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Producing triacetylglycerol with glycerol by two steps: Esterification and acetylation
Fuel Processing Technology 90 (2009) 988–993
Contents lists available at ScienceDirect
Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
Producing triacetylglycerol with glycerol by two steps: Esterification and acetylation Xiaoyuan Liao a,b, Yulei Zhu a,c,⁎, Sheng-Guang Wang a, Yongwang Li a,c a b c
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, 030001, PR China Graduate University of the Chinese Academy of Sciences, Beijing, 100039, PR China Synfuels China Co. Ltd., Taiyuan, 030001, PR China
a r t i c l e
i n f o
Article history: Received 17 December 2008 Received in revised form 21 March 2009 Accepted 23 March 2009 Keywords: Glycerol Resins Esterification Acetylation Acetic acid
a b s t r a c t A two-step method is proposed to obtain high selectivity and high conversion rate for producing additive triacetylglycerol of biofuel from its byproduct glycerol. The esterification of glycerol with acetic acid was carried out over resin and zeolites. Amberlyst-35 was found to be an excellent catalyst. The reaction conditions were optimized by testing catalysts, temperatures, feedstock ratios as well as loads of catalysts. The optimal conditions are temperature of 105 °C and an acetic acid to glycerol molar ratio of 9:1 with 0.5 g catalyst. After the 4 hour reaction of the optimal condition, the selectivity of triacetylglycerol reaches almost 100% in 15 min by adding thereto acid anhydride. Recycling experiments indicate that no significant deactivation of Amerlyst-35 occurred during the reaction. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Biodiesel is a mixture of fatty acid methyl esters (FAME) manufactured by the transesterification of triglycerides with methanol (Scheme 1) [1]. In recent years, there is considerable interest in developing biodiesel as an alternative fuel because of its environmental benefits, and also because it is derived from renewable resources including vegetable oils or animal fats [2]. The U.S. production of biodiesel is expected up to 400 million gallons by the year 2012 with a growth rate of 50–80% per year [3]. However, biodiesel is still more expensive than conventional petroleum derived diesel which is attributed to the higher feedstock and processing costs. Besides, a large surplus of glycerol is formed as a byproduct (10% in weight), which leads to a glut of glycerol in the market [1], as well as higher price of the main product biofuel. The inferior properties of biofuel at low temperatures, high boiling point [4] and low oxidation stability [5] are also the barriers to its expansion and commercial acceptation. This situation has prompted an intensive search for new additives for improving biofuel quality and transference of glycerol to more valuable chemicals. It will be worthy of directly preparing additives for biofuel from its byproduct glycerol. This will not only revalorize glycerol but also increase the
⁎ Corresponding author. State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, 030001, PR China. Tel.: +86 351 7117097; fax: +86 351 7560668. E-mail address: [email protected] (Y. Zhu). 0378-3820/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2009.03.015
gross yield of preferred chemicals in biofuel synthesis process [6]. Logically, this is an approach for both decreasing cost and increasing biofuel quality. Many researchers have been focusing on transference of glycerol. Noureddini et al. [7,8] obtained triether and diethers by glycerol reacting with isobutylene. These compounds decreased biodiesel's viscosity and cloud point when mixed with biodiesel. Some researchers have prepared other derivatives, such as acetals or glycerol carbonates. These derivatives can not only reduce diesel fuel particulate emissions, but also reduce the pour point and the viscosity of biodiesel [9,10]. Likewise, glycerol has also been esterified with acetic acid or transesterified with methyl acetate to yield di- and/ or triacetin [9–12]. It was also found that the production of acetylated glycerol derivatives might be of potential interest to find applications for the excess of glycerol produced from biodiesel (Scheme 1). The monoand diacetylated esters by glycerol reacting with acetic acid are also known as monoacetylglycerol (MAG) and diacetylglycerol (DAG), and have applications in cryogenics and as raw material for production of biodegradable polyesters [13]. The triacetylated derivative is known as triacetylglycerol (TAG), and has applications going from cosmetics to fuel additive [14–16]. Many researchers found that TAG [17,18] results in a final fuel having enhanced cold and viscosity properties, and it has also been used as an antiknock additive for gasoline [12]. Garcia et al.'s recent work [6] also demonstrated that TAG not only improves effectively viscosity but also meets the specification in EN14214 [19] and ASTM D6751 [20] Standards for flash point and oxidation stability.
X. Liao et al. / Fuel Processing Technology 90 (2009) 988–993
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Scheme 1. Reaction for production of biodiesel and glycerol transfers to acetylglycerol.
Although esterification has attracted much interest recently with the development of new synthetic routes to fuel and chemicals based on renewable feedstock, few researches were working on esterification of glycerol with acetic acid. Valter et al. [21] have done pioneer work and found that the glycerol conversion reaches over 90% after 10 min esterification reaction with the TAG selectivity of 13% at 25 min over resins or zeolites. Recently, nearly 37% selectivity to TAG with nearly 80% conversion of glycerol was obtained for esterification of glycerol with acetic acid (T = 125 °C) over acidic SBA-15 [12]. However, the expensive templating copolymer in SBA-15 preparation and complex acid functionalized process are this kind of mesoporous material's bottleneck in widespread industry application. In one word, the main disadvantage of previous works is the low selectivity of TAG, which is preferred additive for biofuels. We therefore focus on enhancing TAG selectivity of esterification of glycerol with acetic acid in the present research.
2. Experimental The esterification reaction was carried out in a roundbottomed glass flask (100 cm3) fitted with a water cooled condenser. The temperature was maintained using an oil bath connected to a thermostat. The reactants glycerol and acetic acid were taken directly into the roundbottomed flask along with the catalyst. The reaction mixture was continuously stirred during the reaction using a magnetic stirrer. The reaction was carried out for a definite period of time after which the catalyst separated from the reaction mixture by filtration. After the optimal conditions were chosen, the quantitative acid anhydride was added drop by drop when the conversion of glycerol reaches almost 100% in 1 h. The products were analyzed using a SP-2000 GC (Shandong Ruihong chromatogram analysis Co., Ltd, China) equipped with a flame ionization detector (FID) and a capillary column (J&W DB-WAX, 30 m × 0.32 mm). The unsured products were identified by GC (6890N, Agilent, USA) with a coupled mass spectrometer (MS, 5973, Agilent, USA). The GC–MS was equipped with a capillary Chromatographic column: J&W DB-WAX (30 m × 0.32 mm). Amberlysts-15 (A-15) and Amberlysts-35 (A-35) from Rohm and Haas (France) were used as esterification catalysts. Wet ion-exchange resins have been dried for 12 h after washing with methanol at 110 °C. Physical characteristics of ion-exchange resin catalysts are summarized in Table 1. Two commercial large-pore zeolites HY and HZSM-5 from the Zeolyst Int. were used as esterification catalysts after activation in the stream of dry air during 6 h at 500 °C. The sorption characteristics of these zeolites were determined by adsorption of nitrogen at the temperature of liquid nitrogen. The characteristics of these zeolites are also listed in Table 1.
