Investigation on Characteristics of Liquefied Products from Solvolysis

Article pubs.acs.org/EF

Investigation on Characteristics of Liquefied Products from Solvolysis Liquefaction of Chlorella pyrenoidosa in Ethanol−Water Systems Xiaowei Peng, Xiaoqian Ma,* and Yousheng Lin Guangdong Province Key Laboratory of Efficient and Clean Energy Utilization, South China University of Technology, Guangzhou 510640, People’s Republic of China S Supporting Information *

ABSTRACT: The current work presented the characteristics of liquefied products from solvolysis liquefaction of Chlorella pyrenoidosa in ethanol−water systems. The effect of different ethanol/water volume ratios on the physical and chemical characteristics of solid residue, dichloromethane-insoluble solid product (DCM-insoluble solid product), aqueous product, light bio-oil, and biocrude oil was systematically carried out. Elemental analysis and scanning electron microscopy revealed that the addition of ethanol contributed to the decomposition of solid residue. However, when the ethanol content exceeded 60%, the repolymerization of solid residue occurred. Moreover, on the basis of Fourier transform infrared spectroscopy, a plausible reaction mechanism of solid residues and DCM-insoluble solid products with respect to ethanol addition and reaction temperature was presented. The main chemical compositions of aqueous product, light bio-oil, and biocrude oil were identified by gas chromatography−mass spectrometry. It indicated that the ethanol−water cosolvent contributed to the generation of nitrogen-containing compounds in aqueous product and short-chain esters in light bio-oil, and decreased the content of toxic nitrogenous organic compounds with two or more methyl groups in aqueous product. The ethanol−water system could facilitate ester formation in biocrude oil through esterification and alcoholysis reactions. Through the thermal gravimetric analysis and thermal stability experiment of biocrude oils, it was demonstrated that the biocrude oil obtained from 60% ethanol content had the highest content of low boiling point components of C10−C20 and performed with the best thermal stability.

1. INTRODUCTION Considering the finite traditional fossil fuel resources and escalating demands for the same, developing sustainable sources of energy have gained great attention in recent decades.1 Microalgae are viewed as a favorable feedstock for next generation bioenergy products due to their higher photosynthetic efficiency, growth rate, and area-specific yield.2,3 Moreover, microalgae can be cultivated in sea water, freshwater, or wastewater, which can reduce competition with food crops for arable lands.4 Typically, microalgae feedstock can be converted into liquid biofuel via fast pyrolysis, transesterification, and hydrothermal liquefaction.5,6 However, fast pyrolysis technology requires dried microalgae feedstock, resulting in much more energy consumption.7 The transesterification technology only requires the high-lipid microalgae as feedstock, and only uses the lipid composition.8 Furthermore, growing relatively pure cultures of high-lipid algae usually has lower biomass productivities compared to low-lipid algal strains.9 In contrast, hydrothermal liquefaction technology dramatically reduces the energy input for dewatering and drying, and converts all organic compounds in microalgae into biocrude oil rather than the lipid content only.10,11 Previous studies9,12,13 had extensively investigated the effect of operating parameters on biocrude oil yield and properties from hydrothermal liquefaction of various microalgae. In order to improve the biocrude oil yield and properties, Xu et al.14,15 found that hydrothermal catalytic liquefaction of algae biomass with Ce/HZSM-5, NaOH, and NaCO 3 significantly increased the bio-oil yield and improved the © 2016 American Chemical Society

physicochemical properties. Furthermore, some researchers have focused on the solvolysis liquefaction of biomass in ethanol−water systems,16−18 due to the low critical value and hydrogen donor capability of ethanol. Wu et al.,18 Zhang et al.,19 and Liu et al.20 found that the addition of ethanol could significantly increase the biocrude oil yield and esters content in biocrude oil. However, these studies mainly focused on the investigation of characteristics of biocrude oil. The solid residue and aqueous product are two important byproducts after solvolysis liquefaction. An in-depth study of these byproducts contributed to the comprehensive understanding of solvolysis liquefaction of microalgae in ethanol−water system. Although Wu et al.18 had investigated the physicochemical characteristics of solid residue and biocrude oil obtained from liquefaction of microalgae in ethanol−water cosolvent, they did not investigate the effect of ethanol addition on the chemical composition of aqueous product. Jena et al.21 had found that the high content of nitrogen, phosphorus, and potassium along with other minerals and essential micronutrients in aqueous phase benefited the algae growth. However, Pham et al.22 concluded that some nitrogenous organic compounds (NOCs) (e.g., 2,2,6,6-tetramethyl-4-piperidone and 2,6-dimethyl-3-pyridinol) also exhibited toxicity to algae and other aquatic organisms. Thus, it is important to investigate the effect of ethanol addition on the yield of NOCs and toxicity of aqueous phase. Wu et al.18 Received: May 7, 2016 Revised: July 12, 2016 Published: July 19, 2016 6475

