Multifunctional heterogeneous catalyst for one step transformation of lignocellulosic biomass into low oxygenated hydrocarbons

Applied Catalysis A: General 495 (2015) 162–172

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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

Multifunctional heterogeneous catalyst for one step transformation of lignocellulosic biomass into low oxygenated hydrocarbons Cherif Larabi a,b,∗ , Walid Al Maksoud a,b , Kai C. Szeto b , Anthony Garron a,b , Philippe P. Arquilliere a,b , Jean J. Walter a , Catherine C. Santini b,∗∗ a b

Synthopetrol, 37 Rue des Maruthins, 75008 Paris 8, France Université de Lyon, ICL, C2P2, UMR 5265 CNRS-ESCPE Lyon, 43 bd du 11 Novembre 1918, 69616 Villeurbanne Cedex, France

a r t i c l e

i n f o

Article history: Received 27 October 2014 Received in revised form 2 February 2015 Accepted 10 February 2015 Available online 18 February 2015 Keywords: Biomass Bimetallic Biofuel Hydrotreatment Heteropolyanion

a b s t r a c t Mono and bimetallic nanoparticles based on ruthenium or copper–ruthenium with controlled size, supported on heteropolyanion were easily synthesized. The partial exchange of the keggin-type heteropolyacid protons by large monovalent cations such as Cs+ leads to the formation of salts with uniform microcavities and high surface area. Heteropolyanions with various amounts of cesium content were synthesized and their specific surface areas were measured. The solids with high surface area were used as supports and functionalized by copper and ruthenium nanoparticles. Cun Rum @Csx PW catalysts thus obtained were characterized by various techniques including BET, TEM, XRD, solid state NMR and elemental analysis. Moreover, the effect of Ru loading and Cs content on the distribution of the particle size has been elucidated. Finally, the activities of the catalysts on the direct hydro-conversion of pine wood into liquids suitable for fuel application were evaluated and the influence of the experimental conditions such as temperature, hydrogen pressure, reaction time and the amount of the catalyst has been exemplified. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Conventional reserves of fossil sources including natural gas and oil are expected to decrease, while the need for energy is rising inexorably, resulting in energy supplies issue by the future. In addition, more drastic regulations are continuously introduced for the environment protection. As result, the need for sustainable sources of energy is emphasized, this implies immediate research to develop clean and renewable source of energy. It is believed that nonedible biomass such as lignocellulose could be a potential source for chemical platform and fuels as well as drop in fuels adaptable with current infrastructure and technology [1–6]. Fuels originated from biomass are carbon-neutral as the carbon dioxide produced during their combustions is recycled by plants through photosynthesis process. Due to the polymeric and complex structure of

∗ Corresponding author at: Université de Lyon, ICL, C2P2, UMR 5265 CNRS-ESCPE Lyon, 43 bd du 11 Novembre 1918, 69616 Villeurbanne Cedex, France. Tel.: +33 04 72 43 18 10; fax: +33 04 72 43 17 95. ∗∗ Corresponding author. Tel.: +33 04 72 43 18 10; fax: +33 04 72 43 17 95. E-mail addresses: [email protected] (C. Larabi), [email protected] (C.C. Santini). http://dx.doi.org/10.1016/j.apcata.2015.02.018 0926-860X/© 2015 Elsevier B.V. All rights reserved.

lignocellulosic biomass, its deconstruction into simpler building block with the aim to produce value added bio-products and biofuel is necessary. Fisher–Tropsch process is a possible route to convert synthesis gas originated from the gasification of biomass into alkanes; this approach is a multistep process, it requires multiple gas conditioning and high operating pressures and temperatures [7]. Fast and slow pyrolysis are considered as promising strategies to convert biomass into chemicals including aromatic compounds and biofuel. Slow biomass pyrolysis leads to the formation of liquid phase and co-product up to 40 wt% of bio-char and more than 10 wt% of gas. Bio-oils produced from fast pyrolysis are characterized by their high oxygen and water contents, further stabilization and hydro-processing are required before being adaptable as biofuel [8–10]. Recent development in the field of heterogeneous catalyst with the aim of fast pyrolysis oil upgrading has been well reviewed by Ruddy et al. [11]. In fact, various heterogeneous catalysts are reported in the literature inter alia transition metal oxide or sulfide, supported noble metal nanoparticles and alternatives metal carbide, nitride and phosphide [11]. In the same optic, our group has reported a novel approach based on catalytic slow hydro-pyrolysis of solid lignocellulosic biomass. The process leads to a formation of 30 wt% of organics fraction containing less than 3 wt% of

