Transesterification kinetics

Compared to the base-catalyzed synthesis of biodiesel, fewer studies have dealt with the subject of acid-catalyzed transesterification of lipid feedstocks. Among acid catalysts, sulfuric acid has been the most widely studied. In the previously mentioned work of Freedman et al., the authors examined the transesterification kinetics of soybean oil with butanol using sulfuric acid. The three reaction regimes observed (in accordance with reaction rate) for base-catalyzed reactions were also observed here. A large molar ratio of alcohol-to-oil, 30 1, was required in this system in order to carry out the reaction in a reasonable time. As expected, transesterification followed pseudo-first-order kinetics for the forward reactions (Figure 2), while reverse reactions showed second-order kinetics. 

As with inorganic solid catalysts, the most extensively studied system was acetic acid—ethanol [428,432,434,444—448]. Other alcohols used in kinetic studies were methanol [430,449,450], 2-propanol [438], 1-bu-tanol [429,431,433,451—458], allyl alcohol [459], 1-pentanol [434] and ethyleneglycol [460] besides acetic acid, the reactions of formic [450], propionic [443,461], salicylic [430,449], benzoic [453—457] and oleic acids [430,451—453] and of phthalic anhydride [462] have been reported. Investigation of a greater variety of reactants is reported in only one paper [463] six alcohols (C4, Cs and C8) and five acids (mainly dicarboxylic were studied. Transesterification kinetic studies were performed with ethyl formate [437,439,441], isobutyrate [437,439—441] acetate [402, 435—437,439—442], methoxyacetate [441] and acrylate [403,404,464, 465] the alcohols used were methanol [402,435,437,439—442,450],... 

Transesterification reactions between poly(ethylene terephthalate) PET, and acetoxybenzoic acid (ABA) were conducted using the melt polymerization technique to understand the transesterification kinetics of a phase segregated system. The transesterification kinetics of two compositions PET 20 / 80 (ABA) and PETIO / 90 (ABA) have been studied at 260, 275, 290 and 305°C using dibutyl tinoxide (0.1 mole percent) as a catalyst. Homopolymerization of acetoxy benzoic acid was also studied at similar temperatures and catalyst concentration. 

A 250 ml glass reactor as shown in Figure 1 was used for the melt transesterification kinetic investigations. 

Bikou, E. A. Louloudi N. Papayannakos. The effect of water on the transesterification kinetics of cotton seed oil with ethanol. Chem. Eng. Technol. 1999, 22, 70—75. 

Yang HH, He J, Liang BR. Transesterification kinetics of poly(ethylene terephthalate) and poly(ethylene 2,6-naphthalate) blends with the addition of 2,2 -bis(l,3-oxazoline). J Polym Sci Part B Polym Phys 2001 39(21) 2607-14. 

PET TCP Transesterification kinetics studied with HNMR. Bis(2-oxaline) added as a chain extender. (LCP = poly(oxybenzoate-p-terephthalate) 272... 

The variety of enzyme-catalyzed kinetic resolutions of enantiomers reported ia recent years is enormous. Similar to asymmetric synthesis, enantioselective resolutions are carried out ia either hydrolytic or esterification—transesterification modes. Both modes have advantages and disadvantages. Hydrolytic resolutions that are carried out ia a predominantiy aqueous medium are usually faster and, as a consequence, require smaller quantities of enzymes. On the other hand, esterifications ia organic solvents are experimentally simpler procedures, aHowiag easy product isolation and reuse of the enzyme without immobilization. 

Transesterifications, also termed ester exchange or ester interchange reactions, include hydroxy-ester, carboxy-ester, and ester-ester reactions. Hydroxy-ester reaction is the most important one and is used for many aromatic-aliphatic and wholly aromatic polyester syntheses. Carboxy-ester interchange is restricted to the synthesis of wholly aromatic polyesters while the ester-ester route is rarely used for polyester preparation due to slow kinetics. All these reactions may take place in comparable experimental conditions and can be catalyzed by similar classes of compounds. 

Scheme 5.11 Dynamic kinetic resolution of alcohol 18 by combination of enzymatic transesterification and ruthenium-catalyzed racemization.

