Hydrogenation of soybean oil over various platinum catalysts: Effects of support materials on trans fatty acid levels

Catalysis Communications 62 (2015) 1–5

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Hydrogenation of soybean oil over various platinum catalysts: Effects of support materials on trans fatty acid levels Hajime Iida ⁎, Daigo Itoh, Satoshi Minowa, Atsushi Yanagisawa, Akira Igarashi Department of Applied Chemistry, Faculty of Engineering, Kogakuin University, 2665-1 Nakano-machi, Hachioji-shi, Tokyo 192-0015, Japan

a r t i c l e

i n f o

Article history: Received 1 November 2014 Received in revised form 28 December 2014 Accepted 29 December 2014 Available online 2 January 2015 Keywords: Hydrogenation of edible oils Trans fatty acid Pt/BaSO4 Electronegativity

a b s t r a c t The effects of various support materials on the catalytic performance of supported platinum catalysts for the hydrogenation of soybean oil were examined. There was a linear relationship between the catalytic activity and the platinum dispersion of the platinum catalysts. Among the examined catalysts, Pt/BaSO4 was effective for the reduction of both trans fatty acid (TFA) and additional saturated fatty acid (ASFA) levels in partially hydrogenated oils (iodine value (IV) = 70). In addition, the relationship between the TFA levels and the electronegativity of the metal ion in the support material was a volcano function. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The hydrogenation of vegetable oils is an important chemical process in the modification of fats and oils. The purpose of this process is to increase the melting characteristics and improve the oxidation stability of the liquid oils [1]. The process was patented in 1903 [2] and first commercialized by Procter & Gamble in 1911. As such, the use of margarine and vegetable shortenings rapidly increased as butter substitutes since the 1940s [3]. The hydrogenation processing of oils involves three reactions; the saturation of double bonds and both geometric (cis–trans) and positional isomerization. The chemical and physical properties of the hydrogenated oils are significantly influenced by the degree of unsaturation and the cis–trans isomerization of fatty acids. Trans fatty acids (TFAs), which are formed by geometric isomerization, have been associated with an increased risk of coronary heart disease (CHD) and several public health organizations have recommended that the intake of TFA be lowered as much as possible. This had led to a demand for reduced TFA formation during hydrogenation [4,5]. In the current hydrogenation technology for fats and oils, nickel catalysts are immersed in vegetable oils at temperatures of 393–473 K in the presence of hydrogen at approximately 0.1–0.5 MPa [6]. Lower temperatures and higher pressures result in lower TFA levels [7]; however, it has not been possible to produce partially hydrogenated oils with low TFA levels. There have been numerous studies for hydrogenation of vegetable oils over supported metal catalysts, with many reports on the various modifications of nickel, palladium, and platinum catalysts to reduce TFA levels during hydrogenation [8–18]. For example, Li ⁎ Corresponding author. E-mail address: [email protected] (H. Iida).

http://dx.doi.org/10.1016/j.catcom.2014.12.025 1566-7367/© 2015 Elsevier B.V. All rights reserved.

et al. reported that a nickel–boron alloy catalyst has a lower TFA selectivity for the hydrogenation of soybean oil [8]. Fernández et al. reported that the cis–isomer selectivity was improved by the molybdenum addition to Pd/Al2O3 prepared by sol–gel method [9]. McArdle et al. reported a platinum catalyst supported on mesoporous silica that had high catalytic activity and low TFA selectivity for the hydrogenation of sunflower oil [10]. In addition, they reported that the addition of nickel to Pt/SiO2 catalyst improved the cis–isomer selectivity [11]. Hsu et al. have reported a Pd/Al2O3 catalyst with higher activity and lower TFA selectivity for the hydrogenation of canola oil than that for Pd/C and Pd/BaSO4 catalysts [12]. The effects of the support material on the catalytic performance for the hydrogenation of vegetable oils over supported metal catalysts have long since been investigated; however there is still room for a systematic investigation of catalyst support materials. The relationship between the TFA levels in partially hydrogenated oil and the electronegativity of metal ions in the support materials was systematically investigated for the hydrogenation of vegetable oils over platinum catalysts. 2. Experimental 2.1. Catalyst preparation Supported platinum catalysts were prepared by a conventional impregnation method. Al2O3, ZrO2, CeO2, TiO2, MgO, and MoO3 − x support materials were obtained by calcining JRC-ALO-6, JRC-ZRO-3, JRC-CEO-2, JRC-TIO-4, JRC-TIO-9, JRC-MGO-4 500A (reference catalysts of the Catalysis Society of Japan), and MoO3 (Wako Pure Chemical Industries) at 673 K for 1 h in a stream of air, respectively. Colloidal silica (CARiACT Q-6, Fuji Silysia)

