Patented Nov. 4, 1952 2,616,791

UNITED STATES PATENT OFFICE 2,616,791 PROCESS FOR PREPARING SELENIUM DOXDE Reuben Roseman, Baltimore, Ralph W. Neptune, Dundalk, and Benjamin W. Allan, Baltimore, t Md., assignors to The Glidden Company, Cleve i: land, Ohio, a corporation of Ohio NoDrawing. Application September 30, 1950, Serial No. 187,842 10 Claims. (CI. 23-139) This invention relates to novel processes for The following are examples of our process, al preparing aqueous solutions of selenious acid and though these examples are not to be construed for preparing selenium dioxide. in a limiting sense. The compound selenium dioxide, SeO2, has a Eacample it variety of uses, among which may be mentioned 5 the following: it is widely used as an oxidizing 200 g. of gray selenium powder (better than agent in the field of organic chemistry as, for 99% pure) were slurried in 100 cc. of water con instance, in the preparation of sex hormone de tained in a 2-liter beaker. To this slurry Were rivatives related to pregnenolone and androste added 387 cc. of 30% hydrogen peroxide solution rone; it has interesting catalytic properties serv 10 (General Chemical Company), equivalent to ing, for example, in the medium of concentrated 75% of the amount theoretically, demanded by Sulfuric acid as a catalyst for the rapid Combus reaction (1), above. The hydrogen peroxide tion of organic matter in the Kjeldahl method of Was added over a period of four hours, with pro analysis; likewise, in concentrated sulfuric acid visions made throughout for cooling up 25-35 Solution, it gives characteristic color reactions 15 C. and for thorough stirring of the reaction mix and organic selenium compounds with alkaloids ture. The mixture was further stirred for about and aromatic amines; it is used in the manu an hour and was then allowed to stand overnight facture of other selenium compounds such as to insure complete reaction of the hydrogen per the selenites; and it is employed in toning proc oxide, following which it was filtered. The resi eSSes in photography. 20 due on the funnel was washed, then dried at Heretofore the best methods for the prepara 105° C. to yield 59 g. of selenium, suitable for tion of selenium dioxide have involved oxidation reuse in a fresh reaction with hydrogen peroxide; of selenium by nitric acid or combustion of sele the filtrate and washings were evaporated to ap nium in the presence of nitrogen dioxide. We proximate dryness on the hot plate, and the re have discovered what we believe to be a new 25 sulting crystals were heated for several hours at process for making selenium dioxide which is, at approximately 150-160° C. to decompose any re the same time, simple, direct, and free of any maining Selenious acid, ground in a mortar, and funne nuisance. - finally dried at 105°s-10° C. (to remove any nois Contrary to all available prior published work ture picked up in the course of grinding), to on the reaction between selenium and hydrogen 30 yield 197 g. of selenium dioxide. This product peroxide, which teaches only oxidation of the analyzed 70.5% Se (theoretical Se content in element by the peroxide to selenic acid, that is, SeO2 is 71.2%); it was found to be entirely free the hexavalent State (See, for example, the all of hexavalent selenium as shown by tests on a thoritative Mellor's "A Comprehensive Treatise portion with barium chloride in the presence on Inorganic and Theoretical Chemistry,” Long 35 of nitric acid; it contained less than 0.15% total mans, Green & Co., 1922, 1930, vol. I, pp. 937 and metallic impurities as shown by spectrographic 941, and vol X, p. 747; and Friend's "A Textbook analysis; and it exhibited an X-ray powder dif of Inorganic Chemistry,' Griffin and Co., 1931, fraction pattern identical in all respects with vol. VII, part II, p.300), we have found that under that of known Seleniurn dioxide, the appropriate conditions oxidation to the quad 40 Eacample 2 rivalent stage exclusively is readily secured, ap parently according to the reaction, In this experiment a slurry of 200 g. of Selenium in 100 cc. of water, contained in a 1500-cc. (1) Se--2H2O2=SeO3--2H2O beaker, was treated with 516 cc. of 30% hydro We have discovered that if a concentrated hydro 45 gen peroxide solution; the hydrogen peroxide, gen peroxide solution is slowly