JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGYVolume 38, Issue 3, June 2010 Online English edition of the Chinese language journal

Received: 26-Oct-2009; Revised: 18-Jan-2010 * Corresponding author. Tel: + 86-20-87057760; Fax: + 86-20-87057737; E-mail: lvpm@ms.giec.ac.cn Foundation item: supported by the National High-Tech Research and Development Program of China (863 Program, 2007AA05Z412).

Copyright 2010, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

RESEARCH PAPERCite this article as: J Fuel Chem Technol, 2010, 38(3), 281 286

Synthesis of biodiesel by Fe(II)-Zn double-metal cyanide complexesYAN Fang1,2, YUAN Zhen-hong1, LÜ Peng-mei1,*, LUO Wen1, YANG Ling-mei1, DENG Li2

1 Key Laboratory of Renewable Energy and Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China

2 School of Life Science and Technology, Beijing University of Chemical Technology, Beijing 100029, China

Abstract: A kind of Fe(II)-Zn double-metal cyanide (DMC) complexes solid catalyst was prepared through coreaction of potassium ferrocyanide, zinc chloride, and complexing agent of tert-BuOH. The catalyst has highly catalytic activity on the simultaneous transesterification of triglycerides and esterification of free fatty acids (FFA) reactions. The effect of various factors on the reaction was studied, including different cocomplexing agents, DMC catalyst amount, reaction temperature, methanol/oil molar ratio, reaction time, and water and fatty acid content in raw materials. High catalytic activity was observed for DMC under relatively higher content of water or FFA. Under the optimum condition, the methyl ester yield can reach 98%. The 93.45% catalyst can be recovered after 6 cycles.Keywords: DMC; solid acid; transesterification; esterification; biodiesel

In recent years, due to the increasing world oil-resource depletion and air pollution caused by oil burning, people have started looking for new alternative energy sources. Biodiesel consists of alkyl esters derived from the transesterification of triglycerides (TGs) from animal or vegetable oils with short-chained alcohols that is becoming increasingly important as a potential substitute for diesel fuel energy sources[1,2].Compared with ordinary diesel, biodiesel has many advantages[3]: it is biodegradable, renewable, and its emission of toxic gases (SOx, etc.) is lesser, and so on. At present, various countries around the world are active in biodiesel research[4–8].

Restaurant waste oil is a by-product of modern catering industry. There are a lot of water and free fatty acids in the waste oil, most of which illegally enter the catering industry to cause great harm, except some of which is used to produce low valued soap. There is more than 2 million tons of waste oil each year in China. Using waste oil as raw materials to produce biodiesel can change wastes into resources and reduce the costs of biodiesel production, which has already been studied[9–12]. The usual chemical method of homogeneous catalysts is used at present in industrial production of biodiesel from waste oil uses. However, homogeneous catalysts (liquid acid or alkali) can seriously corrode facilities, result in

complex post-treatment processes, and generate substantial wastewater that is detrimental to the environment. Heterogeneous solid catalysts are easily separated from products after the reaction of transesterification, which avoids wastewater of product washing process. However, water and free fatty acids in the waste oil easily poison catalysts, which lead to significant decrease of reaction conversion rate. Thus, water separation and acid value reduction of waste oil are needed through the pretreatment.

Double metal cyanide (DMC) with a cage structure of loading zeolite, recently has been of concern because of its magnetic, electrochromic, magneto-optical, and nanomagnetic nature[13]. DMC is not soluble in organic solvents and even aqua regia, gathers Lewis acid and alkali metal together, and is insensitive to water and acid. It is often used as catalysts in copolymerization of epoxides and CO2 and polyether polyol preparation, as well as in carbonate transesterification[14–16].However, there are few reports about its usage in biodiesel production.

In order to explore the effect of water and free fatty acids on the DMC-catalyzed transesterification, in this study, the double metal cyanide complexes were used for both esterification of fatty acids and transesterification of triglycerides to produce biodiesel under a high water content

YAN Fang et al. / Journal of Fuel Chemistry and Technology, 2010, 38(3): 281 286

environment.

1 Experimental

1.1 Materials and equipment

Material: Rapeseed oil (molecular weight 880), oleic acid, methanol, potassium ferrocyanide, zinc chloride, tert-BuOH(Tert-BuOH), polyethylene glycol (PEG), and other chemical reagents, which were analytically pure, were purchased in Tianjin Damao Chemical Reagent Factory.

