HZSM-5 catalysts: Methyl hexadecanoate as the model compound

Catalysis Communications 20 (2012) 80–84

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Short Communication

Hydrodeoxygenation of vegetable oils to liquid alkane fuels over Ni/HZSM-5 catalysts: Methyl hexadecanoate as the model compound Na Shi, Qi-ying Liu ⁎, Ting Jiang, Tie-jun Wang ⁎⁎, Long-long Ma, Qi Zhang, Xing-hua Zhang Key Laboratory of Renewable Energy and Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou, PR China

a r t i c l e

i n f o

Article history: Received 19 September 2011 Received in revised form 10 January 2012 Accepted 12 January 2012 Available online 20 January 2012 Keywords: Hydrodeoxygenation Methyl hexadecanoate Biofuels Ni/HZSM-5 catalyst

a b s t r a c t Ni/HZSM-5 catalysts with different Ni loadings were prepared by impregnation method, and their catalytic performance was evaluated in the hydrodeoxygenation (HDO) upgrading of the model reactant methyl hexadecanoate (MHD) to liquid alkane fuels. The physicochemical properties of the catalysts were characterized by XRD, SEM, NH3-TPD, and BET. Ni loadings were obviously influenced by the acidity, and thus significantly affected their catalytic performance. Over 7 wt.% Ni/HZSM-5 catalyst, MHD conversion of more than 90% and total liquid C5–C16 hydrocarbon selectivity of 83% were obtained under the optimum reaction condition, possibly due to the synergistic effect of metal Ni and acid HZSM-5. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Biodiesel (Fatty Acid Methyl Esters, FAMEs) is presently used as an alternative for fossil fuels. However, because of C_C bonds and oxygen atoms, FAMEs generally show poor fuel performance such as low energy density, low chemical stability and limited miscibility with current fossil fuels [1–4]. For overcoming the above mentioned disadvantages, hydrodeoxygenation (HDO) upgrading of vegetable oils to obtain liquid alkanes called the 2nd generation fuels has attracted much attention [5–8]. The primary HDO catalysts were mainly composed of the supported noble metals Pt and Pd, and transition metals Mo, Ni–Mo, Co–Mo, Ni–W and Co–W [9–12]. Noble metal catalysts showed excellent catalytic performance, but their high cost limited their application on a large scale [13]. Mo or W modified with transition metal Co and Ni catalysts was highly active and stable for HDO of vegetable oil [14–16]. To maintain the activity and stability, however, introduction of extrinsic sulfur containing agents is necessary to keep the sulfurization of these catalysts during catalyst activation and HDO upgrading processes, which inevitably brought about sulfur pollution in final products [17]. As a result, to prevent sulfur contamination, exploring non-sulfided transition metal Co and Ni catalyst is of importance but still an open challenge. HZSM-5 has been widely applied in crude oil refinery and gas adsorption–separation industry due to its strong acidity as well as shape

⁎ Corresponding author. Tel.: + 86 20 87057789; fax: + 86 20 87057737. ⁎⁎ Corresponding author. Tel.: + 86 20 87057751; fax: + 86 20 87057737. E-mail addresses: [email protected] (Q. Liu), [email protected] (T. Wang). 1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2012.01.007

selectivity. The pristine HZSM-5 was previously used to hydrotreat vegetable oils for producing liquid alkane fuels, but significant amount of aromatics and severe carbon deposition were observed [18], while NiMo supported on HZSM-5, significantly cracking C1–C4 alkanes and dehydrogenated olefins were obtained [19,20]. Very recently, investigation in our group showed that transition metal supported HZSM-5 catalysts presented excellent performance in HDO of xylitol to gasoline [21]. However, this catalyst was rarely reported in HDO of vegetable oils to biofuels. In this work, we prepared Ni/HZSM-5 catalysts by wetness impregnation, and evaluated their catalytic performances in HDO of MHD to liquid alkane fuels dependent on the Ni loadings and the processing parameters. It was found that these catalysts showed the promising activity and selectivity to the liquid alkanes.

2. Experimental 2.1. Catalyst preparation The support for HZSM-5 (Si/Al = 38) was commercially purchased from the Catalyst Plant of Nankai University. The catalysts were prepared by impregnating HZSM-5 with aqueous solution of Ni (NO3)2·6H2O of concentration of 0.0089 mol/L (0.027, 0.044, 0.062 and 0.08 mol/L) to obtain the calculated Ni loading of 1 wt.% (3, 5, 7 and 9 wt.%), followed by stirring for 10 h at ambient temperature. Then, the suspension was evaporated and the solid that remained was dried at 393 K and calcined at 823 K for 3 h with a ramp of 10 K/min in the air. The catalysts were reduced at 723 K for 2 h with the heating rate of 10 K/min under 5 vol.% H2/N2 flow before use.

