In situ generated palladium nanoparticles in imidazolium-based ionic liquids: a versatile medium for an efficient and selective partial biodiesel hydrogenation

Abstract

An important drawback to be overcome in biodiesel technology is its low oxidative stability. One approach to improve the oxidative stability of soybean oil biodiesel is the partial hydrogenation of double bonds. In the current work, an efficient two-phase catalytic system using palladium acetate dissolved in BMI·BF₄ ionic liquid to an in situ generation of palladium nanoparticles was developed in order to promote a selective hydrogenation reaction. Upon using this catalytic system it was possible to partially hydrogenate biodiesel into mono-hydrogenated compounds avoiding the formation of saturated compounds. The nanoparticulate system was compared with the traditional heterogeneous Pd/C system and gave far higher selectivity. It was possible to recover and reuse the ionic phase containing the catalyst up to three times without significant loss in its catalytic performance. Indeed, atomic absorption spectroscopy showed an excellent reclaim of the catalyst, which stayed in the ionic phase. Several parameters, such as temperature, hydrogen pressure, metal concentration and reaction time, were also evaluated.

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Introduction

Biodiesel has emerged in the last decades as an elegant alternative solution for increasing energy demand and environmental awareness and it has been considered as a promising and efficient alternative to fossil fuels.¹ For these reasons, several countries, such as United States, Brazil and the European Community, have introduced biodiesel in their fuel market, which sells pure or blended with diesel fuel.² For instance, in Brazil after January 2008, it is mandatory to blend biodiesel in fossil diesel fuel.³ Biodiesel is largely defined as a mixture of fatty acid methyl or ethyl esters, which can be produced after transesterification of triacylglycerides or esterification of fatty acids.³ Nowadays, the most used raw material for biodiesel in USA, Brazil and other countries outside EU is soybean oil, which has a large amount of polyunsaturated fatty acids. It was previously reported that commercial sources of soybean oil in Brazil display on the average 24% of oleic acid, 52% of linoleic acid and 6% of linolenic acid in a typical composition (see Fig. 1 for fatty acid structures).⁴ However, these polyunsaturated fatty acids and their derivatives are extremely susceptible to oxidative degradation due to autoxidative processes involving the allylic and bis-allylic positions in their chain.⁵ This degradation leads to undesirable polymers and oxygenated compounds, diminishing their storage time and their performance as fuel due to deposits and detritions in diesel engines. For this reason, all the specifications for biodiesel include an oxidative stability parameter, typically the Rancimat method.⁶ Thus, soybean oil is less competitive as a raw material for biodiesel when compared to other high oleic oils like rapeseed.⁷

Fig. 1 Structures of the main fatty acids present in soybean oil: oleic, linoleic and linolenic acids.

One approach to improve soybean based biodiesel oxidative stability is the use of antioxidant additives,⁸ which are able to retard the reaction of oxygen triplet with the allylic positions in the fatty acid chains. Regrettably, these additives do not eliminate the double bonds. In some international specifications for biodiesel, such as EN 14214 in the European Community, the iodine index parameter is also specified, which is a direct measurement of olefins.⁹ Therefore, even using antioxidant additives, this regulation eliminates soybean oil as a potential feedstock, as it generally does not pass the test of the iodine index parameter.¹⁰ Besides, it is largely known that the oxidative stability increases with the saturation degree of the fatty acid chains. Therefore, another possible alternative to stabilize biodiesel is the direct hydrogenation of the double bonds.¹⁰

One major drawback to the direct hydrogenation of biodiesel is that upon decreasing the unsaturated chains, a straight increase in the melting point of biodiesel is noted, making less interesting its beneficial properties, thus becoming a hard task to match some specified parameters such as a cold filter plugging point.¹¹ Hence, in order to improve the oxidation stability without compromising biodiesel properties, the partial and selective hydrogenation is a main goal and a real challenge to this emerging technology. Preferentially, the process should result in a higher oleic composition, which displays only one double bond. Besides, it is also well known that during hydrogenation processes, the cis double bonds are partially isomerized into trans isomers.^12–19 This phenomenon is particularly undesirable due to the highest crystallization point of trans isomers when compared with those of a cis conformation, which becomes also a critical issue for the cold filter plugging point parameter especially for monounsaturated fatty acids.¹⁶

In order to overcome major technological problems associated with polyunsaturated biodiesel derivatives, and to avoid selectivity issues in the hydrogenation reaction, we envisage the use of imidazolium-based ionic liquids (ILs, Fig. 2) as a media to promote direct biodiesel hydrogenation. Our main expectation was that this class of reaction medium would be able to allow a higher control in the “oleic-like” ester derivatives formation during the partial hydrogenation process.

