Methanol Mixture: Effect of Solvent

Aug 12, 2013 - Cypress Liquefaction in a Water/Methanol Mixture: Effect of Solvent Ratio on Products Distribution and Characterization of Products. Hu...
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Cypress Liquefaction in a Water/Methanol Mixture: Effect of Solvent Ratio on Products Distribution and Characterization of Products Hua-Min Liu* College of Food Science and Technology, Henan University of Technology, Zhengzhou 450052, China S Supporting Information *

ABSTRACT: The effect of mixed solvents (water/methanol) on product yields obtained from cypress liquefaction and characterization of products was investigated at 180−300 °C. Results showed that the mixed solvents markedly enhanced the biooil yield as compared with pure water and pure methanol. The highest bio-oil yield of 46.7% was obtained in a mixed solvent with the ratio of 5/5 at 280 °C. Hemicelluloses decomposition preceded lignin and cellulose in all tested solvents and lignin was almost totally transformed after liquefaction at 280 °C in pure water. Water had the best effect on the conversion of cellulose as compared with other mixed solvents and methanol tests. The compositions of bio-oils were strongly different depending on the ratios of water/methanol. The higher heating value of solid residues obtained from mixed solvent tests at 280 and 300 °C were 24.7−29.7 MJ/kg, indicating that it would be suitable for combustion as solid fuel.

1. INTRODUCTION With the concern of environmental protection and the depletion of fossil fuel, the utilization of biomass resources has attracted increasing interest around the world.1 Biomass sources, including energy crops, wood, and wood wastes, aquatic plants, agricultural crops, and their waste byproducts, as well as animal and municipal wastes, can be considered as potential sources of fuels.2 Liquefaction is a promising thermochemical conversion route and plays a vital role in biomass conversion. Liquefaction requires a relatively low temperature in comparison with gasification and pyrolysis.3 In addition, in liquefaction the presence of the solvent dilutes the concentration of the products preventing cross-linked and reverse reactions. Water is a cheap and a common solvent as well as an effective liquefaction agent in the liquefaction process, but the high critical value of water (374.3 °C, 22.1 MPa) means that the sub/supercritical water liquefaction process requires challenging operation conditions. In addition, another drawback of hydrothermal liquefaction is the high oxygen concentration in the liquid products, resulting in low-heating value for bio-oil.4 Therefore, many researches have been conducted to investigate the liquefaction conversion of biomass to bio-oil with low oxygen value in low-critical-value organic solvents.4−6 Yan et al.7 reported that some mixed solvents had a synergistic capability to inhibit the formation of the solid residue, and enhance the liquefaction conversion of biomass. These mixtures with synergistic capability were composed of two solvents with different polarity: (1) an electron acceptor solvent, containing hydroxyl group, with high polarity, and (2) an electron donor solvent with middle intensity. Biomass is mainly composed of three main components: cellulose, hemicelluloses, and lignin.8 The lowest temperature of the decomposition shows hemicelluloses, and then cellulose, giving the highest loss of weight; the lignin is the biomass component of the highest thermal resistance.8 The bio-oils are very complex, involving an uncountable number of organic © 2013 American Chemical Society

compounds. Therefore, the degradation of biomass cannot be easily described by detailed chemical reaction pathways with well-defined single reaction steps. The liquefaction process shows similarities with the pyrolysis process but also significant differences due to the presence of solvent as the reactant and reaction medium.9 Liu et al.10 investigated the mechanism of biomass liquefaction in hot-compressed water by examining the effects of reaction conditions on the characteristics of the solid residues remaining after liquefaction, and results showed that the liquefaction process could be described in three steps. However, there are not enough data to study the mechanism of biomass liquefaction in the mixed solvent through the investigation of the characteristics of the solid residues. In this study, methanol was mixed with water, prepared as a synergistic reaction medium. The effect of solvent ratio on the products distribution (water-soluble oil, heavy oil, and solid residue) was examined at different temperatures. The main composition of bio-oils in different solvents was analyzed by FT-IR and GC−MS. In addition, the solid residues obtained from different liquefaction processes were characterized by FTIR, X-ray diffraction, and elemental analysis to help understand the mechanism of the liquefaction process in the mixed solvents.

