Ken-ichi
Shimizu
*a,
Keiichiro
Ohshima
b,
Yutaka
Tai
c,
Masazumi
Tamura
b and
Atsushi
Satsuma
b
aCatalysis Research Center, Hokkaido University, N-21, W-10, Sapporo 001-0021, Japan. E-mail: kshimizu@cat.hokudai.ac.jp; Fax: +81-11-706-9163; Tel: +81-11-706-9240
bDepartment of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
cMaterials Research Institute for Sustainable Development, Chubu Research Base of National Institute of Advanced Industrial Science and Technology (AIST), Nagoya 463-8560, Japan
First published on 21st December 2011
γ-Alumina-supported Pt nanoclusters with an average particle size of 0.8 nm, Pt/Al2O3-0.8, act as an effective heterogeneous catalyst for mono-N-alkylation of amines with different amines. To establish a catalyst design concept, systematic studies on the structure–activity relationship are carried out, combined with characterization by Pt L3-edge XAFS (X-ray absorption fine structure), X-ray photoelectron spectroscopy (XPS), and infrared (IR) study of CO adsorption. By changing the particle size of Pt over the size range of 0.8–24 nm, it is demonstrated that the present reaction is a structure-sensitive reaction, demanding coordinatively unsaturated Pt atoms on metallic nanoclusters. The support also affects the activity and electronic state of Pt. The electron density of Pt increases with basicity of the support oxide, and the support with moderate basicity (Al2O3) gives the highest activity probably due to a moderate electron density of Pt. Kinetic studies suggest that the present reaction proceeds through a “hydrogen-borrowing” mechanism.
Amines are intermediates and products of enormous importance for chemical and life science applications. Pd- and Cu-catalyzed aminations of aryl halides20 and the transition-metal-catalyzed amination of alcohols21,22 represent attractive approaches for the alkylation of amines. The transition-metal-catalyzed alkylation of amines by amines is an attractive alternative method for alkylamine synthesis.23–31 The reaction proceeds through a hydrogen-borrowing (hydrogen auto-transfer) mechanism.21–26 The process begins with the dehydrogenation of an alkylamine to the corresponding imine. The imine undergoes addition of another nucleophilic amine and elimination of ammonia to form an N-alkylimine, which is hydrogenated by the in situ formed hydride species to the secondary amine product. Beller's Ru complexes24,25 and Williams's Ir complexes26 are successful catalytic systems for selective amine cross-coupling of different amines, leaving ammonia as the only by-product. From the environmental and economic viewpoints, it is preferable to accomplish the selective cross-coupling reaction using reusable heterogeneous catalysts. Previous examples of heterogeneous systems for amine coupling21,23 suffer from low selectivity for cross-coupling of different amines, reusability, low turnover number (TON),21,23 limited scope,31b and need for stoichiometric amounts of additives. Some systems require special reaction methods such as microwave heating,28,31a electrocatalysis29 or photocatalysis.30 Recently, Pd/C-catalyzed cross-coupling reaction was reported as the first example of the reusable heterogeneous catalyst for this reaction, but the system requires microwave heating.28 In our recent communication,32 we reported that small Pd metal particles supported on alumina effectively catalysed selective mono-N-alkylation of benzyl amines with different amines, but the scope was still limited mainly to the reaction of benzylamines with cyclic secondary amines.
In this paper, we report a new and atom economical catalytic system using Pt NCs-loaded γ-Al2O3 (Pt/Al2O3) as a recyclable heterogeneous catalyst for mono-N-alkylation of amines with different amines (cross-coupling of different amines), and the catalyst shows higher TON than previously reported homogeneous and heterogeneous catalysts for the selective cross-coupling of amines. To establish a structure–activity relationship, effects of the Pt cluster size, support basicity (acidity) and nature of transition metals on turnover frequency (TOF), defined as the activity per unit of exposed metal surface, are studied, and a design concept of the supported Pt NCs catalyst for the present reaction is discussed.