DOI: 10.1021/acs.energyfuels.6b01103 Energy Fuels 2016, 30, 6475−6485

Article

Energy & Fuels Table 1. Characteristics of Chlorella pyrenoidosa (Dry Basis)25 proximate analysis (wt %)

a

volatiles fixed carbon ashes

80.82 ± 0.22 13.37 ± 0.32 5.81 ± 0.10

HHVa (MJ/kg)

24.09 ± 0.17

HHV,

26

ultimate analysis (wt %) C H N S Ob

50.99 ± 0.05 7.83 ± 0.08 9.48 ± 0.06 1.08 ± 0.01 24.81 ± 0.20

organic matter composition (wt %) crude protein crude fat crude fiber

70.65 ± 0.28 5.43 ± 0.23 2.17 ± 0.19

nonfibrous carbohydratesb

15.94 ± 0.69

higher heating value. HHV = 0.338 wt % C + 1.428 ((wt % H − wt % O)/8) + 0.095 wt % S. bBy difference.

Figure 1. (a) Products separation and extraction procedure and (b) liquefied products distribution.25

these studies did not consider the effect of operating parameters on the component evolution of solid residue under an ethanol−water system. According to the research by Yoo et al.,23 the asphaltene-lean biocrude generated less solid precipitate after thermal stability experiment, performing with a better antiaging property. Therefore, the influence of ethanol−

had analyzed the microscopic structure and surface chemical composition of solid residue through scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Yu et al.9 also studied the effect of retention time on the carbon recovery and nitrogen recovery of solid residue after hydrothermal liquefaction of low-lipid microalgae, whereas 6476


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accounted for as part of the solid residue. Overall, the liquefied products were separated into gas, solid residue, aqueous product, light bio-oil, and biocrude oil, and the yields of liquefied products were shown in Figure 1b. Because the gas fraction was relatively small and the components were simple (mainly CO2, CO, CH4, and H2, referred to in previous studies2,27), the gas was vented into the atmosphere without further analysis. In order to investigate the effect of water− ethanol reaction medium on the liquefied products properties, the operating condition was performed at a reaction temperature of 300 °C, retention time of 60 min, solid/liquid ratio of 18.8/75 g/mL, and the ethanol content variation from 0% to 100% (v/v). Meanwhile, the DCM-insoluble-solid product was deeply analyzed. The DCMinsoluble solid product was obtained from the following conditions: reaction temperature of 220, 240, and 260 °C, retention time of 30 min, solid/liquid ratio of 9.4/75 g/mL, and ethanol content of 40%. 2.3. Analytical Approach. The elemental analysis was based on the ASTM D5373 standard, and the elemental analysis was measured using an elemental analyzer (Vario EL cube, Hanau, Germany). The morphology of solid residue and DCM-insoluble solid product was analyzed by scanning electron microscopy (SEM, Merlin S3700). The functional groups of solid residue, DCM-insoluble solid product and biocrude oil were characterized by a Nicolet iS 10 FTIR spectrometer. FTIR analysis was carried out using a resolution of 4 cm−1 and 8 scans per sampling, with the frequency range of 500−4000 cm−1. The chemical compositions of the aqueous product, light bio-oil, and biocrude oil were analyzed by an Agilent Technologies 7890B gas chromatograph with 5977A mass spectrometer equipped with a capillary column (HP-5 MS; 30 m length, 250 μm i.d., and 0.25 μm film thickness). Helium (99.99% purity) flowing at 1 mL/min was used as the carrier gas. A 10 mL aliquot of aqueous product was removed from water by adding proper amount of anhydrous Na2SO4 powder, and then dissolved in 10 mL of chromatographic grade ethanol. After filtration, 1 μL of aqueous product sample was injected with a split ratio of 20:1, and a 1.84 min solvent delay was set to protect the filament. The sample inlet temperature was set to 250 °C. The GC oven temperature program was as follows: 40 °C (hold for 10 min) → 250 °C (5 °C/min, hold for 5 min). The light bio-oil in the ethanol phase was further diluted to 50% (v/v) in chromatographic grade ethanol, and then a 1 μL light bio-oil sample was injected with a split ratio of 20:1. A 1.84 min solvent delay was set to protect the filament. The sample inlet temperature was set to 250 °C. The GC oven temperature program was as follows: 40 °C (hold for 10 min) → 135 °C (5 °C/min) → 250 °C (20 °C/min, hold for 5 min). The biocrude oil sample was prepared as 3 ± 0.5 wt % solutions in chromatographic grade dichloromethane. A 1 μL aliquot of biocrude oil sample was injected with a split ratio of 60:1, and a 2 min solvent delay was set to protect the filament. The sample inlet temperature was set to 320 °C. The GC oven temperature program was as follows: 40 °C (hold for 0 min) → 300 °C (10 °C/min, hold for 4 min). The compounds in aqueous product, light bio-oil, and biocrude oil were identified by the NIST mass spectral database (NIST14). The boiling point distribution of the biocrude oil was analyzed using TGA (METTLER TOLEDO TGA/DSC1) under N2 atmosphere at a flow rate of 60 mL/min. About 6 mg of biocrude oil sample was heated from 35 to 800 °C at the heating rate of 10 °C/min. The thermal stability of biocrude oil was investigated by heating biocrude oil in a tubular electrical furnace. A 0.2 g amount of biocrude oil sample was loaded into the sample holder and then the sample holder was inserted into the tubular electrical furnace. The furnace was heated from ambient temperature up to 250 °C with 0.06 m3/h of nitrogen as sweeping gas. After heated for 3 h, the sample holder was cooled to room temperature under N2 atmosphere, and the treated biocrude oil was mixed with 15 mL DCM. The DCM-insoluble precipitates were separated by filtration under vacuum through a preweighted filter paper and the solid residue was dried in the oven at 105 °C for 12 h before weighing.

