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Fe molar ratio
Industrial Crops & Products 141 (2019) 111814
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Performance and stability assessment of Mg-Al-Fe nanocatalyst in the transesterification of sunflower oil: Effect of Al/Fe molar ratio Freshteh Naderia, Hamed Nayebzadehb, a b
T
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Department of Chemistry, Shahr-e-Qods Branch, Islamic Azad University, Tehran, Iran Esfarayen University of Technology, Esfarayen, North Khorasan, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Spinel nanocatalyst Heterogeneous catalyst Solution combustion method Sunflower oil Transesterification Biodiesel
Heterogeneously biodiesel production from vegetable oils is recently concerned for reduction the environmental problems of its homogeneous production process. Therefore, synthesis an active and stable catalyst via simple, cost-effective and industrialization method must be proposed. For this purpose, a series of MgO impregnated on MgAlFe mixed metal catalysts with different Al/Fe molar ratio was prepared via solution combustion method for production of biodiesel from sunflower oil. The results of characterization presented that spinel type of MgFe2O4 was successfully synthesized by combustion method. Moreover, present of Al cation into mixture of precursors caused to the combustion reaction transfer from smoldering to flam reaction that is related to lower heat formation of MgAl2O4. This phenomenon prevents the insufficient growth of crystal and particles. Moreover, due to releasing huge amount of gases during combustion reaction, high reaction temperature and well bonding of active phases with the samples containing Al cations, they showed highly porous structure, high surface area, appropriate pore shape and high thermal stability. These advantages clearly increased their activity in the transesterification reaction. According the results, the MgO/MgAl0.4Fe1.6O4 nanocatalyst was selected as optimum sample that converted 93.2% of sunflower oil to biodiesel at the conditions of 110 °C, 12 M ratio of methanol/oil, 3 wt.% of catalyst and 3 h of reaction time. The sample also exhibited high stability for several uses such that preserved its activity at least for five times (conversion > 85%).
1. Introduction Biodiesel is a renewable energy, non-toxic and biodegradable fuel that is concerned as alternative for petroleum fuels. Biodiesel is produced by conversion of triglycerides of oil using methanol to fatty acid methyl ester (FAME) in the present of catalyst (Ambat et al., 2018). For more environmental friendly production of biodiesel, the heterogeneous catalysts have been suggested that acid, base and enzymatic catalysts are extensively studied (Mardhiah et al., 2017). Main researches are focused on the base heterogeneous catalysts due to their higher activity compared to other kind of heterogeneous catalysts (Veljković et al., 2018). However, some problems toward using the heterogeneous catalyst for biodiesel production such as lower reaction rate than homogeneous catalyst, difficult and long preparation procedure, high price and often low stability cause to industrial heterogeneously biodiesel production is postponed (Jamil et al., 2018). Fabrication of catalysts in nanoscale can largely overcome the low reaction rate of heterogeneous catalysts by providing higher surface area. For this purpose, although various methods such as sol-gel, co-
⁎
precipitation, ball-milling, impregnation, etc. were applied for fabrication of the nanocatalyst, these methods require long time and consume high energy for preparing the desirable nanocatalyst (Jamil et al., 2018; Thangaraj et al., 2018). Coriolano et al. (2017) synthesized Mg/ Al layered double hydroxides (LDH) impregnated with potassium for conversion of sunflower oil to biodiesel. Although high activity of the catalyst was reported, more than 24 h was required for preparation of the catalyst. Recently, solution combustion method was suggested for preparation of the nanocatalyst with high surface area, porous and pure structure, low levels of residual carbon and high stability toward deactivation via simple and cost-effective procedure without using a hightemperature calcination step (Varma et al., 2016). Solution combustion method leads to energy-saving and extremely reduction the preparation time (Hashemzehi et al., 2016a). Rahmani Vahid et al. (2018) studied the activity of MgAl2O4 fabricated by different synthesis method in the biodiesel production process. They reported that the sample synthesized by solution combustion method presented higher crystallinity and porous structure, more similarity the elemental compositions of the
Corresponding author. E-mail address: [email protected] (H. Nayebzadeh).
https://doi.org/10.1016/j.indcrop.2019.111814 Received 13 July 2019; Received in revised form 15 September 2019; Accepted 26 September 2019 0926-6690/ © 2019 Elsevier B.V. All rights reserved.
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final powder to parent solution, and better pore shape which caused to its higher activity. One of the most significant advantages relates to the preparation of doped complex catalysts such as spinel structure (AB2O4) with the highest possible structural homogeneity (Sankaranarayanan et al., 2013). In the past decade, these types of nanocatalyst are concerned for biodiesel production process that divalent material is usually chosen from alkaline earth metal oxides (Ca, Mg, Sr, Ba) for their basic properties and trivalent material usually is Al, Fe and Co (Mierczynski et al., 2015a,b; Xue et al., 2014). MgO/MgAl2O4 (Rahmani Vahid and Haghighi, 2016; Wang et al., 2008; Yousefi et al., 2018) and MgO/ MgFe2O4 (Alaei et al., 2018; Ita et al., 2018; Liu et al., 2016) were evaluated in the biodiesel production process. Due to higher activity of spinel catalyst containing Al (da Costa Evangelista et al., 2016d) and simple separation of the catalyst containing Fe for their magnetic properties (Shokrollahi, 2017), it seem that utilizing both cations can lead to a catalyst with high activity along with simple separate-ability. Zhang et.al (2017) synthesized Mg-Fe-Al hydrotalcite catalyst for CO hydrogenation process. They reported the higher activity of the sample by increasing the Al cations into precursor’s solution. Kazemifard et al. (2019) investigated KOH/Al2O3@Fe3O4 via impregnation-coprecipitation method for biodiesel production from microalgae and mentioned the high catalytic activity and increasing in stability by loading alumina. However, no comprehensive data are available on Mg-Al-Fe mixed metal oxides for the methanolysis of vegetable oil, where the samples can consider as a promising catalyst due to its high activity along with simple separation by applying an external magnetic field. Therefore, in this study, a series of Mg-Al-Fe mixed metal oxides with different Al/Fe ratio as structure of MgAlxFe2-xO4 (x = 0, 0.2, 0.4 and 0.6) was synthesized via solution combustion method to obtain a nanocatalyst with high activity, stability and ability for simple separation from reaction medium. Then, MgO as active phases was impregnated on the supports to enhance their activity in the transesterification reaction for conversion of sunflower oil to biodiesel. The samples were precisely investigated by various techniques and their activity and stability were finally evaluated.
(Yousefi et al., 2018). For this purpose, desirable amount of magnesium nitrate hexahydrate was dissolved in distillated water and appropriate amount of support (MgO/MgAlxFe(2-x)O4 ratio of 20 wt.%) was added. The mixture was stirred at 80 °C for 180 min and was then placed in oven overnight to dry. At the end, the samples were calcined at 550 °C for 3 h to obtain final powder as labeled MFA-X, where X is the mole amount of Al cation (x = 0, 0.2, 0.4, and 0.6). 2.2. Catalyst characterization X-Ray Diffraction (XRD) was utilized for crystalline analysis of the samples by using a PW1730 (PHILIPS) apparatus in the range of 10–80°. The thermal stability of the samples and efficiency of combustion method to decompose the precursors were evaluated by Thermogravimetric analysis (TGA). It was performed by heating the sample in a flow of air (100 mL min−1) with rate of 20 °C/min to 800 °C using a Q600 (TA, USA) instrument and the weight loss was recorded. SHIMADZU 4300 (Japan) spectrometer was used for assessment the bonds of element in support and on surface of samples by Fouriertransform infrared spectroscopy (FTIR) method in the range of 400–4000 cm−1. The Brunauer–Emmett–Teller (BET) method for measuring the surface area, pore volume and pore size of the sample using Belsorp mini II (Microtrac Bel Corp, Japan) apparatus was applied. Morphology of the samples was evaluated by the FESEM images took by FEI NOVA Nanosem 450 (Voelte-Keegan Nanoscience Research Center, USA) which was equipped by Energy-dispersive X-ray spectroscopy (EDS) for elemental analysis of the sample. 2.3. Transesterification reaction The transesterification reaction was carried out in the stainless steel reactor (100 mL), which placed in oil bath to control the reaction temperature with error of ± 2 °C, equipped by a thermocouple Type K and a barometer to detect the reaction temperature and pressure, respectively. For each run, the reactor was poured by 20 g canola oil (Mw =850 g/gmole), 11.5 mL methanol (alcohol/oil molar ratio of 12), 3 wt.% of catalyst and the transesterification reaction was performed at 110 °C for 3 h (Alaei et al., 2018). After the reaction, the product mixture was gravitationally separated to two layer containing biodiesel (top layer) and glycerol (bottom layer). After heating the biodiesel to eliminate excess methanol, its fatty acid methyl ester (FAME) content and profile were measured by a gas chromatography (GC-2014, Shimadzu, Japan) equipped with a field emission detector (FID) and a BPX70 (120 m ×0.25 mm ×0.25 μm) column. A sample containing biodiesel/hexane/internal standard at volume ratio of 10/100/1 was injected at split mode (1/100) and the FID and injector temperature were set on 260 °C.
