Acid Bifunctional Catalyst ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Graphene-Based Metal/Acid Bifunctional Catalyst for the Conversion of Levulinic Acid to γ‑Valerolactone Yong Wang, Zeming Rong,* Yu Wang, Ting Wang, Qinqin Du, Yue Wang, and Jingping Qu State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, No. 2 Linggong Road, Ganjingzi District, Dalian 116024, P. R. China S Supporting Information *

ABSTRACT: A facile strategy was developed for the preparation of Ru nanoparticles supported on reduced graphene oxide (Ru/rGO) and its following functionalization with benzenesulfonic acid groups (Ru/rGO-S). The formation of a C−C bond between p-sulfophenyl and sp2 carbon of graphene can be unambiguously confirmed by XPS quantitative analysis. The two catalysts were used to catalyze the conversion of levulinic acid in aqueous phase, which is an important reaction during biorefinery. Not GC but HPLC was found to be the reliable analysis method for this reaction. Both of the catalysts exhibited high hydrogenation activities under mild reaction conditions (50 °C, 2 MPa), although their selectivities were different. The hydrogenated intermediate 4-hydroxyvaleric acid could be accumulated over Ru/rGO with a yield of 82% in 40 min, but the main product over Ru/rGO-S was γ-valerolactone (a yield of 82% within the same time) due to the accelerated dehydration process by strong acid sites. After being reused several times, the hydrogenation activities of Ru in each catalyst and dehydration ability of SO3H could be well kept. This effective method could be a general way to prepare graphene-based bifunctional catalysts, which are expected to have broad application prospects in biomass conversion. KEYWORDS: Ru catalyst, Covalent functionalization, Hydrogenation, Dehydration, Biomass

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nanoarchitecture of a metal/acid bifunctional catalyst,12 which is increasingly researched and applied in biomass conversion.13−16 In fact, bifunctional or multifunctional catalysts have the advantage of integrating several catalytic processes (hydrogenation, oxidation, hydrolysis, or dehydration, etc.) that are usually tandem or parallel in the transformations of various complex biomasses and their derivatives.17,18 Take the conversion of levulinic acid (LA) to gamma-valerolactone (GVL) (a simple but key step during biorefinery)19−24 for example, there are also two different pathways (Scheme 1) over the catalytic system of Ru/C and pressurized hydrogen, and the one via 4-hydroxyvaleric acid (HVA) as intermediate has been experimentally confirmed to be dominant.25−27 Bond et al. found the hydrogenation of LA to HVA to be kinetically beneficial relative to the dehydration of HVA to GVL at low temperature (within 70 °C), and the latter process can be accelerated by introducing acid catalysts or elevating the reaction temperature.27 In light of the low-energy consumption, some solid acids (Amberlyst A70 or A15 etc.) were used as cocatalysts of Ru/C to promote the conversion of LA to

INTRODUCTION With the rapid depletion of fossil fuels, biomass has received significant attention as a sustainable source for releasing the dependence upon nonrenewable energy and reducing greenhouse gas emissions.1 Catalytic conversion of biomass is of vital importance to provide biofuels2 and value-added chemicals.3 For the development of high-performance and cost-effective catalysts, carbon materials, which can also be prepared from biomass,4 have been advocated as prominent catalyst supports or carbocatalysts for biomass transformation due to the large specific surface area, high porosity, excellent electron conductivity, and resistance to acid or basic media.5,6 Graphene is a rising “shining star” among carbon allotropes,7 and its application in catalytic conversion of biomass was recently reviewed.8 In general, graphene-based materials have mainly been used as solid acid catalysts (graphene oxide or sulfonated graphene) for the hydrolysis or dehydration of various carbohydrates.8 Only a handful of graphene-supported metal or metal oxide catalysts have been synthesized for the transformations of biomass-derived platform molecules, such as conversion of polyols to lactic acid over CuPd bimetallic nanoparticles (NPs) loaded on reduced graphene oxide (rGO),9 dehydration of xylose to furfural over TiO2/rGO,10 and hydrogenation of cellulose or cellobiose to sorbitol over Pt/rGO.11 However, graphene has not been explored on the © 2016 American Chemical Society

Received: September 18, 2016 Revised: November 26, 2016 Published: December 16, 2016 1538

DOI: 10.1021/acssuschemeng.6b02244 ACS Sustainable Chem. Eng. 2017, 5, 1538−1548

Research Article

ACS Sustainable Chemistry & Engineering

especially for the conversion of biomass-derived platform molecules.

