Highly selective epoxidation of olefins using vanadium (IV) schiff base- amine-tagged graphene oxide composite

Journal Pre-proof Highly selective epoxidation of olefins using vanadium (IV) schiff baseamine-tagged graphene oxide composite Hassan M.A. Hassan, Mohamed A. Betiha, E.A. El-Sharkawy, Reda F.M. Elshaarawy, N.B. El-Assy, Amr A. Essawy, A.M. Tolba, Abdelrahman M. Rabie

PII:

S0927-7757(20)30113-8

DOI:

https://doi.org/10.1016/j.colsurfa.2020.124520

Reference:

COLSUA 124520

To appear in:

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Received Date:

25 December 2019

Revised Date:

24 January 2020

Accepted Date:

29 January 2020

Please cite this article as: Hassan HMA, Betiha MA, El-Sharkawy EA, Elshaarawy RFM, El-Assy NB, Essawy AA, Tolba AM, Rabie AM, Highly selective epoxidation of olefins using vanadium (IV) schiff base- amine-tagged graphene oxide composite, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2020), doi: https://doi.org/10.1016/j.colsurfa.2020.124520

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Highly selective epoxidation of olefins using vanadium (IV) schiff baseamine-tagged graphene oxide composite Hassan M. A. Hassana,b**, Mohamed A. Betihac,d***, E. A. El-Sharkawyb, Reda F. M. Elshaarawyb, N. B. El-Assyf, Amr A. Essawya,f, A. M. Tolbae, Abdelrahman M. Rabiec* a

Department of Chemistry, College of Science, Jouf University, PO Box 2014, Sakaka, Saudi Arabia Department of Chemistry, Faculty of Science, Suez University, Suez, Egypt c Egyptian Petroleum Research Institute, Cairo 11727, Nasr city-Cairo d Egypt Nanotechnology Center (EGNC), Cairo University, Egypt e Department of Chemistry, Faculty of Science, Arish University, Arish, Egypt f Chemistry Department, Faculty of Science, Fayoum University, 63514 Fayoum, Egypt b

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*Corresponding author. Fax: +20 222747433 E-mail addresses: (Abdelrahman M. Rabie): [email protected] (Hassan M. A. Hassan): [email protected] (Mohamed A. Betiha) : [email protected]

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Graphical abstract

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cyclooctene

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1-Octene

Styrene

Graphene functionalized vanadium (IV) Schiff-base Complex

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Epoxides

Highlights

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 Preparation and characterization of aminopropylsilane–GO (NH2GO) is reported.  Vanadium complex incorporated Schiff base ligand (catalyst) was synthesized.  Catalyst is used for epoxidation of alkenes using NaHCO3/H2O2 system.  Catalyst show good catalytic activity (98%) in the oxidation of olefins to epoxides.  The catalyst shows high durability and selectivity.

Abstract

Catalysts are of great importance in the petrochemical industry, so the design

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and synthesis of heterogeneous catalysts for this purpose is the most important field in catalysts. In this context, 3-aminopropyltriethoxysilane was grafted into

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graphene oxide via the condensation reaction, and then the amine terminal groups were utilized to produce azomethine (–C=N–) group using the 3-formyl 4-hydroxy

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benzoic acid compound. The post-modification approach was used to fabricate the novel vanadium (IV) Schiff-base complex. Characterization was carried out to achieve chemical composition and physical properties such as Raman spectroscopy, FTIR, XRD, XPS, elementary analysis, TGA, FESEM-EDX, AFM, HRTEM, and nitrogen sorption have been performed to realize the grafting vanadium (IV) Schiff base complex on GO. The value of the prepared material as a catalyst was examined for the epoxidation of cyclohexene, cyclooctene, 1-

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octene, styrene, allyl bromide, and ally alcohol, and styrene. Also, the impact of pH, time, temperature, and catalyst dose was observed. The results showed that the novel catalyst for the epoxidation reaction showed distinct catalytic efficiency. Besides, owing to the strong interaction between the V(IV) ions and chelating moieties in GO, the novel catalyst showed outstanding recycling effectiveness after being used for six subsequent cycles. Keywords: Green Epoxidation; Graphene oxide complex; Schiff base; grafting; heterogeneous catalyst.

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1. Introduction

Epoxides are considered one of the most critical intermediate chemicals in the field of industrial chemistry because of their high commercial importance [1, 2]. They are used in the production of detergents, paints, pharmaceuticals,

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pesticides, polymers, surfactants, corrosion inhibitors, textiles, and cosmetics

[3]. Therefore, efforts are being made to develop or produce catalysts that are

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effective in providing epoxides [4-7]. Schiff bases are multilateral ligands that can join transition metal ions to create impressively coordinated complexes for

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different applications comprising biological research [8-11], chemical analysis [12], optics [13], magnetic chemistry [14], photophysical studies [15] and

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catalysis [16].

