Research Article pubs.acs.org/acscatalysis
Enhancing the Catalytic Properties of Ruthenium Nanoparticle-SILP Catalysts by Dilution with Iron Kylie L. Luska,† Alexis Bordet,§ Simon Tricard,§ Ilya Sinev,∥ Wolfgang Grünert,∥ Bruno Chaudret,§ and Walter Leitner*,†,‡ †
Institut für Technische und Makromolekulare Chemie, RWTH Aachen University, Worringerweg 2, 52074 Aachen, Germany Max-Planck-Institut für Kohlenforschung, 45470 Mülheim an der Ruhr, Germany § Laboratoire de Physique et Chimie des Nano-Objets, Université de Toulouse, LPCNO, INSA, UPS, CNRS-UMR5215, 135 Avenue de Rangueil, 31077 Toulouse, France ∥ Lehrstuhl für Technische Chemie, Ruhr-Universität Bochum, Universitätsstraße 150, 44801 Bochum, Germany ‡
S Supporting Information *
ABSTRACT: The partial replacement of ruthenium by iron (“dilution”) provided enhanced catalytic activities and selectivities for bimetallic iron−ruthenium nanoparticles immobilized on a supported ionic liquid phase (FeRuNPs@SILP). An organometallic synthetic approach to the preparation of FeRuNPs@SILP allowed for a controlled and flexible incorporation of Fe into bimetallic FeRu NPs. The hydrogenation of substituted aromatic substrates using bimetallic FeRuNPs@SILP showed high catalytic activities and selectivities for the reduction of a variety of unsaturated moieties without saturation of the aromatic ring. The formation of a bimetallic phase not only leads to an enhanced differentiation of the hydrogenation selectivity, but even reversed the order of functional group hydrogenation in certain cases. In particular, bimetallic FeRuNPs@SILP (Fe:Ru = 25:75) were found to exhibit accelerated reaction rates for CO hydrogenation within furan-based substrates which were >4 times faster than monometallic RuNPs@SILP. Thus, the controlled incorporation of the non-noble metal into the bimetallic phase provided novel catalytic properties that could not be obtained using either of the monometallic catalysts. KEYWORDS: bimetallic nanoparticles, iron, ruthenium, supported ionic liquid phases, selective hydrogenation
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INTRODUCTION Ruthenium is widely used as a metal component in catalysts for hydrogenation and hydrogenolysis processes. Molecular and material systems provide a suitable balance between catalytic activity and stability1 that has been attributed to the ability to undergo relatively thermoneutral exchange reactions with various donor atoms (e.g., oxygen, nitrogen, carbon).2 Its tolerance to oxygen donor functionalities has stimulated intensive research toward the application of ruthenium-based catalysts in the conversion of biomass, as these feedstocks possess high carbon-to-oxygen ratios and are transformed in the presence of water.3−17 Although, ruthenium is considerably cheaper than other noble metals such as platinum, palladium, and rhodium, there has been significant interest in its replacement with the third-row homologue iron as an abundant, inexpensive, and environmentally benign alternative to the noble metal catalysts. Iron nanoparticles (Fe NPs) have long been known to possess activity in Fischer−Tropsch synthesis,18 while recent publications also have demonstrated their activity for the hydrogenation of organic substrates.19−27 Several groups have © XXXX American Chemical Society
reported Fe NPs as effective catalysts for the reduction of C−C multiple bonds in alkenes and alkynes under mild conditions;19−25 however, the hydrogenation of other unsaturated functional groups has been challenging to achieve.26−29 On the contrary, ruthenium is an effective catalyst for the hydrogenation of various polar unsaturated functionalities (e.g., aldehydes, ketones, esters, imines, nitriles, etc.). Thus, the partial replacement (“dilution”) of ruthenium with iron, in the formation of bimetallic catalysts, could provide an appropriate balance between the cost and performance of Ru-based catalysts. Bimetallic FeRu NPs have been reported as catalysts for Fischer−Tropsch synthesis30−33 and the water−gas shift reaction;34 however, only a few reports have investigated their use in hydrogenation reactions.35−39 Chaudret and co-workers have recently reported the preparation of colloidal FeRu NPs and found that the Fe:Ru ratio influenced the selectivity in hydrogenation catalysis.35 Received: March 18, 2016 Revised: May 2, 2016
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DOI: 10.1021/acscatal.6b00796 ACS Catal. 2016, 6, 3719−3726
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achieved according to a previous reported procedure and involved the condensation of [1-butyl-3-(3-triethoxysilylpropyl)-imidazolium]NTf2 with dehydroxylated SiO2.49 The synthesis of bimetallic FeRu NPs immobilized on a SILP (FeRuNPs@SILP) involved the in situ reduction of a mesitylene solution (5 mL) of {Fe[N(Si(CH3)3)2]2}2 and [Ru(cod) (cot)] (total metal loading = 0.20 mmol) in the presence of the SILP (0.50 g) under an atmosphere of H2(g) (3 bar) at 150 °C (Scheme 1). Upon stirring the suspension under
In general, the preparation of bimetallic NPs has been an effective strategy to tune the catalytic properties of heterogeneous catalysts,40−42 in which the major challenge in this field concerns the development of synthetic methodologies that permit control over the structure of the bimetallic NPs (e.g., metal oxidation state, NP size and distribution, atomic distribution) and thus an ability to tailor the catalytic properties of bimetallic NPs.42 In our present work, we have adapted an organometallic approach for the synthesis of bimetallic NPs, previously reported by some of us,35 toward the preparation of iron−ruthenium NPs (FeRu NPs) embedded in a supported ionic liquid phase (SILP) (Figure 1).
