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Quantum chemical study of copper (II) chloride and the Deacon reaction
Chemical Physics Letters 501 (2011) 215–220
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Quantum chemical study of copper (II) chloride and the Deacon reaction Ibrahim A. Suleiman, John C. Mackie ⇑, Eric M. Kennedy, Marian W. Radny, Bogdan Z. Dlugogorski ⇑ Priority Research Centre for Energy, The University of Newcastle, Callaghan, NSW 2308, Australia
a r t i c l e
i n f o
Article history: Received 15 September 2010 In final form 10 November 2010 Available online 13 November 2010
a b s t r a c t A model for the gas phase oxidation of hydrogen chloride in the presence of copper (II) chloride (the Deacon reaction) has been investigated by the density functional theory. Chlorine (Cl2) is produced by the thermal decomposition of CuCl2 generating Cu2Cl2 which reacts with O2 (3Rg) to form several intermediates and complexes which further react with hydrogen chloride. A key step in the mechanism is the fission of a O–O linkage between two Cu2Cl2 moieties. This step possesses a barrier prohibitive to homogeneous gas phase catalysis but one which is expected to be easily overcome on a copper chloride surface. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Handling of hydrochloric acid (HCl), which results from the use of elemental chlorine (Cl2) in many chemical processes, constitutes a worldwide waste disposal problem. Because of its corrosive nature and the limited markets for hydrochloric acid, throughout the late 19th and the 20th centuries, there has been considerable activity towards the development of a safe and economic process for the recovery of chlorine from hydrogen chloride. In 1875, Deacon [1] developed a route for chlorine recovery involving the gas phase catalytic oxidation of HCl with air or oxygen. The Deacon reaction was considered simple to implement and had only modest thermal requirements. The overall reaction can be represented as
2HClðgÞ þ 1=2O2 ðgÞ ! Cl2 ðgÞ þ H2 OðgÞ
ð1Þ
In Deacon’s invention [1], copper (II) chloride was the preferred catalyst although other copper compounds exhibit similar activity [2]. Subsequently, however, serious limitations in the operation of the Deacon process were discovered. Principally, these are associated with corrosion problems as a consequence of incomplete conversion of HCl [3] and with the CuCl2(s) catalyst rapidly deactivating owing to the emission of Cl2 and ensuing sublimation of Cu2Cl2 at temperatures above 400 °C. Recently, there has been renewed interest in the development of more efficient catalytic systems for the oxidation of HCl to Cl2. These include the MT-Chlor process of Mitsuo Toatsu Chemicals [4,5] which employs a Cr2O3/SiO2 catalyst in a fluid bed reactor ⇑ Corresponding authors. Also at: School of Chemistry, The University of Sydney, Australia (J.C. Mackie). Fax: +61 2 4921 6893. E-mail addresses: [email protected] (J.C. Mackie), Bogdan. [email protected] (B.Z. Dlugogorski). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.11.030
at 350–400 °C and, more recently, the Sumitomo Chemicals process using a RuO2/TiO2 catalyst [6]. Despite the continuing interest in development of the process, there have been very few mechanistic studies of the Deacon reaction. Hisham and Benson [7] investigated the thermodynamics of the reaction over several transition metal oxide surfaces as well as MgO(s) and Al2O3(s). Overall reaction (1) is exothermic with DrH°298 = 13.7 kcal. According to Hisham and Benson [7], initially, HCl(g) interacts with the metal oxide surface to form a metal oxychloride on the surface. In the second stage of the process, the oxychloride is oxidized by O2(g) to liberate gaseous chlorine and regenerate the metal oxide. Hisham and Benson considered CuO(s) as the only metal oxide which could undergo a complete catalytic cycle at temperatures below 700 K. It should be emphasized that no identification of elementary reaction steps was carried out in the above analysis nor does the above thermodynamic analysis explain the efficacy with which copper (II) chloride surfaces produce Cl2 (not withstanding their drawbacks as practical catalysts) compared with the metal oxide surfaces. Recently, however, Lopez et al. [8] have studied the oxidation of HCl(g) over a RuO2 surface both experimentally and theoretically using density functional theory (DFT). From X-ray diffraction and photoelectron spectroscopy, Lopez et al. [8] concluded that no ruthenium oxychloride was detectable on the surface, nor was there significant inclusion of chlorine species into the surface. From DFT computations of the surface, they concluded that, as a consequence of dissociative adsorption of O2, abstraction of a hydrogen atom by O(ads) from adsorbed HCl produces adsorbed OH and Cl. This is followed by the desorption of Cl2 recombined on the surface. Surface combination of two OH(ads) radicals produces H2O(ads) + O(ads). Desorption of water and surface regeneration completes the cycle. The conclusions of Lopez et al. [8] on the mechanism of oxidation of HCl on RuO2 strongly suggest that the mechanism of the
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Figure 1. Optimized structures of stable species.
