mesoporous carbon nanocatalysts onto their activity and selectivity

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Deoxygenation of oleic acid: Influence of the synthesis route of Pd/mesoporous carbon nanocatalysts onto their activity and selectivity A. Dragu a , S. Kinayyigit b,c,d , E.J. García-Suárez b,c,e , M. Florea a , E. Stepan f , S. Velea f , L. Tanase g , V. Collière b,c , K. Philippot b,c,∗ , P. Granger g,∗∗ , V.I. Parvulescu a,∗ ∗ ∗ a University of Bucharest, Faculty of Chemistry, Department of Organic Chemistry, Biochemistry and Catalysis, 4–12 Regina Elisabeta Bvd., Bucharest 030016, Romania b CNRS, LCC (Laboratoire de Chimie de Coordination du CNRS), BP 44099, 205 Route de Narbonne, F-31077 Toulouse Cedex 4, France c Université de Toulouse, UPS, INPT, LCC, F-31077 Toulouse, France d Sabanci University Nanotechnology Research and Application Center, Orhanli, Tuzla, Istanbul 34956, Turkey e Instituto Nacional del Carbón, INCAR-CSIC, Francisco Pintado Fe 26, Apartado 73, E-33080 Oviedo, Spain f Scientific Research and Technological Development in Chemical and Petrochemical Industry, Spl. Independentei 202, Bucharest 060021, Romania g Unité de Catalyse et de Chimie du Solide UMR 8181, Université Lille1 Sciences and Technologies, Cité scientifique, bâtiment C3, 59650 Villeneuve d’Ascq, France

a r t i c l e

i n f o

Article history: Received 4 September 2014 Received in revised form 3 January 2015 Accepted 5 January 2015 Available online xxx Keywords: Palladium Supported nanoparticle Mesoporous carbon Catalysis Oleic acid Deoxygenation reaction

a b s t r a c t Supported Pd nanocatalysts were prepared by deposition of Pd nanoparticles (NPs) onto spherical mesoporous carbon beads (MB) functionalized by thermal or acidic treatement. The Pd NPs were synthesized by decomposition of [Pd2 (dba)3 ] (dba: dibenzylideneacetone) under dihydrogen either directly on the carbon supports without stabilizer leading to naked Pd NPs (Pd/MB series) or in solution in the presence of a stabilizer (polymer (PVP series) or triphenylphosphine (TPP series)) to obtain stable colloidal solutions that were further used to impregnate the carbon materials to have carbon-deposited Pd NPs. The NPs deposited on carbon displayed a Pd loading from 0.5 to 14.8 wt.% and were characterized by different techniques (nitrogen physisorption at 77 K, H2 -chemissorption and TPD, XRD, XPS and HRTEM). Their catalytic performance in deoxygenation of oleic acid was evaluated in batch and flow reaction conditions. Flow conditions led to superior results compared to batch. No aromatic compounds were detected as side products, but in the case of the Pd/MB series, octadecanol and octadecane were significantly formed suggesting the involvement of a deoxygenation mechanism in which the hydrocarbons were produced via both decarbonylation/decarboxylation and dehydration steps. Further experiments carried out in H2 /N2 mixture or in pure N2 highlighted the key role of hydrogen. For a N2 /H2 of 2.5:1 the dehydration route was crossing out and even no traces of octadecanol nor octadecane were detected. Then, complete removal of H2 produced heptadecene in a high excess compared to heptadecane (almost 7–1) thus suggesting the decarbonylation/decarboxylation steps as the main route. ICP-OES measurements indicated no leaching of palladium and simple washing of catalysts with mesitylene allowed recycling without any change in conversion or product distribution. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The production of transportation fuels from biomass has gained a growing attention due to diminishing fossil fuel reserves, rising

∗ Corresponding author at: CNRS, LCC (Laboratoire de Chimie de Coordination du CNRS), BP 44099, 205 Route de Narbonne, F-31077 Toulouse Cedex 4, France. ∗∗ Corresponding author. ∗ ∗ ∗Corresponding author. Tel.: +40 214100241; fax: +40 214100241. E-mail address: [email protected] (V.I. Parvulescu).

petroleum prices and increasing concern about global warming. In recent years, renewable hydrocarbons that are completely fungible with fossil fuels have been suggested to be efficiently produced by catalytic deoxygenation of fatty acids and their derivatives via decarboxylation/decarbonylation ([1,2], and references therein). Several triglycerides (tall oil fatty acids) and saturated/unsaturated fatty acids and their corresponding esters were used as feedstocks [3]. Their impact together with the influence of the reaction conditions and the catalyst composition on the nature of the reaction pathways of the deoxygenation of vegetable oils and their derivatives were recently reviewed [4].

http://dx.doi.org/10.1016/j.apcata.2015.01.008 0926-860X/© 2015 Elsevier B.V. All rights reserved.