Instrument: FCZ magnetic drive high-pressure reactor (Mao Experimental Equipment Co., Ltd. Dalian Branch); DF-101S collector-type magnetic stirrer (Jintan Medical Instrument Factory); HM-200 electronic balance (Japan AND Company); Shimadzu GC2010-type gas chromatograph equipped with Shimadzu GC solution chromatography workstation, FID detector and RTX WAX capillary column (30 m × 0. 25 mm × 0. 25 m, the maximum use temperature of 330 C).

1.2 Experimental methods

1.2.1 Catalysts preparation

0.02 mol of K4Fe(CN)6·3H2O was dissolved in 80 mL of distilled water to prepare solution 1. Solution 2 was prepared by dissolving 0.2 mol of ZnCl2 in 20 mL of distilled water and 20 mL of tert-BuOH. Solution 3 was prepared by dissolving 30 g of PEG in 4 mL of distilled water and 80 mL of tert-BuOH. Solution 1 (slow drip) and solution 3 were sequentially dropped into solution 2 at 50 C under magnetic stirring condition, and continually stirred for 1 h. The resulting precipitate was filtered, repeatedly washed with distilled water, and finally filtered and vacuum dried at 50 C for 8 h.

To test the effect of Tert-BuOH and PEG on the catalyst composition and catalytic properties, the catalysts, without PEG, were prepared in the same way by using 80 mL tert-BuOH instead of solution 3. In addition, the catalysts, with

neither PEG nor Tert-BuOH, were prepared by using 40 mL of 5 mol/L ZnCl2 solution instead of solution 2, and without solution 3.

1.2.2 Catalyst characterization

Xport pro X-ray diffractometer (Netherlands Panalytical Company) was used for XRD analysis of the DMC catalyst. The test conditions are reflection, Cu K target, tube voltage of 40 kV, tube current of 40 mA, scan rate of 0.4°/s, and scan range of 5° –100°.

1.2.3 DMC-catalyzed transesterification and esterification

Methanol and rapeseed oil with a certain molar ratio and a certain mass fraction of the DMC catalyst were added in the autoclave reactor. The temperature and rotation rate were set, and recycled water for reflux condensation was used. After the reaction, the catalyst was leached and separated, and then the filtrate was poured into a separatory funnel and put aside overnight. Excess methanol of the upper biodiesel was steamed out for recycling. The content of methyl esters of C16–18 were detected using heptadecanoic acid methyl ester as the internal standard by internal standard method in gas chromatography, then the methyl ester yield of transesterification and esterification reaction was calculated[17].DMC-catalyzed esterification of oleic acid and methanol was carried out under the best conditions of the transesterification.

2 Results and discussion

2.1 Catalyst characterization

Organic ligand is a key factor affecting the morphology of DMC catalyst. The organic ligands (tert-BuOH) and PEG containing electron-rich atom groups have a great influence on the crystalline morphology of DMC, as shown in Fig. 1.

Fig. 1 XRD patterns of DMC catalysts (a): organic ligand tert-BuOH added or not into DMC; (b): organic ligand tert-BuOH added with different cocomplexing agent(last-added) or not

a: DMC without organic ligands; b: DMC with tert-BuOH; c: DMC with tert-BuOH and PEG400; d: DMC with tert-BuOH and PEG2000;

e: DMC with tert-BuOH and PEG4000; : Zn2[Fe(CN)6]·3H2O

YAN Fang et al. / Journal of Fuel Chemistry and Technology, 2010, 38(3): 281 286

Fig. 2 Yield of methyl ester with different DMC catalysts 1: organic ligand - free; 2:organic ligand tert-BuOH added; 3: organic ligand

t-BuOH and cocomplexing agent PEG400 added; 4: organic ligand tert-BuOH

and cocomplexing agent PEG2000 added; 5: organic ligand tert-BuOH and

cocomplexing agent PEG4000 added

reaction conditions: rapeseed oil and methanol, mol ratio of methanol to oil is

16 : 1, 3% catalyst, 433 K, 300 r/min, 3 MPa, 8 h

Catalyst structure and crystalline state of DMC is complicated. The main diffraction peaks show the crystalline state of Zn2[Fe(CN)6]·3H2O is the basic skeleton (Fig. 1). The XRD patterns of DMC catalysts with or without tert-BuOH show that adding tert-BuOH makes worse crystallinity during the preparation of DMC (Fig. 1(a)). This is because of the fact that the strong electro-negativity and the number of oxygen atoms in tert-BuOH affect the formation of DMC. The strong electro-negativity of oxygen atom reduces the crystallization of DMC. Adding PEG in the process of catalyst preparation can further weaken the crystallization of the catalyst (Fig. 1 (b)). The larger the molecular weight and longer the molecular chain of PEG are, the heavier the effect is. That is, the strength of the catalyst crystalline is in the order of tert-BuOH + PEG4000< tert-BuOH + PEG2000< tert-BuOH + PEG400< tert-BuOH. DMC particles produced by two metal salts solution mixture can immediately complex on the long-chain structure of PEG to prevent the particles from continuing to grow, coalesce, and crystallize.

