CNTs catalyst

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Effective hydrodeoxygenation of palmitic acid to diesel-like hydrocarbons over MoO2/CNTs catalyst Ranran Ding, Yulong Wu n, Yu Chen, Junmei Liang, Ji Liu, Mingde Yang Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, PR China

H I G H L I G H T S

G R A P H I C A L


MoO2/CNTs are a novel catalyst for long-chain fatty acid deoxygenation.
MoO2/CNTs showed superior low temperature activity and hexadecane selectivity.
The H2 pressure of the reaction can control the product selectivity.
MoO2/CNTs are a much better catalyst for deoxygenation of palmitic acid than Pd/CNTs.

In this work, hydrodeoxygenation of palmitic acid into C16 hydrocarbons was successfully achieved over 5 % MoO2 /CNTs catalysts at a much lower temperature (220oC). The H2 pressure of the reaction can controls the product selectivity. The low cost and high activity make the 5 % MoO2 /CNTs catalyst more competitive than Pd-based noble metals catalysts. In summary, this material described in this paper provides a new protocol to decrease the oxygen content of bio-oil, especially algae-based, for their utilization in the future. Proposed main reaction pathways for palmitic acid conversion over MoO2 /CNTs in the presence of 4Mp H2 at 220 oC.

art ic l e i nf o

a b s t r a c t

Article history: Received 21 July 2014 Received in revised form 30 September 2014 Accepted 11 October 2014

The liquid-phase deoxygenation reaction of palmitic acid has been investigated in a batch reactor over carbon nanotubes (CNTs)-supported MoO2 and Pd catalysts under hydrogen atmosphere. The results showed that palmitic acid can be converted completely at 220 1C on MoO2/CNTs catalyst with high hexadecane selectivity (92.2%). The H2 pressure of the reaction can control the product's selectivity. Compared with noble metal Pd-based catalyst, MoO2-based catalyst is a much better catalyst because of its high activity, selectivity, stability and low cost. Product distribution revealed that Pd offered high selectivity for pentadecane, whereas MoO2 showed excellent selectivity for hexadecane, which showed more favorable atom efficiency. Therefore, the MoO2/CNTs catalyst can be regarded as an attractive candidate for catalytic hydrodeoxygenation of long-chain fatty acids to produce high-grade transportation biofuels. & 2014 Published by Elsevier Ltd.

Keywords: MoO2/CNTs catalyst Catalytic hydrodeoxygenation Palmitic acid Diesel-like hydrocarbons Reaction mechanism

A B S T R A C T

1. Introduction Fuel production from renewable and carbon-neutral biomass has attracted considerable attention because of concerns for the depletion of fossil fuels and global warming by greenhouse gases (Na et al., 2012). As a new type of energy crop, algae have

n

Corresponding author: Tel.: +86 10 8979 6163; fax: +86 10 6278 4831. E-mail address: [email protected] (Y. Wu).

many advantages over other feedstocks, such as impressive productivity, noncompetition with agriculture, and high carbon dioxide (CO2) absorption and uptake rate (Sani et al., 2013; Zhao et al., 2013). Given these advantages, algae-derived biofuels have been recognized as the “third generation of biomass energy” and the “only current renewable source of oil that could meet the global demand for transport fuels” (Yang et al., 2011). Several technologies are known to produce liquid fuels from algae, and are summarized in recent reviews (Chaiwong et al., 2013; Lestari et al., 2009). The conventional process of biofuel production is the extraction of

http://dx.doi.org/10.1016/j.ces.2014.10.024 0009-2509/& 2014 Published by Elsevier Ltd.

