Zeolites unit cell

Figure 3-3. Silica-alumina ratio versus zeolite unit cell size.

Larger Zeolite Unit Cell Size AddZSM-5 Additive Higher Cat/Oil ratio Higher Mix Zone temperature Split Feed injection Riser Quench... 

All zeolite samples ( 1,4g) were used after dehydration procedure at 773 K under Ar. Then, weighted amount of DPP corresponding to 1 molecule per zeolite unit cell (UC) was added under dry argon at 293 K and the powders were shaken in the dark. Powders were transferred under dry argon in quartz glass Suprasil cell or in cylindrical EPR quartz tube and sealed. 

Other MFl-type zeolite-sorbate systems are known to exhibit similar behavior. In a recent study, Yu et al. [34] reported that at saturated loadings of -hexane a single MFl-type zeolite unit cell has an overall volume expansion of 2.3%, which can correlate to shrinkage in non-zeoUtic pores up to 7 nm for a 1 tm crystal when isotropic expansion is assumed. It was demonstrated that, even in membranes with large number of defects, the crystallite swelling caused the membrane to achieve significant separation between n-hexane and trimethylbenzene, iso-octane and 2,2-dimethylbutane using pervaporation [34]. 

The zeolite unit cell size was determined by X ray diffraction according to ASTM-D-3942-80 at SINTEF (SINTEF, Oslo, Norway). For data see Table 4.2. 

USY Catalysts. USY catalysts are advertised by catalyst vendors, as low coke/high octane catalysts. This behavior results from the smaller zeolite unit cell size due to dealumination (21.221. The controlled dealumination leads to fewer but stronger acid sites resulting in increased cracking relative to H-transfer. The decrease in the extent of the exothermic H-transfer reactions also results in net increase in the endothermic heat of cracking for USY catalysts (211. 

Metals passivation compliments the latest generations of FCC catalysts octane catalysts based on USY zeolite technology and chemical dealumination. Octane catalysts equilibrate at lower unit cell sizes, resulting in minimization of hydrogen transfer reactions (15). Commercial tests have demonstrated that antimony does not affect the zeolite unit cell size (9). 

Fig. 4. Line representations of zeolite structure (a) sodalite cage, or truncated octahedron (b) type A zeolite unit cell (c) unit cell of types X and Y, or faujasite (d) cation sites in type A (there are 8 1,3 II, and 12 III sites per unit cell) (e) cation sites in types X and Y (161,32 Y, 32 II, 32 IT, 48 III, and 32 IIT, sites per unit cell).

The first 10 to 25% of steam has the greatest influence. The zeolite unit cell size reduction, which should give an indication of the zeolite activity loss by dealum[nation [8] is not very sensitive to steam partial pressure, with the exception that some steam is necessary for ceil size shrinkage,... 

The next stage of characterization focuses upon the different phases present within the catalyst particle and their nature. Bulk, component structural information is determined principally by x-ray powder diffraction (XRD). In FCC catalysts, for example, XRD is used to determine the unit cell size of the zeolite component within the catalyst particle. The zeolite unit cell size is a function of the number of aluminum atoms in the framework and has been related to the coke selectivity and octane performance of the catalyst in commercial operations. Scanning electron microscopy (SEM) can provide information about the distribution of crystalline and chemical phases greater than lOOnm within the catalyst particle. Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) can be used to obtain information on crystal transformations, decomposition, or chemical reactions within the particles. Cotterman, et al describe how the generation of this information can be used to understand an FCC catalyst system. 

Commercially deactivated FCC Beats of varying matrix types and containing a wide range of sodium were characterized by t-plot surface area (ASTM D4365-85) to determine the effect of Na on zeolite and matrix area stability. The Beats were also examined by electron microprobe (Cameca SX50) to determine the Na distribution within a catalyst particle. Some of the Beats were separated into eight age fractions based on a modified sink/float procedure described in the literature (13,14). Bach age fraction was analyzed by ICP, t-plot and zeolite unit cell size (ASTM D3942-91). 

