Best Method to Make Methylamine Gas from 40% aqueous solution.

Background

     Methylamine (MeNH2) is commercially produced for sale in three major forms:
•Compressed gas
•Methylamine hydrochloride salt
•40% aqueous solution

     Handling the first two forms has been well discussed in Hive forums.  However, a review of TFSE and the Rhodium chemistry page reveals unsound information regarding how best to extract methylamine gas--MeNH2(g)--from the 40% aqueous solution.

     40% methylamine solution is commercially produced by the absorption of MeNH2(g) in water.  Each liter of solution contains 360 g of MeNH2.  This solution is not explosive, but it is flammable and should be handled with the usual care taken for toxic flammable liquids, especially regarding exposure to open flame or to an ignition source.
 
According to the write-up by Sunlight on Rhodium’s chemistry page, MeNH2(g) is best produced by dripping the 40% solution over solid NaOH.  While this method produces gas, unfortunately it has disadvantages: it is low yielding, slow, and does not lend itself to larger scale production because a waste product containing solid NaOH and a thick solution of MeNH2 forms in the bottom of the reactor flask and cannot be easily removed.  In fact, this method is almost unworkable.  The person using this method is faced with the choice of either throwing away the reaction flask after use, or actually digging in and trying to extract the stinking NaOH cake and MeNH2 liquid in the bottom.   Not a pleasant task.

In response to a recent post, Rhodium suggested converting the 40% solution to MeNH2•HCl (aq) by addition of muriatic acid, boiling off the water, and then reacting the dry methylamine hydrochloride salt with NaOH to generate MeNH2(g).  This method works, but reacting concentrated 40% MeNH2 solution with acid is time-consuming and uses more acid plus NaOH than the method proposed here.   In addition, it leaves a substantial amount of NaCl solution, contaminated with MeNH2, that must be removed and discarded.  It does not allow for easy larger scale production.
 
Through experimentation it was determined that a third method was superior to either of the previous two.  This third method is simple and was derived from experiments based on the suggestions of the members responding to a recent post on the subject.  Thanks go to them and especially to Rhodium for the information provided.  So that nobody will have to suffer through all the trial and error and gas inhalation SWIA endured by naively following the Sunlight write-up and other unsound posted suggestions, and in the interest of establishing clear and definitive information for novice chemists, I offer the following “best” method for converting 40% aqueous solution of MeNH2 to MeNH2(g) with 99% efficiency and in a manner that allows for larger scale production.


Discussion:

Best Method
  Through experimentation it was determined that the best method for extracting the MeNH2 from the aqueous solution is to raise the temperature of the solution while stirring.  Gas is produced immediately upon stirring at standard temperature and pressure and the solution begins to boil at 60ºC.  Copious amounts of MeNH2(g) can be obtained by gradually increasing the temperature of the solution between 60ºC and 80ºC at normal pressure.  A reflux condenser and a gas washing tube filled with anhydrous MgSO4 to pre-dry the gas and 3A molecular sieve to provide a final drying are sufficient to remove any water vapor.  (Note--use of NaOH, as has been suggested in Hive posts, to dry the gas is not recommended for the following reason:  NaOH will form a hard moist cake at the vapor/NaOH interface.  This thin cake will eventually impede gas flow, raising line pressure, and causing a joint to pop or explode--escaping toxic noxious MeNH2 fumes will quickly render the workplace uninhabitable.  This is a fact based on experience, not idle speculation, so avoid the posted suggestion to use NaOH to dry MeNH2 gas: that suggestion is unsound and although a person can “get by” in the short term, in the long term it will eventually lead to catastrophe.)
 
