Effects of ZnO on Characteristics and Selectivity of Coprecipitated Ni

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Article

Effects of ZnO on characteristics and selectivity of co-precipitated Ni/ZnO/AlO catalysts for partial hydrogenation of sunflower oil 2

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Farng Hui Wong, Timm Joyce Tiong, Loong Kong Leong, Kuen-Song Lin, and Yeow Hong Yap Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04963 • Publication Date (Web): 21 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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Industrial & Engineering Chemistry Research

Effects of ZnO on characteristics and selectivity of co-precipitated Ni/ZnO/Al2O3 catalysts for partial hydrogenation of sunflower oil

Farng Hui Wonga, Timm Joyce Tiongb, Loong Kong Leonga, Kuen-Song Linc, Yeow Hong Yapa,* a

Department of Chemical Engineering, Lee Kong Chian Faculty of Engineering and Science,

Universiti Tunku Abdul Rahman, Sungai Long Campus, Jalan Sungai Long, Bandar Sungai Long, Cheras 43000, Kajang, Selangor, Malaysia. b

Department of Chemical and Environmental Engineering, Faculty of Engineering, University of

Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Malaysia. c

Department of Chemical Engineering and Materials Science/Environmental Technology

Research Center, Yuan Ze University, Chungli District, Taoyuan City 32003, Taiwan, ROC.

*

Corresponding author: [email protected]

Abstract Ni/ZnO/Al2O3 catalysts at different molar ratio were synthesised, characterised and the catalytic performance were assessed by partial hydrogenation of sunflower oil. Ni:Zn at various molar ratio have resulted in varying degree of Ni and Zn migration from the bulk to the catalyst surface, which led to different pore characteristics. Catalyst with equimolar Ni:Zn has shown wider pores and higher pore volume, and achieved 23% higher hydrogenation activity than Ni/Al2O3. The presence of ZnO also led to higher Ni surface area and dispersion, and TEM image has indicated that this could be attributed to ZnO that acts as spacer for the Ni crystallites. However, Ni surface area diminish as the ZnO content becomes higher. The presence of ZnO in Ni/ZnO/Al2O3 catalysts is also shown to lower trans-fats formation when wide pores are available.

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Keywords Nickel-Zinc catalyst, partial hydrogenation, trans-fats, saturated fats, pore size, selectivity

1.0

Introduction

Melting point is an important property in edible oils, as it determines the sensory-enhancing properties and shelf-life of a product. One common way to improve the melting profile is to perform partial hydrogenation of edible oil to reduce the number of carbon double bonds on the fatty acids. Two typical products where partial hydrogenation are performed are cocoa butter substitute (for chocolate production) and milk fats replacer (for dairy products production). Here, an iodine value (IV) of 1 – 7 with saturated fatty acids content of 35 – 60 wt% (cocoa butter substitute) or 18 – 25 wt% (milk fats replacer) is achieved, in order to achieve melting point of 33 – 35 °C, which enables them to remain solid at room temperature but would melt and release flavours at human body temperature, achieving the designated texture and taste 1,2. During partial hydrogenation of oils, isomerisation could take place, leading to trans-fats formation. Despite the health concerns arising from high trans-fats consumption

3,4

, trans-fats are desirable in certain

applications for their ability to produce steep melting profile 5. Some examples of trans-fats composition in commercial products include 35 wt% in frying oils 6 and 46 wt% in cocoa butter replacer 7 in terms of percent of total fatty acids. Nickel catalyst is one of the most commonly used catalysts for partial hydrogenation process. Although partial hydrogenation of edible oils with nickel catalyst is widely applied in industrial processes, there are still gaps for improvement in terms of the selectivity and activity of the reaction, with respect to the reduction of unhealthy trans-fats content, enhancement of melting profile and the improvement of process efficiency. Nickel catalyst is also prone to deactivation by free fatty acids and other radicals in the feedstock to form nickel soap 8, which will reduce the catalytic activity and lead to risk of contamination in the end-product. There would be less risk of nickel soap formation and significant savings in the operating cost if nickel content can be reduced or substituted with cheaper catalytic materials without sacrificing the catalytic performance.


