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Al hydrotalcite catalyst
Energy 107 (2016) 523e531
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Mixed methanol/ethanol on transesterification of waste cooking oil using Mg/Al hydrotalcite catalyst Yingqun Ma a, Qunhui Wang a, b, *, Lu Zheng a, Zhen Gao a, Qiang Wang c, Yuhui Ma a a
Department of Environmental Engineering, University of Science and Technology Beijing, Beijing, 100083, China Beijing Key Laboratory on Resource-oriented Treatment of Industrial Pollutants, Beijing, 100083, China c College of Environmental Science and Engineering, Beijing Forestry University, Beijing, 100083, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 March 2015 Received in revised form 22 February 2016 Accepted 13 April 2016
Biodiesel production from waste cooking oil using calcined Mg/Al HT (hydrotalcite) as heterogeneous catalyst was investigated. This study describes the calcined Mg/Al HT prepared under optimal conditions to catalyse waste cooking oil for biodiesel preparation and proposes a plausible catalysis mechanism. The catalysts were characterised by Fourier Transform-Infrared, X-ray diffraction, Thermal Gravity AnalysisDifferential thermal gravity and BrunnerEmmetTeller measurements. Hydrotalcite with Mg/Al ratio of 3:1 showed a uniform mesoporous structure, excellent crystallinity, high surface area (270.5 m2/g) and good catalytic activity (at 500 C calcination). The highest biodiesel yield obtained was 95.2% under optimised conditions of alcohol/oil molar ratio of 6:1, methanol/ethanol molar ratio of 4:2, catalyst content of 1.5%, reaction time of 2.5 h, reaction temperature of 80 C. Mixed methanol/ethanol showed good synergistic effects as an ester exchange agent, and the catalyst was easily separated and recycled. Therefore, Mg/Al hydrotalcite can effectively catalyse waste cooking oil for biodiesel preparation with mixed methanol/ethanol. © 2016 Published by Elsevier Ltd.
Keywords: Biodiesel Mg/Al hydrotalcite Mixed methanol/ethanol Synergistic effect Catalyst reuse
1. Introduction Depletion and scarcity of fossil oil resources inevitably threatens the national energy security, and the increasing demand for energy and environmental awareness has prompted many researchers to embark on alternative fuel platforms that are environmentally acceptable. The SO2, NOx, and a large number of particles produced from combustion of fossil fuels inflict serious environmental problems [1]. Biodiesel, a sulphur-free, nontoxic and biodegradable renewable fuel [2,3], can greatly reduce the emissions of harmful gases and dust to the environment. Biodiesel is generally produced via the transesterification of vegetable oils or animal fats with low molecular weight alcohols [4]. Currently, biodiesel is commercially produced using a homogeneous catalyst (NaOH, alkali catalysis; H2SO4, acid catalysis) for the transesterification process, because of its high reaction activity and low cost. However, the recovery of these soluble catalysts from the products is time consuming and
* Department of Environmental Engineering, University of Science and Technology Beijing, Beijing, 100083, China. Tel.: þ86 10 6233 2778; fax: þ86 10 82376239. E-mail address: [email protected] (Q. Wang). http://dx.doi.org/10.1016/j.energy.2016.04.066 0360-5442/© 2016 Published by Elsevier Ltd.
difficult, meanwhile, produces a large amount of waste water [5]. To eliminate these problems, heterogeneous solid catalysts are developed and introduced [6]. The use of heterogeneous catalysts makes separation of the product easier. Biodiesel production costs are reportedly reduced when a heterogeneous catalyst is used during transesterification [7]. 3þ The general formula of hydrotalcite (HT) is ½M2þ 1x Mx ðOHÞ2 (An)x/n$mH2O [8], where M2þ and M3þ are divalent (e.g., Mg2þ, Zn2þand Ni2þ) and trivalent (e.g., Al3þ, Ga3þ, Fe3þand Mn3þ) cations, respectively. Here, An is the charge-compensating anion (e.g., 2 3þ 2þ 3þ CO2 3 , Cl and SO4 and x is the molar ratio of M /(M þM ), which is normally between 0.2 and 0.33 [9]. The parameter n is the charge number of interlayer anions and m is the number of crystallisation water in the interlayer [10,11]. The ideal formula of typical HT with a structure similar to Mg(OH)2 is Mg6Al2(OH)16(CO3)$4H2O. HT provides electrons or accepts protons from reactant molecules and features a surface anion hole composed of surface O2 or O2eOH, which is called a free electron centre [12]. Given that HT exhibits an active centre, a large surface area and strong alkalinity [13], after high-temperature calcination, calcined HTs or HT-like solids have been extensively studied as catalysts for transesterification reactions. Hydrotalcites synthesized
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using an alkali-free co-precipitation method in the absence of calcinations reportedly provides no catalytic activity for transesterification reaction [14]. Recently, it reported the employment of calcined MgeAl hydrotalcite (HT) as catalysts Structure-reactivity relationships have been involved in the design of useful HT catalysts for the biodiesel preparation [15e17]. A new heterogeneous catalyst for the transesterification of rdova Reyes et al. [18], the calcined canola oil was reported by co MgeAl hydrotalcite doped with nitratine resulted very effective in the transesterification of canola oil and methanol, the MgeAl hydrotalcite loaded with 7 wt% of nitratine content showed the highest conversion to biodiesel reaching up to 91 wt%, compared to the pristine hydrotalcite substrate, the biodiesel yield up to 30 wt%, but the conversion rate is also not very competitive in the biodiesel industry, the reason may be that the methanol has low solubility with oil leading to have a less contact area with oil, the transesterification only can be conducted in the phase interface, so the whole reaction run progressively slower and the biodiesel yield is unsatisfactory. Moreover, the regeneration of the calcined MgeAl hydrotalcite doped with nitratine catalyst was not mentioned. Martin Hajek et al. [19] reported that MgeAl mixed oxides with a different Mg/Al molar ratio (from 1.8 to 7.2) were prepared by thermal pre-treatment of hydrotalcite-like precursors at 450 C and used as catalysts for transesterification of rapeseed oil. The highest ester yield (78%) was observed for MgeAl mixed oxides pre-treated at 450 C (with Mg/Al molar ratio 7.2) with the low basicity (~124 lmol CO2/g), with specific surface area ~211 m2/g and crystallite size of MgO ~6.5 nm. The ester yield was lower and the regeneration of the catalyst was not reported in the study. Z. Helwani et al. [20] studied the calcined HT promotes the transesterification of Jatropha curcas oil with methanol with 75.2% conversion using re-crystallized HT from its mixed oxides when reaction was carried out at 65 C with a methanol:jatropha oil molar ratio of 12:1, a reaction time of 6 h and a catalyst loading of 4 wt%. The conversion rate is unsatisfactory, the reason may be that the bad dispersion of the MgeAl hydrotalcite catalyst. Besides, the catalyst recycling times was not study and the regeneration of the catalyst tested and evaluated was not mentioned in the study. Brito et al. [21] utilised sunflower and waste oils as raw materials and Mg/Al HTs as solid base catalysts to achieve methyl ester yields of over 90%, the researchers further observed that frying oil has suitable characteristics for the transesterification reaction, with the process yielding biodiesel with properties similar to those obtained from sunflower oil. But the catalyst stability and reutilization was not well, it had some small degree of deactivation in the second cycle and the methyl esters yields decrease. The reason may be that the higher reaction temperature (120 C) for the biodiesel production in the study, the higher reaction temperature more damage on the active basic sites [22]. Moreover, the mechanism of HT catalysing waste frying oil for biodiesel preparation was not reported in the study. From the above, we find that the bad dispersion of the MgeAl hydrotalcite catalyst, the lower conversion rate, the bad catalyst stability and reutilization was the main problem in the previous studies, besides, the regeneration of the catalyst tested and evaluated was not mentioned, the relevant catalysis mechanism of HT catalysed waste frying oil for biodiesel preparation are yet available. In this study, the Mg/Al hydrotalcite was filtered with acetone to achieve good dispersion. Mixed methanol/ethanol as a transesterification agent, waste cooking oil as raw material and calcined Mg/Al HT as a catalyst for biodiesel preparation was explored. The catalysts were characterised by Fourier Transform-Infrared, X-ray diffraction, Thermal Gravity Analysis-Differential thermal gravity and BrunnerEmmetTeller measurements to verify the mechanism of transesterification catalysed by Mg/Al HT. Mixed methanol/
ethanol was preliminarily explored as an ester exchange agent to improve the conversion rate. The catalyst recycling times and the regeneration of the catalyst tested and evaluated was study. These results serve as an important theoretical foundation and basic data for the development and optimisation of biodiesel synthesis. 2. Experiment 2.1. Materials Waste cooking oil was obtained from the restaurant of the University of Science and Technology Beijing, soybean oil is the raw material of the restaurant, so the oil derived from soybean. The components of waste cooking oil (fatty acid) were detected by gas chromatograph and mass spectrum (GCeMS), palmitic, oleic, linoleic and stearic acids were the principal components of waste cooking oil [23]. Methanol (purity 99.8%), ethanol (purity 95%), Sodium methoxide, and other chemicals were all purchased from Sinopharm Chemical Reagent Co., Ltd. 2.2. Catalyst preparation The co-precipitation method for preparing Mg/Al HTs is as follows: Exactly 0.075 mol of Mg(NO3)2$6H2O and 0.025 mol of Al(NO3)3$9H2O were added to 100 mL of distilled water, after which the mixture was poured into a 250 mL three-necked round bottom flask. Then, 0.05 mol of Na2CO3 in 100 mL of distilled water was weighed in a separate funnel and added to the mixture above. The solution pH was maintained at 10 using 4 mol/L NaOH. The final mixture was maintained at room temperature for 24 h with continuous stirring. The mixture was then filtered and washed with deionised water until a pH of 7 was achieved. The mixture was filtered with acetone to achieve good dispersion, and the cake obtained was placed in an acetone-filled beaker with continuous stirring for 1 h. Finally, the acetone solution was filtered, and the sample was dried in an oven at 65 C for 24 h. 2.3. Biodiesel preparation Before biodiesel synthesis, the waste cooking oil was pretreated by the following procedure [23]. First, wipe off the bone, paper, plastic, vegetables and other debris of waste cooking oil manually, then added the activated clay (20 g/100 g oil),centrifugation (4000r/min),filtration, at last, deacidification by utilizing the method of extraction (acid value below 2 mg KOH/g) and remove water (moisture below 0.5%). Transesterification reactions were conducted in a 100 mL Erlenmeyer flask fitted with a condenser. The raw material was 50 mL of pretreated waste cooking oil, and the magnetic stirring rate was 300 rpm. The reaction procedure is as follows: The catalyst was first dispersed in a certain proportion of mixed methanol/ethanol under magnetic stirring. Waste cooking oil and a small amount of NaOH were added to achieve a pH > 7. Samples were taken out from the reaction mixture at specific times. The excess methanol/ethanol was distilled in vacuum, and products were centrifuged, thereby forming three layers. The upper layer was biodiesel, the middle layer was glycerol and the lower layer was a mixture of the catalyst and a small amount of glycerol. After separating these layers, the biodiesel layer was repeatedly washed with water. Finally, the sample was dried in an oven to obtain refined biodiesel. The catalyst and a small amount of glycerol in the residua were separated after removing the glycerol layer. The separated catalyst was collected for recycling. Biodiesel yield (%) was determined as the weight percentage of esters recovered divided by the theoretical weight of the esters [24].