Fig. 2 Examples of imidazolium-based ionic liquids. Note that these are derivatives of the 1-butyl-3-methylimidazolium cation.

Our group has a large experience in developing and applying ILs,^20–23 including biodiesel synthesis in these media.^24–27 It is well known that ILs have a beneficial effect on the selectivity of many reactions, and they can also promote reactions that are difficult to occur in classical organic solvents.^28–31 The aim of this work is to study the catalytic activity of palladium acetate dissolved in ILs to an in situ generation of palladium nanoparticles that could be used in the selective hydrogenation of soybean fatty acid methyl esters (FAMEs). The study on the influence of palladium nanoparticles on the selective hydrogenation reaction (and on its parameters, e.g. pressure and temperature) and the catalyst/FAME molar ratio, as well as the recycling reactions is presented herein.

Experimental section

Materials

1-Butyl-3-methylimidazolium tetrafluoroborate (BMI·BF₄)³⁹ and soybean oil fatty acid methyl esters (FAMEs)⁴⁰ were prepared according to procedures previously published in the literature. Hydrogen (analytical grade, 99.99%) and nitrogen (industrial grade) were obtained from White Martins and used without further purification. The catalyst precursor Pd(OAc)₂ (98%) and Pd/C (95%) were obtained from Aldrich and used as received. Standards linolenic (18 ∶ 3), linoleic (18 ∶ 2), oleic (18 ∶ 1 cis), elaidic (18 ∶ 1 trans), stearic (18 ∶ 0) and palmitic (16 ∶ 0) methyl esters were obtained from Aldrich.

Hydrogenation procedure

The hydrogenations were performed in a 100 mL stainless steel autoclave, according to a previous procedure.⁴¹ The reaction system consists of a biphasic catalytic reaction: one composed of Pd(OAc)₂ dissolved in the used IL (bottom phase), and the second of biodiesel (top phase). After hydrogenation reaction, partially hydrogenated soybean biodiesel is also a component of the top phase. Under nitrogen flux, the reactor was charged with the desired amount of catalyst precursor Pd(OAc)₂, previously dissolved in 2 mL of BMI·BF₄. Then, FAME (15 mL) was added and heated at the desired temperature and pressurized with H₂. The zero time of the reaction corresponded to the start of magnetic stirring. After a determined period, the stirring stopped; the reactor was depressurized and the phases separated by simple decantation. For kinetic studies and reuse of the ionic phase, the final steps were carried out under nitrogen flux.

Product analysis

The iodine value (IV) of FAME samples was determined by titration according to the Wijs standard method.⁹¹H NMR and ¹³C NMR data were recorded using a VARIAN spectrometer (Mercury plus model, 7.04 T) operating at 300 and 75 MHz, respectively. FAME sample compositions were determined using a chromatographic method based on a procedure available in the literature.⁴² The analysis was carried out in a CTO-20A (Shimadzu, Tokyo, Japan) high-performance liquid chromatography (HPLC) equipment using an ultraviolet detector (UV) set at 205 nm. Two Shim-Pack VP-ODS C18 reversed-phase columns (250 mm × 4.6 mm, 5 μm) in series kept at 40 °C were used. All samples were dissolved in acetonitrile and injected without previous treatment. The injection volume of 10 μL and the flow-rate of 1 mL min⁻¹ with isocratic elution of acetonitrile for 34 min were used for all experiments, and chromatograms were generated by LabSolutions software (Shimadzu, Japan). Using a UV detector, each FAME has a different response coefficient depending on its unsaturation degree, as previously determined by others.⁴³ Therefore, an external standard method was used for the calibration curve as published elsewhere.⁴⁴Palladium contents in the ionic and organic phases were determined by atomic absorption analysis. All measurements were carried out using an AAS 5EA atomic absorption spectrometer (Analytik Jena AG, Germany), equipped with a transversely heated graphite tube atomizer, and continuum source background correction using a standard procedure for Pd determination.⁴⁵Palladium nanoparticles were characterized using TEM. TEM micrographs were taken on an EM 208S-Philips operating at an acceleration voltage of 80 kV. In order to perform the TEM analysis, a droplet of the black suspensions containing the [Pd(0)]_n nanoparticles embedded in the ILs was dispersed in isopropanol and a small amount of this dispersion was placed on a carbon coated copper grid. The nanoparticles diameter was estimated from ensembles of 100 particles (200 counts) chosen in arbitrary areas of the enlarged micrographs.