Scheme 1. Reaction Pathways for the Conversion of LA to GVL

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EXPERIMENTAL SECTION

Preparation of Ru/rGO. First, graphite powder was oxidized to prepare graphene oxide (GO) by an improved synthesis method.36 Next, 120 mg of GO was dissolved in 100 mL of deionized water aided by ultrasound and agitation and then mixed with 0.60 mL of an aqueous solution of RuCl3•3H2O (0.049 M). The mixture was transferred into a 200 mL stainless-steel autoclave with Teflon lining. The reactor was sealed, purged with N2 and H2 each three times, and pressurized to 1 MPa with H2 before being placed in an oil bath (140 °C). The hydrothermal treatment lasted for 4 h with continuous stirring. Finally, the acquired solid (Ru/rGO) was filtrated and washed with water until reaching neutral. There is no residual Ru3+ in the filtrate as confirmed by ICP-OES, and thus the actual loading amount of Ru is around 5 wt % based on the total weight (∼62 mg) of Ru/ rGO. Preparation of Ru/rGO-S. First, the aryl diazonium salt of sulfanilic acid was synthesized as below: 3.12 g of sulfanilic acid was dissolved in 180 mL of aqueous solution of HCl (1 M), and then 20 mL of an aqueous solution of NaNO2 (1 M) was dropwise added with continuous stirring at 3−5 °C; after allowing the reaction to proceed for 2 h, the white precipitate that was generated was filtered and washed with water for immediate use. Next, 31 mg of fresh Ru/rGO was dispersed in 10 mL of water, and then all of the diazonium salt, 10 mL of ethanol, and 10 mL of H3PO2 (50 wt %) were successively added with continuous stirring at 3−5 °C; after 30 min, another 10 mL of H3PO2 was supplied and reacted for 1 more hour. Finally, the sulfonated catalyst (Ru/rGO-S) was filtered and thoroughly washed with warm water. No leaching of Ru was confirmed by ICP-OES testing on the filtrate, and thus the actual loading amount of Ru is around 4 wt % based on the total weight (∼38 mg) of Ru/rGO-S. Characterization Techniques. Scanning electron microscope (SEM) images were acquired on an FEI Quanta 450 microscope with an energy dispersive spectrometer (EDS) unit operating at an acceleration voltage of 20 kV. Transmission electron microscope (TEM) images were acquired on an FEI Tecnai G2 microscope operating at 200 kV. The TEM sample was prepared by dispersing the catalyst powder in ethanol with the aid of ultrasound, and then the mixture was dropped onto the copper grid with an ultrathin carbon film. Approximately 300−500 Ru NPs were randomly counted to determine the particle size distribution. The mean size in each catalyst was calculated from the following formula: d = (Σnidi)/ni. X-ray diffraction (XRD) patterns of all samples were obtained with a RIGAKU D/MAX 2400 diffractometer using Cu Kα radiation (40 kV, 100 mA) in the range of 5−85°. The X-ray photoelectron spectroscopy (XPS) measurement was carried out on an ESCALAB MKII spectrometer using monochromatic Al Kα radiation (1486.6 eV). The binding energies were calibrated based on the graphite C 1s peak at 284.5 eV. The CASA XPS program with a Gaussian− Lorentzian mix function and Shirley background subtraction was

GVL.27,28 More interestingly, Ru NPs were directly loaded on some polymers with sulfonic groups,29,30 and these metal/acid bifunctional catalysts can yield GVL from LA under mild aqueous conditions with no need for additives. However, such a “direct” way would still have some disadvantages, as others found that the sulfated supports (hydroxyapatite31 and ordered mesoporous carbon32) could inhibit the activity of Ru catalysts on LA hydrogenation, probably due to the poisoning effect of low-valence sulfide species.32 Thus, a “two-step” approach was developed here to prepare Ru/SO3H bifunctional catalyst based on graphene. Ru NPs were first loaded on rGO through a green hydrothermal synthesis, and then a mild sulfonation of the as-prepared Ru/ rGO was achieved by aryl diazonium salt of sulfanilic acid to afford Ru/rGO-S (Figure 1). Both catalysts were employed in the conversion of LA. Ru/rGO performed with higher activity than many other reported catalysts did at relatively low temperature, and the main product was HVA; Ru/rGO-S improved the selectivity to GVL due to acceleration of the dehydration process by SO3H, which hardly influenced the hydrogenation activity of Ru. We previously found the superiority of graphene as a support for Ru33 or Pt34 nanocatalysts on benzene or cinnamaldehyde hydrogenation relative to that on carbon nanotubes (CNTs) and activated carbon (AC) through modulation on the electronic or geometric structures of metal NPs or the enhanced adsorption of substrates and elimination of diffusion resistance, and this work again validated its advantage in such the heterogeneous catalytic reaction. Furthermore, it illuminated that the twodimensional graphitic structure of graphene could allow one to construct various organic−inorganic hybrid nanocomposites through the combination of covalent chemistry35 and nanotechnology. To the best of our knowledge, this is the first example of a graphene-based metal/acid bifunctional catalyst,

