In situ heterogeneous transesterification of microalgae using combined ultrasound and microwave irradiation

Energy Conversion and Management 90 (2015) 41–46

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In situ heterogeneous transesterification of microalgae using combined ultrasound and microwave irradiation Guixia Ma a,b, Wenrong Hu a,c, Haiyan Pei a,c,⇑, Liqun Jiang a, Mingming Song a, Ruimin Mu b a

School of Environmental Science and Engineering, Shandong University, Jinan 250100, China School of Municipal and Environmental Engineering, Shandong Jianzhu University, Jinan 250101, China c Shandong Provincial Engineering Centre on Environmental Science & Technology, Jinan 250061, China b

a r t i c l e

i n f o

Article history: Received 25 June 2014 Accepted 28 October 2014

Keywords: Microalgae Biodiesel Ultrasound and microwave Transesterification Heterogeneous catalyst

a b s t r a c t Heterogeneous KF/CaO catalyst, prepared by the wet impregnation method with different KF loadings (15–35 wt.%) and calcination temperatures (600–1000 °C), was used to assist biodiesel production from microalgae Chlorella vulgaris under the combination of ultrasound and microwave (US–MW) irradiation. It was found that the catalyst calcinated at 900 °C with 25 wt.% KF loading presented the optimum performance, when the test was conducted with 12 wt.% of catalyst and a methanol to biomass ratio of 8:1 at 60 °C for 45 min. The highest fatty acid methyl esters (FAME) yield was 93.07 ± 2.39%, which was significantly higher than that achieved by using US or MW irradiation individually. The catalysts were characterized by X-ray diffraction, scanning electron microscopy and Brunauer–Emmett–Teller. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Bio-fuel is a more attractive sustainable energy source than traditional fossil fuels when we are facing the increasing challenge of energy crisis and environmental pollution. Microalgae have been intensively researched in recent years as a promising feedstock for biodiesel production, owing to the properties including rapid growth, high lipid content, CO2 fixation, and wastewater utilization and disposal [1–3]. One of the most common methods to produce biodiesel is the in situ transesterification of triglyceride (the lipid compound in microalgae) with the presence of methanol and catalysts, and the products are usually fatty acid methyl esters (FAME) or named biodiesel. Catalysts commonly compromise homogeneous and heterogeneous catalysts. However, compared to homogeneous catalysts, heterogeneous catalysts have attracted more attention in biodiesel production from vegetable oils or animal fats, due to the easy separation, recycling, and product purification [3]. Alkaline earth metal oxide – calcium oxide (CaO) has been widely evaluated as the catalyst in biodiesel production, owing to the easy availability and prominent cost effectiveness [4–6]. However, the catalytic activity of CaO is limited because of the low specific surface area and gels formation problem in methanol [7]. Hence, the modifications of ⇑ Corresponding author at: School of Environmental Science and Engineering, Shandong University, 27 Shanda Nan Road, Jinan 250100, China. Tel./fax: +86 531 88392983. E-mail address: [email protected] (H. Pei). http://dx.doi.org/10.1016/j.enconman.2014.10.061 0196-8904/Ó 2014 Elsevier Ltd. All rights reserved.

CaO were essential: for instance, Wen et al. [8] investigated KF/CaO as heterogeneous catalyst producing biodiesel from Chinese tallow seed oil and the high conversion efficiency was more than 96%; Liu et al. [9] found cinder supported CaO/KF exhibited perfect reusability and high biodiesel yield as a catalyst in soybean oil conversion. Table 1 summarizes the heterogeneous catalyst processes of biodiesel production using different kinds of feedstock with conventional heating method, ultrasound or microwaveassisted transesterification. It was found that the reaction time of conventional heating method was usually longer than 120 min [7–18], which could cause problems in heterogeneous (KF/CaO) transesterification of microalgae due to the rigid cell wall. Conversely, it was reported that ultrasound (US) or microwave (MW) irradiation assistance could significantly reduce the reaction time [19–22]. US or MW assisted catalysis could also improve the microalgae conversion efficiency [23–27]. Iqbal and Theegala [23] reported that using a MW – mediated method could reduce energy consumption, accelerate organic synthesis, and enhance reaction rate to short reaction time, because MW radiation delivered heat more effectively than convection and conduction [24]; but MW was not able to provide energy or break bonds [25]. US, especially in the low frequency range from 20 to 50 kHz, could generate acoustic cavitations, increasing the interfacial region of the reactants and disruption of algal cells, enhancing mass transfer, and accelerating the biodiesel production [26,27]. However, the limitation of US was low effective heating transfer ability [28]. Then, the combination of US and MW (US–MW) irradiation could overcome the limitations,

