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Upgrading of fast pyrolysis bio-oil to drop-in fuel over Ru catalysts
Journal of the Energy Institute 92 (2019) 855e860
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Journal of the Energy Institute journal homepage: http://www.journals.elsevier.com/journal-of-the-energyinstitute
Upgrading of fast pyrolysis bio-oil to drop-in fuel over Ru catalysts Zhongyi Ma a, Lin Wei b, *, Wei Zhou c, **, Litao Jia a, Bo Hou a, Debao Li a, ***, Yongxiang Zhao c a
State Key Lab of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China Department of Agricultural and Biosystems Engineering, South Dakota State University, Brookings, SD, 57007, USA c College of Chemistry & Chemical Engineering, Shanxi University, Taiyuan, 030006, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 January 2018 Received in revised form 13 June 2018 Accepted 18 June 2018 Available online 1 September 2018
Catalyst plays a key role in the upgrading of fast pyrolysis bio-oil to advanced drop-in fuel, while the selectivity and deactivation of catalyst still remain the biggest challenge. In this study, three Ru catalysts with activated carbon, Al2O3 and ZSM-5 as supports were prepared and tested in bio-oil hydrotreating process. The physical properties and components of upgraded bio-oil were detected to identify the difference in catalytic performance of three catalysts. The results showed that furan, phenols and their derivatives in fast pyrolysis bio-oil could be hydrogenated to alkanes, alkenes and benzenes over Ru catalysts. The different components of oil phase over three catalysts may be resulted from the surface properties of three supports. Activated carbon supported Ru catalyst showed the best catalytic performance and was suggested to be the most promising catalyst for pyrolysis bio-oil upgrading. © 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Bio-oil Hydrotreating Catalyst Activated carbon Upgrading
1. Introduction Biomass is considered as a renewable and alternative resource to fossil fuels for the production of fuels and chemicals [1,2], such as bio-oil [3] and 5-Hydroxymethylfurfural (5-HMF) [4,5]. Fast pyrolysis as the promising technology to produce bio-oil from solid biomass has been developed for years [6]. However, pyrolysis bio-oil cannot be directly used as transportation fuels because of undesirable properties, such as low calorific value, corrosivity, instability under storage and transportation conditions, immiscibility with hydrocarbon fuels and high viscosity [7]. Catalytic upgrading is the way that can improve the properties of pyrolysis bio-oil by removing oxygen via H2O andCO2 and changing chemical structures to resemble those of petrochemical products. ZSM-5 is wildly used as catalytic cracking catalyst for bio-oil upgrading at moderate operation conditions without hydrogen [8]. However, catalyst deactivation resulted from the extensive carbon deposition is the critical problem [9]. On other hand, the upgraded bio-oil from catalytic cracking has low H/C ratio because no more hydrogen is added. Additionally, the aromatics (benzene, xylene, and indene) are the main composition of upgraded bio-oil from catalytic cracking process, which will affect its compatibility on existing petroleum refinery system [10]. Hydrotreating process is considered as the other promising process to upgrade fast pyrolysis bio-oil. In this process, oxygen is removed by hydrogenation and high grade fuel is produced. Catalysts used in this process are sulfided Co-Mo and Ni-Mo based catalysts, which are traditional hydrodesulphurization (HDS) catalysts [11]. In addition, some noble metal catalysts are also used in this process and show good performance [12]. The hydrotreating catalysts show longer lifetime than the cracking catalysts in presence of hydrogen. Meanwhile, the properties of upgraded bio-oil produced from this process are more similar with petroleum fuels system [13]. In this respect, hydrotreating process has a better potential to current existing fuel infrastructure than that of catalytic cracking process [14].
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (L. Wei), [email protected] (W. Zhou), [email protected] (D. Li). https://doi.org/10.1016/j.joei.2018.06.013 1743-9671/© 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
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Sulfided Co-Mo/Al2O3 is used as hydrotreating catalyst of fast pyrolysis bio-oil, but the instability of alumina and coke deposition are recognized as serious shortcomings [15,16]. Some noble metal catalysts also have been developed for hydrotreating process because of high activity [17]. Ru/AC is considered as a promising catalyst for acetic acid and furfural hydrogenation [18]. Pt-Pd catalyst supported on a phosphorus-containing activated carbon(ACP) has been studied in the hydrodeoxygenation (HDO) of raw bio-oil, and compared with a FCC (Fluid Catalytic Cracking) catalysts acid support. The high stability of Pt-Pd/ACP catalyst shows an outstanding performance in the HDO of raw bio-oil [19]. ZrO2 supported Rh, Pd, and Pt catalysts are also investigated in the guaiacol hydrotreating process and Rh/ZrO2 catalyst shows the