Selective Partial Hydrogenation of Methyl ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Selective Partial Hydrogenation of Methyl Linoleate Using Highly Active Palladium Nanoparticles in Polyethylene Glycol Wei Liu,*,† Lige Xu,† Guanghui Lu,† and Hua Zhang*,‡ †

College of Food Science and Technology, Henan University of Technology, Lianhua Street, Zhengzhou 450001, P. R. China College of Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, P. R. China

‡

S Supporting Information *

ABSTRACT: Partial hydrogenation of carbon−carbon double bonds is among one of the approaches to improve the low oxidative stability of vegetable biodiesel, which is an important drawback in biodiesel technology. In the current work, we developed an efficient two-phase catalytic system utilizing Pd(OAc)2 dissolved in polyethylene glycol (PEG) that in situ generates palladium nanoparticles in order to promote a selective partial hydrogenation reaction of methyl linoleate into mono-hydrogenated compounds while avoiding the generation of saturated compounds. High yield of methyl oleate was also obtained by hydrogenation of sunflower oil biodiesel using the same catalytic system. Through evaluating the palladium nanoparticles by TEM analysis, it is observed that 4 nm palladium nanoparticles generated in situ in PEG4000 showed high selectivity both for the partial hydrogenation of methyl linoleate and sunflower oil biodiesel. KEYWORDS: Biodiesel, Fatty acid methyl ester, Hydrogenation, Palladium nanoparticles, Polyethylene glycol, Methyl linoleate

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INTRODUCTION Biodiesel or fatty acid methyl esters (FAME), mainly derived from polyunsaturated oils (e.g., soybean oil, sunflower oil), are known as an alternative for petroleum-based diesel. However, the high unsaturation degree of polyunsaturated fatty acids in vegetable oils, mainly linoleic acid (>50%), Scheme 1, leads to

properties. And another concern on partial hydrogenation of FAME in biodiesel is the conversion of cis- into the more thermodynamically favored trans-double bonds. In the case of biodiesel, the side isomerization of oleic into elaidic acid is particularly undesirable as compared with those of cis,1 the trans isomers give higher crystallization point and compromises the cold flow properties of the fuel. Ni catalysts are usually used in commercial hydrogenation of oils. Although they are cheaper than noble metal (e.g., Pd, Pt, Ir) catalysts, more severe hydrogenation pressure and harsh reaction temperature are required for the former compared with the latter ones.2 In this respect, noble metal catalysts such as Pd seem to be the most promising one.3 Supported Pd catalysts have been widely employed as catalysts for hydrogenation reaction.4 Considering the catalytic activity and complex preparation conditions of supported Pd catalysts (Table S1), Pd nanocatalysis has received great attention due to its wide applicability, such as alkene hydrogenation,5 C−C coupling reactions 6,7 and reduction of aromatic nitro compounds.8 We are particularly interested in the selective hydrogenation of polyolefins, as the oxidative stability depends on the unsaturation degree of the vegetable oil fatty acid methyl esters (FAMEs). In fact, avoiding complete hydrogenation and cis/trans isomerization is a major challenge in hydrogenation of FAMEs or polyunsaturated oils. It was reported that 4−6 nm

Scheme 1. Main Fatty Acids Present as Glycerol Esters in Polyunsaturated Oils

several problems concerning oxidative and thermal stability.1 Lower oxidative stability was obtained when biodiesel was produced from vegetable oil as it contains higher unsaturated fatty acid composition. On the other hand, higher saturated fatty acid composition led to worse cold flow properties. Complete hydrogenation of polyunsaturated FAME to saturated ones strongly affects the cold flow properties of biodiesel, for example, melting point of methyl stearate is 39− 42 °C). Therefore, partial hydrogenation of polyunsaturated FAMEs to monounsaturated ones is a promising solution to improve oxidative stability, with minimal effect to cold flow © XXXX American Chemical Society

