Simultaneous conversion of triacylglycerides and fatty acids into fatty acid methyl esters using organometallic tin(IV) compounds as catalysts

Applied Catalysis A: General 443–444 (2012) 202–206

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Simultaneous conversion of triacylglycerides and fatty acids into fatty acid methyl esters using organometallic tin(IV) compounds as catalysts Yariadner C. Brito, Daví A.C. Ferreira, Danielle M. de A. Fragoso, Paula R. Mendes, César M.J. de Oliveira, Mario R. Meneghetti, Simoni M.P. Meneghetti ∗ Instituto de Química e Biotecnologia, Universidade Federal de Alagoas, Av. Lourival de Melo Mota, s/no, Maceió-AL-57072-970, Brazil

a r t i c l e

i n f o

Article history: Received 12 May 2012 Received in revised form 29 July 2012 Accepted 30 July 2012 Available online 8 August 2012 Keywords: Transesterification Esterification Simultaneous transesterification and esterification Biodiesel Tin catalysts Chemical computational studies

a b s t r a c t Three Sn(IV) complexes, named butyl stannoic acid (BTA), di-n-butyl-oxo-stannane (DBTO) and dibutyltin dilaurate (DBTDL), were initially tested as catalysts for esterification reaction of fatty acids in the presence of methanol as alcoholysis agent. Parameters like reaction time, temperature, and catalyst amount were systematically evaluated in this work. All complexes were active at relative high reaction temperatures, but BTA displayed the highest activity. Former studies have already demonstrated that these same complexes display good catalytic activity in methanolysis of triacylglycerides (TAGs). These results prompted us to test BTA catalyst also in simultaneous esterification/transesterification reactions from a mixture of free fatty acids (FFAs) and TAGs in the presence of methanol. BTA was able to convert the mixtures in to fatty acid methyl esters (FAMEs) with good yields, with simple isolation processes. The results obtained and discussed in this work can help the development of new catalytic systems to biodiesel production from oils with very high acid content. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The two main liquid biofuels used in Brazil are ethanol, produced from sugarcane and, in increasing scale, biodiesel, mainly produced from transesterification of TGAs of soybean oil in the presence of methanol. Indeed, Brazil is among the largest producers and consumers of biodiesel in the world, with an annual production 2.5 billion liters and production capability of 5.8 billion liters, and about 18% of the whole fuels employed are derived from renewable sources [1]. It is important to mention that nowadays, all diesel commercialized in Brazil contains 5 vol% of biodiesel (B5). Indeed, all biodiesel produced in Brazil is sold as B5 for transport fuel [1]. The catalytic transesterification of TGAs in the presence of a short chain alcohol, such as methanol, leads to biodiesel (a mixture of alkyl monoesters) and glycerol as the final products [2]. Several vegetable oils can be employed as source to obtain biodiesel. One of the main factors affecting their cost of production is the high price of this raw material [3]. Therefore, the use of low-cost feedstocks such as non-edible oil or waste cooking oil should improve the economic feasibility of biodiesel production [4]. However, these low-cost sources have high free fatty acid (FFA) content and in this case, at least, an esterification step must be employed.

∗ Corresponding author. Tel.: +55 82 3214 1703; fax: +55 82 3214 1384. E-mail address: [email protected] (S.M.P. Meneghetti). 0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2012.07.040

The catalysts used in transesterification reactions can be of different natures, like enzymes, or Brønsted acids or bases in homogeneous or heterogeneous form [2,5]. The homogeneous Brønsted base catalysts (hydroxides or alkoxides) are mostly used for their greater efficiency, but their use leads to the formation of soap, if the oil or fat is high in FFA. One alternative could be the utilization of Brønsted acid catalysts, but these systems exhibit lower conversions when compared to former ones at the same reaction conditions. Futhermore, Brønsted acid catalysts are normally associated to reactor corrosion problems. Recent studies have shown that species based on complexes exhibiting Lewis acid sites present promising results in both esterification, transesterification and simultaneous [6–8]. There are some examples of the use of tin(IV)-based homogeneous or heterogeneous catalytic systems in esterification, transesterification and polycondensation reactions, with the aim of producing polymeric and intermediate materials [9–11]. Recently, we have reported the potential use of these types of catalytic systems for biodiesel production [12–15]. However, as far as we are aware, no information is available about the performance of this class of catalysts face to esterification or simultaneous esterification and transesterification reactions. In this context, we carried out a systematic study on esterification and simultaneous esterification/transesterification reactions, and part of the catalytic activity of the complexes studied was discussed at light of chemical computational studies.

