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Fast catalytic hydrogenation of 2,5-hydroxymethylfurfural to 2,5-dimethylfuran with ruthenium on carbon nanotubes. Peter Priecel, Nor Azam Endot, Piera Demma Carà, and Jose Antonio Lopez-Sanchez Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04715 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 22, 2018

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Industrial & Engineering Chemistry Research

Fast catalytic hydrogenation of 2,5hydroxymethylfurfural to 2,5-dimethylfuran with ruthenium on carbon nanotubes Peter Priecel,+, ‡ Nor Azam Endot,+,‡ Piera Demma Cara,&,† Jose Antonio Lopez-Sanchez*,+,& +

Stephenson Institute for Renewable Energy, Department of Chemistry, University of Liverpool,

L69 7ZD Liverpool, UK. &

MicroBioRefinery facility, Department of Chemistry, University of Liverpool, L69 7ZD

Liverpool, UK. *corresponding author: [email protected]

ABSTRACT: We have found that the utilization of carbon nanotubes as support for ruthenium nanoparticles increases hydrogenation activity over 40 times in terms of turn-over frequency (TOF) when compared to activated carbon in the transformation of hydroxymethylfurfural to dimethylfuran. Catalysts based on carbon nanotubes produced 83.5 % yield of dimethylfuran (TOF 819.7 h-1) in under one hour at 150 °C and less than 20 bar hydrogen pressure whereas the activated carbon catalyst required more than 3 hours to give an 80 % yield of dimethylfuran (TOF 36 h-1). The superior accessibility of pores in carbon nanotubes, plus an electronic promotional effect in the carbon nanotubes appear to be responsible for the superior activity of

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the catalysts supported on carbon nanotubes. The catalysts were synthesized by impregnation and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, nitrogen physisorption, temperature programmed reduction, electron microscopy and pulse CO chemisorption to propose the structure-activity relationships. This work highlights the importance of the support in hydrogenating reactions with ruthenium and the potential applicability of carbon nanotubes as supports for the hydrogenation of other bioderivatives.

Keywords: Hydroxymethylfurfural; dimethylfuran; carbon nanotubes; ruthenium; hydrogenation;

1. INTRODUCTION 2,5-Dimethylfuran (DMF) is considered a potential renewable transportation fuel1, 2 thanks to its properties such as immiscibility with water,2 high octane number, low oxygen content, high energy density3 or optimum boiling point.4 Furthermore, there is a growing amount of literature showing that DMF can serve as a substrate in the synthesis of p-xylene and other aromatic compounds via the cycloaddition of olefins.5 The production and recent progress in the development of biofuel 2,5-dimethylfuran has recently been reviewed by Lu et al. 6 DMF is generally produced from 2,5-hydroxymethylfurfural (HMF),2 which is in turn produced from fructose via glucose isomerisation.7, 8 De Vries and co-workers have recently reviewed the literature regarding the synthesis and utilization of HMF as a platform chemical.9 The production of glucose from lignocellulosic biomass offers a pathway for the sustainable production of fuels and aromatics derived from DMF.10


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There have been many reported routes and catalysts for DMF production. Using formic acid as transfer hydrogenating agent, the one-pot conversion of fructose to DMF in 51 % yield was possible,11 but also, the same reaction was able to produce 50 % yield of ring-hydrogenated 2,5dimethyltetrahydrofuran (DMTHF).12 Most researchers focus their efforts in improving the yield of DMF in the last step of the hydrogenation of HMF.8, 10, 13-16 100 % yield of DMF was achieved by Chatterjee et al.17 over Pd/C, but required a supercritical CO2 mixture with water at low temperature (80 °C) and H2 pressure (10 bar) in 2 hours. Almost the same yield of DMF was obtained by Nishimura and co-workers (>99%)18 using PdAu/C at 60 °C and atmospheric pressure of hydrogen in 12 hours, but despite the milder conditions in these experiments the reaction time is very long. Several other authors reported >95 % DMF yield at in only two hours employing PtCo supported on hollow carbon nanospheres (160180°C),19 or using 7 % Ni-30 % W carbide/C at 180 °C, 40 bar H2 after 3 hours reaction.20 In 2015, Luo et al.21 attempted the optimization of reaction conditions and prepared DMF in 68.5 % yield over 10 % Pt/C (180 °C, 33 bar H2, W/F 50 g min g-1) in a flow reactor.21 Within noble metals, Ru is frequently selected as the most suitable metal for various biomass hydrogenations.22-25 The choice of support and reaction conditions widely varies across the literature and whereas some systems produced a DMF yield of 94.7 % after 2 h,22 (5 % Ru/C, 200 °C, 20 bar H2) other systems required 24 h to produce a yield of 93.4 % 4 (5 % Ru/Co3O4, 130 °C, 7 bar H2). Recently, Jae et al.26 studied the oxidation state of ruthenium in the catalytic transfer hydrogenation of HMF and demonstrated that RuO2 is mainly involved in the transformation of aldehyde group of HMF to alcohol forming DHMF (2,5bis(hydroxymethyl)furan), while reduced Ru/C produced DMF among other by-products. Moreover, a physical mixture of the two gave as high as 70 % selectivity to dimethylfuran. It


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was shown that interaction between the two Ru phases and partially oxidized ruthenium surface are responsible for the high activity of Ru-based catalysts. Nevertheless, the production of bioderived DMF still competes with the price of fossil feedstocks and more extensive production of this green bioderived chemical will depend on economic factors. Therefore, there is a strong desire to increase the activity of catalysts to minimize cost in DMF production which is already limited by the relatively high price of HMF as starting material. On the other hand, carbon nanotubes have shown to exhibit different metal-support interactions and often superior catalytic activities when compared to activated carbon,27-31 although both conversion and selectivity has to be considered in these cases.32, 33 Importantly, in studies which compared CNT or other support of similar properties, e.g. graphene oxide34, and AC-based catalysts, support porosity seemed to produce the most pronounced effect34-36 although electronic promotional effect was also suggested.37, 38 In this work, we report highly active and selective ruthenium nanoparticles supported on multiwalled carbon nanotubes (MWCNT) for the hydrogenation of HMF to DMF. We show that simple impregnation of Ru on CNT can give a high dispersion of ruthenium at 5 wt. % loading and achieve full conversion of hydroxymethylfurfural in just one hour at relatively mild conditions (150 °C, 20 bar total pressure).

