Improved Graphene Oxide Derived Carbon Sponge for Effective

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Improved Graphene Oxide Derived Carbon Sponge for Effective Hydrocarbon Absorption and C-C Coupling Reaction Saikat Dutta, Joseph P. Smith, Karl S. Booksh, and Basudeb Saha ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02053 • Publication Date (Web): 23 Jul 2018 Downloaded from http://pubs.acs.org on July 24, 2018

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ACS Sustainable Chemistry & Engineering

Improved Graphene Oxide Derived Carbon Sponge for Effective Hydrocarbon Absorption and C-C Coupling Reaction Saikat Duttaa, Joseph P. Smithb, Karl S. Bookshc, and Basudeb Sahaa* a

Catalysis Center for Energy Innovation, 221, Academy Street, University of Delaware, Newark, DE 19716, USA b

Analytical Research & Development, Merck Research Laboratories, Merck & Co., Inc., 126 East Lincoln Avenue, Rahway, NJ 07065

c

Department of Chemistry & Biochemistry, 102 Brown Laboratory, University of Delaware, Newark, DE 19716, USA Corresponding author’s email: [email protected]

Abstract Graphenic sponges have created tantamount interest due to their special affinity for absorbing a broad range of petroleum oils and solvents resulting from controllable surface wettability. A major challenge is to fabricate such materials with surface hydrophilicity coexisting with hydrophobicity. Herein, we report a scalable self-assembly of randomly oriented improved graphene oxide (IGO) sheets into graphene oxide sponge (GOS-H) with uniform cylindrical shape via a hydrothermal method. Extensive characterization of GOS-H using Raman microspectroscopy, Raman imaging, X-ray photoelectron spectroscopy (XPS), and electron microscopic (SEM and HRTEM) techniques suggests the sponge surface is hydrophobic with some hydrophilic oxygen content and has defect sites and roughness associated with voids formation. These features and conformal C-O coating on the basal plans result in high dieselrange alkanes absorption from pure alkanes as well as from alkane-water mixture, which could enable as-synthesized GOS-H as an efficient and inexpensive sponge for hydrocarbons cleaning from contaminated water. In addition, GOS-H exhibits high catalytic activity for C-C coupling reaction of biomass derived furfural and 2-methylfuran under solventless conditions to produce jet fuel ranged high carbon oxygenates with branched backbone.

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Keywords: Carbon sponge, graphene oxide sponge, hydrocarbon absorption, water cleaning, coupling reaction

Introduction Excellent mechanical stability and tunable surface wettability make graphene-sponge as effective absorbent in hydrophobic and hydrophilic environments1,2. Homogeneous elastic absorbent,3,4 block-copolymer modified graphene foam,1 have been used for reversible absorption and desorption of absorbates by switching surface properties from hydrophobic to hydrophilic. While graphene sponge has shown to be highly repeatable and reversible for absorption of organic absorbates and oils,3-5 the reasons for high absorption capacity, especially the origin of surface wettability for some materials are unclear. Graphene oxide sponge (GOS), derived via hydrothermal reduction of graphene oxide (GO) followed by freeze drying/molding for volume expansion, possessed enhanced interactions with organic adsorbates due to the reduction of surface oxygen functionality. Reduced graphene oxide sponge (rGOS), prepared by hydrazine reduction, has been reported to be a good absorbent for motor oil6; however, rGOS is less hydrophobic than GOS because of its lower water contact angle (~99.8°)6 than GOS (~114°). Superwetting behavior of GOS arises from minimal self-agglomeration of GO sheets during chemical reduction in the absence of any template or spacer

