Effect of support on the NiMo phase and its catalytic hydrodeoxygenation of triglycerides

Fuel 159 (2015) 430–435

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Effect of support on the NiMo phase and its catalytic hydrodeoxygenation of triglycerides Hao Chen a, Qingfa Wang a,b,⇑, Xiangwen Zhang a,b, Li Wang a,b a b

Key Laboratory of Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 The support interacted with the

metals and affected the valence.  NiMo/SAPO-11 has more NiMoO4 and

MoO3 sulfides than NiMo/Al2O3.  NiMo/SAPO-11 decarboxylates more

than 40% of jatropha oil.  NiMo/Al2O3 hydrodeoxygenates

nearly 62% of jatropha oil.

a r t i c l e

i n f o

Article history: Received 31 March 2015 Received in revised form 30 June 2015 Accepted 2 July 2015 Available online 9 July 2015 Keywords: Acidity of support NiMo phase Triglycerides Decarboxylation Hydrodeoxygenation

a b s t r a c t The support effect on the NiMo phase was studied by using two kinds of mesoporous materials, SAPO-11 and Al2O3, which were applied on the hydroconversion of jatropha oil. It was shown that the amounts of strong acidic sites of the NiMo/SAPO-11 catalyst (610 lmol/g) was higher than that of NiMo/Al2O3 catalyst (190 lmol/g), resulting into the different NiMo phase formed on the respective SAPO-11 and Al2O3. The NiMo/SAPO-11 catalyst with larger NiMoO4 and MoO3 sulfided phases showed higher selectivity for decarboxylation reaction (41.2%), while the NiMo/Al2O3 catalyst with more Mo4+ polymeric octahedral sulfided phases led to main hydrodeoxygenation products (62.5%). Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Currently, the necessity of renewable energy sources is highlighted by the exhaustion of fossil resources and the serious environmental problems associated with fuel sources [1,2]. Biofuels derived from biomass (such as triglycerides) have been ⇑ Corresponding author at: Key Laboratory of Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China. Tel./fax: +86 22 27892340. E-mail address: [email protected] (Q. Wang). http://dx.doi.org/10.1016/j.fuel.2015.07.010 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.

considered as a sustainable substitute for fossil resources [3,4]. The hydroconversion of triglycerides to hydrocarbons has attracted great attention due to its green nature and cleanness [5,6]. This conversion includes the following three reaction pathways: (1) hydrodeoxygenation (HDO), (2) decarbonylation (DCO) and (3) decarboxylation (DCO2) [7,8]. Large amount of previous studies have focused on tailoring selectivity for certain reaction pathway [9–14]. It has been shown that the support plays an important role not only for cracking and isomerization but also for selectivity in the deoxygenation pathways. For achieving a controllable reaction pathway, many supports include Al2O3, SiO2, TiO2, mixed oxides,

