Deoxygenation of methyl laurate to hydrocarbons on silica-supported Ni-Mo phosphides: Effect of calcination temperatures of precursor

Journal of Energy Chemistry 24(2015)77–86

Deoxygenation of methyl laurate to hydrocarbons on silica-supported Ni-Mo phosphides: Effect of calcination temperatures of precursor Zhengyi Pana ,

Rijie Wanga , Mingfeng Lib , Yang Chub , Jixiang Chena∗

a. 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, China; b. Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China [ Manuscript received August 11, 2014; revised October 11, 2014 ]

Abstract SiO2 -supported Ni-Mo bimetallic phosphides were prepared by temperature-programmed reduction (TPR) method from the phosphate precursors calcined at different temperatures. Their properties were characterized by means of ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS), H2 temperature-programmed reduction (H2 -TPR), X-ray diffraction (XRD), transmission electron microscopy (TEM), CO chemisorption, H2 and NH3 temperature-programmed desorptions (H2 -TPD and NH3 -TPD). Their catalytic performances for the deoxygenation of methyl laurate were tested in a fixed-bed reactor. When the precursors were calcined at 400 and 500 ◦ C, respectively, NiMoP2 phase could be formed apart from Ni2 P and MoP phases in the prepared C400 and C500 catalysts. However, when the precursors were calcined at 600, 700 and 800 ◦ C, respectively, only Ni2 P and MoP phases could be detected in the prepared C600, C700 and C800 catalysts. Also, in C400, C500 and C600 catalysts, Mo atoms were found to be entered in the lattice of Ni2 P phase, but the entering extent became less with the increase of calcination temperature. As the calcination temperature of the precursor increased, the interaction between Ni and Mo in the prepared catalysts decreased, and the phosphide crystallite size tended to increase, subsequently leading to the decrease in the surface metal site density and the acid amount. C600 catalyst showed the highest activity among the tested ones for the deoxygenation of methyl laurate. As the calcination temperature of the precursor increased, the selectivity to C12 hydrocarbons decreased while the selectivity to C11 hydrocarbons tended to increase. This can be mainly attributed to the decreased Ni-Mo interaction and the increased phosphide particle size. In sum, the structure and performance of Ni-Mo bimetallic phosphide catalyst can be tuned by the calcination temperature of precursor. Key words metal phosphide; calcination temperature; methyl laurate; hydrodeoxygenation; decarbonylation

1. Introduction Nowadays, with the fast development of global economy, the demand for energy is sharply growing. The resource of limited non-renewable fossil energy is dwindling rapidly, meanwhile, its extensive utilization simultaneously results in serious environmental problems. To develop clean and renewable alternative energy resource is very urgent around the world. A renewable and clean environment-friendly energy resource, biomass, has drawn great attention. Through transesterification with methanol, triglycerides, the primary composition of vegetable oil and animal fat can be transformed into biodiesel (i.e., fatty acid methyl ester). As biodiesel contains a large number of oxygen, the removal of oxygen is urgent for meeting the specification as fuel in terms of viscosity, volatility, corrosiveness and stability [1].

The deoxygenation reaction pathways of fatty acid ester include direct hydrodeoxygenation (HDO) and decarboxylation/decarbonylation. The former leads to the hydrocarbon with the same number of carbon atoms as the corresponding fatty acid and oxygen is removed in the form of water, and the latter leads to the hydrocarbon with one carbon less than the corresponding fatty acid while oxygen is removed as CO/CO2 . Compared with HDO pathway, the decarboxylation/decarbonylation pathway consumes less hydrogen but gives lower carbon yield if the methanation of CO/CO2 does not take place. From the practical viewpoint, tuning of the hydrocarbon composition has a very vital significance, which can be achieved via regulating the catalyst compositions and properties. Nowadays, the deoxygenation catalysts mainly include transition metal sulfide, noble metal and metallic Ni [2–7].