Y.C. Brito et al. / Applied Catalysis A: General 443–444 (2012) 202–206

2. Experimental 2.1. Materials All chemicals were used as received without further purification. Butyl stannoic acid (BTA), di-n-butyl-oxo-stannane (DBTO) and dibutyl tin dilaurate (DBTDL) were purchased from Aldrich. Methanol was commercially acquired from Merck (analytical grade) and stored over MgSO4 as desiccant. Soybean oil (commercial grade) was supplied by Bunge Alimentos (Brazil) and was used as received. The free fatty acids found in the soybean oil (characterized as oleic acid content, in percentage, according to the AOCS official method Ca 5a-40) summed up to a total amount equivalent to 0.1%. Fatty acids mixture was obtained through saponification of soybean oil followed by acidification with hydrochloric acid. The mixture was washed with water and dried with magnesium sulfate. 2.2. Transesterification and esterification Transesterification and esterification reactions were performed in a 100-mL batch stainless steel reactor coupled to a manometer, temperature probe and a magnetic stirrer working at 1000 rpm. All reactions were carried out using alcohol:oil (or fat acid):catalyst molar ratio of 400:100:x (x = 1, 2 or 3), at different temperatures (120, 140 or 160 ◦ C) during 1, 2, 3 or 4 h. After appropriate reaction time, the product obtained by transesterification was washed three times with distilled water, dried in the presence of MgSO4 , and centrifuged. The yield of the transesterification reaction was determined by GC and expressed in terms of the percentage of fatty acid methyl esters (% FAMEs) produced [13]. In the case of the product obtained from esterification, the volatiles were removed in a rotary evaporator, and the reaction yield was calculated based on the diminishing of acid index for products in relation to acid index for the initial fatty acids mixture, according to AOCS Cd3d63 standard method. For the simultaneous esterification/transesterification reactions, three mixtures of fatty acids with triacylglycerides (FFA:TAG of 50:50, 30:70 and 70:30 wt%, respectively) were prepared and underwent methanolysis in the presence of BTA as catalyst, at 160 ◦ C, and analyzed after 1, 2, 3 and 4 h of reaction. For all mixtures of FFA:TAG reactions were carried out using alcohol:FFA + (1/3TAG):catalyst molar ratio of approximately 400:100:1. The product in all experiments was treated following the same procedure employed in the transesterification. HPLC was used to analyze the product obtained from the mixtures, determining the content of TAG, FFA, DAG (diacylglyceride), MAG (monoacylglyceride), and FAMEs present in the final product. 2.3. Chemical computational studies All calculations reported in the present study were performed using the Firefly QC package, which is partially based on the GAMESS (US) [16] source code, and molecular structures were drawn using the ORTEP 3 program [17]. For understanding catalytical behavior these organotin systems, we accomplish a theoretical study starting by modeling the electronic structure of catalysts. All these structures were fully optimized in solvent medium at the gradient-corrected Density Functional Theory [18] level using the three-parameter fit of the exchange-correlation potential suggested by Becke, B3, in conjunction with the correlation functional suggested by Lee, Yang and Parr, LYP [19], with base set 321++G(d,p) [20–23]. Local minima were identified by the absence of negative eigenvalues (NIMAG = 0) in the Hessian matrix following vibrational frequency analysis. Here we modeled the catalyst in an environment saturated with CH3 OH, treated with the PCM

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continuum model at 298.15 K and 1.0 atmosphere. As expected we have confirmed that the HOMO–LUMO gap is not affected by change temperature between 298.15 K and 498.15 K, justifying the temperature applied in this study. 3. Results and discussion In previous studies, our research group investigated the catalytic potential of tin(IV) compounds on methanolysis of soybean oil and the results pointed out the reaction yields can be significantly increased with the reaction temperature or when an appropriate phase equilibrium is established inside the closed reactor [12–15]. It is important to highlight that these species, exhibiting Lewis acidity, are receiving more attention as potential catalyst, principally because with their employ no soap formation is observed and the difficulties concerning phase separation, after reaction, are minimized. In addition, they are efficient in the presence methanol, ethanol and other alcohols, even for those with long alkyl chains. Furthermore, they are interesting species to develop new heterogeneous and reusable catalytic systems [2,24–27]. This study prompted us to expand this work to evaluate the catalytic properties exhibited by Sn(IV) complexes on esterification and simultaneous transesterification/esterification reactions. In this context, we first carried out esterification reactions of soybean FFAs with methanol, in the presence of the three Sn(IV) complexes (see Fig. 1). Different from transesterification, the esterification can be self-catalyzed due to the presence of Brønsted acid (the fatty acid itself) in the reaction media. For this reason, the catalytic activity of each metal complex must always be compared with a reaction conducted without the presence of the metal compound. 3.1. Reaction temperature The esterification reactions were tested at different temperatures using the molar ratio 400:100:1 (alcohol:FFA:catalyst), at 1 h of reaction (Fig. 2). All complexes were active since in their presence better conversions were observed when compared with the self-catalyzed reaction. As demonstrated in former transesterification reaction studies [13], those catalysts present better performances at higher reaction temperatures, because at these conditions the catalysts can be effectively activated [28,29], and their solubility in the reaction medium can be improved [13]. Complexes like BTA and DBTO are insoluble in a series of solvents, and organized as stable oligomeric structures at room temperature [30]. However, on increasing the temperature these particular arrangements can be destabilized, resulting in more active molecular species. Besides that, the order of activity observed can not only be related just to the different solubility of the Sn(IV) species on reaction medium, since the most soluble one, DBTDL, is not the most active. The observed differences (for example, yields of 89% for BTA, 60% for DBTO and 57% for DBTDL, at 160 ◦ C and 1 h) can be related to different Lewis acidity of the organometallic species, acting directly in the formation of the Lewis acid–base complex intermediate that is formed by the interaction of the substrate (FFA, TAG, DAG or MAG) with the metal center, via the oxygen of the carbonyl group, see Fig. 3. This interaction/coordination activates the substrate to undergo subsequent nucleophilic attack by the alcohol [31]. 3.2. Reaction time The influence of reaction time on the conversion was evaluated at 160 ◦ C, during 1, 2, 3 and 4 h. It was observed that there is a strong dependence of the conversion with reaction time when the catalyst systems DBTO and DBTDL were used (Fig. 4), but BTA is

