Chemoselective Hydrogenation of Nitrobenzaldehyde to Nitrobenzyl

Article pubs.acs.org/JPCC

Chemoselective Hydrogenation of Nitrobenzaldehyde to Nitrobenzyl Alcohol with Unsupported Au Nanorod Catalysts in Water Gao Li, Chenjie Zeng, and Rongchao Jin* Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States ABSTRACT: We report the chemoselective hydrogenation of 4-nitrobenaldehyde to 4-nitrobenzyl alcohol using the unsupported Au spherical nanoparticle and nanorod catalysts at 80 °C in water. A ∼100% selectivity for the 4-nitrobenzyl alcohol product was obtained when the hydrogenation reaction was catalyzed by the unsupported gold catalysts. The Au nanorod catalysts exhibited an aspect-ratio dependent reactivity and generally performed much better than the Au spherical nanoparticle catalyst. The Au nanorod catalysts showed excellent recyclability in the chemoselective hydrogenation (>99% conversion and 100% selectivity after 5 cycles).

1. INTRODUCTION Chemoselective catalytic hydrogenation is of major importance for achieving controlled reduction of organic compounds under mild conditions.1−4 This process is traditionally catalyzed by nickel, palladium, platinum, and copper catalysts.5 In recent years, more work focuses on the gold nanoparticle-catalyzed hydrogenation.6−11 The gold nanoparticle catalysts have been reported to be capable of reducing the aldehyde group to alcohol in some reports12−14 and the nitro group to amine group in other work.15−19 In terms of chemoselective hydrogenation, Claus et al.12 and Zhu et al.13 reported that Au nanoparticles can selectively hydrogenate α,β-unsaturated aldehydes to α,β-unsaturated alcohols. Corma et al. 12 demonstrated that oxide-supported Au nanoparticles can selectively hydrogenate the substituted nitro compounds with H2. Recently, Ren et al.20 and Yan et al.21 reported that TiO2supported gold nanoparticles catalyzed selective hydrogenation of quinolines with H2. Although the literature work has reported that Au nanoparticle catalysts can be applied for the selective hydrogenation reactions, the reaction conditions are quite harsh (e.g., more than 100 °C) due to the limited reactivity of nanogold catalysts toward H2 activation and dissociation.22 Compared to the traditional platinum group metal-based catalysts, Au catalysts are not as efficient as those metals for the hydrogenation reactions.15,23 The development of more effective Au catalysts for the hydrogenation processes with high activity and high selectivity under mild conditions is still a major challenge. The majority of the reported catalytic hydrogenations are performed in organic, poisonous solvents (e.g., toluene) using reducing agents such as sodium hydrosulfite, sodium borohydride, iron, tin, or zinc in ammonium hydroxide. Such processes produce large quantities of waste, and hence, reactions in water are highly desirable. Besides, compared with the oxide-supported gold nanoparticles,24−28 unsupported Au nanoparticle catalysts offer the opportunity to investigate the intrinsic catalytic activity by eliminating the complicated effect of the catalytic support. From both the green-process and © XXXX American Chemical Society

catalysis points of view, it is highly desirable to develop highly selective and efficient catalytic reduction processes for clean chemical synthesis, especially catalytic hydrogenation with H2 in water. In this work, we report a chemoselective hydrogenation reaction of 4-nitrobenzaldehyde to 4-nitrobenzyl alcohol in water under mild conditions (e.g., 80 °C, 20 bar H2) using unsupported, water-soluble Au nanospheres and nanorods as the catalysts. These two types of Au nanocatalysts offer high activity and chemoselectivity in the hydrogenation reaction. In regard to the reduction products of 4-nitrobenzaldehyde (1, Scheme 1), it can be possibly reduced to three hydrogenation Scheme 1. Possible Products of Hydrogenation Reaction of 4-Nitrobenzaldehyde (1) Catalyzed by Au Nanoparticles Using H2 as the Hydrogen Source

products in principle: 4-nitrobenzyl alcohol (2), 4-aminobenzaldehyde (3), and the fully hydrogenated product 4aminobenzyl alcohol (4), Scheme 1. In our catalytic system, the Au nanoparticle catalysts exclusively yield the 4-nitrobenzyl alcohol in the catalytic hydrogenation. The obtained complete chemoselectivity of the aldehyde reduction against the nitro group reduction is in contrast with the previously reported chemoselective reduction of the nitro group against the aldehyde group in 4-nitrobenzaldehyde.15 We further revealed the nanorod aspect ratio dependence in the catalytic reaction. Special Issue: Current Trends in Clusters and Nanoparticles Conference Received: November 30, 2014 Revised: January 25, 2015

