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Regio- and Chemoselective Hydrogenation of Dienes to Monoenes Governed by a Well-Structured Bimetallic Surface Masayoshi Miyazaki, Shinya Furukawa, and Takayuki Komatsu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b08792 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on November 28, 2017

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Journal of the American Chemical Society

Regio- and Chemoselective Hydrogenation of Dienes to Monoenes Governed by a Well-Structured Bimetallic Surface Masayoshi Miyazaki,† Shinya Furukawa,‡,§,* and Takayuki Komatsu†,* †

Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-E1-10

Ookayama, Meguro-ku, Tokyo 152-8551, Japan ‡

Institute for Catalysis, Hokkaido University, N10 W21, Kita-ku, Sapporo 001-0021, Japan

§

ESICB, Kyoto University, Nishikyo-ku, Kyoto 519-5510, Japan

Corresponding authors Shinya Furukawa Institute for Catalysis, Hokkaido University, N10 W21, Kita-ku, Sapporo 001-0021, Japan E-mail: [email protected], Tel: +81-11-706-9162,

Takayuki Komatsu Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-E1-10 Ookayama, Meguro-ku, Tokyo 152-8551, Japan E-mail: [email protected], Tel: +81-3-5734-3532, Fax: +81-3-5734-2758

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Abstract Unprecedented surface chemistry, governed by specific atomic arrangements and the steric effect of ordered alloys, is reported. Rh-based ordered alloys supported on SiO2 (RhxMy/SiO2, M = Bi, Cu, Fe, Ga, In, Pb, Sn, and Zn) were prepared and tested as catalysts for selective hydrogenation of trans-1,4-hexadiene to trans-2-hexene. RhBi/SiO2 exhibited excellent regioselectivity for the terminal C=C bond and chemoselective hydrogenation to the monoene, not to the overhydrogenated alkane, resulting in a high trans-2-hexene yield. Various asymmetric dienes, including terpenoids, were converted into the corresponding inner monoenes in high yields. This is the first example of a regio- and chemoselective hydrogenation of dienes using heterogeneous catalysts. Kinetic studies and density functional theory calculations revealed the origin of the high selectivity: (1) one-dimensionally aligned Rh arrays geometrically limit hydrogen diffusion and attack to alkenyl carbons from one direction and (2) adsorption of the inner C=C moiety to Rh is inhibited by steric repulsion from the large Bi atoms. The combination of these effects preferentially hydrogenates the terminal C=C bond and prevents overhydrogenation to the alkane.

Keywords: regioselective, chemoselective, diene, hydrogenation, intermetallic compound


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Introduction Selective hydrogenation of unsaturated hydrocarbons, such as alkynes and dienes, is an important chemical transformation in various disciplines such as organic synthesis, pharmacology, and industrial chemistry.1–6 Semihydrogenation of an alkyne to the alkene has been extensively studied using Pd-based bimetallic materials, as represented by Lindlar catalysts.7,8 To this day, a number of efficient catalytic systems for alkyne semihydrogenation have been reported.9–15 The common strategy for obtaining high alkene selectivity is the promotion of alkene desorption to inhibit overhydrogenation to the alkane, which is typically attained by reducing the adsorption energy of the alkene.16 Similar approaches are also available for diene hydrogenation to the monoene.17 However, success in selective diene hydrogenation using heterogeneous catalysts has been limited for symmetric dienes such as 1,3-butadiene.18–22 This is because regioselectivity between the two symmetrically distinct C=C bonds is required for hydrogenation of asymmetric dienes. Traditional solid metal catalysts are hardly capable of distinguishing such differences. In a typical approach, regioselective molecular recognition ability can be provided to heterogeneous systems using micro- or mesoporous materials. Physical restrictions of the adsorption geometry or transition states by the pore walls aid regioselective conversions. For instance, Pt nanoparticles encapsulated in a ZIF-8 metal–organic framework23 or Pd complexes immobilized in MCM-41 mesoporous silica24 have shown higher regioselectivity for terminal C=C bond hydrogenation than that of the inner C=C bond. However, no heterogeneous catalysts affording high monoene yields from asymmetric diene have been reported in the literature. This is partly because undesired overhydrogenation to the alkane seems to be unavoidable over such active metal sites. That is to say, not only regioselectivity but also chemoselectivity is mandatory for achieving highly efficient conversions of dienes to monoenes. For this purpose, a metallic surface itself should recognize the position of the C=C bond and distinguish diene and monoene chemically. It remains a tremendous challenge to construct such a sophisticated reaction environment for high monoene yields using conventional inorganic materials. Ordered alloys, also known as intermetallic compounds, exhibit specific crystal structures and well-defined surface atomic arrangements of their metal components, making them attractive candidates for inorganic materials that enable selective diene conversion.25,26 Their highly ordered and electronically modified bimetallic surfaces would be appropriate reaction environments for 3 ACS Paragon Plus Environment


