Deoxygenation of Methyl Laurate as a Model Compound to

May 20, 2013 - The deoxygenation of methyl laurate as a model compound to diesel-like hydrocarbons was performed on Ni2P/SiO2, Ni2P/MCM-41, and Ni2P/S...
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Deoxygenation of Methyl Laurate as a Model Compound to Hydrocarbons on Ni2P/SiO2, Ni2P/MCM-41, and Ni2P/SBA-15 Catalysts with Different Dispersions Yan Yang, Jixiang Chen,* and Heng Shi Tianjin Key Laboratory of Applied Catalysis Science and Technology, Department of Catalysis Science and Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China S Supporting Information *

ABSTRACT: The deoxygenation of methyl laurate as a model compound to diesel-like hydrocarbons was performed on Ni2P/ SiO2, Ni2P/MCM-41, and Ni2P/SBA-15 catalysts. The effect of Ni2P dispersion on the catalyst structure and performance was investigated. The average Ni2P crystallite sizes varying from 3 to 12 nm were obtained. In correlation with the Ni/P ratio, the catalyst acid amount was mainly determined by the surplus P species. The deoxygenation was tested at 300−340 °C, 2.0 MPa, weight hourly space velocity of 10 h−1, and H2/methyl laurate ratio of 50. For different catalysts, the conversion of methyl laurate followed the different sequence from the turnover frequency (TOF). The TOF increased with the Ni2P crystallite size. The lower TOF on smaller crystallites can be attributed to the stronger interaction between Ni and P. Both hydrodeoxygenation and decarbonylation pathways occurred on the Ni2P catalysts. As indicated by the ratio between n-undecane (n-C11) and n-dodecane (n-C12) being larger than 1.0, the main deoxygenation pathway was decarbonylation. We suggested that the deoxygenation pathway was affected by Brönsted acidity and Ni2P crystallite size (i.e., the interaction between the Ni and P atoms). The Brönsted acid sites because of P−OH groups and the Ni sites having less interaction with P favored the decarbonylation pathway. With an increasing reaction temperature, the conversion, the selectivity to n-C11 and n-C12, and the n-C11/n-C12 ratio increased. At 340 °C, the conversion and the selectivity to n-C11 and n-C12 on all Ni2P catalysts exceeded 97 and 99%, respectively.

1. INTRODUCTION Fossil fuels have played an important role in human progress and social development. With population growth and economic development, the gap between rapid growth of energy demand and limited fossil fuel reserves is continuously expanding. As a result, to develop alternative energy has drawn great attention. As a kind of renewable energy, biodiesel has substituted part of the energy supply and contributes to the reduction of greenhouse gas. Usually, biodiesel refers to fatty acid methyl esters produced by transesterification of vegetable oils or animal fats with alcohol (mostly methanol). However, some properties (such as high oxygen content, high viscosity, high pour point, poor thermal and chemical stability, and low energy density) limit its application. Green diesel is another kind of biodiesel, which has similar composition to fossil diesel. It can be derived from vegetable oils and/or animal fats via hydrodeoxygenation (HDO) and decarbonylation/decarboxylation pathways in the presence of sulfide and metal catalysts.1 Conventional hydrotreating catalysts [sulfide Ni(Co)−Mo(W)] are widely studied and have been industrialized (e.g., NExBTL1). They mainly give the HDO hydrocarbons that have the same carbon number to the corresponding fatty acid.2−6 However, the leaching of sulfur because of the oxidation of the active phase leads to the catalyst deactivation.5,6 The addition of S-containing agents (e.g., H2S and CS2) can improve catalyst stability, whereas the agents react with the reactants and intermediates to form undesirable S-containing products, reducing the performance of products.6,7 Noble metal catalysts also show high deoxygenation activity.8−10 In comparison to sulfide catalysts, they mainly yield decarbonylation/decarbox© 2013 American Chemical Society

ylation hydrocarbons that have one carbon atom less than the corresponding fatty acid.11 Nevertheless, the expensive price of noble metals limits their wide industrial application. Similar to the noble metal catalysts, metallic Ni-based catalysts mainly give the decarbonylation/decarboxylation products. However, they also yield more cracked products.8,12,13 This not only reduces the diesel yield but also increases the H2 consumption. Thus, to develop new types of catalysts with high deoxygenation performance and low cost is extremely urgent. In recent decades, a lot of studies on transition-metal phosphides as new hydroprocessing catalysts have been published. The metal phosphide catalysts reveal excellent reactivity for hydrodesulfurization/hydrodenitrogenation,14,15 hydrodechlorination,16,17 synthesis gas conversion,18,19 hydrazine decomposition,20 and water−gas shift reaction.21 Their special catalytic property is suggested to be attributed to the ensemble and/or ligand effects of phosphorus.21−23 Recently, it has been reported that the metal phosphide catalysts can efficiently remove oxygen from molecules presented in bio-oils (e.g., guaiacol, 4-methylphenol, anisole, and dibenzofuran)24−27 and methyl oleate.12 Yang et al.12 reported that Ni2P/SBA-15 gave a much higher n-C18/n-C17 ratio than Ni/SBA-15 in the deoxygenation of methyl oleate, although it had slightly lower activity. However, the application of phosphide catalysts in the deoxygenation of fatty acids and esters is rare. Moreover, the relationship between the structure and the deoxygenation Received: March 20, 2013 Revised: May 18, 2013 Published: May 20, 2013 3400

