Deactivation mechanism of AuCl₃ catalyst in acetylene hydrochlorination reaction: a DFT study

Abstract

The deactivation mechanism of AuCl₃ catalyst in the reaction of acetylene hydrochlorination was studied by using AuCl₃ dimer model and the density functional theory (DFT) method. Four possible paths for the acetylene hydrochlorination reaction catalyzed by AuCl₃ were illustrated with corresponding transition states. The activation free energies and reaction rate constants of the four paths were also analyzed. It is apparent that when HCl and C₂H₂ coadsorbed on the AuCl₃ dimer, the C₂H₂ was co-catalyzed by HCl and the AuCl₃ dimer to produce C₂H₃Cl and the reaction energy barrier was as low as 23.35 kcal mol⁻¹. If the HCl in the gas phase could not adsorb on the Au site within the set time, the intermediate chlorovinyl was difficult to desorb from the AuCl₃ catalyst as its desorption energy was as high as 41.336 kcal mol⁻¹. As the reaction temperature increased, C₂H₂ became easier to be adsorbed on the AuCl₃ catalyst prior to HCl, which resulted in the side reaction and the rapid deactivation of the AuCl₃ dimer due to the loss of Cl atoms. Our calculations are necessary for us to clearly understand the experimental results, which indicate a great dependence of activity and stability of AuCl₃ catalysts on the HCl : C₂H₂ ratio as well as the temperature.

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1. Introduction

Acetylene hydrochlorination is currently an important industrial process in China for the manufacture of vinyl chloride monomer (VCM) that is polymerized to produce polyvinyl chloride (PVC) plastic. This importance is due to vast domestic coal resources and the relative lack of oil available to China.¹ In the current industrial acetylene hydrochlorination reaction process, acetylene is reacted with anhydrous hydrogen chloride gas over a mercuric chloride (HgCl₂) catalyst supported on activated carbon to give vinyl chloride.² The use of mercuric chloride catalyst results in severe environmental pollution problems because of its strong tendency to sublime during the production of VCM.^3,4 Hence, it is crucial to explore environmentally friendly non-mercury catalysts for acetylene hydrochlorination for sustainable PVC production via the acetylene-based method in China.

In recent decades, many metal complexes have been studied as possible candidates to replace the toxic HgCl₂ catalyst. Hutchings and co-workers^5–9 found that the catalytic activity of the metal complex correlated with the standard electrode potential of the metal cation, and their experimental studies^6,10 confirmed that the catalytic activity of Au³⁺ was the highest among the selected metal complexes, involving Bi³⁺,^11,12 Pd²⁺,¹³ Pt²⁺¹⁴ and Pt⁴⁺.¹⁵. However, the XPS spectra in their experiments showed that Au-based catalysts are easy to deactivate with increasing reaction time due to the reduction of Au³⁺ to Au⁰.^16,17 Conte et al.¹⁷ studied the effect of the second metallic component, including Pd²⁺, Pt⁴⁺, Rh³⁺, Ir³⁺ and Ru³⁺, on the Au-based catalyst, however, the stability of these bimetallic catalysts provided no significant improvement. Wang et al.¹⁸ reported a Au³⁺-Cu²⁺/C catalyst with a stability exceeding 200 h on stream reaction. Dai and co-workers¹⁹ reported that the component of La³⁺ on the Au-based catalyst can inhibit the valence change of gold to improve the stability of the catalyst. Conte et al.²⁰ investigated the effect of the HCl : C₂H₂ ratio on the catalytic activity of Au-based catalysts and suggested that excessive HCl can enhance the activity of Au-based catalysts. Hutchings and coworkers⁷ reported that the deposition of carbonaceous materials on the catalyst is the predominant deactivation process occurring at temperatures lower than 100 °C, while the reduction of Au³⁺ to Au⁰ is the other deactivation pathway at temperatures higher than 120 °C. However, the deactivation mechanism of Au-based catalysts in the acetylene hydrochlorination reaction, which is fundamental to develop highly efficient and environmentally benign non-mercuric catalysts for industrial VCM production, has remained unknown.

