Abstract
The electrochemical reduction of carbon dioxide is an appealing technology that stores renewable electricity in the chemical form and has the potential to transform the way carbon fuels are utilized today. While there have been successes in the electrosynthesis of alkanes, alkenes and alcohols, access to organonitrogen molecules such as alkylamines remains largely beyond the reach of current electrocatalysis. Here we report the first electrochemical reaction that converts carbon dioxide and nitrate to methylamine in aqueous media under ambient conditions catalysed by a cobalt β-tetraaminophthalocyanine molecular catalyst supported on carbon nanotubes. The overall reaction, involving the transfer of 14 electrons and 15 protons to form each methylamine molecule, is an eight-step catalytic cascade process enabled by the coupling of two reactive intermediates near the catalyst surface. The key C–N bond-forming step is found to be the spillover of hydroxylamine from nitrate reduction and its subsequent condensation with formaldehyde from carbon dioxide reduction. This study provides a successful example of sustainable alkylamine synthesis from inorganic carbon and nitrogen wastes, which could contribute to greenhouse gas mitigation for a carbon-neutral future.

Methylamine stinks. Opening a canister of this odiferous gas outside the fume hood will make your lab smell for days. Even cracking the top will aggravate anyone in a ten foot radius. It’s not a smell that anyone would find pleasant. It’s the smell of dead, rotting fish. Considering that it gives off such an offensive odour, the molecular structure of methylamine is a very simple. It is a tertiary amine where the central nitrogen atom is bound to three methyl groups in a trigonal pyramidal geometry. It’s a gas at room temperature but is highly soluble in water, making for potent aqueous solutions. Because of the lone pair of electrons and electronegativity of nitrogen, methylamine is a good nucleophile. Neutral amines are such reactive nucleophiles that nature herself often employs a range of amine side groups that catalyze nucleophilic reactions by accepting protons.

The reason methylamine smells so foul is because it is a common product of plant and animal decomposition (and some infections), and it’s thought that we’ve evolved to find these smells repulsive to prevent us consuming rotting food. The oxygenated precursor to methylamine is present in fish and breaks down to give that characteristic rotting fish smell. It’s thought that fish and other sea creatures contain this molecule to depress the freezing point of their bodily fluids and prevent urea, also present in the bodies of many sea creatures, from degrading proteins.

Methylamine is also released in the bodily fluids of individuals with ‘fish odour syndrome,’ a genetic disorder with the clinical name trimethylaminuria. This unfortunate condition causes body odour, breath, urine, or all three to smell like rotting fish. Those with trimethylaminuria are unable to metabolize methylamine from food, and the molecule is thus released unchanged. It has been estimated that only 1% of the UK population are carriers of the trimethylaminuria–causing gene. As of now, there is no known cure for trimethylaminuria; however, suffers may find some relief by regulating their diet to avoid certain foods, such as fish, eggs, liver, broccoli and other dark green vegetables.