New Hybrid Iron Phosphonate Material as an Efficient Catalyst for the

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A New Hybrid Iron Phosphonate Material as an Efficient Catalyst for the Synthesis of Adipic Acid in Air and Water Piyali Bhanja, Kajari Ghosh, Sk. Safikul Islam, Astam K Patra, Sk Manirul Islam, and Asim Bhaumik ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02023 • Publication Date (Web): 27 Sep 2016 Downloaded from http://pubs.acs.org on September 29, 2016

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ACS Sustainable Chemistry & Engineering

A New Hybrid Iron Phosphonate Material as an Efficient Catalyst for the Synthesis of Adipic Acid in Air and Water Piyali Bhanjaa, Kajari Ghoshb, Sk Safikul Islamb, Astam K. Patra,a Sk. Manirul Islamb and Asim Bhaumik*,a a

Department of Materials Science, Indian Association for the Cultivation of Science, 2A & 2B

Raja S. C. Mullick Road, Jadavpur 700 032, India. *Corresponding author. E-mail: [email protected] b

Department of Chemistry, University of Kalyani, Nadia 741235, West Bengal, India.

ABSTRACT: A new organic–inorganic hybrid iron phosphonate material (FePO-1-2) has been synthesized hydrothermally using etidronic acid (1-hydroxyethylidene-1,1-diphosphonic acid) as an organophosphorus precursor. Under optimized reaction conditions the synthesis has been carried out hydrothermally for three days at 180 ºC temperature and at near neutral pH. The material has been characterized thoroughly by various techniques and its crystal structure has been indexed to new orthorhombic phase with a unit cell parameters of a = 10.995 Å, b = 10.395 Å, c = 11.793 Å and α = β = γ = 90°. Considerably good Brunauer-Emmett-Teller (BET) surface area of 236 m2 g-1, pore volume of 0.229 cc g-1 and robust nature of FePO-1-2 have motivated us to explore its catalytic activity in liquid phase partial oxidation reactions under green conditions. FePO-1-2 exhibits excellent catalytic activity (96% product selectivity, 72% conversion) for

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selective liquid phase oxidation of cyclohexanone to adipic acid in the presence of molecular O2 under atmospheric pressure and in aqueous medium. This selective liquid (aqueous) phase oxidation pathway is highly green and sustainable as it does not involving any need of nitric acid, initiator, peroxides and other organic solvents. KEYWORDS: Metal-organic framework, orthorhombic crystal structure, adipic acid production, liquid phase oxidation. 1. INTRODUCTION In the era of rapidly changing global environment more and more research attempts are directed for the production of bulk chemicals involving eco-friendly and efficient catalytic routes.1-3 Innovation in different frontline methodologies for the production of high yielding and cost effective bulk chemicals is very demanding in the context of catalysis research today.4-6 Nylon-6,6, an essential polymeric material for our daily life and fundamental building block for the manufacture of plasticizers, food additives, fibers etc. is actually produced from adipic acid (1,6-hexanoic acid, AA), a very important dicarboxylic acid.7 Thus, it is one of the most essential components in textile and plastic industry.8,9 The conventional industrial procedure for the production of adipic acid involves two steps, first through the catalytic oxidation of cyclohexane in air which leads to the production of an intermediate called KA oil (ketone/alcohol oil) and secondly, further oxidation of KA oil to desired product AA in the presence of nitric acid as oxidant.10-12 Further, this conversion has a side effect of generating heavy amount of nitrous oxide (almost stoichiometric 1:1 with respect to AA) in the oxidation process which acts as a chemical hazard, depleting the stratospheric ozone layer and thus green house effect.13-15 Additionally, for the synthesis of adipic acid from renewable feedstocks it is highly preferable to design a economic, sustainable and environment-friendly process where hazardous nitric acid is 2 ACS Paragon Plus Environment

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not required for the process. This serious issue of atmospheric imbalance has motivated the researchers to explore more suitable and environmentally sound strategy for production of adipic acid. Sato et al. demonstrated16 an alternative method of adipic acid production through the oxidation of cyclohexanone by using hydrogen peroxide as an oxidizing agent and H2WO4 as a pre-catalyst. Although production of H2O2 has its own adverse effect, this partial oxidation method is quite eco-friendly as its degradation product is only water.17 Oxidation of cyclohexene to adipic acid over WSBA-15 heterogeneous catalysts provides maximum product yield of 45.9%, as mentioned by Cheng et al.18 Recently, Wen et al have reported19 the oxidation of cyclohexene to adipic acid with H2O2 using homogeneous catalyst system involving H2WO4, H3PO4 and H2SO4, where the maximum observed yield has been improved to 93% for to 8 h reaction time. Noyori has reported20 the good catalytic efficiency for oxidation of cyclohexene to adipic acid (91% yield) using homogeneous catalyst H2WO4 with 3.3 molar 30% H2O2. Although, homogeneous catalysts can lead to higher product selectivity in a chemical reaction, due to the advantages like recyclability and green catalytic routes heterogeneous catalysts are very demanding for these liquid phase reactions. Designing the novel nanoporous materials bearing an active transition metal as an integral part of the framework is one of the challenging areas of research due to their wide spectrum of potential application in heterogeneous catalysis.21 Among these newly evolving materials such as porous organic polymers, purely inorganic porous materials like metallo phosphates,22 zeolites, metal oxides, organic-inorganic hybrid frameworks,23 later class of materials are gaining increasing attention in the recent times. The nature of the organic moiety present in those organic-inorganic hybrid materials often plays crucial role for the catalytic reactions.24,25 To direct the architectural micorporosity or mesoporosity usually single molecule