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. 2,5-Hydroxymethylfurfural (HMF) (99 %), 2,5-dimethylfuran (DMF) (99 %), 5-methylfurfural (MF) (99 %), 5-methylfurfuryl alcohol (MFA) (99 %), 2methylfuran (MFUR) (99.5 %), 2,5-dihydroxymethylfuran (97 %) (DHMF), 2,5-


4

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dimethyltetrahydrofuran (DMTHF) (96 %), 2-hexanol (99 %), 1,2-hexanediol (99 %), 1,2,6hexanetriol (96 %), 5 wt. % Ru/C, RuCl3.xH2O (99.98 %), dioxane (>99.5 %), tridecane (99 %) and multi-walled carbon nanotubes (CNT) were purchased from Sigma-Aldrich. 2,5-dihydroxylmethyltetrahydrofuran (DHMTHF) (98 %) was purchased from Carbosynth. Activated carbon (AC) Norit SX Plus was kindly provided by Cabot Corporation. 2.2. Synthesis of catalysts. All catalysts were prepared via incipient wetness impregnation as follows. Typically, for 5 wt. % loading of Ru, 103 mg of RuCl3.xH2O were dissolved in 2 mL of deionized water. The aqueous solution of the metal salt was then added dropwise to 1 g of support (activated carbon, carbon nanotubes). The mixture was mixed in a mortar before being dried under vacuum in an oven at 60 °C overnight. 2.3. Hydrogenation of 2,5-hydroxymethylfurfural (catalytic testing). The hydrogenation of HMF was performed in a 50 mL stainless steel Parr 4590 series reactor equipped with an overhead gas entrainment stirrer. Before reaction, the catalyst was reduced at 400 °C for 4 hours (temperature ramp of 5 K/min) in a tubular furnace in a flow of pure hydrogen (50 mL/min). 60 mg of catalyst were transferred into the reactor vessel together with 30 mL of stock solution of HMF and tridecane (internal standard) in dioxane (40 mM). The reactor was then sealed and purged with N2. When the reaction temperature was reached (130 - 200 °C), the reactor was pressurized with H2 (10 - 40 bar total pressure). Samples were taken at specific times via a sampling valve for time online analysis. The products were separated from the catalyst via centrifugation (5 min at 1300 rpm) and filtered through 0.22 µm syringe membrane filter before being analyzed by gas chromatography.


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2.4. Chromatographic analysis of reaction products. Reaction products were analyzed quantitatively by an Agilent 7890A gas chromatograph (GC) equipped with a SUPELCOWAX 10 capillary column (30 m x 0.32 mm x 0.25 µm) using a flame ionization detector (FID). The following settings for were used for the GC analysis: Tinjector = 270 °C; TFID = 280 °C; flow(N2) = 1 mL/min; method: 40 °C for 2 min, 20 K min-1 ramp to 260 °C and hold for 3 min. Concentrations of all compounds were calculated by using calibrations with pure standards and internal standard (tridecane). Unknown products were identified with a GC/MS (Agilent 7820A GC coupled with 5975 MSD) equipped with a HP-5MS column and hydrogen as carrier gas. If not stated otherwise, the carbon mass balances were always >90 %. 2.5. Calculation of activity parameters. HMF conversion, product yields, carbon mass balance, and turn-over frequencies (TOF) were calculated using the following equations.

Conversion % =

Yield % =

moles of substrate reacted . 100 initial moles of substrate

moles of product . 100 initial moles of substrate

Carbon mass balance % =

TOF =

total moles of products and unreacted substrate . 100 initial moles of substrate

!"# $ #%&#'()'" * +,"('"- +%&"( $ )*'.," #.'"#.'."

!"# $ #%&#'()'" * +,"('"-

= -.#/"(#.

+. !"# $ "')!.'."

[molHMF molRu-1 h-1] or [h-1]

For the calculation of the yields of unidentified over-hydrogenation and ring-opening products, an average response factor of available standards was used as an estimate.


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Dispersion (D) based on TEM (transmission electron microscopy) particle size distribution (PSD)39 (assuming hemispherical particle shape) and CO chemisorption was calculated as follows.

0123 = 6.

567 ⁄867 . 100 :67

where VRu is the volume occupied by Ru atom in the bulk of metal (0.01365 nm3), ARu is the surface area occupied by Ru atom on the polycrystalline surface (0.0635 nm2) and dRu is experimental mean particle size of Ru [nm].

0;< =

=>. 8? . @A=;< B67

where SF is the stoichiometric factor of CO adsorption on Ru surface (1), Ar is Ru relative atomic weight (101.07 g mol-1), CHSCO is the number of moles chemisorbed per gram of catalyst (mol g-1) and cRu is the real metal loading (%). Kinetic reaction rate constants (ki) of consumption (HMF) and formation (DHMF, DMF, DMTHF) were calculated from the linearized form of reaction rate law assuming first order reaction as follows. lnc. = −k. t + lnc.F where ci is concentration of reactant or product [mol L-1], k is rate constant [s-1], t is time [s]. Zero denotes initial conditions. Plotting ln ci vs. time the rate constant was obtained from the slope of the linear regression equation.


