Field of the invention

The present invention is situated in the field of the production of benzoic acid, particularly by the photocatalytic conversion or photooxidation of polystyrene.

Background of the invention

Plastics are inexpensive and durable, which makes them very adaptable for different uses. They are typically prepared from fossil fuels. The chemical structure of most of the plastics renders them resistant to many natural processes of degradation and as a result, they are slow to degrade. Taken together, these two factors contribute to large volumes of plastic to enter the environment as mismanaged waste and to persist in the ecosystem. Today, about 300 million tonnes of plastic waste is produced every year. Plastic pollution is the accumulation of plastic objects and particles in the Earth's environment that adversely affects wildlife, their habitat, and humans.

Among different classes of plastics, polystyrene is widely used in food disposable package boxes, foam, toys, and yogurt boxes, etc. The global production capacity of polystyrene amounted to 15.61 million metric tons in 2020. Additionally, polystyrene accounts for 10% of global waste and causes serious global environmental problems owing to the difficulty towards degradation.

Benzoic acid is a versatile chemical feedstock that plays a pivotal role in modern organic synthesis, food and pharmaceuticals industry. In particular, it serves as a precursor to many aromatic and organic compounds, including but not limited to phenols, terephthalic acid, caprolactam and plasticizers, and is a synthetic intermediate in the production of fine chemicals. In addition, benzoic acid is a highly effective organic acid to inhibit harmful microorganisms. Benzoic acid has a wide antibacterial spectrum, not only for killing spores, cocci, but also for killing fungi and yeast. In addition, its sodium, potassium, and calcium salts are widely employed as preservatives in tobacco, cosmetics, foods, and drinks.

Different methods for the industrial production of benzoic acid have been developed, such as the liquid phase aerobic oxidation of toluene with a cobalt naphthenate catalyst under high pressure and temperature, the hydrolysis of benzotrichloride, obtained by the photochlorination of toluene at 100 - 150 °C, with ZnCh as catalyst, or the oxidation of benzyl halide by potassium permanganate under acidic conditions in the presence of a phase transfer catalyst. All these procedures require an aromatic source as starting material, which typically is provided by fossil-fuel feedstocks.

There is thus a need for a selective, sustainable and cheap conversion of plastic waste to high value chemical compounds. There is also a need for methods and systems for the production of high value chemicals with minimal use of fossil-fuel feedstocks.

Summary of the invention

The inventors have developed an improved photocatalytic or photooxidation method and system for the conversion of polystyrene to benzoic acid that addresses the above indicated needs. The present invention provides an elegant and simple strategy to use polystyrene as starting material to generate the highly valued benzoic acid, and thus to recuperate the aromatic group from polystyrene waste materials in the form of benzoic acid. In particular, the generation of benzoic acid from polystyrene as envisaged herein is able to be performed under mild reaction conditions, such as room temperature and atmospheric pressure conditions, is easily scalable, and can be performed with a low power light source. In addition, a cheap metal-free catalytic system has been developed to make the polystyrene conversion greener and more renewable. Furthermore, the thus obtained “green” benzoic acid can be used as the precursor or intermediate compound in the production of many organic or aromatic compounds.

One aspect of the present invention provides a method for generating benzoic acid from polystyrene, comprising combining polystyrene or a polystyrene containing material, a solvent and a metal-free reactive compound and irradiating the reaction mixture with UV light in the presence of oxygen, thereby generating a composition comprising benzoic acid. In particular embodiments the metal-free reactive compound is selected from a succinimide derivative, a bromide salt of an alkali metal or of an alkaline earth metal, CBr4, and an acid. Accordingly in particular embodiments, the present invention provides a method for generating benzoic acid from polystyrene, comprising:

(i) preparing a reaction mixture by combining polystyrene or a polystyrene containing material, a solvent and a metal-free reactive compound, wherein said metal-free reactive compound is (a) a succinimide derivative, (b) a bromide salt of an alkali metal or of an alkaline earth metal, (c) CBr4, or (d) an acid; and

(ii) irradiating the reaction mixture with UV light in the presence of oxygen, thereby generating a composition comprising benzoic acid. In particular embodiments, the metal-free reactive component as envisaged herein is a:

(a) an N-substituted succinimide derivative, preferably selected from the group N-bromosuccinimide (NBS), N-chlorosuccinimide (NCS) and N-iodosuccinimide (NIS); or

(b) a bromide salt of an alkali metal or of an alkaline earth metal selected from the group LiBr, NaBr, MgBr2, CaBr2 and MgBr2.Et2O; or

(c) CBr ₄ .

In particular embodiments, the molar ratio of polystyrene, expressed on the monomer molecular weight, to the metal-free reactive compound, i.e. the ratio of the polystyrene monomer to the metal-free reactive compound, ranges between 20:1 and 1 :1.

In particular embodiments, the reaction mixture as envisaged herein further comprises a sulfonate or sulfinate as a reaction promoting compound, particularly wherein the reaction promoting compound is selected from the group consisting of sodium trifluoromethanesulfonate, sodium trifluoromethanesulfinate, sodium methylsulfinate and sodium benzenesulfinate. More in particular, the molar ratio of metal-free reactive component to the sulfonate or sulfinate reaction promoting compound ranges between 0.05:1 and 5:1.

In particular embodiments, the metal-free reactive compound is one or more of sulfuric acid, hydrochloric acid, hydrobromic acid, chloric acid, bromic acid, perchloric acid, perbromic acid and/or nitric acid. More in particular, the molar ratio of polystyrene, expressed on the monomer molecular weight, to the acid, i.e. the ratio of the polystyrene monomer to the acid, ranges between 1 :0.5 and 1 :20.

