Heteropoly

Subscriber access provided by - Access paid by the | UCSB Libraries

Article

Synthesis and Characterization of Oligomeric Ionic Liquid/ Heteropoly Acid Composite as a new Heterogeneous Catalyst through Anion-Exchange Method for Selective Cyclohexane Oxidation with Molecular Oxygen under Solvent-free Conditions Neda Sammah, and Mehran Ghiaci Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03071 • Publication Date (Web): 04 Sep 2017 Downloaded from http://pubs.acs.org on September 7, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

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

Industrial & Engineering Chemistry Research

Synthesis and Characterization of Oligomeric Ionic Liquid/Heteropoly Acid Composite as a new Heterogeneous Catalyst through Anion-Exchange Method for Selective Cyclohexane Oxidation with Molecular Oxygen under Solvent-free Conditions Neda Sammaha, Mehran Ghiacia,* a

Department of Chemistry, Isfahan University of Technology, Isfahan, 8415683111, Iran *[email protected]

Abstract: In this work, we investigated solvent-free oxidation of cyclohexane using oligomeric ionic liquid (OIL)/Heteropoly Acid (HPA) composite with molecular oxygen under solvent-free conditions. OIL has six imidazolium cores which synthesized in three successive stages and was anion-exchanged by phosphomolybdic acid (H3PMo12O40, PMo) to synthesize OIL/HPA composite. OIL and OIL/HPA composite were characterized by different techniques such as, FTIR, UV, BET, TGA, XRD, FESEM, EDX and TEM. Optimal reaction conditions were obtained by evaluating the effective parameters on the reaction including: temperature, time, oxygen pressure, and amount of the catalyst. The main product of the reaction in the presence of 20 mg catalyst and 5 mL cyclohexane was cyclohexanone by 21% conversion and 75% selectivity at 110 °C and 20 bar oxygen pressure after 5 h. The OIL/HPA showed good stability under reaction conditions and maintained its catalytic activity up to 5 cycles with little decrease in reaction conversion.

Keywords: Oligomeric ionic liquid; Heteropoly acid; Cyclohexane oxidation; anion exchange; Heterogeneous catalyst.

1 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

Introduction Oxidation of cyclohexane is an extremely important industrial process because of becoming a low-cost material into highly valuable products such as cyclohexanone, cyclohexanol and adipic acid; hence, there is a great attention to this issue from the past to present1-4. Many of the methods5,6, oxidants such as molecular oxygen2,3,7,8, hydrogen peroxide912

, tert-butylhydroperoxide13,14, as well as,different catalysts such as metal mixed oxides2,3,

molecular sieves13,15, (supported) metal complexes11,12,15-17, (supported) metal ions and nanoparticles7,14,18,19, ionic liquids8,12,13,20, heteropoly acids (HPAs) and their related polyoxometals (POMs)8,9,20,21, and so on10 have been investigated and proposed for the oxidation of cyclohexane. HPAs and their related POMs are thermally and oxidatively stable metal–oxygen anionic clusters of transition metals of group 6 (Mo, W) or less commonly group 5 (V, Nb, Ta) in their high oxidation states which make them very strong oxidants in the field of organic oxidation8,9,2023

. The use of these catalysts is usually associated with the main oxidizing agent so that improve

the conversion and selectivity of the reactant9,22. Despite the high oxidation efficiency, the homogeneity of them is the major challenge with these catalysts because of problems such as corrosion of the reactor, catalyst removal, and waste formation, and so on21. For the preparation of insoluble, solid or heterogeneous HPA catalysts, salt formation of them with acidic large cations such as cesium, ammonium, or cerium, as well as, the inclusion of them in silica or other supports have been reported20,21. A serious problem with the solid HPA catalysts (acidic cesium, ammonium, or cerium salt of HPAs) is their deactivation during organic reactions due to the formation of carbonaceous deposit (coke) on the catalyst surface22. Although, supporting HPA on a carrier inhibits the formation of coke, but they have strong acid sites less than solid HPA catalysts which results in lower oxidation reaction efficiency22. Moreover, ionic liquids (ILs) and their supported or modified forms have demonstrated their effective role as solvent or catalyst in the oxidation of cyclohexane and other organics13,20,24-27. To solve the problem of homogeneity of ionic liquids in oxidation reactions, they are stabilized on various substrates such as cross-linked insoluble polymeric matrices26, graphite carbon28, organosilica–titania29, alumina30, etc31. This, beside the particular advantages, has the problem of reducing the catalytic activity due to the reduction of the catalytic active sites.


