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Communication

Reduction of Nitrobenzene to Aniline by CO/H2O in the Presence of Palladium Nanoparticles

by
Agnieszka Krogul-Sobczak
*,
Jakub Cedrowski
,
Patrycja Kasperska
and
Grzegorz Litwinienko
Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland
*
Author to whom correspondence should be addressed.
Catalysts 2019, 9(5), 404; https://doi.org/10.3390/catal9050404
Submission received: 18 March 2019 / Revised: 18 April 2019 / Accepted: 24 April 2019 / Published: 30 April 2019
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Abstract

:
The transformation of aromatic nitrocompounds into amines by CO/H2O is catalyzed by palladium(II) complexes. Recently, we have proposed that the catalytic cycle includes Pd0 as the transient intermediate and herein, for the first time, we describe the application of palladium nanoparticles (PdNPs) stabilized by monodentate N-heterocyclic ligands as nanocatalysts facilitating the reduction of Ar–NO2 into Ar–NH2 by CO/H2O. Among the series—Pd(II) complexes, PdNPs and commercial Pdblack—the highest catalytic activity was observed for PdNPs (3.0 ± 0.5 nm) stabilized by 4-Me-pyridine in the presence of 2-Cl-pyridine. The results may be helpful for mechanistic considerations on the role of metallic nanoparticles as active species in other organic processes.