In particular embodiments, the solvent is ethyl acetate.

In particular embodiments, in step (ii), the oxygen is provided by a gas stream comprising between 20% (v/v) and 100% (v/v) oxygen, such as by air or oxygen enriched air or by oxygen gas.

In particular embodiments, in step (ii) the reaction mixture is illuminated with near UV light, particularly with a wavelength between 300 and 400 nm. In particular embodiments, step (ii) is performed with an artificial light, such as a LED, Kessil lamp, Xenon lamp, XeHg lamp, or a mercury vapor lamp.

In particular embodiments, step (ii) is performed at a temperature ranging between 10 °C and 60 °C, preferably between 15 °C and 50°C, such as between 20 °C and 40 °C. In particular embodiments, step (ii) is performed at a pressure between 1 bar and 2 bar, preferably at atmospheric pressure. In particular embodiments, the method as envisaged herein further comprises the step (iii) of isolating benzoic acid from the composition comprising benzoic acid obtained in step (ii). In particular embodiments, the method as envisaged herein further comprises the step (iv) of further converting the benzoic acid obtained in step (ii) or, optionally in step (iii), into an organic compound, particularly into an aromatic compound, including but not limited to phenol, benzene, toluene, biphenyl, salicylic acid, or terephthalic acid.

A related aspect of the present invention provides a photooxidation or photocatalytic system for the conversion of polystyrene, in particular for the generation of benzoic acid from polystyrene, comprising (i) a reaction mixture as envisaged herein; and (ii) a light source, a means for illuminating the reaction mixture, or a means for exposing the reaction mixture to a light source. More in particular, the system further comprises (iii) a reaction vessel or container for holding the reaction mixture and/or (iv) a means for providing an oxygen-containing gas to the reaction vessel. The reaction mixture as envisaged herein comprises polystyrene, a solvent and a metal-free reactive compound. In particular embodiments said reactive compound is (a) a succinimide derivative, (b) a bromide salt of an alkali metal or of an alkaline earth metal, (c) CBr4, or (d) an acid. In particular, the reaction mixture comprises (a) a succinimide derivative selected from the group N-bromosuccinimide (NBS), N-chlorosuccinimide (NCS) and N-iodosuccinimide (NIS); (b) a bromide salt of an alkali metal or alkaline earth metal selected from the group LiBr, NaBr, MgBr2, CaBr2 and MgBr2.Et2O; (c) CBr4; or (d) an acid, in particular sulfuric acid, hydrochloric acid, hydrobromic acid, chloric acid, bromic acid, perchloric acid, perbromic acid and/or nitric acid. More in particular, the reaction mixture may further comprise a sulfonate or sulfinate reaction promoting compound as envisaged herein.

In particular embodiments, the light source or the means for illuminating the reaction mixture or the reaction vessel is an artificial light generating near UV light, such as a LED, Xenon lamp, Kessil lamp, XeHg lamp, or a mercury vapor lamp.

The present invention also provides for the use of a metal-free reactive compound, such as a succinimide derivative, a bromide salt of an alkali metal or alkaline earth metal, CBr4, or an acid, in the photooxidation or photocatalytic conversion of polystyrene to benzoic acid. More in particular, the present invention also provides for the use of a metal-free reactive compound selected from (a) a succinimide derivative selected from the group N-bromosuccinimide (NBS), N-chlorosuccinimide (NCS) and N-iodosuccinimide (NIS); (b) a bromide salt of an alkali metal or alkaline earth metal selected from the group LiBr, NaBr, MgBr2, CaBr2 and MgBr2.Et2O; (c) CBr4; or (d) an acid, in particular sulfuric acid, hydrochloric acid, hydrobromic acid, chloric acid, bromic acid, perchloric acid, perbromic acid and/or nitric acid, in the photocatalytic conversion of polystyrene to benzoic acid.

Figure 1 shows the reaction schemes for the conversion of polystyrene to benzoic acid, according to certain embodiments.

Figure 2 shows the reaction schemes for the conversion of polystyrene to (a) benzene; (b) toluene; (c) biphenyl; (d) salicylic acid; (e) estrone benzoate; (f) hippuric acid; (g) hexylcaine (1-(cyclohexylamino)-2-propanol benzoate); and (h) cholesteryl benzoate, according to certain embodiments.

Detailed description of invention

Before the present system and method of the invention are described, it is to be understood that this invention is not limited to particular systems and methods or combinations described, since such systems and methods and combinations may, of course, vary. It is also to be understood that the terminology used herein is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.

As used herein, the singular forms "a", "an", and "the" include both singular and plural referents unless the context clearly dictates otherwise.

The terms "comprising", "comprises" and "comprised of' as used herein are synonymous with "including", "includes" or "containing", "contains", and are inclusive or open- ended and do not exclude additional, non-recited members, elements or method steps. It will be appreciated that the terms "comprising", "comprises" and "comprised of' as used herein comprise the terms "consisting of", "consists" and "consists of'.

The recitation of numerical ranges by endpoints includes all numbers and fractions subsumed within the respective ranges, as well as the recited endpoints.

The term "about" or “approximately” as used herein when referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, is meant to encompass variations of +/-10% or less, preferably +/-5% or less, more preferably +/-1% or less, and still more preferably +/-0.1 % or less of and from the specified value, insofar such variations are appropriate to perform in the disclosed invention. It is to be understood that the value to which the modifier "about" or “approximately” refers is itself also specifically, and preferably, disclosed.

Whereas the terms “one or more” or “at least one”, such as one or more or at least one member(s) of a group of members, is clear per se, by means of further exemplification, the term encompasses inter alia a reference to any one of said members, or to any two or more of said members, such as, e.g., any >3, >4, >5, >6 or >7 etc. of said members, and up to all said members.

