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Jor
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ammonium perrhenate reduction
Having bought 75 grams of ammonium perrhenate for about 45 euros (and having 5 more grams as a donation from woelen before this auction was on eBay
 ), I want to at least reduce half of this to high purity Re powder, as i want to
make Re compounds from this.
I have seen that the best method of reduction is with hydrogen at very high temperatures, I think above 500C (and thus autoignition temperature). I
think this is too much risk and I don't want to do this.
Having done many searching in google, the sciencemadness library and the internet library of the university, i cannot find much information, except
reduction with borohydride, wich i do not have.
Does anyone have experience with the reduction of this salt in other ways? If going through aqeous routes i prefer a heavy sponge and not something
such as palladium black wich is very fine and hard to filter.
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not_important
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This is a tough one. Many common reducing agents only take it to the +4 state. Hydrazine or Na-Hg on prolonged contact with perrhenate solutions
give a mix of the metal and dioxide.
Unfortunately the books that I know of that really cover Re chemistry are rare and in copyright...
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Fleaker
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Congratulations on your recent acquisition--interesting enough that you get some so soon :-).
You have to reduce it with hydrogen--it's the best and only way to go. Do the reduction at 700 C in hydrogen atmosphere in a quartz vessel; purge the
tube with hydrogen before it is at temperature. The reduction is smooth, and you can follow its progress by titration of the ammonia you will collect.
Cool under hydrogen, then turn off tank and pull a vacuum and heat to 500C to get the majority of the hydrogen off the sponge. If you do not do this,
it will take fire when you pour it from the tube. Your quartz tube will be plated mirror-bright with rhenium (it always is).
I'm sure you can make quite a few rhenium compounds from perrhenate itself...
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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JohnWW
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It is very difficult to reduce Re from the (VII) oxidation state, unlike Mn(VII). It was once thought that the polarimetric electrolytic reduction of
the ReO4- anion, by 8 redox equivalents, reduced it to the Re- anion; but subsequent work, particularly including that by Prof. Neil F Curtis, who was
the Prof. of Inorganic Chemistry at Victoria University Of Wellington, New Zealand, when I was there, showed that it was really hydrogenated to the
ReH8- anion. (Or it could have been instead, or some of it was additionally, ReH9--, as DJF90 says below, but a coordination number of 9 is extremely
rare, even with the small size of H atoms.).
Re is used largely as a catalyst (experimental). Because of its very high melting-point, it has been used as a substitute for W in the filaments of
incandescent electric light bulbs, and as a carbide it could probably substitute for WC in tips of cutting blades and drill bits; but it is really too
rare for such uses.
[Edited on 29-3-10 by JohnWW]
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woelen
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I agree with JohnWW that it is not easily reduced. Perrhenate is not a strong oxidizer at all. Using sodium hypophosphite combined with some acid and
heating, however, I managed to reduce the colorless solution to a dark (almost black) turbid liquid. I'm not sure though if this contains rhenium or
some oxide at lower than +7 oxidation state.
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Jor
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not_important , I do have hydrazine. Will heating it with excess hydrazine for a few days maybe reduce it all to the metal? And could i wash away the
dioxide maybe with HCl?
Fleaker, thanks for that method, but I do not have any quartz tube at hand, and I think the process is too dangerous.
I think I will try some reducing agents.
-hydrazine
-hypophosphite
-zinc powder, possibly heating large excess of zinc powder with APR in a crucible, and washing out the zinc with HCl or NaOH? I'm not sure if this
reaction is energetic.
-If it is soluble in ethanol, maybe Na/EtOH
-Al/Hg, then remove excess Al with NaOH, and boil away traces of mercury (milligrams at most)
But I am not really confident that any of these works, as according to one article even Li or Na in liq. NH3 gives ReO.2H2O. I do think that a dry
mixture of Mg and APR works, but I don't have Mg powder, and that reaction could proceed to energetically, with the result that the Re flies away as
smoke.
A ReH8- ion, that is quite interesting!
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DJF90
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I thought it was ReH9(2-) with a tricapped trigonal prismatic structure if I remember correctly...
[Edited on 28-3-2010 by DJF90]
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not_important
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I don't think zinc takes it any lower than Re(IV), indeed if Na-Hg doesn't fully reduce it I'd be a bit surprised if Zn did.
I thought of hypophosphite, but couldn't find any mention of a state below IV for it either.