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templates or supramoelcular assembly of ionic/non-ionic surfactants are employed. But keeping the porous architecture unruffled after the removal of template molecules is a major concern. Therefore preparation of a novel porous organic-inorganic hybrid material without using a template during its synthesis is always prudent. For designing metal organic frameworks (MOFs) the functionality of the framework and pore size can easily be tuned by varying the precursor organic ligand.26,27 Among these MOFs based on phosphonate organic ligands,28-30 phosphate site can interact with various metal cations through strong covalent bond. This gives additional durability and robustness in phosphonate based MOFs. Strong binding of the metal centres with the ligands is very crucial for their chemical stability, especially for their applications as a heterogeneous catalyst in liquid phase catalytic reactions.31-37 Thus, phosphonate based metal organic-inorganic frameworks have huge potential to be used in eco-friendly catalysis. Herein, we report a new iron phosphonate framework material FePO-1-2 by using etidronic acid as organophosphorus precursor via hydrothermal synthesis route having orthorhombic crystalline phase. Good BET surface area, nanoscale porosity, high surface acidity and highly reactive iron as an integral part of the framework could be the major driving force for the aerobic oxidation of cyclohexanone to adipic acid under O2 atmosphere over FePO-1-2. The material exhibits high catalytic activity as well as selectivity for the production of adipic acid from cyclohexanone under mild and green reaction conditions. This hybrid material can be easily separated from the reaction mixture and it displayed good recycling ability up to sixth consecutive reaction cycles without significant loss of any catalytic activity. Both conversion of cyclohexanone to adipic acid and its selectivity under aerobic oxidation over FePO-1-2 nanomaterial is very high compared to the other catalysts reported in the literature.38-40 2. EXPERIMENTAL SECTION 4 ACS Paragon Plus Environment

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2.1. Chemicals. Iron(III) chloride anhydrous (M = 162.20 g/mol), etidronic acid 60% aqueous (M = 206.03 g/mol) and aqueous (25%) ammonium hydroxide were purchased from SigmaAldrich, India. Other organic solvents were used as received without further purification. 2.2. Instrumentation. Nitrogen adsorption/desorption isotherms were obtained by using a Quantachrome Autosorb 1-C surface area analyzer at 77 K temperature. Prior to the nitrogen gas adsorption experiments samples were degassed for 12 h at 423 K under high vacuum. By employing non-local density functional theory (NLDFT) pore size distribution was determined from the sorption isotherm using N2 adsorption on carbon slit/cylindrical pore model. The wide angle powder X-ray diffraction pattern has been recorded on a Bruker D8 Advance SWAX diffractometer operated at 40 kV voltage and 40 mA current. The instrument was calibrated with a standard silicon sample, using Ni-filtered Cu Kα (λ=0.15406 nm) radiation. FT-IR spectrum of the solid material was recorded by using a Nicolet MAGNA-FT IR 750 spectrometer Series II and the UV-visible spectrum of the material was recorded using UV 2401PC using an integrating sphere attachment where BaSO4 was used as a background standard. For TEM analysis, approximately 10 mg of material was dispersed into absolute ethanol for 5-6 minutes sonication, then one drop of the dispersed solution was placed onto the carbon coated copper grid and dried before TEM analysis. To get an idea on the morphology and particle size of the sample JEOL JEM 6700 field emission scanning electron microscope (FE SEM) was used. The electron paramagnetic resonance (EPR) spectrum was recorded for the solid sample at room temperature using a JESFA200ESR spectrometer (JEOL). The iron content of the sample was determined by Shimadzu AA-6300 atomic absorption spectrophotometer (AAS). Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of the sample were performed in a TGA instrument thermal analyzer TA-SDTQ-600 under air flow. The carbon, hydrogen and nitrogen



