(628h) Utilizing Metal-Impregnated Hierarchical Zeolites for the Selective Hydrogenation of Long-Chain Ester Wax Generated in Polyurethane Upcycling

Polyurethane (PU) stands as one of the most widely utilized polymers, boasting a global production volume of approximately 30 million metric tons. Amidst the chemical recycling and upcycling processes of PU, a notable byproduct emerges in the form of long-chain ester waxes, constituting 5-20 wt.% of the total output. Within the PU structure, the urethane component plays a pivotal role in the decomposition pathway. Analysis of bond dissociation energies reveals the propensity for the cleavage of the C-N bond in urethane as a primary event. However, certain C-O bonds may persist, giving rise to the formation of long-chain ester waxes as a consequential byproduct.

Due to the limited diffusion of long-chain esters into small pores, the subsequent transformation of such waxes presents a significant challenge. To enhance the feasibility of the upcycling process, it becomes vital to further decompose PU-generated long-chain ester waxes into valuable products. One promising avenue for this decomposition involves hydrogenation reactions, where C-O bonds can be broken, yielding alcohols and glycols.

H-ZSM-5 served as the catalyst for hydrogenating ester waxes at 450 ℃, yielding alcohols and glycols as the primary products. However, a notable quantity of waxes persisted under these conditions. To enhance the interaction between the catalysts and waxes, hierarchical MFI type zeolites with high surface areas were synthesized. This increase in surface area facilitated a reduction in wax output from the reaction significantly. For further development modifications to both catalysts and reaction conditions are necessary to minimize wax output. Hence, various metal precursors, such as palladium, ruthenium, and cobalt, can be incorporated into both commercial H-ZSM-5 and synthesized hierarchical MFI zeolites to enhance the rate of hydrogenation reaction. Moreover, the influence of hydrogen concentration on the reaction is investigated. All reactions are conducted at atmospheric pressure and temperatures ranging from 200 to 500 ℃.