Figure 1. Schematic illustration of the catalyst preparation strategy. 1539


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ACS Sustainable Chemistry & Engineering employed to deconvolute the XPS spectra. The infrared (IR) spectrum was collected on a Thermo Scientific NICOLET 6700 FT-IR spectrometer in the range of 400−4000 cm−1. The Raman spectrum was collected on a Thermo Scientific DXR Raman microscope using a 532 nm laser source and a power of 0.1 mW to avoid damaging the sample. Temperature-programmed reduction (TPR) and temperatureprogrammed desorption (TPD) were performed on a TP-5076 Adsorption Instrument (Tianjin Xianquan Industry and Trading Co., Ltd.). In detail, 95 mg of each vacuum-dried catalyst was heated from room temperature to 800 °C at a rate of 10 °C min−1 in a flow of 5% H2/N2 (30 mL min−1, STP) for H2-TPR or N2 (99.999%) for N2TRD. For NH3-TPD testing, 95 mg of each vacuum-dried catalyst was first heated at 110 °C for 1 h in a flow of N2, and then NH3 (99.999%) was switched to flow through the sample for another 1 h at 110 °C; after that, N2 was switched back to purge physically adsorbed NH3 for 1 h at 110 °C. Finally, once the system was cooled down to room temperature, TPD was performed until reaching 400 °C. Catalytic Conversion of LA. The catalytic reaction was conducted in a 70 mL stainless-steel autoclave. Catalyst (10 mg of Ru/rGO and 12 mg of Ru/rGO-S to ensure the same charging of ∼0.5 mg of Ru), substrate (200 mg of LA, the molar ratio of LA to Ru is 348) and solvent (10 mL of H2O) were directly added to it. The reactor was sealed and replaced with N2 and H2 each three times before being placed in a water bath. When the inner temperature reached 50 °C, H2 was filled into the system with 2 MPa, and then the time was recorded once agitated (∼700 rpm). It was additionally confirmed that the autoclave itself cannot catalyze this reaction under these conditions through conducting contrast experiments with and without Teflon lining. For the catalyst stability testing, each catalyst after 40 min reaction under the above conditions was filtrated and washed six times with water for recycling usage. Samples were extracted at intervals and immediately analyzed by HPLC (Elite, P230) equipped with a refractive index detector and SinoChrom C18 column (ODS-BP, 4.6 mm × 250 mm, 5 μm), using acetonitrile/water (12:88) as mobile phase at a flow of 1.0 mL min−1. GC-MS (Agilent, 5975C), LC-MS (Thermo Scientific, ACCELA SYSTEM, TSQ Quantum Ultra, electrospray ionization), and 1H NMR (Bruker Avance II 400) were combined to do the qualitative analysis.

Figure 2. XRD patterns of GO, Ru/rGO, and Ru/rGO-S.

Figure 3. IR spectra of GO, Ru/rGO, and Ru/rGO-S.