Results and discussions

Different catalytic systems were tested for the selective hydrogenation of FAMEs and the main results are summarized in Table 1.

Table 1 Hydrogenation of FAME using different catalytic systems at 50 °C and 10 atm of H₂ using 15 mL of FAME and 0.045 mmol of Pd

Entry	Solvent	Time/h	Catalyst	FAME (%)
				18 ∶ 3	18 ∶ 3i	18 ∶ 2	18 ∶ 2i	18 ∶ 1 cis	18 ∶ 1 trans	18 ∶ 0
0	—	—	—	6	1	47	1	23	0	4
1	BMI·BF4	24	Pd(OAc)2	0	0	7	4	39	27	17
2	None	15	Pd(OAc)2	0	0	0	0	0	13	72
3	None	24	Pd(OAc)2	0	0	0	0	0	6	76
4	None	2	Pd/C	0	0	0	0	0	0	88

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The starting FAME composition is shown in entry 0. At the end of the reaction, the formation of a fine black powder was observed for entries 1 to 3. In the case of entry 1, a two phase system was obtained: an organic upper layer containing FAME and an ionic lower phase composed of a dispersion of the black powder in BMI·BF₄. For homogeneous conditions (entries 2 and 3) a dispersion of the black powder in FAME was observed. Finally, for entry 4 a typical heterogeneous mixture was observed at the end of the reaction. As depicted from Table 1, all studied systems were active in the hydrogenation of the different unsaturated FAMEs. When comparing entries 1–3, it can be noted that a strong selectivity effect takes place in the presence of IL 1c. Indeed, while 18 ∶ 1 cis and 18 ∶ 1 trans are the most important hydrogenation products using 1c as the reaction media (Table 1, entry 1), the complete hydrogenation compound (C18 ∶ 0) is the major product with no ionic liquid (Table 1, entries 2 and 3) or conventional heterogeneous catalysis (Table 1, entry 4). This IL effect will be better discussed in due course. Moreover, although two-phase conditions showed the highest selectivity, it has also the lowest activity, which can be interpreted as a phase mass transfer phenomenon. Note that the highest activity and the lowest selectivity were observed using Pd/C, leading to the total hydrogenation of biodiesel, resulting in a maximum percentage of 18 ∶ 0, in only 2 hours of reaction.

As already described, in entries 1–3 it was possible to observe the formation of a black precipitate finely divided at the end of the hydrogenation of FAME. This precipitate remains dispersed almost exclusively in the IL phase, which was analyzed by transmission electron microscopy (TEM) to testify palladium nanoparticles formation, shape and size distribution, which is stabilized by IL 1c. The shape of the nanoparticles is discussed later.

In order to study the influence of catalyst concentration, temperature and pressure on the hydrogenation of FAME promoted by palladium nanoparticles anchored in BMI·BF₄1c, different experiments were carried out using Pd(OAc)₂ as the catalyst precursor, which are summarized in Table 2. The catalytic activity could be evaluated by means of FAME composition, iodine value (IV) and selectivity (by FAME composition).

Table 2 Hydrogenation of biodiesel using palladium acetate as a catalyst precursor dissolved in 2 mL of BMI·BF₄ in 1 hour under different reaction conditions using 15 mL of FAME

Entry	Pd(OAc)2^a	Temp./°C	Press.^b	FAME (%)^c							IV^d
				18 ∶ 3	18 ∶ 3i^e	18 ∶ 2	18 ∶ 2i^f	18 ∶ 1 cis	18 ∶ 1 trans	18 ∶ 0
a mmol of palladium acetate. b Pressure (atm). c 1 hour. d Iodine value (IV) = (mg of I2)/(g of biodiesel). e Isomerized methyl linoleate compounds mixture. f Isomerized methyl linoleate compounds mixture.
0	—	—	—	6	1	47	1	23	0	4	131
5	0.1114	25	5	6	1	48	1	27	2	4	115
6	0.0446	25	5	5	1	46	3	26	1	4	120
7	0.0446	25	10	3	0	45	2	31	5	4	111
8	0.1114	50	5	1	0	37	5	30	12	4	108
9	0.1114	50	10	0	0	30	0	36	16	4	106
10	0.0446	50	1	6	0	48	0	25	2	4	126
11	0.0446	50	5	2	0	40	4	26	7	4	117
12	0.0446	50	10	1	0	35	5	32	12	4	110
13	0.0446	50	50	1	0	10	1	44	16	6	90
14	0.0446	50	75	0	0	12	2	50	1	7	73
15	0.1114	80	5	1	0	32	7	30	16	5	107
16	0.1114	80	10	0	0	0	1	20	48	10	70
17	0.0446	80	1	5	1	45	4	25	3	4	125
18	0.0446	80	5	0	0	33	2	32	17	4	107
19	0.0446	80	10	0	0	1	6	38	32	5	81
20	0.0446	80	75	0	0	0	3	57	2	8	61
21	0.0046	80	75	0	0	0	0	39	24	11	75