Page 2 of 23

Page 3 of 23

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


Conversion of ILs to poly ionic liquids (PILs) via chemical modification of existent polymers through ion exchange, N-alkylation, or ligation of ILs results in synthesis of functional ionic polymers with high density of covalently assembled ILs through a polymeric backbone or framework32,33. This group of polymeric materials with unique structural configuration by broadening the property and applicative spectra of ILs have found fundamental and practical research frontiers, including sensing applications32,34, desulfurization35, gas separation36, polymer electrolytes37, water treatment38, and organic oxidation39. Along with their unique features, PILs like most ionic liquids, have high solubility in water and aqueous solutions, and their heterogeneity as catalyst or catalyst support is still a major challenge; however, converting them to heterogeneous or insoluble catalysts is more easier than related ILs by using different methods such as creating a porous mineral coating around them or anion exchange of PILs with coarse anions, such as HAPs, as presented in this paper. In this work, we developed a new system in which combination of two homogeneous catalysts (OIL+HPA) by using anion exchange reaction caused a notable heterogeneous catalyst. The large size of cation (OIL) plays two important roles: (i) eliminating the solubility of the HPA in aqueous systems, (ii) being the support for the main catalyst i.e. HPA by uniform and molecular dispersion of HPA on its oligomeric bed which leads to an increase in the efficiency of the reaction by giving a more hydrophobic nature to the active site of the catalyst for oxidation of cyclohexane. The latter one eliminates the problem of the deactivation of the solid HPA salts with large cations (mentioned above) during organic reactions. The OIL/HPA catalyst has revealed considerable catalytic activity in selective oxidation of cyclohexane under mild conditions by using molecular O2 as oxidant.

2.Experimental 2. 1. Materials and analysis All necessary chemicals such as 1,2-dibromobutane, benzyl chloride, imidazole, phosphomolybdic acid (PMO) and cyclohexane were purchased from Merck Company. Molecular oxygen (99.99 %) was obtained from Argon Company (Iran). All solvents used in this study including carbon tetrachloride, chloroform, ethanol and acetonitrile were analytical grade. FTIR spectra were recorded by FTIR spectrophotometer (Jasco 680-plus) using KBr pellets. X-Ray diffraction analyses were performed using a diffractometer with Cu anode 3 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

(Philips X’pert, Netherland). The surface morphology of the samples was observed by ﬁeld emission scanning electron microscopy (TESCAN MIRA3) with energy dispersive X-ray (EDX) spectrometer and TEM analyzer. TGA curve of the catalyst was obtained under a nitrogen atmosphere at a sample heating rate of 10 °C min-1 using BAHR-STA/TGA-503. Adsorption– desorption isotherms of nitrogen at 77.3 K were measured using a NOVAWin2, version 2.2 (Quantachrome instruments) and surface area was measured by the BET equation. 2.2. Preparation of the Catalyst 2.2.1. Synthesis of the 1,2-di(1H-imidazol-1-yl)ethane(1). Sodium hydride (60% in oil, 400 mg, 10 mmol) was added to imidazole (680 mg, 10 mmol) dissolved in 30 mL acetonitrile at 0 °C and then 0.43 mL (5 mmol) 1,2-dibromoethane was added to the mixture and refluxed for 8 h. The reaction was monitored by TLC, and by completion of the reaction the solid was separated by filtration. By removing the solvent at reduced pressure, the oily product was purified by column chromatography (Solvent: cyclohexane-EtOAc). 1H NMR (400 MHz, DMSO-d6): δ 4.17-4.31 (m, 4H, CH2), 6.85 (m, 2H, CH), 7.01 (m, 2H, CH), 7.37 (m, 2H, CH) (Figure S1). 2.2.2. Synthesis of the 3,3’-(ethane-1,2-diyl)bis(2-bromoethyl-1H-imidazol-3-ium)bromide (IL 1). 2.58 mL (30 mmol) 1,2-dibromoethane was added to an acetonitrile solution of 1,2-di(1Himidazol-1-yl)ethane (810 mg, 5 mmol). The resulting solution was stirred at 80 ºC for 6 h under reflux. After completion of the reaction, the solvent was evaporated under reduced pressure and then by addition of 20 mL H2O the mixture was extracted with CCl4. The IL-1 was extracted from the aqueous solution by ethyl acetate (2x25 mL) and purified by column chromatography on silica gel (solvent: cyclohexane-EtOAc) (Scheme 1).1H NMR (400 MHz, DMSO-d6): δ 4.664.79 (m, 12H, CH2), 7.42 (m, 2H, CH), 7.50 (m, 2H, CH), 7.72 (m, 2H, CH) (Figure S2). 2.2.3. Synthesis of the 1-(2(1H-imidazol-1-yl)ethyl)-3-benzyl-1H-imidazol-3-ium) chloride (IL 2). 0.57 mL (5 mmol) benzyl chloride was added to a solution of acetonitrile containing 810 mg (5 mmol) of 1,2-di(1H-imidazol-1-yl)ethane and then mixture was stirred under reflux for 8 h. After completion of the reaction, 50 mL water was added to the reaction mixture and after extraction with CCl4 (2x25 mL), the aqueous solution was extracted with EtOAc (2x 25 mL) to separate the crude IL-2 which was purified with column chromatography (cyclohexane-EtOAc). 1H NMR (400 MHz, DMSO-d6): δ 5.20 (m, 4H, CH2), 5.48 (m, 2H, CH2), 6.88 (s, 1H, CH), 7.02 (m, 3H, CH), 7.23-7.44 (s, 5H, CH), 7.69 (s, 1H, CH), 7.77 (s, 1H, CH) (Figure S3). 4 ACS Paragon Plus Environment

