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Study of KOH/Al2O3 as heterogeneous catalyst for biodiesel production via in situ transesterification from microalgae ab
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Guixia Ma , Wenrong Hu , Haiyan Pei , Liqun Jiang , Yan Ji & Ruimin Mu
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School of Environmental Science and Engineering, Shandong University, Jinan 250100, People's Republic of China b
School of Municipal and Environmental Engineering, Shandong Jianzhu University, Jinan 250101, People's Republic of China c
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Shandong Provincial Engineering Centre on Environmental Science & Technology, Jinan 250061, People's Republic of China Accepted author version posted online: 13 Aug 2014.Published online: 15 Sep 2014.
To cite this article: Guixia Ma, Wenrong Hu, Haiyan Pei, Liqun Jiang, Yan Ji & Ruimin Mu (2014): Study of KOH/Al2O3 as heterogeneous catalyst for biodiesel production via in situ transesterification from microalgae, Environmental Technology, DOI: 10.1080/09593330.2014.954629 To link to this article: http://dx.doi.org/10.1080/09593330.2014.954629
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Environmental Technology, 2014 http://dx.doi.org/10.1080/09593330.2014.954629
Study of KOH/Al2 O3 as heterogeneous catalyst for biodiesel production via in situ transesterification from microalgae Guixia Maa,b , Wenrong Hua,c , Haiyan Peia,c∗ , Liqun Jianga , Yan Jia and Ruimin Mub a School of Environmental Science and Engineering, Shandong University, Jinan 250100, People’s Republic of China; b School of Municipal and Environmental Engineering, Shandong Jianzhu University, Jinan 250101, People’s Republic of China; c Shandong Provincial Engineering Centre on Environmental Science & Technology, Jinan 250061, People’s Republic of China
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(Received 27 February 2014; final version received 10 August 2014 ) Heterogeneous KOH/Al2 O3 catalysts, synthesized by the wet impregnation method with different KOH loadings (20–40 wt%) and calcination temperatures from 400°C to 800°C, were used to produce biodiesel from Chlorella vulgaris biomass by in situ transesterification. The highest yield of biodiesel of 89.53 ± 1.58% was achieved at calcination temperature of 700°C for 2 h and 35 wt% loading of KOH, and at the optimal reaction condition of 10 wt% of catalyst content, 8 mL/g of methanol to biomass ratio and at 60°C for 5 h. The characteristics of the catalysts were analysed by X-ray diffraction, scanning electron microscopy and Brunauer–Emmett–Teller. Keywords: microalgae; biodiesel; in situ; transesterification; heterogeneous catalyst
1. Introduction Owing to energy crisis and environmental concerns, fatty acid methyl esters (FAME) or named biodiesel produced from microalgae as an alternative biofuel have been attracting considerable attention in recent decades.[1–3] Biodiesel production from microalgae generally involved catalytic transesterification of microalgae lipid with methanol in the presence of a catalyst. Homogenous alkaline catalysts such as sodium hydroxide, potassium hydroxide and alkoxide were usually used as liquid catalysts to produce biodiesel by the in situ transesterification method because of their higher reaction rates under moderate operation conditions compared with acid catalysts.[4,5] However, homogeneous-based catalysts have many drawbacks, for example, their sensitivity to free fatty acid and water would cause undesired saponification, which is difficult to separate from the products, and the neutralization of base catalyst and purification of products would generate a large amount of wastewater.[6–8] Thus, because of the easy separation from the reaction mixture and recycling after being activated, eco-friendly and effective heterogeneous catalysts have been widely used to produce biodiesel from vegetable oil in recent years.[7–12] From previous studies, potassium compounds (KNO3 , KOH, K2 CO3 , KF and KI) supported on different carriers, such as zeolite and alumina were reported to be effective for biodiesel preparation from soybean, sunflower and palm oil because of their high catalytic activity,[9,10,12] although the leaching of active species
*Corresponding author. Email: [email protected] © 2014 Taylor & Francis
on the surface is the main problem during the transesterification process.[13,14] Xie et al. [10] used the impregnation method to prepare a catalyst by loading KNO3 on alumina to produce biodiesel from soybean oil, and found solubility of the catalyst was in relation to the calcination temperature. In fact, calcination temperature was an important factor which significantly influences on the crystal, structure and the catalysis activity of the catalysts.[12] Hence, the selection of optimum calcination temperature of a catalyst would be a crucial step in the heterogeneous transesterification. Thus, this paper focuses on the optimization of calcination temperature of catalysts with different loadings of KOH loaded on alumina through microalgae transesterification process. Due to microalgae’s rigid cell wall, the biodiesel production by heterogeneous catalysts usually includes two-step procedures: lipid extraction followed by lipid transesterification, just as literatures reported by Carrero et al. [15] and Umdu et al. [16] using different heterogeneous catalysts to transesterify Nannochloropsis oil. But few literatures have reported about the in situ heterogeneous transesterification of microalgae. Therefore, the purpose of this study was to (1) investigate the optimal synthesis condition of the heterogeneous catalyst (KOH/Al2 O3 ) for microalgae transesterification, including calcination temperature, calcination time and KOH loading and (2) confirm the optimum reaction conditions of biodiesel production from microalgae Chlorella vulgaris by in situ transesterification. Parameters, such as
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catalyst content, methanol to biomass ratio, reaction time and water content of microalgae on the transesterification and potassium leakage were all evaluated in detail. 2.