best activity among three catalysts [20]. Al2O3 supported noble metal catalysts also have been investigated for bio-oil upgrading and Pt/Al2O3 is found with good activity in this process [21]. The products of HDO process are very complex in nature and contain hundreds of compounds belonging to a variety of organic compound classes [22]. In order to determine the chemical transformations taking place during the HDO process, solvent extraction in combination with other techniques was considered as the very useful approach to gain insights in the composition of HDO products [23]. The analytical methods also need to be updated because of complexity of HDO products. So far, noble metal catalysts showed good catalytic performance in upgrading of fast pyrolysis bio-oil because of high activity and moderate reaction condition. Nevertheless, the influence of supports on catalytic performance was not very clear because different model bio-oils with varied chemicals were used. In this work, three Ru catalysts with activated carbon, Al2O3, and ZSM-5 as supports were prepared and tested in upgrading of fast pyrolysis bio-oil. The components and physical properties of upgraded bio-oils were investigated and compared with fast pyrolysis bio-oil. At the same time, the difference in catalytic performance of three catalysts was discussed. 2. Materials and methods 2.1. Catalyst preparation The activated carbon(Norit Company), Al2O3(Almatis company) and ZSM-5(Zeolyst Company) supports were activated at 120 C for 12 h prior to being used, and then the supports were impregnated in RuCl3 solution with wet impregnation method. The ruthenium loading was 5 wt% with respect to each support. The catalysts were dried at 105 C for 8e12 h and then calcined at 550 C for 3e5 h. 2.2. NH3-TPD test NH3-TPD was carried out to get the acidity of the catalysts. 100 mg of the catalysts were firstly heated in Ar flow at 600 C for 1 h to remove moisture from the sample completely and then cooled to 50 C. After that, the catalysts were exposed to NH3 until saturation and flushed by Ar for 1h in order to eliminate the physical adsorbed NH3. Finally, TPD of the sample was carried out by increasing the temperature to 600 C under the NH3 flow and recorded by the Online CDMC chromatography. 2.3. Catalyst test Mixed bio-oil was used as the feedstock, which was composed of fast pyrolysis bio-oil produced from corn stover and pine sawdust. An autoclave (Parr, Model 4575) connected to a hydrogen cylinder was used as the hydrotreating reactor. 5g catalysts were sealed in the autoclave and reduced at 250 C for 2 h at hydrogen atmosphere. After catalysts reduction, 200g fast pyrolysis bio-oils were carefully poured into the autoclave. Then the hydrogen was filled to remove the air inside and to charge the autoclave until the pressure was 4.8 MPa at room temperature. The reactor was then heated up to 350 C with a heating rate of 10 C/min and held at 350 C for 4hr. The turbine paddle stirrer of the reactor was set at 1000 rpm throughout the test. After each test, the products were discharged and separated at room temperature. Small amounts of coke and catalyst that might exist in the flow liquid were removed by keeping the product at 5 C for 1 h to allow it to precipitate, and then the oil and aqueous phases of the product were separated in a 250 mL separating funnel. 2.4. Product characterizations The pH value was measured with an AB15 pH Meter (Accumet Company). Dynamic viscosity was measured with a viscoanalyzer (REOLOGICA Instruments AB Company), and the results were obtained in units of millipascal-seconds (centipoise, cP) at 20 C. The calorie value was measured by a bomb calorimeter (Parr Instrument Company 1341), and then the higher heating value (HHV) was converted from calories per gram to megajoules per kilogram. The moisture content was measured by the Karl Fischer Titrator V20 (Mettler Toledo Company). GC-MS(Agilent GC-7890A (DB-5column:30 m 0.25 mm 0.25 mm) and MSD-5975C electron ionization at 70eV, mass range of 50e5000 m/z, semi-quantitation based on TIC, mass chromatograms) was used to analyze the chemical composition of samples. The gas chromatograph was programmed at 45 C for 3 min, followed by ramp at 10 C/min to 350 C. The temperature was held for an additional 3 min after each ramp. The injector temperature was 300 C, and the injection size was 1 mL. The flow rate of carrier gas (hydrogen) was 1 mL/min. The compositions were identified through the NIST Mass Spectral library [24]. 3. Results and discussion 3.1. NH3-TPD test The acidity of the supports are characterized by NH3-TPD and the profiles are shown in Fig. 1. It can be seen that no NH3 desorption peak was found on activated carbon and only one peak was detected on Al2O3 support. Two NH3 desorption peaks located at about 200 C and 400 C were observed on ZSM-5 sample. The higher the temperature of the desorption peak is, the stronger acidity of the support is. The larger area of the desorption peak is, the more acidic site of the support is [25]. ZSM-5 showed the most acidic sites and the strongest acidity
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Fig. 1. NH3-TPD profiles of three supports: (a)AC; (b)ZSM-5; (c)Al2O3.