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Attempts to improve selectivity and activity of partial hydrogenation of vegetable oils with nickel catalyst are focused on altering the reaction conditions and addition of different metals in the catalyst. For example, operating at lower temperature, higher hydrogen pressure and higher stirring intensities, which would result in lower trans-fats content. These conditions, though effective, would also result in lower activity and higher saturated fats content 9,10 as well as being limited by mechanical constraints during implementation. Most cases in the industry have a set guidelines on the optimised operating conditions, inhibiting further alterations in reaction pathways for the catalyst. Hence, further improvements in the catalytic performance can only be obtained from modification of the nickel catalyst with promoters or other metals. The addition of other metals were said to alter the pore structure of the catalysts, which would subsequently result in significant change in the activity and selectivity of reaction. Partial hydrogenation of vegetable oils involve bulky triglyceride molecules. Synthesising catalysts with a suitable pore size is essential to facilitate proper access of the triglyceride molecules to their respective active sites 11, which will subsequently manipulate the hydrogenation activities 12. The selectivity of triglyceride, on the other hand, was deemed to be affected by the diffusion rate within the pores of catalyst

13

. It is known that the addition and alteration of different catalyst support

could contribute significantly to the pore size of the catalysts. For example, the pore size of alumina-supported Ni catalyst is half the size smaller than silica-supported Ni 14 as well as Pd/TiO2 which has half the pore size of Pd/ZrO2 15. Pore size is also influenced by different catalyst synthesis methods. It was reported that depositionprecipitation method resulted in Ni/SiO2 with larger pores than incipient wetness method 14 while the pore size of alumina support could also be controlled during synthesis by increasing aging time or by using additives such as acid, organic salts of ammonia or water-soluble polymeric additives to achieve smaller pores 16. Concurrently, certain changes in catalyst formulation and composition would also modify the pore characteristics. For example, varying amount of cerium added into nickel catalyst well as addition of silver into nickel catalyst

18

17

as

were reported to slightly change the pore

characteristics. Incorporation of Zn in Ni catalyst was also observed to substantially change the specific surface area of the catalyst

19

. However, there are also instances whereby there is no

effective change in pore characteristics such as when Ni loadings are varied in pure nickel catalyst 3 ACS Paragon Plus Environment


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, change of catalytic metal from Pd to Pt on the same type of support

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as well as additional

presence of Ag in Ni-Mg catalyst supported on diatomite 21. Modification of catalyst formulation by addition of another element to form bimetallic catalyst was also found to alter the electronic structure of the active phases which helped to enhance the selectivity, activity and stability of the catalysts

22,23

. For nickel catalyst, addition of

cerium 17, silver 21 and boron 24 were reported to reduce trans-fats formation but with some loss of activity compared to single-metallic Ni catalyst. The effect of an additional element on selectivity of Ni catalyst was also demonstrated by incorporation of sulphur as a promoter to increase transselectivity 25, which enabled the partially hydrogenated soybean oil to have steeper melting profile similar to that of cocoa butter in order to act as a low-cost cocoa butter replacer albeit being deleterious to health. The addition of Zn into Ni catalysts for hydrogenation has been studied by several researchers. For example, Zn added in trace amount was found to improve the activity of Ni/SiO2 catalysts for hydrogenation of fish oil and soybean oil 26 but the characteristics of the catalyst were not studied and the melting points of the hydrogenated fats were the only indicator of reaction selectivity with no analysis of fats composition. Ni-Zn catalyst was also found to increase the olefin selectivity in the hydrogenation of acetylene 19,27. Application of Ni/ZnO catalyst in partial hydrogenation of sunflower oil with a thorough analysis of fats composition to determine its selectivity has not been studied. Seeing the potential benefits of the addition of Zn into Ni, this work aims to study the influence of ZnO in Ni catalyst in terms of catalyst characteristics and catalytic behaviour (activity and selectivity) in partial hydrogenation of sunflower oil. To achieve that, Ni/ZnO/Al2O3 catalysts at different stoichiometric molar ratio were synthesised, characterised and their catalytic performance evaluated in partial hydrogenation of sunflower oil.

2.0

Materials and Methods 4 ACS Paragon Plus Environment

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2.1

Materials

Olife Sunflower Oil (Sime Darby Food and Marketing Sdn. Bhd., Malaysia) was used for partial hydrogenation. The sunflower oil has iodine value (IV) of 162 with its compositions listed in Table S1 (Supporting Information). All chemicals used were of analytical grade and obtained from Sigma Aldrich Malaysia, unless otherwise stated. Methylene chloride, methanol, hydrochloric acid and potassium hydroxide used for methylation prior to gas chromatography analysis were obtained from Merck, Malaysia.