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2.4. Characterisation of HTs Thermal gravimetryedifferential thermal gravimetry (TG-DTG) was performed using a thermogravimetric analyser (HCT-3; Hengjiu, China). Powder XRD (X-ray diffraction) analyses were conducted in a rotating anode diffractometer (Rigaku) using a power of 40 kV 150 mA. Diffraction patterns were recorded in the range of 2q ¼ 10 e100 with a step size of 0.02 . BET (BrunnerEmmetTeller) specific surface areas were measured from N2 adsorption and desorption isotherms determined at 77 K using a QuadraSorb SI (Quantachrome Instruments; USA) surface area and pore size analyser. Fresh HT was degassed at 110 C prior to the measurements. 2.5. Properties evaluation of biodiesel product The principal properties of biodiesel product were determined using standard test methods set by the ASTM D-6751 (American Society for Testing and Materials D-6751). 3. Results and discussion 3.1. Preparation and characterisation of the catalyst
4000
3500
1633
667
(b) the calcined sample
3000
2500
2000
1390
3456
1631
Intensity (a.u.)
3506
(a) fresh HT
shifted toward a lower wave number, which indicates the presence of H2O and eOH in lattice interstices after calcination. Water and carbonates were still observed in the calcined sample, which might be because of the adsorption of water and carbon dioxide during KBr tableting or the incomplete decomposition of carbonic acid ester [26]. Fresh HT and mixed Mg/Al oxide phases were confirmed by XRD analyses (Fig. 2). For fresh HT, the characteristic diffraction peaks of layered double hydroxide structure clearly observed at 11.44 , 22.96 , 34.5 , 38.84 , 45.96 , 60.2 , and 61.22 , which can be attributed to the (003), (006), (009), (015), (018), (110), and (113) crystal planes, respectively. The diffraction peaks showed stable baselines and sharp peaks, which indicates high crystallisation and good regularity. Fig. 2(b) shows that the mixed oxide structure was not completely destroyed; instead, this structure retained some peaks characteristic of fresh HT at relatively low temperatures. The (003) peak shifted to the right-hand side, suggesting the shrinkage of the interlayer distance due to the removal of interlayer H2O molecules. Fig. 2(c) shows that the interlayer structure was completely destroyed after calcination at temperatures beyond 400 C because of water evaporation and CO2 decomposition, 3 which causes the characteristic diffraction peaks to disappear and the crystal structure to change [8]. The 2q values of 43.1 and 62.4 indicated visible diffraction peaks with broad shapes and high crystallinity. Fig. 2(c) shows a pattern with stable baselines and characteristic peaks consisting of MgO but not Al2O3. Fig. 2(d) shows a clear non-uniform peak characteristic of Al2O3 at 34.8 ; this peak indicates that the mixed oxide structure is destroyed, that the structure of the mixed oxides is not uniform, and that phase separation occurs after high-temperature calcination. The XRD patterns obtained indicate that Al3þ occupies octahedral sites in HT before calcination and that the structure of the material is rearranged at high temperatures; here, Al3þ transfers from octahedral to tetrahedral sites and Mg2þ is isomorphously substituted in the mixed oxides [27]. Al3þ enters the MgO lattice and partly substitutes for Mg2þ in the Mg/Al oxides [14]. Al3þ is well dispersed in the catalyst structure and forms an aluminium-doped MgO oxide crystal [28,29]. However, it was not a simple mixture of two oxides, but two ions in the same crystal state. Fig. 3 presents the thermal behaviour of Mg/Al HT. Mg/Al HT presented a typical two-stage decomposition profile with the first stage occurring from 50 C to 200 C and the second stage occurring from 300 C to 400 C; thus, two weight loss peaks were observed between approximately 200 and 400 C. The weight losses in these two stages were approximately 17.5% and 20%, respectively. The
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Fig. 1 presents the infrared spectra of fresh and calcined HT samples. A broad band appearing approximately between 3400 and 3600 cm1 can be attributed to the eOH stretch in the brucite-like layer. The band between 1550 and 1700 cm1 is due to the vibration of angular deformation of interlayer H2O molecules and surface absorbed water-molecules. A strong CO2 absorption band 3 appeared between 1250 and 1450 cm1 because of the positive charges on the surface of newly-formed Al(OH)3, which attracts negative charged anions during HT formation [25]. A large number of NO 3 ions were observed in the solution during early HT for2 mation. As the reaction proceeded, NO 3 was substituted by CO3 . The wave number of the CO2 band was lower than that of the 3 CaCO3 band (1425 cm1), which indicates that the interlayer CO2 3 is connected to hydroxyl groups by strong hydrogen bonds, and results in CeO bonds of CO2 3 generating an asymmetric stretching vibration absorption [16]. The absorption band between 584 and 675 cm1 was attributed to AleO and MgeO stretching vibrations. The absorption bands described are typical of HT absorption. Comparing the curves in Fig. 1(a) and 1(b), the absorption peak intensities of CO2 3 and H2O were weakened after calcination and the stretching vibration absorption band of eOH at 3456 cm1
-1
1500
1000
500
wavenumber(cm ) Fig. 1. Infrared Testing of samples.
525
Fig. 2. X-ray diffraction (XRD) patterns for samples.
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Fig. 3. Thermal gravimetryedifferential thermal gravimetry (TG-DTG) curves of Mg/Al hydrotalcites.
first decomposition stage corresponded to the elimination of physically adsorbed water on the HT surface and interlayer water without destruction of the HT layer structure [30]. The second decomposition stage corresponded to the dehydroxylation of vicinal eOH groups in the HT and escape of interlayer CO2 3 as CO2. The release of water and CO2 increases the porosity of the sample because of the dehydroxylation and interlayer carbonate decomposition. Consequently, a certain amount of micropores were created, which provide basic sites for the reaction and promote the high catalytic activity of Mg/Al oxides. A relatively stable double metal oxide forms between approximately 450 and 550 C [31]. The weight of the product changed slightly between 550 and 800 C. Thus, to obtain high activity, the calcination temperature of Mg/Al HT must exceed 400 C but should not be too high to avoid generating spinels. The specific surface area and pore size are essential performance parameters of a catalyst. The specific surface area of Mg/Al HTs before calcination was 118.7 m2/g. After calcination, the interlayer anion decomposition and radical splitting occurred, and the escape of small molecules formed microporous and mesoporous structures, thereby leading to an increase in specific surface area [32]. Table 1 shows the specific surface area of Mg/Al oxides calcined from 300 C to 600 C. The highest specific surface area of 270.5 m2/ g was obtained at 500 C. Fig. 4(a) shows that the sample exhibits the characteristics of a type IV mesoporous material isotherm after calcination at 500 C and presents an H1-type hysteresis loop when P/P0 is between 0.8 and 0.99. The H1 hysteresis loop is uniformly formed by regular pore shapes, which indicates that Mg/Al oxides have uniform mesoporous structures. BJH (BarretteJoynereHalenda) desorption tests showed that the cumulative pore volume of the product is 0.906 cm3/g and its average pore size is 12.596 nm. Fig. 4(b) reveals that the maximum pore volume appears at a pore diameter of 12.3 nm (pore diameter distribution, 5e35 nm); this condition allows oil and alcohol molecules to enter the pores smoothly.
Fig. 4. (a) The N2 adsorptionedesorption isotherms of calcined sample, and (b) the pore size distribution of the calcined sample.
3.2. Influence of reaction conditions on biodiesel preparation The effect of calcination temperature on the catalytic activity was investigated, and the results showed that the catalytic activity was affected significantly by calcination temperature. Fig. 5 shows the influence of catalyst calcination temperature on catalytic activity. Biodiesel yield reached maximum levels at 500 C. This phenomenon is attributed to interlayer and crystallisation water loss as well as CO2 escape from the Mg/Al oxides, which increases the availability of basic sites and catalytic activity. Further increases in temperature damage the crystal structure, thereby causing the oxides to form spinels or other materials. The pore structure becomes clogged and collapses, thereby decreasing biodiesel yield. According to the characterisation results and catalytic performance observed, the optimum catalyst calcination temperature is 500 C. Fig. 6 shows the biodiesel yield obtained by changing the molar ratio of alcohol/oil, molar ratio of methanol/ethanol, amount of catalysts used and reaction time. The molar ratio of alcohol/oil has an important effect on the transesterification rate. In general, transesterification rate could be improved by increasing the amounts of alcohol, so the high molar ratio of alcohol/oil could enhance the transesterification rate. Although the alcohol/oil theoretical molar ratio was 3 in the
Table 1 BrunnerEmmetTeller (BET) Specific surface area of Mg/Al hydrotalcite under different calcinations temperature. Calcination temperature, C
BET specific surface Area, (m2/g)
Fresh HT 300 400 500 600
118.7 151.4 220.3 270.5 242.5
Fig. 5. Influence of calcination temperature on the biodiesel yield. Reaction conditions: Alcohol/Oil molar ratio 6:1, Catalyst amount 1.5%, Reaction temperature 80 C, Reaction time 2.5 h. Data are means of three replicates with error bars indicating standard deviations.
