Platinum Oxide on Silicic Acid Stable, Active Hydrogenation Catalyst F. A VANDENHEUVEL Atlantic Fisheries Experimental Station, Fisherkr Research Board o f Canada, Halifax,
Platinum oxide supported on silicic acid, a moditication of the well-known Adams catalyst, is very active at room temperature yet is a very stable hydrogenation catalyst. The rate data obtained with the catalyst are reproducibleand are of definite analytical value because they are dependent upon the chemical constitution of the material hydrogenated. In the determination of multiple bond unsaturation, its use is of advantage because i t is conducive to short reaction times and sharp end points.
Can.
data obtained through the use of the catalyst in a kinetic investigation to be reported a t a later date. All the data presented here resulted from experiments carried out a t 25' C. and 760-mm. pressure, using the apparatus and techniques described earlier (9). One important feature of the apparatus is to allow control of the rate of hydrogen diffusion to the reaction medium by adjustment of the stirring. When stirring efficiency is increased beyond a certain point, hydrogen concentration in the medium remains constant-Le., a t the saturation value corresponding to the temperature and pressure used. Under these conditions, the variables affecting the rate of hydrogenation are the concentrations of unsaturated compounds initially involved, those of intermediate products, if any, and in some cases those of end products. Application of the known simple tests to the absorption-time data invariably failed to reveal a simple relationship between rate and unsaturated compound concentration. This is in contrast to data obtained with catalysts such as Raney nickel ( 9 ) , where a first-order relationship can be found. However, plotting the rate against the corresponding total unsaturation yielded characteristic curves which were used to establish the dependency of absorption rates on the variables listed above. I n all experiments, the buret was read every minute and the rate a t time t was taken as the average of the volumes absorbed a t times t 1 and t 1. I n spite of a small systematic error, this method proved to be more precise and more expeditious than the application to the time-absorption curves of the graphical tangent procedure. The total unsaturation a t time t was taken as the corresponding volume of hydrogen still to be absorbed for complete reduction-Le., the difference between the volume corresponding to complete saturation and that absorbed a t the time considered.
D
URIKG the early stage3 of an investigation of the kinetics
of catalytic hydrogenation undertaken previously in this laboratory, the main obst,acle to satisfactory experimental work proved to be the instability of catalysts. This study concerned hydrogen absorption rat,es under normal conditions of temperature and pressure. The highly active catalysts needed for this type of work were prepared according to known procedures, most of them being obtained in the form of finely divided metals, such as platinum, palladium, or nickel, generally supported on some incrt material and suspended in a solvent. A comprehensive review of the literature concerning this type of catalyst is nomavailable ( 3 ,4). I n spite of elaborate precautions involving cold storage in an oxygen-free atmosphere, the activity of the catalyst,s declined much too rapidly for comparable data to be secured. Further difficulties were encountered with catalysts such as Raney nickel, which could not be measured in precise amounts even by pipetting from thoroughly stirred and seemingly homogeneous suspensions. Platinum oxide prepared according to Adams ( 1 , 7 ) was thought to offer a solution to this problem. It is easily prepared and can be weighed out accurately. On reduction with hydrogen, it readily provides a highly dispersed and very active form of plat,inum, which can be obtained in ~ i t ujust prior to the introduction of unsaturated compounds. I t is therefore not exposed t o the effects of prolonged storage as is the case Tvith most catalysts. Indeed, as determined from t'he behavior of equal amounts of the same platinum oxide preparation tested over a period of several weeks, only a slonr over-all decline in activity was observed; the variation from run to run, howcver, was appreciable. I t m-as obvious from the examination of samples which had displayed a particularly low activity that a change had occurred in the catalyst structure. The normally dispersed platinum microcrystallites had gathered to form agglomerates of variable size which settled rapidly when stirring was interrupted. The effect on the active area of this uncontrollable phenomenon is similar to that observed with other catalysts as a result of sintering through overheating. It was thought that this tendency might be suppressed or a t least minimized if platinum oxide was bound to a catalytically inert support. Martin (6) had described the preparation of supported platinum oxide catalysts for industrial use. These were not found active enough to warrant their use in this investigation. A suitable catalyst could be prepared by including silicic acid in the preparation of the well-known Adams catalyst and by modifying the classical procedure to some extent. This new catalyst offers potential advantages to the analytical chemist. A few illustrative cases have been selected from the extensive
