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Palladium
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[*] posted on 24-5-2011 at 08:04
Pd/C H2-gas reduction of ß-nitrostyrenes



The reduction of ß-nitrostyrenes resulting in a phenetylamine, PEA. This is not the methylated PEA I'm purposing on, PEA is substance found in chocolate and is sold as nutrition.


References

We can take a look at this report which performing a reduction of ß-nitrostyrenes under very mild conditions:

http://www.lycaeum.org/rhodium/chemistry/ns.hydrogenation.ht...


Experiments

1. I followed the example in the link exactly, but used a normal 5% Pd/C catalyst, not in the report mentioned "5% Pd/C K-type" - which I can't find any further information about anywhere. I can tell you flat out that this process doesn't work - at least not with a normal Pd/C catalyst. Notice that they are using relatively large amounts of catalyst (200 g Pd/C 5% per mole PEA), probably because the Pd/C is undermined in efficiency at 0 °C compared to higher temperatures. I tried this reduction at 2-5 °C in EtOH and HCl (aq) at 1 atm H2-pressure in 3 h. No noticeable reaction; in fact, all of the ß-nitrostyrene was left when solvent was removed - and after heating much of this will polymerize.

2. This time from 50 °C the first half of time and up to 90 °C the second half, at 2 atm pressure but without HCl (aq). Reaction worked satisfactionally, but I haven't counted on the yields accurately.


Discussion

ß-nitrostyrenes are sensitive for heat and pressure What I have heard is that normally the yield is only around 50% to the corresponding PEA. I understand that higher temperatures will polymerize the ß-nitrostyrene - but not very fast - especially at 90 °C, where Pd/C works best (Pd absorbs more than 900 times its own volume of hydrogen at 90 °C).

Pd/C in acidic solution After reading several Pd/C reductions I can't see anywhere that this is performed in acidic solutions, except when it works as a promoter to form an ester - which is the case in a OH-reduction. I counted pH for the solution in the link to 0,5 - very acidic, isn't it? Too much acid can cause different types of tar, but didn't occur in my case at cold conditions.

Amount of solvent This process in the link also used very much solvent (3,9 L per mole PEA), which I also found not necessary. Too less solvent is not good, but I have an idea of using 1L EtOH per mole PEA.


Questions

1. What undesired reaction tends to happen to the ß-nitrostyrenes at higher H2-pressures?

2. Why would HCl in this solution promote the reduction?

3. Any idea about the minimum amount of solution in these reductions?

4. How would the ß-nitrostyrene respond to a Pt02-catalyst?
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[*] posted on 24-5-2011 at 10:15


There's some quite interesting stuff on β-nitrostyrene and its derivatives here.

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[*] posted on 24-5-2011 at 10:58


Quote: Originally posted by Palladium  

1. I followed the example in the link exactly, but used a normal 5% Pd/C catalyst, not in the report mentioned "5% Pd/C K-type" - which I can't find any further information about anywhere. I can tell you flat out that this process doesn't work - at least not with a normal Pd/C catalyst.

I don't understand why it would not work for beta-nitrostyrene when it works for methoxy substituted beta-nitrostyrenes. How sure you are you that your Pd-C is still active? How did you verify the purity and identity of your beta-nitrostyrene? Is your HCl free of H2S?

Quote:
Notice that they are using relatively large amounts of catalyst (200 g Pd/C 5% per mole PEA), probably because the Pd/C is undermined in efficiency at 0 °C compared to higher temperatures. I tried this reduction at 2-5 °C in EtOH and HCl (aq) at 1 atm H2-pressure in 3 h. No noticeable reaction; in fact, all of the ß-nitrostyrene was left when solvent was removed - and after heating much of this will polymerize.