As the temperature of the solution increases to 80ºC  water vapor is observed condensing in the lower half of the reflux condenser.  After a time at the same temperature the production of MeNH2(g) begins to decrease.  At this point the partially spent solution is allowed to cool and is pumped out of the reactor flask (a peristaltic pump is ideal) and into a plastic (HDPE) carboy for further treatment to recover the remaining MeNH2.  The reactor is reloaded with fresh 40% solution (again, a peristaltic pump is ideal--if this type of pump is unfamiliar see http://www.masterflex.com) and stirring and heating are applied, as above, until the temperature reaches 80º and gas production diminishes, whereupon this partially spent solution is added to the contents of the carboy, and the reactor reloaded.  In this fashion, a substantial amount of 40% solution can be processed, without having to take the gas apparatus apart, nor expose the work area to MeNH2 fumes, nor with any solid residue remaining in the boiling flask.

The partially spent solution can then be further treated to obtain practically 100% of the remaining MeNH2.  This is accomplished by addition of muriatic acid according to the reaction MeNH2(aq) + HCl(aq) = MeNH2•HCl(aq).  The reactor should be maintained in an ice bath during acid addition because there is substantial heat generated by acid addition.  Upon neutralization, the MeNH2•HCl solution is brought to a boil, the water and any MeNH2 vapor recovered by condensation, and the dry MeNH2•HCl can then be reacted with saturated NaOH solution to generate MeNH2(g) according to the reaction:

MeNH2•HCl(s) + NaOH(aq) = MeNH2 (g) + NaCl(aq)+ H2O
 
There is an advantage obtained by first boiling off the major part of the MeNH2(g), prior to acid addition, since substantially less acid is consumed and substantially less hydrochloride salt is produced, therefore less NaOH is needed to convert the hydrochloride to a gas, etc., and less mess and hassle overall.  In fact, a person could produce all the MeNH2 he or she might need by simple stirring and boiling of the initial 40% solution, easily unloading and reloading the reactor, and save the task of reacting the spent solution with muriatic acid for some later date.


Absorbtion of MeNH2 in MeOH 
It is assumed that members reading this post understand that one of the reasons for generating MeNH2 gas is to absorb that gas in cold, stirred MeOH.  By weighing the MeOH before and after gas dissolution, the amount of MeNH2 recovered can be calculated, which is required for additional syntheses.  Please note that several posts on the Hive have suggested using a dispersion tube when absorbing MeNH2(g) into MeOH.  That advice is not warranted and is unsound, because it will increase line pressure on the system which could lead to catastrophe.  Do not follow those suggestions--do not use a dispersion tube.  MeNH2(g) is readily absorbed in cold MeOH.  The b.p. of MeNH2 is -6ºC, so a acetone/dry ice bath is sufficient to condense vapors--a water/ice bath will also work, if dry ice is unavailable.  Besides, MeNH2(g) is absorbed in MeOH at just about any temperature short of hot.  1/2”OD polyethylene tubing from the hardware store without any sort of dispersion device on the end of the tubing is sufficient for this purpose.

Suck-back control
Suck-back occurs when the amount of gas being generated is insufficient to compensate for the amount of gas being absorbed.  When the production of gas diminishes, suck-back will occur.  The suck-back of MeOH with MeNH2 gas can be rapid and violent.  Continual suck-back indicates that it is time to replace the spent solution in the reactor with fresh 40% solution.  Suck-back is controlled by stop-cocks to relieve line pressure.  However, a trap must be installed between the gas drying tube and the receiver for those inevitable times when the operator looks the other way and valuable MeNH2/MeOH solution is sucked into the system.  The trap must be larger than the volume of MeOH in the receiver, so that nothing is lost and nothing can reach the reactor.  If MeOH were ever to be sucked back into the hot reactor where the temperature is above the b.p. of MeOH, the resulting explosion of glassware will get everybody’s attention.  But be assured that this is not possible with the described set-up.