2. Materials and methods 2.1. Catalyst preparation The solution combustion method was utilized for fabrication of the Mg-Al-Fe mixed metal nanocatalysts. Based on the principle, the nitrate salt of precursors according to their stoichiometric amount in the final powder was mixed in distillated water. Then, appropriate amount of urea as fuel (1.5 times of stoichiometric ratio as suggested by Alaie et al. (2018)) was added. The amount of precursors for each synthesized catalyst can be obtained from Eq. (1):
3. Results and discussion 3.1. Magnetic catalyst assessment
Mg (NO3 )2 . 6H2 O + x Al (NO3 )3.9H2 O + (2 − x ) Fe (NO3 )3.9H2 O + 1.5n CO (NH2 )2 + m O2 → MgAl x Fe2 − x O4 + 1.5n CO2
3.1.1. XRD analysis The XRD analysis of the synthesized spinel samples with different Al cation content is presented in Fig. 1. MFA-0 nanocatalyst shows the spinel structure of MgFe2O4 (JCPDS No. 73-1720) as main structure that MgO (JCPDS No. 77-2364) as active phases was supported on it. When Al3+ cation was added to spinel structure, the XRD plot was insignificantly changed due to similar valence of aluminum and ferric. However, the different ionic radius of two cations (78.5 pm for Fe3+ and 67.5 pm for Al3+ in octahedral coordination) causes to slightly shift the XRD peak to higher degree (Fernández et al., 1998). By increasing the Al concentration to 0.4 mol (MFA-0.4), the diffraction peaks of MgAl2O4 (JCPDS No. 75-1803) dispart from magnesium ferric structure. When the highest concentration of aluminum (x = 0.6) was used, the spinel structure of MgFe2O4 and MgAl2O4 can separately observes.
+ (4 + 1.5n) N2 + (24 + 3n) H2 O + (10 + m − 2.25n) O2
(1)
where x is the mole of aluminum ions (0, 0.2, 0.4 and 0.6) and n is the mole of urea used in the stoichiometric ratio (6.67). The mixture was heated at 80 °C to form a viscous gel and placed in microwave oven at maximum power (900 W). After evaporation of excess water, the gel started to boil and huge amounts of smoke egressed from reaction medium. Finally, the mixture was initiated from a point and spontaneously spread to all mixture and the foamy nanocatalyst was obtained. To improve the activity of the sample in the biodiesel production process, the MgO as active phases was loaded on the surface of supports 2
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that this process perform in adiabatic condition. Therefore, the temperature generated during the reaction is called adiabatic temperature (Khot et al., 2012). In this study, the fuel rich (1.5 times of stoichiometric ratio) conditions was chosen for fabrication of the spinel catalysts (see Eq. 1) which implies that the reaction require external oxygen to carry out, completely. Ignition condition and initiation of combustion reaction can be theoretically estimated by thermodynamic equations such as enthalpy of reaction (combustion) and flame temperature (adiabatic temperature) as follow (Kermani et al., 2019; Zhu et al., 2019):
(ΔH )combustion = (∑ (nΔHf )products − (∑ nΔHf )reactants
Q = −(ΔH )combustion =
T adiabatic
∫298
(2)
(∑ nCp)products dT
(3)
where n is the number of moles, ΔHf is heat of formation, ΔH is enthalpy of reaction (combustion), Q is the heat absorbed by products during combustion reaction (under adiabatic condition), T is the reference temperature (T = 298 K) and Cp is the heat capacity of the products at constant pressure. It should be noted that the actual reactions are quite complicated and the equations given are theoretical (Zhu et al., 2019). According to the thermodynamic data presented for various reactants and products involved in combustion (Table 2) and Eqs. (2) and (3), the adiabatic flame temperatures and heat absorbed by products for different Al/Fe ratios were calculated. The results revealed that the reaction heat (Q) was decreased from 840 kcal mol−1 for MFA-0 to 833 kcal mol−1 for MFA-0.6 while adiabatic flame temperature (Tadiabatic) was increased from 1639 °C to 1651 °C. Reduction the heat of reaction leads to inhibition of over growth of the crystals whereas increase of Tadiabatic can provide the fast formation of crystals that can be referred to increase of relative crystallinity and separate formation of both spinel crystals with appropriate intensity as observed in XRD plots (Gribchenkova et al., 2018; Khot et al., 2012; Shornikov, 2017). Fig. 1. XRD pattern of magnetic MgO/MgAlxFe(2-x)O4 fabricated by solution combustion method ((a) x = 0, (b) x = 0.2, (c) x = 0.4 and (d) x = 0.6).
3.1.3. TGA analysis The TGA analysis of the nanocatalyst in the range 25–800 °C is illustrated in Fig. 2. All samples show almost same weight loss during heating process. Due to high combustion reaction temperature, most of the precursors were decomposed to crystal from. The stability of the sample was slightly enhanced by increasing the proportion of Al cation to form MgAl2O4. It can be referred to higher ability of Al cation to make bond with Mg, either spinel form or supported on the surface (Alaei et al., 2018; Rahmani Vahid and Haghighi, 2017). The weight loss (2–2.5 wt.%) up to 200 °C can be attributed to the removal of water absorbed on the surface of catalyst that the samples contain Al cation present higher hydrophilic properties (Nayebzadeh et al., 2019). MFA-0 and MFA-0.2 nanocatalysts show higher weight loss (around 2 wt.%) in the range of 200–500 °C, related to decomposition of remained precursors or elimination of CO2 from surface of catalyst compared to MFA-0.4 and MFA-0.6 nanocatalysts (Yousefi
Moreover, no peaks related to Fe2O3 observe in the plot of the samples indicating the formation of pure phase of spinel phase as support. The relative crystallinity and crystalline size of the samples measured according to the highest intensity peak at 33.5° are listed in Table 1. It was mentioned that the crystallite size depends on the conditions of combustion reaction such as adiabatic flame temperature, amount of released gases and enthalpy of reaction (Hwang and Wu, 2004). Therefore, competition between spinel type of Fe and Al cations affect the crystalline size and relative crystallinity, consequently. The heat of formation of MgAl2O4 is higher than MgFe2O4 that effect on the growth of crystal such that the excessive growth of crystals is prevented and broader peak can be detected. It causes to decline the crystalline size of the sample containing aluminum ions such that MFA-0.6 presents the lowest crystalline size and relative crystallinity.
Table 2 The thermodynamic data of reactants and products involved in combustion reaction.
3.1.2. Thermodynamic analysis According to basic of solution combustion method, it can assume Table 1 Physicochemical properties of magnetic MgO/MgAlxFe(2-x)O4 fabricated by solution combustion method. Nanocatalyst
Al/Fe molar ratio
BET (m2/g)
PV (cm3)
Pd (nm)
Relative Crystallinity
Crystallite size (nm)
MFA-0 MFA-0.2 MFA-0.4 MFA-0.6
0.0/2.0 0.2/1.8 0.4/1.6 0.6/1.4
27.33 41.84 38.96 35.32
0.196 0.163 0.134 0.126
28.6 15.6 13.7 14.3
92.5 100 94.3 70.4
8.2 6.8 9.5 5.6
3
Compound
ΔHf (kcal. mol−1)
Cp (cal mol−1 K)
Fe(NO3)3·9H2O (s) Al(NO3)3·9H2O (s) Mg(NO3)2·6H2O (s) CH2NH2COOH (s) H2O (g) CO2 (g) N2 (g) O2 (g) MgFe2O4 (s) MgAl2O4 (s)
−784 −897.6 −622.2 −79.7 −57.79 −94.05 0 0 −343.7 −547.4
– – – – 7.2 + 0.0036 T 10.34 + 0.00274 T 6.5 + 0.001 T 5.92 + 0.00367 T 34.16 34.2
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related to hydrophilic properties of the nanocatalyst (Sistani et al., 2018). Moreover, OeH stretching and bending vibrations observe at 3200 cm−1 and 1640 cm−1 (Timár et al., 2019). These bands represent the sample containing higher amount of Al cations is more hydrophilic due to higher height of these peaks (narrower and sharper). The results completely support the results of TGA analyses. The small peak at 2360 cm−1 is assigned to CO2 adsorbed from atmosphere on the surface of support as detected in the TGA analysis in the range of 200–500 °C (Zhang et al., 2011). 3.1.5. BET-BJH analysis The BET surface area, pore volume and mean pore size of the MgAlxFe2-xO4 nanocatalysts fabricated by solution combustion method are presented in Table 1. It clearly observes that the surface area increases by incorporation of Al cation. It can be related to different atomic radius of Al compared to Fe that cause some change in the spinel structure (Zhang et al., 2017). As discussed in XRD, reduction the heat formation with increase of Al content hinders growth of crystal and probably smaller particles form that lead to increase the surface area. Another important result is the high surface area of the samples synthesized by combustion method such that the surface area was increased around 5 time compared to same MgFe2O4 prepared by other synthesis method (Shahid et al., 2013; Zhang et al., 2011). It is related to number of mole of gases escaped during combustion which lead to formation of lots pores in powder and increase the surface area (Nayebzadeh et al., 2018). The mean pore size as important factor for controlling the diffusion of reactant inside the porosity was decreased by increase of Al content. It is due to smaller crystalline size of the samples containing Al which caused to the particle growth nearer to each other. However, the porosities are large enough for diffusion of the reactants, especially for triglycerides as large molecules. It was reported that the pore size must be greater than 6 nm for simple diffusion of oil molecules into pores (Nayebzadeh et al., 2017; Rahmani Vahid and Haghighi, 2016). The adsorption-desorption hysteresis of the samples along with their pore size distribution are shown in Fig. 4. All samples present Type IV hysteresis loop which is corresponded to mesoporous material (Yazdani et al., 2019; Zhang et al., 2017). However, by loading of Al cations the hysteresis shape was changed from type H3 for MgFe2O4 (slit-shaped pores) to type H2 for MFA-0.2 (pores closed at one end) and type H1 for MFA-0.4 and MFA-0.6 (cylindrical pore channels) (Hashemzehi et al., 2016b; Kazemifard et al., 2018). Type H1 and H3 are not suitable for biodiesel production reaction due to limitation in diffusion of reactant inside the pores and egression of products outside the pores (Rahmani Vahid and Haghighi, 2017). Therefore, according to pores shape of the samples, it seems that MFA-0.4 and MFA-0.6 nanocatalysts have appropriate structure for diffusion and passing the reactants into porosities and it expects that these samples can convert higher amount of oil to biodiesel. The pore size distribution of the samples also proves the ability of the samples fabricated at higher amount of Al in which uniform pore size distribution observes. The samples show the pores with the size of 7 nm have the highest volume that is suitable for transesterification reaction.