There are mentions of electrolysis in fused salts to obtain the bulk metal, although that sounds no more fun than the H2 reduction.
Mentions of perrhenate oxidising HBr to produce Br2, again implied you end up with the IV state.
hmmm...
| Quote: | | the reduction of potassium perrhenate by potassium metal yields the nonahydridorhenate K2ReH9, containing the ReH92− anion in which the
oxidation state of rhenium is actually +7 |
doesn't sound too encouraging for lower temperature chemistry to do the job. However it appears that most Re compounds don't start with the metal,
but the +4 or +7 state so perhaps the metal is not what you want.
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Jor
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Well I at least wanted some Re metal as an element sample, so about 0,5g powder. Maybe I can do the reduction with hydrogen on a very small scale
safely, although even small amounts of the gas can cause dangerous explosions, wich is highly dangerous in glass. I don't have a quartz tube. The only
device I have to lead a gas in and out is a 3-neck RB flask. I could place some of the APR in that and lead a stream of H2 through, but can RB-flasks
be used at temperatures of 700C?
I thought that Zn might work when you melt the zinc together with the APR, because of the high temperatures involved. I will try this on a 100mg scale
when I get home, so 100mg of APR and 500mg Zn (large excess) and melt in a crucible with cover. If H2 works at 500-700C, I don't see why zinc won't at
very high temperatures. And it is safer than H2 gas at autoignition temps I think.
The only problem is the H2 absorbed on the Re. I don't have vacuum, can it removed in other ways, such as simple heating to 500C in a current of CO2?
Or adding some H2O to the cold sample, and then treating this suspension with something with oxidises the H2 but not the Re?
To be honest I don't know how reactive Re is. I know it reacts with HNO3, but does it react (slowly) with HCl? Because it seems it is very hard to get
to the metal, wich would indicate that this metal is not very noble at all (although precious )
If i want to make a compound of Re, I think I will first reduce and precitate an oxide, and dissolve in HCl to make a hydrated chloride.
[Edited on 29-3-2010 by Jor]
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not_important
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I think that the high temperature H2 reduction works because the gas flow removes the H2O formed.
Dry NH3 at the same temperature _might_ be an alternative, especially if first flowed over some iron at the same temperature.
You want a tube, and borosilicate glass is only rated to 500 C or so. Possibly you could salvage the fused quartz tubes in an old radiant electric
heater. Or use a iron/steel tube with a ceramic/fused quartz boat to hold the perrhenate.
I suspect the H2 dissolves in the metallic Re, so a decent vacuum at high temperature would be needed to pull it back out. A flowing gas stream at
somewhat reduced pressure might work, but CO2 would seem likely to serve as an oxidising agent to the metal; N2 should be OK.
Sounds like you need to track down a copy of Rhenium: dvi-manganese, the element of atomic number 75 by Gerald Druce. Also this PDF http://library.lanl.gov/cgi-bin/getfile?rc000051.pdf which states that aqueous HCl and HF do not dissolve the metal, but moist air slowly
converts it to HReO4.
It looks as if wet methods of making chlorides often result in mixtures of halide or complex halide and either oxihalides or HReO4 - Re loves
oxygen it seems. Also
| Quote: | | ReOCl4 will react with ammonia to form ReO(NH2)2Cl. This oxydiamidodichloride compound will decompose at temperatures above 400° to form Re and
Re02. |
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woelen
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Re metal is somewhat air-sensitive. I have 10 grams of the powdered metal and once it was a nice dry powder. Right now, it is a sticky mass which is
very humid. Moisture and oxygen from the air have oxididized the metal somewhat in which HReO4 is formed, which is a hygroscopic material and this
entirely wets the metal. If I had known this, then I would have stored the metal in a better container (it now is in a little glass vial with a flimsy
plastic cap). The powder is a very compact black material, 10 grams is just a little bit more than a spatula full of powder.
I also have the bulk metal (little pieces of foil) and these seem to keep better. Of course, the contact area with air is much lower in this case and
hence the much less pronounced severity of oxidation.
[Edited on 29-3-10 by woelen]
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Jor
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Maybe you could wash away those traces of perrhenic acid with water, wash with acetone and dry?
But thus it seems that Re is quite a reactive metal...
Thanks for the article not_important, I will look into it. Also, if NH3 works, that would be very nice, I might try it.