A

DOI: 10.1021/jp511930n J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

2. EXPERIMENTAL METHODS 2.1. Synthesis of Au/Citrate Nanoparticles. A total of 9.8 mg (0.025 mmol) HAuCl4·H2O was dissolved in 100 mL of nanopure water in a trineck 250 mL round-bottom flask under a N2 atmosphere. The yellow solution was heated to reflux, and 2.5 mL of 38.3 mM trisodium citrate solution was rapidly added all at once. After the yellow solution changed to red, the solution was refluxed for an additional 10 min. The solution was cooled down to room temperature. The Au/citrate nanoparticles were characterized by TEM and UV−vis spectroscopy. 2.2. Synthesis of Au Nanorods. The procedure consists of two primary steps. The first step is for the synthesis of Au seeds. Briefly, 364 mg (1 mmol) cetyltrimethylammonium bromide (CTABr) and 1 mg (0.0025 mmol) HAuCl4·3H2O were dissolved in 10 mL of nanopure water in a trineck 25 mL round-bottom flask. After 10 min, 0.6 mL (10 mM) of NaBH4 was added to the flask that was under vigorous magnetic stirring. The second step was for the seeded synthesis of Au nanorods. A total of 364 mg (1 mmol) CTABr, 2 mg (0.005 mmol) HAuCl4·3H2O, and 4 mM AgNO3 (for sample I, 10 μL; II, 20 μL; III, 50 μL; and IV, 100 μL) were mixed in 10 mL of nanopure water in a trineck 25 mL round-bottom flask. After 10 min, 70 μL (78.8 mM) of ascorbic acid was added to each flask. The flasks were immersed in a warm water bath at 27−30 °C, and 12 μL of Au seeds was added. After 1 h, the reaction was stopped. For catalytic use, excess CTABr was separated by centrifugation (e.g., ∼4000 rpm for 5 min). The as-prepared Au nanorods were characterized by TEM and UV−vis. 2.3. Typical Procedure for Chemselective Hydrogenation of 4-Nitrobenzaldehyde. In a typical selective hydrogenation reaction, 4-nitrobenzaldehyde (0.1 mmol), pyridine (0.1 mmol), and Au nanoparticles (∼0.5 mg in 1 mL water) were added to a high-pressure cylindrical reactor (Parr instrument company, 22 mL capacity, series 4700) under 20 bar H2. The reaction was kept at 80 °C for 8 h, unless otherwise noted. After the reaction, the mixture was extracted by ethyl acetate and the left aqueous phase was used for the next-recycle hydrogenation reaction. The crude product was obtained after removal of solvent. The conversion of 4nitrobenzaldehyde and the selectivity for the 4-nitrobenzyl alcohol product was determined by 1H NMR (300 MHz). In the recycling reaction, the gold nanorod catalyst was collected by centrifugation (10000 rpm for 5 min) after the reaction, and the gold nanorods were redissolved in water for a fresh reaction with fresh reactants.

Figure 1. UV−vis spectrum (A) and TEM image (B) of the Au/citrate nanoparticles.

was analyzed by 1H NMR (300 MHz) spectroscopy. Two species, that is, the residual starting material (4-nitrobenzaldehyde) and 4-nitrobenzyl alcohol (2, Scheme 1), were found in the 1H NMR spectrum of the reaction mixture, while no 4-aminobenzaldehyde and 4-aminobenzyl alcohol products were detected (Figure 2). The residual 4-nitrobenzaldehyde

Figure 2. 1H NMR spectrum of the mixture after the catalytic reaction; residual reactant 4-nitrobenzaldehyde (−CHO at 10.18, the proton on phenyl ring at 8.44 and 8.41, 8.12, and 8.09 ppm) and the exclusive 4nitrobenzyl alcohol product (−CH2 at 4.87, the proton on phenyl ring at 8.26 and 8.23, 7.58, and 7.55 ppm).