among three supports and almost no acidic sites was detected on activated carbon. The surface acidity difference may be result in different catalytic performance in HDO process. 3.2. Hydrotreating test Two phases, oil phase and aqueous phase were detected after fast pyrolysis bio-oil was hydrotreated over three catalysts. Then the two phases were separated and the selectivities of each phase were calculated and listed in Table 1. Coke quantification was also calculated as the increase in catalyst mass, which may include the mass of heavy tar or any other possible particles. Three catalysts showed different catalytic performance in liquid yield and selectivities of oil phase and aqueous phase. Ru/AC catalyst had the highest liquid yield and selectivity of oil phase among three catalysts, which were as much as 84.02% and 45.42%, respectively. The liquid yield was 82.50% for Ru/ZSM-5 catalyst, which was similar to that of Ru/Al2O3 catalyst, while the selectivity of oil phase for Ru/ZSM-5 was lower than that of Ru/Al2O3. Additionally, some coke was found on Ru/ZSM-5 catalyst, while no coke was detected on Ru/AC and Ru/Al2O3 catalysts. Hence, Ru/ZSM-5 catalyst showed the highest selectivities of aqueous phase and coke among three catalysts. ZSM-5 is used traditionally as a cracking catalyst in petroleum refinery because it has suitable acidity and special pore structure [26]. NH3-TPD results showed that ZSM-5 had most acidity and more acidic sites among three supports. The acidity sites on ZSM-5 may play the cracking role during hydrotreating process of bio-oil and large molecular components can be converted to CO, CO2 and some light hydrocarbons [27], which may be accounted for more aqueous phase and coke over Ru/ZSM-5 catalyst. Activated carbon is usually considered as inert support in hydrogenation process and almost no acidity was found in NH3-TPD results, thus more bio-oil was hydrotreated rather than cracked over Ru/AC catalyst and then the highest selectivity of oil phase and liquid yield were obtained. Al2O3 is widely used as support for bio-oil upgrading, while the deactivation in the presence of water is the major shortcoming [28]. Ru/Al2O3 catalyst showed the higher selectivity of oil phase than that of Ru/ZSM-5, which may be also related to different acidity observed from NH3-TPD results. To further understand the different catalytic performance of three Ru catalysts, GC/MS analysis was carried out to determine the compositions of upgraded bio-oils. For comparison, the components of fast pyrolysis bio-oil were also determined. The main compositions of fast pyrolysis bio-oil were furan, furanone, cyclohexanone, phenols and some kinds of acids. In addition, some new compounds such as benzene, naphthalene and other aromatic chemicals were detected in oil phase of upgraded bio-oil, which may be come from the hydrogenation of oxygenate compounds in fast pyrolysis bio-oil over Ru catalysts. Table 2 listed the relative contents of main organic compounds of the fast pyrolysis bio-oil and upgraded oil phase. It can be seen that furans, ketones and phenols were the main compounds in fast pyrolysis bio-oil and the total amount was as much as 90%, which was similar with the composition in literature [3,29]. The total amount of furans and phenols was negligible in upgraded oil phase, while the content of ketones changed little especially for Ru/ZSM-5 catalyst. Phenols can be hydrogenated on bifunctional catalyst in aqueous phase via a hydrogenation-dehydration route [30]. Cyclohexanol was produced on metal surface via hydrogenation of phenols and then dehydrated on acidic sites to form cyclohexene [31]. Sequential hydrogenation of cyclohexene on noble metal surface could produce the cyclohexane. Alkenes and alkanes also came from the hydrogenation of furans [32,33]. The formation of alkenes and alkanes in the oil phase of upgraded bio-oil may be clarified by the hydrogenation of furan and/or phenols.
Table 1 Catalytic performances of three Ru catalysts for fast pyrolysis oil upgrading. Catalysts
Liquid yield (wt%)
Products selectivity (wt%) Oil phase
Aqueous phase
Cokea
Ru/AC Ru/ZSM-5 Ru/Al2O3
84.02 82.53 82.80
45.42 39.97 42.87
54.65 61.78 57.13
0 1.09 0
a
Coke quantification was calculated as the increase in catalyst mass, and this may include the mass of heavy tar or any other possible particles.
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Table 2 Relative content of compositions of the fast pyrolysis bio-oil and upgraded oil phase. Samples
Pyrolysis bio-oil Ru/AC Ru/ZSM-5 Ru/Al2O3
Furan
36.34 0.76 0.66 0
Ketones
Alkenes
12.53 7.52 12.23 3.48
Alkanes
1.16 7.60 8.87 0
Aldehyde
0 3.12 2.12 18.48
1.24 2.33 1.46 1.10
Aromatics phenols
benzenes
others
36.07 0 0.48 0
0 27.59 31.98 13.16
0 40.90 34.80 37.41
Acids
Others
4.24 1.72 4.51 20.67
3.90 8.46 2.89 5.70
Hydrogenation of guaiacol on noble metal catalyst was different from those on CoMo catalysts [34]. The primary product of aqueousphase hydrogenation of guaiacol on noble metal catalysts was the intermediately hydrogenated 2-ethoxycyclohexanone by the metalcatalyzed hydrogenation of the aromatic ring [35], which resulted in the high content of ketones in oil phase and aqueous phase of upgraded bio-oils. It is noted that difference among the compositions of oil phases over three Ru catalysts was also remarkable. Aromatics were the main product in three upgraded oil phases, which was similar with the results in-situ hydrodeoxygenation of bio-oil [3], while the amount was varied with catalysts. The amount of aromatics was as much as 66.78% for Ru/ZSM-5 catalyst while that of Ru/Al2O3 was only 50.57%. Meanwhile, the amount of benzene was as high as 31.98% for Ru/ZSM-5 catalyst. The amounts of alkanes and alkenes were also obviously different. The amount of alkanes was 18.48% in Ru/Al2O3 upgraded bio-oil, while that was 3.12% for Ru/AC and that was 2.12% for Ru/ZSM-5, respectively. For Ru/AC and Ru/ZSM-5 catalysts, the relative contents of alkenes were 87.60% and 87% respectively. While in the case of Ru/AC catalyst, the alkenes could not be detected. The varied surface properties of different catalyst supports may be accounted for these differences. Alkanes could be cracked into small molecular compounds on ZSM-5 surface and aromatics chemicals were also favorable formation on ZSM-5 support because of its special surface properties and pore structure [36]. Alumina structure could be changed and the pore size was narrowed in the presence of water [37], then the formation of large molecular chemicals was limited. Activated carbon and ZSM-5 had large pore size, which favored the formation of aromatics compounds. The absence of alkenes in upgraded bio-oil over Ru/Al2O3 may be related to the high hydrogenation reactivity of Ru/Al2O3 catalyst. According to the catalytic performance of the three catalysts, Ru/AC catalyst showed the best activity for fast pyrolysis bio-oil upgrading. The aqueous phases of upgraded oil were also detected by GC/MS to identify the products difference in three catalysts. The results showed that the compounds in three aqueous phases were almost same though fast pyrolysis bio-oils were treated by different catalysts. The typical compositions of aqueous phase over Ru/AC catalyst were listed in Table 3. It was illustrated that the main compound was cylcopentanone, which was as much as 87%. Meanwhile, some kinds of acids and phenols were detected, which may be come from the unreacted compounds in fast pyrolysis bio-oil.
