process

Abstract

Disclosed herein is a method for upcycling plastic waste comprising (i) contacting a plastic from said plastic waste with an enzyme to degrade said plastic to provide a degraded composition comprising monomers and / or oligomers, and / or derivatives thereof, of said plastic; and (ii) conducting electrolysis on said degraded composition to produce a mixture of products comprising hydrogen gas, and preferably oxidation products of said monomers and / or oligomers, and / or derivatives thereof, of said plastic and / or said monomers and / or oligomers, and / or derivatives thereof, of said plastic. A system for upcycling plastic waste is also disclosed.

Description

[0001] Process

[0002] INTRODUCTION

[0003] The invention relates to the field of upcycling plastic waste to H2 and chemicals. Specifically, the invention relates to a method of upcycling plastic waste using enzymatic degradation and electrolysis. A system for upcycling plastic waste is also described.

[0004] BACKGROUND

[0005] Waste plastics are becoming a growing concern as environmental pollutants. Millions of tons of synthetic plastics are generated annually, and a mere 12 %wt is recycled worldwide. This places significant pressure on landfill sites and represents a huge loss in chemical resources.

[0006] One method that is being explored to process waste plastic is to degrade the polymers using strongly acid or alkaline conditions and then to conduct electrolysis on these degraded materials to generate useful products, such as hydrogen. However, these methods typically employ harsh pH conditions and the cost and environmental penalties from using extreme pH to breakdown waste plastics and carry out electrolysis present a major barrier to the commercialization of this technology.

[0007] Accordingly, there is a need to develop an alternative route for processing waste plastics to generate useful chemical products.

[0008] SUMMARY OF INVENTION

[0009] In general, the present invention proposes a new methodology for upcycling plastic waste by using a combination of enzyme treatment and electrolysis. Specifically, it has been found to be possible to degrade plastic waste with an enzyme and then conduct electrolysis on the degraded composition to produce a mixture of useful products. Advantageously, the method of the invention can be carried out in mild conditions (i.e. mild temperature and pH, and at atmospheric pressure) compared to prior art methods. The invention harnesses technology from two disparate fields of research that typically work entirely independently to realise the beneficial new method.

[0010] Accordingly, in an aspect of the invention there is provided a method for upcycling plastic waste comprising:

[0011] (i) contacting a plastic from said plastic waste with an enzyme to degrade said plastic to provide a degraded composition comprising monomers and / or oligomers, and / or derivatives thereof, of said plastic; and (ii) conducting electrolysis on said degraded composition to produce a mixture of products comprising hydrogen gas, and preferably oxidation products of said monomers and / or oligomers, and / or derivatives thereof, of said plastic and / or said monomers and / or oligomers, and / or derivatives thereof, of said plastic.

[0012] Advantageously, the method allows for the generation of valuable products, including hydrogen, directly from waste plastic. This may include monomers and / or oligomers, of said plastic as well as their oxidation products.

[0013] Advantageously, the method is capable of operating at relatively low temperatures, atmospheric pressure and mild pH conditions, avoiding the necessity for harsh alkaline or acidic conditions employed in prior art methods for plastic degradation as well as industrial electrolysers. This may overcome the drawbacks associated with scaling-up prior art methods for commercial use. It may also provide benefits to the operational safety and operational efficiency of upcycling the plastic waste.

[0014] Advantageously, the use of enzymes also allows high selectivity. For example, enzymes may be tuned to the degradation of particular plastics or may be used to degrade plastics within mixtures that contain other components. As a result, mixed feedstocks can be used without the need for separation steps.

[0015] Advantageously, the enzymes can also be used at lower concentrations to degrade plastics at much lower cost than methods known in the prior art.

[0016] Advantageously, it has also been shown possible to conduct the method of the invention without treatment or modification of the degraded composition for electrolysis. This may increase the operational efficiency compared to prior art methods.

[0017] It is particularly surprising that these biological and electrochemical reactions are compatible and can be combined in the way described. As mentioned above, the invention combines technology from two highly distinct technical fields (microbiology and electrochemistry) to provide a method that is capable, in mild temperature and pH conditions, of generating useful products from a waste material (one which is typically hard to process), in a safe and cost effective manner.

[0018] In another aspect of the invention there is provided a method for upcycling plastic waste comprising: conducting electrolysis on a degraded composition obtained from the degradation of a plastic from plastic waste, said degraded composition comprising monomers and / or oligomers, and / or derivatives thereof, of said plastic; wherein said degraded composition further comprises a buffer solution and has a pH between 5 and 11 ; and wherein said electrolysis produces a mixture of products comprising hydrogen gas and preferably oxidation products of said monomers and / or oligomers, and / or derivatives thereof, of said plastic and / or said monomers and / or oligomers, and / or derivatives thereof, of said plastic.

[0019] In another aspect of the invention there is provided a method for generating hydrogen comprising: conducting electrolysis on an aqueous composition comprising 6- hydroxyhexanoic acid, ethylene glycol, methyenedianiline, bis (2-hydroxyethyl) terephthalate (BHET), mono (2-hydroxyethyl) terephthalate (MHET), 4-hydroxybutanoic acid, terephthalic acid, 1 ,4-butanediol, naphthalene-2,6-dicarboxylic acid, 1 ,3- propanediol, lactic acid, 2-hydroxybutanoic acid, 3-hydroxylbutanoic acid, glycolic acid, formate, adipic acid, amides, diols, polyols, and / or derivatives thereof; wherein said composition further comprises a buffer solution and has a pH between 5 and 11 .

[0020] In another aspect of the invention there is provided a system for upcycling plastic waste comprising:

[0021] (i) a degradation reactor for contacting a plastic from said plastic waste with an enzyme to degrade said plastic to provide a degraded composition comprising monomers and / or oligomers, and / or derivatives thereof, of said plastic;

[0022] (ii) a electrolysis reactor comprising a cathode and anode configured to apply a potential to said degraded composition to produce a mixture of products comprising hydrogen gas and preferably oxidation products of said monomers and / or oligomers, and / or derivatives thereof, of said plastic and / or said monomers and / or oligomers, and / or derivatives thereof, of said plastic, and

[0023] (iii) a means for transporting the degraded composition comprising monomers and / or oligomers, and / or derivatives thereof, of said plastic to the anode.