N. Shi et al. / Catalysis Communications 20 (2012) 80–84

2.2. Catalyst characterization

81

NiO

2.3. Catalyst performance The HDO experiments were performed in a 100 mL semi-continuous stainless autoclave equipped with a mechanical stirrer and a condensing tube at export. Typically, 3.0 g of freshly reduced catalyst and 30.0 g of MHD were loaded in the autoclave. After removing air by flowing Ar, the reaction system was pressurized with H2 and heated to a certain temperature for a certain period. For stability evaluation, the spent catalyst was washed with cyclohexane and dried to remove the cyclohexane residue after each run, and then reused for the next batch. The products were quantified by gas chromatography and identified by GC/MS. The gas products were collected at every 30 min and analyzed by GC 9800 chromatography equipped with a flame ionization detector and a packed column (Porapak-Q column, 3 m × 3 mm) for C1–C4 alkanes and dimethyl ether, and a thermal conductivity detector and a packed column (TDX-01 column, 3 m × 3 mm) for CO, CO2 and CH4. At the end of reaction, the liquid products were collected and analyzed by Agilent Technologies 4890 gas chromatography equipped with a flame ionization detector and a capillary column (DB-1701, 60 m × 0.25 mm× 0.25 μm). The conversion of methyl hexadecanoate (XA) and selectivity to product i (Si) was defined as: X A ðwt:%Þ ¼

w0 −w1  100% w0

ðAÞ

Si ðmol:%Þ ¼

ni  ai  100%: i P ni  ai

ðBÞ

1

Here w0 refers to the initial weight of methyl hexadecanoate, and w1 means the weight of final methyl hexadecanoate after reaction; ni refers to moles of the product i, and a is the carbon atom numbers for product i. 3. Results and discussion 3.1. Catalyst characterization The XRD patterns of Ni/HZSM-5 catalysts were shown in Fig. 1. Although the diffraction intensities of HZSM-5 support decreased with increasing Ni loadings, the diffraction peaks could be seen pronouncedly, indicating that the crystallographic structure of MFI was not largely altered after loading Ni and calcination. With Ni loadings below 3 wt.%, no diffractions corresponding to NiO crystallite were observed, probably due to that such small amounts of Ni loadings dispersed highly on the support with very tiny particles. As the nickel

f

Intensity (a.u)

X-ray powder diffraction (XRD) patterns of the catalysts were obtained using x'pert PRO MPD diffractometer (PANalytical) with a Cu Kα (λ = 0.15406 nm) radiation operated at 40 kV and 40 mA. Scanning electron microscopy (SEM) images were recorded on a Hitachi S4800 instrument operated at 10 kV. The BET surface area was determined by N2 isothermal adsorption using Micrometrics ASAP-2010 automated system. NH3 temperature-programmed desorption (NH3TPD) studies were carried out in a home-made instrument. Catalysts were pretreated in a flow of nitrogen at 873 K for 30 min, followed by cooling to 393 K, and then the catalysts were absorbed by NH3 until saturation state was reached. Physically adsorbed NH3 was removed by purging helium at 393 K. NH3-TPD profiles were recorded by programming the temperature from 393 K to 873 K at a ramp of 10 K/min. TG analyses of spent catalyst were implemented under an air flow rate of 30 mL/min by increasing the temperature from 313 K to 1013 K at 10 K/min.

e d c b a 10

20

30

40

50

60

70

80

2 theta(degree) Fig. 1. XRD patterns of various Ni/HZSM-5 catalysts: (a) HZSM-5, (b) 1 wt.% Ni/HZSM5, (c) 3 wt.% Ni/HZSM-5, (d) 5 wt.% Ni/HZSM-5, (e) 7 wt.% Ni/HZSM-5, and (f) 9 wt.% Ni/HZSM-5.

