- Email: [email protected]
Hydrothermal conversion of microalgae and its waste residue after biofuel extraction to acetic acid with CuO as solid oxidant
J. of Supercritical Fluids 157 (2020) 104717
Contents lists available at ScienceDirect
The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu
Hydrothermal conversion of microalgae and its waste residue after biofuel extraction to acetic acid with CuO as solid oxidant Heng Zhong a,b , Lin Ma a , Yingying Zhu a , Binbin Jin a , Tianfu Wang a,b , Yangang Wang c,∗ , Fangming Jin a,c,∗ a School of Environmental Science and Engineering, State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan RD, Shanghai, 200240, China b Shanghai Institute of Pollution Control and Ecological Security, Shanghai, 200092, China c College of Biological Chemical Science and Engineering, Jiaxing University, Jiaxing, 314001, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• New method for microalgae and • • • •
its bio-waste residue conversion to acetic acid (AA). Moderate oxidation ability of CuO is the key to selective AA generation. New way of using Cu-CuO cycle as indirect O2 input for biomass conversion. 35 % AA yield from microalgae is obtained with CuO, much higher than with H2 O2 . Confirm microalgae component for AA generation obeys protein > carbohydrate > lipid.
a r t i c l e
i n f o
Article history: Received 31 July 2019 Received in revised form 1 December 2019 Accepted 4 December 2019 Available online 9 December 2019 Keywords: Microalgae Bioresource Biomass conversion Solid oxidant Copper extraction High-pressure reaction
a b s t r a c t Production of chemicals from microalgae is a substantial way to contribute to the economic viability of a biorefinery. In this research, we report a new and green method for producing acetic acid from microalgae directly or from its waste residue after biofuel extraction using copper (II) oxide (CuO) as a solid oxidant. Results show that the CuO can significantly facilitate the oxidation of microalgae and its waste residue into acetic acid with the simultaneous reduction of CuO to Cu and Cu2 O. The maximum acetic acid yield reached up to 35 % and 31 % from the microalgae and its waste residue, respectively, which are much higher than previous report (14.9 %) which is obtained with traditional liquid oxidants such as H2 O2 . The reduction of CuO to Cu(0) at a relatively low temperature (300 ◦ C) offered an additional benefit of developing a sustainable method in copper metallurgy at mild conditions. © 2019 Published by Elsevier B.V.
1. Introduction
∗ Corresponding authors at: School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan RD, Shanghai, 200240, China. E-mail addresses: [email protected] (Y. Wang), [email protected] (F. Jin). https://doi.org/10.1016/j.supflu.2019.104717 0896-8446/© 2019 Published by Elsevier B.V.
Reliance on fossil fuels as the main energy source has led to a serious energy crisis and associated environmental issues in current human civilization. Utilization of biomass as a renewable feedstock to produce energy and chemicals is one promising
2
H. Zhong, L. Ma, Y. Zhu et al. / J. of Supercritical Fluids 157 (2020) 104717
Table 1 Element analysis of the microalgae and microalgae waste residue used in this research. Composition (wt%)
Element C O H N Other (P, S, et al.)
Microalgae
Microalgae waste residue
45.0 39.8 6.8 8.3 0.1
45.3 43.8 6.3 8.4 1.3
method to reduce the dependence on petroleum-derived feedstock and atmospheric CO2 rise resulted from the use of fossil fuels [1–4]. In this regard, conversion of microalgae has received increasing attention because of its fast growth, high lipid content, ability to convert CO2 , and the potential for producing useful chemicals [5–9]. In addition, microalgae demand neither rigorous living conditions nor agriculturally productive land, which are necessities for terrestrial plants [10]. Recent interests of using microalgae mainly focus on biofuel extraction [11], however, its utilization efficiency is rather low due to the low oil content of microalgae (only 10 %) [12–15]. On the other hand, production of chemicals from microalgae directly or microalgae waste residue after biofuel extraction (denoted as MWR thereafter) can utilize various components of the microalgae and their intermediates, and therefore maximizing the value derived from the biomass feedstock [9,16,17]. Acetic acid is a widely used industrial chemical, which is generally produced from petrochemical feedstock. It has been reported that the biomass feedstocks, such as carbohydrate, fatty acid, and lignin, can be selectively oxidized into acetic acid under hydrothermal conditions [18–21]. However, only a few reports involved the conversion of microalgae into acetic acid, in which the yield of acetic acid was just 14.9 % using H2 O2 as a liquid oxidant [22]. In the traditional hydrothermal oxidation of biomass into acetic acid, liquid or gaseous oxidants such as H2 O2 , O2 , and air are generally used, which leads to high energy costs due to the gas compression as well as several insecurity hazards. On the other hand, solid oxides, such as CuO, have the potential to be used as a solid oxidant to replace the conventional liquid or gaseous oxidants in biomass oxidation given to their advantages including easy accessibility, economy and recoverability. The purpose of the present research is to study the potential of the conversion of microalgae into acetic acid using metal oxides (such as CuO) as solid oxidant and optimization of reaction conditions to achieve the maximum conversion efficiency.
2. Materials and methods
2.2. Experimental procedure A stainless steel (SUS-316) tubular reactor with a 5.7 mL inner volume was used for all the experiments. The schematic drawing of this reactor can be found in our previous report [24,25]. The reactor was first loaded with 0.035 g microalgae, corresponding amount of CuO and water, making the reaction mixture filled up to 20–60 % of the total inner volume of the reactor. Then, the reactor was sealed and immersed in a salt bath that was preheated to the desired reaction temperature (250–300 ◦ C) to initiate the reaction. At the desired time, the reactor was quickly transferred into a cold-water bath to quench the reaction. The reaction time was defined as the time when the reactor was kept in the salt bath. All experiments were performed with degassed and deionized water and the reactor was purged with nitrogen gas before the reaction. After the reaction, the liquid sample was collected by filtering through a 0.45 m syringe filter, while the solid samples were thoroughly washed with deionized water and alcohol, and then dried at 45−55 ◦ C for 24 h. 2.3. Analytic method Liquid samples after the reactions were analyzed by a gas chromatography analyzer (GC-FID, HP7890) equipped with an HPINNOWax column (30 m ×0.25 mm ×0.3 m). Helium was used as the carrier gas with a flow rate of 1 mL/min, and the injector and FID detector temperatures were 200 ◦ C and 220 ◦ C, respectively. The GC oven was programmed to begin at 40 ◦ C for 4 min, increased at a rate of 7.5 ◦ C/min to 220 ◦ C and then remained at 220 ◦ C for an additional 10 min. The sample injection volume was 1 l. An Agilent 7890 gas chromatography-mass spectrometry (GC–MS) equipped with a 5975C MSD detector was also used to investigate other possible chemicals in the liquid phase. The peak identification was accomplished by the NIST library search. For GC analyses, liquid samples were diluted by water to a ratio of 1:5 and were pH adjusted to 1 with HCl. Total organic carbon (TOC) and inorganic carbon (IC) of the liquid samples were analyzed by a Shimadzu TOC analyzer (TOC-V CPH). Solid samples after reaction were examined by a Shimadzu Xray diffractometer (6100), which was operated at 40 kV and 30 mA with a scan rate of 2 deg/min. All quantitative data reported in this study were the average values of three samples with a relative error less than 5 %. 2.4. Definition The yield of acetic acid is defined as follows: Yield(%) =
Carbon amount in acetic acid(g) × 100% Carbon amount in initial microalgae(g)
2.1. Materials
3. Results and discussion
In this research, food-grade Chlorella Vulgaris (powder, Shandong Binzhou Tian Jian Biotechnology Co., Ltd., China) was used as a representative of microalgae since it is one of the most common cultivated microalgae [23]. Besides microalgae, microalgae waste residue after biofuel extraction (MWR) was also chosen as test materials for acetic acid production. The chemical formulas of microalgae and MWR were determined to be C3.75 H6.79 O2.49 N0.59 and C3.35 H6.30 O2.74 N0.60 , respectively (Table 1), which were calculated from the elemental composition data obtained by an organic element analyzer (Vario EL III). Analytical grade copper oxide (CuO) with a purity of 99.9 % from Sinopharm Chemical Reagent co., LTD was used as the oxidants. Gaseous CO2 (>99.995 %) was purchased from Shanghai Poly-Gas Technology Co., Ltd. Deionized water was used throughout the entire study.
