Facile Synthesis of Magnetically Recoverable Pd and Ru Catalysts for

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Article Cite This: ACS Omega 2018, 3, 14717−14725

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Facile Synthesis of Magnetically Recoverable Pd and Ru Catalysts for 4‑Nitrophenol Reduction: Identifying Key Factors Lennon Gregor,† Austin K. Reilly,† Tomer A. Dickstein,† Sumaira Mazhar,† Stanley Bram,† David Gene Morgan,† Yaroslav Losovyj,† Maren Pink,† Barry D. Stein,‡ Valentina G. Matveeva,§ and Lyudmila M. Bronstein*,†,∥,⊥ †

Department of Chemistry, Indiana University, 800 E. Kirkwood Avenue, Bloomington, Indiana 47405, United States Department of Biology, Indiana University, 1001 E. Third Street, Bloomington, Indiana 47405, United States § Regional Technological Center, Tver State University, Zhelyabova Street, 33, Tver 170100, Russia ∥ A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov Street, Moscow 119991 Russia ⊥ Faculty of Science, Department of Physics, King Abdulaziz University, P.O. Box 80303, Jeddah 21589, Saudi Arabia

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‡

S Supporting Information *

ABSTRACT: This paper reports the development of robust Pd- and Ru-containing magnetically recoverable catalysts in a one-pot procedure using commercially available, branched polyethyleneimine (PEI) as capping and reducing agent. For both catalytic metals, ∼3 nm nanoparticles (NPs) are stabilized in the PEI shell of magnetite NPs, whose aggregation allows for prompt magnetic separation. The catalyst properties were studied in a model reaction of 4nitrophenol hydrogenation to 4-aminophenol with NaBH4. A similar catalytic NP size allowed us to decouple the NP size impact on the catalytic performance from other parameters and to follow the influence of the catalytic metal type and amount as well as the PEI amount on the catalytic activity. The best catalytic performances, the 1.2 min−1 rate constant and the 433.2 min−1 turnover frequency, are obtained for the Ru-containing catalyst. This is discussed in terms of stability of Ru hydride facilitating the surface-hydrogen transfer and the presence of Ru4+ species on the Ru NP surface facilitating the nitro group adsorption, both leading to an increased catalyst efficiency. High catalytic activity as well as the high stability of the catalyst performance in five consecutive catalytic cycles after magnetic separation makes this catalyst promising for nitroarene hydrogenation reactions.

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green chemistry reactions such as dehydrogenation,27 reduction of 4-nitrophenol (4-NP),29,31 the Tsuji−Trost reaction,30 one-pot synthesis of 2-amino-3-cyano-4H-pyran derivatives via a multicomponent reaction,28 and so forth. In a number of cases, magnetic NPs were preformed and then coated with PEI28,29 or added to the premade catalytic nanocomposite,30,31 leading to a multistep catalyst preparation. Here, we report the development of a family of nanoparticulate, magnetically recoverable catalysts based on commercially available, branched PEI. The catalyst components including magnetite and catalytic NPs are formed in a one-pot procedure and stabilized by PEI. The activity of the catalysts synthesized has been tested in a model reactionthe reduction of 4-NP to 4-aminophenol (4-AP) with an excess of NaBH4allowing one to compare catalysts suitable for a nitro

INTRODUCTION Nanoparticulate catalysts bearing magnetic nanoparticles (NPs) along with catalytic NPs received considerable attention because of added capability of magnetic recovery, resulting in greener routes, energy preservation, and less-expensive target products.1−9 Different capping molecules or supports have been used to stabilize catalytic and magnetic NPs, including various polymers.10−20 Polymer-containing functional groups allow for coordination with metal species and stabilization of the forming NP surface. However, to ensure accessibility to catalytic centers, the polymer should form a nondense shell around the catalytic NPs. It was documented that branched polymers/copolymers as stabilizing molecules allow for a better access to catalytic NPs and higher catalytic activities compared to linear polymers.14,21−23 Branched polyethyleneimine (PEI) has been explored for the stabilization of catalytic24−28 or catalytic/magnetic NPs29−31 in the development of various hydrophilic catalysts. These catalysts can be used in water or alcohols for important © 2018 American Chemical Society

Received: September 14, 2018 Accepted: October 25, 2018 Published: November 2, 2018 14717

