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Heterogeneous Catalytic Hydrogenation: Is Double Bond/Surface Coordination Necessary? Franc-oise Delbecq, David Loffreda, and Philippe Sautet* Universite de Lyon, Institut de Chimie de Lyon, Ecole Normale Sup erieure de Lyon and CNRS, 46 All ee d'Italie, 69364 LYON Cedex 07, France
ABSTRACT The hydrogenation of unsaturated molecules is a key reaction in heterogeneous catalysis, with very broad applications in chemistry. The accepted scheme today is that the double bond is first chemisorbed on the catalyst's surface, such as platinum, and that hydrogen is transferred on the adsorbed molecule by a concerted mechanism through a triangular three-member ring transition state. In this Letter, we show that an alternative mechanism is possible, where the double bond is not coordinated with the surface but approaches above the H atom leading to a six-member ring transition state. Such a mechanism is demonstrated from first-principle calculations for the case of butadiene hydrogenation on platinum and on a Pt surface modified by alloying with Sn. The hydrogenation elementary step on an uncoordinated CdC bond of butadiene shows a low activation barrier (∼20 kJ 3 mol-1). The cases where this pathway is globally favorable, with a small energy cost for CdC bond decoordination, are finally discussed. SECTION Surfaces, Interfaces, Catalysis
T
Scheme 1. Mechanism for Ethene Hydrogenation on a Transition-Metal Surfacea
he hydrogenation of unsaturated organic compounds is a textbook reaction in heterogeneous catalysis,1 and it has a wide scope of industrial applications in the synthesis of fine and specialty chemicals and in petroleum or food chemistry. Hence, a detailed knowledge of the mechanism of that reaction is strongly desirable. The hydrogenation of ethylene has served as a classic model. Among the hypotheses proposed over the years, the Horiuti-Polanyi stepwise mechanism2 is now widely admitted. It involves a series of surface-catalyzed atomic hydrogen addition steps on the unsaturated molecule backbone. For the hydrogenation on platinum particles, experimental3,4 and simulation5,6 data propose a preliminary adsorption of ethylene on the catalyst, forming two Pt-C chemical bonds and inducing a carbon hybridization intermediate between sp2 and sp3. The subsequent attack of coadsorbed atomic hydrogen yields a surface ethyl fragment, following a concerted mechanism with the formation of one C-H bond and the simultaneous cleavage of the Pt-C and Pt-H bonds (see Scheme 1). The ethyl surface intermediate is further hydrogenated to ethane in a second step. The specific adsorption mode of the molecule has been the subject of debate. While the most stable adsorption form is di-σ, involving two neighboring Pt atoms on the surface, spectroscopy7 and theory5 showed that, in realistic coverage conditions, the active precursor is the π mode with a lateral interaction of the double bond on a single Pt atom. In the case of ethylene hydrogenation over homogeneous complexes, the C-H bond is also formed at a double bond coordinated to the Pt atom.8,9 This scheme has been directly extended to other unsaturated molecules, such as diolefins or aromatics. It is hence
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a
The molecule is chemisorbed prior to its hydrogenation.
supposed that the double bond to be hydrogenated is initially chemisorbed on the surface and that the C-H bond formation step occurs from a C atom bonded to Pt in a classical three-center transition state (C, Pt, H). The purpose of this Letter is to show that this hypothesis is not always correct. A second pathway exists where the sp2 carbon to be hydrogenated is not interacting with the Pt surface. By considering butadiene as a prototype polyunsaturated molecule, we show from first-principle calculations that this pathway with an uncoordinated CdC bond exists for the second hydrogenation step from an allyl-type molecule to butene on Pt and can even be favored with respect to the coordinated pathway for the first and second steps of butadiene hydrogenation on a Pt surface modified by alloying with Sn. Quantum chemical calculations offer a unique capability to determine transition states for reactive processes. Our calculations are in the framework of density functional theory within the generalized gradient approximation in the implementation of the VASP code (see Supporting Information for Received Date: October 19, 2009 Accepted Date: November 24, 2009 Published on Web Date: December 03, 2009
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Figure 1. Hydrogenation of 2-butenyl on Pt(111) from the adsorbed η3 (R-η3) or partially decoordinated η1 (R-η1) reactant via a three-center transition state (TS-3c) or a six-center one (TS-6c), forming 1-butene in a di-σ (1B-diσ) or π (1B-π) mode, respectively. Energies are indicated in kJ 3 mol-1.
