Catalytic Behavior of Graphene Oxides for Converting CO2 into Cyclic

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Catalytic behavior of graphene oxides for converting CO2 into cyclic carbonates at one atmospheric pressure Sai Zhang, Huan Zhang, Fangxian Cao, Yuanyuan Ma, and Yongquan Qu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04600 • Publication Date (Web): 07 Feb 2018 Downloaded from http://pubs.acs.org on February 9, 2018

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ACS Sustainable Chemistry & Engineering

Catalytic behavior of graphene oxides for converting CO2 into cyclic carbonates at one atmospheric pressure Sai Zhang, † Huan Zhang, † Fangxian Cao, † Yuanyuan Ma*† and Yongquan Qu*† †

School of Chemical Engineering and Technology and Frontier Institute of Science and

Technology, Xi’an Jiaotong University, Xi’an, 710054, China Address: 99Yanxiang Road, Xi'an, Shaanxi710054, P.R. China * Corresponding author: [email protected] and [email protected] KEYWORDS. Heterogeneous catalysis; graphene oxide; CO2 conversion; cycloaddition; hydroxyl group

ABSTRACT. Catalytic conversion of carbon dioxide (CO2) into various highly value-added chemicals is an important challenge for the effective suppression of CO2 emission and practical utilization of CO2. To search a catalyst for the CO2 conversion under mild conditions is a longterm target. Herein, we reported the as-received graphene oxides (GOs) alone as the efficient and green heterogeneous catalysts for the cycloaddition between epoxides and CO2 at 1.0 atm. As-received GOs enabled the styrene oxide conversion of 97.8% with a 97.4% chemoselectivity

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to phenylethylene carbonate after 12 h reaction. Kinetic and X-ray photoelectron spectroscopy analysis revealed that as-received GOs delivered a strong surface-depended catalytic performance, where the GO catalysts with the highest amount of the oxygen-containing groups exhibited the highest reaction rate. Control experiments indicated that the activation of CO2 realized by DMF was independent on the CO2 pressures.

The activation of epoxide was

achieved by the oxygen-containing groups in the GO catalysts as the surface active sites. The overall catalytic efficiency was strongly depended on the surface properties of various GO catalysts for the cycloaddition reaction. Such unique carbon catalysts provide a cost-effective approach for CO2 conversion under mild conditions.

INTRODUCTION Steadily increased release of carbon dioxide (CO2) from the carbon-based fossil fuels, a major component of the greenhouse gases, has become an important concern for the sustainable society.1-9 The developments of the effective strategies for the transformation and utilization CO2 are of a high interest to avoid the excessive CO2 emission and reach the equilibrium of carbon cycle on our planet. Catalytic conversion of CO2 into the highly value-added chemicals attracts much attention today due to their contribution to balance carbon cycle, reduction of climate changes induced by CO2 emission as well as production of many valuable chemicals.1-3, 5 CO2 can be transformed into many gaseous or liquid fuels, such as CO, CH4, HCOOH, HCHO, CH3OH and other hydrocarbon fuels.1-3, 5, 9-13 As an ideal renewable C1 source, the catalytic transformations of CO2 into the value-added chemicals (carbonates, carbonxylates, carbamates and so on) are also desired.14-17 Among various important transformations of CO2, synthesis of the organic cyclic carbonates, the important polar aprotic solvents, electrolytes in lithium-ion


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batteries and useful monomers for polycarbonates, is considered as the most promising pathway for CO2 utilization at a large scale. Generally, the reactions of CO2 require a high-energy consumption due to its chemical inertness and high bond energy. In this aspect, it's not benign or sustainable since the high energy demands lead to the formation of CO2 again if the fossil fuels are employed as the heat resource.4