Deacon reaction over CuCl2 surfaces is qualitatively very different from that taking place over oxide surfaces. It is well known that CuCl2(s) liberates Cl2(g) when heated above 300 °C. Furthermore, it is possible that CuO(s) can be partially converted to surface CuCl2 in the Deacon process. It is therefore important to attempt to unravel the elementary reaction steps in the production of chlorine gas in the presence of copper (II) chloride, as done in the present study. If, indeed, the modus operandi of the Deacon process on copper chloride is different from that on metal oxide surfaces, then this will have important implications for development of improved catalysts for chlorine production. Whilst ultimately the full elucidation of the mechanism requires a detailed surface analysis, a quantum chemical study of the gas phase reaction between HCl, O2 and CuCl2 provides useful mechanistic information on the reaction pathways, intermediates and transition states, elucidating the key differences between copper (II) chloride and ruthenium dioxide as catalysts for oxidation of HCl.
2. Computational method The DFT hybrid functional B3LYP, which employs the three parameter Becke exchange functional, B3 [9] with the Lee–Yang– Parr non-local correctional functional, LYP [10], has been used for all structural optimizations together with the 6-31G(d) basis set [11] for Cl, H and O atoms. The Stuttgart Dresden (SDD) pseudopotential [12] has been used for copper. All calculations were performed with the GAUSSIAN 03 suite of programs [13]. Energy calculations have been made at the B3LYP/6-311+G(3df,2p)// B3LYP/6-31G(d) level of theory for Cl, H and O. Since several of the intermediates, transition states and reaction complexes may be considered to be composites of smaller molecular units, basis set superposition errors (BSSE) require energy corrections. These corrections have been made using the Counterpoise method contained in GAUSSIAN 03. BSS errors ranged from approximately 1.8 to 3.0 kcal mol1. Stationary points located were either minima or transition states (TS) identified by an analysis of the vibrational fre-
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217
Figure 2. Optimized structures of transition states.
quencies where first order saddle points contain one and only one imaginary frequency along the reaction coordinate. IRC calculations have been carried out to confirm that calculated TS connect the reactants and products. For harmonic vibrational frequencies, the program provides the harmonic oscillator-rigid rotor model approximation assuming complete separation of internal energy modes. The reliability of our adopted computational approach for copper chloride compounds has been addressed by several authors [14–16] who tested their computational results against the available experimental geometries. Hargittai et al. [15] compared computed geometries of CunCln species using MP2 and DFT methods (BPW91 and B3LYP), concluding that the B3LYP method gives bondlengths and angles closest to their experimental electron diffraction observations. All atomisation energies, enthalpies of formation, activation barriers, and relative energies are obtained from the calculated total energies for the molecular systems and their constituents at 0 K and at standard conditions (pressure of 1 bar and temperature of 298.15 K). The barriers at 0 K have been evaluated including the zero-point energy.
3. Results and discussion In developing gas-phase potential energy profiles (i.e., potential energy along the reaction path) for the Deacon reaction involving copper (II) chloride, our premise is that Cl2 is produced by the initial thermal decomposition of CuCl2 and, in a series of reactions involving HCl and O2, product H2O is formed and CuCl2 is regenerated. Although the Deacon process involves CuCl2 surfaces, a gasphase analysis described below provides useful insights into chemical species and their formation pathways, explaining the main features of the process and its differences from processes based on RuO2/TiO2 and Cr2O3/SiO2 catalysts. 3.1. Thermal decomposition of copper (II) chloride The first step in the mechanism is the decomposition of copper (II) chloride to form copper (I) chloride and Cl2. We find that this takes place via reaction (2)
2CuCl2 ðgÞ ! Cu2 Cl2 ðgÞ þ Cl2 ðgÞ
ð2Þ
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Reaction (2), found to have an enthalpy of 15.3 kcal at 298 K, is taking place via transition state TS1 with a barrier of 17.1 kcal mol1 above the reactants (at the B3LYP/6-311+G(3df,2p)//B3LYP/ 6-31G(d) level, see Figure 3). Optimized structures of stable molecules are shown in Figure 1 and of transition states in Figure 2. Atomic co-ordinates and key geometrical parameters of all optimized species are given in the Supporting Information. Cu2Cl2 exhibits a cyclic planar optimized structure. Hargittai et al. [15] observed the formation of Cu2Cl2 dimer together with the trimer and tetramer from vaporized copper (I) chloride at 1333 K. From electron diffraction results, they found a bondlength of 2.254 ± 0.011 Å and Cu–Cl–Cu bond angle of 67.3 ± 1.1°. Our computed values of 2.288 Å and 63.6° are in good agreement with the results of their experiments. The potential energy profile for this reaction is shown in Figure 3.
the reaction path, for a triplet surface (multiplicities of all species are available in the Supporting information), is given in Figure 4 for reactions (3), (4a), and (4b). 3.3. Reaction with hydrogen chloride Hydrogen chloride can react with copper oxychloride to form a weak van der Waals complex Cu2OCl2HCl (Cplx1):
Cu2 OCl2 þ HCl ! Cplx1
ð5Þ
The optimized structure of this complex is shown in Figure 1. The complex lies just 0.5 kcal mol1 below the reactants (at 298 K). Cplx1 can then pass through TS3 with a barrier of 1.9 kcal mol1 to form the stable intermediate, IM2 (for structure, see Figure 1) via reaction (6).
