Hydroconversion of Jatropha Oil to Alternative Fuel over Hierarchical

Article pubs.acs.org/IECR

Hydroconversion of Jatropha Oil to Alternative Fuel over Hierarchical ZSM‑5 Hao Chen,† Qingfa Wang,*,†,‡ Xiangwen Zhang,†,‡ and Li Wang†,‡ †

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 ABSTRACT: Hierarchical ZSM-5 catalysts were prepared by desilication with different NaOH concentrations. Their structure and acidity were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N2 adsorption and desorption, ammonia temperature-programmed desorption (NH3-TPD), pyridine Fourier transform infrared (FTIR) spectroscopy, CO chemisorptions, and hydrogen temperature-programmed desorption (H2-TPR). The catalytic conversion of jatropha oil into alternative fuel over the NiMo/hierarchical ZSM-5 catalysts was investigated. The deoxygenation pathways were tuned by controlling the alkaline treatment. The DCO2 reaction was strongly favored by strengthening the support-active phase interaction. The aromatization reaction among C2−C8 olefins led to the increase of organic liquid products, especially the C9− C15 hydrocarbons. In addition, this reaction was significantly improved by partial amorphization of the support but inhibited by the development of intracrystal mesopores. The larger mesoporous structure improved the selectivity of C9−C15 paraffins. Moreover, the intracrystal mesopores had better catalytic cracking selectivity for C9−C15 paraffins than pure microporous structure and amorphous structure.

1. INTRODUCTION The development of renewable and clean fuels has become imperative, because of the increased demand for petroleum with declining resources of crude oil and increasing global environmental concerns on fossil fuels.1 In recent years, biofuels derived from vegetable oil have been considered to be one of the significant renewable sources, because of their alternative sources, renewability, and reuse of CO2.2 Two important procedures have been developed to convert vegetable oil into alternative fuels: transesterification and hydrotreatment. Transesterification is commonly used to produce biodiesel, which is a first-generation biofuel. However, the poor properties of the final products (low energy content, poor storage stability, and cold-flow properties) limit its development to some extent.3 Nowadays, hydroconversion of triglyceride into hydrocarbons has been considered as an alternative way to produce superior fuel. Many works have been conducted on the conversion of vegetable oils into hydrocarbons such as microalgae oil,4 jatropha oil,5,6 palm oil,7 and C18 fatty acids.8 The desirable products from gasoline to diesel fraction with excellent properties have been obtained by regulating the operation conditions (temperature, pressure, H2/ oil ratio, and so on) and the types of catalysts.9,10 Different active metals and supports have been investigated for the conversion of triglycerides into fuel-like hydrocarbons. Noble metals palladium and platinum have been viewed as the best catalysts.11 However, the high cost and limited worldwide supply of these noble metals make their application in viable commercial processes unattractive. Recently, transition metals such as Ni, Mo, W, and Co (especially Ni and Mo) have been widely used for hydroprocessing of vegetable oil.11−13 Among the supported transition-metal catalysts, the support plays a significant role for the hydroconversion of vegetable oil. The © 2014 American Chemical Society

Al2O3, varying widely in textural properties (surface area, pore volume, and pore size), has been predominantly investigated.10,12 Other supports (e.g., SiO2, TiO2, mixed oxides, zeolites, mesoporous-silica-based supports, carbons, MCM-41, SAPO-11, etc.) have also been studied.13−15 It was found that the selectivity of hydroconversion of vegetable oil could be finetuned by support selection and modification. More recently, hierarchical materials have attracted much attention in catalysis.16,17 Verma’s group first investigated the effect of hierarchical ZSM-5 supported NiMo or NiW catalysts on the hydroconversion of jatropha oil.18 High yield of C9−C15 hydrocarbons with excellent isomerization selectivity was obtained (40%−45% with i/n = 2−6 and 40%−50% with i/n = 3−13 over NiW and NiMo catalysts, respectively). Botas et al. studied the influence of hierarchical ZSM-5 catalyst on the catalytic conversion of rapeseed oil.19 The introduction of hierarchical porosity in ZSM-5 increased the proportion of light olefins in the products. Nevertheless, it could be observed that these hierarchical ZSM-5 zeolites were all obtained by direct synthesis (hydrothermal method) using different templates. Usually, the hierarchical materials could also be synthesized by post-synthesis treatments (dealumination or desilication).20 The post-synthesis treatments tend to produce intracrystal mesopores rather than zeolite fragments produced via direct synthesis. Moreover, the desilication by alkali is recognized as a versatile, controllable, and scalable method, compared to hydrothermal synthesis.21 Some work have been done on the biomass conversion using desilicated ZSM-5 zeolite, such as Received: Revised: Accepted: Published: 19916