2. MATERIALS AND METHODS 2.1. Materials. Cypress was obtained from Hennan province in China. It was first ground and sieved. The obtained particles smaller than 40 mesh were used in this work. The cypress flour was extracted with distilled water and ethanol to remove water-soluble and ethanol-soluble compounds, then dried at 105 °C for 24 h and kept in a desiccator at room temperature. The ash, elemental components, chemical Received: Revised: Accepted: Published: 12523

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methanol volume ratios (0/10, 3/7, 5/5, 7/3, and 10/0) were carried out at the various temperatures of 180, 200, 220, 240, 260, 280, and 300 °C. The results are illustrated in Figure 2. Clearly, the yield of bio-oil (WSO and HO) strongly depends on the solvent ratios and the reaction temperature. Among the five tested solvents, the ratio of 5/5 had the best effect on the conversion in the tested temperatures of 240−300 °C and the lowest solid residue yields were 31.7%, 26.1%, 17.0%, 21.9%, and 56.6% under the mixed solvent ratios of 0/10, 3/7, 5/5, 7/ 3, and 10/0, respectively. For all the tested solvents (except pure methanol), the bio-oil and HO yields increased with increasing reaction temperature at first, and then decreased when the temperature further increased. The different bio-oil yields against reaction temperatures can be explained by the pathway of bio-oil formation from liquefaction of biomass, which consists of three steps: biomass breaking down, bio-oil formation, and bio-oil decomposition. High temperature promotes the cracking and repolymerization of the bio-oil, resulting in a decrease in bio-oil yield (WSO and HO) and increase in solid residue yield when the solvent ratios are 7/3, 5/5, and 10/0 at 300 °C. Considering the bio-oil and HO yields, the solvent ratio of 5/5 had the highest bio-oil and HO yields of 46.7% and 36.6% obtained at 280 and 260 °C, respectively. Biomass constitutes three biopolymers: cellulose, hemicelluloses, and lignin, which together form a complex and rigid structure.13,14 The distinctive structural characteristics of biomass make it resistant to attack of sole solvent during the liquefaction process.4 The enhancement of the liquefaction conversion of biomass in mixed solvent was because the synergistic capability to improve the decomposition of biomass. First, hot-compress water accelerated biomass depolymerization by hydrolysis, and cellulose and hemicelluloses broken down to smaller sugar units that eventually caused the breakdown of the entire biomass structure.15,16 Second, methanol impregnated the plant tissue, carrying the reagents to the lignin and the resulting lignin fragments from the inner part of the cell to the solution. Therefore, the synergistic capability of water/methanol mixture allowed the great decomposition of the structure of biomass. 3.2. Characterization of Solid Residue. 3.2.1. FT-IR Analysis of Solid Residue. The FT-IR spectra of the raw cypress and the solid residues after liquefaction with different water/methanol volume ratios and pure solvents are shown in Figure 3. According to the literatures,3,17,18 the bands in the raw cypress were assigned as follows. The bands at 3362 cm−1 was caused by −OH stretching vibration in cypress and water. Xylans of hemicelluloses have characteristic absorption at a wavenumber of 1722 cm−1 (CO stretching vibration, carbonyl, and ester groups). Cellulose has its characteristic absorption peaks at about 1365 cm−1 (−CH3 bending vibration) and 1022 cm−1 (C−O bending vibration). A spectrum of lignin shows some distinct bands. The absorption at 1600−1590 cm−1 (aromatic skeletal CC plus CO stretching vibration), 1520−1500 cm−1 (aromatic skeletal C C stretching vibration), 1314 cm−1 (O−H in-plane bending vibration), and 830−750 cm−1 (C−H bending vibration) represent the lignin. The band at 1722 cm−1 almost disappeared in the spectra of the samples liquefied at 260 °C showing that hemicelluloses decomposition preceded lignin and cellulose in all tested solvents. It was worth noting that after liquefaction with pure methanol and mixed solvents, the band of 1722 cm−1 appeared again when the liquefaction temperatures were increased to 280

composition, and the higher heating value (HHV) of cypress were determined according to previously described methods.10 Analysis showed that the cypress contained about 48.9% carbon, 44.8% oxygen, 6% hydrogen, 0.3% nitrogen, 43.2% cellulose, 26.3% hemicelluloses, 28.2% lignin, and 2.3% ash (on a dry basis). The sample had a HHV of 17.1 MJ/kg. The chemicals used in this work were of analytical grade and purchased from Beijing Chemical Reagent Limited Company. 2.2. Apparatus and Experimental Procedure. The experiments were carried out with a batch reactor system. The stainless steel autoclave (Parr, USA) was heated with an external electrical furnace, and temperature was measured with a thermocouple. The detail of the liquefaction experiment has been described previously.11 The procedure for the separation is shown in Figure 1. Briefly, once the reactor was cooled to