that imparts surface

hydrophobicity.7 Thermal treatment GO forms 3D microstructures.8 Aggregation of GO to GOS of a unique structural properties9-10 via edge-to-edge assembly is of significant interest.11-12 GO contains hydrophobic aromatic domains and hydrophilic domains carrying oxygen functional groups. Strong π-stacking of aromatic domains in GO tend to irreversibly agglomerates, making the surface hydrophobic. For example, π-π and van der Waal13 interactions of hierarchical GO resulted in GOS6 or aerogels14. In addition to oil absorption, GOS can also be used as a carbocatalyst for C-C coupling reactions. For example, we have recently reported that an improved GO (IGO) containing Brønsted acidic oxygen functionality and defect sites on the surface and edges is very effective for hydroxyalkylation/alkylation (HAA) reaction of lignocellulosic biomass derived furans to high carbon oxygenates, suitable precursors for jet fuels, with very high yield (maximum 95%).15,16-19 However, catalytic applications of carbon sponges (e.g. GOS, rGOS) are not well-explored. 2 ACS Paragon Plus Environment

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Herein, we report a hydrothermal method for preparation of a GOS-H from aqueous dispersion of parent IGO (Scheme 1), which we have extensively studied for HAA reaction.15 As-synthesized GOS-H was characterized by X-ray photoelectron spectroscopy (XPS), Raman microspectroscopy, Raman imaging, scanning-electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM), and used for various alkanes (decane, C9-C35 hydrocarbons, octane, and decane-water mixture) absorption and HAA reaction. The results demonstrate high alkanes absorption at room temperature and high catalytic activity for HAA reaction at low temperature (60 °C) under solventless conditions. We illustrate the surface and morphological features of GOS-H to correlate with observed performances.

Scheme 1. Illustration for IGO transformation to GOS-H. Digital image in caption is for assynthesized GOS-H.

Experimental Section Materials. Graphite powder, potassium permanganate, hydrochloric acid (37%), ethanol, diethyl ether, phosphoric acid, hydrogen peroxide (30 wt%), sulfuric acid, 2-methylfuran (2-MF), furfural, decane and octane were purchased from Sigma-Aldrich (USA). C9-C35 alkanes mixture for absorption study was purchased from Absolute Standards Inc, USA. All chemicals were used as received without further purification. IGO, a precursor for GOS-H, was prepared by our reported method15 in which Hummer’s modified method was followed using KMnO4 and phosphoric acid-sulfuric acid mixture to generate highly oxidized surface.

Preparation of GOS-H. GOS-H was synthesized by a hydrothermal method. In this method, 90 mL of a 4 mg mL-1 homogeneous IGO aqueous dispersion was placed in a 150 mL Teflon-lined 3 ACS Paragon Plus Environment


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autoclave and the solution was heated at 180 °C for 12 h in a furnace. Upon cooling the autoclave, produced hydrogel sponge (see picture in Scheme 1) was carefully taken out, washed with ethanol and water, and dried at 25 °C for 24 h.

Oil absorption experiment. A calculated amount of dry GOS-H was placed in known amount of different alkanes (decane, octane, C9-C35 alkanes or decane-water mixture). Alkanes were allowed to absorb on GOS-H for 5 min and wet GOS-H sample was collected in a pre-weighed weighing boat to weigh. The absorption capacity was calculated using equation 1 following a reported method6 =

!"# $%& $%&

(1)

Recycling experiments of GOS-H was performed for absorption of several alkanes following prior report.6 Wet GOS-H was dried in an oven for 3 h at the boiling temperature of alkanes and two successive weight of dry GOS-H was taken to ensure complete evaporation of absorbed alkanes. After 3 h of desorption, dried GOS-H was reused after washing with acetone and drying at 80 °C for overnight.

Hydroxyalkylation/alkylation (HAA) reaction. Furfural (0.770 g, 8 mmol), 2-MF (1.45 g, 17.6 mmol) and GOS-H (50 mg) were added in a 4 mL screw-cap vial equipped with a magnetic spin bar. The vial cap was closed and the mixture was stirred for 10 h at 60 °C. This reaction condition was found to be optimal for IGO catalyzed HAA condensation of furfural and 2-MF in our earlier report.15 After cooling the reaction mixture to room temperature, GOS-H was separated and the resulting condensation product (5,5’-bis(2-methylfuranyl)furan-2-ylmethane; BMFFM, see Figure 6) was analyzed by GC, 1HNMR and GC-MS (Figures S1-S2). For recycling experiments, recovered GOS-H was washed with ethanol (5 mL) for three times and dried under vacuum at room temperature before to reuse.