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Fig. 6. Influence of reaction parameters on yield of biodiesel. Reaction conditions: (a) catalyst amount 1.5%, reaction temperature 80 C, reaction time 2.5 h; (b) Alcohol/Oil molar ratio 6:1, catalyst amount 1.5%, temperature 80 C, reaction time 2.5 h; (c) Alcohol/Oil molar ratio 6:1, Methanol/Ethanol molar ration 4:2, reaction temperature 80 C, reaction time 2.5 h; (d) Alcohol/Oil molar ratio 6:1,Methanol/Ethanol molar ration 4:2, catalyst amount 1.5%, reaction temperature 80 C. (e) Alcohol/Oil molar ratio 6:1, methanol/ethanol molar ration 4:2, amount of catalyst 1.5%, reaction time 2.5 h. Data are means of three replicates with error bars indicating standard deviations.
reaction, the transesterification rate seems to reduce due to a part of alcohol appear in gaseous state during the transesterification process, so the alcohol/oil actual molar ratio need be elevated. As shown in Fig. 6(a), the biodiesel yield reaches a maximum of 95.2% when the alcohol/oil molar ratio is 6:1; when this ratio exceeds 6:1, yield decreases slowly. This phenomenon is attributed to the fact that excess alcohol dilutes the oil and catalyst concentration in the reaction system, which decreases the availability of reaction substrates with active catalyst centres. So the optimum of alcohol/oil molar ratio was 6:1. The influence of methanol/ethanol molar ratio on biodiesel yield was illustrated in Fig. 6(b). When the molar ratio of methanol/ ethanol was 4/2, the biodiesel yield reached 95.2%, it was 8% and 18% respectively higher than that of methanol or ethanol alone as ester exchange agent, this is because waste cooking oil, which is
non-polar, is more soluble in ethanol than in methanol because of the low polarity of the former (0.654) compared with the latter (0.762) [33]. When single methanol as transesterification agent, methanol has low solubility with oil leading to have a less contact area with oil, the transesterification only can be conducted in the phase interface [34], so the biodiesel yield is unsatisfactory. In order to obtain the satisfactory yield, high alcohol/oil molar ratio (24:1) used in the previous study [21], but the excessive alcohol dilutes the oil concentration in the reaction system, weaken the contact of oil and catalyst, so the biodiesel yield was only 90%, moreover, the excess alcohol recycle is power-wasting. When single ethanol as transesterification agent, it not only has long carbon chain and the stronger drawbacks of the steric effect than methanol, but also is easy to happen emulsification effect with oil, lead to the waste cooking oil and ethanol are difficult to touch, so the biodiesel yield
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is lower. However, added a small amount of ethanol as cosolvent, ethanol and oil both have good solubility, enhancing the waste cooking oil and methanol solubility, made alcohol and oil system was more average phase, the high solubility of oil in alcohol mixture and the few mass transfer limitations involved in the reaction mixture contributed to the rapid ester formation. At the same time, methanol alleviate the stronger drawbacks of the steric effect of ethanol and the weakness of easy emulsification effect with oil, mixed methanol/ethanol as ester exchange agent showed a good synergistic effect, consequently improve the yield of biodiesel. When the molar ratio of methanol/ethanol was less than 4/2, due to the characteristics of ethanol, the steric hindrance and emulsification gradually dominate, the negative effect is greater than the positive role of solubility, resulting in a lower of biodiesel yield, and synergy effect is not obvious, so the optimum of molar ratio of methanol/ethanol was 4/2. Due to added a small amount of ethanol as cosolvent, so when the alcohol/oil molar ratio was only 6:1, the biodiesel yield could reach 95.2%. Compare with previous study [21], it obtained the higher yield and save the energy consumption of excess alcohol recycle. As depicted in Fig. 6(c), the influence of catalyst amount on biodiesel yield. Biodiesel yield reached maximum levels when the catalyst weight was 1.5% (w/w of oil). Strong O2 sites increase in number as the catalyst weight increases, and these sites extract Hþ from alcohol to form the catalytic active centre CH3O (Fig. 7). The reaction substrate has superior contact with these catalytic centres under continuous stirring, thereby increasing the yield. As the catalyst amount continues to increase, however, the yield decreases because the reaction reverses as the product concentration increases. Moreover, excess catalysts also cause emulsification or other side reactions, which decrease yields. Thus, a catalyst amount of 1.5% (w/w of oil) was used for biodiesel preparation. The influence of reaction time on biodiesel yield was shown in Fig. 6(d). Yield increased steadily as reaction time increased and then reached a maximum of 95.2% at approximately 2.5 h. The reaction rapidly becomes more positive as reactant molecules collide with and adsorb onto each other. This collision and adsorption of active molecules stabilises after a period of time, and the reaction reaches equilibrium. Thus, we selected 2.5 has the reaction time in subsequent experiments to save energy.
CH3O-
CH3OH
+
In order to investigate the effect of different reaction temperatures on conversion yield, the transesterification processes were carried out at 50, 60, 70, 80, 90and 100 C with 1.5% (w/w of oil) catalyst and a methanol/oil molar ratio of 6 for 2.5 h. In general, the increase in temperature would accelerate the reaction rate for such endothermic reaction. As shown in Fig. 6(e), lower temperatures resulted in a drop of the transesterification because only a small amount of molecules was able to get over the required energy barrier. However, increasing temperature had a positive effect on the conversion of transesterification. Moreover, the reactions became very violent over 80 C, which is in the vicinity of ethanol boiling point, and there would also be a loss of methanol/ethanol at high-temperature. Taking conversion rate, stability and safety into account, a reaction temperature of 80 C was chosen for subsequent tests. This is quite different from the previous reported. Amornmart Chantrasa et al. [22] reported that the best reaction temperature (120 C) for the biodiesel production used the Mg/Al hydrotalcite catalyst. But alcohol was evaporated at the temperatures beyond its boiling point, it will hinder the transesterification reaction process, moreover, the higher reaction temperature more damage on the active basic sites, and lead to the bad catalyst stability and reutilization. 3.3. Reaction mechanism Calcined Mg/Al oxide exhibits strong alkalinity, which makes it a promising catalyst for chemical reactions. According to Di Cosimo et al. [35], pure MgO has strong basic sites that predominantly consist of O2, whilst calcined HTs contain three types of surface low OHe, moderate Mg2þeO2 and Al3þeO2 and strong O2 anion (Lewis) basic sites. Fresh HT presents no substantial catalytic activity, whilst calcination requires dehydration and decarbonisation to activate Lewis basic sites. Based on this theory, we propose a plausible reaction mechanism for oil catalysis by HT (Fig. 7). First, strong surface basic O2 sites extract Hþ from CH3OH to form surface CH3O, which is the primary catalytically active group in the transesterification reaction [36]. Carbonyl carbon atoms of triglyceride molecules then attract CH3O from the surface of the Mg/Al oxides to form a tetrahedral intermediate, which obtains Hþ from the surface of Mg/Al oxides. These triglyceride molecules react with CH3OH to generate CH3O. Finally, rearrangement of tetrahedral
H+ H3CO
O R1
interlayer +
catalyst
OR
surface
C
CH3O
-
R1
C
O
R1
OR
H3 CO
R1
C
O-
+
R1
CH3OH
C
O-
+
CH3O-
ORH+
OR H3 CO
H3CO C ORH+
O-
R1
C
+
C ORH+
H+
H3CO
R1
H3CO -
ROH
O
Fig. 7. Mechanism of calcined Mg/Al HT catalysed transesterification reaction.
O-
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intermediates forms biodiesel and glycerol, where R1 represents the log chain alkyl group.
3.4. Catalyst regeneration The recycling of the catalyst is very important for its commercial application. Therefore, it is quite necessary to evaluate the recyclability of the prepared Mg/Al hydrotalcite catalyst [37,38]. After each experiment, the catalyst was separated from the liquid reaction mixture and washed thoroughly with methanol/ethanol, in order to remove adhered esters and alcohols or other impurities, calcinated in a muffle oven at optimum temperature (500 C) for 1 h before next use. The experiments were repeated under the optimal conditions described in Section 3.2. As shown in Fig. 8, the calcined Mg/Al HT presents high catalytic activity even after seven cycles of reused, the biodiesel yield could still reach 80%, so the catalyst is stable and repeatable. Moreover, the biodiesel yield was gradually decreased as regeneration times increased (Fig. 8). The XRD patterns of the calcined and seven times recycled catalyst (calcination temperature of 500 C) are compared in Fig. 9. It can be seen that all diffraction peaks strength of the sample weakened, crystal plane diffraction peaks widened, that means the diameter of crystal particles diminished. Besides, all diffraction peaks of the sample moved to small angle direction, that means the crystals distance of hydrotalcite structure increased, the reason may be that the dissolution of Mg and Al in the repeated reactions process. Due to the hydrotalcite structure changed, so the catalytic activity declined and lead to the biodiesel yield reduced, the result was accordant with the previous reported by Zheng et al. [39]. As shown in Table 2, The BET surface areas, average pore diameters and basicity densities of the calcined and seven times recycled catalysts (calcination temperature of 500 C) are compared. After the catalyst seven times recycled, BET surface area decreased by 33 m2/g and average pore diameter decreased by 1.8 nm (the reason may be that the catalyst adsorb free fatty acids and glycerol in the repeated reactions process), and the basicity density of the catalyst declined. The previous study illustrated that the catalyst active was related with BET surface area, average pore diameter and basicity density [39], so this may be the reason for the catalytic activity declined and biodiesel yield decreased. Moreover, other reasons for the biodiesel yield declined may be: (1) Catalyst losses in transfer and washing process. Catalyst dosage was 1.5 g in the first time, but only 1.32 g solid catalyst remained after the seventh recycle. (2) Catalyst agglomeration caused by slight saponification. (3) The solid alkali
Fig. 8. Influence of regeneration of catalyst on the yield of biodiesel. Data are means of three replicates with error bars indicating standard deviations.
Fig. 9. XRD patterns of calcined(a) and seven times recycled catalyst(b).
catalyst adsorbed free fatty acids and glycerol strongly and the impurities difficult eliminated in this experiment conditions. In order to keep high yield of biodiesel, it is necessary to add new calcined Mg/Al hydrotalcite catalyst regularly during transesterification process and a more effective catalyst regeneration method should be studied to ensure the catalyst stability and reutilization. 3.5. Biodiesel properties and environmental impact analysis Biodiesel was often used in transportation areas, the property and environmental effect were of crucial importance for this utilization. There have been a lot of research in this area [15,40,41], the index of biodiesel have an important influence for the environment. The Properties of Biodiesel, such as viscosity, density, acid value, moisture, sulphur, congealing point, cetane number, carbon residue et al. were measured. Density is one of the most important parameters of biodiesel. An inherent relationship exists between density and physicochemical properties of biodiesel. Density affects nozzle and oil atomization quality pivotally. Biodiesel density is proportional to the total number of carbon atoms. When the number of carbon atoms increase, a larger output power supply leads to a higher possibility of generating soot particles and contaminate the environment. As shown in Table 3, the ASTM standard indicated that the density should be within 0.82 g/mL to 0.9 g/mL, the biodiesel we produced were all meet this requirement. This limit not only provides effective output power but also decrease the production of quantities of soot particles, which pollute the environment. The SO2 produced from combustion of fossil fuels inflict serious environmental problems. However, we don't found sulphur in the biodiesel which produced by waste cooking oil and mixed methanol/ethanol using Mg/Al hydrotalcite catalyst (Table 3), it may be that the raw material (waste cooking oil, i.e., vegetable oil) contains no sulphur. So the biodiesel produced by waste cooking oil (vegetable oil) is a sulphur-free and nontoxic renewable fuel, can greatly reduce the emissions of harmful gases and dust to the environment. Viscosity has a significant effect on fuel flow and atomization performance. A high kinematic viscosity will cause oil supply difficulties, incomplete combustion, and fuel consumption. Oil fluidity will be extremely high, thus causing incomplete combustion and reduced engine efficiency if the viscosity of biodiesel is low, Table 3 showed that the viscosity of the biodiesel was within the limits of the ASTM
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Table 2 Structure and basicity of calcined and seven times recycled catalyst. Mg/Al hydrotalcitea
BET surface area (m2/g)
Average pore diameter (nm)
Basicity density (mmol/m2)
Calcined Seven times recycled
270.5 237.4
12.6 10.8
1.7 1.5
a
Calcination temperature of 500 C.
