Catalytic Cracking of Tar from Biomass Gasification over a HZSM-5

Energy Fuels , 2015, 29 (12), pp 7969–7974. DOI: 10.1021/acs.energyfuels.5b00830. Publication Date (Web): November 8, 2015. Copyright © 2015 Americ...
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Catalytic Cracking of Tar from Biomass Gasification over a HZSM-5Supported Ni−MgO Catalyst Guan-Yi Chen,*,†,‡ Cong Liu,†,§ Wen-Chao Ma,†,∥ Bei-Bei Yan,*,†,∥ and Na Ji† †

School of Environmental Science and Engineering, State Key Laboratory of Engines, Tianjin University, Tianjin 300072, People’s Republic of China ‡ Tianjin Engineering Center of Biomass-Derived Gas and Oil, Tianjin 300072, People’s Republic of China § Architectural Design Research Institute of Tianjin University, Tianjin 300073, People’s Republic of China ∥ Key Laboratory of Efficient Utilization of Low and Medium Grade Energy (Tianjin University), Ministry of Education, Tianjin 300072, People’s Republic of China ABSTRACT: The catalytic cracking of tar derived from biomass gasification was investigated, focusing on the catalyst performance of HZSM-5 (Si/Al = 25) loading with Ni or Ni and MgO, which was pre-calcined and reduced at 500 °C. The catalysts were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and temperatureprogrammed desorption (TPD). The liquid and gaseous products were analyzed by gas chromatography−mass spectrometry (GC−MS) and gas chromatography (GC). The results showed that the catalytic activity of Ni−MgO/HZSM-5 was better than that of Ni/HZSM-5. When the loadings of Ni and MgO (mass %) were 6 and 2 wt %, respectively, tar conversion attained 91.03 wt % and the heating value of gas released reached 7.64 MJ/Nm3. Ni−MgO/HZSM-5 showed a stronger resistance to carbon deposition (11.69 wt %). Laksmono et al.13 used acidic (zeolite, HZSM-5), neutral (alumina, Al2O3) and basic (magnesium oxide, MgO) compounds as catalysts to convert tar from wood gasification into biodiesel. Zeolite showed the best catalytic performance. Adjaye and Bakhshi14 reported that HZSM-5 was the most effective catalyst for producing the organic distillate fraction after comparing the relative performance of HZSM-5, Hmordenite, H−Y, silicalite, and silica−alumina. The Si/Al ratio determines the acidity of HZSM-5. The lower the Si/Al ratio, the more acidic sites it contains. Moreover, HZSM-5 with a low Si/Al ratio as the catalyst can improve the yield of olefins,15 such as ethylene and propylene. Thus, it can increase the heating value of gas generated. Moreover, the temperature of catalytic cracking with HZSM-5 is lower than that with other catalysts. On the other hand, the acidity of the catalyst support affects the coke accumulation, which could lead to the deactivation of the catalyst. As promoters, alkaline earth metals can neutralize the acidity and then reduce coke deposition on the catalyst surface. Studies showed that almost complete tar conversion can been achieved with synthetic catalysts at above 900 °C.16,17 Because of the high energy consumption, effective decomposition of tar with catalysts in low temperatures becomes attractive. Coincidentally, the temperature of catalytic cracking with HZSM-5 is lower than that with other catalysts. In this work, HZSM-5 (Si/Al = 25) was used as the catalyst support. The performance of HZSM-5 supported with Ni or Ni with different amounts of MgO was investigated in a fixed-bed reactor. The purpose of this study is to maximize the utilization