Scheme 1. Synthesis of Iron−Ruthenium Nanoparticles Immobilized on a Supported Ionic Liquid Phase (FeRuNPs@SILP)
an atmosphere of H2(g) for 18 h, the mixture transformed from a light green to a dark black color indicating the coreduction of the organometallic precursors and the formation of FeRu NPs. This synthetic procedure allowed for a facile variation of the Fe:Ru ratio by adjusting the quantities of {Fe[N(Si(CH3)3)2]2}2 and [Ru(cod)(cot)] used for the synthesis, in which a series of bimetallic FeRuNPs@SILP was prepared: Ru100, Fe20Ru80, Fe25Ru75, Fe33Ru67, Fe60Ru40, and Fe100 (see the Supporting Information (SI) for complete SILP and FeRuNP@SILP synthetic and characterization data). Scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) was used to confirm the Fe:Ru ratio and the total metal loading (1 wt % total metal loading) (Table S1), while TEM confirmed the presence of small and well-dispersed NPs in all cases (2.9−3.5 nm) (Figure S1). Further, both metals appear well-dispersed throughout the particles as evidenced by elemental mapping using scanning transmission electron microscopy with energy dispersive spectroscopy (STEM/EDS) (Figure 2).
Figure 1. Schematic representation of bimetallic iron−ruthenium nanoparticles immobilized on a supported ionic liquid phase (FeRuNPs@SILP) as catalysts for the selective hydrogenation of substituted aromatic substrates.
The design of our bimetallic catalysts involved the covalent attachment of an imidazolium ionic liquid (IL) onto silica to decorate the inorganic support material with a stabilization matrix for FeRu NPs generated from organometallic complexes as precursors. Although these surface layers are of course no longer “liquid phases”, immobilized IL-type structures are commonly known as SILPs43−47 and have been investigated as supports of homogeneous44−48 and heterogeneous catalysts.6,49−54 For NP-based systems, the SILP allows for (a) facilitated access of the substrates and removal of products from the active metal sites; (b) enhanced NP stability through the combination of electrosteric stabilization of the IL and steric protection of the silica support; and (c) molecular tuneability of the NP reactivity by modifying the IL structure.17 The organometallic approach toward the synthesis of FeRu NPs stabilized on a SILP (FeRuNPs@SILP) provides a facile method to tailor the quantity of Fe introduced into the bimetallic FeRu NPs by controlling the ratio between the metal constituents,35 while the SILP permitted a retention of the materials characteristics of the NPs (i.e., NP size, shape, distribution, atomic distribution). This enabled us to systematically vary the Fe:Ru ratio in this matrix, providing intriguing evidence that the resulting catalysts possessed exclusive and partly enhanced catalytic properties that differ from those of the Fe and Ru monometallic counterparts. Extensive characterization of the materials, including X-ray absorption spectroscopy (XAS), allowed for a rationalization of the influence of the Fe:Ru ratio on the catalytic properties of FeRuNPs@SILP.