Please cite this article as: Ding, R., et al., Effective hydrodeoxygenation of palmitic acid to diesel-like hydrocarbons over MoO2/ CNTs catalyst. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.10.024i

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the lipid fraction and subsequent transesterification. However, for low lipid content algae, thermochemical conversion (mainly including hydrothermal liquefaction and pyrolysis) may prove to be an economical method for producing bio-oils (Bai et al., 2014). Nevertheless, extracted algae oil (mainly compose of triglyceride) or bio-oil both contain large amounts of oxygen and results in undesirable biofuel qualities such as oil acidity, polymerization, high viscosity, and low heating value (Zou et al., 2010; Peng et al., 2012a). Therefore, crude oils must be upgraded before being used as transportation fuels. C16–C18 long-chain fatty acids are the main components of bio-oil derived from algae or intermediates of algae oil conversion; therefore, oxygen removal of long-chain fatty acids is an important step in the utilization of algal bio-energy. To upgrade algal-derived oils, transesterification and deoxygenation have been proposed. Although transesterification of algae oil into biodiesel is a mature technology and the resulting fatty acid methyl esters (typical first generation biodiesel) has found relatively widespread commercial application, the unfavorable cold flow properties and poor storage stability, as well as engine compatibility issues, render biodiesel a less than ideal transportation fuel (Santillan-Jimenez et al., 2012). Against this background, a novel technology to produce diesel-like hydrocarbon via catalytic deoxygenation reaction has been intensively investigated during recent years (Snåre et al., 2006; Peng et al., 2012b; Mäki-Arvela et al., 2007). Such biofuels (the so-called second-generation biodiesels) have higher energy densities and higher storage stabilities than first-generation biodiesels because of the absence of oxygen and are fully compatible with existing vehicles (Hollak et al., 2012). The key element to successfully upgrade bio-oils is the development of a catalytic system that can achieve deep deoxygenation. The fundamentals of this process are mostly studied by using longchain fatty acids as model compounds to better understand the catalytic reactions involved in deoxygenation upgrading. Deoxygenation of fatty acids feeds can occur by different pathways depending on reaction conditions and the type of catalyst used. The different pathways for the deoxygenation of fatty acids are shown in Scheme 1 (Gosselink et al., 2013; Peng et al., 2013). Decarbonylation and decarboxylation (DCO) yield hydrocarbon chains with one carbon atom less as compared with the reactant, whereas hydrodeoxygenation (HDO) results in hydrocarbons with the same chain length as the starting compound. The catalyst screening studies revealed that Pd and Pt supported on activated carbon catalysts are the most active and selective catalysts for this purpose (Rozmysłowicz et al., 2012; Han et al., 2010). However, because the deoxygenation of bio-oils is expected to be a large-scale process, using noble metal-based catalysts could significantly increase the costs of processing. In addition, the adsorption of low–boiling point compounds on the active sites of palladium resulted in the deactivation of the catalyst (Han et al., 2011). Hence, there is an urgent need to develop practical non-noble metal catalysts with a high activity for the deoxygenation of bio-crude into diesel-like hydrocarbons and at the same time obtain a sufficient lifetime. The redox properties of certain metal oxides, such as molybdenum oxide (MoOx), have been used as catalysts in the HDO processes of small oxygenates under the premise that a reverse

Scheme 1. Pathways for the deoxygenation of fatty acids.

Mars–van Krevelen reaction would result in the removal of the oxygen atom from the oxy-compound upon the adsorption on the vacancy site with concomitant regeneration of the vacancy with H2 to produce water (Prasomsri et al., 2013; Whiffen et al., 2010; Yakovlev et al., 2009). In this mechanism, the generation of vacancy sites is responsible for the activation of both oxy compound and hydrogen on the catalytic surface (Mortensen et al., 2013). However, the activity and selectivity of MoO2 for the HDO of long-chain fatty acids has not been explored. The role of supports also plays a crucial role in the HDO activity and selectivity. Carbon nanotubes (CNTs) are allotropes of carbon with a distinct structure that gives high thermal conductivity, electrical, and mechanical properties. Compared with active carbon, the relatively few organic groups in the CNTs could decrease the possibility for the occurrence of side reactions, making it a promising catalyst support for deoxygenation (Yang et al., 2013). In addition, the uniform and larger pore diameter of these supports facilitates relatively much easier diffusion of substrates compared with conventional catalysts. Taking these into account, we first synthesized multi-walled CNTs supported by 5% MoO2 catalyst, and tested it in palmitic acid deoxygenation reactions. Results showed that HDO of palmitic acid can be successfully performed over a MoO2/CNTs catalyst with high activity and selectivity at a much lower temperature. Compared with noble metal Pd-based catalysts, the MoO2/CNTs catalyst exhibited much better catalytic activity and selectivity to HDO products, which contained more carbon atoms in the target molecules and showed more favorable atom efficiency. Therefore, the MoO2/CNTs can be deemed as an attractive candidate for catalyzed HDO of fatty acids to produce high-grade secondgeneration transportation biofuels. The effects of the H2 pressure and reaction temperature were also investigated. Based on the resulting products, a detailed reaction mechanism was proposed.

