Protocol for photocatalytic upcycling of non-biodegradable plastics into platform chemicals at ambient conditions

Summary

Upcycling plastics presents an opportunity not only to reduce plastic waste, but also to provide an alternative carbon source to fossil fuels. Herein, we present a protocol to upcycle plastics with resin codes 2–7 using a commercially available base-metal photocatalyst. We first conducted batch reactions, followed by a continuous, segmented flow system for gram-scale upcycling into value-added platform chemicals. This protocol, employing tandem carbon-hydrogen bond oxidation/carbon-carbon bond cleavage reactions, can be useful for photocatalytically transforming plastics at ambient conditions.

For complete details on the use and execution of this protocol, please refer to Li et al. (2023).¹

Graphical abstract

Highlights

• •Protocol for ambient aerobic photocatalytic upcycling of plastics with resin labels 2–7
• •Detailed protocol for setting up batch reactions and gram-scale continuous flow reaction
• •Procedures to determine product yields and for kinetic studies
• •Transformation of plastics into platform chemicals, promoting circular economy

Publisher’s note: Undertaking any experimental protocol requires adherence to local institutional guidelines for laboratory safety and ethics.

Upcycling plastics presents an opportunity not only to reduce plastic waste, but also to provide an alternative carbon source to fossil fuels. Herein, we present a protocol to upcycle plastics with resin codes 2–7 using a commercially available base-metal photocatalyst. We first conducted batch reactions, followed by a continuous, segmented flow system for gram-scale upcycling into value-added platform chemicals. This protocol, employing tandem carbon-hydrogen bond oxidation/carbon-carbon bond cleavage reactions, can be useful for photocatalytically transforming plastics at ambient conditions.

Before you begin

The protocol below describes the photocatalytic conversion of non-biodegradable plastics into useful platform chemicals such as formic acid, acetic acid, benzoic acid, and acetophenone. This photocatalytic upcycling of plastics can take place under ambient conditions – room temperature and atmospheric pressure, utilizing visible light from light emitting diodes (LEDs) as the sole source of input energy to drive the reactions. Before one begins, the following preparations are necessary.

Installation of LED plate base board on a heat sink equipped with a case fan

Timing: 3 h

• 1.
  Attach a LED plate base board on heat sink equipped with a case fan.

  • a.
    Screw the case fan to the bottom of the heat sink.
  • b.
    Apply thermal paste on the surface of the heat sink.
  • c.
    Place the LED plate base board on the thermal paste.
  • d.
    Screw the LED plate base board onto the heat sink.
• 2.
  Soldering wires onto the LED plate base board.

  • a.
    Expose the copper contacts of the wire by cutting off the rubber cladding at the end of the wire.
  • b.
    Heat up the soldering iron and clean the tip.
  • c.
    Place the exposed copper contact on the LED plate base board contact and melt solder onto the contacts.
  • d.
    Continue adding solder until the wire is connected well to the LED light (Figure 1).
• 3.Connect the wires of the LED plate base board contact and the case fan to the connector of power plug.

Figure 1.

LED setup with heat sink and case fan

(A) Photograph of the LED equipped with a heat sink and a case fan and its respective (B) top view, (C) side view, and (D) bottom view.

