MECHANISMS OF ETERIFICATION OF TEREFTALIC ACID WITH ETYLENGLYCOL

ABSTRACT

The article presents the mechanisms of the reaction of esterification of terephthalic acid with ethylene glycol, the effects of the chemical structure of the catalyst on the rate constant of the esterification reaction, and the analysis of changes in the number of unresponsive groups during esterification using different catalysts.

АННОТАЦИЯ

В статье описаны механизмы реакции этерификации терефталевой кислоты этиленгликолем, влияние химического строения катализатора на константу скорости реакции этерификации, а также анализ изменения количества нереагирующих групп при этерификации с использованием различных катализаторы.

Keywords: polyethylene terephthalate (PETP,PET), terephthalic acid (TPA), ethyleneglycol (EG), esterification, tetrabutoxytitanium, lead dibutyl phthalate, nucleophilic component, sulfuric acid.

Ключевые слова: полиэтилентерефталат (ПЭТФ, ПЭТФ), терефталаткислота (ТФК), этиленгликоль (ЭГ), этерификация, тетрабутоксититанат, дибутилфталат свинца, нуклеофильный компонент, серная кислота.

Analyzing the esterification reaction of terephthalic acid with ethylene glycol (Figure 1).

Figure 1. Etherification reaction of terephthalic acid with ethylene glycol

Etherification processes are usually carried out in the presence of catalysts. In practice, acid catalysts (sulfuric acid, p-toluene sulfoxic acid, etc.) and amphoteric catalysts (tetrabutoxytitanate, lead dibutylphthalate, etc.) are used. Sometimes the process is done without a catalyst.

The catalytic activity of strong acids is very large. When used, the reaction rate increases by more than 30 times. This can be seen in the example of the values of the rate constants of the reaction of esterification of TPA with diol in their presence. Table 1 shows the effect of the chemical structure of the catalyst on the rate constant of the esterification reaction.

Table 1.

The effect of the chemical structure of the catalyst on the rate constant of the esterification reaction

However, when strong acids are used in practice as catalysts, they complicate the process due to their simultaneous catalytic acceleration of additional reactions and their interaction with the initial and final products. For example, sulfuric acid sulfates the products of the reaction at high temperatures. Similarly, it catalyzes the intermolecular dehydration reaction of ethylene glycol, resulting in an unacceptable amount of diethylene glycol of 3-5%. Large amounts of 1,4-dioxane were found in the by-products of the reaction.

On fig. 2 is a graphical representation of the number of unreacted carboxyl groups as a function of the catalyst used.

Figure 2. Change in the number of non-reactive groups during esterification using various catalysts (% of the mass of TFA). T=230℃

1- without catalyst; 2- antimony acid; 3- lead dibutyl phthalate; 4- tetrabutoxytitanate; 5- sulfuric acid.

When the temperature rises above 230 ℃, the reaction rate rises to such an extent that it can be carried out without an external catalyst, but with a rapid release of water.

To synthesize PETP, TPA undergoes a reaction to obtain diglycol ester without an external catalyst. In this case, the role of the catalyst is played by the ion pair - TPA dimer, which is formed by autoprolysis of TPA.

The reaction mechanism is shown in fig. 3 and is in good agreement with classical ideas about the process of esterification of carboxylic acids with alcohols.

1

2

3

4                                                              5

6

7

8

9

10

11

Figure 3. Mechanisms of the esterification reaction of carboxylic acids with alcohols

Due to autoprolysis, the protonation of the oxygen atom of the carbonyl group of TPA 1 occurs. The protonated molecule carrying the positive charge then undergoes a polycondensation reaction with 1 EG. In the scheme, the structures (1,2,3,4,5 and 6,7,9,10) interconnected by a sharp arrow on both sides are considered saturated. Their rate of transition is so great that they can be detected spectroscopically as if they existed at the same time. However, only carbokation (electrophilic) 2 and 7 are attacked by the nucleophilic component (EG). The resulting structure redirects the proton within 5 molecules, along with the positive charge, to the carbony oxygen atom of the empty carboxyl group. A protonated molecule of the second carbonyl group of TPA monoglycol ester 6 (7) is formed, which interacts with the next molecule of EG. The esterification process continues until the formation of diglycol ester of TPA 11.

In order to reach the end of the reaction, water must be expelled from the reaction area, if no water is expelled, an equilibrium state is created and the reaction stops before it reaches the end. The release of water accelerates the reaction and shifts the equilibrium towards the formation of TPA esters.

The esterification is carried out using a continuous method by delivering a suspension of TPA in the EG to the reaction area and bringing the esterification to the polycondensation stage. The process is so rapid that the TPA suspension has almost complete reaction from the point of its introduction into the reactor until the reaction mass is removed from the apparatus circulation circuit. (that is, the TPA does not remain in the etheric).

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