[Edited on 29-3-2010 by Jor]
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watson.fawkes
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This is
certain. I've been reading up on the construction of vacuum devices, and the procedure for brazing stainless steel requires a dry hydrogen atmosphere
because of the formation of chromium oxides. "Dry" hydrogen here doesn't mean tank hydrogen (or worse, Kipp-generator hydrogen). It means a hydrogen
source that's been passed through a drying train consisting of first, a catalytic oxidizer to convert any residual oxygen (much lower than LEL; these
are impurities) to water vapor and second, a heated column of molecular sieves to trap all the water vapor. For achieving bulk reduction of rhenium,
all this apparatus (likely) won't be necessary, but it does indicate that complete reduction is pretty sensitive to the steady-state concentrations of
H2 vs. H2O, and that's for chromium. Rhenium is certainly much more sensitive.
Safety for heated hydrogen atmosphere reactions isn't easy to achieve, but it's pretty straightforward. The apparatus is a continuous flow system, not
a sealed system. On the far end of the apparatus, there's an exhaust for the gas (see below). Before applying heat, you purge the chamber with
nitrogen. Then, again before heating, you replace the nitrogen with hydrogen. At this point, you bring the furnace up to heat. When your reaction is
done, you keep the hydrogen flowing while the furnace cools off. Finally, you purge with nitrogen again before opening the furnace.
The exhaust tube is cooled in a water jacket to reduce the possibility of explosion. The LEL (lower explosive limit) of any fuel gas is lower when the
gas is hot. If you've got an adequate ventilation system and can dilute the hydrogen coming out quickly enough (and have no spark sources in the gas
flow), it's fine to just exhaust the hydrogen. It's also possible to flare off the hydrogen by using a gas jet with an adequately small orifice and an
adequately long and thin feed tube (to cool off any flashback below its propagation point). At the beginning of the hydrogen purge, you use an
auxiliary flame to drive combustion. After a while, the hydrogen will burn by itself.
As for using NH3, I would guess that nitriding would be a serious problem.
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not_important
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The references I have at hand indicate that rhenium does not form a nitride, thus my suggested for using ammonia. The sulfides decompose under strong
heating, so precipitating a sulfide followed by drying and then strong heating under reduced pressure might be another route.
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unionised
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Just a thought.
Here's a quote from Sidgwick's Chemical elements and their compounds;
"Rhenium compounds are easily reduced to the metal."
If it was easy in 1962 why isn't it easy now?
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woelen
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From a theoretical point of view it is easy, just pass some dry H2 over the red hot metal
But practice is another thing. Theory does not tell anything about the engineering problems, it does not suffer from safety issues (explosion and fire
risk). That is a problem which I frequently encounter. Many times, text books mention how simple a certain reaction is, but if you try it in a real
system, then a lot of nasty practical engineering problems jump out of the hat and you have a hard time to catch all those beasts 
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watson.fawkes
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OK. Practically speaking, the dryness of the ammonia could matter a lot (I don't know with any certainty one way or another), which
means an anhydrous ammonia source or a drying train. The trouble with drying in this case is that molecular sieves won't distinguish between NH3 and
H2O. Even tank ammonia might have too much water in it. Per Armarego, use a cold trap, chemical drying with CaO, BaO, or Na, and the following gem:
"It can be rendered oxygen-free by passage through a soln of potassium in liquid ammonia".
It's not clear to me how much of this is necessary for the reduction, but it seems like more equipment than a hydrogen furnace requires. Then again,
I'm pretty comfortable mitigating down fire dangers in ways that others are not.
[Added] It appears that rhenium catalyzes the decomposition of ammonia at a temperature range below that needed to reduce the oxide. See Ammonia Decomposition and Related Phenomena on Rhenium Catalysts. If I'm reading this right, it means that use of ammonia doesn't get rid of the
fire hazard, but rather gives a mixed fire + ammonia fume hazard.
[Edited on 30-3-2010 by watson.fawkes]
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not_important
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Even iron will do the same, it's used when a reducing atmosphere is desired and the low volumetric density of H2 is a problem; liquid NH3 (pressure
tank) has 50% more hydrogen atoms per volume than does liquid H2.
It does reduce the fire hazard, as until decomposed NH3 has a smaller flammability range than H2, isn't near so eager to leak through tiny holes and
small leaks are easy to detect.
