Article pubs.acs.org/IECR
Organic-Soluble Palladium Nanoparticles Costabilized by Hyperbranched Polymer and Dispersants as Highly Efficient and Reusable Catalysts in Biphasic Solution Xin Wen,†,‡ Guang Li,† Qingzhi Chen,† Hailei Zhang,† Xinwu Ba,† and Guoyi Bai*,† †
Key Laboratory of Chemical Biology of Hebei Province, College of Chemistry and Environmental Science, Hebei University, Baoding, Hebei 071002, People’s Republic of China ‡ Bioreactor and Protein Drug Research and Development Center of Hebei Universities, Hebei Chemical and Pharmaceutical College, Shijiazhuang, Hebei 050026, People’s Republic of China S Supporting Information *
ABSTRACT: A series of organic-soluble and size-controllable palladium nanoparticles (PdNPs) costabilized by alkyl-modified hyperbranched poly(amide-ester) (HP) and different dispersants were prepared by chemical reduction. Their catalytic activities and stabilities were investigated by the reduction of 4-nitrophenol to 4-aminophenol in an organic/aqueous biphasic solution. It was found that high concentration HP and long alkyl chain dispersants reduce the particle sizes of PdNPs and improve their organic solubility, accounting for their good activity and stability in the organic/aqueous biphasic solution. A HP and oleylamine costabilized PdNPs showed the best catalytic performance in the reduction of 4-nitrophenol, and it can be easily separated from the biphasic solution and recycled 10 times with a conversion efficiency of around 100%. Notably, all the costabilized PdNPs showed excellent time-dependent stability and exhibited good activity and stability even after 35 days of storage. centrifugation or dialysis,20−22 which is a tedious, timeconsuming process. In order to solve this problem, much effort has been devoted to the design and preparation of recoverable catalysts, such as solid-supported,23−27 magnetic,28,29 and responsive catalysts.30,31 Although good results have been achieved, the complicated structures of these catalysts make them difficult to prepare. Another simple and efficient method to separate catalysts is the use of a biphasic solution.32,33 In particular, organic/ aqueous biphasic solutions have attracted much attention due to their convenient catalyst separation and reuse,19 as well as the easy accessibility and the uncomplicated preparation of catalysts.34,35 One basic principle in organic/aqueous biphasic solutions is that the catalyst, substrate, and product cannot exist in the same phase. However, most of the metal NPs and their stabilizers are hydrophilic and these kinds of metal NPs were unstable in organic solvents. Thus, most of the reported metal NPs were only suitable for organic-soluble substrates,36−41 whereas there are few reports on the synthesis of organicsoluble metal NPs for water-soluble substrates. Furthermore, the stabilizers for the organic-soluble metal NPs were usually complicated and costly, thus limiting their further application in industry. For example, Liu et al. and Kojima et al. have designed and prepared hyperbranched polymeric stabilizers terminated with a hydrophobic dendritic shell or quaternary ammonium groups by a multistep organic synthesis.17,42 Therefore, there is a need to develop efficient and simple stabilizers for organic-
1. INTRODUCTION In recent decades, metal nanoparticles (NPs) have attracted much attention due to their superior catalytic performance in hydrogenation, Suzuki coupling, and Heck C−C coupling reactions,1−3 as well as their potential applications in industry.4,5 The high surface-to-volume ratio of metal NPs leads to a shift of the electron affinity and cohesive energy of the atoms,6 which accounts for their high activity. However, one challenge in applying traditional metal NPs as catalysts is that very small NPs generally tend to coagulate to give bulky analogues, due to their high surface energies, resulting in marked reduction or even loss of their catalytic activity. To overcome the kinetically unstable nature of such metal NPs, polymers containing complexation sites for metal NPs, such as amino or carboxylate groups, have been employed to prevent the agglomeration of dispersive metal NPs.7−11 In particular, hyperbranched polymers are often used as stabilizers or embedding agents to enhance the stability and activity of metal NPs due to their unique three-dimensional structures, multifunctionalization, and cost-effective synthesis for largescale productions.12−14 Applications of hyperbranched polymers to stabilize metal NPs transformed solid-state catalysis into liquid-state catalysis, and then from heterogeneous catalysis to homogeneous catalysis.12 It resulted in the decrease of surface energies, subsequently prevented agglomeration of metal NPs, and thus suppressed the deactivation of the catalysts due to agglomeration.15,16 As is well-known, separation and reuse are key factors if catalysts are to be used in industrial applications.17−19 Another challenge in applying metal NPs as catalysts is that they are difficult to separate from reaction mixtures due to their small sizes. Generally, most metal NPs catalysts are separated by © 2014 American Chemical Society
