Meenakshisundaram
Sankar
*ab,
Qian
He
c,
Simon
Dawson
b,
Ewa
Nowicka
b,
Li
Lu
c,
Pieter C. A.
Bruijnincx
a,
Andrew M.
Beale
de,
Christopher J.
Kiely
c and
Bert M.
Weckhuysen
*a
aInorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands. E-mail: B.M.Weckhuysen@uu.nl
bCardiff Catalysis Institute, School of Chemistry, Cardiff University, Cardiff CF10 3AT, UK. E-mail: Sankar@cardiff.ac.uk; Tel: +44 (0)29 2087 5748
cDepartment of Material Science and Engineering, Lehigh University, 5 East Packer Avenue, Bethlehem, PA 18015-3195, USA
dResearch Complex at Harwell, Rutherford Appleton Laboratory, Oxfordshire, OX11 0FA, UK
eDepartment of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK
First published on 23rd March 2016
The synthesis and functionalization of imines and amines are key steps in the preparation of many fine chemicals and for pharmaceuticals in particular. Traditionally, metal complexes are used as homogeneous catalysts for these organic transformations. Here we report gold–palladium and ruthenium–palladium nano-alloys supported on TiO2 acting as highly efficient heterogeneous catalysts for the one-pot synthesis of the imine N-benzylideneaniline and the secondary amine N-benzylaniline directly from the easily available and stable nitrobenzene and benzyl alcohol precursors using a hydrogen auto-transfer strategy. These reactions were carried out without any added external hydrogen, sacrificial hydrogen donor or a homogeneous base. The bimetallic catalysts were prepared by the recently developed modified impregnation strategy, giving efficient control of size and nano-alloy composition. Both bimetallic catalysts were found to be far more active than their monometallic analogues due to a synergistic effect. Based on the turnover numbers the catalytic activities follow the order Ru < Pd < Au ≪ Au–Pd < Ru–Pd. Aberration corrected scanning transmission electron microscopy (AC-STEM) and X-ray absorption spectroscopy (XAFS) studies of these catalysts revealed that the reason for the observed synergistic effect is the electronic modification of the metal sites in the case of the Au–Pd system and a size stabilisation effect in the case of the Ru–Pd catalyst.
Recently, Sankar et al. reported an excess anion modification of the CIm method (the modified impregnation (MIm) method) for the synthesis of supported gold–palladium nano-alloys, which affords a more precise control over size, composition and morphology.8 By adding excess hydrochloric acid to the Au and Pd precursors in the wet-impregnation stage, and subsequent direct gas-phase reduction, Au–Pd alloy particle size can be controlled to be within the 1–6 nm range with no size significant dependent composition variation of Au and Pd. Occasionally, some micron-scale Au-rich particles were found in some of the MIm derived Au–Pd catalysts. Such micron-sized particles could be completely eliminated, though, by increasing the hydrochloric acid concentration during the preparation stage. However, the latter catalyst, i.e., without any micron-sized gold-rich particles, was found to be unstable for catalytic applications, whereas the former catalyst with occasional micron-scale particles was found to be stable for catalytic re-use.8 The stable bimetallic catalysts showed significant improvements in the catalytic activities for the direct synthesis of hydrogen peroxide and the aerobic benzyl alcohol oxidation reactions.10 Very recently, we also reported the synthesis of supported Ru–Pd catalysts, using the same MIm route; no micron-scale particles were detected for these catalysts, suggesting all of the metal precursors are converted into alloyed nanoparticles with controlled size and composition. These supported bimetallic Ru–Pd nano-alloy catalysts were exceptionally active, selective and stable in the hydrogenation of biomass derived levulinic acid to γ-valerolactone.9
