AJKOER - 10-1-2013 at 13:24
Need some input here as I suspect the reaction product may be of interest. Assume one adds Al strips in an open container plus a little NaCl (to alter
pH) to a solution of CH3OH, C2H5OH and H2O.
With time (at least a day), I would suspect/speculate on the following reactions:
2 Al + 6 H2O --NaCl--> 2 Al(OH)3 (s) + 3 H2 (g)
as Aluminum will slowly react with water in the presence of chloride to weaken the protective Al2O3 (a major corrosion issue). To quote one source:
"For aluminum, pitting corrosion is most commonly produced by halide ions, of which chloride (Cl -) is the most frequently encountered in service.
Pitting of aluminum in halide solutions open to the air occurs because, in the presence of oxygen, the metal is readily polarized to its pitting
potential. " See http://www.keytometals.com/Article14.htm
Then, with the removal of the water, the reaction between Methanol and Aluminum increases:
2 Al + 6 CH3OH --> 2 Al(OCH3)3 + 3 H2 (g)
The reaction is also temperature and O2 dependent (see http://books.google.com/books?id=EKrYkYxYgHAC&pg=PA290&a... ).
Also, for mixed alcohol solutions with water one source notes: "Anhydrous alcohol is known to react with aluminum at high temperatures, resulting in
aluminum alkoxide formation. However the reaction when a mixture of two or more alcohols are used has not been reported. Moreover the addition of
minute quantities of water has been shown to prevent this alkoxide reaction. " (see http://sciencelinks.jp/j-east/article/200404/000020040404A00... ). And, as such I am not expecting any reaction between Ethanol and Al until the
water is removed at room temperature. However, this is a not completely clear/certain as in the presence of more corrosive AlCl3 (not NaCl), a
reaction does occur between Al and C2H5OH (see full paper at http://www.jim.or.jp/journal/e/pdf3/50/06/1433.pdf ). This paper also notes that diffusion conditions (like from stirring) are significant with
respect to reaction rate.
Generally speaking, I would also limit the amount of Al to that needed (1/3 of the total moles of H2O and CH3OH) to react plus a safety factor based
on the concentration of CH3OH. In the worst case, Methanol just reacts with Aluminum equal to (but more likely faster) than Ethanol, thereby reducing
the CH3OH presence and possibly concentrating as well. Complete with a vacuum distillation of say 75% of the solution. Some experimenting may be
needed to assess the best concentration of NaCl to maximize CH3CH2OH and reduce CH3OH.
There has been a prior citation in Sciencemadness of the reaction of Mg and Ethanol (see http://www.sciencemadness.org/talk/viewthread.php?tid=7174 ). There is also a patent reference (see http://www.google.com/patents/US6297188 ) on the reaction between Mg and Ethanol at its boiling temperature, so other than the relative cost and
material access considerations, I would expect better results (but more slowly derived) as Mg more slowly reacts with water than does Aluminum.
For related cited preparation of dry ethanol, please see http://www.lookchem.com/Chempedia/Chemical-Technology/Labora...
Feedback, especially on the stability of Al(OCH3)3, welcomed.
[Edited on 11-1-2013 by AJKOER]
chemrox - 10-1-2013 at 14:13
I'd like to see the equations balanced to account for the Na+ which is more reactive toward metallic Al than Cl- is. eg. its well known by most
housekeepers (how's that for PC?) that Al cookware should be kept free of all Na salts including bicarb, carbonate, lye and table salt. Aq. Na+ tends
to form Al(OH)3 and precipitate. Al+++ does not react with Cl- according to one source (http://www.public.asu.edu/~jpbirk/qual/qualanal/aluminum.htm...) so its hard to envision how the pitting occurs without activation by Na+. In a
marine environment there's reduced Cl available and a large amount of Na+. It might be interesting to look at the role of Na+ in pitting which I
believe is caused by Cl2 not Cl-.
bbartlog - 10-1-2013 at 14:54
The water is being transformed into further passivating compounds (Al(OH)3, Al2O3) as it's 'removed'. Its presence is only helpful as a solvating
agent for the NaCl, and in fact, if you get to the point where all the water has reacted and the ethoxides could plausibly be stable, I would expect
that the solubility of the NaCl would already be near zero. You could try some chloride salt that is more soluble in alcohol (like CaCl2) and start
your process with strictly anhydrous conditions, but I'm still skeptical that it will work.
AJKOER - 10-1-2013 at 15:17
Interesting feedback.
Per this source (http://www.jim.or.jp/journal/e/pdf3/50/06/1433.pdf ) to quote:
"The reaction is explained by the superimposition of Al/Al3+ reaction as the anodic reaction and ethanol/ethlate reaction as the cathodic reaction. In
an environment containing chloride ions, an Al/AlCl3 reaction participates in anodic reaction above and then becomes dominant anodic reaction. Rate of
aluminum dissolution by the Al/AlCl3 anodic reaction in ethyl alcohol is accelerated due to the fast system which has low polarization resistance."
As such, I suspect that an electrochemical based explanation is most likely. One might actually look at this as a case of galvanic corrosion.
One more note, this dated source (http://www.fischer-tropsch.org/Tom%20Reels/Linked/TOM%20231%... ) observes that when Cu ion is present, along with NaCl, that the reaction between
Al and C2H5OH is markedly and visibly increased, otherwise, limited (so avoid Cu contamination).
[Edited on 11-1-2013 by AJKOER]