shows signals at δ = 10.19 (-CHO, 1H), 8.42 ppm (2H), and 8.10 ppm (2H), and the chemoselectively hydrogenated product 4-nitrobenzyl alcohol shows signals at 8.25 ppm (2H), 7.56 ppm (2H), and 4.87 ppm (-CH2OH, 2H); of note, the peak at 7.28 ppm belongs to the residual solvent peak of CDCl3 (Figure 2). It is interesting that the selectivity in our catalytic system is different from the previous work by Corma et al.;15 for the latter, a 96.8% selectivity for 4-aminobenzaldehyde (3, Scheme 1) was obtained in the hydrogenation of 4-nitrobenzaldehyde in toluene (as solvent) at 100 °C using supported Au/TiO2 and H2 (10 bar). We further investigated various temperatures for the hydrogenation reaction of 4-nitrobenzaldehyde using the Au/ citrate nanoparticles as the catalyst (Table 1). At r.t., a low conversion (14.5%) of 4-nitrobenzaldehyde (hereafter, the conversion refers to 4-nitrobenzaldehyde) but with an almost 100% selectivity for 4-nitrobenzyl alcohol (hereafter, the selectivity refers to 4-nitrobenzyl alcohol) was observed when the reaction was performed in the presence of 1 equiv pyridine as the base (entry 1, Table 1). When the reaction temperature

3. RESULTS AND DISCUSSION In the present work, citrate-stabilized Au nanoparticles were synthesized via reduction of chloroauric acid (HAuCl4·3H2O) by trisodium citrate.29 The UV−vis absorption spectrum of the Au/citrate nanoparticles exhibits a strong surface plasmon resonance peak at ∼520 nm (Figure 1A). The TEM image shows that the average size of the Au/citrate nanoparticles is ∼18 ± 2 nm (Figure 1B). This colloid was then utilized as the catalyst for hydrogenation of 4-nitrobenzaldehyde. The catalytic reaction was carried out under a 20 bar H2 atmosphere in a Parr reactor using water as the reaction solvent (other conditions: 0.1 mmol 4-nitrobenzaldehyde, 0.1 mmol pyridine, 1 mL water, and 0.5 mg Au/citrate nanoparticles, see Experimental Methods). The reaction mixture after the reaction B

DOI: 10.1021/jp511930n J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

removal of 4-nitrobenzaldehyde and pyridine. The Au/citrate nanoparticles were found to aggregate under such conditions, thus, the main reason for the loss of activity of Au/citrate nanoparticles lies in the inherently insufficient thermal stability of such nanoparticles, instead of the adsorption of reactants and pyridine during the catalytic reaction. To improve the catalyst stability and also investigate the shape dependence, herein we further synthesized cetyltrimethylammonium bromide (CTABr) protected Au nanospheres and nanorods. Such nanostructures were utilized as catalysts for the hydrogenation reaction. We indeed found that these nanogold catalysts possess higher thermal stability than the Au/ citrate nanoparticles. Four different Au nanorods (designated A−D) were synthesized following a literature protocol.30−32 Briefly, the Au nanorod synthesis consists of two primary steps. In the first step, 2−3 nm gold seeds were synthesized via reduction of HAuCl4·3H2O with NaBH4 in the presence of excess CTABr (CTABr/Au(III) = 400:1, mol/mol). The aqueous solution of gold seeds appeared brown and a broad peak was seen at ∼520 nm in the UV−vis spectrum (Figure 3E). In the second step, the gold seeds grew into nanorods in the Au(I)/CTABr (CTABr/Au(I) = 400:1, in water) solution with different amounts of Ag(I) salt. The as-prepared gold colloids (I−IV) were characterized by UV−vis and TEM. The dimensions of Au nanostructures are width 24 ± 5 nm and length 28 ± 6 nm (sample I), width 19 ± 4 and length 35 ± 7 (sample II), width 21 ± 4 nm and length 44 ± 6 (sample III), and width 16 ± 3 and length 42 ± 7 nm (sample IV), as shown in the TEM images (Figure 3A−D). The gold colloid sample I appeared as a red solution in water, and the particles are essentially spherical, consistent with the one peak at ∼530 nm (Figure 3E); sample II appeared violet and two peaks (∼535 and ∼600 nm, overlapped) were shown in the UV−vis spectrum; sample

Table 1. Hydrogenation of 4-Nitrobenzaldehyde to 4Nitrobenzyl Alcohol under Various Conditions Using the Unsupported Gold/Citrate Nanoparticles as Catalystsa

selectivityb (%) b

entry

T (°C)

base

conv. (%)

2

3

4

1 2 3 4

25 55 80 80

pyridine pyridine pyridine none

14.5 33.3 53.2 (50.9)c trace

100 100 100

n.d. n.d. n.d.

n.d. n.d. n.d.