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oxygen and 30 wt% of aqueous fraction with only 2 wt% of carbon [12]. The direct transformation process of lignocellulosic biomass into biofuel, involves first a controlled decomposition of the polymeric structure, followed by desoxygenation reactions through decarbonylation, decarboxylation, cracking or hydrocracking, hydrogenation and hydrodesoxygenation mechanisms [13]. Thus, a multifunctional catalyst that contains one or more transition metal dispersed on suitable supports is needed. To enable the catalytic activity of different species of a multifunctional catalyst in a synergistic way, the number, the distances, the physical and chemical environment have to be tuned [13]. Processes based on the use of multifunctional catalyst are largely overlooked in the field of bio-based chemistry. A direct conversion of furfural into butanediol over a multifunctional platinum nanoparticle supported on mixed oxide TiO2 –ZrO2 is reported, the catalyst undergoes selective oxidation of furfural to furanones and their hydrogenation to butanediol [14]. Tungsten-based catalysts were found to be efficient in the transformation of cellulose to ethylene glycol through successively hydrolysis of cellulose, C C bond cleavage of sugars and hydrogenation of glycolaldehyde [15]. Recently, Satsuma et al. reviewed the importance of the acid catalysis in the environmental friendly processes development [16]. Keggin heteropolyacids are strong Brønsted acids. Owing to their availability as well as their chemical and thermal stability, they are widely adopted in homogenous or supported for heterogeneous catalysis [17–19]. Due to very low surface area of heteropolyacid, to enable their use in heterogeneous catalysis, much efforts were dedicated to their loading on varies carrier including silica, alumina and active carbon [20–22]. The insoluble heteropolyacid salt synthesized by proton exchange with large monovalent cations such as Cs+ increases the acidity, thermal stability as well as the surface area [23]. Therefore, they are employed as heterogeneous catalyst [24] or support for multifunctional catalyst [25]. High catalytic activity for one-step conversion of biomass was found for multifunctional catalyst based on bimetallic nanoparticles supported on heteropolyacid salt (Cs2.5 H0.5 PW12 O40 ) [12]. Herein, the shape and size of nanoparticles versus amount of heteropolyacid proton exchanged with cesium are studied, then their catalytic activities in the conversion of biomass are evaluated, finally the effect of experimental conditions (temperature, pressure, time and the amount of the catalyst) on the catalytic activity are optimized.

2. Experimental 2.1. Materials Prior to use, the pine wood was grounded in a Retcsh type RM100 mortar mill, sieved to give particle size less than 0.7 mm in diameter. In order to distinguish between the water formed during the decomposition process and the initially physisorbed water, the wood was pretreated and dried at 150 ◦ C, then stored in the glove box. Phosphotungstic acid H3 PW12 O40 ·xH2 O (99.9%, Aldrich), Cs2 CO3 (99.9%, Alfa Aesar), RuCl3 ·3H2 O (99.9%, Aldrich) and (Cu(NO3 )2 ·2.5H2 O) (99%, Aldrich) were purchased. The purity of the heteropolyacid was checked by 31 P liquid-state NMR, only the expected signal was observed. Keggin heteropolyacid (H3 PW12 O40 ·xH2 O) was dried at 200 ◦ C under vacuum (10−5 mbar) for 3 h in a glass reactor, dry oxygen was introduced to the reactor and heated at 200 ◦ C for 2 h in order to oxidize the metals atoms that may have been reduced during the thermal treatment. The other metal compounds from the commercials sources were dried at 120 ◦ C under vacuum (10−5 mbar) overnight and stored in the glove box.

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2.2. Instrumentation Prior the catalytic test, all the samples (neat support and the nanoparticles supported catalyst) were analyzed by: powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDX), elemental analysis (EA), magic angle spinning nuclear magnetic resonance (MAS NMR), N2 adsorption–desorption, Fourier transform infrared spectroscopy (FTIRS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The liquids were analyzed with gas chromatography–mass spectrometry (GC–MS) and elemental analysis (EA). X-ray powder diffraction (XRD) patterns were recorded on a D8 Advance Bruker instrument, using a Cu K␣1 radiation source in Bragg-Bretano reflecting geometry. The sample preparation was done by grinding the material until obtaining a fine powder, followed by addition of ethanol and in the end deposition of a suspension on a glass plate. TGA experiments were performed with a thermobalance, Mettler Toledo TGA/DSC1. Approximately 15 mg of material were placed in an Al2 O3 crucible and heated under 30 ml min−1 of argon from room temperature to 1000 ◦ C with a heating rate of 5 ◦ C min−1 . Scanning electron microscopy studies were conducted on an HITAHI S800 FEG scanning electron microscope (SEM). Transmission electron microscopy (TEM) observations were carried out on Philips CM120 instrument with an acceleration voltage up to 120 kV. HRTEM analyses were done with a 200 kV analytical microscope JEM 2100 F from Jeol with an ultrahigh resolution a probe size under 0.5 nm and rapid data acquisition. The electronic microscopy analyses (SEM, TEM and HRTEM) were supported by energy-dispersive analysis of X-ray spectra (EDX) to probe the local composition of the particles as well as the composition of the surface of the support. The samples were suspended in toluene and ultrasonically treated for 2 min. Then, a drop of this suspension was disposed uniformly on a molybdenum grid and dried. Elemental analyses (C, H and O) were performed at the Welience – Pôle Chimie Moléculaire Faculté des Sciences Mirande (Dijon, France), using CHNS/O thermo electron flash 1112 Series elemental analyzer. Metal concentrations (Cu, Ru, W) were carried out using inductively coupled plasma atomic emission spectrometry (ICPAES) apparatus (JOBIN YVON 38 Type III) in the “Laboratoire des Sciences Analytiques, Laboratoire d’Analyse Industrielle Unité CPE Lyon (LSA)” in CPE Lyon (Villeurbanne, France). ICP standards were prepared by dissolving a corresponding metal salts with purity >99.99% in volumetric flasks with up to 5% nitric acid in water. Cs and P were measured at the CNRS central analysis department of the analytical science institute (Villeurbanne, France). 31 P (121.5 MHz), 133 Cs (39.36 MHz) magic angle spinning solid state NMR spectra were collected on a Bruker Avance 300 spectrometer. The impeller zirconia (ZrO2 ) of 4 mm was filled with the desired product and sealed with a kel-f stopper, then transferred into the probe allowing rotation of the rotor at a speed of 10 kHz. The time between two acquisitions was always optimized to allow complete relaxation of the nucleus. Chemical shifts were measured relative to 85% H3 PO4 aqueous solution for 31 P and 1 M of CsCl for 133 Cs. Fourier transform infrared (FTIR) spectra were collected in transmission mode on Nicolet FT-5700. The solid samples were compressed into self-supporting wafers, by deposition of a suspension on a silicon wafer, or diluting the sample in KBr (typically 5 mg of the sample in 500 mg of the KBr). Afterwards, the pellet was placed in the sample holder when the spectra were recorded. The cell is equipped with KBr or CaF2 windows. Typically, 16 scans were