3.3. Physical properties of upgraded bio-oil The physical properties of upgraded oils were listed in Table 4. It was found that pH values and heating values of upgraded oils increased compared with those of fast pyrolysis bio-oils. Meanwhile, the moisture content and viscosity decreased. It was also found that the heating values varied with three catalysts. The heating value of upgraded oil from Ru/ZSM-5 was the lowest among three upgraded oils, which may
Table 3 Major compounds of aqueous phase of upgraded oil over Ru/AC. RT (min)
Compounds
Relative content
3.218 3.339 4.197 4.225 4.534 5.124 5.278 5.473 5.822 6.211 7.029 7.418 8.86
Cyclopentanone, 2-methylCyclopentanone, 3-methylHexanoic acid, 6-bromoCyclopentanone, 2-methyl2-Cyclopenten-1-one, 2-methylCyclopentanone, 2-ethyl5-Hexenoic acid Cyclohexanone, 4-methyl2-Cyclopenten-1-one, 3-methylMethyl-trans-hexahydrophththalide 2-Cyclopenten-1-one, 2,3-dimethyl2-Cyclopenten-1-one, 2,3,4-trimeth Phenol
32.78 12.91 3.93 6.06 4.29 4.36 3.12 2.49 5.33 2.65 15.51 4.22 2.34
Table 4 Physical properties of the upgraded oils and fast pyrolysis bio-oil. Samples
Ru/AC
Ru/ZSM-5
Ru/Al2O3
Fast pyrolysis bio-oil
Heating value(MJ/kg) Moisture content pH value Viscosity
31.6 4.3 4.1 2.3
29.3 4.8 3.9 2.5
31.0 4.6 3.7 2.4
13.08 45.32 2.8 9.7
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Table 5 The reusability test of Ru/AC catalysts for fast pyrolysis oil upgrading. Recycle
1 2 3 a
Liquid yield (wt%)
84.02 83.53 84.20
Products selectivity (wt%) Oil phase
Aqueous phase
Cokea
45.42 45.97 44.87
54.65 54.03 55.13
0 0 0
Coke quantification was calculated as the increase in catalyst mass, and this may include the mass of heavy tar or any other possible particles.
be resulted from its highest ketones content. The lowest pH value of upgraded oil from Ru/Al2O3 may be related to the highest acid compounds. Based on catalytic performance and physical properties of upgraded bio-oil, Ru/AC was suggested to be the most promising catalyst. Ru/ AC catalyst showed the highest selectivity of oil phase among the three catalysts in fast pyrolysis bio-oil upgrading. The upgraded bio-oil produced from Ru/AC catalyst showed the best physical properties, such as lower organic acid, higher aromatic content and higher heating value. In order to check the reusability of Ru/AC catalysts, the spent catalysts were separated from the liquid product and washed with ethanol for three times after every reaction. Then the used catalyst were placed in the autoclave and used in HDO reaction again. The catalytic performance was listed in Table 5. It can be seen no obvious deactivation was observed after three recycle reaction, and the Ru/AC catalyst showed good reusability in bio-oil HDO reaction. 4. Conclusion Three Ru catalysts with activated carbon, Al2O3, and ZSM-5 as support were prepared and applied in fast pyrolysis bio-oils hydrotreating in an autoclave reactor at 350 C. The upgraded bio-oils were separated into oil phase and aqueous phase. The compositions of two phases were detected by GC-MS and compared with fast pyrolysis bio-oil. The results indicated that the furans and phenols in fast pyrolysis bio-oil could be hydrogenated to hydrocarbon, which enhanced the heating value of upgraded bio-oil. It was also concluded that the catalyst support had significant effects on the catalytic performance for fast pyrolysis bio-oil upgrading. According to the catalytic performance and physical properties of upgraded bio-oil, Ru/AC catalyst was considered as the best hydrotreating catalyst for fast pyrolysis bio-oil upgrading among three catalysts. Acknowledgements The authors would like to thank Dr. Douglas Raynie in the Chemical Analytic Lab in the Chemistry Department of SDSU for the helps in chemical (GC/MS) analysis of the bio-oil samples. The US DOT (Grant number: DE-FG36-08GO88073) is acknowledged for funding. All the supports are gratefully acknowledged. However, only the authors are responsible for the opinions expressed in this paper and for any possible error. References [1] G.W. Huber, A. Corma, Synthesis of transportation fuels from biomass: chemistry, catalysts, and engineering, Chem. Rev. 106 (2006) 4044e4098. [2] A.V. Bridgwater, Renewable fuels and chemicals by thermal processing of biomass, Chem. Eng. J. 91 (2003) 87e102. [3] S.Y. Cheng, L. Wei, M.R. Alsowij, F. Corbin, J. Julson, E. Boakye, D. 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Upgrading of fast pyrolysis bio-oil to drop-in fuel over Ru catalysts Zhongyi Ma a, Lin Wei b, *, Wei Zhou c, **, Litao Jia a, Bo Hou a, Debao Li a, ***, Yongxiang Zhao c a
