Catalytic Upgrading of Bio-oil over Ni-Based Catalysts Supported on

Mar 27, 2014 - Wang Yin , Arjan Kloekhorst , Robertus H. Venderbosch , Maria V. Bykova , Sofia A. Khromova , Vadim A. Yakovlev , Hero J. Heeres. Catal...
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Catalytic Upgrading of Bio-oil over Ni-Based Catalysts Supported on Mixed Oxides Xinghua Zhang,† Jinxing Long,† Wei Kong,†,‡ Qi Zhang,† Luangang Chen,† Tiejun Wang,† Longlong Ma,*,† and Yuping Li† †

Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences (CAS), Guangzhou 510640, People’s Republic of China ‡ School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, People’s Republic of China S Supporting Information *

ABSTRACT: Ni-Based catalysts using mixed oxides of Al2O3−SiO2, Al2O3−TiO2, TiO2−SiO2, and TiO2−ZrO2 as supports were evaluated for hydrotreatments using guaiacol as the model compound and characterized by N2 physical adsorption, X-ray diffraction (XRD), temperature-programmed desorption of ammonia (NH3-TPD), and temperature-programmed reduction of hydrogen (H2-TPR) techniques. The influence of the support, solvent, reaction temperature, and pressure on guaiacol conversion and product distributions were determined. Guaiacol conversion of 100% with cyclohexane selectivity of 86.4% was obtained over the Ni/TiO2−ZrO2 catalyst at the conditions of 300 °C, 4.0 MPa H2 pressure, and decalin solvent. Furthermore, this system is also efficient for real bio-oil, where nearly 19.3% of the upgraded bio-oil yield was achieved under the optimal conditions determined for guaiacol. Gas chromatography−mass spectrometry (GC−MS) analysis showed that the principal components were phenolic compounds, while the content of acids and aldehydes was negligible. The pH of bio-oil increased from 2.38 to 4.21, and the high heating value drastically increased from 13.1 to 25.8 MJ/kg.

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overcome these flaws, catalysts supported on SiO2,19,20 TiO2,17 ZrO2,17,21,22 zeolites,23 and various mixed oxides24 have been explored in recent years. For example, it was reported that ZrO2 allowed CoMoS catalysts to reach higher catalytic activities because the ZrO2 support appears to favor activation of O compounds.17,20 Moreover, the carbon deposition on the surface of the ZrO2-supported catalyst was lower than that of the sulfided CoMo/Al2O3 catalyst.17,21,25 More recently, we also found that Ni/SiO2−ZrO2 is an efficient catalyst for the production of cyclic alkanes via HDO of phenol, guaiacol, and lignin-derived phenolic compounds, suggesting that the supports of mixed oxides were attractive for the improvement of catalytic performance.26,27 The solvent also has important influence on the hydrotreatment of bio-oil. Recently, hydrocarbon solvents, such as hexane, dodecane, and tetradecane, were used as effective reaction media for the hydrotreatment of bio-oil and its model compounds.6,13,26 The main reason is that the solvent alkane can act as an effective hydrogen donor via the “hydrogen shuttling” mechanism because it has excellent solubility for H2 at its supercritical state.6 Therefore, the hydrogen-donor solvent is considered to be an excellent solvent and should be investigated in the process of bio-oil hydrotreatment. In this work, several mixed oxides were synthesized as supports and a series of Ni-based catalysts were prepared by the impregnation method. These catalysts were characterized by N2 physical adsorption, X-ray diffraction (XRD), temperatureprogrammed reduction of hydrogen (H2-TPR), and temper-

INTRODUCTION Biomass is a promising alternative energy source, owing to its renewability and zero emissions.1 Fast pyrolysis is considered the most effective technology for conversion of biomass to biooils. However, these oils cannot be directly used as transportation fuels because of their high oxygen and water contents.2,3 The oxygenated compounds in bio-oil cause high viscosity, poor thermal and chemical stability, corrosion, and immiscibility with hydrocarbon fuels.4−6 Fortunately, these flaws can be overcome with catalytic hydrotreating, which improves the property of the bio-oil by removing oxygen.7,8 Catalytic hydrotreatment is considered to be an effective method for bio-oil upgrade, and many kinds of hydrotreatment catalysts have been investigated.3,6,9−12 Most of the investigated catalysts were reported to be bifunctional catalysts, combining the hydrogenation function of active metal with hydrolysis and dehydration of the support. Thereinto, the hydrogenation catalyst usually includes base metals of Ni, Mo, and Co, noble metals of Pt, Rh, and Ru, etc.6,13−15 Hydrogenation can improve the properties of bio-oil obviously. It was reported that the yield of 65 wt % upgraded bio-oil was obtained through hydrotreating over Ru/C in a batch reactor (T, 350 °C; PH2, 20 MPa). Of particular interest is the high yield of the hydrocarbon fraction (21 wt %) in the upgraded oil.16 Apart from the component of metal, the support is also a key factor determining the hydrodeoxygenation (HDO) activity of bifunctional catalysts. Cheap γ-Al2O3 was previously used to hydrotreat oxygenated compounds for producing hydrocarbons, but severe carbon deposition was observed.17 Moreover, alumina is sensitive to water and will partially transform into boehmite under hydrothermal conditions.18 To © 2014 American Chemical Society

