Improved process for producing α,β-unsaturated carboxylic acids from poly(3-hydroxyalkanoates)

Abstract

The present invention relates to a method for producing α,β-unsaturated carboxylic acids by pyrolysis of poly(3-hydroxyalkanoates), which limits the unintended condensation of hot α,β-unsaturated carboxylic acid vapors generated on the walls of the method and the clogging associated with the subsequent solid formation by the radical polymerization reaction. The invention is based on the introduction of a radical polymerization inhibitor into the pyrolysis reactor and the use of specific pyrolysis conditions that partially volatilize the inhibitor during the pyrolysis of poly(3-hydroxyalkanoates).

Description

[Technical Field]

[0001] The present invention relates to a method for producing α,β-unsaturated carboxylic acids by pyrolysis of poly(3-hydroxyalkanoates) while limiting the fouling phenomenon associated with the unintentional condensation of the resulting hot α,β-unsaturated carboxylic acid vapors on the process walls and the subsequent formation of solids by radical polymerization reactions. The invention is based on the introduction of a radical polymerization inhibitor into the pyrolysis reactor and the use of specific pyrolysis conditions that partially volatilize the inhibitor during the pyrolysis of the poly(3-hydroxyalkanoates). Thus, any unintentional condensation of the resulting hot α,β-unsaturated carboxylic acid vapors on the process walls also condenses the radical polymerization inhibitor, thereby protecting the liquid phase that is formed from radical polymerization reactions.

[0002] Prior art and technical issues Today, α,β-unsaturated carboxylic acids are produced industrially, mainly from raw materials of fossil origin: for example, acrylic acid can be obtained by oxidation of propylene, or methacrylic acid can be obtained by oxidation of isobutylene.

[0003] There is a strong market demand for these α,β-unsaturated carboxylic acids, which are used as monomers in many applications, to be obtained from biobased feedstocks, which are derived from renewable organic matter (biomass) of biological origin (microorganisms, plants, or animals).

[0004] One method for obtaining these α,β-unsaturated carboxylic acids is by pyrolysis of the corresponding poly(3-hydroxyalkanoates) at temperatures between 150° C. and 300° C. according to the following reaction: TIFF2025527855000001.tif32170 R1=H or alkyl and R2=H or alkyl; n is a number greater than 30; If R1=R2=H, · Poly(3-hydroxyalkanoate) = poly(3-hydroxypropionate) (P3HP); ·α,β-Unsaturated carboxylic acid = propenoic acid (acrylic acid). If R1=methyl and R2=H, then · Poly(3-hydroxyalkanoate) = Poly(3-hydroxyisobutyrate) (P3HiB); ·α,β-Unsaturated carboxylic acid = isobutenoic acid (methacrylic acid). If R1=H and R2=methyl, then · Poly(3-hydroxyalkanoate) = Poly(3-hydroxybutyrate) (P3HB); ·α,β-Unsaturated carboxylic acid = but-2-enoic acid (crotonic acid). If R1=H and R2=ethyl, then · Poly(3-hydroxyalkanoate) is poly(3-hydroxyvalerate) (P3HV); ·α,β-Unsaturated carboxylic acid = Pent-2-enoic acid.

[0005] These poly(3-hydroxyalkanoates) themselves are not only obtained by chemical conversion of fossil-derived raw materials, but also previously obtained by fermentation of biomass.

[0006] One potential problem with processes for producing α,β-unsaturated carboxylic acids is that these compounds can easily undergo radical polymerization when they are hot and in the liquid phase. This applies to intentionally formed liquid phases (e.g., those present in distillation columns, reactors, or condensers), but it can also occur in unintentionally formed liquid phases (e.g., those formed during the unintentional condensation of hot vapors on walls with cold spots). This process usually results in the deposition of solid polymer on plant equipment, eventually causing blockages and necessitating plant shutdowns for cleaning, which is difficult and costly in terms of unproductive downtime.

[0007] To alleviate these drawbacks, radical polymerization inhibitors have traditionally been added at all stages of the manufacturing process, namely at the synthesis and purification stages.

