unsupported mixed oxides of Ca and Mg as heterogeneous catalysts for transesterification of Nannochloropsis sp. microalga’s oil

Energy Conversion and Management xxx (2014) xxx–xxx

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Alumina supported/unsupported mixed oxides of Ca and Mg as heterogeneous catalysts for transesterification of Nannochloropsis sp. microalga’s oil Siow Hwa Teo, Y.H. Taufiq-Yap ⇑, F.L. Ng Catalysis Science and Technology Research Centre, Faculty of Science, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia

a r t i c l e

i n f o

Article history: Available online xxxx Keywords: Biodiesel CaMgO CaMgO/Al2O3 Heterogeneous Nannochloropsis oculata Transesterification

a b s t r a c t In this study, calcium magnesium (CaMgO) and alumina (Al2O3) supported CaMgO mixed oxide catalysts were prepared via pH-controlled co-precipitation (Na2CO3 and NaOH as a precipitant) for transesterification of crude Nannochloropsis oculata (N. oculata) oil with methanol. The catalysts were characterized by means of Thermogravimetric analyses (TGA), X-ray diffraction (XRD), Fourier transform-infrared (FTIR), Temperature programmed desorption of CO2 (CO2-TPD), Inductively coupled plasma–atomic emission spectrometer (ICP–AES) and Scanning electron microscopy (SEM) analysis. At optimization condition, CaMgO mixed oxide catalyst showed 75.2% of fatty acid methyl ester (FAME) yield with catalyst loading of 20 wt.% at 3 h. Meanwhile, the supported CaMgO mixed oxide catalyst gave a higher FAME yield of 85.3% with catalyst loading of 10 wt.% at same conditions. Besides, the reusability study of catalyst was performed to investigate the stability and durability of supported/unsupported catalysts. The high content of Ca2+ and Mg2+ precipitated on Al2O3 supported CaMgO mixed oxide catalyst tend to increase the total basicity and provide more active sites for transesterification reaction. Moreover, better moisture resistant on the Al2O3 supported CaMgO mixed oxide catalyst compared with CaMgO mixed oxide catalyst, which is favourable for transesterification reaction on high water content microalgae oil. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Concern over the petroleum reserves and environmental impacts, impetus in development of renewable source of energy has resulted in biodiesel development. Biodiesel also known as fatty acid methyl ester (FAME) has many benefits over the fossil diesel fuel such as being nontoxic, biodegradable, renewable, and does not contribute to net accumulation of the green house gases. Also, biodiesel has lower sulfur and aromatic content, higher cetane number and flash point than petroleum diesel fuel [1]. Due to the weaknesses of first and second generation feedstocks, the search of the more sustainable biodiesel feedstock is becoming more important, and the searching yield the third generation of biodiesel which derived from different microbial species such as microalgae, yeast and fungus [2–5]. They can be used as potential sources for biodiesel as they can biosynthesize and store ⇑ Corresponding author at: Catalysis Science and Technology Research Centre, Faculty of Science, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia. Tel.: +60 3 89466809; fax: +60 3 89466758. E-mail address: taufi[email protected] (Y.H. Taufiq-Yap).

high amounts of fatty acid in the biomass. From the environment point of view, biodiesel from microalgae lipid is environmental friendly due to the non-toxicity and high biodegradability [6]. Also, microalgae can diminish carbon dioxide concentration in atmosphere as they intake carbon dioxide as the carbon source for growth via photosynthesis. Homogeneous alkaline catalysts such as NaOH, KOH or NaOCH3 were used to accelerate the methanolysis reaction; the high free fatty acid lipids such as microalgae lipid prevented the use of homogeneous base catalyst for transesterification reaction [7]. At this point in time, explorations on heterogeneous catalyst for the transesterification reaction have also been done extensively due to the reusability and eco-friendly nature of the heterogeneous catalyst lend themselves to easier product separation and better product purity [8]. To our best knowledges, only several reported on heterogeneous system i.e. SrO [9], Mg–Zr [10], zeolite (h-ZSM-5 and h-Beta) [11], lipase supported on alkyl-grafted Fe2O3–SiO2 [12] and CaO/Al2O3 catalysts for converting microalgae oil to biodiesel [13]. In order to achieve commercial production that meet the sustainable of global biodiesel demand, the production process must

http://dx.doi.org/10.1016/j.enconman.2014.04.049 0196-8904/Ó 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Teo SH et al. Alumina supported/unsupported mixed oxides of Ca and Mg as heterogeneous catalysts for transesterification of Nannochloropsis sp. microalga’s oil. Energy Convers Manage (2014), http://dx.doi.org/10.1016/j.enconman.2014.04.049