Received: Revised: Accepted: Published: 11646
March 13, 2014 June 30, 2014 July 2, 2014 July 2, 2014 dx.doi.org/10.1021/ie5021816 | Ind. Eng. Chem. Res. 2014, 53, 11646−11652
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2.3. Reduction of 4-Nitrophenol in Biphasic Solution. A 1 mL volume of the above costabilized PdNPs CH2Cl2 solution was first diluted to 5 mL with extra CH2Cl2. Then, 20 mL of aqueous solution containing 4-NP (2.4 × 10−2 M) and NaBH4 (2.4 M) was poured into the above solution. The solution was then stirred vigorously using a magnetic stirrer. Over a certain period of time, the reaction mixture was first kept still for 8 s, and then 10 μL of the solution was removed from the upper aqueous phase. It was diluted to 3 mL with water to take UV−vis measurements. The reaction was stopped when the absorption at 400 nm in the UV−vis spectra disappeared. 2.4. Separation and Reuse. A representative procedure is shown in Figure 2b. When the reduction of 4-NP was complete, the biphasic solution was left standing for about 10 min until the phases had completely separated. The costabilized PdNPs phase was separated by removing the upper aqueous layer. After the CH2Cl2 phase was diluted to 5 mL with additional CH2Cl2, a new portion of aqueous 4-NP and NaBH4 was added for the next batch of reaction. 2.5. Characterization. Fourier transform infrared (FT-IR) spectra were recorded on a Varian-640 spectrophotometer (KBr pellet technique). Inductively coupled plasma mass spectroscopy (ICP-MS) measurements were carried out using an Agilent7500a. Transmission electron microscopy (TEM) images were obtained with a FEI Tecnai G2 F20 microscope. UV−vis spectra were performed on a Shimadzu WV-2550 spectrophotometer using 1.0 cm quartz cells. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a PHI 1600 spectrometer using Mg Kα X-ray source for excitation.
soluble metal NPs which are suitable for the catalytic reaction involving the water-soluble substrates in organic/aqueous biphasic solutions. In this paper, we report our recent research into the preparation of organic-soluble PdNPs costabilized by alkylmodified hyperbranched poly(amide-ester) (HP) and different dispersants and their applications in the reduction of 4nitrophenol (4-NP) to 4-aminophenol (4-AP) by NaBH4 in an organic/aqueous biphasic solution. The influence of the HP concentration and the alkyl chain length of dispersants on the organic solubility, particle size, and catalytic performance of PdNPs were studied systematically based on necessary characterizations. Organic-soluble PdNPs costabilized by 1.0 mg/mL HP and oleylamine showed an excellent catalytic rate and durable activity. Particularly, it can be recycled for 10 successive cycles, even after a 35 day storage period.
2. EXPERIMENTAL SECTION 2.1. Materials. Unless otherwise noted, all chemicals were of analytical reagent grade and used without further purification. Palladium chloride (PdCl2), 1-butanamine (BA), sodium borohydride (NaBH4), and 4-NP were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd.. 1-Dodecanoyl chloride was purchased from J&K Scientific Ltd. 1-Dodecylamine (DA) was purchased from Tianjin Guangfu Chemical Research Institute. Oleylamine (OA; C18, 80−90%), 2nitrophenol (2-NP), and 3-nitrophenol (3-NP) were purchased from Aladdin Reagent Co. Ltd. Hyperbranched poly(amideester) terminated with hydroxyl groups (HP-OH) and HP with a hydrophilic hyperbranched core and a hydrophobic linear shell (Figure 1) were synthesized according to the method reported in the literature (for details, see the Supporting Information).17,43
