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Catalytic alcohothermal liquefaction of wet microalgae with supercritical methanol
J. of Supercritical Fluids 157 (2020) 104704
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The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu
Catalytic alcohothermal liquefaction of wet microalgae with supercritical methanol Fon Yee Han a , Masaharu Komiyama a,b,c,∗ , Yoshimitsu Uemura b , Nurul Ekmi Rabat a a
Chemical Engineering Department, Universiti Teknologi PETRONAS, 32610, Seri Iskandar, Perak Darul Ridzuan, Malaysia HICoE−Centre for Biofuel and Biochemical Research, Institute for Self-Sustainable Building, Universiti Teknologi PETRONAS, 32610, Seri Iskandar, Perak Darul Ridzuan, Malaysia c Clean Energy Research Center, University of Yamanashi, Takeda, Kofu, 400-8511, Japan b
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
• Wet Chlorella vulgaris was liquefied under supercritical methanol (methanothermal). • Over 60 % of feed carbon is liquefied into bio-oil. • Methanothermal reaction gave twice as much light oil than hydrothermal reaction. • Oxide catalysts such as Zr(WO4 )2 , MnO2 , ZnO2 and ZrO2 were active for the reaction.
a r t i c l e
i n f o
Article history: Received 28 September 2019 Received in revised form 26 November 2019 Accepted 26 November 2019 Available online 28 November 2019 Keywords: Microalgae Chlorella vulgaris Biofuel Heterogeneous catalyst Supercritical methanol Methanothermal liquefaction Alcohothermal liquefaction
a b s t r a c t Microalgae are promising feedstock for the production of biofuel due to their high productivity and low interference with food production for their cultivation and usage. Owing to their aquatic production environment, however, energy-intensive dewatering and drying processes may be required before their conversion to biofuel. In order to avoid these processes, their liquefaction through hydrothermal means has attracted attention, with extensive research works. The present research proposes and examines alcohothermal liquefaction of wet microalgae, in an attempt to modify and improve microalgae liquefaction characteristics. Thus supercritical methanol was employed as a reaction media for liquefaction of wet Chlorella vulgaris, and the effects of water presence as well as the effects of heterogeneous oxide catalysts were examined. This methanothermal liquefaction gave high bio-oil yield of 54.5 C% (feed microalgae basis) compared to 30.5 C% by hydrothermal for non-catalytic reaction at 385 ◦ C. With Zr(WO4 )2 catalyst methanothermal bio-oil yield increased further to 65.6 C%. The presence of water in the methanol medium somewhat decreased the bio-oil yield, while some of the catalysts mitigated the decrease: at 50 wt% water content in methanol, MnO2 , ZnO2 and ZrO2 gave bio-oil yield of 55.5 C%, 59.5 C% and 57.6 C%, respectively. Possible mechanism of catalytic methanothermal liquefaction of wet microalgae is discussed. © 2019 Published by Elsevier B.V.
∗ Corresponding author at: Chemical Engineering Department, Universiti Teknologi PETRONAS, 32610, Seri Iskandar, Perak Darul Ridzuan, Malaysia. E-mail addresses: [email protected], [email protected] (M. Komiyama). https://doi.org/10.1016/j.supflu.2019.104704 0896-8446/© 2019 Published by Elsevier B.V.
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1. Introduction Microalgae, aquatic microscopic photosynthetic unicellular organisms, are promising biomass feedstocks for the production of biofuel due to their high productivity and minimal interference with food production for their cultivation and usage [1]. Potential processes for microalgal biomass conversion to biofuel include: (i) biochemical conversion to ethanol or biogas (methane/CO2 mixture); (ii) thermochemical conversion to bio-oil and gaseous fuel; and (iii) oil extraction to produce biodiesel [2,3]. Except for the biochemical conversion process (i), other processes require energyintensive dewatering and drying steps before their conversion to biofuel. In process (iii) further pretreatments such as cell disruption and oil extraction are necessary before their triglyceride transesterification to biodiesel [4]. In order to avoid these energy-intensive and/or tedious processes, one-path or one step microalgae liquefaction through hydrothermal treatment has attracted attention. It applies high temperature (above 250 ◦ C) and pressure (above 5 MPa) to aqueous microalgae suspension, and bio-oil was produced as the main product together with water soluble component, gas and solid residue. The hydrothermal conversion of microalgal biomass to liquid fuels was examined by Yokoyama and his group in the 1990s [5–7], later followed by a few other groups [8–12]. It is noteworthy that hydrothermal liquefaction of microalgae produce oil more than the amount of natural lipid it contained originally [5], suggesting that the hydrothermal process not only extracts the naturally occurring oils in the algae but also produces oils from the non-lipid components of the algal biomass. It is also noted that energy balance analyses show that hydrothermal liquefaction of microalgae can be a net energy producer [6,7]. Detailed molecular analysis of bio-oil obtained by hydrothermal treatment of microalgae is given by Torri et al. [13]. Since major part of microalgal lipid is triglyceride, it may be natural to attempt direct transesterification of algal lipid without feed algal pretreatments such as cell disruption and lipid extraction. To this end, alcohols at the near critical conditions has been employed. Thus Levine, Pinnarat and Savage [14] devised a two-step direct transesterification of Chlorella vulgaris, in which in the first step wet algal biomass is reacted in subcritical water to hydrolyze intracellular lipids to be retained in conglomerate cells, and in the second step the wet fatty acid-rich solids undergo in situ transesterification with supercritical ethanol to produce biodiesel in the form of fatty acid ethyl esters (FAEEs). Deng et al. [15–17] used supercritical methanol and ethanol to directly extract and transesterify dry Nannochloropsis lipid to biodiesel. Jazzar et al. [18,19] employed supercritical methanol for direct in situ transesterification of dry Nannochloropsis and Chlorella lipids to biodiesel. Bi, He and McDonald [20] performed in situ transesterification of dry Schizochytrium limacinum lipid with sub- and super-critical methanol. Hegel et al. [21] extracted biodiesel from dry Neochloris oleoabundans by supercritical methanol. Shirazi, Karimi-Sabet and Ghotbi [22] examined direct transesterification of wet Spirulina with near critical methanol. In all those trials, transesterification of triglycerides is expected to proceed without acid or base catalyst, due to the nature of the near-critical (sub- or super-critical) alcohols [23]. All the above mentioned near-critical liquefaction work [14–22] were concerned only with fatty acid methyl (or ethyl) ester (FAME or FAEE) biodiesel production and not with bio-oil production. When the catalyst is present with supercritical alcohol, reactions that occur are not only extraction and transesterification of natural lipid present in the feed microalgae, but the whole bio-oil production is affected. Thus with the analogy to “hydrothermal” reaction or treatment [5–13], the overall reaction under near-critical alcohol will be termed here as “alcohothermal” reaction or treatment. To
Table 1 Elemental and chemical compositions of Chlorella vulgaris (dry basis)a . Elemental Composition (wt%) C H N S Ash Ob a b
47.37 9.85 10.89 0.72 3.32 27.85
Chemical Composition (wt%) Ash Lipid Protein Carbohydratesb
3.53 13.21 72.29 10.97
Moisture content as received: 5.85 wt %. Calculated by difference.