[0024] Advantageously, the system is for conducting the method according to the other aspects of the invention.

[0025] DEFINTIONS

[0026] As used herein the term “upcycling” refers to the conversion of material regarded as waste to valuable, typically useable, material.

[0027] As used herein the term “plastic waste” refers to waste comprising one or more polymers and additives. Typically plastic waste is mixed, i.e. it comprises a mixture of different types or classes of polymers. Examples of plastic waste include used bottles, films, carrier bags, trays and other forms of packaging.

[0028] As used herein the term “plastic” refers to the polymer(s) present in plastic waste.

[0029] As used herein the term “derivatives”, which is used in relation to monomers and oligomers, refers to compounds that are derived from monomers or oligomers, by a chemical conversion.

[0030] As used herein the term “borate buffer” refers to any buffer solution with the main pH buffering ingredient being borate or a borate salt.

[0031] DETAILED DESCRIPTION OF THE INVENTION

[0032] Embodiments of the various aspects of the invention are described below. For the avoidance of doubt, it will be appreciated, where appropriate, that any embodiments as described herein in relation to one aspect of the present invention will also apply to the other aspects of the present invention.

[0033] As described above, in an aspect of the invention there is a method for upcycling plastic waste comprising:

[0034] (i) contacting a plastic from said plastic waste with an enzyme to degrade said plastic to provide a degraded composition comprising monomers and / or oligomers, and / or derivatives thereof, of said plastic; and

[0035] (ii) conducting electrolysis on said degraded composition to produce a mixture of products comprising hydrogen gas, and preferably (a) oxidation products of said monomers and / or oligomers, and / or derivatives thereof, of said plastic and / or (b) said monomers and / or oligomers, and / or derivatives thereof, of said plastic.

[0036] Plastic waste

[0037] The plastic waste processed in the method of the present invention is preferably post consumer plastic waste. This is plastic waste produced by an end consumer. A typical example of post consumer plastic waste is packaging.

[0038] Typically plastic waste is collected from consumers along with glass, paper, and / or metal. In preferred methods of the invention, the plastic in the plastic waste is separated from glass, paper and metal prior to step (i) of the method herein. Preferably materials that might damage processing equipment (e.g. stones, wood etc) are also separated from the plastic. Thus a further preferred method of the invention comprises separating the plastic in the plastic waste from non-plastic contaminants prior to step (i) of the method herein. In another preferred method of the invention, the plastic waste is washed (e.g. with water) prior to step (i) of the method. Separation of plastic and washing may be carried out simultaneously or sequentially. Conventional equipment and methods may be used.

[0039] In the method of the present invention the plastic from the plastic waste is preferably in the form of film, powder, pellets, fibers, micro-particles, nano-particles, or combinations thereof. Optionally the plastic from the plastic waste goes through extrusion or heating, and the resultant plastic is shredded, pelletised, or milled prior to step (i) of the method of the present invention. Again, conventional equipment may be used. In prior art methods, the plastic is often converted to fiber or powder form to increase the surface area of the plastic for degradation. However, the need for this additional processing step consumes energy.

[0040] One preferred form of plastic for use in the method of the present invention is film. The ability to directly use films may remove the requirements for preliminary processing steps thereby increasing the efficiency of the process.

[0041] In a preferred method of the present invention, the plastic that undergoes degradation is a condensation polymer.

[0042] In a preferred method of the present invention, the plastic that undergoes degradation comprises polyester, polyamide and / or polyurethane.

[0043] The plastic that undergoes degradation may consist of polyester, polyamide and / or polyurethane.

[0044] More preferably the plastic that undergoes degradation comprises polyester, and still more preferably the plastic that undergoes degradation consists of polyester. The polyester may be an aliphatic polyester or a semi-aromatic polyester. It is another benefit of the method of the present invention that polyesters may be processed since the majority of the commercial recycling operations focus on polyolefins.

[0045] Examples of suitable polyester include poly(ethylene terephthalate) (PET), poly(butylene terephthalate) (PBT), polyethylene naphthalene (PEN), polytrimethylene terephthalate (PTT), polycaprolactone (PCL), polylactic acid (PLA), poly hydroxy butyrate (PHB), polyglycolic acid (PGA), polyethylene adipate (PEA), or combinations thereof. One preferred polyester is poly(ethylene terephthalate) (PET). Another preferred polyester is polycaprolactone (PCL). A further preferred polyester is polyethylene adipate (PEA). In a preferred method of the present invention, the plastic that undergoes degradation comprises poly(ethylene terephthalate) (PET), polyethylene adipate (PEA), polyamide and / or polyurethane (Pll).

[0046] Enzymes

[0047] The purpose of the enzyme is to degrade the plastic from the plastic waste into monomers and / or oligomers, and / or derivatives thereof, which can undergo electrolysis to aid production of further products (e.g. hydrogen). Enzymes are particularly advantageous over the strong alkaline solutions used in the prior art because they can operate in benign pH and can be highly selective for particular polymer types. It is therefore possible to foresee that the method of the present invention could be used to upcycle polyester in mixed plastic from plastic waste, and another process used to upcycle another class of polymer.

[0048] It will be appreciated that the invention is not particularly limited by the form in which the enzyme is provided. For example, the invention is intended to cover the direct use of the enzymes, as well as enzymes provided in cell lysates, cell secretions, or microorganisms. In a preferred method of the invention an enzyme per se is used.

[0049] In preferred methods of the invention, the enzyme degrades the plastic by hydrolysis.

[0050] Preferably the enzyme is selected from depolymerases, laccases, esterases, peroixdases, alkane hydroxylases, amidases, polyurethanases, peptidases (proteases), tannases, lignolytic enzymes, exopolysaccharide-degrading enzymes, lipases, and combinations thereof. Preferably the enzyme is an esterase, amidases and urethanases. More preferably the enzyme is an esterase selected from cutinase, carboxylesterase, PETase and combinations thereof. Still more preferably the enzyme is a PETase or a cutinase. Particularly preferably the enzyme is a cutinase.