3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of Costabilized PdNPs. The organic-soluble HP was first prepared by the acylation of the terminal hydroxyl group of HP-OH using 1dodecanoyl chloride as a capping agent.17,43 The reaction was terminated when the peak for hydroxyl groups at 3200−3600 cm−1 in FT-IR spectra had almost disappeared (Figure S1 of the Supporting Information). HP and different dispersants were then used as costabilizers to prepare organic-soluble PdNPs (Figure 2a). PdCl2 was first dissolved in various dispersants to form PdCl2−dispersant complexes;44−46 HP was then added to the above complexes; finally, HP and dispersant costabilized PdNPs were in situ generated by reduction using NaBH4. UV− vis absorption spectroscopy was used to monitor the reduction process, as shown in Figure 3. The characteristic peaks related to PdCl2−dispersant complexes are at 288 nm (PdCl2−BA and PdCl2−DA) and 273 nm (PdCl2−OA), respectively, due to differing dispersant to metal charge transfer.20 Obviously, these characteristic peaks disappeared after the addition of NaBH4, indicating that Pd(II) was reduced to Pd(0) under the reduction conditions,47 as also confirmed by the XPS result (Figure S2 of the Supporting Information). In order to verify the organic solubility of the above PdNPs, the as-prepared PdNPs−CH2Cl2 solution was extracted with water. The concentrations of Pd in water, leaching from the CH2Cl2 phase, were checked by ICP-MS analysis and the results are shown in Figure 4. It was found that the concentration of Pd in water decreased with the increase of the concentration of HP. All high concentration HP stabilized PdNPs exhibited good organic solubility in CH2Cl2, and the leaching of Pd from CH2Cl2 to water was only 0.01−0.05%, which can almost be ignored. The time-dependent stability of
Figure 1. Schematic representation of the synthesized HP.
2.2. Preparation of Organic-Soluble PdNPs. A representative procedure for the preparation of organic-soluble PdNPs is shown in Figure 2a. A solution of PdCl2 in different dispersants (BA, DA, OA) (2.5 × 10−2 M, 50 μL) was added to 4.85 mL of CH2Cl2 containing HP with different concentrations ([HP0] = 0, [HP1] = 0.1 mg/mL, [HP2] = 1.0 mg/ mL). After 30 min of stirring to form a stable solution, 100 μL of fresh NaBH4 solution in methanol (0.125 M) was injected. The solution then turned from colorless to brown. The mixture was kept at room temperature for 5 h to ensure that all Pd(II) ions have been completely reduced to Pd(0). The costabilized PdNPs are denoted as Pd/HPc-d, in which c refers to the concentration of HP and d refers to the nature of the dispersants. 11647
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Figure 2. Schematic illustration of (a) the preparation of PdNPs costabilized by HP and different dispersants and (b) the separation and reuse of the catalysts.
Figure 4. Concentration of Pd(0) in aqueous extract of costabilized PdNPs in CH2Cl2. Conditions of extraction: H2O (20 mL), CH2Cl2 (5 mL), and [Pd] = 5 × 10−5 M.
did not exhibit any trace of aggregation or precipitation. Therefore, combined with the ICP-MS results, we can draw a conclusion that high concentration polymer and long alkyl chain dispersant can improve both the organic solubility and the time-dependent stability of the costabilized PdNPs. The surface morphology and size distributions of the costabilized PdNPs were recorded by TEM, as shown in Figure 5. It is clear that each sample displayed approximately spherical morphology, consistent with other reported hyperbranched copolymer or dispersant stabilized NPs.42,48 As can be seen, the average diameter of PdNPs decreased from 3.46 to 1.87 nm and the size distributions became narrower with the increase of the concentration of HP (Figure 5a,b,e). At the same time, the average diameter of PdNPs decreased from 3.31 to 1.87 nm, together with more uniform in size distribution, with the increase of the alkyl chain length of dispersant (Figure 5c−e). Thus, the particle size and distribution of PdNPs can be tuned by the concentration of HP and the alkyl chain length of dispersant. 3.2. Catalytic Reduction of 4-NP. As is well-known, 4-AP is an important organic intermediate, and is widely used in the synthesis of drugs, dyes, antioxygen, and photographic
Figure 3. UV−vis spectra of PdCl2−dispersant complexes (a) PdCl2− BA, (b) PdCl2−DA, and (c) PdCl2−OA in CH2Cl2 before and after reduction by NaBH4.
costabilized PdNPs in CH2Cl2 was also investigated. After 35 days of storage at room temperature, the costabilized PdNPs 11648
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Figure 5. TEM images and size distributions of costabilized PdNPs: (a) Pd/HP0-OA, (b) Pd/HP1-OA, (c) Pd/HP2-BA, (d) Pd/HP2-DA, and (e) Pd/HP2-OA.