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regio- and chemoselective molecular recognition. We recently reported that specific atomic arrangements of some intermetallic compounds controlled stereoselective alkene isomerization27 and chemoselective molecular recognition for nitroarene hydrogenation quite well.28 Motivated by these discoveries, in the present study, we developed a highly efficient catalytic system for hydrogenation of asymmetric dienes to monoenes using intermetallic catalysts. The reaction mechanism was investigated in detail using kinetic studies and density functional theory (DFT) calculations to clarify the surface reaction dynamics. Herein, we report the first example of heterogeneous regio- and chemoselective diene hydrogenation and a novel concept using ordered alloys for well-controlled surface chemistry.

Experimental Section Catalyst preparation A series of monometallic catalysts was prepared by pore-filling impregnation using silica as a support. Aqueous solutions of H2PtCl6, Pd(NO3)2, Rh(NO3)3, and Ni(NO3)2 were added to dried silica gel (CARiACT G-6, Fuji Silysia, SBET = 470 m2 g−1) so that the solutions filled the silica pores (pore volume: 0.76 cm3g−1, metal concentration: 185 mM for the synthesis of Rh/SiO2). The concentration of Rh(NO3)3 aq was 4.53 wt%. The mixtures were sealed overnight at room temperature and dried over a hot plate, followed by reduction under flowing H2 at 673 K for 1 h. Silica-supported Rh-based intermetallic catalysts (RhM/SiO2, M = Bi, Fe, Ga, In, Pb, Sb, Sn, and Zn) were prepared by pore-filling co-impregnation. Mixed aqueous solutions of Rh(NO3)3 and a second metal salt (Fe(NO3)3·9H2O (Wako, 99%), Zn(NO3)2·6H2O (Kanto, 99%), Ga(NO3)3·nH2O (Wako, 99.9%),

In(NO3)3·9H2O

(Kanto,

99.9%),

SnCl2

(Kanto,

97%),

SbCl3

(Kanto,

98%),

Pb(CH3COO)2·3H2O (Kanto, 99.5%), and Bi(NO3)3·5H2O (Kanto, 99.5%)) were used in a manner similar to that of the monometallic catalysts. HNO3 or HCl was used to dissolve Bi(NO3)3·5H2O or SnCl2 and SbCl3, respectively. The metal loading and the atomic ratio of Rh/M were adjusted to 3 wt% and 1, respectively. For bimetallic catalysts, the reduction procedure was performed at 1073 K. RhBi catalysts using other supports were prepared by conventional co-impregnation with an excess amount of water (ca. 15 mL of ion exchanged water/g of support).

Catalytic reaction


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Diene hydrogenations were carried out in 50 mL three-necked round-bottomed flasks equipped with a silicone rubber septum and a gas storage balloon (2 L). The catalyst (10 or 20 mg) was reduced under flowing H2 (60 mL min−1) at 673 K for 45 min in the reactor followed by cooling to room temperature. The reaction was initiated by adding a tetrahydrofuran (THF) solution (5 mL) of diene (0.2 mmol) into the reactor under a 1 atm H2 atmosphere at room temperature. The products were quantitated by a flame ionization detection gas chromatograph (FID–GC, Shimadzu, GC-14B) equipped with a capillary column (GL Sciences, TC-WAX, 0.25 mm × 30 m). The partial pressure of H2 was adjusted using Ar as a diluent for the kinetic study. The reaction rates for all kinetic analyses were obtained at conversions of 1,4-hexadiene and H2 lower than 20% and 1%, respectively. For recycling tests, the catalyst (100 mg) after reaction was washed by decantation in the reactor with THF. The THF rinse was completely removed by decantation and subsequent drying under an Ar flow (40 mL min−1) at room temperature. The washed catalyst was reused after the pretreatment step outlined above.

