Zeolite catalysts unit cell size

Steam treatment severity has been varied to match commercial equilibrium catalyst activities and other properties. One measure of a suitably steam-aged catalyst is the surface area which should be in the range of 51 to 200 m /g (Ritter, 1985). Perhaps a more meaningful measure is to use a bulk property of the zeolite, the unit cell size, which is measured by X-ray diffraction. Typically, the unit cell size of USY zeolites are reduced to below 24.26 A whereas RE-USY zeolites equilibrate to 24.26-24.32 A and REY zeolites to 24.5 A in the FCC unit (Scherzer,... 

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. ... 

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

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. 

Because of the low rare earth content and Initially higher zeolitic silica/alumina ratio of catalyst B, its unit cell size after steaming is lower than that for catalyst A. 

The main objective in FCC catalyst design is to prepare cracking catalyst compositions which are active and selective for the conversion of gas-oil into high octane gasoline fraction. From the point of view of the zeolitic component, most of the present advances in octane enhancement have been achieved by introducing low unit cell size ultrastable zeolites (1) and by inclusion of about 1-2 of ZSM-5 zeolite in the final catalyst formulation (2). With these formulations, it is possible to increase the Research Octane Number (RON) of the gasoline, while only a minor increase in the Motor Octane Number (MON) has been obtained. Other materials such as mixed oxides and PILCS (3,4) have been studied as possible components, but there are selectivity limitations which must be overcome. 

Analysis of Fractions. Surface areas and pore size distributions for both coked and regenerated catalyst fractions were determined by low temperature (Digisorb) N2 adsorption isotherms. Relative zeolite (micropore volume) and matrix (external surface area) contributions to the BET surface area were determined by t-plot analyses (3). Carbon and hydrogen on catalyst were determined using a Perkin Elmer 240 C instrument. Unit cell size and crystallinity for the molecular zeolite component were determined for coked and for regenerated catalyst fractions by x-ray diffraction. Elemental compositions for Ni, Fe, and V on each fraction were determined by ICP. Regeneration of coked catalyst fractions was accomplished in an air muffle furnace heated to 538°C at 2.8°C/min and held at that temperature for 6 hr. 

The results for residual carbon on equilibrium catalyst fractions (Figure 2) and for cumene cracking on regenerated equilibrium fractions (Table V) also indicate that cracking activity shows little dependence on zeolite content following completion of framework dealumination (minimum unit cell size). 

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 catalysts studied were prepared by dispersing 20 wt% of a USY zeolite with a 2.433 nm of unit cell size in either silica (Basf (D-ll-11)) or aluminic sepiolite (SA1). The final catalyst was steamed at 750flC in a 100% steam for 6 hours. 

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. 

Catalyst Structural Characteristics. Structural features of AFS and USY materials have been characterized in this work in terms of unit cell size, presence of extraframework material, active-site distributions, and pore-size distributions. These features are similar for both sets of USY and AFS samples which indicates that structural characteristics are not related to the source of Y zeolite. 

Coke selectivity directly influences the rate of catalyst deactivation as seen by comparing coke selectivities in Tables VI and VII with observed rate constants in Table V. Our data indicate calcined AFS zeolites show higher coke selectivities than USY zeolites when compared at similar unit cell sizes. This result suggests that distribution of framework acid sites(as reflected by the distribution of framework silicon) has a strong impact on coke selectivity. In addition, coke selectivity has been shown to correlate with the density of strong acid sites in the framework(20). Our data confirm this and show that steaming decreases the density of such sites which, in turn, leads to decreased coke selectivities. 

Gasoline octane number for steamed USY zeolites has been shown to correlate with unit cell size(24). This concept has been exploited to design USY catalysts for octane production(25). In this work we demonstrate that parameters other than unit cell size have an impact on octane similar conclusions have been reported by others (26). 

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). 

To determine if we could simulate in the laboratory the effect of sodium on conunercially deactivated FCC catalysts, we prepared catalysts containing Na in the range of 0.22 to 0.41 wt% by modifying the catalyst washing procedure and deactivated the samples at 1088 K fa- 4 hours under 1 atm of steam This steaming procedure is commonly used to prepare deactivated catalysts with physical properties (zeolite and matrix surface areas and unit cell size) that match conunercial Beats. 

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. 

This evidence suggests that not all Na species are mobile. Some Na species must in fact have reacted irreversibly with components on the catalyst, leaving it unavailable to poison the acid sites. It is likely that these reactions occur during the early stages of hydrothermal deactivation. The exact mechanism is unclear, but may involve reactions with extraffamework alumina. As the zeolite dealuminates from 24.55 to 24.25A unit cell size, approximately 65% of the initial framework alumina (about 15 wt% of the zeolite) comes out of the zeolite structure. Sodium, which also must leave the exchange sites as the zeolite dealuminates may react with this very reactive form of alumina. The other possibility is that as kaolin undergoes its transition to metakaolin at 800K... 

Under FCC regeneration conditions, the zeolite is quickly deactivated and, at the same time, its unit cell size is modified. The commercial FCC unit has a catalyst inventory made up of a mixture of particles of different ages. The newer particles contribute most of the activity to convert the gas oil. A part of the catalyst inventory has a low zeolite activity, and, notwithstanding, it has a substantial activity in the matrix, because the matrix is more stable than the zeolite. The oldest fraction is responsible for providing the activity to crack the heavy fraction of the feed. It is imperative to pay special attention to the design of the matrix and specifically to its pore size distribution, surface area, activity and stability. 

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]. 

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