And yes it likely takes a good drying, but so would H2. Traces of water shouldn't be a problem, the reduction creates water. The sweeping out by the
gas stream handles the water issue.
Neither should need to be completely free of water or oxygen, so long as the O2 level didn't bring you inside the explosive range (a fairly high level
for contaminant in a gas generator) You can burn the NH3-H2-N2 mix the same way you burn straight H2, feed into a gas flame.
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unionised
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Why worry about water?
The same reference I quoted says you can reduce just about any rhenium compound with hydrogen, that would include the oxide so the reaction
ReOx +x H2 --> Re + x H2O.
If the reaction goes in that direction the it won't be interested in going in the opposite direction so water won't react with rhenium.
BTW "From a theoretical point of view it is easy, just pass some dry H2 over the red hot metal"
There's a hole in my bucket...
I think you could get some relatively inert gas like nitrogen or argon, bubble it through conc. ammonia pass the mixture over the hot Re salt , leave
this gas running while it cools and get the metal. You might want to add a flux so the powdered metal is covered up and doesn't intermediately react
with the air when you open the tube.
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watson.fawkes
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Quote: Originally posted by unionised  | Why worry about water?
The same reference I quoted says you can reduce just about any rhenium compound with hydrogen, that would include the oxide so the reaction ReOx +x H2
--> Re + x H2O. |
The amount of water determine (1) how fast the reaction proceeds and the amount of
reagent gas required and (2) how far along the reaction proceeds to completion before it hits steady-state. My original point was that if you need
highly pure material, you've got to worry about water content. If want a sample for an element collection, as Jor does, I would not doubt that the
oxides could be reduced out adequately for that purpose. (Although if you want it nice and shiny, you might want to do more, and then ampule it.) If
you have a different purpose, say you're using it as an alloying element, oxide purity can matter more.
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watson.fawkes
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| Quote: Originally posted by not_important | Even iron will do the same[...]
It does reduce the fire hazard, [...]
And yes it likely takes a good drying, but so would H2.[...]
Neither should need to be completely free of water or oxygen, [...] |
I agree with all this. My question,
still open to me, is which of these works better in practice in a home lab context. There's the relative cost of reagent gases per gram of reduced
metal. I would guess that, at small quantities of reductant, this has more to do with minimum flow through the apparatus than with molar efficiency.
There's the cost of assembling and operating a drying train, if needed at all. There's the furnace and the plumbing. There's waste gas flare-off and
ventilation. I don't see that one or the other reagent gases (H2 or NH3) is obviously superior; each seems to have different advantages and
disadvantages.
There's one open chemistry question that could tip the balance. It's conceivable that the reduction temperature in NH3 could be significantly lower
than for H2. If it's low enough to do the reduction in borosilicate glass, that would be an advantage.
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Jor
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I will only do the reaction in glass, if that's not possible, I will not do it, because i don't have the time to assemble a metal device for doing the
reaction.
The other real problem is the absorption of hydrogen by rhenium making it pyrophoric. And i don't have a vaccuum.
So likely I will try the zinc route first, or maybe even with molten sodium (very small amounts). Or hydrazine/hypophosphite (although I prefer to
keep the latter). Then I might try the NH3 and H2 routes but I'm really not sure yet, because of safety issues.
And finally if everything fails (wich i doubt) I will try to find someone here in the Netherlands who can do the H2 reduction for me, letting him have
a few grams of the salt.
Oh and is anyone prepared to trade some of the perrhenate for ruthenium/iridium/platinum/palladium? I'm willing to trade up to 30 grams.
I'm also willing to sell about 2-3g of gold powder (99+% pure).
[Edited on 30-3-2010 by Jor]
[Edited on 30-3-2010 by Jor]
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Fleaker
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Jor,
I can trade you Re for your perrhenate.
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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DJF90
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The king of precious metal extraction speaks 
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Fleaker
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King? I think not! My first reduction of ammonium perrhenate failed miserably! I nearly burnt the hell out of myself upon removing it from the quartz
tube when it took fire (hence my advice above to pull a vacuum and heat it up to remove hydrogen!!!) I should mention, the [mixed] oxides of rhenium
are beautiful colours!!
Furthermore, when the stuff was sent out to be ebeam melted, my shoddy and quite unsatisfactory neophyte reduction caused the operator quite some
trouble. I'm sure that guy still wants a piece of me. Lesson learned--always find out how much ammonia has been produced!
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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