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H. Iida et al. / Catalysis Communications 62 (2015) 1–5

Table 1 Catalytic properties and performance of various supported platinum catalysts. Catalyst

BET surface area (m2 g−1)

Metal ion electronegativity in support material (−)

Platinum dispersion (%)

Catalyst concentration CPt × 104 (g−Pt g−1 −oil)

Reaction rate constant k (s−1 g−oil g−1 Pt )

TOF (s−1)

Pt/C Pt/ZrO2 Pt/Al2O3 Pt/SiO2 (Q-6) Pt/SiO2 (MCM41) Pt/SiO2 (SBA15) Pt/TiO2 (TIO-4) P/TiO2 (TIO-9) Pt/CeO2 Pt/MgO Pt/BaSO4 Pt/CaO Pt/MoO3 − x

712 94 177 268 839 644 53 91 122 20 1 8 3

– 12.0 11.3 17.1 17.1 17.1 13.9 13.9 7.8 6.6 4.5 5.5 19.2

40.0 38.5 31.6 23.0 29.9 36.8 14.1 43.8 – 6.0 2.6 1.8 0.05

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 2.0 2.0 2.0 2.0 2.0

14.1 11.1 9.3 9.0 7.5 12.3 10.2 11.8 2.8 1.7 2.8 0.3 0.3

35 29 30 39 25 33 73 27 – 29 108 19 529

and mesoporous silica (SBA15 and MCM41) were used as SiO2 supports. The MCM41 and SBA15 were prepared by a hydrothermal method [19]. A CaO support was obtained by calcining CaCO3 (Wako Pure Chemical Industries) at 1173 K for 1 h in a stream of helium. BaSO4 as a support was prepared by the precipitation method using BaCl2 and Na2SO4 (Wako Pure Chemical Industries). Bis(acetylacetonato) platinum(II) (Tanaka Kikinzoku Kogyo) was used as the platinum precursor and platinum was loaded at 5 wt.%. A commercial nickel catalyst (SO-450, Sakai Chemical) for low TFA levels and a commercial Pt/C catalyst (STD, N. E. ChemCat) were used as reference catalysts.

2.2. Activity test Hydrogenation of soybean oil (iodine value (IV) = 130) was performed using a small stainless steel batch reactor (25 cm3). Prior to the reaction, the catalysts were reduced at 673 K for 1 h. The reaction conditions employed were a catalyst concentration in the soybean oil of 50 or 200 ppm−Pt, a reaction temperature of 413 K, and hydrogen pressure at 0.5 MPa. A stirring rate of 1700 rpm was employed because the apparent reaction rate was constant at the stirring rate above 1500 rpm. The composition of fatty acids in the hydrogenated oil was analyzed using flame ionization detector-gas chromatography (FID-

GC; capillary column, SP-2560, 100 m, 0.25 mm i.d.). Prior to the GC analysis, the triglycerides were converted into their fatty acid methyl esters (FAME) following the American Oil Chemists' Society (AOCS) official method [20]. The iodine value (IV) was estimated by the composition of fatty acids in the hydrogenated oil obtained by the GC analysis, where IV was defined as: IV ¼ M I2

