Thermal desorption and coke deposition studies of spent zeolite catalysts in the hydrocracking of expanded polystyrene waste

The development of sustainable catalysts towards the chemical recycling of waste plastics has become imperative due to the huge amount of plastic waste generated, the associated disposal problems, and the attendant negative effects on human health and the environment. Spent zeolite catalysts used in the hydrocracking of waste polystyrene have been analysed with focus on coke deposition and product distribution via thermogravimetric analysis and thermal desorption studies respectively. The proton forms of the zeolites (HY and HBeta), the platinum-doped forms obtained via 1% wt. impregnation: 1PtHY and 1PtHBeta, which were further thermally reduced to 1Pt0HY and 1Pt0HBeta respectively were used for various hydrocracking process reactions. Results showed that the higher catalyst loading in hydrocracking reactions caused a reduction in the amount of coke deposited on the catalyst surface as coke content decreased from 15 to 11% at a polymer:catalyst of 15:1 and 5:1 respectively. At higher reaction times (60 min), temperatures (330 ℃), and pressures (25 bar H2), the amount of coke decreases indicating thermal stability of the porous catalysts under harsh reaction conditions. The presence of platinum significantly reduced the amount of coke formed from 35% (HY) to 16% (1PtHY), and from 41% (HBeta) to 15% (1PtHBeta) implying that the metal provided high resistance to coking. Repeated catalysts reuse showed the deposition of aromatics on the catalysts surface and a slight increase in the average particle size of the Pt nanoparticle from 3.1 nm for a single run to 4.0 nm for repeated runs using 1Pt0HBeta. Thermal desorption analysis revealed the presence of mostly mono-aromatics, indicating that the more condensed polyaromatics desorbed from the zeolite pores undergo cracking reactions leading to the formation of lower molecular weight hydrocarbon molecules, thus giving insights on the nature of hydrocarbon species on used catalysts.