Received: August 1, 2016 Revised: December 20, 2016

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DOI: 10.1021/acssuschemeng.6b01823 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Hydrogenation of Methyl Linoleate. In a typical experiment, methyl linoleate (4.0 mmol) and Pd-PEGn (2.0 g) were added to the reactor. Then H2 was introduced into the reactor to a desired pressure. The resulting mixture was stirred at 75 °C. After a certain period of time, a certain amount of the sample was taken out and diluted with nhexane, then the sample was analyzed by an Agilent 6890N gas chromatography (GC) instrument with a BPX-70 capillary column. Recycling of Pd-PEG Catalyst. In the catalyst recycling experiments, the Pd-PEGn catalyst system was easily separable from the reaction mixture by extraction of the products with diethyl ether. The PEG layer containing the Pd nanoparticle catalyst was recovered followed by warming up in a rotavapor to remove any solvents. The catalyst thus was recovered and used for the next reaction cycle. GC Analysis of FAMEs. The composition of the FAMEs and hydrogenated products were detected by GC-FID, which is the same as a previous report.25 The FAMEs were identified with known compounds. And the iodine value (IV) could be calculated from the composition of FAMEs based on the GC data.25 Characterization of Pd-PEGn Catalyst. The Pd-PEGn catalyst was dissolved in CH2Cl2. UV−vis spectroscopy (TU1810, Beijing PERSEE) was performed for Pd(II) before and after the reduction with PEG. Meanwhile, the solutions were dropped on a carbon film coated copper grid and left in air for drying, then a TEM (JEM-2100, JEOL) operating at 200 kV was used to analyze the morphological and structure of Pd-PEGn catalyst.

palladium nanoparticles generated in situ in imidazolium-based ionic liquids (ILs) are efficient in the partial hydrogenation of soybean biodiesel under mild temperatures and high pressures (75 atm).9 ILs contain organic cations and inorganic anions with unique properties such as low melting temperature, high thermal stability, wide liquid phase range, nonflammability and low vapor pressure.10,11 However, ILs have some disadvantages, such as complex preparation conditions, expensive, pollution of the environment and recycling, etc.12 Therefore, finding a suitable carrier to load the Pd nanocatalyst is the key part of green and sustainable catalytic reaction process. In this aspect, employing polyethylene glycols (PEGs) as both reaction media and catalyst carrier has attracted much attention.13 PEGs are cheap, nontoxic, and their properties can be tuned by changing molecular weight.14 PEGs are so benign that they are approved for use in food industry.15 Recently, a variety of reactions in PEGs have been studied, such as polymerization,16,17 oxidation reactions,18,19 reduction reactions,20,21 etc. Sahle-Demessie et al.22 developed the selective hydrogenation of olefins using phenanthroline stabilized Pd nanoparticles as the catalyst in PEG400. Han et al.23 reported that Pd nanoparticles in PEGs are very active and selective for hydrogenation of various olefins under mild conditions. In this work, polyethylene glycol (PEG) with different molecular weight as dispersing agent and reducing agent has been used to prepare Pd(0)-PEGn catalysts, and this catalytic system has been successfully evaluated in partial hydrogenation of methyl linoleate into mono-hydrogenated compounds without the formation of saturated compounds. Transmission electron microscope (TEM) analysis of the Pd catalysts indicated that the size of Pd nanoparticles has important effect on the selectivity of hydrogenation methyl linoleate into monohydrogenated methyl oleate. Moreover, Pd(0)-PEGn catalysts has been successfully applied in selective partial hydrogenation of sunflower oil FAMEs, which produced biodiesel with high content of methyl oleate.

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RESULTS AND DISCUSSION Study of PEG Reduction Performance. As reported,24 UV−vis spectra can be used to monitor the complete reduction of Pd(II). The UV−vis spectrum of Pd(OAc)2 in PEG with different MW (400−4000) at different times are shown in Figure S1. As can be seen, the UV−vis spectrum of Pd(OAc)2 (0 min) shows absorption maximum at around 400 nm, which is characteristic of Pd(II).26 As the reaction progress, the peak observed at 400 nm disappeared at around 1 h, indicating the complete conversion of Pd(II) to Pd(0) nanoparticles. Figure 1