∗

Corresponding author. Tel: +86-22-27890865; Fax: +86-22-87894301; E-mail: [email protected] (J. Chen) This work was supported by the National Natural Science Foundation of China (No. 21176177), the Natural Science Foundation of Tianjin (No. 12JCYBJC13200) and State Key Laboratory of Catalytic Materials and Reaction Engineering (RIPP, SINOPEC). Copyright©2015, Science Press and Dalian Institute of Chemical Physics. All rights reserved. doi: 10.1016/S2095-4956(15)60287-X

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For the sulfide catalysts, it is imperative to add sulfurcontaining reagents (e.g. CS2 or H2 S) into feedstock to maintain their activity and stability [8]. However, this leads to the formation of undesirable sulfur-containing products as well, which does not accord with the demand on the clean fuel. Noble metal catalysts possess good performance for deoxygenation [4,5]. However, its high price limits their application on a large scale. Metallic Ni-based catalysts are also active for decarboxylation/decarbonylation. However, they also have high activity for methanation and cracking [7,9], which brings about large amounts of hydrogen consumption and low carbon yield of the desirable products. As new types of hydrodeoxygenation catalysts, transition metal carbide, nitride and phosphide catalysts have been extensively studied [10,11] due to their uses in sulfur-free conditions. Recently, transition metal phosphide catalysts have received the widespread attention in hydrotreating processes [12,13]. Also, they show unique performance in the deoxygenation of fatty acid esters [11,14]. In contrast to metallic Ni, transition metal phosphides have very lower activity for hydrogenolysis and methanation, which is favorable for enhancing carbon yield and reducing H2 consumption. Among the transition metal (Ni, Co, Fe, Mo and W) phosphides, Ni2 P and MoP are more active than Fe, Co and W phosphides. In addition, Ni2 P mainly gives the decarbonylation hydrocarbons, whereas MoP primarily yields the HDO hydrocarbons [14]. Compared with single metallic phosphide catalysts, bimetallic phosphide catalysts have a better and controllable relationship between their structure and activity [15]. In deed, our previous work [16] indicates that there is a strong interaction between Ni and Mo in the phosphide catalysts, which leads to interesting performance in the deoxygenation of methyl laurate, such as obvious change on the deoxygenation pathway of methyl laurate. It is well-known that changing the calcination temperature of precursor is an important method to alter the bimetallic interaction [17,18]. However, to our knowledge, the effect of calcination temperature of catalyst precursor on the property of Ni-Mo bimetallic phosphide catalysts has not been reported. In this study, the effect of calcination temperature of catalyst precursor on the structure and performance of SiO2 supported Ni-Mo bimetallic phosphides for the deoxygenation of methyl laurate was investigated. According to the characterization and test results, the structure-activity relationship of Ni-Mo bimetallic phosphide catalysts was analyzed. The finding may provide important information for tuning the interaction between Ni and Mo and developing catalyst to produce products with different hydrocarbon compositions to meet the practical demand. 2. Experimental 2.1. Catalyst preparation SiO2 -supported Ni-Mo bimetallic phosphide precursors were prepared by the incipient wetness impregnation method,