Fig. 2. TGA plot of magnetic MgO/MgAlxFe(2-x)O4 fabricated by solution combustion method ((a) x = 0, (b) x = 0.2, (c) x = 0.4 and (d) x = 0.6).
et al., 2018). It can be corresponded to lower formation enthalpy of MgAl2O4 than MgFe2O4 that causes to less amount of Mg and Fe cations in the form of separate cations remain in the catalyst (Gribchenkova et al., 2018; Khot et al., 2012). The weight loss over 600 °C is assigned to diffusion of some Mg cations in alumina or ferric lattice that is lower than 1 wt.%. It presents the high ability of combustion method for fabrication of spinel type of materials with a pure phase that is consistent with the XRD result. 3.1.4. FTIR analysis The FTIR spectra of the spinel nanocatalyst fabricated by different Fe/Al ratio are plotted in Fig. 3. The absorption bands in the range of 400-1000 cm−1 correspond to stretching vibration of metal-oxygen bonds at tetrahedral and octahedral sites (Kazemifard et al., 2018). They can prove the formation of spinel type of magnesium aluminate and spinel ferrites. When the mole fraction of Al increased in the structure of final powder, the peak in the range of 400-800 cm−1 becomes broader that can be assigned to AleO bonds in the AlO4 and AlO6 configurations (Ooi et al., 2019; Zhang et al., 2020). Moreover, Metal/ OH vibration appears in 1400-1600 cm−1 and 3400–3700 cm−1 that
3.1.6. FESEM analysis The FESEM images of the samples are illustrated in Fig. 5. The porous structure of the samples and observation of many voids on the surface of nanocatalysts is corresponded to the large volume of gases generated during the combustion, which released among the powder and formed mesoporous structure (Barbosa et al., 2019; Hassan et al., 2019). The particle size was clearly reduced by Al doping that is related to reduction the heat of formation whereby the insufficient growth of crystal was limited. Among the samples, MFA-0.4 shows less agglomerated particle along with uniformly distribution of MgO on the surface of support.
Fig. 3. FTIR spectra of magnetic MgO/MgAlxFe(2-x)O4 fabricated by solution combustion method ((a) x = 0, (b) x = 0.2, (c) x = 0.4 and (d) x = 0.6). 4
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Fig. 4. Adsorption-desorption hysteresis and pore size distribution plots of magnetic MgO/MgAlxFe(2-x)O4 fabricated by solution combustion method ((a) x = 0, (b) x = 0.2, (c) x = 0.4 and (d) x = 0.6).
Fe cations were uniformly distributed while MFA-0.4 and MFA-0.6 only exhibit the well distribution of Mg cation. It can effect on the activity of the sample because MgO as active phase has major role in the transesterification reaction.
3.1.7. EDS and dot-mapping analysis The EDS and dot-mapping analyses of the samples are presented in Fig. 6. All samples show no impurities in their structure that confirm the high ability of combustion method for fabrication the nanocatalyst powder with pure structure. Moreover, the elemental composition of the samples is significantly close to parent solution. It can prove well decomposition of precursors to form mixed metal oxides and ability of spinel structure of support to bond with Mg cations as active phases. The distribution of elements (dot-mapping) reveals that the Al and
3.1.8. Magnetic properties The magnetic properties of the samples toward external magnetic medium are depicted in Fig. 7. As expected, by increasing the Al cation, the magnetic property was reduced due to less magnetic property of Al
Fig. 5. FESEM images of magnetic MgO/MgAlxFe(2-x)O4 fabricated by solution combustion method ((a) x = 0, (b) x = 0.2, (c) x = 0.4 and (d) x = 0.6). 5
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Fig. 6. EDS and dot-mapping images of magnetic MgO/MgAlxFe(2-x)O4 fabricated by solution combustion method ((a) x = 0, (b) x = 0.2, (c) x = 0.4 and (d) x = 0.6).
6
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Fig. 7. Magnetic properties of MgO/MgAlxFe(2-x)O4 fabricated by solution combustion method toward external magnetic medium.
compared to Fe. However, the major reduction visually observes for MFA-0.6 nanocatalyst and other samples containing Al show suitable magnetic properties to simple separate after reaction. 3.2. Catalytic activity and stability assessment 3.2.1. Catalytic activity The catalytic activity of the sample in the transesterification reaction is depicted in Fig. 8. As expected from the results, the samples prepared with high amount of Al cation converted higher amount of sunflower oil to biodiesel at the same reaction conditions. These samples presented suitable surface area and pore shape, large pore size, well crystallinity, low crystalline and particle size, and high thermal stability that lead to meaningfully enhancement their activity in the transesterification reaction. On the other hands, for exploitation the magnetic properties of the sample for simple separation after reaction, the sample with the highest amount of Al cation is not suitable. Therefore, MFA-0.4 was chosen as optimum nanocatalyst.
Fig. 9. Reusability of magnetic MgO/MgAl0.4Fe1.6O4 fabricated by solution combustion method in the transesterification reaction.
3.2.3. Comparing the results with other studies The catalyst activity and stability along with transesterification reaction conditions of the MgO/MgAl0.4Fe1.6O4 as optimum nanocatalyst were compared with other similar magnetic nanocatalyst as tabulated in Table 3. The synthesized magnetic nanocatalyst provided suitable conversion as well as other samples while high stability was obtained for several uses with less reduction in the conversion. It seems that loading of aluminum cations in the lattice structure of ferric oxide improved the surface area, pore volume and size, crystalline size and particle shape and size of MgFe2O4 and led to MgO/MgAl0.4Fe1.6O4 nanocatalyst can be a suitable choice for application in biodiesel production process.
3.2.2. Reusability The reusability of MgO/MgAl0.4Fe1.6O4 nanocatalyst as optimum sample in the transesterification reaction was evaluated, as results are shown in Fig. 9. After each reaction, the sample was separated from the reaction medium and washed twice with methanol/hexane (1:1 Vol./ Vol.) solution to eliminate the unreacted reactants and products. After drying in oven for overnight, it was utilized for next reaction. The results represent the high stability of MFA-0.4 in the transesterification reaction such that the activity preserved for 5 time (FAME > 85%). The reduction in the activity can be referred to surface poisoning of the sample, leaching of some MgO particles with weak bonds and filling of some porosities which were not cleaned during washing step. This catalyst has high potential for utilizing in industrial biodiesel production process after further studies on other effective parameters on its activity and stability.