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S.H. Teo et al. / Energy Conversion and Management xxx (2014) xxx–xxx

be efficient and affordable. Koberg et al. [9] recently produced biodiesel from oleaginous Nannochloropsis sp. (one of the most suitable microalgal oil sources) using microwave, ultrasound radiation and reflux with the aid of a SrO catalyst. However, the FAME yield were very low, which are 32.8%, 18.9% and 6.95% (one-step transesterification), respectively. Similarity, Li et al. [10] developed Mg–Zr solid base catalyst, and tested on the same species of microalgae oil with both two-step conventional and one-step method. A maximum FAME yield of 28% was reached with the catalyst amount of 10 wt.% (with refer to initial biomass weight) using one-step method. Meanwhile, conventional twostep method achieved only a yield of 22.2% methyl ester, which is lower than one-step method. On the other hand, the results obtained by Carrero and co-researchers [11] from Nannochloropsis spesies using hierarchiral ZSM-5 (h-ZSM-5) and Beta (h-Beta) zeolite catalysts for transesterification reaction. It was found that temperature is necessary to reach 115 °C to achieve any activity with both catalysts. However, the FAME productions were found very low at 5% and 25%, respectively. Transesterification was performed on Chlorella vulgaris ESP-31 microalgae for biodiesel synthesis using immobilized Burkholderia lipase on alkyl-grafted Fe2O3– SiO2 support [12]. The results show that one-step method achieved higher biodiesel conversion (97.3 wt.% oil) than two-step methods (72.1 wt.% oil). In the point of view, lipase can be immobilized on supports, enabling easy recovery and reuse of the biocatalyst, but lipase catalysts are expensive than solid metal catalysts and it is also very costly. One excellent breakthrough in the production of biodiesel from Nannochloropsis oculata (N. oculata) was demonstrated by Umdu et al. [13]. The author achieved enhanced conversion of 97.5% by using 80% calcium oxide on alumina; catalyst was synthesized using a modified single step sol–gel method. This high yield obtained further illustrated the promising potential of producing microalgae biodiesel by utilizing solid base catalysts. Though the author alluded on the way of how the catalyst basicity was affected the catalyst activity, the report did not elaborate further and precise regarding the basic site population of the catalyst on catalytic performance. In this study, there is a must for breakthrough invention on heterogeneous supported/unsupported mixed oxide of Ca and Mg catalysts which employed in production of biodiesel derived from yellow green N. oculata derived microalgae oil. Until now, study on microalgae biodiesel production accelerated by heterogeneous catalysts is still very constrained. The foregoing highlights that the reaction conditions such as time, temperature, oil to alcohol ratio and other parameters such as microalgal oil source and basic strength of the catalyst determine the production of the biodiesel [14–16]. For this, various reaction conditions were carried out to examine the effect of the catalyst dosage, methanol to oil molar ratio, reaction temperature and reaction time. The deactivation of the catalysts from the reusability test to produce biodiesel will also be discussed.

2. Experimental section 2.1. Microalgae strain and cultivation system The microalgae N. oculata was purchased from the AlgaeTech Sdn. Bhd., Klang, Malaysia. The artificial seawater medium was prepared by dissolving 165 g of instant ocean salt into 5l distilled water. 0.1 ml of liquid sterilize agent (chlorine bleach) was added to the medium for sterilization purpose. The sterilization was taken place for 4 h. De-chlorination was performed by adding 0.2 ml of Na2S2O3. Photoautotrophic cultivation of N. oculata was initially carried in a Erlenmeyer flask containing a modified F/2 medium, containing the following components (mg l1): NaNO3