[0008] For example, during the industrial production of acrylic acid (AA), radical polymerization inhibitors are added to the absorption tower to absorb AA vapors from the catalytic oxidation of propylene into water, then to each distillation tower, and finally to the final product.

[0009] Radical polymerization inhibitors that have traditionally been used in these manufacturing processes include phenol derivatives such as hydroquinone (HQ) and its derivatives, such as hydroquinone methyl ether (HQME), 2,6-di-tert-butyl-4-methylphenol (BHT), or 2,4-dimethyl-6-tert-butylphenol (Topanol A); phenothiazine and its derivatives; nitroxide compounds such as 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (4-OH-TEMPO); and amino compounds such as paraphenylenediamine derivatives.

[0010] One drawback of these inhibitors is that they are usually considered non-volatile under the conditions for producing α,β-unsaturated carboxylic acids. Therefore, in order to be present in all liquid phases containing α,β-unsaturated acids, these inhibitors must not only be injected into the reaction, but also into the feed, boiler, condenser, and reflux sections of the purification equipment. Inhibitors can be sprayed into solution to protect all surfaces onto which hot α,β-unsaturated acid vapors may unintentionally condense. This problem is well known to those skilled in the art; for example, during the industrial purification of acrylic acid by distillation, polymerization inhibitors are not only added to the feed, condenser, and reflux sections of the distillation column, but are also often sprayed in the form of a spray to protect the dome, gooseneck, manhole, or other column components onto which acrylic acid vapors may condense.

[0011] EP 2398832 describes another solution aimed at preventing polymerization, including in the case of unintentional accidental condensation of AA vapor. Here, a second type of inhibitor (called a fugitive inhibitor, which is volatile under production operating conditions) is used, which has a similar volatility to the α,β-unsaturated acid (here, acrylic acid). This inhibitor then exists in the gas phase and condenses simultaneously with the acrylic acid vapor during unintentional condensation. However, these polymerization inhibitors (here, nitrosobenzene derivatives) have the disadvantage of being highly toxic.

[0012] The problem of polymerization of α,β-unsaturated carboxylic acids during manufacture also exists when these acids are produced by thermal decomposition of the corresponding poly(3-hydroxyalkanoates).

[0013] U.S. Patent No. 2,568,636 (US 2,568,636) describes a method for forming acrylic acid (AA) by pyrolysis of poly(3-hydroxypropionate) (P3HP) at temperatures between 130°C and 300°C, and the use of triaryl phosphate to limit the polymerization of AA in the pyrolysis reactor. U.S. Patent Application No. 3,002,017 (US 3,002,017) describes a similar pyrolysis in which AA vapor is absorbed into cold AA to limit polymerization during the condensation step.

[0014] US Patent No. 9115070 describes the pyrolysis of P3HP to form AA using a tertiary amine catalyst to reduce the reaction temperature. Conventional polymerization inhibitors, such as phenothiazine (PTZ), can potentially be used in the reaction medium in amounts of 10 to 1000 ppm by weight relative to P3HP to reduce the polymerization of AA formed in the pyrolysis medium.

[0015] US Patent No. 10065914 describes the pyrolysis of P3HP to form AA at temperatures between 100°C and 300°C, using a sodium acrylate catalyst to reduce the reaction temperature and limit the risk of polymerization of the AA formed in the pyrolysis medium. Polymerization can also be reduced by using polymerization inhibitors (e.g., PTZ and HQME) in the liquid phase of the pyrolysis reactor and intentionally introducing them into the liquid phase of the distiller or condenser.

[0016] However, the prior art processes have significant drawbacks. Although the prior art describes methods for reducing the risks associated with the polymerization of the liquid phase of α,β-unsaturated acids by injecting polymerization inhibitors into these intentionally formed liquid phases (such as the pyrolysis medium, the condenser, and the liquid phase of the distillation column), it does not provide a solution when the liquid phase is formed unintentionally (for example, during the undesired condensation of α,β-unsaturated acids in the "cold spots" of the plant). Although it is certainly possible to use sprayers to spray these inhibitors onto all the walls of an industrial system, this implementation is complicated. The use of non-traditional inhibitors (such as nitrosobenzene derivatives) is also complicated for industrial implementation.

