Amorphous Solid Sulfonic Acids

To the 8 L of 1 1 methanol/water mixture containing the (Z)-2-carbomethoxy-3-phenyl-2-propene-l-sulfonic acid sodium salt was added 60.0 g of W-24 raney nickel. The resulting suspension was pressurized under 50 psi of hydrogen and was allowed to shake on a Parr shaker for 24 h, at which time an additional 20.0 g of raney nickel catalyst was added. After 6 h under 50 psi of hydrogen, the catalyst was removed by filtration and the solution was concentrated to dryness. To the dry white solid was added ethyl acetate (6 L) and heptane (4 L) and the solution was vigorously stirred with a mechanical stirrer overnight. The white suspension was removed by filtration yielding 530.0 g (88%) of the desired 2-carbomethoxy-3-phenylpropane-l-sulfonic acid sodium salt as an amorphous powder. 

Not only do these acids present an economic advantage over conventional acid technology, but there are also frequent selectivity eidiancements and changes in reactivity associated with the use of these heterogeneous catalysts. A number of different types of solid acids have been involved in synthetically useful reactions. They range from the hydrogen forms of various ion exchange resins and the perflourinated resin sulfonic acid, Nafion-H, to the amorphous acidic oxides, silica and aluminum silicate,the crystalline zeolites > and the natural clays. -9... 

The diamine (2.99 g, 11.1 mmol) and EtsN (1.7 mL, 12.2 mmol) were dissolved in CH2CI2 (50 mL) and cooled in an ice bath. p-Toluenesulfonyl chloride (2.29 g, 12.0 mmol) was added slowly to the solution. The reaction mixture was stirred for 2 h, and then saturated aqueous NaHCOs (50 mL) was added and the solution was stirred overnight to quench excess sulfonic acid. The aqueous solution was drawn off and the CH2CI2 layer was washed (2 x 50 mL) with saturated aqueous NaHCOs and water. The organic solution was dried (Mg2SO4) and decanted, and the solvent was removed in vacuo. The resulting amorphous white solid was recrystallized from LtOAc hexane (3 1) to give the mono-protected product as colorless crystals (4.49 g, 96%). 

Figure 1. Sulfonated amorphous solids (a) sulfonated polystyrene-silica hybrid prepared via the sol-gel technique. Si02-grafted groups (b) alkylphenylsulfonic acid (c) sulfonated polyphenylsilsesquioxane (d) perfluorosulfonic acid (e) benzylsulfonic acids attached to a chlorinated silica (f) sulfonated polysiloxane.

This acid-catalyzed cleavage of the glycosidic bonds is rather complex and often suffers from a lack of selectivity mainly due to side dehydration or recombination reactions of monosaccharides. In the existing literature, four different classes of solid catalysts are reported (1) cation-exchange resins, (2) siliceous-based materials, (3) metal oxides, and (4) sulfonated amorphous carbons. 

Stepwise degradation of sugars under catalytic conditions leads to shorter-chain sugars and such by-products as CO, C02, and H20 depending on the conditions (see also next section).348 Sucrose or other carbohydrates have been used for the preparation of new solid acidic catalysts by partial carbonization followed by sulfonation. This leads to sheets of amorphous carbon bearing hydroxyl, carboxyl, and sulfonic groups. These have been used for the production of biodiesel fuel.349... 

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