Thus, the great challenge in CO2 transformations is overcoming its inherent

thermodynamics stability and kinetic inertness. This topic would be extremely important for the CO2 activation under the mild conditions for the practical applications. Numerous homogeneous catalytic systems have been developed for the catalytic cycloaddition of CO2 and epoxides. Especially, the homogeneous catalytic systems, such as salen complexes,18-19 porphyrin complexes,20-21 amino-phenolate coordinated complexes,22-24 phosphorus ylide,25 ionic liquids systems26-27 and metal complexes,28-31 exhibit the remarkable activity under the mild conditions. The problems for the homogeneous catalysis are their complicated synthesis, intricate difficulty in the catalyst-recycling and exorbitant price of noble metal based catalysts for some catalytic systems, which will limit their practical and/or large-scale applications in industry. Recently, the metal-organic frameworks (MOFs),32-35 grafted graphene oxides,36-38 modified polymers31, 39 and metal oxides40-41 have been reported as the heterogeneous catalysts for the cycloaddition of CO2 and epoxides. The heterogeneous catalysts can effectively overcome the problem of the catalyst separation in the homogeneous catalytic systems. However, the catalytic activity or efficiency of these heterogeneous catalysts is not satisfactory even under high CO2 pressures and reaction temperatures (>120 °C and 3 MPa), as compared with those homogeneous catalysts.35 Besides, the majority of the heterogeneous catalytic systems (MOFs, graphene oxide and modified polymers) require the additional co-catalysts/additives of halide anions to assist their catalytic


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Thus, the development of the greener technologies and metal-free

heterogeneous catalysts that can be operated at the atmospheric pressure or mild reaction conditions in the absence of any co-catalysts or additives is a priority to provide the truly valuable strategies for the cycloaddition reaction between CO2 and epoxides. The oxygen atoms in the structure of CO2 are a weak nucleophilic Lewis base site and the carbon atoms often act as an electrophilic Lewis acid center.10 Therefore, a Lewis acid/surface acidic site or a nucleophile molecule/surface site is necessary and used to attack epoxides or CO2 for the cycloaddition reaction. Owing to the remarkable physical and chemical properties as well as its large specific surface area, graphene oxides (GOs) and their composites have attracted enormous interests as various advanced catalysts.44-46 The surface of GOs is in rich of various reactive oxygen functional groups (-C-OH, -C=O, -COOH, et. al.), which have been recognized as the highly active sites for various organic catalytic reactions including oxidation, reduction, hydrogenation, hydrolysis, ring-opening and coupling reactions.47-55 These surface functional groups of GOs, as the typical acidic or basic sites,48 might exhibit their possibility as the potential heterogeneous catalysts for catalyzing the cycloaddition reaction between CO2 and epoxides. In this study, we report GOs, a readily available and inexpensive commercial material, as an efficient and robust carbon catalyst for the cycloaddition of CO2 and epoxides at one atmospheric CO2 pressure. N,N-dimethylformamide (DMF) as a functional accessory solvent can obviously enhance the catalytic activity of GOs. Kinetic and X-ray photoelectron spectroscopy (XPS) experimental results suggest that the catalytic activity of GO catalysts shows a strong correlation of the amount of the available surface oxygen-containing groups and their performance. Further catalytic mechanism investigations indicate that the activation of CO2 is realized by DMF and


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independent with the CO2 pressures. While, the surface -C-OH species as the major oxygencontaining groups of the GO catalysts is recognized as the main active sites for the activation of epoxides. RESULTS AND DISCUSSION The commercial GOs were washed with water and ethanol alternatively for three times in order to remove the impurities before use. As shown in Figure 1a, the TEM image of asreceived GOs displays a typical two-dimensional layer structure with a size of several micrometers.

XRD pattern shows a broad peak at 10.8o, corresponding to the (002)

characteristics peak of as-received GOs (Figure 1b), similar to the previous report.53 Meanwhile, XPS analysis confirmed that the –C–OH groups with surface fraction of 25.3% is the main oxygen-containing groups in the structure of as-received GOs (Table S1). Catalytic activity of as-received GOs for cycloaddition between CO2 and styrene oxide. The cycloaddition of styrene oxide and CO2 into phenylethylene carbonate was selected as a model reaction to explore the catalytic behaviors of as-received GOs. The optimization of the reaction conditions was shown in Table 1. The cycloaddition reaction could not occur in the absence of GO catalysts due to the natural inertness of CO2 molecule (Entry 1, Table 1). At 140 °C and one atmospheric CO2 pressure, the conversion of styrene oxide was only 12.1% after even 16 h reaction, indicating the slow conversion of styrene oxide in DMF in the absence of any catalysts (Entry 3, Table 1). When 2.5 mg of as-received GOs was added into the catalytic system, the conversion of styrene oxide could reach 57.7% with a 96.6% selectivity of phenylethylene carbonate after 6 h under the identical reaction conditions (Entry 4, Table 1). Trace amount of by-product 1-phenyl-1,2-ethanediol was also detected by the GC-MS and GC analysis. Meanwhile, the conversion would be further enhanced to 75.1% with 5 mg of as-


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received GOs (Entry 7, Table 1). Results suggest that the as-received GOs can serve as the effective catalysts for the cycloaddition reaction between CO2 and epoxides in the absence of any other co-catalysts or additives.