Graphical Abstract

1. Introduction

Aromatic amines are important intermediates and the final products in the chemical and polymer industry [1,2,3,4,5]. Among the commonly applied methods of their synthesis, the reduction of nitroarenes is the most convenient approach. Direct hydrogenation of nitrocompounds undergoes minimal waste generation [6] but is nonselective and expensive [7,8], thus, other methods of reduction are needed for large scale production. One of the most promising methods utilizes the mixture of CO/H2O as an easily accessible reducing agent [7,8,9,10,11] and is generally described by Equation (1).
Catalysts 09 00404 i001
Recently, we have developed the method of reduction of aromatic nitrocompounds to amines with CO/H2O, employing PdCl2(XnPy)2 complexes as pre-catalysts (X = Cl or CH3, n = 0 − 2) in the presence of Fe powder and I2 as co-catalyst and we proposed a plausible mechanism of this process [11,12]. Initially, the traces of aniline are generated from water or ethanol and nitrobenzene (no-catalyst is needed, thus, this step is not included in the main cyclic mechanism presented in Scheme 1) and the formed aniline reacts with the Pd(II) complex [11,12,13,14]:
PdCl2(XnPy2)2 + 2PhNH2 → PdCl2(PhNH2)2 + 2XnPy
The subsequent reaction with CO gives N,N’-diphenylurea and trace amounts of Pd0 (Equation (3)):
PdCl2(PhNH2)2 + (z + 1)CO + mXnPy → (PhNH)2CO + Pd0(XnPy)m(CO)z + 2HCl
where m and z depend on the coordinating ability of Pd(0), amount of XnPy, and pressure of CO. Pdblack is oxidized to Pd2+:
Fe2+ + 5CO + Pd(black) → Fe(CO)5 + Pd2+
by Fe2+ cations generated from Fe (powder) and I2 i.e. co-catalysts [15]. One of the proofs of the Pd0 presence in the system is the partial precipitation of Pdblack [16]. Complex Pd0(XnPy)m(CO)z formed in reaction (3) reacts with nitrobenzene and carbon monoxide and, at this moment, the catalytic cycle starts; see Scheme 1.
Because nitroaromatic compounds having electron deficient Y substituents (Y-C6H4NO2) undergo a faster reaction, we postulated that the electron transfer from Pd0 to Y-C6H4NO2 is the rate determining step (RDS). If it is true, the more basic XnPy ligands (derivatives of pyridine) in PdCl2(XnPy)2 should accelerate the catalytic cycle in Scheme 1. Surprisingly, the overall conversion and yield increased with the decreasing basicity of XnPy, i.e., complexes of chloropyridines were better than methylpyridines [11]. We suppose that increasing the basicity of XnPy is beneficial for the stability of Pd0 during RDS described by Equation (5) (see also step I in Scheme 1) but, on the other hand, the very basic pyridine ligand competes with Y-C6H4NO2 for access to the catalytic centre. In this work, we are testing the hypothesis that Pd0 is involved in the RDS and we are trying to explain the role of XnPy in RDS.
Pd0(XnPy)m + CO + YC6H4NO2 ⇆ [Pd2+(XnPy)2(YC6H4NO2)(CO)] + (m − 2)XnPy
Formation of Pd0 in situ as heterogeneous catalysts is in agreement with the mechanisms postulated for other reactions (mainly, hydrogenations) mediated by transition-metal complexes [17] and the common features of such a kind of catalysis are (i) application of easily reduced pre-catalysts, (ii) forcing reaction conditions, and (iii) the presence of nanocluster stabilizers (here: Pd complexes, 180 °C, 4MPa, and N-donor ligands, respectively). In order to verify this hypothesis, we directly applied Pd nanoparticles (PdNPs). The introduction of Pd0 to the reaction system would increase the rate, as the step of nucleation and formation of Pd0 (Reactions (2) and (3)) is omitted, and it can give a proof that PdNPs are essential for the catalytic process described by Scheme 1.
During our earlier studies on the catalytic activity of PdNPs/4MePy, we employed a model process of reduction of nitrobenzene to aniline by NaBH4 [18]. We noticed that the size and surface area of nanoparticles (dependent on the way of nanoparticles, NPs, preparation) had a significant impact on their activity and NPs previously dried (before catalytic tests) have higher activity than freshly prepared wet NPs that are transferred directly (after preparation) to the reaction vessel [18]. In this work the catalytic activity of PdNPs is tested for an industrially important process, i.e., the reduction of nitrobenzene (NB) to aniline (AN) by CO/H2O. To the best our knowledge, there are only a few publications on the reduction of nitro compounds by CO/H2O performed in the presence of nanoparticles: Fe(OH)x supported Au nanoclusters [7], hydrotalcite-supported metal nanoparticles [19], TiO2-supported Au nanoparticles [20] and Co3O4/nitrogen doped graphene [21]. Two of the above catalytic systems are based on gold as Au-based catalysts have attracted considerable attention in the past, however, the high effectiveness of palladium (II) complexes in the mentioned process encouraged us to investigate the catalytic activity of PdNPs, which should be potentially more active in comparing the Pd (II) complexes. Compared to bulk catalysts, nanoparticles often possess many unique or improved (catalytic) properties due to the large surface to volume ratio [22] but it is a matter of vivid debate whether the intermediate species generated in situ act as homogeneous and heterogeneous catalysts [17,23,24,25,26,27,28,29,30]. Our goal is to connect the form (size and morphology of PdNPs) and oxidation state of palladium with its catalytic activity in the reduction of NB to AN by CO/H2O. There is no information in the literature on the application of palladium nanoparticles in the investigated process, therefore, our work provides new knowledge on the catalytic activity of PdNPs. We also study the role of the pyridine ligand during the catalytic process.

