Size-Dependent Halogenated Nitrobenzene Hydrogenation Selectivity

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Size-Dependent Halogenated Nitrobenzene Hydrogenation Selectivity of Pd Nanoparticles Jinghui Lyu, Jian-guo Wang, Chuanshan Lu, Lei Ma, Qunfeng Zhang, Xiaobo He, and Xiaonian Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp411442f • Publication Date (Web): 09 Jan 2014 Downloaded from http://pubs.acs.org on January 22, 2014

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The Journal of Physical Chemistry

Size-Dependent Halogenated Nitrobenzene Hydrogenation Selectivity of Pd Nanoparticles Jinghui Lyu, Jianguo Wang*, Chunshan Lu, Lei Ma, Qunfeng Zhang, Xiaobo He, Xiaonian Li* Industrial Catalysis Institute of Zhejiang University of Technology, State Key Laboratory Breeding Base of Green Chemistry Synthesis Technology, Hangzhou, 310032, P. R. China

ACS Paragon Plus Environment

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ABSTRACT The selectively hydrogenation of halogenated nitrobenzene (HNB) has been a great important chemical reaction in the fine chemical productions. In this study, the effect of metal particle size on the selective hydrogenation of HNB over Pd/C catalysts has been extensively investigated through the combination of theoretical (density-functional theory calculations(DFT)) and experimental methods. DFT calculations showed that the reaction barriers for dechlorination strongly depend on the type of reaction sites (terrace or edge), while the hydrogenation reaction barriers are nearly same on different sites, which indicates that Pd nanoparticle size significantly affects the catalyst selectivity. Moreover, Pd nanoparticles with different sizes (from 2.1 to 30 nm) supported on activated carbon were synthesized using the methods developed by our group. In a 500 mL reactor, the selectivity is over 99.90% when the Pd nanoparticles are bigger than 25 nm. Finally, the industrial applications of the proposed catalyst were evaluated in several pilot factories. This study provides useful information on controlling the selectivity of other similar reactions catalyzed by noble metal nanocatalysts. KEYWORDS: Size-dependent selectivity, Pd nanoparticles, hydrogenation, halogenated nitrobenzene


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Introduction Selective catalysis has been a challenge issue in the complex catalytic reactions, because the selectivity to the desired product is often affected by various factors, including reaction conditions, reactant composition, and the chemical-physical properties of catalyst.1, 2 Since the discovery of the distinct properties of nanogold catalysts and the development of methods for the experimental preparation, characterization, and theoretical simulation of nanocatalysts, controlling the reactivity or selectivity by regulating the nanocatalyst particle size, shape, and exposed planes has become a hot topic in catalyst design.3-9 Halogenated aromatic amines are important intermediates in the synthesis of organic compounds such as dyes, herbicides, pesticides, medicines, and functional.10 These widely applied organic amines are usually produced by the selective catalytic hydrogenation of halonitrobenzenes (HNBs) to the corresponding haloanilines (HANs) using nickel-, platinumand palladium-based catalysts.10-12 Generally, these metals are active at modest temperature and pressure for HNB hydrogenations, but they are not selective towards HANs due to the occurrence of hydrodehalogenation if the catalysts are not precisely designed.13, 14 Recently, many effective catalysts have been designed to prevent the hydrodehalegentaion in HNBs hydrogenation. For example, a partially reduced Pt/r-Fe2O3 nanocomposite was reported to be effective in the hydrogenation of chloronitrobenzenes to chloroanilines, possibly because the iron oxide supports weaken the extent of electron feedback from Pt particle of electrondeficient state to the aromatic ring in chloroanilines which suppressed the hydrodechlorination.15 Han et al.16 investigated the influence of transition metals modified Pt/CNTs catalyst on the pchloronitrobenzene hydrogenation, which suggested that the PtFe/CNTs catalyst exhibits the highest activity, while the PtMn/CNTs catalyst shows the highest p-CAN yield. Cheng et al.17