1. INTRODUCTION As a carbon-neutral renewable energy source, biomass to energy via gasification has been a renewed interest. Tar is normally associated with the gasification process. Tar condensed may block up the connecting pipes and, meanwhile, lead to the corrosion occurring on the surface of pipes. Furthermore, releasing of tar may reduce the gasification efficiency because tar contains energy. In this context, releasing of tar associated with the gasification process is therefore becoming a major bottleneck for bringing this technology to industries.1−3 There are various methods to remove tar, such as water scrubbing, filtering, electrostatic removal, high-temperature cracking, and mild-temperature catalytic cracking. Catalytic cracking seems to be a high-efficiency and clean way to convert tar components into combustible gas and useful chemicals. For this reason, a lot of effort has been made in understanding tar catalytic cracking. At present, biomass gasification tar or its model compounds have been used as feedstocks for catalytic cracking. Most studies have focused on transition-metal-based catalysts, such as a Ni catalyst.4−8 Moreover, the supported Ni catalysts showed good activity and are less expensive than other metals, such as Pt, Ru, and Rh.9 Liu et al. used Fe−Ni/palygorskite (PG) catalysts to decompose rice hull gasification tar, indicating its excellent performance.10 The performance of PG and goethite for tar decomposition was also investigated by the same researchers.11 PG had a higher catalytic activity than goethite, and the addition of Ni promoted the catalytic activity. Olivares et al.12 verified that nickel-based catalysts are 8−10 times more active than calcined dolomites under the same operating conditions. Ni-supported catalysts have been proven to be efficient. © 2015 American Chemical Society

Received: April 17, 2015 Revised: November 8, 2015 Published: November 8, 2015 7969

DOI: 10.1021/acs.energyfuels.5b00830 Energy Fuels 2015, 29, 7969−7974

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Energy & Fuels

120 mg. The catalyst was flushed at 500 °C under a helium atmosphere with a heating rate of 15 °C/min. Then, the adsorption of ammonia was carried out at 120 °C for 1 h. After that, the programmed desorption of NH3 was run from 120 to 700 °C with a heating rate of 10 °C/min. 2.3.5. Temperature-Programmed Reduction (TPR). The optimum reduction temperature of the catalyst sample was determined by temperature-programmed reduction of hydrogen (H2-TPR). The catalyst sample used was about 150 mg. The temperature was programmed to ramp at 10 °C/min to 500 °C and hold for 30 min under a nitrogen atmosphere. Then, the sample was cooled to 100 °C. Subsequently, the gas line was switched to H2. After the baseline was stable, the programmed reduction of H2 was run from 100 to 700 °C with a heating rate of 7.5 °C/min. 2.4. Experimental Section. The experiments were conducted in the fixed-bed reaction system shown in Figure 1. The reactor, which

of tar with the method of catalytic cracking. The optimum loading amounts of Ni and MgO were determined by evaluating the liquid and gaseous products and coke deposition rate. Finally, exploring how to enhance the quality of combustible gases produced during the reaction at a lower temperature is another goal of this work.

2. EXPERIMENTAL SECTION 2.1. Materials. The tar used was taken from the rice straw gasification process at Tianjin Daming Hengyun Renewable Energy Company, Ltd. The gasification apparatus is a downdraft gasifier. The gasification temperature was about 800 °C, and the air/steam was used as the gasification agent. During the gasification reaction, combustable gas and liquid products (tar) were generated. The gas was stored in the gas tank and then transported to the households. Tar was collected after condensation for further processing. The amount of tar is ranging from 1000 to 2000 mg/Nm3 producer gas at the exit of the gasifier. The tar was a physical appearance of a dark and highly viscous liquid. The ultimate analysis of tar was tested using an elemental analyzer (Vario Micro cube). The results are shown in Table 1.

Table 1. Ultimate Analysis of Tar ultimate analysis (wt %)

a

sample

C

H

Oa

N

S

tar

67.93

6.81

22.71

2.08

0.47

By difference.

