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Research Article pubs.acs.org/acscatalysis

Solventless C−C Coupling of Low Carbon Furanics to High Carbon Fuel Precursors Using an Improved Graphene Oxide Carbocatalyst Saikat Dutta,†,# Ashish Bohre,‡,# Weiqing Zheng,† Glen R. Jenness,† Marcel Núñez,† Basudeb Saha,*,† and Dionisios G. Vlachos*,† †

Catalysis Center for Energy Innovation, University of Delaware, Newark, Delaware 19716, United States Department of Chemistry, University of Delhi, Delhi 110007, India



S Supporting Information *

ABSTRACT: Graphene oxide, decorated with surface oxygen functionalities, has emerged as an alternative to precious-metal catalysts for many reactions. Herein, we report that graphene oxide becomes superactive for C−C coupling upon incorporation of a highly oxidized surface associated with Brønsted acidic oxygen functionality and defect sites along the surface and edges. The resulting improved graphene oxide (IGO) demonstrates significantly higher activity over commonly used framework zeolites for the upgrade of low-carbon biomass furanics to high-carbon fuel precursors. A maximum 95% yield of C15 fuel precursor with high selectivity is obtained at low temperature (60 °C) and neat conditions via hydroxyalkylation/alkylation (HAA) of 2-methylfuran (2-MF) and furfural. Coupling of 2-MF with carbonyl compounds ranging from C3 to C6 produces precursors of carbon numbers 12 to 21 with a high yield. The catalyst regains nearly full activity upon regeneration. Extensive microscopic and spectroscopic characterization of the fresh and reused IGO carbocatalysts indicates that defects and the enhanced oxygen content are strongly correlated with the high activity of IGO. Density functional theory calculations reveal defects at carbonyl sites as suitable Brønsted acidic oxygen functional groups. A plausible reaction mechanism is also hypothesized. KEYWORDS: graphene oxide, C−C coupling, biomass, defects, jet fuels, fuel precursors



INTRODUCTION

The thermochemical processing of lignocellulose-to-jet fuels via pyrolysis or gasification, followed by the catalytic upgrading of the resulting bio-oils or syngas has previously been reported.5 However, these high-temperature processes are nonselective, suffer from heat transfer limitations, and are generally unsuitable for large-scale production. An alternative strategy is the conversion of polysaccharides (cellulose and hemicelluloses) of lignocellulose to branched chain hydrocarbons via the catalytic conversion of sugars to jet (CCSTJ) using a combination of hydrolysis, dehydration, condensation, and hydrodeoxygenation (HDO) reactions.6−8 In this strategy, polysaccharides are first converted to C6 or C5 furanic platforms via hydrolysis and dehydration. As these low-carbon furanics are not suitable for aviation or diesel ranged fuels upon ring opening (RO) and HDO, these intermediates need to be coupled via aldol-condensation or hydroxyalkylation/alkylation (HAA) reactions to form precursors of the desired carbon chain length. The formation of C−C coupling products, via

The environmentally harmful carbon emissions and high price volatility of petroleum, owing to demand-supply imbalance and rapid depletion of fuel reservoirs, necessitates utilization of renewable carbon alternatives for chemicals and fuels in order to control the balance of global ecosystems.1 The utilization of nonfood lignocellulosic biomass, a renewable carbon source, for liquid fuels has received increasing attention2,3 because of its natural abundance and sustainability. This alternative resource is projected to have a significant economic, societal, and environmental impact owing to the reduction of the environmental carbon footprint and the development of a profitable agricultural economy. Despite decades of research on exploring natural biopolymers, composed of C6 and C5 sugars, there are still challenges for chemicals, fuels, and materials. Cost-effective and scalable processes for producing drop-in fuels (e.g., aviation and diesel fuels) with a minimal number of steps are still in early stages of development.2,4 Thus, it is crucial to design efficient chemical methods that offer scalable production of fuel precursors from cellulosic biomass, which can then undergo deoxygenation to branched chain alkanes of high energy density. © XXXX American Chemical Society

Received: November 1, 2016 Revised: February 27, 2017 Published: April 21, 2017 3905

DOI: 10.1021/acscatal.6b03113 ACS Catal. 2017, 7, 3905−3915

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ACS Catalysis Scheme 1. IGO-Catalyzed HAA of 2-MF with Biomass-Derived Carbonyl Compoundsa

The right image shows a 3D structure of IGO surface decorated with oxygen functional groups as obtained from first-principles calculations. The maximum yield of each product is shown in parentheses. a

nanoparticles,25 synthetic polymers,26 carbon allotropes (fullerenes, carbon nanotubes), and quantum dots27 have allowed tailoring GO for applications in energy storage, catalysis, biosensing, and biomedical applications.25,27 Nanocarbon functionalization for catalytic applications is established28 and incorporation of new functionalities in GO surface via chemical modification has been a topic of research.29,30 Surface modification of a parent graphitic framework with new functionalities could enhance reactivity and provide novel materials for developing new chemistry. This alternate carbocatalyst has the potential to reduce our dependence on precious metals (i.e., elimination of the use of precious Pdbased nanoparticulate catalysts for C−C coupling).1b,31 One example of surface modification involves chemical oxidation of regular GO using a refined Hummer’s method,32 which results in the production of improved graphene oxide (IGO). IGO possesses a stronger Brønsted acidity with improved oxygen functionalities and hydrophilicity than that for regular GO. Thus, IGO can be an effective carbocatalyst for HAA of lowcarbon furans for accessing furan derivatives of desired branched-chain carbon backbone and carbon numbers. To our knowledge, the catalytic activity of an IGO-containing extended sheet-like framework and stacked multilayers with highly oxidized surface has not been explored for C−C coupling reactions. Herein, we report a general strategy for C−C coupling of 2MF with biomass derived aldehydes and ketones through HAA using IGO as a catalyst at neat conditions. The results show that IGO has a significantly higher activity when compared to traditional framework catalysts (e.g., HY(80), HY(30), HBEA(300), and ZIF-8) and traditional commercial GO. It enables the selective formation of C12−C21 fuel precursors with high yields (Scheme 1). We elucidate the structural and surface features of IGO by Raman, FTIR, TEM, SEM, XPS, and AFM techniques along with first-principles calculations and demonstrate that its high activity is due to the presence of oxidative defect sites and Brønsted acidic oxygen functionality. The oxygen functionality improves the surface hydrophilicity enabling beneficial adsorption of reactants. Stability studies

