Applied Catalysis A: General 353 (2009) 203–212 Contents lists available at ScienceDirect Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata Simultaneous transesteriﬁcation and esteriﬁcation of unreﬁned or waste oils over ZnO-La2O3 catalysts Shuli Yan, Steven O. Salley, K.Y. Simon Ng * Department of Chemical Engineering and Materials Science, Wayne State University, Detroit, MI 48202, USA A R T I C L E I N F O A B S T R A C T Article history: A single-step method was developed for biodiesel production from unreﬁned or waste oils using a series Received 10 June 2008 of heterogeneous zinc and lanthanum mixed oxides. Effects of metal oxide molar ratio, free fatty acids Received in revised form 17 October 2008 (FFA) and water content in feedstock, molar ratio of methanol and oil, and reaction temperature on the Accepted 27 October 2008 yield of biodiesel were investigated. A strong interaction between Zn and La species was observed with Available online 11 November 2008 enhanced catalyst activities. Lanthanum promoted zinc oxide distribution, and increased the surface acid and base sites. The catalyst with 3:1 ratio of zinc to lanthanum was found to simultaneously catalyze the Keywords: oil transesteriﬁcation and fatty acid esteriﬁcation reactions, while minimizing oil and biodiesel Biodiesel hydrolysis. A reaction temperature window of 170–220 8C was found for the biodiesel formation. A high Unreﬁned and waste oils Transesteriﬁcation yield (96%) of fatty acid methyl esters (FAME) was obtained within 3 h even using unreﬁned or waste oils. Esteriﬁcation Published by Elsevier B.V. Hydrolysis 1. Introduction A conventional operation usually uses strong basic or acidic solutions (i.e., NaOH, KOH and H2SO4) as catalyst and food-grade Biodiesel, a renewable fuel with similar combustion properties vegetable oils as raw material. These homogeneous catalysts are to fossil diesel, is normally produced by transesteriﬁcation of quite sensitive to free fatty acids (FFA) and water in the oil highly reﬁned oils with short-chain alcohols. Biodiesel can feedstocks and alcohols. FFA reacts with the basic catalyst signiﬁcantly decrease the exhaust emission of CO2, SOx and (NaOH, KOH) and forms soaps. This soap formation complicates unburned hydrocarbons from motor vehicles [1,2]. Biodiesel is the glycerol separation, and drastically reduces the methyl ester environmentally beneﬁcial, and therefore, is a promising alter- yield. Water in the feedstock leads to hydrolysis of oils and fatty native to fossil diesel . acid methyl esters (FAME) in the presence of strong basic or Transesteriﬁcation reaction of triglycerides for the production acidic catalysts. Thus, some inexpensive oils, such as crude of biodiesel is as follows: vegetable oils, waste cooking oil, and rendered animal fats, which generally contain a high content of FFA and water, cannot utilize homogeneous catalysts directly. Furthermore, the water * Corresponding author. Tel.: +1 313 577 3805; fax: +1 313 578 5814. content in alcohols is also an important issue in traditional E-mail address: firstname.lastname@example.org (K.Y. Simon Ng). processes. Since alcohols are hygroscopic, the recovered alcohols 0926-860X/$ – see front matter . Published by Elsevier B.V. doi:10.1016/j.apcata.2008.10.053 204 S. Yan et al. / Applied Catalysis A: General 353 (2009) 203–212 must be dried thoroughly to remove the water from the azeo- 2. Experiments tropes. For conventional processes using homogenous catalysts, the FFA content in the feedstock must be lower than 0.50 2.1. Materials (wt%)  and water content lower than 0.06 (wt%) . Usually, highly reﬁned oils are used in conventional methods for bio- Food-grade soybean oil was purchased from Costco warehouse diesel production. According to the calculation of Haas et al. , (Detroit, MI), crude soybean oil from BDI (Denton, TX), crude palm the cost of oil feedstock accounts for up to a total of 88% of oil from Malaysia Palm Oil Board (Selangor, Malaysia) and waste biodiesel production cost in traditional processes. With recent cooking oil was obtained from a local restaurant. The fatty acid increases in reﬁned oil prices, the cost of oil feedstock compositions of