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RESULTS AND DISCUSSION Synthesis and Characterization of FeRuNPs@SILP. The preparation of a supported ionic liquid phase (SILP) was
Figure 2. (a) High-resolution TEM of Fe25Ru75NPs@SILP and STEM/EDS elemental mappings of (b) Fe−K (yellow), (c) Ru−L (purple), and (d) the overlay of Fe−K and Ru−L. 3720
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ACS Catalysis Further characterization of the mono- and bimetallic NPs@ SILP was achieved using X-ray absorption fine structure (XAFS) measurements which showed spectral features that were dependent on the Fe content of the bimetallic NPs (see the SI for complete XAFS data analysis). The Ru K-edge X-ray absorption near-edge structure (XANES) spectra of monometallic Ru100NPs@SILP and bimetallic Fe25Ru75NPs@SILP were similar to a Ru foil reference and thus indicated a predominately zerovalent state for both catalysts (Figure 3a).55
Figure 4. Fe K-edge spectra of the (a) XANES and (b) EXAFS regions for Fe100NPs@SILP (blue), Fe60Ru40NPs (purple), and Fe25Ru75NPs (red).
and EXAFS data for higher Fe loadings provided evidence for the presence of both Fe oxide and metallic Fe within monometallic Fe100NPs@SILP and bimetallic Fe60Ru40NPs@ SILP. In contrast, the EXAFS spectrum of Fe25Ru75NPs@SILP contained a much weaker Fe−O backscattering event at 1.5 Å, as well as two additional signals at 1.95 and 2.45 Å. Modeling of the Fe25Ru75NPs@SILP Fe−K and Ru−K EXAFS spectra (involving Fe−O, Fe−Fe, Fe−Ru, Ru−Ru, and Ru−Fe backscattering) was successful and therefore provided evidence for the formation of Fe0 included in a bimetallic FeRu phase (Figure S8). Lastly, the EXAFS data was employed to determine the atomic distribution (J), also known as alloy extent, within the bimetallic FeRu NPs using an approach developed by Hwang and co-workers (see the SI for details on atomic distribution calculations).58 From these calculations it was determined that the atomic distributions within Fe60Ru40NPs@SILP and Fe25Ru75NPs@SILP were similar and provided evidence for a homophilic bimetallic structure; a bimetallic phase possessing a higher extent of Ru and Fe homoatomic interactions than for a perfect bimetallic structure (Table S3). In conclusion, these results demonstrate that the organometallic approach provides an effective method for the controlled preparation of supported bimetallic FeRu NPs in various compositions. This synthetic procedure allowed for a facile variation of the Fe:Ru ratio by adjusting the quantities of the organometallic precursors, {Fe[N(Si(CH3)3)2]2}2 and [Ru(cod)(cot)]. XAFS measurements showed that the corresponding FeRuNPs@SILP possessed homophilic bimetallic structures independent of the Fe:Ru ratio, whereas the Fe oxidation state varied with the NP composition. While the formation of an FeRu bimetallic phase was dominant at low Fe contents, high Fe contents provided a mixture of Fe oxide and FeRu alloy. This dependence of the Fe oxidation state on the Fe:Ru ratio was not observed for the analogous colloidal FeRu NPs35 and may at least partly result from the presence of the SILP. Thus, this method allowed for supported bimetallic FeRu NPs to be prepared under mild conditions (150 °C, 3 bar H2) with the ability to tune the properties of the NPs, while traditional syntheses of supported FeRu bimetallic catalysts require high temperature reduction using H2 (>450 °C) and high Ru contents to favor the formation of bimetallic FeRu phases.30−33
Figure 3. Ru K-edge spectra of the (a) XANES and (b) EXAFS regions for a Ru foil reference (black), Ru100NPs@SILP (blue), Fe25Ru75NPs (red), Fe60Ru40NPs (purple), and a RuO2 reference (light blue).
