Article pubs.acs.org/OPRD
Hydrogenation of Sodium Oleate in Aqueous Emulsion with the Hoveyda−Grubbs Second-Generation Catalyst Work from the Organic Reactions Catalysis Society Meeting 2016 Hui Wang,† Max Hamilton,‡ and Garry L. Rempel*,‡ †
Institute of Chemical and Engineering Sciences (ICES), Agency for Science, Technology and Research (A*STAR), Singapore, 1 Pesek Road, Jurong Island, 627833, Singapore ‡ Department of Chemical Engineering, University of Waterloo, 200 University Ave. West, Waterloo, Ontario N2L 3G1, Canada S Supporting Information *
ABSTRACT: The catalytic hydrogenation of sodium oleate in emulsion form was carried out using the second-generation Ru Hoveyda−Grubbs catalyst in a semibatch reactor. A fast and complete hydrogenation of the CC bond was observed after a reaction time of 1 h with 0.4 wt % catalyst at 120 °C and 8.27 MPa hydrogen gas. 1H NMR characterization shows that the carboxylic group CO was preserved without any reduction. The hydrogenated product sodium stearate was thus obtained. Our results show that the Hoveyda−Grubbs second-generation catalyst can maintain its activity for the hydrogenation reactions in an organic solvent free environment. The hydrogenation reaction presented here provides an alternate pathway to manufacture sodium stearate and other saturated fatty acid salts in a greater volume by an environmental friendly “green” process. In addition, this research will benefit the commercial process of catalytic hydrogenation of unsaturated elastomers in latex form.
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INTRODUCTION Sodium oleate (or sodium 9-octadecenoate) is the carboxylate sodium salt of monounsaturated fatty oleic acid, an anionic surfactant with excellent amphiphilic capabilities (Figure 1).
workers found that Hoveyda−Grubbs second generation (HG2) catalyst shows superior catalytic activity toward the hydrogenation reaction in the nitrile butadiene rubber (NBR) latex system even in the absence of any organic cosolvent or other chemicals to assist in dissolving/delivering the catalyst.5 A fast catalytic hydrogenation (e.g., TOF > 7000 h−1 with the degree of hydrogenation reaching 95 mol % of NBR substrate) was achieved under a very low catalyst concentration. However, there is one unexplored problem involved in this HG2 hydrogenation process, which is whether the emulsifier sodium oleate (that was employed in the “parent” latex polymerization) was hydrogenated or not in the subsequent hydrogenation stage. This question is also one of the main motivations for the present work. HG2 catalyst as shown in Figure 2 is commercially available and has been shown to have a higher catalytic reactivity than the Grubbs second-generation catalyst at lower temperatures.6−9 Compared to other types of Grubbs-type catalysts, HG2 catalyst is characterized by its improved stability which
Figure 1. Chemical structure of sodium oleate.
Sodium oleate exhibits a 120° rotation in the double link and the presence of a double link which effects the molecular packing in oil/water interfacial membranes.1 Sodium oleate and its hydrogenated product sodium stearate, as emulsifying agents, are some of the most common components of commercial soaps, which have found a broad range of important industrial applications covering emulsification, polymer preparations, detergency, cosmetic, food, and pharmaceutical sections.2,3 In addition to the sodium salts, some other frequently used oleate and stearate salts are those containing potassium, lithium, or calcium.4 To develop a green and economical process for catalytic hydrogenation of diene-based polymers has been standing as a priority pursuit for the today’s rubber industry such as that of Arlanxeo Inc. (formerly Lanxess) and Nippon Zeon Chemicals. Because most of the polymers are commercially produced in latex form, the direct hydrogenation of unsaturated polymers in the emulsion form is thus becoming a very promising route. The hydrogenation reactions of the polymers cannot proceed without the presence of a catalyst. Recently, Rempel and co© XXXX American Chemical Society
Figure 2. Chemical structure of the Hoveyda−Grubbs secondgeneration catalyst. Received: March 7, 2016