2. Results and Discussion

A choice of the stabilizing ligand for PdNPs is extremely important [27,31] because nanocatalysts should effectively interact with both the ligands and with the reacting compound(s). Simple and not bulky derivatives of pyridine (exhibiting a balance between lability and stabilizing properties) seem to be promising candidates for stabilization of NPs [18,32] and this is the first report on the application of PdNPs stabilized by monodentate N-heterocyclic ligands for the reduction of NB to AN by CO/H2O. Basing on the knowledge that Pd(II) complexes with ClyPy ligands (where Y = 1 or 2) exhibited higher catalytic activity than the ones with MeyPy ligands during the reduction of nitrocompounds using CO/H2O [11], we tried to generate palladium nanoparticles stabilized by 2ClPy (PdNPs/2ClPy). Unfortunately, we did not succeed with this ligand as a stabilizing agent [18], probably due to its low nucleophilicity [12]. This observation does not refute the hypothesis that Pd0 is the active species also in the system catalyzed with PdCl2(2ClPy)2 because in such a case, Pd0 generated in situ may be instantaneously stabilized by traces of aniline. Taking into account the inability of ClyPy to stabilize the PdNPs generated separately, in this work we used other pyridine ligands (Py-ligands): 4-methylpyridine (4MePy), N,N-dimethylaminopyridine (DMAP) and 4-ethylpyridine (4EtPy) as the ligands giving stable NPs [18].
Centrifugation and drying is a common post-synthesis treatment of nanoparticles stabilized by strong (covalently bound) ligands [22,33] and, in this work, we checked how those two methods alter the stability, size and morphology of PdNPs stabilized by Py-ligand. Freshly prepared PdNPs were centrifuged from the raw solution and (a) directly re-suspended in 0.7 mL of ultrapure (Milli-Q, 18.2 MΩ·cm resistivity at 25 °C) water or (b) dried in a vacuum oven (30 °C) and then re-suspended in 0.7 mL of Milli-Q water. TG analysis (see Supplementary Material) shows that Py-ligand is partially desorbed from the surface of PdNPs during the drying process and it might cause the agglomeration of PdNPs. TEM images of PdNPs stabilized by 4MePy (Figure 1), DMAP (Figures S1 and S2) or 4EtPy (Figures S3 and S4) indicate that centrifugation does not significantly influence the size and morphology of PdNPs, whereas the formation of agglomerates is observed for nanoparticles subjected to the drying process.
We made efforts to resolve the nature of the active form of Pd catalyst, following the definition given by Schwartz [17,34] for homogeneous catalyst (a metal-complex homogeneous catalyst—monometallic catalyst), and heterogeneous catalyst (nanoclusters, or another metal-particle heterogeneous catalyst—bulk metal). A variety of experiments were proposed to distinguish those two forms of catalysis [17,25,26,35,36,37,38,39] but none of them allows for definitely distinguishing homogeneous catalysis from highly dispersed metal-particle heterogeneous catalysis [17]. Bringing the above discussion into our field, if the mechanism proposed in Scheme 1 is valid, the reduction of NB by CO/H2O should be catalyzed not only by Pd (II) complexes (as homogeneous pre-catalyst) but also by PdNPs (as heterogeneous pre-catalysts) stabilized with Py-ligands (4MePy, DMAP, 4EtPy). Catalytic tests were performed according to the general procedure described elsewhere [11]. This system requires water as a reactant (molar ratio of water to nitrobenzene = 1:1) but an excess of water added to the NB/ethanol mixture leads to the formation/separation of the second liquid phase. In order to keep the optimal amount of water in the reacting system, the nanoparticles (0.056 mmol of PdNPs i.e. 0.14%mol vs NB) were previously isolated, purified and re-suspended in 0.7 mL of water and added to the autoclave. The results listed in Table 1 and presented in Figure 2 clearly show that agglomerated NPs were less active catalysts that non-agglomerated (see turnover frequency, TOF, values, entries 4–6,9,10,13,14 for agglomerated and entries 1–3,7,8,11,12 for non-aglomerated NPs). Next, a series of experiments were carried out in order to determine the role of pyridine derivatives in the reaction kinetics. PdNPs stabilized with pyridine ligands were used alone (see entries 