All used chemicals (acetic acid, acetic anhydride and glycerol) were of analytical grade purity. 3. Results and discussion 3.1. Designing the reaction Glycerol is a highly functionalized molecule and an advantageous plateau to use as feedstock for the production of bioadditive derivatives. The reaction rate and acetylating ability are considerably higher than that of the esterification [22]. But the price of the acetic anhydride is as high as four times than acetic acid each ton. The acetylation is very fast and is hard to be handled in practice [6]. However, it is very difficult to compress the production of MAG and DAG during esterification if just the acetic acid was used to react with glycerol. In order to reduce production costs and maximize the head product (TAG), we employ a two-step method to obtain high selectivity TAG. Firstly, with cheap acetic acid, the glycerol is completely esterified to the mixture of MAG, DAG and TAG over a catalyst. We then transfer both MAG and DAG into TAG in the second step (Scheme 1). 3.2. The first reaction step The first thing is to find a cheap and suitable catalyst for this reaction. On the catalytic esterification knowledge, it is well known that sulfonated resins (e.g., Amberlyst-15 [23,24], Amberlyst-35 [24]) are the most widespread and selective acidic catalytic sources to carry out either gas- and liquid-phase esterification reactions at industrial level. Indeed, Amberlyst polymer based catalyst strong base anionic ion-exchange resins involve mostly the use of functionalized styrene divinylbenzene or polyacrylic copolymers with different surface properties and porosities. The functional group is generally of a quaternized ammonium group. These resins have been used for 40 years in a wide variety of reactions and purification processes [25]. Some authors have also pointed out the ability of high-silica Hexchanged zeolites (e.g., HY [26,27], HZSM-5 [27]) to catalyze this reaction. These resins or zeolites catalysts are non-corrosive and ecofriendly as well as easy to separate from the reaction mixture. They
Table 1 Physical and chemical properties of ion-exchange resins and zeolites. Catalyst
Average pore diameter (nm)
SBET (m2/g)
A-35 A-15 HY HZSM-5(Si/Al = 25)
30 30 0.74 0.56
50 53 400 730
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X. Liao et al. / Fuel Processing Technology 90 (2009) 988–993
Fig. 1. (a) Conversion of glycerol and (b) selectivity to TAG for esterification of glycerol with acetic acid, effect of time on various catalysts. (T = 105 °C, acetic acid/ glycerol = 0.6 mol:0.1 mol, 0.5 g of catalyst).
can also be used repeatedly over a prolonged period without any difficulty in handling and storing them. Fig. 1 shows the comparison between these catalysts. The esterification reaction was carried out for 4 h with the acid: alcohol molar ratio of 6:1 and the catalyst weight of 0.5 g (5% of glycerol weight). The reaction rates vary on different catalysts. The conversion reaches 99% in 2 h when catalyzed by A-35 and A-15. However, the conversion is about 80% in 4 h when catalyzed by HY and HZSM-5. By comparing with the blank test, HY and HZSM-5 do not apparently exhibit catalytic property during the reaction. Although zeolites were reported to show good selectivities towards esterification reactions (e.g., HY), they are less active than ion-exchange resins because of their lower number of acidic active sites and due to configurational diffusion effects, in fact, their catalytic activity is only ascribed to mesopores in the borderline between adjacent zeolite particles. Compared with A-15, A-35 has slightly better conversion of glycerol (99.0% vs 98.6%) and higher selectivity of TAG (25.9% vs 25.0%) at the same reaction condition (Fig. 2). Indeed, A-35 has larger acid capacity compared to A-15 (5.3 vs 4.8 mequiv of H+/g) [28]. Parra et al suggested that the catalytic activity of these ion-exchange resins is strongly dependent on their acid capacity [29].Therefore, it is easily extrapolated that A-35 has better acid activity than A-15, this result also has been confirmed by methyl tertbutyl ether (MTBE) production [28]. In MTBE production process, the conversion of isobutene over A35 is higher than over A-15 by 5–10% under the same reaction conditions [28]. Based on above reasons, A-35 was chosen for further research. From plot (b) in Fig. 1, can we also notice that, although 99% glycerol has been converted within 2 h, it will take longer time for the reaction to reach equilibrium? For example, the selectivity of TAG catalyzed by A-35 keeps increasing during the reaction, and we only obtained 25.9% selectivity in 4 h. We can also see the values in Fig. 2, which show the conversion of glycerol and selectivity of TAG in 4 h. It is obvious that A-35 and A-15 exert better conversion activity than HY,
HZSM-5 as well as without catalyst. This result in agreement with acid exchange resins have better activity than HZSM-5 in the esterification of acetic acid with butanol [23]. Again, we can find that the zeolites, especially the HZSM-5, have no significant catalytic activity than no any catalyst. In fact, acidic site predominantly inside the pores of the zeolites is responsible for the esterification [27], maybe the diameter of reactant is greater than the pore size of the zeolites. This result was also proved by that the HY has better activity than the HZSM-5 for HY has larger pore than HZSM-5. Notably, the A-35 holds the highest selectivity to TAG (25.9%) with the 99.0% conversion of glycerol in line with its largest pore diameter. The reaction was optimized over A-35 catalyst. We tested the influence of temperature first, as shown by plot (a) of Fig. 3. The esterification reaction was carried out in the temperature from 95 to 115 °C while keeping the acid–glycerol molar ratio at 6:1 and the catalyst weight at 0.5 g (Fig. 3). In general, it has been found that the conversion of the glycerol is almost 100% after 4 h. The selectivity of TAG is increasing with decreasing selectivities of MAG and DAG. This also explains that the TAG is formed through consecutive esterification reactions. It should be mentioned that the reaction mixture became suspended after the 2 hour reaction at 115 °C. The above phenomenon shows that the A-35 may break up, for the reaction temperature is close to its limited temperature (120 °C) [28]. Plot (b) of Fig. 3 shows the optimization of molar ratio. The reaction was carried out over A-35 using different molar ratios of acetic acid to glycerol. The conversion of glycerol was found to near 100% after 4 h with molar ratio of both 6/1 and 9/1. The selectivity to TAG increases with increasing in concentration of acetic acid in the reaction mixture. This implies that an increase of the acetic acid or decrease of the glycerol concentration promotes the esterification reaction. Likewise, it can be deduced that the amount of acetic acid is a positive influential parameter in the formation of TAG. On the other hand, this plot demonstrates that the optimal conditions for DAG production are an intermediate acetic acid to glycerol molar ratio (6/1). This shows clearly that continuously increasing the amount of acetic acid supports a negative effect on the selectivity toward DAG, likely due to the transformation into TAG. Finally, the selectivity to TAG is favored using high acetic acid to glycerol molar ratios, which is well in agreement with acetylation of glycerol over acidic mesoporous silica [12]. Fig. 3. shows that high acetic acid to glycerol molar ratios and high temperatures result in higher selectivities to TAG, as could be expected prior considering that high reaction conversions would favor the
Fig. 2. Conversion of glycerol (blue color) and selectivity of TAG (red color) of esterification of glycerol with acetic acid over various catalysts. (T = 105 °C, t = 4 h, acetic acid/glycerol = 0.6 mol:0.1 mol, 0.5 g of catalyst, blank means uncatalyzed reaction). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. Conversion of glycerol for esterification of glycerol with acetic acid, effect of catalyst concentration (T = 105 °C, t = 4 h).
