambient air or water. The structure change would significantly reduce the hydrogen adsorption capacity in MOF-177.4,12,15 Therefore, it is necessary to investigate the structural stability of MOF-177 or other MOFs under various conditions because this crucial information will not only provide us the service time of MOFs as hydrogen storage media, it could also give us valuable insight into the structural variation of MOFs and help us improve the stability of this new type of materials. In this study, a newly synthesized MOF-177 sample was exposed to ambient air for 5 weeks and monitored for its structural changes by collecting X-ray diffraction (XRD) pattern data each week. A separate MOF-177 sample was immersed in liquid water at ambient temperature, then dried and analyzed
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Received Date: September 22, 2009 Accepted Date: October 19, 2009 Published on Web Date: November 06, 2009
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DOI: 10.1021/jz900028u |J. Phys. Chem. Lett. 2010, 1, 73–78
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Figure 1. (a) XRD pattern of MOF-177 after extensive evacuation of guest molecules. (b) Zoom-in view of panel a. (c) XRD pattern of as-synhtesized MOF-177 without any evacuation. Table 1. Summary of Chronological Structural Property Changes of an MOF-177 Sample crystallographic properties
fresh sample
after 1 week
cell type
hexagonal
orthogonal
monoclinic
monoclinic
monoclinic
monoclinic
space group lattice parameter (Å)
P63(173) a = 20.905
Pbam(55) a = 19.161
P2/c (13) a = 18.700
P2/c(13) a = 18.878
P21(4) a = 18.641
P2/m(10) a = 19.874
b = 20.905
b = 23.691
b = 17.870
b = 15.307
b = 16.223
b = 13.066
c = 22.718
c = 17.527
c = 21.540
c = 18.947
c = 23.724
c = 24.785
R = 90°
R = 90°
R = 90°
R = 90°
R = 90°
R = 90°
β = 90°
β = 90°
β = 123.2°
β = 97.7°
β = 138.7°
β = 114°
γ = 120°
γ = 90°
γ = 90°
γ = 90°
γ = 90°
γ = 90°
8598.4
7956.5
6025.8
5425.2
4739.1
5879.9
lattice angle
cell volume (Å3)
after 2 weeks
Figure 2a-e shows the XRD patterns and chronological shifts of MOF-177 crystal structure when it is exposed to ambient air with a relative humidity of 16% and 25 °C for 1-5 weeks. It is observed that, after 1 week (Figure 2a), there is a drastic change in the peak intensity as well as the crystallographic configuration. The intensity of the first peak shifted from 485 counts to 145 counts. The intensity of the second peak also decreased, but with a lower rate, resulting in almost the same intensities as the first two peaks at the end of first week. The optimized crystal structure shows that MOF-177 crystals are transformed into an orthogonal type with a space group Pbam(55). The cell volume decreases from 8598.4 Å3 to 7956.5 Å3, suggesting a decrease of the overall crystal size. After 2 weeks, the unit cell further changed to a new crystallographic configuration of a monoclinic type (Figure 2b) and the change in crystallographic properties from 2 to 4
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after 3 weeks
after 4 weeks
after 5 weeks
weeks resembles a general trend of decrease in cell volume while maintaining the monoclinic nature. In space group determination, it is also revealed that the cell exhibits primitive centering of the motif in all the cases. In the XRD patterns (Figure 2b-d), it is observed that the intensity of first two peaks decrease in almost in periodic nature; however, the nature of the rest of the peaks does not resemble any periodicity. After 5 weeks, the XRD pattern shows a drastic change (Figure 2e) from the previous week. The intensity of first peak lessened to be more than half from the previous week (fourth week), but the second peak totally vanished from its position. At the higher angle, several prominent peaks become visible in the overall pattern. In the crystal structure analysis, it is revealed that crystal still maintains its monoclinic symmetry with space group P2/m(10). The change in the trend of unit cell volume is reversed after 5 weeks as it increased to
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Figure 2. XRD patterns of MOF-177 after different durations of exposure to ambient air: (a) 1 week; (b) 2 weeks; (c) 3 weeks; (d) 4 weeks; (e) 5 weeks.
Figure 3. XRD pattern of MOF-177 after exposure to direct water.
