CF3I Synthesis Catalyzed by Activated Carbon: A ... - ACS Publications

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CF3I Synthesis Catalyzed by Activated Carbon: A Density Functional Theory Study Yingjie Hu,†,‡ Taiping Wu,§ Weizhou Liu,‡ Liyang Zhang,‡ and Renming Pan*,† †

School of Chemical Engineering, Nanjing University of Science anad Technology, Nanjing 210094, China Nanjing Xiaozhuang University, Nanjing 211171, China § Machinery and Electronics Industries Co., Ltd, Beijing 100000, China ‡

ABSTRACT: A revised reaction mechanism of CF3I synthesis catalyzed by activated carbon is investigated with quantum chemistry methods using density functional theory (DFT). The adsorption configurations of possible intermediates are carefully examined. The reaction pathway and related transition states are also analyzed. According to our calculations, first, the dehydrofluorination of CHF3 is catalyzed by −COOH groups, which possesses the highest barrier and is accordingly identified as the rate-determining step. Second, the difluorocarbene disproportionation over graphite (001) surface proceeds instead of dimerization. The next reaction steps involving the association of fluoromethine and trifluoromethyl, the fluorine abstractions between intermediates and the iodine abstractions by the desorbed CF3 and CF2CF3 from molecular iodine are also feasible over graphite (001) surfaces. It is also found that the coke deposition in experiments is due to the fluorine abstraction from fluoromethine. This revised mechanism is in agreement with available experimental data and our theoretical computations.

1. INTRODUCTION

Currently a continuous vapor-phase catalytic process for transformation of CHF3 into trifluoroiodomethane (CF3I) has successfully been developed by reacting CHF3 with I2 in the presence of a catalyst.7 As a promising replacement for halon and other halohydrocarbons in the application of fire extinguishing agent, freezing medium, and etching gas, CF3I has a weak C−I bond, which makes it chemically and photochemically active, and possess a short atmospheric lifetime and low GWP.8 Nagasaki et al. have studied the mechanism for CF3I synthesis on the vapor-phase catalytic reaction and suggested that the metal salts catalyzed the dehydrofluorination of CHF3 to produce CF2 carbene, and the supporter of activated carbon (AC) catalyzed the disproportionation of CF2 carbene to afford CF3 radicals, then the CF3 radicals reacted with I2 to give CF3I.9 Recently, Yang et al. have done a further investigation on the CF3I synthesis mechanism.10−12 In their experiments, CF3I was formed in the reaction of CHF3 and I2 only in the presence of AC without other catalyst components, and they proposed that AC is

Global warming is one of the gravest issues concerning the earth’s environment. Although a large part of global warming is thought to have been caused by an increased concentration of CO2 gas in the atmosphere, trifluoromethane (CHF3, HFC-23), which is the major byproduct during the manufacture of HCFC22 (CHClF2), may have also contributed. It was reported that the global warming potential (GWP) of CHF3 is roughly 11,700 times as high as that of CO2.1 CHF3 has limited applications, generally for refrigeration, dry etching in the semiconductor industry, or fire inhibition agent. Its emission is now being regulated by the Kyoto Protocol, and its recycling is urgently needed. At the present time, many routes for the treatment of CHF3 have been developed: destroy CHF3 by thermal oxidation,2 catalytically destruct CHF3 by phosphates3 and ZrO2−SO4,4 and pyrolyze CHF3 into CO2 and HF by plasma technology.5,6 A major problem associated with these methods is that fluorine is simply removed as HF. It would be environmentally more sustainable and economically more viable to utilize waste HFC23, rather than simply destroying it. © 2014 American Chemical Society

Received: October 12, 2013 Revised: December 28, 2013 Published: February 3, 2014 1918

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fixed and the other layer is relaxed with a vacuum region of approximately 16 Å. The k-points set for such a supercell was 3 × 3 × 1 during all the calculations. All computed stationary points and transition states on the potential energy surface were confirmed by frequency analysis and minimum energy pathway (MEP) search based on nudged-elastic band (NEB) algorithm23 in Dmol3 package.

not only the catalyst supporter to form CF3I, but also a cocatalyst to promote the formation of CF2 carbene and the disproportionation of CF2, as illustrated in Scheme 1. However, by comparing Scheme 1. Proposed Mechanism for the Formation of CF3I and Byproducts