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2.6. Characterization of catalysts. 2.6.1. X-ray diffraction (XRD). XRD patterns were recorded on a Panalytical X’Pert PRO HTS X-ray diffractometer using Cu-Kα radiation (λ = 0.154 nm) over the 2θ range of 4 - 90°. 2.6.2. Inductively-coupled plasma optical emission spectroscopy (ICP-OES). ICP-OES was used to determine real loading of Ru in the catalysts. Catalysts were digested in aqua regia ( a mixture of HCl and HNO3) in CEM Discover SP microwave reactor at 180 °C for 10 minutes. 2.6.3. Nitrogen physisorption. Nitrogen physisorption isotherms of supports and catalysts were measured in the range of relative pressures 0.05-0.99 on Quantachrome NOVA 4200e analyser at the temperature of liquid nitrogen. Total pore volume was taken at adsorption point of 0.99 p/p0. Specific surface areas were estimated by BET theory (Brunauer–Emmett–Teller) from adsorption branch points between 0.08 and 0.35 p/p0. Pore size distribution was analyzed by BJH method (Barrett-Joyner-Halenda) from desorption branch of the isotherm. Supports were heat treated in nitrogen flow at 400 °C for 4 hours and metal-containing catalysts were reduced in hydrogen flow at 400 °C for 4 hours. All samples were degassed overnight at 300 °C before physisorption experiments. Unfortunately, due to the vacuum limitation, micropore analysis was not possible. Typical statistical surface area measurement error is ±20 m2/g. 2.6.4. Temperature programmed reduction (TPR) with H2. TPR profiles were obtained with Micromeritics AutoChem II 2920 analyzer equipped with a thermal conductivity detector (TCD). For TPR experiments, 50 mg of the sample was placed in a U-shaped quartz reactor and heated at 10 °C/min from room temperature to 800 °C under 5 vol. % H2/N2 flow (total flow rate of 50


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cm3/min, STP) (standard temperature and pressure). Water formed during the reduction process was trapped by isopropanol and liquid nitrogen mixture to prevent it from interfering with hydrogen consumption signal. 2.6.5. CO chemisorption. Total molar chemisorbed amounts of gas were determined from pulse chemisorption of carbon monoxide or hydrogen on Micromeritics AutoChem II 2920 analyzer equipped with TCD. Typically, 50 mg of the sample were placed in a U-shaped quartz reactor and reduced in-situ at 400 °C for 4 h (5 K/min ramp from room temperature) in 50 mL/min flow of 5 vol. % H2/N2 mixture. Afterwards, the sample was flushed with inert gas (nitrogen or helium) and cooled to 50 °C. 10 vol. % CO/He was pulsed on the sample until saturation was achieved. 2.6.6. Transmission electron microscopy. Samples were supported on a 300 mesh holey carbon copper grid and micrographs were acquired on JEOL 3010 TEM operated at 300 kV (Nanoinvestigation Centre at University of Liverpool (NiCaL)). Approximately 200 particles were counted to determine the particle size distributions. 2.6.7. X-ray photoelectron spectroscopy. XPS spectra were recorded in an ESCALAB 250 spectrometer and Al Kα radiation was used as the X-ray source. The C1s peak at 285 eV was used as a reference for the calibration of the binding energy (BE). Binding energy maxima were determined by fitting the peaks with asymmetric line-shape function.

3. RESULTS AND DISCUSSION 3.1. Characterisation.


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3.1.1. X-ray diffraction. To obtain phase characteristics of our catalysts, XRD analysis was performed (Figure 1). Diffractions at 20.8, 25.5, 26.6, 36.3, 39.2, 40, 42.8, 45.5, 67 and 73° were attributed to the carbon phase based on the JCPDS files (001-0640, 003-0401) and comparison with bare supports. No diffractions of metallic Ru or ruthenium oxides were observed in the patterns, which gives a first indication of a good dispersion of the active phase.

100

♦ ♦

♦

5%Ru/CNT 3%Ru/CNT

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

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1%Ru/CNT ♦

CNT

♦

♦ 5%Ru/AC 3%Ru/AC 1.5%Ru/AC

♦

♦ ♦

AC

♦ 10

20

30

40

50

60

70

80

90

2theta (°)

Figure 1. XRD patterns of activated carbon (AC), carbon nanotubes (CNT), reduced catalysts: 1.5, 3 and 5%Ru/AC, 1, 3 and 5%Ru/CNT. Full diamond () stands for carbon phase.

3.1.2. Nitrogen physisorption. Nitrogen adsorption-desorption isotherms and particle size distributions for bare supports and reduced catalysts with the highest loading (5%Ru/AC and 5%Ru/CNT) are shown in Figure 2 with specific surface areas, total pore volumes and average


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pore diameter presented in Table 1. It can be seen that activated carbon has a higher specific surface area of 664 m2 g-1 as compared to 330 m2 g-1 for CNT. However, the pore volume of AC (0.79 cm3 g-1) is ca. 35 % lower than that of CNT (1.23 cm3 g-1). Along with this, the pore diameter of CNT (17.4 nm) is >3.6 times bigger than that of AC (4.8 nm).

0.020

800

0.020

700

0.015 0.015

600

dV(r) (cm3 g-1 nm-1)

volume adsorbed (cm3 g-1)

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


500 400 300 200

0.010 0.005

0.010

0.000

3

4

5

6

7

8

9

10

0.005

100 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 relative pressure (p/p0)

0.000 0

10

20

30 40 50 60 70 pore diameter (nm)

80

90 100 110

Figure 2. Nitrogen adsorption-desorption isotherms (left) and pore size distributions (right) for bare supports (activated carbon, AC; carbon nanotubes, CNT) and reduced catalysts (5%Ru/AC and 5%Ru/CNT). Legend: AC (), CNT (), 5%Ru/AC (), 5%Ru/CNT ( ); pore size distribution plot: black line (AC (solid), 5%Ru/AC (dash dot)), red line (CNT (dash), 5%Ru/CNT (dot)). It is also relevant to note that impregnation of 5 % ruthenium and reduction at 400 °C in hydrogen flow has a small effect in surface area when using both supports, and decreases by less than a 5%. Pore volume and pore size distribution remain fundamentally unchanged in the synthesis of the 5%Ru/AC catalyst, whereas the average pore diameter slightly decreases from 17,4 to 15.7 nm when supporting Ru on the CNTs.


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Table 1. Specific surface area (SBET), total pore volume (Vtotal) and average pore diameter (davg) for bare supports and reduced 5%Ru/AC and 5%Ru/CNT. Sample g-1

SBET, m2

g-1

Vtotal, cm3

davg, nm

AC

664

0.79

4.8

CNT

330

1.23

17.4

5%Ru/A

636

0.79

4.9

5%Ru/C NT

317

1.25

15.7

C

3.1.3. Temperature programmed reduction. Redox properties of the catalysts were studied by temperature programmed reduction (Figure 3). Peaks in the low-temperature range below 300 °C can be attributed to the simultaneous decomposition of the RuCl3 (reduction of unsupported chloride at 160 °C)40, 41 and reduction of the metal precursor to the metallic state.42 A broad feature at the temperature >300 °C corresponds to the reduction of carbon species or carbonrelated functional groups43 or to the methanation of carbon as it was reported in the presence of supported metals.40, 42, 44


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TCD Signal (a.u.)