2.2

Catalyst synthesis

Five catalyst samples were prepared based on stoichiometric molar ratios shown in Table 1. The catalysts with different nickel and zinc precursor molar ratio were prepared by slight modification of the co-precipitation method reported by Ganguli et al. 28 for producing industrial hydrogenation catalyst. Briefly, nickel nitrate hexahydrate (Ni(NO3)2·6H2O) and zinc nitrate hexahydrate (Zn(NO3)2·6H2O) solutions were each prepared according to the molar ratio and the volume of the solutions were varied to obtain concentration of 0.3 M each. The mixture of the solutions was then adjusted and maintained at pH 9.4 by the addition of 4.5 M sodium carbonate (Na2CO3) at room temperature, followed by a dropwise addition of 1.0 M aluminium nitrate nonahydrate (Al(NO3)3·9H2O) solution and aged in the mother liquor at 97 °C for 90 min after which pH 9 was observed, with the formation of Ni-Zn-Al hydrotalcite-like precursor. Throughout precipitation and aging, the slurry was vigorously stirred. The two-step co-precipitation enabled customisation of the conditions for each step whereby the first precipitation step was carried out in excess of precipitant at low temperature to promote the formation of highly dispersed metal nanoparticles while the second step involves ageing with support precursor at high temperature to favour narrower distribution of pore size 29. The slurry formed was filtered and thoroughly washed with 1.5 L of hot deionised water and 300 mL of acetone. The wet cake was then dried in the oven at 100 °C for 1 hour and subsequently activated by hydrogen reduction at 450 °C for 5 hours. Prior to hydrogen reduction process, the reduction tube furnace containing the samples was heated up to 450 °C in nitrogen gas flow for 45 minutes. Finally, the reduced catalyst powder to be used for reaction was coated with saturated fats to prevent oxidation, forming pellets with 22 wt% Ni. 5 ACS Paragon Plus Environment


2.3

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Hydrogenation

Hydrogenation reactions were conducted in a pressure slurry reactor (1.5 L Buchiglasuster EcoClave) connected to a temperature regulator (Huber Unistat Tango Nuevo). Each reaction was carried out with 700 ml of sunflower oil and 2 g of fat-coated catalyst pellets at 180 °C, 5 bar and stirring intensity of 2000 rpm. Reaction temperature was set at 180 °C in consideration of certain catalysts that are inactive at lower temperatures. At pre-set time intervals of 10 to 30 minutes, 5 ml of oil samples were collected from the sampling port. Reaction was ended when IV 70 was achieved or when the reaction time reached 8 hours, whichever comes first.

2.4

Catalyst Characterisation and Product Analyses

Three samples for each catalyst formulation in powder form were pre-reduced and kept in ambient condition to be used for characterisation. X-ray photoelectron spectroscopy (XPS) was conducted with Thermo Fisher Scientific K-Alpha X-ray Photoelectron Spectrometer at the excitation energy of Al Ka (1486.6 eV) to measure the composition of catalyst surface (1 – 12 nm depth). Energy dispersive X-ray spectroscopy (EDXS) analysis was performed with Hitachi S-3400N at 20 kV to probe the elementary composition in the sub-surface (1 μm depth) of the catalysts. Inductivelycoupled plasma optical emission spectroscopy (ICP-OES) analysis was conducted with Perkin Elmer Optical Emission Spectrometer Optima 7000 DV to obtain the bulk composition of the catalyst samples. Three readings were taken for each sample and wash time of 120 seconds was allocated between analyses. Nitrogen adsorption-desorption analysis was performed with Thermo Scientific Sorptomatic to obtain pore information. Samples were degassed at 200 °C overnight prior to analysis and the analysis was carried out at -196 oC (77 K). Average pore size was calculated assuming cylindrical pores, d = 4V/S, where V is the specific pore volume and S is the BET surface area. Pore size distribution curve were determined using BJH method from the desorption branch of isotherm. X-ray diffraction (XRD) analysis was conducted with Shimadzu LabX XRD-6000 with Cu Kα radiation (𝜆 = 1.54 Å) operated at 30 mA and 40 kV. The samples were analysed at a rate of 2o min-1 in the range of 2θ = 3o to 80o. Transmission electron microscopy


