Fe2O3 Nanohybrids for Catalytic

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Kinetics, Catalysis, and Reaction Engineering 2

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Poly-ionic Resin Supported Pd/FeO Nanohybrids for Catalytic Hydrodehalogenation: Improved and Versatile Remediation for Toxic Pollutants Kinkar Biswas, Shreyasi Chattopadhyay, Yunke Jing, Renchao Che, Goutam De, Basudeb Basu, and Dongyuan Zhao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04464 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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Industrial & Engineering Chemistry Research

Poly-ionic Resin Supported Pd/Fe2O3 Nanohybrids for Catalytic Hydrodehalogenation: Improved and Versatile Remediation for Toxic Pollutants Kinkar Biswas,† Shreyasi Chattopadhyay,‡ Yunke Jing,§ Renchao Che,§ Goutam De,*,‡‖⊥ Basudeb Basu,*†# and Dongyuan Zhao*§ † ‡

Department of Chemistry, North Bengal University, Darjeeling 734013, India

CSIR‒Central Glass & Ceramics Research Institute, 196, Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India

§

Department of Chemistry, State Key Laboratory of Molecular Engineering of Polymers and Advanced MaterialsLaboratory, Fudan University, Shanghai 200433, P. R. China ‖

Institute of Nano Science and Technology, Mohali 166062, Punjab, India #

Raiganj University, Raiganj 733134, India

ACS Paragon Plus Environment

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: A series of Pd/Fe2O3 nanohybrids with low metal content supported with Amberlite resin formate (Pd/Fe2O3@ARF) was prepared and characterized by FTIR, XRD, XPS, EELS, SEMEDAX and HRTEM. The coexistence of mainly crystalline Pd and Fe2O3 nanoparticles (NPs) of average size ~4‒5 nm in the resin matrix was confirmed. These nanohybrids were used for hydrodehalogenation of polyhaloaromatics using NaBH4 as a reducing agent in water. Notably, the composite Pd/Fe2O3@ARF‒110

exhibits excellent catalytic performance in

the

hydrodehalogenation of different haloaromatics. High TOF (in comparison to other related heterogeneous catalysts), recyclability and chemoselectivity between halide and C═C bond make this nanohybrid catalyst very attractive for the degradation of persistent organic pollutants originated from industries. The experimental observations and other analytical studies suggest that the enhanced catalytic activity could be due to strong interactions between Fe 2O3 and Pd NPs that facilitate the cleavage of B–H bond and subsequent hydride generation.

KEYWORDS: Amberlite resins; Hydrodehalogenation; Metal-metal oxide nanohybrid; Nanocatalysts; Polyhaloarenes.


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1. INTRODUCTION Halogenated aromatic compounds (HACs), particularly polyhalogenated and polyaromatic compounds (PPCs), have deleterious impacts on the environment and in living organisms. 1‒3 Polychlorinated dibenzofurans (PCDFs) or dibenzo‒p‒dioxins (PCDDs) are considered to be as worst contaminants that are emitted to the atmosphere as by-products of industrial processes and during combustion of municipal waste. They impart many harmful effects on human and environmental health.4‒5 Polychlorinated biphenyls (PCBs) are actually a mixture of congeners and have been used to manufacture different industrial products. Similarly, polybrominated biphenyls (PBBs) are also found as environmental contaminants showing cyto‒ and genotoxicity and long‒term health hazards.6,7 Owing to persistent toxicological effects, HACs in general and higher chlorinated PCBs pose serious threats to global ecosystem.8 Degradation of such HACs to environmentally friendly and commercially valuable products has been considered as fruitful strategy.9‒14 Microbial treatment of toxic haloaromatic compounds leads to dehalogenation via aerobic oxidation forming another class of toxic metabolites and higher chlorinated congeners are found to be difficult to degrade.9,10 Besides photocatalytic degradation,11 the metal‒catalyzed reductive hydrodehalogenation (HDH) of haloarenes through the cleavage of sp2C–halogen bond to form sp2C–H bond has been considered as an effective alternative for their removal. 12‒14 It is well known that the HDH of halogenated aromatic compounds occurs in the order of C–I > C–Br > C–Cl >> C–F, which is ascribed to the dissociation energy of the C–halogen bonds.14,15 Although different transition metal catalysts such as Pd, Ni, Pt, Au, Rh, Ru and Fe have been used in the HDH reaction, the success of the reaction is also dependent of the reducing sources.16‒22 Even bi‒ or trimetallic catalysts, either in alloys or in intermetallic forms, often show better performance than monometallic catalysts, 23‒26 Efforts towards developing bimetallic


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catalysts for HDH have therefore been made. Supported bimetallic catalysts involving two coinage metals (Ag/Au),27 and in particular Pd/Fe have been prepared and employed in HDH reaction.28‒32 Poly(vinylidene fluoride)‒alumina membrane‒supported Pd/Fe nanocatalyst was used in the dechlorination of only monochloroacetic acid,28 Pd/Fe NPs impregnated with granular activated charcoal,29 and Pd/Fe bimetallic nanotubes, composed of Fe nanotubes as core coated with shell of Pd NPs, were studied for hydrodechlorination of only polychlorobiphenyls, 30 or Pd/Fe bimetallic catalyst with micron sizes was employed specifically for the HDH of tetrabromo‒ or tetrachlorobisphenol A.31,32 It is evident that the applicability of these supported catalytic systems has been often found to be limited and presumably stems from the fact that these catalytic systems do not comply with actual shape or facets, spatial distribution and requisite proximity for interaction between two metals on the surface, among others, which could activate the reducing sources for further reactions. Moreover, in the case of Pd/Fe‒based catalytic systems, it has been found that Pd‒Fe electronic interactions and consequent synergism are important in hydrodeoxygenation catalysis. 33 Apart from supported Pd/Fe‒based catalytic systems, bare bimetallic Pd/Fe NPs (B-NPs) microemulsion was also employed as the catalyst in HDH using ammonium formate as the reducing source.26 It was observed that such catalyst gets easily deactivated significantly even after the first run due to the cumulative effects of (i) accumulation of NH4Cl on the surface of the NPs and (ii) Fe(II) ions produced in the course of reaction are unable to be reduced to the original Fe(0). To overcome this, Pt/Pd/Fe trimetallic alloy NPs (T-NPs) were prepared and much better catalytic efficiency was observed. Considering above, it remains a major challenge to develop suitably fabricated and stabilized Pd/Fe based nanocatalysts that could reside in close proximity into polymeric matrices for facile


