Thomas D. Bennett

Chemical structure, network topology, and porosity effects on the mechanical properties of Zeolitic Imidazolate Frameworks.

The mechanical properties of seven zeolitic imidazolate frameworks (ZIFs) based on five unique network topologies have been systematically characterized by single-crystal nanoindentation studies. We demonstrate that the elastic properties of ZIF crystal structures are strongly correlated to the framework density and the underlying porosity. For the systems considered here, the elastic modulus was found to range from 3 to 10 GPa, whereas the hardness property lies between 300 MPa and 1.1 GPa. Notably, these properties are superior to those of other metal–organic frameworks (MOFs), such as MOF-5. In substituted imidazolate frameworks, our results show that their mechanical properties are mainly governed by the rigidity and bulkiness of the substituted organic linkages. The framework topology and the intricate pore morphology can also influence the degree of mechanical anisotropy. Our findings present the previously undescribed structure-mechanical property relationships pertaining to hybrid open frameworks that are important for the design and application of new MOF materials.

Negative Linear Compressibility of a Metal–Organic Framework

A 3D hybrid zinc formate framework, [NH(4)][Zn(HCOO)(3)], possessing an acs topology, shows a high degree of mechanical anisotropy and negative linear compressibility (NLC) along its c axis. High-pressure single-crystal X-ray diffraction studies and density functional theory calculations indicate that contraction of the Zn-O bonds and tilting of the formate ligands with increasing pressure induce changes in structure that result in shrinkage of the a and b axes and the NLC effect along c.

Structure and properties of an amorphous metal-organic framework.

ZIF-4, a metal-organic framework (MOF) with a zeolitic structure, undergoes a crystal-amorphous transition on heating to 300 degrees C. The amorphous form, which we term a-ZIF, is recoverable to ambient conditions or may be converted to a dense crystalline phase of the same composition by heating to 400 degrees C. Neutron and x-ray total scattering data collected during the amorphization process are used as a basis for reverse Monte Carlo refinement of an atomistic model of the structure of a-ZIF. The structure is best understood in terms of a continuous random network analogous to that of a-SiO2. Optical microscopy, electron diffraction and nanoindentation measurements reveal a-ZIF to be an isotropic glasslike phase capable of plastic flow on its formation. Our results suggest an avenue for designing broad new families of amorphous and glasslike materials that exploit the chemical and structural diversity of MOFs.

Facile mechanosynthesis of amorphous zeolitic imidazolate frameworks.

A fast and efficient mechanosynthesis (ball-milling) method of preparing amorphous zeolitic imidazolate frameworks (ZIFs) from different starting materials is discussed. Using X-ray total scattering, N(2) sorption analysis, and gas pycnometry, these frameworks are indistinguishable from one another and from temperature-amorphized ZIFs. Gas sorption analysis also confirms that they are nonporous once formed, in contrast to activated ZIF-4, which displays interesting gate-opening behavior. Nanoparticles of a prototypical nanoporous substituted ZIF, ZIF-8, were also prepared and shown to undergo amorphization.

Reversible pressure-induced amorphization of a zeolitic imidazolate framework (ZIF-4).

We report the reversible pressure-induced amorphization of a zeolitic imidazolate framework (ZIF-4, [Zn(Im)(2)]). This occurs irrespective of pore occupancy and takes place via a novel high pressure phase (ZIF-4-I) when solvent molecules are present in the pores. A significant reduction in bulk modulus upon framework evacuation is also observed for both ZIF-4 and ZIF-4-I.