these four kinds of oil were determined by GC–MS accounts for an even higher fraction of the total production (Table 1). Oleic acid and methyl alcohol were obtained from the cost. Thus, it is important to develop new catalytic processes Mallinckrodt Chemicals (Phillipsburg, NJ), with water contents of which can handle unreﬁned and waste oils directly to lower the 0.02% and 0.03%, respectively. Zinc nitrate hexahydrate (98%), cost of biodiesel. lanthanum nitrate hydrate (98%), and urea (99%) are of analysis Recently, an acid- and alkali-catalyzed two-step method for grade, and were purchased from Sigma–Aldrich Company (St. biodiesel production using some unreﬁned or waste oils as raw Louis, MO). materials has been reported [7,8]. In this method, an acidic catalyst (H2SO4, HCl) is initially used to convert FFA to the esters, and then 2.2. Catalyst preparation and characterization in the second stage, transesteriﬁcation of oil is performed using an alkaline catalyst (NaOH, KOH). Although this method can utilize A homogeneous-coprecipitation method  was used to unreﬁned or waste oils for biodiesel production, the process prepare catalyst samples. First, 2 M Zn(NO3)2 and 1 M La(NO3)3 requires multiple reactions, washing, and separation stages. The were prepared with distilled water. Then, solutions with varying strong acidic or basic catalysts used are highly corrosive, and must ratios of Zn–La (1:0, 1:1, 3:1, 9:1, 0:1) were mixed with a 2 M urea be removed from the biodiesel product by multiple washing. Thus, solution. The mixture was boiled for 4 h, and then dried at 150 8C a signiﬁcant amount of waste water is generated, together with a for 8 h, followed by step-rising calcination at 250 8C, 300 8C, 350 8C, loss of catalyst [9,10]. 400 8C, ﬁnally at 450 8C for 8 h. The catalysts are noted as Zn10La0, Therefore, it is advantageous to develop a new class of Zn1La1, Zn3La1, Zn9La1, Zn0La10 according to their catalyst heterogeneous catalysts, which has a higher tolerance to water compositions. and FFA in oils, and can simultaneously catalyze both the Powder X-ray diffraction (XRD) patterns were taken with a esteriﬁcation and transesteriﬁcation reactions. There are reports Rigaku RU2000 rotating anode powder diffractometer (The of heterogeneous catalytic esteriﬁcation of fatty acids [11,12] Woodlands, TX) equipped with CuKa radiation (40 kV, 200 mA). and transesteriﬁcation of highly reﬁned oils [13–16]. Moreover, Scanning electron microscopy (SEM) and energy dispersive Omota et al.  reported a process of fatty acid esteriﬁcation spectrometer (EDS) were taken with a Hitachi S-2400 Scanning using zirconium salts as catalysts. Suppes et al.  reported Electron Microscope (San Jose, CA). Maximum operating voltage that some salts and oxides of magnesium, calcium, and zinc are used was 25 kV. N2 adsorption and desorption isotherms were active in transesteriﬁcation. Serio et al.  used Mg–Al calcined measured at 77 K with a Quantachrome AS-1MP volumetric hydrotalcites in the tranesteriﬁcation of reﬁned soybean oil, adsorption analyzer. Before adsorption measurements, all the which contained 0.1 (wt%) FFA, at high temperature. They found samples were outgased for 12 h at 300 8C. that this type of heterogeneous catalyst is more tolerant to XPS analysis was performed with a PHI 5500 system water and the catalytic performance was not affected by the (PerkinElmer, Wellesley, MA), using a monochromatic Al Ka X- presence of 1 (wt%) of water in oils. Recently, Yan et al.  ray radiation source (1486.6 eV) and AugerScan system control reported that supported CaO catalysts directly catalyzed the (RBD Enterprises, Bend, OR). Elemental concentration on the reaction of some crude oils into biodiesel when water content in sample surface was measured by XPS multiplex scan (spot size: the oils was less than 2 (wt%) and FFA content less than 3.5 $1 mm diameter). (wt%). However, further increases of FFA and water content in oils could inhibit the transesteriﬁcation. The mechanism for the 2.3. Biodiesel reactions and product analysis improved tolerance to water and FFA of this type of catalysts was not fully elucidated. Bournay and Hillion  stated in a Catalytic