Dehydrogenation reactions are often carried out in the presence of stoichiometric amounts or a large excess of oxygen, peroxides, iodates, metal oxides or sacrificial hydrogen acceptors to circumvent the thermodynamic restrictions associated with such dehydrogenations.10 In all these processes, the liberated hydrogen is “wasted” and ends up in hydrogenated by-products (e.g., H2O). An example of this kind of reaction is the oxidative dehydrogenation of benzyl alcohol to benzaldehyde, where oxygen is used as the sacrificial hydrogen acceptor to form water.11 From an atom-economic perspective, the expensive hydrogen produced by the dehydrogenation reaction should be more effectively used (in situ), for instance in a coupled hydrogenation reaction. Here, we demonstrate such an efficient use of the hydrogen liberated in the dehydrogenation of benzyl alcohol (1), by using nitrobenzene (3) as the hydrogen-acceptor rather than O2 thus reducing nitrobenzene effectively to aniline (4). The products of these reactions, i.e., benzaldehyde (2) and aniline, readily couple to form N-benzylideneaniline (5). When 1 is used in excess, 5 is further hydrogenated to N-benzylaniline (6) (Fig. 1). Furthermore, the rate of the overall reaction increases with the increase in the amount of alcohol. This sequence of reactions has been previously reported and are known as acceptor-less dehydrogenation reactions, hydrogen auto-transfer reactions or as a borrowing hydrogen strategy.10,12,13 Such reactions have proven extremely valuable for the synthesis of an assortment of useful compounds, including amines and amides, without stoichiometric reagents and harmful by-products. However in order to compete with the economically more advantageous conventional strategies using molecular H2, the catalysts for these hydrogen-auto transfer processes have to be exceptionally active and selective. Various heterogeneous catalysts have been reported for the N-alkylation of aniline using benzyl alcohol using this strategy.14,15 However, only a few catalytic systems have been reported for the direct synthesis of imines and N-alkylamines from nitroarenes and alcohols using this hydrogen auto transfer strategy. Most of these use homogeneous Ru, Ir or Pd-based metal complexes.15–18 To the best of our knowledge, only a few research groups have used heterogeneous catalysts for this transformation process, and most of these systems require a homogeneous base for this direct transformation of nitrobenzene and benzyl alcohol to the corresponding benzyl imines and amines.19,20 Two groups have reported the use of supported monometallic gold nanoparticles for this one-pot synthesis.21,22
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Fig. 1 Sequence of reactions involved in the one-pot tandem synthesis of N-benzylideneaniline (5) and N-benzylaniline (6) from nitrobenzene (3) and benzyl alcohol (1). |
Here, we show that supported gold–palladium and ruthenium–palladium nano-alloys are efficient catalysts for this hydrogen auto-transfer reaction, greatly surpassing the catalytic activities of their supported monometallic counterparts when prepared and tested under identical reaction conditions. Using abberation corrected scanning transmission electron microscopy (AC-STEM) and X-ray absorption spectroscopy (XAFS) characterisation data, we rationalise the observed catalytic behaviour of these supported gold–palladium and ruthenium–palladium nano-alloy catalysts.
Total TON = [mol(3) converted/moltotal metal in the catalyst]. |
The amine turnover number (Amine TON) was calculated using the formula
Amine TON = [mol(6) formed/moltotal metal in the catalyst]. |
Product selectivity is defined as the fraction of the identified products. The products from benzyl alcohol dehydrogenation and disproportionation (benzaldehyde and toluene) have not been quantified.