water reaction medium on the antiaging property of biocrude oil needs further study. In fact, during the ethanol separation process, the light bio-oil escaped easily, accompanying the removal of excess ethanol in a rotary evaporator. However, many previous studies18,19,24 had not analyzed the components of the light bio-oil. In our pervious study,25 when the reaction temperature was below 260 °C, there existed a soft and unsticky product which was insoluble in dichloromethane (DCM) during the extraction process. The physicochemical characteristic of this DCM-insoluble product also needs further investigation. In this study, the experiment of solvolysis liquefaction of Chlorella pyrenoidosa (C. pyrenoidosa) in an ethanol−water system was performed. Elemental analysis (EA), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR) were used to verify the liquefaction behavior on the basis of microalgae feedstock, solid residues, and DCM-insoluble products. The main chemical compositions of aqueous product, light bio-oil, and biocrude oil were identified by gas chromatography−mass spectrometry (GC− MS) and FTIR. Furthermore, the boiling point distribution and thermal stability of biocrude oil were studied by thermal gravimetric analysis (TGA) and thermal stability experiment. To the best of our knowledge, this present study is the first of its kind to systematically evaluate the characteristics of solid residue, DCM-insoluble product, aqueous product, light bio-oil, and biocrude oil from solvolysis liquefaction of microalgae in an ethanol−water system. Through the deep study of properties of different liquefied products can provide deep understanding of the ethanol−water liquefaction mechanism for microalgae and give guidance for further resource utilization of liquefied products.

