Surface acidity catalytic activity correlation

In some instances, there still exist conflicting reports about the surface acidity-catalytic activity correlation. These differences may arise not only from the use of different reaction conditions and different approaches to preparing or modifying the catalysts but also from a poor characterization of the materials employed. Indeed, the detailed physicochemical characterization of the catalytic materials, as well as the study of their interaction with reagents and products, still represents well-recognized problems in the use of heterogeneous catalysis for organic syntheses. 

Amorphous Sn-, Si-, and Al-containing mixed oxides with homogeneous elemental distribution, elemental domains, and well-characterized pore architecture, including micropores and mesopores, can be prepared under controlled conditions by use of two different sol-gel processes. Sn-Si mixed oxides with low Sn content are very active and selective mild acid catalysts which are useful for esterification and etherification reactions [121]. These materials have large surface areas, and their catalytic activity and selectivity are excellent. In the esterification reaction of pentaerythritol and stearic acid catalytic activity can be correlated with surface area and decreasing tin content. The trend of decreasing tin content points to the potential importance of isolated Sn centers as active sites. 

In all above mentioned applications, the surface properties of group IIIA elements based solids are of primary importance in governing the thermodynamics of the adsorption, reaction, and desorption steps, which represent the core of a catalytic process. The method often used to clarify the mechanism of catalytic action is to search for correlations between the catalyst activity and selectivity and some other properties of its surface as, for instance, surface composition and surface acidity and basicity [58-60]. Also, since contact catalysis involves the adsorption of at least one of the reactants as a step of the reaction mechanism, the correlation of quantities related to the reactant chemisorption with the catalytic activity is necessary. The magnitude of the bonds between reactants and catalysts is obviously a relevant parameter. It has been quantitatively confirmed that only a fraction of the surface sites is active during catalysis, the more reactive sites being inhibited by strongly adsorbed species and the less reactive sites not allowing the formation of active species [61]. 

In summary, CO poisoning is held responsible for slow formic acid oxidation rates. Activity improvements should therefore correlate well with the CO tolerance of the catalytic surface. However, to date, no systematic study has been presented comparing the catalytic activity of different binary and ternary alloy surfaces under similar conditions for testing the model predictions of Figure 6.31. 

The conversion rates of n-hexane are shown as a function of the crystallinity parameter Qai for different temperatures. We found that the catalytical activity increases simultaneously with the increased crystallinity of the composites, the crystallization products. According this linear correlation it can be concluded that the catalytical active sites, the acidic centers in the zeolitic framework, are always, independent of the crystal content of the composite material, accessible for educt of the test reaction, the n-hexane molecules. This leads to the assumption that the crystallization must start on the interface (at the phase border) between the solution (contains the alkalinity and the template) and the solid (porous glass) surface and has to carry on to the volume phase of the glass resulting finally in complete transformed granules. 

Liquid-phase hydrogenation of 1,4 butynediol to cis-1,4-butenediol and 1,4-butanediol has been carried out on nickel catalysts supported on thirteen different supports. Some commercial nickel catalysts were used as references. Furthermore, metal loading and Ni-Cu alloying have also been studied. The results obtained indicates that catalytic activity, selectivity and metal surface area of catalysts are closely correlated to some textural and/or acid-base properties of the corresponding support. Similarly, the influence of Cu as a second metal in catalyst behaviour is also related to the nature of the support. 

A correlation between the acid amount of the surface and the catalytic activity for the Cs salts of H3PW12O40 is shown in Fig. 29 128). The number of surface... 

The lack of correlation between isosteric heats of adsorption and catalytic activity illustrates the point we have been stressing the strength of attachment of a base to a catalyst surface is not a valid index of catalytic activity when a substantial portion of the base is strongly bonded to inactive portions of a catalyst surface. Putting it more explicitly, isosteric heats of adsorption are useful indexes for the prediction of acid-catalyzed... 

Catalytic activity measurements and correlations with surface acidity have been obtained by numerous investigators. The reactions studied most frequently are cracking of cumene or normal paraffins and isomerization reactions both types of reactions proceed by carbonium ion mechanisms. Venuto et al. (219) investigated alkylation reactions over rare earth ion-exchanged X zeolite catalysts (REX). On the basis of product distributions, patterns of substrate reactivity, and deuterium tracer experiments, they concluded that zeolite-catalyzed alkylation proceeded via carbonium ion mechanisms. The reactions that occurred over REX catalysts such as alkylation of benzene/phenol with ethylene, isomerization of o-xylene, and isomerization of paraffins, resulted in product distribu-... 