Page 4 of 23

Page 5 of 23

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


2.2.4. Synthesis of the 1,1’-(ethane-1,2-diyl)bis(3-(2-(1-(2-(3-benzyl-1H-imidazol-3-ium-1yl)ethyl)-1H-imidazol-3-ium-3-yl)ethyl)-1H-imidazol-3-ium) tetrabromide dichloride (IL 3). For preparation of the ionic liquid IL-3, in a 100 mL round-bottom flask 534 mg (1 mmol) of IL-1 and 579 mg (2 mmol) of IL-2 and 30 mL dry DMF was poured and the mixture was refluxed for 24 h. The progress of the reaction monitored by TLC and by completion of the reaction, solvent was evaporated under vacuum at 100 °C. The IL-3 ionic liquid was precipitated by adding ethanol to the aqueous solution of crude IL-3 and then characterized by 1H NMR spectroscopy. 1

H NMR (400 MHz, DMSO-d6): δ 4.55-4.59 (m, 6H, CH2), 4.61-4.62 (m, 4H, CH2), 4.82-4.85

(m, 14H, CH2), 6.89 (m, 4H, CH), 7.22 (m, 4H, CH), 7.83-7.88 (m, 16H, CH), 8.13 (s, 2H, CH) (Figure S4). Anal. Calcd. for C42H52N12Cl2Br4: C 45.20, H 4.66, N 15.07%. Found: C 45.38, H 4.73, N 14.89%. 2.2.5. Synthesis of the IL3/phosphomolybdate (OIL/PMo) To prepare OIL/HPA catalyst, i.e. IL3/phosphomolybdate (OIL/PMo) 0.33 g (0.3 mmol) of IL3 was dissolved in 5 mL of deionized water and subsequently an aqueous solution of 1 g (0.6 mmol) of PMA in 10 mL of H2O was slowly added to it and the mixture putted under ultrasonic radiation for 2x15 min. Then, the produced solids was separated by filtration, dried at room temperature, and renamed hereafter as OIL/PMO.



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

Scheme 1. Preparation process for synthesis of the catalyst. 2.2.6. General Procedure for Oxidation of Cyclohexane A typical procedure for the cyclohexane oxidation experiments was adopted somewhat from the Xiao et al work7 as follows: A titanium batch reactor (50 mL) was charged with a 6 ACS Paragon Plus Environment

Page 6 of 23

Page 7 of 23

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


proper amount of the catalyst (20 mg) and cyclohexane (5 mL) under solvent-free conditions. The oxygen pressure and the temperature were set at 20 bar and 110 °C, respectively. After completion of the reaction and cooling the reactor, two phases, an organic phase containing unreacted cyclohexane and the liquid products, and a solid phase containing the catalyst and adipic acid were separated by filter paper. Then, ethanol was added to the solid phase to dissolve adipic acid and the catalyst was separated again by filtration. The organic phase was analyzed quantitatively by GC using p-xylene as internal standard and the components were identified by GC-MS. The yield of adipic acid was defined as the ratio of the weight of adipic acid, as determined by titration, to the weight of adipic acid theoretically produced upon oxidation of cyclohexane. 3. Results and Discussion 3.1. Characterization of the catalyst 3.1.1. FTIR analysis The FTIR spectra of the phosphomolybdic acid (PMA), IL1, IL2, IL3, and IL3/phosphomolybdate catalyst are shown in Figure 1. The IL1 (Figure 1a), IL2 (Figure 1b), and IL3 (Figure 1c) show strong bands centered at 3500 cm-1which are attributed to the –OH stretching vibrations of the water molecules present in the hydrophilic ILs, and the peaks at 3020 to 3060 cm-1 are assigned to the C-H stretching vibrations of the imidazolium rings. The peaks in the region of 2850-3000 cm-1 are attributed to the aliphatic C-H stretching vibrations. The peak at 1158 cm-1 is attributed to out-of-plane C-H bending vibrations of the imidazolium rings40. Also, the peaks centered at 1630, 1560 cm-1 are ascribed to the breathing vibrations of C=N bonds. In the FTIR spectrum of the PMo (Figure 1d) four characteristic bands are observed in 1160, 950, 880 and 800 cm-1 which are due tothe stretching vibrations of (P–O), (Mo=O), and (Mo–O–Mo), respectively and are characteristic of the Keggin unit of the HPA41. As can be seen, in the spectrum of IL3/phosphomolybdate (OIL/HPA) (Figure 1e) the both characteristic bands of the PIL and HPA are clearly visible; only, the peak intensities are slightly decreased. Also, electrostatic interaction between oxygen of the Mo=O bond in the Keggin oxoanion with the positive charge of the imidazolium ring of the surrounding ionic liquids leads to a red shift in peak regions42.