Fig. 3. (a) Selectivity of TAG for esterification of glycerol with acetic acid, effect of reaction temperature. (t = 4 h, acetic acid/glycerol = 0.6 mol:0.1 mol, 0.5 g of A-35). (b) Selectivity of TAG for esterification of glycerol with acetic acid, effect of molar ratio of the reactants. (T = 105 °C, t = 4 h, 0.5 g of A-35).
formation of the di- and tri-substituted derivatives. It has been reported that this enhancement in the acid strength leads to an improvement in the catalytic activity in several acid-catalyzed reactions. However, because A35 has temperature restriction, mild reaction temperature (105 °C or so) and high acetic acid–glycerol molar ratios appear to favor the evolution of the final product distribution in equilibrium to the most substituted glycerol acetylated derivatives (TAG). For the process interest, the influence of amount of catalyst is also investigated. The amount of the catalyst was varied from 0 to 1.0 g while keeping the molar ratio of acid–glycerol of 6:1. The reaction was carried out at 105 °C for 4 h. The conversion of the glycerol is at 73.6% conversion in the absence of a catalyst and always kept at almost 100% conversion when the high or low load catalyst is added (Fig. 4). In the absence of a catalyst the reaction yielded 97.6% of MAG with 2.2% selectivity to TAG. However, with the catalyst load increases from 0 to 0.5 g, the selectivity of MAG decreases with increasing of selectivity to DAG and TAG. This also confirms that esterification of glycerol is a consecutive reaction. However, an insignificant change in selectivity to MAG, DAG and TAG was observed for increasing the catalyst weight from 0.5 to 1.0 g, this means that much of the catalyst that was added has no positive influence on catalytic activity. In all, when the reaction time is up to 4 h, the amount of catalyst does not influence very much the result. From the above discussion, the optimal reaction condition for the first step reaction is as follows: 0.5 g A-35 as catalyst (each 10 g glycerol as reactant), reaction time 4 h, acetic acid/glycerol 9, and reaction temperature 105 °C.
known that acetylation has better activity than esterification and some researchers found that acetylation have great ability over the acid catalyst [6,22]. This raises an idea of rapid increasing selectivity of TAG by acetylation. In order to test this idea, we investigated acetylation by acetic acid and the acetic anhydride by comparing with esterification. Practically, we divided reaction mixture into three parts equally after reaction 4 h with molar ratio of acetic acid/ glycerol = 3:1 (0.3 mol and 0.1 mol respectively) at 105 °C. One part reacts continuously without any change of conditions. The other two parts were added respectively acetic acid (0.2 mol) or acetic anhydride (0.1 mol) in order to reach optimal reaction condition as mentioned above (acetic acid/glycerol = 9/1). As was shown in Fig. 5, the selectivity to TAG sharply increases to almost 100% in 15 min after 0.1 mol acetic anhydride was added. While acetic was added, TAG selectivity was slightly raised. We can also see from Fig. 5, that it is impossible to obtain high selectivity without acetylation. This result is in agreement with acetic anhydride that has better acetylation ability than the acetic acid [6]. Indeed, it is well-established that the acylation reaction proceeds through an acylium intermediate RCO+, generated from the adsorption of the acylating agent [30], i.e., the anhydride onto the Bronsted acidic sites of the A-35 catalyst. This intermediate RCO+ has considerably higher acetylation activity than that of the
3.3. The second reaction step From the first step, we found that the highest selectivity to TAG is 34.8%, and produced MAG and DAG are more than preferred. It is well
Fig. 5. Selectivity to TAG of esterification of glycerol, (1) previous 4 h, T = 105 °C, t = 4 h, acetic acid/glycerol = 0.3 mol:0.1 mol, 0.5 g of catalyst; (2) later 1 h, (a) regular reaction running, (b) adding acetic acid, and (c) adding acetic anhydride.
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and acetic acid to glycerol molar ratios have shown that it is necessary to use high acetic acid excesses in order to push the equilibrium toward the simultaneous enhancement of the conversion of glycerol and the selectivity toward the most valuable triacetylated derivatives. Likewise, to obtain optimal glycerol conversions, it is advantageous to work with moderate temperatures for high temperature that does harm to resin catalyst. Within the studied range, optimal conditions have been found to be a temperature of 105 °C and an acetic acid to glycerol molar ratio of 9:1. Under these reaction conditions, glycerol conversions of almost 100% and combined selectivities toward triacetylglycerol of over 34.8% were achieved after 4 h of reaction over Amberlyst-35. Then, by adding thereto acid anhydride, the selectivity of TAG reaches almost 100% in 15 min. Recycling experiments indicate that no significant deactivation of Amerlyst-35 occurred during the reaction. We expect that this method would be easy for industrial use with its high selectivity, high conversion rate, mild reaction temperature and fast reaction. Fig. 6. Reuse of Amberlyst-35 catalyst in esterification glycerol, the reaction condition of each run was given as follow: (1) previous 4 h, T = 105 °C, t = 4 h, acetic acid/ glycerol = 0.3 mol/0.1 mol, 0.5 g of catalyst; (2) later 15 min, adding acetic anhydride 0.1 mol.
