Sciencemadness Discussion Board

Reusing Column Silica

dermolotov - 27-12-2015 at 13:48

That stuff is expensive and it'd be quite effective to find a method to reuse silica.

Has anyone done this before? There's always the little disk of junk on the top layer but I'm talking more about soaking it in a giant vat of solvent and perhaps distilling it off then rewashing it. Much like a soxhlet extractor but reverse?

I already do that with my TLC plates (just let them develop in a chambre overnight). Finding a method to clean silica would be quite nice.

BromicAcid - 27-12-2015 at 14:29

Depends on what you are running through your column and how tightly it is bound. Even though you are leaving a layer at the top you don't know what is between the top and the bottom, sometimes just throwing a black light on it will show you plenty of bands. I have seen columns setup and after the target compound is off them the polarity of solvent is increased or decreased to 'flush' the column and then it is used in place as is (usually for the same thing). You could wash it and filter it, I'm sure that would help, but you wouldn't want to distill it off, you'll leave your crap in with your silica gel. Instead of running something "soxhlet extractor but reverse" a soxhlet extractor is exactly what you would be looking for except that your solvent probably won't percholate through your silica gel that quickly.

PS: If you really are capturing your impurities at the top of your column then don't bother with the column, just shoot it through a silica gel plug.

A better explanation.

dermolotov - 27-12-2015 at 14:50

Quote: Originally posted by BromicAcid  
Depends on what you are running through your column and how tightly it is bound. Even though you are leaving a layer at the top you don't know what is between the top and the bottom, sometimes just throwing a black light on it will show you plenty of bands. I have seen columns setup and after the target compound is off them the polarity of solvent is increased or decreased to 'flush' the column and then it is used in place as is (usually for the same thing). You could wash it and filter it, I'm sure that would help, but you wouldn't want to distill it off, you'll leave your crap in with your silica gel. Instead of running something "soxhlet extractor but reverse" a soxhlet extractor is exactly what you would be looking for except that your solvent probably won't percholate through your silica gel that quickly.

PS: If you really are capturing your impurities at the top of your column then don't bother with the column, just shoot it through a silica gel plug.

My explanation was inept. Sorry.
If my mixture was unburnt and stable (most lab procedures like demethylation, PCC, high temperature DMSO reflux, etc leaves behinds the black gunk that stays at the top of the silica), could I just stir the silica in excess aluent, decant into a rotovap, then pour the remaining solution back into the silica vessel to soak/stir and repeat that to wash it through? That should release any adsorbed spots, right?
I guess you are right with the whole "case by case" situation.

Dr.Bob - 27-12-2015 at 16:51

Years ago, my professor used to save the bulk of the used silica gel (most guys would ditch the top 1/2" of the material, as that is where most of the trash gets bound), wash it well, heat it to a high temp in a vacuum oven, and then reuse it for non-critical stuff.

dermolotov - 27-12-2015 at 20:23

Quote: Originally posted by Dr.Bob  
Years ago, my professor used to save the bulk of the used silica gel (most guys would ditch the top 1/2" of the material, as that is where most of the trash gets bound), wash it well, heat it to a high temp in a vacuum oven, and then reuse it for non-critical stuff.

Good to know it kinda works. Much like reusing silica plates, I suppose it's worth salvaging for non-critical stuff, as you mentioned. I guess he wasn't working with fluorescing materials, too that'd be adsorbed and dye the stuff everywhere.

If only I could get academia prices for their drums of silica gel. It's $100 for 2kilos here! :(. Ah well, 'tis the price of the trade.

How did he efficiently remove all the top 1cm anyway? Especially when plopped in the column like so.

Dr.Bob - 28-12-2015 at 08:00

It was not an exact science to get rid of the to bit of silica. If the column is damp, and you blow the silica out of it slowly with air, you just let the first glop go to the solid waste, then collect the rest. That also gets rid of any sand from the top. If it is try, it just takes a careful rotation of the column to keep the top material coming out first, and that helps some as well. If you heat the silica enough in a drying oven, it will burn off most of the remaining organics, but you want to make sure no solvent is left in it when you do that. Even "academic" prices for silica are very high, at least from my experience, so I try to use a plug to remove most of the baseline material when possible, and then crystallize products or use the smallest column that I think will work for products. Often if you can get a material mostly pure with a small column, then it will readily crystallize and then the mother liqueurs can be re-purified later if needed. A lot depends on if you are scaling up the same things over and over, or making a new compound each reaction. Lots of product variety makes it much harder to save silica gel.

Have you tried Silicycle? They are in Canada, make their own silica gel, and sometimes are flexible on pricing. You might be able to get some old materials or an off spec batch cheap. I have used their products before, and they have some great stuff, but more expensive in the US than US made materials.

dermolotov - 1-1-2016 at 01:17

Quote: Originally posted by Dr.Bob  
It was not an exact science to get rid of the to bit of silica. If the column is damp, and you blow the silica out of it slowly with air, you just let the first glop go to the solid waste, then collect the rest. That also gets rid of any sand from the top. If it is try, it just takes a careful rotation of the column to keep the top material coming out first, and that helps some as well. If you heat the silica enough in a drying oven, it will burn off most of the remaining organics, but you want to make sure no solvent is left in it when you do that.

Darn it! The issue is that I have a solvent reservoir so it's pretty hard to skim wet silica. I only have one column (3cm) that is a straight column with no bulb at the end. I'll figure a way out. I seem to always do..

Thanks for the recommendation as well as the source of silica. I'll check into them. I'll check them out!
Currently, I pay $100/ 500g of 60-200mesh silica. :o

macckone - 1-1-2016 at 09:49

There is no universal way to reuse silica as others have noted. The same rules that apply to fritted filters apply here. If it is just relatively low boiling point organics then slow baking is your friend. If it is stuff that will carbonize or inorganics then it is really difficult to clean. Organometalics are practically impossible to remove. The stuff that will carbonize can be cleaned out with pirahna solution made from vaccuum distilled hydrogen peroxide and vacuum distilled sulfuric acid. But then you have to remove the sulfuric acid. Triple distilled water using glassware is good for this as silica will be the only major contaminant. If you are cleaning the stuff up with a solvent you are likely to spend more on solvent than new silica. Chromatography grade solvent is required and that is expensive.

Morkva - 2-1-2016 at 08:59

Will ozone destroy C/H/N/O compounds in enough time? Silica catalyzes the decomposition of ozone, and there is a lot of silica, but you might still get it to work.

[Edited on 2-1-2016 by Morkva]

macckone - 3-1-2016 at 22:43

Ozone will react with carbonized crap but it isn't going to penetrate into silica due to the catalytic action. So you would need a really thin layer.