2. Experimental 2.1. Materials CNTs were generously provided by Xianfeng Advanced Material Supplier (Nanjing, China). Decane (4 99%), hexadecanoic acid (99.0%) and cetyl alcohol (4 98%) were purchased from Guangfu Fine Chemical Research Institute (Tianjin, China). Ammonium molybdate (99.0%) was purchased from Guangfu Technology Development Co., Ltd. (Tianjin, China). Pd (NO3)2  2H2O was purchased from Civi-Chem Technology Co., Ltd. (Shanghai, China). Hexadecanal (497%) was purchased from Tokyo Chemical Industry. All reagents were used as-received without further purification. 2.2. Catalyst preparation The theoretical MoO2 and Pd loading were approximately 5% based on CNTs support. The catalysts consisting of MoO2 and Pd supported on CNTs were synthesized by the incipient wetness impregnation method. In a representative procedure, 5 wt% MoO2/ CNTs was prepared by first dissolving (NH4)6Mo7O24  4H2O (1.02 g) in water (10 mL) and then slowly dropping this solution onto CNTs (2 g) with continuous stirring. After allowing the metal to incorporate into the support over 4 h at ambient temperature, the catalyst was first dried overnight at 105 1C for 12 h. Thereafter, it was calcined in air at 500 1C for 3 h (ramp: 10 mL min  1) and reduced at 500 1C for 4 h (ramp: 10 1C min  1) in hydrogen (flow rate: 80 mL min  1).

Please cite this article as: Ding, R., et al., Effective hydrodeoxygenation of palmitic acid to diesel-like hydrocarbons over MoO2/ CNTs catalyst. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.10.024i

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2.3. Catalyst characterization XRD patterns were measured on a D8 ADVANCE diffractometer using Cu Kα radiation (λ ¼0.1541 nm, 36 kV, 2 mA, scanning step¼21 min  1). The diffraction patterns were recorded by scanning at an angle ranging from 01 to 801. Specific surface area analysis of the catalyst was performed by nitrogen sorption isotherms at 196 1C in a Micromeritics Tristar analyzer. The surface areas were calculated by the Brunauer–Emmett–Teller (BET) method, and the pore size distribution was calculated by the Barrett–Joyner–Halenda (BJH) method. Pretreatment of samples for surface area measurement was done by flowing N2 for 6 h at 300 1C. TEM observations were taken on a FEI Tecnai F20 Field Emission Electron Microscope, operated at 200 kV. Inductively coupled plasma with atomic emission spectroscopy (ICP-OES) measurements were performed on an IRIS Intrepid II XSP atomic emission spectrophotometer. The final liquid phase product was analyzed in an Agilent GC–MS (6820) with a GsBP-Inowax (30 m  0.32 mm  0.25 mm) column and a flame ionization detector. 1 ml of the sample was injected into the GC at a split ratio of 10:1, and the carrier gas (helium) flow rate was 10 mL/min. The oven temperature program started at 120 1C for 2 min, and then ramped to 260 1C at a rate of 10 1C min  1 held for 10 min at this temperature. Quantitative analysis of liquid products was conducted using the external standard method. The vapor phase was analyzed online by a gas chromatograph with TCD detector and capillary column (Porapak Q). 2.4. Catalytic tests Catalytic experiments with palmitic acid, 1-hexadecanol, and hexadecanal were carried out in a 150 mL autoclave in batch mode. A typical reaction was conducted as follows: reactant (0.5 g), decane (50 mL), and catalyst (0.25 g) were loaded onto the autoclave, which was then purged with the gas at ambient temperature and the pressure was adjusted to 4 MP prior to the reaction. Finally, the mixture was heated to 220 1C under stirring at 300 rpm to start the reaction. Conversion ¼(moles of converted palmitic acid/mole of the starting palmitic acid)  100%. Selectivity ¼(moles of each product/mole of total product)  100%. 2.5. Catalyst recycling When the reaction was complete, 5% MoO2/CNTs catalyst was deposited at the bottom of the autoclave. The precipitate was separated by filtration and then dried in an oven at 105 1C for 10 min. The catalyst was recovered and reused in the next run without further activation treatment.

