Biofuels through electrochemical transformation of pyrolysis oil
Talal Ashraf is a PhD student in the Department of Photocatalytic Synthesis. Promotors are prof.dr. G. Mul from the Faculty of Science & Technology and prof.dr. B. Mei from the Ruhr University Bochum.
This thesis explores electrochemical strategies for upgrading carboxylic acids mixtures derived from pyrolysis oil using platinum (Pt) and boron-doped diamond (BDD) electrodes. A range of electrochemical techniques was employed to analyze reaction selectivity, electrolyte effects, and the transition zones of Kolbe electrolysis. Additionally, the in-situ production of intermediates was detected, and the underlying reaction mechanisms were examined.
The study investigates the electrooxidation of acetic, formic, and propionic acids on BDD electrodes as a potential alternative to Pt, with functionalization using Pt nanoparticles shifting reaction selectivity from indirect oxidation to C-C coupling. The use of acid mixtures introduced complexity, as overoxidation led to the loss of valuable products. To address this challenge, pulsed and flow electrolysis were developed as mitigation strategies. Furthermore, 3D BDD nano-pillars were fabricated via nanofabrication techniques and tested for their efficiency in hydrogen peroxide formation and acetate decarboxylation.
Key Findings and Contributions:
Biomass Utilization and Electrochemical Conversion:
- Biomass-derived pyrolysis oil was evaluated as a sustainable alternative to fossil fuels.
- Electrochemical conversion of carboxylic acids into upgraded fuel components was explored using renewable energy-derived “green” electrons.
- Literature on Pt and BDD electrodes for electrochemical decarboxylation was reviewed, forming the basis for experimental investigations.
Kolbe Electrolysis and Oxygen Evolution Reaction (OER):
- Rotating ring disc electrode (RRDE) measurements revealed competition between Kolbe electrolysis (C-C coupling) and OER.
- On Pt electrodes, the transition from OER to Kolbe electrolysis was influenced by electrolyte pH and foreign anions/cations.
- Sulfate anions and lithium cations promoted OER, whereas BDD electrodes exhibited inhibition of OER, favoring hydroxyl radical-mediated decarboxylation.
- Spin trapping agent was used with RRDE revealing OH radicals formation on the BDD electrode
Electrolyte Composition and Cation Effects:
- Selectivity for acetic acid decarboxylation was strongly influenced by the choice of supporting electrolyte( not only anions but cations too).
- K and Cs cations improved faradaic efficiency by stabilizing the carboxylate layer, while Li cations led to a local decrease in pH, affecting reaction rates.
- At high current densities (>50 mA/cm²), cation effects became negligible.
- These findings suggest that if carboxylic acids from pyrolysis oil are separated as precipitated salt in form K and Cs acetate will aid in decarboxylation efficiency.
Mechanisms of Decarboxylation on BDD Electrodes:
- Compared to Pt (DPCET), BDD electrodes favored hydroxyl radical-mediated oxidation of acetic acid, leading to methanol formation instead of C-C coupling products.
- The formation of methanol was largely independent of pH (for pH >5) and current density, with minimal overoxidation.
- Functionalization with Pt nanoparticles shifted the mechanism from indirect oxidation to Kolbe electrolysis. However, Pt nanoparticles exhibited instability and required effective stabilization.
Electrooxidation of Propionic Acid and Overoxidation Control:
- BDD electrodes facilitated the production of ethylene and ethanol from propionic acid, but overoxidation reduced faradaic efficiency by converting ethanol to acetaldehyde.
- In mixed electrolytes (acetic, propionic, and formic acid), propionic acid was preferentially oxidized due to its higher reaction rate with hydroxyl radicals.
- Overoxidation was mitigated using pulsed electrolysis and flow electrolysis, leading to improved faradaic efficiencies without altering acid selectivity.
3D Nanostructured BDD Electrodes:
- Nanofabricated 3D BDD pillars were tested for anodic hydrogen peroxide formation and acetate decarboxylation.
- Despite their larger surface area, these electrodes exhibited lower faradaic efficiencies than planar BDD, attributed to faster decomposition of intermediates (e.g., peroxydicarbonate).
- The non-uniformity of BDD crytals clusters across the 3D pillars showed the ineffective heat and reactants distribution in the CVD reactor across the structures.
This research advances the electrochemical upgrading of biomass-derived carboxylic acids by optimizing electrode materials and process conditions. It highlights the feasibility of using BDD electrodes as an alternative to platinum for sustainable bio-oil valorization, while addressing challenges such as overoxidation and electrode stability. The findings contribute to the development of more efficient electrochemical pathways for biofuel production using renewable energy sources.