Table 3 Properties of biodiesel derived from waste cooking oil method. Property
Density at 20 C Viscosity at 40 C Acid value Moisture Congealing point Sulphur Heating Value Cetane number Free glycerol content Total glycerin content Carbon residue a
Unit
Valuea
ASTM D-6751
g/mL mm2/s mgKOH/g wt% C % MJ/kg e wt% wt% wt%
0.84 3.7 0.05 e 15 e 40.42 51 0.006 0.044 0.08
0.82e0.9 1.9e6.0 <0.8 0.05 e 0.005 e 49 0.020 0.240 0.3
Testing methods specified in ASTM D-6751.
standard (1.9 mm2/s to 6.0 mm2/s), i.e., the biodiesel had good fluidity and combustion efficiency. Acid value, cetane number, and carbon residue et al. were all within the limits of the ASTM standard. No moisture, or sulphur was found in the biodiesel detection process. The result indicates that the biodiesel prepared from waste cooking oil (vegetable oil) was high quality, and environmental friendliness. 4. Conclusion This study demonstrates the effects of different calcination temperatures on the physical, chemical and catalytic properties of Mg/Al hydrotalcite. The TG-DTG curve obtained presented typical two-stage weight loss phenomena with a high degree of crystallinity, BJH desorption pore volume of 0.906 cm3/g and average pore diameter of 12.596 nm. The curve exhibited a uniform mesoporous HT structure after calcination at 500 C. Moreover, the catalytic effect (yield of biodiesel) at 500 C calcination was higher than that at other calcination temperatures. The maximum biodiesel yield reached 95.2% under optimised conditions of alcohol/oil molar ratio of 6:1, methanol/ethanol molar ratio of 4:2, catalyst content of 1.5% and reaction time of 2.5 h. We primarily studied the mechanism of HT catalysing waste cooking oil for biodiesel preparation. Methoxide anions are bonded to the carbonyl carbon atoms of triglyceride molecules to form a tetrahedral intermediate and then recombine Hþ on the HT surface to complete the reaction. Mixed methanol/ethanol as an ester exchange agent generated good synergistic effects and increased biodiesel yield. The properties of biodiesel, such as acid value, viscosity, density, and pour points, were within the limits of the ASTM standards. The catalyst showed high activity and was easily separated and recycled for reaction. Therefore, Mg/Al hydrotalcite effectively catalyses waste cooking oil with mixed methanol/ethanol for biodiesel preparation. Acknowledgements The project was supported by the National Environmental Protection Public Welfare Science and Technology Research Program of China (Project No.201309023) and International cooperation project (2013DFG92600)
References [1] Lim C, Lee J, Hong J, Song C, Han J, Cha JS. Evaluation of regulated and unregulated emissions from a diesel powered vehicle fueled with diesel/biodiesel blends in Korea. Energy 2014;77:533e41. [2] Pullen J, Saeed K. Factors affecting biodiesel engine performance and exhaust emissions e part II: experimental study. Energy 2014;72:17e34. [3] Lund MT, Berntsen TK, Fuglestvedt JS. Climate impacts of short-lived climate forcers versus CO2 from biodiesel: a case of the EU on-road sector. Environ Sci Technol 2014;48:14445e54. [4] Dias JM, Araujo JM, Costa JF, Alvim-Ferraz M, Almeida MF. Biodiesel production from raw castor oil. Energy 2013;53:58e66. [5] Hajek M, Skopal F, Capek L, Cernoch M, Kutalek P. Ethanolysis of rapeseed oil by KOH as homogeneous and as heterogeneous catalyst supported on alumina and CaO. Energy 2012;48:392e7. [6] Meher LC, Sagar DV, Naik SN. Technical aspects of biodiesel production by transesterification: a review. Renew Sustain Energy Rev 2006;10:248e68. [7] Kiss AA, Omota F, Damian AC, Rothenberg G. The heterogeneous advantages biodiesel by catalytic reactive distillation. Top Catal 2006;40:141e50. [8] Cantrell DG, Gillie LJ, Lee AF, Wilson K. Structure-reactivity correlations in Mg/ Al hydrotalcite catalysts for biodiesel synthesis. Appl Catal A General 2005;287:183e90. [9] Palmer SJ, Frost RL, Nguyen T. Thermal decomposition of hydrotalcite with molybdate and vanadate anions in the interlayer. Therm Anal Calorim 2008;92:879e86. [10] Bone JS. Report: development and characterisation of the interlayer chemistry of layered double hydroxides. University of Exeter; 1995. p. 2e7. [11] Wang Q, Tay HH, Guo ZH, Chen LW, Liu Y, Chang Zhong ZY, et al. Morphology and composition controllable synthesis of Mg-Al-CO2 3 hydrotalcites by tuning the synthesis pH and the CO2 capture capacity. Appl Clay Sci 2012;55:18e26. [12] Jiang SL, Zhang FX, Guan NJ. Progress on application of solid base catalysts in catalytic reactions. Petrochem Technol 2006;35:1e10. [13] Newman SP, Jones W. Synthesis, characterization and applications of layered double hydroxides containing organic guests. New J Chem 1998;22:105e15. [14] Xie WL, Peng H, Chen LG. Calcined MgeAl hydrotalcites as solid base catalysts for methanolysis of soybean oil. J Mol Catal A Chem 2006;246:24e32. [15] Sun CJ, Qiu FX, Yang DY, Ye B. Preparation of biodiesel from soybean oil catalyzed by AleCa hydrotalcite loaded with K2CO3 as heterogeneous solid base catalyst. Fuel Process Technol 2014;126:383e91. [16] Reyeroa I, Velascob I, Sanzb O, Montesb M, Arzamendia G, Gandíaa LM. Structured catalysts based on MgeAl hydrotalcite for the synthesis of biodiesel. Catal Today 2013;216:211e9. [17] Adewale P, Dumont M, Ngadi M. Recent trends of biodiesel production from animal fat wastes and associated production techniques. Renew Sustain Energy Rev 2015;45:574e88. rdovaReyes I, Salmones J, Zeifert B, Contreras JL, Rojas F. Transesterification [18] Co of canola oil catalized by calcined MgeAl hydrotalcite doped with nitratine. Chem Eng Sci 2014;119:174e81. [19] Hajek M, Kutalek P, Smolakova L, Troppova I, Apek L, Kubi cka D, et al. Transesterification of rapeseed oil by MgeAl mixed oxides with various Mg/Al molar ratio. Chem Eng J 2015;263:160e7. [20] Helwani Z, Aziz N, Bakar MZA, Mukhtar H, Kim J, Othman MR. Conversion of Jatropha curcas oil into biodiesel using re-crystallized hydrotalcite. Energy Convers Manag 2013;73:128e34. [21] Brito A, Borges ME, Garin M, Hernandez A. Biodiesel production from waste oil using MgeAl layered double hydroxide catalysts. Energy & Fuels 2009;23: 2952e8. [22] Chantrasa A, Phlernjai N, Goodwin JG. Kinetics of hydrotalcite catalyzed transesterification of tricaprylin and methanol for biodiesel synthesis. Chem Eng J 2011;168:333e40. [23] Ma YQ, Wang QH, Zheng L, Gao Z, Yang YJ, Wang N, et al. Biodiesel production using unrefined methanol as transesterification agent and the research of individual effect of impurities. Energy 2015;82:361e9. [24] Shibasaki-Kitakawa N, Kanagawa K, Nakashima K, Yonemoto T. Simultaneous production of high quality biodiesel and glycerin from Jatropha oil using ionexchange resins as catalysts and adsorbent. Bioresour Technol 2013;142: 732e6. [25] Mckenzie AL, Fishel CT, Davis RJ. Investigation of the surface structure and basic properties of calcined hydrotalcites. J Catal 1992;138:547e61. [26] Shen JY, Tu M, Hu C. Structural and surface acid/base properties of hydrotalcite-derived MgAIO oxides calcined at varying temperatures. Solid State Chem 1998;137:295e301.
Y. Ma et al. / Energy 107 (2016) 523e531 [27] Trakampruk W, Pomtangjitlikit S. Palm oil biodiesel synthesized with potassium loaded calcined hydrotalcite and effect of biodiesel blend on elastomer properties. Renew Energy 2012;33:1558e63. [28] Reichle WT, Kang SY, Evethardt DS. The nature of the thermal decomposition of a catalytically active anionic clay mineral. J Catal 1986;101: 352e9. [29] Rey F, Fornes V, Rojo JM. Thermal decomposition of hydrotalcites. An infrared and nuelear magnetic resonance spectro-scopic study. Faraday Trans 1992;88:2233e8. [30] Liu XJ, He HY, Wang YJ, Zhu SL, Piao XL. Transesterification of soybean oil to biodiesel using CaO as a solid base catalyst. Fuel 2008;87:216e21. [31] Navajas A, Campo I, Arzamendi G, Hernandez WY, Bobadilla LF, Centeno MA, et al. Synthesis of biodiesel from the methanolysis of sunflower oil using PURAL(R) MgeAl hydrotalcites as catalyst precursors. Appl Catal B Environ 2010;100:299e309. [32] Ngamcharussrivichai C, Nunthasanti P, Tanachai S, Bunyakiat K. Biodiesel production through transesterification over natural calciums. Fuel Process Technol 2010;91:1409e15. [33] Zakaria ZA, Somehit MN, Sulaiman MR. Preliminary investigation on the antinociceptive properties of haruan (Channa striatus) fillet extracted with various solvent systems. Pakistan J Biological Sci 2004. [34] Ma YQ, Zheng L, Wang QH, Ma HZ, Niu RX, Gao Z. Synergistic Effect of mixed methanol/ethanol on transesterification of waste food oil using p-toluenesulfonic acid as catalyst. Environ Prog Sustain Energy 2015;34:1547e53.
531
[35] Di cosimo JI, D(i)ez VK, Xu M, Iglesia E, Apestegui CR. Structure and surface and catalytic properties of Mg-AI basic oxides. J Catal 1998;178:499e510. [36] Gomes JFP, Puna JFB, Goncalves LM, Bordado JCM. Study on the use of MgAl hydrotalcites as solid heterogeneous catalysts for biodiesel production. Energy 2011;36:6770e8. [37] Yadav GD, Kadam AA. Selective engineering using MgeAl calcined hydrotalcite and microwave irradiation in mono-transesterification of diethyl malonate with cyclohexanol. Chem Eng J 2013;230:547e57. [38] Zheng LP, Xia SX, Hou ZT, Zhang MY, Hou ZY. Transesterification of glycerol with dimethyl carbonate over Mg-Al hydrotalcites. Chin J Catal 2014;35: 310e8. [39] Portugal-Pereira J, Nakatani J, Kurisu KH. Comparative energy and environmental analysis of Jatropha bioelectricity versus biodiesel production in remote areas. Energy 2015;83:284e93. [40] Rakopoulos DC, Rakopoulos CD, Giakoumis EG. Impact of properties of vegetable oil, bio-diesel, ethanol and n-butanol on the combustion and emissions of turbocharged HDDI diesel engine operating under steady and transient conditions. Fuel 2015;156:1e19. [41] Tse H, Leung CW, Cheung CS. Investigation on the combustion characteristics and particulate emissions from a diesel engine fueled with diesel-biodieselethanol blends. Energy 2015;83:343e50.