Supported Pt catalysts (entries 1–10 in Table 1, Pt = 5 wt%, support oxides (MOx) = γ-Al2O3, CeO2, ZrO2, SiO2) were prepared by impregnating MOx with an aqueous HNO3 solution of Pt(NH3)2(NO3)2 (Tanaka Kikinzoku), followed by evaporation to dryness at 80 °C, drying at 120 °C for 12 h, calcination in air, and reduction in 100% H2 for 10 min. Supported Pd (1 wt%), Rh, (5 wt%), Ag (5 wt%), and Cu (5 wt%) catalysts were prepared by the impregnation method using aqueous HNO3 solution of Rh(NO3)3 or Pd(NO3)2 or aqueous solution of Ag(I) and Cu(II) nitrates. To control the metal particle size, temperatures of calcination (Tcal) and reduction (TH2) were changed as summarized in Table 1. Note that the rate of temperature increase was about 30 °C min−1 for the calcination and reduction treatments. The catalysts are designated as Pt/Al2O3-x, where x is the metal particle size (nm). Pt/Al2O3-0.8 was used as a standard catalyst. The preparation method and characterization results for Au/SiO2-6.0 (Au = 1 wt%) are shown in our previous report.4 Pd/C (Pd = 5 wt%) was purchased from Kawaken Fine Chemicals.
Entry | Catalysts-xa | T cal b/°C | T H2 c/°C | D a/nm |
---|---|---|---|---|
a Average particle size (nm) of the supported metal estimated from the CO adsorption experiment. b Temperatures of calcination. c Temperatures of reduction in H2. d Metal particle size was estimated from XRD using the Scherrer formula. e Size of Au particle was estimated from TEM in our previous study.4 f Size of Ag particle was estimated from EXAFS in our previous study.6 | ||||
1 | Pt/Al2O3-0.8 | — | 200 | 0.82 |
2 | Pt/Al2O3-1.3 | — | 500 | 1.3 |
3 | Pt/Al2O3-2.2 | 550 | 500 | 2.2 |
4 | Pt/Al2O3-4.3 | 600 | 500 | 4.3 |
5 | Pt/Al2O3-7.8 | 650 | 500 | 7.8 |
6 | Pt/Al2O3-24 | 700 | 500 | 23.5 |
7 | Pt/CeO2-1.8 | 500 | 500 | 1.8 |
8 | Pt/ZrO2-2.3 | 500 | 200 | 2.3 |
9 | Pt/SiO2-2.5 | 500 | 500 | 2.5 |
10 | Pt/SiAl-2.1 | 500 | 500 | 2.1 |
11 | Pt/SiO2-8.7 | 600 | 500 | 8.7 |
12 | Pd/SiO2-5.3 | 500 | 300 | 5.3 |
13 | Rh/SiO2-5.2 | 500 | 200 | 5.2 |
14 | Ru/SiO2-5.1 | 500 | 200 | 5.1 |
15 | Ag/SiO2-5.0 | 500 | 500 | 5.0d |
16 | Au/SiO2-6.0 | 300 | — | 6.0e |
17 | Cu/SiO2-14 | 250 | 250 | 13.8d |
18 | Ag/Al2O3-0.8 | 600 | 300 | 0.84f |
19 | Pt black-25 | — | — | 25.0 |
20 | PtO2-28 | — | — | 27.5d |
Pt L3-edge in situ XAFS measurement was carried out at BL01B1 of SPring-8 (Hyogo, Japan). The storage ring energy was operated at 8 GeV with a typical current of 100 mA. A self-supported wafer form (pressed pellet) of the pre-reduced Pt/Al2O3 samples with ca. 10 mm diameter was placed in a quartz in situ cell in a flow of 100% H2 (100 cm3 min−1) for 10 min at 200 °C under atmospheric pressure, then the sample was cooled to 40 °C under a flow of He. The analyses of the extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structures (XANES) were performed using the REX version 2.5 program (RIGAKU). For EXAFS analysis, the spectra were extracted by utilizing the cubic spline method and normalized to the edge height. The Fourier transformation of the k3-weighted EXAFS from k space to R space was carried out over the k range 3–15 Å−1 to obtain a radial distribution function. The inversely Fourier filtered data (in the R range of 1.5 Å–3.3 Å) were analyzed with a usual curve fitting method in the k range of 3.3–14.7 Å−1 using the empirical phase shift and amplitude functions for Pt–Pt and Pt–O shells extracted from the data for Pt foil and PtO2, respectively. During the fitting procedure the absorber–scatterer distances, Debye–Waller factors or the coordination numbers were refined using a least squares refinement procedure.