1,4,7,9,11,13 in Table 1) or used in the presence of excess 4MePy or 2ClPy (6.2 mmol per 0.056mmol of PdNPs, see footnote a in Table 1). The latter derivative was applied because our previous studies showed that PdCl2(2ClPy)2 complex was a more active catalyst than PdCl2(4MePy)2 although 2ClPy is a very poor stabilizing ligand for PdNPs (fast precipitation occurs when other ligands are not present). In this work, when PdNPs stabilized by 4MePy, DMAP, or 4EtPy are used as pre-catalyst, the addition of 2ClPy causes a 7 and 11% increase the yield of aniline (entries 2 and 5, respectively). The opposite effect, i.e., a strong decrease of the yield, is observed when the excess of 4MePy is added (see entries 3 and 6 in Table 1).
Although the positive effect of an excess of 2ClPy (entries 2,5 in Table 1) and the negative effect of an excess of 4MePy (entries 3,6 in Table 1) seems to be counterintuitive, it can be rationalized by the much stronger binding of PdNPs by 4MePy than by 2ClPy ligand. The addition of 100-fold stoichiometric excess of 4MePy over PdNPs makes them perfectly stable, but access to the surface of NPs is strongly restricted. On the other hand, the addition of 2ClPy into the system containing PdNPs stabilized with a small amount of 4MePy results in an equilibrium between chloropyridine and methylpyridine at the metal surface, and some fraction of 2ClPy, as more labile, can be easily replaced by the reacting molecule (see Equation (5)). This interplay of a stronger (in our work 4MePy) and weaker (2ClPy) ligand can be compared to the test described by Finke, with CS2 being a good binding agent that covers the surface of nanocatalyst and decreases its catalytic activity (however, the CS2 test is recommended for low temperatures and cannot be used above 50 °C) [39]. Certainly, the presence of both ligands, strong (MePy) and weak (2ClPy), is desirable because the lability of 2ClPy can facilitate the access of substrate to the active surface of the metal. The role of pyridine derivatives is also explained by differences in the catalytic activity of PdNPs stabilized by various pyridine ligands (4MePy, DMAP or 4EtPy). Such results (different yields) cannot be explained by different sizes of PdNPs because, regardless of the stabilizing agent, the size of PdNPs is between 3–4 nm. Perhaps the lowest catalytic activity of PdNPs/DMAP is related to the too-strong electrostatic interactions of NPs with the DMAP ligand.
We also compared the effectiveness of PdNPs/4MePy with other two pre-catalysts: PdCl2(4MePy)2 complex and the commercially available Pdblack and centrifuged PdNPs/4MePy (with a calculated surface of 161 m2/g(PdNPs), see Supplementary Material) still exhibited the highest activity (entries 1, 15 and 19 in Table 1 and Figure 3). The yields of AN obtained for the two other forms of Pd0: agglomerated (dried) PdNPs (because of agglomeration a histogram of size distribution was not obtained, thus, the surface was not calculated) and Pdblack (with a surface = 40–60 m2/g, as specified by the producer) are lower compared to the PdCl2(4MePy)2 complex (entries 1, 4, 15 and 19 in Table 1). The results also indicate that the presence of 2ClPy increases the yield of aniline for those two heterogeneous nanocatalysts, similarly to the system with PdNPs.
If PdNPs are the real active species, the main benefit from the application of PdNPs should be the acceleration of the reaction because nucleation is omitted. However, the yields are comparable to the yields obtained in the presence of PdCl2(3,5Cl2Py)2 and PdCl2(2ClPy)2 [11], and such results indicate that Pd nanoparticles probably act as reservoirs for the formation of homogeneous complexes Pd0(XnPy)m(CO)z triggering the mechanism presented in Scheme 1.
Although the main scope of our work was to investigate the efficiency of Pd0 in the catalytic cycle, we also tried to test whether PdNPs can be re-used. The experiments showed that PdNPs/4MePy isolated after a reaction (entry 1 Table 1) by centrifugation and washed with EtOH can be reused in the next process (the same 73% yield was obtained). However, after the second recovery of PdNPs/4MePy, the yield of the process decreased to half of the starting value. Probably, using supported PdNPs would have been more effective and easier for recovery.

