Efficient Production of the Liquid Fuel 2,5-Dimethylfuran from 5

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Efficient production of the liquid fuel 2,5-dimethylfuran from 5-hydroxymethylfurfural in the absence of acid additive over bimetallic PdAu supported on graphitized carbon Feng Zhang, Yunfei Liu, Fulong Yuan, Xiaoyu Niu, and Yujun Zhu Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 01 May 2017 Downloaded from http://pubs.acs.org on May 8, 2017

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Efficient production of the liquid fuel 2,5-dimethylfuran from 5-hydroxymethylfurfural in the absence of acid additive over bimetallic PdAu supported on graphitized carbon Feng Zhang, Yunfei Liu, Fulong Yuan, Xiaoyu Niu*, Yujun Zhu* Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education, School of Chemistry and Materials, Heilongjiang University, Harbin, 150080 P. R. China * Corresponding Author: Yujun Zhu, Email: [email protected], Xiaoyu Niu, Email: [email protected], Tel: +86-451-86609650; Fax: +86-451-86609650

Abstract A series of carbon supports (GC) with different graphitization degree were prepared and characterized. The study clearly showed that GC800 was the most proper support after loading the noble metal in the hydrogenation of biomass-derived HMF to liquid fuel DMF under mild reaction conditions. Over the Pd/GC800 catalysts, the relatively high selectivity to DMF was achieved, whereas the Au/GC800 catalysts displayed extremely low selectivity to DMF. Furthermore, PdAux/GC800 (x=1-4) catalysts with different Au/Pd mass ratio were synthesized which showed excellent catalytic activity without any activator or acidic media. More gratifyingly, the PdAu4/GC800 catalyst gave 94.4% DMF selectivity with 86.8% HMF conversion at 150 °C for 4 h and kept good stability after five cycles, which was attributed to the intimate contact and intensive interaction between the Pd and Au nanoparticles. As a result, the dispersion of particles was improved and the content of active Pd0 species was increased, as evidenced in the TEM and XPS results. In addition, the influence of varied reaction

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parameters was also investigated. Keywords:

graphitized

carbon;

hydrogenation;

bimetallic

Pd

and

Au;

5-hydroxymethylfurfural; 2,5-dimethylfuran

1. Introduction As is well known, rapid depletion of nonrenewable petroleum reserves, high volatility of the crude oil price and global warming caused by increased carbon dioxide emissions indicate the development of renewable alternatives is needed in the near future.1-6 In recent years, a growing interest in transportation fuels and fine chemicals from biomass is being considered.7,8 5-hydroxymethylfurfural (HMF), one of the top 10 bio-based platform molecules with significant market potential,9 produced from a variety of biomass-derived carbohydrates,10,11 can be further transformed into a series of valuable chemicals due to its polyfunctionality.12-19 Among those derivatives, 2,5-dimethylfuran (DMF) is particularly attractive because of its superior energy density (30 KJ·cm-3), high octane number (119), low oxygen content (O/C=0.17) and nearly ideal boiling point (92-94 °C).20-22 Additionally, DMF is immiscible with water and easier to blend with gasoline compared to the market-leading bioethanol, all these excellent properties make DMF a more appropriate and promising biomass-derived renewable liquid transportation fuel.23 However, many side products are easily formed during the hydrogenation of HMF to DMF owing to the different functionalities of HMF (Scheme 1), which decrease the selectivity to DMF and increase the costs of product purification.24 Herein, improving the selectivity and yield is still a key challenge in converting HMF to DMF.25 There were several recent reports on the hydrogenation of HMF to DMF over various

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catalysts mainly including Cu, Ni, Ru, Pt and Pd metals. For Cu based catalysts, Cu-PMO,21 Cu/ZrO2,26 Cu-Ni/γ-Al2O3, 27 Cu-Co®/NGr/α-Al2O328 gave a relatively low yield of DMF. For Ni based catalysts, Ni/Co3O4,29 Ni-W2C/AC30 and Ni-Fe/CNT25, showed either a low yield of DMF or a high selectivity to DMF with high hydrogen pressure, which was unfavorable for the practical production of DMF. NiCo/C31,32 exhibited good catalytic activity for the hydrogenation of HMF to DMF, however, it need a long reaction time. Similarly, the Ru based catalysts such as Ru/C,33 CuRu/C,17 RuSn/C,34 Ru-HT,35 Ru-NaY36 and Ru/Co3O437 presented either a low DMF yield or a excellent selectivity to DMF with high catalyst loading or high reaction temperature. Nevertheless, for Pt based catalyst, Pt-Ni nanocrystals,38 Pt-Co nanocrystals39 and PtCo@HCS-based bimetallic catalysts24 exhibited a superior yield of DMF compared with Pt/rGO40 catalyst, proving that the formation of alloy is crucial for the hydrogenation of HMF to DMF. Furthermore, the hydrogenation of HMF to DMF was also conducted over Pd based catalysts. For example, Pd/C in supercritical carbon dioxide can show effective catalytic hydrogenation of HMF to DMF.41 81% selectivity to DMF was obtained over Pd-CsDTP/K-10 by Gawade et al.42 Other Pd catalysts such as Pd/C43-45 and PdAu/C46 gave a marvelous yield of DMF. It should be pointed out that hydrogenation of HMF to DMF was carried out over most of catalysts in the presence of Lewis acid, hydrochloric acid (HCl) formic acid (FA) or H2SO4, whereas the using of corrosive acid made this reaction process less eco-friendly. Importantly, in our previous work, it was found that the carbon support may play a critical role in the dispersion of noble metal and the adsorption of reactant containing conjugate structure such as cinnamaldehyde in the hydrogenation reaction.47,48