Materials and methods
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2.1. Materials C. vulgaris biomass was provided by Tianjian biotechnology company (Binzhou, China) with lipid content of 17.09 ± 1.02 wt% (of dried microalgae biomass). Heptadecanoic acid methyl ester and FAME standards (10 mg/mL) were purchased from Sigma-Aldrich (USA). All reagents and solvents were either of chromatographic pure or analytical grade purchased from commercial sources. 2.2. Catalyst preparation A series of KOH/Al2 O3 catalysts with KOH loadings (20%, 25%, 30%, 35% and 40%; wt) were prepared by the impregnation method.[9,14] Because Al2 O3 was difficult to dissolve in water, Al(NO3 )3 ·9H2 O was used as its precursor in this process. The prepared catalyst was synthesized in a two-neck round bottom flask (100 mL) equipped with magnetic stirring and a temperature indicator. The impregnation process was performed at 60°C under continuous stirring until completion of the preparation, and subsequently dried in an oven at 105°C to remove surplus water. The catalysts were then calcinated in a muffle furnace at different temperatures (400°C, 500°C, 600°C, 700°C and 800°C) for 2 h, to determine the optimum calcination temperature. Additionally, calcination time from 1 to 4 h was also optimized under the optimal calcination temperature. 2.3. Catalyst characterization To identify the crystalline phases of the calcinated catalyst composition, X-ray diffraction (XRD) was performed using a Siemens D-500 diffract-meter with Cu kα irradiation, over a 2θ from 4° to 70° with a step width of 0.05° every 3 s. The surface morphology of the calcinated catalysts was characterized by scanning electronic microscopy (SEM; S570, Hitachi, Japan) at 15 kV. Samples were mounted on copper stubs and sputter coated with gold–palladium by a sputter coater (IB-3, EIKO, Japan). The specific surface area and pore diameter were measured by the Brunauer–Emmett–Teller (BET) method at liquid nitrogen temperature ( − 196°C) using a Micrometitics ASAP 2000 analyzer. 2.4. Lipid composition analysis The lipid extraction method was the same as described by Song et al. [1] and 1.00 g dried algae powder was extracted in this procedure. Fractionation and analysis of
lipid was performed by thin layer chromatography. Silica gel (10 × 20 cm) matrix with aluminium support was used to separate the compositions of biomass. The detail experimental method was according to Dong et al. [6], with minor modification. Spots were visualized by staining with iodine and then different compositions were incubated by methanol solution (5% H2 SO4 , v/v) at 60°C for 4 h. The composition of FAME was analysed by gas chromatography-mass spectrometry (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 μm, Agilent). The column temperature was kept at 150°C for 1 min, raised to 165°C at a rate of 1°C/min. The yield of FAME was expressed as its weight compared with the total transesterifiable lipids of biomass. 2.5. In situ transesterification In this transesterification, 5 g of dry microalgae with different methanol to biomass molar ratios (4–12 mL/g) and various amounts of catalysts (2–12 wt%) were employed. All the experiments were conducted in a 100 mL twoneck round flask equipped with a thermometer and a water-cooled condenser and carried out in a water bath at a constant temperature of 60°C under magnetic stir at 300 rpm. After each experiment, hexane (6 mL) was added to the solution and it was separated into two layers. The upper layer contained hexane and FAME and the lower layer contained the formation of glycerine, water and some residues. The upper layer solution (400 μL) was taken out and poured into a clean vial (2 mL) and then 100 μL heptadecanoic acid methyl ester (C17: 0, 2 mg/mL) was added as the internal standard of methyl ester, and then it was analysed by the method described in Section 2.4. Triplicate experiments and standard deviation were reported to check the reliability of the results. 3.
Results and discussion
3.1. Catalyst characterization 3.1.1. X-ray diffraction The XRD patterns of 35 wt% KOH/Al2 O3 catalysts calcinated at different temperatures (400–800°C) were examined to determine the changes of crystal structure of the catalysts, and the corresponding results are depicted in Figure 1. As can be seen, when the calcination temperature was 400°C, 500°C and 600°C, the main components were Al2 O3 and KAlO2. The characteristic diffraction peaks of Al2 O3 at 2θ value were 19°, 27°, 42° and 44°, and the 2θ value was 24° and 29° which were ascribed to KAlO2 according to the powder diffraction file database. With the increase in calcination temperature, the intensities of typical diffraction peaks (KAlO2 , Al2 O3 ) almost disappeared, which agreed with low-temperature form of KAlO2 [14] and a new diffraction peak at 2θ value of 31° and 59°
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Table 1. Surface area of catalysts calcinated at 700°C with different KOH loadings. Catalysts (calcinated at 700°C) Al2 O3 20% KOH/Al2 O3 25% KOH/Al2 O3 30% KOH/Al2 O3 35% KOH/Al2 O3 40% KOH/Al2 O3
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Figure 1. XRD of 35% wt. KOH loading catalyst at different calcination temperatures: 400–800°C.
appeared, which was due to new phase of K2 O species formed on the catalyst surface, consisting with K2 O formed at high temperature.[10] 3.1.2. SEM analysis The microscopic features of catalysts with 35 wt% KOH loading calcinated at 600°C, 700°C and 800°C are shown in Figure 2(a)–(c). With the increase in the temperature, the surface morphology of the catalysts changed distinctly. When the calcination temperature was 700°C, there was a good distribution of KOH on the support surface, but when temperature increased further to 800°C (shown in Figure 2(c)); an obvious change was found due to the particles congregated into a huge agglomeration. 3.1.3. BET surface area measurement The BET surface areas of KOH/Al2 O3 with different KOH loadings from 20 to 40 wt% calcinated at 700°C are listed in Table 1. The BET surface area of alumina was 121.02 m2 /g, while with the increase in KOH loading the surface area decreased significantly. The surface area reduced from 51.92 to 6.09 m2 /g when the KOH amount increased from 20 to 40 wt%. This was due to the surface (a)
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Surface area (m2 /g)
Pore volume (cm3 /g)
121.02 51.92 24.42 19.68 17.04 6.09
1.07 0.59 0.42 0.35 0.29 0.21
and pores of the catalyst being covered by the impregnation composition.
