Cyrax (Hive Bee)
09-08-02 16:45
No 354291
      Adam's catalyst preparation  Bookmark   

Platinum Dioxide (Adam's Catalyst)

Platinum dioxide for use in hydrogenations is available commercially.  It may alternatively be prepared by either of the following methods.

Method 1 (from ammonium chloroplatinate).

Place 3.0 g of ammonium chloroplatinate and 30 g of sodium nitrate (AnalaR) (1) in a Pyrex beaker or porcelain dish and heat gently at first until the evolution of gas slackens and then more strongly until a temperature of 300 C is reached.  This occupies about 15 minutes, and there is no spattering.  Maintain the fluid mass at 500 - 530 C for 30 minutes, and allow the mixture to cool.  Treat the solid mass with 50 ml of water.  The brown precipitate of platinum oxide (PtO2.H2O) settles to the bottom.  Wash it once or twice by decantation, filter through a hardened filter paper and wash on the filter until practically free from nitrates.  Stop the washing process immediately when the precipitate tends to become colloidal (2): traces of sodium nitrate do not affect the efficiency of the catalist.  Dry the oxide in a dessicator, weigh out portions of the dried material a required.

Method 2 (from chloroplatinic acid).

Dissolve 3.5 g of the purest commercial chloroplatinic acid in 10 ml of water contained in a 250-ml Pyrex beaker or porcelain basin, and add 35 g of sodium nitrate (AnalaR) (1).  Evaporate the mixture to dryness by heating gently over a Bunsen flame while stirring with a glas rod.  Then raise the temperature to 350-370 C within about 10 minutes: fusion will occur accompanied by the evolution of brown oxides of nitrogen and the gradual separation of a precipitate of brown platinum oxide.  If foaming occurs, stir the mixture more vigorously and direct an additional flame at the top of the reaction mixture, if necessary.  If the burned beneath the beaker is removed when frothing commences, the top of the fused mass solidifies and material may be carried over the sides of the vessel.  After 15 minutes, when the temperature has reached 400 C, the evolution of gas decreases considerably.  Continue the heating until at the end of 20 minutes the temperature is 500 - 550 C; at this stage the evolution of oxides of nitrogen has practically ceased and there is gentle evolution of gas.  Maintain the temperature at this point (best with the full force of a Bunsen burner) for about 30 minutes, by which time fusion is complete.  Allow the mass to cool (the Pyrex beaker may crack), add 50 ml of water and proceed as in Method 1.


Notes:
 (1) The use of an equivalent quantity of potassium nitrate (AnalaR) is said to produce a more active catalyst.
 (2) It is advisable to test a small portion of the filtrate from platinum by acidifying with hydrochloric acid and adding a few drops of SnCl2 solution: a yellow or brown colour develops according to the quantity of platinum present.  The yellow colour is soluble in ether, thus rendering the test more sensitive.  If platinum is found, treat the filtrate with excess of formaldehyde and NaOH solution and heat; platinum black separates on standing and may be filtered and worked up with other platinum residues.

Platinum residues from hydrogenation reactions should be carefully preserved and subsequently recovered by conversion into ammonium chloroplatinate by the following method.  Dissolve the platinum or platinum residues in aqua regia, evaporate just to dryness several times with concentrated hydrochloric acid, dissolve the final residue in a little water and filter.  Precipitate ammonium chloroplatinate from the filtrate by addition of excess of a saturated solution of ammonium chloride.  Filter and dry the precipitate at 100 C

ref.: Vogel's Textbook of practical organic chemistry (5 th ed) p 459 - 460

Further info about the preparation of this catalyst can be found in JACS, sept. 1923 p 2171 - 2179  (damn, I forgot to write down the vol. and n ... well, sorry about that ...).  However, this is a very interesting article: read it!)


So, you might wonder: why the hell do I need this catalyst.  You can oxidize alcohols with it an all kinds of usefull stuff, however you might also check out this:

Freifelder, Catalytic Hydrogenation in Organic Synthesis: Procedures and Commentary (John Wiley & Sons, Inc., 1978) 101-2; Freifelder, Practical Catalytic Hydrogenation: Techniques and Applications (John Wiley & Sons, Inc., 1971) 366.

This appears to be the highest yielding published procedure for synthesizing methamphetamine from phenyl-2-propanone and methylamine. This is not surprising, considering Mr. Freifelder's vast knowledge of and experience with heterogenous catalytic hydrogenation and his many years as head of catalytic hydrogenation at Abbott Laboratories, maker of Desoxyn(r) brand d-methamphetamine.