from which the phosphide catalysts were prepared by H2 temperature-programmed reduction (H2 -TPR). The detail of the catalyst preparation was described as follows. SiO2 was incipiently impregnated with an aqueous solution containing NH4 H2 PO4 , (NH4 )6 Mo7 O24 and Ni(NO3 )2 , whose pH value was adjusted to 1.87 using HNO3 , followed by drying at 120 ◦ C for 12 h. Afterward, the sample was divided into five parts which were calcined at 400, 500, 600, 700 and 800 ◦ C for 4 h, respectively. In the resulting precursors, the P/metal (Ni and Mo) molar ratios were 1.0. After that, the precursors were reduced from 20 to 650 ◦ C at a heating rate of 1 ◦ C·min−1 and maintained at 650 ◦ C for 3 h with a H2 (>99.9%) flow rate of 320 mL·min−1 per gram of the precursor. The prepared catalyst was cooled to room temperature under H2 flow and then passivated with a 0.5 vol% O2 /N2 flow at a flowrate of 320 mL·min−1 for 4 h. The mass ratio between Ni and SiO2 was 12%, and the Mo/Ni molar ratio was 1.0. The phosphide catalysts prepared from the precursors calcined at 400, 500, 600, 700 and 800 ◦ C were denoted as C400, C500, C600, C700 and C800, respectively. Additionally, NiO, MoO3 , nickel phosphate and molybdenum phosphate used in the ultraviolet and visible absorption spectra as references were also prepared. NiO and MoO3 were derived from the calcinations of Ni(NO3 )2 and (NH4 )6 Mo7 O24 at 500 ◦ C for 4 h, respectively. The aqueous solution of Ni(NO3 )2 or (NH4 )6 Mo7 O24 and NH4 H2 PO4 (nominal Ni(or Mo)/P ratio = 1.0) was vaporized and dried at 393 K for 12 h. The dried sample was then calcined at 500 ◦ C for 4 h to obtain nickel or molybdenum phosphate. 2.2. Catalyst characterization To distinguish the metal-oxygen species in the catalyst precursors, ultraviolet and visible diffuse reflectance spectra (UV-Vis DRS) were obtained using a Perkin-Elmer Lambda 750S UV-Vis-NIR spectrometer over a wavelength range of 200∼800 nm. The catalysts precursors were ground into a powder with a particle size less than 2 µm and loaded into the sample holder with BaSO4 as the white standard. The reducibility of the catalyst precursors was characterized by H2 temperature-programmed reduction (H2 -TPR) on a home-made instrument. 50 mg sample was loaded in a quartz U-shaped tube (inner diameter of 4 mm) and reduced in a 10 vol% H2 /N2 flow with a flowrate of 60 mL·min−1 at a heating rate of 10 ◦ C·min−1 . The hydrogen consumption was monitored by a thermal conductivity detector (TCD). X-ray diffraction (XRD) patterns were obtained on a D8 Focus powder diffractometer using Cu Kα radiation (λ = 0.1541 nm) operated at 40 kV and 40 mA. Transmission electron microscopy (TEM) images were obtained on a JEM-2100F instrument, operated at 200 kV. The powder sample was ultrasonically dispersed in alcohol and then deposited on the micrograte with carbon grid. CO chemisorption uptakes were used to measure the surface density of metal sites, using the same equipment as H2 TPR. 100 mg passivated catalyst was loaded in the reactor and

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reduced in a H2 flow (60 mL·min−1) at 450 ◦ C for 1 h. And then, the sample was flushed with a He flow (40 mL·min−1) at 450 ◦ C for 1 h to remove the hydrogen adsorbed on the surface. Afterward, it was cooled to 30 ◦ C. When the TCD signal was stable, pulses of CO (50 µL) were passed through the sample until the effluent areas of consecutive pulses were constant. Subsequently, the total dynamic CO uptake was calculated. H2 temperature-programmed desorption (H2 -TPD) was performed using the same apparatus as H2 -TPR. 100 mg 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 swept with a N2 flow (20 mL·min−1) until the TCD signal was stable. H2 -TPD was conducted at a heating rate of 15 ◦ C·min−1 . The desorbed H2 was detected by a TCD. Before the detector, the gas was passed through a trap containing solid NaOH to remove water. NH3 temperature-programmed desorption (NH3 -TPD) measurements were carried out with the same equipment as H2 -TPR. 70 mg 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 adsorbed NH3 . Subsequently, NH3 -TPD was implemented at a heating rate of 15 ◦ C·min−1 with a TCD to detect the desorbed NH3 . Before the detector, a trap containing solid NaOH was installed to remove water. To analyze the amount of desorbed NH3 quantitatively, the apparatus was calibrated by measuring the corresponding signal of the thermal decomposition of known amount of [Ni(NH3 )6 ]Cl2 . 2.3. Activity test The catalyst reactivity was tested on a continuous-flow stainless-steel fixed-bed reactor (inner diameter of 12 mm). 0.5 g passivated catalyst (0.15∼0.25 mm in diameter) mixed with 4 g quartz sand in the same size to dilute the catalyst was loaded in the reactor. 2 g quartz sand with diameter of 0.43∼0.85 mm was put on the catalyst bed to preheat the reactants. Before reaction, the passivated catalyst was re-reduced at 450 ◦ C for 1 h in a H2 flow (>99.9%, 100 mL·min−1). And then, the temperature and H2 pressure were adjusted to desired values. Subsequently, methyl laurate was fed into the reactor with a liquid micro pump. The weight hourly space velocity (WHSV) of methyl laurate was 10 h−1 and the H2 /methyl laurate molar ratio was 25. The liquid products were identified using gas chromatograph (GC) standards and the identification was further confirmed by an Agilent GC6890-MS5973N. The liquid products were analyzed on a SP-3420 gas chromatograph equipped with a flame ionization detector (FID) and a HP-5 capillary column (30 m×0.32 mm×0.5 µm). Tetrahydronaphthalene was used as an internal standard. The gas products (CO and CH4 ) were quantitatively analyzed on an on-line 102 GC equipped with a thermal conductivity detector (TCD) and a TDX-101 packed column. N2 was used as an internal standard. The mass balance ((the mass of liquid effluent from reactor per hour+the