Earlier, Sankar et al. have reported that, for the oxidative dehydrogenation of 1, TiO2-supported Au–Pd nano-alloys also catalyse an unwanted disproportionation reaction, resulting in the production of toluene and benzaldehyde, besides the desired dehydrogenation reaction to form benzaldehyde.23 It was further reported that MgO-supported gold–palladium catalysts exclusively favour the dehydrogenation reaction by switching-off the disproportionation reaction.23 A 1% AuPd/MgO (MIm) catalyst was therefore also tested for the current hydrogen auto-transfer reaction, but was found to have a much lower activity (23% conversion) compared to the TiO2-supported catalyst (99% conversion). An activated carbon-supported Ru–Pd catalyst also proved to be less active (9% conversion) (Table 1). In addition to the choice of support, the preparation method is also expected to strongly influence the activity, selectivity and stability of these supported bimetallic catalysts.3,8 Prati et al. demonstrated the sol-immobilization (SIm) technique as an effective way of preparing supported Au–Pd catalysts.4 Hence, 1% Au–Pd/TiO2 (SIm) and 1% AuPd/MgO (SIm) catalysts were also therefore tested in the hydrogen auto-transfer reaction for comparative purposes, but neither of these catalyst preparations proved to be very effective (Table 1) displaying only 4% and 3% conversion levels respectively. The catalytic results thus clearly show that the TiO2-supported bimetallic catalysts prepared by the MIm route perform best in the direct synthesis of 5 and 6 from 1 and 3. To further understand the role that the amount of 1 has on the resultant activity, we performed reactions with different 1 to 3 molar ratios (i.e. 5 & 3) using the best catalyst 1% RuPd/TiO2 (MIm) and the results are presented in Table 1. The results clearly indicate that the activity depends on the amount of benzyl alcohol present, or in other words, the extent of the dehydrogenation reaction. For the reaction with a 1 to 3 molar ratio of 5, the molar conversion of 3 is 60% with a selectivity of 93% to product 5. For the corresponding reaction performed with a molar ratio of 3, the conversion is found to be 32% with 97% selectivity to product 5. It is important to note that for all the other standard reactions reported in this article a molar ratio of 10 was used. Another approach to synthesize substituted amines is the reductive alkylation of nitrobenzene using benzyl alcohol (equimolar) as the alkylating agent under H2 atmosphere. To test our bimetallic 1% RuPd/TiO2 (MIm) catalyst for this reductive alkylation reaction, we used an equimolar mixture of 1 & 3 under 20 bar of H2. After 3 h of reaction, a 78% molar conversion of 3 was achieved with an 84% selectivity to product 6. This result clearly indicates that this bimetallic catalyst is effective for the reductive alkylation reaction as well. Further optimization studies are currently in progress for this reaction.
Catalyst | Preparation method | Nitrobenzene conversion (%) | Product selectivity (%) | |
---|---|---|---|---|
5 | 6 | |||
a Reaction conditions: catalysts: 0.1 g; nitrobenzene: 4.5 mmol; benzyl alcohol: 45 mmol; mesitylene (solvent): 5 mL; Ar: 20 bar; T: 433 K; t: 3 h. b Nitrobenzene: 4.5 mmol; benzyl alcohol: 22.5 mmol (benzyl alcohol vs. nitrobenzene = 5). c Nitrobenzene: 4.5 mmol; benzyl alcohol: 13.5 mmol (benzyl alcohol vs. nitrobenzene = 3). d Nitrobenzene: 4.5 mmol; benzyl alcohol: 4.5 mmol (benzyl alcohol vs. nitrobenzene = 1); reaction performed at 20 bar H2. | ||||
1% AuPd/TiO2 | SIm | 4 | >99 | — |
1% Au–Pd/MgO | SIm | 3 | >99 | — |
1% Au–Pd/TiO2 | MIm | 99 | 88 | 12 |
1% Au–Pd/MgO | MIm | 23 | >99 | — |
1% Ru–Pd/TiO2 | MIm | 99 | 54 | 45 |
1% Ru–Pd/MgO | MIm | 26 | >99 | — |
1% RuPd/C | MIm | 9 | >99 | — |
1% RuPd/TiO2b | MIm | 60 | 93 | 7 |
1% RuPd/TiO2c | MIm | 32 | 97 | 3 |
1% RuPd/TiO2d | MIm | 78 | 16 | 84 |
The heterogeneous nature of the most active catalyst (1% Ru–Pd/TiO2 (MIm)) was demonstrated by three methods; namely (a) hot filtration, (b) ICP-MS analysis of the reaction mixture for metal content and (c) reusability of the recovered catalyst. These catalytic tests were performed in a 50 mL Radley's glass reactor held at 413 K and a pressure of 1 bar of He. In the hot filtration method, the catalyst was filtered off after 30 min (8% conversion) and the reaction mixture was allowed to react for a further 150 min under standard reaction conditions. No increase in conversion (9%) and selectivity was seen after filtration (Fig. 4a). The reaction without catalyst removal had a conversion of 23% over the same time-period. ICP-MS measurements showed the filtered reaction mixture to contain negligible amounts of Ru and Pd (1.5% for Ru and 0.6% for Pd of the original amounts used in the reaction). Furthermore, the catalyst could be re-used three times without any loss in activity (8–9%). Interestingly however, the selectivity to 6 increased progressively from 27% for the fresh catalyst to 40% for the three-times used catalyst at the expense of 5 for the spent catalysts (Fig. 4b).