(225), NaH2PO4H2O (5.65), Na2-EDTA (2.38), FeCl36H2O (1.8), CuSO45H2O (0.006), ZnSO47H2O (0.013), CoCl26H2O (0.006), MnCl24H2O (0.103), Na2MoO42H2O (0.003), at 24–28 °C with air flowing under 16 h on/8 h off artificial light cycle (light intensity: 60 W m2). Dissolved carbon dioxide concentration was controlled by increasing agitation speed and airflow. The strain must constantly agitate while grown because of the ease of sediment build-up which hinders growth. The cultivation process took 14 days for one cycle. 2.2. Harvesting and lipid extraction The microalgae cell was collected by flocculation and washed with distilled water to remove the coagulant, then dried at 70 °C in vacuum oven for 24 h and triturated to get dried microalgae powder for experiment. The lipids extraction was performed using the modified bligh and dyer method [17]. Crude dried microalgae were mixed with chloroform-methanol (2:1 v/v) and sonicated in a sonicator (S60 h Elmasonic) for 5 min. Then, the extraction process was preceded with continue stirred and extracted in 1 l volumetric flask for 72 h. The extracted lipid was separated from the microalgae powder by filtration and crude lipid was vacuum dried, then stored in the freezer. 2.3. Characterization of dried microalgae and crude lipids 2.3.1. CHNS elemental analysis The total carbon, hydrogen, nitrogen and sulfur in dried microalgae were determined using CHNS Analyzer model LECO TruSpec. For this analysis, 2 mg of dried microalgae were weighed and placed into a tin capsule. The combustion temperature in oven was 1100 °C. 2.3.2. Gas chromatography–mass spectrometric analysis According to a typical sample preparation method used by Ariffin et al. [18], about 0.05 g crude lipid was initially converted into FAME form by refluxing with 10 ml methanolic sodium hydroxide in round bottom flask for 4 h. The reaction mixture was then shaken vigorously with 2 ml of hexane in a vortex mixer. The clear, separated methyl ester layer was injected in to Shimadzu GCMS-QP2010 Plus (GC–MS) for analysis. Analysis of the fatty acid profile was carried out with a GC-2010 series gas chromatography equipped with flame ionization detector and coupled to a mass spectrometer. A HP-5 column was used with He as a carrier gas. The oven temperature is isothermal at 320 °C and the injector and detector temperature was set to 280 °C. Molecular weight (MWoil) of microalgae oil was calculated using the following correlation; MWoil = [(3  MWfatty acid) + MWglycerol]  3  MWwater. 2.4. Catalyst preparation 2.4.1. Synthesis of CaMgO catalyst via co-precipitation Ca–Mg mixed oxides catalyst were prepared by conventional co-precipitation method prepared by Taufiq-Yap et al. [8] with slightly modification. Typically, 2 M of Ca(NO3)24H2O and 1 M of Mg(NO3)26H2O were dissolved in distilled water and the solution was mixed and stirred vigorously for 1 h to achieve homogeneous mixing. Then, basic aqueous solution that containing Na2CO3 and NaOH was added into the mixed metal nitrates slowly. Precipitation was controlled at pH 9–10. The resulting white suspended slurry was performed under vigorous stirring for overnight at temperature 60 °C. The precipitates were filtered and washed thoroughly with hot water until the filtrate remains pH 7. The dried solid obtained was calcined at 800 °C for 6 h with the ramp at 5 °C min1.

Please cite this article in press as: Teo SH et al. Alumina supported/unsupported mixed oxides of Ca and Mg as heterogeneous catalysts for transesterification of Nannochloropsis sp. microalga’s oil. Energy Convers Manage (2014), http://dx.doi.org/10.1016/j.enconman.2014.04.049

S.H. Teo et al. / Energy Conversion and Management xxx (2014) xxx–xxx

2.4.2. Synthesis of CaMgO supported on Al2O3 catalyst via homogeneous co-precipitation deposition 5 g support material, Al2O3 was stirred and suspended in distilled water for 1 h to form a homogeneous solution. Then, Na2CO3 was added to the solution to prepare a basic aqueous solution at pH 11. 2 M of Ca(NO3)24H2O and 1 M of Mg(NO3)26H2O were mixed and stirred vigorously for 1 h to achieve homogeneous mixing, then drop wised into the prepared basic aqueous solution. Precipitation was controlled at pH 9–10 and at temperature 60 °C. NaOH was used adjust the pH when needed. The resulting precipitations were stirred vigorously for overnight. The precipitates were filtered and washed thoroughly with hot water until the filtrate remains pH 7. The dried solid obtained was calcined at 800 °C for 6 h with the ramp at 5 °C min1.

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catalysts. The 1000 ppm stock solutions (Ca, Mg and Al) were then diluted to 100 ppm. The 100 ppm stock solutions were further diluted down to 40, 20, 10 and 5 ppm. On the other hand, the catalyst was digested under heating with appropriate amount of nitric acid and dissolved in distilled water. The blank solution was prepared by adding adequate amount of nitric acid into distilled water. 2.5.6. Scanning electron microscopy The morphology observation of supported/unsupported mixed oxides of Ca and Mg catalysts were observed using variable pressure scanning electron microscopy (VPSEM, model JEOL JSM6400 SEM). The catalysts powder were dispersed well on the sticky carbon tape, then were coated with Au (gold) for protecting the induction of electric current using a BIO-RAS Sputter Coater.

