Acid Bifunctional Catalyst for the Conversion

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Graphene-based metal/acid bifunctional catalyst for the conversion of levulinic acid to #-valerolactone Yong Wang, Zeming Rong, Yu Wang, Ting Wang, Qinqin Du, Yue Wang, and Jingping Qu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02244 • Publication Date (Web): 16 Dec 2016 Downloaded from http://pubs.acs.org on December 19, 2016

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ACS Sustainable Chemistry & Engineering

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 [email protected] (Yong Wang); [email protected] (Zeming Rong*); [email protected] (Yu Wang); [email protected] (Ting Wang); [email protected] (Qinqin Du); [email protected] (Yue Wang); [email protected] (Jingping Qu) 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 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

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°C, 2 MPa), while their selectivities are different. The hydrogenated intermediate 4hydroxyvaleric acid can be accumulated over Ru/rGO with a yield of 82% in 40 minutes, but the main product over Ru/rGO-S is γ-valerolactone (a yield of 82% within the same time) due to the accelerated dehydration process by strong acid sites. After being reused for several times, the hydrogenation activities of Ru in each catalyst and dehydration ability of SO3H can 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. 1. Introduction With the rapid depletion of fossil fuels, biomass has received much attention as a sustainable source for releasing the dependence upon non-renewable energy and reducing greenhouse gas emissions.1 Catalytic conversion of biomass is of vital importance to provide biofuels2 and value-added chemicals3. In order to develop 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 were mainly 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 were 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


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hydrogenation of cellulose or cellobiose to sorbitol over Pt/rGO.11 However, graphene has not been explored on the nanoarchitecture of 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 biomass and its 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 is 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 reaction temperature.27 In view of the low-energy consumption, some solid acids (Amberlyst A70 or A15 etc.) were used as co-catalysts of Ru/C to promote the conversion of LA to 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. Scheme 1. Reaction Pathways for the Conversion of LA to GVL


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However, such the “direct” way would still have some disadvantages, as others found the sulphated 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 two catalysts were employed in the conversion of LA. Ru/rGO performed a higher activity than many other reported catalysts did at relatively low temperature, and the main product is HVA; Ru/rGO-S improved the selectivity to GVL due to the acceleration of dehydration process by SO3H, while which hardly influenced the hydrogenation activity of Ru. We previously found the superiority of graphene as support for Ru33 or Pt34 nanocatalysts on benzene or cinnamaldehyde hydrogenation relative to carbon nanotubes (CNTs) and activated carbon (AC) through the 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. What’s more, it illuminated that the two-dimensional 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 graphene-based metal/acid bifunctional catalyst, especially for the conversion of biomass-derived platform molecules.


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Figure 1. Schematic illustration of catalyst preparation strategy. 2. Experimental 2.1 Preparation of Ru/rGO First, graphite powder was oxidized to prepare graphene oxide (GO) by an improved synthesis method.36 Next, 120 mg GO was dissolved in 100 mL deionized water aided by ultrasound and agitation, and then mixed with 0.60 mL 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 for 3 times, and pressurized to 1 MPa with H2 before being placed in an oil bath (140 °C). The hydrothermal treatment lasted for 4 hours with continuous stirring. Finally, the acquired solid (Ru/rGO) was filtrated and washed with water till to neutral. There is no residual Ru3+ in the filtrate 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. 2.2 Preparation of Ru/rGO-S First, the aryl diazonium salt of sulfanilic acid was synthesized as below: 3.12 g sulfanilic acid was dissolved in 180 mL aqueous solution of HCl (1 M), and then 20 mL aqueous solution of NaNO2 (1 M) was dropwise added with continuous stirring under 3-5 °C; after reaction for 2 hours, the generated white precipitate was filtered and washed with water for immediate use. Next, 31 mg of fresh Ru/rGO was dispersed in 10 mL water, and then all the diazonium salt, 10