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which has been successfully designed and well documented in product synthesis, owing to reaction time reduction, decrease in energy consumption, improved and enhanced yield, and selection of products [29,30]. However, there has been little research on the combining of US–MW technology employed in heterogeneous transesterification of microalgae. The objectives of this paper were (1) to identify whether KF/CaO was an efficient catalyst for biodiesel production from microalgae and (2) to confirm the optimum in situ conversion conditions for the US–MW reactor, including the parameters, such as the calcination temperature of KF/CaO catalyst, KF loading, catalyst amount, methanol to biomass ratio and reaction time. Furthermore, the effect of US or MW alone on microalgae conversion was also investigated for comparison.

2. Materials and methods

2.3. Catalyst characterization To identify the crystalline phases of the catalysts calcinated at different temperature, X-ray diffraction (XRD) was performed by a Siemens D500 X-ray powder diffractometer, using a Cu Ka irradiation, over a 2h angle varying from 4° to 70° with a step width of 0.05° every 3 s. The morphologies of the calcinated catalysts were investigated using Scanning Electron Microscope (SEM; S570, Hitachi, Japan). The samples were mounted on copper stubs and sputter-coated with gold–palladium by a sputter coater (IB-3, EIKO, Japan) and photographed at a 15 kV accelerating voltage. The specific surface area was measured by the Brunauer– Emmett–Teller (BET) multipoint N2 adsorption and desorption method at liquid nitrogen temperature ( 196 °C) using a Micrometitics ASAP 2000 analyzer. 2.4. In situ transesterification

2.1. Materials Microalgae Chlorella vulgaris was provided by Tianjian biotechnology company (Binzhou, China) with the physical and chemical properties displayed in Table 2. Heptadecanoic acid methyl ester and FAME standards (10 mg/mL) were purchased from Sigma– Aldrich (USA). All reagents and solvents were either of HPLC or analytical grade purchased from local commercial suppliers. 2.2. Catalyst preparation A series of KF/CaO catalysts with different KF loadings (15–35wt.%) were prepared by the wet impregnation method. Because CaO was difficult to dissolved in water, Ca (NO3)2
4H2O was used as the precursor instead. The catalysts were prepared in a two-neck round bottom flask (100 mL) equipped with a temperature indicator and a magnetic stirrer. The impregnation was carried out at 60 °C under continuous stirring until the completion of the preparation process, and subsequently dried in an oven at 105 °C. The catalysts were then transferred to a muffle furnace and calcinated at different temperatures (600, 700, 800, 900 and 1000 °C) for 2 h for the optimum calcination temperature determination.

In the transesterification experiments, the mixture of dry microalgae (5 g) with a fixed amount of methanol and catalyst was employed in a US–MW synergistic extraction apparatus (CW2000, China), with a 100 mL flask and a cooling condenser, as shown in Fig. 1. The frequency of US and of MW was 40 kHz and 2450 MHz, respectively, and the power was 50 W and 10–80 W (self-regulated by the fixed temperature of 60 °C, there was a temperature indicator at the back of US–MW reactor). After the reaction was complete, the mixture was separated into two layers by adding 6 mL hexane. The upper layer (400 lL) contained hexane and FAME, which was then transferred into a clean vial (2 mL) with 100 lL heptadecanoic acid methyl ester (C17: 0, 2 mg/mL) as internal standard. The obtained FAME was qualitatively and quantitatively analyzed by GC–MS (Trace GC ultra and DSQ II) equipped with an automatic sampler (Thermo Fisher, USA) and a capillary column of VF-23 ms (30 m  0.25 mm  0.25 lm, Agilent). Helium was used as carrier gas. The oven temperature was set at 150 °C for 1 min, and sequentially heated up to 165 °C at a rate of 1 °C/min. The yield of FAME was expressed as its weight to the transesterifiable lipid presented in the microalgae biomass and the lipid extraction method and triglyceride analysis were according to literature reported by Ma et al. [31].