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Energy journal homepage: www.elsevier.com/locate/energy
Mixed methanol/ethanol on transesterification of waste cooking oil using Mg/Al hydrotalcite catalyst Yingqun Ma a, Qunhui Wang a, b, *, Lu Zheng a, Zhen Gao a, Qiang Wang c, Yuhui Ma a a
Department of Environmental Engineering, University of Science and Technology Beijing, Beijing, 100083, China Beijing Key Laboratory on Resource-oriented Treatment of Industrial Pollutants, Beijing, 100083, China c College of Environmental Science and Engineering, Beijing Forestry University, Beijing, 100083, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 March 2015 Received in revised form 22 February 2016 Accepted 13 April 2016
Biodiesel production from waste cooking oil using calcined Mg/Al HT (hydrotalcite) as heterogeneous catalyst was investigated. This study describes the calcined Mg/Al HT prepared under optimal conditions to catalyse waste cooking oil for biodiesel preparation and proposes a plausible catalysis mechanism. The catalysts were characterised by Fourier Transform-Infrared, X-ray diffraction, Thermal Gravity AnalysisDifferential thermal gravity and BrunnerEmmetTeller measurements. Hydrotalcite with Mg/Al ratio of 3:1 showed a uniform mesoporous structure, excellent crystallinity, high surface area (270.5 m2/g) and good catalytic activity (at 500 C calcination). The highest biodiesel yield obtained was 95.2% under optimised conditions of alcohol/oil molar ratio of 6:1, methanol/ethanol molar ratio of 4:2, catalyst content of 1.5%, reaction time of 2.5 h, reaction temperature of 80 C. Mixed methanol/ethanol showed good synergistic effects as an ester exchange agent, and the catalyst was easily separated and recycled. Therefore, Mg/Al hydrotalcite can effectively catalyse waste cooking oil for biodiesel preparation with mixed methanol/ethanol. © 2016 Published by Elsevier Ltd.
Keywords: Biodiesel Mg/Al hydrotalcite Mixed methanol/ethanol Synergistic effect Catalyst reuse
1. Introduction Depletion and scarcity of fossil oil resources inevitably threatens the national energy security, and the increasing demand for energy and environmental awareness has prompted many researchers to embark on alternative fuel platforms that are environmentally acceptable. The SO2, NOx, and a large number of particles produced from combustion of fossil fuels inflict serious environmental problems [1]. Biodiesel, a sulphur-free, nontoxic and biodegradable renewable fuel [2,3], can greatly reduce the emissions of harmful gases and dust to the environment. Biodiesel is generally produced via the transesterification of vegetable oils or animal fats with low molecular weight alcohols [4]. Currently, biodiesel is commercially produced using a homogeneous catalyst (NaOH, alkali catalysis; H2SO4, acid catalysis) for the transesterification process, because of its high reaction activity and low cost. However, the recovery of these soluble catalysts from the products is time consuming and
* Department of Environmental Engineering, University of Science and Technology Beijing, Beijing, 100083, China. Tel.: þ86 10 6233 2778; fax: þ86 10 82376239. E-mail address: [email protected] (Q. Wang). http://dx.doi.org/10.1016/j.energy.2016.04.066 0360-5442/© 2016 Published by Elsevier Ltd.
difficult, meanwhile, produces a large amount of waste water [5]. To eliminate these problems, heterogeneous solid catalysts are developed and introduced [6]. The use of heterogeneous catalysts makes separation of the product easier. Biodiesel production costs are reportedly reduced when a heterogeneous catalyst is used during transesterification [7]. 3þ The general formula of hydrotalcite (HT) is ½M2þ 1x Mx ðOHÞ2 (An)x/n$mH2O [8], where M2þ and M3þ are divalent (e.g., Mg2þ, Zn2þand Ni2þ) and trivalent (e.g., Al3þ, Ga3þ, Fe3þand Mn3þ) cations, respectively. Here, An is the charge-compensating anion (e.g., 2 3þ 2þ 3þ CO2 3 , Cl and SO4 and x is the molar ratio of M /(M þM ), which is normally between 0.2 and 0.33 [9]. The parameter n is the charge number of interlayer anions and m is the number of crystallisation water in the interlayer [10,11]. The ideal formula of typical HT with a structure similar to Mg(OH)2 is Mg6Al2(OH)16(CO3)$4H2O. HT provides electrons or accepts protons from reactant molecules and features a surface anion hole composed of surface O2 or O2eOH, which is called a free electron centre [12]. Given that HT exhibits an active centre, a large surface area and strong alkalinity [13], after high-temperature calcination, calcined HTs or HT-like solids have been extensively studied as catalysts for transesterification reactions. Hydrotalcites synthesized
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using an alkali-free co-precipitation method in the absence of calcinations reportedly provides no catalytic activity for transesterification reaction [14]. Recently, it reported the employment of calcined MgeAl hydrotalcite (HT) as catalysts Structure-reactivity relationships have been involved in the design of useful HT catalysts for the biodiesel preparation [15e17]. A new heterogeneous catalyst for the transesterification of rdova Reyes et al. [18], the calcined canola oil was reported by co MgeAl hydrotalcite doped with nitratine resulted very effective in the transesterification of canola oil and methanol, the MgeAl hydrotalcite loaded with 7 wt% of nitratine content showed the highest conversion to biodiesel reaching up to 91 wt%, compared to the pristine hydrotalcite substrate, the biodiesel yield up to 30 wt%, but the conversion rate is also not very competitive in the biodiesel industry, the reason may be that the methanol has low solubility with oil leading to have a less contact area with oil, the transesterification only can be conducted in the phase interface, so the whole reaction run progressively slower and the biodiesel yield is unsatisfactory. Moreover, the regeneration of the calcined MgeAl hydrotalcite doped with nitratine catalyst was not mentioned. Martin Hajek et al. [19] reported that MgeAl mixed oxides with a different Mg/Al molar ratio (from 1.8 to 7.2) were prepared by thermal pre-treatment of hydrotalcite-like precursors at 450 C and used as catalysts for transesterification of rapeseed oil. The highest ester yield (78%) was observed for MgeAl mixed oxides pre-treated at 450 C (with Mg/Al molar ratio 7.2) with the low basicity (~124 lmol CO2/g), with specific surface area ~211 m2/g and crystallite size of MgO ~6.5 nm. The ester yield was lower and the regeneration of the catalyst was not reported in the study. Z. Helwani et al. [20] studied the calcined HT promotes the transesterification of Jatropha curcas oil with methanol with 75.2% conversion using re-crystallized HT from its mixed oxides when reaction was carried out at 65 C with a methanol:jatropha oil molar ratio of 12:1, a reaction time of 6 h and a catalyst loading of 4 wt%. The conversion rate is unsatisfactory, the reason may be that the bad dispersion of the MgeAl hydrotalcite catalyst. Besides, the catalyst recycling times was not study and the regeneration of the catalyst tested and evaluated was not mentioned in the study. Brito et al. [21] utilised sunflower and waste oils as raw materials and Mg/Al HTs as solid base catalysts to achieve methyl ester yields of over 90%, the researchers further observed that frying oil has suitable characteristics for the transesterification reaction, with the process yielding biodiesel with properties similar to those obtained from sunflower oil. But the catalyst stability and reutilization was not well, it had some small degree of deactivation in the second cycle and the methyl esters yields decrease. The reason may be that the higher reaction temperature (120 C) for the biodiesel production in the study, the higher reaction temperature more damage on the active basic sites [22]. Moreover, the mechanism of HT catalysing waste frying oil for biodiesel preparation was not reported in the study. From the above, we find that the bad dispersion of the MgeAl hydrotalcite catalyst, the lower conversion rate, the bad catalyst stability and reutilization was the main problem in the previous studies, besides, the regeneration of the catalyst tested and evaluated was not mentioned, the relevant catalysis mechanism of HT catalysed waste frying oil for biodiesel preparation are yet available. In this study, the Mg/Al hydrotalcite was filtered with acetone to achieve good dispersion. Mixed methanol/ethanol as a transesterification agent, waste cooking oil as raw material and calcined Mg/Al HT as a catalyst for biodiesel preparation was explored. The catalysts were characterised by Fourier Transform-Infrared, X-ray diffraction, Thermal Gravity Analysis-Differential thermal gravity and BrunnerEmmetTeller measurements to verify the mechanism of transesterification catalysed by Mg/Al HT. Mixed methanol/
ethanol was preliminarily explored as an ester exchange agent to improve the conversion rate. The catalyst recycling times and the regeneration of the catalyst tested and evaluated was study. These results serve as an important theoretical foundation and basic data for the development and optimisation of biodiesel synthesis. 2. Experiment 2.1. Materials Waste cooking oil was obtained from the restaurant of the University of Science and Technology Beijing, soybean oil is the raw material of the restaurant, so the oil derived from soybean. The components of waste cooking oil (fatty acid) were detected by gas chromatograph and mass spectrum (GCeMS), palmitic, oleic, linoleic and stearic acids were the principal components of waste cooking oil [23]. Methanol (purity 99.8%), ethanol (purity 95%), Sodium methoxide, and other chemicals were all purchased from Sinopharm Chemical Reagent Co., Ltd. 2.2. Catalyst preparation The co-precipitation method for preparing Mg/Al HTs is as follows: Exactly 0.075 mol of Mg(NO3)2$6H2O and 0.025 mol of Al(NO3)3$9H2O were added to 100 mL of distilled water, after which the mixture was poured into a 250 mL three-necked round bottom flask. Then, 0.05 mol of Na2CO3 in 100 mL of distilled water was weighed in a separate funnel and added to the mixture above. The solution pH was maintained at 10 using 4 mol/L NaOH. The final mixture was maintained at room temperature for 24 h with continuous stirring. The mixture was then filtered and washed with deionised water until a pH of 7 was achieved. The mixture was filtered with acetone to achieve good dispersion, and the cake obtained was placed in an acetone-filled beaker with continuous stirring for 1 h. Finally, the acetone solution was filtered, and the sample was dried in an oven at 65 C for 24 h. 2.3. Biodiesel preparation Before biodiesel synthesis, the waste cooking oil was pretreated by the following procedure [23]. First, wipe off the bone, paper, plastic, vegetables and other debris of waste cooking oil manually, then added the activated clay (20 g/100 g oil),centrifugation (4000r/min),filtration, at last, deacidification by utilizing the method of extraction (acid value below 2 mg KOH/g) and remove water (moisture below 0.5%). Transesterification reactions were conducted in a 100 mL Erlenmeyer flask fitted with a condenser. The raw material was 50 mL of pretreated waste cooking oil, and the magnetic stirring rate was 300 rpm. The reaction procedure is as follows: The catalyst was first dispersed in a certain proportion of mixed methanol/ethanol under magnetic stirring. Waste cooking oil and a small amount of NaOH were added to achieve a pH > 7. Samples were taken out from the reaction mixture at specific times. The excess methanol/ethanol was distilled in vacuum, and products were centrifuged, thereby forming three layers. The upper layer was biodiesel, the middle layer was glycerol and the lower layer was a mixture of the catalyst and a small amount of glycerol. After separating these layers, the biodiesel layer was repeatedly washed with water. Finally, the sample was dried in an oven to obtain refined biodiesel. The catalyst and a small amount of glycerol in the residua were separated after removing the glycerol layer. The separated catalyst was collected for recycling. Biodiesel yield (%) was determined as the weight percentage of esters recovered divided by the theoretical weight of the esters [24].