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reported that 99.7% selectivity to o-chloroaniline (o-CAN) could be obtained at 100% conversion of o-chloronitrobenzene (o-CNB) using PEG-stabilized Pt nanoparticles as the catalyst. However, these catalytic systems are still far from industrial application mainly due to the difficulty in recovering the noble metal from such complex systems. Activated carbon supported catalysts are one of the most promising candidates for the selective HNB hydrogenation to HAN in industry since the noble metal can be easily recovered through simply burn out the carbon support. Previous research in our group have found that the Pd/C catalysts showed high activity in the hydrogenation of nitrobenzene and HNBs,18, 19 both activity and selectivity for these reactions are found to be highly related with the Pd particle sizes. Coq et al.20 also have reported that the catalytic hydrodehalogention activity increases with the decreasing of Pt particle size in hydrogenation CNB to CAN. Therefore, it is expected that the selective hydrogenation of HNB over supported Pd catalysts must be a structure-sensitive reaction.21-29 However, the size effect of the supported Pd on hydrodehalogenation reactions remains highly controversial.20, 30-32 In this study, the effect of metal particle (crystal) size on the selective hydrogenation of HNB over Pd/C catalysts has been extensively investigated through the combination of theoretical (first principal density-functional theory (DFT) calculations) and experimental methods. First, by means of DFT calculations, we found that the reaction barriers of hydrogenation of pchloronitrobenzene (p-CNB) to p-chloroaniline (p-CAN) are nearly same on two kinds of Pd reaction sites (teerace and step). While, the reaction barriers of dechlorination of both p-CNB and p-CAN on step sites are much smaller than those on teerace. It is well known that small/large particles have large/small edge sites. The results indicate that Pd nanocatalyst size have much larger affects on dechlorination reaction than hydrogenation one. Second, activated carbon-


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supported Pd nanoparticles with different sizes were synthesized using the methods developed by our group. Third, the correlation between the selectivity of p-CNB hydrogenation to p-CAN over Pd/C and the different Pd particle sizes was experimentally confirmed. Finally, the industrial applications of the proposed catalyst were evaluated in several pilot factories.

Methods Computational Details All of calculations were performed using the Vienna ab initio simulation package (VASP),33-35 a periodic density functional theory (DFT) code with projector augmented wave (PAW) potentials. The including vdw interactions in VASP code was implemented via a self-consistent vdw-DFT functional

36, 37

In this study, the vdw-DF functional with PBE exchange were used,

which has been successfully applied to polycyclic aromatic hydrocarbons38 and chlorobenzene on Au and Pt surfaces.39 The flat Pd(111) surfaces, stepped Pd(211) surfaces and icosahedral Pd55, Pd13 were used as substrates, which might represent the terrace, step, and corner sites of Pd nanoparticles. The Pd(211) surfaces can be described as a step-terrace structure consisting of three-atom-wide terraces of (111) orientation and a monatomic step with a (100) orientation, or 3(111) × (100) in microfacet notation. Four layers (4×4) Pd(111) and five layers (1×4) Pd(211) were used, in which the two bottom layers were fixed during the optimizations and the vacuum layer of Pd(111) and Pd(211) is 15 Å. Pd55 and Pd13 clusters in a 20×20×20 unit cell were fully relaxed. The Brillouin zone integration was performed using the Monkhorst-Pack scheme with 4×4×1 and 4×2×1 mesh for the flat Pd(111) and stepped Pd(211) surfaces The gamma points were used for Pd55 and Pd13 clusters. All structures were optimized with a convergence criterion of 10 meV/