[0051] Example enzymes that are suitable for use in the method of the present invention include enzymes selected from native sequences IsPETase (A0A0K8P6T7), Mono(2- hydroxyethyl) terephthalate hydrolase (A0A0K8P8E7), leaf-branch compost cutinase (LCC, G9BY57), Thermobifida fusca cutinase (TfCut, E5BBQ3_THEFU), Thermobifida fusca hydrolase (TfH, Q6A0I4), NylA (NYLA_PSES8) and NylB from strain NK87, Est1 from T.alba AHK119 (D4Q9N1); and their variants with sequence identity greater than 75%. Such enzymes are commercially available.

[0052] In preferred methods of the invention, the plastic is PET and the enzyme is a PET-hydrolysing enzyme (PHE). Enzyme degradation step

[0053] In preferred methods of the invention, step (i) is carried out at a temperature of 0 to 120 °C, such as 30 to 75 °C. These relatively mild conditions are beneficial in that they are cheap to operate, and straight-forward to scale.

[0054] In preferred methods of the invention, step (i) is carried out for 1 hr to 7 days. More preferably 12 to 48 hrs, and even more preferably about 24 hrs.

[0055] In some methods of the invention, the plastic is in an aqueous solution or a dispersion. In such methods the concentration of the plastic is preferably 0.1 to 1500 mg per ml, more preferably 0.5 to 500 mg per ml and still more preferably 1 to 30 mg per ml. In some methods of the invention, the plastic is in an aqueous solution or dispersion with a concentration of 5 mg of plastic per ml or less, such as 1 mg of plastic per ml or less, or 0.5 mg of plastic per ml or less.

[0056] Preferably the aqueous solution is stirred or agitated during step (i). For example, the aqueous solution may be stirred at 30 to 1000 rpm.

[0057] In preferred methods of the invention, the enzyme and the plastic are present in a weight ratio of 0.001 to 1.0% (weightenzyme / weightpiastic). More preferably, the enzyme and the plastic are present in a weight ratio of 0.05 to 0.4%.

[0058] Buffers and pH

[0059] Preferably, the step of conducting electrolysis on the degraded composition occurs in the presence of a buffer solution. Particularly preferably, an aqueous buffer solution. Especially preferably the step of conducting electrolysis on the degraded composition occurs in the presence of the same buffer solution used to control the pH of the enzyme degraded composition.

[0060] Preferably, the buffer solution controls the pH of the degraded composition within the range of 4 to 11. The buffer solution may control the pH of the degraded composition within the range of 5 to 11. The buffer solution may control the pH of the degraded composition within the range of 5.5 to 11 . The buffer solution may control the pH of the degraded composition within the range of 6 to 11 . The buffer solution may control the pH of the degraded composition within the range of 7 to 11 . The buffer solution may control the pH of the degraded composition within the range of 7.5 to 10.5. The buffer solution may control the pH of the degraded composition within the range of 8 to 10.5. The buffer solution may control the pH of the degraded composition within the range of 8.5 to 10. The buffer solution may control the pH of the degraded composition within the range of 8.5 to 9.5. The buffer solution may control the pH of the degraded composition of about 9.

[0061] Analogously, the step of contacting the plastic with an enzyme may occur in the presence of a buffer solution. Particularly preferably, an aqueous buffer solution.

[0062] Preferably, the buffer solution controls the pH of a solution comprising the enzyme used to degrade the plastic within the range of 4 to 11 . The buffer solution may control the pH of a solution comprising the enzyme used to degrade the plastic within the range of 5 to 11. The buffer solution may control the pH of a solution comprising the enzyme used to degrade the plastic within the range of 5.5 to 11. The buffer solution may control the pH a solution comprising the enzyme used to degrade the plastic within the range of 6 to 11. The buffer solution may control the pH of a solution comprising the enzyme used to degrade the plastic within the range of 7 to 11. The buffer solution may control the pH of a solution comprising the enzyme used to degrade the plastic within the range of 7.5 to 10.5. The buffer solution may control the pH of a solution comprising the enzyme used to degrade the plastic within the range of 8 to 10.5. The buffer solution may control the pH of a solution comprising the enzyme used to degrade the plastic within the range of 8.5 to 10. The buffer solution may control the pH of a solution comprising the enzyme used to degrade the plastic within the range of 8.5 to 9.5. The buffer solution may control the pH of a solution comprising the enzyme used to degrade the plastic about 9.

[0063] In preferred methods of the invention, both the steps of contacting the plastic with an enzyme and the step of conducting electrolysis on the degraded composition occur in the presence of buffer solution. In particularly preferred embodiments, the steps of contacting the plastic with an enzyme and the step of conducting electrolysis on the degraded composition occurs in the presence of same buffer solution.

[0064] In further preferred methods of the invention the buffer solution controls the pH of the enzyme used to degrade the plastic, the pH of the degraded composition and / or the pH during electrolysis, within the range of 4 to 11 , preferably 7 to 10. Preferably the same buffer solution is used.

[0065] Accordingly, the buffer solution may control the pH of a solution comprising the enzyme, the subsequent degraded composition and the pH during electrolysis within the range of 4 to 11. The buffer solution may control the pH of a solution comprising the enzyme, the subsequent degraded composition and the pH during electrolysis within the range of 5 to 11. The buffer solution may control the pH of a solution comprising the enzyme, the subsequent degraded composition and the pH during electrolysis within the range of 5.5 to 11. The buffer solution may control the pH of a solution comprising the enzyme, the subsequent degraded composition and the pH during electrolysis within the range of 6 to 11. The buffer solution may control the pH of a solution comprising the enzyme, the subsequent degraded composition and the pH during electrolysis within the range of 7 to 11 . The buffer solution may control the pH of a solution comprising the enzyme, the subsequent degraded composition and the pH during electrolysis within the range of 7.5 to 10.5. The buffer solution may control the pH of a solution comprising the enzyme, the subsequent degraded composition and the pH during electrolysis within the range of 8 to 10.5. The buffer solution may control the pH of a solution comprising the enzyme, the subsequent degraded composition and the pH during electrolysis within the range of 8.5 to 10. The buffer solution may control the pH of a solution comprising the enzyme, the subsequent degraded composition and the pH during electrolysis within the range of 8.5 to 9.5. The buffer solution may control the pH of a solution comprising the enzyme, the subsequent degraded composition and the pH during electrolysis about 9.

