Oxidation of Silanes to Silanols on Pd Nanoparticles: H2 Desorption

Oct 14, 2013 - Catalysis Research Center, Hokkaido University, N-21, W-10, ... on Pd Nanoparticles: H2 Desorption Accelerated by Surface Oxygen Atom...
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Oxidation of Silanes to Silanols on Pd Nanoparticles: H2 Desorption Accelerated by Surface Oxygen Atom Takashi Kamachi,† Ken-ichi Shimizu,‡,§ Daisuke Yoshihiro,∥ Kazunobu Igawa,∥ Katsuhiko Tomooka,∥ and Kazunari Yoshizawa*,†,§ †

Institute for Materials Chemistry and Engineering and International Research Center for Molecular Systems, Kyushu University, Fukuoka 819-0395, Japan ‡ Catalysis Research Center, Hokkaido University, N-21, W-10, Sapporo 001-0021, Japan § Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, Katsura, Kyoto 615-8520, Japan ∥ Institute for Materials Chemistry and Engineering and Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga 816-8580, Japan S Supporting Information *

ABSTRACT: The oxidation of silane to silanol on the clean and oxygen-covered Pd(111) surface is investigated with periodic density functional theory calculations to gain a better understanding of the effect of surface oxygen atom on Pd nanoparticle catalysts. The calculations confirmed that this catalytic reaction is initiated by the dissociative adsorption of silane on the Pd surface. The resultant silyl group is attacked by a water molecule to form silanol and an H atom on the Pd surface with inversion of configuration at the Si center. An activation energy of 11.3 kcal/ mol is required for the water addition, and the transition state for this step is energetically highest in the entire reaction profile. These computational results are in good agreement with our stereochemical and kinetic studies. The H atoms on the Pd surface inhibit further reaction, and therefore, they should be removed to achieve the catalytic activity experimentally. We found that the role of the surface oxygen atom is to facilitate the desorption of H2 from the Pd surface without the formation of OH and H2O. The introduction of surface oxygen atoms can enhance the catalytic ability of metal nanoparticles for green organic reactions. conditions by reusable heterogeneous catalysts18 would be ideal from an environmental viewpoint. Among various heterogeneous catalysts reported to date,18−22 a carbon-supported Pd nanoparticle catalyst14 developed by one of the present authors shows relatively high activity in the hydration of various silanes to silanols and H2 with a turnover number (TON) of >99000 and a turnover frequency (TOF) of 20000 h−l (eq 1). The catalyst, prepared from readily available materials through simple synthetic procedures, was reusable at least 10 times without any loss of activity, and various silanes were selectively transformed into corresponding silanols. Interestingly, the catalytic activity depends strongly on the coverage of adsorbed oxygen atoms. When the prereduced catalyst was used without exposing it to air, the catalyst showed low activity. However, exposure of the clean metal surface to air (O2) at room temperature led to the formation of the atomic oxygen on the metal surface, and the catalyst showed significantly high activity. Combined with kinetic results, a cooperative mechanism between metallic Pd and the surface oxygen was suggested.14

1. INTRODUCTION The rational design of next-generation catalysts for organic synthesis requires a design concept of multifunctional catalysis1,2 on the basis of fundamental knowledge of the correlation between structure, chemical composition, and reactivity. Several fundamental studies of metal surfaces3−13 have demonstrated that various adatom (X = O, S, or C) preadsorbed metal surfaces show different reactivity from clean surfaces. Recent surface science studies have shown that oxygen atoms on well-defined Au(111) and Ag(111) surfaces act as a basic cocatalyst that promotes various organic reactions.7−12 The preadsorbed X may also change local electronic states of the metal surface.5 Shimizu and co-workers have applied the concept to develop highly effective metal nanoparticle catalysts with surface oxygen adatoms for a wide variety of organic reactions.14−16 Silanols have found widespread use in organic synthesis, particularly in cross-coupling reactions, and in industry as building blocks for silicon-based polymeric materials.17 Among various preparation methods, such as hydrolysis of the corresponding chlorosilanes, treatment of siloxanes with alkali reagent, and oxidation of silanes with oxidants under alkaline or neutral conditions, oxidation of silanes with water under neutral © 2013 American Chemical Society

Received: August 19, 2013 Revised: October 7, 2013 Published: October 14, 2013 22967