As the experimenters at the Hyperlab forum demonstrated, the amount of the catalyst can be reduced to 5 mol% (or even less, highly depending on the substrate) and still obtaining good yields provided you use pressure in the range of 2-5 bar and start with a cold mixture. My memory is not very good, but I think they used slightly above 1 equivalent of conc. HCl in methanol. I do well remember that lots of substrates have been tested and no reports of failure mentioned. Check by yourself as I have no time to search info for you. I remember there is also an example of an arylacetonitrile hydrogenation with Pd-C. Most posts are in English so you should have no troubles with the search engine.

Anyway, the HCl is used to prevent the formation of secondary amines by in situ amine protection. Secondary amines form during the reduction of oximes and nitriles in the absence of added acid (see Hydrogenation Methods by Rylander). Since the reduction of beta-nitrostyrenes gives first the phenylacetaldoximes which are then the difficult ones to reduce (can be even preparatively isolated in good yields as is described in some articles), the acid can cause troubles due to partial oxime hydrolysis, especially if the temperature is higher. Aldoximes reduce slowly with Pd-C (contrary to their reduction with Raney nickel which is very fast) while, as far as I know, ketoximes are nearly impossible to hydrogenate with Pd-C. This slow reduction step is probably the reason for the high dependence of yields on the catalyst loading. If there is not enough Pd-C, much of the oxime intermediate hydrolyzes before getting reduced to the amine. I'm mostly guessing from the results described in the few articles on the topic, that I read (years ago, so don't rely to much on what I say).




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[*] posted on 24-5-2011 at 14:30




Quote:

I don't understand why it would not work for beta-nitrostyrene when it works for methoxy substituted beta-nitrostyrenes. How sure you are you that your Pd-C is still active? How did you verify the purity and identity of your beta-nitrostyrene? Is your HCl free of H2S?


I believe it works, but maybe the reason is because they have a "K-type" Pd/C catalyst? I don't have and I don't know what K-type is.

I'm very sure that the Pd/C works, and I know how to test it before. It's also pre-reduced and then tested before I start the reduction. Stirring was excellent.

The ß-nitrostyrene was highly purified through crystallization the day before.



Quote:

Aldoximes reduce slowly with Pd-C (contrary to their reduction with Raney nickel which is very fast) while, as far as I know, ketoximes are nearly impossible to hydrogenate with Pd-C.


I believe Ni/C do this reduction of ketoximes almost as well as RaNi, at least better than Pd/C?

However, thanks a lot for your answers.

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[*] posted on 24-5-2011 at 15:33


What do you think of this patent?
http://www.google.com/patents?id=G3AcAAAAEBAJ&printsec=a...
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[*] posted on 24-5-2011 at 21:24


Does anyone think it's a good idea to prepare my own Pd/C? I have some PdCl2 solution and activated carbon.
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[*] posted on 25-5-2011 at 04:23


Quote: Originally posted by Melgar  
Does anyone think it's a good idea to prepare my own Pd/C? I have some PdCl2 solution and activated carbon.


You almost can't failure that process. Just buy some cheap "Norit" activated carbon, and reflux this in 10% HNO3 (aq) for 3 h. Then you wash this with H2O several times until pH is neutral and then a few times more.

Then you solve NaAc in H2O together with pre-dissolved PdCl2 into an hydrogenation vessel, and add the HNO3-washed charcoal. Hydrogenate this with agitation until absorption ends. Filter off and wash several times with H2O. Drying isn't necessary of one rinse a few times with EtOH under inert atmosphere. If it's not inert avoid using EtOH at all, otherwise it's a great way to start a fire.
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[*] posted on 25-5-2011 at 12:06




Quote:

Anyway, the HCl is used to prevent the formation of secondary amines by in situ amine protection. Secondary amines form during the reduction of oximes and nitriles in the absence of added acid (see Hydrogenation Methods by Rylander). Since the reduction of beta-nitrostyrenes gives first the phenylacetaldoximes which are then the difficult ones to reduce (can be even preparatively isolated in good yields as is described in some articles), the acid can cause troubles due to partial oxime hydrolysis, especially if the temperature is higher.