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

Figure 1. IR spectra of (a) IL1, (b) IL2, (c) IL3, (d) PMA, (e)

IL3/phosphomolybdate. 3.1.2. XRD and TG analyses The XRD patterns of IL3, PMo and the IL3/phosphomolybdate catalyst are shown in Figure 2. The broad peaks in the XRD pattern of IL3 (Figure 2a) at 2θ = 26°, 32° and 43° might


Page 8 of 23

Page 9 of 23

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


be interpreted as a semi-crystalline feature of the synthesized poly(ionic liquid). Moreover, in the XRD pattern of the PMo (Figure 2b), the peaks in the range of 2θ = 8-40° clearly demonstrate a crystalline structure for H3PMo12O40 13H2O40. In the case of IL3/phosphomolybdate sample, the composite has lost its crystallinity to a large extent, and formed an amorphous-porous solid (Figure 2c); however, the location of the characteristic peaks is also clear. Thermal gravimetric analysis was also carried out to study the thermal stability of the prepared OIL/HAP catalyst (Figure 3). Thermal analysis of the hybrid organic-inorganic composite showed three stages of weight loss. The first weight loss (2%) in the temperature below 230 °C is related to removal of the absorbed water. The second weight loss (4%) between 380 and 460 °C could be attributed to degradation of alkyl, imidazole and benzyl groups. The third weight loss from 500 to 720°C (7%) was attributed to Keggin oxoanion decomposition. Therefore, it was concluded that about 95 wt% of the catalyst used in each run was HPA, and also the catalyst should have desired stability in the operational condition.

Figure 2. XRD patterns of (a) IL3, (b) PMA, and (c) IL3/phosphomolybdate. 9 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

Page 10 of 23

Figure 3. TGA analysis of the IL3/phosphomolybdate.

3.1.3. FESEM, EDX and TEManalysis The FESEM images of the catalyst are shown in Figure 4a. As can be seen, a uniform aggregate of spherical-shape particles with size range between 50-100 nm has built a porous circumscribed aggregation. Energy-dispersive X-ray (EDX) spectroscopy of the IL3/PMo catalyst was carried out to be evidence for the presence of C, O, N, Mo and very low present of bromide ions in the material (Figure 4b). Through the EDX mapping, one could clearly see that Keggin oxoanion was highly dispersed into the oligomeric ionic liquid (IL3). This successfully confirms well anion exchange of ionic liquid with the Keggin oxoanion and nearly homogeneous distribution of P and Mo on the catalyst. The TEM image (Figure 4c) shows a closer look at the nanoparticles that might reflect the idea of a good dispersion of Keggin oxoanion in the polyionic chains.


Page 11 of 23

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




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 12 of 23

Figure 4. (a) FE-SEM images (b) EDAX spectrum and mapping for analysis phase

information (c) TEM images of IL3/phosphomolybdate. 3.1.4. N2 adsorption–desorption isotherm N2 adsorption–desorption and the BJH pore size distribution graphs depict the textural properties of the IL3/PMo catalyst (Figure 5). A specific surface area of 32 m2/g and a pore volume of 0.38 cm3/g were calculated for the catalyst by BET method (Figure 5a). The nitrogen adsorption–desorption isotherm of the catalyst was recognized as type II according to IUPAC classification which might be classified as a macroporous solid41. The H3 hysteresis loop of IL3/PMo catalyst is characteristic of solids consisting of aggregates or agglomerates of particles forming slit shaped pores with non-uniform size and/or shape42 that are more confirmed by FESEM images (Figure 4a). The pore size distribution of the catalyst was calculated using the BJH method in the range of 1.8-12 nm (Figure 5b).


Page 13 of 23

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


Figure 5. BET curve of IL3/phosphomolybdate (a) Adsorption/desorption isotherm,

(b), BJH information. 3.2. Catalytic Performance Investigations 3.2.1. Effect of temperature Table 1 shows the effect of temperature on oxidation of cyclohexane. The reaction was extremely sensitive to temperature. The conversion of the reaction was very low at 90 °C and 100 °C but by increasing the temperature to 110 °C the conversion reached to 21% with desire selectivity to cyclohexanone (75%) and white crystalline adipic acid (8%). When the temperature increased to 120 °C, although the conversion was increased, but the reaction mixture turned dark and giving a dark solid phase; probably over-oxidation through radical chain reactions causes a series of highly exothermic reactions that ends up to unknown carbonization products which reduces the selectivity of the main products i.e. cyclohexanone and adipic acid. Once that we tried to perform the reaction at 150 °C, when the reactor was opened the only detectable material in the reaction vessel was tar. On the basis of these data, the appropriate temperature was set 110 °C. 13 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

Page 14 of 23

Table 1. Oxidation of cyclohexane on IL3/phosphomolybdate. Selectivity Entry catalyst T (°C) Conv. (%)

A1 K2 CHHP3 Acids Others TOF

1

IL3-Mo

90

1

-

98

-

-

2

10

2

IL3-Mo

100

5

-

98

-

-

2

53

3

IL3-Mo

110

21

-

75

10

8

7

201

4

IL3-Mo

120

32

-

65

2

2

21

307

5

blank

110

N.R

-

-

-

-

-

-

6

IL3

110

N.R

-

-

-

-

-

-

7

HPAa

110

N.R

-

-

-

-

-

-

Reaction conditions: cyclohexane: 5 mL; catalyst: 20 mg; O2 pressure: 20 bar; reaction time: 5 h. a 1 Alcohol; 2 Ketone; 3cyclohexylhydroperoxide; phosphomolybdic acid.