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some possible damage on the structure of Ru/rGO. Unlike the methods using chlorosulfonic acid or sulfuric acid, which require harsh conditions, the handling of 4-benzenediazonium sulfonate is more simple and feasible to prepare sulfonated graphene.46 As seen from the IR spectra (Figure 3), the peaks at 1179, 1121, 1035, 1005, and 835 cm−1 (two νSO, one νS‑phenyl, and two δphenyl‑H) confirmed the presence of benzenesulfonic acid groups in Ru/rGO-S,47,48 the stronger absorption band at 3432 cm−1 (νO−H) should result from the SO3H groups, and the blue shift of νCC (from 1548 to 1572 cm−1) might be due to the contribution of the newly introduced phenyl rings. XPS full spectra (Figure 4) again proved the successful introduction of S in Ru/rGO-S. The single peak of S 2p emerging at 168.1 eV (inset in Figure 4) confirmed the existence of SO3H, and no low-valence sulfide species (∼164 eV)32 formed during sulfonation. Incidentally, the fluorine element emerging at 689.5 eV for both samples should be derived from Teflon components (impurity 540 °C) probably due to the inferior stability of sp3 carbon and SO3H. H2-TPR profiles could also confirm this issue: the broad peaks at higher temperature should be associated with the hydrogasification of supports catalyzed by Ru NPs, and the peak value for Ru/rGO-S (∼290 °C) is much

into six bands based on the literature.52,56,57 The main peak at 284.5 eV originates in sp2-hybridized graphite-like carbon atoms. A peak at 285.1 eV is assigned to sp3-hybridized carbon atoms. Peaks with higher binding energies at 286.2, 287.5, and 288.9 eV are considered as C−O (phenols and ethers), CO (ketones and quinones), O−CO (carboxyls and esters), or C−SO3H (a strong electron-withdrawing group in Ru/rGO-S only), respectively. The π−π* transition loss peak at 291.0 eV corresponds to the characteristic shakeup line of carbon in aromatic structures. The relative atomic concentrations of these

Table 1. Relative Atomic Concentration of Peaks in the C 1s Spectra of Ru/rGO and Ru/rGO-S relative atomic concentration (%)

a

catalyst

2

sp C

3

sp C

C−O and CO

O−CO and C−SO3H

π−π*

Ru/rGOa Ru/rGO-S

66.0 (67.9) 51.1

11.0 (11.3) 26.1

15.6 (16.1) 15.9

4.6 (4.7) 6.9

2.8 0

Values in parentheses are percentages after excluding the one of π−π*. 1542


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ACS Sustainable Chemistry & Engineering lower than that for Ru/rGO (∼470 °C). The peaks around 100 °C should be attributed to the hydrogenation of some oxygen species on the surface of Ru NPs and hydrogen spillover to the carbon support. The higher temperature (112 °C) for Ru/rGOS might be also due to its greater proportion of sp3 carbon, which may retard the hydrogen spillover effect to a certain extent. Regardless, Ru/rGO-S should be quite stable up to 200 °C under a H2 atmosphere, ensuring a wide application scope of such a graphene-based metal/acid bifunctional catalyst. Catalytic Conversion of LA. The two catalysts were employed in the conversion of LA to GVL, one important biomass-derived platform molecule to another.2,17,19−24 As shown in Figure 9, the yield of GVL is much higher over Ru/

Ru/rGO-S promoted the dehydration of the intermediate HVA and thus rendered a higher yield of GVL. For Ru/rGO-S, the rate of HVA dehydration slowed down after 40 min due to the competitive diffusion between HVA and GVL, and 100% yield of GVL could still be obtained within 80 min. Whereas for Ru/ rGO, a much longer time (5 h) was needed to reach a nearly 100% yield of GVL. Recently, more and more efforts have been focused on the improvement of catalytic activity toward this reaction at relatively low temperature. Table 2 summarizes various catalytic systems in this category. Usually, supported Ru catalysts are preferred due to their intrinsically high activity for LA hydrogenation62 and low cost relative to other noble metals,63 and aqueous solutions of LA were mostly adopted under batch conditions.64 As shown in Table 2, both Ru/rGO and Ru/rGOS performed with higher activities compared with those of previously reported results. Their TOFs (640−660 h−1) are almost twice as much as the higher ones (290−380 h−1) among others. This should be because of the benefit of the small sizes of Ru NPs and especially the unique properties of graphene as the catalyst support.65,66 Just as we found for cinnamaldehyde hydrogenation, Pt/rGO showed the best activity and selectivity for the hydrogenation of the CO bond compared with those of Pt/CNTs and Pt/AC, probably due to its special surface chemistry for adsorption/desorption and its open planar structure for diffusion.34 Table 2 also shows that the selectivity to GVL is much lower for Ru/rGO than that of others. The hydrogenated intermediate HVA can be accumulated as the sole byproduct at low temperature, which is in accordance with the research of Bond’s group27,51 and Heeres’s group,67,68 whereas others scarcely found the existence of HVA even at lower conversion of LA. For the influence of LA concentration on the selectivity to GVL to be investigated, 1000 mg instead of 200 mg of LA was used to perform the catalytic reactions over Ru/rGO and Ru/rGO-S. Thus, the molar ratio of LA to Ru was increased from 348 to 1742, far higher than others in Table 2. As seen in Figure 10, a higher concentration of LA could indeed improve the selectivity to GVL to a certain extent, probably due to the self catalysis. However, we can also observe the accumulation of HVA, and the rate of its intramolecular dehydration is much

