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Biomass-Derived Activated Carbon Supported Fe3O4 Nanoparticles as Recyclable Catalysts for Reduction of Nitroarenes Pitchaimani Veerakumar, Irulandi Panneer Muthuselvam, ChinTe Hung, King Chuen Lin, Shang-Bin Liu, and Fang-Cheng Chou ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01727 • Publication Date (Web): 03 Oct 2016 Downloaded from http://pubs.acs.org on October 5, 2016

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ACS Sustainable Chemistry & Engineering

Biomass-Derived Activated Carbon Supported Fe3O4 Nanoparticles as Recyclable Catalysts for Reduction of Nitroarenes Pitchaimani Veerakumar,†,§,* Irulandi Panneer Muthuselvam,‡ Chin-Te Hung,† King-Chuen Lin,§ Fang-Cheng Chou,‡ and Shang-Bin Liu,†, ⊥,* †

Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan §

‡

Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan

Center for Condensed Matter Sciences and Department of Physics, National Taiwan University, Taipei 10617, Taiwan

⊥

Department of Chemistry, National Taiwan Normal University Taipei 11677, Taiwan

* E-Mails: [email protected] (P. Veerakumar); [email protected] (S. B. Liu)

ABSTARCT: Highly

porous beetroot-derived

activated carbons incorporated with

well-dispered magnetite nanoparticles (Fe3O4 NPs; average size ca. 3.8 ± 0.5 nm) were fabricated via a microwave-assisted synthesis route. The magnetic Fe3O4@BRAC catalysts so fabricated were characterized by a variety of diffent physicochemical teniques, viz. XRD, FE-TEM, VSM, gas physisorption/chemisorption, TGA, XPS, Raman, ICP-AES, and FT-IR spectroscopy. The as-prepared catalysts were exploited for heterogenous-phase reduction of a series of nitroaromatics (RNO2; R = H, OH, NH2, CH3, and COOH) under KOH as a base, isopropanol acting as a hydrogen donar as well as solvent and also tested with other solvents. The reaction system not only exhibit excellent activity with high anilines yield but also represents a green and durable catalytic process, which facilitates facile operation, easy separation, and catalyst recycle.

KEYWORDS: Porous carbon, Fe3O4, Microwave irradaition, Heterogeneous catalysis, Nitroaromatics

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INTRODUCTION Metal incorporated mesoporous carbon-based catalysts have garnered significant attention, because of their broad applications in sensors, catalysis, and energy-related applications.1−10 Recently, biomass-derived carbon porous materials (CPMs) have attracted considerable research interest due to their unique physicochemical properties as well as cost effectiveness.11,12 As such, metal nanoparticles (MNPs) incorporated CPMs have been considered as efficient composite catalysts to replace conventional homogeneous catalysts, which are normally hampered by separation and recyclability, hence, persecute operation process and cost. On the other hand, MNP@CPM composite catalysts also suffer from severe problems, such as aggregation and/or leaching of MNPs. The latter may be prevented by effective dispersion of MNPs on the supports.13 In this context, CPMs with high surface areas and enriched surface functionalities are desirable candidates as catalyst supports.14,15 It is worth pointing out that, nitroarenes reduction has been recognized as a trusted model reaction for evaluating the catalytic activities of nanoparticle (NP) catalysts.16 Aromatic amines are key intermediates for the productions of important chemicals in dyes, pharmaceuticals, and agricultural industries.17 Conversion of anilines are mostly rely on homogenous phase catalytic reduction of nitro compounds, mostly in the presence of a solvent and/or an activation agent.18−23 Among various catalyst studied, carbon supported iron oxide (i.e., magnetite; Fe3O4) NPs have attracted considerable attentions due to: (i) ease of separation, (ii) high catalytic activity, (iii) good solvent resistance, (iv) thermostability, and (v) facile magnetic recovery and recyclability.24−28 Such types of nanocomposites have long been used for organic transformation reactions.29−32 2 ACS Paragon Plus Environment

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In terms of CPMs, activated carbons derived from biomass feedstocks, which are readily available in nature, are not only eco-friendly and cost-effective but also possess high porosity, good electrical conductivity, and oxygen surface functional groups and heteroatoms desirable as catalyst supports for applications in sensors,15,33−37 catalysis,38,39 and energy storage devices.36,40,41 The synthesis of biomass-derived CPMs normally invokes a chemical activation process during which activating agents, such as ZnCl2, KOH, NaOH, or H3PO4, are introduced along with the carbon precursor.42,43 Upon completing the subsequent carbonization treatment, the substrate was then washed with concentrated HCl to remove undesirable impurities.44 Beetroot red (or betanin), a natural antioxidant whose color depends on pH, is usually obtained from the extract of beet juice and commonly exploited as pigment in food industry.45,46 It is known that betanin contains sugar and indole moieties47 and its aglycone, betanidin, may be obtained by removing the glucose molecule by hydrolyzation.48 The abundance of sugar moieties and nitrogen content in the beetroots juice makes it a useful raw material for the fabrication of CPMs. The beetroot-derived activated carbons (hereafter denoted as BRACs) so prepared, for the first time, are used as supports for magnetite (Fe3O4) NPs and the catalytic activity of such Fe3O4@BRACs composite catalysts were assessed by the reduction of nitroarene. The BRAC supports exploited herein were prepared from a low-cost bio-waste carbon precursor, namely beetroot (Beta vulgaris subsp. vulgaris var. conditiva) juice, which are abundant in nature. It will be shown later that the renewable BRACs possess both micro- and meso-porosities with high surface areas desirable for the dispersion of MNPs. The as prepared porous BRACs were chemically activated using the ZnCl2 salt at different carbonization temperatures. Moreover, the fabricated Fe3O4@BRAC catalysts were characterized by several analytical and spectroscopic techniques, and were found to exhibit excellent catalytic activity for the reduction of nitro aromatics using isopropanol or 2-propanol as the sole hydrogen source as well as the reaction medium during the additive-free reduction reaction, as illustrated in Scheme 1. 3 ACS Paragon Plus Environment