2. MATERIALS AND METHODS 2.1. Raw Materials. Chlorella pyrenoidosa was provided by Wudi lv Qi Bioengineering Co. Ltd. (Shangdong Province, China) in powdered form. Before experiments for various reaction conditions, the microalgae powder was further dried at 105 °C for 12 h. Characteristics of C. pyrenoidosa were shown in Table 1. The reagent/ chromatographic grade ethanol and dichloromethane were purchased from Sinopharm Chemical Reagent Co. Ltd. 2.2. Experimental Procedure. The detailed experimental procedure was described in our previous study.25 Briefly, the microalgae sample and ethanol−water mixed solvent were fed into three 250 mL batch autoclaves. The reactors were heated to the desired temperature for a designated reaction time with 0.69 MPa of initial nitrogen pressure. In each experiment, the volume of solvent accounted for 30% of the reactor capacity. The product separation was carried out according to the previously reported method,16 as shown in Figure 1a. During the liquefied products separation process, the excess ethanol was removed by rotary evaporator and collected in a sample bottle. It was interesting that the colorless and transparent ethanol phase became light yellow overnight, which is shown in Figure S1a in the Supporting Information. This indicated that the light bio-oil might have escaped into the ethanol phase. Interestingly, when the reaction temperature was below 260 °C, there existed a kind of soft and unsticky product (as shown in Figure S1b in the Supporting Information) that was insoluble in DCM (dichloromethane) and adhered on the separating funnel wall during the DCM extraction process. After being dried in the air, this kind of soft and unsticky product changed into a fragile solid. This solid product was defined as a DCM-insoluble solid product in this study. The proportion of DCMinsoluble solid product was less than 1%. Moreover, because the DCM-insoluble solid product was only produced when the reaction temperature was less than 260 °C, and other liquefying conditions did not occur this phenomenon, the DCM-insoluble solid product was not 6477


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Energy & Fuels Table 2. Elemental Analysis of Solid Residues and DCM-Insoluble Solid Products (Dry Basis) elemental analysis (wt %) sample SR0 SR20 SR40 SR60 SR80 SR100 DCM220 DCM240 DCM260 a

C 73.74 36.45 26.76 21.37 23.86 41.58 67.95 66.63 65.61

± ± ± ± ± ± ± ± ±

H 0.02 0.02 0.12 0.05 0.03 0.09 0.05 0.24 0.14

7.73 4.25 3.08 3.11 2.78 4.15 7.10 7.37 7.27

± ± ± ± ± ± ± ± ±

N 0.02 0.01 0.01 0.03 0.03 0.02 0.02 0.09 0.02

6.59 3.19 2.69 3.66 3.37 6.29 9.17 10.40 9.45

± ± ± ± ± ± ± ± ±

Oa

S 0.01 0.01 0.01 0.02 0.02 0.02 0.01 0.04 0.03

0.98 0.78 0.80 1.14 2.09 1.50 0.87 0.95 1.34

± ± ± ± ± ± ± ± ±

0.03 0.00 0.00 0.00 0.00 0.01 0.00 0.01 0.01

8.61 8.60 3.90 9.72 12.07 9.10 4.71 13.82 14.00

± ± ± ± ± ± ± ± ±

0.07 0.04 0.14 0.09 0.08 0.14 0.08 0.38 0.19

By difference.

3. RESULTS AND DISCUSSION 3.1. Characteristics of Solid Products. 3.1.1. Elemental Analysis. The solid residues were marked as SR0, SR20, SR40, SR60, SR80, and SR100 in accordance with ethanol contents of 0%, 20%, 40%, 60%, 80%, and 100%, respectively. The DCMinsoluble solid products were marked as DCM220, DCM240, and DCM260 in accordance with reaction temperatures of 220, 240, and 260 °C, respectively. Table 2 showed the elemental analysis of the solid residues and DCM-insoluble solid products. The Van Krevelen diagram of solid residues and DCM-insoluble solid products was presented in Figure 2. As

