Screening of common synthetic polymers for depolymerization by subcritical hydrothermal liquefaction

Abstract

Hydrothermal liquefaction could potentially utilize mixed plastic wastes for sustainable biocrude production, however the fate of plastics under HTL is largely unexplored for the same reaction conditions.In this study, we evaluate how synthetic waste polymers can be depolymerized to bio-crude or platform chemicals using HTL at typical conditions expected in future commercial applications with and without alkali catalyst (potassium hydroxide). We evaluate different characteristics for HTL processing of polyacrylonitrile-butadiene-styrene (ABS), Bisphenol-A Epoxy-resin, high-density polyethylene (HDPE), low density PE (LDPE), polyamide 6 (PA6), polyamide 66 (PA66), polyethylene terephthalate (PET), polycarbonate (PC), polypropylene (PP), polystyrene (PS) and polyurethane (PUR) at 350 ºC and 20 minutes residence time. Polyolefins and PS showed little depolymerization due to lack of reactive sites for hydrolysis. HTL of PC and Epoxy yielded predominantly bisphenol-A in oil fraction and phenols in aqueous phase. PA6 and PA66 yielded one of its monomers caprolactam and a range of platform chemicals in the aqueous phase. PET produces both original monomers. PUR yields a complex oil containing similar molecules to its monomers and longer hydrocarbons. Our results show how HTL can depolymerizes several different synthetic polymers and highlights which of those are the most attractive or are unsuitable for subcritical processing.

Batch Hydrothermal liquefaction of end-of-life plastic and oil characterization

“…For both oils, the GC-MS analysis does not show the presence of C6-C11 paraffins and C6-C13 olefins, typical products of the degradation of polyolefins [17]. This result is in agreement with the literature [8], since supercritical conditions are required for polyolefins degradation [17].…”

“…Sodium hydroxide also causes an increase in oxygenated compounds (5,34% w/w) and compounds containing in their structure more than one benzene ring (3,72% w/w). As suggested by literature [8], basic catalysts promote the decomposition of nylon, giving rise to caprolactam (0.72% w/w). Through the combined use of GC-MS and GC-FID, 13.6% w/w of the compounds in the oil were characterized and quantified.…”

“…The spectrum underlines the presence of oxygencontaining compounds (1688 cm -1 C=O stretching; 1488 cm -1 O-H bending, 1252 cm -1 C-O stretching) and aromatic rings (706 cm -1 C-H bending of monosubstituted benzene derivatives). GC-MS analysis of the oil from test 1 (figure 7), reveals the presence of a high amount of benzoic acid (18% w/w), direct product of terephthalic acid (TA) decarboxylation [8]. TA comes from the hydrolysis of PET.…”

“…In this context, plastic waste can be recycled for the remanufacturing of new plastics or conversion into other valuable products. Among the different types of chemical recycling, Hydrothermal Liquefaction (HTL) appears as a very promising method for the sustainable production of biocrude [8]. This technique has high potential as an environmentally friendly green process due to the superior properties as chemical reaction media [9].…”

“…It was used without further purification. Two reactions were carried out working under subcritical conditions (340 ° C, 5 h), as suggested by recent literature [8]. Since a significant part of the feedstock was PET, a test was conducted with 2 wt.% of NaOH, to facilitate the breakdown of ester bonds, as occurs in basic hydrolysis processes [13].…”