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

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(sp(3))-C(sp(3)) bonds. In this report, a tandem conversion process, i.e., hydropyrolysis and subsequent vapor-phase hydrocracking of primary intermediates (C-n > 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 (C-5-C-12) and jet-fuel (C-8-C-16) 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 similar to 73.1 wt% gasoline yield and a 54.7 wt% C-8-C-16 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.

Automated summary

The extensive use of plastics in modern life has led to a global waste crisis. This study proposes a tandem conversion process to recycle polyethylene into liquid fuels. The results show that under optimized conditions, the process achieves high conversion yields and the catalyst remains active over a long period of time.