Photocatalytic reduction of carboxylic acids using gallium nitride

Abstract

There is provided a photocatalyst for converting aliphatic carboxylic acids into ethanol, methanol, formic acid, CO and / or H2, the photocatalyst comprising GaN optionally doped with gold nanoparticles and cobalt oxide. There is also provided a method of producing ethanol, methanol and / or formic acid from aliphatic carboxylic acids. An aqueous solution comprising the aliphatic carboxylic acids is contacted with a photocatalyst comprising GaN. And, the photocatalyst is irradiated with light to active the photocatalyst and convert the aliphatic carboxylic acids into ethanol, methanol and / or formic acid.

Description

PHOTOCATALYTIC REDUCTION OF CARBOXYLIC ACIDS USING GALLIUM NITRIDETECHNICAL FIELD

[0001] This disclosure relates to the field of ethanol production and catalysts for producing the same, particularly the production of ethanol in the context of recycling plastics such as polyolefins.BACKGROUND OF THE ART

[0002] Plastic pollution is a serious and growing concern, with plastics found everywhere from urban areas to remote locations such as deserts, seafloors, and polar regions. Furthermore, projections indicate that plastic production will continuously increase, with estimates exceeding 500 million metric tons by 2050. Among the various types of plastics, polyolefins such as polyethylene (PE) and polypropylene (PP) present significant recycling challenges due to their inert chemical properties. Ironically, these plastics constitute the majority of overall plastic production. The rapid increase in their production also coincides with the global adoption of singleuse packaging, making them evermore pervasive as possible pollutants. Additionally, much of the recycling efforts involve mechanical techniques that result in lower-value secondary products due to loss of function and these products ultimately end up as pollutants as they are not recycled repeatedly. Therefore, there is a pressing need for alternative strategies to mitigate the vast plastic waste and decrease its mismanagement.

[0003] Chemical plastic upcycling has garnered attention as a means of extracting valuable chemicals from plastic waste. This approach not only minimizes waste but also contributes to a sustainable, circular economy by converting plastics into different types of polymers or breaking them down further into valuable chemical building blocks. In this regard, a tandem, multi-step method is a efficient and practical approach to increase the versatility of traditionally difficult chemical recycling.

[0004] Recently, Hakkarainen et al. (Backstrdm, E.; Odelius, K.; Hakkarainen, M. Trash to Treasure: Microwave-Assisted Conversion of Polyethylene to Functional Chemicals. Industrial & Engineering Chemistry Research 2017, 56, 14814-14821.) reported a microwave-assisted oxidation of low-density PE (LDPE) to various (di)carboxylic acids in the presence of nitric acid. Although this method is effective at the plastic breakdown for feedstock recovery, the range of products obtained is non-economical, as it requires intense separation and purification for downstream applications.

[0005] Accordingly, improvements in the recycling of plastics, such as polyolefins, to obtain desirable and economical products are still needed. One such economical product is ethanol which can be used to manufacture bioplastics or as a biofuel.SUMMARY

[0006] In one aspect, there is provided a photocatalyst for converting aliphatic carboxylic acids into ethanol, methanol and / or formic acid, the photocatalyst comprising GaN doped with gold nanoparticles and cobalt oxide.

[0007] In some embodiments, the gold nanoparticles and the cobalt oxide are provided in a total concentration of 4.5 to 5.5 wt. % of the total weight of the photocatalyst.

[0008] In some embodiments, the gold nanoparticles and the cobalt oxide are present in a weight ratio of from 2:1 to 1 :2.

[0009] In some embodiments, the gallium nitride has rod-shaped pores having a length of from 1 to 5 pm.

[0010] In some embodiments, the gold nanoparticles have a diameter of from 5 to 20 nm.

[0011] In some embodiments, the cobalt oxide is a mixture of Co(ll) oxide and Co(lll) oxide.

[0012] In one aspect, there is provided a method of producing ethanol, methanol and / or formic acid from aliphatic carboxylic acids, the method comprising: contacting an aqueous solution comprising the aliphatic carboxylic acids with a photocatalyst comprising GaN; and irradiating the photocatalyst with light to activate the photocatalyst and convert the aliphatic carboxylic acids into ethanol, methanol and / or formic acid.

[0013] In some embodiments, the photocatalyst consists of GaN. In such embodiments, the light can be a UV light having a wavelength of from 100 to 380 nm.

[0014] In some embodiments, the photocatalyst is doped with gold, platinum, ruthenium, and / or iridium. In such embodiments, the photocatalyst can be doped with gold nanoparticles. In such embodiments, the photocatalyst can be doped with gold nanoparticles and cobalt oxide.

[0015] In some embodiments, the light is visible light having a wavelength of from 380 to 780 nm.

[0016] In some embodiments, the method is performed at a pressure of 0.95 - 1.05 atm.

[0017] In some embodiments, the method is performed at a temperature of from 2 to 30 °C.

[0018] In some embodiments, the aliphatic carboxylic acids are provided in a concentration of from 5 to 15 v / v % in the aqueous solution.

[0019] In some embodiments, the photocatalyst is provided in the aqueous solution and is present in a concentration of from 0.2 to 5 w / v %.

[0020] In some embodiments, the aliphatic carboxylic acids comprise acetic acid, succinic acid, glutaric acid, adipic acid, and azelaic acid.

[0021] In a further aspect, there is provided a method for converting polyolefin polymers into ethanol, methanol and / or formic acid, the method comprising: converting polyolefin polymers into aliphatic carboxylic acids by microwave-assisted oxidation; and converting the aliphatic carboxylic acids into ethanol, methanol and / or formic acid by performing the method as herein. In some embodiments, the polyolefin polymers are selected from polyethylene, polystyrene, polyvinyl chloride, polypropylene, polymethylpentene, polybutene-1 , and polyisobutylene.

[0022] Many further features and combinations thereof concerning the present improvements will appear to those skilled in the art following a reading of the instant disclosure.DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 is a1H nuclear magnetic resonance (NMR) spectra of post-microwave- treatment of low density polyethylene (LDPE).

[0024] FIG. 2 is a close up of Fig. 1.

[0025] FIG. 3 is a high resolution mass spectrometry (HRMS) of post-microwave-treatmentLDPE aqueous solution.

[0026] FIG. 4 is a graph showing the production rate of (di)carboxylic acids for different plastic polymers (for each polymer from left to right the yield of acetic acid, succinic acid, glutaric acid, adipic acid, and azelaic acid).

[0027] FIG. 5 is a UV-Vis spectra of a GaN catalyst doped with Au and Co oxide (Au / CoOx@GaN) and commercial GaN (c-GaN).

[0028] FIG. 6A is a transmission electron microscopy (TEM) image of Au / CoOx@GaN.

[0029] FIG. 6B is a high-angle annular dark-field imaging (HAADF)-scanning transmission electron microscopy (STEM)-energy dispersive X-ray spectroscopy (EDS) elemental mapping profile of Au / CoOx@GaN showing nitrogen atoms.

[0030] FIG. 6C is a HAADF-STEM-EDS elemental mapping profile of Au / CoOx@GaN showing gallium atoms.

[0031] FIG. 6D is a HAADF-STEM-EDS elemental mapping profile of Au / CoOx@GaN showing gold atoms.

[0032] FIG. 6E is a HAADF-STEM-EDS elemental mapping profile of Au / CoOx@GaN showing cobalt atoms.

[0033] FIG. 6F is a HAADF-STEM-EDS elemental mapping profile of Au / CoOx@GaN showing the combined nitrogen, gallium, gold and cobalt.

[0034] FIG. 6G is a high-resolution TEM image of AuNPs on Au / CoOx@GaN.

[0035] FIG. 6H is a fast-Fourier-transform (FFT) in TEM pattern of an AuNP onAu / CoOx@GaN.

[0036] FIG. 7 is a photoluminescence (PL) spectra of Au / CoOx@GaN and c-GaN.

[0037] FIG. 8A is a X-ray photoelectron spectroscopy (XPS) survey scan of Au / CoOx@GaN.

[0038] FIG. 8B is a Au 4f region X-ray photoelectron spectroscopy spectrum ofAu / CoOx@GaN.

[0039] FIG. 8C is a Co 2p region X-ray photoelectron spectroscopy spectrum ofAu / CoOx@GaN.

[0040] FIG. 9 shows powder X-ray diffraction (pXRD) patterns of pristine Au / CoOx@GaN, commercial GaN (GaN), and reference wurtzite GaN structure.

