Heterogeneous acid catalysis

Hydroxymethylation of 2-Methoxyphenol with Formaldehyde to Yield Vanillic Alcohols A Comparison Between Homogeneous and Heterogeneous Acid Catalysis... 

Overall there are benefits and limitations to all of the common methods for immobilising Lewis acids. This helps illustrate the apparently inescapable conclusion in heterogeneous acid catalysis that there are likely to be as many optimum catalysts as there are acid-catalysed processes. 

Isatin-3-ketals are obtained by reaction with diols under homogeneous356 or heterogeneous acid catalysis, employing the strongly acidic resin Dowex 50X-X2357 or by... 

Activation of glycosyl sulfoxides by heterogeneous acid catalysis using Nafion-H or sulfated zirconia has also been described [256],... 

Here a surface Lewis acid (denoted by j) abstracts a hydride ion from the methylene group adjacent to the double bond. This mechanism is in accord with the essential Lewis acid nature of the silica-alumina surface and is consistent with the previously demonstrated ability of this surface to abstract hydride ions from tertiary hydrocarbons. Since an alkenyl carbonium ion is stabilized by resonance to a greater extent than is a saturated carbonium ion, it may well be the most stable species which could form in the chemisorption of an aliphatic olefin or its precursor. It seems reasonable, therefore, to presume that such species may be involved in heterogeneous acid catalysis to a greater extent than has been generally recognized. This chemisorption process does not, of course, exclude the more conventional acid addition to the double bond which may occur under suitable circumstances but rather, it introduces an alternate path which may well exert a considerable influence on the overall course of catalytic reactions. Thus, for example, since a substituted ally lie carbonium ion may be converted to a conjugated diene by loss of a proton, it may be an important intermediate in the formation... 

The rate of reactions occurring through heterogeneous acid catalysis is obviously determined by the characteristics of the acid sites (i.e. their number, nature, strength and density). For skeletal transformations of hydrocarbons (and cokipg is obviously of this type) the rate depends essentially on the protonic acidity of the catalysts [10]. The Lewis acid sites alone do not seem to be active. However, when located in the vicinity of protonic sites they can increase their strength and consequently their activity [11]. Therefore for skeletal transformations of hydrocarbons the C/P ratio must depend only on the strength and on the density of the protonic sites. 

In the area of heterogeneous acid catalysis, zeolites (microporous, crystalline aluminosilicates) have had a massive impact on the conversion of small molecules, usually at high temperatures, and their role in the conversion of crude oil to smaller, typically unsaturated molecules has been a cornerstone of chemistry for almost half a century. This relies on a combination of tunable (and potentially very strong) acidity, shape selectivity due to small and very uniform pores, and exceptional thermal stability, which allows regeneration to be carried out by burning off heavy byproducts. 

Zeolites have been used extensively in heterogeneous acid-catalysis involving hydrocarbon transformation. Their high acidity is derived from protons that are required to maintain electrical neutrality, as shown below ... 

Heterogeneous acid catalysis 1.2.1.2.1 Metal ion complexes as catalysts... 

Production of GSCB by homogeneous and heterogeneous acid catalysis has been evaluated in terms of sustainabihty including process economics, process safety. 

Kozhevnikov IV (2007) Sustainable heterogeneous acid catalysis by heteropoly acids. J Mol Catal A 262 86-92... 

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