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60 min to prepare Ni/SiO2 and Pd/SiO2. In addition, a POx/SiO2 sample with the nominal P content of 6.9 wt % was prepared by impregnating SiO2−H with an aqueous solution of NH4H2PO4, followed by drying at 120 °C for 12 h and calcination at 500 °C for 4 h. 2.2. Catalyst Characterization. H2 temperature-programmed reduction (H2-TPR) was carried out in a quartz U-tube reactor (inner diameter of 4 mm) with 50 mg of precursor loading in the thermostatic zone. The reduction was conducted in a 10 vol % H2/ N2 flow (60 mL min−1) at a heating rate of 10 °C min−1. The hydrogen consumption was determined by a thermal conductivity detector (TCD). X-ray diffraction (XRD) patterns were obtained on a D8 Focus powder diffractometer operated at 40 kV and 40 mA using Cu Kα radiation (λ = 0.154 06 nm). The Ni2P crystallite size was calculated by the Scherrer equation. Transmission electron microscope (TEM) images were obtained on a Tecnai G2 F20 instrument, working at 200 kV. The powder sample was ultrasonically dispersed in ethanol and then deposited on a holey carbon film supported on a copper grid. N2 adsorption−desorption isotherms were measured on a Quantachrom QuadraSorb SI at −196 °C. The BET equation was used to calculate the specific surface area. The pore volume was estimated at a relative pressure of 0.99. The pore structure was determined by the Barrett−Joyner−Halenda (BJH) method using the desorption branch of the isotherm. The Ni and P contents were measured by a Varian VISTA-MPX inductively coupled plasma−atomic emission spectrometer (ICP− AES). CO chemisorption uptakes were obtained on the same apparatus as used for H2-TPR. A total of 100 mg of passivated catalyst was rereduced with a H2 flow (60 mL min−1) at 450 °C for 1 h. Subsequently, the sample was swept with a He flow (80 mL min−1) at 450 °C for 1 h to remove the hydrogen adsorbed on the surface and then cooled to 30 °C. When the TCD signal was stable, pluses of CO (50 μL) were passed through the sample until the effluent areas of consecutive pulses were constant. Then, the total dynamic CO uptake was calculated. H2 temperature-programmed desorption (H2-TPD) was used to measure the states of hydrogen species using the same apparatus as with H2-TPR. A total of 100 mg of passivated catalyst was re-reduced with a H2 flow (60 mL min−1) at 450 °C for 1 h and then cooled to 30 °C. After H2 adsorption for 30 min, the sample was flushed with a N2 flow (20 mL min−1) until the TCD signal was stable. H2-TPD was performed at a heating rate of 15 °C min−1. The desorbed H2 was detected by a TCD. A trap containing soild NaOH was set to remove water before detection. Using the same apparatus as with H2-TPR, NH3-TPD was adopted to measure the catalyst acidity. A total of 70 mg of passivated catalyst was re-reduced with a H2 flow (60 mL min−1) at 450 °C for 1 h and then cooled to 100 °C. After NH3 adsorption for 30 min, the sample was flushed with a He flow (60 mL min−1) to remove the physically adsobed NH3. NH3-TPD was executed at a heating rate of 15 °C min−1 using a TCD to detect the desorbed NH3. A trap containing soild NaOH was set to remove water before detection. 2.3. Activity Test. The deoxygenation of methyl laurate was carried out on a continuous flow stainless reactor (inner diameter of 12 mm and length of 380 mm). A total of 0.4 g of passivated catalyst (60−100 mesh) blended with 3.2 g of quartz sand (60−100 mesh) was loaded in the reactor. A total of 2.0 g of quartz sand (20−40 mesh) was loaded on the catalyst bed to preheat the reactants. The passivated catalyst was rereduced at 450 °C for 1 h in a H2 flow (100 mL min−1). After the reduction, the temperature and H2 pressure were adjusted to the desired values and methyl lautate was then fed into the reactor with a liquid pump. The weight hourly space velocity (WHSV) and H2/methyl laurate molar ratio were 10 h−1 and 50, respectively. When the turnover frequencies (TOFs) were calculated, the reaction was found to operate in the absence of heat-/mass-transfer limitations. The liquid products were identified using gas chromatograph (GC) standards, and the identification was further confirmed by an Agilent