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(CHN) contents in solid material were determined in a Vario EL III elemental analyzer. The temperature-programmed desorption of ammonia (NH3-TPD) experiment was performed on a flow apparatus (Micrometrics, ChemiSorb 2720). For the TPD-NH3 experiment before taking that material in the U-type glass cell the sample was allowed to keep for outgassing at 130 °C temperature under inert atmosphere (helium) for 3 h. After cooling down to room temperature, the ammonia (NH3) gas flow was started at 30 ml/min for 30 min to reach at saturation condition and again for 45 min helium gas was purged to flush out the additional amount of NH3 gas from the glass cell. Then NH3-TPD desorption profile of this material was obtained using a Thermal Conductivity Detector (TCD) by increasing the temperature at 5 ºC per minute ramp. After the catalytic reaction the products are identified by 1H NMR and quantified by using a Varian 3800 gas chromatograph fitted with a capillary column and flame ionization detector. 2.3. Synthesis of porous iron phosphonate framework. In a typical synthesis, first 1.03 g (3 mmol) of etidronic acid (60% aqueous solution) was taken in a 100 mL glass beaker containing 10 mL distilled water. In another beaker 0.972 g (6 mmol) of anhydrous FeCl3 was dissolved in 5 mL of distilled water. Then both the solutions are mixed well and allowed to stir for 15 minutes under vigorous stirring conditions. At this point the pH of the reaction mixture was very acidic ca. 1.0. Then aqueous ammonium hydroxide solution (25% in water) was added to the final reaction mixture dropwise to maintain final pH of the synthesis gel at ~6.0. The reaction mixture was stirred for overnight to get homogeneous brown colored slurry. Finally, the resulting slurry was transferred to stainless steel teflon line autoclave and kept it inside the oven under static condition at 180 °C temperature for three days. After hydrothermal treatment, the solid product was filtered through simple filtration technique using whatmann-41 filter paper and washed with distilled water for several times to get rid of un-reacted reactants. The collected solid product


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was dried in an air oven at 75 °C and observed yield of the product was 1.4 g. The color of this iron phosphonate solid was pale green. This solid material was denoted as FePO-1-2 where ligand to metal ratio was 1:2 and characterized thoroughly by several techniques. Schematic representation for the synthesis of hybrid iron phosphonate is shown in Scheme 1. We have also synthesized another iron phosphonate material by varying the molar ratio of organic ligand to metal (L : M = 1:3) and this material has been designated as FePO-1-3. However, it does not show any sharp diffraction peak in the powder XRD pattern and displayed very low BET surface area. Thus, we have chosen FePO-1-2 material to characterize thoroughly and employed as acid catalyst in the liquid phase selective partial oxidation reaction. 2.4. Procedure for the synthesis of adipic acid from cyclohexanone. In a typical catalytic run the oxidation of cyclohexanone to adipic acid was carried out in a round bottomed flask fitted with a water cooled condenser under liquid phase reaction conditions. For this, 5.0 mmol (0.5 g) of cyclohexanone was taken in 10 ml of distilled water and the suspension was allowed to stir continuously. Then 25 mg of FePO-1-2 catalyst was added to the reaction mixture. The flask was purged with a balloon filled with air to make an aerobic atmosphere and the mixture was then heated at 75 °C temperature for 10 h. After completion of reaction, under hot conditions the mixture was filtered and the collected filtrate was analyzed by a capillary gas chromatography (Varian 3800 fitted with capillary column and flame ionization detector). Then the filtrate was evaporated through rotary evaporator to get white solid product and it was washed with organic solvents. This white solid product was also characterized through FT-IR spectroscopy, 1H and 13

C NMR spectroscopy (DMSO-d6 solvent).

3. RESULTS AND DISCUSSION



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Powder X-Ray diffraction analysis. The wide angle powder X-ray diffraction pattern of FePO1-2 material is shown in Figure 1. As seen from this pattern that several sharp medium to high intensity peaks are present in the wide angle XRD pattern suggesting crystalline porous architecture of FePO-1-2 material. The crystallinity of the organic-inorganic hybrid material is sufficiently high and this XRD pattern does not match with any reported X-ray pattern of standard JCPDS literature profiles. We have employed the REFLEX software for indexing the crystal planes of the new iron phosphonate phase. The CELSIZ unit cell refinement program has been employed to verify the extent of matching and analyze the unit cell parameter and volume. The X-ray diffraction peak of FePO-1-2 material can be assigned with a new orthorhombic phase having unit cell parameters a = 10.995 Å, b = 10.395 Å, c = 11.793 Å and α = β = γ = 90° (Table 1). The calculated unit cell volume is 1348.056 Å3 with a very small deviation (ESD = 5.610). On the other hand, the wide angle XRD pattern of FePO-1-3 material synthesized with 1:3 molar ratio of Fe : ligand etidronic acid is shown in Figure S1 of the supporting information (ESI). Here no sharp peak is observed in the diffraction pattern, indicating the amorphous nature of the material. Porosity and surface area measurement. The BET (Brunauer-Emmett-Teller) surface area, pore size distribution and pore volume of FePO-1-2 material are estimated from nitrogen adsorption/desorption isotherms. Figure 2 represents the N2 adsorption/desorption isotherm, which can be classified as type I for micropores at low P/P0 together with type IV isotherm and a H3 type hysteresis loop at high P/P0.41,42 At low pressure region 0.02-0.19 P/P0 the isotherm is almost flat, indicating the presence of microporosity. However, a capillary uptake in the high pressure region from 0.69 to 0.98 P/P0 is observed, suggesting the substantial amount of interparticle mesoporosity in the material. The observed BET surface area of FePO-1-2 material is


