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Effect of chemically reduced palladium supported catalyst on sunflower oil hydrogenation conversion and selectivity
⁎Corresponding author. Tel.: +60 1123526746; fax: +60 389216148. Majeedsh70@yahoo.com (Abdulmajid Alshaibani)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Catalytic hydrogenation of sunflower oil was studied in order to improve the conversion and to reduce the trans-isomerization selectivity. The hydrogenation was performed using Pd–B/γ-Al2O3 prepared catalyst and Pd/Al2O3 commercial catalyst under similar conditions. The Pd–B/γ-Al2O3 catalyst was prepared by wet impregnation and chemical reduction processes. It was characterized by Brunauer–Emmett–Teller surface area analysis (BET), X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The result of sunflower oil hydrogenation on Pd–B/γ-Al2O3 catalyst showed a 17% higher conversion and a 23% lower trans-isomerization selectivity compared to the commercial Pd/Al2O3 catalyst. The chemical reduction of palladium supported catalyst using potassium borohydride (KBH4) has affected the Pd–B/γ-Al2O3 catalyst’s structure and particle size. These most likely influenced its catalytic performance toward higher conversion and lower trans-isomerization selectivity.
Keywords
Hydrogenation
Palladium
Sunflower oil
Trans-fatty acids
1 Introduction
The hydrogenation of vegetable oils is important for both food and non-food applications (Ravasio et al., 2002). Hydrogenation increases oxidation resistance, improves shelf life and changes the thermal behavior of the oil, making it suitable for specific uses (Fernandez et al., 2009).
As a result of the undesirable health effects of trans-fatty acids (TFAs), customers’ awareness of hydrogenated oils has increased. The issues with TFAs have increased the impetus to discover viable alternatives that can be accessed through catalyst development (Belkacemi et al., 2006). Palladium (Pd) has been reported to have unique features for catalytic reactions due to its high conversion under moderate conditions and the ability to alter its behavior by adding modifiers (Polshettiwara et al., 2009).
Alloy materials have recently been the focus of much study as new catalysts for many reactions, especially hydrogenation. Alloys have different chemical and physical properties than the individual component metals (Takahashi and Kai, 1999).These properties provide a more active and selective catalytic performance (Takahashi and Kai, 1999; Wang et al., 2004). Nickel-based alloy catalysts that exhibit excellent activity in hydrogenation reactions have been reported by many researchers (Wang et al., 2004; Li et al., 2009). The use of Pd alloy materials, however, is rare reported in the field of hydrogenation. A Pd–B/SiO2 amorphous catalyst was prepared and used for nitrobenzene hydrogenation for the first time in 2000 (Yu et al., 2000). However, Pd–B/γ-Al2O3 has been reported previously in the field of hydrocarbon hydrogenation (Xiong et al., 2007; Zou et al., 2007a).
Many studies have considered the benefits of Pd catalysts for edible oil hydrogenation especially for high conversion and moderate trans-fatty acids production (Fernandez et al., 2009; Tonetto et al., 2009). Modified Pd can achieve compromise between the catalyst activity and trans-isomerization selectivity Si (Belkacemi et al., 2007). A literature search found no studies for using Pd–B/γ-Al2O3 catalysts in the field of edible oil hydrogenation.
The aim of this study is to investigate the use of a Pd–B/γ-Al2O3 alloy catalyst to increase conversion and reduce Si for hydrogenation of sunflower oil.
2 Materials and methods
2.1 Catalyst preparation
The Pd/γ-Al2O3 catalyst was prepared by the wet impregnation method used previously by Tonetto et al. (2009). The method was altered slightly by preparing the catalyst in the absence of nitrogen (N2) or argon (Ar) gases during drying of the support and calcination of the catalyst. γ-Al2O3 (Alfa Aesar, 26 Parkridge Rd Ward Hill, USA) was used as a support and Pd (acac)2 (Johnson Matthey, New West Drive, Pasadena, Texas, USA) was used as a Pd precursor. The Pd–B/γ-Al2O3 was prepared by the chemical reduction of the Pd/γ-Al2O3 catalyst using 0.2 M KBH4 (Acros Organics, Belgium, USA) solution. For 1% theoretical Pd content in the Pd/γ-Al2O3 catalyst, 0.31% boron was used to achieve total reduction for palladium oxide (PdO). The KBH4 solution was added dropwise to Pd/γ-Al2O3 under good agitation in a cold bath until the hydrogen bubbles ceased. The catalyst was washed with distilled water and then with alcohol before thermal treatment (Xiong et al., 2007). Pd–B/γ-Al2O3 annealing was performed at 873 K for 2 h in the presence of air.
It is worth mentioning that, unlike other oil hydrogenation catalysts, Pd–B/γ-Al2O3 is a highly stable catalyst and does not require storage in an inert atmosphere or saturated fat to prevent oxidation.