September 25, 2014 November 25, 2014 November 27, 2014 November 27, 2014 dx.doi.org/10.1021/ie503799t | Ind. Eng. Chem. Res. 2014, 53, 19916−19924

Industrial & Engineering Chemistry Research

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xylose dehydration to furfural,22 ethanol/methanol dehydration to hydrocarbons,23,24 and catalytic fast pyrolysis (CFP) of lignocellulosic biomass.25 However, few works have been devoted to exploring the influence of zeolites desilication on hydroconversion of vegetable oil, especially on the deoxygenation pathways. In this work, the hierarchical ZSM-5 zeolites were prepared by desilication using various concentrations of NaOH. The hydroconversion of jatropha oil over the hierarchical ZSM-5 supported NiMo catalysts was carried out. The object was to investigate the influence of desilication on the metal−support interaction, metal dispersion, acidity, and, consequently, the deoxygenation pathways, as well as the product distribution.

2.3. Catalyst Characterizations. X-ray diffraction (XRD) was used to determine the structural properties of the hierarchical ZSM-5 samples on D/MAX-2500 X-ray diffractometer with Cu Kα (λ = 1.541 Å) radiation at 40 kV and 140 mA. Each sample was scanned at a speed of 4°/min over a range from 5° to 90°. The relative crystallinity was determined based on the intensity of the characteristic peaks in the range between 6° and 9° and between 22.5° and 25.0°. The Si/Al ratio was determined by X-ray fluorescence spectrometry (Model S4 Pioneer, Bruker). Specific surface areas and pore size distribution were determined by N2 adsorption and desorption isotherms using a Micromeritics Model ASAP 2020 volumetric instrument. The total pore volume was derived from the amount of adsorbed N2 at P/P0 = 0.99. The mesopore volumes were calculated by subtracting the micropore volume. The t-plot method was used to discriminate between the microporosity and mesoporosity and the specific surface area was calculated using the Brunauer−Emmett−Teller (BET) method. In order to avoid the Tensile Strengthening Effect, the mesopore size distributions were derived from the adsorption branch of the isotherm using the Barrett−Joyner−Halenda (BJH) model. TEM images were obtained on a JEM Model 2010FEF. Ammonia temperature-programmed desorption (NH3-TPD), temperature-programmed reduction (TPR), and CO chemisorption were performed on a Chemisorption Physisorption Analyzer (Model AMI-300, Altamira Instruments) equipped with a thermal conductivity detector (TCD). For NH3-TPD experiments, all the samples were pretreated at 800 K in helium for 1 h and then cooled to 373 K. NH3 in helium was adsorbed at 373 K for 30 min, followed by purging with pure helium at the same temperature for 2 h to remove weakly adsorbed NH3. NH3-TPD was monitored in the range of 400−800 K. For the CO chemisorption, the sample (100 mg) was pretreated in helium at 393 K for 1 h, reduced in H2 flow (50 cm3/min) at 723 K for 2 h, evacuated at 723 K for 2 h, and then cooled to 313 K in vacuum. Afterward the CO adsorption isotherm was recorded at 313 K based on the amount of adsorbed CO at different pressures. In H2-TPR experiments, all the measurements were carried out by heating the catalyst (100 mg) at 10 K/min from room temperature to 1173 K with a mixture of 10% H2 in helium at a constant flow rate of 30 mL/min. Before the experiment, all the samples were pretreated at 723 K (10 K/min) in helium for 1 h. The distribution of Lewis and Brønsted acids was determined by infrared spectroscopy of adsorbed pyridine (Vertex 70, Bruker). The quantitative analysis of Brønsted and Lewis acid sites was carried out using the absorption at 1545 and 1454 cm−1, respectively.27 2.4. Catalytic Experiments. The hydrotreatment of jatropha oil was conducted in a fixed-bed reactor. The reaction temperature was controlled by three thermocouples on the reactor wall and monitored with a thermocouple in the catalyst bed. Ten grams (10 g) of catalyst diluted with SiC was loaded in the reactor. The feedstock was injected into the reactor, using a high-pressure pump. The reactions were carried out at 653 K under 3 MPa with the liquid hourly space velocity of 3.8 h−1 and a H2/oil ratio (v/v) of 500. All the catalysts were presulfided in situ at 593 K and 3.0 MPa for 4 h, using 3.0 wt % CS2 in cyclohexane before use. The products of each experiment were collected for 4 h after the 2-h reaction under the given conditions. The selectivity and activity versus the time-on-stream for each catalyst changed very little (see the inset in Figure 7a, presented later in this