3.1. Potential of CuO in converting microalgae into acetic acid Reactions of chlorella in the presence and absence of CuO powder were conducted at 300 ◦ C for 2 h to investigate the possibility of microalgae oxidation to acetic acid with CuO as the solid oxidant. Reaction conditions were selected based on the optimized conditions obtained in our previous research on the hydrothermal oxidation of carbohydrate into acetic acid [24,26]. Analysis of the liquid samples after the reaction showed that the major liquid products were several carboxylic acids, including acetic acid, formic acid, propionic acid, butyric acid, and isovaleric acid in both cases (with and without CuO) (Fig. 1). Interestingly, GC/MS peaks of these acids, particularly for acetic acid, were much higher in the presence of CuO than those in the absence of CuO, suggesting that
H. Zhong, L. Ma, Y. Zhu et al. / J. of Supercritical Fluids 157 (2020) 104717
Fig. 1. GC/MS chromatographs of liquid samples after the reactions of chlorella with and without CuO at 300 ◦ C for 2 h (C1 = formic acid, C3 = propionic acid, C5 = isovaleric acid; reaction conditions: 0.035 g chlorella, 0.55 g CuO, 2 mol/L NaOH, 50 % water filling).
3
Fig. 3. Effect of NaOH on the yield of acetic acid (300 ◦ C, 2 h, 0.035 g chlorella, 0.55 g CuO, 50 % water filling).
Fig. 4. Carbon distribution in the liquid samples after the reaction of microalgae with CuO (300 ◦ C, 2 h, 2 mol/L NaOH, 0.035 g chlorella, 50 % water filling).
Fig. 2. XRD pattern of solid sample after the reaction of chlorella with CuO (300 ◦ C, 2 h, 2 mol/L NaOH, 0.035 g chlorella, 0.55 g CuO, 50 % water filling).
CuO could promote the production of acetic acid from microalgae. The reduced products of CuO were determined to be Cu and Cu2 O by the XRD analysis (Fig. 2), confirming the oxidative role of CuO in promoting the production of acetic acid from chlorella. Also, possible dissolved Cu ion in solution after the reaction was analyzed by ICP analysis, and the result showed that the concentration of dissolved Cu ion was 153 mg/L, which corresponds to 0.1 wt% of the original Cu added to the reaction. This result indicates that the leaching of Cu in the alkaline solution is limit. Our previous research on the oxidation of carbohydrate into acetic acid have shown that the yield of acetic acid is significantly enhanced in alkaline conditions [24,26]. Thus, the effect of alkaline on the acetic acid yield from microalgae in the presence of CuO was studied then. As illustrated in Fig. 3, the yield of acetic acid greatly increased from 8 % to 31 % when the concentration of NaOH increased from 0 to 2 mol/L, indicating that the presence of alkaline (OH− ) can enhance the oxidation of chlorella to acetic acid. The increase of acetic acid yield in the presence of NaOH is probably due to the increasing hydrolysis rate of chlorella and high solubility of CuO to form hydroxo complex in the alkaline medium. Next, carbon distribution in the liquid sample was examined. As shown in Fig. 4, the percentage of total organic carbon (TOC) in the liquid samples was 60 %, in which acetic acid contributed about 50 %. The inorganic carbon (IC) in liquid samples should be attributed to the dissolved CO2 or carbonate in the alkaline solution. The total dissolved carbon (TOC + IC) in the liquid sample accounts for 78 wt% of the original carbon contained in chlorella. Furthermore, CO2 was detected in gaseous samples in the presence of both CuO and NaOH.
Fig. 5. Effect of CuO amount on the formation (red square) and decomposition (blue circle) of acetic acid (300 ◦ C, 2 h, 50 % water filling, 2 mol/L NaOH and 0.035 g chlorella for the formation of acetic acid, 200 mmol/L acetic acid for the decomposition of acetic acid).
These results indicated that CuO also promoted the oxidation of chlorella to CO2 , particularly in the presence of alkali. 3.2. Effect of experimental parameters on the acetic acid yield Fig. 5 shows the effect of CuO amount on the yield of acetic acid from chlorella. The acetic acid yield increased clearly from 9 % to 31 % with the increase of CuO amount from 0 to 0.55 g. However, a further increase in the amount of CuO led to a decrease in the yield of acetic acid, which is probably due to a further decomposition of acetic acid to CO2 in presence of an excess amount of CuO. To verify this speculation, a reaction with acetic acid (200 mmol/L) in the presence of CuO was performed. As expected, a decomposition
4
H. Zhong, L. Ma, Y. Zhu et al. / J. of Supercritical Fluids 157 (2020) 104717
Fig. 6. (a) Effect of reaction temperature (blue square) and reaction time (red circle) on the yield of acetic acid (2 mol/L NaOH, 0.035 g chlorella, 0.55 g CuO, 50 % water filling, 2 h for the effect of reaction temperature, 300 ◦ C for the effect of reaction time); (b) Effect of water filling rate on the yield of acetic acid (2 h, 300 ◦ C, 2 mol/L NaOH, 0.55 g CuO, 0.035 g chlorella).
of acetic acid from 98 % to 64 % was observed with the increase of CuO amount from 0 to 2.25 g (Fig. 5). Thus, an optimum amount of CuO exists in the conversion of chlorella to acetic acid. Fig. 6a depicts the acetic acid yield obtained at different reaction times and temperatures. Results revealed that an increase in the reaction temperature from 250 to 300 ◦ C promoted the acetic acid yield from 17 % to 31 %. The reaction pressures were ca. 4.0 and 8.4 MPa at 250 and 300 ◦ C, respectively, which is almost the same as the water vapor pressure at related temperature, suggesting that the reaction pressure was mainly contributed by the water vapor pressure. However, a further increase in the temperature caused a decline in the acetic acid yield, which is probably resulted by the decomposition of acetic acid at higher temperatures as already discussed. A similar trend was found for the effect of reaction time, which was optimized as 2 h to achieve the highest acetic acid yield. Water filling rate is a key factor in the hydrothermal reactions because the water filling rate affects the pressure of reaction system as well as the water density. As illustrated in Fig. 6b, water filling rate had a great effect on promoting the acetic acid yield, particularly for water filling exceeding 40 % (Fig. 6b). An optimum acetic acid yield of 35 % was achieved at the water filling of 60 %. This is probably because the hydrolysis rate of microalgae was enhanced at higher water fillings due to the increase in reaction pressure caused by high water filling [27–30]. Based on Fig. 6a, the acetic acid yield can probably be further enhanced by increasing the water filling rate, however, experiments with higher water filling rates (>60 %) were not conducted due to the experimental limitation and safety issues.
