ORIGINAL PAPER Mechanistic insights into the

Chemical Papers 66 (11) 1059–1064 (2012) DOI: 10.2478/s11696-012-0205-8

ORIGINAL PAPER

Mechanistic insights into the reaction of CF3 CCl3 with SO3 : Theory and experiment Zhi-Zheng Liu, Zhi-Rong Chen, Hong Yin*, Shen-Feng Yuan Department of Chemical and Biochemical Engineering, Zhejiang University, Hangzhou, 310027, China Received 15 November 2011; Revised 6 March 2012; Accepted 6 March 2012

Reaction mechanism of 1,1,1-trifluorotrichloroethane (CF3 CCl3 ) and sulphur trioxide (SO3 ) in the presence of mercury salts (Hg2 SO4 and HgSO4 ) was studied applying the density functional theory (DFT) at the UB3LYP/6-31+G(d,p) level. It was found that this reaction occurs in the free radical chain path as follows: mercury(I) sulphate free radical is generated by heat, causing CF3 CCl3 to produce the CF3 CCl2 free radical which reacts with SO3 leading to the formation of CF3 CCl2 OSO2 decomposing into CF3 COCl and SO2 Cl. The SO2 Cl free radical triggers CF3 CCl3 to regenerate CF3 CCl2 which recycles the free radical growth reaction. This elementary reaction has the highest energy barrier and it is therefore the rate control step of the whole reaction path. Experiment data can confirm the existence of the mercury(I) salt free radical and the free radical initiation stage. So, mercury salts play the role of initiators not that of catalysts. The results agree well with our hypothesis. c 2012 Institute of Chemistry, Slovak Academy of Sciences Keywords: reaction mechanism, mercury salts, density functional theory, free radical chain

Introduction The reaction of 1,1,1-trifluorotrichloroethane (CF3 CCl3 ) with sulphur trioxide (SO3 ) is the most common industrial way to prepare trifluoroacetyl chloride (CF3 COCl) which is an advantageously typical intermediate in the preparation of trifluoroacetic acid (CF3 COOH), ethyl trifluoroacetate (CF3 COOCH2 CH3 ), and other trifluoroacetyl derivatives (March, 1992). Benning and Park (1946) first invented this method in Du Pont Company which provided the basis for the current industrial process of CF3 COCl production. It is generally accepted that this method requires some substances, such as mercury salts, halogens, halogenated sulphonic acid, etc., to promote the reaction; these substances are thought to play the role of catalysts in this reaction system (Paucksch et al., 1973; Ruudorufu et al., 1981; Anello et al., 1982; Yoshida et al., 1985). Furthermore, the effect of mercury salts (Hg2 SO4 and HgSO4 ) is better than that

of other substances such as some halogens including Br2 or I2 (Marangoni et al., 1980). This method was developed nearly seventy years ago and it has been widely used for industrial production since. However, attention has been focused on industrial application research such as the optimisation of process parameters (Marangoni et al., 1980), improvement of the process route (Yoshida et al., 1985), etc. A few researches have focused on the theoretical fundamental study such as the reaction mechanism. Therefore, the purpose of this paper is to shed light on the catalytic mechanism details of mercury salts in this reaction system. In this study, the most feasible reaction mechanism is proposed using the density functional theory calculation. In this system, common instruments cannot be used to analyse the reaction process because of strong corrosion properties of SO3 . Therefore, we must depend on the information obtained by theoretical calculation to some extent. One advantage of computational chemistry is that it can significantly reduce the

*Corresponding author, e-mail: [email protected]

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Z. Z. Liu et al./Chemical Papers 66 (11) 1059–1064 (2012)

number of laboratory experiments that are not easy to be performed but are easy to be identified by calculation. Thermodynamic and kinetic simulations were calculated to obtain more detailed research data. At the same time, experiments to verify our hypothesis were designed and the factors affecting the reaction were attempted to be explained theoretically and systematically according to the proposed mechanism.