[0017] The present inventors have surprisingly discovered that it is possible to significantly reduce polymerization phenomena associated with the unintended condensation of hot vapors in the process for synthesizing α,β-unsaturated carboxylic acids from poly(3-hydroxyalkanoates) without the use of unconventional inhibitors. Specifically, the parameters of the pyrolysis reaction of poly(3-hydroxyalkanoates) to yield α,β-unsaturated carboxylic acids can be adjusted to achieve significant volatility with several conventional polymerization inhibitors. These inhibitors then exist in the gas phase and condense simultaneously with the α,β-unsaturated carboxylic acids during unintended condensation in the cold spot, instantly protecting the formed liquid phase. Complex systems for delivering the inhibitor at several points in the process, as well as expensive and toxic inhibitors, are not required.

[0018] It is therefore proposed by the present invention to provide a simple and easily implementable solution for reducing fouling phenomena while maintaining high reliability and improved productivity in processes for producing α,β-unsaturated carboxylic acids from poly(3-hydroxypropionate).

[0019] Summary of the Invention The present invention relates to a process for producing α,β-unsaturated carboxylic acids by pyrolysis of poly(3-hydroxyalkanoates) in the presence of one or more polymerization inhibitors, which is carried out in a pyrolysis reactor from which the carboxylic acid vapors produced enter a condenser, characterized in that the pressure in the reactor is adjusted to be less than twice the vapor pressure of at least one inhibitor at the temperature at which the pyrolysis is carried out.

[0020] According to the present invention, the pyrolysis conditions (pressure and temperature) used render one of the polymerization inhibitors significantly more volatile, thereby achieving the desired effect.

[0021] According to various implementations, the process has the following characteristics, in appropriate combinations: All values ​​given are expressed by weight unless otherwise indicated. Value ranges given include the extreme values.

[0022] According to one embodiment, the poly(3-hydroxyalkanoate) used in the pyrolysis process contains one type of 3-hydroxyalkanoate unit, and therefore the product formed is composed of one α,β-unsaturated carboxylic acid.

[0023] According to one embodiment, the poly(3-hydroxyalkanoate) used in the pyrolysis process contains several different 3-hydroxyalkanoate units, and thus the product formed is composed of a mixture of various α,β-unsaturated carboxylic acids. Examples of P3HA copolymers are poly-3-hydroxybutyrate-co-3-hydroxypropionate (poly-3HB-co-3HP) or poly-3-hydroxybutyrate-co-3-hydroxyvalerate (poly-3HB-co-3HV).

[0024] In one embodiment, the poly(3-hydroxyalkanoates) used in the pyrolysis process are obtained from fossil-derived feedstocks.

[0025] In one embodiment, the poly(3-hydroxyalkanoate) used in the pyrolysis process is obtained from renewable or at least partially renewable raw materials. According to this embodiment, the poly(3-hydroxyalkanoate) is more than 50% by weight, preferably more than 80% by weight, and advantageously 100% by weight, derived from renewable raw materials.

[0026] According to one embodiment, the poly(3-hydroxyalkanoates) used in the pyrolysis process are obtained by chemical reaction, for example P3HP is obtained by polymerization of β-propiolactone (itself obtained from ethylene oxide and carbon monoxide).

[0027] According to one embodiment, the poly(3-hydroxyalkanoates) used in the pyrolysis process are obtained by biological reactions, in particular fermentation.

[0028] According to one embodiment, the poly(3-hydroxyalkanoate) used in the pyrolysis process is purified prior to the pyrolysis reaction.

[0029] According to one embodiment, the poly(3-hydroxyalkanoates) used in the pyrolysis process are used without prior purification, in particular without separation of the cell membranes if obtained by fermentation.