With the rapid development of computer technology and quantum chemistry, theoretical studies on the mechanism of the catalytic reaction as well as catalyst deactivation have become an area of great interest.^21–26 Studies at the electronic level via density functional theory (DFT) are considered valuable in order to provide theoretical guidance for catalyst design.^27–29

In this article, a systemic study of the catalytic mechanism of AuCl₃ in the reaction of acetylene hydrochlorination was carried out by using DFT. A better understanding of the deactivation mechanism of the AuCl₃ catalyst will not only help to improve the performance of Au-based catalysts, but also shed light on the development of efficient industrial non-mercury catalysts for the reaction of acetylene hydrochlorination.

2. Computational methods

All calculations were carried out with the Gaussian 03 program package.³⁰ The geometrical optimizations of the reactants, products, intermediates and transition states were performed using Beeke’s three-parameter exchange functional³¹ and the nonlocal correlation functional of Lee, Yang, and Parr³² (B3LYP) with the 6-31G(d) basis set for all atoms except for Au, which has been described with Lanl2dz pseudo-potential basis set. Harmonic vibrational frequency calculations were performed at the same level in order to confirm the various stationary points to be either a minimum or a transition structure (TS). Intrinsic reaction coordinate (IRC)^33,34 calculations were carried out to confirm the connection of each TS to its corresponding reactants and products. All the charge analyses reported were calculated by natural bond orbital (NBO) analysis.^35,36 The discussed energies are relative Gibbs free energies (ΔG_298K). The relative enthalpies (ΔH_298K) and ZPE corrected electronic energies (ΔE_0K) are also provided for reference.

The reaction rate constants (k) of all the discussed reactions were calculated using transition state theory (TST) (equation (1)).^37,38

where h, K_B and R are the Planck, Boltzmann, and ideal gas constants, respectively. T represents the reaction temperature. And ΔG is the activation free energy, which is the free energy difference between the transition state and reactant.

3. Results and discussion

3.1 The adsorption of acetylene and hydrogen chloride on gold trichloride.

The structures of gold chlorides, especially those of trichlorides, differed from the structures of many other metal trichlorides. AuCl₃ forms planar dimeric molecules in both the low-temperature gas phase and its crystal structure.^39,40 Since the reaction of acetylene hydrochlorination catalyzed by gold trichloride is a gas-solid reaction system, it is more reasonable to use the AuCl₃ dimer structure to study its deactivation mechanism here. According to our calculation result, the dimer of gold trichloride has a planar D_2h-symmetry chloride-bridged structure, which is in agreement with the 5d⁸ structure of Au³⁺. Relativistic effects bring about the contraction of the 6s orbital and the expansion of the 5d orbital. Due to this effect, the 5d orbital is the major contributor to the valence shell. The shapes of these orbitals favor the planar arrangement found in AuCl₃.⁴¹ In the planar structure, the angle of Cl_side-Au-Cl_bridge is 177.6°. The bond lengths of Au-Cl_bridge and Au-Cl_side are 2.467 and 2.333 Å, respectively. The lowest unoccupied molecular orbital (LUMO) of this structure exhibits a large lobe on the six chlorines of the complex, especially on the four side chlorines (shown in Fig. 1).

Fig. 1 The calculated LUMO state for Au₂Cl₆ at the B3LYP/6-31G(d) (Lanl2dz for Au) level.

The electron affinity of the Au³⁺ decreased because of the planar four-coordinate structure. Since the electron affinity of the Cl⁻ was low, it is difficult for C₂H₂ and HCl to form a stable complex structure with the planar AuCl₃ dimer. The favored adsorption structures of the C₂H₂ molecule and the HCl molecule on the AuCl₃ dimer are shown in Fig. 2. They differ from the adsorption of C₂H₂ and HCl on a single AuCl₃.¹⁷E_ad shown in the figure represents the adsorption energy between the reactants and the catalyst. It was calculated using equation (2).