3.2.
Influence of catalyst preparation conditions on the conversion Calcination temperature was a crucial factor which determined the crystal and structure of catalysts. Thus, series of catalysts with KOH loading from 20 to 40 wt% calcinated at 400–800°C for 2 h were investigated at the reaction conditions: 12 wt% of catalyst content, 10 mL/g of methanol to biomass and at 60°C for 4 h. As shown in Figure 3, the FAME yield increased with the increase in calcination temperature from 400°C to 700°C and KOH loading from 20 to 35 wt%, and the maximum yield of 81.74 ± 2.38% was provided by the catalyst calcinated at 700°C with 35 wt% KOH loading. When the calcination temperature was 400–600°C, the yield of FAME obtained was low. This was presumably due to the lower catalytic ability of Al2 O3 and KAlO2 formed under these temperatures as descript in Figure 1. It is in agreement with literature reported by Xie and Li [11] and Li et al. [14] that Al2 O3 itself had no catalytic capacity on the transesterification and the basic and catalytic activity of KAlO2 was very low. However, as the temperature was 700°C, the main component of K2 O formed, which was attributed to the strong basic activity and catalytic performance.[14,17] But higher temperature above 700°C resulted in a drop of FAME yield, probably (c)
Figure 2. SEM images of 35 wt% loading KOH catalysts calcinated at different temperatures: (a) 600°C, (b) 700°C and (c) 800°C.
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FAME yield (%)
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caused by surface sintering at a higher temperature, leading to the decrease in surface area and causing the inefficiency catalytical ability of active sites. It further demonstrated that the catalytic activities significantly depended on the calcination temperature. In addition, KOH loading also significantly influence the yield of FAME, with the KOH amount increasing more active sites formed, which greatly accelerated the transesterification of FAME. However, too much KOH loading above 35 wt%, there was even a drop of FAME yield. It was probably caused by the agglomeration and the cover of basic active sites (K2 O) by the excess KOH, leading to the decrease in surface area of catalyst and then the catalytic activity.[7,10] Thus, the catalytic ability was proportional to the amount of active sites, which was responsible for conversion. Therefore, 35 wt% KOH loading was the optimum condition for catalyst preparation, which was similar to the literature reported by Soetaredjo et al. [9] and Noiroj et al. [17]. Calcination time was another important factor which would influence the catalyst morphological, structure and further affect the catalytic performance. Series of catalysts with 35 wt% KOH calcinated at 700°C were evaluated at different calcination times from 1 to 4 h under the same transesterification condition described above. As shown in Figure 4, FAME yield provided by catalyst calcinated time of 2 h was higher than that of 1 h, but had the similar results to that of 3 and 4 h. From the point of economy, calcination time of 2 h was selected for further investigation, and it also illustrated that calcination time was not a dominant factor compared with calcination temperature.
3.3. Influence of transesterification parameters 3.3.1. Influence of catalyst amount Figure 5 depicts the profile of microalgae conversion at different catalyst loadings from 2% to 12% (wt. of
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Figure 4. Influence of calcination time of catalyst calcinated at 700°C on FAME yield at the reaction conditions: methanol to biomass ratio of 10 mL/g, catalyst content of 12 wt% and at 60°C for 4 h. 90 80
FAME yield (%)
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Figure 3. Influence of calcination temperature and KOH loading on FAME yield (%) at the reaction conditions: methanol to biomass ratio of 10 mL/g, catalyst content of 12 wt% and at 60°C for 4 h.
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Figure 5. Influence of catalyst content on FAME yield at the reaction conditions: methanol to biomass ratio of 10 mL/g, KOH loading of 35 wt% and at 60°C for 4 h.
biomass). The reactions were performed at a methanol to biomass ratio of 10 mL/g and at 60°C for 4 h. As can be seen, FAME yield was increased from 17.21 ± 2.07% to 82.72 ± 1.62% when the catalyst loading increased from 2% to 10%. However, with further increase in catalyst loading above 10%, the conversion of FAME decreased. It was probably because lower catalyst amount could not provide enough catalytic active sites, resulting in a lower reaction rate and precluding the production of FAME, but too much catalyst amount might lead to aggregation of catalyst particles, thus, resulting in the mass transfer limitation and also decreasing the interaction among the reactants (active sites, reactants and methanol) and the lower yield of FAME. Thus, 10% of catalyst was the optimal amount in this study. 3.3.2. Influence of the ratio of methanol to biomass The influence of methanol to biomass ratio (4–12 mL/g) on microalgae transesterification was investigated. The ratio
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Table 2. Influence of water content on FAME yield and potassium ion leakage.