Characterization of GOS-H, Powder X-ray diffraction (XRD) pattern was collected at room temperature using a Bruker D8 Advance X-ray diffractometer equipped with monochromatized Cu Kα radiation (λ = 1.54056 Å) source operating at 45 V and 40 mA. HRTEM images were collected on a JEM-2010F (Keck CAMM) aberration corrected transmission electron microscope


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(TEM) operating at an accelerating voltage of 120 kV. Samples for TEM images were prepared by applying one drop of dilute suspension of GOS-H dispersed in acetone onto the carbon coated Cu grid and allowing the solvent to evaporate at room temperature. Scanning Electron Microscope (SEM) images of GOS-H were collected using a cross-beam SEM (Auriga-60, ZEISS) equipped with a Ga+ ion source FIB (Focused Ion Beam). A thermo-fisher K-alpha+ xray photoelectron spectrometer equipped with a monochromatic aluminum K-alpha x-ray source (400 nm) was used for X-ray Photoelectron Spectroscopy (XPS) analysis. Raman microspectroscopy and Raman imaging were performed using a Senterra Raman microscope (Bruker Optics, Massachusetts, USA). All Raman spectra were collected using a 532 nm excitation wavelength generated from a frequency-doubled Nd:YAG laser. The laser beam was focused onto the solid samples using a 50X objective lens (Olympus, New York, USA). The probed area for Raman imaging was ~2 µm in diameter. The laser power was held constant at 5 mW during Raman spectral acquisition. Integration times of 5 to 10 seconds with 3 to 5 co-averages were employed. A spectral range of 70 to 3700 cm-1 and spectral resolution of 3 to 5 cm-1 was used. Excitation wavelength calibration, Raman shift calibration, and background measurements were performed prior to the collection of each Raman spectrum. Raman imaging was accomplished by generating a rectangular grid of Raman spectra on the surface of the given sample. This rectangular grid was comprised of 100 total Raman spectra generated within the probed area. Furthermore, a 10 by 10 Raman spectral grid, with a step-size between Raman spectra of ~5 µm, was used. The specific x and y locations of spectral acquisition within this grid were achieved using the controllable stage within the Raman microscope, in which the z direction was held constant. Contact angle measurements were performed using a Dino-Lite digital microscope camera with magnification of 1-50X. For each contact angle measurement, water droplets of ~5 µL volume were placed onto the GOS-H surface, and the microscope camera was used to capture the water-surface angle. All contact angle measurements were conducted at ambient conditions and calculated using Dino-Lite digital microscope camera software. Average contact angles of at least 4 measurements are reported with standard deviations of ≤±2°.20

Analysis of catalysis product. The yield of HAA condensation product was obtained by analyzing the product solution on a gas chromatography (GC; model Agilent 7890A) equipped with an FID detector and HP-INNOWAX capillary column of dimension 0.25 mm ID × 0.25 µm 5 ACS Paragon Plus Environment


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× 50 micron. The essential parameters of the GC analysis were as follows: injection volume 1.0 µL, inlet temperature 250 °C, detector temperature 250 °C and split ratio 1:10. The initial column temperature was 40 °C (5 min) with a temperature ramp of 15 °C min-1 and the final temperature was 250 °C. The peak of desired condensation product, BMFFA, in GCchromatogram was identified by analyzing the product solution further using a GC-MS (Shimadzu-QP2010 Plus Mass spectrometer). BMFFM peak area was properly integrated and its actual concentration was obtained from a pre-calibrated plot of peak area against concentrations. The calibration plot of BMFFA was developed using a sample of pre-determined concentration from 1H NMR analysis following our earlier report.15 The typical electron energy of GC-MS was 70 eV with the ion source temperature maintained at 250 °C. A 30 meter HP-INNOWAX capillary column (250 µm ID × 0.50 µm thickness × 25 mm diameter) was used for GC-MS analysis. The initial column temperature of GC-MS was set at 40 °C (for 5 min) and programmed to reach 250 °C at a heating rate of 10 °C min-1. Carrier gas flow rate was set at 1 mL min-1. The injection temperature was set at 250 °C.