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Based on above descriptions, we prepared a series of graphitized carbon (GC) samples with different graphitization degree, and then selected a proper GC sample as the support according to their characterizations. Bimetallic Pd and Au supported on GC800 with different Au/Pd mass ratio (PdAux/GC800, x=1-4) were prepared and characterized to evaluate the activity for the hydrogenation of HMF to DMF without any activator or acidic media under mild reaction conditions. The effect of the interaction between Pd and Au particles on the catalytic activity was particularly investigated.

2. Experimental 2.1 Catalyst preparation As described previously, the graphitized carbon (GC) support was prepared by an in situ self-generating template route.49 Typically, 16 g polyacrylic weak acid cation-exchanged resin (PWAC) and 12 g FeCl2·4H2O were added into the water (400 mL), more notably, a small amount of trisodium citrate was also needed in the aqueous solution to avoid the oxidation of Fe2+. Then, the solution was stirred for 3 h under nitrogen protection and washed. Similarly, another 12 g FeCl2·4H2O and trisodium citrate were added into the water (400 mL) again and stirred for 3 h. After two washing/centrifugation times, the solid was collected and dried under vacuum. Next, the solid was calcined at 700 °C (800, 900 and 1000 °C) for 1 h under N2. Then, the obtained sample was treated with 10% hydrochloric acid for 8 h to remove the Fe species. After washing with deionized water several cycles, the solid was separated and dried in a vacuum oven at 80 °C for 6 h. The graphitized carbon sample was obtained and denoted as GC700, GC800, GC900 and GC1000 according to the carbonization temperature, respectively.

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The bimetallic Pd and Au supported on GC were synthesized by a sodium borohydride (NaBH4) reduction method according to the previous report.50 In a typical procedure, graphitized carbon (0.20 g) was dispersed into deionized water (20 mL) and ultrasonicated for 30 min. Then, 0.34 g PdCl2 solution (Pd content of 6.0 mg·g-1) and 0.41 g (0.82, 1.23 and 1.64 g) HAuCl4 aqueous solutions (Au content of 4.9 mg·g-1) were added to the above graphitized carbon slurry, the slurry was further ultrasonicated for another 30 min. Afterwards, the slurry was cooled in an ice bath (2-4 °C) with vigorous stirring for 30 min. Subsequently, the pH value of slurry was adjusted to 10.0 by using NaOH (0.1 mol·L-1) solution. Then, the excessive NaBH4 (0.1 mol·L-1) was gradually added to the mixture and further stirred for 3 h to reduce the high oxidation state of noble metals. The mixture was statically kept in the ice bath for 10 h. Finally, after filtrating, washing with deionized water until no chloride ions were detected by the mixture of AgNO3 and HNO3, and then washing with ethanol. The PdAux/GC (x=1-4) catalysts were obtained after drying at 60 °C for 10 h. For the PdAux/GC800 (x=1-4) catalysts, the loading of Pd kept constant of 1 wt%, whereas the x value from 1 to 4 presents the metal percentage of Au from 1 wt% to 4 wt%. In other word, the Au/Pd mass ratio varied from 1 to 4. As references, Pd or Au supported on GC with different Pd or Au contents were also prepared under similar conditions. 2.2 Catalyst characterizations Wide-angle X-ray diffraction (XRD) patterns were obtained with a Rigaku D/max-IIIB diffractometer by using Cu-Kα (λ =1.5418 Å) radiation (40 KV, 20 mA) in the range of 10-80°. A Jobin Yvon HR 800 micro-Raman spectrometer equipped with a 50 × objective lens and a 457.9 nm laser beam was employed for Raman spectroscopy. Nitrogen

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adsorption-desorption isotherms were measured with a Micromeritics Tristar II at -196 °C, and the BET specific surface areas were determined from the adsorption data in the relative pressure (P/P0) range of 0.05-0.25. X-Ray photoemission spectroscopy (XPS) measurements were carried out in a Kratos-AXIS ULTRA DLD equipped with a monochromatic Al Kα radiation source. Transmission electron microscopy (TEM) experiments were taken on a JOEL model JEM-2100 electron microscope with an acceleration tension of 200 kV. Scanning electron microscopy (SEM) images were recorded on a Hitachi S-4800 field emission electron microscope at 20 kV. The content of palladium and gold in all catalysts was determined by inductively coupled plasma-optical emission spectrometry (ICP-OES), which was performed by a PerkinElmer Optima 7000DV analyzer. In each case, the sample was dissolved in a diluted HF and chloroazotic acid solution before the measurement. 2.3 Hydrogenation of HMF to DMF Hydrogenation reactions were carried out in a stainless steel autoclave (100 mL) fitted with a magnetic stirring bar. Briefly, the mixture of HMF (0.252 g), catalyst (0.050 g), THF (15 mL) and tridecane (0.065 g, internal standard) were introduced into the sealed autoclave equipped with a temperature-controlled furnace. Subsequently, the reactor was purged three times with hydrogen to remove air and filled with required hydrogen pressure (8-14 bar) and heated to the required temperature (150 °C) under continuous stirring for 0.5-6 h. After reaction, the reactor was cooled down to room temperature by quenching in ice water. The solution was filtered to remove the solid catalyst and the liquid products in the filtrate were analyzed by gas chromatography (BFRL SP-3420, China) equipped with a FID detector and a HP-5 column (30m×0.32mm×0.25um). The products in liquid phase were further identified

