Framework charge balance zeolite

Flanigen believed that the components of alkali metal cations of the synthetic system would play an important role in two aspects in the synthesis of zeolites [55] (1) supply of base, i.e., OH and (2) structure directing. Recently, Dutta and colleagues concluded that the hydrated cations can stabilize aluminosilicate ions via static electric and steric interactions.[56] In a word, alkali metal cations or alkaline earth metal cations mainly play the roles of structure direction, framework charge balancing, and pH adjustment in the synthesis of aluminosilicate zeolites. 

Zeolites with the FAU structure type can be prepared by direct synthesis with Si/Al ratios over a much narrower range (from 1 to ca. 5). The higher values refer to syntheses that use the crown ether 15-erown-5 as an additive. In these cases the sodium complex of 15-crown-5 is included in the structure. The lower charge density of the included complex compared to hydrated sodium cations requires less framework charge for charge balancing. Zeolite Y with much higher Si/Al ratios, required for catalytic applications, must be prepared by post-synthetic treatment (Chapter 6). 

In contrast, reflection was obtained at 20=14.6° when the CdCl2 salt and the zeolite NH4-Y was contacted (Fig. 3). The reflection was shown to be related with the presence of Cd2+ cations in the lattice, balancing negative framework charge. The intensity of the reflection increased in time parallel with the increasing degree of Cd-exchange. 

Catalytic oxidative dehydrogenation of propane by N20 (ODHP) over Fe-zeolite catalysts represents a potential process for simultaneous functionalization of propane and utilization of N20 waste as an environmentally harmful gas. The assumed structure of highly active Fe-species is presented by iron ions balanced by negative framework charge, mostly populated at low Fe loadings. These isolated Fe sites are able to stabilize the atomic oxygen and prevent its recombination to a molecular form, and facilitate its transfer to a paraffin molecule [1], A major drawback of iron zeolites in ODHP with N20 is their deactivation by accumulated coke, leading to a rapid decrease of the propylene yield. 

Zeolites are crystalline aluminosilicates with porous, framework structures made up of linked [Si04] and [A104] tetrahedra that form channels and cages of discrete size [24]. The framework structures of zeolites bear a net negative charge, which must be balanced by positively charged species, typically alkali or alkaline earth metal cations these cations maybe exchanged for one another under appropriate experimental conditions. Zeolites are capable of... 

As a numerical example, consider a partially dealuminated H-Y zeolite that contains 32 Alf/u.c., all of which are isolated, and 8 extraframework Al cations/u.c. This example is similar to the case of the partially dealuminated H-ZSM-20 zeolite in Figure 6 if one assumes that most of the extraframework Al is present in the cationic form. If the extraframework Al is complexed such that each Al has an equivalent charge of +2, 16 protons would be required to balance the framework charge. Here it is assumed that the cationic extraframework... 

An additional family of organometallic materials is the cyanometallates, which are Prussian blue analogues. These are microporous materials, similar to zeolites, with relatively large adsorption space and small access windows [237-241], These Prussian blue analogues develop zeolite-like structures based upon a simple cubic (T[M(CN)6]) framework, in which octahedral [M(CN)6]" complexes are linked via octahedrally coordinated, nitrogen-bound Tm+ ions [237], In the prototypic compound, that is, Prussian blue, specifically (Fe4[Fe(CN)6]3 14H20), charge balance with the Fe3+ ions conducts to vacancies at one-quarter of the [Fe(CN)6]4 complexes [242],... 

A new family of crystalline molecular sieves, 2) having aluminophosphate frameworks was synthesized. Strict alternation of A1 and P on the tetrahedral nodes yields neutral oxygens in contrast to the aluminosilicate zeolites, and non-framework cations are not needed for charge balance. Whereas a microporous silica (silicalite, 3 ) with neutral oxygens is hydrophobic, the aluminophosphate sieves are moderatley hydrophilic. 

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