performance of Ni2P catalysts needs to be explored. Oyama et al.28 have reported that two types of Ni sites [tetrahedral Ni(1) sites and square pyramidal Ni(2) sites] are located on Ni2P crystallites and contribute to different desulfurization pathways. Also, the number of Ni(1) sites or Ni(2) sites varies with the Ni2P crystallites; that is, the number of Ni(2) sites increases with the reduction of the crystallite size. Simakova et al.29 observed that Pd dispersion remarkably influenced the reactivity of Pd/C for the deoxygenation of palmitic and stearic acids. Therefore, the effect of Ni2P dispersion as well as the roles of different Ni sites on the deoxygenation is worth investigating. This is favorable for insight into the optimal catalyst for the production of green diesel. In this work, MCM-41, SBA-15, and SiO2 were adopted as supports to prepare the Ni2 P catalysts with different dispersions. Methyl laurate was used as a model reactant to evaluate the deoxygenation performance of the Ni2P catalysts. The effect of Ni2P dispersion on the deoxygenation performance was investigated associating with the structural properties of the Ni2P catalysts. The influence of the temperature on the deoxygenation was also examined.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. MCM-41 [The Catalyst Plant of Nankai University, with a Brunauer−Emmett−Teller (BET) specific surface area (SBET) = 1000 m2/g], SBA-15 (Nanjing XianFeng Nano Material Technology Co., Ltd., with SBET = 650 m2/g), and SiO2 (Qingdao Haiyang Chemicals Co., Ltd., with SBET = 548 and 341 m2/ g, denoted as SiO2−H and SiO2−L, respectively) were used as supports. The Ni2P/MCM-41, Ni2P/SBA-15, Ni2P/SiO2−H, and Ni2P/SiO2−L catalysts were prepared by the temperature-programmed reduction (TPR) method. First, 5.0 g of support was incipiently impregnated with an aqueous solution containing 4.37 g (15.03 mmol) of Ni(NO3)2·6H2O and 1.73 g (15.03 mmol) of NH4H2PO4. After drying at room temperature for 48 h and at 120 °C for 12 h, the sample was calcined at 500 °C for 4 h to obtain the precursor. Second, the precursor was reduced with H2 (>99.9%, 320 mL min−1 g−1 of precursor) from 20 to 650 °C at a rate of 1 °C min−1 and then maintained at 650 °C for 3 h. After that, the sample was cooled to room temperature and passivated in a 0.5 vol % O2/N2 flow for 6 h. In addition, a highly dispersed Ni2P catalyst (s-Ni2P/SiO2) was prepared according to ref 30. First, a solution of 14.55 g (50.03 mmol) of Ni(NO3)2·6H2O, 10.5 g of urea, and 560 mL of H2O was kept in a flask at 30 °C, and its pH value was adjust to 2.5 by adding HNO3 solution. Then, a solution of 56 mL of tetraethyl orthosilicate (TEOS) and 50 mL of EtOH was dropwise added to the above solution under stirring to form a sol. The sol was stirred for 4 h, and it was then heated to 90 °C. The pH value of the mixture was increased because of urea hydrolysis, leading to a gel containing Ni2+ ions. When the pH value reached 8.0, the gel was filtered and washed with ethanol and water. After the sample dried at 50 °C for 48 h and at 120 °C for 12 h, it was calcined at 500 °C for 4 h to obtain the nickel-containing silica (s-Ni/SiO2). Second, 5 g of s-Ni/SiO2 (60−100 mesh) was incipiently impregnated with an aqueous solution of 1.31 g (11.39 mmol) of NH4H2PO4. After the sample dried at room temperature for 48 h and at 120 °C for 12 h, it was clained at 500 °C for 4 h to obtain the precursor. Third, the precursor was reduced with H2 (>99.9%, 320 mL min−1 g−1 of precursor) from 20 to 650 °C at a rate of 1 °C min−1 and then maintained at 650 °C for 3 h. The sample was cooled to room temperature and passivated in a 0.5 vol % O2/N2 flow for 6 h. For comparison, Ni/SiO2 (nominal Ni content of 15 wt %) and Pd/ SiO2 (nominal Pd content of 5 wt %) were prepared according to the following procedure: SiO2−H was incipiently impregnated with an aqueous solution of Ni(NO3)2 or PdCl2, followed by drying at 120 °C for 12 h and calcination at 500 °C for 4 h. The resulting Ni/SiO2 and Pd/SiO2 precursors were respectively reduced at 450 and 300 °C for 3401

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GC6890-MS5973N. The liquid products (time on stream = 3−4 h) were quantitatively analyzed on a SP-3420 gas chromatograph equipped with a flame ionization detector (FID) and a HP-5 capillary column (30 m × 0.33 mm × 0.5 μm). The tetrahydronaphthalene was selected as an internal standard. Gaseous products were analyzed by a 102 gas chromatograph equipped with a TCD and a TDX-101 packed column, and N2 was selected as an internal standard to quantify the amounts of CO and CH4. The conversion of methyl laurate and the product selectivities were caculated as follows:

is, the existence of nickel species promoted the reduction of the P−O bond because of the hydrogen spillover effect. The peak maximum slightly shifted to a higher temperature as the support surface area increased. This is related to more highly dispersed nickel phosphate on the support with a larger surface area, giving rise to a stronger interaction between the nickel and phosphorus species.28 Because there was a strong interaction between nickel and silica derived from the sol−gel process, the s-Ni2P/SiO2 precursor required a higher reduction temperature than other catalyst precursors.30 Figure 2A shows the wide-angle XRD patterns. The broad peak at around 22.5° is ascribed to amorphous silica. The

⎛ n⎞ conversion (%) = ⎜1 − ⎟ × 100% n0 ⎠ ⎝ selectivity (Si , %) =

ni × 100% n0 − n

where n0 and n denote the moles of methyl laurate in the feed and product, respectively, and ni is the moles of product i (e.g., n-undecane, n-dodecane, and oxygenated intermediates).

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. H2-TPR was used to characterize the reducibility of precursors. The profiles are illustrated in Figure 1. As a reference, the TPR profiles of POx/

Figure 1. H2-TPR profiles of catalyst precursors.

