FR3134392A1 - PROCESS FOR DEPOLYMERIZATION OF PET INTO TEREPHTHALATE ESTER AND MONOETHYLENE GLYCOL AT ROOM TEMPERATURE - Google Patents

Abstract

The present invention relates to the field of recycling plastics of the polyethylene terephthalate (PET) type commonly used for the manufacture of disposable plastic bottles, food trays, textiles, etc. More particularly, it relates to a process allowing the recycling of PET into dimethyl terephthalate (DMT) in less than an hour and without a pretreatment step. In addition, this process does not use toxic products. It is therefore particularly advantageous from an industrial point of view.

Description

PROCESS FOR DEPOLYMERIZATION OF PET INTO TEREPHTHALATE ESTER AND MONOETHYLENE GLYCOL AT ROOM TEMPERATURE

The present invention relates to the field of recycling plastics of the polyethylene terephthalate (PET) type commonly used for the manufacture of disposable plastic bottles, food trays, textiles, etc. More particularly, it relates to a process allowing the recycling of PET into dimethyl terephthalate (DMT) in less than an hour and without a pretreatment step. In addition, this process does not use toxic products. It is therefore particularly advantageous from an industrial point of view.

Field of the invention

PET recycling is an important environmental topic and thus represents a business opportunity due to its widespread use, abundance and sustainability. However, plastics recycling is complex and varies depending on polymer type, packaging design and product type.

The main obstacle to the use of recycled plastics is the contamination of waste streams with different types of polymers that are not compatible with each other. Therefore, it is often not possible to add recycled PET-like plastic to virgin polymer without diminishing certain quality attributes, such as color, clarity or impact resistance. Thus, the ability to replace virgin polymer with recycled PET is highly dependent on the purity of the recycled product and the requirements of the final product.

According to the principle of chemical recycling, PET can be depolymerized by solvolysis such as methanolysis or glycolysis, or by hydrolysis and the monomers thus obtained can be reused to generate new PET polymers called “recycled PET”.

Depending on industrial need, some PET resin manufacturing technologies resort to the use of dimethanol ester of terephthalic acid (DMT).

Conventional methanolysis techniques use processes that are very demanding in terms of energy and equipment costs; these processes use a supercritical phase at temperatures above 300°C and 5 to 10 bars of pressure, which induces structural changes in the molecular units of PET, in particular isomerization or degradation (US 6,706,843; WO2021/126661) . These modified molecules can be toxic or cause disruptions during the production of recycled PET, they harm the quality of the depolymerized product for these future applications.

Document WO2020/128218 describes a process for depolymerizing PET by alcoholysis using a monoalcohol such as methanol or ethanol and a base chosen from sodium methoxide, KOH or NaOH in a stoichiometric quantity relative to the PET.

It is known that the use of a base in a catalytic quantity relative to the mass of the PET makes it possible to obtain DMT but the kinetics of the reaction are quite slow; the reaction time is greater than 10h30, during which the reaction solution is heated continuously. As an example, we can cite documents US2019/0256450 and WO2020/188359 which describe the depolymerization of PET into DMT in the presence of methanol and an alkoxide such as sodium methoxide. These methanolysis reactions take place at temperatures between 25°C and 100°C. These processes necessarily include a first phase of swelling the PET with chlorinated or polar solvents such as DMSO or DMF or methanol. Document US2019/0256450 proposes reacting PET with a base, sodium methoxide in a catalytic quantity, and methanol. The process described in document WO2020/188359 is characterized by the sequential addition of methanol and methoxide solutions several times after addition of sodium methoxide. The authors describe high PET production yields. Document US2019/390035 describes another depolymerization approach by adding glycolate salt; the preparation of this salt includes isolation and drying stages which extend over a week.

Document WO2021/126661 describes an improved process for depolymerization of PET by methanolysis using catalysts chosen from sodium carbonate, magnesium methoxide, DBU and TBD. This process is carried out at temperatures of at least 110 – 140°C by applying a pressure of 15 bars.

For those skilled in the art, the implementations of the processes described above obviously present problems of industrial operability and feasibility with regard to the safety aspect of an ATEX environment such as that of reflux methanol which requires complex precautions and expensive devices to introduce flammable products during the process.