Key resources table

REAGENT or RESOURCE	SOURCE	IDENTIFIER
Chemicals, peptides, and recombinant proteins
2,2′-Azobisisobutyronitrile	Sinopharm Chemical Reagent	CAS: 78-67-1
2,3-Dimethyl-2-butanol	Sigma-Aldrich	CAS: 594-60-5
2,3-Dimethylbutane	Tokyo Chemical Industry Co., Ltd.	CAS: 79-29-8
4-Trifluoromethylstyrene	Apollo	CAS: 402-50-6
Acetophenone	Merck	CAS: 98-86-2
Air gas cylinder	Leeden National Oxygen Ltd.	CAS: 132259-10-0
Argon gas cylinder (99.99%)	Leeden National Oxygen Ltd.	CAS: 7440-37-1
Benzoic acid	Fluorochem	CAS: 65-85-0
Caesium carbonate	Alfa Aesar	CAS: 534-17-8
Deuterated dichloromethane	Cambridge Isotope Laboratories	CAS: 1665-00-5
Deuterium oxide	Cambridge Isotope Laboratories	CAS: 7789-20-0
Dichloromethane (DCM), HPLC	J.T. Baker	CAS: 75-09-2
Diethyl ether	Fisher Scientific	CAS: 60-29-7
Ethylene vinylacetate (54,500 g/mol)	Sigma-Aldrich	CAS: 24937-78-8
High density polyethylene (139,000 g/mol)	Sigma-Aldrich	CAS: 9002-88-4
Hydrogen chloride	Sigma-Aldrich	CAS: 7647-01-0
Liquid nitrogen	Air Products	https://www.airproducts.com.sg/gases/nitrogen
Low density polyethylene (183,000 g/mol)	Sigma-Aldrich	CAS: 9002-88-4
Methanol	Fisher Scientific	CAS: 67-56-1
Nitrobenzene	Sigma-Aldrich	CAS: 98-95-3
Nitrogen gas cylinder (99.99%)	Leeden National Oxygen Ltd.	CAS: 7727-37-9
Oxygen gas cylinder (99.8%)	Leeden National Oxygen Ltd.	CAS: 7782-44-7
Pivalic acid	Alfa Aesar	CAS: 75-98-9
Polypropylene (22,900 g/mol)	Sigma-Aldrich	CAS: 9003-07-0
Polypropylene takeaway box	Nanyang Technological University recycling bin	N/A
Polypropylene-ethylene vinyl alcohol-propylene multilayer packaging sheet	Desu Plastics	N/A
Polystyrene (35,000 g/mol)	Sigma-Aldrich	CAS: 9003-53-6
Polystyrene-ethylene vinyl alcohol-polystyrene yogurt container	Nanyang Technological University recycling bin	N/A
Polyvinyl acetate (43,200 g/mol)	Sigma-Aldrich	CAS: 9003-20-7
Polyvinyl chloride (92,200 g/mol)	Sigma-Aldrich	CAS: 9002-86-2
Pure medical oxygen gas cylinder (≥99.0%)	Leeden National Oxygen Ltd.	CAS: 7782-44-7
Sec-butylbenzene	Sigma-Aldrich	CAS: 135-98-8
Sodium hydroxide	VWR Chemicals	CAS: 1310-73-2
Sodium sulfate	Unichem	CAS: 7757-82-6
Styrofoam containers	Nanyang Technological University recycling bin	N/A
Tetrahydrofuran	Fisher Scientific	CAS: 109-99-9
Vanadium(V) oxide	Sigma-Aldrich	CAS: 1314-62-1
Vanadyl acetylacetonate	Sigma-Aldrich	CAS: 13476-99-8
Vanadyl isopropoxide	Sigma-Aldrich	CAS: 5588-84-1
Software and algorithms
Gaussian	Gaussian, Inc.	Gaussian 16c
Mnova	Mestrelab Research	Mnova 14.2
OriginPro	OriginLab	Origin 2021
SPECS	SPECS	SPECS GmbH
XPSPEAK	Raymund Kwok	Version 4.1
Others
12 V/50 W 6000 K white LED	Tmall	https://detail.tmall.com/item.htm?spm=a1z0d.6639537/tb.1997196601.114.2cf574847WcvGL&id=601986959362&sku_properties=13381687:10122
12 V/50 W sapphire blue LED	Tmall	https://detail.tmall.com/item.htm?spm=a1z0d.6639537/tb.1997196601.114.2cf574847WcvGL&id=601986959362&sku_properties=13381687:10122
Acid digestion vessel	Parr Instrument Company	Model 4748, 125 mL
Aluminum heat sink	Tmall	https://detail.tmall.com/item.htm?abbucket=20&id=681566500433&ns=1&spm=a21n57.1.0.0.204e523czhRuTL
Ball mill machine	Retsch	MM400
Balloons	Tmall	https://m.tb.cn/h.565RoKr?tk=nVA0WbFNI9w
Case fan	Tmall	https://detail.tmall.com/item.htm?_u=7283072c395b&id=617283045473&spm=a1z09.2.0.0.4db92e8dB9IDVz
Centrifuge	Thermo Scientific	Sorvall Legend X1
Differential refractometer	Shimadzu	RID-20A
Electron paramagnetic resonance spectrometer	Bruker BioSpin	ELEXSYS-IIE500 EPR
Elemental analyzer	PerkinElmer	2400 Series II CHNS/O
Fiber optic spectrometer	Ocean Optics	USB4000
FT-IR spectrometer	PerkinElmer	Spectrum 100
Fused silica column	Restek	RTX-5 fused silica column (30 m; 0.53 mm ID)
Gas chromatograph	Shimadzu	GC-2010 Plus
Gel permeation chromatograph	Agilent	PL-GPC 220
Glovebox	Vacuum Atmospheres Company	OMNI-LAB glovebox
LED plate base board	Linfu Electronics Inc.	LFS0812GL
Liquid chromatograph	Shimadzu	LC-2030C Plus
MALDI-TOF mass spectrometer	Applied Biosystems	ABI 4800
Mass flow controller	Masterflex	32907-55
Mixed gel column	Shodex	KF-804L mixed gel column (300 × 8.0 mm; bead size = 7 μm; pore size = 1500 Å)
Mixed gel column	Shodex	LF-804 mixed gel column (300 × 8.0 mm; bead size = 6 μm; pore size = 3000 Å)
Mixed-B column	Agilent	PLgel Mixed-B column (7.5 mm × 300 mm)
Mixed-B guard column	Agilent	PLgel Mixed-B Guard column (7.5 mm × 50 mm)
Nuclear magnetic resonance spectrometer	JEOL	ECA400SL 400 MHz spectrometer
Oven	Memmert	UNB500
Packed molecular sieves column	Restek	MS-5A 60/80 2.0 m 2.0 mm ID 1/8 in OD packed molecular sieves column
Peristaltic pump	Masterflex	07522-20
Poly(tetrafluoroethylene) syringe filter	Porefil	Hydrophobic; 13 mm; pore size = 0.45 μm
PoraPLOT column	Restek	PP-Q 80/100 2.0 m 2.0 mm ID 1/8 in OD PoraPLOT column
Programmable logic controller	Siemens	S7-200 SMART CPU SR20
Quartz coil	Customed made	Outer diameter = 5 mm; inner diameter = 3 mm; 20 coils
Reaction tube (15 mL)	Synthware	Schlenk tube, 15 mL
Schlenk cuvette	Custom made	Path length = 10 mm; PTFE tap with one gas outlet
Temperature controller	PolyScience	9106A12P
Thermal paste	Dow Corning	DC340
T-junction mixer	Cole-Parmer	01356-02 solenoid valve
UV-Vis-NIR spectrometer	Shimadzu	UV-3600
X-ray photoelectron spectrometer	SPECS	PHOIBOS 150

