(183c) Low-Temperature, Energy-Efficient Hydrocracking of Polyolefins to Fuels

Plastics waste (PW) contamination of the environment is a major global ecological and economic challenge. Polyolefines, including polyethylene (PE) and polypropylene (PP), which constitute about 57% of all PW are hard to recycle using currently offered technologies due to the rigidity of C-C bond in the polymer backbone. A promising approach to break PE and PP down to smaller fuel range hydrocarbons in an energy-efficient way is hydrocracking [1]. This study aims to develop an optimal hydrocracking catalyst. Another goal is to provide guidelines in tuning process selectivity from gasoline to lubricants by changing catalyst properties/composition or by altering reaction conditions.

Hydrocracking was performed in a batch reactor at 10-50 bar H₂ pressure for 1-12 h using different virgin and waste polymers as a feedstock. Platinum (Pt) on tungstated zirconia (WO₃/ZrO₂) was found to be active in the conversion of PE with a molecular weight of 250 kDa to C₇-C₁₂ alkanes with a fraction of branched isomers close to 90%. When mixed with HY zeolite activity increases 4-fold with a shift in selectivity to lighter C₅-C₇ hydrocarbons. Detailed investigation of reaction network showed that zeolite plays an important role in cracking of primary olefins originated from Pt/WO₃/ZrO₂. The acid site density and mesoporosity of the zeolite also significantly affect performance. Lower acidity together with enhanced micropores accessibility favor less deep cracking. The catalysts showed nearly complete reusability after calcination. Fuel-grade hydrocarbons yield reaches 80-85% at 250-275 °C within 2-4 h of reaction. Major mechanistic insights allow us to propose further development of plastic waste hydrocracking.