X

Dið100 Cw; i=MiÞ

ð1Þ

where MI2 (=253.8 g mol−1) is the molar mass of I2, Di is the number of double bond in fatty acid i, and Cw, i is the mass concentration of fatty acid i (wt.%). Assuming a first order rate equation with respect to IV, the reaction rate constant k, was calculated using: − ln ðIV t =IV 0 Þ ¼ k C Pt t;

ð2Þ

where IV0 and IVt are the iodine values at t = 0 and t = t (s), 1 −1 k (s− 1 g− oil g− − Pt) is the reaction rate constant, and CPt (g−Pt g−oil) is the catalyst concentration based on platinum. Linear relationships between −ln (IVt/IV0) and CPtt were observed for all of the examined catalysts (data not shown).

Reaction rate constant k (s-1 g-oil g-Pt-1)

2.3. Catalyst characterization

15 SBA15 TIO-4

10 Q-6

5

TIO-9 ZrO2 Al2O3 MCM41

BaSO4

MoO3-x MgO CaO

0

0

10

20 30 40 Pt dispersion (%)

50

Fig. 1. Relationship between the catalytic activity and platinum dispersion of supported platinum catalysts.

The Brunauer–Emmett–Teller (BET) surface area of the newlyprepared catalysts was determined by N2 adsorption at 77 K using a flow absorption apparatus (Flow Sorb II 2300, Micromeritics). The composition of the flow gas was N2:He = 30:70. The catalyst was degassed at 473 K for 15 min prior to the measurement. The textural properties of Pt/SiO2 catalysts were determined from nitrogen adsorption/desorption isotherm measurements at 77 K using a volumetric absorption apparatus (ASAP2010, Micromeritics). The pore size distribution was generated from a Barrett–Joyner–Halenda (BJH) analysis of the adsorption branches. The amount of chemisorbed CO on the catalysts reduced at 673 K was estimated using a volumetric absorption apparatus (ASAP2010, Micromeritics) with a chemisorption unit. Adsorption isotherms for CO adsorbed on the catalysts were obtained at 308 K. The platinum dispersion (DPt) of the catalysts was calculated from the amount of chemisorbed CO, where DPt was defined as: n o DPt ¼ V CO f CO=Pt =0:0224 =fLPt =MPt g;

ð3Þ

1 where VCO (m3−STP g− − cat) is the amount of chemisorbed CO on the catalyst, LCO (−) is the Pt content in the catalyst, fCO/Pt (= 1.0) is the stoichiometric factor for CO chemisorption on Pt, and MPt (=195.1 g mol−1) is the molar mass of Pt.

H. Iida et al. / Catalysis Communications 62 (2015) 1–5

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Table 2 Fatty acid composition in soybean oil hydrogenated (IV = 70) over various Pt catalysts. Fatty acid composition (%)

Soybean oil Pt/BaSO4 Pt/CaO Pt/MgO Pt/CeO2 Pt/Al2O3 Pt/ZrO2 Pt/TiO2 (TIO-9) Pt/TiO2 (TIO-4) Pt/SiO2 (Q-6) Pt/SiO2 (SBA15) Pt/SiO2 (MCM41) Pt/MoO3 − x Pt/C Commercial Ni catalyst