Y. Ma et al. / Energy 107 (2016) 523e531
2.4. Characterisation of HTs Thermal gravimetryedifferential thermal gravimetry (TG-DTG) was performed using a thermogravimetric analyser (HCT-3; Hengjiu, China). Powder XRD (X-ray diffraction) analyses were conducted in a rotating anode diffractometer (Rigaku) using a power of 40 kV 150 mA. Diffraction patterns were recorded in the range of 2q ¼ 10 e100 with a step size of 0.02 . BET (BrunnerEmmetTeller) specific surface areas were measured from N2 adsorption and desorption isotherms determined at 77 K using a QuadraSorb SI (Quantachrome Instruments; USA) surface area and pore size analyser. Fresh HT was degassed at 110 C prior to the measurements. 2.5. Properties evaluation of biodiesel product The principal properties of biodiesel product were determined using standard test methods set by the ASTM D-6751 (American Society for Testing and Materials D-6751). 3. Results and discussion 3.1. Preparation and characterisation of the catalyst
4000
3500
1633
667
(b) the calcined sample
3000
2500
2000
1390
3456
1631
Intensity (a.u.)
3506
(a) fresh HT
shifted toward a lower wave number, which indicates the presence of H2O and eOH in lattice interstices after calcination. Water and carbonates were still observed in the calcined sample, which might be because of the adsorption of water and carbon dioxide during KBr tableting or the incomplete decomposition of carbonic acid ester [26]. Fresh HT and mixed Mg/Al oxide phases were confirmed by XRD analyses (Fig. 2). For fresh HT, the characteristic diffraction peaks of layered double hydroxide structure clearly observed at 11.44 , 22.96 , 34.5 , 38.84 , 45.96 , 60.2 , and 61.22 , which can be attributed to the (003), (006), (009), (015), (018), (110), and (113) crystal planes, respectively. The diffraction peaks showed stable baselines and sharp peaks, which indicates high crystallisation and good regularity. Fig. 2(b) shows that the mixed oxide structure was not completely destroyed; instead, this structure retained some peaks characteristic of fresh HT at relatively low temperatures. The (003) peak shifted to the right-hand side, suggesting the shrinkage of the interlayer distance due to the removal of interlayer H2O molecules. Fig. 2(c) shows that the interlayer structure was completely destroyed after calcination at temperatures beyond 400 C because of water evaporation and CO2 decomposition, 3 which causes the characteristic diffraction peaks to disappear and the crystal structure to change [8]. The 2q values of 43.1 and 62.4 indicated visible diffraction peaks with broad shapes and high crystallinity. Fig. 2(c) shows a pattern with stable baselines and characteristic peaks consisting of MgO but not Al2O3. Fig. 2(d) shows a clear non-uniform peak characteristic of Al2O3 at 34.8 ; this peak indicates that the mixed oxide structure is destroyed, that the structure of the mixed oxides is not uniform, and that phase separation occurs after high-temperature calcination. The XRD patterns obtained indicate that Al3þ occupies octahedral sites in HT before calcination and that the structure of the material is rearranged at high temperatures; here, Al3þ transfers from octahedral to tetrahedral sites and Mg2þ is isomorphously substituted in the mixed oxides [27]. Al3þ enters the MgO lattice and partly substitutes for Mg2þ in the Mg/Al oxides [14]. Al3þ is well dispersed in the catalyst structure and forms an aluminium-doped MgO oxide crystal [28,29]. However, it was not a simple mixture of two oxides, but two ions in the same crystal state. Fig. 3 presents the thermal behaviour of Mg/Al HT. Mg/Al HT presented a typical two-stage decomposition profile with the first stage occurring from 50 C to 200 C and the second stage occurring from 300 C to 400 C; thus, two weight loss peaks were observed between approximately 200 and 400 C. The weight losses in these two stages were approximately 17.5% and 20%, respectively. The
1386
Fig. 1 presents the infrared spectra of fresh and calcined HT samples. A broad band appearing approximately between 3400 and 3600 cm1 can be attributed to the eOH stretch in the brucite-like layer. The band between 1550 and 1700 cm1 is due to the vibration of angular deformation of interlayer H2O molecules and surface absorbed water-molecules. A strong CO2 absorption band 3 appeared between 1250 and 1450 cm1 because of the positive charges on the surface of newly-formed Al(OH)3, which attracts negative charged anions during HT formation [25]. A large number of NO 3 ions were observed in the solution during early HT for2 mation. As the reaction proceeded, NO 3 was substituted by CO3 . The wave number of the CO2 band was lower than that of the 3 CaCO3 band (1425 cm1), which indicates that the interlayer CO2 3 is connected to hydroxyl groups by strong hydrogen bonds, and results in CeO bonds of CO2 3 generating an asymmetric stretching vibration absorption [16]. The absorption band between 584 and 675 cm1 was attributed to AleO and MgeO stretching vibrations. The absorption bands described are typical of HT absorption. Comparing the curves in Fig. 1(a) and 1(b), the absorption peak intensities of CO2 3 and H2O were weakened after calcination and the stretching vibration absorption band of eOH at 3456 cm1
-1
1500
1000
500
wavenumber(cm ) Fig. 1. Infrared Testing of samples.
525
Fig. 2. X-ray diffraction (XRD) patterns for samples.
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Fig. 3. Thermal gravimetryedifferential thermal gravimetry (TG-DTG) curves of Mg/Al hydrotalcites.
first decomposition stage corresponded to the elimination of physically adsorbed water on the HT surface and interlayer water without destruction of the HT layer structure [30]. The second decomposition stage corresponded to the dehydroxylation of vicinal eOH groups in the HT and escape of interlayer CO2 3 as CO2. The release of water and CO2 increases the porosity of the sample because of the dehydroxylation and interlayer carbonate decomposition. Consequently, a certain amount of micropores were created, which provide basic sites for the reaction and promote the high catalytic activity of Mg/Al oxides. A relatively stable double metal oxide forms between approximately 450 and 550 C [31]. The weight of the product changed slightly between 550 and 800 C. Thus, to obtain high activity, the calcination temperature of Mg/Al HT must exceed 400 C but should not be too high to avoid generating spinels. The specific surface area and pore size are essential performance parameters of a catalyst. The specific surface area of Mg/Al HTs before calcination was 118.7 m2/g. After calcination, the interlayer anion decomposition and radical splitting occurred, and the escape of small molecules formed microporous and mesoporous structures, thereby leading to an increase in specific surface area [32]. Table 1 shows the specific surface area of Mg/Al oxides calcined from 300 C to 600 C. The highest specific surface area of 270.5 m2/ g was obtained at 500 C. Fig. 4(a) shows that the sample exhibits the characteristics of a type IV mesoporous material isotherm after calcination at 500 C and presents an H1-type hysteresis loop when P/P0 is between 0.8 and 0.99. The H1 hysteresis loop is uniformly formed by regular pore shapes, which indicates that Mg/Al oxides have uniform mesoporous structures. BJH (BarretteJoynereHalenda) desorption tests showed that the cumulative pore volume of the product is 0.906 cm3/g and its average pore size is 12.596 nm. Fig. 4(b) reveals that the maximum pore volume appears at a pore diameter of 12.3 nm (pore diameter distribution, 5e35 nm); this condition allows oil and alcohol molecules to enter the pores smoothly.
Fig. 4. (a) The N2 adsorptionedesorption isotherms of calcined sample, and (b) the pore size distribution of the calcined sample.
3.2. Influence of reaction conditions on biodiesel preparation The effect of calcination temperature on the catalytic activity was investigated, and the results showed that the catalytic activity was affected significantly by calcination temperature. Fig. 5 shows the influence of catalyst calcination temperature on catalytic activity. Biodiesel yield reached maximum levels at 500 C. This phenomenon is attributed to interlayer and crystallisation water loss as well as CO2 escape from the Mg/Al oxides, which increases the availability of basic sites and catalytic activity. Further increases in temperature damage the crystal structure, thereby causing the oxides to form spinels or other materials. The pore structure becomes clogged and collapses, thereby decreasing biodiesel yield. According to the characterisation results and catalytic performance observed, the optimum catalyst calcination temperature is 500 C. Fig. 6 shows the biodiesel yield obtained by changing the molar ratio of alcohol/oil, molar ratio of methanol/ethanol, amount of catalysts used and reaction time. The molar ratio of alcohol/oil has an important effect on the transesterification rate. In general, transesterification rate could be improved by increasing the amounts of alcohol, so the high molar ratio of alcohol/oil could enhance the transesterification rate. Although the alcohol/oil theoretical molar ratio was 3 in the
Table 1 BrunnerEmmetTeller (BET) Specific surface area of Mg/Al hydrotalcite under different calcinations temperature. Calcination temperature, C
BET specific surface Area, (m2/g)
Fresh HT 300 400 500 600
118.7 151.4 220.3 270.5 242.5
Fig. 5. Influence of calcination temperature on the biodiesel yield. Reaction conditions: Alcohol/Oil molar ratio 6:1, Catalyst amount 1.5%, Reaction temperature 80 C, Reaction time 2.5 h. Data are means of three replicates with error bars indicating standard deviations.
