Preparation, characterization and application of heterogeneous solid base catalyst for biodiesel production from soybean oil

Preparation, characterization and application of heterogeneous solid base catalyst for biodiesel production from soybean oil Yihuai Li a , Fengxian Qiu a, *, Dongya Yang a , Xiaohua Li b , Ping Sun b a School of Chemistry and Chemical Engineering, Jiangsu University, Xuefu Road 301, 212013 Zhenjiang, PR China b Jiangsu Provincial Key Laboratory of Power Machinery and Application of Clean Energy, 212013 Zhenjiang, PR China article info Article history: Received 24 September 2010 Received in revised form 22 February 2011 Accepted 4 March 2011 Available online 24 March 2011 Keywords: Heterogeneous catalyst Transesterification Biodiesel Potassium hydroxide Neodymium oxide abstract A solid base catalyst was prepared by neodymium oxide loaded with potassium hydroxide and investigated for transesterification of soybean oil with methanol to biodiesel. After loading KOH of 30 wt.% on neodymium oxide followed by calcination at 600  C, the catalyst gave the highest basicity and the best catalytic activity for this reaction. The obtained catalyst was characterized by means of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM), Thermogravimetric analysis (TGA), N 2 adsorptionedesorption measurements and the Hammett indicator method. The catalyst has longer lifetime and maintained sustained activity after being used for five times, and were noncorrosive and environmentally benign. The separate effects of the molar ratio of methanol to oil, reaction temperature, mass ratio of catalyst to oil and reaction time were investigated. The experimental results showed that a 14:1 M ratio of methanol to oil, addition of 6.0% catalyst, 60  C reaction temperature and 1.5 h reaction time gave the best results and the biodiesel yield of 92.41% was achieved. The properties of obtained biodiesel are close to commercial diesel fuel and is rated as a realistic fuel as an alternative to diesel. ª 2011 Elsevier Ltd. All rights reserved. 1. Introduction As conventional fuels are diminishing and environmental pollution is aggravating, alternative fuels have gained signifi- cant attention [1]. Biodiesel fuel, as a promising alternative diesel fuel to conventional fossil diesel produced by a catalytic transesterification of vegetable oils, animal fats and waste cooking oils with short chain alcohol, is becoming a favorable biofuel in many regions of the world [2,3], Compared to conventionaldiesel from petroleum, biodiesel is technicallyand economically more competitive because of its renewability, biodegradability, low emission profiles, high Flash point, excellent lubricity and superior cetane number [4]. In addition, the use of biodiesel has the potential to reduce both the levels of pollutants and potential or probable carcinogens [5]. Biodiesel can be produced through transesterification of vegetable oils and fats with methanol in the presence of a suitable catalyst. In conventional homogeneous method of fatty acid methyl ester (FAME) synthesis, the removal of cata- lysts after reaction is unwanted step of biodiesel synthesis, where a large amount of wastewater is produced during neutralization the catalyst (NaOH or KOH) and FAME washing during separation from side products (glycerol, salt). Acid- catalyzed process often uses sulfonic acid and hydrochloric * Corresponding author. Tel.: þ86 51188791800. E-mail address: fxqiuchem@163.com (F. Qiu). Available at www.sciencedirect.com http://www.elsevier.com/locate/biombioe biomass and bioenergy 35 (2011) 2787e2795 0961-9534/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2011.03.009 acid as catalysts, however, the reaction time is very long (48e96 h) even at reflux of methanol, and a high molar ratio of methanol to oil is needed (30e150:1) [6]. Compared with homogeneous catalysts, heterogeneous catalysts can provide green and recyclable catalytic systems [7,8]. The advantage of heterogeneous catalyst usage is its fast and easy separation from the reaction mixture without requiring the use of neutralization agent. There are many solid heterogeneous acid- andalkali-catalysts for biodiesel synthesis. Tungstated zirconia (WO 3 /ZrO 2 ) was prepared by method of impregnation was a promising heterogeneous acid catalyst [9]. Various carbohydrate-derived and a carbon-based solid acid catalyst [10,11] have good catalytic activity to high free fatty acid-containing waste oils. Unfortunately, the performances of these acid catalysts are still inferior compared with the base catalysts. For thisreason, a widevariety of solid bases havebeen examined for transesterification reactions for biodiesel synthesis. Example include CaO [12],SrO[13],KNO 3 