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EXPERIMENTAL SECTION Materials and Reagents. Iron(III) acetylacetonate, Fe(acac)3 (98%), isopropanol (98%) were purchased from Acros, whereas zinc chloride (ZnCl2) and potassium hydroxide (KOH) were obtained from Sigma-Aldrich. Fresh beetroots (Beta vulgaris subsp. vulgaris var. conditiva), also known as red beet, were obtained from a local market. All solutions were prepared using double distilled water and all chemicals were used as received.

Scheme 1. Schematic Illustration for the Preparation Route of Magnetic Fe3O4@BRAC Catalysts.

Preparation of Fe3O4@BRAC Catalysts. The beetroot activated carbons (BRACs) were prepared by chemical activation method using ZnCl2 as the activating agent and beetroot juice 4 ACS Paragon Plus Environment

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as the carbon source (see Scheme 1). In brief, clean beetroots (0.5 kg) were peeled, sliced and homogenised in a centrifugal juice extractor (5000 rpm), filtered, and evaporate to concentrate at low temperature. The obtained dark-red colour beet colloidal resins were mixed with ZnCl2 (2.0 g). Subsequently, the betanin-containing substrate was dried in an oven at 100 oC for 24 h, then, grinded evenly to form dispersed mixture. The mixture was placed in porcelain boat, followed by a graphitization treatment in a tube furnace under N2 atmosphere. The graphitization treatment was carried out with a ramping temperature of 5 oC min−1 till reaching a maximum temperature (600−900 oC), then, maintained at the temperature for 2 h. The carbon substrate was allowed to cool slowly, then, washed thoroughly with 1 M HCl and deionized water till a neutral pH value was reached. The resulting carbon powders so prepared were denoted as BRAC-x, where x represents the final carbonization temperature in oC. For the preparation of Fe3O4@BRACs composites, Fe3O4 NPs were immobilized on the BRAC-900 support by direct thermal reduction of Fe(acac)3 under N2 atmosphere. In brief, the as-synthesized BRAC-900 powder (0.5 g) and Fe(acac)3 (20 mg) were dispersed in an ethanol solution (10 mL) under ultrasonic vibration for at least 1 h, followed by irradiation by microwave heating (Milestone’s START MW power: 300 W) at 100 °C for 2 h. The sample was then heated in a furnace at 900 °C for 3 h under inert gas atmosphere. Finally, the resultant magnetic nanocomposite catalyst, denoted as Fe3O4@BRAC. Catalyst Characterization. All powdered X-ray diffraction (XRD) experiments were recorded on a PANalytical (X’Pert PRO) diffractometer using Cu Kα radiation (λ = 0.1541 nm). Raman spectra were recorded on a Jobin Yvon T64000 spectrometer equipped with a charge coupled device (CCD) detector cooled by liquid nitrogen. The backscattering signal was collected with a microscope using an Ar+ laser (centered at 488 nm) as the excitation source. The morphology of the sample was studied by field emission transmission electron microscopy (FE-TEM; JEOL JEM-2100F) operating with 200 kV at room temperature (25 °C). Elemental composition of various samples were analysed with an energy-dispersive X-ray (EDX) analyser, 5 ACS Paragon Plus Environment


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which is an accessory of the FE-TEM apparatus. Nitrogen porosimetry measurements were carried out with a Quantachrome Autosorb-1 volumetric adsorption analyzer at −196 °C (77 K). Prior to each measurement, the sample was purged with flowing N2 gas at 150 °C for at least 12 h. The pore size distributions of various samples were derived from density functional theory (DFT) calculations. X-ray photoelectron spectroscopy (XPS) measurements were performed using an Ulvac PHI 5000 VersaProbe apparatus. Thermogravimetric analysis (TGA) was conducted on a Netzsch TG-209 instrument under air atmosphere. Room-temperature Fourier-transform infrared (FT-IR) spectra were acquired using a Bruker IFS28 spectrometer in the region of 4000−400 cm−1 with a spectral resolution of 2 cm−1 using a KBr pellet. Hydrogen temperature-programmed reduction (H2-TPR) measurements were performed utilizing an Autochem-2920 under a flow of 10% H2/Ar gas mixture and a heating rate of 10 °C min‒1 from room temperature to 900 °C. Prior to each measurement, the sample was pre-treated by argon gas with a flow rate of 30 mL min‒1 at 600 °C for 2 h to remove impurities, then, the system was cooled to room temperature. The amount of H2 uptake during the reduction was measured continuously with a thermal conductivity detector (TCD). The iron contents of the Fe3O4@BRAC sample were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a Perkin Elmer Optima 5300 DV instrument. Magnetic properties of the Fe3O4@BRAC sample was characterized by a superconducting quantum interference device (SQUID) based vibrating sample magnetometer (VSM; Quantum design; maximum applied continuous field 50k G) at room temperature. Reduction of Nitroarenes. Nitrobenzene (1.0 mmol), KOH (1.5 mmol), and Fe3O4@BRAC catalyst (10 mg) were added to 5 mL of isopropanol in a flask (25 mL) with magnetic stirrer. The reaction mixture was first sonicated, and then placed in a focused microwave (MW) synthesis system (Discover series, CEM GmbH) operating at 100 °C with a power of 300 W for 15 min. The solvent was then evaporated and then the crude products were recrystallized under vacuum. The residue so obtained was loaded on a silica gel column and 6 ACS Paragon Plus Environment