The above procedure of incorporating sodium to fresh catalyst has an inherent shortcoming. Sodium from FCC feedstock accumulate on catalysts which have been hydrothermally aged. During hydrothermal aging, the zeolite unit cell size decreases from above 24.50 A to typically lower than 24.30 A, the surf ace area of both zeolite and matrix decreases and transformation of kaolin clay to metakaolin occurs. 

Effect of Na on Fresh and Steam Deactivated Catalysts Properties of the two USY silica sol catalyst samples, having different method of sodium incorporation, are shown in Table 3. Both samples had similar zeolite and matrix surface areas and zeolite unit cell size after 4 hours at 1088K steaming. 

Surface areas of catalysts were determined by N2 adsorption using an ASAP 2000 analyzer from Micromeritics. Matrix and zeolite surface areas were calculated by the t-plot method accordingly to the ASTM-D-4365 standard test [11]. Zeolite unit cell size (UCS) was determined by X-Ray diffraction using a SIEMENS D-500 automated analyzer according to the ASTM-D-3942-80 standard [11]. 

Clinoptilolite, Zeolite, Unit Cell Na,(AI02),(Si02)so 2413 0... 

The number of acetic acid molecules sorbed per zeolite unit cell was determined by tga for three,ZSM-5 samples with different aluminium contents (Table 1). 

The number of molecules of acetic acid sorbed per zeolite unit cell was comparable to the number of aluminium atoms per unit cell (determined by Atomic Absorption analysis), and hence [7] the number of Bronsted sites per unit cell, suggesting that the strongly sorbed acetic acid is associated with the acid sites. 

The amount of armtonia that desorbed (2.4 0.1 molecules per zeolite unit cell) approached the number of zeolite acid sites per unit cell (2.5 0.1). Very little acetic acid desorbed. This implies that armtonia, being the stronger base, displaces acetic acid from the Bronsted sites. However, more than simple displacement of acetic acid by armtonia may have occurred, as acetic acid and armtonia reacted to form the salts... 

Zeolite Unit cell formula Nepal v Vm Nh Nioo... 

Fig. 3. Influence of USY zeolite unit cell size on the initial (TOS= 1 min) 2-butene conversion and TMP/DMH ratio during isobutane/2-butene alkylation at 50°C, 2.5 MPa total pressure, i-C4/2-C4 molar ratio of 15, and W1ISV (referred to the olefin) of 1-4 h 1.

Figure 9. Isoinerisation/hydrogen transfer as a function of zeolite unit cell size iX) NHAY CSY ZSM20 x CSY-S OSAPO-37138) REUSY (Cheng J Catal, 1969)...

Table IV. P-A Zeolite Unit Cell Dimensions, a vs. P2O5 Wt %...

Increasing the Si/Al ratio also decreases the average size of the zeolite unit cell. The use of the unit cell size as the independent variable of zeolite behavior was pioneered by Pine et al. The technique has simplicity and convenience to recommend it, but must be used with caution since many factors may affect the unit cell size. 

Calculations with point-charge arrays representing zeolite unit cells have, indeed, confirmed that the BE trends described above are closely mirrored by the trends of fhe average Madelung potential felt by the emitting atom type (Si, 0, Al,... 

The active surface of a zeolite is internal and intrinsic to the crystal structure. Diffraction techniques can therefore yield direct data on those structural features that control catalytic or sorptive performance. However, zeolite characteristics hamper the effective application of diffraction methods. Zeolite constituents have, generally, low atomic numbers and the normalized scattering power of a zeolite unit cell is relatively small. Zeolites have open firework structures supporting accessible void volumes which can be as much as 50% of the total crystal volume [1-3]. The void spaces are either empty (and hence contributing no scattered intensity to the measured diffraction pattern) or filled with species that are have positional or dynamic disorder and hence contribute to the diffraction peaks almost exclusively at low scattering angles. 

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