3. RESULTS AND DISCUSSION According to the experimental results, we propose a revised reaction mechanism that is primarily composed of 1−14 steps, as illustrated in Scheme 2. Scheme 2. Revised Mechanism for the Formation of CF3I and Byproducts

the pyrolysis of CHF3 over AC treated in HNO3 with that over AC treated in H2, Wenfeng Han et al. suggested that the oxygen groups on surface of carbon may play a major role for the decomposition of CHF3.1 However, CF2 carbene, the most important intermediate for CF3I synthesis, has not been observed directly in the above experiments. The CF2 disproportionation mechanism is still controversial. The mechanisms of formation of the products (CF3I, CF4, C2F6, C3F8, C2HF5, and C2F5I) in synthesis of CF3I over AC have not been experimentally or theoretically clarified in detail. In the present work, a revised reaction mechanism is proposed and is theoretically investigated with the DFT method. Our purposes here are (1) to elucidate the reaction mechanism and (2) to provide detailed knowledge of characteristic features of this reaction.

2. COMPUTATIONAL METHODS DFT calculations with the GGA-PBE13 functional were performed with Materials Studio Dmol314 from Accelrys. The wave function was expanded in terms of numerical basis sets of double numerical quality (DNP15) with d-type polarization functions on each atom. DNP includes a polarization p-function on all hydrogen atoms, which accurately describes the hydrogenbonded system.16 The core electrons for the palladium atoms here were modeled using effective core pseudopotentials (ECP) by Dolg17 and Bergner,18 which explicitly treat scalar relativistic corrections. All calculations were performed spin unrestricted. The real space cutoff radius was 0.37 nm. The convergence tolerance for SCF was 1.0 × 10−6 Ha. Those of geometry optimization for energy and maximum force were 1.0 × 10−5 Ha and 0.002 Ha/Å, respectively. A transition state (TS) between two immediate stable structures was first identified by linear synchronous transit19 and then cyclically refined by quadratic synchronous transit and conjugate gradient methods. Each TS was converged within 0.002 Ha/Å. AC is believed to be composed of tiny graphite-like platelets with oxygen-containing organic functional groups, which are located mostly at the edges of broken graphitic ring systems.20−22 −COOH (carboxylic acid group) is the most common oxygencontaining group. In this study, C38H15−COOH is selected for the catalytic dehydrofluorination of CHF3, and graphite (001) surface is selected for the next steps. The graphite (001) surface is represented by the graphite(001)-(5 × 4) surface model in all calculations. The surface was constructed by a periodically repeated two-layer slab of carbon atoms, in which one layer is

First, difluorocarbene (CF2) is formed via the HF elimination from CHF3 catalyzed by the carboxyl groups in AC (step1). Second, the disproportionation of CF2 affords fluoromethine (CF) and trifluoromethyl (CF3) (step 2). Subsequently, the association of CF and CF3 affords tetrafluoroethylidene (CFCF3) (step 3). The fluorine abstractions from CF and CF2 by CF2 and CF3 afford CF3 (steps 2 and 4) and CF4 (steps 6 and 9). CFCF3 and CF2CF3, having similar structure and reaction behavior to CF2 and CF3, respectively,24 abstract fluorine atom from CF and CF2 to produce pentafluoroethyl (CF2CF3) (steps 5 and 8) and C2F6 (steps 7 and 10) respectively. Also, the fluorine abstractions from CF (steps 4−7) produce carbon radicals (C). In the case of I2 existing, the desorbed CF3 and CF2CF3 react with I2 to afford CF3I and CF5I (steps 11 and 12). In this work, this revised reaction mechanism is examined, and the surface adsorption properties of the reaction intermediates on a graphite (001) surface are also studied. All these reaction steps are performed on the graphite (001) surface. 1919

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Figure 1. Calculated reaction pathways and transition state structures for the catalytic dehydrofluorination of CHF3 over C38H15−COOH (step 1) and homogeneous gas phase dehydrofluorination of CHF3. The dark orange, red, white, and light cyan spheres represent carbon, oxygen, hydrogen, and fluorine atoms, respectively. The marked distances are in Å.