In contrast, Fe60Ru40NPs@SILP showed spectral features similar to RuO2, which suggests that the Ru may have been partly oxidized in NPs with high Fe content. The Fouriertransformed Ru K-edge extended X-ray absorption fine structure (EXAFS) spectra for mono- and bimetallic NPs displayed dominate signals originating from Ru−Ru backscattering (2.30 Å, Figure 3b). The bimetallic NPs also exhibited a shoulder signal at 1.80 Å resulting from Ru−Fe backscattering, where the magnitude of the Ru−Fe scattering signal was larger for Fe60Ru40NPs@SILP in comparison to Fe25Ru75NPs@SILP. The Fe K-edge XANES spectra of bimetallic Fe25Ru75NPs@ SILP (Figure 4a) possessed a broad pre-edge signal (7115 eV) similar to that of Fe foil (Figure S4); however, the spectral features above the absorption edge differed from that of the Fe reference. Similar changes within Fe K-edge XANES spectra were reported for Fe in bimetallic NPs with other noble metals such as Pt56 and Rh57 and were ascribed to formation of alloytype structures. In the present series, the XANES of samples with higher Fe loadings, Fe60Ru40NPs@SILP and Fe100NPs@ SILP, contained clear indications for oxidation of Fe as the spectra contained sharp pre-edge signals and an absorption edge with a stronger intensity drop above a more intense white line. While the pre-edge region was similar to that of a Fe2O3 reference, the characteristics of the absorption edge differed, and thus support for FeRu bimetallic formation was sought in the EXAFS region. Fe K-edge EXAFS showed a significant Fe−O scattering signal at 1.5 Å similar to that for the Fe2O3 reference (Figure S5) and a weak signal for metallic Fe−Fe backscattering at 1.95 (shoulder signal) and 2.30 Å for Fe60Ru40NPs@SILP and Fe100NPs@SILP, respectively (Figure 4b). Thus, the XANES 3721
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ACS Catalysis Hydrogenation of Substituted Aromatic Substrates. The hydrogenation of CC and CO functional groups in substituted aromatic substrates was used to probe the catalytic properties of monometallic Fe-, Ru-, and bimetallic FeRu-based SILP materials (Table 1). Monometallic Fe100NPs@SILP
The selectivity of the CC, CO, and aromatic hydrogenation in the α,β-unsaturated ketone benzylideneacetone (5) to the products 5a−d was used to gain further insight into the catalytic properties of the bimetallic FeRuNPs@SILP (Table 2). The monometallic Ru100NPs@SILP proved a highly active
Table 1. Hydrogenation of Substituted Aromatic Substrates Using Monometallic and Bimetallic NPs@SILPa
Table 2. Hydrogenation of Benzylideneacetone (5) Using FeRuNPs@SILPa
product yield (%)b entry
catalyst
conversion (%)
5a
5b
5c
5d
1 2 3 4 5 6c
Ru100 Fe20Ru80 Fe25Ru75 Fe33Ru67 Fe60Ru40 Fe100 + Ru100
>99 >99 >99 96 5 >99
0 0 0 84 5 0
0 0 0 0 0 0
0 >99 99 12 0 0
>99 0 0 0 0 >99
a
Reaction conditions: FeRuNPs@SILP (40 mg, 0.016 mmol total metal loading), mesitylene (0.5 mL), benzylideneacetone (0.4 mmol), H2 (20 bar), 120 °C, 18 h. bDetermined by GC using tetradecane as an internal standard. cPhysical mixture of Fe100NPs@SILP (10 mg) and Ru100NPs@SILP (30 mg) (0.016 mmol total metal loading, Fe:Ru ratio = 25:75).
catalyst for the deep hydrogenation of 5 toward the completely saturated product 5d (entry 1). The introduction of small quantities of Fe into the Ru NPs, Fe20Ru80NPs@SILP and Fe25Ru75NPs@SILP, controlled the reaction toward the formation of the aromatic alcohol 5c in quantitative yields under identical conditions (entries 2 and 3). Further increasing the quantity of Fe in Fe33Ru67NPs@SILP provided high selectivity toward the production of aromatic ketone 5a at almost quantitative conversion (entry 4). However, the catalytic activity collapsed almost completely at Fe:Ru ratios above 50:50, as evidenced with Fe60Ru40NPs@SILP (entry 5). The hydrogenation of 5 was also performed using a physical mixture of the monometallic Fe100NPs@SILP and Ru100NPs@ SILP catalysts (Table 2, entry 6). Using a Fe:Ru ratio of 25:75 provided a quantitative yield of the completely saturated alcohol 5d, similar to those results obtained using only Ru100NPs@SILP (Table 2, entry 1). Furthermore, a hot filtration test using Fe25Ru75NPs@SILP demonstrated that hydrogenation ceased immediately once the reaction mixture was separated from the solid material, excluding the leaching of soluble molecular clusters as the active species. Therefore, the unique catalytic properties of the FeRuNP@SILP catalysts are directly correlated with the presence of the bimetallic FeRu phase in the SILP matrix. Next, a competition experiment was performed in which the CC and CO moieties of benzylideneacetone (5) were separated into individual substrates (Scheme 2). The hydrogenation of β-methylstyrene (1) and 4-phenyl-2-butanone (5a) over Fe25Ru75NPs@SILP produced propylbenzene (1a) and 4phenyl-2-butanol (5c) in >99% and 86% yields, respectively. Thus, the bimetallic catalyst can simultaneously hydrogenate CC and CO moieties, wherein the reduction of CC bonds occurs preferentially. Overall, these results indicate that the dilution of Ru by Fe greatly enhances the differentiation of the hydrogenation activity for the functional moieties vs the aryl rings. Fe contents
a
Reaction conditions: NPs@SILP (40 mg, 0.016 mmol total metal loading), mesitylene (0.5 mL), substrate (0.4 mmol), H2 (20 bar), 120 °C (except for substrate 4 which used 100 °C), 18 h. bDetermined by GC using tetradecane as an internal standard. cDecomposition to mixture of unidentified products. dRemainder of reaction mixture was composed of N,N-dibenzylmethylamine.