Mostly, the metal Schiff base catalysts are needed to improve the performance of hydrogen peroxide as an oxidant for alkene epoxidation [17-

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19]. Among transition metal complexes, expanded research has performed on vanadium Schiff base complexes owing to its amazing structural features and

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catalytic purpose [20-22]. In the catalytic oxidation process, the preference of high oxidation state vanadium is promoted by taking into account (a) the different coordination numbers, (b) the facile interchangeable of the oxidation states of vanadium (where V (IV) and V (V) are the most stable ones), (c) the Lewis acid characteristics of the vanadium central ion, and (d) the high oxygen affinity of the metal ion [23, 24].

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Considerable development has performed with an assortment of homogeneous vanadium complexes for epoxidation reactions [25-27]. In spite of this, homogeneous catalysis offers outstanding catalytic performance and selectivity; however, dealing with the homogenous catalyst is associated with separation and recyclable problems from the reaction mixture. Therefore, heterogeneous catalysts fasten on the applying of nano-sized vanadium oxide nanoparticles are well known, owing to their charming properties as facile separation and recovery. However, nano-sized vanadium oxide nanoparticles

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expose to agglomerate, which results in catalyst deactivation. An enthusiastic approach to solving this problem could be the design of heterogeneous catalysts

where the vanadium Schiff base complex grafted on solid supports such as

organic polymers [28], mesoporous silica [5], zeolites [29], clays [30] and

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boehmite [31]. These solid supports can distribute the active sites of the catalyst

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and assist the catalytic efficiency and recovery.

Currently, carbon-based materials in many fields receive unusual

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attention because they derive from easily accessible natural sources and are easy to address with functional groups that enable them to use in different areas such as environmental catalysis [32-34], photocatalysis [35, 36], epoxidation

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reaction [37] energy storage [38], and biological uses [39]. Graphene oxide and reduced graphene occupy a significant position in the interests of researchers,

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as it has two-dimensional graphene oxide (GO) as carbon-based materials with a hexagonal lattice of sp2-hybridized carbon atoms display unrivaled features

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like high surface area, distinct adsorption capacity, and stability [40, 41]. Furthermore, GO containing different functional moieties on the edges and basal planes such as carboxyl, epoxy, and hydroxyl groups, which render the surface of GO with different materials such as metal complexes and nanoparticles more facilely [32, 42-44]. Among different ligands, Schiff base ligands have known as appropriate ones for stabilizing various metals owing to the electronic features 4

and facile synthesis. Recently, many active and well-tailored Schiff base-metal catalysts have progressed [23, 24, 45]. It would, therefore, be attractive to create an extremely effective recyclable catalyst based on the vanadium Schiff base complex for the epoxidation reaction. Continuing research into the synthesis of heterogeneous catalytic constituents for epoxidation reactions, we reported for the first time a catalyst consisting of the vanadium-Schiff base complex resulted from the reaction of 3-formyl-4-hydroxybenzoic acid with 3-aminopropyltriethoxysilane, which has

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already been fixed on GO. The vanadium-Schiff-base complex is used as a catalyst for epoxidation of various olefin substrates. This system is not limited

to the advantages resulting from the characteristics of graphene oxide as

nanostructure support, but also from the presence of vanadium complex on the

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GO surface that is responsible for good catalytic activity.

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2. Experimental 2.1 Materials

All materials include 3-aminopropyltriethoxysilane (3-APTES, 99%),

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graphite, toluene (anhydrous, 99.8%), ethanol (≥99.8%), (sulfuric acid (99.99%), phosphoric acid (85 wt. % in H2O), KMnO4 (≥99.0%), hydrogen peroxide (30 wt.

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% in H2O), 3-formyl-4-hydroxybenzoic acid (97%), cyclohexene (≥99.0%), and sodium bicarbonate (≥99.7%) are purchased from Sigma Aldrich and used without any purification.

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2.2 Characterization

On Bruker Vector 22 spectrometer, the FTIR spectra of the prepared

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samples (KBr disc) was registered in the 400-4000 cm-1 range. On Shimadzu 60 thermal analyzer, the thermal analysis was recorded under a stream of nitrogen (20 ml/min) and heating rate of 5 °C/min from ambient temperature to 1000 °C. The elemental analysis was determined on a CHN-analyzer (2400 PerkinElmer, USA). Powder XRD patterns were performed on an advanced X-ray diffractometer (X’Pert Philips Materials Research Diffractometer), using Cu-Kα line as a

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radiation source (λ =1.54056 Å). The surface morphologies and EDX analyses were examined by scanning electron microscopy (SEM) (JEOL JSM-7100F). High-resolution transmission electron microscopy (HRTEM) images were obtained from a JEOL model 2100 instrument operating at 200 kV. XPS analysis was performed on a JEOL JPS-9200 X-ray photoelectron spectrometer.