None of these processes is satisfactory. It is therefore desirable to have improved PET recycling processes that are low cost and easily operable industrially in order to facilitate the generalization of this recycling and broaden the fields of use of recycled PET.

The inventors have developed a new, particularly efficient process for depolymerization by alcoholysis under mild conditions for the recycling of polyethylene terephthalate (PET) polymer into terephthalate and monoethylene glycol (MEG) ester monomers. This process is very fast while being much more environmentally friendly than state-of-the-art processes. It provides access to a product in solid form that is directly reusable due to its purity, in particular DMT in crystalline form.

Thus, the invention relates to a process for recycling polyethylene terephthalate plastic waste into terephthalate ester monomers comprising two steps:

has. a step of grinding the waste to produce fragments, and

b. a step of depolymerization of PET into terephthalate ester and monoethylene glycol (MEG), in the presence:

(i) a catalyst chosen from a metal or organic etheroxide base, a metal acetate, a metal oxide, a metal hydroxide or a metal carbonate and

(ii) a polar solvent of the cyclic ester or etheroxide type

(iii) an alcohol chosen from a monoalcohol or a diol

characterized in that:

• said base is present in a catalytic quantity relative to the quantity of PET
• said depolymerization step is carried out at room temperature or by heating up to 70°C for a period of between 1 minute and 4 hours.

Advantages of the invention

The process according to the invention proposes to combine (i) a catalyst in a catalytic quantity relative to PET and (ii) an alcohol which is either a monoalcohol or a diol, and to react them under mild conditions. It has several advantages compared to the processes described previously, which are explained below.

A first remarkable advantage: this process does not require pretreatment, a step which requires the use of toxic products. The depolymerization reaction is sufficiently efficient to allow complete depolymerization without prior swelling of the material to be treated. Thus, the process according to the invention is simpler (one step less), more environmentally friendly (no toxic product therefore no effluent to treat), faster and less expensive.

The process being at very moderate industrial risk, the industrial installations for its implementation can therefore be set up more easily, the level of security of these installations being less restrictive. Regulatory compliance is simplified during factory installation and throughout the production cycle. CAPEX is thus significantly reduced.

Any type of solvent can be used for depolymerization although ester type solvents are preferred. The latter are in fact devoid of any toxicity, they are products which are particularly used in the food industry, in the field of aromas.

Remarkably, the depolymerization reaction is complete, very rapid and produces a terephthalate ester of high purity. This is particularly advantageous when PET is depolymerized into DMT because the latter is then easily recyclable and has industrial outlets and a recognized market.

The process can be described as “very rapid” since the reaction is complete in less than 4 hours at room temperature, and in less than 20 minutes under optimized heating conditions, in particular between 55 and 70°C. It starts instantly and can lead to complete depolymerization within 1 min.

The depolymerization reaction is simple. Depolymerization and purification can be done in a single step. In a preferred embodiment, the depolymerization is carried out by methanolysis. After completion of the reaction, the product obtained is directly a terephthalate ester (like DMT) in the form of crystals. Washing makes it possible to remove intermediate or degradation products which would require tedious distillation operations in conventional processes in order to separate them from the product of interest.

The process also makes it possible to obtain DET or BHET when the depolymerization of PET is carried out by ethanolysis or glycolysis respectively.

This process can be applied to any type of plastic including PET, pure or mixed, transparent or colored, whatever the thickness of the material... The reaction is selective with respect to PET and does not modify the other components possible; separation is thus easy between the depolymerized PET in monomer form and the other larger components; these can be eliminated by simple filtration. Then, the cooled mixture allows the precipitation of DMT. One wash is enough to obtain directly usable DMT. The washing solvents are advantageously the alcohols used during depolymerization.

The yield of the process is high: at least 85%, particularly for the depolymerization of PET into DMT.

