A new study inquires to a previously misunderstood phenomenon occurring between chemical reactions used to create plastics and pharmaceuticals - and this could help chemical manufacturing become more environmentally friendly.
In the production of drugs and plastics, chemical manufacturers often use toxic solvent materials like benzene and alcohol. With the new study, researchers aim to offer new insight into catalytic chemistry and improve existing manufacturing processes. The study is led by members from the University of Illinois Urbana-Champaign, University of Minnesota Twin Cities, and Virginia Tech, publishing their report in the journal Science, February 5.
An Overlooked Part of Chemical Manufacturing
In the manufacturing industry, the combination of solvents and metal nanoparticles often catalyzes several chemical reactions and optimizes the process - ensuring maximum yield and profit for the manufacturers. The problem is that many of these solvents are toxic and require specialized procedures and facilities for safe handling and disposal. While water, the universal solvent, can also be used, it is not nearly as efficient as organic solvents. Researchers noted that this difference in performance lies in the solubility of reactants with water.
However, in the new study, researchers report several irregularities in their experimental data, leading them to understand that the solvents' differences were not entirely understood.
To illuminate this phenomenon, researchers experimented on oxygen reduction to hydrogen peroxide. In one setup, they used water, while they used methanol on the other. Some setups included a mix of both. In all setups, the metal used was palladium nanoparticles.
"In experiments with methanol, we observed spontaneous decomposition of the solvent that leaves an organic residue, or scum, on the surface of the nanoparticles," explains David Flaherty, an author of the study and a chemical and biomolecular engineering professor at UI, in a press release from the University. He adds that this "scumlike residue" attaches itself to nanoparticles in some cases, further increasing reaction rates and the amount of output hydrogen peroxide. Flaherty said that this observation prompted their team to inquire more about how this scum could help the process.
The Mysterious Organic Scum
The "scumlike residue" - or formally called surface redox mediators - is capable of holding compounds with oxygen, such as the key component called hydroxymethyl. According to the published report, this residue accumulates on the palladium nanoparticles' surface, opening new chemical reaction pathways.
"Once formed, the residue becomes part of the catalytic cycle and is likely responsible for some of the different efficiencies among solvents reported over the past 40 years of work on this reaction," Flaherty explains. He adds that their latest work strongly suggests that the redox mediators also form in alcohol solvents and could solve the mysteries behind this phenomenon.
Additional experiments and simulations revealed that the redox mediators also effectively transfer protons and electrons to the reactant materials. Reactions with water as the solvent could also transfer protons only, but not electrons. Redox mediators also affect the metal nanoparticles' surface, lowering their energy barriers and making them more amenable for proton and electron transfer.
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