Characterization The crystal structure of the prepared catalyst was examined by powder X-ray diffraction (XRD) with a Rigaku RINT2400 using an X-ray source of Cu Kα. Difference XRD patterns were obtained by subtraction of the silica support pattern from those of the supported catalysts so that the base line was flattened. Transmission electron microscopy (TEM) was conducted using a JEOL JEM-2010F microscope. High angle annular dark field scanning TEM microscopy (HAADF-STEM) was carried out using a JEOL JEM-ARM200M microscope equipped with an energy dispersive X-ray (EDX) analyzer (EX24221M1G5T). Both TEM analyses were performed at an accelerating voltage of 200 kV. To prepare the TEM specimen, all samples were sonicated in ethanol and then dispersed on a Cu grid supported by an ultrathin carbon film. Fourier-transformed infrared (FT-IR) spectra of adsorbed CO were obtained with a JASCO FT/IR-430 spectrometer in transmission mode. A self-supporting wafer (50 mg cm−2) of catalyst was placed in a quartz cell with CaF2 windows and attached to a glass circulation system. The catalyst was reduced under flowing H2 at 400 °C for 1 h, evacuated at the same temperature for 0.5 h, and cooled to room temperature. After the pretreatment, a spectrum was recorded as the base line for subsequent measurements. A pulse of CO, typically of 100–103 Pa order, was introduced in a stepwise manner at room temperature. All spectra were recorded at 1 cm−1 5 ACS Paragon Plus Environment


resolution. CO pulse chemisorption was performed to estimate the Pd, Pt, Rh, and Ni dispersion in the prepared catalysts. Prior to chemisorption, the catalyst was pretreated under a H2 flow (60 mL min−1) for 0.5 h. After the reduction pretreatment, He was introduced at the same temperature for 10 min to remove the chemisorbed hydrogen, followed by cooling to room temperature. A 5% CO/He pulse was introduced into the reactor, and the passed supplied CO was quantified downstream by a thermal conductivity detector. For example, for RhBi/SiO2, Rh dispersion was estimated as 0.26%. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis of the catalysts was performed using a Shimadzu ICPE-9000 spectrometer.

DFT calculations Periodic DFT calculations were performed using the CASTEP code29 with Vanderbilt-type ultrasoft pseudopotentials30 and the revised version of the Perdew–Burke–Ernzerhof exchange–correlation functional31,32 based on the generalized gradient approximation. The plane-wave basis set was truncated at a kinetic energy of 350 eV. A Fermi smearing of 0.1 eV was utilized. DFT-D correlations were considered using the Tkatchenko–Scheffler method33 with parameters of sR = 0.94 and d = 20.0. The reciprocal space was sampled using a k-point mesh with a spacing of typically 0.05 Å−1, as generated by the Monkhorst−Pack scheme.34 Geometry optimizations and transition state (TS) searches were performed on supercell structures using periodic boundary conditions. The surfaces were modeled using metallic slabs with a thickness of typically four atomic layers with 13 Å of vacuum spacing. Unit cells were (2 × 6) for RhZn, RhGa, RhIn, and RhFe, and (2 × 3) for RhBi. The convergence criteria for structure optimization and energy calculation were set to (a) an SCF tolerance of 2.0 × 10−6 eV per atom, (b) an energy tolerance of 1.0 × 10−5 eV per atom, (c) a maximum force tolerance of 0.05 eV Å−1, and (d) a maximum displacement tolerance of 1.0 × 10−3 Å. The adsorption energy was defined as follows: Ead = EA-S–(ES + EA) where EA-S is the energy of the slab together with the adsorbate, EA is the total energy of the free adsorbate, and ES is the total energy of the bare slab. The adsorption energy for a hydrogen-preadsorbed slab was calculated using ESH, the total energy of the hydrogen-adsorbed slab, instead of using ES.