[0030] According to one embodiment, poly(3-hydroxyalkanoates) are obtained intracellularly by a fermentation reaction, the biomass is washed and dried, but the poly(3-hydroxyalkanoates) are not separated from the cell membranes prior to the pyrolysis step.

[0031] According to one embodiment, poly(3-hydroxyalkanoates) are obtained intracellularly by a fermentation reaction, the biomass is washed and dried, and the poly(3-hydroxyalkanoates) are separated from the cell membranes, for example by extraction, prior to the pyrolysis step.

[0032] According to one embodiment, the poly(3-hydroxyalkanoate) pyrolysis reaction occurs in the absence of a solvent, in which case the product is in solid form or in a molten state.

[0033] According to one embodiment, the poly(3-hydroxyalkanoate) pyrolysis reaction occurs in solution.

[0034] According to one embodiment, the poly(3-hydroxyalkanoate) pyrolysis reaction occurs in suspension.

[0035] According to one embodiment, the poly(3-hydroxyalkanoate) pyrolysis reaction occurs batchwise.

[0036] According to one embodiment, the poly(3-hydroxyalkanoate) pyrolysis reaction occurs continuously.

[0037] According to one embodiment, the poly(3-hydroxyalkanoate) pyrolysis reaction occurs in the absence of a catalyst.

[0038] The polymerization inhibitors used in the process according to the present invention are selected from inhibitors conventionally used in existing industrial processes for producing α,β-unsaturated carboxylic acids, including phenol derivatives such as hydroquinone (HQ) and its derivatives, phenothiazine and its derivatives, nitroxide compounds, and amino compounds such as paraphenylenediamine derivatives.

[0039] According to one embodiment, the poly(3-hydroxyalkanoate) comprises 3-hydroxypropionate units and at least one of the α,β-unsaturated carboxylic acids produced is acrylic acid.

[0040] According to one embodiment, the poly(3-hydroxyalkanoate) is poly(3-hydroxypropionate) and the α,β-unsaturated carboxylic acid produced is acrylic acid.

[0041] According to one embodiment, the poly(3-hydroxyalkanoate) comprises 3-hydroxybutyrate units and at least one of the α,β-unsaturated carboxylic acids produced is crotonic acid.

[0042] According to one embodiment, the poly(3-hydroxyalkanoate) is poly(3-hydroxybutyrate) and the α,β-unsaturated carboxylic acid produced is crotonic acid.

[0043] According to one embodiment, the poly(3-hydroxyalkanoate) comprises 3-hydroxyisobutyrate units and at least one of the α,β-unsaturated carboxylic acids produced is methacrylic acid.

[0044] According to one embodiment, the poly(3-hydroxyalkanoate) is poly(3-hydroxyisobutyrate) and the α,β-unsaturated carboxylic acid produced is methacrylic acid.

[0045] Another subject of the present invention relates to a method for purifying α,β-unsaturated carboxylic acids obtained by a poly(3-hydroxyalkanoate) pyrolysis process carried out at pyrolysis temperatures and at a pressure less than twice the vapor pressure of at least one polymerization inhibitor, characterized in that it comprises a step of condensing the vapors of one or more α,β-unsaturated carboxylic acids thus obtained, followed by one or more purification steps.

[0046] The present invention fulfills the needs expressed in the prior art. The present invention makes it possible to prevent the risk of fouling due to the unintended condensation of α,β-unsaturated carboxylic acid vapors in cold spots when α,β-unsaturated carboxylic acids are generated by the thermal decomposition of poly(3-hydroxypropionate). In particular, the present invention makes it possible to protect the area located between the thermal decomposition reactor and the condenser. Since the polymerization inhibitor becomes volatile in the thermal decomposition medium, the polymerization inhibitor will be present in the gas phase throughout the part of the plant where one or more α,β-unsaturated carboxylic acids are present in the gas phase. The present invention also makes it possible to avoid the formation of polymers in the reaction medium.

[0047] The present invention will now be described in more detail below.

[0048] Detailed Description of the Invention The present invention aims to produce α,β-unsaturated carboxylic acids on an industrial scale by pyrolysis of poly(3-hydroxyalkanoate) without facing the problem of fouling of the equipment used due to condensation of the vapor of the α,β-unsaturated carboxylic acid in cold spots during polymerization of the vapor.