X-Ray diffraction (XRD) patterns of the powdered catalysts were recorded with a Rigaku MiniFlex II/AP diffractometer with Cu Kα radiation. The average metal particle size was calculated from the half-width of the peak from the XRD pattern using the Scherrer equation.
The X-ray photoelectron spectroscopy (XPS) measurements were carried out using a JEOL JPS-900MC with an AlKα anode operated at 20 mA and 10 kV. The oxygen 1s core electron levels in support oxides were recorded. Binding energies were calibrated with respect to C1s at 285.0 eV.
In situ FTIR spectra were recorded at room temperature on a JASCO FT/IR-620 equipped with a quartz IR cell connected to a conventional flow reaction system. The sample was pressed into 10–30 mg of a self-supporting wafer and mounted into the quartz IR cell with CaF2 windows. Spectra were measured accumulating 15 scans at a resolution of 4 cm−1. A reference spectrum of the catalyst wafer in He was subtracted from each spectrum. Prior to each experiment the catalyst disk was heated in H2(2%)/He flow (100 cm3 min−1) at temperatures shown in Table 1 for 10 min, followed by cooling to room temperature under He flow. Then, the catalyst was exposed to a flow of CO(250 ppm)/He for 100 s.
Fig. 1 (A) EXAFS Fourier transforms and (B) XANES spectra at Pt L3-edge recorded in situ under He at 40 °C after flowing H2 for 10 min at 200 °C. Ex situ XANES spectra of reference compounds (Pt foil and PtO2) are included in (B). |
Entry | Catalysts-xb | Conv. (%) | 3 yield (%) | 4 yield (%) | TOFc/h−1 |
---|---|---|---|---|---|
a Conversion of 1 and yields were determined by GC. b Average particle size (nm) of the supported metal. c TOF calculated using the number of surface metal atoms and the initial rate of formation of 3 measured under the condition where conversions were below 46%. | |||||
1 | Pt/Al2O3-0.8 | 100 | 93 | 2 | 101 |
2 | Pt/Al2O3-1.3 | 86 | 73 | 3 | 91 |
3 | Pt/Al2O3-2.2 | 60 | 50 | 3 | 65 |
4 | Pt/Al2O3-4.3 | 33 | 25 | 3 | 50 |
5 | Pt/Al2O3-7.8 | 20 | 11 | 3 | 25 |
6 | Pt/Al2O3-24 | 5 | 2 | 2 | 10 |
7 | Pt/CeO2-1.8 | 19 | 10 | 4 | 13 |
8 | Pt/ZrO2-2.3 | 45 | 36 | 3 | 48 |
9 | Pt/SiO2-2.5 | 41 | 36 | 1 | 43 |
10 | Pt/SiAl-2.1 | 39 | 31 | 2 | 41 |
11 | Pt/SiO2-8.7 | 37 | 16 | 17 | 30 |
12 | Pd/SiO2-5.3 | 30 | 23 | 5 | 32 |
13 | Rh/SiO2-5.2 | 14 | 9 | 4 | 12 |
14 | Ru/SiO2-5.1 | 8 | 4 | 3 | 6 |
15 | Ag/SiO2-5.0 | 3 | 0 | 0 | 0 |
16 | Au/SiO2-6.0 | 3 | 0 | 0 | 0 |
17 | Cu/SiO2-14 | 4 | 4 | 0 | 8 |
18 | Ag/Al2O3-0.8 | 2 | 2 | 0 | 0 |
19 | Pt black-25 | 6 | 2 | 2 | 7 |
20 | PtO2-28 | 2 | 0 | 0 | 0 |
21 | Pd/C | 39 | 31 | 5 | 22 |
22 | Pd black | 0 | 0 | 0 | 0 |
With Pt/Al2O3-0.8 as a standard catalyst, we examined the catalytic properties of the Pt NCs for the selective amine cross-coupling reaction. Fig. 2 shows a time course of the reaction of aniline 1 with 2 equivalents of iPr2NH 2 in the presence of Pt/Al2O3-0.8 (1 mol% of Pt). A typical time–conversion profile of the consecutive reaction was observed; the imine intermediate 4, formed at an initial period (t = 0.5 h), was consumed, and then yield of the hydrogenated product 3 increased with time. After 3 h, conversion of aniline 1 reached 100% and the desired amine 3 was obtained in 93% yield (Table 4, entry 1). The reaction was completely stopped by the removal of the catalyst from the reaction mixture; after stirring the reaction mixture for 0.5 h (3 yield = 44%), the catalyst was filtered off, and then the reaction did not proceed further when the reaction mixture