All references cited in the present specification are hereby incorporated by reference in their entirety. In particular, the teachings of all references herein specifically referred to are incorporated by reference.

Unless otherwise defined, all terms used in disclosing the invention, including technical and scientific terms, have the meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. By means of further guidance, term definitions are included to better appreciate the teaching of the present invention.

In the following passages, different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments. Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the appended claims, any of the claimed embodiments can be used in any combination.

In the present description of the invention, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration only of specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilised and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.

In the present application, the percentages are given by weight, unless otherwise stated. The inventors have developed new methods and systems for the highly selective generation of benzoic acid from polystyrene by photocatalytic conversion or photooxidation, which involves the use of a metal-free reactive compound, which is contacted with polystyrene dissolved in a suitable solvent to obtain a reaction mixture, which, is then irradiated with UV light.

Polystyrene is a synthetic aromatic hydrocarbon polymer made up of styrene monomers, with (C6H5-CH-CH2) as repeating unit. Polystyrene has the chemical formula (CsH8)n, with n being the degree of polymerization, i.e. the number of monomers or repeating units in the polystyrene polymer. A polystyrene polymer may comprise up to multiple thousand monomers or repeating units, giving a molecular weight typically in the range of 100,000- 400,000 g/mol. The determination of the molecular weight or molecular weight range of polystyrene is known in the art and may be performed by size exclusion chromatography or by viscometry, as known in the art. Dividing the polystyrene molecular weight by 104, i.e. the molecular weight of the monomer/repeating unit, gives the degree of polymerization. Dividing the polystyrene mass by 104, i.e. the molecular weight of the monomer/repeating unit, gives the mole amount of polystyrene monomers/repeating units.

The methods and systems according to the present invention, as further detailed below, involve the use of a metal-free reactive compound.

In particular embodiments, the metal-free reactive compound as envisaged herein, also referred to herein as the metal-free catalyst or photocatalyst, is (a) a succinimide derivative, (b) a bromide salt of an alkali metal or of an alkaline earth metal, (c) CBr4, or (d) an acid.

When the metal-free reactive compound as envisaged herein is a succinimide derivative, the succinimide derivative is in particular an N-substituted succinimide. Preferably, the N-substituted succinimide comprises a halogen as substituent. The term “halogen” as used herein corresponds to an element of group 17 of the periodic table, according to current IIIPAC nomenclature. More preferably, the N-substituted succinimide is selected from the group N-bromosuccinimide (NBS), N-chlorosuccinimide (NCS) and N-iodosuccinimide (NIS). Most preferably, the N-substituted succinimide is NBS or NCS.

When the reactive compound as envisaged herein is a bromide salt of an alkali metal or of an alkaline earth metal, said bromide salt of an alkali metal or of an alkaline earth metal is preferably selected from the group consisting of LiBr, NaBr, MgBr2, CaBr2 and MgBr2.Et2O. When the reactive compound as envisaged herein is an acid, the acid is preferably selected from the group consisting of sulfuric acid, hydrochloric acid, hydrobromic acid, chloric acid, bromic acid, perchloric acid, perbromic acid and nitric acid.

An aspect of the present invention provides a method for the conversion or depolymerization of polystyrene. In particular embodiments, the method is used for the generation of benzoic acid from polystyrene. Optionally, the method further involves the generation of organic compounds.

In particular embodiments, the method comprises preparing a reaction mixture by combining polystyrene, a solvent and a metal-free reactive compound and irradiating the reaction mixture with UV light in the presence of oxygen. In particular embodiments, the method comprises:

(i) preparing a reaction mixture by combining polystyrene, a solvent and a metal-free reactive compound as envisaged herein, wherein said metal-free reactive compound is (a) a succinimide derivative, (b) a bromide salt of an alkali metal or of an alkaline earth metal, (c) CBr4, or (d) an acid; and

(ii) irradiating the reaction mixture with UV light in the presence of oxygen. In particular embodiments, the method generates a composition comprising benzoic acid. In particular embodiments, the method further comprises converting the benzoic acid obtained in step (ii) into an organic compound.

It is understood that, in the context of the present invention, any source of polystyrene is suitable. It is also understood that the degree of polymerization of the polystyrene polymer or the random or non-random distribution of the phenyl groups on both sides of the polystyrene polymer is not limiting.

In preferred embodiments, the polystyrene used in the present invention originates from and/or is isolated from plastic material, such as plastic waste, in particular polystyrene waste material. Depending on the nature and the size of the plastic material, the generation of small-sized particles for use in the reaction may be of interest, such as in pieces smaller than 5cm ³ , 2cm ³ , 1cm ³ , 0.5 cm ³ or less. Advantageously, the present invention thus allows to recover the valuable aromatic functional group from polystyrene (waste) material via benzoic acid, a valuable and versatile chemical. In certain embodiments, the present invention thus provides a method for the conversion or depolymerization of polystyrene containing plastics, in particular for the generation of benzoic acid from the polystyrene containing plastics, comprising: (i) preparing a reaction mixture by combining polystyrene containing plastic, a solvent and a metal-free reactive compound as envisaged herein and (ii) irradiating the reaction mixture with UV light in the presence of oxygen, thereby generating or obtaining a composition comprising benzoic acid. In particular embodiments, said metal-free reactive compound is (a) a succinimide derivative, (b) a bromide salt of an alkali metal or of an alkaline earth metal, (c) CBr4, or (d) an acid;. Polystyrene containing plastics or plastic material, in particular polystyrene containing plastic waste, is well known and includes but is not limited to plastic cups and food packages, foams, such as styrofoam, toys, transparent CD cases, and the like. The polystyrene containing plastic material may be shredded or otherwise reduced in size prior to step (i).

