Efficient Hydroliquefaction of Sawdust over a Novel Silica-Supported

Jul 25, 2016 - School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People,s Republic of China. •S Supporting Informat...
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Efficient Hydroliquefaction of Sawdust over a Novel Silica-Supported Monoclinic Molybdenum Dioxide Catalyst Min Li,† Dong Liu,*,† Ping-Ping Wu,† Xing-Shun Cong,‡ Lin-Hua Song,† Qing-Tai Chen,† Jing Liu,† Hao Wu,§ and Zi-Feng Yan† †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao, Shandong 266580, People’s Republic of China ‡ Department of Chemistry, Zaozhuang University, Zaozhuang, Shandong 277160, People’s Republic of China § School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China S Supporting Information *

ABSTRACT: A novel synthesis method was developed to prepare a silica-supported monoclinic molybdenum dioxide (MoO2/ SiO2) catalyst via calcination of molybdenum disulfide (MoS2)/SiO2 in N2 flow with 0.1 vol % O2. MoO2/SiO2 was employed as a catalyst for sawdust hydroliquefaction in ethanol at 320 °C under an initial H2 pressure (IHP) of 2−6 MPa, and the bio-oil yield as high as 72.3% was obtained under an IHP of 6 MPa. The high catalytic activity of MoO2/SiO2 should be attributed to the efficient cleavage of H2 on the surface of highly dispersed MoO2, which shows an unusual metallic characteristic. The obtained bio-oil was analyzed with gas chromatography/mass spectrometry (GC/MS), and the main organic compounds detected by GC/ MS were phenolics and ethyl esters.

use. Other materials, such as reagents and high-pressure gas, were commercially purchased and used as received. 2.2. Catalyst Preparation. 2.2.1. Synthesis of the SiO2 Support. About 10 mL of ethanol was mixed with 150 mL of deionized water. A proper amount of cetyltrimethylammonium bromide and triethanolamine were added to the mixture and stirred at 30 °C until totally dissolved to form a solution. A total of 20 mL of tetraethoxysilane was added dropwise under stirring to form a light blue gel. The gel was moved into an oven and maintained at 60 °C for 4 h. After that, a proper amount of ethanol was added to the gel, and some precipitation was formed. The precipitation was separated by centrifugation and then dried to offer the SiO2 support with a pore size of 1−2 nm. 2.2.2. Synthesis of Ammonium Tetrathiomolybdate [ATTM, (NH4)2MoS4]. A total of 10 g of ammonium heptamolybdate tetrahydrate [(NH4)6Mo7O24·4H2O] and 60 mL of concentrated ammonia were added to a 250 mL round-bottom flask to form a solution, and then 90.6 mL of ammonium sulfide solution (S > 8 wt %) was added to the flask. Then, the flask was heated to 68 °C and refluxed with stirring for 60 min. After that, the flask was cooled to 4 °C, and the crystal of ATTM grew while quietly standing for 24 h. The crystal was separated by filtration and washed repeatedly with deionized water and ethanol. The obtained crystal of ATTM was dried at 40 °C for 4 h before use. 2.2.3. Preparation of MoS2/SiO2, MoO2/SiO2, and MoO3/SiO2. ATTM (Mo/SiO2 = 10 wt %) was loaded onto SiO2 with the incipient wetness impregnation method, and the resulting mixture was dried at 120 °C for 12 h and then calcined in N2 at 450 °C for 3 h to give MoS2/SiO2. MoS2/SiO2 was calcined at 400 °C in the atmosphere of N2 with a volume ratio of 0.1% O2 to offer MoO2/SiO2 with a Mo content of ca. 8.7 wt %. MoO3/SiO2 was prepared by calcination of (NH4)6Mo7O24/SiO2 at 500 °C for 3 h. 2.3. Sawdust Hydroliquefaction. As Figure S1 of the Supporting Information shows, in a typical run, 1 g of sawdust, 0.1 g of catalyst,