[0041] FIG. 10 is a schematic of the photoinduced reaction mechanism of converting di(carboxylic acids) to ethanol with the Au / CoOx@GaN photocatalyst.

[0042] FIG. 11 is a graph showing the production rate of (di)carboxylic acids for postconsumer plastics (for each polymer from left to right: the yield of acetic acid, succinic acid, glutaric acid, adipic acid, and azelaic acid).

[0043] FIG. 12 is a graph showing the change in ethanol production based on the number of uses of the Au / CoOx@GaN catalyst.

[0044] FIG. 13A is a scanning electron microscopy (SEM) image of pristine Au / CoOx@GaN.

[0045] FIG. 13B is a close-up of Fig. 13A.

[0046] FIG. 13C is a SEM image of spent Au / CoOx@GaN catalyst (after 5 iterations).

[0047] FIG. 13D is a close-up of Fig. 13C.

[0048] FIG. 14 shows pXRD patterns of pristine and spent Au / CoOx@GaN samples.

[0049] FIG. 15A shows the XPS spectra of the Ga 3d region of pristine and spent heterogeneous catalysts.

[0050] FIG. 15B shows the XPS spectra of the N 1s region of pristine and spent heterogeneous catalysts.

[0051] FIG. 15C shows the XPS spectra of the Au 4f region of pristine and spent heterogeneous catalysts.

[0052] FIG. 15D shows the XPS spectra of the Co 2p region of pristine and spent heterogeneous catalysts. A weak satellite peak between the doublet Co 2p3 / 2 and Co 2pi / 2 in the CoO XPS are typical in cobalt oxide formation.

[0053] FIG. 16A is a TEM image of lrOx / GaN.

[0054] FIG. 16B is a close up of Fig. 16A.

[0055] FIG. 17A is a HAADF- scanning TEM of lrOx / GaN.

[0056] FIG. 17B is a HAADF-STEM-EDS elemental mapping profile of lrOx / GaN showing gallium atoms.

[0057] FIG. 17C is a HAADF-STEM-EDS elemental mapping profile of lrOx / GaN showing nitrogen atoms.

[0058] FIG. 17D is a HAADF-STEM-EDS elemental mapping profile of lrOx / GaN showing iridium atoms.

[0059] FIG. 17E is a HAADF-STEM-EDS elemental mapping profile of lrOx / GaN showing the combined profiles of Figs. 17B-17D.

[0060] FIG. 17F is a EDS spectrum of lrOx / GaN.

[0061] FIG. 18 is a XPS survey scan of lrOx / GaN.

[0062] FIG. 19 is a XPS spectrum of the Ir 4f region of lrOx / GaN.

[0063] FIG. 20A is a TEM image of PtOx / GaN.

[0064] FIG. 20B is a close up of Fig. 16A.

[0065] FIG. 21A is a HAADF- scanning TEM of PtOx / GaN.

[0066] FIG. 21 B is a HAADF-STEM-EDS elemental mapping profile of PtOx / GaN showing gallium atoms.

[0067] FIG. 21C is a HAADF-STEM-EDS elemental mapping profile of PtOx / GaN showing nitrogen atoms.

[0068] FIG. 21 D is a HAADF-STEM-EDS elemental mapping profile of PtOx / GaN showing iridium atoms.

[0069] FIG. 21 E is a HAADF-STEM-EDS elemental mapping profile of PtOx / GaN showing the combined profiles of Figs. 21 B-21 D.

[0070] FIG. 21 F is a EDS spectrum of PtOx / GaN.

[0071] FIG. 22 is a XPS survey scan of PtOx / GaN.

[0072] FIG. 23 is a XPS spectrum of the Pt 4f region of PtOx / GaN.

[0073] FIG. 24A is a TEM image of RuOx / GaN.

[0074] FIG. 24B is a close up of Fig. 24A.

[0075] FIG. 25A is a HAADF- scanning TEM of RuOx / GaN.

[0076] FIG. 25B is a HAADF-STEM-EDS elemental mapping profile of RuOx / GaN showing gallium atoms.

[0077] FIG. 25C is a HAADF-STEM-EDS elemental mapping profile of RuOx / GaN showing nitrogen atoms.

[0078] FIG. 25D is a HAADF-STEM-EDS elemental mapping profile of RuOx / GaN showing iridium atoms.

[0079] FIG. 25E is a HAADF-STEM-EDS elemental mapping profile of RuOx / GaN showing the combined profiles of Figs. 25B-25D.

[0080] FIG. 25F is a EDS spectrum of RuOx / GaN.

[0081] FIG. 26 is a XPS survey scan of RuOx / GaN.

[0082] FIG. 27 is a XPS spectrum of the Ru 3d region of RuOx / GaN (the spectrum contained C 1s signals from the adventitious carbon present during XPS, the C 1 s signal (285 eV) overlaps with the Ru 3d signal).

[0083] FIG. 28 illustrates EPR spectra of radical trapping reaction with DMPO with acetic acid, succinic acid, and simulated for the corresponding DMPO species.

[0084] FIG. 29 is a photograph of the circulating flow system involving a fixed bed, a rotary pump, a gas reservoir, placed in a cold-water bath with reflective mirrors to improve light penetration in the fixed bed.DETAILED DESCRIPTION

[0085] Light-mediated photochemical methods are sustainable and green strategies for chemical synthesis. However, direct photoinduced plastic degradations often suffer from poor conversions and prolonged reaction times which can be avoided by using a tandem approach. To improve conversion rates and reaction times, gallium nitride (GaN) catalyst is provided herein asa suitable photocatalyst for the production of ethanol due to its high electromigration rate and breakdown voltage providing both efficient and stable properties. Additionally, GaN has a wide band gap (3.4eV) and modifiable Fermi levels to promote selectivity for a desired chemical transformation. The GaN photocatalyst is used in a two-step process that combines a first step of microwave-assisted polymer oxidation to produce aliphatic carboxylic acids from polyolefins, and a second step of converting the aliphatic carboxylic acids with the GaN-based photocatalyst under visible light to produce ethanol via the aliphatic carboxylic acid intermediates.

[0086] The term “aliphatic carboxylic acids” as used herein refers to C2-C10 aliphatic carboxylic acids. Preferably, the aliphatic carboxylic acids are a mixture of mono-carboxylic acids and dicarboxylic acids. The term “(di)carboxylic acids” as used herein refers to a mixture containing both mono-carboxylic acids and dicarboxylic acids that are aliphatic. The dicarboxylic acids, in some embodiments, are defined as an organic molecule with a molecular weight of up to 300 g / mol and comprising two carboxylic acid groups. The dicarboxylic acids can be of formula I:

[0087] where R is a Ci-Os alkyl, that is optionally substituted by methyl, ethyl, propyl or butyl. Examples of dicarboxylic acids include but are not limited to succinic acid, glutaric acid, adipic acid, and azelaic acid. In some embodiments, the aliphatic carboxylic acids or the (di)carboxylic acids comprise acetic acid, succinic acid, glutaric acid, adipic acid, and azelaic acid in a concentration of 80% or more, preferably in a concentration of 90 % or more, more preferably in a concentration of 95 % or more. In some embodiments, the term “(di)carboxylic acids” is defined as a mixture consisting or consisting essentially of acetic acid, succinic acid, glutaric acid, adipic acid, and azelaic acid.

[0088] The present disclosure provides a method for recycling polyolefins into ethanol. The polyolefins are first converted into (di)carboxylic acids by a microwave-assisted oxidation of the polyolefins. The microwave-assisted oxidation can be performed as described in Hakkarainen et al. An oxidation agent to assist the oxidation, for example nitric acid (up to 0.5 g / mL) or sulfuric acid (0.01 - 0.03 g / mL). The oxidation reaction is performed at high temperature such as 160 - 180 °C at a pressure of 30 - 50 bar. The polyolefins are preferably treated to obtain a powderbefore performing the microwave-assisted oxidation. The powder can be obtained by mechanical means such as grinding or milling.

[0089] In some embodiments, polyolefin polymers are selected from polyethylene, polystyrene, polyvinyl chloride, polypropylene, polymethylpentene, polybutene-1 , and polyisobutylene. The polyethylene is for example low density polyethylene (LDPE) with a molecular weight of 917-930 kg / m3. The polyethylene can also be high density polyethylene (HDPE) with a molecular weight of 930-970 kg / m3.