SiO2 and the Ni/SiO2 precursor are also presented. The reduction of POx/SiO2 started at about 660 °C and was not complete until 1000 °C. Such a high temperature is attributed to the high thermally stable P−O bond.31 The Ni/SiO2 precursor gave a main peak at 367 °C with two shoulders at lower and higher temperatures. The main peak and the shoulder at a lower temperature are attributed to the reduction of bulk NiO, while the shoulder at a higher temperature is due to nickel silicate.26 For the Ni2P/MCM-41, Ni2P/SBA-15, and s-Ni2P/SiO2 precursors, there were minor peaks below 600 °C, which may be ascribed to the reduction of nickel oxide and/or nickel silicate without interacting with the phosphorus species.32 In each profile of Ni2P catalyst precursor, the predominant peak because of nickel phosphate is broad and asymmetrical between 600 and 900 °C. The shoulder at a lower temperature and the main peak are attributed to the reductions of nickel species and the P−O bond, respectively.25,33 During the H2-TPR, nickel species were first reduced and then H2 dissociated from metallic Ni sites reduced the P−O bond. That

Figure 2. (A) Wide-angle XRD patterns of different Ni2P catalysts and (B) low-angle XRD patterns of MCM-41, Ni2P/MCM-41, SBA-15, and Ni2P/SBA-15.

diffraction peaks at 40.9°, 44.7°, 47.4°, and 54.6° (PDF 030953) are attributed to Ni2P. The intensity of Ni2P diffraction peaks varied on different catalysts, indicative of different Ni2P dispersions. On the base of Ni2P (111) reflection (2θ = 40.9°), the average crystalline sizes of Ni2P were calculated by the Scherrer formula, and the results are presented in Table 1. The Ni2P average crystalline sizes on Ni2P/SiO2−L, Ni2P/SiO2−H, Ni2P/SBA-15, and s-Ni2P/SiO2 were about 12.0, 11.4, 9.3, and 4.3 nm, respectively. Because of the indistinct peaks, the Ni2P 3402

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Table 1. Physical and Chemical Properties of Ni2P Catalysts

a

catalyst

Ni content (wt %)

Ni/P molar ratio

Ni2P crystallite size (nm)

BET surface area (m2/g)

average pore diameter (nm)

pore volume (cm3/g)

CO uptake (μmol/g)

TOFa (s−1)

Ni2P/MCM-41 s-Ni2P/SiO2 Ni2P/SBA-15 Ni2P/SiO2−H Ni2P/SiO2−L

11.6 12.3 12.7 12.6 13.7

1/0.76 1/0.66 1/0.63 1/0.66 1/0.58

3.0b 4.3 9.3 11.4 12.0

163 211 354 411 238

1.6 12.3 7.8 5.6 7.9

0.39 0.74 0.58 0.57 0.58

105 141 67 72 57

0.064 0.076 0.123 0.136 0.151

Reaction temperature of 300 °C. bDetermined from TEM images.

unreduced phosphate. In addition, the pore damage gave rise to a much lower surface area of Ni2P/MCM-41 than that of MCM-41. As seen in Table 1, the Ni contents and the Ni/P molar ratios in different catalysts were 11.6−13.7 wt % and from 1:0.76 to 1:0.58, respectively. The Ni/P ratios were lower than the stoichiometry value (2.0) of Ni2P, indicating that there was surplus unreduced phosphate on the catalysts. Meanwhile, the Ni/P ratios were larger than that in the precursors (1.0). Some phosphorus was lost in the form of PxHy (for example, PH3) during the reduction of phosphate to phosphide.22 Ni2P/ MCM-41 had the lowest Ni/P ratio among the catalysts. The more P that remained on Ni2P/MCM-41 may be due to the strong interaction between nickel and phosphorus with high dispersion in the precursor.28 CO chemisorption was used to determine the density of exposed Ni sites. The CO uptakes are listed in Table 1. s-Ni2P/ SiO2, Ni2P/SBA-15, and Ni2P/SiO2−H had similar Ni content and Ni/P ratio, whereas s-Ni2P/SiO2 gave a larger CO uptake because of its smaller Ni2P crystallites, as indicated by the XRD and TEM results. Ni2P/MCM-41 had smaller Ni2P crystallites than s-Ni2P/SiO2, whereas it had slightly lower CO uptake. This is related to more surplus P species on Ni2P/MCM-41 that covered the nickel sites.22 The lowest CO uptake of Ni2P/ SiO2−L might be related to the largest Ni2P particles. H2-TPD was used to investigate the state of hydrogen species adsorbed on the catalysts. The results are presented in Figure 4 and Table 2. The peaks below 400 °C are attributed to desorption of hydrogen species adsorbed on the surface of Ni2P particles.17,25,35 Several peaks below 400 °C indicate that there were different states of hydrogen species adsorbed on the Ni2P particles. Liu et al.36 have suggested that H2 may adsorb on Ni sites, Ni−P bridge sites, and P sites with different interactions. As shown in Table 2, Ni2P/MCM-41 and s-Ni2P/SiO2 had larger H2 desorption amounts than other catalysts, which is attributed to their higher Ni2P dispersion. Above 400 °C, two peaks centered at around 570 and 800 °C are attributed to the spilt-over hydrogen species located at the metal−support interface and distant from the Ni2P particles.17,25,35 Ni2P/ MCM-41 had the largest spilt-over hydrogen. This can be explained by that more surplus P species on Ni2P/MCM-41, providing more P−OH groups. It has been proposed that −OH groups can stabilize spilt-over hydrogen.37 NH3-TPD was used to investigate the acidity of the different Ni2P catalysts. The results are presented in Figure 5 and Table 2. In each NH3-TPD profile, there was a prominent peak at around 200 °C with a shoulder at a higher temperature, indicating that the supported Ni2P catalyst contained both weak and medium strength acid sites, especially the weak strength acid sites. For comparison, the NH3-TPD profiles of POx/SiO2 and Ni/SiO2 are also presented in Figure 5. POx/SiO2 gave a peak at about 200 °C that is ascribed to the P−OH groups.38 Ni/SiO2 had two peaks centered at about 463 and 603 K, which

