Solid Acids and Bases as Catalysts

An obvious solution to this salt generation problem is the widespread replacement of traditional Bransted and Lewis acids with recyclable solid acids and bases [3-7]. This will obviate the need for hydrolytic work-up and eliminate the costs and environmental burden associated with the neutralization and disposal of e.g. liquid acids such as H2S04. The use of solid acids and bases as catalysts provides additional benefits  

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A simple oxide catalyst can be used in either the bulk state or supported on an inert oxide support material. The bulk oxides are usually prepared using a precipitation-calcination sequence similar to those described in Chapter 9 for the preparation of support oxides. " In general, the simple semiconductor oxides are not very good catalysts for synthetic reactions. The insulator oxides, however, can be used as solid acids and bases for a number of reactions. Alumina has been used as an acid catalyst for the vapor phase rearrangement of cyclohexanone oxime to caprolactam (Eqn. 10.9). Modification of the y-alumina surface by the addition of 10-20% of B2O3 increased its activity for this reaction, giving caprolactam in 80% selectivity even after several hours of continuous operation. "... 

Heterogeneous catalysts are defined as solids or mixture of solids that are used to accelerate a chemical reaction without undergoing change themselves. The types of solids used in industry as heterogeneous catalysts include simple oxides, mixed oxides, metal salts, solid acids and bases, metals, and dispersed metals. Catalysts are used in a wide variety of chemical and environmental processes worldwide. The global value of fuels and chemicals produced by catalytic routes is about US 2.4-3 trillion per year. About 20% of all products produced in the United States are derived from a catalytic process of some form. As important as catalysis is to the world economy, the number of various chemicals used as a catalyst as well as the form and shape of the material vary as much as the number of processes that use catalysts. Fig. 1 is a picture of a number of various types of catalysts and illustrates the numerous possibilities of shapes and sizes. Naturally, the preparation processes of such a wide variety of products is also numerous. 

Sulfonic acid resins can be used as solid catalysts for esterifications and other acid-catalyzed reactions. Am-berlyst 15 was a more effective catalyst for the preparation of esters of phenethyl alcohol and cyclohexanol than sulfated zirconia, an acid clay, and dodecatungstophos-phoric acid.113 (Amberlyst and Amberlite are trademarks of Rohm Haas.) (See Chap. 6 for more detail on solid acids and bases.) The same catalyst gave 86-96% yields of hydroxyesters when a lactone was stored with a hy-droxyacid.114 Diols can be monoacylated in 58-92% yields by transesterification with ethyl propionate in the presence of Dowex 50W (a product of the Dow Chemical Co.).115 Modification of the sulfonic acid resin with 2-mercaptoethylamine produced a catalyst for the reaction of phenol with acetone to produce bisphenol A (5.30) in 99.5% yield.116 After 20 cycles the yield was still 98.7%. When used as catalysts, ion-exchange resins can last for 6 months to 2 years. 

Many types of solid acid and base catalysts are known.11 Superacids are those that are at least as strong as 100% sulfuric acid.12 The acid strengths are measured using basic indicators and are assigned a Hammett acidity function, H0- Table 6.1 lists some superacids, with the strongest at the top. 

If you have never run a catalytic vapor-phase reaction in a hot tube, pick one that could use a solid acid or base as a catalyst and try it. 

Needless to say, solid acid and base catalysts play a key role in the transformation of biomass-derived materials to value-added compounds such as carbonyl compounds [21-33]. For example, lactic acid could be obtained from cellulose using tungstated alumina as a Lewis add catalyst [137], and from glucose using HT as a solid base catalyst [138]. In this parL selective conversions of biomass-sourced materials using solid acid and base catalysts are surveyed alongside mechanistic considerations. 

In accordance with the above definitions, a summarized list of solid acids and bases is given in Tables 1.1 and 1.2. The first group of solid acids in Table 1.1 includes naturally occurring clay minerals. The main constituents are silica and alumina. Various types of synthetic zeolites such as zeolites X,Y,A, ZMS-5, ZSM-11, etc. have been reported to show characteristic catalytic activities and selectivities. The well-known solid acid, synthetic silica-alumina, is listed in the seventh group, which also includes the many oxide mixtures which have recently been found to display acidic properties and catalytic activity. In the fifth and sixth groups are included many inorganic chemicals such as metal oxides, sulfides, sulfates, nitrates, phosphates and halides. Many have been found to show characteristic selectivities as catalysts. 

This reaction proceeds readily over solid acids and bases. The mechanism, selectivity, and stereochemistry of dehydrochlorination and dehydrobromination have been extensively studied in relation to the acid-base properties of catalysts, as have those in homogeneous liquid phase.It has been fairly well established that the... 

As in the case of other solid catalysts, the deactivation of solid acid and base catalysts occurs during use for catalytic reactions by the following mechanisms. Adsorption of poisonous compounds Basic molecules are poisons or inhibitors for acidic... 

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