Tandem methanolysis and catalytic transfer hydrogenolysis of polyethylene terephthalate to p-xylene over Cu/ZnZrOx catalysts

We demonstrate a novel approach of utilizing methanol (CH3OH) for both the methanolysis of polyethylene terephthalate (PET) to form dimethyl terephthalate (DMT) at near-quantitative yields (~97%) and catalytic transfer hydrogenolysis (CTH) of DMT to p-xylene (PX, ~63% at 240 °C and 16 h) on a reducible ZnZrOx supported Cu catalyst (i.e., Cu/ZnZrOx). Pre- and post-reaction surface and bulk characterization, along with density functional theory (DFT) computations, explicate the role of the metal-support interface of Cu/ZnZrOx in providing active sites for the activation of both CH3OH and DMT and facilitating a lower free-energy pathway for both CH3OH dehydrogenation and DMT hydrogenolysis, compared to Cu supported on redox-neutral SiO2 support. DFT calculations further reveal that the rate-determining step for CTH of DMT is the cleavage of the C-O linkages in –(C=O)-OR) of DMT. Loading studies and thermodynamic calculations showed that, under reaction conditions, CH3OH in the gas phase, rather than in the liquid phase, is critical for CTH of DMT. Interestingly, the Cu/ZnZrOx catalyst was also effective for the methanolysis and hydrogenolysis of C-C bonds (compared to C-O bonds for PET) of waste polycarbonate (PC), largely forming xylenol (~38%) and methyl isopropyl anisole (~42%) demonstrating the versatility of this approach toward valorizing a wide range of condensation polymers.