Heterogeneous Acid-Catalyzed Reactions

In this section, these influences will be described. Besides the acidic properties, the absorption properties of solid heteropolyacids for polar molecules are often critical in determining the catalytic function in pseudoliquid phase behavior. This is a new concept in heterogeneous catalysis by inorganic materials and is described separately in Section VI. With this behavior, reactions catalyzed by solid heteropoly compounds can be classified into three types surface type, bulk type I, and bulk type II (Sections VII and IX). Softness of the heteropolyanion is important for high catalytic activity, although the concept has not yet been sufficiently clarified. 

The influence of the heteroatom on the acid strength is shown, for example, in Fig. 28. Here the rates of alkylation of 1,3,5-trimethylbenzene and the decomposition of cyclohexyl acetate catalyzed by the acid forms of several 12-tungstates are plotted against the negative charge of polyanion for solid heteropolyacids (63, 183). The catalytic activities correlate well with the acid strength in solution (Fig. 14). This correlation indicates that the acid strength of 

TOSHIO OKUHARA, NOR1TAKA MIZUNO, AND MAKOTO MISONO 

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 

TOSHIO OKUHARA, NORITAKA MIZUNO, AND MAKOTO M1SONO 

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The previous results led us to investigate the reactivity of other phosphonic acids bearing functional groups. It thus seemed interesting to study the behaviour of the carboxylic function in the reaction process and to know if these groups were accessible to organic compounds for heterogeneous acid catalyzed reactions such as deacetalization. The first results have been obtained with the 2-carboxyethylphosphonic acid. 

Acidic, high area silica-almnina had received substantial attention in ICC 1, 52-58). Perhaps the most dramatic change in the subsequent catalytic literature was the debut of zeolites. Why acid catalyzed reactions are so much faster on zeolites than on silica-alumina has been extensively discussed but probably not conclusively. One should be able to know the exact structures of catalytic sites in zeolites, but initial hopes that this would do wonders for mechanistic imderstanding have not been fully realized. Super acids and carbonium ions came into heterogeneous catalysis from homogeneous chemistry and in special cases reaction via carbonium ions seems to occur. 

Heterogenous reactions, Sh/Nu ratio, 27 64 Heteroligand complex, 32 260-262 Heteropolyacids defined, 41 117 heteroatoms, 41 118, 120, 121 Prins reaction, 41 156 supported, 41 149-150 Heteropolyanions, 41 113, 117, 119-121 Heteropoly blues, 41 191 Heteropoly compounds absorption, 41 179-180, 190-191 acid-catalyzed reactions heterogeneous, 41 161-178 liquid phase, 41 150-161 acidic properties in solid state, 41 141-150 in solution, 41 139—14] catalysis, 41 114, 116-117, 190-191 as catalyst, 41 113-116, 117, 223-232... 

Transesterification Reactions. The heterogeneous acid-catalyzed transesterification of TGs has not been investigated as much as its counterpart, the base-catalyzed reaction. Various solids are available with sufficient acid strength to be effective catalysts for the named reaction. Among the solid acids available are functionalized polymers, such as the acid forms of some resins, as well as inorganic materials, such as zeolites, modified oxides, clays, and others. Some of these solids have already been found to be effective in transesterification reactions of simple esters and (3-ketoesters. 

Solid acid catalysts such as mixed oxides (chalcides) have been used extensively for many years in the petroleum industry and organic synthesis. Their main advantage compared with liquid acid catalysts is the ease of separation from the reaction mixture, which allows continuous operation, as well as regeneration and reutilization of the catalyst. Furthermore, the heterogeneous solid catalysts can lead to high selectivity or specific activity. Due to the heterogeneity of solid superacids, accurate acidity measurements are difficult to carry out and to interpret. Up until now, the most useful way to estimate the acidity of a solid catalyst is to test its catalytic activity in well-known acid-catalyzed reactions. 

Superacids Immobilized on Solid Supports. The considerable success of Magic Acid and related superacids in solution chemistry and interest to extend the scope and utility of acid-catalyzed reactions, particularly hydrocarbon transformations, logically led to the attempts to adopt this chemistry to solid systems allowing heterogeneous catalytic processes. 

Like the AVADA and the AlkyClean processes, these two processes also replace the liquid acid/base catalysts with solid acids and bases [192]. Although the reaction mechanism for the heterogeneous acid-catalyzed esterification is similar to the homogeneously catalyzed one [207,208], there is an important difference concerning the relationship between the surface hydrophobicity and the catalyst s activity. This is especially true for fatty acids, which are very lipophilic compounds. One can envisage three cases First, if there are isolated Bronsted acid sites surrounded by a... 