2.2. Preparation of Catalysts. HZSM-5 used was supplied by Tianjin Nankai Catalyst Company. The particle size was 0.18−0.38 mm after extrusion, crushing, and sieving. HZSM-5 loading with nickel and magnesium oxide was prepared by incipient wetness impregnation with aqueous solutions of Mg(NO3)2·6H2O and Ni(NO3)2·6H2O, followed by calcination in air for 5 h at 550 °C. The calcined catalyst was then reduced in hydrogen with a flow rate of 50 mL/min, held at 500 °C for 1.5 h, and then cooled to the temperature for experiments or further characterization. The different amounts of MgO on the catalyst were loaded by changing the concentration of Mg(NO3)2· 6H2O aqueous solutions in the procedure of impregnation. The asprepared catalysts were noted as Ni−xMgO/HZSM-5, where x represents different MgO loadings. To decompose tar and convert it into useful components, the catalytic performance of Ni−MgO/HZSM-5 was tested. The loading amount of Ni was 6 wt %, whereas the loading amount of MgO was varied (0, 1, 2, 3, or 4 wt %). 2.3. Characterization of the Catalysts. 2.3.1. X-ray Diffraction (XRD). The XRD of the catalyst was performed on a D8 Advanced Xray diffractometer with Cu Kα radiation. The tube voltage was 40 kV, and the current was 40 mA. The XRD patterns were taken in the range of 10−90° at a scan speed of 4°/min. Phase identification was carried out by comparison to a database. 2.3.2. Brunauer−Emmett−Teller (BET) Surface Area. N2 adsorption/desorption isotherms of the catalyst were obtained using an automated gas sorption analyzer (Quantachrome). The amount of catalysts used was 150 mg. Prior to the measurement, the samples were outgassed at 150 °C under N2 flow for 6 h. Then, the surface areas, pore volumes, and pore diameter were analyzed by N2 physisorption at −200 °C. The results of characterization, such as BET specific area and pore volumes, were analyzed by BET and Barrett−Joyner−Halenda (BJH) methods. 2.3.3. Transmission Electron Microscopy (TEM). The morphology of the catalysts after reduction was determined by TEM. The measurements were performed on the JEM-2100f apparatus. The samples were mixed with alcohol and deposited on a Cu grid covered with a perforated carbon membrane. 2.3.4. Temperature-Programmed Desorption (TPD). The acidity of catalysts were investigated by temperature-programmed desorption of ammonia (NH3-TPD). The catalyst sample used was approximately

Figure 1. Schematic drawing of the fixed-bed reactor for tar catalytic cracking.

was heated by an electrical heating system, is made of stainless steel with a 50 cm height and 9 cm diameter. The catalyst bed was supported by quartz wool in the middle of the reactor. The temperature was measured and controlled with a thermocouple placed in the center of the reactor and connected to a temperature controller. The experimental conditions were fixed at atmospheric pressure. Before reaction, the catalysts were reduced at 500 °C under the atmosphere of mixed gases (5 vol % H2 and 95 vol % N2 at 50 mL/ min). After reduction, the catalyst activity was tested. The operating temperature was 500 °C, and the reaction time was 1 h. A total of 5 g of catalyst was put in the catalyst bed, and the weight hourly space velocity (WHSV) was 0.65 h−1. Nitrogen was used as the carrier gas with a flow rate of 60 mL/min. The biomass gasification tar was heated and introduced into the reactor with a peristaltic pump (BT100L, provided by Baoding Lead Fluid Technology Co., Ltd.). The pattern of the pump head is Y15, and the flow rate is 0.05−120 mL/min. The tar rate was 0.65 h−1, determined by the WHSV. The liquid product was collected after the condensing device, as shown in Figure 1. Thus, the amount of liquid product can be calculated by the subtraction method. Non-condensable gases (methane, carbon monoxide, carbon dioxide, hydrogen, nitrogen, light alkanes, and olefin) after the condensing device were collected by a gas collection bag. The tar conversion rate (Ytar) can be calculated by eq 1, where min and mout represented the mass of tar pumped into the reactor and the liquid products collected in the container (Erlenmeyer flask with a standard mouth), respectively. min − mout represented the amount of converted tar during the reaction. The sample of tar was put into the beaker, and the total mass of them is m1. The mass of the rubber tube mounted on the peristaltic pump is m2. After reaction, the total masses of the tar and beaker and the rubber tube are m′1 and m′2, respectively. Therefore, min = (m1 + m2) − (m1′ + m2′). 7970

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Energy & Fuels The mass of equipment 9 before and after reaction in Figure 1 was m3 and m4, respectively. Therefore, mout = m4 − m3. Actually, mout was the mass of liquid products. The coke deposition of the catalyst was measured by thermogravimetric analysis under an O2 atmosphere. The coke deposition rate was calculated by eq 2, where mcat represented the weight of the catalysts ′ represented the total weight of the catalyst before reaction and mcat and coke after reaction. m − mout Ytar = in × 100% m in (1)

Ycoke =

′ − mcat mcat × 100% m in − mout

Figure 3 shows the heating value of gas produced during the reaction. When MgO loading is 2 wt %, the heating value and

(2)

3. RESULTS AND DISCUSSION 3.1. Catalyst Performance. The effects of catalysts with different MgO loadings on Ni/HZSM-5, labeled as Ni−xMgO/ HZSM-5, were tested. Figure 2 presents the results of tar Figure 3. Heating values of gas released during biomass tar cracking.

