Flexible zeolite framework

It is known that the flexibility of the zeolite framework arises from the flexibility of the Si-O-Si joints linking rigid Si04 tetrahedra. This feature allows for... 

Theoretical studies on the Beckmann rearrangement mechanism over zeolite catalyst supported by experimental data have increased. The catalytic activity of the zeohte is determined by Brpnsted and Lewis acid sites created by protonation or activation by metallic cations. The reactivity of the acid sites is strongly influenced by the geometry and flexibility of the zeolite framework ". ... 

June et al. (12) used TST as an alternative method to investigate Xe diffusion in silicalite. Interactions between the zeolite oxygen atoms and the Xe atoms were modeled with a 6-12 Lennard-Jones function, with potential parameters similar to those used in previous MD simulations (11). Simulations were performed with both a rigid and a flexible zeolite lattice, and those that included flexibility of the zeolite framework employed a harmonic term to describe the motion of the zeolite atoms, with a force constant and bond length data taken from previous simulations (26). 

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... 

It is clear that in the case of MFI, the zeolite pore entrances should not be considered as rigid apertures. Instead, zeolite framework topologies can show flexibility. While the O-Si-0 angle in the tetrahedral unit is rigid (109 + 1 °), the Si-O-Si angle between the units can vary between 145 and 180°. Based on isomorphous substitution of Si by other T-atoms in the framework [18], framework defects [19], cation positions, changes in the water content [16], external forces on the crystalline material [20] and upon adsorption of guest molecules [21] phase transitions can occur that have a dramatic influence in particular cases on the framework atom positions. 

The pores of zeolites are of molecular dimensions (Fig. 1). In principle molecules can be excluded from the pores if their diameter is larger than the pore apertures. The zeolite framework is, however, not a rigid structure. Especially at higher temperatures, the pores become more flexible. As a result, molecules with larger diameters than the dimensions of the pores will be able to penetrate the channels. The maximum diameter of molecules that are able to adsorb within the zeolite is called the adsorption cutoff diameter. This diameter is about 0.95 nm for zeolite X and Y [30], 0.65 nm for ZSM-5 [30], and 0.4 nm for zeolite-4 A [1] at 3(X) K. This means that, in principle, all molecules listed in Fig. 1 can permeate through a zeolitic membrane made from zeolite X, Y or ZSM-5, except for the large amines. Complete exclusion of molecules from the pores will therefore occur only when using a zeolite-A membrane. 

Both the template molecules and the zeolite frameworks were treated as flexible, using the zeolite force field developed in Delft together with the MM3 force field developed by Allinger et Calculations were carried... 

Ti(OH)2(OSi)2, while Ti-oxides incorporated within the zeolite framework have a closed structure of Ti(OSi)4, as suggested by Coluccia et al. [15] Moreover, it is likely that the molecular environment of the Ti-oxides such as the rigidity or flexibility of the zeolite framework causes a significant and pronounced effect on the chemical properties of the Xi04 unit. Taking these results into consideration, the different band positions of the observed photoluminescence of the various Ti-oxide catalysts may be attributed to the differences in the molecular environment of the Ti-oxides such as the rigidity or flexibility of the zeolite framework and the local stmctures of the Ti04 unit, /. e., Ti(OSi)4,... 

Unique and efficient photocatalytic systems incorporating transition metal oxides (Mo, Ti, V) have been designed using the cavities and frameworks of zeolites and mesoporous molecular sieves. The present results have demonstrated the unique physicochemical properties of the local stmcture of such anchored metal oxides, which were the result of the rigidity or flexibility of the zeolite framework, as well as their photocatalytic reactivities. These well-defined photocatalysts can be considered one of the most promising candidates for use in the environmentally-friendly reaction systems. 

Our work highlights the need for reliable atomistic potentials for other cations and other adsorbates that would allow this mechanism to be studied further. Potentials that would allow for a flexible zeolite are also needed. We believe the potential used here is adequate to shed light onto the behavior, but its inability to describe motion of the framework is a significant shortcoming. Experimentally it... 

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