transesteriﬁcation, esteriﬁcation and hydrolysis reac- patent that oils with high content of FFA can be directly used tions were conducted in a 500 mL stainless steel stirred reactor with a Lewis acid catalyst (zinc aluminate) for biodiesel (Parr 4575 HT/HP Reactor). For transesteriﬁcation, 126 g of oil, production. However, that catalyst was quite sensitive to water 180 g of methanol, and 3 g of catalyst were used. During the and the limitation of water content in oils is 0.15 (wt%). Sreeprasanth et al.  prepared other types of Lewis acid Table 1 catalysts containing zinc and iron and used them in transester- Fatty acid methyl esters, FFA and water content of food-grade soybean oil, crude soybean oil, crude palm oil and waste cooking oil. ifying unreﬁned and used oils. He found that the catalyst is active in both esteriﬁcation and transesteriﬁcation. However, Fatty acid Food-grade Crude Crude Waste the catalyst is also active in the hydrolysis of FAME to FFA, thus components soybean soybean palm cooking oil (%) oil (%) oil (%) oil (%) the overall yield of biodiesel is decreased. In this paper, we prepared and characterized a new class of ZnO-La2O3 hetero- C 14:0 0 0.27 0.21 0 geneous catalysts with different Zn:La ratios. The effects of C 16:0 11.07 13.05 41.92 11.58 C 16:1 0.09 0.39 0.23 0.18 metal oxide ratio on the Lewis acid and base sites, and on the C 18:0 3.62 4.17 3.85 4.26 transesteriﬁcation of triglyceride, esteriﬁcation of fatty acid, C 18:1 20.26 22.75 42.44 24.84 and hydrolysis of triglyceride and fatty acid methyl esters, C 18:2 57.60 52.78 11.30 53.55 were investigated. These mixed oxide catalyst shows very C 18:3 7.36 6.59 0.04 5.60 FFA content 0.02 3.31 0.24 3.78 promising results for processing unreﬁned and waste oils dire- Water content 0.02 0.27 0.04 0.06 ctly into biodiesel. S. Yan et al. / Applied Catalysis A: General 353 (2009) 203–212 205 reaction, 15 mL of reaction mixture was collected via a two-stage liquid sampler at different time intervals. When the reaction was completed, the catalyst was ﬁltered out. The liquid product obtained was vaporized to remove excessive methanol, and then settled in a separating funnel. The upper layer in the separating funnel (mainly containing fatty acid methyl esters) was char- acterized by a GC–MS spectrometer (Clarus 500 MS System, PerkinElmer, Shelton, CT) equipped with a capillary column (Rtx- WAX Cat. no.12426) (Bellefonte, PA). Methyl arachidate (Nu-Chek Prep Inc, Elysian, MN) was used as an internal standard. Esteriﬁcations were performed with 126 g of oleic acid, 180 g of methanol, and 3 g of Zn3La1 catalyst. Yield of oleic acid methyl ester were determined by GC–MS. And oleic acid concentration was determined using a Brinkman/Metrohm 809 titrando (West- bury, NY) according to ASTM D 664. Hydrolysis of oil and hydrolysis of biodiesel were conducted with 285 g of food-grade soybean oil or soybean biodiesel (Wacker Oil, Manchester, MI), 15 g of water, and 2.3 (wt%) of Zn3La1 catalyst. Yield of FFA was determined by Brinkman/Metrohm 809 titrando (Westbury, NY) according to ASTM D 664. Water content Fig. 2. Transesteriﬁcation activities of Zn10La0, Zn9La1, Zn3La1, Zn1La1 and was analyzed using a Brinkman/Metrohm 831 KF Coulometer Zn0La10 at 200 8C. Reaction mixtures are 126 g of oil, 180 g of methanol, 3 g of catalyst. Zn3La1 shows the highest activity compared to other catalysts. (Westbury, NY) according to ASTM D 6304-00. 3. Results and discussion To investigate the effect of catalyst composition on transester- iﬁcation, mixed oxides with different molar ratio of zinc to 3.1. Catalytic activity of zinc lanthanum mixed oxides in oil lanthanum were tested at 200 8C (Fig. 2). The times needed to reach transesteriﬁcation chemical equilibrium were about 60 min, 80 min, 120 min, 150 min and 200 min for Zn3La1, Zn0La10, Zn1La1, Zn9La1 and Triglycerides are the major component of vegetable oils. In this Zn10La0, respectively. The catalyst with 3:1 molar ratio of zinc to study, a food-grade soybean oil, which contained 99.5 (wt%) of lanthanum showed the highest activity in oil transesteriﬁcation. triglycerides, was used as the model for most of the studies. The effect of reaction