2.3 Synthesis

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2.3.1 Synthesis of graphene oxide (GO) Modified Hummers method, with some minor modifications, was adopted

to prepare graphene oxide, GO, using graphite powder [46]. In this method, 27 ml

of concentrated sulfuric acid is mixed with 3 ml of phosphoric acid under stirring,

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and then 0.29 g of graphite is added to the mixture. After stirring the mixture for a quarter of an hour, the KMnO4 compound (1.35 g) is added portion-by-portion

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with great care to avoid the sudden rise of temperature or gas retention, and after complete addition, the temperature is elevated to 50 °C. The stirring continues until

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the dark green is seen (this step needs about three hours). Then, the mixture is transferred to 30 ml ice containing 1.5 ml H2O2, and the heating was elevated slowly to remove the remaining permanganate. After centrifugal separation, 30 mL

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of hydrochloric acid and distilled water (1: 4, V/V) are added. Finally, the solid was soaked in diethyl ether for a day, filtered, and dried at 50 °C overnight.

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2.3.2 Synthesis of amino-functionalized graphene oxide (NH2-GO) Amino-functionalized graphene oxide has been synthesized by grafting 3-

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aminopropyltriethoxysilane (3-APTES) on graphene oxide to hydroxyl and carboxylic moieties [47]. Typically, 100 mg of graphene oxide was dispersed in 50 mL anhydrous toluene with an ultrasonic technique. Afterward, 2.3 mmol of 3APTES was added to the suspended GO solution, and the mixture was refluxed at 110 °C under N2 atmosphere for 24 h. The product was centrifuged and the solid was washed with toluene 4 times to remove non-reacted APTES. The functionalized graphene oxide, known as NH2-GO, was dried in a 50 °C. 6

2.3.3 Synthesis of vanadium (IV) Schiff base Using an ultrasonic bath, the NH2-GO (500 mg) was suspended in 100 mL ethanol and then added 1.0 g of 3-formyl-4-hydroxybenzoic acid. Under N2 gas flow, the reaction mixture was refluxed for 8 h. The resulting powder was separated after completion of the reaction and rinsed by ethanol to eliminate the undigested reagent. Finally, 0.5 g of the as-prepared GO- immobilized Schiff base was distributed in ethanol and refluxed under a nitrogen environment (Scheme 1) for 24 hours with 1.0 g vanadium sulfate (VOSO4). After the product was separated

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and washed with methanol followed by dry in an oven at 50 °C. The black powder product complex was used as a catalyst for the epoxidation of alkenes. 2.4 Epoxidation reaction

The catalytic reaction was conducted to produce 1,2-cyclohexanediolin Pyrex

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triple-neck flask equipped with nitrogen gas inlet and condenser. After obtaining

the heat of the set degree, a mixture of hydrogen peroxide and sodium bicarbonate

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is added drop by drop to the reaction. Temperature, pH (hydrogen peroxide-tosodium bicarbonate ratio), solvent type (acetonitrile, dimethylformamide, and

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ethyl alcohol), and catalyst percent were changed to achieve the highest conversion and selectivity. Each of the previous trials was conducted three times to ensure the accuracy of the data, and the average results were recorded. During the scheduled

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time of each experiment, a gas chromatographic analysis (Agilent 6890-N equipped with DB-17 column), and the different by-products were identified using

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GC-MS Agilent equipment. The possibility of reusing the catalyst more than once, the catalyst was separated and washed with a solvent used several times, dried and

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then reused in the same reaction conditions. Kinetic analysis was done for the understanding of the epoxidation reaction. The cyclohexene conversion and cyclohexene oxide (1,2-epoxycyclohexane)

selectivity were calculated according to the following equations Conversion (Conv, %) =

Cyclohexene oxide (mole) 100 Cyclohexene (mole)

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(1)

Cyclohexene oxide Selectivity (Sel, %) =

Yield of 1,2-epoxy cyclohexane (%) =

Cyclohexene oxide (mole) (mole) 100 (2) Converted cyclohexene (mole)

Conversion (%)  Selectivity (%) 100

(3)

3. Results and discussion 3.1 Characterization of the synthesized materials Table 1 reveals the percentages of elemental analysis like carbon, hydrogen, and nitrogen, detected by CHN analysis of the fabricated samples. The

