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Mar 2, 2015 - The deactivation of bifunctional Ni/HZSM-5 catalysts is essentially due to the formation of heavy secondary products (i.e., coke) in the...
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Coke Deposition on Ni/HZSM‑5 in Bio-oil Hydrodeoxygenation Processing Yu Li,†,‡ Changsen Zhang,†,‡ Yonggang Liu,†,‡ Xiaoxue Hou,†,‡ Ruiqin Zhang,*,†,‡ and Xiaoyan Tang‡ †

College of Chemistry and Molecular Engineering, and ‡Research Institute of Environmental Science, Zhengzhou University, Zhengzhou, Henan 450001, People’s Republic of China ABSTRACT: The deactivation of bifunctional Ni/HZSM-5 catalysts is essentially due to the formation of heavy secondary products (i.e., coke) in the hydrodeoxygenation (HDO) of bio-oil. Coke deposited on the Ni/HZSM-5 catalyst was characterized using analytical techniques, including Fourier transform infrared spectroscopy (FTIR), thermogravimetric (TG) analysis, and transmission electron microscopy (TEM). The type and quantitative distribution of the deposited coke on the Ni/HZSM-5 catalyst were determined by the dissolution of the aluminum−silicate matrix in a hydrofluoric acid solution. The composition of soluble coke was obtained by gas chromatography−mass spectrometry (GC−MS) measurements. It is found that coke can be divided into soft coke, hard coke, and graphite at different reaction temperatures. Organic reactants can be transformed into soft coke and hard coke by different chemical reactions, such as alkylation, aromatization, hydrogen transfer, and dehydrogenation. The quantitative distribution between soluble and insoluble coke on the Ni/HZSM-5 catalyst in the HDO of bio-oil and the related kinetic characteristics of coke were presented in the paper.

1. INTRODUCTION The awareness of global warming and decreasing crude oil reserves have highlighted the need for alternative sustainable fuel supplies. The biomass to liquid (BtL) process is a promising route for producing high-energy-density fuels.1 Fast pyrolysis is a good method for converting biomass to liquid products known as bio-oils. However, these oils cannot be directly used as transportation fuels because of their high oxygen and water contents. Consequently, upgrading such biooils is necessary to reduce the oxygen content.2 Hydrodeoxygenation (HDO) is considered the most effective method for upgrading bio-oil. Noble metal catalysts, such as Pd, Rh, Ru, and possibly Pt, could be used as potential catalysts for HDO synthesis; however, the high price of these metals makes them unattractive.3 Co−MoS2 and Ni−MoS2 have been the most frequently used catalysts for HDO;4 however, their activity decreases during prolonged operations because of their transformation from a sulfide form to an oxide form.5 An improvement to the method by co-feeding H2S to the system can regenerate sulfide sites and stabilize the catalyst, but the production of compounds, such as thiols and sulfides, contaminates the environment. Zeolite catalysts are largely employed as catalysts in refining and petrochemical processes, owing to their shape selectivity and the remarkable properties of their active sites.3 The transition metals can activate hydrogen, and zeolite played a very good cracking effect because of its acid sites and porous structure; hence, we try to combine them in the process of biooil HDO. Among them, a Ni/Zeolite Socony Mobil-Five (Ni/ HZSM-5) catalyst seems effective, scalable, and stable for HDO processes.2 However, catalyst deactivation is a pronounced problem in HDO. It is well-known that catalyst deactivation is a very complicated process. For the Ni/HZSM-5 catalyst, deactivation is strongly dependent upon the zeolite pore structure, the acidic properties (the principle function of Lewis acid sites is to bind © 2015 American Chemical Society