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reaction vessel was open to the atmosphere to release the gas (H2) evolved. Conversion of silane and yields of products were determined by GC using n-dodecane as an internal standard, and first-order rate constants for the silanol (Et3SiOH) formation were determined. To determine the stereochemical course of the reaction, Pd/C-100Hox (0.5 mol % Pd with respect to silane) was added to the mixture of optically active (S)-tert-butyl(butyl)phenylsilane (iii)26 (0.169 mmol, 58% ee) and water (0.254 mmol) in a reaction vessel equipped with a condenser, and the mixture was stirred at 333 K for 20 min under argon atmosphere. The isolated yield of tert-butyl(butyl)phenylsilanol (iv),27 thus obtained, was determined after purification by silica-gel chromatography. The enantiopurity and absolute stereochemistry of the silanol iv was determined by HPLC analysis using chiral stationary phase according to the reported method.27

Kershner and Medlin13 presented a surface science study of the reaction of silane on Pd(111). Temperature-programmed desorption studies after the dissociative adsorption of SiH4 on Pd(111) showed an H2 desorption peak around 380−390 K.13 Combined with our previous result14 that the hydration of silanes at 308 K by oxygen-preadsorbed Pd nanoparticle catalyst resulted in the formation of silanols with a stoichiometric amount of H2, it is deduced that an important role of the preadsorbed atomic oxygen may be a promotion of H2 formation at low temperature. In this work, we performed a periodic slab DFT investigation of the oxidation of silanes to silanols on clean and oxygen-covered Pd(111) surfaces to propose a reaction mechanism. We found that the formation of H2 is facile at a higher coverage of the surface oxygen atom, and therefore, the removal of Pd−H species by molecular oxygen is unnecessary in the oxygen-adsorbed Pd nanoparticle catalyst.

3. RESULTS AND DISCUSSION 3.1. Dissociative Adsorption of Trimethylsilane on the Pd(111) Surface. The dissociative adsorption of silanes on metal surfaces has been widely studied.13,28,29 Kershner and Medlin13 studied SiH4 adsorption and decomposition on the Pd(111) surface with HREELS, TPD, and AES methods. SiH4 was found to adsorb dissociatively on the Pd surface at 150 K, resulting in SiH3. Jeon et al.18 observed the partial hydrogenation of the alkynyl group as a side reaction in the oxidation of alkynylsilanes by a Pd nanoparticle catalyst. This implies that the Si−H bond of silane is cleaved to produce an H atom and a silyl group on the Pd surface. Thus, we assume that the dissociative adsorption of silanes takes place in the initial stage of the reaction. Figure 1 shows a potential-energy curve for the

2. COMPUTATIONAL AND EXPERIMENTAL METHODS All calculations were performed with the DMol3 program23 in Material Studio of Accelrys Inc. Perdew−Burke−Ernzerhof (PBE) generalized gradient functional was employed for the exchange-correlation energy. The method applied here is the same as those in our previous work.15 The wave functions were expanded in terms of numerical basis sets. We employed the DND basis set (double numerical basis set with the d-type polarization functions) for geometry optimization. Single-point energy calculations were performed with the larger DNP basis set (double numerical basis set with the d-type polarization functions for heavy atoms and the p-type polarization functions for hydrogen atoms). Brillouin zone integrations are performed on a Monkhorst−Pack24 k-point grid with a k-point spacing of 0.04 Å−1, unless otherwise noted. The transition state was determined using the linear and quadratic synchronous transit (LST/QST) complete search method.25 The Pd nanoparticle was modeled by a supercell slab that consists of a 5 × 5 surface unit cell with three atomic (111) surface layers (lattice constants a = b = 13.8 Å) unless otherwise noted. The average size of the Pd nanoparticles estimated from TEM analysis is 6.1 nm,14 which indicates that a majority of the surface metal atoms are at plane sites. Thus, the Pd nanoparticles are reasonably modeled by the Pd(111) periodic slab. The slab was separated by a vacuum space with a height of 20 Å. The top layer was fully relaxed, whereas the bottom two layers were fixed at the corresponding bulk positions. To confirm the reliability of the selected model, we considered the formation of H2 and OH with extended models (6 × 6 surface unit cell, four surface layers, and k-point spacing of 0.03 Å−1). As summarized in Table S1, the small quantitative differences have no impact on the conclusions of the present paper. Pd-carbon (Pd = 5 wt %) was purchased from Kawaken Fine Chemicals. The Pd/C-100Hox catalyst was prepared by reducing a commercial Pd-carbon under a flow of 100% H2 (flow rate = 60 cm3 mn−1) at 100 °C for 10 min and cooled to room temperature in a flow of He, followed by exposing the powder to air at 25 °C. For catalytic tests, Pd/C-100Hox (0.0083 mol % Pd with respect to silane) was added to the mixture of Et3SiH (6 mmol), water (9 mmol), and dimethoxyethane (6 mL) in a reaction vessel equipped with a condenser, and the mixture was stirred at 278, 283, 288, or 308 K. The

