Catalysis by solid acids

Oxidic surfaces in particular develop acid or basic properties which are important in catalysis. We will approach this subject first by taking as a starting point the ionic bond model [2]. The lattice is considered to consist of cations and anions held together by electrostatic interactions. Later we will discuss a more balanced theory that also accounts for covalent bonding aspects. 

Surface Lewis acidic or basic sites are created on ionic surfaces because surface atoms have a lower coordination number compared to their bulk values. A 

Where Q is the formal ion charge and C its coordination number. For the Mg2+ ion, the Pauling valency equals 

On the surface the charge excess of an ion is defined as the formal charge Q of the ion considered plus the sum of the Pauling valencies, computed with respect to the other ion  

A negative value is computed for e on the oxygen anion. A positive charge excess implies Lewis acidity and a negative charge excess implies Lewis basicity. When the surface becomes exposed to NH3, ammonia will be adsorbed to the Mg2+ ion. 

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Catalysis by Solid Acids. Two aspects are considered here. The first aspect is concerned with transesterification reactions catalyzed by solid acids. Unfortunately, little research dealing with this subject has been reported in the literature. The second aspect deals with esterification reactions of carboxylic acids (or FFAs). This second part addresses an important characteristic of inexpensive TG feedstocks, i.e., high FFA content. Ideally, an active solid catalyst should be able to carry out transesterification and esterification simultaneously, thus eliminating pretreatment steps. It is likely that heterogeneous catalysts that perform well in esterification should also be good candidates for transesterification since the mechanisms for both reactions are quite similar. 

The prospects of obtaining a detailed molecular-level understanding of heterogeneous catalysts would appear to be best for solid acids (1). Catalysis by solid acids often involves an appreciable concentration of reasonably uniform sites and restricted roles for defect structures. Furthermore, the great number of reaction studies, physical property measurements, and spectroscopic studies of solid acids provides a background (2-4) for the design and evaluation of further experimental and theoretical work. 

Previous sections have shown that catalysis by solid acids has received much attention due to its importance in petroleum refining and petrochemical processes. Conversely, relatively few studies have focused on catalysis by bases, even if acid and base are paired concepts. Base catalysts, however, play a decisive role in several reactions essential for fine-chemical syntheses [248-251]. Solid-base catalysts have many advantages over liquid bases. Examples of successfijl reactions include isomerization, aldol condensation, Knoevenagel condensation, Michael condensation, oxidation and Si—C bond formation. Various reviews have discussed catalysis by solid bases [248-255]. 

Iglcsia. E Barton, D(i Biscardi, JA Gines. MJL Soled, SL, Bifunctional pathw ays in catalysis by solid acids and bases. Catalysis Today. 1997 38, 339-360. 

Catalysis by Solid Acids and Bases By S Malinowski and M Marezewski... 

Because catalysis by solid acids and bases is central to a variety of industrial processes, relationships between solid acid-base behavior and catalytic properties have been explored [68,120]. However, many concepts, either qualitative or empirical, demand closer scrutiny. 

Catalysis by solid acids is of paramount importance in industrial chmnistry, namely due to its application in catalytic cracking, one of the most important processes in the world. However, despite its enormous importance, only recently have practical and quantitative relationship between the acidity of the catalyst and its catalytic activity began to appear, unlike homogeneous acid catalysis, which has made use of the Brdnsted relations for many years. The difficulties to be overcome are of various nature but it was found by some of the authors that Bronsted-like relationships also apply to solid acid catalysts [1]. 

Solid Acids and Bases, Proceedings of the DGMK Conference on Catalysis by Solid Acids... 

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