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Author: Subject: Cu/SiO2 a cheap versatil hydrogenation catalyst?
Jor
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[*] posted on 3-12-2008 at 14:02


Klute, when will your lab be up running again? You already working on your hood? ;)
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bfesser
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[*] posted on 3-12-2008 at 15:08


I've been wondering if it would be possible to use L-ascorbic acid to reduce the copper instead of hydrogen (skipping the calcining step altogether) and then to dry the catalyst in vacuo. Of course, the L-ascorbic acid would have to be eluted from the silica prior to drying. Do you think this would work?
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Klute
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[*] posted on 4-12-2008 at 00:18


@Jor: Unfortunaly, not in any close futur, my lab is packed in boxes, 1000km from where I now live (14m2), so no more personal lab work until easily 6-12month.. Sigh..

It's a pity, I had started to collect some nice elements for the hood, including a large 180x110cm large window pane, some porcelain panes for the bench, a N2 trap and dewar, etc

But I'm going to be more than busy at work, I will be working in two seperate teams that have been waiting for another member since at least 2 years! So plenty of things to do...


@Bfesser: Well, even if I guess some metalli copper will be obtained, I think it will be no where as active. Indeed, the catalyst is very active because it can form copper hydride in a stabilized form, that will exist long enough to be able to reduce the substrate. Using ascorbic acid will surely give a less finely divided metal surface, and perhaps not as organised (apparently the Cu is formed as largish (at a microscopic scale) cristallites that are necessary to complete the adsorption-reduction-desorbtion cycle. Also, chances are copper hydride will react with the ascorbic acid function, etc There is a very complete article on the preparation of coppe rhydride on various supports, which has some very valuable info concerning the first steps in the reduction, with lots of complete data. I will try to find the ref, I thinkI left the printed paper at my old town.

ANyway, It wouldn't be possible to elute the ascorbic acid from the catalyst, as after reduction the catalyst needs to be protected form traces of oxygen, so unless you directly transfert the catalyst to a degassed column via schlenk technics, and elute with degassed solvents, the catalyst will be ruined or at least partially desactivated IMHO, no worth the effort.

A good alternative would be using IPA a a H2 donor in CTH condiitons, surely other donors can be used too! But H2 is still needed to reduce the catalyst, and I"m pretty sure calcination is required to obtain an active catalyst. I'm not familiar enough with the theory surround this, but I have seen that in nearly all metal SiO2- or Al2O3- supported catalyst preparation, there is a calcination, I guess to produce metal silicates and have more than just an adsorbed metal, that will leach off gradually, but a truely bonded metal. Even the reduction requires miniam temps to give an active catalyst, most of the time above 300°C...


So Ni borides do look a more practical alternative, provided they work, and with which substrats..




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[*] posted on 4-12-2008 at 07:46


Klute,

If I ever find my silica gel I'll try to make the same copper catalyst using sodium borohydride in MeOH instead of dihydrogen at elevated temperature for the last step.
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[*] posted on 4-12-2008 at 09:58


Is it possible to prepare the same catalist by adding water solution of CuSO4 to solution of Na2SiO3 ???
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Klute
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[*] posted on 4-12-2008 at 10:31


@ Barium: that might indeed be worth a try. But I think you will obtain Cu boride, which could be a perfectly suitable hydrogenation catalyst. there isn't much in the litt on this compound...

@Outer: I don't think there would be any advantage in forming the silica in presence of a copper salt, as most will stay in solution, and you would need to add lots of NH3 and then dilute to a huge volume before you can precipitate the hydroxide/oxide.. It would be more reasonable to just form the silica, wash and filter it, then add the copper complex.
In any case, i think adding the Cu 2+ salt to a solution of Na silicate will form insoluble Cu silicates, isn't there a demonstration where various transition metal salts are added to a silicate solution and it forms colorfull trailings?




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[*] posted on 4-12-2008 at 10:55


Somewhere I read about using of sodium hypophosphite (NaH2PO2) to reduce Cu(2+) salts to metal copper. May be, it can be used instead of H2 or NaBH4.

About insoluble copper silicates. As I know, CuSiO3*x H20 that forms in this reaction, is a mixture of Cu(OH)2 gel and aqueus Silica gel. May be, after reducing, it may be useful as a catalist?

[Edited on 4-12-2008 by Outer]
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Klute
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[*] posted on 4-12-2008 at 12:00


Well, it could be worth a try. I am a little sceptical I must admit, although copper metal will surely be formed, it might not have the catalytic properties the calcined, H2-reduced catalyst has. I don't think you can reduce it in aqueous conditions and still expect copper hydride to form.

The silicate method could work, but you will have to add more silica to obtain a suitable loading. Also, the fact that the copper salt will immediatly precipitate will surely not give a good dispersion on the surface of the silica, but large agregates. It could be possible to gradually add a dilute sodium silicate solution to a suspension of fine silica gel in a dilute copper salt solution, gradually precipitating the hydroxide/silicate on the surface of the silica...




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[*] posted on 5-12-2008 at 01:23


Quote:
Originally posted by Klute
Well, it could be worth a try. I am a little sceptical I must admit, although copper metal will surely be formed, it might not have the catalytic properties the calcined, H2-reduced catalyst has. I don't think you can reduce it in aqueous conditions and still expect copper hydride to form.

There is a paper where the NaBH4/CuSO4 system is used in H2O/EtOH solvent (if I remember correctly). A brown precipitate immediately forms after adding CuSO4 to NaBH4. This combination is claimed to reduce a some functional groups that NaBH4 alone can't (like the nitro group, for example). I would imagine some highly active catalytic copper or copper hydride could be responsible for this change in borohydride chemoselectivity since borane formation is ruled out due to protic solvent conditions. I don't have the reference here, but I guess you can find it (it is quite possible the paper was already uploaded somewhere around this forum).
Of course, loading this active copper species on SiO2 for use in hydrogenations could prove quite possible and is certainly worth trying.