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Earlier in 1930s, Bertram [5] successfully decarboxylated stearic acid to heptadecane by a homogeneous catalytic reaction over selenium. Heterogeneous catalytic decarboxylation of aliphatic and aromatic carboxylic acids in the gas phase was then reported in 1982, when octanoic and benzoic acids were decarboxylated over Pd/SiO2 and Ni/Al2 O3 with very high alkane yields over Pd catalysts [6]. However, these authors did not observe decarboxylation activity under N2 , despite the absence of H2 in the decarboxylation reaction stoichiometry. Recently, new catalysts were proposed for deoxygenation of fatty acids. From a wide variety of metals (Ni, Ni/Mo, Ru, Pd, Pd/Pt, Pt, Ir, Os and Rh) supported on carbon and metal oxides [7] the screening results revealed that Pd on carbon was the most effective catalyst, rendering over 90% selectivity towards deoxygenation products (n-heptadecane, 1-heptadecene and other C17 products) [8]. To avoid internal diffusion limitation and to afford the production of branched hydrocarbons, Pd was deposited on larger porous supports such as SBA-15 [9], ultra-porous silica mesocellularfoam [10], SAPO-31 [11], silica [12] and mesoporous Si–C [13]. The deoxygenation of fatty acids was also carried out utilizing a classical hydrotreating catalyst with the risk of possible poisoning effects due to strong adsorption of reaction products and catalysts deactivation induced by water [14–16]. Although less active, nonprecious metal catalysts comprising supports other than carbon such as Ni on oxidic supports and hydrotalcite materials were also used for the conversion of lipids into fuel-like hydrocarbons via deoxygenation [17–19]. These studies also revealed the role of the support on the catalyst activity; for instance the rate of deoxygenation of palmitic acid was higher on Ni/ZrO2 than that on Ni/SiO2 or Ni/Al2 O3 , but slower than that on Ni/H-zeolite [20]. In addition to Ni [21], zeolites were proven to be efficient supports for some noble metals (Pt–Re/HZSM-5) for the hydrodeoxygenation of fatty acids to alkanes at 523–573 [22]. Noble metals supported on heteropoly acid materials were also reported for the same scope [23]. As mentioned above the role of the support in this reaction is crucial. The comparison of 1 wt.% Pt/SiO2 with 1 wt.% Pt/C catalysts was in favour of the activated carbon and the increase of the metal loading was accompanied by an increase of the conversion [24]. Pd/C is more active than Pd/Al2 O3 and Pd/SiO2 for fatty acid deoxygenation [25]. It was also suggested that the dispersion of palladium on the support is extremely important and for an optimum dispersion a catalyst with 1 wt.% Pd may lead to equivalent results with those of catalysts containing 5 wt.% Pd [8,3]. This may suggest that the metal support interface also plays an important role. The polarity of the carbon support is a key-parameter in the catalytic behaviour of the Pd catalysts. Oxygen containing groups of the carbon support influence reactions of amphiphilic reactants as fatty acids (they contain a polar carboxylic group and a nonpolar hydrocarbon tail) due to a preferential adsorption as it has been demonstrated for Pd/CNF (CNF; carbon nanofibers) in deoxygenation of stearic acid [26]. One challenge in the catalytic deoxygenation of fatty acids is the catalyst deactivation [27]. This was associated with the poisoning of the active sites aromatic compounds formed during the reaction and the coking of the metal surface. The solvent was found to participate in this process. But the performances of the catalysts were reported to be influenced as well by the kinetic regime in which they were investigated. Semi-batch experiments with oleic acid under H2 atmosphere indicated that the deoxygenation occurs towards n-heptadecane. On the other hand, oleic acid deoxygenation occurs primarily via decarbonylation to diunsaturated heptadecene isomers in an inert He atmosphere [28]. In both cases, cracking to shorter hydrocarbons, aromatization to C17 aromatics, dehydrogenation, isomerization and polymerization were secondary reactions. By comparison, no clear result was obtained in a plug flow reactor. Lestari et al. reported that in neat

oleic and stearic acid in a trickle bed reactor the catalyst gave stable performances with time-on-stream also producing n-heptadecane as the main product [29]. The selectivities were higher than 70%, whereas the formation of hydrocarbons, mainly olefins and aromatics, was below 10%. However, working under continuous condition Arend et al. observed a strong catalyst deactivation [30]. The performances of the catalysts are also correlated to the reaction route. Murzin et al. [31] and Lamb et al. [25] suggested that the deoxygenation of C10–C18 fatty acids follow the decarboxylation route, whereas Boda et al. [32] proposed the decarbonylation route as more probable. A microkinetic model derived from DFT calculations and transition state theory showed that it was likely the decarbonylation route for Pd as in which the first step is the dehydroxylation of the fatty acid [33,34]. In fact, recent studies indicated that the reaction route is controlled either by the nature of the metal or the support. It was shown that Pd/C favours the direct decarboxylation ( CO2 ), while Pt/C and Raney Ni catalyze the direct decarbonylation pathway ( CO) [20]. Lercher and co-workers also indicated that each type of support favours a different primary reaction route [20]. A neutral support like carbon will orientate the reaction towards the direct decarbonylation/decarboxylation route whereas the primary route will change to one of sequential hydrogenation–dehydration with a solid Bronsted acid support. Finally, the literature mentions the influence of the reaction atmosphere. Changing the gas from H2 to an inert one altered the individual reaction rates and the specific pathways. Thus, experiments conducted under H2 atmosphere suppressed the dehydrogenation step [35]. Low partial pressure of H2 also enhanced the catalyst activity; the experiments performed under H2 –Ar led to higher TOF values compared to experiments carried out under either pure H2 or in the absence of H2 at all [35,36]. Other studies indicated that, contrarily, when H2 was diluted in N2 , the deoxygenation rate decreased [30]. In addition to different inert gases, various supercritical fluids containing CO2 (scCO2 ), propane (scC3 H8 ) and n-hexane (scC6 H14 ), were introduced with H2 and a fatty oil into a fixed-bed reactor with the same scope [37]. From the process-engineering perspective, however, there are fundamental limitations that hinder the effective conversion of triglycerides at low reaction temperatures using small amounts of H2 . Herein, we report a comparative investigation of the catalytic activity of three series of catalysts made of carbon-supported Pd NPs. These catalysts were prepared by decomposition of Pd2 (dba)3 complex under dihydrogen either directly on the carbon supports without stabilizer leading to naked carbon-supported Pd NPs (Pd/MB series) or in solution in the presence of a polymer (PVP) or a ligand (triphenylphosphine; TPP) as stabilizer to obtain stable colloidal solutions that were further used to impregnate the carbon materials to have deposited carbon-PdNPs (respectively Pd/PVP/H2 and Pd/TPP/H2 series). In a comparative study, the so-obtained nanocatalysts were investigated in the deoxygenation reaction of oleic acid, both in batch and continuous-flow conditions as well as in different reaction atmospheres.