esterification, which then reacts with the MAG or DAG and transfer them to DAG maximally. In sum, the above experimental results have shown that it is necessary to use high acetic acid excess and add acetic anhydride in order to push the reaction equilibrium toward head product (TAG). In fact, esterification of carboxylic acids with alcohols is a typical example of an equilibrium-limited reaction [31]. Traditionally, two main methods have been carried out for equilibrium displacement: either using an excess of reactant or trying to separate the formed water from the reaction medium by means of reactive distillation [27]. In the present work, excess acid and strong acid catalysts are employed in the reaction mixture to accelerate the reaction rate. It should be mentioned that the thereto acetic anhydride consumes the water produced during the esterification reaction, and also pushes the reaction toward to head product (TAG). 3.4. Reusability of catalyst Recycling experiments were investigated over A-35. The results are presented in Fig. 6. After the reaction finished, the catalyst was separated from the reaction mixture by filtration and washed with methanol and then reused after drying at 100 °C overnight. It is seen that the conversion of glycerol has no obvious drop during the five times reactions. A-35 has excellent reusability without significant deactivation under the reaction conditions. We also extend reaction time to as long as 12 h, and also find that A-35 has good stability. Combined with the result we mention above, the A-35 catalyst breaks up when the reaction is performed near its limited temperature (120 °C), we can safely insure that resin A-35 is sensitive to the temperature rather than reaction time. This is in line with resin established characters [25]. 4. Conclusions A two-step method is proposed to obtain high selectivity and high conversion rate for producing additive of biofuel from its byproduct. The esterification and acetylation of glycerol with acetic acid was carried out over resin and zeolites. Amberlyst-35 is found to be an excellent catalyst. The product of esterification of glycerol with acetic acid is acetylated compounds which perform well as bioadditives for petrol fuels. The reaction conditions were optimized. The experiments were carried out for different levels of temperature
Acknowledgements This work was supported by the National Outstanding Young Scientists Foundation of China (Grant No. 20625620) and Synfuels China. Co., Ltd. References [1] C.-H. Zhou, J.N. Beltramini, Y.-X. Fan, G.Q. Lu, Chemoselective catalytic conversion of glycerol as a biorenewable source to valuable commodity chemicals, Chem. Soc. Rev. 37 (2008) 527–549. [2] A. Kawashima, K. Matsubara, K. Honda, Acceleration of catalytic activity of calcium oxide for biodiesel production, Bioresour. Technol. 100 (2009) 696–700. [3] C.-W. Chiu, Catalytic Conversion of Glycerol to Propylene Glycol: Synthesis and Technology Assessment. University of Missouri-Columbia Ph.D Dissertation (2006) Page 1. [4] R.O. Dunn, M.O. Bagby, Low-temperature properties of triglyceride-based diesel fuels: Transesterified methyl esters and petroleum middle distillate/ester blends, J. Am. Oil Chem. Soc. 72 (1995) 895–903. [5] R.L. McCormick, T.L. Alleman, M. Ratcliff, L. Moens, R. Lawrence, Survey of the quality and stability of biodiesel and biodiesel blends in the United States in 2004, Technical Report of NREL/TP 540-38836, Oct. 2005. [6] E. Garcia, M. Laca, E. Perez, A. Garrido, J. Peinado, New class of acetal derived from glycerin as a biodiesel fuel component, Energy Fuel 22 (2008) 4274–4280. [7] H. Noureddini, W.R. Dailey, B.A. Hunt, Chemical and Biomolecular Engineering Research and Publications, 1998. [8] H. Noureddini, U.S. Patent 6015440 (2001). [9] J. Delgado, U.S. Patent 0167681 (2003). [10] B. Delfort, I. Duran, A. Jaecker, T. Lacome, X. Montagne, F. Paille, U.S. Patent 6890364 (2005). [11] B. Delfort, G. Hilion and I. Durand, FR Patent 2866654 (2004). [12] J.A. Melero, R. van Grieken, G. Morales, M. Paniagua, Acidic mesoporous silica for the acetylation of glycerol: synthesis of bioadditives to petrol fuel, Energy Fuels 21 (2007) 1782–1791. [13] Y. Taguchi, A. Oishi, Y. Ikeda, K. Fujita, T. Masuda, JP Patent 298099 (2000). [14] H. Nabeshima, K. Ito, JP Patent 276787 (1995). [15] P. Hofmann, DE Patent 3512497 (1985). [16] S. Nomura, T. Hyoshi, JP Patent 203429 (1995). [17] B. Delfort, G. Hillion, I. Durand, FR Patent 2866654 (2004). [18] J. Delgado, SP Patent 2201894 (2002). [19] European Standard EN 14214, Automotive Fuels. Fatty Acid Methylesters (FAME) for Diesel Engines. Requirements and Test Methods, 2003. [20] American Society for Testing and Materials (ASTM) Standard D6751. Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels, Dec. 2001. [21] V.L.C. Goncalves, B.P. Pinto, J.C. Silva, C.J.A. Mota, Acetylation of glycerol catalyzed by different solid acids, Catal. Today 133 (2008) 673–677. [22] N. Bremus, G. Dieckelmann, L. Jeromin, W. Rupilius and H. Schutt, US Patent 4381407 (1983). [23] T.A. Peters, N.E. Benes, A. Holmen, J.T.F. Keurentjes, Comparison of commercial solid acid catalysts for the esterification of acetic acid with butanol, Appl. Catal. A 297 (2006) 182–188. [24] N. Özbay, N. Oktar, N.A. Tapan, Esterification of free fatty acids in waste cooking oils (WCO): role of ion-exchange resins, Fuel 87 (2008) 1789–1798. [25] http://www.amberlyst.com/sba.htm, Dec., (2008). [26] A. Corma, H. Garcia, S. Iborra, J. Primo, Modified faujasite zeolites as catalysts in organic reactions: esterification of carboxylic acids in the presence of HY zeolites, J. Catal. 120 (1989) 78–87.
X. Liao et al. / Fuel Processing Technology 90 (2009) 988–993 [27] S.R. Kirumakki, N. Nagaraju, S. Narayanan, A comparative esterification of benzyl alcohol with acetic acid over zeolites Hβ, HY and HZSM5, Appl. Catal. A 273 (2004) 1–9. [28] M.D. Girolamo, M. Lami, M. Marchionna, E. Pescarollo, L. Tagliabue, F. Ancillotti, Liquid-phase etherification/dimerization of isobutene over sulfonic acid resins, Ind. Eng. Chem. Res. 36 (1997) 4452–4458. [29] D. Parra, J.F. Izquierdo, F. Cunill, J. Tejero, M. Iborra, C. Fite, Catalytic activity of acidic ion exchange resins in methyl tert-butyl ether liquid phase synthesis,
993
Proceedings of the First European Congress on Chemical Engineering, Florence, Italy, vol. 4, 1997, pp. 3001–3004. [30] U. Freese, F. Heinrich, F. Roessner, Acylation of aromatic compounds on H-beta zeolites, Catal. Today 49 (1999) 237–244. [31] O. Iglesia, R. Mallada, M. Menendez, J. Coronas, Continuous zeolite membrane reactor for esterification of ethanol and acetic acid, Chem. Eng. J. 131 (2007) 35–39.