loadings increased to 5 wt.%, the characteristic diffraction peaks of NiO (2θ = 37.2°, 43.2° and 62.8°) were observed [22], and its diffraction intensities increased with the Ni loading up to 9 wt.%. Previous research showed that, for the supported Ni catalysts, the crystallite sizes of NiO varied depending on nickel loadings, and their sizes grew with the incremental nickel contents [23]. Apparently, these fortified diffraction peaks of NiO were certainly due to its increasing crystallite sizes as the nickel loadings increased. Fig. 2 showed the SEM images of Ni/HZSM-5 catalysts. The particle sizes of support HZSM-5 are presented as several micrometers. With all Ni loadings, the size of HZSM-5 remained almost unchanged, but the sizes of NiO crystallites increased gradually with increasing Ni loadings. At low Ni loadings (1 wt.% and 3 wt.%), the NiO particles were hardly seen, which was due to the high dispersion of NiO phase on the HZSM-5 surface. This was well consistent with XRD patterns (Fig. 1). Very occasionally, NiO particles with the size of about 100 nm were observed isolatedly on 3 wt.% Ni/HZSM-5. As the Ni loading increased to 7 wt.%, the NiO particle sizes were increased to about 150 nm and significant agglomeration occurred. However, as the Ni loading was further increased to 9 wt.%, the NiO particle sizes were not obviously changed. For spent 7 wt.% Ni/HZSM-5, severe agglomeration of Ni particles was observed after the fifth run (Fig. 2f). It's noted that the large sizes of NiO were possibly ascribed to the very low external surface of HZSM-5 with the size of several micrometers. BET analyses revealed that the surface area of Ni/HZSM-5 catalysts monotonously decreased with incremental Ni loadings (HZSM-5, 309 m 2/g; 1 wt.% Ni/HZSM-5, 230 m 2/g; 7 wt.% Ni/HZSM5, 173 m 2/g), indicating that the NiO particles located on the external surface blocked the micropores of HZSM-5 and induced obvious inaccessibility for N2 molecules. The NH3-TPD profiles of Ni/SiO2 and Ni/HZSM-5 were shown in Fig. 3. No characteristic peak of NH3 desorption was observed over 7 wt.% Ni/SiO2, indicating that no acidic sites exist on this catalyst surface. For HZSM-5, the two distinct desorption peaks of NH3 appeared at about 515 K and 730 K, indicating that HZSM-5 presents weak and strong acidic sites simultaneously. When Ni loadings of no more than 5 wt.% were implemented, the peak position of weak and strong acidic sites kept almost unchanged but their areas gradually decreased, indicating the continuous reduction of both weak and strong acid sites on the Ni/HZSM-5 surfaces. This decrease of overall acid sites in 1–5 wt.% Ni/HZSM-5 catalysts is due to the fact that NiO particles located on HZSM-5 partially blocked the HZSM-5 microchannels and decreased the accessibility for NH3. With the Ni loadings exceeding 5 wt.%, however, the peaks relative to strong acid sites were enhanced significantly, demonstrating that NiO particles located on HZSM-5 surface create strong acidity. This is well consistent with the previous reports [24,25].

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Fig. 2. SEM images of various Ni/HSM-5 catalysts: (a) 1 wt.% Ni/HZSM-5, (b) 3 wt.% Ni/HZSM-5, (c) 5 wt.% Ni/HZSM-5, (d) 7 wt.% Ni/HZSM-5, (e) 9 wt.% Ni/HZSM-5, and (f) 7 wt.% Ni/HZSM-5 used in the fifth run.

g f e

400

500

600

700

97 96 95 94 93 92 91 60

97 96 95 94 93 92 91 60

CO CO2 DME C1-C4 C5-C16 Hexadecanoic acid

50

50

40

40

d

30

30

c b

20

20

10

10

a

0

800

Temperature (K) Fig. 3. NH3-TPD profiles of Ni/HZSM-5 catalysts for (a) 7 wt.% Ni/SiO2, (b) HZSM-5, (c) 1 wt.% Ni/HZSM-5, (d) 3 wt.% Ni/HZSM-5, (e) 5 wt.% Ni/HZSM-5, (f) 7 wt.% Ni/ HZSM-5, and (g) 9 wt.% Ni/HZSM-5.

/H

1%

Ni

3%

M ZS

5%

Ni

M ZS

/H

/H

/H

Ni

M ZS

7%

-5

-5

-5

-5

-5

M ZS

Ni

9%

Si (mol %)

Intensity (a.u.)

Over 7 wt.% Ni/HZSM-5, we firstly investigated the external mass transfer effect by varying the stirring rate of 400, 600, 800, and 1000 rpm in this batch reactor under 493 K, 4 MPa of H2 pressure and 160 min, and found that MHD conversions and product selectivities kept unchanged as the stirring rate was more than 600 rpm, demonstrating the elimination of external mass transfer effect. The maximal stirring of 1000 rpm was used for all HDO evaluations.