[0049] The present invention proposes to provide a process that reduces or eliminates this fouling risk. The invention is based on the addition of a polymerization inhibitor and the selection of pressure and temperature conditions in a poly(3-hydroxyalkanoate) pyrolysis reactor, so that the inhibitor has significant volatility under the reaction conditions. The pressure in the reactor is typically adjusted to be less than twice the vapor pressure of the inhibitor at the pyrolysis temperature.

[0050] The term "thermolysis" of poly(3-hydroxyalkanoates) refers to their chemical decomposition into α,β-unsaturated carboxylic acids under the influence of temperature. This term is synonymous with high-temperature pyrolysis.

[0051] According to IUPAC, "saturation vapor pressure" is the pressure exerted by a pure substance (at a given temperature) in a system containing only the vapor and condensed phases (liquid or solid) of that substance (Pure and Applied Chemistry, 1990, Volume 62, No. 11, pp. 2167-2219 & Glossary of atmospheric chemistry terms (Recommendations 1990), p. 2212).

[0052] In the present specification, the term "vapor pressure" is used interchangeably with the term "saturated vapor pressure." The term "vapor pressure" is also synonymous with "vapor tension."

[0053] In the poly(3-hydroxyalkanoate) pyrolysis process according to the present invention, the poly(3-hydroxyalkanoate) is heated to a temperature of 130°C to 300°C, preferably 170°C to 230°C.

[0054] The reaction medium in the pyrolysis reactor contains at least one polymerization inhibitor, preferably in a proportion of 50 ppm by weight to 5% by weight, more preferably 0.01 to 3% by weight, based on the weight of poly(3-hydroxyalkanoate). If there are two or more inhibitors, their total content does not exceed 5% by weight.

[0055] The polymerization inhibitor is selected from inhibitors conventionally used in existing industrial processes for producing α,β-unsaturated carboxylic acids, including phenol derivatives such as hydroquinone (HQ) and its derivatives, such as hydroquinone methyl ether (HQME), 2,6-di-tert-butyl-4-methylphenol (BHT), or 2,4-dimethyl-6-tert-butylphenol (Topanol A); phenothiazine and its derivatives; nitroxide compounds such as 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (4-OH-TEMPO); and amino compounds such as paraphenylenediamine derivatives.

[0056] In the poly(3-hydroxyalkanoate) pyrolysis process according to the present invention, the temperature and pressure conditions in the pyrolysis reactor are selected so that the α,β-unsaturated carboxylic acid formed is in vapor form and at least one of the inhibitors is volatile, which is achieved when the pressure in the reactor is less than twice the vapor pressure of one of the inhibitors at the pyrolysis temperature.

[0057] According to one embodiment, at least one of the polymerization inhibitors is hydroquinone methyl ether (HQME).

[0058] For example, the vapor pressure of HQME is: 20 kPa at 190 ° C; the pressure in the reactor is adjusted to below 40 kPa for pyrolysis at 190 ° C; 28.5 kPa at 200 °C; the pressure in the reactor is adjusted to below 57 kPa for pyrolysis at 200 °C; 39 kPa at 210 °C; the pressure in the reactor is adjusted to below 78 kPa for pyrolysis at 210 °C.

[0059] Thus, the method according to the invention makes it possible to protect the operation in the event of undesired condensation of hot α,β-unsaturated carboxylic acid vapors in one cold spot on the wall of the installation by adopting specific pressure conditions to obtain volatility of the inhibitor.

[0060] According to one embodiment of the present invention, the pyrolysis reaction is carried out in the presence of a solvent, either in solution or suspension, and the solvent is selected so that its vapor pressure at the pyrolysis temperature of the poly(3-hydroxyalkanoate) is less than three-quarters of the pressure at which the pyrolysis is carried out, in order to limit the evaporation of the solvent by the α,β-saturated carboxylic acid vapors generated during the pyrolysis of the poly(3-hydroxyalkanoate).