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by GC-MS (Agilent 6890/5973N). The conversion of HMF and the selectivity to each product were calculated by using the following equations: Conversion (%) =

Moles of HMF reacted ×100 % Moles of HMF added

Selectivity (%) =

Moles of product produced ×100 % Moles of HMF reacted

3. Results and discussion Carbon supports with different graphitization degree (GC) were successfully prepared by the different carbonization temperature from 700 °C to 1000 °C and noted as GC700, GC800, GC900 and GC1000, respectively. Figure S1 displays the XRD patterns, the diffraction peaks located at 26.4, 42.5, 54.6 and 77.4° can be assigned to the graphite (002), (100), (004) and (110) plane, indicating that all samples have the graphitic structure.51 As listed in Table S1, the intensity ratio of G band to D band (IG/ID) values decreased from 4.2 of GC1000 to 1.4 of GC700 calculated from the Raman spectra (Figure S2), implying a decrease in graphitization degree and an increase in the structural disorder and defective sites.52,53 Meanwhile, the SEM images in Figure S3 demonstrated that all GC supports adopted preferential flake morphology which mostly piled up together. Moreover, as shown in Table S1 and Figure S4, the BET surface area (SBET) firstly increased to a maximum of 120 m2·g-1 and then decreased with an increase in carbonization temperature from 700 °C to 1000 °C, meanwhile, the GC800 sample possessed the largest SBET value of 120 m2·g-1 and pore volume of 0.51 cm3·g-1. Furthermore, as displayed in Figure S5, the XPS of C 1s was deconvoluted into five peaks for every different support.54 The main peak appeared at around 284.6 eV, corresponding to sp2 hybridized graphite-like carbon.55 In detail, Table S2 summarized the fractions of the different

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species. It can be clearly observed that the percentage of sp2 hybridized C-C bonds increased from 52.3% of GC700 to 64.3% of GC1000, whereas other species gradually decreased as the carbonization temperature increased, confirming an increase of graphitization degree with temperature, which was in accordance with the Raman results. The above results demonstrate the GC supports have different degree of graphitization, which means the different surface nature of GC supports. In order to select a right GC support, the Pd supported on GC with different degree of graphitization as catalysts (3 wt% Pd/GC) were synthesized and used in the hydrogenation of HMF to DMF. The catalytic activities are described in Figure S6. Obviously, GC800 supported 3 wt% Pd catalyst (3 wt% Pd/GC800) showed much higher conversion of HMF and selectivity to DMF than the other graphitized carbons supported 3 wt% Pd, owning to the difference in the natures of these GC supports. In order to better illustrate the effect of different degree of graphitization, 3 wt% Pd/GC catalysts were also characterized by XPS, as illustrated in Figure S7. For 3 wt% Pd/GC catalysts, the Pd 3d regions include two spin-split states. From a viewpoint of the spectrum and the fractions of the surface species (Table S3), it was found that the amount of the Pd0 species in 3 wt% Pd/GC700, 3 wt% Pd/GC800, 3 wt% Pd/GC900 and 3 wt% Pd/GC1000 were about 54.0%, 59.3%, 46.5% and 48.1%, respectively. Obviously, the metallic Pd0 content in 3 wt% Pd/GC800 was higher than it in other 3 wt% Pd/GC catalysts, suggesting that carbon supports with different graphitization degree could give rise to the different physicochemical characteristics of the catalysts. All these results confirmed that GC800 support was a superior support for the hydrogenation of HMF to DMF due to its high surface area and proper degree of graphitization, which may be favor of the

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dispersion of noble metal and the adsorption of HMF. However, the selectivity to DMF over 3 wt% Pd/GC800 was still relatively low compared with other reports about Pd based catalysts.41-45 Hence, to further improve the conversion of HMF and selectivity to DMF, a series of bimetallic Pd and Au catalysts supported on GC800 with different mass ratio of Au to Pd (PdAux/GC800, x=1-4) were prepared and evaluated in terms of the hydrogenation of HMF to DMF. 3.1 Structural Characteristics of PdAux/GC800 XRD patterns of PdAux/GC800, 1 wt% Pd/GC800 and 4 wt% Au/GC800 are presented in Figure 1(A). It can be clearly seen that all samples exhibited the diffraction peaks of GC800 (Figure S1). Besides, the XRD pattern of 1 wt% Pd/GC800 showed the typical reflections at 40.11 and 46.65°, corresponding to the Pd (111) and (200) planes, respectively (PDF#65-2867). The XRD pattern of 4 wt% Au/GC800 displayed characteristic peaks at 38.19, 44.39 and 64.58°, matching the Au (111), (200) and (220) planes with face-centered cubic (fcc) structure, respectively (PDF#65-2870).56 Figure 1(B) is the enlarged regional patterns between 36° and 42°, it could be seen that the reflection peak of Pd (111) was not visible in PdAux/GC800 catalysts, whereas the Au (111) reflection was obvious and the peak intensity of Au (111) plane increased along with the Au content, demonstrating presence of strong interaction between Pd and Au particles which was responsible for the high dispersion of Pd nanoparticles. On the other hand, as listed in Table 1, it was significant that the average particle sizes of 1 wt% Pd/GC800 and 4 wt% Au/GC800 were 7.3 and 14.9 nm calculated from Pd (111) and Au (111) plane, respectively. However, the average particle size was in range of 5.9-6.7 nm for PdAux/GC800 catalysts calculated by using the Au (111) reflection

