Electrocatalytic Conversion of Phenol to Cyclohexane Using Pt and Ru-Based Catalysts
Authors: Page Jeffrey R. , Pophali Amol , Kim Taejin , Lopez-Ruiz Juan A. , Bliznakov Stoyan , Valla Julia A. 
Published: August 2024

The production of renewable fuels has become necessary as fossil fuels continue to deplete.1 Pyrolysis of biomass to liquid fuels has emerged as a promising solution due to the possibility of conversion of high carbon content feedstocks into fuel. Pyrolysis oils, however, struggle from low yields and increased heteroatom content.2 In order to solve this problem a second upgrading step is needed to increase the fuel’s hydrogen content and remove heteroatoms. Conventional oil upgrading through thermochemical processes is an option using high temperatures (>300 °C) and high-pressure hydrogen (10-20MPa).3 Electrochemical upgrading of bio-oils represents a novel avenue for the conversion of bio-oils to usable fuels or commodity chemicals under mild condition which requires milder temperatures (30-80 °C) and generates the hydrogen in situ through the water splitting reaction.3 Though initial results on electrocatalytic upgrading have been promising, few studies report appreciable yields of fully deoxygenated products and the efficiency remains low at high current densities.3

This work focuses on the upgrading of phenol as a bio-oil model compound. ECH of phenol was investigated in a custom electrochemical cell (H-Cell), with a solvent trap for the collection and quantification of volatile organic products. Efficient ECH conversion of phenol to cyclohexane was demonstrated and the primary factors impacting the selective production of cyclohexane were studied. Multiple variables, including catalyst type, electrolyte composition and concentration, and cathodic potential were investigated to determine their influence on ECH reaction rate, selectivity, and efficiency. The results obtained show that lab-synthesized, bi-metallic PtRu-C catalyst results in the highest specific ECH rate of 23.52 mol h-1 g-1
metal in comparison to 19.92 mol h-1 g-1
metal, 15.84 mol h-1 g-1
metal and 4.64 mol h-1 g-1
metal, measured with a commercial PtRu-C and mono-metallic Pt-C and Ru-C catalysts, respectively. In addition, the lab synthesized PtRu-C catalyst achieved > 30% selectivity towards cyclohexane, which is among the highest reported in literature to date. Constant potential experiments showed that the cyclohexane was 4.6% higher at -0.46 V vs Ag/AgCl than -0.28 V vs Ag/AgCl indicating cyclohexane selectivity is weakly dependent on cathodic potential. To improve the mechanistic understanding of phenol ECH, operando Raman spectroscopy was explored, finding strong evidence for cyclohexanone reaction intermediates.

References

(1) Zhang, X.; Lei, H.; Chen, S.; Wu, J. Catalytic Co-Pyrolysis of Lignocellulosic Biomass with Polymers: A Critical Review. Green Chemistry. 2016. https://doi.org/10.1039/c6gc00911e.

(2) Zhang, B.; Zhong, Z.; Min, M.; Ding, K.; Xie, Q.; Ruan, R. Catalytic Fast Co-Pyrolysis of Biomass and Food Waste to Produce Aromatics: Analytical Py-GC/MS Study. Bioresour. Technol.
2015, 189. https://doi.org/10.1016/j.biortech.2015.03.092.

(3) Chen, G.; Liang, L.; Li, N.; Lu, X.; Yan, B.; Cheng, Z. Upgrading of Bio-Oil Model Compounds and Bio-Crude into Biofuel by Electrocatalysis: A Review. ChemSusChem. 2021. https://doi.org/10.1002/cssc.202002063.

Figure 1