species to the catalyst surface; Brønsted sites function by donating protons to the compounds of relevance, forming carbocations, which are believed to be responsible for coking), the reaction temperature, and the nature of the reactant.3,6 The deactivation could occur by nitrogen or water poisoning, catalyst sintering, metal deposition (specifically alkali metals), or carbon deposition (i.e., coking).7 Carbon deposition has proven to be a widespread problem and is the primary path of catalyst deactivation. Carbon deposition is also very complex and involves many successive steps, in which the intra- and intermolecular condensation reactions of reactants and/or products play key roles. These reactions are generally reversible under specific operating conditions, and the concentration of the products is limited by thermodynamic equilibrium. Their products are called “reversible coke”. These condensation reactions generally undergo nearly irreversible secondary reactions involving dehydrogenation or hydrogen transfer to form stable reaction products, such as oligomers or graphite carbon, which are difficult to remove from an acid catalyst. Such products have low volatility and solubility and are often retained within pores or on the outer surface of the catalyst. Unlike model compounds, bio-oil is directly used to study carbon deposition on zeolite catalysts in HDO. In this paper, coke formation, its quantitative distribution on the Ni/HZSM-5 catalyst, and the related mechanism in the bio-oil HDO process are reported.

2. EXPERIMENTAL SECTION 2.1. Chemicals. All reagents used were analytical-grade. Toluene, n-butyl alcohol, and hydrogen fluoride were obtained from Sinopharm Chemical Reagent Co., Ltd. HZSM-5 was purchased from the Nankai University Catalyst Factory. Bio-oil was supplied by a fast pyrolysis of Received: November 4, 2014 Revised: March 1, 2015 Published: March 2, 2015 1722

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Energy & Fuels corn at 500 °C in a bench-scale fluidized-bed reactor at Zhengzhou University. Properties of raw and upgraded bio-oil are presented in Table 1.

matrix in a hydrofluoric acid solution (40%) at room temperature.11 Finally, because the carbonaceous compounds are insoluble, a final solvent extraction step using CH2Cl2 recovered the black particles (“insoluble coke”).9 2.5. Analytical Methods. Transmission electron microscopy (TEM) images were recorded on a Tecnai G2 20 S-TWIN transmission electron microscope (FEI, Netherlands) at an accelerating voltage of 200 kV. A Micromeritics Nova 1000e system (Quantachrome) was employed for the determination of the specific surface area and porosity by nitrogen adsorption and desorption. The surface area of the catalyst was calculated according to the Brunauer−Emmett−Teller (BET) equation. The micropore volume was calculated using the tplot method. The elemental compositions of both bio-oil and deactivated catalyst were determined by a Thermo Electron Corporation Flash EA 1112 analyzer (Delft, Netherlands). Gas chromatography−mass spectrometry (GC−MS) analyses were performed for different organic compositions on the spent catalyst in an Agilent 7890A-5975C GC equipped with a DB-FFAP capillary column (30 m × 0.25 mm × 0.25 μm). The GC split was 1:100, and the injector temperature was set at 250 °C with an injection volume of 1 μL. The oven temperature was kept at 50 °C for 3 min, increased to 200 °C at a rate of 3 °C min−1, and then held at 200 °C for 50 min. Helium was used as the carrier gas with a constant flow rate of 1 mL min−1. The mass spectrum analyses employed a 70 eV electron impact ionization source. The compositions of HDO products were compared to the NIST08 spectrogram database. Fourier transform infrared (FTIR) spectrograms of the oil phase and catalysts were recorded by a Bruker Alpha Class 1 instrument. Typical quality assurance/control procedures were closely followed to ensure data adequacy. Thermogravimetric (TG) studies of the used catalysts were performed under an air flow rate of 60 mL min−1 with a STA 409 PC thermal analyzer (NETZSCH) using 5 mg samples at a ramp rate of 10 °C min−1.

