Zeolite unit cell size

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... 

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). 

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]. 

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)...

Pine, L.A., Maher, P.J. and Wachter, W.A., "Prediction of cracking catalyst behavior by a zeolite unit cell size model", J. Catal., 85, 466-476 (1984). 

Aluminum distribution in zeolites is also important to the catalytic activity. An inbalance in charge between the silicon atoms in the zeolite framework creates active sites, which determine the predominant reactivity and selectivity of FCC catalyst. Selectivity and octane performance are correlated with unit cell size, which in turn can be correlated with the number of aluminum atoms in the zeolite framework. ... 

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. 

Unit Cell Size (UCS). The UCS is a measure of aluminum sites or the total potential acidity per unit cell. The negatively-charged aluminum atoms are sources of active sites in the zeolite. Silicon atoms do not... 

It has already been mentioned that the formation of ultrastable Y zeolites has been related to the expulsion of A1 from the framework into the zeolite cages in the presence of steam (8,9), and the filling of framework vacancies by silicon atoms (11,12). This results in a smaller unit cell size and lower ion- exchange capacity (6). It also results in a shift of X-ray diffraction peaks to higher 20 values. Ultrastable Y zeolites prepared with two calcination steps (USY-B) have a more silicious framework than those prepared with a single calcination step (USY-A). Furthermore, since fewer aluminum atoms are left in the USY-B framework, its unit cell size and ion-exchange capacity are also lower and most of the nonframework aluminum is in neutral form (18). 

Aluminum-deficient Y zeolites prepared by partial removal of aluminum with a chelating agent (e.g. EDTA) also show improved thermal and hydrothermal stability compared to the parent zeolite. The optimum stability was found in the range of 25 to 50 percent of framework A1 extraction (8). However, the maximum degree of dealumination is also affected by the SiO /Al O ratio in the parent zeolite a higher ratio appears to allow more advanced dealumination without loss of crystallinity (8,25,45). Above 50 or 60 percent dealumination, significant loss of crystallinity was observed. Calcination of the aluminum-deficient zeolite resulted in a material with a smaller unit cell size and lower ion-exchange capacity compared to the parent zeolite. 

Dealumination processes are usually used in conjunction with production of the acid form of zeolite Y for many catalytic apphcations, and zeolites A and X are in most cases no longer used in acid catalytic applications because the high amount of aluminum in their frameworks makes them difficult to stabilize using various dealumination techniques. Successful dealumination and at least partial annealing of defects, resulting from movement of silicon cations to the aluminum vacancies, can be assessed by measurement of the reduction of the unit cell size of the zeolite. This unit cell reduction is a consequence of the relative ionic radii of AF (0.54 A) and Si + (0.40A). 

J.H., and O Donnell, D.J. (1985) Determination of framework aluminum content in dealuminated Y-type zeolites a comparison based on unit cell size and wavenumber of i.r. bands. Zeolites, 6, 225-227. 

Figure 16.n Hydrocracking catalyst performance in single stage recycle as a function of zeolite content and unit cell size. 

Tables 5.1 and 5.2 hst the main physicochemical properties of the modified zeolite characterized by a series of analyzing methods. XRF, XRD, and Al NMR results listed in Table 5.1 showed that with the increasing intensity of CP treatment, nonframework aluminum was ranoved gradually with httle influence on zeohte framework (unit cell size (UCS) changed little), thus the relative crystallinity increased. The removal of nonframework aluminum can also be verified by the FT-IR results shown in Figure 5.1, in which it can be seen that after CP treatment the intensity of the small peak at wave number 3660-3690 cm characterizing nonframework hydroxyl groups decreased step by step.

When the zeolite surface area is plotted as a function of catalytic coke the correlation improves. The best correlation between the physicochemical properties of the catalyst and catalytic coke is the one involving an amount of aluminums in the framework estimated from the unit cell size by Equation 10.1 [1], as it is evidenced in Figure 10.2. 

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