(532z) Role of Solid Acid in Low-Temperature Hydrocracking of Polyolefins to Fuels

Plastics waste (PW) is a huge threat to the environment due to accumulation of the PW in landfills with massive leakage into the ocean and soil. Packaging material which is a major contributor to landfills consists of polyolefins (PO), namely polypropylene and polyethylene (PE), which are especially energy-demanding to recycle back to produce monomers. Hydrocracking can be successfully utilized to convert PO to diesel and jet range alkanes. Hydrocracking is conducted at mild conditions in presence of metal-acid bifunctional catalyst. Our recent report shows that rational catalyst design can increase activity and shift selectivity from low-value gasoline to more valuable and sustainable jet fuel. In this work, we aim at understanding how the type and structure of acid component will change the outcome of bifunctional hydrocracking of PE.

Several solid acids were used in this study to elucidate the effect of acid strength, concentration and site accessibility on outcome of hydrocracking. Zeolites BEA, MOR, FAU, MFI, mesoporous Al-MCM-41, WO₃/ZrO₂ were doped with 0.5% Pt as metal component. Reaction was performed in batch system under stirring at 250 °C at different H₂ pressures. Zeolites were modified using desilication. This enables us to compare zeolites with similar acid sites concentration and different porosity.

Our results show that major faction in zeolite-catalyzed hydrocracking is interpore diffusion constraints. Medium-sized reaction intermediates cannot escape porous network without further cracking to small C₅-C₇ fraction. Upon introduction of relatively narrow 3-4 nm mesopores in FAU zeolite the yield of C₁₃₊ alkanes drastically increases. In case of zeolites, initial polymer binding occurs mainly on the outer surface of zeolite crystal.

Our findings provide a blueprint for further improvement in plastic waste hydrocracking and it shows how tailoring of acid component can drastically change overall activity and selectivity.