base-catalyzed aldol condensation, and their subsequent HDO, via tandem metal-acid catalysis, has been reported.9 In addition to frequently explored aldol condensation, the HAA condensation of 2-methylfuran (2-MF) with carbonyl containing molecules using an acid catalyst is an efficient alternative strategy for producing fuel precursors.9b,10−12 The reaction involving the selective condensation of 2-MF at its C5 position with carbonyl containing compounds (e.g., furfural, HMF, acetone, butanal, pentanal, levulinic ester) yields C12− C21 fuel precursors in liquid form. Subsequent HDO of the precursors over bifunctional metal-acid catalysts produces alkanes.9a,13 HAA is also a key step for C−C coupling of many organic14,15 and biorenewable molecular building blocks.13 HAA condensation of 2-MF has been reported with homogeneous Brønsted acid catalysts (e.g., p-toluenesulfonic acid, sulfuric acid).16,17 Transition-metal catalysts have also been used for conventional C−C coupling in synthetic chemistry.18 The homogeneous catalyzed condensation poses challenges in the separation of catalysts and recycling. Thus, development of effective heterogeneous catalysts for performing HAA condensation under neat conditions is a desirable strategy for promoting C−C coupling of furanics. Solid catalysts containing framework acidic sites, such as Nafion-212 and zeolites, have been attempted for HAA between 2-MF and carbonyl compounds but have resulted in moderate yields of desired products.16,19 The condensation of 2-MF with furfural using Nafion-212 produced 5,5′-bis(2methylfuranyl)furan-2-ylmethane (BMFFM) with 75% yield.19 In contrast to the reported homo- and heterogeneous catalysis, carbon-framework-based catalysts (e.g., graphene oxide (GO)) are attractive owing to their promising physical and chemical properties.20 GO features several oxygen functionalities, such as epoxy, hydroxyl, carbonyls, and carboxyl groups, which contribute to the material’s Brønsted acidity and hydrophilic/ oxophilic properties21,15b and induce beneficial oxidated defect sites.22,23 These properties can further be tuned via advanced synthesis techniques.24 The unique features of functionalized graphene and GO are reflected in their various applications. Specifically, noncovalent functionalization of GO with metal oxide and hydroxide 3906

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ACS Catalysis demonstrate that IGO can be regenerated, and thus, it is a very promising catalyst for C−C coupling at industrial scale.

Table 1. Catalytic Effectiveness of IGO for Synthesis of 1a from HAA of 2-MF (1.45 g; 17.6 mmol) and Furfural (0.77 g; 0.8 mmol) under Neat Condition



EXPERIMENTAL SECTION Chemicals. Graphite powder, HMF, furfural, acetone, palladium chloride, 2-methylfuran, potassium permanganate, hydrochloric acid (37%), ethanol, ether, phosphoric acid, hydrogen peroxide (30%), and sulfuric acid were purchased from Sigma-Aldrich (U.S.A.). HY(30), HY(80), and HBEA(300) were purchased from commercial sources. ZIF-8 materials were synthesized by reported methods.33 Commercial GO was purchased from Sigma-Aldrich (catalog number 796034-1G). These chemicals were used as received without further purification. IGO preparation and regeneration procedures are detailed in the Supporting Information. Catalyst Characterization. Powder X-ray diffraction (XRD) pattern of IGO, recycled IGO, and commercial GO were 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 40 V and 40 mA. TEM images were collected on a JEM-2010F (JEOL, Japan) transmission electron microscope operating at an accelerating voltage of 200 kV. Samples for TEM images were prepared by applying one drop of dilute suspension of as-synthesized IGO dispersed in acetone onto the carbon-coated Cu grid and allowing the solvent to evaporate at room temperature. Raman analysis was conducted on a Renishaw Raman RM 1000 spectrometer (Renishaw plc, Gloucestershire, U.K.), equipped with a Leica research microscope and a 20× magnifying lens. All measurements were performed at an excited wavelength of 514.5 nm using an argon ion laser source (maximal output 20 mW). FTIR spectra (KBr disk, 4000−400 cm−1) were recorded on a PerkinElmer FTIR 2000 spectrophotometer. SEM images of IGO samples were collected using a cross-beam SEM microscope (Auriga-60, ZEISS) equipped with a Ga+ ion source FIB (Focused Ion Beam). A thermo-fisher Kα + X-ray photoelectron spectrometer equipped with a monochromatic aluminum Kα X-ray source (30−400 nm) was used for XPS analysis. A Bruker MultiMode AFM equipped with a Nanoscope V controller was used for AFM tomography. NMR analysis of HAA products were performed on Ascend 850 Bruker 400 MHz and JEOL JNM ECX-400 P 400 MHz nuclear magnetic resonance (NMR) spectrometers. GC and GC-MS analysis were conducted on an Agilent 7890A GC and Shimadzu-QP2010 Plus Mass spectrometer, respectively. Hydroxyalkylation−Alkylation (HAA) Reactions. Synthesis of 5,5′-Bis(2-methylfuranyl)furan-2-ylmethane (1a). Furfural (0.77 g, 8 mmol), 2-MF (1.45 g, 17.6 mmol), and IGO (60 mg) were charged in a 4 mL screw-cap vial containing a magnetic spin bar. The vial was sealed, and the mixture was stirred for a set reaction time and temperature as listed in Tables 1 and 2 under oil bath heating. After completion of the reaction, the mixture was cooled to room temperature with the catalyst being separated by centrifugation, and the resultant liquid HAA product (1a) was analyzed by NMR and GC. Both 1 H and 13C NMR spectra of the liquid product revealed the formation of 1a (Figures S1 and S2). 1H NMR (400 MHz, CDCl3), δ 2.30 (s, 6H), 5.47 (s, 1H), 5.95 (dq, 2H), 6.03 (d, 2H), 6.16 (dt, 1H), 6.36 (dd, 1H), 7.40 (dd, 1H). 13C NMR (400 MHz, CDCl3) δ 13.5, 39.0, 106.1, 107.0, 107.8, 110.2, 141.8, 150.2, 151.5, 152.5. Total isolated yield of product 1a is 2.05 g (96%).