EXPERIMENTAL SECTION

Materials. Palladium acetate(>98.0%) was purchased from TCI (Shanghai, China). PEG400, PEG600, PEG2000 and PEG4000 are chemical grade and were purchased from TCI (Shanghai, China). PEG1000 is chemical grade and was purchased from Sigma (Shanghai, China). Methyl linoleate (>99.5%) was purchased from Sigma (Shanghai, China). H2 (>99.999%) was purchased from Beijing Praxair Corporation, China. Sunflower oil fatty acid methyl esters were prepared through methylation with NaOH−methanol from refined sunflower oil, which was purchased from a local market. Preparation of Pd-PEGn Catalyst. The procedures were similar to those reported in previous literature.24 A different molar ratio of Pd(II) acetate [Pd(OAc)2] was added into PEG (4 g) with different molecular weights (400, 600, 1000, 2000 and 4000) in a 25 mL roundbottomed flask. The mixture was heated in an oil bath at different temperature under vigorous stirring for 2 h. The light yellow color of the solution turned brown and finally to gray dark, suggesting the formation of Pd(0). And PEG was comfirmed by HNMR analysis24 to role as reducing agent through the oxidation of the hydroxyl group and generate the aldehyde group during the reduction of Pd(OAc)2. Then the mixture was solidified when cooling to room temperature. The asprepared catalyst was named as Pd-PEGn. Importantly, the asprepared nano-Pd catalysts can be preserved for months without changes of properties. Compared with conventional supported Pd catalysts, the presented synthesis of Pd nanoparticles in PEG is faster and simple, for example, 30 min is enough for the whole preparation process, no special atmosphere is required, and the experimental skill for such type of catalyst preparation is also not stringent (Table S1).

Figure 1. UV−vis spectrum of Pd-PEGn at 5 min.

displays the experimental UV−vis spectra of the Pd nanoparticles by using different molecular weights of PEG as reducing agent at 75 °C for 5 min. It is clear that the reducing reactivity of PEG was markedly increased with the enhancement of its average molecular weight. The exact reason behind such behavior of different PEG molecular weights is not very clear. However, it might be related to the formation of metal complexes. Compared with the large molecular weight of PEG, the small molecular weight of PEG is easier to connect with transition metal ions to form complexes, which will hinder the reduction of metal cation. Study of Pd-PEGn System Catalytic Hydrogenation Performance. According to the catalyst preparation conB


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Figure 2. (a) Curve of iodine value (IV) of the hydrogenation of methyl linoleate; (b) content of methyl oleate during the hydrogenation.

and Pd-PEG4000 were chosen as the catalyst for the following optimization. Palladium Ion Concentration Screening. The concentration of Pd ion will affect the catalyst preparation time. UV− vis spectra was used to determine the Pd(II) ion reduction during the preparation of Pd-PEG2000 and Pd-PEG4000 with different concentration of Pd(II) ion (Figure S2). Figure S2 reflected the UV−vis spectrum of Pd-PEG2000 and PdPEG4000, which were prepared with three different concentration of Pd(II) (0.005−0.02 mmol/g). It could be concluded from the spectra that the concentration of Pd(II) was gradually reduced after reaction for 30 min, and 1 h was conducted to ensure that Pd(II) ion would be fully reduced to Pd(0) and formed stable Pd nanoparticles. Then we investigated the efficiency and selectivity in hydrogenation of methyl linoleate catalyzed by Pd-PEG2000 with Pd concentration in the range of 0.005 mmol/g to 0.04 mmol/g. Figure 4a reflects the IV during the hydrogenation of methyl linoleate catalyzed by Pd-PEG2000 catalyst, which showed that with the Pd concentration increased from 0.005 to 0.02 mmol/g, hydrogenation reaction rates were accelerated. However, with the Pd concentration further increased from 0.02 to 0.04 mmol/g, hydrogenation reaction rate of methyl linoleate was sharply reduced, which may be due to the high concentration of Pd lead to the size of Pd particle increases, reducing the contact area of hydrogen gas with the substrates. Figure 4b reflects the content of methyl oleate generated during the hydrogenation of methyl linoleate, which showed that hydrogenation catalyzed by different concentration of Pd catalyst have good catalytic hydrogenation selectivity. And hydrogenation using Pd-PEG2000 (CPd = 0.02 mmol/g) as catalyst exhibited slightly better selectivity to methyl oleate than others (Figure S5). Meanwhile, hydrogenation using PdPEG2000 with different concentration of Pd generated similar amount of trans isomers (75−90%) at IV of 86 (Figure S6). Therefore, using Pd-PEG2000 with Pd concentration of 0.02 mmol/g as the catalyst, the hydrogenation rate is the fastest and methyl oleate selectivity is the best. Figure 5a reflects the IV during the hydrogenation of methyl linoleate catalyzed by Pd-PEG4000 catalyst. It can be seen that hydrogenation using Pd-PEG4000 with 0.005 mmol/g Pd concentration was the fastest. And further decreasing the Pd concentration (e.g., 0.0025 mmol/g) led to reduced hydrogenation rate. When Pd-PEG4000 (CPd = 0.005 mmol/g) was used as catalyst, the content of methyl oleate formed reached maximum (92.0%) (Figure 5b). And hydrogenation using PdPEG4000 (CPd = 0.005 mmol/g) as catalyst exhibited slightly better selectivity to methyl oleate than others (Figure S7).