was refluxed for 2.5 h. This result excludes a possible contribution of homogeneous catalysis of leached Pt species. We also studied the reuse of the Pt/Al2O3-0.8 catalyst. The catalyst can be easily separated from the reaction mixture by centrifugation. The separated catalyst was washed with acetone (5 mL) and distilled water (5 mL), followed by drying at 200 °C for 1 h, and by reducing in H2 at 200 °C for 10 min. The catalyst showed high yield of 3 (86–89%) during the successive three catalyst recycles (entries 2–4 in Table 4). The initial rate (TOF) for the third reuse test (91 h−1) was only slightly lower than the TOF for the fresh catalyst (101 h−1). The total turnover number (TON) based on the total Pt content is 338, which is higher than that of homogeneous Ir catalyst (TON = 98) for the same reaction.26 Changing the 1/2 ratio from 1:2 to 1:1 did not result in a marked decrease in the selectivity toward 3; the yield of 3 was 89% (Table 4, entry 5), which demonstrates a highly atom efficient feature of the present catalytic system. The above results clearly demonstrate that Pt/Al2O3-0.8 is a highly efficient heterogeneous catalyst for the present reaction.
Fig. 2 Yields of (△) unreacted 1, (○) 3 and (●) 4 for the reaction of aniline with iPr2NH using Pt/Al2O3-0.8 vs. reaction time. Reaction conditions are shown in Table 3. |
To study the scope and limitation of the present reaction using Pt/Al2O3-0.8, the alkylation of aniline with various aliphatic amines was tested (Table 4). Primary amines (entries 6–8) acted as effective amine donors except for 3-pentylamine (entry 8). The reaction of 1 with a secondary amine, di-sec-butylamine (entry 9), led to the formation of the corresponding N-alkylated amine in very good yield (93%). Ethyl transfer with triethylamine (entry 10) resulted in a moderate yield (62%). Then, we investigated the transfer of the isopropyl group from diisopropylamine to a range of anilines (Table 5). The reaction of electron-rich aniline derivatives (Table 5, entries 1–3, 5) proceeded smoothly and selectively to give the corresponding N-alkylated products in good yields (80–96%). The result for an electron-deficient aniline (entry 4) was not successful. The amination of the aniline derivative with a sterically hindered substituent at the ortho position was also effective (entry 6). Recently, Williams and co-workers26 reported the first example of selective amine alkylation using a [Cp*IrI2] complex even when both the amines are capable of undergoing oxidation to an imine. Thus, we carried out the alkylation of amines when both the amines could undergo oxidation. The reaction of benzylamine (entry 7) or aliphatic amines (entries 8 and 9) with iPr2NH 2 yielded mono-N-alkylated cross-coupling products with moderate yields (70–76%). This is the first example of heterogeneously catalyzed selective amine alkylation when both the amines are able to undergo oxidation. Note that by-products were produced by homo-coupling of benzyl and aliphatic amines (entries 7–9). Recently, we reported that an Al2O3-supported Pd cluster with an average Pd size of 1.8 nm (Pd/Al2O3-1.8) was a highly effective catalyst for N-alkylation of benzylamines with cyclic secondary amines.32 To compare the catalytic activity of Pt/Al2O3-0.8 and Pd/Al2O3-1.8, the substrate scope of Pd/Al2O3-1.8 catalyzed reactions is shown in Table S1 and Schemes S1 and S2 (ESI†). In contrast to Pt/Al2O3-0.8, Pd/Al2O3-1.8 was totally ineffective for the reaction of aniline with