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electron transfer so as to exhibit high catalytic efficiency and recyclability in wide varieties of HDH reactions. In hydrodehalogenation reactions, the common hydride sources are mixed metal hydrides, ammonium formate, silane derivatives and molecular hydrogen. Sodium borohydride (NaBH4) is recognized as a green, efficient, environment-friendly and safe reducing agent.34 The use of NaBH4 has some advantages as it forms hydrogen gas with water or with polar hydroxylic solvents at a faster rate on metal surfaces. The homogeneous metallic systems along with NaBH 4 reagent have shown profound effect on HDH reaction,35,36 and other sources of hydrogen with heterogeneous metals catalysts are known. 26,31,32,37-39 However, the combination of solidsupported metal catalysts with NaBH4 for HDH reaction is not known and this paper reports new polymer supported Pd and iron oxide nanocomposites for catalytic application in HDH with enhanced efficiency and recyclability. Macroporous polystyrene based amberlite resins with counter formate (COO¯) anions were supported with palladium and iron oxide NPs, designated as Pd/Fe 2O3@ARF. The amberlite resin formate (ARF) has been chosen owing to its poly–ionic microenvironment, stability, low cost and reusability from the perspectives of green chemistry and industrial application. It can be easily synthesized through ion exchange of commercially available amberlite resin chloride.40 In the present study, five different composites of Pd/Fe 2O3@ARF were prepared at varying temperatures and/or capping agents (oleic acid and sodium oleate). The Fe2O3@ARF (without Pd) was also prepared as a control. One of the catalytic systems prepared via co‒impregnation mode at 110 °C (denoted as Pd/Fe2O3@ARF–110) exhibited versatile catalytic efficiency with reusability in HDH of a broad range of haloaromatics using NaBH4‒H2O. Comparative analyses of the catalytic efficiency with other metal‒based or Pd/Fe‒based heterogeneous catalysts in


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respect of diversity, reaction conditions and turnover frequency (TOF) clearly establish its high performance over other systems. Moreover, it is easy to recover such type of composite catalysts after the reaction through simple filtration. Overall experimental observations and analyses by XPS, SEM-EDS, HRTEM and EELS have revealed that the co-existing palladium and iron oxide NPs in the porous polymeric surface makes facile electron transfer between them under the reaction conditions, which is likely to play the key role in making the catalytic system as a robust and versatile with improved activity and recyclability. 2. EXPERIMENTAL SECTION Amberlite IRA 900 (chloride form) resin is a macroreticular polystyrene type 1 strong base anion exchange resin containing quaternary ammonium groups. This allows complete removal of all anions, including weakly dissociated ions such as silica. The macroreticular structure combined with the strong basicity permits the removal of large size soluble organic molecules. Amberlite IRA 900 (chloride form) was purchased from Acros Organics, Belgium and used after washing with water and acetone followed by drying under vacuum. Other chemicals were purchased and used directly. The amberlite resin formate (ARF) was prepared from commercially available Amberlite IRA 900 (chloride form) by rinsing with 10% aqueous sodium formate solution until free from chloride ions.40 The resin beads were then washed with water followed by acetone, dried under vacuum, and used for the preparation of heterogeneous monometallic (with iron) and metal-metal oxide hybrid nanocomposites. 2.1 Preparation of Fe2O3@ARF–110 The ARF (500 mg) was added to a clear solution of FeCl 3 (162 mg, 1 mmol) in dry and distilled DMF (8 mL), stirred initially for 10 min. at room temperature, and then heated at 110 °C in a screw‒capped sealed tube for 8 h with occasional shaking. The grey colored resin turned


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red due to the impregnation of Fe in to the resin. The solid material was then filtered, washed successively with DMF (3 × 5 mL), DI water (2 × 5 mL) and acetone (2 × 5 mL). The red colored composite was dried under vacuum and characterized. 2.2 Preparation of Pd/Fe2O3@ARF–110 and other nanohybrids To a solution of PdCl2 (42 mg, 0.237 mmol) and FeCl3 (154 mg, 0.947 mmol) in dry and distilled DMF (8 mL), ARF (500 mg) was added, and the mixture was heated in a screw‒capped sealed tube at 110 °C for 8 h with occasional shaking (Figure 1). The supernatant liquid appeared almost colorless by this time and the ARF turned deep brown. The reaction mixture was cooled to room temperature and the solid composite was filtered off, washed successively with DMF (3 × 5 mL) DI water (2 × 5 mL) and then with acetone (2 × 5 mL). The resulting dark brown composite materials were dried under vacuum and used for analysis and catalytic reactions. The samples Pd/Fe2O3@ARF–80 and Pd/Fe2O3@ARF–140 were prepared similarly according to the above procedure at 80 and 140 oC, respectively (Figure 1). Other two nanohybrids (Pd/Fe2O3@ARF–110–OA and Pd/Fe2O3@ARF–110–NaOA) were also prepared following the same procedure in the presence of oleic acid (OA) (1 mmol) or sodium oleate (NaOA) (1 mmol) using ARF (500 mg), PdCl2 (42 mg, 0.237 mmol) and FeCl3 (154 mg, 0.947 mmol) in DMF (8 mL) at 110 oC for 8 h. After cooling to room temperature, the solid materials were filtered off, washed as above and designated as Pd/Fe2O3@ARF–110–OA and Pd/Fe2O3@ARF–110–NaOA (Figure 1), respectively.