Received: December 9, 2013 Revised: March 27, 2014 Published: March 27, 2014 2562

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were carried out under an air flow rate of 30 mL/min with analyzers using a 10−15 mg sample and a temperature rise rate of 10 °C/min. Catalytic Activity Test. HDO reactions of guaiacol were carried out in a 250 mL stainless autoclave equipped with an electromagneticdriven impeller. The schematic diagram was shown in Figure S1 of the Supporting Information. For each run, 1.0 g of catalyst, 5.0 g of guaiacol, and 50.0 g of solvent were loaded into the autoclave. After air was displaced, the reactor was pressurized with H2 to 4.0 MPa at room temperature and sealed. The autoclave was heated to the desired temperature, while the reagents were vigorously stirred at 800 rpm. The reaction time was 8 h. Liquid samples were withdrawn from the reactor for subsequent off-line analysis when the reaction was completed. The conversions of guaiacol and the product distribution were calculated on the basis of the following formulas:

ature-programmed desorption of ammonia (NH3-TPD) and evaluated by the hydrodeoxygenaton reaction. Guaiacol was chosen as the probe reactant for initial screening and then investigation of the hydrotreatment of crude bio-oil over Nibased catalysts supported on mixed oxides. The aim was to obtain a stable upgraded bio-oil with high heating value through hydrotreatment.

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EXPERIMENTAL SECTION

Materials. The bio-oil used in this study was a viscous, dark brown liquid with a smoky odor product, obtained from fast pyrolysis of rice husk.28 The feedstock of bio-oil was filtered through the filter paper with a mesh size of 45 μm prior to the hydrotreatment experiment. All chemicals used in the study (with AR grade) were commercially available and used without further purification. Preparation of Catalysts. The supports of mixed oxides were prepared according to the following methods. Al2O3−SiO2. Appropriate amounts of Al(NO 3)3·9H2O were dissolved in deionized water. With continuously stirring, ammonia solution was dropped gradually into Al(NO3)3 solution until the pH value of solution closed approximately to 8.0, by which the Al(OH)3 precipitate was prepared. According to the similar procedure, the Si(OH)4 precipitate was prepared using Na2SiO3·9H2O and NH4NO3 as material. The two kinds of precipitates were mixed and whisked. The mixed precipitate was aged for 3 h at 60 °C. Subsequently, the precipitate was filtered and washed with deionized water to remove impurities. The obtained solid was dried overnight at 120 °C and then calcined at 500 °C for 5 h, by which the complex oxide Al2O3−SiO2 (Si/Al = 3) was prepared. TiO2−SiO2. TiCl4 was dissolved in deionized water. With the continuous stirring, the solution of ammonia was dropped gradually into the TiCl4 solution until the pH of the solution reached an approximate value of 8.0, by which the Ti(OH)4 precipitate was prepared. Si(OH)4 and Ti(OH)4 were mixed and whisked, then aged, washed, and calcined, from which the mixed oxide TiO2−SiO2 (Ti/Si = 3) was prepared. Al2O3−TiO2. The mixed oxide of Al2O3−TiO2 (Ti/Al = 1) was prepared using the precipitates of Al(OH)3 and Ti(OH)4 as material according to the same procedure mentioned previously. TiO2−ZrO2. The mixed oxide of TiO2−ZrO2 with a Ti/Zr ratio of 3 was prepared according to the literature.29 Appropriate amounts of TiCl4 and ZrOCl2 were dissolved in deionized water, and a sufficient amount of urea was added. The solution was refluxed at 95 °C until the pH of the solution reached an approximate value of 7. The precipitate was filtered, washed, dried overnight, and then calcined at 500 °C for 5 h. An appropriate support was dipped into the Ni(NO3)2 solution, followed by agitating and evaporating to dryness. The solid that remained was dried in air at 120 °C overnight and then calcined at 550 °C for 4 h. The Ni loading was 10 wt % for all catalysts tested in this work. The catalyst was crushed and sieved to 120−200 BSS mesh and reduced at 500 or 650 °C for 4 h in a flow of reducing gas (5% H2 + 95% N2) before use. Characterization of Catalysts. The Brunauer−Emmett−Teller (BET) specific surface area, average pore diameter, and pore volume of catalysts were determined by N2 isothermal (77 K) adsorption using a QUADRASORB SI analyzer equipped with a QuadraWin software system. The catalysts were characterized by XRD [X’Pert Pro MPD with Cu Kα (λ = 0.154) radiation, Philip]. Scanning electron microscopy (SEM) images were recorded on a Hitachi S-4800 instrument operated at 20 kV. H2-TPR and NH3-TPD studies of the different catalysts were carried out in a quartz tube reactor with a thermal conductivity detector (TCD). Hydrogen consumption (from 300 to 750 °C) was calculated by an external standard method using H2-TPR of CuO as the standard. The amount of reduced NiO was calculated according to the hydrogen consumption, and the reduction degree was obtained by calculation.30 Thermogravimetry (TG) studies of the used catalysts