date, there exists only a few study on catalytic alcohothermal treatment of microalgae using near-critical alcohol. Huang et al. [24] treated dry Spirulina with near-critical ethanol and found that FeS increased bio-oil yield while decreasing solid residue. Zhang et al. [25] liquefied dry Chlorella pyrenoidosa in sub- and super-critical ethanol with HZSM-5 and Raney-Ni as heterogeneous catalysts. Jin et al. [26] examined the effect of metal chloride Lewis acid catalysts on the liquefaction characteristics of Chlorella pyrenoidosa under supercritical ethanol. They also examined the effect of moisture content in the feed microalgae in the range of 0−80 wt% and found that the moisture content in the feed is the most influential factor affecting the yield and properties of bio-oil produced. In this paper, catalytic alcohothermal liquefaction of wet Chlorella vulgaris with supercritical methanol was examined with various catalysts that are known to be stable under supercritical water conditions [27]. The effects of water content, reaction time and temperature, and the types of catalysts were examined. Possible microalgal alcohothermal liquefaction mechanism is also discussed. 2. Experimental 2.1. Materials The feed microalgae, Chlorella vulgaris, supplied by Chlorella Industry Co., Ltd. (Japan) was used as received in this research. It was in the form of fine green powder (average particle size: 76 m). Its elemental composition was determined using a Perkin Elmer CHNS/O analyzer Series II 2400, and chemical composition with standard analytical procedures, and are listed in Table 1. As alcohothermal solvents, methanol obtained from Merck (purity >99.9 %) and distilled water were used. Other chemicals employed were mostly of analytical grade and obtained from Merck, Sigma-Aldrich and Wako Pure Chemicals. Eight heterogeneous oxide catalysts, ␣-Al2 O3 , Fe2 O3 , MnO2 , ZnO, ZrO2 , Zr(WO4 )2 , LaFeO3 and LaMnO3 were examined. The choice of the catalysts is due to their stability in other supercritical water reactions [27]. Details of the catalyst source and preparation, as well as their BET surface area and metal oxide crystallite sizes are found elsewhere [27]. 2.2. Reaction procedure Methanothermal liquefaction of the microalgae was performed using a tube type batch reactor with a high-pressure valve connected for gas retrieval as described elsewhere [28]. A molten salt bath was used for heating purpose. A flowchart describing the reaction procedure as well as product analysis is given in Fig. 1. For the reaction, pre-determined amount of dried Chlorella vulgaris powder, catalyst, methanol and distilled water were charged into the batch reactor and closed tightly. Typical experiments were done with 4.0 g of reaction medium (methanol and/or distilled water), 0.4444 g of microalgae (1/9 feed/medium ratio), 0.0222 g of catalyst (5 wt% of feed) charged into an 8.5 mL volume reactor. The
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Fig. 1. Flowchart of methanothermal reaction and product analysis in the present catalytic supercritical microalgae liquefaction (DCM: dichloromethane).
Table 2 Critical temperature and pressure for respective reaction media. Water fraction in methanol-water medium (wt%)
Critical temperature (◦ C)
Critical pressure (MPa)
0 (methanol only) 50 100 (water only)
240.05 352.10 373.95
7.95 15.77 22.06
reactor was then immersed in the molten salt bath set at a desired temperature. In all the experiments, the reaction time was measured from the time the reactor was immersed in the salt bath. The reactor reached the set reaction temperature within 5 min [27]. Table 2 shows the critical temperature and pressure for pure solvents and for a methanol-water mixture calculated on the bases of the Lorentz-Berthelot type mixing rules. After running the reaction for a desired time (commonly 60 min), the reactor was taken out from the salt bath and put under running tap water to quench the reactor and terminate the reaction. For a comparison purpose, conventional oil extraction of the present microalgae was performed by the methanol-chloroform method, and then the obtained oil fraction was separated to hexane solubles (light oil) and nonsolubles (heavy oil).
20 mL distilled water and was centrifuged to separate the three phases (hexane phase, water phase, and non-dissolved phase) at 3000 rpm for 10 min. The hexane phase containing light oil was dried under an air stream and then in an oven at 105 ◦ C for 15 min. Water phase and non-dissolved phase were separated and dried at 90 ◦ C for 12 h. Here bio-oil that is soluble to hexane is termed “light oil” and dichloroethane-soluble but hexane-insoluble phase, “heavy oil” (Fig. 1). Both light oil and heavy oil are grouped together as “bio-oil” where necessary. The conversions, product yields and carbon yields of various products such as gas, light oil, heavy oil and solid residue were defined as follows. Conversion (C%) = 100 − Product yield (wt%) = Carbon yield (C%) =
g of Csolid residue g of Cdried algae
g of product × 100 g of dry algae input
g of Cproduct g of Cdry algae
× 100
(1) (2) (3)
3. Results and discussion 3.1. Effects of reaction temperature, catalyst amount and reaction time
2.3. Product analysis Once the reactor was cooled to room temperature, the connected high-pressure valve was opened, and the gas product was retrieved by using a gas bag. Its volume was measured by the water displacement method and its composition by a gas chromatograph (Shimadzu GC-8A with TCD detector and MolSieve-5A packed column). Then, the reactor was opened, and the reaction products were filtered (funnel with a folded filter paper, 2.5 m), and the liquid product was collected in a 50 mL glass vial. This way unreacted microalgae cells are separated as solid residue, along with ash and solid catalyst. The reactor was rinsed thrice with dichloromethane to ensure all the reaction products were collected. The liquid product was air-dried, washed thrice by mixing a 5 mL n-hexane and
First the effect of reaction temperature on the methanothermal liquefaction of dry and wet (as 50 wt% water mixed to reaction media) Chlorella vulgaris with ␣-Al2 O3 catalyst (5 wt% to the feed microalga) was examined in the temperature range of 185 ◦ C and 385 ◦ C, and the results are shown in Fig. 2. As apparent in Fig. 2(a), at a lower temperature (185 ◦ C), microalgae conversions (expressed by solid diamonds in the figure) are low (59 C% for 0 wt% water media and 71 C% for 50 wt% water media, respectively). As temperature increased to 235 ◦ C and above, solid residue yield (expressed with gray shaded bar graphs) suddenly decreased, and the conversion shot up and stayed at above 98 C%. This conversion increase seems to indicate that above this temperature and in near-critical methanol/water mixture, microalgae cells are almost
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Fig. 2. The effect of reaction temperature with ␣-Al2 O3 catalyst in 0 wt% water (methanol only medium) and 50 wt% water (methanol-water medium) on (a) product yield (bar graphs, left axis) and conversion (solid diamond, right axis); (b) carbon yield among the methanothermal reaction products. Reaction time is 60 min and catalyst loading, 5 wt%. Symbols: , water soluble component; 䊐, gas; , light oil; , heavy oil; , solid residue.