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enables it to work under most reaction conditions.10−12 Considering that the HG2 has been used in the hydrogenation of NBR latex described above, HG2 catalyst was selected for the hydrogenation of sodium oleate in aqueous form. In the past few decades, there have been numerous studies investigating the hydrogenation of oleic acid and/or its esters and other unsaturated fatty acids using various catalyst systems. These reports have focused either on the hydrogenation of the CC olefin units to form the stearic acid13−17 or on the selective hydrogenation of the CO bond to prepare the oleyl alcohol.18−23 The CC group naturally will be hydrogenated in the first place due to the lower activation energy required. Therefore, the selective hydrogenation of the CO bond without attacking the CC requires more complex catalysts and precise reaction conditions. Zhu et al. investigated the performance of LaNi4.8Cu0.2 alloy catalyst in the hydrogenation of oleic acid to the stearic acid and reported that the hydrogenation proceeded fast at 90 °C and 0.5 MPa hydrogen pressure.13 Pootawang et al. prepared a Ag nanoparticleincorporated mesoporous silica catalyst via a solution plasma process and examined its catalytic activity for the hydrogenation of oleic acid to stearic acid.14 The unsaturated oleic acid can be mostly hydrogenated to the saturated stearic acid in ethanol with a 90.56% conversion. Mendes et al. studied the performance of bimetallic TiO2-supported Ru−Sn catalysts for the selective hydrogenation of oleic acid to the unsaturated oleyl alcohol. A near total suppression of the hydrogenation of the olefin bond was observed in favor of the activation of the hydrogenation of the carboxylic bond, which thus leads to the selective formation of unsaturated alcohols.18 Pieck and coworkers examined the selectivity and activity of Ru−Sn−B/ Al2O3 catalysts for the hydrogenation of oleic acid to the unsaturated oleyl alcohol and found that the incorporation of Sn is the key leading to the catalysts capability of producing oleyl alcohol.21 Specially, the Ru−Sn−B/Al2O3 catalyst is selective to the oleyl alcohol while Ru or Ru−B/Al2O3 catalysts are not selective to produce oleyl alcohol. In contrast to the wealth of information on the hydrogenation of oleic acid, no example has been reported to the authors’ knowledge in which the oleate salt has been hydrogenated even when the hydrogenation is performed in a single aqueous phase without the assistance of any organic solvents. In addition, the reactions at the oil/water interface play a large part with respect to the development of economical and environmental “green” processes. Therefore, the objective of this study is to investigate the hydrogenation behavior of the oleate salt in an aqueous emulsion environment assuming that the hydrogenation will occur along the interface. It is believed that the results presented can be extended to the salts of other similar fatty acids, such as the linoleic acid with 18 carbon atoms and two double bonds and palmitoleic acid with 16 carbon atoms and one double bond. In addition, the results should hold for fatty acid salts containing other metals such as the potassium salt.
and used as received. The pure water (18.2 MΩ; Millipore Milli-Q system) was provided by the Department of Chemical Engineering, University of Waterloo, Canada. Hydrogenation Procedures of Sodium Oleate in an Aqueous Emulsion. In a 300 mL Parr 316 stainless steel semibatch reactor, 0.54 g of sodium oleate was dissolved in 100 mL of pure water. The resulting solution was cooled in an ice− water bath for 1 h, and then 0.00216 g (0.4 wt % on the basis of the weight of the sodium oleate) HG2 catalyst was accurately weighted in a nitrogen glovebox with less than 1 ppm oxygen and transferred into the reactor. The reactor was then sealed. The reactor was purged with 275 kPa nitrogen gas for three quick cycles and then continuously for 2 h with a slow nitrogen flow rate. This operation was performed under a constant agitation of 200 rpm and at a temperature of 3−4 °C using an ice−water bath. After completion of the purging, the reactor was heated up to 120 °C under the constant agitation of 450 rpm and then stabilized for 30 min. The reactor was then charged with hydrogen gas at a pressure of 8.27 MPa (1200 psi) and allowed to react for 5 h while maintaining the temperature of 120 °C and agitation speed of 450 rpm. One sample was taken before charging the hydrogen, and the rest of the samples were taken with the proceeding of the reaction. After the hydrogenation for 5 h, the reactor was cooled down to room temperature using tap water bath, and the hydrogen was then released. The reactor was disassembled, and the product was transferred directly into a clean glass jar. Characterization. Fourier Transform Infrared Spectroscopy (FTIR). The samples were first dried to obtain the solid product. Then the samples were dissolved in methanol and cast onto a sodium chloride disk. A film 99%) and Hoveyda−Grubbs second-generation catalyst (97%) were purchased from SigmaAldrich (Oakville, CA) and used as received. Ultrahigh purity hydrogen (99.999% oxygen-free) and ultrahigh purity nitrogen (99.999% oxygen-free) were purchased from Praxair Inc. (Mississauga, CA) and used as received. Deuterated chloroform (CDCl3) and dimethyl sulfoxide (DMSO) were purchased from Cambridge Isotope Laboratories (Massachusetts, USA)