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Figure 1. Digital images of (a) different steps during the preparation of Pd/Fe2O3@ARF‒110; (b) other nanohybrids prepared under various conditions as mentioned in the experimental section.

2.3. Characterization The FT‒IR analyses of the samples were carried out with Nicolet 380 FT‒IR spectrometer (resolution 4 cm–1) using KBr pellet method. NMR spectra were taken in CDCl3 using a Bruker Avance AV‒300 spectrometer operating for 1H at 300 MHz and for 13C at 75 MHz. The X‒ray diffraction (XRD) patterns were recorded on RigakuSmartLab (9 kW) diffractometer using CuKα radiation. X‒ray photoelectron spectroscopy (XPS) analysis was done with a PHI 5000 Versa probe II XPS system having a source of Al Kα and charge neutralizer at room temperature with a base pressure at 6 x 10−10 mbar. Scanning electron microscopy (SEM) images were recorded on a


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Philips S4800 microscope operated at 10 kV. Energy‒dispersive X‒ray scattering (EDS) measurements were done in JEOL JSM‒IT 100 scanning electron microscope operated at 2 kV. Transmission electron microscopy (TEM), EDS and high‒angle annular dark‒field scanning TEM (HAADF‒STEM) analyses were conducted on a JEOL JEM‒2100F and JEM‒2100 microscopes (Japan) operated at 200 kV. The samples for TEM measurements were suspended ultrasonically in ethanol and supported onto a carbon‒coated copper grid.

2.4. General procedure for hydrodehalogenation using NaBH4 and Pd/Fe2O3@ARF‒110 To a suspension of Pd/Fe2O3@ARF‒110 (50 mg) in THF:H2O (2:1, 2 mL), the haloarene (1.0 mmol), TMEDA (4 mmol), and NaBH4 (4 mmol) were added and the reaction mixture was heated with gentle magnetic stirring for hours as mentioned in Table 2. The progress of the reaction was monitored by thin layer chromatography (TLC). After completion of the reaction, the mixture was diluted with water (5 mL), and the catalyst was filtered off. The filtrate was extracted with dichloromethane (4 × 10 mL) and the combined organic extracts were washed with brine (1 × 5 mL), dried over anhydrous Na2SO4 and concentrated under vacuum. The residue was purified by passing through a short column of silica gel and eluted with light petroleum or a mixture of ethyl acetate–light petroleum to afford the desired hydrodehalogenated products. All products were characterized by 1H‒,

13

C‒NMR and FT‒IR spectral data, and also

compared with the reported melting points (for known solid compounds). The conversion to low molecular weight compounds like benzene, pyridine, aniline or phenol were checked by HPLC analysis and compared with authentic samples.


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2.5. Procedure for hydrodehalogenation using H2 gas and Pd/Fe2O3@ARF‒110 To a suspension of Pd/Fe2O3@ARF‒110 (50 mg) in THF:H2O (2:1, 2 mL), the 9,10dibromoanthracene (335 mg, 1 mmol) was added. The flask containing the reaction mixture fitted with a short condenser was evacuated and purged with H2 gas repeatedly for three times and then a balloon filled with H2 gas was fixed at the top. The reaction mixture was heated at 60 o

C with gentle magnetic stirring. After 8 hours, the reaction was cooled to room temperature, the

set up was opened and the mixture was diluted with water (5 mL). The catalyst was filtered off and washed with dichloromethane. The filtrate was extracted with dichloromethane (4 × 5 mL). The combined organic extracts were washed with brine (1 × 5 mL), dried over anhydrous Na2SO4 and concentrated. The TLC of the solution showed two spots corresponding to the starting dibromoanthracene and the reduced product, as also confirmed by co-spotting on TLC. The desired product anthracene was isolated by separation over silica gel column and eluting with light petroleum (74 mg, 41%).

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization The FT–IR spectra (Figure 2) show that the carboxylate anion (HCOO‾) of the ARF exhibits both symmetric and anti‒symmetric stretching vibrations at 1349 and 1590 cm −1, respectively, while those of Pd/Fe2O3@ARF‒110 display similar absorptions at 1420 and 1611 cm −1. Such significant shifting of the absorption bands (71–21 cm−1) could be attributed to the metal–oxygen attachment in the composites.40 Similar spectra and shifting of stretching vibrations of carboxylate anion (HCOO−) were also observed for other five nanocomposites Fe 2O3@ARF, Pd/Fe2O3@ARF‒80, Pd/Fe2O3@ARF‒140, Pd/Fe2O3@ARF‒OA and Pd/Fe2O3@ARF‒NaOA.


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The C‒N str. bands observed in ARF and other nanocomposites indicated that the poly-ionic resins did not decompose during the preparation of nanocomposites.

ARF

1349 1590 Fe2O @ARF 3

Pd/Fe2O @ARF-80 3

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1664 Pd/Fe2O @ARF-110 3

Pd/Fe2O @ARF-140 3

Pd/Fe2O @ARF-OA 3

Pd/Fe2O @ARF-NaOA 3

1630 1611

2000

1750

1572 1480 1500

1380 1420

1250

-1 Wavenumber (cm )

1000

Figure 2. FT‒IR spectra of ARF, Fe2O3@ARF and Pd/Fe2O3@ARF nanohybrids prepared under different conditions.