However, the catalytic activity was decreased to 20.3%

conversion of styrene oxide at a low reaction temperature of 120 °C in presence of 2.5 mg of the as-received GO catalysts (Entry 6, Table 1). Therefore, the reaction temperature of 140 °C and 1.0 atm pressure of CO2 were selected as the optimized conditions for the cycloaddition reaction. Under the optimized reaction conditions, a near linear catalytic behavior of the as-received GO catalysts was observed (Figure 2a), where the conversions of styrene oxide were plotted as a function of reaction times. After 12 h, the conversion of styrene oxide reached 97.8% with a 97.4% selectivity to phenylethylene carbonate. Meanwhile, the as-received GO catalysts also exhibited the excellent catalytic recyclability for the cycloadditon reaction. At the end of the catalytic reaction, the as-received GO catalysts can be easily separated from the reaction system by centrifugation and reused for the next cycle without any additional treatments. During the next several cycles, the catalytic actvity can be well maintained at the range of 86.5% to 90.3% with the high selectivity (Figure 2b), domenstrating the high catalytic stability of the as-received GO catalysts. No obvious morphological and structural changes of the spent GO catalysts under the optimized reaction conditions suggest the structural robustness of the catalysts (Figure S1a and S1b). The surface fraction of –C–OH groups for the spent GO catalysts (26.2%) also exhibited the similar value of the fresh GO catalysts (25.3%), which also indicated the catalytic stability of GO for the cycloaddition (Figure S1c and Table S1). The catalytic performance of other carbon materials of acetylene black and activated carbon was also tested under the same reaction conditions as a comparison. As shown in Figure 3a, the acetylene black and activated carbon exhibited much lower catalytic activity for the


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cycloaddition reaction, compared with that of the as-received GO catalysts. After 6 h reaction, the conversions of styrene oxides were only 18.7% and 17.1% for acetylene black and activated carbon, respectively. Their selectivities of phenylethylene carbonate were 98.6% and 98.8%, respectively, similar to that catalyzed by the as-received GO catalysts. For all carbon catalysts, their common characteristics contain the oxygen-containing groups (-OH, -COOH, et.al.), ensuring their catalytic activity for the cycloaddition of CO2 and epoxides. However, since their surface areas and densities of the oxygen defects of the as-received GOs, acetylene black and activated carbon are quite different, the direct comparison on their catalytic activity might be unfair. In order to understand the correlation between the surface properties of the carbon materials and their catalytic activity, the post-treated GO catalysts with various densities of the surface oxygen-defects were synthesized through either the chemical reduction by NaBH4 or further oxidation by oxygen plasma treatment.56-57 Thus, the GO catalysts with the controllable surface properties were realized. After the reduction and oxidation processes, the obtained oxidized and reduced GO catalysts showed the well preserved morphology (Figure S2a and S2b) as that of asreceived GO catalysts. As shown in XRD spectra (Figure 1b), compared with the as-received GO or oxidized GO catalysts, the characteristics peak at 10.8o became very weak for the reduced GO catalysts, indicating the successful chemical reduction of GOs by NaBH4. Due to their similar structural and morphological features, the enhanced or decreased catalytic activity of the oxidized or reduced GO catalysts can be derived from the different surface functional groups characteristics. Through this approach, it provides a reasonable platform to investigate the correlation between the surface properties of the GO catalysts and their catalytic activity for the cycloaddition reaction between CO2 and epoxides. For the cycloaddition reaction of styrene


