Framework flexibility

Fixed framework-flexible adsorbate calculations were also reported by Dumont and Bougeard (68, 69). The diffusion coefficient calculated from a 42-ps calculation of 4 molecules per unit cell at 300 K was 1.60 x 10 8 m2/s. Once again the anisotropy of the diffusive process was calculated to be strong transport through the straight channel was found to be twice as fast as through the sinusoidal channel and an order of magnitude faster than motion parallel to the z-axis. 

Bull et al. (97) reported a systematic 2H-NMR and MD study of siliceous faujasite. MD calculations were performed for 1 molecule of benzene adsorbed in a single unit cell of faujasite. Full framework flexibility was incorporated, using potential parameters from MSI s cff91 force field (5). Simulations were performed for diffusion at 298, 350,400, and 450 K, using a time step of 1 fs for a 25-ps calculation run (following 5 ps of equilibration). 

It has already been mentioned that zeolites are shape selective with respect to molecular adsorption. This property relates to their micropores stmcture. The zeolite framework shows a limited flexibility, which is essential. For instance, Yashonath et al. have shown in their classical dynamic simulations study of molecular diffusion within zeolite micropore that the zeolite framework flexibility affects significantly diffusion when the molecules have a size comparable with the micropore size. To get an idea of the order of magnitude of this flexibility, one can consider the hybrid semi-empirical DFT periodic study of chabazite zeolite of Ugliengo et al. V They introduced in the unit cell of chabazite Br0nsted acidic sites which are known to induce an increase of the volume of around 10 This increase of the volume relates with the difference of volume between a Si04 tetraheron and a... 

In all of these giant skeletons, the glassy state arises from the easy lack of periodicity. This means that a small rotation or bending of the bonds does not markedly affect the free enthalpy of the system because of framework flexibility. 

Because different biopolymer gel systems can be encountered, different gelation mechanisms also can be encountered. Because of variations in the number and nature of the cross-links, framework flexibility, attractions and repulsions between framework elements, and interactions with solvent, different properties of the formed gels... 

In conclusion, separations foreseen, must also be based on the possible framework flexibility upon guest molecule adsorption, and not only on crystallographic diameters of as-synthesized or even calcined/activated zeolite structures in some of the frequently studied zeohtes. 

Garcia-Sanchez, A., Dubbeldam, D., Calero, S. Modeling adsorption and self-dUfusion of methane in LTA zeolites the influence of framework flexibility. J. Phys. Chem. C114,15068-15074 (2010)... 

Fig. 9 Two kinds of framework flexibility observed in MAF-2. (a, b) Temperature-induced closed and open states of the ethyl blocked aperture, (c, d) Guest-induced distortion of the Cu-triazolate framework...

We can anticipate that, by further efforts and, in particularly collaborating with engineers, researchers will be able to design and prepare more practically applicable MOF-based materials with unique pore surface, sizes, shapes, and framework flexibilities for excellent molecular recognition abilities and higher performance than other porous materials, in adsorption, separation, sensing, catalysis and other aspects, and to finely modify and fabricate these materials into practical devices. 

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