2.5. Catalyst characterization 2.5.1. Thermogravimetric analysis Thermogravimetric analyses (TGA) of supported/unsupported mixed oxides of Ca and Mg catalysts were performed using a Mettler Toledo thermogravimetric analyzer. The instrument was operating under continuous nitrogen gas flow at flow rate of 50 ml min1 and at 10 °C min1 heating rate from room temperature to 1000 °C. 2.5.2. X-ray diffraction analysis The powder X-ray diffraction analysis (XRD) was employed to identify the crystallography of supported/unsupported mixed oxides of Ca and Mg catalysts. The analysis was carried out using a Shimadzu diffractometer model XRD6000. The diffractometer employing Cu Ka radiation (2.7 kW and 30 mA) with wavelength (k) of 1.54 Å to generate diffraction patterns from powder crystalline samples at ambient temperature. The data were recorded over a 2h range of 10–80° with a steps of 0.02° and count time 1 s. Likewise, the average crystallite sizes were calculated from the peaks of the diffraction peaks employing Scherrer’s equation [8]. 2.5.3. Fourier transform infrared spectroscopic analysis Infrared spectra of supported/unsupported mixed oxides of Ca and Mg catalysts were measured by using attenuated total reflection-Fourier transform-infrared (ATR-FTIR) on a PerkinElmer (PC) Spectrum 100 FTIR spectrometer to identify surface functional groups presenting on the catalyst at room temperature. Each spectrum was average of 128 scans analysed over the scanning over a wavelength of 400–4000 cm1 at a resolution of 4 cm1. 2.5.4. Temperature – programmed desorption of CO2 analysis Basicity of the catalysts was determined using temperature programmed desorption of CO2 with Thermo Finnigan TPDRO 1100. In typical experiment, 0.1 g of catalyst was pretreated in nitrogen gas flow (20 ml min1) at 250 °C for 10 min (10 °C min1). The temperature was then risen to 500 °C and flow of CO2 at flow rate of 30 ml min1 was introduced to the catalysts for 90 min. The nitrogen gas flow (20 ml min1) was again introduced for 30 min in order remove excess CO2 which presents in the system. Subsequently, desorption of the CO2 from catalysts was performed by flushing with helium gas flow (30 ml min1) which acts as carrier gas over a temperature range of 50–900 °C at heating rate of 10 °C min1. 2.5.5. Inductively coupled plasma–atomic emission spectroscopic analysis Inductively coupled plasma–atomic emission spectrometer model Perkin–Elmer Optima 2000 DV was used to determine the concentration of trace metals in the solution. This analysis was performed in order to determine the bulk chemical composition of the

2.6. Transesterification reaction and analysis of biodiesel Microalgae oil was reacted with methanol via heterogeneous catalyst transesterification by method as shown in Supp. 1. Transesterification reaction was performed in a microreactor by using 1 g crude lipid extracted from N. oculata with methanol to lipid molar ratio ranged from 15 to 90 molar ratios, different reaction time of 3 h and 6 h, reaction temperature of 60 °C and 100 °C and varied concentration of powder catalysts (3–20% weight). After the reaction was completed, the mixture was filtered to remove the catalyst and solid particles. Biodiesel was obtained by evaporation to remove the excess solvent. The methyl ester yield was calculated from its content in biodiesel as analysed by gas chromatography (GC) (Agilent technologies 7890A) with flame ionization detector (FID) and a fused silica capillary HP-88 column (300 m  0.25 mm i.d., 0.20 lm film thickness) from Agilent (USA). The oven temperature was set at 140 °C and held for 5 min, raised to the final temperature at 240 °C at rate of 4 °C min1 and held for 15 min. The sample size injected was 1 ll; with helium as the carrier gas, flow at rate of 1 ml min1. The methyl esters in the sample were identified by comparing their retention times with the reference standard components of FAME mixtures. Methyl heptadecanoate was used as an internal standard and hexane was used as a solvent. The gas chromatogram of the biodiesel product was shown in Supp. 2. The FAME content was determined in agreement with European regulated procedure EN 14103 [19]. All values reported were the average of three measurements. 2.7. Reusability test The reusability of catalysts for transesterification of microalgae oil was examined. At the end of the reaction, the mixture solvents were discarded without removing the catalyst from the microreactor, the additional amount of crude lipid and required volume of methanol were added to the microreactor. The reaction was carried out under optimized conditions. 3. Results and discussion 3.1. Characterization of dried biomass and crude lipids from of N. oculata 3.1.1. Elemental analysis of N. oculata microalgae The result of elemental analysis of dried microalgae powder was listed in Table 1. The nitrogen (0.6%) content in dried N. oculata was correlated to its protein content [20]. Then, carbon (41.4%) content was suggested due to carbon storage products such as carbohydrates, fatty acids and lipids. Meanwhile, the high content of

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Table 1 Elemental composition of microalgae. Material

N. oculata a

Elemental composition (%) C

H

N

S

Oa

41.4 ± 0.11

7.8 ± 0.05

1.8 ± 1.02

–

49.0 ± 0.33

Calculated by difference.

