Theoretical investigation of isotope exchange reaction in tritium-contaminated mineral oil in vacuum pump

Journal of Hazardous Materials 287 (2015) 42–50

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Theoretical investigation of isotope exchange reaction in tritium-contaminated mineral oil in vacuum pump Liang Dong a , Yun Xie a , Liang Du a,c , Weiyi Li b,∗ , Zhaoyi Tan a,∗ a b c

Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, PR China School of Physics and Chemistry, Xihua University, Chengdu 610065, PR China School of Radiation Medicine and Protection (SRMP), School for Radiological and Interdisciplinary Sciences (RAD-X), Suzhou 215000, PR China

h i g h l i g h t s • • • •

This is the first theoretical investigation about T–H exchange in vacuum oil. T–H isotope exchange is accomplished through two different change mechanisms. Isotope exchange is selective, molecules with OH and COOH exchange more easily. The methyl and methylene radicals in waste oil were observed by 1 HNMR.

a r t i c l e

i n f o

Article history: Received 1 October 2014 Received in revised form 9 January 2015 Accepted 10 January 2015 Available online 13 January 2015 Keywords: Hydrogen Tritium Isotope exchange Vacuum pump oil DFT

a b s t r a c t The mechanism of the isotope exchange reaction between molecular tritium and several typical organic molecules in vacuum pump mineral oil has been investigated by density functional theory (DFT), and the reaction rates are determined by conventional transition state theory (TST). The tritium–hydrogen isotope exchange reaction can proceed with two different mechanisms, the direct T–H exchange mechanism and the hyrogenation–dehydrogenation exchange mechanism. In the direct exchange mechanism, the titrated product is obtained through one-step via a four-membered ring hydrogen migration transition state. In the hyrogenation–dehydrogenation exchange mechanism, the T–H exchange could be accomplished by the hydrogenation of the unsaturated bond with tritium followed by the dehydrogenation of HT. Isotope exchange between hydrogen and tritium is selective, and oil containing molecules with OH and COOH groups can more easily exchange hydrogen for tritium. For aldehydes and ketones, the ability of T–H isotope exchange can be determined by the hydrogenation of T2 or the dehydrogenation of HT. The molecules containing one type of hydrogen provide a single product, while the molecules containing different types of hydrogens provide competitive products. The rate constants are presented to quantitatively estimate the selectivity of the products. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Tritium is a very useful radioactive isotope in fusion reactions, which lead to the formation of radioactive tritium-containing waste during normal operations in most nuclear facilities [1]. Mineral oil, mainly comprising hydrocarbons, naphthenes, and aromatic compounds with side chains, is usually used as the working liquid in vacuum pumps [2,3]. Tritium contamination of vacuum pump oil can result from an isotope exchange reaction between tritium and organic compounds in the oil [4]. The energy released upon tri-

∗ Corresponding author. Tel.: +86 816 2480807; fax: +86 816 2495280. E-mail addresses: [email protected], [email protected] (Z. Tan). http://dx.doi.org/10.1016/j.jhazmat.2015.01.030 0304-3894/© 2015 Elsevier B.V. All rights reserved.

tium decay could breakdown the oil into smaller fragments, such as methane (CH4 ), ethylene (C2 H4 ), propane (C3 H8 ), and propylene (C3 H6 ) [5]. Furthermore, the interaction of tritium with the oil can lead to the formation of various organic tritiated species in spent vacuum oils, which are much more toxic than tritium gas. Since the content of tritium in oil used in diffusion and forevacuum pumps can reach 103 Ci/kg [6], proper handling of tritium-contaminated waste is crucial. When the pumped tritium gas comes into contact with hydrocarbons in the oil, tritium-containing hydrocarbons are formed by isotope exchange. In the presence of oxygen, radiation-chemical oxidation of tritium and hydrocarbons occurs, leading to the formation of peroxide and hydroperoxides radicals. Many reactions of these radicals with other particles result in the formation of stable