2. Experimental 2.1. Catalysts preparation 2.1.1. Materials All reactions were carried out using standard Schlenk tube or Fischer–Porter bottle techniques under inert and dry atmosphere. All the chemicals were of analytical purity and were used without any additional purification. THF (Sigma–Aldrich, anhydrous, 99.9%) was freshly distilled over sodium/benzophenone under inert atmosphere prior to use and degassed by three freeze-pump cycles. Toluene and pentane were purified under

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nitrogen atmosphere using solvent purification equipment (Braun) and degassed by three freeze-pump cycles prior to use. Tris(dibenzylidene-acetone)dipalladium(0) complex [Pd2 (dba)3 ] and triphenylphosphine (PPh3 ; TPP) were purchased from Strem Chemicals and Sigma–Aldrich, respectively. Polyvinylpyrrolidone (PVP) was purchased from Sigma–Aldrich and dried over P2 O5 at 353 K under vacuum for three days prior to use. Argon and H2 gases were purchased from Air Liquid (99.999% purity). Spherical mesoporous carbon beads (MB; commercially available phenolic resin-based activated carbon with a bi-modal pore texture) with the particle size range of 0.5–1.0 mm were used as the starting carbon support [38]. MB was then treated as previously reported either by heating at 1773 K (MB-1500) or under hydrogen peroxide conditions (MB-H2 O2 ) to have three different supports [39]. 2.1.2. Preparation of carbon-supported Pd nanocatalysts 2.1.2.1. Carbon supported naked Pd NPs. A THF suspension of the corresponding carbon support (MB-1500 or MB-H2 O2 ; 400 mg in 22 mL of THF) was mixed with a THF solution (18 mL) of [Pd2 (dba)3 ] (274.7 mg). The reaction mixture was then stirred for 3 h before evaporation of the solvent under vacuum. The solid was then dried under vacuum overnight. The reaction bottle was pressurized with 3 atm H2 and placed under stirring for 24 h at room temperature (r.t.). After this period of time, H2 was evacuated under vacuum. The solid residue containing carbon-supported Pd NPs was washed with freeze-thawed pentane (3 mL × 20 mL) and then dried overnight under vacuum. Yields: Pd/MB-1500: 365 mg; Pd content (ICP): 14.2%. Pd/MB-H2 O2 : 368.6 mg; Pd content (ICP): 14.4%. Pd/MB: 520 mg; Pd content (ICP): 14.8%. 2.1.2.2. Carbon supported Pd/PVP/H2 NPs:. 1.332 g of PVP (40,000 MW) were dissolved in 40 mL of THF. This colourless solution was transferred into a Fischer–Porter bottle containing a solution of [Pd2 (dba)3 ] (549.3 mg, 599.9 ␮mol) in 41 mL of THF. The reaction bottle was pressurized with 3 atm H2 and placed under stirring at r.t. for 24 h. After this period of time, H2 was evacuated under vacuum and the solvent was evaporated to 20 mL. The solid residue (Pd/PVP/H2 NPs) was washed with freeze-thawed pentane (5 mL × 20 mL) and then dried overnight in the vacuum line. Total yield: 1.51 g. 500 mg of previously prepared Pd/PVP/H2 nanoparticles dissolved in 12 mL of freeze-thawed THF were added to 200 mg of the corresponding carbon support (MB, MB-1500 or MB-H2 O2 ) in a Fischer–Porter bottle. The reaction mixture was stirred during 3 h at r.t. in order to have a complete impregnation of the support with the colloidal solution of Pd NPs. The solvent was subsequently evaporated to dryness and the solid was dried under vacuum overnight. The so-obtained nanocatalysts were denoted as Pd/PVP/H2 (MB), Pd/PVP/H2 (MB-1500), Pd/PVP/H2 (MB-H2 O2 ), related to the stabilizing agent and the support used for their preparation. Yields: Pd/PVP/H2 (MB): 620 mg; Pd content (ICP): 3.6%. Pd/PVP/H2 (MB-1500): 580 mg; Pd content (ICP): 3.6%. Pd/PVP/H2 (MB-H2 O2 ): 600 mg; Pd content (ICP): 3.7%. 2.1.2.3. Carbon supported Pd/TPP/H2 NPs:. 49.6 g of triphenylphosphine (TPP) were dissolved in 10 mL of THF and the solution was transferred into a Fischer-Porter bottle containing a solution of [Pd2 (dba)3 ] (173.1 mg, 189.1 ␮mol) in THF (102.5 mL) at 223 K. The reaction bottle was pressurized with 3 bar of H2 and placed under stirring at r.t. for 24 h. After this period of time, H2 was evacuated under vacuum and the solvent was evaporated to 10 mL. The remaining solution was washed with freeze-thawed pentane

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(5 mL × 30 mL) and the obtained solid was dried overnight in the vacuum line. 22.3 mg of previously prepared Pd/TPP/H2 NPs were dissolved in 36 mL of freeze-thawed THF and the obtained solution was divided into three equivalent volumes. A volume of colloidal solution was added to 200 mg of the corresponding carbon support (MB, MB-1500 or MB-H2 O2 ) in a Fischer–Porter bottle. The reaction mixture was stirred for 3 h at r.t. in order to get a complete impregnation of the support with the colloidal solution of Pd NPs. The solvent was subsequently evaporated to dryness and the solid was dried under vacuum overnight. The so-obtained nanocatalysts were denoted as Pd/TPP/H2 (MB), Pd/TPP/H2 (MB-1500), Pd/TPP/H2 (MBH2 O2 ), related to the stabilizing agent and the support used for their preparation. Yields: Pd/TPP/H2 (MB): 147.4 mg; Pd content (ICP): 0.52%. Pd/TPP/H2 (MB-1500): 263 mg; Pd content (ICP): 0.47%. Pd/TPP/H2 (MB-H2 O2 ): 165.8 mg; Pd content (ICP): 0.71%. 2.2. Characterization of catalysts TG-DTA experiments were performed on a Shimadzu device, using a Pt crucible. The heating rate was 10 K/min, starting from r.t. to 1073 K and under a flow of nitrogen of 10 mL/min. The N2 adsorption–desorption isotherms were recorded at 77 K using a Micromeritics ASAP2020 automated instrument. Prior to analysis, the catalysts pre-treated in H2 at 600 K were firstly exposed to argon and then degassed for 15 h at 423 K and 1.3 × 10−9 atm. Surface areas were estimated according to the BET model, and pore size dimensions were calculated using the BJH method. Powder X-ray diffraction patterns were recorded with a Shimadzu XRD-7000 – diffractometer using CuK␣ radiation. Patterns were collected in steps of 0.02◦ (2) over the angular ranges 1◦ –10◦ (2) or 10◦ –80◦ (2) for 25 s per step. H2 -pulse chemisorption measurements were performed by an AutoChemII 2920 station from Micromeritics. The samples (∼20 mg), placed in a U-shaped quartz reactor with an inner diameter of 0.5 cm, were preheated under He (Purity 5.0, from Linde) at 393 K for 1 h. Then the sample was cooled to r.t. under a helium flow of 50 mL/min and then exposed to pulses of H2 at r.t. until surface saturation. The number of adsorbed molecules was determined from a calibration curve of H2 . The diameter of the particles, d, and the metal surface area were calculated assuming a Pd:H stoichiometry of 1:1 according to the formula below [40–42]: d(Å ) =