kBT 6πηD
where RH is the hydrodynamic radius, kB is the Boltzmann constant (1.38 × 10−23 J/K), T is the temperature, η is the viscosity, and D is the diffusion coefficient. Zeta Potential. The zeta potential was determined using a Zetasizer Nano ZS (Malvern Instruments, Worcestershire, UK). A clear, folded, disposable zeta cell (DTS-1070 from Malvern, Worcestershire, UK) with a volume of ∼1 mL was B
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used. The sample was analyzed at 25 °C at a constant voltage. The same dilute solution as used for the DLS was used to find the zeta potential. The measurement was performed twice, and the average value was reported. Using Henry’s equation, the zeta potential was automatically calculated by the instrument from the electrophoretic mobility distribution. The Henry equation is as follows: UE =
2εzf (Ka) 3η
where UE is the electrophoretic mobility, ε is the dielectric constant, z is the zeta potential, f(Ka) is Henry’s function, and η is the viscosity. Transmission Electron Microscopy (TEM). The size and morphology of the micelles were observed using a Tecnai G2 300 kV transmission electron microscope (TEM) (FEI, Eindhoven, Netherlands). A portion of 10 μL of the sodium oleate solution, which has the same concentration as the starting emulsion for the hydrogenation, was placed on a 400mesh copper grid at room temperature, and excess solution was removed from the edge using tissue paper. The grid was then negatively stained using 2% (w/v) uranyl acetate, and excess stain was removed using tissue paper. After air drying for 30 s, the grid was delivered to the TEM chamber for imaging.
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RESULTS AND DISCUSSION Organic solvents are usually added to dissolve the series of ruthenium catalysts during reactions like olefin metathesis. The challenge of performing such reactions using water as the dispersing agent is mainly concerned with the stability of the catalyst and the mass transfer of the catalyst toward the substrate. However, the surfactant micellar solution maybe an exception which however can act as a nanoholder for the catalyst molecules. Mingotaud and co-workers carried out an interesting experiment in which the UV−visible absorbance spectra were recorded to examine the solubility of Hoveyda− Grubbs first-generation catalyst in the surfactant micellar solutions of two types of ammonium surfactants, namely, dodecyltrimethylammonium and cetyltrimethylammonium chlorides, respectively.24 The absorbance intensity showed that the hydrophobic Hoveyda−Grubbs’ catalyst can be solubilized into the micelles and the maximum solubility of catalyst is proportional to the concentration of surfactant. The solubilization of the hydrophobic Hoveyda−Grubbs’ catalyst in the micellar aqueous phase provides great potentiality for the emulsion catalytic hydrogenation of sodium oleate using HG2 in the aqueous phase as described in this work. Figure 3 shows the morphology and dimensions of the micelles formed by the sodium oleate using the 300 kV TEM. Uranyl acetate was used to negatively stain the sample in order to enhance contrast by shadowing the background with a metallic deposit. The uranium ion attracts to the negative carboxylic group present on the exterior of the micelle so that there are clearly defined edges produced around the micelles. It can be observed that the micelles are roughly spherical in shape. The concentration of sodium oleate has an important impact on the phase behavior of the sodium oleate/water system. Coppola and co-workers reported that the sodium oleate micelles appear as the spherical morphology at a concentration ranging from the critical micelle concentration (CMC) of sodium oleate (about 0.008 wt %) up to 10 wt %.1 With a further increase in the concentration, the sodium oleate micelles start to grow from the spheres to
Figure 3. TEM images of the starting sodium oleate micelles under two magnification scales using a Tecnai G2 TEM under the operation voltage of 300 kV. The scale bar in the second TEM image is 50 nm. The samples are negatively stained with 2% (w/v) uranyl acetate before sending to the chamber of TEM.