L. Dong et al. / Journal of Hazardous Materials 287 (2015) 42–50

tritium-containing oxidation products, such as alcohols, carbonyls, and carboxylic acids [7]. Thus, various forms of organic tritium exist in spent mineral oil due to tritium–hydrogen (T–H) isotope exchange. To determine the safest and the most efficient method for removing tritium from oil, it is necessary to understand the chemical forms of tritium dissolved in the oil after T–H isotope exchange as well as the T–H exchange processes. To the best of our knowledge, the investigations concerning isotope exchange of tritium in oil are rare. Sazonov et al. [3] established that tritium can be present as dissolved molecular tritium, tritium-containing hydrocarbons, tritium-containing water, and oxidation products in which tritium is bound to carbon and oxygen atoms in VM-5 vacuum oil. They proposed a scheme for handling tritium-contaminated wastes by combining the adsorption

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of tritium-containing impurities with the isotope exchange desorption. This method can efficiently decrease the radiation hazard while recycling the tritium gas. Magomedbekov et al. [2] removed tritium from oil by isotope exchange with concentrated sulfuric acid and oleum at 80 ◦ C and extracted 93% of the tritium over 5 h. Although many efforts have been made to explore the T–H isotope exchange reaction in experiments, the detailed information at the molecular level is still lacking. Furthermore, experimental investigations have proposed that isotope exchange is selective. The probability of tritium to undergo isotope exchange relates to the reactivity of the organic molecule. However, the reactivity of the organic molecules with different functional groups is not clear. A better understanding of the isotope exchange reaction and the chemical forms of tritium in vacuum oil after T–H exchange would

Table 1 Relative energies (kcal/mol) of the TSs of isotope exchange based on the different types of hydrogens in typical organic molecules found in vacuum pump oil.

1 2 3 4 5 6 7 8 9

Alkane

CH4

CH4 C2 H6 C3 H8 Alkene C2 H4 C3 H6 Hydrocarbon oxide CH3 CH2 OH CH3 COOH CH3 CHO CH3 COCH3 Aromatic compound

105.8

CH3 100.8 CH3 104.7 99.7 98.6 99.5 Ortho-

CH3 104.7 105.0 CH2 102.4 100.5 CH2 104.9

CH2

103.8 CH 101.9 OH 64.8

COOH

CHO

72.0 103.6 Meta-

Para-

CH3

103.3

103.1

102.7

100.7

103.5

103.1

102.9

103.6

103.9

104.5

102.5

102.2

102.8

105.0

104.1

104.6

CH2

OH

100.1

64.6

COOH

CHO

103.4

10

C6H6 CH3 11

C6H5-CH3 CH2OH 12

C6H5CH2OH COOH 13

72.0

C6H5COOH O C 14

CH3

97.8

C6H5COCH3 O C 15

H

C6H5CHO

104.4

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L. Dong et al. / Journal of Hazardous Materials 287 (2015) 42–50