yield of mixed gas obtained reached the maximum (7.64 MJ/ Nm3 and 6.87 L/h). Then, the heating value reduced with the MgO loading increasing. Overall, it can be inferred that the catalytic activity of Ni−2MgO/HZSM-5 was better than other catalysts, and the optimum adding amount of MgO was 2 wt %. 3.2. Liquid Products. Components of liquid products were analyzed by gas chromatography−mass spectrometry (GC− MS, QB2010 Plus). The column was HP-5MS at 30 m × 0.25 mm × 0.25 μm. The oven temperature was programmed to hold at 50 °C for 3 min, ramp at 15 °C/min to 280 °C, and hold at 280 °C for 5 min. The carrier gas was helium with a flow rate of 0.95 mL/min. The injector temperature was 280 °C, and the injector split ratio was 15:1. Area normalization was used for semi-quantitative analyses.19,20 The components of liquid products were analyzed by GC− MS. We selected 60 kinds of higher contents and classified by functional groups. The compounds mainly consist of carboxylic acids, phenols, aromatic hydrocarbons, ethers, esters, ketones, alkanes, and furans. The detailed composition of components identified in the liquid products is shown in Table 2. It can be found that the effect of different catalysts on the composition of liquid products is noticeable. The types of compounds produced by tar catalytic cracking with Ni−2MgO/HZSM-5 are the least. The amount of carboxylic acids reaches the maximum, and the contents of phenols and aromatic hydrocarbons are relatively lower. Compounds on tar converting to hydrocarbons over HZSM-5 mainly go through the following reactions: dehydration, decarboxylation, decarbonylation, hydrogen transfer, alkylation, isomerization, and aromatization.21−24 The reaction with Ni−2MgO/HZSM-5 is thorough, and the performance of Ni−2MgO/HZSM-5 was the most efficient. In the further study, we will consider the distribution of liquid products. Because the main components of liquid products are similar to bio-oil, liquid products may be upgraded to use as a potential liquid fuel for the boiler and engines. 3.3. Gaseous Products. Gas produced during the reaction was analyzed by gas chromatography (GC, Agilent 7980A) equipped with a thermal conductivity detector (TCD). The column was a packed column with Molsieve 5Å, 6 ft × 1/8 in. × 2 mm. The oven was programmed to hold at 50 °C for 3 min,

Figure 2. Effect of catalysts with different MgO loadings on the catalytic cracking of biomass tar.

conversion and coke deposition rate over the catalysts with different MgO loadings. The results of the tar conversion rate, coke deposition rate, and heating value were obtained through the average of three sets of parallel experiments. The tar conversion rates were 90.52, 91.54, 91.03, 89.78, and 87.19 wt %, respectively. When the MgO loadings were 1 and 2 wt %, the tar conversion rates were very close. While the loading was beyond 2 wt %, the tar conversion rate decreased to below 90 wt %. The coke deposition rate decreased and then increased with MgO loading increasing. The coke deposition rates were 19.20, 14.66, 11.69, 13.39, and 15.82 wt %, respectively. The tar conversion rate of Ni−2MgO/HZSM-5 is higher than that of Ni/HZSM-5, and the coke deposition rate of Ni−2MgO/ HZSM-5 is lower than that of Ni/HZSM-5. An appropriate amount of promoter (MgO) can enhance the performance of Ni/HZSM-5. However, excessive additives may cause the physical blockage of pores or channels of HZSM-5 and lead to a lower catalytic activity. That is why the tar conversion rate presents this variation trend. The reason for coke deposition was mainly due to the acidity of HZSM-5. Catalyst support acidity affects coke accumulation and catalyst deactivation18 and also causes physical blockage. 7971

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Energy & Fuels Table 2. Effect of Different Catalysts on Components and Their Relative Content of Liquid Products component

Ni/HZSM-5

Ni−1MgO/HZSM-5

Ni−2MgO/HZSM-5

carboxylic acids phenols esters aromatic hydrocarbons ketones ethers alkanes alcohols furans nitriles

37.306 14.566 45.175

0.877 67.186 2.070 25.496 1.953

42.783 19.284 36.535

1.579 1.374

Ni−3MgO/HZSM-5

Ni−4MgO/HZSM-5

56.608 1.520 38.760

0.996 63.397 2.443 27.811 2.411

1.398 0.568 0.535 0.698

1.343

1.139 0.552

1.769

1.254

Table 3. Gas Components and Their Relative Content over Different Catalysts gas component