XGUA (%) =

Si (%) =

moles(GUA)in − moles(GUA)out × 100 moles(GUA)in

moles(product)i × 100 ∑ moles(product)

(1)

(2)

where XGUA represents the conversion of guaiacol and Si represents the product selectivity. Hydrotreatments of Bio-oil. The hydrotreatments of bio-oil were also performed according to the similar conditions mentioned previously. For each run, a total amount of 50.0 g of mixed reactants (15.0 g of bio-oil and 35.0 g of decalin) and 1.0 g of catalyst was loaded into the reactor. The reactor was cooled to room temperature and opened when the reaction was completed. The gas product was collected by vacuum bag for subsequent off-line analysis. The obtained liquid product was filtered and weighed. The amount of upgraded biooil was calculated by subtracting the weight of decalin from the weight of the liquid product. The solid residues were dried at 85 °C for 12 h and then weighed. Similarly, the amount of coke formed during the hydrotreatment of bio-oil was determined by subtracting the weight of catalyst used in each run from the weight of solid residues. Product Analysis. The components for the product of guaiacol HDO were confirmed by gas chromatography−mass spectrometry (GC−MS). Quantitative analysis of product was performed by GC [Shimadzu GC2010 with a flame ionization detector (FID) and a SE30 column] with benzyl alcohol as an internal standard. The vaporization temperature was 250 °C, and the oven temperature program ranged from 50 to 250 °C at a rise rate of 10 °C/min. The qualitative and semi-quantitative analysis of the crude bio-oil and upgraded bio-oil were carried out by GC−MS (Agilent 7890A/ 5975C, Agilent, Santa Clara, CA) equipped with a column of HP-5 (30 m × 0.32 mm × 0.25 μm) and determined by the National Institute of Standards and Technology (NIST) library. The carrier gas was He with 99.995% purity, and the oven temperature program ranged from 40 °C (holding for 5 min) to 250 °C (holding for 10 min) at a rise rate of 10 °C/min. The ion trap detector had a mass range of m/z 50−500 with scan times of 1 s. The mass spectrometer ion source was 250 °C with a 70 eV ionization potential. The components of CO, CO2, and CH4 in the gas product were analyzed by GC with a TCD and TDX-01 (3 m × 3 mm) column, and C2H4, C2H6, and C3 were analyzed by GC with a FID and Porapak-Q (3 m × 3.175 mm) column. The water content of the bio-oil was determined by Karl Fischer titration (ASTM D1744, GB11146-89) with a Metrohm 787 KF Titrino. The acidity was determined with a PHC-3C precision pHmeter from Shanghai REX Instrument Factory. The high heating value was determined by a calorimeter (IKA C2000).

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RESULTS AND DISCUSSION Characterization of Catalysts. The textural structure of Ni catalysts supported on different mixed oxides was summarized in Table 1 and Figure 1. The pore size distributions of these catalysts are all in the range of 5−30

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NiO supported on Al2O3−TiO2 and TiO2−ZrO2 are weaker and broader than those of TiO2−SiO2 and Al2O3−SiO2. This suggests that the strong interaction between NiO and supports of Al2O3−TiO2 and TiO2−ZrO2 could inhibit the enlargement of the crystallite size of NiO. The crystallite sizes of NiO calculated by the Scherrer equation are listed in Table 1. It can also be found that the crystallite size of NiO supported on TiO2−ZrO2 was the smallest. The NH3-TPD profiles are presented in Figure 3. The observed desorption peaks show the acidity of Ni-based