loaded on flyash [14],ZnOeLa 2 O 3 [15] and zinc aluminate [16].But,these heterogeneous catalysts require a high temperature to achieve the high conversion. Other heterogeneous base catalysts like CaMnO 3 [17],KNO 3 /Al 2 O 3 [18],andMgeAlhydrotalcites[19]have also demonstrated some potential for activity in production of biodiesel. However, these catalysts need more time (more than 3 h) to reach the higher biodiesel yield. The result will increase the production cost due to the requirements for high tempera- tures and a long time operation. Neodymium oxide (Nd 2 O 3 ) or rare earth sesquoxides is widely used in various applications such as photonic, lumi- nescent materials, catalyst for automotive industry, UV absorbent, glass-polishing materials, and protective coatings. However, in this work, a new type of catalyst for biodiesel synthesis withKOH asactive componenton neodymium oxide support was synthesized using the way of impregnation, and reported the activity and selectivity of the basic solids for the transesterification of soybean oil with methanol. A screening of the reaction conditions has been carried out by examining the effectof theconcentration ofcatalyst, theinitial methanol/ oil, catalyst/oil molar ratio, reaction temperature and time. 2. Experimental 2.1. Materials Soybean oil was purchased from Jinlongyu Company (Fujian, China). Methanol, zirconium dioxide (ZrO 2 ), titanium dioxide (TiO 2 ), alumina (Al 2 O 3 ), neodymium oxide (Nd 2 O 3 ), potassium hydroxide (KOH), potassium iodide (KI), potassium bromate (KBrO 3 ), potassium hydrogen phthalate (C 8 H 5 O 4 K) and potas- sium nitrate (KNO 3 ) were obtained from Sinopharm Chemical Reagent Co. Ltd., (Shanghai, China). All solvents were AR grade and were used without purification. 2.2. Preparation of catalyst All the catalysts were prepared by incipient wetness impreg- nation of different porous medium supports with solution of potassium compounds. For this purpose, the required amount of aqueous KOH solution was slowly added to the support and kept 24 h. The catalytic carrier was previously calcined in a mufflefor12hat600  C.Afterimpregnation,thecatalysts were dried for12hat 100  C andthenthe solid was calcinedinamuffle furnace at designed temperature for 12 h before use for the reaction. 2.3. Characterization of the catalyst FT-IR spectra of the samples were obtained between 4000 and 400 cm À1 on a KBr powder with an FTIR spectrometer (AVATAR 360, Nicolet, Madison, USA). A minimum of 32 scans was signal- averaged with a resolution of 2 cm À1 in the 4000e400-cm À1 range. Scanning electron microscopy (SEM) images were obtained with 20-kV accelerating voltage with a field emission scanning electron microscope (S-4800, HITACHI Corp., Tokyo, Japan). X-ray diffraction (XRD) patterns of selected samples were obtained were recorded by the reflection scan with nickel- filtered Cu Ka radiation (D8, Bruker-AXS, Germany). The X-ray generator was run at 40 kV and 70 mA. All the XRD measure- ments were performed at 2q values between 10 and 80  . Thermogravimetric analysis (TGA) was performed on a Netz- sch instrument ( STA 449C, Netzsch, S eligensta dt, Germa ny). The programmed heating range was from room temperature to 1300  C, at a heating rate o f 10  C/min under a nitrogen atmo- sphere. The measurement was taken using 6e10 mg samples. The nitrogen adsorption and desorption isotherms were measured at À196  C using a NDVA2000e analytical system made by Quntachrome Corporation (USA). The specific surface area was calculated by Brunauer-Emmett-Teller (BET) method. Pore size distribution and pore volume were calculated by Barrett-Joyner-Halenda (BJH) method. Hammett indicator experiments were conducted to deter- mine the basic strength of each catalyst. The Hammett indica- tors used were bromothymol blue (pKa ¼ 7.2), phenolphthalein (pKa ¼ 9.8), 2,4-dinitroaniline (pKa ¼ 15), and 4-nitroaniline (pKa ¼ 18.4). Typically, 300 mg of the catalyst was mixed with 1 mL of a solution of Hammett indicators diluted in 10 mL methanol and allowed to sit for at least 2 h. After the equili- bration,thecolor of thecatalystwasnoted. The basicstrength of the catalyst was taken to be higher than the weakest indicator that underwent a color change and lower than the strongest indicator that underwent no color change. To measure the basicity of solid bases, the method of Hammett indicator- benzene carboxylic acid (0.02 mol/L anhydrous ethanol solu- tion) titration was used. 2.4. Transesterification of soybean oil and chemical analyses The transesterification reactions were performed at 60  Cin a 125 ml three-neck reaction flask equipped with a condenser by refluxing 10 mL of methanol (247 mmol) with 15.82 g of soybean oil (commercial edible grade, acid value ¼ 0.976 mg KOH/g, saponification index ¼ 188.6 mg KOH/g, and average molecular weight ¼ 896.88 g/mol)and 0.95g of catalyst(6 wt.