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eluted with n-hexane and ethyl acetate to yield the final products. Subsequently, the products were recrystallized in ethanol to obtain pure crystals during which some of the acidic groups may be converted into corresponding salts. The products were characterized by FT-IR, 1H and 13

C NMR, and by comparing their melting points with reported literature values. Moreover, for

recycling purposes, the magnetic catalyst was separated by using an external magnet, thoroughly washed with ethanol, then, dried under a reduced pressure and reused for subsequent cycles.

RESULTS AND DISCUSSION Physicochemical Properties. The structural properties of the Fe3O4@BRAC catalyst was compared to that of as-prepared BRAC-x (x = 600−900 oC) and the bulk Fe3O4, as illustrated for the XRD profiles in Figure 1a. The BRAC-x series carbons exhibited two main broad diffractions at 2θ = 23.4 and 43.3° anticipated for the (002) and (100) planes of the graphitic structure.49 Similar peaks were also observed for the Fe3O4@BRAC catalyst in addition to several sharp peaks at 2θ = 30.1, 35.6, 43.2, 53.6, 56.9, 62.7, and 74.2°, which may be attributed to facets of the (220), (311), (400), (422), (511), and (440) planes of Fe3O4, respectively.50 The above results indicate a successful incorporation of Fe3O4 NPs on the biomass-derived activated carbon (BRAC-900) substrate. That no diffraction peaks responsible for the pure ZnO lattice (JCPDF No. 36-1451) were observed in the BRAC-x nor in the Fe3O4@BRAC, indicating a thorough removal of the Zn species by acid washing.36 The

textural

properties

of

the

Fe3O4@BRAC

catalyst were

studied

by

N2

adsorption/desorption isotherm measurements carried out at ‒196 oC. Similar to that of the bare BRAC series samples, the isotherm observed for the Fe3O4@BRAC catalyst exhibited the typical Type IV isotherm with H4 hysteresis loop (IUPAC classification) at a partial pressure (P/P0) of ca. 0.5 (Figure 1b), reflecting the presence of mesoporosity. Moreover, the notable increases in adsorption capacity in the Henry’s law (P/P0 < 0.1) region reveal the co-existence 7 ACS Paragon Plus Environment


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of micropores. Table 1 depicts the textural parameters, viz., BET surface area (SBET), pore volume (V), and pore size (dDFT), of various samples. It is clear that the pore size and microand mesopore surface area

Figure 1. (a) XRD spectra, (b) N2 adsorption/desorption isotherms, (c) variations of total and microporous surface areas vs carbonization temperature, (d) Raman spectra, and (e) TGA curves of the as-prepared BRAC-x (x = 600, 700, 800, and 900 oC) and the Fe3O4@BRAC catalyst. Insert in (c): expanded region of Raman spectra accountable for bulk Fe3O4.


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Table 1. Textural Properties of the Pristine BRAC and the Fe3O4@BRAC Catalyst Sample

SBET (m2 g−1)c

Dm b

V (cm3 g−1)d

IG/ID

(nm) f

0.01

0.05

3.1

1.10

0.36

0.05

0.31

3.4

1.01

368

0.48

0.18

0.30

3.4

1.00

1587

427

0.91

0.23

0.68

3.8

0.99

---

2281

539

1.50

0.31

1.19

4.1

0.98

3.8 ± 0.5

2026

425

1.15

0.28

0.87

4.0

0.98

STot

SMicro

BRACa

---

107

BRAC-600

---

BRAC-700

VTot

VMicro

54

0.06

647

122

---

889

BRAC-800

---

BRAC-900 Fe3O4@BRAC a

dDFT VMeso

(nm)

e

e

Bare BRAC without ZnCl2 activation. b Average size of Fe3O4 nanoparticle determined by FE-TEM

analysis. c Brunauer-Emmet-Teller (BET) surface areas. d Pore volumes calculated as the amount of N2 adsorbed at P/P0 = 0.99. e Microporous surface areas (SMicro) and pore volumes (VMicro) obtained from t-plot analyses; mesopore volume (Vmeso = VTot − Vmicro). f Average pore size determined by non-local DFT calculations.

and pore volume of BRACs increase with increasing carbonization temperature (Figure 1c). The BRAC-900 was found to possess a total BET surface area (STot) and pore volume (VTot) of 2281 m2 g−1 and 1.50 cm3 g−1, and a microporous surface area (Smicro) and pore volume (VMicro) of 539 m2 g−1 and 0.31 cm3 g−1, respectively. The pore size distributions estimated by means of DFT calculation based on the adsorption branch of the isotherm are displayed in Figure S1 of the Supporting Information (SI). Clearly, the size of mesopores also increases with increasing carbonization temperature. Upon incorporating Fe3O4 NPs onto the BRAC substrate, notable decreases in textural parameters were evident compared to the bare support, namely the BRAC-900. Again, this indicates that the Fe3O4 NPs were mostly dispersed in the pore cavities or channels of the BRAC support. As a result, an average pore size of 4.0 nm may be inferred for the Fe3O4@BRAC catalyst (Table 1 and Figure S1).



