PET Glycolysis by Modulating the Crystal Facets of PW₁₂@UiO-67: Understanding the Activation of Dual Substrates

Polyethylene terephthalate (PET) glycolysis presents an effective solution to address plastic pollution while promoting the utilization of renewable resources. It is highly important to gain in-depth insights into the identification of the well-defined active sites and the structure-activity relationships in PET glycolysis. Herein, PW₁₂@UiO-67 with different exposed crystal facets, i.e., octahedral PW₁₂@UiO-67-[111] and cubic PW₁₂@UiO-67-[001], are successfully synthesized by using a coordination modulation strategy. In situ DRIFTS, XPS, and XAFS characterizations confirmed that, by fine-tuning the dosages of HAc and PW₁₂, PW₁₂@UiO-67-[001], endowed with the abundant exposed PW₁₂ sites and coordination-unsaturated Zr sites, is induced by the missing-linker defects. The PW₁₂@UiO-67-[001] exhibited excellent catalytic performance in PET glycolysis, i.e., >99.9% PET conversion and >95.0% bis(2-hydroxyethyl) terephthalate (BHET) yield, exceeding PW₁₂@UiO-67-[111] (with buried PW₁₂ sites, 69.4% BHET yield). The results of in situ experiments revealed that the PW₁₂ sites can simultaneously facilitate EG adsorption and activation, whereas the Zr sites played the crucial role in PET activation during glycolysis. Characterizations and DFT calculations verified that the abundant exposed PW₁₂ sites exhibited more electron-rich states, thereby optimizing EG adsorption behaviors. Notably, facet engineering-induced missing-linker defects effectively modulated the d-electron configuration of Zr, shifting its d-band center closer to the Fermi level and consequently enhancing its binding affinity with PET. The synergistic effects decreased the energy barriers for the activation of PET and EG, thereby facilitating mass transport and accelerating reaction kinetics. This work provides an atomic-level insight into facet-sensitivity catalytic mechanisms in PET glycolysis.