developer.49,50 It is therefore worthwhile to develop an efficient and durable catalytic system to produce 4-AP. Thus, the reduction of 4-NP to 4-AP by NaBH4 in an organic/aqueous biphasic solution was chosen as a model reaction to evaluate the catalytic properties of the as-prepared PdNPs. Considering the dissolving characteristic of PdNPs (organic soluble), reactant 4-NP, and product 4-AP (water soluble), vigorous stirring will enhance collision probability between the PdNPs and 4-NP, then improving the conversion of 4-NP and the yield of 4-AP. Moreover, due to the immiscibility of CH2Cl2 and water, PdNPs can be rapidly separated from the reaction mixture via simple phase separation, once the reduction is finished. As shown in Figure 6, 4-NP solution has a maximum absorption at about 317 nm. This absorption peak shifted immediately from 317 to 400 nm with the addition of alkaline NaBH4 solution, due to the formation of 4-nitrophenolate ions under alkaline conditions.49 Further reduction of these 4nitrophenolate ions to the desired 4-AP will not occur in the absence of PdNPs. In contrast, after the addition of Pd/HP2OA to the 4-NP and NaBH4 solution, the color of the solution
Figure 6. Typical time-dependent UV−vis spectra on the reduction of 4-NP with NaBH4 catalyzed by Pd/HP2-OA. The inset is the color change during the reaction. Reaction conditions: [4-NP] = 2.4 × 10−2 M, [NaBH4] = 2.4 M, [Pd] = 5.0 × 10−5 M, and VCH2Cl2:VH2O = 5:20.
changed from yellow to colorless within 2.5 min (the inset of Figure 6), indicating the reduction of 4-NP. Meanwhile, the absorption peak at 400 nm related to 4-nitrophenolate ions gradually decreased in intensity along with the increase of a 11649
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Figure 7. (a) Relationship between ln(C/C0) and reaction time of the reduction of 4-NP with NaBH4 catalyzed by OA stabilized PdNPs. (b) Relationship between kapp and dispersants (I, II, and III refer to HP0, HP1, and HP2, respectively).
Pd/HP2-OA also exhibited excellent activity not only in the reduction of 4-NP, but also in other nitrophenols, such as 2-NP and 3-NP (Figure S5 of the Supporting Information). Figure 8 shows the influence of the number of cycles of the costabilized PdNPs on the reaction time for the full conversion
new absorption peak at about 297 nm for 4-aminophenolate ions. Furthermore, based on the comparison of the full reaction time, Pd/HP2-OA showed the best catalytic rate in all the catalysts studied: almost 3 times higher than those of HP and BA costabilized PdNPs (Figure S3 of the Supporting Information). Hence, combined with the TEM results, we suggested that higher HP concentration and longer alkyl chains of dispersants favored the formation of smaller and more active PdNPs. From the results of reduction times, Pd/HP2-OA catalyst exhibited the best activity under the reaction conditions adopted in this paper, but it is only a qualitative expression of the catalytic activities of the catalysts. To quantitatively evaluate their catalytic activities, the apparent rate constant (kapp) of the reduction of 4-NP to 4-AP was chosen as an important parameter. The relationships between ln(C/C0) and the reaction time of Pd/HP2-OA are shown in Figure 7a (for the results of Pd/HP2-BA and Pd/HP2-DA, see Figure S4 of the Supporting Information). As can be seen, all values of ln(C/ C0) are linear with respect to t, suggesting that all the reductions are first-order reactions under the conditions adopted. The slope of the fitting lines of the points in Figure 7a is defined as kapp according to eq 1, and the results are shown in Figure 7b. ln(A /A 0) = ln(C /C0) = −kappt
Figure 8. Relationship between number of cycles of costabilized PdNPs and the time of full conversion of 4-NP. Reaction conditions: [4-NP] = 2.4 × 10−2 M, [NaBH4] = 2.4 M, [Pd] = 5.0 × 10−5 M, and VCH2Cl2:VH2O = 5:20.