3.2.2. Effect of oxygen pressure The effect of the initial O2 pressure on the cyclohexane oxidation is shown in Table 2. As can be seen, a slight enhancement in the cyclohexane conversion and selectivity to cyclohexanone was detectable but by increasing the oxygen pressure more than 20 bar a brownish reaction mixture was obtained. Therefore, the pressure of oxygen was set to 20 bars. Table 2. Influence of other reaction conditions on oxidation of cyclohexane. Selectivity

O2

Cat.

Time

Conv.

(bar)

(mg)

(h)

(%)

1

20

20

3

5

2

20

10

5

6

-

3

15

20

5

10

4

20

20

5

5

20

20

6

20

50

Entry

A1 K2 CHHP3 Acids Others TOF 40 50

-

-

10

80

98

-

-

2

115

-

60

8

5

27

96

21

-

75

10

8

7

201

8

28

-

65

10

5

20

168

5

24

15 60

7

8

10

92

1

2

Reaction conditions: amount of cyclohexane: 5 mL; T=110 °C;

hydroperoxide. 14 ACS Paragon Plus Environment

Alcohol;

Ketone;

3

cyclohexyl

Page 15 of 23

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


3.2.3. Effect of reaction time The effect of reaction time on the oxidation reaction is summarized in Table 2. The cyclohexane conversion and cyclohexanone selectivity were reached to their maximum values after 5 h. Perhaps, the time has shown the same effect of increasing the temperature because by increasing the reaction time to 8 h, over-oxidation did occur and the reaction mixture was brownish after this period of time. 3.2.4. The effect of amount of catalyst The effect of the amount of catalyst on cyclohexane oxidation is summered in Table 2 too. A range of 10-50 mg of the catalyst was used in this study while the best conversion and selectivity to cyclohexanone obtained when 20 mg catalyst was used. We have used a magnetic stirrer in these reactions because of our limitations but definitely rate of mixing could have drastic effect on the conversion of this reaction. 3.2.5. Reusability of the catalyst By finishing the reaction and separating the adipic acid from the catalyst by dissolving it in ethanol, and filtration, the catalyst washed three times with ethanol, once with chloroform and dried at room temperature. As shown in Figure 6, the catalytic activity of the regenerated catalyst was subsequently obtained after 5 runs. It is clear that the conversion and selectivity to the desired products did not changed too much by the repeated runs. The amount of molybdenum in the fresh catalyst and after 5 runs was determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES); difference in Mo loading was in the range of experimental error. Therefore, we think that decrease in activity of the catalyst could be due to the tar formation around the active sites. These results demonstrate that the IL3/HPA catalyst has a desirable reusability. The data in Table 3 compares the results obtained over IL3/HPA catalyst and other catalysts used for this reaction in similar conditions. As shown in Table 3, the catalyst used in the present study shows a reasonable activity in respect of oxidation of cyclohexane. The extension of application of this type of modified heteropoly acids to different oxidation reactions is currently under investigation in our laboratory.



25 21

21 19

19

19

Run 3

Run 4

run 5

20

Conversion %

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 16 of 23

15

10

5

0 Run 1

Run 2

Fig. 6. Recycling of the catalyst. Table 3. Catalytic activities of IL-3/HPA and some previously reported catalysts in the cyclohexane oxidation. Entry

Catalyst

1 2 3 4 5 6 7 8 9 10

Mn2CO3Ox Au@TiO2/MCM41 Mn-TNPP/BM Au-Pd/MgO Au/HAP Co-SAOP-5-0.6 Ni-ZSM-5 Co3O4 Co-ZSM-5 IL3/HPA

Amount of catalyst/mg 50 20 50 6 100 20 200 200 200 20

Atmosphere/MPa

conversion

Reference

O2/0.5 O2/1 O2/1 O2/3 O2/1 O2/1 O2/1 O2/1 O2/1 O2/2

10.4 10.3 14.0 11 14.9 19.1 2.4 11.1 7.2 21

46 47 48 49 50 51 52 52 52 this work

3.3. The proposed mechanism The nature of polyoxometalates such as phosphomolybdate oxoanion (PMo) in solution cannot be predicted because of existence of some pH-dependent equilibria. Therefore, there is not really a good knowledge about the mechanism of oxidation reactions that are catalyzed by Keggin oxoanion although different attempts have been reported in the literature43,44. In the prepared catalyst, i.e., IL3/PMO the oxoanion can be either anion, H-anion or partially protonated anion that by breaking a C-H bond producing a cyclohexyl radical, a proton and the 16 ACS Paragon Plus Environment

Page 17 of 23

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


reduced oxoanion. In the next step, molecular oxygen through a series of radical chain reactions oxidizes the cyclohexane; also oxidize the reduced oxoanion and returning it to the redox cycle. 4. CONCLUSION OIL/HPA composite was synthesized by using simple procedure and its nano-size was demonstrated by FESEM and TEM techniques. Results of catalytic activity of the PIL/HPA composite for oxidation of cyclohexane by molecular oxygen are very interesting in optimum conditions. The maximum conversion of cyclohexane was 21% and the selectivity for the main product i.e., cyclohexanone was 75%. We have succeeded in conversion of two important homogeneous catalysts for oxidation of cyclohexane into a single heterogeneous catalyst by conducting a smart reaction between them which creates synergy between the two catalysts without causing previous problems as a result of heterogeneity of the catalysts. The results of this study can be used to guide other oxidative reactions in organic chemistry.

