Ultrasound as a tool for reducing energy consumption in electrocatalytic hydrogenation of aromatic ketones using graphite as catalyst support

Abstract

Benzylic alcohols have an important role in organic synthesis as target molecules of biological interest, which can be produced from the reduction of aromatic carbonyl substrates. In this work, the sonoelectrocatalytic hydrogenation (SECH) of aryl ketones was studied in water. The ultrasonic irradiation was carried out at frequency of 20 kHz ±500 Hz with a cylindrical titanium probe (MS 73 microtip; Ti-6AI-4V alloy; 3.0 mm diameter; Pacoustic = 2.11 W). The results showed that the ultrasound irradiation enhances the mass-transport of the substrates from the bulk solution to the electrode surface, increasing the substrate availability to react with the adsorbed H, leading to higher yields. Regarding the electrode material, the following hydrogenation yields of a group of acetophenones was observed: Cu (79.0 ± 1.5%) > Ni (73.0 ± 2.0%) ≈ Fe/Ni (72.0 ± 1.7%) > graphite (69.0 ± 1.7%) > Fe (67.5 ± 2.3%). The SECH presented a slight variation in the reactivity of some reagents, and its behaviour depends on the catalytic nature of the electrode support. Besides the lower overpotential and higher exchange current density of the graphite, it was surprisingly active under ultrasound irradiation, showing higher yields and current efficiencies than others electrocatalytic hydrogenation (ECH) procedures. The ultrasound irradiation decreases more than 6-folds the energy consumption of the ECH method, making it an effective and inexpensive way to produce alcohols from the respective aryl ketones in water.