8.85 (H/Pd)

D (%) =

1.1 d

S (m2 /g) = 3.9p

(1) (2)

H Pd

(3)

where p is the percent in the metal. Subsequently, H2 TPD experiments were carried out with the same instrument once saturation was reached at a heating rate of 5 K/min up to 1073 K. The amount of H2 evolved in the gas phase was quantified by using a calibration curve. Transmission electron microscopy (TEM) analyses were performed at the Service Commun de Microscopie Electronique de l’Université Paul Sabatier (TEMSCAN). High resolution transmission electron microscopy (HRTEM) and EDX studies were performed with JEOL JEM 2010 Electron Microscope working at a 200 kV with ˚ FFT analyses were done with Gatan a resolution point of 2.35 A. Digital Micrograph Version 1.80.70. HRTEM images were obtained on Pd/PVP/H2 and Pd/TPP/H2 NPs samples prepared by dropping the colloidal solution on holey carbon covered copper grids. The

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samples for the supported NPs were prepared using ultramicrotomy technique. Thin slices were obtained by cutting down the middle of nanocatalysts. They were deposited on carbon copper grids for the analysis. Mean sizes of nanoparticles with or without the supports were calculated by manually counting at least 200 nanoparticles on enlarged TEM micrographs. XPS spectra for both fresh and used catalysts were recorded at r.t. using a SSX-100 spectrometer, Model 206 from Surface Science Instrument. The pressure in the analysis chamber during the analysis was 1.33 mPa. Monochromatized Al-K␣ radiation (h = 1486.6 eV) generated by bombarding the Al anode with an electron gun operated with a beam current of 12 mA and acceleration voltage of 10 kV was used. The spectrometer energy scale was calibrated using the Au 4f7/2 peak centred at 83.98 eV. Charge correction was made with the C1s photopeak of adventitious carbon (C–C or C–H bonds) located at 284.8 eV. The atomic surface compositions were calculated using the sensitivity factors provided with the apparatus software, applied to the surface below the corresponding fitted XPS signals. An estimated error of ±0.1 eV on the Binding Energy (B.E.) values was assumed for all measurements. Elemental analyses (ICP) for the Pd contents of the nanocatalysts were performed at the Service Central d’Analyse du CNRS in Vernaison (France). 2.3. Catalytic reactions The batch catalytic experiments were carried out under stirring conditions in a 16 mL autoclave from HEL. The autoclave was charged in a nitrogen-filled dry box with the catalyst (30 mg), and then flushed several times with H2 . The catalyst was reduced with H2 at 600 K for 30 min. under a pressure of 10 atm. Upon the completion of reduction, the autoclave was cooled to r.t. and a solution

of oleic acid (282 mg, 0.1 mmol) in mesitylene as a solvent (5 mL) was introduced under N2 atmosphere. Thereafter, the autoclave was again flushed with hydrogen and the pressure was adjusted to 20 atm prior to reaction. The reaction mixture was heated at 573 K for 9 h, then cooled to r.t., filtered and the solids were washed with mesitylene (5 mL). The continuous catalytic experiments were carried out in a fixedbed tubular reactor from PID&Eng Tech (length of 300 mm and i.d. of 9 mm, Hasteloy X tube), equipped with a thermo well in the centre of the catalyst bed. Reactions were conducted at 573 K, at 4 atm and using a H2 flow of 10 mL/min. The catalyst (30 mg) was placed between two layers of quartz wool and was first activated for 0.5 h under flowing H2 at 573 K. The experiments were typically carried out in a down-flow system with the liquid flow of 0.1 mL/min by using a high performance liquid chromatography pump (HPLC). The concentration of the oleic acid in mesitylene was 0.2 M. The reaction started when the first drop of product mixture was collected from the reactor. 2.4. Product analysis The liquid products were analyzed by a gas chromatograph (Shimadzu GC-2014) equipped with a flame ionization detector and a capillary column DB-5 (length, 60 m; internal diameter, 0.32 mm; film thickness, 0.5 um). The following temperature programme was used for analysis: 313 K hold for 2 min, 523 K (5 K/min) hold for 10 min. Samples were silylated with N,O-bis(trimethyl)trifluoroacetamide (BSTFA). Generally, BSTFA (30 ␮L) and pyridine (30 ␮L) were added to the sample (100 ␮L). After addition of silylation agent, the samples were kept in an oven at 333 K for 30 min. The identification of the reaction products was confirmed by gas chromatography-mass spectrometry, using a Trace GC

Fig. 1. TEM image and the corresponding distribution of (a) Pd/PVP/H2 NPs and (b) Pd/TPP/H2 NPs.

