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J. Phys. Chem. B 2001, 105, 1562-1572
A First-Principle Analysis of Ethylene Chemisorption on Copper Chloride Clusters Matthew Neurock* and Xinyuan Zhang Department of Chemical Engineering, UniVersity of Virginia, CharlottesVille, Virginia 22902-2442
Michael Olken, Mark Jones, Dan Hickman, Ted Calverley, and Robert Gulotty Dow Chemical Company, 1776 Building, Midland, Michigan 48674 ReceiVed: September 16, 2000; In Final Form: NoVember 29, 2000
Ethylene chemisorption on model copper chloride [CuxCly, CuxCly(OH)z] and supported copper chloride [CuxCly(OH)z/Alr(OH)s] clusters was examined using spin-polarized gradient corrected density functional theory. Both the mode and the energy of ethylene chemisorption are affected by the oxidation state, the coordination number, and the ligand field at the Cu center. In addition, the specific location of the vacant site (atop vs in-plane), adsorbate orientation, and support interactions were also found to be important in dictating the strength of the bond between ethylene and Cu. Ethylene weakly physisorbs atop a central Cu atom, in an axial ligand position, with an energy of less than 2 kcal/mol on the fully saturated Cu2+ adsorption site. The more favorable adsorption state is one which contains a defect site within the CuxCly plane. Ethylene strongly chemisorbs at this site, with its CdC bond oriented perpendicular to the CuxCly plane, with an energy of -18.7 kcal/mol. Two intermediate states also exist whereby ethylene chemisorbs less strongly, at -9.3 and -10.9 kcal/mol. These energies are consistent with the ethylene physisorption and chemisorption states reported experimentally at -2.3, -10.3, and -17.5 (to -19.8) kcal/mol.29 The interaction between ethylene and copper follows the classic Dewer-Chatt donation/back-donation model. Hydroxyl ligands at the Cu center act as trans-directing agents for ethylene chemisorption. They increase the binding energy of ethylene if they sit trans to ethylene; otherwise, they weaken the interaction between Cu and ethylene. These interactions are, in general, on the order of 5 kcal/mol. The interaction of the CuxCly complex with the alumina support strongly depends on the nature of the ligands that anchor the cluster to the support. Bridging oxygen atoms form a much stronger bond between the CuxCly complex and the support than the bonds formed by Cl or OH bridges. The alumina support increases the negative charge on the bridging oxygen species, which in turn increases the positive charge on the Cu center. This leads to an increase in the binding energies of the supported Cu centers over the unsupported centers of 3-5 kcal/mol.
Introduction The vinyl chloride monomer (VCM) is currently manufactured in a balanced, three-step process which involves the chlorination of ethylene (i), the dehydrochlorination of ethyl dichloride (EDC) (ii), and the oxychlorination of ethylene (iii), as shown below.
CH2dCH2 + Cl2 f ClCH2-CH2Cl (EDC) ClCH2-CH2Cl f CH2dCHCl + HCl (VCM) (EDC)
(i) (ii)
CH2dCH2 + 2 HCl + 1/2 O2 f ClCH2-CH2Cl + H2O (EDC) (iii) Oxychlorination utilizes the hydrogen chloride that is produced in the dehydrochlorination process to convert ethylene to EDC without the further addition of chlorine. This offers a significant economic benefit in that the HCl that would have gone on to form waste is instead used in the oxychlorination step to form additional EDC. Of the three processes, oxychlorination offers the greatest incentive for improving selectivity and overall yield. Despite more than 30 years of commercial practice, the
mechanism of oxychlorination is still not very well understood. This is due to an inability to characterize the active surface structure. It is generally agreed that oxychlorination involves a redox process in which copper cycles between the Cu2+ and Cu+ states. The literature suggests that the active site involves an isolated CuxCly complex that is anchored to the γ-Al2O3 support.1-7 Potassium chloride is typically used as a promoter,8-10 to enhance selectivity in the reaction. The interface between CuxCly and Al2O3 is thought to play an important role. Little, however, is known about the size of the CuxCly particles that are formed or how they are anchored to the support. Monomer, dimer, and trimer clusters of Cu, as well as bulk phases of CuxCly, have all been suggested