ditions, the influence of catalyst preparation conditions mainly include the molecular weight of PEG, the concentration of Pd ion and reaction temperature. Considering methyl linoleate (>50%) is the main polyunsaturated compound in FAMEs, hydrogenation of methyl linoleate was chosen as the model reaction to evaluate the prepared Pd-PEGn catalysts. The evaluation factors are the amount of methyl oleate generated and hydrogenation reaction rate during the hydrogenation of methyl linoleate. Molecular Weight of PEG Screening. First, using hydrogen (H2) at 1.0 atm, we studied PEG with different molecular weights as the reducing agent of the preparation of Pd-PEGn catalyst for catalytic hydrogenation of methyl linoleate (Figure 2). Figure 2a shows the hydrogenation rates of the reactions catalyzed by Pd-PEG400, Pd-PEG600, PdPEG1000, Pd-PEG2000 and Pd-PEG4000, respectively. Figure 2b shows the content of methyl oleate generated during the hydrogenation of methyl linoleate. It is clear that using PdPEG600, Pd-PEG2000 and Pd-PEG4000 as catalyst for hydrogenation of methyl linoleate, the reaction rates were faster than the hydrogenation catalyzed by Pd-PEG400 and PdPEG1000. Meanwhile, the contents of methyl oleate in the reactions catalyzed by Pd-PEG600, Pd-PEG2000 and PdPEG4000 were higher than that in the reactions catalyzed by Pd-PEG400 and Pd-PEG1000. And hydrogenation using PdPEG2000 or Pd-PEG4000 catalyst exhibited better selectivity to methyl oleate than Pd-PEG600 catalyst (Figure S3). Meanwhile, hydrogenation using Pd-PEG4000 catalyst generated a less amount of trans isomers than Pd-PEG2000 and PdPEG600 catalyst (55% vs 90%) at IV of 86 (Figure S4). However, when Pd-PEG600 dissolved in CH2Cl2, a massive black particles precipitate was observed (Figure 3), which illustrates that the Pd nanoparticles in PEG600 is unstable. Interestingly, Pd-PEG2000 and Pd-PEG4000 can both completely dissolve in CH2Cl2 after vibration and exhibits as a homogeneous solution (Figure 3). Therefore, Pd-PEG2000

Figure 3. Photo of Pd-PEGn (n = 600, 2000, 4000) catalysts dissolved in CH2Cl2. C


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Figure 4. (a) Curve of IV of the hydrogenation of methyl linoleate; (b) content of methyl oleate during the hydrogenation.