completely broken to be soluble to dichloromethane. Concurrent to this conversion increase (or solid residue decrease), the bio-oil yield (expressed by cross hatched bars for heavy oil and diagonally hatched bars for light oil in Fig. 2(a)) increased with the increase of temperature and reached the maximum of 38.9 wt% at 335 ◦ C for 0 wt% water media and 31.5 wt% at 385 ◦ C for 50 wt% water media, respectively. This finding is in agreement with the results of Zhang et al. [25] for the catalytic ethanothermal liquefaction of dry Chlorella pyrenoidosa. The decrease of bio-oil yield at 385 ◦ C for 0 wt% water media is apparently due to the sudden increase of gas component (expressed by blank bars) at that temperature at the cost of heavy oil. Note that this gas production was not observed with 50 wt% water media, and it is a marked effect of water presence in the present methanothermal microalgae liquefaction. The major component of the produced gas is always CO2 (> 50 %), with minor amount of methane, H2 and higher hydrocarbons. No significant difference of gas composition with the addition of water to the reaction medium was observed. It is also worth noting here that the bio-oil yield is almost three-times higher than the initial lipid content of feed Chlorella vulgaris (13.2 wt% on dry algae basis, Table 1). Both in 0 wt% and 50 wt% water mixed media, reaction temperature rise increased the yield of light oil at the cost of heavy oil.
The same observations hold in terms of carbon yields shown in Fig. 1(b). As may be apparent in the figure, almost half of the microalgae carbon was retained in bio-oil at higher reaction temperatures (> 235 ◦ C). The carbon distributed to the bio-oil inclined from 27.8 C% to 53.8 C% with increasing reaction temperature from 185 ◦ C to 385 ◦ C with 0 wt% water (methanol only) media and from 29.8 C% to 53.4 C % in the presence of 50 wt% water in the media. Furthermore, above 235 ◦ C with media water content of both 0 wt% and 50 wt%, gradual increase of light oil carbon is observed at the cost of heavy oil carbon and water-soluble carbon. When the reaction time is varied from 15 to 60 min at 385 ◦ C (data not shown), the conversion remained above 99 %, indicating that cell disruption occurs at the early stage of reaction. For 0 wt% water content, carbon yield of bio-oil varied little from 54.3 C% to 53.8 C% between the reaction time of 15–60 min, while for 50 wt% water content it increased from 42.5 C% to 53.4 C%. Apparently the liquefaction rate is slower with 50 wt% water present, but reaches at the similar level of conversion of methanol-only reaction at 60 min. Thus as the reaction time, 60 min is used throughout the present experiments. The effect of catalyst loading was also tested between 5 wt% to 50 wt% of feed microalgae using ␣-Al2 O3 catalyst (data not shown).
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Fig. 3. The effect of water content (0: methanol only; 50: 50 wt% water-methanol; 100: water only) in the reaction media and heterogeneous catalyst on carbon yield of each products in methanothermal liquefaction of Chlorella vulgaris. Reaction conditions: 385 ◦ C, 60 min, 5 wt% catalyst loading. Symbols: , water soluble component; 䊐, gas; , light oil; , heavy oil; , solid residue, * conventional extraction.
In terms of solid residue and bio-oil yield, 5 wt% catalyst outperformed 25 wt% and 50 wt%. Thus as catalyst loading 5 wt% of feed is employed throughout the present experiment. 3.2. Characteristics of catalytic methanothermal liquefaction Fig. 3 compares the carbon yields of methanothermal liquefaction products at 385 ◦ C from catalysis with eight metal oxides (␣-Al2 O3 , Fe2 O3 , MnO2 , ZnO, ZrO2 , LaFeO3 , LaMnO3 and Zr(WO4 )2 ) and non-catalytic reaction (blank), along with the conventionally extracted oil. First of all, it is immediately noticed by comparing 0 wt% water content (methanol only) and 100 % water content (water only) reactions for blank reaction that the methanothermal liquefaction gives much higher bio-oil C yield (sum of heavy oil and light oil C yields) than hydrothermal liquefaction: it gave bio-oil yield of 54.5 C% for methanothermal compared to 30.5 C% by hydrothermal. Similar phenomenon is observed for all of the catalytic reactions performed here. For instance, ZnO gave biooil yield of 63.5 C% for methanothermal compared to 33.6 C% by hydrothermal and Zr(WO4 )2 , 65.6 C% vs. 34.7 C%, respectively. When 0 wt% water (methanol only) cases are compared among the non-catalytic and catalytic reactions, first it is noted that gasification amount is relatively high: in all the cases examined here, 25–30 C% of the alga biomass is gasified. At the same time, above 50 C% of biomass is retained as bio-oil, and the following catalysts showed higher bio-oil C yield than non-catalytic (54.5 C% bio-oil yield): Fe2 O3 (60.9 C%), MnO2 (59.0 C%), ZnO (63.5 C%), ZrO2 (58.5 C%) and Zr(WO4 )2 (65.6 C%). When 50 wt% water is added to the methanol medium, it is noticed that the gasification is greatly suppressed to less than 6 C%. At the same time, it tended to decrease the bio-oil yield, too, from 54.5 C% to 46.3 C% in the case of non-catalytic. However, some of the catalysts mitigated the decrease: MnO2 , ZnO and ZrO2 gave bio-oil yield of 55.5 C%, 59.5 C% and 57.6 C%, respectively. This trend may suggest that the employed catalysts improve the hydrogen transfer from the methanol (hydrogen donor solvent) to the fragments derived from the microalgae decomposition, resulting in stabilizing of those fragment and later cause increment of bio-oil yield
[24]. Overall, it may be said that many oxide catalysts contribute toward methanothermal liquefaction of microalgae under supercritical methanol environment, while with the presence of water (also supercritical), active catalysts for the present reaction may be limited, in the present case to MnO2 , ZnO and ZrO2 . 