Zeolites electrostatic interactions

S.2 SiOHAl, S103HAP Bridging OH groups in small cages of zeolites, electrostatic interaction 218,221... 

Study [23] Jacobsen s complex was entrapped in the final step of the zeohte synthesis (method C). This process was possible because MCM-22 zeohte is prepared by condensation of a layered precursor, which is exchangeable by the catalytic complex. Leaching of Mn was not observed in these systems, which is not unexpected bearing in mind that the complex is also bovmd to the zeolite structure through an electrostatic interaction. 

Two methods for including explicit electrostatic interactions are proposed. In the first, and more difficult approach, one would need to conduct extensive quantum mechanical calculations of the potential energy variation between a model surface and one adjacent water molecule using thousands of different geometrical orientations. This approach has been used in a limited fashion to study the interaction potential between water and surface Si-OH groups on aluminosilicates, silicates and zeolites (37-39). 

A number of techniques have been employed to model the framework structure of silica and zeolites (Catlow Cormack, 1987). Early attempts at calculating the lattice energy of a silicate assumed only electrostatic interactions. These calculations were of limited use since the short-range interactions had been ignored. The short-range terms are generally modelled in terms of the Buckingham potential,... 

Of course, concerns about periodicity only relate to systems that are not periodic. The discussion above pertains primarily to the simulations of liquids, or solutes in liquid solutions, where PBCs are a useful approximation that helps to model solvation phenomena more realistically than would be the case for a small cluster. If the system truly is periodic, e.g., a zeolite crystal, tlien PBCs are integral to the model. Moreover, imposing PBCs can provide certain advantages in a simulation. For instance, Ewald summation, which accounts for electrostatic interactions to infinite length as discussed in Chapter 2, can only be carried out within the context of PBCs. 

Snurr et al. (192) used biased GC-MC simulations to predict isotherms, isosteric heats of adsorption, and locations of benzene and p-xylene at various concentrations. The suitability of a bias method is clear, at low coverages to prevent trial insertions overlapping with the zeolite walls and at high coverages to prevent overlap with other sorbate molecules. (Slightly different bias schemes were used for the two extremes of concentrations.) Interactions between sorbates and zeolites—both of which were considered to be rigid—were modeled with parameters from the literature (79, 87). Electrostatic interactions were included to account for the quadrupole moment of the sorbates. Sorbate-sorbate interaction parameters were taken from Shi and Bartell (194) for benzene and from Jorgensen et al. (195) for p-xylene. 

Hydrophilic and Hydrophobic Surfaces. Polar adsorbents such as most zeolites, silica gel, or activated alumina adsorb water (a small polar molecule) more strongly than they adsorb organic species, and, as a result, such adsorbents are commonly called hydrophilic. In contrast, on a nonpolar surface where there is no electrostatic interaction, water is held only very weakly and is easily displaced by organics. Such adsorhenis, which are the only practical choice for adsorption of organics from aqueous solutions, are termed hydrophobic. 

Quaternary amines, such as tetraalkylammonium bromides and hydroxides (the alkyl group being Q to C4) are the typical zeolite templates. Quaternary amines fulfill the above-mentioned requirements of stability, specific interaction with the precursor (electrostatic interaction between quaternary amines and silicate), and easy removal (by calcination). 

Using the results obtained in the previous section, one can state that as a rule, for the ion-exchange reaction (Equation 7.1) in the case of zeolites with high Si/Al ratio, that is, zeolites with low TEC, AH is not very important. Because the aluminum content in the zeolite is low, the negative charge in the framework is low, and, consequently, the framework electrostatic interaction with the... 

In the case of alkane cracking, dispersion forces between the alkane molecules and the siliceous walls of the zeolites and perhaps other nanoporous crystalline and ordered materials are possibly the most important interactions for stabilizing adsorption in the cavities, since the proton affinity of alkanes is low [104] and the electrostatic interactions between the alkane and the adsorbent are negligible [97],... 

This effect may be based on a favorable electrostatic interaction between the large cations and the condensed DnR silicates. The observed shift parallels the often-facilitated formation of Si-rich zeolites in the presence of organics. 

Zeolites also lend themselves particularly well to reactions where the catalytically active species are cationic. Under these circumstances there is a strong electrostatic interaction between the active entity and the support which minimizes activity loss via leaching processes. 

The physisorption of alkanes in zeolites is essentially due to van der Waals forces. The main contributions to the stabilization are the induction energy, roughly speaking the electrostatic interaction of the dipole moment induced by the electric field of zeolite with the framework charge distribution and the... 

POMs can display very strong Br0nsted acidity and are efficient oxidants, exhibiting fast and reversible multi-electron redox transformations under mild conditions. This acidity is very strong, as the negative charge is delocalized over much larger anions than for mineral acids or solids such as zeolites. Consequently, the electrostatic interaction between protons and the anion is much less for POMs. 

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