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Author: Subject: Catalytic Hydrogenation comments......
solo
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[*] posted on 31-1-2008 at 19:46
Catalytic Hydrogenation comments......


I've been trying to figure out how much hydrogen a reaction will take up when all one has at disposition is a pressure gauge that one can use to count the amount of psi's used up in the reaction,.........but how does that help in figuring out the total amount of hydrogen used up.....I would like to figure this one out as my Parr Hydrogenator is very useful but I want to calculate amount usage vs. catalyst poisoning and needing to replenish the catalyst as opposed to thinking the reaction has been fully hydrogenated.........solo



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smuv
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[*] posted on 31-1-2008 at 20:44


Look at this. The only thing slightly inconvenient about this is you need to know with reasonable accuracy the volume of the headspace above the hydrogenation solvent.

So it would be something like moles used = moles initial - moles final.
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[*] posted on 31-1-2008 at 21:38


Solo,

PV=nRT should be useful here. I'm assuming you know what this means. If not let me (us) know.




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[*] posted on 1-2-2008 at 04:54


Actually I would like to hear from members that are familiar with my dilemma ......those that have done catalytic hydrogenations and have figured exactly how much hydrogen will be taken up and have managed to know that by the only parameter available on the equipment....a pressure gauage! I have in the past, while doing similar reduction with similar molar content, been able to estimate the number of psi that need to be taken up before the reaction was complete.........but that was by observation no real calculated parameters or values of psi to guide me, only after doing tlc was I able to determine if the reaction was complete....... but then that's probably the only way to do this , hydrogenate till no more is taken up do a tlc and then decide if the uptake stopped because the catalyst got poisoned or because the reaction is complete.......solo



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[*] posted on 1-2-2008 at 05:41


Based on the formulas we have given you, if you know the volume of gas in the pressurized vessel, its temp and the pressure you can calculate the number of moles of gas in the vessel. So I guess you need to invest in a thermometer and a ruler to measure the temp and volume accordingly.

Any formula that does not rely on temperature makes no sense (assuming the temp is not fixed). Any formula that calculates moles without a volume makes no sense. If you just think about it for a minute, you should understand that there is no logical way of calculating the moles of gas without pressure,volume and temperature.


[Edited on 1-2-2008 by smuv]
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[*] posted on 1-2-2008 at 06:11


Perhaps they did things differently than you think they did? Perhaps they calculated the theoretical amount of hydrogen necessary, but used the gauge only to tell when no more hydrogen was being absorbed? Perhaps while they were at it, they also calculated yields based on weight gain or isolated product? Maybe you could give us an example, a quote?
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[*] posted on 1-2-2008 at 09:43


Estimating the PSI/mmol value by using PV=nRT is one way of doing it.
The other one is doing a calibration with an easily reducible alkene (styrene, cyclohexene, etc.) using the same volume of solvent as in later reductions (and preferably the same solvent anyway). I did this once on my Parr, just to get a rough picture of how much the PSI/mmol value is. I don't see any reason to get an accurate measurement. After all, you generally don't use the same amount of solvent for every hydrogenation you do.
If I remember correctly, there are also some estimates on the Parr institute homepage (assuming you use their standard size vessels for shaker type apparatuses).




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[*] posted on 1-2-2008 at 14:53


You're working with two pressures. The pressure being supplied to the reaction vessel and the pressure on the supply tank. The pressure on the tank is supposed to account for the use given constant temperature. The smaller the supply tank the easier its going to be to callibrate the process. Another approach to consider, I have not done this, is to weigh the supply tank before and after the reaction. Weigh the H2 tank before connecting it. As per usual, watch for the pressure to stop changing, disconnect the tank and weigh it again. Temperature no longer a worry this way and it offers another way of callibrating. All you need is a balance with precision in the range of the tank weight. Physicists and engineers tend to have things like that.

My guages don't have the kind of precission one would need for what you want. Given a large face, finely marked guage, you could keep a thermometer in line with the gas and account for any T change. This would take you back to the gas law method. It should work out the same.




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[*] posted on 31-3-2008 at 06:21


Well I've given up in trying to calculate the amount of hydrogen used based on the PSI's used, since it depends on head space , solvent......so I continue to use the method I've used for many years.....hydrogenate till no more.