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5%Ru/AC 5%Ru/CNT

3%Ru/CNT 1%Ru/CNT 100

200

300

400

500

600

700

Temperature (°C) Figure 3. H2-TPR profiles for ruthenium supported on activated carbon (AC) and carbon nanotubes (CNT). Legend (top to bottom): 5%Ru/AC, 5%Ru/CNT, 3%Ru/CNT, 1%Ru/CNT.

3.1.4. Microscopy. To further investigate the active phase in the catalysts, average Ru particle sizes were measured and are shown in the TEM images (Figure 4). It can be seen that all measured particle sizes are between 1.5 and 2 nm and catalysts have typical Gaussian particle size distribution as it is expected from incipient wetness impregnation (Figure 4A,C). Also, from the measured distributions, carbon nanotubes-based catalysts seem to have slightly lower average particle size of 1.53-1.58 nm as compared to the activated carbon-based (1.85-2.03 nm) although 2.03 nm belongs to the commercial 5%Ru/C.


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A

B

Mean = 1.85± ±0.73 nm

Particle Count

30

20

10

0

0

1

2

3

4

5

Diameter (nm)

C

Mean = 1.53± ±0.48 nm

50 40

D

Particle Count

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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30 20 10 0

0

1

2

3

4

Diameter (nm)

Figure 4. TEM images of 5%Ru/AC (A, B), 5%Ru/CNT (C, D) and the corresponding histograms with particle size distributions.


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3.1.5. X-ray photoelectron spectroscopy. First, it should be noted that the spectra should be corrected even though they still show a ca. 3 eV shift to higher binding energies (Figure 5).

Ru 3p3

466.54 eV

476

C 1s + Ru 3d

464.95 eV

474

472

470

468

466

287.91 eV

Intensity (a.u.)

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


464

462

460

458

298

287.27 eV

296

294

binding energy (eV)

292

290

288

286

284

282

binding energy (eV)

Figure 5. X-ray photoelectron spectra of 5%Ru/AC (grey) and 5%Ru/CNT (black) shown as Ru(3p3) (left) and C(1s)/Ru(3d) (right) core levels showing corresponding maxima of binding energies.

It must however be noted that both reduced samples were temporarily exposed to air during the transfer to XPS chamber. However, in Figure 5, the relative position and the main difference between binding energies in core levels of 5%Ru/AC and 5%Ru/CNT is evident in both Ru(3p3) and C(1s)/Ru(3d) XPS spectra. In both cases there is a shift to lower binding energies when carbon nanotubes are used as supports. A -1.59 eV shift is appreciated in Ru(3p3) (left) and 0.64 eV in C(1s)/Ru(3d) core levels (right). Within Ru(3p3) spectra this points to weaker interaction of Ru with the support and the narrower width of the convoluted peak of 5%Ru/CNT indicates a higher relative ratio of Ru0 compared to 5%Ru/AC. Combined C(1s)/Ru(3d) spectra also suggest a different distribution of carbon environment for the two samples, with CNT being


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more varied and AC having significantly more of single type of carbon species. These observations are in agreement with the TPR section which later describes differences in the redox properties of the two catalysts which could underpin electronic promotional effects.

3.2. Hydrogenation of hydroxymethylfurfural. Mechanistically, the first step in the hydrogenation of HMF is the reduction of the aldehyde group to alcohol to give DHMF, which proceeds readly at mild conditions.45 This is followed by consecutive hydrogenolysis of the two hydroxyl groups producing DMF and water. We used a batch reactor with a gas entrainment stirrer at 1100 rpm to ensure no external diffusion limitations in our study and optimum distribution of hydrogen in the liquid. Furthermore, we performed control experiments with AC and CNT reduced the same way as all catalysts, which yielded 40 % similarly to the commercial catalyst (Figure 9). This is expected, as the commercial 5%Ru/C catalyst provided


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by Sigma-Aldrich has been commercialized because of its excellent activity and the difference in activity between this catalyst and the homemade Ru/AC catalyst could be caused by multitude of factors but most probably is the result of the optimized synthesis of the commercial sample. Our Ru/AC catalysts have been produced to be compared to the Ru/CNTs catalysts and therefore share the same preparation methods, precursors and pre-treatments and neither set of samples has been optimized. Nevertheless, conversion rates and DMF yields increase with Ru loading as expected from 1.5 to the 5% ruthenium range.

50

50

A

B

40

40

30

30

DMF Yield (%)

Conversion (%)

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


20

20

10

10

0

0 0

1

2

3 Time (h)

4

5

0

1

2

3 Time (h)

4

5

Figure 9. Conversion of HMF (A) and DMF yield up to its maximum (B) at 150 °C and 20 bar total pressure in reaction time profile over activated carbon-supported (AC) catalysts with different Ru loadings. Reaction conditions: 150 °C, 30 mL of 40 mM HMF in dioxane; 60 mg catalyst; 20 bar total pressure; 1100 rpm stirring. Legend: conversions (A)/ DMF yield (B): 1.5%Ru/AC (), 3%Ru/AC (), 5%Ru/AC ().

3.2.4. The use of CNTs as support.


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Initially, we prepared a 5%Ru/CNT catalyst following the same incipient wetness impregnation procedure as the catalysts prepared on AC. Comparison of Figure 9 and Figure 10 immediately shows the extraordinary promoting effect of the support. A vast improvement in the reaction rates of both HMF conversion and DMF formation can be observed.

100 90 80

Conversion, Yields (%)

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 60 50 40 30 20 10 0 0

1

2

3

4

5

6

Time (h) Figure 10. Time online reaction of HMF hydrogenation over 5%Ru/CNT at 150 °C. Reaction conditions: 30 mL of 40 mM HMF in dioxane; 60 mg of catalyst; 20 bar total pressure; 150 °C; 1100 rpm. Legend: conversion (), yields: 2,5-dimethylfuran (), 2,5-bis(hydroxymethyl)furan (), methylfuran (), 5-methylfurfurylalcohol (), 2,5-dimethyltetrahydrofuran (⊳), 2hexanol (), other ring opening and hydrogenation products ().


