thermal oxidation of two malonamides, extractants for minor actinides in nuclear fuel reprocessing

Journal of Analytical and Applied Pyrolysis58–59 (2001) 589–603

Thermal oxidation of two malonamides,extractants for minor actinides in nuclear fuel

reprocessing

F. Delavente a, J.M. Guillot a,*, O. Thomas a, L. Berthon b,C. Nicol b

a Ecole des Mines d’Ales, Laboratoire Genie de l’En6ironnement Industriel, 6 a6enue de Cla6ieres,F-30319 Ales, France

b CEA Valrho Marcoule, DCC/DRRV/SEMP/LNE, Bat 399, F-30200 Bagnols/Ceze, France

Received 7 April 2000; accepted 30 August 2000

Abstract

In this study, the thermal destruction of two potential extractants — the N,N %-dimethyl,N,N %-dibutyl, tetradecyl malonamide (DMDBTDMA) and the N,N %-dimethyl, N,N %-dibutyl,dodecylethoxy malonamide (DMDBDDEMA) — for minor actinides in nuclear fuel repro-cessing was compared. The thermal destruction of a main by-product, the larger monoamideformed, was also studied for each extractant. Experiments were carried out in small reactors(closed or open) with different oxidising atmospheres. The recuperation of the syrupydegradation mixture, with ethyl acetate, was the first step of analysis before separation byGas Chromatography (GC). The quantitation of separated by-products was performed witha Flame Ionisation Detector (FID) and the identification was realized by both FourierTransform Infrared Spectroscopy (FTIR) and Mass Spectrometry (MS). Several by-productsare identified and are obtained by cleavages of covalent bond and/or oxidation. After 1 h at250°C, under oxygen flow in an open reactor, residual levels of DMDBTDMA andDMDBDDEMA pure solutions are 10 and 12% respectively, showing that the behaviour ofboth diamides seems similar. However, the destruction level of initial molecule do not informon the global degradation of such complex structures. Initial diamides can lose their methyland/or butyl groups and then give another malonamides. Its can also lead to monoamide bycleavage of C�CO malonamide bond. One of these monoamide, the major by-product, canrepresent 13% of conversion from initial diamide with conditions described previously. An

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* Corresponding author. Tel.: +33-4-66782780; fax: +33-4-66782701.E-mail address: [email protected] (J.M. Guillot).

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590 F. Dela6ente et al. / J. Anal. Appl. Pyrolysis 58–59 (2001) 589–603

index of degradation based on the molecular weight of residual products, was calculated soas to compare the efficiency of thermal oxidation. Behaviour of diamides was also followedby thermal differential analysis (TDA) and thermal gravimetric analysis (TGA) coupled withFTIR. These techniques has shown that carbon dioxide can be produced at temperaturearound 250°C. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Thermal destruction; Malonamide; Extractant; GC/MS; GC/FTIR; GC/FID; TGA/FTIR

1. Introduction

To improve the management of radioactive wastes produced after spent nuclearfuel reprocessing, CEA (Commissariat a l’Energie Atomique) has launched theSPIN program with the objective, in particular, to separate the minor actinides(Am(III) and Cm(III)) from the high level liquid waste. One separation strategy isbased on a two-step process using incinerable extractants (made only with C, H, O,and N atoms). The first step consists of extracting trivalent minor actinides andlanthanides with a diamide (malonamide) molecule, this is the so-called DIAMEXprocess. The semi-developed formula of malonamide is

with R, R % and R¦ alkyl or oxyalkyl groups. The feasibility of the DIAMEXprocess has been shown in counter-current ‘hot’ tests with N,N %-dimethyl, N,N %-dibutyl tetradecyl malonamide (DMDBTDMA): ((C4H9(CH3)NCO)2CH(C14H29))which was the first reference molecule of the process [1]. These extractants aredegraded by hydrolysis and radiolysis during extraction process and this partialdegradation implies the solvent regeneration to eliminate degradation products andmetallic cations (which are not eliminate by scrubbing steps). However, it isnecessary to know how destroy the partially degraded solvent. This kind ofmolecules [2] contains only carbon, hydrogen and nitrogen atoms and hencepotentially incinerable. Previous studies carried out on tertiary amides have shownthermal instability of tertiary amides [3–6].