average crystalline size of Ni2P/MCM-41 was determined at about 3.0 nm from the TEM images (Figure 3). In addition, as indicted by the XRD patterns (not shown here), metallic Ni and Pd were formed on Ni/SiO2 and Pd/SiO2, respectively. Figure 3 displays the representative TEM images and the Ni2P particle size distributions. The dark spheres are Ni2P particles. The diameter ranges of Ni2P particles are mainly about 2−5, 2−7, and 3−13 nm for Ni2P/MCM-41, s-Ni2P/SiO2, and Ni2P/ SBA-15, respectively. The Ni2P particles on Ni2P/SiO2−H and Ni2P/SiO2−L ranged from about 3 to above 20 nm. Clearly, the Ni2P particles were smaller and more uniform on Ni2P/ MCM-41 and s-Ni2P/SiO2 than on other catalysts. For s-Ni2P/ SiO2, the high Ni2P dispersion is ascribed to its preparation process.30 To prepare the s-Ni2P/SiO2 precursor, s-Ni/SiO2 with high dispersion was first prepared via the sol−gel method and the P species was then introduced to s-Ni/SiO2. In s-Ni/ SiO2, the nickel species existed in the form of nickel silicate, leading to a strong interaction between nickel and the support. This inhibited nickel sintering at a high reduction temperature and, therefore, gave small Ni2P dispersion. For Ni2P/MCM-41, the formation of small and uniform Ni2P particles may be derived from the high surface area and uniform pore (about 2.8 nm in diameter) of MCM-41. During the preparation, nickel phosphate in the pores of MCM-41 was reduced. The uniform pores restrained the formation of larger Ni2P particles. However, the high reduction temperature at 650 °C eventually destroyed the pore structure of MCM-41, as indicated by the following result. As shown in Figure 3, the ordered channels are visible for Ni2P/SBA-15 rather than for Ni2P/MCM-41, indicating that the hexagonal pore structure of SBA-15 was still maintained. This is also confirmed by the low-angle XRD. Figure 2B shows the low-angle XRD patterns. A major (100) reflection and two minor (110) and (200) reflections are observed for MCM-41 and SBA-15 because of the highly ordered hexagonal pore structure. No characteristic peaks are visible for Ni2P/MCM-41, implying that the ordered hexagonal pore structure of MCM-41 was severely destroyed. This is ascribed to the pore wall collapse because of the low thermal stability of MCM-41. Here, the high Ni2P loading (about 21.2 wt % Ni2P) might aggravate the pore damage of MCM-41, because the pore structure of MCM-41 was maintained for 12.2 wt % Ni2P/MCM-41.34 For Ni2P/SBA-15, the (100) reflection is visible, whereas (110) and (200) reflections disappear. Thus, the ordered hexagonal pore structure is still retained for Ni2P/ SBA-15, although its order was reduced. This is consistent with the TEM result. Textural properties of the catalysts were obtained from the N2 adsorption−desorption isotherms (not shown here), and they are summarized in Table 1. The BET surface area and pore volume for each catalyst were lower than those of the corresponding support. This is ascribed to the coverage of surfaces and the blockage of pores by Ni2P particles and 3403

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Figure 3. TEM images of (a) Ni2P/MCM-41, (b) s-Ni2P/SiO2, (c) Ni2P/SBA-15, (d) Ni2P/SiO2−H, and (e) Ni2P/SiO2−L.

mainly ascribed to the P−OH groups with weak strength acidity, while the shoulder may be mostly related to the Niδ+ sites with medium strength acidity. As indicated from Tables 1 and 2 and Figure 5, the lower the Ni/P ratio, the more the total amount of acid sites and the amount of the weak strength acid sites. Ni2P/MCM-41 had the largest acid amounts. This can be explained as follows: (a) highly dispersed Ni2P particles exposed more Ni sites and (b) more unreduced P species provided more P−OH groups. However, s-Ni2P/SiO2 with high

are probably related to acidities of SiO2 and the unreduced nickel silicate,39 respectively. It is known that there are both Brönsted and Lewis acid sites on Ni2P/SiO2.38 The Brönsted acidity is ascribed to the P−OH groups, while the Lewis acidity may be associated with the electron-deficiency Niδ+ (0 < δ < 1) species, which were caused by electron transfer from Ni to P.38,40 Abu and Smith41 reported that the source of Brönsted acid sites is unreduced phosphate species. Associated with the NH3-TPD profile of POx/SiO2, the peak at about 200 °C is 3404

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and 2.0 MPa. For all Ni2P catalysts, CO and CH4 were found in the gaseous products and the detected liquid products include n-undecane (n-C11), n-dodecane (n-C12), oxygenated intermediates (lauryl alcohol, lauraldehyde, lauric acid, and lauryl laurate), methanol, and cracked hydrocarbons. In addition, there was some light yellow insoluble substance in the liquid at low conversion, which is the polymers with high boiling points and could not be detected by a GC. To explore the reaction pathway, the effect of WHSV on the performance of Ni2P/ SiO2−H was tested and the relationship between the methyl laurate conversion and the selectivities to different products was obtained (see Figure 1S of the Supporting Information). As the WHSV decreased, the conversion and the selectivity to C11 and C12 increased, while the selectivities to lauric acid, lauryl alcohol, and lauryl laurate decreased. This indicates that C11 and C12 were final products, while lauric acid, lauryl alcohol, and lauryl laurate were intermediate products. Although the selectivity to lauraldehyde did not obviously change, it should also be an intermediate. This indicates that lauraldehyde was highly reactive and easy to further convert. During the reaction, the primary products may include lauric acid and lauraldehyde. As shown in Scheme 1, lauric acid can be produced via

Figure 4. H2-TPD profiles of different Ni2P catalysts.