mass of gaseous products per hour)/the mass of methyl laurate entered reactor per hour) was better than 95%. The conversion of methyl laurate (X) and the selectivity to product i (Si ) are calculated based on the following equations: X (%) = (n0 –n)/n0 ×100% Si (%) = ni /(n0 –n)×100% where, n0 and n denote the moles of methyl laurate in the feedstock and product, respectively, and ni is the mole of product i (such as n-undecane, n-dodecane and oxygenated intermediates). Additionally, the turnover frequency (TOF) of methyl laurate is calculated as follows: TOF (s−1 ) = Amount of converted methyl laurate per second (µmol·s−1 )/[CO uptake (µmol·g−1)×catalyst weight (g)] 3. Results and discussion 3.1. Catalyst characterization 3.1.1. UV-Vis DRS To obtain the coordination information around metal atoms of different metal-oxygen species in the catalyst precursors, UV-Vis spectroscopy was applied in a diffuse absorbance mode. The diffuse reflectance UV-Vis absorption spectra of the catalyst precursors and some references are shown in Figure 1. Various metal-oxygen species gave absorption bands in UV-Vis region due to ligand-metal charge transfer. For Mo phosphate and MoO3 (Figure 1(7) and (9)), there was a broad band between 230 and 440 nm assigned to Mo6+ -O charge transfer transitions, which can be ascribed to the overlap of the spectral regions of tetrahedrally coordinated species (230∼295 nm), octahedrally coordinated species (270∼330 nm) and connected molybdenum oxide centers (250∼290 nm) as reported in Ref. [19]. In contrast to MoO3 , Mo phosphate also gave a broad band above 500 nm, which may be attributed to octahedrally coordinated Mo6+ in phosphate. For NiO (Figure 1(8)), two strong adsorption bands at around 262 and 320 nm are due to Ni-O charge-transfer transitions, and the weak bands at about 419 and 720 nm are assigned to the octahedrally coordinated Ni2+ [20,21]. For Ni phosphate (Figure 1(6)), the adsorption bands between 400∼600 nm and above 700 nm are related to octahedrally coordinated Ni2+ in nickel phosphate [20]. On the basis of the above analysis, for the catalyst precursors (Figure 1(1)−(5)), the strong adsorption band around 260 nm is associated with the tetrahedrally coordinated Mo6+ , and the shoulder adsorption band at around 320 nm belongs to the octahedrally coordinated Mo6+ . The adsorption bands between 418 and 429 nm are related to the octahedrally coordinated Ni2+ in nickel phosphate. Additionally, the band above 500 nm is related to octahedrally coordinated Mo6+ as well as Ni2+ . With the calcination temperature rising from 400 to

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800 ◦ C, the adsorption band due to the octahedrally coordinated Ni2+ in nickel phosphate red-shifted and became obvious, indicating that there was a separation between Ni and Mo phosphates. In other word, there was a good mixture of Ni and Mo phosphates (perhaps partially forming solid solution) in the precursor calcined at low temperature. However, as the calcination temperature increased, Ni and Mo species in precursors were separated and their interaction became weaker and weaker. This is also confirmed by the following XRD results.

metal and phosphate increased and the metal phosphate particles were aggregated. This can account for that all the initial reduction temperature and the reduction peak shifted to high temperature.

