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Author: Subject: γ-MnO2 vs MnO2 vs Activated MnO2
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[*] posted on 6-3-2018 at 11:21
γ-MnO2 vs MnO2 vs Activated MnO2

I've encountered a number of reactions over the years that seem to specifically require γ-MnO2, which is easily prepared by reacting Manganese Sulfate and Permanganate. The stated reasons being that the gamma-form has higher reactivity and is supposedly completely necessary for many reactions.

What's confusing to me is that I can't really seem to locate any chemical suppliers that sell γ-MnO2. Even Sigma only really seems to sell normal MnO2. Sigma sells "Activated MnO2", but I've read the Activated just means it has been dried in an oven.

If this gamma form is so unique in its reactivity and so widely used, why does it seem no suppliers sell it? Is standard MnO2 just less reactive, retaining the same degree of reactivity? I can't imagine that would be the only reason if so many papers make it a point to use the gamma form.
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[*] posted on 6-3-2018 at 12:11

My understanding is that the super active MnO2 is formed when ppt'd from permanganate, but that that material eventually becomes less active, perhaps due to crystal form, dryness, pore size, or a hundred other factors that I have never seen explained. I have found that the "activated" MnO2 from Sigma Aldrich works well for some, if not many, reactions, even older bottles, but that for a few reactions it was necessary to make it fresh. Heterogeneous reagents are the bane of chemists everywhere, just like finding a good bottle of Pd/C is always a crap shoot. But once you find it, hide it well... Since both MnO2 and permanganate are cheap , I just try MnO2 and if it works, I use it, if not, I make it fresh.
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