Cplx1 ! IM2 3.2. Oxidation of copper (I) chloride Two molecules of Cu2Cl2 react with molecular oxygen to form the intermediate Cu2Cl2O–OCu2Cl2 (IM1) according to 3
2Cu2 Cl2 þ O2 Rg ! IM1
ð3Þ
The optimized structure of IM1 is depicted in Figure 1. In IM1, the distance between oxygen atoms is elongated from 1.215 in O2 to 1.344 Å. Reaction (3) has an exothermicity of 16.0 kcal (at 298 K). Although possessing a relatively weak O–O bond, IM1 does not directly fission into two copper oxychloride molecules. Instead, it undergoes a rearrangement to another intermediate, IM1a, which has approximate C2v symmetry. The barrier for this rearrangement is very small, 0.1 kcal mol1 and the transition state (TS2), depicted in Figure 2, is very reactant-like. IM1a, whose structure is given in Figure 1, lies 3.3 kcal mol1 below IM1 and can be described as a dimer of Cu2OCl2. Fission takes place according to
IM1 ! IM1a
ð4aÞ
ð6Þ
The enthalpy for reaction (6) was found to be 20.3 kcal at 298 K. The final step in the reaction with HCl is the rearrangement of intermediate IM2 to Cu2Cl3OH.
IM2 ! Cu2 Cl3 OH
ð7Þ
Reaction (7) has an exothermicity of 34.2 kcal at 298 K and passes through TS4 and has a barrier of 2.3 kcal mol1 (see Figure 5). The structure of Cu2Cl3OH is given in Figure 1 and those of TS3 and TS4 in Figure 2. The potential energy along the reaction coordinate for the reaction between copper oxychloride and HCl is shown in Figure 5. It may be seen that, overall, this is a highly exothermic process with only small intermediate barriers along the pathway. 3.4. Formation of product H2O Generation of the product, H2O(g), takes place via three steps. The first of these is the formation of a weak van der Waals complex between Cu2Cl3OH and HCl.
Cu2 Cl3 OH þ HCl ! Cplx2 IM1a ! 2Cu2 OCl2
ð4bÞ
Reaction (4) has an overall endothermicity of 56.9 kcal (at 298 K). We have been unable to locate transition states either for reaction (3) or (4b) and it appears that these are barrierless addition and fission reactions, respectively. The potential energy along
ð8Þ
This complex, whose structure is depicted in Figure 1, lies 6.2 kcal mol1 below the reactants (at 298 K) in reaction (8). Complex Cplx2 can undergo intramolecular rearrangement through TS5 to a further complex, Cplx3, via a barrier of only 2.0 kcal mol1 (see Figure 6).
Cplx2 ! Cplx3
ð9Þ
20
15
Cu2Cl2+Cl2
15.4
10
5
2CuCl2 0
0.0 Figure 3. Potential energy along the reaction coordinate for the thermal decomposition of copper (II) chloride (relative to 2CuCl2) at 0 K. CuCl2 and Cu2Cl2 species are shown in Figure 1 and TS1 is shown in Figure 2. TS denotes transition state.
38.1 Relative Energy (kcal/mol)
Relative Energy (kcal/mol)
17.1
2Cu2OCl2
40
TS1
30
20
10
2Cu2Cl2+O2 0
0.0 -10
-20
IM1
TS2
-16.4
-16.3
IM1a -19.7
Figure 4. Potential energy along the reaction coordinate for oxidation of copper (I) chloride (relative to reactants in reaction (3)) at 0 K. CuCl2, Cu2OCl2, IM1 and IM1a species are shown in Figure 1 and TS2 is shown in Figure 2. IM and TS denote intermediate and transition state, respectively.
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Cu2OCl2
Relative Energy (kcal/mol)
0
+ HCl
Cplx1
0.0
-0.2
Table 1 Reaction enthalpies for individual reactions in the gasphase Deacon reaction of CuCl2 calculated at the B3LYP/ 6-311+G(3df,2p)//B3LYP/6-31G(d) level of theory.
TS3 1.7
-10
IM2
-20
-19.9
TS4 -17.6
-30
-40
Cu2Cl3OH
-50
Reaction
DrH298/kcal
2CuCl2 ? Cu2Cl2 + Cl2 2Cu2Cl2 + O2(3Rg) ? IM1 IM1 ? IM1a IM1a ? 2Cu2OCl2 Cu2OCl2 + HCl ? Cplx1 Cplx1 ? IM2 IM2 ? Cu2Cl3OH Cu2Cl3OH + HCl ? Cplx2 Cplx2 ? Cplx3 Cplx3 ? Cu2Cl4 + H2O Cu2Cl4 ? 2CuCl2
15.3 16.0 3.4 60.3 0.5 20.3 34.2 6.2 19.7 10.6 26.1
-54.2 -60
Figure 5. Potential energy along the reaction coordinate for formation of Cu2Cl3OH (relative to reactants in reaction (5)) at 0 K. Cu2OCl2, Cplx1, IM2 and C2Cl3OH species are shown in Figure 1 and TS3 and TS4 are shown in Figure 2. Cplx, IM and TS denote complex, intermediate and transition state, respectively.
be located. From examination of the optimized structure of Cu2Cl4 in Figure 1, it might be concluded that simple concerted fission of two Cu-Cl bonds in the structure would lead to the formation of the two CuCl2 molecules, so that a barrierless fission would seem likely. 3.6. The overall deacon reaction with CuCl2
Cu2Cl3OH + HCl
This step is barrierless and has an endothermicity of 9.6 kcal at (298 K). However, as may be seen from the reaction coordinate diagram in Figure 6, the overall process is exothermic.