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We observe a maximum yield close to 85 % of DMF before one hour reaction, but similarly to the other catalysts tested, DMF yield does not increase further because of the reactivity of DMF under our reaction conditions. It appears that when the conversion of HMF is close to 90 %, DMF is hydrogenated to by-products such as ring-hydrogenated dimethyltetrahydrofuran or ringopened hexanol. This observation suggests that competitive adsorption mechanisms exist for the primary reaction with the consecutive hydrogenation of the furan ring. It is however highly remarkable that after one hour reaction the 5%Ru/CNT gave 83.5 % yield of DMF at 96.5 % conversion of HMF, whereas the 5%Ru/AC catalyst produced only 5% yield of DMF and only 14 % conversion of HMF at the same reaction conditions. We carried out some initial recyclability studies with the 5%Ru/CNT catalyst at 150 °C for one hour, and we found that activity gradually decreases from 100 % conversion of DMF to 76 % in the third test. In these experiments we did merely filter the catalyst without adding a pre-treatment step and, we believe that the exposure of the ruthenium nanoparticles to air could result in an apparent deactivation in the catalyst that might not occur under the actual reaction conditions (highly reductive environment). This deactivation requires dedicated detailed studies. We do however anticipate that any deactivation due to the oxidation of the metallic ruthenium can easily be overcome by a reductive treatment47 or by optimising the impregnation process to locate the nanoparticles on the inside of the nanotubes. Finally, we decided to increase reaction temperature aiming for higher reaction rates with the 5%Ru/CNT catalyst. When the reaction was performed over 5%Ru/CNT at 200 °C (Figure S1 in ESI) the overall reaction rate did not improve as compared to 150 °C (Figure 10). The conversion of HMF is almost identical for both temperatures, however, at 200 °C we do not observe much of the reaction intermediate (DHMF), but we observe significant yields of ring opening and over-hydrogenation products which are negligible


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at lower temperatures. Therefore, the yield of DMF is lower than expected and decreases quickly due to the formation of more by-products with time on line. This product distribution is reflected in the shape of the by-product yield curve in the time on-line plot in Figure S1 in the ESI.

3.2.5. Preliminary kinetic analysis of hydrogenation of HMF. We have carried out a preliminary kinetic analysis with the data presented above to determine reaction rate constants for HMF conversion and formation of DHMF, DMF and DMTHF (where applicable) as our substrate and intermediates/(by-)products, respectively, to identify the rate limiting step for 5 % Ru catalysts. We used kinetic data at 150 °C and 20 bar total pressure from 5%Ru/AC (Figure 9), 5%Ru/C (Figure 8) and 5%Ru/CNT (Figure 10) and first order approximation of absolute values of reaction rate constants is presented in Table 2. It should be noted that they do not represent initial rate constants (apart from conversion of HMF) because these rate constants are determined from the previously discussed time online data for HMF conversion.

Table 2 First-order reaction rate constants for reaction steps HMF DHMF DMF DMTHF (where applicable). Minus sign (-) denotes consumption of the substrate and plus (+) denotes formation of the product. Numbers in brackets are coefficients of determination (R2) for respective linear regression from determination of displayed constants. All reaction rate constants are multiplied by 104 and have unit s-1.

molecule

5%Ru/AC

5%Ru/C

5%Ru/CNT

(-) HMF

0.56 (0.9721)

1.29 (0.9976)

7.55 (0.9944)

(+) DHMF

could not be determined

5.35 (0.9789)

6.19 (0.8443)

(+) DMF

6.61 (0.8694)

7.22 (0.9479)

5.97 (0.9526)


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(+) DMTHF

not formed

2.42 (0.9925)

11.5 (0.9925)

For both activated carbon-based 5 % Ru catalysts, the conversion of HMF is the step with the lowest rate constant and therefore, the rate determining step (RDS). The formation of DMF is the fastest step. Indeed, the superior performance of the 5%Ru/CNTs is not the result of a faster DMF formation from DHMF and, does not present a superior rate constant for DMF formation. The key to the superior activity of the 5%Ru/CNT must be in the much faster activation of the reaction substrate (HMF). The 5%Ru/CNT catalyst displays a rate constant for HMF conversion which is 14 times higher than the 5%Ru/AC catalyst. Indeed, rate constants for the conversion of HMF are in the increasing order of 5%Ru/AC < 5%Ru/C < 5%Ru/CNT which is in agreement with presented catalytic activities. Comparison of rate constants for the formation of DHMF or DMF for the three catalysts does not show major differences between them and, these are clearly faster steps for the two carbon supported catalysts when compared to HMF activation. This preliminary kinetic analysis suggests that the main differences in the catalytic performances of 5%Ru/AC, 5%Ru/C and 5%Ru/CNT catalysts are due to differences in the catalyst’s ability to promote the initial conversion of HMF, and we have found that CNTs are exceptionally good realising such promotion.

3.2.5. The use of CNTs as support: effect of ruthenium loading. In view of the excellent promoting effect of the CNTs, we decided to synthesize lower Ru loadings (1 and 3 wt. %) which would give us an indication of the ability of our preparation method to produce highly dispersed nanoparticles and also to optimise dispersion. Figure 11


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displays the time on-line plots for the two newly tested catalysts. A comparison of reaction rates reveals that the conversion of HMF increases with the increase in the ruthenium loading in the catalysts. This suggests that the dispersion is good within this whole range from 1 to 5% Ru loadings. The effect of metal loading is particularly evident when the product distribution is taken into account. After the relatively fast hydrogenation of the aldehyde group has occurred forming 2,5-bis(hydroxymethyl)furan, the intermediate desorbs and requires re-adsorption for the two hydrogenolysis steps to proceed and result in the production of DMF. For the 1%Ru/CNT catalyst, DHMF is the primary reaction product, probably as DHMF does not find sufficient active sites to re-adsorb and to convert further, whereas the 3%Ru/CNT catalyst offers sufficient active sites for DHMF to convert further to methylfurfuryl alcohol and to 30% of DMF after 6 hours (Figure 11). Nevertheless, only the 5%Ru/CNT produces DMF as the main product from the onset of the reaction as displayed in Figure 10. These observations suggest that increasing the loading does indeed result in well-dispersed ruthenium and the increased catalytic activity. It could be argued that the product distribution could also be affected by differences in the redox properties of these catalysts (Figure 3) which show only one distinct and broad reduction peak for 1 % Ru (only DHMF produced) while some DMF is being formed at 3 % Ru (one reduction peak with visible shoulder) and mainly DMF is formed at 5 % Ru when the TPR peak separation is clearly visible (2 peaks). Figure 3 also reveals how the increasing catalyst loading shifts the main reduction peak to higher temperatures. It is therefore arguable that higher loadings result in catalysts that present lower reducibility as compared to the lower metal loadings. Unfortunately, the change in metal loading and reduction profile are interrelated, and this is probably a limitation of our synthetic procedure. Notwithstanding, it is remarkable that the