Supporting information The Supporting Information file contains: Figures S1-S4 reporting 1H NMR spectra of 1,2di(1H-imidazol-1-yl)ethane, IL1, IL2, and IL3.

AUTHORS INFORMATION Corresponding Author *Tel. and FAX +98-031-33913254 (2350) E-Mail: [email protected] WEB site: www.ghiaci.iut.ac.ir ORCID The authors declare no competing financial interest.

ACKNOWLEDGMENTS We are grateful for the financial support from the Research Council of Isfahan University of Technology (IUT), Iran.



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 23

References (1) Schuchardt, U.; Carvalho, W.A.; Spinace, E.V. Why is it interesting to Study Cyclohexane Oxidation. Synlett. 1993, 713. (2) Hao, J.; Cheng, H.; Wang, H.; Cai, S.; Zhao, F. Oxidation of cyclohexane-A

significant impactof stainless steel reactor wall. J. Mol. Catal. A: Chem. 2007, 271, 42. (3) Wu, M.; Zhan, W.; Guo, Y.; Guo, Y.; Wang, Y.; Wang, L.; Lu, G. An effective Mn-Co mixed oxide catalyst for the solvent-free selective oxidation of cyclohexane with molecular oxygen. Appl. Catal. A: Gen. 2016, 523, 97. (4) Berezin, I.V.; Denisov, E.T.; Emanuel, N.M. The Oxidation of Cyclohexane, Pergamon Press. Oxford, 1966, 294 pages. (5) Karande, R.; Debor, L.; Salamanca, D.; Bogdahn, F.; Engesser, K.H.; Buehler, K.; Schmid, A. Continuous Cyclohexane Oxidation to Cyclohexanol Using a Novel Cytochrome P450Monooxygenase From Acid ovorax sp. CHX100 in Recombinant P. taiwanensis VLB120 Biofilms. Biotechnol. Bioeng. 2016, 113, 52. (6) Almeida, A.R.; Moulijn, J.A.; Mul, G. Photocatalytic Oxidation of Cyclohexane over TiO2: Evidence for a Mars-van Krevelen Mechanism. J. Phys. Chem. C, 2011, 115, 1330. (7) Xiao, Z.; Zhan, W.; Guo, Y.; Gong, X.; Lu, G. The synthesis of Co-doped SAPO-5 molecular sieve and its performance in the oxidation of cyclohexane with molecular oxygen. Chin. J. Catal. 2016, 37, 273. (8) Kuznetsova, L.I.; Kuznetsova, N.I.; Koscheeva, O.S. Promoting effect of ionic liquids in liquid-phase oxidation of cyclohexane with a mixture of О2 and Н2. Catal. Commun. 2017, 88, 50. (9) Mizuno,N.; Kamata, K.; Yamaguchi, K. Oxidative functional group transformations with hydrogen peroxide catalyzed by a di-vanadium-substituted phosphotungstate. Catal. Today. 2012, 185, 157. (10) Santra, C.; Shah, S.; Mondal, A.; Pandey, J.K.; Panda, A.B.; Maity, S.; Chowdhury, B. Synthesis, characterization of VPO catalyst dispersed on mesoporous silica surface and catalytic activity for cyclohexane oxidation reaction. Microp.and Mesop. Mat. 2016, 223, 121. (11) Ferre, F.T.; Resende, J.A.L.C.; Schultz, J.; Mangrich, A.S.; Faria, R.B.; Rocha, A.B.; Scarpellini, M. Catalytic promiscuity of mononuclear copper(II) complexes in mild conditions: Catechol and cyclohexane oxidations. Polyhedron 2017, 123, 293. 18 ACS Paragon Plus Environment

Page 19 of 23

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


(12) Ribeiro, A.P.C.; Martin, L. M.D.R.S.; Hazra, S.; Pombeiro, A.J.L. Catalytic oxidation of cyclohexane with hydrogen peroxide and a tetracopper(II) complex in an ionic liquid. C.R. Chimie. 2015, 18, 758. (13) Hu, Y.; Wang, J.; Zhao, R.; Liu, Y.; Liu, R.; Li, Y. Catalytic Oxidation of Cyclohexane over ZSM-5 Catalyst in N-alkyl-N-methylimidazolium Ionic Liquids. Chin. J. Chem. Eng. 2009, 17, 407. (14) Pereira, R.; Rufo, M.; Schuchardt, U. Copper(II) Catalyzed Oxidation of Cyclohexane by tert-Butyl Hydroperoxide. J. Braz. Chem. Soc. 1994, 5, 83. (15) Pinto, V.H.A.; Reboucas, J.S.; Ucoski, G.M.; de Faria, E.H.; Ferreira, B.F.; Gil, R.A.S.S.; Nakagaki,

S.