C16:0

C18:0

ASFA

C18:1t

C18:2t

TFA

C18:1c

C18:2c

C18:3

11.7 11.6 14.3 13.2 12.8 13.2 12.2 12.8 14.3 11.9 11.5 11.8 12.0 11.7 12.4

4.1 24.3 19.3 16.5 18.6 15.8 13.5 17.5 15.5 16.6 19.5 20.0 27.0 19.2 12.8

0.0 20.1 17.8 13.9 15.6 13.2 9.9 14.5 14.0 12.7 15.2 16.0 23.2 15.1 9.4

0.0 9.0 11.9 18.8 16.6 25.2 32.1 24.5 24.3 19.7 20.0 18.2 9.0 21.0 23.0

0.0 2.0 2.3 2.5 3.4 5.8 3.0 5.2 4.0 4.8 3.8 5.4 2.0 3.6 0.5

0.0 11.0 14.2 21.3 20.0 31.0 35.1 29.7 28.3 24.5 23.8 23.6 11.0 24.6 23.5

24.5 39.0 39.7 40.3 39.1 34.2 35.0 34.0 33.8 41.6 37.0 36.8 33.0 35.9 45.5

53.0 13.1 12.3 8.7 9.5 5.8 4.2 6.0 8.1 5.4 8.0 7.5 16.0 8.3 5.8

6.7 1.0 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.2 0.3 1.0 0.3 0.0

The turn over frequency (TOF) at t = 0 s was defined as:   −1 ¼ −r C¼C;0 =ðDPt =M Pt ÞNA; −rC¼C;0 ¼ k IV 0 NA =ð100  M I2 Þ; TOF s

ð4Þ

where −rC_C,0 (s−1) is the initial reaction rate, NA (mol−1) is the Avogadro constant, and MI2 (=253.8 g mol−1) is the molar mass of I2. X-ray photoelectron spectroscopy (XPS) measurements were performed using a spectrometer (Quantum 2000, Ulvac Phi) with an X-ray Al anode. The beam diameter was 100 μm and the acceleration voltage was 15 kV. The electronegativity of metal ions in the support materials was defined as follows: χi ¼ ð1 þ 2z Þχ0 ;

ð5Þ

where χ0 and χi are the electronegativity of the metal and electronegativity of the metal ion, respectively, and z is the number of valence electrons of the metal ion [21]. 3. Results and discussion The catalytic properties and performance of the supported platinum catalysts are summarized in Table 1. The platinum dispersion was

0.3

Incremental volume (cm3g-1)

‫ۑ‬: Pt/SiO2 (Q-6) ‫ە‬: Pt/SiO2 (SBA15) 0.2

‫ڧ‬: Pt/SiO2 (MCM41)

0.1

0.0 1

10 Pore diameter (nm)

Fig. 2. Pore volume distributions for Pt/SiO2 (Q-6, SBA15, MCM41) catalysts.

100

decreased in the following order: Pt/TiO2 (TIO-9) N Pt/C N Pt/ZrO2 N Pt/SiO2 (SBA15) N Pt/Al2O3 N Pt/SiO2 (MCM41) N Pt/SiO2 (Q-6) N Pt/ TiO2 (TIO-4) N Pt/MgO N Pt/BaSO 4 N Pt/CaO N Pt/MoO 3 − x . In the case of Pt/CeO2, CO is consumed by the reduction of CeO2, so that the platinum dispersion cannot be precisely estimated from CO adsorption. The platinum dispersion is not exactly dependent on the BET surface area, because the platinum dispersion is substantially influenced by the interaction between the platinum precursor and the surface of the support material. In addition, the reaction rate constant decreased in the following order: Pt/C N Pt/SiO 2 (SBA15) N Pt/TiO 2 (TIO-9) N Pt/ZrO 2 N Pt/TiO 2 (TIO-4) N Pt/Al 2 O 3 N Pt/SiO 2 (Q-6) N Pt/SiO 2 (MCM41) N Pt/CeO 2 , Pt/BaSO 4 N Pt/MgO N Pt/CaO, Pt/MoO 3 − x . Fig. 1 shows the relationship between the catalytic activity and the platinum dispersion. The catalytic activity of the supported platinum catalysts was generally dependent on the platinum dispersion. Although the platinum catalysts generally exhibited TOFs of approximately 30 s−1, the Pt/MoO3 − x, Pt/BaSO4, and Pt/TiO2 (TIO-4) catalysts exhibited higher TOFs of 529, 108, and 73 s−1, respectively. Table 2 shows the fatty acid composition in partially hydrogenated oil (IV = 70) obtained using the supported platinum catalysts. The soybean oil used as a feedstock was composed of 6.7% linolenate (C18:3c), 53.0% linoleate (C18:2c), 24.5% oleate (C18:1c), and 15.8% saturated fatty acids (SFAs; stearate (C18:0) and palmitate (C16:0)). TFAs (C18:1t and C18:2t) and additional SFA (ASFA) were generated by the hydrogenation reaction and were contained in the hydrogenated oils produced using supported platinum catalysts and a commercial nickel catalyst. The levels of TFA generally decreased with increasing ASFA, C18:3, and C18:2c levels. Several researchers have reported the effects of the support on the levels of TFA in hydrogenated oils. In most cases, different support characteristics have little to significant effect on the TFA levels [22–24]. However, Nohair et al. reported that Pd/MgO, Pd/ZnO, Pd/CeO2, and Pd/CeZrO2 produced higher levels of TFA than Pd/TiO2 and Pd/SiO2, although these differences could not be directly related to the physical and chemical properties of the support materials, such as their acid–base properties and specific surface areas [25]. In the present study, the levels of TFA in the hydrogenated oils (IV = 70) were decreased using Pt/BaSO4, Pt/CaO, and Pt/MoO3 catalysts among the examined platinum catalysts. Fig. 2 shows the pore volume distributions for the Pt/SiO2 catalysts (Q-6, SBA15, and MCM41), which those for SBA15 and MCM41 are sharper than that for Q-6. The mode pore diameters for SBA15, MCM41, and Q-6 were 5.2, 2.6, and 6.1 nm, respectively. However, the fatty acid composition of the hydrogenated oils (IV = 70) catalyzed with Pt/Q-6 was similar to that obtained using the Pt/SBA-15 and Pt/ MCM-41 catalysts. These results indicate that the fatty acid composition is not directly related to the microstructure of the catalyst support materials.