Fig. 2 The adsorption structures of reactant (C₂H₂, HCl, HCl–C₂H₂ complex) on Au₂Cl₆.

E _ad = E_{absorption-state} − (E_reactant + E_catalyst) (2)

There are three different adsorption sites for acetylene on the AuCl₃ dimer, which are denoted as Cl_bridge site, Au_top site and Cl_side site (shown in Fig. 2 (a,b,c)). In the gas phase, the HCl and C₂H₂ can form the complex structure by σ-π coordinate bond. This complex adsorbs onto the AuCl₃ catalyst via the interaction of Au and the Cl from HCl (shown in Fig. 2 (d)). Another coadsorption of HCl and C₂H₂ on AuCl₃ dimer is shown in Fig. 2 (e), in which the AuCl₃ dimer plain is distorted after HCl and C₂H₂ coadsorb on the Cl_side and Au_top of the AuCl₃ dimer. The adsorption energies of these structures are very low, and the distances between the adsorbate and the AuCl₃ dimer are longer than 3 Å, indicating that the molecules of C₂H₂ and the HCl–C₂H₂ complex are physically adsorbed on the AuCl₃ dimer. If the C₂H₂ molecule and HCl–C₂H₂ complex are much closer to the AuCl₃ dimer, they would be activated by the AuCl₃ dimer catalyst and begin to react during the chemical adsorption process. However, our results show that HCl can not be stably adsorbed on the AuCl₃ dimer and be activated efficiently by the catalyst. The adsorption energy of HCl on the AuCl₃ dimer is only 0.30 kcal mol⁻¹.

3.2 Reaction mechanisms of acetylene hydrochlorination over Au₂Cl₆

In order to better understand the deactivation mechanism of the AuCl₃ catalyst in the reaction of acetylene hydrochlorination, the reaction mechanism of acetylene hydrochlorination catalyzed by Au₂Cl₆ was systematically studied. The reaction paths beginning with the HCl and C₂H₂ coadsorption structures (shown in Fig. 2 (d,e)) were denoted as path 1 and path 2, respectively. When the gas phase around the Au catalyst is lacking HCl, the co-absorption of HCl with C₂H₂ on the catalyst surface can not occur, and the acetylene will react with the AuCl₃ catalyst first. When C₂H₂ adsorbed at the Cl_bridge site, it was difficult for C₂H₂ to react with the AuCl₃ dimer because the Cl_bridge atoms were very stable. But the adsorbed C₂H₂ can easily transfer from the Cl_bridge site to the Au_top site and begin to react with the AuCl₃ dimer. The reaction paths beginning with adsorbed C₂H₂ at Au_top site and Cl_side site were denoted as path 3 and path 4, respectively. The reaction mechanism of these paths is discussed in detail below.

3.2.1 Reaction mechanism of path 1. The HCl–C₂H₂ complex forms the coordination structure with the Au center through the interaction between Cl (HCl) and Au. Then, the reaction occurs as shown in Fig. 3. The π electron of C₂H₂ is transferred to Cl 14, resulting in the negative charge of Cl 14 increasing, and the nucleophilicity of Cl 14 is strengthened. After a nucleophilic adsorption between Cl 14 and Au 2, the complex R₁ is formed (shown in Fig. 4). In R₁, the bond lengths of H 13–Cl 14 (HCl) and C 9–C 10 (C₂H₂) are longer than in gas phase, implying that the HCl and C₂H₂ are partially activated. The structure of R₁ can convert into P_1-1 with a transition state TS₁ which has a six-membered ring structure. In TS₁, electrophilic attack of C 10 (+0.06 e) on the Cl 3 (−0.17 e) occurrs, generating a six-membered ring coordinate structure. The π electron on the Au which comes from C₂H₂ transfers back to C₂H₂ through the C 10–Cl 3 bond. This electron transfer process can clearly be deduced by the difference in the positive charge on Au shown in Fig. 4. The activation free energy of this reaction pathway is 29.22 kcal mol⁻¹, and 5.55 kcal mol⁻¹ free energy is released during the reaction process. The product P_1-1 is the adsorption state of the chloride ethylene and Au₂Cl₆. It is noted that the Cl in chloride ethylene comes from the AuCl₃ catalyst rather than HCl, while the Cl in HCl is supplied to the AuCl₃ catalyst to prevent the reduction of Au³⁺ during the reaction. However, the Cl substitution results in the distortion of the AuCl₃ dimer plane. In the final product P_1-2, the chloride ethylene desorbs from the Au₂Cl₆ with 23.81 kcal mol⁻¹ free energy released, and the AuCl₃ dimer catalyst recovers to the planar structure.

