Catalytic Solvothermal Liquefaction of Polyolefin Plastic Waste in Ghana

Abstract

Global plastic production exceeds 460 million tonnes per year, with polyolefins constituting the dominant fraction of post-consumer waste. In Ghana, plastics account for approximately 17 % of municipal solid waste, yet the formal recycling rate remains around 10 %. Clogged waterways, overflowing landfills and open burning are visible consequences of a system overwhelmed by growing consumption and insufficient infrastructure. Polyolefins are particularly challenging to recycle because they resist degradation under the mild conditions of conventional hydrothermal liquefaction, while supercritical water processing requires pressures exceeding 22 MPa, limiting deployment in low-resource contexts. Solvothermal liquefaction (STL) with an organic solvent and a heterogeneous catalyst offers a potentially lower-severity alternative, but its feasibility for polyolefin-rich waste streams remains poorly understood, particularly in low- and middle-income country settings.

This thesis investigates the technical and socio-technical feasibility of catalytic STL for polyolefin plastic waste in Ghana through two complementary studies. The technical study comprised two experimental campaigns on a mixed polyolefin feedstock (50 wt% PP, 30 wt% HDPE, 20 wt% LDPE) sourced from
Accra in Ghana: a subcritical campaign using decanoic acid as solvent with ZSM-5 as catalyst (335–345 °C), and a supercritical campaign using both water and decanoic acid at 455 °C. The socio-technical study conducted 14 stakeholder interviews in Ghana, mapped the plastic waste value chain and identified conditions under which chemical recycling could feasibly be integrated into the existing system.

The subcritical STL campaign with decanoic acid proved technically infeasible in its current form. Visual inspection and analytical characterisation revealed that the original plastic morphology was fully disrupted. No pieces retaining the feedstock structure were recovered from either the solid or the liquid phase,
which both appeared homogeneous, suggesting physical interaction between the polyolefins and the solvent. The nature of this interaction could not be determined from the available evidence. Critically, the solvent could not be separated from the organic phase, making a mass balance impossible and preventing any quantitative assessment of product yields.

The supercritical campaign with water demonstrated effective conversion, achieving an oil yield of 54.01 wt% at 455 °C and 60 min, with o-xylene and mesitylene as the dominant products. The catalyst markedly promoted monomer formation in the gas phase, increasing the combined ethane and ethylene fraction from 19.90 to 29.05 wt% and propylene from 9.80 to 12.76 wt%, opening a pathway toward plastic-to-plastic recycling. In contrast, the supercritical experiments with decanoic acid as solvent showed that at 455 °C, decanoic acid decomposes rather than acting as a stable reaction medium. To confirm this, a decanoic acid blank experiment without plastic was conducted. Ultra-high resolution APCI-FT-Orbitrap MS analysis, evaluated through a heteroatom class histogram, Van Krevelen diagram, DBE versus carbon number plot and KNM versus O/C ratio plot, showed that the decanoic
acid experiment with plastic was virtually indistinguishable from the blank across all four representations, confirming that the oil phase is governed by solvent decomposition products rather than plastic conversion.

The field research identified six structural complexities shaping Ghana’s plastic waste management system and concluded that existing policy frameworks, including the NPAP roadmap, provide a foundation for chemical recycling deployment but require translation into binding legislation, explicit chemical
recycling targets and dedicated financing. A phased Plastic-to-Fuel followed by Plastic-to-Polymer deployment strategy is proposed, leveraging Ghana’s position as a net fuel importer and the growing volumes of otherwise unrecoverable mixed plastic waste. The proposed deployment strategy positions chemical recycling as complementary to, rather than competitive with, mechanical recycling, forming a
cascading system that maximises resource recovery across the full waste stream.