I don't have detailed literature or sources about Pd/C, but I know its life if different depending on what it's used to - of course. At least 4-5 times according to some producers of Pd/C.

How about when the Pd/C is used in HCl (aq)? I guess the PdCl2 is dissolves out from C in that case, but not in e.g. GAA as the only acid in solution. I heard from someone that Pd/C is not reusable without processing after one time when a strong acid was present in a reduction. Is the Pd solved out when using HCl at pH below 1 in solution? Is this negligible when only GAA is the acid?


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[*] posted on 25-5-2011 at 13:22


I have never really reused Pd-C. It is more or less one use only under laboratory conditions. It can however be reactivated. I think it can be done by washing with alcohols, then NaOH(aq) or NH3(aq) and plenty of water. I don't see why palladium leaching would be much of a problem with HCl. The chloride anions are quite good ligands for Pd(II), but as far as I know they are not good enough for Pd(0). Though Pd-C tends to leach lots of Pd as colloidal particles in any solvent.

Quote:
I believe it works, but maybe the reason is because they have a "K-type" Pd/C catalyst? I don't have and I don't know what K-type is.

The posts at Hyperlab mention no special type of Pd-C. I believe the generic 5% Pd-C as can be found in any lab was used, or else the experimenters would specify it and I would certainly remember such a peculiarity. Your failure in repeating the literature method is most likely connected with other factors. Try the reaction on p-methoxy-beta-nitrostyrene or some other substrate that is verified in that Bull. Chem. Soc. Jpn. article. If you still don't succeed, then something is wrong with either one of the reactants/solvents/catalyst/HCl you use, or your technique.
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[*] posted on 25-5-2011 at 16:21


http://www.sciencemadness.org/talk/viewthread.php?tid=12792
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[*] posted on 26-5-2011 at 09:43


Quote: Originally posted by Palladium  
Quote: Originally posted by Melgar  
Does anyone think it's a good idea to prepare my own Pd/C? I have some PdCl2 solution and activated carbon.


You almost can't failure that process. Just buy some cheap "Norit" activated carbon, and reflux this in 10% HNO3 (aq) for 3 h. Then you wash this with H2O several times until pH is neutral and then a few times more.

Then you solve NaAc in H2O together with pre-dissolved PdCl2 into an hydrogenation vessel, and add the HNO3-washed charcoal. Hydrogenate this with agitation until absorption ends. Filter off and wash several times with H2O. Drying isn't necessary of one rinse a few times with EtOH under inert atmosphere. If it's not inert avoid using EtOH at all, otherwise it's a great way to start a fire.

Thanks. I have some Norit activated carbon already actually, I just need to acquire some nitric acid. The only thing is, I'm not sure how pure or what concentration my PdCl2 solution is. Some calcium chloride and sulfuric acid got in the solution by accident, but only a very small amount. It's a pretty opaque reddish-brown, but I doubt that indicates much. You think I should just guess, or is there an easy way to measure the concentration? Maybe by how much hydrogen it takes to reduce the PdCl2 to Pd?
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[*] posted on 27-5-2011 at 10:54


Quote: Originally posted by Melgar  
Quote: Originally posted by Palladium  
Quote: Originally posted by Melgar  
Does anyone think it's a good idea to prepare my own Pd/C? I have some PdCl2 solution and activated carbon.


You almost can't failure that process. Just buy some cheap "Norit" activated carbon, and reflux this in 10% HNO3 (aq) for 3 h. Then you wash this with H2O several times until pH is neutral and then a few times more.

Then you solve NaAc in H2O together with pre-dissolved PdCl2 into an hydrogenation vessel, and add the HNO3-washed charcoal. Hydrogenate this with agitation until absorption ends. Filter off and wash several times with H2O. Drying isn't necessary of one rinse a few times with EtOH under inert atmosphere. If it's not inert avoid using EtOH at all, otherwise it's a great way to start a fire.