Step-by-step method details

Photocatalytic conversion of plastics in batch reactions

Timing: 4–7 days

The conversion of non-biodegradable plastics into useful platform chemicals is achieved through base-metal photocatalysis by following these steps.

Note: Polystyrene (PS), polyvinyl acetate (PVAc), polyvinyl chloride (PVC), and ethylene vinyl acetate (EVA) are readily soluble in DCM, whereas polypropylene (PP), low-density polyethylene (LDPE), high-density polyethylene (HDPE), and PP-ethylene vinyl alcohol-PP (PP-EVOH-PP) are poorly soluble in DCM.

• 1.
  For the polymers that are soluble in DCM (PS, PVAc, PVC, and EVA), the polymers can be used as received without pretreatment.

  • a.
    Weigh 144 μmol of the single repeat unit of the plastic resin into an oven-dried 15 mL reaction tube.
• 2.
  Prepare a stock solution of the vanadium acetylacetonate (V(O)(acac)₂) photocatalyst.

  • a.
    Add 7.6 mg of V(O)(acac)₂ to a conical flask.
  • b.
    Add 70 mL of dichloromethane (HPLC grade) to the conical flask.
  • c.
    Stir the resulting mixture to achieve a homogenous solution.
• 3.Add 7 mL of the V(O)(acac)₂ stock solution to a reaction tube containing the plastic resin.
• 4.
  Prepare the balloon adapter.

