Cu/SiO2 a cheap versatil hydrogenation catalyst?

While seaching information on the selective 1,4-hydrogenation of unsaturated ketones ("Raspberry ketone" thread), I stumbled on the use of 8% Cu/SiO2 catalyst as a selective, easily made hydrogenation catalyst. Further enquiry showed that by changing reaction conditions, preparation conditions, etc, this catalyst could be usefull for reduction of ketone to alcohols, deoxygenation of aromatic ketone to methylen compounds (so long dirty Clemmesen and Wolf-Kishner!), and of course selective 1,4- hydrogenation of unsaturated ketones, without producing the corresponding alcohol or other side products...

The catalyst is easily prepared, by adding silica to a solution of Cu(NH3)4 2+ (made by adding conc. ammonia to Cu(NO3)2.3H2O solution, and diluting the slurry to hydrolyse the complex and deposit a finely dispersed precipitate of Cu(OH)2. Filtration, washing and drying prior to calcination at 350°C and reduction by H2 at 270°C under atm pressure afford a finely dispersed Cu/SiO2, non pyrophoric or pyrogenic catalyst.

This catalyst is commonly used in the industry, and can be recycled several times. I'm sure it can be usefull for other applications than those cited previously, possibly dehalogeantion, reduction of nitro compounds, etc. By modifying the loading and the preparation, it's selectivity can be changed.

This could turn out to be a versatil, easily made catalyst for the home-chemist, opening doors to reactions that otherwise require expensive and/or pyrophoric catalyst such as 10% Pd/C, raney nickel, etc.

I decided on trying out this catalyst on the 1,4-hydrogenation of an unsaturated ketone, to compare the selectivity and efficienty to Pd/C, and then try other applications. Different grades of silica will be tried.

Preparation of 8% Cu/SiO2

Preparation of the unreduced catalyst


Cu(NO3)2.3H2O solution was prepared by adding fine 3.5g (55.12 mmol) Cu pellets (Panreac) (washed with acetone and dH2O prior to addition) to 13mL (143.27 mmol) 53% w/w HNO3 (technical) (excess Cu). there was vigorous evolution of NO2, and significant heating up as the solution quickly turned blue. the erlenmeyer was then heated to 50°C for 12H, and left to rest at RT for another 24H.
This resulted in a slightly viscous cyan blue solution, with a little amount of unreacted copper. The solution was decanted of the metal, and completed with a little dH2O to 10mL (~12g/10mL : 1200g/L solution).

Concentrated solution (1200g/L):


Diluted solution (160g/L):




3.3mL of this solution were diluted to 25mL with dH2O (160g/L solution). 30% NH4OH (Panreac) was then added until the Cu(OH)2 precipitate that initially formed redissolved. pH was >9, and a very dark blue solution was obtained.

30% NH4OH:


Cu(OH)2 precipitate:


Cu(NH3)4 2+ solution:




10g of chromatographical grade silica gel (60-200mesh, Acros) were then added in portions, with slow magnetic stirring. there was significant warming up of the solution. The dark blue slurry was stirred for 30min at RT.


Silica gel:


Slurry:




The slurry was then transfered to a 1L beaker placed in a ice bath, and slowly diluted to 1L with strong magnetic stirring. The blue color cleared up to a dark cyan blue colour. After 10min stirring, the blue solid was left to decant, leaving a very slightly blue supernatant. Half of the solution was filtered off, and the remaining supernatant diluted to 1l with stirring. After decantation, the now colorless supernatant was filtered off, leaving the solid in the beaker, and the first filtrate added, and diluted to 1L with stirring. The solid was then vacuum filtered, thoroughly washed with 200mL, and dried by suction for 5min.


First dilution:


Decanted solid:


Filtered solid:





The solid was then transfered to a pyrex dish, and heated on the hotplate to ~100°C. The caked solid quickly dried back to a very fluid powder, as dried silica gel, and was left at 100°C overnight. There were a few light white particules, which was considered to be talc from the latex gloves..


Wet solid:


Dried solid:




In the morning, the solid still was cyan blue. It was transfered to a 250mL beaker, heating increased to 150°C for 1H, the solid turning slightly darker, then placed on a heating mantle and slowly heated to 350°C. The solid gradually turned dark green, and was kept at that temperature for 4H.


After 12H drying:


Calcination:





The catalyst will be reduced prior to the reaction, by atmospheric H2 at 270°C. The formed water will be removed either by vacuum distn, or by refluxing toluene with a Dean Stark (the reaction will be performed in toluene at 90°C)


References: (Available in the Ref's forum)

Thank very much to Nicodem, who very kindly recovered most of the articles.