In the particular case of the depolymerization of PET into DMT using methanol, the product obtained is 99.9% pure at the end of the reaction (after filtration and washing); there is therefore no need for further purification. DMT can be used directly after washing with methanol. Given its level of purity, it can be used in many applications, to remake PET or any other type of technical resin using this monomer. The choice of reagents and the implementation of mild conditions ensure that no isomerization reaction occurs, nor the formation of degradation products which harm the quality of the product obtained. When present, these secondary reaction molecules disrupt the polymerization reaction and purification of the crude DMT is therefore necessary before its use.

This process is more economical and more environmentally friendly than existing processes due to the fact that the bases are used in catalytic quantities relative to the quantity of PET to be recycled, and that the reaction temperatures are below 80°C, generally between room temperature (around 25°C) and 60°C and that the reaction times are very short compared to those of the PET depolymerization processes described in the literature.

In particular, alcohol is used in proportions ranging from 1.1 to 3 molar equivalents relative to PET, which is a substantial improvement compared to conventional methanolysis technologies in which proportions of 25 times molar are required.

The proportions of the polar acetate solvent are also reduced, varying from 1:1.5 to 1:4 with regard to the mass of PET: volume of solvent.

From an ecological point of view, it should be noted that the depolymerization bath containing the solvent can be reused for a new treatment cycle once the product has been filtered. The bath can be used at least 2 times without affecting the efficiency of the reaction. Once the reaction is complete, the solvents can be recovered by simple, energy-efficient distillation given their low boiling point.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a process for recycling polyethylene terephthalate plastic waste into terephthalate ester monomers comprising two steps:

vs. a step of grinding the waste to produce fragments, and

d. a step of depolymerization of PET into terephthalate ester and monoethylene glycol (MEG), in the presence:

(i) a catalyst chosen from a metal or organic etheroxide base, a metal acetate, a metal oxide, a metal hydroxide, a metal carbonate or a metal ester, and

(ii) a polar solvent of the cyclic ester or etheroxide type

(iii) an alcohol chosen from a monoalcohol or a diol

characterized in that:

• said base is present in a catalytic quantity relative to the quantity of PET
• said depolymerization step is carried out at room temperature or by heating up to 70°C for a period of between 1 minute and 4 hours.

The catalyst is a base chosen from:

(i) an etheroxide is of the sodium methoxide, magnesium methoxide, potassium methoxide or ammonium methoxide type,

(ii) a metallic carbonate of the sodium carbonate or potassium carbonate type,

(iii) a metal hydroxide of the sodium hydroxide or potassium hydroxide type, and

(iv) a metal acetate of the zinc acetate type Zn(OAc) ₂ ,

(v) a metal oxide,

(vi) a metal ester of the Ti(OiPr) ₄ titanium ester _, Mn(OR) ₂ manganese ester, or Sb(OR) _{2 antimony ester type.}

In a preferred embodiment of the invention, the catalyst is chosen from sodium methoxide, magnesium methoxide, potassium methoxide or ammonium methoxide.

The catalyst is present in a molar ratio of less than 35% relative to the PET, preferably between 1 and 20%.

The ester type solvent preferably corresponds to formulas A or B:

in which R₁and R₂are identical or different and are chosen (independently) from an aryl C_notH_2n , C alkyl_notH_2n+1Or VS_notH_2n-1with n = 1 to 10

In a preferred embodiment, the polar solvent is of the ester type because it is non-toxic. The ester type solvent can be chosen from methyl, ethyl, propyl, butyl, isopropyl acetate.

Table 2 (experimental part) describes different embodiments of the invention depending on the base used.

The solvent can also be of the cyclic etheroxide type, such as dioxane.

In a preferred embodiment of the invention, the PET:solvent ratio is between 1:1.5 and 1:5.

The quantity of alcohol involved in the depolymerization reaction varies. The alcohol can either be provided by the base in solution (in an alcohol), or is added as such to the reaction medium. The alcohol can thus be in excess, in an equivalent quantity or in default compared to the quantity of PET. This parameter will be adjusted by a person skilled in the art.

In another preferred embodiment of the invention, the alcohol: PET molar ratio is between 0.25 and 3, preferably between 1.1 and 3.

Advantageously, the process is implemented by applying a PET:solvent ratio of between 1:1.5 and 1:5 and an alcohol:PET molar ratio of between 0.25 and 3.

