Radiation stability of cis-isomers of dicyclohexano-18-crown-6

Radiat. Phys. Chem. Vol. 48, No. 4, pp. 463--472, 1996 Copyright © 1996 Elsevier Science Ltd 80969-806X(96)00012-6 Printed in Great Britain. All fights reserved 0969-806X/96 $15.00+ 0.00

Pergamon

RADIATION STABILITY OF CIS-ISOMERS OF DICYCLOHEXANO- 18-CROWN-6 V. M. ABASHKIN, l D. W. WESTER,2t J. A. CAMPBELL 2 and K. E. G R A N T 2 tAll-Russian Research Institute of Chemical Technology, 33 Kashirskoe Avenue, Moscow 115230, Russia and 2pacific Northwest Laboratory, MSIN P7-25, Richland, WA 99352, U.S.A. (Received I 1 September 1995; accepted 5 January 1996) Abstract--The ?-ray initiated decomposition of the pure cis-syn-cis and cis-anti-cis stereoisomers of dicyclobexano-18-crown-6 (DCH-6) and its strontium nitrate complex was studied by Fourier-transform infrared spectrophotometry and gas-chromatography-mass-spectrometry with integrated doses up to 1.5 MGy and dose rate varying from 3 to 8 Gy/s. The proposed scheme of radiolysis of DCH-6 that is derived from the experimental data is consistent with symmetric initial cleavage of the C--g) bonds between the terminal C of the bridging diethylether groups and the O of the cyclobexane-l,2-diol groups. The cis-syn-cis isomer of DCH-6 is more stable to ?-radiation than the cis-anti-cis isomer. Copyright © 1996 Elsevier Science Ltd

1. INTRODUCTION

Beginning in the 1960s with the discovery of macrocyclic polyethers, or crown ethers, these stereoselective complexones have attracted great attention primarily as specific ligands exhibiting high selectivity for nearly all kinds of cations (Pederen and Frensdorff, 1972; Melson, 1979). Solvent extraction seemed one of the most successful fields for crown ether application. At first, the main attention was directed toward analytical applications. Numerous publications and patents reported the recovery, preconcentration and separation of different cations including radionuclides (Filippov et al., 1978; Gerow and Davis, 1979; Horwitz et aL, 1990; Moyer et al., 1991; Filippov et al., 1992). Unfortunately, the limited availability of crown ethers and especially a lack of experimental data for reprocessing of highly irradiated and concentrated aqueous solutions using crown ethers as extractants have somewhat inhibited the evaluation and recognition of crown ethers as new agents in the reprocessing of industrial nuclear fuel and radioactive waste. Nevertheless, the possible application of crown ethers for recovery of radionuclides [Sr, Cs, transuranium elements (TUE)] from real radioactive waste solutions has been reported. The most promising properties belong to DCH-6 and its derivatives, e.g. the SREX process developed by Horwitz et al. in 1990 in addition to 9°Sr and 137Cs solvent extraction from nitric acid solutions reported by Filippov et al. in 1992. The extractant used for liquid radioactive waste decontamination is exposed to as much as 40 kGy per single extraction-stripping cycle, depending on the tTo whom all correspondence should be addressed.

extraction regime and equipment. For a cyclic scheme, the total radiation doses may reach tens of M G y (Egorov, 1986). Such consequences of radiolysis as the degradation of the extractant and the lowering of its extraction efficiency, formation of destruction products that reduce the selectivity and phase separation, and/or the production of corrosive acids and oligomers may strongly influence the real efficiency of the solvent-extraction process. The most complete data concerning the radiation chemistry of crown ethers are collected in a review (Makhliarchuk and Zatonsky, 1992). The degradation of crown ethers was investigated primarily by analyzing gaseous products. Possible schemes of radiodegradation were reconstructed on that basis. The crown ether DCH-6, which attracted the attention of researchers, was used as a commercial product consisting of a mixture of stereoisomers, mainly cis-syn-cis and cis-anti-cis, without any purification or separation. However, these isomers are known to have very different metal-complexing properties and liquid-liquid extraction behavior (Lamaire et al., 1991; Yakshin et al., 1989). In our work, we used infrared (IR) spectroscopy and gas-chromatographymass-spectrometry (GC-MS) to determine the relative radiation stability of the pure stereoisomers cis-syn-c/s-dicyclohexano-18-crown-6 (isomer A) and cis-anti-cis-dicyclohexano-18-crown-6 (isomer B) in addition to dibenzo-18-crown-6 (DB-6) and the Sr complex of the former. 2. EXPERIMENTAL