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Compared to the BRAC-900 material, a somewhat amorphous-like carbons with much lower STot and VTot were observed for the bare BRAC prepared in the absence of activation treatment by ZnCl2 (see Figures S2a and b; SI). This reveals that, upon activated by ZnCl2, the BRAC-x (x = 600−900 oC) indeed possess micro-and-meso-porosities. The notable increases in surface area and pore volume may be ascribed due to voids left by ZnCl2 after acid and water washing treatments. During the activation process, the impregnated ZnCl2 tends to promote dehydration of the carbon substrate, leading to charring and aromatization along with the creation of porosities. As proposed in an earlier report,40 the formation of pores in BRACs invoked the following route: (i) initially, mobile liquid ZnCl2 (m.p. ∼ 283 oC) is anticipated to form during the activation at low temperature, (ii) as the activation temperature further increased beyond 750 oC (b.p. of ZnCl2 ∼ 730 oC), strong interactions between carbon atoms and Zn species occur, (iii) considerable collapses between the carbon interlayers take place, creating meso- and micro-porosities after thorough washing treatments by acid (1 M HCl) and distilled water. Likewise, the Raman spectrum observed for various BRACs and the Fe3O4@BRAC catalyst are rather similar, except that additional absorption bands in 800−250 cm−1 region were observed for the nanocomposite sample (Figure 1d). These weak bands centering at ca. 680 cm‒1 are coincide to the characteristic band associated with the A1g mode of Fe3O4.51,52 On the other hand, the two predominant peaks at ca. 1600 and 1340 cm−1 may be ascribed to the in-plane bond stretching ordered sp2 carbon atoms (i.e., the E2g photons; known as the G band) and defects or disordered carbon (known as the D band), respectively.53 By comparison, bare BRAC without ZnCl2 activation showed only weak G- and D-bands at 1593 and 1353 cm−1, respectively (Figure S2c; SI). Moreover, an additional (2D) band at ca. 2715 cm−1 responsible for the overtone of the D band was observed only for both graphitized at elevated temperature,54 viz., the pyrolytic BRAC-900, and Fe3O4@BRAC composite (Figure 1c). Accordingly, the relative intensity between the G- and D-band (IG/ID), which is normally used 10 ACS Paragon Plus Environment

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to assess the degree of crystallinity of carbon materials, may be derived, as depicted in Table 1. Note that a rather high IG/ID value observed for all BRACs (≥ 0.98), indicating the high degree of graphitization provoked during activation treatment in the presence of ZnCl2.55,56 The thermal stability of various samples were further studied by TGA. The TGA profile of the bare BRAC prepared without ZnCl2 activation procedure showed ca. 10.4% weight loss during the initial stage (50−105 °C) of thermal degradation (Figure S2d) due to desorption of physisorbed water.38 This is followed by a more drastic weight loss starting at ca. 387°C, revealing a nearly complete oxidation of the activated carbon at temperature beyond ca. 445 °C. Similarly, as shown in Figure 1e, the activated BRACs showed a notable weight-loss in the temperature range of ca. 500−625 oC due to pyrolysis of the carbon network. Whereas that occurred above 600 oC is most likely due to the decomposition of carbon frame work moieties.1,40 By comparison, the weight-loss of the Fe3O4@BRAC catalyst occurred at a somewhat broader range of ca. 400–700 oC. In any case, it is indicative that the Fe3O4@BRAC catalyst remains stable at least up to 400 oC. Structural Properties. The FE-TEM images of the as-prepared BRAC-900 (Figure 2a) and the Fe3O4@BRAC (Figures 2b−d) reveal that both samples show highly porous structure abundant with micropores as well as mesopores. Moreover, the Fe3O4 NPs are highly dispersed on the surfaces of the BRAC and exhibit nearly spherical morphology with uniform size of 3.8 ± 0.5 nm in diameter. The particle size distribution for the Fe3O4 NPs was illustrated in Figure S3 (SI). Further study by EDX provides additional information on structural compositions of the Fe3O4@BRAC catalyst. As shown in Figure 2e, the nanocomposite catalyst contains elements of carbon (72.3%), oxygen (14.2%), nitrogen (5.4%), and iron (8.1%). The copper signals arising from diffuse scattering of the supporting copper grid were excluded during elemental analysis. As anticipated, the atomic Fe in magnetite NPs displayed signals at ca. 0.8, 6.4, and 7.1 keV, respectively. Moreover, the contents of elements C, O, N, and Fe contents present in various samples were also analysed, as summarized in Table S1 (SI). 11 ACS Paragon Plus Environment


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Figure 2. FE-TEM images of (a) the pristine BRAC-900, (b-d) Fe3O4@BRAC catalyst, and (e) its

corresponding EDX spectrum. Inset in (d) shows the SAED pattern of a typical Fe3O4 NP. Table in (e) depicts the elemental compositions of the Fe3O4@BRAC catalyst in atom%.