2.2 Catalyst characterization
Catalyst characterization was performed for the catalyst before and after chemical reduction (for Pd/γ-Al2O3 and Pd–B/γ-Al2O3, respectively). The Brunauer–Emmett–Teller (BET, Micromeritics ASAP 2020) surface area was determined using a method similar to that reported in Belkacemi et al. (2006). The structure of the prepared catalyst was characterized by X-ray powder diffraction (XRD, Bruker, D8-Advance with Cu Kα radiation). Its surface morphology was observed by scanning electron microscopy (SEM, INCAx-sight-7353, Oxford Instruments), including the energy-dispersive X-ray spectroscopy (EDX) application for catalyst metal content measurement. Transmission electron microscopy (TEM) was performed on a CM12 transmission electron microscope (Philips).
2.3 Catalyst activity measurements
The catalyst test was performed in a 400 ml batch reactor using refined, bleached and deodorized sunflower oil supplied by the Yemen Company for Ghee and Soap Industry (YCGSI). The initial iodine value of the oil was 125 (g iodine per 100 g oil), and the fatty acid composition (%) was C18: 0 = 3.0, C18: 1 = 30.22, C18: 2 = 56.42 and C18: 3 = 0.1, where the first number indicates the total carbon number of the fatty acid and the second number indicates the total number of C⚌C double bonds. The study focus was the C18: x group of fatty acids because they are influenced by the hydrogenation reaction. Differences between the C18: x distributions in the hydrogenated and non-hydrogenated sunflower oils were used to determine the overall hydrogenation conversion.
The reactor was supplied with a constant pressure of chromatographic grade hydrogen gas, as required for the test.
The catalytic test for Pd–B/γ-Al2O3 was performed at 373 K, 413.5 kPa and 1400 rpm for 1 h using 80 ml of sunflower oil. A commercial Pd/Al2O3 catalyst (BASF, Netherlands) was used for comparison purpose. The catalyst dose employed in the reaction was 0.1 mg Pd per ml oil.
Contrary to the normal process for supported palladium catalysts, pre-reduction was not performed for the Pd–B/γ-Al2O3 catalyst due to its high oxidative stability.
The C⚌C conversion and trans-isomerization selectivity (Si) were calculated according to the methods previously reported by Tonetto et al. (2009) and Ramirez et al. (2011), respectively. The analyses of fatty acid composition, trans-fatty acids and iodine value (IV) were performed using the corresponding American Oil Chemists’ Society test methods (AOCS, 2011).
3 Results and discussion
3.1 Catalyst characterization
Fig. 1 demonstrates the nitrogen adsorption–desorption isotherms of the Pd–B/γ-Al2O3 catalyst sample. The shape of the adsorption isotherms is a Type 4 in the Brunauer, Deming and Teller classification corresponding to a mesoporous structure (Fernandez et al., 2009). The results showed that the BET surface area was reduced from164 m2/g for Pd/γ-Al2O3 to 135 m2/g for Pd–B/γ-Al2O3. This reduction in surface area could be attributed to (1) an increase in the metal loading resulting from chemical reduction, or (2) the thermal treatment at 873 K causing the sintering and gathering of Pd–B alloy particles (Bernas et al., 2002; Piqueras et al., 2006). The pore diameter, however, increased from 6.3 nm for Pd/γ-Al2O3 to 7.4 nm for Pd–B/γ-Al2O3. This increase can be attributed to the thermal treatment performed for Pd–B/γ-Al2O3. The pore diameter of Pd–B/γ-Al2O3 was considered to be suitable for oil hydrogenation according to the Coenen classification, in which the triglyceride molecule easily enters and exits a slot of the pore of the support (Coenen, 1986).
Nitrogen adsorption–desorption isotherms for the Pd–B/γ-Al2O3 catalyst.
Fig. 2 shows the XRD patterns of γ-Al2O3, Pd/γ-Al2O3 and Pd–B/γ-Al2O3 where the peaks at 2θ = 37°, 45.8° and 67.3° are representing Al2O3 (Gao et al., 2008; Chen et al., 2010). No noticeable difference was observed between the pattern of alumina (a) and Pd/γ-Al2O3 (b), possibly due to the low Pd content in Pd/γ-Al2O3 (Gao et al., 2008). Diffraction peak at 2θ = 33.8° appeared in the pattern (c) shows the Pd2B alloy was formed as a result of chemical reduction of Pd/γ-Al2O3 by KBH4. Appearance of this peak was a result of Pd–B crystalline structure formation which took place at temperature higher than 573 K (Ma et al., 2010).
XRD diffraction patterns of (a) γ-Al2O3, (b) Pd/γ-Al2O3, (c) Pd–B/γ-Al2O3.
The morphology structure of the Pd–B/γ-Al2O3 catalyst was further studied by SEM. Pure, clean, and small particles appeared on the surface of Pd/γ-Al2O3, as shown in Fig. 3a. This finding reveals a high degree of gathering of the Pd/γ-Al2O3 particles due to the post-thermal treatment for this catalyst sample at 773 K. However, Pd–B/γ-Al2O3 (Fig. 3b) exhibited a large crack on the catalyst surface, indicating a high degree of crystallization, which was attributed to the chemical reduction and the thermal treatment performed at 873 K. The Pd–B/γ-Al2O3 exhibited a good distribution for Pd–B sites on the alumina support as shown in Fig. 3c and Fig. 3d. The EDX analysis detected both Pd and B, confirming the existence of Pd–B alloys on the catalyst particles, the average content of palladium and boron was 0.73% and 0.075%, respectively.