Table 2. H2-TPD and NH3-TPD Data of Supported Ni2P Catalysts relative H2 desorption amounta catalyst Ni2P/MCM-41 s-Ni2P/SiO2 Ni2P/SBA-15 Ni2P/SiO2−H Ni2P/SiO2−L

400 °C 2.69 2.97 1.15 1.28 1.00

2.64 1.73 1.24 1.67 1.23

Scheme 1. Proposed Reaction Pathway of Methyl Laurate

total

relative NH3 desorption amountb

5.33 4.70 2.39 2.95 2.23

3.00 1.43 1.47 1.38 1.00

Designating the H2 desorption amount of Ni2P/SiO2−L below 400 °C as 1.00. bDesignating the NH3 desorption amount of Ni2P/SiO2−L as 1.00. a

hydrogenolysis (reaction a) and/or hydrolysis (reaction b), while lauraldehyde can be produced via hydrogenolysis (reaction c). It is reasonable that there was a consecutive reaction pathway as methyl laurate → laurate acid → lauraldehyde → lauryl alcohol → C 12 , during which lauraldehyde was also a secondary product. In addition, laurate acid and lauraldehyde can also be converted to C11 via decarbonylation.42 It has been suggested that the decarbonylation of carboxylic ester is easier than that of carboxylic acid.43 Thus, the reaction that methyl laurate was directly decarbonylated to C11 (reaction d) cannot be excluded. The formation of methanol indicates that reactions b−d could take place. Because no CO2 was detected in gaseous effluence and the mole of CO was close to that of n-C11 on all Ni2P catalysts at different temperatures, n-C11 was produced from the decarbonylation of methyl laurate as well as lauric acid and lauraldehyde. To testify no decarboxylation, we also evaluated the hydrogenation of CO2 on Ni2P/SiO2−H and found that Ni2P/SiO2− H had very low activity, even at 340 °C (no shown here). In addition, lauryl laurate was generated via the esterification of lauric acid with lauryl alcohol on acid sites. It is reasonable that lauryl laurate was deoxygenated via a similar mechanism to methyl laurate.

Figure 5. NH3-TPD profiles of different Ni2P catalysts, POx/SiO2, and Ni/SiO2.

Ni2P dispersion had a similar Ni/P ratio and acid amount to Ni2P/SBA-15 and Ni2P/SiO2−H. Thus, the acid amount may be primarily determined by the surplus P content. Associated with the result reported by Abu and Smith,41 the more surplus unreduced P species would generate more Brönsted acid sites. 3.2. Catalyst Reactivity. The deoxygenation of methyl laurate on the Ni2P catalysts was carried out at 300−340 °C 3405

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Figure 6. Deoxygenation of methyl laurate over different Ni2P catalysts.

smaller Pd particles for the deoxygenation of palmitic and stearic acids is associated with the stronger interactions between the Pd species and the support, resulting in the changes of the Pd structure required for effective deoxygenation. On the basis of the EXAFS analysis, Oyama et al.28 suggest that the square pyramidal Ni(2) sites have a stronger interaction with P atoms than tetrahedral Ni(1) sites and their number increases with reducing Ni2P crystallites. Thus, the stronger interaction between the Ni and P atoms may account for the lower TOF of smaller Ni2P crystallites. We speculate that the stronger interaction between Ni and P atoms

Figure 6A illustrates the conversions of methyl laurate on different Ni2P catalysts. At 300 °C, the conversions followed the sequence: Ni2P/MCM-41 (52.5%) < Ni2P/SBA-15 (63.7%) < Ni2P/SiO2−L (66.7%) < Ni2P/SiO2−H (76.3%) < s-Ni2P/SiO2 (82.6%). The TOFs of different catalysts were estimated using CO uptakes (shown in Table 1). The TOF increased in the order of Ni2P/MCM-41 < s-Ni2P/SiO2 < Ni2P/SBA-15 < Ni2P/SiO2−H < Ni2P/SiO2−L, which is corresponding to the sequence of the Ni2P crystallite size. That is, the larger the Ni2P crystallite, the higher the TOF. Simakova et al.29 have proposed that the lower activity of 3406

