Improving pyrolysis oil processing via electrochemistry for plastics and biomass feedstock recycling

Abstract

The environmental pressures of plastic waste accumulation and fossil fuel dependency have driven interest in technologies that enable the sustainable conversion of waste into valuable products. Pyrolysis offers a promising route for recycling biomass and plastic feedstocks into liquid fuels and chemical intermediates. However, the resulting pyrolysis oils are typically unstable, oxygen-rich, and incompatible with existing fuel infrastructure. This thesis investigates electrochemical hydrogenation (ECH) as a low-temperature, electrically-driven method for upgrading these oils under mild conditions using water as a hydrogen source, supporting circular economy goals. The research combines model compound studies with real pyrolysis oil experiments, using a PtRu/ACC (platinum–ruthenium on activated carbon cloth) catalyst. Benzoic acid exhibited complete selectivity for cyclohexane carboxylic acid (100%) under optimised conditions. Notably, the presence of phenol enhanced benzoic acid conversion by up to 10%, due to a novel hydrogen-bond- assisted mechanism proposed in this work. This mechanism, which facilitates adsorption and lowers activation barriers, was supported by density functional theory (DFT) calculations and represents a previously unreported pathway in electrochemical hydrogenation. When applied to real bio-oils derived from pinewood and wheat straw, ECH reduced oxygenated species and increased alcohol content in the aqueous phase potentially improving both stability and energy content. The oily phase was treated in methanol with conductivity enhancers. While NaCl improved reactivity, it caused significant catalyst degradation. Tetrabutylammonium hexafluorophosphate (TBAHFP), initially considered less corrosive, led to greater catalyst degradation but higher conversion of identifiable compounds, highlighting a trade-off between catalyst durability and product yield. Advanced characterisation (GC-MS, FTIR, SEM, EDS, XRD, Raman) confirmed these transformations and catalyst changes.iii This work offers mechanistic insight and practical guidance for advancing electrochemical upgrading of pyrolysis oils, providing a foundation for scalable, low-carbon processes that integrate waste into the energy and chemical sectors.