(Figure 6) of the two catalysts were very similar. Meanwhile, hydrogenation using the above two catalysts also produced lower amount of trans isomers (32−36%). Thus, Pd-PEG4000 (CPd = 0.005 mmol/g) was chosen as an optimal catalyst from the economic point of view. Catalyst Preparation Temperature Optimization. As one of the main influence factors of catalyst preparation process, the preparation temperature is also very important to affect the formation of Pd nanoparticles. Therefore, we studied the influence of temperature on characterization of PdPEG4000 catalyst. Considering the melting point of PEG4000 is 53−56 °C, the hydrogenation of methyl linoleate were evaluated by different Pd-PEG4000 catalysts, which were prepared at 60, 75, 90 and 105 °C, respectively (Figure 7). The Pd-PEG4000 catalyst prepared at 75 °C showed the fastest hydrogenation reaction rate. Because higher preparation temperature makes the Pd nanoparticles deactivate or lead to reduction faster from Pd(II) to Pd(0), which tends to precipitate out palladium from the colloidal state. The hydrogenation reaction rate obviously became slower when the preparation was conducted at 105 °C. Figure 7b reflects the content of methyl oleate generated during the hydrogenation catalyzed by Pd-PEG4000 prepared at different temperatures (60−105 °C). Using Pd-PEG4000 (CPd = 0.005 mmol/g) prepared at 75 °C as the catalyst led to high content of methyl oleate (92.0%). Futhermore, the calculated methyl oleate selectivities of Pd-PEG4000 prepared at 60, 75 and 90 °C were very similar and better than the catalyst prepared at 105 °C (Figure S9). And hydrogenation using Pd-PEG4000 (CPd = 0.005 mmol/g) prepared at 105 °C as the catalyst led to the lowest content of methyl oleate (61.2%). Therefore, 75 °C was

Meanwhile, hydrogenation using Pd-PEG4000 with different concentrations of Pd generated the amount of trans isomers in the range from 74% to 89% (Figure S8). Based on the above results, using Pd-PEG4000 with 0.005 mmol/g Pd concentration as the catalyst, the hydrogenation rate is the fastest and methyl oleate selectivity is the best. Finally, the hydrogenation of methyl linoleate using PdPEG2000 (CPd = 0.02 mmol/g) and Pd-PEG4000 (CPd = 0.005 mmol/g) catalysts are compared in Figure 6. Both of the hydrogenation rates (Figure 4, 5) and methyl oleate selectivity

Figure 6. Methyl oleate selectivity during the hydrogenation catalyzed by two types of Pd-PEGn catalysts (selectivity = methyl oleate (mol)/ converted methyl linoleate (mol); IV0 corresponded to the iodine value of methyl linoleate (172), IV corresponded to the iodine value of hydrogenated product). D


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Figure 8. TEM photos of Pd-PEG nanoparticles prepared at different conditions. (A: Pd-PEG4000 (CPd = 0.005 mmol/g), 60 °C; B: Pd-PEG4000 (CPd = 0.005 mmol/g), 75 °C; C: Pd-PEG4000 (CPd = 0.005 mmol/g), 90 °C; D: Pd-PEG4000 (CPd = 0.005 mmol/g), 105 °C; E: Pd-PEG4000 (CPd = 0.01 mmol/g), 75 °C; F: Pd-PEG4000 (CPd = 0.02 mmol/g), 75 °C; G: Pd-PEG4000 (CPd = 0.04 mmol/g), 75 °C; H: Pd-PEG2000 (CPd = 0.02 mmol/g), 75 °C).

Table 1. Particle Size and the trans Isomers Content at the IV of 86a Entry

PEG

[Pd] (mmol/gPEG)

Temp. (°C)

Mean particle size (nm)

Trans-methyl oleate (%)

C18:1 (%)b

C18:0 (%)

C18:2 (%)d

1 2 3 4e 5 6 7 8

4000 4000 4000 4000 4000 4000 4000 2000

0.005 0.005 0.005 0.005 0.01 0.02 0.04 0.02

60 75 90 105 75 75 75 75

1.4 4.0 6.8 −c 2 7.2 5.4 3.9

45 32 56 −c 42 57 60 36

90 92 93 61 88 89 79 93

3.3 3.9 3.7 0.2 6.9 7.0 11.8 1.6

6.7(0.4) 4.1(0.3) 3.3(0.3) 38.8(3.8) 5.1(0.3) 4.0(0.3) 9.2(0.3) 5.4(0.4)

a

IV is commonly used to evaluate the sum of unsaturated CC bonds in FAMEs. The IV of methyl linoleate is 172, and the IV of methyl oleate is 86. Therefore, we compared the results at the point of IV of 86. bMethyl oleate (%) = Cis-methyl oleate (%) + Trans-methyl oleate (%), and the yields were determined by GC. cPreparation temperature at 105 °C partly inactivated the Pd catalyst, which can decelerate the hydrogenation rate. Thus, the content of trans isomer at the IV of 86 was not calculated. dC18:2 (%) = methyl linoleate (%) + Trans-methyl linoleate (%); value in parentheses indicates the amount of methyl linoleate. eThe IV of the hydrogenation product is 119.