Additional XPS measurements on the Fe3O4@BRAC catalyst confirmed the presences of the Fe 2p, O 1s, N 1s, and C 1s core levels, as revealed by the survey spectrum in Figure 3a. The wide scan spectra in Figure 3b for the Fe 2p level showed two absorption peaks with binding energies of 712.2 and 728.3 eV, which may be unambiguously assigned due to Fe 2p3/2


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and Fe 2p1/2, respectively,57 revealing the existence of both Fe2+ and Fe3+ species in the Fe3O4 NPs. The presence of a weak satellite peak at ca. 720.8 eV indicates the existence of mixed iron oxides with divalent (Fe2+) and trivalent (Fe3+) ions anticipated for magnetite (Fe3O4). The results obtained from XPS core level spectrum of Fe 2p are in good agreement with the XRD data (Figure 1a), thus, verifying the formation of Fe3O4 NPs via the microwave-assisted synthesis route. Likewise, the core level spectrum of N 1s also showed three overlapping absorption peaks at398.6, 399.7, and 401.1 eV (Figure 3c), corresponding to the presences of pyridinic, pyrrolic, and quaternary N (or graphitic N) species, respectively.58

Figure 3. (a) XPS survey spectrum of the Fe3O4@BRAC catalyst, and corresponding core level spectra

of (b) Fe 2p, (c) N 1s, (d) C 1s, and (e) O 1s. 13 ACS Paragon Plus Environment


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The spectrum in the C 1s region of the Fe3O4@BRAC catalyst showed multiple absorption peaks (Figure 3d), which may be assigned due to the presences of C–C (284.6 eV), C−OH (285.5 eV), and C=O (288.5 eV) functional groups. The C–C peak is mainly arising from the BRAC-900 carbon, whereas the other two species (C–OH and C=O) are due to partially dehydrated residues, whose presence greatly affect the surface properties of the Fe3O4@BRAC catalyst. On the other hand, the XPS spectrum in the O 1s region gave rise to a broad single peak at 532.4 eV (Figure 3e), revealing the presence of lattice oxygen in Fe3O4.59 Moreover, the structural and chemical properties of the BRAC-900 support and the Fe3O4@BRAC composite catalyst were also investigated by FT-IR spectroscopy. As shown in Figure S4 (SI), the FT-IR spectrum observed for the Fe3O4@BRAC catalyst is rather similar to that of the BRAC-900, indicating a well-dispersed Fe3O4 NPs on the carbon support. The broad absorption band centering at 3378 cm−1 should be associated with the O–H stretching vibration mode of the hydroxyl functional groups in BRAC. The weak bands at 2973 and 2933 cm−1 confirm the presence of CH2 groups, while the bands at 925 and 1572 cm−1 revealed the presences of oxygen-containing functional groups. Whereas the absorptions at 1076, 1244, 1420, and 1572 cm−1 may be ascribed due to stretching vibrations of C−OH, C−O, ‒CH2, and C=O, respectively. Moreover, an additional band accountable for the N-H bending vibrations of the amine groups was also observed at 1571 cm−1.60 On the basis of FT-IR results, it is indicative that the Fe3O4 was indeed distributed on the surfaces of the porous carbon substrate that are abundant with various functional moieties such as hydroxyl, carbonyl, and amine groups. Reduction and Magnetic Properties. The reduction behaviour of the Fe3O4@BRAC catalyst was studied by H2-TPR technique. For comparison, pure hematite (Fe2O3) and magnetite (Fe3O4) NPs prepared by using similar procedures were also examined. The pure Fe2O3 sample showed three distinct reduction peaks at 308, 503, and 607 °C (Figure 4a), which may be attributed to reduction of Fe2O3 to 14 ACS Paragon Plus Environment

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Fe3O4¸ Fe3O4 to FeO, and FeO to Fe, respectively.61,62 Unlike Fe2O3, the H2-TPR profiles observed for the as-prepared Fe3O4 and the Fe3O4@BRAC catalyst are rather similar, revealing only two overlapping peaks in the temperature range of 400–680 °C. Thus, it is indicative that the reduction was accelerated from of Fe3O4 to FeO rather than from Fe2O3 and that the Fe3O4 NPs were successfully anchored on the BRAC material.

Figure 4. Comparisons of (a) H2-TPR profile and (b) magnetic plot of Fe2O3, Fe3O4, and the Fe3O4@BRAC catalysts. (c) Pictures of the Fe3O4@BRAC catalyst before (left) and after (right) reaction. The latter clearly illustrates facile recovery of the catalyst by an external magnetic field.

The magnetic properties of the pristine Fe3O4 and the Fe3O4@BRAC catalyst were investigated by VSM method at room temperature. As shown in Figure 4b. the two substrates exhibited similar correlation between the magnetization (M) and the applied filed, revealing the 15 ACS Paragon Plus Environment


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soft-ferromagnetic behaviour with nearly no hysteresis, remanence, and coercivity. Accordingly, a saturation magnetization (Ms) of 61.6 and 58.8 emu g−1 may be determined for the pristine Fe3O4 NPs and the Fe3O4@BRAC nanocomposite catalyst, respectively, which is low compared to that of bulk Fe3O4 (Ms = 90–100 emu g−1).63 The slightly lower Ms value found for the Fe3O4@BRAC than the pristine Fe3O4 is attributed to the shielding of the carbon matrix on the surfaces of the magnetic Fe3O4 NPs. This, in turn, provide additional support to the anchoring of magnetite NPs on the BRAC matrix.64 As pointed out earlier, one of the advantages of exploiting such Fe3O4@BRAC catalyst is that the magnetic nanocomposite may be facilely recovered by applying an external magnetic field; a simple procedure that may be accomplished within matters of a few seconds. As shown in Figure 4c, the clear supernatant can be decanted practically with 5 s upon applying an external magnet to the reaction vessel. As a result, the recovered Fe3O4@BRAC catalyst may be reused repeatedly without significant loss in activity (vide infra). Catalytic Property. The effects of experimental parameters such as type and amount of solvent, catalyst loading, and reaction time on catalytic activity reduction of nitroarenes were also investigated. A wide variety of reagents, namely formic acid (HCOOH),65 ammonium formate (HCOONH4),66 hydrogen gas (H2),67 sodium borohydride (NaBH4),2,13,68,69 hydrazine hydrate (NH2-NH2),70 thiourea (NH2-CS-NH2),71 glucose (C6H12O6),72 CO/H2O-EtOH,73 glycerol,74 and sodium hypophosphite,75 have been employed for reduction of nitro compounds. The use of some of the reagents may be drawback by issues such as safety,67 toxicity,70 and operation costs. To evaluate catalytic activity of the Fe3O4@BRAC catalyst and to optimize the reaction conditions, reduction of nitrobenzene was chosen as a model reaction herein. Prior to process optimization, catalytic activity of the Fe3O4@BRAC nanocomposite in the presence of different solvents was compared. Since an aqueous system is considered more environmentally benign,76 reduction reaction was first carried out in aqueous medium with and without the presence of a 16 ACS Paragon Plus Environment