provide guidance for decontaminating tritium-contaminated oil more safely and effectively. Here, we choose a variety of typically organic molecules representative of those found in pump oil and present a theoretical study concerning the isotope exchange reaction mechanism between T2 and the model organic compounds in the gas-phase. Furthermore, we clarify the reactivity of several functional groups of organic molecules in vacuum pump oil. 2. Computational details All calculations were carried out using Gaussian 09 program [8]. Full geometry optimizations were run to locate all stationary points and transition states (TSs) using the hybrid density functional B3LYP [9–11] method with the 6-311++G(d,p) basis set [12–15]. Zero-point vibrational energy (ZPVE) corrections were applied to the relative energies. To evaluate the reliability of the function theory levels, a few structures which have small energy differences were reoptimized at the B3LYP/6-31+G(d,p), BPW91/6-311++G(d,p) and M06/6-311++G(d,p) levels (details in Supplementary data, part C). The calculations indicate that B3LYP method could provide highly accurate energies and the B3LYP/6311++G(d,p) level would be suitable for studying the titled system. Vibrational frequencies were obtained at the same level, and the species were characterized as a minimum (no imaginary frequency) or a transition state (unique imaginary frequency). Intrinsic reaction coordinate (IRC) calculations were performed to further confirm that the optimized transition state (TS) correctly connects the relevant reactant and product. To obtain an in-depth insight into the electronic properties of the reaction, natural bond orbital (NBO) analysis [16,17] at the B3LYP/6-311++G(d,p) level was performed. Rate constants were also calculated according to conventional transition state theory (TST) [18], including a tunneling correction based on Wigner’s formulation [19]. 3. Results and discussion In this work, we discuss the isotope exchange reaction in tritium-containing mineral oil. Since the oil contains hydrocarbons, aromatic compounds, alcohols, carbonyl compounds, and carboxylic acids, we chose a variety of molecules as reactants, as shown in Table 1, to investigate the T–H isotope exchange reaction. Our calculations indicate that two mechanisms were considered for the isotope exchange reaction. The isotope exchange between these organic molecules and T2 proceed through a direct isotope exchange mechanism via a four-membered ring TS. Moreover, for the molecules with unsaturated double bond, the isotope exchange may proceed through a hydrogenation–dehydrogenation mechanism. It is noted that, small amounts of unsaturated hydrocarbons could not inconceivably occur under normal operating conditions of a vacuum pump. However, the amount of the unsaturated hydrocarbons is small but not insignificant, considering the integrity of the research, it is necessary to study all the possible reaction paths between tritium and the molecules might exist in the pump oil.

Fig. 1. Reaction path of T–H isotope exchange of methane and the corresponding optimized structures along the reaction path (bond length in Å).

As shown in Fig. 1, the predicted reaction starts with T2 attacking the C1 atom of CH4 . Subsequently, T–H exchange between T2 and CH4 occurs via hydrogen migration followed by the expulsion of HT to release the product CH3 -T. As shown in Fig. 1, T–H exchange occurs via a four-membered ring hydrogen migration transition state TSCH4 (involving the C1, H1, T1 and T2 atom). The C1 H1, C1 T1, H1 T2 and T1 T2 bond lengths in TSCH4 are 1.496, 1.513, 1.218 and 1.215 Å, respectively. The vibrational mode of TSCH4 involves the T1 atom moving toward the C1 atom, while the H1 atom simultaneously moves toward the T2 atom with a vibrational frequency of 2479i. The relative energy of hydrogen migration in TSCH4 is calculated to be 105.8 kcal/mol. The relatively high calculated energy corresponds to the increased ring strain in the four-membered hydrogen migration transition state. Seewald [20] studied the reactions between ethane, ethene, water and inorganic redox-sensitive minerals by redox-buffered hydrothermal experiments. He found that thermodynamically unstable species would be allowed to persist in a metastable state for geologically significant periods of time. The reversible metastable thermodynamic equilibrium is attained between ethane, ethene, water and inorganic minerals. Because water participates directly in this equilibrium, it may represent a reactive and abundant source of hydrogen for hydrocarbon generation. Although the Seewald study is focused on hydrocarbons dissolved in an aqueous solution, which is not quite the same as an idealized mineral oil, the similar metastable equilibrium may be attained in the mineral oil. Furthermore, as mentioned above, the mineral oil is chemically inert, but a slow exchange reaction is possible even at room temperature because the energy emitted from tritium decay could breakdown long-chain hydrocarbons in the oil. Actually, the signals of methyl

3.1. Direct T–H isotope exchange mechanism. The direct T–H exchange mechanisms for multiple hydrogens on different organic molecules were studied. The relative energies of the transition states are summarized in Table 1. As the simplest hydrocarbon, methane was chosen to illustrate the detailed direct isotope exchange mechanism of the isotope exchange reaction. The calculated potential energy surface (PES) at the B3LYP/6–311 + +G(d,p) level for the T–H exchange reaction path of methane and the corresponding optimized structures are given in Fig. 1.