[0066] It will be appreciated that in methods wherein both steps comprise the same buffer solution, the degraded composition provided from contacting the plastic with the enzyme will contain the buffer from step (i) and can be used directly in step (ii) without a need to change the buffer or pH.

[0067] It has been advantageously found that hydrogen and other valuable chemical products can still be produced despite the relativity mild pH conditions described above. These relatively mild conditions are suitable for the enzyme degradation and therefore there is no requirement to change the pH between the two steps leading to an efficient, safe and cost effective process. This is surprising as industrial and academic electrolysers typically work under extreme pH conditions, either strongly acidic (PEM electrolysis) or basic pH (alkaline electrolysis) to achieve acceptable current density.

[0068] Preferably, the buffer solution comprises a borate buffer, a sulphate buffer, a carbonate buffer, a phosphate buffer, a bicine buffer, a Bis-Tris buffer, a Tris buffer or combinations thereof.

[0069] Alternatively or additionally, the buffer solution may comprise a biological buffer. Particularly preferably, the buffer solution comprises a borate buffer.

[0070] The buffer solution may consist of a borate buffer.

[0071] Preferably the buffer solution has a pKa of 5-11 .

[0072] Preferably, the buffer solution has a buffer salt concentration of 1 nM to 10M.

[0073] More preferably, the buffer solution may have a buffer salt concentration of the 1 mM to 1M. The buffer solution may have a buffer salt concentration of from 0.01 M to 1 M. The buffer solution may have a buffer salt concentration of from 0.01 M to 0.5M. The buffer solution may have a buffer salt concentration of from 0.02M to 0.3M. The buffer solution may have a buffer salt concentration of from 0.03M to 0.2M. The buffer solution may have a buffer salt concentration of from 0.05M to 0.1 M.

[0074] Composition obtained from degrading plastic

[0075] In preferred methods of the invention, the composition obtained by degradation of the plastic comprises monomers and / or oligomers of the plastic. In some cases, derivatives of monomers and / or oligomers may be obtained. In preferred methods the composition obtained by degradation predominantly comprises monomers. The monomers and / or oligomers may be oxidised to produce further products in the electrolysis step. As shown in the examples, enzymes are effective at producing monomers from plastic and these monomers can optionally be oxidised into useful organic compounds.

[0076] Examples of monomers include 6-hydroxyhexanoic acid, ethylene glycol, methyenedianiline, bis (2-hydroxyethyl) terephthalate (BHET), mono (2-hydroxyethyl) terephthalate (MHET), 4-hydroxybutanoic acid, terephthalic acid, 1 ,4-butanediol, naphthalene-2,6-dicarboxylic acid, 1 ,3-propanediol, lactic acid, 2-hydroxybutanoic acid, 3-hydroxylbutanoic acid, glycolic acid, adipic acid and / or derivatives thereof, and combinations thereof.

[0077] Monomers may also include diols, diisocyanates, caprolactams, amides, a, coamino acids and / or a stoichiometric mixture of a diamine and a diacid. Another possible monomer is formate.

[0078] In a preferred method of the invention, the monomers comprise ethylene glycol, and more preferably the monomers comprise terephthalic acid and ethylene glycol.

[0079] In some methods of the invention, the method comprises removing any solid residues from the composition comprising monomers and / or oligomers, and / or derivatives thereof, of said plastic prior to step (ii). Solid residues may, for example, be removed by centrifugation of the composition.

[0080] In some methods of the invention, one or more of the degradation products of the plastic is removed from the composition. This may be advantageous if one of the products is found to interfere with the electrolysis thereby reducing the yield of useful products generated.

[0081] In some methods of the invention, the concentration of the monomers and / or oligomers, and / or derivatives thereof, of said plastic in the degraded composition is of from 0.01 mM to 18 M. Preferably the concentration of the monomers and / or oligomers, and / or derivatives thereof, of said plastic in the degraded composition is of from 0.1 mM to 1M. For example, the concentration of the monomers and / or oligomers, and / or derivatives thereof, of said plastic in the degraded composition may be of from 0.5 mM to 100mM. In some methods of the invention the concentration of the monomers and / or oligomers, and / or derivatives thereof, in the degraded composition is of from 0.5 mM to 10mM.

[0082] Electrolysis step

[0083] In preferred methods of the invention, the composition which undergoes electrolysis in step (ii) is an aqueous solution.

[0084] In some further preferred methods of the invention, the degraded composition comprising monomers and / or oligomers, and / or derivatives thereof, of said plastic is used directly in step (ii). Put another way, electrolysis may be conducted directly on the degraded composition without purification thereof. Preferably the degraded composition is a solution (i.e. the monomers and / or oligomers, and / or derivatives thereof, of said plastic are water soluble). In other preferred methods, and as mentioned above, solids and / or specific compounds may optionally be removed.

[0085] Preferably, the step of conducting electrolysis comprises using a foil, a mesh, or a porous cathode.

[0086] Preferably, the step of conducting electrolysis comprises using a cathode comprising Ni, Pd, Au, CuO, Pt, CuxPdy, or combinations thereof, where X is between 10 and 40 and y is between 40 and 90. Optionally CuxPdyis Cu27Pd?3.

[0087] Particularly preferably, the cathode is a Ni |Cu27Pd?3 electrode

[0088] Preferably, the step of conducting electrolysis comprises using a pure metal anode or a metal alloy anode.

[0089] Preferably, the anode is metal anode or a metal alloy anode

[0090] Preferably the anode comprises Pd, Au, CuO, Pt, CuxPdy, or combinations thereof, where X is between 10 and 40 and y is between 40 and 90.

[0091] Particularly preferably, the anode is Pt.