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mL ethanol and 10 mL H3PO2 (50 wt.%) were successively added with continuous stirring under 3-5 °C; after 30 minutes, another 10 mL H3PO2 were supplied and reacted for 1 more hour. Finally, the sulfonated catalyst (Ru/rGO-S) was filtered and thoroughly washed with warm water. None of Ru leaching 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. 2.3 Characterization Techniques Scanning electron microscope (SEM) images were acquired on a 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 a 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 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°. XRay 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 employed to deconvolute the XPS spectra. Infrared (IR) spectrum was collected on a Thermo Scientific NICOLET 6700 FT-IR spectrometer in the range of 400-4000 cm-1. 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 the damage on the sample. Temperature programmed reduction (TPR)


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and temperature programmed desorption (TPD) were performed on a TP-5076 Adsorption Instrument (Tianjin Xianquan Industry and Trading Co., Ltd.). In detail, 95 mg of each vacuumdried 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 N2-TRD. For NH3-TPD testing, 95 mg of each vacuum-dried catalyst was first heated at 110 °C for 1 hour in a flow of N2, and then NH3 (99.999%) was switched to flow through the sample for another 1 hour at 110 °C; after that, N2 was switched back to purge physically-adsorbed NH3 for 1 hour at 110 °C; finally, once the system was cooled down to room temperature, TPD was performed till to 400 °C. 2.4 Catalytic Conversion of LA The catalytic reaction was conducted in a 70-mL stainless-steel autoclave. Catalyst (10 mg Ru/rGO and 12 mg Ru/rGO-S to make sure the same charging of ~0.5 mg Ru), substrate (200 mg LA, the molar ratio of LA to Ru is 348) and solvent (10 mL H2O) were directly added into it. The reactor was sealed and replaced with N2 and H2 each for 3 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 (ca. 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-minutes reaction under the above conditions was filtrated and washed 6 times with water for recycling usage. Samples were extracted at intervals and immediately analyzed by HPLC (Elite, P230) equipped with refractive index detector and SinoChrom C18 column (ODSBP, 4.6mm×250mm, 5µm), 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


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Quantum Ultra, electrospray ionization), and 1H-NMR (Bruker Avance II 400) were combined to do the qualitative analysis. 3. Results and discussion 3.1 Coreduction of GO and Ru3+ Although the coreduction of GO and metal salts can be achieved by some strong reductants (N2H4,37 NaBH4,38 LiAlH439 etc.), the organic solvents or surfactants are essential to keep the reduced metal particles in nanoscale. Here, we adopted H2 as weak reductant and GO itself as stabilizer to form Ru NPs in aqueous solution, and the GO can be simultaneously reduced probably due to the combined effect of thermal reduction33, 40-42 and hydrogen spillover43-44. As shown in the XRD patterns (Figure 2), the broad peak at around 25° for Ru/rGO which is corresponding to the graphite (002) plane reflection indicated the removal of most oxygenated surface groups (OSGs) and the disordered stacking of rGO33, 38, 41, 44. The diffraction peak of graphite (100) plane again reflected the π-π stacking interaction of the formed sp2-carbon domains in Ru/rGO.44

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


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Also from the IR spectra (Figure 3), a significant reduction of various OSGs for Ru/rGO can be observed, and a new band emerged at 1548 cm-1 corresponding to the C=C stretching vibration of aromatic rings clearly confirmed the reduction of GO. It should be note that the peak at 1630 cm-1 for GO has been proved to be the bending vibration of water molecules (δO-H) but not the skeletal vibration of unoxidized graphitic domains in GO.45

Figure 3. IR spectra of GO, Ru/rGO, and Ru/rGO-S. 3.2 Sulfonation of Ru/rGO Due to the pre-loaded Ru NPs on graphene, a mild sulfonation process is preferred to avoid some possible damage on the structure of Ru/rGO. Unlike the methods by using chlorosulfonic acid or sulfuric acid which need harsh conditions, the handling of 4-benzenediazoniumsulfonate is more simple and feasible to prepare sulfonated graphene.46 As seen from the IR spectra (Figure 3), the peaks at 1179 cm-1, 1121 cm-1, 1035 cm-1, 1005 cm1

, 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 and the stronger absorption band at 3432 cm-1 (νOH)

should be resulted from the SO3H groups, also the blue shift of νC=C (from 1548 cm-1 to 1572

cm-1) might be due to the contribution of the newly introduced phenyl rings.