4. Conclusion In this study, transesterification of sunflower oil to biodiesel fuel via a novel magnetic, active and stable nanocatalyst was assessed. For this regard, the solution combustion method as cost effective and simple method was utilized for fabrication a series of MgO/MgAlxFe2-xO4 spinel nanocatalysts with different Al/Fe molar ratio. Due to different atom radius of Al and Fe, formation heat of MgAl2O4 and MgFe2O4, and interaction between MgO as active phases with support surface, the different structural and textural properties were observed. The high content of Al was benefit for fabrication of a catalyst with well crystallinity along with small crystalline size that is related to less formation heat of MgAl2O4 compared with MgFe2O4. Moreover, the BET surface area was significantly increased by loading Al as well as the thermal stability according to increasing the adiabatic flame temperature. Al cations also effect on the pore shape such that the cylindrical porosities were formed at high levels of Al/Fe ratio. These advantages of the sample containing appropriate amount of Al caused to it could convert high amount of sunflower oil to biodiesel due to simple diffusion of reactants into porosities and maximum accessibility to internal surface area. The results revealed that MgO/MgAl0.4Fe1.6O4 nanocatalyst can
Fig. 8. Catalytic activity of magnetic MgO/MgAlxFe(2-x)O4 fabricated by solution combustion method (x = 0, 0.2, 0.4, 0.6) in the transesterification reaction. 7
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Table 3 Comparison of catalytic performance and stability of various magnetic catalysts in biodiesel production process. Catalyst
MgO/MgAl0.4Fe1.6O4 MgO/MgAl2O4 MgO/MgFe2O4 MgO/CaFe2O4 KOH/Fe3O4@γ‐Al2O3 sodium silicate /Fe3O4–MCM-41 MgFe2O4@CaO CoMgFe2O4
Feedstock
Sunflower Sunflower Sunflower Soybean Canola Soybean Soybean Jatropha
Transesterification condition a
T (°C)
MOR
110 110 110 100 65 65 70 60
12 12 12 15 16.2 25 12 7
b
c
Yield (%)
Stability
Ref.
93.2 95 91.2 83.5 97.4 99.2 98.3 92
5 3 3 2 2 4 3 –
This study (Rahmani Vahid and Haghighi, 2017) (Alaei et al., 2018) (Xue et al., 2014) (Ghalandari et al., 2019) (Xie et al., 2018) (Xie and Huang, 2018) (Ita et al., 2018)
d
C (wt.%)
t (h)
3 3 4 4 6.5 3 1 0.5
3 3 4 1 5.5 8 3 3
Abbreviations: T = Reaction temperature; MOR = Methanol/oil molar ratio; C = Catalyst concentration; t = Reaction time.
concern as a useful catalyst for industrial biodiesel production process or other reaction in the liquid phases due to its high stability. It presented appropriate conversion at least for five times without requirement to costly treatment process after each run.
Kazemifard, S., Nayebzadeh, H., Saghatoleslami, N., Safakish, E., 2018. Assessment the activity of magnetic KOH/Fe3O4@Al2O3 core–shell nanocatalyst in transesterification reaction: effect of Fe/Al ratio on structural and performance. Environ. Sci. Pollut. Res. - Int. 25, 32811–32821. Kazemifard, S., Nayebzadeh, H., Saghatoleslami, N., Safakish, E., 2019. Application of magnetic alumina-ferric oxide nanocatalyst supported by KOH for in-situ transesterification of microalgae cultivated in wastewater medium. Biomass Bioenergy 129, 105338. Kermani, F., Mollazadeh, S., Vahdati Khaki, J., 2019. A simple thermodynamics model for estimation and comparison the concentration of oxygen vacancies generated in oxide powders synthesized via the solution combustion method. Ceram. Int. 45, 13496–13501. Khot, V.M., Salunkhe, A.B., Phadatare, M.R., Pawar, S.H., 2012. Formation, microstructure and magnetic properties of nanocrystalline MgFe2O4. Mater. Chem. Phys. 132, 782–787. Liu, Y., Zhang, P., Fan, M., Jiang, P., 2016. Biodiesel production from soybean oil catalyzed by magnetic nanoparticle MgFe2O4@CaO. Fuel 164, 314–321. Mardhiah, H.H., Ong, H.C., Masjuki, H.H., Lim, S., Lee, H.V., 2017. A review on latest developments and future prospects of heterogeneous catalyst in biodiesel production from non-edible oils. Renew. Sustain. Energy Rev. 67, 1225–1236. Mierczynski, P., Chalupka, K.A., Maniukiewicz, W., Kubicki, J., Szynkowska, M.I., Maniecki, T.P., 2015a. SrAl2O4 spinel phase as active phase of transesterification of rapeseed oil. Appl. Catal. B 164, 176–183. Mierczynski, P., Ciesielski, R., Kedziora, A., Maniukiewicz, W., Shtyka, O., Kubicki, J., Albinska, J., Maniecki, T.P., 2015b. Biodiesel production on MgO, CaO, SrO and BaO oxides supported on (SrO)(Al2O3) mixed oxide. Catal. Lett. 145, 1196–1205. Nayebzadeh, H., Haghighi, M., Saghatoleslami, N., Alaei, S., Yousefi, S., 2019. Texture/ phase evolution during plasma treatment of microwave-combustion synthesized KOH/Ca12Al14O33-C nanocatalyst for reusability enhancement in conversion of canola oil to biodiesel. Renew. Energy 139, 28–39. Nayebzadeh, H., Saghatoleslami, N., Haghighi, M., Tabasizadeh, M., 2017. Influence of fuel type on microwave-enhanced fabrication of KOH/Ca12Al14O33 nanocatalyst for biodiesel production via microwave heating. J. Taiwan Inst. Chem. Eng. 75, 148–155. Nayebzadeh, H., Saghatoleslami, N., Haghighi, M., Tabasizadeh, M., Binaeian, E., 2018. Comparative assessment of the ability of a microwave absorber nanocatalyst in the microwave-assisted biodiesel production process. C. R. Chim. 21, 676–683. Ooi, X.Y., Oi, L.E., Choo, M.-Y., Ong, H.C., Lee, H.V., Show, P.L., Lin, Y.-C., Juan, J.C., 2019. Efficient deoxygenation of triglycerides to hydrocarbon-biofuel over mesoporous Al2O3-TiO2 catalyst. Fuel Process. Technol. 194, 106120. Rahmani Vahid, B., Haghighi, M., 2016. Urea-nitrate combustion synthesis of MgO/ MgAl2O4 nanocatalyst used in biodiesel production from sunflower oil: Influence of fuel ratio on catalytic properties and performance. Energy Convers. Manage. 126, 362–372. Rahmani Vahid, B., Haghighi, M., 2017. Biodiesel production from sunflower oil over MgO/MgAl2O4 nanocatalyst: effect of fuel type on catalyst nanostructure and performance. Energy Convers. Manage. 134, 290–300. Rahmani Vahid, B., Haghighi, M., Toghiani, J., Alaei, S., 2018. Hybrid-coprecipitation vs. combustion synthesis of Mg-Al spinel based nanocatalyst for efficient biodiesel production. Energy Convers. Manage. 160, 220–229. Sankaranarayanan, T.M., Shanthi, R.V., Thirunavukkarasu, K., Pandurangan, A., Sivasanker, S., 2013. Catalytic properties of spinel-type mixed oxides in transesterification of vegetable oils. J. Mol. Catal. A Chem. 379, 234–242. Shahid, M., Jingling, L., Ali, Z., Shakir, I., Warsi, M.F., Parveen, R., Nadeem, M., 2013. Photocatalytic degradation of methylene blue on magnetically separable MgFe2O4 under visible light irradiation. Mater. Chem. Phys. 139, 566–571. Shokrollahi, H., 2017. A review of the magnetic properties, synthesis methods and applications of maghemite. J. Magn. Magn. Mater. 426, 74–81. Shornikov, S.I., 2017. Thermodynamic properties of spinel MgAl2O4: a mass spectrometric study. Russ. J. Phys. Chem. 91, 287–294. Sistani, A., Saghatoleslami, N., Nayebzadeh, H., 2018. Influence of calcination temperature on the activity of mesoporous CaO/TiO2–ZrO2 catalyst in the esterification reaction. J. Nanostructure Chem. 8, 321–331. Thangaraj, B., Solomon, P.R., Muniyandi, B., Ranganathan, S., Lin, L., 2018. Catalysis in biodiesel production—a review. Clean Energy 3, 2–23. Timár, Z., Sztankovics, D., Varga, G., Sipos, P., Pálinkó, I., 2019. Esterification reactions with acetate- or benzoate-containing CaAl-layered double hydroxide samples. J. Mol.