temperature on Zn3La1– catalyzed A step-rising heating method (heating rate of 2 8C/min, and held transeseteriﬁcation process was shown in Fig. 3. Since the at the target temperature for 1 min) was used to compare the methanol concentration was kept in excess, a power rate law catalytic activities of mono-metal oxide (Zn10La0, Zn0La10) and model can be written as: mixed metal oxides (Zn3La1) for the oil transesteriﬁcation reaction a Àg ¼ k1 Coil (Fig. 1). All three catalysts show activities in catalyzing the transesteriﬁcation reaction when the reaction temperature is where r is the reaction rate (for <50.00% FAME yield), k1 is the higher than 170 8C; while without any catalyst, the transester- apparent reaction rate constant, Coil is the oil concentration, and a iﬁcation reaction starts at 220 8C. The mixed oxides catalyst clearly is the reaction order. Based on the data in Fig. 3, the apparent shows an enhanced activity as compared to the pure oxides. reaction order to the oil was found to be 1.2. The apparent activation energy Eapp was 90.9 KJ molÀ1, which was signiﬁcantly higher than the reported activation energy (19.9 KJ molÀ1) using Fig. 1. Transesteriﬁcation activities of Zn10La0, Zn3La1 and Zn0La10 as a function of temperature. Reaction mixtures were heated at 2 8C/min and maintained at target Fig. 3. Transesteriﬁcation with different molar ratio of methanol to oil with Zn3La1 temperatures for 1 min. Zn3La1 shows the highest activity compared to Zn10La0 at 200 8C. Reactants are 3 g of catalyst and 300 g of oil and methanol mixture. 36–42 and Zn0La10. of molar ratio of methanol to oil shows high yield of FAME. 206 S. Yan et al. / Applied Catalysis A: General 353 (2009) 203–212 NaOH catalyst . The higher activation energy is expected of a heterogeneous catalytic system. The effect of molar ratio of methanol to oil was shown in Fig. 4. In traditional homogeneous processes, a 6:1 molar ratio of methanol to oil is usually used for biodiesel production. However, in Fig. 4 it gave poor yields of FAME in Zn3La1 catalyzed process, and higher molar ratios of methanol to oil (36:1 to 42: 1) led to yields higher than 95% within 60 min. The optimal molar ratio is suggested to be 36:1. The excess methanol promoted the transesteriﬁcation reaction forward, and extracted products (FAME and glycerin) from reactants to renew the catalyst surface . 3.2. Catalytic activity of zinc lanthanum mixed oxides in fatty acid esteriﬁcation In unreﬁned or waste oils, FFA concentration can be very high (0.5–30%) . Therefore, developing a catalyst which can simultaneously esterify FFA and transesterify triglyceride into biodiesel is important. As fatty acids have similar chemical Fig. 4. Transesteriﬁcation at different reaction temperatures with Zn3La1. Reactant properties, oleic acid was used as a model FFA for the esteriﬁcation mixtures are 126 g of oil, 180 g of methanol, 3 g of catalyst. study. The esteriﬁcation reaction with methanol is as follows: Fig. 5a presents the results of using Zn3La1 in esterifying pure 3.3. Catalytic activity of zinc lanthanum mixed oxides in biodiesel and oleic acid with methanol using a step-rising heating method oil hydrolysis (heating rate of 2 8C/min, and held at the target temperature for 1 min). When the temperature was higher than 140 8C, esteriﬁca- Unreﬁned or waste oils generally contain a high content of tion activity was observed. At 200 8C, a 96.7% yield of oleic acid water. Thus, fatty acid methyl esters and triglycerides hydrolysis methyl ester can be obtained within 10 min (Fig. 5b). are undesirable side reactions that may decrease the yield of FAME Since FFA can exist in considerable amount in unreﬁned or and increase the acidity of reaction mixtures. Fig. 7 illustrates that, waste oil feedstock, investigation of esterifying FFA with methanol without zinc and lanthanum mixed oxides catalyst hydrolysis in the presence of triglycerides is important. A mixture oil reactions were not observed even for reaction temperature up to containing 5.4 (wt%) of oleic acid and 94.6 (wt%) of food-grade 250 8C. In the presence of Zn3La1, hydrolysis occurs at tempera- soybean oil was prepared and reacted with excess methanol at tures