Steam-mediated evolution of reaction intermediates in catalytic pyrolysis of polyethylene: A method to diminish coking and to enhance formation of aromatics.

Catalytic pyrolysis of plastics or biomass inevitably forms coke, while simultaneous feeding steam could diminish coking but dilutes liquid product. Intermittent feeding steam (feeding feedstock or steam in sequence/cycles but not simultaneous) might be a solution. This hypothesis was verified herein by conducting catalytic pyrolysis of polyethylene (PE) over Ni/HZSM-5 with no feeding, constant feeding or intermittent feeding of steam at 650-700 °C. The results showed that liquid was the main product (yield: 84 %) with no feeding steam while gas became the main one (yield: 69 %) with constant feeding in catalytic pyrolysis. Intermittent steam feeding enhanced formation of CO, CO2 and H2 from gasification of carbonaceous intermediates. Feeding steam was beneficial for production of aromatic hydrocarbons, especially the BTX (benzene, toluene and xylene) organics, as H2O-derived hydrogen radicals induced formation of olefin intermediates that were further converted via aromatization. Intermittent feeding steam could rapidly eliminate nascent coke to expose acidic sites, forming BTX with even higher yields than constant feeding steam. Content of coke in Ni/HZSM-5 from catalytic pyrolysis of PE was very high (0.80 g/gcat versus 0.003 g/gcat with constant feeding steam), due to insufficient gasification from absence of steam. Catalytic pyrolysis at 650 °C plus gasification at 700°C with intermittent steam feeding also formed much less coke (0.341 g/gcat) with higher aliphatic nature. In comparison, catalytic pyrolysis at 700 °C plus gasification at 700°C generated more coke with higher C/H ratio and higher aromaticity. Coking at 700 °C proceeded very quickly. In-situ IR characterization confirmed formation of abundance of oxygen-containing species through oxidation of carbonaceous intermediates with steam, which further affected evolution of coke into nanotube structures.