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made for each spectrum with a resolution of 2 cm−1 for the study of solids and 32 acquisitions with a resolution of 1 cm−1 for the study of gases (CO and CO2 ). DRIFT spectra were recorded in diffuse reflectance mode on Nicolet FT-6700 instrument, equipped with CaF2 windows and MCT detector. Typically, 64 acquisition scans were made for each spectrum with a resolution of 2 cm−1 and in Kubelka–Munk model. Adsorption–desorption isotherms of nitrogen at 77 K were measured for all the materials with Micromeritics ASAP 2020 surface and porosity analyzer. Cumulative pore volume was calculated using BJH model for adsorption isotherm. Before the adsorption analysis, the samples were degassed for 3 h at 200 ◦ C. Light hydrocarbons analyzes and quantification were performed on a HP 5890 gas chromatograph, equipped with a flame ionization detector (FID) and a KCl/Al2 O3 on fused silica column (50 m × 0.32 mm). Heavier organic hydrocarbons formed were separated by a MS compatible HP5 column (30 m × 0.25 mm) and analyzed by GCMS (Agilent GC 6850 MS 5975C).

40 bar of hydrogen was introduced in the reactor, before heating to 470 ◦ C (2 ◦ C min−1 ) under stirring (400 rpm). The temperature was maintained at 470 ◦ C for 3 h. Note that the pressure inside the autoclave reached 90–100 bar. After the heat treatment, five fractions, gas phase, two liquid phases (organic and aqueous), two solid phases (coke and acetone soluble polymer) were isolated and weighed. The liquid phase products were condensed in a cold trap placed in liquid nitrogen under vacuum. The organic phase (bio-oil) was separated by decantation. The solid residue left in the reactor consisted mainly of coke, soluble polymers and catalysts, was washed with acetone. The soluble polymer was weighed after evaporation of the later acetone. The amount of coke was estimated after subtracting the initial catalyst mass. Finally, the laws of conservation of matter allowed estimating the gas phase. The water content of the bio-oil was determined by the KarlFischer method. The organic phase was analyzed by GC–MS using an Agilent 6850/5975C gas chromatograph–mass spectrometer.

2.3. Catalyst synthesis

3. Results and discussions

First of all, 0.4 M solution of cesium carbonate and 0.8 M solution of heteropolyacid were prepared by dissolving 13 g of dry Cs2 CO3 in 100 ml of water and 115.4 g of H3 PW12 O40 in 50 ml of water respectively.

3.1. Preparation and characterization of cesium-substituted phosphotungstic acid

2.3.1. Heteropolyacid salt with various amount of cesium cation In a beaker, heteropolyacid salts with various quantities of cesium cation were synthesized by dropwise addition of a desired stoichiometric amount of the aqueous solution of Cs2 CO3 (0.4 M) to 2 ml of H3 PW12 O40 solution (0.8 M). The spontaneously white precipitates obtained were stirred until the water evaporated, then dried in an oven at 100 ◦ C for 6 h. Six samples of heteropolyacid salts with different amount of cesium (0.5, 1, 1.5, 2, 2.5 and 3 equiv. of HPW) were synthesized (see Table A.1 Supporting information for experimental details). The supports were calcined under continuous flow of dry air (30 ml min−1 ) for 3 h at 400 ◦ C (heating rate of 3 ◦ C min−1 ). Afterwards, the samples were reduced under a continuous flow of hydrogen (30 ml min−1 ) for 3 h at 400 ◦ C (heating rate of 3 ◦ C min−1 ). The solids were recovered and characterized after each step. 2.3.2. Heteropolyacid salt with various amounts of cesium and metal (Cu and Ru) In a beaker, 2 ml of H3 PW12 O40 solution (0.8 M) was introduced. Copper nitrate (Cu(NO3 )2 ·2.5H2 O) and/or ruthenium chloride (RuCl3 ·3H2 O) was (were) added under stirring, followed by dropwise addition of desired stoichiometric amount of the aqueous solution of Cs2 CO3 (0.4 M). The mixture was then stirred until water evaporated, and dried in an oven at 100 ◦ C for 6 h. Several series of catalyst were synthesized by varying the ruthenium content (1, 2, 3 wt%) and the amount of the proton exchanged (1.5, 2, 2.5 and 3 equiv.), (see Table A.2 Supporting information for experimental details). The resulted catalysts were calcined under continuous flow of dry air (30 ml min−1 ) for 3 h at 400 ◦ C (heating rate of 2 ◦ C min−1 ). Afterwards, the samples were reduced under a continuous flow of hydrogen (30 ml min−1 ) for 3 h at 400 ◦ C. 2.4. Catalytic tests In a glove box, 10 g of the pine wood powder and 1 g of catalyst were introduced into a stainless steel batch reactor from Parr instrumentation (capacity: 300 ml, T (max.) 550 ◦ C and P (max.) ∼ 140 bar). The batch reactor was then connected to high pressure line of hydrogen. The air and the water that may be adsorbed were flashed under hydrogen and vacuum. After that,