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As already described elsewhere,¹⁹ at all studied temperatures, by increasing the pressure, the catalytic activity is enhanced (compare in Table 2, entries 6 and 7, and entries 10–14, 15–16 and entries 17–20). Indeed, using 0.0446 mmol of Pd(OAc)₂ at 25 °C, increasing the pressure from 5 atm (Table 2, entry 6) to 10 atm (Table 2, entry 7), IV decreases from 120 to 111. This behavior was even more intense at 80 °C (Table 2, entries 17–19), when we observed iodine values of 125, 107 and 81, respectively, using 1, 5 and 10 atm of pressure of H₂. This effect of pressure can be easily understood in terms of the availability of hydrogen, which enhances with temperature, because the viscosity and density of the IL strongly decrease.³² Thus, phase mass transfer issues are much less important.

In order to have a better evaluation of the effect of pressure in the reaction selectivity, the chart is plotted as shown in Fig. 3, with the percentage of 18 ∶ 1 trans formed in entries 10–14 (Table 2).

Fig. 3 Effect of hydrogen pressure on 18 ∶ 1 trans formation at 50 °C (▼) and 80 °C (△).

As depicted in Fig. 3, increasing the hydrogen pressure from 1 to ∼50 atm increases the formation of 18 ∶ 1 trans. At higher pressures, formation of this isomer is diminished. According to a previous study conducted by Coenen,¹² the lifetime of the FAME intermediate semi-hydrogenated depends on the concentration of hydrogen atoms adsorbed. For low-pressures (Table 2, entries 10–12), the amount of hydrogen available on the catalytic surface is not sufficient to promote hydrogenation. Thus, the rotation of the CH₂R fragment around the C–C bond followed by hydrogen atom elimination results in C=C double bond isomerization. Yet, at higher pressures (Table 2, entries 14 and 20), the amount of hydrogen is sufficient and reductive elimination takes place affording the hydrogenated product and the elimination of an isomerized unsaturated product is inhibited. This behavior was equally noted at 50 °C and 80 °C, being more evident at higher temperatures, despite the fact that it is not easy to rationalize.

A plausible explanation is due to the presence of in situ generated stabilized metallic nanoparticles (as described before), where the catalytic active species consumes high amounts of hydrogen and is favoured by both temperature and pressure. Thus, increasing the pressure up to 50 atm, more active species are formed and due to diffusion issues there is not enough hydrogen available, making the isomerization more competitive than hydrogenation. For high pressures, hydrogen diffusion does not compromise its availability in the catalytic sites and the isomerization becomes less important than hydrogenation. Nonetheless, it is important to highlight that even with this increasing isomer formation for pressures down to 50 atm, the system presented here is highly selective when compared with other works reported previously in the literature. For instance, Deliy et al.¹⁴ obtained 40% of 18 ∶ 1 trans at 10 atm and 30 °C and in this current work, at similar conditions, only 5% (Table 2, entry 7) of the trans isomer was observed. Note that even at high temperatures (80 °C and 10 atm—Table 2, entry 19), that usually promote isomerization, only 32% of 18 ∶ 1 trans was obtained.

It was also observed that the amount of 18 ∶ 0 did not increase significantly upon increasing the pressure. Indeed, it seems that the 18 ∶ 3 and 18 ∶ 2 had a greater selectivity by catalytic site than 18 ∶ 1 chains. As already highlighted, the high selectivity in mono-hydrogenated products from dienes using ILs was claimed to be due to the higher solubility of dienes in BMI·BF₄ when compared to mono-enes and paraffins.³⁴ In the last few years some papers have been published relating the hydrogenation of polyunsaturated compounds using metal complexes dissolved in ILs.^33,34,39 It was claimed in these papers that the highest solubility of dienes when compared with mono-enes in imidazolium based ILs provides high selectivity using molecular compounds^34,39 or nanoparticles (such as in our case),³⁵ always leading preferentially to partially hydrogenated products. Besides enhancing the selectivity of the catalytic systems, ILs have been highlighted as environmental friendly media useful for two phase catalysis, providing easy separation of the catalysts from the products and high activity due to charged intermediates stabilization.^36,37 Actually, the IL effect observed for our selective hydrogenation is that mono-enes formed during the reaction are separated (solubility effect) from the reaction medium (ionic phase). Additionally, the IL effect is responsible to (co-)promote the formation and stabilization of ionic intermediates of the hydrogenation reaction through different types of ion pairs formation.