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oxides and CO2, the oxidized GO catalysts delivered the conversion of styrene oxides of 65.3% with a 98.8% selectivity of phenylethylene carbonate after 6 h under the identical reaction conditions (Figure 3a), which was higher than that catalyzed by the as-received GO catalysts (57.7%). On the contrary, the obvious decreased catalytic activity of the reduced GO catalysts was observed with only 25.7% conversion of styrene oxide and 98.6% selectivity of phenylethylene carbonate (Figure 3a). The results suggest that the catalytic activity of various GO catalysts for the cycloaddition reaction follows an order of the oxidized GOs > as-received GOs > reduced GOs. To further understand the catalytic behaviors of GOs under ambient pressure, the kinetic experiments were executed to explore their intrinsic activity in the oxidized GO, as-received GO and reduced GO catalysts at the temperature range from 120 °C to 150 °C. The catalytic performance of the three GO catalysts for the cycloaddition of CO2 and styrene oxide was shown in Figure S3. The moles of styrene oxide were decreased near linearly with the reaction time during the initial period for all catalysts, where the conversions of styrene oxides were in the range of 0−50%. The catalytic behaviors suggest a zero-order reaction for the cycloaddition of CO2 and styrene oxide (Figure S4), indicating that the cycloaddition reaction is not limited by the external diffusion of reactants in the present catalytic system. Thus, the reaction rates can be easily determined and calculated from the slope of the styrene oxide conversion curves during the initial period, as shown in Figure 3b. The reaction rates as described by the conversion of styrene oxide showed a strong dependence on the oxidation degree of GOs and followed an order of oxidized GOs > as-received GOs > reduced GOs at all reaction temperatures. Taking 140 °C as an example, the reaction rate was 214 mmol g-1 h-1 for the oxidized GO catalysts, much higher


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than that of as-received GO catalysts (189 mmol g-1 h-1) and that of the reduced GO catalysts (102 mmol g-1 h-1). Due to the zero-order characteristics in the initial period, the reaction rate constants (k) could be obtained from the slopes of the fitting lines (Figure S4). As shown in Figure 3b, each catalyst follows a good linearity (R2 > 0.9789) between the logarithm of the as-obtained k (lnk) and the reciprocal absolute temperature (1/T). The apparent activation energy (Ea) also can be easily derived from the Arrhenius Equation (1): Lnk = -Ea/RT + lnA (1) Where k is the reaction rate constant, Ea is the intrinsic activation energy, T is the reaction temperature, and A is the pre-exponential factors. As shown in Figure 3c, the Ea values of all GO catalysts were in the range of 68-73 kJ mol1

. Different from their reaction rates, the Ea values were insensitive of the oxidation degree of

the GO catalysts. The nearly unchanged Ea could further conclude the similar catalytic behavior for the surface active sites on various GO catalysts. Therefore, their different catalytic activities or reaction rates might be determined by their various amounts of the surface oxygen-containing groups of the GO catalysts. Generally, there are several different oxygen-containing groups in the structure of GOs, which can be characterized by the XPS analysis (Figure 4a). The C1s spectra consist five peaks that correspond to sp2 carbon at 284.6 eV, sp3 carbon at 285.8 eV, –C–OH group at 286.6 eV, – C=O group at 288.2 eV and O–C=O group at 289.0 eV, respectively. The fractions of each peak were summarized in Table S1. Compared with as-received GO catalysts, the amount of oxygencontaining groups increased from 35.4% to 37.5% for the oxidized GO catalysts. While, the amount of oxygen-containing decreased to 15% for the reduced GO catalysts. Clearly, the


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catalytic activity decreased with the decreased fraction of the oxygen-containing groups in various GO catalysts. Based on XPS analysis (Table S1), the surface –C–OH species of the received GO and oxidized GO catalysts are the major surface oxygen-containing groups with acidic characteristics. Therefore, the –C–OH groups in the GO catalysts can be used as the descriptor for the cycloaddition of CO2 and styrene oxide to ethylene carbonate. As shown in Figure 4b, the conversions of styrene oxide delivered a good linear relationship of the fractions of –C–OH group with a correlation coefficient of 0.9962, suggesting a strong interfacialdependent catalytic activity of the GO catalysts in the present catalytic system. In contrast, no linear relationship was revealed between the conversions of styrene oxide and the amount of – C=O/–O–C=O groups, as shown in Figure S5. Thus, the higher amount of the surface oxygencontaining groups of GOs leads to the higher catalytic activity. Mechanistic investigations. The hydroxyl groups as the main oxygen-containing groups in the structure of GOs can promote the cycloaddition reaction. The similar phenomena was also observed in the ionic liquid catalytic system.58 However, a unsatisfactory catalytic performance was yielded in the presence of free hydroxyl groups by addition of trace amount H2O in the reaction system, as shown in Figure S6. More importantly, the cycloaddition reaction catalyzed by hydroxyl groups cannot be highly efficient without the help of halide anions as co-catalysts in previous reports.43,