N. S.,
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+
PREPARATION
Seven grams of C.P. chloroplatinic acid are dissolved in 25 ml. of distilled water in a 100-ml. borosilicate glass beaker. Twenty grams of 200-mesh fraction S o . 2847 silicic acid (Mallinckrodt) are added, and the whole is stirred with a glass rod until a smooth paste is obtained. The beaker, supported by a metal gauze, is placed over a burner and cautiously heated with continuous stirring until the mass is completely desiccated. The resulting polvder is added by small portions with continuous and rapid stirring to 70 grams of molten C.P. sodium nitrate a t 350" C. This is achieved by heating a 250-ml. beaker containing the nitrate with a strong flame, somewhat broader than the bottom of the beaker so that the walls are heated as well. Each addition of powder to the melt induces a temporary release of reddish nitrogen oxide fumes and a frothing of the mass. Additions are so spaced as to avoid any rising of the froth higher than about 1 inch above the original level. When all the powder has been added, any material which has solidified on the walls is melted, by use of an auxiliary heater, and is pushed down into the melt with the stirring rod. Heating and stirring are continued until gas evolution has practically ceased. The beaker is held with tongs while the contents are s l o d y poured into a 2-liter beaker containing 1.5 liters of distilled water, agitated by a mechanical stirrer. Another 20 grams of sodium nitrate are melted in the 250-ml. beaker. The melt is used to rinse the mvalls with the help of the glass rod, and then poured into the vater. The beaker is allowed to cool, then it is washed with distilled water. The vashings are added to the bulk of the preparation, which is stirred for 2 hours and allowed to settle overnight. As much clear supernate as possible is siphoned off and the residue equally distributed in centrifuge tubes. Water is added and the contents of each tube are well stirred with a glass rod. After centrifuging, the clear supernate is poured off and replaced by distilled water. The preceding operation is repeated
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363
V O L U M E 2 8 , NO. 8, M A R C H 1 9 5 6 until the washings show just a faint pink coloration with phenolphthalein. The preparation is then transferred to a mediumporosity sintered-glass funnel with a minimum of water, and the liquid is filtered off. I t is twice replaced by 95% alcohol, then ether. After being air-dried for 1 hour, the product is transferred to a vacuum desiccator containing phosphorus pentoxide. The dried material is transferred to a small mortar, where it is easily pulverized, and then in small portions to a 200-mesh sieve. A camel’s-hair brush is used to move the powder over the gauze. Any residue is reground and sieved until everything has passed through. The light-brown powder is stored in a screw-capped bottle and well mixed before use. The yield is 21.0 grams, containing 0.14 gram of platinum per gram of the material. STAB1 LITY
Microscopic examination of the reduced catalyst (PS 14) shows the platinum microcrystallites firmly embedded in the support. This markedly increases their stability. A sample of the catalyst, stored in a frequently uncorked bottle and exposed to full daylight for more than 2 years, has not sustained any appreciable loss of activity. Moreover, structural changes, such as previously observed with unsupported platinum, do not occur while the catalyst is being used. One immediate consequence of this high stability is the reproducibility of hydrogenation rate data, which is required for reliable analytical results.
acid (curve I) and pure fumaric acid (curve 11) are hydrogenated in 95% ethyl alcohol. Both are typical of most curves obtained under these conditions. Curve 11, for instance, displays two branches, OA and A B , distinct in character. Branch AB, which is a straight line, allows the initial rate P R to be obtained by extrapolation, as shown by the dotted lines. Thus P R is the ordinate corresponding to zero time for fumaric acid; PR’ is the corresponding value for maleic acid. When these initial rates are plotted against amounts of catalyst used, straight lines passing through the origin are obtained, as shown in Figure 2. I n the case of fumaric acid, somewhat higher values of the initial rates are obtained with higher initial concentrations of this acid; a 10% variation is observed when the amount of acid used varies between 50 and 200 mg. or when the volume of alcohol varies between 10 and 20 ml. With maleic acid only a 2% variation of the initial rate is observed within the same range of conditions. Thus, the initial rate corresponding to maleic or fumaric acid for one unit weight of catalyst affords a convenient way of defining the activity of a given sample.