Declaration of Competing Interest None. Acknowledgments The authors gratefully acknowledge the financial support from the Research Council of Islamic Azad University, Shahr-e-Qods branch (Grant Number: 32879), as well as Esfarayen University of Technology (Grant Number: 2225). References Alaei, S., Haghighi, M., Toghiani, J., Rahmani Vahid, B., 2018. Magnetic and reusable MgO/MgFe2O4 nanocatalyst for biodiesel production from sunflower oil: influence of fuel ratio in combustion synthesis on catalytic properties and performance. Ind. Crops Prod. 117, 322–332. Ambat, I., Srivastava, V., Sillanpää, M., 2018. Recent advancement in biodiesel production methodologies using various feedstock: a review. Renew. Sustain. Energy Rev. 90, 356–369. Barbosa, W.T., Nascimento, I.V.S.R., Garcia-Carrodeguas, R., Fook, M.V.L., Rodríguez, M.A., 2019. Combustion synthesis and characterization of Sr3Al2O6. Int. J. Appl. Ceram. Technol. 16, 595–601. Coriolano, A.C.F., Alves, A.A., Araujo, R.A., Delgado, R.C.O.B., Carvalho, F.R., Fernandes, V.J., Araujo, A.S., 2017. Thermogravimetry study of the ester interchange of sunflower oil using Mg/Al layered double hydroxides (LDH) impregnated with potassium. J. Therm. Anal. Calorim. 127, 1863–1867. da Costa Evangelista, J.P., Gondim, A.D., Souza, L.D., Araujo, A.S., 2016d. Aluminasupported potassium compounds as heterogeneous catalysts for biodiesel production: a review. Renew. Sustain. Energy Rev. 59, 887–894. Fernández, J.M., Ulibarri, M.A., Labajos, F.M., Rives, V., 1998. The effect of iron on the crystalline phases formed upon thermal decomposition of Mg-Al-Fe hydrotalcites. J. Mater. Chem. 8, 2507–2514. Ghalandari, A., Taghizadeh, M., Rahmani, M., 2019. Statistical optimization of the biodiesel production process using a magnetic core-mesoporous shell KOH/Fe3O4@γAl2O3 nanocatalyst. Chem. Eng. Technol. 42, 89–99. Gribchenkova, N.A., Smorchkov, K.G., Kolmakov, A.G., Alikhanyan, A.S., 2018. Enthalpy of formation of MgAl2O4 · 0.793Al2O3 nonstoichiometric spinel at 1900 K from hightemperature mass spectrometry data. Inorg. Mater. 54, 575–580. Hashemzehi, M., Saghatoleslami, N., Nayebzadeh, H., 2016a. Microwave-assisted solution combustion synthesis of spinel-type mixed oxides for esterification reaction. Chem. Eng. Commun. 204, 415–423. Hashemzehi, M., Saghatoleslami, N., Nayebzadeh, H., 2016b. A study on the structure and catalytic performance of ZnxCu1−xAl2O4 catalysts synthesized by the solution combustion method for the esterification reaction. C. R. Chim. 19, 955–962. Hassan, H.S., Madcour, W.E., Elmaghraby, E.K., 2019. Removal of radioactive cesium and europium from aqueous solutions using activated Al2O3 prepared by solution combustion. Mater. Chem. Phys. 234, 55–66. Hwang, C.-C., Wu, T.-Y., 2004. Synthesis and characterization of nanocrystalline ZnO powders by a novel combustion synthesis method. Mater. Sci. Eng. B 111, 197–206. Ita, B.I., Magu, T.O., Ehi-Eromosele, C.O., 2018. Physico-chemical properties of biodiesel obtained from jatropha curcas seeds oil using CoMgFe2O4 and MgFe2O4 as nanocatalysts. J. Ind. Technol. 3, 1–16. Jamil, F., Al-Haj, L., Al-Muhtaseb Ala’a, H., Al-Hinai Mohab, A., Baawain, M., Rashid, U., Ahmad Mohammad, N.M., 2018. Current scenario of catalysts for biodiesel production: a critical review. Rev. Chem. Eng. 34, 267–297.
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heterogeneous base nanocatalysts supported on a spray dried gamma alumina applying optimized production of biodiesel from waste cooking oil. Biofuels 1–7. Yousefi, S., Haghighi, M., Rahmani Vahid, B., 2018. Facile and efficient microwave combustion fabrication of Mg-spinel as support for MgO nanocatalyst used in biodiesel production from sunflower oil: fuel type approach. Chem. Eng. Res. Des. 138, 506–518. Zhang, J., Wang, X., Ma, L., Yu, X., Ma, Q., Fan, S., Zhao, T., 2017. Preparation of layered K/Mg-Fe-Al catalysts and its catalytic performances in CO hydrogenation. J. Fuel Chem. Technol. 45, 1489–1498. Zhang, L., He, Y., Wu, Y., Wu, T., 2011. Photocatalytic degradation of RhB over MgFe2O4/ TiO2 composite materials. Mater. Sci. Eng. B 176, 1497–1504. Zhang, R., Zhu, F., Dong, Y., Wu, X., Sun, Y., Zhang, D., Zhang, T., Han, M., 2020. Function promotion of SO42−/Al2O3–SnO2 catalyst for biodiesel production from sewage sludge. Renew. Energy 147, 275–283. Zhu, X., Cheng, J., Chen, P., Wei, B., Gao, Y., Gao, D., 2019. Preparation and characterization of nanosized W-Cu powders by a novel solution combustion and hydrogen reduction method. J. Alloys. Compd. 793, 352–359.
Struct. 1186, 303–306. Varma, A., Mukasyan, A.S., Rogachev, A.S., Manukyan, K.V., 2016. Solution combustion synthesis of nanoscale materials. Chem. Rev. 116, 14493–14586. Veljković, V.B., Biberdžić, M.O., Banković-Ilić, I.B., Djalović, I.G., Tasić, M.B., Nježić, Z.B., Stamenković, O.S., 2018. Biodiesel production from corn oil: a review. Renew. Sustain. Energy Rev. 91, 531–548. Wang, Y., Zhang, F., Xu, S., Yang, L., Li, D., Evans, D.G., Duan, X., 2008. Preparation of macrospherical magnesia-rich magnesium aluminate spinel catalysts for methanolysis of soybean oil. Chem. Eng. Sci. 63, 4306–4312. Xie, W., Han, Y., Wang, H., 2018. Magnetic Fe3O4/MCM-41 composite-supported sodium silicate as heterogeneous catalysts for biodiesel production. Renew. Energy 125, 675–681. Xie, W., Huang, M., 2018. Immobilization of Candida rugosa lipase onto graphene oxide Fe3O4 nanocomposite: characterization and application for biodiesel production. Energy Convers. Manage. 159, 42–53. Xue, B.-j., Luo, J., Zhang, F., Fang, Z., 2014. Biodiesel production from soybean and Jatropha oils by magnetic CaFe2O4–Ca2Fe2O5-based catalyst. Energy 68, 584–591. Yazdani, F., Akia, M., Khanbolouk, F., Hamze, H., Arandiyan, H., 2019. Novel
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Performance and stability assessment of Mg-Al-Fe nanocatalyst in the transesterification of sunflower oil: Effect of Al/Fe molar ratio Freshteh Naderia, Hamed Nayebzadehb, a b
T
⁎
Department of Chemistry, Shahr-e-Qods Branch, Islamic Azad University, Tehran, Iran Esfarayen University of Technology, Esfarayen, North Khorasan, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Spinel nanocatalyst Heterogeneous catalyst Solution combustion method Sunflower oil Transesterification Biodiesel
Heterogeneously biodiesel production from vegetable oils is recently concerned for reduction the environmental problems of its homogeneous production process. Therefore, synthesis an active and stable catalyst via simple, cost-effective and industrialization method must be proposed. For this purpose, a series of MgO impregnated on MgAlFe mixed metal catalysts with different Al/Fe molar ratio was prepared via solution combustion method for production of biodiesel from sunflower oil. The results of characterization presented that spinel type of MgFe2O4 was successfully synthesized by combustion method. Moreover, present of Al cation into mixture of precursors caused to the combustion reaction transfer from smoldering to flam reaction that is related to lower heat formation of MgAl2O4. This phenomenon prevents the insufficient growth of crystal and particles. Moreover, due to releasing huge amount of gases during combustion reaction, high reaction temperature and well bonding of active phases with the samples containing Al cations, they showed highly porous structure, high surface area, appropriate pore shape and high thermal stability. These advantages clearly increased their activity in the transesterification reaction. According the results, the MgO/MgAl0.4Fe1.6O4 nanocatalyst was selected as optimum sample that converted 93.2% of sunflower oil to biodiesel at the conditions of 110 °C, 12 M ratio of methanol/oil, 3 wt.% of catalyst and 3 h of reaction time. The sample also exhibited high stability for several uses such that preserved its activity at least for five times (conversion > 85%).
