Hi Ritter and everybody else! Thank you for welcoming me here, I´m sure I´ll enjoy this place. Don´t worry, I just love a good and lively discussion.
First of all, I just love catalytic hydrogenations/dehydrogenations/hydrogenolysis. It has to be the most versatile transformation process in all chemistry. Parr was the first company to make a robust and smart hydrogenation equipment commercially avalible. To this day it has still(pretty much)the same design it had when it was marketed around 1920. This is truly something they can be proud of. Still today it does it´s job well, but what I ment in my previous posts is that the job can be done better with our modern technology and knowledge.
Parr used a very smart way, shaking, to overcome the mass transfer barrier with the technology they had avalible then. But today we have better materials and technology. The type of reactors most in use today are stirred reactors and the loop reactor. The latter mainly for continous production. We can use a dispersed catalyst or a fixed catalyst. The latter again mainly in continous production.
I could go on the whole day babbling about the advatages/disadvatages between diffrent types of reactors, but let´s go back to the topic.
In a gas/liquid hydrogenation reactor we have to overcome one big obstacle, the mass transfer barrier. Gas-liquid-solid. Those three stops has the hydrogen molecule to pass before it can be utilised to reduce something. Hydrogen has quite low solubility in most used solvents. This means that at every given moment there is only a small amount of hydrogen solvated and avalible to the catalyst. The solubility in water for example is only 1:50, 1ml H2 in 50ml water @ 0 deg C. This is the big barrier. Why? Because hydrogen has to compete with the substrate, and now and then some nasty poisons, for the sites on the catalyst suface. sometimes we even add poisons to moderate the activity of the catalyst, lindlar catalyst for example. Those poisons has higher affinity for the sites of the catalyst than both the substrate and hydrogen. This is very much the same way as drugs are competing with neurotransmittors over receptor sites.
If we have substrates which has to go through a number of intermediate steps to become the product we want, like phenylnitroalkenes, and those intermediates can do freaky shit when they are left alone for a while, then we must minimise then time of their existense. That is by reducing them further as soon as possible.
Now if we have a catalyst which is depraved of hydrogen there is going to be more intermediates present, and freaky shit happens, like as big glob of tar as the product.
If we on the other hand have substrates like imines, enamines, alkenes, alkynes, aromatic nitros, N- and O-benzyls to reduce. Then there is way less of a problem to have a hydrogen depraved catalyst, since none of these forms any particulary kinky intermediates.
What type of reactor gives a hydrogen depraved catalyst then? Any type of reactor where you don´t have the solvent saturated with hydrogen, and lots and lots of finely dispersed hydrogen bubbles ready to go into solution as soon as they are allowed. By increasing the pressure inside the reactor you can get the solvent oversaturated with hydrogen. By increasing the amount of dispersed gas bubbles together with increased pressure(5-200 bar)you can make sure that the catalyst is surrounded by hydrogen at all times. This will speed up the reaction rate and lessen the life-span of the intermediates. The drawback is that you can get a runaway reaction if you don´t keep a close watch on whats going on inside. A very good proof of how big an obstacle the mass transfer barrier really is and how quick the catalyst becomes hydrogen depraved, is what happens when you have a reaction which is about to become a runaway and the temperature is skyrocketing. Just stop the mixing...No more hydrogen gets solvated, and the reaction dies immediately.
Of course one can run almost any kind of hydrogenation in a Parr shaker. The litterature is a good proof of that. But I´m simply saying that there are better ways to mix the contents and overcome the mass transfer barrier than shaking. This is the hollow shaft stirrer I mentioned earlier. It is not expensive at all, since it´s just a regular stirrer shaft but hollow, and perforated at the upper part, where gas gets sucked in, and at the lower end, where gas gets pressed out, all by the spinning of the shaft.
I have a couple of reactors myself. But the one I use most is a low-pressure(1-8 bar)vessel with a hollow shaft stirrer. With this baby I can run reactions which normally takes 5-24 hours and 5-20% w/w catalyst/substrate loading in a Parr shaker, in 0,5-5 hours and with 0,1-3% w/w catalyst loading only, and it is actually cheaper than a Parr shaker...
I have never done anything illegal nor will I, but let´s just for arguments sake say that I were to perform a reductive alkylation of methylamine with 3,4-MDP-2-P. PtO2 or Pt black are the most used catalysts in the litterature for this type of reduction. The amounts of those expensive catalysts used are almost silly. Ritter told us he were using 1,5g pure Pt(reused 4 times)for the reduction of 100g ketone. With my baby I would use 1 to abso-fucking-lutely-max-2g 5%Pt/C or Pd/C. 50-100mg versus 1,5g pure metal. This may perhaps seem a bit cheap considering the value of 500g product compared to the cost of the catalyst. But it also reflects the efficiency of the reactor(it also has to do with the surface area). My reaction time is also a bit less 1,5-2 hours @ 40-55 deg C versus 2,5-3 hours @ room temp.
I would have the following ratios: 1 mol ketone, 1,05 mol amine and 1,5 mol glacial acetic acid, all in some 200ml EtOH or MeOH. Dump in the catalyst first then the ketone and acetic acid. Mix the alcohol solvent with the aqueous amine and add this in 4-5 portions to the ketone over 4-5 minutes. Purge with argon(my choice) and pressurise to 2 bar with hydrogen. The reaction mixture should have reached 40 deg C by itself, if needed heat in a waterbath to that temperature. Start stirring at 1000 rpm. Keep the pressure constant at 2 bar. Check the progress of the reaction from time to time by closing the hydrogen feed-valve and observe the pressure drop. When the reaction ceases to consume hydrogen it´s done. But don´t do this too early. The very last bit of imine has a hard time to find the catalyst. So the reaction might appear to have ceased. Just let it stir for another 30 min @ 2 bar to reduce the last bit of imine. This last step is the diffrence between a 90-95% yield and a 98-99,9% yield.
Add 4-5-teaspoons celite to the mixture ad give it a good stir. Filter at the water pump through a regular filter paper in a buchner funnel. Re-run the filtrate once again using the same filter cake to get rid of the last catalyst. Strip off the alcohol in a rotovap. There will be a aqueous solution of the amine acetate in some acetic acid. Wash with 500 ml toluene, basify and there will be close to a theoretical yield of amine ready to be crystalised.