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amount of nitrogen included in the catalysts was 7.07% upon grafting of amino groups, confirming the successful grafting of amino moieties. Moreover, the vanadium content of 1.2 mmol/g was obtained from ICP-MS analysis. Fig 1a

shows XRD patterns for oxidized graphite (GO), NH2-GO, and the grafted vanadium (IV) Schiff-base complex on GO. It is known that graphite shows a peak

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at 2θ of 26°, corresponding to a d-spacing of 0.343 nm, and this peak is moved

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toward lower 2θ value (11.13°, 0.782 nm) after conducting the Hummers reaction, confirming the success of the oxidation process to a large extent. After the

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silylation of GO by 3-APTES, the peak at 11.13° disappeared to a large extent due to the exfoliation of GO-layers, and a broad peak appeared at about 21.9° due to the presence of amorphous silica. There are no additional peaks that appeared after

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complexation with vanadium species due to the good dispersion of metallic species on the catalyst support. In addition, the continued appearance of the peak at ~4.5° in all samples confirms that the internal structure of graphene has not altered either

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after the silylation or complex formation. To further investigate the grafting of Schiff-base on GO surface, FTIR was carried out. Fig. 1b displays the FT-IR

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spectral analysis of GO, amine-tagged graphene oxide, and vanadium (IV) Schiffbase complex grafted on GO. For GO, three typical peaks at 3443, 1631, and 1032 cm-1 are assigned to oxygenated functional moieties hydroxyl, carbonyl, and epoxy groups, respectively. The shoulder at 3100 cm-1 was associated with C-H aromatic rings stretching in GO structure. The spectrum of amine-tagged GO exhibits additional peaks, which can be relied upon to prove the NH2 moiety

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functionalization process; stretching (N-H) peak at 3442 cm-1 and bending (N-H) peak at 1517 cm-1. Besides, the peaks at 2960 and 2925 cm-1 corresponding to the symmetric and asymmetric CH2 stretching of the APTES alkyl chains [48]. Successful grafting of GO with aminosilane moieties via chemical bonding results in the appearance of peaks at 1111 and 1034 cm-1, which assigned to the Si–O-Si and Si–O–C bonds. The V (IV) Schiff-base complex on GO shows a new peak at ~1620 cm-1 allocated the stretching of the imine (C=N), which confirms the Schiff base’s chemical grafting was on GO surface.

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The Raman spectra of GO and V (IV) Schiff-base complex are shown in Fig. 1c. The Raman spectrum of GO is featured by two main modes (Fig. 1c), the D mode (1352 cm-1) corresponding to the disorder of carbon, and the G mode

(1584 cm-1) assigned to the graphitic lattice (sp2 carbon skeleton). A redshift was

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observed in the G mode in the case of V-complex in comparison with GO,

confirming the successful grafting of the complex on GO sheets. ID/IG is a unique

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feature that can be adopted as a graphitic structure quality measurement where the ID/IG value for defect-free graphene is close to zero [47]. The value of ID/IG for V-

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complex (1.14) rises compared to the GO value (0.88), suggesting an enhanced disturbance in graphene lattice after functionality. The successful synthesis of vanadium (IV) Schiff-base complex/GO was

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further confirmed by XPS (Fig. 2). It primarily demonstrates peaks connected with C1s, N 1s, O 1s, and V 2p, confirming the existence of a metal complex in GO that

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is in excellent agreement with the above results. The XPS spectrum of highresolution C1s (Fig. 2a) shows the existence of three peaks located at 284.4, 286.3,

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and 287.4 eV, respectively corresponding to C=C, C–N, and C=O groups, indicating that GO has been functionalized effectively. The O1s (Fig. 3a) demonstrates the respective carbonyl (C = O), (C-O-C), and hydroxyl (OH) binding energies 530.9, 531.9, and 532.8 eV, respectively. In the high-resolution assessment of N 1s XPS (Fig. 2c), the spectrum is divided into two peaks at 399.9 and 401.6 eV ascribed to amines and imine (C = N), suggesting that the complex was anchored to GO. The V 2p XPS spectrum of vanadium (IV) Schiff-base 9

complex / GO (Fig. 2d) shows peaks at ca. 516.3 eV and 521.9 eV, which can be ascribed to the vanadium (IV) oxidation state in the catalyst. The TGA profiles of GO, amine-tagged graphene oxide, and vanadium (IV) Schiff-base complex/GO are carried out to investigate their thermal stability (Fig. 3). The graphene oxide TG curve points out three featured stages. The first weight loss of approximately 25 percent below 150 °C, owing to the evaporation of water molecules captured in the material. Due to the thermal degradation of oxygencontaining functional groups [49], the second notable weight loss is observed in