Fig. 2. Visualization of the orbital interaction of CH4 and T2 in the T–H exchange transition state TS CH4 .

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and methylene radicals in waste oil were observed by 1 HNMR in the experiments (see the Supplementary data, Fig. S1), which is consistent with the previous investigation [4]. NBO calculations were performed to explore the orbital interaction of the T–H exchange process in detail. Visualization of the orbital interaction of CH4 with T2 in TSCH4 is presented in Fig. 2. CH4 is tightly bound to T2 with a stabilization energy of 211.2 kcal/mol for BD()C–H(2) → BD*()T(6)–T(7). Meanwhile, NBO analysis gives the stabilization energy of BD()T(6)–T(7) → BD*()C–H(2) is 179.0 kcal/mol. The results indicate that a strong interaction between CH4 and T2 during T–H exchange. In addition, only one type of hydrogen exists in CH4 , therefore, only one T–H exchange reaction path is considered, where CH3 -T and HT are formed by isotope exchange. However, there are many different types of hydrogens in other organic molecules, such as C3 H8 , C3 H6 and CH3 CH2 OH. Various tritiated products are formed from an identical reactant when T–H exchange occurs at different positions. Furthermore, for the direct T–H exchange mechanism, the isotope exchange reaction proceeds through only one transition state. Thus, the transition state determines which hydrogen is most likely to exchange with tritium, which determines the tritiated product. In other words, the transition state determines the reactivity of a functional group of an organic molecule along with the exchange rate. The isotope exchange reactions proceeding via a direct isotope exchange mechanism with various organic molecules in vacuum pump oil containing different types of hydrogens were investigated. The optimized structures along the T–H exchange paths of typical organic molecules in vacuum pump oil, except CH4 , are given in Fig. 3. The relative energies of the T–H exchange TS in each reaction are listed in Table 1. As shown in Table 1, different types of hydrogens in alkanes have different relative energies (entries 1–3). The relative transition state energy of T–H exchange in methane is ca. 106 kcal/mol, which is higher than that of the CH3 in ethane or propane, which is then higher than that of the CH2 in propane. This result indicates that isotope exchange is selective and the activity of the H atom in alkanes is CH2 > CH3 > CH4 . In general, the relative transition state energy of alkenes is lower than that of alkanes. The relative transition state energy of T–H exchange for CH2 in C2 H4 is 102.4 kcal/mol, while those for the different hydrogens in C3 H6 are slightly lower at ca. 101 kcal/mol (entries 4–5). For the hydrocarbon oxides (entries 6–9), the relative transition state energies of oxygen-containing functional groups are ordered as OH (64.5 kcal/mol) < COOH (72.0 kcal/mol) < CHO (103.6 kcal/mol), which are much lower than the relative energies of alkyl groups. The energies of non-oxygenated alcohol hydrogens (CH3 and CH2 ) are ca. 105.0 kcal/mol, while those of the CH3 hydrogens in the other molecules are ca. 99 kcal/mol. Thus, tritium exchange occurs more easily with the OH and COOH groups of alcohols and acids, respectively. It should be noted that for the aldehyde the relative transition state energy of T–H exchange of CH3 is lower than that of CHO, which indicates that tritium exchange may occur with the CH3 group in aldehydes. Furthermore, in the ketone, the relative transition state energy of T–H exchange with CH3 is 99.5 kcal/mol. From the calculations, based on the direct isotope exchange mechansim, the ability for T–H isotope exchange is ordered as CH3 CH2 OH > CH3 COOH > CH3 CHO > CH3 COCH3 > alkene > alkane. For the aromatic compounds, the calculations indicate that the ability for T–H exchange is similar to that for the non-aromatic compounds. The relative transition state energies of T–H exchange with hydrogens at different positions on the benzene ring are very similar. However, T–H exchange with the hydrogen of the oxygen-containing functional group is quite different from that with the benzene ring. As shown in Table 1, the relative transition state energy of T–H exchange with hydrogen on benzene is ca.