H. Chen et al. / Fuel 159 (2015) 430–435

zeolites, mesoporous silica based supports, carbons, MCM-41, SAPO-11, etc. have been extensively investigated [10,15–20]. Kubicˇka et al. [10] investigated the effects of the active phase cluster size and pore size distribution of the catalysts on the hydrotreatment of rapeseed oil using SiO2, TiO2 and Al2O3 as support. It was found that the large cluster size of the active phase and the broad pore size distribution contributed to the HDO pathway. SBA-15 and hydrotalcite were also investigated as supports for the deoxygenation of triglycerides compared with SiO2, TiO2 and Al2O3 [19]. The high selectivity of the HDO reaction was attributed to the high specific surface area and organized structure. Gong et al. [13] found that the total acidity of the catalyst had a positive effect on the DCO2 and/or DCO (DCOx) pathways. In particular, the presence of strong acidic sites is beneficial for DCO2 reaction [21]. Verma et al. [22] studied the hydroprocessing of jatropha oil over the hierarchical SAPO-11 with different acidities, finding that the increased acidity in catalyst increased the DCO2 and/or DCO reaction selectivity. Although many supports have been investigated, the nature of support effect on deoxygenation reaction is still unclear. The active metal in the catalysts is well known to influence the deoxygenation pathways [14,23,24]. In our previous work, we found that the deoxygenation pathways mainly depended on the active phase [25]. The distribution of the active metal is suspected to be affected by the acidity of support. In this work, SAPO-11 and Al2O3 will be used as supports for NiMo with the aim of investigating the effect of support acidity on the active metal phase that is anticipated to lead to different deoxygenation selectivity. 2. Materials and methods 2.1. Materials (NH4)6Mo7O24
4H2O (P99.0 wt%, J&K) and Ni(NO3)2
6H2O (P98.0 wt%, Alfa Aesar) were used as the Mo and Ni precursors, respectively. The SAPO-11, Al2O3 and cyclohexane were used as received. Jatropha oil was purchased from Jiangsu Donghu Bioenergy Co., Ltd, the physical and chemical properties of which are summarized in Table 1. 2.2. Catalyst preparation NiMo/SAPO-11 and NiMo/Al2O3 (4 wt% Ni and 12 wt% Mo) were synthesized using the incipient wetness co-impregnation method. The supports (SAPO-11 and Al2O3) were calcinated at 450 °C and then dried at 120 °C in vacuum overnight to remove the impurities in the supports. Then the supports (SAPO-11 and Al2O3) were immersed in an aqueous solution of (NH4)6Mo7O24
4H2O and Ni(NO3)2
6H2O and kept overnight at room temperature to achieve sufficient impregnation. After drying at 120 °C for 12 h, the catalysts were obtained by calcination at 450 °C (5 °C/min) for 4.5 h.

431

2.3. Catalyst characterization The textural properties and pore size distribution of the two supports were determined by N2 adsorption and desorption isotherms using Micromeritics ASAP 2020. All of the samples were outgassed in vacuum for 24 h. The mesopore size distributions were obtained using the Barrett–Joyner–Halenda (BJH) algorithm. Ammonia temperature programmed desorption (NH3-TPD) and hydrogen temperature programmed reduction (H2-TPR) were performed on a Chemisorption Physisorption Analyzer (AMI-300, Altamira Instruments) equipped with a thermal conductivity detector (TCD). For NH3-TPD, after pre-treatment at 450 °C (10 °C/min) in He for 1 h, the adsorption of NH3 was controlled at 120 °C for 30 min. To remove the physically adsorbed NH3, the sample was subsequently purged with He for 2 h. NH3-TPD was monitored in the range of 100–600 °C (10 °C/min). In the H2-TPR experiments, the sample (100 mg) was pretreated at 450 °C (10 °C/min) in He for 1 h and then cooled to room temperature. A gas mixture of H2 (10%) + He (90%) was used as the reducing agent with a total flow rate of 30 mL/min. The samples were heated to 900 °C (10 °C/min) and maintained at this temperature for 30 min, and then the TPR profile was recorded. X-ray photoelectron spectroscopy (XPS) spectra were recorded using a PHI5000 VersaProbe. Binding energy was calibrated using adventitious carbon at 284.6 eV. 2.4. Catalytic experiments The hydroconversion of jatropha oil was conducted in a fixed-bed flow reactor (1.2 cm I.D. and 45 cm in length). The reaction temperature was controlled by three thermocouples on the reactor wall and monitored with a thermocouple in the catalyst bed. Catalyst (10 g) was loaded and fixed with SiC to obtain a sufficient catalyst-bed length in the reactor. Jatropha oil (25 wt% in cyclohexane) was used as the feedstock and supplied at a flow rate of 0.8 mL/min with a high-pressure pump. The experiments were carried out at 380 °C under 3 MPa in a flowing of H2 at 400 mL/min. All catalysts were presulfided in situ at 320 °C and 3.0 MPa for 4 h using CS2 (3.0 wt% in cyclohexane) under H2 atmosphere prior to each experiment. The gaseous products were analyzed using an Agilent 3000 gas chromatograph equipped with three columns (molecular sieve, plot U and alumina) and TCD detectors. The organic liquid products (OLPs) were qualitatively determined using gas chromatography/ mass spectroscopy (GC/MS) (Agilent 6890N gas chromatograph coupled with an Agilent 5975N mass spectrometer). A gas chromatograph (Agilent, 7890A) equipped with a flame ionization detector (FID) and a commercially column (PONA, 50 m  0.2 mm  0.5 lm) was used to quantitatively analyze the hydrocarbons using tetracosane as the internal standard. 3. Results and discussion 3.1. Catalysts properties