2.1. Materials DCH-6 (98% mixture of cis-syn-cis and cis-anticis isomers) and DB-6 were obtained commercially (Aldrich Chemical Co., Milwaukee, WI). All organic

463

V.M. Abashkin et al.

464

solvents used were Baker Analyzed Reagents. Deionized water was prepared by passing distilled water through an ion-exchange column. 2.2. Separation of isomers The precipitation of the perchlorate oxonium complex was used to isolate the pure isomers. The cis-syn-cis isomer (isomer A) was produced according to the procedure described earlier (Izatt et al., 1972) by adding aqueous perchloric acid (0.1 M) to DCH-6 dissolved in deionized water. The solid oxonium complex of isomer A was filtered, washed with CC14, and dissolved in CHC13. The residue containing the cis-anti-cis isomer (isomer B) was dissolved in water, extracted into CCI4 (phase ratio = 1:2), and mixed with 2.0 M aqueous perchloric acid (1:1) in separatory funnel. The mixture was shaken vigorously for 20 min at room temperature until three layers formed. The middle layer, which is a suspension and contains isomer B as the oxonium complex, was separated by filtration and dissolved in CHC13. After washing with excess deionized water (until the pH = 7), the CHCI 3 solutions of each isomer are centrifuged, evaporated under vacuum to dryness, and recrystallized from a CH2C12-CC14 mixture. The determined melting points of the isolated isomers A and B (59-60 and 69-70°C, respectively) are practically the same as the literature values (Frensdorff,

1971). Both isomers were separated with good yield (75-80%). The remaining CC14 solution was washed with deionized water, centrifuged and evaporated under vacuum. The weight of dry residue (impurities in commercial DCH-6) was near 1.5% of the initial quality.

2.3. Strontium nitrate anhydrous complex formation Equiomolar amounts (10-3M) of cis-syn-cis DCH-6 (isomer A) and Sr(NO3)2 were dissolved in a minimum amount of dry ethanol. After allowing the mixture to stand overnight, the solvent was evaporated to one third of its original volume under vacuum. The resulting precipitate was separated by filtration, washed with CC14, and recrystallized from a CH2C12--CC14 mixture to yield the anhydrous Sr(NO3)2 complex of isomer A. The determined Sr content (inductively-coupled-plasma method) is consistent with the equimolar composition of the produced complex. The crystal and molecular structure of that complex was reported earlier (Krasnova et al., 1985). The complex decomposes on heating in an Ar atmosphere near 270°C without melting.

2.4. IR spectra The IR spectra were measured using a Nicolet 510P Fourier-transform infrared (FTIR) spectrophotometer with a resolution of 4 crn-~, a DTGS KBr

0.6

0.5

(~

0.4

02 • 0

0 WAVE NUMBER (cm-1)

Fig. 1. IR spectra of cis-syn-cis DCH-6 (isomer A). Lower line--initial sample; upper line---sample irradiated at 1500kOy (3.0 kGy/s).

Radiation stability of dicyclohexano-18-crown-6

465

0.5

0.5 U,I

O

(~ 0.4

0.3

0.2 4000

I

I

]

I

I

I

3500

3000

2500

2000

1500

1000

50q

WAVE NUMBER (CM-1) Fig. 2. IR spectra of cis-anti-cis DCH-6 (isomer B). Lower line---initial sample; upper linel-sample irradiated at 1500 kGy (3.0 kGy/s).