added to a con representing the chemical equivalent of the sele-- centrated water slurry of powdered elementary nium, was added over a period of 2% hours, with selenium, with vigorous stirring and with cooling good agitation of the reaction nixture and cool of the mixture, and in such amount as not to ing to 25°-35° C. Stirring was continued for an exceed that stoichiometrically demanded by re 50 additional four-hour period and the mixture was action (1), above, then simple dehydration of the allowed to stand overnight to insure complete re filtrate obtained from the thoroughly reacted re action of the hydrogen peroxide. The steps of action mixture will yield crystals of selenium filtration, Washing, and dehydration were cone dioxide (the anhydride of selenious acid) entirely ducted as in the previous example. The residual free of selenic acid or selenium trioxide. 55 selenium weighed 26 g. and was re-usable, and 2,616,791 3 4. the yield of selenium dioxide crystals was 245 g. ing the temperature of the reaction mass below The latter product analyzed 70.5% Se, was en about 35 C., until the total effective quantity tirely free of hexavalent selenium, and, as in the of added hydrogen peroxide amounts to no more case of the product of Example 1, was well suited than 2 moles thereof per mole of selenium in for use in the various-types of chemical reactions said slurry; thereafter discontinuing the addi calling for selenium dioxide. tion of hydrogen peroxide and continuing to stir In general, in the practice of our invention it the reaction mass and to maintain its tempera is best ture below about 35° C. until all of the hydrogen (a) To employ such an amount of hydrogen peroxide in said mass has been reduced; then peroxide as is not in excess of that equivalent 0 removing any undissolved selenium from the to the selenium in the reaction, reaction mass and recovering an aqueous Solu tion of selenium dioxide in the form of selenious acid. (b) To conduct the reaction in the cold, and 2. The process as claimed in claim 1 which (c) To provide for the absence of any residual 5 includes the further step of evaporating the re hydrogen peroxide in the reaction mixture just covered solution approximately to dryness and prior to the filtration and dehydration steps. then decomposing any remaining selenious acid Such absence may be secured by continued stir in the resulting residue into selenium dioxide ring of the reaction mixture for a suitable period and Water vapor by heating the residue at tem after the last addition of hydrogen peroxide. 20 peratures sufficiently high to effect such decom Inattention to the above conditions will lead position but below about 160° C. to end products which are contaminated With 3. The process as claimed in claim 1 wherein hexavalent selenium, as the following experi the temperature of the reaction mass is continu ment will show. ously maintained between about 25° C. and 35° C. A 50-g. sample of gray selenium powder, such 25 4. The process as claimed in claim 3 which as was used in the preceding examples, Was includes the further step of evaporating the re slurried in 25 cc. of water, and the slurry was COvered Solution approximately to dryness and treated with 194 cc. of 30% H2O2. The peroxide, then decomposing any remaining selenious acid representing a 50% excess according to reaction in the resulting residue into selenium dioxide (1) above, was added rapidly (in less than 10 30 and Water vapor by heating the residue at tem minutes), with stirring, and no attempt was peratures sufficiently high to effect such decom made to cool the reaction mixture. The mix position but below about 160° C. Was further heated on the hot plate for about 5. The process of preparing selenium dioxide an hour, at the end of which time it was observed which comprises the steps of treating an aqueous that all of the selenium was in solution. The 35 Slurry of selenium, while stirring, with aqueous filtered solution, unlike those of Examples 1 and hydrogen peroxide at temperatures below about 2, could not be dried by evaporation on the hot 35° C. until at least a part of the selenium has plate followed by heating overnight at a tem been dissolved, the hydrogen peroxide employed perature of 150°-160° C.; the end product of in Said treatment corresponding to not more than this experiment, unlike the excellent, dry crystal 40 2 moles thereof per mole of selenium being line preparations obtained in the preceding ex treated; continuing stirring of the reaction mix amples, was a pasty mass which contained the ture for a sufficient period to insure complete re equivalent of more than 27% SeO3. duction of the hydrogen peroxide; thereafter It will be understood that hydrogen peroxide separating any undissolved selenium from the of any convenient concentration may be em- . 