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Herein, the catalytic amount of the GO catalysts can realize the

cycloaddition reaction at 1.0 atm of CO2 in the absence of any co-catalysts or additives. DMF solvent is believed to play an important role in the GO catalyzed cycloaddition reaction. As shown in Table 1, only 10.1 % conversion of styrene oxides was yielded after 6 h reaction in presence of GOs without DMF as the solvent (Entry 8, Table 1). In order to confirm the role of DMF, various amounts of DMF were added in the reaction system. When the reaction time was


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controlled at 6 h, the conversion of styrene oxide was increased to 32.1% in the presence of the catalytic amount of DMF (0.1 mL, Entry 9, Table 1). When the amount of DMF was further increased to 1 mL, the conversion of styrene oxide after 6 h reaction was further enhanced to 53.3% (Entry 10, Table 1), which was similar to the conversion obtained in the presence of 4 mL of DMF. In contrast, the low catalytic efficiency was also observed in the absence of GOs, when DMF was used as the solvent (Entry 2, Table 1). Therefore, the highly efficient cycloaddition reaction was realized with the synergistical effects between GOs and DMF. As described in the previous report,23 the DMF solvent has been proved to enable the activation of CO2. Based on these, the proposed reaction process was shown in Figure 5a. Firstly, the H atom of the hydroxyl group in the GO catalysts can contact with the O atom of styrene oxide through a hydrogen bond, giving the intermediate I with the polarization of C–O bonds in styrene oxide. At the same time, the activated CO2 by DMF (IV) nucleophilically attacks the less sterically hindered carbon atom of intermediate I. Then, the ring of styrene oxide is opened to form compound II. After that, DMF is liberated and forms another IV with another CO2 molecule. The intermediate III undergoes a cyclization to form the phenylethylene carbonate product and recovers the surface active sites of the GO catalysts. The activation of CO2 by DMF and activation of styrene oxide by hydroxyl groups on the surface of GOs are two key processes for the cycloaddition reaction from the proposed reaction mechanism. In order to reveal the correlation between the CO2 activation and their pressures, the cycloaddition reactions were performed at various CO2 pressures. As shown in Figure 5b, the conversions of styrene oxide were achieved between 54.9% and 58.3% in the rage of 0.1 MPa to 2.0 MPa of CO2 pressures. The almost unaltered conversions of styrene oxide under various pressures indicate that the activation of CO2 by DMF is independent on the CO2 pressures for the


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cycloaddition reaction between CO2 and styrene oxide. In contrast, the conversions increased obviously with the increasing amount of the fresh GO or reduced GO catalysts (Figure 5c). To further reveal the relationship between the surface hydroxyl groups and the conversions of styrene oxide, the available amount of the surface hydroxyl groups of 2.5 mg of the reduced GO catalysts is defined as 1 unit. Based on their XPS spectra, the surface amounts of hydroxyl groups for the as-received GO and oxidized GO catalysts with the same mass are normalized as 5.06 and 5.76 unit, respectively. As shown in Figure 5d, the conversions of styrene oxide exhibit a strong near linear correlation with the increase of the available amount of the surface hydroxyl group (R2=0.9590). To be noted, the correlation does not pass through the zero point due to the catalytic contribution of other oxygen-containing groups on the surface of various GO catalysts. Therefore, the activation of styrene oxide by the GO catalysts exhibits a strong interfacialdepended catalytic behavior for the cycloaddition reaction in the present catalytic system. The catalytic performance of the GO catalysts is increased with the increased surface fractions of the oxygen-containing groups, when the same amount of the catalysts is used. Scope. The substrate scopes were then screened on various epoxides using GOs as catalyst. As shown in Table 2, the as-received GO catalysts were found to be effective for a variety of terminal epoxides (Entries 1−5). Specifically, chloroethylene oxide exhibited a high activity catalyzed by GOs. The >99.9 % conversion of chloroethylene oxide was yielded with 95.6% selectivity of cyclic chloroethylene carbonate after 12 h reaction at a lower temperature of 100 °C (Entry 1). For 1-bromo-2,3-epoxypropane (Entry 2), the >99.9 % conversion and 96.1% selectivity were also yielded after 10 h reaction at 100 °C. The reaction temperature of 140 °C was required for the cyclohexene oxide, isobutylene oxide and propylene oxide. After 10 h reaction, their conversions were 39.7%, 89.9% and 96.6%, respectively (Entries 3-5).