3.3. Proposed mechanism for the conversion of microalgae to acetic acid with CuO It is known that microalgae have three body components, which are protein, lipid, and carbohydrate. Therefore, contribution of these three major components for acetic acid production and their interactional effects were studied. Glutamic acid, hexadecanoic acid, and glucose were used as model compounds for protein, lipid and carbohydrate, respectively. Results showed that the yields of acetic acid were 34 %, 5 % and 45 % from glutamic acid, hexadecanoic
Table 2 Acetic acid yield from the oxidation of different starting materials.a Entry
Substance
Acetic acid yield (%)
1 2 3 4
Glutamic acid (GA) Hexadecanoic acid (HA) Glucose (GL) GA+HA + GLb
34 5 45 32
a b
reaction conditions: 300 ◦ C, 2 h, 2 mol/L NaOH, 0.55 g Cu, 50 % water filling. GA:HA:GL = 6:1:3.
acid, and glucose, respectively under the same reaction condition as using microalgae for obtaining the optimum acetic yield (Table 2). Considering that the mass ratio of protein, lipid, and carbohydrate in microalgae is 6:1:3, a mixture of glutamic acid, hexadecanoic acid and glucose with the same mass ratio was reacted. An acetic acid yield of 32 % was attained at the optimal conditions (Table 2, Entry 4), which was very close to the highest value of 35 % from microalgae. Interestingly, by multiplying the acetic acid yields obtained from glutamic acid, hexadecanoic acid, and glucose (34 %, 5 % and 45 %) with their corresponding ratio (60 %, 10 %, 30 %), a 34.4 % acetic acid was obtained, which is similar to that obtained with the chlorella directly. These results suggested that the contribution of the constituents of microalgae to the production of acetic acid was in the order of protein > carbohydrate > lipid, and there was no significant interaction among these three components in the hydrothermal conversion of microalgae into acetic acid. With these experimental results, a tentative conversion pathway of microalgae was postulated in Fig. 7. First, microalgae are hydrolyzed to protein, lipid, and carbohydrate, which then undergo further hydrolysis to form amino acid, fatty acid, and glucose, respectively, under the alkaline condition. Subsequently, the protein, lipid, and carbohydrate are oxidized in the presence of dissociated Cu(II) ions formed in the alkaline solution. For the protein, dissociated Cu(II) ions probably coordinate with amino acid to form a comparatively active coordination compound. The short distance between oxygen and Cu atom in the coordination compound is favorable for the electron transfer from oxygen atom to Cu(II) ion, resulting in the reduction of Cu(II) into Cu(0) or Cu(I) and the formation of acetic acid. Then, the decomposition products of
Table 3 Distribution of nitrogen element in liquid products after the reaction of chlorella with different amount of CuO.a Entry
CuO(g)
NH4 + (mg/L)
NO2 − (mg/L)
NO3 − (mg/L)
TNb (mg/L)
NH4 + /TN(%)
1 2
0.55 1.10
612 528
1 1
199 54
896 932
68 57
a b
Reaction conditions: 300 ◦ C, 2 h, 2 mol/L NaOH, 0.035 g chlorella, 50 % water filling. TN = total nitrogen.
H. Zhong, L. Ma, Y. Zhu et al. / J. of Supercritical Fluids 157 (2020) 104717
5
Fig. 7. Proposed reaction pathways of the conversion of microalgae into acetic acid with CuO oxidant.
Table 4 Acetic acid yield obtained from the oxidation of chlorella with different solid oxidants.a Oxidant
Amount (g)b
Acetic acid yield (%)
CuO MnO2 TiO2 ZrO2 Fe2 O3 Al2 O3
0.55 0.30 0.28 0.43 0.37 0.24
16.8 10.5 10.0 10.6 9.1 9.2
a Reaction conditions: 300 ◦ C, 0.5 h, 2 mol/L NaOH, 0.035 g chlorella, 50 % water filling. b the amount is selected to keep the oxygen content the same.
4. Conclusion We have demonstrated a new method for the conversion of microalgae and its waste residue from biofuel extraction to acetic acid with CuO as a solid oxidant. The highest acetic acid yields from microalgae and microalgae waste residue were 35 % and 31 %, respectively, which is much higher than previous work (14.9 %) using H2 O2 as a liquid oxidant. The proposed method not only provides a new way for the conversion of microalgae and microalgae waste residue into value-added chemicals but also offers a new concept of developing a green copper metallurgy at low temperature.
Declaration of Competing Interest the coordination compound undergo hydrolysis to form ammonia, water and carboxylic acids. As showed in Table 3, the total dissolved nitrogen (TN) was mainly contributed by the NH4 + , suggesting that the major nitrogen of microalgae were converted to NH4 + . For the lipid conversion path, since fatty acids are readily oxidized on the carbon to form C−OH in the presence of OH− [31,32], it is suggested that fatty acids are first oxidized on the -carbon by CuO to form 2-hydroxyl fatty acid. Then the dissociated Cu(II) ions coordinate with 2-hydroxyl fatty acid to form Cu complex, which is oxidized to acetic acid. For the carbohydrate oxidation path, glucose first reacts with Cu(II) ions to form a stable complex [33], and then a redox reaction occurs as a result of electron transfer from the hydroxyl oxygen atom in glucose to the Cu(II) ions. Cu(0) or Cu(I) is obtained and glucose is decomposed to small molecules such as lactic acid, which is further oxidized to acetic acid via a similar transformation with the formation of Cu complex and the release of CO2 . 3.4. Production of acetic acid from microalgae waste residue (MWR) Since the current utilization of microalgae is mainly focused on biofuel extraction, a large amount of MWR is produced in this process. Therefore, conversion of MWR was further investigated, in which the residue with about 10 % oil content was used. Results showed that a high acetic acid yield (31 %) could be achieved with CuO at 300 ◦ C for 2 h, which was almost the same as that obtained from microalgae directly. These results indicate that the MWR can also be treated as potential feedstock to produce acetic acid. To test the application of the proposed method of microalgae oxidation into acetic acid with solid oxidant, various possible metal oxides were further tested. As shown in Table 4, all the tested metal oxides had the ability to convert microalgae into acetic acid, among which, CuO exhibited the highest activity. These results suggest that using solid oxidants for the microalgae oxidation into acetic acid has wide applicability.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments The authors thank the funding support from the State Key Program of Natural Science Foundation of China (No. 21436007), the Natural Science Foundation of Shanghai (No. 19ZR1424800), Startup Fund for Youngman Research at SJTU, and the Center of Hydrogen Science, Shanghai Jiao Tong University, China.