Pd Catalyst Characterization. To have a better understanding of the relationship between Pd nanoparticles size and

chosen as the best temperature for the preparation of Pd nanoparticles in PEG4000. E


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ment indicated that hydrogenation rate of methyl oleate was much slower than that of methyl linoleate (Figure S10). Recycling of Pd-PEG4000. The recyclability of PdPEG4000 (CPd = 0.005 mmol/g) was examined (Figure 9).

hydrogenation reaction rate and methyl oleate selectivity, Pd catalysts prepared under different conditions have been characterized by the transmission electron microscopy (TEM), and the results are shown in Figure 8 and Table 1. Figure 8 reflects the TEM photos of Pd catalysts that were prepared at different conditions. Meanwhile, we calculated the particle size of Pd catalysts formed under different conditions (Table 1). The Pd catalysts prepared at the temperature of 60, 75 and 90 °C can form spherical nanoparticles with a better crystal forms and dispersity. And with the increase of preparation temperature, Pd particle size increases gradually. Moreover, Pd catalysts prepared under this condition have good hydrogenation activity and hydrogenation selectivity (Table 1, entries 1−3). When the preparation temperature of catalyst reached 105 °C, spherical Pd nanoparticles with lattice fringe and a better dispersity could not be found from the TEM photo, and the catalytic activity during the hydrogenation of methyl linoleate is low (Table 1, entry 4). Thus, it can be concluded that high preparation temperature will make the Pd catalyst partly deactivated. Furthermore, catalysts prepared with the Pd amount of 0.02 mmol/g PEG4000, 0.01 mmol/g PEG4000 and 0.005 mmol/g PEG4000 can form spherical nanoparticles with a better crystal forms and dispersity. Meanwhile, catalysts prepared under this condition also have good hydrogenation activity and selectivity. However, when the concentration of Pd reached as high as 0.04 mmol/g PEG4000, the TEM photo showed that less amount of spherical nanoparticles formed. Therefore, we proposed that a part of free Pd(0) generate during the preparation of the catalyst, which may lead to the poor methyl oleate selectivity (Table 1, entry 7). Importantly, hydrogenation reaction using Pd catalyst with the average size of nanoparticles in 4 nm led to lower trans isomers (32−36%) generated during the hydrogenation (Table 1, entries 2 and 8). Therefore, Pd-PEG4000 (CPd = 0.005 mmol/g) prepared at 75 °C was chosen as the optimal catalyst. On the basis of the above results, we proposed that this methyl oleate selectivity derived from the coordination of the Pd nanoparticles (∼4 nm) with the carbon−carbon double bonds in methyl linoleate and methyl oleate (Scheme 2). The

Figure 9. Conversion and yield of methyl oleate at IV of 86 for the recycling of Pd-PEG4000 at 75 °C and 1.0 atm of H2.

The nano Pd-PEG catalyst was easily separated from the reaction mixture by extraction with diethyl ether and can be reused five times without losing any activity or methyl oleate selectivity. And the Pd-PEG nanocatalyst has be characterized by TEM after the recycling, and the results showed that the particle size of Pd nanocatalyst is 4.2 nm (Figure S11), which is similar to the Pd-PEG nanocatalyst used before. The main reason is that the soft stability of PEG4000 can prevent the aggregation of the Pd nanoparticles. However, the catalytic activity was then dropped in the sixth run yielding 72% conversion. The agglomeration of Pd nanoparticles is likely to take place without any ligands (e.g., phenanthroline) stabilization,22 which led to the deactivation of Pd-PEG catalyst during the recycling procedure. Selective Hydrogenation of Sunflower Oil FAMEs. To evaluate the Pd-PEG4000 catalyst in selective hydrogenation of biodiesel, we turned to investigate hydrogenation of sunflower oil fatty acid methyl esters (SFFAME) (Table 2). SFFAME was composed of 11.7% of saturated FAMEs (C16:0 and C18:0), 25.0% of methyl oleate (C18:1) and 63.0% of methyl linoleate (C18:2), which was required to hydrogenated to more stable biodiesel product (mainly C18:1). With the optimized PdPEG4000 catalyst in hand, we studied selective hydrogenation of SFFAME under different H2 pressures (1−10 atm). At 1.0 atm of H2, the hydrogenation of sunflower oil FAMEs could afford the product with as high as 87.1% methyl oleate (C18:1) with a very slightly increase of saturated FAMEs (11.7% vs 12.4%). And continuing increasing the H2 pressures (10 atm) could drastically decrease the trans isomer of methyl oleate (44.5% to 24.0%) with very slightly decrease of methyl oleate (87.1%−85.8%). Compared with supported Pd-catalyst in the literature (Table S2), current Pd-PEG catalyst produced slightly lower contents of trans isomers and C18:0 than Pd/SiO2 catalyst27 (24%/6% vs 29.9%/8.5%) under similar conditions. And the hydrogenation using current Pd-PEG catalyst produced similar amount of methyl stearate (C18:0) with Pd(0)/BMI·BF4 catalyst.9