Experimental Computational methods All time-independent computational studies reported in this work were performed using the Gaussian 09 program (Frisch et al., 2010) on a ThinkCentre M8200t computer with the density functional theory (DFT) framework and UB3LYP hybrid functionals (Farnell et al., 1983; Schlegel & Sosa, 1988). The Lanl2dz basis set was used for mercury atoms (Liu et al., 2007) and the 6-31+G(d,p) basis set for the other ones (Qiu et al., 2004). A scale of 0.9614 was used for the zero point energies to eliminate systematic errors. Calculations above this value were carried out using the default convergence criteria. Frequency analysis was performed to confirm that all obtained structures were corresponding to stationary points. For transition state geometry determination, quasi-Newton transitguided (QSTN) computations were used. Moreover, correct transition states were confirmed by intrinsic reaction coordinate (IRC) calculations. According to the Eyring transition state theory of elementary reactions, the activation enthalpy, activation energy, and reaction rate (Wang et al., 2001) can be obtained as the following equations: ∆r H = = HTS − HR

(1)

Experimental methods In order to determine the role of mercury salts, the reaction was performed with an initial mixture of 5.65 g H2 SO4 and mercury salts (0.442 g of Hg2 SO4 and 0.438 g of HgSO4 ) followed by an addition of 53.52 g of CF3 CCl3 and 25.16 g of SO3 , the prepared mixture was heated at 333.15 K for 6 h. The mixture of mercury salts after the reaction was analysed as follows: the crude mixture was cooled to room temperature and filtered to give white and black insoluble catalyst, washed three or four times with sulphuric acid (98 %), dissolved in diluted nitric acid (1 M), excessive solid sodium hydroxide was used to neutralise nitric acid and the solution was centrifuged. The KI method and the Mohr method were used to detect the content of mercury(I) sulphate (House & House, 2010) and chlorine (Christian, 2003), respectively. To calculate the reaction mechanism theoretically, the single factor method was also used to determine the process parameters affecting the reactions. The reactions were studied applying the following typical procedures: 38.6 g of SO3 and 52.3 g of SO2 Cl2 were mixed in a 250 mL round-bottom flask equipped with magnetic stirring, a thermometer and a condenser under nitrogen atmosphere conditions; and the flask was heated at 343.15 K for 24 h. After the addition of H2 SO4 (9.1 g) and mercury salts (0.2273 g of Hg2 SO4 and 0.2273 g of HgSO4 ), CF3 CCl3 and SO3 (ϕr = 1.1 : 1) were dropped in at 338.15 K to start the reaction and the reaction mixture was stirred at this temperature for 6 h to yield approximately 81 % of the product. The yield of CF3 COCl was measured by the derivative method (Wei & Zhang, 1990).

Results and discussion Reaction path and geometry parameters

Ea = ∆r H = + nRT

(2)

k T ∆r G= k= exp − h RT

(3)

where ∆r H= is the activation enthalpy of the elementary reaction (J mol−1 ), HTS and HR are the enthalpies of the transition state and the reactant, respectively, (J mol−1 ), Ea is the activation energy (J mol−1 ), n is the number of reactant molecules of the elementary reaction (mol), R is the universal gas constant (J mol−1 K−1 ), T is the reaction temperature (K), k is the Boltzmann constant (J K−1 ), h is the Plank constant (J s−1 ), and ∆r G= is the activation Gibbs free energy of the elementary reaction (J mol−1 ).

The Mohr and KI methods showed that a part of Hg2 SO4 changed to a divalent mercury salt after the reaction. We assumed that it was (HgCl)2 SO4 . The composition change of mercury salts demonstrates that mercury salts are not catalysts, as it will be discussed more in detail later. The existence of a mercury(II) salt containing chlorine confirms the formation of a mercury(I) free radical. According to this finding, it can be assumed that O2 S(OHg)2 is produced via a free radical mechanism initiated by mercury salts (Hg2 SO4 and HgSO4 ). Then, thermodynamic and kinetic data of different reaction routes based on this hypothesis were calculated. Comparing these data it was concluded that the most favourable reaction mechanism is the following



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Fig. 1. UB3LYP/6-31+G(d,p) optimised bond lengths (nm) and bond angles (◦ ) of some reactants, products, intermediates, and transition states.

one:

(10) (4)

(5)

(6) (7)