Typical acid-catalyzed reactions like the dehydration of alcohols and double bond shifts in olefins have been mentioned occasionally as reactions catalyzed by organic heterogeneous catalysts. An extensive kinetic study of the dehydration of tertiary butyl alcohol over pyrolized polyacrylonitrile has been describ-... 

They found that at room temperature using benzene as the solvent only 3% consisted of the diketone 25 and mainly the ring-contracted products were obtained 33% of keto aldehyde 24 and 28% of ketone 26. From an industrial point of view the desired compound is the keto aldehyde 24, which is an interesting intermediate for the synthesis of other cyclopentanone derivatives with floral and fruity smells. The acid catalyzed reaction mechanism leading to the synthesis of keto aldehyde 24 had been discussed earlier (25). Therefore, it is of interest whether the product distribution changes in the presence of a heterogeneous catalyst system and also whether the decarbonylation of the compound 24 to compound 26 can be suppressed. 

The WGSR is normally practised as a heterogeneously metal-catalyzed reaction Fe is the most commonly used catalyst. However other metals are also active, for example the homogeneous Rh/H catalyst in the Monsanto acetic acid process (Section 4.2.4) concurrently catalyzes the WGSR via a Rh(I)/ Rh(III) cycle (Equations 8 and 9),... 

FIGURE 4.14. Heterogeneous conditions to limit acid-catalyzed reactions... 

It was substituted by the new process of oxidation of propylene to acrylic add via acrolein using heterogeneous Bi-molybdate based catalysts followed by acid-catalyzed reaction of acrolein with the alcohol ... 

Results are summarized in Fig. 5. Over SOaCrOz and SA, the reaction took place smoothly at 373 K, though the disproportionation of E2 proceeded higher temperature. Modified clay minerals also catalyzed the reaction at an appreciable rate. Alumina and HY showed low activity and other catalysts were almost inactive for the reaction. This clearly shows that strong solid acids can catalyze the alkylation reaction. The heterogeneous alkylation of E2 is an acid catalyzed reaction. 

The classical model of solid acid and bifimctional heterogeneous catalysis basically assumed that the acid catalyzed reactions followed the laws known for liquid acids. It also assumed that catalysis over bifunctional catalysts, exposing both metal sites and acid sites, could be described by assuming simple additivity, reaction intermediates were thought to shuttle fi equently between metal sites and acid sites. More recent research has, however, shown that both assumptions are inadequate. Some reactions which are not catalyzed by liquid acids are fast over solid acids, moreover bifimctional catalysts display substantial deviations fi"om the additivity model. 

Aldol condensations can be efficiently carried out in the liquid or gas phase. These processes are in some cases catalyzed by supported transition metals, occasionally by acids, and more frequently by bases (1,9). The choice of a appropriate catalyst is necessary for obtaining the desired final product Aldol condensations catalyzed by supported metals in the presence of hydrogen yield a saturated carbonyl (10), whereas the acid-catalyzed reaction leads mainly to hydrocarbons and the base-catalyzed reaction is suitable for the synthesis of heavier a,j8-unsaturated carbonyl polymers. The majority of the heterogeneous catalysts are acidic or basic metal oxides or mixtures of metal oxides. 

In this chapter, we focus on acid catalysis in water. While there are numerous examples of catalytic reactions in water, the main body of these involves acid catalysis. Homogeneous catalysis, heterogeneous catalysis, and micellar catalysis, including catalytic enantioselective reactions, are discussed in detail. Acid-catalyzed reactions using a small amount of water may not be included unless they are crucial for the further development of the field. 

Hydrogenations and oxidations form two important classes of catalytic reactions. In heterogeneous catalysis, the metals from the groups Vin and IB of the periodic system, as well as oxides or sulfides, catalyze such reactions. In view of their unique reaction mechanisms, acid-catalyzed reactions are considered as a separate class, while a fourth category is formed by reactions that are catalyzed by coordination complexes or organometallic complexes in solution, as in homogeneous catalysis. Heterogeneous catalytic reactions will be the focus, however. 

Equation (1.20) is frequently used to correlate data from complex reactions. Complex reactions can give rise to rate expressions that have the form of Equation (1.20), but with fractional or even negative exponents. Complex reactions with observed orders of 1/2 or 3/2 can be explained theoretically based on mechanisms discussed in Chapter 2. Negative orders arise when a compound retards a reaction—say, by competing for active sites in a heterogeneously catalyzed reaction—or when the reaction is reversible. Observed reaction orders above 3 are occasionally reported. An example is the reaction of styrene with nitric acid, where an overall order of 4 has been observed. The likely explanation is that the acid serves both as a catalyst and as a reactant. The reaction is far from elementary. 

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