Table 1 Physical and fatty acid composition of jatropha oil. Jatropha oil

Percentage

Free fatty acid content (wt%) Glyceride content (wt%) Fatty acid composition (wt%) Stearic acid (C18:0) Oleic acid (C18:1) Linoleic acid (C18:2) Palmitic acid (C16:0) Other acids

8.3 91.7 6.9 40.1 36.3 14.7 2.0

The pore size distributions of the supports are shown in Fig. 1 and the textural properties of the supports are summarized in Table 2. It can be seen that the Al2O3 and SAPO-11 showed a similar mesopore size distribution centered at 3–4 nm (Fig. 1). The specific surface area of Al2O3 and SAPO-11 calculated from Brunauer–Em met–Teller (BET) method were 141.9 m2/g and 94.6 m2/g, respectively. The acidity of the two catalysts was obtained by NH3-TPD. As shown in Fig. 2, The NH3-TPD profiles were fitted into three peaks corresponding to the weak, medium, and strong acid sites [13] and the detail properties are shown in Table 3. For

432

H. Chen et al. / Fuel 159 (2015) 430–435 0.040

Table 3 H2-TPR and NH3-TPD characteristic data for the NiMo catalysts.

Al2O3 SAPO-11

0.035

Catalyst

Acidity (lmol/g) Weak and/or Medium

Strong

Peak 1

Peak 2

Peak 3

NiMo/Al2O3 NiMo/SAPO-11

190 360

190 610

1010.4 584.2

782.8 826.8

864.7 1422.9

0.030

dV/dD

0.025

H2 consumption (lmol/g)

0.020 0.015 40

0.010 0.005

MoO3

30

0.000 2

4

6

8

10

Signal a.u.

0

Pore size (nm) Fig. 1. Pore size distribution of the mesoporous supports.

NiMo/SAPO-11

20

NiMo 4 6+ Mo

10

Table 2 Physicochemical properties of the mesoporous supports. SBETa (m2 g Al2O3 SAPO-11 a b

141.9 94.6

1

)

Vtotalb (cm3 g

1

)

0.21 0.11

4+ Mo

NiMo/Al2O3 dave (nm)

Vmicro

4.0 4.1

– 0.03

b

0

200

400

600

800

o

Temperature ( C)

Specific surface area. Pore volume.

Fig. 3. H2-TPR results for the NiMo-supported catalysts.

NiMo/Al2O3 catalyst, the amount of weak and/or medium acidic sites (190 lmol/g) was equivalent to that of strong acidic sites (190 lmol/g). In the case of NiMo/SAPO-11 catalyst, the weak and/or medium and strong acidity were about two, three times higher than that of NiMo/Al2O3, respectively. The TPR results for two catalysts are shown in Fig. 3. The H2-TPR profile of each catalyst exhibited three main reduction peaks, which are related to the reduction of Mo6+ to Mo4+ for the polymeric octahedral Mo species, the NiMoO4 -like phase and the MoO3 species [26]. The reduction peak of NiMoO4 and the MoO3-like phase for the NiMo/SAPO-11 catalyst were higher than the NiMo/Al2O3 catalyst, indicating that the amount of NiMoO4 and MoO3 species in NiMo/SAPO-11 was larger (Fig. 3 and Table 2). However, the Mo4+ polymeric octahedral phase was the dominant component in the NiMo/Al2O3 catalyst. According to the characterization of BET and NH3-TPD, it was found that the major difference between the two catalysts was the acidity of the support. It has been [27] reported that the interaction between

the support and metal increased as the acidity of the support increased. Thus, the higher acidity of SAPO-11 led to stronger interactions with the NiMo, forming much more NiMoO4 and MoO3 active phase. The chemical structures and surface chemical states were further investigated by XPS. The Mo3d and Ni2p XPS spectra of the catalysts are shown in Fig. 4. The binding energies and the atom distributions are summarized in Table 4. Mo signals were fitted into two peaks, which corresponded to the IV, V oxidation state, respectively [28,29]. The Ni 2p core level spectra (Fig. 4(c) and (d)) were reasonably fitted into two peaks at 856.2 eV and 862.0 eV corresponding to the Ni 2p3/2 level [30,31]. The main binding energy peak of Ni 2p3/2 at 862.0 eV was a signature of II oxidation state [30]. From Table 4, it was observed that the amount of NiII in the two catalysts was similar, while the Mo distribution was different. For NiMo/SAPO-11 catalyst, the MoV (88%) species were predominant, which were attributed to the presence of some difficult-to-reduce species formed by