Our model for the gas-phase Deacon reaction commences with the formation of Cl2 from thermal decomposition of CuCl2. Examination of the reaction coordinate diagrams of Figs. 3–6 indicates that, with one exception, the overall mechanism involves only modest energy barriers. The exception is reaction (4b) in which a peroxy linkage between two Cu2Cl2 entities is ruptured. This step would be a stumbling-block to a homogeneously catalysed gas phase process. Indeed, it was considered that the Deacon process could take place homogeneously in the gas phase with vaporized copper chloride [17]. However, this reaction was shown to take place heterogeneously on deposited solid copper chloride [18]. Our gas phase modeling therefore identifies the rupture of an O–O bond as the crucial transformation and on a copper chloride surface, unlike the gas phase, this is expected to be a facile process. Therefore, we believe that the prevailing mechanism commencing with a CuCl2 surface comprises loss of Cl2 and associated production of surface Cu2Cl2 regions adjacent to which dissociative adsorption of O2 takes place. In unpublished DFT surface studies of adsorption of O2 on copper chloride surfaces carried out in our laboratories, barrierless and exothermic dissociative adsorption of O2 has been computed. Hence, processes analogous to reactions (5)–(11) can take place readily on the modified CuCl2 surface. Concerning the accuracy of our computational approach, at the studied level of theory (B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) for H, O and Cl atoms; SDD pseudopotential for Cu), the overall computed enthalpy for reaction (1) at 298 K is 8.1 kcal mol1 compared with the experimental value of 13.7 kcal mol1. Computed enthalpy changes for individual reactions are given in Table 1. Better agreement with experiment would no doubt result from computations at a higher level of theory. However, we do not expect the qualitative conclusions drawn to be affected by the level of theory used in the present work.
3.5. Regeneration of CuCl2
4. Conclusions
Relative Energy (kcal/mol)
0
0.0
-5
TS5 Cplx2 -5.1
-3.1
-10
Cu2Cl4
-15
+ H2O -15.6
-20
Cplx3 -25
-25.2 -30 Figure 6. Potential energy along the reaction coordinate for H2O formation (relative to reactants in reaction 8) at 0 K. Cu2Cl3OH, Cplx2, Cplx3 and Cu2Cl4 species are shown in Figure 1 and TS5 is shown in Figure 2. Cplx and TS denote complex and transition state, respectively.
The optimized structure of this latter complex is shown in Figure 1 and that of TS5 in Figure 2. The structure of Cplx3 can be construed as that of a complex between H2O and a Cu2Cl4 entity. The enthalpy of the rearrangement (9) is 19.7 kcal at 298 K. Water is finally produced by removal from its complex with Cu2Cl4.
Cplx3 ! Cu2 Cl4 þ H2 O
ð10Þ
In this step, CuCl2 is regenerated via the reaction,
Cu2 Cl4 ! 2CuCl2
ð11Þ
thereby completing the catalytic cycle. This reaction is 26.1 kcal endothermic (at 298 K). No transition state for this reaction could
Gas phase quantum chemical computations indicate that copper chloride catalyses the Deacon reaction in a qualitatively different way from oxide surfaces such as RuO2 or other transition metals. Cl2 arises from decomposition of copper (II) chloride. The computations highlight the breaking of a peroxy linkage between
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two Cu2Cl2 moieties as the crucial barrier to homogeneous gas phase catalysis – a barrier which should easily be overcome on a copper chloride surface. Acknowledgements This research project has been funded by the Australian Research Council and supported by a grant of computing time from the Australian Centre of Advanced Computing and Communications (ac3). I.A.S. thanks the University of Newcastle for a postgraduate research scholarship. J.C.M., M.W.R., E.M.K. and B.Z.D. acknowledge the Australian Research Council (ARC) for support (Project No. DP0988907). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2010.11.030.
