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Borohydride reduction of the perrhenate ion
Notes
1375
log [p/(p¢--p) ] = kt + c
(1)
where p is the gas pressure at time t, pC is the final pressure and k and c are constants. It is of interest to note that at lower temperature viz. 195°C the constant k is different for acceleratory and decay period, but with increase in temperature the difference between the two k values falls eventually to zero, i.e. the Prout-Tompkins plot gives a continuous straight line for both acceleratory and decay periods. The Arrhenius plot for the acceleratory period (log k against I/T) for both unirradiated and irradiated material gives good straight lines. The activation energy was found to be 36-7 kcal/mole for unirradiated and 30.8 kcal/mole for irradiated material. The activation energy is appreciably affected by irradiation. A full account of work, mechanism, analysis of decomposition products (thermal and radiation), will be presented in later publication. We are indebted to Dr. J. Shankar, Head of Chemistry Division, and Dr. K. N. Rao, Head of Radiation Chemistry Section, BARC, Trombay, Bombay, for irradiation facilities. Our thanks are also due to Dr. R. H. Sahasrabudhey, Head of the Department of Chemistry, Nagpur University, for laboratory facilities. Department o f Chemistry NagpurUniversity Nagpur, India
M . N . RAY* N.D. SINNARKAR
*To whom correspondence to be addressed.
J. inorg, nucl. Chem., 1973, Vol. 35, pp. 1375-1377. Pergamon Press.
Printed in Great Britain
Borohydride reduction of the perrhenate ion (First received 15 May 1972; in revised form 22 June 1972) THIS WORK was undertaken because of the uncertain nature of the reduction product when rhenium (VII) is reduced by the borohydride ion. Johnson[l] reported the product to be Re~Oa; prior synthesis of Re203 had been unknown, inasmuch as species containing rhenium in an oxidation state less than + 4 are readily air-oxidized to Re(IV). The equation for the reaction in acetic acid/sodium acetate buffer is: 2 NH4ReO4 + NaBH4 --} RezO~ + NaBOz + 2 NH4OH + H20. Broadbent and Johnson[2] described the synthesis in some detail, but no conclusive analysis of the product was made to show that RezO~ had indeed been prepared. The % Re in Re203 is 88.6%; analysis showed the % Re in their product to vary from 87' 1 to 89.1%. It is possible that the actual product was ReO2 (which contains 85.4% Re) contaminated with rhenium metal. Unfortunately, the equivalent weight of the product was not determined, which however would have been more conclusive. Re203 has an equivalent weight of 52.6 while the equivalent weight of ReOz is 72-7, based on oxidation to the +7 state. Hurst et al. [3] also reduced ammonium perrhenate with sodium borohydride and obtained an oxide which was reported to "correspond closely to Re203-3 H~O," based upon X-ray diffraction and thermogravimetric analysis. Upon heating under vacuum, the product was dehydrated and converted to metallic rhenium, ReOz, and oxygen. In no case was anhydrous Re203 obtained. This leads one to question whether or not Re203 is the reduction product when Re(VII) is reduced 1. J. H. Johnson, U.S. Dept. Com., Office Tech. Serv., P. B. Rept. 133802, 99 pp., 1960. 2. H. Broadbent and J. Johnson, J. org. Chem. 27, 4400 (1962). 3. F. Hurst. P. Gibart and K. Traore, C.r. hebd. S~anc. Acad. Sci., Paris C263(2), 97 (1966).
