Commercial Zeolites

Each zeolite type has a typical silica/alumina ratio related to the crystal stmcture. It is possible, to increase the silica/alumina ratio however, by removing aluminum atoms from the Y-zeolite framework, with no effect on crystallinity. This, of course, modifies catalyst performance by changing the nature and 

TABLE 5.5. Shrinkage of Zeolite Unit Cell as Silica/Almnina Ratio Increases. 

Crystalline Y-zeolite is formed from framework silicort and aluminum atoms in tetrahedral coordination with oxygen atoms and linked to other tetrahedra. Certain rules apply  

Aluminum atom sites with no next nearest neighbors have less electronic interaction with other sites and more acidity— maximum acidity of individual sites is reached with about 9-12 aluminum atoms in the unit cell. 

Typical properties and the composition of zeolites used in FCC catalysts are listed in Table 5.8, with the molecular diameters of relevant hydrocarbons given in Table 5.6. 

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Currently, benzene alkylation to produce ethylbenzene and cumene is routinely carried out using zeohtes. We performed a study comparing a zeohte Y embedded in TUD-1 to a commercial zeolite Y for ethylbenzene synthesis. Two different particle diameters (0.3 and 1.3 mm) were used for each catalyst. In Figure 41.7, the first-order rate constants were plotted versus particle diameter, which is analogous to a linear plot of effectiveness factor versus Thiele modulus. In this way, the rate constants were fitted for both catalysts. 

As can be seen in the graph, the Y/TUD-1 catalyst was twice as active as the commercial Y catalyst. This is primarily due to its very high calculated diffusivity of 131x10 cm /sec, which is over 10 times the diffusivity calculated for commercial zeolite Y, 11x10 cm /sec. Extrapolation of the curve to zero particle size shows that the commercial Y zeolite is in fact intrinsically more active than the Y zeolite embedded in the TUD-1. If the Y zeolite in TUD-1 had been optimized for this reaction like the commercial Y catalyst, one should expect an even greater boost in performance. 

A commercial zeolite NaZSM-5 (Zeocat, Si/Al = 26) is used as a starting material. The HZSM-5 zeolite form is prepared by three aqueous exchanges at 80°C for 24h with a HNO3 solution (0.1M). 

Most of the commercial zeolite catalyzed processes occur either through acid catalysis fluid catalytic cracking (FCC), aromatic alkylation, methanol to olefins (MTO),... 

A proven solution to the binder problem is to use water insoluble organic polymer binders instead of clay. For example cellulose acetates and cellulose ethers binders are successfully employed to make commercial zeolitic adsorbents for sugar separation in aqueous solutions [154, 205, 218-223, 225-226, 231-232, 238]. This technique allows the use of zeoHte adsorbents in aqueous separation processes. 

The unique properhes of zeolite materials combined with the conhnuous separahon properhes of membranes make zeolite membranes very attrachve for a wide range of separahon and catalysis applications. Zeolite membranes, however, have poor processability, poor mechanical stability and are much more expensive than the commercial polymer membranes with current state-of-the-art membrane manufacturing process. So far, the only large-scale commercial zeolite membrane is the A-type zeolite membrane and it has been used for dehydrahon of alcohols [22]. Further advancement in making thinner zeolite membranes and continuous improvement in membrane produchon techniques and reproducibility will make zeolite membranes more successful in commercial applicahons. 

Catalytic testing. The samples were exchanged by Ce ions, then calcined at 680°C in dry air. In two types of catalytic tests, a commercial zeolitic catalyst (super D from Grace Davison) was taken as reference. 

The catalyst chosen for this study was a low metal, equilibrium, commercial zeolite-containing cracking catalyst obtained from Phillips Petroleum Company. No specific characterization of the catalyst is available. 

Aside from the question of the precise model by which relaxation times are interpreted there is the more practical problem of isolating that part of the relaxation specifically caused by diffusion. The contributions of exchange processes (see below), spin-rotation interaction (9), and spin diffusion (9) can be identified by temperature dependences different from that which is solely the result of the motionally modulated nuclear dipolar interaction as sketched above, and corrections can be made. The molecular rotation contributions to dipolar relaxation can be removed or corrected for by (a) isotopic substitution methods (19), (b) the fact that rotation is in some cases much faster than diffusion, and its relaxation effects are shifted to much lower temperatures (7, 20), and (c) doping with paramagnetic impurities as outlined above. The last method has been used in almost all cases reported thus far, more by default than by design, because commercial zeolites are thus doped by their method of preparation this... 

In commercial zeolites the proton relaxation of adsorbed molecules is controlled by magnetic interaction with paramagnetic impurities (Fe8+) of the zeolite lattice (9). The relaxation rate 1/Ti is proportional to the number of these paramagnetic centers. If the lattice is covered with diamagnetic metal atoms, this interaction should be reduced according to the amount and dispersity of the metal. 

We assume that the adsorbent mass used in the kinetic test consists of a sphere of radius R. It may be composed of several microsize particles (such as zeolite crystals) bonded together as in a commercial zeolite bead or simply an assemblage of the microparticles. It may also be composed of a noncrystalline material such as gels or aluminas or activated carbons. The resistance to mass transfer may occur at the surface of the sphere or at the surface of each microparticle. The heat transfer inside the adsorbent mass is controlled by its effective thermal conductivity. Each microparticle is at a uniform temperature dependent on time and its position in the sphere. 