Table 1 Heterogeneous catalyst used in biodiesel production. Number

Feedstock

Heterogeneous catalysts

Calcination temperature (°C)

Optimal reaction condition

Maximum yield (%)

References

1 2 3 4 5 6 7 8 8 9 10 11

Zeolite supported CaO KF/CaO CaO/KF supported on cinder TiO2–ZnO2 Alumina/silica supported K2CO3 Cs/Al/Fe3O4 Ca/Al composite oxide MgAl hydrotalcites KF/CaO–Fe3O4 Mg–Zr solid base catalyst Hierarchical zeolites CaO/Al2O3 and Mg/Al2O3

600 450, 600 and 700 500 400 300, 600 and 1000 600 120–1000 507 and 700 600 600 550 500

180 min, 65 °C 150 min, 65 °C 20 min, 65 °C 300 min, 60 °C 300 min, 120 °C 120 min, 58 °C 180 min, 65 °C 240 min, 65 °C 180 min, 65 °C 240 min, 60 °C 240 min, 115 °C 240 min, 50 °C

95 >96 99.9 92.2 About 93 94.8 >94 97.1 95 – 47 97.5

[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

12

Soybean oil Chinese tallow seed oil Soybean oil Palm oil Sunflower oil Sunflower oil Rapeseed oil Vegetable oil Stillingia oil Nannochloropsis sp. Nannochloropsis oil Nannochloropsis oculata lipid Crude jatropha oil

–

50 min, 65 °C

84

[19]

13 14 15

Jatropha curcus oil Free fatty acid Vegetable oil

Alumina-supported heteropolyacid Na/SiO2 Sulfated zirconia ZnO/La2O2CO3

600 – 550

15 min, 50–70 °C 20 min, 60 °C 5 min, <100 °C

98.53 >90 >95

[20] [21] [22]

Note: Numbers 1–11 conducted by conventional method, 12–13 by ultrasound and 14–15 by microwave irradiation.

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G. Ma et al. / Energy Conversion and Management 90 (2015) 41–46 Table 2 Properties of microalgae Chlorella vulgaris. Properties

FFA content (wt.%)

Lipid content (wt.%)

Value

1.91

21.13

Fatty acid compositional profiles (% of total) C14: 0

C16: 0

C16: 1

C18: 0

C18: 1

C18: 2

C18: 3

1.98

43.85

8.195

4.355

5.55

36.08

1.98

of calcination temperature. The finest and most uniform distribution of particles on the support surface was observed at 900 °C. The nitrogen adsorption–desorption isotherm of catalyst calcinated at 900 °C, presented in Fig. 4, was similar to Type IV with mesoporous, which was probably the reason for high catalytic ability of catalyst and was consistent with the results reported by Hu et al. [32] and the high specific area and large pore were the major reason for high catalytic activity of KF/CaO. Additionally, the BET surface areas of KF/CaO from 15% to 35% calcinated at 900 °C are listed in Table 3. With an increase of KF loading from 15% to 35%, the surface area decreased from 68.04 to 29.45 m2/g, which could be attributed to the surface covering by the deposition of impregnation composition. 3.2. Effect of KF loading and calcination temperature on biodiesel yield

Fig. 1. Schematic diagram of US–MW synergistic extraction apparatus: (1) condenser, (2) flask (with energy conversion equipment), (3) US contactor and (4) control panel.