  • a.
    Cut around an inch of laboratory tubing.
  • b.
    Attach the laboratory tubing to the glass stopcock adapter.
  • c.
    Attach two layers of balloons to the tubing and secure it with a rubber band.
• 5.Fill up the balloon with oxygen (O₂) and secure the balloon to the reaction tube to flush the reaction tube with O₂ in order to create an O₂-rich atmosphere for photocatalytic reactions. Secure the reaction tube with a clamp (Figure 2A).

Note: Inflate the balloons to similar sizes to ensure that the reaction mixtures are saturated with O₂.

• 6.Immerse the reaction tube into a beaker containing 250 mL of water in the batch reaction set-up to maintain the reactions at ambient temperature (24–29°C).
• 7.Turn on the LED lights and stirrer for the photocatalytic reactions to proceed for 4–7 days. Figure 2B shows the set-up for the batch reactions.
• 8.
  For the polymers that are poorly soluble in DCM (PP, LDPE, HDPE, PP-EVOH-PP), pre-treatments of the plastics are necessary.

  • a.
    Weigh 1.44 mmol of the single repeat unit of the plastic resin into a stainless-steel autoclave with a Teflon liner.
  • b.
    Add 60 mL of DCM to the autoclave.
  • c.
    Heat the autoclave to 110°C for 20 min.

    Note: Elevated temperature and pressure experiment with a non-flammable but volatile organic solvent.

  • d.
    Let the autoclave cool down naturally to room temperature (<30°C).
  • e.
    Pour the well-dispersed plastic colloidal suspension solution into a conical flask.
  • f.
    Add 7.6 mg of V(O)(acac)₂ photocatalyst to the conical flask.
  • g.
    Add 6 mL of the resulting mixture into an oven-dried 15 mL reaction tube.
• 9.Repeat Step 4–7 to set up the photocatalytic reactions in batch.

Figure 2.

Photocatalytic setup for plastic conversion in batch reactions

(A) A photograph of the reaction tube attached to an O₂-filled balloon. The liquid phase contains the V(O)(acac)₂ photocatalyst and the highly soluble PS in DCM.

(B) A photograph of the photocatalytic reaction set-up in batch. A piece of aluminum foil is placed in the gap between the LEDs to ensure good reflection of light onto the sides of the reaction tubes and also prevent light interference from adjacent LEDs.

Product analysis by ¹H nuclear magnetic resonance (¹H NMR)

Timing: 1 day

• 10.
  After the end of the photocatalytic reactions, add 24.5 μmol of pivalic acid as an internal standard into the reaction tube.

  • a.
    Stir the mixture thoroughly to ensure a homogeneous solution.
• 11.
  Take 0.5 mL aliquot out from the resulting mixture using a micropipette.

  • a.
    Transfer the aliquot to an NMR tube of 176 mm (length) x 5 mm (diameter).
• 12.
  Analyze the sample using ¹H NMR spectroscopy in the presence of the DCM solvent.

  Note: No rotary evaporation is necessary for solvent removal due to the volatile nature of formic and acetic acid. The sample can be directly analyzed without the need for any deuterated solvent since the product peaks are sufficiently separated from the solvent signals.

  • a.
    Conduct the ¹H NMR analyses on a JEOL ECA400 (ECA400SL) 400 MHz spectrometer.
  • b.
    Collect the spectra of the sample (Figure 3).
  • c.
    Calibrate the chemical shift for the standard residual undeuterated solvent signal in ¹H NMR spectroscopy at 5.33 ppm for DCM.

    Note: Single peaks appear at 1.19 ppm, 2.07 ppm, and 8.00 ppm, corresponding to pivalic acid, acetic acid, and formic acid, respectively.

• 13.
  Isolate carboxylates from the remaining solution in the reaction tube.

  • a.
    Dilute the remaining solution by adding 20 mL of DCM.
  • b.
    Extract the organic phase with 1 M aqueous NaOH.
  • c.
    Repeat the extraction three times.
• 14.
  Recover the remaining oligomers in the organic phase.