above 220 8C. As hydrolysis is not desirable, a reaction 200 8C. The results are shown in Fig. 6. Oleic acid content in mixed temperature lower than 220 8C is necessary. A ﬁnal yield of 95.0% oil quickly decreased in the ﬁrst 10 min. The yield of FAME FAME can be obtained with a mixture of 5.3 (wt%) water and 94.7 approached 95.3% after 60 min. Fig. 6 suggests that, on Zn3La1, the (wt%) food-grade soybean oil at 200 8C (Fig. 8). The water contents esteriﬁcation reaction is rapid and occurs simultaneously with the in the reactant mixture were found to maintain at about 5.2% transesteriﬁcation of triglycerides in a single-step process. during the process. The FFA contents were very low (<0.01%), Fig. 5. Esteriﬁcation activity of Zn3La1 in pure oleic acid (a) yield of oleic methyl esters as a function of temperature. Reaction mixtures were heated at 2 8C/min and maintained at target temperatures for 1 min. Zn3La1 shows the esteriﬁcation activity over 140 8C; (b) yield of oleic methyl esters at 200 8C. S. Yan et al. / Applied Catalysis A: General 353 (2009) 203–212 207 3.4. Effects of FFA and water on transesteriﬁcation FFA and water are usually considered as poisons to both homogeneous acidic and basic catalysts in traditional biodiesel production processes [4,5]. In order to examine the effects of FFA and water, the activity of Zn3La1 catalyst on food-grade soybean oils with 5.20%, 10.13%, 15.21% and 30.56% of oleic acid, with 1.03%, 3.12% and 5.07% of water, and with 5.16% of oleic acid and 3.10% of water were examined. Fig. 9a also shows that the addition of oleic acid accelerated the reaction rate and shortened the time to a high yield of FAME. For example, without FFA addition, 60 min is needed to reach completion; while for oil with 5.20% FFA addition, the time to reach completion is only about 20 min. With further increase in FFA addition, the time to reach completion decreased. In Fig. 9b, the effect of FFA addition on the equilibrium yields of FAME using H2SO4, NaOH and Zn3La1 as catalyst is compared. A sharp decrease in FAME yield was observed in the processes that use H2SO4 or Fig. 6. The FAME yield and oleic acid content as a function of time at 200 8C. Reaction NaOH as catalyst [23–25]. Fig. 9b shows that even with 5.20% FFA, mixtures are 126 g of the oil containing 5.4 (wt%) oleic acid and 94.6 (wt%) FAME yield decreased to 78% and 88% in NaOH catalyzed and triglycerides, 180 g of methanol, 3 g of Zn3La1 catalyst. H2SO4 catalyzed processes, respectively. However, FAME yield was maintained at 96.6% in Zn3La1 catalyzed process. More difference can be observed for 30.56% FFA addition, FAME yield decreased to 10% and 60% in NaOH catalyzed and H2SO4 catalyzed processes, respectively; while FAME yield was still maintained at 96.0% in Zn3La1 catalyzed process. The result indicates that in comparison with NaOH and H2SO4, Zn3La1 has a remarkable tolerance to FFA in the transesteriﬁcation reaction. Thus, this class of mixed metal oxides is very promising for direct conversion of acidic oils to FAME in a single-step process. Effects of water on FAME yield using Zn3La1 as catalyst are shown in Fig. 9c and d. It was found that the addition of water prolonged the time to reach completion. For example, with 3.12% water added to oil, the time to completion was prolonged from 60 min to 90 min. Further increase of water led to the increase of reaction time to equilibrium. However, the equilibrium yield of FAME was around 97% regardless of the water content. On the other hand, for H2SO4 and NaOH catalyzed processes, water showed considerable effect on transesteriﬁcation activity (Fig. 9d) [23–25]. When water addition in oils was 5.20%, FAME yields decreased to 78% and 11% for NaOH and H2SO4 catalyst, respectively. Fig. 7. Hydrolysis of biodiesel and oil in presence or absence of Zn3La1 catalyst. Reaction mixtures are 285 g of oil or biodiesel and 15 g of distilled water. 