Catalyst supports zeolites

The section on catalyst preparation (Chapters 8 and 9) is concerned with the preparation of catalyst supports, zeolites, and supported catalysts, with an emphasis on general principles and mechanistic aspects. For the supported catalysts the relation between the preparative method and the surface chemistry of the support is highlighted. The molecular approach is maintained throughout. 

Preparation of catalyst supports, zeolites and mesoporous materials... 

Catalysis by Metal Oxides and Zeolites. Metal oxides are common catalyst supports and catalysts. Some metal oxides alone are industrial catalysts an example is the y-Al202 used for ethanol dehydration to give ethylene. But these simple oxides are the exception mixed metal oxides are more... 

Typical conditions would be to pass a mixture of SO2 and air over Pt gauze or more commonly a V20,s/K20 contact catalyst. supported on Kiesel-guhr or zeolite. 

What is a zeolite How does it differ from a catalyst support ... 

Partial oxidation reactions are usually carried out over transition metal oxides capable of changing their valent state during their interaction with reacting molecules. Naturally, zeolites with their alumina-silicate composition did not prove themselves as good oxidation catalysts. They failed also to serve as efScient catalyst supporters, since transition metals being introduced into the zeolite matrix lose their ability to activate dioxygen [3,4],... 

Zeolites have ordered micropores smaller than 2nm in diameter and are widely used as catalysts and supports in many practical reactions. Some zeolites have solid acidity and show shape-selectivity, which gives crucial effects in the processes of oil refining and petrochemistry. Metal nanoclusters and complexes can be synthesized in zeolites by the ship-in-a-bottle technique (Figure 1) [1,2], and the composite materials have also been applied to catalytic reactions. However, the decline of catalytic activity was often observed due to the diffusion-limitation of substrates or products in the micropores of zeolites. To overcome this drawback, newly developed mesoporous silicas such as FSM-16 [3,4], MCM-41 [5], and SBA-15 [6] have been used as catalyst supports, because they have large pores (2-10 nm) and high surface area (500-1000 m g ) [7,8]. The internal surface of the channels accounts for more than 90% of the surface area of mesoporous silicas. With the help of the new incredible materials, template synthesis of metal nanoclusters inside mesoporous channels is achieved and the nanoclusters give stupendous performances in various applications [9]. In this chapter, nanoclusters include nanoparticles and nanowires, and we focus on the synthesis and catalytic application of noble-metal nanoclusters in mesoporous silicas. 

Because the size of the interlayers can be easily varied by incorporation of complex moieties of different sizes, these clays (montmorillonite, hectorite) may nevertheless compete as catalyst supports with zeolites which have a rigid, predetermined cavity size. 

Previously, we have developed several techniques for platinum supported zeolite catalysts to improve the benzene product purity, including on-line sulfiding [3], precoking [6], and dual-bed catalyst system [7]. We report herein an in-depth investigation on the synergism of proton zeolite and platinum supported ZSM-12 catalyst (Pt/Z12) in a cascade dual-catalyst system. 

A dual-bed catalyst system has been developed to tackle the key problems in benzene product impurity during heavy aromatics transalkylation processing over metal-supported zeolite catalysts. It was found that by introducing zeolite H-Beta as a complementary component to the conventional single-bed Pt/ZSM-12 catalyst, the cascaded dual-bed catalyst shows synergistic effect not only in catalytic stability but also in adjustments of benzene product purity and product yields and hence should represent a versatile catalyst system for heavy aromatics transalkylation. 

The dispersion and solid-state ion exchange of ZnCl2 on to the surface of NaY zeolite by use of microwave irradiation [17] and modification of the surface of active carbon as catalyst support by means of microwave induced treatment have also been reported [18]. The ion-exchange reactions of both cationic (montmorillonites) and anionic clays (layered double hydroxides) were greatly accelerated under conditions of microwave heating compared with other techniques currently available [19.]... 

The CVD catalyst exhibits good catalytic performance for the selective oxidation/ammoxida-tion of propene as shown in Table 8.5. Propene is converted selectively to acrolein (major) and acrylonitrile (minor) in the presence of NH3, whereas cracking to CxHy and complete oxidation to C02 proceeds under the propene+02 reaction conditions without NH3. The difference is obvious. HZ has no catalytic activity for the selective oxidation. A conventional impregnation Re/HZ catalyst and a physically mixed Re/HZ catalyst are not selective for the reaction (Table 8.5). Note that NH3 opened a reaction path to convert propene to acrolein. Catalysts prepared by impregnation and physical mixing methods also catalyzed the reaction but the selectivity was much lower than that for the CVD catalyst. Other zeolites are much less effective as supports for ReOx species in the selective oxidation because active Re clusters cannot be produced effectively in the pores of those zeolites, probably owing to its inappropriate pore structure and acidity. 

This section describes catalytic systems made by a heterogeneous catalyst (e.g., a supported metal, dispersed metals, immobilized organometaUic complexes, supported acid-base catalysts, modified zeolites) that is immobilized in a hydrophilic or ionic liquid catalyst-philic phase, and in the presence of a second liquid phase—immiscible in the first phase—made, for example, by an organic solvent. The rationale for this multiphasic system is usually ease in product separation, since it can be removed with the organic phase, and ease in catalyst recovery and reuse because the latter remains immobilized in the catalyst-philic phase, it can be filtered away, and it does not contaminate the product. These systems often show improved rates as well as selectivities, along with catalyst stabilization. 