[0090] The (di)carboxylic acids intermediates are then converted into methanol, ethanol and / or formic acid with a GaN photocatalyst when exposed to light. The GaN catalyst (without dopants) converts the (di)carboxylic acids when exposed to LIV light (200-380 nm). The GaN catalyst when doped with gold nanoparticles (AuGaN or Au@GaN), can advantageously operate under visible light (e.g. 380 - 780 nm). Moreover, it was advantageously found that further doping the AuGaN photocatalyst with cobalt oxide (CoOx) can improve the selectivity for ethanol under visible light. The term cobalt oxide as used herein means the combination of Co(ll) and Co(lll) oxides, which are CoO and CO3O4 species. The cobalt oxide dopant is dispersed on the surface of the catalyst.

[0091] The (di)carboxylic acids intermediates are provided in an aqueous solution (such as water). The concentration of the (di)carboxylic acids can vary based on the scale of operation, shape of the reactor and efficiency of the mixing. Generally, the (di)carboxylic acids are provided in a concentration of from 5 to 15 v / v %. When performing a batch operation, the photocatalyst can be provided in the solution in a concentration of from 0.2 to 5 w / v %. However, it is also possible to anchor the photocatalyst on a surface particularly when considering scale up operations. In such embodiments, the concentration of the photocatalyst is not a pertinent parameter, and one must simply ensure that the surface covered by the photocatalyst needs to sufficiently come into contact with the aqueous solution containing the (di)carboxylic acids to initiate the photochemical reaction.

[0092] The GaN photocatalyst (without dopants), in some embodiments, is as described in W02024020685. GaN is characterized by a regular wurtzite crystal structure, which is the thermodynamically stable phase of GaN. In this crystal structure, the exposed surfaces of the GaN nanoparticles are composed of c-planes and m-planes. The m-plane is a one dimensional rectangular configuration of the Ga and N atoms and the c-plane is a one dimensional hexagonalconfiguration of the Ga and N atoms. The overall m-plane of GaN is nonpolar since it is composed of equal numbers of Ga and N atoms which are tetrahedrally coordinated with each other, whereas the polar c-plane comprises only one type of atom (either Ga or N) which exhibits piezoelectric polarization along the c-axis.

[0093] In some embodiments, the GaN is in the form of a powder which may have a grain size of from 100 to 500 A, from 150 to 350 A or from 200 to 250 A. In other embodiments, the GaN is in the form of a nanoparticle which is defined as having a diameter in the nanoscale. For example, the GaN nanoparticles can have a diameter of between 10 and 1000 nm, between 20 and 900 nm, between 30 and 800 nm, or between 40 and 700 nm. In further embodiments, the GaN is supported by a catalyst support, for example zeolite or silica gel solid supports. At industrial scale the use of a flow reactor is generally more cost effective than a batch system, accordingly, it is preferred to have the catalyst on a solid support rather than in suspension.

[0094] In some embodiments, the gallium nitride powder is porous and is characterized by rod-shaped pores. The rod-shaped pores can have a length of from 1 to 5 pm. In some embodiments, the GaN powder is characterized by a porosity of from 5 to 20 m2 / g, from 7 to 15 m2 / g or from 11 to 14 m2 / g as measured by the Brunauer-Emmet-Teller (BET) method. Accordingly, the GaN is not considered mesoporous.

[0095] As previously explained, in some embodiments, the GaN photocatalyst is doped with gold, iridium, platinum or ruthenium in the form of nanoparticles (e.g. AuNP or Au). The AuNP are localized on the surface of the GaN. In some embodiments, the nanoparticles have a diameter of from 5 to 20 nm and preferably from 10 to 20 nm. This can be measured by transmission electron microscopy or dynamic light scattering. Generally, gold nanoparticles with a diameter above 20 nm have weak localized surface plasmon resonance (LSPR) and a lower transfer efficiency of hot holes to the semiconductor. With a diameter of at least 5 or at least 10 nm, the gold nanoparticles achieve efficiency and selectivity for ethanol by favoring carbon-carbon couplings. In some embodiments, the gold nanoparticles are present in a concentration of from 2.5 to 10 wt. %, from 3 to 7 wt. %, from 4 to 6 wt. %, from 4.5 to 5.5 wt. % or about 5 wt. % with respect to the total weight of the photocatalyst. The gold nanoparticles can be characterized by X-ray photoelectron spectroscopy (XPS) and have a 4f? / 2 binding energy peak of from 83.95 to 84.05 eV or 83.97 to 83.01 eV and a 4fs / 2 binding energy peak of from 87.65 to 87.75 eV or 87.67 to 87.71 eV.

[0096] In preferred embodiments, the Au@GaN photocatalyst is also doped with cobalt oxide (CoOx). The resulting photocatalyst was labelled Au / CoOx@GaN. Indeed, as explained above and demonstrated in the example section below, the addition of cobalt oxide dopant can improve the selectivity for ethanol under visible light. Cobalt oxides are efficient aggregators of electron holes, improving the photocurrent and stability of the heterogeneous catalyst and therefore its reactivity. The Au and CoOx can be provided in a weight ratio of from 3:1 to 1 :3, from 2.5:1 to 1 :2.5, 2:1 to 1 :2, from 1.5:1 to 1 :1.5, from 1.25:1 to 1 :1.25 of Au to CoOx. The total content of dopant (combined weight of Au and CoOx) in the Au / CoOx@GaN photocatalyst can be provided in a concentration of from 4.5 to 5.5 wt. % with respect to the total weight of the photocatalyst.

[0097] Although the present disclosure and results are focused on Au, there are other dopants such as oxides of Ir, Pt, and Ru that can be used with GaN. These other dopants similarly to Au, form nanoparticles at the surface of the GaN (e.g. lrOxnanoparticles, PtOxnanoparticles, and / or RuOxnanoparticles). The Ir, Pt, and Ru oxide nanoparticles are of a similar size as the Au nanoparticles and are provided in similar proportions. For example, the nanoparticles have a diameter of from 5 to 20 nm and preferably from 10 to 20 nm. This can be measured by transmission electron microscopy or dynamic light scattering.EXAMPLE

[0098] Experiments under the microwave reactor were performed using an Anton Paar Monowave microwave reactor. Experiments under light irradiation was performed using a 427 nm PR160L™ Kessil lamp equipped with a constant water bath for temperature control. The temperature of the water bath was set to 20 °C.

[0099] Reagents were purchased from the following vendors: Sigma-Aldrich, Thermo Fisher Scientific, and Fisher Scientific, and were used without further purification unless otherwise indicated. Specifically, low density polyethylene (LDPE) (1000 micron, Alfa Aesar), high density polyethylene (HDPE) (average Mw:125 000, Alfa Aesar), and polypropylene (PP) (isotactic, average Mw: 250,000, Sigma Aldrich). All the water used as the reaction solvent was Milli-Q™ water unless otherwise indicated.

[0100] The experiment began with microwave-assisted plastic degradation and the commercial, sole plastic polymer, LDPE as the model substrate. The complete decomposition of LDPE was realized under a microwave reaction with the optimized conditions of 2 mL of 1.6 M HNO3 (aq) for 20 min at 190 °C followed by 2 h at 180 °C .

[0101] More specifically, the microwave-assisted reactions were performed as follows unless otherwise stated. The reaction was performed in an Anton Paar Monowave microwave reactor. To a 10 mL Pyrex™ microwave tube equipped with a Teflon™-coated magnetic stirring bar was added 50 mg of plastic in 2 mL of 1.6 M HNO3 (aq) then sealed with an aluminium cap with a septum. The temperature program was set to 20 min at 190 °C followed up by 2 h at 180 °C. The pressure was kept at 12 bar for the duration of the reaction. After completion, the solution was neutralized with saturated KOH (aq) and analyzed with1H nuclear magnetic resonance (NMR) in D2O with imidazole as the internal standard. High resolution mass spectrometry (HRMS) analysis was completed with the reaction solution prior to neutralization.

[0102] The calculation of carbon yield was performed as follows: moles of carbon in liquidphase products (acetic acid + succinic acid + glutaric acid + adipic acid + azelaic acid) per moles of carbon in the polymer substrate. Specifically -CH2CH2- was considered a molecular formula for PE, and -(CH3)CHCH2- was considered a molecular formula for PP.

