Chemical recycling: Production of BTX by catalytic reforming of n-Heptane

The generation of plastic waste is increasing worldwide causing several environmental impacts. In 2021 was estimated 850 Mt of greenhouse gases were globally generated by the production and incineration of plastic (WWF Australia 2021). In 2019, 6,1 Mt of plastic waste leaked into aquatic environments and 1,7 Mt flowed into oceans causing marine ecotoxicity (OECD 2022). Unfortunately the mechanical technologies come up short due to the intermixture of various types and grades of polymeric materials, the content of other substances such as additives or or fillers, which are normally added to optimize its properties. Methods for treating plastic waste include pyrolysis, which is an alternative that values plastic waste as a feedstock to produce a crude oil type. This oil is rich in hydrocarbons, thus is becoming as potential source to produce lighter aromatics such as: benzene, toluene, and xylene and cover their increasing demand by the chemical industry. In this work, n-heptane was chosen as model component of the paraffinic fraction of pyrolysis oil to find out the effectiveness of its aromatization under different catalytic conditions. As such, different types of catalysts were prepared by wetness impregnation and activated by calcination under air flow at 350 and 550 °C are compared in the dehydrocyclization of n-heptane. ZSM-5 zeolite and activated carbon were used as supporters. While Gallium (Ga) and zinc (Zn) were the active components applied on the carriers in three different proportions (1, 3 and 5 wt.%). Furthermore, treated coke was physically mixed with dual metal impregnated ZSM-5. To achieve high yields of BTX the optimal temperature in most of the experiments was 550 °C, optimal load for Zn was 1 wt.% and for Ga 3 wt.%. Activated carbon as support did not show considerable aromatization (10 wt.% selectivity at 550°C, but seemed to have properties for cracking and isomerization of n-heptane when impregnated with 5 wt.%Ga. Another important finding in this work was the positive effect on the catalytic activity of impregnated catalysts when it was mixed with treated pyrolysis coke. Zn-ZSM-5 catalysts displayed the highest selectivity towards BTX production (above 30 wt.%). Moreover, it seemed that this metal provided dehydrogenation function as well as the reduction of coking rate. Finally, by means of a relatively easy and effective method as wet impregnation is, fractions of plastic pyrolysis can be converted into valuable aromatics.