primary and secondary amines (entries 1–5 in Table S1†). The reaction of p-methoxyaniline and iPr2NH 2 using Pd/Al2O3-1.8 yielded a mono-N-alkylated product with moderate yields (71%) after 20 h (Scheme S1†), while in the case of Pt/Al2O3-0.8 excellent yield (96%) was attained after 3 h. The reaction of benzylamine with iPr2NH 2 (Scheme S2†) resulted in the formation of undesired byproducts, mainly formed via self-coupling of benzylamine, while in the case of Pt/Al2O3-0.8 moderate yield of the desired cross-coupling product (71%) was achieved. These results indicate that Pt/Al2O3-0.8 has higher activity and improved substrate scope compared with Pd/Al2O3-1.8, which is the most effective catalyst in our previous report.32
Entry | Amine substrate | Yielda (%) |
---|---|---|
a Yields determined by GC are based on the amine substrate. b Pt= 5 mol%. c t = 10 h. d t = 6 h. e Yield of amine produced by self-coupling the amine substrate. f Yield of imine produced by self-coupling the amine substrate. | ||
1 | 4-MeOC6H4NH2 | 96 |
2 | 4-MeC6H4NH2 | 92 |
3 | 4-tBuC6H4NH2 | 93 |
4b | 4-ClC6H4NH2 | 22 |
5c | 80 | |
6c | 69 | |
7 | 71 (14e, 3f) | |
8 | 76 (18e, 2f) | |
9d | 70 (19e, 9f) |
Fig. 3 Formation rates of 3 (○, □) and 4 (●, ■) as a function of the concentration of (A) iPr2NH (CiPr2NH = 0.12 to 2.0 M) and (B) aniline (Caniline = 0.12 to 2.0 M) for the reaction of aniline with iPr2NH using Pt/Al2O3-0.8. |
Fig. 4 Hammett plot for the reaction of p-substituted anilines with iPr2NH using Pt/Al2O3-0.8. |
Fig. 5 Mechanism of Pt/Al2O3-catalyzed alkylation of aniline with iPr2NH. |
Fig. 6 (□) TOF based on the number of surface Pt atoms and selectivity for (○) 3 or (●) 4vs. particle size of Pt in Pt/Al2O3. |
The selectivity at similar conversions (20–45%) is also plotted in Fig. 6. It is shown that a decrease in the Pt particle size results in an increase in the selectivity to 3 (decrease in the selectivity to 4). Chen et al. reported a theoretical study on dissociative adsorption of H2 on a Pt6 cluster and showed that, at low H coverage, the reaction on the Pt6 cluster is more exothermic with the chemisorptions energy twice that on a Pt(111) surface.37 This indicates larger bond energy of Pt–H on a small Pt cluster than that on a flat surface. Therefore, it is reasonable to assume that reactivity of Pt–H to the imine 4 should decrease with a decrease in the coordination numbers of Pt. This hypothesis is against the fact that hydrogenation of imine 4 to amine 3 is more favorable for the Pt catalyst with smaller size (Fig. 6). Thus, geometrical and electronic effects cannot explain why smaller Pt size results in the higher selectivity to amine 3. Knowing that Pt/Al2O3 catalysts with smaller Pt particle size have longer metal-oxide perimeter, the higher amine 3 selectivity for the smaller Pt size may be explained by larger number of Pt sites at the metal–support interface, which act as catalytically important sites for hydrogenation of the imine 4. In the literature of the transfer hydrogenation of imines with homogeneous catalysts, a cooperation mechanism of coordinatively unsaturated metal center and adjacent acid/base centers is widely accepted; H− in metal hydrides and H+ at a OH or NH group of the ligand transfer to the C and N atoms of the polar CN bond, respectively.38 The polar surface of Al2O3 at the metal–support boundary may help the formation of polar hydrogen species (Hδ−, Hδ+), which are effective for the transfer hydrogenation of the polar CN bond.