[0103] Nuclear magnetic resonance (NMR) spectra,1H NMR was recorded on a Bruker 500 MHz spectrometer under automatic solvent suppression technique to suppress the dominant H2O signal. The chemical shifts are references to the solvent residual signal (D2O: 1 H: 5 4.80 ppm). NMR data are reported as follows: multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, dd = doublet of doublet, etc.), coupling constant (J, Hz), and integration. All NMR spectra were recorded at room temperature.

[0104] High-resolution mass spectrometry (HRMS) was performed on a Thermo-Fisher Exactive Plus Orbitrap, using atmospheric pressure chemical ionization (APCI) and electrospraying ionization (ESI), in positive and negative modes. High mass accuracy measurements were used to confirm the elemental composition of relevant ions.

[0105] The water-soluble products were determined by1H NMR and HRMS after neutralization as; succinic, glutaric, adipic, azelaic, and acetic acids in order of decreasing concentrations (Figs. 1-3). A carbon yield of 56% was achieved, similar to that of the Hakkarainen et al. (Fig. 4). Changes in the HNO3 loading or the duration and temperature of the reaction resulted in a less optimal generation of (di)carboxylic acids, potentially due to the increased generation of gaseous products. Next, other polymers were subjected to depolymerization, high- density polyethylene (HDPE) and polypropylene (PP) fully decomposed and formed various concentrations of (di)carboxylic acids with carbon yields of 46% and 6% respectively (Fig. 4). Thereduced carbon yield with PP is likely due to its different chemical composition forming more gaseous and non-acid products. As different polymers all generated carboxylic acids, we looked at the degradation of the polymer mixtures. To our delight the mixture of all the polymers fully degraded and formed the desired (di)carboxylic acids with a carbon yield of 39% with full degradation (Fig. 4).Table 1. Acid production rates from microwave-assisted reaction with polyolefin(s)aYields obtained by1H NMR with imidazole as the internal standard.

[0106] To prepare 5 wt. % Au / GaN, 3 mg GaN was added into 3 mL water, followed by 30 minutes of sonication. Next, 32 pL of 24 mM HAuC SFLO aqueous solution was added to the GaN suspension, followed by 30 mins of vigorous stirring. Finally, 100 pL of freshly prepared ice- cold 0.05 mg / mL NaBFL aqueous solution was added to the mixture dropwise. The mixture was immediately brought to a water-bath sonication and sonicated for 10 s, followed by 3 h of vigorous stirring. The solution was then left aging overnight without disturbance, followed by purification by rinsing with water 3 times.

[0107] To prepare 5 wt. % Au / CoOx(weight ratio = 1 :1)@GaN. In 3 mL of H2O is added 3 mg GaN and sonicated for 30 min. 16 pL of 24 mM HAuCkSFLO (aq) and 32 pL of 42 M COCI2 6H2O (aq) was added to the solution and stirred for 30 min. Followed up with 100 pL freshly prepared ice-cold 1.3 M NaBFL (aq) added dropwise to the solution. The mixture was immediately sonicated for 10 s and an additional 3 hr of stirring. The resulting solution was then left to settle and washed with 3 times with H2O (10 mL). The resulting 5 wt. % Au / CoOx(weight ratio = 1 :1)@GaN had a deep grey and red hue in physical appearance.

[0108] Characterizations of the heterogeneous catalyst were done to gain insight into its reactivity. Diffuse UV-Vis spectra was one of the methods used to characterize the catalyst and was performed as follows. After water removal by a centrifuge, the catalyst powder was dried under a vacuum chamber at room temperature. GaN powder was analyzed as received. The diffuse UV-Vis spectrum was performed on a UV-Vis-NearIR spectrophotometer Cary 5000 from Agilent with Praying mantis diffuse reflectance holder. The UV-Vis spectra of the Au / CoOx@GaN catalyst compared to c-GaN showed an increase in absorbance in the visible region around 550 nm, suggesting the presence of the plasmonic resonance peak from the gold nanoparticles (Au NPs) (Fig. 5).

[0109] Transmission electron microscopy (TEM) and high-angle annular dark-field imaging (HAADF)-STEM was also used to characterize the Au / CoOx@GaN (Fig. 6A). As can be seen in Fig. 6A, it was observed in the transmission emission microscopy image that the Au NPs were smaller in size than the incident wavelength to realize localized surface plasmon resonance. An aqueous suspension of the catalyst was drop casted onto a 200 mesh Cu grid (SPI Supplies) with carbon coating and left to dry under vacuum. The characterization was performed on a Thermo Scientific Talos F200X G2 (S)TEM, with the beam convergence angle 10.5 mrad and the collection angle 58-200 mrad in HAADF imaging. Spectroscopic mapping by energy dispersive X-ray (EDS) spectroscopy was performed using a 60 mm2silicon drift detector from Bruker. (HAADF)-scanning transmission electron microscopy (STEM)-energy dispersive X-ray spectroscopy (EDS) elemental mapping profile of Au / CoOx@GaN results are shown in Figs. 6B- 6F. Notably, Fig. 6E shows that cobalt oxide is dispersed on the surface of A fragment of the catalyst was chosen to facilitate STEM imaging process. This observation displays the homogeneous distribution of CoOxand the formation of AuNPs on the GaN surface. Finally, an identification of Au lattice in Au / CoOx@GaN via using fast-Fourier-transform (FFT) in TEM was confirmed (Figs. 6G-6H).

[0110] The photoluminescence (PL) measurement was performed with either a 405 nm laser or 325 nm He-Cd laser (Kimmon Koha) as the excitation source. The PL spectra of the Au / CoOx@GaN catalyst showed a reduced intensity of PL emissions compared to c-GaN (Fig. 7). The decrease in PL emissions eludes a Mott-Schottky barrier at the interface of the GaN and the dopant.

[0111] X-ray photoelectron spectroscopy (XPS) was also used to characterize the catalyst. The XPS analysis was performed on a Thermo-Scientific K-Alpha with an Al Ka micro-focusedmonochromator, an X-ray spot size of 400 pm, and the flood gun on to avoid sample charging. Spectral energies were calibrated by setting the C-C binding energy of C1s at 284.8 eV. Peak fitting was performed using the Thermo Avantage software (version 4.60). The XPS survey scan shows the presence of Au, Co, Ga, N, and O (Fig. 8A). X-ray photoelectron spectroscopy analysis of the catalyst at the Au 4f region (Fig. 8B) fits the formation of metallic gold and the Co 2p region (Fig. 8C) represents the formation of cobalt oxide.

[0112] Powder X-ray diffraction (pXRD) was used to further characterize the catalyst. 6 mg of the material, suspended in 500 pL water, was drop casted on a thin glass slide and then was dried under a vacuum chamber at room temperature. The pXRD analysis was performed on a Bruker D8 Advance powder X-ray diffractometer with a Cu Ka source. Fig. 9 presents the XRD patterns of pristine Au / CoOx@GaN and commercial GaN samples. There are no obvious changes to the pXRD patterns after deposition of Au / CoOxon the GaN. This suggests the active regions of GaN remains after the deposition

[0113] The GaN photocatalyst (with and without dopants) was investigated for the conversion of the (di)carboxylic acid solution obtained from the microwave reaction to other products. The following two methods were used to characterized the products in the gas.

[0114] Gas phase analysis by gas chromatography - thermal conductivity detection (GC- TCD): experiments were performed on an Agilent Technologies Gas Chromatograph System (Agilent GC 6890) equipped with a thermal conductivity detector (TCD). Two columns were used. Column 1 was an Agilent HP PLOT-Q™ column composed of bonded polystyrene- divinylbenzene. Column 2 was an Agilent HP-PLOT™ MoleSieve™ composed of zeolite. The analysis is done with 1 mL of the gas phase atmosphere with a gas-tight sampling syringe (Hamilton 1700 Series Syringes).

[0115] Gas phase analysis by gas chromatography - mass spectroscopy (GC-MS): experiments were performed on an Agilent Technologies Gas Chromatograph System equipped with a 5973 Inert Mass Selective Detector. The column Rtx-5 (30 m length, 0.25 mm internal diameter, 0.25 pm df) from Restek Corporation was used.

[0116] The photoreaction was performed as follows unless otherwise stated. To a 10 mL quartz tube equipped with a TeflonTM-coated magnetic stirring bar was added 100 pL of the neutralized digested plastic solution with 1 mg of 5 wt. % Au / CoOx(weight ratio 1 :1)@GaN in 1 mL of H2O. The tube was sealed, evacuated, and backfilled with argon three times using thefreeze-pump-thaw technique. The reaction vial was stirred under the irradiation of a 427 nm Kessil lamp under a constant water bath for 24 h. The distance between the light source and the reaction tubes was 4 cm apart at a 100% lamp intensity. After completion, the solution was centrifuged at 10k rpm for 3 min to separate the solution from the heterogeneous catalyst. The supernatant was analyzed by1H NMR in D2O with imidazole as the internal standard.