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H. Iida et al. / Catalysis Communications 62 (2015) 1–5

40

40

ZrO2

(a)

35 TIO-9

Al2O3

30

30

SBA15 Q-6 MgO CeO MCM41 TIO-4

25 20

ASFA (%)

TFA (%)

(b)

35

2

CaO

15

BaSO4

10

MoO3-x

25

BaSO4

20

MCM41 CeO2 TIO-9 CaO Al2O3 SBA15 Q-6 MgO TIO-4 ZrO2

15 10

MoO3-x

5

5

0

0 0

5 10 15 20 Metal ion electronegativity (-)

25

0

5 10 15 20 25 Metal ion electronegativity (-)

Fig. 3. Relationship between the TFA and ASFA levels and the electronegativity of metal ions in the support materials; (a) TFA, (b) ASFA.

Fig. 3 shows the relationship between the TFA and ASFA levels and the electronegativity of the metal ion in the support. There is a volcano function relationship between the TFA levels and the metal ion electronegativity, whereas the supported platinum catalysts with moderate electronegativity had lower ASFA levels among the examined platinum catalysts. The change in the ASFA levels with the electronegativity of the metal ion in the support material is smaller than that observed for the TFA levels. Fig. 4 shows XPS spectra of the Pt4f region for the supported platinum catalysts. For the Pt/C catalyst, the peaks in this region were assigned to Pt0 (4f7/2 = 71.2 eV, 4f5/2 = 74.5 eV). The spectrum for Pt/ Al2O3 contained peaks from Pt0 and an overlapping Al2p peak. For most of the examined platinum catalysts, the peaks were shifted to slightly lower binding energy with respect to Pt0. From these results, it may be inferred that electrons are donated from the support to platinum, and that platinum is present as (Pt)σn − on the catalyst support [26]. Fig. 5 shows the relationship between the binding energy of Pt4f7/2 and the metal ion electronegativity of the support. The binding energy