5879.9 Å3, which is higher than the cell volume after 3 weeks. The results discussed above clearly demonstrated that MOF177 is not stable in moist ambient air; its crystal structures will gradually deteriorate if it is exposed to it. This is possibly caused by the slow oxidation of the metal clusters and organic linker molecules in the MOF-177 structure. XRD data shown in Figure 3 were collected on a fresh MOF177 sample after immersing in liquid water for 12 h and drying at 150 °C under a vacuum for another 12 h. It can be seen from Figure 3 that the MOF-177 sample soaked with water has basically collapsed because it did not show any characteristic peaks of MOF-177. The Brunauer-Emmett-Teller (BET) specific surface area of the water-soaked MOF-177 after extended degassing is less than 1 m2/g, which is much smaller than that of fresh MOF-177 (3814 m2/g).13 This further confirms that liquid water molecules can completely destroy the crystal structure of MOF-177.
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A few explanations were given in the literature regarding the degradation of zinc-based MOFs in the presence of water.18,19 Molecular dynamics simulation performed by Greathouse and Allendorf18 showed that the bonds between the zinc ion and the oxygen atom could be attacked by water molecules and cause the MOF structures to collapse. Huang et al.19 suggested that Zn4O clusters in the MOF material undergo hydrolysis in the presence of water to form the zinc ion and the corresponding organic linker acid (benzenedicarboxylic acid for MOF-5, and BTB for MOF-177). However, they also proposed that the hydrolysis reaction occurs favorably in the presence of an acid. On the basis of the possible reaction for MOF-5 proposed by Greathouse and Allendorf,18 Li and Yang15 suggested a hydrolysis reaction for MOF-177: Zn4 OðBTBÞ2 þ 4H2 O ¼ ½ðZn4 OÞðH2 OÞ4 ðBTBÞ3þ þ BTB3 ð1Þ
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DOI: 10.1021/jz900028u |J. Phys. Chem. Lett. 2010, 1, 73–78
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It is quite possible the following reaction may also occur in series with the above reaction:
molecules from the pores of MOF-177. However, a significant weight loss (approximately 74.7 wt %) was observed at temperatures from 330 to 420 °C for the oxygen-treated sample. The phase structure of the final product was confirmed as zinc oxide (ZnO) by the analysis of XRD. For the evacuated sample, the final mass loss at 400 °C (55 wt %) was found to be lower than that of the oxygen-treated one (74.7 wt %) because of the deposition of carbon soot on the remaining mass, unlike the oxygen-treated sample where the elemental carbon could not form because of the presence of an oxidizing atmosphere. Besides the carbon soot, the remaining mass was identified as ZnO. In order to monitor the structural change of MOF-177 after heat treatment, XRD data were collected on two MOF-177 samples after TGA at temperatures up to 330 and 490 °C, respectively. Figure 5a shows the XRD pattern of MOF-177 samples after TGA at temperatures up to 330 °C. Figure 5b is the XRD pattern of the MOF-177 samples after TGA at temperatures up to 490 °C. The XRD pattern shown in Figure 5a is slightly different from the XRD pattern of fresh MOF-177 after complete evacuation shown in Figure 1a, which suggests phase structure change during the TGA. The detailed crystallographic data refinement reveals that a unit cell can be indexed as a monoclinic type with a space group of P21(4) and a cell volume of 4192.2 Å3. Table 2 The type of unit cell as well as the primitive type of lattice is similar to those of the MOF-177 sample after exposure to ambient air after 2 weeks. As shown in Figure 5b, the sample obtained after TGA at temperatures up to 490 °C has a very different XRD pattern, showing that the crystal phase structure of the final product is different from that of MOF-177. The XRD pattern in Figure 5b matches well with that of zinc oxide (ZnO). This confirms the TGA result shown in Figure 4 that MOF-177 converts to zinc oxide at about 420 °C in the presence of oxygen.
½ðZn4 OÞðH2 OÞ4 ðBTBÞ3þ f ZnðOHÞ2 þ 3ZnO þ BTBð3HÞ þ 3Hþ
ð2Þ
The protons generated in the reaction above will further destroy the ZnO4 clusters, as suggested by Huang et al.18 Figure 4 show the weight loss versus temperature for MOF177 obtained in the TGA under oxygen flow and evacuated condition. The weight loss of the MOF-177 sample from the initial temperature (25 °C) to 330 °C is very small for oxygentreated MOF-177; however, no weight loss was observed for the evacuated sample. It is believed that the initial small weight loss for the oxyen-treated sample (