/
i
P
ACTIVITY
Weight for weight, the platinum oxide in PS 14 is 2.5 times more active than the best Adams catalyst prepared in this laboratory, as judged by the time required for the complete saturation of a variety of olefinic compounds.
A 2.C
P S 14
WITH
1 DO MILLIGRAMS
I N 1 8 4 SS%ETHYLALCDW
FUMARIC ACID
I 150 OF C A T A L Y S T
,
200
Figure 2. Amount of catalyst us. initial rate plots for maleic and fumaric acids, showing simple proportionality
l[f
~
I
MALEIC
I
ACID,
1. 95% ETHYL 3 c
.
!
0
I
ALCOHOL ~Q ;a I j IO 20 3 40 UNREDUCED M A T E R I A L EXPRESSED IN M I L L I L I T E R S O F HYDROGEN
I
;p
Figure 1. Hydrogenation rate curves of maleic and fumaric acids obtained under the same conditions
A comparison made with Raney nickel W-5 ( 2 ) in the reduction of long-chain unsaturated fatty esters shows PS 14 to be much more active. With only 20 mg. ( 3 mg. of platinum), the time required to reduce 256 mg. of methyl oleate was 11 minutes; with Raney nickel containing 191 mg. of nickel, the time to reduce 192 mg. of methyl oleate was 61 minutes. Half saturation of 264 mg. of methyl linoleate was attained in 5 minutes using 20 mg. of PS14, while a sample of Raney nickel containing 200 mg. of nickel required 10 minutes to half saturate 192 mg. of the same ester. These data were selected from a series of experiments carried out in 95% ethyl alcohol. Under these same conditions the benzene ring in phenol, benzoic acid, cinnamic acid, and benzamide is completely reduced in less than 2 hours, provided larger amounts of PS 14 are used. Figure 1 shows the absorption rate curve when pure maleic
The advantage of maleic acid in yielding initial rate values practically independent of the initial acid concentration within relatively wide limits is offset, however, by the somewhat elaborate procedure (6) required to obtain the acid in a pure state. (The chemical which is sold as pure maleic acid is a mixture of the two stereo isomers.) Fumaric acid is stable, easily purified, and chemically well defined, and is thus to be preferred as a standard. As a measure of activity, called “fumaric acid value,” the initial rate of absorption expressed in milliliters of hydrogen has been adopted. This is the value observed when 100 mg. of catalyst is used with 100 mg. of pure fumaric acid in 15 to 20 ml. of 96% ethyl alcohol under normal pressure a t 25’ C. The use of fumaric acid values should allow a direct comparison of results obtained in different laboratories. When the activities for the same unsaturated compound of two different samples of PS 14 (measured as initial rates obtained with equal amounts of the two samples) are determined, the ratio of these activities is usually found equal to the ratio of the corresponding fumaric acid values. SELECTIVITY
Although the benzene ring in cinnamic acid is completely reduced, the reaction is much slower than the reduction of the side chain. A sharp break appears on the hydrogenation rate curve, clearly indicating the disappearance of the aliphatic double bonds; thus, it is possible to determine both aliphatic and aromatic unsaturation from the same experiment.
364
ANALYTICAL CHEMISTRY
The benzene ring in phenol, benzoic acid, and benzamide is also reduced; it is left intact in phenol ethers and in alkyl benzoates. I n aryl substituted alkenes and aryl substituted olefinic esters, only the side chain is reduced. I n 95% alcohol, PS 14 displays little selectivity in the reduction of polyunsaturated fatty acids and esters. If, however, the hydrogenation is carried out in dibutyl ether, there is a marked selectivity comparable to that observed with Raney nickel in 95% alcohol. This is exemplified in Figure 3, which shows three rate curves obtained with methyl linoleate. Curve I corresponds t o PS 14 in 95% alcohol; i t does not show the characteristic break indicating saturation of the 12-13 double bond observed in curves I1 and 111, which were obtained with PS 14 in dibutyl ether and with Raney nickel in 95% ethyl alcohol, respectively.