[0117] The post-consumer plastic package (LDPE) was mechanically cut into pieces to approximately 1 cm X 0.5 cm (L X W). The post-consumer disposable cup (PP) was mechanically cut into pieces with approximately 0.5 cm X 0.3 cm (L X W). Both post-consumer plastic polymers was subject to the optimized microwave-assisted reaction conditions and followed by the optimized photoinduced reaction conditions.

[0118] The preliminary test was performed with 100 pL of (di)carboxylic acid solution from the LDPE oxidation in 1 mL of water with 1 mg of the commercial GaN (c-GaN) under an argon atmosphere irradiated for 24 h under a broad-spectrum xenon lamp. The formation of ethanol, methanol, and formic acid was observed (Table 2, entry 1). To improve the photocatalytic property under visible light, metallic gold as a dopant for the GaN catalyst was explored as it exhibits surface plasmonic resonance. This behaviour is an efficient strategy to harvest visible light energy to semiconductors limited by large band-gaps by injecting hot-carriers. Moreover, it can heighten the photocatalytic ability by altering the electromigration rate, improving the charge transfer, and suppressing electron-hole recombination. The Au / GaN catalyst was active under visible light at converting the acids with greater selectivity for ethanol (Table 2, entry 3). The presence of cobalt oxide as a second dopant was found to enhance the selectivity and formation of ethanol (Table 2, entry 4). Cobalt oxides are efficient aggregators of electron holes, improving the photocurrent and stability of the heterogeneous catalyst and therefore its reactivity. Furthermore, the selectivity for ethanol is highly desirable as it is a renewable fuel and the building block to synthesize biopolyethylene (bioPE), a green, sustainable alternative to PE, contributing to the circular economy. Control reactions showed the reaction did not occur in the dark or without the catalyst (Table 2, entries 8 and 9).Table 2. Photoreaction conditions and results

[0119] To a 10 mL quartz tube equipped with a TeflonTM-coated magnetic stirring bar was added 100 pL of the neutralized digested plastic solution with 1 mg of 5 wt. % Au / CoOx(weight ratio = 1 :1)@GaN in 1 mL of H2O. The tube was sealed, evacuated, and backfilled with argon three times using the freeze-pump-thaw technique. The reaction vial was stirred under the irradiation of a 427 nm Kessil lamp in a constant water bath for 24 h. The distance between the light source and the reaction tubes was 4 cm apart at a 100% lamp intensity. After completion, the solution was centrifuged at 10k rpm for 3 min to separate the solution from the heterogeneous catalyst. The supernatant was analyzed by1H NMR in D2O with imidazole as the internal standard. The heterogeneous catalyst was collected and washed with H2O three times then dried under a vacuum overnight before the next use.Table 3: Optimization of the photoinduced reactions

[0120] The influence of each carboxylic acid was investigated by preparing individual solutions of each (di)carboxylic acid. The concentration of each intermediate followed the experimental values of the microwave-reaction step. The concentration of acetic acid was based on the values of polypropylene reaction and the rest of the acids was based on the values of low- density polyethylene reaction. The concentrations were as followed: acetic acid (88 mM), succinic acid (60 mM), glutaric acid (52 mM), adipic acid (44 mM), and azelaic acid (20 mM). The final volume of each solution was 100 pL in H2O and concentration KOH (aq) was added dropwise to neutralize the solution.

[0121] Each (di)carboxylic acid intermediate was treated independently under the optimized photoreaction conditions to observe the formation of products. The formation of the desired product (i.e. ethanol) was observed in all the intermediates, suggesting the combination of all the (di)carboxylic acids take part in the formation of ethanol in the reactions.

[0122] The individual acids were subjected to the optimized photocatalytic conditions. In all cases, the formation of ethanol was observed, suggesting the observed acids contributed to product formation (Table 4). Additional studies suggest methanol serves as the intermediate to ethanol which is supported by previous literature demonstrating direct methanol to ethanol conversion over photoinduced GaN via methyl carbene (Table 5). Based on the observed data, a plausible mechanism for the photoinduced reaction was proposed (Fig. 10, scheme 1). The tandem reaction begins with the plastic polymers oxidized to (di)carboxylic acids under the microwave reactor in acid. The generated (di)carboxylic acids (1) are then subject to the photoreaction with Au / CoOx@GaN catalyst. Under visible light irradiation, hot charge carriers are induced in the Au NPs, activating the catalyst. The generated hot electron will reduce the acid (1) to methanol (3). The methanol (3) is directly converted to ethanol (2). The accompanying electron-hole promotes the oxidation of water, which could occur at the cobalt oxide, which is a reported photocatalytic water oxidation catalyst.Scheme 1.Au / CoOx@GaN (1 mg) (Di)carboxylic acid (100 uL) - ► EtOH + MeOH + HCOOH1 427 nm (Kessil), 2 3 41 mL H2O, Ar, 24 hTable 4. Intermediate study under photoreactionaNMR with imidazole as internal standard

[0123] To examine the possibility of methanol as the intermediate to ethanol formation the following reaction was conducted. Considering 100 pL of digested polypropylene solution contains majority of acetic acid (4.75 pmol), equivalent to 9.5 pmol of methanol in carbon equivalence. The standard reaction was completed with 9.5 pmol of methanol as the starting carbon source. The reaction performed similarly to the typical photoreaction of the digested polypropylene solution (Table 5).Photoreaction procedure

[0124] As a representative of the photoreactions, unless otherwise stated. To a 10 mL quartz tube equipped with a Teflon-coated magnetic stirring bar was added 100 pL of the neutralized digested plastic solution with 1 mg of 5wt% Au / CoOx(weight ratio 1 :1) / GaN in 1 mL of H2O. The tube was sealed, evacuated, and backfilled with argon three times using the freeze-pump-thawtechnique. The reaction vial was stirred under the irradiation of a 427 nm Kessil lamp in a circulating water bath for 24 h. The distance between the light source and the reaction tubes was 4 cm apart at a 100% lamp intensity. After completion, the solution was centrifuged at 10k rpm for 3 min to separate the solution from the heterogeneous catalyst. The supernatant was analyzed by1H NMR in D2O with imidazole as the internal standard.Table 5. Methanol intermediate study under optimized photoreaction condition

[0125] With the optimized conditions, the photoconversion of the (di)carboxylic acid intermediates was evaluated from other polymers and the polymer mixtures. With both HDPE and PP as starting polymers, ethanol formed as the major product with PP showing the highest ethanol production (Table 6, entries 2 and 3). Reactions were conducted under an inert atmosphere for 24 h in 1 mL H2O. Yields were obtained by1H NMR with imidazole as internal standard. The acids derived from the polymer mixtures also gave the desired product albeit with lower ethanol formation than the individual counterparts (Table 5, entries 4 and 5).Table 6. Different sources of polymers

[0126] The reaction application was extended to post-consumer single-use packaging (LDPE) and cups (PP) with the same reaction conditions (Table 7). Yields were obtained by1H NMR with imidazole as internal standard. In both cases, microwave-assisted oxidation completely degraded the plastics into (di)carboxylic acids with carbon yields of 33% and 6% respectively. As well, the photoconversion step generated similar amounts of ethanol compared to the commercially available polymer variant, suggesting the robust reactivity of the Au / CoOx@GaN catalyst (Fig. 11).Table 7. Post-consumer package (LDPE) and cups (PP) sourced polymersTable 8. Acid production rates from microwave-assisted reaction with post-consumer polyolefinaYields obtained by1H NMR with imidazole as the internal standard.