This paper deals with the thermal destruction of malonamide extractants withoutmetallic cations in relatively ‘soft’ conditions, at temperature below 250°C. Twomalonamides molecules were compared: the DMDBTDMA and the N,N %-dimethylN,N %-dibutyl dodecyloxyethyl malonamide (DMDBDDEMA): ((C4H9(CH3)NCO)2-CH(C2H4OC12H25)).

Among different kinds of devices available for thermal destruction [6–10], aclosed and an open reactor were both tested. Derivative monoamides from malon-amides were also studied.

591F. Dela6ente et al. / J. Anal. Appl. Pyrolysis 58–59 (2001) 589–603

2. Experimental

2.1. Compounds

The diamides and monoamides were provided by Panchim (France), their purityare up to 99%. The developed formulae of diamides DMDBTDMA and DMDBD-DEMA are:

The developed formulae of monoamides, N-butyl N-methyl, hexadecanamide(BMHDA) and N-butyl N-methyl, dodecyloxobutylamide (BMDDOBA).

Diluted solutions of diamides were prepared with pure dodecane (Acros, 99%),ethyl acetate (Rathburn, hplc quality) and hexadecane (Fluka, up to 98%) wererespectively used as a solvent of recuperation and as internal standard (I.S.).

2.2. Thermal oxidation de6ices

Thermal oxidation experiments were carried out both in closed and openreactors. The closed reactor was a 7.4 mL Wheaton tube capped with a septum.The amount of diamide introduced, under atmospheric air, was between 10 and 110mg. This closed tube was then heated in an oven, at the desired temperature (150,200 or 250°C) for a period between 10 min and 8 h. After cooling to roomtemperature, IS and ethyl acetate (6 mL) were added in order to solubilize thedegradation residues.

The open reactor was a 15 mL glass flask. Pure diamides or solutions indodecane were introduced and then heated with an oil bath up to 250°C (Fig. 1).Pure oxygen, air or ozonated air flow could go through the whole solution.Ozonated air is produced by a Trailigaz laboratory ozonator. An adsorbent (TenaxTA) was placed at the top of the condenser to trap volatile molecules. At the endof the thermal oxidation experiment, ethyl acetate (12 mL) with I.S. was added tothe syrupy residues to collect them.


All values given in results are obtained by the average of two replicates.

2.3. Analysis of by-products

2.3.1. GC/MSBoth electronic impact (EI) and chemical ionisation (CI) with methane GC/MS

analysis were carried out with a Varian model 3400 chromatograph equipped witha septum programmable injector (SPI) and a Varian Saturn II mass detector.Electron energy for both ionisation mode were respectively 70 and 10 eV. Twocolumns were employed: a J&W DB5-MS 60 m×0.25 mm-id, 0.25 mm filmthickness to separate monoamides and a SGE HT5 25 m×0.22 mm-id, 0.1 mm filmto separate diamides and compounds with high molecular weight. Temperatureconditions are given in Table 1. The mass range was 35–549 a.m.u. and electronemission was set to 40 mA and 10 mA for EI and CI analysis respectively.

Analysis of Tenax cartridges was carried with a thermal desorber (Perkin ElmerATD 400) coupled with a Perkin Elmer CG/MS. The desorption, in two steps withan overall split ratio of 1:10, is carried out by firstly heating the cartridges at 250°Cfor 5 min and transfering the compounds to a cryogenic trap at −30°C. Then, thiscold trap was quickly heated to 300°C to bring the sample to the GC (Autosystem)via a transfer line at 200°C. Separation was performed on a CP-Sil 8CB column(Chrompack), 60 m×0.32 mm-i.d., 1 mm film thickness with an helium flow fixedat 1 mL min−1. The oven was initially at 35°C for 5 min, then programmed to300°C at 10°C min−1 and kept at this temperature for 7 min while the GC/MStransfer line was heated at 220°C. Spectrometric detection (Q-mass), in EI mode,was performed at 1.5 scan s−1 with a mass range from 12 to 344 a.m.u.

