Catalyst Structure Nature of the Active Site

Microporous catalysts are heterogeneous catalysts used in catalytic converters and for many other specialized applications, because of their very large surface areas and reaction specificity. Zeolites, for example, are microporous aluminosilicates (see Section 14.19) with three-dimensional structures riddled with hexagonal channels connected by tunnels (Fig. 13.38). The enclosed nature of the active sites in zeolites gives them a special advantage over other heterogeneous catalysts, because an intermediate can be held in place inside the channels until the products form. Moreover, the channels allow products to grow only to a particular size. 

One of the most interesting results of this work is that properly prepared AU/Y-AI2O3 are effective lean NO, reduction catalysts in the presence of 1.5 % H2O and 4.7 % O2. Their activities are stable, and comparable or higher than a Cu-ZSM-5 catalyst under similar reaction conditions. Another interesting result is the observation that the activity depends strongly on the preparation procedure, which must be related to the detailed structure of the catalyst and the nature of the active sites. 

The present review summarizes contemporary views of the problems, achievements, and prospects involved in the deep desulfurization of gas oils, including identification and reactivity of sulfur species in the feed, the reaction pathways and mechanisms, activity and selectivity of the conventional catalysts, and concerns of fluorescence color production. Process schemes and guidelines for the development of the next-generation catalysts for improved deep desulfurization technology based on these discussions are also proposed. The structure and nature of the active sites of current catalysts will not be extensively covered in this review, because several excellent reviews have been published on these subjects within the past two years (1-3). 

Although numerous investigations have been performed on methanol synthesis catalysts, the structure of the active catalysts, the nature of the active sites, and the reaction mechanism are still subjects of considerable controversy. 

In spite of much effort, the nature of the active sites on acid—base inorganic catalysts is still not completely understood. However, the work on this problem has shown how complicated the surface structure may be and that several types of active centres may be simultaneously present on the surface the question is then which type plays the major role in a particular reaction. Also, the catalytic activity may be influenced to a large extent by impurities present in the feed (catalytic poisons) or by-products of the reaction. The last point is often not taken into account and it will be discussed specially in Sect. 1.2.6. First, the models of surface sites on the most important and best-studied catalysts will be described. 

The hydrogenolysis and isomerization of methyloxirane were studied over various Pt catalysts in order to determine the number and nature of the active sites. The steps were found to be the probable active sites and the transformation is structure-sensitive. The regioselectivity is not affected by variation in the catalyst structure, so it is determined by the nature of the metal. 

The investigation of the mechanism of olefin oxidation over oxide catalysts has paralleled catalyst development work, but with somewhat less success. Despite extensive efforts in this area which have been recently reviewed by several authors (9-13), there continues to be a good deal of uncertainty concerning the structure of the reactive intermediates, the nature of the active sites, and the relationship of catalyst structure with catalytic activity and selectivity. Some of this uncertainty is due to the fact that comparisons between various studies are frequently difficult to make because of the use of ill-defined catalysts or different catalytic systems, different reaction conditions, or different reactor designs. Thus, rather than reviewing the broader area of selective oxidation of hydrocarbons, this review will attempt to focus on a single aspect of selective hydrocarbon oxidation, the selective oxidation of propylene to acrolein, with the following questions in mind ... 

Figure 1 summarizes the main differences and objectives between the major preparation strategies. A collection of the major individual reaction steps for the synthesis of unsupported catalysts can be found in Table 1. One fundamental insight from this rather schematic comparison is that differences in the reaction kinetics of the synthesis of a given material will lead to different mesoscopic and macroscopic structures which considerably affect the catalytic performance. It is necessary to control these analytically difficult-to-describc parameters with much the same precision as the atomic arrangement or the local electronic structure. Whereas these latter parameters influence the nature of the active site, it is the mcso/macrostructure which controls the distribution and abundance of active sites on a given material. It is necessary in certain cases to apply the costly method of fusion as there is no other way to... 

However, the structural identity of polybutadienes prepared with bis(7r-crotylnickel chloride) and with (C4H7NiX)2+ Lewis acids system suggests that the nature of the active sites is similar for these catalysts. 

V2O5 catalysts are structure-sensitive for various reactions. The V=0 group located in the (010) plane plays an essential role in many reactions. Another aspect of these catalysts is the nature of the active site that leads to the SCR reaction or the oxidation of ammonia when ammonia is used as reductant. 

Polyethylenes synthesized by metallocene/MAO catalysts have a molecular weight distribution of M /M = 2. The molecular weight can easily be lowered by increasing the temperature, increasing the metallocene concentration, or decreasing the ethene concentration. The narrow molecular weight distribution is characteristic for a single site catalyst. Nearly every zirconocene forms an active site of a cationic metallocene - and an anionic MAO compound or a complex of both. The nature of the active site would be clearer if more details were known about the structure of the alumoxane. 

The precise nature of the active sites and the exact mechanism of the reaction are, after two decades of active research, still the subjects of investigation and lively debate. The detailed description of the polymerization process given 20 years ago by Arlman and Cosee (29) still has current validity, although a great deal more detail and some further insight have been added subsequently. A review of catalyst structures and reaction mechanisms is the subject of a separate chapter. 

Molecular weight measurements are far less sensitive to variables which are by their nature most affected by experimental error, such as catalyst amount or catalyst/monomer purity, and, more important, on the number of active centers, but are highly sensitive to the catalyst structure, monomer concentration, and polymerization temperature. Hence, reliable molecular weights can give much information on the nature of the active sites. Studies of this kind have been reported by Miilhaupt and Brintzinger on catalysts C2-T24 and 25, by Kaminsky and Werner on three Ci-symmetric zirconocenes, and... 

The variety and complexity of the structures of most catalysts make it difficult to determine the nature of the active sites and whether activation and deep oxidation occur at the same site. For alkali-doped MgO and CaO, Lunsford has shown a correlation between methane activation and the existence of O- species, identified by Under... 

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