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1.58 1.54 1.37 2.14 2.10 1.33 1.96
97.75 135.45 153.87 178.93 253.08 274.10 298.13
± ± ± ± ± ± ±
5.13 4.54 3.28 5.87 4.08 3.78 3.21
3.96 5.49 6.23 7.25 10.25 11.10 12.08
Methanol to biomass ratio (mg/L)
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Figure 6. Influence of methanol to biomass ratio on FAME yield at the reaction conditions: catalyst amount of 10 wt% and at 60°C for 4 h.
of methanol to biomass less than 4 mL/g was not investigated because it was the minimum amount to immerge the biomass. As shown in Figure 6, when the methanol was 4 mL/g, the yield was only 55.75 ± 1.38%, as methanol had dual functions both as a solvent to extract lipid from microalgae and as a reactant to convert it into biodiesel in this process. Therefore, with the increase in the methanol ratio, the FAME yield increased quickly. The maximum yield of 83.07 ± 0.65% was obtained when the methanol to biomass ratio was 8 mL/g in this approach, as it was a reversible reaction and high ratio of methanol could shift the equilibrium to get more products. However, when the methanol amount increased above 10 mL/g, the FAME yield decreased, because excess methanol would dilute the lipid and catalyst concentration [18] and tend to extract more polar compounds such as phospholipids, protein, carbohydrate and sterols, which might hinder the FAME conversion [19] and decrease the contact opportunity between lipid and catalyst.[20–22] Therefore, the optimum methanol amount for this reaction was 8 mL/g. 3.3.3. Influence of reaction time The reaction time has a significant effect on the microalgae transesterification. To study the effect of reaction time on the FAME yield, the reaction time was varied from 1 to 6 h at the condition of methanol to biomass ratio of 8 mL/g, KOH loading of 35 wt%, catalyst amount of 10 wt% and at 60°C. As this was a three-phase (methanol, catalyst and lipid) reaction system, the microalgae lipid penetrated by methanol was low at the beginning of the reaction, resulting in mass transfer limitations and a lower FAME yield of 20.66 ± 1.34%. But with time increasing, the yield increased quickly and the maximum yield of 89.53 ± 1.58% was achieved at 5 h. Longer reaction time of 6 h was inefficient (FAME yield of 87.74 ± 1.93%) as it might result in solvent loss and by-product formation such as olefins and fatty alcohol.[6] Therefore, 5 h was considered as the optimum reaction time.
From the above analysis, it could be concluded that the optimum reaction conditions for this heterogeneous catalyst transesterification reaction were: 10 wt% of catalyst, 8 mL/g of methanol to microalgae biomass ratio and at 60°C for 5 h. 3.3.4.
Water tolerance of heterogeneous catalyst
In order to investigate the water tolerance of heterogeneous catalyst, different water contents from 0 to 6 wt% (to biomass) were conducted at the optimum reaction conditions. After each experiment, the solution was filtered and then digested with HNO3 in a microwave digestion system (Thermo Fisher, USA). The potassium concentration was detected by inductively coupled plasma-atomic emission spectrometer (Thermo Fisher, USA) and the potassium ion released by microalgae itself was deducted as background. The effect of water on KOH/Al2 O3 catalytic activity is listed in Table 2. The results illustrated that a higher FAME yield (more than 80%) was obtained when the water content was no more than 3 wt%. But afterwards, the yield rapidly decreased when the water content increased from 3 to 6 wt%. At the same time, the potassium concentration increased quickly. From this point, it could be concluded that the potassium leaching was probably the main reason of the reduction of FAME yield and the active sites (K2 O) in the presence of water would form KOH, which has no tolerance to any moisture in converting microalgae to biodiesel.[23] In fact, active species leakage was an unavoidable issue in the heterogeneous transesterification. Verziu et al. [24] reported that after fourth runs 1.5% potassium leached from KF/Al2 O3 . Chen et al. [25] found that approximately 29% of potassium leached from K/γ -Al2 O3 after five successive cycles. Noiroj et al. [17] investigate that large amount of 51.26% of potassium leached from KOH/Al2 O3 during soybean oil transesterification. Compared with the data listed in Table 2, the leaching of potassium was not seriously with higher water content of 6 wt% during transesterification reaction. It could be reasonable concluding that the catalyst of KOH/Al2 O3 was stable for a limited numbers of reusability. As described above, the catalyst of KOH/Al2 O3 calcinated at a higher temperature featured
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good water tolerance than KOH itself and more suitable to produce biodiesel from microalgae.
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4. Conclusion Heterogeneous KOH/Al2 O3 catalysts synthesized by the wet impregnation method exhibited high tolerance to water and excellent catalytic ability on in situ transesterification of C. vulgaris microalgae. Regarding the catalysts performance, the most efficient was calcinated at 700°C with 35 wt% KOH loading. The highest FAME yield of 89.53 ± 1.58% was achieved at 5 h when the reaction was carried out at 60°C with 10 wt% catalyst and a methanol to biomass ratio of 8 mL/g. The main composition of K2 O formed by the KOH thermal decomposition was responsible for the catalytic activity to this reaction. Acknowledgements 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) and The Program for New Century Excellent Talents in University of the Ministry of Education of China (Grant No. NCET-20341). The authors thank Findlay A. Nicol of Shandong University of Finance and Economics for revising the English in the manuscript.