DOI: 10.1021/acsomega.8b02382 ACS Omega 2018, 3, 14717−14725

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group hydrogenation.32−34 It is noteworthy that 4-AP is a building block in organic syntheses including the industrial synthesis of paracetamol.35 There are numerous studies of this reaction using heterogeneous catalysts containing transition metals (Au, Pd, Ag, Ru, etc.).34,36−43 It was demonstrated that the catalytic activity in the case of NPs strongly depends on a number of parameters such as NP size, structure, stabilizing molecules, support, environment, and so forth.44−48 Various magnetically recoverable catalysts for the 4-NP reduction have been reported,41,49−51 but in the majority of cases, the catalyst synthesis requires several steps, making it a complicated process. In this work, the PEI-based magnetically recoverable catalyst containing Ru NPs showed exceptional activity in the 4-NP reduction exceeding that of other reported catalysts by ∼58%.20 Excellent catalytic properties, sustainable preparation conditions, and easy magnetic recovery make these robust catalysts promising for nitroarene hydrogenation processes.

partial reduction of Fe3+ to Fe2+ is quite likely. The crystallite size from the Scherrer equation is 8.8 nm, which is close to the NP size. After iron oxide NPs are prefabricated, the reaction temperature was lowered to 60 °C before the addition of a catalytic metal compound to let the latter coordinate with amino groups of PEI for about 24 h. Subsequently, the reaction temperature was increased to TEG boiling and the catalytic metal compound was allowed to decompose/reduce. If the metal precursor is added at the TEG boiling temperature, very large Pd or Ru NPs are formed, which are unattached to the iron oxide NPs (the image is not shown). This is very different from the synthesis of Pd- and Pt-containing magnetically recoverable catalysts formed in the presence of hydrophobic polymers in benzyl ether, where the Pd and Pt compound solutions were injected at ∼285 °C and ∼3 nm Pd and Pt NPs were stabilized in the polymer shell of Fe3O4 NPs.12 Apparently, TEG (a strongly coordinating solvent) interacts with catalytic metal species, preventing immediate coordination with the PEI amino groups. After the coordination of metal ions with PEI and the decomposition of Pd acetate at 285 °C, the catalytic NPs are formed in the PEI shell of iron oxide NP aggregates (Figure 1). The Fourier-transform infrared (FTIR) data (see Figure S2 and the text in Supporting Information) show that the NH2-scissoring band of PEI is significantly perturbed after the nanocomposite formation, indicating the interaction with NPs present in the nanocomposite. Structure of Pd-Containing Catalysts. Figure 2a shows a typical TEM image of a Pd-containing magnetically recover-

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RESULTS AND DISCUSSION Catalyst Syntheses. Syntheses of the magnetically recoverable catalysts containing Pd or Ru species were carried out in a one-pot solvothermal procedure (Figure 1). In the first

Figure 1. Schematic representation of the catalyst synthesis.

Figure 2. (a) TEM image and (b) XRD pattern of IO-Pd-2. Red notations in (b) are for the Pd reflection, while black notations are for spinel reflections. Red arrows in (a) show small, potentially Pd NPs.

step, iron oxide NPs are formed by the thermal decomposition of Fe(acac)3 in the presence of PEI as the capping polymer at the triethylene glycol (TEG) boiling temperature. The transmission electron microscopy (TEM) image of the sample isolated at this stage (Figure S1a, the Supporting Information) shows the formation of NP aggregates that ensures fast magnetic separation. The individual NP diameter in this sample is 9.6, but it can vary between 8.6 and 10.0 depending on the amount and the molecular weight of PEI used (Table S1, Supporting Information). The X-ray diffraction (XRD) pattern of IO-PEI (Figure S1b, Supporting Information) shows a set of reflections which are typical of spinel and thus magnetite.52,53 However, the resemblance of the XRD patterns of both magnetite and maghemite NPs due to line broadening makes this assignment tentative. This hypothesis is based on the inert medium and the presence of PEI, which is both reducing and stabilizing agent,54,55 so the