detector, and an aperture of 35.00. ZnSe glasses were used for IR spectrophotometry of neat species of irradiated isomers. 2.5. Gas chromatography~mass spectrometry An HP 5988A mass spectrometer was equipped with an HP 5980 gas chromatograph operated in the splitless mode. Separation was obtained with a DB-5 capillary column (30 m x 0.25 mm i.d., 0.25 Im-film thickness, J • W Scientific). The injector port was heated to 250°C; mass spectrometric interfaces, 280°C; and source, 200°C. The oven temperature was typically programmed in the following manner: 50°C for 1 min, 8°C/min to 300°C, and hold at 300°C for 5 min. The mass spectrometer was scanned from 50 to 500 ainu. Mass spectra were also obtained using positive ion chemical ionization with isobutane as the reagent gas. The conditions were the same as listed above. The mass range scanned was 70-500 amu. All crown ether samples were dissolved in CHCI 3. The injection volume was 1-2/~1. The concentration range was 0.002-0.02 mg/ml. 2.6. Irradiations Pure samples (near 20 mg) of each DCH-6 isomer, DB-6, and the Sr complex of isomer A in glass vials were irradiated using a ~°Co ~,,-source. The dose rate was 3.0Gy/s (1.08.106R/h). The maximum total dose was 1.5. 106Gy (1.5.108 R). The IR and G C - M S spectra of the irradiated samples were determined.

3. R E S U L T S A N D D I S C U S S I O N

3. I. Changes in IR spectra The qualitative changes in the IR spectra are similar for both DCH-6 isomers illustrated in Figs 1 and 2. They can be summarized as follows: (1) at 1630cm -1, the growth of a small band presumably due to an unsaturated species of the cyclohexane type; (2) at 1735 cm -~, the appearance and growth of a peak in the carbonyl region where aldehydes predominate; (3) at 3550 cm -1, the appearance and growth of a wide band probably involving hydroxyls of alcohols.

0,10 -

0.0(5

0.04

0.02

0,00 0

T 2O0

i 4O0

i ~0

i emo

f lOO0

i 't2oo

r .t4oo

leSOO

Total D o w (kOy)

Fig. 3. Absorbance changes at 1735 crn-~ for different total doses. Isomer A--bottom line; isomer B--upper line.

466

V.M. Abashkin et al. O.OS

to a C - - H stretch in a CHO group and a C--C---OH stretch in an alcohol (Lin-Vien et aL, 1991).

•

0.04-

3.2. G C - M S data 0.03

<

0.02

0.01

0

200

400

600

800

1000

1200

1400

1600

TOtal Dcee (kGy)

4.

cm-t

Fig. Absorbance change at 3550 for different total doses. Isomer A--bottom line; isomer B--upper line.

The kinetic curves of the absorbance changes are presented in Fig. 3 for the carbonyl band and Fig. 4 for the hydroxyl band. The appearance of peaks at 2820 and 1060cm -~ for isomer B exposed to 1.5 MGy are presumably due

The gas chromatograms do not indicate formation of any products after irradiation doses of < 500 kGy. Starting at 500 kGy, the GC trace of each irradiated sample (dissolved in CHCI3) exhibits a peak in the range 26.5-27.5 min, which represents the only major radiolysis product, in addition to the main peak of the starting compound at 30-31.5 min (Fig. 5). The mass spectra of the corresponding products are shown in Fig. 6. Since the intensity of other GC peaks up to 1500 kGy is negligibly small ( < 1%), the extent of radiolysis was determined and the degradation products were analyzed by using the aforementioned peaks. We used the peak-area ratio of the degradation products (peak near 27 min) and the starting isomer (peak near 31 rain) as a criterion of the extent of radiolytic decomposition. As expected, the radiolysis

Lv.o. o.,

0

(M] +

M

-

186

H H'~-'~

L '~CH 3

U

-

M

158

-

142

M

-

8Z

Scheme I

/0 +

H 0

0~

)¢

[~0

CH3-CHz-CH-OH(÷ )

0

\

/

m/e

177

m/e

89 ~heme2

m/,

59

Radiation stability of dicyclohexano-18-crown-6

467

(a)

g,. o~

12

16

20

24

32

28

36

Retention time (rain)

(b)

e~

o

U g,.

l m

" ' | ' ' ' | ' "

12

" " gl ' ~ " . . .l . .

16

I'"

" I

20

"'"

!