Results and discussion GOS-H characterization. We choose IGO as a precursor for GOS-H because of its (1) multilayer structure which could enable curve outward for volume expansion, and (2) high surface hydrophilicity enabling high dispersion in aqueous phase. The hydrothermal heating of the aqueous dispersion of IGO gives GOS-H with a mass loss of about 3 times of IGO and volume expansion as seen in Scheme 1. The 2D conjugated structure (SP2) of graphene21 partly restores in 3D GOS-H during its formation via H-bonding interactions among IGO sheets and simultaneous removal of surface oxygen functionality (epoxy, carboxylic, carbonyl, and hydroxyl groups).15, 22-23 High and low resolution SEM images of GOS-H (Figures 1a to c) show that twisted sheets of parent IGO has expanded outward to enable volume expansion and the resulting GOS-H has disordered arrangement of interconnected void spaces. Figure 1a shows enhanced surface roughness of GOS-H containing large voids (Figures 1b-1c) as compared to parent IGO, which has smooth graphenic layers with minimum roughness as revealed from the HRTEM (Figures S3a and S3b), SEM (Figure S3c) and AFM (Figure S3d) images, respectively.15 Higher surface


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roughness can give rise to higher surface hydrophobicity and lower surface energy,24-25 and hence can have higher surface interactions with hydrophobic alkanes.

Figure 1. SEM images of GOS-H from lower to higher resolutions. Voids formation and surface roughness can be seen in Figures b and c.

We have reported that IGO contains high amounts of oxygen functional groups on the basal planes and edges with associated defect sites, and has crystallinity as well as high aspect ratio.15 The oxygen functionality of IGO suppressed in GOS-H as evidenced from a comparison of C1s peak intensities of IGO and GOS-H in the oxidized region (285.5-290 eV) containing C-O, C=O and O-C=O functionalities (Figure 2a). Deconvoluted C 1s XPS spectrum of GOS-H (Figure 2b) reveals several oxygen functional groups, which were assigned according to their binding energies (BE) from prior reports.15,

26-27

The C1s intensity and XPS survey spectral data are

compared in Table S1, which show less surface oxygen in GOS-H (24%) than its parent IGO (37%), indicating hydrothermal removal of some oxygen occurred during GOS-H formation. Noteworthy, GOS-H surface contains more oxygen than commercial GO or highly ordered pyrolytic graphite (HOPG) (Table S1). A comparison of atomic percentages of different surface functional groups of GOS-H and parent IGO from their deconvoluted C 1s spectra is given in Table S2. It corroborates with the results of Table S1 that GOS-H contains less surface oxygen functional groups than that of IGO. The XRD pattern of GOS-H shows a broad diffraction peak at 24.9°, which can be assigned to (002) plane of stacked graphene sheets (Figure 2c). GOS-H is amorphous as opposed to its parent IGO, which exhibited crystallinity with enhanced interlayer spacing and lattice disruption (d200=9.67 Å at 9.5°).15, 28 This peak of IGO at 9.5° is absent in GOS-H.



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Figure 2. (a) Overlay C1s XPS spectra of GOS-H and IGO, (b) deconvoluted C1s XPS spectra of GOS-H (all spectra were calibrated according to the asymmetric graphitic peak at 284.4 eV compared to the reference data of HOPG. The fitting were done by fixing the peak position within ±0.2eV for all spectra and constraining the full width half maximum (FWHM) of 0.9-1.3 eV), and (c) XRD patterns of GOS-H and IGO.