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Fig. 2. (a) HRTEM image and (b) inverted micrograph image of Pd/PVP/H2 NPs. (c) HRTEM image and (d) inverted micrograph image of Pd/TPP/H2 NPs.

2000 system with MS detector (Thermo Electron Scientific 65 Corporation, USA) incorporating a TR-WAX capillary column. 3. Results and discussions Pd/PVP/H2 NPs and Pd/TPP/H2 NPs were obtained from the decomposition of Pd2 (dba)3 complex in THF under H2 atmosphere in the presence of a polymer (polyvinylpirrolidone, PVP) and a monophosphine ligand (triphenylphosphine; TPP), which provides sterical and chemical stabilization, respectively. 2.96 ± 0.20 nm PdNPs were obtained in PVP (Fig. 1(a)). TPP ligand led to 1.46 ± 0.20 nm (Fig. 1(b)). Colloidal solutions of these PdNPs were used to impregnate mesoporous carbon materials (MB, MB-1500 and MB-H2 O2 ) as supports. Such carbon materials were chosen for their high chemical stability, good mechanical properties and functionalized surfaces with optimum pore size distributions. No change in size was observed after the impregnation step on the mesoporous carbon supports. High-resolution transmission electron microscopy (HRTEM) analyses of Pd NPs are presented in Fig. 2. Fast Fourier transformation (FFT) analysis on individual nanoparticles of Pd/PVP/H2 and Pd/TPP/H2 samples reveal the fcc structure in addition to

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some amorphous formations (Fig. 2(b) and (d)). EDX analysis on Pd/TPP/H2 NPs sample displays the presence of phosphorous atoms due to the ligands on the NP surface (ESI, Fig. 2). Thermal analysis under N2 indicated that the heating of the supported Pd NPs catalysts, even at low temperatures, is accompanied by a weight loss that differs as a function of the catalysts composition (Fig. 3). While carbon supported naked Pd NPs lose only the derivatives coming from the Pd2 (dba)3 complex used as metal source, the catalysts containing TPP or PVP lead a larger weight loss due to the organic part decomposition. Hence, since the investigated reaction requires higher temperatures, the catalysts were treated in hydrogen at 600 K, to avoid any further decomposition of the supported NPs under reaction conditions. Textural characterization of these catalysts led to the results compiled in the Table 1. Starting MB, oxidized MB-H2 O2 and high temperature treated MB-1500 carbon materials showed relatively high surface areas [39]. The loss of BET surface after the thermal treatment is due to a graphitization process. This explains the enhancement of the specific surface area after the H2 O2 treatment due to functionalization. The removal of the organic stabilizer from the Pd metal surface by heating in hydrogen has no significant influence on the surface area. Thus, the Pd/TPP series shows surface areas pretty close to those of the parent MB supports. The mesopore diameter decreases as an effect of the metal deposition. MB-supported Pd catalysts prepared directly from [Pd2 (dba)3 ], where the loading was over 14 wt.% Pd, showed much smaller surface areas due to the very high metal content. However, except Pd/MB-H2 O2 where the blockage of the pores was very high, BET surfaces values remain in the range 130–270 m2 /g. An important decrease is also observed for the surface areas of all Pd catalysts stabilized in PVP. This decrease is not associated to the metal loading (<0.7 wt.% Pd) but likely to a polymer blocking effect. Thus, while in Pd/MB catalysts the decrease of the mesopore pore size is important due to the penetration of Pd in the mesopores, for Pd/PVP type supported catalysts, part of the pores were completely blocked by the polymer (leading to smaller surface areas). The other catalysts display diameters close to the initial pores. Although they were mainly mesoporous, all catalysts present pores with diameters smaller than 4 nm. The pore distribution is bimodal (Table 1). Table 2 reports the H2 -chemisorption and H2 -TPD results. H2 chemisorption represents a common technique for the evaluation of the particle size and dispersion of supported noble metal particles, and is largely reported for the characterization of supported palladium [43–45]. Most of these studies confirms a good concordance between the H2 -chemisorption and TEM results. H2 -chemisorption and H2 -TPD lead to very similar up-take results, indicating a reproducible chemisorption process. These

Fig. 3. TG and DTA analysis of Pd/MB and Pd/PVP/H2 (MB) catalysts exposed to nitrogen.

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6 Table 1 Textural characterization of the investigated catalysts. Catalyst

Pd loading (wt.%)

BET surface area (m2 /g)

MB MB-1500 MB-H2 O2 Pd/MB Pd/MB-1500 Pd/MB-H2 O2 Pd/TPP/H2 (MB) Pd/TPP/H2 (MB-1500) Pd/TPP/H2 (MB-H2 O2 ) Pd/PVP/H2 (MB) Pd/PVP/H2 (MB-1500) Pd/PVP/H2 (MB-H2 O2 )

– – – 14.2 14.4 14.8 3.6 3.6 3.7 0.52 0.47 0.71

1294 909 1211 274 129 43 945 589 1083 128 128 101

Langmuir surface area (m2 /g)

374 179 61 1257 807 1445 175 178 140

measurements demonstrate that the deposition of naked Pd NPs onto the mesoporous carbons followed by the thermal treatment leads to an increase of the particle size compared to the NPs as-prepared and then impregnated onto the MB supports. This increase depends on both the pretreatment conditions of the support prior to NP impregnation and the preparation/stabilization conditions of the Pd NPs. Thus, both heating of the MB carbon material at 1773 K and the treatment with hydrogen peroxide (30 wt.% in water) at 323 K lead to an enlargement of the supported Pd NPs compared to the parent MB support. On this basis, we may appreciate that the support functionalization would lead to a greater interaction of Pd particles with the support. Pd would wet the support more significantly with a greater interfacial particle perimeter compared to NPs. This behaviour is more evident for MB calcined at 1773 K. The incorporation of PVP or TPP-stabilized Pd NPs in PVP or TPP generates a better preservation of the initial particle size and also a better dispersion (Table 2). With the stabilized Pd NPs, the effect of the MB support pre-treatment is insignificant. XRD patterns confirm the presence of the reduced palladium (Fig. 4). The XRD particle sizes calculated with the Sherrer formula are included in Table 2. The differences between these values and those calculated from the chemisorption H2 up-take are dependent on the preparation and support used. These differences diminish from the Pd/MB series to Pd/TPP/MB and Pd/PVP/MB series. An explanation of this behaviour could be supported by the effect of hydrogen spillover [46]. According to this effect, the particle size measurements from H2 -chemisorption can be affected by the dissociative chemisorption of H2 on metal sites, then further migration onto the support surface, leading to an additional H2 consumption, and thus to an apparent higher Pd dispersion. The presence of the stabilizer (TPP, and even more of PVP) limits the direct interaction of Pd NPs with the support, and as a consequence reduces the

t-Plot surface area (m2 /g)