[0127] Finally, the capacity to reuse the catalyst was evaluated. The catalyst was recycled 4 times, for a total of five reactions and the change in the ethanol production of each reaction is reported in Fig. 12. After five iterations the catalyst reactivity did not significantly decrease, suggesting a robust catalytic structure (Fig. 12). The structure of the catalyst was evaluated by scanning electron microscope (SEM). SEM was performed with FEI Quanta 450 Environmental Scanning Electron Microscope (FE-ESEM) with EDAX Octane Super 60 mm2 SDD and TEAM EDS Analysis System. A layer of 4 nm Pt was sputtering on the sample surface to facilitate the imaging process. Figs. 13A-13B show the pristine catalyst before reaction and Figs. 13C-13Dshow the spent catalyst (i.e. after five iterations of the reaction). As can be seen in Figs. 13A-13D there was no significant change in the structure of the spent catalyst from the pristine catalyst. XRD was also used to compare the pristine and spent Au / CoOx@GaN samples (Fig. 14). There were no changes observed between the pristine and spent catalyst, demonstrating good stability of the heterogeneous catalyst structure. Further, XPS was used to compare the pristine and spent catalysts (Figs. 15A-15D). XPS spectra of the Co 2p region of pristine and spent heterogeneous catalysts show the same spectra (Fig. 15D). A weak satellite peak between the doublet Co 2p3 / 2 and Co 2pi / 2 / in the CoO XPS are typical in CoOxformation (Fig. 15D). Fig. 15D shows the XPS spectra of the Co 2p region of pristine and spent heterogeneous catalysts. A weak satellite peak between the doublet Co 2p3 / 2and Co 2p2show the presence of both Co(ll) and Co(lll) oxides (presence of both binding energies of the two oxidation states and the characteristic satellite peaks of CoO and CO3O4).

[0128] The experiment reaction was repeated with conditions as presented in Table 9, to demonstrate the variability in the ratio between Au and CoOx as well as the variability of the total dopant concentration. More specifically, for Entry 1 of Table 9, to a 10 mL quartz tube equipped with a TeflonTM-coated magnetic stirring bar was added 100 pL of the neutralized digested plastic solution with 1 mg of catalyst in 1 mL of H2O. The tube was sealed, evacuated, and backfilled with argon three times using the freeze-pump-thaw technique. The reaction vial was stirred under the irradiation of a 427 nm Kessil lamp for 24 h in a constant water bath. The distance between the light source and the reaction tubes was 4 cm apart at a 100% lamp intensity. After completion, the solution was centrifuged at 10k rpm for 3 min to separate the solution from the heterogeneous catalyst. The supernatant was analyzed by1H NMR in D2O with imidazole as the internal standard. For entries 2-12, to a 10 mL quartz tube equipped with a Teflon-coated magnetic stirring bar was added 200 pL of the neutralized digested plastic solution with 2 mg of corresponding catalyst in 2 mL of H2O. In Entries 3 and 4, the resulting solution was adjusted by a minimal volume of HNO3 or KOH solutions to pH = 4 and pH = 11 respectively. The tube was then sealed under an air atmosphere. The reaction vial was stirred under the irradiation of a broad-spectrum Xenon lamp for 48 h in a constant water bath. After completion, the solution was centrifuged at 10k rpm for 3 min to separate the solution from the heterogeneous catalyst. Ir / GaN, Ru / GaN, Pt / GaN were obtained by the same chemical reduction method to produce Au / GaN as described above. The precursors were IrCh, RuCh, and H2PtCl6'6H2O. The supernatant was analyzed by1H NMR in D2O with imidazole as the internal standard.Table 9. Additional photoreaction conditions and resultsn.d. = not detected

[0129] In summary, the present example reports plastic upcycling into ethanol, a sustainable fuel and a building block to ethylene which is used industrially to produce bioPE, thereby completing a circular plastic economy. A tandem strategy was demonstrated involving microwave- assisted oxidation and GaN-based photocatalysis for effective plastic degradation and selective ethanol formation via the (di)carboxylic acid intermediate from multiple plastic sources (LDPE, HDPE, and PP) and its mixtures without the need for pre-washing and pre-sorting. The heterogeneous catalyst, Au / CoOx@GaN was able to harness visible-light energy via the hot charge carriers from the surface plasmonic resonance of Au NPs. Most notably, post-consumer plastics were upcycled with similar efficiency to their plain counterparts, demonstrating the protocol's tolerance to commercial impurities.

[0130] To evaluate the concentration range of Au / CoOx, the concentration was varied from 5 wt. % to 3 or 7 wt. % as follows. To a 10 mL quartz tube equipped with a TeflonTM-coated magnetic stirring bar was added 100 pL of the neutralized digested plastic solution with 1 mg of catalyst in 1 mL of H2O. The tube was sealed, evacuated, and backfilled with argon three times using the freeze-pump-thaw technique. The reaction vial was stirred under the irradiation of a 427 nm Kessil™ lamp for 24 h in a constant water bath. The distance between the light source and the reaction tubes was 4 cm apart at a 100% lamp intensity. After completion, the solution was centrifuged at 10k rpm for 3 min to separate the solution from the heterogeneous catalyst. The supernatant was analysed by1H NMR in D2O with imidazole as the internal standard. The weight loading of the dopants is 1 :1 (Au:CoOx).Table 10. Expansion on Au:CoOxweight loading on GaN

[0131] The two different loadings of AuCoOx(entries 1 and 3) resulted in no obvious production of ethanol and did not result in a higher production rate of any other products (methanol and formic acid). This suggests that the optimal loading for the dopants remains at 5 wt% (1 :1 wt% ratio) on GaN.

[0132] Photoreaction conditions optimized for formic acid production rate were chosen. To a 10 mL quartz tube equipped with a TeflonTM-coated magnetic stirring bar was added 200 pL of the neutralized digested plastic solution with 2 mg of catalyst in 2 mL of H2O. The tube was then sealed under an air atmosphere. The reaction vial was stirred under the irradiation of a broadspectrum Xenon lamp for 48 h in a constant water bath. After completion, the solution was centrifuged at 10k rpm for 3 min to separate the solution from the heterogeneous catalyst. The supernatant was analysed by1H NMR in D2O with imidazole as the internal standard.Table 11. Expansion on Au:CoOxweight ratio on GaN under a second reaction condition

[0133] In the second reaction condition, a higher formic acid space-time yield was observed with 3 wt% AuCoOx / GaN at 14 pmohgCataiyst'1 ,h'1(entry 2 table 11).

[0134] To build on the above results and demonstrate the extrapolation from gold nanoparticles to other nanoparticles additional experiments were performed with the oxides of Ir, Pt and Ru.

[0135] To prepare 5 wt% PtOx@GaN, in 3 mL of H2O, 3 mg GaN was added then degassed by bubbling argon for 15 mins, then sealed and sonicated for 30 min. This was followed up with the addition of 336 pL of freshly prepared 1 mg / mL K2PtCI4aqueous solution to the GaN mixture under constant stirring for 30 mins. After the stirring 100 pL of freshly prepared ice-cold 0.05 g / mL NaBH4aqueous solution was added dropwise. The mixture was immediately sonicated for 10 s and an additional 3 hr of stirring. The solution was left aging overnight without disturbance, followed by purification by rinsing with water 3 times. The resulting solution of 5 wt. % PtOx@GaN had a brownish-grey physical appearance.

[0136] To prepare 5 wt% RuOx@GaN, an aqueous solution of 3 mL of 3.3 mM RuCh was added to a concentrated KOH aqueous solution until the pH of the solution reached pH 7. This was followed with the addition of 19 mg of GaN into the neutralized solution under constant stirring. While maintaining the stirring of the solution, 1 mL of freshly prepared ice-cold 0.1 M NaBH4aqueous solution was added dropwise. The stirring was maintained for an additional 30 mins and afterwards, the precipitate was filtered and rinsed with 10 mL of H2O three times. The resulting precipitate of 5 wt% RuOx@GaN had a dark grey physical appearance.

[0137] To prepare 5 wt% lrOx@GaN, in 3 mL of H2O, 18 mg GaN and 1.2 mg IrCh x^O was added. The pH of the solution was set to pH 10 with concentrated ammonia under constant stirring. While maintaining the stirring of the solution, 1 mL of freshly prepared ice-cold 0.1 M NaBH4aqueous solution was added to the mixture dropwise. The solution was stirred for an additional 2 days and afterwards, the precipitate was filtered and rinsed with 10 mL of H2O three times. The resulting precipitate of 5 wt% lrOx@GaN had a light grey physical appearance.