3. Results and discussion 3.1. Synthesis of catalysts The XRD patterns of the two catalysts are shown in Fig. 1. It can be seen that the sample (a) has diffraction peaks at 2θ¼261, 371, 531, 601, and 671, which are assigned to the planes of (011), (020), (022), (031), and (  231) of MoO2, respectively (JCPDS: 65-1273). No other crystalline phases are observed besides the expected MoO2 and CNTs are detected by XRD. It can be assured that the sample (a) has formed MoO2/CNTs catalyst. The peaks of Pd (JCPDS: 65-6174) are clearly observable besides those ascribed to CNTs in the sample (b). It indicates that sample (b) formed Pd/ CNTs catalyst. In addition, narrow and intensive diffraction peaks of the samples (a) and (b) indicate a well crystallized structure.

Fig. 1. X-ray diffraction patterns of the catalysts: (a) MoO2/CNT; (b) Pd/CNT. Legend: (●) Pd, (♦) MoO2 and (Δ) CNT.

Table 1 Lattice parameters and textural properties of the catalysts. Catalysts

Crystal system

Mean Dhkl (nm)

o ε2hkl 4 1/2 BET (m2/g)  102

Pore size (nm)

Mo2C/CNTs Pd/CNTs

Monoclinic Cubic

13.5 18.7

1.63 0.68

2.4 2.7

130.0 234.5

From the crystallite and lattice constants, we can see that MoO2 and Pd belong to monoclinic and cubic crystal structures, respectively. The values of lattice distortion were calculated by the formula (Niu et al., 2005): (2β)2 cos2θ¼4/π2(λ/Dhkl)2 þ 32 o 2hkl 4 sin2θ, where Dhkl is the average thickness of the lattice face, λ is the wavelength of the X-ray used (0.15406 nm), θ is the diffraction angle of the (hkl), and β is the corrected full-width at halfmaximum of the diffraction peak, and oε2hkl 4 1/2 is the lattice distortion. The lattice distortions of MoO2 and Pd are estimated as 1.63  10  2 and 0.68  10  2, respectively. Because the products are nanometer materials with smaller grain size and larger surface area that is extremely disorderly, the crystal structures deviate from its bulk and show a certain amount of lattice distortion. The mean grain size was calculated by Debye–Scherrer equation (Ding et al., 2013): D ¼0.89 λ/βcosθ. As shown in Table 1, the crystallite size of MoO2 is smaller than Pd. Nitrogen adsorption–desorption isotherms of MoO2/CNTs and Pd/CNTs exhibit representative type IV curves with sharp capillary condensation steps at relative pressures of 0.8–0.98 (Fig. 2a) for both catalysts (Selvaraj et al., 2014). Highly ordered arrangements of mesopores are confirmed by the presence of an H1-type hysteresis loop in both catalysts (Biabani-Ravandi et al., 2013). As shown in Fig. 2b, MoO2/CNTs and Pd/CNTs possess narrow pore size distributions centered at 2.4, and 2.7 nm, respectively. The pore volumes of MoO2- and Pd-supported catalysts are 0.42 and 0.67 cm3/g, respectively. It may be because of the partial blockage of pores after the MoO2/CNTs preparation; hence, it has reduced textural properties. As shown in Table 1, the BET surface area of MoO2/CNTs is lower than that of Pd/CNTs catalyst, consistent with pore volume measurements. In order to investigate the shape, size, and particle distribution, we analyzed the TEM images of samples, as shown in Fig. 3. TEM images display a good distribution of nanoparticles on the CNTs

Please cite this article as: Ding, R., et al., Effective hydrodeoxygenation of palmitic acid to diesel-like hydrocarbons over MoO2/ CNTs catalyst. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.10.024i

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Fig. 2. N2 sorption–desorption isotherms (a) and pore size distributions (b) of MoO2/CNTs and Pd/CNTs catalysts.