cylinders and then to long cylindrical aggregates. As a consequence of above, a hexagonal liquid crystal is formed. The concentration of sodium oleate used in this research is 0.54 wt %, which is in good agreement with Coppola and coworkers’ report. The spherical micelles formed by the sodium oleate create an ideal reaction platform for the localization of the HG2 catalyst in the water-based system. Figure 4 shows the zeta potential distribution of the starting sodium oleate micellar dispersion. The zeta potential of a colloid is indicative of its stability with values greater than +30 mV or less than −30 mV as the characteristic of a stable colloid. C
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Figure 4. Zeta potential distribution of sodium oleate emulsion (concentration of sodium oleate emulsion = 0.54 wt %, pH of sodium oleate emulsion = 10.9, and temperature at measurement = 25 °C).
Table 1. Particle Size of Sodium Oleate Dispersion peak type size (d/nm) standard deviation (d/nm)
DI
DV1
shoulder 178.35 132.3
86.605 47.86
DV2
DN
364.85 124.75
single 54.235 20.945
two
Table 2. Particle Size of Sodium Stearate Dispersion peak type size (d/nm) standard deviation (d/nm)
DI
DV
DN
single 181.0 56.30
single 152.1 61.23
single 100.4 39.27
Colloids with a zeta potential value greater than +60 mV or less than −60 mV can be considered to have good stability. The measured average zeta potential value for the 0.54 wt % sodium oleate emulsion (pH = 10.9) is −83.1 mV at 25 °C, which indicates an excellent colloidal stability. This value agrees with the observed stability where there was no flocculation for the 0.54 wt % sodium oleate emulsion after more than half a year in a sealed bottle at room temperature. Figure S1 presents one set of FTIR spectra characterizing the hydrogenation progress of sodium oleate from 1 to 5 h. The samples were taken every 1 h throughout the reaction. The ∼1560 cm−1 peak is characteristic of the CO stretch. The ∼3010 cm−1 peak is assigned to the alkene C−H stretch, and the ∼965 cm−1 peak corresponds to the CC stretch. After an elapsed time of 1 h as shown in Figure S1ii, it was found that the alkene C−H stretch peak (∼3010 cm−1) and CC stretch peak (∼965 cm−1) both disappeared, which indicated that the CC has been completely hydrogenated. In addition, the existence of CO stretch at ∼1560 cm−1 peak demonstrates that the carboxylic group is preserved during the hydrogenation of sodium oleate, i.e., sodium stearate. It has been previously reported that the hydrogenation of the CO bond begins after the hydrogenation of the CC bond over the aluminasupported ruthenium catalyst (Ru/Al) and titania-supported ruthenium catalyst (Ru/Ti) systems.18 For both catalysts Ru/Al and Ru/Ti, the primary reaction is an almost instantaneous hydrogenation of the CC bond of the oleic acid to stearic acid, which is then followed by a slower consecutive hydrogenation of the carboxyl group of the stearic acid to the saturated stearyl alcohol. From this point of view, we extended the hydrogenation up to 5 h in order to examine whether the CO will be reduced following the complete hydrogenation of CC. With the continuing reaction of up to 5 h (Figure S1iii−
Figure 5. 1H NMR spectra for the hydrogenation of sodium oleate in the emulsion form sampled at 5, 10, 20, and 40 min (Bruker NMR at 300 MHz and DMSO as a solvent). Hydrogenation conditions: HG2/ sodium oleate = 0.4 wt %, PH2 = 8.27 MPa (1200 psi), agitation = 450 rpm, T = 120 °C.
Figure 6. Conversion of emulsion hydrogenation of sodium oleate with the evolution of reaction time. Hydrogenation conditions: HG2/ sodium oleate = 0.4 wt %, PH2 = 8.27 MPa (1200 psi), agitation = 450 rpm, T = 120 °C.
D
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Figure 7. Particle size distributions based on the intensity, volume, and number for (i) sodium oleate, (ii) sodium stearate.