Fig. 4 compared the catalytic performance of Ni/HZSM-5 catalysts in HDO upgrading of MHD. Considering the Ni content in stainless steel reactor used here, we implemented blank experiment without catalyst at the same reaction condition to those over Ni/HZSM-5 catalysts. The result showed that the conversion of methyl hexadecanoate was only 0.7%, indicating that the steel autoclave effect can be

XA (wt. %)

3.2. Catalyst performance

0

M ZS

/H

Ni

Fig. 4. Effect of Ni loadings of Ni/HZSM-5 catalysts on the activities and product distributions in HDO of MHD at the reaction condition of 220 °C, H2 pressure at 4 MPa and 160 min.

N. Shi et al. / Catalysis Communications 20 (2012) 80–84

100 98

Mass (%)

ruled out. All Ni/HZSM-5 catalysts showed MHD conversions of more than 94%, indicating that these catalysts were highly active for this reaction. The detectable products were mainly comprised of the gaseous CO, CO2, dimethyl ether (DME) and C1–C4 alkanes, liquid C5– C16 alkanes and hexadecanoic acid. From Fig. 4, it can be seen that Ni loadings obviously affected the product selectivity. As the Ni loadings rose from 1 wt.% to 9 wt.%, the gaseous products CO, CO2 and DME gradually increased but with the selectivities being below 5%. For the C1–C4 alkanes, however, the highest selectivity of 55% was observed at 5 wt.% Ni/HZSM-5 catalyst. With increasing the Ni loadings, the selectivities for hexadecanoic acid significantly decreased and gained the lowest percentage of about 3% at the Ni loading of 9 wt.%. For obtaining higher selectivities to the liquid C5–C16 alkanes, the Ni loadings of more than 7 wt.% are desirable. Table 1 summarized the influence of process parameters (the reaction temperature, the H2 pressure and the reaction time) on HDO of MHD over 7 wt.% Ni/HZSM-5 catalyst. With increasing the reaction temperature, the MHD conversion noticeably raised with the significantly decreased selectivities for the liquid C5–C16 alkanes and sharply increased for the gaseous C1–C4 hydrocarbons, indicating that high reaction temperature promoted the HDO of MHD but also enhanced cracking of MHD and primary alkanes. As increasing the H2 pressures, the conversions of MHD changed a little but the selectivities of the products decreased slightly aside from normal C5–C16 alkanes and hexadecanoic acid, indicating that high H2 pressure slightly suppresses the volume-expanded HDO process. Additionally, the reaction time significantly influenced the conversion of MHD and the product distributions. MHD could be converted into the products with nearly 100% conversion as the reaction time lengthened to 360 min. Meanwhile, the selectivities to the C5–C16 alkanes dramatically decreased, implying the significant promotion of deteriorated cracking of the produced long chain alkanes. This is proven by the phenomenon that the gaseous C1–C4 alkanes drastically increased as the reaction time lengthened. At the reaction time of 60 min, the highest selectivity of 83% for the liquid C5–C16 alkanes was obtained. Hexadecanoic acid was observed in this HDO process, but its content decreased with the temperature enhanced and the time lengthened, indicating that it presented as an intermediate and was further deoxygenated to the alkanes. CO, CO2 and DME were detected with very low selectivities, which originated from decarbonylation/decarboxylation of MHD and/or hexadecanoic acid intermediate, and dehydration of methanol intermediate, respectively [26]. The iso-alkanes, which could improve the low temperature performance and octane number of alkane fuels [27], were also observed in the final products. This was ascribed to the synergistic effect of Ni/HZSM-5 catalysts with the bifunctional metal (Ni) and acidity (HZSM-5). The maximal yield of 76% for liquid C5–C16 alkanes was obtained under the reaction condition of 493 K, 2 MPa of H2 pressure and 60 min (Table 1, No. 6), which is comparable to the previous report that the methyl heptanoate

83

96 94 92 90 88 86 400

500

600

700

800

900

1000

Temperature (K) Fig. 5. TG curve of 7 wt.% Ni/HZSM-5 catalyst with the fifth run.