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reducing agent. However, only a satisfactory nitrobenzene yield of 86% was achieved for reduction of nitrobenzene over the Fe3O4@BRAC catalyst (Table 2). Subsequently, hydroxyl (OH) group-containing organic solvents were exploited for the reduction reactions, which were carried out under microwave irradiation (MW power: 300 W, for 15 min) over 10 mg of the Fe3O4@BRAC catalyst (entries 2‒6). In this case, it was found that solvents with lower boiling points such as methanol and ethanol (entries 2 and 3) led to a lower product yield of 42 and 60%, respectively, compared to that of ethylene glycol (93%), 1,2-propanediol (92%), glycerol (90%), and isopropanol (> 94%) as depicted in Table 2 (entries 4‒6, 15, and 16). This is most likely due to inadequate hydrogen release during the reaction. It is noteworthy that solvents with lower boiling points (b.p.) such as methanol and ethanol, which possesses no α-H atom, appear to result in lower yields over a reaction time of 15 min (entries 2 and 3). This may be attributed to the slow release of hydrogen atoms during the reaction. On the other hand, the use of isopropanol as solvent led to a 99% yield (entry 13). The above results confirm that the type of alcohol plays an important role for the reduction reaction. For comparison, the catalytic performances during reduction of nitrobenzene over different kinds of BRACs without Fe3O4 NPs were also investigated. As depicted in Table S2 (SI), in the absence of Fe3O4 NPs, bare BRAC (without ZnCl2 activation) showed no activity while acid-washed BRAC-x (x = 600−900 oC) with trace amount of Zn (typically ≤ 1.3 ppm, Table S2; SI) resulted in modest yields, typically ca. 30−50%. Thus, these ZnCl2-activated BRAC-x are affordable as catalysts for reduction of nitrobenzene with moderate yields, which is due to the presence of residual Zn species77 as well as the enhanced surface areas and active functionalities in the substrates.4,78,79 By comparison, the same reaction carried out over the iron precursor Fe(acac)3, pristine Fe3O4, and bare BRAC-900 using isopropanol as solvent were found to proceed very slowly and all resulted in poor nitrobenzene yield (typically < 75%; see Table 2, entries 7−9).79 In the absence of a hydrogen donating solvent, a null product yield was obtained (entry 11). Likewise, for reaction done without the catalyst (entry 10). Among the various experimental conditions 17 ACS Paragon Plus Environment


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examined, an optimal nitrobenzene yield of 99% was achieved over the Fe3O4@BRAC catalyst under the conditions: solvent, isopropanol; catalyst amount, 10 mg; reaction time, 15 min; reaction temperature, 85 °C under microwave irradiation (entry 13). In this context, it is clear that isopropanol acts not only as a solvent but also the hydrogen donor. The use of isopropanol is also beneficial compared to the other solvents; as may be specified below: (i) it is a simple and inexpensive solvent, (ii) could donate hydrogen at relatively lower temperature (85 °C), (iii) non-toxic and volatile in nature, and (iv) the final product was acetone, which was environmentally friendly and easy to remove from the reaction system.80‒83 Moreover, an increase in product yield with the catalyst amount may be inferred. In this context, the effect of reaction time appears to play a minor role, especially when the duration exceeds 15 min; as may be seen from the results depicted in Table 2 (entries 12‒16).

Table 2. Reduction of Aromatic Nitro Compounds over the Fe3O4@BRAC Catalysta

Entry Solvent

B.p. (oC)

Catalyst

Amount (mg)

Time (min)

Yield (%)b

1 Water 100 Fe3O4@BRAC 10.0 25 86 2 Methanol 64.7 Fe3O4@BRAC 10.0 15 42 15 60 3 Ethanol 78.4 Fe3O4@BRAC 10.0 4 Ethylene glycol 197.3 Fe3O4@BRAC 10.0 15 93 5 1,2-propanediol 188.2 Fe3O4@BRAC 10.0 10 92 6 Glycerol 290 Fe3O4@BRAC 10.0 15 90 7 Isopropanol 82.6 Fe(acac)3 10.0 15 55 8 Isopropanol 82.6 Fe3O4 10.0 15 75 9 Isopropanol 82.6 BRAC-900 10.0 15 50 10 Isopropanol 82.6 ----15 --15 --11 ----Fe3O4@BRAC 10.0 12 Isopropanol 82.6 Fe3O4@BRAC 2.5 15 90 13 Isopropanol 82.6 Fe3O4@BRAC 10.0 15 99 14 Isopropanol 82.6 Fe3O4@BRAC 5.0 15 94 15 Isopropanol 82.6 Fe3O4@BRAC 5.0 25 94 16 Isopropanol 82.6 Fe3O4@BRAC 10.0 25 98 a Reaction conditions: nitrobenzene (1 mmol), KOH (1.5 mmol), solvent (5 mL), temperature 85 °C. b

Derived from gas chromatography.