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highest 5% loading produces such high activity, even when its higher loading implies some loss in the reducibility of the catalyst as shown in the TPR profiles.

100

100

90

90

80


80


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


70 60 50 40 30

70 60 50 40 30

20

20

10

10 0

0 0

1

2

3

4

5

6

0

1

2

3

4

5

6

Time (h)

Time (h)

Figure 11. Conversion of HMF and product yields in HMF hydrogenation with 1 % (left) and 3 % (right) loading of Ru/CNT. Reaction conditions: 30 mL of 40 mM HMF in dioxane; 60 mg of catalyst; 20 bar total pressure; 150 °C; 1100 rpm. Legend: conversion (), yields: 2,5dimethylfuran (), 2,5-bis(hydroxymethyl)furan (), methylfuran (), 5-methylfurfurylalcohol (), 2,5-dimethyltetrahydrofuran (⊳), 2-hexanol (), 2,5-bis(hydroxymethyl)tetrahydrofuran ().

3.2.6. The use of CNTs as support: TOF values and origin of the promotion effect It is particularly relevant to compare TOF values to elucidate the origin of the promotion by CNTs. Therefore, we first carried out CO chemisorption studies and calculated dispersion as shown in Table S1 in the supplementary information where we have also displayed the calculated TOF values at 30 % HMF conversion. To allow for easy comparison, we have plotted


29


bar diagram of HMF conversion and DMF yield at reaction time of 50 minutes for the three 5 % Ru catalysts on the three different carbon supports and we also display the TOF values for each catalyst calculated at 30 % HMF conversion in Figure 12.

Yield of DMF, Conversion (%)

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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819.7 h-1

100 80 60 40 20 0

57.9 h-1 36.1 h-1 5%Ru/AC

5%Ru/C

5%Ru/CNT

Figure 12. Conversion (blue bar) and DMF yield (orange bar) in HMF hydrogenation over 5%Ru/AC, 5%Ru/C and 5%Ru/CNT at 150 °C after 50 minutes. Turn-over frequencies (number of active sites from dispersion measured by CO chemisorption) obtained from HMF converted at 30 % conversion are shown over the bars in red text. Reaction conditions: 30 mL of 40 mM HMF in dioxane; 60 mg of catalyst; 20 bar total pressure; 150 °C; 50 minutes; 1100 rpm stirring rate.

Figure 12 clearly shows the effect of the carbon nanotubes in enhancing activity regarding conversions and yields, but also enhancing the intrinsic activity of the ruthenium as shown by the TOF values in red text. DMF yield increases one order of magnitude and TOF increases from 36 to 820 h-1 by changing the support from AC to CNT. Furthermore, Table S1 in the ESI also


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shows that 1%Ru/CNT displays even higher intrinsic activity (TOF=1226 h-1). Such effect is unprecedented and extraordinary in this reaction. Three main possible explanations are typically proposed for such activity enhancement: particle size effects, an enhancement of the porosity profile and electronic effects. There have been previous reports indicating superior activity for metals supported on CNTs due to more suitable particle size. J. Pan et al.37 showed carbon nanotubes were better supports than Al2O3 and SiO2 to support ruthenium for glucose hydrogenation to sorbitol and ascribed this enhancement to a better particle size distribution and potentially a strong metal support interaction. They also suggest that the semi-conductive properties of the CNTs make them easily lose electrons that make the ruthenium nanoparticles more electron-enriched. They suggest that this electron transfer might confer superior activity to ruthenium deposited on carbon, but also suggest that CNTs could enhance the spillover effect also beneficial for the reaction. In our case, XPS results indeed suggest shift in Ru and carbon binding energies which might induce a change in electron transfer on ruthenium nanoparticles and their interaction with carbon surface. R.M. Mironenko et al. do also suggest the better dispersion of ruthenium on CNTs as the main reason for promotion in the hydrogenation of furfural.32 In our case, the PSD of our 5%Ru/CNT, 5%Ru/AC and commercial 5%Ru/C were found to be similar within the error margin at 1.53 ± 0.48 nm, 1.85 ± 0.73 nm and 2.03 ± 0.44 nm, respectively, while the order of catalytic activity was 5%Ru/AC < 5%Ru/C < 5%Ru/CNT. Clearly, differences in catalytic activity cannot be explained on the basis of differences in particle size and it seems that other factors might play a stronger role than PSD in the promotion effect by CNTs. The work of M. Jahjah et al.48 shows that immobilizing the ruthenium nanoparticles provides better activity than the activity of the unsupported nanoparticles or a Ru/C catalyst in the hydrogenation of a wide range of unsaturated