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(TEM) analysis was conducted with a 200 kV JEOL JEM-ARM200F Transmission Electron Microscope and the image analysis was performed using ImageJ. Temperature-programmed reduction (TPR) analysis was performed with ThermoScientific TPDRO 1100. Sample pre-treatment was done with nitrogen gas flow of 20 cm3/min with heating from room temperature to 200 °C with heating ramp of 10 °C/min and held for 10 minutes. During analysis, 5.47 % hydrogen in nitrogen gas was injected at 25 cm3/min with temperature starting from room temperature to 1000 °C with heating ramp of 5 °C/min. Pulse chemisorption analysis was also conducted with ThermoScientific TPDRO 1100. Samples were degassed in nitrogen gas flow, reduced with hydrogen gas at 450 °C for 5 hours and degassed again prior to analysis. During analysis, 20 pulses of hydrogen gas were performed with 15 minutes interval between each pulse. The metal dispersion and surface area were calculated according to the method published by Bergeret and Gallezot 30. Gas chromatography (GC) analysis was performed on the oil samples with a Perkin Elmer Clarus 500 Gas Chromatography, with a Perkin Elmer COL-ELITE-2560 capillary column (100 m × 0.25 mm ID × 0.20 µm df), operating with 20 cm/s of helium as the carrier gas. Before analysis, the oil samples were esterified into fatty acid methyl esters according to the method by Jham et al. 31

at the optimal conditions reported. Temperatures of the oven, injector and flame ionisation

detector (FID) were set at 175 °C, 210 °C and 250 °C respectively. Volume per injection was 1µL with split ratio of 100:1. The composition of each compound was calculated by taking the relative area of their respective peaks. The iodine value (IV) of each hydrogenated oil sample was tested with Wijs solution according to the American Oil Chemist Society (AOCS) Official Method Tg 1a-64.

3.0

Results and Discussion

3.1

Catalyst Composition

Table 1 summarises the elemental composition for the synthesised catalyst samples prior to fats coating assessed under ICP-OES, XPS, and EDXS. As shown in Table 1, the samples are ranked 7 ACS Paragon Plus Environment


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according to increasing total surface metal Ni+Zn content based on XPS analysis results, as metals on catalyst surface are more directly involved and influential in catalytic properties than those deeper in the core. It can be seen that the composition of Ni, Zn and Al from ICP-OES analysis show that the relative amount of Ni:Zn:Al followed the molar ratio trend of the metal precursors added during synthesis. ICP-OES and EDXS analyses show that the combined atomic percentage of Ni+Zn and Ni+Zn+Al for the catalyst samples is consistent, indicating the homogeneity of the catalysts. However, XPS analysis does not show the same consistency in the combined Ni+Zn+Al atomic percentage for Ni/Al2O3 (21.44 At%) compared to other ZnO-bearing samples (31.72 – 36.58 At%). This is mainly due to the formation of oxide on Ni prior to the XPS analysis, as supported by the high NiO content from the deconvoluted XPS fitting (Figure 1a) and TEM image (Figure 3b) of the same sample. Despite this consistency, Table 1 shows that the sum of Ni+Zn+Al percentage on the surface (21.44 – 36.58 At% in XPS) is lower than the bulk (41.49 – 44.48 At% in ICP-OES), which in turn is lower than the sub-surface (50.45 – 56.36 At% in EDXS). This difference indicates that there is a significant movement of different species from surface to subsurface, and vice versa.

Table 1 Elemental composition from XPS, EDXS and ICP-OES analyses for synthesised catalyst samples prior to fats coating and XPS binding energy for Ni 2p3/2

Catalyst Sample

Ni/Al2O3

1_Ni/ZnO

2_Ni/ZnO

3_Ni/ZnO

ZnO

/Al2O3

/Al2O3

/Al2O3

/Al2O3


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Molar Ratio a (Ni:Zn:Al)

ICP -OES (At %)

XPS (At %)

EDXS (At %)

Ni Zn Al Na Ni+Zn Ni+Zn+Al Zn/Ni ratio Ni 2p3/2 Zn 2p3/2 b Al 2p Na Ni+Zn Ni+Zn+Al Zn/Ni ratio Ni Zn Al Na Ni+Zn+Al Zn/Ni ratio

XPS Ni 2p3/2 Binding Energy (eV)