Table 1. Properties of Raw and Upgraded Bio-oil upgraded bio-oil water content (wt %) heat value (MJ kg−1) viscosity (mm2/s) yield of oil (%) elemental analysis (wt %) C H O N

raw bio-oil

250 °C

280 °C

300 °C

330 °C

31.28 13.03 7.10

3.26 34.04 3.86 52.8

2.54 35.82 2.76 43.89

2.03 36.51 1.90 33.1

1.18 37.28 1.46 29.5

34.91 7.64 56.75 0.34

67.97 9.56 23.53 0.42

66.98 9.34 23.15 0.41

66.89 9.16 22.05 0.41

75.89 8.63 15.28 0.20

2.2. Catalyst Preparation. Ni/HZSM-5 catalysts containing 15 wt % Ni were prepared through the wet impregnation of HZSM-5 (Si/Al = 50) with Ni(NO3)2·6H2O. During the preparation, ethylene glycol (EG) was added to the metal nitrate aqueous solution at a Ni/EG molar ratio of 1:1.8 Then, the solution was evaporated, and the residues were dried at 110 °C overnight, calcined in air at 400 °C, and reduced in 60% H2/N2 at 460 °C for 4 h. 2.3. Apparatus and Experimental Procedures. Reactants were stirred at 650 rpm with a magnetic stirrer in a 500 mL autoclave (Weihai Automatic Control). The temperature was controlled using an electric jacket combined with a thermocouple, and the pressure was regulated by a back-pressure valve. The reactor was filled with 60 g of bio-oil, 40 g of organic solvents (i.e., 20 g of toluene and 20 g of nbutyl alcohol), and 5 g of Ni/HZSM-5 catalyst. Subsequently, the reactor was purged 3 times at 3 MPa with H2 and eventually pressurized with 2.0 MPa H2 at room temperature. The reactor was heated to the intended reaction temperature at a heating rate of 3.0 °C min−1. The volume of off-gas was metered by a wet-type gas meter, and off-gas was collected using a gas bag during the reaction for the detection of the gaseous composition by gas chromatography (GC). After completion of the reaction, the reactor was cooled to ambient temperature.9 The catalysts were separated from the liquid phase by filtration. The liquid phase was then separated via a separator funnel into 2 or 3 phases (i.e., light/heavy oils and a water phase). 2.4. Carbon Deposition Analysis. To determine the coke composition and distribution, the carbonaceous compounds were separated by the following method, which is limited to a molecular sieve catalyst with a porous structure:10 first, the compounds were separated by ethyl alcohol (CH3CH2OH) evaporation to remove biooil retained on the outer surface of the deactivated catalyst sample. Second, the compounds were treated with dichloromethane (CH2Cl2) to recover the coke precursor and any soluble coke retained on the spent catalyst.10 Then, the carbonaceous compounds were liberated from the micropores of the zeolite by dissolution of the aluminosilicate

3. RESULTS AND DISCUSSION 3.1. TEM Imaging. The morphology of the coke on the spent catalyst was imaged by TEM and is shown in Figure 1. At 250 °C, the spent catalyst was a gray color (i.e., the same color as unspent catalyst), and at 280 °C, it was a dark gray color. When the temperature reached 330 °C, the catalyst turned black. TEM images of catalysts at 280 °C revealed stacked features and many filament-like carbon strands similar to carbon nanotubes.12 As the temperature increased, the filamentous carbon disappeared and a flat plate containing large carbon-rich molecules was observed to form. The TEM images also revealed that most of the deposited filamentous carbon was converted to the carbon-rich flat plate within the temperature range of 280−330 °C.13 Filamentous carbon and carbon-rich flat plate always have low volatility and solubility

Figure 1. TEM photographs of the spent catalyst at different reaction temperatures. 1723

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Energy & Fuels Table 2. Surface Area (SBET), Micropore Surface (Smicro), Total Pore Volume (Vtotal), and Micropore Volume (Vmicro) of the Investigated Catalystsa catalyst

SBET (m2/g)

Smicro (m2/g)

Vtotal (cm3/g)

Vmicro (cm3/g)

Z0 Z1 Z2 Z3 Z4

279.1 155.7 81.0 15.7 1.1

254.9 114.6 55.1 4.4 0.4

0.154 0.126 0.071 0.022 0.004

0.104 0.059 0.029 0.002 0.002

a

Catalyst, 15 wt % Ni/HZSM-5 (Si/Al = 50): Z0, fresh; Z1, coked at 250 °C; Z2, coked at 280 °C; Z3, coked at 300 °C; and Z4, coked at 330 °C. Reaction conditions: time, 1 h; n, 650; P, 2 MPa; and K, 3 °C min−1.