Thanks. I have some Norit activated carbon already actually, I just need to acquire some nitric acid. The only thing is, I'm not sure how pure or what concentration my PdCl2 solution is. Some calcium chloride and sulfuric acid got in the solution by accident, but only a very small amount. It's a pretty opaque reddish-brown, but I doubt that indicates much. You think I should just guess, or is there an easy way to measure the concentration? Maybe by how much hydrogen it takes to reduce the PdCl2 to Pd?


I would rinse PdCl2 from CaCl2 and H2SO4 first. I'm not quite sure, but I think this process would work: Evaporate the solution with PdCl2 completely with heat. Chop up the solid with a mortar-and-pestle-set to dust. Reflux this dust in EtOH/IPA (CaCl2 is soluble in alcohol) a moment and filter off. Dry the solid again and dissolve it in HCl (aq), evapore to almost dryness, add more HCl - reapeat this a few times and lastly dry completely. Weight the solid, which should be only PdCl2. Maybe one day of work but Pd is expensive nowadays.

I don't know the amount H2 it takes, but I would give the pre-reduction more pressure than 1 atm to 2 atm. Someone can correct me if I'm wrong about the pressure of pre-reduction, but I think it's similar to PtO2 (2 atm).

If you don't have H2-gas just build your own generator and accumulate it in another tank. One day of work.
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[*] posted on 28-5-2011 at 11:45


Sorry, I had wrong about rinse PdCl2 from CaCl2 through dissolve CaCl2 in EtOH. PdCl2 is also soluble in alcohols, so just forget what I wrote in the previous post.

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[*] posted on 30-5-2011 at 05:08


I'm guessing that the H2SO4 probably reacted with the CaCl2 to form insoluble CaSO4. Either that or the CaCl2 reacted with the PdCl2 to form calcium tetrachloropalladate, if that's possible. Anyway, I made some nitric acid then cleaned and rinsed my carbon, so I'm almost ready for the hydrogenation part.

I'm thinking after I hydrogenate the palladium chloride, I'll just rinse the stuff with water a few times to get any of those salts off. I don't have a source of EtOH, so I'll probably use MeOH. I can use isopropyl alcohol too, but I have more MeOH and it's cheaper. Plus, its properties are closer to EtOH.
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[*] posted on 30-5-2011 at 15:51


Palladium: you mention Pd/C being relatively easy to prepare, but Sunlight posted extremely poor results for a CTH reduction when using their own Pd/C. Do you think it was just an error in their preparation?
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[*] posted on 2-6-2011 at 08:12


Quote: Originally posted by Melgar  
I'm guessing that the H2SO4 probably reacted with the CaCl2 to form insoluble CaSO4. Either that or the CaCl2 reacted with the PdCl2 to form calcium tetrachloropalladate, if that's possible. Anyway, I made some nitric acid then cleaned and rinsed my carbon, so I'm almost ready for the hydrogenation part.

I'm thinking after I hydrogenate the palladium chloride, I'll just rinse the stuff with water a few times to get any of those salts off. I don't have a source of EtOH, so I'll probably use MeOH. I can use isopropyl alcohol too, but I have more MeOH and it's cheaper. Plus, its properties are closer to EtOH.


MeOH works fine also. In some cases with Pd/C the conditions should be changed depending on what solvent used. For example, the range for MeOH is preferred to be 25 < C < 50 but in the same reaction with EtOH it is 45 < C < 80.

In some cases, with reduction with PtO2, the time for a reduction to complete takes three times with MeOH than with EtOH.

Even though you don't have pure PdCl2, I think it's worth a try. I believe - nut not sure - that the reduction of PdCl2 to Pd(0) will affix the Pd but not the other salts. This is only my guess.

To wash the Pd/C, I used only H2O. That catalyst worked fine, but I can't promise it couldn't work better. Nicodem has far more knowledge than me, and he mentioned to wash after reduction (not after pre-reduction) with NaOH/H2O. Personally, I wash first with EtOH, then with toluene, then with EtOH again until the liquid gets clear.