Mn-porphyrins

immobilized

on

non-modified

and

chloropropyl-

functionalized mesoporous silica SBA-15 as catalysts for cyclohexane oxidation. Appl. Catal. A: Gen. 2016, 526, 9. (16) Nesterova, O.V.; Nesterov, D.S.; Krogul-Sobczak, A.; da Silva, M.F.C.G.; Pombeiro, A.J.L. Synthesis, crystal structures and catalytic activity of Cu(II) and Mn(III)Schiff base complexes: Influence of additives on the oxidation catalysis of cyclohexane and 1phenylehanol. J. Mol. Catal. A: Chem. 2017, 426, 506. (17) Wang, T.; She, Y.; Fu, H.; Li, H. Selective cyclohexane oxidation catalyzed by manganese porphyrins and co-catalysts. Catal. Today. 2016, 264, 185. (18) Yang, D.; Liu, M.; Zhao, W.; Gao, L.A. Comparative oxidation of cyclohexane catalyzed by N-hydroxyphthalimide and ZSM-5 supported Co(II), Mn(II), Ni(II), Zn(II), Fe(III) with molecular oxygen in the absence of solvents and reductants. Catal. Commun. 2008, 9, 2407. (19) Wu, P.; Bai, P.; Yan, Z.; Zhao, G.X.S. Gold nanoparticles supported on mesoporous silica: origin of high activity and role of Au NPs in selective oxidation of cyclohexane. Sci. Reports, 2016, 6, 18817. (20) Dai, C.; Zhang, J.; Huang, C.; Lei, Z. Ionic Liquids in Selective Oxidation: Catalysts and Solvents. Chem. Rev. 2017, 117, 6929. (21) Mizuno, N.; Misono, M. Heteropoly acid catalysts Curr. Opin. Solid State. Mater. Sci. 1997, 2, 84. (22) Kozhevnikov, I.V. Catalysis by Heteropoly Acids and Multicomponent Polyoxometalatesin Liquid-Phase Reactions. Chem. Rev. 1998, 98, 171. 19 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

Page 20 of 23

(23) Shringarpure, P.; Patel, A. Undecatungstophospho(aqua)ruthenate(II): One pot synthesis, characterization and non-solvent liquid phase aerobic oxidation of alkenes. Inorg. Chim. Acta. 2009, 362, 3796. (24) Welton, T. Ionic liquids in catalysis, Coord. Chem. Rev. 2004, 248, 2459. (25) Karthikeyan, P.; Bhagat, P.R.; Kumar, S.S.A. novel, green 1-glycyl-3-methyl imidazolium chloride–copper(II)complex catalyzed C–H oxidation of alkyl benzene and cyclohexane. Chin. Chem. Lett. 2012, 23, 681. (26) Restrepo, J.; Lozano, P.; Burguete, M.I.; García-Verdugo, E.; Luis, S.V. Gold nanoparticles immobilized onto supported ionic liquid-like phases for microwave phenylethanol oxidation in water. Catal. Today. 2015, 255, 97. (27) Cognigni, A.; Kampichler, S.; Bica, K. Surface-active ionic liquids in catalysis: Impact of structure and concentration on the aerobic oxidation of octanol in water. J. Colloid Interf. Sci. 2017, 492, 136. (28) Jiang, W.; Zheng, D.; Xun, S.; Qin, Y.; Lu, Q.; Zhu, W.; Li, H. Polyoxometalate-based ionic liquid

supported

on

graphite

carboninduced

solvent-free

ultra-deep

oxidative

desulfurization of model fuels. Fuel, 2017, 190, 1. (29) Elhamifar, D.; Yari, O.; Karimi, B. Highly Ordered Mesoporous Organosilica–titania with Ionic Liquid Framework as Very Efficient Nanocatalystfor Green Oxidation of Alcohols. J. Colloid Interf. Sci. 2017, 500, 212. (30) Azimzadeh, H.; Akbari, A.; Omidkhah, M. Catalytic oxidative desulfurization performance of immobilized NMP FeCl3 ionic liquid on γ-Al2O3 support. Chem. Eng. J. 2017, 320, 189. (31) Wu, P.; Dai, B.; Chao, Y.; Wu, Y.; Jiang, W.; Li, C.; Zhu, W.; Li, H. Graphene-like boron nitride anchored Brönsted acid ionic liquids asmetal-free catalyst for advanced oxidation process. Mol. Catal. 2017, 436, 53. (32) Guterman, R.; Ambrogi, M.; Yuan, J. Harnessing Poly(ionic liquid)s for Sensing Applications. Macromol. Rapid Commun. 2016, 1. (33) Eftekharia, A.; Saito, T. Synthesis and properties of polymerized ionic liquids. Euro. Polym. J. 2017, 90, 245. (34) Mao, H.; Liang, J.; Zhang, H.; Pei, Q.; Liu, D.; Wu, S.; Zhang, Y.; Song, X.M. Poly-ionic liquids functionalized polypyrrole/graphene oxide nanosheets for electrochemical sensor to detect dopamine in the presence of ascorbic acid. Biosens. Bioelectr. 2015, 70, 289. 20 ACS Paragon Plus Environment