and are often retained within pores or on the outer surface of the catalyst.7 This phenomenon seemed to be the main factor for catalyst deactivation because of relatively low surface areas and total pore volumes, which can be seen in Table 2. 3.2. FTIR Characterization. Figure 2 shows the FTIR spectra of Ni/HZSM-5 catalysts after bio-oil HDO processing at different reaction temperatures. The experimental conditions were as follows: an experiment time (t) of 1 h, a stirring speed (n) of 650 rpm, an initial hydrogen pressure (P) of 2 MPa, and a heating rate (K) at 3 °C min−1. Several absorption bands can be found between 500 and 4000 cm−1.2 The strong absorption band at 600 and 1095 cm−1 reflects T−O−T (T = Al or Si) bending vibrations from the catalyst support.14 The bands at 1450−1630 cm−1 represent the stretching vibrations of aromatic rings.13 The bands at 2921 and 2963 cm−1 represent aliphatic groups. The peak at 3413 cm−1 is associated with OH.2 Additionally, as the temperature increased from 200 to 280 °C, the strength of the peak at 3473 cm−1 slowly decreased, indicating that the HZSM-5 pores were increasingly blocked. Coke molecules trapped within the pores of HZSM-5 could potentially block reactant molecules from reaching acid sites; this may be the main reason for catalyst deactivation. The bands at 2963 and 2921 cm−1 represent the stretching

Figure 3. TG profiles of the catalysts at different temperatures. Reaction conditions: organic solvents (50 g of toluene and 50 g of nbutyl alcohol); catalyst, 15 wt % Ni/HZSM-5 (Si/Al = 50) (5 g); t, 1 h; n, 650; P, 2 MPa; and K, 3 °C min−1.

vibrations of CH3 and CH2, respectively. As the reaction temperature increased, these peaks increased in strength, indicating strong alkylation reactions. The bands at 1530 and 1630 cm−1 represent CC stretching vibrations in aromatic rings. The peaks at 1630 cm−1 disappeared, and a band at 1600 cm−1 appeared, as the temperature increased from 280 to 330 °C. These observations likely correlate with the formation of soluble coke. After bond rearrangements and condensation, the soluble coke was then transformed into hydrogen-poor aromatic species via alkylation reactions and hydrogen-transfer steps as the reaction temperature increased. 3.3. TG Analysis. To explore the source of coke formation, the control experiments were performed in the first place. As shown in Figure 3, when only solvents (1-butanol and toluene) existed in the reaction system, the weight losses (indicate that coke formation from the solvent itself) were 4.2, 4.9, 6.8, and

Figure 2. FTIR spectra of spent catalysts at different reaction temperatures. Reaction conditions: catalyst, 15 wt % Ni/HZSM-5 (Si/Al = 50); t, 1 h; n, 650; P, 2 MPa; and K, 3 °C min−1. 1724

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Figure 4. TG profiles of the catalysts at different temperatures. Reaction conditions: catalyst, 15 wt % Ni/HZSM-5 (Si/Al = 50); t, 1 h; n, 650; P, 2 MPa; and K, 3 °C min−1.