Raman microspectroscopy and Raman imaging analyses of GOS-H and parent IGO were conducted to understand defects. Raman spectra of GOS-H type carbon materials can be divided into first order (1100-1800 cm-1) and second order (2200-3400 cm-1) spectral regions. The first order Raman spectrum of GOS-H (Figure 3a) displays primary Raman bands at 1344 cm-1 (Dband), attributed to the A1g mode from breakdown of translational and local lattice symmetries 29 and at 1580 cm-1 (G-band), attributed to the E2g mode of graphitic carbon from the stretching vibration in the aromatic layers. The ratio of intensities of the D and G bands (ID/IG) of the first order Raman spectra indicates the extent of microstructural disorder and defects of carbon frameworks30-31 The ID/IG value of GOS-H (0.84) is slightly lower than that of its parent IGO (0.89), indicating GOS-H has a slightly higher degree of disorder 29, 32 and graphitic sp2 carbon.29, 33-35

G′ and D′ bands in the Raman spectrum of GOS-H are observed at 2668 cm-1 and 2920 cm-1,

respectively, and these are attributed to the presence of disorder and multilayers36 features that are retained from the parent IGO. D′-band, a defect-induced Raman mode, is attributed to the presence of disorder in carbon framework.36 Raman mapping of G′ and D′ bands are deposited in Figure S4. Chemical images of GOS-H, collected via Raman imaging within the probed area (~2 µm diameter; see instrumentation section), is shown in Figure 3b. Detailed methodology for Raman imaging is discussed in the instrumentation section. Raman imaging of the D and G bands 8 ACS Paragon Plus Environment

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produced chemical images from their integrated intensities (Figure 3c). The chemical image shows disordered region (D/G ≥ 1; red-yellow zone) and graphitic region (D/G ~ 0; blue zone).

Figure 3. (a) First order Raman spectra of GOS-H (blue), and IGO (red). (b) The probed area of GOS-H with corresponding chemical images displaying the D- and G-bands. (c) The chemical image resulting from integrated D and G bands ratio of GOS-H with marked disordered area (D/G ≥ 1, intense yellow-red color zone) and graphitic area (D/G ~ 0, intense blue color zone).

HRTEM images show twisted sheets of GOS-H (Figures 4a to 4d). Yellow index areas in Figures 4c and 4d represent the layer structures of GOS-H, possibly associated with cross-linking of IGO layers via H-bonding interactions that occurred during hydrothermal heating of IGO. Comparison of HRTEM images of IGO and GOS-H (Figure S5) reveals layer formation and wrinkles appearance at the edges of GOS-H from cross-linkages of carbonyl and carboxyl groups IGO sheets (Figure 2b), as ascribed in prior report.37 This observation corroborates with a decrease in C1s intensity of GOS-H in the BE region of carbonyl and carboxyl groups (287.4 288.7 eV; Figure 2a). AFM image of GOS-H (Figures S6a and 6b) further evidences stacked layers and wrinkles at the layer edges. In addition, a slight blue shift of the first order Raman spectral G band from IGO (1586 cm-1) to GOS-H (1593 cm-1) serves as an evidence of interactions between IGO sheets during formation of GOS-H.



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Figure 4. HRTEM images of GOS-H. Images (a) and (b) show layer structures of large twisted sheets and images (c) and (d) show rough surfaces at different magnifications.