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Study of KOH/Al2O3 as heterogeneous catalyst for biodiesel production via in situ transesterification from microalgae ab
ac
ac
a
a
Guixia Ma , Wenrong Hu , Haiyan Pei , Liqun Jiang , Yan Ji & Ruimin Mu
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School of Environmental Science and Engineering, Shandong University, Jinan 250100, People's Republic of China b
School of Municipal and Environmental Engineering, Shandong Jianzhu University, Jinan 250101, People's Republic of China c
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Shandong Provincial Engineering Centre on Environmental Science & Technology, Jinan 250061, People's Republic of China Accepted author version posted online: 13 Aug 2014.Published online: 15 Sep 2014.
To cite this article: Guixia Ma, Wenrong Hu, Haiyan Pei, Liqun Jiang, Yan Ji & Ruimin Mu (2014): Study of KOH/Al2O3 as heterogeneous catalyst for biodiesel production via in situ transesterification from microalgae, Environmental Technology, DOI: 10.1080/09593330.2014.954629 To link to this article: http://dx.doi.org/10.1080/09593330.2014.954629
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Environmental Technology, 2014 http://dx.doi.org/10.1080/09593330.2014.954629
Study of KOH/Al2 O3 as heterogeneous catalyst for biodiesel production via in situ transesterification from microalgae Guixia Maa,b , Wenrong Hua,c , Haiyan Peia,c∗ , Liqun Jianga , Yan Jia and Ruimin Mub a School of Environmental Science and Engineering, Shandong University, Jinan 250100, People’s Republic of China; b School of Municipal and Environmental Engineering, Shandong Jianzhu University, Jinan 250101, People’s Republic of China; c Shandong Provincial Engineering Centre on Environmental Science & Technology, Jinan 250061, People’s Republic of China
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(Received 27 February 2014; final version received 10 August 2014 ) Heterogeneous KOH/Al2 O3 catalysts, synthesized by the wet impregnation method with different KOH loadings (20–40 wt%) and calcination temperatures from 400°C to 800°C, were used to produce biodiesel from Chlorella vulgaris biomass by in situ transesterification. The highest yield of biodiesel of 89.53 ± 1.58% was achieved at calcination temperature of 700°C for 2 h and 35 wt% loading of KOH, and at the optimal reaction condition of 10 wt% of catalyst content, 8 mL/g of methanol to biomass ratio and at 60°C for 5 h. The characteristics of the catalysts were analysed by X-ray diffraction, scanning electron microscopy and Brunauer–Emmett–Teller. Keywords: microalgae; biodiesel; in situ; transesterification; heterogeneous catalyst
1. Introduction Owing to energy crisis and environmental concerns, fatty acid methyl esters (FAME) or named biodiesel produced from microalgae as an alternative biofuel have been attracting considerable attention in recent decades.[1–3] Biodiesel production from microalgae generally involved catalytic transesterification of microalgae lipid with methanol in the presence of a catalyst. Homogenous alkaline catalysts such as sodium hydroxide, potassium hydroxide and alkoxide were usually used as liquid catalysts to produce biodiesel by the in situ transesterification method because of their higher reaction rates under moderate operation conditions compared with acid catalysts.[4,5] However, homogeneous-based catalysts have many drawbacks, for example, their sensitivity to free fatty acid and water would cause undesired saponification, which is difficult to separate from the products, and the neutralization of base catalyst and purification of products would generate a large amount of wastewater.[6–8] Thus, because of the easy separation from the reaction mixture and recycling after being activated, eco-friendly and effective heterogeneous catalysts have been widely used to produce biodiesel from vegetable oil in recent years.[7–12] From previous studies, potassium compounds (KNO3 , KOH, K2 CO3 , KF and KI) supported on different carriers, such as zeolite and alumina were reported to be effective for biodiesel preparation from soybean, sunflower and palm oil because of their high catalytic activity,[9,10,12] although the leaching of active species
*Corresponding author. Email: [email protected] © 2014 Taylor & Francis
on the surface is the main problem during the transesterification process.[13,14] Xie et al. [10] used the impregnation method to prepare a catalyst by loading KNO3 on alumina to produce biodiesel from soybean oil, and found solubility of the catalyst was in relation to the calcination temperature. In fact, calcination temperature was an important factor which significantly influences on the crystal, structure and the catalysis activity of the catalysts.[12] Hence, the selection of optimum calcination temperature of a catalyst would be a crucial step in the heterogeneous transesterification. Thus, this paper focuses on the optimization of calcination temperature of catalysts with different loadings of KOH loaded on alumina through microalgae transesterification process. Due to microalgae’s rigid cell wall, the biodiesel production by heterogeneous catalysts usually includes two-step procedures: lipid extraction followed by lipid transesterification, just as literatures reported by Carrero et al. [15] and Umdu et al. [16] using different heterogeneous catalysts to transesterify Nannochloropsis oil. But few literatures have reported about the in situ heterogeneous transesterification of microalgae. Therefore, the purpose of this study was to (1) investigate the optimal synthesis condition of the heterogeneous catalyst (KOH/Al2 O3 ) for microalgae transesterification, including calcination temperature, calcination time and KOH loading and (2) confirm the optimum reaction conditions of biodiesel production from microalgae Chlorella vulgaris by in situ transesterification. Parameters, such as
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catalyst content, methanol to biomass ratio, reaction time and water content of microalgae on the transesterification and potassium leakage were all evaluated in detail. 2.
