Volanschemia
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Potassium Phthalate from multiple Xylene isomers
So since my recent attempts at getting phthalates from the plastisizers in vinyl product have failed miserably (due to the plastisizer required being
banned in Australia), I had a look around for some other possible routes.
Research and advice pointed me in the direction of oxidizing o-Xylene with Potassium Permanganate to obtain Potassium Phthalate.
The idea looked good, seeing as I can get both Xylene and KMnO4 OTC and have the required glassware. The only problem is that the OTC
Xylene is sold as mixed isomers (p-Xylene and m-Xylene as well) and separating the isomers is widely known as being a major PITA.
My destination with the phthalates is to end up at Phthalic Anhydride, by decomposing Phthalic Acid, and it appears that only o-Phthalic Acid will
decompose, the rest merely burning.
So armed with this information, I have formulated a plan to get to Phthalic Anhydride by oxidizing the OTC Xylene with KMnO4 in a process
similar to this video to obtain a mixture of Potassium Phthalate isomers, acidify them all to obtain a mixture of Phthalic Acid isomers, and then heat it up
and sublimate off only Phthalic Anhydride from the decomposing o-Phthalic Acid and leave the rest behind.
Does anyone foresee any problems with this idea, or has anyone actually tried this before?
If anyone has some input for me, it'd be much appreciated.
"The chemists are a strange class of mortals, impelled by an almost insane impulse to seek their pleasures amid smoke and
vapor, soot and flame, poisons and poverty; yet among all these evils I seem to live so sweetly that may I die if I were to change places with the
Persian king" - Johann Joachim Becher, 1635 to 1682.
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UC235
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If you've never run a toluene or xylene oxidation with KMnO4 prepare to be blown away by the sheer quantity of oxidant needed to consume all the
xylene. Xylene's molar mass is 106.16 g/mol. To (theoretically) react it entirely, you need 4eq of KMnO4. This is 632g.
Not all of the xylene will react. In my experience, there are still some unreacted xylenes present even if KMnO4 is used up. Some ring-opening
oxidation occurs (at least with o-phthalic acid) and to turn even a small amount entirely into CO2 uses up a large amount of KMnO4. After
recrystallization (and starting from pure o-xylene) I only was getting 55-60% yield of phthalic acid.
Further complicating your plan is that o-xylene is a minority product in commercial xylene mix. (See http://www3.epa.gov/airtoxics/hlthef/xylenes.html ) It's primarily m-xylene which has no major commercial uses (except that small amounts of
isophthalic acid added to PETE makes the polymer harder to crystallize leading to easier to process, clear plastic). The o-xylene is oxidized to
phthalic anhydride for plasticizers mainly. p-xylene is feedstock for PETE plastic and is the most valuable isomer. Additionally, there is some
ethylbenzene content, which will oxidize to benzoic acid.
You also have the problem of reaction volume. I don't think I was running more than 20g in a 300ml reaction flask. So, lets say you scale up and run a
1L reflux for several hours to process a half mole of xylenes. This will use 316g of KMnO4. Going by the EPA reference above, let's say that you got
lucky and have a typical maximum of o-xylene present at 20%. We're now at 0.1mol of o-xylene. Due to all the other isomers present workup is not going
to be smooth. Lets say that you manage the 60% I was getting with isolated o-xylene. You're only getting 10g of the acid for all your time and effort
and expense. And that's a best case scenario. It could be half that of there's less o-xylene in the mix and your workup is difficult.
Another issue I can raise is with the plans to sublimate. terephthalic acid sublimes at 400C. isophthalic acid melts at 300C so it will have a
significant vapor pressure while you're subliming the phthalic anhydride. Benzoic acid will definitely be volatile at that temperature. An initial hot
water extraction will leave most terephthalic acid behind and give you benzoic and phthalic with some isophthalic. That concentrate may be easier to
figure out a workup for.
Alternately, there is the oft-mentioned but (AFAIK) as-of-yet undemonstrated KMnO4 oxidation of naphthalene to phthalic acid. Even though the KMnO4
consumption per mole is higher than for o-xylene, if it can be made to work it will be better than the mixed xylenes option.
Is there any way you could just buy/import potassium acid phthalate? It's a standard monoprotic acid for titrations in every lab since ever and has no
obvious dodgy uses.
[Edited on 10-12-2015 by UC235]
[Edited on 10-12-2015 by UC235]
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Volanschemia
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Something I should have mentioned in the OP is that I only intend on making a small amount (likely <3g) and it has to be OTC.
Thanks for that UC235, and for your video as well. I didn't realise that there is so little o-Xylene in the paint thinner stuff, that could be an
annoyance. I checked the MSDS for my supplier of OTC Xylene and it doesn't say anything about ethylbenzene, it just says <0.1%w/w Benzene, which
isn't a lot.
Phthalic Anhydride sublimates at just under 300C, so I don't think other byproducts vapourizing will pose too much of an annoyance, as long as the
temperature is kept down, and assuming there isn't going to be much, if any, benzoic acid present.
The yields do look quite dismal for this method, I will look into the naphthalene pathway you mentioned, I think i have heard it discussed briefly
before. If it works, I should be able to use it since naphthalene is OTC as well.
"The chemists are a strange class of mortals, impelled by an almost insane impulse to seek their pleasures amid smoke and
vapor, soot and flame, poisons and poverty; yet among all these evils I seem to live so sweetly that may I die if I were to change places with the
Persian king" - Johann Joachim Becher, 1635 to 1682.
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byko3y
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I can actually buy a relatively cheap o-xylene, because it's the only isomer that can be separated by a simple distillation on something like 30-50
plates column.
I've tried ractional crystallization, but only few percents of all the stuff crystallized at -40°C. As far as I can remember, tertiary eutectic is
something like 10% p-, 25% o-, 65% m-xylene, and it melts way below -50°C.
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Boffis
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@TAuSc I mentioned a paper that I had found regarding oxidation of xylene and separation of the phthalic acid isomers in my thread on terephthalic
acid but I didn't reference it at the time as it wasn't particularly relevant to the reactions of terephthalic acid suitable for home chemistry.
However, on the first page or two the paper below describes the oxidation of mixed isomer xylene with potassium permanganate and the separation of the
resulting isomers. It goes on to describe Friedel Craft reactions of various product. I haven't tried the process so good luck. I think you will need
to start out with quite a large quantity otherwise you will not have enough raw product to carry out the crystallisations with.
Check out:
M. E. Smith; J. America Chem. Soc.; 1921; v43(is8); p1820: ja01441a020
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HeYBrO
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I have some potassium hydrogen phthalate if you want?
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Volanschemia
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Thanks for the paper Boffis, I'll check it out when I get the chance.
@HeYBrO Thanks for the offer, I appreciate it. I don't need any at the moment, but I'll keep it in mind.
"The chemists are a strange class of mortals, impelled by an almost insane impulse to seek their pleasures amid smoke and
vapor, soot and flame, poisons and poverty; yet among all these evils I seem to live so sweetly that may I die if I were to change places with the
Persian king" - Johann Joachim Becher, 1635 to 1682.
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Boffis
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Here is the paper I referred to above, reading it again its actually more relevant than I had previously given it credit inspite of its use of rather
unobtainable acid chlorinating agants. Again on closer reading they use fractional distillation to remove most of the o-isomer first, still some
experimentation would be worthwhile on the full mixed isomer mixture and the paper also gives the conditions for esterifying the isomers.
Note the amount of potassium permanganate per unit of xylene; 300g KMnO4 to 60g Xylene.
Attachment: Oxidation of mixed xylene and separation of phthalic acids JACS01441a020.pdf (75kB)
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