exhibited different properties. At 250−350 °C, the spent catalysts coked at 250 °C showed obvious weightlessness, indicating the deposition of soft carbon, which is soluble in organic solvents.15 At 300−450 °C, the spent catalyst coked at 280 °C showed that some soluble carbon burned off or was transformed into derivatives with higher molecular weights. The spent catalyst coked at 300 °C showed evidence of deposited hard coke (or amorphous graphite), which is insoluble in organic solvents. The spent catalysts coked at 330 °C showed evidence of deposited graphite or amorphous graphite.16 On the basis of these results, different species of carbon were deposited on the spent catalysts at different temperatures. Figure 4b shows the heat release curve of the used catalysts, revealing the types of carbon deposited. Catalysts coked at 250 °C exhibited three exothermic peaks at 350, 450, and 600 °C. This indicates that three different types of carbon were deposited on the spent catalysts: the peak at 350 °C is primarily attributed to aliphatic derivatives; the peak at 450 °C is attributed to soft coke; and the peak at 600 °C is characteristic of hard coke.17 Catalysts coked at 280 °C exhibited two exothermic peaks at 350 and 450 °C. Catalysts coked at 300 °C exhibited two exothermic peaks at 400 and 500 °C, which represent soft coke and hard coke (amorphous graphite), respectively. Catalysts coked at 330 °C showed only one exothermic peak at 650 °C; the asymmetric peak may indicate multiple coke compositions deposited on the catalyst and is likely a hard coke and graphite mixture.18 3.4. Soluble Coke Composition. Figure 5 shows the quantitative distribution of soluble and insoluble coke deposited on Ni/HZSM-5 catalysts based on GC−MS analyses. The spent catalyst sample at 250 °C contained 82.2% soluble coke and 17.8% insoluble coke; the sample at 280 °C contained 66.7% soluble coke and 33.3% insoluble coke; the sample at 300 °C contained 36.6% soluble coke and 63.4% insoluble coke; and the sample at 330 °C contained 7.3% soluble coke and 92.7% insoluble coke. Carbonaceous compounds are mostly soluble in CH2Cl2 at lower temperatures; as such, the mass fraction of insoluble coke increased as the reaction temperature increased. This indicated that organic reactants were transformed into soluble coke, and subsequently, soluble coke was transformed into insoluble coke by some chemical reaction, such as alkylation, hydrogen transfer, or dehydrogenation. The effect of the temperature on the species and

Figure 5. Distribution of soluble and insoluble coke. Reaction conditions: catalyst, 15 wt % Ni/HZSM-5 (Si/Al = 50); time, 1 h; n, 650; P, 2 MPa; and K, 3 °C min−1.

Table 3. Elemental Composition of Coke T (°C)

H (wt %)

C (wt %)

atomic ratio H/C

250 280 300 330

1.23 1.68 2.85 2.71

8.46 16.42 42.2 48.6

1.74 1.18 0.79 0.67

11.6% under 250, 280, 300, and 330 °C reaction temperatures, respectively. However, when the bio-oil was involved in the reaction, the weight losses were 7.8, 21.9, 46.4, and 65.6% at the same reaction temperatures, respectively (Figure 4a). This indicated that the coke formation mainly comes from bio-oil composition. Figure 4a shows four stages of the observed mass loss from 50 to 250 °C, from 250 to 450 °C, from 450 to 600 °C, and from 600 to 750 °C, which correspond to the loss of moisture and physical absorbents and the formation of soft coke, hard coke, and graphite, respectively. These ranges are in agreement with the reported literature, in which the different coke species would form at different reaction temperatures.13 There are no measured weights in the 0−250 °C region because all spent catalysts were dried at 100 °C before the experiment. As the temperature increased, the spent catalysts 1725

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Energy & Fuels Table 4. Components of Soluble Coke As Determined by GC−MSa surface adsorption

area (%)

trapped within a micropore

aldehydes ketones 2-cyclopenten-1-one, 3,4,4-trimethyl2-cyclopenten-1-one, 3,4-dimethylcyclopentanone, 2-methyl2-cyclopenten-1-one, 3-methylbenzene, (1-methylbutyl)2-cyclopenten-1-one, 2-methyl2-cyclopenten-1-one, 2,3-dimethylalcohols 1-butanol phenols phenol, 3,5-dimethylphenol, 2,5-bis(1,1-dimethylethyl)phenol, 4-methylphenol phenol, 4-ethylesters acetic acid, trichloro-, ethyl ester propanoic acid, butyl ester acetic acid, butyl ester hydrocarbons benzene, 1-ethyl-4-methoxytoluene

