Sciencemadness Discussion Board

Methanol Dehydration

mbrown3391 - 14-6-2007 at 14:42

This isn't for school or anything, but im trying to synthesize ethylene gas from ethanol. Im using the denatured stuff from the hardware store, which is 4% methanol, and dehydrating it with conc. sulfuric acid and heat, which i know produces ethylene, co2 and some so2. I then pass this through NaOH to destroy the latter two gasses and collect it in a baloon.

Anyway, my question is, what gasses would methanol produce by this method of dehydration, or does methanol even dehydrate. I thought I read somewhere that it does, but there is no oxygen in its structure so how can a water molecule be removed?


[Edited on 14-6-2007 by mbrown3391]

garage chemist - 14-6-2007 at 14:48

Dimethyl ether will likely be the main product from methanol and H2SO4.
Also, it will partially be oxidised to formaldehyde and formic acid, the latter immediately dehydrates to CO under those conditions (hot mineral acid).

Dimethyl ether will be an important contaminant of your ethylene.
If you plan to add chlorine to the ethylene to obtain 1,2-dichloroethane, be aware that mono- and dichlorodimethyl ether are very toxic, the latter being the strongest known human carcinogen. So consider the dimethyl eher in every synthesis you are doing with the ethylene. Dimethyl ether itself is quite harmless apart from its flammability.

mbrown3391 - 14-6-2007 at 15:30

do you think the Dimethyl ether would dissolve in the sodium hydroxide solution? it has a solubility in water of 328 g/100 mL (20°C). If not, is there some other chemical i could put in the sodium hydroxide solution which would react with the dimethyl ether?

garage chemist - 14-6-2007 at 15:51

Where do you have that solubility data? It sounds like bullshit.
Dimethyl ether boils at -24°C, there is no other gas that dissolves to only nearly 100g/100ml in water.
Maybe it was milligram instead of gram?

mbrown3391 - 14-6-2007 at 16:09

Wikipedia says 328g/100ml
http://en.wikipedia.org/wiki/Dimethyl_ether

however its likely a mistake because i found 6.9g/100ml on another site
http://www.chemicalland21.com/petrochemical/DIMETHYL%20ETHER...

Edit:
Still, 100g of denatured alcohol should only produce 2.88g of dimethyl ether--not an unreasonable amount to dissolve.

[Edited on 15-6-2007 by mbrown3391]

garage chemist - 14-6-2007 at 16:20

I looked it up in Ullmann. Correct is:

at 1 bar 5,7wt-% (that means 5,7g per 100ml of solution)
and at 4,8 bar 36wt-% (that means 36g per 100ml of solution).
Both at 20°C.

The second value is probably the cause of the incorrect figure on wikipedia: they had the value wrong by a factor of ten (probably the figure was given as g per liter of water, and they wrote it as g per 100ml) AND they forgot to state that this figure is only true at 4,8bar of pressure!

Anyone who trusts wikipedia with numbers or critical information is a fool, and so is everyone who doesnt question the things he reads on the internet, regardless of the source.

Dimethyl ether is only moderately polar, and everyone who knows just a little chemistry will think that it can't have a higher solubility than a highly polar gas like ammonia, which dissolves to the extent of 35wt-% in water at 1 bar.

EDIT: The dimethyl ether will not be dissolved by the water. Remember the concept of partial pressure- the solubility in water is only true for an atmosphere of pure dimethyl ether above it.
When the dimethyl ether is diluted, the solubility will be much lower.
That is the reason why you can remove dissolved gases from water or other liquids with an air or inert gas stream- you lower the partial pressure of the dissolved gases above the solution.



[Edited on 15-6-2007 by garage chemist]

mbrown3391 - 14-6-2007 at 16:27

Then it would be reasonable to assume that 2.88g dimethyl ether could dissolve in the water? I would never be using more than 100g of the denatured alcohol anyway since 100g should make around 46 L of ethylene

garage chemist - 14-6-2007 at 16:32

No, it is not reasonable to assume that the dimethyl ether will dissolve in the water. I explained it in my last posting here, in the edit.
The dimethyl ether will stay with the ethylene, and you will not be able to remove it easily.

What do you want to do with the ethylene anyway? That is the question here.

mbrown3391 - 14-6-2007 at 16:41

im doing some experiments with the effects of ethylene on various plants. ethylene is one of the basic plant hormones and has very different effects on different plants such as causing a plant to flower prematurely, causing fruit to ripen faster, among other things. The presence of other gasses could ruin the experiment however.

garage chemist - 14-6-2007 at 16:48

Dimethyl ether is nontoxic to all forms of life and has no biological role. You can use your ethylene just as it is.

