Binuclear Metal-Based Covalent Organic Framework Mimicking Metallohydrolases for Direct Photoreforming of PET Plastic.

Abstract

Direct photoreforming of waste poly(ethylene terephthalate) (PET) plastic into high-value-added chemicals is a promising method that improves resource utilization. However, the traditional method for PET depolymerization was mostly harsh alkali pretreatment (C_OH^- = 5-10 M), which largely impeded direct reforming due to the diversity of the conditions and thus caused inefficiency and discontinuity for plastic upgrading. Binuclear metallohydrolases have been proven to cause maximal acceleration of hydrolytic processes via the proximity effect under mild conditions but are unable to realize further reforming reaction due to monofunctionality. Integrating binuclear metallohydrolases and reforming active units into covalent organic frameworks (COFs) will be a promising solution to resolve the above issues. Herein, we develop two Robson-type binuclear metal-based COFs (ZnZn-Salen-Ni COF and CuCu-Salen-Ni COF) by introducing binuclear mimicking enzymatic sites and water photolysis sites to realize direct photoreforming of PET under mild alkaline conditions without chemical pretreatment. The integrated system successfully utilized ethylene glycol intermediates from depolymerization to promote high-value-added chemicals (formic acid, 421.46 μmol g_cat^-1 h^-1) and hydrogen (923.25 μmol g_cat^-1 h^-1) production with the one-pot-one-catalyst method over ZnZn-Salen-Ni COF and achieved an overall specific activity of 0.125 g_PET g_cat^-1 h^-1, with ∼100% conversion toward PET photoreforming. The mechanism of PET depolymerization and intermediates promoting hydrogen evolution was studied through density functional theory calculations. This work provides a new idea and a sustainable route for the rational design and development of function-integrated materials for upgrading plastic waste into value-added chemicals.