NiMo/Al 2O3

TCD signal (a.u.)

TCD signal (a.u.)

NiMo/SAPO-11

200

300

400

500

600

100

200

300

Temperature (°C) Fig. 2. NH3-TPD results for the NiMo-supported catalysts.

400

500

Temperature (°C)

600

700

800

433

H. Chen et al. / Fuel 159 (2015) 430–435 V

Mo 3d5/2

V

Mo 3d5/2 Mo-Al2O3

Mo-SAPO-11 V

Mo 3d3/2

V

Mo 3d3/2

IV

Mo 3d3/2 IV

Mo 3d5/2

IV

Mo 3d3/2

IV

Mo 3d5/2

225

230

235

240

225

230

Binding energy eV II

240

II

Ni

Ni-Al2O3

235

Binding energy eV Ni Ni-SAPO-11

0 855

860

865

850

855

Binding energy eV

860

865

Binding energy eV

Fig. 4. XPS results for the NiMo-supported catalysts.

Table 4 XPS parameters of the different contributions of Mo3d and Ni2p obtained for the oxide NiMo catalysts.

a b

Binding energies (eV) IV

DCO

50 V

II

Mo 3d

Mo

Mo

Ni 2p3

Ni

NiMo/Al2O3

232.4 232.5

232.4 (0.73)a 232.6 (0.88)a

856.0

NiMo/SAPO-11

231.1 (0.27)a 231.2 (0.12)a

856.1 (0.64)b 856.2 (0.58)b

856.2

Atomic ratios, calculated from total Mo atom. Atomic ratios, calculated from total Ni atom.

mol %

Sample

HDO DCO2

60

40 30 20 10

the strong interaction with the support, in agreement with the H2-TPR results (Fig. 3). Interestingly, the atomic content of MoIV in the NiMo/Al2O3 catalyst was about two times higher (ca. 27%) then NiMo/SAPO-11 (12%), resulting from the weaker interaction between Mo species and Al2O3 support. 3.2. Hydrotreatment of jatropha oil The catalytic performances of the sulfided NiMo catalysts were investigated on the hydroconversion of jatropha oil, and the result is shown in Fig. 5. Full conversions were achieved for all the catalysts under given conditions, which was in line with previous reports [16,32]. For the two catalysts, the HDO and DCOx reactions occurred simultaneously (Fig. 5) due to the incorporation of both Ni and Mo [14], but with different selectivity of reaction pathways. The ratio of C17/C18 was usually used to estimate the selectivity of the deoxygenation pathways, in particular selectivity of HDO [19]. For the NiMo/Al2O3 catalyst, the C17/C18 ratio was approximately 0.6, indicating that the main deoxygenation of jatropha oil is HDO reaction (62.5%) with higher consumption of hydrogen. In

0

NiMo/SAPO-11

NiMo/Al2O3

Fig. 5. Deoxygenation results for the NiMo-supported catalysts.

contrast, the products of SAPO-11 supported catalyst showed higher C17/C18 ratio (1.8), indicting lower HDO selectivity (37.7%) and higher DCOx selectivity (62.3%). During deoxygenation, the methanation reactions were much slower than deoxygenation and could be ignored in the fixed-bed flow reactor under the experimental conditions [13]. Thus, the selectivity of DCO2 and DCO can be evaluated by CO2/CO. The CO2/CO ratio of 2.0 for NiMo/SAPO-11 catalyst indicated that the DCO2 reaction (41.2%) was preferred compared with NiMo/Al2O3 (18.8% of DCO2 selectivity). Above all, the main pathway for NiMo/Al2O3 catalyst was HDO, while the DCO2 was much more favored for NiMo/SAPO-11. In this work, the effect of support on the distribution of the active phase was investigated. The oxidation state of molybdenum

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H. Chen et al. / Fuel 159 (2015) 430–435

Table 5 The product distribution of the hydroconversion of jatropha oil for the two catalysts.