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XPS full spectra (Figure 4) again proved the successful introduction of S in Ru/rGO-S, and the single peak of S 2p emerged at 168.1 eV (inset in Figure 4) confirmed its existence form of SO3H and no low-valence sulfide species (~164 eV)32 formed during sulfonation. Incidentally, fluorine element emerged at 689.5 eV for both samples should be derived from Teflon components (impurity < 1 wt.%).

Figure 4. XPS full spectra of Ru/rGO and Ru/rGO-S with an inset of S 2p narrow spectrum. Figure 5 shows the EDS elemental mappings from SEM image of Ru/rGO-S. The image was taken from a random region, and the homogeneous distribution of S can be validated. Quantitative analysis by EDS revealed that the atomic ratio of C, O, S, and Ru is 76.6:22.2:1.1:0.1, which is similar to the result from XPS analysis (80.2:18.0:1.6:0.2). The density of SO3H groups in Ru/rGO-S is calculated to be 0.84 and 1.22 mmol/g according to EDS and XPS, respectively. Furthermore, such the strong acid sites can also be identified by the hightemperature region in NH3-TPD profiles (Figure S4),49-50 while it is difficult to determine the acid density according to this, because there is no obvious boundary between the lowtemperature and high-temperature regions.


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Figure 5. SEM image of Ru/rGO-S and EDS spectrum with elemental mappings. 3.3 Characterization on Ru NPs First, the particle size distributions (PSD) of Ru in the fresh and used catalysts were determined by TEM. As shown in Figure 6c and 6d, the difference of size distributions between Ru/rGO and Ru/rGO-S is insignificant, and their mean sizes of Ru NPs are 2.0 nm and 2.1 nm, respectively, meaning that our coreduction method is sufficient for the synthesis of nanocatalysts and the mild sulfonation process would not affect the existed Ru NPs too much. After reused for several times, although their mean particle sizes had no big changes (2.2 nm for 10-times used Ru/rGO and 2.3 nm for 8-times used Ru), more Ru NPs larger than 4 nm were generated as shown from the PSD histograms in Figure 6e and 6f. This phenomenon is more severe for Ru/rGO-S. It seems that the existence of benzenesulfonic acid groups would affect the stability of Ru NPs during the conversion of LA to GVL. This may be attributed to the increase of


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support electronegativity, which could lower the surface electron density of graphene, weaken the metal-support interaction, and thus accelerate the sintering of Ru NPs toward such an aqueous-phase reaction.51

Figure 6. TEM images of fresh (a-d) and used (e-f) Ru/rGO (a, c, e) and Ru/rGO-S (b, d, f). In order to verify whether there is any modification on the electronic structure of Ru after sulfonation, narrow spectra of XPS were scanned in the regions of Ru 3d5/2 and Ru 3p (Figure S5). The binding energy of Ru 3d5/2 for Ru/rGO is around 280.8 eV, which is 0.2 eV higher than what we recently found for Ru NPs loaded on graphene by high-temperature gas-phase reduction method.33 It should be due to the higher content of OSGs and relatively amorphous structure of


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Ru NPs for Ru/rGO, and both aspects could make Ru electron deficient.52 But unfortunately, the signals of Ru for Ru/rGO-S are too low to do such a comparison between Ru/rGO and Ru/rGO-S (Figure S5). Actually, the loading amount of Ru in Ru/rGO-S calculated from XPS analysis is 1.5 wt.%, which is inconsistent with the result based on ICP-OES (4 wt.%) and far below the one in Ru/rGO (4.7 wt.% from XPS). Thus the large discrepancy between 1.5 wt.% and 4 wt.% should be attributed to a certain shielding effect on Ru by S during XPS testing, which may be similar to the situations of metals confined inside the channels of CNTs or CNFs.53-54 This could in turn illustrate that the upper SO3H groups should be closely associated with Ru NPs. 3.4 Covalent functionalization with p-sulfophenyl Although IR spectrum ensured the existence of benzenesulfonic acid groups on graphene, the formation of C-C bond between p-sulfophenyl and graphene cannot yet be confirmed. Some researchers employed