Contents lists available at ScienceDirect
Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
Producing triacetylglycerol with glycerol by two steps: Esterification and acetylation Xiaoyuan Liao a,b, Yulei Zhu a,c,⁎, Sheng-Guang Wang a, Yongwang Li a,c a b c
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, 030001, PR China Graduate University of the Chinese Academy of Sciences, Beijing, 100039, PR China Synfuels China Co. Ltd., Taiyuan, 030001, PR China
a r t i c l e
i n f o
Article history: Received 17 December 2008 Received in revised form 21 March 2009 Accepted 23 March 2009 Keywords: Glycerol Resins Esterification Acetylation Acetic acid
a b s t r a c t A two-step method is proposed to obtain high selectivity and high conversion rate for producing additive triacetylglycerol of biofuel from its byproduct glycerol. The esterification of glycerol with acetic acid was carried out over resin and zeolites. Amberlyst-35 was found to be an excellent catalyst. The reaction conditions were optimized by testing catalysts, temperatures, feedstock ratios as well as loads of catalysts. The optimal conditions are temperature of 105 °C and an acetic acid to glycerol molar ratio of 9:1 with 0.5 g catalyst. After the 4 hour reaction of the optimal condition, the selectivity of triacetylglycerol reaches almost 100% in 15 min by adding thereto acid anhydride. Recycling experiments indicate that no significant deactivation of Amerlyst-35 occurred during the reaction. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Biodiesel is a mixture of fatty acid methyl esters (FAME) manufactured by the transesterification of triglycerides with methanol (Scheme 1) [1]. In recent years, there is considerable interest in developing biodiesel as an alternative fuel because of its environmental benefits, and also because it is derived from renewable resources including vegetable oils or animal fats [2]. The U.S. production of biodiesel is expected up to 400 million gallons by the year 2012 with a growth rate of 50–80% per year [3]. However, biodiesel is still more expensive than conventional petroleum derived diesel which is attributed to the higher feedstock and processing costs. Besides, a large surplus of glycerol is formed as a byproduct (10% in weight), which leads to a glut of glycerol in the market [1], as well as higher price of the main product biofuel. The inferior properties of biofuel at low temperatures, high boiling point [4] and low oxidation stability [5] are also the barriers to its expansion and commercial acceptation. This situation has prompted an intensive search for new additives for improving biofuel quality and transference of glycerol to more valuable chemicals. It will be worthy of directly preparing additives for biofuel from its byproduct glycerol. This will not only revalorize glycerol but also increase the
⁎ Corresponding author. State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, 030001, PR China. Tel.: +86 351 7117097; fax: +86 351 7560668. E-mail address: [email protected] (Y. Zhu). 0378-3820/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2009.03.015
gross yield of preferred chemicals in biofuel synthesis process [6]. Logically, this is an approach for both decreasing cost and increasing biofuel quality. Many researchers have been focusing on transference of glycerol. Noureddini et al. [7,8] obtained triether and diethers by glycerol reacting with isobutylene. These compounds decreased biodiesel's viscosity and cloud point when mixed with biodiesel. Some researchers have prepared other derivatives, such as acetals or glycerol carbonates. These derivatives can not only reduce diesel fuel particulate emissions, but also reduce the pour point and the viscosity of biodiesel [9,10]. Likewise, glycerol has also been esterified with acetic acid or transesterified with methyl acetate to yield di- and/ or triacetin [9–12]. It was also found that the production of acetylated glycerol derivatives might be of potential interest to find applications for the excess of glycerol produced from biodiesel (Scheme 1). The monoand diacetylated esters by glycerol reacting with acetic acid are also known as monoacetylglycerol (MAG) and diacetylglycerol (DAG), and have applications in cryogenics and as raw material for production of biodegradable polyesters [13]. The triacetylated derivative is known as triacetylglycerol (TAG), and has applications going from cosmetics to fuel additive [14–16]. Many researchers found that TAG [17,18] results in a final fuel having enhanced cold and viscosity properties, and it has also been used as an antiknock additive for gasoline [12]. Garcia et al.'s recent work [6] also demonstrated that TAG not only improves effectively viscosity but also meets the specification in EN14214 [19] and ASTM D6751 [20] Standards for flash point and oxidation stability.
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Scheme 1. Reaction for production of biodiesel and glycerol transfers to acetylglycerol.
Although esterification has attracted much interest recently with the development of new synthetic routes to fuel and chemicals based on renewable feedstock, few researches were working on esterification of glycerol with acetic acid. Valter et al. [21] have done pioneer work and found that the glycerol conversion reaches over 90% after 10 min esterification reaction with the TAG selectivity of 13% at 25 min over resins or zeolites. Recently, nearly 37% selectivity to TAG with nearly 80% conversion of glycerol was obtained for esterification of glycerol with acetic acid (T = 125 °C) over acidic SBA-15 [12]. However, the expensive templating copolymer in SBA-15 preparation and complex acid functionalized process are this kind of mesoporous material's bottleneck in widespread industry application. In one word, the main disadvantage of previous works is the low selectivity of TAG, which is preferred additive for biofuels. We therefore focus on enhancing TAG selectivity of esterification of glycerol with acetic acid in the present research.
2. Experimental The esterification reaction was carried out in a roundbottomed glass flask (100 cm3) fitted with a water cooled condenser. The temperature was maintained using an oil bath connected to a thermostat. The reactants glycerol and acetic acid were taken directly into the roundbottomed flask along with the catalyst. The reaction mixture was continuously stirred during the reaction using a magnetic stirrer. The reaction was carried out for a definite period of time after which the catalyst separated from the reaction mixture by filtration. After the optimal conditions were chosen, the quantitative acid anhydride was added drop by drop when the conversion of glycerol reaches almost 100% in 1 h. The products were analyzed using a SP-2000 GC (Shandong Ruihong chromatogram analysis Co., Ltd, China) equipped with a flame ionization detector (FID) and a capillary column (J&W DB-WAX, 30 m × 0.32 mm). The unsured products were identified by GC (6890N, Agilent, USA) with a coupled mass spectrometer (MS, 5973, Agilent, USA). The GC–MS was equipped with a capillary Chromatographic column: J&W DB-WAX (30 m × 0.32 mm). Amberlysts-15 (A-15) and Amberlysts-35 (A-35) from Rohm and Haas (France) were used as esterification catalysts. Wet ion-exchange resins have been dried for 12 h after washing with methanol at 110 °C. Physical characteristics of ion-exchange resin catalysts are summarized in Table 1. Two commercial large-pore zeolites HY and HZSM-5 from the Zeolyst Int. were used as esterification catalysts after activation in the stream of dry air during 6 h at 500 °C. The sorption characteristics of these zeolites were determined by adsorption of nitrogen at the temperature of liquid nitrogen. The characteristics of these zeolites are also listed in Table 1.