[0092] Preferably, the step of conducting electrolysis comprises applying of voltage between 1 and 1.5. The voltage of between 1 and 1.5 may be vs. RHE (Reversible Hydrogen Electrode)

[0093] In the methods of the present invention, steps (i) and (ii) may be carried out separately or may be carried out concurrently. In some methods steps (i) and (ii) are carried out sequentially in a single reactor. In other methods, steps (i) and (ii) are carried out concurrently in a single reactor. Conducting the steps in a single reactor may increase the operational efficiency compared to prior art methods and it is surprising that these biological and photocatalytic reactions are compatible and can occur concurrently.

[0094] Products

[0095] As described above, the method of the present invention produces hydrogen and preferably said monomers and / or oligomers, and / or derivatives thereof, of said plastic. The method also produces oxidation products of said monomers and / or oligomers, and / or derivatives thereof, of said plastic.

[0096] Preferably the oxidation products of said monomers and / or oligomers, and / or derivatives thereof, of said plastic are selected from alcohols, aldehydes, ketones, amines, organic acids, carbon dioxide, derivatives therefrom, or combinations thereof.

[0097] For example, the oxidation products of said monomers and / or oligomers, and / or derivatives thereof, of said plastic may be selected from formic acid, glycolic acid, oxalic acid, acetic acid, glyoxal, glycolaldehyde, pentanal, hexanal, butanal, propanal, ethanal, carbon dioxide, derivatives therefrom, and / or combinations thereof.

[0098] The method preferably yields monomers of the plastic (i.e. non-oxidised versions of the monomers) as an overall product. These monomers may be commercially useful for further applications, such as the producing of further polymeric materials. These monomers may include 6-hydroxyhexanoic acid, ethylene glycol, methyenedianiline, bis (2-hydroxyethyl) terephthalate (BHET), mono (2-hydroxyethyl) terephthalate (MHET), 4- hydroxybutanoic acid, terephthalic acid, 1 ,4-butanediol, naphthalene-2,6-dicarboxylic acid, 1 ,3-propanediol, lactic acid, 2-hydroxybutanoic acid, 3-hydroxylbutanoic acid, glycolic acid, formate, oxalate, adipic acid, amides, diols, polyols, and / or derivatives thereof. Preferably these monomers include 6-hydroxyhexanoic acid, ethylene glycol, methyenedianiline, bis (2-hydroxyethyl) terephthalate (BHET), mono (2-hydroxyethyl) terephthalate (MHET), 4-hydroxybutanoic acid, terephthalic acid, 1 ,4-butanediol, naphthalene-2,6-dicarboxylic acid, 1 ,3-propanediol, lactic acid, 2-hydroxybutanoic acid, 3-hydroxylbutanoic acid, glycolic acid, oxalate, adipic acid, amides, diols, and / or derivatives thereof. Particularly preferably, these monomers included ethylene glycol and / or terephthalic acid.

[0099] System The present invention also relates to a system for upcycling plastic waste comprising: i) a degradation reactor for contacting a plastic from said plastic waste with an enzyme to degrade said plastic to provide a degraded composition comprising monomers and / or oligomers, and / or derivatives thereof, of said plastic;

[0100] (ii) a electrolysis reactor comprising a cathode and anode configured to apply a potential to said degraded composition to produce a mixture of products comprising hydrogen gas, and preferably oxidation products of said monomers and / or oligomers, and / or derivatives thereof, of said plastic and / or said monomers and / or oligomers, and / or derivatives thereof, of said plastic, and

[0101] (iii) a means for transporting the degraded composition comprising monomers and / or oligomers, and / or derivatives thereof, of said plastic to the anode.

[0102] In preferred systems, the electrolysis reactor comprises multiple compartments. For example, the cathode and anode may be disposed in separate compartments of the electrolysis reactor. The electrolysis reactor may further comprise an ion exchange membrane between the compartments comprising the anode and the cathode.

[0103] In preferred systems, the cathode comprises Ni Pd, Au, CuO, Pt, CuxPdy, or combinations thereof.

[0104] In preferred systems, the anode comprises Ni Pd, Au, CuO, Pt, CuxPdy, or combinations thereof.

[0105] In preferred systems, the degraded composition is soluble in aqueous solution.

[0106] In preferred systems, the degradation reactor and the electrolysis reactor are integrated.

[0107] In preferred systems, degradation reactor is for contacting a plastic from said plastic waste with an enzyme in the presence of a buffer, preferably a borate buffer.

[0108] BRIEF DESCRIPTION OF FIGURES

[0109] Figure 1 shows the hydrogen evolution obtained from electrolysis of a solution comprising 1 mM ethylene glycol dissolved in a 0.05M, pH 9, borate buffer solution.

[0110] Figure 2 shows a comparison of the hydrogen evolution from electrolysis at different pH valves. Specifically, it shows the hydrogen evolution obtained from electrolysis of solutions comprising 1 mM ethylene glycol dissolved in either a pH 8, 9, 10 or 11 , 0.05M, borate buffer solution.

[0111] Figure 3 shows a comparison of the hydrogen evolution from electrolysis at different buffer concentrations. Specifically, it shows the hydrogen evolution obtained from electrolysis of solutions comprising 1 mM ethylene glycol dissolved in either a 0.05M, pH 9, borate buffer solution or a 0.1 M, pH 9, borate buffer solution.

[0112] Figure 4 shows the hydrogen evolution obtained from electrolysis of a solution comprising 1 mM ethylene glycol dissolved in a 0.05M, pH 8, ammonium sulphate buffer solution.

[0113] Figure 5 shows the hydrogen evolution over 24 h obtained from electrolysis of a solution produced from the enzymatic degradation of a PET film within a 0.05M, pH 9, borate buffer.

[0114] Figure 6 shows the 1 H-NMR spectrum of a sample of borate buffer + PET after enzymatic treatment. Both ethylene glycol and terephthalic acid are produced, indicating degradation of the PET

[0115] Figure 7 shows the results of electroreforming of chemoenzymatic treated PEA. The hydrogen evolution achieved over 4 h is also shown and is compared to PEA control (PEA in phosphate buffer without enzyme degradation) and KOH control.