Page 21 of 23

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


(35) Balinge, K.R.; Khiratkar, A.G.; Krishnamurthy, M.; Patle, D.S.; Cheralathan, K.K.; Bhagat, P.R. Deep-desulfurization of the petroleum diesel using the heterogeneous carboxyl functionalized poly-ionic liquid. Resour. Effic. Technol. 2016, 2, 105. (36) Zarca, G.; Horne, W.J.; Ortiz, I.; Urtiaga, A.; Bara, J.E. Synthesis and gas separation properties of poly(ionicliquid)-ionic Liquid composite membranes containing a copper salt. J. Membr. Sci. 2016, 515, 109. (37) Shaplov, A.S.; Marcilla, R.; Mecerreyes, D. Recent Advances in Innovative Polymer Electrolytes based on Poly(ionic liquid)s. Electrochim. Acta. 2015, 175, 18. (38) Mi, H.; Jiang, Z.; Kong, J. Hydrophobic Poly(ionic liquid) for Highly Effective Separation of Methyl Blue and Chromium Ions from Water. Polymers. 2013, 5, 1203. (39) Bo, X.; Bai, J.; Ju, J.; Guo, L. Highly dispersed Pt nanoparticles supported on poly(ionic liquids) derived hollow carbon spheres for methanol oxidation. J. Power Sources. 2011, 196, 8360. (40) Surmacki, J.; Brozek-Pluska, B.; Kordek, R.; Abramczyk, H. The lipid-reactive oxygen species pheno-type of breast cancer. Raman spectroscopy and mapping, PCA and PLSDA for invasive ductal carcinoma and invasive lobular carcinoma. Molecular tumorigenic mechanisms beyond Warburg effect. Analyst. 2015, 140, 2121. (41) Mansilla, D.S.; Torviso, M.R.; Alesso, E.N.; Vazquez, P.G.; Caceres, C.V. Synthesis and characterization of copper and aluminum salts of H3PMo12O40 for their use as catalysts in the eco-friendly synthesis of chromanes. Appl. Catal. A: Gen. 2010, 375, 196. (42) Chen, D.; Deng, Z.; Liu, X.; Wang. Synthesis, Characterization and Catalytic Polymerization of N-Methyl Imidazolium Phosphotungstic Catalyst. Catalysts 2015, 5, 1862. (43) Leofanti, G.G.; Padovan, M.; Tozzola, G. and Venturelli, B. Surface area and pore texture of catalysts. Catal. Today. 1998, 41, 207. (44) Okuhara, T.; Mizuno, N.; Misono, M. Catalytic chemistry of heteropoly compounds. Adv. Catal. 1996, 41, 113. (45) Hill, C.L.; Prosser-McCartha, C.M. Homogeneous catalysis by transition metal oxygen anion clusters, Coord. Chem. Rev. 1995, 143, 407.



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 22 of 23

(46) Wu, M.; Zhan, W.; Guo, Ya.; Guo, Y.; Wang, Y.; Wang, L.; Lu.; G. An effective Mn-Co mixed oxide catalyst for the solvent-free selective oxidation of cyclohexane with molecular oxygen. Appl. Catal. A: Gen. 2016, 523, 97. (47) Zhou, J.; Yang, X.; Wang, Y.; Chen, W. An efficient oxidation of cyclohexane over Au@TiO2/MCM-41 catalyst prepared by photocatalytic reduction method using molecular oxygen as oxidant. Catal. Commun. 2014, 46, 228. (48) Feng, Z.; Xie, Y.; Hao, F.; Liu, P.; Luo, H. Catalytic oxidation of cyclohexane to KA oil by zinc oxide supported manganese 5,10,15,20-tetrakis(4-nitrophenyl)porphyrin. J. Mol. Catal. A: Chem. 2015, 410, 221. (49) Liu, X.; Conte, M.; Sankar, M.; He, Q.; Murphy, D.M.; Morgan, D.; Jenkis, R.L.; Knight. D.; Whiston. K.; Kiely, C.J.; Hutching, G.J. Liquid phase oxidation of cyclohexane using bimetallic Au–Pd/MgO catalysts. Appl. Catal. A: Gen. 2015, 504, 373. (50) Liu, Y.; Tsunoyama, H.; Akita, T.; Xie, S.; Tsukuda, T. Aerobic Oxidation of Cyclohexane Catalyzed by Size-Controlled Au Clusters on Hydroxyapatite: Size Effect in the Sub-2 nm Regime. ACS Catal. 2012, 1, 2. (51) Xiao, Z.; Zhan, W.; Guo, Y. A.; Guo, Y.; Gong, X.; Lu, G. The synthesis of Co‐doped SAPO‐5 molecular sieve and its performance in the oxidation of cyclohexane with molecular oxygen. Chin. J. Catal. 2016, 37, 273. (52) Zhang, J.L.; Yu, A.A.; Zhou, D.; Xia, Q.H.; Selective oxidation of cyclohexane to KA-oil with oxygen over active Co3O4 in a solvent-free system. Indian J. Chem. Section. A. 2012, 51, 420.


Page 23 of 23

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


For Table of Contents Only 180x138mm (300 x 300 DPI)

ACS Paragon Plus Environment

Heteropoly

Recommend Documents