3: 0: 1

1: 2: 1

2:1:1

1.5: 1.5: 1

0: 3: 1

32.37 9.50 0.40 32.37 41.87 11.67 9.77 11.67 21.44 46.66 9.70 56.36 -

9.83 21.71 9.95 0.80 31.54 41.49 2.21 6.82 15.78 13.98 22.60 36.58 2.31 16.44 22.87 11.14 50.45 1.39

22.17 11.94 10.37 0.42 34.11 44.48 0.54 11.53 9.80 11.77 21.33 33.10 0.85 26.10 10.37 15.98 52.45 0.40

15.73 17.20 11.47 0.79 32.93 44.40 1.09 10.00 12.66 9.06 22.66 31.72 1.27 27.19 14.97 13.89 56.05 0.55

32.05 12.31 0.74 32.05 44.36 24.97 9.21 24.97 34.18 43.96 9.55 53.51 -

857.63

856.09

856.26

856.32

-

Degree of ZnO 1.05 1.57 1.17 Migration c a Stoichiometric molar ratio of Ni(NO3)2·6H2O to Zn(NO3)2·6H2O and Al(NO3)3·9H2O during synthesis. b

Core electron at 2p energy level with aligned angular momentum and spin.

c

Ratio of surface Zn/Ni ratio (from XPS) to bulk Zn/Ni ratio (from ICP-OES).

The composition of Ni and Zn from ICP-OES analysis show that the amount of Ni relative to Zn followed the molar ratio trend of the metal precursors added during synthesis. However, further analysis of the elemental composition on the catalyst surface and sub-surface do not follow the trend of pre-set molar ratio, indicating the difference in Zn content on the surface and sub9 ACS Paragon Plus Environment


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surface. It can be seen that the Zn/Ni ratio increased when moving from the sub-surface to the surface of the catalyst, implying that there was migration of Zn from the sub-surface to the surface, resulting in a deficiency of Zn in the sub-surface and enrichment of Zn on the surface, while Ni is seen to move in the opposite direction. The degree of ZnO migration calculated by the ratio of surface Zn/Ni ratio (from XPS) to bulk Zn/Ni ratio (from ICP-OES), decreased with higher Zn content present in the catalyst. It is thought that when ZnO present in low quantity such as in 2_Ni/ZnO/Al2O3, the movement of ZnO outwards would be more prevalent. However, when ZnO becomes the major constituent, as seen in 1_Ni/ZnO/Al2O3, the outward migration is very limited. Similar observations on enrichment of Zn at catalyst surfaces were also reported for hydrogenreduced Cu/ZnO catalyst by several researchers 32–34, in which the migration of ZnO to the surface was attributed to the lower surface free energy of ZnO compared to Cu. In the current case, the samples were reduced by hydrogen and the surface free energy of ZnO (0.74 J/m2) significantly lower than Al2O3 (1.54 J/m2)

36,37

35

is also

and Ni (2.080 J/m2) 38, enabling the tendency of

ZnO to migrate to the catalyst surface. As shown in Figure 1, the XPS peak for Ni 2p3/2 in single metal Ni/Al2O3 catalyst was detected at binding energy of 857.63 eV but for the Ni/ZnO/Al2O3 catalysts, the peak shifted to 856.09 – 856.32 eV. This implies that addition of ZnO to Ni catalyst results in electronic structure alteration of Ni, leading to a lower binding energy for the Ni/ZnO/Al2O3 catalysts. Fittings for XPS spectra in Figure 1 show that in samples containing Ni, the species present were Ni, NiO and NiAl2O4 while in samples containing Zn, the species present were ZnO and ZnAl2O4. For samples containing Ni, a satellite peak due to presence of Ni2+ species in paramagnetic state was also detected. The satellite peak at binding energy of 861.3 eV is typical in the spectra of samples containing NiO and NiAl2O4 compounds at binding energy of 856.0 eV and 855.4 eV respectively 39

. It serves as a fingerprint for the identification of these compounds, thus further confirming the

presence of NiO and NiAl2O4.