entry

IGO (mg)

time (h)

temperature (°C)

2-MF conversion (%)

1a yield (%)

1 2 3 4 5 6 7 8 9

60 60 60 60 10 30 60 60 60

1 3 6 12 6 6 6 6 6

60 60 60 60 60 60 25 40 80

90 100 100 100 72 75 76 77 100

83 91 95 95 63 71 71 71 95

Table 2. Comparison of the Catalytic Activity of IGO and Zeolites for the Synthesis of 2a from HAA of 2-MF (1.45 g) and Butanal (0.66 g) at Neat Condition and Varying Reaction Parameters entry

catalyst

loading (mg)

time (h)

T (°C)

yield (%)

1 2 3 4 5 6

IGO HY80 HY80 HY80 HY80 HY30

60 60 120 120 200 100

12 12 12 40 12 12

60 85 60 60 60 60

83 81 75 77 84 65

Synthesis of 5,5′-(Butane-1,1-diyl)bis(2-methylfuran) (2a). Butanal (0.634 g, 8.8 mmol), 2-MF (1.45 g, 17.6 mmol), and IGO (60 mg) were mixed in a 4 mL screw-cap vial containing a magnetic spin bar. The mixture was stirred, and the product was recovered and analyzed by following a similar procedure described for 1a. 1H NMR spectrum (Figure S3) of the liquid product reveals the formation of 2a with high purity. 1H NMR (400 MHz, CDCl3), δ 0.94 (t, 3H), 1.33 (q, 2H), 1.94 (q, 2H), 2.25 (s, 6H), 3.98 (t, 1H), 5.88 (d, 2H), 5.94 (d, 2H). Synthesis of 5,5′-(Propane-2,2-diyl)bis(2-methylfuran) (3a). Acetone (0.232 g, 4 mmol), 2-MF (0.722 g, 8.8 mmol), and IGO (60 mg) were mixed in a 4 mL screw-cap vial equipped with a magnetic spin bar. The mixture was sealed and stirred at 50 °C for 12 h under oil bath heating. The resultant liquid product, recovered by following the procedure described for compound 1a, was analyzed by NMR (Figure S4). 1H NMR (400 MHz, CDCl3), δ 1.59 (s, 6H), 2.25 (s, 6H), 5.86 (dq, 2H), 5.87 (d, 2H). 13C NMR (400 MHz, CDCl3) δ 13.5, 26.4, 37.1, 104.4, 105.6, 150.6, 158.5. Synthesis of 5,5′((5-((5-Methylfuran-2-yl)methyl)furan-2yl)methylene)bis(2-methylfuran) (4a). HMF (0.126 g, 1 mmol), 2-MF (0.246 g, 3 mmol), and IGO (50 mg) were mixed in a 4 mL screw-cap vial. The mixture was sealed and stirred using a magnetic spin bar at 63 °C for 12 h. The resultant liquid product (4a), recovered by following the procedure described for compound 1a, was analyzed by NMR (Figure S5). 1H NMR (400 MHz, CDCl3), δ 2.25 (bs, 9H), 3.91 (s, 2H), 5.39 (s, 1H), 5.878(dq, 1H), 5.92 (dq, 2H), 5.94 (d, 1H), 5.97 (d, 2H), 6.01 (m, 2H), 13C NMR (400 MHz, CDCl3) δ 13.4, 13.5, 27.4, 39.0, 105.9, 106.1, 106.5, 106.9, 107.8, 107.8, 149.6, 150.4, 150.8,151.1, 151.2, 151.3. Synthesis of 5,5′-(1-Hydroxypropane-2,2-diyl)bis(2-methylfuran) (5a). Acetol (0.296 g, 4.0 mmol), 2-MF (0.722 g, 8.8 mmol), and IGO (50 mg) were mixed in a 4 mL screw-cap vial. The mixture was stirred at 63 °C for 12 h. The condensation 3907

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Figure 1. (a) XRD pattern of IGO and commercial GO, (b) normalized Raman spectra of IGO and commercial GO, (c) FTIR spectrum of IGO, and (d) XPS C 1s spectra of IGO, commercial GO, and HOPG.

mmol) in the presence of IGO (60 mg), the catalyst was separated from the product by centrifugation and washed three times with ethanol (5 mL each time). The recovered IGO was dried under vacuum at room temperature and used for the HAA of furfural with 2-MF in the next cycle under comparable reaction conditions. Similarly, the catalyst was reused for four consecutive cycles with the product from each cycle being analyzed by NMR and GC techniques. The recovered catalysts were analyzed by XRD, XPS, Raman, and SEM techniques for determination of surface oxygen functionalities and structural properties. The characterization data of the fresh and recovered catalysts were used for correlation of the changes of the catalytic properties with observed HAA activities. Measurement of Acid Density of IGO. The acid density of IGO was measured by a titration method suitable for solid acids.35 In this method, 50 mg IGO was mixed with an aqueous solution of sodium hydroxide (0.05 mM, 20 mL), which was subsequently ultrasonicated for 1 h at room temperature. After separation of solid IGO by centrifugation, the supernatant was titrated by a dilute aqueous solution of hydrochloric acid (50 mM) using phenolphthalein as an indicator. The concentration of NaOH that was neutralized by IGO was taken as a measure of the total acid density of IGO. Computational Methodology. Density functional theory (DFT) calculations were carried out with the GPAW ab initio code, with core electrons represented with the projector augmented wave function (PAW) method and the valence electrons being represented with the optPBE-vdW density functional. Initial configurations were generated utilizing a combined kinetic Monte Carlo (KMC)/DFT scheme. Full details regarding the computational methodology and the KMC/DFT scheme can be found in the Supporting Information.