References [1] H. Li, A. Riisager, S. Saravanamurugan, A. Pandey, R.S. Sangwan, S. Yang, R. Luque, Carbon-increasing catalytic strategies for upgrading biomass into energy-intensive fuels and chemicals, ACS Catal. 8 (2018) 148–187. [2] W.W. Lin, H. Chen, J. Li, K.Q. Chen, X.Y. Lu, P.K. Ouyang, J. Fu, Enhanced stability of Pt/C by the atomic layer deposition of porous MOx for the decarboxylation of oleic acid, Catal. Commun. 123 (2019) 59–63. [3] Z.H. Zhang, H. Cheng, H. Chen, J. Li, K.Q. Chen, X.Y. Lu, P.K. Ouyang, J. Fu, Catalytic fast pyrolysis of rice straw to aromatics over hierarchical HZSM-5 treated with different organosilanes, Energy Fuels 33 (2019) 307–312. [4] Z.H. Zhang, Q.W. Yang, H. Chen, K.Q. Chen, X.Y. Lu, P.K. Ouyang, J. Fu, J.G.G. Chen, In situ hydrogenation and decarboxylation of oleic acid into heptadecane over a Cu-Ni alloy catalyst using methanol as a hydrogen carrier, Green Chem. 20 (2018) 197–205. [5] V. Abrahamsson, L.P. Cunico, N. Andersson, B. Nilsson, C. Turner, Multicomponent inverse modeling of supercritical fluid extraction of carotenoids, chlorophyll A, ergosterol and lipids from microalgae, J. Supercrit. Fluids 139 (2018) 53–61. [6] R. Feller, Â.P. Matos, S. Mazzutti, E.H.S. Moecke, M.V. Tres, R.B. Derner, J.V. Oliveira, A.F. Junior, Polyunsaturated -3 and -6 fatty acids, total carotenoids and antioxidant activity of three marine microalgae extracts obtained by supercritical CO2 and subcritical n-butane, J. Supercrit. Fluids 133 (2018) 437–443. [7] A. Raheem, P. Prinsen, A.K. Vuppaladadiyam, M. Zhao, R. Luque, A review on sustainable microalgae based biofuel and bioenergy production: recent developments, J. Clean. Prod. 181 (2018) 42–59.
6
H. Zhong, L. Ma, Y. Zhu et al. / J. of Supercritical Fluids 157 (2020) 104717
[8] Y. Yang, H. Zhong, R.T. He, X.G. Wang, J. Cheng, G.D. Yao, F.M. Jin, Synergetic conversion of microalgae and CO2 into value-added chemicals under hydrothermal conditions, Green Chem. 21 (2019) 1247–1252. [9] T. Mathimani, A. Baldinelli, K. Rajendran, D. Prabakar, M. Matheswaran, R.P. van Leeuwen, A. Pugazhendhi, Review on cultivation and thermochemical conversion of microalgae to fuels and chemicals: process evaluation and knowledge gaps, J. Clean. Prod. 208 (2019) 1053–1064. [10] A.B. Ross, P. Biller, M.L. Kubacki, H. Li, A. Lea-Langton, J.M. Jones, Hydrothermal processing of microalgae using alkali and organic acids, Fuel 89 (2010) 2234–2243. [11] L. Brennan, P. Owende, Biofuels from microalgae – a review of technologies for production, processing, and extractions of biofuels and co-products, Renew. Sust. Energy Rev. 14 (2010) 557–577. [12] X.L. Miao, Q.Y. Wu, Biodiesel production from heterotrophic microalgal oil, Bioresour. Technol. 97 (2006) 841–846. [13] X. Meng, J. Yang, X. Xu, L. Zhang, Q. Nie, M. Xian, Biodiesel production from oleaginous microorganisms, Renew. Energy 34 (2009) 1–5. [14] R. Halim, B. Gladman, M.K. Danquah, P.A. Webley, Oil extraction from microalgae for biodiesel production, Bioresour. Technol. 102 (2011) 178–185. [15] Y.-P. Xu, P.-G. Duan, F. Wang, Hydrothermal processing of macroalgae for producing crude bio-oil, Fuel Process. Technol. 130 (2015) 268–274. [16] Y.D. Lu, R.B. Levine, P.E. Savage, Fatty acids for nutraceuticals and biofuels from hydrothermal carbonization of microalgae, Ind. Eng. Chem. Res. 54 (2015) 4066–4071. [17] Y.F. Zan, Y.Y. Sun, L.Z. Kong, G. Miao, L.W. Bao, H. Wang, S.G. Li, Y.H. Sun, Formic acid-induced controlled-release hydrolysis of microalgae (scenedesmus) to lactic acid over Sn-beta catalyst, ChemSusChem 11 (2018) 2492–2496. [18] F.M. Jin, J. Yun, G.M. Li, A. Kishita, K. Tohji, H. Enomoto, Hydrothermal conversion of carbohydrate biomass into formic acid at mild temperatures, Green Chem. 10 (2008) 612–615. [19] F.M. Jin, H. Enomoto, Application of hydrothermal reaction to conversion of plant-origin biomasses into acetic and lactic acids, J. Mater. Sci. 43 (2008) 2463–2471. [20] F.M. Jin, H. Enomoto, Hydrothermal conversion of biomass into value-added products: technology that mimics nature, Bioresources 4 (2009) 704–713. [21] F.M. Jin, H. Enomoto, Rapid and highly selective conversion of biomass into value-added products in hydrothermal conditions: chemistry of acid/base-catalysed and oxidation reactions, Energy Environ. Sci. 4 (2011) 382–397.
[22] Z. Shen, J.F. Zhou, X.F. Zhou, Y.L. Zhang, The production of acetic acid from microalgae under hydrothermal conditions, Appl. Energy 88 (2011) 3444–3447. [23] C. Safi, B. Zebib, O. Merah, P.-Y. Pontalier, C. Vaca-Garcia, Morphology, composition, production, processing and applications of Chlorella vulgaris: a review, Renew. Sust. Energy Rev. 35 (2014) 265–278. [24] F.M. Jin, A. Kishita, T. Moriya, H. Enomoto, Kinetics of oxidation of food wastes with H2 O2 in supercritical water, J. Supercrit. Fluids 19 (2001) 251–262. [25] H. Zhong, G.D. Yao, X. Cui, P. Yan, X.G. Wang, F.M. Jin, Selective conversion of carbon dioxide into methane with a 98% yield on an in situ formed Ni nanoparticle catalyst in water, Chem. Eng. J. 357 (2019) 421–427. [26] F.M. Jin, Z.Y. Zhou, T. Moriya, H. Kishida, H. Higashijima, H. Enomoto, Controlling hydrothermal reaction pathways to improve acetic acid production from carbohydrate biomass, Environ. Sci. Technol. 39 (2005) 1893–1902. [27] S.H. Townsend, M.A. Abraham, G.L. Huppert, M.T. Klein, S.C. Paspek, Solvent effects during reactions in supercritical water, Ind. Eng. Chem. Res. 27 (1988) 143–149. [28] L.A. Torry, R. Kaminsky, M.T. Klein, M.R. Klotz, The effect of salts on hydrolysis in supercritical and near-critical water - reactivity and availability, J. Supercrit. Fluids 5 (1992) 163–168. [29] T.M. Aida, Y. Sato, M. Watanabe, K. Tajima, T. Nonaka, H. Hattori, K. Arai, Dehydration of D-glucose in high temperature water at pressures up to 80 MPa, J. Supercrit. Fluids 40 (2007) 381–388. [30] T.M. Aida, A. Ikarashi, Y. Saito, M. Watanabe, R.L. Smith, K. Arai, Dehydration of lactic acid to acrylic acid in high temperature water at high pressures, J. Supercrit. Fluids 50 (2009) 257–264. [31] S.V. Mohan, M.V. Reddy, G.V. Subhash, P.N. Sarma, Fermentative effluents from hydrogen producing bioreactor as substrate for poly(beta-OH) butyrate production with simultaneous treatment: an integrated approach, Bioresour. Technol. 101 (2010) 9382–9386. [32] T.G. Volova, N.O. Zhila, G.S. Kalacheva, V.A. Sokolenko, E.J. Sinski, Synthesis of 3-hydroxybutyrate-CO-4-hydroxybutyrate copolymers by hydrogen-oxidizing bacteria, Appl. Biochem. Microbiol. 47 (2011) 494–499. [33] Y.H. Ni, B.A. Wang, R. Liu, J.M. Hong, A simple glucose reduction route for the synthesis of sheet-like copper dendrites, Cryst. Res. Technol. 44 (2009) 1293–1296.