2.2 DMC-catalyzed transesterification and optimization of reaction conditions

2.2.1 Effect of different DMC on transesterification

In the DMC, Zn2+, organic ligands (tert-BuOH), and cocomplexing agent (PEG) have impacts on its catalytic activity. The influence of different DMC on the transesterification is shown in Fig. 2.

As shown in Fig. 2, DMC without organic ligands (catalyst 1) also has certain activity, which provides the evidence for the viewpoint that Zn2+ is the catalytic active site. Adding organic ligands (tert-BuOH) into DMC (catalyst 2) causes a substantial increase of the catalytic activity, which illustrates that the performance of DMC is closely related to the organic ligands

(tert-BuOH). Through comparing five kinds of DMC, it is shown that addition of cocomplexing agents (PEG) can improve the catalytic activity of DMC to some extent (catalyst 3–5). The long chain of PEG can reduce the crystallization and increase tert-BuOH content of DMC, which improves the catalytic activity of DMC. The longer the PEG molecular chain, the more significant the effect is. Catalyst 5 with PEG4000 as cocomplexing agents has the best catalytic activity where methyl ester yield is more than 98%. Therefore, catalyst 5 is selected to be used in the following experiments of condition optimization of transesterification reaction, esterification reaction, activity test (water or free fatty acids existed in system), and catalyst life test. The methyl ester yield achieves 84.22% for the esterification of oleic acid and methanol under the reaction conditions of the catalyst 5 amount of 3% of oil, reaction temperature of 433 K, nitrogen pressure of 3 MPa, stirring speed of 300 r/min, molar ratio of methanol to oil of 16:1, and reaction time of 8 h.

2.2.2 Effect of catalyst amounts on transesterification

The effect of catalyst amounts on transesterification is shown in Fig. 3. In the chosen range of catalyst amount, the final ester yield increases with the increasing amount of catalyst within the reaction system. When the amount of catalyst is less than 3% of the oil quality, it has a more significant influence on the reaction. This is because when the amount of catalyst is lesser, it is inadequate of raw materials to access catalysts, causing inadequate transesterification and low yield of methyl ester production. With the increase in mass fraction of the catalyst, the contacting probability of the catalyst and raw material increases, and the catalytic efficiency is significantly improved. Continuing to increase the amount of catalyst, catalytic effect is no longer significantly increased. It determines that the most appropriate amount of catalyst is 3% of oil.

2.2.3 Effect of reaction temperature on ransesterification

Reaction temperature has a greater impact on the reaction rate of transesterification. In a certain temperature range, high temperature promotes oil and methanol to dissolve fully and increases molecular motion in the reaction system to put forward the movement of chemical balance. However, beyond a certain range, high temperature may also cause side reactions such as grease polymerization and low methyl ester yield. The effect of reaction temperature on transesterification is shown in Fig. 4. The methyl ester yield increases gradually with the increase of reaction temperature below 433 K. However, when the temperature is above 433 K, the yield of methyl ester decreases. Therefore, the suitable reaction temperature is 433 K.

YAN Fang et al. / Journal of Fuel Chemistry and Technology, 2010, 38(3): 281 286

Fig. 3 Effect of catalyst amounts on DMC-catalyzed transesterification

reaction conditions: rapeseed oil, methanol and catalyst 5, mol ratio of

methanol to oil is 16 : 1, 433 K, 300 r/min, 3 MPa, 8 h

Fig. 4 Effect of temperature on DMC-catalyzed transesterification

reaction conditions: rapeseed oil and methanol, mol ratio of methanol to oil is

16 : 1, 3% catalyst 5, 300 r/min, 3 MPa, 8 h

Fig. 5 Effect of mol ratio of methanol to oil on DMC-catalyzed transesterification

reaction conditions: rapeseed oil and methanol, 3% catalyst 5, 433 K, 300 r/min,

3 MPa, 8 h

Fig. 6 Effect of reaction time on DMC-catalyzed transesterification

reaction conditions: rapeseed oil and methanol, mol ratio of methanol to oil is

16 : 1, 3% catalyst 5, 433K, 300 r/min, 3 MPa

2.2.4 Effect of molar ratio of methanol to oil on transesterification

The effect of molar ratio of methanol to oil on transesterification is shown in Fig. 5.