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Fig. 6. Influence of reaction parameters on yield of biodiesel. Reaction conditions: (a) catalyst amount 1.5%, reaction temperature 80 C, reaction time 2.5 h; (b) Alcohol/Oil molar ratio 6:1, catalyst amount 1.5%, temperature 80 C, reaction time 2.5 h; (c) Alcohol/Oil molar ratio 6:1, Methanol/Ethanol molar ration 4:2, reaction temperature 80 C, reaction time 2.5 h; (d) Alcohol/Oil molar ratio 6:1,Methanol/Ethanol molar ration 4:2, catalyst amount 1.5%, reaction temperature 80 C. (e) Alcohol/Oil molar ratio 6:1, methanol/ethanol molar ration 4:2, amount of catalyst 1.5%, reaction time 2.5 h. Data are means of three replicates with error bars indicating standard deviations.
reaction, the transesterification rate seems to reduce due to a part of alcohol appear in gaseous state during the transesterification process, so the alcohol/oil actual molar ratio need be elevated. As shown in Fig. 6(a), the biodiesel yield reaches a maximum of 95.2% when the alcohol/oil molar ratio is 6:1; when this ratio exceeds 6:1, yield decreases slowly. This phenomenon is attributed to the fact that excess alcohol dilutes the oil and catalyst concentration in the reaction system, which decreases the availability of reaction substrates with active catalyst centres. So the optimum of alcohol/oil molar ratio was 6:1. The influence of methanol/ethanol molar ratio on biodiesel yield was illustrated in Fig. 6(b). When the molar ratio of methanol/ ethanol was 4/2, the biodiesel yield reached 95.2%, it was 8% and 18% respectively higher than that of methanol or ethanol alone as ester exchange agent, this is because waste cooking oil, which is
non-polar, is more soluble in ethanol than in methanol because of the low polarity of the former (0.654) compared with the latter (0.762) [33]. When single methanol as transesterification agent, methanol has low solubility with oil leading to have a less contact area with oil, the transesterification only can be conducted in the phase interface [34], so the biodiesel yield is unsatisfactory. In order to obtain the satisfactory yield, high alcohol/oil molar ratio (24:1) used in the previous study [21], but the excessive alcohol dilutes the oil concentration in the reaction system, weaken the contact of oil and catalyst, so the biodiesel yield was only 90%, moreover, the excess alcohol recycle is power-wasting. When single ethanol as transesterification agent, it not only has long carbon chain and the stronger drawbacks of the steric effect than methanol, but also is easy to happen emulsification effect with oil, lead to the waste cooking oil and ethanol are difficult to touch, so the biodiesel yield
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is lower. However, added a small amount of ethanol as cosolvent, ethanol and oil both have good solubility, enhancing the waste cooking oil and methanol solubility, made alcohol and oil system was more average phase, the high solubility of oil in alcohol mixture and the few mass transfer limitations involved in the reaction mixture contributed to the rapid ester formation. At the same time, methanol alleviate the stronger drawbacks of the steric effect of ethanol and the weakness of easy emulsification effect with oil, mixed methanol/ethanol as ester exchange agent showed a good synergistic effect, consequently improve the yield of biodiesel. When the molar ratio of methanol/ethanol was less than 4/2, due to the characteristics of ethanol, the steric hindrance and emulsification gradually dominate, the negative effect is greater than the positive role of solubility, resulting in a lower of biodiesel yield, and synergy effect is not obvious, so the optimum of molar ratio of methanol/ethanol was 4/2. Due to added a small amount of ethanol as cosolvent, so when the alcohol/oil molar ratio was only 6:1, the biodiesel yield could reach 95.2%. Compare with previous study [21], it obtained the higher yield and save the energy consumption of excess alcohol recycle. As depicted in Fig. 6(c), the influence of catalyst amount on biodiesel yield. Biodiesel yield reached maximum levels when the catalyst weight was 1.5% (w/w of oil). Strong O2 sites increase in number as the catalyst weight increases, and these sites extract Hþ from alcohol to form the catalytic active centre CH3O (Fig. 7). The reaction substrate has superior contact with these catalytic centres under continuous stirring, thereby increasing the yield. As the catalyst amount continues to increase, however, the yield decreases because the reaction reverses as the product concentration increases. Moreover, excess catalysts also cause emulsification or other side reactions, which decrease yields. Thus, a catalyst amount of 1.5% (w/w of oil) was used for biodiesel preparation. The influence of reaction time on biodiesel yield was shown in Fig. 6(d). Yield increased steadily as reaction time increased and then reached a maximum of 95.2% at approximately 2.5 h. The reaction rapidly becomes more positive as reactant molecules collide with and adsorb onto each other. This collision and adsorption of active molecules stabilises after a period of time, and the reaction reaches equilibrium. Thus, we selected 2.5 has the reaction time in subsequent experiments to save energy.
CH3O-
CH3OH
+
In order to investigate the effect of different reaction temperatures on conversion yield, the transesterification processes were carried out at 50, 60, 70, 80, 90and 100 C with 1.5% (w/w of oil) catalyst and a methanol/oil molar ratio of 6 for 2.5 h. In general, the increase in temperature would accelerate the reaction rate for such endothermic reaction. As shown in Fig. 6(e), lower temperatures resulted in a drop of the transesterification because only a small amount of molecules was able to get over the required energy barrier. However, increasing temperature had a positive effect on the conversion of transesterification. Moreover, the reactions became very violent over 80 C, which is in the vicinity of ethanol boiling point, and there would also be a loss of methanol/ethanol at high-temperature. Taking conversion rate, stability and safety into account, a reaction temperature of 80 C was chosen for subsequent tests. This is quite different from the previous reported. Amornmart Chantrasa et al. [22] reported that the best reaction temperature (120 C) for the biodiesel production used the Mg/Al hydrotalcite catalyst. But alcohol was evaporated at the temperatures beyond its boiling point, it will hinder the transesterification reaction process, moreover, the higher reaction temperature more damage on the active basic sites, and lead to the bad catalyst stability and reutilization. 3.3. Reaction mechanism Calcined Mg/Al oxide exhibits strong alkalinity, which makes it a promising catalyst for chemical reactions. According to Di Cosimo et al. [35], pure MgO has strong basic sites that predominantly consist of O2, whilst calcined HTs contain three types of surface low OHe, moderate Mg2þeO2 and Al3þeO2 and strong O2 anion (Lewis) basic sites. Fresh HT presents no substantial catalytic activity, whilst calcination requires dehydration and decarbonisation to activate Lewis basic sites. Based on this theory, we propose a plausible reaction mechanism for oil catalysis by HT (Fig. 7). First, strong surface basic O2 sites extract Hþ from CH3OH to form surface CH3O, which is the primary catalytically active group in the transesterification reaction [36]. Carbonyl carbon atoms of triglyceride molecules then attract CH3O from the surface of the Mg/Al oxides to form a tetrahedral intermediate, which obtains Hþ from the surface of Mg/Al oxides. These triglyceride molecules react with CH3OH to generate CH3O. Finally, rearrangement of tetrahedral
H+ H3CO
O R1
interlayer +
catalyst
OR
surface
C
CH3O
-
R1
C
O
R1
OR
H3 CO
R1
C
O-
+
R1
CH3OH
C
O-
+
CH3O-
ORH+
OR H3 CO
H3CO C ORH+
O-
R1
C
+
C ORH+
H+
H3CO
R1
H3CO -
ROH
O
Fig. 7. Mechanism of calcined Mg/Al HT catalysed transesterification reaction.
O-
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intermediates forms biodiesel and glycerol, where R1 represents the log chain alkyl group.
3.4. Catalyst regeneration The recycling of the catalyst is very important for its commercial application. Therefore, it is quite necessary to evaluate the recyclability of the prepared Mg/Al hydrotalcite catalyst [37,38]. After each experiment, the catalyst was separated from the liquid reaction mixture and washed thoroughly with methanol/ethanol, in order to remove adhered esters and alcohols or other impurities, calcinated in a muffle oven at optimum temperature (500 C) for 1 h before next use. The experiments were repeated under the optimal conditions described in Section 3.2. As shown in Fig. 8, the calcined Mg/Al HT presents high catalytic activity even after seven cycles of reused, the biodiesel yield could still reach 80%, so the catalyst is stable and repeatable. Moreover, the biodiesel yield was gradually decreased as regeneration times increased (Fig. 8). The XRD patterns of the calcined and seven times recycled catalyst (calcination temperature of 500 C) are compared in Fig. 9. It can be seen that all diffraction peaks strength of the sample weakened, crystal plane diffraction peaks widened, that means the diameter of crystal particles diminished. Besides, all diffraction peaks of the sample moved to small angle direction, that means the crystals distance of hydrotalcite structure increased, the reason may be that the dissolution of Mg and Al in the repeated reactions process. Due to the hydrotalcite structure changed, so the catalytic activity declined and lead to the biodiesel yield reduced, the result was accordant with the previous reported by Zheng et al. [39]. As shown in Table 2, The BET surface areas, average pore diameters and basicity densities of the calcined and seven times recycled catalysts (calcination temperature of 500 C) are compared. After the catalyst seven times recycled, BET surface area decreased by 33 m2/g and average pore diameter decreased by 1.8 nm (the reason may be that the catalyst adsorb free fatty acids and glycerol in the repeated reactions process), and the basicity density of the catalyst declined. The previous study illustrated that the catalyst active was related with BET surface area, average pore diameter and basicity density [39], so this may be the reason for the catalytic activity declined and biodiesel yield decreased. Moreover, other reasons for the biodiesel yield declined may be: (1) Catalyst losses in transfer and washing process. Catalyst dosage was 1.5 g in the first time, but only 1.32 g solid catalyst remained after the seventh recycle. (2) Catalyst agglomeration caused by slight saponification. (3) The solid alkali
Fig. 8. Influence of regeneration of catalyst on the yield of biodiesel. Data are means of three replicates with error bars indicating standard deviations.
Fig. 9. XRD patterns of calcined(a) and seven times recycled catalyst(b).
catalyst adsorbed free fatty acids and glycerol strongly and the impurities difficult eliminated in this experiment conditions. In order to keep high yield of biodiesel, it is necessary to add new calcined Mg/Al hydrotalcite catalyst regularly during transesterification process and a more effective catalyst regeneration method should be studied to ensure the catalyst stability and reutilization. 3.5. Biodiesel properties and environmental impact analysis Biodiesel was often used in transportation areas, the property and environmental effect were of crucial importance for this utilization. There have been a lot of research in this area [15,40,41], the index of biodiesel have an important influence for the environment. The Properties of Biodiesel, such as viscosity, density, acid value, moisture, sulphur, congealing point, cetane number, carbon residue et al. were measured. Density is one of the most important parameters of biodiesel. An inherent relationship exists between density and physicochemical properties of biodiesel. Density affects nozzle and oil atomization quality pivotally. Biodiesel density is proportional to the total number of carbon atoms. When the number of carbon atoms increase, a larger output power supply leads to a higher possibility of generating soot particles and contaminate the environment. As shown in Table 3, the ASTM standard indicated that the density should be within 0.82 g/mL to 0.9 g/mL, the biodiesel we produced were all meet this requirement. This limit not only provides effective output power but also decrease the production of quantities of soot particles, which pollute the environment. The SO2 produced from combustion of fossil fuels inflict serious environmental problems. However, we don't found sulphur in the biodiesel which produced by waste cooking oil and mixed methanol/ethanol using Mg/Al hydrotalcite catalyst (Table 3), it may be that the raw material (waste cooking oil, i.e., vegetable oil) contains no sulphur. So the biodiesel produced by waste cooking oil (vegetable oil) is a sulphur-free and nontoxic renewable fuel, can greatly reduce the emissions of harmful gases and dust to the environment. Viscosity has a significant effect on fuel flow and atomization performance. A high kinematic viscosity will cause oil supply difficulties, incomplete combustion, and fuel consumption. Oil fluidity will be extremely high, thus causing incomplete combustion and reduced engine efficiency if the viscosity of biodiesel is low, Table 3 showed that the viscosity of the biodiesel was within the limits of the ASTM
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Table 2 Structure and basicity of calcined and seven times recycled catalyst. Mg/Al hydrotalcitea
BET surface area (m2/g)
Average pore diameter (nm)
Basicity density (mmol/m2)
Calcined Seven times recycled
270.5 237.4
12.6 10.8
1.7 1.5
a
Calcination temperature of 500 C.