3.3. Mechanistic considerations and the role of catalysts It may be worthwhile to conjecture the mechanism of the present methanothermal liquefaction of Chlorella vulgaris. The reaction undoubtedly starts from the cell rapture to release its contents (lipids, proteins and carbohydrates) to reaction media. In the present study this happens at the temperature above 235 ◦ C (cf. Section 3.1). The cell contents released in the reaction media will then go through thermal and/or catalytic decomposition, methanolysis and hydrolysis (when water is present). These complex chains of reactions seem to occur faster with methanol-only media than in methanol-water mixed media (Section 3.1). Maybe partly due to this, methanothermal liquefaction of microalgae produces higher amount of bio-oil compared to hydrothermal (100 % water) liquefaction (cf. Fig. 3). Without catalysts, methanothermal reaction retains about 50 C% carbon in oil phase, while with active catalysts carbon retention in the oil phase increases to above 60 C%. Whether it is hydrothermal or alcohothermal, the amount of biooil produced far exceeds the amount of lipid present in the feed microalgae at the beginning, indicating that the present methanothermal liquefaction also converts proteins and carbohydrates into oil components. In order to examine the liquefaction mechanism further, correlations between carbon yields of heavy and light oils and their N and O contents are plotted in Fig. 4. When hydrothermal and methanothermal reactions are compared, there exist a distinct difference in light oil production between the reaction media: compare, for instance, open triangles (light oil yield from hydrothermal reaction) and open circles and squares (light oil yield from methanothermal reactions) in Fig. 4(a) or (b). Hydrothermal reactions give light oil yields between 16 and 21 wt% (open triangles) while methanothermal reactions give 31–44 wt% yields (open circles for 100 %
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Fig. 4. The correlations between the C yield of light (open symbols) and heavy oils (solid symbols) and N (a) and O (b) contents in the respective oil. Symbols: methanol only reaction media: 䊉 heavy oil and light oil; 50 wt% water-methanol reaction media: 䊏 heavy oil and 䊐 light oil; water only reaction media (within dotted circles): heavy oil and light oil.
methanol and open squares for 50 % methanol). Obviously, the presence of supercritical methanol in the reaction media (whether it is 100 % or 50 %) increases the lighter oil component about twice as much compared to hydrothermal reactions. This indicates active participation of supercritical methanol toward the production of the present light oil. Note that this is not the case for heavy oil components (closed symbols in Fig. 4(a) or (b)). Whether it is hydrothermal (closed triangles) or methanothermal (closed circles and squares), heavy oil yield falls within the range between 7 and 24 wt%. Let us further examine the reaction by following the variations of N content (which should originate from protein component in microalgae) and O content (majority of which should originate from carbohydrates in microalgae) in Fig. 4. As may be obvious in the figure, heavy oil (closed symbols) contains 5.9–8.0 wt% N, about 1.5–2 times higher than light oil (open symbols, 2.6–5.1 wt%) (Fig. 4(a)). Since the naturally occurring oil in the present feed (conventionally extracted oil, the rightmost bar graph in Fig. 3) is expected to contain very small amount of N, it is natural to conjecture that almost all the N in oils shown in Fig. 4(a) comes from the molecular components classified as protein in the feed. The N content in heavy oil, however, is 8.0 wt% at most in the present study, about one half of that of common protein which is 16 wt% (Nitrogen-protein conversion factor of 6.25). Thus, it may be said that about half of the heavy oil and about a quarter of the light oil obtained here originate from protein present in the feed microalga. Since naturally occurring heavy oil component is only 3.2 C% and light oil 14.5 C% (Fig. 3), the rest ((each produced oil) minus (naturally occurring oil) minus (oil originating from protein)) should have come from carbohydrate components of the feed microalga. The contribution from carbohydrates into these oil components seems complicated. Since the naturally occurring heavy oil is not much (cf. Fig. 3) roughly half of the heavy oil might have come from carbohydrates (and the other half from protein). However, O content of heavy oil (Fig. 4(b), 5.3 wt% to 15.9 wt%) seems much lower than expected for pristine carbohydrates. In case of light oil, this tendency of lower O content is more apparent: O content in light oil is even lower than in heavy oil, ranging from 0.7 wt% to 8.8 wt%. Considering the fact that naturally occurring triglycerides in this feed microalgae (14.5 C%, Fig. 3) should contain ca. 10 wt% of O, it seems that the low O content in light oil indicates possible deoxygenation occurring during the present supercritical liquefaction of microalga. This could happen in a way that aliphatic chain of O- or
N-containing molecules will be broken down and end up in light oil, while heteroatom-containing parts move to aqueous/alcohol phase. Note that this assumed deoxygenation seems to be there in 100 % water hydrothermal reactions (open triangles), too. The effect of active catalysts in the present methanothermal liquefaction appears to be to increase the bio-oil yield, particularly light oil yield, compared to non-catalytic reactions. Behavior of each catalyst, however, seems to depend on the reaction environment: under methanol only environment many oxide catalysts such as Fe2 O3 , MnO2 , ZnO, ZrO2 and Zr(WO4 )2 showed activity (higher bio-oil production), while under 50 wt% water environment active catalytic species are limited to MnO2 , ZnO and ZrO2 . How those catalysts work toward increasing bio-oil is not clear from the data obtained in the present experiments. Nevertheless, it is apparent that under the present supercritical media conditions alga cells are easily ruptured, as apparent from very low solid phase remaining after the reaction, and the cell contents are released in the supercritical phase for subsequent reaction. After the reaction the products are separated into less polar oil phase and more polar methanol or methanol/water soluble phase. Thus, increase of oil phase (bio-oil) necessarily means decrease of molecules in the polar phase, and this seems to happen by decomposition of O-containing molecule decomposition into polar and non-polar fragments.
4. Conclusions Alcohothermal liquefaction of wet Chlorella vulgaris was attempted by using supercritical methanol as reaction media, and the effects of water content as well as the effects of heterogeneous oxide catalysts were examined. This methanothermal liquefaction gave higher bio-oil yield compared to hydrothermal for non-catalytic and oxide-catalyzed reactions. Addition of water to the alcohothermal system somewhat retarded bio-oil formation. Bio-oil produced were found to be the mixture of natural lipid present in the feed microalgae and depolymerized protein and carbohydrates. Solid catalysts active for increasing bio-oil may be promoting decomposition of O-containing molecules into polar and non-polar components resulting in an increase in the non-polar components (bio-oil) and decreasing their loss to polar solvent phase.