Figure 2. Hydrogenation of butadiene on Pt2Sn/Pt(111) from the adsorbed (BD-σπσ) or partially decoordinated (BD-diσ) reactant, via a three-center transition state (TS-3c) or a six-center one (TS-6c), forming 2-butenyl in an η3 mode (R-η3). Energies are indicated in kJ 3 mol-1.
details). Transitions states are located with the nudged elastic band method, and they are accurately converged with a quasiNewton algorithm and validated with a vibrational analysis. Let us start with the hydrogenation of butadiene on the Pt(111) surface. The multistep mechanism has been explored in details in a previous publication.10 Let us suppose that butadiene has already been hydrogenated at the terminal carbon, forming 2-butenyl, and focus on the second hydrogenation step to form 1-butene. 2-Butenyl is a substituted allyl, and its most stable adsorption proceeds via an η3 π-σ mode (Figure 1, black line). The classical concerted pathway proceeds from that η3 adsorption by formation of a three-center transition state, with a barrier of 70 kJ.mol-1, to form 1-butene in a di-σ coordination. However, another mechanism is possible which counterintuitively starts by the partial decoordination of the allyl moiety to an η1 mode, where one end carbon, sp3-hybridized, interacts with a Pt atom and the CdC double bond to be hydrogenated is free and almost planar (CdC = 1.35 Å, dihedral angle CCCC = 178°) (Figure 1, red line). This partial desorption costs 58 kJ 3 mol-1. The subsequent hydrogenation step on the uncoordinated CdC bond shows a low activation barrier (17 kJ 3 mol-1) and proceeds through a six-center transition state [Pt-HCdC-C-Pt]#, involving two Pt, one H, and the three carbon atoms of the allyl moiety. This TS shows a quasi-linear Pt-H 3 3 3 C configuration (168°), which contrasts with the bent structure for the usual three-center TS (93°). The CdC
bond which receives the H atom is initially sp2-hybridized, not interacting with Pt, and its out-of-plane deformation remains modest in the TS (6-14°). This nonclassical pathway forms 1-butene in a π mode on the surface, which can diffuse to the most stable di-σ mode. The second case deals with the first step of butadiene hydrogenation on a Pt2Sn/Pt(111) surface alloy. Again, two different pathways have been identified. The first one is classical; the two double bonds of butadiene are chemisorbed on the surface, and hydrogen transfer proceeds via a threecenter transition state, involving a C atom initially coordinated to the surface (TS-3c in Figure 2, black line). It shows a barrier of 87 kJ 3 mol-1 and is similar to the one found on Pt(111). However, the butadiene chemisorption energy is weakened on the alloy, as evidenced by desorption experiments and calculations.11-13 Indeed, the cost to decoordinate one CdC double bond and to form a di-σ adsorbed butadiene is reduced from 71 kJ 3 mol-1 on Pt(111) to 47 kJ 3 mol-1 on Pt2Sn/Pt(111). This partially uncoordinated butadiene is very active for hydrogenation, in a pathway similar to the one shown before (Figure 2, red line). Hydrogen approaches below the nonadsorbed sp2 double bond and is transferred via a six-center TS with a small activation energy of 21 kJ 3 mol-1. The total barrier for the process is 68 kJ.mol-1, and hence, this noncoordinated pathway is clearly favored compared to the classical three-center one. The TS is earlier, compared to the one of Figure 1, with a shorter Pt-H and longer C 3 3 3 H bond
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DOI: 10.1021/jz900159q |J. Phys. Chem. Lett. 2010, 1, 323–326
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and a CdC moiety with an almost planar geometry (Table S1, Supporting Information). The Pt-H 3 3 3 C angle is again very open. That pathway for the first hydrogenation of butadiene is however not favored on Pt(111). Since the CdC desorption is more expensive (71 kJ 3 mol-1), the overall barrier, including the hydrogenation step (∼20 kJ 3 mol-1), would be indeed 91 kJ 3 mol-1, 10 kJ 3 mol-1 higher than that of the three-center pathway. On Pt2Sn/Pt(111), the second hydrogenation step from 2-butenyl to 1-butene also preferentially follows a noncoordinated path, with first a transformation from η3 to η1 2-butenyl, which only costs 35 kJ 3 mol-1. The