Oil Absorption Study of GOS-H GOS has received significant attentions as absorbents38-39 for alkanes, toluene, and other organic solvents.6 The XPS results (Tables S1 and S2) show that the GOS-H surface has less oxygen than its parent IGO, meaning GOS-H surface is more hydrophobic. SEM images evidenced cross-linkages of parent IGO which produced porous framework with voids in GOSH. Water contact angle measurement shows higher contact angle of GOS-H (86.5°) than highly oriented pyrolytic graphite (HOPG; 67.2°) which was used as a standard (Table S4), confirming the GOS-H surface is hydrophobic. Important to note that HOPG contains a flat homogeneous surface while GOS-H surface is rough and heterogeneous. In case of HOPG, hydrophobic domains will pin the motion of the water front as it advances, and the same domain of surface will hold back to contracting motion of the water drop front when recedes, which results is a decrease in water contact angle on HOPG. In case of GOS-H, the surface roughness plays a role for generating hysteresis where the motion of the contact line of water is pinned due to a barrier resulting from microscopic variation of slope on a rough surface. This causes to increase in water contact angle on a rough surface,40 although its surface is less hydrophobic can HOPG. Because of features discussed above of GOS-H, we evaluated its performance for absorption of alkanes, e.g., octane, decane, mixture of C9-C35 alkanes and decane-water (1:2 v/v). Experimental proof for decane absorption by GOS-H from a mixture of a drop of decane and a drop of water on a clean glass surface (with paper background) is shown in Figure 5a. Decane was colored using a Sudan III dye for visualization. A high-resolution camera was used to capture pictures during decane absorption. A cylindrical piece of GOS-H (10 mm × 10 mm × 10 mm; see Scheme 1) was placed into the mixture (Figure 5a; zero time). Three pictures in Figure 10 ACS Paragon Plus Environment

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5a show gradual absorption of decane by GOS-H and complete absorption in 68 sec, leaving the water droplet on the glass surface intact. Figure 5b illustrates decane absorption from a mixture of decane and water. The top colored phase is decane and the bottom phase is water, and they formed two distinct phases (picture at zero time). The middle picture shows some absorption of decane by GOS-H after 20 sec and that the GOS-H stays in the decane phase. After 77 sec, the decane phase is completely absorbed and the water phase remained unabsorbed. The absorption capacity (κ), described in the experimental section, is 42 (wt of decane absorbed/wt of dry GOS-H), which is high.

Figure 5. The results of alkanes absorption by GOS-H (10 mm×10 mm×10 mm size) from (a) a mixture of a drop of decane and a drop of water, and (b) decane-water mixture. Figure (c) compares κ values (g g-1) of GOS-H for absorption of different alkanes and decane from decanewater mixture. Figure (d) shows recycling results of GOS-H for absorption of different alkanes.

Figure 5c compares κ values of GOS-H for octane, decane and a C9-C35 alkanes mixture. It shows the absorption capacity of decane is the best. Next, alkanes absorbed GOS-H was kept in an oven at the boiling temperature to desorbed alkanes and reuse GOS-H (see experimental 11 ACS Paragon Plus Environment


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section). A small drop in k values (Figure 5d) is observed for all alkanes from the 1st cycle to the 3rd cycle, which is likely due to the blockage of some internal or external pores of GOS-H by alkanes or cokes formed during drying. Next, we compare κ values of GOS-H with reported carbon sponges for alkanes absorption and their contact angels in Table S5. A graphene sponge (Table S5, entry 1) exhibits significantly high κ value (86 g g-1) for alkanes and pump oils because of its high water contact angle and surface roughness. In contract, a rGOS is a weaker absorbent (κ = 37 g g-1) for oil and organic solvents (Table S5, entry 2) likely because of its open pores structures.41 A hybrid graphene and carbon nanotube (CNT) sponge has high κ value (80-95 g g-1; Table S5, entry 5) for absorption of compressed and sesame oils in spite of its lower water contact angle (89°) than graphene sponge (114°) (Table S5, entry 1), which might be attributed to the influence of high hydrophobic property of CNT through its nano-texture network with graphene.42-43 A graphene aerogel performed poorly for vegetable and paraffin oils absorption (κ = 15-17 g g-1) (Table S5, entry 4). We found GOS-H is a moderate absorber for diesel ranged alkanes due to its voids, surface roughness, lower surface energy10 and surface hydrophobicity. Deconvolution C1s spectrum evidenced the presence of C-O functional groups (C-O conformal coating) on the basal plans (Figure 2b), as observed in reported carbon materials.44 Such coating might increase surface interaction with non-polar alkanes, resulting in alkanes absorption.