1376
Notes
by borohydride ion. The RezOa prepared by Broadbent and Johnson/4] was observed to be an effective catalyst for hydrogenation reactions. The fact that ReOz adsorbs hydrogen, as observed by Tribalat and Mofidi[4], leads one to wonder whether the catalytic ability might actually be due to hydrogen adsorbed on ReO2. EXPERIMENTAL The reduction of perrhenate by borohydride ion, as described by Broadbent and Johnson [2], was repeated. The colloidal, black product was isolated by centrifugation but no attempt was made to recover it quantitatively, in view of the danger of air oxidation. The reduction was carried out in a glove bag in a N2 atmosphere. The product was immediately transferred to a vacuum desiccator, (also in the glove bag) previously charged with Mg(CIO4)z and N2, and allowed to dry for one week. The dried product was then removed and subjected to analyses for % Re and for equivalent weight. The % Re was found by dissolving a portion of the sample in ammoniacal H2Oz and precipitating Re(VII) as the tetraphenylarsonium salt, as described by Broadbent and Johnson/2]. The equivalent weight was determined by adding an excess of Ce(1V) to a second portion of the sample, allowing sufficient time for r e a c t i o n - a s indicated by the complete disappearance of all black particulate m a t t e r - a n d back titrating with standard Fe(II). The results, given below, were compared with calculated values for possible rhenium oxide compounds. The results, to - one standard deviation, show an average % Re of 84.3_+ 2"0, and an average equivalent weight of 48.9-+ 4.2. The compound which gives the best fit with the data is ReO.H20, followed in turn by RezO3"H20. However, when i.r. spectra of sample products were taken, the characteristic H20 bands at 3756, 3652, and 1545 cm -1 were absent. By way of reference, a sample of NiCI2.6H20 of comparable mass showed strong H20 bands at the wave numbers indicated. Because of the remote possibility that a change in oxidation state of rhenium could take place during centrifugation and/or while transferring the products to the desiccator, a determination of the number of electrons transferred per perrhenate ion was carried out immediately at the end of the reaction. In a glove bag, a known amount of KReO4 was treated with an excess of NaBH4 solution (in 0' 1 M NaOH), followed by dropwise addition of 1 M CH3COOH. After effervescence ceased, dilute H2SO4 was added and the solution was bubbled with Nz f o r 0 - 3 0 min to remove Hz. Next, a known excess of standard Ce(IV) was added and the flask was stoppered. After reoxidation of the rhenium species to Re(VII), as evidenced by the complete disappearance of all black color, the excess Ce(IV) was back-titrated with standard Fe(ll). The average number of electrons transferred per perrhenate ion, n, was calculated. The results show no dependence on the time of bubbling with N2. Excess H2 evidently escapes readily from the solution and, if it is adsorbed by the product, bubbling with N2 has no effect. The results show clearly that an effective oxidation state is obtained which is less than +3. The fact that a non-integral n-value is obtained also indicates that a mixture of two or more species is obtained. Table 1. Analyses of the prepared rhenium oxide Run
% Re
Equiv. wt.
1
81.6 81.5 85-6 83.1 84.5 84"7 87"0 86'2 84.3
57.1 49-1 46.9 51.9 45-4 48.1 43"5 49"2 48.9
2 3 4 5 6 7 8 Ave
Table 2. Determination of the average number of electrons transferred per perrhenate ion, n Time of bubbling with N~ (min)
n
0 5 10 15 22 30
4.83 6.46 5.49 4.55 4.72 4.88
4. S. Tribalat and D. Mofldi, Bull. Soc. Chim. France 3684 (1965).
Notes
13 7 7
The fact that n values > 5 were obtained in two runs favors the assignment of rhenium metal as the lower oxidation state species. One cannot distinguish between Re~Oa or ReOs as the higher oxidation state species on the basis of the data available. Since ReO2 does not catalyze hydrogenation reactions [4], if ReO2 is the higher oxidation state species, adsorbed hydrogen must also be p r e s e n t - u n l e s s rhenium metal itself is the catalyst in question. CONCLUSION The reduction of perrhenate ion by borohydride ion produces a mixture of rhenium species, rather than a single compound. The effective overall oxidation state of the products is between + 2 and +3. The black solid isolated as a product of the perrhenate-borohydride reaction was found to contain 84.3 - 2.0% Re and to have an equivalent weight of 48.9 ± 4.2. The reaction products consist of rhenium metal and either ReO2 or Re203. It is uncertain whether the catalytic ability of the reaction products is due to rhenium metal, Re2Oz, or adsorbed hydrogen.
Department of Chemistry Purdue University, Fort Wayne Campus Fort Wayne, Indiana 46805
R I C H A R D A. PACER
J. inorg, nucl. Chem., 1973, Vol. 35, pp. 1377-1379.
Pergamon Press.