The yield of methyl ester shows sharp increase initially and then gets moderate with increase in alcohol-oil molar ratio. As the reaction temperature is much higher than the boiling point of methanol, the methanol state changes between gas and liquid phase in a balance during the reaction process, which causes the concentration of methanol in the actual reaction solution to be low. When the molar ratio of methanol to oil increases, it can improve the concentration of methanol in reaction solution and promote the reaction. Continuing to increase the alcohol-oil ratio for more than 16 : 1, the yield of methyl ester undergoes a few changes. The appropriate molar ratio of alcohol to oil is 16 : 1.

2.2.5 Effect of reaction time on transesterification

The effect of reaction time on transesterification is shown in Fig. 6.

At the beginning the yield of methyl ester rapidly increases with reaction time. This is because the reaction does not yet reach equilibrium in the initial stage. It can promote the reaction by moving toward the positive direction to extend the reaction time. After 8 h reaction, the chemical equilibrium of transesterification is almost achieved, and extending the reaction time further cannot benefit the yield of methyl ester.

2.2.6 Effect of free fatty acids (FFA) on transesterification

The effect of free fatty acids on transesterification is shown in Fig. 7.

YAN Fang et al. / Journal of Fuel Chemistry and Technology, 2010, 38(3): 281 286

Fig. 7 Effect of fatty acid content on DMC-catalyzed transesterification

reaction conditions: rapeseed oil and methanol, mol ratio of methanol to oil is

16 : 1, 3% catalyst 5, 433 K, 300 r/min, 3 MPa, 8 h

Fig. 8 Effect of water content on DMC-catalyzed transesterification reaction conditions: rapeseed oil and methanol, mol ratio of methanol to oil is

16 : 1, 3% catalyst 5, 433 K, 300 r/min, 3 MPa, 8 h

Fig. 9 Lifetime test of DMC catalyst reaction conditions: rapeseed oil and methanol, mol ratio of methanol to oil is

16 : 1, 3% catalyst 5; 433 K; 300 r/min; 3 MPa; 8 h

Generally, a preremoving process is needed for raw materials to reduce the free fatty acid (FFA) content to less than 1% in the use of solid base catalyst[18]. Otherwise, the FFA will result in deactivation of the catalyst. In order to examine the impact of FFA content on the reaction, oleic acid was chosen to be added into the raw materials in this experiment because it is one of the major fatty acids in vegetable oils. As shown in Fig. 7, the yield of methyl ester is slightly decreased, but relatively stable. This is because DMC can catalyze esterification of FFA with methanol to generate methyl ester. Some FFA can also esterify with the glycerol to generate intermediates and increase reverse reaction, which result in a low yield of methyl ester. However, after 8 h reaction, the yield of methyl ester can still be maintained at a high level. Compared with the solid base, DMC can be applied to a number of high acid value oil materials.

2.2.7 Effect of water content on transesterification

Waste oils usually contain a certain amount of water. It is necessary to lower the water content to less than 0.03% by pretreatment before the transesterification, if homogeneous catalysts are used to produce biodiesel. At present, the activities of most heterogeneous solid catalysts are also affected by moisture content. In alkaline condition, the water existing in the system will poison the catalyst surface structure, increase the consumption of the catalyst, and sometimes conglomerate the catalyst particles. The presence of water also causes hydrolysis of oil with water to generate fatty acids and to easily generate soap, making the product to emulsify and difficult to separate. DMC with its unique structure and the feature of gathering Lewis acid and alkali metal together strengthens its tolerance to water. The effect of water content on transesterification is shown in Fig. 8.

The catalytic activity is almost stable within the water content of 10% of raw materials.

2.3 Catalyst stability and reusability

Used DMC catalysts are separated and reused for another batch without any treatment for the research of stability of the catalyst. The result is shown in Fig. 9.

There is less decrease of the catalytic activity of recycled DMC catalysts. However, the catalyst recovery is still up to 93.45% after 6 cycles. DMC is a kind of relatively stable catalyst for which catalytic activity could be maintained for long-term by its repeated use.