2. MATERIAL AND METHODS 2.1. Materials. The ZSM-5 (Si/Al = 54.0) was purchased from the catalyst plant of Nankai University. (NH4)6Mo7O24 (≥99 wt %, J&K), Ni(NO3)2 (≥98 wt %, Alfa Aesar), cyclohexane, ammonium nitrate (NH4NO3), and sodium hydroxide (NaOH) were used as received. Jatropha oil was purchased from Jiangsu Donghu Bioenergy Co., Ltd. The physical and chemical properties of jatropha oil are presented in Table 1. The triglycerides content was up to 91.7 wt %. The main fatty acids in jatropha oil were linoleic, oleic, palmitic, and stearic acids. Table 1. Physical Properties and Fatty Acids Composition of Jatropha Oil property density (g/cm3) viscosity @ 304 K (mPa s) acid value (mg of KOH/g) free fatty acid content (wt %) glyceride content (wt %) fatty acid composition (wt %) stearic acid (C18:0) oleic acid (C18:1) linoleic aid (C18:2) palmitic acid (C16:0) other acids

value 0.9 40.4 8.2 8.3 91.7 6.9 40.1 36.3 14.7 2.0

2.2. Catalyst Preparation. The hierarchical ZSM-5 samples were prepared by a simple alkaline treatment method.26 In a typical run, 20 g of ZSM-5 powders were added into 200 mL of NaOH aqueous solution with different concentrations (0.2, 0.35, 0.5, and 0.8 M). The mixture was heated to 343 K for 1 h with agitating. The slurry then was cooled to room temperature. The zeolites were recovered by filtration and washed using deionized water to neutral. The obtained samples were dried at 393 K for 12 h and then were converted to the H-form by three consecutive exchanges in 1.0 M NH4NO3 solution at 363 K. Finally, the samples were dried at 393 K for 12 h and further calcinated at 773 K for 4 h. The hierarchical samples were denoted as Z5−0.2, Z5−0.35, Z5− 0.5, Z5−0.8, respectively, corresponding to the NaOH concentration. Ni (4.0 wt %) and Mo (12 wt %) were loaded on the hierarchical ZSM-5 zeolites by incipient wetness coimpregnation with an aqueous solution containing (NH4)6Mo7O24 and Ni(NO3)2. After impregnation, the samples were kept overnight at room temperature and dried at 393 K for 12 h. Finally, the samples were calcinated at 723 K (5 K/min) for 4.5 h. 19917

dx.doi.org/10.1021/ie503799t | Ind. Eng. Chem. Res. 2014, 53, 19916−19924


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hierarchical samples showed the same characteristic peaks assigned to ZSM-5. This meant that the framework topology of the ZSM-5 zeolite was retained after desilication, which was consistent with the literature.28 However, the intensity of the peaks changed dramatically. The relative crystallinity of alkalitreated ZSM-5 catalyst increased slightly for the Z5−0.2 and Z5−0.35 samples, whereas a noticeable loss of crystallinity was observed for the Z5−0.5 sample and much more for the higher alkali-treated sample (Z5−0.8) (see Table 2). This could be attributed to the removal of silicon from the framework of ZSM-5 without complete destruction of the crystal lattice at a low alkaline concentration during partial amorphization, with the generation of intracrystalline mesoporosity at a high alkaline concentration.29 The Si/Al ratio of the zeolites decreased from 54.0 for the parent ZSM-5 sample to 20.3 for the Z5−0.8 sample (see Table 2), because of the selective removal of silicon entities. (This was because the Si−O−Si bond was easier for cleavage than the Si−O−Al bond in the presence of OH−, because of the existence of AlO4− tetrahedron. 30) The intracrystalline mesoporosity formed by desilication in NaOH solution was analyzed by N2 adsorption−desorption isotherms (see Figure 2). As shown in Figure 2a, the parent ZSM-5 zeolite presented an I-type isotherm with the plateau starting at a very low relative pressure, which implied that the parent zeolite was dominated by the microporous structure. This was also confirmed by the porous properties in Table 2, which showed that 80% of the pore volume was related to microporosity. However, the type IV isotherm with a remarkable hysteresis loop, which is a known fingerprint of a hierarchical porous system, appeared after alkaline treatment (see Figure 2a). It could be inferred that mesopores were formed. The uptake at low relative pressures decreased, which was coupled to an increased uptake at medium to high relative pressures. This indicated that the loss in microporosity and the development of mesoporosity occurred upon the alkaline treatment. Especially, the isotherms of the hierarchical ZSM-5 samples presented a rapid increase of absorption with P/P0 > 0.9, which was indicative of the damage of the internal pores and the capillary condensation on the surface or intracrystalline portion of zeolite. The mesopore distributions of the ZSM-5 catalysts from the adsorption branch of isotherm curve are shown in Figure 2b. The mesopore sizes for the hierarchical ZSM-5 catalysts were centered at ca. 4 and 15 nm, indicating that a three-gradient porous structure was formed. Moreover, the amount of larger mesopores at ca. 15 nm increased with increasing NaOH concentration. These changes were expected to have a positive effect on the catalytic activity for the conversion of jatropha oil. The hierarchy factor (HF) was considered as a suitable tool to classify hierarchical porous zeolites by desilication.31 The