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[*] posted on 5-12-2008 at 01:46


yes, I remeber some discussion on this, I think it was Isomeric Fred that tried out the reaction with some nitro compound and got satisfying results after a little optimization. It would be nice to compare the selectivity between Cu boride/hydride and Ni boride.



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[*] posted on 5-12-2008 at 01:58


Quote:
Originally posted by Klute I think it was Isomeric Fred that (...) got satisfying results after a little optimization.


Nope, he didnt. I was also pretty interested in that but as of late, i came to the conclusion that all M+/BH4- combinations that claim reductions which NaBH4 alone won't do on its own are either very picky or unreliable.




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[*] posted on 5-12-2008 at 02:37


Ok, thanks for clearing that out. A pity, though..



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[*] posted on 5-12-2008 at 05:56


In Tet. Lett. 1986, vol. 27, No. 10, pp 1205-1208 they compare a Cu2+/borohydride-system with the more well known Co2+ and Ni2+-systems. They also claim the active species to be a copper boride, which I personally doubt.
In Synth. Comm., 1989, 19(17), 3047-3050 a Cu(I)Cl/KBH4-system is evaluated for the reduction of aromatic nitro groups and found to be superior to Ni2+ and Co+2/NaBH4-systems. Nothing is mentioned about the active copper species though.


[Edited on 5-12-2008 by Barium]

[Edited on 5-12-2008 by Barium]
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[*] posted on 5-12-2008 at 07:27


Reference Information


Cu2+/BH−4 reduction system: Synthetic utility and mode of action
J. A. Cowan
Tetrahedron LettersVolume 27, Issue 10, Pages 1205-1208, (1986)

Abstract
Cu2+/BH−4 offers a mild, method of reduction for aromatic and aliphatic tertiary nitro groups. Its mode of action has been studied, in comparison with related reagents, by deuterium isotope experiments.

Attachment: Cu2+:BH−4 reduction system- Synthetic utility and mode of action.pdf (175kB)
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[*] posted on 7-10-2010 at 00:34


I would like to note that we carried out a couple of tests on the selective hydrogenation of 4-(4'-hydroxy-3-methoxyphenyl)but-3-ene-2-one to Zingerone and 4-(4'-hydroxyphenyl)but-3-en-2-one to Rheosmin by using Cu/SiO2 as already described for other -unsaturated ketones[1].
The reaction carried out by klute probably failed due to the use of AcOEt as solvent. Actually we have some evidences that these kind of esters lead to a poisoning of the metal centres, thus inhibiting the hydrogenation activity. In order to use solvents suitable for the substrates, but compatible with the catalysts we employed a mixture of toluene and 2-propanol with 4-(4'-hydroxy-3-methoxyphenyl)but-3-ene-2-one and pure 2-propanol with 4-(4'-hydroxyphenyl)but-3-en-2-one.
We used the same catalyst over silica prepared by klute according to the procedure described in Tetrahedron Letters [2], under 1 atm of H2 at 80°C observing in both cases complete conversion of the substrate after 1.5 h and high selectivity in the corresponding saturated ketone. The reaction mixtures were separated from the catalyst by simple filtration and raw reaction mixtures characterised by 1HNMR, comparing the spectra with those recodered for the references sent by klute.

Hydrogenation of hydrogenation of 4-(4'-hydroxy-3-methoxyphenyl)but-3-ene-2-one in toluene/2-propanol gave after 1.5 h 94% zingerone. Spectrum of raw reaction mixture available.


Experimental conditions:
Catalyst prereduction- The catalyst was activated before use by treatment at 270°C in air for 20 min, under reduced pressure at the same temperature and then under H2 at the same temperature by removing the water formed by the reduction under reduced pressure. Reaction conditions: 100 mg cat (8% Cu), 100 mg substrate, 1 atm H2, 90°C, magnetic stirring-

*In the GC-MS analysis the saturated ketone is the only product. The 1HNMR of the raw reaction mixture shows some impurities but seems to be better respect to the reference sent by Vincent .

Nicoletta Ravasio

References
[1] N. Ravasio, F. Zaccheria, M. Guidotti, R. Psaro, Topics in Catal. 27, 2004, 1-4;
N. Ravasio, F. Zaccheria, P. Allegrini, M. Ercoli, Catalysis Today121, 2007, 2-5
[2] N. Ravasio, M. Antenori, A. Gargano, P. Mastrorilli, Tetrahedron Lett. 37, 1996, 3529-3532
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[*] posted on 7-10-2010 at 07:14


Thanks for reporting your findings and taking the time to check your reduction system on this substrate. Klute has not been around in the last few months, but I'm sure he would appreciate.
Your catalyst seems very useful for certain reductions, but preparing it just to test if it is able to perform a desired reduction is a bit annoying. Are there any chances of it becoming commercially available? I don't know how much interest there would be in general, but I'm sure I would order some if it was made available trough the common chemical companies. Perhaps you should check if there is any company willing to manufacture it? Is the prereduced, ready for use, catalyst stable for storage and still maintains its activity?
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[*] posted on 14-10-2010 at 13:57


There is very similar catalyst being in use in chemical industry.
It is CuO promoted with Fe/Cr (packed in steel drums). Fresh catalyst is reduced in stream of H2 to Cu.
Unfortunately, in reduced state it is very sensitive to air.
Recently we have reached more than 700 C during deactivation (with air) of used up catalyst.
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