All used chemicals (acetic acid, acetic anhydride and glycerol) were of analytical grade purity. 3. Results and discussion 3.1. Designing the reaction Glycerol is a highly functionalized molecule and an advantageous plateau to use as feedstock for the production of bioadditive derivatives. The reaction rate and acetylating ability are considerably higher than that of the esterification [22]. But the price of the acetic anhydride is as high as four times than acetic acid each ton. The acetylation is very fast and is hard to be handled in practice [6]. However, it is very difficult to compress the production of MAG and DAG during esterification if just the acetic acid was used to react with glycerol. In order to reduce production costs and maximize the head product (TAG), we employ a two-step method to obtain high selectivity TAG. Firstly, with cheap acetic acid, the glycerol is completely esterified to the mixture of MAG, DAG and TAG over a catalyst. We then transfer both MAG and DAG into TAG in the second step (Scheme 1). 3.2. The first reaction step The first thing is to find a cheap and suitable catalyst for this reaction. On the catalytic esterification knowledge, it is well known that sulfonated resins (e.g., Amberlyst-15 [23,24], Amberlyst-35 [24]) are the most widespread and selective acidic catalytic sources to carry out either gas- and liquid-phase esterification reactions at industrial level. Indeed, Amberlyst polymer based catalyst strong base anionic ion-exchange resins involve mostly the use of functionalized styrene divinylbenzene or polyacrylic copolymers with different surface properties and porosities. The functional group is generally of a quaternized ammonium group. These resins have been used for 40 years in a wide variety of reactions and purification processes [25]. Some authors have also pointed out the ability of high-silica Hexchanged zeolites (e.g., HY [26,27], HZSM-5 [27]) to catalyze this reaction. These resins or zeolites catalysts are non-corrosive and ecofriendly as well as easy to separate from the reaction mixture. They
Table 1 Physical and chemical properties of ion-exchange resins and zeolites. Catalyst
Average pore diameter (nm)
SBET (m2/g)
A-35 A-15 HY HZSM-5(Si/Al = 25)
30 30 0.74 0.56
50 53 400 730
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Fig. 1. (a) Conversion of glycerol and (b) selectivity to TAG for esterification of glycerol with acetic acid, effect of time on various catalysts. (T = 105 °C, acetic acid/ glycerol = 0.6 mol:0.1 mol, 0.5 g of catalyst).
can also be used repeatedly over a prolonged period without any difficulty in handling and storing them. Fig. 1 shows the comparison between these catalysts. The esterification reaction was carried out for 4 h with the acid: alcohol molar ratio of 6:1 and the catalyst weight of 0.5 g (5% of glycerol weight). The reaction rates vary on different catalysts. The conversion reaches 99% in 2 h when catalyzed by A-35 and A-15. However, the conversion is about 80% in 4 h when catalyzed by HY and HZSM-5. By comparing with the blank test, HY and HZSM-5 do not apparently exhibit catalytic property during the reaction. Although zeolites were reported to show good selectivities towards esterification reactions (e.g., HY), they are less active than ion-exchange resins because of their lower number of acidic active sites and due to configurational diffusion effects, in fact, their catalytic activity is only ascribed to mesopores in the borderline between adjacent zeolite particles. Compared with A-15, A-35 has slightly better conversion of glycerol (99.0% vs 98.6%) and higher selectivity of TAG (25.9% vs 25.0%) at the same reaction condition (Fig. 2). Indeed, A-35 has larger acid capacity compared to A-15 (5.3 vs 4.8 mequiv of H+/g) [28]. Parra et al suggested that the catalytic activity of these ion-exchange resins is strongly dependent on their acid capacity [29].Therefore, it is easily extrapolated that A-35 has better acid activity than A-15, this result also has been confirmed by methyl tertbutyl ether (MTBE) production [28]. In MTBE production process, the conversion of isobutene over A35 is higher than over A-15 by 5–10% under the same reaction conditions [28]. Based on above reasons, A-35 was chosen for further research. From plot (b) in Fig. 1, can we also notice that, although 99% glycerol has been converted within 2 h, it will take longer time for the reaction to reach equilibrium? For example, the selectivity of TAG catalyzed by A-35 keeps increasing during the reaction, and we only obtained 25.9% selectivity in 4 h. We can also see the values in Fig. 2, which show the conversion of glycerol and selectivity of TAG in 4 h. It is obvious that A-35 and A-15 exert better conversion activity than HY,
HZSM-5 as well as without catalyst. This result in agreement with acid exchange resins have better activity than HZSM-5 in the esterification of acetic acid with butanol [23]. Again, we can find that the zeolites, especially the HZSM-5, have no significant catalytic activity than no any catalyst. In fact, acidic site predominantly inside the pores of the zeolites is responsible for the esterification [27], maybe the diameter of reactant is greater than the pore size of the zeolites. This result was also proved by that the HY has better activity than the HZSM-5 for HY has larger pore than HZSM-5. Notably, the A-35 holds the highest selectivity to TAG (25.9%) with the 99.0% conversion of glycerol in line with its largest pore diameter. The reaction was optimized over A-35 catalyst. We tested the influence of temperature first, as shown by plot (a) of Fig. 3. The esterification reaction was carried out in the temperature from 95 to 115 °C while keeping the acid–glycerol molar ratio at 6:1 and the catalyst weight at 0.5 g (Fig. 3). In general, it has been found that the conversion of the glycerol is almost 100% after 4 h. The selectivity of TAG is increasing with decreasing selectivities of MAG and DAG. This also explains that the TAG is formed through consecutive esterification reactions. It should be mentioned that the reaction mixture became suspended after the 2 hour reaction at 115 °C. The above phenomenon shows that the A-35 may break up, for the reaction temperature is close to its limited temperature (120 °C) [28]. Plot (b) of Fig. 3 shows the optimization of molar ratio. The reaction was carried out over A-35 using different molar ratios of acetic acid to glycerol. The conversion of glycerol was found to near 100% after 4 h with molar ratio of both 6/1 and 9/1. The selectivity to TAG increases with increasing in concentration of acetic acid in the reaction mixture. This implies that an increase of the acetic acid or decrease of the glycerol concentration promotes the esterification reaction. Likewise, it can be deduced that the amount of acetic acid is a positive influential parameter in the formation of TAG. On the other hand, this plot demonstrates that the optimal conditions for DAG production are an intermediate acetic acid to glycerol molar ratio (6/1). This shows clearly that continuously increasing the amount of acetic acid supports a negative effect on the selectivity toward DAG, likely due to the transformation into TAG. Finally, the selectivity to TAG is favored using high acetic acid to glycerol molar ratios, which is well in agreement with acetylation of glycerol over acidic mesoporous silica [12]. Fig. 3. shows that high acetic acid to glycerol molar ratios and high temperatures result in higher selectivities to TAG, as could be expected prior considering that high reaction conversions would favor the
Fig. 2. Conversion of glycerol (blue color) and selectivity of TAG (red color) of esterification of glycerol with acetic acid over various catalysts. (T = 105 °C, t = 4 h, acetic acid/glycerol = 0.6 mol:0.1 mol, 0.5 g of catalyst, blank means uncatalyzed reaction). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. Conversion of glycerol for esterification of glycerol with acetic acid, effect of catalyst concentration (T = 105 °C, t = 4 h).