as the active Cu ensembles. In addition to the size of the active complex, a number of other factors such as the oxidation state of Cu, the number of ligands, the chemical properties of each ligand, the spatial arrangement of each ligand about the active Cu center, and the properties of the catalyst support are also important in dictating catalyst activity and selectivity. As a first step toward understanding the behavior of Cu in these systems, we examine how each of these factors affects the chemisorption of ethylene, by performing first-principle quantum chemical calculations on model CuxCly clusters. We first review the experimental literature on supported and unsupported copper chloride clusters and the
10.1021/jp003318m CCC: $20.00 © 2001 American Chemical Society Published on Web 01/26/2001
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Figure 1. Model structure of proposed active CuxCly phase supported on Al2O3.13
properties of these materials in order to outline a series of different CuxCly, CuxCly(OH)z, and CuxCly(OH)z/Alr(OH)s cluster models that can be used in this study. Background. Most of the work reported in the literature that concerns oxychlorination of ethylene has focused on catalyst synthesis and characterization, with the aim of correlating structure and composition with reactivity. These characterization results provide the integral input required to construct model Cu clusters that can be used for computational studies. Characterization efforts have focused on identifying the active phase of the fresh catalyst which forms as the result of reaction between cupric chloride and alumina support. Avila and coworkers11 speculated that the active interface is formed by the following reaction between the impregnated cupric chloride and the alumina support:
y Al-OH + x CuCl2 f y Al-Cl + Cux(OH)yCl2
(1)
The bridging hydroxyl groups, oxygen atoms, and/or chlorine atoms that anchor the CuxCly cluster to the Al2O3 support are thought to control the copper’s activity. Avila and co-workers suggested that the catalyst may have weak Cu-Cl bonds. Under reaction conditions, the chlorine atoms may readily react with hydrocarbon surface intermediates. Valle et al.,12 however, presented a different view of the active phase of the CuCl2 catalyst. They extracted the catalyst after reaction using acetone to determine the amount of soluble Cu2+ species. The amount of Cu “fixed” to the surface was then calculated by subtracting the soluble Cu2+ result from the total level of Cu2+. They found a linear relationship between the reaction rate and the concentration of “fixed” Cu2+ species on the catalyst surface. They suggest that during catalyst synthesis, CuCl2 reacts with the hydroxyl groups on the surface of alumina to form “fixed” Cu2+ species. This is thought to be the active Cu-Cl-O-Al2O3 phase:
CuCl2 + Al2O3-OH T Al2O3-O-Cu-Cl + HCl (2) The speculation of an active phase, made by Valle et al., was later confirmed by Garcia and Resasco,9 who found that stable Cu-O-Al surface species were indeed present. Pandey et al.13 proposed that the active phase contains the features that were suggested by both the Avila and Valle models. They concluded that the active phase for the CuCl2-KCl/γ-Al2O3 oxychlorination catalyst contains both CuCl2‚3Cu(OH)2 as the major phase and Cu(OH)Cl as the minor phase. The model that they proposed as the active complex involves Cu2+ fixed to the support via oxygen bridge sites on the Al2O3 surface. Hydroxyl groups and chlorine atoms can also act as the bridging ligands for the Cu2+ centers. A schematic of this structure is shown in Figure 1. In an effort to elucidate the nature of the active sites of the unsupported CuxCly phase, as well the CuxCly/Al2O3 interface,
