Framework charge

Material class Materials Chemical formula Framework charge Electronic prop Ionic Exch. prop p.z.c Ref... 

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. 

The higher thermal stability of the exchanged phase is a direct consequence of the presence of NH4 molecules in the channels the decomposition of the molecules, at about 450 °C, and the loss of NFI3" ions, leave in the channels H+ protons to balance the framework charges. While Na2+ ions tend to weaken the Si-O bonds, the presence of FI+ protons contribute to stabilize the structure balancing framework negative charges. 

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. 

Figure 4.3 Compensation of a negative framework charge by a Na+ cation.

Because of the variety of Si locations (isolated Si and Si islands) in SAPO molecular sieves, frequently no correlation exists between Si content and the number of acid sites [105]. However, for SAPO with low Si content. Si sites are usually isolated and there is one acid site per Si. In general, the framework charge and, thus, the maximum number of acid sites in a SAPO should be related to the value of framework (Al-P) [106]. This relationship is true of zeolites too, because (Al-P) is equal to A1 if there is no framework P. Based on this relationship, the... 

The incorporation of Al atoms into the framework of zeolites occurs in a tetrahedral oxygen coordination and leads to negative framework charges. These framework charges are compensated by protons in acidic hydroxyl groups or by extra-framework cations such as Li , Na, Cs", Mg ", etc. Accordingly, these surface sites are responsible for the chemical behavior of zeolites in separation processes and in catalysis (199,200). 

In the zeolite, most of the TMA ions are encaged in the gmelinite units, their charge being satisfied by the negative framework charge. We designate this cation site I. 

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... 

Table II. Chemical conpceition, synthesis efficiency, pore filling and framework charge balance of seme pure Nu-10 sarples, synthesized under various experimental...

Sarrple Unit cell conpceiticn of Synthesis efficiency Pore filling Framework charge balance ... 

When the incorporation of Si is again considered as a replacement of A1 and/or P in an hypothetical aluminum phosphate lattice, the MCM formula precludes isomorphic substitution via mechanism 3 and allows only for mechanism 1 and 2 to occur. According to the MCM formula, Si02 tetrahedra replace A102 and introduce positive framework charge, which is neutralised by the anions Nn or replaces P02 and creates negative framework charge, which is neutralised by the cations Mm+. 

Framework charge calculated from chemical analyses. 

Chemical Analysis. The chemical analyses of these and other MeAPO molecular sieves are consistent with tetrahedral frameworks in which Me and A1 occupy 50% of the T sites and P, the other 50%. All of the available structural evidence on A1P0< and MeAPO also supports frameworks based on alternating (Al.Me) and P in the lattice T sites. The negative framework charge produced by such an incorporation pattern is balanced primarily by the positively charged template and, secondarily, by other occluded species, such as H or Me5. ... 

The framework charge-compensating cations in a zeolite, which for synthetic zeolites are normally sodium ions, can be exchanged for other cations of different type and/or valency. However, care must be taken during ion exchange to avoid strongly acidic solutions which can lead to proton exchange with the zeolite metal cations or even structure collapse. For example, zeolites A, X, and Y decompose in 0.1 N HCI. The more silica-rich zeolites such as mordenite are, however, stable under such conditions. Acidity can be introduced into a zeolite in a number of different ways ... 

---