3.5. Single-step conversion of unreﬁned and waste oils which are beyond the detection limit of the titrator. Fig. 8 suggests Zn3La1 was demonstrated to be active in the transesteriﬁcation that at 200 8C and in the presence of Zn3La1, there was little or no reaction above 170 8C, in FFA esteriﬁcation above 140 8C, and in hydrolysis of biodiesel and triglyceride, while a high yield of FAME biodiesel and oil hydrolysis over 220 8C. Thus, in order to obtain a was obtained. The triglyceride hydrolysis (1) and FAME hydrolysis high FAME yield, the reaction temperature in this system should be (2) reactions are: limited to the range of 170–220 8C to enhance simultaneous (1) (2) 208 S. Yan et al. / Applied Catalysis A: General 353 (2009) 203–212 re-examine the temperature windows for the three reactions in ﬁxed bed reactors to optimize the FAME yield. Fig. 10 [26,27] illustrates the possible reaction pathway in converting unreﬁned and waste oils into biodiesel in the presence of Zn3La1 at 200 8C. FAME is formed through triglyceride transesteriﬁcation and FFA esteriﬁcation reactions; while being consumed through FAME hydrolysis reaction. Triglyceride hydrolysis can also occur which lead to lower FAME yield and higher total acid number of the resulting products. At 200 8C, the reaction rates of FAME hydrolysis and triglyceride hydrolysis were not signiﬁcant. Several unreﬁned and waste oils without any pretreatment were converted directly using the Zn3La1 catalyst at 200 8C. Fig. 11 illustrates the FAME yields from waste cooking oil, unreﬁned soybean oil, unreﬁned palm oil, food-grade soybean oil with 5.2% oleic acid and 3.1% water, and food-grade soybean oil. The FFA and water contents of these oils are shown in Table 1. It is remarkable that the equilibrium yield of the different oils were all very high around 96%. For this Zn3La1 catalyst, the presence of FFA and water Fig. 8. The FAME yield and water content as a function of time at 200 8C. Reaction mixtures are 126 g of the oil containing 5.3 (wt%) oleic acid and 94.7 (wt%) did not signiﬁcantly affect the equilibrium yield. triglycerides, 180 g of methanol, 3 g of Zn3La1 catalyst. 3.6. Effect of metal oxide composition on catalyst structure transesteriﬁcation and esteriﬁcation, while limiting hydrolysis 3.6.1. XRD, SEM, EDS and BET reactions. It should be noted that in a ﬁxed bed reactor con- The XRD patterns and EDS of zinc and lanthanum metal oxides ﬁguration, the optimal operating temperature will be a function are given in Fig. 12 and Table 2. The XRD pattern of Zn10La0 of contact time with the catalyst. Therefore, it is necessary to corresponds with hexagonal wurtzite structure of zinc oxide. The Fig. 9. Effect of FFA and water additions on transesteriﬁcation (a) yield of FAME in the presence of different FFA addition; (b) effect of FFA content on equilibrium yield of FAME; (c) yield of FAME in the presence of different water addition; (d) effect of water addition on equilibrium yield of FAME. Reaction conditions: (1) acidic catalysis process, sulfuric acid amount is 3%, molar ratio of methanol to oil is 6:1, reaction temperature is 60 8C, and reaction time is 96 h ; (2) alkaline catalysis process, NaOH amount is 1%, molar ratio of methanol to oil is 6:1, reaction temperature is 25 8C, and reaction time is 8 h ; (3) heterogeneously catalytic process, catalyst amount of Zn3La1 is 2.3 (wt%), molar ratio of methanol to oil is 36:1, reaction temperature is 200 8C, and reaction time is 1.5 h. S. Yan et al. / Applied Catalysis A: General 353 (2009) 203–212 209 Fig. 10. Reactions pathways of transesteriﬁcation, esteriﬁcation, and hydrolysis of unreﬁned and waste oils. Fig. 11. FAME yield of crude palm oil, crude soybean oil, waste cooking oil, food- grade soybean oil and food-grade soybean oil with 3.1% water and 5.2% oleic acid Fig. 12. XRD patterns of pure and mixed zinc and lanthanum mixed metal oxides. addition. Reaction mixtures are 126 g of oil, 180 g of methanol, 3 g of catalyst. Note Note the transition of bulk ZnO structures to mixed ZnO-La2O3 structures as La that 96% yield can be obtained within 3 h. content increases. pattern of Zn0La10 shows a mixture of La2CO5 and LaOOH. The Zn1La1) have higher activities than pure zinc oxide (Zn10La0), diffraction patterns observed for Zn9La1, Zn3La1 and Zn1La1 show which correlate well with the effects of lanthanum on enhancing lower intensity than Zn10La0, and mixed ZnO, La2CO5 and LaOOH the dispersion of ZnO. are