product, recovered by following the procedure described for compound 1a, was analyzed by NMR (Figure S6). 1H NMR (400 MHz, CDCl3), δ 1.59 (s, 3H), 2.24 (s, 6H), 3.92 (S, 2H), 5.93 (d, 2H). 13C NMR (400 MHz, CDCl3) δ 13.5, 20.6, 43.5, 68.4, 106.3, 151.0, 154.9. Determination of Yields of Products 1a−5a. The yields of condensation products were determined by analyzing the product solution on a gas chromatograph (GC, Agilent 7890A) equipped with a FID detector and an HP-INNOWAX capillary column of dimension 0.25 mm ID × 0.25 μm × 50 m. The essential parameters for the GC analysis are as follows: injection volume of 1.0 μL, inlet temperature of 250 °C, detector temperature of 250 °C, and a split ratio of 1:10. The initial column temperature was 40 °C (5 min), which was ramped at a rate of 15 °C min−1 with a final temperature of 250 °C. 1a was identified by its retention time (17.7 min), obtained from GC-MS analysis of the same solution. 1a peak in the GC chromatogram was properly integrated, and its concentration was obtained from a precalibrated plot of peak area against concentrations. The calibration plot of 1a was developed by using a sample of 1a of known concentration, which was determined from 1H NMR analysis by the internal standard method in which the intensity of aromatic proton of mesitylene (internal standard) was compared with a −CH (furylmethane) proton peak of 1a.34 GC-MS analysis was carried out on a Shimadzu-QP2010 Plus Mass spectrometer. The typical electron energy was 70 eV with the ion source temperature maintained at 250 °C. Other parameters for GC-MS operation are as follows: 30 m HP-INNOWAX capillary column of dimension 0.25 mm ID × 0.25 μm × 50 m, initial column temperature hold at 40 °C for 5 min and ramped at the rate of 10 °C min−1 to 250 °C, flow rate = 1 mL min−1 and injection temperature = 250 °C. The yields of other condensation products were determined by following the similar procedure. Unless otherwise mentioned, yields were calculated on molar basis. IGO Recycling for HAA. After completion of a reaction between furfural (0.77 g, 8 mmol) and 2-MF (1.45 g, 17.6



RESULTS AND DISCUSSION Imparting Hydrophilic and Oxygen Functionalities to Graphene Oxide. The chemical oxidative exfoliation of graphite in the modified method forms IGO with a highly 3908

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samples, which allows for the decomposition of the spectral features assigned to different bands.48 Although the decomposition of the Raman spectra is simplistic given the complexity of carbon materials, it provides a useful comparative tool to measure surface and edge defects for IGO. Therefore, we analyzed the D and G Raman bands of Figure 1b by curvefitting (Origin 8.5), which generated a combination of three Lorentzian-shaped bands (G, D1, D2) at 1590, 1350, and 1610 cm−1, respectively, with an additional Gaussian-shaped band (D3) at ∼1550 cm−1 (Figure 2). These derived band

oxidized surface, crystallinity, high aspect ratio, and defect sites.36 The appearance of IGO (dark-brown) is also noticeably different from regular GO obtained from Hummer’s method of oxidation. The IGO sample exhibits enhanced surface and bulk oxidation of graphite, which is evidenced from the combination of XRD, Raman, FTIR, and XPS characterizations. The presence of a highly oxidized surface of IGO is supported by XRD results (Figure 1a). The interlayer spacing of IGO as measured by XRD is proportional to the degree of oxidation. Commercial GO shows a lattice spacing similar to graphite (d200 = 3.35 Å at 26.5°); in contrast, the interlayer spacing of IGO is significantly enhanced (d200 = 9.67 Å at 9.5°), which is most likely due to the increased degree of oxidation and lattice disruptions on graphite. The structure and properties of IGO and commercial GO were further assessed by Raman spectroscopy, which is particularly sensitive to the microstructure of carbon materials.37 The Raman spectrum of IGO shows characteristic D and G bands (Figure 1b). The D band at ∼1352 cm−1 is associated with the breathing mode of sp3 carbon and the G band (∼1600 cm−1) is generally assigned to the E2g phonon of the sp2 bond of carbon atoms.38 The intensity ratio of D and G bands provides information about defect formation during the chemical processing of graphite.39 A comparison of ID/IG values of IGO (0.897) and commercial GO (0.379) confirms IGO has a greater number of lattice defects compared to the commercial samples. Furthermore, the FTIR spectrum of IGO (Figure 1c) features a strong vibrational band at 3400 cm−1 that is associated with the O−H stretching mode. Bands at 1718 and 1620 cm−1 are assigned to −COOH and CO vibrations.40 This spectrum also shows two types of peaks for C−O stretching vibration; peaks at 1071 and 820 cm−1 are assigned to −C-O stretching of the epoxy groups, while those at 1470 and 1015 cm−1 correspond to the −C−O stretch of the C−OH groups.41 The FTIR data further confirm that oxygen functionalities are present on the IGO surface.42 Table S1 summarizes the surface atomic contents of IGO and commercial GO. It is clear that the fresh IGO contains much higher oxygen species (36.6%) compared with the commercial GO (9.8%). Figure 1d shows the overlay of the C 1s spectra for IGO, commercial GO, and a reference sample, highly ordered pyrolytic graphite (HOPG). The contribution of oxygen in the C 1s peak at >285 eV corresponds to multiple oxygen functional groups on the IGO surface.31a In order to study these functional groups, we have quantitatively fitted C 1s spectra (vide infra),43 with the results summarized in Table S2. The asymmetric peak at around 284.4 eV is ascribed to sp2 carbon, which is characteristic of graphitic carbon.44 Two symmetric peaks at approximately 284.0 and 284.8 eV are assigned to disordered carbons of both sp3-like configuration and a defective graphitic carbon surface.45 The C−O single bonded oxygenates are fitted with two symmetric peaks, where the C−O in alcohols, phenols, and ethers are at 285.5 eV, and the C−O in keto−enolic equilibria or furans is around 286.2 eV.44−46 Highly oxidized peaks at around 287.4 eV (CO) and 288.7 eV (COO) are assigned according to the literature BE values.47 The minor peak at around 289.5 eV is assigned to carbonate.46b Table S2 shows that the surface oxygen content of IGO is higher than that of the commercial GO, which corroborates with total the surface oxygen content summarized in Table S1. A recent paper by Sadezky et al. outlines a method for the spectral analysis of Raman spectra obtained from carbon

