Selective Oxidation of Benzylic CâH Using Nanoscale Graphene

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Selective Oxidation of Benzylic C-H using Nano-scale Graphene Oxide as Highly Efficient Carbocatalyst: Direct Synthesis of Terephthalic acid Masoud Heidari, alireza sedrpoushan, and Farajollah Mohannazadeh Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.7b00056 • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 22, 2017

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Organic Process Research & Development

Selective Oxidation of Benzylic C-H using Nano-scale Graphene Oxide as Highly Efficient Carbocatalyst: Direct Synthesis of Terephthalic acid.

Masoud Heidaria , Alireza Sedrpoushana,∗, Farajollah Mohannazadeha a

Institute of Industrial Chemistry, Iranian research Organization for Science and Technology (IROST), P.O. Box 33535-111, Tehran, Iran.

∗

Corresponding author: Tel: +98 21 56276031. E-mail address: [email protected] (Alireza Sedrpoushan). 1 ACS Paragon Plus Environment


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Table of Contents Graphic

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Abstract: Nano-scale graphene oxide sheets (NGO), as activated carbocatalysts, were synthesized by reduction size strategy and was used in chemo-selective oxidative conversion of benzylic C-H to the corresponding carboxylic acid. Based on the results of the optimization process of different parameters, 3eq of H2O2 for each C-H group, 100wt% of NGO in aqueous medium, acetone as co-solvent and reaction temperature of 100oC was selected as optimum-parameters. In this optimum condition xylenes and toluene during 24h, with good yield were converted to the corresponding carboxylic acid, and in the cases diphenylmethane and ethylbenzene, these substrates with excellent yield were converted to benzophenone and acetophenone. Keywords: Carbocatalysis, oxidation of hydrocarbons, Synthesis of Terephthalic acid, metal-free catalysis



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1. Introduction: In now days, transformation of hydrocarbons as an important basic chemical raw material to the corresponding carbonyl compounds, especially carboxylic acid groups as value-added oxygenated chemical feed stock products, based on environmental and economic concerns have attracted much attention1–4. In accordance with the wide and increasing application of terephthalic acid family as precursors in the manufacturing the polymeric products (especially polyethylene terephthalate (PET)), oxidation of xylenes to corresponding carboxylic acid groups have earned a particular place among the hydrocarbon family5. Traditional approaches in the direct oxidation of xylenes to corresponding acid groups, have been focused on the usage of transition-metals based catalytic system and/or reagent6–17, among these methods the AMOCO oxidation process of xylenes involving homogeneous Co-Mn-Br catalyst system in the acetic acid medium widely have been used in the chemical industry. But these methods usually have suffered from some or all of the disadvantages such as need to non-environmentally solvents, high temperature, high pressure, bromide promoter, costly equipment and longtime of reaction, in addition to the fundamental disadvantages of heavy metals from both economic and environmental point of view. Therefore, in the direction of green chemistry, it would be highly desirable to develop metal-free catalysts systems for the direct oxidation of xylene to the corresponding carboxylic acid group. Recently the carbocatalyst concept as one of the metal-free alternatives18–29 have been studied in the oxidation reactions, although based on our best knowledge the application of carbocatalyst systems in the oxidation of hydrocabons especially benzene, toluene and xylene (BTX) were very rare30–35. Even, In the cases that the oxidative conversion of BTX have been investigated the conversion yield of reactions were less than the yield of other derivatives of hydrocarbons, also the chemo-selectivity of these protocols weren’t


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toward the carboxylic acid groups31 (especially for xylenes). Therefore, 1) in accordance with the above literature review, 2) due to this confirmed fact that the defects of edges of Nano sized carbon materials (with having different active sites such as lone-unpaired electrons at the zigzag format of sp2 bonds at vacancies of edges) have the peroxidase like ability36–42 and 3) based on the our previous successful experiences in the use of reduction size strategy (for synthesizing active carbocatalyst system based-graphene materials in oxidative conversion of alcohols to corresponding carboxylic acid) (CJC-2016-10-052.R343), in this work were used the reduction size strategy for designing a new active carbocatalyst system in oxidation of hydrocarbones from graphene oxide (GO). By using this strategy, the Nanoscale graphene oxide (NGO) was synthesized and examined in the oxidation of xylenes, toluene, diphenylmethane, ethylbenzene and naphthalene by H2O2. We expected the NGO with having smaller sheets have more edges, higher density of defects and consequently higher catalytic performance to starting GO and other carbonaceous materials. 2. Experimental: 2.1. Synthesis of NGO: The NGO sheets were synthesized by a modified hummers' method44 coupled with a long time sonication45. In details, after adding the 100 ml deionized water to the solid products of hummers' method, the sample was sonicated three times (each time for 6h). After the first and second time the sample was centrifuged at 5000 rpm for 15 min, and after third sonication the sample was centrifuged at 10000 rpm for 20 min to separate large particles. 2.2. Study the chemical and physical properties of synthesized NGO: For determination the chemical map of NGO sheets, the samples of the NGO were investigated by FTIR, UV-visible and Raman spectroscopy. Also the different physical parameters such as 5 ACS Paragon Plus Environment