13

C NMR47-48,

55

or Raman48,

55

technique to verify this point. In

consideration of the ambiguity on chemical shifts between the carbon bonding with SO3H and the carbon in graphene bonding with p-sulfophenyl (both around 140 ppm), Raman would be more preferred. In a typical Raman spectra of sulfonated graphene and rGO,55 both the slightly increased intensity ratio of ID/IG and the blue shift of G band after sulfonation were regarded as indications of the successful covalent functionalization with p-sulfophenyl on sp2 carbon of rGO. However, this is not true in our case (Figure S7), as no obvious changes can be observed on either the shift of G band or the ratio of ID/IG (Table S3). It seems that Raman is not suitable to do such a confirmation here, probably due to the pre-loaded Ru on rGO. The XPS quantitative analysis of C 1s spectra is thought to be an effective means to achieve this purpose, based on the fact that the covalent functionalization with p-sulfophenyl would change the atomic ratio of sp2 carbon to sp3 carbon in graphene. As shown in Figure 7, C 1s


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peaks were deconvoluted into six bands based on the literatures.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 eV, 287.5 eV 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 is corresponding to the characteristic shakeup line of carbon in aromatic structures. The relative atomic concentrations of these peaks in C 1s spectra of Ru/rGO and Ru/rGO-S were summarized in Table 1.

Figure 7. XPS spectra of C 1s for Ru/rGO and Ru/rGO-S. As shown in Table 1, the most significant difference between the two catalysts lies in the relative atomic concentrations of sp2 carbon and sp3 carbon. The atomic ratio of sp2 carbon to sp3 carbon is 6 for Ru/rGO, while it is just 2 for Ru/rGO-S. This undoubtedly indicated the covalent functionalization with p-sulfophenyl on sp2 carbon of Ru/rGO. Also the disappearance of π-π* band for Ru/rGO-S demonstrated the massive loss of sp2-carbon domains, which is consistent with the degradation of C (100) diffraction peak in XRD patterns (Figure 2). Moreover, the relative atomic concentration of C-SO3H could be estimated by the subtraction from the total


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carbons bonding with OSGs in Ru/rGO-S (Table 1) to those in Ru/rGO (data in parentheses), that is 2.0%. It is quite coincident with the atomic ratio of S to C (1.6:80.2) calculated from the integral areas in XPS full spectrum of Ru/rGO-S (Figure 4), indicating that the XPS quantitative analysis of C 1s spectra should be a convincing and reliable way to confirm the covalent functionalization of graphene35 or related materials with sp2 carbon (CNTs and CNFs etc.). Table 1. Relative atomic concentration of peaks in C 1s spectra of Ru/rGO and Ru/rGO-S. Catalyst Ru/rGO

a

Relative Atomic Concentration (%) sp2 C

sp3 C

C-O&C=O

O-C=O&C-SO3H

π-π*

66.0 (67.9)

11.0 (11.3)

15.6 (16.1)

4.6 (4.7)

2.8

Ru/rGO-S 51.1 26.1 15.9 6.9 a The values in parentheses are percentages after excluding the one of π-π*.

0

In order to evaluate the thermal stability of the covalent-bonded sulfophenyl, especially under the atmosphere of H2, N2-TPD and H2-TPR testings were performed on Ru/rGO-S and Ru/rGO for comparison. As shown in Figure 8, the curves of N2-TPD indicated that both two catalysts have fine thermal stabilities and Ru/rGO is more stable under inert atmosphere. This is consistent with our previous work that graphene-supported Ru could endure a heat treating at 700 °C in N2 flow even without changing its particle size too much.33 While for Ru/rGO-S, decomposition would occur at higher temperature (> 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 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 carbon support. The higher temperature (112 °C) for Ru/rGO-S might be also due to its greater proportion of sp3 carbon which may retard the hydrogen spillover effect to a certain extent. Anyhow, Ru/rGO-S should be quite stable up to 200 °C under H2


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atmosphere, ensuring a wide application scope of such the graphene-based metal/acid bifunctional catalyst.