State Key Lab of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China Department of Agricultural and Biosystems Engineering, South Dakota State University, Brookings, SD, 57007, USA c College of Chemistry & Chemical Engineering, Shanxi University, Taiyuan, 030006, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 January 2018 Received in revised form 13 June 2018 Accepted 18 June 2018 Available online 1 September 2018
Catalyst plays a key role in the upgrading of fast pyrolysis bio-oil to advanced drop-in fuel, while the selectivity and deactivation of catalyst still remain the biggest challenge. In this study, three Ru catalysts with activated carbon, Al2O3 and ZSM-5 as supports were prepared and tested in bio-oil hydrotreating process. The physical properties and components of upgraded bio-oil were detected to identify the difference in catalytic performance of three catalysts. The results showed that furan, phenols and their derivatives in fast pyrolysis bio-oil could be hydrogenated to alkanes, alkenes and benzenes over Ru catalysts. The different components of oil phase over three catalysts may be resulted from the surface properties of three supports. Activated carbon supported Ru catalyst showed the best catalytic performance and was suggested to be the most promising catalyst for pyrolysis bio-oil upgrading. © 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Bio-oil Hydrotreating Catalyst Activated carbon Upgrading
1. Introduction Biomass is considered as a renewable and alternative resource to fossil fuels for the production of fuels and chemicals [1,2], such as bio-oil [3] and 5-Hydroxymethylfurfural (5-HMF) [4,5]. Fast pyrolysis as the promising technology to produce bio-oil from solid biomass has been developed for years [6]. However, pyrolysis bio-oil cannot be directly used as transportation fuels because of undesirable properties, such as low calorific value, corrosivity, instability under storage and transportation conditions, immiscibility with hydrocarbon fuels and high viscosity [7]. Catalytic upgrading is the way that can improve the properties of pyrolysis bio-oil by removing oxygen via H2O andCO2 and changing chemical structures to resemble those of petrochemical products. ZSM-5 is wildly used as catalytic cracking catalyst for bio-oil upgrading at moderate operation conditions without hydrogen [8]. However, catalyst deactivation resulted from the extensive carbon deposition is the critical problem [9]. On other hand, the upgraded bio-oil from catalytic cracking has low H/C ratio because no more hydrogen is added. Additionally, the aromatics (benzene, xylene, and indene) are the main composition of upgraded bio-oil from catalytic cracking process, which will affect its compatibility on existing petroleum refinery system [10]. Hydrotreating process is considered as the other promising process to upgrade fast pyrolysis bio-oil. In this process, oxygen is removed by hydrogenation and high grade fuel is produced. Catalysts used in this process are sulfided Co-Mo and Ni-Mo based catalysts, which are traditional hydrodesulphurization (HDS) catalysts [11]. In addition, some noble metal catalysts are also used in this process and show good performance [12]. The hydrotreating catalysts show longer lifetime than the cracking catalysts in presence of hydrogen. Meanwhile, the properties of upgraded bio-oil produced from this process are more similar with petroleum fuels system [13]. In this respect, hydrotreating process has a better potential to current existing fuel infrastructure than that of catalytic cracking process [14].