When very small amounts of sodium hydroxide are added to the 95% ethyl alcohol used as a solvent for the reduction of fumaric acid, an increase in activity is firat observed. A maximam increase is reached for a 0.002Y alkali concentration in the alcohol: There is a definite swelling of the support and an expansion in active area of about 100% as judged from the increase in hydrogenation rate. However, a3 the alkali concentration is further increased, the support gradually deteriorates and the activity decline?. With a 0.05.V solution, the catalyst no longer disperser normally throughout the reacting mist'ure, but forms a heavy flocculate having less than one fifth t,he activity of the normal catalyst. Thus, both inorganic acids and bases affect the activity, apparently by inducing structural changes in the support. S o satisfactory substitute for silicic acid has yet been found. Therefore the use of PS 14 should be avoided in strongly basic or acid reaction mixtures. APPLICATIONS
p/
/ /'
a
METHYL L I N O L E A T E
I
WITH P S I 4 I N 95% E T H Y L A L C O H O L
(=J W I T H PS14 IN D I B U T Y L E T H E R
c
@ WITH R A N E Y NI IN E T H Y L A L C O H O L 5 IO 15 20 UNREDUCED M A T C R I A L EXPRESSED I N M I L L I L I T E R S O f HYDROGEN
Figure 3. Hydrogenation rate curves obtained with methyl linoleate, showing differences in selectivity
Another type of selectivity is indicated by curves I and I1 in Figure 1. These were obtained with the same amounts of PS I4 and of the acids, in the same volume of 95% ethyl alcohol. Thus, maleic acid is reduced about twice as fast a s its trans isomer. Ethyl esters of these acids are reduced more slowly than the corresponding acids. FACTORS INFLUEYCING SELECTIVITY
The marked solvent effect upon selectivity described above for methyl linoleate is but one particular case of a more general phenomenon. The hydrogenation rate of any double bond is influenced by the reaction medium. For instance, while 20 mg. of PS 14 requires only 11 minutes t o reduce 256 mg. of methyl oleate in 95% alcohol, 19 minutes are required in dibutyl ether t o saturate 152 mg. of the same compound. This solvent effect may stem from many different cause?. I n this particular case i t is due to the stronger adsorptive competltlon on the catalyst surface of the fully reduced compound. lT7hen increasing amounts of methyl stearate are deliberately added t o identical mixtures of methyl oleate, dibutyl ether, and reduced catalyst, the observed initial hydrogenation rates are found to decrease with increasing amounts of the additive. Only a small decrease in rate is observed, on the other hand, when succinic acid is added to a reaction mixture containing either fumaric or maleic acid in 95% alcohol. EFFECT OF INORGANIC ACIDS AND BASES
Attempts were made to speed up the removal of alkali from the freshly prepared unwashed catalyst, by neutralizing an aqueous suspension with inorganic acids t o pH 7. This accelerated the final washing, but resulted in a very poor catalyst.