Figure 2. H2 -TPR profiles of different catalyst precursors. (1) C400, (2) C500, (3) C600, (4) C700, (5) C800

3.1.3. XRD

Figure 1. UV-Vis absorption spectra recorded on reference substances and different catalyst precursors. (1) C400, (2) C500, (3) C600, (4) C700, (5) C800, (6) Ni phosphate, (7) Mo phosphate, (8) NiO, (9) MoO3

3.1.2. H 2 -TPR Figure 2 shows the H2 -TPR profiles of different catalyst precursors. The peak between 340∼554 ◦ C is due to the coreduction of NiO, nickel silicate and Mo6+ reduction to Mo4+ [22,23]. The main perk at about 554−685 ◦ C is attributed to the reduction of Mo4+ and Ni2+ in the phosphate [24]. The peaks appeared at higher 685 ◦ C belongs to the reduction of metal (Ni and Mo) phosphates [16]. As the calcination temperature of precursor increased, the reduction peaks shifted to high temperature, indicating that the reduction of precursor became difficult. This may be caused by the following reasons. First, as indicated by UV-Vis results, the interaction between Ni and Mo decreased as the calcination temperature increased. It is known that Ni species is more easily reduced than Mo species, while the reduced metallic Ni can dissociate H2 and subsequently the reactive atomic hydrogen spills over and reduces the adjacent Mo species. However, as the interaction between Ni and Mo decreased due to high calcination temperature, the promoting role of Ni for the reduction of Mo species became small. Second, what is more important may be the sintering of metal phosphate particles. With increasing the calcination temperature, the interaction between

XRD patterns of the as-prepared catalysts are shown in Figure 3. The broad peak between 15o and 35o is associated to amorphous silica. Both Ni2 P (2θ = 40.7o, 44.6o, 47.3o and 54.1o, PDF#65-3544) and MoP (2θ = 32.0o , 43.0o and 57.1o, PDF#65-6487) phases were detected in all the catalysts, while NiMoP2 (2θ = 31.0o, 45.0o and 55.1o, PDF#33-0927) phase existed only in C400 and C500. Obviously, the crystalline phases of the catalyst are affected by the calcination temperature of the precursor. Relative to standard peaks of Ni2 P, the peaks due to Ni2 P for C400, C500 and C600 were at the low angle, indicating that Mo atom entered Ni2 P lattice by a homogeneous replacement of Ni atom to form solid solution because the radius of Mo atom (0.145 nm) is larger than that of Ni atom (0.135 nm). Also, in the order of C400, C500 and C600, the peaks due to Ni2 P shifted to the standard position, indicating that the amount of Mo entered Ni2 P lattice became less. For C700 and C800, the diffraction peaks due to Ni2 P were located at the standard position and no NiMoP2 phase was found. This is consistent with the above-mentioned UV-Vis results, that is, the seperation of Ni and Mo species at high calcination temperature led to the formation of separated Ni2 P and MoP. Combining the UV-Vis and XRD results, it can be concluded that the interaction between Ni and Mo in the precursors as well as in the catalysts became less as the calcination temperature of precursor increased. Additionally, the average Ni2 P particle sizes were calculated using Scherrer equation (Table 1). The particle sizes of both MoP and NiMoP2 could not be calculated accurately because their diffraction peaks are not well separated from others. The average Ni2 P particle size increased in the sequence

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of C500
Figure 3. XRD patterns of the as-prepared catalysts. (1) C400, (2) C500, (3) C600, (4) C700, (5) C800