Heterogeneous catalysts are often composed of an active species dispersed on high surface area solid support. Herein, the catalytic activity is attributed to both metal nanoparticles and the residual protons of the heteropolyanion salt. Thus, the characterization of the support as well as the active species are essential to tune the activity, selectivity and reusability of the catalyst. Heteropolyacid salt with various amount of cesium (0.5, 1, 1.5, 2, 2.5 and 3 equiv.) are prepared. Figures and Tables of Supplementary information are provided in a separate file and identified by A. The different supports are analyzed by SEM technique, to observe the shape and the dispersion of the crystals. The micrographs obtained are represented in Table A.3. We notice that in the pictures (Tables A.3, column 2) of different samples containing 1 equiv. of cesium or less, the crystals show almost the same shape regardless of size and surface which are well defined and smooth. For 1.5 equiv. of cesium and above, the crystal surfaces became porous and some small sphere are observed as well as their number increases with the rise of cesium amount. This can be attributed to the exchange of the cesium cations with the surface accessible protons, then diffuse inside the crystals and form small amorphous spherical agglomerates. SEM analyses of the same samples after thermal treatment (Table A.3, column 3) showed that the morphological structure is preserved and not affected by the treatments. ICP and EDX analyses were used to measure different element content (P, Cs, W and O) of heteropolyacid (HPW) and heteropolayacid salts (Csx PW). As shown in Table 1, P, Cs and W content (wt%) in the supports based on HPW are close to expected values. The discrepancies observed are attributed to the presence of approximately ten water molecules per HPW unit. In addition, results obtained by EDX analyses showed that unless the pure HPW, the other samples have non-homogenous surface composition. The given results in Table 1, column 4, are averaged between different results gathered from various region of the surface. A good concordance is observed with the theoretical calculated value, based on the dry samples. Thermogravimetric analyses were performed in order to measure the amount of moisture adsorbed and to get some information on their thermal stability. The results of the TGA profiles are displayed in Fig. A.1. Around 6.5% weight loss is measured at 80 ◦ C attributed to the removal of physisorbed water. Assuming that H3 PW12 O40 . 6H2 O is the molecular structure of the resulted solid at 80 ◦ C, 12 water molecules are removed during this first step.

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Table 1 ICP and EDX elemental analysis (P, Cs and W) of the HPA based supports. Samples

HPW Cs0.5 PW Cs1 PW Cs1.5 PW Cs2 PW Cs2.5 PW Cs3 PW a

Expected (Wt%)

Measured (ICP) (wt%)

Measured (EDX) (wt%)

Cs

P

W

O

Cs

P

W

Oa

Cs

P

W

O

– 2.2 4.4 6.5 8.4 10.3 12.2

1.1 1.0 1.0 1.0 0.9 0.9 0.9

76.6 74.9 73.2 71.7 70.2 68.7 67.3

22.2 21.7 21.2 20.8 20.3 19.9 19.5

– 2.2 4.2 6.3 8.1 10.1 12.0

1.0 1.0 1.0 0.9 0.9 0.9 0.9

72.0 69.7 69.0 67.7 66.4 65.1 63.4

26.1 25.6 25.1 24.6 24.1 23.6 23.1

0 1.7 4.2 6.9 9.6 12.2 13.2

1.2 1.3 1.4 1.2 1.0 1.1 1.1

78.0 76.1 74.0 72.9 70.6 66.4 64.8

20.8 20.9 20.2 19.0 18.8 20.3 20.9

The amount of the oxygen is estimated assuming that the amount of H (wt%) is negligible and P wt% + Cs wt% + W wt% + O wt% = 100 wt%.

Additional weight loss is observed at 160 ◦ C, ascribed to the dehydration of the six water molecules per keggin unit, which are in interaction with the protons of H3 PW12 O40 . Minor weight loss at 450 ◦ C resulting from the loss of acid protons and oxygen from the polyoxometalates structure, followed by the reduction of the metal centers and structure decomposition [26]. The thermal analyses of Csx PW solids presented in Fig. A.1 showed a weight loss at 180 ◦ C, due to the elimination of the water molecules which are bonded to multiple primary structures [27]. In addition, the water loss is decreasing when the cesium content increases as already observed owing to the decrease of the hydration [24]. Bulk heteropolyacid (HPW) and heteropolyacid salts with various amount of cesium (Csx PW) were examined using Fourier transform infrared spectroscopy (FTIRS) technique. Fig. A.2 shows the FTIR spectrum of dried HPW in the region between 785 and 1200 cm−1 . Peaks at 1080, 986, 892 and 840 cm−1 are observed, attributed to (P–Oa), (W = Ot), (W–Oc–W) and (W–Oe–W), respectively where a, t, c and e correspond to different oxygen position atoms in Keggin structure (internal, terminal, corner and edge-shared) as already reported [28]. Resemblances have been observed between IR spectrum of HPW and Csx PW like absorbance bands at 1080 cm−1 for P O in the central tetrahedron, 986 cm−1 for terminal W O and 840 cm−1 for O W related to the asymmetric vibration in the Keggin polyanion (Fig. A.2). The IR spectra of the supports after their treatment under oxygen and hydrogen at 400 ◦ C match with the IR spectra of the as synthesized samples and the fresh keggin heteropolyacid. These suggest that the proton exchange with cesium atoms and the thermal treatment of the samples do not affect the kegging structure. The powder XRD analyses of the solids in the range of 4–70◦ (2 scale) are provided in Fig. A.3. In general, all the prepared supports exhibited clear diffraction peaks in accordance to the literature, the diffraction pattern of HPW displayed the reflections of cubic Pn3m structure as reported [29]. After doping with Cs and at relatively low loading (Cs0.5 , Cs1 , Cs1.5 ), the intensities of the reflection characteristic of HPW structure decreased by the increasing of the cesium content (peaks in the range for 2 of 4–8). New set of peaks characteristic of Cs3 PW structure appeared and well observed for loadings higher than 1 equiv. of cesium. This observation is in agreement with the proposed model in which H3 PW12 O40 is dispersed on the particle of Cs3 PW12 O40 [30]. Csx PW doped supports after treatment under oxygen and hydrogen at 400 ◦ C for 3 h showed no modification, suggesting that the crystal structures of Csx PW remain unchanged. The specific surface area and the pore volumes are measured using the BET and BJH methods respectively. The BET isotherms are depicted in Fig. A.4 and the obtained surfaces and pores volumes are summarized in Table A.4. Up to x = 1.5, the surface area of the solids are very low, in the range of 2–8 m2 g−1 . For x higher or equal to two, the surface area increased significantly to 78 m2 g−1 for Cs2 PW and 157 m2 g−1 for Cs3 PW. These results are in agreement with the literature [31]. The porosity of the materials increases with the Cs content. Particularly for Cs2.5 PW and Cs3 PW where their

