Catalyst Activity and Selectivity

The product distrihution is influenced hy the catalyst properties as well as the various reaction parameters. The catalyst activity and selectivity are functions of acidity, crystalline size, silica/alumina ratio, and even the synthetic procedure. Since the discovery of the MTG process. 

The breakthrough in FCC catalyst was the use of X and Y zeolites during the early 1960s. The addition of these zeolites substantially increased catalyst activity and selectivity. Product distribution with a zeolite-containing catalyst is different from the distribution with an amorphous silica-alumina catalyst (Table 4-3). In addition, zeolites are 1,000 times more active than the amorphous silica alumina catalysts. 

Promotion, electrochemical promotion and metal-support interactions are three, at a first glance, independent phenomena which can affect catalyst activity and selectivity in a dramatic manner. In Chapter 5 we established the (functional) similarities and (operational) differences of promotion and electrochemical promotion. In this chapter we established again the functional similarities and only operational differences of electrochemical promotion and metal-support interactions on ionic and mixed conducting supports. It is therefore clear that promotion, electrochemical promotion and metal-support interactions on ion-conducting and mixed-conducting supports are three different facets of the same phenomenon. They are all three linked via the phenomenon of spillover-backspillover. And they are all three due to the same underlying cause The interaction of adsorbed reactants and intermediates with an effective double layer formed by promoting species at the metal/gas interface (Fig. 11.2). 

It is well known also that higher alkanes suffer radical gas phase oxidation above 723 K. Therefore, their use requires catalysts active and selective for deNOx at lower temperatures. The mechanism of NOx elimination is still debated a redox mechanism involving Cu ions is probable, and isolated Cu cations exchanged into MFI [4,5] or mordenite [6] have been found to be more active than CuO clusters. It must be emphasized, however, that acid zeolites exhibit good activity at high temperature, and acid mechanisms have been proposed [7-10]. In presence of Cu this acid mechanism disappears probably due to the decrease of the acidity of mordenite upon Cu exchange [6]. According to... 

The used Pd/ACF catalyst shows a higher selectivity than the fresh Lindlar catalyst, for example, 94 1% versus 89 + 2%, respectively, at 90% conversion. The higher yield of 1-hexene is 87 + 2% with the used catalyst versus 82 + 3% of the Lindlar in a 1.3-fold shorter reaction time. Higher catalyst activity and selectivity is attributed to Pd size and monodispersity. Alkynes hydrogenation is structure-sensitive. The highest catalytic activity and alkene selectivity are observed with Pd dispersions <20% [26]. This indicates the importance of the Pd size control during the catalyst preparation. This can be achieved via the modified ME technique. 

In 1968 Wilkinson discovered that phosphine-modified rhodium complexes display a significantly higher activity and chemoselectivity compared to the first generation cobalt catalyst [29]. Since this time ligand modification of the rhodium catalyst system has been the method of choice in order to influence catalyst activity and selectivity [10]. 

Addition of ammonium hydroxide and water were explored to evaluate their influence upon catalyst activity and selectivity. The data in this study suggest that there was little influence of ammonium hydroxide on reaction rate and selectivity. The data, however, were not sufficient to definitively define the role of these additives and investigation of these effects will be the subject of future exploration. Examination of Figure 3 may lead to the conclusion that water is actually harmful to the life of the catalyst but such a preliminary hypothesis is overly simplistic, acknowledging that the ammonium hydroxide additive comprises 70% water. 

The task of developing a suitable catalyst for commercial applications involves many considerations, ranging from obvious factors like catalyst activity and selectivity to variables like the catalyst shape and the composition of the binder used in a pelletizing process. This section is devoted to a discussion of these considerations and of the techniques involved in manufacturing industrial catalysts. 

A comparison of catalyst activity and selectivities to hydrocarbon and C02 over the reduced air calcined and nitric oxide calcined 15% Co/Si02 catalysts is shown in Figures 3.1 through 3.4 and Table 3.3. Initial CO conversion at an SV of 10 Nl/g-cat/h over the reduced air calcined sample was 33%, but was significantly... 

To summarize, from literature there does not seem to be much consensus on whether bulk cobalt carbide forms during realistic FTS conditions. Bulk carbide is generally considered a metastable species. However, it is clear that it may form under upset conditions. Furthermore, there is strong evidence to show that if bulk cobalt carbide is present, it is deleterious in terms of both catalyst activity and selectivity. With this in mind, it would be prudent to operate the catalyst in a regime (sufficiently high H2/CO ratio) where bulk carbide formation is avoided. 

In this work, a detailed kinetic model for the Fischer-Tropsch synthesis (FTS) has been developed. Based on the analysis of the literature data concerning the FT reaction mechanism and on the results we obtained from chemical enrichment experiments, we have first defined a detailed FT mechanism for a cobalt-based catalyst, explaining the synthesis of each product through the evolution of adsorbed reaction intermediates. Moreover, appropriate rate laws have been attributed to each reaction step and the resulting kinetic scheme fitted to a comprehensive set of FT data describing the effect of process conditions on catalyst activity and selectivity in the range of process conditions typical of industrial operations. 

Effect of the Process Conditions on the Catalyst Activity and Selectivity... 

One of the reasons for the low selectivity of the mesoporous Ti silicates is their surface hydrophilicity, which is caused by the presence of a large number of surface Si-OH and Ti-OH groups. Because these mesoporous materials are better suited than TS-1 to the oxidation of large, bulky molecules, the passivation of these OH groups (e.g., by silylation) may improve catalyst activity and selectivity. Attempts have been made to reduce the concentrations of such OH groups by silylating them with various alkyl silanes (Table LI) (273). 

The process operates in the liquid phase by dissolving the ethylene in an inert solvent such as cyclohexane or isopentane. The metallocene catalyst is also injected to the mix. The solvent has several important functions. It keeps in solution the alpha olefins produced as well as the ethylene and catalyst. It also enhances the catalyst activity and selectivity. 

The control of the activity and selectivity of cracking catalyst is the key to optimum yields and profitability. Currently, refineries employ two different methods of control the addition of fresh catalyst and the addition of good quality equilibrium catalyst. Onsite FCCU catalyst demetalization, called Demet, is a third alternative which was originally developed by ARCO and then improved by ChemCat Corporation workers (1). The Demet procedures are used to remove active metal contaminants from the surface of equilibrium catalysts, thus improving catalyst activity and selectivity. Demet procedures are applicable to all types of amorphous and zeolitic catalysts. 

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