In this work we tried to determine the differences in the solubility of C18 ∶ 0, C18 ∶ 1, C18 ∶ 2 and C18 ∶ 3 in the ionic 1c but the very low solubility and high viscosity and density of the biodiesel mixture led to unprecise results. However, despite the fact that it was not possible to determine with accuracy the comparative solubility of the different C18 FAMEs, by analogy with the different studies available in the literature and discussed before (see ref. 33–35,39) it is more than reasonable to assume that the selectivity in this case can also be attributed to the higher solubility of polyunsaturated FAME in IL 1c in comparison to monounsaturated and saturated FAMEs. It means, the formation of the saturated product was inhibited by diffusion problems of 18 ∶ 1 isomers to the catalytic site. In order to check this hypothesis, a reaction with no IL was performed and no selectivity was observed. The natural course is to form saturated products that in fact occurs very fast. Another important result shown in Table 2 is for the reactions performed at 75 atm of H₂ pressure (entries 14 and 20). We observed a high removal of polyunsaturated FAME without an increasing level of 18 ∶ 1 trans and 18 ∶ 0. Thus, it was possible to maintain its beneficial properties, due to a higher oleic composition, naturally, also increasing its oxidation stability.

ILs display both high organization and the presence of ionic channels (for instance, see ref. 38), which can also be responsible for a faster hydrogenation than isomerization. When the substrate (biodiesel) is inside the three dimensional well-organized supramolecular structure of BMI·BF₄1c, the steric hindrance effect has an enhanced importance, and hydrogen transfer is favored than isomerization (which is avoided), mainly due to the long size chain of the substrate that is stacked inside the used IL (see Scheme 1). Consequently, hydrogen transfer may occur faster than isomerization reaction. Moreover, after mono-ene formation, a solubility effect takes place, favoring its maintenance as a mono-ene compound.

Scheme 1 Ionic liquid effect favoring hydrogenation reaction faster than isomerization reaction.

Note in Scheme 1 that the well-organized 3D structure of BMI·BF₄ probably avoids an easy isomerization due to the steric hindrance effect. Also, note that hydrogen transfer is facilitated inside the 3D structure of IL since it stabilizes the palladium nanoparticles and the charged intermediates from hydrogen transfer.

The temperature effect on biodiesel hydrogenation was equally studied at 25, 50 and 80 °C as previously summarized in Table 2. Using the same amount of the catalyst, it was observed that by increasing the temperature the catalytic activity was increased (compare entries 5, 8 and 15; entries 6, 11 and 18; entries 7, 12 and 19; entries 9 and 16; entries 14 and 21). Results are in perfect agreement with those related in the literature.¹⁹ It was also observed, at least for the first reaction hour, that upon temperature increase, the percentage of 18 ∶ 1 trans raises and has almost no influence on the formation of 18 ∶ 0. The result confirms previous works described in the literature.^12,14,15

In Fig. 4 we see the shape and size distribution of the generated palladium nanoparticles. Note that even at different temperatures both nanoparticles shape and size are statistically the same. Indeed, the diameters were calculated at different temperatures, resulting in 5.1 ± 0.7 nm at 25 °C, 4.7 ± 0.9 nm at 50 °C and 4.2 ± 0.8 nm at 80 °C. It is also important to note that the best nanoparticle distribution was obtained at 50 °C. A long term reaction (96 h) was carried out at 10 atm and 50 °C using 0.0446 mmol of Pd. Fig. 5 shows the composition of samples collected at different times.

Fig. 4 MET analysis of palladium nanoparticles imbibed in BMI·BF₄: (a) 25 °C (5.1 ± 0.7 nm); (b) 50 °C (4.7 ± 0.9 nm); and (c) 80 °C (4.2 ± 0.8 nm).

Fig. 5 Biodiesel hydrogenation products as a function of time. Reaction conditions: P(H₂) = 10 atm, T = 50 °C; 2 mL of BMI·BF₄.