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based on Scherer equation,57 which were much smaller than that of monometallic catalysts. It implies that the addition of Au can inhibit the growth of Pd particles and achieve the much smaller metal particle size of Au and Pd. Moreover, ICP-OES analysis of the catalysts mentioned above illustrated that the contents of palladium and gold were almost the same as the theoretical amounts. The nitrogen adsorption-desorption isotherms of the PdAux/GC800 catalysts are displayed in Figure 2. It was found that all these isotherms belonged to type Ⅳ with obvious hysteresis loops of type H1, the same as the isotherms of GC supports (Figure S4), which were the typical characteristic isotherms for mesoporous materials.51 Meanwhile, the BET specific surface areas of all samples are displayed in Table 1. It can be concluded that all samples possessed the similar surface area, whereas increasing metal loading resulted in a slight decrease in the surface area compared to the GC800 support. 3.2 Morphological Characteristics of PdAux/GC800 Morphological and structural details of the PdAux/GC800 catalysts were characterized by SEM measurements. As shown in Figure 3, SEM images of the PdAux/GC800 catalysts displayed similar plate-like morphology as the support (Figure S3). Notably, the particles can be observed with uniform dispersion on the graphitized carbon for the PdAux/GC800 catalysts except PdAu/GC800, which was attributed to the low loadings of Pd (1%) and Au (1%). The TEM and STEM-EDS elemental micrographs were conducted to further study the morphology, metal distribution and particle size of the PdAux/GC800 catalysts. Figure 4 depicted a uniform dispersion of Pd and Au nanoparticles with average particle size of 5.8, 5.5, 5.9 and 6.1 nm for the PdAu/GC800, PdAu2/GC800, PdAu3/GC800 and PdAu4/GC800

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catalysts, respectively, which was consistent with the values derived from the XRD data (Table 1). The elemental mapping of the PdAu2/GC800 and PdAu4/GC800 (Figure 5) also exhibited the parallel densities of Pd and Au on the GC800, revealing their homogenous dispersion. For 1 wt% Pd/GC800 and 4 wt% Au/GC800 catalysts, the TEM images (Figure 6) showed the average particle sizes of Pd and Au were about 6.4 and 14.7 nm, respectively, which were larger than that of PdAux/GC800. The results are consistent with the XRD data (Table 1). Moreover, from the HRTEM images of PdAux/GC800 in Figure 4 b, e, h, k, it could be clearly seen that one form of the lattice fringe was measured as 0.235 nm, corresponding to the (111) crystal planes of the Au particles,58 another form of crystal lattice was 0.225 nm, which was ascribed to Pd particles with (111) crystal planes.59 Besides, in every PdAux/GC800 catalyst, Pd and Au particles were in close contact with each other on the GC800 (Figure 4 h, k, Figure S8). However, some isolated Pd and Au particles could be observed in PdAu/GC800 and PdAu2/GC800 due to the much smaller content of Au (Figure 4 b, e), resulting in the week interaction between the Pd and Au particles. In the TEM images of PdAu3/GC800 and PdAu4/GC800, most of Pd and Au particles were in intimate contact with each other, leading to the strong interaction between them. Especially, three adjoining particles of Au-Pd-Au were obviously observed for PdAu4/GC800 (Figure 4k) as a result of the more Au amount. Based on these evidences, it is rational to conclude that the coexistence of Pd and Au in the PdAux/GC800 catalysts showed distinctly intimate contact of Pd with Au, which could lead to charge transfer between them. In addition, sheet thicknesses of few layers assigned to GC800 were observed for PdAux/GC800 catalysts in Figure 4 a, d, g, j, nevertheless, the faint and incomplete lattice fringes of the carbon were also observed in