of Pt4f decreased with the electronegativity of the metal ion. Therefore, the platinum was negatively charged by electron donation from the lower electronegativity metal in the support material. From these results, the volcano function relationship in Fig. 3(a) can be explained as follows. The two important factors that influence the TFA levels are the adsorption strength of hydrogen and adsorption strength of the double bonds of fatty acids over platinum particles. Oudenhuijzen et al. conducted density functional theory (DFT) calculations and reported that the adsorption energy of hydrogen increases for Pt particles on basic support materials [27]. Therefore, on the left-hand side of the volcano curve, the adsorption strength of hydrogen over platinum is increased with a decrease in the electronegativity of the metal ions in the support. Simultaneously, the adsorption strength of fatty acid double bonds is decreased with increasing electron density of platinum particles because the double bonds act as Lewis base sites. Nohair et al. also reported that an increase in the electron density of the metal particles weaken the adsorption of unsaturated fatty acid on the surface of catalysts [13,25]. In addition, Boitiaux et al. reported that the electron-attracting additives cause the increase of the adsorption

71.0

(c)

(d) (e) (f) (g) (h)

Binding energy of Pt4f7/2 (eV)

(a) (b)

Intensity (a. u.)

Intensity (a. u.)

ZrO2

70.8 CeO2

Al2O3

70.6

TiO2 (TIO-9)

CaO 70.4

SiO2 (Q-6)

BaSO4

70.2

70.0

80 75 70 65 Binding Energy (eV)

80 75 70 65 Binding Energy (eV)

Fig. 4. XPS spectra for Pt4f of the supported platinum catalysts; (a) Pt/BaSO4, (b) Pt/TiO2 (TIO-4), (c) Pt/C, (d) Pt/CeO2, (e) Pt/CaO, (f) Pt/ZrO2, (g) Pt/SiO2 (Q-6), and (h) Pt/Al2O3.

0

5

10

15

20

Metal ion electronegativity (-) Fig. 5. Relationship between the binding energy of Pt4f7/2 in the XPS spectra and the metal ion electronegativity of the support.

H. Iida et al. / Catalysis Communications 62 (2015) 1–5

strength of the hydrocarbon [28]. Therefore, the strength of double bonds is decreased with a decrease in the electronegativity of the metal ions in the support because the electron density of platinum particles is increased with a decrease in the electronegativity of the metal ions in support. McArdle et al. proposed that the an increase in the electron donor characteristic by the addition of Ni to Pt would weaken the adsorption strength of double bonds, which result in the decrease in the TFA levels [11]. Thus, stronger hydrogen adsorption and weaker unsaturated fatty acid adsorption would result in the lower TFA levels because the half-hydrogenated intermediates are hydrogenated before the intermediates loose the hydrogen and form trans double bonds. In contrast, on the right-hand side of the volcano curve, an increase in the metal ion electronegativity causes an increase in the electron deficiency of platinum particles, which results in too strong adsorption of the fatty acid double bonds. In this case, the intermediates are retained over platinum particles until complete hydrogenation of the intermediates, which results in a decrease of the TFA levels. For supported platinum catalysts prepared using supports with lower electronegativity metal ions, the stronger adsorption strength of hydrogen and weaker adsorption strength of double bonds of hydrocarbon cause a decrease in the TFA levels. On the other hand, the supported platinum catalysts prepared using supports with higher electronegativity metal ions cause stronger adsorption strength of fatty acid double bonds and thus a decrease in TFA levels. Consequently, the TFA levels were the highest for the supports with medium electronegativity metal ions.

Acknowledgments The authors would like to thank Prof. Tsuyoshi Kugita of the Teikyo University of Science & Technology for preparation of the mesoporous silica (SBA15 and MCM41). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2014.12.025. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

4. Conclusion The effects of the support material on the catalytic performance of supported platinum catalysts for the hydrogenation of soybean oil were examined. There was a linear relationship between the catalytic activity and the platinum dispersion of the platinum catalysts. The Pt/ BaSO4 catalyst was the most effective for the reduction of TFA and ASFA levels in hydrogenated oils at IV = 70 among the examined platinum catalysts. In addition, there was a volcano function relationship between the TFA levels and the electronegativity of the metal ions in the support materials. The TFA levels were the highest for the medium electronegativity metal ion supports. These results indicate that the electronic interactions between the support and platinum have a significant effect on the TFA levels in partially hydrogenated oils.

5

[15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]

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