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The catalytic performances during reduction of various aromatic nitro compounds over the Fe3O4@BRAC catalyst were assessed. Typically, reactions were carried out in the presence of a solvent (namely, isopropanol, which also served as the hydrogen donor) and a reductant (KOH) at 85 °C (under microwave heating) for 10−25 min. The yields of corresponding aromatic amine products were then derived from GC analyses; as summarized in Table 3. An excellent phenylamine (aniline) yield of 99% was achieved when nitrobenzene was employed as the reactant (entry 1). As for the other derivatives of nitrobenzene, for examples, those link with a hydroxyl, nitro, methyl, or carboxyl groups at various positions of the aromatic ring, the corresponding aromatic amine products may also be obtained with good to excellent yields (75−99%; entries 2‒8). In particular, a good yield of 99% was achieved when 4-nitroanilne was employed as the starting material (entry 6), surpassing that observed using 2-nitro and 3-nitroanilines, which gave rise to a yield of 88 and 91% (entries 4 and 5), respectively. For reduction of 4-nitrotoluene by isopropanol (entry 7), only moderate yield of the main product (i.e., 4-toluidine; ca. 75%) was observed. As the reaction time was increased from 15 to 30 min, the yield was further increased to 94%. Interestingly, the reduction of 4-nitrobenzoicacid (entry 8) took place without affecting the carboxylic group. It is worth pointing out that 4-aminobenzoic acid is a key ingredient for biosynthesis of folic acid, which is a constituent of Vitamin B complex commonly found in animal and plant tissues. Here, the sodium salt of 4-aminobenzoic acid was purified by adding a desirable amount of sodium hydroxide, then, washed repeatedly (3 times) with 95% alcohol before drying at 100

o

C. Moreover,

2-nitro-8-hydroxyquinoline and 5-nitro-8-hydroxyquinoline may also be reduced with a desirable product yield of 90% and 88% (entries 9 and 10). The latter two products are key ingredients for pharmaceutical and biological applications.84



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Table 3. Catalytic Performances of the Fe3O4@BRAC Catalyst for Reduction of Various Aromatic Nitro Compounds using Isopropanol as Solvent and KOH as the Basea Time (min)

Yield (%)b

1

15

99

2

15

96

3

15

99

4

15

88

5

15

91

6

10

99

7

15

75

8

25

78

9

25

90

10

20

88

Entry

a

Reactant

Product

Reaction conditions: Reactant, 1 mmol; KOH, 1.5 mmol; isopropanol, 5 mL; catalyst amount, 10

mg; MW power, 300 W; reaction temperature, 85 oC. b

Derived from gas chromatography results. 20 ACS Paragon Plus Environment

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On the basis of the above results and available literature data,80−83,85 a plausible reaction pathway for reduction of nitrobenzene is proposed, as shown in Scheme 2. In view of the fact that the magnetic substrates should interact strongly with the magnetic field created during the microwave irradiation, the reaction pathway may be summarized in brief: Step 1, nitrobenzene and isopropanol are chemisorbed on the surfaces of the magnetic Fe3O4@BRAC catalyst. This provokes an interfacial electron transfer between the solvent and the substrate, leading to simultaneous transfer of hydrogen in form of hydride ion to the substrate (Step 2). Subsequently, the reaction proceed while in the presences of a base (KOH, which served as active proton donor)86 and the Fe3O4@BRAC catalyst, leading to conversion of alcohol molecules to alkoxide species.86 The active nitro groups on the surfaces of the catalyst were then reduced to nitrosobenzene, followed by hydrogen transfer from isopropanol (which is adsorbed on an adjacent site), leading to a rapid reaction between the reactive H and nitrosobenzene (Step 3) similar to the Meerwein-Ponndorf-Verley (MPV) reduction. Consequently, aniline was formed as the final product and desorbed from the catalyst surfaces (Step 4). It is noteworthy that, here, the formation of aniline from nitrobenzene proceeds via the formation of nitrosobenzene and N-phenylhydroxylamine as reaction intermediates. Thus, it is conclusive that the Fe3O4@BRAC catalyst readily promote the transfer of hydrogen from interacting isopropanol and KOH. Finally, the catalyst may be facilely recovered by an external magnet after the reaction and reused for subsequent runs.



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Scheme 2. Proposed Mechanism for Reduction of Nitrobenzene in Isopropanol over the Fe3O4@BRAC Catalyst.

The recyclability of the Fe3O4@BRAC catalyst during reduction of nitrobenzene was assessed under the optimized reaction conditions: nitrobenzene 1.0 mmol, KOH 1.5 mmol, isopropanol 5 mL, catalyst 10 mg, reaction temperature 85 oC, and reaction time 15 min. After each experimental run, the catalyst was washed with ethanol, dried under vacuum at 80 °C for 1−2 h, then, exploited for subsequent cycles. In terms of aniline yield, the catalytic performances observed for the Fe3O4@BRAC catalyst after five consecutive runs are depicted 22 ACS Paragon Plus Environment

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in Figure 5a. Although the aniline yield was found to gradually decrease with experimental cycle, it descended only marginally from 99% of the initial run to 98% after five repeated cycles. Nevertheless, it is noteworthy that the gradual decrease in catalytic activity with experimental cycle is attributed to the inevitable loss of catalyst and, in-part, due to the progressive increase in reaction time required for complete reduction of nitrobenzene to aniline. In any case, the aniline yield maintained at a satisfactory level well beyond 90% after five consecutive running cycles with a fixed reaction time of 15 min each, indicating an excellent recyclability of the Fe3O4@BRAC catalyst for reduction of nitroarenes.