Figure 2. Curve-fitting of Raman spectra of IGO and commercial GO.

characteristics can provide precise defect features of the graphene layers in IGO. The first order D band in Figure 1b is characteristic of a disordered (sp3) graphitic carbon, while the D1 (for A1g symmetry at 1350 cm−1) and D2 (for E2g symmetry at ∼1610 cm−1) bands correspond to disorder at the edges and on the surface of the graphene layers. Comparison of the ID1/IG and ID2/IG values allows for a further elucidation of the nature of the defect sites as removal of the ID2 contribution from IG peak allows for edge and surface defects to be distinguished. The higher ID1/IG values of IGO (based on peak intensities) as compared with the commercial GO (1.57 vs 0.38, respectively; Figure 2) suggest a higher concentration of edge defects in IGO (Table S3). A similar trend is observed in the concentration of surface defects of IGO as measured by ID2/IG (Table S3). The HRTEM images show a continuous sheet like structure of IGO (Figure 3a) consisting of 15−20 layers (Figure 3b). The image also exhibits wrinkles and folded regions, owing to the surface exfoliation of graphite (Figure 3b). Rolling layers of graphene are also present, which is possibly due to the loss of solvent after drying the sample (Figure 3c). The SEM analysis of the top surface of IGO reveals that the surface is slightly folded into a coarse structure, and the sheets are crumpled with multiple wrinkles (Figure 3d). The cross-sectional SEM image exhibits a compact layer-by-layer stacking arrangement (Figure 3e), with a thickness of ∼10 nm for each layer (determined from the cross-section SEM image, Figure 3f), with the morphology of the nanosheets being continuous. This suggests that nascent GO interacts with other GO sheets through interlayer van der Waals forces and stacks during IGO formation. Titration35 of IGO shows a high acid density of 2.1 mmol g−1, which supports the XPS profile (Table S2) showing a high percentage of Brønsted acidic oxygen functional groups (hydroxyl and carboxyl) anchored on the basal plane and edges of IGO. 3909

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Figure 3. TEM images of IGO showing a continuous sheet-like structure (a), wrinkles and folded regions for surface exfoliation (b), and low magnification TEM image (c). SEM images show a coarse structure (d), stacking of multilayers (e), and a cross section (f) of IGO.

HAA Activity of IGO. We next explore the effectiveness of IGO as a carbocatalyst for the C−C coupling of 2-MF with different biomass-derived carbonyl molecules for upgrading of low carbon furanic intermediates to advanced fuel precursors (see Scheme 1). Although there are limited reports of the use GO as an acidic carbocatalyst,49−51 the catalytic activity of IGO containing an extended sheet-like framework and stacked multilayers with a high concentration of oxygen functionalities and defects sites has not been explored, especially for C−C coupling reactions. We first examined the activity of IGO for the HAA reaction of 2-MF and furfural under various reaction conditions in a solventless environment, and we compared its activity with framework solid acid and homogeneous acid catalysts. The results presented in Table 1 show 83% 1a yield at 60 °C for 1 h. The yield improved to 93% upon continuing the reaction for 3 h. Extending the reaction to 6 h shows only a marginal increase in yield (95%), and the yield remains constant as the reaction time is increased to 12 h. Varying the temperature between 25−80 °C and the 2-MF/IGO (w/w) in the range of 24−145 enabled a maximum 95% yield of 1a. Noticeably, a roomtemperature (25 °C) reaction achieved 71% yield of 1a at 2MF/IGO ratio of 24 (w/w) under neat condition. The framework-zeolites HY80, HY30, and BEA300 produced 30%, 28%, and 22% of 1a, respectively, at a 2-MF-to-catalyst ratio of 24 (w/w; 60 mg of catalyst) and 60 °C for 6 h (Figure 4 and Table S4). These results demonstrate the superior catalytic activity of IGO compared to the other solid acid catalysts. Higher HY80 loading (200 mg; 2-MF/catalyst = 7.2 (w/w)) improved the yield of 1a to 63% for a reaction carried out for a longer reaction time (12 h); however, this maximum yield is still significantly lower than that of IGO. The highly Brønsted acidic Amberlyst-15 (Amb-15; acid density 2.2 mmol g−1)52 demonstrated higher activity than the zeolites (Figure 4), with the yield being ∼28% lower than IGO. The initial rates calculated at low 2-MF conversion (9−15%) for IGO, Amb-15, HY80, HY30, and BEA300 catalysts are 774, 704, 43, 29, and 29 mmol g−1 h−1, respectively (Table S5). Their TOFs are 369, 320, 70, 101, and 154 h−1, respectively (Table S5). The rates followed a similar trend for IGO (147 mmol g−1 h−1) and Amb-

Figure 4. Comparison of the catalytic activity of IGO with zeolites and Amb-15 for synthesis of 1a from the HAA of 2-MF (1.45 g; 17.6 mmol) and furfural (0.77 g; 0.8 mmol) using 60 mg catalysts. All reactions were conducted at 60 °C for 6 h. 2-MF to catalyst ratio is 24 (w/w). The initial rates and TOFs for all catalysts at low conversion of 2-MF are summarized in Table S5.