  • a.
    Add 1.0 M aqueous HCl.
  • b.
    Separate the DCM layer.
  • c.
    Remove the remaining DCM by drying in vacuum.
  • d.
    Label the resulting sample as “soluble oligomers”.

Figure 3.

1H NMR spectra for the aliquot collected from the photocatalytic transformation of EVA

(Top) The complete 1H NMR spectrum. (Bottom) To omit the intense DCM signal, an enlarged spectrum with an axis break is included. For quantification of product formation, 24.5 μmol of pivalic acid was added as the internal standard.

Gram-scale photoconversion of plastics in a continuous flow system

Timing: 5–7 days

This part describes the photocatalytic conversion of plastic waste on a gram-scale basis to investigate the scalability of our protocol. In this flow system, the concentration of the plastic waste increased by about nine-fold. Only plastics that are readily dissolved in the DCM were used to avoid clogging up the photoreactor and building up pressure in the system. The design of the photocatalytic system follows several criteria: i) compact design of the photoreactor using a horizontal spiral of quartz coils for efficient use of space, while also providing sufficient residence time under light illumination; ii) coils with a smaller diameter allow a higher photon flux to reach the reaction mixture due to a shorter irradiation path length; and iii) segmented flow creates more gas-liquid interfaces for better accessibility to O₂.

• 15.
  Customize a horizontal spiral of quartz coils.

  • a.
    The specifications for the quartz coils: inner diameter (ID) of 3 mm; outer diameter (OD) of 5 mm; 20 coils in total.
• 16.
  Recover a post-consumer Styrofoam container and a PS-EVOH-PS yoghurt container from a recycling bin in Nanyang Technological University, Singapore.

  • a.
    Wash the plastic containers and allow it to dry naturally.
  • b.
    Cut the plastic containers into pieces.
• 17.
  Preparation of reaction mixture for up-scale photocatalytic conversion of plastic waste.

  • a.
    Weigh 0.96 g of plastic waste pieces and transfer them to a round bottom flask.
  • b.
    Add 49 mg of V(O)(acac)₂ photocatalyst into a conical flask containing 50 mL of DCM.
  • c.
    Pour the solution containing the V(O)(acac)₂ photocatalyst into a round bottom flask containing the plastic waste.

    Note: Styrofoam container dissolves immediately on contact with DCM.

  • d.
    Swirl the mixture to form a homogeneous solution.
• 18.
  Photocatalytic upcycling of plastics in a continuous system with segmented flow for improved gas-liquid mixing and O₂ saturation in liquid phase (Figure 4).

  • a.
    Transfer the reaction mixture to a 25 mL jacketed reservoir using a peristaltic pump at a flow rate of 4.6 mL/min.
  • b.
    Stir the reaction mixture in the jacketed reservoir consistently.
  • c.
    Circulate the reaction mixture to the T-junction mixer using the same peristaltic pump at a flow rate of 0.80 mL/min.
  • d.
    Direct high-purity medical O₂ gas to the T-junction mixer using a mass flow controller.
  • e.
    Link the mass flow controller and peristaltic pump using a programmable logic controller (PLC) system.
  • f.
    Direct the gas-liquid mixture from the outlet of T-junction mixer to the inlet of the horizontal spiral of quartz coils.
  • g.
    Place nine units of LED on the top of the horizontal spiral of quartz coils, which serves as a photoreactor.

    Note: To avoid excessive evaporation of DCM due to the heat generated from LEDs, place the horizontal spiral of quartz coils on an aluminum cooling block for heat dissipation. A table fan can be used to cool down the reaction mixture.

  • h.
    Shield the photoreactor with black covers to minimize the light interference from the surroundings.
  • i.
    Install a needle valve at the outlet of the photoreactor to regulate the pressure in the photoreactor.
  • j.
    After exiting the photoreactor, direct the gas-liquid segmented mixture back into the reservoir to achieve a closed-loop, continuous flow process.

    Note: Cool the reaction mixture in the reservoir using running cooling water at 26°C supplied from a temperature controller. Allow the cooling water to flow through the jacket of the reservoir for heat exchange.