work), indicating that no catalyst deactivation occurred during the reactions. The gaseous products were analyzed online with an Agilent Model 3000 gas chromatograph equipped with three columns (molecular sieve, plot U, and alumina). The liquid fraction was divided in two parts: water and organic liquid products (OLPs). The water was separated and weighted. The organic liquid products were qualitatively determined via gas chromatography/mass spectroscopy (GC/MS) (Agilent Model 6890N gas chromatograph, coupled with an Agilent Model 5975N mass spectrometer). A gas chromatograph (Agilent, Model 7890A), equipped with a flame ionization detector (FID) and a commercially column (PONA, 50 m × 0.20 mm × 0.5 μm), was used to quantitatively analyze the hydrocarbons using tetracosane as the internal standard. The effect of solvent on the product distribution was identified by a preliminary experiment using the solvent cyclohexane as the reactant. The conversion and selectivity then were calculated according to eqs 1 and 2: mJO feed − mJO products conversion = mJO feed (1) selectivity(C H ) = x

y

(CH)n products ∑ ((CH)n products )

(2)

where mJO feed and mJO products are the weights of jatropha oil in the feed and in the products, respectively. (CH)n products is the weight of Cn hydrocarbons in the products.

3. RESULTS AND DISCUSSION 3.1. Catalyst Texture Properties. The XRD patterns of different ZSM-5 samples are shown in Figure 1. All the

Figure 1. XRD patterns for parent and alkali-treated ZSM-5 samples.

Table 2. Elemental Analysis and Physical Properties of ZSM-5 Treated with NaOH Specific Surface Area (m2 g−1) Z5 Z5−0.2 Z5−0.35 Z5−0.5 Z5−0.8 a

Pore Volume (cm3 g−1)

Si/Al ratio

SBET

Sext/mes

Smicro

Vmesopore

Vmicro

hierarchy factor, HFa

relative crystallinity (%)

54.0 53.5 46.3 31.9 20.3

357.2 407.1 446.6 337.5 271.2

39.0 138.9 261.7 198.1 176.0

318.2 268.1 184.9 139.4 95.2

0.035 0.226 0.258 0.217 0.178

0.134 0.104 0.082 0.065 0.046

0.0866 0.1075 0.1413 0.1353 0.1333

85.70 100.00 86.85 64.94 59.29

The hierarchy factor (HF) = (Vmicro/Vpore) × (Smeso/SBET). 19918



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ZSM-5 samples were higher than that of the parent one. It was increased from 1.0 to 1.413 and then slightly decreased as the OH− concentration increased, which was also in accordance with the results presented in ref 32. The acidity of the hierarchical ZSM-5 samples was studied by NH3-TPD. As shown in Figure 3, the spectra exhibited three

Figure 3. NH3-TPD profiles for parent and alkali-treated ZSM-5 supported NiMo catalysts.

peaks at ∼500 K, 550 K, and 650 K, which represented weak, medium, and strong acidity, respectively.15 The acid properties of different ZSM-5 samples determined by NH3-TPD are summarized in Table 3. The number of acid sites in the hierarchical ZSM-5 samples was much higher than that of the parent one. This was attributed to the higher Al content in the alkali-treated zeolites, which gave a higher density of acid sites. Moreover, the total acidity of the hierarchical samples increased with the increase of alkali concentration and a maximum value was obtained over the NiMo/Z5−0.5 sample. For the NiMo/ Z5−0.8 sample, the acid sites density decreased due to the collapse of zeolite framework and/or the presence of Al atoms in nonaccessible positions or nonframework octahedral Al of the pore walls.33 The distribution of Lewis acidity (L) and Brønsted acidity (B) was further investigated by infrared spectroscopy of adsorbed pyridine. It was found that the ratio B/L decreased dramatically and a large number of Lewis acid sites were formed after desilication. This suggested that the alkaline treatment converted some of the Brønted acid sites into Lewis acid sites, because of the removal of silica, which was consistent with the result presented in ref 22. In addition, the formation of mesopores at the consumption of micropores might also decrease the number of Brønsted acid sites. The influence of alkaline treatment on the active phase was investigated by CO chemisorption, transmission electron microscopy (TEM), and hydrogen temperature-programmed desorption (H2-TPR). The CO chemisorptions of different catalysts are shown in Table 3. The absorbed CO amount increased quickly from the NiMo/Z5 catalyst to the NiMo/ Z5−0.35. This indicated that the dispersion of active metals, Ni and Mo, increased for these catalysts. As the NaOH concentration increased up to 0.5 and 0.8 M, the adsorbed