15 (117 mmol g−1 h−1) at lower temperature (25 °C, Table S5). We attribute the observed high activity of IGO to its high concentration of surface oxygen functionalities, leading to Brønsted acidity and defect sites. Given the bulky nature of the products formed from the HAA of furfural and 2-MF,16 the lower activity of zeolites can be attributed to their microporosity; consequently, we analyzed the microporous features and the acid density of our commercial zeolites (HY80, HY30, BEA300) in addition to zeolitic imidazolate framework (ZIFs) materials (Tables 2 and S5). HY80 offered the best yield of 1a (at about 30% conversion of 2-MF) among the Faujasite and Beta zeolites. The initial reaction rates and TOFs for zeolites at low 2-MF conversion and their microporous features are shown in Tables S5 and S6. The results show lower HAA rates and TOFs for the zeolites than IGO and Amb-15. The rates are comparable for HY30 and BEA300 (29 mmol g−1 h−1) and slightly higher for HY80 (43 mmol g−1 h−1). However, the TOF of BEA300 with lower acid density (0.19 mmol g−1) is higher (154 h−1) than 3910

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ACS Catalysis HY30 (101 h−1) and HY80 (70 h−1) having acid densities of 0.29 mmol g−1 and 0.63 mmol g−1, respectively (Table S5). This analysis suggests that TOFs over microporous zeolites are controlled by accessibility of their acid sites. A previous report demonstrated higher HAA activity for delaminated zeolites (ITQ-2)16 having high sites accessibility, owing to large external surface areas (800 m2 g−1) and large cavities on the surface possessing 2.5 nm depth of layers with ∼0.5 nm open channels.53 To further examine this effect, we employed high external surface area materials, i.e., ZIFs; these materials contain framework − NH− groups that act as weak Brønsted acid sites in conjunction with −OH base sites. ZIFs exhibit no HAA activity despite their high surface area, which suggests that the activity of ZIFs is controlled by their weak acidity and microporous features (pore diameter ∼0.34 nm) and not by their high external surface areas. This analysis suggests the catalysts with a larger pore network and channel structure (i.e., ITQ-216) and a high acid density would be effective for HAA coupling. IGO also demonstrates high activity for the coupling of 2-MF with butanal, enabling a maximum 83% yield of 2a at 60 °C for 6 h, while the zeolite catalysts require either higher temperature (85 °C) or 2- to 3-fold higher catalyst loadings or longer reaction times to achieve a similar yield of 2a (Table 2). Similar coupling reactions of 2-MF with acetone, HMF, and acetol using IGO gave 90%, 89%, and 87% yields of 3a, 4a, and 5a, respectively. Comparison of IGO with homogeneous acids is shown in Figure 5. Among the homogeneous catalysts, p-toluenesulfonic

two catalytic cycles under optimal conditions (Table 3). The initial rates at low 2-MF conversion are also comparable for Table 3. Results of IGO Recycling for HAA of 2-MF and Furfurala entry

cycle number

2-MF conversion (%)

1a yield (%)

1 2 3 4 5

1 2 3 4 commercial GO

100 100 70 20 10

95 95 58 0 0

6 h, 60 °C, neat reaction, 60 mg IGO or commercial GO, 1.45 g of 2MF, and 0.77 g of furfural. Fresh IGO was used in cycle 1.

a

IGO and IGOcyc2 (Table S5). Significant deactivation of IGO is noted in the third cycle (58% 1a yield), followed by complete deactivation in the fourth cycle. The initial rates of the catalysts in the third and fourth cycles (IGOcyl3 and IGOcyl4) at low conversion of 2-MF (Table S5) also corroborated with observed deactivation of the catalysts in terms of yields of product 1a. Visual observation shows a significant change in IGO texture after each cycle. These changes of the recycled IGO are also corroborated with the HRTEM images (Figure S7), which reveal that the framework extended sheets-like structure remain the same in the recovered IGO; however, the material lost its layered structure upon recycling and becomes amorphous (Figure S8) because of the surface exfoliation55 by absorbed reactants. To elucidate the surface oxygen functionalities of the recycled materials, we compared the XPS of IGO after cycles 1 and 2 (Figure 6). An elemental analysis according to the XPS spectra (Tables S1 and S2) of IGO, IGOcycl2,

Figure 5. Homogeneous acid catalyzed synthesis of 1a from HAA coupling of 2-MF (0.72 g, 8.8 mmol) and furfural (0.38 g, 4 mmol) at 65 °C for 12 h.

acid (p-TSA) is the most effective (90% 1a yield). H2SO4 shows lower activity, which is possibly due to immiscibility of aqueous H2SO4 phase with the hydrophobic 2-MF.54 Under comparable reaction conditions, weak Brønsted acidic phenol and acetic acid (HOAc) are inactive. The acid concentrations of the homogeneous catalysts, p-TSA (0.29 mmol) and H2SO4 (0.51 mmol), used in the present reactions are higher than the total Brønsted acid density of 60 mg IGO (0.13 mmol). Yet, IGO gives a higher yield of 1a than the two homogeneous acids. Thus, the structural features and surface/edge defects of IGO are believed to play an important role in the enhanced activity in addition to the acid density. Recyclability of IGO. Recycling experiments show comparable activity of IGO, in terms of 1a yield, in the first