As can be depicted from Fig. 5, the polyunsaturated compounds are quickly hydrogenated into monounsaturated compounds. Indeed, 18 ∶ 3 and 18 ∶ 2 are consumed after 60 and 1055 min, respectively. Note that C18 ∶ 0 starts to be formed only after 340 min of reaction and in 1055 min only 5% of the unsaturated compounds were completely hydrogenated. Thus, using the catalytic system proposed herein, it is possible to convert almost all the polyunsaturated compounds into the so-desired monounsaturated products, avoiding the formation of the paraffinic FAME.

Therefore, it is also possible to depict from Fig. 5 that the hydrogenation of the monounsaturated compounds has a slower velocity when compared with the hydrogenation of polyunsaturated ones. The IV against the time curve shown in Fig. 6, which displays two distinct regions, allows a better visualization of the observed behavior. Indeed, from first to the sixth hour of reaction, a high hydrogenation rate is observed (IV drops by approximately six units per minute, which is almost six times higher to the values observed in an industrial process)¹⁵ and after 10 h a very low hydrogenation rate is observed (IV drops by 10⁻³ units per minute). As already pointed out, this is possibly a result of diffusion issues due to differences in the solubility of the C18 ∶ 3, C18 ∶ 2 and C18 ∶ 1. It is interesting to compare the results from this study with those shown in Table 1 in the absence of any IL. Note that with no IL (Table 1, entry 1), biodiesel was completely hydrogenated in 1440 min. To check this hypothesis, at the end of the reaction, the organic phase was separated and mixed with a new charge of Pd(OAc)₂ (0.446 mmol) dissolved in 2 mL of BMI·BF₄ and the system was once more pressurized with 10 atm of H₂ for additional 24 h at 50 °C. The final product composition was 0% of 18 ∶ 3; 0% of 18 ∶ 2; 29% of 18 ∶ 1 cis; 27% of 18 ∶ 1 trans, and 17% of 18 ∶ 0; and the IV was 64. It means the IV dropped by only 4%, showing that the low reaction rate is not a result of catalyst deactivation but probably a low hydrogenation activity of the system for monounsaturated compounds as explained before.

Fig. 6 Iodine value as a function of time.

As highlighted, one of the advantages in the use of ILs as a reaction medium in a two-phase catalysis is the possibility of recover and reuse of the catalysts. For instance, some of us hydrogenated 1,3-butadiene with the palladium acetylacetonate dissolved in BMI·BF₄1c and could carry out up to 15 recycles without loss of catalytic activity.³⁹ Recycling of the ionic catalytic phase was achieved in the current work by simple phase separation and recovering the black ionic viscous suspension (containing palladium nanoparticles). It was recharged for further runs and the results are shown in Table 3. Note that this study was conducted using the reaction parameters similar to those previously used (Table 2, entry 14: 10 mg of the catalyst precursor, 2 mL of BMI·BF₄, 15 mL of biodiesel, 90 min, 75 atm of H₂ and 50 °C).

Table 3 Hydrogenation of biodiesel (recycling reactions) using the same ionic phase under 50 °C and 75 atm using 15 mL of FAME and 0.0446 mmol of Pd

Entry	Percentage of FAME (%)							IV
	18 ∶ 3	18 ∶ 3i	18 ∶ 2	18 ∶ 2i	18 ∶ 1 cis	18 ∶ 1 trans	18 ∶ 0
Starting biodiesel	7	1	52	1	23	0	3	135
22	0	0	0	3	57	4	8	61
23	0	0	0	2	56	3	9	60
24	0	0	7	4	50	3	7	68
25	1	0	21	0	41	2	7	80
26	0	1	29	2	33	1	5	86
27	3	1	38	1	24	1	4	106
28	6	0	51	1	23	0	4	136

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As can be depicted from Table 3, the catalytic activity and selectivity remained almost the same in the two first charges of biodiesel. However, a continuous drop was observed in the catalytic activity from the third run on, being verified complete deactivation of the catalytic system in the seventh run. This behavior can be better visualized in Fig. 7.

We envisage that part of palladium nanoparticles could be migrating to the FAME phase. Actually, atomic absorption experiments confirmed that 3% of the palladium content can be found in the FAME phase after recycling reaction, which does not explain the loss of activity during recharge reactions. Despite the partial possibility of recycling the system, the obtained results are considerably interesting, mainly because until the moment, to the best of our knowledge, it is the most selective and efficient system to perform selective hydrogenation of biodiesel towards oleic composition of the final product. However, TEM analysis (data not shown) performed after catalysis demonstrated that the palladium nanoparticles are not agglomerating during recharge reactions. To obtain a plausible explanation to the observed behavior (loss of activity), BET analysis was performed. Interestingly, we could observe after the first run a superficial area of 171.685 m² g⁻¹, a porous volume of 0.3854 cc g⁻¹ and a porous size smaller than 735.8 Å. After the sixth run, the same analysis was performed and we could observe a superficial area of 199.615 m² g⁻¹, a porous volume of 0.4726 cc g⁻¹ and a porous size smaller than 412.8 Å. If the porous size is decreasing during the reaction, it is now comprehensive the reason for the loss of activity. The substrate cannot access the catalyst and the reaction does not take place, especially because the tested FAMEs are much higher in van der Waals volume than 1,3-butadiene.