I'm also going to do a reduction with GAA instead of HCl (aq) these days. I'll write about this soon.
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[*] posted on 2-6-2011 at 08:19


Quote: Originally posted by jwarr  
Palladium: you mention Pd/C being relatively easy to prepare, but Sunlight posted extremely poor results for a CTH reduction when using their own Pd/C. Do you think it was just an error in their preparation?


What molecule was reduced?
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[*] posted on 4-6-2011 at 02:55


Hi everyone i have a question thats not related to catalytic transfer hydrogenations but i didn't think it should have a new thread started for it.
This has been troubling me for some time so i thought i would ask someone with more knowledge than me.
In relation to the reduction of beta unsaturated nitro compound
(ex) phenyl 2 nitropropene ,i understand that the reduction with metal salts or acid and metal affords oximes, and that the reduction with metal and ammonium chloride forms hydroxylamines but what i don't understand is why amalgams or catalytic hydrogenations reduce nitroalkenes directly to amines ,my thoughts are that the nitroalkene gets attracted to the surface of the catylist where it joins the hydrogen to the double bond which stops the formation of oximes .
could someone explain whether i am wrong and explain why this happens.
And why is this not the case with terminal nitroalkenes.


regards azo

[Edited on 4-6-2011 by azo]
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[*] posted on 4-6-2011 at 06:14


Quote: Originally posted by azo  
and that the reduction with metal and ammonium chloride forms hydroxylamines

Could please give the reference where a reduction of any 2-nitropropene to a hydroxylamine is described using such a metal dissolving reaction? That would be quite useful.
Quote:
my thoughts are that the nitroalkene gets attracted to the surface of the catylist where it joins the hydrogen to the double bond which stops the formation of oximes .

If the double bond would be the first to reduce, then oximes would not be intermediates, wouldn't they? It would be unusual for such a highly electron poor double bond to be hydrogenated faster than the nitro group when using Pd-C. The oximes are intermediates, most probably exactly because the double bond refuses to get hydrogenated as rapidly (N-hydroxyenamines are tautomeric to oximes!).
Quote:
And why is this not the case with terminal nitroalkenes.

I already gave a plausible explanation above thread. Aldoximes reduce much more easily with Pd-C. Ketoximes require either too harsh conditions, or more apropriate catalysts (like Raney Ni or Rh-C). See the conditions listed in the patent US3458576 cited above by Vogelzang and compare that to the conditions used in the Bull. Chem. Soc. Jpn. article.
By the way, aluminium amalgam reduces both, aldoximes and ketoximes, extremely easily, so I guess this answer also your other question.




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[*] posted on 4-6-2011 at 23:50


Thank you for clearing that up for me nicodem now it makes sence greatly appreciated for your responce.

REGARDS AZO
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[*] posted on 5-6-2011 at 07:40



Anyone knows how much the pressure of H2 influences the reaction using 5% Pd/C? Of course it does, but I'm more thinking of e.g. 30 psi when it's preferred with 60 psi - can I compensate this with longer reaction time instead? Too much pressure I can imagine sometimes will crush the molecule to an undesired product (hydrogenolysis instead of hydrogenation), but in cases when the H2-pressure is lower than preferred, and I simply drive the reaction until no more H2 is adsorbed - what risk might appear?


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[*] posted on 5-6-2011 at 12:42


It pretty much depends on what you are hydrogenating. For example, if you are hydrogenating an isolated olefinic double bonds which reduces already at 1 atm, increasing the pressure is not going to help much. Nitroarenes also don't need increased pressure and 1 atm does just fine. But if you are reducing groups difficult to reduce using Pd-C (e.g., nitriles, oximes, debenzylations, some aryl alkyl ketones, dechlorinations), then it is well worth speeding up the reaction by increasing the pressure. But in essence, whatever works in a Parr shaker apparatus (designed for 60 PSI = 4.14 bar) will work also using a balloon as long as you load it up with some more catalyst (and/or use the 10% Pd-C) to compensate for the slowing down.