3.1. HF Elimination from CHF3. In this section, the optimized structure of C38H15−COOH combined with CHF3, 1, is selected as the initial structure to investigate the dehydrofluorination of CHF3 catalyzed by carboxyl acid group of AC, as shown in Figure 1. For comparation, the homogeneous gas phase dehydrofluorination of CHF3 is also investigated at the same calculation level. In 1, the F1···HCOOH hydrogen bond (2.03 Å) is formed, making the C−F1 bond slightly lengthened from 1.35 Å in free CHF3 to 1.38 Å. Also, the hydrogen atom of CHF3 approaches the oxygen atom of carbonyl group to form hydrogen bond (2.34 Å), due to the remarkable electronegativity of the three fluorine atoms attaching carbon atom. Consistent with these features, the combination energy is −4.38 kcal/mol. Starting from 1, the dehydrofluorination reaction takes place through TS1−2 to afford CF2 carbene and adsorbed HF over carboxyl acid group, 2. In the transition state TS1−2, the C−F1 bond lengthens by 0.90 Å and the F1 atom considerably approaches the H−COOH atom with a distance of 1.44 Å, while the C−H bond little lengthens (1.15 Å) and the F−H distance is still very long (1.45 Å). These features are analogous to that of the transition state in homogeneous dehydrofluorination, TS1′‑2′. As a result, the energy barrier (ΔE*) of the catalystic dehydrofluorination (57.94 kcal/mol) is smaller by 8.06 kcal/mol than that of homogeneous dehydrofluorination, due to the FHcarbonyl hydrogen bonds that compensate the energy destabilization by the C−H bond and primarily by the C−F1 bond weakening. 3.2. Surface Adsorption. We study the surface adsorption of the reaction intermediates involved in the suggested mechanism, including CF2, CF3, CF, CFCF3, CF2CF3, and C. As it should be, the carbon atom is the only adsorption site for all the intermediate radicals over the graphite (001) surface. In physisorption mode, each intermediate binds on several sites of the surface with subequal adsorption energy (less than 1.7 kcal/mol) and the Cintermediate−Cgraphite(001) distances are longer than 3.0 Å. In chemisorption mode, CF2, CFCF3, CF, and carbon radical bind directly over a conjugated π bond (C···C) of graphite surface to form three-membered ring structures (CCC) analogous to difluoro-cyclopropane compound (Figure 2). The bond lengths of CCF2−Cgraphite, CCFCF3−Cgraphite, CCF−Cgraphite, and CC−Cgraphite are all shorter than 1.55 Å, closing to those of cyclopropane, while the C···C bonds lengthen from 1.42 Å in free graphite to about 1.55 Å, closing to the carbon−carbon single bond length (1.54 Å). Because of the strong combination, the graphite (001) surface is considerably distorted. As for CF3 and

Figure 2. Optimized structures of chemisorbed species involved into the CF3I synthesis mechanism. The marked distances are in Å.

CF2CF3, the one and only adsorption site is the top site of the carbon atom with one CCF3−Cgraphite bond (1.62 Å) and one CCF2CF3−Cgraphite bond (1.66 Å) respectively. Also, the graphite (001) surface is considerably distorted. 3.3. Disproportionation of CF2. As indicated by the proposed reaction mechanism, the CF2 carbenes adsorbed over graphite (001) and disproportionate to yield CF and CF3. The optimized structure of two chemisorbed CF2 carbenes on graphite (001), 3, is chosen as the initial geometry for the disproportionation, as shown in Figure 3. One transition state was found, and the only one imaginary frequency of 298i cm−1 was calculated. The eigenvector with this imaginary frequency mainly involves the approach of F1 to C2 and the elimination of 1920

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Figure 3. Calculated reaction pathway and transition state structure for the disproportionation of CF2 (step 2). The marked distances are in Å.

Figure 4. Calculated reaction pathway and transition state structure for the association of CF3 and CF (step 3). The marked distances are in Å.

Figure 5. Calculated reaction pathways and transition state structures for the fluorine abstractions from CF (a) by CF2 (step 4) and (b) by CFCF3 (step 5). The marked distances are in Å.