Figure 5. (a) Recycle test of the Fe3O4@BRAC catalyst towards nitrobenzene reduction reaction, (b) XRD profile, (c) TEM image, and (d) N2 adsorption/desorption isotherm of the spent catalyst recovered after five consecutive experimental cycles. Inset in (c) shows the SAED pattern of the Fe3O4 NP. Inset in (d) displays the pore size distribution of the spent catalyst.



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Catalyst Stability. The stability of the Fe3O4@BRAC catalyst was also examined by using XRD, TEM, and N2 adsorption/desorption measurements. That the XRD patterns of the recovered spent catalysts shown in Figure 5b are identical to diffraction peaks observed for the fresh catalyst (Figure 1a), indicates that the structure of the catalyst remained stable even after repeated experimental runs. Based on the FE-TEM image obtained from the spent Fe3O4@BRAC catalyst recovered after five consecutive runs (Figure 5c), a slight agglomeration of the Fe3O4 NPs may be inferred. Nevertheless, the spent catalyst gave rise to a comparable BET surface area (2012 m2 g−1) compared to that of the fresh catalyst (2026 m2 g−1; Table 1). Likewise, similar pore size distributions were observed for the spent (inset, Figure 5d) and fresh (Figure S1; SI) catalysts. Thus, it is conclusive that the Fe3O4@BRAC catalyst is highly stability; its structural and textural properties remain practically unchanged after repeated experimental cycles. The active and stable Fe3O4 NPs together with the high BET surface area and micro-and meso-porosities of the BARC support both contribute to the high catalytic activity observed. The size distribution was indicate that the Fe3O4 NPs has an average particle size of ca. 4.1 ± 0.1 nm for the spent catalyst (Figure S5; SI). The heterogeneity of the Fe3O4@BRAC catalyst was also examined. This is carried out by a hot filtration procedure using nitrobenzene as the reactant and isopropanol as the solvent under optimized conditions (vide supra; see Table 3). After the reaction the catalyst was filtered directly under hot conditions (at 85 oC). Subsequently, the solid catalyst filtrate was again subjected to reduction reaction under the same conditions for additional reaction time till no yield of aniline was observed. The spent catalyst so prepared by the hot filtration procedure was examined by ICP-AES, which revealed no significant leaching of Fe3O4 NPs. Only about 12 ppm loss of Fe species was obtained compared to that of the fresh (ca. 8.13 wt%; Figure 2e) Fe3O4@BRAC catalyst and a spent catalyst obtained after five consecutive runs (ca. 8.10 wt%; Figure S6, SI). These results clearly demonstrate that the Fe3O4@BRAC catalyst is truly robust and heterogeneous in nature. 24 ACS Paragon Plus Environment

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For comparison, available results in the literature on reduction of nitrobenzenes in the presence of isopropanol solvent over various catalysts were depicted in Table S3 (SI). It is indicative that, with a less amount of catalyst (10 mg) employed in this study, the Fe3O4@BRAC catalyst studied herein shows superior catalytic performance for reduction of nitroarenes compared to other catalyst systems reported earlier in the literature. Moreover, the fabrication procedure of such Fe3O4@BRAC catalyst from biomass precursors is not only simple but also more environmental friendly, hence, renders practical industrial applications.

CONCLUSIONS The Fe3O4@BRAC catalyst prepared by immobilizing active Fe3O4 nanoparticles on beetroot-derived activated carbons (BRAC), which possesses high surface area and porosities, were found to exhibit excellent activity for reduction of nitroarenes under microwave-assisted heating. The catalyst so prepared by a facile route from a biomass feedstock is not only economical but also eco-friendly. The anchoring of active Fe3O4 NPs on high surface area porous carbon support readily renders the reduction reaction to take place effectively within a short period of time and under relatively mild reaction conditions to produce aromatic amines with high yields. In particular, when isopropanol is employed as the solvent as well as the hydrogen donor required for the reaction, the heterogeneous catalyst may be easily separated from the reaction mixture by using an external magnetic device. The cost-effective Fe3O4@BRAC catalyst reported herein was also found to be highly stable and robust for recycle use, hence, should be practical for industrial production of important aromatic amine compounds and intermediates with desirable functionalities.



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ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.xbxxxxx. Additional XRD, N2 physisorption, TGA, pore size and metal particle size distributions, EDX, FT-IR, and NMR results of assorted catalyst samples and/or amine products, and comparative catalytic performances on reduction of nitrobenzene over various catalysts (PDF).

AUTHOR INFORMATION Corresponding Authors * E-mails: [email protected] (P. Veerakumar); [email protected] (S. B. Liu) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful for the financial support (MOST 104-2113-M-001-019) from the Ministry of Science and Technology (MOST), Taiwan.

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Table of Content Biomass-Derived Activated Carbon Supported Fe3O4 Nanoparticles as Recyclable Catalysts for Reduction of Nitroarenes Pitchaimani Veerakumar, Irulandi Panneer Muthuselvam, Chin-Te Hung, King-Chuen Lin, Fang-Cheng Chou, and Shang-Bin Liu

Porous carbon derived from biomass feedstocks shows excellent performances as efficient supported catalyst for the reduction of nitro compounds.


Biomass-Derived Activated Carbon Supported ... - ACS Publications

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