conversion of more than 90% and the total alkane selectivity of 76% were presented over sulfided NiMo/γ-Al2O3 catalyst [28]. The bifunctionality of Ni/HZSM-5 catalysts played the crucial role in governing the final products and their distributions in HDO of MHD. As indicated by the NH3-TPD measurements (Fig. 3), the acidity of Ni/HZSM-5 catalysts gradually decreased with increasing the Ni loadings until 5 wt.%. This acidity decrease resulted in the decrease of hexadecanoic acid selectivities and significant improvement for the gaseous C1–C4 and liquid C5–C16 alkanes (Fig. 4), seeming that HDO of MHD mainly occurred on the supported metallic Ni surface. It's noted that the acidic sites of HZSM-5 play the synergistic role together with the metallic Ni and significantly promote HDO of MHD [15,17,23,24,26]. This was demonstrated by the fact that the conversion of MHD was only 60% as the neutral 7 wt.% Ni/SiO2 was used (No. 9, Table 1). Due to the absence of acidic function over the catalyst, the cracking reaction was significantly suppressed, resulting in the much higher selectivity of 89% for liquid alkanes, as compared to the Ni/HZSM-5 catalysts. To investigate the stability of catalyst, 7 wt.% Ni/HZSM-5 was used repeatedly for HDO of MHD at 220 °C, 240 min and 2 MPa of H2 pressure. Both the MHD conversions and the C5–C16 alkane selectivities significantly dropped from the initial 99.3% and 65.7% to 89.1% and 45.6%, respectively, after the catalyst was reused for five times. Besides the severe agglomeration of Ni particles as indicated in Fig. 2f, this catalyst deactivation might be mainly attributed to the coke formation. As shown in Fig. 5, about 13 wt.% of carbon deposition was observed after the catalyst was reused for the fifth run. The carbon formed covered on Ni and acidic sites, and induced a fast deactivation in HDO of MHD. To lengthen the catalyst lifetime, further studies for reducing the Ni aggregation and carbon deposition is in progress by rationally mediating surface area, pore size, and acidity of Ni based catalysts.

Table 1 Effect of reaction parameters on HDO of MHD over 7 wt.% Ni/HZSM-5. Item

No. 1

No. 2

No. 3

No. 4

No. 5

No. 6

No. 7

No. 8

No. 9a

Reaction temperature (K) H2 pressure (MPa) Reaction time (min) XA (%) Si (mol.%) CO CO2 DME C1–C4 i-C5–C16 n-C5–C16 Hexadecanoic acid

433 4 160 68.8

493 4 160 95.0

513 4 160 97.7

493 2 160 95.8

493 6 160 95.2

493 2 60 92.3

493 2 240 99.3

493 2 360 99.7

493 2 360 60.1

5.4 0.5 9.8 25.0 1.1 45.1 13.0

2.3 0.4 2.5 40.9 11.3 39.3 3.3

1.4 0.4 0.2 59.6 10.5 26.1 1.9

2.0 0.4 2.2 38.0 13.3 41.2 2.9

0.5 0.5 1.1 35.1 10.1 47.6 5.1

1.7 0.3 0.8 9.9 12.2 71.0 4.1

1.9 0.2 1.3 30.0 8.8 56.9 0.8

1.3 0.3 1.1 36.5 8.7 51.9 0.3

1.8 0.1 0.1 9.2 1.3 87.5 –

a

The employed catalyst was 7 wt.% Ni/SiO2.

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4. Conclusions The Ni/HZSM-5 catalysts with different Ni loadings were prepared by wet impregnation. At low Ni loadings of below 3%, the NiO could highly disperse on the surface of HZSM-5, and gradually aggregate into the relatively large particles of more than 100 nm as the Ni loading exceeds 3 wt.%. Supporting Ni on HZSM-5 significantly changed the acidic properties of Ni/HZSM-5 based on NH3-TPD measurements. Highly dispersing (the low Ni loadings) Ni on HZSM-5 decreased the acid sites of Ni/HZSM-5 catalysts because of the blockage of HZSM-5 microchannels by NiO crystallites. As Ni loadings exceeded 5 wt.%, large NiO particles formed on the surface of HZSM-5 and originate the strong acidic sites. Catalytic evaluation of these Ni/HZSM-5 samples revealed that HDO upgrading of MHD contained hydrogenation, dehydration, decarbonylation/decarboxylation, cracking, and isomerization, presenting the complex reaction network. The metallic and acidic functionalities of Ni/HZSM-5 catalysts were proven to show the synergistic effect in HDO of MHD process and obtained the high yields for liquid C5–C16 alkanes. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51076157), the Key Natural Science Fund of China (No. 51036006) and the Key Orientation Project of Knowledge Innovative, Chinese Academy of Sciences (No. KSCX2-YW-G-063). References [1] O. . Senol, E.M. Ryymin, T.R. Viljava, A.O.I. Krause, Journal of Molecular Catalysis A: Chemical 277 (2007) 107–112. [2] Y.Q. Yang, H.A. Luo, G.S. Tong, K.J. Smithb, C.T. Tye, Chinese Journal of Chemical Engineering 16 (2008) 733–739. [3] B. Donnis, R. Gottschalck, P. Blom, Topics in Catalysis 52 (2009) 229–240.

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