45 reaction mixture; and dehydrating the resulting ployed, but that concentrated solutions are pre Solution at temperatures not in excess of about ferred since they introduce less Water and con 160° C. Sequently reduce the amount of water which 6. The process of preparing selenium dioxide must ultimately be evaporated to bring about from selenium, which comprises: slurrying about recovery of crystalline selenium dioxide. For 50 200 parts of selenium in about 100 parts of Water; the same reason we prefer to use concentrated While maintaining the reaction mixture in a slurries of comminuted selenium. Enough water liquid state but below a temperature of about should be present, however, to permit adequate 35° C., and while constantly stirring, slowly add control of the temperature of the reaction mix ing about 30% aqueous hydrogen peroxide to said ture by stirring and cooling. 55 slurry until the total effective quantity of hydro gen peroxide; SO added is not more than 2 moles Eacample 3 per mole of selenium; continuing stirring of the A slurry of selenium-100 g. in 50 cc. of water - reaction mixture for a sufficient period to insure was added to 258 cc. of 30%.H2O2, over a period complete reduction of the hydrogen peroxide; of 2 hours, with vigorous agitation of the reac 60 thereafter filtering off any undissolved selenium; tion mixture and cooling to below 35° C. Beyond evaporating the resulting filtrate to approximate this point the procedure was the same as that dryness; and heating the residue attemperatures of Examples 1 and 2. The residual selenium not in excess of about 160° C. to complete its Weighed 16.4 g., and the yield of selenium dioxide dehydration to selenium dioxide. crystals was 117 g. The latter material analyzed 65 7. The process of converting selenium selec 70.5% Se and it was entirely free of hexavalent tively to Selenium dioxide, Which comprises the Selenium. steps of: reacting comminuted selenium with not Having now disclosed our invention, what We more than two moles of hydrogen peroxide per claim is: mole of selenium in the presence of Water and 1. The process for converting selenium selec 70 With agitation at temperatures below. about 35° tively to selenium dioxide which comprises the C. until all of the hydrogen peroxide has been steps of: providing an aqueous slurry of com reduced; thereafter separating any undissolved minuted selenium; gradually adding aqueous hy Selenium from the reaction mass and recovering drogen peroxide to said slurry while stirring the an aqueous solution of selenium dioxide in the reaction mass and while continuously maintains 75 form of selenious acid. 2,616,791 5 6 8. The process as claimed in claim 7 which in the resulting residue into selenium dioxide and includes the further step of evaporating the re Water vapor by heating the residue at tempera covered Solution approximately to dryness and tures sufficiently high to effect such decomposi then decomposing any remaining selenious acid tion but below about 160° C. in the resulting residue into selenium dioxide and REUBEN ROSEMAN. Water vapor by heating the residue at tempera RALPH. W. NEPTUNE. tures sufficiently high to effect such decomposi BENJAMIN W. ALLAN. tion but below about 160° C. REFERENCES CITED 9. The process as claimed in claim 7 wherein the comminuted selenium is introduced slowly O The following references are of record in the into aqueous hydrogen peroxide. file of this patent: 10. The process as claimed in claim 9 which Mellor, 'A Comprehensive Treatise on Inor includes the further step of evaporating the re ganic and Theoretical Chemistry,' Longmans, covered solution approximately to dryness and Green & Co., N. Y., vol. X (1930), pages 808-810, then decomposing any remaining selenious acid l3 816, 817, 843-845.