Figure 8. H2-TPR and N2-TPD profiles of Ru/rGO and Ru/rGO-S. 3.5 Catalytic conversion of LA The two catalysts were employed in the conversion of LA to GVL, one important biomassderived platform molecule to another.2, 17, 19-24 As shown in Figure 9, the yield of GVL is much higher over Ru/rGO-S than that over Ru/rGO, which surely confirmed the effectiveness of our bifunctional catalyst. Actually, the conversions of LA at 30 minutes are both over 90% for the two catalysts, and full conversions can be reached within 40 minutes, but their selectivities are completely different (Entry 1 and 2 in Table 2). The change of selectivity to GVL between the two intervals is more obvious for Ru/rGO-S (from 57% to 82%) than Ru/rGO (from 13% to 18%). The strong acidity of SO3H in 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 would slow down after 40 minutes due to the competition diffusion between HVA and GVL, and 100% yield of GVL can still be obtained within 80 minutes. While for Ru/rGO, much longer time (5 h) is needed to reach a near 100% yield of GVL.


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

TOFc/h-1

1d

2.0

2

2d

3d 4

5 6 7 8 9

Ru/rGO

Ru/rGO-S

2.1

Ru/rGO + S-rGO Ru/C + A70

Ru/SMS

58

Ru/HAP

59

Ru/OMC-P

28

32

Ru/DOWEX

29 60

Ru/MIL-101(Cr) 30

10

Ru/SPES

11

Ru/TiO261

348

348

50

50

2

0.5

95

13

661

0.67

100

18

522

0.5

92

57

641

0.67

100

82

522

2.0

348

50

2

0.5

90

51

627

-

1000 50

3

3

93

99.1

310

70

0.5

3

98

99.5

327

1.5

200

70

0.5

4

99.2

96.4

50

10-20

348

70

0.5

4

99

99

86

1.6

1000 70

0.7

6

98

94

163

2.8

420

70

1

4

98.3

99

102

2.4

333

70

1

5

99

99

67

3.0

856

70

3

2

87.9

99

376

2.9

286

70

5

1

100

100

286

a

Average particle size of Ru NPs. Obtained from TEM. The molar ratio of substrate (LA) to catalyst (Ru) c Turnover frequency, as (mol LA converted)/(mol Ru×h). Calculated from literature data at the conversion indicated and on bulk Ru content. d This work. b


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Page 18 of 35

Recently, more and more efforts have been focused on the improvement of catalytic activity toward this reaction at relatively low temperature. Table 2 summarized various catalytic systems in this category. Usually, supported Ru catalysts are preferred due to its intrinsic high activity for LA hydrogenation62 and its low cost relative to other noble metals63, and aqueous solution of LA were mostly adopted under batch conditions.64 As shown in Table 2, both Ru/rGO and Ru/rGO-S performed higher activities compared with 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 benefited from the small sizes of Ru NPs and especially the unique properties of graphene as catalyst support.65-66 Just as we found for cinnamaldehyde hydrogenation, Pt/rGO showed the best activity and selectivity for the hydrogenation of C=O bond compared with 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 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 group67-68, while others scarcely found the existence of HVA even at lower conversion of LA. In order to investigate the influence of LA concentration on the selectivity to GVL, 1000 mg instead of 200 mg LA was used to do 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 from Figure 10, 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 lower over Ru/rGO than Ru/rGO-S. It indicated that the obvious difference in selectivity between our work and others (Table 2)


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might be attributed to the intrinsic kinetic factor. Bond et al. found 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 while Ruppert et al. supposed the formation of HVA is most probably the rate-limiting step in their study.61 Besides, Heeres et al. recently confirmed the big influence of intra-particle diffusion limitation on this reaction.67-68 These kinetic factors should greatly depend on the adopted catalytic systems. In our case, due to 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.