scheme is similar to Figure 1, with a hydrogenation from the η1 2-butenyl occurring through a six-center TS with a barrier of 21 kJ 3 mol-1 and hence a total barrier for the second step of 56 kJ 3 mol-1. The rate-limiting step is then the first hydrogenation with a barrier of 68 kJ 3 mol-1, in very good agreement with the experimental activation energy of 72 kJ 3 mol-1 determined from temperature-programmed desorption on the Pt2Sn/Pt(111) surface alloy.12 The classical pathway from fully chemisorbed butadiene and through a three-center TS leads to a higher overall barrier (89 kJ 3 mol-1). The partial decoordination of unsaturated hydrocarbons hence allows hydrogenation with a low-energy barrier on the CdC bond that is not interacting with the catalyst. The partially coordinated butadiene structures (BD-diσ) or 2-butenyl (R-η1) are metastable minima on the potential energy surface, and hence, they are connected to the fully coordinated mode by a transition state. However the barrier from a partially to a fully coordinated structure is very small (from 3 to 5 kJ 3 mol-1 on Pt2Sn/Pt(111)), and the transition state is very close to the partially coordinated mode. The decoordination barrier of the molecule is hence not significantly larger than the energy difference used above and does not represent a kinetic bottleneck for the reaction. The pathway might be seen as stepwise, with a preliminary desorption of the CdC bond, followed by H transfer from the surface. It is however globally favorable only if the energy cost for decoordination is moderate. There are several ways to reduce that cost. The first one is an internal compensation within the molecule, where the desorption of one double bond is attenuated by the strengthening of the interaction between other carbon atoms on the unsaturated hydrocarbon and the surface. Decoordination of one double bond for butadiene is, for example, clearly less difficult than ethylene desorption. The second way is to decrease the overall adsorption energy by modifying the catalyst surface. The noncoordinated pathway is clearly favored when chemisorption is weak and is expected to dominate on metals such as Au or Ag catalysts. Alloying is another important channel to control the chemisorption strength. Alloying Pt with Sn results in a small charge transfer toward Pt, increasing the Pauli repulsion with the π orbitals of the unsaturated molecule and hence weakening the adsorption. As shown above, this not only modifies the available ensembles of surface Pt atoms but also changes the hydrogenation pathway. Another possibility to weaken the molecular adsorption is by positioning coadsorbates on the surface. A high coverage of
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hydrogen or of the molecule itself significantly weakens the adsorption of the unsaturated hydrocarbons,14,15 hence favoring the noncoordinated mechanism. Finally, molecules such as aldehydes or unsaturated aldehydes, with smaller adsorption energy for the CdO bond, are prone to favor a hydrogenation with a noncoordinated double bond.16,17 The present study therefore questions the need for an activation of the CdC bond by chemisorption on the catalyst as a prerequisite for hydrogenation. In contrast, the barrier of the hydrogenation elementary step is always found to be larger on chemisorbed double bonds (70-90 kJ 3 mol-1) compared to that on free noncoordinated ones (20 kJ 3 mol-1). Chemisorption results in an overall stabilization of all intermediates and TSs, but the balance is in favor of the chemisorbed pathway only for strong adsorption cases. As a simple rule, if the CdC adsorption energy is lower than 50-70 kJ 3 mol-1, the noncoordinated path should be favored. This non-necessary coordination of the olefin for its hydrogenation hence opens new mechanistic insights for catalytic hydrogenation of unsaturated hydrocarbons.
SUPPORTING INFORMATION AVAILABLE Computational details and structures for initial and transition states. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: Philippe.
[email protected].
ACKNOWLEDGMENT The authors thank Dr Fabienne Vigne for helpful discussions. They thank also IDRIS at Orsay, CINES at Montpellier (Project 609), and PSMN at Lyon for CPU time and assistance.