(8)

where Eqs. (4)–(6) represent the free radical chain initiation, Eqs. (7)–(9) the free radical chain growth, and Eq. (10) represents the free radical chain termination. From the equations above follows that the reaction is initiated by the mercury(I) sulphate free radical resulting from Hg2 SO4 in the presence of H2 SO4 and Hg2 SO4 at temperatures higher than 323.15 K. Therefore, we can postulate that Hg2 SO4 plays the role of an initiator instead of that of an catalyst. The CF3 CCl2 free radical is the key intermediate which recycles the free radical chain growth to maintain the reaction. Geometry parameters of reactants, products, intermediates and transition states in elementary reactions are summarised and given in Fig. 1. Imaginary frequencies of important transition states TSeq4 , TSeq5 , and TSeq6 are –443.35 cm−1 , –365.26 cm−1 , and – 305.08 cm−1 , respectively. Analysis of thermodynamic functions

(9)

As it can be seen from Table 1, the calculated values of standard reaction enthalpy (∆r H Θ ) of most el-


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Table 1. Calculated ∆r H Θ , ∆r GΘ , and ∆r S Θ (at 273.15 K and 101.325 kPa) of equations at the UB3LYP/6-31+G(d,p) level Eq. No. 4 5 6 7 8 9 10

∆r H Θ /(kJ mol−1 )

∆r GΘ /(kJ mol−1 )

−18.31 −25.96 −16.49 −84.24 −64.48 45.74 −181.98

∆r S Θ /(J mol−1 K−1 )

−25.63 −39.95 −31.15 −38.95 −117.15 42.22 −114.73

ementary reactions are lower than 0, except for that of Eq. (9). Therefore, the elementary reaction related to Eq. (9) is an endothermic one and it is promoted by temperature increase. Also, the ∆r H Θ value of Eq. (10) is the most negative, which means that high temperature is harmful to the free radical chain termination. From Table 1, the values of standard reaction enthropy (∆r S Θ ) and ∆r H Θ of Eqs. (7) and (10) are lower than 0 indicating that these reactions can be promoted by entropy increase and enthalpy decrease, and that higher temperatures are harmful to Eqs. (7) and (10). According to the equilibrium state theory, lower value of the standard reaction Gibbs free energy (∆r GΘ ) favours the reaction and the conversion of reaction while the equilibrium is reached (Rogers, 2011). The ∆r GΘ values of Eqs. (4)–(8), and (10) are much lower than that of Eq. (9) which means that they are more likely to occur than Eq. (9). From the reaction path above, it can be concluded that the CF3 CCl2 free radical is the key intermediate formed in Eqs. (5) and (6) during the free radical initiation stage, and in Eq. (9) during the free radical chain growth. Eqs. (5) and (6) are favoured as they are exothermic and spontaneous reactions. Eq. (9) is an endothermic reaction and the formation of CF3 CCl2 cannot take place spontaneously. Therefore, reaction conditions should be set to be beneficial to Eq. (9) so as to increase the rate of the CF3 CCl2 free radical recycle promoting thus the whole reaction. Analysis of kinetic functions Calculated kinetic data of the whole reaction are shown in Table 2. During the free radical initiation stage, the Ea value of Eq. (4) is much lower than those of Eqs. (5) and (6) indicating that Hg2 SO4 can easily turn into the mercury(I) sulphate free radical under heat conditions. The free radical triggers CF3 CCl3 to generate the CF3 CCl2 free radical, which is the rate control step of this stage. Barrier energies of Eqs. (5) and (6) are nearly the same, illustrating that one mole of the mercury(I) sulphate free radical can initiate two moles of CF3 CCl3 , which corresponds to the formation of a mercury(II) salt containing chlorine. So,

24.55 46.94 49.18 −151.93 176.66 11.81 −225.57

Table 2. Calculated ∆r H= , ∆r G= , and Ea (at 273.15 K and 101.325 kPa) of elementary reactions at UB3LYP/631+G(d,p) level Equation ∆r H = /(kJ mol−1 )∆r G= /(kJ mol−1 )Ea /(kJ mol−1 ) 4 5 6 7 8 9

