Zeolite guest molecules

For microporous compounds with special compositions, calcination effects are even more severe. As compared with zeolites, these compounds have lower thermal stability. Strictly speaking, most of them are nonporous since removal of the occluded guest molecules by calcination usually results in collapse. This is due to strong H-bonds with the framework, coordination bonds, and sometimes the templating molecule is shared with the inorganic polyhedra. Relevant examples of low-stability microporous compounds with interesting structural features are zeolitic open-framework phosphates made of Ga [178], In [179], Zn [180], Fe [181],... 

Corma, A., Garcia, H., Sastre, G. and Viruela, P.M. (1997). Activation of molecules in confined spaces an approach to zeolite-guest supramolecular systems. J. Phys. Chem. B 101, 4575-4582... 

Positively charged stopcocks can be plugged in the zeolite channels by ion exchange, whereas neutral stopcocks can be added by dehydration of the zeolite channels and adsorption from a nonaqueous solution or from the gas phase. The zeolite s external surface consists of a coat and a base. These two surfaces differ in a number of properties so that the interactions can be tuned. For MFI- and FAU-type zeolites, as an example, it was reported that guest molecules bind to the holes on the external surface much more strongly than on the framework between the holes [38,39]. 

A number of clay-like 2D layered and zeolite-like 3D framework host structures have been materialized using isopolycyanocadmate as the linkage unit. The 2D layered and the 3D framed hosts accommodate a variety of organic guest molecules and onium cations of different geometrical characters. 

The concept zeolites conventionally served as the synonym for aluminosilicates with microporous host lattice structures. Upon removal of the guest water, zeolites demonstrate adsorptive property at the molecular level as a result they are also referred to as molecular sieves. Crystalline zeosils, AlPO s, SAPO s, MAPO s (M=metal), expanded clay minerals and Werner compounds are also able to adsorb molecules vitally on reproval of any of the guest species they occlude and play an Important role in fields such as separation and catalysis (ref. 1). Inclusion compounds are another kind of crystalline materials with open framework structures. The guest molecules in an inclusion compound are believed to be indispensable to sustaining the framework structure their removal from the host lattice usually results in collapse of the host into a more compact crystal structure or even into an amorphous structure. 

Dendrimer 23 has been viewed as a potential organic zeolite because it could serve as a reaction chamber for small guest molecules. The three-dimensional branched structures of 22 and 23, as revealed by computer models, are shown in Figures 3a and 3b, respectively. ... 

Other flexible framework calculations of methane diffusion in silicalite have been performed by Catlow et al. (64, 66). A more rigorous potential was used to simulate the motion of the zeolite lattice, developed by Vessal et al. (78), whose parameters were derived by fitting to reproduce the static structural and elastic properties of a-quartz. The guest molecule interactions were taken from the work of Kiselev et al. (79), with methane treated as a flexible polyatomic molecule. Concentrations of 1 and 2 methane molecules per 2 unit cells were considered. Simulations were done with a time step of 1 fs and ran for 120 ps. 

The configurational-bias Monte Carlo method (CB-MC) (112) was developed to overcome these sorts of problems. Instead of a random insertion into the zeolite host, the guest molecule is grown atom by atom within the host in a way that avoids unfavorable overlap with the zeolite atoms. 

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