From our mentor-by-literature Neal Anderson comes talk of using activated charcoal in your process (from "Practical Process Research and Development", page 215.):
Polar impurities can be removed by stirring a solution of the product with 1-2 wt% of activated carbon relative to the solute, adsorbing these impurities to the finely divided solid. Impurities are trapped in the pores of the activated carbon by van der Waals attractive forces. There are three categories of pore sizes: macroporous (1000 - 100,000 Å), mesoporous (100-1000 Å) and microporous (<100 Å)...Yeah, I hate the stuff. You can't clean it off from anything, and you're always sure it's in there somewhere. Celite does well at getting most of it; I haven't used any of the in-line filters, so I can't speak to those. But when I think about it, I think to myself, "NO." (I'm probably too harsh -- it is useful stuff.)
Some tips:
Activated carbon treatment can cause difficulties in cleaning equipment, as the finely divided solid is moderately electrostatic and tend to adhere to almost every surface it contacts. No solvent effectively dissolves charcoal, although vessels may be cleaned by boiling out with aqueous NaOH. For this reason, activated charcoal treated on large scale is often limited to dedicated vessels. Solution may also be passed through in-line filters containing granular, spherical, or pelletized activated carbon; these filters retain the activated carbon and avoid many of the cleaning issues.
- Adsorption of a molecule to activated carbon may change the pH of the resulting filtrate.
- Adsorption to activated carbon is based on equilibria. Short contact times decrease the efficiency of impurity adsorption.
- Performance of activated carbons can vary greatly.
I like charcoal, but I agree it's a pain to clean off your glassware (and everything else).
ReplyDeleteActivated charcoal is great if you don't feel like going to church on Ash Wednesday.
ReplyDeleteMy view is the opposite and think YES as would use carbon treatment as SOP for all process, particularly prior to any crystallizations. Correct there are physical/clean up issues to consider (definitely agree about Celite trapping) however the actions have never been much more extensive than requirements for dealing with other residues. Commercial in-line filter cartridges are OK if they work but can be relatively costly, have limited types of C available (as in point 3 selection and matching is critical to find best) and can be slow (many more recirculations, due to less surface area than powders). I have seen chemical operators jury-rig in-line filter systems that perform great. The reward:work ratio is highly favorable in my experience.
ReplyDeleteCMCguy
CMCguy: I stand corrected!
ReplyDeleteI wonder if anyone tried to use C-18 silanized silica, the standard reverse phase material, in the same way as charcoal. Sure it is more expensive but it is recyclable and it is white and should have less problem with microparticulates
ReplyDeletemilkshake I have often seen/used pass or recycle through a silica or alumina plug as an simple means to remove "baseline" impurities (including inorganics) even at larger scale (actually even good for post C treatment much like Celite). Like everything else must do good upfront work to determine right combination of solvents and absorption media for particulars and do extensive pre-wash as can get significant leaching in some systems.
ReplyDeletePlease do not forget charcoals awesome properties as a carrier material for your standard hydrogenation catalyst. In that case the welcome property of C for polish filtrations to absorb your high molecular weight trace impurities can sure give you a headache when hydrogenating on scale. Not very pleasing when you have to continuously readd catalyst to drive the reaction to completion after switching to a new batch of that waaaay cheaper starting material
ReplyDeleteI agree with the sentiment that it is a pain for most purifications. Think it is a little sadistic that it is often used in intro organic chemistry teaching labs. Students usually think, the more I use, the cleaner my stuff! They often forget their yield also decreases with increased activated charcoal. Or perhaps that is the lesson that is being passed on in those lab experiments....
ReplyDeleteAnother of the benefits of activated charcoal is that it helps eliminate bad breath, because it cleanses both the mouth and the digestive tract. It also helps to purify the blood. use of activated carbon
ReplyDelete