Pore volume (cm3 /g)

Pore size diameter (nm)

160 113 40 225 378 273 68 113 66

1.14 1.10 0.99 0.52 0.64 0.24 0.71 0.87 0.85 0.31 0.64 0.26

<4.0 and 24.1 <4.0 and 27.2 <4.0 and 24.1 <4.0 and 15.2 <4.0 and 17.6 <4.0 and 19.4 <4.0 and 23.1 <4.0 and 17.7 <4.0 and 21.8 <4.0 and 18.1 <4.0 and 22.7 <4.0 and 21.3

hydrogen spillover. As a result, the values measured for the Pd/PVP/MB series using the two methods were almost identical. The overestimation of the H2 -chemisorption can also result from the formation of the Pd hydride that decomposes around 343 K. Ambient temperature H2 absorption is known to generate ␤phase Pd hydride. H2 -TPD profiles make this behaviour very clear, especially for TPP and PVP stabilized Pd NPs (Fig. 5). They also corroborate very well with the chemisorption conclusions. Thus, for the Pd/MB series, the desorption of chemisorbed H2 merely occurs at higher temperatures which is typical for a chemisorption process. Higher desorption temperatures may denote a stronger interaction of the Pd metal with the support. ␤-phase Pd hydride was also formed in the case of supported naked Pd/MB series but to a less extent. Based on these results, it appears that tuning the support treatment, the nature of the stabilizing agent and the concentration of the supported Pd metal, it was possible to produce catalysts with average particle size between 2.5 and 10 nm and dispersions between 3 and 35%. Fig. 6 shows the characteristic XPS spectra for the Pd 3d core level recorded on the investigated Pd NPs/MB catalysts. The binding energies of the Pd 3d5/2 photopeak, after deconvolution, are compiled in Table 3. The spectra of the NPs stabilized by PVP shows only one contribution that, according to literature, corresponds to Pd0 [47]. Differently, the spectra of Pd NPs deposited directly from [Pd2 (dba)3 ] or from TPP-stabilized NPs were deconvoluted with two components separated by BE of 1.6–2.3 eV. The first component located at 335.6–335.8 eV corresponds to Pd0 component while the second one at 337.7–337.9 eV to oxidic Pd species in the non-decomposed [Pd2 (dba)3 ] complex [48]. Finally, the Pd/MB series show components with binding energies assigned to Pd0 species, but also to Pd2+ (337.0–337.6 eV) generated from the decomposition of the complex and stabilized by the support [49].

Table 2 H2 up-take, desorbed H2 from H2 -TPD, desorption temperature, metal surface area, dispersion, particle size for the investigated supported catalysts. Catalyst

Pd/MB Pd/MB-1500 Pd/MB-H2 O2 Pd/TPP/H2 (MB) Pd/TPP/H2 (MB-1500) Pd/TPP/H2 (MB-H2 O2 ) Pd/PVP/H2 (MB) Pd/PVP/H2 (MB-1500) Pd/PVP/H2 (MB-H2 O2 )

H2 up-take from chemisorption (mmol H2 /g)

0.027 0.016 0.017 0.017 0.005 0.009 0.002 0.002 0.004

Desorbed H2 from H2 -TPD (mmoles H2 /g)

0.030 0.013 0.016 0.020 0.003 0.005 0.002 0.003 0.003

Desorption temperature (K)

327, 380, 462 375, 450 400–500 320–340, 370–395 330–350, 360–380 320–350 320–360, 390–440 320–350, 355–380 320–370, 375–400

Metal surface area (m2 gPd−1 )

23.1 13.3 14.0 14.0 4.1 7.5 5.8 5.1 5.1

Dispersion (%)

5.0 3.0 3.1 12.4 5.7 3.6 35.5 34.4 22.9

Particle size

Chemisorption (nm)

XRD Pd(1 1 1) (nm)

22.1 36.8 35.4 8.9 30.5 17.0 3.1 3.2 4.8

5.1 10.1 5.3 6.4 <2.0 3.3 2.9 3.0 2.5

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TCD signal (a.u.)

Pd/MB Pd/MB-H2O2 Pd/MB-1500

300

350

400

450

500

Temperature (K)

TCD signal (a.u.)

Pd/TPP/H2(MB) Pd/TPP/H2(MB-H2O2) Pd/TPP/H2(MB-1500)

300

320

340

360

380

400

Temperature (K)

TCD signal (a.u.)

Pd/PVP/H2(MB) Pd/PVP/H2(MB-H2O2) Pd/PVP/H2(MB-1500)

300

320

340

360

380

400

Temperature (K) Fig. 5. Representative H2 -TPD profiles of the supported Pd NPs.

octadecane are produced in large extents. However, the extent of the reaction depends, firstly, on the loading of Pd metal and then on the dispersion of the catalysts and reduction of palladium, both influenced by the way the support has been treated before the NP deposition and by the preparation/stabilization methodology of Pd NPs. To explain the formation of octadecanol/octadecane, Lercher et al. proposed an intermediate esterification step of the resulted

Fig. 4. XRD patterns of the H2 thermally treated supported Pd NPs catalysts (AC active carbon lines).