Figure 1. Calculated potential-energy curve and optimized geometries for the dissociative adsorption of trimethylsilane on the Pd(111) surface; distances in Å.

dissociative adsorption of trimethylsilane on the Pd(111) surface. The calculations were performed on a 3 × 3 surface unit cell with three atomic (111) surface layers to reduce the computational cost. The energy was computed by varying the distance between the Si atom of silane and top Pd surface (d = 2.5−5.5). As shown in Figure 1, the dissociative adsorption takes place in a virtually barrierless fashion. The silane molecule is strongly adsorbed on the Pd surface at a distance of 3.1 Å. A computed Si−H distance of 1.584 Å is much longer than that of 22968

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Figure 2. Optimized geometries and computed energies for the addition of a water molecule to the silyl group on the Pd(111) surface. Only important part is displayed for clarity. Relative energies are measured from 1ret; units in Å and kcal/mol.

proceed via backside attack of a water molecule to the silicon on catalyst surface.31

1.5 Å in free silane molecule, which indicates that the Si−H bond is significantly activated at the minimum. The optimized structure of the transition state for the dissociative adsorption has a close resemblance to that of the local minimum, as expected from the very low activation energy of 0.6 kcal/mol. The Si−H dissociation is exothermic by 15.7 and 28.1 kcal/mol measured from the local minimum and the dissociation limit, respectively. These computational results are in good agreement with an observed zero-order rate dependence on the silane concentration.14 The resulting silyl group and the H atom are located on the top and the 3-fold positions, respectively. 3.2. Addition of a Water Molecule to the Silyl Group on the Pd(111) Surface. As described above, the dissociative adsorption of silane precedes the reaction of silane with water to produce silanol. A water molecule would attack the resultant silyl group in the first step of the formation of silanol. To clarify the detailed mechanism, we need to reveal the stereochemical course on silicon at the attack of a water molecule. Sommer’s group30 reported that the reaction of optically active (R)methyl(1-naphthyl)phenylsilane (i) with water in the presence of a common Pd/C afforded (R)-methyl(1-naphthyl)phenylsilanol (ii) with 81% of inversion of configuration at the silicon center (eq 2). This result indicates that the reactions

We also determined the stereochemical course of the presented oxidation using optically active (S)-tert-butyl(butyl)phenylsilane (iii) (58% ee)26 as a substrate. Although the oxidation of (S)-iii with water in the presence of Pd/C-100HOX (0.5 mol %) did not proceed at ambient temperature, H2 gas generation was observed at 333 K and the silanol iv was produced within 20 min in 96% yield (eq 3). The absolute stereochemistry of the silanol iv, thus obtained, was determined to (S)-configuration (47% ee) by comparison of the retention time with an authentic sample in HPLC analysis using chiral stationary phase.27 This result means that the present reaction mainly proceeds with inversion of configuration at the silicon center and implies that a water molecule attacks from the backside of the silicon on Pd nanoparticle catalyst, in accord with Sommer’s result. 22969