Ni/HZSM-5

Ni−1MgO/HZSM-5

Ni−2MgO/HZSM-5

Ni−3MgO/HZSM-5

Ni−4MgO/HZSM-5

H2 CH4 CO CO2 C2H4 C2H6 C3H6 C3H8 N2

16.263 4.539 3.349 1.244 3.113 0.801 1.330 0.383 68.978

24.543 4.896 8.245 1.495 1.715 0.421 1.244 0.641 56.801

27.342 4.627 10.751 1.602 1.394 0.369 1.135 0.415 52.365

20.528 4.278 4.808 1.588 1.470 0.622 1.340 0.664 64.701

20.083 4.71 5.377 1.428 1.622 0.482 1.101 0.893 64.305

Figure 4. XRD patterns of catalysts.

ramp at 15 °C/min to 190 °C, and hold at 190 °C for 5 min. The TCD temperature was set at 250 °C, and reference gas was He with a flow rate of 35 mL/min. An external standard method was adopted for qualitative and quantitative analyses, with standard gas provided by Tianjin Lianbo Chemical Co., Ltd. The standard gas consisted of CH4 (5.00 mol %), C2H4 (2.08 mol %), C2H6 (1.02 mol %), C3H6 (1.02 mol %), C3H8 (0.5 mol %), H2 (1.92 mol %), CO (38.85 mol %), and CO2 (49.61 mol %). The software of Aspen Plus 8.0 was used to calculate the heating values. The gas yield (Ygas) was calculated by eq 3, where PN2 represented the proportion of N2 in the mixed gas. The reaction time was 1 h, and the flow rate of N2 was 60 mL/min. Therefore, the amount of N2 used in each experiment was 3.6 L.

Ygas =

3.6 PN2

Gaseous products mainly consist of CO, CO2, CH4, and H2 and a small amount of C2−C3 alkanes and olefins (C2H4, C2H6, C3H6, and C3H8). This is consistent with other study results.7,13 Table 3 shows the effect of different catalysts on gas composition. The addition of MgO made a significant increase in the content of hydrogen and carbon monoxide. However, C2−C3 alkanes and olefins decreased with the addition of it. The catalyst of HZSM-5 provides a strong acid proton. The proton combines with carbon (macromolecules in tar) and the carbocation forms. Because the structure is unstable, the bond of carbocation breaks. During the process, gaseous products are generated, such as H2, CH4, and short-alkene hydrocarbons. Hydrogen mainly came from dehydrogenation and aromatic cyclization of paraffins.25,26 Carbon monoxide was derived from the decomposition product of ethers and the oxygen-containing heterocyclic ring. Moreover, coke deposited on the catalyst reacting with carbon dioxide also generates carbon monoxide. Methane came from the cleavage of fat chains and aromatic side

(3) 7972

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Energy & Fuels chains with methyl groups. The generation of carbon dioxide depended upon the decomposition of carboxyl.27 3.4. Characterization of Catalysts. 3.4.1. XRD Results. The XRD patterns of catalysts (Ni/HZSM-5 and Ni−2MgO/ HZSM-5) are presented in Figure 4. The differences of the two curves are not obvious. The reflection centered at 2θ = 39.51°, 41.46°, 44.47°, 51.72°, and 76.21° belongs to Ni. NiO was not found, which means that it was completely reduced or a small amount of NiO existed and was highly dispersed on the surface of the catalyst, thereby leading to the difficulty of detecting it. The catalyst of Ni−2MgO/HZSM-5 exhibited the major peaks of MgO at 2θ = 28.85°, 38.26°, and 64.33°. It indicated that MgO was not reduced to Mg. The reason is mainly attributed to the low reduction temperature (500 °C). However, HZSM-5 impregnated with nickel nitrate or nickel nitrate and magnesium nitrate was transformed into silicon aluminum magnesium compounds by reaction in hydrogen at 500 °C. The existence of MgO can neutralize part of the acidity of HZSM-5. Thus, it decreases the coke deposition rate. The result of TPD can also prove it. 3.4.2. TEM Results. TEM images of Ni/HZSM-5 and Ni− 2MgO/HZSM-5 after H2 reduction were shown in Figures 5 and 6. The black particles dispersed on the surface of the catalyst were Ni. However, MgO was not obvious. It was mainly due to the small loading amount.