3.1.4. TEM Figure 4 shows the TEM images and corresponding phosphide particle size distributions of different catalysts. The dark spherical particles in TEM images are assigned to metal phosphides [16]. The crystal lattice fringe due to metal phosphide was clearly observed in the magnified TEM images. For C400 and C500, the phosphide particle sizes mainly distributed in the range of 2−6 nm, while C500 had smaller particle size. For C600, the phosphide particle size ranged from 4 to 13 nm, and the average diameter was about 7 nm. For C700 and C800, the phosphide particles mainly distributed between 11 and 23 nm, while C800 had larger particles. The average particle sizes of the catalysts are also

shown in Figure 4. In short, with increasing the calcination temperature of precursors, the metal phosphide particle sizes in the prepared catalysts followed the sequence: C500
Table 1. Ni2 P particle size, CO uptake and H2 desorption, acid amount and TOF of different catalysts Catalysts C400 C500 C600 C700 C800 a d

Ni2 P sizea (nm) 3.5 2.7 7.8 18.8 37.4

CO uptake (µmol·g−1 cat ) 48 58 25 − −

TOFb (s−1 ) 0.20 0.17 0.40 − −

TOFc (s−1 ) 0.25 0.21 0.45 − −

Relative H2 desorption amountd (µmol·g−1 cat ) below 400 ◦ C above 400 ◦ C total 5.1 8.1 13.3 6.0 8.2 14.3 5.9 5.9 11.8 2.4 3.9 6.3 1.0 3.0 4.0

Acid amounte (µmol·g−1 cat ) 443 557 300 129 43

Calculated by Scherre equation based on the Ni2 P(111) reflection; b At reaction temperature of 300 ◦ C; c At reaction temperature of 320 ◦ C; The H2 desorption amount of C800 below 400 ◦ C was designated as 1.00; e Obtained from NH3 -TPD

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Figure 4. TEM images and correspongding phosphide particle size distributions on different catalysts. (a) C400, (b) C500, (c) C600, (d) C700, (e) C800

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C500>C400>C600>C700>C800. This is consistent with the results of CO uptake and H2 -TPD. It is reasonable that the metal sites can adsorb both CO (or H2 ) and NH3 , and POH and Mo-OH groups also contribute to the spilt-over hydrogen [16].

Figure 5. H2 -TPD profiles of C400 (1), C500 (2), C600 (3), C700 (4), C800 (5)

consistent with that of CO uptake. In addition, with the calcination temperature of catalyst precursor rising from 400 to 800 ◦ C, the amount of spilt-over hydrogen species decreased, which is closely associated to the increasing tendency in the average particle sizes. On the whole, the smaller the phosphide particle size, the more amounts of adsorbed H2 on metal sites and the spilt-over hydrogen are found. 3.1.7. NH 3 -TPD Figure 6 presents the NH3 -TPD profiles of different catalysts. As indicated in our the previous work [16], there are many different acid sites on Ni-Mo bimetallic phosphide catalysts such as P-OH, Mo-OH, Niδ+ (0<δ<1), Moδ+ (0<δ<4) as well as Mon+ (n = 4 and 6). In general, POH and Mo-OH groups belong to Brønsted acid sites, while Niδ+ , Moδ+ as well as Mon+ are Lewis acid sites. The main peak at about 230 ◦ C is mostly due to P-OH as well as Mo-OH [27–29], while the shoulder peak at higher temperature may be assigned to the unreduced molybdenum species (Mo4+ and Mo6+ ) and the reduced metal sites with positive charge (such as Niδ+ and Moδ+ ) [30]. As shown in Table 1, the total acid amount followed the order of

Figure 6. NH3 -TPD profiles of different catalysts. (1) C400, (2) C500, (3) C600, (4) C700, (5) C800