2.3.2. GC/FTIRAn Hewlett-Packard model HP-5890 series II chromatograph was linked to a 510

Nicolet FTIR spectrometer and data collection was carried out on a PC withOmnic software. HT-5 column was used with the same chromatographic conditionsreported in Table 1.

Fig. 1. Thermal oxidation device (open reactor).


Table 1Oven and injector temperature ranges for GC/MS analysis

ProgrammingFinal temperatureInitial temperature (°C) Sequence time Total time (min)(°C) (min)(°C.min−1)

Column HT-580 0 1 180

11.11 or 12.580 12.11 or 13.5180 9 or 810 22.11 or 23.56180 240

240 27.89 or 26.5240 500

Septum programmableinjector connected toHT-5

1100 1100 0100 10 4.25 5.25260

44.75 50260 260 0

Column DB5-MS180 180 0

17.77 18.77980 240240 0 36.23 56240

Septum programmableinjector connected toDB5-MS

90 190 104 54090 250

50 56250 250 0

2.3.3. GC/FIDAn Hewlett-Packard model HP-5890 series II chromatograph equipped with an

on-column injector and a flame ionization detector (FID) was employed forquantitative analysis. The HT-5 column, describe previously, was used in the sameconditions. FID temperature was kept at 280°C.

2.3.4. TDA-TGAThermal differential analysis (TDA) and thermal gravimetric analysis (TGA)

were carried out with a Setaram TGDTA 92 apparatus connected, via a transferline maintained at 200°C, to a gas cell of a FTIR Bruker ISF 66 spectrophotometer.Conditions are given in Table 2. Residues from crucibles were recuperated withethyl acetate were injected in GC/MS and GC/FID.

2.4. Calculation of response coefficient

Because degradation products were not available as pure compounds, responsecoefficients were estimated. The calculation of the response coefficient was based oncoefficients of pure initial diamides, pure monoamides and carbon number in themolecule.


2.5. Destruction indicator

In order to follow the mechanisms of destruction, an indicator was calculated:the Global Residual Percentage (GRP). This percentage is based on the conversionratio Ti of the initial diamide to product i taking in account molecular weights.

GRP=%i

(Mi/Md)·Ti

With MI, Molecular weight of molecule i, Md, Molecular weight of initialdiamide, TI, Ratio (moles of molecule i/moles of initial diamide).

When Ti is lower than 1%, such a conversion is not included in the calculation.

3. Results and discussion

3.1. Identification of by-products

The identification, by FTIR and MS analysis, of about thirty major compoundshas shown the way of formation of these different kinds of by-products. A CG/MSchromatogram is shown in Fig. 2 and identification data of both MS and FTIR arereported in Table 3.The by-products are:� Primary or secondary diamides resulting of the cleavage of a N�C bond (C

depending of an alkyl group methyl or butyl).� Monoamides due to the cleavage of an OC�CHR bond. This last family can also

present N�C bond cleavage and then gives primary or secondary monoamides.� Diamides or monoamides with shortened alkyl groups obtained by C�C bond

cleavage. These products have the same functionalities than precursors but witha lower molecular weight.

� Carboxylic acids such as butanoic, undecanoic or dodecanoic acids resultingfrom the oxidation of alkyl chains.

� Imides due to the oxidation of an a-carbon from nitrogen in monoamide.

Table 2Conditions for TDA and TGA analysis

Parameters Values

Range of studied temperature 30–250°CInitial level 5 min at 25 or 30°CGradient of temperature 5°C min−1

Final level From 1 to 3 hMass of samples From 20 to 50 mgType of crucibles Platinum (0.1 mL)

Reconstituted air (17 mL min−1)Carrier gasThermocouples Platinum


Fig. 2. GC/MS Chromatogram of thermal degradation mixture of DMDBTDMA (3 h, 200°C, oxygenflow in open reactor). TOT signifies Total ion current.