into naphthene ring systems via hydrogenation. Further, the naphthene ring systems were converted into alkyl chains. Therefore, as the liquefaction temperature increased, the yield of DCM-insoluble solid product decreased,25 due to the decomposition reaction. 3.1.2. SEM Analysis. To investigate the effect of solvolysis liquefaction in the ethanol−water system on the microscopic structure of solid residues, the SEM images of microalgae feedstock and solid residues were shown in Figure 3. After solvolysis liquefaction, the cell structures of microalgae were destroyed completely. It was obvious that a large proportion of the SR0 surface was smooth and dense. This phenomenon suggested that the molecular chains of the solid residue were not destroyed completely, resulting in the high degree of polymerization and high yield of solid residue after liquefaction in sole water solvent. With the addition of ethanol in the reaction solvent, the surfaces of SR20, SR40, SR60, and SR80 became more coarse and porous. This phenomenon may be explained in that ethanol can provide an active hydrogen in the process of microalgae liquefaction and then break the large molecules into small molecules. Yuan et al.30 had revealed that the intermolecular hydrogen bonds in ethanol would be decomposed into hydrogen free radical (H•) under hightemperature and -pressure condition. Subsequently, the reactive H• would hydrocrack the long-chain polymers (such as protein, lipid, and carbohydrate) into low-molecular-weight fragments. Meanwhile, Xu and Etcheverry31 pointed out that H• effectively stabilized the free radicals formed during decomposition of biomass at high temperature. For example, the aromatic free radicals (Ar•) degraded from microalgae may be stabilized by H• to form stable biocrude oil (Ar• + H• → Ar), preventing the repolymerization/condensation reactions of small-biomassderived fragments/intermediates to form solid residues. Furthermore, because of the relatively lower dielectric constant of ethanol, it can readily dissolve the relatively high-molecularweight products.24 Thus, the addition of ethanol contributed to the decomposition of the solid residue. However, it was observed that the particles of SR60 occurred from aggregation to form larger particles, which indicated that the larger volume of ethanol would lead to repolymerization. Moreover, as the ethanol content reached 80%, it further generated some particles with a compact structure among the solid residue. Subsequently, when the ethanol content was 100%, SR100 showed a distinctly blocky structure, due to the enhanced dehydration (seen from the Van Krevelen diagram in Figure 2) and condensation reactions. However, the structure of SR100 was still less compact than that of SR0. The evolution process of the microscopic structure of solid residues was consistent

Figure 2. Van Krevelen diagram of solid residues and DCM-insoluble solid products.

shown in Table 2, the C content of SR0 reached 73.74%, indicating that the composition of SR0 was mainly coke. When microalgae was liquefied in the ethanol−water system, the C content of solid residue sharply decreased, and the SR60 had the lowest C content (21.37%). With the ethanol content increasing from 60% to 100%, the C content of solid residue increased from 21.37% to 41.58%, suggesting that the repolymerization reaction occurred. The proportions of C, H, N, and S in DCM-insoluble solid products were similar. However, it was clear that there was an increase in the H/C atomic ratio when the liquefaction temperature increased. According to the research by Ortiz-Moreno et al.,28 the DCMinsoluble solid product might be carbene/carboid. Previous study29 revealed that the increase in H/C atomic ratio of asphaltene constituents would result in a decrease of aromaticity. Therefore, the aromatic rings were converted 6478


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Figure 3. SEM images of microalgae and solid residues from different ethanol content.

with the change tendency of the C content of solid residues. Obviously, the surface morphology of DCM-insoluble solid products had great difference with solid residues (as shown in Figure S2 in Supporting Information). The DCM-insoluble solid products had a flat smooth surface and dense matrix. Furthermore, there was not an obvious difference on the surface morphology and structure between the DCM-insoluble solid products obtained from different reaction temperatures. 3.1.3. FTIR Analysis. As shown in Figure 4, the FTIR spectra of solid residues obtained from ethanol−water cosolvent had an obvious difference with that obtained from a solely water solvent. A strong absorption band of 600−1300 cm−1 existed in SR20, SR40, SR60, SR80, and SR100 but was not seen in SR0. The strong absorption band of 950−1300 cm−1 was ascribed to C−O stretching in esters and ethers. The band of 950−830 cm−1 was due to the C−O symmetric vibration in alicyclic ethers. This indicated that ethanol promoted the generation of esters and ethers. The CO stretching at 1700 cm−1 and -OH stretching in the range of 3000−3700 cm−1 revealed the presence of carboxylic acids. Notably, the absorbance intensity of CO decreased with the addition of ethanol. Thus, the esters derived from the esterification between carboxylic acids and ethanol. Meanwhile, two carboxyl groups (-COOH) in dicarboxylic acids might occur in the dehydration reaction to form anhydride (containing C−O).32 However, this reaction process might only occur when the ethanol content exceeded 80%, because only SR100 had a dehydration process according to the Van Krevelen diagram in Figure 2. Actually, Yoo et al.23

Figure 4. FTIR spectra of solid residues obtained from different ethanol content.

had revealed that the dehydration pathway generating H2O should be suppressed when H2O was used as a reaction medium for hydrothermal liquefaction. Therefore, only the ethanol−water reaction medium reaching enough low water content (in other words, increasing the ethanol content) would impel the dehydration reaction. The broad band between 3700 and 3000 cm−1 was due to -OH and N−H stretching vibrations. However, it was obvious that there existed two absorption 6479


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Figure 5. Chemical transformation of the functional groups on solid residues by addition of ethanol.

