Zeolitization Parameters

The composition and the basicity of the synthesis gel, the presence of mineralizing agents and salts or ion pairs and template molecules, the synthesis temperature and time, are all important parameters determining the outcome of a synthesis experiment in terms of nature of the crystalline phase, composition and compositional homogeneity, purity, size, and morphology of the crystals [98]. The typical chemical composition of a synthesis hydrogel, commonly expressed in terms of molar composition of the oxides, is as follows  

In aluminosilicate gels, silicate species become more deprotonated at higher alkalinity, resulting in a decreased condensation rate. The condensation rate of silicate with aluminate at higher alkalinity, however, is much less affected [133,137]. 

Added F anions take the role of mineralizing catalyst in acid medium, via complexation with Si(Al) atoms [137]. Other elements, viz. Ga andTi, forming complexes with fluoride, can be substituted in the zeolite framework. High F concentrations may also impart coordination ability from tetrahedral into octahedral. As in such conditions, the degree of supersaturation is rather limited, nucleation and growth is reduced and slow growth occurs with formation of a decreased number of structural defects. 

Zeolite crystallization is thermally activated and enhanced at higher temperatures, viz. from 373 to 473 K. If the micropore-filling agents like water or organics are becoming too volatile at high temperatures, zeolite formation ceases at the expense of layered structures like clay minerals [93]. 

Very high temperature synthesis of porous open structures with pore fillers of low volatility is expected to be possible [98]. 

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Figure 3. Equilibrium sorption data for CHt on pure 5A zeolite parameter is temperature data of (x) Roberts, (O) Derrah, COJ Loughlin, (A) Lederman (---------) theoretical curves calculated using Equation 4 and data in Table I...

Figure 4. Equilibrium sorption data for Kr on pure 5A zeolite parameter is...

Another zeolite parameter which may have a strong effect on the conversion, is the silicon to aluminium ratio. Some ZSM-5 samples with approximately the same crystal size, but different silicon to aluminium ratios were tested. It appeared, that with increasing silicon to aluminium ratio, the yield of aminopicoline decreased probably because of the decreasing number of active sites (Figure 3). 

Vapor phase synthesis of MTBE over zeolite Beta is very efficient. For example. Beta zeolite is three times more active than Amberlyst-15 for MTBE vapor phase synthesis at 50°C. The better catalytic performance of H-Beta was verified in liquid phase. The external surface area, the amount of bridging AlOHSi, and silanol groups are important zeolite parameters for the ether synthesis. The reaction occurs on bridging AlOHSi acid sites. The highest yields are reached for low SiOH/AlOHSi ratios where methanol clusters bonded to silanol groups allow accessibility of isobutene to the active AlOHSi groups. 

Figure 12.2 Schematic overview of zeolite parameters determining acidity in silicate-based zeolites [28]. M, trivalent element in tetrahedral coordination in the framework.

I have carried out widespread studies on the application of a sensitive and selective preconcentration method for the determination of trace a mounts of nickel by atomic absorption spectrometry. The method is based on soi ption of Cu(II) ions on natural Analcime Zeolit column modified with a new Schiff base 5-((4-hexaoxyphenylazo)-N-(n-hexyl-aminophenyl)) Salicylaldimine and then eluted with O.IM EDTA and determination by EAAS. Various parameters such as the effect of pH, flow rate, type and minimum amount of stripping and the effects of various cationic interferences on the recovery of ions were studied in the present work. 

Having chosen a suitable refrigerant, the best adsorbent must be found. Zeolites, silica gels and chemical adsorbents have been used as well as carbons, but this chapter will concentrate on the carbon adsorbents. An indication as to the range of cop s that can be expected and the influence of the type of carbon used can be obtained by modelling the perfonnance of carbons with a range of adsorption parameters. For this purpose it is preferable to use the Dubinin-Raduschkevich... 

The range of the parameters which covers most carbons (and zeolites) is ... 

The elementary building block of the zeolite crystal is a unit cell. The unit cell size (UCS) is the distance between the repeating cells in the zeolite structure. One unit cell in a typical fresh Y-zeolite lathee contains 192 framework atomic positions 55 atoms of aluminum and 1atoms of silicon. This corresponds to a silica (SiOj) to alumina (AI.O,) molal ratio (SAR) of 5. The UCS is an important parameter in characterizing the zeolite structure. 

Various analytical tests determine zeolite properties. These tests supply information about the strength, type, number, and distribution of acid sites. Additional tests can also provide information about surface area and pore size distribution. The three most common parameters governing zeolite behavior are as follows ... 

UCS, rare earth, and sodium are just three of the parameters that are readily available to characterize the zeolite properties. They provide valuable information about catalyst behavior in the cat cracker. If required, additional tests can be conducted to examine other zeolite properties. 

In the absence of steric constraints in Eq. (1.24a) Eact will not vary. Eads and 9 are the parameters that significantly change with hydrocarbon chain length or zeolite. 

Fig. 1. a) Standard protonation enthalpy in secondary carbenium ion formation on H-(US)Y-zeolites with a varying Si/Al ratio, b) Effect of the average acid strength for a series of H-(US)Y zeolites experimental (symbols) versus calculated results based on the parameter values obtained in [11] (lines) for n-nonane conversion as a function of the space time at 506 K, 0.45 MPa, Hj/HC = 13.13 (Si/Al-ratios 2.6, 18, 60)... 

Figure 5 shows such an Auger parameter plot for a series of aluminum compounds. Due to crowding, several values given in Table I are omitted from this plot. Most of the compounds are grouped to the lower left, whereas aluminum metal is at the upper right. Intermediate between these are sodium zeolite and zinc aluminate (ZnAl204). 

Table 1 Structural Parameter as Derived from the Mo K-Edge EXAFS for Mo and Co-Mo Sulfide Catalysts Encaged in a NaY Zeolite...

Fichtner-Schmittler, H., Lohse, U., Engelhardt, G. et al. (1984) Unit-cell constants of zeolites stabilized by dealumination - determination of al-content from lattice-parameters, Cryst. Res. Technol., 19, Kl. 

Irrespectively of the iron content, the applied synthesis procedure yielded highly crystalline microporous products i.e. the Fe-ZSM-22 zeolite. No contamination with other microporous phases or unreacted amorphous material was detected. The SEM analysis revealed that size and morphology of the crystals depended on the Si/Fe ratio. The ZSM-22 samples poor in Fe (Si/Fe=150) consisted of rice-like isolated crystals up to 5 p. On the other hand the preparation with a high iron content (Fe=27, 36) consisted of agglomerates of very small (<0.5 p) poorly defined crystals. The incorporation of Fe3+ into the framework positions was confirmed by XRD - an increase of the unit cell parameters with the increase in the number of the Fe atoms introduced into the framework was observed, and by IR - the Si-OH-Fe band at 3620 cm 1 appeared in the spectra of activated Fe-TON samples. 

Interaction energies, geometrical parameters and shifts in vibrational frequencies for various AN and CO adsorption complexes found in zeolites Na-A and Na-FER. 

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