3. Results and discussion 3.1. Catalyst characterization Fig. 2 shows that the XRD of 25% KF loaded catalysts calcinated at different temperatures from 600 to 1000 °C. The results presented that the formed compounds on the surface were KCaF3 (2h angles of 28.7°, 41.3°, 51.7° and 64.1°), CaO (32.1°, 37.3°, 53.8° and 67.3°) and Ca(OH)2 (18.0°, 23.6°, 34.0°, 46.9° and 55.8°) according to the powder diffraction file (PDF) database. As calcination temperature rose, the Ca(OH)2 diffraction peak decreased, while the KCaF3 diffraction peak increased dramatically. It further indicated that the calcination temperature had great influence on the compositions of catalyst. The SEM photographs of catalysts are shown in Fig. 3. It was found that particle size and morphology varied with the increase

Calcination temperature was one of the major factors, affecting the crystal structure of catalysts [13]. In order to evaluate the effect of calcination temperature on microalgae conversion, 15–35% KF loadings of KF/CaO catalysts calcinated from 600 to 1000 °C for 2 h were investigated to evaluate the effect of calcination temperature on microalgae conversion at the reaction condition of catalyst content of 10%, methanol to biomass ratio of 10:1 and at 60 °C for 30 min. As shown in Fig. 5, the FAME yield increased with the increase of calcination temperature from 600 to 900 °C and KF loading from 15% to 25%, and the maximum yield of 76.45 ± 1.13% was obtained at 900 °C and 25% KF loading. It could be explained by the fact that, Ca(OH)2 diminished and KCaF3 increased with higher calcination temperature, just as Kouzu et al. [4] and Wu et al. [7] reported that Ca(OH)2 caused saponification formation and performed less efficiently than CaO, consequently exhibiting low catalytic ability in biodiesel production from soybean oil [4– 6]. Additionally, KCaF3 could improve saponification resistance, catalyst stabilization, and catalytic activity [7,8]. As the temperature was 900 °C, the main components of the catalyst were KCaF3, CaO, and, especially, KCaF3, which was reported to be the main active component of catalytic activity in transesterification reaction [8]. However, a higher calcination temperature of 1000 °C resulted in a lower FAME yield, which can be attributed to surface sintering at higher temperature (surface area decreased from 67.19 to 35.34 m2/g with temperature increasing from 600 to 1000 °C), leading to the decrease of surface area and causing the inefficiency in the catalytic ability of active sites. In addition, with the KF loading increasing, more active sites were formed, which significantly accelerated the conversion of FAME. However, overdosed KF loading such as above 25% caused a slight drop of FAME yield. It was probably caused by the agglomeration and the cover of basic active sites by the excess KF, leading to the decrease of surface area and the catalytic activity. Thus, the number of active sites was proportional to the catalytic performance and biodiesel conversion [33]. Therefore, the optimum KF loading was 25%, similar to the research reported by Wen et al. [8]. 3.3. Effect of reaction parameters

Fig. 2. XRD of 25 wt.% KF loaded catalyst at different calcination temperatures: 600–1000 °C.

3.3.1. Effect of catalyst content Catalyst concentration was an important factor directly influencing the biodiesel recovery. To confirm the optimum catalyst

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Fig. 3. SEM images of 25% KF loaded catalysts calcinated at different temperatures: 600–900 °C (a-d).

80

80

a b c d e

70 60

FAME yield (%)

Volume adsorbed (cm3/g)

70

50 40 30 20

60

50

40

10 30 0 0.0

0.2

0.4

0.6

0.8

1.0 15

Relative pressure (P/P0 )

20

25

30

35

KOH loading (%)

Fig. 4. N2 adsorption–desorption isotherm of catalyst calcinated at 900 °C.

Fig. 5. Influence of calcination temperature (a–e: 600–1000 °C) and KF loading on FAME yield with reaction conditions: methanol to biomass ratio of 10:1 and catalyst content of 10% at 60 °C for 30 min.