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compounds. This suggests that carbon nanotubes can enhance the hydrogenation activity of ruthenium nanoparticles without affecting particle size distribution as it is shown in this paper. There are instances in which the functionality of the support contributes to the reaction to be catalysed, and we should consider the possible participation of the support in some steps in our multistep mechanism. W. Deng et al. showed the superior catalytic performance of ruthenium nanoparticles supported on CNTs as compared to a range of metal oxides in the cellobiose to sorbitol one-pot reaction and they explain that this is due to the functionality of CNTs being able to catalyse the hydrolysis of cellulose by itself.49 In a subsequent publication, they explain how the hydrogenation reaction and sorbitol production depend on particle size, but the effect of the CNTs compared to AC is not studied. More recently, L.S. Ribeiro et al. compared the activity of Ru, Ni and Ru-Ni nanoparticles supported on CNTs and AC in the one-pot conversion of cellulose to sorbitol.50 Their reaction tests include ball milling of cellulose with the catalysts, which is substantially different to our system, but they find consistently superior results for catalysts supported on CNTs regarding conversion and activity as compared to activated carbon and explain their results by better dispersion of the metal on CNT-based catalysts. We must conclude that in the hydrogenolysis of HMF to DMF, we cannot exclude the participation of CNTs functionality as it is possible that the functionality in CNTs plays a role in some of the steps of the reaction. Nevertheless, we found negligible HMF conversion by the CNTs alone and our experimental observations point to changes in the metallic particles redox properties as the most probable explanation for promotion. We also find some experimental support to relate the observed promotion with CNTs with the enhanced porous system of carbon nanotubes. Our surface area analysis is displayed in Table 1 and points out to a different porosity profiles. Carbon nanotubes represent a 58 % increase in


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total pore volume and more than three times larger average pore diameter compared to the activated carbon used in this study. The fact that CNT-based catalysts have significantly bigger pores, and that the majority of the overall porosity is located within them, could be the reason why CNT (or similar mesoporous carbon-based material) supported catalysts can be more active than those supported on activated carbon as others have reported in the literature for similar systems.27-30, 34, 35 In the case of HMF hydrogenation, a recent study explored the use of graphene as support for Pt and compared it to AC. 34 They found that the reduced graphene oxide catalyst decreased the reaction time and increased the yield of DMF as compared to the catalyst based on activated carbon. This was attributed to both better accessibility of the support mesoporosity and the presence of oxygen-containing surface groups. Although we did not purposely oxidize the CNTs, it is possible that similar phenomena could contribute to the enhanced activity of our ruthenium catalysts. The authors obtained 100 % conversion of HMF and 73.2 % yield of DMF over Pt/rGO after 2 hours in comparison with 67.4 and 32.6 % over Pt/AC, respectively. Electronic effects have also extensively been claimed as responsible for the the enhanced activity of CNT-supported catalysts. A. Borgna and co-workers have studied in detail the deposition and dispersion of ruthenium nanoparticles on CNTs and their activity towards cellobiose conversion to sugar alcohols. Their work highlights the importance of the functionalization of the carbon walls to optimise metal dispersion which in turn conditions catalytic activity.51 Also, they found that dispersing the ruthenium nanoparticles inside the nanotubes conferred higher activity and stability due to the higher reducibility of nanoparticles located inside the tubes.52 This confinement effect was enhanced by reducing the CNT channel size,38 which further emphasizes that electronic effects and, not only dispersion, accessibility and PSD participation in the promotion effect by CNTs as support.


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Although it is out of the scope of this work to fully determine the promotion mechanism of the CNTs, it is evident that the observed enhancement in activity in our experiments cannot be attributed to metal particle size effects and, although the porosity of the CNTs might offer advantages in terms of accessibility, it is unlikely that it can explain on its own the vast differences in TOFs calculated at lower conversions as displayed in Figure 12. Our results do however favor that both textural (porosity measurements) and electronic effects (binding energies in XPS and TPR reduction profile) play an essential role and that the enhancement in the reducibility of ruthenium nanoparticles located inside the CNTs might be a key to the observed catalytic performances. Further support for the presence of electronic effects can be drawn by inspection of the calculated TOFs for the Ru/CNT catalyst as a function of metal loading. 1%Ru/CNT gives the highest TOF of 1225.9 h-1 which gradually decreases with increasing Ru loading down to TOF of 819.7 h-1 for 5%Ru/CNT (see Table S1 in SEI). This loss in intrinsic activity is also correlated with a decrease in the reducibility of the catalyst as shown by the TPR studies in Figure 3. However, why higher ruthenium loadings resulted in less reducible nanoparticles of similar PSD? Although speculative, it is conceivable that the impregnation technique we utilized favors deposition of metal inside the nanotubes at low loadings, but as higher metal loadings are produced more metal is deposited in the external surface of the nanotubes. The higher the metal loading results therefore in relatively less ruthenium being promoted by the confinement effects within the nanotubes and slowly becoming less reducible in average as suggested by the TPR data. The decrease in TOF by increasing metal loading will, therefore, be explained by a decrease in the reducibility of the supported metal. As a result of these observations, future work must be directed towards producing the dispersion of


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ruthenium exclusively within the nanotubes, which in turn, is expected to enhance stability and further increase activity. 4. CONCLUSIONS We have shown that Ru supported on carbon nanotubes makes excellent catalysts for the selective production of DMF by hydrogenation of HMF. Catalysts based on carbon nanotubes produced 83.5 % yield of dimethylfuran in under one hour at 150 °C and less than 20 bar hydrogen pressure in comparison with the activated carbon catalyst which required more than 3 hours to give 80.3 % yield of dimethylfuran. The observed enhancement in activity as compared to analogous catalysts supported on activated carbons, cannot be explained exclusively regarding particle size distribution nor dispersion as both catalysts presented similar PSD. TOFs calculated at 30 % HMF conversion were 1226, 902 and 820 h-1 for Ru/CNT of loadings of 1, 3 and 5 % respectively, whereas a 5%Ru/AC catalyst produced a TOF of only 36 h-1. We suggest that this enhanced performance of ruthenium on CNTs can be explained by a combination of the superior porosity of CNT with electronic promotion within the carbon nanotubes able to increase the reducibility of ruthenium.

AUTHOR INFORMATION Corresponding Author * Jose A. Lopez-Sanchez*,+,&


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+

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Stephenson Institute for Renewable Energy, Department of Chemistry, University of Liverpool,

L69 7ZD Liverpool, UK. &

MicroBioRefinery facility, Department of Chemistry, University of Liverpool, L69 7ZD

Liverpool, UK. [email protected]

Present Addresses † Piera Demma Cara, IOP Publishing, Temple Circus House, Temple Way, Bristol BS1 6HG, United Kingdom Author Contributions The manuscript was written through contributions of all authors. All authors have approved the final version of the manuscript. ‡These authors contributed equally. Funding Sources EPSRC grant EP/K014773/1 UK Department of Business Skills and Innovation (Regional Growth Fund, MicroBioRefinery). Malaysian government MARA for Ph.D. scholarship of Nor Azam Endot and financial support.