1. INTRODUCTION Hydroliquefaction of lignocellulosic biomass (LCBM), including wood, grass, and agricultural residue, is the most promising way to solve the energy crisis and environmental impact caused by fossil fuel combustion. The catalyst plays a key role for the LCBM hydroliquefaction process. Hence, many catalysts, such as alkali,1 metal,2 zeolite,3 ionic liquid,4 etc., were investigated to enhance the bio-oil production. Liu et al.5 studied catalytic hydroliquefaction of sawdust in petroleum ether and found that the maximum bio-oil yield reached 47.69%. The subsequent investigation revealed that the bio-oil yield could be enhanced by simultaneous catalysis of 1-butyl-3-methylimidazolium bromide and NiCl2 in ethanol.6,7 Grilc et al.8 investigated liquefaction of three kinds of sawdust over NiMo/Al2O3, Pd/ Al2O3, and zeolite Y catalysts and found that liquefaction and hydrodeoxygenation of sawdust could happen simultaneously. The alkali catalyst could obviously enhance the bio-oil yield of corncob hydroliquefaction from 49.0 to 57.5%.9 However, alkali is corrosive, and ionic liquid is costly. MoO2 is a cost-effective and non-corrosive catalyst. The reports about MoO2 usually focused on battery material10 and catalytic isomerization of normal alkanes.11 However, to the best of our knowledge, no reports are issued on the catalytic hydroliquefaction of LCBM over a MoO2/SiO2 catalyst. In this paper, a novel synthetic method of MoO2/SiO2 was proposed, and the application of MoO2/SiO2 to sawdust hydroliquefaction was first investigated. 2. EXPERIMENTAL SECTION 2.1. Materials. Sawdust, collected from a wood processing factory in China, was pulverized to pass through a 60-mesh sieve and dried in vacuum at 80 °C for 24 h. As listed in Table S1 of the Supporting Information, the contents of cellulose, hemicellulose, lignin, and extractives in the sawdust are 52.68, 17.02, 22.68, and 7.62%, respectively. Ethanol and acetone were purified by distillation before © XXXX American Chemical Society

Received: May 14, 2016 Revised: July 14, 2016

A

DOI: 10.1021/acs.energyfuels.6b01166 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels and 10 mL of ethanol were put into a microreactor. The reactor was sealed and purged with N2 3 times. After that, the reactor was pressurized to the desired initial H2 pressure (IHP, such as 2, 3, 4, 5, or 6 MPa) and then heated to 320 °C for 40 min. After the reaction, the reactor was quickly cooled in a cool water bath. The reaction mixture was filtrated, and the filter residue was fractionally extracted with ethanol, and acetone. The combined solutions were evaporated to yield bio-oil. The bio-oil yield is defined as follows: bio‐oil yield (wt %) =