Figure 1. Procedure for separation of liquefaction products.

room temperature, the gas was vented. The solid and liquid products in autoclave were poured into a beaker, and the wall of the autoclave was washed with methanol (for water liquefaction test, the autoclave was washed with water). Methanol was first removed from the solid and liquid mixture, and then 50−100 mL of deionized water was added to the mixture. After removal of the water under reduced pressure at 85 °C in a rotary evaporator, the aqueous phase product was designated watersoluble oil (WSO). The water-insoluble fraction was washed with acetone, and the contents were separated by filtration under vacuum. The acetone was evaporated in a rotary evaporator, and this fraction was designated heavy oil (HO). The acetone-insoluble fraction was dried at 105 °C and called the solid residue (SR). All experiments were repeated twice, and the relative errors were within 4%. 2.3. Analysis. The X-ray diffraction (XRD), elemental composition (C, H, N, and O), and Fourier transform infrared spectrometer (FT-IR) analysis of the products were determined as described previously.11,12 The bio-oils were analyzed by a GC−MS (HP-5 capillary column; Agilent 7890A/5978, USA). The GC was programmed at 40 °C for 2 min and then increased at 5 °C/min to 300 °C. The carrier gas He velocity was 1 mL/min and the injection size was 0.1 μL.

3. RESULTS AND DISCUSSION 3.1. Effect of Solvent Ratios on Bio-oil Yield. Experiments of cypress liquefaction in solvent with different water/ 12524

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Figure 2. Effect of solvent ratios on the yield of bio-oil (10 g of cypress, reaction time of 0 min, and 100 mL of mixture solvent).

and 300 °C. This indicated that carbonyl and ester groups were produced in the process of liquefaction with pure methanol and mixed solvents at higher temperatures. A possible explanation for the observed spectral changes was that cellulose and hemicelluloses were decomposed to acid compounds at the initial stage, and then these acids and hydroxyl in solid residue formed carbonyl and ester groups at higher temperatures. The effects of mixed solvents, pure methanol, and water on cypress liquefaction can be clearly seen from the cellulose and lignin decomposition. As shown from Figure 3, pure water and the solvent ratio of 7/3 compared with other mixed solvents and pure methanol gave the best cellulose decomposition at 280 and 300 °C, respectively. This can be confirmed by the substantial decrease of absorption intensities of cellulose (1365 and 1022 cm−1). The disappearance of bands at 1314 and 1520−1500 cm−1 showed that the lignin was almost totally transformed after liquefaction at 280 °C in pure water. The only weak bands were at 1600−1590 cm−1, and 830−750 cm−1 bands implied the existence of lignin fragments in the solid residues obtained from liquefaction cypress in the various mixed solvents and pure methanol. 3.2.2. Residue Crystallinity. Thermo-chemical processes can change cellulose crystalline structure by disrupting inter/intra hydrogen bonding of cellulose chains,19 and X-ray measurements of the crystallinity index (CrI) are the best option to estimate their impacts on biomass crystallinity. To examine the crystalline species in the raw cypress and solid residues from the liquefaction processes with different solvents, X-ray

diffraction measurements were carried out. Figure 4 shows the X-ray diffraction patterns of the raw cypress before and after liquefaction in different solvents at temperatures varying from 180 to 300 °C. The X-ray diffraction pattern of raw cypress showed two primary peaks at 2θ = 15.8° and 22.0°, typical of cellulose I. It has been well documented that these peaks correspond to the (110) and (200) planes of cellulose.20,21 As shown in the Figure, the peak at 22.0° present in the raw cypress diffraction pattern were significantly shifted to 22.6° in the solid residues in all liquefaction experiments, indicating that there was structural order in the cellulose after liquefaction with different solvents. The raw cypress has a broad diffraction pattern, which indicates the presence of both amorphous and crystalline materials. After the liquefaction processes at 180− 260 °C for all the tests, the X-ray diffraction pattern of the solid residue shows the presence of highly crystalline cellulose compared to that raw cypress. This observation is a clear indication that the amorphous part of cypress (lignin and hemicelluloses) is reacting at the operating conditions of 180− 260 °C. The solvent effect on cypress decomposition can be clearly seen from Figure 4. Among the five tested solvents, pure water and mixed solvents at the ratios of 5/5 and 7/3 had the better effect on the conversion of cellulose at 280 and 300 °C. In those solid residues, the diffraction lines of both cellulose (110) and (200) planes disappeared, suggesting a complete decomposition of cellulose and the organic materials in the cypress. 12525

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Figure 3. FT-IR spectra of raw cypress and solid residues (A, 260; B, 280 °C; and C, 300 °C).