3. Materials and Methods

3.1. Materials

NaBH4, PdCl2, NaCl were used as received. Liquid compounds: pyridine (Py) and substituted pyridines, nitrobenzene (NB), and ethanol, were distilled over the drying agent and stored under argon.

3.2. Synthesis of Palladium Nanoparticles

PdNPs were prepared by the procedure described elsewhere [18]. Briefly, a freshly prepared solution of ligand (a derivative of pyridine; 0.628 mmol in 9 mL of ultrapure water) was added to a stirred solution of NaCl (0.188 mmol; 0.11g) and PdCl2 (0.084 mmol; 0.015 g) in 6 mL of ultrapure water at room temperature. After 20 min of continuous stirring, the mixture was reduced by NaBH4 (1% w/v, 1.1 mL) and added in 20 μL portions. The progress of the reaction was visually monitored as the mixture progressively changed colour from light orange to dark brown (almost black). The resulting PdNPs were stirred for 30 min.

3.3. Analysis of PdNPs

The size of NPs was measured by Transmission Electron Microscopy (TEM), the observations were carried out using a JEM 1400 microscope (JEOL Co. Tokyo, Japan) at a 120 kV acceleration voltage. The samples were obtained by casting aqueous solution of palladium nanoparticles (0.6 mg/ml−1) onto a carbon coated nickel microgrid (200 mesh) and air-dried overnight.
The composition of Pd/ligand expressed as a percentage of the organic ligand and the metal in palladium nanoparticles stabilized by pyridine ligands was determined by thermogravimetry (TG). The measurements of PdNPs/4-MePy were performed by means of a termogravimeter Q50—1261 TA Instruments (New Castle, DE, USA)—under a nitrogen flow of 6 dm3/h, with a heating rate = 10 K/min in platinum cells and the weight of the sample was about 8 mg. Weight loss during thermal decomposition of PdNPs/4-MePy was determined in the temperature range 40–600 °C (see Figures S5 and S6 in Supplementary Material). The results presented in this paper is the arithmetic mean of 3 repetitions and the difference of results in a series of determinations of the sample is up to 2%.
X-ray photoelectron spectroscopic (XPS) measurements were performed using a PHI 5000 VersaProbe (ULVAC-PHI, Chigasaki, Japan) spectrometer with monochromatic Al Kα radiation (hν = 1486.6 eV). The X-ray beam with a power of 25 W was focused to a diameter of 100 µm with the measured area defined as 250 × 250 µm. The survey spectra were taken in the energy range of 1350–0 eV at the energy step size of 0.4 eV and the pass energy of 117.4 eV. The high-resolution (HR) XPS spectra were collected with the hemispherical analyzer at the pass energy of 23.5 eV, the energy step size of 0.1 eV and the photoelectron take-off angle 45° with respect to the surface plane. The CasaXPS software (v.2.3.19, Casa Software Ltd, Teignmouth, UK) was used to evaluate the XPS data. All survey spectra were analyzed using the MULTIPAK v.9 software (Chanhassen, MN, USA). The spectra were calibrated against 284.6 eV for the C 1s region. High-resolution spectra of the Pd 3d region were collected for (i) nanoparticles from raw solution, (ii) NPs centrifuged and suspended in ultrapure water, (iii) NPs dried and re-suspended (see Figure S7 in Supplementary Material). For all samples, it was found that the Pd 3d spectrum is an effect of two pairs of spin−orbit components. The Pd 3d5/2 and 3d3/2 peaks found at binding energies (BE) = ~335.1 and ~340.3 eV, respectively, are attributed to the metallic Pd0 [40], while the second pair of Pd 3d signals, at higher BE values (~337.6 and ~342.8 eV), have been associated with Pd2+ [40]. Comparison of the peak areas of Pd0 and Pd2+ (after deconvolution, see Figure S7), indicates that 85% of Pd atoms are in the form of Pd0. However, the XPS results are limited to the surface of NPs and the real content of Pd0 in the whole sample might be higher than 85%.
Calculation of the surface of PdNPs. Assuming the spherical shape of PdNPs, the surface per gram of Pd [m2/g] was calculated as a ratio of surface to mass calculated for single PdNPs, see Supplementary Material.