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Contents lists available at ScienceDirect
The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu
Catalytic alcohothermal liquefaction of wet microalgae with supercritical methanol Fon Yee Han a , Masaharu Komiyama a,b,c,∗ , Yoshimitsu Uemura b , Nurul Ekmi Rabat a a
Chemical Engineering Department, Universiti Teknologi PETRONAS, 32610, Seri Iskandar, Perak Darul Ridzuan, Malaysia HICoE−Centre for Biofuel and Biochemical Research, Institute for Self-Sustainable Building, Universiti Teknologi PETRONAS, 32610, Seri Iskandar, Perak Darul Ridzuan, Malaysia c Clean Energy Research Center, University of Yamanashi, Takeda, Kofu, 400-8511, Japan b
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
• Wet Chlorella vulgaris was liquefied under supercritical methanol (methanothermal). • Over 60 % of feed carbon is liquefied into bio-oil. • Methanothermal reaction gave twice as much light oil than hydrothermal reaction. • Oxide catalysts such as Zr(WO4 )2 , MnO2 , ZnO2 and ZrO2 were active for the reaction.
a r t i c l e
i n f o
Article history: Received 28 September 2019 Received in revised form 26 November 2019 Accepted 26 November 2019 Available online 28 November 2019 Keywords: Microalgae Chlorella vulgaris Biofuel Heterogeneous catalyst Supercritical methanol Methanothermal liquefaction Alcohothermal liquefaction
a b s t r a c t Microalgae are promising feedstock for the production of biofuel due to their high productivity and low interference with food production for their cultivation and usage. Owing to their aquatic production environment, however, energy-intensive dewatering and drying processes may be required before their conversion to biofuel. In order to avoid these processes, their liquefaction through hydrothermal means has attracted attention, with extensive research works. The present research proposes and examines alcohothermal liquefaction of wet microalgae, in an attempt to modify and improve microalgae liquefaction characteristics. Thus supercritical methanol was employed as a reaction media for liquefaction of wet Chlorella vulgaris, and the effects of water presence as well as the effects of heterogeneous oxide catalysts were examined. This methanothermal liquefaction gave high bio-oil yield of 54.5 C% (feed microalgae basis) compared to 30.5 C% by hydrothermal for non-catalytic reaction at 385 ◦ C. With Zr(WO4 )2 catalyst methanothermal bio-oil yield increased further to 65.6 C%. The presence of water in the methanol medium somewhat decreased the bio-oil yield, while some of the catalysts mitigated the decrease: at 50 wt% water content in methanol, MnO2 , ZnO2 and ZrO2 gave bio-oil yield of 55.5 C%, 59.5 C% and 57.6 C%, respectively. Possible mechanism of catalytic methanothermal liquefaction of wet microalgae is discussed. © 2019 Published by Elsevier B.V.
∗ Corresponding author at: Chemical Engineering Department, Universiti Teknologi PETRONAS, 32610, Seri Iskandar, Perak Darul Ridzuan, Malaysia. E-mail addresses: [email protected], [email protected] (M. Komiyama). https://doi.org/10.1016/j.supflu.2019.104704 0896-8446/© 2019 Published by Elsevier B.V.
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1. Introduction Microalgae, aquatic microscopic photosynthetic unicellular organisms, are promising biomass feedstocks for the production of biofuel due to their high productivity and minimal interference with food production for their cultivation and usage [1]. Potential processes for microalgal biomass conversion to biofuel include: (i) biochemical conversion to ethanol or biogas (methane/CO2 mixture); (ii) thermochemical conversion to bio-oil and gaseous fuel; and (iii) oil extraction to produce biodiesel [2,3]. Except for the biochemical conversion process (i), other processes require energyintensive dewatering and drying steps before their conversion to biofuel. In process (iii) further pretreatments such as cell disruption and oil extraction are necessary before their triglyceride transesterification to biodiesel [4]. In order to avoid these energy-intensive and/or tedious processes, one-path or one step microalgae liquefaction through hydrothermal treatment has attracted attention. It applies high temperature (above 250 ◦ C) and pressure (above 5 MPa) to aqueous microalgae suspension, and bio-oil was produced as the main product together with water soluble component, gas and solid residue. The hydrothermal conversion of microalgal biomass to liquid fuels was examined by Yokoyama and his group in the 1990s [5–7], later followed by a few other groups [8–12]. It is noteworthy that hydrothermal liquefaction of microalgae produce oil more than the amount of natural lipid it contained originally [5], suggesting that the hydrothermal process not only extracts the naturally occurring oils in the algae but also produces oils from the non-lipid components of the algal biomass. It is also noted that energy balance analyses show that hydrothermal liquefaction of microalgae can be a net energy producer [6,7]. Detailed molecular analysis of bio-oil obtained by hydrothermal treatment of microalgae is given by Torri et al. [13]. Since major part of microalgal lipid is triglyceride, it may be natural to attempt direct transesterification of algal lipid without feed algal pretreatments such as cell disruption and lipid extraction. To this end, alcohols at the near critical conditions has been employed. Thus Levine, Pinnarat and Savage [14] devised a two-step direct transesterification of Chlorella vulgaris, in which in the first step wet algal biomass is reacted in subcritical water to hydrolyze intracellular lipids to be retained in conglomerate cells, and in the second step the wet fatty acid-rich solids undergo in situ transesterification with supercritical ethanol to produce biodiesel in the form of fatty acid ethyl esters (FAEEs). Deng et al. [15–17] used supercritical methanol and ethanol to directly extract and transesterify dry Nannochloropsis lipid to biodiesel. Jazzar et al. [18,19] employed supercritical methanol for direct in situ transesterification of dry Nannochloropsis and Chlorella lipids to biodiesel. Bi, He and McDonald [20] performed in situ transesterification of dry Schizochytrium limacinum lipid with sub- and super-critical methanol. Hegel et al. [21] extracted biodiesel from dry Neochloris oleoabundans by supercritical methanol. Shirazi, Karimi-Sabet and Ghotbi [22] examined direct transesterification of wet Spirulina with near critical methanol. In all those trials, transesterification of triglycerides is expected to proceed without acid or base catalyst, due to the nature of the near-critical (sub- or super-critical) alcohols [23]. All the above mentioned near-critical liquefaction work [14–22] were concerned only with fatty acid methyl (or ethyl) ester (FAME or FAEE) biodiesel production and not with bio-oil production. When the catalyst is present with supercritical alcohol, reactions that occur are not only extraction and transesterification of natural lipid present in the feed microalgae, but the whole bio-oil production is affected. Thus with the analogy to “hydrothermal” reaction or treatment [5–13], the overall reaction under near-critical alcohol will be termed here as “alcohothermal” reaction or treatment. To
Table 1 Elemental and chemical compositions of Chlorella vulgaris (dry basis)a . Elemental Composition (wt%) C H N S Ash Ob a b
47.37 9.85 10.89 0.72 3.32 27.85
Chemical Composition (wt%) Ash Lipid Protein Carbohydratesb
3.53 13.21 72.29 10.97
Moisture content as received: 5.85 wt %. Calculated by difference.