" .

.n

24

.

.

w.

.

.i

28

- .' "

.

I.

.

.I

32

. L .. ! .

: -. "

.I[

36

Retention time (min)

Fig. 5. GC traces of highly irradiated (1500 kGy) isomer A (a) and isomer B (b).

|

"

!

40

468

V . M . Abashkin et al. 157

(a) 81

263 !

,m

187 .,, 127 /

~ / i

I

I

i

160

120

80

i

i

J

i

|

i

i

200

!

w i

|

i

i

I

r i

240

I|

I

i

!

i

i

!"111

280

i

i

!

llJl

!

:320

(b) ~9

81

89

/ 141

\ 157

,107

•

•

|

•

w

!

I

8O

•

,

i

"

120

'

..J

I ' ' '

160

20"/ 217

I

"w

,

,,.I, "

200

M/Z Fig. 6(a)-(b).

'-*

-

231

•

/,

269

.

240

W , l , W

t

wl."

280

Radiation stability of dicyclohexano-18-crown-6

(c)

/

157

469

263 /

81

171

o >,

/

127

8O

407 /

]ii

", - !

I

160

120

'

I

200

"

!

I

•

'

I

280

240

•

i"

s

320

|

' •

i

'

360

|

'

I

400

I "

l

)

t. _~

440

M/Z

(d) 59

81

o ),

a u

99 /

141 / 127

,w, ! 80

155 //

185 20

120

4 ),

217

~

160

200

244

261

(

/,,

240

i

,..

280

M/Z Fig. 6(c)-(d) Fig. 6. Mass spectra of radiolysis products (near 27 rain) of isomer A [(a) chemical ionization, Cl; (b) electronic impact] and isomer B, (c) CI; (d) EI.

470

V.M. Abashkin et al.

electron-impact mass spectra, such a transformation involves an initial cleavage of one or two symmetric C---O bonds near the cyclohexane ring owing to 0.08?-radiolysis and subsequent recyclization (Scheme 4). A low-temperature electron-paramagnetic-reson~ 0.04ance (EPR) study of the initial radiolysis products could elucidate the difference in the radiolysis mechanisms of these two isomers with very similar ~ 0.0~' structures. The discrepancy in the yields of H~ and resulting 0.00 .v. ~ ~ ~ , , , , , 200 400 600 800 1000 1200 1400 1600 paramagnetic species and the complicated nature of Total ~ (kGy) the EPR spectra of irradiated crown ethers has previously been thought to be a consequence of the Fig. 7. Ratios of peak areas of degradation products (26.8 min) and pure isomers (30.7 rain) for different doses. simultaneous occurrence during radiolysis of several Isomer A--bottom curve; isomer B---middle curve; primary processes. The principal one is probably Sr(NO3)2 (isomer A complex)--top curve. cleavage of the macrocycle at the C--g) bonds (Grigor'ev et al., 1987). However, others (Kurac and Sersen, 1990) consider the formation of radicals increases with increasing total y-radiation dose containing the ---4)---CH--CH2-- fragment to be (Fig. 7). However, isomer A under the irradiation most probable, in analogy with reactions in dioxane conditions was slightly more stable than isomer B. and tetrahydrofuran. Mass-spectral analysis of the major radiolysis Evidently the reasons for the different stability of products isolated by chromatography suggested the the isomers lie in the more subtle vibrational mechanmost probable radiolysis scheme. The electron- isms of the polyether ring, primarily in the molecular impact (EI) and chemical-ionization (CI) mass spec- structures of the isomers. These have not yet been tra of the radiolysis products of both isomers are experimentally determined. shown in Fig. 6. From a practical viewpoint, the preliminary conThe most probable radiolysis mechanism that is clusion that the cis-syn-cis isomer is more radiation consistent with the product appearing in the chro- stable is rather unexpected. However, only further matogram is the symmetric cleavage of two bonds investigations of the radiolysis of organic solutions of near the cyclohexane rings of DCH-6 (Scheme 1). the isomers (for example, in alcohols) can prove its Partial de-hydrogenation of the cyclohexane ring and validity. loss of ethylene oxide fragments in the polyether Simultaneously we studied the radiation stability of molecule under the influence of gamma radiation can, DB-6 and the complex of DCH-6 (isomer A) with according to the literature (Myasoedova and Za- Sr(NO3)2. As it turned out, DB-6 was unexpectedly gorets, 1986), produce the corresponding fragments. less radiation stable than DCH-6. In particular, Electron impact in the mass spectrometer exper- practically only one peak near 31 min is seen in iment produces the enol form of this same cyclo- addition to the starting material in the GC starting at polyether fragment that loses ethylene oxide moieties 100 kGy. The mass spectra of this species is identical until cyclohexane appears in the mass spectrum at to pure DCH-6. The integrated intensity of this m / e = 81. peak increases in proportion to the y-radiation dose. The electron-impact and chemical-ionization (iso- As a rule, adding aromatic substituents to a butane) mass spectra clearly show the corresponding polyether ring increases the radiation stability fragment ions ( M - 1)+ (Fig. 6). The peaks in the through intramolecular energy transfer from the mass spectra at role = 59 and 89 represent recycli- macrocycle to the substituent (Grigor'ev et al., 1987). zation products of the corresponding polyether However, in our instance DB-6 is probably partially fragments (Scheme 2). With respect to the peak at m / e = 73, which is practically absent for unsubstituted crown ethers and + was previously detected in mass spectra in dibenzo18-crown-6 and dibenzo-24-crown-8 (De Souza and Oliveira, 1977), the structure ~ H - - - C H 2 - - O : = CH2( + ) is postulated. However, a mechanism of ° such fragmentation cannot yet be suggested. A less probable structure for the parent fragment with m / e ffi 185 is the cyclic polyether with three O atoms separated by ethylene bridges (Scheme 3). In analogy with the previously published scheme (De Souza and Oliveira, 1977; Gray et al., 1977) for the fragmentation of aromatic analogs of DCH-6 in Scheme 3 0.00 -

