Relationship between catalytic activity

Mg/Me (Me=Al, Fe) mixed oxides prepared from hydrotalcite precursors were compared in the gas-phase m-cresol methylation in order to find out a relationship between catalytic activity and physico-chemical properties. It was found that the regio-selectivity in the methylation is considerably affected by the surface acid-basic properties of the catalysts. The co-existence of Lewis acid sites and basic sites leads to an enhancement of the selectivity to the product of ortho-C-alkylation with respect to the sole presence of basic sites. This derives from the combination of two effects, (i) The H+-abstraction properties of the basic site lead to the generation of the phenolate anion, (ii) The coordinative properties of Lewis acid sites, through their interaction with the aromatic ring, make the mesomeric effect less efficient, with predominance of the inductive effect of the -O species in directing the regio-selectivity of the C-methylation into the ortho position. 

Metal catalysts composed of more than two different metal elements are of interest from both technological and scientific viewpoints for improving the catalyst quality or properties (20). In fact, bimetallic (or multimetallic) catalysts have long been valuable for in-depth investigations of the relationship between catalytic activity and catalyst particle structure (21). Sinfelt et al. have made a series of studies on bimetallic nanoparticle catalysts supported on inorganic supports, for example,... 

In the present chapter, we have attempted to illustrate how surface bonding and catalytic activity are closely related. One of the main conclusions is that adsorption energies of the main intermediates in a surface catalyzed reaction is often a very good descriptor of the catalytic activity. The underlying reason is that we find correlations, Brpnsted-Evans-Polanyi relations, between activation barriers and reaction energies for a number of surface reactions. When combined with simple kinetic models such correlations lead to volcano-shaped relationships between catalytic activity and adsorption energies. 

It is clear that the influence of surface geometry upon catalytic activity is extremely complex and many more studies are required before any definitive relationship between catalytic activity and metal particle size can be established. Such studies will require to take cognisance of such factors as the perturbation of surface structure due to the formation of carbidic residues, as noted by Boudart [289] and by Thomson and Webb [95], and by the modification of catalytic properties on adsorption, as noted by Izumi et al. [296—298] and by Groenewegen and Sachtler [299] in studies of the modification of nickel catalysts for enantioselective hydrogenation. Possible effects of the support, as will be discussed in Sect. 6.3, must also be taken into account. 

Radioactive tracer experiments reported by Lombardo and Hall (4) showed that each butene isomer can be directly interconverted into the other two. These results are consistent with a common intermediate being in operation in this reaction. In Figure 3 the linear relationship between catalytic activity and percentage of Na+ replaced by H+ strongly favors a Bronsted acid catalyzed mechanism in which the common intermediate could be a secondary carbonium ion. This conclusion is also supported by the tracer experiments. 

Fig. 44. Relationships between catalytic activity and bulk acidity, (a) (O) Dehydration of 2-propanol, (A) decomposition of formic acid, ( ) conversion of methanol, (ta) ( ) Isomerization of cjs-2-butene after treatment at 423 K, ( ) isomerization of m-2-butene after treatment at 573 K. (From Ref. 46b.)...

As the number of Alf/u.c. is increased, the probability of having isolated Alf atoms decreases thus, the activity would decrease. It is at the lower Alf concentrations, however, that a simple statistical treatment fails to predict the observed linear relationship between catalytic activity and Alf concentration. For example, at 8 Al /u.c. it is difficult to imagine how there would be a significant concentration of strongly acidic centers. There must be 3-cages that contain at least 3 Alf atoms and one A1(0H)2+ cation in order for there to be a strongly acidic proton. But this proton may be even more acidic than in the case described previously.. Even within the more acidic materials, there may be subsets of strongly acidic sites. The net effect may be the observed linear relationship between catalytic activity and Alf content. Clearly, more theoretical and experimental work is needed to test the validity of these models. 

Kushch, S.D., Fursikov, P.V., Kuyunko, N.S., Kulikov, A.V. and Savchenko, V.I. (2001) Fullerene black relationship between catalytic activity in w-alkanes dehydrocyclization and reactivity in oxidation, bromination and hydrogenolysis. Eurasian Chemico-Technol. Journal, 3, No. 2, 131-139. 

The importance of catalysis in chemical technology provides a strong motivation for determination of relationships between catalytic activity and catalyst structure at the atomic scale. Spectroscopic techniques for characterization of catalysts in the working state are powerful, because they provide fundamental information about catalyst structures, including surface structures, under the appropriate conditions (Burch, 1991 Clausen et al., 1998 Dumesic and Topsoe, 1977 Hunger and Weitkamp, 2001 Niemantsverdriet, 1993 Somorjai, 1999 Thomas and Somorjai, 1999 Thomas, 1980 Topsoe, 2000 Weckhuysen, 2002). Such characterizations have permitted major advances in catalysis, as they can be the basis for the design or discovery of new catalysts. 

In one of the first papers on the application of XAFS spectroscopy to catalysis (Lytle et al., 1974) is the statement "... these results demonstrate that the EXAFS technique can be a powerful tool for studying catalysis in order to determine the precise structural relationship between catalytically active sites and the surrounding atoms." It is exactly this precise structural relationship that is the critical kind of information needed if true structure-property relationships are to be developed. 

