11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

Tundo et al. [15] reported an efficient catalytic detoxification method for toxicpolychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans(PCDFs) under mild conditions (50 C and 1 atm of hydrogen) with a supported metalcatalyst modified by the PT agent Aliquat 336. Their results show that the methodologyproved successful for hydrodechlorinating the toxic samples to yield mixtures containingconcentration of contaminants lower than the experimentally detectable limit by gas chromatography–high-resolutionmass spectrometry. This method has the potential to bepractically applied in the detoxification of PCDDs and PCDFs.PTC is also widely used in polymerization reactions. The main function of thequaternary ammonium salts is that they can transfer the diphenolate from the aqueousphase into the organic phase to react with the diacid chloride. Hodget et al. [16] presentedthe synthesis of polyesters by the reaction of dicarboxylic acid salts with bishalides ortosylates or by the self-condensation of salts of bromocarboxylic acids under liquid–liquidPTC. With benzyltrimethylammonium salts and halides in dry acetonitrile as solvent,using sodium or potassium salts, the yields of polyesters are, in degrees of polymerization(DP), in the range 17–47, and the rate of dissolution of salts is very slow and rate limiting;while in a liquid–liquid system, the DP is in the range 22–161. Liquid–liquid PTC is morefavorable in the synthesis of polyesters [16]:RCOX þ R 0 OH ! RCOOR 0 þ HXRCOO M þ þ R 0 X ! RCOOR 0 þ M þ Xð4Þð5Þwhere X ¼ -Cl, -Br, -I, -OSO 2 CH 3 , or -OSO 2 C 6 H 4 CH 3 .The applications of PTC in polymerization are gradually increasing. Tagle and coworkers[17,18] synthesized poly(amide ester)s from diphenols with the amide group in theside chain, using PT catalysts such as benzyltriethylammonium chloride, with good results.The use of anhydrous potassium carbonate as the base is to promote the organic reactionunder solid–liquid PTC. Albanese et al. [19] described some recent applications in thisarea, and the reactions of aza anions with 2-bromocarboxylic esters and expoxidesafforded protected -amino acids and -amido alcohols. Sirovski [20] described someexamples of PTC applications in organochlorine chemistry. Using a polymeric crownether the results of m-phenoxytoluene chlorination are also reported. Carboxylic acidsand picric acid act as inhibitors, while benzyl alcohol behaves as a strong promoter. Inthe absence of the promoter, the reaction is conducted either at the interface or in the thirdphase that is a border liquid film between the organic and aqueous phases.The importance of triphase catalysis in industry grows continuously. The supportsfor immobilizing the triphase catalyst are mostly of organic type, i.e., copolymers ofpolystyrene. Yadav and Naik [21] reported that clay could be used as support for thePT catalyst; benzoic anhydride was prepared from benzoyl chloride and sodium benzoateusing a clay-supported quaternary ammonium salt at 30 C. The polymer-supportedcatalysts are less active than the clay-supported catalyst for this reaction system.Desikan and Doraiswamy [22] investigated the enhanced activity of polymer-supportedPT catalysts for the esterification of benzyl chloride with aqueous sodium acetate. Theyfound that the reactivity using a triphase catalyst is higher than that using a solubleone. They hypothesized that the enhancement due to increased lipophilicity of thepolymer-supported catalyst was more than compensated by the decreased diffusionalresistance.Jayachandran and Wang [23] prepared a new PT catalyst, 2-benzilidine-N,N,N,N 0 ,N 0 ,N-hexaethylpropane-1,3-diammonium dibromide (Dq-Br), to investigateCopyright © 2003 by Taylor & Francis Group, LLC