Fig. 3. TEM images of (a) MoO2/CNTs and (b) Pd/CNTs, HRTEM images of (c) MoO2, and (d) Pd nanoparticles.

Please cite this article as: Ding, R., et al., Effective hydrodeoxygenation of palmitic acid to diesel-like hydrocarbons over MoO2/ CNTs catalyst. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.10.024i

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Fig. 4. Conversion and selectivity for the main products as a function of reaction temperature of (a) MoO2/CNTs and (b) Pd/CNTs catalysts.

Table 2 Conversion of palmitic acid and selectivity of products over MoO2/CNTs and Pd/CNTs catalysts at different reaction temperatures Catalyst

MoO2/CNTs MoO2/CNTs MoO2/CNTs MoO2/CNTs MoO2/CNTs MoO2/CNTs blank Pd/CNTs Pd/CNTs Pd/CNTs Pd/CNTs

Temperature (1C)

190 200 210 220 240 260 220 220 240 260 280

Conversion (%)

53.8 96.1 98.6 100 100 100 10.3 64.2 80.2 93.3 95.8

Product distribution Pentadecane

Hexadecane

Hexadecanol

Light alkanes

2.4 5.8 5.9 7.6 14.2 15.4 3.4 58.0 74.1 85.4 78.3

4.1 20.1 64.9 92.2 78.7 72.4 2.0 2.0 2.5 3.1 12.1

47.3 70.0 27.7 0 0 0 0 4.2 2.1 0 0

0 0 0.2 0.2 7.1 12.2 4.9 0 1.5 4.8 5.4

Reaction conditions: palmitic acid (0.5 g), decane (50 mL), MoO2/CNTs (0.25 g), H2 (4 MP), 4 h, and stirring at 300 rpm.

surface in both MoO2/CNTs and Pd/CNTs. Nanostructured spherical Pd particles were well distributed on the outer surface of CNTs, and no evident agglomerations were observed (Fig. 3b). The mean size of Pd particles was about 10 nm. It could be seen that MoO2 particles were highly dispersed and most of the MoO2 particles were embedded in the carbon walls (Fig. 3a). It is considered that the CNTs-supported MoO2 and Pd catalysts reveal a model of “support dispersion.” On the one hand, CNTs can disperse MoO2 and Pd particles adequately, as the majority of them are present on the CNTs surface; on the other hand, CNTs act as sintering barriers effectively restraining the sintering of MoO2 and Pd particles. The individual MoO2 and Pd nanoparticles can be well observed in MoO2/CNTs and Pd/CNTs by high-resolution TEM (HRTEM). The image shown in Fig. 3c displays a lattice distance of 0.34 nm, which can be assigned to the (011) plane of monoclinic MoO2 phase. The interplanar distance of 0.22 nm agrees well with the lattice spacing of the (111) plane of the Pd (Fig. 3d). These suggest that our synthetic strategy is facile for uploading individual MoO2 and Pd nanoparticles on CNTs. 3.2. Deoxygenation of palmitic acid at different temperatures Temperature is an important parameter that influences the reaction rate and equilibrium. Catalytic reaction over MoO2/CNTs at different reaction temperatures showed that the conversion was very low at 190 1C, but increased dramatically with temperature to

100% at 220 1C (Fig. 4a). Analysis of product distribution revealed that when the reaction temperature increased to 220 1C, the highest selectivity for n-hexadecane was achieved at 92.2%. Further elevating the temperature to 260 1C, the selectivity of hexadecane decreased from 92.2% to 72.4%. A more detailed overview of the product selectivity can be found in Table 2. As shown in Table 2, large amounts of hexadecanol could be observed when performing the reaction at a lower temperature (200 1C). This result indicates that dehydration of the alcohol has the highest temperature barrier in the overall reaction when MoO2/ CNTs is used as a catalyst at 200 1C and 4 MP H2. The products of DCO increased with temperature, but to a smaller extent (from 2.4% at 190 1C to 15.4% at 260 1C). The products of hydrocracking were detected only at temperatures higher than 220 1C. This suggests that temperatures higher than 220 1C tend to favor DCO and hydrocracking at the expense of HDO. Hence, 220 1C was selected as the desired reaction temperature. These observations also suggest that the apparent activation energies for the processes leading to DCO and hydrocracking should be higher than the apparent activation energies of the steps that lead to HDO when MoO2/CNTs is used as catalyst. The palmitic acid conversion over Pd/CNTs catalysts was found to be a strong function of temperature. As the temperature increased, the conversion increased. As shown in Fig. 4b, the conversion was only 64.2% at 220 1C; when the reaction proceeded at 280 1C, the conversion increased to 95.8%. Analysis of the