found in Zn9La1, Zn3La1 and Zn1La1. The XRD pattern of Zn3La1 shows a mixture of ZnO, La2CO5 and The mean grain size of ZnO in Zn9La1, Zn3La1 and Zn1La1 LaOOH phases. Various polar crystal planes of ZnO, such as ZnO was calculated by the Deby-Scherrer equation based on the (1 0 2), (1 0 3) and (1 1 2), could be observed. Methanol molecules reﬂection peak of ZnO (1 0 1) in Fig. 12. The bulk molar ratios of prefer to stick on ZnO (1 0 2) and (1 0 3) which contain higher zinc to lanthanum were determined by EDS (Table 2). The mean concentration of oxygen atoms than zinc. While on ZnO (1 1 2), grain size of ZnO was found to decrease with the addition of La where concentration of zinc atoms is higher, adsorption of (Table 2), suggesting that a strong interaction between the carbonyl groups are favored [28,29]. Therefore, these polar La and Zn species enhances the ZnO dispersion. Figs. 1 and 2 surfaces can be attributed as the active centers for transesteriﬁca- show that the mixed ZnO-La2O3 catalysts (Zn9La1, Zn3La1 and tion and esteriﬁcation reactions. Table 2 Speciﬁc surface area, mean grain size and lattice constants of zinc lanthanum oxides as a function of Zn:La ratio. Note the Zn:La ratio. Note the decrease of ZnO grain size as La content increase. Catalyst XRD structure Mean grain Lattice constants for ZnO phase Zn:La Speciﬁc size of ZnO (nm) (bulk molar ratio) surface area (m2/g) ˚ a (A) ˚ c (A) ˚ Vol (A3) Density (C) Zn10La0 ZnO >100 3.25 5.21 47.63 5.68 1.0: 0 16.3 Zn9La1 ZnO 27.6 3.25 5.36 48.62 5.56 8.9: 1.0 16.8 Zn3La1 ZnO, La2CO5, LaOOH 17.1 3.25 5.23 47.81 5.65 3.5: 1.0 15.7 Zn1La1 ZnO, La2CO5, LaOOH 9.8 3.33 5.10 49.12 5.50 1.2: 1.0 14.9 Zn0La10 La2CO5, LaOOH – – – – – 0: 1.0 12.2 210 S. Yan et al. / Applied Catalysis A: General 353 (2009) 203–212 The XRD patterns of Zn10La0, Zn9La1, Zn3La1 and Zn1La1 show a shift of the diffraction peaks of ZnO (1 0 0), (0 0 2), (1 0 1) based on MDI’s JADE and DATASCAN. Lattice constants of ZnO crystal were calculated (Table 2). The increase of lattice constants in a and c directions suggests partial incorporation of La 3+ ion with ZnO crystal resulting in lattice distortion . Thus, crystal growth of wurtzite ZnO was inhibted and ZnO was highly dispersed . Speciﬁc surface area of catalysts ranges from 12.2 m2/g to 16.8 m2/g (Table 2), and do not show a direct correlation with lanthanum loading. The particle size and morphology of Zn3La1 are shown in Fig. 13. Small particles and some big aggregations are observed. 3.6.2. XPS The XPS data of Zn, La and O elements on the surface of Zn10La0, Zn9La1, Zn3La1, Zn1La1 and Zn0La10 are shown in Table 3. The binding energy of 1021.9 eV, 835.0 eV, 530.6 eV and 528.9 eV can Fig. 13. A representative SEM image of Zn3La1 showing aggregations of catalyst be attributed to Zn 2+, La 3+, Oad (adsorbed oxygen) and Olat (lattice particles. oxygen), respectively. For metal oxides, lattice oxygens on the Table 3 The binding energy and surface percentage of Olat, Zn2+, La3+ as a function of Zn:La ratio. Note the highest total surface percentage of Olat, Zn2+ and La3+ observed for the Zn3La1 catalyst. Samples Binding energy (eV) Surface percentage (at.%) Surface atom ratio 2+ 3+ 2+ 3+ 2+ 3+ La3d Olat Oad Zn2p Olat Zn La Zn and La Olat, Zn and La Zn2+:La3+ (Zn2+ + La3+):Olat Zn10La0 – 528.9 530.6 1021.9 19.9 10.2 – 10.2 30.1 – 0.5 Zn9La1 836.8 530.2 531.8 1021.6 23.2 24.4 2.1 26.5 49.7 11.6 1.1 Zn3La1 835.7 530.2 531.5 1021.4 30.5 26.8 2.8 29.6 60.1 9.6 1.0 Zn1La1 835.3 530.1 531.4 1021.1 32.9 4.2 2.9 7.1 40.0 1.4 0.2 Zn0La10 835 530.3 531.7 – 37.5 – 7.9 7.9 45.4 – 0.2 Fig. 14. Schematic representation of possible mechanism for esteriﬁcation of fatty acid with methanol. S. Yan et al. / Applied Catalysis A: General 353 (2009) 203–212 211 Fig. 15. Schematic representation of possible mechanism for transesteriﬁcation of triglyceride with methanol. surface are Lewis base sites and metal ions are Lewis acid sites . tetrahedral intermediate (Fig. 13). During esteriﬁcation the Base sites are considered catalytic sites for transesteriﬁcation tetrahedral intermediate eliminates water molecule to form one reactions [33,34], and acid sites are considered as the active sites