Figure 6. Deconvoluted C 1s XPS spectra of IGO, IGOcyl2, and IGOreg. All spectra were calibrated according to the asymmetric graphitic peak at 284.4 eV compared to the reference data of HOPG. The fittings were done by fixing the peak position within ±0.2 eV for all spectra and constraining the full width at half-maximum (fwhm) of 1.1−1.3 eV. 3911

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Figure 7. Overlaid first order Raman spectra of IGO and IGOcyl2 (a) and curve fitting spectrum of IGOcyc2 (b).

erated IGO confirms the regeneration of the surface oxidized carbons and the associated defect sites. The regenerated IGO has a slightly higher graphitic carbon content (CC sp2) than the original IGO (Table S2). To evaluate the catalytic activity of the regenerated IGO, we carried out the HAA of 2-MF and furfural under the conditions shown in entry 3 of Table 1. The results show a full conversion of 2-MF with maximum 93% yield of 1a, suggesting the regenerated IGO regains the majority of its activity. In addition, IGOreg exhibits comparable initial rate as the fresh IGO at low conversion of 2-MF (Table S5). Oxygen Functionality and Defects. It is evident from the deconvoluted C 1s and FTIR spectra that IGO surface comprises of different functional groups such as aromatic CC, aliphatic C−C, hydroxyl (C−O), epoxy (C−O−C), carbonyl (CO), and carboxyl (O−CO) carbons. The percentage of oxidized carbons as well as of surface oxygen atoms decreased (Tables S1) upon recycling of IGO. The first order Raman spectra and their derivative spectra after curvefittings evidenced significant edge and surface defects of IGO when compared with commercial GO. These defects also decreased upon recycling. Thus, the surface oxygen functionalities and defect sites, which are coupled with each other, are important for IGO activity, and the decreasing amount of these features corroborates with the loss of catalytic activity. This interpretation also is consistent with the IGO regaining, upon regeneration, similar functional and structural features and comparable activity as that of the fresh IGO. To further visualize the structural features, we compared the tapping mode of AFM tomography of IGO, IGOcyl2 and commercial GO surfaces and their corresponding height profiles (Figure S11). The profile reveals that IGO and IGOcyl2 surfaces are more patterned and rough than the commercial GO. This essentially indicates that IGO surface is more oxidized with defect sites than the commercial GO. In order to gain atomistic insights into the nature of the surface defect, kinetic Monte Carlo (KMC) and density functional theory (DFT) calculations were carried out in order to create stable structures with various functional groups consistent with XPS data (see SI for details of the computational methodology). We predict several defects (shown as the nonhexagonal regions in Figure 8a) forming at locations of high concentration of carbonyl (CO) species (see Figure 8a for reference); non defected structures are energetically unstable. These results indicate that defects are associated with this oxygen moiety and the concentration of carbonyl species should play an important role in determining the activity of the oxidized carbocatalyst.

IGOcycl4 reveals a loss of oxygen content and functional groups. During recycling, the total percentage of surface oxygen atoms decreased from 36.6% (in IGO), to 28.3% (in IGOcyc2), to 20.2% (in IGOcyc4) (Table S1). While a decrease in the percentage of oxidized carbon and concentration of oxygen atoms corresponds to decrease in the number of acid and defect sites (with a subsequent decrease in the catalytic activity), IGOcyl2 contained a sufficient amount of oxidized carbon to retain a comparable activity of the fresh IGO. Commercial GO is catalytically inactive due to its significantly lower degree of oxidation. As shown in Table S1, IGO contains about 3.17% of sulfur on the surface; the S 2p1/2 peak at 170.08 eV from XPS (Figure S9) reveals an oxidation state of +6, which can be attributed to the presence of a sulfonate groups. This amount of sulfur corresponds to about 1 mmol sulfonate groups per gram of IGO, which is lower than the total measured acid density of IGO (2.1 mmol per gram). While the contribution of sulfur species toward the total HAA activity is not clear, IGOcyl2 retains a similar activity to that of IGO in terms of yield of 1a, despite its significantly lower amount of sulfur (1.62%). This suggests the presence of a small amount of residual sulfur in IGO is not significant in its catalytic activity. In addition, we have shown that 60 mg IGO containing less acid sites (0.13 mmol) exhibit a slightly higher activity (in terms of product 1a yield) than the homogeneous p-TSA, which contains a higher acid concentration (0.29 mmol). This analysis further confirms that, aside from the acidic oxygen functionality, the structural features and defect sites of IGO play an important role in its enhanced activity. Analysis of the Raman spectrum of IGOcyl2 reveals changes in the defected nature of the material as the ID/IG ratio is lower for IGOcyl2 compared to fresh IGO (0.714 vs 0.897; Figure 7a). To further elucidate the changes of defects on the surface and edges, curve fitting of the first order Raman spectra of IGOcyl2 was performed (Figure 7b). This analysis reveals a lower ID1/IG (1.06) and ID2/IG (0.61) values of IGOcyl2 than the fresh IGO sample (ID1/IG (1.57) and ID2/IG (0.84) (based on peak intensities; Table S3), suggesting that the defect sites decreased both on the surface and edges upon recycling. Regeneration of IGO. In order to regain the activity of IGOcyl4, we regenerated the spent catalyst following the procedure described in the SI methods section. The XRD pattern of the regenerated IGO (Figure S10) is similar as that of the fresh IGO with a characteristic peak at 9.5° and an interlayer spacing (d200) of 8.70 Å, suggesting that amorphous IGOcyl4 regained its original surface features to a significant extent upon regeneration. Deconvoluted XPS of the regen3912