F1 from C1. In transition state TS3−4, the bond lengths of C1−F1 and C2−F1 are equal to 1.74 Å, and CCC still keep a threemembered ring structure with C1−C1a and C1−C1b bond

shortened by 0.03 Å and 0.06 Å respectively, and C1a−C1b lengthened to 1.62 Å. While the C2F2 carbene significantly moves toward F1 and is located on the top site of C2a atom, with C2−C2b 1921

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Figure 6. Calculated reaction pathways and transition state structures for the fluorine abstractions from CF (a) by CF3 (step 6) and (b) by CF2CF3 (step 7). The marked distances are in Å.

distance considerably lengthened by 0.89 Å, C2−C2a bond slightly lengthened by 0.07 Å, and C2a−C2b bond shortened to 1.50 Å. These features indicate that CCC has been destructed, but the new C2−F1 bond has not been completely formed. As a result, a relatively high energy barrier (ΔE* = 36.82 kcal/mol) is calculated. In the product, 4, CF and CF3 are coadsorbed on the graphite(001) surface with C1−C1a bond (1.61 Å) and C2−C2a bond (1.65 Å), respectively. The calculated reaction energy of −1.38 kcal/mol indicate that this step is thermodynamically feasible. 3.4. Association of CF3 and CF. According to step 3, the CFCF3 intermediate is formed by association of the CF and CF3 species. Starting from 4, the reaction takes place through TS4−5 to afford a chemisorbed CFCF3 on graphite (001), 5 (Figure 4). In TS4−5, only one imaginary frequency of 115i cm−1 is calculated, and its eigenvector mainly involves the approach of C2F3 to C1F and the elimination of C2F3 from C2a. In TS4−5, the C2F3 considerably moves upward from the graphite (001) plane with C2−C2a bond lengthened by 0.57 Å, while the C1F moves toward C1b atom and is located on the top site of C1a···C1b, forming a three-membered ring structure with bond lengths of 1.47 Å (C1−C1a), 1.52 Å (C1−C1b), and 1.66 Å (C1a−C1b). In product 5, the CFCF3 has been formed completely and binds directly over C1a···C1b with bond lengths of 1.53 Å. The calculated heavily exothermic reaction energy (−70.06 kcal/ mol) and moderate energy barrier (18.84 kcal/mol) suggest that this step is kinetically and thermodynamically favorable.

3.5. Fluorine Abstraction from CF. To investigate the fluorine abstraction from CF by CF2 (step 4), we optimize a geometry of chemisorbed CF2 (top side of C2a···C2b) and CF (top side of C1a) on graphite (001) surface, 6, as shown in Figure 5a. From 6, the CF2, approaching CF at a distance of 2.33 Å, moves to the top site of C2a with C2−C2a bond (1.54 Å), and the CF moves to the top site of C1a···C1b with C1−C1a and C1−C1b bonds shortened to 1.50 and 1.59 Å, respectively, which afford a precursor complex, 7. The C1−F1 bond, influenced by the nearer CF2, is slightly lengthened to 1.39 Å. Consistent with these features, the destabilization energy is 18.82 kcal/mol. Starting from 7, the abstraction reaction takes place through TS7−8 to afford chemisorbed carbon radical and CF3 on graphite (001) surface, 8. In TS7−8, only one imaginary frequency of 186i cm−1 is calculated, and its eigenvector mainly involves the approach of F1 to C2 and elimination of F1 from C1. In this transition state, the bond length of C2−F1 lengthens to 1.53 Å and C1−F1 shortens to 1.82 Å, while the geometry of the other parts little change. Consistent with the small geometry changes, the abstraction easily occurs with a much smaller energy barrier of 2.00 kcal/mol. In the product, 8, the chemisorbed CF3 on graphite (001) surface with C2−C2a bond (1.62 Å) has been formed completely. Also, the C1 radical combined with C1a···C1b is generated. This step is thermodynamically favorable with a little exothermic reaction energy of 13.78 kcal/mol. We investigate fluorine abstraction from CF by CFCF3 (step 5), using a model analogous to step 4, as shown in Figure 5b. 1922

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Figure 7. Calculated reaction pathway and transition state structure for the fluorine abstraction from CF2 by CFCF3 (step 8). The marked distances are in Å.