showed almost negligible activity for the hydrogenation of βmethylstyrene (1) or acetophenone (2) as only a 4% conversion of 1 to propylbenzene (1a) and no conversion of 2 were observed. As expected, monometallic Ru100NPs@SILP were highly active catalysts, leading to the hydrogenation of not only the olefin and carbonyl groups, but also of the aromatic moieties. Thus, 1 and 2 were hydrogenated to the completely saturated products propylcyclohexane (1b) and 1-cyclohexylethanol (2b) in quantitative yields, respectively. In sharp contrast, bimetallic Fe25Ru75NPs@SILP possessed high activities for CC and CO hydrogenation, but not for the aromatic ring as 1 and 2 were converted almost completely to propylbenzene (1a) and 1-phenylethanol (2a) over the bimetallic FeRu catalyst. Monometallic and bimetallic catalysts were also tested for the hydrogenation of N-benzylidenemethylamine (3) and furfural (4) (Table 1). Fe100NPs@SILP were inactive for the hydrogenation of both 3 and 4. Treatment of 3 under the standard reaction conditions in the presence of Ru100NPs@SILP led only to complex mixtures of unidentified decomposition products, while Fe25Ru75NPs@SILP produced high yields of Nbenzylmethylamine (3a) (83%). Hydrogenation of 4 using Ru100NPs@SILP provided a mixture of furfuryl alcohol (4a) (53%) and tetrahydrofurfuryl alcohol (4b) (47%), whereas a quantitative yield of 4a was obtained using the bimetallic Fe25Ru75NP@SILP catalyst. 3722
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ACS Catalysis Scheme 2. Competition Experiment between βMethylstyrene (1) and 4-Phenyl-2-Butanone (5a)a
under shaken and not magnetically stirred conditions in order to reduce mechanical degradation of the catalyst. Under shaken conditions, the catalytic activity and selectivity for Fe25Ru75NPs@SILP toward aromatic products 6a and 6c were maintained for at least two cycles of catalysis (Table 3, entries 6 and 7). Characterization of Fe25Ru75NPs@SILP after catalysis showed no significant NP growth or aggregation by TEM (Table S2) and a stable bimetallic structure as evidenced by EXAFS (Figures S6−S7 and Table S3). The reaction profiles for the hydrogenation of 6 using Fe25Ru75NPs@SILP and Ru100NPs@SILP reveal that the different selectivities are not caused by a simple decrease of the general hydrogenation activity upon dilution with iron, but result f rom a concurrent increase and decrease of the activity for certain functional groups (Figure 5). Both catalysts possess a
a
Reaction conditions: Fe25Ru75NPs@SILP (40 mg, 0.016 mmol total metal loading), mesitylene (0.5 mL), 4-phenyl-2-butanone (0.4 mmol), β-methylstyrene (0.4 mmol), H2 (20 bar), 120 °C, 9 h.
in the range 25−33% essentially shut down the hydrogenation of aromatic rings while maintaining practically useful activities for the CC and CO groups. This allows excellent selectivities in the order CC > CO ≫ arene. Next, we moved to multifunctional heteroaromatic substrates, where the intrinsic reactivity of the functional groups over monometallic Ru NP catalysts follow the order CC > heteroarene > C O.59 Furfuralacetone (6) was selected to examine the selective hydrogenation of furan-based substrates as encouraging results were obtained in the reduction of furfural (4) using the bimetallic FeRuNPs@SILP. Furfuralacetone also represents an important lignocellulose-derived platform chemical obtained as the primary aldol condensation product of furfural and acetone; a strategy that allows for pentose-derived intermediates to be upgraded to higher carbon products and fuel components.17,59−62 Under the standard conditions of the present study, hydrogenation of 6 catalyzed by Ru100NPs@SILP provided a mixture of the tetrahydrofuran products 6b and 6d in 35% and 65% yields, respectively (Table 3, entry 1). In Table 3. Hydrogenation of Furfuralacetone (6) using FeRuNPs@SILPa
product yield (%)b entry
catalyst
conversion (%)
6a
6b
6c
6d
1 2 3 4 5 6c 7d
Ru100 Fe20Ru80 Fe25Ru75 Fe33Ru67 Fe60Ru40 Fe25Ru75(first cycle) Fe25Ru75(second cycle)
>99 >99 >99 >99 4 >99 >99
0 0 0 84 4 13 13
35 0 0 0 0 0 0
0 94 94 16 0 81 83
65 6 6 0 0 6 4
a
Reaction conditions: FeRuNPs@SILP (40 mg, 0.016 mmol total metal loading), mesitylene (0.5 mL), furfuralacetone (0.4 mmol), H2 (20 bar), 100 °C, 18 h. bDetermined by GC using tetradecane as an internal standard. c1st cycle, 100 °C, 18 h, shaken at 240 rpm. d2nd cycle, 100 °C, 18 h, shaken at 240 rpm.