asymmetrical and symmetrical stretching vibrations in methyl, methylene, and methyne groups. And the region of 720−810 cm−1 showed the existence of alkyl chains. The peak at 1667 cm−1 was ascribed to CO stretching, which was probably from secondary amides and carboxylic acids. Both -OH stretching and CO stretching vibrations revealed the presence of carboxylic acids. The peaks at 1667, 1518, and 1450 cm−1 were attributed to the aromatic skeletal stretching vibrations. And the bands at 1450 and 1380 cm−1 were assigned to C−H bending vibration in -CH3 groups, which existed in alkyl aromatic compounds. At 1263 cm−1, the C−O stretch band was detected. The bands between 900 and 650 cm−1 indicated the presence of substituted aromatic groups. As the reaction temperature increased from 220 to 260 °C, the intensity of C−H, -CH3, and C−O increased, while the -OH, N−H, CO, and aromatic ring experienced a loss in absorbance intensity. This indicated the removal of -OH, N− H, and CO groups, and the aromatic skeleton was converted into alkyl chains. Furthermore, the alkyl chains fell off from the aromatic skeleton, resulting in the decline of DCM-insoluble solid product yield. The change in the functional groups of the DCM-insoluble solid product with respect to reaction temperature might lead to a plausible reaction mechanism, as shown in Figure 7. 3.2. Characteristics of Aqueous Product and Light Bio-oil. The aqueous products were marked as AP0, AP20, AP40, AP 60, AP80, and AP100 in accordance with ethanol contents of 0%, 20%, 40%, 60%, 80%, and 100%, respectively. The light bio-oils were marked as LO0, LO20, LO40, LO60, LO80, and LO100 in accordance with ethanol contents of 0%, 20%, 40%, 60%, 80%, and 100%, respectively. In order to detect the composition of aqueous product and light bio-oil, GC−MS analysis of aqueous product and light bio-oil was performed. Due to a wide range of different compounds that existed in the aqueous product and light bio-oil, these compounds were categorized into several chemical groups including straight amine derivatives, cyclic amine derivatives, and hydrocarbons. Especially, compounds containing more than one functional group were classified into only one category. For instance, 2pyrrolidinone consists of both CO and N−H functional

peaks in IR spectra of SR40, SR60, SR80, and SR100, which indicated the existence of primary amides. Furthermore, the band of 670−760 cm−1 was due to N−H out-of-plane swing vibration in primary amides. The addition of ethanol would promote the generation of primary amides. C−H stretching contributed to the absorption band of 2800−3000 cm−1. The CC adsorption band at 1650 cm−1 was likely from benzene backbone vibrations. And the bands at 1450 and 1380 cm−1 were assigned to -CH3 bending. These peaks suggested the existence of aliphatic and alkyl aromatic compounds in solid residue. After liquefaction in ethanol−water cosolvent, the intensity of C−H, CO, and benzene backbone decreased, but the intensity of N−H and C−O increased obviously. The change in the functional groups of the solid residues with respect to ethanol addition might lead to a plausible reaction mechanism, as shown in Figure 5. Figure 6 presented the FTIR spectra of DCM-insoluble solid products. The broad band between 3700 and 3000 cm−1 was due to -OH and N−H stretching vibrations. The absorbance peaks in the range of 2800−3000 cm−1 were caused by C−H

Figure 6. FTIR spectra of DCM-insoluble solid products obtained from different reaction temperatures. 6480


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Figure 7. Chemical transformation of the functional groups on DCM-insoluble solid products by increasing reaction temperature.