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236 m2 g-1 with a pore volume of 0.229 cc g-1. The non-local density functional theory (NLDFT) has been employed42 to measure the average pore diameter of FePO-1-2. Corresponding pore size distribution plot has been shown in the inset of the Figure 2. A peak pore width of 1.1 nm and a weak one appeared around 8.9 nm due to inter-particle porosity of the material which matched well with the electron microscopic images. The De Boer statistical thickness (t-plot) revealed the different surface areas for FePO-1-2 sample due to microporosity and mesoporosity contributions as 38 and 198 m2 g-1, respectively. For FePO-1-3 material, the nitrogen adsorption/desorption isotherm is of type IV without any hysteresis loop having BET surface area of 56 m2 g-1 (ESI: Figure S2a). Spectroscopic analysis. The Fourier transform infrared (FT-IR) spectrum of porous FePO-1-2 material is shown in Figure 3. The two broad bands are appeared at 3428 and 3223 cm-1 due to the presence of defected P-OH groups and adsorbed water molecule on the surface of material, respectively. The peak is observed at 1632 cm-1 could be assigned for the bending vibration of water molecule. Additional two peaks noticed at 1097 and 905 cm-1 could be assigned to the stretching vibration of the tetrahedral -CPO3 groups.24 On the other hand the sharp and broad peaks appeared at 1411 and 2976 cm-1 could be attributed to the bending and stretching vibration of methyl group. Further, in this figure a distinct band at 557 cm-1 is observed, which could be attributed to the stretching frequency of Fe-O bond. FTIR spectrum of FePO-1-3 material is shown in Figure S3, suggesting similar framework peaks as that of FePO-1-2. Microscopic analysis. The scanning electron microscopic image can provide useful information about the particle morphology and size of this hybrid iron phosphonate material. FE-SEM images as shown in Figures 4a and 4b suggested flake like particle morphology of FePO-1-2 material with sharp edges. Size of those flake particles are varied in the range of 150 to 250 nm. 9 ACS Paragon Plus Environment


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These flakes are agglomerated and the aggregation of the flake-like nanoparticles could be responsible for the mesoporosity in this hybrid framework. On the other hand, the ultra high resolution transmission electron microscopic (UHR-TEM) images for FePO-1-2 sample are shown in Figure 5. As seen from these images that micropores are spread over the entire specimen having a dimension of ca. 1.0 nm. The FePO-1-3 sample also showed almost similar type of flake like morphology (Figure S4a) and small micropores (Figure S4b) as that of FePO1-2. The EDAX pattern of the sample is shown in Figure S5, which represent the presence of iron, phosphorous and oxygen in the framework. The aggregation of flake like morphology creates the interparticle mesoporosity in the matrix, which agrees very well with the pore size distribution plot shown in Figure 2. Thermal stability (TG/DTA). In order to investigate the thermal stability of FePO-1-2 and FePO-1-3 material we have carried out thermogravimetric analysis at 10 °C per min temperature ramp under air flow. Figure S6A (ESI) represents the TGA/DTA plot of FePO-1-2 sample, where first weight loss up to 101 °C is attributed to the evaporation of adsorbed free water molecule bound at the outer and inner surface of FePO-1-2. Above this temperature region the further weight loss could be assigned to the cleavage of C-C, C-H and C-P bonds. Beyond 200 °C temperature steep weight loss observed and this could be attributed to decomposition of residual organic fraction involving the cleavage of C-C, C-H and C-P bonds. Also, Figure S6B indicates the TG/DTA profile of FePO-1-3 material where first weight loss up to 98 °C due to evaporation of the surface adsorbed water molecules and secondly, weight loss up to 292 °C for decomposition of organic part. Beyond this temperature weight loss could be attributed to the burning of the remaining part of the framework. So TGA/DTA analysis suggests the material possess considerably good thermal stability up to 200 °C. We have carried out the atomic