Contents lists available at ScienceDirect
The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu
Hydrothermal conversion of microalgae and its waste residue after biofuel extraction to acetic acid with CuO as solid oxidant Heng Zhong a,b , Lin Ma a , Yingying Zhu a , Binbin Jin a , Tianfu Wang a,b , Yangang Wang c,∗ , Fangming Jin a,c,∗ a School of Environmental Science and Engineering, State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan RD, Shanghai, 200240, China b Shanghai Institute of Pollution Control and Ecological Security, Shanghai, 200092, China c College of Biological Chemical Science and Engineering, Jiaxing University, Jiaxing, 314001, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• New method for microalgae and • • • •
its bio-waste residue conversion to acetic acid (AA). Moderate oxidation ability of CuO is the key to selective AA generation. New way of using Cu-CuO cycle as indirect O2 input for biomass conversion. 35 % AA yield from microalgae is obtained with CuO, much higher than with H2 O2 . Confirm microalgae component for AA generation obeys protein > carbohydrate > lipid.
a r t i c l e
i n f o
Article history: Received 31 July 2019 Received in revised form 1 December 2019 Accepted 4 December 2019 Available online 9 December 2019 Keywords: Microalgae Bioresource Biomass conversion Solid oxidant Copper extraction High-pressure reaction
a b s t r a c t Production of chemicals from microalgae is a substantial way to contribute to the economic viability of a biorefinery. In this research, we report a new and green method for producing acetic acid from microalgae directly or from its waste residue after biofuel extraction using copper (II) oxide (CuO) as a solid oxidant. Results show that the CuO can significantly facilitate the oxidation of microalgae and its waste residue into acetic acid with the simultaneous reduction of CuO to Cu and Cu2 O. The maximum acetic acid yield reached up to 35 % and 31 % from the microalgae and its waste residue, respectively, which are much higher than previous report (14.9 %) which is obtained with traditional liquid oxidants such as H2 O2 . The reduction of CuO to Cu(0) at a relatively low temperature (300 ◦ C) offered an additional benefit of developing a sustainable method in copper metallurgy at mild conditions. © 2019 Published by Elsevier B.V.
1. Introduction
∗ Corresponding authors at: School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan RD, Shanghai, 200240, China. E-mail addresses: [email protected] (Y. Wang), [email protected] (F. Jin). https://doi.org/10.1016/j.supflu.2019.104717 0896-8446/© 2019 Published by Elsevier B.V.
Reliance on fossil fuels as the main energy source has led to a serious energy crisis and associated environmental issues in current human civilization. Utilization of biomass as a renewable feedstock to produce energy and chemicals is one promising
2
H. Zhong, L. Ma, Y. Zhu et al. / J. of Supercritical Fluids 157 (2020) 104717
Table 1 Element analysis of the microalgae and microalgae waste residue used in this research. Composition (wt%)
Element C O H N Other (P, S, et al.)
Microalgae
Microalgae waste residue
45.0 39.8 6.8 8.3 0.1
45.3 43.8 6.3 8.4 1.3
method to reduce the dependence on petroleum-derived feedstock and atmospheric CO2 rise resulted from the use of fossil fuels [1–4]. In this regard, conversion of microalgae has received increasing attention because of its fast growth, high lipid content, ability to convert CO2 , and the potential for producing useful chemicals [5–9]. In addition, microalgae demand neither rigorous living conditions nor agriculturally productive land, which are necessities for terrestrial plants [10]. Recent interests of using microalgae mainly focus on biofuel extraction [11], however, its utilization efficiency is rather low due to the low oil content of microalgae (only 10 %) [12–15]. On the other hand, production of chemicals from microalgae directly or microalgae waste residue after biofuel extraction (denoted as MWR thereafter) can utilize various components of the microalgae and their intermediates, and therefore maximizing the value derived from the biomass feedstock [9,16,17]. Acetic acid is a widely used industrial chemical, which is generally produced from petrochemical feedstock. It has been reported that the biomass feedstocks, such as carbohydrate, fatty acid, and lignin, can be selectively oxidized into acetic acid under hydrothermal conditions [18–21]. However, only a few reports involved the conversion of microalgae into acetic acid, in which the yield of acetic acid was just 14.9 % using H2 O2 as a liquid oxidant [22]. In the traditional hydrothermal oxidation of biomass into acetic acid, liquid or gaseous oxidants such as H2 O2 , O2 , and air are generally used, which leads to high energy costs due to the gas compression as well as several insecurity hazards. On the other hand, solid oxides, such as CuO, have the potential to be used as a solid oxidant to replace the conventional liquid or gaseous oxidants in biomass oxidation given to their advantages including easy accessibility, economy and recoverability. The purpose of the present research is to study the potential of the conversion of microalgae into acetic acid using metal oxides (such as CuO) as solid oxidant and optimization of reaction conditions to achieve the maximum conversion efficiency.
2. Materials and methods
2.2. Experimental procedure A stainless steel (SUS-316) tubular reactor with a 5.7 mL inner volume was used for all the experiments. The schematic drawing of this reactor can be found in our previous report [24,25]. The reactor was first loaded with 0.035 g microalgae, corresponding amount of CuO and water, making the reaction mixture filled up to 20–60 % of the total inner volume of the reactor. Then, the reactor was sealed and immersed in a salt bath that was preheated to the desired reaction temperature (250–300 ◦ C) to initiate the reaction. At the desired time, the reactor was quickly transferred into a cold-water bath to quench the reaction. The reaction time was defined as the time when the reactor was kept in the salt bath. All experiments were performed with degassed and deionized water and the reactor was purged with nitrogen gas before the reaction. After the reaction, the liquid sample was collected by filtering through a 0.45 m syringe filter, while the solid samples were thoroughly washed with deionized water and alcohol, and then dried at 45−55 ◦ C for 24 h. 2.3. Analytic method Liquid samples after the reactions were analyzed by a gas chromatography analyzer (GC-FID, HP7890) equipped with an HPINNOWax column (30 m ×0.25 mm ×0.3 m). Helium was used as the carrier gas with a flow rate of 1 mL/min, and the injector and FID detector temperatures were 200 ◦ C and 220 ◦ C, respectively. The GC oven was programmed to begin at 40 ◦ C for 4 min, increased at a rate of 7.5 ◦ C/min to 220 ◦ C and then remained at 220 ◦ C for an additional 10 min. The sample injection volume was 1 l. An Agilent 7890 gas chromatography-mass spectrometry (GC–MS) equipped with a 5975C MSD detector was also used to investigate other possible chemicals in the liquid phase. The peak identification was accomplished by the NIST library search. For GC analyses, liquid samples were diluted by water to a ratio of 1:5 and were pH adjusted to 1 with HCl. Total organic carbon (TOC) and inorganic carbon (IC) of the liquid samples were analyzed by a Shimadzu TOC analyzer (TOC-V CPH). Solid samples after reaction were examined by a Shimadzu Xray diffractometer (6100), which was operated at 40 kV and 30 mA with a scan rate of 2 deg/min. All quantitative data reported in this study were the average values of three samples with a relative error less than 5 %. 2.4. Definition The yield of acetic acid is defined as follows: Yield(%) =
Carbon amount in acetic acid(g) × 100% Carbon amount in initial microalgae(g)
2.1. Materials
3. Results and discussion
In this research, food-grade Chlorella Vulgaris (powder, Shandong Binzhou Tian Jian Biotechnology Co., Ltd., China) was used as a representative of microalgae since it is one of the most common cultivated microalgae [23]. Besides microalgae, microalgae waste residue after biofuel extraction (MWR) was also chosen as test materials for acetic acid production. The chemical formulas of microalgae and MWR were determined to be C3.75 H6.79 O2.49 N0.59 and C3.35 H6.30 O2.74 N0.60 , respectively (Table 1), which were calculated from the elemental composition data obtained by an organic element analyzer (Vario EL III). Analytical grade copper oxide (CuO) with a purity of 99.9 % from Sinopharm Chemical Reagent co., LTD was used as the oxidants. Gaseous CO2 (>99.995 %) was purchased from Shanghai Poly-Gas Technology Co., Ltd. Deionized water was used throughout the entire study.