[0116] Figure 8 shows the results of electroreforming of chemoenzymatic treated Pll. The hydrogen evolution achieved over 4 h is also shown and is compared to Pll control (Pll in phosphate buffer without enzyme degradation) and KOH control.

[0117] Figure 9 shows the results of electroreforming of chemoenzymatic treated nylon 66. The hydrogen evolution achieved over 4 h is also shown and is compared to nylon 66 control (Nylon 66 in phosphate buffer without enzyme degradation) and KOH control.

[0118] Figure 10 shows the water suppression 1 H-NMR spectrum of a sample obtained after electrolysis of solution comprising ethylene glycol. Some key oxidation products (glyoxal, glycolate, acetate) are marked.

[0119] EXAMPLES

[0120] The materials used in the examples were all commercially available, unless otherwise stated.

[0121] Preparation of Borate Buffer

[0122] To prepare a 0.05 M borate buffer, boric acid (310 mg), sodium chloride (220 mg) and sodium tetraborate (250 mg) were dissolved in 200 mL ultrapure water.

[0123] The pH of the buffer was then adjusted to the target value using either 1 M hydrochloric acid or 1 M sodium hydroxide. pH 8 buffer solutions required adjustment using 1 M hydrochloric acid, whereas pH 9 or greater buffer solutions required adjustment using 1M sodium hydroxide. Higher concentration buffers were prepared analogously to 0.05 M buffers with the exception of varying the amount of ultrapure water.

[0124] Preparation of Ammonium Sulphate Buffer

[0125] To prepare a 0.05 M Ammonium Sulphate buffer, ammonium sulphate (1.32 g) was dissolved in 200 mL ultrapure water.

[0126] The pH of the buffer was then adjusted to the target value using 1 M sodium hydroxide.

[0127] Preparation of ethylene glycol solutions

[0128] To provide a test system that mimics solutions obtained from the enzymatic treatment of plastic waste, ethylene glycol (a monomer obtained from degradation of PET) was dissolved in one of the above-mentioned buffer solutions.

[0129] An initial stock solution was prepared by dissolving the ethylene glycol in the buffer at concentration of 0.15 M. A portion of stock solution was then further diluted with the buffer solution to achieve a 1 mM concentration of ethylene glycol for the electrolysis experiments, as further described below.

[0130] Electrolysis experiments on ethylene glycol solutions

[0131] Electrolysis experiments were run at room temperature under stirring in a 2- electrode set-up using Ni-foam|Cu2?Pd73 electrodes (as described in Nat. Synth. 2023, 2, 182-192) as the cathode and Pt-mesh as the anode.

[0132] A buffer (15 mL), e.g. a 0.05 M, pH 9, borate buffer, was added into a 3-necked round-bottom flask. The electrodes were secured in the necks, and the flask was then sealed and purged for 20 minutes using N2 gas with 2 % CH4 as the internal standard for gas chromatography measurements. After purging, ethylene glycol (100 pL, 0.15 M) dissolved in the same buffer solution was added to the flask, providing an overall concentration of ethylene glycol of 1 mM. Electrolysis was run at 1.2 V applied voltage vs. RHE (reversible hydrogen electrode) for 2 hours. Headspace samples (50 pL) were taken every 30 min to track H2 production via gas chromatography.

[0133] Control data was obtained using an analogous set up but without the introduction of the ethylene glycol.

[0134] PET-hydrolysing enzymes preparation The PET-hydrolysing enzymes (PHE) were used as purchased (CAS number: 9001-62-1 , Strem, Novozymes; 62301-5G-F, Sigma-Aldrich) after a buffer exchange step into borate buffer using PD-10 desalting columns (GE Healthcare) and stored at 4 °C for up to 4 days.

[0135] Polyamide-degrading enzymes preparation

[0136] Amano acylase (CAS 9012-37-7, Merck, 534862) was used as purchased at 100 mg per 20 mL reaction.

[0137] PEA-degrading and Polyurethane-degrading enzymes preparation

[0138] Lipase from Candida rugosa (CAS 9001-62-1 , Merck L1754) was used as purchased and at 100 mg per 20 mL reaction.

[0139] Preparation of PET films

[0140] PET films were directly cut from post-consume PET packaging box. The average dimensions of the films used in the experiments was 10 cm x 5 cm x 0.02 cm.

[0141] Preparation of PEA flakes and polyamide pellets

[0142] Each of poly(ethyleneadipate) (CAS 24938-37-2) flakes, av. mol. weight 10,000, Sigma Aldrich 181919) and Nylon 6 / 6 (pellets, Aldrich 429201) were used as purchased.

[0143] Preparation of Polyurethane powder

[0144] Polyurethane (domestic yellow sponge) was mechanically pre-treated by cutting it into approximately 1x1x1 (cm) cubes, which were subsequently frozen in liquid nitrogen and processed in a Krups coffee grinder for 10 sec. The result was yellow PU powder.

[0145] Enzymatic treatment of plastic materials

[0146] Enzyme stock solution and polymer film were prepared as described above. The polymer films and the corresponding enzyme solution were then combined within a borate buffer and incubated prior to electrolysis. A representative protocol for PET is described below. The same protocol was used for the other enzymes and plastics.

[0147] An enzyme stock solution and PET film was prepared as described above. The PHE was diluted to 1 pM concentration with borate buffer (50 mL, 0.05 M, pH9) in 50 mL volume; a PET film (545 mg) was then added to the enzyme suspension. The solution was incubated for 24 h at 60 °C. This enzyme solution was then used for electrolysis without further purification.

[0148] 1H-NMR (400 MHz) was run in D2O before and after incubation (200 pL sample) to quantify the production of ethylene glycol. 0.75 wt% TSP in D2O (50 pL) was used as the internal standard. Figure 6 shows the results of the1H-NMR (400 MHz) on a PET film, showing the production of both terephthalic acid (peak at approx. 7.8) and ethylene glycol (peak at approx. 3.6)

[0149] Enzyme stocks of two of 100 mg of dry powder lipase, and one of amano acylase, were incubated with 300 ug of mechanically pretreated Pll, 1 g PEA flakes and 1 g Nylon 6, respectively, in 20 ml of 100 mM phosphate buffer pH 8.0, at 50 °C for the amano acylase and at 37 °C for the lipase.