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concentration of H2O. These computational and experimental results lead us to conclude that the water addition is the ratedetermining step in the catalytic reaction. The 2inv would collapse into stable 2ret by rotating the protonated silanol moiety. The produced protonated silanol molecule is weakly bound through the interaction between the Pd surface and the water moiety of the product in 2ret; the Si−Pd and O−Pd distances are 4.523 and 3.010 Å, respectively. A proton is readily transferred from the protonated silanol molecule to the Pd surface via TS(2→3) with a low barrier of 0.4 kcal/mol to form silanol and an H atom, 3. Judging from the low activation barrier, the proton would be readily transferred into the Pd surface through the network of solvent. The produced silanol molecule is released into the solvent. In our previous work,15 we demonstrated that the surface oxygen atoms on the Pd catalyst promote water decomposition and the produced OH group attacks nitriles to produce amides. Thus, we considered a possible reaction pathway where the OH group generated by the water decomposition is responsible for the silanol formation. Optimized geometries and computed energies for the addition of an OH group to the silyl group on the oxygen-covered Pd surface are shown in Figure S1 in the Supporting Information. This mechanism is initiated by the water decomposition (1′→2′), followed by the addition of the produced OH group to the silyl group on the Pd surface (2′→ 3′). This mechanism would be expected to lead to retention of configuration. The barrier height of the reaction was computed to be 18.1 kcal/mol, which is 7.9 kcal/mol lower and 6.8 kcal/ mol higher than that in the retention and inversion pathways, respectively, without the water decomposition. Thus, the silanol formation can be promoted by the water decomposition, but the backside attack of the water molecule is most favored in energy. We conclude that the water decomposition is not essential for the silane oxidation and the silyl group on the Pd surface is directly attacked by the water molecule from the backside to produce silanol with inversion of configuration. 3.3. H2 Desorption Accelerated by Surface Oxygen Atom. After the release of produced silanol into solvent, two H atoms remain stoichiometrically on the Pd(111) surface; one comes from silane and the other from water. Jeon et al.22 experimentally demonstrated that the H atoms should be removed from the Pd surface to achieve the catalytic activity and that they can be removed by the reaction with molecular oxygen. DFT studies34−38 show that O2 is readily hydrogenated to lead to the nonselective formation of water on the Pd surface. On the other hand, Shimizu et al. reported that a stoichiometric amount of H2 was produced in the silane oxidation on the oxygen-covered Pd nanoparticles and that the catalyst is active under either aerobic or anaerobic conditions. These results indicate that molecular oxygen is not necessary in the removal of Pd−H species to regenerate Pd surface with the desorption of H2. However, one may wonder why the hydrogenation of the surface oxygen does not take place, leading to the formation of OH and H2O and eventually to the inactivation of the catalyst. Thus, we investigated the formation of H2 and OH on the oxygen-covered Pd(111) surface with DFT computations. To examine the coverage dependence of the reactions, we considered the Pd(111) surface at 1/16, 1/8, 3/16, and 1/4 coverage of the surface oxygen atom with a 4 × 4 surface unit cell because the exposure of the Pd(111) surface to O2 at temperature between 300 and 575 K forms a simple (2 × 2) overlayer.39,40 As shown in Figure 4, two H atoms are coadsorbed with four surface oxygen atoms on the Pd(111)

With this information in mind, we considered two pathways where the water addition to the sliyl group proceeds with inversion (1inv→2inv) or retention (1ret→2ret) of configuration, using DFT calculations, as shown in Figure 2. Preliminary calculations suggest that these reactions are significantly affected by solvent effects. Here we incorporated the dielectric effect of CH2Cl2 solvent using the conductor-like screening model (COSMO) for surfaces.32 The dielectric constant of 8.9 for CH2Cl2 is closest to that of 7.2 for dimethoxyethane among solvents supported in the DMol3 program. Sommer and coworkers30 used CH2Cl2 as solvent for the oxidation of i catalyzed by Pd/C. The water molecule approaches the Si atom from the backside in the inversion pathway. In 1inv, the water molecule is weakly bound to the silyl group; the Si···O distance is 3.916 Å. As the reaction progresses, the bond between the water molecule and the silyl group grows, and the bond between the silyl group and the Pd atom is weakened. In TS(1→2)inv, the configuration of the Si atom of the silyl group becomes inverted to form silanol, 2inv. This reaction is endothermic by 8.4 kcal/mol with an activation barrier of 11.3 kcal/mol. On the other hand, the water molecule is coadsorbed with the silyl group at a distance of 3.783 Å between the Si atom of silyl group and the O atom of the water molecules in 1ret. The distance is shortened to 2.148 Å in the transition state, TS(1→2)ret to form 2ret. A barrier height of 26.0 kcal/mol for the retention pathway is much higher than that for the inversion pathway, which shows that the silanol formation proceeds with inversion of configuration on the Pd surface. To support the computational results, we have determined the activation energy from the slope of Arrhenius plot for the oxidation of triethylsilane by Pd/C-100Hox as shown in Figure 3. A calculated activation energy of 11.3 kcal/

Figure 3. Arrhenius plot for the oxidation of triethylsilane by Pd/C100Hox. Conditions: Et3SiH (6 mmol) and H2O (9 mmol) in dimethoxyethane (6 mL), T = 278, 283, 288, or 308 K.

mol for the inversion mechanism agrees qualitatively with the experimentally determined value of 18 kcal/mol because DFT methods tend to underestimate barrier highs for chemical reactions.33 TS(1→2)inv is energetically highest in the most favorable pathway after the dissociative addition of silane. Shimizu et al. showed that the reaction rate of the oxidation of Et3SiH catalyzed by the Pd catalyst is dependent only on the 22970

