Article pubs.acs.org/EF
Study on Pyrolysis of Pine Sawdust with Solid Base and Acid Mixed Catalysts by Thermogravimetry−Fourier Transform Infrared Spectroscopy and Pyrolysis−Gas Chromatography/Mass Spectrometry Huiyan Zhang,† Jian Zheng,† Rui Xiao,*,† Yixuan Jia,† Dekui Shen,† Baosheng Jin,† and Guomin Xiao‡ †
Key Laboratory of Energy Thermal Conversion and Control, Ministry of Education, School of Energy and Environment, Southeast University, Nanjing, Jiangsu 210096, People’s Republic of China ‡ School of Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu 211189, People’s Republic of China ABSTRACT: Catalytic fast pyrolysis of biomass with microporous ZSM-5 to produce aromatics is a promising technology for biomass use. To improve the aromatic yield, meso- and macroporous [CaO, MgO, and fluidized catalytic cracking (FCC)] catalysts were added in a microporous catalyst (ZSM-5). The added catalysts can crack large-molecule oxygenates from pyrolysis into small-molecule oxygenates, and then these small-molecule oxygenates were converted to aromatics over ZSM-5. The experiments of pine wood catalytic pyrolysis with two mixed catalysts were performed and analyzed using thermogravimetry− Fourier transform infrared spectroscopy (TG−FTIR) and pyrolysis−gas chromatography/mass spectrometry (Py−GC/MS). The TG−FTIR results show that CaO and ZSM-5 mixed catalysts produced more aromatic rings and C−H bonds than pure ZSM-5. The Py−GC/MS results show that CaO and MgO mixed with ZSM-5 improved the aromatic yield significantly. The maximum aromatic yield that boosted 30% of that with pure ZSM-5 was obtained with CaO as the additive. It was obtained with a biomass/CaO/ZSM-5 mass ratio of 1:4:4. According to the disposal mode of feedstock and two catalyst studies, the completely separated mode of feedstock, CaO, and ZSM-5 (mode 3) produced the highest yield of indene and its derivatives and other hydrocarbons. This paper provides a simple way to improve the aromatic yield from biomass catalytic pyrolysis. been proposed by our previous study.19 Rice stalk was used as the feedstock for catalytic pyrolysis with two mixed catalysts in an internally interconnected fluidized bed. The addition of cracking catalysts shows higher hydrocarbon yields compared to that with a pure microporous LOSA-1 catalyst. In this work, the catalytic pyrolysis experiments were conducted in thermogravimetry−Fourier transform infrared spectroscopy (TG−FTIR) and pyrolysis−gas chromatography/mass spectrometry (Py−GC/MS) over mixed catalysts [CaO, MgO, or fluidized catalytic cracking (FCC) mixed with ZSM-5] for comparison and obtaining more information. The aim of this work is to prove the validity of this method from the functional groups (via TG−FTIR experiments) and catalytic pyrolysis products (via Py−GC/MS experiments) using pine wood as biomass. The effects of the percentage of two catalyst in the mixed catalysts on product yields and selectivities were studied. The optimal conditions of the cracking catalysts were compared. Also, the effects of disposal modes of feedstock and two catalysts on product yields and selectivities were investigated.