3.4. The Procedure of the Reduction of Nitrobenzene

The reaction was carried out in a 200 mL stainless-steel autoclave equipped with a magnetic stirrer. Pd-based catalyst (0.056 mmol) and Fe powder (2.68 mmol) were subsequently placed in the autoclave, the air was evacuated and the system was filled with purified argon. Then, other reagents and solvents were added under an argon stream: I2 (0.12 mmol), XPy (6.2 mmol), nitrobenzene (40 mmol), 0.7 mL of water or an aqueous solution of PdNPs, 20 mL of ethanol. The cover was closed and the autoclave was directly filled with carbon monoxide to reach a pressure of 4 MPa, fixed, placed in a hot oil bath and kept at 180 °C for 60 minutes. After the reaction was completed, the autoclave was cooled in the water bath, then vented, and a liquid sample of the reaction mixture was analysed. The yield of the reaction was calculated on the basis of the gas chromatography with Flame Ionization Detector (GC-FID) using n-decyl alcohol as the standard.

4. Conclusions

Comparison of the experiments with PdNPs (stabilized by pyridine ligands) with experiments with Pd(II) complexes (comprising the same pyridine ligands as stabilizing agent for PdNPs), applied respectively as pre-catalysts, showed that Pd0 is a real active species responsible for the observed catalytic effect, thus, confirming the mechanism proposed in Scheme 1. A significantly higher yield observed for PdNPs compared to Pdblack indicates that the catalytically active species are not heterogeneous bulk metal. We suggest that Pd nanoparticles stabilized by pyridine ligands are the most similar to catalytically active reaction intermediates (in agreement with several works indicating the importance of metal-particle heterogeneous catalysis [17,25,27,28]) or that Pd nanoparticles act as reservoirs for the formation of homogeneous complexes (with Pd0) which are the real catalysts. Based on the obtained results, we postulate that the role of pyridine ligands (with electron-donating substituents) is to stabilize PdNPs whereas the most probable role of ClPy, as the labile ligand, is to facilitate access of the nitro compound to the active surface of the metal. The PdNPs (i) can be obtained with minimal synthetic effort, (ii) exhibit higher catalytic activity than Pd(II) complexes with the same ligand (which is used as a stabilizing agent) or than Pd(black) and, therefore, they have great potential to be applied as a pre-catalyst facilitating reduction by CO/H2O.
The presented results may be also helpful for mechanistic considerations on the role of metallic nanoparticles in other organic processes based on the application of Pd(II) complexes as pre-catalysts.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/9/5/404/s1, Figure S1. TEM images of raw PdNPs/DMAP and histogram of particle size distribution; Figure S2. TEM images of centrifuged PdNPs/DMAP and histogram of particle size distribution; Figure S3. TEM images of raw PdNPs/4EtPy and histogram of particle size distribution; Figure S4. TEM images of centrifuged PdNPs/4EtPy and histogram of particle size distribution; Figure S5. TG curves for centrifuged PdNPs stabilized by 4MePy, DMAP, and 4EtPy; Figure S6. TG curves for centrifuged and dried PdNPs stabilized by 4MePy, DMAP, and 4EtPy; Figure S7. XPS Pd 3d spectra of: A) nanoparticles from raw solution, B) NPs centrifuged and suspended in ultrapure water, C) NPs dried and re-suspended in ultrapure water.

Author Contributions

Investigation, A.K.-S., P.K., and J.C.; conceptualization, conceiving, and planning the experiments, A.K.-S. and G.L.; characterization analysis, A.K.-S.; supervision, original draft preparation, G.L. and A.K.-S. All authors discussed the results and contributed to the final manuscript.