contributed to hydrogen-donating effect; thus the cyclic amine derivatives were converted into straight amine derivatives by hydrogenation. As the ethanol content exceeded 40%, amino acids and sugars derived from the hydrolysis of protein and polysaccharides in microalgae would react with each other through the Maillard reaction,33,34 promoting the formation of nitrogen-containing cyclic organic compounds such as pyridines and pyrroles. Pham et al.22 found that the nitrogenous organic compounds (NOCs) with two or more methyl groups were more toxic than those with one or no alkyl substitutions, which increased the resistance of the compound to biological degradation. According to Table S1 in the Supporting Information, the NOCs in aqueous product were mainly 2pyrrolidinone (retention time (RT, min), 20.286), 2-ethylpiperidine (RT, 37.138), thiazole, 4,5-dimethyl- (RT, 39.668), 2-ethyl-1,3,4-trimethyl-3-pyrazolin-5-one (RT, 39.956), and pyrrolo[1,2-a] pyrazine-1,4-dione, hexahydro-3-(2-methylpropyl)- (RT, 41.914). The contents of NOCs with two or more methyl groups were 18.49%, 3.51%, 0%, 8.18%, 8.02%, and 5.27% for AP0, AP20, AP40, AP60, AP80, and AP100, respectively. This observation suggested that ethanol addition significantly decreased the number of methyl substitutions in NOCs, consequently reducing the toxicity of aqueous product. Notably, AP40 had the lowest content of NOCs with more than two methyl groups, indicating that there existed significant synergistic effect between ethanol and water. Therefore, solvolysis liquefaction of microalgae in an ethanol−water system not only increased the content of N nutrient in aqueous phase for culturing microalgae but also significantly decreased the toxicity of aqueous phase. Additionally, the addition of ethanol decreased the concentration of ketones and phenols. Figure 9 presented that the major compounds of LO0 were nitrogenated aromatics, which reached to 62.96%. Twenty-six nitrogenated aromatic compounds were found in the light biooil (seen in Table S2 in Supporting Information), most of which were pyridine (RT, 4.09), pyrazine, methyl- (RT, 6.677), pyrazine, 2,5-dimethyl- (RT, 12.278), pyrazine, 2,6-dimethyl(RT, 12.403), pyrazine, ethyl- (RT, 12.581), pyrazine, 2-ethyl6-methyl- (RT, 16.886), pyrazine, 2-ethyl-5-methyl- (RT, 16.987), pyrazine, 2-ethyl-3-methyl- (RT, 17.142), and pyrazine, 3-ethyl-2,5-dimethyl- (RT, 20.198). With the addition of ethanol, the nitrogenated aromatic rings were decomposed into straight amine derivatives by hydrogenation, resulting in the obvious decline of nitrogenated aromatics in light bio-oil

groups, and it was classified as cycle amine derivatives based on its chemical property. Only those compounds with peak area (%) higher than 1% were summarized in the Supporting Information, and more than 80% of the peak area for identified compounds was grouped. Figure 8 presented the major compounds in the aqueous product. Notably, aqueous product contained a large amount of

Figure 8. Effect of ethanol content on the composition of aqueous products.

nitrogen-containing compounds (such as cyclic amine derivatives and straight amine derivatives, 63.46%−76.36%), which were derived from the decomposition of protein. This revealed that the aqueous product had excellent further utilization for culturing microalgae as a N nutrient source. Moreover, the content of nitrogen-containing compounds in AP40 (76.36%) and AP60 (73.28%) was higher than AP0 (65.71%) and AP100 (68.63%), indicating that solvolysis liquefaction in ethanol− water cosolvent promoted the transformation of nitrogencontaining compounds into aqueous phase compared to liquefaction in sole water and ethanol solvent. Interestingly, the cyclic amine derivatives content first decreased from 50.22% at AP0 to 46.95% at AP40, and then increased to 62.95% at AP100. While the content of straight amine derivatives performed the inverse tendency, increasing from 15.49% at AP0 to 29.41% at AP40 and then decreasing to 5.68% at AP100. At the beginning, the addition of ethanol 6481


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Figure 9. Effect of ethanol content on the composition of light biooils.

Figure 10. FTIR spectra of biocrude oils obtained from different ethanol content.

obtained from the ethanol−water system. Meanwhile, the content of ketones decreased gradually with the increasing ethanol content. Obviously, the concentration of esters increased greatly as the ethanol−water medium was employed. Moreover, most of the esters compounds were short-chain esters (C70) than that obtained from solely water solvent. This might be due to the fact that repolymerization was promoted more for ethanol−water cosolvent. Interestingly, BO60 had the highest content of C10−C16 (kerosene, 8.97%) and C14−C20 (diesel oils, 30.53%). This phenomenon indicated that there existed significant synergistic effect between ethanol and water. Therefore, the biocrude oil obtained from ethanol−water cosolvent with 60% ethanol content might have a favorable fuel property. 6483


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Energy & Fuels Table 3. Boiling Point Distribution of Biocrude Oils Identified by TGA boiling point of biocrude oils (wt %)

a

distillate range (°C)

carbon no.

coke oil typea

600

C1−C9 C5−C10 C10−C16 C14−C20 C20−C50 C20−C70 >C70

gases and naphtha gasoline kerosene diesel oils lubricating oils fuel oils residue

BO0 0.22 2.31 7.59 21.09 57.85 0.98 1.00

± ± ± ± ± ± ±

BO20 0.00 0.10 0.20 0.34 0.72 0.20 0.13

0.23 2.42 8.94 25.43 51.22 1.03 1.37

See ref 43.