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Figure 4. Formation of H2 and OH on the Pd(111) surface at 1/4 coverage of the surface oxygen atom; distances in Å.

surface oxygen atoms. Thus, the repulsive interaction between the H atom and the surface oxygen atom is also weaker at the higher coverage, which is advantageous for the formation of OH. However, this effect is offset by the lower proton affinity of the surface oxygen atoms at the higher coverage, and therefore, the activation energy is insensitive to the coverage of the surface oxygen atoms. These computational results are in good agreement with the experimental findings that H2 is formed in stoichiometric amounts on the oxygen-covered Pd nanoparticle without the undesired formation of OH and that the reaction rate of the silane oxidation increases with the amount of the surface oxygen atoms. The role of the surface oxygen atoms in the silane oxidation is to facilitate the desorption of H2 from the Pd surface but not the water decomposition.

surface at 1/4 coverage of the surface oxygen atom. The H atoms are located on two neighboring 3-fold positions. In the formation of H2, the H atoms are driven off the surface toward the nearest top site, leading to the H−H bond formation. In the transition state, the distance between the H atoms is 1.303 Å and the H atoms are 1.43 Å high above the Pd surface. The formed H2 molecule is adsorbed at the top site, being 1.92 Å away from the Pd surface. The H2 formation is endothermic by 8.3 kcal/mol with an activation energy of 16.3 kcal/mol. In the course of the formation of OH, an H atom gets close to a surface oxygen atom on the nearest 3-fold position; the O−H distance decreases to 1.541 Å in the transition state. The produced OH group moves toward a bridge site. The reaction is strongly exothermic by 10.5 kcal/mol and has a high activation energy of 23.1 kcal/mol. Interestingly, the H2 formation is more kinetically favorable than the formation of OH although the formation of OH is thermodynamically favored as expected. As summarized in Table 1, the activation energy for the formation of H2 is lowered by the increase of the number of the

H2 formation

OH formation

0 1/16 1/8 3/16 1/4

22.9 22.5 22.3 17.9 16.3

25.7 25.5 25.4 23.1

a

charge (O)a charge (H)a −0.335 −0.333 −0.330 −0.328

CONCLUSIONS

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ASSOCIATED CONTENT

In the present study, we have discussed the oxidation of silanes to silanols on Pd nanoparticles. The proposed mechanism of catalysis begins with the dissociative adsorption of silane on the Pd surface. The dissociation of silane to an H atom and a silyl group is very facile nearly without activation barrier. The silyl group is subsequently attacked by a water molecule from the backside to produce silanol and an H atom on the Pd surface. The water addition is the rate-determining step in the catalytic reaction. This mechanism is in good agreement with the stereochemical and kinetic studies. The H atoms should be removed from the Pd surface to achieve the catalytic activity experimentally. Our computational and experimental results demonstrate that oxygen atoms on the Pd surface promote the desorption of H2 without the formation of OH and H2O. The mechanistic findings offer a deeper insight into the development of new metal nanoparticle catalysts by using surface contaminants.

Table 1. Barrier Height (kcal/mol) for the Formation of H2 and OH on the Pd(111) Surface at Different Coverage of the Surface Oxygen Atom coverage (ML)

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−0.132 −0.130 −0.128 −0.120 −0.112

Averaged atomic charge on the O and H atoms.

surface oxygen atom, whereas that for the OH formation is not affected. The activation barrier is well correlated with the computed atomic charges on the H atoms; the H atoms are less negatively charged at a higher coverage due to the high electronegativity of the surface oxygen atoms. The repulsive interaction between the negatively charged H atoms is weaker at the higher coverage so that the activation energy for the formation of H2 is reduced. The surface oxygen atom is also less negatively charged at a higher coverage, which indicates that the surface oxygen atoms have lower proton affinity at the higher coverage. This is because the negative charge supplied from the H atoms and the Pd surface is evenly shared among coexisting

* Supporting Information S

One figure of optimized geometries and computed energies for the addition of a water molecule to the silyl group on the oxygen-covered Pd surface, and one table for the formation of H2 and OH. This material is available free of charge via the Internet at http://pubs.acs.org. 22971

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-92-802-2529. E-mail: [email protected]. jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Grants-in-Aid for Scientific Research (Nos. 22245028, 24109014, and 24550190) from the Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), the Kyushu University Global COE Project, the Nanotechnology Support Project, the MEXT Projects of “Integrated Research on Chemical Synthesis” and “Elements Strategy Initiative to Form Core Research Center”, and CREST of the Japan Science and Technology Cooperation.

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