Less than 20 minutes are usually needed to complete a quantitative determination of multiple bond unsaturation. 1Iuch larger amounts of other less active catalysts normally require 1 hour under the same conditions. This rapid and complete reaction is also characterized by a very sharp end point; there is no dragging of the absorption t o m r d the end as with other catalysts. T h a t this occurs with Raney nickel W-5 is shown in the following example: fifteen hours were required with 20.3 mg. of nickel (in the form of Raney nickel W-5) to reduce 250 mg. of methyl oleate in alcohbl. The last 0.18 nil. of hydrogen was absorbed in just over 1 hour. When 10 times that amount of catalyst was used, the total time was reduced to 61 minutes, 0.24 ml. being absorbed in the last 10 minutes. In contrast with this, only 3 mg. of platinum (20 mg. of PS 14) were needed to hydrogenate 286 mg. of the same ester in 11 minutes, 0.51 ml. being absorbed in the last 2 minutes. On the basis of rate of hydrogen absorbed and weight of metal used, it. would appear that the platinum in PS 14 is over 500 time3 more active than the nickel in Raney nickel. Admittedly approximate, this figure indicates, however, a con,siderably higher activity at low concentration of unsaturated compound. Such a property is particularly desirable for hydrogen value detcrminations. Comparative experiments carried out on a variety of compounds have shown it t o be general in respect to double bonds under the conditions used. The superiority of platinum catalysts over nickel catalysts has been explained (8)on the basis of closer matching between atom spacing in the met,al and carbon-carbon spacing of the unsaturated bond. The extensive literature on hatch-type catalytic hydrogenation under normal temperature and pressure includes an appreciable amount of hydrogenation rate data. Whether these data have a n y significance other than a qualitative one is doubtful. Most of them were obtained with catalyst3, the activity of which could not be maintained constant long enough to allow theae data to be duplicated. Furthermore, they were seldom recorded under all desirable conditions. By using an apparatus which allows such conditions to be established ( 4 ) it is possible to demonstrate t,hat hydrogenation rate curves cannot be duplicated when these catalysts are used. Those obtained with PS 14, on the other hand, can be duplicated even after many months. Such curves have interejting characteristics. Curve I1 in Figure 1 is typical of mono-olefinic compounds in ethyl alcohol. The initial rate P R , the coordinates of point 8 , the intercept OD, and the slope of line A B are all related to the particular type of double bond involved in the reaction. The position of this bond in the molecule; the presence, nature, and location of substituents; and the presence of activating functional groups influence the values of the above characteristics. For the initial rate alone
V O L U M E 28, NO. 3, M A R C H 19.56 .there is a iyide range of values from zero (benzenic unsaturation, in some cases) to a very high one (allylic unsaturation). K i t h test materials containing different types of double bonds, the curve may, vhen the proper medium is used, show charact,eristic breaks such as in curves I1 and 111,Figure 3. Such curves allo\y the quantitative estimation of the different double bonds by comparison with standard curves, which are established by using mixtures containing known amounts of the same constituents. The full advantage of these properties may not be generally appreciated until a reasonably complete and systematic exploration of PS 14 behavior toward unsaturated compounds has been accomplished. Such a task requires access to a considerable number of pure. not commercially available, chemicals.
365 LITERATURE CITED (1) Adams, R., Voorhees, V., Shriner, R. L., “Organic Synthesis,” Collective Vol. I, p. 463, Wiley, New York, 1948. (2) ddkins, H., Billica, H. R., J . Am. Chem. SOC.70, 695 (1948). (3) Ciapetta, F. G., Plank, C. J., “Catalysis,” Vol. 1 , pp. 315-52, ed. by Paul H. Emmet, Reinhold, iYew York, 1954. (4) Feuge, R. P., Ibid., Vol. 3, pp. 413-31. J . O i g . C h e n . 2, 314 (5) Hurd, C. D., Roe, A. S., Williams, J. W., (1937). (6) Martin, R. TV., U. S. Patent 2,207,868 (July 16, 1940). 46, 1683 (1924). (7) Shriner, R. L., ddams, R., J . Am. Chmna. SOC. (8) Twigg, G. H., Rideal, E. K., Trans. Faraday SOC.36, 533-7 (1940). (9) Vandenheuvel, F. -I., ;INAL. CHEM.24, 847 (1952).
RECEIVED for review January 4, 1955.
Accepted December 19, 1955,
Separation of Radioactive Silver-I 11 from Pile-Irradiated Palladium FRED SlClLlO and M. D. PETERSON, Department o f Chemistry, Vanderbilt University, Nashville, Tenn. GUILFORD G. RUDOLPH, Radioisotope Unit, Thayer Veterans Administration Hospital, Nashville, Tenn.
and
Radioactive silver-111, a 1-m.e.v. beta emitter with a 7.5-day half life, made by neutron irradiation of palladium metal, w-as separated in a remotely controlled, shielded, glass apparatus. The palladium metal was dissolved in aqua regia and 20 mg. of silver carrier as the chloride complex was added. The separation involved precipitation of silver chloride by dilution, dissolving again in ammonium hydroxide, followed by reprecipitation. The product was recovered in ammonium hydroxide solution or, if silver nitrate was desired, by metathesis to silver oxide, which was then dissolved in nitric acid. The yields were greater than 90yc. The products contained less than 1 y of palladium, and no activities other than that of silver-111 have been detected, even after decaying for 10 half lives. The silver-111 content was about 50 mc. in the highest activity product, and tests indicated that the method may be suitable for much larger quantities of silver-111.