Recently I've had to hydrogenate some experimental material that doesn't take hydrogen so rapid and in the literature the hydrogenation can take 18 hours at nl pressure, I would suppose that under a higher pressure the reaction should take hydrogen much faster ....... but it doesn't, another thing I've noticed by using the wet Pd/C the actual amount of Pd is reduced to 50% by weight, true it saves the experimenter from flame ups and can be shipped by plane since there is no risk but it doesn't seem to work as well as the dry Pd/C that I've used for so many years. I also found that the wet Pd seems to get poisoned much easier.....

Getting back to why some reactions that claim to be catalytic hydrogenation reactions take so long.....inpatient........solo




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[*] posted on 2-4-2008 at 11:56


You have ur own Parr Hydrogenator? That's not bad going.

I bought a Manometer for similar reasons but dont think thats much good.

I think I read that the reaction is completed when the pressure doesnt keep dropping every few hours and stays constant.

I thought the way you can work out if the theoretical amount of hydrogen has been absorbed must be done using a volumetric technique. For instance a burette is partially filled with water and the reading before the reaction is noted. After the reaction the reading will have changed.

Infact, in Vogel it shows 2 burettes hooked up so that it works as a manometer. That is for atmospheric pressure hydrogenation though where u calculate it based on volume.

Maybe if you know the volume of your vessel, then you can plug the pressure drop recording into an equation and use the data to calculate amount of hydrogen absorbed?

Man, you gott read some of those books like Rylander etc.




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[*] posted on 2-4-2008 at 18:39


Standardization: http://www.parrinst.com/doc_library/members/271m.pdf

The apparatus can be standardized by making a preliminary run with a known amount of any compound that can be completely and quantitatively reduced. The pressure drop per mole of hydrogen
consumed in such tests is then used as a basis for estimating the progress of a reaction with unfamiliar materials. A procedure is described in reference (3) for standardizing the apparatus by reducing
11.6 grams (0. 1 mole) of pure maleic acid dissolved in 150 mL of 95% ethanol using 0.1 gram of catalyst. The reaction is carried out as previously described with shaking continued until no more hydrogen
is consumed. This usually takes twenty to thirty minutes, after which the pressure drop in the tank is recorded. Since exactly 0.1 mole of hydrogen has been consumed in this run, this decrease in
tank pressure can be used as a basis for measuring or regulating the amount of hydrogen consumed when treating other compounds.


APPENDIX F. Molar amount of each gas per batch in pdf file: BATCH AQUEOUS-PHASE REFORMING OF LIGNOCELLULOSIC. BIOMASS FOR HYDROGEN PRODUCTION
http://etd.gatech.edu/theses/available/etd-07102006-141653/u...




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[*] posted on 4-4-2008 at 10:25


^Yeah, that sounds like a very sensible way of doing it.

It's important that you have an accurate analytical piece of apparatus to record the pressure drop though. If it's just a gauge then im not sure if it would allow you to do it with that level of precision.




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[*] posted on 4-4-2008 at 11:08


If you are doing hydrogenations that consume more than just a couple mmols of hydrogen then the normal gauge showing deltaP of 1 PSI is just fine. Like I already wrote somewhere above, I did such a calibration once using acetophenone (takes up 2 mol H2 per mol substrate, getting reduced to ethylbenzene). If I remember correctly it took approximately 2 PSI per mmol hydrogen for a 100ml filled 250ml Parr bottle. Such calibrations are important if you do hydrogenations selective on one double bond over some other, otherwise you just hydrogenate till constant pressure. However, sometimes summing up the pressure drop helps to estimate if a reduction is not going as it should or that the catalyst got poisoned.
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[*] posted on 30-8-2008 at 08:42


Within the scope of problems with catalytic hydrogenations....there is a recurrent problem with poisoning of the Palladium catalyst....an older chemist suggested to a friend that it lies in the imine, in the case where reductive amination is being employed...., it was suggested to add some activated charcoal to the imine solution and filter then add the desired Pd catalyst and hydrogenate, without the fear of the catalyst being poisoned.......and perhaps it may be the reason why only 50 % of the imine in reductive amination of phenylacetone stops there and won't go any further....or in that case is there a different reason ....?.........solo



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[*] posted on 30-8-2008 at 09:56


Palladium should not be the first choice of catalyst for that reduction to begin with. Platinum is much more effective. Different carriers can change the activity and selectivity as well. Carbon is the normal choice but acidic alumina usually gives great yields in this type of hydrogenations. Activated carbon should suffie to remove any poisons before the hydrogenation commences. If sulfur is the suspected poison, then a little raney nickel is the best choice since it will form NiS irreversibly. After removal of the nickel the noble metal catalyst can be added and the process started as usual.