%). The catalyst was activating at 773 K for 12 h before use for the reaction. After the reaction completion, the samples were separated from catalyst and glycerol by centrifuge. The glyc- erol could be separated because it was insoluble in the esters biomass and bioenergy 35 (2011) 2787e27952788 and had a much higher density. Then methanol was removed using rotary evaporation and the obtained product was analyzed by gas chromatography (GC) to determine the bio- diesel yield (fatty acid methyl ester, FAME). Reference materials and samples were analyzed by a 7890A gas chromatograph (Agilent Technology Inc. USA), equipped with a flame-ionization detector (FID) and a HP-5 capillary column (30 m  0.32 mm  0.25 mm). Helium was used as the carrier gas. The oven temperature ramp program was 135  Cfor 10 min, 170  Cat10  C/min, and held for 10 min. 250  Cat25  C/ min, and held 2 min. The flow rate of hydrogen was 30 ml/min and the flow rate of air was about 400 ml/min. Temperatures of the injector and detector were 280 and 300  C. The injection was performed in splitmode with a split ratioof100:1. Biodieselyield was quantified in the presence of tricaprylin as an internal standard. The analysis of biodiesel for each sample was carried out by dissolving 1 ml of biodiesel sample into 5 ml of n-hexane and injecting 0.5 ml of this solution in GC, in the same condition described as above. The biodiesel yield was calculated from the content of methyl esters analyzed by GC with the following equation: Biodiesel yield ¼ m tricaprylin A B f tricaprylin A tricaprylin m s  100% (1) where m tricaprylin ¼ weight of the internal standard, A B ¼ peak area of FAME, f tricaprylin ¼ response factor, A tricaprylin ¼ peak area of the internal standard, and m s ¼ weight of the sample. Determination of sulfur content of biodiesel was measured by Inductively Coupled Plasma Emission Spectrometer (ICP) using Intrepid XP Radial ICP-OES (VISTA-MPX, Varian, USA) with a concentric nebulizer and CCD detectors technology. Flash point was determined by a closed-cup tester (BF-02, Dalian NorthAnalytical InstrumentsCo., Ltd.),using ASTM D 93. 3. Results and discussion 3.1. Screening of catalyst The catalytic activity screening of Nd 2 O 3 loaded with different potassium compounds (KOH,KI, KBrO 3 ,C 8 H 5 O 4 KorKNO 3 )inthe soybean oil transesterification was performed. The results were summarized Table 1. To make direct comparisons, the same reaction conditions, as shown in Table 1,wereemployedfor each catalyst in all experiments. The reaction conditions were not optimized for the highest reaction yield; however, they provided a way to compare the activities of the catalysts. Obvi- ously, it is observed from Table 1 that pure Nd 2 O 3 and KOH catalyst exhibited no activity and serious saponification phenomenon due to excess KOH amount [20], respectively. However, when potassium compounds were loaded on Nd 2 O 3 and activated at high temperatures, the supported catalysts except KNO 3 /Nd 2 O 3 showed catalytic activities. Thus, it is essential to support potassium compounds on Nd 2 O 3 to generate the catalytic activities for the transesterification reac- tion. Among the catalysts tested, Nd 2 O 3 loaded with KOH, KI, KBrO 3 or C 8 H 5 O 4 K exhibited comparatively high activities, giving biodiesel yields higher than 80%. Especially, KOH/Nd 2 O 3 demonstrated the superior catalytic activity compared to the other catalysts. When the transesterification was conducted overtheKOH/Nd 2 O 3 catalyst,the highest biodieselyield of 89.7% was achieved. Over KBrO 3 /Nd 2 O 3 , KI/Nd 2 O 3 and C 8 H 5 O 4 K/Nd 2 O 3 catalysts, however, the lower biodiesel yields in the range of 17.05e74.06% were obtained, attributable to their relatively low catalytic activities. Based on these results, the catalytic activity is in the following order: KOH/Nd 2 O 3 > KBrO 3 /Nd 2 O 3 > C 8 H 5 O 4 K/ Nd 2 O 3 > KI/Nd 2 O 3 > KNO 3 /Nd 2 O 3 . The base strengths of Nd 2 O 3 modified with different potas- sium compounds were measured by using Hammett indica- tors. As evident in Table 1, loading of KBrO 3 ,orC 8 H 5 O 4 Konthe surface of Nd 2 O 3 generated the weaker basic sites with pK BHþ in the range of 7.2e9.8. Taking both the base strength and the catalytic activity into account, we can conclude that the observed activities of Nd 2 O 3 -supported catalysts seem to be related to their base strengths, i.e. the higher base strengths of the catalysts result in the higher conversions. In particular, the KI/Nd 2 O 3 or KNO 3 /Nd 2 O 3 sample possessed the weakest base strength in the range of pK BHþ <7.2, consequently