Table 3.6 shows that zeolites Na-X and Na-Y have been produced depending on the reaction time. These materials are similar in phase composition to commercial Na-X and Na-Y zeolites, because, are composed of about 80 wt % of the zeolitic phase. In commercial zeolites because part of the phases are binders [124], the amount of zeolite is about 80 wt %. Applications of these type of materials are described elsewhere [11,25,52,125,126],... 

This methods depends on the implicit assumption that the uptake rate is controlled entirely by intracrystalline diffusion in an isothermal system, with all other resistances to either mass or heat transfer negligible. This is a valid approximation if diffusion is sufficiently slow or if the zeolite crystals are sufficiently large but the dominance of intracrystalline diffusional resistance should not be assumed without experimental verification. In many practical systems, particularly with small commercial zeolite crystals, the external heat and mass transfer resistances are in fact dominant. A detailed discussion of such effects has been given by Lee and Ruthven(5-7). 

Commercial zeolite based hydroisomerization catalysts comprise alumina bound and platinum impregnated dealuminated mordenite. The activity and selectivity of the hydroisomerization of n-paraffins is strongly influenced by acid leaching. The influence of silica to alumina ratio has been studied for pentane isomerization over platinum mordenite many times since one of the first papers published (6). 

Commercial zeolites are usually mixed with a binder such as y-alumina and shaped into pellets. The shaping procedure causes changes in binder porosity to give zeolites with different physical properties. 

Sophisticated catalysts, such as ZSM-5 or HZSM-5 [22] and other zeolites are also suggested in numerous papers, e.g. KEY [23], HY and H-mordenite [24], Re-zeolite-based Engelhardt FCC commercial catalyst [25], and steamed commercial zeolite catalyst [26]. These investigations are mainly devoted to fundamental studies and the correlation between feed composition, catalyst properties, process parameters and efficiency connected with prodnct distribution. Iron supported on silica-alumina, mesoporous silica and active carbons serves as the next example of materials applied in the waste plastics cracking [27, 28]. On the other hand, according to some results [29] application of cracking catalysts such as Zn-13X, Fe-5A and CoMo-HY are ineffective in waste plastics cracking. 

Ercan et al. studied the alkylation of ethylbenzene, EB, with light olefins (ethylene and propylene) over a commercial zeolite Y catalyst in a fixed-bed reactor with recycle [C. Ercan, F. M. Dautzenberg, C. Y. Yeh, and H. E. Earner, Ind. Eng. Chem. Res., 37 (1998) 1724]. The solid-catalyzed liquid-phase reaction was carried out in excess ethylbenzene at 25 bar and 190°C. Assume... 

In order to verify the diffusion through the crystals, we investigated the steady-state diffusion of a number of organic vapors in granules of zeolite CaA and NaX 12, 13). The experiments were carried out in the presence of carrier gas at atmospheric pressure. Granules of commercial zeolite samples were secured with hermetizing paste into the orifice of an aluminum membrane, and powdered crystallic zeolite was pressed into the orifice. 

Besides the commercial zeolites that are widely used as acid or bifunctiona> catalysts in the industrial applications, there is a need for obtaining new specific products. This implies, together with the improvement of already utilized zeolites, the study of other types previously synthesized and characterized that ara potentially applicable as catalysts for specific organic reactions. 

Figure 5.14. Al triple-quantum MAS NMR spectra of commercial zeolite mordenite in the hydrogen form, unheated and heated to various temperatures followed by hydration. Spinning side bands are marked by asterisks. The final spectrum is of a sample heated in two steps, of 650°C and 500°C with an intermediate rehydration step. After Chen et al. (2000), by permission of copyright owner.

Airborne erionite fibers are generally respirable. Erionite is reported to be a minor component in some commercial zeolites. 

In a study of selective adsorption of sulfur compounds and aromatic compounds in a hexadecane on commercial zeolites, NaY, USy, HY, and 13X by adsorption at 55 °C and flow calorimetry techniques at 30 °C, Ng et al. found that a linear correlation between the heat of adsorption and the amount of S adsorbed for NaY.162 Competitive adsorption using a mixture of anthracene, DBT, and quinoline indicates that NaY selectively adsorbs quinoline, while anthracene and DBT have similar affinity to NaY, indicating that NaY is difficult to adsorptively separate sulfur compounds from aromatic hydrocarbons with the same number of the aromatic rings. 

The commercial zeolites mordenite, X, Y and erionite as well as y-AljOj (microporous material with an irregular pore system) were used as supports in catalyst preparation, having comparable crystallite and particle sizes (1-2 pm) that were determined by SEM. 

Fig. 5 illustrates that the catalysts Pt/NaY, Pt/mordenite and Pt/erionite give rise to defined peaks between 370 K and 570 K. This is due to an appropriate range of Pt particle sizes, which are induced by the regular structures of the supports during the catalyst preparation. Pt/support interaction is negligible in this case (commercial zeolites as supports). 

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