Isotope Separation by Selective Photodissociation of Glyoxal

United States Patent im [in 3,947,335 Marling [45] Mar. 30, 1976

[54] ISOTOPE SEPARATION BY SELECTIVE Primary Examiner—Howard S. Williams PHOTODISSOCIATION OF GLYOXAL Attorney, Agent, or Firm—Dean E. Carlson; F. A. Robertson; Irene S. Croft [75] Inventor: John B. Marling, Pleasanton, Calif. [73] Assignee: The United States of America as represented by the United States [57] ABSTRACT Energy Research and Development Administration, Washington, D.C. Dissociation products, mainly formaldehyde and car- [22] Filed: Mar. 4, 1975 bon monoxide, enriched in a desired isotope of carbon, oxygen, or hydrogen are obtained by the selec- [21] Appl. No.: 555,315 tive photodissociation of glyoxal wherein glyoxal is subjected to electromagnetic radiation of a predeter- [52] U.S. Cl 204/158 R mined wavelength such that photon absorption excites [51] Int. Cl.2 B01J1/10 and induces dissociation of only those molecules of [58] Field of Search 204/158 R glyoxal containing the desired isotope.

[56] References Cited 6 Claims, No Drawings OTHER PUBLICATIONS Moore, Account of Chemical Research, Vol. 6, 1973, p. 323. 3,947,335 1 2 ISOTOPE SEPARATION BY SELECTIVE SUMMARY OF THE INVENTION PHOTODISSOCIATION OF GLYOXAL In accordance with the present invention, there is provided a method of separating isotopes by the selec- BACKGROUND OF THE INVENTION 5 tive photodissociation of glyoxal. The invention described herein was made in the More particularly, it has been found that a dissocia- course of, or under, ERDA Contract No. W-7405- tion product enriched in a desired isotope of carbon, ENG-48 with the University of California. oxygen, or hydrogen is obtained by subjecting glyoxal This invention relates to a method of separating iso- in the vapor phase to electromagnetic radiation, prefer- topes of carbon, oxygen, or hydrogen, particularly to ably laser radiation, having a wavelength such that such processes which make use of the so-called "iso- photon absorption excites and induces dissociation of tope shift", i.e., a slight shift in the absorption spectra only those molecules of glyoxal containing the desired of isotopes of the same element caused by the small isotope. The required wavelength is predeterminable from experimental studies of the absorption profiles of difference in nuclear mass. More particularly, this in- 15 vention relates to a method of isotope separation the various isotopic species of glyoxal. Preferably, gly- wherein selective excitation is followed by chemical oxal is subjected to electromagnetic radiation having a dissociation to produce compounds enriched in the predetermined Wavelength in the range of from about desired isotope. The term "dissociation" as used herein 3500 to about 4600A, specifically from about 4300 to refers to the breakup of a molecule with the resultant 20 about 4600A, to obtain a dissociation product enriched in a desired isotope of carbon, oxygen, or hydrogen. formation of molecular and/or radical products. The isotopically enriched dissociation product can be Isotope enrichment by photochemical dissociation is separated from the reaction mixture by standard chem- accomplished by tuning the emission of a monochro- ical methods. matic spectral source, such as a laser, to a wavelength such that photon absorption selectively excites and 25 DETAILED DESCRIPTION OF THE INVENTION induces dissociation of only those molecules containing the desired isotope. In general, the dissociation prod- It has been found that glyoxal fulfills all of the above- ucts are smaller molecules of greatly enriched isotopic cited requirements for an ideal photodissociation mole- content which are easily separated from the reaction cule. mixture by standard chemical methods. 30 The photodissociation of glyoxal is represented by the following reactions: The ideal molecule for isotopic enrichment by photochemical dissociation should have the following prop- OCH—CHO + hf (4550A) -» HCHO + CO (1) erties: (1) Unit quantum efficiency. One hundred per- cent of those molecules absorbing light will dissociate: OCH—CHO + hi* (4550A) -» H2 + 2CO (2) M + hv -* A + B, where M, A, B are discrete mole- 35 cules. (2) Discrete absorption lines. This permits a Reaction (1), which occurs about 60% of the time, has highly selective single photon dissociation process. (3) a quantum efficiency of 0.6; the quantum efficiency of Well resolved isotope shift. The molecular absorption reaction (2) is much lower, of the order of about 1%. coefficient must differ significantly between the various Thus, the major dissociation mode fulfills the require- isotopic species. (4) Stable dissociation products. ment of high quantum efficiency. Photo-products A and B (see above) should be stable Both formaldehyde and carbon monoxide are stable molecules and not radicals; radicals may attack other molecules. No radicals are produced in the glyoxal molecules M in an isotopically nonselective manner, dissociation; hence, there is no problem of isotope mixing by radical reaction. thereby reducing the isotopic separation factor of the 45 process. (5) Convenient absorption wavelength. The A significant advantage in using glyoxal as the photo- best absorption band would be in the visible range dissociation molecule is that a major absorption band is where highly efficient dye lasers are available. (6) Eas- centered in the visible blue (4500A), a spectral region ily achieved high vapor pressure as well as stability in which matches existing highly efficient coumarin dye lasers. Therefore, frequency doubling is not required, the vapor phase. (7) Ready availability. 