1. INTRODUCTION Lignocellulosic biomass can be converted into bio-oil by pyrolysis technology with a very high yield.1 However, the biooil has many disadvantages, such as high oxygen content, high acid content, and instability and needs to be updated to expand its application range.2 Catalytic fast pyrolysis (CFP) is a promising method for biomass conversion because it can realize biomass pyrolysis and the upgrading of vapor in one reactor.3 The catalyst is the key factor for the CFP process. Many kinds of catalysts have been synthesized and tested for CFP of biomass. These catalysts include microporous,4−6 mesoporous,7−9 and macroporous catalysts.10,11 ZSM-5 shows good characteristics for aromatic production because of its threedimensional pore structure, with a pore size of 5.5−5.6 Å.12 Actually, it has been proven that ZSM-5 is one of the best catalysts for producing hydrocarbons because of its special pore structure and activity.13−16 The low hydrocarbon yield and rapid deactivation are the main problems of microporous catalysts.17 This is because the heavy compounds from biomass pyrolysis cannot enter the small pore of microporous catalysts and deposit on its surfaces. The meso- and macroporous catalysts with strong cracking characteristics (such as CaO and MgO) are used for biomass pyrolysis and gasification for cracking heavy compounds (tars). The results showed that they cracked heavy compounds into smaller oxygenates.11,18 However, these catalysts have no shape-selective catalytic characteristics. A method of using mesoand macroporous catalysts to crack heavy compounds into smaller oxygenates, followed by microporous catalyst catalytic conversion to convert these oxygenates into hydrocarbons, has © 2014 American Chemical Society
Special Issue: International Biorefinery Conference Received: January 17, 2014 Revised: April 25, 2014 Published: April 25, 2014 4294
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Figure 1. FTIR analysis results of several main products evolving from pine wood pyrolysis with sand, ZSM-5, or CaO/ZSM-5 as catalysts. modes of the feedstock and two kinds of catalysts are different and studied in the following section. The pyrolysis temperature was set at 600 °C and held for 20 s, with the heating rate of 20 °C/ms. The injector temperature was kept at 300 °C. The chromatographic separation was performed using a HP-5MS capillary column (30 m × 0.25 mm × 0.25 μm). Helium (99.999%) was used as the carrier gas with a constant flow rate of 1 mL/min and a 1:60 split ratio. The oven temperature was programmed from 50 °C (2 min) to 290 °C (1 min) at the heating rate of 8 °C/min. The temperature of the GC/MS interface was held at 280 °C. Identification of chromatographic peaks was achieved according to the National Institute of Standards and Technology (NIST) MS library and the literature data of bio-oils.
2. MATERIALS AND METHODS 2.1. Materials. Pine wood, which was collected from Nanjing, Jiangsu, China, was chosen as the feedstock. Prior to all experiments, pine wood was ground in a high-speed rotary cutting mill, sieved to 0.075−0.098 mm, and then dried at 80 °C until a constant weight. The elemental composition of pine wood (air-dried basis) was 41.9 wt % carbon, 4.2 wt % hydrogen, and 53.0 wt % oxygen (by difference). Its proximate analysis (air-dried basis) was 6.2 wt % moisture, 70.2 wt % volatiles, 22.7 wt % fixed carbon, and 0.9 wt % ash. ZSM-5, CaO, MgO, and spent FCC catalysts were used in this study. The ZSM-5 catalyst (CBV 3024E, SiO2/Al2O3 = 30) was supplied by the Zeolyst Company. CaO and MgO was bought from Shanghai Jiuyi Chemical Reagent Co., Ltd. Spent FCC was recovered from a conventional FCC unit of Sinopec Yangzi Petrochemical Company, Ltd. These catalysts were dried at 120 °C for 2 h to remove moisture and kept in a desiccator for experiments. 2.2. Experimental Setup. 2.2.1. TG−FTIR Experiments. The TG− FTIR instrument consists of a thermogravimetric analyzer (Setaram, Setsys-1750) coupled with FTIR (Bruker, Vector22) for investigating the weight loss of pine wood and analyzing the evolved pyrolysis vapors. In each experiment, 5 mg of pine wood was mixed with 5 mg of catalysts and placed in the crucible of the thermogravimetric analyzer. The mixture was heated from room temperature to 600 °C with the heating rate of 90 °C/min in an inert atmosphere. High-purity nitrogen (99.999%) was used as the carrier gas with a flow rate of 65 mL/min to maintain the inert atmosphere for pine wood pyrolysis. The evolving volatile products were carried into a gas cell of the FTIR spectrometer, where the volatiles were analyzed through a transfer tube. The cell was heated to about 180 °C to prevent the condensation of pyrolysis vapors on the tube wall. The scanning range was set to be 4000−600 cm−1. The area integral of one functional group presents the relative amount of compounds that contains the functional group. 2.2.2. Py−GC/MS Experiments. Pyrolysis was performed using a CDS Pyroprobe 5200 pyrolyser coupled with GC/MS (Agilent, 7890A5975C). In each experiment, 0.5 mg of pine wood was mixed or separated with different amounts of catalysts and placed in a quartz filler tube. Then, the tube was filled with some quartz wool. The disposal
3. RESULTS AND DISCUSSION 3.1. Products Evolving of Pine Wood Non-catalytic and Catalytic Pyrolysis over Pure ZSM-5 and CaO Mixed with ZSM-5 Using TG−FTIR. Figure 1 shows the FTIR analysis results of aromatic rings, C−H bonds, CO2, and CO produced from pine wood pyrolysis with sand, ZSM-5, or CaO/ZSM-5 as catalysts. Aromatic rings and C−H bonds represent hydrocarbon compounds. The appearance time of aromatic ring and C−H bond peaks increases in the following order: without catalyst < ZSM-5 < CaO/ZSM-5. Catalytic pyrolysis is pyrolysis followed by catalytic conversion. Therefore, the appearance time of aromatic ring and C−H bond peaks is higher than that without a catalyst run. When CaO was added, heavy compounds experienced the cracking process; therefore, the appearance time is higher than that in the pure ZSM-5 run. The use of a catalyst produced more aromatic rings and C−H bonds compared to that without a catalyst. Furthermore, adding CaO in the ZSM-5 catalyst produces much more aromatic rings and C−H bonds compared to that using pure ZSM-5. The results display the synergistic effect between the two catalysts. The addition of CaO made more heavy oxygenates (especially from lignin pyrolysis) crack to small oxygenates.11,18 These oxygenates 4295
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Figure 2. Aromatic yields of pine wood catalytic pyrolysis over different catalysts and percentages mixed with ZSM-5.