Please cite this article as: Ding, R., et al., Effective hydrodeoxygenation of palmitic acid to diesel-like hydrocarbons over MoO2/ CNTs catalyst. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.10.024i

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product selectivity revealed that Pd offered high selectivity for pentadecane, which are formed by DCO (Scheme 1). When the reaction temperature increased to 260 1C, the selectivity of pentadecane achieved 85.4%. Further elevating temperature to 280 1C, the selectivity of pentadecane decreased to 78.3%. In order to obtain information about the contributions of thermal cracking, a blank experiment was also performed under the selected reaction conditions. The results indicate that only little light alkanes, pentadecane, and hexadecane occur in the absence of catalysts. Gas products obtained over MoO2/CNTs and Pd/CNTs catalysts were also analyzed. For gas products generated at 220 1C reactions, the main oxygen-containing product is water over the MoO2/CNTs and CO over the Pd/CNTs catalysts, respectively. Trace amounts of CO2 and CH4 were also detected in the reaction system. This confirms the experimental observations showing that the MoO2/CNTs catalyst is a much better catalyst for the conversion of palmitic acid to diesel-like hydrocarbons than Pd/CNTs. The different product distributions suggest that different deoxygenation mechanisms prevail over the two catalyst types. An important feature of the MoO2 catalyst is that the main reaction products contain the same number of carbon atoms as that of the parent reactant, thus indicating that C–O bonds are selectively cleaved without cleaving C–C bonds.

under the reaction condition employed. These results also demonstrate that the formation of hexadecene from 1-hexadecanol would be the rate-limiting step in the overall reaction. According to product distributions, the hydrogenation–dehydration–hydrogenation reactions are the main route for producing C16 alkanes without carbon loss (92.2% selectivity), whereas decarbonylation of the intermediately formed C16 aldehyde is the minor route. To better understand the HDO step of palmitic acids to alkanes, the representative intermediate, hexadecanol, was converted in an independent experiment on MoO2/CNTs at conditions identical to palmitic acid conversion. As shown in Fig. 6, the selectivity of nhexadecane increased almost linearly with 1-octadecanol conversion, attaining 96% after 1 h, which was formed via a tandem dehydration–hydrogenation route together with 3% of C15 alkane. In addition, the 1-hexadecanol and hexadecanal are concluded to be equilibrated because the selectivity of individually produced nC16 and n-C15 alkanes increased linearly with increasing conversion. Only traces of n-hexadecene were detected during the whole course because of the fast hydrogenation of the double bond. In order to verify the formation of 1-hexadecanol from hexadecanal, the hexadecanal was subjected to reaction conditions. According to product distributions shown in Fig. 7, the selectivity

3.3. Deoxygenation mechanism of palmitic acid on MoO2/CNTs To elucidate the mechanism of palmitic acid deoxygenation, we traced the changes of conversion and selectivity as a function of reaction time (depicted in Fig. 5). It could be seen that nearly complete conversion of palmitic acid was obtained after reaction for 40 min. The yield of n-hexadecane (C16) increased continuously to 92.2% at 4 h, whereas that of n-pentadecane (C15) grew slowly to 7.8% after 4 h. Fig. 5 shows that the selectivity of hexadecanol gradually increased to 40%, and then gradually decreased, accompanied by an increase in alkane yields (mainly including C16 and C15 alkanes) as a function of time. Because palmitic acid disappeared earlier than hexadecanol, it is likely that the reaction proceeded initially by a fast hydrogenation reduction of the COOH group to alcohol, which eventually underwent slow dehydration to give the major reaction product. By tracing the palmitic acid conversion course, intermediate hexadecanal was confirmed. Because only traces of hexadecanal were detected in the overall reaction, the hexadecanal reduction rate to the hexadecanol was concluded to be faster than its production rate over MoO2/CNTs

Fig. 5. Palmitic acid conversion and selectivity for the main products over MoO2/ CNTs catalysts at 220 1C as a function of reaction time.