References [1] H. Deacon, US Patent 165, 802 (1875). [2] M. Altarawneh, B.Z. Dlugogorski, E.M. Kennedy, J.C. Mackie, Prog. Energy Combust. Sci. 33 (2009) 245. [3] H.Y. Pan, R.G. Minet, S.W. Benson, T.T. Tsotsis, Ind. Eng. Chem. Res. 33 (1994) 2996. [4] H. Itoh, Y. Kono, M. Ajioka, S. Takezaka, M. Katzita, US Patent 4803,065 (1989). [5] T. Kiyoura, Y. Kogure, T. Nagayama, K. Kanaya, US Patent 4822,589 (1989). [6] T. Hibi, T. Okuhara, K. Seki, H. Abekawa, H. Hamamatsu, PCT Int. Appl. (2001) WO2001010550A120010215. [7] M.W.M. Hisham, S.W. Benson, J. Phys. Chem. 99 (1995) 6194. [8] N. Lopez, J. Gomez-Sigura, R.P. Marin, J. Perez-Ramirez, J. Catal. 255 (2008) 29. [9] A.D. Becke, J. Chem. Phys. 7 (1993) 5648. [10] C.T. Lee, W.T. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [11] J.A. Montgomery, J.W. Ochterski, G.A. Petersson, J. Chem. Phys. 101 (1994) 5900. [12] M. Dolg, H. Stoll, H. Preuss, R.M. Pitzer, J. Phys. Chem. 97 (1993) 5852. [13] M.J. Frisch et al., GAUSSIAN 03 revision A.11 ed., GAUSSIAN, Inc., Pittsburgh, PA, 2001. [14] J. El-Bahraoui, J.A. Dobado, J.M. Molina, THEOCHEM 493 (1999) 249. [15] M. Hargittai, P. Schwerdtfeger, B. Reffy, R. Brown, Chem. Eur. J. 9 (2003) 327. [16] C. Hasselgren, S. Jagner, I. Dance, Chem. Eur. J. 8 (2002) 1269. [17] F. Wolf, F. Runge, R. Korn, Z. Anorg. Allg. Chem. 304 (1960) 48. [18] I. Slama, Z. Kodejs, Collect. Czech. Chem. Commun. 34 (1969) 2378.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Quantum chemical study of copper (II) chloride and the Deacon reaction Ibrahim A. Suleiman, John C. Mackie ⇑, Eric M. Kennedy, Marian W. Radny, Bogdan Z. Dlugogorski ⇑ Priority Research Centre for Energy, The University of Newcastle, Callaghan, NSW 2308, Australia
a r t i c l e
i n f o
Article history: Received 15 September 2010 In final form 10 November 2010 Available online 13 November 2010
a b s t r a c t A model for the gas phase oxidation of hydrogen chloride in the presence of copper (II) chloride (the Deacon reaction) has been investigated by the density functional theory. Chlorine (Cl2) is produced by the thermal decomposition of CuCl2 generating Cu2Cl2 which reacts with O2 (3Rg) to form several intermediates and complexes which further react with hydrogen chloride. A key step in the mechanism is the fission of a O–O linkage between two Cu2Cl2 moieties. This step possesses a barrier prohibitive to homogeneous gas phase catalysis but one which is expected to be easily overcome on a copper chloride surface. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Handling of hydrochloric acid (HCl), which results from the use of elemental chlorine (Cl2) in many chemical processes, constitutes a worldwide waste disposal problem. Because of its corrosive nature and the limited markets for hydrochloric acid, throughout the late 19th and the 20th centuries, there has been considerable activity towards the development of a safe and economic process for the recovery of chlorine from hydrogen chloride. In 1875, Deacon [1] developed a route for chlorine recovery involving the gas phase catalytic oxidation of HCl with air or oxygen. The Deacon reaction was considered simple to implement and had only modest thermal requirements. The overall reaction can be represented as
2HClðgÞ þ 1=2O2 ðgÞ ! Cl2 ðgÞ þ H2 OðgÞ
ð1Þ
In Deacon’s invention [1], copper (II) chloride was the preferred catalyst although other copper compounds exhibit similar activity [2]. Subsequently, however, serious limitations in the operation of the Deacon process were discovered. Principally, these are associated with corrosion problems as a consequence of incomplete conversion of HCl [3] and with the CuCl2(s) catalyst rapidly deactivating owing to the emission of Cl2 and ensuing sublimation of Cu2Cl2 at temperatures above 400 °C. Recently, there has been renewed interest in the development of more efficient catalytic systems for the oxidation of HCl to Cl2. These include the MT-Chlor process of Mitsuo Toatsu Chemicals [4,5] which employs a Cr2O3/SiO2 catalyst in a fluid bed reactor ⇑ Corresponding authors. Also at: School of Chemistry, The University of Sydney, Australia (J.C. Mackie). Fax: +61 2 4921 6893. E-mail addresses: [email protected] (J.C. Mackie), Bogdan. [email protected] (B.Z. Dlugogorski). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.11.030
at 350–400 °C and, more recently, the Sumitomo Chemicals process using a RuO2/TiO2 catalyst [6]. Despite the continuing interest in development of the process, there have been very few mechanistic studies of the Deacon reaction. Hisham and Benson [7] investigated the thermodynamics of the reaction over several transition metal oxide surfaces as well as MgO(s) and Al2O3(s). Overall reaction (1) is exothermic with DrH°298 = 13.7 kcal. According to Hisham and Benson [7], initially, HCl(g) interacts with the metal oxide surface to form a metal oxychloride on the surface. In the second stage of the process, the oxychloride is oxidized by O2(g) to liberate gaseous chlorine and regenerate the metal oxide. Hisham and Benson considered CuO(s) as the only metal oxide which could undergo a complete catalytic cycle at temperatures below 700 K. It should be emphasized that no identification of elementary reaction steps was carried out in the above analysis nor does the above thermodynamic analysis explain the efficacy with which copper (II) chloride surfaces produce Cl2 (not withstanding their drawbacks as practical catalysts) compared with the metal oxide surfaces. Recently, however, Lopez et al. [8] have studied the oxidation of HCl(g) over a RuO2 surface both experimentally and theoretically using density functional theory (DFT). From X-ray diffraction and photoelectron spectroscopy, Lopez et al. [8] concluded that no ruthenium oxychloride was detectable on the surface, nor was there significant inclusion of chlorine species into the surface. From DFT computations of the surface, they concluded that, as a consequence of dissociative adsorption of O2, abstraction of a hydrogen atom by O(ads) from adsorbed HCl produces adsorbed OH and Cl. This is followed by the desorption of Cl2 recombined on the surface. Surface combination of two OH(ads) radicals produces H2O(ads) + O(ads). Desorption of water and surface regeneration completes the cycle. The conclusions of Lopez et al. [8] on the mechanism of oxidation of HCl on RuO2 strongly suggest that the mechanism of the
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Figure 1. Optimized structures of stable species.