Table 1. Textural Structure of Ni-Based Catalysts

sample

BET surface area (m2 g−1)

pore volume (cm3 g−1)

average pore diametera (nm)

Ni/Al2O3−SiO2 Ni/Al2O3−TiO2 Ni/TiO2−SiO2 Ni/TiO2−ZrO2

231.4 112.3 258.8 88.0

0.80 0.27 0.75 0.33

13.8 9.6 11.6 15.0

DNiO (nm)b 30.98 20.63 27.53 17.70

(0.2676) (0.4015) (0.3011) (0.4684)

Calculated by the formula: (pore volume × 4)/BET surface area. The NiO crystal sizes of the catalysts were calculated by the Scherrer equation, and the corresponding full width at half maximum (fwhm) values were presented in parentheses.

a b

Figure 3. NH3-TPD profiles of different catalysts.

catalysts supported on different mixed oxides. It can be found that the positions for all NH3 desorption peaks of Ni/Al2O3− TiO2 and Ni/TiO2−ZrO2 are below 400 °C. The desorption peaks of Ni/Al2O3−SiO2 and Ni/TiO2−SiO2 moved to a higher temperature, implying a stronger acid strength. It could be deduced that the order of acid strengthen for the four catalysts was Ni/TiO2−SiO2 > Ni/Al2O3−SiO2 > Ni/TiO2− ZrO2 > Ni/Al2O3−TiO2 according to the position of the NH3 desorption peak. The H2-TPR results of different catalysts are given in Figure 4 and Table 2. The profiles of Ni/Al2O3−SiO2 and Ni/TiO2− SiO2 show a two-stage reduction behavior. The peak centered at the lower temperature can be assigned to the reduction of NiO that interacts weakly with the support, and the peak centered at the higher temperature can be assigned to the reduction of NiO strongly interacting with the support.

Figure 1. Pore size distribution of different Ni-based catalysts.

nm, suggesting that they are mesopore materials. It should be noticed that the average pore diameter of Ni/TiO2−ZrO2 is the biggest, although its BET surface area and pore volume are relatively small. Figure 2 exhibited the XRD profiles of Ni-based catalysts supported on different mixed oxides. Sharp peaks assigned to

Figure 2. XRD patterns of different Ni-based catalysts.

TiO2 (2θ = 5.4°, 48.1°, 53.9°, and 55.1°) were observed in the XRD profiles of Ni/Al2O3−TiO2 and Ni/TiO2−ZrO2. On the contrary, the characteristic peak of TiO2 was not found in the XRD profile of Ni/TiO2−SiO2 because TiO2 was welldispersed on the surface of amorphous SiO2. The characteristic peaks of NiO (2θ = 37.4°, 43.5°, 63.2°, and 75.4°) can be clearly observed for all catalysts. The characteristic peaks of

Figure 4. H2-TPR profiles of different catalysts. 2564

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catalysts. The catalyst with a bigger pore diameter can facilitate the diffusion of reactants and products. Moreover, some small particles of metal Ni can enter into the pore of the catalyst. Hence, more HDO reactions were completed over catalyst Ni/ TiO2−ZrO2. In addition, to reduce NiO completely, these Ni-based catalysts supported on different mixed oxides were reduced at a higher temperature (650 °C) in a flow of reducing gas. The results for guaiacol HDO are also presented in Table 3. It can be clearly seen that the guaiacol conversion and cyclohexane selectivity increased obviously, as expected for all tested catalysts. Especially, the highest cyclohexane yield was obtained over the catalyst Ni/TiO2−ZrO2. This suggests that the catalyst Ni/TiO2−ZrO2 has a higher catalytic activity than that of other catalysts for the guaiacol HDO. Thus, the catalyst Ni/TiO2− ZrO2 reduced at 650 °C was chosen for the rest of the experiments. Effect of the Solvent on Guaiacol HDO. To explore the influence of the solvent, guaiacol HDO reactions were carried out using water, octane, and decalin as the solvent. The results are presented in Figure 5. It can be seen that the conversion of

Table 2. Results for the H2-TPR of Various Catalysts peak position (°C) sample

θ1

θ2

Ni/Al2O3−SiO2 Ni/Al2O3−TiO2 Ni/TiO2−SiO2 Ni/TiO2−ZrO2

405 625 450 525

580 670

H2 consumption (mmol g−1)

degree of reduction (%)

