Framework composition

When acid zeolite catalysts must be prepared, trivalent elements which can be in tetrahedral coordination are introduced to the synthesis media with silica. Then a structure with framework groups of the type  

Another way of controlling the acid strength of the Br0nsted acid sites is by changing the nature of the T atoms. For instance, a low acidity Boron ZSM-5 zeolite containing Ce is active and selective for isomerization of 2-alkylacroleins into 2-methyl-2-alkenals without skeleton isomerization (Eq. 1) [10], 

The control of zeolite acidity is of special importance when catalyzing reactions involving strong bases such as NH3 or pyridines. For such reactions a zeolite catalyst with excessive acidity will be rapidly poisoned by adsorption of the reactant or the basic products. For instance, in the aldol condensation of aldehydes and ketones with ammonia for the production of pyridine and 3-methylpyridine, an important intermediate in the synthesis of vitamin B3, milder acidities are preferred [11]. 

There is no doubt that zeolite electronegativity plays a predominant role in determining the final acidity of the zeolite Brpnsted sites. When, however, different structures with similar framework compositions were compared, different acidity, as measured by use of probe molecules, was found. For example, the binding energy of NH3 on H-Mordenite and H-ZSM-5 is 160 and 145 kJ moff respectively 

Today we tend to see the influence of zeolite structure on reactivity for acid-catalyzed reactions, not only because of different intrinsic acidity, but because of the effect of the structure and the local surroundings on adsorption, and on the stabilization of the activated complex. It seems logical that the structure will determine both spatial conformation and the number of hydrogen-bonds that the protonated transition complex can form with the framework anion. This hydrogen-bond-acceptor ability is an important component of the zeolites as microcatalytic reactors 

There are many reports of Si substitutions in SAPO-5, SAPO-11, SAPO-31, SAPO-34, and SAPO-37 in the literature.[20 24] 

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Silica Oxide to Alumina Oxide Ratio (SAR) is used to describe the framework composition of zeolite. 

In addition to the influence of neighbors on 29Si chemical shifts, the geometrical effects (such as Si-O-T angles) already described above are also evident of mixed frameworks with elements other than Si on tetrahedral positions. This is reflected by the broadness of the bars shown in Fig. 1. Multinuclear NMR investigations on a large set of sodalite structures with various framework compositions show that T-O-T bond angle (T = Si, Al, Ga) and dTT distance chemical shift dependences exist, and mutual correlations between chemical shift of these NMR nuclei can be observed [68],... 

Fries rearrangement, 18 336, 337 isomerization and transalkylation of alky-laromatics, 18 329 epoxide transformations, 18 351-352 hydration and ammonolysis of ethylene oxide, 18 351, 352 isomerization, 18 351 framework composition, 33 226-228 hydrogenation, dehydrogenation, and related reactions, 18 360-365 dehydrocyclization of s-ethylphenyl using zeolites and carbonyl sulfide, 18 364, 365... 

The characteristics of aluminophosphate molecular sieves include a univariant framework composition with Al/P = 1, a high degree of structural diversity and a wide range of pore sizes and volumes, exceeding the pore sizes known previously in zeolite molecular sieves with the VPI-5 18-membered ring material. They are neutral frameworks and therefore have nil ion-exchange capacity or acidic catalytic properties. Their surface selectivity is mildly hydrophilic. They exhibit excellent thermal and hydrothermal stability, up to 1000 °C (thermal) and 600 °C (steam). 

In the metal aluminophosphate (MeAPO) family the framework composition contains metal, aluminum and phosphorus [27]. The metal (Me) species include the divalent forms of Co, Fe, Mg, Mn and Zn and trivalent Fe. As in the case of SAPO, the MeAPOs exhibit both structural diversity and even more extensive composihonal variation. Seventeen microporous structures have been reported, 11 of these never before observed in zeoUtes. Structure types crystallized in the MeAPO family include framework topologies related to the zeolites, for example, -34 (CHA) and -35 (LEV), and to the AIPO4S, e.g., -5 and -11, as well as novel structures, e.g., -36 (O.Snm pore) and -39 (0.4nm pore). The MeAPOs represent the first demonstrated incorporation of divalent elements into microporous frameworks. 

Quinary and senary framework compositions have been synthesized containing aluminum, phosphorus and silicon, with additional combinations of divalent (Me) metals. In the ElAPO and ElAPSO compositions the additional elements Li, Be, B, Ga, Ge, As and Ti have been incorporated into the AIPO4 framework [27]. 