Table 3 The surface area of different KF loaded catalysts calcinated at 900 °C. Surface area (m2/g)

15 20 25 30 35

68.04 61.57 54.33 40.16 29.45

percentage, 4–14% (weight of catalyst/microalgae biomass) was studied at the test conditions of methanol to biomass ratio of 10:1 and temperature of 60 °C for 30 min. The results of FAME conversion are shown in Fig. 6, in which FAME yield increasing from 27.73 ± 2.23% to 86.35 ± 1.03% was observed with the increase of the catalyst amount from 4% to 12%. However, when the catalyst amount was below 12%, some reduction of FAME yield was noticed, probably because lower catalyst amount could not provide sufficient catalytic active sites, resulting in lower reaction rate and precluding the production of FAME; on the other hand, too great a catalyst amount of 14% might result in aggregation of catalyst particles, thus, leading to the limitation of mass transfer and also reducing the contact between active sites of catalysts and reactants [34]. Therefore, 12% catalyst was selected as the optimum catalyst content in this study. 3.3.2. Effect of methanol to biomass ratio As the production of biodiesel is a reversible reaction, which proceeds between triglyceride and methanol, and one mole of triglyceride reacts with three moles of methanol to produce three moles of biodiesel and one mole of glycerol (by-product), so extra

100

80

FAME yield (%)

KF loadings (%) (calcinated at 900 °C)

60

40

20

0 4

6

8

10

12

14

Catalyst content (wt. %) Fig. 6. Influence of catalyst amount on FAME yield with reaction conditions: methanol to biomass ratio of 10:1 at 60 °C for 30 min.

methanol is essential to accelerate the reaction to forward direction and support the synthesis of biodiesel. To investigate the effect of methanol to biomass ratio on the transesterification of microalgae, the methanol/biomass ratio varying from 4:1 to 12:1 (vol/wt) was carried out at the same reaction conditions of 12% catalyst and temperature of 60 °C for 30 min. As illustrated in Fig. 7, with the increase of methanol to biomass ratio, the biodiesel yield increased

G. Ma et al. / Energy Conversion and Management 90 (2015) 41–46

100

FAME yield (%)

80

60

40

20

0 4:1

6:1

8:1

10:1

12:1

Methanol ratio (vol/wt) Fig. 7. Influence of methanol to biomass ratio on FAME yield with reaction conditions: catalyst amount of 12% and at 60 °C for 30 min.

quickly and the maximum yield of 89.52 ± 1.88% was achieved at the ratio of 8:1. However, after exceeding 8:1, with further increase of methanol ratio, the yield decreased slightly. The reason was probably that methanol had two functions in the direct conversion of microalgae, one as extractor to extract intracellular lipid and another as reactant to produce biodiesel. Just as Velasquez-Orta et al. [35] reported, the recovery of FAME depended not only on the lipid content but also on the approachability to retrieve it from the microalgae cell to promote the release of lipid from intracellular structure of microalgae. Thus, a small quantity of methanol was not efficient to accelerate the release of lipid and lipid conversion, resulting in the lower yield of biodiesel at the beginning of the reaction. However, excess methanol would dilute the concentration of reactants (lipid and active sites of catalyst), reduce the contact among them and retard the conversion rate to give a lower yield [36]. Thus, the optimal methanol to biomass ratio was 8:1 and it would be employed for the subsequent research.

3.3.3. Effect of reaction time Reaction time was a crucial factor, which could not only determine the efficiency of biodiesel formation but also the quality of final products. In order to fully understand the effect of reaction

199

100 90

US-MW US MW

FAME yield (%)