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Figure 1 Ni 2p3/2 and Zn 2p3/2 XPS spectra fitting for (a) Ni/Al2O3 (b) ZnO/Al2O3 (c)-(d) 1_Ni/ZnO/Al2O3 (e)-(f) 2_Ni/ZnO/Al2O3 and (g)-(h) 3_Ni/ZnO/Al2O3 11 ACS Paragon Plus Environment


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The presence of NiO alongside metallic Ni species implies that after reduction, partial oxidation occurred through exposure to ambient condition before characterisation. Hence it is necessary to coat the catalysts with a layer of saturated fats to prevent oxidation before reaction is carried out. On the other hand, ZnO was present instead of Zn because at the present reduction temperature of 450 °C, reduction of ZnO to Zn did not occur. The hydrogen reduction of ZnO to metallic Zn only occurs at high temperature (above 600 °C)

40

. With Ni and ZnO as the main

constituents, it is therefore the reason why catalyst samples are denoted as Ni/ZnO/Al2O3 in this work. It is also noted that small atomic percentage ranging from 0.4 – 0.8 At% of sodium originating from the use of sodium carbonate as precipitating agent, is detected in the bulk samples via ICP-OES analysis, but is absent on the surfaces or near-surfaces (XPS and EDXS analyses). This may be attributed to the thorough washing of wet precipitates with hot deionised water and acetone which are good solvents for sodium ions

41,42

. However, the removal of sodium residing

in the inner cores of the precipitates are harder by washing hence contributing to the residual sodium detected in the bulk of the samples.

3.2

Pore Structures

From the nitrogen adsorption study shown in Table 2, Ni/Al2O3 and ZnO/Al2O3 have shown not so different pore size and pore size distribution (Figure S1 in Supporting Information), with ZnO/Al2O3 exhibiting larger pore volume. However, when Ni and Zn are mixed together for the synthesis of catalyst, it is clear that their variation in Zn content has significant effects on the specific surface area and also the pore size distribution of the catalysts. The pore characteristics were found to follow the ratio of surface Zn/Ni to sub-surface Zn/Ni in the catalyst, whereby a higher ratio (more Zn compared to Ni at surface due to more Zn migration to the surface) led to larger surface area and pore volume. It is known that the decrement of Zn at sub-surface of catalyst, as previously reported in Shimomura et al. 43 and Friedrich et al. 44, have increased the pore size, which resonates with the results obtained in this work. This may be due to the shortage of ZnO molecules in the interior of catalyst following ZnO migration to the surface, leaving behind cavities or more loosely packed structure in the interior, causing wider and higher amount of pores. 12 ACS Paragon Plus Environment

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Table 2 Specific surface area, SBET, specific pore volume, Vp, average pore diameter, dp and pore size distribution of the catalysts obtained from nitrogen adsorption-desorption analysis (Zn/Ni)surface/ Catalyst

(Zn/Ni)Sub-surface a

Ni

b

SBET

Vp

(m2/g)

(cm3/g)

dp

c

(nm)

Mesopore (vol %) d

Micropore

50-20

20-10

10-2

< 2%) (vol

(nm)

(nm)

(nm)

(nm)

-

42.94

0.278

25.90

37.42

39.47

23.1

0

1.67

24.86

0.085

13.68

20.66

40.68

38.7

0

2.13

98.05

0.402

16.39

27.80

56.56

15.6

0

2.31

112.66

0.927

32.93

53.73

27.41

18.9

0

-

67.63

0.415

24.59

47.72

20.73

31.6

0

/Al2O3 1_Ni/ZnO /Al2O3 2_Ni/ZnO /Al2O3 3_Ni/ZnO /Al2O3 ZnO /Al2O3 a

Ratio of Zn/Ni from XPS analysis over Zn/Ni from EDXS analysis.

b

Specific pore volume measured at P/P0 = 0.99

c

Average pore size calculated by dp = 4Vp/SBET

d

Pore diameter calculated with the desorption branch of isotherm using Kelvin equation for critical

meniscus radius and Harkin Jura equation to estimate the thickness of adsorbed multilayer. Graph of pore size distribution based on BJH procedure employing desorption branch is included as Figure S1 in Supplementary Information.

Table 2 shows that mesopores in the range of 20 – 50 nm increases when the catalyst ZnO content was increased from 2_Ni/ZnO/Al2O3 to 3_Ni/ZnO/Al2O3. Interestingly, it can be observed that the specific surface area and pore volume for 2_Ni/ZnO/Al2O3 and 3_Ni/ZnO/Al2O3 surpassed the single-metal catalysts of Ni/Al2O3, demonstrating the strong influence of ZnO in the catalyst matrix. Since 1_Ni/ZnO/Al2O3 exhibits lowest percentage of mesopores in range of 20 – 50 nm 13 ACS Paragon Plus Environment