able catalyst (IO-Pd-2) prepared with 0.1 mmol of Pd acetate (Table 1). Iron oxide NP aggregates are preserved and show two populations of NPs with mean diameters of 10.5 and 3.2 nm, which we tentatively assigned to Fe3O4 and Pd NPs, respectively. The XRD diffraction pattern of this sample (Figure 2b) shows a set of reflections typical for magnetite (see Figure S1b, Supporting Information) and a very broad signal marked in red which could be assigned to the (111) reflection of Pd0.56 The Fe3O4 crystallite size is 9.6 nm, which is in good agreement with the TEM data. The Pd crystallite size cannot be determined due to a broad signal and overlap with a weak reflection from magnetite. The high-resolution (HR) TEM image of IO-Pd-2 is presented in Figure S3 (Supporting Information). It clearly shows crystalline lattice of both Pd (smaller) and Fe3O4 (larger) NPs. The scanning TEM (STEM) dark-field image of IO-Pd-2 (Figure 3a) demonstrates that Pd NPs (brighter spots due to a higher electron density) are located on/in the 14718


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Table 1. Reaction Conditions of Nanocomposite Synthesesa sample notation

PEI, g

Pd(CH3COO)2, mmol

IO-Pd-1 IO-Pd-2 IO-Pd-3 IO-Pd-4 IO-Ru-1 IO-Ru-2 IO-Ru-3

0.15 0.113 0.113 0.15 0.15 0.113 0.15

0.1 0.1 0.2 0.2

RuCl3, mmol

0.05 0.1 0.1

IO NP size, nm 11.7 11.4 11.0 10.8 11.3 9.8 8.7

± ± ± ± ± ± ±

1.3 1.9 1.4 1.3 1.6 1.5 1.2

Pd or Ru NP size, nm 2.6 2.9 2.9 2.9 3.5 3.6 2.8

± ± ± ± ± ± ±

0.6 0.4 0.5 0.5 0.6 0.3 0.4

a

Fe(acac)3 (0.353 g, 1 mmol), TEG (7 mL).

Figure 3. (a) STEM dark-field image and EDS maps of IO-Pd-2 for (b) Fe, (c) O, (d) Pd, (e) Fe−O mix, and (f) Fe−Pd mix.

Fe3O4 NP aggregates (larger gray spots). Moreover, energy dispersive X-ray spectroscopy (EDS) maps of Fe and O (Figure 3) fully coincide, while the Pd map has a different shape, indicating that Pd NPs most probably do not contain oxygen. It is noteworthy that Pd NPs tend to cluster in the Fe3O4 NP aggregates but they are not aggregated and should be available for a substrate in a catalytic reaction. X-ray photoelectron spectroscopy (XPS) has been employed to evaluate the catalyst composition and further elucidate its structure (Figure 4). The HR XPS Fe 2p spectrum of IO-Pd-2 (Figure 4a) displays a main peak at a BE of 710.5 eV, characteristic of iron oxides.57 A satellite, which might be seen for Fe3+ ions at a BE higher than the main peak, is lacking, demonstrating excess of the Fe3+ species beyond the magnetite Fe3+/Fe2+ = 2:1 ratio.58−60 In the case of Fe3O4, the Fe3+ and Fe2+ satellites result in a plateau between the Fe 2p3/2 and Fe 2p1/2 peaks,61 similar to our case. The HR XPS Pd 3d spectrum (Figure 4b) was deconvoluted into a single peak in the Pd 3d region with a BE of 335.2 eV, which is characteristic of Pd0.62−64 This is in good agreement with the XRD data. Structure of Ru-Containing Catalysts. The TEM image and XRD pattern of the Ru-containing catalyst prepared with 100 μmol of RuCl3 are shown in Figure S4 (Supporting Information). Similar to the Pd-containing sample, a population of small (∼3.0 nm) NPs is seen in the TEM image, but the XRD pattern shows only reflections of magnetite, indicating that Ru-containing NPs are amorphous. The STEM dark-field image of IO-Ru-3 (Figure 5a) displays brighter spots on the iron oxide aggregates, which could be assigned to NPs with a higher electron contrast due to the Ru presence. The Ru NP images are not resolved on the Ru map because of the low Ru content and the NP close proximity, but they are clearly located in/on the Fe3O4 NP aggregate.

Figure 4. HR XPS (a) Fe 2p and (b) Pd 3d of IO-Pd-2. The deconvolution data are presented in Tables S2 and S3 (Supporting Information).

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Figure 5. (a) STEM dark-field image and EDS maps of IO-Ru-3 for (b) Fe, (c) O, (d) Ru, (e) Fe−O mix, and (f) Fe−Ru mix.