� Volatile compounds such as 2-butenal, butanal, 2-pentenal, propanamide, bu-tanamide. They are also found in atmosphere of degradation (analysis of VOCs).In higher conditions of temperature, substituted tertiary amides can give another

products such as olefins [11,12].An experimental pathway for DMDBDDEMA thermal degradation, showing the

possible reactions, is presented in Fig. 3. Such a pathway is similar to thus obtainedfor DMDBTDMA because the same kinds of cleavages are observed with bothdiamides. DMDBDDEMA which has an oxo-alkyl group can present a greaterinstability due to C�O cleavages but such cleavages are minors comparatively tothose describe in Fig. 3.

3.2. Influence of operating conditions in closed reactor

First quantitative results are obtained in closed reactor with small quantity ofdiamide and controlled initial atmosphere. As shown on Fig. 4, both diamides havea similar behaviour. The presence of oxo-alkyl group can explain the little differ-ence between these two malonamides. The formation of monoamide from diamideincreases up to a conversion rate around 20%. The decrease of monoamide level canbe explained by the own degradation of monoamide and also by the decrease offormation. This evolution of monoamide levels is followed in a degradation mixturefrom diamide. In order to compare their own stability, experiments have beencarried out on pure monoamides (Table 4). Such experiments prove that degrada-tion rates are similar for both monoamides.


Table 3Identification of by-products from DMDBTDMAa

GC/MS Molecular weightGC/FTIR NameNumber(g mol−1)Retention(see Fig. 2) Retention

time (s) time (s)

285 17032 59 N-Methyl formamide87 Butanamide31 174n.d.

101 N-Butyl formamide30 201312N-Butyl N-methyl formamide326 220 11529N-Butyl N-methyl acetamide375 272 15728

171 N-Butyl N-methyl propanamide31927 426226 Undecanone26 366n.d.

484 N-Butyl N-methyl butanamide25 372 185Dodecanonen.d. 445 18424

226 Hexadecane (IS)n.d.23 560240 Isocyanate R1-N.C.O50422% n.d.223 Isocyanate R2-N.C.On.d.21% 666186 Undecanoic acidn.d.22 577

n.d. N-Butyl N-methyl octanamide21 630 213Pentadecanone792 668 22620

242 Pentadecanoic acid91819 777256 Hexadecanoic acid18 995 814

1119 Tetradecyl malonamide323108017N-Butyl N-methyl decanamiden.d. 1006 25516

255 Hexadecanamide115215 1026255 N-Butyl undecanamiden.d.14 1039269 N-Methyl hexadecanamide106713 1182297 N-Butyl N-methyl tridecanamiden.d.12 1101

n.d. N-Ethyl hexadecanamide11 1172 283N-Methyl ethyl hexadecanamide1404 1230 29710

311 N-Butyl hexadecanamide14289 12648 1458 1281 325 N-Butyl N-methyl hexadecanamide (BMHDA)7 Pollution of analytical de6ice1651 1304

Imide (CH3)(C3H7CO)NCOC15H31n.d. 1312 3396Imide (HOC)(C3H7CO)NCOC15H31n.d. 1379 3395Pollution of analytical de6ice14824 n.d.

3 1710 1538 382 N-Butyl N,N %-dimethyl tetradecyl malonamide424 N,N %-Dibutyl N-methyl tetradecyl malonamide18902 1745

N,N %-Dimethyl N,N %-dibutyl tetradecyl1962 1900 4381malonamide (DMDBTDMA)

a major compounds are written in bold; n.d.: not detected.