ACKNOWLEDGMENTS Authors thank the EPSRC (grant EP/K014773/1) and the UK Department of Business Skills and Innovation (Regional Growth Fund, MicroBioRefinery). Nor Azam Endot thanks Malaysian


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government MARA for Ph.D. scholarship and financial support. Authors also thank CABOT Corporation for providing the activated carbon (NORIT®SX PLUS). The Institute of Chemical and Engineering Sciences (ICES) in Singapore and Dr Ziyi Zhong are acknowledged for XPS measurements. ASSOCIATED CONTENT Supporting Information. Supporting information file includes additional time online plot for 5%Ru/CNT at 200 °C and table with various physicochemical properties and TOFs of tested catalysts. RuCNT ACS ESI.docx ABBREVIATIONS 2HAO, 2-hexanol; AC, activated carbon; Ar, atomic weight; BET, Brunauer–Emmett–Teller; BJH, Barrett-Joyner-Halenda ; C, carbon; CHS, chemisorption; CNT, carbon nanotubes; cRu, real Ru loading; D, dispersion; davg, average pore diameter; DHMF, 2,5-dihydroxymethylfuran or 2,5-bis(hydroxymethyl)furan; DHMTHF, 2,5-bis(hydroxymethyl)tetrahydrofuran; DMF, 2,5dimethylfuran; DMTHF, 2,5-dimethyltetrahydrofuran; FA, furfurylalcohol; FID, flame ionisation detector; GC, gas chromatography; HDN, hexanedione; HMF, 2,5hydroxymethylfurfural; ICP-OES, Inductively-coupled plasma optical emission spectroscopy; MF, 5-methylfurfural; MFA, 5-methylfurfurylalcohol; MS, mass spectrometer; MTHFA, 5methyltetrahydrofurfurylalcohol; MWCNT, multi-walled carbon nanotubes; NiCaL, Nanoinvestigation Centre at University of Liverpool; PSD, particle size distribution; rGO, reduced graphene oxide; rpm, revolutions per minute; SBET, specific surface area determined by BET theory; SF, stoichiometric factor; STP, standard temperature and pressure; TCD, thermal


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conductivity detector; TEM; Transmission electron microscopy; TOF, turn-over frequency; TPR, Temperature programmed reduction; Vtotal, total pore volume; X30, at 30 % conversion; XRD, X-ray diffraction; Y, yield; XPS, X-ray photoelectron spectroscopy REFERENCES (1) Bergthorson, J. M.; Thomson, M. J., A review of the combustion and emissions properties of advanced transportation biofuels and their impact on existing and future engines. Renew. Sust. Energ. Rev. 2015, 42, 1393-1417. (2) Saha, B.; Abu-Omar, M. M., Current Technologies, Economics, and Perspectives for 2,5Dimethylfuran Production from Biomass-Derived Intermediates. ChemSusChem 2015, 8, 113342. (3) Barlow, M. T.; Smith, D. J. H.; Stewart, D. G. Fuel composition. EP0082689 A2, 1983. (4) Zu, Y.; Yang, P.; Wang, J.; Liu, X.; Ren, J.; Lu, G.; Wang, Y., Efficient production of the liquid fuel 2,5-dimethylfuran from 5-hydroxymethylfurfural over Ru/Co3O4 catalyst. Appl. Catal. B Environ. 2014, 146, 244-248. (5) Maneffa, A.; Priecel, P.; Lopez‐Sanchez, J. A., Biomass‐Derived Renewable Aromatics: Selective Routes and Outlook for p‐Xylene Commercialisation. ChemSusChem 2016, 9, 27362748. (6) Qian, Y.; Zhu, L.; Wang, Y.; Lu, X., Recent progress in the development of biofuel 2,5dimethylfuran. Renew. Sust. Energ. Rev. 2015, 41, 633-646. (7) Roman-Leshkov, Y.; Barrett, C. J.; Liu, Z. Y.; Dumesic, J. A., Production of dimethylfuran for liquid fuels from biomass-derived carbohydrates. Nature 2007, 447, 982-5. (8) De, S.; Saha, B.; Luque, R., Hydrodeoxygenation processes: Advances on catalytic transformations of biomass-derived platform chemicals into hydrocarbon fuels. Biores. Technol. 2015, 178, 108-118. (9) de Vries, J. G., Chapter Eight - Green Syntheses of Heterocycles of Industrial Importance. 5Hydroxymethylfurfural as a Platform Chemical. In Advances in Heterocyclic Chemistry, Eric, F. V. S.; Christopher, A. R., Eds. Academic Press: 2017; Vol. Volume 121, pp 247-293. (10) Tong, X.; Ma, Y.; Li, Y., Biomass into chemicals: conversion of sugars to furan derivatives by catalytic processes. Appl. Catal. A Gen. 2010, 385, 1-13. (11) Thananatthanachon, T.; Rauchfuss, T. B., Efficient Production of the Liquid Fuel 2,5Dimethylfuran from Fructose Using Formic Acid as a Reagent. Angew. Chem. Int. Ed. 2010, 49, 6616-6618. (12) Jackson, M. A.; Appell, M.; Blackburn, J. A., Hydrodeoxygenation of Fructose to 2,5Dimethyltetrahydrofuran Using a Sulfur Poisoned Pt/C Catalyst. Ind. Eng. Chem. Res. 2015, 54, 7059-7066. (13) Hu, L.; Lin, L.; Liu, S., Chemoselective Hydrogenation of Biomass-Derived 5Hydroxymethylfurfural into the Liquid Biofuel 2,5-Dimethylfuran. Ind. Eng. Chem. Res. 2014, 53, 9969-9978. (14) Nakagawa, Y.; Tamura, M.; Tomishige, K., Catalytic Reduction of Biomass-Derived Furanic Compounds with Hydrogen. ACS Catal. 2013, 3, 2655-2668. (15) Besson, M.; Gallezot, P.; Pinel, C., Conversion of Biomass into Chemicals over Metal Catalysts. Chem. Rev. 2013, 114, 1827-1870.


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