29.90 53.97 57.20 41.68 44.34 99.86

35.79 93.03 96.04 85.33 45.06 138.51

32.38 58.93 62.15 46.64 46.82 104.82

mercury(I) salt (Hg2 SO4 ) plays the role of an initiator instead of that of a catalyst in this mechanism. This result will be verified more specifically by experimental data later. In the free radical growth stage, Eq. (9) overcomes a high activation barrier of about 100 kJ mol−1 which is the highest in the whole reaction path. This demonstrates that the SO2 Cl free radical triggering CF3 CCl3 to regenerate CF3 CCl2 is the rate determining reaction. Considering that its ∆r H Θ value is higher than 0, correspondingly higher temperature is required to promote the entire reaction. Analysis of experimental results The Mohr and KI methods showed that there are 12.18 × 10−5 mol of chlorine in the mercury salts after the reaction, which means that about 6.85 % of the mole number of Hg2 SO4 changes to (HgCl)2 SO4 under specific conditions. The composition change of mercury salts demonstrates that mercury salts are not catalyst as reported in literature (Paucksch et al., 1973; Ruudorufu et al., 1981; Anello et al., 1982; Yoshida et al., 1985). The existence of a mercury(II) salt containing chlorine confirms the formation of a mercury(I) free radical. However, the other free radicals in this system are not easy to be detected because SO3 is very corrosive causing corrosion to the measuring instruments. This is the reason that direct verification of the free radical chain growth is not yet possible. However, the calculated data above offer the most possible path determined theoretically.


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We also attempted to identify the by-product CF3 CCl2 CCl2 CF3 which results from the free radical chain termination. Unfortunately, it has not been detected probably due to the highly exothermic calculated ∆r H Θ value of –181.98 kJ mol−1 of this elementary reaction. This means that the by-product is not easily formed (Lei et al., 2009). Therefore, the concentration of CF3 CCl2 CCl2 CF3 is low and hard to detect. Factors affecting this reaction as determined by the single factor method are: reaction temperature, CF3 CCl3 /SO3 mole ratio, and HgSO4 and H2 SO4 content. Adding a certain amount of SO2 Cl2 as the substrate can significantly improve the yield of the reaction. These findings are consistent with the results reported in literature (Paucksch et al., 1973). Hg2 SO4 is the initiator which triggers the reaction, but it is easily disproportionated into Hg and HgSO4 under heat conditions. So, HgSO4 and H2 SO4 are needed to stop the disproportionation reaction. The mechanism is as follows: Hg2 SO4 Hg + HgSO4

(11)

2Hg + 2H2 SO4 → Hg2 SO4 + SO2 + 2H2 O

(12)

Higher temperature is beneficial for the promotion of Eq. (9) which is the key elementary reaction in the whole reaction path. Apparently, the addition of H2 SO4 and SO2 Cl2 can lead to higher temperatures compensating thus for the negative impact of the low boiling point of CF3 CCl3 and SO3 . But, improperly higher temperature is harmful because most elementary reactions except Eq. (9) are exothermic. Therefore, the CF3 CCl3 /SO3 mole ratio must be optimised; otherwise the temperature of the reaction system will be too high or too low because of the low boiling point of CF3 CCl3 and SO3 . These experimental phenomena can be sufficiently explained by the calculated data.

Conclusions First, the most probable reaction mechanism was calculated applying the DFT theory at the UB3LYP/ 6-31+G(d,p) level. It was postulated that the mechanism is the free radical chain path: Hg2 SO4 is heated to produce a mercury(I) sulphate free radical which causes CF3 CCl3 to form the CF3 CCl2 free radical which reacts with SO3 creating the corresponding CF3 CCl2 OSO2 free radical which decomposes into CF3 COCl and the SO2 Cl free radical. The latter initiates CF3 CCl3 to regenerate the CF3 CCl2 free radical, which is the rate controlling step of the process. Also, mercury(I) salt was found to play the role of an initiator instead of that of a catalyst in the reaction, which can be verified by both calculations and experiments. Thus, the free radical initiation stage can also be identified. Finally, process parameters in industrial appli-

cation were explained theoretically. It was revealed that temperature and the presence of initiators are the key factors for the reaction. The solvent effect was not considered in the calculations, however, calculation data can still provide many qualitative results on the whole reaction mechanism which can be determined by experiments. This paper provides a theoretical basis for further study; more detailed theoretical research has to be done. Acknowledgements. One of the authors (Zhi-Zheng Liu) thanks his fellow Cheng-Jun Liang. United Laboratory of Chemical Reaction Engineering of the Zhejiang University is gratefully acknowledged for the permission to publish the results.

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