Figure 2. (a) N2 adsorption and desorption isotherms and (b) BJH pore size distribution for parent and alkali-treated ZSM-5 samples.

textural properties and HF values of all the catalysts are summarized in Table 2. Vmesopore gradually increased with the formation of mesopores, especially larger mesopores. For the hierarchical Z5−0.2 and Z5−0.35 catalysts, the mesopore surface area as well as the BET area increased with the increase of alkali concentration, because of the development of mesopores. But it decreased at alkali concentrations of >0.5 M. It was also noted that the value of Smicro was gradually decreased with increasing alkali concentration. In this work, the HF value of the parent ZSM-5 catalyst was 0.0866, which agreed with the literature.31 The HF values of the alkali-treated 19919



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Table 3. Acidity Properties and CO Chemisorption of Series NiMo/Z5 Catalysts Weak

catalyst NiMo/Z5 NiMo/ Z5−0.2 NiMo/ Z5− 0.35 NiMo/ Z5−0.5 NiMo/ Z5−0.8

Medium

Strong

temperature, T(K)

acidity (μmol/g)

temperature, T(K)

acidity (μmol/g)

temperature, T(K)

acidity (μmol/g)

total acidity (μmol/g)

Lewis acidity (μmol/g)

Brønsted acidity (μmol/g)

adsorbed CO (μmol/g)

470 477

68.4 80.9

526 528

117.6 139.2

666 651

119.1 129.6

305.1 349.7

49.1 235.0

256.0 114.7

1.4041 2.0913

487

91.8

535

204.0

706

180.8

476.5

348.8

127.7

2.4732

491

95.3

541

214.5

672

174.6

484.4

320.2

164.2

1.8207

500

80.7

546

185.1

693

169.1

434.9

260.1

174.8

1.5574

peak at 977 K comprised the deep reduction of all Mo sulfides, including highly dispersed tetrahydral Mo species. The peak at 886 K might be assigned to the intermediate-reducible nickel sulfide and the mixed Ni−Mo−S phase. As shown in Figure 4, the reduction of Ni and Mo species mainly occurred at the range of high temperature. This indicated that the Ni and Mo species were mainly in the form of the Ni−Mo−S phase, and the octahedral and/or tetrahedral Mo phase. For the alkalitreated samples, all the reduction peaks shifted to higher temperature. This indicated that the active phase cluster size became smaller which strengthened the active metal−support interaction.14 The change of the active phase cluster size was further confirmed by TEM characterization. As shown in Figure 5, the particle size of the metals decreased from 10.02 ± 0.85 nm for the NiMo/Z5 catalyst to 7.25 ± 1.1 nm for the NiMo/ Z5−0.35 catalyst and then increased gradually up to 9.9 ± 1.05 nm for the NiMo/Z5−0.8 catalyst. Whereas, for the NiMo/ Z5−0.5 and NiMo/Z5−0.8 catalysts, the reduction peaks shifted to lower temperature. The active phase cluster size became larger, weakening the active metal−support interaction. The smaller the active phase cluster size was, the higher the dispersion of active metals would be. Therefore, it could also be observed that the metal dispersion increased first and then decreased as the alkaline concentration increased. The NiMo/ Z5−0.35 catalyst showed the highest reduction temperature, indicating it had the smallest active phase cluster size and best active metal dispersion. This was consistent with the CO chemisorption results. 3.2. Catalytic Conversion of Jatropha Oil. The catalytic hydroconversion of jatropha oil to hydrocarbons on the hierarchical ZSM-5 samples were investigated. For the deoxygenation of triglyceride, three pathways have been accepted: hydrodeoxygenation (HDO), decarbonylation (DCO), and decarboxylation (DCO2).2,11 In this work, it was observed that the oxygen removed from the reactants was almost presented in carbon monoxide, carbon dioxide, and water. No other oxygenated compounds such as fatty acids, fatty alcohols, and fatty esters were detected in the products. Moreover, comparing with the deoxygenation reactions, the methanation reactions (eqs 3 and 4) could be ignored in the fixed-bed flow reactor under the experimental conditions.15

CO amount decreased sharply. As the alkaline treatment underwent, the increased surface area led to the higher metals dispersion. However, for the higher alkali-treated samples, the metals dispersion decreased due to the reduction of surface area derived from partial amorphization. Figure 4 shows the H2-TPR

Figure 4. H2-TPR profiles for parent and alkali-treated ZSM-5 supported NiMo catalysts.

profiles of different samples. The alkali-treated catalysts exhibited different reduction behaviors of surface Ni and Mo sulfide species, indicating the different active metal−support interaction. The strength of the active metal−support interaction could reflect the support nature and the active phase cluster size. The main reduction peak corresponding to the active phase reduction was located between ∼600 K and ∼1200 K (see Figure 4). For the parent sample (NiMo/Z5), the TPR profile showed four characteristic peaks, at ca. 470, 642, 886, and 977 K. The low-temperature peak at 470 K was due to the hydrogenation of weakly bonded S species. The temperature peak at ca. 673 K could be assigned to the partial reduction (Mo6+ to Mo4+) of polymeric octahedral Mo species weakly bound to the catalyst surface.34 The high-temperature