Materials and methods
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2.1. Materials C. vulgaris biomass was provided by Tianjian biotechnology company (Binzhou, China) with lipid content of 17.09 ± 1.02 wt% (of dried microalgae biomass). Heptadecanoic acid methyl ester and FAME standards (10 mg/mL) were purchased from Sigma-Aldrich (USA). All reagents and solvents were either of chromatographic pure or analytical grade purchased from commercial sources. 2.2. Catalyst preparation A series of KOH/Al2 O3 catalysts with KOH loadings (20%, 25%, 30%, 35% and 40%; wt) were prepared by the impregnation method.[9,14] Because Al2 O3 was difficult to dissolve in water, Al(NO3 )3 ·9H2 O was used as its precursor in this process. The prepared catalyst was synthesized in a two-neck round bottom flask (100 mL) equipped with magnetic stirring and a temperature indicator. The impregnation process was performed at 60°C under continuous stirring until completion of the preparation, and subsequently dried in an oven at 105°C to remove surplus water. The catalysts were then calcinated in a muffle furnace at different temperatures (400°C, 500°C, 600°C, 700°C and 800°C) for 2 h, to determine the optimum calcination temperature. Additionally, calcination time from 1 to 4 h was also optimized under the optimal calcination temperature. 2.3. Catalyst characterization To identify the crystalline phases of the calcinated catalyst composition, X-ray diffraction (XRD) was performed using a Siemens D-500 diffract-meter with Cu kα irradiation, over a 2θ from 4° to 70° with a step width of 0.05° every 3 s. The surface morphology of the calcinated catalysts was characterized by scanning electronic microscopy (SEM; S570, Hitachi, Japan) at 15 kV. Samples were mounted on copper stubs and sputter coated with gold–palladium by a sputter coater (IB-3, EIKO, Japan). The specific surface area and pore diameter were measured by the Brunauer–Emmett–Teller (BET) method at liquid nitrogen temperature ( − 196°C) using a Micrometitics ASAP 2000 analyzer. 2.4. Lipid composition analysis The lipid extraction method was the same as described by Song et al. [1] and 1.00 g dried algae powder was extracted in this procedure. Fractionation and analysis of
lipid was performed by thin layer chromatography. Silica gel (10 × 20 cm) matrix with aluminium support was used to separate the compositions of biomass. The detail experimental method was according to Dong et al. [6], with minor modification. Spots were visualized by staining with iodine and then different compositions were incubated by methanol solution (5% H2 SO4 , v/v) at 60°C for 4 h. The composition of FAME was analysed by gas chromatography-mass spectrometry (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 μm, Agilent). The column temperature was kept at 150°C for 1 min, raised to 165°C at a rate of 1°C/min. The yield of FAME was expressed as its weight compared with the total transesterifiable lipids of biomass. 2.5. In situ transesterification In this transesterification, 5 g of dry microalgae with different methanol to biomass molar ratios (4–12 mL/g) and various amounts of catalysts (2–12 wt%) were employed. All the experiments were conducted in a 100 mL twoneck round flask equipped with a thermometer and a water-cooled condenser and carried out in a water bath at a constant temperature of 60°C under magnetic stir at 300 rpm. After each experiment, hexane (6 mL) was added to the solution and it was separated into two layers. The upper layer contained hexane and FAME and the lower layer contained the formation of glycerine, water and some residues. The upper layer solution (400 μL) was taken out and poured into a clean vial (2 mL) and then 100 μL heptadecanoic acid methyl ester (C17: 0, 2 mg/mL) was added as the internal standard of methyl ester, and then it was analysed by the method described in Section 2.4. Triplicate experiments and standard deviation were reported to check the reliability of the results. 3.
Results and discussion
3.1. Catalyst characterization 3.1.1. X-ray diffraction The XRD patterns of 35 wt% KOH/Al2 O3 catalysts calcinated at different temperatures (400–800°C) were examined to determine the changes of crystal structure of the catalysts, and the corresponding results are depicted in Figure 1. As can be seen, when the calcination temperature was 400°C, 500°C and 600°C, the main components were Al2 O3 and KAlO2. The characteristic diffraction peaks of Al2 O3 at 2θ value were 19°, 27°, 42° and 44°, and the 2θ value was 24° and 29° which were ascribed to KAlO2 according to the powder diffraction file database. With the increase in calcination temperature, the intensities of typical diffraction peaks (KAlO2 , Al2 O3 ) almost disappeared, which agreed with low-temperature form of KAlO2 [14] and a new diffraction peak at 2θ value of 31° and 59°
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Table 1. Surface area of catalysts calcinated at 700°C with different KOH loadings. Catalysts (calcinated at 700°C) Al2 O3 20% KOH/Al2 O3 25% KOH/Al2 O3 30% KOH/Al2 O3 35% KOH/Al2 O3 40% KOH/Al2 O3
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Figure 1. XRD of 35% wt. KOH loading catalyst at different calcination temperatures: 400–800°C.
appeared, which was due to new phase of K2 O species formed on the catalyst surface, consisting with K2 O formed at high temperature.[10] 3.1.2. SEM analysis The microscopic features of catalysts with 35 wt% KOH loading calcinated at 600°C, 700°C and 800°C are shown in Figure 2(a)–(c). With the increase in the temperature, the surface morphology of the catalysts changed distinctly. When the calcination temperature was 700°C, there was a good distribution of KOH on the support surface, but when temperature increased further to 800°C (shown in Figure 2(c)); an obvious change was found due to the particles congregated into a huge agglomeration. 3.1.3. BET surface area measurement The BET surface areas of KOH/Al2 O3 with different KOH loadings from 20 to 40 wt% calcinated at 700°C are listed in Table 1. The BET surface area of alumina was 121.02 m2 /g, while with the increase in KOH loading the surface area decreased significantly. The surface area reduced from 51.92 to 6.09 m2 /g when the KOH amount increased from 20 to 40 wt%. This was due to the surface (a)
(b)
Surface area (m2 /g)
Pore volume (cm3 /g)
121.02 51.92 24.42 19.68 17.04 6.09
1.07 0.59 0.42 0.35 0.29 0.21
and pores of the catalyst being covered by the impregnation composition.