Scheme 2. Proposed Mechanism for the Pd-PEGn Catalyzed Selective Hydrogenation of Methyl Linoleate

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coordination of the suitable size of Pd nanoparticles with methyl linoleate (diunsaturated FAME) is much stronger than that of methyl oleate (monounsaturated FAME), which make the hydrogenation rate of methyl linoleate much faster than the hydrogenation rate of methyl oleate. Indeed, control experi-

CONCLUSIONS In conclusion, we have developed an efficient Pd-PEGn nanoparticles catalyzed selective partial hydrogenation of F


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ACS Sustainable Chemistry & Engineering Table 2. Selective Hydrogenation of Sunflower Oil FAMEsa Catalyst

Pressure (atm)

C16:0 (%)

C18:0 (%)

C18:1 (%)

C18:2 (%)

C18:3 (%)

Trans isomer (%)

IV (I2/100 g)

1.0 5.0 10.0

7.6 7.5 7.4 7.6

4.1 4.9 5.4 6.0

25.0 87.1 86.9 85.8

63.0 0.5 0.3 0.6

0.3 0.0 0.0 0.0

0 44.5 37.1 24.0

132.0 77.0 77.0 77.0

Starting composition Pd-PEG4000 Pd-PEG4000 Pd-PEG4000 a

IV is commonly used to evaluate the sum of unsaturated CC bonds in FAMEs. The IV of sunflower oil FAMEs is 132, and the IV is 77 when polyunsaturated FAMEs hydrogenated into methyl oleate. Therefore, we compared the results at the point of IV of 77. The content of fatty acid methyl esters was determined by GC. (3) Philippaerts, A.; Jacobs, P. A.; Sels, B. F. Is there still a future for hydrogenated vegetable oils? Angew. Chem., Int. Ed. 2013, 52, 5220− 5226. (4) Hoffmann, G. The Chemistry and Technology of Edible Oils and Fats and Their High Fat Products; Academic Press, 1989. (5) Yan, N.; Xiao, C. X.; Kou, Y. Transition metal nanoparticle catalysis in green solvents. Coord. Chem. Rev. 2010, 254, 1179−1218. (6) Prechtl, M. H. G.; Scholten, J. D.; Dupont, J. Carbon-Carbon Cross Coupling Reactions in Ionic Liquids Catalysed by Palladium Metal Nanoparticles. Molecules 2010, 15, 3441−3461. (7) Liu, J.; Deng, Y.; Wang, H.; Zhang, H.; Yu, G.; Wu, B.; Zhang, H.; Li, Q.; Marder, T. B.; Yang, Z.; Lei, A. Effective Pd-Nanoparticle (PdNP)-Catalyzed Negishi Coupling Involving Alkylzinc Reagents at Room Temperature. Org. Lett. 2008, 10, 2661−2664. (8) Mei, Y.; Lu, Y.; Polzer, F.; Ballauff, M.; Drechsler, M. Catalytic activity of palladium nanoparticles encapsulated in spherical polyelectrolyte brushes and core-shell microgels. Chem. Mater. 2007, 19, 1062−1069. (9) Carvalho, M. S.; Lacerda, R. A.; Leao, J. P. B.; Scholten, J. D.; Neto, B. A. D.; Suarez, P. A. Z. In situ generated palladium nanoparticles in imidazolium-based ionic liquids: a versatile medium for an efficient and selective partial biodiesel hydrogenation. Catal. Sci. Technol. 2011, 1, 480−488. (10) Welton, T. Room-temperature ionic liquids. Solvents for synthesis and catalysis. Chem. Rev. 1999, 99, 2071−2084. (11) Del Pópolo, M. G.; Voth, G. A. On the structure and dynamics of ionic liquids. J. Phys. Chem. B 2004, 108, 1744−1752. (12) Bubalo, M. C.; Radošević, K.; Redovniković, I. R.; Halambek, J.; Gaurina Srček, V. A brief overview of the potential environmental hazards of ionic liquids. Ecotoxicol. Environ. Saf. 2014, 99, 