we examine how different ligands such as chlorine, oxygen, and hydroxyl groups influence the chemical properties of supported and unsupported model copper clusters by using firstprinciple density functional theory (DFT) quantum chemical calculations. In particular, we probe the chemisorption of ethylene on different CuxCly, CuxClyOHz, and CuxClyAlr(OH)s model clusters. Ethylene Chemisorption. Although ethylene chemisorption is an important step in the oxychlorination process, there have been relatively few experimental adsorption studies which characterize the nature of the active surface or chemisorption energies. Hall and co-workers14 examined ethylene chemisorption on CuCl and CuCl2 using gas-adsorption chromatography and found three distinct states which were suggested to be due to a weak physisorption mode on CuCl, an intermediate chemisorption mode, and a strong chemisorption mode at anion vacancy sites. The strong chemisorption sites for ethylene on CuCl2 were speculated to be anion vacancies at the surface.15 Adsorption at these sites is quite strong, with measured energies reported to be between -17.5 and -19.8 kcal/mol. The convention used herein is that negative adsorption energies refer to stable adsorption modes. An intermediate adsorption state was found at -10.3 kcal/mol. On CuCl, they found that the interaction with ethylene was much weaker, with a heat of adsorption of -2.3 kcal/mol. The chemisorption sites for ethylene on CuCl2 were speculated to be anion vacancy surface sites.15 Cai and co-workers15 studied the adsorption of ethylene, oxygen, and HCl on the CuCl2/γ-Al2O3 system. They indicate that the ability of the catalyst to adsorb HCl, ethylene, and oxygen decreases in this same order. The heat of adsorption of ethylene was found to be slightly weaker (-14 kcal/mol) than that found for the anion vacancy site models proposed by Hall et al.29 They suggest that the adsorption sites for HCl reside on the surface of the alumina support, rather than on CuCl2. This was later confirmed by Xie et al.,16 who suggested that the adsorption sites for ethylene involved Cu2+ ions and the nearby vacancies. Computational Details. First-principle density functional theory (DFT) quantum chemical calculations were carried out to optimize the geometric structures for a series of supported and unsupported CuxCly and CuxCly(OH)z cluster models. Calculations were performed in an effort to probe the electronic structure of these complexes and its effect on the chemisorption of ethylene. All calculations were carried out using the Amsterdam density functional theory (ADF) program, developed by Baerends and co-workers.17-21 ADF employs a fragmentoriented approach to solve the n-particle Kohn-Sham equations and to determine the electronic structure. Polyatomic systems are built up from fragments, which in the calculations discussed below were the individual atoms. All calculations performed were spin-polarized with nonlocal gradient corrections for the correlation and exchange energies. The set of Kohn-Sham equations were iteratively solved to converge upon the selfconsistent field and the overall electronic energy of the system.22-24 Analytical derivatives were computed at each SCF cycle and were used to establish the changes in the nuclear coordinates for subsequent geometry optimization steps. The exchange-correlation potential was modeled using the VoskoWilk-Nusair (VWN) functional.25 Nonlocal gradient corrections to the exchange and correlation energies were computed using the functionals developed by Becke26,27 and Perdew,28 respectively. Nonlocal gradient corrections were computed at each iteration within the SCF cycle. The density and energy of each
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SCF cycle were converged to within 1 × 10-3 and 1 × 10-5 au, respectively. Geometries for all structures were optimized to within 1 × 10-3 au. Triple-ζ Slater basis sets which include spin polarization were employed for all atoms. The innermost core electrons on each atom were fixed using frozen core potentials. This involves the 1s electrons on carbon, hydrogen, and oxygen atoms, and up to the 2p electrons for Cu and Al. The spin states for the bare cluster and the adsorbate/cluster were fully optimized for all systems reported herein. The geometry of each cluster was then allowed to fully relax. To more closely mimic the extended Al2O3 surface, an AlrOs model was cut from the surface structure. The dangling oxygen atoms were subsequently capped with hydrogen atoms, which were aligned in the same direction as the truncated Al atoms. The O-H distances were allowed to optimize, but the Al-O-H bond angles and the O-Al-O-H torsional angles were held in a fixed position to avoid the formation of hydrogen bonds. The energy liberated upon ethylene chemisorption was calculated by subtracting the energies of the optimized gasphase ethylene and CuxCly(OH)z/Alr(OH)s cluster from the energy of the optimized adsorbate-cluster complex, as shown in eq 3.