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Figure 4 h, indicating that the support had some defects and a relatively low degree of graphitization, as evidenced in Raman results. 3.3 Surface properties of PdAux/GC800 To investigate the surface properties and the chemical states of Pd and Au species, the PdAux/GC800 catalysts were further analyzed by XPS. As illustrated in Figure 7(A), the XPS of the PdAux/GC800 catalysts in the range of 333-345 eV ascribed to Pd 3d regions, together with their deconvolution into two spin-split states obtained by fitting Gaussian peaks after Shirley-background subtraction. The Pd 3d5/2 and Pd 3d3/2 doublets at higher binding energy (337.6 and 342.8 eV) were attributed to the Pd2+ species, the Pd 3d5/2 and Pd 3d3/2 at lower binding energy (335.6 and 341.0 eV) were assigned to the metallic Pd0.56 It was observed that both Pd 3d5/2 and Pd 3d3/2 peaks showed a gradual shift to low binding energy with increasing the content of Au in PdAux/GC800 with respect to the 1 wt%Pd/GC800. Meanwhile, the binding energy of Au around at 84.1 and 87.7 eV shifted to high binding energy depending on the amount of Au adding as presented in Figure 7(B). These tendencies are attributable to the charge transfer from Au to Pd, revealing that the surface electronic density structure of Pd increases with the addition of Au.60,61 Additionally, in terms of the quantitation of the surface Pd species, the fractions of the surface Pd0 and Pd2+ species were estimated by the corresponding peak area listed in Table 2. The amounts of the Pd0 and Pd2+ species in 1 wt% Pd/GC800 were about 27% and 73%, respectively. It is noteworthy that the metallic Pd0 content with respect to the Pd2+ content increased from 58% to 72% with increasing the Au/Pd mass ratio from 1 to 4, confirming that the reduction of surface Pd2+ to metallic Pd may be prone to be proceeded as more Au was

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added. It was consistent with the results of Qian et al.62 Therefore, the results disclosed the charge transfer from Au to Pd in view of their strong interaction, which is conducive to the formation of more Pd0 species. Figure 8 presented a good liner relationship between the amount of the surface Pd0 species and the value of x in the PdAux/GC800 catalysts, it could be found that the amount of surface Pd0 increased along with the increase of the Au content in the catalysts, suggesting that there was charge transfer between Pd and Au particles. 3.4 Catalytic activity evaluation of PdAux/GC800 3.4.1 Hydrogenation of HMF to DMF over PdAux/GC800 Generally, the hydrogenation of HMF to DMF was carried out in the presence of acidic media such as HCl 46,63 and H2SO4.43 It was worth mentioning that our studies were conducted without any activator or acidic media. The hydrogenation of HMF to DMF was tested over PdAux/GC800 prepared with different Au/Pd mass ratio displayed in Table 3. It was observed that the conversion of HMF reduced from 91.7% to 86.8% with increasing amount of Au over the PdAux/GC800 catalysts. However, the selectivity to DMF continuously increased from 59.3% to a maximum of 94.4% when the x value was varied from 1 to 4. In contrast, for monometallic Pd or Au catalysts supported on GC800 (Table S4), it could be clearly seen that the Pd/GC800 catalysts gave a low activity under the same reaction conditions. Although the content of Pd was improved from 1 wt% to 5 wt%, 76.4% DMF selectivity with 84.6% HMF conversion was the best activity for the 4 wt% Pd/GC800 catalyst (Table S4). Unfortunately, for the Au/GC800 catalysts, the conversion of HMF and the selectivity to DMF were extremely low whatever the Au content was. Moreover, when using Pd/GC800 as the catalysts,

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the main side product was MFA which was different from the Au/GC800 catalysts, indicating that the ring hydrogenation may be facilitated to drive the formation of other side products such as MFU and MTHF over Au/GC800 catalysts (Scheme 1). More notably, as shown in Table 3, the TOF value for PdAu4/GC800 (469 h-1) was also higher than that for the 1 wt% Pd/GC800 catalyst (342 h-1). Furthermore, it should be pointed out that the bimetallic Pd and Au catalysts can inhibit the occurrence of side reactions, because the selectivity to other side products decreased to 0.8% as the Au/Pd mass ratio increased to 4. In a word, among the PdAux/GC800 catalysts, PdAu4/GC800 exhibited the best catalytic activity that the conversion of HMF and the selectivity to DMF were 86.8% and 94.4%, respectively, which was much greater than its corresponding monometallic catalysts (1 wt% Pd/GC800 and 4 wt% Au/GC800). In detail, 41.6% yield and 72.7% selectivity to DMF were obtained over the 1 wt% Pd/GC800 catalyst, whereas 4 wt% Au/GC800 only presented 3.7% yield and 7.6% selectivity to DMF. Importantly, the yield of DMF presented obviously increase from 41.6% for 1 wt% Pd/GC800 to 81.9% for PdAu4/GC800 with Au addition. Thus, the bimetallic catalyst consisting of a small amount of Pd with the addition of Au can tremendously improve the activity for the hydrogenation of HMF to DMF under mild reaction conditions without any activator or acidic media. Compared with some results by other researchers over the Pd based catalysts summarized in Table S5, PdAu4/GC800 catalyst still presented the excellent catalytic activity without any activator or acidic media. Pd/C catalyst designed by Chatterjee et al. gave 100% DMF yield with 100% HMF conversion in supercritical CO2.41 When the reaction was conducted over Pd-CsDTP/K-10 catalyst, the conversion of HMF with 98% and the selectivity to DMF with