Cu/SiO2: an improved catalyst for the chemoselective hydrogenation of a,b-unsaturated ketones
Nicoletta Ravasio, Marisa Antenori, Michele Gargano and Piero Mastrorilli
Tetrahedron Letters 37(20), 3529-3532 (1996)


Selective hydrogenation of 4-(6-methoxy-2-naphtyl)-3-buten-2-one to Nabumetone®
Nicoletta Ravasio, Federica Zaccheria, Pietro Allegrini and Mauro Ercoli
Catalysis Today, Volume 121, Issues 1-2, 15 March 2007, Pages 2-5


Heterogeneous selective catalytic hydrogenation of aryl ketones to alcohols without additives
Federica Zaccherianext , Nicoletta Ravasiob, Rinaldo Psarob and Achille Fusi
Tetrahedron Letters 46(21); 3695-3697 (2005)


Cu/SiO2-catalyzed hydrogenation of cyclohexanones under very mild conditions
Ravasio, N.; Psaro, R.; Zaccheria, F.
Tetrahedron Letters 43(21); 3943–3945 (2002)


Heterogeneous Cu-catalysts for the reductive deoxygenation of aromatic ketones without additives
Federica Zaccheriaa, Nicoletta Ravasiob, Mauro Ercolic and Pietro Allegrini
Tetrahedron Letters 46(45), 7743-7745 (2005)

Attempted reduction of dehydrozingerone by Cu/SiO2-catalyzed hydrogenation







The catalyst prepared above was used in the atm hydrogenation of 4-(4'-hydroxy-3-methoxyphenyl)but-3-en-2-one ("dehydrozingerone") according to Ravasio et al. (see ref above).

The catalyst was easily reduced by atm hydrogen at >250°C, and the substrate as a toluene/AcOEt solution under argon was introduced. reduction was performed at 90°C with atm hydrogen for 24h.


Reduction of the catalyst


A 100mL 3-neck 14/19 flask was equipped with the following: a simple distn setup with no vacuum inlet (receiver directly attached to the condenser), a claisen adaptator fitted with: a gas inlet connected to a hydrogen generator (pressure equalized addition funnel on 4-neck 250mL RBF), a gas admission tube connected to an inverted 250mL cylinder immersed in a beaker of water; a septum.





0.90g (Ratio substrate/catalyst ~5% mol/mol) of the previously prepared catalyst were weighed and introduced in the flask, with a stir bar.



[IMG]http://img.photobucket.com/albums/v622/Amine
/Zingerone/CuSiO2_Hydrogenation/Cat_Red1.jpg[/IMG]


The whole system was purged with argon 3 times, then the H2SO4 was drippe donto the Zn slurry, and the system purged with H2 two times. The cylinder was filled with H2, and the flask was heated with a mantle to 250-300°C, stirring and shaking the fluid catalyst, which gradaully darkened over 15min. H2 absorption started very quickly, and stopped after ~15min. Heating and stirring was continued for another 15min. A few small water droplets had deposited in the top of the necks.








Under a stream of argon, the distn setup was removed, and the condenser fitted between the flask and the claisen. The setup was purged with H2 3 times.


Reduction of the unsaturated ketone

The unsaturated ketone was introduced in a schlenk, and 30mL distilled toluene were added. Not all the ketone dissolved, so ~5mL AcOEt were added to acheive complete dissolution. The very light yellow solution was degassed 3 times, and transfered to the reduced catalyst by canula. The system was briefly purged with H2, and the cylinder refilled.





The flask was heated to 90°C in an oil bath, and vigorously stirred under H2 atmospher for 24h, withdrawing samples with a a syringe evry 6h. After 24H, the mixture had taken a green colour.




Unfortuanly, TLC revealed that no reduction had taken, not the slightest little spot wher ehte saturated ketone should ahve been... No alcohol, unsaturated alcohol, just a tiny un-eluted spot from condensation products I guess.....

I'm not sure about the reason for this complete inactivity of the catalyst. I can only suppose the kind of silica is truely determinating... Most of the articles mention "fumed silica", I never thought it would have such different properties from chromatographic silica... IIRC, chromatographic silica is prepared by acidification of sodium silicate, or hydrolsyis of Si(OCH3)4, while pyrogenic silica is prepared by combustion/hydrolysis of SiCl4. Which properties one has that the other one hasn't is behond me, but I thinkit has something to do with the stabilisation of copper hydride species.
I have written to Mrs. Ravasio asking her if she has encoutered similar problems with different grades of silica...

Until then, I will start looking elsewhere, NiB catalyst to be precise. These could be easily made from Ni salts and NaBH4,a nd are said to be very active, non-pyrophoric selective catalyst. An improved caatlyst can even be prepared from used NiMH batteries! the variosu lanthanes, Cobalt, Mn species present exhalte the reducting properties.. Until then, back to paperwork...

See:

Hydrogenation catalysts from used nickel metal hydride batteries
Mark R. St J. Foreman, Christian Ekberg and Arvid Ödegaard Jensen
Green Chem. 2008, 10, 825 - 826,

Abstract
A synthesis of a safe and effective hydrogenation catalyst has been developed with used nickel metal hydride electrical cells as the starting material.

availble at the ref forum.

Is that right? Because I think it's either a typo ( incrediably likely compared to the other option I'm about to propose) or you're using a DEhydrogenation catalyst for hydrogenation?

Not allowed to say I'd much rather use Zn/H+ any day...

Quote:

Originally posted by stoichiometric_steve Quote: Originally posted by Barium Steve, which substrate have you tried nickel boride on? Not allowed to say I'd much rather use Zn/H+ any day...

Quote:

Originally posted by stoichiometric_steve Quote: Originally posted by Barium Steve, which substrate have you tried nickel boride on? Not allowed to say I'd much rather use Zn/H+ any day...

Klute, when will your lab be up running again? You already working on your hood?