SEM image of: (a) Pd/γ-Al2O3, (b) Pd–B/γ-Al2O3, (c) palladium distribution on the alumina support, and (d) boron distribution on the alumina support.
As shown in the TEM images in Fig. 4a, the average particle size of Pd/γ-Al2O3 (Fig. 4a) was 5.2 nm which increased to 8.2 nm for Pd–B/γ-Al2O3 (Fig. 4b) after chemical reduction. This difference can be attributed to the aggregation and growth of metal particles during chemical reduction and thermal treatment.
TEM image of (a) Pd/γ-Al2O3, and (b) Pd–B/γ-Al2O3 catalyst.
3.2 Catalyst activity measurement
Fig. 5 depicts the C18: x fatty acid composition for the non-hydrogenated and hydrogenated sunflower oil on Pd–B/γ-Al2O3 and commercial Pd/Al2O3. Due to its small content (less than 1%), the linolenic acid C18: 3 was completely hydrogenated shortly after beginning the reaction on both catalysts. Linoleic acid C18: 2 exhibited a higher depletion on Pd–B/γ-Al2O3 than on Pd/Al2O3, whereas oleic acid C18: 1 exhibited the inverse behavior. Furthermore, the Pd–B/γ-Al2O3 produced significantly more stearic acid C18: 0 than Pd/Al2O3 under similar reaction conditions, indicating a higher saturation conversion. The content of TFAs in sunflower oil was negligible, but it increased to 4.5 and 4.9% as a result of isomerization on Pd–B/γ-Al2O3 and Pd/Al2O3 commercial catalyst respectively, which affects the trans-isomerization selectivity.
Fatty acid composition of non-hydrogenated sunflower oil (A), and hydrogenated samples on both Pd–B/γ Al2O3 (B) and on commercial Pd/Al2O3 (C). Reaction conditions are Temperature of 393 K, Pressure of 413.5 kPa, agitation of 1400 rpm, and catalyst dose of 0.1 mg Pd/ml oil for one hour.
The results of hydrogenation conversion and Si for both catalysts are shown in Table 1. The IV and reaction conversion reveal a higher activity for Pd–B/γ-Al2O3 compared to the commercial Pd/Al2O3 catalyst. In addition, the results of Si for Pd–B/γ-Al2O3 show a lower tendency to form TFAs. The previous results were also compared with the published results of sunflower oil hydrogenation on magnesium modified palladium (Pd–Mg/γ-Al2O3) under similar conditions by Tonetto et al. (2009). As shown in Table 1, the Pd–B/γ-Al2O3 exhibits much higher activity and lower Si than the palladium modified and non-modified catalysts. Experimental conditions: T = 373 K, P = 413.5 kPa, agitation = 1400 rpm, catalyst dose = 0.1mgPd/
and time = 1 h.
Parameter
Unit
Catalyst
Pd–B/γ-Al2O3 (Prepared)
Pd/γ-Al2O3 (Commercial)
Pd/γ-Al2O3a (Published)
Pd–Mg/γ-Al2O3a (Published)
Pd content
%
0.73
5
0.78
0.78
Catalyst dose
mgPd/mloil
0.1
0.1
0.1
0.1
Iodine value
giodine/100goil
34
48
99.2
109.4
Conversion
%
73
62
17.4
8.8
Trans-selectivity (Si)
%
4.9
6.4
16.7
19.1
The higher Pd–B/γ-Al2O3 activity was attributed to its higher capability for hydrogen adsorption and the higher concentration of active sites resulting from the nature of the Pd–B alloy (Zou et al., 2007a; Ding et al., 2003). However, the lower Si of Pd–B/γ-Al2O3 may correlate with the effect of the metalloid element (boron) as an electron donor that changes the electron density of the Pd atom (Zou et al., 2007b).
Furthermore, the Pd–B/γ-Al2O3 catalyst activity was consistent with the reported results about the impact of the particle size in a similar range from 1.9 to 12.1 nm (Piqueras et al., 2006).
4 Conclusion
Pd–B/γ-Al2O3 was prepared by wet impregnation and chemical reduction. It exhibited higher conversion and lower trans-selectivity than commercial Pd/Al2O3 and the previously published Pd–Mg/γ-Al2O3. Pd–B/γ-Al2O3 behaves differently than the palladium modified and non-modified catalysts that it does not need an inert atmosphere during preparation and calcination. It is also highly stable against oxidation. Unlike other catalysts, it does not need to be stored in highly saturated fat before application.
Acknowledgements
The authors acknowledge the financial support provided by the Universiti Kebangsaan Malaysia (UKM) by fund (MOSTI 03-01-02-SF0696), the Yemen Company for Ghee and Soap Industry (YCGSI) and the Nutrition Unit, Product Development and Advisory Services Division, the Malaysian Palm Oil Board (MPOB).
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