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81% were reached by Gawade et al.42 Van Bekkum et al. gave 36% yield of DMF by using a combination of Pd/C catalyst and HCl as an activator.63 Rauchfuss et al. obtained successfully higher than 95% yield of DMF using Pd/C as catalyst in the presence of H2SO4 and FA.43 Nishimura et al. attempted the hydrogenation of HMF to DMF by using PdAu/C catalyst in the presence of hydrochloric acid (HCl), giving 96% yield of DMF.46 Saha et al. reported that the Pd/C catalyst exhibited 85% selectivity to DMF in the presence of Lewis-acidic ZnCl2.44 Besides, non-precious catalysts such as transition metal Cu and Ni based catalysts were also made a comparison,25-32 as listed in Table S6. It was observed that the catalytic activity over the non-noble catalysts was relatively high, but the reaction conditions were very harsh, either high temperature or high H2 pressure was needed to achieve good performance. Recently, Wang’s group31,32 obtained a high yield of DMF under low temperature over the NiCo/C catalyst, but the long reaction time was required. In addition, PdAu4 supported on activated carbon catalyst (PdAu4/AC) was synthesized in order to clarify the effect of GC and its catalytic performance was also evaluated. The PdAu4/AC catalyst only showed the conversion of HMF and the selectivity to DMF were 47.2% and 35.6%, respectively (Table S5), which was much lower than that of PdAu4/GC800 catalyst. It was explained that the graphitized carbon possessed more sp2 hybridized graphite-like carbon and giant π-system which may promote the reactant-graphene interaction and pointedly enhance the catalytic performance, especially for the reactants possessing a π-system.64 Whereas the activated carbon possessed more carbon defects and functional groups like -OH, -COO, -COOH. Thus, the OH and aldehyde groups of HMF were easily adsorbed on the surface of activated carbon by hydrogen bond. As a result, these oxygen-containing functional groups can greatly influence the

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adsorption of HMF and the process of further hydrogenation of HMF over PdAu4/AC.40,65 Obviously, the interaction between Pd and Au particles in PdAux/GC800 had a significant effect on the catalytic activity. The PdAu/GC800 and PdAu2/GC800 catalysts showed a higher conversion of HMF and a lower selectivity to DMF compared with 1 wt% Pd/GC800 catalyst. Combined with the results of TEM and XPS, there was weak interaction between Pd and Au particles in the PdAu/GC800 and PdAu2/GC800 catalysts, which led to the small content of Pd0 species. As a result, the catalytic performance for the two catalysts were much lower owing to the isolated Au particles which were responsible for the formation of a large of side products and the low selectivity to DMF. Interestingly, the PdAu3/GC800 and PdAu4/GC800 catalysts displayed a good performance due to the intensive interaction between Pd and Au nanoparticles. The more content of Au particles possesses more chance to touch with Pd. Thus, the more content of Pd0 species were formed by their interaction. Through further investigating the relationship between the yield of DMF and the amount of Pd0 in 1 wt% Pd/GC800 and PdAux/GC800 catalysts, it could be concluded that the yield of DMF increased along with the amount of Pd0 increase especially for PdAux bimetallic catalysts, implying that Pd0 plays an important role in the hydrogenation of HMF to DMF. In our previous works, we found the more Pd0 species was benefit for the improvement of the catalytic hydrogenation activity for cinnamldehyde,48,66 In this work, the Pd0 species can not only promote the activation of H2 molecules, but also enhance the reaction pathway during the hydrogenation process, resulting in the high activity and selectivity.46 In summary, the excellent performance of the bimetallic PdAu4/GC800 catalyst can be attributed to the intensive interaction between Pd and Au particles which is beneficial for the improvement of

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the particle dispersion, the decrease in nanoparticle size and the increase in the amount of the active Pd0 species. Consequently, the effects of various reaction conditions including H2 pressure and reaction time on the hydrogenation of HMF to DMF have been also investigated over the PdAu4/GC800 catalyst based on its superior catalytic performance. 3.4.2 Effect of H2 pressure over PdAu4/GC800 The effect of H2 pressure on the hydrogenation of HMF to DMF over the PdAu4/GC800 catalyst was investigated by varying the pressure in the range of 8-14 bar. As given in Figure 9, the results clearly showed that the catalytic activity was heavily H2 pressure dependent. When using 8 bar of H2, the conversion of HMF and the selectivity to DMF were 77.5% and 92%, respectively, meaning that the incomplete hydrogenation of HMF to DMF occurred at a low H2 pressure. With the increase in H2 pressure from 8 bar to 10 bar, the conversion of HMF and the selectivity to DMF correspondingly increased to 86.8% and 94.4%, respectively, which might due to the fact that the high solubility of H2 in THF can facilitate the hydrogenation of HMF to DMF.33 When the H2 pressure was further increased, the conversion of HMF increased to near 100%, but the selectivity to DMF showed a sharp decrease as a result of the further opening and excessive hydrogenation of the furan ring.33 Hence, the preferred H2 pressure was 10 bar in the present work. 3.4.3 Effect of reaction time over PdAu4/GC800 The influence of reaction time on the catalytic activity over PdAu4/GC800 was studied by varying the reaction time from 0.5 h to 6 h. As described in Figure 10, with the increase in the reaction time from 0.5 h to 4 h, the conversion of HMF increased from 43.9% to 86.8%, and