[0138] The three catalysts: lrOx / GaN, PtOx / GaN, and RuOx / GaN were characterized with TEM, HAADF, EDS and XPS (Figs. 16A-16B, 17A-17F, 18, 19, 20A-20B, 21A-21 F, 22, 23, 24A- 24B, 25A-25F, 26 and 27). The characterizations of the catalysts suggest that all three catalysts had even distributions of the dopants and the species of the dopant was majorly the oxidant. As shown in Fig. 19, the spectrum shows the presence of lrO2 in the catalyst. The binding energy for Ir 4f 7 / 2 and Ir 4f 5 / 2 are observed at 61 .9 eV and 64.7 eV respectively which are closely related to that of I rO2, suggesting that lrO2 is the major species. Fig. 23 shows the XPS spectrum of the Pt 4f region of PtOx / GaN. The binding energy for Pt 4f 5 / 2 and Pt 4f 7 / 2 were observed at 75.1 eV and 71.9 eV respectively, which are closely related to that of PtO2, suggesting that PtO2 is the major species. Finally, Fig. 27 shows the XPS spectrum of the Ru 3d region of RuOx / GaN. Thebinding energy was observed at 285.7 eV, which suggests a hydrated oxide Ru in the catalyst (RuO2*XH2O / GaN).Electron paramagnetic resonance (EPR) experiment

[0139] EPR experiment with both acetic acid and succinic acid as substrates to represent both the mono- and dicarboxylic acids provided evidence for the presence of OH and CH3radicals via the observation of the corresponding adducts with dimethyl-1-pyrroline-N-oxide (DMPO), as the corresponding EPR spectra was consistent with previous report and simulation.EPR experiment with acetic acid and succinic acid with DMPO

[0140] To a 10 mL quartz tube equipped with Teflon-coated magnetic stirring bars were added 2 mL of acetic acid (20 pmol, 10 mM) or succinic acid (20 pmol, 10 mM) in H2O, 5,5-dimethyl-1- pyrroline-N-oxide (DMPO, 11.3 mg, 0.10 mmol, 1.0 equiv), and AuCoOx / GaN (5 mg). The tube was sealed, evacuated, and backfilled with argon three times using the freeze-pump-thaw technique. The reaction vial was stirred under the irradiation of a 427 nm Kessil lamp in a constant water bath for 1 h. After 1 h, the solution was centrifuged at 10k rpm to separate the solution from the heterogeneous catalyst and taken for electron paramagnetic resonance (EPR) analysis.

[0141] The resulting EPR spectra (Fig. 28) from both acetic acid and succinic acid shows the presence of both DMPO-CH3 adduct corresponding to six peaks with intensities of 1 :1 :1 :1 :1 :1 and DMPO-OH adduct corresponding to four peaks with intensities of 1 :2:2: 1 which is consistent with previous literature and that of our simulation.Upcycling of Polyolefins to Alcohols - Flow Setup1. Adaption of the batch reaction condition to the one-way flow system

[0142] To test the existing reaction system in a flow system, 50 mg of 2.5wt%Au2.5wt%Co / GaN was suspended in 2 mL acetone, then loaded onto 50 mg of glass woolinside a 1=10 cm d= 8mm quartz tubular reactor. The reactor was placed in a 110-120 °C oven for 1 hour to remove all the acetone. After cooling down, the 50 mg catalyst / 50 mg glass wool fixed bed was installed into the flow system.

[0143] 10 mL diluted neutralized LDPE digestion solution (the LDPE digestion solution, its neutralization, and its dilution were prepared in an identical way as the batch reaction, e.g. 2 mL neutralized LDPE digestion solution + 20 mL Milli-Q water) was de-oxygened by bubbling argon for 15 minutes. Then this solution was injected into the fixed bed at 1 mL / min via a syringe pump, while the fixed bed reactor was immersed in a 10-15 °C water bath under 427 nm Kessil lamp (distance from the light source to the fixed bed was 5 cm, and the depth of the fixed bed underneath the water was 1 cm). Experimental results (see Table 12) showed that compared to the batch condition which favored ethanol formation, the flow condition exclusively favored formic acid formation with a ~7x improvement in production efficiency.Table 12. Comparison between the batch and the flow setup

[0144] Various parameters were then investigated in the above flow system, including the flow rate (Table 13), concentration of the starting material (Table 14), light source (Table 15), and switching from a one-way flow system to a circulation flow system. These results indicated thatthe flow reaction requires a different input (such as light sources with shorter wavelengths) compared to the batch reaction in order to produce alcohol products, likely due to the vast difference in their catalyst-reagent contact time and mass transfer.Table 13. Flow rateTable 14. Starting material concentration and one-way / circulation setupTable 15. Light source2. Adaption of the batch reaction condition to the circulation flow system

[0145] Catalytic gasification of plastic on fixed bed to syngas usually requires multiple stages and high temperature above 800°C (https: / / doi.Org / 10.1016 / i.iihydene.2025.06.190), thus, it would be worthwhile to examine the potentials of the system described herein in producing syngas at ambient temperature and pressure. In the batch condition where the reaction was carried out under a mild and visible LED light source (427 nm Kessil), the reaction favored the production of liquid products, namely formic acid, methanol, and ethanol; no obvious gaseous product wasdetected by GC. Since a shift of product selectivity was observed in the one-way flow system above, it was then decided to investigate the possibility of catalytic production of syngas from LDPE in a circulation flow system.

[0146] The circulation flow system was set up as shown in Fig. 29. A fixed bed, which is a d=8 mm 1=10 cm quartz tubular reactor filled with 50 mg catalyst supported on 50 mg glass wool, was connected to a gas reservoir (a glass vial with rubber septum) to facilitate the post-reaction gas sampling. The volume of the glass vial can be adjusted to change the system head volume. The fixed bed was placed underneath a Xe lamp (the distance between the lamp and the fixed bed is 6 cm) and immersed inside a cold-water bath (the fixed bed depth is 1 cm below the water surface), where the temperature was maintained at 10-15 °C by adding ice regularly. The bottom of the water bath was covered with mirrors to reflect the Xe lamp light to improve the light penetration inside the fixed bed. The injection port of the circulation system was a T-connector, where a syringe can be attached to the lines (right upper corner in Fig. 29). The flow rate of the circulation system was controlled by a rotary pump.

[0147] Firstly, the impact of the reaction headspace was investigated. To facilitate the production of gaseous products, the reaction must have enough headspace to allow the gaseous product to expand without raising the system pressure extensively. This hypothesis was validated by the experiments (Table 16), where a larger headspace resulted in a higher production of carbon monoxide (CO) .Table 16. Impact of the reaction headspace on the gaseous product in the circulation flow systemNotes: Reaction conditions in Table 16: light source was Xe lamp, unfiltered, distance between the light source and the fixed bed was 6 cm. The circulation time was 4 hours. The starting material was 12 mL diluted {dilution factor being v(original solution): v(water) = 1 :10} neutralized LDPE digestion solution (O2removed). The rate of the rotary pump was 1 .5mL / min.

[0148] Detailed analysis (Table 17) of the 17 mL headspace entry in Table 16 showed excellent selectivity towards carbon monoxide (CO) in the gas phase, as well as the production of acid and alcohol products in the liquid phase. The presence of hydrogen gas (H2), quantified by GC-TCD, showed that the LDPE was converted to syngas. The presence of carbon dioxide (CO2) was also observed by GC-TCD. The overall carbon conversion from the original LDPE solid powders to the carbon monoxyde is -14% in the 17 mL headspace case, and -41% in the 33 mL headspace case.Table 17. Detailed analysis of the 17 mL headspace entry (Table 16, Entry 2)

[0149] Considering the current limitations in precise quantification of hydrogen gas, the following experiments focused on the quantification of carbon-containing products in both liquid and gas phase. Unless otherwise mentioned, all the catalysts presented in the following tableswere prepared by wet chemical reduction method, the one described in the batch condition section above at paragraph

[0106] ,

[0150] To understand which metal decoration was the determining factor in terms of syngas production, single metal decorated GaN were tested as catalysts (Table 18). The result showed that among the gold (Au) and the cobalt in a oxide form, the Au is the crucial catalyst component to produce carbon monoxide (CO), and the Co (oxide) is the cocatalyst to improve the yield of the reaction.Table 18. Impact of metal (oxide) decoration in the circulation flow systemNotes: Reaction conditions in Table 18: light source was Xe lamp, unfiltered, distance between the light source and the fixed bed was 6 cm. The circulation time was 4 hours. The starting material was 12 mL diluted {dilution factor being v(original solution): v(water) = 1 :10} neutralized LDPE digestion solution (O2removed). The rate of the rotary pump was 1.5mL / min. The headspace of the reaction was 33 mL and filled with atmospheric pressure argon or nitrogen gas (no obvious difference in the experiment outcome was found between these two gases).