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adsorption spectroscopic (AAS) analysis to measure iron content in the material framework and the estimated Fe content was 17.2 %. From EDAX analysis the iron content was found to be 18.5%, which agrees well with the estimated amount obtained from AAS analysis. Surface acidity measurement by NH3-TPD analysis. To determine the surface acidic sites of FePO-1-2 material, temperature programmed desorption of ammonia (TPD-NH3) experiment has been carried out in the temperature range of 25-600 ºC. Figure 6 (blue) represents the NH3-TPD desorption profile (TCD signal versus temperature) of the FePO-1-2 sample where two strong peaks are observed at 74 ºC and 532 ºC temperature. The first peak could be attributed due to the existence of acidic sites in the material, whereas the second peak appeared due to decomposition of phosphate based organic functional groups, which is further confirmed from the thermogravimetric analysis (Figure S6A). The total acid strength of the FePO-1-2 material estimated from area under the peak is 1.75 mmolg-1. In addition to that similarly we have estimated the total surface acidity of FePO-1-3 material to be 1.23 mmolg-1 from Figure 6 (purple). Thus, high surface acidity of hybrid iron phosphonate (FePO-1-2) together with the presence of framework redox FeII/III site (see EPR and XPS analysis below) has motivated us to explore its catalytic potential in one pot aerobic oxidation of cyclohexanone to adipic acid. X-ray photoelectron spectroscopic (XPS) studies. To investigate the specific oxidation state and the coordination environment of metal center (iron) of this hybrid metal phosphonate framework, X-ray photoelectron spectroscopic (XPS) analysis has been performed on freshly prepared solid material FePO-1-2 at room temperature. The survey XPS profile (Figure 7) revealed that the FePO-1-2 sample is composed of Fe, P, O and C and the atomic percentage of the element are 13.23, 14.14, 43.17 and 29.45% respectively. The high-resolution Fe 2p1/2 (Fe 2p is show in inset of Figure 7b) and Fe 3p spectra are shown in Figure 7 (b) and (c), respectively. 11 ACS Paragon Plus Environment


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The spectra showed the presence of majority of iron in Fe(II) oxidation state in the FePO-1-2 material. The spectrum after deconvolution showed two major peaks located at 721.4 and 725.1 eV in Fe 2p1/2 spectrum, 56.4, and 58.7 eV in Fe 3p spectrum. These are attributed to Fe(II) and Fe(III) oxidation state respectively. After deconvolution of the data it clearly indicated the presence of 62.1% of the iron in the material is in the Fe(II) oxidation state, whereas remaining 37.9% iron is in Fe(III) state.44,45 Etidronic acid is known to act as a reducing agent to convert majority of the precursor Fe(III) to Fe(II) and form more stable Fe(II) phosphate network under the synthesis conditions.46 Our analysis agreed well with the previously reported XPS data for Fe(II) and Fe(III) centers.47 Figure 7d showed the high resolution O1s spectra. Three kinds of oxygen species are recognized here. The peak centered at 530.8 eV can be assigned to the oxygen atoms binding with phosphorous (P-O),48 the peak centered at 532.6 eV can be assigned to bridging oxygen (Fe(II)-O/Fe(III)-O),49 whereas the peak centered at 534.2 eV is attributed to the surface bound hydroxyl oxygen (C-OH).50 The high resolution P 2p XPS spectrum is shown in Figure 7e. Three deconvolved contributions appear at 132.9, 134.5 and 135.6 eV. The peak centered at 132.9 eV is assigned to the carbon-phosphorus bond (C-P) present in etidronic acid,51 the peak centered at 532.6 eV is assigned to bridging divalent phosphonate group (-PO32-),49 whereas the peak centered at 534.2 eV is attributed to monovalent phosphonate group ((-HPO31-) which is present very less amount in the materials.52,53 The XPS analysis showed that the material contains large amount of divalent phosphonate group. The high-resolution C 1s spectrum (Figure 7f) show three peaks and these three peaks are located at 284.3eV for carbonphosphorous (C-P) bond,54 285.7 eV for carbon-carbon (C-C) bond and 287.0 for carbon-oxygen (C-O) bond.50 The XPS analysis show that the material contains large amount of Fe(II) centers that are bound with etidronic acid through bridging divalent phosphonate group (-PO32-).


