One-pot photothermal upcycling of polylactic acid to hydrogen and pyruvic acid

doi:10.1016/S1872-2067(23)64638-8

Abstract

Abstract:

While biodegradable polymers such as polylactic acid (PLA) are widely used as alternatives to non-biodegradable polymers to address the plastic crisis, their biodegradation is difficult to control, and the process emits carbon. Conversion of PLA waste into value-added products via thermal and photocatalytic pathways is a promising solution. Herein, we describe a one-pot photothermal catalytic method that efficiently converts PLA into hydrogen and other valuable chemicals without requiring a concentrated KOH solution. Examining the catalytic properties of Pt/A-V-PCN (atomic-layered g-C₃N₄), Pt/TiO₂, and Pt/CdS for the photothermal reforming of lactic acid (LA) indicated superior hydrogen production for all the employed catalysts owing to the promotion effects of external heat on the excitation and utilization of carriers, as characterized by electrochemical impedance spectroscopy Nyquist and transient photocurrent (I-t) tests. We found that the cadmium sulfide (CdS) possesses excellent selectivity for converting LA into pyruvic acid (PyA). Using in situ electrochemical electron spin resonance spectroscopy experiments and density functional theory computations, we found that the high PyA selectivity for LA oxidation on the CdS surface was attributed to the different affinities of the active sites for C- and O-adsorbed species, resulting in enhanced dehydrogenation and hindered C-C cleavage. Furthermore, Pt/CdS demonstrated increased reactivity and achieved 98.1% selectivity for high-value pyruvic acid in liquid products during the one-pot photothermal upcycling of commercial PLA plastics. Hence, this study provides a strategy for developing more efficient catalytic routes for upcycling PLA and other polyesters.

Key words: Plastics upcycling, Photothermal catalysis, H₂ production, Polylactic acid, Pyruvic acid

Yuan Xiang, Jin Zhang, Fei Huang, Nantian Xiao, Yiyi Fan, Junhao Zhang, Heng Zheng, Jinwei Chen, Fan Zhang. One-pot photothermal upcycling of polylactic acid to hydrogen and pyruvic acid[J]. Chinese Journal of Catalysis, 2024, 59: 149-158.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(23)64638-8

https://www.cjcatal.com/EN/Y2024/V59/I4/149

Figures/Tables 6

Fig. 1. HRTEM images of Pt/A-V-PCN (a), Pt/TiO2 (b), and Pt/CdS (c) after photo-deposition, with size distribution of Pt clusters presented in the inset. (d) XRD patterns of Pt/A-V-PCN, Pt/TiO2, and Pt/CdS. Light colors are A-V-PCN, TiO2, and CdS.

Fig. 2. (a) Hydrogen production rates at 10, 30, 60, 90, 120, and 150 °C over Pt/A-V-PCN, Pt/TiO2, and Pt/CdS (Inset: thermal catalytic (150 °C), photocatalytic (25 °C), and photothermal (150 °C) catalytic hydrogen production rates of Pt/A-V-PCN, Pt/TiO2, and Pt/CdS. Conditions: 30 mg catalyst, 135 μL H2PtCl6·6H2O, 9% LA solution, pH = 7.0, 300 W Xe lamp, 987 mW·cm-2, and 1 h). Wavelength-dependent AQY of Pt/A-V-PCN (b), Pt/TiO2 (c) and Pt/CdS (d) under irradiation with and without heating (Conditions: 30 mg catalyst, 135 μL H2PtCl6·6H2O, 9% LA solution, pH = 7.0, 300 W Xe lamp, 987 mW·cm-2, and 1 h).

Fig. 3. Electrochemical impedance spectroscopy and transient photocurrent response of Pt/A-V-PCN (a,b), Pt/TiO2 (c,d), and Pt/CdS (e,f) measured at different temperatures between 10 to 60 °C.

Fig. 4. (a) 1H NMR spectrum after photothermal catalytic reaction. (b) Liquid product selectivity of catalysts after reaction over three catalysts. (c) In situ ESR spectra in the presence of LA and DMPO. (d) Proposed mechanism of photocatalytic hydrogen production under visible-light irradiation of catalyst.

Fig. 5. (a) Optimized structures of the intermediates in LA oxidation on CdS (110) surface. (b) Energy diagram for catalytic LA oxidation. X-axis illustrates the reaction coordination; Y-axis illustrates the relative energy of each stage.

Fig. 6. (a) Distribution of products after upcycling of commercial plastics over Pt/CdS (Conditions: 0.15 g commercial PLA, 30 mg Pt/CdS, 40 mL H2O, 135 μL H2PtCl6·6H2O, NaOH 0.1 mL, He, 150 °C, 300 W Xe lamp, and 987 mW·cm-2). (b) Carbon yield after PLA photothermal reaction (Conditions: 0.15 g PLA, 30 mg Pt/CdS, 40 mL H2O, 135 μL H2PtCl6·6H2O, NaOH 0.1 mL, He, 150 °C, 300 W Xe lamp, and 987 mW·cm-2). (c) Time course of photocatalytic hydrogen production over Pt/CdS. (Conditions: 30 mg CdS, 135 μL H2PtCl6·6H2O, 0.15 g PLA, 40 mL H2O, NaOH 0.1 mL, 300 W Xe lamp, and 987 mW·cm-2). (d) Comparison of photocatalytic production rate of H2 and pyruvic acid in LA solution reported in the literature (I: [40], II: [15], III: [41], IV: [42], details in Table S5).

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