Printed in Great Britain
A basic formate of uranium(VI) (Received 15June 1972) TREATMENT of solutions of uranyl acetate and formate with hydroxides of alkali metals yields products of indefinite composition. The formation of a basic acetate of uranium during electrolytic reduction of uranyl acetate has been reported[l]. During our investigations on the thallium(I) uranium(VI) formates[2, 3] it was observed that a suspension of the red-orange basic double formate, TIUO2(OH)z(HCOO), on continued agitation in hot water in dark gave rise to an yellow coloured solid which answered only for U(VI) and did not contain any thallium. On complete characterisation, the solid was found to be UO2(OH)(HCOO)H20, a basic formate of uranium(VI) having a definite composition. Its preparation and some of its characteristics are described below. When equal volumes of equimolar aqueous solutions of thallous formate and uranyl formate were mixed in cold in absence of free formic acid and stirred, the compound TIUO~(OH)2(HCOO) was formed as a red-orange precipitate. The suspension of the precipitate (2-3 g) in about 100 ml water was stirred vigorously in the absence of light for 4 hr at 70-80 ° when the solid turned yellow. The yellow solid was washed with water, alcohol and then dried at 60 °. It did not answer the test for thallium and all the thallium was quantitatively present in the filtrate. The composition of the yellow solid was ascertained by determining the uranium by ignition to 800 ° and weighing as UaOs and the formate content by permanganate method [4]. All the uranium was in oxidation state + 6 as the yellow compound was nonreducing towards acid solutions of vanadium(V) and cerium(IV). The analytical value obtained corresponded to the solid having the composition, UO2(OH) (HCOO)HzO (U, found: 1. L. R. Philips, C. D. Hardin, W. L. Harper and C. D. Montgomery, U.S.A.E.C. Report y-1308, p. 11, 1960. 2. S. Sampath, Ph. D. Thesis, Indian Institute of Technology, Madras, India, 1970. 3. G. Aravamudan and S. Sampath, Proc. Symposium in Chem., Dept. o f Atomic Energy (India), VoL I, p. 299 (1970). 4. A. I. Vogel, Text book of Quantitative Inorganic Analysis, p. 301. E.L.B.S. Longmans (1961).
J | N C Vol. 35, No. 4 - K
1375
log [p/(p¢--p) ] = kt + c
(1)
where p is the gas pressure at time t, pC is the final pressure and k and c are constants. It is of interest to note that at lower temperature viz. 195°C the constant k is different for acceleratory and decay period, but with increase in temperature the difference between the two k values falls eventually to zero, i.e. the Prout-Tompkins plot gives a continuous straight line for both acceleratory and decay periods. The Arrhenius plot for the acceleratory period (log k against I/T) for both unirradiated and irradiated material gives good straight lines. The activation energy was found to be 36-7 kcal/mole for unirradiated and 30.8 kcal/mole for irradiated material. The activation energy is appreciably affected by irradiation. A full account of work, mechanism, analysis of decomposition products (thermal and radiation), will be presented in later publication. We are indebted to Dr. J. Shankar, Head of Chemistry Division, and Dr. K. N. Rao, Head of Radiation Chemistry Section, BARC, Trombay, Bombay, for irradiation facilities. Our thanks are also due to Dr. R. H. Sahasrabudhey, Head of the Department of Chemistry, Nagpur University, for laboratory facilities. Department o f Chemistry NagpurUniversity Nagpur, India
M . N . RAY* N.D. SINNARKAR
*To whom correspondence to be addressed.