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Electron paramagnetic resonance (EPR) studies. The environment of metal center (iron) of this hybrid metal phosphonate framework is further studied by EPR analysis. As confirmed from the XPS analysis that Fe(III) and Fe(II) with d5 and d6 configurations respectively are present in the FePO-1-2 material. Ground state of Fe(III) is 6S. The EPR signal of Fe(III) could be observed in room temperature because S state ion has long spin–lattice relaxation time.55 As shown in Figure 8, three peaks are observed for the sample in room temperature, indicating the presence of Fe(III) oxidation state in the orthorhombic framework, where gx = 2.03, gy = 2.09 and gz = 2.15 correspond to magnetic field strength of A = 332 mT, B = 322 mT and C = 314 mT, respectively. Fe(II) is also paramagnetic but EPR of Fe(II) can be observed only at very low temperatures due to fast spin-lattice relaxation, characteristic of orbitally degenerate electronic ground states.52 So Fe(II) does not have any significance in this EPR spectrum. Further, the XPS analysis data suggested Fe/P molar ratio in FePO-1-2 of 0.992 whereas that obtained from EDAX analysis was 0.76. XPS analysis data being more accurate to measure the chemical composition of a solid these results suggested 1:1 correspondence of Fe and P sites in the hybrid framework of FePO-12. Catalytic activity. To examine the catalytic activity of FePO-1-2 catalyst, we have performed the selective liquid phase oxidation of cyclohexanone to adipic acid under O2 atmosphere. Reaction parameters like solvent, temperature, time and amount of catalyst are screened to select the most optimized reaction conditions. The choice of solvent stands out to be one of the most important parameters that determine the conversion of substrate and desired product selectivity. Here we have performed the aerobic oxidation reaction of cyclohexanone (5.0 mmol) using same reaction conditions solvent (10 mL), temperature (75 °C), time (10 h) and 25 mg catalyst with three different solvents. The reaction has been performed in 1,4-dioxane, acetonitrile and water, 13 ACS Paragon Plus Environment


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and corresponding product selectivities are listed in Table 2. It is seen that reaction proceeded well with increasing polarity of solvent. Because the accessibility of the iron per-oxo species at the vicinity of the reactant will increase with increasing polarity of the solvent, and this could facilitate the immediate oxidation reaction. Since, water is more polar solvent among these solvents, it displayed highest conversion and product selectivity. Further, water is a non-toxic and easily available solvent and this partial oxidation reaction involving molecular O2 as single oxidant for the synthesis of adipic acid is highly environment friendly and sustainable. Temperature plays a crucial role in catalytic activity of FePO-1-2 for adipic acid production on oxidation of cyclohexanone in aqueous medium. At low temperature range up to 50 °C, cyclohexanone conversion is very little together with 100% adipic acid selectivity. As temperature is increased, conversion gradually increased with simultaneous drop in adipic acid selectivity (Figure 9). Finally at 75 °C, the conversion is found to be 72% with 96% adipic acid selectivity. Beyond this temperature, although little increase in conversion is observed but adipic acid selectivity dropped down. This may be due to further oxidation of adipic acid to other byproducts.56 Thus, 75 °C is chosen as the optimum reaction temperature for this organic transformation. FTIR, 1H and

13

C NMR spectra of produced adipic acid is provided in the

supporting information (Figures S7, S8 and S9, respectively). After optimization of reaction temperature, it is necessary to optimize the reaction time. The conversion of cyclohexanone increased in a continuous manner with time up to 6 h and then increased at a faster rate. Maximum product yield is observed after 10 h reaction time. The amount of solid catalyst is also responsible for high product selectivity. High catalyst amount resulted to negative effect on the conversion of cylohexanone and also for the selectivity of adipic acid. Thus, 25 mg of catalyst is taken as an optimized catalyst amount for this reaction. 14 ACS Paragon Plus Environment

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Therefore, the best optimized reaction conditions for selective aerobic oxidation of cyclohexanone to adipic acid in water are 75 °C for 10 h with the addition of 25 mg of catalyst. Optimized reaction condition for aerobic oxidation reaction and obtained product selectivity by varying both time and catalyst amount have been listed in Table 3. We have carried out the same experiment by varying different catalysts to understand the potential of FePO-1-2 material in this liquid phase oxidation reaction. We employed iron(II) acetate and iron(II) sulphate as homogeneous catalyst for this catalytic oxidation reaction. But they did not provide any satisfactory product yield while using iron(III) chloride and iron(II) chloride and these are shown in bar diagram of Figure 10 (ESI: Table S1). When FePO-1-3 is employed as catalyst for same catalytic oxidation of cyclohexanone low product yield as well as selectivity of AA is observed (Table S1). This could be attributed to very low surface area and poor crystallinity of FePO-1-3 over FePO-1-2. We have previously reported iron containing heterogeneous mesoporous core-shell Fenton nanocatalyst for production of adipic acid from cyclohexanone in the presence of H2O2 as oxidant, where selectivity of adipic acid was 86% at the cyclohexanone conversion level of 69%.57 It is interesting to note that this liquid phase oxidation reaction proceeds smoothly in the presence of molecular oxygen alone as the oxidant. Scheme 2 represents the reaction pathway for the oxidation of cyclohexanone over FePO-1-2 catalyst. In Scheme 3 we have proposed the possible reaction mechanism for synthesis of adipic acid over our hybrid iron phosphonate catalyst. Possibly Fe(II) sites activates the O2 molecule through the formation of metal-oxo species.34 Although high valent Fe-compounds/materials are often employed as powerful oxidant to oxidize organic compounds, Fe(II)-complexes in some unique cases shown to activate molecular oxygen.58 The catalyst structure, the oxygen atmosphere and the other optimized 15 ACS Paragon Plus Environment