Funding

This work was funded by the National Science Centre-Poland, grant OPUS decision no. 2014/15/B/ST4/04835. Publication costs were partially covered by University of Warsaw.

Acknowledgments

The authors acknowledge the support from the National Science Centre-Poland, grant OPUS decision no. 2014/15/B/ST4/04835. We also thank Wiktor Lewandowski for helpful discussion on the XPS spectra.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Catalysts 09 00404 sch001 550
Scheme 1. The proposed mechanism of the reduction of nitrobenzene by CO/H2O in the presence of Pd0 formed from PdCl2(XnPy)2 complexes [11]. Pd0(XnPy)m(CO)z is a hypothetical species (with one atom of Pd0 or it might also be a Pd0 nanoparticle or cluster stabilized by a non-stoichiometric number of XnPy ligands and/or CO) that is converted (step I) into a complex of Pd2+ as the central ion.
Scheme 1. The proposed mechanism of the reduction of nitrobenzene by CO/H2O in the presence of Pd0 formed from PdCl2(XnPy)2 complexes [11]. Pd0(XnPy)m(CO)z is a hypothetical species (with one atom of Pd0 or it might also be a Pd0 nanoparticle or cluster stabilized by a non-stoichiometric number of XnPy ligands and/or CO) that is converted (step I) into a complex of Pd2+ as the central ion.
Catalysts 09 00404 sch001
Catalysts 09 00404 g001 550
Figure 1. The TEM images of the PdNPs stabilized by 4-methylpyridine (PdNPs/4MePy): raw nanoparticles (NPs) (A), centrifuged and suspended (B), dried and re-suspended (C and F: both images present the same NPs but with different magnification). Histograms (evaluated from more than 300 NPs) of size distribution for raw (D), and centrifuged (E) PdNPs/4MePy. Due to the formation of agglomerates, the histogram was not obtained for ”dried” PdNPs. See the Experimental Section for the synthesis conditions. TEM images and particle size distribution determined for PdNPs stabilized by N,N-dimethylaminopyridine and 4-ethylpyridine are provided in Figures S1–S4 in Supplementary Material.
Figure 1. The TEM images of the PdNPs stabilized by 4-methylpyridine (PdNPs/4MePy): raw nanoparticles (NPs) (A), centrifuged and suspended (B), dried and re-suspended (C and F: both images present the same NPs but with different magnification). Histograms (evaluated from more than 300 NPs) of size distribution for raw (D), and centrifuged (E) PdNPs/4MePy. Due to the formation of agglomerates, the histogram was not obtained for ”dried” PdNPs. See the Experimental Section for the synthesis conditions. TEM images and particle size distribution determined for PdNPs stabilized by N,N-dimethylaminopyridine and 4-ethylpyridine are provided in Figures S1–S4 in Supplementary Material.
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Figure 2. The yield of aniline obtained from nitrobenzene reduction with CO/H2O in the presence of 0.056 mmol PdNPs stabilized by 4MePy, 4EtPy or DMAP.
Figure 2. The yield of aniline obtained from nitrobenzene reduction with CO/H2O in the presence of 0.056 mmol PdNPs stabilized by 4MePy, 4EtPy or DMAP.
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Figure 3. The yield of aniline obtained from nitrobenzene reduction with CO/H2O in the presence of 0.056 mmol PdNPs/4MePy (3 nm), PdCl2(4MePy)2 complex, PdNPs (agglomerated) and Pdblack/4MePy.