The alcohol used during the depolymerization step is preferably a monoalcohol chosen from methanol, ethanol, propanol or butanol, or a diol such as ethylene glycol.

In a particular embodiment of the invention, an alcohol and an ester of the same rank are used during the depolymerization reaction.

This combination of an alcohol and an ester of the same rank has the advantage of allowing a complete depolymerization reaction. The terephthalate monomers are thus solubilized. Simply cool the solution to precipitate them and recover a product of high purity (at least 99%).

If the material contains a mixture of PET and other components, the latter will not be modified, will remain in suspension and will be easily removed by filtration.

For example, we can combine methanol and methyl acetate, we obtain DMT (methanolysis reaction) or ethanol and ethyl acetate, we obtain DET (diethyl terephthalate diester) ( ethanolysis reaction). If bis-hydroxyethylene glycol is used, BHET (bis(2-Hydroxyethyl) terephthalate) is obtained (glycolysis reaction).

The benefit of DET is for example illustrated in document WO2007/076384 which describes a PET ethanolysis reaction. The production of DET is described as advantageous in that DET is easier to dissolve than DMT. The DET obtained can be oxidized and then used to produce terephthalic acid.

Alternatively, another embodiment according to the invention may consist of combining an alcohol and an ester of different rank. For example, we can combine ethyl acetate and methanol, two commonly used reagents. The depolymerization reaction takes place efficiently and completely and a majority product is obtained corresponding to the alcohol used, in this example DMT due to the presence of methanol, but also secondary products such as DET and other monomers. terephthalate.

The base involved in the depolymerization reaction must be in a catalytic quantity relative to the quantity of PET to be treated.

By “catalytic quantity”, we mean a quantity of non-stoichiometric base, that is to say in a molar ratio of 1% to 49% relative to the quantity of PET to be treated. The term “catalytic” also applies to a reagent that is found in its initial form at the end of the reaction (catalyst).

In a preferred embodiment of the invention, the catalytic quantity of etheroxide base is less than 35 mole%. The catalytic quantity of etheroxide base can vary from 1 molar% to 35 molar%, preferably from 1% to 20 molar%, or even from 5% to 20%. Extended reaction times can be applied to further reduce this quantity, thereby reducing the cost of the reaction.

The reaction temperature may vary. The reaction medium can be heated up to 70°C. The mixture can in particular be advantageously heated between 50°C and 70°C, preferably at a temperature below 60°C. It is however very interesting to note that the reaction works very well at room temperature (around 25°C) while being fast since complete depolymerization is obtained in 3 to 4 hours. Not heating the reaction simplifies implementation and reduces cost.

The present invention will be better understood on reading the examples which follow, provided by way of illustration and should in no way be considered as limiting the scope of the present invention.

EXAMPLES

EXAMPLE 1: Depolymerization of PET by methanolysis

A quantity (500 g) of pieces of polyethylene terephthalate (PET) from different sources (food trays, water bottles, etc.), 2L of methyl acetate and 120 mL of a sodium methoxide solution (25% in methanol) corresponding to a molar ratio of 20% of sodium methoxide relative to the PET introduced and 200 mL of methanol are added to the pieces. The reaction starts instantly. After 30 minutes of reaction at 55°C, all of the pieces of PET have disappeared, giving way to a white solid slightly suspended in solution. The reaction crude is filtered through Buchner in order to retain the unreacted material, the recovered medium gels almost instantly. It contains DMT, the monoethylene glycol product of the depolymerization reaction as well as the base reacted initially and the solvent. The white solid (DMT) which is recovered (410 g, 82%) and is washed with methanol.

EXAMPLE 2: Depolymerization of PET by methanolysis

A quantity (500 g) of PET pieces from different sources (food trays, water bottles, etc.), 2L of methyl acetate and 210 mL of a sodium methoxide solution (25% in methanol ) corresponding to a molar ratio of 35% of sodium methoxide relative to the PET introduced and are added to the pieces. The reaction starts instantly. After 30 minutes of reaction at 55°C, all of the pieces of PET have disappeared, giving way to a white solid slightly suspended in solution. The reaction crude is filtered through Buchner in order to retain the unreacted material, the recovered medium gels almost instantly. It contains DMT, the monoethylene glycol product of the depolymerization reaction as well as the base reacted initially and the solvent. The white solid (DMT) which is recovered (400 g, 80%) and is washed with methanol.