1. Introduction Biodiesel is a renewable energy, non-toxic and biodegradable fuel that is concerned as alternative for petroleum fuels. Biodiesel is produced by conversion of triglycerides of oil using methanol to fatty acid methyl ester (FAME) in the present of catalyst (Ambat et al., 2018). For more environmental friendly production of biodiesel, the heterogeneous catalysts have been suggested that acid, base and enzymatic catalysts are extensively studied (Mardhiah et al., 2017). Main researches are focused on the base heterogeneous catalysts due to their higher activity compared to other kind of heterogeneous catalysts (Veljković et al., 2018). However, some problems toward using the heterogeneous catalyst for biodiesel production such as lower reaction rate than homogeneous catalyst, difficult and long preparation procedure, high price and often low stability cause to industrial heterogeneously biodiesel production is postponed (Jamil et al., 2018). Fabrication of catalysts in nanoscale can largely overcome the low reaction rate of heterogeneous catalysts by providing higher surface area. For this purpose, although various methods such as sol-gel, co-
⁎
precipitation, ball-milling, impregnation, etc. were applied for fabrication of the nanocatalyst, these methods require long time and consume high energy for preparing the desirable nanocatalyst (Jamil et al., 2018; Thangaraj et al., 2018). Coriolano et al. (2017) synthesized Mg/ Al layered double hydroxides (LDH) impregnated with potassium for conversion of sunflower oil to biodiesel. Although high activity of the catalyst was reported, more than 24 h was required for preparation of the catalyst. Recently, solution combustion method was suggested for preparation of the nanocatalyst with high surface area, porous and pure structure, low levels of residual carbon and high stability toward deactivation via simple and cost-effective procedure without using a hightemperature calcination step (Varma et al., 2016). Solution combustion method leads to energy-saving and extremely reduction the preparation time (Hashemzehi et al., 2016a). Rahmani Vahid et al. (2018) studied the activity of MgAl2O4 fabricated by different synthesis method in the biodiesel production process. They reported that the sample synthesized by solution combustion method presented higher crystallinity and porous structure, more similarity the elemental compositions of the
Corresponding author. E-mail address: [email protected] (H. Nayebzadeh).
https://doi.org/10.1016/j.indcrop.2019.111814 Received 13 July 2019; Received in revised form 15 September 2019; Accepted 26 September 2019 0926-6690/ © 2019 Elsevier B.V. All rights reserved.
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final powder to parent solution, and better pore shape which caused to its higher activity. One of the most significant advantages relates to the preparation of doped complex catalysts such as spinel structure (AB2O4) with the highest possible structural homogeneity (Sankaranarayanan et al., 2013). In the past decade, these types of nanocatalyst are concerned for biodiesel production process that divalent material is usually chosen from alkaline earth metal oxides (Ca, Mg, Sr, Ba) for their basic properties and trivalent material usually is Al, Fe and Co (Mierczynski et al., 2015a,b; Xue et al., 2014). MgO/MgAl2O4 (Rahmani Vahid and Haghighi, 2016; Wang et al., 2008; Yousefi et al., 2018) and MgO/ MgFe2O4 (Alaei et al., 2018; Ita et al., 2018; Liu et al., 2016) were evaluated in the biodiesel production process. Due to higher activity of spinel catalyst containing Al (da Costa Evangelista et al., 2016d) and simple separation of the catalyst containing Fe for their magnetic properties (Shokrollahi, 2017), it seem that utilizing both cations can lead to a catalyst with high activity along with simple separate-ability. Zhang et.al (2017) synthesized Mg-Fe-Al hydrotalcite catalyst for CO hydrogenation process. They reported the higher activity of the sample by increasing the Al cations into precursor’s solution. Kazemifard et al. (2019) investigated KOH/Al2O3@Fe3O4 via impregnation-coprecipitation method for biodiesel production from microalgae and mentioned the high catalytic activity and increasing in stability by loading alumina. However, no comprehensive data are available on Mg-Al-Fe mixed metal oxides for the methanolysis of vegetable oil, where the samples can consider as a promising catalyst due to its high activity along with simple separation by applying an external magnetic field. Therefore, in this study, a series of Mg-Al-Fe mixed metal oxides with different Al/Fe ratio as structure of MgAlxFe2-xO4 (x = 0, 0.2, 0.4 and 0.6) was synthesized via solution combustion method to obtain a nanocatalyst with high activity, stability and ability for simple separation from reaction medium. Then, MgO as active phases was impregnated on the supports to enhance their activity in the transesterification reaction for conversion of sunflower oil to biodiesel. The samples were precisely investigated by various techniques and their activity and stability were finally evaluated.
(Yousefi et al., 2018). For this purpose, desirable amount of magnesium nitrate hexahydrate was dissolved in distillated water and appropriate amount of support (MgO/MgAlxFe(2-x)O4 ratio of 20 wt.%) was added. The mixture was stirred at 80 °C for 180 min and was then placed in oven overnight to dry. At the end, the samples were calcined at 550 °C for 3 h to obtain final powder as labeled MFA-X, where X is the mole amount of Al cation (x = 0, 0.2, 0.4, and 0.6). 2.2. Catalyst characterization X-Ray Diffraction (XRD) was utilized for crystalline analysis of the samples by using a PW1730 (PHILIPS) apparatus in the range of 10–80°. The thermal stability of the samples and efficiency of combustion method to decompose the precursors were evaluated by Thermogravimetric analysis (TGA). It was performed by heating the sample in a flow of air (100 mL min−1) with rate of 20 °C/min to 800 °C using a Q600 (TA, USA) instrument and the weight loss was recorded. SHIMADZU 4300 (Japan) spectrometer was used for assessment the bonds of element in support and on surface of samples by Fouriertransform infrared spectroscopy (FTIR) method in the range of 400–4000 cm−1. The Brunauer–Emmett–Teller (BET) method for measuring the surface area, pore volume and pore size of the sample using Belsorp mini II (Microtrac Bel Corp, Japan) apparatus was applied. Morphology of the samples was evaluated by the FESEM images took by FEI NOVA Nanosem 450 (Voelte-Keegan Nanoscience Research Center, USA) which was equipped by Energy-dispersive X-ray spectroscopy (EDS) for elemental analysis of the sample. 2.3. Transesterification reaction The transesterification reaction was carried out in the stainless steel reactor (100 mL), which placed in oil bath to control the reaction temperature with error of ± 2 °C, equipped by a thermocouple Type K and a barometer to detect the reaction temperature and pressure, respectively. For each run, the reactor was poured by 20 g canola oil (Mw =850 g/gmole), 11.5 mL methanol (alcohol/oil molar ratio of 12), 3 wt.% of catalyst and the transesterification reaction was performed at 110 °C for 3 h (Alaei et al., 2018). After the reaction, the product mixture was gravitationally separated to two layer containing biodiesel (top layer) and glycerol (bottom layer). After heating the biodiesel to eliminate excess methanol, its fatty acid methyl ester (FAME) content and profile were measured by a gas chromatography (GC-2014, Shimadzu, Japan) equipped with a field emission detector (FID) and a BPX70 (120 m ×0.25 mm ×0.25 μm) column. A sample containing biodiesel/hexane/internal standard at volume ratio of 10/100/1 was injected at split mode (1/100) and the FID and injector temperature were set on 260 °C.
2. Materials and methods 2.1. Catalyst preparation The solution combustion method was utilized for fabrication of the Mg-Al-Fe mixed metal nanocatalysts. Based on the principle, the nitrate salt of precursors according to their stoichiometric amount in the final powder was mixed in distillated water. Then, appropriate amount of urea as fuel (1.5 times of stoichiometric ratio as suggested by Alaie et al. (2018)) was added. The amount of precursors for each synthesized catalyst can be obtained from Eq. (1):
3. Results and discussion 3.1. Magnetic catalyst assessment
Mg (NO3 )2 . 6H2 O + x Al (NO3 )3.9H2 O + (2 − x ) Fe (NO3 )3.9H2 O + 1.5n CO (NH2 )2 + m O2 → MgAl x Fe2 − x O4 + 1.5n CO2
3.1.1. XRD analysis The XRD analysis of the synthesized spinel samples with different Al cation content is presented in Fig. 1. MFA-0 nanocatalyst shows the spinel structure of MgFe2O4 (JCPDS No. 73-1720) as main structure that MgO (JCPDS No. 77-2364) as active phases was supported on it. When Al3+ cation was added to spinel structure, the XRD plot was insignificantly changed due to similar valence of aluminum and ferric. However, the different ionic radius of two cations (78.5 pm for Fe3+ and 67.5 pm for Al3+ in octahedral coordination) causes to slightly shift the XRD peak to higher degree (Fernández et al., 1998). By increasing the Al concentration to 0.4 mol (MFA-0.4), the diffraction peaks of MgAl2O4 (JCPDS No. 75-1803) dispart from magnesium ferric structure. When the highest concentration of aluminum (x = 0.6) was used, the spinel structure of MgFe2O4 and MgAl2O4 can separately observes.