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lateral dimensions, the height and numbers of layers of NGO sheets were studied by SEM, TEM, AFM and DLS. And at the end of synthesis of NGO, the presence of heavy metal such as Mn arising from KMnO4 using ICP-OES was investigated. 2.3. Oxidative conversion of benzylic C-H by NGO: In order to investigate the catalytic effect of NGO sheets on oxidation of benzylic C-H, oxidative conversion of 1mmol p-xylene was studied in the presence of various concentrations of the NGO sheets from 50 to 200wt%, various amounts of H2O2 (0, 1, 3 and 7eq), at different reaction temperature of 80 and 100oC in the presence co-solvents such as acetone, acetonitrile and ethanol in the aqueous medium. In the first experiment, p-xylene was heated at 100oC in the presence of H2O as solvent, excess amount of H2O2 (7eq) as oxidant and 200wt% NGO for 24h. Then for highlighting the catalytic effect and optimize the concentration of NGO, the p-xylene was heated at 100oC in the presence of 0, 50 and 100wt% of NGO carbocatalyst, and H2O2 as oxidant for 24h. The effect of co-solvents, which can increase the miscibility of benzylic C-H in aqueous medium of catalytic systems, were studied in the oxidative conversion of p-xylene in presence acetone, acetonitrile and ethanol as co-solvent (separately), excess amount of H2O2 as oxidant, 100wt% of NGO at temperature of 100oC for 24h. In following, optimization the concentration of NGO in the presence of acetone as co-solvent was done at p-xylene oxidation in presence 50 and 200wt% of NGO and excess amount of H2O2 at 100oC for 24h. To study the role of H2O2 agent, which can determine whether NGO in addition the catalytic effect have any performance as oxidation reagent or not, the oxidation of p-xylene was also monitored at different concentration of H2O2 (0, 1, 3 and 7eq), in the presence 100wt% NGO, acetone as cosolvent at 100oC for 24h. Finally, the oxidative conversion of p-xylene in optimum condition was studied at a longer period time of 24h, and then the effect of temperature on yield of reaction in


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optimum condition was investigated by oxidation of p-xylene at 80oC in presence H2O2, acetone and 100wt% NGO for 24h. In order to investigate the universality of this protocol in the oxidative conversion of different derivatives of benzylic C-H, we used o-xylene, m-xylene,

ethylbenzene,

toluene,

diphenylmethane and naphthalene as substrate in oxidation reaction by the NGO. The progress of the reaction was monitored by thin layer chromatography (TLC). After the reaction, in the cases of para, ortho and meta xylenes the products were filtered and was characterized by NMR, and in the other cases the products after extraction with CH2Cl2 (3×7 ml) the yield of them were determined by gas chromatography (GC). 3. Result and discussion: 3.1. Characterization of NGO: The results of FTIR and UV-visible spectroscopy for studying the chemical map of NGO (Figure S1a and b) showed the NGO sheets are the oxygen containing functional groups-rich structures, with varied functional groups. In addition, based on the result of Raman spectroscopy (see Figure S1c), it was cleared the NGO sheets except the oxygen functional groups, have a high density of defects on the carbon skeleton of NGO sheets (see supplementary section). The investigation of size and morphology of synthesized NGO was followed by TEM, SEM, AFM and DLS analyze (see Figure 1). Figure 1a, show the TEM image of neat GO as a huge integrated sheet with little wrinkles. Figure 1b, for NGO, in wide and close up window show the sheet like features with lateral dimension mostly less than 100 nm, although these sheets have been accumulated and converted into particle-like structures and clusters. A comparison between the Figures 1a for neat GO and 1b for NGO clearly confirm the high efficiency of used method for reducing the lateral dimension size of GO sheets in this work. Figure 1c, show the SEM



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image of NGO particle-like structures (d

Selective Oxidation of Benzylic CâH Using Nanoscale Graphene

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