EXAMPLE 3: Depolymerization of PET by e thanolysis

A quantity (500 g) of PET pieces from different sources (food trays, water bottles, etc.) 3L of ethyl acetate and 49.22 g of sodium methoxide corresponding to a molar ratio of 35% of sodium methoxide compared to the PET introduced 300 mL of ethanol are added to the pieces. The reaction starts instantly. After 30 minutes of reaction at 70°C, all of the PET pieces have disappeared, giving way to a white solid slightly suspended in solution. The reaction crude is filtered through a Buchner in order to retain the unreacted material, the recovered medium contains the DET, the monoethylene glycol product of the depolymerization reaction as well as the base reacted initially and the solvent. The DET (400 g) is recovered in the form of a pasty solid following the evaporation of the reaction solvents and is washed with ethanol.

EXAMPLE 4: Conversion rate as a function of time and temperature

Table 1 shows the effect of reaction time and temperature on the conversion rate of PET to DMT.

The reaction conditions implemented are as follows: 10 g of PET are incubated in a solution of sodium methoxide (diluted to 25% in MeOH) in a ratio of 20% (mol:mol PET), in the presence of 45ml of methyl acetate.

Table 1 : Effect of time/temperature on conversion rate

EXAMPLE 5: Conversion rate depending on the type of solvent and alcohol

Table 2 presents the effect of reaction time and temperature on the conversion rate of PET to DMT.

The reaction conditions are the same as those of Example 4.

The ester type solvent corresponds to the formula below:

Table 2: Effect of varying the type of solvent/alcohol on the conversion rate

Claims (11)

Process for recycling polyethylene terephthalate plastic waste into terephthalate ester monomers comprising two stages:

1. a step of grinding the waste to produce fragments, and
2. a step of depolymerization of PET into terephthalate ester and monoethylene glycol (MEG), in the presence:

(i) a catalyst chosen from a metal or organic etheroxide base, a metal acetate, a metal oxide, a metal hydroxide, a metal ester or a metal carbonate,
(ii) a polar solvent of the cyclic ester or etheroxide type
(iii) an alcohol chosen from a monoalcohol or a diol
characterized in that:

• said base is present in a catalytic quantity relative to the quantity of PET
• said depolymerization step is carried out at room temperature or by heating up to 70°C for a period of between 1 minute and 4 hours.

Process according to claim 1 in which said catalyst is chosen from (i) an etheroxide is of the sodium methoxide, potassium methoxide, magnesium methoxide or ammonium methoxide type (ii) a metal carbonate of the sodium carbonate or ammonium carbonate type potassium, (iii) a metal hydroxide such as sodium hydroxide or potassium hydroxide, (iv) a metal acetate such as zinc acetate Zn (v) a metal oxide (vi) or a metal ester or (vii) a metal ester of titanium ester type. Process according to claim 2 in which said catalyst is a metal ester chosen from titanium ester, manganese ester, antimony ester, or zinc acetate. Process according to one of the preceding claims in which said catalyst is present in a molar ratio of less than 35% relative to the PET. Process according to one of the preceding claims in which said ester-type polar solvent corresponds to one of formulas A or B:


in which R₁and R₂are identical or different and are chosen from an aryl C_notH_2n , C alkyl_notH_2n+1Or VS_notH_2n-1with n = 1 to 10 Process according to claim 5 in which said ester is chosen from methyl, ethyl, propyl, butyl, isopropyl acetate. Process according to one of the preceding claims in which the PET:solvent ratio is between 1:1.5 and 1:5. Process according to one of the preceding claims in which said monoalcohol is chosen from methanol, ethanol, propanol or butanol and said diol is ethylene glycol. Process according to one of the preceding claims in which the alcohol: PET molar (equivalent) ratio is between 0.25 and 3. Process according to one of the preceding claims in which an alcohol and an ester of the same rank are used for the same depolymerization reaction. Process according to one of claims 1 to 9, in which an alcohol and an ester of different rank are combined.