High Pressure Study On Metal Organic Framework Zif-8 And The Confined Nitrogen In ZIF-8

As a member of new porous materials, metal organic framework ZIF-8 is constructed by metal zinc ions and 2-methylimidazole and has an inner cavity with diameter of 11.6? connected via 6-ring apertures and 4-ring apertures. Due to its flexibility and high porosity, it has advantages in gas adsorption, separation and calalysis over traditional zeolites. Particularly, ZIF-8 has received much attention from scientists as an ideal nanoconfinement template. The structural stability is very important for its potential applications. It has been shown that the methyl imidazole ring swinging around Zn-Zn in the 4-ring windows to open the gate at 1.47 GPa when methanol\ethanol mixture(volume 4:1) is used as pressure transmitting media(PTM). However, ZIF-8 transforms to irreversible amorphization at pressure higher than 0.3GPa when molecules with large size are used as PTM or no PTM is used. Now, several basic scientific issues are still unsolved in the high pressure study of ZIF-8. For example, the factors affecting the entrance of molecules in PTM into the inner cavities of ZIF-8、the interaction of the molecules in PTM with ZIF-8 when they enter the inner cavities of ZIF-8 and the structural transformation and stability of ZIF-8 under higher pressure after molecules enter ZIF-8 all need further study. N2 is a typical diatomic molecule and has the highest binding energy and, except for H2, the shortest bond length and it shows rich structures under high pressure. Combining high pressure technology with confinement effect and studying the structural transformation of confined nitrogen under high pressure is a new scientific field with great significance.In view of the above questions, we carried out high pressure Raman measurements on ZIF-8 with different PTM to study the pressure-induced structural transformation; we chose ZIF-8 as nanoconfinement template and explored the high pressure behavior of nitrogen confined in ZIF-8. The results are listed as follows:1. In situ Raman and synchrotron X ray diffraction studies show that when the size of molecules in PTM are smaller(H2O 2.9?) than、close(N2 3.6?) to or slightly bigger(CH3OH 4.3?,C2H5 OH 5.1 ?) than the size of the 6-ring apertures(3.5?) in ZIF-8, a new Raman band appears on the low frequency side of out-of-plane ring bending mode due to imidazole ring at around 1.1GPa in all of these experiments. This suggests that the imidazole rings swing away from each other to open the gate at this pressure. High pressure XRD experiments shows that the gate-opening effect leads to an expanded lattice volume, which indicates that the small molecules(CH3OH\ C2H5 OH or N2) in PTM enter the inner cavities and support the framework. However, no gate-opening effect is observed when no PTM is used or silicone oil is used as PTM. After the small molecules in PTM enter the inner cavities of ZIF-8, the Raman bands corresponding to the imidazole ring show an obvious blueshift, which suggests that the interaction between the small molecules and imidazole rings at this site is strong and the possible site of the small molecules in ZIF-8 is near the imidazole ring. The CH3OH\ C2H5 OH or N2 which enter the inner cavities of ZIF-8 can improve the structural stability remarkably and amorphization occurs at pressure higher than 20 GPa. The ZIF-8 released from 20 GPa retain chemical stability and initial nanosized morphology due to supporting effects and protection of the molecules in PTM. This study is important for the understanding of structural transformation and stability of ZIF-8 and it will expand the application of this porous material.2. We have carried out detailed high pressure Raman spectroscopy study on the confined nitrogen in the inner cavities of ZIF-8. After the gate-opening effect occurs in ZIF-8, a new Raman band ν′, which can be assigned to confined nitrogen, appears on the low frequency side of the Raman band due to bulk nitrogen. As the pressure is increased, the population of confined nitrogen increases. With the increasing of pressure, ZIF-8 shows an anisotropic contraction with the(110) plane having the largest pressure coefficients and this makes the confined nitrogen experience stronger external interaction. In comparison with the pressure coefficients of bulk nitrogen in different pressure ranges, the pressure coefficient of the confined nitrogen in the pressure range of 1.1 to 2.4GPa is close to that of the solid β phase, that is to say, the confined nitrogen is not in the state of liquid but in the state of highly compressed β phase-like state. Under higher pressure(>2.9GPa), the pressure coefficient of confined nitrogen approaches that of the solid ε phase, which suggests that it has probably transformed to β phase-like state. The confined nitrogen in ZIF-8 shows different high pressure behaviors from that in AFI and the bulk nitrogen. The confined nitrogen can not be captured and totally escapes from the inner cavities upon decompression. This study indicates that we can mediate the insertion and extrusion of nitrogen molecules with respect to ZIF-8 reversibly by compressing and decompressing. Our work demonstrates that the confinement environment is crucial to the high pressure behavior and structural stability of confined nitrogen. This study riches our knowledges on the structural transformation of nitrogen and provides new insight into the capture and storage of gas molecules.