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O O Sn

O Sn

O Sn

OH

O O

DBTO

BTA

DBTDL Fig. 1. Molecular structure of catalytic complexes.

100 DBTDL

90

DBTO 80

BTA Without catalyst

Yield of FAMEs (%)

70 60 50 40 30 20 10 0 120

140

160

Temperature (ºC) Fig. 2. Production of FAMEs (percentage yield) by the esterification in the presence of BTA, DBTO, DBTDL and without catalyst at different temperatures. The reaction conditions were: molar proportions of fatty MeOH:FFA:catalyst = 400:100:1; reaction time = 1 h.

more active and in the first 1 h of reaction yields around 90% were achieved. However, after 2 h BTA and DBTO display practically the same yields in terms of FAMEs, and after 3 h all catalysts show the virtually the same tendency, indicating that the system reach the reaction equilibrium. 3.3. Molar ratio Finally, the influence of the molar ratio of the different catalysts was evaluated (see Fig. 5). As expected, in all cases the reaction yields were increased by increasing catalyst amount and, clearly, these results can be understood since there are more catalytic sites available. According to the results here presented, and others described elsewhere [12–15], it is possible to verify that these catalytic systems are active both in esterification as in transesterification reactions. In order to investigate the possibility of run simultaneously esterification and transesterification reactions, three

mixtures of FFA:TAG (50:50, 30:70 and 70:30 wt%) underwent methanolysis in the presence BTA as catalyst, at 160 ◦ C, during 1, 2, 3 and 4 h (Table 1). The results presented in Table 1 show that the BTA also exhibits catalytic activity in the methanolysis of the FFA:TGA mixtures, reaching conversions up to 90% at 1 h. These observations demonstrate that this type of catalyst is quite active for both esterification and transesterification reactions, as well under simultaneous reaction conditions, opening important possibilities of use in reactions involving raw materials at low cost, and high FFA content. In more details, for FFA:TAG mixtures of 30:70 and 70:30 the content of TAG, FA, DAG, MAG and FAMEs present in the reaction medium were determined at 0, 1, 2, 3 and 4 h (using BTA at 160 ◦ C) and depicted in the Figs. 6 and 7. It is clear that the BTA is very efficient at 1 h reaction, and just small amounts of TAG and FFA are detected in both mixtures studied. However, in the case of mixture FFA:TAG (70:30), which high FFA concentration are present, better yields are obtained because

Fig. 3. Schematic illustration of oxygen species activation via acid–base Lewis type interactions. M: metal center, Sn(IV); R : long hydrocarbonic chain; RO: HO or a glycerol moiety.

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Table 1 Yields in terms of FAMEs after methanolysis of soybean FFAs, TAG and their mixtures, using BTA as catalyst, and temperature of 160 ◦ C. Reaction time (h)

1 2 3 4

Transesterification

Mixtures (FFA:TGA, wt%)

0:100

30:70

50:50

70:30

Esterification 100:0

52 59 67 –

61 69 90 93

71 85 91 –

91 92 93 95

89 91 93 –

100 100 90 80

80 FAMEs

Yield (%)

Yield of FAMEs (%)

70 60 50 40

BTA

30

DBTO

20

Without catalyst

60

FFA TAG DAG

40

MAG

20

DBTDL 0 0

1

10

2

3

4

Reaction time (h)

0 1

2

3

4

Fig. 6. Yield (%) of the TAG, FFA, DAG, MAG and FAMEs for the FFA:TAG (30:70) mixture (BTA at 160 ◦ C).