Fig. 3. (a) Selectivity of TAG for esterification of glycerol with acetic acid, effect of reaction temperature. (t = 4 h, acetic acid/glycerol = 0.6 mol:0.1 mol, 0.5 g of A-35). (b) Selectivity of TAG for esterification of glycerol with acetic acid, effect of molar ratio of the reactants. (T = 105 °C, t = 4 h, 0.5 g of A-35).
formation of the di- and tri-substituted derivatives. It has been reported that this enhancement in the acid strength leads to an improvement in the catalytic activity in several acid-catalyzed reactions. However, because A35 has temperature restriction, mild reaction temperature (105 °C or so) and high acetic acid–glycerol molar ratios appear to favor the evolution of the final product distribution in equilibrium to the most substituted glycerol acetylated derivatives (TAG). For the process interest, the influence of amount of catalyst is also investigated. The amount of the catalyst was varied from 0 to 1.0 g while keeping the molar ratio of acid–glycerol of 6:1. The reaction was carried out at 105 °C for 4 h. The conversion of the glycerol is at 73.6% conversion in the absence of a catalyst and always kept at almost 100% conversion when the high or low load catalyst is added (Fig. 4). In the absence of a catalyst the reaction yielded 97.6% of MAG with 2.2% selectivity to TAG. However, with the catalyst load increases from 0 to 0.5 g, the selectivity of MAG decreases with increasing of selectivity to DAG and TAG. This also confirms that esterification of glycerol is a consecutive reaction. However, an insignificant change in selectivity to MAG, DAG and TAG was observed for increasing the catalyst weight from 0.5 to 1.0 g, this means that much of the catalyst that was added has no positive influence on catalytic activity. In all, when the reaction time is up to 4 h, the amount of catalyst does not influence very much the result. From the above discussion, the optimal reaction condition for the first step reaction is as follows: 0.5 g A-35 as catalyst (each 10 g glycerol as reactant), reaction time 4 h, acetic acid/glycerol 9, and reaction temperature 105 °C.
known that acetylation has better activity than esterification and some researchers found that acetylation have great ability over the acid catalyst [6,22]. This raises an idea of rapid increasing selectivity of TAG by acetylation. In order to test this idea, we investigated acetylation by acetic acid and the acetic anhydride by comparing with esterification. Practically, we divided reaction mixture into three parts equally after reaction 4 h with molar ratio of acetic acid/ glycerol = 3:1 (0.3 mol and 0.1 mol respectively) at 105 °C. One part reacts continuously without any change of conditions. The other two parts were added respectively acetic acid (0.2 mol) or acetic anhydride (0.1 mol) in order to reach optimal reaction condition as mentioned above (acetic acid/glycerol = 9/1). As was shown in Fig. 5, the selectivity to TAG sharply increases to almost 100% in 15 min after 0.1 mol acetic anhydride was added. While acetic was added, TAG selectivity was slightly raised. We can also see from Fig. 5, that it is impossible to obtain high selectivity without acetylation. This result is in agreement with acetic anhydride that has better acetylation ability than the acetic acid [6]. Indeed, it is well-established that the acylation reaction proceeds through an acylium intermediate RCO+, generated from the adsorption of the acylating agent [30], i.e., the anhydride onto the Bronsted acidic sites of the A-35 catalyst. This intermediate RCO+ has considerably higher acetylation activity than that of the
3.3. The second reaction step From the first step, we found that the highest selectivity to TAG is 34.8%, and produced MAG and DAG are more than preferred. It is well
Fig. 5. Selectivity to TAG of esterification of glycerol, (1) previous 4 h, T = 105 °C, t = 4 h, acetic acid/glycerol = 0.3 mol:0.1 mol, 0.5 g of catalyst; (2) later 1 h, (a) regular reaction running, (b) adding acetic acid, and (c) adding acetic anhydride.
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and acetic acid to glycerol molar ratios have shown that it is necessary to use high acetic acid excesses in order to push the equilibrium toward the simultaneous enhancement of the conversion of glycerol and the selectivity toward the most valuable triacetylated derivatives. Likewise, to obtain optimal glycerol conversions, it is advantageous to work with moderate temperatures for high temperature that does harm to resin catalyst. Within the studied range, optimal conditions have been found to be a temperature of 105 °C and an acetic acid to glycerol molar ratio of 9:1. Under these reaction conditions, glycerol conversions of almost 100% and combined selectivities toward triacetylglycerol of over 34.8% were achieved after 4 h of reaction over Amberlyst-35. Then, by adding thereto acid anhydride, the selectivity of TAG reaches almost 100% in 15 min. Recycling experiments indicate that no significant deactivation of Amerlyst-35 occurred during the reaction. We expect that this method would be easy for industrial use with its high selectivity, high conversion rate, mild reaction temperature and fast reaction. Fig. 6. Reuse of Amberlyst-35 catalyst in esterification glycerol, the reaction condition of each run was given as follow: (1) previous 4 h, T = 105 °C, t = 4 h, acetic acid/ glycerol = 0.3 mol/0.1 mol, 0.5 g of catalyst; (2) later 15 min, adding acetic anhydride 0.1 mol.