can enter the small pores of ZSM-5 catalysts and be converted into some compounds with aromatic rings and C−H bonds. Catalytic pyrolysis of pine wood also produced more CO and CO2. 3.2. Aromatic Relative Amounts of Pine Wood Catalytic Pyrolysis over Different Catalysts and Different Percentages Mixed with ZSM-5. Figure 2 shows the aromatic yields of pine wood catalytic pyrolysis over CaO, MgO, or FCC catalysts mixed with ZSM-5 at different percentages using Py− GC/MS. In these experiments, pine wood and ZSM-5 were kept at 0.5 and 2 mg, respectively. The mass of the cracking catalysts ranged from 0 to 6 mg. The relative amount was calculated via the corresponding product yield of two catalyst catalytic runs divided by only the ZSM-5 catalytic run. The aromatic yield increased first and then decreased with the increasing amount of the cracking catalyst. With an increasing amount of cracking catalysts, more heavy compounds were converted into smaller oxygenates, which were further converted into aromatics; thus, the aromatic yield increased first. However, if more cracking catalyst was used, more small oxygenates were converted into CO2, CO, H2O, and other non-condensable gases, which led to the decrease of aromatic yields. The best percentages of different cracking catalysts in the mixed catalyst were different. A total of 50 and 11.1% of CaO and MgO in the mixed catalysts were obtained as the optimal conditions, respectively. 3.3. Comparison of the Catalytic Effects of Different Cracking Catalysts at Their Optimal Conditions. The relative amounts of different chemical groups in aromatics produced from pine wood pyrolysis over different catalysts mixed with ZSM-5 at their optimal percentages are shown in Figure 3. Aromatics are divided as four types, including benzene, indene,
Figure 3. Relative amounts of different chemical groups in aromatics produced from pine sawdust pyrolysis over different catalysts mixed with ZSM-5 under their optimal conditions.
and naphthalene and their derivatives and other hydrocarbons. ZSM-5 mixed with CaO produced the highest yield of all types of aromatics. The total aromatic yield was boosted more than 30%. ZSM-5 mixed with CaO or MgO produced more benzene and its detivatives. ZSM-5 mixed with FCC decreased all types of aromatics. Mante and his co-researchers conducted catalytic pyrolysis of hybrid poplar over mixed catalysts of ZSM-5 and Yzeolite.20 They found that the addition of FCC decreased aromatic yields, which is in the same as that obtained in this study. Figure 4 shows the selectivities of different chemical groups in aromatics produced from pine sawdust pyrolysis over 4296
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Figure 4. Selectivities of different chemical groups in aromatics produced from pine sawdust pyrolysis over different catalysts mixed with ZSM-5 under their optimal conditions.
Figure 5. Different disposal modes of feedstock and two catalysts in the pyrolysis tube.