(i.e., conversion of CC) increases with the reaction time under our reaction conditions. These results indicate that the emulsifier sodium oleate additionally consumed the amount of HG2 when the HG2 was interacting with the CCs in the NBR during the hydrogenation of the NBR latex. The size properties of the sodium oleate micelles and the hydrogenated product sodium stearate micelles were determined by the DLS technique. Table 1 and Table 2 show the diameter of the sodium oleate micelles and sodium stearate micelles, respectively, in which the z-average (Dz), volume (DV), and number (DN) average sizes are reported. Figure 7i and ii present the particle size distributions for the sodium oleate micelles and sodium stearate micelles, respectively. The micelle size was found to increase after the hydrogenation reaction as evidenced by the z-average value increasing from 178.35 to 231.8 nm, which shows that the size of the sodium stearate micelles is larger than the size of the sodium oleate under the same concentration. After the hydrogenation of the double link in the oleate, the molecular packing array at the interfacial membranes will be different between the oleate and stearate. This may be the reason for the increase in the micelle size from the sodium oleate to the sodium stearate micelles. In short, the hydrogenation process shown in this research is attractive because it is not only a simple process without complicated reaction procedures but particularly a “green” process to use water as a solvent as to minimize the negative impact on the environment. Further research will be carried out to recycle/reuse the HG2 catalyst and to explore an economic and efficient catalyst for this emulsion hydrogenation system.
vi), there is no perceptible new peak observed in the spectra, which provides the prima facie evidence that there is no further reaction occurring. The 1H NMR as will be shown later has provided strong evidence to support this result. On the other hand, Apesteguiá and co-workers reported that the silicasupported HG2 catalysts (HG/SiO2) can efficiently promote the self-metathesis of the methyl oleate to yield the 9octadecene and 9-octadecene-1,18-dioate in the cyclohexane liquid.25 Indeed, the HG2 catalyst is known as one of the best metathesis catalysts. However, our previous study showed that the degree of metathesis reaction using HG2 catalyst in the organic solvent free process was very limited.26 Thus, the selfmetathesis of the sodium oleate in this emulsion system can be regarded as being negligible. The complete hydrogenation of CC in the sodium oleate while keeping the carboxylic group intact can be further confirmed from the 1H NMR characterization and analysis. Figure S2I shows the 1H NMR spectrum for the sodium oleate without any hydrogenation. As indicated in this figure, the peak at ∼5.3 ppm is assigned to the hydrogen atoms on the CC, and the peak at ∼2 ppm is assigned to the hydrogen atoms on the carbon atoms adjacent to the CC bond. Figure S2ii shows that the hydrogen signals at ∼5.3 ppm and ∼2 ppm are no longer observed after 1 h hydrogenation. No other peaks were formed or eliminated with a reaction time of up to 5 h, as shown in Figure S2iii−vi. It is difficult to determine whether the CO bond started to be reduced from the FTIR. However, the 1H NMR data is clear. The primary alcohol group, i.e., R−CH2−O−, would be produced if CO was hydrogenated, where the signal of the two hydrogen atoms on the carbon atoms adjacent to the oxygen group should exhibit signals at ∼3.5 ppm. No peaks were found that would indicate the presence of a primary alcohol group. As a consequence, the hydrogenation reaction of CC dominated the reaction, and the hydrogenated product is sodium stearate. Following the research progress presented above, we investigated the rate of this emulsion hydrogenation within the first hour under the studied reaction conditions. The hydrogenated samples at 5, 10, 20, and 40 min were taken for the characterization, and their 1H NMR spectra are shown in Figure 5. The conversions of the hydrogenation were thereafter calculated on the basis of the integral area for peaks of CC at ∼5.3 ppm and the terminal methyl group at ∼0.9 ppm. The quantitative conversions as a function of the reaction time are shown in Figure 6. It can be seen that the hydrogenation degree