3 Conclusions

DMC solid catalysts were prepared using water-soluble metal cyanide complexes of potassium ferrocyanide and metal compounds of zinc chloride, and complexing organic ligands of tert-BuOH, which have a desirable effect on catalyzing

YAN Fang et al. / Journal of Fuel Chemistry and Technology, 2010, 38(3): 281 286

transesterification of triglycerides and esterification of fatty acid to synthetic biodiesel. DMC catalysts show high activity to catalyze transesterification and esterification reactions to produce biodiesel in the presence of water or FFA. The methyl ester yield can go up to 98% under the optimum reaction condition, which is catalyst mass of 3% of the oil quality, the reaction temperature of 433 K, alcohol-oil molar ratio of 16 : 1, and reaction time of 8 h.

References

[1] Boehman A L. Biodiesel production and processing. Fuel Process Technol, 2005, 86(10): 1057–1058.

[2] Demirbas A. Importance of biodiesel as transportation fuel. Energy Policy, 2007, 35(9): 4661–4670.

[3] Srivastava A, Prasad R. Triglycerides-based diesel fuels. Renewable Sustainable Energy Rev, 2000, 4(2): 111–133.

[4] Kim H-J, Kang B-S, Kim M-J, Park Y M, Kim D-K, Lee J-S, Lee K-Y. Transesterification of vegetable oil to biodiesel using heterogeneous base catalyst. Catal Today, 2004, 93/95: 315–320.

[5] Leclercq E, Finiels A, Moreau C. Transesterification of rapeseed oil in the presence of basic zeolites and related solid catalysts. J Am Oil Chem Soc., 2001, 78(11): 1161–1165.

[6] Suppes G J, Dasari M A, Doskocil E J, Mankidy P J, Goff M J. Transesterification of soybean oil with zeolite and metal catalysts. Appl Catal A, 2004, 257(2): 213–223.

[7] Furuta S, Matsuhashi H, Arata K. Biodiesel fuel production with solid superacid catalysis in fixed bed reactor under atmospheric pressure. Catal Commun, 2004, 5(12): 721–723.

[8] Shieh C J ,Liao H F, Lee C C. Optimizationof lipase-catalyzed biodiesel by response surface methodology. Bioresour Technol, 2003, 88(8): 103–106.

[9] Wang J X, Huang Q D, Zhu J, Huang Q J, Huang F H. Lipase-catalyzed esterification of high acid value waste oil to produce biodiesel with diesel oil as solvent. Journal of Fuel Chemistry and Technology, 2008. 36(3): 291–296.

[10] Wang Y, Ou S, Liu P. Preparation of biodiesel from waste cooking oilvia two-step catalyzed process. Energy Convers Manage, 2006, 31(5): 59–62.

[11] Guo P M, Huang F H, Huang Q D. Preparation of biodiesel from waste oil with high acid value. China Oils and Fats, 2006, 31(7): 66–69.

[12] Felizardo P, Joana Neiva Correia M, Raposo I, Mendes J F, Berkemeier R, João Moura Bordado. Production of biodiesel from waste frying oils. Waste Manage, 2006, 26(5): 487–494.

[13] Verdaguer M, Girolami G S. Magnetism: molecules to materials. Miller J S, Drillon M ed. Weinheim, Wiley-VCH, 2005: 284–344.

[14] Chen S, Qi G R, Hua Z J, Yan H Q. Double metal cyanide complex based on Zn3[Co(CN)6]2 as highly active catalyst for copolymerization of carbon dioxide and cyclohexene oxide. J Polym Sci Part A, 2004, 42(20): 5284–5291.

[15] Huang Y J, Qi G R, Feng L X. Preparation, characterization and catalytic performance of double metal-cyanide complexes. Chinese Journal of Catalysis, 2002, 23(2): 114–117.

[16] Srivastava R, Srinivas D, Ratnasamy P. Fe–Zn double-metal cyanide complexes as novel, solid transesterification catalysts. Journal of Catalysis, 2006, 241(1): 34–44.

[17] Zhang Z Q, Xiao G, Guan C Y. Determining biodiesel yield by gas chromatography inner standard method. Journal of the Chinese Cereals and Oils Association, 2009, 24(5): 139–142.

[18] Ma F, Clements L, Hanna M. The effects of catalysts, free fatty acids and water on transesterification of beef tallow. Trans ASAE, 1998, 41(4): 1261–1264.