In many instances for metal oxide electrodes, high electrocatalytic activity is accompanied by poor catalyst stability, i.e. mainly poor resistance to dissolution. It may be possible for certain oxide systems to establish some type of general relationship between catalytic activity and catalyst stability, e.g. based on extent of hydration, crystallization and so on. 

The relationship between catalytic activity and TMA content in MAO is shown in Figure 17.5. Residual TMA in the MAO decreases the catalytic activity. The reactivity of TMA is too strong to control the active site formation and decomposes the active sites for polymerization. 

The relationship between catalytic activity and molecular weight of MAO is shown in Figure 17.6. The catalytic activity showed the maximum around a molecular weight of 400g/mol. It seems probable that a rapid alkylation of the metallocene by MAO takes place first, and the active species arises from a methyl transfer reaction between the metallocene alkyls and MAO. The active species formed from these reactions are probably complexed by a bulky MAO-coordinating non-quenching anion. 

Supported vanadium catalysts, whereby vanadium oxide is dispersed on a support such as alumina or titania are of particular importance in, for instance, the oxidative dehydrogenation of alkanes [58-64]. Such materials have attracted considerable interest in the direct dehydrogenation of butane, where a key driver is to identify the relationship between catalytic activity and structural properties [5, 6, 65-68]. In the pure (solid) metal oxides the coordination of vanadium is well defined. However, this is not necessarily true in the case of supported catalysts. Vanadium may be present on the support surface as isolated vanadium ions dimeric or polymeric species one- and two-dimensional chains of vanadium ions ... 

High-temperature superconducting oxides are of interest for electrocatalysis since they represent materials that are comprehensively characterized by various physical methods. They therefore hold promise for obtaining new correlative relationships between catalytic activity and the bulk properties of materials. 

Hydroisomerization/ hydrocracking of n-decane A1 Ga-(Be, M) impr. The order of activity found was USY >A1-Be > Ga-M > Al-M. A direct relationship between catalytic activity and Bronsted acidity associated with the Al and Si in the tetrahedral layers of Be. 52... 

Physically, the relationship between catalytic activity and Z f can be understood from a study of single phase bismuth cerium molybdate solid solutions. The results show that maximum activity is achieved when there exists a maximum number and optimal distribution of all the key catalytic components bismuth, molybdenum and cerium in the solid. Therefore, it reasonably follows that the low catalytic activity observed for the two phase compositions where Af Af(min) results from the presence of interfacial regions in the catalysts where the compositional uniformity deviates significantly from the equilibrium distribution of bismuth and cerium cations present in the solid solutions. These compositions may contain areas in the interfacial region which are more bismuth-rich or cerium-rich than the saturated solid solutions. Conversely, at Af(min), the catalyst is similar to an ideal mixture of the two optimal solid solutions. The compositional homogeneity of the interfacial region approaches that of the saturated solid solutions. Therefore, the catalytic behavior of compositions at Af(min) is similar to that of the saturated solid solutions. 

Fig, 6.7. a Site time yield in the processes described in the figure as a function of radius of the metal cluster involved in the catalytic processes. No particle size effect is observed down to small clusters, b Particle size effect for Pt clusters dispersed on Si02 in the recombination reaction of oxygen and hydrogen, c Particle size effect for Fe clusters dispersed on MgO for the ammonia synthesis at atmospheric pressure and 570° K. d Relationship between catalytic activity, dispersion of the metal cluster on support and bulk metal character of the cluster for metals of the group VIII... 

The activity of the zeolite Beta catalyst was found to increase upon increasing activation temperature and deep-bed calcination conditions resulted in higher catalytic activity than a shallow-bed procedure, implying a relationship between the catalytic activity and the extent of framework dealumination, because the former method results in more auto-steaming. Infrared results indicated, moreover, a relationship between catalytic activity and the amount of partially hydrolyzed framework aluminum. This was further supported by a detailed Si and " Al MAS NMR and FTIR study [29]. In the proposed mechanism, the first step is chemisorption of f-PrOH on a Lewis-acid site, consisting of co-ordinatively unsaturated framework-attached Al. This results in the formation of a surface alkoxide considered to be the H-donor. Co-ordinative interaction of the ketone to the same Al... 

This paper introduces (1) the detailed behaviour of the N0-H2 reaction under both static and dynamic conditions over three kinds of noble metal catalysts (Pt, Pd, and Rh/a-Al 2O3), and (2) the relationships between catalytic activity and periodic operation effects obtained from the various binary gas systems over the same catalysts. Finally, in order to explore the role of periodic operation, the concept of self-poisoning of reactants contained in the binary gas systems is postulated, allowing reasonable interpretation of our results obtained under the cycling conditions. 

Even though most solid catalysts are porous bodies, few research reports have been concerned with the relationships between catalytic activity and pore structure. Chemical engineers are now studying the relationships between mass and heat transfer and the pore structure of catalysts used in reactor devices. The usual models of pore structure, however, are too simple to provide fundamental knowledge of the catalyst. The pore structure of a catalyst is related not only to the dispersion state of the solid but also to its surface structure, and so we believe that a study of the porous structure of a solid catalyst is an important fundamentgil research. 

Possible Relationships Between Catalytic Activity and Surface Acidity... 

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