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CONCLUSIONS The fast and complete hydrogenation of the olefin unit of sodium oleate in an emulsion form was successfully achieved after 1 h using 0.4 wt % HG2 catalyst at 120 °C with a hydrogen pressure of 8.27 MPa. Meanwhile, the carboxylic group CO was preserved intact without any reduction. Sodium stearate was thus prepared as the hydrogenated product. The results demonstrate that the HG2 catalyst has maintained its catalytic efficacy and is still active in the hydrogenation of small molecules in the aqueous emulsion form. The hydrogenation reaction examined here provides a new and “green” way to convert unsaturated long chain fatty acid salts into their saturated counterparts without the assistance of any organic solvent. This study may provide E
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using a thin hydride layer of hydrogen: Storage alloy LaNi4.8Cu0.2 as catalyst. J. Alloys Compd. 1997, 253−254, 689−691. (14) Pootawang, P.; Saito, N.; Takai, O. Ag nanoparticle incorporation in mesoporous silica synthesized by solution plasma and their catalysis for oleic acid hydrogenation. Mater. Lett. 2011, 65, 1037−1040. (15) Nielsen, K.; Hansen, H. J. M.; Nielsen, V. R. Selectivity in the hydrogenation of oleic-linoleic acid oils with commercial nickel catalysts. J. Am. Oil Chem. Soc. 1960, 37, 271−274. (16) Aylward, F.; Rao, C. V. N. Use of hydrazine as a reducing agent for unsaturated compounds. J. Appl. Chem. 1956, 6, 248−252. (17) Kemp, P.; Lander, D. J.; Gunstone, F. D. The hydrogenation of some cis- and trans-octadecenoic acids to stearic acid by a rumen Fusocillus sp. Br. J. Nutr. 1984, 52, 165−170. (18) Mendes, M. J.; Santos, O. A. A.; Jordão, E.; Silva, A. M. Hydrogenation of oleic acid over ruthenium catalysts. Appl. Catal., A 2001, 217, 253−262. (19) Cheah, K. Y.; Tang, T. S.; Mizukami, F.; Niwa, S.; Toba, M.; Choo, Y. M. Selective hydrogenation of oleic acid to 9-octadecen-1-ol: Catalyst preparation and optimum reaction conditions. J. Am. Oil Chem. Soc. 1992, 69, 410−416. (20) Kluson, P.; Cerveny, L. Selective hydrogenation over ruthenium catalysts. Appl. Catal., A 1995, 128, 13−31. (21) Sánchez, M. A.; Pouilloux, Y.; Mazzieri, V. A.; Pieck, C. L. Influence of the operating conditions and kinetic analysis of the selective hydrogenation of oleic acid on Ru−Sn−B/Al2O3 catalysts. Appl. Catal., A 2013, 467, 552−558. (22) Costa, C. M. M.; Jordão, E.; Mendes, M. J.; Santos, O. A. A.; Bozon-Verduraz, F. Hydrogenation of oleic acid over sol-gel ruthenium catalysts. React. Kinet. Catal. Lett. 1999, 66, 155−162. (23) Richter, J. D.; van den Berg, P. J. Hydrogenation of unsaturated fatty acids to unsaturated fatty alcohols: I. Study of Cu and Cd oleates as catalysts. J. Am. Oil Chem. Soc. 1969, 46, 155−157. (24) Mingotaud, A. F.; Mingotaud, C.; Moussa, W. Characterization of the micellar ring opening metathesis polymerization in water of a norbornene derivative initiated by Hoveyda−Grubbs’ catalyst. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 2833−2844. (25) Zelin, J.; Trasarti, A. F.; Apesteguía, C. R. Self-metathesis of methyl oleate on silica-supported Hoveyda−Grubbs catalysts. Catal. Commun. 2013, 42, 84−88. (26) Liu, Y. Versatile routes for acrylonitrile butadiene rubber latex hydrogenation. PhD Thesis, University of Waterloo, Canada, 2012.
some insights for the catalytic hydrogenation of rubber latex and thereby facilitate the commercialization of “green” latex hydrogenation in industry.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.oprd.6b00074. IR and 1H NMR spectra characterizing the hydrogenation progress of sodium oleate from 1 to 5 h (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: +1 519 8884567 ext. 32702. Fax: +1 519 7464979. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge LANXESS Deutschland GmbH for supporting the current project. We thank Mr. Robert Harris (University of Guelph, Canada) for his assistance with the TEM operation.
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REFERENCES
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