isotherms correspond to type IV according to IUPAC classification and display H3 type hysteresis loop characteristic of capillary condensation between the aggregates composing the solid. Their pores volume still very low compared to other conventional industrial solid supports it is only ca. 0.14 cm3 g−1 . Solid state 31 P NMR of the central atom in the heteropolyanion structure of the HPW and Csx PW are displayed in Fig. A.5 and showed a sharp peak at −15 ppm [32]. The spectra of the same solid when treated at 400 ◦ C under oxygen or hydrogen gave other peaks at −13 ppm and −10 ppm due to the partial or total dehydration of the polyanion crystals [31]. This phenomenon is reversible and is not observed for all the samples because of the moisture contamination during the preparation of the sample for NMR analyses. Solid state 133 Cs NMR spectra of the acidic Cs salts are presented in Fig. A.6, showing only one peak at −51 ppm flanked with spinning sidebands, as already observed [12,33]. When the samples are treated under oxygen and hydrogen at 400 ◦ C, an upfield shift of 8 and 10 ppm respectively are observed, due to the dehydration process of the polyanion structure. It is well known that 133 Cs NMR is very sensitive to water [34] as confirmed by liquid state NMR, different concentration of CsCl (0.01, 0.05 and 0.1) gave different Cs shift.

3.2. Cs heteropolaynion salt functionalized by nanoparticles Cu and Ru supported bimetallic nanoparticles over Cs2.5 PW was found to be an efficient multifunctional catalyst for one-step conversion of solid biomass into biofuel. The combination of both Cu and Ru leads to the formation of higher amount of organics (32 wt%) with low oxygen content (less than 4 wt%) [12]. Cs heteropoly acid salts are known to have high thermal stability and very strong Brønsted acidity. As demonstrated above, the exchange of between 2 and 3 equiv. of protons by Cs+ offers non-soluble material with high surface area. Accordingly, these materials seem to be usable as catalyst supports. Thus, three solids containing three different amounts of Cs (Cs2 , Cs2.5 and Cs3 ) were selected to be functionalized by bimetallic nanoparticles. 3 series of 4 samples were prepared by varying the amount of ruthenium content (1, 2, 3 and 4 wt%) and the cesium of each series was screened (2, 2.5 and 3 equiv.) (see Table A.2). The samples were investigated by BET, XRD, solid state NMR, elemental analyses, TEM and HRTEM. The textural properties of the different catalysts were examined by nitrogen adsorption–desorption isotherm measurement. The isotherm profiles of the materials are depicted in Fig. 1. The surface areas for all the catalysts are increasing with the increasing of cesium content. Addition of bimetallic nanoparticles on Csx PW lowers the specific surface area (BET). As expected, when the metal loading (ruthenium content) increased the surface area decreased for each cesium value, ca. SBET (CuRu1 @Cs2.5 PW) = 119 m2 g−1 and SBET (CuRu4 @Cs2.5 PW) = 92 m2 g−1 . This is due first to the change in the weight of the sample and to the progressive filling of the pores between the aggregates of the support.

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Fig. 1. Specific surface area evolution of the various catalysts with cesium and ruthenium content.

The XRD patterns of the bimetallic catalysts are shown in Fig. A.7. The diffractograms show only one phase close to the cubic crystalline structure of H3 PW12 O40 as discussed above and did not reveal the presence of any crystalline metallic phase even at higher metal loading (4 wt% of ruthenium). This is probably due to small size and well dispersed bimetallic nanoparticles (CuRu) on Csx PW. These results indicate also that the structure of the supports are preserved after impregnation and thermal treatments. The 31 P, 133 Cs MAS NMR spectra of the samples are displayed in Fig. 2. 31 P spectra are characterized by one main peak at −15 ppm assigned to central atom of the crystalline polyanionic species. The spectrum of CuRu4 @Cs3 PW catalyst shows an additional shoulder at −13 ppm due to the partial dehydration of the heteropolyanion as already observed for the support. 133 Cs MAS NMR spectra of Cs+ cations in the catalyst (Fig. 2) show only one peak at −44 ppm surrounded by spinning sidebands. This corresponds to the expected shift [35] and assigned by comparison to the chemical shift of the cesium found with nearly planar sixfold oxygen coordination in CsNaY zeolite. This is justified by the fact that the incorporation of the Cs+ associate the primary structures to form a secondary structure and being bounded by terminal oxygen and water molecules depending on the hydration. The upfield of the chemical shift is the result of the sample dehydration as discussed above. As for the XRD analyses, the chemical shift of P and Cs are not affected by the functionalization of the supports suggesting that the structure of the heteropolyanion is preserved. The catalysts were analyzed by transmission electron microscopy. The obtained micrographs and the dispersion of the nanoparticles are highlighted in Fig. 3. All the bimetallic catalysts show the presence of well dispersed metal nanoparticles on the support (Csx PW). The sizes as well as the dispersions decreased with the increasing of the cesium amount. In fact, the average size of the particles observed for sample CuRu2 @Cs2 PW is ca. 2.1 ± 0.5 nm, decreases to 1.8 ± 0.5 nm for the catalyst loaded on Cs2.5 PW and 1.4 ± 0.3 nm when deposited on Cs3 PW. This is attributed to the porosity and the surface area which increases with the cesium content leading to well repartition on the surface of the support. Fig. 4 displayed the TEM micrographs and nanoparticles dispersion of catalyst with different ruthenium content (1, 2, 3 and 4 wt%) supported on the Cs2.5 PW. For easier comparison, the distribution is reported in the same figure (Fig. 5). As expected, the particle size distribution increases with the Ru content from narrow repartition (1.4 ± 0.3 nm) of particles in the case of the first catalyst (CuRu1 @Cs2.5 PW) to higher