Characterization techniques continue to develop and will impact their application to zeolitic systems. Aberration corrected electron microscopes are now being used to improve our understanding of catalysts and other nano-materials and will do the same for zeolites. For example, individual Pt atoms dispersed on a catalyst support are now able to be imaged in the STEM mode [252]. The application of this technique for imaging the location of rare-earth or other high atomic number cations in a zeolite would be expected to follow. Combining this with tomography... 

Organic molecules spontaneously form corresponding cation-radicals on inclusion within activated zeolites (Yoon and Kochi 1988, Yoon 1993, Pitchumani et al. 1997). Zeolites are crystalline alu-mosilicate minerals that are widely used as sorbents, ion exchangers, catalysts, and catalyst supports. As zeolites act as electron acceptors due to the presence of Lewis- or Broensted-acid sites, confined organic compounds occur to be electron donors. Frequently, the interaction of electron donor with electron acceptor centers spontaneously generates cation-radicals and traps the ejected electrons. 

Zeolites have attracted much attention as cobalt catalyst supports ]151-155]. Co2(CO)8 reacts rapidly from the vapor phase with X and Y faujasite type zeolites Co4(CO)i2, subcarbonyl species and [Co(CO)4] species form inside the pores. Further migration of Co4(CO)i2 carbonyl is inhibited because of pore size hmita-tions, and subsequent decarbonylation can take place only above 150 °C. In contrast, the reaction of Co2(CO)g with an A-type zeolite is limited to the surface due to the inability of the carbonyl precursors to pass through the apertures of the cavities of the structure. 

Theoretical studies on the Beckmann rearrangement mechanism over zeolite catalyst supported by experimental data have increased. The catalytic activity of the zeohte is determined by Brpnsted and Lewis acid sites created by protonation or activation by metallic cations. The reactivity of the acid sites is strongly influenced by the geometry and flexibility of the zeolite framework ". ... 

Recently, we reported that an Fe supported zeolite (FeHY-1) shows high activity for acidic reactions such as toluene disproportionation and resid hydrocracking in the presence of H2S [1,2]. Investigations using electron spin resonance (ESR), Fourier transform infrared spectroscopy (FT-IR), MiJssbauer and transmission electron microscopy (TEM) revealed that superfine ferric oxide cluster interacts with the zeolite framework in the super-cage of Y-type zeolites [3,4]. Furthermore, we reported change in physicochemical properties and catalytic activities for toluene disproportionation during the sample preparation period[5]. It was revealed that the activation of the catalyst was closely related with interaction between the iron cluster and the zeolite framework. In this work, we will report the effect of preparation conditions on the physicochemical properties and activity for toluene disproportionation in the presence of 82. ... 

Three Fe supported zeolites were prepared by modifying NH Y with 0.25M Fe(N03)3 at various temperatures from 293K to 373K, Figure 2 shows the influence of preparation temperature on physicochemical properties and catalytic activity of the obtained catalysts. 

Silica is of particular importance because of its use as a stable catalyst support with low acidity and its relationship to zeolite catalysts, which will be discussed in chapter 4. Silicon is an abundant material in the earth s crust and occurs in various forms including silica. Silica is also polymorphous with the main forms being quartz, cristobalite and trydimite. The stable room temperature form is quartz (Si02). Recently, a new family of stable silica-based ceramics from chemically stabilized cristobalites has been described using electron microscopy (Gai et al 1993). We describe the synthesis and microstructures of these ceramic supports in chapters 3 and 5. 

Most of the adsorbents used in the adsorption process are also useful to catalysis, because they can act as solid catalysts or their supports. The basic function of catalyst supports, usually porous adsorbents, is to keep the catalytically active phase in a highly dispersed state. It is obvious that the methods of preparation and characterization of adsorbents and catalysts are very similar or identical. The physical structure of catalysts is investigated by means of both adsorption methods and various instrumental techniques derived for estimating their porosity and surface area. Factors such as surface area, distribution of pore volumes, pore sizes, stability, and mechanical properties of materials used are also very important in both processes—adsorption and catalysis. Activated carbons, silica, and alumina species as well as natural amorphous aluminosilicates and zeolites are widely used as either catalyst supports or heterogeneous catalysts. From the above, the following conclusions can be easily drawn (Dabrowski, 2001) ... 

For the detailed study of reaction-transport interactions in the porous catalytic layer, the spatially 3D model computer-reconstructed washcoat section can be employed (Koci et al., 2006, 2007a). The structure of porous catalyst support is controlled in the course of washcoat preparation on two levels (i) the level of macropores, influenced by mixing of wet supporting material particles with different sizes followed by specific thermal treatment and (ii) the level of meso-/ micropores, determined by the internal nanostructure of the used materials (e.g. alumina, zeolites) and sizes of noble metal crystallites. Information about the porous structure (pore size distribution, typical sizes of particles, etc.) on the micro- and nanoscale levels can be obtained from scanning electron microscopy (SEM), transmission electron microscopy ( ), or other high-resolution imaging techniques in combination with mercury porosimetry and BET adsorption isotherm data. This information can be used in computer reconstruction of porous catalytic medium. In the reconstructed catalyst, transport (diffusion, permeation, heat conduction) and combined reaction-transport processes can be simulated on detailed level (Kosek et al., 2005). 

Patents cover a new reforming catalyst based on L-zeolite which gives a significantly higher yield of BTX, especially benzene, from light paraffinic feeds (11). Other new zeolites (12) may also offer advantages over the traditional reforming catalyst supports. 

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