The HR XPS spectrum of IO-Ru-3 in the Ru 3d region is shown in Figure 6. The deconvolution of the Ru 3d spectrum

Table 2. Table of Calculated Rate Constants and TOF Values for Ru- and Pd-Containing Catalysts in the Reduction of 4-NP to 4-AP

elemental analysis data, wt %a

a

Figure 6. HR XPS Ru 3d spectrum of IO-Ru-3. In the Ru 3d region, the deconvolution peaks at 279.80 and 283.97 eV are assigned to Ru0, while the peaks at 280.61 and 284.1 eV are ascribed to Ru4+. In the C 1s region, the peak at 284.81 eV is due to C−C of PEI and adventitious carbon, the peak at 285.92 is due to C−N of PEI, the peaks at 286.25 and 288.16 eV are due to C−OH and COOH, respectively.67 Oxygen containing groups are most likely due to adsorption of CO and CO2 from atmosphere. The deconvolution data are presented in Table S4.

catalyst notation

rate constant, min−1

Pd

IO-Pd-1 IO-Pd-2 IO-Pd-3 IO-Pd-4 IO-Ru-1 IO-Ru-2 IO-Ru-3 Mn@Cs@Ru20

0.45 0.65 0.75 0.38 0.27 1.2 0.75 9

4.95 5.2 8.7 7.9

Ru

TOF, min−1

0.5 1.05 0.95 1.08

36.3 49.9 34.4 19.2 204.7 433.2 299.2 273.9

From X-ray fluorescence (XRF) measurements.

appreciable rate by sodium borohydride. In the presence of the catalyst, the reaction proceeds very quickly, forming 4-AP. Considering that in all nanocomposites both Pd and Ru NP sizes are nearly the same (∼3 nm), the NP size influence was decoupled from other parameters, allowing us to evaluate the influence of the catalytic metal type and the catalytic metal and polymer loading on the activity of the catalysts synthesized. A significant absorption peak at 315 nm appears in the absorption spectrum of pure 4-NP (not shown). Upon addition of the NaBH4 aqueous solution to the 4-NP, the absorption peak position shifts to 400 nm because of the formation of 4-nitrophenolate ions. After the addition of 4-NP in the reaction medium containing a catalyst, the reduction of 4-NP began. It was monitored utilizing UV−vis spectroscopy and following a continuing decrease in intensity of the 400 nm peak, which is accompanied by the appearance of a new peak at 301 nm, characteristic for 4-AP.68 Typical UV−vis spectra of the reaction progression for Pd- and Ru-containing catalysts with different amounts of catalytic metals are shown in Figure S5 (Supporting Information). It is noteworthy that the reaction occurred very fast and no induction period was observed. It was reported that the induction period is caused by the dynamic restructuring of the NP surface.32,34 Apparently, no restructuring takes place in the case of the

shows two components with BEs of 279.80 and 280.61 eV, which are assigned to Ru0 and Ru4+, respectively, with the ratio of Ru0/Ru4+ = 7:1 (Table S4, Supporting Information). The BE values obtained are in good agreement with those reported for Ru NPs.65 It has been reported that RuO2 forms on the Ru0 NP surface upon exposure to air.66 The assignments of the C 1s spectrum deconvolution (it overlaps with the Ru 3d spectrum) are presented in Table S4 (Supporting Information). The HR XPS Fe 2p (not shown) of IO-Ru-3 is very similar to that of IO-Pd-2 (Figure 4), indicating the magnetite NP surface. Catalytic Data. The catalytic properties of the nanocomposites synthesized were tested in the reduction of 4-NP to 4-AP with excess of NaBH4 (Table 2). This is a model reaction, which is normally used to compare catalysts. It is noteworthy that in the absence of a catalyst or in the presence of iron oxide NPs stabilized by PEI, 4-NP is not reduced at an 14720


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could be due to hydrogen spillover, as was recently demonstrated even for nonreducible supports.74 To study the catalyst recovery and performance in the repeated catalytic reactions, we carried out consecutive catalytic experiments with the same catalyst load, separating the reaction solution using a rare earth magnet (Figure S7, Supporting Information), sonicating the catalyst in the cuvette in water for 15 min and adding new portions of NaBH4 and 4NP. The data (see Tables S5 and S6) demonstrate remarkable stability of the catalyst performance in five consecutive reactions. Moreover, the ICP analysis of the supernatants after first and fifth reactions shows only traces of Ru or Fe: below 0.03 ppm for Ru and 0.42 ppm for Fe, indicating that at least 97.5% of Ru has been retained in the catalyst. It is noteworthy that the catalyst separation occurs within 60 s, but if sonication is not used before the following reaction, the catalyst activity drops by ∼20%, indicating that the catalyst is aggregated on the stir bar upon magnetic separation and needs to be redispersed to maintain its activity.