The effect of oxygen in the closed reactor on both monoamides is reported inTable 5. With addition of oxygen, the increase of degradation rate is about 20–30%even if the molar ratio ([monoamide]/[oxygen]) was not very high due to the


Fig. 3. Experimental reaction pathway for the degradation of DMDBDDEMA (Levels 1–3 correspondrespectively to malonamides, monoamides and formamides, main oxidfation products are placedbetween levels 1 and 2 on the right of the pathway and several compounds are given with their molecularweight).

presence of a large amount of monoamide. In such a thermal oxidation, the level ofoxygen is a limiting factor for the reaction. This fact was observed in previous studywith tertiary amides [3,6] and is confirmed by results shown in Fig. 5. In the smallclosed reactor, if the initial amount of diamide exceeds 60 mg, the destruction rateis limited to 20%.


Fig. 4. Thermal oxidation at 250°C, in closed reactor, of 45 mg of DMDBTDMA and DMDBDDEMA.

3.3. Influence of operating conditions in open reactor

Second quantitative results are obtained in open reactor with higher quantity ofdiamide than in closed reactor and purging atmosphere or not. This kind ofexperiments have been carried out on pure and diluted diamides. The reason fordilution is that such extractants will not be used pure but diluted in industrialprocess. Such a dilution will be performed in dodecane.

3.3.1. Pure diamidesResults presented in Fig. 6 show that degradation rate is quite important after 1

h, with a value around 90%. If the thermal oxidation is carried out for 2 h,degradation rate increase to 95% and 98% for DMDBDDEMA and DMDBTDMArespectively.

Behaviour of both diamides is similar. If the conversion rate to monoamide isconsidered, new similarities are still observed. In fact, the small difference betweenboth diamides can explain the small difference for both monoamides because suchconversion from di- to monoamide is a main way of initial molecule degradation.Relative stabilities of pure monoamides seem identical.

The destruction of initial molecule leads to monoamide and a lot of otherby-products as shown in the reaction pathway (Fig. 2). When the decrease ofmolecular weight is taken in account, global residual percentage (GRP) shows thatdestruction level is not as good as shown by diamide percentage only. For a low

Table 4Thermal oxidation at 250°C, in closed reactor, of both monoamides (50 mg)

Residual BMHDA (%) Residual BMDDOBA (%)Reaction time

20% 70 686540% 646360% 62


Table 5Thermal oxidation at 200°C, in closed reactor, of both monoamides (100 mg) as a function of oxygenlevel

Time Residual BMHDA (%) Residual BMDDOBA (%)

Air Air with oxygenAir with oxygenAir

20% 64 88 658984 6040% 85 6182 578060% 60

destruction level, lower than 50%, the GRP is, of course, essentially due to residualstudied diamide (Table 6). In such a case, GRP value is not very different fromresidual diamide percentage. On the contrary, when the destruction of initialmolecule seems quite complete such as 98%, GRP has a value of 25.6% showingthat the destruction yield over the whole pathway is not 98% but 74.4%.

3.3.2. Diluted diamidesIn dodecane solutions, diamide with oxo-alkyl chain is degradated more quickly

than its analogue with C14 alkyl group (Table 7). In these conditions, the relativeweakness of C�O bond, comparatively to C�C bond, can explain such a behaviour.This difference is observed whatever the time of degradation and the type ofoxidation (oxygen or ozonated air). Contrary to the case of pure compounds,important degradation rate do not signify important conversion rate tomonoamide. The weakness of C�O bond previously observed in diamide seems tobe similar in monoamide. The oznonation is less effective than oxygenation, evenwith humid solutions.

Results given in Table 8 show the influence of several factors on degradation withdifferent experiments A to F. A can be considered as reference. The experiments A

Fig. 5. Influence of initial mass of BMHDA on thermal degradation (30 min at 250°C under oxygenatmosphere).


Fig. 6. Thermal oxidation of pure diamides (500 mg at 250°C, oxygen at 3 mL min−1 flow rate).

and B, carried out in open reactor without gas flow confirm the influence of initialmass and therefore the influence of molar ratio [diamide]/[oxygen]. The experimentC, performed in closed reactor comparatively to A, confirms that results arecomparable between both kinds of devices if there is no flow rate in open reactor.The experiment D shows the influence of temperature because at 200°C, degrada-tion is quite weak. When air flow (15 mL min−1) is added in open reactor, thedegradation can increase to reach 95%. With pure oxygen (experiment F), degrada-tion level is still important at about 96%

Because no real amelioration was observed with utilization of ozonated air (at 40mL min−1) as oxidant, the same levels than wthose obtained with oxygen wereobtained and therefore are not presented in Table 8. This point confirms previousstudy based on experimental design and carried out in closed reactor [13,14].