Other Framework Compositions GrystaUine microporous frameworks have been reported with composihons of beryUophosphate [43], aluminoborate [44], alumi-noarsenate [45], galloarsenate [46], gallophosphate [47], antimonosilicate [48] and germanosilicate [49],... 

Si NMR provides quantitative information about the framework composition, and framework Si/Al ratio is an important parameter used to tune the catalyst property. Zeolite acidity is directly related to the amount of framework Al. Framework Si/Al ratio can directly be obtained from just Si NMR alone. Si/Al ratio can be calculated from Si NMR intensities if the resonances due to different Q (nAl) species are well-resolved using Eq. (4.10), assuming there is no Al-O-Al bonds present ... 

Figure 1 The framework composition of a faujasite (e.g., NaY or NaX) depicting the 13-A-diameter supercage surrounded by tetrahedrally disposed 7.5- A-diameter windows.

It has already been reported that the weight loss of as-synthesized MMSs depends on the kind of the template used in the synthesis [17]. This is an obvious consequence of the fact that different templates decompose and thermodesorb at different temperatures. However, it was somewhat unexpected that the decomposition/desorption of the same kind of the template may be dramatically influenced by the framework composition of materials [4,10-14]. This can be understood as an influence of the framework structure on the process of Hoffmann elimination of alkylammonium to the corresponding alkene and low molecular weight amine [4,8], This decomposition process leads not only to the elimination of the electrostatic framework-template interactions but also to the formation of decomposition products of lower molecular weight than that of the surfactant. Thus, the framework-surfactant interactions are crucial factors determining the thermogravimetric behavior. 

Equation (IS) provides the zeolite chemist with a powerful quantitative method for the determination of framework composition of zeolites. By comparing (Si/A1)NMR values with the results of chemical analysis, which gives bulk composition, the amount of nonframework (six-coordinated) aluminum can be calculated. This is of particular value in the study of chemically modified zeolites (see Sections III,J-III,M). 

Structure type code Type of material Name Formula Framework composition Channel Pore system opening Cage Comments... 

The chemical analysis of the synthesized samples expresses in the framework composition, that is, (MeIAlyPz)02 provides the as-synthesized sample framework composition, which is shown in Table 3.7, indicating the presence of about 1% of Me in the synthesized aluminophosphate [29], From the x-ray powder diffractograms (Figure 3.14), it is noted that the crystallized products exhibit all the characteristic reflections of the MeAPO-5 molecular sieves [140] and a high crystallinity and degree of purity [29],... 

The total exchange capacity (TEC) of an aluminosilicate zeolite is a function of the framework composition that is, the Si/Al ratio. It is easy to obtain a numerical relation between the TEC, C, in milliequivalent per gram, and the number of A1 atoms per framework unit cell, NAl [5]... 

Despite the variation in framework composition, both the 16 and 35 structure-types show characteristic ratios of approximately 10 and 9, respectively, for the number of T02 units per template. These values are completely consistent with structural knowledge. For the 16 structure (lj)> there are 10 T02 per cage and room for 1 QUIN 1n each. For the 35 structure there are 9... 

Further support for the conclusion that extensive realumination has taken place comes from the increase in unit cell parameter and from IR spectra (4). The spectra of treated samples show shifts to lower frequencies in the framework vibration region with respect to the untreated samples, except for the T-O bending at ca. 450 cm 1 which is known (17) to be insensitive to framework composition. These changes are considerable and consistent with the increase in the framework aluminium content. The band at ca. 730 cm 1, related to the symmetric Si-O-Al or to "isolated" AIO4 tetrahedra, increases in intensity following the treatment. Furthermore, the band at ca. 812 cm-1 which is known to shift to lower frequencies and decrease in intensity with an increase in framework aluminium is clearly observed in the realuminated product. 

Third, the relative intensities of the Si(nAl) signals in realuminated samples are strikingly different from those in the as-prepared zeolites with the same framework composition, which means that the distribution of Si and A1 in the treated zeolites is different. This is a consequence of the different site selectivities discussed above, but also of the fact that both the original Si(OAl) sites and the Si(OAl) sites created during ultrastabilization are available for A1 substitution. 

Chemical analysis overestimates framework composition because this sample contained amorphous Si02 ... 

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