[0150] Electrolysis experiments on enzyme treated solution of PET

[0151] Electrolysis experiments were run at room temperature under stirring in a 2- electrode set-up using Ni-foam|Cu2?Pd73 electrodes as the cathode and Pt-mesh as the anode.

[0152] A solution comprising enzyme degraded PET within a borate buffer solution (15 mL, 0.05 M, pH9), obtained following the method described above, was added into a 3- necked round-bottom flask. The electrodes were secured in the necks, and then the flask was sealed and purged for 20 minutes using N2 gas with 2 % CH4 as the internal standard for gas chromatography measurements. Electrolysis was run at 1.2 V applied voltage vs. RHE (reversible hydrogen electrode) for 2 hours. Headspace samples (50 pL) were taken every 30 min to track H2 production via gas chromatography. An additional final measurement was taken at 24 h.

[0153] Electrolysis experiments on enzyme treated solution of PEA, Polyamide (Nylon 66) and Polyurethane

[0154] Electrolysis experiments for PEA, Polyamide and Polyurethane were generally run in a similar manner to the experiments described above for PET.

[0155] A 2 M KOH solution was mixed with each sample solution in a 1 :1 volume ratio (14 mL) and loaded into the anodic chamber, while a 1 M KOH solution was added to the cathodic chamber. The two chambers were separated by a Nation membrane. Chronoamperometric tests were conducted by applying a discrete potential of 1.23 V vs. RHE. Cyclic voltammetry (CV) was performed at a scan rate of 10 mV s-1within a non- Faradaic potential window ranging from -0.8 to 0.6 V vs. Ag / AgCl. Hydrogen generation was quantified using a gas chromatography (GC) system. Specifically, 50 pL of gas was extracted from the cathodic chamber at 2 and 4 hours after the reaction and injected into a Shimadzu GC system for analysis.

[0156] Results

[0157] Hydrogen production from ethylene glycol test solutions

[0158] Figure 1 shows the production of hydrogen from the electrolysis of a solution comprising 1 mM ethylene glycol as described in the method section above. The buffer used was a 0.05 M borate buffer adjusted to a pH of 8. As can be seen, hydrogen gas was observed at time periods from 30 mins onwards, and at a significant level compared to the control (blank) sample. This indicates redox reactions involving both ethylene glycol and water occurring at the electrodes. The buffer conditions and monomers are representative of an enzyme-degraded sample and therefore support the concept of hydrogen generation from electrolysis of an enzyme degraded plastic.

[0159] Varying the pH of the Buffer

[0160] Figure 2 shows the production of hydrogen from the electrolysis of 1 mM ethylene glycol solutions with different pH values. 0.05 M borate buffer was used and adjusted to pH 8, 9, 10 or 11 . A control (blank) data set was collected for each sample and subtracted from the experimental data. As can be seen, hydrogen gas was observed at all pH values. As mentioned previously, prior art methods typically employ highly alkaline environments which are not be suitable for enzyme degradation. It is therefore surprising that significant electrolysis occurs using the relatively mild pH conditions of the samples in Figure 2. It is particularly surprising that higher quantities of hydrogen gas were observed across all time points for the sample with a buffer at pH 9 compared to the buffer with pH values of 8, 10 & 11. Buffers, and in particular borate buffers, with a pH around 9 may therefore represent an optimum point where significant H2 generation is achieved, while still being suitable for the initial enzymatic degradation process. In addition to avoiding extra process steps after enzyme degradation, using a mild pH for the electrolysis provides a safer, more efficient and more sustainable methodology.

[0161] Varying the concentration of the buffer.

[0162] Figure 3 shows the production of hydrogen from the electrolysis of two 1 mM ethylene glycol test solutions with different concentrations of buffer solution. In particular, 0.05 M and 0.1 M borate buffers both adjusted to a pH of 9. A control (blank) data set was collected for each sample and subtracted from the experimental data. As can be seen, hydrogen gas was observed for both samples with the highest hydrogen production being observed for the 0.1 M buffer after 120 minutes. Without wishing to be bound by theory, it is believed that the higher ionic strength of electrolyte improves conductivity and can result in higher system activity.

[0163] Other buffer solutions.

[0164] Figure 4 shows the production of hydrogen from the electrolysis of a 1 mM ethylene glycol test solution with a different buffer solution. The buffer used was a 0.05 M ammonium sulfate buffer solution adjusted to a pH of 8. As can be seen, hydrogen gas was observed at time periods beyond 60 mins, however, the amount is relatively minor in view of the control (blank) sample, particularly at longer time periods. This indicates that while other buffer solutions may be used for the invention, borate buffers are surprisingly effective.

[0165] Hydrogen production from an enzyme treated solution

[0166] Figure 5 shows the production of hydrogen from electrolysis on a solution obtained directly from the enzyme degradation of PET, as described in the method section above. The buffer used was a 0.05 M borate buffer adjusted to a pH of 9. As can be seen, hydrogen gas was observed at all time periods. This indicates that the electrolysis can be conducted directly on the enzymatically degraded solution to produce hydrogen, without the need for further purification or modification steps.

[0167] Figures 7, 8 and 9 show the production of hydrogen from electrolysis on solutions obtained directly from the enzyme degradation of PEA (Figure 7), polyurethane (Figure 8) and polyamide (Figure 9) respectively. The results are shown in comparison to a blank comprising a 1 M KOH solution. As can be seen from the data, high yields of hydrogen can be obtained across a range of plastic substrates.

[0168] Products

[0169] Figure 10 shows the production of various oxidation products (glyoxal, glycolate and acetate) from electrolysis on a borate buffered solution at pH 9 comprising ethylene glycol (1 mM). This indicates that the electrolysis step can produce valuable oxidation products in addition to hydrogen. Overall, the combined enzymatic degradation and electrolysis process presents a straightforward and sustainable system for plastic reforming into hydrogen fuel. It also represents a process to generate value-added organic compounds including feedstock compounds such as acetate, formate, glycolate and glyoxal from the oxidation of the monomers.

