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I have to reason whether silica or alumina would be a better choice of stationary phase for separating a mixture of pyridine and cyclohexanone with column chromatography.

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I would expect both pyridine and cyclohexanone to be slightly polar (silica because it has the $Si(OH)$ groups and alumina I suppose because it has a metal attached to highly electronegative Oxygen), but the main difference I'm getting between the two is that pyridine (according to Wikipedia) is considered to be slightly basic, whereas cyclohexanone is neither really acidic or basic. So I suppose alumina is somehow a proton acceptor?

Another Google search shows that pyridine has a $pK_b \approx 8$, which doesn't seem particularly basic to me, but I suppose it's enough.

My intuition would tell me that since alumina is basic it would be best for separating basic pyridine from non-acidic/basic cyclohexanone. But I'm having some confusion wrapping my head around about why using an acidic or basic stationary phase would be helpful (or perhaps it's not helpful and I'm heading on the wrong track -- but even then I'm confused in justifying how it wouldn't be helpful). This brings me to my questions:

  1. How would having an acidic or basic stationary phase be useful for chromatography? Chromatography, as I understand it, relies in things adsorbing and sticking to the stationary phase for different retention times, whereas acid/base properties just have to do with proton transfer. If alumina is a basic stationary phase, I suppose it could be accepting protons from the solution -- but how it would this lead it to adsorb and retain one of pyridine vs cyclohexanone stronger? I would guess that maybe having a protonated stationary phase leads to the basic compound (pyridine) competing for those protons on alumina and somehow the pyridine ends up stuck to the stationary phase where all the protons are ... but I'm not sure if this would be logical.
  2. How does pH of the stationary phase affect the quality of separation? Retailers seem to be selling alumina stationary phases in various pH ranges: pH 6-7.5 (called neutral); pH 4-5.5 (called acidc...); pH 8-9.5 (called basic). How could the pH range which the alumina is kept affect its separation properties? I'm thinking of things in a "Henderson-Hasselbalch amino-acid titration way," but I suppose alumina may be more protonated at lower pHs than higher ones ... does this affect it's separation properties?
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  • $\begingroup$ See if this helps: dynamicadsorbents.com/alumina/what-is-alumina $\endgroup$
    – AChem
    Sep 29, 2022 at 1:25
  • $\begingroup$ @Achem The source says "active sites used for separation consist of acidic alumina ions, basic oxide ions and polar Al-OH groups" but also says " the hydroxyl groups at the surface are responsible for the selective retention of adsorbed materials." $\endgroup$ Sep 29, 2022 at 13:28
  • $\begingroup$ The latter statement implies alumina's OH groups are responsible for separation quality. But how do the acid/basic properties of alumina affect its separations of other acidic/basic compounds (if that is the case at all). What's the "mechanism" for how acid/base properties would help a stationary phase? That's what I'm confused on because I don't understand how proton transfer leads to adsorption. $\endgroup$ Sep 29, 2022 at 13:32
  • $\begingroup$ "Retailers seem to be selling alumina stationary phases in various pH ranges: pH 6-7.5 (called neutral); pH 4-5.5 (called acidc...); pH 8-9.5 (called basic). " That rings my absurd detector. What the heck is that even supposed to mean in such a solid? $\endgroup$
    – Mithoron
    Sep 29, 2022 at 20:49
  • $\begingroup$ @Mithoron I really don't know. It confused me as well which was part of the reason I asked here to get some more information. You can look at the hyperlinks and investigate if you wish. $\endgroup$ Sep 29, 2022 at 23:22

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pH of alumina

Alumina for chromatography is not "just" simple aluminum oxide. It is made by heating aluminum oxide particles in a carbon dioxide stream at very high temperatures:

When aluminum hydroxide is screened for particle size and heated in a carbon dioxide stream at about 900°C, it is converted to individual particles of aluminum oxide that are coated with a thin layer of aluminum oxycarbonate (with the approximate formula: $\ce{Al2(OH)5]2CO3@H2O}$. The alumina particles are between 70-290 mesh (50-200 mm), and most are "approximately 150 mesh". Water content and alkalinity are then adjusted by washing with dilute acids.

So the high-temperature (900 °C!) treatment with $\ce{CO2}$ alkalizes the alumina surface by forming "aluminum oxycarbonate". These alkaline surface groups can be removed by washing with acid. Without washing, the alumina will give a basic pH on addition to water.

Why is this useful for chromatography?

Altering the proton affinity of the solid (aka stationary) phase in chromatography can alter the way it interacts with analytes. I'd suppose that since pyridine is a weak base, it exists in most solutions in equilibrium with some amount, perhaps a very small one, of pyridinum ion, and that these pyridinium ions would interact much more strongly with a basic surface than would non-ionic species.

However, it's worth noting the same document I excerpted above cautions against using basic alumina with ketones like acetone. It can cause polymerization reactions. Whether the same thing happens to your cyclohexanone may be concentration dependent.

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