esterification, which then reacts with the MAG or DAG and transfer them to DAG maximally. In sum, the above experimental results have shown that it is necessary to use high acetic acid excess and add acetic anhydride in order to push the reaction equilibrium toward head product (TAG). In fact, esterification of carboxylic acids with alcohols is a typical example of an equilibrium-limited reaction [31]. Traditionally, two main methods have been carried out for equilibrium displacement: either using an excess of reactant or trying to separate the formed water from the reaction medium by means of reactive distillation [27]. In the present work, excess acid and strong acid catalysts are employed in the reaction mixture to accelerate the reaction rate. It should be mentioned that the thereto acetic anhydride consumes the water produced during the esterification reaction, and also pushes the reaction toward to head product (TAG). 3.4. Reusability of catalyst Recycling experiments were investigated over A-35. The results are presented in Fig. 6. After the reaction finished, the catalyst was separated from the reaction mixture by filtration and washed with methanol and then reused after drying at 100 °C overnight. It is seen that the conversion of glycerol has no obvious drop during the five times reactions. A-35 has excellent reusability without significant deactivation under the reaction conditions. We also extend reaction time to as long as 12 h, and also find that A-35 has good stability. Combined with the result we mention above, the A-35 catalyst breaks up when the reaction is performed near its limited temperature (120 °C), we can safely insure that resin A-35 is sensitive to the temperature rather than reaction time. This is in line with resin established characters [25]. 4. Conclusions A two-step method is proposed to obtain high selectivity and high conversion rate for producing additive of biofuel from its byproduct. The esterification and acetylation of glycerol with acetic acid was carried out over resin and zeolites. Amberlyst-35 is found to be an excellent catalyst. The product of esterification of glycerol with acetic acid is acetylated compounds which perform well as bioadditives for petrol fuels. The reaction conditions were optimized. The experiments were carried out for different levels of temperature
Acknowledgements This work was supported by the National Outstanding Young Scientists Foundation of China (Grant No. 20625620) and Synfuels China. Co., Ltd. References [1] C.-H. Zhou, J.N. Beltramini, Y.-X. Fan, G.Q. Lu, Chemoselective catalytic conversion of glycerol as a biorenewable source to valuable commodity chemicals, Chem. Soc. Rev. 37 (2008) 527–549. [2] A. Kawashima, K. Matsubara, K. Honda, Acceleration of catalytic activity of calcium oxide for biodiesel production, Bioresour. Technol. 100 (2009) 696–700. [3] C.-W. Chiu, Catalytic Conversion of Glycerol to Propylene Glycol: Synthesis and Technology Assessment. University of Missouri-Columbia Ph.D Dissertation (2006) Page 1. [4] R.O. Dunn, M.O. Bagby, Low-temperature properties of triglyceride-based diesel fuels: Transesterified methyl esters and petroleum middle distillate/ester blends, J. Am. Oil Chem. Soc. 72 (1995) 895–903. [5] R.L. McCormick, T.L. Alleman, M. Ratcliff, L. Moens, R. Lawrence, Survey of the quality and stability of biodiesel and biodiesel blends in the United States in 2004, Technical Report of NREL/TP 540-38836, Oct. 2005. [6] E. Garcia, M. Laca, E. Perez, A. Garrido, J. Peinado, New class of acetal derived from glycerin as a biodiesel fuel component, Energy Fuel 22 (2008) 4274–4280. [7] H. Noureddini, W.R. Dailey, B.A. Hunt, Chemical and Biomolecular Engineering Research and Publications, 1998. [8] H. Noureddini, U.S. Patent 6015440 (2001). [9] J. Delgado, U.S. Patent 0167681 (2003). [10] B. Delfort, I. Duran, A. Jaecker, T. Lacome, X. Montagne, F. Paille, U.S. Patent 6890364 (2005). [11] B. Delfort, G. Hilion and I. Durand, FR Patent 2866654 (2004). [12] J.A. Melero, R. van Grieken, G. Morales, M. Paniagua, Acidic mesoporous silica for the acetylation of glycerol: synthesis of bioadditives to petrol fuel, Energy Fuels 21 (2007) 1782–1791. [13] Y. Taguchi, A. Oishi, Y. Ikeda, K. Fujita, T. Masuda, JP Patent 298099 (2000). [14] H. Nabeshima, K. Ito, JP Patent 276787 (1995). [15] P. Hofmann, DE Patent 3512497 (1985). [16] S. Nomura, T. Hyoshi, JP Patent 203429 (1995). [17] B. Delfort, G. Hillion, I. Durand, FR Patent 2866654 (2004). [18] J. Delgado, SP Patent 2201894 (2002). [19] European Standard EN 14214, Automotive Fuels. Fatty Acid Methylesters (FAME) for Diesel Engines. Requirements and Test Methods, 2003. [20] American Society for Testing and Materials (ASTM) Standard D6751. Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels, Dec. 2001. [21] V.L.C. Goncalves, B.P. Pinto, J.C. Silva, C.J.A. Mota, Acetylation of glycerol catalyzed by different solid acids, Catal. Today 133 (2008) 673–677. [22] N. Bremus, G. Dieckelmann, L. Jeromin, W. Rupilius and H. Schutt, US Patent 4381407 (1983). [23] T.A. Peters, N.E. Benes, A. Holmen, J.T.F. Keurentjes, Comparison of commercial solid acid catalysts for the esterification of acetic acid with butanol, Appl. Catal. A 297 (2006) 182–188. [24] N. Özbay, N. Oktar, N.A. Tapan, Esterification of free fatty acids in waste cooking oils (WCO): role of ion-exchange resins, Fuel 87 (2008) 1789–1798. [25] http://www.amberlyst.com/sba.htm, Dec., (2008). [26] A. Corma, H. Garcia, S. Iborra, J. Primo, Modified faujasite zeolites as catalysts in organic reactions: esterification of carboxylic acids in the presence of HY zeolites, J. Catal. 120 (1989) 78–87.
X. Liao et al. / Fuel Processing Technology 90 (2009) 988–993 [27] S.R. Kirumakki, N. Nagaraju, S. Narayanan, A comparative esterification of benzyl alcohol with acetic acid over zeolites Hβ, HY and HZSM5, Appl. Catal. A 273 (2004) 1–9. [28] M.D. Girolamo, M. Lami, M. Marchionna, E. Pescarollo, L. Tagliabue, F. Ancillotti, Liquid-phase etherification/dimerization of isobutene over sulfonic acid resins, Ind. Eng. Chem. Res. 36 (1997) 4452–4458. [29] D. Parra, J.F. Izquierdo, F. Cunill, J. Tejero, M. Iborra, C. Fite, Catalytic activity of acidic ion exchange resins in methyl tert-butyl ether liquid phase synthesis,
993
Proceedings of the First European Congress on Chemical Engineering, Florence, Italy, vol. 4, 1997, pp. 3001–3004. [30] U. Freese, F. Heinrich, F. Roessner, Acylation of aromatic compounds on H-beta zeolites, Catal. Today 49 (1999) 237–244. [31] O. Iglesia, R. Mallada, M. Menendez, J. Coronas, Continuous zeolite membrane reactor for esterification of ethanol and acetic acid, Chem. Eng. J. 131 (2007) 35–39.