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103 kcal/mol, while that with an alkyl group attached to the ring is ca. 100.5 kcal/mol (entries 10–12). In acetophenone, the relative energy of T–H exchange with the CH3 of the ketone is calculated to be 97.8 kcal/mol, much lower than that of the alkyl group attached the benzene ring (entry 14), which indicates that isotope exchange is more likely to occur with the CH3 group in acetophenone. The relative energy of T–H exchange with oxygen-containing groups on the aromatic compounds is similar to that with hydrocarbon oxides. The relative transition state energy of T–H exchange with benzoic acid is lower than that with benzaldehyde, while that with benzyl alcohol is the lowest (entries 12–13, 15). Furthermore, the relative energy of exchange with the CHO group is calculated to be 104.4 kcal/mol, which is a little higher than that with the hydrogen at the meta-position of the benzene ring. This indicates that T–H isotope exchange proceeds direct exchange is most likely to occur with the hydrogen at the metaposition of the benzene ring in benzaldehyde. In benzyl alcohol and benzoic acid, however, T–H isotope exchange will likely occur with the oxygen-containing group. Thus, for the aromatic compounds, based on the direct T–H isotope exchange mechanism, the ability for T–H exchange is ordered as benzyl alcohol > benzoic acid > acetophenone > toluene > benzene > benzaldehyde. 3.2. Hydrogenation-dehydrogenation T–H isotope exchange mechanism. For the unsaturated organic molecules, another T–H exchange mechanism, involving hydrogenation–dehydrogenation of the double bonds, was considered. The T–H exchange reaction invloved the hydrogenation–dehydrogenation mechanism is composed of two steps as follows: (1) the hydrogenation of the T2 adds to the C C or C O bond on the organic molecules, and (2) the dehydrogenation of the HT on the different positions. It is noted that after the dehydrogenation of HT, an enol structure is obtained. Then, keto-enol tautomerism would carry out and provide the tritiated product. The hydrogenation–dehydrogenation exchange reaction pathways of C2 H4 , C3 H6 , CH3 COOH, CH3 CHO, CH3 COCH3 , C6 H5 COOH, C6 H5 COCH3 and C6 H5 CHO are presented in Fig. 4. The optimized structures are shown at the B3LYP/6-311++G(d,p) level. From calculations, it is shown that the hydrogenation–dehydrogenation reactions accomplish through two or three transition states, thus, the transition state which is the energy top determines the reactivity of the organic molecules. In general, as shown in Fig. 4, the relative energies of the structures involved in the hydrogenation–dehydrogenation T–H exchange mechanism are lower than those involved in the direct T–H exchange mechanism. From the calculations, it is clear that the ability of T–H exchange is still alkene > alkane. However, in the CH3 COOH and C6 H5 COOH, the T–H exchange occurring on the COOH group involved in the hydrogenation–dehydrogenation exchange mechanism is more difficult than in the direct exchange mechanism. Unfortunately, the calculations failed to locate the structures correspond to the T–H exchange occurring on the CH3 group on CH3 COOH. Considering the hydrogen on the COOH group is more active than that on the CH3 group, we consider it would not affect our determination concerning the reactivity of the molecules. Furthermore, for the aromatic compounds, in virtue of the position of the unsaturated double bond, the T–H exchange would not likely to occur on the benzene ring involved in the hydrogenation–dehydrogenation exchange mechanism. For the aldehydes and ketones, the T–H exchange involved in the hydrogenation–dehydrogenation exchange mechanism are easier than in the direct exchange mechanism. Especially, the T–H exchange occuring on the CHO group becomes very easy in the hydrogenation–dehydrogenation exchange mechanism on CH3 CHO and C6 H5 CHO. The relative TS energy of hydrogenation of

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L. Dong et al. / Journal of Hazardous Materials 287 (2015) 42–50

Fig. 3. Optimized structures in the T–H exchange of typical organic molecules in vacuum pump oil, except CH4 .