NiMo/SPAO-11 NiMo/Al2O3

C (%)

SC9-15 (%)

SC15-18 (%)

YOLP (%)

i/n

94.4 100.0

24.5 12.3

38.6 93.5

87.8 90.1

2.3 0.1

NiMo/SAPO-11 NiMo/Al2O3

40 35

Selectivity %

4. Conclusion In summary, the support played an important role in the fatty acid hydroconversion process in catalysts. SAPO-11 and Al2O3 supported NiMo were used as catalysts in the hydroconversion of jatropha oil. In regard to deoxygenation, the NiMoO4 and MoO3 sulfided phases led to the preferential DCO2 reaction, while the Mo4+ sulfided phase favored the HDO reaction. As a result, the NiMo/SAPO-11 catalyst showed high selectivity for the formation of hydrocarbons by DCOx, especially DCO2. At the same time, the NiMo/Al2O3 catalyst produced more HDO products. Because of the lack of strong acid sites and the zeolite-like pore structure, the product distributions obtained from NiMo/Al2O3 were mainly diesel oil fractions with low isomerization.

C, conversion of the triglycerides. S, selectivity of the products.

45

of n-paraffin [38]. Thus, the high concentration of strong acid in SAPO-11 gave a higher iso/n-paraffin ratio.

30 25 20 15

Acknowledgements

10 5 0

7

8

9

10

11

12

13

14

15

16

17

18

Carbon number Fig. 6. Product distribution selectivity of the paraffins for the two catalysts.

exhibited a dramatic change in the different NiMo catalysts. From the result, it was found that the amount of the NiMoO4 and MoO3 species were higher in NiMo/SAPO-11 due to the high acidity while the amount of Mo4+ species was more in NiMo/Al2O3. The larger the amount of the NiMoO4 and MoO3 sulfided active phase was, the higher the DCO2 activity of the catalyst showed. In contrast, NiMo/Al2O3 with more Mo4+ species showed higher HDO selectivity. So the Mo4+ sulfided species contributed to the better catalytic performance of HDO reaction and the NiMoO4 and MoO3 sulfided species preferred the DCO2 reaction. The acidity of the support has a significant effect on the hydrocracking and isomerization of the products. In this work, experiments were conducted under the same condition and the results are shown in Table 5 and Fig. 6. The acidity, specifically the strong acidity, was much lower for NiMo/Al2O3 than that of NiMo/SAPO-11. The product distribution for NiMo/Al2O3 was mainly in the range from C15 to C18 hydrocarbons (93.5%), which were comprised of n-C17 (30.6%) and n-C18 (44.6%) hydrocarbons. Under the same conditions, the NiMo/SAPO-11 catalyst gave high selectivity in the cracking fraction hydrocarbons due to the large number of acid sites, specifically the strong acid sites [13,33]. The yield of OLPs was also decreased to 87.8% (90.1% for NiMo/Al2O3). The isomerization is another significant reaction required to obtain excellent fuel properties, and the results of C5–C18 hydrocarbons isomerization are listed in Table 5. Significant differences were observed with the catalysts using different types of supports. The NiMo/Al2O3 catalyst had lower amount of acidic sites, and thus fewer isomerization products (i/n  0.1) were obtained. The SAPO-11 supported catalyst exhibited higher selectivity for isomerization (i/n  2.3) compared with the NiMo/Al2O3 catalyst. Among the zeolites, SAPO-11 is known for its high selectivity for hydroisomerization because the SAPO-11 is a tubular 10-MR zeolite and its appropriate acidity (number and strength distribution) [34–37]. Recently, it was reported that isomerization was more favored by weak acid [28]. However, Iliopoulou et al. suggested that strong acid sites are acquired during hydroisomerization because the acidity should be strong enough for carbonium ions, which are suitable for stable branching

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