Figure 5. Reaction profile for the hydrogenation of furfuralacetone (6) using (a) Ru100NPs@SILP and (b) Fe25Ru75NPs@SILP. Reaction conditions: Conversion >99%, FeRuNPs@SILP (40 mg, 0.016 mmol total metal loading), mesitylene (0.5 mL), furfuralacetone (0.4 mmol), H2 (20 bar), 100 °C.
sharp contrast, the bimetallic catalysts Fe20Ru80NPs@SILP and Fe25Ru75NPs@SILP remained highly active catalysts, but now produced the aromatic alcohol 6c in excellent yields of 94% (entries 2 and 3). Again, increasing the Fe content further allowed a shift in the product selectivity toward the saturated ketone 6a with Fe33Ru67NPs@SILP (entry 4), but an almost complete lose in activity for Fe contents above 50% (Fe60Ru40NPs@SILP, entry 5). The recyclability of the bimetallic Fe25Ru75NP@SILP catalyst was evaluated briefly for the hydrogenation of 6 conducted
very high activity toward the hydrogenation of the CC bond as >99% conversion of 6 was achieved after 1 h. The monometallic Ru100NPs@SILP catalyst exhibited a substantial arene hydrogenation activity from the beginning of the reaction as a mixture of the furanic (6a, 28%) and tetrahydrofuranic ketone (6b, 62%) was already formed after 1 h. The yield of the fully saturated ketone 6b reached a maximum after 4 h and a slow consumption of 6b to aliphatic alcohol 6d occurred over the course of the reaction. In stark contrast, the Fe25Ru75NP@ 3723
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group was significantly enhanced with the bimetallic catalyst in case of furan-based substrate 6. This indicates that dilution of an active noble metal catalyst by a non-noble metal, which cannot form an active catalyst by itself, can result not only in improved relative selectivities, but also in enhanced absolute activities at least for hydrogenation catalysts. Taking this into account may open intriguing opportunities in the current quest for the replacement of noble metals in catalysis (e.g., in the area of biomass conversion). Naturally, direct substitution of noble metals will often not be possible due to lack of catalytic activity of the corresponding third-row metals. The results presented here show that controlled dilution through formation of bimetallic catalysts can not only help to reduce the required amount of noble metals, but also may be used as a strategy to access catalytic materials with improved catalytic properties that cannot be achieved by either of the individual metals alone.
SILP catalyst showed a very selective formation of aromatic ketone 6a after 1 h, which was then smoothly converted to aromatic alcohol 6c. Only small quantities of the tetrahydrofuran-based ketone 6b and alcohol 6d were observed throughout the course of the reaction and >90% yield of aromatic alcohol 6c was obtained after 12 h. Reaction rates for the CO hydrogenation of intermediates 6a and 6c by Fe25Ru75NPs@SILP and Ru100NPs@SILP were determined to be 0.107 and 0.025 M/h, respectively (Figure S9). These data demonstrate that that the hydrogenation of the heteroarene is suppressed and the ketone hydrogenation is enhanced at the same time, when ca. one-fourth of ruthenium is replaced with iron. As summarized in Scheme 3, the Scheme 3. Major Reaction Pathways for the Hydrogenation of Furfuralacetone (6) Using Bimetallic Fe25Ru75NPs@SILP (Blue) and Monometallic Ru100NPs@SILP (Red) Catalystsa
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EXPERIMENTAL SECTION Safety Warning. High-pressure experiments with compressed H2(g) must be carried out only with appropriate equipment and under rigorous safety precautions. General. The supported ionic liquid phase (SILP) was synthesized as previously reported.49 If not otherwise stated, the synthesis of the SILP and FeRu NPs immobilized on SILPs (FeRuNPs@SILP) were carried out under an inert atmosphere using standard Schlenk techniques or within a glovebox. After synthesis, SILPs and FeRuNPs@SILPs were stored under an inert atmosphere. All catalyst solutions were prepared under an inert atmosphere. Furfuralacetone was synthesized according to a known literature method.63 {Fe[N(Si(CH3)3)2]2}2 and [Ru(cod)(cot)] were obtained from NanoMePS. Furfuralacetone and benzylideneacetone were purified by sublimation, and furfural was distilled, prior to use. Mesitlyene (from VWR Prolabo, 99%) was dried over alumina desiccant and degassed via freeze−pump−thaw cycling. All other chemicals and solvents were purchased from commercial sources and used without purification. Analytics. Solution-state nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AV300, 400, or 600 MHz spectrometer. 