Reaction time (h) Fig. 4. Production of FAMEs (percentage yield) by the esterification in the presence of BTA, DBTO, DBTDL and without catalyst at different reaction times. The reaction conditions were: molar proportions of fatty acid:MeOH:catalyst = 100:400:1; temperature = 160 ◦ C.

100

Yield (%)

80 FAMEs FFA TAG DAG MAG

60

40

20

0 0

1

2

3

4

Reaction time (h) Fig. 5. Production of FAMEs (percentage yield) by the esterification in the presence of different amount of BTA, DBTO, DBTDL and without catalyst. The reaction conditions were: molar proportions of fatty acid:MeOH:catalyst = 100:400:x (x = 1, 2 or 3); temperature = 160 ◦ C; reaction time = 1 h.

under these experimental conditions, the esterification is kinetically favored. 3.4. Chemical computational studies During the kinetic control of the esterification reaction, i.e. in the first 2 h, the reactivity order observed is BTA  DBTO > DBTDL (see Fig. 4). After this time, the equilibrium conditions are practically reached. We believe that one of the reasons of the high activity of BTA is related to the electronic feature of the Lewis acid–base interaction. Considering orbital interactions, the formation of the Lewis acid–base complex is generally view as a result of an overlap between the lower unoccupied molecular orbital (LUMO) of the Lewis acid (metal center), and the higher occupied molecular

Fig. 7. Yield (%) of the TAG, FFA, DAG, MAG and FAMEs for the FFA:TAG (70:30) mixture (BTA at 160 ◦ C).

orbital (HOMO) of the Lewis base species (in this particular case, the oxygen of the carbonyl groups). In energetic terms, these interactions are more effective if the energies of the HOMO and LUMO of respective species are quite close, as illustrated on Fig. 8. In order to confirm this hypothesis, chemical computational studies were carried out (Table 2). These organotin compounds normally show Lewis acid character since they present empty Table 2 Frontiers molecular orbitals of catalysts and hexanoic acid. Catalysts

LUMO (eV)

E of HOMOcarbonyl –LUMOcomplex (eV)a

BTA DBTO DBTDL

+0.580 +0.824 +1.363

12.978 13.222 13.761

a EHOMO–LUMO is a modular measure of gap HOMO–LUMO involving HOMO of hexanoic acid (fatty acid model; value is −12.398 eV) and LUMO of catalyst.

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Energy B)

A)

LUMO (Lewis Acid)

LUMO (Lewis Acid)

HOMO (Lewis Base) HOMO (Lewis Base)

HOMO: Higher Occupaid Molecular Orbital of the ester or acid (Lewis Base) LUMO: Lower Molecular Orbital of the metal center (Lewis acid) Fig. 8. Two different relative levels of Lewis acid–base interactions. Interaction of type A is more effective due to the lower energy difference of orbitals involved.

orbitals able to interact with electron donor substrates. The Lewis acid–base interaction will be more favored if the energy gap between HOMOcarbonyl –LUMOcomplex are quite close, as already mentioned. Using the HOMO energy value of −12.398 eV for hexanoic acid, as a fatty acid model, it is possible to verify that the lower energy gap between the respective orbitals occurs with BTA, in comparison with DBTO and DBTDL. It thus means that the strongest Lewis Acid-Base interaction occurs with BTA and the carbonyl group of the fatty acid, activating this group to undergo the nucleophilic attack of the alcohol, and consequently improving kinetic factors of the catalyzed esterification reaction. Of course other features can play a role, like steric effects, in this activation, but the electronic effect seems to be the most important feature in these examples here studied. It is also worth of noting that almost the same energy gap between HOMOcarbonyl and LUMOcomplex is observed for DBTO and DBTDL and this fact can be related with their catalytic activity (see Fig. 4 and Table 2). 4. Conclusions The results obtained and discussed are quite important to the development of catalytic systems for biodiesel production that can be very suitable for esterification, transesterification, and simultaneous esterification/transesterification reactions. This observation is very important since the catalytic systems here evaluated, mainly BTA, are surprisingly flexible in terms of the quality of the fatty acid oil source. Sources, like FFAs with high content of TAG, for example, can be employed, using this family of catalyst; as well a source based on vegetable oils (TAGs) with high content of FFA, for example some palm oils, recycled oils, and animal fats [4] can be easily also used. Differently to the catalysts commonly used at the industry in conventional transesterification process (alkoxide and hydroxide), that cannot be applicable in esterification or with raw materials containing high free fatty acid due to saponification reaction, the tin(IV) catalysts tested in this work can be used as an excellent alternative to the development of new biodiesel production processes. Acknowledgements The authors wish to thank FINEP, CAPES and CNPq for financial support. YCB, DACF, DMAF, PRM and CMJO express their appreciation for fellowships granted by CAPES and CNPq. SMPM and MRM thank CNPq for research fellowships.

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