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and also low pore volume, reaction could be severely impaired for this catalyst. As the partial hydrogenation of oils involve large molecules of triglyceride, the size of the pores will imperatively influence the diffusion and access of reactants to the active sites. Coenen 11 showed that pores narrower than 2 nm prohibit the diffusion of fats molecules, while pores wider than 3.5 nm are necessary to allow access by the fats molecules to the nickel active sites. At this pore size, diffusion of the fats molecules are possible but still limited by the frequent collision with the pore wall. As exemplified by Witoon et al. 45, diffusion limitation of palm oil was observed for catalysts with average pore size around 12 nm. Abundance of wider pores in 3_Ni/ZnO/Al2O3 catalyst alleviates diffusion limitation and facilitates the diffusion of triglyceride molecules in pores, which may enhance the hydrogenation activity. The relationship between hydrogenation activity and pore size will be discussed in Section 3.6.

3.3

XRD and TEM analyses

In order to provide a clearer view on the crystalline phases present, Figure 2 compares the XRD spectra of the pre-reduced catalysts. It can be seen that sharp and defined peaks representing Ni and ZnO were obtained, indicating the presence of crystalline Ni (ICDD card No. 00-004-0805), NiO (ICDD Card No. 00-044-1159) and ZnO (ICDD Card No. 00-036-1451) after reduction. The XRD spectra of the samples are also in agreement with the spectra of Ni/ZnO/Al2O3 catalyst reported in literature

46

. The Ni(111), Ni(200), NiO(101) and ZnO(110) lattices represented by

peaks at 44.5°, 51.7°, 37.2° and 37.3° respectively are also captured in TEM image (Figure 3).


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Figure 2 Comparison of XRD spectra for pre-reduced catalyst samples with their respective identified compounds

With the presence of large amount of ZnO, the peaks corresponding to Ni species diminished as seen in sample 1_Ni/ZnO/Al2O3, with only a small peak of Ni(111) plane at 44.5° being visible. When ZnO is a minor constituent as exhibited by sample 2_Ni/ZnO/Al2O3, the XRD spectra closely resembles the spectra of Ni/Al2O3 and the peaks corresponding to ZnO are hardly visible, which is probably due to limitation of detection for species in small amount. The crystallinity of ZnO could also be overshadowed by the prominent Ni and NiO peaks, resulting in seemingly amorphous ZnO state in sample 2_Ni/ZnO/Al2O3. Peaks corresponding to both Ni and ZnO species are sharp and prominent in sample 3_Ni/ZnO/Al2O3 which has equimolar Ni and ZnO content. Calculation of crystallite size with Scherrer’s equation for Ni(111) shows that the crystallite size decreased by half from 6.8 nm for Ni/Al2O3 to 3.2 nm for 2_Ni/ZnO/Al2O3. This could be attributed to the small amount of ZnO that acts as physical spacer between Ni crystallites, preventing the formation of bulky crystallites. When ZnO content increases as seen in 3_Ni/ZnO/Al2O3, the effect of ZnO being a spacer becomes less prominent with the increase of crystallite size to 11.9 nm. On the contrary, when Ni becomes the minor constituent in 1_Ni/ZnO/Al2O3, the crystallite size is at 4.9 nm, which is similar but slightly smaller than 15 ACS Paragon Plus Environment


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Ni/Al2O3, possibly due to the significantly lower Ni content that resulted in more limited crystallite growth. The crystal plane and particle size of 3_Ni/ZnO/Al2O3 and Ni/Al2O3 were magnified and captured in their TEM images as shown in Figure 3. It is evident that the catalyst surface are crystalline as observed from the lattice fringes, with crystallite sizes that range from 5 - 10 nm. Unlike the well-defined nanoparticles seen in typical supported metal catalysts, there is low contrast between the nanoparticles and the support, which necessitates a contrast enhancement as seen in this figure. Measurement on the lattice spacing indicates that Ni species present as Ni(111), Ni(200) and NiO(101) while ZnO exhibits (110) crystal lattice, which resonates with the crystal lattices detected in XRD spectra. It can be observed that crystallites of ZnO are sandwiched between the Ni crystallites in 3_Ni/ZnO/Al2O3, which indicates the role played by ZnO to stabilize the active Ni phase in a dispersed state and suggesting that the presence of ZnO acts as structural promoter to improve the Ni dispersion in 3_Ni/ZnO/Al2O3. Compared to Ni/Al2O3 which shows relatively less metallic Ni species, the larger presence of Ni crystallites in between ZnO crystallites probably implies that ZnO has the ability to suppress the oxidation of Ni.