a Reaction conditions: ∼0.5 mg Au nanoparticle catalysts in 1 mL water, 0.1 mmol 4-nitrobenzaldehyde, 0.1 mmol pyridine, 20 bar H2, 8 h. bThe conversion (conv.) of 4-nitrobenzaldehyde and selectivity for 2 was determined by 1H NMR spectrum. cIsolated conversion of 4nitrobenzaldehyde; n.d. = not detectable.

was raised to 55 °C, the conversion was improved to 33.3%, and a 100% selectivity was again attained. At the 80 °C reaction temperature, a higher conversion (53.2%) was seen, with the selectivity maintained at 100%. But the stability of the catalyst at 80 °C was not satisfactory. Unlike the room temperature or 55 °C cases, we found that the Au/citrate nanoparticle catalyst started to aggregate and precipitate out at 80 °C in the catalytic process, as the reaction solution turned from the initial red (the inherent color of ∼18 nm Au nanoparticles) to finally colorless and a black solid was formed in the reactor (note: the solid could not be dissolved in fresh nanopure water) after the catalytic reaction. To identify the reason why the Au/citrate nanoparticle catalyst aggregated during the hydrogenation, we investigated the thermostability of the Au/citrate nanoparticles dispersed in water in the reactor with 20 bar H2 at 80 °C for 8 h, that is, identical with the reaction conditions except the

Figure 3. (A−D) TEM images of gold colloids I−IV, (E) UV−vis spectra of the gold colloids I−IV and gold seeds (in water). C

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The Journal of Physical Chemistry C III appeared deep blue with peaks at ∼530 and ∼670 nm in the spectrum; finally, sample IV appeared pink in water, and two peaks were at ∼530 and ∼750 nm (Figure 3E). The Au colloids in water (I−IV) were directly used as catalysts for the catalytic hydrogenation of 4-nitrobenzaldehyde. Under the catalytic reaction conditions (i.e., at 80 °C for 8 h using pyridine as the base and water as the solvent), Au colloid I gave a 27.0% conversion, but the selectivity was 100% (entry 1, Table 2). When the hydrogenation reaction was catalyzed by

results, we propose the following catalytic mechanism for the hydrogenation reaction. At the first step, the reactants are adsorbed onto the {111} facet of the gold nanorod. The H−H bond is activated with the aid of the pyridine promoter, and the pyridine group binds one proton from H2 to generate pyridinium (PyH+), with the H− being left on the Au nanorod. On the other hand, the other reactant (i.e., nitrobenzaldehyde) should absorb onto the surface of the Au nanorod via interactions of −CHO and −NO2 groups with PyH+ and H− species. Finally, H+ (PyH+) and H− would be transferred to NO2PhCHO and selectively reduce it to the NO2PhCH2OH product. Insight into the chemoselectivity (i.e., NO2 as opposed to CHO) and more details on the mechanism require future experimental and theoretical work. Previous work by Cao and co-workers16 reported that the mixture of CO and H2O gas can be used as the hydrogen source (CO + H2O → CO2 + H2) for the gold nanoparticlecatalyzed hydrogenation reaction of nitro compounds. Therefore, we herein compare the CO/H2O with the H2 system by replacing the H2 with CO gas (10 bar) with other reaction conditions the same (i.e., ∼0.5 mg Au nanorod II catalyst in 1 mL water, 0.1 mmol 4-nitrobenzaldehyde, 0.1 mmol pyridine, 8 h). Interestingly, the Au nanorod II catalyst gave a very low conversion (4.9%). In addition, the Au nanorod II catalyst was found to aggregate under the CO atmosphere, evidenced by the observation that the reaction solution turned colorless and a black solid was formed after the hydrogenation reaction as in the case of Au/citrate nanoparticles. These results implied that the gold nanorod catalysts are aggregated in the CO/H2O system. We further investigated the reuse of the recycled catalysts. Au nanorod II was chosen for the recyclability test. The nanorods were separated by centrifugation of the reaction mixture at 10000 rpm for 5 min. With the recycled catalyst, a fresh reaction with H2 as the hydrogen source was performed with fresh reactants under identical reaction conditions, except the catalyst. Remarkably, the nanorod catalyst was found to be very stable and no appreciable loss of catalytic activity for the hydrogenation was found after five cycles (Table 3). Thus, the Au nanorod catalyst shows an excellent recyclability.