exhibiting weak or no catalytic activity. As for the catalytic sites on KOH/ Nd 2 O 3 sample, it can also be proposed that the K 2 Ospecies, which was possibly formed by dehydroxylation of the OH groups, was at least a part of catalytically active sites. As remarked above, it seems that the transesterification reaction needs strongly basic sites. The effect of supports on the activity of the catalyst was listed in Table 2. Obviously, when KOH was supported on Table 1 e Catalytic activity and base strength of Nd 2 O 3 loaded with different potassium compounds. Catalyst Basic strength (pK BHþ ) Biodiesel yield (%) Nd 2 O 3 <7.2 No reaction KOH 15e18.4 Serious saponification phenomenon KOH/Nd 2 O 3 9.8e15 89.7 KBrO 3 /Nd 2 O 3 7.2e9.8 74.06 KI/Nd 2 O 3 <7.2 17.05 C 8 H 5 O 4 K/Nd 2 O 3 7.2e9.8 32.03 KNO 3 /Nd 2 O 3 <7.2 No reaction Transesterification condition: methanol/oil molar ratio, 12:1; cata- lyst amount 6 wt.%; reaction time, 3 h; reaction temperature, 60  C. All catalysts were activating at 600  C for 12 h before use for the reaction. Table 2 e Catalytic activities and base strengths of KOH supported on the different carriers. Catalyst Basic strength (pK BHþ ) Biodiesel yield (%) KOH/TiO 2 9.8 < H < 15 86.47 KOH/ZrO 2 9.8 < H < 15 85.43 KOH/Al 2 O 3 9.8 < H < 15 88.87 KOH/Nd 2 O 3 9.8 < H < 15 89.70 Transesterification condition: methanol/oil molar ratio, 12:1; cata- lyst amount, 6 wt.%; reaction time, 3 h; reaction temperature, 60  C. All catalysts were activating at 600  C for 12 h before use for the reaction. biomass and bioenergy 35 (2011) 2787e2795 2789 different carriers, the base strengths were almost equality, but the activity of the catalyst was different greatly. KOH/Nd 2 O 3 was the most active catalyst for the transesterification reac- tion, giving a conversion of 89.70%. Over KOH/ZrO 2 , KOH/ Al 2 O 3 and KOH/TiO 2 catalysts, even though they possessed different centers of a base strength, the high biodiesel yields of 85.43%, 88.87% and 86.47% were also achieved, respectively. Thus, Nd 2 O 3 can be regarded as the best support. From these discussions, KOH/Nd 2 O 3 showed the best catalytic activity. On account of the high activity of the catalysts in the trans- esterification reaction, KOH/Nd 2 O 3 was, therefore, selected for further investigation and its properties were studied in more detail. A 14 wt.% (KOH to Nd 2 O 3 weight ratio) KOH on Nd 2 O 3 support was prepared by an impregnation method and the following procedure: a solid Nd 2 O 3 support (25 g) was mixed with KOH (3.5 g) in 15 mL of water, and the resulting solid was dried in an oven at 90  C for 24 h. The solid was then crushed and calcined in air at 600  C for 12 h. Similarly, 17, 25, 30 and 32 wt.% KOH loaded Nd 2 O 3 were prepared. The effect of KOH loading amounts on the biodiesel yield was shown in Table 3.It can be seen from Table 3 that when the loading amount of KOH increased from 14 wt.% to 30 wt.%, the biodiesel yield increased from 80.47% to 89.52%. Then, the biodiesel yield decreased with the loading amount of KOH. This is because the base strength of catalyst increases with the loading amount of KOH. On the other hand, the catalytic activity and activity sites also increase with the loading amount of KOH. But, with further increase in the amount of loaded KOH, the basicity may decrease the surface basic sites, which resulted in a drop of the catalytic activity towards the reaction. This is presumably due to the coverage of surface basic sites by the excessive KOH. These sites are inaccessible to incoming reactants when the amount of loaded KOH exceeded 30 wt.%. Therefore, catalytic activity and biodiesel yield decreased. On the basis of the results, the optimum loading amount of KOH was 30 wt.%. Moreover, the biodiesel yield of 30 wt.% KOH/Nd 2 O 3 sample calcined at different temperatures was measured by the same method, and the results are presented in Table 4. From the Table 4, it can be observed that the maximum biodiesel yield, reaching 90.02%, was obtained at a calcination temperature of 600  C. But, a low level of biodiesel yield was observed below 512 and above 700  C. Obviously, the biodiesel yield changes with calcination temperatures parallel the changes in the catalytic activity for the transesterification reaction. Based on these results, the optimal preparation conditions of the catalyst are load KOH, support Nd 2 O 3 , loading amount 30% and calcination temperature of 600  C. Therefore, 30 wt.% KOH/Nd 2 O 3 catalyst was selected for further investigation of transesterification of soybean oil. 3.2. Catalyst characterizations A series of catalysts were synthesized by incipient wetness impregnation method and calcined at 600  C. In this high temperature, KOH decomposed into K 2 O. The powder X-ray diffraction patterns of KOH/Nd 2 O 3 samples with various loading amounts of KOH were presented in Fig. 1. As can be seen, when the loading amount of KOH was 14 wt.