In particular embodiments, the polystyrene monomer concentration in the reaction mixture and/or in the solvent, i.e. the polystyrene concentration in the reaction mixture and/or in the solvent expressed based on the mole amount of monomer/repeating unit, ranges between 0.01 M and 2.0M, preferably between 0.05M and 1.0M, such as between 0.05M and 0.5M. Lower concentrations would result in a dilute reaction mixture, which decreases the efficiency of the reaction, while higher concentrations may give rise to a too viscous reaction mixture.

In particular embodiments, the solvent is ethyl acetate.

As detailed above, the reaction mixture as envisaged herein may comprise as a metal- free reactive compound or as a photocatalyst:

(a) a succinimide derivative, preferably a N-substituted succinimide, preferably selected from the group N-bromosuccinimide (NBS), N-chlorosuccinimide (NCS) and N-iodosuccinimide (NIS); or

(b) a bromide salt of an alkali metal or of an alkaline earth metal particularly selected from the group LiBr, NaBr, MgBr2, CaBr2 and MgBr2.Et2O; or

(c) CBr ₄ .

In particular embodiments, the molar ratio of polystyrene monomers/repeating units to the photocatalyst or metal-free reactive compound in the reaction mixture, or stated differently the molar ratio of polystyrene expressed on monomer molecular weight, ranges between 20:1 and 1 :1 , preferably between 10:1 and 1 :1 , such as 9:1 , 8:1 , 7:1 , 6:1 , 5:1 , 4:1 , 3:1 or 2:1.

In particular embodiments, the reaction mixture as envisaged herein may further comprise a sulfonate or sulfinate as a reaction promoting compound. Accordingly, in particular embodiments, step (i) of the method as envisaged herein may further comprise adding a sulfonate or sulfinate as a reaction promoting compound. Advantageously, the combination of a photocatalyst or metal-free reactive compound as detailed herein with a sulfonate or sulfinate reaction promoting compound increases the reaction yield, allowing to obtain higher amounts of benzoic acid. Preferably, the sulfonate or sulfinate reaction promoting compound is selected from the group consisting of sodium trifluoromethanesulfonate, sodium trifluoromethanesulfinate, sodium methylsulfinate and sodium benzenesulfinate.

In particular embodiments, the molar ratio of photocatalyst or metal-free reactive component to the sulfonate or sulfinate reaction promoting compound in the reaction mixture ranges between 0.05:1 and 5:1 , preferably between 0.1 :1 and 2:1 , such as between 0.2:1 and 1 :1.

In particular embodiments, the method for the conversion of polystyrene, particularly for the generation of benzoic acid from polystyrene comprises:

(i) preparing a reaction mixture by combining polystyrene, ethyl acetate and a metal-free reactive compound, and, optionally, a sulfonate or sulfinate reaction promoting compound, wherein said metal-free reactive compound is preferably selected from (a) a succinimide derivative selected from the group consisting of NBS and NCS, (b) a bromide salt of an alkali metal or of an alkaline earth metal, selected from the group consisting of LiBr, NaBr, MgBr2, CaBr2 and MgBr2.Et2O, or (c) CBr4, and wherein said sulfonate or sulfinate reaction promoting compound is selected from the group consisting of sodium trifluoromethanesulfonate, sodium trifluoromethanesulfinate, sodium methylsulfinate and sodium benzenesulfinate, and

(ii) irradiating the reaction mixture with UV light in the presence of oxygen, thereby generating or obtaining a composition comprising benzoic acid.

More in particular, the polystyrene concentration expressed based on the mole amount of monomer/repeating unit, ranges between 0.05M and 1.0M; and/or the molar ratio of polystyrene monomers/repeating units to the reactive component in the reaction mixture ranges between 10:1 and 1 :1 ; and/or the molar ratio of the reactive component to the sulfonate or sulfinate reaction promoting compound, if present, ranges between 0.2:1 and 1 :1.

As detailed above, alternatively, the reaction mixture as envisaged herein may comprise an acid, in particular a strong acid. In particular embodiments, the acid is selected from the group consisting of sulfuric acid, hydrochloric acid, hydrobromic acid, chloric acid, bromic acid, perchloric acid, perbromic acid and nitric acid.

In particular embodiments, the molar ratio of the polystyrene monomer or repeating unit to the acid ranges between 1 :0.5 and 1 :20, preferably between 1 :1 and 1 :10, such as between 1 :1 and 1 :5.

Accordingly, in particular embodiments, the method for the conversion of polystyrene, particularly for the generation of benzoic acid from polystyrene comprises: (i) preparing a reaction mixture by combining polystyrene or a polystyrene containing plastic material, ethyl acetate and a metal-free reactive compound, wherein said metal-free reactive compound is an acid, selected from the group consisting of sulfuric acid, hydrochloric acid, hydrobromic acid, chloric acid, bromic acid, perchloric acid, perbromic acid and nitric acid; and

(ii) irradiating the reaction mixture with UV light in the presence of oxygen, thereby obtaining or generating a composition comprising benzoic acid.

More in particular, the polystyrene concentration, expressed based on the mole amount of monomer/repeating unit, ranges between 0.05M and 1.0M; and/or the molar ratio of polystyrene monomers/repeating units to the acid in the reaction mixture ranges between 1 :1 and 1 :5.

In particular embodiments, in step (ii) of the method envisaged herein, oxygen is provided to the reaction mixture by a gas stream comprising between 20% (v/v) and 100% oxygen, such as by air or oxygen enriched air or by oxygen gas.