To discuss the role of the support in the Pt catalyzed alkylation of aniline, we prepared a series of supported Pt catalysts with the same Pt content (5 wt%) and a similar particle size (1.8–2.5 nm), but with a variety of supports. To estimate the electronic effect of the support, we measured the binding energy of the O1s electron in the support oxide by XPS analysis. It is established that the O1s binding energy of metal oxides decreases with increase in the electron density of oxygen in the metal oxide, or in other words, basicity of the metal oxide surface.18,39 XPS spectra of the O1s region in the supports are given in Fig. 7. The O1s binding energy depended on the oxides and decreased in the order of SiO2Al2O3 > SiO2 > Al2O3 > ZrO2 > CeO2. Catalytic data are plotted in Fig. 8 as a function of the O1s binding energy of the support oxides. The selectivity and TOF are estimated at similar conversions (19–45%). There is a volcano-type relationship between the catalytic activity (TOF) and the O1s binding energy, and Al2O3 as a moderately basic (moderately acidic) support gives the highest activity. The support with high O1s binding energy (basic oxides) gave lower selectivity to amine 3 (higher selectivity to imine 4).
Fig. 7 XPS spectra of the O1s core level region of the support materials. |
Fig. 8 (□) TOF and selectivity for (○) 3 or (●) 4 for Pt/MOx catalysts as a function of O1s binding energy of support oxides (from Fig. 7). |
The effect of acid–base property of support oxides on the electron density of platinum has been investigated by Yazawa et al.16 Based on the results of the CO adsorption IR experiment and Pt L3-edge XANES of Pt catalysts supported on various metal oxides, they showed that the electron density of the supported Pt decreased with an increase in the acid strength of the support materials. It is widely recognized that the wavenumber of CO stretching increases with an increase in the electron deficiency of the metal.39 We measured CO adsorption IR spectra of Pt/MOx catalysts to discuss the effect of the support acidity on the electronic state of Pt (Fig. 9). In a region characteristic to linearly coordinated CO on metallic Pt (2100–2000 cm−1), the wavenumber of the maximum intensity depended on the kind of the support material and increased in the order of CeO2 < Al2O3 < ZrO2 < SiO2 < SiO2Al2O3. This trend is consistent with the order of the support acidity estimated from O1s binding energy (Fig. 7) except for ZrO2, whose IR band has a relatively broad feature. This result indicates that an acidic support (with electron poor surface oxygen atom) decreases the electron density of metallic Pt, which is consistent with the conclusion of Yazawa et al.16 We believe that the exceptional data for Pt/ZrO2-2.3 may result from its relatively broad IR feature. To discuss a possible reason for the support acidity–basicity dependent catalytic properties in Fig. 8, catalytic data in Fig. 8 were re-plotted as a function of the wavenumber of the maximum intensity of CO stretching (Fig. 10). There is a volcano-type relationship between TOF and the wavenumber of CO, indicating that Pt with moderate electron density is advantageous to the activity. Among the catalysts tested, Pt on the most basic support, CeO2, showed the highest electron density and it showed the lowest selectivity to amine 3 (highest selectivity to imine 4). The time–conversion profile of Pt/CeO2 catalyzed reaction of 1 and 2 (Fig. S3, ESI†) showed that the yield of the unreduced product 4 continuously increased with time even after 24 h, indicating that hydrogenation of 4 to 3 is a slow step in the case of Pt/CeO2. This is in contrast to the result for Pt/Al2O3-0.8 (Fig. 2) that the imine 4 once produced is hydrogenated to 3 after 0.5 h, indicating that hydrogenation of 4 to 3 is not slow.