mole of methyl ester. The transesteriﬁcation takes place between for esteriﬁcation reactions [34,35]. Therefore, it is interesting to see the adsorbed methanol and triglyceride. The transesteriﬁcation if there is a correlation between surface concentrations of Olat, mechanism can be extended to di- and mono-glyceride. Methanol Zn2+, La3+ and the activities of esteriﬁcation and transesteriﬁcation is adsorbed on the Lewis base site (BÀ) of the catalyst and forms reactions. oxygen anion. The nucleophilic attack of alcohol to the esters The effect of La3+ concentration on the fraction of basic and acid produces a tetrahedral intermediate (Fig. 14). Then the hydroxyl sites is shown in Table 3. Percentages of surface lattice oxygen group breaks and forms two kinds of esters. In both cases, (base site) increased with the lanthanum content. On the other esteriﬁcation and transesteriﬁcation use of excess methanol favors hand, surface content of Zn2+ and La3+ in Zn3La1 are higher than forward reaction and thus maximizes the FAME yield. Zn10La0, Zn9La1, Zn1La1 and Zn0La10, suggesting that there is an optimal La loading to maximize surface acid sites. Combining both 4. Conclusion the acid and base sites, Zn3La1 again show the highest total surface percentage of Olat, Zn2+ and La3+, as compared to other catalysts. The synthesis of FAME from unreﬁned and waste oils was This correlates well with the transesteriﬁcation activities (Figs. 1 investigated using a series of zinc and lanthanum mixed oxides and 2) and XRD ﬁndings (Table 2). catalysts. There was a strong interaction between zinc and Shifts of binding energies of Olat, Zn2p and La3d are presented in lanthanium species, and the catalyst with 3:1 of zinc to lanthanum Table 2. Binding energies of Olat in Zn9La1, Zn3La1 and Zn1La1 are which has shown a higher activity than pure metal oxides. We higher than Zn10La0, while Zn2p in Zn9La1, Zn3La1 and Zn1La1 have found that at 200 8C, Zn3La1 was highly tolerant to FFA and are lower than Zn10La0. Moreover, binding energies of La3d water, active in both transesteriﬁcation and esteriﬁcation decrease with the La content. The shift of binding energy can be reactions, and with no hydrolysis activity. Within 3 h, 96% yield attributed to the electron transfer from lattice oxygen atoms to of FAME was obtained even with crude palm oil, crude soybean oil, metal atoms. This suggests that La3+ acts as an electron donor, waste cooking oil, food-grade soybean oil with 3% water and 5% enhancing the interaction of reactant molecules with catalyst oleic acid addition. A temperature window was suggested surfaces . between 170 8C and 220 8C to change unreﬁned and waste oils into FAME based on Zn3La1 catalyst. Lanthanum promoted ZnO 3.7. Reaction mechanism of simultaneous transesteriﬁcation and dispersion, increased the surface amounts of acid and base sites, esteriﬁcation thus enhanced the catalyst ability in both transeseteriﬁcation and esteriﬁcation reaction. The zinc and lanthanum mixed oxides The reaction mechanism of simultaneous esteriﬁcation and catalyst allows the direct use of unreﬁned and waste oils for transesteriﬁcation is suggested in Figs. 14 and 15. The esteriﬁca- biodiesel production. Using this class of catalysts, which is tion takes place between the adsorbed fatty acids and free relatively inexpensive because of low raw materials and methanol. The interaction of the carbonyl oxygen of fatty acid with manufacturing cost, signiﬁcantly simpliﬁes the oil pretreatment the Lewis acidic site (L+) of the catalyst forms carbocation. The process and product puriﬁcation process, and greatly decreases nucleophilic attack of alcohol to the carbocation produces a the production cost of biodiesel. 212 S. Yan et al. / Applied Catalysis A: General 353 (2009) 203–212 Acknowledgements  C.V. McNeff, L.C. McNeff, B. Yan, D.T. Nowlan, A.E. Gyberg, B.J. Krohn, R. Fedie, T.R. Hoye, M. Rasmussen, Appl. Catal. A: Gen. 343 (2008) 39–48.  T. Lacome, G. Hillion, B. Delfort, R. Revel, S. Leporg, G. Acakpo, US, 2005. Financial support from the Department of Energy (grant  S.L. Yan, H.F. Lu, B. Liang, Energy Fuels 22 (2007) 646–651. 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