Table 4. Surface Area, Pore Volume, and Average Pore Diameter of Prepared Catalysts catalyst

surface area (m2/g)

pore volume (cm3/g)

pore diameter (nm)

Ni/HZSM-5 Ni−2MgO/HZSM-5

316.068 281.389

0.149 0.153

1.872 1.880

are attached to the surface of the catalysts. The images of TEM also prove it. According to the images, the black particles dispersed on the surface of catalysts. Considering the results of XRD, the particles should be Ni, MgO, and their compounds. 3.4.4. TPD Results. Figure 7 shows NH3-TPD results of the acidity of catalysts with different loading amounts. Two main

Figure 7. NH3-TPD patterns with different Ni and MgO loadings.

peaks (A and B) appear at temperature ranges of 200−300 and 450−550 °C, which represented the weak and strong acid sites, respectively.28 With the increasing loading amount of MgO, peak A shifted toward a high temperature and peak B becomes more and more unobvious. The phenomena indicate that an increased loading amount of MgO resulted in neutralization and blockage of surface acid sites.28 Thus, the coke deposition rate decreased, and simultaneously, the tar conversion rate increased. 3.4.5. TPR Results. Figure 8 shows H2-TPR results of Ni/ HZSM-5. The temperature reduction peak of the catalyst appears at about 500 °C. It can be determined that the in situ reduction temperature of the catalyst was 500 °C.

Figure 5. TEM images of Ni/HZSM-5 after reduction.

4. CONCLUSION Ni/HZSM-5 showed good activity for decomposing biomass gasification tar. Because of the acidic sites, the coke deposition rate of Ni/HZSM-5 is higher. Then, Ni (6 wt %) and different amounts of promoter MgO (0, 1, 2, 3, and 4 wt %) were loaded on HZSM-5 to prepare a series of modified catalysts. The tar conversion rate, coke deposition rate, heating value, and components of gas released were investigated by experiments. With the addition of an appropriate amount of promoter MgO (2 wt %), the acidity of catalysts became weaker and then the coke deposition rate seemed to decrease. The heating value of gas generated reached the maximum. When MgO loading was more than 2 wt %, the coke deposition rate continues to increase and the tar conversion rate and gas heating value

Figure 6. TEM images of Ni−MgO/HZSM-5 after reduction.

3.4.3. BET Results. N2 adsorption−desorption analysis of Ni/ HZSM-5 and Ni−2MgO/HZSM-5 was conducted using an automated gas sorption analyzer. The surface areas and pore volumes were obtained by BET and BJH methods, respectively. Table 4 shows the BET surface area, pore volume, and average pore diameter of the two catalysts. The surface area decreased with the addition of MgO. The pore volumes and pore diameter exhibited the opposite effect; however, it is not obvious. Therefore, we can speculate that most of the particles 7973

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Figure 8. H2-TPR pattern of Ni/HZSM-5.

become lower. This is mainly attributed to the weakness of the acidity and blockage of channels in the catalysts. On the basis of the overall results, we can infer that the catalyst of HZSM-5 provides a strong acid proton. The proton combines with carbon (macromolecules in tar) and the carbocation forms. Because the structure is unstable, the bond of the carbocation breaks. During the process, gaseous products generate, such as H2, CH4, and short-alkene hydrocarbons. Ni and MgO can enhance the performance of HZSM-5 during the reaction. Additionally, MgO and acid protons can go through the neutralized reaction and lead to carbon reduction. The reactions of dehydration, decarboxylation, decarbonylation, hydrogen transfer, alkylation, isomerization, and aromatization occur throughout the whole process.

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AUTHOR INFORMATION

Corresponding Authors

*Telephone/Fax: +86-22-87402075. E-mail: [email protected]. *Telephone/Fax: +86-22-87402075. E-mail: yanbeibei@tju. edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This paper is financially supported by the Project 2012AA051801 in the National High Technology 863 Plan of China and the National Natural Science Foundation of China through Projects 51036006 and 51076158.

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DOI: 10.1021/acs.energyfuels.5b00830 Energy Fuels 2015, 29, 7969−7974