I am aware that the standard way to remove MEHQ from Methacrylic Acid is to perform reduced pressure distillation. The papers I see suggest using ~ 10 Torr pressure to perform distillation. Unfortunately I do not have access to such a pump. I am very aware of the fact that I can just add a little extra initiator, I was wondering if there was a way of removing MEHQ apart from the reduced pressure technique.
While flash chromatography is a viable method for small quantities, I have used activated granular carbon to remove the MEHQ. The carbon was 4-8 mesh (large granular for easy handling) that was placed in a vacuum drying oven at 120C for 24 hours. This was then added to the acrylate to adsorb the MEHQ. The slurry was gently mechanically agitated for good mixing. Please use a mechanical agitator, not a magnetic spin bar (as the bar tends to 'grind' the carbon to smaller powder against the bottom of the beaker/pot). You can monitor the MEHQ content, adding additional carbon and agitation time until you reach the desired inhibitor content. Then simply filter the carbon off of the acrylate. If you totally deplete all of the inhibitor, use the monomer as soon as possible. Acrylic acid polymerization is self accelerating (giving off a lot of heat of polymerization) and can result in a violent (read explosive) reaction (conditions).
I recommend removal of inhibitor on only amounts you can use within 24 hours of filtering. So long as the monomer is in contact with the carbon-MEHQ, I have not had any stability problems. Once filtered, the monomer is VERY reactive.
I have used this method with a number of different acrylate and methacrylate monomers, with sizes of 100 grams up to 10 kilos in an industrial setting.
Flash chromatography
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thanks your replies Peter Farrington and Ali Delibas i have methacrylic acid (MAA) and not methylmethacrylalate (MMA)
Yes, MAA melts at about 15degC. Warm it up and flash it through the column.
I'm unsure by what you mean by flashing it through a column. Do you mean run it through a neutral silica column at RT ?
Flash chromatography uses air pressure to drive the purified liquid through the column rather than hydro static pressure. It takes seconds instead of minutes.
While flash chromatography is a viable method for small quantities, I have used activated granular carbon to remove the MEHQ. The carbon was 4-8 mesh (large granular for easy handling) that was placed in a vacuum drying oven at 120C for 24 hours. This was then added to the acrylate to adsorb the MEHQ. The slurry was gently mechanically agitated for good mixing. Please use a mechanical agitator, not a magnetic spin bar (as the bar tends to 'grind' the carbon to smaller powder against the bottom of the beaker/pot). You can monitor the MEHQ content, adding additional carbon and agitation time until you reach the desired inhibitor content. Then simply filter the carbon off of the acrylate. If you totally deplete all of the inhibitor, use the monomer as soon as possible. Acrylic acid polymerization is self accelerating (giving off a lot of heat of polymerization) and can result in a violent (read explosive) reaction (conditions).
I recommend removal of inhibitor on only amounts you can use within 24 hours of filtering. So long as the monomer is in contact with the carbon-MEHQ, I have not had any stability problems. Once filtered, the monomer is VERY reactive.
I have used this method with a number of different acrylate and methacrylate monomers, with sizes of 100 grams up to 10 kilos in an industrial setting.
Peter Farrington Thank you for clarifying and pardon me for my ignorance- I have only packed wet columns. It's basically a column of dry alumina packed in a sintered funnel placed on a Buckner flask with vacuum ? Is there a suggested ratio of analyte to be filtered and alumina to be used ? I am planning to flash through the column ~ 2 mL of MAA, is that too small an amount ? I need only 600 uL for my reaction currently.
Paul Schmalzl thank you for your suggestion! The thought of using activated charcoal didn't cross my mind at all! I shall definitely use it when I scale up my reaction.
Paul Schmalzl how do you detect the removal of MEHQ from your acrylic acid monomer ? Do you perform UV-Vis ? I found this online, where they check for an absorbance peak around 220 nm but its for acryonitrile. I think MAA has a shoulder that extends over the said region. Frequent NMRs are kinda out of my reach.
Hi Uma, for small volumes a glass pipette works fine. Jam a 2mm ball of glass wool into the constriction and the add 2cm column of alumina. Then add you monomer carefully and then apply some air pressure and collect the purified monomer.
Uma, The colorimetric determination of MEHQ in Acrylates is easily performed by ASTM D-3125 method. This involves sodium intrite, 0.1 N HCl solution and a visible spectrometer. The calibration is linear over 0.5 to 120 ppm MEHQ concentration. The procedure is quick, like < 20 minutes, and accurate to +/_ 0.25 ppm. The ASTM procedure has been used for a long time in industry, reference DOW/DUPONT: https://dowac.custhelp.com/app/answers/detail/a_id/2960/related/1
article. You will have to develop the calibration curve for the spectrometer you wish to use.
Also, the DOW answer page (above) has several references relating to Acrylates, particularly when stripped of inhibitors. Please read these and be aware of all safety precautions you will need to take when removing inhibitors from reactive monomers.
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