1H and 13C NMR spectra were calibrated to TMS using the residual solvent signal. Highpressure experiments were performed using in-house engineered 10 and 20 mL stainless steel finger autoclaves. Catalytic reactions were performed in glass inlets using a magnetic stirbar (600 rpm) and an aluminum heating block. Recyclability experiments were performed using a GFL Orbital Shaker (Model 3017) operating at 240 rpm. Gas chromatography (GC) was performed on a Thermo Scientific Chromatograph Tace GC Ultra equipped with a CP-Wax 52 CB column from Agilent. Brunauer−Emmett−Teller (BET) measurements were performed on a Quadrasord SI automated Surface Area and Pore Size Analyzer from Quantachrome Instruments and data analysis using QuadraWin 5-04. Transmission electron microscopic (TEM) images were collected using a JEOL JEM 1400 operated at 120 kV and using a tungsten filament. Scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) was performed on a JEOL JSM 7800F operated at 10 kV with a thermally assisted Schottky electron gun and equipped with a Bruker XFlash 6|60 detector (silicon drift detector technology). Scanning transmission electron microscopy with energy dispersive spectroscopy (STEM/EDS) was obtained on a Hitachi S-5500 ultrahigh-resolution cold field
a
Note: thickness of the arrows qualitatively reflects the relative rates of the individual steps.
bimetallic catalyst thus opens an alternative parallel reaction pathway in the consecutive hydrogenation sequence. The “diluted” catalyst enables the highly selective and efficient synthesis of compound 6c, a species that could not be targeted with monometallic Ru-based catalysts due to the intrinsic functional group selectivity of the Ru NPs.59
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SUMMARY AND CONCLUSIONS The present study demonstrates that partial substitution of iron for ruthenium can result in enhanced catalytic activities and selectivities for the hydrogenation of multifunctional aromatic and heteroaromatic substrates. An organometallic synthetic approach to prepare bimetallic iron−ruthenium nanoparticles immobilized on a supported ionic liquid phase (FeRuNPs@ SILP) provided a molecular tool to control the ratio of Fe:Ru with all other material characteristics remaining largely identical. At high Fe contents (above 50%), a high degree of oxidized Fe- and Ru-species was found within these materials, while at Fe contents in the range of ca. 20−30% the metallic state was retained for both metals in what appears to be a welldispersed FeRu bimetallic phase. The structural characteristics are also reflected in the catalytic properties of these materials. The monometallic Fe and the bimetallic FeRuNPs@SILP with iron content >50% were practically inactive for all hydrogenation reactions under investigation. In contrast, the materials with ca. 20−30% Fe content showed excellent hydrogenation activity, but with a very distinct reactivity profile as compared to the monometallic Ru particles. The typically high catalytic activity for arene and heteroarene groups of the ruthenium metal was suppressed almost completely upon dilution with iron, whereas very significant activities for CC, CO, and CN groups were still observed with the bimetallic particles. Most significantly, the activity for the hydrogenation of the ketone to the alcohol 3724
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ACS Catalysis
reactor was cooled in an ice bath, carefully vented and the organic phase was removed by decantation. The reaction mixture was analyzed via GC using tetradecane as an internal standard. The determination of the reaction profile for the hydrogenation of furfuralacetone was performed as individual experiments as outlined above and were stopped after the desired reaction time. Reaction rates for the CO hydrogenation of intermediates 6a and 6c were determined from the GC data of the reaction profiles. For recycling experiments, the reaction was performed using a shaker plate operating at 240 rpm. After catalysis, the reaction mixture was decanted from the catalyst under an inert atmosphere and additional furfuralacetone and mesitylene were added directly to the catalyst. Note: care must be taken to prepare all catalyst solutions under an inert atmosphere as exposure of the bimetallic catalysts to atmospheric conditions prior to catalysis can alter their catalytic properties (Table S4).