Methanation: CO + 3H 2 ↔ CH4 + H 2O

(3)

Methanation: CO2 + 4H 2 ↔ CH4 + 2H 2O 19920

(4)



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Figure 5. TEM image (top) and the corresponding size distribution of the metal particle (bottom) of (a) NiMo/Z5, (b) NiMo/Z5−0.2, (c) NiMo/ Z5−0.35, (d) NiMo/Z5−0.5, and (e) NiMo/Z5−0.8.

Meanwhile, the water-gas shift reaction (eq 5) was sensitive to temperature, and the reaction equilibrium constant under the same temperature was identical. Therefore, the variation of CO2, CO, and H2O in this work could be considered from different deoxygenation reactions. Water-gas shift: CO + H 2O ↔ CO2 + H 2

(5)

The distribution of CO, CO2, and H2O over different catalysts is shown in Figure 6. When the parent ZSM-5 zeolite was used as support, it was found that H2O (60 mol %) and CO (30 mol %) were the main products and only 9 mol % CO2 was obtained. This meant that the three oxygen-removal pathways occurred simultaneously in the hydroconversion of jatropha oil and DCO was the dominant reaction. The amount of H2O produced (in mol %) was greater than the amount of CO2 and CO, which meant that a large amount of hydrogen was consumed. The oxygen-containing products distribution was significantly influenced by the hierarchical supports. In the case of NiMo/Z5−0.2, the main oxygen-containing products were also water and carbon monoxide. The selectivity of CO2 (13 mol %) and H2O (61 mol %) increased and the CO (26 mol %) decreased. The maximum selectivity of CO2 (15 mol %) was obtained over the NiMo/Z5−0.35 sample with a decrease of water selectivity. For the NiMo/Z5−0.5 and NiMo/Z5−0.8 samples, the selectivity of CO2 decreased gradually, but that of CO increased. The content of H2O increased very little. Moreover, water was still the dominant product. Considering the influence of the water-gas shift reaction, the selectivity of pathways of oxygen removal could use the ratio of (H2O − CO)/2 to (CO + CO2) to identify the hydrodeoxygenation (HDO) to decarbonylation and/or decarboxylation (DCOx), according to the reaction pathways. As shown in Figure 6b, the molar ratio of HDO to DCOx was ca. 1:2.5 for the NiMo/Z5 catalyst and changed only slightly over the NiMo/Z5−0.2 catalyst, while it increased dramatically up to 1:3.2, because of the formation of three-gradient hierarchical structure. The molar ratio further increased to 1:3.9 for the NiMo/Z5−0.5 catalyst and to 1:4.4 for the NiMo/ Z5−0.8 catalyst. These results indicated that the larger mesopore could inhibit the HDO reaction and favor the DCOx reaction.

Figure 6. (a) Distribution of H2O, CO, and CO2 parent and alkalitreated ZSM-5-supported NiMo catalysts and (b) the variation of the DCOx/DH2O ratio for parent and alkali-treated ZSM-5-supported NiMo catalysts.

Since the active phase and its composition was the same for all of the catalysts, the variation of deoxygenation pathways over the sulfided NiMo catalysts could be assigned to the change in active components. For the deoxygenation of vegetable oils, the bimetallic NiMo sulfide catalyst showed a 19921