Table 3 Properties of biodiesel derived from waste cooking oil method. Property
Density at 20 C Viscosity at 40 C Acid value Moisture Congealing point Sulphur Heating Value Cetane number Free glycerol content Total glycerin content Carbon residue a
Unit
Valuea
ASTM D-6751
g/mL mm2/s mgKOH/g wt% C % MJ/kg e wt% wt% wt%
0.84 3.7 0.05 e 15 e 40.42 51 0.006 0.044 0.08
0.82e0.9 1.9e6.0 <0.8 0.05 e 0.005 e 49 0.020 0.240 0.3
Testing methods specified in ASTM D-6751.
standard (1.9 mm2/s to 6.0 mm2/s), i.e., the biodiesel had good fluidity and combustion efficiency. Acid value, cetane number, and carbon residue et al. were all within the limits of the ASTM standard. No moisture, or sulphur was found in the biodiesel detection process. The result indicates that the biodiesel prepared from waste cooking oil (vegetable oil) was high quality, and environmental friendliness. 4. Conclusion This study demonstrates the effects of different calcination temperatures on the physical, chemical and catalytic properties of Mg/Al hydrotalcite. The TG-DTG curve obtained presented typical two-stage weight loss phenomena with a high degree of crystallinity, BJH desorption pore volume of 0.906 cm3/g and average pore diameter of 12.596 nm. The curve exhibited a uniform mesoporous HT structure after calcination at 500 C. Moreover, the catalytic effect (yield of biodiesel) at 500 C calcination was higher than that at other calcination temperatures. The maximum biodiesel yield reached 95.2% under optimised conditions of alcohol/oil molar ratio of 6:1, methanol/ethanol molar ratio of 4:2, catalyst content of 1.5% and reaction time of 2.5 h. We primarily studied the mechanism of HT catalysing waste cooking oil for biodiesel preparation. Methoxide anions are bonded to the carbonyl carbon atoms of triglyceride molecules to form a tetrahedral intermediate and then recombine Hþ on the HT surface to complete the reaction. Mixed methanol/ethanol as an ester exchange agent generated good synergistic effects and increased biodiesel yield. The properties of biodiesel, such as acid value, viscosity, density, and pour points, were within the limits of the ASTM standards. The catalyst showed high activity and was easily separated and recycled for reaction. Therefore, Mg/Al hydrotalcite effectively catalyses waste cooking oil with mixed methanol/ethanol for biodiesel preparation. Acknowledgements The project was supported by the National Environmental Protection Public Welfare Science and Technology Research Program of China (Project No.201309023) and International cooperation project (2013DFG92600)
References [1] Lim C, Lee J, Hong J, Song C, Han J, Cha JS. Evaluation of regulated and unregulated emissions from a diesel powered vehicle fueled with diesel/biodiesel blends in Korea. Energy 2014;77:533e41. [2] Pullen J, Saeed K. Factors affecting biodiesel engine performance and exhaust emissions e part II: experimental study. Energy 2014;72:17e34. [3] Lund MT, Berntsen TK, Fuglestvedt JS. Climate impacts of short-lived climate forcers versus CO2 from biodiesel: a case of the EU on-road sector. Environ Sci Technol 2014;48:14445e54. [4] Dias JM, Araujo JM, Costa JF, Alvim-Ferraz M, Almeida MF. Biodiesel production from raw castor oil. Energy 2013;53:58e66. [5] Hajek M, Skopal F, Capek L, Cernoch M, Kutalek P. Ethanolysis of rapeseed oil by KOH as homogeneous and as heterogeneous catalyst supported on alumina and CaO. Energy 2012;48:392e7. [6] Meher LC, Sagar DV, Naik SN. Technical aspects of biodiesel production by transesterification: a review. Renew Sustain Energy Rev 2006;10:248e68. [7] Kiss AA, Omota F, Damian AC, Rothenberg G. The heterogeneous advantages biodiesel by catalytic reactive distillation. Top Catal 2006;40:141e50. [8] Cantrell DG, Gillie LJ, Lee AF, Wilson K. Structure-reactivity correlations in Mg/ Al hydrotalcite catalysts for biodiesel synthesis. Appl Catal A General 2005;287:183e90. [9] Palmer SJ, Frost RL, Nguyen T. Thermal decomposition of hydrotalcite with molybdate and vanadate anions in the interlayer. Therm Anal Calorim 2008;92:879e86. [10] Bone JS. Report: development and characterisation of the interlayer chemistry of layered double hydroxides. University of Exeter; 1995. p. 2e7. [11] Wang Q, Tay HH, Guo ZH, Chen LW, Liu Y, Chang Zhong ZY, et al. Morphology and composition controllable synthesis of Mg-Al-CO2 3 hydrotalcites by tuning the synthesis pH and the CO2 capture capacity. Appl Clay Sci 2012;55:18e26. [12] Jiang SL, Zhang FX, Guan NJ. Progress on application of solid base catalysts in catalytic reactions. Petrochem Technol 2006;35:1e10. [13] Newman SP, Jones W. Synthesis, characterization and applications of layered double hydroxides containing organic guests. New J Chem 1998;22:105e15. [14] Xie WL, Peng H, Chen LG. Calcined MgeAl hydrotalcites as solid base catalysts for methanolysis of soybean oil. J Mol Catal A Chem 2006;246:24e32. [15] Sun CJ, Qiu FX, Yang DY, Ye B. Preparation of biodiesel from soybean oil catalyzed by AleCa hydrotalcite loaded with K2CO3 as heterogeneous solid base catalyst. Fuel Process Technol 2014;126:383e91. [16] Reyeroa I, Velascob I, Sanzb O, Montesb M, Arzamendia G, Gandíaa LM. Structured catalysts based on MgeAl hydrotalcite for the synthesis of biodiesel. Catal Today 2013;216:211e9. [17] Adewale P, Dumont M, Ngadi M. Recent trends of biodiesel production from animal fat wastes and associated production techniques. Renew Sustain Energy Rev 2015;45:574e88. rdovaReyes I, Salmones J, Zeifert B, Contreras JL, Rojas F. Transesterification [18] Co of canola oil catalized by calcined MgeAl hydrotalcite doped with nitratine. Chem Eng Sci 2014;119:174e81. [19] Hajek M, Kutalek P, Smolakova L, Troppova I, Apek L, Kubi cka D, et al. Transesterification of rapeseed oil by MgeAl mixed oxides with various Mg/Al molar ratio. Chem Eng J 2015;263:160e7. [20] Helwani Z, Aziz N, Bakar MZA, Mukhtar H, Kim J, Othman MR. Conversion of Jatropha curcas oil into biodiesel using re-crystallized hydrotalcite. Energy Convers Manag 2013;73:128e34. [21] Brito A, Borges ME, Garin M, Hernandez A. Biodiesel production from waste oil using MgeAl layered double hydroxide catalysts. Energy & Fuels 2009;23: 2952e8. [22] Chantrasa A, Phlernjai N, Goodwin JG. Kinetics of hydrotalcite catalyzed transesterification of tricaprylin and methanol for biodiesel synthesis. Chem Eng J 2011;168:333e40. [23] Ma YQ, Wang QH, Zheng L, Gao Z, Yang YJ, Wang N, et al. Biodiesel production using unrefined methanol as transesterification agent and the research of individual effect of impurities. Energy 2015;82:361e9. [24] Shibasaki-Kitakawa N, Kanagawa K, Nakashima K, Yonemoto T. Simultaneous production of high quality biodiesel and glycerin from Jatropha oil using ionexchange resins as catalysts and adsorbent. Bioresour Technol 2013;142: 732e6. [25] Mckenzie AL, Fishel CT, Davis RJ. Investigation of the surface structure and basic properties of calcined hydrotalcites. J Catal 1992;138:547e61. [26] Shen JY, Tu M, Hu C. Structural and surface acid/base properties of hydrotalcite-derived MgAIO oxides calcined at varying temperatures. Solid State Chem 1998;137:295e301.
Y. Ma et al. / Energy 107 (2016) 523e531 [27] Trakampruk W, Pomtangjitlikit S. Palm oil biodiesel synthesized with potassium loaded calcined hydrotalcite and effect of biodiesel blend on elastomer properties. Renew Energy 2012;33:1558e63. [28] Reichle WT, Kang SY, Evethardt DS. The nature of the thermal decomposition of a catalytically active anionic clay mineral. J Catal 1986;101: 352e9. [29] Rey F, Fornes V, Rojo JM. Thermal decomposition of hydrotalcites. An infrared and nuelear magnetic resonance spectro-scopic study. Faraday Trans 1992;88:2233e8. [30] Liu XJ, He HY, Wang YJ, Zhu SL, Piao XL. Transesterification of soybean oil to biodiesel using CaO as a solid base catalyst. Fuel 2008;87:216e21. [31] Navajas A, Campo I, Arzamendi G, Hernandez WY, Bobadilla LF, Centeno MA, et al. Synthesis of biodiesel from the methanolysis of sunflower oil using PURAL(R) MgeAl hydrotalcites as catalyst precursors. Appl Catal B Environ 2010;100:299e309. [32] Ngamcharussrivichai C, Nunthasanti P, Tanachai S, Bunyakiat K. Biodiesel production through transesterification over natural calciums. Fuel Process Technol 2010;91:1409e15. [33] Zakaria ZA, Somehit MN, Sulaiman MR. Preliminary investigation on the antinociceptive properties of haruan (Channa striatus) fillet extracted with various solvent systems. Pakistan J Biological Sci 2004. [34] Ma YQ, Zheng L, Wang QH, Ma HZ, Niu RX, Gao Z. Synergistic Effect of mixed methanol/ethanol on transesterification of waste food oil using p-toluenesulfonic acid as catalyst. Environ Prog Sustain Energy 2015;34:1547e53.
531
[35] Di cosimo JI, D(i)ez VK, Xu M, Iglesia E, Apestegui CR. Structure and surface and catalytic properties of Mg-AI basic oxides. J Catal 1998;178:499e510. [36] Gomes JFP, Puna JFB, Goncalves LM, Bordado JCM. Study on the use of MgAl hydrotalcites as solid heterogeneous catalysts for biodiesel production. Energy 2011;36:6770e8. [37] Yadav GD, Kadam AA. Selective engineering using MgeAl calcined hydrotalcite and microwave irradiation in mono-transesterification of diethyl malonate with cyclohexanol. Chem Eng J 2013;230:547e57. [38] Zheng LP, Xia SX, Hou ZT, Zhang MY, Hou ZY. Transesterification of glycerol with dimethyl carbonate over Mg-Al hydrotalcites. Chin J Catal 2014;35: 310e8. [39] Portugal-Pereira J, Nakatani J, Kurisu KH. Comparative energy and environmental analysis of Jatropha bioelectricity versus biodiesel production in remote areas. Energy 2015;83:284e93. [40] Rakopoulos DC, Rakopoulos CD, Giakoumis EG. Impact of properties of vegetable oil, bio-diesel, ethanol and n-butanol on the combustion and emissions of turbocharged HDDI diesel engine operating under steady and transient conditions. Fuel 2015;156:1e19. [41] Tse H, Leung CW, Cheung CS. Investigation on the combustion characteristics and particulate emissions from a diesel engine fueled with diesel-biodieselethanol blends. Energy 2015;83:343e50.