(111h) Electrocatalytic Conversion of Phenol to Cyclohexane

The production of renewable fuels has become necessary as fossil fuels continue to deplete past the point of no return.¹ Pyrolysis of biomass to liquid fuels has emerged as a promising solution due to the possibility of conversion of high carbon content feeds 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 in order to increase the fuel’s hydrogen content and remove heteroatoms. Conventional oil upgrading has been done through thermochemical processes using high temperatures (>300 °C) and high-pressure hydrogen (10-20MPa).^3,4 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.⁴ Though initial results on electrocatalytic upgrading have been promising few reports appreciable yields of fully deoxygenated products and the efficiency remains low at high current densities.⁴

This work presents the upgrading of phenol as a bio-oil model compound. ECH of phenol was investigated in a custom electrochemical cell (H-Cell), wherein the anode and cathode are separated by a Nafion® 117 membrane. Preliminary ECH results using a commercial PtRu-C catalyst showed that after 4 hours of electrolysis at 55 mA cm^-2_, phenol conversion reaches 80%. The primary products of phenol ECH are cyclohexanol and cyclohexane with selectivity of 59% and 27%, respectively. Further ECH experiments were performed using lab synthesized PtRu, Pt and Ru catalysts to investigate the role of active metal on cyclohexane selectivity. These results (figure 1b) showed that the lab synthesized PtRu catalyst produced the highest cyclohexane selectivities (31%), highest conversion (100%) and efficiencies (35%). ECH of phenol was further investigated using operando Raman spectroscopy to understand the mechanism of cyclohexane formation. Preliminary results from these experiments showed that the cleavage of the C-OH bond depends on the potential.

(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) Page, J. R.; Manfredi, Z.; Bliznakov, S.; Valla, J. A. Recent Progress in Electrochemical Upgrading of Bio-Oil Model Compounds and Bio-Oils to Renewable Fuels and Platform Chemicals. Materials (Basel). 2023, 16 (1), 1–33.

(4) 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.