Fig. 3 DFT calculated energy surface for C₂H₂ hydrochlorination through path 1.

Fig. 4 The optimized key structures for C₂H₂ hydrochlorination through path 1 (the unit of bond length is Å).

3.2.2 Reaction mechanism of path 2. The reaction path 2 which starting with the coadsorption structure of Fig. 2(e) is shown in Fig. 5 and Fig. 6. The two C atoms of C₂H₂ and Au³⁺ form a compound structure via a π-σ coordination bond. The π electron of C₂H₂ transfers to the unoccupied molecular orbital of Au³⁺, thus the C=C bond is weakened and the bond length stretched from 1.205 to 1.225 Å. The planar structure of the AuCl₃ dimer catalyst is distorted. The Cl 4-Au 2 bond reverses downward and forms an angle of 85.25° with the plane. Meanwhile, the H atom of HCl interacts with the Cl 4 atom of the AuCl₃ dimer to form a H 14–Cl 4 bond (shown in the Fig. 6 R_2-1). The distance of Au 2 and Cl 5 is 3.214 Å, implying that the Au 2–Cl 5 bond is broken. The AuCl₃ dimer catalyst displays a monomer T-shape feature. The complex of R_2-1 can convert into the intermediate P_2-1 with a six-membered ring transition structure TS_2-1. In this reaction process, nucleophilic attack occurs between the adsorbed HCl and C₂H₂. The Cl 13 atom (−0.19 e) from HCl and C 10 atom (+0.05 e) from C₂H₂ begin to interact with each other and their distance is shortened from 4.055 to 2.511 Å in R_2-1 and TS_2-1, respectively. Thus, a six-membeedr ring is formed in TS_2-1, which is similar to that in the TS₁ of path 1. The π electron on the Au is transferred back to C₂H₂ through the six-member ring. A nucleophilic addition reaction finished with producing the intermediate chlorovinyl. Contrary to path 1, the Cl of the intermediate chlorovinyl is supplied by HCl, and then the H 14 combines with Cl 4 from the AuCl₃ dimer to form a new adsorbed HCl molecule on the AuCl₃ dimer. When the chlorovinyl intermediate is produced, the AuCl₃ dimer loses a Cl atom, while a HCl molecule exists before and after the production of chloride vinyl. It seems as though the AuCl₃ dimer acts as a reactant and the HCl acts as a catalyst in this reaction pathway. In the intermediate P_2-1, the Au catalyst becomes a Au₂Cl₅ coordinate structure by losing a Cl atom, but it still maintains its planar structure due to the adsorption of chlorovinyl. The TS_2-1 transition state requires an activation free energy of 23.35 kcal mol⁻¹, and a 36.57 kcal mol⁻¹ free energy is released during this reaction (shown in Fig. 5).

Fig. 5 DFT calculated energy surface for C₂H₂ hydrochlorination through path 2.

Fig. 6 The optimized key structures for C₂H₂ hydrochlorination through path 2 (the unit of bond length is Å).