catalysts studied in this work because the catalytic NP surface is already modified with NH and NH2 groups of PEI. To determine the rate constant of each reaction, a graph of ln(At/A0) versus time was plotted (see example for IO-Pd-2 in Figure S6, Supporting Information). A linear function that fits the data was added and the negative of the slope was determined to be the rate constant. The rate constants characterizing the catalyst activity in the 4-NP reduction and turnover frequency (TOF) values calculated using elemental analysis data are presented in Table 2. The Table 2 data show that the rate constants depend on the catalytic metal type and metal and polymer loading. An increase of the polymer loading by ∼33% leads to slowing the reaction by 60 and 90% for Ru- and Pd-containing catalysts, respectively. This is most likely due to poor access of the substrate to the catalytic centers under the thicker polymer coating. When the loading of Pd was doubled, the rate constant of the reaction increased but only by ∼64%. This could be caused by NPs shielding each other at the higher Pd loading. Taking this information into account, the Rucontaining catalysts were obtained at lower metal compound loadings: 0.05 and 0.1 mmol of Ru. The most notable difference in the rate constants was observed for different catalytic metals, Pd and Ru, with the latter being ∼1.5 times more active than the former (comparing rate constants), while the TOF values differ by a factor of 8.7. The literature data for Pd- and Ru-containing catalysts are not consistent with the above finding which could be assigned to several factors. Liu et al. compared catalytic efficiency of click-dendrimer-stabilized late transition metal NPs in 4-NP reduction.69 The highest rate constant was found for Rh, while the second best performance was demonstrated for Pd. The Ru NPs showed a much lower rate. It is noteworthy, however, that the NP sizes were different for different metals and the data for Pd NPs were obtained in earlier work.70 In our study, the NP sizes are ∼3 nm for Ruand Pd-containing catalysts, so the higher activity of Rucontaining catalysts is not due to the NP size difference. In order to understand the possible causes of such a behavior, we considered differences between the two metals taking into account a well-established mechanism in terms of the Langmuir−Hinshelwood model for the reduction of 4-NP with the NaBH4 excess.32,34 The borohydride ions formed by hydrolysis of NaBH4 transfer surface-hydrogen in a reversible manner to the NP surface. At the same time, 4-NP is also adsorbed on the surface and the rate-determining step involves the reduction of 4-NP by the surface-hydrogen species.32,34 We assume that when hydrogen species are transferred to the metal surface, most probably metal hydride complexes are formed. One of the possible explanations of the higher activity of the Ru-containing catalysts is a higher probability for transfer of the hydrogen species to the nitro group of 4-NP because of stability of Ru hydride complexes. In contrast to the abundant and frequently characterized ruthenium hydrides,71 palladium hydrides are rarer and often used more as solid state materials for capture and sensing of H2.72,73 The other likely cause of the higher activity is the presence of Ru4+ species on Ru NPs, which should be beneficial for the nitro group adsorption because of Lewis acid properties, thus facilitating hydrogenation. The presence of magnetite NPs in close proximity to catalytic NPs may enable the 4-NP adsorption for both Pd- and Ru-containing catalysts, thus facilitating the catalytic reaction with magnetically recoverable catalysts. This

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CONCLUSIONS Robust magnetically recoverable nanocomposites containing ∼3 nm Pd or Ru NPs and stabilized by commercially available branched PEI have been synthesized using a one-pot procedure and characterized by a combination of physicochemical methods. PEI functions as a stabilizing support for the nanocomposite, allowing for coordination with catalytic metal ions, their reduction and stabilization of magnetite and catalytic NPs. XPS revealed that Pd NPs consist solely of Pd0, while Ru NPs contain both Ru0 and Ru4+ with the ratio Ru0/ Ru4+ = 7:1, indicating partial surface oxidation of the Ru NPs. STEM EDS mapping showed that for both Pd- and Rucontaining nanocomposites, Pd or Ru NPs are located within magnetite NP aggregates making them magnetically recoverable within 60 s. It was demonstrated that the Ru-containing nanocomposites are more catalytically active in the reduction of 4-NP to 4-AP than their Pd-containing analogs most likely due to a facilitated surface-hydrogen transfer and the presence of Ru4+ species, enabling the NO2 group adsorption. Considering that Ru is approximately 4.5 times cheaper than Pd, the Ru-containing magnetic catalysts synthesized in this work seem to be especially beneficial for hydrogenation reactions of nitroarenes.