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Å for the forces and 0.01 meV for the energy. Transition states were searched using the climbing image nudged elastic band (CI-NEB) method. The adsorption energies of molecules on substrates (surfaces or clusters) were defined as Ead=Etotal-Emol-Esub, where Etotal, Emol and Esub are total energies of the adsorbed system, the isolated molecule, and the clean substrate system, respectively. According to the above definition, a negative value indicates the process is exothermic, whereas positive values are endothermic. The activation barrier was defined as Eac=E(transition state)-E(initial state), where E(transition state)-E(initial state) are total energies of the transition and initial (adsorbed) states in each elementary reaction step. Catalyst preparation Activated carbon-supported Pd catalysts were prepared by the impregnation technique according to a previously described procedure.18 The commercially available activated carbon with particle size smaller than 61 µm (250 mesh), Brunauer–Emmett–Teller (BET) surface area of 1733 m2/g, and 0.48 wt% ash content (Fujian Xinsen Carbon Co., Ltd., P. R. China) was used in our study. The activated carbon was pretreated with different HNO3 concentrations (from 1 to 14 M) at 366 K for 5 h, filtered, rinsed in distilled water until the pH became neutral, and eventually dried at 383 K for 10 h. The activated carbon was also pretreated with different KI concentrations (2.5 M) at 333 K for 6 h, filtered, and then rinsed in distilled water until no precipitation was found in the filter liquor. AgNO3 solution was added to the activated carbon, which was subsequently dried at 383 K for 10 h. A desired volume of aqueous H2PdCl4 solution was added into an aqueous suspension of the pretreated activated carbon at 353 K. After stirring for 6 h, 10% NaOH solution was added drop-wise to the suspension to maintain the pH within the range of 9 to 10 for 30 min. The formed catalyst was then washed and the precipitated


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Pd(OH)2 was reduced using hydrazine hydrate. The Pd/C catalyst was filtered, rinsed in distilled water until pH = 7, and eventually outgassed under vacuum at 383 K for 10 h. Catalyst characterization The catalysts were characterized by X-ray diffraction (XRD) using an X’Pert PRO diffractometer (PNAlytical Co.) equipped with a Cu Kα radiation source that was operated at 60 kV and 55 mA. Diffraction patterns were recorded at a scanning rate of 2°/min and at a step of 0.02°. The mean size of the Pd particles was calculated using the Scherrer equation. The particle size and morphology of Pd on the Pd/C catalyst surface were determined by transmission electron microscopy (TEM) using a Tecnai G2 F30 S-Twin Microscope (Philips-FEI Co., Netherlands). At least 500 individual Pd particles were counted for each catalyst. The mean Pd particle size of the catalyst, ds, was calculated using the following equation: ds=Σnidi3/Σnidi2, where the visible particle size di on the micrographs was measured by a computerized system, ni is the number of particles with diameter di, and Σni > 500. Laboratory-scale catalyst evaluation Solvent-free hydrogenation of halogenated nitroaromatic compounds was performed in a 500 mL stainless steel autoclave, which was charged with 200 g of the halogenated nitroaromatic compound and 1 g of the Pd/C catalyst. The batch reactor was initially purged five times with pure N2 and five times with H2 to replace the gas in the system. The reactor was then heated to the desired temperature at the desired H2 pressure and at a stirring rate of 1200 rpm. After the complete conversion of the reactant, the Pd/C catalyst was filtered from the mixture for subsequent use, and the corresponding aromatic haloamine was separated from the aqueous phase using a separatory funnel. The products were identified by gas chromatography-mass spectrometry (GC-MS, Agilent 5973N) and analyzed by a GC (Agilent 7890A) equipped with a


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flame ionization detector (FID) and a DB-1 capillary column (30 m×0.32 mm×3 µm). Quantitative analysis was conducted using the area-normalization method. Pilot-scale industrialization The solvent-free hydrogenation of halogenated nitroaromatic compounds was performed in a 2000 or 3000 L stainless steel autoclave, which was charged with a number of halogenated nitroaromatic compounds and a certain amount of the Pd/C catalyst. The batch reactor was initially purged twice with pure N2 and twice with H2 at 0.2 MPa to replace the gas in the system. The reactor was then heated to the desired temperature at the desired H2 pressure and at a stirring rate of 300 to 400 rpm. Cooling water was used to control the temperature during the reaction. When the amount of consumed hydrogen reached the theoretical level of consumption and the reaction rate decreased, stirring was discontinued, and the samples were analyzed by GC. The reaction was considered to be concerted completely when the raw material (halogenated nitroaromatic compound 6-CNT or 3,4-DCNB) concentration was below 0.1%. The Pd/C catalyst was filtered from the mixture for subsequent use, and the corresponding aromatic haloamine was separated by phase separation. The products were analyzed by a GC (Shimadzu, GC-2014) equipped