When the intermediate P_1-2 adsorbs a HCl on the Au 2 site (shown in Fig. 6 R_2-2), it can generate product P_2-2 with a transition state TS_2-2. In R_2-2, the Au 2–Cl 5 bond is broken due to the adsorption of HCl. The Cl of the adsorbed HCl bonds with Au 2 and the bond length is 2.429 Å. In addition, the H atom forms a weak hydrogen bond with the nearby Cl 5. Then, the H 14–Cl 13 bond rotates due to the H 14–C 8 interaction, which forms a four-membered ring in the transition state TS_2-2 structure. In TS_2-2, an electrophilic collision occurs between H 14 (+0.3 e) and C 8 (−0.62 e). The H 14–Cl 13 bond is broken and the H 14–C 8 bond is formed in the product P_2-2. In P_2-2, Cl 13 from HCl bonds to Au 2 and the Au₂Cl₅ recovers to the Au₂Cl₆ coordinate structure. The planar structure of Au₂Cl₆ was distorted after reaction. The activation free energy from R_2-2 to P_2-2 is 22.56 kcal mol⁻¹ and the process is almost thermodynamically neutral (the exergonic energy is only 1.39 kcal mol⁻¹). The distorted dimer Au catalyst is a non-stable structure,⁴¹ and it can convert into the planar structure spontaneously.

3.2.3 Reaction mechanism of path 3. This reaction path starts with the adsorbed structure of C₂H₂ on Au₂Cl₆ shown in Fig. 2 (b), denoted as R₃ here. R₃ can convert into the intermediate P₃ through the transition state TS₃ (shown in Fig. 7 and 8). When the adsorbed C₂H₂ molecule in R₃ moves closer to the AuCl₃ dimer, the C 10 would interact with Cl 4, which causes the Au 2–Cl 4 bond stretched and rotated downward to form an angle of 49.97° with the AuCl₃ dimer plane (shown in Fig. 8 TS₃). In TS₃, nucleophilic attack of Cl 4 (−0.36 e) on the C 10 (+0.02 e) occurs, so the C=C bond is weakened. The distance between C 10 and Cl 4 shortens from 3.734 to 2.745 Å and the bond of Au 2–Cl 4 is stretched to 2.699 Å. The conformation of transition state TS₃ through R₃ needs an activation free energy of 29.84 kcal mol⁻¹, which is larger than that in path 2. In the intermediate product P₃, the Cl 4 from AuCl₃ dimer is added to the C₂H₂ to form the intermediate chlorovinyl, which causes the Au₂Cl₆ to become reduced to Au₂Cl₅. In P₃, the structure of the adsorbed chlorovinyl on Au₂Cl₅, had the same structure feature as the intermediate P_2-1 in path 2. Therefore, it can convert into the final product P_2-2 along the same path shown in Fig. 5, when sufficient HCl adsorbs on the site of Au 2.

Fig. 7 DFT calculated energy surface for C₂H₂ Hydrochlorination through path 3.

Fig. 8 The optimized key structures for C₂H₂ hydrochlorination through path 3 (bond length measured in Å).

3.2.4 Reaction mechanism of path 4. The fourth reaction path for C₂H₂ hydrochlorination is shown in Fig. 9. In the reactant R₄ of path 4 (shown in Fig. 10), the C=C bond of C₂H₂ is slightly stretched from 1.205 to 1.211 Å. When the partly activated C₂H₂ molecule moves close to the AuCl₃ catalyst, the interaction of C 9–Cl 4 is enhanced and C 10 forms a bond with Cl 3. Then, the product of P_4-1 is generated through the transition state TS₄. In the structure of TS₄, the distance C 9–Cl 4 is shortened from 2.956 to 2.031 Å. Meanwhile, electrophilic adsorption between C 10 (+0.05 e) and Cl 3 (−0.38 e) occurs and the C=C bond length of C₂H₂ is further stretched to 1.25 Å. The activation free energy for this path is 23.97 kcal mol⁻¹. In P_4-1, the bond length of C 10–Cl 3 is 1.760 Å, demonstrating that P_4-1 is actually an adsorption structure of 1,2-dichloroethane on Au₂Cl₄. 1,2-Dichloroethane would need a minimum energy of 17.54 kcal mol⁻¹ to be desorbed from Au₂Cl₄. In path 4, the Au catalyst would be reduced because of the loss of Cl.