In particular embodiments, in step (ii) of the method envisaged herein, the reaction mixture as envisaged herein is illuminated with near UV light. Preferably, the near UV light has a wavelength between 300 and 400 nm. The present invention is not particularly limited to a specific source of UV light. Any light source may be used so long as at least part of the incident light is UV light, particularly near UV light. In certain embodiments, step (ii) is performed with an artificial light, such as a LED, Xenon lamp, XeHg lamp, or a mercury vapor lamp. A LED light source may be particularly preferred, at least in part due to their very long lifespan and high efficiency. The period for exposure to light can vary widely, there being no upper limit on the time of exposure from an operational point of view.

In particular embodiments, step (ii) is performed at a temperature ranging between 10 °C and 60 °C, preferably between 15 °C and 50°C, such as between 20 °C and 40 °C; and/or at a pressure between 1 bar and 2 bar, preferably at atmospheric pressure. It is a particular advantage of the present invention that the conversion of polystyrene as envisaged herein can be performed under mild reaction conditions.

In particular embodiments, the method as envisaged herein further comprises the step

(iii) of isolating and/or purifying benzoic acid from the composition comprising benzoic acid obtained in step (ii). Benzoic acid may be isolated and/or purified by methods known in the art, such as by distillation or crystallization. In certain embodiments, the benzoic acid may be isolated and/or purified from the composition comprising benzoic acid by an isolation method comprising a liquid-liquid extraction step, wherein the benzoic acid is obtained in a volatile organic solvent, such as diethyl ether, which, upon evaporation of the solvent, yields purified benzoic acid. Advantageously, the latter purification method does not require the high energy consumption of purification by distillation, and does not generate a lot of wastewater as in the case of crystallization.

In particular embodiments, the method as envisaged herein further comprises the step (iv) of converting the benzoic acid obtained in step (ii) or, optionally in step (iii), into an organic compound, particularly into an aromatic compound, such as phenol, benzene, toluene, biphenyl, salicylic acid, or terephthalic acid. Stated differently, the present invention further provides a method for the production of an organic compound, particularly an aromatic compound, from polystyrene, the method comprising the steps (i) and (ii), and optionally (iii) of the method for generating benzoic acid from polystyrene as envisaged herein, and further comprising the step of converting the benzoic acid obtained in step (ii) into an organic compound, particularly into an aromatic compound, such as phenol, benzene, toluene, biphenyl, salicylic acid, or terephthalic acid.

Methods to convert benzoic acid into an organic compound, particularly into an aromatic compound are generally known in the art. Synthesized benzoic acid can be converted into benzene, toluene, salicylic acid, and biphenyl. Additionally, benzoic acid can be converted into benzene containing drugs such as hippuric acid, hexylcaine, cholesteryl benzoate, estrone benzoate. For instance, benzoic acid may be converted to phenol using e.g. copper benzoate as catalyst. Decarboxylation of benzoic acid, as known in the art, yields benzene. Salicylic acid can be synthesized via further hydroxylation and biphenyl can be synthesized via cross coupling reaction. All the drug/bioactive molecules can be synthesized by adding benzoic acid directly with the corresponding molecules.

A further related aspect of the present invention provides a photooxidation or photocatalytic system for the conversion of polystyrene, particularly for the generation of benzoic acid from polystyrene comprising

(i) a reaction mixture comprising polystyrene, a solvent and a metal-free reactive compound as envisaged herein, wherein said reactive compound is (a) a succinimide derivative, (b) a bromide salt of an alkali metal or of an alkaline earth metal, (c) CBr4, or (d) an acid; and one or more of

(ii) a light source, a means for illuminating the reaction mixture, or a means for exposing the reaction mixture to a light source;

(iii) a reaction vessel or container for holding the reaction mixture; and

(iv) a means for providing an oxygen-containing gas to the reaction vessel.

As discussed elsewhere herein, in particular embodiments, the reaction mixture comprises (a) a N-substituted succinimide, preferably selected from the group N-bromosuccinimide (NBS), N-chlorosuccinimide (NCS) and N-iodosuccinimide (NIS); (b) a bromide salt of an alkali metal or alkaline earth metal, preferably selected from the group LiBr, NaBr, MgBr2, CaBr2 and MgBr2.Et2O; (c) CBr4; or (d) an acid, preferably selected from sulfuric acid, hydrochloric acid, hydrobromic acid, chloric acid, bromic acid, perchloric acid, perbromic acid and nitric acid. As discussed elsewhere herein, in particular embodiments, the reaction mixture further comprises a sulfinate or sulfonate reaction promoting compound, preferably selected from the group consisting of sodium trifluoromethanesulfonate, sodium trifluoromethanesulfinate, sodium methylsulfinate and sodium benzenesulfinate.

In particular embodiments, the photooxidation or photocatalytic system comprises at least elements (i) and (ii) as recited above. In further embodiments, the photooxidation or photocatalytic system comprises at least elements (i), (ii) and (iii) as recited above. In particular embodiments the photooxidation or photocatalytic system comprises each of (i), (ii), (iii) and (iv) recited above. Further combinations of elements of (i) with (ii), (iii) and (iv) are also envisaged.

The reaction vessel may be any vessel defining an enclosed volume wherein the photooxidation reaction as considered herein takes place. It may be equipped with heating and/or cooling means, and/or other standard chemical process equipment, such as mixing means, temperature and pressure sensors, valves for controlling the input and output flows, and the like. In particular embodiments, the reaction vessel can include a transparent portion or an opaque portion or a combination thereof, for instance, for allowing the UV light or the artificial light source to illuminate the reaction mixture.