• 19.Collect 0.5 mL aliquots from the reservoir and perform 1H NMR spectroscopic analyses as mentioned in Step 12.

Figure 4.

Flow reactor system for gram-scale photocatalytic conversion of plastic

(A) A photograph of the continuous, segmented flow system for the photocatalytic conversion of plastics on a gram-scale.

(B) A photograph of the photoreactor, which consists of nine units of LEDs placed on top of the horizontal spiral of quartz coils.

(C) A photograph showing the horizontal spiral of quartz coils with segmented flow of a gas-liquid mixture.

Kinetic studies with gas chromatography

Timing: 5 days

This part focuses on the kinetic studies to monitor the progress of PS conversion and investigate the rate laws governing each reaction parameter. To facilitate the rate law investigations, we selected sec-butylbenzene as the model compound for PS owing to the slow conversion of PS, which requires a few days, and the impracticality of tracking polymer species via gas chromatography (GC). In the kinetic studies, nitrobenzene serves as the ideal internal standard, as it maintains chemical inertness under the prevailing conditions and exhibits efficient vaporization in GC.

• 20.
  A control experiment was conducted to show that nitrobenzene could be used as an internal standard without reacting as follows:

  • a.
    Add 3 μL nitrobenzene and 0.3 mg V(O)acac₂ in 1 mL CD₂Cl₂ in an oven-dried reaction tube.
  • b.
    Bubble the solution with O₂ for 30 s.
  • c.
    Fill up a balloon with O₂.
  • d.
    Secure the balloon to the reaction tube.
  • e.
    Flush the reaction tube with O₂ from the balloon.
  • f.
    Stir the reaction mixture continuously and keep the reaction tube within a beaker containing 250 mL of deionized water at about 25°C.
  • g.
    Switch on LEDs for 10 h.
  • h.
    Transfer the sample into an NMR tube for the ¹H NMR spectroscopic experiment.
  • i.
    The ¹H NMR spectrum showed no new signal from the decomposition of nitrobenzene.
• 21.
  Set up photoreactions using sec-butylbenzene as the substrate because the photoconversion of PS is slow. Since sec-butylbenzene is a model compound for PS, the conversion is relatively fast compared to PS. Specifications of the amount of sec-butylbenzene added, catalyst loading, number of LED(s), and the atmosphere used are shown in Table 1.

  • a.
    Add the specified amount of sec-butylbenzene and V(O)(acac)₂ photocatalyst to an oven-dried 15 mL reaction tube.
  • b.
    Add 7 mL of DCM containing 72 μmol of nitrobenzene into the reaction tube.
  • c.
    Fill up a balloon with O₂.
  • d.
    Secure the balloon to the reaction tube.
  • e.
    Flush the reaction tube with O₂ from the balloon.
  • f.
    Stir the reaction mixture continuously and keep the reaction tube within a beaker containing 250 mL of deionized water at about 25°C.
  • g.
    Switch on LEDs for 8 h.
  • h.
    Collect samples (200 μL aliquot) at 2 h intervals.
  • i.
    Repeat the experiments three times.

Note: Use the same set of two LEDs as the reaction set-up for plastic conversion. Cover 25%, 50%, and 75% of the light dots of both LEDs using aluminum foils to acquire 0.5, 1, and 1.5 LEDs, respectively.

• 22.
  Analyze the collected samples using a GC equipped with a flame ionization detector (FID) held at 50–180°C with a heating rate of 20°C/min, a hold time of 7 min, and a flow rate of 5 mL/min.

  • a.
    Inject the samples in a split mode (split ratio = 20) at 200°C.
  • b.
    Detect the products using a Shimadzu FID at 290°C.
• 23.
  Precise quantification of products generated from photocatalytic conversion of sec-butylbenzene requires calibration curves for the respective products.

  • a.
    Each calibration curve establishes a linear relationship between product concentrations and the peak areas detected by GC. Therefore, the concentration of products formed in the photoreaction can be identified based on the peak area observed in GC. Similarly, the consumption of the substrate can also be determined from the peak area observed in GC.
  • b.
    Prepare five different concentrations of commercial sec-butylbenzene, nitrobenzene, benzoic acid, and acetophenone.
  • c.
    Use Origin 2021 to determine the linear fits of each calibration curve. The calibration curves are shown in Figures 5A–5C.
  • d.
    Figures 5D–5F show the GC traces. The concentrations of substrates and products are obtained using the calibration curves.
• 24.
  Determine the rate order of the reactants using Equation 1.

