(305a) H2-Free Conversion of Condensation Polymers with Organic H2 Carriers - Kinetic Coupling of Hydrogenolysis and Dehydrogenation Pathways

Hitherto catalytic approaches to maintain resource circularity of plastics utilize the learnings in hydrocarbon chemistries and often use noble metals for the C-C bond cleavage for converting polymers into chemicals, and fuels. Reductive methods requiring high-pressure H₂ (>20 bar) have emerged as promising low-temperature routes for their chemical deconstruction. However, H₂ is expensive and derived from fossil sources. In 2021, only 1% of H₂ was classified as low emissions. As such, H₂-free routes are desired to deconstruct plastic wastes effectively and sustainably. H₂-free approaches for condensation polymers, of which the polyester, polyethylene terephthalate (PET) alone comprises of 10% of the total waste, includes chemolysis (e.g.,alcoholysis) of ester bonds with the use of excess solvents (e.g.,methanol) and homogenous catalysts.

In this work, we utilized CH₃OH as a solvent and H-donor on metal-oxide interfaces of Cu metal on reducible metal-oxide (e.g.,Zn_0.2Zr_0.8O₂) to kinetically couple dehydrogenation of CH₃OH and hydrogenolysis of dimethyl terephthalate (DMT) to p-xylene. We investigate the role of direct and indirect H-transfer with reactivity analysis and density functional theory (DFT) computations. This was coupled with microscopic and spectroscopic investigation with TEM and SEM, XPS, EPR, and H₂-TPR to explicate the role played by the interface on the mechanism and kinetic coupling.

Table 1 shows that the interface of Cu on a Zn_0.2Zr_0.8O_2-ö support (not Cu or the Zn_0.2Zr_0.8O_2-ö support alone) shows activity for the hydrogenolysis of DMT to p-xylene. Our mechanistic hypothesis is further supported by the fact that under our reaction conditions where CH₃OH is in the gas-phase, the duality of CH₃OH is broken, and it functions solely as a H₂ carrier (Figure 1).

Table 1. Catalytic activity for CH₃OH dehydrogenation, DMT conversion, and H₂ formation in the gas-phase.

Catalyst	CH3OH consumption	DMT conversion	Final H2 formation
Cu/SiO2	18%	0%	3%
Zn0.2Zr0.8O2	2%	0%	0%
Cu/Zn0.2Zr0.8O2	46%	100%	21%