date, there exists only a few study on catalytic alcohothermal treatment of microalgae using near-critical alcohol. Huang et al. [24] treated dry Spirulina with near-critical ethanol and found that FeS increased bio-oil yield while decreasing solid residue. Zhang et al. [25] liquefied dry Chlorella pyrenoidosa in sub- and super-critical ethanol with HZSM-5 and Raney-Ni as heterogeneous catalysts. Jin et al. [26] examined the effect of metal chloride Lewis acid catalysts on the liquefaction characteristics of Chlorella pyrenoidosa under supercritical ethanol. They also examined the effect of moisture content in the feed microalgae in the range of 0−80 wt% and found that the moisture content in the feed is the most influential factor affecting the yield and properties of bio-oil produced. In this paper, catalytic alcohothermal liquefaction of wet Chlorella vulgaris with supercritical methanol was examined with various catalysts that are known to be stable under supercritical water conditions [27]. The effects of water content, reaction time and temperature, and the types of catalysts were examined. Possible microalgal alcohothermal liquefaction mechanism is also discussed. 2. Experimental 2.1. Materials The feed microalgae, Chlorella vulgaris, supplied by Chlorella Industry Co., Ltd. (Japan) was used as received in this research. It was in the form of fine green powder (average particle size: 76 m). Its elemental composition was determined using a Perkin Elmer CHNS/O analyzer Series II 2400, and chemical composition with standard analytical procedures, and are listed in Table 1. As alcohothermal solvents, methanol obtained from Merck (purity >99.9 %) and distilled water were used. Other chemicals employed were mostly of analytical grade and obtained from Merck, Sigma-Aldrich and Wako Pure Chemicals. Eight heterogeneous oxide catalysts, ␣-Al2 O3 , Fe2 O3 , MnO2 , ZnO, ZrO2 , Zr(WO4 )2 , LaFeO3 and LaMnO3 were examined. The choice of the catalysts is due to their stability in other supercritical water reactions [27]. Details of the catalyst source and preparation, as well as their BET surface area and metal oxide crystallite sizes are found elsewhere [27]. 2.2. Reaction procedure Methanothermal liquefaction of the microalgae was performed using a tube type batch reactor with a high-pressure valve connected for gas retrieval as described elsewhere [28]. A molten salt bath was used for heating purpose. A flowchart describing the reaction procedure as well as product analysis is given in Fig. 1. For the reaction, pre-determined amount of dried Chlorella vulgaris powder, catalyst, methanol and distilled water were charged into the batch reactor and closed tightly. Typical experiments were done with 4.0 g of reaction medium (methanol and/or distilled water), 0.4444 g of microalgae (1/9 feed/medium ratio), 0.0222 g of catalyst (5 wt% of feed) charged into an 8.5 mL volume reactor. The
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Fig. 1. Flowchart of methanothermal reaction and product analysis in the present catalytic supercritical microalgae liquefaction (DCM: dichloromethane).
Table 2 Critical temperature and pressure for respective reaction media. Water fraction in methanol-water medium (wt%)
Critical temperature (◦ C)
Critical pressure (MPa)
0 (methanol only) 50 100 (water only)
240.05 352.10 373.95
7.95 15.77 22.06
reactor was then immersed in the molten salt bath set at a desired temperature. In all the experiments, the reaction time was measured from the time the reactor was immersed in the salt bath. The reactor reached the set reaction temperature within 5 min [27]. Table 2 shows the critical temperature and pressure for pure solvents and for a methanol-water mixture calculated on the bases of the Lorentz-Berthelot type mixing rules. After running the reaction for a desired time (commonly 60 min), the reactor was taken out from the salt bath and put under running tap water to quench the reactor and terminate the reaction. For a comparison purpose, conventional oil extraction of the present microalgae was performed by the methanol-chloroform method, and then the obtained oil fraction was separated to hexane solubles (light oil) and nonsolubles (heavy oil).
20 mL distilled water and was centrifuged to separate the three phases (hexane phase, water phase, and non-dissolved phase) at 3000 rpm for 10 min. The hexane phase containing light oil was dried under an air stream and then in an oven at 105 ◦ C for 15 min. Water phase and non-dissolved phase were separated and dried at 90 ◦ C for 12 h. Here bio-oil that is soluble to hexane is termed “light oil” and dichloroethane-soluble but hexane-insoluble phase, “heavy oil” (Fig. 1). Both light oil and heavy oil are grouped together as “bio-oil” where necessary. The conversions, product yields and carbon yields of various products such as gas, light oil, heavy oil and solid residue were defined as follows. Conversion (C%) = 100 − Product yield (wt%) = Carbon yield (C%) =
g of Csolid residue g of Cdried algae
g of product × 100 g of dry algae input
g of Cproduct g of Cdry algae
× 100
(1) (2) (3)
3. Results and discussion 3.1. Effects of reaction temperature, catalyst amount and reaction time
2.3. Product analysis Once the reactor was cooled to room temperature, the connected high-pressure valve was opened, and the gas product was retrieved by using a gas bag. Its volume was measured by the water displacement method and its composition by a gas chromatograph (Shimadzu GC-8A with TCD detector and MolSieve-5A packed column). Then, the reactor was opened, and the reaction products were filtered (funnel with a folded filter paper, 2.5 m), and the liquid product was collected in a 50 mL glass vial. This way unreacted microalgae cells are separated as solid residue, along with ash and solid catalyst. The reactor was rinsed thrice with dichloromethane to ensure all the reaction products were collected. The liquid product was air-dried, washed thrice by mixing a 5 mL n-hexane and
First the effect of reaction temperature on the methanothermal liquefaction of dry and wet (as 50 wt% water mixed to reaction media) Chlorella vulgaris with ␣-Al2 O3 catalyst (5 wt% to the feed microalga) was examined in the temperature range of 185 ◦ C and 385 ◦ C, and the results are shown in Fig. 2. As apparent in Fig. 2(a), at a lower temperature (185 ◦ C), microalgae conversions (expressed by solid diamonds in the figure) are low (59 C% for 0 wt% water media and 71 C% for 50 wt% water media, respectively). As temperature increased to 235 ◦ C and above, solid residue yield (expressed with gray shaded bar graphs) suddenly decreased, and the conversion shot up and stayed at above 98 C%. This conversion increase seems to indicate that above this temperature and in near-critical methanol/water mixture, microalgae cells are almost
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Fig. 2. The effect of reaction temperature with ␣-Al2 O3 catalyst in 0 wt% water (methanol only medium) and 50 wt% water (methanol-water medium) on (a) product yield (bar graphs, left axis) and conversion (solid diamond, right axis); (b) carbon yield among the methanothermal reaction products. Reaction time is 60 min and catalyst loading, 5 wt%. Symbols: , water soluble component; 䊐, gas; , light oil; , heavy oil; , solid residue.