[0061] According to one embodiment, for operating conditions of 200° C. and 20 kPa, the solvent must have a vapor pressure at 200° C. of less than 15 kPa and may be selected from: Higher alkanes containing more than 14 carbon atoms; for example, the vapor pressure of n-hexadecane (C16) at 200°C is 10 kPa. When the solvent is an alkane, the pyrolysis reaction occurs in suspension. Fatty acids containing more than 8 carbon atoms; for example, the vapor pressure of capric acid (C10) at 200°C is 11.2 kPa. When the solvent is a fatty acid, the pyrolysis reaction occurs in suspension. Polyglycol dimethyl ether (glyme) starting from tetraglyme; for example, the vapor pressure of tetraglyme at 200°C is 10.2 kPa. When the solvent is glyme, the thermal decomposition reaction occurs in suspension. The vapor pressure of sulfolane at 200°C is 10.3 kPa. When the solvent is sulfolane, the thermal decomposition reaction occurs in solution.

[0062] According to one embodiment, when the poly(3-hydroxyalkanoate) pyrolysis reaction is carried out in suspension in a solvent or in solution in a solvent, the operating pressure is between 1.5 times the vapor pressure of the solvent at the pyrolysis temperature and 2 times the vapor pressure of the at least one inhibitor at the pyrolysis temperature.

[0063] The preferred operating pressure for the poly(3-hydroxyalkanoate) pyrolysis reaction is just below the vapor pressure of the inhibitor.

[0064] According to a preferred embodiment, the polymerization inhibitor is hydroquinone methyl ether and the run is carried out in solution in a solvent (for example sulfolane or tetraglyme).

[0065] According to one embodiment, the pyrolysis of poly(3-hydroxyalkanoate) is carried out in the absence of a catalyst. The use of a catalyst can accelerate the pyrolysis rate and / or lower the pyrolysis temperature. However, the use of a catalyst makes the process more complicated and more difficult to implement on an industrial scale.

[0066] The present invention also relates to a method for purifying one or more α,β-unsaturated carboxylic acids obtained by a poly(3-hydroxyalkanoate) pyrolysis process carried out at a pyrolysis temperature and a pressure less than twice the vapor pressure of at least one polymerization inhibitor, characterized in that the method comprises a step of condensing the vapors of one or more α,β-unsaturated carboxylic acids thus obtained, followed by one or more purification steps. Purification operations may generally involve distillation, liquid / liquid extraction, separation using a thin-film evaporator, or recrystallization, or a combination of these techniques.

[0067] The following examples illustrate the present invention but are not intended to limit the scope of the invention.

[0068] Experimental Department The thermal decomposition test of poly(3-hydroxypropionic acid) (P3HP) to produce acrylic acid (AA) is carried out in a laboratory assembly. 2 g of pure P3HP is placed in a two-neck round-bottom flask (25 ml).

[0069] Optionally, an inhibitor (PTZ or HQME) is added in an amount of 20 mg.

[0070] Optionally, a solvent is added in an amount of 10 g.

[0071] The side neck of the round-bottom flask is equipped with a thermometer to monitor the reaction temperature. The upper neck of the round-bottom flask is equipped with an isolating bridge leading to a water-cooled side condenser, which itself leads to a receiver consisting of a second round-bottom flask (25 ml). A tap between the condenser and receiver allows the pressure in the assembly to be reduced.

[0072] At the start of the experiment, the system is brought to the desired pressure, and then the flask containing P3HP and, optionally, inhibitors and / or solvents is placed in a heating system (oil bath or electric heating mantle) capable of establishing the desired pyrolysis temperature. The receiver is cooled in an ice bath. The separating bridge between the pyrolysis round-bottom flask and the side condenser is left uninsulated to simulate the presence of a cold spot.

[0073] As soon as the pyrolysis reactor reached a temperature above 170 °C, AA vapor formation was observed, condensing primarily in the side condenser but also in the cold spot of the separating bridge. After 4 hours of heating, the formation of AA vapor in the pyrolysis reactor tapered off, after which the experiment was stopped.