+ (4 + 1.5n) N2 + (24 + 3n) H2 O + (10 + m − 2.25n) O2
(1)
where x is the mole of aluminum ions (0, 0.2, 0.4 and 0.6) and n is the mole of urea used in the stoichiometric ratio (6.67). The mixture was heated at 80 °C to form a viscous gel and placed in microwave oven at maximum power (900 W). After evaporation of excess water, the gel started to boil and huge amounts of smoke egressed from reaction medium. Finally, the mixture was initiated from a point and spontaneously spread to all mixture and the foamy nanocatalyst was obtained. To improve the activity of the sample in the biodiesel production process, the MgO as active phases was loaded on the surface of supports 2
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that this process perform in adiabatic condition. Therefore, the temperature generated during the reaction is called adiabatic temperature (Khot et al., 2012). In this study, the fuel rich (1.5 times of stoichiometric ratio) conditions was chosen for fabrication of the spinel catalysts (see Eq. 1) which implies that the reaction require external oxygen to carry out, completely. Ignition condition and initiation of combustion reaction can be theoretically estimated by thermodynamic equations such as enthalpy of reaction (combustion) and flame temperature (adiabatic temperature) as follow (Kermani et al., 2019; Zhu et al., 2019):
(ΔH )combustion = (∑ (nΔHf )products − (∑ nΔHf )reactants
Q = −(ΔH )combustion =
T adiabatic
∫298
(2)
(∑ nCp)products dT
(3)
where n is the number of moles, ΔHf is heat of formation, ΔH is enthalpy of reaction (combustion), Q is the heat absorbed by products during combustion reaction (under adiabatic condition), T is the reference temperature (T = 298 K) and Cp is the heat capacity of the products at constant pressure. It should be noted that the actual reactions are quite complicated and the equations given are theoretical (Zhu et al., 2019). According to the thermodynamic data presented for various reactants and products involved in combustion (Table 2) and Eqs. (2) and (3), the adiabatic flame temperatures and heat absorbed by products for different Al/Fe ratios were calculated. The results revealed that the reaction heat (Q) was decreased from 840 kcal mol−1 for MFA-0 to 833 kcal mol−1 for MFA-0.6 while adiabatic flame temperature (Tadiabatic) was increased from 1639 °C to 1651 °C. Reduction the heat of reaction leads to inhibition of over growth of the crystals whereas increase of Tadiabatic can provide the fast formation of crystals that can be referred to increase of relative crystallinity and separate formation of both spinel crystals with appropriate intensity as observed in XRD plots (Gribchenkova et al., 2018; Khot et al., 2012; Shornikov, 2017). Fig. 1. XRD pattern of magnetic MgO/MgAlxFe(2-x)O4 fabricated by solution combustion method ((a) x = 0, (b) x = 0.2, (c) x = 0.4 and (d) x = 0.6).
3.1.3. TGA analysis The TGA analysis of the nanocatalyst in the range 25–800 °C is illustrated in Fig. 2. All samples show almost same weight loss during heating process. Due to high combustion reaction temperature, most of the precursors were decomposed to crystal from. The stability of the sample was slightly enhanced by increasing the proportion of Al cation to form MgAl2O4. It can be referred to higher ability of Al cation to make bond with Mg, either spinel form or supported on the surface (Alaei et al., 2018; Rahmani Vahid and Haghighi, 2017). The weight loss (2–2.5 wt.%) up to 200 °C can be attributed to the removal of water absorbed on the surface of catalyst that the samples contain Al cation present higher hydrophilic properties (Nayebzadeh et al., 2019). MFA-0 and MFA-0.2 nanocatalysts show higher weight loss (around 2 wt.%) in the range of 200–500 °C, related to decomposition of remained precursors or elimination of CO2 from surface of catalyst compared to MFA-0.4 and MFA-0.6 nanocatalysts (Yousefi
Moreover, no peaks related to Fe2O3 observe in the plot of the samples indicating the formation of pure phase of spinel phase as support. The relative crystallinity and crystalline size of the samples measured according to the highest intensity peak at 33.5° are listed in Table 1. It was mentioned that the crystallite size depends on the conditions of combustion reaction such as adiabatic flame temperature, amount of released gases and enthalpy of reaction (Hwang and Wu, 2004). Therefore, competition between spinel type of Fe and Al cations affect the crystalline size and relative crystallinity, consequently. The heat of formation of MgAl2O4 is higher than MgFe2O4 that effect on the growth of crystal such that the excessive growth of crystals is prevented and broader peak can be detected. It causes to decline the crystalline size of the sample containing aluminum ions such that MFA-0.6 presents the lowest crystalline size and relative crystallinity.
Table 2 The thermodynamic data of reactants and products involved in combustion reaction.
3.1.2. Thermodynamic analysis According to basic of solution combustion method, it can assume Table 1 Physicochemical properties of magnetic MgO/MgAlxFe(2-x)O4 fabricated by solution combustion method. Nanocatalyst
Al/Fe molar ratio
BET (m2/g)
PV (cm3)
Pd (nm)
Relative Crystallinity
Crystallite size (nm)
MFA-0 MFA-0.2 MFA-0.4 MFA-0.6
0.0/2.0 0.2/1.8 0.4/1.6 0.6/1.4
27.33 41.84 38.96 35.32
0.196 0.163 0.134 0.126
28.6 15.6 13.7 14.3
92.5 100 94.3 70.4
8.2 6.8 9.5 5.6
3
Compound
ΔHf (kcal. mol−1)
Cp (cal mol−1 K)
Fe(NO3)3·9H2O (s) Al(NO3)3·9H2O (s) Mg(NO3)2·6H2O (s) CH2NH2COOH (s) H2O (g) CO2 (g) N2 (g) O2 (g) MgFe2O4 (s) MgAl2O4 (s)
−784 −897.6 −622.2 −79.7 −57.79 −94.05 0 0 −343.7 −547.4
– – – – 7.2 + 0.0036 T 10.34 + 0.00274 T 6.5 + 0.001 T 5.92 + 0.00367 T 34.16 34.2
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related to hydrophilic properties of the nanocatalyst (Sistani et al., 2018). Moreover, OeH stretching and bending vibrations observe at 3200 cm−1 and 1640 cm−1 (Timár et al., 2019). These bands represent the sample containing higher amount of Al cations is more hydrophilic due to higher height of these peaks (narrower and sharper). The results completely support the results of TGA analyses. The small peak at 2360 cm−1 is assigned to CO2 adsorbed from atmosphere on the surface of support as detected in the TGA analysis in the range of 200–500 °C (Zhang et al., 2011). 3.1.5. BET-BJH analysis The BET surface area, pore volume and mean pore size of the MgAlxFe2-xO4 nanocatalysts fabricated by solution combustion method are presented in Table 1. It clearly observes that the surface area increases by incorporation of Al cation. It can be related to different atomic radius of Al compared to Fe that cause some change in the spinel structure (Zhang et al., 2017). As discussed in XRD, reduction the heat formation with increase of Al content hinders growth of crystal and probably smaller particles form that lead to increase the surface area. Another important result is the high surface area of the samples synthesized by combustion method such that the surface area was increased around 5 time compared to same MgFe2O4 prepared by other synthesis method (Shahid et al., 2013; Zhang et al., 2011). It is related to number of mole of gases escaped during combustion which lead to formation of lots pores in powder and increase the surface area (Nayebzadeh et al., 2018). The mean pore size as important factor for controlling the diffusion of reactant inside the porosity was decreased by increase of Al content. It is due to smaller crystalline size of the samples containing Al which caused to the particle growth nearer to each other. However, the porosities are large enough for diffusion of the reactants, especially for triglycerides as large molecules. It was reported that the pore size must be greater than 6 nm for simple diffusion of oil molecules into pores (Nayebzadeh et al., 2017; Rahmani Vahid and Haghighi, 2016). The adsorption-desorption hysteresis of the samples along with their pore size distribution are shown in Fig. 4. All samples present Type IV hysteresis loop which is corresponded to mesoporous material (Yazdani et al., 2019; Zhang et al., 2017). However, by loading of Al cations the hysteresis shape was changed from type H3 for MgFe2O4 (slit-shaped pores) to type H2 for MFA-0.2 (pores closed at one end) and type H1 for MFA-0.4 and MFA-0.6 (cylindrical pore channels) (Hashemzehi et al., 2016b; Kazemifard et al., 2018). Type H1 and H3 are not suitable for biodiesel production reaction due to limitation in diffusion of reactant inside the pores and egression of products outside the pores (Rahmani Vahid and Haghighi, 2017). Therefore, according to pores shape of the samples, it seems that MFA-0.4 and MFA-0.6 nanocatalysts have appropriate structure for diffusion and passing the reactants into porosities and it expects that these samples can convert higher amount of oil to biodiesel. The pore size distribution of the samples also proves the ability of the samples fabricated at higher amount of Al in which uniform pore size distribution observes. The samples show the pores with the size of 7 nm have the highest volume that is suitable for transesterification reaction.