1-Phenyl-2-propanone, 68.5 g. (~0.5 mole) in 150 ml ethanol was reacted with 51.8 g. (0.5 mole) 30% w/w aqueous methylamine solution, (fn.1),(fn.2) and hydrogenated at 3 atm. pressure (fn.3) with 1.4 g. of platinum oxide. (fn.4) There was a lag period of 1-2 hours, during which time there was little or no uptake of hydrogen (prereduction of catalyst did not change the lag period). Thereafter uptake was usually complete in an additional 2-4 hours. (fn.5) After removal of catalyst and concentration of filtrate and washings, high yields of the racemic N- methylphenylisopropylamine (methamphetamine) were obtained (90% or greater yield).

This procedure is taken from ../rhodium/chemistry /reductive.alkylation.html
 
 
 
 
    Rhodium
(Chief Bee)
09-08-02 17:23
No 354298
      Pt Catalysts  Bookmark   

Great, I have added the preparations to ../rhodium/chemistry /pd-catalyst.faq.html
 
 
 
 
    Cyrax
(Hive Bee)
09-08-02 23:51
No 354384
      JACS article  Bookmark   

These are the most important topics from the JACS article:

* During the fusion of sodium nitrate and chloroplatinic acid fumes of nitrogen dioxide (yukk!) are evolved and the oxide of platinum is precipitated.  It is probable that the following reactions take place:
 6 NaNO3 + H2PtCl6 --> 6 NaCl + Pt(NO3)4 + 2 HNO3
 Pt(NO3)4 --> PtO2 + (NO2)4 + O2

* To determine the fusion temperature that gives the most active catalyst, the following experiments are done:
A solution of 4.2 g of chloroplatinic acid in 10 ml of water was mixed with 40 g of C.P. sodium nitrate and evaporated to dryness in a casserole or beaker.  The mass was heated with a Bunsen or Meker burner until fusion took place.  The mixture and melt were stirred continuously with a thermocouple encased in a Pyrex glass tube and the temperature was read on a pyrometer.  After the fusion was complete, the melt was allowed to cool and treated with water until the filtrates were free of nitrates and nitrites.  The oxide was then dried in a desiccator.
The temperatures used in the fusion for the preparation of the oxide ranged from 310 C to 700 C.
In the first experiment the mixture was heated, during the course of 5 minutes, to 270-280 C at which temperature melting began.  The temperature was gradually raised, the reaction mixture reaching 300 C at the end of about 8 minutes, and evolution of nitrogen dioxide commenced.  The heating was continued until at the end of 10 minutes it reached 310 C, then it ws increased rapidly to 320 C where it was held until the end of 20 minutes, after wihich the mixture was allowed to cool.  Gas was evolved slowly from the time a temperature of 300 C was reached and at the same time a brown precipitate gradually appeared.  The reaction mixture was then worked up as described and the yield of 0.3 g (1.69 g os the theoretical amount) of PtO2 was obtained.
In the second experiment, the mixture was heated more rapidly, the fusion reaching 350-370 C in 10 minutes.  At this temperature a very vigorous evolution of oxides of nitrogen took place.  The temperature was raised to 400 C by the end of 15 minutes, by which time the evolution of gas had slackened.  The temperature was then held at 400 C until 20 minutes had elapsed when heating was discontinued.  Evolution of gas had practically stopped at the end of this time.  The precipitation of the oxide of platinum was rapid from 350 C on.  The yield of dry product was 1.4 g.
In subsequent experiments sufficient heat was applied so that a temperature of 350-370 C was reached in 10 minutes.  The temperature was then raised and controlled over the period between 10 and 25 minutes.  In each case, at the end of 15 minutes the evolution of gas had stopped. In experiment 3, a temperature of 400 C was reached at the end of 15 minutes and a temperature of 490-500 C at the end of 20 minutes, at which point it was remained until 25 minutes had elapsed.
In experiment 4 the temperature was aboot 500 C at the end of 15 minutes, 580 C at the end of 20 minutes, and was maintained at 590-600 C until 25 minutes had elapsed.
In experiments 5 and 6 the conditions were similar to those in experiments 3 and 4, except that in experiment 5 a temperature of 650 C, and in experiment 6 a temperature of 700 C was reached at the end of 20 minutes and maintained at this point until 25 minutes had elapsed.  At a temperature of 650 C or higher the melt boiled.

The yield of oxide in experiments 3, 4, 5 and 6 was practically quantitative.  The only difference in the oxides obtained at different temperatures was in their appearance, the color changing from light brown in experiments 1 and 2 to very deep brown in experiment 6.  It is probable that the difference in the color is due merely to a difference in the size of the particles.  A microcopic examination showed that the oxide is in every case non-crystalline in character and is composed of amorphous, approximately round granulles.