carbon also suggested that N. oculata having high capacity of carbon setting. However, hydrogen (7.8%) content was low compared to carbon content which suggested the presence of polyunsaturated fatty acids i.e. eicosapentaenoic acid (EPA) [21]. Surprisingly, sulfur content was not detected which could suggested that the biodiesel derived from microalgae will be an eco-friendly biofuel. 3.1.2. Lipid extraction and fatty acid composition of N. oculata In this current work, Bligh and Dyer method [14] with slightly modification was use to extract the highest lipid amount. In this case, mixture of CH3Cl and CH3OH (2:1, v/v) is suitable to extract lipid from N. oculata microalgae. This is because these combinations of chemical solvents provide high selectivity and solubility towards lipid, especially for microalgae lipid containing mainly polar unsaturated fatty acid compounds [18]. The content of fatty acid compositions from N. oculata was given in Table 2. The main chemical components are 35.43% of palmitic acid (C16:0), 24.54% of palmitoleic acid (C16:1), 8.62% of oleic acid (C18:1), and 8.29% of EPA (C20:5), in which the unsaturated fatty acids were the main components. Others methyl esters such as myristic acid (7.69%), stearic acid (2.50%), linoleic acid (5.22%), arachidonic acid (2.47%) and DHA (2.24%) are also presence in the oil. Molecular weight (MWoil) of microalgae oil was calculated as 831.62 g mol1. The high value of MW indicating the high concentration of triglycerides, and therefore N. oculata microalgae derived oil is highly suitable feedstock for the production of FAME. 3.2. Catalyst characterization 3.2.1. Thermal decomposition The TGA pattern (Supp. 3b) of uncalcined CaMgO showed two distinct stages of weight losses: one at temperature below 410 °C and another one between 580 to 785 °C. The first state can be attributed to the decomposition of MgCO3 precursor to form magnesium oxide by liberation of CO2; the weight lost was about 4.6%. The second stage exhibited the major weight loss at 775 °C, corresponding to 34.3 wt.% was due to the change of CaCO3 phase to CaO phase which confirmed by XRD and the FTIR results (Supp. 4 and 5, respectively). The resulting products from decomposition of CaMgO precursor are CaO and MgO, following this reaction:

CaMgðCO3 Þ2 ! Cao þ MgO þ 2CO2 " ðexothermicÞ

Table 2 Fatty acid content of crude N. oculata microalgae based methyl esters using GC–MS analysis. Substituent

Fatty acid in lipid

C

Content (%)

Saturated lipid

Myristic acid Palmitic acid Stearic acid

14:0 16:0 18:0

7.69 ± 0.52 35.43 ± 0.06 2.50 ± 0.26

Monounsaturated lipid

Palmitoleic acid Oleic acid

16:1 18:1

27.54 ± 0.15 8.62 ± 0.02

Linoleic acid Arachidonic acid EPA DHA

18:2 20:4 20:5 22:6

5.22 ± 0.23 2.47 ± 0.05 8.29 ± 0.08 2.24 ± 0.16

Polyunsaturated lipid

On the other hand, TGA curve of uncalcined CaMgO/Al2O3 (Supp. 3a) showed four stages of weight lost spontaneously. The first weight lost about 5.8% from 39 °C to 219 °C which corresponded to desorption of physically adsorbed molecules such as water and carbon dioxide, then, 1.3 wt.% of weight loss over 220–280 °C because of small amount of chemisorbed water or crystal water molecules. Subsequently, decomposition temperature between 284 to 557 °C was due to decomposition of the MgCO3 and formation of MgO by liberation of carbon dioxide, the weight lost was about 6.7 wt.%. Lastly, the fourth stage weight lost about 15.4 wt.% ranging from 561 to 753 °C was regarded the decomposition of CaCO3 precursor to form CaO by liberation of CO2. As the sample weight remained constant after 780 °C, the 800 °C was then consider suitable as calcination temperature for the method. 3.2.2. Structure X-ray diffraction patterns of CaMgO/Al2O3 and CaMgO catalysts (Supp. 4a and 4b) both showed the CaO phase (JCPDS File No. 00037-1497) with 2h at 32.2°, 37.3°, 53.8°, 64.1°, 67.3° and the MgO phase (JCPDS File No. 00-004-0829) gave 2h at 42.8° and 62.3°, respectively. Whereas, the broadness peaks from CaMgO/Al2O3 catalyst was attributed to present of amorphous phase of Al2O3 (JCPDS File No. 01-080-0956). By using Debye–Scherer’s equation, the crystallite sizes of the catalysts were calculated and summarized in Table 3. The average crystallite sizes of CaO phase from CaO, CaMgO and CaMgO/Al2O3 catalysts were 66.3, 44.7 and 41.9 nm, respectively. The average crystallite sizes of MgO phase was smaller compared to CaO as i.e. 15.9, 22.0 and 35.4 nm, respectively for MgO, CaMgO and CaMgO/Al2O3 catalysts. The crystallite sizes of CaO phase was arranged in sequence of CaMgO/Al2O3 < CaMgO < CaO catalysts, while crystallite sizes of MgO were arranged in sequence of MgO < CaMgO < CaMgO/Al2O3 catalysts. This result was demonstrated the well dispersion of CaO on surface alumina during the method preparation. 3.2.3. Functional group Supp. 5a showed the IR spectrum of uncalcined CaMgO precursor. The broad transmission band at approximately 3500 cm1 can be attributed to OH stretching vibration from physisorbed moisture. The C–O stretching vibration gave sharp peak at 1400 cm1 and peak at 871 cm1 corresponding to C–O out of plane bending shown the presence of carbonate functional groups. This indicated the uncalcined CaMgO precursor was CaCO3 and MgCO3 which was in agreement with the TGA result (Supp. 3a). On the other hand, Supp. 5c revealed the spectrum of uncalcines CaMgO/Al2O3 precursor. The existence of strong peaks at 1476 cm1 and 871 cm1 were corresponding to C–O stretching vibration and C–O out of plane bending. This C–O stretching suggesting that the adsorbed precursor species on aluminium oxide are carbonates of Ca and Mg. Furthermore, the vibration signals at 592 and 403 cm1 were assigned to be the symmetric and asymmetric vibrations of AlO6 [22]. Upon calcination, the C–O stretching vibration (Supp. 5b) at approximately 1410 cm1 still can be seen suggesting that the

