Dechlorination and upgrading of waste plastics pyrolysis oil over MWW and TUN zeolites

Abstract

Pyrolysis is a promising technique for converting waste plastics into oil, but the resulting liquid often contains halogens, particularly chlorine, which pose environmental and corrosion risks when used as fuel or refinery feedstock. These halogens primarily originate from materials such as PVC and other Cl-containing compounds present in the waste. This study evaluates the performance of several zeolites (TNU-9, MCM-22, MCM-49, MCM-56, and MCM-36) in the catalytic dehalogenation and upgrading of a real pyrolysis oil derived from waste plastics with a chlorine content of 290 ppm. Experiments were conducted using a fixed-bed catalytic reactor at 450 °C. All the tested catalysts improved the oil properties, decreasing its paraffin content and favouring the formation of aromatic, cyclic and olefinic hydrocarbons. This is positive regarding the possible use of this oil fraction in the formulation of transportation fuels or as a source of raw chemicals. The deepest modification was observed over MCM-36 zeolite, achieving the highest cracking activity into light olefins (mainly C3 and C4). Its superior performance is attributed to its high external surface area, mesopore volume, and accessible Brønsted acid sites, resulting from its pillarized structure. Moreover, MCM-36 was also the zeolite exhibiting the best oil dechlorination capability, which remained relatively stable along the time on stream in contrast to other zeolite catalysts. Analyses of the spent catalysts indicated that the deposited carbonaceous matter accumulates most of the Cl-containing species removed from the oil; hence, the overall oil upgrading process can be considered a combination of catalytic and adsorption effects. This material could be effectively regenerated by a sequential combination of Soxhlet extraction and calcination treatments.