16.4

0.50 0.51 0.64 0.72 1.08 1.26 1.84 alcohols 1-butanol phenols phenol, 2,5-bis(1,1-dimethylethyl)-

6.90 0.72 0.91 0.97 1.43 1.68

7.20 9.35

esters 0.61 1.07 6.34

acetic acid, butyl ester

n-butyl ether

0.87

trichloromethane

1.46

3.34

hydrocarbons benzene, 4-ethyl-1,2-dimethylcyclohexane, (1-methylpropyl)benzene, 1,3,5-trimethyltrichloromethane benzene, 1,2,3,4-tetramethyltoluene ethers n-butyl ether others carbon tetrachloride

0.61 64.6

ethers others a

area (%)

aldehydes benzaldehyde, 3,5-dimethylketones

0.53 1.29 1.69 2.42 6.93 38.67 0.80 0.78

Reaction conditions: catalyst, 15 wt % Ni/HZSM-5 (Si/Al = 50); T, 330 °C; time, 1 h; n, 650; P, 2 MPa; and K, 3 °C min−1.

Figure 6. Arrhenius curve of Rc (μg mg−1 of catalyst h−1) for 15 wt % Ni/HZSM-5 (Si/Al = 50) catalyst. Reaction conditions: time, 1 h; n, 650; P, 2 MPa; and K, 3 °C min−1.

Figure 7. Kinetic curves of coke burning for the catalysts after reaction: L1, coked at 280 °C; L2, coked at 300 °C; and L3, coked at 330 °C. Reaction conditions: time, 1 h; n, 650; P, 2 MPa; and K, 3 °C min−1.

composition (e.g., H/C ratio) of coke is shown in Table 3 for coke transformations over Ni/HZSM-5 in the HDO of bio-oil. Table 4 shows that the components of the surface-soluble coke were very different from those of the coke trapped within the catalyst micropores. In addition to the excessive amount of

solvents (i.e., toluene and n-butyl alcohol), large quantities of oxygenated chemicals, such as ketones, phenols, and their derivatives, were adsorbed on the acid sites of the carrier, 1726

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Energy & Fuels Table 5. Coke-Burning E and A for Different Catalysts line

temperature (°C)

slope

intercept

E (kJ/mol)

A (s−1)

R

L1 L2 L3

427−527 427−527 427−527

3.00 4.08 5.52

9.43 9.06 7.40

24.96 33.95 45.85

7.65 × 102 4.52 × 103 3.37 × 102

0.9980 0.9984 0.9986

However, coked catalysts can also be damaged during calcination at high temperatures because of the loss of aluminum.21 Insoluble coke typically has a high ignition point; hence, a higher calcination temperature should be used to recover catalyst activity. There is an optimal temperature at which the amount of insoluble coke is minimized. Cokeburning kinetic parameters for the regeneration of coked catalyst can be obtained (as shown in Figure 6) based on the TG results19

causing direct polymerization reactions of the reactants; hence, macromolecular carbon products were deposited on the carrier. The molecular size of mononuclear aromatics is comparable to those of zeolite micropores; this indicates that zeolite pores were likely plugged with derivatives of mononuclear aromatics. The derivatives of mononuclear aromatics can be transformed into macromolecular products by alkylation, hydrogen transfer, or dehydrogenation. It is clear that monomolecular aromatics and oxygenated compounds, which are regarded as coke precursors, play significant roles in the formation of carbon deposits. 3.5. Kinetic Characteristics of Coke. The rate of coke deposition (Rc) provides a good basis for assessing the performance of a catalyst. The effect of the temperature on the carbon deposition rate in HDO has been examined in detail by Guisnet et al.7 According to the relationship between the deposition rate of coke and temperature, the Arrhenius equation of the related coke formation reaction can be obtained

R c = Ae(−E / RT )