The XRD patterns of all six different nanohybrids (Figure 3) confirm the presence of mainly α‒Fe2O3, Pd and a trace amount of Fe3O4,33 along with amorphous characteristics of the organic polymeric resins showing a broad peak at 2θ of 20°.41 The XRD patterns of the sample


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Fe2O3@ARF‒110 showed diffraction peaks at 2θ of 25, 33, 35, 42, 50, 54, 63 and 65° corresponding to the (012), (104), (110), (113), (024), (116), (214) and (300) planes of α‒Fe2O3

 (220)  





 

3

10

4

20



Pd/Fe O @ARF-80   2 3





Pd (Cubic) Unidentified

30

40

  



 



Fe2O3@ARF





 

+ Fe O 2 3  Fe O





 

(200)

(111)



(214) (300) (125) (119) (217)



(116)



(024)

(012) 

(113)

(104) (110)

(JCPDS#01‒087‒1166), respectively.42

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Pd/Fe2O3@ARF-110   Pd/Fe2O3@ARF-140

Pd/Fe2O3@ARF-110-OA Pd/Fe2O3@ARF-110-NaOA

50

60

70

80

2 (degree) Figure3. XRD patterns of Fe2O3@ARF and Pd/Fe2O3@ARF nanohybrids prepared under different conditions.

Whereas in the case of nanohybrids of Pd/Fe2O3@ARF‒80, the diffraction peaks related to only cubic Pd were observed at 2θ values of 40 and 46° correspond to (111) and (200) planes (JCPDS#01‒087‒0643) but no peaks of α‒Fe2O3 could be detected. On the other hand, XRD patterns of both Pd/Fe2O3@ARF‒110 and Pd/Fe2O3@ARF‒140 exhibited co‒existence of α‒Fe2O3 and cubic Pd. A trace of Fe3O4 (JCPDS #01–075–0449) was also found for the sample Pd/Fe2O3@ARF‒110. Similarly, existence of α‒Fe2O3 along with cubic Pd were also observed for

both

the

composites

Pd/Fe2O3@ARF‒110‒OA

and


Pd/Fe2O3@ARF‒110‒NaOA.

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Furthermore, the sizes of the metal NPs in the composite nanohybrid materials (Pd/Fe2O3@ARF–80, Pd/Fe2O3@ARF–110, Pd/Fe2O3@ARF–140 and Pd/Fe2O3@ARF–110– OA) were evaluated using Scherer equation from the respective Pd (111) peak and found to be ~4–5 nm in all cases. However, no signature of any metallic Fe(0) NPs was observed in the XRD patterns of any of the studied metal-metal oxide nanohybrids.43,44 The XPS study of the Pd/Fe2O3@ARF–110 indicated the electronic environment of Fe and Pd as well as confirmed the formation of metal-metal oxide nanohybrid. The high‒resolution spectrum of Fe 2p (Figure 4a) showed two distinct peaks at ~710.9 for 2p3/2 and ~725 eV for 2p1/2 with a satellite peak in the range of 717–721 eV. Further, deconvolution of the spectrum gave rise to four peaks with binding energy values 710.5, 712.6 eV for Fe 2p3/2 and 724.93 for Fe 2p1/2 along with the satellite peak at 719.01 eV, confirming the presence of both Fe 2O3 and Fe3O4 in the composite material.45,46 The deconvoluted high resolution Pd 3d spectrum of the sample (Figure 4b) showed the presence of four distinct peaks. The higher binding energy values of 336.70 and 342.07 eV could be assigned to the 3d 5/2 and 3d3/2 of metallic Pd(0), respectively engaged in the interaction with ARF surface. 46,47 It is known that the presence of iron oxide (mainly Fe2O3) leads to the increase in electron density around Pd making it more electron reach.48,49 As a result, shifting of binding energy towards lower region can be attributed to the formation of metal-metal oxide nanohybrids Pd–Fe2O3.48,49 Thus, appearance of other set of two peaks at 335.26 (Pd 3d5/2) and 340.57 eV (Pd 3d3/2) confirmed Pd metal–Fe2O3 oxide nanohybrids formation within the Pd/Fe2O3@ARF‒110. Therefore, thorough XPS analysis assured strong interaction between iron oxide and Pd as well as the facile electron transfer from Fe2O3 to Pd in the hybrid catalyst, which is the key factor for enhanced catalytic activity of Pd/Fe2O3@ARF‒110.48


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(a)

Raw data Fitted curve Fe 2p3/2

Intensity

Fe 2p1/2

710

715

720

725

730

Binding energy (eV) Raw profile Bimetallic Pd(0) 3d5/2

(b)

Free Pd(0) 3d5/2 Bimetallic Pd(0) 3d3/2 Free Pd(0) 3d3/2

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Fitted profile

342

340 338

336

334

Binding energy (eV)

Figure 4. High resolution XPS spectra of (a) Fe 2p and (b) Pd 3d obtained from Pd/Fe2O3@ARF–110.

The surface morphology of Pd/Fe2O3@ARF nanohybrids prepared at different temperatures was examined by scanning electron microscopy (SEM) and compared with that of pure ARF and Fe2O3@ARF (prepared separately under similar conditions and using only FeCl 3). Distinct variations of the surface morphologies could be observed from the SEM images (Figure 5 and S1 in SI) because of the deposition of metal/metal oxide NPs on the surface of poly‒ionic resinous materials. Several dots could be seen in the images of the composites at same magnification,


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which might be due to the deposition of metal NPs on the resin surface. Further, Pd/Fe atomic ratio of the fresh Pd/Fe2O3@ARF–110 catalyst was estimated by the SEM-EDS technique. For this purpose, the average values of Pd and Fe data acquired from three different regions are used. The representative EDS spectrum of the catalyst showed the estimated average Pd/Fe atomic ratio as ~1:1 (Figure S2a and Table S1 of the Supporting Information).