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Surface energy calculations were conducted for densely packed low-index planes of RhBi ((100), (101), (001), (110), (102), (103), and (211)) and RhZn ((100), (110), (111), (210), (211), and (221)), where X-ray diffraction were observed. Surface energies of stoichiometric surfaces were estimated using γ = lim

1

n→∞ 2A

[ES − NEB ]

where EB is the energy of bulk unit cell, A is the surface area, n is the number of layers, and N is the number of unit cells in the slab. The surface energies converged typically within 6 or 7 atomic layers for the considered surfaces. Surface energies of non-stoichiometric surfaces were calculated using a method in literature.27 The TS search was performed using the complete linear synchronous transit/quadratic synchronous transit (QST) method.35,36 Linear synchronous transit maximization was performed followed by energy minimization in the directions conjugating to the reaction pathway. The approximated TS was used to perform QST maximization with conjugate gradient minimization refinements. The cycle was repeated until a stationary point was found. Convergence criteria for the TS calculations were set to root-mean-square forces on an atom tolerance of 0.10 eV Å−1. Frequency calculations of adsorbed CO molecules were conducted from geometry optimized structures using the DMol3 code.37 These calculations involved the RPBE functional, a double-numeric quality basis set with polarization functions except that no p functions are used on hydrogen (DND, comparable to Gaussian 6-31G*)38 with a real-space cutoff of 4.2 Å, DFT semi-core pseudopotential core treatment,39 and a Fermi smearing of 0.1 eV. The SCF convergence was accelerated using the iterative scheme proposed by Kresse and Furthmüller.40 The partial Hessian matrix including C and O atoms was computed to evaluate the harmonic frequencies for adsorbed CO. All computed harmonic frequencies were scaled by an empirical factor of 1.0386, which corresponds to the ratio of experimental41 and as-calculated values for gas-phase CO (2143 cm−1/2063.4 cm−1).

Results Catalyst characterization A series of Rh-based intermetallic catalysts was prepared using a co-impregnation method with SiO2



as the support (RhM/SiO2, M = Bi, Fe, Ga, In, Pb, Sb, Sn, and Zn). Crystallite phases of the prepared catalysts were analyzed using XRD (Figure 1). For all the catalysts, the desired intermetallic phases were formed with high phase purities. The crystallite sizes of the intermetallic phases, estimated by Scherrer’s equation, were 2–10 nm. Figures 2a and 2b show the transmission electron microscopy (TEM) image of RhBi/SiO2 and the size distribution of the nanoparticles, respectively. Particle sizes ranged from 2 to 6 nm (mean diameter: 4.1 nm), which agreed with the crystallite size estimated using XRD (4 nm). Figures 2c and 2d show the high-resolution (HR)-TEM image of a single nanoparticle. Lattice fringes with 0.222 nm spacing were clearly observed, which is consistent with the interplanar distance of the RhBi(102) plane (0.221 nm). The fast Fourier transform of the HR-TEM image (Figure 2e) indicates that the nanoparticle is of a RhBi single crystal oriented along the [22ത 1ത ] direction. These results indicate that the observed nanoparticles are single crystals of the intermetallic RhBi phase. We also performed high-angle annular dark field scanning TEM (HAADF-STEM) analysis for RhBi/SiO2. The HAADF-STEM image of RhBi/SiO2 was shown in Figure 2f. The Z contrast between metal nanoparticles, SiO2 support, and the background was clearly observed, showing that the RhBi nanoparticles are dispersed on SiO2. Elemental maps of Rh and Bi acquired using energy dispersive X-ray (EDX) analysis on this field revealed that Rh and Bi atoms composing the nanoparticles were homogeneously dispersed (Figures 2g, 2h, and 2j). This suggests that the observed nanoparticles comprise both Rh and Bi without segregation. The presence of Rh and Bi on SiO2 in 1:1 ratio was also confirmed by ICP-AES analysis (Rh/Bi = 1.04). The EDX mapping with Si allows elemental identification on the HAADF-STEM image more clearly (Figures 2i and 2k). Figure 2l shows the HR-HAADF-STEM image of a single RhBi nanoparticle. Hexagonally aligned Rh and/or Bi atoms were observed, which can be assigned to intermetallic RhBi (space group: P63/mmc) viewed along [001] direction (Figure 2m). Note that superposition of Figures 2l and 2m shows a good agreement (Figure S1). At the edge region of this nanoparticle, bulk-terminated structures were observed, implying that {110} planes are exposed. This result is consistent with our recent experimental and theoretical study suggesting that (110) plane dominates the surface of RhBi.28

Diene hydrogenation We first tested the catalytic performances of monometallic catalysts (M/SiO2, M = Pt, Pd, Rh, and 8 ACS Paragon Plus Environment