Table 2. Hydrogenation of 4-Nitrobenzaldehyde to 4Nitrobenzyl Alcohol Catalyzed by Unsupported Gold Nanoparticles I−IV as Catalystsa

selectivityb (%) entry

catalyst

base

conv.b (%)

2

3

4

1 2 3 4 5 6

Au I Au II Au III Au IV Au II CTABr

pyridine pyridine pyridine pyridine none pyridine

27.0 99.9 99.9 99.4 trace n.r.

100 100 100 100

n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d.

a Reaction conditions: ∼0.5 mg Au nanoparticle catalysts in 1 mL water, 0.1 mmol 4-nitrobenzaldehyde, 0.1 mmol pyridine, 20 bar H2, 8 h. bThe conversion of 4-nitrobenzaldehyde and selectivity for 2 was determined by 1H NMR; n.r. = no reaction; n.d. = not detectable.

Au nanorods II−IV, the conversion was significantly increased to 99.9% and the selectivity was ∼100% (entries 2−4, Table 2). Unlike the Au/citrate catalyst, the Au/CTABr catalysts (I−IV) did not aggregate during the hydrogenation process, evidenced by the UV−vis spectra. The Au/CTABr nanorod catalysts, except the Au colloid I, performed much better than the Au/ citrate nanoparticle catalyst under the same reaction temperature (80 °C). Control experiment (using CTABr only) gave no conversion of 4-nitrobenzaldehyde, thus, the gold nanoparticles constitute the catalytically active species in the catalytic processes. Comparing sample I with samples II−IV, we note that the nanorod catalysts with higher aspect ratios all offer significantly higher conversions of 4-nitrobenzaldehyde (>99% with nanorod catalysts versus 27% with nanosphere catalyst). This drastic shape effect can be attributed to the effect of surface Au atom arrangement in nanorod catalysts. High resolution TEM analysis reveals that the Au nanorods are bound by {100} and {111} facets.32 The extended {100} and {111} faces on the nanorods serve as extraordinary catalytic sites, as opposed to the small patches of {100} and {111} faces in the case of nanospheres. The base pyridine plays an important role in the hydrogenation reaction. Yan et al.33 found that the interaction between pyridine and Au nanoparticle surface was weak according to XPS analysis. On the other hand, FT-IR spectral analysis suggested strong interaction between pyridine and water through hydrogen-bond formation in the hydrogenation.33 In our system, it was found that the hydrogenation reaction did not occur in the absence of pyridine, no matter the catalyst was Au/citrate nanoparticle or Au nanorod catalysts (Tables 1 and 2), thus, pyridine serves as the important promoter in the catalytic reaction. Based on these experimental

Table 3. Recovery and Reuse of Au Nanorod II Catalyst for Hydrogenation Reaction of 4-Nitrobenzaldehydea selectivity (%) entry

cycle

conv. (%)

2

3

4

1 2 3 4 5

first second third fourth fifth

99.9 99.4 99.7 99.0 99.1

100 100 100 100 100

n.d. n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d. n.d.

Reaction conditions: ∼0.5 mg Au nanorod II catalyst in 1 mL water, 0.1 mmol 4-nitrobenzaldehyde, 0.1 mmol pyridine, 20 bar H2, at 80 °C for 8 h; conv. = conversion. a

4. CONCLUSIONS In summary, we have investigated the catalytic properties of the unsupported Au/citrate and Au/CTABr colloids for the chemoselective hydrogenation of 4-nitrobenaldehyde in water with H2 (20 bar) as the hydrogen source. Remarkably, the gold nanoparticle catalysts exclusively reduce the aldehyde group in 4-nitrobenaldehyde to yield 4-nitrobenzyl alcohol with ∼100% D

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The Journal of Physical Chemistry C

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chemoselectivity, which is in contrast with the reduction of the nitro group in 4-nitrobenaldehyde to give rise to 4-aminobenzaldehyde as reported previously. The Au/CTAB nanorod catalysts performed much better than the ∼18 nm Au/citrate nanoparticle catalyst and also showed excellent recyclability. Overall, compared to the supported Au catalysts reported in the literature, our work of gold colloids explicitly demonstrates the intrinsic activity of gold nanospheres and nanorods for the chemoselective hydrogenation without any complication by the catalytic support.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by U.S. Department of Energy−Office of Basic Energy Sciences (Grant DE-FG0212ER16354).

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REFERENCES

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DOI: 10.1021/jp511930n J. Phys. Chem. C XXXX, XXX, XXX−XXX