with an FID detector and

an AT.XE 60 capillary column

(30 m×0.20 mm×0.25 µm). Quantitative analysis was conducted using the area-normalization method.

Results and discussion In general, the shapes of supported noble metal (particularly Au, Pd, and Pt) nanoparticles are octahedral, truncated octahedral, decahedral, and icosahedral because of thermodynamic and kinetic equilibrium.5, 40 Depending on the coordination environment, various types of reaction sites that are independent of the nanoparticle shape exist on these nanoparticles. For example, for


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ideal truncated octahedral nanoparticles, terraces and edges (including steps and corners) are two types of reaction sites (Figure 1a). The ratio of terrace sites to edge sites can be controlled by changing the nanoparticle sizes. The relationship between the percentages of different reaction sites (terraces, steps, and corners) and the nanoparticle size is plotted in Figure 1b. As the nanoparticle size increases, the percentage of terrace sites dramatically increases, levels off at 99% when the particle size reaches 25 nm, and then shows negligible changes as the nanoparticle size continues to increase. By contrast, the percentage of edge sites increases with decreasing particle size. The edge sites become predominant ones when the particle size is below ~2 nm.

Figure 1. (a) The model of ideal truncated octahedron nanoparticle. (b) Ratio of three kinds of reactions sites (terraces, step and corner) on different size ideal cubooctahedral Pd nanoparticles. (c) Zoom in HR-TEM images for 5.2, 11.3 and 26.9 nm Pd nanoparticles, respectively.


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Figure 2. The optimized structures of p-CNB on Pd(111), Pd(211), Pd55 and Pd13.

Although molecular adsorption and reactivity on differently sized nanoparticles (ranging from 2 to 30 nm) cannot be simulated, simulations can be performed on the adsorption and reactivity on different reaction sites. As previously discussed, the site distribution on Pd nanoparticles is size-dependent. Therefore, the reactivity of differently sized nanoparticles can be predicted by simulating their reactivity on different sites. In this study, Pd(111) and Pd(211) surfaces and icosahedral Pd55 and Pd13 clusters were used to represent the terrace, step, and corner sites of Pd nanoparticles. Figure 2 shows the two stable p-CNB configurations based on these models. On the Pd(111) surface, the benzene ring is either on the bridge (most stable) or in the hollow (second most stable) site. These two sites have nearly the same adsorption energies. On the Pd(211) surface, the preferred adsorption site is the step edge. The benzene ring in the second most stable structure is similar to that found on the Pd(111) surface. On the icosahedral Pd55 and Pd13 clusters, the benzene ring remains bonded to low-coordinated Pd atoms. It is seen that the distance between Cl and Pd is 2.98 and 2.65 Å on Pd(111) and Pd(211). The adsorption energy of p-CNB in the Pd13 cluster is higher than that in the Pd (111) surface by more than 1.0 eV. In


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other words, the adsorption energy in the low-coordinated Pd edge sites is higher than that in the terrace surfaces sites by more than 1.0 eV. Furthermore, the atomic hydrogen adsorption energy shows negligible changes in the different models. Figure 1b shows a large amount of edge sites on the small nanoparticles (e.g., smaller than 5 nm), whereas terrace sites are predominant on larger nanoparticles (e.g., larger than 10 nm). This result indicates that the adsorption of p-CNB on small nanoparticles is significantly stronger than that on larger nanoparticles. The stoichiometric reaction of p-CAN formation from p-CNB hydrogenation is ClC6 H 4 NO2 + 3H 2 = ClC6 H 4 NH 2 + 2 H 2O Based on previous studies,10,