± ± ± ± ± ± ±

0.01 0.02 0.07 0.09 0.49 0.22 0.01

■

3.3.4. Thermal Stability of Biocrude Oil. The intermolecular reactions between various molecular species within the biocrude oil would result in the increase of molecular weights and viscosity of biocrude oil as the thermal aging enhanced. This limited stability caused problems in the storage and application of the biocrude oil. Furthermore, low chemical stability that resulted in the catalytic upgrading of biocrude oil became difficult, due to char formation inducing rapid fouling of catalysts.23 In order to investigate the thermal stability of the biocrude oils obtained from solvolysis liquefaction of microalgae in ethanol−water solvent with different ethanol content, the thermal stability experiments were carried out in this context. The results showed that the yields of DCM-insoluble solid precipitation were 4.42%, 6.23%, 3.55%, 2.37%, 3.55%, and 5.48% for 0%, 20%, 40%, 60%, 80%, and 100% ethanol content in ethanol−water cosolvent, respectively. It indicated that either low or high ethanol content (20% or 100%) contributed to higher DCM-insoluble solid precipitation yield compared with sole water solvent, resulting in poor thermal stability. This could be attributed to various condensation and polymerization reactions during the thermal aging period. In contrast, the lowest DCM-insoluble solid precipitation yield was obtained from 60% ethanol content, indicating the better thermal stability.

BO40 0.26 2.50 8.77 23.37 53.79 0.93 1.02

± ± ± ± ± ± ±

0.03 0.11 0.43 0.31 1.44 0.49 0.03

BO60 0.24 2.02 8.97 30.53 47.53 0.94 1.50

± ± ± ± ± ± ±

0.01 0.03 0.14 1.28 1.84 0.35 0.06

BO80 0.24 1.61 7.72 27.18 50.99 1.10 1.31

± ± ± ± ± ± ±

0.01 0.04 0.21 1.18 1.49 0.00 0.10

BO100 0.16 1.34 6.88 28.34 44.52 1.69 1.82

± ± ± ± ± ± ±

0.01 0.02 0.06 0.06 0.29 0.02 0.23

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01103. Apparent characteristics of ethanol phase and DCMinsoluble solid product (Figure S1), SEM images of DCM-insoluble solid products (Figure S2), total ion current (TIC) of the aqueous products, light bio-oils and biocrude oils determined by GC−MS (Figures S3−S5), and identified compounds in aqueous products, light biooils, and biocrude oils by GC−MS (Tables S1−S3) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 20 87110232. Fax: +86 20 87110613. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant 51476060); the National Basic Research Program of China (973 Program; Grant 2013CB228100); the Guangdong Province Key Laboratory of Efficient and Clean Energy Utilization (Grant 2013A061401005); the Key Laboratory of Efficient and Clean Energy Utilization of Guangdong Higher Education Institutes (Grant KLB10004); and the China Postdoctoral Science Foundation (Grant 2015M582382).

4. CONCLUSIONS It was shown in this study that solvolysis liquefaction of microalgae in an ethanol−water system could sever as a favorable approach for biofuel production. The addition of ethanol promoted the decomposition of solid residue and increased the esters yield in solid residue. However, excess ethanol addition (>60%) would lead to repolymerization. The DCM-insoluble solid products had a flat smooth surface and dense matrix, and the aromaticity decreased as the reaction temperature increased. GC−MS analysis demonstrated that the ethanol−water cosolvent contributed to the generation of nitrogen-containing compounds in aqueous product and shortchain esters in light bio-oil and decreased the content of toxic nitrogenous organic compounds with two or more methyl groups in aqueous product. On the basis of FTIR and GC−MS analysis, the generation of esters from esterification and alcoholysis reactions improved the quality of biocrude oil. The biocrude oil obtained from 60% ethanol content had the highest content of low boiling point components of C10−C20 and performed the best thermal stability.

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Investigation on Characteristics of Liquefied Products from Solvolysis

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