P
ALLADIUiU-110, of 0.135 natural abundance, is transformed by sloiv neutron capture (cross section about 0.4 barn) t o r a d i o a d v e palladium-111, which decays with a 22minute half life by 2-m.e.v. beta emission to radioactive silver-111. The silver-111 is a 1-m.e.v. beta emitter wit,h a 7.5-day half life, decaying t o stable cadmium-111. Also, palladium-108 (natural abundance 0.268, s l o neut’ron ~ absorpt’ion cross section ahout 11 barns) forme radioactive palladium-109, a 1-n1.e.v. I,et,a emitter with 13-hour half life n-hich decays t o stable silver109 (3).
Griess and Rogers (1) obtained carrier-free silver-111 by electrolysis from dilute palladium solution. Haymond and others , ( 2 ) carried the silver-111 on a mercurous chloride precipitate, and later removed the carrier by evaporation a t 450’ C. Zimen (8)carried the silver-111 on a silver chloride precipitate, reduced the precipitate to silver using hydrogen a t 500” C., and dissolved the silver in nitric acid. Rouser and Hahn ( 4 ) reduced the animonia complex of t,he silver-111 and carrier silver in the palladium solution Tyith vitamin C, and dissolved the metallic silver in nitric acid. Schweit,zer and Nehls (6)utilized a method based .on the formation of radiocolloids in basic solution, and Sunderman and Meinke ( 7 ) exchanged t,he silver-111 from a solution with the silver of a silver chloride film on platinum gauze. I n the work reported here, 1-gram foils of palladium metal,
irradiated in an Oak Ridge Sational Laboratory reactor for a week, were allowed to “cool” for 4 days before processing, after m-hich time the more intense palladium-109 activity had decreased to about the same level as that of the silver-111. The separation was performed by dissolving the palladium foil in aqua regia, adding inactive silver carrier, then concentrated h j drochloric acid to assure complete solution of all the silver as its complex chloride ion. Silver chloride was precipitated by dilution with water, then dissolved in ammonium hydroxide and reprecipitated for further pui ification. The silver chloride 1% as dissolved in ammonium hydroxide for the final product However, if silver nitrate v a s desired as the final product, sodium h>droxide was added t o precipitate silver oxide, which v a s then dissolved in nitric acid. The silver chloride from the first precipitation always showed the same slight tan discoloration, indicating palladium contamination. Hon-ever, the amount of palladium adsorbed on the precipitate was clearly shonm t o be less than 10 y by dissolving one of the precipitates in 4 ml. of concentrated hldrochloric acid and comparing the color of the solution and its absorption (at the 460-mp absorption band of palladium-h) diochloric acid solutions) n-ith similar solutions containing known amounts of palladium. The palladium decontamination factor for a single precipitation is about lo5, therefore, the palladiuni contamination after reprecipitation should be only a very small fraction of a microgram. Sone of the reprecipitated silver chloride products showed any t a n discoloration, and a concentrated hydrochloric acid solution of one showed no absorbancy a t 460 mp, under such conditions that the limit of detection n-as about 1 y of palladium. The color of the silver chloride precipitate is therefore a very delicate indication of the palladium contaminatione.g., if the precipitate is white, the palladium content is surely less than a fexv micrograms. No foreign radioactivities have been detected in the products. Gamma spectra were recorded automatically Kith a sodium iodide (Tl) crystal spectrometer, and the spectrum was not changed by further purification. The activity of a product has been folloned for 10 half lives, with no departure from linearity in the log activity us. time curve. The separation procedure was followed using unirradiated palladium and silver carrier containing purified silver-1 10 (2iO-day half life) rrith an activity of about 20 mr. per hour a t 10 em. The activity of all solutions was measured v i t h a Geiger-Muller survey meter, and also by aliquot counting. The yield was greater than 90% in both cases. The jield m-as also