3.4. Effects of the Disposal Modes on the Aromatic Relative Amounts and the Selectivities of Produced Chemical Groups. The reason for a different disposal mode study is to compare different modes in a real reaction system, such as (1) using a fluidized bed with two mixed catalysts as bed materials, (2) using a fluidzed bed with a base cracking catalyst as
different catalysts mixed with ZSM-5 under their optimal conditions. Toluene, xylenes, indene, 1-methyl-1H-indene, naphthalene, 1-methylnaphthalene, and 2,7-dimethylnaphthalene are the main compounds in the aromatic products. FCC gave the highest 1-methylmaphthalene selectivity, while CaO gave the highest xylene selectivity. 4297
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bed materials, followed by another fixed-bed reactor with the ZSM-5 catalyst for catalytic conversion, etc. Figure 5 shows different disposal modes of feedstock and two catalysts in the pyrolysis tube. These modes include only with the ZSM-5 catalyst (modes 1 and 2) and with two catalysts (modes 3, 4, 5, and 6). Biomass is mixed (modes 2, 4, and 6) or separated (modes 1, 3, and 5) with catalysts. The two catalysts were mixed (modes 5 and 6) or separated (modes 3 and 4) in the tube. The masses of biomass and ZSM-5 catalyst are 0.5 and 2 mg in each run, respectively. CaO mass is 1 mg if a run used it. The relative amounts of different chemical groups in aromatics produced from pine sawdust pyrolysis with different disposal modes of feedstock and two catalysts are shown in Figure 6. The
separated mode of feedstock, CaO, and ZSM-5 (mode 3) produced the highest of yield indene and its derivatives and other hydrocarbons, while the separated mode of ZSM-5 and feedstock mixed with CaO (mode 4) gave the highest yield of naphthalene and its derivatives. The feedstock and two mixed catalyst run (mode 6) shows a lower aromatic yield compared to that obtained with pure ZSM-5. This result can be attributed to excessive catalysis of pyrolysis vapors and can be adjusted by reducing the adding mass of CaO. Figure 7 shows the selectivities of different chemical groups in aromatics produced from pine sawdust pyrolysis with different disposal modes of feedstock and two catalysts. The selectivities of the main compounds in benzene and its derivatives have little changes between different modes. Mode 2 presented the highest selectivity of indene. Mode 5 produced the highest selectivity of 1-methylnaphthalene in naphthalene and its derivatives group.
3. CONCLUSION In this paper, pine wood catalytic pyrolysis with two physically mixed catalysts (one is ZSM-5 and the other is CaO, MgO, or FCC) has been investigated. The following conclusions can be obtained: (1) The addition of CaO or MgO in ZSM-5 can improve the aromatic yield. (2) A total of 50% of CaO in mixed catalysts produced the maximum aromatic yield, which boosted 30% of that with pure ZSM-5. (3) The separated mode of feedstock, CaO, and ZSM-5 produced the highest hydrocarbon yield.
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Figure 6. Relative amounts of different chemical groups in aromatics produced from pine sawdust pyrolysis with different disposal modes of feedstock and two catalysts.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
Figure 7. Selectivities of different chemical groups in aromatics produced from pine sawdust pyrolysis with different disposal modes of feedstock and two catalysts. 4298
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Notes
derived compounds with chemical liquid deposition (CLD) modified ZSM-5. Bioresour. Technol. 2014, 155, 57−62. (16) Zhang, H. Y.; Nie, J. L.; Xiao, R.; Jin, B. S.; Dong, C. Q.; Xiao, G. M. Catalytic co-pyrolysis of biomass and different plastics (PE, PP, PS) to improve hydrocarbon yield in a fluidized bed reactor. Energy Fuels 2014, 28 (3), 1940−1947. (17) Zhang, H. Y.; Shao, S. S.; Xiao, R.; Shen, D. K.; Zeng, J. M. Characterization of coke deposition in the catalytic fast pyrolysis of biomass derivates. Energy Fuels 2014, 28 (1), 52−57. (18) Li, R.; Zhong, Z. P.; Jin, B. S.; Zheng, A. J. Application of mineral bed materials during fast pyrolysis of rice husk to improve water-soluble organics production. Bioresour. Technol. 2012, 119, 324−330. (19) Zhang, H. Y.; Xiao, R.; Jin, B. S.; Xiao, G. M.; Chen, R. Biomass catalytic pyrolysis to produce olefins and aromatics with a physically mixed catalyst. Bioresour. Technol. 2013, 140, 256−262. (20) Mante, O. D.; Agblevor, F.; Oyama, S.; McClung, R. Catalytic pyrolysis with ZSM-5 based additive as co-catalyst to Y-zeolite in two reactor configurations. Fuel 2014, 117, 649−659.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the financial support of the National Natural Science Foundation of China (Grant 51306036), the National Basic Research Program of China (973 Program) (Grant 2010CB732206), the Major Research Plan of the National Natural Science Foundation of China (Grant 91334205), and the Jiangsu Natural Science Foundation (Grant BK20130615). The authors are grateful to the kind support from the Committee of the 4th International Conference on Biorefinerytowards Bioenergy (ICBB 2013) in Xiamen, China.
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