of 4-NP. In order to exclude or minimize the effect of timedependent stability on the results of the cycle experiments, all cycles of the reduction over an individual PdNPs catalyst were carried out within 12 h. As can be seen, the reduction time for the full conversion of 4-NP decreased with the increase of the HP concentration regardless of the nature of the dispersants, indicating that the increase of the HP concentration not only increased the catalytic activity of PdNPs, but also weakened their aggregation17 and inhibited the loss of active species of PdNPs during the cycle experiments. Considering that the cycle number increased with the increase of the alkyl chain length of the dispersants while there was no HP used in their preparation, the long alkyl chains of dispersants are suggested to enhance the activity and stability of PdNPs. Furthermore, ICP-MS and TEM were carried out to clarify the reasons for prolonged reaction time with the increase of cycles. ICP-MS analysis of the water phase after the reaction showed that only small amounts of Pd (for instance, 0.31% Pd in the case of Pd/ HP2-OA) has leached to water phase; thus it should not be the main reason. In contrast, the sizes of the used PdNPs markedly increased, as demonstrated by TEM (Figure S6 of the Supporting Information), indicating the aggregation of the PdNPs and resulting in the decrease of their activities. Thus, longer reaction time was needed for the full conversion of 4-NP in successive cycles. However, the full conversion time for the reduction of 4-NP over Pd/HP2-OA catalyst was 35 min in the
(1)
where A0 and C0 are the initial absorbance and concentration of 4-NP, respectively; A and C are the absorbance and concentration of 4-NP at a certain reaction time, respectively; t is the reaction time. In the case of the constant dispersant (Figure 7b), it is found that kapp increased with an increase of the HP concentration. The reduction of 4-NP catalyzed by Pd/HP2-OA showed the highest kapp (3.27 × 10−2 s−1), which was also higher than most of the previous reported results (Table S1 of the Supporting Information). We have ascribed this result to the following two reasons. One is the increase of the HP concentration made the size of PdNPs small and uniform, thus enhancing their activity; the other is the increase of the HP concentration also led to strong interactions between HP and PdNPs, inhibiting the aggregation of PdNPs and hence slowing the loss in their activities. Noticeably, although only a small amount of HP (0.1 mg/mL) was used in the preparation of Pd/HP1-OA, its kapp values are much higher than that of Pd/HP0-OA (without any HP added during its preparation), indicating the markedly costabilizing effect of OA and HP on PdNPs, compared to BA. It also suggested that the long alkyl chain dispersant should be more beneficial to the catalytic activity of PdNPs. Furthermore, 11650
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S1). This material is available free of charge via the Internet at http://pubs.acs.org.
tenth cycle; in contrast, the full conversion time over a similar Au-based catalyst exceeded 200 min only in the third cycle.17 Furthermore, Pd/HP2-OA exhibited similar activities in the 10 times amplified reactions (Figure S7 of the Supporting Information). Significantly, we have succeeded in obtaining highly active PdNPs by a common reagent OA and a costeffective and easily synthesized polymeric HP, exhibiting potential applications in industry. Finally, the catalytic performance of the aged PdNPs, which have been stored for 35 days, was investigated to evaluate their time-dependent stability (Figure 9). It was found that the aged
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86-312-5079359. Fax: +86-312-5937102. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Prof. David Knight for his kind help. Financial support by the National Natural Science Foundation of China (21376060) and the Natural Science Foundation of Hebei Province (B2014201024) is gratefully acknowledged.