L. Dong et al. / Journal of Hazardous Materials 287 (2015) 42–50

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Fig. 4. The hydrogenation–dehydrogenation exchange reaction pathways of C2 H4 , C3 H6 , CH3 COOH, CH3 CHO, CH3 COCH3 , C6 H5 COOH, C6 H5 COCH3 and C6 H5 CHO. Relative energies (kcal/mol) of the corresponding species relative to the reactants are shown in square brackets.

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L. Dong et al. / Journal of Hazardous Materials 287 (2015) 42–50

Table 2 The involved mechanism, rate constants (in cm3 mol−1 s−1 ), product, and branching ratio (%) of the different molecules of the T–H exchange reaction. Species

Mechanism

Rate constants

Product

Branching ratio

CH4 C2 H4 C6 H6 CH3 COCH3 C2 H5 OH

Direct exchange Hydrogenation-dehydrogenation exchange Direct exchange Hydrogenation-dehydrogenation exchange Direct exchange

CH3 COOH

Direct exchange Direct exchange Hydrogenation-dehydrogenation exchange

k1 = 1.6 × 108 exp(−438,340/RT) k2 = 7.8 × 106 exp(−378,081/RT) k3 = 3.7 × 107 exp(−428,503/RT) k4 = 8.5 × 103 exp(−373,746/RT) k5 = 5.3 × 107 exp(−434,622/RT) k6 = 8.0 × 107 exp(−436,618/RT) k7 = 6.7 × 106 exp(−265,414/RT) k8 = 2.0 × 107 exp(−412,979/RT) k9 = 1.6 × 107 exp(−296,679/RT) k10 = 3.0 × 107 exp(−408,582/RT) k11 = 6.0 × 105 exp(−287,122/RT)

CH3 T C2 H3 T C6 H5 T CH3 COCH2 T CH2 T-CH2 OH CH3 CHT-OH CH3 CH2 OT CH2 T-COOH CH3 COOT CH2 T-CHO CH3 CTO

– – – – Almost zero Almost zero 100 Almost zero 100 Almost zero 100

CH3 CHO

C O bond is 70.4 kcal/mol (or 72.7 kcal/mol), sightly higher than that of dehydrogenation of HT on aldehydes. Thus, the ability of T–H exchange of CHO group is determined by the hydrogenation of C O bond. The relative TS energy of dehydrogenation of HT is 91.2 kcal/mol (or 94.5 kcal/mol), much higher than that of hydrogenation of C O bond on ketones. This means the ability of T–H exchange of ketone is determined by the dehydrogenation of HT. In summary, according to the calculations involved in the direct exchange mechanism and the hydrogenation–dehydrogenation exchange mechanism, it is concluded that T–H exchange is sensitive and selective. The ability for T–H isotope exchange is ordered as alcohol > acid > arene > alkene > alkane. Oil containing molecules with the OH and COOH functional groups can easily exchange tritium for hydrogen with the direct T–H exchange mechanism. The results are consistent with the previous investigations proposed by Schimmelmann et al. [25] and Sessions et al. [26]. They specifically studied the D–H isotope exchange, and the isotope abundance of D is much greater than T. Although the investigation objects are not quite the same as those in our systems, T–H isotope exchange might proceed by the similar mechanisms. For aldehydes or ketones, the T–H exchange proceeds more favorably through hydrogenation–dehydrogenation exchange mechanism and the ability of T–H exchange is determined by the hydrogenation of T2 or the dehydrogenation of HT.

