The role of carbon dioxide in hydrocarbon conversion during catalytic pyrolysis of plastic mixture over Ni catalyst

Abstract

Pyrolysis converts plastic waste to fuel range hydrocarbons (HCs) and industrial chemicals but requires high purity polyolefin waste. However, practical plastic waste is a heterogeneous mixture, enabling a limited amount of high purity plastic recycling. Pyrolysis of mixed plastic produces low quality plastic oil, necessitating a series of refining processes to obtain low yield of target chemicals. In contrast, catalytic pyrolysis can directly convert mixed plastic into higher value syngas, but rapid catalyst deactivation by carbon deposition is a problem. In this study, a sustainable platform for mixed plastic to syngas was developed by introducing Ni/Al2O3 as a catalyst and CO2 as a feedstock gas. Volatile HCs evolved from pyrolysis of plastic were decomposed over Ni catalyst to produce shorter chained HCs and H2, but catalyst deactivation was shown by carbon deposition under N2. When CO2 was introduced, HCs were further converted into syngas (H2 and CO) by catalytic reactions with CO2, suppressing carbon deposition on catalyst. The best yield of gaseous products from catalytic pyrolysis of a polypropylene was 377 % at 650 ◦C under CO2, which was more than 7 times higher gas yield under N2. When heterogeneous plastic mixture was used as a reactant, the same trend was shown, and the presence of aromatic plastics in plastic mixture increased the yield of aromatic compounds. This study experimentally demonstrated that low quality plastic mixture and CO2 could be simultaneously converted into syngas by catalytic pyrolysis with suppression of carbon deactivation.