Figure 9. Yield of GVL over Ru/rGO and Ru/rGO-S at various intervals. Reaction conditions: each catalyst with 0.5 mg of Ru, 200 mg of LA, 10 mL of H2O, 2 MPa H2, and 50 °C.

rGO-S than that over Ru/rGO, which surely confirms the effectiveness of our bifunctional catalyst. Actually, the conversions of LA at 30 min are both over 90% for the two catalysts, and full conversions can be reached within 40 min, but their selectivities are completely different (entries 1 and 2, Table 2). The change in selectivity to GVL between the two intervals is more obvious for Ru/rGO-S (from 57 to 82%) than for Ru/rGO (from 13 to 18%). The strong acidity of SO3H in

Table 2. Comparison of LA Hydrogenation Performance over Supported Ru Catalysts at Relatively Low Temperature under Batch Conditions entry

catalyst

sizea/nm

S:Cb

T/°C

P(H2)/MPa

1d

Ru/rGO

2.0

348

50

2

2d

Ru/rGO-S

2.1

348

50

2

3d 4

Ru/rGO + S-rGO Ru/C + A7028

2.0

348 1000

5 6 7 8 9 10 11

Ru/SMS58 Ru/HAP59 Ru/OMC-P32 Ru/DOWEX29 Ru/MIL-101(Cr)60 Ru/SPES30 Ru/TiO261

1.5 10−20 1.6 2.8 2.4 3.0 2.9

200 348 1000 420 333 856 286

50 50 70 70 70 70 70 70 70 70

2 3 0.5 0.5 0.5 0.7 1 1 3 5

time/h

conv./%

sel.(GVL)/%

TOFc/h−1

0.5 0.67 0.5 0.67 0.5 3 3 4 4 6 4 5 2 1

95 100 92 100 90 93 98 99.2 99 98 98.3 99 87.9 100

13 18 57 82 51 99.1 99.5 96.4 99 94 99 99 99 100

661 522 641 522 627 310 327 50 86 163 102 67 376 286

a

Average particle size of Ru NPs obtained from TEM. bMolar ratio of substrate (LA) to catalyst (Ru). cTurnover frequency, as (mol LA converted)/ (mol Ru × h) as calculated from literature data at the conversion indicated and on bulk Ru content. dThis work. 1543


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HVA, which had been identified by 1H NMR and LC-MS analysis, was largely or completely converted to GVL during GC or GC-MS analysis. Although HVA can be detected by GC analysis elsewhere,25,28,70 it is still questionable to determine its real content. Thus, HPLC operated at room temperature is suggested to be the reliable analysis method for this reaction.27,29,32,51,61 For the dehydration of HVA to form GVL at relatively low temperature to be promoted, strong acid sites are needed to cooperate with Ru NPs. After introducing benzenesulfonic acid groups on Ru/rGO, the selectivity to GVL was notably improved (Figures 9 and 10). The strong acid density of Ru/ rGO-S was calculated to be 0.84 and 1.22 mmol/g based on EDS and XPS analyses, respectively. This is comparable to the ones of reported sulfonated graphene (0.5−2 mmol/g summarized in Table S4), which usually exhibited higher activities for various acid-catalyzed reactions than did some commercial solid acid catalysts. Here, a similar sulfonated graphene, S-rGO, was also synthesized according to the literature55 for comparison. It served as cocatalyst with Ru/ rGO with equivalent mass to catalyze LA conversion (entry 3 in Table 2), and the selectivity to GVL was improved from 13 to 51% after 0.5 h, indicating the significant role of benzenesulfonic acid groups. Meanwhile, the selectivity to GVL over Ru/rGO-S (57%) was slightly higher than that over the blended catalytic system (51%), which may suggest that a certain synergistic effect between the adjacent metal sites and acid sites should be present in this bifunctional catalyst. Furthermore, for both Ru/rGO-S and the blended one, the hydrogenation activities of Ru were scarcely influenced by SO3H, benefitting from the inexistence of low-valence sulfide species in our catalysts (Figure 5). The catalyst stability was evaluated for both catalysts under the same conditions. As shown in Figure 11, the main product over Ru/rGO was HVA with the selectivity always higher than 83% in each 40 min run, whereas the conversion of LA declined by 18% after 10× reuse. Considering that the Ru leachings in solution for both cycles over the two catalysts are negligible (ICP-OES), the growth of Ru NPs (Figure 6) should be responsible for this, and the mass loss of catalysts during filtration after each run might be another reason. In the case of Ru/rGO-S, the conversion of LA in the eighth run declined to 78%, which is 10% lower than that over Ru/rGO (88% in the eighth run), which should be partly ascribed to the more severe sintering of Ru NPs (Figure 6). Meanwhile, the selectivity to GVL was 82% in the first run and reached 90% in the second run, but it dropped to 68% after 8× reuse, indicating a gradual deactivation of acid sites. Additional XPS spectra (Figure S6) confirmed that the sulfur is unlikely to be leached during the reaction under relatively mild conditions (50 °C, 2 MPa H2), and this is consistent with the characterization of H2-TPR

