Efficient catalytic conversion of polyethylene terephthalate to dimethyl terephthalate over mesoporous beta zeolite supported zinc oxide

The escalating global plastics crisis, exemplified by the >31 million tons of PET production in 2019, necessitates efficient recycling strategies as PET waste persists as harmful microplastics. Among various recycling approaches, catalytic methanolysis to dimethyl terephthalate (DMT) shows particular promise due to its direct integration into PET synthesis cycles. While supercritical methanol processes (9.0–11.0 MPa, 260–270 ℃) achieve high conversion (> 99.9%), their substantial energy requirements without catalysts limit industrial viability. Similarly, homogeneous metal acetate catalysts, despite their high activity, face separation challenges that impede practical implementation.

In this work, we report the synthesis of a hierarchically porous Zn-Beta-meso catalyst via facile impregnation. Comprehensive characterization confirmed the catalyst structure: XRD patterns exhibited characteristic *BEA framework peaks without ZnO diffraction signals (31.7°, 34.4°, 36.2°), while XPS analysis revealed Zn²⁺ species (Zn2p peaks at 1045.0 and 1022.2 eV). N₂ physisorption demonstrated dual micro-mesoporous architecture through sharp uptakes at P/P₀ < 0.05 and 0.4-0.8. The reduced surface area and pore volume post-Zn loading, coupled with STEM-EDS mapping, confirmed highly dispersed Zn species within the hierarchical pore network.

Under optimized conditions (180℃), the catalyst achieved quantitative PET conversion with exceptional DMT selectivity (>99.9%). Comparative studies revealed the synergistic effect between Zn species and mesoporosity: H-Beta-meso and ZnO yielded <1% and 72% DMT respectively, while microporous Zn-Beta, Zn-ZSM-5, and Zn-Y showed 61-86% yields. The catalyst demonstrated remarkable versatility across various PET substrates, including pigmented bottles, polyester fabric, adhesive tape, and soundproofing cotton (>99% conversion, >99% DMT yield).

Mechanistic investigations utilizing BHET dimer as a model compound elucidated the reaction pathway. Initial simultaneous methanolysis of terminal and internal ester bonds (k = 0.028 and 0.037 min^-1) generates MHET intermediates, followed by rate-determining conversion to DMT (k = 0.018 min^-1). Despite literature suggestions of acid site catalysis, in-situ FTIR studies using 2,4,6-tri-tert-butylpyridine and kinetic correlations identified Zn species as the primary active centers.

The catalyst maintained excellent stability through three cycles (>99% DMT yield) with slight deactivation in the fourth cycle (91% yield) attributed to coking (8.4 wt% mass loss above 300℃). Full activity was restored through calcination at 550°C. Hot filtration experiments confirmed the heterogeneous nature of the active species, with negligible contribution from leached Zn species.

This work establishes an efficient catalytic system for PET chemical recycling while providing molecular-level mechanistic insights. The demonstrated efficiency, versatility, and recyclability offer promising directions for industrial-scale PET chemical upcycling.

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