Fig. 9 DFT calculated energy surface for C₂H₂ hydrochlorination through path 4.

Fig. 10 The optimized key structures for C₂H₂ hydrochlorination through path 4 (bond length measured in Å).

3.3 Dynamic analysis of the four reaction paths

According to transition state theory (TST), the reaction rate constant is related with the activation free energy. It could be calculated by equation 1 described in section 2, which is simplified as the following (equation 3).

k_TST = 2.08 × 10¹⁰Te^−ΔG/RT (3)

Here, the rate constants of the reaction paths discussed in section 3.2 are calculated using equation 3 and the changes of k_TST with reaction temperature are shown in Fig. 11.

Fig. 11 Reaction rate constants versus the reaction temperature via TST calculation.

Among the four reaction paths, path 2 and path 3 are two-step reactions. The results in Fig. 11 show that the rate constant of the second step reaction in paths 2 and 3 is far larger than that of the first step reaction, so the first step reaction is the rate-limiting step in paths 2 and 3. It is noted that both the rate constants of paths 1 and 3 are in a magnitude of 10⁻³, while the rate constants of paths 2 and 4 are in a magnitude of 10⁰. The rate constants of paths 2 and 4 are much larger than those of the paths 1 and 3, indicating that the reaction of acetylene hydrochlorination catalyzed by Au₂Cl₆ mainly occurs through paths 2 and 4. In addition, the reaction rate constant of path 4 is larger than that of path 2. Furthermore, the difference becomes more apparent as the reaction temperature is increased. It is suggested that a higher temperature causes more rapid deactivation of the AuCl₃ catalyst, which is consistent with Hutchings and coworkers’ experimental results.⁷ They reported that the conversion rate of HCl decreased with a corresponding increase in temperature when the temperature is higher than 100 °C.

3.4 Deactivation mechanism of AuCl₃ catalyst during the reaction of acetylene hydrochlorination

The dynamic analysis above shows that the rate constants of paths 2 and 4 are much larger than those of paths 1 and 3. The activation free energy comparison among the four paths gives the same conclusion. The activation free energies of paths 2 and 4 are 23.35 and 23.97 kcal mol⁻¹, respectively, whereas paths 1 and 3 need 29.22 and 29.84 kcal mol⁻¹, respectively. Thus, the reaction of acetylene hydrochlorination catalyzed by AuCl₃ would be much easier to occur via paths 2 and 4 rather than paths 1 and 3. Since the activation free energies of paths 2 and 4 are nearly equal, they become the competitive reaction paths.

When the reaction of acetylene hydrochlorination occurs via path 2, the AuCl₃ catalyst still keeps the Au₂Cl₆ coordinate structure after the reaction and can recover to the initial planar configuration. Therefore, its catalytic activity would not decrease. If the HCl in the gas phase could not adsorb on the Au 2 site efficiently, the second step of path 2 would be more difficult. In this case, chlorovinyl is harder to be desorbed from the catalyst as its desorption energy is as high as 41.336 kcal mol⁻¹, which causes the catalytic activity of AuCl₃ to decrease due to the active Au sites being occupied. Therefore, the sufficient supply of HCl and its efficient adsorption on the Au site is the key to maintain the activity of the AuCl₃ catalyst during the reaction of acetylene hydrochlorination. Our theoretical analysis was supported by the experimental works of Hutchings and co-workers.^7,20 Their results showed that the activity and stability of the AuCl₃ catalyst were sensitive to the ratio of HCl : C₂H₂. When the HCl : C₂H₂ ratio was 1 : 1, the conversion of C₂H₂ decreased by 10% after 2 h and the activity of the AuCl₃ catalyst also decreased. When the HCl : C₂H₂ ratio is 1.5 : 1, the activitiy of the AuCl₃ catalyst increased and the conversion of C₂H₂ was still high after 2 h.²⁰ Hutchings and co-workers also reported that purging the reaction system with HCl gas during the reaction process could improve the activity of the AuCl₃ catalyst. This phenomenon is also well explained by our theoretical results. When HCl gas is added in the reaction system during the reaction process, HCl is adsorbed at the Au 2 site to form R_2-2 (shown in Fig. 5). Then it would react with the adsorbed chlorovinyl through the second step of path 2. Therefore, the additional HCl could help chlorovinyl to be desorbed from the Au site and release the active site, which reactivated the AuCl₃ catalyst.