⎛ X⎞ AR E − ln⎜ −ln 2 ⎟ = ln ⎝ ⎠ βE RT T

where X is the mass percentage of the unburned coke, A is the frequency factor, E is the activation energy, R is the gas constant, T is the temperature, and β = dT/dt. According to the results of the TG analysis, as shown in Figure 7, straight lines can be obtained for each catalyst. The slopes and intercepts of the lines in Figure 7 were calculated by linear regressions to obtain the coke-burning data for each catalyst, as shown in Table 5. It can be seen that the intercept E of L3 (45.85 kJ mol−1) is greater than those of L2 (33.95 kJ mol−1) and L1 (24.96 kJ mol−1). This illustrates that the coke deposited during HDO at higher temperatures was more difficult to remove.22 It can also be seen that a portion of soft carbon was converted to hard coke or graphite carbon, which was consistent with previous results.22,23

(1) −1

−1

where Rc is the coke deposition rate (μg mg of catalyst h ), A is the frequency factor (s−1), E is the activation energy (J/ mol), R is the gas constant (J mol−1 K−1), and T is the temperature (K). Using the Coats and Redfern method,19 eq 1 becomes ln R c = ln A −

E ⎛⎜ 1 ⎞⎟ R ⎝T ⎠

(3)

(2)

4. CONCLUSION The character of coke deposited on the spent catalysts from the HDO of bio-oil was investigated. The following conclusions can be made: (1) The reaction temperature affects the coke deposition on the catalysts dramatically. TEM analyses reveal that the morphologies of coke deposited during HDO comprised primarily of carbon-rich flat plates at higher reaction temperatures (330 °C) and filamentous carbon at lower reaction temperatures (280 °C). (2) FTIR analyses reveal that the coke components (mainly formed by aromatization, dehydrogenation, and hydrogen transfer) are mainly polyaromatics at higher temperatures (>280 °C), while the coke is aliphatic compounds (mainly formed by aromatization and rearrangement steps) at lower temperatures. (3) TG measurement peaks were observed for the combustion of soft coke at moderate temperatures (250−450 °C) and hard coke at high temperatures (450−750 °C). The TG measurements revealed that the peaks evolved toward hard coke profiles as the temperature increased. (4) The concept of soluble and insoluble carbon species was introduced in this paper. The distinct separation of these coke forms is useful for understanding coke formation and coke-burning kinetics and can provide some guidelines for selecting the appropriate parameters for the regeneration of spent catalysts during and after the HDO of bio-oil.

To obtain the relationship between the temperature and the coke deposition rate, plots of ln Rc versus 1/T are shown in Figure 6. As shown in Figure 6, the coke deposition rate slowly increased when the reaction temperature increased from 250 to 280 °C, which is a range in which the gasification of deposited carbon may play a major role.20 Soluble coke was generated for reaction temperatures below 280 °C. The soluble coke was easy to volatilize off of the spent catalyst because of its low boiling point. The gasification of deposited carbon partially reduced the deposited carbon. However, the coke deposition rate increased rapidly when the reaction temperature increased from 280 to 300 °C. A significant amount of amorphous carbon was observed on catalyst surfaces when the reaction temperature was above 280 °C. These products have low volatility and solubility and were typically retained within the pores or on the outer surface of the catalyst. Under these conditions, coke desorption becomes difficult and leads to a rapid increase in the coke deposition rate. When the reaction temperature increased to 300 °C, graphite-like carbon or carbon graphite formed around the catalysts. This implies that the catalysts were quickly deactivated. On the basis of the above results, bio-oil HDO processing should occur at 280 °C to avoid rapid catalyst deactivation. The presence of coke on the catalysts after HDO was the result of higher rates of coke formation compared to coke desorption. The activity of coked catalysts can be partially restored by calcination with oxygen-containing mixtures.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 1727

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Energy & Fuels Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors sincerely acknowledge the financial support by the Key Programs of Science and Technology of Henan Province and Zhengzhou City (Projects 10ZDGG121 and 111PCXTD165). The authors also thank Xingmin Xu and Peng Liu for their assistance in laboratory analysis.

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