(511d) Electrocatalytic Hydrogenation of Biogenic Compounds: Reaction Networks and Mechanisms

Increasing
availability of sustainable electricity and its intermittent character is
stimulating the interest in storing electrical energy into chemical bonds. In
this work, we aim to couple electrolytic H₂ production with chemical
transformations of organic compounds aiming valuable chemicals and fuels. Thus,
a series of carbon-supported metals have been tested for the electrocatalytic reduction of phenolic compounds, ethers,
and aldehydes in ideal aqueous conditions.

Electrocatalytic hydrogenation (ECH) of
series of phenolic compounds, di-aryl ethers and aromatic aldehydes, has been
performed in C-supported Pt, Pd, Rh, and Ni as
working electrodes (cathodes), whereas a Pt mesh is used as counter electrode (anode)
and an Ag/AgCl electrode is used as reference. H₂ evolution (HER) is the
prevalent side reaction on all metals, thus we define efficiency as the
fraction of electrons used to reduce the organic compounds. Another series of experiments has been performed, where cathodic
potential is replaced by a flow of H₂ at atmospheric pressure. This
is denoted as thermal catalytic hydrogenation (TCH).

The electrocatalytic
hydrogenation of aromatic rings and carbonyl groups readily occur on Pt, Rh,
and Ni, whereas Pd is highly efficient for reduction
in the reduction of carbonyl groups in aldehydes. The rates of carbonyl
hydrogenation are higher than those of the saturation of aromatic rings. The
rates of ECH of all tested compounds, as well as HER, increase with increasingly
negative potentials. However, the efficiency is a strong function of the metal.
Pt and Ni, for instance, strongly favor HER leading to low efficiencies. Other
strategies to increase reaction rates at room temperature are increasing the
concentration of hydronium ions and pressure.

In
the conversion of substituted phenols and diaryl
ethers, the dominant reaction pathway is hydrogenation to cyclic alcohols and cycloalkyl ethers. Methoxy and benzyloxy groups undergo C-O bond cleavage via hydrolysis
(minor route) and hydrogenolysis, which opens
pathways to the formation of hydrocarbons. In stark contrast to the series of
phenolic compounds, aromatic and pseudo-aromatic aldehydes and ketones undergo
only reduction of the carbonyl groups without exhibiting any hydrogenation of
the aromatic ring.  For instance,
benzaldehyde and furfural are selectively converted to benzyl alcohol and furfuryl alcohol, respectively. Increasing the
concentration of hydronium ions enable C-O bond cleavage of the alcohols through
a bifunctional (acid-metal) mechanism.

The
results show that electrocatalysis drives the desired
transformations at mild conditions and allows a better control over the
reaction pathways than thermal catalysis. Cathodic potentials leads to high
rates through increasing the availability of surface H and by providing
alternative pathways with low-energy barriers.