Polyethylene upcycling to fuels: Narrowing the carbon number distribution in n-alkanes by tandem hydropyrolysis/hydrocracking

Abstract

The extensive use of plastics in modern life has resulted in a global waste crisis to the environment. Polyethylene (PE) is one of the most popular and hardest plastics to recycle because of its strong C(sp3)-C(sp3) bonds. In this report, a tandem conversion process, i.e., hydropyrolysis and subsequent vapor-phase hydrocracking of primary intermediates (Cn>5 alkenes and long-chain alkanes), was conducted via a two-step pressurized flow-through fixed-bed reactor over a CoAl2O4 spinel-derived catalyst. The product distribution could be flexibly tuned by regulating operating parameters in the cascade fixed-bed reactor and Co/Al molar ratio in CoAl2O4 spinel catalysts. Under optimal reaction conditions (0.2 MPa H2, 550 °C for hydropyrolysis in the 1st reactor, 300–325 °C for hydrocracking in the 2nd reactor, 20 s−1 of gas hourly space velocity), the maximum single-pass yields of gasoline (C5-C12) and jet-fuel (C8-C16) range n-alkanes reached 86.0 wt% and 68.1 wt%, respectively. The CoAl2O4 spinel catalysts also gained high activity in degrading realistic post-consumer plastics such as linear low-, low-, and high-density PE, and a ∼ 73.1 wt% gasoline yield and a ∼ 54.7 wt% C8-C16 yield were retained even after 3 cycles in-situ regeneration of deactivated CoAl2O4 catalysts. This work provides an efficient and tunable approach to upcycle PE wastes into liquid fuels with an ideal carbon length.