Claims

CLAIMS1 . A method for upcycling plastic waste comprising:(i) contacting a plastic from said plastic waste with an enzyme to degrade said plastic to provide a degraded composition comprising monomers and / or oligomers, and / or derivatives thereof, of said plastic; and(ii) conducting electrolysis on said degraded composition to produce a mixture of products comprising hydrogen gas, and preferably oxidation products of said monomers and / or oligomers, and / or derivatives thereof, of said plastic and / or said monomers and / or oligomers, and / or derivatives thereof, of said plastic.

2. A method as claimed in claim 1 , wherein electrolysis is conducted directly on said degraded composition, without purification thereof.

3. A method as claimed in claim 1 or 2, wherein conducting electrolysis on said degraded composition occurs in the presence of a buffer solution, preferably at a pH within the range 4 to 11.

4. A method as claimed in any one of claims 1 to 3, wherein contacting the plastic with an enzyme occurs in the presence of a buffer solution.

5. A method as claimed in claim 3 or 4, wherein the buffer solution controls the pH of the enzyme used to degrade the plastic, the pH of the degraded composition and / or the pH during electrolysis, within the range of 4 to 11 , preferably 7 to 10.

6. A method as claimed in any one of claims 3 to 5, wherein the buffer solution comprises a borate buffer, a sulphate buffer, a carbonate buffer, a phosphate buffer, a bicine buffer, a Bis-Tris buffer, a Tris buffer, a biological buffer, or combinations thereof; preferably wherein the buffer solution comprises a borate buffer.

7. A method as claimed in any one of claims 3 to 6, wherein the buffer solution comprises a buffer salt concentration of 1 nM to 10 M, preferably 0.01 M to 1 M.

8. A method as claimed in any preceding claim, wherein said plastic comprises, preferably consists of, polyester, polyurethane and / or polyamide.

9. A method as claimed in claim 8, wherein the plastic comprises polyester and wherein said polyester is selected from poly(ethylene terephthalate) (PET), poly(butylene terephthalate) (PBT), polyethylene naphthalene (PEN), polytrimethylene terephthalate (PTT), polycaprolactone (PCL), polylactic acid (PLA), poly hydroxy butyrate (PHB), polyglycolic acid (PGA), polyethylene adipate (PEA), or combinations thereof.

10. A method as claimed in claim 9, wherein said polyester is poly(ethylene terephthalate) (PET) and / or polyethylene adipate (PEA).

11. A method as claimed in any preceding claim, wherein said enzyme is selected from depolymerases, laccases, esterases (such as cutinases, caboxylesterases, PETases), peroixdases, alkane hydroxylases, amidases, polyurethanases, peptidases (proteases), tannases, lignolytic enzymes, exopolysaccharide-degrading enzymes, lipases and combinations thereof.

12. A method as claimed in any preceding claim, wherein the plastic is PET and said enzyme is a PHE.

13. A method as claimed in any preceding claim, wherein said degraded composition comprises monomers of said plastic.

14. A method as claimed in any preceding claim, wherein electrolysis comprises using a cathode and / or an anode comprising Pd, Au, CuO, Pt, CuxPdy, or combinations thereof.

15. A method as claimed in any preceding claim, wherein electrolysis comprises applying of voltage between 1 and 1 .5 V.

16. A method as claimed in any one of claims 1 to 15, wherein steps (i) and (ii) are carried out sequentially in a single reactor.

17. A method as claimed in any one of claims 1 to 15, wherein steps (i) and (ii) are carried out concurrently in a single reactor.

18. A method for upcycling plastic waste comprising:conducting electrolysis on a degraded composition obtained from the degradation of a plastic from plastic waste, said degraded composition comprising monomers and / or oligomers, and / or derivatives thereof, of said plastic; wherein said degraded composition further comprises a buffer solution and has a pH between 4 and 11 ; and wherein said electrolysis produces a mixture of products comprising hydrogen gas and preferably oxidation products of said monomers and / or oligomers, and / or derivatives thereof, of said plastic and / or said monomers and / or oligomers, and / or derivatives thereof, of said plastic.

19. A method as claimed in claim 18, wherein the buffer is a borate buffer.

20. A method as claimed in claim 18 or 19, wherein said monomers and / or oligomers, and / or derivatives thereof, of said plastic comprise ethylene glycol.21 . A method as claimed in any one of claims 18 to 20, wherein the pH is of from 8 to 10.

22. A system for upcycling plastic waste comprising:(i) a degradation reactor for contacting a plastic from said plastic waste with an enzyme to degrade said plastic to provide a degraded composition comprising monomers and / or oligomers, and / or derivatives thereof, of said plastic;(ii) a electrolysis reactor comprising a cathode and anode configured to apply a potential to said degraded composition to produce a mixture of products comprising hydrogen gas and preferably oxidation products of said monomers and / or oligomers, and / or derivatives thereof, of said plastic and / or said monomers and / or oligomers, and / or derivatives thereof, of said plastic, and(iii) a means for transporting the degraded composition comprising monomers and / or oligomers, and / or derivatives thereof, of said plastic to the anode.

23. A system as claimed in claim 22, wherein said degradation reactor and said electrolysis reactor are integrated.

24. A system as claimed in claims 22 or 23, wherein said degradation reactor is for contacting a plastic from said plastic waste with an enzyme in the presence of a buffer solution, preferably a borate buffer.

25. A method for generating hydrogen comprising: conducting electrolysis on an aqueous composition comprising 6- hydroxyhexanoic acid, ethylene glycol, methyenedianiline, bis (2-hydroxyethyl) terephthalate (BHET), mono (2-hydroxyethyl) terephthalate (MHET), 4-hydroxybutanoic acid, terephthalic acid, 1,4-butanediol, naphthalene-2,6-dicarboxylic acid, 1,3- propanediol, lactic acid, 2-hydroxybutanoic acid, 3-hydroxylbutanoic acid, glycolic acid, formate, adipic acid, amides, diols, polyols, and / or derivatives thereof; wherein said composition further comprises a buffer solution and has a pH between 5 and 11.

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