Figure 5. SEM images of (a) ARF and (b) Pd/Fe2O3@ARF–110 nanohybrid catalyst. Further, TEM analysis (Figure 6) and (Figure S3 of the Supporting Information) of different nanohybrid samples showed the presence of Pd metal/Fe oxide NPs immobilized in the amberlite resin polymeric matrices. In the case of Fe2O3@ARF–110, the presence of α‒Fe2O3 NPs with maximum population at 3.13 to 4.17 nm (Figure S3a of the Supporting Information) was observed. For the sample Pd/Fe2O3@ARF–110–OA most of the NPs were found at 4.78 nm (inset Figure S3b of the Supporting Information). TEM of Pd/Fe2O3@ARF–110 was studied in detail and presented in Figure 6. TEM clearly (Figure 6a,c) revealed the presence of uniformly dispersed NPs populating mostly in between 4.28 to 5.0 nm in the Pd/Fe 2O3@ARF–110 nanohybrid. The EDS analysis (Figure 6b) of Pd/Fe2O3@ARF‒110 showed the presence of elemental Pd and Fe with a molar ratio of ~1:1, which also corroborates the


15


(a)

Fe2O3 (110)

Distribution (%)

25

4.28 nm 5.0 nm

20 15 10 5 0

2

3

4

5

6

7

8

d= 0.29 nm Fe3O4 (220) d=0.25 nm

Fe2O3 (104)

Pd (111)

(c)

20 nm

Cu

Fe2O3 (012)

9

Average diameter (nm)

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Fe2O3 (104) d=0.27 nm

Fe

(b)

Fe2O3 (104)

Pd Fe

Pd 1

2

3

4

5

6

Pd (111) d=0.22 nm

7

Energy (keV)

Fe

Pd

(d)

(e)

(f)

Figure 6. TEM analysis of Pd/Fe2O3@ARF‒110 nanohybrid: (a) Low‒resolution TEM image with particle size distribution of NPs (in the inset); (b) EDS analysis; Cu peak is from the grid; (c) HRTEM showing the lattice fringes of Pd and Fe-oxide NPs. Inset shows the first Fourier diffractogram (FFT) of the area highlighted with dotted lines. The existence of Fe and Pd in close proximity can be understood from HRTEM analysis; (d‒f) STEM image and EDS elemental mapping of (e) Pd, and (f) Fe in the composite.

SEM-EDS data (Table S1 of the Supporting Information). The peaks for Cu are from the carbon coated Cu grid used for the analysis. HRTEM analysis of Pd/Fe 2O3@ARF–110 (Figure 6c) was further performed to confirm the co‒existence of Pd and Fe 2O3 NPs in a close proximity. The


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lattice fringes measured from the HRTEM image and the corresponding Fourier diffractogram (FFT) (Figure 6c and inset) clearly revealed the existence of crystalline Pd and iron oxide NPs in the close vicinity within the ARF resin matrix. Moreover, all the crystal planes of Pd, Fe 2O3 and Fe3O4 observed in HRTEM and FFT analysis fully corroborated the XRD analysis of Pd/Fe2O3@ARF–110 nanohybrid. Furthermore, elemental mapping of Pd/Fe2O3@ARF–110 shown in Figure 6(d‒f) confirmed homogenous distribution of Pd and iron oxide in the polymeric resin matrix. For the purpose of qualitative analysis of the NPs and to explore the electronic interaction between them, the electron energy‒loss spectroscopy (EELS) has been performed ascomplementary to XPS analysis by focusing the electron beam on the NPs. The energy loss spectrum without background correction is shown in SI, Figure S4. The background‒corrected spectrum (Figure 7) showed peaks at ~334, 532 and 706 eV, which are the characteristic fine structures originated from Pd‒M4,5, O‒K and Fe‒L2,3 orbitals, respectively. It is noteworthy that the energy loss value of Pd (~334 eV) supports the interaction with Fe, and such lowering in binding energy value was also observed in XPS analysis for metal-metal oxide hybrid Pd (335.26 eV) in Pd‒Fe2O3 (Fig. 4b). Moreover, the shifting of energy loss value of iron (oxide), 50,51 and the observed fine structures of O‒K edge and Fe‒L2,3 also reveal a clear charge/electron coupling between Pd and Fe (Figure 7).


17


FeL2,3

O-K

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 38

PdM4,5

300

400

500

600

700

800

900

1000

Energy loss (eV)

Figure 7. Background‒corrected EELS spectrum of Pd/Fe 2O3@ARF–110, revealing electron transfer from Fe2O3 to Pd NPs.