Fig. 2. TGA plot of magnetic MgO/MgAlxFe(2-x)O4 fabricated by solution combustion method ((a) x = 0, (b) x = 0.2, (c) x = 0.4 and (d) x = 0.6).
et al., 2018). It can be corresponded to lower formation enthalpy of MgAl2O4 than MgFe2O4 that causes to less amount of Mg and Fe cations in the form of separate cations remain in the catalyst (Gribchenkova et al., 2018; Khot et al., 2012). The weight loss over 600 °C is assigned to diffusion of some Mg cations in alumina or ferric lattice that is lower than 1 wt.%. It presents the high ability of combustion method for fabrication of spinel type of materials with a pure phase that is consistent with the XRD result. 3.1.4. FTIR analysis The FTIR spectra of the spinel nanocatalyst fabricated by different Fe/Al ratio are plotted in Fig. 3. The absorption bands in the range of 400-1000 cm−1 correspond to stretching vibration of metal-oxygen bonds at tetrahedral and octahedral sites (Kazemifard et al., 2018). They can prove the formation of spinel type of magnesium aluminate and spinel ferrites. When the mole fraction of Al increased in the structure of final powder, the peak in the range of 400-800 cm−1 becomes broader that can be assigned to AleO bonds in the AlO4 and AlO6 configurations (Ooi et al., 2019; Zhang et al., 2020). Moreover, Metal/ OH vibration appears in 1400-1600 cm−1 and 3400–3700 cm−1 that
3.1.6. FESEM analysis The FESEM images of the samples are illustrated in Fig. 5. The porous structure of the samples and observation of many voids on the surface of nanocatalysts is corresponded to the large volume of gases generated during the combustion, which released among the powder and formed mesoporous structure (Barbosa et al., 2019; Hassan et al., 2019). The particle size was clearly reduced by Al doping that is related to reduction the heat of formation whereby the insufficient growth of crystal was limited. Among the samples, MFA-0.4 shows less agglomerated particle along with uniformly distribution of MgO on the surface of support.
Fig. 3. FTIR spectra of magnetic MgO/MgAlxFe(2-x)O4 fabricated by solution combustion method ((a) x = 0, (b) x = 0.2, (c) x = 0.4 and (d) x = 0.6). 4
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Fig. 4. Adsorption-desorption hysteresis and pore size distribution plots of magnetic MgO/MgAlxFe(2-x)O4 fabricated by solution combustion method ((a) x = 0, (b) x = 0.2, (c) x = 0.4 and (d) x = 0.6).
Fe cations were uniformly distributed while MFA-0.4 and MFA-0.6 only exhibit the well distribution of Mg cation. It can effect on the activity of the sample because MgO as active phase has major role in the transesterification reaction.
3.1.7. EDS and dot-mapping analysis The EDS and dot-mapping analyses of the samples are presented in Fig. 6. All samples show no impurities in their structure that confirm the high ability of combustion method for fabrication the nanocatalyst powder with pure structure. Moreover, the elemental composition of the samples is significantly close to parent solution. It can prove well decomposition of precursors to form mixed metal oxides and ability of spinel structure of support to bond with Mg cations as active phases. The distribution of elements (dot-mapping) reveals that the Al and
3.1.8. Magnetic properties The magnetic properties of the samples toward external magnetic medium are depicted in Fig. 7. As expected, by increasing the Al cation, the magnetic property was reduced due to less magnetic property of Al
Fig. 5. FESEM images of magnetic MgO/MgAlxFe(2-x)O4 fabricated by solution combustion method ((a) x = 0, (b) x = 0.2, (c) x = 0.4 and (d) x = 0.6). 5
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Fig. 6. EDS and dot-mapping images of magnetic MgO/MgAlxFe(2-x)O4 fabricated by solution combustion method ((a) x = 0, (b) x = 0.2, (c) x = 0.4 and (d) x = 0.6).
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Fig. 7. Magnetic properties of MgO/MgAlxFe(2-x)O4 fabricated by solution combustion method toward external magnetic medium.
compared to Fe. However, the major reduction visually observes for MFA-0.6 nanocatalyst and other samples containing Al show suitable magnetic properties to simple separate after reaction. 3.2. Catalytic activity and stability assessment 3.2.1. Catalytic activity The catalytic activity of the sample in the transesterification reaction is depicted in Fig. 8. As expected from the results, the samples prepared with high amount of Al cation converted higher amount of sunflower oil to biodiesel at the same reaction conditions. These samples presented suitable surface area and pore shape, large pore size, well crystallinity, low crystalline and particle size, and high thermal stability that lead to meaningfully enhancement their activity in the transesterification reaction. On the other hands, for exploitation the magnetic properties of the sample for simple separation after reaction, the sample with the highest amount of Al cation is not suitable. Therefore, MFA-0.4 was chosen as optimum nanocatalyst.
Fig. 9. Reusability of magnetic MgO/MgAl0.4Fe1.6O4 fabricated by solution combustion method in the transesterification reaction.
3.2.3. Comparing the results with other studies The catalyst activity and stability along with transesterification reaction conditions of the MgO/MgAl0.4Fe1.6O4 as optimum nanocatalyst were compared with other similar magnetic nanocatalyst as tabulated in Table 3. The synthesized magnetic nanocatalyst provided suitable conversion as well as other samples while high stability was obtained for several uses with less reduction in the conversion. It seems that loading of aluminum cations in the lattice structure of ferric oxide improved the surface area, pore volume and size, crystalline size and particle shape and size of MgFe2O4 and led to MgO/MgAl0.4Fe1.6O4 nanocatalyst can be a suitable choice for application in biodiesel production process.
3.2.2. Reusability The reusability of MgO/MgAl0.4Fe1.6O4 nanocatalyst as optimum sample in the transesterification reaction was evaluated, as results are shown in Fig. 9. After each reaction, the sample was separated from the reaction medium and washed twice with methanol/hexane (1:1 Vol./ Vol.) solution to eliminate the unreacted reactants and products. After drying in oven for overnight, it was utilized for next reaction. The results represent the high stability of MFA-0.4 in the transesterification reaction such that the activity preserved for 5 time (FAME > 85%). The reduction in the activity can be referred to surface poisoning of the sample, leaching of some MgO particles with weak bonds and filling of some porosities which were not cleaned during washing step. This catalyst has high potential for utilizing in industrial biodiesel production process after further studies on other effective parameters on its activity and stability.
4. Conclusion In this study, transesterification of sunflower oil to biodiesel fuel via a novel magnetic, active and stable nanocatalyst was assessed. For this regard, the solution combustion method as cost effective and simple method was utilized for fabrication a series of MgO/MgAlxFe2-xO4 spinel nanocatalysts with different Al/Fe molar ratio. Due to different atom radius of Al and Fe, formation heat of MgAl2O4 and MgFe2O4, and interaction between MgO as active phases with support surface, the different structural and textural properties were observed. The high content of Al was benefit for fabrication of a catalyst with well crystallinity along with small crystalline size that is related to less formation heat of MgAl2O4 compared with MgFe2O4. Moreover, the BET surface area was significantly increased by loading Al as well as the thermal stability according to increasing the adiabatic flame temperature. Al cations also effect on the pore shape such that the cylindrical porosities were formed at high levels of Al/Fe ratio. These advantages of the sample containing appropriate amount of Al caused to it could convert high amount of sunflower oil to biodiesel due to simple diffusion of reactants into porosities and maximum accessibility to internal surface area. The results revealed that MgO/MgAl0.4Fe1.6O4 nanocatalyst can
Fig. 8. Catalytic activity of magnetic MgO/MgAlxFe(2-x)O4 fabricated by solution combustion method (x = 0, 0.2, 0.4, 0.6) in the transesterification reaction. 7
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Table 3 Comparison of catalytic performance and stability of various magnetic catalysts in biodiesel production process. Catalyst
MgO/MgAl0.4Fe1.6O4 MgO/MgAl2O4 MgO/MgFe2O4 MgO/CaFe2O4 KOH/Fe3O4@γ‐Al2O3 sodium silicate /Fe3O4–MCM-41 MgFe2O4@CaO CoMgFe2O4
Feedstock
Sunflower Sunflower Sunflower Soybean Canola Soybean Soybean Jatropha
Transesterification condition a
T (°C)
MOR
110 110 110 100 65 65 70 60
12 12 12 15 16.2 25 12 7
b
c
Yield (%)
Stability
Ref.
93.2 95 91.2 83.5 97.4 99.2 98.3 92
5 3 3 2 2 4 3 –
This study (Rahmani Vahid and Haghighi, 2017) (Alaei et al., 2018) (Xue et al., 2014) (Ghalandari et al., 2019) (Xie et al., 2018) (Xie and Huang, 2018) (Ita et al., 2018)
d
C (wt.%)
t (h)
3 3 4 4 6.5 3 1 0.5
3 3 4 1 5.5 8 3 3
Abbreviations: T = Reaction temperature; MOR = Methanol/oil molar ratio; C = Catalyst concentration; t = Reaction time.
concern as a useful catalyst for industrial biodiesel production process or other reaction in the liquid phases due to its high stability. It presented appropriate conversion at least for five times without requirement to costly treatment process after each run.
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