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mixture of DCO2 and HDO products, which were yielded from the active Ni species and Mo species, respectively.13 From the TPR result, it could be obtained that the Ni and Mo species were present mainly in the Ni−Mo−S phase and tetrahedral/ octahedral Mo phase. These active species contributed to the formation of CO2, CO, and H2O through different pathways. The sizes of these active phase cluster over the alkali-treated support decreased and then increased with the severity of alkaline treatment. The smallest active cluster size was obtained over the NiMo/Z5−0.35 sample (see Figure 5). As shown in Figure 6a, the selectivity of CO2 showed identical behavior with the active cluster size, whereas H2O did the contrary. As mentioned previously, the active Ni species, Ni−Mo−S, mainly contributed to the product of CO2. These results strongly indicated that the smaller the active Ni−Mo−S phase cluster was, the better the decarboxylation activity of supported NiMo catalysts became. In contrast, the larger tetrahedral Mo species contributed to a better catalytic performance of the HDO reaction. The amount of CO may be dependent on the combined effect of the Ni and Mo species, especially on the Mo species. Therefore, it showed a behavior similar to that of H2O over the different NiMo catalysts. Interestingly, the selectivity of H2O over the NiMo/Z5−0.2 catalyst was higher than the NiMo/Z5−0.35 catalyst, but very close to that of the NiMo/Z5 sample. Although the active phase for all the catalysts were mainly composed of the Ni−Mo−S phase and the tetrahedral/ octahedral Mo phase, the Ni−Mo−S phase was dominant for the NiMo/Z5−0.2 sample, but the dominant one for the other samples was the tetrahedral Mo phase. The Mo species in the Ni−Mo−S phase may have a better HDO performance than in the tetrahedral/octahedral Mo phase. So, the NiMo/Z5−0.2 sample showed a different distribution of CO and H2O. It was also considered that the strong acidic sites might influence the oxygen removal pathways because they could cause the polarization of adsorbed carboxyl groups of the acid and ester leading to decarboxylation.35 Comparing the NiMo/Z5−0.2 catalyst with NiMo/Z5−0.8 catalyst, it could be observed that the size of these two samples was similar to that of the active phase. However, the strong acid sites of the NiMo/Z5−0.8 catalyst were much more than that of the NiMo/Z5−0.2 catalyst. Nevertheless, the selectivity of CO2 was also similar. Therefore, it could be inferred that the deoxygenation pathways were mainly dependent on the variation of active phase. The conversion of jatropha oil over the ZSM-5 catalysts is given in Figure 7a. Almost complete conversion was achieved over all the catalysts. The yields of organic liquid products were ca. 70% for the high crystallinity samples (NiMo/Z5, NiMo/ Z5−0.2, and NiMo/Z5−0.35) and then increased significantly, up to 73.2% for the NiMo/Z5−0.5 sample and 83.7% for the NiMo/Z5−0.8 sample. This result indicated that, although the hierarchical structure was developed, the selectivity of organic liquid products was not improved until the zeolite structure was partially destroyed. This may be because the collapsed structure could supply more accessible active sites for gaseous olefins to be converted to larger molecules, as described in the following discussion. The liquid products distributions over the series catalysts are also shown in Figure 7b. The NiMo/Z5 catalyst showed a high selectivity (62.1%) for C4−C8 hydrocarbons, with 32.7% for C9−C15 hydrocarbons and 5.2% for C16−C18 hydrocarbons. After alkaline treatment, the product distribution changed greatly. The fraction of C4−C8 hydrocarbons increased slowly and then quickly decreased with a maximum selectivity of

Figure 7. (a) Yield of OLP, (b) hydrocarbon product distribution, and (c) distribution of aromatics and different paraffins over parent and alkali-treated ZSM-5-supported NiMo catalysts.

65.2% over the NiMo/Z5−0.35 catalyst. Notably, the C9−C15 hydrocarbons increased from 29.2% for the NiMo/Z5−0.2 catalyst up to 48.0% for the NiMo/Z5−0.8 catalyst. The fraction of C16−C18 hydrocarbons was gradually reduced over the higher alkali-treated catalysts. The difference of these products distribution could be mainly attributed to the variation of the accessibility and the acid properties of ZSM-5 zeolite modified by desilication. The medium and strong acidity of the catalyst played a very important role in the catalytic cracking activity of paraffins.36 The low yield of organic liquid hydrocarbons and high selectivity to gasoline over the NiMo/ Z5 catalyst were expected, because of its high medium and strong acidity. In comparison with the NiMo/Z5 catalyst, the product distribution changed only slightly for the NiMo/Z5− 19922



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products was inhibited, probably because of the enhancement of mass transfer by the mesopores. Moreover, the aromatic compounds were mainly C9 aromatics and/or more heavy aromatics. For the NiMo/Z5−0.5 and NiMo/Z5−0.8 catalysts, the selectivity of aromatics showed a quick increase, although the acid site density and HF values were very similar to those of the NiMo/Z5−0.35 catalyst. This could be attributed to the different compositions of acid sites. As the collapse of ZSM-5 structure, more nonframework acid sites (amorphous Al and Si species) were formed. Accordingly, the aromatization reaction was enhanced. This indicated that the amorphous acid sites had much higher activity for the aromatization reaction than framework acid sites. After alkaline treatment, the selectivity of C9−C15 paraffins did not change for the NiMo/Z5−0.2 catalyst, although the small mesopore was formed. As much larger mesopores were formed, the selectivity of C9−C15 paraffins significantly increased for the NiMo/Z5−0.35 catalyst. With the structure of intracrystal mesopores being destructed into the amorphous structure, the selectivity of C9−C15 paraffins increased very slowly for the NiMo/Z5−0.5 and NiMo/Z5−0.8 catalysts. This result indicated that the hierarchical structure with larger intracrystal mesopores had better shape-selective catalytic cracking for C9−C15 paraffins than pure microporous structure and amorphous structure.