Fig. 7 Biodiesel hydrogenation yields during recycling reactions.

Conclusions

In summary, we have developed a new, efficient, clean, selective and eco-friendly system to promote the partial hydrogenation of FAMEs derived of soybean. The system could be reused at least three times with no loss in its activity. The in situ formation and stabilization of palladium nanoparticles in IL media is an efficient and simple method that can overcome many experimental drawbacks such as manipulation. Additionally, the system promotes hydrogenation reaction faster than isomerization reaction, mainly due to a very pronounced IL effect. The IL effect functions as (i) a media for an additional stabilization of charged intermediates, (ii) a media to avoid isomerization due to a steric effect, and (iii) a media to allow a higher solubility of dienes (or poly-enes) instead of mono-enes, that results in a high and important selection, mostly based on solubility parameters. The decrease of the porous size explains the loss of activity of the catalyst and allows a better understanding of palladium nanoparticle catalysis in ILs.

Acknowledgements

We would like to thank CAPES, FAPDF, CNPq, and Petrobras for partial financial support. PAZ Suarez, BAD Neto and MS Carvalho are indebted with CNPq for their research fellowship. The authors are deeply indebted to Professor Jaïrton Dupont (Laboratory of Molecular Catalysis, UFRGS) for atomic absorption analysis, TEM analysis, and for insightful discussions and suggestions.