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Meanwhile, their high chemoselectivity to the corresponded cyclodicarbonate was well preserved over 96.3% after 10 h reaction. CONCLUSION In summary, the commercial GO catalysts prepared by Hummers’ method can activate epoxides and successfully enable the cycloaddition reaction between CO2 and epoxides at 1.0 atm of CO2. The as-received GO catalysts deliver high catalytic activity as well as the robust catalytic stability. Their catalytic activity for the cycloaddition reaction is strongly correlated to the surface properties of GOs. Kinetic and XPS analysis reveal two important features of the current catalytic system: (1) The activation of CO2 molecules by DMF is independent on the CO2 pressures and (2) The surface oxygen-containing groups of GOs is recognized as the surface active sites for the activation of epoxide oxides. Overall reaction exhibits a strong interfacialdepended catalytic behavior for the cycloaddition reaction under the equal amount of GOs in the present catalytic system, where the catalytic performance of the GO catalysts is increased with the increase surface fractions of the oxygen-containing groups. It is envisaged that the GO catalysts with the controllable surface functional groups will emerge as the promising and costeffective heterogeneous catalysts for CO2 transformations and utilizations. EXPERIMENTAL SECTION All chemicals (AR grade) were obtained form the Energy Chemical. The graphene oxides, acetylene black and activated carbon were obtained form the Alibaba Group. All glassware were thoroughly washed by aqua regia (a volume ratio of 1: 3 of concentrated nitric acid and hydrochloric acid) to avoid any possible contamination.


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Synthesis of the oxidized GO and reduced GO catalysts. The oxidized GO catalysts were obtained by oxygen plasma treatment. Typically, ~ 50 mg of as-received GOs were deposited on a glass substrate and then exposed to oxygen RF plasma (200 W) for 60 s (1 mbar O2 pressure).

After that, the oxidized GO catalysts were washed with water and alcohol

alternatively for three times, collected by centrifugation and dried for the future use. The reduced GO catalysts were synthesized by the chemical reduction with NaBH4 as reducing reagents. Briefly, ~ 50 mg of GOs were dispersed in 25 mL of water. Then, 5 mL of the freshly prepared NaBH4 aqueous solution containing 10 mg of NaBH4 was added dropwisely into the GOs solution under the vigorous stirring at room temperature. After 1 h reaction, the reduced GO catalysts were separated from the solution with the centrifugal treatment, washed with water and alcohol alternatively for three times and dried for the catalytic reactions. Characterization of Catalysts.

The phase evolution of various GO catalysts was

monitored by powder X-ray diffraction (XRD). The XRD patterns with diffraction intensity versus 2θ were recorded in a Shimadzu X-ray diffractometer (Model 6000) using Cu Kα radiation. Transmission electron microscopy (TEM) studies were conducted on a Hitachi HT7700 transmission electron microscope with an accelerating voltage of 120 kV. XPS were acquired using a Thermo Electron Model K-Alpha with Al Kα as the excitation source. Catalytic cycloaddition of CO2 and epoxides. For a typical catalytic reaction, 5 mmol of styrene oxide (substrates) and 2.5 mg of various GO catalysts were firstly mixed in 4 mL of high boiling DMF. Then, the solution was transferred into the air-tight reactor with the filled CO2. Afterwards, the reaction system was heated to 140 °C for the cycloaddition reaction. After the reaction, the air-tight reactor was cooled down to room temperature and the products were analyzed by GC-MS and GC with m-xylene as the internal standard. The catalytic reactions


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under high CO2 pressures were performed in a stainless steel autoclave equipped with the pressure control system.

ASSOCIATED CONTENT Supporting Information. This file provides more detailed information regarding structural characterizations of the used GO catalysts, oxidized GO catalysts, reduced GO catalysts, the catalytic behavior of various GO catalysts for cycloaddition of styrene oxide and CO2 at various reaction temperatures, styrene oxide conversions as a function of reaction time at various temperatures, optimization of the GO catalysts for cycloaddition reaction, and summary the fractions of each oxygen-containing group in various GO catalysts from the XPS analysis of C1s. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * [email protected] and [email protected] Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT We acknowledge the financial support from the National 1000-Plan program.