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crystallites favor HDO or that the larger Ni2P crystallites favor decarbonylation. Oyama et al.28 suggest the different roles of the Ni(2) and Ni(1) sites in the hydrodesulfurization of 4,6DMDBT. The Ni(1) sites are responsible for the DDS route, while the Ni(2) sites are highly active sites for the HYD route because they can activate H2. It is reasonable that the high hydrogenation ability of Ni(2) sites may favor the HDO pathway. In other word, the strong interaction between the Ni and P atoms may restrain decarbonylation. This is obviously indicated by the fact that Ni/SiO2 gave the n-C11/n-C12 ratio of 23.7 at 300 °C. Ni/SiO2 had a much higher decarbonylation ability than the Ni2P catalysts. This may be ascribed to the difference in the electron properties of Ni and Ni2P. It is known that the ligand effect of P reduces the electron density of the Ni site in Ni2P.23,26 That is, the electron density of the Ni site in Ni2P is lower than that in metallic Ni. Thus, the Ni site in Ni2P has a higher electrophilicity than that in metallic Ni. The higher electrophilicity of the Ni site may favor the HDO pathway via adsorption of oxygen of the ester group, followed by hydrogenation. Dupont et al.48 proposed that the oxygen atom of the CO bond or the C−O−C group is preferentially adsorbed on the Mo site compared to the Ni site on NiMoS because of the larger positive charge on the Mo site (with higher electrophilicity). Similarly, Chen et al.49 have found that the addition of Mo to the Ru catalyst promotes the hydrogenation of the CO bond, owing to the decrease of the electron density on the Ru centers derived from the interaction between MoOx species. On Ni2P, the Ni(2) sites may slightly favor the HDO pathway in comparison to the Ni(1) sites because of the stronger interaction between the Ni(2) sites and the P atoms. This may account for a slightly lower n-C11/n-C12 ratio on s-Ni2P/SiO2 than on Ni2P/SiO2−H. Ni2P/SiO2−L had a similar Ni2P crystallite size to Ni2P/SiO2− H, whereas it gave a lower n-C11/n-C12 ratio. This may be related to its less acid sites. Thus, for the different Ni2P catalysts, the n-C11/n-C12 ratio is slightly influenced by the Ni2P crystallite size (i.e., the interaction between Ni and P atoms) but obviously by Brönsted acidity. However, the role of the P− OH groups on Ni2P may not separate from that of the Ni sites. There may be a synergism between the Ni sites and the P−OH groups. This effect has been found for the hydrodechlorination of chlorobenzene.50 Simakova et al.29 also found that the Pd dispersion did not affect the product distribution during the deoxygenations of palmitic and stearic acids. In the liquid products, cracked products (n-decane, nnonane, n-octane, and n-heptane) and oxygenated intermediates (lauryl alcohol, lauraldehyde, lauric acid, and lauryl laurate) were also detected. The total selectivity to cracked products was very low, and it was less than 0.5%, even at 340 °C, on Ni2P. The low activity for the cracking reaction on Ni2P catalysts was similar to that on Pd/SiO2. The selectivity to cracked products on Pd/SiO2 was only about 0.2% at 300 °C. However, Ni/SiO2 gave the selectivity of about 2.1% at 300 °C. The high hydrogenolysis activities of metallic Ni catalysts were also indicated in the deoxygenation of stearic acid,8 methyl oleate,12 and biodiesel [fatty acid methyl esters (FAMEs)].13 In a previous work,51 we suggested that, in comparison to that of Ni/SiO2, the lower activity of Ni2P/SiO2 for glycerol hydrogenolysis can be ascribed to the ligand (electronic) and ensemble (geometrical) effects of P. Similarly, the Ru site with lower electron density had lower activity for the cleavage of C− C bonds.49 Also, the ensemble effect of P may make steric hindrance to the adsorption and cleavage of the carbon chain.

may further lower the electron density of the Ni site because of the ligand effect of P. However, the high electron density of the metal site is favorable for the electron transfer from the metal site to the lowest unoccupied molecular orbital (LUMO) of the C−O band, which facilitates the dissociation of the C−O bond.25 As the temperature increased, the conversion increased on all catalysts, particularly evident on Ni2P/MCM-41. At the temperature of 340 °C, the conversion exceeded 97% on all Ni2P catalysts. n-C11 and n-C12 were the final products and derived from decarbonylation and HDO pathways, respectively. Figure 6B shows the total selectivity (SC11 + C12) to n-C11 and n-C12 on different catalysts. At 300 °C, Ni2P/MCM-41 gave the lowest SC11 + C12 (about 48%). s-Ni2P/SiO2, Ni2P/SBA-15, and Ni2P/ SiO2−H gave similar SC11 + C12 (about 87%), and Ni2P/SiO2−L gave SC11 + C12 of about 80.1%. SC11 + C12 increased as the reaction temperature rised, especially on Ni2P/MCM-41. At 340 °C, SC11 + C12 was higher than 99% on all Ni2P catalysts. Figure 6C shows the molar ratio between n-C11 and n-C12 (nC11/n-C12). On all Ni2P catalysts, the n-C11/n-C12 ratios exceeded 1.0, implying that decarbonylation was the main deoxygenation pathway. At 300 °C, the n-C11/n-C12 ratio on Ni2P/MCM-41 was the largest (about 4.0), while it was between 2.4 and 3.0 on other catalysts. The n-C11/n-C12 ratio increased from 2.4−4.0 to 5.3−7.3 with an increasing temperature from 300 to 340 °C. That is, an increasing temperature promotes the decarbonylation reaction.44 For different catalysts, the n-C11/n-C12 ratio followed a similar sequence between 300 and 340 °C. To explore the factors influencing the deoxygenation pathway, Ni/SiO2, Pd/SiO2, and POx/SiO2 were tested for the deoxygenation at 300 °C. On Ni/ SiO2, Pd/SiO2, and POx/SiO2, the total yields of C11 and C12 hydrocarbons were 73.5, 29.4, and 0.3%, respectively, while the C11/C12 ratios were 23.7, 10.8, and 4.0, respectively. Clearly, the function of metal Ni and Pd is the determining factor for the deoxygenation in comparison to the acidity of POx/SiO2. The main deoxygenation pathways on Ni/SiO2, Pd/SiO2, and POx/SiO2 were decarbonylation. Generally, the metal function favors the decarbonylation/decarboxylation.8 It is known that POx/SiO2 possesses weak Brönsted acidity because of P−OH groups. Thus, the decarbonylation and the HDO pathways on POx/SiO2 are mainly attributed to Brönsted acidity. Laurent et al.45 and Şenol et al.46 have found that the SH− groups (i.e., Brönsted acid sites) primarily promote the decarbonylation/ decarboxylation pathway and slightly influence the HDO pathway. The decarbonylation catalyzed by Brönsted acidity can be explained by the addition of H+ to one of the oxygens of the carboxylic group followed by a β elimination of the whole carboxylic group to produce n-C11.45 Also, P−OH groups provide hydrogen to hydrogenate the ester group that was adsorbed on the metal site, giving rise to the HDO product (C12). Contrarily, Zuo et al.47 have reported that the HDO pathway is promoted by medium strength Brönsted acidity of Ni/SAPO-11. The stronger Brönsted acid site more easily releases the H+ ion. Here, perhaps because of its weak Brönsted acidity, POx/SiO2 gave lower ability for hydrogenation than decarbonylation. The more surplus P leading to more Brönsted acid sites may account for the largest n-C11/n-C12 ratio on Ni2P/MCM-41. s-Ni2P/SiO2 had a similar acid amount to Ni2P/SiO2−H, whereas it gave a slightly lower n-C11/n-C12. This can be explained by the fact that the smaller Ni2P 3407