Deacon reaction over CuCl2 surfaces is qualitatively very different from that taking place over oxide surfaces. It is well known that CuCl2(s) liberates Cl2(g) when heated above 300 °C. Furthermore, it is possible that CuO(s) can be partially converted to surface CuCl2 in the Deacon process. It is therefore important to attempt to unravel the elementary reaction steps in the production of chlorine gas in the presence of copper (II) chloride, as done in the present study. If, indeed, the modus operandi of the Deacon process on copper chloride is different from that on metal oxide surfaces, then this will have important implications for development of improved catalysts for chlorine production. Whilst ultimately the full elucidation of the mechanism requires a detailed surface analysis, a quantum chemical study of the gas phase reaction between HCl, O2 and CuCl2 provides useful mechanistic information on the reaction pathways, intermediates and transition states, elucidating the key differences between copper (II) chloride and ruthenium dioxide as catalysts for oxidation of HCl.
2. Computational method The DFT hybrid functional B3LYP, which employs the three parameter Becke exchange functional, B3 [9] with the Lee–Yang– Parr non-local correctional functional, LYP [10], has been used for all structural optimizations together with the 6-31G(d) basis set [11] for Cl, H and O atoms. The Stuttgart Dresden (SDD) pseudopotential [12] has been used for copper. All calculations were performed with the GAUSSIAN 03 suite of programs [13]. Energy calculations have been made at the B3LYP/6-311+G(3df,2p)// B3LYP/6-31G(d) level of theory for Cl, H and O. Since several of the intermediates, transition states and reaction complexes may be considered to be composites of smaller molecular units, basis set superposition errors (BSSE) require energy corrections. These corrections have been made using the Counterpoise method contained in GAUSSIAN 03. BSS errors ranged from approximately 1.8 to 3.0 kcal mol1. Stationary points located were either minima or transition states (TS) identified by an analysis of the vibrational fre-
I.A. Suleiman et al. / Chemical Physics Letters 501 (2011) 215–220
217
Figure 2. Optimized structures of transition states.
quencies where first order saddle points contain one and only one imaginary frequency along the reaction coordinate. IRC calculations have been carried out to confirm that calculated TS connect the reactants and products. For harmonic vibrational frequencies, the program provides the harmonic oscillator-rigid rotor model approximation assuming complete separation of internal energy modes. The reliability of our adopted computational approach for copper chloride compounds has been addressed by several authors [14–16] who tested their computational results against the available experimental geometries. Hargittai et al. [15] compared computed geometries of CunCln species using MP2 and DFT methods (BPW91 and B3LYP), concluding that the B3LYP method gives bondlengths and angles closest to their experimental electron diffraction observations. All atomisation energies, enthalpies of formation, activation barriers, and relative energies are obtained from the calculated total energies for the molecular systems and their constituents at 0 K and at standard conditions (pressure of 1 bar and temperature of 298.15 K). The barriers at 0 K have been evaluated including the zero-point energy.
3. Results and discussion In developing gas-phase potential energy profiles (i.e., potential energy along the reaction path) for the Deacon reaction involving copper (II) chloride, our premise is that Cl2 is produced by the initial thermal decomposition of CuCl2 and, in a series of reactions involving HCl and O2, product H2O is formed and CuCl2 is regenerated. Although the Deacon process involves CuCl2 surfaces, a gasphase analysis described below provides useful insights into chemical species and their formation pathways, explaining the main features of the process and its differences from processes based on RuO2/TiO2 and Cr2O3/SiO2 catalysts. 3.1. Thermal decomposition of copper (II) chloride The first step in the mechanism is the decomposition of copper (II) chloride to form copper (I) chloride and Cl2. We find that this takes place via reaction (2)
2CuCl2 ðgÞ ! Cu2 Cl2 ðgÞ þ Cl2 ðgÞ
ð2Þ
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Reaction (2), found to have an enthalpy of 15.3 kcal at 298 K, is taking place via transition state TS1 with a barrier of 17.1 kcal mol1 above the reactants (at the B3LYP/6-311+G(3df,2p)//B3LYP/ 6-31G(d) level, see Figure 3). Optimized structures of stable molecules are shown in Figure 1 and of transition states in Figure 2. Atomic co-ordinates and key geometrical parameters of all optimized species are given in the Supporting Information. Cu2Cl2 exhibits a cyclic planar optimized structure. Hargittai et al. [15] observed the formation of Cu2Cl2 dimer together with the trimer and tetramer from vaporized copper (I) chloride at 1333 K. From electron diffraction results, they found a bondlength of 2.254 ± 0.011 Å and Cu–Cl–Cu bond angle of 67.3 ± 1.1°. Our computed values of 2.288 Å and 63.6° are in good agreement with the results of their experiments. The potential energy profile for this reaction is shown in Figure 3.