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between the adsorbate quadrupole and surface dipole, and the enhanced binding of the perpendicular configuration is due to the lack of this repulsive interaction and the favorable dipole− dipole interaction.57 These results, however, do not explain the difference in activity between IGO and the oxide with a reduced oxygen concentration (i.e., commercial GO and reused IGO). To better understand the effect of a reduced oxygen concentration, the procedure utilized to generate the IGO surface in Figure 8 was repeated; ∼75% of the oxygen functionality was removed. The binding energies and configurations for furfural and 2-MF on the GO surface are shown in Figures S14 and S15. Comparison of GO and IGO reveals that while IGO prefers a vertical binding motif, GO prefers a flatter configuration (in line with the reduced number of surface dipoles that would be present on the GO surface). Additionally, our calculations reveal the GO surface has a larger graphitic region that can accommodate our heterocylic aromatics. Interestingly, the binding energy for furfural for this region is within DFT error of the binding energy to a Bronsted acid site. These results indicate binding to nonreactive graphene regions would be competitive with the binding to the acidic active sites; thus, we can ascribe the difference in reactivity between IGO and GO being due to this competitive binding between graphitic regions and the Bronsted acid sites. From a mechanistic standpoint, these results show that in order to form the requisite electrophilic carbenium ion, the furfural must first interact with the surface in a perpendicular configuration (A of Scheme 2). The resultant carbenium ion can then readily react with the 2-MF after the formation of a carbanion at the ω-position (B of Scheme 2). The resulting charge−charge interaction between furfural and 2-MF results in formation of the furanic dimer, with an alcohol group as a linker (C of Scheme 2). As the resulting alcohol species dehydrates, a carbenium ion forms,58 which can then react with another 2MF carbanion.

Figure 8. (a) Representative DFT optimized geometry of IGO: carbon-gray; oxygen-red; hydrogen-white. (b) Outline of the carbon backbone with nonhexagons denoting defected regions and black dots denoted the location of the carbon atoms. Comparison of (b) with (a) reveals defects are associated with the carbonyl functional groups. The solid line around both figures denotes the presence of the periodic unit cell used in the DFT calculations.

HAA Mechanism on the IGO Surface. HAA condensation of furans with carbonyl molecules has been reported with homogeneous acid catalysts and Cu(OTf)2.56 It has been proposed that an oxocarbenium intermediate is formed first at the carbonyl center via interaction with acid sites. Subsequently, 2-MF readily interacts with the electrophilic oxocarbenium species to form a dimer. The hydroxyl group of the dimer further propagates the reaction with another molecule of furan to form a trimer. In order to understand the interaction of furanics with the defect sites in Figure 8, we examined the adsorption of 2-MF and furfural on the IGO surface, with the configurations and binding energies shown in Figures S12 and S13. The DFT binding energies reveal the aromatic furan molecule prefers to be bound to the surface in a perpendicular configuration, with the binding energy being enhanced by a factor of 2−3 over that of the parallel configuration. These results can be rationalized on the basis of the intermolecular interactions between the adsorbate and the IGO surface. Because of the presence of the oxygen functionalities, the IGO surface would carry a dipole in the direction normal to the surface; in a similar vein cyclical aromatics carry an out-of-plane quadrupole moment, with the addition of a heteratom into the ring forming a dipole in plane. This information indicates that the weaker binding for the parallel configuration arises due to a repulsive interaction



CONCLUSIONS

We report an efficient approach for the production of high carbon, branched-chain fuel precursors of C12−C21 from biomass-derived low-carbon building blocks. In this approach, a lattice defected multilayer improved graphene oxide (IGO) is synthesized via a refined Hummer’s method. The results suggest that IGO is highly active for low-temperature C−C

Scheme 2. Plausible Mechanism for HAA of 2-MF and Furfural on the IGO Surface (A) Interaction of Furfural and 2-MF in Perpendicular Orientation on IGO, (B) Resulting Carbenium Ion of Furfural Reacts with Carbanion of 2-MF, and (C) Furanic Dimer Formation

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Technologies resources at the University of Delaware, specifically the high-performance computing resources.

coupling under neat conditions, giving a maximum 95% C15 condensation product with high selectivity via hydroxyalkylation/alkylation (HAA) of 2-MF with furfural. Similar activity is observed for HAA of other biorenewable carbonyl molecules. A comparative study with framework Brønsted acidic zeolites reveals IGO has a higher activity. Extensive solid-state characterization of IGO and recycled materials using spectroscopic techniques (XRD, IR, Raman, XPS, and FTIR) alongside of microscopic imaging (SEM, HRTEM, AFM) reveals that IGO is crystalline and the surface is highly oxidized, and features Brønsted acid functionalities and defect sites on the surface and edges. The catalyst after the fourth cycle becomes inactive with a loss of its surface oxygen functionalities and defects. However, the catalyst regains the surface features upon regeneration. Characterization and activity data confirm that the high degree of surface oxidation with associated defect sites and the multilayer crystalline features are responsible for the high catalytic activity of IGO in C−C coupling. Furthermore, we elucidate defect formation at the surface carbonyl sites and propose a plausible surface reaction mechanism through first-principles calculations. The high activity of IGO under neat conditions and its low cost open up opportunities to further explore its catalytic activity for other C−C coupling reactions.





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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b03113. IGO preparation and regeneration methods; curve-fitting procedure; IGO surface atomic percentages and functional groups; initial rates; 1H and 13C NMR spectra of products and additional TEM, XRD, AFM, and XPS images and spectra of IGO; recycled and regenerated IGO; DFT calculation methods (PDF)



REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Basudeb Saha: 0000-0002-3591-3227 Author Contributions #

S.D. and A.B. contributed equally.

Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was 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. A.B. acknowledges the University Grant Commission for a DS Kothari Postdoctoral Research Fellowship. The authors acknowledge support for XPS (funded by NSF award number 1428149). We also acknowledge the resources of the National Energy Research Scientific Computing Center (which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC0205CH11231) and support through the use of Information 3914

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