14,

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(1)

the reaction intermediates are nitroso- and n-

phenylhydroxylamino compounds. Therefore, the elementary steps of p-CAN formation are as follows:

ClC6 H 4 NO2 * +6 H * = ClC6 H 4 NO2 H * +5H *

(2)

ClC6 H 4 NO2 H * +5H * = ClC6 H 4 NO * + H 2O + 4 H *

(3)

ClC6 H 4 NO * + H 2O + 4 H * = ClC6 H 4 NOH * + H 2O + 3H *

(4)

ClC6 H 4 NOH * + H 2O + 3H * = ClC6 H 4 N * +2 H 2O + 2 H *

(5)

ClC6 H 4 N * +2 H 2O + 2 H * = ClC6 H 4 NH * +2 H 2O + 1H *

(6)

ClC6 H 4 NH * +2 H 2O + 1H * = ClC6 H 4 NH 2 * +2 H 2O

(7)

The reactions for nitrobenzene and aniline formation are as follows: ClC6 H 4 NO2 + H 2 = C6 H 5 NO2 + HCl

(8)

ClC6 H 4 NH 2 + H 2 = C6 H 5 NH 2 + HCl

(9)

The dechlorination of p-CNB(p-CAN) (R10, R12) and its hydrogenation to nitrobenzene (aniline) (R11, R13) results in the formation of HCl from the dissociated chlorine and the adsorbed hydrogen (R14), as follows:


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ClC6 H 4 NO2 * = C6 H 5 NO2 * +Cl *

(10)

C6 H 4 NO2 * + H * = C6 H 5 NO2 *

(11)

ClC6 H 4 NH 2 * = C6 H 4 NH 2 * +Cl *

(12)

C6 H 4 NH 2 * + H * = C6 H 5 NH 2 *

(13)

Cl * + H * = HCl *

(14)

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These elementary reaction steps have been investigated on flat Pd(111) and stepped Pd(211) surfaces. The reaction energy diagrams of p-CAN and nitrobenzene (aniline) formation are summarized in Figure 3a. The optimized structures for the corresponding species and the transition states on Pd(111) and Pd(211) surfaces are shown in Figure S1-S2 and Figure 4a-4b. The reactions for the formation of p-CAN are highly exothermic on the Pd(111) and Pd(211) surfaces, as indicated by the -4.82 and -4.86 eV reaction energies, respectively. Only two elementary reactions (R2 and R5) are slightly endothermic on the two surfaces. Moreover, the thermodynamic properties (exothermic and endothermic) of each elementary reaction have the same signs, but the reaction energies strongly depend on the surfaces. In particular, the major differences between the flat Pd(111) and stepped Pd(211) surfaces are the the elementary reaction R6 and R7. On the Pd(111) surface, the formation of sub p-CAN is highly exothermic, whereas p-CAN formation is nearly neutral. On the Pd(211) surface, p-CAN formation is highly exothermic. These results can be attributed to the different adsorption geometries of the corresponding reactant and product in each elementary step on the two surfaces (Figure S1 and S2). The reaction barriers for H2O and HCl formation were not considered in this study. In these hydrogenation steps, the preferential adsorption sites of hydrogen and dissociated hydroxyl are the hollows or bridges. The reaction barriers of R2, R4, R6, and R7 on the two surfaces are nearly same (Figure 3a). For the hydroxyl dissociation steps, the favourable adsorption site for hydroxyl is the top or bridge site. The rate-determining step, which is the hydroxyl dissociation


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from phenylhydroxylamino, is nearly the same on the Pd(111) and Pd(211) surfaces. The reaction barriers for each elementary step, including the rate-determining step, on Pd(111) and Pd(211) are nearly the same.