Figure 8. Calculated reaction pathways and transition state structures for the fluorine abstraction from CF2 (a) by CF3 (step 9) and (b) by CF2CF3 (step 10). The marked distances are in Å.

higher barrier of 22.67 kcal/mol and a positive reaction energy of 11.79 kcal/mol. According to step 6, the CF4 and carbon radical are formed through fluorine abstraction by CF3 from CF, as shown in Figure 6a. An optimized configuration of chemisorbed CF and physisorbed CF3 on graphite (001), 12, is chosen as the initial geometry for this reaction step. Starting from 12, the abstraction

Compared with step 4, analogous configuration of reactant (9), transition state (TS10−11), and product (11) are obtained, except for the precursor complex (10). In 10, the CFCF3 is still located on the top side of C2a···C2b due to the steric repulsion. This precursor stage gives rise to a destabilization energy of 6.33 kcal/ mol. From 10 to 11, the abstraction proceeds with a relatively 1923

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CF3 (or CF2CF3) desorbs from graphite (001) surface and then abstracts iodine from molecular iodine to yield CF3I (or CF2CF3I). As shown in Figure 9a, chemisorbed CF3 desorbs

reaction takes place through TS12−13 to afford a combined carbon radical with graphite (001) surface and a free CF4, 13. In TS12−13, only one imaginary frequency of 228i cm−1 is calculated, and its eigenvector mainly involves the approach of F1 to C2 and elimination of F1 from C1. The C2F3 moves toward C1F at a distance of 1.84 Å, while the C1−F1 bond lengthens to 1.59. In the product, 13, the CF4 has been formed completely and departs from the graphite (001) surface by 4.87 Å. Also, the chemisorbed carbon radical has been generated completely. This reaction proceeds with a moderate barrier of 7.22 kcal/mol and a negative reaction energy of −28.92 kcal/mol, which suggests that this step is thermodynamically and kinetically favorable. We model the fluorine abstraction by CF2CF3 from CF (step 7) based on the configuration of step 6, as shown in Figure 6b. Compared to step 6, analogous configuration of reactant (14), transition state (TS14−15), and product (15) are obtained from the DFT calculation. The eigenvector with the only imaginary frequency (329i cm−1) exhibits geometry changes consistent with the fluorine abstraction by CF2CF3 from CF. This step proceeds with a relatively little higher barrier of 11.95 kcal/mol and a relatively less negative reaction energy of −22.66 kcal/mol and is also thermodynamically and kinetically favorable. 3.6. Fluorine Abstraction from CF2. A model analogous to the disproportionation of CF2 is used to investigate the fluorine abstraction from CF2 by CFCF3 (step 8), as shown in Figure 7. The calculation results show that all the geometrical features in this reaction resemble well those in the disproportionation of CF2 carbene (section 3.3). The eigenvector with the only imaginary frequency (262i cm−1) exhibits geometry changes consistent with the fluorine abstraction from CF2 by CFCF3. Compared to the disproportionation of CF2, this step has analogous reaction barrier (36.30 kcal/mol) and small endothermic reaction energy (8.77 kcal/mol). According to step 9, the CF4 and CF are formed through fluorine abstraction by CF3 from CF2. We optimize a configuration of chemisorbed CF2 and physisorbed CF3 on graphite (001), 18, which is chosen as the initial geometry for this reaction step (Figure 8a). In 18, the physisorbed CF3 approaches chemisorbed CF2 on the graphite (001) surface at a distance of 3.21 Å. Starting from 18, the abstraction reaction takes place through TS18−19 to afford an adsorbed CF and a free CF4, 19. In TS18−19, only one imaginary frequency of 149i cm−1 is calculated, and its eigenvector mainly involves the approach of F1 to C2 and elimination of F1 from C1. In this transition state, the CF3 moves toward chemisorbed CF2 at a distance of 1.86 Å, while the C1−F1 bond lengthens to 1.73 Å. In the product, 19, the CF4 has been formed completely and departs from the graphite (001) surface by 4.3 Å. Also, the CF is adsorbed on the graphite (001) surface with CCF−Cgraphite distances of 2.82 and 2.78 Å. This step proceeds with a moderate barrier of 23.41 kcal/mol and a negative reaction energy of −18.41 kcal/mol, which suggests that this step is thermodynamically and kinetically favorable. A similar model to step 9 is used to investigate fluorine abstraction by CF2CF3 from CF2, as shown in Figure 8b. Analogous configurations of reactant (20), transition state (TS20−21), and product (21) are obtained from the DFT calculation. The eigenvector with the only imaginary frequency (300i cm−1) exhibits geometry changes consistent with the fluorine abstraction by CF2CF3 from CF2. Compared to step 9, this step proceeds with a subequal barrier (23.51 kcal/mol) and a less exothermic reaction energy of −12.22 kcal/mol. 3.7. Iodine Abstractions by the Desorbed CF3 and CF2CF3 from Molecular Iodine. According to steps 11−14,