Ethylene is employed commercially for ripening bananas, and is used in very dilute form for this purpose. It is a hormone, and active in small amounts.
The gas mixture for ripening bananas is 5% ethylene in nitrogen, which is nonflammable and therefore easier to handle.
This is mixed with a large amount of air in use.

mbrown3391 - 14-6-2007 at 16:53

I need a better storage system. it went right through the balloon in minutes.

Edit:
Also, upon further inpspection, the water appears to have collected oily dropplets, probably ether. How can i limit the production of ether and force the reaction to produce mostly ethylene.

[Edited on 15-6-2007 by mbrown3391]

garage chemist - 14-6-2007 at 17:09

Alum (potassium aluminium sulfate), or simple aluminium sulfate, is a good additional catalyst that makes the dehydration of ethanol give ethylene as the main product instead of ether.

A methode for ethylene preparation as part of another synthesis I have here says to mix 25g alum, 25g ethanol and 150g conc. H2SO4.
The mix is heated until ethylene production starts, and then a mix of 150g ethanol and 300g conc. H2SO4 is slowly dropped in during heating order to keep the ethylene evolution constant.

The resulting gas is washed with conc H2SO4, then with 10% NaOH and again with conc. H2SO4. (three washing bottles in series)
The H2SO4 dissolves and binds ether very well (oxonium salt formation). It will also remove the dimethyl ether this way.

[Edited on 15-6-2007 by garage chemist]

[Edited on 15-6-2007 by garage chemist]

mbrown3391 - 14-6-2007 at 17:15

thanks, ill try that

mbrown3391 - 14-6-2007 at 18:10

is it ok to breathe the gasses that come out of this system? because a fair amount of it escapes through the baloon

garage chemist - 14-6-2007 at 19:36

I looked it up in another book (Sartori, The War Gases, available in the Sciencemadness library, Preparation of dichloroethyl sulphide), and they indeed use alum instead of aluminium. So this was a typo, and you need to use potassium aluminium sulfate, not aluminium.

Ethylene is of low toxicity, but you should really check your balloon, its connection most likely isnt airtight, or there is a leak on some other part of the apparatus.

Ozone - 14-6-2007 at 20:26

Or... your gas is diffusing *through* you balloon. What is your balloon made from? Latex will definitely leak.

Cheers,

O3

mbrown3391 - 15-6-2007 at 06:38

its definately diffusing through the balloon. the balloon begins to inflate but at a certain point it just stops getting any bigger even though gas is still bubbling though the water. there are no leaks, so the pressure in the balloon must be forcing the ethylene out at a certain point.

Edit:
I also read something about using aluminum oxide as a reducing agent. You boil the ethanol and pass the vapor over hot aluminum oxide then run the gas through water. Any thoughts on that?

[Edited on 15-6-2007 by mbrown3391]

roamingnome - 15-6-2007 at 08:38

Quote:

I also read something about using aluminum oxide as a reducing agent. You boil the ethanol and pass the vapor over hot aluminum oxide then run the gas through water. Any thoughts on that?



maybe you read something about it in the
"ethylene production" thread found here

if you are working with plants etc... purchasing everclear or 95% consumable alcohol will completly eliminate methanol concerns....

dehydrating methanol is interesting dont get me wrong... maybye as a far out methylation tool......

garage chemist - 15-6-2007 at 19:36

The standard method for ethylene preparation from ethanol is to run ethanol vapor through a tube filled with aluminum oxide (acting as catalyst) and heated to ca. 500°C.
The industry once did this on large scale, for manufacture of e.g. ethylene chlorohydrin and dichloroethane.
The catalyst tube can be conveniently made of copper and be heated by a bath of molten potassium nitrate.

The resulting gas is cooled to condense unreacted ethanol and byproduct water, and the ethylene is washed with H2SO4 and used right away.

guy - 15-6-2007 at 23:26

Quote:
Originally posted by garage chemist
The standard method for ethylene preparation from ethanol is to run ethanol vapor through a tube filled with aluminum oxide (acting as catalyst) and heated to ca. 500°C.
The industry once did this on large scale, for manufacture of e.g. ethylene chlorohydrin and dichloroethane.
The catalyst tube can be conveniently made of copper and be heated by a bath of molten potassium nitrate.

The resulting gas is cooled to condense unreacted ethanol and byproduct water, and the ethylene is washed with H2SO4 and used right away.


Won't the molten nitrate eat at the copper?

mbrown3391 - 16-6-2007 at 16:20

I would buy consumable alcohol, but unfortunately im a minor, so i cant buy the non denatured stuff... at least not easily that is