Sustainable Transport Energy: From Waste & CO₂ to Drop‑In Fuels and Cleaner Engines

Post-consumer plastic waste is typically composed of mixed polypropylene (PP) and polyethylene (PE) rather than single polymers which creates significant challenges for efficient chemical upcycling. In this study the thermo-catalytic pyrolysis of a realistic mixed plastic feedstock consisting of PP and PE was investigated using Fe and FeRu impregnated H-mordenite (HM) zeolite catalysts to produce olefins, light hydrocarbons jet-fuel-range hydrocarbons and hydrogen. The catalysts were characterized by N₂ adsorption–desorption for surface area and porosity, NH₃-TPD for acidity (bronsted and Lewis), H₂-TPR for metal–support interactions as well as reduction temperature, XRD and XPS for structural and oxidation state properties. The thermal degradation behavior of the mixed plastic feedstock was examined by thermogravimetric analysis, and the evolved products were identified using GC-MS via TCD and MS detectors. Catalytic performance was evaluated for HM, Fe-HM and FeRu-HM catalysts at different reaction temperatures (°C), inert gas N₂ flow rate (ml/min) and catalyst-to-plastic ratios (w/w). The results show that temperature strongly influences product distribution under mixed-plastic conditions with the FeRu-HM catalyst exhibiting superior activity compared to mono-metallic and parent catalysts. At 500°C the FeRu-HM catalyst achieved more than 81% conversion of the mixed plastic feedstock with approximately 30% selectivity toward jet-fuel-range hydrocarbons highlighting the potential of FeRu-modified HM catalysts for practical upgrading of mixed polyolefin waste into value-added fuels.

The development of efficient and cost-effective chemical recycling methods for plastic waste, particularly polyethylene terephthalate (PET), is essential for advancing a circular economy. Significant progress over recent decades has led to large-scale recycling technologies, including methanolysis, glycolysis, and hydrolysis, which depolymerise PET into monomers such as ethylene glycol and terephthalic acid or esters for virgin PET production.¹ However, the economic viability of chemically recycled PET remains a challenge, as its cost often exceeds that of PET derived from crude oil. Upcycling PET into higher-value products offers a potential solution. Catalytic hydrogenation has gained attention as a cleaner approach for PET depolymerisation, yielding ethylene glycol and 1,4-benzenedimethanol under mild conditions. Although recent studies report high turnover numbers, the limited market demand for 1,4-benzenedimethanol restricts its utility. Thus, new methods to convert this intermediate into more valuable compounds are crucial. In this study, we explore the transformation of 1,4-benzenedimethanol into novel alcohols, which serve as key intermediates for synthesising carboxylic acids, esters, and amides. The Guerbet coupling reaction, involving a hydrogen-borrowing mechanism, has been extensively applied to upgrade simple alcohols like ethanol to higher alcohols such as butanol. We hypothesised that a similar strategy could be applied to 1,4-benzenedimethanol, obtained via PET hydrogenative depolymerisation, ¹ and report herein a catalytic method to upgrade 1,4-benzenedimethanol to 3-(4-(hydroxymethyl)phenyl)propan-1-ol by coupling 1,4-benzenedimethanol with ethanol using a ruthenium catalyst. Through systematic optimisation of catalytic conditions, a high TON of up to 400,000 has been achieved.² In future these precursors will be further investigated for synthesising long-chain aromatic alkyl components used in sustainable jet fuel.
Reference
1) Sci. Adv., 2018, 4, eaat9669.
2) Ghosh et al., RSC Advances, 2025, 15, 21424– 21428.

