Catalytic performances reaction

The high value of catalytically performed reactions as compared to non-catalytic variants is particularly evidenced in the field of enantioselective reactions. Chemists cannot complete enantioselective reactions without certain chiral information in the reacting system. This information is regularly derived from the chiral compounds present in nature, collectively named the chiral pool of the nature. Their availability is often limited, which is not an issue when they are used as catalysts, but causes significant costs of non-catalytic reactions when they are needed in equimolar quantities. The practical value of the catalytic approach to enantioselective processes cannot be overestimated. Asymmetric catalysis characterizes the amplification of chirality one chiral molecule of the catalyst generates an enormous number of chiral molecules of the product in the optically pure form. This results with high chiral economy of catalytically performed enantioselective syntheses. 

The Car-Parrinello quantum molecular dynamics technique, introduced by Car and Parrinello in 1985 [1], has been applied to a variety of problems, mainly in physics. The apparent efficiency of the technique, and the fact that it combines a description at the quantum mechanical level with explicit molecular dynamics, suggests that this technique might be ideally suited to study chemical reactions. The bond breaking and formation phenomena characteristic of chemical reactions require a quantum mechanical description, and these phenomena inherently involve molecular dynamics. In 1994 it was shown for the first time that this technique may indeed be applied efficiently to the study of, in that particular application catalytic, chemical reactions [2]. We will discuss the results from this and related studies we have performed. 

The precious metals possess much higher specific catalytic activity than do the base metals. In addition, base metal catalysts sinter upon exposure to the exhaust gas temperatures found in engine exhaust, thereby losing the catalytic performance needed for low temperature operation. Also, the base metals deactivate because of reactions with sulfur compounds at the low temperature end of auto exhaust. As a result, a base metal automobile exhaust... 

As already discussed in Chapter 1, a promoter is a substance which when added to a catalyst, usually on its surface, enhances its catalytic performance, i.e. it increases the rate, r, of a catalytic reaction or the selectivity to a desired product. 

CO2, N2 and N2O production as a function of the catalyst potential, UWR> obtained at 62IK for fixed inlet pressures of NO and CO. A sharp increase in reaction rate and product is observed as the catalyst potential is reduced below 0 V, i.e., upon Na supply to the Pt catalyst. The selectivity to N2, Sn2, is enhanced from 17% to 62%. This dramatic enhancement in catalytic performance is due to (a) enhanced NO vs CO chemisorption on Pt with decreasing potential and (b) Na-induced dissociation of chemisorbed NO. 

The development of new and improved catalysts requires advances in our understanding of how to make catalysts with specified properties the relationships between surface stracture, composition, and catalytic performance the dynamics of chemical reactions occurring at a catalyst surface the deployment of catalytic surface within supporting microstracture and the dynamics of transport to and from that surface. Research opportmuties for chemical engineers are evident in four areas catalyst synthesis, characterization of surface stracture, surface chemistry, and design. 

As shown in Table 2.1, the improved catalytic performance of alkaline-treated zeolites compared to the parent purely microporous counterparts has been demonstrated decidedly by different groups active in academia and in industry. The positive effect is reflected in the enhanced activity, selectivity, and/or lifetime (coking resistance) of the hierarchical systems. The examples listed embrace not only a variety of zeohte topologies (MFl, MOR, MTW, BEA, and AST) but also reactions involving hghter hydrocarbons as well as bulky molecules. This illustrates the potential of the desihcation treatment, although more work is to be done in optimizing the catalytic system for the wide variety of applications. 

We investigated the catalytic performance of the CU2O coated copper nanoparticles for Ullmann coupling reactions. When the coupling reactions using aryl bromides such as 2-... 

In this paper, the preparation, characterization and the catalytic performance of the Moo.esVoasWo.ioOx-mixed oxide as a partial oxidation catalyst for the methanol to formaldehyde reaction was studied. 

To improve selectivity towards phenol 0.5 wt% of Sn was added as a promoter while preparing 5.0Fe/AC catalyst. The catalytic performance of 5.0Fe-0.5Sn/AC catalyst was investigated under similar reaction conditions. The addition of Sn to Fe/AC catalyst seems to enhance phenol selectivity by 33% (Fig. 7). TOF and physical properties of iron loaded catalysts are shown in Table 1. 

