Surfactant-type catalysts

Figure 9.1 Direct esterification by dehydration in the presence of a surfactant-type catalyst in water.

CHAPTER 13. Development of Surfactant-Type Catalysts for Organic... 

The concept of surfactant-type catalysts described above was also found to be applicable to catalytic systems other than Lewis acid-catalysed reactions. For example, we have developed palladium-catalysed allylic substitution reactions using a combination of Pd(PPh3)4 and a non-ionic surfactant, Triton X-100 [32]. 

The amounts of DBSA used were also found to affect the equilibrium position (Table 13.6). Each equilibrium position was confirmed by conducting both esterification of the carboxylic acid with the alcohol and hydrolysis of the ester. Table 13.6 clearly shows that increase of the amount of DBSA resulted in decrease of the yield of the ester at the equilibrium position. This result may be attributable to the size difference of the emulsion droplets that were formed by the hydrophobic substrates and the surfactant in water. As the amount of the surfactant-type catalyst increases, the size of each droplet may decrease, because the emulsion system may become a microemulsion system where the substrates are solubilized in water by a large amount of the surfactant. In fact, while 10 mol% DBSA gave the white turbid mixture, the reaction mixture was almost clear in the presence of 200 mol% DBSA, indicating that the size of the droplets became smaller. The smaller the droplets, the larger the sum of surface area of the droplets. As a result. 

The esterification is successful primarily because the surfactant-type catalysts and organic substrates (carboxylic acids and alcohols) in water form droplets whose interiors is hydrophobic. The surfactants concentrate a catalytic species such as a proton onto the droplet surface, where the reaction takes place. 

Chavan, H. V. Bandgar, B. P. Aqueous extract of Acacia concinna pods An efficient surfactant type catalyst for synthesis of 3-carboxycoumarins and cinnamic acids via Knoevenagel condensation. ACS Sustainable Chem. Eng. 2013,1, 929-936. 

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