US REPORT R-610 Conference Paper October 1972

ON THE PREPARATION OF PURE XENON TETRAFLUORIDE

M.BOHINC, J.SLIVNIK, B.ŽEMVA

univerza v Ijubljanl We regret t-.hat some of the pages in the microfiche copy of this report may not be up to the proper legibility standards, even though the best possible copy was used for preparing the master fiche. KEYWORDS

XENON TETRAFLUORIDE SYNTHESIS

Odobrile Oddelek ta kemijo in Konifija sa ti»k US (natisnjeno na US v 200 izvodih) US REPORT R-610

Oi* THE PREPARATION OF PURE XEUON TETRAFLUORIDE*

M.Bohinc, J.Slivnik and B.Žemva

Institute "J.Stefan" and Faculty of Natural Sciences and Technology, Ljubljana, University of Ljubljana, Yugoslavia

ABSTRACT. Methods for preparation of xenon tetrafluoride, which have been published up to now, yield product which are usually contamined by xenon difluoride and hexafluoride. The main problem of the purification of xenon tetrafluoride is the removal of traces of difluoride. So far, only one method for chemical purifi­ cation of XeFi, has been published. For the preparation of pure xenon tetrafluoride the thermal dissociation of hexafluoride can be employed. As we have reported earlier, it proceeded according to the following equation:

XeF6 «•"* XeFi, + F2. Xenon tetrafluoride of purity bether than 99 vt> was obtained by dissociation of the compound formed between sodium fluoride and xenon hexafluoride, published elsewhere, at 300 C. The possibility of the removal of traces of XeF2 observed in products thus obtained, using cobalt difluoride will be dis­ cussed. The purity of XeFi, was determined by checking its melting point and infrared spectra of vapours.

Nowadays the preparation of xenon fluorides does not repre­ sent any appreciable problem. These syntheses are in principle similar to the syntheses of some higher fluorides. One of course should keep in mind that the most of the methods described so far yield besides of the main product also a small amounts, up to few percent, of the others two xenon fluorides.

* Presented at 4tn European Symposium on Fluorine Chemistry, Ljubljana, August 2bth - Sept. 1st, 1972. The simplest is the preparation of xenon difluoride:

hv Xe • F2 ^7 • XeF2 room temp. Its simple photosynthesis was reported by Streng ( ) and Holloway (2*3) already in 1965 and 1966. Xenon hexafluoride can be separated from the mixture of xenon fluorides, because of its com­ paratively higher vapour pressure, by trap to trap sublimation. The method reported by Malm (") and Sheft (5) is very suitable for the preparation of somewhat larger quantities, up to few grams of xenon hexafluoride:

50°C NaF • XeF + XeF«, + XeF J T + flaF.XeF* + XeFi, + 69 2 ove^- night ^^ 120°C pump off

NaF+XeF*

* XeF* ^^ ^ XeF* + XeFa pump off

It is based on the selective absorption of xenon hexafluoride on sodium fluoride at 50 C and desorption at 120 C. The preparation of pure xenon tetrafluoride, however, seems to be very difficult, specially so since it form already at room temperature an addition molecular compound with xenon difluoride having lower vapour pressure than xenon tetrafluoride (*):

TOOm tem XeF2 • XeF% ?' > XeF2.XeF„

Preparation of pure xenon tetrafluoride which involves purifi­ cation with arsenic pentafluoride in the solution of bromine pentafluoride was reported by Bartlett and Sladky (7):

XeF. • XeF, • XeF, "Z™™™* • *f W •

• Xe2F3/AsFft/ • XeF„ '>vtk(%> XeF* dyn« vacuum - 2 - In our experiments very often larger quantities up to several ten grams of pure xenon tetrafluoride are needed. Therefore we have decided to investigate the possibility of synthetizing larger quantities of xenon tetrafluoride utilizing the experience gained in our study of the thermal dissociation of xenon hexafluoride. This work was reported on the 5 International Symposium on Fluorine Chemistry in Moscow (8). The dissociation was found to o follow at 330 C the following equation:

XeFfc 9*± XeF* + F2

It should be emphasized here, that at the conditions employed we did not obtain any indication of the presence of xenon difluoride which could have been possibly formed by the thermal dissociation of xenon tetrafluoride. Our work was planned along the following lines: * preparation of mixture of xenon tetrafluoride and hexafluoride which would not contain any xenon difluoride, - separation of both by the sorption of xenon hexafluoride on sodium fluoride. Xenon hexafluoride was prepared at 300 C starting with the mixture of xenon and fluorine having a molar ratio 1:20 with the total pressure amounting 60 atm. It was purified by the already mentioned sodium fluoride method ( • ). Before the use, sodium fluoride was heated at 500°C for 18 hours, then treated with fluorine at 10 atm pressure for 20 hours at 200 C. Subsequently fluorine was thoroughly pumped off, the solid in addition treated with xenon hexafluoride at 150°C and finally pumped on in high vacuum at 350°C. In a typical experiment 13 grama of sodium fluoride (treated as it was mentioned before) was put into 120 ml nickel reaction vessel and 11 grams of xenon hexafluoride were added by subli­ mation. The reaction vessel was then put into a furnace at 350 • 5 C for two and a half hours. The evolved fluorine was

- 3 - pumped away at the liquid nitrogen temperature, vhile in the re­ action vessel; sodium fluoride, xenon tetrafluoride and hexa- fluoride remained:

350 ± 5 c NaF.XeF* * NaF • XeF* + XeF* + F 7 time 2,5 hours 2 *-196°C I-pump off F2 (1,7 g)

The decomposition with the subsequent taking fluorine avay vas repeated seven times. From the diagram (Fig. 1) shoving the time dependence of xenon hexafluoride thermal dissociation, it is evident that the dissociation is practically concluded after the seventh run.

T«350**5*C

NoF XtF,—^ XeFt •IfciF *F,

2.S 7.5 12.5 IS 17,5 20 timifh]

- 1»,- In order to bind the possibly remaining traces of xenon hexafluo­ ride the mixture in the reaction vessel was heated at 50 C overnight and the volatiles were sublimed away, afterwords at the same tempera­ ture. In the infrared spectrum of their vapour, however, we observed besides of the strong absorption band of xenon tetrafluoride also a band of xenon hexafluoride. The product was then in addition pumpied by trap to trap sublimation:

50°C > HaF • XeF6 • XeF., ^° ? UJ.» NaF.XeF* • XeF* -—- over night dyn.vacuum (0,6 g) (9,lg)

—* XeF„ + some XeF6 room temp. ( x trap to trap subl. ^ (9M) (8.5 g)

The purified product had melting point at 117,1 t 0,05 C, exactly as at the temperature reported by Schreiner and coworkers: 117*10 C (9) for the melting point of xenon tetrafluoride, Also, the infrared spectrum of the vapour revealed only the presence of xenon tetrafluoride. The yield of the procedure was found to be 89»5 %'

From 11,3 g of XeF* 95»5 % disscc. Q • ° after trap to trap subl. • & v

In the course of this work we have examine also another method of preparation of pure xenon tetrafluoride. It was found in our laboratory, that cobalt difluoride is oxidized to the trifluoride at room temperature very rapidly by xenon difluoride, whereas the rate of oxidation is very low when xenon tetra­ fluoride is used for that purpose. This difference in rates of oxidation could be possibly employed for the separation of xenon di fluoride and tetrafluoride. The purity of xenon tetrafluoride obtained after this purification is, nevertheless, lower as com­ pound by the purity of xenon tetrafluoride obtained by the thermal dissociation of xenon hexafluoride. Mixtures of xenon

- 5 - difluoride and tetrafluoride is shaken at roor temperature with a larger excess of cobalt difluoride. At -80 C the evolved xenon is pumped away, and xenon tetrafluoride is then sublimed in dynamic vacuum from the reaction mixture at room temperature. The method could be used only for the preparation of smaller amounts of xenon tetrafluoride. With greater amounts there is danger of xenon difluoride inclusion in the crystals of xenon tetrafluoride. In conclusion we can say that the thermal dissociation of xenon hexafluoride in the presence of sodium fluoride is suitable for the preparation of pure xenon tetrafluoride.

ACKNOWLtiDGEMMT - The financial support of the "Boris Kidrič" Foundation, Ljubljana is greatefully acknowledged.

REFERENCES:

(*) L.V.Streng, A.G.Streng, Inorg.Chem., U_9 1370 (1965) (2) J.H.Holloway, J.Chem.Educ., j+3, 202 O966) (3) J.H.Holloway, Chem.Commun., 1966. 22 (**) J.G.Malm, F.Schreiner, D.W.Osborne, Inorg.Nucl.Chem.Lett., 1, 97 (1965) (5) I.Sheft, T.M.Spittler, F.Martin, Science, J]£, 701 ( 196*0 (6) J.H.Burns, R.D.Ellison, H.A.Levy, J.Phys.Chem,, 6J, 1569 (1963) (7) N.Bartlett, F.O.Sladky, J.Am.Chem.Soc.,