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (L. Wei), [email protected] (W. Zhou), [email protected] (D. Li). https://doi.org/10.1016/j.joei.2018.06.013 1743-9671/© 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
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Sulfided Co-Mo/Al2O3 is used as hydrotreating catalyst of fast pyrolysis bio-oil, but the instability of alumina and coke deposition are recognized as serious shortcomings [15,16]. Some noble metal catalysts also have been developed for hydrotreating process because of high activity [17]. Ru/AC is considered as a promising catalyst for acetic acid and furfural hydrogenation [18]. Pt-Pd catalyst supported on a phosphorus-containing activated carbon(ACP) has been studied in the hydrodeoxygenation (HDO) of raw bio-oil, and compared with a FCC (Fluid Catalytic Cracking) catalysts acid support. The high stability of Pt-Pd/ACP catalyst shows an outstanding performance in the HDO of raw bio-oil [19]. ZrO2 supported Rh, Pd, and Pt catalysts are also investigated in the guaiacol hydrotreating process and Rh/ZrO2 catalyst shows the best activity among three catalysts [20]. Al2O3 supported noble metal catalysts also have been investigated for bio-oil upgrading and Pt/Al2O3 is found with good activity in this process [21]. The products of HDO process are very complex in nature and contain hundreds of compounds belonging to a variety of organic compound classes [22]. In order to determine the chemical transformations taking place during the HDO process, solvent extraction in combination with other techniques was considered as the very useful approach to gain insights in the composition of HDO products [23]. The analytical methods also need to be updated because of complexity of HDO products. So far, noble metal catalysts showed good catalytic performance in upgrading of fast pyrolysis bio-oil because of high activity and moderate reaction condition. Nevertheless, the influence of supports on catalytic performance was not very clear because different model bio-oils with varied chemicals were used. In this work, three Ru catalysts with activated carbon, Al2O3, and ZSM-5 as supports were prepared and tested in upgrading of fast pyrolysis bio-oil. The components and physical properties of upgraded bio-oils were investigated and compared with fast pyrolysis bio-oil. At the same time, the difference in catalytic performance of three catalysts was discussed. 2. Materials and methods 2.1. Catalyst preparation The activated carbon(Norit Company), Al2O3(Almatis company) and ZSM-5(Zeolyst Company) supports were activated at 120 C for 12 h prior to being used, and then the supports were impregnated in RuCl3 solution with wet impregnation method. The ruthenium loading was 5 wt% with respect to each support. The catalysts were dried at 105 C for 8e12 h and then calcined at 550 C for 3e5 h. 2.2. NH3-TPD test NH3-TPD was carried out to get the acidity of the catalysts. 100 mg of the catalysts were firstly heated in Ar flow at 600 C for 1 h to remove moisture from the sample completely and then cooled to 50 C. After that, the catalysts were exposed to NH3 until saturation and flushed by Ar for 1h in order to eliminate the physical adsorbed NH3. Finally, TPD of the sample was carried out by increasing the temperature to 600 C under the NH3 flow and recorded by the Online CDMC chromatography. 2.3. Catalyst test Mixed bio-oil was used as the feedstock, which was composed of fast pyrolysis bio-oil produced from corn stover and pine sawdust. An autoclave (Parr, Model 4575) connected to a hydrogen cylinder was used as the hydrotreating reactor. 5g catalysts were sealed in the autoclave and reduced at 250 C for 2 h at hydrogen atmosphere. After catalysts reduction, 200g fast pyrolysis bio-oils were carefully poured into the autoclave. Then the hydrogen was filled to remove the air inside and to charge the autoclave until the pressure was 4.8 MPa at room temperature. The reactor was then heated up to 350 C with a heating rate of 10 C/min and held at 350 C for 4hr. The turbine paddle stirrer of the reactor was set at 1000 rpm throughout the test. After each test, the products were discharged and separated at room temperature. Small amounts of coke and catalyst that might exist in the flow liquid were removed by keeping the product at 5 C for 1 h to allow it to precipitate, and then the oil and aqueous phases of the product were separated in a 250 mL separating funnel. 2.4. Product characterizations The pH value was measured with an AB15 pH Meter (Accumet Company). Dynamic viscosity was measured with a viscoanalyzer (REOLOGICA Instruments AB Company), and the results were obtained in units of millipascal-seconds (centipoise, cP) at 20 C. The calorie value was measured by a bomb calorimeter (Parr Instrument Company 1341), and then the higher heating value (HHV) was converted from calories per gram to megajoules per kilogram. The moisture content was measured by the Karl Fischer Titrator V20 (Mettler Toledo Company). GC-MS(Agilent GC-7890A (DB-5column:30 m 0.25 mm 0.25 mm) and MSD-5975C electron ionization at 70eV, mass range of 50e5000 m/z, semi-quantitation based on TIC, mass chromatograms) was used to analyze the chemical composition of samples. The gas chromatograph was programmed at 45 C for 3 min, followed by ramp at 10 C/min to 350 C. The temperature was held for an additional 3 min after each ramp. The injector temperature was 300 C, and the injection size was 1 mL. The flow rate of carrier gas (hydrogen) was 1 mL/min. The compositions were identified through the NIST Mass Spectral library [24]. 3. Results and discussion 3.1. NH3-TPD test The acidity of the supports are characterized by NH3-TPD and the profiles are shown in Fig. 1. It can be seen that no NH3 desorption peak was found on activated carbon and only one peak was detected on Al2O3 support. Two NH3 desorption peaks located at about 200 C and 400 C were observed on ZSM-5 sample. The higher the temperature of the desorption peak is, the stronger acidity of the support is. The larger area of the desorption peak is, the more acidic site of the support is [25]. ZSM-5 showed the most acidic sites and the strongest acidity
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Fig. 1. NH3-TPD profiles of three supports: (a)AC; (b)ZSM-5; (c)Al2O3.