Mechanical Properties of Dense Zeolitic Imidazolate Frameworks (ZIFs): A High-Pressure X-ray Diffraction, Nanoindentation and Computational Study of the Zinc Framework Zn(Im)(2), and its Lithium-Boron Analogue, LiB(Im)(4)

The dense, anhydrous zeolitic imidazolate frameworks (ZIFs), Zn(Im)(2) (1) and LiB(Im)(4) (2), adopt the same zni topology and differ only in terms of the inorganic species present in their structures. Their mechanical properties (specifically the Youngs and bulk moduli, along with the hardness) have been elucidated by using high pressure, synchrotron X-ray diffraction, density functional calculations and nanoindentation studies. Under hydrostatic pressure, framework 2 undergoes a phase transition at 1.69 GPa, which is somewhat higher than the transition previously reported in 1. The Youngs modulus (E) and hardness (H) of 1 (E≈8.5, H≈1 GPa) is substantially higher than that of 2 (E≈3, H≈0.1 GPa), whilst its bulk modulus is relatively lower (≈14 GPa cf. ≈16.6 GPa). The heavier, zinc-containing material was also found to be significantly harder than its light analogue. The differential behaviour of the two materials is discussed in terms of the smaller pore volume of 2 and the greater flexibility of the LiN(4) tetrathedron compared with the ZnN(4) and BN(4) units.

Ball-Milling-Induced Amorphization of Zeolitic Imidazolate Frameworks (ZIFs) for the Irreversible Trapping of Iodine

The I2-sorption and -retention properties of several existing zeolitic imidazolate frameworks (ZIF-4, -8, -69) and a novel framework, ZIF-mnIm ([Zn(mnIm)2 ]; mnIm=4-methyl-5-nitroimidazolate), have been characterised using microanalysis, thermogravimetric analysis and X-ray diffraction. The topologically identical ZIF-8 ([Zn(mIm)2]; mIm=2-methylimidazolate) and ZIF-mnIm display similar sorption abilities, though strikingly different guest-retention behaviour upon heating. We discover that this guest retention is greatly enhanced upon facile amorphisation by ball milling, particularly in the case of ZIF-mnIm, for which I2 loss is retarded by as much as 200 °C. It is anticipated that this general approach should be applicable to the wide range of available metal-organic framework-type materials for the permanent storage of harmful guest species.

Thermal Amorphization of Zeolitic Imidazolate Frameworks

Zeolitic imidazolate frameworks (ZIFs) are a family of metal–organic frameworks (MOFs) that display network topologies analogous to those seen in zeolites whereby the zeolitic building blocks of corner-sharing SiO4 tetrahedra are replicated by MN4 tetrahedra (M=metal) linked by imidazolate anions. Over 100 distinct ZIF phases adopting 40 network types currently exist. Interest has focused mainly on the tuneable gas sorption and separation properties of these porous materials, though their potential for catalytic activity is starting to be explored. The retention of thermal stability derived from their zeolitic structures makes them particularly attractive candidates for practical applications. Inorganic zeolites are known to undergo pressureor temperature-induced amorphization. Depending on the heating/pressurization rate, the amorphous materials thus formed can retain some aspects of crystalline topology, and consequently possess a lower configurational entropy than true glasses. Polyamorphism (the presence of structurally isomeric amorphous phases differing in density and entropy) has been identified both experimentally and theoretically in these materials. Given the comparison often drawn between between ZIFs and zeolites (ascribed to the common subtended angles of ca. 1458 at the metal-bridging species, see Figure 1a), it is not surprising that reports of pressure-induced ZIF phase transitions and amorphization exist, albeit at pressures far lower than those of their zeolitic counterparts. Recently, we reported an amorphous ZIF (a-ZIF) with a network topology comparable to that of silica glass, formed by thermal amorphization of the crystalline Zn-based ZIF-4 framework. Further heating of the a-ZIF yielded the dense ZIF-zni. The mechanical properties of the a-ZIF, studied using nanoindentation, were found to be isotropic and intermediate between ZIF-4 and ZIF-zni. Remarkably, the amorphization temperature is comparable to that of purely inorganic zeolites. Here, we show that a-ZIF and crystalline ZIF-zni can also be prepared by heating two structural isomers of ZIF-4 (see Figure 1c). Zn-based ZIF-1 and ZIF-3, possessing the BCT and DFT zeolitic topologies, undergo amorphization and recrystallization to ZIF-zni at similar temperatures to that of ZIF-4. The same process occurs in a cobalt analogue of ZIF-4 (Co-ZIF-4), yielding an amorphous MOF containing a spinactive transition-metal ion. We also show that five ZIFs incorporating substituted imidazolate bridging ions do not undergo thermal amorphization. These frameworks—ZIF-8, -9, -11, -14, and ZIF-bqtz—adopt four different network topologies and possess three different substituted imidazolate species. Solvothermal reaction of Zn(NO3)2 and imidazole (Im) under varying conditions yielded single-crystal samples of ZIF-1, ZIF-4, Co-ZIF-4, ZIF-8, ZIF-9, and ZIF-11 of typical size 0.2! 0.2! 0.1 mm [3,19] whilst a polycrystalline powder sample of ZIF-3 was prepared by a liquid-mixing method. Liquid-assisted grinding was used to synthesize polycrystalline samples of ZIF-14 and ZIF-bqtz. For ZIF-1, -3, and -4 (Co, Zn) thermogravimetric analysis shows that the structure-directing agent and solvent molecules trapped within the porous cavities of the frameworks are Figure 1. a) The similar Si-O-Si and Zn-Im-Zn linkages in zeolites and ZIFs, respectively. b) A snapshot of the continuous random network (CRN) topology of the a-ZIF gained from reverse Monte Carlo (RMC) modeling. c) Representative views of the expanded unit cells of ZIF1 (left), ZIF-3 (center), and ZIF-4 (right).