Fig. 2. 31 P and 133 Cs solid-state MAS NMR spectra of the catalysts: (a) CuRu1 @Cs2 PW, (b) CuRu2 @Cs2 PW, (c) CuRu3 @Cs2 PW, (d) CuRu4 @Cs2 PW, (e) [email protected], and (f) CuRu4 @Cs3 PW.

(1.8 ± 0.5 nm) for the second one (CuRu2 @Cs2.5 PW). The particle size of the catalyst loaded with 4 wt% of Ru (CuRu4 @Cs2.5 PW) is larger and is ca. 2.1 ± 0.5 nm as observed in the literature, the dispersion of ruthenium particles on ZnAl2 O4 spinel increases with the rise of ruthenium content [36]. In order to further reveal the detailed structure of the supported bimetallic particles, HRTEM analyses were performed. Fig. A.9 shows typical HRTEM images of the samples and the corresponding FFT estimation of the lattice parameters. Their comparison leads to the observation of metal oxides (monoclinic CuO, tetragonal RuO2 and cubic RuO4 ). However, no evidence of the presence of bimetallic alloy (CuRu) could be assessed. Electron energy loss spectroscopy (EELS) and Extended X-Ray Absorption Fine Structure (EXAFS) techniques could be used to investigate the composition of the particles. 3.2.1. Catalytic hydro-treatment of lignocellulosic biomass Previous study shows that pine wood decomposes faster under hydrogen than under argon. The decomposition can be divided in two steps: (i) transformation of cellulose and hemicelluloses into cyclic oxygenated derivates such as furan and furfural around 275 ◦ C; (ii) degradation of lignin structure to phenolic compounds from 350 ◦ C [37]. In addition, the decomposition temperature of the holocellulose of pine wood is reduced by 100 ◦ C in the presence of keggin-type heteropolyacid [33]. In the present study, the hydro-conversion of biomass into liquid biofuel in the presence of multifunctional catalyst described above (Table A.2) is investigated

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Fig. 3. TEM micrographs of the bimetallic catalyst CuRu1 loaded on various supports, Cs2 PW, Cs2.5 PW, Cs3 PW from top to down respectively.

in a batch reactor at 470 ◦ C under 40 bar of hydrogen. The tested catalysts (Table A.2) contain different amount of ruthenium supported on various supports (CuRun @Csx PW/n = 1, 2, 3, 4 and x = 2, 2.5, 3). The comparison between the amounts of organic fraction yielded for each catalyst represented by histograms (Fig. 6) indicates clearly that for the same amount of ruthenium, the catalytic hydroconversion of biomass rises with the rise of the cesium content. A minor decrease in the yields is also observed while the cesium augmented to 3 equiv. per keggin unit. This is due to the complete loss of the support acidity. Furthermore, the four different catalyst containing various amount of ruthenium (1, 2, 3 and 4 wt%) supported on Cs2.5 PW were deeply investigated.

The five isolated fractions (gas phase, organic, aqueous, coke and acetone soluble polymer) are weighed, the amount of each phase obtained in the presence of the forth aforementioned catalyst are summarized in Table 2. The amount of ruthenium and copper presented in different catalysts are measured by inductively coupled plasma (ICP) atomic emission spectroscopy. The experimental result obtained (Table 2, column 4) are very close compared to the expected ones (Table 2, column 3), the small discrepancies are due to moisture adsorbed on metal sources, the amount of the water adsorbed can vary and create some uncertainty of measurements. In the absence of catalyst (Table 2, entry 1), very low conversion of wood has been observed. 33 wt% of liquid phase is obtained (with

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Fig. 4. TEM micrographs of the bimetallic catalyst CuRu1 , CuRu2 , CuRu3 , CuRu4 from top to down respectively supported on the same support (Cs2.5 PW).

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169

Table 2 Yield in the different phases obtained during the catalytic hydro treatment of pine wood (reaction performed at 470 ◦ C under 40 bar of hydrogen, 10 g of the pine wood, 10 wt% of catalyst). Entries

Catalysts

1 2 3 4 5 6

Blank reaction Cs2.5 WP [email protected] WP CuRu2 @Cs2.5 WP CuRu3 @Cs2.5 WP CuRu4 @Cs2.5 WP

a b

Cu/Ru (wt%)a

1/1 1/2 1/3 1/4

Cu/Ru (wt%)b

Aqueous phase

Organic phase

Gas

Coke

Polymer

1.1/0.8 1.1/1.9 1.1/2.7 0.9/3.8

33 30 39 36 39 42

– 7 21 29 32 28

22 21 20 17 14 15

42 38 13 11 9 10

3 3 7 7 6 5

Theoretical expected amount of Cu and Ru in catalyst. Experimental measured amount of Cu and Ru in catalyst.