For acid catalysis, the rates of bulk-type reactions show close correlations with the bulk acidity, while the catalytic activities for surface-type reactions are related to the surface acidity which is sensitive to the surface composition and often change randomly. Similarly, in the case of oxidation catalysis, good correlations exist between the oxidizing ability of catalyst and the catalytic activity for oxidation in both bulk-type and surface-type reactions. Acid and redox bifunctionality is another characteristic of HPAs. For example, the acidity and oxidizing ability work cooperatively for the oxidation of mcthacrolcin, whereas they function competitively for the oxidative dehydrogenation of isobutyric acid [5]. Interestingly, the former is of surface type and the latter of bulk type. 

The redox properties can also be controlled by the formation of salts. However, the relationship between these properties and the catalytic activity for oxidation is not sufficiently clarified. For example, it was reported that the activity order for alkali salts was reversed by the reaction temperature [35]. Nevertheless, good correlations are obtained if the concept of surface- and bulk-type catalysis is appropriately considered [4, 36-38]. The reduction by H2 of free acid and group A salts proceeds both on the surface and in the... 

The effect of Ca loading on the acid-base and redox properties of chromia catalysts supported on alumina has been investigated by microcalorimetry of NH3 adsorption and TPR. This alkaline promoter strongly decreases the acidity of the chromia catalyst, particularly suppressing the medium and strong acid sites. No clear correlations were found between the surface acidic properties and the catalytic behavior of the investigated samples in the oxidative dehydrogenation of isobutene, while clear trends were observed between reducibihty and catalytic activity [52]. 

The sol-gel synthesis of a V205-Si02 catalyst and its application in the oxidative dehydrogenation of w-butane to give CO, CO2 or dehydrogenated compounds was described by Sham and coworkers using V(acac)3 and Si(OEt)4 (teos) as precursors. Calcination at 500 °C resulted in the formation of a solid with a high surface area, which allows a better dispersion of active species. Furthermore, a direct correlation between the catalytic activity and the Bronsted acidity was also observed. 

As discussed above, there have been few systematic studies in which the acid or basic strength of materials relevant to catalysis has been correlated on a quantitative scale. The utility of microcalorimetric measurements of the heats of adsorption of various molecules is evident. These measurements can be used to determine the acid or basic strength of surfaces and establish their effect on the catalytic behavior of the materials. If we desire to control these acid-base properties to tailor and improve catalysts for existing processes and to design improved catalysts for new catalytic processes, a quantitative scale of the acid-base interactions is required. Appropriate correlations, perhaps involving electronegativity scales, would allow the prediction of the acid-base strength of the surface sites which can then be related to the catalytic activity of those sites. Additional research in this area is required. 

We should note that the calorimetric results obtained in studies at room temperature did not correlate with the catalytic activity at 573 K. Samples with high activity showed essentially the same values of heat of adsorption of NH3 as did samples that showed considerably lower activity. In other cases, samples with lower activity showed higher heats than the sample with highest activity. On the other hand, more recent studies (158) demonstrated that the differential heat of adsorption of NH3 at 473 K on the same samples displayed the same trend as seen for the catalytic cracking at 523 K. Moreover, the cracking activity agreed better with the acidity determined by adsorption of pyridine at 473 K. Thus, when attempting to correlate the activity for an acid-catalyzed reaction with the surface acidity, it is advisable to study the adsorption process at the temperature used for the catalytic reaction, and the basic molecule used should have a similar size to that of the reactants and should interact selectively with the active sites. 

Since tin-containing co-catalysts are essential for the metathesis of functionalized olefins [26], it was soon discovered that 1 supported on acidic metal oxides forms metathesis catalysts that are active without additives even for functionalized olefins [26]. Standard supports are Al203-Si02, or Nb205 and the activity is related to the surface acidity [2, 3, 26]. A high metathesis activity is observed when MTO is chemisorbed on the surface. No evidence for a surface carbene species was obtained, but there appears to be a correlation between the catalytic activity and the presence of an alkyl fragment on the surface [26a-c]. 

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