the reaction path, for a triplet surface (multiplicities of all species are available in the Supporting information), is given in Figure 4 for reactions (3), (4a), and (4b). 3.3. Reaction with hydrogen chloride Hydrogen chloride can react with copper oxychloride to form a weak van der Waals complex Cu2OCl2HCl (Cplx1):
Cu2 OCl2 þ HCl ! Cplx1
ð5Þ
The optimized structure of this complex is shown in Figure 1. The complex lies just 0.5 kcal mol1 below the reactants (at 298 K). Cplx1 can then pass through TS3 with a barrier of 1.9 kcal mol1 to form the stable intermediate, IM2 (for structure, see Figure 1) via reaction (6).
Cplx1 ! IM2 3.2. Oxidation of copper (I) chloride Two molecules of Cu2Cl2 react with molecular oxygen to form the intermediate Cu2Cl2O–OCu2Cl2 (IM1) according to 3
2Cu2 Cl2 þ O2 Rg ! IM1
ð3Þ
The optimized structure of IM1 is depicted in Figure 1. In IM1, the distance between oxygen atoms is elongated from 1.215 in O2 to 1.344 Å. Reaction (3) has an exothermicity of 16.0 kcal (at 298 K). Although possessing a relatively weak O–O bond, IM1 does not directly fission into two copper oxychloride molecules. Instead, it undergoes a rearrangement to another intermediate, IM1a, which has approximate C2v symmetry. The barrier for this rearrangement is very small, 0.1 kcal mol1 and the transition state (TS2), depicted in Figure 2, is very reactant-like. IM1a, whose structure is given in Figure 1, lies 3.3 kcal mol1 below IM1 and can be described as a dimer of Cu2OCl2. Fission takes place according to
IM1 ! IM1a
ð4aÞ
ð6Þ
The enthalpy for reaction (6) was found to be 20.3 kcal at 298 K. The final step in the reaction with HCl is the rearrangement of intermediate IM2 to Cu2Cl3OH.
IM2 ! Cu2 Cl3 OH
ð7Þ
Reaction (7) has an exothermicity of 34.2 kcal at 298 K and passes through TS4 and has a barrier of 2.3 kcal mol1 (see Figure 5). The structure of Cu2Cl3OH is given in Figure 1 and those of TS3 and TS4 in Figure 2. The potential energy along the reaction coordinate for the reaction between copper oxychloride and HCl is shown in Figure 5. It may be seen that, overall, this is a highly exothermic process with only small intermediate barriers along the pathway. 3.4. Formation of product H2O Generation of the product, H2O(g), takes place via three steps. The first of these is the formation of a weak van der Waals complex between Cu2Cl3OH and HCl.
Cu2 Cl3 OH þ HCl ! Cplx2 IM1a ! 2Cu2 OCl2
ð4bÞ
Reaction (4) has an overall endothermicity of 56.9 kcal (at 298 K). We have been unable to locate transition states either for reaction (3) or (4b) and it appears that these are barrierless addition and fission reactions, respectively. The potential energy along
ð8Þ
This complex, whose structure is depicted in Figure 1, lies 6.2 kcal mol1 below the reactants (at 298 K) in reaction (8). Complex Cplx2 can undergo intramolecular rearrangement through TS5 to a further complex, Cplx3, via a barrier of only 2.0 kcal mol1 (see Figure 6).
Cplx2 ! Cplx3
ð9Þ
20
15
Cu2Cl2+Cl2
15.4
10
5
2CuCl2 0
0.0 Figure 3. Potential energy along the reaction coordinate for the thermal decomposition of copper (II) chloride (relative to 2CuCl2) at 0 K. CuCl2 and Cu2Cl2 species are shown in Figure 1 and TS1 is shown in Figure 2. TS denotes transition state.
38.1 Relative Energy (kcal/mol)
Relative Energy (kcal/mol)
17.1
2Cu2OCl2
40
TS1
30
20
10
2Cu2Cl2+O2 0
0.0 -10
-20
IM1
TS2
-16.4
-16.3
IM1a -19.7
Figure 4. Potential energy along the reaction coordinate for oxidation of copper (I) chloride (relative to reactants in reaction (3)) at 0 K. CuCl2, Cu2OCl2, IM1 and IM1a species are shown in Figure 1 and TS2 is shown in Figure 2. IM and TS denote intermediate and transition state, respectively.
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Cu2OCl2
Relative Energy (kcal/mol)
0
+ HCl
Cplx1
0.0
-0.2
Table 1 Reaction enthalpies for individual reactions in the gasphase Deacon reaction of CuCl2 calculated at the B3LYP/ 6-311+G(3df,2p)//B3LYP/6-31G(d) level of theory.