Figure 9. Calculated reaction pathways and transition state structures for the desorption of (a) CF3 (step 11) and (b) CF2CF3 (step 13). The marked distances are in Å.

from graphite (001) to form physisorbed CF3. Our calculations show that this desorption easily proceeds via a transition state TSdesorption of CF3 with a small activation barrier of 10.00 kcal/mol (an imaginary frequency of 132i cm−1) and a much smaller endothermic reaction energy (1.10 kcal/mol). As for the desorption of CF2CF3, similar configuration of TSdesorption of CF2CF3, and activation barrier (an imaginary frequency of 238i cm−1) are obtained from the DFT calculation, as shown in Figure 9b. Subsequently, physisorbed CF3 and CF2CF3 desorb as gaseous CF3 and CF2CF3 with endothermic reaction energies of 1.14 and 1.33 kcal/mol, respectively. These processes have no transition state. The iodine abstractions by CF3 (step 12) and by CF2CF3 (step 12) from molecular iodine in gaseous phase are also simulated, and the results show that these reactions (steps 12 and 14) are spontaneous. 3.8. Energy Barrier and Reaction Energy after Enthalpy Correction. The energies reported above correspond to only electronic energies under 0 K, and as a result, the contributions from the vibrational, rotational, and translational energies were not taken into account. Considering explicitly the temperature dependence of the activation barriers and reaction energies gives a more realistic picture of the catalytic reaction. The reaction barriers and reaction energies were corrected by calculating the enthalpy correction and add to the original electronic energies as shown in Table 1. According to previous study, reactions with a barrier of 21 kcal/mol or less will proceed readily at room temperature.25 Reaction step 1, step 2, and step 8 possess high barriers, indicating that the predicted reactions should be feasible at high 1924

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Table 1. Original and Corrected Barrier of Transition States (kcal/mol) and Reaction Energies (kcal/mol) under Different Temperatures reaction energy corrected under different temperatures

barrier corrected under different temperatures reaction step

original barrier

298.15 K

500 K

825 K

1000 K

original reaction energy

298.15 K

500 K

825 K

1000 K

step 1 step 2 step 3 step 4 step 5 step 6 step 7 step 8 step 9 step 10 step 11 step 13

57.94 36.82 18.84 2.00 22.67 7.22 11.95 36.30 23.41 23.51 11.08 10.73

57.401 37.576 19.35 2.259 23.449 7.572 12.081 37.103 23.178 23.956 11.987 11.656

56.763 37.719 19.34 2.625 24.08 7.949 12.119 37.377 23.059 23.854 12.275 12.078

55.409 37.707 19.217 3.258 24.777 8.577 12.12 37.55 22.825 23.225 12.469 12.674

54.633 37.681 19.159 3.603 25.021 8.919 12.112 37.594 22.73 22.751 12.521 12.996

50.70 −1.38 −70.06 −13.78 11.79 −28.92 −22.66 8.77 −18.41 −12.22 1.10 −0.12

50.636 −1.442 −69.815 −13.87 12.618 −30.314 −23.546 10.347 −20.222 −13.892 0.895 −0.995

50.37 −1.886 −69.76 −13.897 13.454 −31.336 −24.221 11.341 −22.063 −15.735 −0.033 −2.186

49.634 −3.051 −69.525 −13.861 14.624 −32.795 −25.146 12.599 −25.395 −19.071 −1.567 −4.422

49.154 −3.828 −69.325 −13.84 15.162 −33.538 −25.589 13.16 −27.296 −20.973 −2.528 −5.704

As for the other byproducts in Yang’s experiments, C3F8 may be yielded by the combination of CF3 and C2F5. CF3 and C2F5 can easily abstract hydrogen from the carboxyl acid (−COOH),34,35 hydroxyl (−OH),36 aldehyde (−CHO),37,38 benzene,39 and cyclane.40,41 These imply that CHF3 and C2HF5 may be yielded via hydrogen abstraction from the hydrogen containing groups in AC. Notably, the hydrogen abstraction from carboxyl group will eliminate carboxyl acid group in AC. It might be an important reason for the catalyst deactivation since the carboxyl acid groups play an important role in the dehydroflourination of CHF3.

