Catalytic activity Micro

EVALUATION OF CATALYTIC ACTIVITY Micro-reactor tests... 

The steam treatment does however affect the Al-surroundings in the zeolite crystal. As seen in Fig. 2b, the intensity of both the tetragonally (at 0 ppm) and octahedrally (at 55 ppm) coordinated aluminum species decreases considerably after steam treatments for more than 4 h. Steam treatment for more than 8 h did not lead to a further decrease in the signal intensities. The decreases confirm that aluminum is extracted from the framework during the steaming process, as was also concluded from the 29Si MAS-NMR spectra (Fig. 1 b). This may lead to the formation of additional (micro) porosity, but the aluminum extraction could negatively affect the catalytic activity. 

Partial recrystallization of zeolites into composite micro/mesoporous materials leads to 1,3-2 fold increase of n-octane conversion and 2-3 fold increase of the yield of target products - branched octanes, indicating improved accessibility of active sites and transport of bulky molecules provided by mesopores. In the case of BEA series recrystallization in mild conditions leads to remarkable increase in selectivity to i-octane from 40 to 67%. On the contrary, complete recrystallization results in low catalytic activity, comparable with MCM-41 catalyst. 

Composite zeolite/mesoporous materials show remarkably high catalytic activity, stability and selectivity in the isomerization of n-octane due to high zeolitic acidity combined with improved accessibility of active sites and easier transport of bulky molecules provided by mesopores. The best catalyst performance can be achieved by the optimisation of the contributions of micro- and mesopores in the composite material. 

Nowadays synthesis of mesoporous materials with zeolite character has been suggested to overcome the problems of week catalytic activity and poor hydrothermal stability of highly silicious materials. So different approaches for the synthesis of this new generation of bimodal porous materials have been described in the literature like dealumination [4] or desilication [5], use of various carbon forms as templates like carbon black, carbon aerosols, mesoporous carbon or carbon replicas [6] have been applied. These mesoporous zeolites potentially improve the efficiency of zeolitic catalysis via increase in external surface area, accessibility of large molecules due to the mesoporosity and hydrothermal stability due to zeolitic crystalline walls. During past few years various research groups emphasized the importance of the synthesis of siliceous materials with micro- and mesoporosity [7-9]. Microwave synthesis had... 

In view of the numerous advantages of POMs the development of strategies for converting them to solid catalysts is of primaiy interest. First, catalytically active POMs can be heterogenized in the form of insoluble salts using Cs, Ag, K, NH/ and some organic cations [37,49, 58-64]. Such salts possess micro/mesoporous structure and their smface area is typically in the range of 10-150 mVg. 

If one can understand what the basic parameters of the reactants and the surface are that determine the reaction dynamics (activation barriers etc.) then given a micro-kinetic model one has a knowledge of the factors determining the catalytic activity of the catalyst. 

In spite of the shortcomings of the modelling, the real strength is that they can be used to understand ( em variations in the catalytic activity from one system to another. The stability of the intermediates and the activation barriers are among the input parameters for the micro-kinetic model, and it is straight forward to calculate the effects of changes in stability for some or all the intermediates. 

The environmental effects are caused by the micro-environments constituted by the domain of a polymer ligand. The electrostatic domain of a polymer-metal complex was demonstrated in the reaction of the polymer-Co(ni) complex with ionic species (Section IVA), and was shown to be utilized in the catalytic activity of the polymer-Cu complex (Section VIA). In another case, the hydrophobic domain was predominant, ie. in the reaction with hydrophobic substrates (Sections IVB and VIIC). The environmental effects of a polymer ligand also include dynamic effects, Which vary with the solution conditions (Section IIIC). 

Details about preparation and characterization of dispersed microcrystals can be found in review chapters [322] and will not be dealt with here. All investigations indicate that the properties of microcrystals differ considerably from those of bulk metals (and from those of adatoms and thin films as well) [328], and that they can also be influenced by the nature and texture of the support. In particular, micro-deposits of precious metals on various inert supports (Ti, Ta, Zr, Nb, glassy carbon etc.) exhibit enhanced electrocatalytic effects as evaluated per metal atom, while the mechanism of H2 evolution remains the same [329], and the enhancement increases as the crystallite size decreases [326, 331] (Fig. 17). However, while this is the case with Rh, Pt, Os and Ir, Pd shows only an insignificant increase, whereas for Ru even a drastic decrease is observed [315, 332]. Thus, the effect of crystal size on the catalytic activity appears to depend on the nature of the catalyst (without any relation with the crystal structure group) [330]. 