On the other hand, hydrogenations in autoclaves at >100 bar with heating up are something quite different. Usually things that refuse to get hydrogenated in a good ol'shaker apparatus, will eventually give up under such forcing conditions. But these are usually employed when using catalysts like copper chromite, or for the reduction of benzene rings with Ni or Pt based catalysts.

For the more common transformations, you can always choose a more appropriate catalyst instead of forcing conditions. For example, if neutral or basic conditions are tolerated, you can often choose Raney Nickel over Pd-C. As an example, nitriles reduce sluggishly with Pd-C in the presence of HCl, but the same transformation can be performed better with Raney Ni in the presence of ammonia at the same pressure. Furthermore, since the Raney Ni is cheap, you can use lots of it, thus making the reaction proceed faster at a much lower pressure.

In the case of beta-nitrostyrenes, I would expect a slight complication when the reaction is too slow. Lowering the pressure without increasing the catalyst load, will slow down the hydrogenation, but it will not slow down any side reactions that do not involve hydrogen (like the hydrolysis of some intermediate, for example). I believe this might be the reason why the Japonese authors perform the reaction at 0 °C. I also believe this is why, in the above mentioned patent, increased temperatures can be used to force the reaction on a nitropropene - no acid is being used. This probably reduces the selectivity for the primary amine, but at least it allows for harsher conditions. The claim that the reduction goes better with a mixture of Ni and Pd based catalysts might have a rationale as well. The Pd catalyst might reduce to a certain stubborn intermediate which is on turn further reduced by the Ni catalyst (the oxime?).




…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)

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[*] posted on 6-6-2011 at 07:59


Another good thing about raney nickel is you can do reductions on dirtier starting materials without as much worry about poisoning. It takes very little 'poison' to stop a few 10's of mgs of Pd in it's tracks, but the large quantities of raney nickel that are use make poisoning less of an issue.



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[*] posted on 7-6-2011 at 00:17


After studying about the preparation of rainey nickel i was supprised to no that there is additional metals like zinc cessium and others added as a catylist promotor.And i have to ask myself what happens to the promotor metal during the leaching of the aluminium when treated with sodium hydroxide.
This in some way must also activate the promotor catylist as well.
And ovuesly the leaching process is directly related to the amount of hydrogen that the rainey nickel can carry on its surface which would tell me that you would be limited on what acid or base that would be used during the hydrogenation.
It appears to me that the preparation of the catylist don't seem to be that hard if one had a high tempurature furnace.
There also seems to be endless different types of rainey nickel that can be prepaired.
So i haved added this patent to provide further information.


regards azo

Attachment: Raney nickel catalysts, a method for producing said raney nickel catalysts and the use of the same for hydrogenating org (255kB)
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[*] posted on 7-6-2011 at 05:11


Quote: Originally posted by azo  
After studying about the preparation of rainey nickel i was supprised to no that there is additional metals like zinc cessium and others added as a catylist promotor.And i have to ask myself what happens to the promotor metal during the leaching of the aluminium when treated with sodium hydroxide.
This in some way must also activate the promotor catylist as well.
And ovuesly the leaching process is directly related to the amount of hydrogen that the rainey nickel can carry on its surface which would tell me that you would be limited on what acid or base that would be used during the hydrogenation.
It appears to me that the preparation of the catylist don't seem to be that hard if one had a high tempurature furnace.
There also seems to be endless different types of rainey nickel that can be prepaired.
So i haved added this patent to provide further information.


regards azo



I don't see the preparation of RaNi as the challange, even though it seems not to be easy. The agitation is not possible with magnetic stirring with RaNi; it must be performed in a shaker. This reactor must also have simultaneous heating in addition to agitation. Also, the pressure required is very high.
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