1−12. (13) Chen, J.; Spear, S. K.; Huddleston, J. G.; Rogers, R. D. Polyethylene glycol and solutions of polyethylene glycol as green reaction media. Green Chem. 2005, 7, 64−82. (14) Sheftel, V. O. Indirect Food Additives and Polymers: Migration and Toxicology; Lewis Publishers, Inc., 2000; p 1114. (15) Code of Federal Regulations, Title 21, Vol. 3, CITE21CFR172.820; FDA, 2001. (16) Perrier, S.; Gemici, H.; Li, S. Poly(ethylene glycol) as solvent for transition metal mediated living radical polymerisation. Chem. Commun. 2004, 604−605. (17) Ding, M.; Jiang, X.; Peng, J.; Zhang, L.; Cheng, Z.; Zhu, X. An atom transfer radical polymerization system: catalyzed by an iron catalyst in PEG-400. Green Chem. 2015, 17, 271−278. (18) Hou, Z.; Theyssen, N.; Brinkmann, A.; Leitner, W. Biphasic Aerobic Oxidation of Alcohols Catalyzed by Poly(ethylene glycol)Stabilized Palladium Nanoparticles in Supercritical Carbon Dioxide. Angew. Chem., Int. Ed. 2005, 44, 1346−1349. (19) Li, B.; Liu, A.-H.; He, L.-N.; Yang, J.; Gao, Z.-Z.; Chen, K.-H. Iron-catalyzed selective oxidation of sulfides to sulfoxides with the polyethylene glycol/O2 system. Green Chem. 2012, 14, 130−135. (20) Heldebrant, D. J.; Jessop, P. G. Liquid Poly(ethylene glycol) and supercritical crbon dioxide: A benign biphasic solvent system for use and recycling of homogeneous catalysts. J. Am. Chem. Soc. 2003, 125, 5600−5601. (21) Harraz, F. A.; El-Hout, S. E.; Killa, H. M.; Ibrahim, I. A. Palladium nanoparticles stabilized by polyethylene glycol: Efficient,

methyl linoleate to afford mono-hydrogenated compounds. 92% yield of methyl oleate with full conversion of methyl linoleate was obtained by using PEG4000 as reducing agent and stabilizer and a 0.005 mmol/g concentration of Pd ion in PEG4000 at 75 °C. The optimized Pd-PEG4000 nanocatalyst could also promote the selective hydrogenation of sunflower oil FAMEs. In addition, through characterizing by TEM, we find out that high temperature could make the catalyst deactivate during the process of catalyst preparation and the Pd nanoparticles catalyst with average size of 4 nm showed the best hydrogenation selectivity. Our protocol provides an efficient method toward solving the biodiesel or fatty acid methyl esters (FAMEs) problems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01823. UV−vis spectra of various Pd-PEG Catalysts; methyl oleate selectivity at different conditions; trans isomer selectivity at different conditions; characterization of PdPEG catalyst after recycling; information for comparation of presented work and wetness impregnation; information for comparison of trans isomers produced in hydrogenation of FAMEs (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel/Fax: 086-371-67758022. E-mail: [email protected] (Wei Liu). *E-mail: [email protected] (Hua Zhang). ORCID

Wei Liu: 0000-0003-1706-1208 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Young-aged Backbone Teacher Funds of Henan Province of China (No. 2014GGJS-058) and Project of Henan University of Technology Excellent Young Teachers (No. 2014003) for financial support.

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REFERENCES

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