3.2 Catalytic activity The catalytic activity of Pd/Fe2O3@ARF‒110 was evaluated in the hydrodehalogenation reaction of various haloaromatic compounds. Initially, as a model, we studied the catalytic activity and reaction conditions in the hydrodebromination of 9,10‒dibromoanthracene using NaBH4 in THF. The results are presented in Table 1. At room temperature, the reaction did not proceed smoothly even after 16 hours (entry 1). Further increase in temperature showed minor increase in yield (entry 2). However, carrying out the reaction in a mixture of solvents [THF:H2O; (2:1)] resulted in the course of the reaction profoundly, and the hydrodebrominated product was obtained in excellent yield (96%) (entry 3). While decreasing the quantity of the catalyst from 100 to 50 mg mmol‒1 gave rise to similar conversion (entry 4), and further lowering of the catalyst to 25 mg mmol ‒1 could not afford good conversion (entry 5). The reaction did not proceed in the absence of NaBH4 and the catalyst, keeping other conditions unchanged (entries 7 & 8). Further studies with Pd@ARF40 and Fe2O3@ARF‒110 as the


18

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catalysts also afforded hydrodehalogenation product in relatively lower yields (entries 9 & 10). We also carried out hydrodebromination of 9,10‒dibromoanthracene using molecular H2 instead of NaBH4, and in the absence of TMEDA (entry 11). We however obtained the desired product in relatively poor yield (41%) even after continuing the reaction for longer period. Among different conditions attempted, the best conversion was achieved in the presence of the catalyst (50 mg mmol‒1) and NaBH4 using an external ligand [N,N,N',N'‒tetramethylethylenediamine; (TMEDA)] in a solvent system THF:H2O (2:1) at 70 °C under aerobic condition (entry 4).

Table 1. Optimization 9,10‒dibromoanthracene.

of

reaction

conditions

for

the

hydrodebromination

of

Br Pd/Fe2O3@ARF-110, NaBH4 Base, Solvent, Temp. Br

Entry

Solvent

Catalyst (mg)

Temperature (oC)

Time (h)

Yield(%)a

1

THF

100

RT

16

10

2

THF

100

70

16

25

3

THF:H2O (2:1)

100

70

4

96

4

THF:H2O (2:1)

50

70

5

96

5

THF:H2O (2:1)

25

70

10

60

6b

THF:H2O (2:1)

50

70

7

80

7c

THF:H2O (2:1)

50

70

22

Nil

8d

THF:H2O (2:1)

00

70

16

Nil

9e

THF:H2O (2:1)

50

70

5

83

10f

THF:H2O (2:1)

50

70

5

42

11g

THF:H2O (2:1)

50

60

8

41


19


Page 20 of 38

Reaction conditions. 9,10‒dibromoanthracene (1 mmol), NaBH4 (4 mmol), TMEDA (4 mmol), THF:H2O (2:1, 2 mL), Pd/Fe2O3@ARF‒110 catalyst (50 mg). aIsolated yield after purification by column chromatography over silica. bEt3N used as a base. cNo NaBH4 was added. dNo catalyst was used. ePd@ARF,40 was used as the catalyst. fFe2O3@ARF‒110 was used as the catalyst. gH2 gas was used instead of NaBH4 and in the absence of TMEDA.

With the optimized condition (Table 1, entry 4), we attempted similar HDH of a range of mono‒ and poly‒substituted haloarenes. In each case, nearly complete conversion was achieved (Table 2). Different aromatic halides (‒Cl, ‒Br and ‒I) were smoothly reduced under the catalytic

conditions.

In

the

case

of

poly‒substituted

bromoaromatics

such

as

2,4,6‒tribromophenol or tetrabromobisphenol A (TBBPA), the reaction, however, took a longer reaction time (9‒12 hours) (Table 2, entries 4 and 5). Mixed aromatic halide like 3‒bromochlorobenzene also underwent easy reaction with complete conversion (entry 9). However, the sp2C‒F bond could not be removed with this catalyst under the condition (entry 11), possibly due to much higher bond dissociation energy. Another notable feature of the catalyst is its chemoselectivity that was observed in the reaction of haloarene bearing unsaturated C═C bonds. Actually, conjugated C═C bonds are easily reducible under the conditions but an example of n-butyl p-bromocinnamate, bearing both double bond and bromide group, was subjected to HDH (entry 12). Interestingly, we found complete hydrodebromination keeping the conjugated C═C bond unchanged, which qualifies for the chemoselectivity (entry 12).


20

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Table 2. Hydrodehalogenation of haloarenes in the presence of the nanocomposite catalyst Pd/Fe2O3@ARF‒110.a Entry Haloarene 1

Time (h) Product

Br

NHCOCH3

2

Br

4.5

Yield (%) 95

NHCOCH3

5

96

5

97

Br

3

Br

Br

4

9

Br Br

96

OH

OH Br

5

Br OH Br

6

8

CH3 C CH3

OH

OH

Br

N

Cl

OH

90 N

NH2

Cl

90

Br

4.5 Br

7

12

Br CH3 C CH3

OH

5 5

NH2

OH

96 91

Cl

9 Br

10

5

96

4

94

Cl

I


21


11b 12

F

Br

12

NH2

O

NR

5

Bun

Page 22 of 38

NR

O

87

Bun

a

Haloarene (1 mmol), NaBH4 (4 mmol), TMEDA (4 mmol), THF:H2O (2:1, 2 mL), Pd/Fe@ARF‒110 (50 mg), heating the reaction mixture at 70 oC under aerobic condition. bNot detected in HPLC analysis. (NR = not reacted)

To check the recycling potentiality, we isolated the catalyst (Pd/Fe2O3@ARF‒110) by simple filtration, followed by washing with water, acetone and then drying under vacuum. The recovered catalyst was also used for the hydrodebromination of 9,10‒dibromoanthracene. The catalyst was found to be efficient for consecutive five runs tested without any significant drop in the conversion (Figure 8). To understand the stability of the embedded NPs, the Pd/Fe atomic ratios of the recovered catalysts (after first and second runs) were checked by the SEM-EDS analysis. It has been found that while the fresh sample has Pd/Fe ~1:1, the recovered catalysts (after first and second runs) showed slightly lower content of Fe (Table S1, Supporting Information). We assume that there could be slight leaching or loss of iron species from the nanohybrid catalysts, though it does not affect the catalytic performance giving nearly similar yields of the dehydrohalogenated product in subsequent runs (Figure 8). Moreover, TEM bright field image of the recovered catalyst after the first run shows the existence of well dispersed NPs of maximum population from 4.0–6.1 nm revealing no significant agglomeration of NPs after the catalysis (Figure S5 of the Supporting Information).