0.45 0.19 0.54 0.48

33.84 14.31 40.17 36.07

It was reported that only a reduction peak centered at 450 °C was observed for the catalyst Ni/TiO2.31 The reduction peak of NiO shifted to 525 °C in the case of Ni/TiO2−ZrO2 and 625 °C in the case of Ni/Al2O3−TiO2, suggesting an intensified interaction between NiO and the supports of mixed oxides. Moreover, the reduction peak of Ni/Al2O3−TiO2 was very small, and its degree of reduction is only 14.31%, implying the inefficiency of the H2 reduction for NiO that interacted strongly with the support. Catalytic Activity. The catalytic activity was evaluated by the guaiacol HDO reaction under various conditions. Guaiacol was chosen as a model compound for bio-oil because it contains three types of C−O bonds for CAR−OH, CAR−OCH3, and CARO−CH3, which are frequently found in phenolic compounds derived from bio-oils. Moreover, guaiacol and its derivatives are the main components of bio-oil. Table 3 gathers the results of guaiacol conversion and product distribution over different catalysts in solvent octane. It can be clearly seen that guaiacol conversion increased with the acid strength of the catalysts. At 300 °C, guaiacol was converted completely with 93.8% of phenol selectivity over catalysts of Ni/TiO2−SiO2. It could be speculated that the acid sites play a vital role for the high conversion and phenol selectivity. The C−O bond is adsorbed and activated on the acid sites, which proceeded hydrogenolysis of the C−O bond to form catechol, followed by the elimination of the hydroxyl group to produce phenol.26,27 In addition, trimethylphenol was detected in the product over the catalyst Ni/TiO 2 −SiO 2, which suggested that the intermolecular transmethylation reaction occurred because of the strong acid of Ni/TiO2−SiO2.2,32 Of these tested catalysts, the cyclohexane selectivity over Ni/ TiO2−ZrO2 was the highest. One reason is that the crystallite size of NiO supported on TiO2−ZrO2 is smaller than those of the other catalysts, as discussed in the section of XRD analysis. The catalyst with a smaller size can improve the catalytic activity for hydrogenation. Another reason is that the pore diameter of Ni/TiO2−ZrO2 is bigger than those of the other

Figure 5. Influence of the solvent on guaiacol conversion and cyclohexane selectivity. Reaction conditions: T, 300 °C; P, 4 MPa; t, 8 h; and catalyst, Ni/TiO2−ZrO2.

guaiacol was lower when water was used as the solvent. Two possible reasons could afford this: (i) The solubility of guaiacol in water is very low, which is unfavorable for the HDO reaction of guaiacol. (ii) It was reported that the oxygenated compound will adsorb much more strongly on the acid site than the nonpolar or less polar compounds because of its strong polarity and well-accessible basic oxygen electronic doublet.33 Thus, competition adsorption occurred on the acid sties of the

Table 3. Catalytic Performance of the Different Ni-Based Catalysts for Guaiacol HDOa product selectivity (%) sample

conversion (%)

cyclohexane

cyclohexanol

phenol

othersb

Ni/Al2O3−SiO2 Ni/Al2O3−TiO2 Ni/TiO2−SiO2 Ni/TiO2−ZrO2 Ni/Al2O3−SiO2c Ni/Al2O3−TiO2c Ni/TiO2−SiO2c Ni/TiO2−ZrO2c

75.5 30.8 100 60.3 86.9 56.5 100 94.1

21.2 5.6 1.8 29.9 36.2 26.9 33.7 46.5

0.2 12.0 0.2 8.8 1.1 12.9 0.6 15.8

56.4 75.3 93.8 45.2 46.1 50.7 56.7 26.9

22.2 7.1 4.2 16.1 16.6 9.5 9.0 10.8

Reaction conditions: T, 300 °C; P, 4 MPa; solvent, octane; and t, 8 h. bOthers including anisole, dimethoxybenzene, methylguaiacol, and some compounds that cannot be determined by chromatography. cCatalysts were reduced 4 h by H2 under 650 °C.

a

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be successfully converted into cyclohexane when the reaction temperature further increased to 300 °C, thus resulting in an evident decrease for the yield of phenol. In decalin, the hydrotreatment of guaiacol over various H2 pressures was also carried out over Ni/TiO2−ZrO2. As shown in Figure 7, the conversion of guaiacol increased from 51.6 to

catalyst between H2O and guaiacol, which could inhibit the guaiacol HDO reaction because the polarity of H2O is stronger than that of the C−O bond of the guaiacol molecule. Contrary to water, octane and decalin are nonpolar solvents. Thus, guaiacol could be activated because of the adsorption of the C− O bond on the acid sites, favoring the HDO reaction. Noticeably, in decalin, 100% of guaiacol conversion with 86.4% of selectivity for cyclohexane was obtained over Ni/ TiO2−ZrO2. This result is comparable to that of guaiacol HDO over noble metals of Pt, Pd, and Rh.25 Decalin is an excellent hydrogen-donor solvent. Dissociated hydrogen with high reactive activity can be easily obtained from the solvent decalin, which could promote the hydrogenation of the aromatic ring, resulting in high guaiacol conversion and cyclohexane selectivity. Therefore, the next experiments for the hydrotreatment of guaiacol were carried out in solvent decalin. Effects of the Temperature and Pressure on Guaiacol HDO. The temperature is a key factor for the HDO of guaiacol. Figure 6 shows the guaiacol conversion and product