3.1. Potential of CuO in converting microalgae into acetic acid Reactions of chlorella in the presence and absence of CuO powder were conducted at 300 ◦ C for 2 h to investigate the possibility of microalgae oxidation to acetic acid with CuO as the solid oxidant. Reaction conditions were selected based on the optimized conditions obtained in our previous research on the hydrothermal oxidation of carbohydrate into acetic acid [24,26]. Analysis of the liquid samples after the reaction showed that the major liquid products were several carboxylic acids, including acetic acid, formic acid, propionic acid, butyric acid, and isovaleric acid in both cases (with and without CuO) (Fig. 1). Interestingly, GC/MS peaks of these acids, particularly for acetic acid, were much higher in the presence of CuO than those in the absence of CuO, suggesting that
H. Zhong, L. Ma, Y. Zhu et al. / J. of Supercritical Fluids 157 (2020) 104717
Fig. 1. GC/MS chromatographs of liquid samples after the reactions of chlorella with and without CuO at 300 ◦ C for 2 h (C1 = formic acid, C3 = propionic acid, C5 = isovaleric acid; reaction conditions: 0.035 g chlorella, 0.55 g CuO, 2 mol/L NaOH, 50 % water filling).
3
Fig. 3. Effect of NaOH on the yield of acetic acid (300 ◦ C, 2 h, 0.035 g chlorella, 0.55 g CuO, 50 % water filling).
Fig. 4. Carbon distribution in the liquid samples after the reaction of microalgae with CuO (300 ◦ C, 2 h, 2 mol/L NaOH, 0.035 g chlorella, 50 % water filling).
Fig. 2. XRD pattern of solid sample after the reaction of chlorella with CuO (300 ◦ C, 2 h, 2 mol/L NaOH, 0.035 g chlorella, 0.55 g CuO, 50 % water filling).
CuO could promote the production of acetic acid from microalgae. The reduced products of CuO were determined to be Cu and Cu2 O by the XRD analysis (Fig. 2), confirming the oxidative role of CuO in promoting the production of acetic acid from chlorella. Also, possible dissolved Cu ion in solution after the reaction was analyzed by ICP analysis, and the result showed that the concentration of dissolved Cu ion was 153 mg/L, which corresponds to 0.1 wt% of the original Cu added to the reaction. This result indicates that the leaching of Cu in the alkaline solution is limit. Our previous research on the oxidation of carbohydrate into acetic acid have shown that the yield of acetic acid is significantly enhanced in alkaline conditions [24,26]. Thus, the effect of alkaline on the acetic acid yield from microalgae in the presence of CuO was studied then. As illustrated in Fig. 3, the yield of acetic acid greatly increased from 8 % to 31 % when the concentration of NaOH increased from 0 to 2 mol/L, indicating that the presence of alkaline (OH− ) can enhance the oxidation of chlorella to acetic acid. The increase of acetic acid yield in the presence of NaOH is probably due to the increasing hydrolysis rate of chlorella and high solubility of CuO to form hydroxo complex in the alkaline medium. Next, carbon distribution in the liquid sample was examined. As shown in Fig. 4, the percentage of total organic carbon (TOC) in the liquid samples was 60 %, in which acetic acid contributed about 50 %. The inorganic carbon (IC) in liquid samples should be attributed to the dissolved CO2 or carbonate in the alkaline solution. The total dissolved carbon (TOC + IC) in the liquid sample accounts for 78 wt% of the original carbon contained in chlorella. Furthermore, CO2 was detected in gaseous samples in the presence of both CuO and NaOH.
Fig. 5. Effect of CuO amount on the formation (red square) and decomposition (blue circle) of acetic acid (300 ◦ C, 2 h, 50 % water filling, 2 mol/L NaOH and 0.035 g chlorella for the formation of acetic acid, 200 mmol/L acetic acid for the decomposition of acetic acid).
These results indicated that CuO also promoted the oxidation of chlorella to CO2 , particularly in the presence of alkali. 3.2. Effect of experimental parameters on the acetic acid yield Fig. 5 shows the effect of CuO amount on the yield of acetic acid from chlorella. The acetic acid yield increased clearly from 9 % to 31 % with the increase of CuO amount from 0 to 0.55 g. However, a further increase in the amount of CuO led to a decrease in the yield of acetic acid, which is probably due to a further decomposition of acetic acid to CO2 in presence of an excess amount of CuO. To verify this speculation, a reaction with acetic acid (200 mmol/L) in the presence of CuO was performed. As expected, a decomposition
4
H. Zhong, L. Ma, Y. Zhu et al. / J. of Supercritical Fluids 157 (2020) 104717
Fig. 6. (a) Effect of reaction temperature (blue square) and reaction time (red circle) on the yield of acetic acid (2 mol/L NaOH, 0.035 g chlorella, 0.55 g CuO, 50 % water filling, 2 h for the effect of reaction temperature, 300 ◦ C for the effect of reaction time); (b) Effect of water filling rate on the yield of acetic acid (2 h, 300 ◦ C, 2 mol/L NaOH, 0.55 g CuO, 0.035 g chlorella).
of acetic acid from 98 % to 64 % was observed with the increase of CuO amount from 0 to 2.25 g (Fig. 5). Thus, an optimum amount of CuO exists in the conversion of chlorella to acetic acid. Fig. 6a depicts the acetic acid yield obtained at different reaction times and temperatures. Results revealed that an increase in the reaction temperature from 250 to 300 ◦ C promoted the acetic acid yield from 17 % to 31 %. The reaction pressures were ca. 4.0 and 8.4 MPa at 250 and 300 ◦ C, respectively, which is almost the same as the water vapor pressure at related temperature, suggesting that the reaction pressure was mainly contributed by the water vapor pressure. However, a further increase in the temperature caused a decline in the acetic acid yield, which is probably resulted by the decomposition of acetic acid at higher temperatures as already discussed. A similar trend was found for the effect of reaction time, which was optimized as 2 h to achieve the highest acetic acid yield. Water filling rate is a key factor in the hydrothermal reactions because the water filling rate affects the pressure of reaction system as well as the water density. As illustrated in Fig. 6b, water filling rate had a great effect on promoting the acetic acid yield, particularly for water filling exceeding 40 % (Fig. 6b). An optimum acetic acid yield of 35 % was achieved at the water filling of 60 %. This is probably because the hydrolysis rate of microalgae was enhanced at higher water fillings due to the increase in reaction pressure caused by high water filling [27–30]. Based on Fig. 6a, the acetic acid yield can probably be further enhanced by increasing the water filling rate, however, experiments with higher water filling rates (>60 %) were not conducted due to the experimental limitation and safety issues.