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the corresponding selectivity to DMF increased from 46.9% to 94.4%, whereas the selectivity to MFA decreased from 22.8% to 4.8%. The above results indicated that the MFA formed during the reaction course was gradually converted to DMF with increasing reaction time. Once the reaction time was further prolonged to 6 h, the conversion of HMF remained constant, however, the selectivity to DMF decreased, which implied that more undesired side products were possibly formed if the time exceeded a certain point.33 Therefore, 4 h was selected as the suitable reaction time. 3.4.4 Recyclability study over PdAu4/GC800 It should be pointed out that the stability is of great importance for practical application in the hydrogenation of HMF to DMF. The recyclability of the PdAu4/GC800 catalyst was evaluated through five repeated reactions and the results are presented in Figure 11. The used PdAu4/GC800 catalyst was separated by centrifugation and reused without any reactivation after every testing. It was found that the conversion of HMF and the selectivity to DMF decreased slightly run by run up to the fifth cycle and finally kept 79.5% and 86.6%, respectively. Evidently, these results suggest that the catalyst remains high activity even after five cycles, indicating good stability of the catalyst.

4. Conclusion In conclusion, carbon supports with different graphitization degree (GC) were synthesized. Typically, GC800 was found to be the most superior support after loading the noble metal due to its high surface area and proper degree of graphitization. Afterwards, a series of PdAux/GC800 (x=1-4) catalysts were prepared and evaluated for the hydrogenation of HMF to DMF without any activator or acidic media. The excellent catalytic performance

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was observed, especially for PdAu4/GC800 catalyst which displayed a high selectivity to DMF of 94.4% with 86.8% HMF conversion. Moreover, the conversion of HMF and the selectivity to DMF decreased slightly after the fifth cycle. This enhanced catalytic activity was attributed to the intimate contact and intensive interaction between the Pd and Au nanoparticles, leading to the charge transfer from Au to Pd and the formation of more amounts of active Pd0 species. In addition, the intensive interaction can improve the dispersion and decrease the size of Pd and Au particles, which may be in favor of the catalytic efficiency to some extent.

Acknowledgements This work is supported by Natural Sciences Fund of Heilongjiang Province (B2015009), Foundation of Educational Commission of Heilongjiang Province of China (11531286), the Innovative Research Project of Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education (2015)

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Table 1. Physical properties of the PdAux/GC800 catalysts Catalysts

DXRD(nm)a

SBET(m2·g-1)d

DTEM(nm)e

1 wt% Pd/GC800

7.3b

119

6.4

4 wt% Au/GC800

14.9c

112

14.7

PdAu/GC800

6.2c

119

5.8

PdAu2/GC800

5.9c

118

5.5

PdAu3/GC800

6.4c

118

5.9

PdAu4/GC800

6.7c

117

6.1

a

Average particle sizes calculated from XRD patterns.

b

Calculated from Pd (111) based on Scherrer’s equation.

c

Calculated from Au (111) based on Scherrer’s equation.

d

Derived from N2 adsorption-desorption isotherms.

e

Average particle sizes calculated from at least 100 individual crystallites in TEM images.

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Table 2. XPS data of the Pd 3d levels of the different catalysts Pd Catalysts

1 wt% Pd/GC800

PdAu/GC800

PdAu2/GC800

PdAu3/GC800

PdAu4/GC800

species

Pd species BE (eV)

Content of Pd

3d5/2

3d3/2

species (%)

Pd0

335.6

341.0

27

Pd2+

337.6

342.8

73

Pd0

335.4

340.8

58

Pd2+

337.4

342.6

42

Pd0

335.2

340.6

64

Pd2+

337.1

342.5

36

Pd0

335.3

340.7

67

Pd2+

337.3

342.5

33

Pd0

335.2

340.6

72

Pd2+

337.2

342.3

28

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Table 3. Catalytic activity of the PdAux/GC800 catalysts a Selectivity (%)

Conversion

Yield of

-1 b

Catalysts

TOF (h ) (%)

DMF

MFA

Others c

DMF (%)

1 wt% Pd/GC800

57.2

342

72.7

17.7

9.6

41.6

4 wt% Au/GC800

48.4

—

7.6

1.4

91

3.7

PdAu/GC800

91.7

295

59.3

5.8

34.9

54.4

PdAu2/GC800

91.2

294

62.4

3.5

34.1

56.9

PdAu3/GC800

89.7

336

67.9

4.3

27.8

60.9

PdAu4/GC800

86.8

469

94.4

4.8

0.8

81.9

a

b

Reaction conditions: under 10 bar H2 at 150 °C for 4 h. Turnover frequency (TOF) = [moles of DMF produced]/[(moles of Pd loading)×(dispersion of

Pd)×(reaction time)]. c

Includes 2-methylfuran (MFU), 2-methyltetrahydrofuran (MTHF), 2,5-dimethyltetrahydrofuran (DMTHF)

and other products that could not be identified by GC-MS.