Figure 7. Influence of the pressure on guaiacol conversion and cyclohexane selectivity. Reaction conditions: T, 300 °C; solvent, decalin; t, 8 h; and catalyst, Ni/TiO2−ZrO2.

100% when the initial H2 pressure increased from 3.0 to 4.0 MPa. Meanwhile, the selectivity of cyclohexane also obviously increased with the increased H2 pressure. The solubility of H2 in the solvent decalin would be increased with the increased H2 pressure, which could promote the hydrogenation reaction. However, the selectivity of cyclohexane decreased slightly when the initial H2 pressure further increased from 4.0 to 5.0 MPa. The possible reason is that a higher H2 pressure promotes the cracking of cyclohexane during the HDO process, resulting in the decrease of cyclohexane selectivity.35 Resistance of Coking. TG curve and SEM image of the used Ni/TiO2−ZrO2 catalyst are exhibited in Figure 8. There

Figure 6. Conversion of guaiacol and selectivity of phenol and cyclohexane at different temperatures. Reaction conditions: P, 4 MPa; solvent, decalin; t, 8 h; and catalyst, Ni/TiO2−ZrO2.

distributions over the Ni/TiO2−ZrO2 catalyst at various temperatures. It can be found that the conversion of guaiacol was only 19.6% and the cyclohexane selectivity was only 4.3% at 240 °C. However, in the investigated temperature, the guaiacol conversion and cyclohexane yield increased with the increased temperature. This suggests that a higher temperature can promote the guaiacol HDO reaction.34 It is interesting to find that the yield of phenol exhibits a maximum at 270 °C. This can be explained as this: guaiacol was converted into phenol intermediate first and then converted into cyclohexane through HDO (Scheme 1). The phenol intermediate increased with the increased temperature. However, at a lower temperature (240 and 270 °C), the HDO of phenol (step 2) is difficult to occur, leading to the increase of the phenol yield. Phenol can Scheme 1. Tentative Reaction Pathways for the Guaiacol HDO

Figure 8. TG curve and SEM images of the used Ni/TiO2−ZrO2 catalyst: (a) SEM image before reaction, (b) SEM image after reaction, and (c) EDS image after reaction. 2566

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Figure 9. Hydrotreatment of bio-oil over the Ni/TiO2−ZrO2 catalyst in decalin.

Two sharp phases for the aqueous and organic phases were obtained after hydrotreatment. The yield of water (57.3%) was higher than the water content of crude bio-oil (51.5%). The increased water is derived from the undesirable repolymerization reaction and the catalytic HDO reaction.36,37 The organic phase, which included solvent decalin and upgraded bio-oil, was analyzed by GC−MS. The changes for main compositions of raw bio-oil and upgraded bio-oil can clearly be seen from Table 4. It can be seen that organic acids, such as acetic and propanoic acids, significantly decreased after hydrotreating. These organic acids were converted into CO and CO2 through decarbonylation and decarboxylation. The detected CO, CO2, and C2−C3 alkanes in gas products are strong evidence for these reactions. The compositions of aldehydes and ketones were significantly changed after hydrotreatment. As shown in Table 4, hydroxyacetaldehyde, hydroxylpropanone, and 5-(hydroxymethyl)-2-furancarboxaldehyde (HMF) identified in the crude bio-oil completely disappeared from the upgraded bio-oil. Interestingly, many methyl-substituted cyclopentanones were observed in the upgraded bio-oil. This is in agreement with the literature, in which 2-methylcyclopentanone was found in the upgraded bio-oil obtained over the Pd/C catalyst.38 These cyclopentanones were reported to be produced from furfurals via hydrogenation.38,39 Phenolics are the principal components for the upgraded biooil. It can be clearly found that the content of phenolics (especially the alkyl-substituted phenolics) drastically increased after hydrotreating. It is speculated that the increased alkylsubstituted phenolics were produced from the hydrogenation/ HDO of phenolic compounds contained in crude bio-oil. As shown in Table 5, vanillin could be converted into guaiacol because of the decarbonylation under cracking conditions40 or converted into 4-methyl-2-methoxyphenol via hydrogenation of the aldehyde group. Homovanillyl alcohol (4-hydroxy-3methoxybenzeneethanol) could be converted into 4-ethyl-2methoxyphenol through dehydroxylation over the bifunctional catalyst Ni/TiO2−ZrO2. 4-Allyl-2-methoxyphenol and 2methoxy-4-(1-propenyl)-phenol could be converted into 2methoxy-4-propylphenol through hydrogenation of the double bond. 1-(4-Hydroxy-3-methoxyphenyl)-ethanone, which is a representative component with a high content in bio-oil,38 could be theoretically converted into 4-ethyl-2-methoxyphenol through HDO of the carbonyl group. However, this does not appear to be the case. Only a slight increase for the content of 4-ethyl-2-methoxyphenol was observed in the upgraded bio-oil. The possible reason is that part of 1-(4-hydroxy-3-methoxyphenyl)-ethanone could polymerize with aldehydes or ketones contained in the bio-oil through aldol condensation because of its active α-hydrogen.