In addition to the coating techniques discussed below, one may also consider making the whole micro reactor out of the catalytically active material [44]. However, as precious metals are frequently used, this is not a cost-competitive option for mass production, of course. 

One reason to use micro structured reaction chambers is certainly the possibility of describing the fluid dynamic behavior in these structures due to the laminar flow regime. With the following calculations the reactive gas flow in a square micro structure with coated catalytically active walls will be studied in detail. The task was to find a channel arrangement and to calculate the residence time distribution of this arrangement numerically (Figure 4.93). 

Figure 5.3.9 (A) Simplified geometric model [46, 89] for the preparation of industrial Cu/ZnO catalysts comprising subsequent meso- and nanostructuring of the material from [56], In a first micro structure directing step (mesostructuring), the Cu,Zn coprecipitate crystallizes in the form of thin needles of the zincian malachite precursor, (Cu,Zn)2(0H)C03. In a second step, the individual needles are decomposed and demix into CuO and ZnO. The effectiveness of this nanostructuring step depends critically on a high Zn content in the precursor, which in zincian malachite is limited to Cu Zn ca. 70 30 due to solid-state chemical constraints [75]. Finally, interdispersed CuO/ZnO is reduced to yield active Cu/ZnO. (B) Chemical memory Dependence of catalytic activity in methanol synthesis on the conditions of the coprecipitation and aging steps, from [85].

The 27A1 and 29Si NMR measurements (7) showed that after treatment with 0.01 molar HC1 most of the amorphous silica-containing material is removed from the parent catalyst A. This can be understood easily since the maximum solubility of silica (16) is reached at pH = 2. Although the improved performance of the treated catalyst cannot be entirely explained by the removal of less active material, i.e. the increase of the number of Lewis acid sites per mass unit, it is believed that these silica species block most of the catalytically active centers, i.e. the highly dispersed Lewis acidic alumina sites in the micro- and mesopores of the parent US-Y zeolite. 

Type A includes the classical approach to chiral polymeric catalysts a monomeric ligand is synthesized and attached to a random coil polymer. If a binding position for the metal is incorporated in every constitutional repeating unit, the catalytically active centres are randomly oriented, which is quite a delicate situation. Their micro-environment is dependent on the position on the polymer, and it can be expected that this is true for their asymmetric induction as well. Such a catalyst can hardly be optimized rationally. 

Figure 7. Polybinaphthols as stereoregular polymers with a well-defined micro-environment of the catalytically active centers.

There are two extreme views in modeling zeolitic catalysts. One is based on the observation that the catalytic activity is intimately related to the local properties of the zeolite s active sites and therefore requires a relatively small molecular model, including just a few atoms of the zeolite framework, in direct contact with the substrate molecule, i.e. a molecular cluster is sufficient to describe the essential features of reactivity. The other, opposing view emphasizes that zeolites are (micro)crystalline solids, corresponding to periodic lattices. While molecular clusters are best described by quantum chemical methods, based on the LCAO approximation, which develops the electronic wave function on a set of localized (usually Gaussian) basis functions, the methods developed out of solid state physics using plane wave basis sets, are much better adapted for the periodic lattice models. 

Catalytic Activity Measurement. The reaction was carried out in a stainless steel microflow reactor. In each run, 2 g catalyst was placed in the reactor and heated to 520 °C under a nitrogen stream. The nitrogen stream was replaced by a light naphtha vapor fed by a micro plunger pump. The reaction was carried out at 520 °C, under various pressures and WHS Vs without any hydrogen addition. The products were analyzed periodically by gas chromatography. The properties of the light naphtha are shown in Table I. 

As is shown in Section XII, it is possible to produce electrodeposited composite materials, for example, of Ni and Mo (75) or Ni and W, that have high specific real areas and exhibit quite different Tafel slopes (much lower values) from those of corresponding bulk alloys having the same nominal compositions. It is believed that this arises on account of the much lower effective real current densities that then obtain at ordinary practical current densities and possible involvement of micro-metal clusters having intrinsically better catalytic activity as referred to above in the case of Raney materials. Codeposited, sorbed H may also be important for HER catalysis, giving rise to hydridic phases (75,134). 

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