  • a.
    Plot the graph of the initial reaction rate against varying conditions to obtain the rate order of the reaction.

$$-\frac{d\left[P\right]}{dt}=k\left[P\right]\left[O_2\right]\left[catalyst\right]\left[hv\right]$$ (Equation 1)

Table 1.

Parameters for the rate law studies

Entry	Sec-butylbenzene (μmol mL−1)	Catalyst loading (mol %)	No. of LED lights	Atmosphere
1	0	2	2	O2
2	10.3	2	2	O2
3	17.5	2	2	O2
4	20.6	2	2	O2
5	41.1	2	2	O2
6	20.6	2	2	Ar
7	20.6	2	2	Air
8	20.6	0	2	O2
9	20.6	1	2	O2
10	20.6	3	2	O2
11	20.6	4	2	O2
12	20.6	5	2	O2
13	20.6	2	0	O2
14	20.6	2	0.5	O2
15	20.6	2	1	O2
16	20.6	2	1.5	O2

Figure 5.

Kinetic studies for the photo-oxidation reactions via GC analysis

Calibration curves of (A) sec-butylbenzene, (B) benzoic acid, and (C) acetophenone versus an internal standard, nitrobenzene.

(D) An example of a full GC trace of kinetic study, (E) an example of the integration area for the kinetic study of the sec-butylbenzene model compound, and (F) an example of the integration area for the kinetic study of the PS conversion.

Expected outcomes

By following this protocol, it is expected that a series of non-biodegradable plastics, including conventional polymers, multilayered packaging, and post-consumer polymers, covering resin codes 2-7, can be photocatalytically upcycled into useful platform chemicals, including formic acid, acetophenone, benzoic acid, and acetic acid. Using our customized continuous flow reactor, yoghurt containers made from PS-EVOH-PS and Styrofoam containers can be processed on a gram-scale.

Limitations

Our developed continuous flow system can only process plastics with excellent solubility in DCM. Poorly soluble plastics such as PP, LDPE, HDPE, and PP-EVOH-PP cannot be applied in this system because the solid particles might clog the polytetrafluoroethylene (PTFE) tubings and quartz coils, leading to pressure buildup and eventual damage to the system.

Troubleshooting

Problem 1

The reaction product, such as formic acid, is volatile. If the collected samples are left at ambient conditions for extended periods, some of the products might evaporate, leading to lower product yields according to the ¹H NMR analyses.

Potential solution

The collected samples should be sent for ¹H NMR analyses as soon as they are collected. In instances where immediate NMR analysis is not possible due to equipment unavailability, store the samples in a freezer at −20°C.

Problem 2

Some DCM and formic acid might evaporate due to the heat dissipated by the LEDs during the reaction.

Potential solution

For a batch reaction, keep the reaction tubes in a water bath for cooling purposes. Ensure timely topping up of water in the bath to maintain an optimal cooling environment. For continuous flow reactions, incorporate a powerful fan directed toward the reactor and an aluminum fin for effective heat dissipation. In addition, ensure a continuous flow of cooling water to the reservoir for constant heat exchange.

Problem 3

Due to the high solubility of most plastics in DCM, some fittings and connectors in the flow system might be damaged by DCM, leading to leaks.

Potential solution

Use stainless steel connectors and fittings whenever possible. If the use of plastic connectors is unavoidable due to its advantageous properties of elasticity for connection, opt for plastic connectors made of PTFE exclusively.

Resource availability

Lead contact

Further information and requests for resources and reagents should be directed to and will be fulfilled by the lead contact, Han Sen Soo (hansen@ntu.edu.sg).

Technical contact

Technical questions about this protocol should be directed to the technical contact, Chenfei Li (chenfeili@sjtu.edu.cn).

Materials availability

This study did not generate new unique reagents.

Data and code availability

This study did not generate any datasets.