50 and the energy efficiency of the overall enrichment An isotope enrichment process by selective photodis- process is correspondingly increased, generally by a sociation of formaldehyde is reported by C. Bradley factor of about ten. Moore, Accounts of Chemical Research, 6 323, 1973. In addition, glyoxal has a high vapor pressure (284 However, the major absorption bands of formaldehyde torr at 26°C) and is stable in the vapor phase. Finally, are in the ultraviolet (3000-3500A) rather than the 55 glyoxal is easily prepared by passing ethylene over visible range, thus requiring irradiation by frequency- selenium dioxide at 150° - 200°C, thereby fulfilling doubled dye lasers which is less efficient than using requirement of ready availability. tunable light sources emitting directly in the visible The selective photodissociation of glyoxal can be range. used to obtain isotopic enrichment of deuterium, trit- It is, therefore, an object of this invention to provide 60 ium, carbon-13, carbon-14, oxygen-17, and oxygen-18. a method of separating isotopes which makes use of Any given sample of glyoxal molecules consists mainly "isotope shift". ofthe isotopic species 160IH,2C,2C,Hle0 since 'H, ,2C, More particularly, it is an object of this invention to and ,60 are the most abundant isotopes of their respec- provide a method of separating isotopes of carbon, tive elements. A much fewer number of glyoxal mole- oxygen, or hydrogen by selective photodissociation, 65 cules has a deuterium atom substituted for one of the specifically, by selective photodissociation of a mole- hydrogen atoms, the isotopic composition being cule whose major absorption bands are in the visible 160'HlaCt2CDls0, where D represents deuterium (2H). range. Extending this idea, the glyoxal sample contains nu- 3,9' 1-7,335 3 4 merous isotopic species representing all possible ways separating it from the gaseous deuterated formalde- that the isotopes of H, C, and O can combine to form hyde. Because of the highly selective nature of the OHC—CHO. The fraction of each isotopic species photodissociation, the resulting deuterium enrichment present depends on the natural abundance of the par- is correspondingly high. ticular isotopes comprising that species. Thus, for ex- 5 Any source of monochromatic radiation which is ample the isotopic species OHC—CTO where T repre- capable of being tuned to the narrow bandwidth and sents tritium (3H) would be present only in trace exact wavelength requirements can be used in the pre- amounts because tritium is only a trace component of sent process. Lasers are by far the most desirable radia- naturally occurring hydrogen. Each of the isotopic tion sources by virtue of their high monochromaticity. species comprising the glyoxal sample is a distinct en- 10 Although the invention has been described with ref- tity having its own unique absorption spectrum. By erence to preferred embodiments, various modifica- choosing a single strong absorption line which does not tions and changes will be apparent to those skilled in overlap absorption lines of other species and tuning the the art, all of which may be achieved without departing emission of a monochromatic spectral source, such as a from the spirit and scope of the invention as defined by laser, to precisely that wavelength, one can induce a '5 the following claims. highly selective photodissociation of the chosen isoto- What I claim is: pic species. 1. A method of separating isotopes which comprises subjecting glyoxal vapor to electromagnetic radiation EXAMPLE having a predetermined wavelength such that photon As a specific example of isotopic enrichment accord- 20 absorption selectively excites molecules of glyoxal con- ing to the present invention, consider monodeuterated taining a desired isotope of carbon, oxygen, or hydro- glyoxal (OHC—CDO), a compound whose absorption gen and induces dissociation of the selectively excited spectrum has been extensively studied. One particular molecules into a product enriched in the desired iso- line in the spectrum of OHC—CDO, at 4545A, 2g tope, and separating the isotopicaHy enriched dissocia- matches the emission wavelength of an argon laser. tion product from the reaction mixture. Glyoxal, containing a mixture of naturally occurring 2. A method according to claim 1 wherein the elec- isotopic species, is allowed to vaporize at room temper- tromagnetic radiation is laser radiation. ature to a total pressure of about 10 Torr in a meter- 3. A method according to claim 1 wherein the prede- long, quartz absorption cell. Argon laser light at a termined wavelength is in the range of from about wavelength of 4545 A is passed through the vapor in the 3500A to about 4600A. cell. Only the OHC—CDO molecules absorb radiation 4. A method according to claim 3 wherein the prede- of that wavelength. The selectively excited OH- termined wavelength is in the range of from about C—CDO molecules dissociate to produce deuterated 4300A to about 4600A. formaldehyde. The reaction is represented as follows: 35 5. A method according to claim 1 wherein the dissociation product comprise formaldehyde and carbon OHC—CDO + hi* (4545A) —• HDCO + CO monoxide. 6. A method according to claim 1 wherein the de- The resulting vaporous mixture is passed through a sired isotope is deuterium and the dissociation product cold trap at a temperature of about —70°C to about comprises formaldehyde enriched in deuterium. —$0°C to freeze out undissociated glyoxal, thereby