TS3 1.7
-10
IM2
-20
-19.9
TS4 -17.6
-30
-40
Cu2Cl3OH
-50
Reaction
DrH298/kcal
2CuCl2 ? Cu2Cl2 + Cl2 2Cu2Cl2 + O2(3Rg) ? IM1 IM1 ? IM1a IM1a ? 2Cu2OCl2 Cu2OCl2 + HCl ? Cplx1 Cplx1 ? IM2 IM2 ? Cu2Cl3OH Cu2Cl3OH + HCl ? Cplx2 Cplx2 ? Cplx3 Cplx3 ? Cu2Cl4 + H2O Cu2Cl4 ? 2CuCl2
15.3 16.0 3.4 60.3 0.5 20.3 34.2 6.2 19.7 10.6 26.1
-54.2 -60
Figure 5. Potential energy along the reaction coordinate for formation of Cu2Cl3OH (relative to reactants in reaction (5)) at 0 K. Cu2OCl2, Cplx1, IM2 and C2Cl3OH species are shown in Figure 1 and TS3 and TS4 are shown in Figure 2. Cplx, IM and TS denote complex, intermediate and transition state, respectively.
be located. From examination of the optimized structure of Cu2Cl4 in Figure 1, it might be concluded that simple concerted fission of two Cu-Cl bonds in the structure would lead to the formation of the two CuCl2 molecules, so that a barrierless fission would seem likely. 3.6. The overall deacon reaction with CuCl2
Cu2Cl3OH + HCl
This step is barrierless and has an endothermicity of 9.6 kcal at (298 K). However, as may be seen from the reaction coordinate diagram in Figure 6, the overall process is exothermic.
Our model for the gas-phase Deacon reaction commences with the formation of Cl2 from thermal decomposition of CuCl2. Examination of the reaction coordinate diagrams of Figs. 3–6 indicates that, with one exception, the overall mechanism involves only modest energy barriers. The exception is reaction (4b) in which a peroxy linkage between two Cu2Cl2 entities is ruptured. This step would be a stumbling-block to a homogeneously catalysed gas phase process. Indeed, it was considered that the Deacon process could take place homogeneously in the gas phase with vaporized copper chloride [17]. However, this reaction was shown to take place heterogeneously on deposited solid copper chloride [18]. Our gas phase modeling therefore identifies the rupture of an O–O bond as the crucial transformation and on a copper chloride surface, unlike the gas phase, this is expected to be a facile process. Therefore, we believe that the prevailing mechanism commencing with a CuCl2 surface comprises loss of Cl2 and associated production of surface Cu2Cl2 regions adjacent to which dissociative adsorption of O2 takes place. In unpublished DFT surface studies of adsorption of O2 on copper chloride surfaces carried out in our laboratories, barrierless and exothermic dissociative adsorption of O2 has been computed. Hence, processes analogous to reactions (5)–(11) can take place readily on the modified CuCl2 surface. Concerning the accuracy of our computational approach, at the studied level of theory (B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) for H, O and Cl atoms; SDD pseudopotential for Cu), the overall computed enthalpy for reaction (1) at 298 K is 8.1 kcal mol1 compared with the experimental value of 13.7 kcal mol1. Computed enthalpy changes for individual reactions are given in Table 1. Better agreement with experiment would no doubt result from computations at a higher level of theory. However, we do not expect the qualitative conclusions drawn to be affected by the level of theory used in the present work.
3.5. Regeneration of CuCl2
4. Conclusions
Relative Energy (kcal/mol)
0
0.0
-5
TS5 Cplx2 -5.1
-3.1
-10
Cu2Cl4
-15
+ H2O -15.6
-20
Cplx3 -25
-25.2 -30 Figure 6. Potential energy along the reaction coordinate for H2O formation (relative to reactants in reaction 8) at 0 K. Cu2Cl3OH, Cplx2, Cplx3 and Cu2Cl4 species are shown in Figure 1 and TS5 is shown in Figure 2. Cplx and TS denote complex and transition state, respectively.
The optimized structure of this latter complex is shown in Figure 1 and that of TS5 in Figure 2. The structure of Cplx3 can be construed as that of a complex between H2O and a Cu2Cl4 entity. The enthalpy of the rearrangement (9) is 19.7 kcal at 298 K. Water is finally produced by removal from its complex with Cu2Cl4.
Cplx3 ! Cu2 Cl4 þ H2 O
ð10Þ
In this step, CuCl2 is regenerated via the reaction,
Cu2 Cl4 ! 2CuCl2
ð11Þ
thereby completing the catalytic cycle. This reaction is 26.1 kcal endothermic (at 298 K). No transition state for this reaction could
Gas phase quantum chemical computations indicate that copper chloride catalyses the Deacon reaction in a qualitatively different way from oxide surfaces such as RuO2 or other transition metals. Cl2 arises from decomposition of copper (II) chloride. The computations highlight the breaking of a peroxy linkage between
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two Cu2Cl2 moieties as the crucial barrier to homogeneous gas phase catalysis – a barrier which should easily be overcome on a copper chloride surface. Acknowledgements This research project has been funded by the Australian Research Council and supported by a grant of computing time from the Australian Centre of Advanced Computing and Communications (ac3). I.A.S. thanks the University of Newcastle for a postgraduate research scholarship. J.C.M., M.W.R., E.M.K. and B.Z.D. acknowledge the Australian Research Council (ARC) for support (Project No. DP0988907). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2010.11.030.
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