Figure 3. The yield of aniline obtained from nitrobenzene reduction with CO/H2O in the presence of 0.056 mmol PdNPs/4MePy (3 nm), PdCl2(4MePy)2 complex, PdNPs (agglomerated) and Pdblack/4MePy.
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Table
Table 1. The yield of aniline (YAN) obtained from the nitrobenzene reduction with CO/H2O in the presence of 0.056 mmol Pd-species and values of TOF. a,b.
Table 1. The yield of aniline (YAN) obtained from the nitrobenzene reduction with CO/H2O in the presence of 0.056 mmol Pd-species and values of TOF. a,b.
EntryPd-species/LigandAdditive (XPy)YAN [%]TOF c
1PdNPs d/4MePy-73521
2PdNPs/4MePy2ClPy80571
3PdNPs/4MePy4MePy35250
4PdAgglom e/4MePy-27193
5PdAgglom/4MePy2ClPy38271
6PdAgglom/4MePy4MePy1071
7PdNPs/DMAP-40286
8PdNPs/DMAP2ClPy48343
9PdAgglom/DMAP-20143
10PdAgglom/DMAP2ClPy29207
11PdNPs/4EtPy-54386
12PdNPs/4EtPy2ClPy62443
13PdAgglom/4EtPy-24171
14PdAgglom/4EtPy2ClPy37264
15Pdblack/4MePy f-1286
16Pdblack/4MePy f2ClPy25179
17Pdblack/4MePy f4MePy1071
18 g-2ClPy-0
19PdCl2(4MePy)2-45321
20PdCl2(4MePy)22ClPy50357
21PdCl2(4MePy)24MePy34243
a Reaction conditions: Pd-species/Fe/I2 = 0.056/2.68/0.12 mmol; 6.2 mmol XPy (2ClPy or 4MePy); 40 mmol NB; 20 mL ethanol; 0,7 mL water; 180 °C, 40 MPa CO, 60 min. 4MePy = 4-methylpyridine; 4EtPy = 4-ethylpyridine; DMAP = N,N-dimethylaminopyridine; 2ClPy = chloropyridine; AN = aniline; NB=nitrobenzene. b Selectivity = 100% (aniline is the only product). c TOFAN (turnover frequency for AN) = (mmol of AN formed)(mmol of Pd-species used)−1h−1, where mmol of PdNPs corresponds to mmol of Pd in PdNPs/Py-ligand and the composition of Pd/Py-ligand was determined by thermogravimetry (TG) under nitrogen atmosphere with heating rate = 10 K/min, see Figures S5 and S6 in Supplementary Material. d PdNPs with average size 3 nm (see Figure 1B) prepared by the method “a”. e Agglomerated PdNPs (see Figure 1C prepared by the method “b”. f 4MePy equal to the percentage of 4MePy in PdNPs/4MePy. g Control experiment showed that the reduction of nitrobenzene does not occur in the absence of Pd-based pre-catalyst.

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Krogul-Sobczak, A.; Cedrowski, J.; Kasperska, P.; Litwinienko, G. Reduction of Nitrobenzene to Aniline by CO/H2O in the Presence of Palladium Nanoparticles. Catalysts 2019, 9, 404. https://doi.org/10.3390/catal9050404

AMA Style

Krogul-Sobczak A, Cedrowski J, Kasperska P, Litwinienko G. Reduction of Nitrobenzene to Aniline by CO/H2O in the Presence of Palladium Nanoparticles. Catalysts. 2019; 9(5):404. https://doi.org/10.3390/catal9050404

Chicago/Turabian Style

Krogul-Sobczak, Agnieszka, Jakub Cedrowski, Patrycja Kasperska, and Grzegorz Litwinienko. 2019. "Reduction of Nitrobenzene to Aniline by CO/H2O in the Presence of Palladium Nanoparticles" Catalysts 9, no. 5: 404. https://doi.org/10.3390/catal9050404

APA Style

Krogul-Sobczak, A., Cedrowski, J., Kasperska, P., & Litwinienko, G. (2019). Reduction of Nitrobenzene to Aniline by CO/H2O in the Presence of Palladium Nanoparticles. Catalysts, 9(5), 404. https://doi.org/10.3390/catal9050404

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