22

Page 23 of 38

100

96

94

92

93

3

4

91

80

Isolated yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40

20

0 1

2

5

No. of runs

Figure 8. Recycling experiments using Pd/Fe2O3@ARF‒110 catalyst in hydrodebromination of 9,10‒dibromoanthracene.

3.3 Comparative performance A comparison of the catalytic performance of the present system (Pd/Fe2O3@ARF‒110) with related heterogeneous catalysts employed in HDH reactions was made, which is presented in Table 3. It can be seen that other catalytic systems used for such HDH suffer from various disadvantages such as long reaction time, higher temperature, the need for using strong bases, phosphine‒based ligands and most of the reported catalysts are non‒recyclable. The chemoselective catalytic efficiency is an added feature for the present catalytic system, which has not been reported so far. It is noteworthy to mention that previous studies involving Pd/Fe‒based catalysts in HDH reaction were not conducted using aqueous NaBH4 as the hydrogen source. In the case of using monometallic Pd or Ni complexes,35,52‒54 such reactions were carried out in the presence of aqueous NaBH4 and other reducing sources. These monometallic complexes however worked as homogeneous catalysts and were not recoverable and reusable. Bare Pd/Fe or Pt/Pd/Fe NPs were used as catalysts in HDH using ammonium formate as the reducing source but with limited


23


Page 24 of 38

success because of deactivation (Pd/Fe NPs) or regeneration (Pt/Pd/Fe NPs) problems (Table 3, entry 5).26 On the other hand, in comparison to the reported TOF values for HDH using supported Pd/Fe‒based catalysts,28‒32,55 our catalytic system showed significantly high TOF value (3.24 h‒1) (Table S2 of the Supporting Information).

Table 3. Comparison of various metal-based catalytic hydrodehalogenation with the present catalyst (Pd/Fe2O3@ARF‒110) highlighting its advantages. Entry Catalytic system

Applicability

Remarks

Reference

1

Ni-precatalyst (5 mol%), 1-benzoyl-5-hydroxypyrazoline and PPh3 ligands; iPrZnBr (1.5 equiv.); THF, 70 oC, 24 h.

With aryl iodide and Requires long bromide; aryl reaction time; chlorides gave poor catalyst is not yields recyclable; side products are formed.

52

2

Pd2(dba)3 (2.5 mol%), phosphite ligand, base NaOBut; 80-120 oC; 310 h

With bromo- and chloroarenes, aryl bromides required high temperature (120 oC) and longer reaction time (10 h).

Strong base is required (NaOBut and Cs2CO3); Phosphine based ligand; Not recyclable.

53

3

Pd(OAc)2 (1 mol%), Tested for chloro-, PPh3 (4 mol%), K2CO3 bromo- and iodo(2 equiv.), 12 h under arenes. N2.

Inorganic base and phosphine ligand are used and not recyclable.

54

4

1,1’-bis(diphenylTested only for phosphino) ferrocene hydrodebromination complexes of Pd; of bromobiphenyl. NaBH4 (reductant), TMEDA (base), room temperature.

Phosphine based ligand was used; NaBH4 and TMEDA were used in excess; catalytic system was not recyclable.

35

5

Pt/Pd/Fe-TNP water in Tested for chloro-, oil microemulsion, and bromo- and iodoPd/Fe-BNP system; arenes. NH4OOCH, iPrOH, 22

Relatively mild conditions; Pt/Pd/Fe-TNP can be effective for

26


24

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o

C

6

Pd/Fe2O3@ARF‒110 reductant NaBH4; 70 oC, 5 h

sixth run but for the Pd/Fe-BNP system, significant loss of catalytic activity was seen after 1st run. Chloro-, bromo- and iodoarenes and other polyhalo-arenes are reacted smoothly under mild condition with high conversion.

New type of heterogeneous Pd/Fe2O3-based nanohybrids. Recycling was examined for four times without any loss of efficiency.

Present catalyst

3.4 Plausible mechanism Literature reports towards the mechanism of the hydrodehalogenation reaction with bimetallic catalysts reveal that the reaction goes through the reductive catalytic pathway where one metal (particularly iron or zinc) generates hydrogen through corrosion with water and the second metal is used as a dopant to form metal hydride. Nano‒sized bimetallic particles with zero‒valent iron, such as Ni/Fe or Pd/Fe with a largesurface area has profound effect on the degradation of haloaromatics due to the increase of the availability of surface reaction sites. 56 On the other hand, it is well known that the reaction of NaBH4 with water liberates hydrogen slowly at room temperature, and can be accelerated with the aid of metal NPs coated on metal oxides or on carbon.57,58 For example, Pt NP coated on LiCoO2, Pt/C or Pd/C, due to their (Pt or Pd) high d‒band centre, was found to be excellent catalysts to generate hydrogen from aqueous NaBH 4 solution.57,58 Here, NaBH4 seems to act as the reducing source in combination with TMEDA and water in the presence of our catalyst (Table 1, entries 2, 3 and 7). Water has also an important role as it was observed experimentally that its presence dramatically increases the conversion to


25


Page 26 of 38

the desired product. However, the catalytic system consists of Pd/Fe(III), where iron oxide cannot singly produce hydrogen from the reaction of NaBH4 and water effectively.58 It is known that combination of metal-metal oxide could facilitate the reaction of NaBH4 with water.57 In our catalytic system, we presume that the presence of Fe 2O3 and trace quantity of Fe3O4 could generate electron by redox process, as outlined below: Fe2+ + 2e‒ → Fe0