was only a weight loss of about 4.5% for the used catalyst, which could be attributed to the decomposition/combustion of residual polymers and coke deposited on the surface of the catalyst. The aggregation and agglomeration of Ni particles were not observed from the SEM images of the fresh and used catalysts. Their surface morphologies did not differ significantly from each other. About 5.34% carbon was found on the surface of the used catalyst according to the SEM−energy-dispersive spectrometry (EDS) analysis. It was approximately consistent with TG analysis. To investigate the effect of deposited carbons, a batch of Ni/TiO2−ZrO2 was used repeatedly for guaiacol HDO at 300 °C and under an initial pressure of H2 of 4.0 MPa. The guaiacol conversions for the two runs were 100 and 90.2%, respectively. A significant drop in conversion of guaiacol was observed over the reused catalyst. It can be induced that the deposited carbons cover on Ni and acidic sites, leading to a decrease for the catalytic activity.5,25 Upgrading of Bio-oil. The upgrading of bio-oil was carried out under the optimal conditions established for guaiacol (catalyst, Ni/TiO2−ZrO2; T, 300 °C; solvent, decalin; and PH2, 4.0 MPa). As shown in Figure 9, it can be seen that the crude bio-oil was recovered from the system as gas, water, upgraded bio-oil (organic phase), and coke after hydrotreating. The yield of the gas product was only 2 wt %, and CO2, CO, CH4, and C2 hydrocarbons were considered as the principal components (Figure 10). It should be noted that the sum of the gas product, water, upgraded bio-oil, and coke was 95.3%. Solid residues that adhered to the reactor wall and stirrer were the key factor for the 4.7% weight loss.

Figure 10. Composition of the gas product obtained from the hydrotreatment of bio-oil. 2567

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Table 4. Main Components of Raw Bio-oil and Upgraded Bio-oil as Determined by GC−MS

Table 5. Corresponding Components in Crude Bio-oil and Upgraded Bio-oil via Hydrotreatment

relative content (%) compounda acids acetic acid propanoic acid butanoic acid benzeneacetic acid, 4-hydroxy-3-methoxyaldehydes furfural acetaldehyde, hydroxy2-furancarboxaldehyde, 5-(hydroxymethyl)4-hydroxy-2-methoxycinnamaldehyde ketones 2-propanone, 1-hydroxy2-cyclopenten-1-one 1,2-cyclopentanedione, 3-methyl2-cyclopentan-1-one, 2,3,4-trimethylethanone, 1-(2-furanyl)2-cyclopentan-1-one, 3-methyl2-cyclopentan-1-one, 2,3-dimethyl2-cyclopentan-1-one, 2-methylortho-hydroxypropiophenone phenols phenol, 2-methoxyphenol, 2-methoxy-4-methylphenol, 2-methylphenol phenol, 4-ethyl-2-methoxyphenol, 3,5-dimethylphenol, 4-methylphenol, 2-methoxy-4-propylphenol, 3-ethylphenol, 2,4,6-trimethylphenol, 4-propylvanillin 3-allyl-6-methoxyphenol phenol, 2-methoxy-4-(1-propenyl)ethanone, 1-(4-hydroxy-3-methoxyphenyl)homovanillyl alcohol others benzene, butyl-

crude bio-oil

upgraded bio-oil

25.28 20.74 1.95 1.22 1.37

3.64 2.45

12.51 2.80 5.88 1.83

2.16 2.16

2.00 12.57 9.27 1.06 2.24

40.82 4.73 5.75 1.51 1.81 2.20 2.16 0.51

1.19

9.45

1.13 1.58 1.09 1.38 3.17 1.10 36.94 5.67 5.61 3.02 2.19 2.80 5.64 1.88 3.39 3.91 1.00 1.83

aqueous phase. In Figure 9, it can be seen clearly that the upgraded bio-oil can be blended with hydrocarbon (decalin) because hydrocarbons and more hydrophobic groups were produced during hydrotreatment. To evaluate the properties of the upgraded bio-oil, pH value, high heating value, and water content of upgraded bio-oil were also quantified and presented in Table 6. After hydrotreating Table 6. Properties of Crude Bio-oil and Upgraded Bio-oil