completely broken to be soluble to dichloromethane. Concurrent to this conversion increase (or solid residue decrease), the bio-oil yield (expressed by cross hatched bars for heavy oil and diagonally hatched bars for light oil in Fig. 2(a)) increased with the increase of temperature and reached the maximum of 38.9 wt% at 335 ◦ C for 0 wt% water media and 31.5 wt% at 385 ◦ C for 50 wt% water media, respectively. This finding is in agreement with the results of Zhang et al. [25] for the catalytic ethanothermal liquefaction of dry Chlorella pyrenoidosa. The decrease of bio-oil yield at 385 ◦ C for 0 wt% water media is apparently due to the sudden increase of gas component (expressed by blank bars) at that temperature at the cost of heavy oil. Note that this gas production was not observed with 50 wt% water media, and it is a marked effect of water presence in the present methanothermal microalgae liquefaction. The major component of the produced gas is always CO2 (> 50 %), with minor amount of methane, H2 and higher hydrocarbons. No significant difference of gas composition with the addition of water to the reaction medium was observed. It is also worth noting here that the bio-oil yield is almost three-times higher than the initial lipid content of feed Chlorella vulgaris (13.2 wt% on dry algae basis, Table 1). Both in 0 wt% and 50 wt% water mixed media, reaction temperature rise increased the yield of light oil at the cost of heavy oil.
The same observations hold in terms of carbon yields shown in Fig. 1(b). As may be apparent in the figure, almost half of the microalgae carbon was retained in bio-oil at higher reaction temperatures (> 235 ◦ C). The carbon distributed to the bio-oil inclined from 27.8 C% to 53.8 C% with increasing reaction temperature from 185 ◦ C to 385 ◦ C with 0 wt% water (methanol only) media and from 29.8 C% to 53.4 C % in the presence of 50 wt% water in the media. Furthermore, above 235 ◦ C with media water content of both 0 wt% and 50 wt%, gradual increase of light oil carbon is observed at the cost of heavy oil carbon and water-soluble carbon. When the reaction time is varied from 15 to 60 min at 385 ◦ C (data not shown), the conversion remained above 99 %, indicating that cell disruption occurs at the early stage of reaction. For 0 wt% water content, carbon yield of bio-oil varied little from 54.3 C% to 53.8 C% between the reaction time of 15–60 min, while for 50 wt% water content it increased from 42.5 C% to 53.4 C%. Apparently the liquefaction rate is slower with 50 wt% water present, but reaches at the similar level of conversion of methanol-only reaction at 60 min. Thus as the reaction time, 60 min is used throughout the present experiments. The effect of catalyst loading was also tested between 5 wt% to 50 wt% of feed microalgae using ␣-Al2 O3 catalyst (data not shown).
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Fig. 3. The effect of water content (0: methanol only; 50: 50 wt% water-methanol; 100: water only) in the reaction media and heterogeneous catalyst on carbon yield of each products in methanothermal liquefaction of Chlorella vulgaris. Reaction conditions: 385 ◦ C, 60 min, 5 wt% catalyst loading. Symbols: , water soluble component; 䊐, gas; , light oil; , heavy oil; , solid residue, * conventional extraction.
In terms of solid residue and bio-oil yield, 5 wt% catalyst outperformed 25 wt% and 50 wt%. Thus as catalyst loading 5 wt% of feed is employed throughout the present experiment. 3.2. Characteristics of catalytic methanothermal liquefaction Fig. 3 compares the carbon yields of methanothermal liquefaction products at 385 ◦ C from catalysis with eight metal oxides (␣-Al2 O3 , Fe2 O3 , MnO2 , ZnO, ZrO2 , LaFeO3 , LaMnO3 and Zr(WO4 )2 ) and non-catalytic reaction (blank), along with the conventionally extracted oil. First of all, it is immediately noticed by comparing 0 wt% water content (methanol only) and 100 % water content (water only) reactions for blank reaction that the methanothermal liquefaction gives much higher bio-oil C yield (sum of heavy oil and light oil C yields) than hydrothermal liquefaction: it gave bio-oil yield of 54.5 C% for methanothermal compared to 30.5 C% by hydrothermal. Similar phenomenon is observed for all of the catalytic reactions performed here. For instance, ZnO gave biooil yield of 63.5 C% for methanothermal compared to 33.6 C% by hydrothermal and Zr(WO4 )2 , 65.6 C% vs. 34.7 C%, respectively. When 0 wt% water (methanol only) cases are compared among the non-catalytic and catalytic reactions, first it is noted that gasification amount is relatively high: in all the cases examined here, 25–30 C% of the alga biomass is gasified. At the same time, above 50 C% of biomass is retained as bio-oil, and the following catalysts showed higher bio-oil C yield than non-catalytic (54.5 C% bio-oil yield): Fe2 O3 (60.9 C%), MnO2 (59.0 C%), ZnO (63.5 C%), ZrO2 (58.5 C%) and Zr(WO4 )2 (65.6 C%). When 50 wt% water is added to the methanol medium, it is noticed that the gasification is greatly suppressed to less than 6 C%. At the same time, it tended to decrease the bio-oil yield, too, from 54.5 C% to 46.3 C% in the case of non-catalytic. However, some of the catalysts mitigated the decrease: MnO2 , ZnO and ZrO2 gave bio-oil yield of 55.5 C%, 59.5 C% and 57.6 C%, respectively. This trend may suggest that the employed catalysts improve the hydrogen transfer from the methanol (hydrogen donor solvent) to the fragments derived from the microalgae decomposition, resulting in stabilizing of those fragment and later cause increment of bio-oil yield
[24]. Overall, it may be said that many oxide catalysts contribute toward methanothermal liquefaction of microalgae under supercritical methanol environment, while with the presence of water (also supercritical), active catalysts for the present reaction may be limited, in the present case to MnO2 , ZnO and ZrO2 . 