Table 3 Binding energies of the Pd3d5/2 level for the investigated catalysts. Catalyst

The formation of carbon supported Pdn+ species has already been reported in the literature when working with Pd NPs [50,51]. Fig. 7 and Table S1 present the catalytic results in the batch deoxygenation reaction of oleic acid. After 6 h, the conversion of oleic acid is complete on all the investigated catalysts. For the naked Pd NPS /MB series, the reaction was more advanced leading to deoxygenated hydrocarbons and also to cracking compounds (Fig. 7a). Differently to other reports, no aromatic compounds are detected in the reaction products while octadecanol and

Pd/MB Pd/MB1500 Pd/MBH2 O2 Pd/TPP/MB Pd/TPP/H2 (MB-1500) Pd/TPP/H2 (MB-H2 O2 ) Pd/PVP/H2 (MB) Pd/PVP/H2 (MB-1500) Pd/PVP/H2 (MB-H2 O2 )

Binding energy (eV) Pd3d5/2

Pd3d5/2

335.7 335.4 335.8 335.8 335.6 335.8 335.0 334.9 334.9

337.6 337.0 337.6 337.7 337.9 337.7 – – –

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Fig. 6. XPS spectra over the Pd 3d region for the investigated Pd NPs/MB catalysts.

a

100

100

100

100

b

98 80

80

96

96

Conversion (%)

40

octadecanol

Conversion stearic acid octadecane heptdecane

94 92

60

octadecanol

40

90 88

20

20 86

0

94 0

1

2

3

4

5

6

7

8

9

84

10

0 0

1

2

3

4

Time (h)

c

5

6

7

8

9

Time (h) 100

100

98 96

80

92

60

Conversion stearic acid octadecane heptdecane

90 88

40

octadecanol

86

Selectivity (%)

Conversion (%)

94

20

84 82

0

80 0

1

2

3

4

5

6

7

8

9

10

Time (h) Fig. 7. Time evolution of the conversion and selectivity for batch experiments: (a) Pd/MB; (b) Pd(TPP)/H2 /MB; (c) Pd(PVP)/H2 /MB.

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Selectivity (%)

60

Conversion stearic acid octadecane heptdecane

Selectivity (%)

Conversion (%)

98

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a

100

100.0

90 Conversion stearic acid

99.5

80

Conversion (%)

70 60 99.0 50 40 98.5

30

Selectivity (%)

20 98.0

10 0 0

1

2

Time (h)

b

100

100 90

98

80

Conversion (%)

70 96

60 Conversion stearic acid

94

50 40 30

92

Selectivity (%)

20 10

90

0 0

1

2

3

Time (h)

c

100

100 90

98

80

Conversion (%)

70 96

60 Conversion stearic acid heptadecane

94

50 40 30

92

Selectivity (%)

alcohol [20]. However, we did not identify the presence of the esters and therefore the simple dehydration is more plausible like that proposed by the same authors using zeolites as support. Actually, on the basis of previous studies carried out on the same support it appears they exhibit surface oxygen functional groups that can allow the sequential dehydration steps represented in Scheme 1 [39]. Calcination of MB at 1773 K may generate an increased content in such species, as demonstrated by the higher content in octadecanol and octadecane, but also cracking is observed on Pd/MB-1500. Changing the preparation procedure concomitantly with a decrease of the Pd loading leads to lower activity in the stearic acid deoxygenation for the Pd/TPP series (Fig. 7b and Table SI1), while for Pd/PVP set of catalysts (Fig. 7c and Table SI1), hydrocarbons are produced only in trace amounts. However, since neither octadecanol nor octadecane have been detected for these catalyst series, it appears that there is no cooperation between Pd NPs and support in these cases, and therefore the parallel dehydration route has been completely blocked and only decarbonylation/decarboxylation is effective. Thus, these results strongly support that decarbonylation/decarboxylation is the main route on these catalysts occurring on the metal particles and that it is not influenced neither by the particle size nor by the support properties. The decrease of the Pd loading also corresponds to a decrease of the NP size and to a complete reduction of palladium to Pd0 that is in good correlation with the previous reports [8,24]. The dehydration route requires the cooperation of large metal particles and of an oxygenated support, like in the case of naked Pd NPs deposed onto MB. On this basis we may assume that the deoxygenation of the fatty acids on the investigated catalysts follows the routes described in Scheme 1. Fig. 8 and Table SI2 show the results collected with the same catalysts for the flow deoxygenation reaction conditions. Working under flow, the pressure was decreased from 20 to 4 atm keeping the same reaction temperature of 573 K. Under these reaction conditions, the conversion of stearic acid to heptadecane is superior to all the catalysts. Thus, even for the Pd/PVP series, the product was composed of over 20% heptadecane. It is important to note that heptadecane is the only reaction product; no octadecanol, octadecane and aromatic compounds have been detected among the reaction products. Again, the most efficient catalysts were those from the Pd/MB series. The maximum in performances is observed after 0.5 h. After that, the yields in heptadecane suffers from a severe decrease until 3 h reaction time. However, at that time, stopping feeding the reactor with oleic acid and flushing the catalyst only with mesitylene lead to a recovery of the initial activity and selectivity of the catalysts. TOF values calculated for these catalysts under both batch and flow reaction conditions showed an increased activity in hydrogenation of oleic acid to stearic acid with the dispersion (Tables 2, S1 and S2). Comparable values were calculated for the two types of reactors thus showing a limited mass transfer effect. The deoxygenation of the fatty acid requires however a smaller metal dispersion and therefore the Pd/MB series exhibited the higher activity (Figs. 7 and 8 and Tables S1 and S2). Theoretically, deoxygenation of fatty acids does not require H2 . Therefore, for comparison, experiments in different atmospheres have been performed using as reference the most active catalyst, i.e. Pd/MB (Table 4). The collected results highlight the strong effect of H2 . Thus, working with a 2.5: 1 N2 /H2 mixture is crossing out the dehydration route and even no trace of octadecanol nor octadecane is detected. When removing H2 completely, the analysis reveals the presence of heptadecene in a high excess compared to heptadecane (almost 7–1) thus suggesting decarbonylation as the main route (Scheme 1). Beside this, the presence of hydrogen is very important for the catalysts stability. XPS analysis of the Pd/PVP/H2 (MB) sample showed only a photopeak with a binding energy of 335.0 eV

9

20 10 0

90 0

1

2

3

Time (h) Fig. 8. Time evolution of the conversion and selectivity for flow experiments: (a) Pd/MB; (b) Pd(TPP)/H2 /MB; (c) Pd(PVP)/H2 /MB.