(E0 = ‒0.44)

……….

eqn. 1

Fe3+ + 3e‒ → Fe0

(E0 = ‒0.04)

……….

eqn. 2

Fe2+ → Fe3+ + e‒

(combining two equations)

(E0 stands for standard reduction potential) This presumably increases the electron density on the Pd surface that is responsible for making faster B‒H bond‒breaking of BH4‒ ion and easy hydride generation and quicker formation of hydrogen in the presence of water subsequently, which eventually results in the enhancement of the overall catalytic performance in HDH reaction. All these phenomena are possible due to the co-existence of Pd and iron oxide NPs in close proximity, which was further corroborated by HRTEM and EDS mapping analysis (Figure 6). The combining effect of TMEDA and water in enhancing hydrogen production is reported previously,35 and believed to play an important role in this catalytic process as well. However, such accelerating effect is not observed in the case of using H2 gas directly, though NaBH4 in contact with water gives slowly hydrogen gas and sodium metaborate. We believe that the B‒H bond‒breaking of BH4‒ ion is significantly facilitated by the metal nanohybrid based catalytic system resulting an active reductant that works more efficiently than molecular H2. The proposed catalytic cycle is presented in scheme 1.


26

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H O

Fe2O3 Pd

O n

N(CH3)3

Pd- Fe2O3 @ARF-110

m

NaBH4 + 2H2O

Ref. 57 H O Pd- Fe2O3 @ARF-110 O H H NaBO2 H

Pd- Fe2O3 @ARF-110

Ar H

Reductive elimination Ar H

H H B H H

Ar X

Oxidative addition Ar

Pd- Fe2O3 @ARF-110

X

4H2 Pd- Fe2O3 @ARF-110 Pd- Fe2O3 @ARF-110

Ref. 35

Hydride transfer Hydride formation TMEDA+NaBH4

Hydrogen transfer

Hydrogen addition

Ref. 26

H

Pd- Fe2O3 @ARF-110 H Active metal hydride complex

Scheme 1. Plausible mechanism for the catalytic activity in HDH reaction using NaBH4 in water. 4. CONCLUSION In summary, we have demonstrated that the close proximity of Pd and Fe2O3 NPs supported on amberlite resin formate (ARF) could exhibit enhanced and versatile catalytic activity in the hydrodehalogenation of haloaromatic compounds in the presence of NaBH4 in water. While remediation of persistent organic pollutants (POPs), particularly polyhaloarenes remains a problem, the present catalytic system shows promising results and could be attractive towards environment and industry.The basic novelty of this catalytic system involves in the facile electron transfer between iron oxide and palladium nanoparticles, co-existed in the porous polymeric surface that facilitates faster B‒H bond‒breaking from the BH4‒ ion and active H formation resulting in exhibiting excellent catalytic activity. Such effect is however not noticed


27


Page 28 of 38

when the reaction was carried out using H2 gas instead of NaBH4. Furthermore, the heterogeneous catalyst is easily separable and recyclable with nearly equal efficiency tested for five consecutive runs. The newly developed catalytic system is found to be significantly better as compared to other similar catalysts used for this reaction in terms of easy method of preparation, cheap starting materials, no precious ancillary ligands, low loading of metals, mild reaction conditions, as well as applicable to various aromatic halides, chemoselectivity, and relatively high turnover frequency. 5. ASSOCIATED CONTENT Supporting Information SEM images of all composite nanohybrid catalysts (Figure S1), SEM-EDS spectra of fresh and recovered catalysts (Figure S2), TEM images of other composite nanohybid catalysts (Figure S3), EELS spectrum of Pd/Fe2O3@ARF–110 (Figure S4), Pd/Fe atomic ratios estimated by SEM-EDS (Table S1), Comparison of turnover frequency of various catalytic systems tested in the hydrodehalogenation of haloarenes (Table S2). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected], [email protected] ⊥Superannuated

from CSIR-Central Glass and Ceramic Research institute on January 31, 2018

(G.D.). ORCID Goutam De: 0000-0003-0271-1634 Basudeb Basu: 0000-0002-7993-2964 Dongyuan Zhao: 0000-0001-8440-6902


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Financial support from SERB, New Delhi, India is gratefully acknowledged [Grant No. EMR/2015/000549]. KB and SC thank UGC, New Delhi for fellowship under FDP and NET‒JRF, respectively. REFERENCES (1) Sun; J.‒L.; Zeng, H.; Ni, H.‒G. Halogenated Polycyclic Aromatic Hydrocarbons in the Environment. Chemosphere 2013, 90, 1751–1759. (2) Ohura, T. Environmental Behavior, Sources, and Effects of Chlorinated Polycyclic Aromatic Hydrocarbons. Scientific World J. 2007, 7, 372–380. (3) Safe, S. H. Chlorocarbons and Chlorohydrocarbons, Toxic Aromatics. Kirk‒Othmer Encyclopedia of Chemical Technology, Wiley: 2003. (4) Zhang, Q.; Gao, L.; Zheng, M.; Liu, L.; Li, C. Polychlorinated Dibenzo-p-dioxins (PCDDs) and Dibenzofurans (PCDFs) and Polychlorinated Biphenyls (PCBs) in Water Samples from the Middle Reaches of the Yangtze River, China. Bull. Environ. Contam. Toxicol. 2014, 92, 585– 589.


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Huang,

Q.;

Liu,

W.; Peng,

P.;

Huang,

W. Reductive Debromination of

Tetrabromobisphenol A by Pd/Fe Bimetallic Catalysts. Chemosphere 2013, 92, 1321‒1327. (32)

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