Any light source may be used so long as at least part of the incident light is UV light, particularly near UV light. The light source or the means for illuminating the reaction vessel may be an artificial light generating near UV light, such as a LED, Xenon lamp, Kessil lamp, XeHg lamp, or a mercury vapor lamp. A LED light source may be particularly preferred, at least in part due to their very long lifespan and high efficiency.

In particular embodiments, the photooxidation system comprises suitable conduits as the means for providing an oxygen-containing gas to the reaction vessel.

Another aspect of the present invention relates to the use of a metal-free reactive compound, more particularly selected from (a) a succinimide derivative, preferably selected from the group N-bromosuccinimide (NBS), N-chlorosuccinimide (NCS) and N-iodosuccinimide (NIS); (b) a bromide salt of an alkali metal or alkaline earth metal, preferably selected from the group LiBr, NaBr, MgBr2, CaBr2 and MgBr2.Et2O; (c) CBr4, or (d) an acid, preferably selected from sulfuric acid, hydrochloric acid, hydrobromic acid, chloric acid, bromic acid, perchloric acid, perbromic acid and/or nitric acid, as a photocatalyst or active compound in the photocatalytic conversion or photooxidation of polystyrene to benzoic acid.

Examples

Example 1 - NBS mediated conversion of polystyrene

Commercially available polystyrene was used as starting material to explore the reaction conditions under the irradiation of 390 nm light. Since ethyl acetate can dissolve PS well and can be easily removed, ethyl acetate was used as the reaction solvent. In the present example, N-bromosuccinimide (NBS) was used as the catalyst. In addition, CFsSC^Na (sodium trifluoromethanesulfinate) was added as well to promote the PS conversion. The reaction scheme is shown in Figure 1 , path a.

First, PS (20.8 mg, corresponding to 0.2 mmol when based on the monomer MW) was dissolved in ethyl acetate (2.0 mL) and ultrasonicated for about 5 minutes to get a homogeneous solution (0.1 M polystyrene solution). Next, NBS (0.04 mmol, 7.1 mg), which serves as the catalyst, and CFsSC^Na (0.1 mmol, 15.6 mg) as an additive to promote the reaction, were added to the PS solution, thus obtaining the reaction mixture. In the reaction mixture, the molar ratio between PS: NBS: CFsSC^Na is 1 : 0.2: 0.5. An O2 balloon was equipped to provide an oxygen atmosphere over the reaction mixture.

The Schlenk flask filled with the reaction mixture was irradiated under a Kessil-lamp (Kessil 160 L PR, A = 390 nm) for 16 h, at room temperature. This way, BA was obtained with a 72% yield.

More in particular, in the labscale photooxidation setup, the UV light with total power of 80 W was fixed on an iron stand. The Schlenk flask was kept on a blender and was irradiated with two 40 W blue UV lamps which were cooled by a fan (2 cm away from the Schlenk flask). The oxygen balloon was used to maintain the O2 atmosphere throughout the reaction.

Example 2 - Sulfuric acid mediated conversion of polystyrene

Commercially available polystyrene was used as starting material to explore the reaction conditions under the irradiation of 390 nm light. Since ethyl acetate can dissolve PS well and can be easily removed, ethyl acetate was used as the reaction solvent. By adding two equivalents of H2SO4 (to one equivalent of PS, based on monomer molecular weight) a good conversion to BA was also achieved, although a mixture of BA and the corresponding ethyl ester product was obtained. The reaction scheme is shown in Figure 1 , path b. First, PS (0.2 mmol, 20.8 mg) was dissolved in ethyl acetate (1.0 mL) and ultrasonicated for about 5 minutes to get a homogeneous solution. Next, H2SO4 (0.4 mmol, 0.01 mL) was added to maintain an acidic environment. The general setup was further as in example 1. An O2 balloon was equipped to provide an oxygen atmosphere over the reaction mixture. The Schlenk tube flask with the reaction mixture was irradiated under a Kessil-lamp (Kessil 160 L PR, A = 390 nm). In addition, a fan was installed to keep the reaction at room temperature (25 °C) throughout the reaction. After 48h of reaction, the combined yield of BA and its ethyl ester was 77%, with a 4:1 ratio of BA : BA ester.

Example 3 - metal-free catalytic conversion of polystyrene

Using a setup similar as in example 1 and 2 above, but without the addition of CFsSC^Na or another reaction promoting compound in the reaction mixture, different compounds were evaluated as catalyst in the photocatalytic conversion of polystyrene to benzoic acid. The results are shown in Table 1 below.

Example 4 - Purification of benzoic acid

The benzoic acid obtained in example 1 and 2 could easily and efficiently be purified.

After the reaction was finished, the pH value of the reaction mixture was adjusted to 12.0 with 1 M NaOH. The resulting solution was washed three times with 20 mL dichloromethane. The aqueous phases were collected and the pH value was adjusted to 2.0 with 1 M HCI. Then five times aqueous work up was done with 50 mL of diethyl ether and the combined organic phase was dried over anhydrous magnesium sulphate and evaporated to get the pure benzoic acid.

This purification method has considerable advantages over prior art BA purification methods, such as distillation, which requires high energy consumption, and crystallization. In the case of crystallization, such as to obtain pharmaceutical grade BA, it will produce a lot of wastewater. In addition, as the solubility of BA in boiling water is only about 10% (mass ratio), the raw material consumption is huge, and the cost is also high.

Example 5 - Conversion of polystyrene to benzoic acid and further conversion of the benzoic acid to aromatic compounds

The benzoic acid generated by certain embodiments of the present invention was further converted to different aromatic compounds, according to the reaction schemes shown in Figure 2a-2h.