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Ni) in the hydrogenation of dienes (Table 1). trans-1,4-Hexadiene (hereafter 1,4-hexadiene) was used as a simple model diene with both terminal and internal C=C bonds. In all cases, trans-2-hexene (hereafter, 2-hexene), 1-hexene, and n-hexane were formed by hydrogenation of terminal, internal, and both C=C bonds of 1,4-hexadiene, respectively. The metal dispersion measured by CO chemisorption and the corresponding turnover frequency (TOF) for each catalyst are also listed in Table 1. Although Pt/SiO2 showed high catalytic activity, the alkene selectivity was decreased by significant overhydrogenation to the alkane. Pd/SiO2 gave high alkene selectivity, and substantial amounts of 1-hexene were generated, which decreased the regioselectivity. When Rh/SiO2 was used, moderate catalytic activity was obtained and the 2-hexene selectivity was higher than that observed with Pt and Pd. The use of Ni/SiO2 resulted in very low catalytic activity. This is probably due to insufficient reduction of Ni in this condition (pretreated by H2 at 400°C), which is also reflected in the very low Ni dispersion. Thus, based on these results, we chose Rh as the main active metal for developing bimetallic catalysts that would be regio- and chemoselective. Then, a series of Rh-based intermetallic catalysts was tested in the same reaction. Figure 3 shows the conversion–selectivity curves obtained in 1,4-hexadiene hydrogenation over Pt and some Rh-based catalysts. Bimetallic catalysts showed 2-hexene selectivities higher than that of monometallic Rh and Pt. RhBi showed the highest 2-hexene selectivity over the whole conversion region. For most catalysts, the selectivity decreased as the conversion increased, corresponding to overhydrogenation of the hexenes to n-hexane. In contrast, surprisingly, RhBi retained the initial high selectivity until complete conversion. This indicated that overhydrogenation of 2-hexene to n-hexane was well inhibited, even at the high conversion regions. Moreover, no 1-hexene was detected during the hydrogenation over RhBi (Figure S2), suggesting selective hydrogenation of the terminal C=C bond. Thus, RhBi showed high regio- and chemoselectivities in diene hydrogenation. These selectivities should be evaluated quantitatively so that individual contributions to the overall 2-hexene selectivity are distinguished more clearly. The intrinsic regioselectivity was expected to appear at the initial stage of hydrogenation, where overhydrogenation of monoene to alkane would be little. In this study, we used the relative formation rate of 1-hexene to that of 2-hexene (r1h/r2h) at low conversions (99

9

>99

6

Styrene +

β-methylstyrene

Ethylbenzene + propylbenzene

10

Reaction conditions: catalyst, 10 mg (RhBi/SiO2); substrate, 0.2 mmol; temperature, 25 ˚C; solvent, THF (5 mL); atmosphere, 1 atm H2.


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Figure 1. X-ray diffraction (XRD) patterns of Rh-based bimetallic catalysts supported on SiO2 and corresponding references (ICDD-PDF, open and filled symbols). For clarity, diffraction of the SiO2 support was subtracted from the original patterns. 75x74mm (600 x 600 DPI)

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Figure 2. (a) Transmission electron microscopy (TEM) image of RhBi/SiO2 and (b) size distribution of the nanoparticles. (c) High-resolution TEM image of a single nanoparticle. (d) Expansion of the region designated by the orange square in (c). (e) Fast Fourier transform (FT) of the single nanoparticle in (c). (f) HAADF-STEM image of RhBi/SiO2. Elemental maps of (g) Rh (L-edge), (h) Bi (M-edge), (i) Si (K-edge), (j) Rh + Bi, and (k) Rh + Bi + Si acquired using EDX. (l) HR-HAADF-STEM image of one RhBi nanoparticle. (m) Crystal structure of intermetallic RhBi viewed along [001] direction as a structural model of (l). 152x115mm (300 x 300 DPI)


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Figure 3. Conversion–selectivity curves obtained from 1,4-hexadiene hydrogenation over Pt and selected Rhbased catalysts. 66x55mm (300 x 300 DPI)



Figure 4. Regioselectivity (r1h/r2h) and chemoselectivity (∆S1+2) for monometal catalysts (Pd, Pt, Ni, Rh) and Rh-based intermetallic catalysts. Regioselectivity (r1h/r2h) is the relative formation rate of 1-hexene to that of 2-hexene at low conversions (

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