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the range of 150-350 °C (30 percent). The third weight loss in the temperature range of 450-650 °C (32 %) is ascribed to the removal of more stable oxygen

functionality, which generally decomposes at greater temperatures [49]. Due to

losing the adsorbed water moieties (8 %), the APTES grafted graphene oxide, and

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the immobilized oxo-vanadium Schiff base catalyst shows a small weight loss around 150 and 180 °C, respectively, but this mass loss is considerably low

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compared to GO. There were two more important mass losses in the aminofunctionalized graphene oxide. The first loss is attributed to the degradation of

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oxygen-functionalities (23 percent) within the range of 250-400 °C, which did not participate in linkage with APTES. Then, another main weight loss begins at 400 °C due to the degradation of APTES molecules (37 %). The V-

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complex/GO catalyst TGA profile shows a significant weight loss (60 %) over a wide 180-600 °C temperature range due to the slow degradation of the Schiff

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base complex [50]. The weight loss above 700 °C for GO and functionalized samples was approximately 95 and 72 percent, respectively.

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The morphology of the prepared samples is studied by field emission

scanning electron microscopy (Fig.4). The grafting of the vanadium complex coordinated to the amine group is obtained in each layer as well as on the edges of the GO sheets with heavy crumpling features with a disordered network (Fig. 4b). EDX analysis (Fig. 4c) displays the peaks for V along with the peaks characteristic for C, O, and Si of V-complex/GO catalyst. On the other hand, the HRTEM image

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of the V (IV) complex reveals that the layered structure of graphene remained unaltered after the grafting of the V (IV) complex (Fig. 4d). AFM analysis provides verification of the surface morphology of the catalytic sample (Fig. 5). It is easy to distinguish between graphene oxide, silica, and vanadium species. The graphene sample appears as abundant flat species while silica accumulates on its surface. Evaluate the thickness of graphene oxide by about 3 nm because amino-silicate acts as a crosslinking agent between the GO layers while silica aggregation was 26 nm. Vanadium complex on silica

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aggregation also shows at the highest silica species in clusters of 50 nm. N2 adsorption-desorption isotherms were performed at 77 K° owing to

investigate the textural properties of GO after the grafting of V (IV) complex (Fig.

6). The surface area was 80 m2/g for GO and decreases to 55 m2/g for the

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synthesized complex. Moreover, all sorption isotherms are Type IV hysteresis of

a mesoporous feature, according to IUPAC classification. Obviously, the

outstanding of the GO stability.

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3.2 Epoxidation-Like Activity

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mesoporous structure was unchanged after the grafting of the complex due to the

The creation of new heterogeneous catalysts that are extremely effective

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reveals a particular interest in the production of fine chemicals. Many attempts are being made in terms of distinct changes to promising durable materials, offering a wide range of surface functionalities, notable characteristics, and unprecedented

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physical characteristics such as porosity, and considerable surface area. Critical difficulties include making complete use of this collection of materials to push for

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sustainable manufacturing. The V(IV) Schiff-base/GO complex was tested in cyclohexene (CHE) oxidation with 2.5 M-NaHCO3/H2O2 as an oxidant in acetonitrile solvent. The results of the epoxidation process of cyclohexene showed that 1,2-epoxy cyclohexane and 1,2-cyclohexanidiol are the main products of the epoxidation process and small amounts of 2-cyclohexenol, 2-cyclohexenone, and 2,3-epoxycyclohexanone compounds as by-products. Figure 7a illustrates the

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conversion of cyclohexene conversion Vs. time at a temperature of 298 K° (room temperature). The cyclohexene conversion is increased by almost constant rate with the reaction time up to 12 h. The cyclohexane conversion reaches ~96% during the 12 h, then the conversion is continuously slowly increased up to ~98 %, and reaching a plateau within 14 h. However, selectivity toward 1,2epoxycyclohexane does not match the behavior of cyclohexene conversion (Figure 7b). Selectivity decreases after 4 h of reaction reaching ~96.5% and then decreases after to 27.1% after 14 h. The inverse relationship with passing time is

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attributed to successive oxidation or hydrolysis of 1,2-epoxycyclohexane into 1,2cyclohexanediol compound. The results indicate that the selectivity value obtained

in 8 h is ~90%, which opposes the conversion of cyclohexane 9.5 %, indicating the efficiency of the V(IV)Schiff-base on a surface of silica doped graphene.

