Selective hydrocracking of polystyrene waste over Pt- and Ru-supported zeolite catalysts into high yield LPG and ethylbenzene

Abstract

This work investigates the chemical upcycling of non-biodegradable polystyrene waste into useful industrial products via catalytic hydrocracking. Zeolite-Y and ZSM-5 catalysts were impregnated with platinum and ruthenium and used in the conversion of polystyrene waste into the highly-desirable liquified natural gases and liquid ethylbenzene. Reactions carried out in a Parr reactor gave C₃$-$C₄ gas selectivity of 60–67% for Pt-doped and 41–74 % for Ru-doped HY catalyst, while the Pt-doped ZSM-5 gave 94–96% C₂$-$C₄ selectivity of the total gas products. Liquid analysis revealed both Pt and Ru were selective towards single-ring aromatics, notably ethylbenzene: 37–44%. However, Pt-ZSM-5 gave negligible liquid products due to its relatively larger pore sizes (3.8 nm) which promotes faster mass transport leading to more cracking and less secondary reactions. Higher pressures significantly increased feedstock to fluid conversion from 37 to 73% at 15 and 25 bar H₂ respectively. Longer reaction times and lower polymer : catalyst ratios enhanced hydrogenation and ring opening reactions to give lower molecular weight compounds $\le$C₈ and cycloalkanes. Spent catalysts were reused four times and similar ethylbenzene selectivity obtained, hence indicating excellent catalyst stability. We present a waste management control strategy in the feedstock recycling of plastic waste using bifunctional catalysts to produce fuels, whilst promoting alternative sources to the growing energy demand and environmentally benign synthetic routes in the petrochemical industry.