Fig. 6. Hexadecanol conversion and selectivity for the main products over MoO2/ CNTs catalysts at 220 1C as a function of reaction time.

Fig. 7. Hexadecanal conversion and selectivity for the main products over MoO2/ CNTs catalysts at 220 1C as a function of reaction time.

Please cite this article as: Ding, R., et al., Effective hydrodeoxygenation of palmitic acid to diesel-like hydrocarbons over MoO2/ CNTs catalyst. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.10.024i

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for HDO products (hexadecane) was more than 80%, which indicated that HDO was the main reaction route for hexadecanal conversion on MoO2/CNTs catalysts. On the contrary, the selectivity of decarbonylation products (pentadecane) was less than 17% on MoO2/CNTs catalysts. Fig. 7 shows that the selectivity of hexadecanol gradually increased to 38% and then gradually decreased, accompanied by an increase in alkane selectivity (mainly including C16 and C15 alkanes) as a function of time. This implies that the intermediate hexadecanol obtained from hexadecanal is formed through a hydrogenation mechanism. Combining these experiments allows us to formulate the overall reaction pathway for palmitic acid transformation (Scheme 2). The reaction pathway proceeds through hydrogenation of the carboxylic group of fatty acid leads to the corresponding aldehyde, for example, hexadecanal, followed by either decarbonylation of hexadecanal to n-heptadecane and carbon monoxide (minor route) or hydrogenation of hexadecanal to 1-hexadecanol. Subsequently, the produced 1-hexadecanol undergoes sequential dehydration (C–O bond cleavage, rate-determining step) and hydrogenation leading to the final n-hexadecane (major route). CO may react with H2 to produce methane and water. A similar hydrogenation–dehydration mechanism is known to occur over the Ni/HBeta (10 wt%, Si/Al ¼180) catalyst when using stearic acid as model compounds at 260 1C (Peng et al., 2012a). The difference is that the fatty acid hydrogenation is the rate-determining step for Ni/HBeta catalyst, whereas dehydration is the rate-determining step for MoO2/CNTs catalysts in the overall reaction. It can also be deduced that the individual reaction rates follow the order of r4 (hydrogenation of CQC double bonds in the alkyl chain) 4r2 (hydrogenation of hexadecanal) 4r1 (hydrogenation of fatty acid) 4 r3 (dehydration of hexadecanol, rate-determining step). In addition, the decarbonylation and hydrocracking of palmitic acid leads to pentadecane and light alkanes (minor routes), respectively. The hydrogenation–dehydration route is shown to be feasible and superior to the decarbonylation pathway because of the higher atom efficiency. Further investigation of the mechanism and conditions of MoO2/CNTs catalyzed deoxygenation of fatty acid are ongoing in our group.

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hydrogen applied (Fig. 8). When the initial hydrogen pressure increased from 0 to 1 MPa (Table 3, entries 1–3), selectivity for intermediates, such as hexadecanol and hexadecanal, increases first and then decreases. Further elevating the pressure of hydrogen to 4 MP, offered complete conversion, and high selectivity of hexadecane was achieved. Consistent with previous reports, low hydrogen pressures increased the selectivity for decarbonylation and hydrocracking, whereas high hydrogen pressures favored HDO (Kandel et al., 2014; Boda et al., 2010). This dependence of selectivity on hydrogen pressure is likely the result of the participation of H2 in the equilibrium between hexadecanal and 1-hexadecanol, the branching step in the process. Because hydrogen is required to convert the hexadecanal into 1-hexadecanol, increasing the amount of H2 shifts the equilibrium toward the alcohol favoring the route to n-hexadecane, whereas decreasing the amount of the H2 has the opposite effect leading to the npentadecane pathway. This confirmed the experimental observations showing that higher hydrogen pressure favors the HDO pathway over the decarbonylation pathway for 5% MoO2/CNTs catalyst.