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reaction parameters might help in the forming a cyclic ester which may resulted from the rearrangement of a cyclic 6-membered transition state. The ester hydrolyses to form 6-hydroxohexanoic acid. Oxidation of this acid leads to our desired product, adipic acid. The small amount of by-product ε-caprolactone is also observed, suggesting that the reaction proceeds through the formation of ε-caprolactone.34,59 Catalyst life time study. Under optimized reaction conditions the aerobic oxidation reaction of cyclohexanone was carried out in order to investigate the reusability as well as recyclability of FePO-1-2 material during adipic acid synthesis. After completion of oxidation reaction the catalyst was collected through simple filtration and washed with distilled water for several times. Further, the recovered catalyst is washed with ethyl acetate to get rid of extra organic molecules and dried over in oven at 75 °C for 6 h. After sixth consecutive reaction cycles the FePO-1-2 catalyst showed good catalytic efficiency without significant loss of its catalytic activity. We have plotted the adipic acid yield and its selectivity for different reaction cycles in Figure 11. Corresponding conversion of cyclohexanone and adipic acid selectivities are provided in ESI Table S2. It is noticed that a very small decrease in adipic acid selectivity (%) and cyclohexanone conversion (%) for every successive catalytic cycle. This result suggested considerably good catalytic efficiency of FePO-1-2 hybrid iron phosphonate for the synthesis of adipic acid. In order to address this issue, the reused catalyst after completion of sixth reaction cycle has been characterized by wide angle XRD and N2 sorption analysis. The wide angle XRD pattern of reused catalyst is shown in Figure S10, suggested only marginal decrease in the crystallinity of material after sixth reaction cycle. Also, from Figure S2b, it is observed that only very small decrease in BET surface area of reused catalyst was observed (232 m2 g-1) without


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any significant change in the nature of the adsorption/desorption isotherm. Thus, our hybrid iron phosphonate catalyst is highly robust for carrying out the liquid phase catalytic reactions. Leaching test. To investigate the heterogeneous nature of catalyst we have performed hot filtration experiment followed by leaching test.60 The dispersed catalyst has been recovered from the reaction mixture after 5 h reaction time and the reaction has been continued with the filtrate for another 5 h at 75 °C. After 5 h the cyclohexanone conversion was 49.2%, which changed only very little to 49.6% after another 5 h reaction time. Further, the iron content in that filtrate has been analyzed by using atomic absorption spectroscopic (AAS) technique, which suggested no detectable Fe in the filtrate. This result suggested no leaching of iron from the hybrid iron phosphonate catalyst surface during the course of liquid phase catalytic oxidation reaction. 4. CONCLUSION A new phosphonate based porous organic-inorganic hybrid material has been synthesized hydrothermally by using 1-hydroxyethylidene-1,1-diphosphonic acid as an organic precursor and ferric chloride as a metal source without any aid of structure directing agent. The wide angle powder XRD data of iron phosphonate revealed a new orthorhombic phase with unit cell parameter of a = 10.995 Å, b = 10.395 Å, c = 11.793 Å and α = β = γ = 90°, and the unit cell volume of 1348.056 Å3. XPS analysis revealed the presence of mixed valance (FeII/FeIII) state of iron in the material. This material exhibited excellent catalytic activity and selectivity for the liquid phase partial oxidation of cyclohexanone to adipic acid using molecular O2 as oxidant at atmospheric pressure and in aqueous medium. It also displayed very high recycling efficiency for several reaction cycles. Protocol presented for the synthesis of adipic acid through this ecofriendly route using molecular oxygen as the oxidant and avoiding the production of the



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greenhouse gas NO is environmentally sound, sustainable and safe approach for the selective organic transformations. ASSOCIATED CONTENT Supporting Information Powder XRD, N2 sorption, FT IR, FE SEM. TEM, EDAX patterns, TG-DTA data of the materials, 1H and 13C NMR of adipic acid, and catalytic data. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENTS PB thanks to CSIR, New Delhi for junior research fellowship. AB wishes to thank DST, New Delhi for instrumental facilities through DST Unit on Nanoscience, DST-SERB and DSTUKIERI project grants. SMI acknowledges DST-SERB, UGC (New Delhi, Govt. of India) and DST-W.B. (West Bengal, India) for financial support. SMI is also gratefully acknowledging DST and UGC for providing support to the Department of Chemistry, University of Kalyani under FIST, PURSE and SAP programme.