[0151] To test if there are other cocatalysts that can boost the catalytic properties of GaN supported Au, various metal decorations were investigated to replace Cobalt as the co-catalyst, including Iron (Fe), nickel (Ni), Magnesium (Mg) and Scandium (Sc) (Table 19). It was found that among all the cocatalysts investigated, Iron, Scandium and Cobalt have shown results providing significant conversion to carbon monoxide. Cobalt was the optimal one in terms of both gas phase and liquid phase productivity. The catalysts presented in the table 19 were prepared by wet chemical reduction method, as described above at paragraph

[0106] , The result is decorated GaN with the various metal or metal oxide on the surface.Table 19. Investigation of cocatalysts in the circulation flow systemNotes: Reaction conditions in Table 19: light source was Xe lamp, unfiltered, distance between the light source and the fixed bed was 6 cm. The circulation time was 4 hours. The starting material was 12 mL diluted {dilution factor was v (original solution): v(water) = 1 :10} neutralized LDPE digestion solution (O2removed). The rate of the rotary pump was 1.5mL / min. The headspace of the reaction was 33 mL and filled with atmospheric pressure argon or nitrogen gas (no obvious difference in the experiment outcome was found between these two gases).

[0152] The impact of the catalyst support on the reaction activities was further investigated. Various catalyst supports, of which the majority were semiconductors, were tested in Table 20. It was found that among all the cocatalysts investigated, GaN was the optimal support in terms of CO productivity.Table 20. Investigation of supports in the circulation flow systemNotes: Reaction conditions in Table 20: light source was Xe lamp, unfiltered, distance between the light source and the fixed bed was 6 cm. The circulation time was 4 hours. The starting material was 12 mL diluted {dilution factor: v (original solution): v(water) = 1 :10} neutralized LDPE digestion solution (O2removed). The rate of the rotary pump was 1.5mL / min. The headspace of the reaction was 33 mL and filled with atmospheric pressure argon or nitrogen gas (no obvious difference in the experiment outcome was found between these two gases). No ethanol was detected in all the entries presented in Table 20.

[0153] Finally, the inventors investigated if the current circulation flow reaction can be run at milder conditions (e.g. visible light radiation instead of full spectra, air atmosphere instead of inert atmosphere), as well as the catalyst reusability in the conversion of LDPE to CO reaction (Table 21).

[0154] Comparison between Table 21 Entry 1 and Entry 2 showed that the conversion of digested LDPE to CO can be done at visible light, with a 48% decrease in CO productivity. Comparison between Table 21 Entry 1 and Entry 4 showed that the production of CO was not compatible with an air atmosphere; the reaction needs to be carried out under an inert atmosphere (such as argon or nitrogen gas) to realize efficient CO production.

[0155] In terms of the reusability test, after a regular 4-hour reaction, the used 2.5wt%Au2.5wt%Co / GaN supported on glass wool was cleaned with water three times to remove any water-soluble impurities. Then the catalyst was cleaned with acetone three times to remove water and dried inside a 110-120 °C oven. After cooling down to room temperature, the used fixed bed was ready to use. Reusability test (Table 21 , Entry 3) showed a 63% decrease in CO productivity in the second 4-hour run, indicating a moderate reusability of 2.5wt% Au2.5wt%Co / Ga N .Table 21. Alternation and reusability test of the current optimal condition in the circulation flow systemNotes: Reaction conditions in Table 8: light source was Xe lamp, filtered or unfiltered, distance between the light source and the fixed bed was 6 cm. The circulation time was 4 hours. The starting material was 12 mL diluted {dilution factor was v (original solution): v(water) = 1 :10} neutralized LDPE digestion solution (O2removed, except Entry 4). The rate of the rotary pump was 1 .5mL / min. The headspace of the reaction was 33 mL and filled with atmospheric pressure argon or nitrogen gas (no obvious difference in the experiment outcome was found between these two gases), except Entry 4.3. Conclusion

[0156] The above experiments showed that the batch system, which is the photocatalytic conversion of digested LDPE to value-added products (formic acid, methanol, and ethanol) via metal-decorated GaN, can be transferred to a flow system. The one-way flow system involving fixed beds gave a 7x improvement in terms of the formic acid production efficiency with a >99% liquid phase selectivity.

[0157] Moreover, this conversion, when done in a circulated flow system, can be alternated to produce syngas (CO and hydrogen gas) with high efficiency. The current best condition described herein with 2.5wt%Au2.5wt%Co / GaN can give a 41 % carbon conversion from original LDPE powders to CO with a gas phase selectivity at >99%, at ambient temperature and pressure within 4 hours. This system can also be reused and can be done under visible light radiation instead of a full spectrum radiation. What’s worthwhile is that the new set of circulating flow systems still produced formic acid and alcohol products in the aqueous phase, adding additional values to the plastic upcycling process.

Claims

WHAT IS CLAIMED IS:

1. A photocatalyst for converting aliphatic carboxylic acids into ethanol, methanol, formic acid and / or syngas, the photocatalyst comprising GaN optionally doped with gold nanoparticles and cobalt oxide.

2. The photocatalyst of claim 1 , wherein the gold nanoparticles and the cobalt oxide are provided in a total concentration of 1.25 to 5.5 wt. % of the total weight of the photocatalyst.

3. The photocatalyst of claim 1 or 2, wherein the gold nanoparticles and the cobalt oxide are present in a weight ratio of from 2:1 to 1 :2.

4. The photocatalyst of any one of claims 1 to 3, wherein the gallium nitride has rod-shaped pores having a length of from 1 to 5 pm.

5. The photocatalyst of any one of claims 1 to 4, wherein the gold nanoparticles have a diameter of from 5 to 20 nm.

6. The photocatalyst of any one of claims 1 to 5, wherein the cobalt oxide is a mixture of Co(ll) oxide and Co(lll) oxide.

7. A method of producing ethanol, methanol and / or formic acid from aliphatic carboxylic acids, the method comprising: contacting an aqueous solution comprising the aliphatic carboxylic acids with a photocatalyst comprising GaN; and irradiating the photocatalyst with light to activate the photocatalyst and convert the aliphatic carboxylic acids into ethanol, methanol and / or formic acid.

8. The method of claim 7, wherein the photocatalyst consists of GaN.

9. The method of claim 8, wherein the light is LIV light having a wavelength of from 200 to 780 nm.

10. The method of claim 7, wherein the photocatalyst is doped with gold, platinum oxide, ruthenium oxide, and / or iridium oxide.11 . The method of claim 7, wherein the photocatalyst is doped with gold nanoparticles.

12. The method of claim 7, wherein the photocatalyst is doped with gold nanoparticles and cobalt oxide.

13. The method of claim 11 or 12, wherein the light is visible light having a wavelength of from 380 to 780 nm.

14. The method of any one of claims 7 to 13, wherein the method is performed at a pressure of 0.95 - 1.05 atm.

15. The method of any one of claims 7 to 14, wherein the method is performed at a temperature of from 2 to 30 °C.

16. The method of any one of claims 7 to 15, wherein the aliphatic carboxylic acids are provided in a concentration of from 5 to 15 v / v % in the aqueous solution.

17. The method of any one of claims 7 to 16, wherein the photocatalyst is provided in the aqueous solution and is present in a concentration of from 0.2 to 5 w / v %.

18. The method of any one of claims 7 to 17, wherein the aliphatic carboxylic acids comprise acetic acid, succinic acid, glutaric acid, adipic acid, and azelaic acid.

19. A method for converting polyolefin polymers into ethanol, methanol and / or formic acid, the method comprising: converting polyolefin polymers into aliphatic carboxylic acids by microwave- assisted oxidation; and converting the aliphatic carboxylic acids into ethanol, methanol and / or formic acid by performing the method as defined in any one of claims 7 to 18.

20. The method of claim 19, wherein the polyolefin polymers are selected from polyethylene, polystyrene, polyvinyl chloride, polypropylene, polymethylpentene, polybutene-1, and polyisobutylene.

21. A method of producing carbon monoxide (CO) and hydrogen (H2), from aliphatic carboxylic acids, the method comprising:contacting in a circulated flow system an aqueous solution comprising the aliphatic carboxylic acids with a photocatalyst comprising GaN and / or InN optionally doped with gold nanoparticles and a co-catalyst in the form of metal or metal oxide, including cobalt;, iron or scandium, and irradiating the photocatalyst with light to activate the photocatalyst and convert the aliphatic carboxylic acids into carbon monoxide (CO)and hydrogen gas (H2).

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