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(1) Calò, V.; Nacci, A.; Monopoli, A.; Cotugno, P. Heck reactions with palladium nanoparticles in ionic liquids: coupling of aryl chlorides with deactivated olefins. Angew. Chem., Int. Ed. 2009, 48, 6101. (2) Zhu, Y.; Qian, H.; Drake, B. A.; Jin, R. Atomically precise Au25(SR)18 nanoparticles as catalysts for the selective hydrogenation of α,β-unsaturated ketones and aldehydes. Angew. Chem., Int. Ed. 2010, 49, 1295. (3) Han, J.; Liu, Y.; Guo, R. Facile synthesis of highly stable gold nanoparticles and their unexpected excellent catalytic activity for Suzuki−Miyaura cross-coupling reaction in water. J. Am. Chem. Soc. 2009, 131, 2060. (4) Chen, R.; Jiang, Y.; Xing, W.; Jin, W. Preparation of palladium nanoparticles deposited on a silanized hollow fiber ceramic membrane support and their catalytic properties. Ind. Eng. Chem. Res. 2013, 52, 5002. (5) Bai, G.; Wen, X.; Zhao, Z.; Li, F.; Dong, H.; Qiu, M. Chemoselective hydrogenation of benzoic acid over Ni−Zr−B− PEG(800) nanoscale amorphous alloy in water. Ind. Eng. Chem. Res. 2013, 52, 2266. (6) Toshima, N.; Yonezawa, T. Bimetallic nanoparticles novel materials for chemical and physical applications. New J. Chem. 1998, 22, 1179. (7) Nishikata, T.; Tsutsumi, H.; Gao, L.; Kojima, K.; Chikama, K.; Nagashima, H. Adhesive catalyst immobilization of palladium nanoparticles on cotton and filter paper: applications to reusable catalysts for sequential catalytic seactions. Adv. Synth. Catal. 2014, 356, 951. (8) Evangelisti, C.; Panziera, N.; D’Alessio, A.; Bertinetti, L.; Botavina, M.; Vitulli, G. New monodispersed palladium nanoparticles stabilized by poly-(N-vinyl-2-pyrrolidone): preparation, structural study and catalytic properties. J. Catal. 2010, 272, 246. (9) Kassube, J. K.; Wadepohl, H.; Gade, L. H. Immobilisation of the BINAP ligand on dendrimers and hyperbranched polymers: dependence of the catalytic properties on the linker unit. Adv. Synth. Catal. 2009, 351, 607. (10) Maity, P.; Yamazoe, S.; Tsukuda, T. Dendrimer-encapsulated copper cluster as a chemoselective and regenerable hydrogenation catalyst. ACS Catal. 2013, 3, 182. (11) Wu, L.; Ling, J.; Wu, Z. A highly active and recyclable catalyst: phosphine dendrimer-stabilized nickel nanoparticles for the Suzuki coupling reaction. Adv. Synth. Catal. 2011, 353, 1452. (12) Hajji, C.; Haag, R. Hyperbranched polymers as platforms for catalysts. Top. Organomet. Chem. 2006, 20, 149. (13) Rajesh, R.; Venkatesan, R. Encapsulation of silver nanoparticles into graphite grafted with hyperbranched poly(amidoamine) dendrimer and their catalytic activity towards reduction of nitro aromatics. J. Mol. Catal. A: Chem. 2012, 359, 88. (14) Mecking, S.; Thomann, R.; Frey, H.; Sunder, A. Preparation of catalytically active palladium nanoclusters in compartments of
Figure 9. Influence of storage time on the catalytic performance of the costabilized PdNPs. Reaction conditions: [4-NP] = 2.4 × 10−2 M, [NaBH4] = 2.4 M, [Pd] = 5.0 × 10−5 M, and VCH2Cl2:VH2O = 5:20.
PdNPs showed similar catalytic performance to the freshly prepared ones. Thus, we can conclude that the costabilized PdNPs showed not only good activity but also excellent timedependent stability in the reduction of 4-NP to 4-AP.
4. CONCLUSIONS In summary, HP and different dispersants have been used as costabilizers in preparing organic-soluble PdNPs. The organicsoluble PdNPs showed excellent catalytic performance in the reduction of 4-NP, a water-soluble substrate, to 4-AP in an organic/aqueous biphasic solution. These catalysts and products can be separated effectively after the reaction and recycled 10 times in this biphasic solution. The long alkyl chain dispersant and high concentration of HP can decrease the particle size of PdNPs, enhancing their organic solubility, catalytic activity, and stability. Pd/HP2-OA exhibited the best catalytic performance in the catalytic reduction of 4-NP. In addition, all costabilized PdNPs showed excellent timedependent stabilities without loss in their activities even after 35 days. Owing to their excellent catalytic efficiency, easy separation, and cost effectiveness, this biphasic catalytic solution involving costabilized PdNPs catalysts is proposed as a potential system for the catalytic reduction of water-soluble substrates in large-scale processes.
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REFERENCES
ASSOCIATED CONTENT
S Supporting Information *
Preparation and characterization of HP; synthetic procedure of HP (Scheme S1); FT-IR spectra of HP and HP−OH (Figure S1); XPS spectrum (Figure S2); time-dependent UV−vis spectra of the reduction of 4-NP (Figure S3); relationship between ln(C/C0) and reaction time (Figure S4); reductions of 2-NP and 3-NP catalyzed by Pd/HP2-OA (Figure S5); TEM images of the used PdNPs (Figure S6); 10 times amplified reduction of 4-NP (Figure S7) and comparison of kapp (Table 11651
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dx.doi.org/10.1021/ie5021816 | Ind. Eng. Chem. Res. 2014, 53, 11646−11652