% (curve a), diffraction peaks (2q ¼ 27.4  , 30.9  , 40.5  , 47.6  and 57.2  ) assigned to the amorphous Nd 2 O 3 support were registered on the XRD patterns, and only a specie such as K 2 O(2q ¼ 29.6  ) was observed, indicating the good dispersion of K 2 OonNd 2 O 3 in the form of a monolayer due to the interaction between K 2 O and the surface of the support at a low loading of KOH. And when the loading amount of KOH was further increased to 25 wt. % (curve b), the new phase of K 2 O can be observed at 32.1  and 51.6  . But, when the loading amount of KOH is further increased to over 30 wt. % (curves c and d), a new phase of a compound containing potassium and neodymium elements could be observed at 2q ¼ 25.8  , 38.8  and 41.2  [21]. The phenomena can be a result from the incorporation of K þ ions into the vacancies in the structure of the neodymium oxide, or K þ ions may react with hydroxyl groups to form NdeOeK on the surface during heat treatment. The result will confirm by FT-IR spectrum of catalyst. Moreover, the intensi- ties of some diffraction peaks (2q ¼ 29.6  , 32.1  , 38.8  , 41.2  and 51.6  ) increased with increase of the loading amount of KOH. On the other hand, the characteristic peaks of Nd 2 O 3 (27.4  , 30.9  , 40.5  , 47.6  and 57.2  ) were almost unchanged on the XRD patterns regardless of the loading amount of KOH. It is note- worthy that the solidestate reaction between the guest compound and the surface of the support in the activation process is favorable for the catalyst to get a high catalytic Table 3 e The effect of KOH loading amounts on the biodiesel yield. KOH loading amount (%) 14 17 25 30 32 Basic strength (pK BHþ ) 9.8 < H < 15 9.8 < H < 15 9.8 < H < 15 9.8 < H < 15 9.8 < H < 15 Biodiesel yield (%) 80.47 81.52 85.72 89.52 73.45 Transesterification condition: methanol/oil molar ratio, 15:1; catalyst amount, 6 wt.%; reaction time, 3.0 h; reaction temperature, 60  C. Table 4 e The effect of calcination temperature on the biodiesel yield. Calcination temperature (  C) 311 422 512 600 700 790 Basic strength (pK BHþ ) 9.8 < H < 15 9.8 < H < 15 9.8 < H < 15 9.8 < H < 15 9.8 < H < 15 9.8 < H < 15 Biodiesel yield (%) 78.80 81.23 81.00 90.02 77.83 73.50 Transesterification condition: methanol/oil molar ratio, 12:1; catalyst amount, 6 wt.%; reaction time, 3.0 h; reaction temperature, 60  C. biomass and bioenergy 35 (2011) 2787e27952790 activity. In the case of KOH/Nd 2 O 3 , the K þ ion of KOH could insert in the vacant sites of Nd 2 O 3 , accelerating dissociative dispersion and decomposition of KOH to form basic sites in the activation process. The more potassium compounds are loaded on the Nd 2 O 3 , the more free vacancies decrease, which results in the surface enrichment of potassium species that is probably considered to be the active sites for base-catalyzed reactions. When the amount of potassium cations loaded on Nd 2 O 3 was below the saturation uptake of K þ , it could be well dispersed. As a result, the number of basic sites together with the activities of the catalysts would increase with the potas- sium contents. However, if Nd 2 O 3 was loaded with too much KOH, the KOH could not be well dispersed and, for this reason, not all but only a part of the loaded KOH could be decomposed. Moreover, as mentioned in the preceding sections, the excess KOH would cover the basic sites on the surface of the catalysts resulting in a lowered catalytic activity (Table 3). SEM images of Nd 2 O 3 and 30 wt.% KOH/Nd 2 O 3 catalyst were shown in Fig. 2. The SEM photographs of Nd 2 O 3 and KOH/ Nd 2 O 3 samples showed the crystallites of 0.2e1 mm size. Evidently, as shown in Fig. 2, no important difference was observed between Nd 2 O 3 and KOH/Nd 2 O 3 samples, thus sug- gesting a good dispersion of KOH on the surface of Nd 2 O 3 . Based on these results, after loading of KOH, Nd 2 O 3 retained its structure that was important for catalysis and therefore the potassium species was found highly distributed upon the surface of the support. FTIR spectra of Nd 2 O 3 and KOHeNd 2 O 3 catalyst were recorded and shown in Fig. 3. The spectrum of the support shows sharp peak at 1475 cm À1 . From the spectrum of catalyst, the new peaks at 880 and 706 cm À1 are attributed to the KeO and KeOeNd bonds, respectively. Furthermore, the broad band at around 3100e3300 cm À1 region could be partly assigned to the stretching vibration of NdeOeK groups [22] which K þ ions could replace the protons of isolated hydroxy groups to form NdeOeK groups in the activation process and were probably considered to be the active species of this cata- lyst. This achieves the same results as the XRD analysis. However, this vibration is well overlapped with the broad vibration band of OH groups which is ascribed to OH stretching vibration of thehydroxylgroupsattached to thectalystsurface, in addition to water molecules absorbed from the atmosphere. The BET surface area, pore volume, and pore diameter of Nd 2 O 3 and30 wt.