To characterize the composition and nanostructure of these highly active bimetallic MIm catalysts and to arrive at a structure/activity correlation, aberration corrected scanning transmission electron microscopy (AC-STEM) studies were carried out; the SIm catalysts have previously been extensively characterized and are not reported here.6,7,23 Analysis of the 1% AuPd/TiO2 MIm sample by high-angle annular dark-field (HAADF) – STEM imaging indicated that the mean size of the supported metal particles was ∼1.5 nm (Fig. 5(a) and (b)). Even smaller sub-nm clusters were also apparent in this sample (Fig. S2(a)†) as were occasional very large μm-scale Au particles. X-ray energy dispersive spectroscopic (XEDS) analysis of individual nm-scale particles confirmed them to be Au–Pd alloys (Fig. 5(c) and (d)). A comparative HAADF-STEM study of the corresponding 1% RuPd/TiO2 MIm sample shows that the mean size of the supported metal particles was slightly smaller at ∼1.2 nm (Fig. 5(e) and (f)). Once again, many sub-nm clusters were apparent (Fig. S2(b)†), but in this case SEM analysis showed that the sample was devoid of any μm-scale metal particles. XEDS analysis confirmed that the nm-scale particles were indeed Ru–Pd alloys (Fig. 5(g) and (h)). To better understand how representative these structures are of the entirety of the catalyst sample, and to complement the STEM data, these catalysts were also characterised by X-ray absorption spectroscopy (XAS).26
The STEM observations of the presence of bimetallic species are confirmed by the XAFS data recorded at the Au L3, Pd K and Ru K edges, respectively.9 The Au and Pd XANES data for selected catalysts are given in Fig. 6, whilst the results from the analysis of the EXAFS data are given in Table 2 along with some example EXAFS spectra (including the results from the least squares fitting of the data) given in the ESI† (Fig. S3). The EXAFS data for the 1% Au–Pd/TiO2 (MIm) and 1% Ru–Pd/TiO2 catalysts are reported in our recent article.9 The Au L3-edge data shown arise from a dipole-allowed transition of an initial 2p3/2 electron to a 5d state and are therefore sensitive to changes in the electronic density of states (both initial and final).27–29 When comparing the reference 1% Au/TiO2 (MIm) sample with that of the bimetallic 1% Au–Pd/TiO2 samples (MIm and SIm) we observe an enhancement of the feature at ∼11935 eV in the latter two samples which appears consistent with the formation of bimetallic species. This signal is proposed to arise as a result of charge transfer/electron donation from Au to the more electropositive Pd. Such charge transfer between Au and Pd leading to a slight positive charge build-up on Pd has previously been confirmed by XPS analysis of these materials.9 This is accompanied by a ‘blue shift’ in the EXAFS oscillations, as well as a reduced oscillation frequency consistent with the formation of shorter Au–Pd bonds (as compared to the longer Au–Au bonds ∼2.85 Å). That the particles are bimetallic can be seen immediately from the Fourier transform (FT) data. Two intense peaks in the FTs are observed as a consequence of a ‘π phase flip’ in the backscattering amplitude from 6 Å−1 for Au (or indeed for all elements where Z > 78) resulting in a splitting of the major contribution in the FT into a high and low R (distance) component. This occurs when two elements are present in equivalent amounts; often the splitting and intensity of the low r-space contribution becomes more intense with an increasing number of bimetallic bonds.30–32 A similar effect is seen in the Pd K-edge FTs of the EXAFS data although a mismatch in the total coordination number from an analysis of the two edges has previously been shown to be due to the presence of large (∼μm sized) Au particles in addition to bimetallic species and for the 1% Au–Pd/TiO2 (MIm) sample, atomically dispersed Pd species.9 The Pd K-edge XANES data are less revealing regarding the extent of alloy formation, but are however, sensitive to the degree of Pd–O interactions. A greater rising absorption edge (lower density of states) is observed in the 1% Ru–Pd/TiO2 (MIm) sample which EXAFS data suggest contains the highest number of Pd–O neighbours (they are therefore the smallest particles and possess fewest number of metal–metal (M–M) bonds). It should be noted however that the similarity in X-ray scattering contrast between Pd and Ru precludes distinguishing between the two components of the bimetal in the EXAFS data although a first shell analysis assuming one M–M scatterer type could be performed on the basis that XEDS confirms the bimetallic nature of the sample.