Prepared MoO3/SiO2, MoS2/SiO2, and MoO2/SiO2 and purchased CaO and NaOH were used as the catalyst for sawdust hydroliquefaction, and the product yields were listed in Table 1. A blank test without a catalyst was also carried out for reference. Every catalyst used could increase the bio-oil yield and lower the biochar yield; however, more gas products were produced over MoO3/SiO2 and NaOH. MoO2/SiO2 had the best catalytic performance and largely enhanced bio-oil production. The possible reason is that the metallic sites on MoO2 are able to dissociate H2 and produce active hydrogen atoms.14 As listed in Table 1, the bio-oil yield is gradually increased from 60.1 to 72.3% along with the increment of IHP from 2 to 6 MPa; meanwhile, the biochar yield is decreased from 16.3 to 10.1%. A high IHP can also lower the gaseous product yield. Some sawdust liquefaction data reported previously are summarized in Table S2 of the Supporting Information. The bio-oil yield over MoO2/SiO2 is obviously higher than those previously reported, indicating that monoclinic MoO2 is of high activity for sawdust hydroliquefaction, even higher than NaOH. To examine the stability of the catalyst, MoO2/SiO2 was treated in ethanol at 320 °C under an IHP of 2 MPa for 40 min and the treatment was repeated 3 times to obtain treated MoO2/SiO2. The bio-oil yield over treated MoO2/SiO2 is almost the same with untreated catalyst, indicating that the catalyst is stable in ethanol at 320 °C under H2. 3.2. Mass Spectrometry (MS) Analysis of Bio-oil. Biooils obtained under 2 and 6 MPa IHP over MoO2/SiO2 are denoted as B1 and B2, respectively. The chemical composition of B1 and B2 was analyzed by gas chromatography/mass spectrometry (GC/MS). Figure 3 shows total ion chromatograms (TICs) of B1 and B2. Detailed TICs and information are depicted in Figure S8 of the Supporting Information and listed in Table S3 of the Supporting Information. In total, 75 organic compounds in B1 and B2 were isolated with GC/MS, and 56 of them were tentatively identified. They could be classified into phenolics, fatty acid esters, ketones, alkanols, unknown compounds, and others. Phenolics are the main GC/MSdetectable compounds in both B1 and B2, accounting for 43.9 and 41.8%, respectively. Phenol with relative contents (RCs) of 5.2−5.5% is the most abundant compound in both B1 and B2. Methoxyphenols with 1−2 methoxy(s) are also the main phenolic compounds. Fatty acid esters are linear fatty acid ethyl esters with a carbon number of C18−30, including a small amount of unsaturated fatty acid ethyl esters and alkanedioic acid diethyl esters. The RCs of esters in B1 and B2 are 28.9 and 29.3%, respectively. The origin and formation of a lot of longchain fatty acid esters from sawdust need further investigation. Although GC/MS is a powerful tool to identity a complex mixture,15,16 there are still 19 unidentified compounds as a result of the co-eluting and insufficient mass spectral database. For example, there are three isomers (including peaks 30 and 35) of C11H14O3 with a base peak of m/z 194, and two of them were detected in bio-oil from biomass pyrolysis.17 The retention times of compounds 30 and 35 are 25.8 and 28.5 min. Another isomer with a retention time of 27.2 min is coeluted with other compounds, and its clear mass spectrum could not be obtained. Compounds 30 and 35 were tentatively identified as 2,6-dimethoxy-4-(2-propenyl)phenol and trans2,6-dimethoxy-4-(1-propenyl)phenol, respectively, according to their retention times, RCs, and origins. Another isomer at 27.2 min should be cis-2,6-dimethoxy-4-(1-propenyl)phenol. Figure S9 of the Supporting Information depicts the time-of-flight

mass of bio‐oil × 100 mass of sawdust on a dry basis

Each bio-oil yield was calculated on the basis of the data of three repeated experiments. The biochar yield was calculated similar to the bio-oil yield by replacing the mass of bio-oil with the mass of biochar.

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization and Sawdust Hydroliquefaction. As described in the Supporting Information, the catalysts were characterized with X-ray diffraction (XRD), highresolution transmission electron microscopy (HR-TEM), scanning transmission electron microscopy (STEM), energydispersive X-ray spectroscopy (EDXS) elemental mapping, etc. As shown in Figure 1, the broad diffraction peak at 2θ range of

Figure 1. XRD pattern of MoO2/SiO2.

22−24° was attributed to the diffraction of the SiO2 support. Most diffraction peaks of monoclinic MoO2 are clearly presented in the XRD pattern of MoO2/SiO2, and no diffraction peaks of MoO3 are observed, indicating that highpure monoclinic MoO2 with a mean size of 74.1 nm (calculated by the Scherrer formula) was successfully synthesized on the SiO2 support. However, very little amount of MoS2 may still remain as a result of the presence of a small diffraction peak at 2θ of 14.1°. The XRD results reveal that a supported MoO2/ SiO2 catalyst could be prepared from MoS2/SiO2 directly without the formation of MoO3 when the O2 content and calcination time are carefully controlled. As Figure 2 shows, as a result of the atomic number of Mo being the biggest atomic number among Si, O, S, and Mo, the brightest regions on the STEM image are Mo, indicating that MoO2 is well-dispersed on the SiO2 support. STEM results correspond well with EDXS elemental mapping of Mo. MoO2 is of interest as a catalyst, especially in hydrocarbon reforming processes, and its traditional preparation method is direct H2 reduction at a high temperature for a long time.12 It needs 20 h to totally reduce MoO3 to MoO2 at 550 °C using H2 and He mixture flow (7:3).13 Development of a mild and facile approach to synthesize the MoO2 catalyst is necessary for the science of catalyst preparation. The novel synthetic method proposed in this paper has some advantages, such as mild (below 500 °C), quick, and no consumption of H2. B

DOI: 10.1021/acs.energyfuels.6b01166 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 2. HR-TEM and STEM images of MoO2/SiO2 and EDXS mapping of O, Mo, and Si elements.