J. inorg, nucl. Chem., 1973, Vol. 35, pp. 1375-1377. Pergamon Press.
Printed in Great Britain
Borohydride reduction of the perrhenate ion (First received 15 May 1972; in revised form 22 June 1972) THIS WORK was undertaken because of the uncertain nature of the reduction product when rhenium (VII) is reduced by the borohydride ion. Johnson[l] reported the product to be Re~Oa; prior synthesis of Re203 had been unknown, inasmuch as species containing rhenium in an oxidation state less than + 4 are readily air-oxidized to Re(IV). The equation for the reaction in acetic acid/sodium acetate buffer is: 2 NH4ReO4 + NaBH4 --} RezO~ + NaBOz + 2 NH4OH + H20. Broadbent and Johnson[2] described the synthesis in some detail, but no conclusive analysis of the product was made to show that RezO~ had indeed been prepared. The % Re in Re203 is 88.6%; analysis showed the % Re in their product to vary from 87' 1 to 89.1%. It is possible that the actual product was ReO2 (which contains 85.4% Re) contaminated with rhenium metal. Unfortunately, the equivalent weight of the product was not determined, which however would have been more conclusive. Re203 has an equivalent weight of 52.6 while the equivalent weight of ReOz is 72-7, based on oxidation to the +7 state. Hurst et al. [3] also reduced ammonium perrhenate with sodium borohydride and obtained an oxide which was reported to "correspond closely to Re203-3 H~O," based upon X-ray diffraction and thermogravimetric analysis. Upon heating under vacuum, the product was dehydrated and converted to metallic rhenium, ReOz, and oxygen. In no case was anhydrous Re203 obtained. This leads one to question whether or not Re203 is the reduction product when Re(VII) is reduced 1. J. H. Johnson, U.S. Dept. Com., Office Tech. Serv., P. B. Rept. 133802, 99 pp., 1960. 2. H. Broadbent and J. Johnson, J. org. Chem. 27, 4400 (1962). 3. F. Hurst. P. Gibart and K. Traore, C.r. hebd. S~anc. Acad. Sci., Paris C263(2), 97 (1966).
1376
Notes
by borohydride ion. The RezOa prepared by Broadbent and Johnson/4] was observed to be an effective catalyst for hydrogenation reactions. The fact that ReOz adsorbs hydrogen, as observed by Tribalat and Mofidi[4], leads one to wonder whether the catalytic ability might actually be due to hydrogen adsorbed on ReO2. EXPERIMENTAL The reduction of perrhenate by borohydride ion, as described by Broadbent and Johnson [2], was repeated. The colloidal, black product was isolated by centrifugation but no attempt was made to recover it quantitatively, in view of the danger of air oxidation. The reduction was carried out in a glove bag in a N2 atmosphere. The product was immediately transferred to a vacuum desiccator, (also in the glove bag) previously charged with Mg(CIO4)z and N2, and allowed to dry for one week. The dried product was then removed and subjected to analyses for % Re and for equivalent weight. The % Re was found by dissolving a portion of the sample in ammoniacal H2Oz and precipitating Re(VII) as the tetraphenylarsonium salt, as described by Broadbent and Johnson/2]. The equivalent weight was determined by adding an excess of Ce(1V) to a second portion of the sample, allowing sufficient time for r e a c t i o n - a s indicated by the complete disappearance of all black particulate m a t t e r - a n d back titrating with standard Fe(II). The results, given below, were compared with calculated values for possible rhenium oxide compounds. The results, to - one standard deviation, show an average % Re of 84.3_+ 2"0, and an average equivalent weight of 48.9-+ 4.2. The compound which gives the best fit with the data is ReO.H20, followed in turn by RezO3"H20. However, when i.r. spectra of sample products were taken, the characteristic H20 bands at 3756, 3652, and 1545 cm -1 were absent. By way of reference, a sample of NiCI2.6H20 of comparable mass showed strong H20 bands at the wave numbers indicated. Because of the remote possibility that a change in oxidation state of rhenium could take place during centrifugation and/or while transferring the products to the desiccator, a determination of the number of electrons transferred per perrhenate ion was carried out immediately at the end of the reaction. In a glove bag, a known amount of KReO4 was treated with an excess of NaBH4 solution (in 0' 1 M NaOH), followed by dropwise addition of 1 M CH3COOH. After effervescence ceased, dilute H2SO4 was added and the solution was bubbled with Nz f o r 0 - 3 0 min to remove Hz. Next, a known excess of standard Ce(IV) was added and the flask was stoppered. After reoxidation of the rhenium species to Re(VII), as evidenced by the complete disappearance of all black color, the excess Ce(IV) was back-titrated with standard Fe(ll). The average number of electrons transferred per perrhenate ion, n, was calculated. The results show no dependence on the time of bubbling with N2. Excess H2 evidently escapes readily from the solution and, if it is adsorbed by the product, bubbling with N2 has no effect. The results show clearly that an effective oxidation state is obtained which is less than +3. The fact that a non-integral n-value is obtained also indicates that a mixture of two or more species is obtained. Table 1. Analyses of the prepared rhenium oxide Run
% Re
Equiv. wt.