emission scanning microscope with a Thermo Scientific NORAN System 7 X-ray Microanalysis unit at an acceleration voltage of 30 kV. X-ray absorption fine structure (XAFS) measurements were carried out on the CLÆSS beamline of the ALBA synchrotron facility (Barcelona, Spain). Synthesis of Iron:Ruthenium Nanoparticles. Fe60Ru40NPs@SILP. A solution of {Fe[N(Si(CH3)3)2]2}2 (45.2 mg, 0.060 mmol) in mesitlyene (2 mL) was combined with a solution of [Ru(cod)(cot)] (25.2 mg, 0.080 mmol) in mesitlyene (2 mL) in a Fischer−Porter bottle (70 mL). The SILP (500 mg) was added to the solution of metal precursors along with mesitylene (1 mL), and the suspension was stirred under argon at rt for 30 min. The Fischer−Porter bottle was evacuated and backfilled with H2(g) and the suspension was stirred at 150 °C under H2(g) (3 bar) for 18 h. Under this reducing environment, a dark black powder and clear supernatant were obtained indicating the immobilization of FeRu NPs onto the SILP. FeRuNPs@SILP were isolated by decantation of mesitlyene. The black powder was then washed with fresh mesitylene (5 mL) and dried in vacuo at 45 °C for 1 h. Samples were prepared for TEM by ultrasound dispersion of FeRuNPs@SILP in mesitylene under ambient atmosphere. A drop of the suspension was deposited onto a copper TEM grid with an amorphous carbon support film and dried in vacuo prior to analysis. The NP size and distribution was determined from the measurement of >150 spherical particles chosen in arbitrary areas of enlarged micrographs. Reaction profile data was obtained from individual experiments stopped after the desired time. Fe33Ru67NPs@SILP. A solution of {Fe[N(Si(CH3)3)2]2}2 (24.8 mg, 0.033 mmol) in mesitlyene (2 mL) was combined with a solution of [Ru(cod)(cot)] (42.2 mg, 0.134 mmol) in mesitlyene (2 mL) in a Fischer−Porter bottle (70 mL). The remainder of the reaction was performed as outlined for Fe60Ru40@SILP. Fe25Ru75NPs@SILP. A solution of {Fe[N(Si(CH3)3)2]2}2 (18.8 mg, 0.025 mmol) in mesitlyene (2 mL) was combined with a solution of [Ru(cod)(cot)] (47.3 mg, 0.150 mmol) in mesitlyene (2 mL) in a Fischer−Porter bottle (70 mL). The remainder of the reaction was performed as outlined for Fe60Ru40@SILP. Fe20Ru80NPs@SILP. A solution of {Fe[N(Si(CH3)3)2]2}2 (15.1 mg, 0.020 mmol) in mesitlyene (2 mL) was combined with a solution of [Ru(cod)(cot)] (50.5 mg, 0.160 mmol) in mesitlyene (2 mL) in a Fischer−Porter bottle (70 mL). The remainder of the reaction was performed as outlined for Fe60Ru40@SILP. Ru100NPs@SILP. A solution of [Ru(cod)(cot)] (63.1 mg, 0.200 mmol) in mesitlyene (4 mL) was combined with SILP (500 mg) and mesitlyene (1 mL). The remainder of the reaction was performed as outlined for Fe60Ru40@SILP. Fe100NPs@SILP. A solution of {Fe[N(Si(CH3)3)2]2}2 (75.3 mg, 0.100 mmol) in mesitlyene (4 mL) was combined with SILP (500 mg) and mesitlyene (1 mL). The remainder of the reaction was performed as outlined for Fe60Ru40@SILP. Hydrogenation of Substituted Aromatic Substrates. As an example, FeRuNPs@SILP (40 mg, 0.016 mmol total metal loading), furfuralacetone (55 mg, 0.4 mmol), and mesitylene (0.5 mL) were combined in a glass insert within a glovebox and placed in a high-pressure autoclave. Upon pressurization with H2(g), the reaction mixture was magnetically stirred (600 rpm) at 100 °C in an aluminum heating block under 20 bar H2(g) for 18 h. Once the reaction was finished, the
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b00796. Complete synthesis, characterization, and catalysis data for FeRuNPs@SILP (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: (+49) 241-8022177. Tel.: (+49) 241-80-26481. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was performed as a part of the Cluster of Excellence “Tailor-Made Fuels from Biomass”, which is funded by the Excellence Initiative of the German federal and state government to promote science and research at German universities. The authors would like to thank Simon Cayez (INSA de Toulouse) for his assistance with SEM/EDS measurements, Karl-Josef Vaeßen (ITMC, RWTH Aachen University) for N2(g) adsorption measurements, Hans-Josef Bongard (MaxPlanck-Institut für Kohlenforschung) for STEM/EDS analysis, and Dr. Nils Theyssen (Max-Planck-Institut für Kohlenforschung) for his generous support. K.L.L. would like to thank Deutscher Akademischer Austaush Dienst (DAAD) for financial support.
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REFERENCES
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