Table 3 Crystallize sizes of CaO, MgO, CaMgO and CaMgO/Al2O3 catalysts. Catalysts

CaMgO CaMgO/Al2O3 CaO MgO

Crystallize size (nm) CaO

MgO

44.7 ± 2.37 41.9 ± 3.42 66.3 ± 3.20 –

22.0 ± 2.13 35.4 ± 3.11 – 15.9 ± 0.60

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possibility CaMgO mixed oxide catalyst can react with the CO2 in the air as CaO and MgO has high tendency to react with the CO2 in the air. However, the supported CaMgO mixed oxide catalyst (Supp. 5d) is less sensitive to moisture as compared to the unsupported CaMgO mixed oxide catalyst. 3.2.4. Basicity The basic site distributions and the total basicity of synthesized catalysts were show in Supp. 6 and the results summarized in Table 4, respectively. The CaO gave strong basic strength with an intense desorption peak at 788 °C, whereas the MgO showed only a small weak desorption peak with strong basic site at 723 °C. The narrower band occurred at high temperature range between 427 to 627 °C attributed to the presence of much stronger basic site [23]. However, no CO2 desorption peak was found in the TPD spectra for Al2O3. This showed that there is no basic site on the surface of Al2O3. On the other hand, CaMgO gave strong basic strength with desorption peak at 725 °C and desorption volume of carbon dioxide equal to 2471.8 lmol g1, whereas the CaMgO/Al2O3 gave stronger basic strength yet broader desorption peak at 733 °C with higher desorption amount of 3218.9 lmol g1 as compared to the unsupported CaMgO as illustrated in Supp. 6. The high dispersity in CaMgO/Al2O3 shown through SEM image can lead to the better exposure of the basic site per unit mass of the catalyst and hence increase the basic strength and intensity of the catalyst [24]. 3.2.5. Morphology The particle of bulk (CaO, MgO and Al2O3) catalysts (Supp. 7a–7c) were in aggregates of cubic crystal, hexagonal shaped flakes and long shaped rods, respectively. The morphology of CaMgO catalyst was illustrated in Supp. 7d. The less dense MgO flakes were distributed heterogeneously on the CaO particles [8]. Conversely, the aggregate cubic crystal of CaO particles and MgO flakes were well dispersed on the long shape alumina (Supp. 7e). The supported catalyst had shown the increase in dispersity for the mixed metal oxide catalyst. 3.2.6. Bulk chemical composition The experimental Ca/Mg molar ratios (Table 5) for CaMgO and the CaMgO/Al2O3 catalysts were 0.95 and 1.28, respectively. The experimental Ca/Mg molar ratios were different with the theoretical Ca/Mg molar ratios which were 2.0 for both catalysts. The results suggested that Ca2+ was favourably precipitated at higher pH than 9 [19].

CO2 desorbed (lmol/g)

Temperature range (°C)

Peak temperature (°C)

3219 ± 0.12 2472 ± 0.18 167 ± 0.15 20 ± 0.22 –

568–827 572–813 650–830 625–780 –

733 725 788 723 –

Table 5 Molar ratio of CaMgO and CaMgO/Al2O3 catalysts. Catalysts

CaMgO CaMgO/ Al2O3

Concentration (ppm) Ca

Mg

Al

35.13 ± 0.13 15.69 ± 0.32

22.42 ± 0.24 7.442 ± 0.18

– 25.79 ± 0.26

FAME yield (%)

