Model-assisted Identification of the Reaction Mechanism and Kinetics of Cyclohexanone Reduction in a PEM-Cell

The kinetics of electroorganic synthesis reactions is usually not analyzed, which prevents knowledge-driven de
sign and operation. This study examines the electrochemical cis-selective reduction of 4-tert-butylcyclohexanone
to 4-tert-butylcyclohexanol using an Rh/C catalyst in a proton exchange membrane (PEM) cell. The reaction
mechanism and kinetics are identified using a combination of experimental cyclic voltammetry (CV) and model
based kinetic analysis. Among three proposed reaction mechanisms– one based on Langmuir-Hinshelwood and
two based on Eley-Rideal theory– the Langmuir-Hinshelwood-type mechanism (LHM), involving the reaction
of adsorbed 4-tert-butylcyclohexanone with adsorbed hydrogen, was identified as the most plausible. Dynamic
simulations based on the LHM showed strong agreement with the experimental data, accurately capturing key
electrochemical features and reproducing Faradaic efficiencies. The kinetic model accounts for reaction kinetics
for the electroreduction and the competitive hydrogen evolution reaction, as well as mass transport processes.
Our findings highlight that ketone adsorption influences both catalytic activity and selectivity, whereas alcohol
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desorption is identified as the rate-limiting step. The accumulation of alcohol at the catalyst surface reduces ac
tive site availability and promotes H2 evolution, thereby lowering Faradaic efficiency at higher current densities.
Simulations under steady-state conditions predict that increasing ketone concentrations reduces side reactions
and increase the Faraday efficiency for the desired product. These insights emphasize the importance of op
timizing catalyst properties to enhance overall conversion rate, and of maintaining high ketone concentration
to achieve high selectivity, energy efficiency, and Faradaic efficiency. Kinetic analysis is thus shown to be a
powerful prerequisite for understanding and improving synthesis processes.