Unlocking a Nano-Aluminum Oxide Lewis Acid Layer as the Electrocatalyst for Hydrogenation of Thiophene and Other Arenes

Abstract

Owing to the ready formation of an insulating oxide layer, there is an intrinsic obstacle to the use of aluminum material as the cathode in the electrocatalytic reaction. In this report, a new Al-based material with a surface aluminum oxide/chloride nanolayer (nAlClO@Al) is prepared. Via the quantum tunneling effect, this material realizes conductivity through the nano-Al-O layer. This material exhibits unusual inertness toward the hydrogen evolution reaction, leaving a wide reduction window for the substrate. Primed with the native Lewis acidity of the Al-Cl structure, this material achieves the electrocatalytic hydrogenation of thiophenes with up to >95% conversion, >95% selectivity, and 90% Faraday efficiency. This catalysis is applicable to heteroarene and benzene rings. In particular, the chemoselectivity is controlled by the nAlClO@Al catalyst instead of the substrate, which is complementary to electrochemical Birch chemistry.

1. Factors for Hydrogenation of Thiophene and Electrocatalyst Design

1

The preparation, morphological, and structural characterization of the cathode material nAlClO@Al. (a) The illustration of the preparation of nAlClO@Al. (b) The sensitivity assessment of the formation of the catalyst. (c) The XRD pattern of nAlClO@Al and other Al materials. (d) The XPS spectra of the Al_2p orbital of nAlClO@Al and Al. (e) XPS depth profile analysis in the Al_2p region for the nAlClO@Al material. (f) The SEM and TEM images of nAlClO@Al. (g) The EDS-mapping images showing the distribution of Al, O, and Cl.

2. The Discovery of nAlClO@Al-Catalyzed Electro-hydrogenation of Thiophene

2

The electrocatalytic performance of the cathode material in hydrogenation of thiophene 1a. (a) The linear sweep voltammetry (LSV) and Tafel plots of Al and nAlClO@Al in 2.0 equiv. Et₄NCl and 0.1 mL of ethanol in an ammonia atmosphere; the scan rate is 10 mV/s. (b) Reaction performances in the hydrogenation of 1a with nAlClO@Al (left) and Al (right) cathodes with various charges under the same conditions. (c) The yield of the reaction using the Al material electro-processed in different protonic media. (d) Recycles of nAlClO@Al and corresponding conversion, yield, and Faraday efficiency of each cycle. Con. = conversion, FE = Faraday efficiency. The conversion and yield are determined by ¹H NMR. (e) The chronopotentiometry (CP) measurement of the nAlClO@Al cathode. (f) The reproducibility of nAlClO@Al-catalyzed electro-hydrogenation of 1a. (g) The sensitivity assessment of the catalytic reaction.

3. Experiments on the Investigation of the Mechanism

4. Scope of Thiophenes in the AlClO@Al-Catalyzed Electro-hydrogenation

5. Scope of Arenes in the nAlClO@Al-Catalyzed Electro-hydrogenation

6. Synthetic Exploration of the Electrocatalytic Hydrogenation and Corresponding Applications