References

1. F. R. Abreu, D. G. Lima, E. H. Hamu, S. Einloft, J. C. Rubim and P. A. Z. Suarez, J. Am. Oil Chem. Soc., 2003, 80, 601.
2. Y. Zang, M. A. Dubé, D. D. McLean and M. Kates, Bioresour. Technol., 2003, 89, 1.
3. G. P. A. G. Pousa, A. L. F. Santos and P. A. Z. Suarez, Energy Policy, 2007, 35, 5393.
4. F. R. Abreu, D. G. Lima, E. H. Hamu, C. R. Wolf and P. A. Z. Suarez, J. Mol. Catal. A: Chem., 2004, 209, 29.
5. E. N. Frankel, Lipid Oxidation, The Oily Press, Bridgwater (UK), 2003.
6. AOCS: Oil stability index (OSI). American Oil Chemists Society Official Method Cd 12b-92, AOCS, Champaign, IL (USA), 1999.
7. N. A. Porter, S. E. Cadwell and K. A. Mills, Lipids, 1995, 30, 277.
8. A. K. Domingos, W. Vecchiatto, H. M. Wilhelm and L. P. Ramos, J. Braz. Chem. Soc., 2007, 18, 416.
9. AOCS: Iodine value of fats and oils: Wijs method. American Oil Chemists Society Official Method Cd 1-25, AOCS, Champaign, IL (USA), 1999.
10. B. R. Moser, M. J. Haas, J. K. Winkler, M. A. Jackson, S. Z. Erhan and G. R. List, Eur. J. Lipid Sci. Technol., 2007, 109, 17.
11. G. Knothe, J. Krahl and J. Van Gerpen, The Biodiesel Handbook, AOCS Press, Champaign, IL (USA), 2005.
12. J. W. E. Coenen, J. Am. Oil Chem. Soc., 1976, 53, 382.
13. P. Melanie, B. Khaled and A. Joseph, Ind. Eng. Chem. Res., 2004, 43, 2382.
14. I. V. Deliy, N. V. Maksimchuk, R. Psaro, V. dal Santo, S. Recchia, E. A. Paukshtis, A. V. Golovin and V. A. Semikolenov, Appl. Catal., A, 2005, 279, 99.
15. N. Hsu, L. L. Diosaday, W. F. Graydon and L. J. Rubin, J. Am. Oil Chem. Soc., 1986, 63, 1036.
16. K. Ihsan, K. Muammer and Y. Semra, Food Chem., 2003, 81, 453.
17. G. R. List, W. E. Neff, R. L. Holliday, J. W. King and R. Holser, J. Am. Oil Chem. Soc., 2000, 77, 311.
18. B. F. Marıa, M. T. Gabriela, H. C. Guillermo and E. D. Daniel, J. Food Eng., 2007, 82, 199.
19. R. C. Hastert, J. Am. Oil Chem. Soc., 1979, 56, 732A.
20. P. A. Z. Suarez, J. E. L. Dullius, S. Einloft, R. F. de Souza and J. Dupont, Polyhedron, 1996, 15, 1217.
21. J. Dupont, P. A. Z. Suarez, A. P. Umpierre and R. F. de Souza, Catal. Lett., 2001, 73, 211.
22. R. A. Pilli, L. G. Robello, N. S. Camilo, J. Dupont, A. A. M. Lapis and B. A. D. Neto, Tetrahedron Lett., 2006, 47, 1669.
23. A. C. Pinto, A. A. M Lapis, B. V. da Silva, L. A. Basso, J. Dupont and B. A. D. Neto, Tetrahedron Lett., 2009, 49, 5639.
24. B. A. D. Neto, M. B. Alves, A. A. M. Lapis, F. M. Nachtigall, M. N. Eberlin, J. Dupont and P. A. Z. Suarez, J. Catal., 2007, 249, 154.
25. A. A. M. Lapis, L. F. de Oliveira, B. A. D. Neto and J. Dupont, ChemSusChem, 2008, 1, 759.
26. F. R. Abreu, M. B. Alves, C. C. S. Macêdo, L. F. Zara and P. A. Z. Suarez, J. Mol. Catal. A: Chem., 2005, 227, 263.
27. J. Dupont, P. A. Z. Suarez, M. R. Meneghetti and S. M. P. Meneghetti, Energy Environ. Sci., 2009, 2, 1258.
28. C. de Bellefon, E. Pollet and P. Grenouillet, J. Mol. Catal. A: Chem., 1999, 145, 121.
29. J. F. Dubreuil and J. P. Bazureau, Tetrahedron Lett., 2000, 41, 7351.
30. Y. Chauvin, L. Mussmann and H. Olivier, Angew. Chem., Int. Ed. Engl., 1996, 34, 2698.
31. J. E. L. Dullius, P. A. Z. Suarez, S. Einloft, R. F. de Souza and J. Dupont, Organometallics, 1998, 17, 815.
32. P. A. Z. Suarez, J. E. L. Dullius, S. Einloft, R. F. de Souza and J. Dupont, J. Chim. Phys. Phys.-Chim. Biol., 1998, 95, 1626.
33. P. A. Z. Suarez, J. E. L. Dullius, S. Einloft, R. F. de Souza and J. Dupont, Inorg. Chim. Acta, 1997, 255, 207.
34. C. S. Consorti, A. P. Umpierre, R. F. de Souza, J. Dupont and P. A. Z. Suarez, J. Braz. Chem. Soc., 2003, 14, 401.
35. A. P. Umpierre, G. Machado, G. H. Fescher, J. Morais and J. Dupont, Adv. Synth. Catal., 2005, 347, 1404.
36. J. Dupont and P. A. Z. Suarez, Phys. Chem. Chem. Phys., 2006, 8, 2441.
37. R. A. Pilli, L. G. Robello, N. S. Camilo, J. Dupont, A. A. M Lapis and B. A. D. Neto, Tetrahedron Lett., 2006, 47, 1669.
38. U. Schroder, J. D. Wadhawan, R. G. Compton, F. Marken, P. A. Z. Suarez, C. S. Consorti, R. F. de Souza and J. Dupont, New J. Chem., 2000, 24, 1009.
39. J. Dupont, P. A. Z. Suarez, A. P. Umpierre and R. F. de Souza, J. Braz. Chem. Soc., 2000, 11, 293.
40. C. C. Cassol, G. Ebeling, B. Ferrera and J. Dupont, Adv. Synth. Catal., 2006, 348, 243.
41. B. Freedman, E. H. Pryde and T. L. Mounts, J. Am. Oil Chem. Soc., 1984, 61, 1638.
42. C. R. Scholfield, J. Am. Oil Chem. Soc., 1975, 52, 36.
43. L. M. L. Nollet, Food Analysis by HPLC, 2000, 2nd edn, p. 175.
44. M. S. Carvalho, M. A. Mendonça, D. M. M. Pinho, I. S. Resck and P. A. Z. Suarez, submitted for publication.
45. M. G. R. Vale, M. M. Silva, B. Welz and É. C. Lima, Spectrochim. Acta, Part B, 2001, 56, 1859.

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