3.2.3. Elemental Analysis of Solid Residue. The elemental composition of solid residues obtained from liquefaction under all the experimental conditions are presented in Table S2 in the Supporting Information. The different mixed solvents had an important effect on the solid residue composition. The main elemental compositions of raw cypress were as follows: carbon (48.2%), oxygen (44.8%), and hydrogen (6%). After liquefaction in all the tested solvents, the carbon content of the solid residue increased, and the oxygen content decreased. The HHVs of the solid residues, which were calculated by Dulong’s formula and increased sharply as the temperature increased from 180 to 300 °C. Compared to the pure water and methanol experiments, the O/C ratios of solid residues obtained from mixed solvents experiments were lower, resulting in the higher HHVs. The lower O/C ratios and oxygen content in solid residues might be attributed to the synergistic capability

The crystallinity index (CrI) of all samples are calculated from the XRD data, and the results are summarized in Table S1 in Supporting Information. According to the results of Table S1, the cellulose conversions with different solvents were different. The calculated CrI changed weakly for the pure methanol tests, which indicated that the liquefaction reaction occurred simultaneously in the crystalline and amorphous zones. More interestingly, the crystalline (CrI) of solid residues from the cypress liquefaction with mixed solvents were higher than that of pure water and methanol liquefaction experiments at 240 and 260 °C. This result suggested that the amorphous zones (such as lignin and hemicelluloses) broke down more under the mixed solvents but these liquefaction conditions were unable to break apart effectively the inter/intra-chain hydrogen bonding in cellulose fibrils. 12526

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Figure 4. X-ray diffraction patterns of cypress before and after liquefaction in different solvents at various temperatures (A, 180; B, 200; C, 220; D, 240; E, 260; F, 280; and G, 300 °C).

were analyzed by FT-IR, and the FT-IR spectra of the samples are given in Figure 5. As to the spectra of bio-oils, the O−H stretching vibrations at the band of 3376 cm−1 indicate the presence of alcohols and phenols. The C−H stretching vibrations between 2800 and 3000 cm−1 and the C−H deformation vibrations between 1350 and 1490 cm−1 indicate the presence of alkanes. The bands between 1650 and 1750 cm−1 are attributed to the CO group and indicate the presence of aldehyde, acids, or ketones. In addition, the vibrations between 1675 and 1575 cm−1 are due to the stretching vibrations of CC groups in aromatics and alkenes. The peaks at about 1513 and 600−900 cm−1 are typical

of water/methanol mixture as explained in section 3.1. The HHV of the solid residues from mixture solvents at 280 and 300 °C fell in the range of 24.7−29.7 MJ/kg, much closer to that of the Loy yang coal22 (26.4 MJ/kg) and the Xuzhou coal23 (23.22 MJ/kg). Therefore, the solid residues were suited for combustion as a solid fuel. 3.3. Characterization of Bio-oil. 3.3.1. FT-IR Analysis of Bio-oils. Characterization of the solid residue showed that the major cypress had decomposed at 300 °C, indicating that the composition of bio-oil obtained at a higher temperature was more representative for a further investigation. Therefore, the characteristics of the bio-oils in different solvents at 300 °C 12527

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liquefaction in the five solvents contained a lot of complex compounds such as acids, phenols, esters, etc., and the compositions of the bio-oils were strongly different and depended on the ratios of water and methanol. All of the compositions found by GC−MS can be divided into aromatic and nonaromatic compounds. The aromatic compounds primarily originated from the decomposition of lignin, such as toluene, 2-methoxy-4-methyl-phenol, 3-methyl-benzenediol, vanillin, and ethyl vanillin, although they could also form from cellulose through hydrolysis to sugar followed by dehydration and ring-closure reactions.5 In the biomass liquefaction process, the major role of solvent is to fragment the biomass by neuclephilic substitution reactions to stabilize the fragmented products.24 The bio-oil from hydrothermal liquefaction contained a significant amount of acids, such as 4-oxopentanoic acid and 4-hydroxy-3-methoxy-benzeneacetic. When the methanol was used as the liquefaction solvent, the bio-oil contained an amount of methyl esters. The ester compounds were possibly produced via an esterification reaction between methanol and acids in the liquefaction process. Despite the different mixture solvents used, several components with the same compositions were obtained, including 2-methoxy-phenol, 1,2-benzenediol, 5-(hydroxyl methyl-)-2-furancarboxaldehyde, 4-ethyl-2-methoxy-phenol, 2methoxy-4-(1-propenyl)-phenol, 1-(4-hydroxy-3-methoxyphenyl)-2-propanone, and 4-hydroxy-3-methoxy-benzeneacetic acid. The results indicated that resemblances in the liquefaction mechanism exist among the liquefaction processes although various ratios of water and methanol were used as solvents.