3.3. Proposed mechanism for the conversion of microalgae to acetic acid with CuO It is known that microalgae have three body components, which are protein, lipid, and carbohydrate. Therefore, contribution of these three major components for acetic acid production and their interactional effects were studied. Glutamic acid, hexadecanoic acid, and glucose were used as model compounds for protein, lipid and carbohydrate, respectively. Results showed that the yields of acetic acid were 34 %, 5 % and 45 % from glutamic acid, hexadecanoic
Table 2 Acetic acid yield from the oxidation of different starting materials.a Entry
Substance
Acetic acid yield (%)
1 2 3 4
Glutamic acid (GA) Hexadecanoic acid (HA) Glucose (GL) GA+HA + GLb
34 5 45 32
a b
reaction conditions: 300 ◦ C, 2 h, 2 mol/L NaOH, 0.55 g Cu, 50 % water filling. GA:HA:GL = 6:1:3.
acid, and glucose, respectively under the same reaction condition as using microalgae for obtaining the optimum acetic yield (Table 2). Considering that the mass ratio of protein, lipid, and carbohydrate in microalgae is 6:1:3, a mixture of glutamic acid, hexadecanoic acid and glucose with the same mass ratio was reacted. An acetic acid yield of 32 % was attained at the optimal conditions (Table 2, Entry 4), which was very close to the highest value of 35 % from microalgae. Interestingly, by multiplying the acetic acid yields obtained from glutamic acid, hexadecanoic acid, and glucose (34 %, 5 % and 45 %) with their corresponding ratio (60 %, 10 %, 30 %), a 34.4 % acetic acid was obtained, which is similar to that obtained with the chlorella directly. These results suggested that the contribution of the constituents of microalgae to the production of acetic acid was in the order of protein > carbohydrate > lipid, and there was no significant interaction among these three components in the hydrothermal conversion of microalgae into acetic acid. With these experimental results, a tentative conversion pathway of microalgae was postulated in Fig. 7. First, microalgae are hydrolyzed to protein, lipid, and carbohydrate, which then undergo further hydrolysis to form amino acid, fatty acid, and glucose, respectively, under the alkaline condition. Subsequently, the protein, lipid, and carbohydrate are oxidized in the presence of dissociated Cu(II) ions formed in the alkaline solution. For the protein, dissociated Cu(II) ions probably coordinate with amino acid to form a comparatively active coordination compound. The short distance between oxygen and Cu atom in the coordination compound is favorable for the electron transfer from oxygen atom to Cu(II) ion, resulting in the reduction of Cu(II) into Cu(0) or Cu(I) and the formation of acetic acid. Then, the decomposition products of
Table 3 Distribution of nitrogen element in liquid products after the reaction of chlorella with different amount of CuO.a Entry
CuO(g)
NH4 + (mg/L)
NO2 − (mg/L)
NO3 − (mg/L)
TNb (mg/L)
NH4 + /TN(%)
1 2
0.55 1.10
612 528
1 1
199 54
896 932
68 57
a b
Reaction conditions: 300 ◦ C, 2 h, 2 mol/L NaOH, 0.035 g chlorella, 50 % water filling. TN = total nitrogen.
H. Zhong, L. Ma, Y. Zhu et al. / J. of Supercritical Fluids 157 (2020) 104717
5
Fig. 7. Proposed reaction pathways of the conversion of microalgae into acetic acid with CuO oxidant.
Table 4 Acetic acid yield obtained from the oxidation of chlorella with different solid oxidants.a Oxidant
Amount (g)b
Acetic acid yield (%)
CuO MnO2 TiO2 ZrO2 Fe2 O3 Al2 O3
0.55 0.30 0.28 0.43 0.37 0.24
16.8 10.5 10.0 10.6 9.1 9.2
a Reaction conditions: 300 ◦ C, 0.5 h, 2 mol/L NaOH, 0.035 g chlorella, 50 % water filling. b the amount is selected to keep the oxygen content the same.
4. Conclusion We have demonstrated a new method for the conversion of microalgae and its waste residue from biofuel extraction to acetic acid with CuO as a solid oxidant. The highest acetic acid yields from microalgae and microalgae waste residue were 35 % and 31 %, respectively, which is much higher than previous work (14.9 %) using H2 O2 as a liquid oxidant. The proposed method not only provides a new way for the conversion of microalgae and microalgae waste residue into value-added chemicals but also offers a new concept of developing a green copper metallurgy at low temperature.
Declaration of Competing Interest the coordination compound undergo hydrolysis to form ammonia, water and carboxylic acids. As showed in Table 3, the total dissolved nitrogen (TN) was mainly contributed by the NH4 + , suggesting that the major nitrogen of microalgae were converted to NH4 + . For the lipid conversion path, since fatty acids are readily oxidized on the carbon to form C−OH in the presence of OH− [31,32], it is suggested that fatty acids are first oxidized on the -carbon by CuO to form 2-hydroxyl fatty acid. Then the dissociated Cu(II) ions coordinate with 2-hydroxyl fatty acid to form Cu complex, which is oxidized to acetic acid. For the carbohydrate oxidation path, glucose first reacts with Cu(II) ions to form a stable complex [33], and then a redox reaction occurs as a result of electron transfer from the hydroxyl oxygen atom in glucose to the Cu(II) ions. Cu(0) or Cu(I) is obtained and glucose is decomposed to small molecules such as lactic acid, which is further oxidized to acetic acid via a similar transformation with the formation of Cu complex and the release of CO2 . 3.4. Production of acetic acid from microalgae waste residue (MWR) Since the current utilization of microalgae is mainly focused on biofuel extraction, a large amount of MWR is produced in this process. Therefore, conversion of MWR was further investigated, in which the residue with about 10 % oil content was used. Results showed that a high acetic acid yield (31 %) could be achieved with CuO at 300 ◦ C for 2 h, which was almost the same as that obtained from microalgae directly. These results indicate that the MWR can also be treated as potential feedstock to produce acetic acid. To test the application of the proposed method of microalgae oxidation into acetic acid with solid oxidant, various possible metal oxides were further tested. As shown in Table 4, all the tested metal oxides had the ability to convert microalgae into acetic acid, among which, CuO exhibited the highest activity. These results suggest that using solid oxidants for the microalgae oxidation into acetic acid has wide applicability.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments The authors thank the funding support from the State Key Program of Natural Science Foundation of China (No. 21436007), the Natural Science Foundation of Shanghai (No. 19ZR1424800), Startup Fund for Youngman Research at SJTU, and the Center of Hydrogen Science, Shanghai Jiao Tong University, China.