3.2.
Influence of catalyst preparation conditions on the conversion Calcination temperature was a crucial factor which determined the crystal and structure of catalysts. Thus, series of catalysts with KOH loading from 20 to 40 wt% calcinated at 400–800°C for 2 h were investigated at the reaction conditions: 12 wt% of catalyst content, 10 mL/g of methanol to biomass and at 60°C for 4 h. As shown in Figure 3, the FAME yield increased with the increase in calcination temperature from 400°C to 700°C and KOH loading from 20 to 35 wt%, and the maximum yield of 81.74 ± 2.38% was provided by the catalyst calcinated at 700°C with 35 wt% KOH loading. When the calcination temperature was 400–600°C, the yield of FAME obtained was low. This was presumably due to the lower catalytic ability of Al2 O3 and KAlO2 formed under these temperatures as descript in Figure 1. It is in agreement with literature reported by Xie and Li [11] and Li et al. [14] that Al2 O3 itself had no catalytic capacity on the transesterification and the basic and catalytic activity of KAlO2 was very low. However, as the temperature was 700°C, the main component of K2 O formed, which was attributed to the strong basic activity and catalytic performance.[14,17] But higher temperature above 700°C resulted in a drop of FAME yield, probably (c)
Figure 2. SEM images of 35 wt% loading KOH catalysts calcinated at different temperatures: (a) 600°C, (b) 700°C and (c) 800°C.
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70
80 70
FAME yield (%)
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FAME yield (%)
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20 % 25 % 30 % 35 % 40 %
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0 400
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Calcination temperature (°C)
caused by surface sintering at a higher temperature, leading to the decrease in surface area and causing the inefficiency catalytical ability of active sites. It further demonstrated that the catalytic activities significantly depended on the calcination temperature. In addition, KOH loading also significantly influence the yield of FAME, with the KOH amount increasing more active sites formed, which greatly accelerated the transesterification of FAME. However, too much KOH loading above 35 wt%, there was even a drop of FAME yield. It was probably caused by the agglomeration and the cover of basic active sites (K2 O) by the excess KOH, leading to the decrease in surface area of catalyst and then the catalytic activity.[7,10] Thus, the catalytic ability was proportional to the amount of active sites, which was responsible for conversion. Therefore, 35 wt% KOH loading was the optimum condition for catalyst preparation, which was similar to the literature reported by Soetaredjo et al. [9] and Noiroj et al. [17]. Calcination time was another important factor which would influence the catalyst morphological, structure and further affect the catalytic performance. Series of catalysts with 35 wt% KOH calcinated at 700°C were evaluated at different calcination times from 1 to 4 h under the same transesterification condition described above. As shown in Figure 4, FAME yield provided by catalyst calcinated time of 2 h was higher than that of 1 h, but had the similar results to that of 3 and 4 h. From the point of economy, calcination time of 2 h was selected for further investigation, and it also illustrated that calcination time was not a dominant factor compared with calcination temperature.
3.3. Influence of transesterification parameters 3.3.1. Influence of catalyst amount Figure 5 depicts the profile of microalgae conversion at different catalyst loadings from 2% to 12% (wt. of
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Figure 4. Influence of calcination time of catalyst calcinated at 700°C on FAME yield at the reaction conditions: methanol to biomass ratio of 10 mL/g, catalyst content of 12 wt% and at 60°C for 4 h. 90 80
FAME yield (%)
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Figure 3. Influence of calcination temperature and KOH loading on FAME yield (%) at the reaction conditions: methanol to biomass ratio of 10 mL/g, catalyst content of 12 wt% and at 60°C for 4 h.
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Calcination time (h)
70 60 50 40 30 20 10 2
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Figure 5. Influence of catalyst content on FAME yield at the reaction conditions: methanol to biomass ratio of 10 mL/g, KOH loading of 35 wt% and at 60°C for 4 h.
biomass). The reactions were performed at a methanol to biomass ratio of 10 mL/g and at 60°C for 4 h. As can be seen, FAME yield was increased from 17.21 ± 2.07% to 82.72 ± 1.62% when the catalyst loading increased from 2% to 10%. However, with further increase in catalyst loading above 10%, the conversion of FAME decreased. It was probably because lower catalyst amount could not provide enough catalytic active sites, resulting in a lower reaction rate and precluding the production of FAME, but too much catalyst amount might lead to aggregation of catalyst particles, thus, resulting in the mass transfer limitation and also decreasing the interaction among the reactants (active sites, reactants and methanol) and the lower yield of FAME. Thus, 10% of catalyst was the optimal amount in this study. 3.3.2. Influence of the ratio of methanol to biomass The influence of methanol to biomass ratio (4–12 mL/g) on microalgae transesterification was investigated. The ratio
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Table 2. Influence of water content on FAME yield and potassium ion leakage.
