Acid-base properties catalysis

Another definition of acids and bases is due to G. N. Lewis (1938). From the experimental point of view Lewis regarded all substances which exhibit typical acid-base properties (neutralisation, replacement, effect on indicators, catalysis), irrespective of their chemical nature and mode of action, as acids or bases. He related the properties of acids to the acceptance of electron pairs, and bases as donors of electron pairs, to form covalent bonds regardless of whether protons are involved. On the experimental side Lewis definition brings together a wide range of qualitative phenomena, e.g. solutions of BF3, BC13,... 

The aim of this study is to develop model reaction for the characterization of the acidity and basicity of various transition aluminas, the experimental conditions being close to that for catalysis use. Among various model reactions, the transformation of cyclopentanol and cyclohexanone mixture was chosen for this work. Indeed, this reaction was well known for estimating simultaneously the acid-base properties of oxide catalysts [1], Two reactions take place the hydrogen transfer (HT) on basic sites and the alcohol dehydration (DEH) on acid sites. The global reaction scheme is shown in Figure 1. 

Electronically excited states of organic molecules, acid-base properties of, 12,131 Energetic tritium and carbon atoms, reactions of, with organic compounds, 2, 201 Enolisation of simple carbonyl compounds and related reactions, 18,1 Entropies of activation and mechanisms of reactions in solution, 1,1 Enzymatic catalysis, physical organic model systems and the problem of, 11, 1 Enzyme action, catalysis of micelles, membranes and other aqueous aggregates as models of, 17. 435... 

As discussed in the previous section, metal oxides have both acidic and basic properties. The acid-base properties of metal oxides have led to many interesting catalytic reactions. Catalytic reactions such as H2-D2 exchange, hydrogenation, isomerization, dehydrogenation, dehydrohalo-genation, and benzylation can be considered as examples of acid-base catalysis reactions.31-36 These reactions will be briefly discussed in the following section. The remarkable properties of MgO as a catalyst have been well documented in the literature and we shall discuss some of these unique catalytic properties. 

The substitution of oxygen by nitrogen in PO4 tetrahedron has allowed the synthesis of a new family of solids with original properties the nitrided phosphates. These systems (e.g., AlPON, AlGaPON) with tunable acid-base properties are used in a growing number of intermediate and fine chemistry production processes [204] as well as supports in heterogeneous catalysis (e.g., dehydrogenation reactions) [205]. 

Bi/Mo = 1. The possible significance of acid—base properties in oxidation catalysis is more generally discussed in Sect. 3. 

It took some time to adopt a similar view of other heterogeneous elimination and substitution reactions. Most efficient experimental tools have been found in stereochemical studies, correlation of structure effects on rates and measurement of deuterium kinetic isotope effects. The usual kinetic studies were not of much help due to the complex nature of catalytic reactions and relatively large experimental error. The progress has been made possible also by the studies of surface acid—base properties of the solids and their meaning for catalysis (for a detailed treatment see ref. 5). 

Campbell JM, von Sonntag C, Schulte-Frohlinde D (1974) Photolysis of 5-bromouracil and some related compounds in solution. Z Naturforsch 29b 750-757 Candeias LP, Steenken S (1989) Structure and acid-base properties of one-electron-oxidized deoxy-guanosine, guanosine, and 1-methylguanosine. J Am Chem Soc 111 1094-1099 Candeias LP, Steenken S (1992a) Electron adducts of adenine nucleosides and nucleotides in aqueous solution protonation at two carbon sites (C2 and C8) and intra- and intermolecular catalysis by phosphate. J Phys Chem 96 937-944... 

However, the above mentioned work on the chemisorption of reactants is for (111) metal surfaces, and in the real world of catalysis, the metal particles are preferably highly dispersed, with particles containing 5-500 atoms, and these particles have a considerable amount of edges, steps and low-coordinated surface sites. Furthermore, as has been mentioned above the electronic structure of these metal particles are influenced by the acid/base properties of the support material. 

The most important feature of zeolites (and zeotypes), in the context of catalysis, is not their range of acid-base properties, since that is also available with amorphous alumino-silicates. It is the presence of a regular structure containing... 

It is known [41] that partial oxidation reactions in heterogeneous catalysis involves redox properties of the solid catalysts, allowing the well known Mars-van Krevelen mechanism [42] to occur, or at least to be facilitated. Acid-base properties are also an important feature, as they play a determining role in the activation of the reactants and in the desorption of the intermediate compounds, for instance, an acid surface will favor desorption of acid products, thus avoiding further over-oxidation, while a basic surface will favor desorption of basic products as olefins. It follows that heteropolyoxometallate compounds, in particular TMSP, appear as potential... 

The two parts of the present volume contain seventeen chapters written by experts from eleven countries. They cover computational chemistry, structural chemistry by spectroscopic methods, luminescence, thermochemistry, synthesis, various aspect of chemical behavior such as application as synthons, acid-base properties, coordination chemistry, redox behavior, electrochemistry, analytical chemistry and biological aspects of the metal enolates. Chapters are devoted to special families of compounds, such as the metal ynolates and 1,2-thiolenes and, besides their use as synthons in organic and inorganic chemistry, chapters appear on applications of metal enolates in structural analysis as NMR shift reagents, catalysis, polymerization, electronic devices and deposition of metals and their oxides. 

Reactions 2 and 3 have been proposed as the primary mode of catalysis for Co (30), Mn (31), and Cr (32). However, it must be pointed out that metal reactivity can change tremendously with complexing agent, which shifts redox potentials, and with solvent, which alters acid/base properties and electron transfer efficiency. Electron transfer oxidations to generate L are extremely rapid in nonpolar media (33, 34), including neat oils, and are less efficient in aqueous or polar protic solvents. 

The main objective of this article is to present a survey of theoretical and applied aspects of microcalorimetry to heterogeneous catalysis with particular emphasis on the determination of acid-base properties of metal oxides and mixed metal oxides. This review is not meant to be comprehensive but to provide an overview of recent work done in the area. Additional applications can be found in recent reviews 1-4). 

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. 

Formation of the tetrahedral intermediate carbinolamine and subsequent elimination of water are amenable to acid-base catalysis and do not require a metal surface. The relative rates of adduct formation and subsequent dehydration to imine or enamine depend on the structure of alcohol and amine, and on the nature and strength of acidic and basic sites on the catalyst surface. It must be stressed that several side-reactions (e. g. dimerization and oligomerization, dehydration) are also acid or base-catalyzed, and good selectivity for the desired product requires proper tuning of the redox and acid-base properties of the catalyst. This is crucial in catalyst development when choosing a suitable support, additive, or modifier. Even traces of impurities remaining on the surface from the catalyst precursor can strongly influence product distribution [10]. 

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