temperatures. The other reaction steps have barriers of less than 25 kcal/mol and will proceed readily in mild conditions. 3.9. Disscussion in Relation to Experimental results. HF elimination of CHF3 having the lowest dissociation energy compared with H elimination or F elimination26 is the initial step of homogeneous gas-phase CHF3 pyrolysis at the temperatures above 1023 K.27,28 Yang’s experiments show that AC can catalytically bring down the pyrolysis temperature to 823 K.10 Furthermore, liquid phase oxidation of AC, increasing mainly the concentration of carboxylic acids,20,29 can raise conversation of CHF3.1 In agreement with these experimental results, our calculations show that the surface COOH group of AC can catalyze the dehydrofluorination of CHF3 (step 1) with the reaction barrier lowered to 57.94 kcal/mol from 66.00 kcal/mol (in homogeneous gas-phase pyrolysis). However, as a product of this dehydrofluorination, CF2 can hardly be captured in the presence of AC by trapping reagents such as hydrogen, 2-methyl2-butene, or itself, in Yang’s experiments.10,11 The calculated strong combination between CF2 carbene and graphite (001) surface gives reason for this experimental phenomenon. Instead, the CF2 disproportionates to CF3 and CF. Owning to much negative reaction heat and moderate energy barrier, the association reaction of CF and CF3 (step 3) proceeds easily to produce CFCF3. CFCF3 can rearrange easily to form tetrafluoroethene30,31 or be captured by carbene trapping reagents. Additionally, in the absence of carbene trapping reagents, the CFCF3 affords perfluoro-2-butenes by carbene dimerization.32,33 However, in Yang’s experiments, no tetrafluoroethene, perfluoro-2-butenes, or relevant captured products were detected.10 These can be interpreted as due to the strong combination of CFCF3 and graphite(001) surface, which is analogous to CF2. Our calculations indicate that the fluorine abstraction from CF by CFCF3 (step 5) or CF2CF3 (step 7) has higher energy barriers than that by CF2 (step 4) or CF3 (step 6), which result in the predomination of CF4 in the pyrolysis of CHF3. These calculation results are in accordance with the experimental result that CF4 is the main product of CHF3 pyrolysis over AC.1,10 Since the iodine abstractions by CF3 and CF2CF3 from molecular iodine in gaseous phase are spontaneous, CF3I as a main product and C2F5I are produced spontaneously in the presence of I2. Consensus exists in the literature that the coke deposition was observed.1,7,9,10 It is considered as the cause of catalyst deactivation. Our calculations indicate that the coke deposition is due to the fluorine abstractions from CF (step 4−7).

4. CONCLUSIONS The revised mechanism of CF3I catalytic synthesis over AC was suggested on the basis of experimental observations and systematically examined using periodic DFT. The calculated adsorption configurations, energy barriers, and transition state structures were used to elucidate the reaction pathway. First, the dehydrofluorination of CHF3 catalyzed by the surface −COOH group produce CF2 with the first highest energy barrier (57.94 kcal/mol). Owing to the strong combination between CF2 and graphite (001) surface, the CF2 dimerization cannot proceed. Instead, the disproportionation of CF2 over AC generates CF3 and CF with the second highest energy barrier (36.82 kcal/mol). Subsequently, the association of CF and CF3 is kinetically and thermodynamically favorable. The fluorine abstraction by CFCF3 or CF2CF3 is more difficult than that by CF2 or CF3. These result in the predomination of CF4 in the pyrolysis of CHF3, and when I2 exits, CF3I is the main product. Furthermore, our study indicated that coke formation is caused by the fluorine abstraction from CF. The revised mechanism is in excellent agreement with available experimental data and our theoretical computations.

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AUTHOR INFORMATION

Corresponding Author

*(R.P.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the support of this work by Doctoral Fund of Ministry of Education of China (Grant No. 1925

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20123219120017) and the Foundation of Nanjing Xiaozhuang University (grant no. 2006NXY18).

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dx.doi.org/10.1021/jp410133v | J. Phys. Chem. A 2014, 118, 1918−1926