We also

acknowledge the China Postdoctoral Science Foundation Grant 2017M620453. Y. Qu is also supported by the Cyrus Tang Foundation through Tang Scholar program.


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(59) Ma, J.; Liu, J.; Zhang, Z.; Han, B. The catalytic mechanism of KI and the co-catalytic mechanism of hydroxyl substances for cycloaddition of CO2 with propylene oxide. Green Chem. 2012, 14, 2410-2420. DOI: 10.1039/c2gc35711a.


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Figure 1. Structural characterization of the GO catalysts. (a) TEM image of as-received GO catalysts, and (b) XRD spectra of the oxidized, as-received and reduced GO catalysts.


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Figure 2. Catalytic performance of as-received GO catalysts for cycloaddition of CO2 and styrene oxide. (a) Plot of the conversion of styrene oxide and selectivity to phenylethylene carbonate as a function of reaction time. (b) Catalytic recyclability of the as-received GO catalysts. Reaction conditions: 5 mmol of styrene oxide, 2.5 mg of as-received GOs, 4 mL of DMF, 140 °C and one atmospheric CO2 pressure. The catalytic reactions were performed for 9 h for the test of recyclability.


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Figure 3. Catalytic performance and kinetic analysis of the oxidized, as-received and reduced GO catalysts. (a) The catalytic activity of acetylene black, activated carbon and various GOs catalysts. Reaction conditions: 5 mmol of styrene oxide, 4 mL of DMF, 2.5 mg of catalysts, 140 °C and 1.0 atm of CO2. (b) Plot of lnk as a function of (1/T) for various GO catalysts, derived from styrene oxide reaction rates vs. reaction time. Inset is the plot of the initial styrene oxide reaction rates at various reaction temperatures. (c) The calculated activation energy of various GO catalysts.


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Figure 4. Structural characterizations of the oxidized, as-received and reduced GO catalysts. (a) XPS analysis of C element of various GO catalysts. (b) Conversions of styrene oxide as a function of the fraction of the surface –C–OH group in various GO catalysts.


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Figure 5. Mechanism investigations. (a) The proposed reaction process of the cycloaddition reaction between CO2 and styrene oxide catalyzed by the as-received GO catalysts.

The

conversions of styrene oxide under (b) various CO2 pressures and (c) various amount of asreceived and reduced GO catalysts. (d) Conversions of styrene oxide as a function of the normalized total amount of the surface –C–OH active sites of various GO catalysts.


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Table 1. Optimization of the cycloaddition reactions catalyzed by the as-received GO catalysts* O

O

O

CO2

O

Entry

GO (mg)

DMF (mL)

Time (h)

T (°C)

Con.(%)

Sel.(%)

1

0

0

6

140

0.48

95.6

2

0

4

6

140

5.6

99.9

3

0

4

16

140

12.1

99.3

4

2.5

4

6

140

57.7

96.6

5

2.5

4

6

100

5.1

99.8

6

2.5

4

6

120

20.3

99.7

7

5

4

6

140

75.1

98.2

8

2.5

0

6

140

10.1

75.7

9

2.5

0.1

6

140

32.1

94.2

10

2.5

1

6

140

53.3

95.4

* Reaction conditions: styrene oxide (5 mmol) with one atmospheric CO2 pressure.


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Table 2. Cycloaddition of CO2 and epoxides to cyclic carbonates by the as-received GO catalysts* Temp. Entry

Substrates

Products

Sel.

(%)

(%)

Time (h) (°C)

O

O O

1

O

Cl

Conv.

100

12

>99.9

95.6

100

12

>99.9

96.1

140

10

39.7

99.8

140

10

89.9

96.3

140

10

96.6

98.3

Cl

O

2

O

O

Br

O

Br O

3

O

O O

O

4

O

5

O

O

O

O O

O

* Reaction conditions: Substrates (5 mmol), DMF (4 mL), as-received GO catalysts (2.5 mg) and 1.0 atm of CO2.


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For Table of Contents Use Only

Synopsis A readily available and inexpensive commercial graphene oxide (GO) catalysts can activate epoxides and successfully enable the cycloaddition reaction between CO2 and epoxides in DMF at 1.0 atm of CO2 pressure in absence of co-catalysts or additives.


32

Catalytic Behavior of Graphene Oxides for Converting CO2 into Cyclic

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