Zeolite catalysts physicochemical properties

The physicochemical properties of Schiff-base complexes encapsulated in zeolite70 and the surface chemistry of zeolite-encapsulated CoSalen and [Fe(bpy)3]2+ catalysts were studied and published.71... 

Chromium zeolites are recognised to possess, at least at the laboratory scale, notable catalytic properties like in ethylene polymerization, oxidation of hydrocarbons, cracking of cumene, disproportionation of n-heptane, and thermolysis of H20 [ 1 ]. Several factors may have an effect on the catalytic activity of the chromium catalysts, such as the oxidation state, the structure (amorphous or crystalline, mono/di-chromate or polychromates, oxides, etc.) and the interaction of the chromium species with the support which depends essentially on the catalysts preparation method. They are ruled principally by several parameters such as the metal loading, the support characteristics, and the nature of the post-treatment (calcination, reduction, etc.). The nature of metal precursor is a parameter which can affect the predominance of chromium species in zeolite. In the case of solid-state exchange, the exchange process initially takes place at the solid- solid interface between the precursor salt and zeolite grains, and the success of the exchange depends on the type of interactions developed [2]. The aim of this work is to study the effect of the chromium precursor on the physicochemical properties of chromium loaded ZSM-5 catalysts and their catalytic performance in ethylene ammoxidation to acetonitrile. 

An increase in the zeolite crystallites size would very likely produce substantial changes in the physicochemical properties of the catalyst and consequently on the selectivity for hydroisomerisation. Since the effect of the zeolite crystallites size in the nanoscale range cannot be predicted theoretically, n-hexadecane hydroisomerization was carried out on PtHBEA catalysts with different zeolite crystallites sizes. 

Steam is invariably present in a real exhaust gas of motor vehieles in relatively high concentration due to the fuel combustion. The influence of water vapor on catalytic performances should not be ignored when dealing with the aim to develop a practical TWCs. Cu/ZSM-5 catalysts once were regarded as suitable substitutes to precious metal catalysts for NO elimination[78], nevertheless, they are susceptible to hydrothermal dealumination leading to a permanent loss of activity[79], Perovskites have a higher hydrothermal stability than zeolites[35]. Although perovskites were expected to be potential autocatalysts in the presence of water[80], few reports related to the influence of water on the reactants adsorption, the perovskite physicochemical properties, and the catalytic performance in NO-SCR were previously documented. The H2O deactivation mechanism is also far from well established. 

When the zeolite surface area is plotted as a function of catalytic coke the correlation improves. The best correlation between the physicochemical properties of the catalyst and catalytic coke is the one involving an amount of aluminums in the framework estimated from the unit cell size by Equation 10.1 [1], as it is evidenced in Figure 10.2. 

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. 

As described above, it was found that physicochemical properties of the iron cluster supported on zeolite and the catalytic activity for toluene disproportionation were significantly affected by the preparation conditions. The catalyst which was prepared by modifying NH Y with 0.25M Fe(N03)3 solution at 323K showed the highest activity among the samples obtained. 

Qynthetic and natural zeolites are becoming increasingly important as catalysts, carriers of catalysts, and adsorbents. Zeolites are especially suited to these purposes because their properties can be modified by cation exchange. The literature describes several studies which show characteristic changes in physicochemical properties resulting from cation exchange— e.g., catalytic activity (1,2), acidic properties (3), adsorption behavior (4), structure of solid (5,6), and thermal stability (7,8). 

The synthesis of these titanium-substituted zeolites has been described to occur by a secondary synthesis process involving the reaction of [NH4]2TiF6 with the preformed corresponding zeolite (Section IV.G). The chemical and physicochemical properties described are not sufficient to establish the presence of Tiiv ions in framework positions. The titanium concentrations reported are much higher than the maximum values observed in titanium silicates for which isomorphous substitution has been demonstrated. The possible presence of Ti02 particles has not been investigated. No indication of the properties of these materials as catalysts in reactions typical of titanium silicates has been provided. It is therefore very doubtful that real isomorphous substitution has been obtained (Skeels et al., 1989 Skeels, 1993). 

We have shown that the changes in the shape selectivity can be explained by changes in diffusivity by using ZSM-5 (MFI type) and Y type zeolites as model zeolites. However, it is very difficult to derive the model equations for representing the deactivation mechanisms for every types of zeolites, since each type of zeolite has different pore structure Hence, the mechanism of deactivation should be clarified for each type of zeolites. Reports on the activity of zeolites which were determined experimentally are omitted here. However, it is still impossible to evaluate physicochemical properties of a catalyst from the spectrum of ammonia TPD, which is usually employed to evaluate the acidic properties of a catalyst, since the spectrum is affected by various factors. Therefore, it is difficult to obtain the exact relationship between acidic properties and the change in activity due to deactivation. However, if an accurate method to evaluate the acidic properties is developed, it is expected that we can clarify whether the coverage of acid sites or pore blockage is the dominant factor of decrease in the activity due to coke deposition. 

Table 9 Physicochemical properties of MOR structure zeolite catalysts with and without a hydrothermal aging... 

Beneficial Modification of HC-SCR DeNO Catalysts to Improve Hydro-thermal Stability. - Impairment of the hydrothermal durability of HC-SCR catalysts, particularly metal-exchanged zeolites which are of interest in high-temperature deNOx applications, can be either prevented or at least reduced by selecting appropriate parent zeolite structure, employing suitable preparation technique, modifying physicochemical properties, and adding subsequent cation. The former two approaches have been extensively described in the previous section and the other ones will be mainly discussed. 

A natural zeolite (NZ) containing mainly mordenite-type zeolite was treated with a 1.0 N HCl solution at 93°C for 20 h in order to remove impurities contained in the ore and to stabilize the pore openings of the zeolite structure [9]. The CuNZA catalyst was prepared by further treatment of NZA (natural zeolite treated wiA acid) with a 1.5 N NH4NO3 solution followed by drying and calcining at 500°C for 10 h and by repeatedly soaking in a 1.0 N Cu(NOj)2 solution at 93°C for 40 h. The physicochemical properties of the catalysts prepared in this study are listed in Table 1. 

It is not proposed to review the properties of zeolites in general. Structural aspects related to catalysis are, however, discussed in detail. Accessibility of sites and other related factors basic to an understanding of zeolite catalytic behavior are also examined. Considerable emphasis is given to the chemistry of activation processes, the characterization of acid zeolite catalysts, and the physicochemical interactions of adsorbed molecules with zeolite surfaces. 

Botella, R, Corma, A., Lopez-Nieto, J. M., Valencia, S., and Jacquot, R. 2000. Acylation of toluene with acetic anhydride over Beta zeolites influence of reaction conditions and physicochemical properties of the catalyst. /. Catal. 195 161-168. 

Although pillared clays could generate low cost fluidized cracking catalysts (FCC) with unique selectivity properties, they have not yet been accepted by the petroleum industry. In fact, refiners (to date) have been reluctant to field test these new catalysts because, in addition to a high tendency for coke generation, they exhibit hydrothermal stability inferior to that of those zeolites used in hydrocarbon conversion processes. The physicochemical properties of pillared clays have been reviewed elsewhere (1,2). 

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