Table 4 Batch deoxygenation of oleic acid over Pd/MB catalyst under different atmospheres. Entry

Atmosphere

Conversion (%)

Products (selectivity, %)

1

H2 (20 atm)

100

2

2.5: 1 N2 /H2 (20 atm)

100

3

N2 (20 atm)

Cracking hydrocarbons (8) Heptadecane (70.5) Octadecane (13) Octadecanol (8.5) Heptadecane (17) Stearic acid (83) Heptadecenes (46) Heptadecane (7) Stearic acid (47)

30

Reaction conditions: 282 mg oleic acid in 5 mL mesitylene, 30 mg Pd/MB catalyst, 573 K, 9 h.

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Scheme 1. Steps involved in deoxygenation of oleic acid over the investigated catalysts.

corresponding to the oxidation zero of the Pd3d5/2 level (Table 3). Recycling this catalyst in hydrogen did not change the XPS analysis. However, recycling this catalyst three times in batch deoxygenation experiments carried out in nitrogen atmosphere indicated a partial oxidation of the Pd NPs. The presence of a second component with a binding of 337.9 eV represented a good confirmation. ICP-OES measurements indicated no leaching of palladium and simple washing of catalysts with mesitylene allowed ten times recycling without any change in conversion or product distribution. This is in line with previous results assuming the formation of aromatic compounds as a main reason for the catalysts deactivation [27]. Noteworthy, mesitylene as solvent exhibited no effect in this reaction. It was completely recovered in the reaction products. 4. Conclusions The deposition of Pd NPs onto mesoporous carbon supports lead to active and stable catalysts for the deoxygenation of oleic acid with different results depending on the reaction conditions, namely, batch and flow conditions. Under batch conditions (200 atm; 573 K), the extent of the reaction depends, firstly, on the Pd loading and then on the metal dispersion and the oxidation state of palladium, both influenced by the way the support has been treated before the NPs deposition and by the preparation/stabilization methodology of Pd NPs. No aromatic compounds were detected in the reaction products but in the case of the Pd/MB series, octadecanol and octadecane were observed in large extents. These data indicate a deoxygenation mechanism in which the hydrocarbons are produced via both decarbonylation/decarboxylation and dehydration routes. For the Pd/TPP and Pd/PVP series, the decarbonylation/decarboxylation was the only deoxygenation route. Under flow conditions (4 atm; 573 K), except for the Pd/MB series, the conversion of stearic acid was superior to that observed in batch conditions. Thus, even for the Pd/PVP series, the product mixture contained over 20% heptadecane. No octadecanol, octadecane and aromatic compounds were detected. Again, the most efficient catalysts were those from the Pd/MB series. The maxima in performances are obtained after 0.5 h. After that, the yields in heptadecane suffers from a severe decrease until 3 h reaction time. However, at that time, stopping feeding the reactor with oleic acid

and flushing the catalyst only with mesitylene recovers the activity and the selectivity of the catalysts. Further results obtained in mixtures of H2 /N2 or in pure N2 show the strong influence of H2 . Working with a 2.5: 1 N2 /H2 mixture is crossing out the dehydration route and even no traces of octadecanol or octadecane were detected. With the complete removal of H2 , the analysis reveals the presence of heptadecene in high excess compared to heptadecane (almost 7 to 1), thus suggesting decarbonylation as the main route. ICP-OES measurements indicated no leaching of palladium and simple washing of catalysts with mesitylene allowed recycling without any change in conversion or product distribution. Noteworthy, mesitylene as solvent exhibited no effect in this reaction. It was completely recovered in the reaction products. This comparative study highlights the influence of the preparation method of the catalysts on their catalytic performance in deoxygenation reaction and opens the way towards the design of new NPs type catalysts for this reaction. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apcata. 2015.01.008. References [1] D.Yu. Murzin, P. Mäki-Arvela, Thermochemical Conversion of Biomass to Liquid Fuels and Chemicals, RSC Publishing, Cambridge, 2010, pp. 496–510. [2] E. Santillan-Jimenez, M. Crocker, Chem. Technol. Biotechnol. 87 (2012) 1041–1050, and references therein. [3] I. Simakova, B. Rozmysłowicz, O. Simakova, P. Maki-Arvela, A. Simakov, D.Yu. Murzin, Top. Catal. 54 (2011) 460–466. [4] R.W. Gosselink, S.A.W. Hollak, S.-W. Chang, J. van Haveren, K.P. de Jong, J.H. Bitter, D.S. van Es, ChemSusChem 6 (2013) 1576–1594. [5] S.H. Bertram, Chem. Weekblad (1936) 457–459. [6] W.F. Maier, W. Roth, I. Thies, P.V. Rague Schleyer, Chem. Ber. 115 (1982) 808–812. [7] M. Snåre, I. Kubiˇcková, P. Mäki-Arvela, K. Eränen, D.Yu. Murzin, Ind. Eng. Chem. Res. 45 (2006) 5708–5715. [8] I. Simakova, O. Simakova, P. Maki-Arvela, A. Simakov, M. Estrada, D.Yu. Murzin, Appl. Catal. A: Gen. 355 (2009) 100–108. [9] S. Lestari, P. Maki-Arvela, K. Eranen, J. Beltramini, G.Q. Max Lu, D.Yu. Murzin, Catal. Lett. 134 (2010) 250–257.

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