%KOH/Nd 2 O 3 catalystwere measured.TheBET surface area as well as the pore volume decreased with loading potassium hydroxide, and this tendency was more outstanding in the case of potassium. The thermal behavior of 30 wt. % KOH/ Nd 2 O 3 sample was shown in Fig. 4. This figure showed that the first weight loss at lower temperature (<200  C) corresponds to the water loss from internal and external surfaces of the samples. The second weight loss (200e400  C), is due to the decomposition of the KOH and K 2 CO 3 . The last weight loss at above 400  C is attributed to the decomposition of the K 2 Oand the residual hydroxyl groups bonded to the oxide lattice. The decomposition products of KOH, probably forming both K 2 O speciesandNdeOeK groupsinthecomposite,werepossiblythe main active sites for the transesterification reaction. In this study, effect of repeated use of KOH/Nd 2 O 3 catalyst onbiodiesel yield was investigated in the optimal transesterification reac- tionconditions. The resultin Table 5indicated that the biodiesel Fig. 1 e XRD patterns of (a) 14 wt.% KOH/Nd 2 O 3 , (b) 25 wt.% KOH/Nd 2 O 3 , (c) 30 wt.% KOH/Nd 2 O 3 , (d) 32 wt.% KOH/Nd 2 O 3 . Fig. 2 e SEM images of (a) Nd 2 O 3 and (b) 30 wt.% KOH/Nd 2 O 3 . biomass and bioenergy 35 (2011) 2787e2795 2791 yields had no significant changes and were in excess of 90% during the repeated experiments. It maintained sustained activity even after being used for five times and the biodiesel yield was only slightly decreased from 90.11% to 90.05%. This was because neodymium oxide compounds are dissolvable in methanol. 3.3. Influence of the transesterification reaction conditions The transesterification process consists of a sequence of three consecutive reversible reactions where the triglyceride is successively transformed into diglyceride, monoglyceride, and finally intoglycerin andthe FAME. The molar ratio of methanol to soybean oil is one of the important factors that affect the conversion to methyl esters. Stoichiometrically, 3 mol of methanol are required for each mole of triglyceride, but in practice a higher molar ratio is employed in order to drive the reaction towards completion and produce more methyl esters as products. This is because that the biodiesel yield could be improved by introducing excess amounts of methanol to shift the equilibrium to the right-hand side. As represented in Fig. 5, the biodiesel yields grew as the methanol-loading molar ratio increased, and the biodiesel yield was increased considerably. The maximum biodiesel yield (90.59%) was obtained when the molar ratio was very close to 14:1. In comparison, the biodiesel yield increased from 77.49% to 90.59% when the molar ratio was increased from 6:1 to 14:1. However, beyond the molar ratio of14:1,the excessivelyaddedmethanol had nosignificant effect on the production yield and the biodiesel yield was 90.12% at 16:1. The reason is that the catalyst content decreased with increase of methanol content. Therefore, we could concludethatto elevatethe biodiesel productionyield an excess methanol feed was effective to a certain extent and the optimum molar ratio of methanol to oil was 14:1. The dependence of the biodiesel yield on the reaction time was investigated. The reaction time was varied in the range 0.5e8h.Fig. 6 revealed that the transesterification reaction was strongly dependent on reaction time, at the beginning (<0.5 h), the reaction was slow due to the mixing and the dispersion of methanol into oil, and the biodiesel yield was increased very fast in the reaction time range between 0.5 and 1.5 h. Moreover, excess reaction time leaded to a bit reduction in the product yield due to the backward reaction trans- esterification, resulting in a loss of esters as well as causing more fatty acids to form soaps [23,24]. So the optimum reac- tion time was obtained at 1.5 h. Fig. 3 e FT-IR spectra of Nd 2 O 3 and catalyst. Fig. 4 e Thermogravimetric analysis of 30 wt. % KOH/ Nd 2 O 3 . Fig. 5 e Effect of different molar of methanol to oil on the biodiesel yield (catalyst amount, 6 wt.%; reaction time, 3.0 h; reaction temperature, 60  C). Table 5 e Effect of repeated use of KOH/Nd 2 O 3 catalyst on biodiesel yield. Repeated times 12345 Biodiesel yield (%) 90.11 90.07 90.06 90.04 90.05 Transesterification condition: methanol/oil molar ratio, 12:1; cata- lyst amount, 6 wt.