In order to determine the relative activity of the various samples of oxide as catalysts, standard experiments were carried out with each sample in the reduction of benzaldehyde.  The procedure was to dissolve 20 g of benzaldehyde in 150 ml of 95 % EtOH.  To the benzaldehyde solutions in each case, was added 1 ml of 0.0001 M ferrous chloride.  The solutions were reduced with 0.25 g of catalyst.  Table 1 gives the time for the reduction in each case.

TABLE 1:

Catalyst    Max. Temp. of     Time for reduction after
Sample      fusion (in C)    reduction started (in min.)
1            310-320               40
2            390-400               22
3            490-500               10
4            590-600               11
5            650                     20
6            700                     20

The lag in the conversion of the platinum oxide to platinum black differed in each sample of catalyst (cfr Table 2).  To find the total time necessary for the reduction of benzaldehyde, it is necessary to add to the time given in Table 1 the corresponding lag time given in Table 2.

TABLE 2:

Catalyst    Max. Temp.     Time elapsing before platinum
Sample      of fusion      oxide was reduced to Pt black
1            310-320             10
2            390-400             5
3            490-500             1
4            590-600             2
5            650                   7
6            700                   11

These experiments show that a temperature of about 500 C is most satisfactory for the fusion in order to obtain a catalyst of maximum activity and minimin lag.  For this temperature one Bunsen burner turned on as high as possible was necessary.

* Platinum oxide is insoluble in aqua regia, concentrated nitric or concentrated hydrochloric acid.  It is readily soluble in hydrobromic acid or hydrochloric acid containing reducing agents such as sulfur dioxide (yukk!).  It decomposes hydrogen peroxide without itself being changed.  It oxidizes alcohols to the corresponding aldehydes.

* It has been shown that the nitrates of Li, K, Ca, Ba and Sr are not nearly so satisfactory for the fusion with chloroplatinic acid as sodium nitrate.


Voil, now there is no need for you bees to run to the library and fetch the article. smile

 
 
 
 
    Cyrax
(Hive Bee)
09-09-02 09:51
No 354553
      Correction  Bookmark   

Remark: I was a little bit confused yesterday: The two Tables had the same heading: Time for reduction after reduction started.  Obviously, this was a mistake (copy - paste ...).  I have corrected this, so that the post makes sense now.

I was fed up with typing the previous night, but this is the experimental to back up the statement:

* It has been shown that the nitrates of Li, K, Ca, Ba and Sr are not nearly so satisfactory for the fusion with chloroplatinic acid as sodium nitrate.

All of the nitrates of the other alkali metals and of the alkaline earth metals fuse below 650 C.
The procedure for these fusions was essentially the same as with sodium nitrate except that 3 g of chloroplatinic acid and 20 g of each of the nitrates were used.  The yields of product were quantitative in every case.

Nitrate   Time for reduction   Time for reduction    Temp of
            of PhCHO after     of oxide to Pt black   fusion
          reduction started
LiNO3       12                     9                490-500
NaNO3      10                     1                490-500
KNO3        25                     3                490-500
Ca(NO3)2   80                    58               500-530
Ba(NO3)2   135                   28               600-630
Sr(NO3)2   no red.                ..               630-640

The lithium nitrate worked very satisfactorily in the fusion and yielded an active catalyst.  It is noticeable, however, that the oxide produced is reduced to platinum black more slowly than that obtained by fusion with sodium nitrate.
The potassium nitrate was not so satisfactory as sodium nitrate.  During the evaporation of the mixture of chloroplatinic acid and potassium nitrate to dryness, excessive foaming took place caused probably by the precipitate of potassium chloroplatinate.  The oxide of platinum was produced in good yields but was not so active a catalyst as that obtained by the use of sodium nitrate.  (Note that this contradicts what is said in Vogel's textbook)
Calcium nitrate itself decomposes just above its melting point into calcium nitrite and then into calcium oxide so that during the fusion with chloroplatinic acid the melt became very thick and viscid.  The catalyst produced was very light yellow in colour and did not resemble the oxide obtained by the fusion with the alkali metal nitrates.  It was rather fluffy in appearance.  It seems probable that this was an oxide of the formula PtO2.2H2O.
Both barium and strontium nitrates melt at so high a temperature that the platinum chloride, for the most part, decomposes to metallic platinum before the fusion occurs, and hence the products from these fusions would not be expected to be so active as catalysts.  That obtained from the barium nitrate caused the reduction of benzaldehyde only very slowly, and that from the strontium nitrate caused no reduction at all.