among three supports and almost no acidic sites was detected on activated carbon. The surface acidity difference may be result in different catalytic performance in HDO process. 3.2. Hydrotreating test Two phases, oil phase and aqueous phase were detected after fast pyrolysis bio-oil was hydrotreated over three catalysts. Then the two phases were separated and the selectivities of each phase were calculated and listed in Table 1. Coke quantification was also calculated as the increase in catalyst mass, which may include the mass of heavy tar or any other possible particles. Three catalysts showed different catalytic performance in liquid yield and selectivities of oil phase and aqueous phase. Ru/AC catalyst had the highest liquid yield and selectivity of oil phase among three catalysts, which were as much as 84.02% and 45.42%, respectively. The liquid yield was 82.50% for Ru/ZSM-5 catalyst, which was similar to that of Ru/Al2O3 catalyst, while the selectivity of oil phase for Ru/ZSM-5 was lower than that of Ru/Al2O3. Additionally, some coke was found on Ru/ZSM-5 catalyst, while no coke was detected on Ru/AC and Ru/Al2O3 catalysts. Hence, Ru/ZSM-5 catalyst showed the highest selectivities of aqueous phase and coke among three catalysts. ZSM-5 is used traditionally as a cracking catalyst in petroleum refinery because it has suitable acidity and special pore structure [26]. NH3-TPD results showed that ZSM-5 had most acidity and more acidic sites among three supports. The acidity sites on ZSM-5 may play the cracking role during hydrotreating process of bio-oil and large molecular components can be converted to CO, CO2 and some light hydrocarbons [27], which may be accounted for more aqueous phase and coke over Ru/ZSM-5 catalyst. Activated carbon is usually considered as inert support in hydrogenation process and almost no acidity was found in NH3-TPD results, thus more bio-oil was hydrotreated rather than cracked over Ru/AC catalyst and then the highest selectivity of oil phase and liquid yield were obtained. Al2O3 is widely used as support for bio-oil upgrading, while the deactivation in the presence of water is the major shortcoming [28]. Ru/Al2O3 catalyst showed the higher selectivity of oil phase than that of Ru/ZSM-5, which may be also related to different acidity observed from NH3-TPD results. To further understand the different catalytic performance of three Ru catalysts, GC/MS analysis was carried out to determine the compositions of upgraded bio-oils. For comparison, the components of fast pyrolysis bio-oil were also determined. The main compositions of fast pyrolysis bio-oil were furan, furanone, cyclohexanone, phenols and some kinds of acids. In addition, some new compounds such as benzene, naphthalene and other aromatic chemicals were detected in oil phase of upgraded bio-oil, which may be come from the hydrogenation of oxygenate compounds in fast pyrolysis bio-oil over Ru catalysts. Table 2 listed the relative contents of main organic compounds of the fast pyrolysis bio-oil and upgraded oil phase. It can be seen that furans, ketones and phenols were the main compounds in fast pyrolysis bio-oil and the total amount was as much as 90%, which was similar with the composition in literature [3,29]. The total amount of furans and phenols was negligible in upgraded oil phase, while the content of ketones changed little especially for Ru/ZSM-5 catalyst. Phenols can be hydrogenated on bifunctional catalyst in aqueous phase via a hydrogenation-dehydration route [30]. Cyclohexanol was produced on metal surface via hydrogenation of phenols and then dehydrated on acidic sites to form cyclohexene [31]. Sequential hydrogenation of cyclohexene on noble metal surface could produce the cyclohexane. Alkenes and alkanes also came from the hydrogenation of furans [32,33]. The formation of alkenes and alkanes in the oil phase of upgraded bio-oil may be clarified by the hydrogenation of furan and/or phenols.
Table 1 Catalytic performances of three Ru catalysts for fast pyrolysis oil upgrading. Catalysts
Liquid yield (wt%)
Products selectivity (wt%) Oil phase
Aqueous phase
Cokea
Ru/AC Ru/ZSM-5 Ru/Al2O3
84.02 82.53 82.80
45.42 39.97 42.87
54.65 61.78 57.13
0 1.09 0
a
Coke quantification was calculated as the increase in catalyst mass, and this may include the mass of heavy tar or any other possible particles.
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Table 2 Relative content of compositions of the fast pyrolysis bio-oil and upgraded oil phase. Samples
Pyrolysis bio-oil Ru/AC Ru/ZSM-5 Ru/Al2O3
Furan
36.34 0.76 0.66 0
Ketones
Alkenes
12.53 7.52 12.23 3.48
Alkanes
1.16 7.60 8.87 0
Aldehyde
0 3.12 2.12 18.48
1.24 2.33 1.46 1.10
Aromatics phenols
benzenes
others
36.07 0 0.48 0
0 27.59 31.98 13.16
0 40.90 34.80 37.41
Acids
Others
4.24 1.72 4.51 20.67
3.90 8.46 2.89 5.70
Hydrogenation of guaiacol on noble metal catalyst was different from those on CoMo catalysts [34]. The primary product of aqueousphase hydrogenation of guaiacol on noble metal catalysts was the intermediately hydrogenated 2-ethoxycyclohexanone by the metalcatalyzed hydrogenation of the aromatic ring [35], which resulted in the high content of ketones in oil phase and aqueous phase of upgraded bio-oils. It is noted that difference among the compositions of oil phases over three Ru catalysts was also remarkable. Aromatics were the main product in three upgraded oil phases, which was similar with the results in-situ hydrodeoxygenation of bio-oil [3], while the amount was varied with catalysts. The amount of aromatics was as much as 66.78% for Ru/ZSM-5 catalyst while that of Ru/Al2O3 was only 50.57%. Meanwhile, the amount of benzene was as high as 31.98% for Ru/ZSM-5 catalyst. The amounts of alkanes and alkenes were also obviously different. The amount of alkanes was 18.48% in Ru/Al2O3 upgraded bio-oil, while that was 3.12% for Ru/AC and that was 2.12% for Ru/ZSM-5, respectively. For Ru/AC and Ru/ZSM-5 catalysts, the relative contents of alkenes were 87.60% and 87% respectively. While in the case of Ru/AC catalyst, the alkenes could not be detected. The varied surface properties of different catalyst supports may be accounted for these differences. Alkanes could be cracked into small molecular compounds on ZSM-5 surface and aromatics chemicals were also favorable formation on ZSM-5 support because of its special surface properties and pore structure [36]. Alumina structure could be changed and the pore size was narrowed in the presence of water [37], then the formation of large molecular chemicals was limited. Activated carbon and ZSM-5 had large pore size, which favored the formation of aromatics compounds. The absence of alkenes in upgraded bio-oil over Ru/Al2O3 may be related to the high hydrogenation reactivity of Ru/Al2O3 catalyst. According to the catalytic performance of the three catalysts, Ru/AC catalyst showed the best activity for fast pyrolysis bio-oil upgrading. The aqueous phases of upgraded oil were also detected by GC/MS to identify the products difference in three catalysts. The results showed that the compounds in three aqueous phases were almost same though fast pyrolysis bio-oils were treated by different catalysts. The typical compositions of aqueous phase over Ru/AC catalyst were listed in Table 3. It was illustrated that the main compound was cylcopentanone, which was as much as 87%. Meanwhile, some kinds of acids and phenols were detected, which may be come from the unreacted compounds in fast pyrolysis bio-oil.