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 Ru, 1000 mg LA, 20 mL H2O, 2 MPa H2, 50 °C. 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 graphenesupported Ru catalysts are quite similar, but the selectivity to GVL (24%) over our Ru/rGO catalyst is still much lower. We found that the methods of product analysis in the three works are


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the same GC analysis,69-71 while the HPLC analysis was adopted in our case. When we used GC to do the analysis, the selectivity to GVL was 92%, much higher than 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 temperature27 which is inevitable during GC analysis. As we (Section S2 in SI) and others72 found, 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. So HPLC operated at room temperature is suggested to be the reliable analysis method for this reaction.27, 29, 32, 51, 61 Table 3. Comparison of LA hydrogenation performance over graphene-supported Ru catalysts near room temperature under batch conditions. Entry Catalyst 1d 2 3 4

Ru/rGO Ru/RGO Ru/FLG

69

70 71

Ru/graphene

Sizea/nm S:Cb T/°C

P(H2)/MPa Time/h Conv./% Sel.(GVL)/%

TOFc/h-1

2.0

1742 25

2

10

99

24

174

2.0

1449 20

4

8

100

99.9

181

1.1

1460 20

4

8

99.3

97.7

181

2-6

1037 30

6

6

100

96

173

a

Average particle size of Ru NPs. Obtained from TEM. The molar ratio of substrate (LA) to catalyst (Ru) c Turnover frequency, as (mol LA converted)/(mol Ru×h). Calculated from literature data at the conversion indicated and on bulk Ru content. d This work. b

In order to promote the dehydration of HVA to form GVL at relatively low temperature, 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 (Figure 9 & 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 analysis, respectively. This is comparable to the ones of reported sulfonated graphene (0.5-2


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mmol/g summarized in Table S4) which usually exhibited higher activities for various acidcatalyzed reactions than some commercial solid acid catalysts did. Here a similar sulfonated graphene, S-rGO, was also synthesized according to the literature55 for comparison. It was served as co-catalyst of 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%) is 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, benefited 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 is HVA with the selectivity always higher than 83% in each 40-min run, while the conversion of LA declined by 18% after 10-times 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%, 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 would drop to 68% after 8-times 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


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Page 22 of 35

characterization of H2-TPR (Figure 8) that the benzenesulfonic acid groups in Ru/rGO-S are quite stable up to 200 °C under 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 usage here or other reactions.

Figure 11. Catalyst stability testing over Ru/rGO (upper) and Ru/rGO-S. Reaction conditions: each catalyst with 0.5 mg Ru, 200 mg LA, 10 mL H2O, 2 MPa H2, 50 °C, 40 min for each cycle. As discussed above, the graphene-based metal/acid bifunctional catalyst was successfully synthesized, exactly for the first time. Although some similar nanocomposites with both metal


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NPs and benzenesulfonic acid groups on graphene have been reported elsewhere,48, 73-74 all of them just 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 from Section S1 in SI). Unlike the reported procedures that loading metal on pre-sulfonated 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 existed Ru NPs. For comparison, Ru/S-rGO prepared by directly loading Ru NPs on S-rGO was also used for LA conversion (Table S2), and its conversion of LA (25%) and selectivity to GVL (20%) within 30 minutes 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 either. 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 graphene35. Recently, many other SO3H-contained 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, so such the bifunctional catalysts with the “genius of graphene” would have great potential applications in biomass conversion.


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4. CONCLUSIONS A facile way to prepare 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 catalysts,65-66 and opened up new possibilities for biomass conversion.5, 8

ASSOCIATED CONTENT Supporting Information. Influence factors on catalysts preparation (Table S1&S2), qualitative analysis on products (1H NMR, LC-MS&GC-MS spectra), and complementary characterizations on catalysts (NH3-TPD, XPS&Raman spectra, Table S3&S4). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], Tel: +86 411 8498 6242. ACKNOWLEDGMENT 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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Table of Contents Title: Graphene-based metal/acid bifunctional catalyst for the conversion of levulinic acid to γvalerolactone Authors: Yong Wang, Zeming Rong*, Yu Wang, Ting Wang, Qinqin Du, Yue Wang, and Jingping Qu

Synopsis: Graphene-based metal/acid bifunctional catalyst was successfully synthesized for efficiently producing γ-valerolactone from levulinic acid aqueous solution, one important biomass-derived platform molecule to another.


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Acid Bifunctional Catalyst for the Conversion

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