Thermocatalytic Conversion of Plastics into Liquid Fuels over Clays

Abstract

Recycling polymer waste is a great challenge in the context of the growing use of plastics. Given the non-renewability of fossil fuels, the task of processing plastic waste into liquid fuels seems to be a promising one. Thermocatalytic conversion is one of the methods that allows obtaining liquid products of the required hydrocarbon range. Clays and clay minerals can be distinguished among possible environmentally friendly, cheap, and common catalysts. The moderate acidity and the presence of both Lewis and Brønsted acid sites on the surface of clays favor heavier hydrocarbons in liquid products of reactions occurring in their pores. Liquids produced with the use of clays are often reported as being in the gasoline and diesel range. In this review, the comprehensive information on the thermocatalytic conversion of plastics over clays obtained during the last two decades was summarized. The main experimental parameters for catalytic conversion of plastics according to the articles’ analysis, were the reaction temperature, the acidity of modified catalysts, and the catalyst-to-plastic ratio. The best clay catalysts observed were the following: bentonite/spent fluid cracking catalyst for high-density polyethylene (HDPE); acid-restructured montmorillonite for medium-density polyethylene (MDPE); neat kaolin powder for low-density polyethylene (LDPE); Ni/acid-washed bentonite clay for polypropylene (PP); neat kaolin for polystyrene (PS); Fe-restructured natural clay for a mixture of polyethylene, PP, PS, polyvinyl chloride (PVC), and polyethylene terephthalate (PET). The main problem in using natural clays and clay minerals as catalysts is their heterogeneous composition, which can vary even within the same deposit. The serpentine group is of interest in studying its catalytic properties as fairly common clay minerals.

Aluminosilicate Clay Minerals: Kaolin, Bentonite, and Halloysite as Fuel Additives for Thermal Conversion of Biomass and Waste

“…When applied to the pyrolysis process, they improve the product yields, allow obtaining liquid products of the required hydrocarbon range, and reduce tar formation. The comprehensive review of the thermocatalytic conversion of plastics over clays has already been summarized by Seliverstov et al [88] and Fadillah et al [89]. In some of the works reviewed, kaolin was found to act as a catalyst during the pyrolysis of different plastic feedstocks: virgin high-density polyethylene (HDPE), HDPE waste, and mixed plastic waste (MPW) by Auxilio et al [90].…”

“…The highest liquid yields of 100 wt.% for HDPE pyrolysis over bentonite (50 wt.%) and spent fluid catalytic cracking catalyst (FCC) were obtained by Elordi et al [92], and 99.82 wt.% for pyrolysis of low-density polyethylene (LDPE) on neat kaolin powder were obtained by Erawati et al [93]. The reviewed research reveals that Seliverstov et al [88] concluded that the modification of clay minerals (e.g., acid-washing or pillaring) was helpful to achieve a higher yield of the liquid fraction in some reviewed cases, but in others, pure clays also showed excellent catalytic activity.…”