Table 6Thermal oxidation of pure DMDBTDMA (500 mg, oxygen at 3 mL min−1 flow rate) in open reactor

BMHDA (%)Time/Temperature of thermal oxidation GRP (%)DMDBTDMA (%)

1 h/200°C 51.5 3.2 61.510.3 13.1 32.21 h/250°C31.0 50.42 h/200°C 6.7

25.813.52.32 h/250°C


Table 7Thermal oxidation of malonamide solutions (1g, 0.5 mol L−1 at 250°C) in open reactor

Dodecane (%) GRP (%)Monoamide (%)Diamide (%)

Compound/Time of oxygenation71.0 45.33.438.3DMDBTDMA/1 h64.0 36.8DMDBDDEMA/1 h 31.6 2.162.0 31.86.0DMDBTDMA/2 h 20.2

3.816.1 60.6 24.8DMDBDDEMA/2 h

Compound/Time of ozonationhumid DMDBTDMA/2 h 10.3 66.7 39.626.9

62.1 30.9humid DMDBDDEMA/2 h 21.1 7.9

3.4. Results of thermal analysis

Thermal gravimetric analysis (TGA) has shown the mass decrease as a functionof time and temperature. These results (Fig. 7) for pure diamides and puremonoamides indicate a stabilization for values between 20 and 30% of initial mass.

A more important mass decrease is observed for diamide and monoamide withoxo-alkyl chain comparatively to those with alkyl chain. The oxygen bond canexplain the relative weakness of these derivatives. TGA is an efficient method tostudy thermal behaviour and can be combined with separation or detection method[8,15]. In this case, FTIR detection has shown the formation of carbon dioxide ata temperature lower than 250°C. TGA can allow calculation of reaction’s kineticparameters [16–18] but the Coats and Redfern method [19] did not give the globalorder and activation energy of these complex degradation.

Table 8Influence of several factors on 2 h degradation

FExperiment BA C D E

250250 250 200 250Temperature (°C) 250Pure oxygenAirAirComposition of atmosphere Air Air Pumped air

Open Closed OpenOpen OpenType of reactor Open0 0 No 0 15Flow rate (mL min−1) 30

119.5 105Initial quantity of diamide 1047.5114.7 45.2 118.6(mg)

4Residual DMDBTDMA (%) 11.5 55.5 9.5 57.5 5.0


Fig. 7. Mass evolution of pure diamides and monoamides (25–30 mg) during ATG experiment from 25to 250°C at 5°C min−1.

4. Conclusion

The identification of numerous by-products has shown that thermal and oxida-tive destruction of diamides leads to several molecules such as lower diamides,monoamides, ketones, aldehydes and carboxylic acids. These by-products areproduced by cleavage of N-alkyl bonds. In closed reactor, the influence of oxidanthas been confirmed showing that destruction processes were essentially due tothermal oxidation. In open reactor, several factors were followed and the mostimportant seem to be the alimentation of oxidant. It can be air or oxygen that allowdestruction of diamides. Ozonated air does not improve a lot such degradation.Both diamides have a similar behaviour. This kind of treatment does not allowreducing as wished the volume of solvent or partially degraded solvent but leads tomonofunctional compounds which are less complexant for fission species. Next stepcould be an easier separation of extractant and extracted species making possible afinal incineration. Therefore, thermal oxidation could be envisaged as a pre-treat-ment in a solvent destruction process.

Acknowledgements

This work has been realized at the ‘Ecole des Mines d’Ales’ for a contractsubscribed by CEA-Marcoule.


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thermal oxidation of two malonamides, extractants for minor actinides in nuclear fuel reprocessing

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