1st run 2nd run 3rd run

CaMgO

CaMgO/Al2O3

73.2 ± 2.88 28.6 ± 2.27 16.1 ± 0.52

85.5 ± 1.56 51.5 ± 0.71 17.8 ± 1.41

3.3. Effect of transesterification parameters on N. oculata biodiesel yield 3.3.1. Methanol to oil molar ratio Transesterification using CaMgO and CaMgO/Al2O3 catalysts with various amount of methanol catalyzed by 3% catalyst was carried out at 60 °C for 3 h (Fig. 1). In both reactions, when the methanol–lipid ratio was increased from 15:1 to 40:1, the FAME yield increased from 8.5% to 13.1% and 5.8% to 11.1%, respectively. Further addition of molar ratio of methanol to 60:1 increased the FAME yields to 50.2% (CaMgO) and 18.4% (CaMgO/Al2O3). However, both were decreased to 48.0% and 17.8% when using 90:0 molar ratio methanol/oil. In these cases, the presence of excess methanol in transesterification was essential for i.e. homogenize the high viscosity of the microalgae crude lipids and break glycerine–fatty acid linkages, which could able to promote high conversion efficiency [25]. The slightly increased of FAME yield with CaMgO/Al2O3 most probably due to insufficient of the catalyst concentration to form more active species for high FAME yield production [10]. 3.3.2. Reaction time Fig. 2 shows FAME yield for the different reaction time. For CaMgO catalyst, the result showed that the FAME yield decreased from 49.9% to 17.5% when increment of reaction time from 3 to 6 h. Similarly, the FAME yield was decreased from 19.1% to 14.1% by using the alumina supported CaMgO catalyst. Reaction time was the significant factor that affects the heterogeneously catalyzed transesterification reaction rate due the presence of three immiscible phases (oil–alcohol–solid catalyst) at the initial stage of reaction [26]. Conversely, the appearance of white emulsion and gel in biodiesel was significant observed which resulted from further prolongation of reaction time. This increased the viscosity of biodiesel and might difficulties in the downstream process.

60 50

FAME yield (%)

CaMgO/Al2O3 CaMgO CaO MgO Al2O3

Run time

3.3.3. Reaction temperature The effect of reaction temperature at 60 and 100 °C on the FAME yield of transesterification of microalgae oil was investigated. As

Table 4 Total basicity of CaO, MgO, CaMgO and CaMgO/Al2O3 catalysts. Catalysts

Table 6 Reusability test.

40

CaMgO CaMgO/Al2O3

30 20 10

Ca/Mg molar ratio 0.95 ± 0.11 1.28 ± 0.21

0

15

21

40

60

90

MeOH ratio (%) Fig. 1. Effect of methanol molar ratio on the FAME yield of crude microalgae lipid by CaMgO and CaMgO/Al2O3 catalysts. Reaction condition: catalyst dosage = 3 wt.%, reaction time = 3 h and reaction temperature = 60 °C.

Please cite this article in press as: Teo SH et al. Alumina supported/unsupported mixed oxides of Ca and Mg as heterogeneous catalysts for transesterification of Nannochloropsis sp. microalga’s oil. Energy Convers Manage (2014), http://dx.doi.org/10.1016/j.enconman.2014.04.049

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S.H. Teo et al. / Energy Conversion and Management xxx (2014) xxx–xxx

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CaMgO CaMgO/Al2O3

40 30 20

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FAME yield (%)

FAME yield (%)

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shown in Fig. 3, FAME yield was achieved at 18.1% using alumina supported CaMgO catalyst at reaction temperature of 60 °C, and increased inappreciable to 24.0% at 100 °C. Transesterification of oil with methanol in the presence of heterogeneous catalyst is a three-phase reaction system. Raising the reaction temperature favoured the transesterification due to the enhancement of miscibility at high temperature [27]. However, CaMgO catalyst showed FAME yield of 51.0% at 60 °C, and reduced to 14.6% at the reaction temperature of 100 °C. This is because if the reaction temperature exceeds the boiling point of methanol (65 °C), the methanol will be vaporize and a large number of bubbles was formed, which will restrain the reaction in the three-phase reaction system [27]. It is necessary to reach higher temperature (>65 °C) to achieve higher activity with both catalysts. Nevertheless, CaMgO catalyst showed a higher FAME yield than supported CaMgO/Al2O3 catalyst. This might due to the synergetic effect between Ca2+ and Mg2+ active phases in binary system of CaMgO mixed oxide catalyst [8,11]. 3.3.4. Catalyst loading The concentration of both catalysts to crude microalgae lipid was studied ranging from 0 to 20 wt.% (with refer to starting oil weight), carried out for 3 h at 60 °C. The results indicate that high concentration of catalyst facilitates to increase the total number of available active catalytic sites for the reaction. As shown in Fig. 4, the yield of FAME increased with enhancing loading of CaMgO/ Al2O3 catalyst, and reached a highest value of 85.3% when 10% catalyst was used. Then, the yield of FAME decreased when the

60

FAME yield (%)

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CaMgO CaMgO/Al2O3

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Fig. 4. Effect of catalyst loading on the FAME yield of crude microalgae lipid by CaMgO and CaMgO/Al2O3 catalysts. Reaction condition: n(methanol):n(lipid) = 60:1, reaction time = 3 h and reaction temperature = 60 °C.