Thermochemistry of zeolitic imidazolate frameworks of varying porosity.

The first thermochemical analysis by room-temperature aqueous solution calorimetry of a series of zeolite imidazolate frameworks (ZIFs) has been completed. The enthalpies of formation of the evacuated ZIFs-ZIF-zni, ZIF-1, ZIF-4, CoZIF-4, ZIF-7, and ZIF-8-along with as-synthesized ZIF-4 (ZIF-4·DMF) and ball-milling amorphized ZIF-4 (a(m)ZIF-4) were measured with respect to dense components: metal oxide (ZnO or CoO), the corresponding imidazole linker, and N,N dimethylformamide (DMF) in the case of ZIF-4·DMF. Enthalpies of formation of ZIFs from these components at 298 K are exothermic, but the ZIFs are metastable energetically with respect to hypothetical dense components in which zinc is bonded to nitrogen rather than oxygen. These enthalpic destabilizations increase with increasing porosity and span a narrow range from 13.0 to 27.1 kJ/mol, while the molar volumes extend from 135.9 to 248.8 cm(3)/mol; thus, almost doubling the molar volume results in only a modest energetic destabilization. The experimental results are supported by DFT calculations. The series of ZIFs studied tie in with previously studied MOF-5, creating a broader trend that mirrors a similar pattern by porous inorganic oxides, zeolites, zeotypes, and mesoporous silicas. These findings suggest that no immediate thermodynamic barrier precludes the further development of highly porous materials.

Improving the mechanical stability of zirconium-based metal–organic frameworks by incorporation of acidic modulators

The ability to retain structural integrity under processing conditions which involve mechanical stress, is essential if metal–organic frameworks (MOFs) are to fulfil their potential as serious candidates for use in gas sorption, separation, catalysis and energy conversion applications. A series of zirconium dicarboxylates, predicted to be amongst the more mechanically robust MOFs, have been found to undergo rapid collapse upon ball-milling, resulting in catastrophic losses of porosity. An inverse relationship between collapse time and framework porosity has been found. Addition of acidic modulator ligands (e.g. trifluoroacetic acid) to UiO-66 provided a striking increase in mechanical robustness, the degree of which is inversely related to modulator pKa. This effect, caused by an increased strength of the zirconium–carboxylate bond, provides an important concept to design microporous hybrid frameworks capable of retaining their structure under harsh processing conditions.