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3.2.1 Influence of contact time

The influence of reaction time and on the yield of cyclohexene epoxidation

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over the V(IV) Schiff-base/GO catalyst (50 mg) using H2O2-NaHCO3 at 298 K⁰ is reported in Fig. 7a. The conversion of cyclohexene was found to be 9.5% in 2 h

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and progressively raised to 29.5% in 4 h, 43% in 8 h, 72% in 10 h, and 56.5% in 16 h. The yield starts to decline after 12 h. Over time, part of the formed 1,2-epoxy

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cyclohexane and turns 1,2- into 1,2-cyclohexanidiol as indicated by GC analysis due to the over oxidation or hydrolysis of the product.

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3.2.2 Influence of reaction temperature It is known that the temperature has a very significant effect on both

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conversion and selectivity, so converting cyclohexene to its epoxide counterpart at temperatures of 283 K°, 298 K°, and 313 K° is plotted in Figure 7a. The high conversion of cyclohexene is obtained at 313 K° compared to other temperature, while selectivity shows an opposite behavior. As the temperature rises, the reaction kinetics increases and consequently, more collision occurs, which leads to a higher rate of reaction or because the heat promotes desorption of reactant from the surface of the catalyst. However, increasing the temperature can promote the

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formation of other secondary compounds that may be undesirable. 2-cyclohexenol,

2-cyclohexenone, and 2,3-epoxycyclohexanone were detected only at 313 K°. Room temperature shows the optimal heat for the reaction. In addition, few control experiments have been conducted to gain more understanding of the mechanism of epoxidation. The reactions were performed in the absence of graphene complex under the mimic conditions; no significant epoxidation reaction was observed. Additional control experiments adopting

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graphene complex in the absence of H2O2- NaHCO3, the results displayed that no epoxidation reaction takes place, indicating that both constituents are needed for the reaction to perform. For better results, the influence of various

parameters such as temperature, contact time, pH, and catalyst dose was

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explored by introducing cyclooctene as a model substrate. The plotting of the yield % of 1,2-epoxy cyclohexane VS. time shows a volcanic curve with the

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3.2.3 Influence of catalyst dose

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highest attainable ratio estimated of ~73 at room temperature after 12 h (Fig.

The impact of the catalyst quantity on cyclohexene epoxidation at 298 K⁰

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and pH 8 after 8 h is shown in Fig. 8a. The dose of the catalyst, V (IV) Schiffbase/GO complex, was started at 10 mg and then increased to 25 mg. The corresponding conversion was found 17% and 46%, respectively. When the

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catalyst amount reached 50 mg, the yield of the epoxide generated risen to a value of ~94 %. This is because of the rise in the number of active catalyst centers

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required to perform the reaction when the catalyst is increased. With a further increase in the catalyst dose from 50 to 100 mg, a significant decrease in the conversion percentage from ~94% to ~67% was detected. Moreover, both selectivity and yield were observed at catalyst amount of 50 mg, that's where selectivity was ~99, and the yield was of ~93 % 3.2.4 Influence of reaction pH

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Firstly, by adding a mixture of H2O2 and NaHCO3 to the mixture of cyclohexene and acetonitrile, an efference is noticed in absence of catalyst, and by analyzing the resulting gas, it was found to be oxygen (disproportionation of hydrogen peroxide into oxygen gas and water), which explains the weakness of the epoxidation process. No such effervescences are observed with the V(IV) Schiff-base/GO complex catalyst. Similar resulted is reported by Escande et al. [51]. The variation of the ratio of H2O2 (30 mmol) to x-mol of NaHCO3 to get pH ranged from 5 to 10 is presented in Fig. 8b. The significance of reaction pH on the

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catalytic epoxidation of cyclooctene at 298 K° is obtained after 8 h at pH 8. The addition of NaHCO3 was entirely effective in the epoxidation reaction catalyzed

by V(IV) Schiff-base/GO complex in order to activate hydrogen peroxide, and

about 1.5 % conversion obtained in the absence of sodium bicarbonate after 2 h.

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When the reaction pH rose from 5 to 7, the yield of cyclohexene oxide achieved

an increase from ~42% to ~73% after 8 h. Moreover, at pH 8, the conversion was

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found a sudden increase of up to ~94%. However, the epoxidation dropped to about ~64 % when the NaHCO3 concentration increased to attain a pH value of 10.