1
81.6 81.5 85-6 83.1 84.5 84"7 87"0 86'2 84.3
57.1 49-1 46.9 51.9 45-4 48.1 43"5 49"2 48.9
2 3 4 5 6 7 8 Ave
Table 2. Determination of the average number of electrons transferred per perrhenate ion, n Time of bubbling with N~ (min)
n
0 5 10 15 22 30
4.83 6.46 5.49 4.55 4.72 4.88
4. S. Tribalat and D. Mofldi, Bull. Soc. Chim. France 3684 (1965).
Notes
13 7 7
The fact that n values > 5 were obtained in two runs favors the assignment of rhenium metal as the lower oxidation state species. One cannot distinguish between Re~Oa or ReOs as the higher oxidation state species on the basis of the data available. Since ReO2 does not catalyze hydrogenation reactions [4], if ReO2 is the higher oxidation state species, adsorbed hydrogen must also be p r e s e n t - u n l e s s rhenium metal itself is the catalyst in question. CONCLUSION The reduction of perrhenate ion by borohydride ion produces a mixture of rhenium species, rather than a single compound. The effective overall oxidation state of the products is between + 2 and +3. The black solid isolated as a product of the perrhenate-borohydride reaction was found to contain 84.3 - 2.0% Re and to have an equivalent weight of 48.9 ± 4.2. The reaction products consist of rhenium metal and either ReO2 or Re203. It is uncertain whether the catalytic ability of the reaction products is due to rhenium metal, Re2Oz, or adsorbed hydrogen.
Department of Chemistry Purdue University, Fort Wayne Campus Fort Wayne, Indiana 46805
R I C H A R D A. PACER
J. inorg, nucl. Chem., 1973, Vol. 35, pp. 1377-1379.
Pergamon Press.
Printed in Great Britain
A basic formate of uranium(VI) (Received 15June 1972) TREATMENT of solutions of uranyl acetate and formate with hydroxides of alkali metals yields products of indefinite composition. The formation of a basic acetate of uranium during electrolytic reduction of uranyl acetate has been reported[l]. During our investigations on the thallium(I) uranium(VI) formates[2, 3] it was observed that a suspension of the red-orange basic double formate, TIUO2(OH)z(HCOO), on continued agitation in hot water in dark gave rise to an yellow coloured solid which answered only for U(VI) and did not contain any thallium. On complete characterisation, the solid was found to be UO2(OH)(HCOO)H20, a basic formate of uranium(VI) having a definite composition. Its preparation and some of its characteristics are described below. When equal volumes of equimolar aqueous solutions of thallous formate and uranyl formate were mixed in cold in absence of free formic acid and stirred, the compound TIUO~(OH)2(HCOO) was formed as a red-orange precipitate. The suspension of the precipitate (2-3 g) in about 100 ml water was stirred vigorously in the absence of light for 4 hr at 70-80 ° when the solid turned yellow. The yellow solid was washed with water, alcohol and then dried at 60 °. It did not answer the test for thallium and all the thallium was quantitatively present in the filtrate. The composition of the yellow solid was ascertained by determining the uranium by ignition to 800 ° and weighing as UaOs and the formate content by permanganate method [4]. All the uranium was in oxidation state + 6 as the yellow compound was nonreducing towards acid solutions of vanadium(V) and cerium(IV). The analytical value obtained corresponded to the solid having the composition, UO2(OH) (HCOO)HzO (U, found: 1. L. R. Philips, C. D. Hardin, W. L. Harper and C. D. Montgomery, U.S.A.E.C. Report y-1308, p. 11, 1960. 2. S. Sampath, Ph. D. Thesis, Indian Institute of Technology, Madras, India, 1970. 3. G. Aravamudan and S. Sampath, Proc. Symposium in Chem., Dept. o f Atomic Energy (India), VoL I, p. 299 (1970). 4. A. I. Vogel, Text book of Quantitative Inorganic Analysis, p. 301. E.L.B.S. Longmans (1961).
J | N C Vol. 35, No. 4 - K