0.2 catalyst. This may be due to the similar strong acidity. The increased selectivity of C16−C18 hydrocarbons could be assigned to the difference of Lewis acid/Brønsted acid (L/B) in strong acidity. The L/B ratio for the NiMo/Z5−0.2 catalyst was 2.05, which was much higher than that of NiMo/Z5 catalyst (0.19), which meant that the Lewis acid became the dominant one. Generally, the cracking activity of the Lewis acid sites was lower than that of the Brønsted acid sites. Therefore, the selectivity of C16−C18 hydrocarbons over the NiMo/Z5− 0.2 catalyst increased. Meanwhile, the strong acidity increased slightly (119.1 to 129.6 μmol/g), leading to an appreciable increase of C4−C8 hydrocarbons. Among the cracking products, the selectivity of C9−C15 hydrocarbons increased with the decrease of C4−C8 hydrocarbons over the higher alkali-treated catalysts. This might be ascribed to (1) the inhibition of deep cracking of C9−C18 hydrocarbons into C4−C8 hydrocarbons by the introduction of mesopore, especially the larger mesopore; or (2) the secondary reaction to transform C4−C8 hydrocarbons into C9−C15 hydrocarbons. The HF value of the NiMo/Z5−0.35 catalyst was higher (0.1413) than that of the NiMo/Z5−0.2 catalyst (0.1075). From the XRD result, it could be observed that the crystal structure of these two catalysts was prone to be perfect, which meant that less nonframework species existed and the acidity mainly came from the framework Al and Si species. As shown in Table 3, the acidity of the NiMo/Z5−0.35 catalyst, especially the medium and strong acidity, increased remarkably (ca. 64.8 and 69.2 μmol/g, respectively). But the C4−C8 paraffins only increased slightly (∼1%); meanwhile, the C9−C15 paraffins increased significantly by ∼13.7% (see Figure 7c). Therefore, it could be reasonably concluded that the mesopore, especially the larger one, enhanced the mass transfer to suppress the consecutive secondary cracking and improved the shape selectivity for large cracked molecules. The HF values of the NiMo/Z5−0.5 and NiMo/Z5−0.8 catalysts were almost the same with the NiMo/Z5−0.35 catalyst. Moreover, the total acidity as well as medium and strong acidity for the NiMo/Z5−0.5 catalyst was also similar to the NiMo/Z5−0.35 catalyst. However, the selectivity of C9−C15 hydrocarbons over the NiMo/Z5−0.5 and NiMo/Z5−0.8 catalysts significantly increased (∼6.5% and 16%, respectively), compared to the NiMo/Z5−0.35 catalyst. It could be strongly inferred that the transformation from C4−C8 hydrocarbons into C9−C15 hydrocarbons occurred. Meanwhile, it also could be observed that, although the total acidity decreased for the NiMo/Z5−0.8 catalyst, the selectivity of C9− C15 hydrocarbons still increased, accompanied by a significant decrease in C4−C8 hydrocarbons. This further confirmed that the increased selectivity for C9−C15 hydrocarbons came from the transformation of C4−C8 hydrocarbons. According to the literature, aromatic hydrocarbons could be eventually produced in the pores of the zeolite catalysts, as result of the aromatization reaction of C2−C10 olefins, and the conversion of olefins to aromatics increased with acid site density.37 In order to certify what reactions happened among the C4−C8 hydrocarbons, the products of different catalysts were carefully analyzed. It was found that the aromatic compounds were also produced besides paraffins. Figure 7c shows the distribution of aromatics and C4−C15 paraffins over different catalysts. The acid site density of these catalysts increased as the HF values for the NiMo/Z5, NiMo/Z5−0.2, and NiMo/Z5−0.35 catalysts each increased. However, the selectivity of aromatics gradually decreased. This result strongly indicated that the aromatization reaction of the cracking

4. CONCLUSIONS A three-gradient porous hierarchical ZSM-5 structure was formed by desilication using different NaOH concentrations. The deoxygenation pathways of triglycerides were tuned by varying the active phase-support interaction. The larger mesopore could inhibit the hydrodeoxygenation (HDO) reaction and favor the decarbonylation and decarboxylation (DCOx) reactions. The small Ni−Mo−S phase cluster mainly contributed to the DCO2 reaction; meanwhile, the large tetrahedral Mo species benefited the HDO reaction. The selectivity of C9−C15 hydrocarbons was evidently improved by the hierarchical structure, because of the inhibition of the deep cracking and the aromatization reaction among the C4−C8 olefins.

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: 86-22-27892340. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial supports by National Natural Science Fundation of China (Grant Nos. 21476169, 21476168) are gratefully acknowledged. The authors thank Ms. Yuhan Yang for her help with catalytic evaluation and Mr. Qiang Deng for his guidance for alkali treatment.

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REFERENCES

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