85 80
FAME yield (%)
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Water content (wt%)
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FAME yield (%)
Potassium ion concentration (mg/L)
Potassium leaching (wt%)
70 65 60 55 50 4
6
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89.53 88.39 84.55 80.08 54.32 33.11 29.14
± ± ± ± ± ± ±
1.58 1.54 1.37 2.14 2.10 1.33 1.96
97.75 135.45 153.87 178.93 253.08 274.10 298.13
± ± ± ± ± ± ±
5.13 4.54 3.28 5.87 4.08 3.78 3.21
3.96 5.49 6.23 7.25 10.25 11.10 12.08
Methanol to biomass ratio (mg/L)
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Figure 6. Influence of methanol to biomass ratio on FAME yield at the reaction conditions: catalyst amount of 10 wt% and at 60°C for 4 h.
of methanol to biomass less than 4 mL/g was not investigated because it was the minimum amount to immerge the biomass. As shown in Figure 6, when the methanol was 4 mL/g, the yield was only 55.75 ± 1.38%, as methanol had dual functions both as a solvent to extract lipid from microalgae and as a reactant to convert it into biodiesel in this process. Therefore, with the increase in the methanol ratio, the FAME yield increased quickly. The maximum yield of 83.07 ± 0.65% was obtained when the methanol to biomass ratio was 8 mL/g in this approach, as it was a reversible reaction and high ratio of methanol could shift the equilibrium to get more products. However, when the methanol amount increased above 10 mL/g, the FAME yield decreased, because excess methanol would dilute the lipid and catalyst concentration [18] and tend to extract more polar compounds such as phospholipids, protein, carbohydrate and sterols, which might hinder the FAME conversion [19] and decrease the contact opportunity between lipid and catalyst.[20–22] Therefore, the optimum methanol amount for this reaction was 8 mL/g. 3.3.3. Influence of reaction time The reaction time has a significant effect on the microalgae transesterification. To study the effect of reaction time on the FAME yield, the reaction time was varied from 1 to 6 h at the condition of methanol to biomass ratio of 8 mL/g, KOH loading of 35 wt%, catalyst amount of 10 wt% and at 60°C. As this was a three-phase (methanol, catalyst and lipid) reaction system, the microalgae lipid penetrated by methanol was low at the beginning of the reaction, resulting in mass transfer limitations and a lower FAME yield of 20.66 ± 1.34%. But with time increasing, the yield increased quickly and the maximum yield of 89.53 ± 1.58% was achieved at 5 h. Longer reaction time of 6 h was inefficient (FAME yield of 87.74 ± 1.93%) as it might result in solvent loss and by-product formation such as olefins and fatty alcohol.[6] Therefore, 5 h was considered as the optimum reaction time.
From the above analysis, it could be concluded that the optimum reaction conditions for this heterogeneous catalyst transesterification reaction were: 10 wt% of catalyst, 8 mL/g of methanol to microalgae biomass ratio and at 60°C for 5 h. 3.3.4.
Water tolerance of heterogeneous catalyst
In order to investigate the water tolerance of heterogeneous catalyst, different water contents from 0 to 6 wt% (to biomass) were conducted at the optimum reaction conditions. After each experiment, the solution was filtered and then digested with HNO3 in a microwave digestion system (Thermo Fisher, USA). The potassium concentration was detected by inductively coupled plasma-atomic emission spectrometer (Thermo Fisher, USA) and the potassium ion released by microalgae itself was deducted as background. The effect of water on KOH/Al2 O3 catalytic activity is listed in Table 2. The results illustrated that a higher FAME yield (more than 80%) was obtained when the water content was no more than 3 wt%. But afterwards, the yield rapidly decreased when the water content increased from 3 to 6 wt%. At the same time, the potassium concentration increased quickly. From this point, it could be concluded that the potassium leaching was probably the main reason of the reduction of FAME yield and the active sites (K2 O) in the presence of water would form KOH, which has no tolerance to any moisture in converting microalgae to biodiesel.[23] In fact, active species leakage was an unavoidable issue in the heterogeneous transesterification. Verziu et al. [24] reported that after fourth runs 1.5% potassium leached from KF/Al2 O3 . Chen et al. [25] found that approximately 29% of potassium leached from K/γ -Al2 O3 after five successive cycles. Noiroj et al. [17] investigate that large amount of 51.26% of potassium leached from KOH/Al2 O3 during soybean oil transesterification. Compared with the data listed in Table 2, the leaching of potassium was not seriously with higher water content of 6 wt% during transesterification reaction. It could be reasonable concluding that the catalyst of KOH/Al2 O3 was stable for a limited numbers of reusability. As described above, the catalyst of KOH/Al2 O3 calcinated at a higher temperature featured
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good water tolerance than KOH itself and more suitable to produce biodiesel from microalgae.
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4. Conclusion Heterogeneous KOH/Al2 O3 catalysts synthesized by the wet impregnation method exhibited high tolerance to water and excellent catalytic ability on in situ transesterification of C. vulgaris microalgae. Regarding the catalysts performance, the most efficient was calcinated at 700°C with 35 wt% KOH loading. The highest FAME yield of 89.53 ± 1.58% was achieved at 5 h when the reaction was carried out at 60°C with 10 wt% catalyst and a methanol to biomass ratio of 8 mL/g. The main composition of K2 O formed by the KOH thermal decomposition was responsible for the catalytic activity to this reaction. Acknowledgements 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) and The Program for New Century Excellent Talents in University of the Ministry of Education of China (Grant No. NCET-20341). The authors thank Findlay A. Nicol of Shandong University of Finance and Economics for revising the English in the manuscript.
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