Figure 10. Conversion of LA and selectivity to HVA and GVL over Ru/rGO (left column) and Ru/rGO-S (right column) at various intervals. Reaction conditions: each catalyst with 0.5 mg of Ru, 1000 mg of LA, 20 mL of H2O, 2 MPa H2, and 50 °C.

lower over Ru/rGO than over Ru/rGO-S. This indicates that the obvious difference in selectivity between our work and others (Table 2) might be attributed to an intrinsic kinetic factor. Bond et al. found that the hydrogenation of LA to HVA is kinetically beneficial compared with the dehydration of HVA to GVL at relatively low temperature (within 70 °C),27 whereas Ruppert et al. supposed the formation of HVA is most likely the rate-limiting step in their study.61 Moreover, Heeres et al. recently confirmed the large influence of intraparticle diffusion limitations on this reaction.67,68 These kinetic factors should greatly depend on the adopted catalytic system. In our case, because of the higher activity of Ru and the two-dimensional open structure of rGO, HVA can be quickly formed and easily diffused into the liquid bulk. Recently, some analogous graphene-supported Ru catalysts were reported to catalyze this reaction near room temperature.69−71 For comparison, the catalytic reaction over Ru/rGO under comparable conditions was conducted. As shown in Table 3, the TOFs of these graphene-supported Ru catalysts were quite similar, but the selectivity to GVL (24%) over our Ru/rGO catalyst was still much lower. We found that the methods of product analysis in the three works are the same GC analysis,69−71 whereas the HPLC analysis was adopted in our case. When we used GC to do the analysis, the selectivity to GVL was 92%, which is much higher than the 24% by HPLC analysis. Thus, this oft-ignored analysis method should be the major reason for the lower selectivity to GVL over Ru/rGO. In fact, the cyclization of HVA to GVL is thermodynamically favorable,51 and this process can be accelerated under elevated temperature,27 which is inevitable during GC analysis. As we (section S2 in Supporting Information) and others72 found,

Table 3. Comparison of LA Hydrogenation Performance over Graphene-Supported Ru Catalysts near Room Temperature under Batch Conditions entry d

1 2 3 4

catalyst

sizea/nm

S:Cb

T/°C

P(H2)/MPa

time/h

conv./%

sel.(GVL)/%

TOFc/h−1

Ru/rGO Ru/RGO69 Ru/FLG70 Ru/graphene71

2.0 2.0 1.1 2−6

1742 1449 1460 1037

25 20 20 30

2 4 4 6

10 8 8 6

99 100 99.3 100

24 99.9 97.7 96

174 181 181 173

a

Average particle size of Ru NPs as obtained from TEM. bMolar ratio of substrate (LA) to catalyst (Ru). cTurnover frequency as (mol LA converted)/(mol Ru × h) as calculated from literature data at the conversion indicated and on bulk Ru content. dThis work. 1544


