Dual-functional ionic liquid catalysts for efficient photooxidative upcycling of polystyrene to benzoic acid

doi:10.1016/S1872-2067(26)64984-4

Abstract

Abstract:

Polystyrene, a widely used yet chemically inert plastic, poses major recycling challenges due to its low degradability and global recovery rate below 1%, calling for innovative upcycling strategies under mild conditions. Herein, we report a dual-functional ionic liquid catalyst, [BSPy][OTf]-Fe(OTf)₃, that enables room-temperature photooxidative upcycling of PS into benzoic acid with a yield of 76.43% in 24 h under an oxygen atmosphere. This catalytic performance is substantially higher than that of control systems using individual components or their physical mixture, indicating a strong synergistic effect. Mechanistic investigations revealed that Fe³⁺ in the ionic liquid initiates chain scission via a single-electron transfer process, generating oxidized intermediates. These intermediates are subsequently converted to benzoic acid through singlet oxygen, as supported by kinetic isotope effect studies and density functional theory calculations. The generation of singlet oxygen is facilitated by the ionic liquid catalyst, as confirmed by ultraviolet-visible and electron paramagnetic resonance spectroscopy. The approach is applicable to post-consumer Polystyrene products, with benzoic acid yields ranging from 56.01% to 74.57%. This study establishes a mechanistically guided strategy for low-energy, selective upcycling of PS, offering a viable alternative to conventional high-temperature or harsh-oxidant approaches.

Key words: Polystyrene upcycling, Acidic ionic liquid catalyst, Photooxidative degradation, Singlet oxygen, Single-electron transfer, Post-consumer plastic waste

Haoyi Wang, Yankai Zhang, Chunying Si, Yunbiao Qi, Quanxing Zhang, Wei Jiang. Dual-functional ionic liquid catalysts for efficient photooxidative upcycling of polystyrene to benzoic acid[J]. Chinese Journal of Catalysis, 2026, 83: 231-243.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(26)64984-4

https://www.cjcatal.com/EN/Y2026/V83/I4/231

Figures/Tables 11

Scheme 1. (a) Previously reported pathways for PS photooxidation. (b) Photooxidation of alkyl aromatics mediated by BAILs. (c) This work: dual-pathway PS photooxidation enabled by ionic liquid catalyst.

Table 1 Reaction conditions optimization for PS photooxidation with Brønsted acidic ILs a.

Entry	Catalyst ^b (mol%)	Solvent (3 mL)	T (h)	M_w ^c (g/mol)	PDI ^c	Yield ^d (mol%)
1	[BSPy][OTf] (100)	MeCN/CH₂Cl₂(1:2)	24	—^e	—	25.38
2	[BSPy][HSO₄^‒] (100)	MeCN/CH₂Cl₂(1:2)	24	21193	5.76	—
3	[BSPy][OTf] (100)	MeCN	24	260912	5.03	—
4	[BSPy][OTf] (100)	CH₂Cl₂	24	204066	6.32	—
5	[BSPy][OTf] (20)	MeCN/CH₂Cl₂(1:2)	24	—	—	9.21
6	[BSPy][OTf] (50)	MeCN/CH₂Cl₂(1:2)	24	—	—	17.28
7	[BSPy][OTf] (200)	MeCN/CH₂Cl₂(1:2)	24	—	—	28.46
8	[BSPy][OTf] (100)	MeCN/CH₂Cl₂(1:2)	48	—	—	48.35

Fig. 1. (a) Visual comparison of samples with varying Fe3+ loadings and chemical structure. (b) FT-IR spectra of [BSPy][OTf] with different Fe3+ loadings. Spectroscopic comparison of “pure IL” and “Fe-IL”: Raman spectra (c) and XPS survey (d) and high-resolution Fe 2p (e), C 1s (f), O 1s (g), and S 2p (h) spectra.

Fig. 2. (a) Benzoic acid yield with a Fe3+ loading range at 100 mol% [BSPy][OTf] loading. (b) Benzoic acid yield with a [BSPy][OTf] loading range at 2 mol% Fe3+ loading. (c) Catalytic performance comparison between structured catalyst and physical mixture. (d) 3D response surface of benzoic acid yield versus [BSPy][OTf] and Fe(OTf)3. (e) Comparison of PS photooxidation catalytic performance over [BSPy][OTf]-Fe(OTf)3 and catalysts from other literature. The data points, labeled Ref. 1-9 within the plot, are categorized as follows: Transition metal catalysis (blue diamonds), corresponding to: Ref. 1 (FeBr3) [30]; Ref. 2 (FeCl2) [31]; Ref. 3 (FeCl3) [32]. Selective acid catalysis (yellow circles), corresponding to: Ref. 4 (HOTf) [50]; Ref. 5 (PPOP-1) [51]. HAT catalysts (gray triangles), corresponding to: Ref. 6 (NBS) [28]; Ref. 7 (Fluorenone) [29]; Ref. 8 (anthraquinone) [55]; Ref. 9 (Ph-Acr-Ph) [56].