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Bareda et al., [52] have found the same behavior and stated that by increasing the concentration of NaHCO3 leads to an increase in pH 8, which would reduce efficiency towards the epoxidation reaction. Thus, the optimum NaHCO3

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concentration as the buffer medium in the olefins epoxidation by V (IV) Schiffbase/GO catalyst was 0.2.5 mol/L. The selectivity for 1,2-epoxy cyclohexane at

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different pH is polluted in Fig.8b. The volcano-curve with pH may be attributed to the formation of sufficient quantities of HCO4– species at a pH of 8. Yao and

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Richardson [53] pointed out the mixture of bicarbonate and oxygen water shows significant efficiency towards the formation of epoxide due to the fact that peroxymonocarbonate (HCO4−) is the most effective in accelerating reaction. 3.2.5. Effect of the solvents Selection of the appropriate solvent for epoxidation reaction is one of the most important factors affecting the conversion of cyclohexane to the

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corresponding epoxide due to the possibility of adsorption of the solvent on the catalyst surface and thus may change or be affected or impede the reactant to reach active sites on the catalyst surface. The effect of some solvents on the conversion of cyclohexene to cyclohexene epoxide, as well as selectivity is plotted in Fig. 8c. As evident, the highest conversion rate, as well as selectivity, were found using acetonitrile solvent. Upon using ethanol as a solvent, the conversion Vs. time in alcohol is more rapid the other solvent (reaching ~80% in 6h compared to ~47% and 36% in case of using acetonitrile and DMF, respectively), while the selectivity

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toward 1,2-epoxy cyclohexane was ~98.5 % while ~80% and ~71% by DMF and ethyl acetate, respectively. Over time, a sharp drop in selectivity occurs if ethyl

alcohol, maybe due to the alcohol promotes the ring-opening of epoxide [54]. Moreover, it has been reported that the slightly basic solvent like acetonitrile can

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inhibit the conversion of styrene oxide to benzaldehyde [55]. Therefore, the best yield is obtained from using acetonitrile as a solvent.

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3.2.6 Catalytic epoxidation of some olefins

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The conversion and selectivity of some olefins at conditions pH 8, room temperature, and acetonitrile were solvent identified in Table 2. It is clear from the table that the conversion of cyclooctene and cyclohexene is the highest that

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contains an aromatic ring or ally group. The behavior may be due to the steric hindrance or low basicity of the π-electron group or tautomerization effect which

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decreasing the electrophilic cycloaddition in the epoxidation [56]. The conversion of was of 98 % (cyclooctene), 96% (1-octene), 91% (styrene), 77% (allyl bromide),

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and 54% (ally alcohol).

3.2.7 Kinetics and the proposed mechanism Kinetics experiments of the V (IV) Schiff-base/GO catalyst at different temperatures display that pseudo-first-order model can be used to evaluate the catalytic rate as experimentally obtained results can fit well with the calculated data (Fig. 9a). It is, therefore, possible to express the kinetic equation for the reaction as 15

− ln(

𝐶 𝐶𝑜

) = 𝑘𝑡

(4)

where k is the rate constant; t is the time of reaction; Co and C are the initial cyclohexene concentration and cyclohexene concentration at time t, respectively. The k values for the epoxidation process at 10, 25, and 40 °C can be estimated from the slopes of these fitted lines, to be 0.0365, 0.106 and 0.2042 h–1, respectively. In addition, the Arrhenius equation can calculate the activation energy (Ea). 𝐸𝑎

(5)

𝑅𝑇

ro of

ln 𝑘 = ln 𝐴 −

where (R = 8.314 J/(K·mol)) is the gas constant, A is the pre-exponential factor, and T is the temperature of the epoxidation reaction. The Ea value for the

-p

cyclohexene oxidation reaction with the V (IV) Schiff-base/GO catalyst can be

calculated from the slope of the fitted line with respect to lnk versus 1/T (Fig. 9b).

re

The Ea value obtained is 42.2kJ/mol, which is analogous to the values previously disclosed [3, 57]. Based on current results, Scheme 2, proposed a possible reaction

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mechanism for the cyclohexene epoxidation with H2O2-NaHCO3 catalyzed by V(IV) Schiff-base/GO complex. The mechanism undertaken through three feasible approaches (i) Low yield of epoxide was obtained in the presence of NaHCO3/H2O2, which enhance

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the catalytic performance owing to the formation of peroxo-monocarbonate (HCO4-) species [45], (ii) in the presence of H2O2/NaHCO3 and V(IV) Schiff-base/GO complex, the oxo-vanadium [VIV(=O)] complex is oxidized to form the dioxo-vanadium [VV(=O)2]

ur

or oxo-hydroxo-vanadium(V) [VV(=O)(OH)] species. These species further interact with oxidant to generated active fragment of peroxo-vanadium (V), (iii) A nucleophilic

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cyclohexene attack is then carried out on the electrophilic oxygen atom covalently attached to vanadium, resulting in the formation of peroxo-metallocycle, which breaks into cyclohexene epoxide and restore the initial active catalyst. 3.2.8 Recycling of the V(IV) Schiff-base/GO catalyst Catalyst Recycling is an important problem in heterogeneous catalysis because it allows for more economic and environmentally friendly operations. Hence, we studied the Catalyst’s durability in cyclohexene epoxidation (Fig. 10). The catalyst can be