∆EAds ) EC2H4/CuxCly(OH)z/Alr(OH)s - EC2H4 - ECuxCly(OH)z/Alr(OH)s (3) Model Clusters. A series of different model CuxCly(OH)z (x ) 1-3, y ) 1-3, and z ) 1-3) clusters were used to study ethylene chemisorption on unsupported cupric chloride ensembles. We have analyzed the influence of Cu coordination, specific adsorption sites, structural orientation of the CdC bond, and ligand effects. Cupric chloride is known to have a distorted CdI2 structure, which is like a NaCl cubic closest-packed structure with one-half of the Na atoms removed. Alternate layers of metal atoms that lie parallel to the (111) plane are removed from the salt-like structure. This leaves Cl with a pyramidal arrangement with three Cu neighbors and three vacant sites. On each (100) layer, copper ions take on a square-planar structure, which repeats itself in one-dimension to form a CuCl2 chain along the repeat axis. The basic structure is shown in Figure 2 where part a depicts the layered CuCl2 structure, whereas part b provides an atomic representation of the surface. The Cu-Cl bond lengths in the square-planar repeat unit of the (100) plane are 2.3 Å. The Cu-Cl distance between each of the (100) layers is 2.95 Å. The active copper chloride ensemble resides in the (100) plane. This, however, has been debated in the literature for some time now. The weight of the evidence suggests that small supported CuxCly clusters, rather than bulk CuCl2, are active at lower operating temperatures. We cannot, however, rule out bulk CuCl2 as also being active. To understand these systems, we carried out calculations over a series of square-planar CuxCly(OH)z (x ) 1-3) clusters. The clusters examined were composed of the basic monomeric, dimeric, and trimeric CuxCly structures with different ligands attached and with different Cu coordination numbers. Chemisorption on the fully periodic CuCl2 structure was also probed in order to analyze the effects of cluster size. The fully optimized CuxCly chain is depicted in Figure 3. Gradient corrected periodic DFT calculations were carried out to compute the geometric structure of the periodic CuCl2 slab. The predicted intralayer Cu-Cl bond length and interlayer Cu-Cl distances of 2.27 and 2.97 Å agree very well with the experimental values of 2.23 and 2.95 Å.
Figure 2. Bulk CuCl2 crystal structure. (A) A view along the crystal planes which distinguishes the rows of Cu, Cl, and vacancy sites. (B) The atomic structure for CuCl2 which shows the (100) plane, which is a repeating unit of the CuCl2 dimer.
Figure 3. DFT-optimized periodic results for the CuCl2 slab.
Results and Discussion The size of the CuxCly cluster, the coordination number about Cu, the Cu oxidation state, the nature of the ligands, and their specific position and orientation about the metal center can all significantly influence the strength of the metal-adsorbate bond and its reactivity. In addition to the terminal ligands, the bridging ligands that anchor the Cu complex to the alumina support, as well as the properties of the support itself, can govern chemisorption and reactivity. In the sections that follow, we systematically probe the effect of each of these factors on the chemisorption of ethylene. Effect of Cu Coordination Number. The coordination number of the cupric ion has a strong influence on the chemisorption of ethylene. The number of ligands about the central Cu ion for different monomer, dimer, and trimer Cu complexes was varied in order to examine the effect of coordination number on ethylene adsorption. The results of these
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Figure 4. Ethylene chemisorption on monomeric Cu1Cly clusters. The effect of the number of Cu ligands on ethylene chemisorption. All bond lengths are given in angstroms.
Figure 5. Ethylene chemisorption on dimeric Cu2Cly clusters. The effect of the number of Cu ligands. All bond lengths are given in angstroms.
TABLE 1: Effect of Cu Coordination Number on Ethylene Chemisorption on CuxCly Complexes
cluster type
Cu coordination number
Cu oxidation state
C2H4 binding energy (kcal/mol)
C-C bond length (Å)
monomer monomer monomer monomer dimer dimer dimer dimer trimer periodic slab
1 2 3 4 2 3 4 4 4 4
+1 +2 +2 +2 +2 +2 +2 +2 +2 +2
-28.8 -24.6 -14.5 +12.3 (unbound) -25.1 -18.7 -6.3 -4.3
