Transesterification activation energy

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. 

BHET formation is conducted at temperatures of 200 to 250 °C to achieve reasonable reaction rates. The activation energies of the two reactions are of the order of 25 000-30 000 cal/mol [4, 5], The BHET formation is usually conducted under pressure to keep the ethylene glycol in the liquid state. Terephthalic acid is slurried with ethylene glycol for the esterification reaction. Dimethyl terephthalate is dissolved in ethylene glycol and BHET for a liquid-phase transesterification reaction. The synthesis of BHET is driven to this material by the removal of water or methanol. The reactions are reversible at reasonable rates if the concentrations of water or methanol reactants are held high. 

Reactions with anhydrides and acid chlorides are more rapid and can occur in an essentially nonreversible fashion. But, anhydrides and acid chlorides are considered high-energy reactants since they often involve additional energy-requiring steps in their production, and are thus less suitable for large-scale production of materials. The activity energies for direct esterification and transesterification are on the order of 30 kcal/mol (120 kJ/mol) while the activation energies for anhydride and acid chloride reaction with alcohols are on the order of 15-20 kcal/mol (60-80 kJ/mol). 

Activation energies for the transesterification reactions involving methanol have been reported in the range of 6-20 kcal/mol. Reaction rate constants... 

Rate coefficients and activation energies for uncatalysed and catalysed polycondensations by transesterification... 

HPA is very soluble in oxygen-containing polar solvents such as water, alcohol, ether, and ketone. In these polar solvents, HPA works as an efficient acid catalyst for alkene hydration, the Prins reaction, nucleophilic cleavage of alicyclic and cyclic ethers, esterification, hydrolysis, transesterification, and acetalization. The catalytic activity of HPA in these reactions is much higher than that of ordinary protonic acids such as sulfuric acid and p-toluenesulfonic acid, and the activation energies of the HPA-catalyzed reactions are remarkably reduced, owing to stabilization of the cationic reaction intermediates by the heteropoly anion [1,4]. 

The molecular dynamics of similar chemical structure blends of poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) were investigated using the TSDC technique [74]. Transesterification reactions between the neat components developed during the melt-mixing process. When the a-relaxation processes of the reactive blends were analyzed into their elementary modes by means of relaxation map analysis, the activation energies of the a-relaxation process were found not to be significantly affected by the transesterification reaction. However, the polarizability of the blend was considerably decreased as the PEN content increased, due mainly to the increased stiffness of the polymer backbone. 

Activation Energy of the Rate-Determining Step of Transesterification of Mustard Oil... 

Figure 14-4. Intrinsic reaction coordinate for the transesterification of the dinucleotide model with B3LYP and M06-2X functionals. Relative free energies of reaction and activation are provided in kcal/mol...

Figure 2.11 Transesterification of a racemic mixture of a secondary alcohol (1 -phenoxy-2-propanol, 1 in Table 2.1) with a butanoic acyl donor follows a ping-pong bi-bi mechanism in which Substrate 1 (acyl donor) enters the enzyme, forms an acyl enzyme expelling Product 1 (the leaving alcohol from the acyl donor). Then another Substrate 2 (the enantiomers of the alcohol to be resolved) reacts with the acyl enzyme to liberate Product 2 (the enantiomers of the produced esters), leaving the enzyme in its original form. In a kinetic resolution one of the enantiomeric alcohols reacts faster than the other to form an excess of one enantiomer of the esters (ideally enantiopure, for 1 the (R)-ester was formed with very high ee). The success of the resolution is expressed by the enantiomeric ratio E, which depends on the difference in free energy of activation of the two diastereomeric transition states. These are in turn related to the two tetrahedral intermediates.

Studies of reaction media, and of conditions in organic reactions that improve selectivity and minimize energy to decrease the damage to the endocrine system, are having a big impact. A recent finding of enzyme and/or antibody activity in ionic liquids is expected to open a road to the rich use of environmentally benign solvents [1]. Especially interesting is the reuse of the enzyme or antibody in an ionic liquid solvent system that enables the asymmetric resolution and/or transesterification [2],... 

Microbial fermentation of CSM, which was proposed to detoxify FG in the CSM (Zhang et ah, 2006a,b), seems promising because some exoenzymes such as cellulolytic enzymes, amylase, protease, and lipolytic enzymes that are secreted by certain microorganisms, and some vitamins, as well as some unknown active substances are produced in the fermented CSM (Brock et ah, 1994), which adds nutritional value of the fermented CSM. Recently, Qian et ah (2008) reported that in situ alkaline-catalyzed transesterification could produce a CSM with FG and TG contents below the FAO standard. However, the requirement for a high amount of methanol usage in the in situ transesterification and the potential energy consumption to remove the methanol in the meal may be an obstacle for its practical application. 

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