Fig. 9 IR spectra of CO adsorbed on Pt/MOx catalysts measured at room temperature. |
Fig. 10 (□) TOF and selectivity for (○) 3 or (●) 4 for Pt/MOx catalysts as a function of the wavenumber of the linear CO on Pt (from Fig. 9). |
The support effect on the electronic properties of Pt particles has been investigated by Koningsberger and co-workers, using experimental (XAFS) and DFT calculations.17,18 For hydrogen adsorption on K doped Pt/Al2O3, they concluded that the Pt–H bond strength increases with basicity of the support oxide.17 They proposed that the shift in the Pt 6s and p and 5d orbitals upon interaction with the support is the origin of the higher Pt–H bond strength on ionic (basic) supports. This model has explained why Pt on an acidic support gave higher activity in hydrogenolysis and hydrogenation reactions; the less stable Pt–H species has higher reactivity.18 Taking this trend into account, we propose the following hypothesis on a possible role of acidity/basicity of the support in the catalytic activity and selectivity. Pt on an acidic support has relatively low electron density and consequently a weak Pt–H bond. According to linear free energy relationships, the weak Pt–H bond should result in the low activity for the C–H bond dissociation of iPr2NH as an initial step of the catalytic cycle. In contrast, the Pt–H bond on a basic support is too stable, and hydrogenation of 4 to 3 by Pt–H is slow, which results in low rate of H removal from Pt sites to regenerate coordinatively unsaturated Pt sites. Therefore, Al2O3 with moderate acidity shows the highest rate of formation of 3. Another possible explanation for the support dependent activity and selectivity might be based on the metal–support bi-functional mechanism, where acid–base sites on the support take part in a certain step of the catalytic cycle. Basic sites of the support might play a role in C–H abstraction of iPr2NH, and acid sites might be effective for a polarization CN bond of imine and thus may promote hydrogenation of imine.
To discuss the reason why Pt and Pd show higher activity than other metals, TOF and selectivity for various metal-loaded SiO2 (from entries 11–17 in Table 3) are plotted in Fig. 11 as a function of the d-band center (εd) relative to the Fermi energy (EF) for the clean metal surface (Hammer–Nørskov model).35,41 Note that the metal particle size of the M/SiO2 catalysts is in the range of 5.0–8.7 nm except for Cu catalyst (13.8 nm). The activity (TOF) shows a volcano-type dependence on the εd − EF value. Taking into account a frequently observed tendency that the further the d-band center is from EF the weaker is the M–H bond,40 the result indicates that the moderate M–H bond strength is favorable for the present reaction. We assume that the following explanations will account for these trends. Metals with weak M–H bond strength (Ag, Au, Cu) cannot effectively promote the dehydrogenation of iPr2NH. For the metals with strong M–H bond strength (Rh, Ru), hydrogenation of 4 to 3 by surface M–H species and regeneration of coordinatively unsaturated metal sites are slow. Consequently, Pt and Pd with moderate M–H bond strength gave the highest efficiency for the present catalytic reaction.
Fig. 11 TOF based on the number of surface metal atoms for SiO2-supported metal catalysts (from entries 11–17 in Table 3) vs. center of d-band relative to Fermi level (εd − EF). |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cy00476c |
This journal is © The Royal Society of Chemistry 2012 |