Conversion to benzene

A dry 15 mL Schlenk tube containing a stirring bar was charged with polystyrene (35.6 mg or 0.35 mmol; 1.0 equiv.) and ethyl acetate (EA) (2.0 mL). The mixture was sonicated for about 5 min to make a homogeneous solution. After this, NBS (15 mol %), CFsSC^Na (50 mol %) and ethyl acetate (1 .5 mL) were added to the tube, and sonicated for another 30 seconds. An oxygen balloon was equipped to keep the oxygen atmosphere and the whole reaction tube was irradiated for 16 h under Kessil lamp (390 nm). After the reaction was completed, the solution was concentrated under reduced pressure (keeping the temperature of the water bath under 30 °C) and monitored by 1 H NMR (iodoform as an internal standard) and LC-MS (1- tetralone as an internal standard). The benzoic acid yield was 73% and the mixture was further dried under low pressure.

Subsequently, a 25 mL Schlenk tube equipped with a stir bar was charged with a mixture comprising the synthesized benzoic acid (0.25 mmol), K2S2O8 (5.0 equiv.) and AgNCh (5 mol%) in a glovebox. Next, MeCN (3.0 mL) was added by syringe and the tube was sealed by a teflon screwcap and placed in an oil bath at 120 °C. The reaction was performed for 24h under continuous stirring. After cooling down, the solvent was removed in vacuo and the residue was purified by chromatography on silica gel to provide the corresponding product. The total yield of compound 3a (benzene) is 21 %. The reaction scheme is shown in Figure 2a.

Conversion to toluene

A dry 15 mL Schlenk tube containing a stirring bar was charged with polystyrene (20.8 mg or 0.2 mmol; 1.0 equiv.) and ethyl acetate (EA) (1.0 mL). The mixture was sonicated for about 5 min to make a homogeneous solution. After this, NBS (15 mol %), CFsSC^Na (50 mol %) and ethyl acetate (1 .0 mL) were added to the tube, and sonicated for another 30 seconds. An oxygen balloon was equipped to keep the oxygen atmosphere and the whole reaction tube was irradiated for 16 h under Kessil lamp (390 nm). After the reaction was completed, the solution was concentrated under reduced pressure (keeping the temperature of the water bath under 30 °C) and monitored by 1 H NMR (iodoform as an internal standard) and LC-MS (1- tetralone as an internal standard). The benzoic acid yield was 73% and the mixture was further dried under low pressure.

Subsequently, benzoic acid mixture (0.15 mmol) was weighed into a three-neck flask which was then closed with a septum and purged with argon. The catalyst B(C6Fs)3 (5 mol%) was introduced via syringe as a freshly prepared solution in 5 mL of anhydrous dichloromethane. Stirring was continued for 30 min and then 2 equiv. of n-butylsilane (n-BS) was added into the reaction solution. The reaction mixture was stirred for about 24 h and after complete conversion of the starting material, the reaction mixture was quenched with 0.1 mL of triethylamine and solvent was removed in vacuo. The crude mixture was purified over silica gel. The total yield of compound 3b (toluene) is 43 %. The reaction scheme is shown in Figure 2b.

Conversion to biphenyl

A dry 15 mL Schlenk tube containing a stirring bar was charged with polystyrene (35.6 mg or 0.35 mmol; 1.0 equiv.) and ethyl acetate (EA) (2.0 mL). The mixture was sonicated for about 5 min to make a homogeneous solution. After this, NBS (15 mol %), CFsSC^Na (50 mol %) and ethyl acetate (1 .0 mL) were added to the tube, and sonicated for another 30 seconds. An oxygen balloon was equipped to keep the oxygen atmosphere and the whole reaction tube was irradiated for 16 h under Kessil lamp (390 nm). After the reaction was completed, the solution was concentrated under reduced pressure (keeping the temperature of the water bath under 30 °C) and monitored by 1 H NMR (iodoform as an internal standard) and LC-MS (1- tetralone as an internal standard). The benzoic acid yield was 73% and the mixture was further dried under low pressure.

Subsequently, a 25 mL Schlenk tube equipped with a stir bar was charged with a mixture comprising the synthesized benzoic acid (0.25 mmol), K2S2O8 (3.0 equiv.) and AgNCh (5 mol%) in a glovebox. Next, benzene (0.5 mL) and MeCN (0.5 mL) was added by syringe and the tube was sealed by a teflon screwcap and placed in an oil bath at 120 °C. The reaction was performed for48h under continuous stirring. After cooling down, the solvent was removed in vacuo and the crude residue was purified by chromatography on silica gel to provide the corresponding product. The total yield of compound 3c (biphenyl) is 30 %. The reaction scheme is shown in Figure 2c.

Conversion to salicylic acid A dry 15 m L Schlenk tube containing a stirring bar was charged with polystyrene (71.2 mg, 0.68 mmol; 1.0 equiv.) and EA (5.0 mL). The mixture was sonicated for about 5 min to make a homogeneous solution. Next, NBS (15 mol %), CFsSC^Na (50 mol %) and EA (1.8 mL) were added to the tube, and sonicated for another 30 seconds. An oxygen balloon was equipped to keep the oxygen atmosphere and the whole reaction tube was irradiated for 16 h under Kessil lamp (390 nm). After the reaction was completed, the solution was concentrated under reduced pressure (keeping the temperature of the water bath under 30 °C) and monitored by 1 H NMR (iodoform as an internal standard) and LC-MS (1-tetralone as an internal standard). The yield was 73 % and the mixture was further dried under low pressure.