Fig. 5. The particles size distribution diagrams of the catalysts.

a low amount of organics ∼1 wt%) and residual solid of 42 wt%. In the presence of support Cs2.5 PW (Table 2, entry 2) the amount of the organic phase obtained increases to 7 wt% which is not negligible but still low. Compared to the non-catalyzed experiments, all the bimetallic catalysts increased the conversion by yielding higher amount of liquids and decreasing the amount of the solid residue. The first bimetallic catalyst composed of 1.1 wt% Cu and 0.8 wt% Ru supported on Cs2.5 PW (Table 2, entry 3) yields 60 wt% of liquids (39 wt% aqueous and 21 wt% organic) and only 14 wt% of residual solid is remained. The conversion of lignocellulosic biomass increases with the ruthenium and reaches 32 wt% of organic phase

when CuRu3 @Cs2.5 PW is used. However, further increase of the ruthenium content results in decrease of the organic phase to 28 wt% for CuRu4 @Cs2.5 PW catalyst. It is clearly seen that the yield of aqueous phase obtained with CuRu4 @Cs2.5 PW catalyst (42 wt%) is higher than the one collected for CuRu3 @Cs2.5 PW catalyst (39 wt%). The content of O, C, and H in the organic phase is a very important parameter for fuel application. Thus, their amounts in the organic fraction are provided in Table 3. The organic phase produced in the presence of the bimetallic CuRu1 @Cs2.5 PW catalyst contained 84, 11.4 and 4 wt% of C, H and O respectively (Table 3, line 1). The amount of oxygen is very low in all the obtained organic fractions, and it decreases with the rise of ruthenium content. These observations are in concordance with the evolution of the yields of organic and aqueous phases, when the HDO catalytic activity increases. This means that the oxygenated organics are converted into chemical with lower molecular weight and water thus the weight of organic decreases while the aqueous one increases. This is observed for the last two examples, the use of CuRu4 @Cs2.5 PW catalyst produced lower amount of organics with lower oxygen content (2.6 wt%) than with CuRu3 @Cs2.5 PW catalyst (3.3 wt%). Ruthenium particles favorize the formation of the organic phase and catalyze the hydrodesoxygenation reaction. Note that further increase of ruthenium content is driving the hydrodesoxygenation reaction and thereby results in additional removal of oxygen atoms in the organic phase, in the same time, decrease of the absolute yield (in weight) of the organic phase accompanied by concomitant increase of the aqueous phase (Table 2, columns 5 and 6). Consequently, the isolated organic fraction, despite lower yield, contains less oxygen and has a higher heating value. The carbon content in the aqueous

Fig. 6. The amount (wt%) of the organic phase obtained during the hydro-conversion of biomass for all the catalyst under 40 bars of H2 at 470 ◦ C.

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Table 3 C, H and O content of aqueous, organic and acetone soluble (polymer) fractions obtained after catalytic hydrotreatment of biomass over bimetallic CuRu based catalyst. Catalysts

Particle size (nm)

CuRu1 @Cs2.5 WP CuRu2 @Cs2.5 WP CuRu3 @Cs2.5 WP CuRu4 @Cs2.5 WP

0.9 1.3 2.1 2.5

a

± ± ± ±

0.2 0.3 0.5 0.7

Organic phase C/H/Oa (%)

C in aqueous phasea (%)

Polymer phase C/H/Oa (%)

84.0/11.4/4.0 84.0/11.5/3.9 84.3/11.9/3.3 84.4/12.2/2.6

2.3 2.2 2.1 1.4

– 82.6/8.4/8.25 83.6/8.9/6.9 –

The % of N between 0.4 and 0.8.

phase is measured (Table 3, column 4) and found to be ca. 2 wt% and decreases with ruthenium loading. The promoting effect of the copper might be related to the high dispersion of the NPs as already reported [12]. In fact, it has been demonstrated that ruthenium particle size and shape have a great influence on hydrogenation and hydrodesoxygenation reactions of lignocellulosic material [38]. The polymeric phase is mainly composed of carbon (83.1 wt%), and exhibited less amount of hydrogen and oxygen 9 and 8 wt% respectively (Table 3, column 5). The polymeric phase is soluble in some organic solvent such as acetone, it is originated from the coupling reaction between the furan moieties as reported by Mealares et al. [39]. The water content in organic phase is determined by the KarlFischer method and shows that the liquid organic phase contains less than 1 wt% of H2 O. The GC–MS analysis of the organic phase resulting from reaction of lignocellulosic biomass with CuRu2 @Cs2.5 PW catalyst is depicted in Fig. A.10. The different peaks in the chromatograph are successfully ascribed (Fig. A.10). 3.2.2. The influence of different experimental conditions (T, P, t and the catalyst) The effect of the experimental condition including temperature, hydrogen pressure, time and the amount of the catalyst used in the reaction are investigated. According to the previously presented results the catalyst CuRu2 @Cs2.5 PW is chosen to perform this study. Effect of temperature: The temperature has an effect on liquefaction of the solid lignocellulosic biomass [37] and on the catalytic hydrotreatment reaction [40]. The conversion of biomass to biofuel in the presence of 10 wt% of the catalyst under 40 bar of hydrogen is performed at different temperatures (350, 400, 450, 470 and 500 ◦ C). The yield evolutions of various fractions (gas, solid and liquid) are reported in Fig. 7 part a). The amount of solid residue decreased by the temperature from 29 to 9 wt% when the temperature increased from 350 ◦ C to 500 ◦ C, suggesting that the conversion of biomass increases with the temperature. The aqueous phase and the soluble polymer remains almost unchanged, while the organic phase increases from 11 to 31 wt% when the temperature reaches 470 ◦ C and then decreases for higher temperatures with concomitance increasing of the amount of gas phase. The results clearly point out the positive influence of temperature on the conversion of pine wood. The increase of the temperature impact the depolymerization of the wood and the catalyst activity. The best higher activity for the bimetallic catalyst is achieved at 470 ◦ C. Effect of the catalyst amount: The cost of the catalyst is important during the optimization of any chemical process, thus the catalyst amount necessary to perform the reaction has to be optimized. The production of biofuel from biomass loaded with 40 bar of H2 and concomitant heating at 470 ◦ C, over various amounts of supported bimetallic catalysts CuRu2 @Cs2.5 PW described above (1, 3, 5, 10 and 20 wt% of the starting biomass) is carried out. The obtained results are highlighted in Fig. 7 part b). In the presence of 1 wt% of the catalyst, the conversion of wood is considerably improved compared to the blank reaction (without catalyst). The amount of organic liquids increases from 3 wt% without catalyst to reach a plateau at 31 wt% in the presence of 10 wt% of the catalyst. In the other hand, while the quantities of acetone soluble polymer remain constant