[0074] The degree of fouling of the separating bridge (representing unintended condensation zones of AA at cold spots in industrial plants) is then visually determined. The AA recovered in the receiver is also analyzed by gas chromatography to identify the presence or absence of inhibitors optionally introduced into the pyrolysis reactor and to verify their volatility or non-volatility under the experimental conditions tested.

[0075] The main results obtained are shown in Table 1.

[0076] Comparative tests 1, 2, 8, 9, 15, and 16, carried out without polymerization inhibitors, show severe fouling of the separating bridge, where hot AA vapor condenses at the cold spot, whether without solvent (1, 2), in suspension (8, 9), in solution (15, 16), and at atmospheric pressure (1, 8, 15), or under reduced pressure of 20 kPa (2, 9, 16).

[0077] Comparative tests 3, 6, 7, 10, 13, 14, 17, 20, and 21, performed in the presence of a polymerization inhibitor but at operating pressures in the assembly exceeding twice the inhibitor's vapor pressure at pyrolysis temperatures, show severe fouling of the separation bridge where hot AA vapor condenses at the cold spot, whether performed without solvent (3, 6, 7), in suspension (10, 13, 14), or in solution (17, 20, 21). It should also be noted that the AA recovered in the receiver showed no trace of the inhibitor, indicating its lack of volatility under pyrolysis operating conditions.

[0078] Tests 4, 5, 11, 12, 18, and 19, performed in the presence of a polymerization inhibitor and at an operating pressure in the assembly of less than twice the vapor pressure of the inhibitor, whether performed in bulk (4, 5), in suspension (11, 12), or in solution (18, 19), demonstrate a significant reduction in fouling of the separation bridge, where hot AA vapor condenses at cold spots. It should also be noted that the AA recovered in the receiver contains at least trace amounts of the inhibitor, indicating that AA was volatile under the pyrolysis operating conditions. The reduction in fouling, and thus the presence of the inhibitor in the formed AA, is small but significant when the operating pressure is slightly below twice the vapor pressure of the inhibitor at the pyrolysis temperature (4, 11, 18) and even more pronounced when the operating pressure is below the vapor pressure of the inhibitor at the pyrolysis temperature (5, 12, 19).

[0079] TIFF2025527855000002.tif169170

[0080] Additional tests 22-31 (Table 2), performed in the presence of HQME in solvent at 200 °C and 20 kPa (i.e., under the conditions of the present invention), demonstrate the absence of fouling of the isolation bridge where hot AA vapor condenses at the cold spot. Note also the presence of inhibitors in the AA recovered in the receiver, indicating that the inhibitors are volatile under the pyrolysis operating conditions.

[0081] These findings demonstrate the importance of the solvent selection during the pyrolysis of poly(3-hydroxyalkanoates) carried out in a solvent medium according to the present invention. Thus, if the vapor pressure of the solvent at the pyrolysis temperature (here, 200 °C) is more than three-quarters of the operating pressure (here, 20 kPa, i.e., the vapor pressure of the solvent at 200 °C is less than 15 kPa), the AA recovered in the receiver is heavily contaminated by the solvent used. (22, 26, 29) This phenomenon is significantly limited when using solvents whose vapor pressure at the pyrolysis temperature is less than three-quarters of the operating pressure. (23, 24, 25, 27, 28, 30, 31)

[0082] C14 = n-tetradecane; C16 = n-hexadecane; C18 = n-octadecane; C20 = n-eicosane C8 acid = octanoic acid; C10 acid = decanoic acid; C12 acid = dodecanoic acid TIFF2025527855000003.tif110170

Claims

1. 1. A process for producing an α,β-unsaturated carboxylic acid by pyrolysis of a poly(3-hydroxyalkanoate) carried out in the presence of at least one polymerization inhibitor, comprising: A method characterized in that the operating pressure in the reactor is less than two times the vapor pressure of one of the inhibitors at the pyrolysis temperature.

2. 2. The method of claim 1, wherein the pyrolysis temperature is between 130°C and 300°C, preferably between 170°C and 230°C.