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EXPERIMENTAL PART Materials. Iron (III) acetylacetonate [Fe(acac)3, 99+%] was purchased from Acros Organics and used as received. PEI with M.W. 1800 (99%) and 10 000 (99%) was purchased from Alfa Aesar and used as received. TEG (C6H14O4, 99%), ruthenium(III) chloride (RuCl3, 95%), palladium(II) acetate [Pd(OAc)2, 98%], hydrazine hydrate (N2H4 × H2O, 98%), 4NP (99%), and 4-AP (98%) were purchased from SigmaAldrich and used without purification. Acetone (99.5%) was purchased from Macron Chemicals and used as received. Ethanol (95%) was purchased from Aaper Alcohol and used as received. Synthesis of Iron Oxide NPs Stabilized by PEI. Syntheses of iron oxide NPs in the presence of PEI were carried out according to the following protocol. In a typical experiment, a three-neck round-bottom flask (with elongated necks) equipped with a magnetic stir bar, a reflux condenser, and two septa, one of which contained an inserted temperature 14721


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catalyst.20 The metal contents in supernatants after magnetic separation were measured using ICP−OES (NexION 300Q, PerkinElmer). Characterization. TEM was utilized for imaging of all samples. To prepare electron-transparent NP samples, a drop of a purified solution was placed onto a carbon-coated Cu grid. All images were acquired at an accelerating voltage of 80 kV on a JEOL JEM1010 TEM system. HRTEM and STEM images and EDS maps were acquired at an accelerating voltage of 300 kV on a JEOL 3200FS transmission electron microscope equipped with an Oxford Instruments INCA EDS system. The same TEM grids were used for all analyses. XRD patterns were collected using an Empyrean from PANalytical. A copper target with a scattering wavelength of 0.154 nm generated the X-rays. Soller slits, antiscatter slits, divergence slits, and a nickel filter were in the beam path. During the measurement in reflection mode, the sample was spinning with a revolution time of 2 s. The measurement was performed with a step-size of 0.02 and a counting time of 1800 s/step. XPS experiments were performed using PHI Versa Probe II instrument equipped with a monochromatic Al Kα source. The X-ray power of 50 W at 15 kV was used for a 200 micron beam size. The instrument work function was calibrated to give a binding energy (BE) of 84.0 eV for the Au 4f7/2 line for metallic gold, and the spectrometer dispersion was adjusted to give BEs of 284.8, 932.7, and of 368.3 eV for the C 1s line of adventitious (aliphatic) carbon present on the nonsputtered samples, Cu 2p3/2 and Ag 3d5/2 photoemission lines, respectively. The PHI double charge compensation system was used on all samples. The ultimate Versa Probe II instrumental resolution was determined to be better than 0.125 eV using the Fermi edge of the valence band for metallic silver. XPS spectra with an energy step of 0.1 eV were recorded using software SmartSoft-XPS v6.3 and processed using PHI MultiPack v9.0 at the pass energies of 23.5 eV for Fe 2p, C 1s, Pd 3d, Ru 3d, and O 1s regions. Peaks were fitted using GL line shapes and/or an asymmetric line shape A(0.2,0.8,0) GL(10), that is, a combination of Gaussians and Lorentzians with 10−50% of Lorentzian content. A Shirley background was used for curve-fitting. The samples were prepared by drop casting of the NP solution in chloroform on a native surface of a Si wafer. XRF measurements to determine the Pd and Ru contents were performed with a Zeiss Jena VRA-30 spectrometer (Mo anode, LiF crystal analyzer and SZ detector). Analyses were based on the Co Kα line and a series of standards prepared by mixing 1 g of polystyrene with 10−20 mg of standard compounds. The time of data acquisition was constant at 10 s.

