Zeolite crystallization with

As described in the previous section, the silica-alumina catalyst covered with the silicalite membrane showed exceUent p-xylene selectivity in disproportionation of toluene [37] at the expense of activity, because the thickness of the sihcahte-1 membrane was large (40 pm), limiting the diffusion of the products. In addition, the catalytic activity of silica-alumina was not so high. To solve these problems, Miyamoto et al. [41 -43] have developed a novel composite zeohte catalyst consisting of a zeolite crystal with an inactive thin layer. In Miyamoto s study [41], a sihcahte-1 layer was grown on proton-exchanged ZSM-5 crystals (silicalite/H-ZSM-5) [42]. The silicalite/H-ZSM-5 catalysts showed excellent para-selectivity of >99.9%, compared to the 63.1% for the uncoated sample, and independent of the toluene conversion. 

Large zeolite crystals with dimensions of tens and hundreds of micrometers have proven to be irreplaceable as model materials for reactivity and diffusion studies in the field of zeolite science and heterogeneous catalysis [1-3], These large crystallites often possesses complex structures consisting of several intergrown subunits and since the pore orientations of the different elements are not always aligned, this phenomenon can have a considerable effect on the accessibility of the pores in different crystallite regions [4]. 

Figure 1.5. Schematic view of some channels in a hexagonal zeolite crystal with cylinder morphology.

We also prepared a wide range of Py loading ranging from 0.007 to 0.182 for zeolite crystals with a length of approximately 650 nm. The crystals were modified with Ox+ acceptors as before. Figure 13b shows the emission spectra upon excitation at 470 nm. The spectra are scaled to the same height at the Py emission maximum, which allows better comparison. The Ox emission has its... 

In this section, we detail our results on the nucleation and growth of zeolite crystals with Si/Al ratios between 1 and 2. Various perturbations, including the effects of reaction time, D20, CH30H and C2Hs0H on the zeolite process are examined. A narrow range of starting compositions and reaction conditions are chosen, so that the effects of the perturbations can be evaluated with a minimum set of variables. These results are discussed in the context of present theories of zeolite growth in the next section. 

Dehydrated zeolite crystals with well-defined pore sizes to admit molecules smaller than the pores. Often used to adsorb water from solvents or reactions, (p. 964)... 

For the higher beam currents necessary for microanalysis in a reasonable time, mass-loss damage at analysis points is likely to occur. For zeolite crystals with low Si Al ratios, mass loss was evident as holes in the specimen at high beam currents (see Figure 2). For such specimens very low beam currents were required for analysis. However, even in cases where mass loss is evident, the trends in compositional variations across particles will still be valid. 

In type a., the separating zeolite layer is equipped with catalytic sites (Bronsted add sites, Lewis acid sites (cations, special Al-sites), metal clusters, catalytic complexes). In type b., the non-supported side of the zeolite layer serves as a support for catalytic entities, e.g. metal crystallites. In type c., zeolite crystals with catalytic power are embedded in a matrix, e.g. a polymer membrane. 

Finally, Vroon et al. [82,97] reported the synthesis of continuous porous films of ZSM5 on top of y-alumina supported membranes (pore diameter 4 nm) by slip-casting with a zeolite crystal suspension. The porous zeolite layers (thickness 1-2.5 pm) consist of densely packed zeolite crystals with a diameter of 70-80 nm and with micropores in the zeolite and mesopores (diameter 8-24 nm) between the zeolite particles. This zeolite layer can be used as a support for further processing, e.g., pore filling of the mesopores or deposition of catalysts. First experiments by Vroon et al. to fill the mesopores by in situ crystallisation of MFI in the pores did not result in gas-tight membranes... 

Polycrystalline zeolite membranes consist of inter-grown zeolite crystals with no apparent cracks or pinholes (Fig. lA). These films are composed of only zeolite (i.e., there are no non-zeolite components such as amorphous silica or polymer). They are normally supported on a substrate although free-standing films have also been synthesized. Membranes can be prepared on different substrates such as silicon wafer, quartz, porous alumina, carbon, glass, stainless steel (SS), gold, etc. Polycrystalline films are primarily prepared by hydrothermal synthesis methods including in situ crystallization, seeded growth,and vapor transport, " and have potential use in all of the applications discussed in this entry. 

A procedure able to modify the lattice composition of a preformed zeolite leaving the framework topology relatively unchanged is indicated with the terms indirect synthesis or secondary synthesis . The method consists in contacting the zeolite crystals with a suitable compound of the element to be inserted In the framework. This procedure has been mainly used to substitute silicon for aluminum atoms in Y zeolite [40]. Several examples describing the indirect synthesis of titanium containing zeolites are reported in the literature (Table 1). 