Figure 3. Reaction energy diagram for (a) the hydrogenation of p-CNB to p-CAN on Pd(111) and Pd(211) surfaces (b) dechlorination of p-CNB (p-CAN) on Pd(111) and Pd(211) surfaces and Pd13 clusters.


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Figure 4. The transition states geometries of (a) the hydrogenation of p-CNB to p-CAN on Pd(111) (b) the hydrogenation of p-CNB to p-CAN on Pd(211) surfaces (c) dechlorination of pCNB (p-CAN) on Pd(111) and Pd(211) surfaces and Pd13 clusters. As previously reported,10 the rate-determining step in nitrobenzene (aniline) formation is pCNB (p-CAN) dechlorination (R10 or R12). The reaction barriers for hydrogenation (R2, R4, R6, and R7) step was nearly same. Therefore, we only focus on dechlorination step (R10, R12) for the formation of nitrobenzene and aniline. Figure 3b shows the reaction energy diagrams of p-CNB (p-CAN) dechlorination on the Pd(111) and Pd(211) surfaces as well as on the Pd13 clusters. The corresponding optimized species and transition states are shown in Figure S3 and Figure 4C. It shows that the reaction barriers for this dechlorination strongly depend on the surface. For both p-CNB and p-CAN, the reaction barrier for dechlorination on the Pd(111)


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surface is significantly greater than those on the stepped Pd(211) surface and Pd13 cluster. This result is consistent with those in the literature.41 These results indicate that p-CNB and p-CAN dechlorination is much easier on the edge sites than that on the terrace ones. In addition, p-CNB and p-CAN dechlorination may be more facile on small nanoparticles than on larger ones. However, the reaction barriers for p-CNB and p-CAN hydrogenation (a rate-determing step) are nearly the same on the different reaction sites.

Figure 5. TEM micrograph and Pd particles size distribution of Pd (2 wt%)/C catalysts pretreated with HNO3 or KI. a) 1 mol/L HNO3, b) 2 mol/L HNO3, c) 4 mol/L HNO3, d) 8 mol/L HNO3, e) 16 mol/L HNO3, f) 2.5 mol/L KI


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Figure 6. XRD pattern of the Pd(2%)/C catalyst pretreated by HNO3 or KI. a) 1 mol/L HNO3, b) 2 mol/L HNO3, c) 4 mol/L HNO3, d) 8 mol/L HNO3, e) 16 mol/L HNO3, f) 2.5 mol/L KI, H) treated by H2 at 1123 K, G) carbon after thermal treatment at 2173 K

Based on these theoretical findings, we prepared activated carbon-supported Pd particles with various sizes (from 2.1 to 30 nm) using the methods developed by our group.18 The TEM results for these particles are shown in Figure 5. As the concentration of HNO3, which was used in the pretreatment of activated carbon, increases, the average-sized, 99.90% selectivity. The industrial applications of the size-dependent selectivity of the nanocatalysts were then evaluated in several different pilot factories. We successfully controlled the reaction selectivity of nanocatalysts by regulating the nanoparticle size of the activated metal. Furthermore, this study provides useful information on controlling the selectivity of other similar reactions catalyzed by noble metal nanocatalysts.

Supporting Information The DFT optimized structures, the recycling results of Pd/C(28.4 nm) catalyst, the structural paramters and surface oxide gropus of the activated carbon with different pretreatments, and characterization of activated carbons preadsorbed with various halogen ions are provided. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *

Corresponding author (J.G. Wang):

Tel: +86-571-88871037; Fax: +86-571-88871037. E-mail address: [email protected] *

Corresponding author (X.N. Li):


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Tel: +86-571-88320002; Fax: +86-571-88320259. E-mail address: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from the National Basic Research Program of China (973 Program) (2011CB710800, 2013CB733501) and National Natural Science Foundation of China (NSFC-20976164, 21176211, 21136001 and 91334103).

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Size-Dependent Halogenated Nitrobenzene Hydrogenation Selectivity

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