Subsequently, a 25 mL Schlenk tube equipped with a magnetic stir bar was charged with Pd(OAc)2 (10 mol%) followed by the benzoic acid mixture (0.5 mmol), benzoquinone (1.0 equiv), KOAc (2.0 equiv) and N,N-dimethylacetamide (1.5 mL). The reaction tube was evacuated and back-filled with O2. After the reaction mixture was stirred at 115 °C for 15 h, it was allowed to cool to room temperature. The reaction mixture was diluted with ethyl acetate and water and then filtered through a small pad of Celite. The organic phase was dried and concentrated under low pressure. The residue was purified by silica gel flash column chromatography to give the corresponding product. The total yield of compound 3d (salicylic acid) is 57 %. The reaction scheme is shown in Figure 2d.

Conversion to estrone benzoate

A dry 25 mL three neck round bottom flask containing a stirring bar was charged with polystyrene (104 mg or 1 mmol; 1.0 equiv.) and ethyl acetate (EA) (10 mL). The mixture was sonicated for about 5 min to make a homogeneous solution. After this, NBS (26.7 mg, 0.15 mmM; 15 mol %) and CFsSC^Na (7.8 mg, 0.5 mmol; 50 mol %) were added to the flask, and sonicated for another 30 seconds. Two oxygen balloons were equipped to keep the oxygen atmosphere. The flask was kept in a water bath at room temperature and irradiated with Kessil lamps (390 nm), which were cooled by a fan (set up 2-3 cm away from the flask). The yield was determined by 1 H NMR (with iodoform as an internal standard) after 60 h. The yield was 68% and the mixture was further dried under low pressure.

Subsequently, benzoic acid mixture (1.0 equiv.), DCM (0.5 M), oxalyl chloride (1.2 equiv.) and one drop of DMF were added into the flask. The reaction mixture was stirred at room temperature for 3h. After the reaction to benzoyl chloride was completed, the mixture was evaporated under reduced pressure.

Then, to a solution of estrone (0.5 mmol) in pyridine (5 mL) under an ice bath, benzoyl chloride (0.6 mmol) formed from the benzoic acid mixture was added to the solution. The solution was stirred at room temperature for 4h. After the reaction was finished, the mixture was extracted with dichloromethane (10 mL) and washed with a solution of diluted HCI (10 mL). The organic phase was then washed for three times with water, dried over MgSC , filtrated, and evaporated under vacuum. Finally, the solid residue was washed with 10 mL of cold methanol and dried under low pressure. The crude mixture was purified over silica gel. The total yield of compound 3e (estrone benzoate) was 60 %. The reaction scheme is shown in Figure 2e.

Conversion to hippuric acid

The benzoic acid was prepared as in the previous sections of example 5.

The benzoic acid mixture (1 .0 equiv), DCM (0.5 M), oxalyl chloride (1.2 equiv) and one drop of DMF were added to a flask. The reaction mixture was stirred at room temperature for 3 h. After the reaction to benzoyl chloride was completed, the mixture was evaporated under reduced pressure. Next, in a round bottomed flask, glycine (1.0 equiv) and NaOH (2.0 equiv) were dissolved in H2O (1 .0 M). Next, the mixture was cooled to 0 °C and the benzoyl chloride was added dropwise. The reaction was stirred at room temperature for 2 h. Then the reaction was washed with EA twice, separated, and the pH of the aqueous phase was adjusted to 2~3 with 3.0 M HCI. The precipitation was filtered and dried to obtain 3f. The crude mixture was purified over silica gel. The total yield of compound 3f (hippuric acid) is 42 %. The reaction scheme is shown in Figure 2f.

Conversion to 1-(cyclohexylamino)-2-propanol benzoate

The benzoic acid was prepared and converted to benzoyl chloride as in the previous sections of examples.

As shown in Figure 2g, a 1 M solution of 2-propanolamine (1) in absolute ethanol was stirred for 15 min at room temperature with 1.5 equiv. of cyclohexanone. The solution was treated with sodium borohydride at 0 °C for 5 min, then the mixture was quenched with water, diluted with methylene chloride, and filtered, and the solvents were evaporated under reduced pressure. The residue was dissolved in HCI and washed with methylene chloride. The aqueous portion was made basic with 5% NaOH, extracted with methylene chloride, dried, and evaporated to compound (2) (1-(cyclohexylamino)-2 propanol). This product can be used for next step without further purification.

A 25 mL flask was charged with compound (2), benzoyl chloride (1.2 equiv) formed from the benzoic acid mixture, Et3N (1.2 equiv) and DCM (5 mL). The solution was stirred at 25 °C and the reaction continued until the thin layer chromatography (TLC) monitoring showed total consumption of (2). Then, the reaction mixture was quenched with water and extracted with dichloromethane. The organic phase was then dried over MgSO4, filtrated, and evaporated under vacuum. The product was purified by column chromatography on silica gel to obtain the pure products. The total yield of compound 3g was 55 %. The reaction scheme is shown in Figure 2g. Conversion to cholesteryl benzoate

A benzoic acid mixture (2a) was prepared as in the previous sections of this example 5.

A 25 mL Schlenk tube equipped with a magnetic stirring bar was charged with cholesterol (1.0 equiv) followed by the benzoic acid mixture (1.1 equiv, 0.68 mmol), and PPh3 (1.1 equiv) in THF under N2 atmosphere at 0 °C. The resulting solution was treated with diethylazodicarboxylate (1.1 equiv.) and the reaction was continued overnight at room temperature under continuous stirring. The solvent was evaporated and the residue was dissolved in ether, then the filtrate was evaporated under reduced pressure. The residue was purified by silica gel flash column chromatography to achieve the corresponding product. The total yield of compound 3h was 47 %. The reaction scheme is shown in Figure 2h.