Catalyst shape formation

The final size and shape of the catalyst particles are determined in the shape formation process, which may also affect the pore size and pore size distribution. Larger pores can be introduced into a catalyst by incorporating in the mixture 5 to 15 % wood, flour, cellulose, starch, or other materials that can subsequently be burned out. As a result, bidisperse catalyst particles are obtained. 

The formation of di- and tri-alkyl aromatics and b henyls from their aromatic precursor is a consecutive reaction, in A hich first the mono-substituted aromatic conopound is formed which subsequently reacts to form the di-alkyl compund. This was observed in the shape selective ethylation of bphenyl [91] isopropjdation of naphthalene [92] over H-Mordenite and the isopropylation of napthalene over HY [63], Therefore, if a dialkyl-isomer is the desired product, the reaction conditions (reaction time/readence time, partial pressures and tenperature) have to be optimized to obtain the the maximum yield of this de ed product. With shape selective catalysts the formation of poly-substituted aromatic compounds can be suppressed because they are not able to diffuse out of the channels of the zeolites [63]. 

Well developed fibrillar structures were formed when the polymerization was carried out with M0CI5 [27], whereas PPP aggregates of indefinite shape were observed with FeCE as catalyst. The formation of the fibrils was interpreted as resulting from simultaneous polymerization and crystallization, and was attributed to the fact that benzene reacts with polyphenylene chain ends aligned on the crystalline surface of already precipitated PPP [27]. In the case of FeCl3, the crystallinity of the PPP is less pronounced, and it was suggested that benzene could form polyphenyl side-chains or polynuclear structures, which would prevent the formation of the fibrillar structures [24]. 

The new catalyst shapes that were under development gave some compensation for these disadvantages. Higher activity, particularly at lower temperature, could help to avoid carbon formation. Carefully optimized shapes also gave a lower pressure drop and provided better heat transfer within the tubes to decrease the temperature of the tube wall. The use of manaiuite tubes also helped to improve operation. 

Mobil MTG and MTO Process. Methanol from any source can be converted to gasoline range hydrocarbons using the Mobil MTG process. This process takes advantage of the shape selective activity of ZSM-5 zeoHte catalyst to limit the size of hydrocarbons in the product. The pore size and cavity dimensions favor the production of C-5—C-10 hydrocarbons. The first step in the conversion is the acid-catalyzed dehydration of methanol to form dimethyl ether. The ether subsequendy is converted to light olefins, then heavier olefins, paraffins, and aromatics. In practice the ether formation and hydrocarbon formation reactions may be performed in separate stages to faciHtate heat removal. 

The type of manufacturing process, reaction conditions, and catalyst are the controlling factors for the molecular structure of the polymers [4-8]. The molecular features govern the melt processability and microstructure of the solids. The formation of the microstructure is also affected by the melt-processing conditions set for shaping the polymeric resin [9]. The ultimate properties are, thus, directly related to the microstructural features of the polymeric solid. 

Although cracking also occurs on chlorine-treated clays and amorphous silica-aluminas, the application of zeolites has resulted in a significant improvement in gasoline yield. The finite size of the zeolite micropores prohibits the formation of large condensed aromatic molecules. This beneficial shape-selectivity improves the carbon efficiency of the process and also the lifetime of the catalyst. 

Besides electronic effects, structure sensitivity phenomena can be understood on the basis of geometric effects. The shape of (metal) nanoparticles is determined by the minimization of the particles free surface energy. According to Wulffs law, this requirement is met if (on condition of thermodynamic equilibrium) for all surfaces that delimit the (crystalline) particle, the ratio between their corresponding energies cr, and their distance to the particle center hi is constant [153]. In (non-model) catalysts, the particles real structure however is furthermore determined by the interaction with the support [154] and by the formation of defects for which Figure 14 shows an example. 

The liquid-phase reduction method was applied to the preparation of the supported catalyst [27]. Virtually, Muramatsu et al. reported the controlled formation of ultrafine Ni particles on hematite particles with different shapes. The Ni particles were selectively deposited on these hematite particles by the liquid-phase reduction with NaBFl4. For the concrete manner, see the following process. Nickel acetylacetonate (Ni(AA)2) and zinc acetylacetonate (Zn(AA)2) were codissolved in 40 ml of 2-propanol with a Zn/Ni ratio of 0-1.0, where the concentration of Ni was 5.0 X lO mol/dm. 0.125 g of Ti02... 

For the amino-borane dehydrocoupling using [Rh(l,5-cod)(p-Cl)]2 as starting catalyst, an induction period and a sigmoid-shaped kinetic curve (plot of substrate conversion versus time) were also observed, consistent with metal-particle formation. But, for Ph2PH BH3... 

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