Save any old inactive raney nickel for this very purpose instead of destroying it.
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[*] posted on 30-8-2008 at 13:58


Quote:
For example, desoxyephedrine was synthesized by reductive alkylation of phenyl-2-propanone and methylamine in the presence of hydrogen and palladium on carbon. Mixing of an equal volume (mole ratio 1:1.7) of P2P (7) and 40% methylamine in methanol immediately produced the imine (8) shown in Fig. 2. After 24 h, desoxyephedrine (9) is the main component (50%). The mass spectrum of the imine compound, 1-phenyl-2-methyl-2-(methylimino)-propane, is shown in Fig. 3. The imine compound is fairly stable since it does not readily reduce under the reaction conditions (50 lb hydrogen, Pd/C).



........................source,


http://psychedelichosting.info/Ionium/Rhodium/chemistry/meth...

.........so then the remaining imine that can't be reduced via Pd/C hydrogenation lends itself to be processed by using PtO Adam's Catalyst?....or is there a way to improve the reaction using Raney Nickle ? .....nice to see you here Barium , this is Java from the Hive.................solo

[Edited on 30-8-2008 by solo]




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[*] posted on 31-8-2008 at 11:35


Thank you Solo/Java

In that article the author did not specify which catalyst he used. So it is really hard to comment on that. But I can guarantee that way better yield and shorter reaction times would be the result by using, pretty much, any type of Platinum on carbon. For God's sake don't use Adams catalyst, unless you have enough money to whipe you ass with. Always use prescious metal catalysts deposited on a carrier. I know university labs still use those old catalysts but that is either because they (a) don't know any better, or (b) have too much money on their hands.

If you can make a really active Raney nickel you should be able to use it at such low pressures for the reduction of a imine.
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[*] posted on 31-8-2008 at 11:55


nickel catalysts.

https://www.hyperlab.info/inv/index.php?act=ST&f=17&...
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[*] posted on 1-9-2008 at 04:27


I have never had any success with any Urushibara type of catalyst. I would sure like to know why since I tried several different types of catalyst on several different functional groups. The insensitivity of the catalysts towards storage and ease of preparation made them highly interesting. But the lack of success made me somewhat puzzled.
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[*] posted on 1-9-2008 at 14:08


Yep, I've also tried many different methods for using Urushibara catalysts, none of which worked well at all. The difficulties in getting them to work led me to pursue other types of reducing systems.
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[*] posted on 1-9-2008 at 16:45


I have got excellent results with Urushibara - A catalyst.

For reduction of C = O to C- H.lI have used 3 different catalysts systems:

1) 10 % Pd/C : which was best for the job. I could recycle the catalyst 10 times without any problem. The temperature and pressure conditions were also very mild.

2) U-Ni-A : Was nearly as good, could be recycled 10 times but T/P conditons were higher as compared to Pd/C

3) Raney Ni: Could complete the reduction at all. Inspite of using drastic T/P conditions , the reduction proceeded only upto C - OH

gsd
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[*] posted on 1-9-2008 at 17:29


Was this the desoxygenation of aromatic ketones, or any aliphatic ketone?

What kind of pressures and temps were used?




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[*] posted on 1-9-2008 at 21:56


Only aromatic ketones (Ar-CO-R; where R is alkyl not containing any heteroatoms at the alpha position) can be hydrogenated to the methylene group (A-CH2-R). This is generally done with Pd-C at very mild conditions (even 1 atm H2 pressure at rt works). For, example acetophenone is reduced in less than one minute in methanol with ~1 mol% 5% Pd-C at ~2 bar of H2 pressure. Up to now I was not aware Urushibara nickel can do the same (Gsd, can you describe the conditions you used in more detail?), but I do know Raney nickel can't (it is actually used to reduce ketones to alcohols).
Aliphatic ketones can't be reduced that way. For their reduction via hydrogenation you can make the thioacetals (using HSCH2CH2SH or similar) and desulfurize with Raney nickel, but that is not particularly practical.




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[*] posted on 2-9-2008 at 05:12


Thank you for the clarification, Nicodem. Now that i think of it, I think it can be done by CTH with Pd/C and formate too.

I guess for the complete desoxygenation of aliphatic ketones, a Wolf Kishner or Clemmensen is still in order...




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[*] posted on 3-9-2008 at 02:23


Klute, it can. There is a paper on it, although i dont have access to it right now. They used Pd/C with NH4COOH in hot GAA. The yields weren't too good, though.



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