In this work, various Ru-BINAP catalysts immobilized on the phosphotungstic acid(PTA) modified alumina were prepared and the effects of the reaction variables (temperature, H2 pressure, solvent and content of triethylamine) on the catalytic performance of the prepared catalysts were investigated in the asymmetric hydrogenation of dimethyl itaconate (DMIT). 

Fig. 3. Evolution of catalytic performances with time for CNF catalysts reaction temperature, 550 C W/F=37.9 (mol CjHsl/g h...

The prafoimance of foe catalyst for foe CTA hj hopurification was evaluated in a batch autoclave r ictor under conditions similar to those in the indtistry. 90g of CTA containing about 3000 ppm o f 4-CBA and 240 ml of water were chaigrf to foe reactor with Ig catalyst loaded. Hydropurification of foe CTA was conducted at 280ti in foe reactor under stirring (800 rpm) and 0.7 MPa hydrogen pressure. Samples takra after 0.5 h of reaction were analyzed with HPLC [4]. The catalytic performance of foe Pd/CNF catalyst was characterized by 4-CBA s conversion. 

A highly detailed picture of a reaction mechanism evolves in-situ studies. It is now known that the adsorption of molecules from the gas phase can seriously influence the reactivity of adsorbed species at oxide surfaces[24]. In-situ observation of adsorbed molecules on metal-oxide surfaces is a crucial issue in molecular-scale understanding of catalysis. The transport of adsorbed species often controls the rate of surface reactions. In practice the inherent compositional and structural inhomogeneity of oxide surfaces makes the problem of identifying the essential issues for their catalytic performance extremely difficult. In order to reduce the level of complexity, a common approach is to study model catalysts such as single crystal oxide surfaces and epitaxial oxide flat surfaces. 

These experiments clearly showed that it is a-oxygen participation that provides FeZSM-5 zeolites with such a remarkable catalytic performance in the reaction of benzene to phenol oxidation. Equations (1-3) written above are the main stages of the reaction mechanism. 

The interactions between metals and supports in conventional supported metal catalysts have been the focus of extensive research [12,30]. The subject is complex, and much attention has been focused on so-called strong metal-support interactions, which may involve reactions of the support with the metal particles, for example, leading to the formation of fragments of an oxide (e.g., Ti02) that creep onto the metal and partially cover it [31]. Such species on a metal may inhibit catalysis by covering sites, but they may also improve catalytic performance, perhaps playing a promoter-like role. 

Even more recently, Grisi and co-workers reported two catalysts bearing NHC ligands with syn (43) and anti (44) methyl gronps on the A-heterocyclic backbone [58]. These two catalysts represent the possible configurations of the same NHC ligand nsed as isomeric mixture in catalyst 42b (Fig. 3.16). The catalytic performance of both 43 and 44 was evaluated in the RCM reactions of 1, 3, and 5, with catalyst 29a taken as benchmark system. It was found that iyn-catalyst 43 was more active than the parent pre-catalyst 29a, while anft-catalyst 44 was less active than 29a. 

Olefin metathesis is one of the most important reaction in organic synthesis [44], Complexes of Ru are extremely useful for this transformation, especially so-called Grubbs catalysts. The introduction of NHCs in Ru metathesis catalysts a decade ago ( second generation Grubbs catalysts) resulted in enhanced activity and lifetime, hence overall improved catalytic performance [45, 46]. However, compared to the archetypal phosphine-based Ru metathesis catalyst 24 (Fig. 13.3), Ru-NHC complexes such as 25 display specific reactivity patterns and as a consequence, are prone to additional decomposition pathways as well as non NHC-specific pathways [47]. 

Figure 9. Catalytic performance of the used Pd/ACF catalyst in the sixth run and fresh Lindlar catalyst. Reaction conditions see Figure 7.

For the Pt(llO) electrode, there are some contradictory results regarding its catalytic performance compared with Pt(lOO) some studies indicate that the activity is higher for Pt(llO), whereas others suggest the opposite [Chang et al., 1990 Clavilier et al., 1981 Lamy et al., 1983]. The differences are probably associated with different surface states of the Pt(l 10) electrode. The acmal surface strucmre of the Pt(llO) electrode is strongly dependent on the electrode pretreatment. Since formic acid oxidation is a surface-sensitive reaction, different electrocatalytic behavior can be obtained for the same electrode after different treatments. 

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