A significant portion of this secondary raw material continues to be mismanaged: 38% of waste plastics were still disposed of in landfills. It is therefore necessary to unlock the potential value of these waste, avoid traditional linear consumption patterns (‘take-make-dispose’), and reduce the need for virgin fossil-based raw materials through effective recovering, reusing and recycling. The production of H₂-rich syngas from pyrolysis-catalytic gasification of plastic waste bottles has been investigated with the designed hybrid-functional materials (Ni-CaO/ Ca₂SiO₄). The highest H₂ production of 59.15 mmol g^-1_{of plastic} was obtained in the presence of a catalyst with 20 wt.% Ni loading, which amounts to H₂ purity as high as 54.2 vol.% in gas production. The Ni-CaO-Ca₂SiO₄ hybrid-functional material is a very promising catalyst in the pyrolysis-catalytic gasification process by capturing CO₂ as it is produced, therefore shifting the water gas shift reaction to enhance H₂ production from plastic waste. The finding could guide the industry for future large-scale application to convert abundant plastic waste into H₂-rich syngas, therefore contributing to the global ‘net zero’ ambition. Despite the considerable potential of Ni-Fe based catalysts, the current research landscape remains relatively under-explored. A significant knowledge gap persists regarding their catalytic activity and stability under diverse operational conditions, which substantially hinders the optimisation of the overall reaction process and their progression toward practical industrial application. In this study, Ni-Fe bimetallic catalysts were employed to upgrade plastic waste into CNTs and H₂ with a two-stage fixed-bed reactor. An investigation of key reaction parameters (Ni-Fe ratio, catalysis temperature, gas flow rate, and catalyst-to-feedstock ratio) was conducted to investigate their effects on CNTs yield and structure with multi-scaled characterisations. Under optimised conditions, the highest H₂ production of 28.49 mmol g^-1_{plastics PE} and abundant growth of multi-walled CNTs of 25.10 wt.% have been achieved. These findings highlight the potential of Ni-Fe bimetal catalysts to efficiently upcycle waste plastics into H₂ and functional carbon nanomaterials, contributing to improved resource recovery and environmental sustainability.

Hydrothermal carbonization (HTC) converts wet biomass waste into energy-dense products by leveraging the properties of subcritical water. HTC generates process water (HTC-PW) which is an aqueous-organic waste. Yet, recovering these organics from the HTC-PW as value-added products could enhance biorefinery process economics. While the literature is replete with the use of conventional liquid-liquid extraction (LLE) to accomplish this separation, LLE requires a large quantity of solvent owing to low extraction efficiencies. The liquid-liquid equilibrium of conventional LLE can be disrupted by the addition of neutral salts in a process known as salt-assisted liquid-liquid extraction (SALLE). Neutral salts strengthen water’s hydrogen-bonding network and reduce solute solubility, disrupting the liquid-liquid equilibria between the HTC-PW and the extracting solvent by raising the chemical potentials of the solute in the HTC-PW. In this work, we use the HTC-PW resulting from HTC of microcrystalline cellulose – a model compound abundant in lignocellulosic biomass – to compare the efficacy of SALLE using three solvents (ethyl acetate, hexane, and acetone) and four salts (sodium chloride, sodium sulphate, potassium chloride, and potassium sulphate) across the partition ratio (K_D) and selectivity of each compound present. Among the four salts, Na₂SO₄, which has the greatest hydration energy and contributes the most entropy, exhibits the largest improvements in K_D, outperforming NaCl and potassium salts. Compound properties also influence separation. Furfural, being less polar, partitions more easily to the organic phase than 5-hydroxymethylfurfural. Longer-chain acids outperform highly hydrophilic acids. Increasing the salt amount beyond its solubility limit initially improves partitioning, but K_D flatlines and decreases beyond specific salt concentrations. Ethyl acetate with 4M Na₂SO₄ is optimal, where all detected compounds in the extracted HTC-PW fall below 200 mg/L, including polar hydrophilic carboxylic acids. Thus, SALLE enhances the extraction efficiency of valuable organics from what is currently considered a waste stream, with up to 50% of the feedstock carbon that ends up in it.

