Catalytic pyrolysis of plastic wastes with two different types of catalysts: ZSM-5 zeolite and Red Mud

The influence of ZSM-5 zeolite and Red Mud in the pyrolysis of plastic wastes has been studied. Both catalysts have been thoroughly characterized; the zeolite shows weak and strong acid sites and great BET surface area (412.0 m2 g−1), while Red Mud contains lower acidity, with also weak and strong acid sites, meso-macropores and BET surface area of 27.49 m2 g−1. Both catalysts have been tested in pyrolysis of a mixture of plastics which resembles municipal plastic wastes, at 440 and 500 °C in a 3.5 dm3 semi-batch reactor. The results have been compared with those of the thermal process. It has been proved that ZSM-5 zeolite has a strong effect in the characteristics and distribution of pyrolysis products. It generates at both temperatures a greater proportion of gases and liquids with a higher content of aromatics than without catalyst. Red Mud needs higher temperatures than ZSM-5 zeolite to exert a catalytic effect in pyrolysis, since similar results to those obtained without catalyst are obtained at 440 °C, while at 500 °C a higher yield of gases and a greater proportion of aromatics in the liquids is obtained.

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► Pyrolysis transforms plastic wastes in valuable liquids and gases useful as fuels or source of chemicals.
► The use of ZSM-5 zeolite in pyrolysis favours the production of gases and of lighter and more aromatic liquids.
► Red Mud, an industrial by-product, exerts a noticeable catalytic effect in pyrolysis, though lower than ZSM-5 zeolite.
► ZSM-5 enables to operate at lower pyrolysis temperatures while Red Mud only modifies the products characteristics.
► Red Mud is a mesoporous material with both strong and weak acid sites.