3.3. Mechanistic considerations and the role of catalysts It may be worthwhile to conjecture the mechanism of the present methanothermal liquefaction of Chlorella vulgaris. The reaction undoubtedly starts from the cell rapture to release its contents (lipids, proteins and carbohydrates) to reaction media. In the present study this happens at the temperature above 235 ◦ C (cf. Section 3.1). The cell contents released in the reaction media will then go through thermal and/or catalytic decomposition, methanolysis and hydrolysis (when water is present). These complex chains of reactions seem to occur faster with methanol-only media than in methanol-water mixed media (Section 3.1). Maybe partly due to this, methanothermal liquefaction of microalgae produces higher amount of bio-oil compared to hydrothermal (100 % water) liquefaction (cf. Fig. 3). Without catalysts, methanothermal reaction retains about 50 C% carbon in oil phase, while with active catalysts carbon retention in the oil phase increases to above 60 C%. Whether it is hydrothermal or alcohothermal, the amount of biooil produced far exceeds the amount of lipid present in the feed microalgae at the beginning, indicating that the present methanothermal liquefaction also converts proteins and carbohydrates into oil components. In order to examine the liquefaction mechanism further, correlations between carbon yields of heavy and light oils and their N and O contents are plotted in Fig. 4. When hydrothermal and methanothermal reactions are compared, there exist a distinct difference in light oil production between the reaction media: compare, for instance, open triangles (light oil yield from hydrothermal reaction) and open circles and squares (light oil yield from methanothermal reactions) in Fig. 4(a) or (b). Hydrothermal reactions give light oil yields between 16 and 21 wt% (open triangles) while methanothermal reactions give 31–44 wt% yields (open circles for 100 %
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Fig. 4. The correlations between the C yield of light (open symbols) and heavy oils (solid symbols) and N (a) and O (b) contents in the respective oil. Symbols: methanol only reaction media: 䊉 heavy oil and light oil; 50 wt% water-methanol reaction media: 䊏 heavy oil and 䊐 light oil; water only reaction media (within dotted circles): heavy oil and light oil.
methanol and open squares for 50 % methanol). Obviously, the presence of supercritical methanol in the reaction media (whether it is 100 % or 50 %) increases the lighter oil component about twice as much compared to hydrothermal reactions. This indicates active participation of supercritical methanol toward the production of the present light oil. Note that this is not the case for heavy oil components (closed symbols in Fig. 4(a) or (b)). Whether it is hydrothermal (closed triangles) or methanothermal (closed circles and squares), heavy oil yield falls within the range between 7 and 24 wt%. Let us further examine the reaction by following the variations of N content (which should originate from protein component in microalgae) and O content (majority of which should originate from carbohydrates in microalgae) in Fig. 4. As may be obvious in the figure, heavy oil (closed symbols) contains 5.9–8.0 wt% N, about 1.5–2 times higher than light oil (open symbols, 2.6–5.1 wt%) (Fig. 4(a)). Since the naturally occurring oil in the present feed (conventionally extracted oil, the rightmost bar graph in Fig. 3) is expected to contain very small amount of N, it is natural to conjecture that almost all the N in oils shown in Fig. 4(a) comes from the molecular components classified as protein in the feed. The N content in heavy oil, however, is 8.0 wt% at most in the present study, about one half of that of common protein which is 16 wt% (Nitrogen-protein conversion factor of 6.25). Thus, it may be said that about half of the heavy oil and about a quarter of the light oil obtained here originate from protein present in the feed microalga. Since naturally occurring heavy oil component is only 3.2 C% and light oil 14.5 C% (Fig. 3), the rest ((each produced oil) minus (naturally occurring oil) minus (oil originating from protein)) should have come from carbohydrate components of the feed microalga. The contribution from carbohydrates into these oil components seems complicated. Since the naturally occurring heavy oil is not much (cf. Fig. 3) roughly half of the heavy oil might have come from carbohydrates (and the other half from protein). However, O content of heavy oil (Fig. 4(b), 5.3 wt% to 15.9 wt%) seems much lower than expected for pristine carbohydrates. In case of light oil, this tendency of lower O content is more apparent: O content in light oil is even lower than in heavy oil, ranging from 0.7 wt% to 8.8 wt%. Considering the fact that naturally occurring triglycerides in this feed microalgae (14.5 C%, Fig. 3) should contain ca. 10 wt% of O, it seems that the low O content in light oil indicates possible deoxygenation occurring during the present supercritical liquefaction of microalga. This could happen in a way that aliphatic chain of O- or
N-containing molecules will be broken down and end up in light oil, while heteroatom-containing parts move to aqueous/alcohol phase. Note that this assumed deoxygenation seems to be there in 100 % water hydrothermal reactions (open triangles), too. The effect of active catalysts in the present methanothermal liquefaction appears to be to increase the bio-oil yield, particularly light oil yield, compared to non-catalytic reactions. Behavior of each catalyst, however, seems to depend on the reaction environment: under methanol only environment many oxide catalysts such as Fe2 O3 , MnO2 , ZnO, ZrO2 and Zr(WO4 )2 showed activity (higher bio-oil production), while under 50 wt% water environment active catalytic species are limited to MnO2 , ZnO and ZrO2 . How those catalysts work toward increasing bio-oil is not clear from the data obtained in the present experiments. Nevertheless, it is apparent that under the present supercritical media conditions alga cells are easily ruptured, as apparent from very low solid phase remaining after the reaction, and the cell contents are released in the supercritical phase for subsequent reaction. After the reaction the products are separated into less polar oil phase and more polar methanol or methanol/water soluble phase. Thus, increase of oil phase (bio-oil) necessarily means decrease of molecules in the polar phase, and this seems to happen by decomposition of O-containing molecule decomposition into polar and non-polar fragments.
4. Conclusions Alcohothermal liquefaction of wet Chlorella vulgaris was attempted by using supercritical methanol as reaction media, and the effects of water content as well as the effects of heterogeneous oxide catalysts were examined. This methanothermal liquefaction gave higher bio-oil yield compared to hydrothermal for non-catalytic and oxide-catalyzed reactions. Addition of water to the alcohothermal system somewhat retarded bio-oil formation. Bio-oil produced were found to be the mixture of natural lipid present in the feed microalgae and depolymerized protein and carbohydrates. Solid catalysts active for increasing bio-oil may be promoting decomposition of O-containing molecules into polar and non-polar components resulting in an increase in the non-polar components (bio-oil) and decreasing their loss to polar solvent phase.
F.Y. Han, M. Komiyama, Y. Uemura et al. / J. of Supercritical Fluids 157 (2020) 104704
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