The rapid increase in atmospheric CO2 emissions from fossil fuel use has intensified the need for efficient and sustainable CO2 utilization technologies. Catalytic hydrogenation of CO2 to valueadded products is attractive but remains challenging due to the molecule’s high thermodynamic stability under conventional thermal conditions. In this work, non-thermal plasma (NTP)-assisted CO2 hydrogenation was investigated using a dielectric barrier discharge (DBD) reactor coupled with iron-based catalysts. Four catalysts—FeCo (1:1), FeCo (3:1), FeCoZn (1:1:1), and FeCoZn (3:1:1)—were synthesized via co-precipitation to evaluate the effects of cobalt and zinc incorporation. Catalyst characterization using EDS, XRD, H2-TPR, and TEM confirmed uniform metal dispersion, formation of Fe2O3, Co3O4, and ZnO phases, and modified reducibility due to metal interactions. The DBD reactor, operated at 12 kV and 850 Hz, enabled effective CO2 activation through energetic electrons and reactive plasma species. Among the tested catalysts, FeCoZn (3:1:1) exhibited the highest CO2 conversion (68.91%) at 200°C, with CO as the dominant product. Reaction optimization using a design of experiments (DOE) approach identified optimal conditions of 209.71°C, 0.09 g catalyst loading, and 0.51 ml/min CO2 flow rate, achieving 65.58% conversion. The results demonstrate a strong plasma–catalyst synergy, enabling efficient CO2 hydrogenation under mild conditions.

Plastic waste remains a persistent environmental challenge due to its low degradability and high production rates. Chemical recycling via pyrolysis offers a promising route to convert polyolefin-rich plastic waste into liquid hydrocarbons that can be upgraded and used as feedstock for steam crackers to produce new virgin-grade plastics. However, the resulting oils often fail to meet specifications because of heteroatom contamination. Among these, chlorine is problematic as its thermal decomposition leads to hydrogen chloride, causing severe corrosion and catalyst poisoning. In particular, the purification of alkyl halides is challenging because of their similar polarity to the surrounding hydrocarbon matrix.
This study evaluates reactive extraction for removing alkyl halides, using 1-chloropentane as a model compound. A solvent mixture containing 1-methylimidazole (MIM) was used to extract 1-chloropentane from the oil phase, after which it reacted with MIM to form a water-soluble imidazolium chloride salt. Reactive extraction model was developed by combine single-phase kinetic in oil and solvent phases with liquid–liquid equilibrium estimated via COSMO-RS.
In batch reactors, 1-chloropentane removal ranged from 46% to 77%, with DMSO and DMF enhancing performance at comparable MIM loadings. The batch reactor model captured relative solvent performance but overpredicted absolute removal; accounting for non-ideal liquid–liquid equilibrium reduced the average relative error from 31.8% to 18.0%. In the continuous mixer–settler model, a three-stage configuration achieved 91.0% removal. Under the same configuration, solvent recycling to maximize product recovery reduced performance to 75.6–81.1% due to partial solvent loading. Overall, reactive extraction with MIM emerges as a promising strategy for industrial-scale chlorine removal from pyrolysis oil and provides a framework for scaling laboratory data.

The transition to lower-emission diesel fuels requires understanding how alternative fuel chemistries influence engine thermal management and emissions infrastructure. This study presents a direct comparison of exhaust gas recirculation (EGR) fouling and heat transfer behavior for petroleum diesel, renewable diesel (R100), and biodiesel (B100) in a 1.9 L turbodiesel engine equipped with a custom shell-and-tube EGR cooler. Unlike prior single-fuel investigations, this work evaluates multiple fuels under identical operating conditions to reveal the role of fuel composition in deposit formation and heat transfer performance. Six-hour steady-state engine tests were performed for each fuel, with hourly measurements of deposit accumulation. Deposit properties were evaluated using thermal characterization techniques, and EGR cooler effectiveness was quantified through thermal performance metrics. Soot concentration trends in the exhaust were monitored throughout the experiments to assess relative fouling behavior across fuels. Diesel generated the highest total fouling mass (45 mg), followed by renewable diesel (27 mg) and biodiesel ( 23mg), with corresponding reductions in heat transfer effectiveness to 72%, 78%, and 77% after one hour. Lower aromatic content in renewable diesel and oxygenated combustion in biodiesel contributed to reduced soot and hydrocarbon deposits, resulting in sustained heat transfer efficiency (≈5%). These results demonstrate that fuel chemistry directly affects EGR cooler performance and fouling rates, with implications for emissions control, engine durability, and sustainable fuel adoption. By linking chemical properties to operational outcomes, this work provides actionable insights for optimizing diesel powertrain components to support reduced emissions and improved thermal management, contributing to greener transportation systems.