80 70 60 50 40 30 20

45

time on biodiesel conversion, reaction times varying from 5 to 60 min (at 10 min interval) were investigated at catalyst content of 12%, methanol to oil ratio of 8:1 and 60 °C. As depicted in Fig. 8, within the first 45 min, the FAME yield increased rapidly with the extension of reaction time and the maximum yield of 93.07 ± 2.39% was obtained at 45 min by US–MW irradiation. This was probably because longer reaction time favored the disruption of cell wall, accelerated the release of triglyceride of microalgae and provided sufficient contact of the reactant mixtures, and then enhanced the formation of biodiesel [37]. However, after 45 min, there was a decrease of the yield, a probable reason being that too long a reaction time could induce excessive energy from US and MW irradiation, which would not only result in lipid oxidation and the formation of secondary products but also increase the operation cost [26]. Thus, 45 min was the optimal reaction time. To fully perceive the efficiency of US and MW irradiation on the biodiesel recovery, another two experiments were investigated, one was performed by MW irradiation (fixed temperature of 60 °C) and the other was conducted by US (the starting temperature being rom temperature at about 30 °C) with the optimum reaction parameters described above. From the data listed in Fig. 8, it was interesting to note that only 63.49 ± 4.69% and 58.12 ± 2.84% of yields were achieved at 45 min by US and MW irradiation alone. These two values were far lower than the 93.07 ± 2.39% of biodiesel yield given by the combination of US–MW, and at the even longer reaction time of 60 min, only 71.02 ± 3.28% and 67.34 ± 2.20% of yield were provided by US and MW irradiation, separately. It illustrated that US had higher efficiency than MW in assisting transesterification of microalgae, from which it could be concluded that the acoustic cavitations produced by US was responsible for the higher yield, and the same result was reported by Guldhe et al. [38] in biodiesel production from Scenedesmus sp. Additionally, the results also indicated that US or MW alone was inefficient in biodiesel production, which was not consistent with the literature reported by Koberg et al. [39] that 99.9% conversion was reached by MW irradiation only in 5 min. The probable reason was that 5 g of microalgae of C. vulgaris was investigated in our study which was far more than 1 g microalgae of Nannochloropsis studied by Koberg et al. [39]. It was known that MW radiation was a non-contact heating and the energy supplied would be utilized to heat the reaction mixture simultaneously due to methanol acting as a strong microwave absorbing solvent [40], which could accelerate the breakage of cells and enhance the release of triglyceride [23], but it could not provide sufficient contact among catalyst and reactants, especially in this reaction (a three-phase system) where mass transfer was limited. However, the cavitations and acoustic stream produced by US irradiation facilitated the mixing of the reactants and improved the production of biodiesel [41], but the temperature was low during the whole process, resulting in the low FAME yield. The same conclusion was reported by Martinez-Guerrea and Gude [40] that US lacks the ability to improve the reaction temperature and is less effective at low temperatures in FAME preparation. Thus, the simultaneous application of US and MW irradiation not only provided enough contact of reactants but also supplied the kind of reaction temperature that biodiesel production required. Therefore, heterogeneous catalysis combined with US and MW technique offered transesterification with a higher yield in a shorter reaction time.

10 0

10

20

30

40

50

60

70

Reaction time (min) Fig. 8. Influence of three different reaction methods at different reaction time on the FAME yield with the reaction conditions: catalyst amount of 12%, methanol to biomass ratio of 8:1.

4. Conclusion KF/CaO catalyst prepared by the wet impregnation method calcinated at 900 °C with 25 wt.% KF loading exhibited excellent catalytic activity on microalgae transesterification with the

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combination of US-MW technology. The highest FAME yield of 93.07 ± 2.39% was achieved under US–MW reactor while only 63.49 ± 4.69% and 58.12 ± 2.84% of FAME yield were offered by US and MW irradiation separately. This study demonstrated that combined US–MW irradiation would be an effective, energy efficient and time reducing technique in heterogeneous catalysis of microalgae to produce biodiesel.

Acknowledgments Funding for this research was provided by Natural Science Foundation of China (51078221), National Science Fund for Excellent Young Scholars (51322811), Science and Technology Development Planning of Shandong Province (2012GGE27027), the Program for New Century Excellent Talents in University of the Ministry of Education of China (Grant No. NCET-12-0341) and the Program for New Century Excellent Talents, Ministry of Education (Grant No. NCET-11-1030). The authors thank Findlay A. Nicol of Shandong University of Finance and Economics for revising the English in the manuscript.

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