3.3. Possible Mechanism of Catalytic Hydroliquefaction over MoO2/SiO2. As a special metal oxide, MoO2 has a metal−metal bond and free electrons and exhibits unusual metallic electrical conductivity, whereas other metal oxides usually do not have these characteristics. The free electrons in MoO2 are considered to enhance the catalytic activity of Mo4+. On the contrary, all of the valence electrons of the metal in MoO3 are bonded to the oxygen atoms.14 As demonstrated in Figure 4, the pyrolysis of the sawdust component, especially hemicellulose, occurs at 320 °C,18 and some free radicals are produced along with sawdust pyrolysis.19 H2 is adsorbed by the metallic sites on the MoO2 surface. The H−H bond is weaken and even dissociated to produce active hydrogen atoms.14 The resulting active hydrogen atoms are quickly bonded with the free radicals produced from pyrolysis of sawdust. Hence, a higher H2 pressure and more active hydrogen atoms could decrease the polymerization of free radicals and lower the biochar yield. Sawdust contains many oxygen-containing linkages, such as ester and ether bridges. Ethanol could nucleophilically attack these ester and ether bridges20 to degrade the macromolecules in sawdust, and the process is called ethanolysis. Alkaline metal oxide could usually enhance alkanolysis. It is reported that

Table 1. Effects of the Catalyst and IHP on Hydroliquefaction Yields of Sawdust product yield (%)a catalyst

IHP (MPa)

gasb

bio-oil

biochar

blank test CaO NaOH MoO3/SiO2 MoS2/SiO2 MoO2/SiO2 treated MoO2/SiO2 MoO2/SiO2 MoO2/SiO2 MoO2/SiO2 MoO2/SiO2

2 2 2 2 2 2 2 3 4 5 6

31.7 30.2 43.1 36.5 31.2 23.6 23.9 23.5 22.3 21.8 17.6

47.0 48.3 50.0 48.1 50.2 60.1 59.6 60.4 62.0 64.3 72.3

21.3 21.5 6.9 15.4 18.7 16.3 16.5 16.1 15.7 13.9 10.1

a

Reaction conditions: 1 g of sawdust, 0.1 g of catalyst, 320 °C, and 40 min. bBy difference.

mass spectrum of B2. The results show that the main peaks are located between m/z 110 and 450, and trace signals are still observed at m/z 1000, indicating that the composition of biooil is extremely complex. C

DOI: 10.1021/acs.energyfuels.6b01166 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 3. TICs and group component distribution of B1 and B2.

Figure 4. Possilbe catalytic hydroliquefaction of sawdust over MoO2/SiO2 in ethanol. (∗) Nucleophilic attack.

oxygen in MoO2 could form a Mo−OH structure via capturing a hydrogen atom from H2.14 Hhydroxyl in ethanol is more active than that in H2. Hence, a hydrogen bond could be formed between Hhydroxyl of ethanol and oxygen in MoO2, which makes ethanol more nucleophilic and promotes the ethanolysis process of sawdust to produce phenolics and ethyl esters.

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biomass liquefaction (Table S2), mass spectral analyses of bio-oil (Figures S8 and S9 and Table S3), and infrared spectral analysis of bio-oil (Figure S10) (PDF)

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-532-86984629. E-mail: [email protected].

4. CONCLUSION A supported monoclinic MoO2 catalyst was successfully synthesized from MoS2/SiO2 for the first time. This method has some advantages, such as mild reaction conditions, high reaction rate, and no consumption of H2. A high bio-oil yield as high as 72.3% is obtained over MoO2/SiO2 at 320 °C under an IHP of 6 MPa. MoO2/SiO2 exhibits high catalytic activity for sawdust hydroliquefaction possibly as a result of the efficient cleavage of H2 to form active hydrogen atoms.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was subsidized by National Natural Science Foundation of China (Grant 21176259), Natural Science Foundation of Shandong Province (Grants ZR2015BM003 and BS2015NJ001), and the Fundamental Research Funds for the Central Universities (Grant 15CX05009A).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01166. Nomenclature, characterization methods, sawdust analysis (Table S1), liquefaction procedure (Figures S1), pore size analysis of SiO2 (Figure S2), reaction pathways of ATTM (Figure S3), XRD (Figure S4), morphology and EDXS analyses (Figures S5−S7), summary of

