Electrocatalytic amino acid synthesis from biomass-derivable keto acids over ball milled carbon nanotubes

Abstract

Electrocatalytic reductive amination (ERA) offers an attractive way to synthesise organonitrogen chemicals from renewable feedstocks. Here, we report carbon nanotubes (CNTs) as an effective catalyst for the ERA of biomass-derivable α-keto acids into amino acids using NH₃ as the nitrogen source. Through a facile ball milling (BM) treatment, the intrinsic defects in the CNTs were increased while the electrocatalytic activity of CNTs converting 2-ketoglutaric acid into glutamic acid was enhanced by approximately seven times. A high faradaic efficiency (FE) of ∼90% with a corresponding glutamic acid formation rate of up to 180.9 mmol g⁻¹_cat h⁻¹ was achieved, and ∼60% molar yield of glutamic acid was obtained after 8 h of electrolysis. Electrokinetic analyses indicated that the BM–CNT catalysed ERA exhibits a first-order dependence on the substrate and NH₃, with a rate-determining step (RDS) involving the first electron transfer. Following this protocol, a number of amino acids were prepared with moderate to high FEs and formation rates. Significantly, we synthesised long carbon chain amino acids, which are typically obtained in lower yields using the existing methods.