ca. 5 wt% the solid residue (coke) decreases to reach a minimum of 9 wt% when 10 wt% of the catalyst is used. Finally the gas phase presents a minimum of 14 wt% for the same amount of the catalyst (10 wt%). As a conclusion, this series of experiments showed that the optimum amount of the catalyst is about 10 wt%. Effect of pressure: The hydrogen pressure has no effect on the decomposition of the biomass as reported [37], but hydrogen adsorption on the catalytic active phase has a key role for hydrogenation and hydrodesoxygenation activity [41]. Therefore a high capacity of hydrogen chemisorption is desirable. This propriety depends on the shape and the size of metallic particles as well as the hydrogen pressure. The biomass transformation into biofuel in the presence of 10 wt% of the catalyst at 470 ◦ C is carried out at different hydrogen pressures (20, 40, 60 bar). The yield evolutions of various fractions (gas, solid and liquid) are depicted in Fig. 7 part c). While the aqueous phases remain constant when hydrogen pressure increases, the yield of organic phase increases from 16 wt% under 20 bar of hydrogen to 31 wt% under 40 bar and remains constant for pressures higher than 40 bar. The acetone soluble phase remains unchanged with the pressure, the solid residue (coke) decreases continuously with the pressure and more gases are produced. As it is aimed to obtain more organic liquids, no need to use higher hydrogen pressures to convert more solid to gas. As result 40 bar is the optimum operating pressure. Effect of reaction time: The influence of the reaction time on the yield of biofuel is performed in the presence of 10 wt% of the catalyst at 470 ◦ C and 40 bar of hydrogen at different times (1, 3 and 5 h). The variations of different fractions (gas, solid and liquid) are displayed in Fig. 7 part d). The reaction for 1 h yields about 36 wt% of aqueous fraction and increases slightly with time due to the further desoxygenation of organics and formation of higher amount of water, reaching 38 wt% after 5 h of reaction. The organic phase profile is characterized by an optimum yield of 29.5 wt% between 2.5 h and 3.5 h. The decrease of yield with time is attributed to two main reactions: (i) hydrodesoxygenation which transforms part of the oxygenated organic compounds into non-oxygenated molecules along with the formation of water; (ii) degradation of organics through hydrogenolysis process [42]. The residue (coke) decreases with the time (20–10 wt% from 1 to 3 h; 10–7.5 wt% from 3 to 5 h) while the soluble polymer remains constant and is about 7 wt%. The solid residue obtained after 1 h of reaction is ca. 20 wt%, additionally 10 wt% of the residue are converted for other 2 h (10 wt% for 3 h of total reaction time). For further 2 h of reaction, the residue decreased with 2.5 wt% (7.5 wt% for 5 h of total reaction time), suggesting that the maximum is reached and around 7 wt% of is constituted of charcoal and ashes. The residue (coke) decreases constantly with the time while the soluble polymer remains constant and is about 7 wt%. As a conclusion for this series of experiment, the optimal experimental conditions are temperature (470 ◦ C), pressure (40 bar), time (3 h) and 10 wt% of the catalyst. The reusability of the catalyst is studied and reported previously, it is found that the catalytic activity decreased slightly due to the rise of the metallic nanoparticles distribution with the increase of the number of catalytic cycles [12].

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Fig. 7. Yield of different phases obtained during catalytic hydro treatment of lignocellulosic biomass over CuRu2 @Cs2.5 PW, under various experimental conditions (a) different temperatures, (b) different catalyst amount, (c) various hydrogen pressures and (d) different reaction times.

4. Conclusion In this work, hydrotreatment of solid biomass for biofuels over multifunctional heterogeneous catalyst was investigated. Main efforts were dedicated to the use of bimetallic copper and ruthenium nanoparticles supported on cesium heteropolyacid salt. First of all, the supports which have relatively high thermal stability and very strong Brønsted acidity (Csx PW) with various amounts of cesium were synthesized and characterized before and after thermal treatment under oxygen and hydrogen (catalytic conversion). It is found that the supports were stable for temperatures higher than 400 ◦ C and the specific surface area of the salts increased with the cesium loading (between 2 and 3 equiv.). Afterwards, three solids with different cesium content were extensively studied (Cs2 PW, Cs2.5 PW and Cs3 PW). The main trend was that the surface area increased with the rise of cesium content and simultaneously lowered the size distribution of the bimetallic nanoparticles. Finally, the support containing 2.5 equiv. of cesium was chosen as support and loaded with different amounts of ruthenium. After their characterization, the catalytic activity toward the direct conversion of biomass into biofuel was evaluated. The yield of the organic compounds produced during the hydrotreatment of wood was influenced by the amount of the ruthenium presented in the catalyst. In fact, the catalyst loaded with 2.4 wt% of ruthenium yielded about 31 wt% of organics liquids containing less than 3.5 wt% of oxygen. Furthermore, the catalytic conversion of wood was affected by the experimental conditions (temperature, pressure, time and

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