Science1 publisher3 min readPublished
Yokohama chemists hold cobalt between metal and oxide to hydrogenate pyridine with electricity
Yokohama National University chemists converted pyridine to piperidine at over 99% yield with a cobalt catalyst kept between metal and oxide. That state drifts during electrolysis, so the balance depends on how the cell is run as well as on how the catalyst is made.
The Scientist · Science desk
What happened
- Catalysts with too much metallic cobalt or too much cobalt oxide were less active, and the best performance came from an intermediate Co(0)/CoOx ratio.
- The same catalyst hydrogenated pyridines, quinolines, pyrazines, nitriles and nitroarenes, and it suppressed side reactions seen with rhodium-based catalysts.
- At gram scale, with the electrolysis run intermittently, the catalyst gave piperidine in 89% yield while the cell voltage stayed stable.
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Why it matters
- decision Anyone adopting this catalyst has to specify the electrolysis schedule along with the material, because continuous current pushes the cobalt out of its best oxidation state.
- constraint Larger and longer runs will be judged on how well the oxide balance holds over time; at gram scale that already cost more than 10 points of yield.
- contradiction The release calls the headline figure a yield but describes it as selectivity, so how much pyridine was actually converted, and with it throughput, stays unconfirmed until the paper is checked.
- capability For nitrogen compounds where rhodium opens unwanted side reactions, an earth-abundant metal now offers cleaner selectivity, demonstrated so far at lab and gram scale.
"We found that the catalytic performance of cobalt is determined not simply by its elemental composition, but by its dynamic oxidation state during electrolysis," Atobe said [14]. The catalyst starts as cobalt sulfate calcined at 750 degrees Celsius and runs in an anion-exchange membrane electrolyzer [3]. Under current it moves between metallic Co(0) and the oxide CoOx, and the ratio the team linked to activity is the one present while the reaction runs [6].
"We wanted to understand how the oxidation state of cobalt changes under operating conditions and whether controlling the balance between metallic cobalt and cobalt oxide could provide an effective catalyst," said Naoki Shida, a co-corresponding author [13]. That question needs measurement during electrolysis. The team paired standard characterization and theoretical calculations with in situ X-ray spectroscopy [7]. A sample examined after the current stops would show only where the cobalt ended up. Ratio and activity moved together across the catalysts they made [6]. The release says the combined results "suggest" that metallic cobalt beside residual oxide helps pyridine adsorb [7].
The headline figure needs a denominator check. The release reports a pyridine-to-piperidine yield above 99% under ambient electrolysis conditions [4], then explains it as almost all of the pyridine that reacted becoming the desired product [5]. Those are two different quantities. Yield is normally counted against the pyridine put in; the explanation describes selectivity, counted against the pyridine consumed. They agree only if conversion was complete, and the paper in the Journal of the American Chemical Society [2] is where that gets settled.
Piperidine is a common building block in synthetic and medicinal chemistry [17]. The evidence the release offers against precious metals is about selectivity: the cobalt suppressed side reactions that rhodium-based catalysts allow [9]. Atobe described the field's problem as replacing platinum-group metals "without sacrificing activity or selectivity" [12]. The release gives no activity comparison.
Time is the other problem. Prolonged electrolysis over-reduces the cobalt and pushes it out of its best state, so the team switched to intermittent electrolysis to hold the balance [10]. The gram-scale run under that regime came in more than 10 percentage points below the small-scale yield [1]. Electrocatalytic hydrogenation draws its hydrogen equivalents from water using electricity [16], yet the release reports no current density, Faradaic efficiency, run length, energy per kilogram of product, or activity figure measured against rhodium or platinum.
In my view the Co(0)/CoOx ratio is a real design variable, with one condition. It is set by the operating schedule as much as by the synthesis, because the team's fix for over-reduction was a change in how the cell is run [10]. "Maintaining an appropriate balance between metallic Co and residual CoOx enables highly selective hydrogenation," Atobe said [15].
What to watch
- A long-duration test showing how many on-off electrolysis cycles the cobalt survives before its Co(0)/CoOx balance can no longer be restored.
- Replication of the intermediate-ratio optimum by another group, ideally with the ratio varied while particle size and surface area are held fixed.