3.4. Deoxygenation of palmitic acid at different hydrogen pressures Initial hydrogen pressure was an important parameter that influenced the HDO rate of palmitic acid. The pressure variation experiments were investigated using 5% MoO2/CNTs as a catalyst under 220 1C. As expected, the conversion of palmitic acid and liquid hydrocarbon selectivity was proportional to the pressure of

Fig. 8. Effect of H2 pressure on palmitic acid conversion and distribution of hydrocarbons over MoO2/CNTs catalyst at 220 1C.

Scheme 2. Proposed main reaction pathways for palmitic acid conversion over MoO2/CNTs in the presence of 4 MP H2 .

Please cite this article as: Ding, R., et al., Effective hydrodeoxygenation of palmitic acid to diesel-like hydrocarbons over MoO2/ CNTs catalyst. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.10.024i

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Table 3 Conversion of palmitic acid and selectivity of products over MoO2/CNTs catalyst at different H2 pressures. Catalyst

MoO2/CNT MoO2/CNT MoO2/CNT MoO2/CNT MoO2/CNT MoO2/CNT

H2(MP) pressure

0 0.5 1 2 3 4

Conversion (%)

14.1 74.3 98.4 100 100 100

Product distribution Pentadecane

Hexadecane

Hexadecanal

Hexadecanol

Others

2.6 17.1 59.0 29.2 19.1 7.6

0 6.5 33.1 66.1 78.4 92.2

0 6.5 0 0 0 0

0 20.9 0 0 0 0

11.5 23.3 6.3 4.7 2.5 0.2

Reaction conditions: palmitic acid (0.5 g), decane (50 mL), MoO2/CNTs (0.25 g) at 220 1C, 4 h, and stirring at 300 rpm.

Fig. 9. (a) Cycling runs tests of 5% MoO2/CNTs catalyst (reaction temperature at 220 1C) and (b) XRD patterns of freshly prepared and used 5% MoO2/CNTs.

3.5. Stability tests of MoO2/CNTs catalyst The stability of the MoO2/CNTs catalyst was evaluated and five successive runs were carried out without any regeneration treatment under the same reaction conditions. In the catalyst recycling tests, nearly no deactivation was observed after consecutive reactions for the fifth time, which confirmed the conclusion that MoO2 exhibited much better resistance to carbon monoxide, water, and short-chain hydrocarbon poisoning (Fig. 9a). ICP-OES measurements manifested that the leaching amount of Pd (Pd/ CNTs catalyst, 0.29 mg L  1) was three times higher than that of Mo (MoO2/CNTs catalyst, 0.096 mg L  1), which demonstrated that MoO2 showed much better resistance to leaching than Pd, revealing good stability. The XRD patterns of MoO2/CNTs catalyst before and after the deoxygenation reaction are shown in Fig. 9b. After five recycling reactions, the position and the intensity of peaks were nearly the same to the fresh MoO2/CNTs, the framework of MoO2 structure remained intact without any signs of collapse, clearly indicating the relative stability of MoO2/CNTs catalyst.

4. Conclusions In summary, an effective approach was disclosed for the transformation of palmitic acid to alkanes by selectively cleaving C–O bonds with CNTs-supported MoO2 catalysts. This pathway gives full HDO of palmitic acid with the highest carbon economy. The low cost, high activity, and stability make the MoO2/CNTs catalyst more competitive than Pd-based noble metal catalysts. The H2 pressure of the reaction controls the equilibrium between aldehyde and alcohol intermediates directing it to two main

pathways: either decarbonylation of the aldehyde or dehydration of the alcohol. The process that involves dehydration of the alcohol has a lower temperature barrier than that of decarbonylation of aldehyde and hydrocracking when 5% MoO2/CNTs is used as a catalyst at 40 bar H2. The material described in this paper provides a new protocol to decrease the oxygen content of bio-oils, especially algae-based fuels, for their utilization in the future.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 21376140 and 21176142), Program for New Century Excellent Talents in University (No. NCET-12–0308) and the Independent Research Programs of Tsinghua University (No. 20111081067).

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Please cite this article as: Ding, R., et al., Effective hydrodeoxygenation of palmitic acid to diesel-like hydrocarbons over MoO2/ CNTs catalyst. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.10.024i