3.3. Physical properties of upgraded bio-oil The physical properties of upgraded oils were listed in Table 4. It was found that pH values and heating values of upgraded oils increased compared with those of fast pyrolysis bio-oils. Meanwhile, the moisture content and viscosity decreased. It was also found that the heating values varied with three catalysts. The heating value of upgraded oil from Ru/ZSM-5 was the lowest among three upgraded oils, which may
Table 3 Major compounds of aqueous phase of upgraded oil over Ru/AC. RT (min)
Compounds
Relative content
3.218 3.339 4.197 4.225 4.534 5.124 5.278 5.473 5.822 6.211 7.029 7.418 8.86
Cyclopentanone, 2-methylCyclopentanone, 3-methylHexanoic acid, 6-bromoCyclopentanone, 2-methyl2-Cyclopenten-1-one, 2-methylCyclopentanone, 2-ethyl5-Hexenoic acid Cyclohexanone, 4-methyl2-Cyclopenten-1-one, 3-methylMethyl-trans-hexahydrophththalide 2-Cyclopenten-1-one, 2,3-dimethyl2-Cyclopenten-1-one, 2,3,4-trimeth Phenol
32.78 12.91 3.93 6.06 4.29 4.36 3.12 2.49 5.33 2.65 15.51 4.22 2.34
Table 4 Physical properties of the upgraded oils and fast pyrolysis bio-oil. Samples
Ru/AC
Ru/ZSM-5
Ru/Al2O3
Fast pyrolysis bio-oil
Heating value(MJ/kg) Moisture content pH value Viscosity
31.6 4.3 4.1 2.3
29.3 4.8 3.9 2.5
31.0 4.6 3.7 2.4
13.08 45.32 2.8 9.7
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Table 5 The reusability test of Ru/AC catalysts for fast pyrolysis oil upgrading. Recycle
1 2 3 a
Liquid yield (wt%)
84.02 83.53 84.20
Products selectivity (wt%) Oil phase
Aqueous phase
Cokea
45.42 45.97 44.87
54.65 54.03 55.13
0 0 0
Coke quantification was calculated as the increase in catalyst mass, and this may include the mass of heavy tar or any other possible particles.
be resulted from its highest ketones content. The lowest pH value of upgraded oil from Ru/Al2O3 may be related to the highest acid compounds. Based on catalytic performance and physical properties of upgraded bio-oil, Ru/AC was suggested to be the most promising catalyst. Ru/ AC catalyst showed the highest selectivity of oil phase among the three catalysts in fast pyrolysis bio-oil upgrading. The upgraded bio-oil produced from Ru/AC catalyst showed the best physical properties, such as lower organic acid, higher aromatic content and higher heating value. In order to check the reusability of Ru/AC catalysts, the spent catalysts were separated from the liquid product and washed with ethanol for three times after every reaction. Then the used catalyst were placed in the autoclave and used in HDO reaction again. The catalytic performance was listed in Table 5. It can be seen no obvious deactivation was observed after three recycle reaction, and the Ru/AC catalyst showed good reusability in bio-oil HDO reaction. 4. Conclusion Three Ru catalysts with activated carbon, Al2O3, and ZSM-5 as support were prepared and applied in fast pyrolysis bio-oils hydrotreating in an autoclave reactor at 350 C. The upgraded bio-oils were separated into oil phase and aqueous phase. The compositions of two phases were detected by GC-MS and compared with fast pyrolysis bio-oil. The results indicated that the furans and phenols in fast pyrolysis bio-oil could be hydrogenated to hydrocarbon, which enhanced the heating value of upgraded bio-oil. It was also concluded that the catalyst support had significant effects on the catalytic performance for fast pyrolysis bio-oil upgrading. 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