Catalytic Activity for Hydroxyalkylation/Alkylation Reaction C-C coupling of furans (e.g. 2-MF, furfural) is a good synthetic strategy for upgrading low carbon biomass furans into high-carbon and high value renewable products. We have recently reported that IGO is an effective catalyst for the HAA reaction15 because of the presence of high degree of surface oxidized carbons, associated defect sites and Brønsted acid sites arising from hydroxyl and carboxyl groups of oxidized carbons.22,

45-46

The catalyst exhibited superior

performance when compared with microporous zeolites and yielded up to 95% C12-C15 oxygenates at low temperature (60 °C) without requiring any solvents.15 Herein we explored the use of GOS-H as a catalyst for the HAA reaction for the following reasons: (1) Parent IGO has unique surface features and microstructure, (2) porous structure with voids (Figure 1) could enable absorption of reactants and facilitate the reaction, and (3) surface oxidized carbon containing some Brønsted acidic oxygen functionality (e.g. hydroxyl, carboxyl; Figure 2 and 12 ACS Paragon Plus Environment

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Table S2) and (4) easy separation. In addition, GOS materials have not been tested for C-C coupling reactions. Under optimal reaction conditions as reported for IGO catalysis,15 GOS-H exhibits good HAA activity for a reaction between 2-MF and furfural (Figure 6). Maximum 81% yield of BMFFM is obtained at 91% conversion of 2-MF. Detailed reaction conditions are in the experimental section. High HAA activity of GOS-H is likely arise from the surface hydroxyl and carboxyl groups (Table S2) on the basal planes and facile adsorption of furans, which are fairly hydrophobic. After reaction, the recovered GOS-H was washed with ethanol, dried and reused. A slight decrease in activity is observed in the 2nd cycle, while the activity drop is significant in the 3rd cycle. The catalytic activity loss can be attributed to the surface exfoliation of the GOS-H by the absorbed reactants and the product, BMFFM. Similar loss of the catalytic activity was observed for the parent IGO in the 3rd cycle but it regained full activity upon regeneration of oxidized surface by oxidation.15



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Figure 6. The results of GOS-H catalyzed HAA of 2-MF and furfural under solventless conditions at 60 °C for 10 h.

Conclusions In conclusion, we found an improved graphene oxide (IGO) derived carbon sponge (GOS-H) with hydrophobic and rough surface exhibits high and repeatable absorption of diesel-ranged alkanes from pure alkanes and alkane-water mixture. GOS-H retained microscopic structural features of graphenic walls from its parent IGO; however, the surface becomes rough and hydrophobic upon hydrothermal treatment of IGO because of loss of surface oxygen. Microscopic (SEM, TEM) and spectroscopic (XPS, Raman) characterizations of GOS-H reveal the presence of surface defects and microscale voids in addition to surface roughness and hydrophobicity, which are the critical factors for observed absorption of alkanes. Furthermore, co-existence of surface oxygen groups (hydroxyl, carboxyl, epoxy) enabled GOS-H as an effective catalyst for C-C coupling reaction of 2-MF and FUR under solventless conditions to produce high-carbon oxygenate with branched backbone.

Supporting Information. XPS spectral analysis results, Raman bands information, contact angles, comparative analysis of graphene sponge’s oil absorption capacity, 1H NMR of BMFFM, GC-MS chromatogram of BMFFM, HRTEM and SEM of IGO, HRTEM and SEM of GOS-H, AFM image of IGO and GOS-H, Raman mapping for D and G bands of GOS-H are deposited.

Acknowledgements This work is conducted with financial support as part of the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001004. The support of this research by the Delaware Space Grant College and Fellowship Program (NASA Grant NNX15A119H) is also gratefully acknowledged. Authors acknowledge Professor Dion Vlachos, Catalysis Center for Energy Innovation, University of Delaware for his valuable time to provide helpful technical suggestions.


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TOC

Synopsis Hydrothermally prepared graphene oxide sponge is effective for oil absorption from oil-water mixture and selective C-C coupling reaction to produce high-carbon performance products


Improved Graphene Oxide Derived Carbon Sponge for Effective

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