3. 3. The method according to claim 1, wherein the content of inhibitor is from 50 ppm to 5% by weight, in particular from 0.01% to 3% by weight, based on the weight of poly(3-hydroxyalkanoate).

4. 4. The method according to claim 1, wherein at least one of the polymerization inhibitors is a compound selected from a phenol derivative, a phenothiazine derivative, a nitroxide derivative, or a paraphenylenediamine derivative.

5. 5. The method of any one of claims 1 to 4, wherein at least one of the polymerization inhibitors is hydroquinone methyl ether (MEHQ).

6. 6. The method of any one of claims 1 to 5, wherein the poly(3-hydroxyalkanoate) is of petrochemical or at least partially renewable origin.

7. 6. The method according to any one of claims 1 to 5, wherein the poly(3-hydroxyalkanoate) is more than 50% by weight, preferably more than 80% by weight, advantageously 100% by weight, of renewable origin.

8. The method according to any one of claims 1 to 7, wherein the poly(3-hydroxyalkanoate) is obtained by chemical reaction.

9. The method of claim 7, wherein the poly(3-hydroxyalkanoate) is obtained by fermentation.

10. 10. The method of claim 9, wherein the poly(3-hydroxyalkanoate) is separated from the biological medium, for example by extraction, prior to the pyrolysis step.

11. 11. The method of any one of claims 1 to 10, wherein the pyrolysis reaction is carried out in the absence of a solvent.

12. 11. The method of any one of claims 1 to 10, wherein the pyrolysis reaction is carried out in suspension in a solvent.

13. 11. The method of any one of claims 1 to 10, wherein the pyrolysis reaction is carried out in solution in a solvent.

14. 14. The method of claim 12 or 13, wherein the solvent has a vapor pressure at the temperature of the pyrolysis reaction that is less than three-quarters of the operating pressure.

15. 14. The method of claim 13, wherein the solvent is sulfolane.

16. 16. The method of claim 15, wherein at least one of the polymerization inhibitors is hydroquinone methyl ether.

17. 17. The method of any one of claims 1 to 16, wherein the pyrolysis of the poly(3-hydroxyalkanoate) is carried out batchwise.

18. 17. The method of any one of claims 1 to 16, wherein the pyrolysis of the poly(3-hydroxyalkanoate) is carried out continuously.

19. 19. The method of any one of claims 1 to 18, wherein the poly(3-hydroxyalkanoate) comprises 3-hydroxypropionate units and at least one of the α,β-unsaturated carboxylic acids produced is acrylic acid.

20. 20. The method according to any one of claims 1 to 19, wherein the poly(3-hydroxyalkanoate) is poly(3-hydroxypropionate) and the α,β-unsaturated carboxylic acid produced is acrylic acid.

21. 19. The method of any one of claims 1 to 18, wherein the poly(3-hydroxyalkanoate) comprises 3-hydroxybutyrate units and at least one of the α,β-unsaturated carboxylic acids produced is crotonic acid.

22. 22. The method according to any one of claims 1 to 18 and 21, wherein the poly(3-hydroxyalkanoate) is poly(3-hydroxybutyrate) and the α,β-unsaturated carboxylic acid produced is crotonic acid.

23. 19. The method of any one of claims 1 to 18, wherein the poly(3-hydroxyalkanoate) comprises 3-hydroxyisobutyrate units and at least one of the α,β-unsaturated carboxylic acids produced is methacrylic acid.

24. 24. The method according to any one of claims 1 to 18 and 23, wherein the poly(3-hydroxyalkanoate) is poly(3-hydroxyisobutyrate) and the α,β-unsaturated carboxylic acid produced is methacrylic acid.

25. 25. A method for producing an α,β-unsaturated carboxylic acid obtained by the method according to any one of claims 1 to 24, characterized in that it comprises a step of condensing the product thus obtained, followed by one or more purification steps selected from distillation, liquid-liquid extraction, separation using a thin-film evaporator, or recrystallization, or a combination of these techniques.

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