3.3. Rate constants To quantitatively estimate the reactivity of various organic molecules and selectivity of the different products, the rate constants have been evaluated according to conventional transition state theory (TST) [18], including a tunneling correction based on Wigner’s formulation [19]. According to the TST, the rate constant can be shown that:  = kw (T) × K(T) The Wigner correction (W (T)) for tunneling assumes a parabolic potential for the nuclear motion near the TS. Therefore, it can not be considered as an accurate correction, but the expression is satisfactory when the correction is small. Even for large quantum effects in certain system which is considered not well justified, the Wigner correction is still surprisingly accurate [21–24]. The Wigner transmission coefficient is given by [24] 2

W (T ) = 1 +

1 ω =/ | | 24 kB T

where kB is the Boltzmann’s constant, ω =/ is the imaginary frequency of the unbound normal mode at the saddle point. The

conventional TST rate constant (K(T)) without tunneling correction of a reaction, A + BC = AB + C can be written as

K(T ) = 

= /

Qx =/ G exp(− ) QA QBC RT

 is the symmetry factor. QA , QBC and Qx =/ are the partition functions of the particles A, BC and transition state, respectively. G =/ is the potential barrier, and R is the gas universal constant. Rate constants for tritium exchange with CH4 , C2 H4 , C6 H6 , CH3 COCH3 , C2 H5 OH, CH3 COOH, and CH3 CHO were calculated because they are the simplest molecules present in mineral oil that contain the desired functional groups. The involved mechanism, rate constants, product, and branching ratio of the different molecules of the T–H exchange reaction are presented in Table 2. As shown in Table 2, the reactants CH4 , C2 H4 , C6 H6 , and CH3 COCH3 produce only CH3 T, C2 H3 T, C6 H5 T, and CH3 COCH2 T, respectively. The formation of CH3 T and C6 H5 T proceeds direct exchange mechanism and the formation of C2 H4 and CH3 COCH3 proceeds the hydrogenation–dehydrogenation exchange mechanism. In the 300–800 K temperature range, the rate constants of CH3 T (k1 ), C2 H3 T (k2 ), C6 H5 T (k3 ), and CH3 COCH2 T (k4 ) formation can be described with the expressions k1 –k4 (in cm3 mol−1 s−1 ) in Table 2. However, the reactant C2 H5 OH leads to the competitive formation of CH2 T-CH2 OH, CH3 CHT-OH, and CH3 CH2 OT. Compared the reaction pathways proceed with the direct and hydrogenation–dehydrogenation exchange mechanism, it is found that the T–H exchange of C2 H5 OH is kinetically favored by the direct exchange mechanism. As shown in Fig. 3, the energy reaction pathways of each product pass through TS1CH3CH2OH , TS2CH3CH2OH , and TS3CH3CH2OH , respectively. In the 300–800 K temperature range, the rate constants of CH2 T–CH2 OH (k5 ), CH3 CHT–OH (k6 ), and CH3 CH2 OT (k7 ) formation can be described with the expressions k5 –k7 (in cm3 mol−1 s−1 ) in Table 2. The branching ratio of CH3 CH2 OT is calculated to be 100.0%, while the branching ratios for CH2 T–CH2 OH and CH3 CHT–OH are almost zero. Thus, the reaction channel for CH3 CH2 OT + HT formation is kinetically predominant, while those for CH2 T-CH2 OH and CH3 CHT-OH formation are so small to be negligible. Similarly, the reactant CH3 COOH leads to the competitive formation of CH2 T–COOH and CH3 COOT. Compared the reaction pathways proceed with the different exchange mechanism, it is found that the T–H exchange of CH3 COOH is kinetically favored by the direct exchange mechanism. As shown in Fig. 3, their energy reaction pathways pass through TS1CH3COOH and TS2CH3COOH , respectively. In the 300–800 K temperature range, the rate constants of CH2 T–COOH (k8 ) and CH3 COOT (k9 ) formation can be described with the expressions k8 ∼k9 (in cm3 mol−1 s−1 ) in Table 2. The branching ratio for CH3 COOT formation is 100.0%, while that for CH2 T-COOH + HT is almost zero. In other words, the CH3 COOT + HT