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FIGURES

Figure 1. Powder X-ray diffraction pattern of FePO-1-2 material.



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Figure 2. Nitrogen adsorption/desorption isotherm for FePO-1-2 material. Filled circle indicates the adsorption and empty circle represents the desorption points. Non-local density functional theory (NLDFT) has been employed to obtain the pore size distribution plot, which is shown in the inset of the Figure 2.


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Figure 3. FT-IR spectrum of FePO-1-2 material.



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Figure 4. Scanning electron microscopic images of FePO-1-2 material at two different places.


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Figure 5. Transmission electron microscopic images of FePO-1-2 at four different magnifications.



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Figure 6. NH3-TPD profile diagram of FePO-1-2 and FePO-1-3 materials in the temperature range of 35 to 600 °C.


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Figure 7. XPS analysis of FePO-1-2 material: (a) survey XPS and High resolution XPS of (b) Fe2p (c) Fe3p (d) O1s (e) P2p (f) C1s.



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Figure 8. EPR spectrum of FePO-1-2.


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Figure 9. Effect of temperature on oxidation of cyclohexanone to adipic acid.



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Figure 10. Dependence of product yield on various catalysts for aerobic oxidation of cyclohexanone at 75 °C temperature.


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Figure 11. Reusability of FePO-1-2 in the arobic oxidation of cyclohexanone to adipic acid, conversions (red bars) and that for selectivity (blue bars).



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SCHEMES

Scheme 1. Schematic representation for the synthesis of FeII/III-phosphonate.


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Scheme 2. Reaction pathway for the oxidation of cyclohexanone over FePO-1-2 catalyst.



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Scheme 3. Possible reaction mechanism for the synthesis of adipic acid through the oxidation of cyclohexanone over FePO-1-2 under O2 atmosphere.


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TABLES. Table 1. Indexing of orthorhombic structure of FePO-1-2 catalyst. h

k

l

2θ in degree

d in Å

0

0

1

7.5108

11.850

1

0

1

11.1814

7.908

1

1

1

13.9534

6.366

2

0

0

16.0431

5.552

2

1

0

18.2927

4.859

1

2

2

24.2098

3.676

0

3

0

25.7770

3.453

0

3

1

26.7472

3.331

2

0

3

27.8027

3.209

1

2

3

29.6685

3.008

2

3

2

34.2316

2.617

4

1

2

37.0036

2.427

2

4

1

39.0293

2.261

5

0

2

44.0258

2.055

Unit cell parameters Parameter

Deviations

a = 10.99558 Å

0.01937

b = 10.39565 Å

0.01815

c = 11.79338 Å

0.03941 41 ACS Paragon Plus Environment


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

V = 1266.454 Å3

Page 42 of 44

ESD = 5.788

Table 2. Variation of different solvents for aerobic oxidation of cyclohexanonea

a

Entry

Solvent

Conversionb (%)

Selectivity of adipic acidc (%)

1

1,4 dioxane

33

46

2

acetonitrile

58

70

3

water

72

96

Reaction conditions: cyclohexanone (5.0 mmol), catalyst (25 mg), O2 (0.1 MPa), temperature

(75°C), time (10 h), solvent (10 ml). bCyclohexanone conversion based on cyclohexanone consumed. cProduct selectivity of main product (adipic acid).


Page 43 of 44

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


Table 3. Optimization table for aerobic oxidation reaction by varying both time and catalyst amounta.

a

Entry

Time (hrs)

Catalyst amount (mg)

Conversionb (%)

Selectivity of adipic acidc (%)

1

4

20

32

39

2

6

20

38

42

3

8

20

53

68

4

8

25

65

79

5

10

25

72

96

6

10

30

70

88

7

12

30

61

64

Reaction conditions: cyclohexanone (5.0 mmol), O2 (0.1MPa), temperature (75°C), solvent (10

ml). bCyclohexanone conversion based on cyclohexanone consumed. cProduct selectivity of main product (adipic acid).



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 44 of 44

For Table of Contents Use Only A New Hybrid Iron Phosphonate Material for Facile Synthesis of Adipic Acid in Air and Water Piyali Bhanja, Kajari Ghosh, Sk Safikul Islam, Astam K. Patra, Sk. Manirul Islam and Asim Bhaumik*

A new FeII/III-phosphonate has been synthesized hydrothermally using etidronic acid as legand and it showed excellent catalytic activity in liquid phase oxidation of cyclohexanone to adipic acid in the presence of molecular O2 under atmospheric pressure and in aqueous medium


New Hybrid Iron Phosphonate Material as an Efficient Catalyst for the

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