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Scheme 1. Reaction network for hydrogenation of HMF

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Figures Captions Figure 1. XRD patterns (A) and enlarged regional patterns between 36° and 42° (B) of the catalysts (a: 4 wt% Au/GC800, b: PdAu4/GC800, c: PdAu3/GC800, d: PdAu2/GC800, e: PdAu/GC800, f: 1 wt% Pd/GC800) Figure 2. Nitrogen adsorption-desorption isotherms of a: PdAu/GC800, b: PdAu2/GC800, c: PdAu3/GC800, d: PdAu4/GC800 Figure 3. SEM images of a: PdAu/GC800, b: PdAu2/GC800, c: PdAu3/GC800, d: PdAu4/GC800 Figure 4. TEM images and particle size distribution of PdAu/GC800 (a, b, c), PdAu2/GC800 (d, e, f), PdAu3/GC800 (g, h, i) and PdAu4/GC800 (j, k, l) Figure 5. STEM-EDS elemental mappings of PdAu2/GC800 (a) and PdAu4/GC800 (b) Figure 6. TEM images and particle size distribution of 1 wt% Pd/GC800 (a, b) and 4 wt% Au/GC800 (c, d) Figure 7. XPS of Pd 3d (A) and Au 4f (B) of the different catalysts. a: 1 wt% Pd/GC800, b: PdAu/GC800, c: PdAu2/GC800, d: PdAu3/GC800, e: PdAu4/GC800 Figure 8. Plot of the amount of surface Pd0 versus the value of x in the PdAux/GC800 catalysts Figure 9. Effect of H2 pressure on activity over the PdAu4/GC800 catalyst (Reaction conditions: 50 mg PdAu4/GC800, 0.252 g HMF, 15 mL THF, 150 °C, 4 h) Figure 10. Effect of reaction time on activity over the PdAu4/GC800 catalyst (Reaction conditions: 50 mg PdAu4/GC800, 0.252 g HMF, 15 mL THF, 10 bar H2, 150 °C) Figure 11. Recyclability experiments of the PdAu4/GC800 catalyst (Reaction conditions: 50

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mg PdAu4/GC800, 0.252 g HMF, 15 mL THF, 10 bar H2, 150 °C, 4 h)

30


Graphite Pd Au

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

f e d c b a 10

111

100

200

111 30

004

f

40

50

110

60

70

e d c b a

220

2 θ (degree)

B

Pd (111)

Au (111)

200

002

20

A

Intensity (a.u.)

Page 31 of 41

80

36

37

38

39

40

41

42

2 θ (degree)

Figure 1. XRD patterns (A) and enlarged regional patterns between 36° and 42° (B) of the catalysts (a: 4 wt% Au/GC800, b: PdAu4/GC800, c: PdAu3/GC800, d: PdAu2/GC800, e: PdAu/GC800, f: 1 wt% Pd/GC800)

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300

Volume adsorbed/(cm3/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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250

a b c d

200 150 100 50 0.0

0.2

0.4

0.6

0.8

1.0

P/P0 Figure 2. Nitrogen adsorption-desorption isotherms of a: PdAu/GC800, b: PdAu2/GC800, c: PdAu3/GC800, d: PdAu4/GC800

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Energy & Fuels

Figure 3. SEM images of a: PdAu/GC800, b: PdAu2/GC800, c: PdAu3/GC800, d: PdAu4/GC800

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Energy & Fuels

Figure 5. STEM-EDS elemental mappings of PdAu2/GC800 (a) and PdAu4/GC800 (b)

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Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 6. TEM images and particle size distribution of 1 wt% Pd/GC800 (a, b) and 4 wt% Au/GC800 (c, d)

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A

Pd 3d5/2

Pd 3d3/2

Au 4f7/2

Au 4f5/2

B

d c Ⅱ

Pd

344

342

340

0

Pd

338

336

Binding energy (eV)

Intensity (a.u.)

e

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

e d

b

c

a

b

334

92

90

88

86

84

82

Binding energy (eV)

Figure 7. XPS of Pd 3d (A) and Au 4f (B) of the different catalysts. a: 1 wt% Pd/GC800, b: PdAu/GC800, c: PdAu2/GC800, d: PdAu3/GC800, e: PdAu4/GC800

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Energy & Fuels

75

0

Amount of surface Pd (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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70

65

60

55 1

2

3

4

Au/Pd mass ratio Figure 8. Plot of the amount of surface Pd0 versus the value of x in the PdAux/GC800 catalysts

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Energy & Fuels

Conversion and selectivity (%)

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100 80 60

HMF Conversion DMF Selectivity MFA Selectivity

40 20 0 8

9

10

11

12

13

14

H2 pressure (bar) Figure 9. Effect of H2 pressure on activity over the PdAu4/GC800 catalyst (Reaction conditions: 50 mg PdAu4/GC800, 0.252 g HMF, 15 mL THF, 150 °C, 4 h)

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Conversion and selectivity (%)

Energy & Fuels

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100 80 60

HMF Conversion DMF Selectivity MFA Selectivity

40 20 0 1

2

3

4

5

6

Time (h) Figure 10. Effect of reaction time on activity over the PdAu4/GC800 catalyst (Reaction conditions: 50 mg PdAu4/GC800, 0.252 g HMF, 15 mL THF, 10 bar H2, 150 °C)

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Energy & Fuels

Conversion and Selectivity (%)

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100

C(HMF)

S(DMF)

80 60 40 20 0 1

2

3

4

Run numbers (time)

5

Figure 11. Recyclability experiments of the PdAu4/GC800 catalyst (Reaction conditions: 50 mg PdAu4/GC800, 0.252 g HMF, 15 mL THF, 10 bar H2, 150 °C, 4 h)

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Efficient Production of the Liquid Fuel 2,5-Dimethylfuran from 5

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