4. CONCLUSIONS The products distribution and characterization of products coming from the cypress liquefaction in pure water and methanol at different temperatures were compared to those obtained at different water/methanol mixtures. The results showed that the water/methanol mixture had a synergistic effect on the decomposition of cypress. Mixed solvent at the ratio of 5/5 had the highest bio-oil yield, and the highest bio-oil yield of 46.7% was obtained at 280 °C. The HHV of the solid residues from mixed solvents at 280 and 300 °C fell in the range of 24.7−29.7 MJ/kg and they were suited for combustion as a solid fuel. The bio-oils obtained from the five solvents were different from each other in their components. Hemicelluloses decomposition preceded lignin and cellulose in all tested solvents, and the lignin was almost totally transformed after liquefaction at 280 °C in pure water. Water had the best effect on the conversion of cellulose as compared with other mixed solvents and methanol tests.

Figure 5. FT-IR spectra of bio-oils at 300 °C.

evidence for the presence of single, polycyclic, and substituted aromatic groups. The bands between 1300 and 900 cm−1 may be attributed to the O−H deformation vibrations and the C−O stretching existing in the phenols. The FT-IR spectra of bio-oils obtained at different solvents were compared. The increasing peak intensity between 600 and 900 cm−1, as shown in the spectra of the bio-oils from mixed solvents liquefaction, could be attributed to the high content of aromatic groups. The aromatic groups (such as phenolic compounds) primarily originated from the decomposition of lignin, although they might also form from cellulose through hydrolysis to sugars followed by ring closure and dehydration reactions.8 This result indicated that more lignin was decomposed due to the synergistic capability of the mixed solvent system. For the water solvent tests, the peaks at 2925 and 1595 cm−1 became weaker as compared to the mixed solvents and pure methanol. In addition, the new peaks at 966 and 939 cm−1 were observed in the spectra of bio-oil from water liquefaction. The difference showed that there had been noticeable changes in the bio-oil products from different mixed solvents liquefaction. 3.3.2. GC-MS Analysis of Bio-oil. The bio-oils that were in different solvents at 300 °C were also characterized by GC−MS for identification of the chemical compositions. Identification of the GC−MS peaks was based in most cases on comparison with spectra from the NIST library. The percent area for each compound was defined by percentage of the compound’s chromatographic area out of the total area, and the results of the percent area are shown in the Table S3 in Supporting Information. Clearly, the bio-oils generated from cypress

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ASSOCIATED CONTENT

S Supporting Information *

Three additional tablesas described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-0371-67789937. Fax: ++86-0371-67789937. Notes

The authors declare no competing financial interest. 12528

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(21) Xu, C. B.; Lad, N. Production of heavy oils with high caloric values by direct of liquefaction of woody biomass in sub/near-critical water. Energy Fuel. 2008, 22, 635. (22) Kuchonthara, P.; Bhattacharya, S.; Tsutsumi, A. Combination of thermochemical recuperative coal gasification cycle and fuel cell for power generation. Fuel 2005, 84, 1019. (23) Xiao, R.; Zhang, M. Y.; Jin, B. S.; Huang, Y. J. High-temperature air/steam-blown gasification of coal in a pressurized spout-fluid bed. Energy Fuel 2006, 20, 715. (24) Akhtar, J.; Amin, N. A. S. A review on process conditions for optimum bio-oil yield in hydrothermal liquefaction of biomass. Renew. Sust. Energy. Rev. 2011, 15, 1615.

ACKNOWLEDGMENTS We sincerely acknowledge the financial support by the Doctor Research Fund of Henan University of Technology (2013BS018) and Major State Basic Research Projects of China (973-2010CB732204/3).

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REFERENCES

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