Sample | Au–Au R (Å) | (N) | 2σ2 (Å2) | Au–Pd (Å) | (N) | 2σ2 (Å2) | Pd–Pd R (Å) | (N) | 2σ2 (Å2) | Pd–Au R (Å) | N | 2σ2 (Å2) | Ref. |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
E
f ∼ ±15 eV; R values for all data range from 28–38%.
a A Pd–O contribution at 2.02 Å, N = 0.3, 2σ2 = 0.013 Å2 is also present.
b The EXAFS data and the associated Fourier transform data for this catalyst is presented in the ESI (Fig. S3).
c A Pd–O contribution at 2.02 Å, N = 1, 2σ2 = 0.0013 Å2 is also present.
d A Pd–O contribution at 1.99 Å, N = 1.3, 2σ2 = 0.013 Å2 is also present.
e A Pd–O contribution at 1.99 Å, N = 2, 2σ2 = 0.007 Å2 is also present.
Afac values, 0.94 (Pd/Ru) and 0.98 Au. Debye–Waller factors were initially determined from the AuPd/TiO2 SIm sample and not refined for the remaining AuPd samples. |
|||||||||||||
1% Au–Pd/TiO2 (SIm)a | 2.8 | 7.8 | 0.017 | 2.78 | 4.1 | 0.014 | 2.75 | 4 | 0.014 | 2.78 | 3.9 | 0.016 | Current Workb |
1% Au–Pd/TiO2 (MIm)c | 2.8 | 4.8 | 0.017 | 2.77 | 2.5 | 0.014 | 2.75 | 1 | 0.014 | 2.78 | 2.0 | 0.016 | 9 |
1% Au–Pd/MgO (MIm)d | 2.79 | 8.0 | 0.017 | 2.73 | 3.3 | 0.014 | 2.72 | 3.5 | 0.014 | 2.75 | 2.9 | 0.016 | Current Work |
Ru–O | Ru–Ru(Pd) | Pd–O | Pd–Pd(Ru) | ||||||||||
1% Ru–Pd/TiO2 (MIm)e | 2.0 | 3.0 | 0.011 | 2.69 | 2 | 0.023 | 2.0 | 2.7 | 0.009 | 2.72 | 1.8 | 0.019 | 9 |
The closeness in the coordination numbers of both species from an analysis of both edges supports the notion that the Pd and Ru exist in a similar environment (i.e. within intimately mixed bimetallic particles). Furthermore, the low coordination numbers obtained are consistent with the STEM analysis in that the particles are on average smaller than those seen for 1% Au–Pd/TiO2 (MIm). These observations from EXAFS and electron microscopy clearly indicate that they are complimentary techniques for characterizing these supported bimetallic nano-alloys. From the above characterization data, it is evident that the bimetallic catalysts have a homogeneous random alloy structure.
Previously, we applied the most active bimetallic catalysts (1% AuPd/TiO2 (MIm), 1% RuPd/TiO2) in a different reaction, namely the selective hydrogenation of levulinic acid, and reported in detail on their characterization.9 Methods used included X-ray photoelectron spectroscopy (XPS), FF-IR of CO adsorption in addition to XAS and AC-STEM data of this material. FT-IR CO adsorption data clearly revealed that the electronic structure of both Au and Pd are modified by their close proximity to each other. This modification of the electronic structures of Au and Pd, where the Au atoms have a slight negative charge and the Pd atoms have slight positive charge, could be a contributing factor for the observed enhanced catalytic activity for the bimetallic Au–Pd catalyst. In the case of the Ru–Pd catalyst, it was very difficult to differentiate between Ru and Pd, however indirect evidence from all the characterisation techniques employed suggest that this material has a slightly smaller particle size compared to the Au–Pd catalyst. This stabilization effect, where the second metal stabilizes the smaller size of the first metal, could be the underlying reason for the enhanced catalytic activity of this latter material.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6cy00425c |
This journal is © The Royal Society of Chemistry 2016 |