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(Table S2), and its conversion of LA (25%) and selectivity to GVL (20%) within 30 min are much lower than the corresponding ones of Ru/rGO-S (entry 2 in Table 2), indicating that the strong interaction between metal and sulfonic acid sites76−78 could inhibit the activity of Ru NPs and the acidity of sulfonic acid groups. Thus, we believe that the synthetic strategy of Ru/rGO-S should be a simple, general, and effective method for constructing metal/acid (or base or ionic liquid) bifunctional catalysts by covalent chemistry on graphene.35 Recently, many other SO3H-containing bifunctional catalysts13−16 or blended catalytic systems28,79−81 (e.g., Ru/C + A70)28 have been increasingly developed and employed in the transformations of biomass or its derivatives, such that the bifunctional catalysts with the “genius of graphene” would have great potential applications in biomass conversion.

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CONCLUSIONS A facile way to prepare a Ru/SO3H bifunctional catalyst was developed on rGO, and it was used for the catalytic conversion of LA in aqueous phase. The strong acidity of sulfonic acid groups could promote the dehydration of HVA to form GVL at low temperature (50 °C) but not severely weaken the hydrogenation activity of Ru. This work enriched and expanded the field of graphene-based heterogeneous catalysts65,66 and opened new possibilities for biomass conversion.5,8

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Figure 11. Catalyst stability testing over Ru/rGO (upper) and Ru/ rGO-S. Reaction conditions: each catalyst with 0.5 mg of Ru, 200 mg of LA, 10 mL of H2O, 2 MPa H2, 50 °C, and 40 min for each cycle.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02244. Influence of factors on catalyst preparation (Tables S1 and S2), qualitative analysis of products (1H NMR, LCMS, and GC-MS spectra), and complementary characterization of catalysts (NH3-TPD, XPS, and Raman spectra; Tables S3 and S4) (PDF)

(Figure 8), that the benzenesulfonic acid groups in Ru/rGO-S are quite stable up to 200 °C under a H2 atmosphere. Thus, a similar reversible deactivation caused by “site blocking”51 might be responsible here, which can simultaneously inhibit the hydrogenation activity of Ru sites. We suppose that both hydrogenation and dehydration reactions conducted over Ru/ rGO-S would result in a more severe coverage of carbohydrate on the surface of catalyst than the situation of the single hydrogenation reaction conducted over Ru/rGO. Although the stability of Ru/rGO-S is not as good as that of Ru/rGO, it should be quite satisfactory for use here or other reactions. As discussed above, the graphene-based metal/acid bifunctional catalyst was successfully synthesized precisely for the first time. Although some similar nanocomposites with both metal NPs and benzenesulfonic acid groups on graphene have been reported elsewhere,48,73,74 all of them only utilized the hydrophilicity of the “water-soluble graphene”47 to achieve the handy immobilization of metal NPs and subsequently to make the catalysts well-dispersed in aqueous media for reaction. In this thesis, the strong acidity of benzenesulfonic acid groups is emphasized to cooperate with Ru sites on rGO, and the key factor in preparing such a bifunctional catalyst is the sequence of metal loading and sulfonation (various factors during catalyst preparation were investigated thoroughly, which can be seen in section S1 of the Supporting Information). Unlike the reported procedures that load metal on presulfonated graphene,48,73,74 we first load Ru NPs on rGO by a green coreduction method during which Ru NPs would be mostly anchored on the defective sites of rGO,75 and then the sp2-carbon domains of rGO would be open to enabling the mild sulfonation reaction with the aryl diazonium salt47 without prejudice to the existing Ru NPs. For comparison, Ru/S-rGO prepared by directly loading of Ru NPs on S-rGO was also used for LA conversion

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; tel: +86 411 8498 6242. ORCID

Zeming Rong: 0000-0003-0799-0871 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21206015), Specialized Research Fund for the Doctoral Program of Higher Education (20120041120022), and the Fundamental Research Funds for the Central Universities (DUT16LK21).

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REFERENCES

(1) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of transportation fuels from biomass: chemistry, catalysts, and engineering. Chem. Rev. 2006, 106 (9), 4044−4098. (2) Alonso, D. M.; Bond, J. Q.; Dumesic, J. A. Catalytic conversion of biomass to biofuels. Green Chem. 2010, 12 (9), 1493−1513. (3) Besson, M.; Gallezot, P.; Pinel, C. Conversion of biomass into chemicals over metal catalysts. Chem. Rev. 2014, 114 (3), 1827−1870.

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