Fig. 3. (a) Molecular weight (Mw)-time and yield-time curves under optimized reaction conditions. (b) GPC characterization of PS photooxidative degradation. (c) Mw evolution of PS over time under different catalysts. (d) Color changes during degradation. (e) Water contact angle measurements. (f) FT-IR characterization of PS photooxidative degradation.

Fig. 4. Investigation of reaction mechanism. (a) Quenching experiment. (b) UV-vis absorption spectrum. (c) EPR spectra under in-situ irradiation at 405 nm with TEMP and DMPO spin traps, respectively. A nitroxyl radical (TEMPO, from 1O2) [g = 2.0056, aiso(14N) = 1.50 mT], an oxygen-centered DMPO adduct (DMPO-•O2-) [g = 2.0057, aiso (14N) = 1.31 mT, aiso (β-1H) = 0.81 mT], an oxygen-centered DMPO adduct (DMPO-•OH) [g = 2.0058, aiso (14N) = 1.55 mT, aiso (β-1H) = 1.55 mT], and a carbon-centered DMPO adduct (DMPO-•R) [g = 2.0055, aiso (14N) = 1.60 mT, aiso (β-1H) = 2.19 mT]. (d) ¹H NMR spectra of PS before and after deuteration; (e) Schematic illustration of 1O2 generation. (f) Mass spectra of benzoic acid products from PS and d-PS photodegradation. (g) Schematic comparison of KIE effects in Fe3+ induced SET and 1O2 oxidation pathways.

Table 2 Summary of deuteration rates in PS and d-PS determined by ¹H NMR.

Peak No.	Chemical shift (δ, ppm)	Assigned proton position	Abbreviation	PS integration	d-PS integration	Deuteration rate
	~3.8	internal standard trimethoxybenzene	IS (6H)	1	1	—
1	6.3-6.6	aromatic proton (para)	Ar-H(1)	1.76	0.2	88.63%
2	6.6-7.3	aromatic protons (meta/ortho)	Ar-H(2)	1.19	0.7	41.17%
3	1.85-2.05	α-hydrogen	α-H	0.93	0.21	77.42%
4	1.35-1.85	β-hydrogen	β-H	1.57	0.62	60.50%

Table 3 Kinetic isotope effect in PS/d-PS degradation.

Entry	Substrate	Product	m/z (‒)	Intensity	Yield (%)	KIE_int^a
1	PS	M1	121.0329	10166	76.4	—
2	D-PS	M2	124.0049	3633	23	10.3
		M3	123.9056
		M4	125.0042

Fig. 5. (a) Gibbs free energy profile and optimized structures for the oxidation of a polystyrene intermediate by 1O2. Energies are given in kcal/mol relative to intermediate 1. Structures were optimized at B3LYP/6-31G(d), energies refined at cc-pVTZ level. (b) Mechanistic pathway for dual functional ionic liquid catalyst enabling cooperative activation through SET and singlet oxygen oxidation in polystyrene upcycling.

Table 4 Photooxidation capacity of post-consumer PS plastic waste by the IL catalyst.

Entry	Name	Molecular weight ^b	Conv.^c(%)	Yield ^d(%)
1	EPS foam waste	M_w = 244439 M_n = 96707	100	66.57
2	Polystyrene culture dish waste from lab	M_w = 241155 M_n = 70543	100	69.54
3	Polystyrene cup waste	M_w = 233715 M_n = 68616	100	74.57
4	Polystyrene food container waste	M_w = 278450 M_n = 95631	100	63.74
5	Polystyrene ruler	M_w = 245133 M_n = 69682	100	63.28
6	Polystyrene mousse cup waste	M_w = 232466 M_n = 79055	100	64.45
7	Polystyrene cup lid waste (white)	M_w = 174668 M_n = 80913	100	60.12
8	Polystyrene cup lid waste (black）	M_w = 174490 M_n = 76745	100	56.01
9	Styrene-butadiene-styrene	M_w = 140579 M_n = 57674 styrene 30 wt.%	100	60.17

Fig. 6. (a) Process for separating and purifying IL catalyst and photooxidation products. (b) FT-IR spectrum of IL catalyst before and after reaction. (c) Reusability of IL catalyst in PS photooxidation. *Cycle 5 was performed after replenishing the catalyst to its initial amount. (d) 1H NMR (DMSO-d6) of recrystallized benzoic acid.

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