16

isolated by easy filtration after each reaction run, rinsed with ethanol, then dried and reuse immediately for subsequent reactions. Obviously, without significant loss of catalytic efficiency, the catalyst can be reused at least six times. Distinctly, the catalyst offered good durability as well as high efficiency for the epoxidation reaction. Conclusion We have successfully for the first time synthesized a novel vanadium (IV) complex immobilized on GO as a promising catalyst for the epoxidation process. The catalyst is effective for the epoxidation reaction. In particular, the catalytic activity was almost

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unaltered after six times reuse. The outstanding stability and durability of the fabricated catalyst appear to be due to the structural robustness of the GO and great linkage between vanadium and conjugated ligand.

All authors have an equal contribution.

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Declaration of interests

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Authors contribution:

The authors declare that they have no known competing financial interests or personal

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na

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Journal of Chemical Engineering, 13 (2018) e2206.

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ro of -p re lP na

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Scheme 1. Fabrication of V(IV) Schiff-base GO catalyst.

24

ro of

Jo

ur

na

lP

re

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Scheme 2. Reaction mechanism for the epoxidation of cyclooctene by V(IV) Schiff-

25

ro of -p re lP na ur Jo Fig. 1 (a) X-ray diffraction (XRD), (b) FTIR spectra, and (c) Raman spectra of pristine GO and V(IV) Schiff-base/GO complex (the inset is the 2D band of complex)

26

ro of -p re lP

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ur

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Fig. 2 (a) C 1sXPS spectrum (b) O 1sXPS spectrum (c) N 1sXPS spectrum (d) V 2p XPS spectrum of V(IV) Schiff-base/GO complex

27

Weight loss %

100

GO NH2-GO V(IV) Schiff-base/GO

80

60

40

0

100

200

300

400

500



ro of

20

600

Temperature ( C)

700

800

Jo

ur

na

lP

re

-p

Fig. 3. TGA profiles of the samples pristine GO, amine-tagged GO and V(IV) Schiff-base/GO complex

28

ro of -p re lP

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na

Fig. 4. (a) FESEM image of GO (b) FESEM image (c) EDX analysis, and (c) TEM image of V(IV) Schiff-base/GO complex

29

ro of -p

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na

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Fig. 5. AFM of V(IV) Schiff-base/GO complex

30

60

40

20

0

ro of

Volume Adsorbed (cm3/g STP)

GO

80

V(IV)-complex/GO 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/Po)

Jo

ur

na

lP

re

-p

Fig. 6 N2 adsorption-desorption isotherms at 77 K for GO and V(IV) Schiff-base/GO catalyst

31

ro of -p re lP na ur Jo Fig. 7. (a) Conversion % (b) product selectivity % and (c) yield % of cyclohexene by V(IV) Schiff-base/GO complex

32

ro of -p re lP na ur Jo

Fig. 8. Optimum reaction conditions of the catalytic epoxidation of cyclohexene by V(IV) Schiff-base/GO complex (a) influence of catalyst dose, (b) influence of pH, (c) effect of solvent

33

-1.2

(a)

0.0

(b)

-1.6

y = -0.0365x + 0.0193 R² = 0.9486

-0.4

y = -0.1062x + 0.0477 R² = 0.9675

-2.4

y = -5100x + 14.763

ro of

-0.8

y = -0.2042x + 0.0096 R² = 0.9978

lnK

ln C/Co

-2.0

R² = 0.99

-2.8 283 K 298 K 313 K 1

2

3

4

5

6

7

re

0

-3.2

-p

-1.2

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Time (h)

0.0032 0.0033 0.0034 0.0035

1/Tk

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Fig. 9. (a) ln(C/C0) versus reaction time and (b) Arrhenius plot (b) of the cyclohexene oxidation at various temperatures.

base GO catalyst.

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ro of -p re

80

60

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Conversion (%)

100

40

na

20

0

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Run 1

Run 2

Run 3

Run 4

Run 5

Run 6

Number of cycles

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Fig. 10. Recycling experiment of V(IV) Schiff-base/GO catalyst, in epoxidation of cyclohexene

35

Table 1. Elemental analysis of CHN analysis of the prepared samples

Catalyst GO NH2-GO V(IV) Schiff-base GO

C

H

N

33.77% 23.04% 29.39%

2.70% 4.85% 3.39%

0 7.07% 4.60%

98

96

94

12

91

89

81

12

96

93

89.3

16

54

80

43.2

14

77

86

66.22

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ur

na

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8

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Table 2. Catalytic epoxidation of different olefin substrates Olefin Time Conversion Selectivity Yield (%) (h) (%) (%) 10 95 92.5 87.8

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