%; reaction time, 1.5 h; reaction temperature, 60  C. biomass and bioenergy 35 (2011) 2787e27952792 In the presence of heterogeneous catalysts, the reaction mixture constitutes a three-phase system, oilemethanol-cata- lyst, inwhich the reactionwouldbe sloweddown because of the diffusion resistance between different phases. However, the reaction rate can be accelerated at higher reaction tempera- tures. In this paper, the synthesis of biodiesel from soybean oil was conducted atvarious temperatures (40  C, 50  C, 60  C, 65  C and 70  C). As shown in Fig. 7, the reaction rate was slow at low temperatures, but the biodiesel yield first increased and then decreased with the increase of the reaction temperature. Generally, a more rapid reaction rate could be obtained at high temperatures, which is due to the endothermic nature of transesterification reaction [25], but at high temperatures, methanol was vaporized and formed a large number of bubbles, which inhibited the reaction on the three-phase interface. The optimum reaction temperature was 60  C and biodiesel yield arrived at 92.41%. When increasing the amount of loading catalyst, the slurry (mixture of catalyst and reactants) was become too viscous giving rise to a problem of mixing and a demand of higher power consumption for adequate stirring. On the other hand, when the catalyst loading amount was not enough, maximum biodiesel yield could not be reached. To avoid this kind of problem, an optimum amount of catalyst loading had to be investigated. The influence of the catalyst amounts was studied at a 14:1 M ratio of methanol to soybean oil at reflux of methanol for 2 h. The catalyst amount was varied in the range of 1.0% and 9.0%. These percentages were weight fractions of the oil supplied for this reaction. The reaction profile of Fig. 8 indicated that the transesterification reaction was strongly dependent upon the catalyst applied. As is evident from Fig. 8, when the catalyst amount increased from 1.0% to 6.0%, the biodiesel production yield was increased. However, with further increase in the catalyst amount the biodiesel yield was Fig. 6 e Effect of reaction time on the biodiesel yield (methanol/oil molar ratio, 14:1; catalyst amount, 6 wt.%; reaction temperature, 60  C). Fig. 7 e Effect of reaction temperature on the biodiesel yield (methanol/oil molar ratio, 14:1; catalyst amount, 6 wt.%; reaction time, 1.5 h). Fig. 8 e Effect of catalyst amount on the biodiesel yield (methanol/oil molar ratio, 14:1; reaction time, 2.0 h; reaction temperature, 60  C). Table 6 e The various absorption peaks of biodiesel. Wavenumber (cm À1 ) Group attribution Vibration type Absorption intensity 3462.27 eOH Stretching Weak 3008.91 ¼ CeH Stretching Strong 2925.76 eCH 2 Asymmetric stretching vibration Strong 2855.00 eCH 2 Symmetric stretching vibration Strong 1743.54 eC]O Stretching Strong 1461.48 eCH 2 Shear-type vibration Middling 1360.75 eCH 3 Bending vibration Middling 1017.78 CeOeC Anti-symmetric stretching vibration Weak 1171.41 CeOeC Symmetric stretching vibration Middling 722.75 eCH 2 Plane rocking vibration Weak biomass and bioenergy 35 (2011) 2787e2795 2793 decreased, which was possibly due to a mixing problem involving reactants, products and solid catalyst. The optimum catalyst loading amount was found to be 6.0% in this system and the maximum biodiesel yield reached to 92.40%. From the above results, the reaction does not require too much time to dispose of the products, for example, neutrali- zation, washing and drying. If the catalyst can be used commercially, filtration is a possible way to recycle the cata- lyst and decrease the cost. As a heterogeneous solid base catalyst, the prepared KOH/Nd 2 O 3 catalyst has a longer cata- lyst lifetime and better stability than current homogeneous catalysts. It is noncorrosive and environmentally benign. It can be applied to produce biodiesel commercially. 3.4. Characterization and properties of biodiesel FT-IR spectrum of the obtained biodiesel was listed in Table 6. From the analysis of the Table 6, we could get the sample including all groups which we needed. At the same time, it proved that the compound was the kind of structures having long-chain fatty acid esters. The content of sulfur and its proper determination play an important role regarding fuels and products of petrochemical industry. The problem of appropriate determination of sulfur is important both from environmental and analytical aspects, because some specifications order to the compulsion decrease of the concentration of sulfur (e.g. from 2005 their maximum concentration is 50 mg/kg in fuels in the countries of European Union). Over the past few decades, there are numerous spec- troscopic techniques