If the gas phase around the catalyst is lacking in HCl, the reaction occurs through path 4 by adsorbing C₂H₂ and the by-product dichloride ethylene would be generated. What is worse is that the AuCl₃ catalyst is deactivated by losing two Cl atoms. The NBO analysis shows that the charge of Au in the planar AuCl₃ dimer was +0.54 e, while it reduced to +0.41 e in P_4-1, indicating the Au is reduced through path 4. In order to analyze whether the HCl purging method can reactivate the reduced Au₂Cl₄, the adsorption of HCl on Au₂Cl₄ is investigated here and the result is shown in Fig. 12. Although the HCl molecule can adsorb on the Au₂Cl₄, no apparent changes in either the charge or the structure of the Au₂Cl₄ are observed. The result indicates that the Au₂Cl₄ can not be reactivated by the simple purging of HCl once it has formed. If the AuCl₃ catalyst is pretreated by purging HCl gas and the HCl is kept in at an appropriate excess during the reaction process, the reaction via path 4 will be avoided. This is also proved by the experiments of Hutchings and co-workers.²⁰

Fig. 12 The optimized structures for Au₂Cl₄ purged by HCl (bond length measured in Å).

4. Conclusions

In this work, the deactivation mechanism of AuCl₃ catalyst in the reaction of acetylene hydrochlorination has been studied using DFT calculations. Our results show that there are four paths for the acetylene hydrochlorination reaction catalyzed by AuCl₃. The activation energy analysis demonstrated that two of these reaction paths are more likely to occur. One is a main reaction and the other is a side reaction, and they compete with each other due to having similar activation free energy.

In the two reaction paths mentioned above, two different deactivation mechanisms are discussed. In the main reaction, HCl and the AuCl₃ dimer co-catalyze C₂H₂ and the intermediate chlorovinyl is produced. If the HCl in the gas phase could not adsorb on the Au site, chlorovinyl is difficult to desorb from the catalyst by converting to C₂H₃Cl as its desorption energy is as high as 41.336 kcal mol⁻¹. Thus, the catalytic activity of AuCl₃ would be decreased owing to the active Au sites being occupied. The timely and sufficient supply of HCl can release the active site to increase the catalytic activity. In the side reaction, the by-product 1,2-dichloroethane, which further produces other by-products, is generated. The AuCl₃ catalyst deactivates rapidly by losing Cl atoms and the reduced state of catalyst, Au₂Cl₄, is formed and can not be reactivated by the HCl purging method.

The dynamic analysis showed that the side reaction occurs more easily when the reaction temperature is higher than 140 °C. Therefore, the appropriate reaction temperature must be selected in the actual experiment to avoid the side reaction occurring. Furthermore, if the AuCl₃ catalyst—after modification—can efficiently activate a HCl molecule rather than C₂H₂, the side reaction will be suppressed, while the main reaction will be promoted. As a result of this, the deactivation of the AuCl₃ catalyst may be effectively avoided .

Acknowledgements

This work was supported by the Special Funds for Major State Basic Research Program of China (2012CB720300), and NSFC (21176174, 20836005).

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