REFERENCES (1) (2)
(3)
(4)
(5)
(6)
(7)
325
Bond, G. C. Metal-Catalyzed Reactions of Hydrocarbons; Springer: New York, 2005. Horiuti, J.; Polanyi, M. Exchange Reactions of Hydrogen on Metallic Catalysts. Trans. Faraday Soc. 1934, 30, 1164– 1172. Zaera, F. On the Mechanism for the Hydrogenation of Olefins on Transition-Metal Surfaces: The Chemistry of Ethylene on Pt(111). Langmuir 1996, 12, 88–94. Mohsin, S. B.; Trenary, M.; Robota, H. J. Infrared Identification of the Low-Temperature Forms of Ethylene Adsorbed on Pt/ Al2O3. J. Phys. Chem. 1988, 92, 5229–5233. Neurock, M.; van Santen, R.A. A First Principles Analysis of C-H Bond Formation in Ethylene Hydrogenation. J. Phys. Chem. B 2000, 104, 11127–11145. Hirschl, R.; Eichler, A.; Hafner, J. Hydrogenation of Ethylene and Formaldehyde on Pt(111) and Pt80Fe20(111): A DensityFunctional Study. J. Catal. 2004, 226, 273–282. Cremer, P. S.; Su, X.; Shen, Y. R.; Somorjai, G. A. Ethylene Hydrogenation on Pt(111) Monitored in Situ at High Pressures Using Sum Frequency Generation. J. Am. Chem. Soc. 1996, 118, 2942.
DOI: 10.1021/jz900159q |J. Phys. Chem. Lett. 2010, 1, 323–326
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(8) (9)
(10)
(11)
(12) (13)
(14)
(15)
(16)
(17)
Thorn, D. L.; Hoffmann, R. J. Olefin Insertion Reaction. J. Am. Chem. Soc. 1978, 100, 2079–2090. Siegbahn, P. E. M. The Olefin Insertion Reaction into a Metal-Hydrogen Bond for 2nd Row Transition-Metal Atoms, Including the Effects of Covalent Ligands. J. Am. Chem. Soc. 1993, 115, 5803–5812. Valcarcel, A.; Clotet, A.; Ricart, J. M.; Delbecq, F.; Sautet, P. Selectivity Control for the Catalytic 1,3-Butadiene Hydrogenation on Pt(111) and Pd(111) Surfaces: Radical versus Closed-Shell Intermediates. J. Phys. Chem. B 2005, 109, 14175–14182. Becker, C.; Haubrich, J.; Wandelt, K.; Delbecq, F.; Loffreda, D.; Sautet, P. Adsorption of Simple Alkenes on Pt(111) and Pt-Sn Surface Alloys: Bond Strength versus Heat of Adsorption. J. Phys. Chem. C 2008, 112, 14693–14695. Zhao, H; Koel, B. E. Hydrogenation of 1,3-Butadiene on Two Ordered Sn/Pt(111) Surface Alloys. J. Catal. 2005, 234, 24–32. A similar weakening of butadiene adsorption with Sn addition was also shown on Pd(111): Breinlich, C.; Haubrich, J.; Becker, C; Valcarcel, A; Delbecq, F.; Wandelt, K. Hydrogenation of 1,3-Butadiene on Pd(111) and PdSn/Pd(111) Surface Alloys under UHV Conditions. J. Catal. 2007, 251, 123–130. Miura, T.; Kobayashi, H.; Domen, K. Density Functional Study of Ethylene Hydrogenation on Pt(111) Surface. J. Phys. Chem. B 2000, 104, 6809–6814. Neurock, M.; Pallassana, V.; van Santen, R. A. The importance of Transient States at Higher Coverages in Catalytic Reactions. J. Am. Chem. Soc. 2000, 122, 1150–1153. Loffreda, D.; Delbecq, F.; Vign e, F.; Sautet, P. Chemo-Regioselectivity in Heterogeneous Catalysis: Competitive Routes for CdO and CdC Hydrogenations from a Theoretical Approach. J. Am. Chem. Soc. 2006, 128, 1316–1323. Loffreda, D.; Delbecq, F.; Vign e, F.; Sautet, P. Fast Prediction of Selectivity in Heterogeneous Catalysis from Extended Br€ onsted-Evans-Polanyi Relations: A Theoretical Insight. Angew. Chem., Int. Ed. 2009, 48, 8978.
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