3.2. Catalyst activity evaluation Catalytic performances of the deoxygenation of methyl laurate were carried out over different catalysts. The main liquid products were n-dodecane (n-C12), n-undecane (n-C11) and the oxygenated intermediates, such as lauric acid, lauraldehyde and lauryl alcohol and lauryl laurate. In addition, there were trace alkenes, isomeric hydrocarbons and cracked hydrocarbons (C6–C10). n-C12 and n-C11 hydrocarbons were produced from the decarbonylation and hydrodeoxygenation (HDO) pathways, respectively. The HDO pathway includes several continuous and consequent reactions as follows: methyl laurate → lauric acid → lauraldehyde → lauryl alcohol → C12 hydrocarbons [14,31]. Not only methyl laurate but the oxygenated intermediates can also be transformed to C11 hydrocarbons via decarbonylation [14]. As reported in our previous studies [23], the active sites on the phosphide

Scheme 1. Proposed reaction pathway of methyl laurate

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catalyst surface include metal sites (Niδ+ , Moδ+ ) and Brønsted acid sites, especially the metal ones are more important. The deoxygenation reaction of methyl laurate can be briefly summarized as follows: (1) hydrogenation, hydrogenolysis and decarbonylation mainly occurred on Niδ+ and Moδ+ metal sites [14,16]; (2) hydrolysis, dehydration and esterification reaction mainly took place on Brønsted acid sites. Therefore, based on the previous work [14], the deoxygenation pathway of methyl laurate is proposed, as shown in Scheme 1. 3.2.1. Activity Figure 7 presents the conversions of methyl laurate on different catalysts at 300, 320 and 340 ◦ C. It is clear that with the increase of reaction temperature the conversion of methyl laurate increased. If the reaction temperature was as high as 340 ◦ C, the difference in conversion of methyl laurate over different catalysts was not obvious. However, if the reaction temperatures were at 300 and 320 ◦ C, respectively, the conversions of methyl laurate decreased in the following sequences: C600>C500>C400>C700>C800. This is not consistent with the CO uptake or H2 -TPD. That is, although C500 and C400 had larger CO uptakes, they did not give higher conversions than C600. The TOFs of methyl laurate on C400, C500 and C600 were also calculated based on the CO uptakes. As shown in Table 1, TOF increased in the order of C500, C400 and C600, indicating that the larger the phosphide particles were, the higher the TOF was. This is consistent with our previous work reported in Ref. [31].

Figure 7. Methyl laurate conversion under different reaction temperatures on different catalysts

3.2.2. Product selectivity Figure 8 illustrates the total selectivity to C11 and C12 hydrocarbons (SC11+C12 ) and the C11/C12 molar ratio on different catalysts. As the calcination temperature of precursor increased, SC11+C12 tended to decrease. The C11/C12 molar ratio indicates the selectivity between the decarbonylation and HDO pathways [14]. As shown in Figure 8,

with the increase of the reaction temperature the decarbonylation pathway was favored and the HDO pathway was suppressed. It is reasonable since the decarbonylation is thermodynamically endothermic and HDO pathway is thermodynamically exothermic, respectively [32]. At any temperature, the C11/C12 ratios on C700 and C800 were larger than 1.0, while those on C400, C500 and C600 were less than 1.0, implying that the decarbonylation pathway was in majority on C700 and C800, while the HDO pathway mainly took place on C400, C500 and C600. In addition, the C11/C12 ratio followed the sequence: C700≈C800>>C600>C500>C400. It seems that the catalyst prepared from higher precursor calcination temperature will more favorably catalyzed the decarbonylation pathway. The factors influencing deoxygenation pathways include the kinds of Ni-Mo bimetallic phosphide phases and the interaction between Ni and Mo. As indicated by the XRD patterns, in C400 and C500, Ni2 P, MoP and NiMoP2 existed and Mo atoms was entered in Ni2 P lattice during the precursor calcination. Although no NiMoP2 could be detected in C600, Mo atoms also entered in Ni2 P lattice as reflected by the XRD result. Therefore, there was an interaction between Ni and Mo and the extent of interaction was in the order of C400
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step during the HDO pathway. As shown in Scheme 1, lauryl alcohol could be converted to dodecene via dehydration, to lauryl laurate via esterification with lauric acid, to C11 hydrocarbons via dehydrogenation followed by decarbonylation, or to C12 hydrocarbons via C-O direct hydrogenolysis [14]. Among them, the reactions on the acid sites are much easier. As the calcination temperature of precursor rose, the acid amount of catalyst decreased. This is not favorable to the further conversion of lauryl alcohol [16]. 4. Conclusions