100 90 CaMgO

80

FAME yield (%)

Fig. 2. Effect of reaction time on the FAME yield of crude microalgae lipid by CaMgO and CaMgO/Al2O3 catalysts. Reaction condition: catalyst dosage = 3 wt.%, n(methanol):n(oil) = 60:1 and reaction temperature = 60 °C.

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CaMgO/Al2O3

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Run times Fig. 5. Stability and reusability test.

catalyst dosage increased to 20%. This result is probably due to resistant of mixing involving reactant, product and solid catalyst that inhibited the conversion [8]. However for CaMgO catalyst, the FAME yield was low (51.3% and 57.6%) at catalyst loading of 3 and 10 wt.%, respectively. A highest FAME yield of 75.5% was achieved when catalyst loading of 20 wt.%. Strong basic in CaMgO catalyst enhanced transesterification reaction at the lower concentration of catalyst. This is similar to the reported by Kouzu et al. [28]. However, sufficient high amount of catalyst is required for transesterification of high free fatty acid microalgae derived lipid. Although CaMgO is strong basic solid catalyst, it given lower FAME yield compared to CaMgO/Al2O3 catalyst, when using 10 wt.% of catalyst amount in the reaction. This phenomenon might be corresponding to the basic site density of CaMgO catalyst was significant lower than CaMgO/Al2O3 catalyst as seen in Table 4.

30

3.4. Catalyst stability and reusability 20 10 0

60

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Reaction temperature ( C) Fig. 3. Effect of reaction temperature on the FAME yield of crude microalgae lipid by CaMgO and CaMgO/Al2O3 catalysts. Reaction condition: catalyst dosage = 3 wt.%, n(methanol):n(oil) = 60:1 and reaction time = 3 h.

The catalyst recycling is an important step as it minimizes the cost of the process. The reusability of catalysts on transesterification of high moisture content of microalgae oil was investigated through subsequent reaction cycles. The spent catalysts were reused at the end of the reaction without undergone washing treatment or re-calcination. The results for consecutive reaction cycles were shown in Fig. 5 and summarized in Table 6. The catalytic activity decreased through consecutive reaction cycles, which was probably attributed to some active sites of catalysts being

Please cite this article in press as: Teo SH et al. Alumina supported/unsupported mixed oxides of Ca and Mg as heterogeneous catalysts for transesterification of Nannochloropsis sp. microalga’s oil. Energy Convers Manage (2014), http://dx.doi.org/10.1016/j.enconman.2014.04.049

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References

Transmittance (%)

(a)

(b)

4000

3000

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Wavenumber (cm-1) Fig. 6. FTIR sprectra of deactivated CaMgO/Al2O3 (a) and CaMgO (b) catalysts.

covered by the resultant come from microalgae, lipid such as phospholipids and moisture [13]. This can be seen from FTIR spectra of spent catalysts shown in Fig. 6. A sharp absorption band at 3639 cm1 due to the surface of spent catalyst was covered by hydrate. The C–O stretching vibration at 1424 cm1 became more intense after the reaction. However, the supported CaMgO catalyst is less sensitive to moisture as compared to the CaMgO catalyst. The sustainable and moisture less sensitive of CaMgO on Al2O3 catalyst found in this study is parallel with literature which reported that no observed significant adsorption of water was found on supported alumina catalyst [13,29]. Hence, this may be the reason of CaMgO supported on Al2O3 catalyst was used for more than 6 h of the reaction time.

4. Conclusions Al2O3 supported and unsupported CaMgO mixed oxide catalysts were successfully prepared by the pH-controlled co-precipitation method. Both, thermally activated at 800 °C, are active in the transesterification reaction of N. oculata microalgae oil with methanol. Both catalysts gave 75–85% of FAME yield at 3 h of reaction time. Since microalgae fatty acids are naturally found in oil bearing crops, in which the fatty acids are suitable for biodiesel production. Therefore, using N. oculata microalage derived oil demonstrated a potential method for producing high quality biodiesel fuel. The excellent breakthroughs in current study are (i) alumina supported CaMgO mixed oxide catalyst revealed more sustainable compared to CaMgO mixed oxide catalyst due to less susceptible to the moisture from microalgae oil. (ii) CaMgO/Al2O3 catalyst was synthesized with a simple homogeneous co-precipitation deposition method, which showed a superior catalytic performance compared to other solid catalysts such as SrO, Mg–Zr and zeolite catalysts [9–11].

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.enconman.2014. 04.049.

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Please cite this article in press as: Teo SH et al. Alumina supported/unsupported mixed oxides of Ca and Mg as heterogeneous catalysts for transesterification of Nannochloropsis sp. microalga’s oil. Energy Convers Manage (2014), http://dx.doi.org/10.1016/j.enconman.2014.04.049