2.52 2.37 3.36 11.09

pH water content (wt %) high heating value (MJ kg−1)

crude bio-oil

upgraded bio-oila

2.38 51.4 13.1

4.21 1.5 25.8b

a

The upgraded bio-oil was not separated from decalin. bThe high heating value of decalin was subtracted from that of the tested samples.

2.81 2.18 2.18

a

Compounds listed are those represented by more than 1% of the total peak area.

over the Ni/TiO2−ZrO2 catalyst, the pH value of bio-oil increased from 2.38 to 4.21. Two reasons may account for the significant increase in the pH value of upgraded bio-oil. The first reason is, as referred anteriorly, that carboxylic acid groups contained in organic acids (such as acetic and propanoic acids) were converted into CO and CO2. The second reason is that most organic acids were moved to the aqueous phase because of their hydrophilic group. More interestingly, the water content of upgraded bio-oil is almost negligible, resulting in a sharp increase for the high heating value. In comparison to the result reported by Xu et al.42 that the pH value of the bio-oil increased slightly from 2.33 to 2.77 and the high heating value increased slightly from 13.96 to 14.17 MJ kg−1 after the hydrotreating process over MoNi/γ-Al2O3, this result is more attractive because the upgraded bio-oil is more suitable to be used as fuel.

In addition, depolymerization of the lignin oligomer is another source of increased phenolics.33,38 Many lignin-derived oligomers that could not be detected by GC−MS were contained in the bio-oil. They would be further depolymerized into light-molecule phenolics under the process of hydrotreatment over the catalyst Ni/TiO2−ZrO2. For example, the detected components of 4-propyl-phenol in upgraded bio-oil may originate from the unit of p-coumaryl alcohol. Crude bio-oil, immiscible with hydrocarbon fuels, has a complex microstructure of microemulsion.41 The microemulsion would be destroyed because of the decrease of hydrophilic compounds (such as organic acid) during the process of hydrotreatment, forcing water to move to the 2568

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Unfortunately, coke-like polymers deposited on the surface of the catalyst could be observed by the naked eye. A total of 18% of the coke yield was obtained during the process of hydrotreatment. These coke-like polymers were derived from the polymerization of a unstable component and lignin-derived oligomers. It could be induced here that the desired hydrotreatment reactions are in competition with polymerization reactions, which can lead to the formation of coke because the rate of polymerization is faster than the rate of the hydrotreatment reaction.37,38 The formed coke deposits on the surface of the catalyst, resulting in the decline of the catalytic activity. This is the reason why the yield of hydrocarbons was low in the upgraded oil. Thus, improvements and optimization of catalysts and reaction conditions to avoid coke are required in further studies.

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CONCLUSION Ni-Based catalysts supported on mixed oxides of Al2O3−SiO2, Al2O3−TiO2, TiO2−SiO2, and TiO2−ZrO2 were evaluated for hydrotreatment using guaiacol as the model compound. Ni/ TiO2−ZrO2 was found to be the most efficient, where 100% of guaiacol conversion with 86.4% of cyclohexane selectivity can be obtained in the hydrogen-donor solvent decalin at 300 °C and 4 MPa H2 pressure. Furthermore, it can also upgrade the crude bio-oil significantly. Under the optimal conditions, the yield of upgraded bio-oil reached 19.3 wt % with pH, water content, and high heating value of 4.21, 1.5 wt %, and 25.8 MJ kg−1, respectively. GC−MS analysis demonstrated that the content of acids and aldehydes was reduced obviously after hydrotreatment, whereas phenolic compounds were detected to be the main components in the upgraded bio-oil. This result implied that the properties of bio-oil can be effectively improved though hydrotreatment.

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ASSOCIATED CONTENT

S Supporting Information *

Schematic diagram of the reactor. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-20-8705-7673. Fax: +86-20-8705-7673. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from the National Natural Science Foundation of China (Projects 51306191 and 51276183), the International Science and Technology Coorperation Program of China (Project 2012DFA61080), and the National Science and Technology Pillar Program (2014BAD02B01).

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