The treatment of the zeolite crystals with titanium compounds are made either through the contact with a liquid phase [41], or with a gas phase [42-44]. The titanium salts used for the secondary synthesis are gaseous TiCl or an aqueous solution of (NH4)2TlFg. 

MTT zeolite crystals with siliceous layer on their external surface were synthesized using a two step hydrothermal procedure in which cores prepared from an aluminosilicate gel were transferred into a siliceous gel for farther crystal growth. The thickness of the siliceous shell was varied by varying the proportion of the two gels. The samples were characterized using physicochemical methods (XRD, XPS, Al MAS NMR, SEM, Na adsorption). The zeolites were converted into bifunctional catalysts and evaluated in hydroisomerization of decane. 

Zeolite materials with tunable size and volume of mesopores can be prepared by using dispersed carbon black particles with narrow distribution of their sizes as inert mesoporous matrix or as secondary template. In such confined space for synthesis the crystallization of zeolite gel occurs inside the interparticle voids of carbon matrix [10,11,12]. In the case of generation of mesopores by secondary templating by means of addition of carbon black into the reaction mixture, zeolite crystals are formed around carbon particles [13]. After burning off a carbon matrix or carbon particles, zeolite crystals with a controlled pore size distribution and a crystalline micro-mesoporous hierarchical structure are prepared. 

P-31 - The fitting equation for zeolite crystallization with seeds... 

Figure 4. Scheme for the growth of coke a lomerates on the outer surface of zeolite crystals with corresponding changes in the strengths of the acid sites. 

As an alternative way to study diffusion anisotropy in zeolite crystallites, it is possible to analyze the shape of the NMR signal attenuation with increasing field gradient intensity. Since in a powder sample all orientations of the zeolite crystals with respect to the field gradient direction are possible, the signal attenuation results as a superposition of exponentials of the type of Eq. 9 with diffusivities determined by the orientation of any individual crystallite. All information about the diffusion tensor must be contained, therefore, in the shape of the echo attenuation. 

In addition to the techniques already discussed above (e.g., XRD, IR) Vedrine emphasized the need of measuring adsorption capacities and adsorption rates. The former reveals the useful volume available for catalysts. The latter is equally important because it shows how accessible this volume is for reacting molecules. (E.g., adsorption into a zeolite crystal with most of its pores blocked by amorphous materials or coke will be much slower than into a crystal with all pores open.) Several other speakers have commented on the importance of adsorption measurements. 

Fig. 9.5-1 Balances of a molecular sieve fixed bed (left, fixed bed, center, pellet, right. zeolite crystal with some cages) (c) zeolite crystals (d) macro pore (e) outer boundary layer (f) micro pore...

Jordi, R.G. and Do, D.D., Frequency response analysis of sorption in zeolite crystals with finite intracrystal reversible mass exchange,. Chem. Soc., Faraday Trans., 88, 2411-2419, 1992. 

Cl. A molecular sieve zeolite adsorbent consists of pellets that are agglomerates of zeolite crystals with density p ystai scattered in a continuous phase of clay binder with a density p gy. In this case, there is an interpellet porosity 8g (between pellets—this is normal Eg) an intercrystal porosity Sp, (which is the porosity in the binder), and an intracrystal porosity p2, (inside the crystals). If the fraction of the particle volume that is crystals (including porosity within the crystals) is f jy, derive formulas for the total porosity, Vavajjabjg, and the particle and bulk densities. 

The shape selectivity of ZSM-5 is modified significantly by treatment with a variety of chemical reagents. For example, modification with phosphorus or boron was made by impregnating the zeolite crystals with aqueous phosphoric acid or orthoboric acid, followed by calcination in air to convert the acid into the oxides. Selected results are summarized in Table 4.3. Though the selectivity iorpara isomer in alkylation with ordinary ZSM-5 is close to that expected from the thermal equilibrium, selectivity as high as 97% is achieved with the modified ZSM-5. 

The traditional way to seek new zeolites was primarily based on a trial-and-error synthesis process. With the growing needs for applications of zeolite materials, the rational synthesis of zeolites with desired structures and functions has become the objective of synthetic chemists. However, the rational synthesis of zeolites with desired structures is still a great challenge due to the unclear formation mechanism of zeolite crystals. With the advancement of understanding the synthesis chemistry and strucmre chemistry of zeolites in recent... 

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