Upgrading pyrolysis liquid from high-density polyethylene to fuel-like hydrocarbons by catalytic transfer hydrogenation with a NaBH4 hydrogen donor

Abstract

The pyrolysis of plastic waste is a promising upcycling process that recovers high-value chemicals and fuel-like hydrocarbons. However, the high content of unsaturated hydrocarbons in the pyrolysis liquid produces low-stability fuels that are highly polluting and damage engines. Unsaturated hydrocarbons can be converted to saturated hydrocarbons by hydrogenation, which upgrades the pyrolysis liquid and enhances its value. In this work, a pyrolysis liquid obtained from high-density polyethylene was upgraded by the catalytic transfer hydrogenation of alkenes in a one-pot reaction over metal catalysts using sodium borohydride (NaBH₄) as a liquid hydrogen source in place of hydrogen gas. Both single metal catalysts (Co, Cu, Fe, Ni, and Mn) and binary metal catalysts were used. The binary catalysts were all based on Cu (Co/Cu, Fe/Cu, Ni/Cu, and Mn/Cu). Co and Ni showed significant catalytic activity for upgrading the pyrolysis liquid with alkene conversion of 61 % and 52 %, respectively. Co/Cu introduced synergetic catalytic activity and increased the alkene conversion to 66 %. While Fe and Mn/Cu catalysts did not support the hydrogenation of alkenes. The ratio between Co and Cu at 4:1 in the binary metal catalyst led to the highest alkene conversion, and the NaBH₄ loading also influenced the conversion of alkenes. The feasibility of hydrogenating alkenes at room temperature was investigated. This simple, green, and economic hydrogenation process converted up to 77 % of unsaturated hydrocarbons as alkenes to saturated hydrocarbons as alkanes. The alkane content in the processed pyrolysis liquid was as high as 80 %.