References [1] H. Li, A. Riisager, S. Saravanamurugan, A. Pandey, R.S. Sangwan, S. Yang, R. Luque, Carbon-increasing catalytic strategies for upgrading biomass into energy-intensive fuels and chemicals, ACS Catal. 8 (2018) 148–187. [2] W.W. Lin, H. Chen, J. Li, K.Q. Chen, X.Y. Lu, P.K. Ouyang, J. Fu, Enhanced stability of Pt/C by the atomic layer deposition of porous MOx for the decarboxylation of oleic acid, Catal. Commun. 123 (2019) 59–63. [3] Z.H. Zhang, H. Cheng, H. Chen, J. Li, K.Q. Chen, X.Y. Lu, P.K. Ouyang, J. Fu, Catalytic fast pyrolysis of rice straw to aromatics over hierarchical HZSM-5 treated with different organosilanes, Energy Fuels 33 (2019) 307–312. [4] Z.H. Zhang, Q.W. Yang, H. Chen, K.Q. Chen, X.Y. Lu, P.K. Ouyang, J. Fu, J.G.G. Chen, In situ hydrogenation and decarboxylation of oleic acid into heptadecane over a Cu-Ni alloy catalyst using methanol as a hydrogen carrier, Green Chem. 20 (2018) 197–205. [5] V. Abrahamsson, L.P. Cunico, N. Andersson, B. Nilsson, C. Turner, Multicomponent inverse modeling of supercritical fluid extraction of carotenoids, chlorophyll A, ergosterol and lipids from microalgae, J. Supercrit. Fluids 139 (2018) 53–61. [6] R. Feller, Â.P. Matos, S. Mazzutti, E.H.S. Moecke, M.V. Tres, R.B. Derner, J.V. Oliveira, A.F. Junior, Polyunsaturated -3 and -6 fatty acids, total carotenoids and antioxidant activity of three marine microalgae extracts obtained by supercritical CO2 and subcritical n-butane, J. Supercrit. Fluids 133 (2018) 437–443. [7] A. Raheem, P. Prinsen, A.K. Vuppaladadiyam, M. Zhao, R. Luque, A review on sustainable microalgae based biofuel and bioenergy production: recent developments, J. Clean. Prod. 181 (2018) 42–59.
6
H. Zhong, L. Ma, Y. Zhu et al. / J. of Supercritical Fluids 157 (2020) 104717
[8] Y. Yang, H. Zhong, R.T. He, X.G. Wang, J. Cheng, G.D. Yao, F.M. Jin, Synergetic conversion of microalgae and CO2 into value-added chemicals under hydrothermal conditions, Green Chem. 21 (2019) 1247–1252. [9] T. Mathimani, A. Baldinelli, K. Rajendran, D. Prabakar, M. Matheswaran, R.P. van Leeuwen, A. Pugazhendhi, Review on cultivation and thermochemical conversion of microalgae to fuels and chemicals: process evaluation and knowledge gaps, J. Clean. Prod. 208 (2019) 1053–1064. [10] A.B. Ross, P. Biller, M.L. Kubacki, H. Li, A. Lea-Langton, J.M. Jones, Hydrothermal processing of microalgae using alkali and organic acids, Fuel 89 (2010) 2234–2243. [11] L. Brennan, P. Owende, Biofuels from microalgae – a review of technologies for production, processing, and extractions of biofuels and co-products, Renew. Sust. Energy Rev. 14 (2010) 557–577. [12] X.L. Miao, Q.Y. Wu, Biodiesel production from heterotrophic microalgal oil, Bioresour. Technol. 97 (2006) 841–846. [13] X. Meng, J. Yang, X. Xu, L. Zhang, Q. Nie, M. Xian, Biodiesel production from oleaginous microorganisms, Renew. Energy 34 (2009) 1–5. [14] R. Halim, B. Gladman, M.K. Danquah, P.A. Webley, Oil extraction from microalgae for biodiesel production, Bioresour. Technol. 102 (2011) 178–185. [15] Y.-P. Xu, P.-G. Duan, F. Wang, Hydrothermal processing of macroalgae for producing crude bio-oil, Fuel Process. Technol. 130 (2015) 268–274. [16] Y.D. Lu, R.B. Levine, P.E. Savage, Fatty acids for nutraceuticals and biofuels from hydrothermal carbonization of microalgae, Ind. Eng. Chem. Res. 54 (2015) 4066–4071. [17] Y.F. Zan, Y.Y. Sun, L.Z. Kong, G. Miao, L.W. Bao, H. Wang, S.G. Li, Y.H. Sun, Formic acid-induced controlled-release hydrolysis of microalgae (scenedesmus) to lactic acid over Sn-beta catalyst, ChemSusChem 11 (2018) 2492–2496. [18] F.M. Jin, J. Yun, G.M. Li, A. Kishita, K. Tohji, H. Enomoto, Hydrothermal conversion of carbohydrate biomass into formic acid at mild temperatures, Green Chem. 10 (2008) 612–615. [19] F.M. Jin, H. Enomoto, Application of hydrothermal reaction to conversion of plant-origin biomasses into acetic and lactic acids, J. Mater. Sci. 43 (2008) 2463–2471. [20] F.M. Jin, H. Enomoto, Hydrothermal conversion of biomass into value-added products: technology that mimics nature, Bioresources 4 (2009) 704–713. [21] F.M. Jin, H. Enomoto, Rapid and highly selective conversion of biomass into value-added products in hydrothermal conditions: chemistry of acid/base-catalysed and oxidation reactions, Energy Environ. Sci. 4 (2011) 382–397.
[22] Z. Shen, J.F. Zhou, X.F. Zhou, Y.L. Zhang, The production of acetic acid from microalgae under hydrothermal conditions, Appl. Energy 88 (2011) 3444–3447. [23] C. Safi, B. Zebib, O. Merah, P.-Y. Pontalier, C. Vaca-Garcia, Morphology, composition, production, processing and applications of Chlorella vulgaris: a review, Renew. Sust. Energy Rev. 35 (2014) 265–278. [24] F.M. Jin, A. Kishita, T. Moriya, H. Enomoto, Kinetics of oxidation of food wastes with H2 O2 in supercritical water, J. Supercrit. Fluids 19 (2001) 251–262. [25] H. Zhong, G.D. Yao, X. Cui, P. Yan, X.G. Wang, F.M. Jin, Selective conversion of carbon dioxide into methane with a 98% yield on an in situ formed Ni nanoparticle catalyst in water, Chem. Eng. J. 357 (2019) 421–427. [26] F.M. Jin, Z.Y. Zhou, T. Moriya, H. Kishida, H. Higashijima, H. Enomoto, Controlling hydrothermal reaction pathways to improve acetic acid production from carbohydrate biomass, Environ. Sci. Technol. 39 (2005) 1893–1902. [27] S.H. Townsend, M.A. Abraham, G.L. Huppert, M.T. Klein, S.C. Paspek, Solvent effects during reactions in supercritical water, Ind. Eng. Chem. Res. 27 (1988) 143–149. [28] L.A. Torry, R. Kaminsky, M.T. Klein, M.R. Klotz, The effect of salts on hydrolysis in supercritical and near-critical water - reactivity and availability, J. Supercrit. Fluids 5 (1992) 163–168. [29] T.M. Aida, Y. Sato, M. Watanabe, K. Tajima, T. Nonaka, H. Hattori, K. Arai, Dehydration of D-glucose in high temperature water at pressures up to 80 MPa, J. Supercrit. Fluids 40 (2007) 381–388. [30] T.M. Aida, A. Ikarashi, Y. Saito, M. Watanabe, R.L. Smith, K. Arai, Dehydration of lactic acid to acrylic acid in high temperature water at high pressures, J. Supercrit. Fluids 50 (2009) 257–264. [31] S.V. Mohan, M.V. Reddy, G.V. Subhash, P.N. Sarma, Fermentative effluents from hydrogen producing bioreactor as substrate for poly(beta-OH) butyrate production with simultaneous treatment: an integrated approach, Bioresour. Technol. 101 (2010) 9382–9386. [32] T.G. Volova, N.O. Zhila, G.S. Kalacheva, V.A. Sokolenko, E.J. Sinski, Synthesis of 3-hydroxybutyrate-CO-4-hydroxybutyrate copolymers by hydrogen-oxidizing bacteria, Appl. Biochem. Microbiol. 47 (2011) 494–499. [33] Y.H. Ni, B.A. Wang, R. Liu, J.M. Hong, A simple glucose reduction route for the synthesis of sheet-like copper dendrites, Cryst. Res. Technol. 44 (2009) 1293–1296.