o..)

Radiation stability of dicyclohexano-18-crown-6

(1)/. / +

471

~/e

528

-CH2CH20

(2)

m/e

186

-CH2CH20

\ m/e

/

230

m/e

142

Scheme 4

hydrogenated, although this must be verified and explained. The complex with St(NO3)3 was less radiation stable compared with the free crown ether. Thus, the relative intensity of the band near 1735 cm -~, which is typical of the carbonyl radiolysis product, increases (in absorption units) from 0.03 to 0.07 for 500 kGy and to 0.15 for 1500 kGy (see Fig. 3). The GC-MS analysis also confirms such a scenario. The ratio of the integrated intensities of the decomposition products to the principal peak is 1.2% at 500 kGy and 7.9% at 1.5 MGy. This is several times greater than the relative concentration of the radiolysis products for the pure isomers. In addition, the complex itself decomposes as judged by the appearance and intensity increase of the stretching band of nonbonded nitrate (1385 cm -I) and the corresponding decrease for the peaks of bidentate nitrate 0320 and 1420 cm-I). The greater radiation stability of the cis-syn-cis isomer of DCH-6 provides additional confirmation of

its higherefficiency in the solvent extraction of Sr and transuranium elements (TUE) from aqueous solutions (Yakshin et al., 1989; Horwitz et al., 1990; Lamaire et al., 1991). Our future studies will investigate the extraction of radionuclides from aqueous solutions by pure isomers of DCH-6 in order to refine the optimal regions and conditions for using each isomer and the limits of their radiation stability. 4. CONCLUSIONS

Complexation with Sr significantly decreases the radiation stability of the crown ether (rapid backextraction is necessary). The main sources of degradation are monocyclohexyl polyethers (alcohols and aldehydes) with a partially dehydrogenated cyclohexane ring. The benzene rings of DB-6 are hydrogenated by 7-radiation to form DCH-6. Acknowledgements--Pacific Northwest Laboratory is operated for the U.S. Department of Energy by Battelle

V . M . Abashkin et aL

472

Memorial Institute under contract DE-AC06-76RLO 1830. We would like to thank the Efficient Separations and Processing Integrated Program of the U.S. Department of Energy, EM-50, and the Special American Business Internship Training program of the U.S. Department of Commerce for supporting VMA during this work.