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channel is predominant, whereas the CH2 T-COOH + HT channel should be kinetically ruled out. Once again, CH3 CHO leads to the competitive formation of CH2 T–CHO and CH3 CTO. Compared the reaction pathways proceed with the different exchange mechanism, it is found that the T–H exchange occurring on the CH3 group is kinetically favored by the direct exchange mechanism via TS1CH2T-CHO , while that occurring on the CHO group is favored by the hydrogenation–dehydrogenation exchange mechanism via hdTS1CH3CHO . In the 300–800 K temperature range, the rate constants of CH2 T-CHO (k10 ) and CH3 CTO (k11 ) formation can be described with the expressions k10 ∼k11 (in cm3 mol−1 s−1 ) in Table 2. The branching ratios for CH3 CTO are calculated to be 100%, while that for CH2 T-CHO is almost zero. Thus, the CH3 CTO channel is predominant, whereas the CH2 T-CHO channel should be kinetically ruled out. In summary, the results concerning the rate constants and the branching ratio indicate that CH4 , C2 H4 , C6 H6 , and CH3 COCH3 produce CH3 T, C2 H3 T, C6 H5 T, and CH3 COCH2 T, respectively. C2 H5 OH, CH3 COOH and CH3 CHO might only produce CH3 CH2 OT, CH3 COOT and CH3 CTO. The determination of the chemical forms of tritiated species in the pump oil could help workers using the more targeted and proper methods to handle the tritium-contaminated pump oil. According to our calculations, it is found that tritium more easily replaces the most mobile hydrogen atoms in organic molecule, i.e., OH and COOH group. Using the isotope exchange to return tritium into production and decrease the specific activity of the pump oil, which decreases its radiation hazard is an efficient method. The strong acidic or basic reagents (i.e., NaOH, NH4 OH and H2 SO4 ) employed to remove tritium from the pump oil by isotope exchange might achieve significant effects.

4. Conclusions The mechanism of isotope exchange between tritium and many typical organic molecules in vacuum pump mineral oil has been investigated theoretically. The reactivity of the different functional groups in the organic molecules and the rate constants of T–H exchange were also interpreted. The present calculations suggest that T–H isotope exchange may proceed with two different mechanisms. One is the direct T–H exchange, where the titrated product is obtained via a four-membered ring hydrogen migration transition state. Another one is the hydrogenation–dehydrogenation T–H exchange, which accomplished through the hydrogenation of the unsaturated bond with tritium followed by the dehydrogenation of HT. T–H isotope exchange is selective, and molecules with OH and COOH functional groups proceeding with the direct T–H exchange mechanism more easily exchange tritium for hydrogen. For aldehydes or ketones, the T–H exchange proceeds more favorably through the hydrogenation–dehydrogenation exchange mechanism and the ability of T–H exchange is determined by the hydrogenation of T2 or the dehydrogenation of HT. The reactants CH4 , C2 H4 , C6 H6 , and CH3 COCH3 lead to the sole formation of CH3 T, C2 H3 T, C6 H5 T and CH3 COCH2 T, respectively. For the C2 H5 OH + T2 reaction, in the 300–800 K temperature range, CH3 CH2 OT is the kinetically favored product with the rate constant of k7 = 6.7 × 106 exp(−265,414/RT) cm3 mol−1 s−1 . In the CH3 COOH + T2 reaction, CH3 COOT is the kinetically favored product with the rate constant of k9 = 1.6 × 107 exp (–296,679/RT) cm3 mol−1 s−1 . In the CH3 CHO + T2 reaction, CH3 CTO is the kinetically favored product with the rate constant of k11 = 6.0 × 105 exp (–287,122/RT) cm3 mol−1 s−1 . For a small amount of water exists in the pump oil, the tritiated water (HTO) is easily produced when tritium interacts with water. Recent investigation [27] of isotope exchange between

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low molecular weight (
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