probe protected with a glass shield and the other had a long inserted needle for argon bubbling, was loaded with 0.353 g (1 mmol) of Fe(acac)3, 0.1125 g (2.613 mmol) of PEI, and 7 mL of TEG. The flask was fitted in a Glas-Col heating mantle, which was placed on a magnetic stirrer. The mantle was attached to a digital temperature controller. The flask was degassed by argon bubbling for 15 min under stirring. Then, the temperature was raised to 60 °C at 10 °C/min and kept under stirring at this temperature for 30 min to allow solubilization of all reagents. Then, the temperature was increased with a heating rate 10 °C/min, and the flask was allowed to reflux for 1 hour at 280−285 °C. After that, the heating mantle was removed and the flask was allowed to cool to room temperature. For the NP isolation, a portion of the reaction solution was placed into a test tube and precipitated with acetone (5−10 fold excess). The mixture was then centrifuged for 15 min. Following the centrifugation, a pellet was formed and the supernatant was removed. The sample was then washed with acetone three times, each followed by the 15 min centrifugation and removal of the supernatant. Then, the sample was dispersed in ethanol, followed by sonication for 10 min. The reaction conditions for iron oxide (IO) NP syntheses are presented in Table S1 (Supporting Information). One-Pot Syntheses of Pd- and Ru-Containing Catalysts. In a typical experiment, first, iron oxide NPs were synthesized as discussed above. After the 1 hour reaction, the flask was allowed to cool to 60 °C. A solution of 0.1 mmol of Pd acetate (0.0302 g) in 1 mL of TEG was prepared in a small vial by sonication for 15 min and injected into the reaction flask. The flask was stirred at 60 °C for 24 h to allow coordination of the Pd compound with PEI. After that, the temperature was increased to 285 °C with a heating rate 10 °C/min and the flask was allowed to reflux for 1 hour. After that, the heating mantle was removed and the flask was allowed to cool to room temperature. In the case of the Ru-containing catalyst, a solution of 0.0206 g (0.1 mmol) of RuCl3 in 1 mL of TEG was prepared similar to the solutions of Pd acetate. The other reaction conditions were similar to those described above for the Pdcontaining catalysts. The reaction conditions for Pd- and Rucontaining samples are presented in Table 1. Reduction of 4-NP to 4-AP. Before the reduction, the catalyst was dried under vacuum overnight. The catalyst (0.7 mg) was dispersed in 2.67 mL of H2O by sonication for 1 h. In a cuvette, 30 μL of the catalyst dispersion, 300 μL of the NaBH4 (10−1 M) aqueous solution, 2.64 mL of H2O, and a stir bar were combined. Then, the reaction solution was degassed by argon bubbling for 30 min. An Agilent Cary 60 UV−vis spectrophotometer was calibrated so that the instrument would read the spectrum from 500 to 200 nm, once every 12 s. The cuvette prepared was zeroed on the machine for 400 nm and 30 μL of the 4-NP (10−2 M) aqueous solution (degassed by argon bubbling) was added into the cuvette upon stirring, after which readings were started. High excess of NaBH 4 makes the reduction rate independent of the NaBH4 concentration; thus, the reaction is considered of pseudo-first-order. The rate constant (k) was obtained from the slopes of the linear part of the ln(At/A0) = −kt plots. A0 and At are designated as the initial 4-NP concentration and the 4-NP concentration at time t, respectively.20,32,33 The TOF values were derived from the equation: TOF = k·n0(4‑NP)/ncat, where n0(4‑NP) is the initial 4NP amount and ncat is the molar amount of Ru or Pd in the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02382. Reaction conditions of iron oxide NP syntheses; FTIR study of complexation with Pd acetate and Pd NP formation; TEM and HRTEM images; XRD patterns; EDS maps; FTIR and XPS data; kinetic data; fitting parameters for the samples; magnetic separation photo; and calculated rate constants and TOF values for 14722


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catalytic recovery experiments using the samples (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.M.B.). ORCID

Lyudmila M. Bronstein: 0000-0002-0438-8132 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.B. and V.M. thanks the Russian Foundation for Basic Research (grants 17-03-00578 and 18-58-80008) for funding. We also thank the IU Nanoscale Characterization Facility for access to the instrumentation as well as NSF grant #CHE1048613 which funded the Empyrean from PANalytical. IU Bloomington XPS facility was funded by NSF MRI grant (NSF DMR 1126394).

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