REFERENCES

Egorov G. F. (1986) Radiochemistry of Solvent Extraction Systems. Energoizdat, Moscow. Filippov E. A., Yakshin V. V., Belov V. A., Arkhipova G. G., Abashkin V. M. and Laskorin B. N. (1978) "Crowns" in the extraction of uranium and actinides from nitric acid solutions. Dokl. Akad. Nauk SSSR 241, 159. Filippov E. A., Dzekun E. A. and Nardova A. K. et al. (1992) Application of crown ethers and ferrocyanide based inorganic material for cesium and strontium recovery. In Proc. Ann. Syrup. Waste Mint at Tucson Ariz., Vol. 1021. Frensdorff H. K,(1971) Salt complexes of cyclic polyethers. Distribution equilibria. J. Am. Chem. Soc. 93, 4684. Gerow I. H. and Davis M.W. Jr. (1979) The use of macrocylic polyethers to remove cesium-137 from acidic nuclear wastes by solvent extraction. Separ. Sci. Technol. 14, 395. Gray R. T., Reinhoudt D. N., Spaargaren Kees and de Bruijn J. (1977) Chemistry of crown ethers: the mass spectra of macrocyclic polyethers. J. Chem. Soe. Perkin Trans. H 2, 206. Grigor'yev E. I., Myasoedova T. G., Nesterov S. V. and Trakhtenberg L. I. (1987) Influence of the structure of crown ethers on their radiation stability. Khim. Vys. Energ. 21, 437. Horwitz E. P., Dietz M. L. and Fisher D. E. (1990) SREX: A new process for the extraction and recovery of strontium from acidic nuclear waste streams. Solvent Extra. Ion Exch. 8, 199.

Izatt R. M., Haymore B. L. and Christenscn J. J. (1972) A stable OHj ~ polyether complex characteriscd by infrared spectroscopy. J. Chem. Soc. Chem. Commun. p. 1308. Krasnova N. F., Simonov Y. A., Dvorkin A. A., Abashkin V. M. and Yakshin V. V. (1985) Crystal and molecular structure of complex of strontium nitrate with cis-syn-cis dicyclohexyl-18-crown-6. Kristallografiya 30, 86. Kuruc J. and Serscn F. (1990) Gamma-radiolysis of some crown ethers and their analogs. J. Radioanal. Nucl. Chem. 145, 197. Lamaire M., Guy A., Chomel R and Foos J. (1991) Dicyclohexano-18-crown-6 ether: A new selective extractant for nuclear fuel reprocessing. J. Chem. Soc. Chem. Commun. p. 1152. Lin-Vien D., Colthup N. B., Fately W. G. and Grasseli J. G. (1991) The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules. Academic Press, New York. Makhliarchuk V. V. and Zatonsky S. V. (1992) Radiochemistry of crown compounds. Usp. Khim. 61, 883. Melson G. A. (Ed.) (1979) Coordination Chemistry of Macroeyclic Compounds. Plenum Press, New York and London. Moyer B. A., Baes C. F., Bryan S. A. et al. (1991) Developing new chemical tools for solvent extraction. In Proc. First Hanford Separation Sci. Workshop, Vol. II, 39. Myasoedova T. G. and Zagorets P. A. (1986) Mass spectrometry and radiolysis of macrocyclic polyethers. Kh/m. Vys. Energy 20, 436. P¢dersen C. J. and Frensdorff H. K. (1972) Macrocyclic polyethers. Dibenzo-18-crown-6 polyether and dicyclohexyl-18-crown-6. Angew. Chem. Int. Ed. Engng 11, 16. de Souza G. A. and Oliveira C. M. F. (1977) Electron impact induced fragmentation of dibenzo-18-crown-6 polyether and its nitro derivatives. Org. Mass Spectr. 12, 407. Yakshin V. V., Myasocdov B. F., Vilkova O. M. et al. (1989) The use of dicyciohexyl-18-crown-6 for extraction of radiactive strontium from water. Radiokhimiya 31, 67.