to analyze the qualitative and quantita- tive elemental composition of fuels. For example, inductively coupled plasma atomic emission spectroscopy (ICPeAES), inductively coupled plasma mass spectroscopy (ICPeMS) and flame or graphic furnace atomic absorption spectroscopy (AAS) were adopted. Each technique has advantageous properties in terms of analytical figures of merit. The atomic absorption and emission techniques are typically used for analysis of the products of hydrocarbon industry. The ICP technique is a fast analytical method, but needs preliminary sample preparation (e.g. digestion). In this work, the sulfur content of biodiesel was carried out by Inductively Coupled Plasma Emission Spec- trometer (ICP) using Intrepid XP Radial ICP-OES (VISTA-MPX, Varian, USA) with a concentric nebulizer and CCD detectors technology. After performing the background equivalent concentration experiment to test the instrument sensitivity, the ICP operating conditions applied are presented in Table 7. In order to determine the sulfur content, the biodiesel was carried out by nitrification firstly. A certain amount of biodiesel was added to concentrated hydrochloric acid (8 mL) and concentrated nitric acid (2 mL), and placed overnight. Subse- quently, the mixture was gently filtered and got clear liquid, then the clear liquid was evaporated in the fuming cupboard about 20 min. Calibration standards were made up from some standard solutions of sulfur. The ranges of the calibration curves (5 points) were selected to match the expected different concentrations standard solution for the sulfur element of the sample investigated. Linearity was checked in the range of 0e40 mg/g. From the calibration curve, the sulfur content of biodiesel was obtained. The sulfur content of the obtained biodiesel was listed in Table 8. The properties of biodiesel, density, cetane number, flash point, cold filter plugging point, acid number, water content, ash content and total glycerol content, were determined and listed in the Table 8. Table 8 also showed comparisons of the obtained biodiesel and the standards of biodiesel in china, Europe and the United States. The properties of the obtained biodiesel, in general, show many similarities, and therefore, Table 8 e Comparison of properties of the obtained biodiesel and the standards of biodiesel in china, Europe and the United States. Item Obtained biodiesel China GB/T 20828e2007 USA ASTM D 6751e03 Europe EN 14214 Density (kg L À1 ) 0.896 (20  C) 0.82e0.90 (20  C) 0.82e0.90 (20  C) 0.86e0.90 (15  C) Flash point (  C) 168 !130 ＞130 ＞120 Cold filter plugging point (  C) À5.0 eeSpring:0 Summer:À10 Autumn:À20 Sulfur content (w/w,%) 0.0065 0.05 0.0015 ＜0.001 Cetane value 56 !49 !47 ＞51 Acid value (KOH) (mg g À1 ) 0.6 0.8 ＜0.8 ＜0.5 Water content (w/w,%) 0.04 0.05 0.05 0.05 Ash content (w/w,%) 0.018 0.05 0.02 0.02 Total glycerol content (w/w,%) 0.020 0.024 0.024 0.025 Table 7 e The properties of ICP-OES spectrometer. Type Intrepid XP Radial ICP-OES Nebulizer Concentric, with a cyclonic spray chamber RF-generator 40.68 MHz crystal-controlled Power 1200 W Reflected power 20 W Observation high 6 mm above the coil Optical system Czerny-Turner vacuum-monochromator Grating Holographic, 1800 groves/mm Focal length 0.75m Optical range 160e800 nm Resolution 1st order: 0.018 nm Detector Photomultiplier Emission lines used l 1 ¼ 180.731 nm For the S analysis l 2 ¼ 181.970 nm biomass and bioenergy 35 (2011) 2787e27952794 the properties of obtained biodiesel from the soybean oil is rated as a realistic fuel as an alternative to diesel. 4. Conclusions Nd 2 O 3 loaded with KOH, which was prepared by impregnation of powdered Nd 2 O 3 with an aqueous solution of KOH followed by calcination at a high temperature, showed high catalytic activities for the transesterification reaction. Both the K 2 O species formed by the thermal decomposition of loaded KOH, and the surface KeOeNd groups formed by saltesupport interactions, were probably the main reasons for the catalytic activity towards the reaction. The activities of the heteroge- neous base catalysts correlated with their corresponding basic properties. The catalyst with 30 wt.% KOH loading on Nd 2 O 3 and calcined at 600  C for 12 h was found to be the optimum catalyst, which gave the best catalytic activity. When the reaction was carried out at reflux of methanol, with a molar ratio of methanol to oil of 14:1, a reaction time 1.5 h, a reaction temperature 60  C and a catalyst amount 6.0%, the highest biodiesel yield reached 92.41%. The properties of obtained biodiesel from soybean oil are close to commercial diesel fuel and is rated as a realistic fuel as an alternative to diesel. 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