Mikolajczyk and coworkers have summarized other methods which lead to the desired sulfmate esters These are asymmetric oxidation of sulfenamides, kinetic resolution of racemic sulfmates in transesterification with chiral alcohols, kinetic resolution of racemic sulfinates upon treatment with chiral Grignard reagents, optical resolution via cyclodextrin complexes, and esterification of sulfinyl chlorides with chiral alcohols in the presence of optically active amines. None of these methods is very satisfactory since the esters produced are of low enantiomeric purity. However, the reaction of dialkyl sulfites (33) with t-butylmagnesium chloride in the presence of quinine gave the corresponding methyl, ethyl, n-propyl, isopropyl and n-butyl 2,2-dimethylpropane-l-yl sulfinates (34) of 43 to 73% enantiomeric purity in 50 to 84% yield. This made available sulfinate esters for the synthesis of t-butyl sulfoxides (35). 

The lipases demonstrated very high stability in media partially or totally composed of organic solvent. In such media, the lipases catalyze esterification, transesterification, and resolution of enantiomers [19,45,75,97-100]. Nevertheless, several biphasic systems (organic-aqueous) are used for hydrolysis of lipid and fats [7,34,101]. Kinetic studies in biphase media or in inverted micelles demonstrate that the lipase behavior is different... 

Improved control was observed, however, upon addition of benzyl alcohol to the dinuclear complexes.887 X-ray crystallography revealed that whereas (296) simply binds the alcohol, (297) reacts to form a trinuclear species bearing four terminal alkoxides. The resultant cluster, (298), polymerizes rac-LA in a relatively controlled manner (Mw/Mn=1.15) up to 70% conversion thereafter GPC traces become bimodal as transesterification becomes increasingly prevalent. NMR spectroscopy demonstrates that the PLA bears BnO end-groups and the number of active sites was determined to be 2.5 0.2. When CL is initiated by (298) only 1.5 alkoxides are active and kinetic analysis suggests that the propagation mechanisms for the two monomers are different, the rate law being first order in LA, but zero order in CL. 

In spite of the above mentioned Co(EII) compounds, kinetically labile metal complexes may provide fast product/substrate exchange and some of these systems show real catalytic activity. In native dinuclear phosphatases Mg(II), Mn(II), Fe(II/III), or Zn(II) ions are present in the active centers. Although the aqua complexes of the weakest Lewis acids, Mg(H) and Mn(II), show measurable acceleration of e.g. the transesterification of 2-hydroxypropyl p-nitrophenyl phosphate HPNP, [Mn(II)] = 0.004 M, kobs/ uncat = 73 at pH 7 and 310 K, [38] or the hydrolysis of S -uridyluridine (UpU) [39], only a few structural [40] but no functional phosphatase-mimicking dinuclear complexes have been reported with these metal ions. 

Reinhoudt at al. have recently reported [48] the preparation of a calix[4]arene functionalized with two Zn(II) centers 25, which is highly efficient on transesterification of HPNP. The dimer complex is reported to be 50 times more active (in 50 (v/v)% acetonitrile-water at pH = 7.4 and I = 298 K) than the corresponding monomer (26) which is itself 6 times more active than 27, implying the contribution of the calix[4]arene moiety in the mechanism. The saturation kinetic experiments showed high association constant for the catalyst-substrate complex (K -... 

Biocatalytic hydrolysis or transesterification of esters is one of the most widely used enzyme-catalyzed reactions. In addition to the kinetic resolution of common esters or amides, attention is also directed toward the reactions of other functional groups such as nitriles, epoxides, and glycosides. It is easy to run these reactions without the need for cofactors, and the commercial availability of many enzymes makes this area quite popular in the laboratory. 

Takaya and co-workers (256) disclosed that chiral copper alkoxide complexes catalyze the transesterification and kinetic resolution of chiral acetate esters. Selec-tivities are very poor (E values of 1.1-1.5) but it was noted that the Lewis acid BINAP CuOTf was not an effective catalyst. The observation thatp-chlorophcnyl-BINAP-CuOf-Bu complex gave faster rates than BINAP-CuOt-Bu suggests that both the Lewis acidic and Lewis basic properties of the copper alkoxide are required for optimal reactivity. 

Many publications have appeared on the kinetics of transesterification, dealing with either PET or model compounds. A selection of these papers is summarized in Table 2.5. The overall reaction order of polycondensation is 3, being 1 each for ester, alcohol, and catalyst [43], The reaction rate of poly condensation is generally limited by the rate of removal of EG from the reaction mixture. A... 

The published values for the activation energies and pre-exponential factors of transesterification and glycolysis vary significantly. Catalysts and stabilizers influence the overall reaction rate markedly, and investigations using different additives cannot be compared directly. Most investigations are affected by mass transport and without knowledge of the respective mass transport parameters, kinetic results cannot be transferred to other systems. 

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