Equipment drawing
A drawing of a gas generator follows.  Two options are not shown.  What is not shown is that an equalizing addition funnel can be added to the gas generator at the joint on the reactor used for the pump in/pump out connection by adding a three-way claisen adapter to the joint (if these items of glassware are not familiar to you, see the post on “best method for HCl gas” for a drawing of an equalizing addition funnel and three-way claisen adapter--or go to http://www.aceglass.com  for descriptions of lab glassware).  Also not shown is that a special two-way adapter with a stop-cock take-off can be inserted between the reactor and the reflux condenser to convert the vertical reflux condenser into a regular condenser by opening the stop-cock. With this addition funnel and special adapter for the condenser, muriatic acid can be added to spent MeNH2 solution to form MeNH2•HCl (aq), without changing condensers.  The spent solution can then be boiled dry to recover solid MeNH2•HCl--all without taking the shown apparatus apart, thereby avoiding any release of MeNH2 fumes into the workplace.  The reason a kettle is preferred over a RB flask as the reactor is precisely in order to recover solid MeNH2•HCl from the spent solution.  (The kettle opens up for removal of the solid, the RB flask does not.)   However, a kettle is not necessary--a 3N RB is perfectly adequate, if only liquids are being processed and the final hydrochloride solution is not being boiled dry in the RB flask.  The reader can be assured that the described set-up is based on experience, not idle speculation.  By pumping 40% solution in and out, a 10L reactor (kettle or RB) and 5L receiver can easily process 20L of 40% MeNH2(aq) in a day, with corresponding production of MeNH2/MeOH solution.  This is more than adequate for most laboratories.     
Not shown on the drawing is that the receiver flask and magnetic stirrer and acetone bath are all supported on a lab jack/stand.  This allows for the receiver to be removed and replaced between batches, in order to weigh the contents and replace MeOH, without adjusting the rest of the set-up.  If using a kettle, either place an asbestos or ceramic cloth pad between the hotplate and bottom of the kettle, or slowly heat the flask, in order to reduce the possibility of uneven glass expansion at the base and possible breakage.   An RB flask would require a heating mantle.
   
Glassware should be clamped or secured to a lab frame.  All standard taper joints must be secured with Keck clamps: all hoses must be fastened with hose clamps.  The contents of the reactor and receiver must be stirred at all times: magnetic stirrers are best.

Many members will think this set-up too elaborate or too expensive, but should they ever be in the position of having to extract the MeNH2 from a substantial amount of 40% solution, experience will quickly teach them that anything less invites disaster.  The equipment shown is based on an actual set-up and is guaranteed to work as described-- efficiently, rapidly, safely.

Conclusion
I realize that most members will never face the task of processing quantities of 40% MeNH2 solution, but I hope this information will prove useful to the few who do. 

Regards
Argox

The drawing for this file and the previous hydrochloric acid file could not be posted with the machine I am using.  I have sent them to Rhodium, hopefully he can post them for me.


THIS IS FROM OLD HIVE, thinking it could be handy here

but I'll try to make a gas generator the next time

You should study what's been posted in:

https://the-collective.ws/forum/index.php?topic=11258

and try a lot harder if you want to avoid so much negativity  The end results from the effort applied 

i

Quote from: mdhard on October 05, 2012, 11:07:42 PM

excellent explanation ...

I tried to make a gas generator methylamine i could not ..

I managed to make 10% methylamine in dry MeOH from ..

sodium methoxide 30% in methanol, thus generated is not h2o. Naoh water is generated in this way does not work for this method ..

only then decant the solids left over from MeNH2.HCl .. the solution will stay completely dry ..

I also managed to find one in china 30% solution of methylamine in MeOH anhydrous ... with it will get easier

but I'll try to make a gas generator the next time

dont understand? sodium methoxide? why you need that to make Menh2?

sorry my english ...

used sodium methoxide to make the solution of methylamine in methanol, without making a gas generator, and without the formation of water in the solution.

to keep odor down you can bung the recieving flask with a moist rag to absorb odor or better yet one soak in dilute acid.
like vinegar.

actually ive bought once GAA and it said on the bottle "to eliminate fishy odors at seafood-meals" for uses!
so yeah, even cheaper than muriatic i guess.