Figure 8. Changes of (C11+C12) selectivities and molar ratios of C11/C12 with reaction temperature over different catalysts

The selectivity to the oxygenated intermediates (Soxy ) is presented in Figure 9. As the reaction temperature increased, Soxy on different catalysts declined obviously. This is attributed to the enhanced deoxygenation activity of the catalyst at high reaction temperature. As the calcination temperature of precursor increased, Soxy increased. Associated with the catalyst activity, the more active the catalyst was, the less the Soxy gave. The main oxygenated intermediates were lauryl alcohol and lauryl laurate. For instance, at the reaction temperature of 300 ◦ C, the selectivities to lauryl alcohol and lauryl laurate on C800 were about 17.4% and 5.7%, respectively, while the total selectivity to lauric acid and lauraldehyde was about 1.2%. It is well-known that lauraldehyde is very reactive which can be easily hydrogenated to form laurayl alcohol. The largest selectivity to lauryl alcohol indicates that the conversion of laurayl alcohol may be the rate-determining

Figure 9. Changes of selectivity to oxygenated intermediates with temperature over different catalysts

H2 -TPR results reveal that as the precursor calcination temperature increased, the reduction of precursor became difficult because of the decreased interaction between Ni and Mo in the precursors and the sintering of metal phosphate particles. For phosphide catalysts, the phosphide particle size, surface metal site density and the acid amount decreased in the sequence of C500>C400>C600>C700–C800. In the deoxygenation of methyl laurate, C600 showed the best performance. TOF tended to increase as the phosphide particles became large. With increasing the precursor calcination temperature, the C11/C12 ratio on the phosphide catalyst increased. This is mainly ascribed to the reduced interaction between Ni and Mo. Moreover, for C400 and C500, there may be a synergism between Ni and Mo sites on NiMoP2 as well as the Mo-entered Ni2 P phase for the HDO pathway. In a word, the structure and subsequent performance of Ni-Mo bimetallic phosphide catalyst can be regulated by controlling the calcination temperature of the precursor. This finding may give valuable information for designing more effective deoxygenation catalyst to meet the demand in the practice of industry. References [1] Kumar P, Yenumala S R, Maity S K, Shee D. Appl Catal A, 2014, 471: 28 [2] S¸enol O ˙I, Ryymin E M, Viljava T R, Krause A O I. J Mol Catal A, 2007, 277(1-2): 107 [3] Toba M, Abe Y, Kuramochi H, Osako M, Mochizuki T, Yoshimura Y. Catal Today, 2011, 164(1): 533 [4] Centeno A, Maggi R, Delmon B. Stud Surf Sci Catal, 1999, 127: 77 [5] Wildschut J, Mahfud F H, Venderbosch R H, Heeres H J. Ind Eng chem Res, 2009, 48(23): 10324 [6] Zhang X H, Wang T J, Ma L L, Zhang Q, Jiang T. Bioresource Technol, 2013, 127: 306 [7] Zuo H L, Liu Q Y, Wang T J, Ma L L, Zhang Q, Zhang Q. Energy Fuels, 2012, 26(6): 3747 [8] Kubiˇcka D, Hor´acˇ ek J. Appl Catal A, 2011, 394(1-2): 9 [9] Wu S K, Lai P C, Lin Y C, Wan H P, Lee H T, Chang Y H. ACS Sustain Chem Eng, 2013, 1(3): 349 [10] Boullosa-Eiras S, Lødeng R, Bergem H, St¨ocker M, Hannevold L, Blekkan E A. Catal Today, 2014, 223: 44 [11] Yang Y X, Ochoa-Hern´andez C, de la Pe˜na O’Shea V A, Coronado J M, Serrano D P. ACS Catal, 2012, 2(4): 592 [12] Prins R, Bussell M E. Catal Lett, 2012, 142(12): 1413

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