IRMOFs framework materials

Metal organic frameworks are the crystalline materials with the lowest density. As an example, MOF-5 possesses a crystal density of only 0.61 gcm [93]. The volumetric hydrogen storage capacity of MOFs, which is referred to the volume of the material, is therefore affected by these low densities. In spite of this, Muller et al. from BASF recently showed the real potential of MOFs for industrial gas storage applications [81]. They showed that a container filled with MOF-5, IRMOF-8 or a porous Cu-based MOF exceeds, at 77 K, the volumetric hydrogen storage capacity (referred to the whole system) of a pressurized empty container filled with hydrogen. 

The presence of water vapour will define which materials can be used based on their hydrolytic stability. As an example metal-organic frameworks (MOFs) and covalent organic framework (COFs) are crystalline microporous materials which can exhibit exceptionally high surface areas and gas sorption capacities and, as such, have been proposed as potential materials for CCS. However, some of these materials can be unstable in the presence of moisture. The IRMOF series and the more recently produced COF materials are particularly unstable (e.g., loss of porosity at room temperature in air). Clearly, materials of this type would be unsuitable for CCS application. If such materials are to be used, more stable types are required for example, one class of MOF known as zeolitic imidazole frameworks (ZIFs) are reported to have greatly improved hydrolytic stability. 

An ideal class of materials to study in this context are the isoreticular metal-organic frameworks synthesised by Yaghi and co-workers [5, 6], which consist of oxide-centered Zn40 tetrahedra linked by six dicarboxylates as illustrated in Fig. 1 b. The resulting materials feature extended, open, three-dimensional frameworks. Because all IRMOFs are built from the same comer unit and have the same network topology and therefore only vary in the linker molecules, they allow the systematic study of the influence of the linker molecules on adsorption. 

The materials included in this study are listed in Fig. 1 b. Two of the materials studied consist of two interpenetrated networks IRMOF-9 and lRMOF-15 are the interpenetrated forms of IRMOF-10 and IRMOF-16 respectively (see Fig. 1 c). Intopenetration results in two opposite effects for adsorption, namely a gain in surface area and stronger en getic interactions between the sorbate molecules and the framework but also a loss of porosity and an increase in the crystal density which affects the gravimetric uptake negatively. 

In this paper, molecular simulations were used to study methane adsorption in IRMOF materials and to examine which properties of the materials influence methane uptake. The materials were carefully characterised using Monte Carlo methods to get pore size distributions, accessible surface areas and pore volumes. Depending on the pressure, three distinct regimes can be distinguished. At low loading (10 kPa), the amount adsorbed is proportional to the isosteric heat of adsorption which is a measure for the strength of the interaction between the methane molecules and the framework. Here, the methane molecules are located preferentially in the energetically favourable comer sites of the materials. Materials with... 

Sonochemical treatment during the synthesis also speeds up the process due to the cavitation effect, also providing conditions for control of interpenetration of different frameworks (catenation) [38,39]. If separate frameworks are self-assembled within each other, this effect is known as framework catenation [40]. It is divided into interpenetration and interweaving. In interpenetrated MOFs, frameworks are maximally displaced from each other in interwoven MOF structures, the distance between both frameworks is minimized (eg, IRMOF-13, Table 1). Catenation results in the enhancement of the rigidity of the material at the expense of its porosity, but increases the stability as well as may help in the case of adsorption and storage of small molecules due to the size selectivity. 

A unique feature of MOFs, which distinguishes these systems from other materials (such as metal oxides), is the absence of an inaccessible volume (so-called dead volume) [52], Moreover, owing to their open architecture, the intrinsic diffusion coefficients of reactant molecules are only slightly less than in the bulk of the solvent, that is, mass transfer in the porous framework system of MOFs is not hindered. The pore sizes in MOFs vary in a tunable manner in a range from 0.4 to 3 nm (eg, for the IRMOF series), with the proper choice of the organic linker determining the pore diameter [4]. 

MIL-53 materials provide a convenient model for comparison with traditional sorbents with similar geometric characteristics, such as activated charcoals and zeoUtes. The specific surface area estimated for MIL-53,1100 mVg (BET method), is similar to the average value for nanocarbon materials and exceeds that for zeolites. The framework of MIL-53 comprises unidimensional channels of 8.5 A in diameter, which is similar to the pore diameter in zeolites (6-12 A) and smaller than that in the IRMOF series, including MOF-5 (12-15 A). At the same time, the hydrogen adsorption capacity of MIL-53 is somewhat higher than that of the zeolite CaX (2.19wt.%, Table 2) and activated charcoals (2.15 wt.%) [179]. It is possible that this parameter is affected by the channel geometry, because zeolites more often have a 3D channel system, as opposed to the unbranched unidimensional channels in MIL-53. However, additional experiments on the adsorption mechanism are required to draw more definitive conclusions. 

One of the first MOFs investigated for H storage was the cubic carbox-ylate-based framework MOF-5 or IRMOF-1, IRMOF-6, and IRMOF-8 (Fig. 3.5) [28]. With complete activation, MOF-5 can adsorbs 7.1 wt% H at 77 K and 40 bar and 10.0 wt% at 100 bar. The latter value is corresponding to a record volumetric storage density of 66 g L" which is near the density of liquid Hj at 20.4 K and 1 bar [29]. This MOF material is the best cryogenic storage material currently known. In addition, it was demonstrated that Hj can... 

Isoreticular metal organic framework (IRMOF) materials were part of the series of MOF materials used in the identification of the correlation between hydrogen saturation amount and Langmuir surface area determined from nitrogen adsorption at 77Hydrogen adsorption on... 

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