REFERENCES

(1) Durak, H.; Aysu, T. Bioresour. Technol. 2014, 166, 309−317. (2) Kloekhorst, A.; Heeres, H. J. ACS Sustainable Chem. Eng. 2015, 3 (9), 1905−1914. (3) Zhang, J.; Chen, W. T.; Zhang, P.; Luo, Z.; Zhang, Y. Bioresour. Technol. 2013, 133, 389−397. (4) Lu, Z.; Zheng, H.; Fan, L.; Liao, Y.; Ding, B.; Huang, B. Bioresour. Technol. 2013, 142, 579−584. (5) Liu, D.; Song, L.; Wu, P.; Liu, Y.; Li, Q.; Yan, Z. Bioresour. Technol. 2014, 155, 152−160.

D

DOI: 10.1021/acs.energyfuels.6b01166 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels (6) Li, Q.; Liu, D.; Song, L.; Wu, P.; Yan, Z. Energy Fuels 2014, 28 (11), 6928−6935. (7) Liu, D.; Li, Q.; Zhao, A.; Song, L.; Wu, P.; Yan, Z. Chem. Eng. J. 2015, 279, 921−928. (8) Grilc, M.; Likozar, B.; Levec, J. ChemCatChem 2016, 8 (1), 180− 191. (9) Khampuang, K.; Boreriboon, N.; Prasassarakich, P. Biomass Bioenergy 2015, 83, 460−466. (10) Zhu, J.; Tang, H.; Tang, Z.; Ma, C.; Xu, Q.; Zhang, X. Electrochim. Acta 2015, 166, 183−189. (11) Al-Kandari, H.; Al-Kharafi, F.; Katrib, A. Appl. Catal., A 2010, 383 (1−2), 141−148. (12) Ellefson, C. A.; Marin-Flores, O.; Ha, S.; Norton, M. G. J. Mater. Sci. 2012, 47 (5), 2057−2071. (13) Lalik, E.; David, W. I. F.; Barnes, P.; Turner, J. F. C. J. Phys. Chem. B 2001, 105 (38), 9153−9156. (14) Marin Flores, O. G.; Ha, S. Appl. Catal., A 2009, 352 (1−2), 124−132. (15) Cong, X. S.; Zong, Z. M.; Wei, Z. H.; Li, Y.; Fan, X.; Zhou, Y.; Li, M.; Zhao, Y. P.; Wei, X. Y. Energy Fuels 2014, 28 (11), 6745−6748. (16) Cong, X. S.; Zong, Z. M.; Li, M.; Gao, L.; Wei, Z. H.; Li, Y.; Fan, X.; Zhou, Y.; Wei, X. Y. Fuel Process. Technol. 2015, 134, 399−403. (17) Mourant, D.; Lievens, C.; Gunawan, R.; Wang, Y.; Hu, X.; Wu, L.; Syed-Hassan, S. S. A.; Li, C. Z. Fuel 2013, 108, 400−408. (18) Yang, H.; Yan, R.; Chen, H.; Lee, D. H.; Zheng, C. Fuel 2007, 86 (12−13), 1781−1788. (19) Liu, M.; Yang, J.; Liu, Z.; He, W.; Liu, Q.; Li, Y.; Yang, Y. Energy Fuels 2015, 29 (9), 5773−5780. (20) Lu, H. Y.; Wei, X. Y.; Yu, R.; Peng, Y. L.; Qi, X. Z.; Qie, L. M.; Wei, Q.; Lv, J.; Zong, Z. M.; Zhao, W.; Zhao, Y. P.; Ni, Z. H.; Wu, L. Energy Fuels 2011, 25 (6), 2741−2745.

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DOI: 10.1021/acs.energyfuels.6b01166 Energy Fuels XXXX, XXX, XXX−XXX