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Recently, Yakovlev et al.13 have found that the Ni−Cu catalysts give much lower hydrogenolysis activities than the Ni catalysts in the deoxygenation of FAME, which is attributed to the formation of solid solution Ni1−xCux. The formation of the Ni− Cu alloy reduces the Ni ensemble size and, therefore, diminishes the C−C hydrogenolysis activity.52 The total selectivity to the oxygenated intermediates (Soxy) is shown in Figure 6D. It is reasonable that the catalyst with higher activity and SC11 + C12 gave lower selectivity to oxygenated intermediates. As the reaction temperature increased, Soxy decreased obviously. At 340 °C, Soxy was less than 0.4% on all catalysts. In the gaseous products, CO and CH4 were detected on the different Ni2P catalysts and Pd/SiO2. However, only CH4 was detected on Ni/SiO2, indicating that the produced CO and/or CO2 were totally hydrogenated to CH4. Clearly, the Ni2P catalysts had much lower activity for the methanation. This should be ascribed to the role of P. It has been reported that the high electron density of the Ni site facilitates the dissociation of adsorbed CO by the enhanced back donation of electrons from the Ni atom to the anti-bonding orbital,53 and the dissociated C species are then hydrogenated to form alkanes (for example, CH4). Because of the lower electron density of the Ni site in the Ni phosphides, CO and H2 have less interaction with Ni2P than with metallic Ni.36 Song et al.19 also found that the nickel phosphide catalysts showed much lower activity than the Ni catalyst for the CO hydrogenation. Similar to that of Pd/SiO2, the lower methanation activity of the Ni2P catalysts gives rise to less hydrogen consumption. This is an advantage of the lowering operational cost. On all catalysts, the CO/CH4 molar ratios were larger than 1.0 (Figure 6E). This is due to the formation of methanol. As indicated in Scheme 1, if there was no methanol formation via reactions b−d, the CO/CH4 ratio would be between 0 and 1.0 when both decarbonylation and HDO pathways took place. Actually, methanol was formed during the deoxygenation, and therefore, the CO/CH4 ratio increased. Apart from the case on Ni2P/MCM-41, Ni2P/SBA-15, and Ni2P/SiO2−L that had lower CO/CH4 ratios at 300 °C than at 320 °C, the CO/CH4 ratio tended to decrease with an increasing temperature, although the decarbonylation pathway became predominating. This may indicate that the hydrogenolysis of the O−CH3 bond and/or the hydrogenation of formed methanol to CH4 became predominant with an increasing temperature.

having less interaction with P favored the decarbonylation pathway. The formation of methanol led to CO/CH4 larger than 1.0. As the reaction temperature increased, the conversion, SC11 + C12, and n-C11/n-C12 ratio increased. At 340 °C, the conversion and the selectivity on all Ni2P catalysts exceeded 97 and 99%, respectively. On the whole, considering the catalyst cost and performance, SiO2 is a suitable support. In comparison to Ni/SiO2, the Ni2P catalysts had lower activities for methanation and the cracking reaction because of the ligand and ensemble effects of P. This is similar to the performance of Pd/SiO2 and gives rise to less H2 consumption and higher product yield.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Selectivities to final hydrocarbons and oxygenated intermediates on Ni2P/SiO2−H as a function of conversion (Figure 1S). This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*Telephone: +86-22-27890865. Fax: +86-22-87894301. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the support from the National Natural Science Foundation of China (21176177), the Natural Science Foundation of Tianjin (12JCYBJC13200), and the Program of Introducing Talents to the University Disciplines (B06006).

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REFERENCES

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