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Figure 3 Contrast-enhanced TEM image of (a) 3_Ni/ZnO/Al2O3 (b) Ni/Al2O3 showing the magnified view of the crystalline planes and different species based on measured spacing between lattice fringes where dNi(111) = 2.03 Å, dNi(200) = 1.76 Å, dNiO(101) = 2.41 Å, dZnO(110) = 2.29 Å

3.4

TPR Profiles

Figure 4 compares the H2-TPR profiles for all the catalyst samples. For comparison purpose, TPR profile for NiO which was obtained from calcination of unsupported nickel precursor is included. From the TPR profile for NiO, it is ascertained that a peak at 290°C indicates the reduction of NiO. For Ni-bearing catalysts, the TPR profiles generally exhibit two reduction peaks: a distinct peak at low temperature and a broad peak at high temperature. For Ni/Al2O3, the low temperature peak at 305 °C represents the reduction of NiO that interacts weakly with Al2O3 support and the 17 ACS Paragon Plus Environment


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reduction peak at 608 °C corresponds to reduction of metal aluminate spinel 47 which were formed through intimate contact between the metal and support 48. Metal aluminate has high stability and very low reducibility due to the incorporation of the metal ion into alumina crystallite structure 47.

Figure 4 Temperature programmed reduction profiles for NiO, Ni/Al2O3, 1_Ni/ZnO/Al2O3, 2_Ni/ZnO/Al2O3, 3_Ni/ZnO/Al2O3 and ZnO/Al2O3 across temperatures from 150 – 1000 oC with inset showing amount of hydrogen consumed during H2-TPR


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The Ni/ZnO/Al2O3 samples also show low temperature peaks corresponding to reduction of NiO albeit at a lower reduction temperature than 290 °C (reduction peak of NiO). The aluminate spinel species present in the samples are NiAl2O4 and ZnAl2O4 as detected by XPS fitting as shown in Figure 1, however it is unlikely that the high temperature peaks are due to reduction of ZnAl2O4. This is because there was barely any reduction occurring in ZnO/Al2O3 sample which was free of NiAl2O4 species, as seen from its broad and stunted profile at very low intensity, hence the high temperature peaks in all Ni/ZnO/Al2O3 samples were resulted by reduction of NiAl2O4. Besides, the stunted TPR profile of ZnO/Al2O3 sample also affirms the presence of ZnO instead of metallic Zn in the samples as there was barely any reduction detected. Figure 4 shows that the incorporation of ZnO improves the reducibility of Ni catalysts, as seen in the shift of the reduction peaks to lower temperatures in the Ni/ZnO/Al2O3 catalysts. This suggests that the presence of ZnO probably alters the metal-support interaction, resulting in weaker binding of Ni on Al2O3 support. The decline in low temperature reduction peak from H2-TPR (Ni/Al2O3 > 1_Ni/Zn/Al2O3 > 2_Ni/Zn/Al2O3 > 3_Ni/Zn/Al2O3) does not follow the amount of the bulk ZnO content in the catalyst, which suggests that different amount of ZnO will lead to different Ni-Zn-Al interaction, as confirmed by the different binding energy shown by XPS (in Table 1). The effect of ZnO in improving reducibility of alumina-supported catalyst was also observed for Cu/ZnO/Al2O3 catalyst in past literature

49–51

, whereby the easier reducibility was correlated to

larger fraction of Cu sites being in close vicinity with ZnO in the system 52. Comparing between the three Ni/ZnO/Al2O3 catalysts, 1_Ni/ZnO/Al2O3 exhibits a broad TPR profile, indicating the reduction of several intermediate species in addition to NiO and NiAl2O4 species. The presence of these intermediate species is probably due to the higher ZnO content where the physical and chemical interaction between Ni-Zn-Al is likely to be stronger and more complex which gives rise to low pore volume, and thus low surface area. On the other hand, 2_Ni/ZnO/Al2O3 and 3_Ni/ZnO/Al2O3 exhibit peaks that resemble Ni/Al2O3 though at lower temperatures. In particular, 3_Ni/ZnO/Al2O3 shows a sharper peak at lower temperature, which indicates a Ni-rich species, which also helps to explain why hydrogenation activity is highest for this sample, as will be discussed in Section 3.6. The hydrogen consumption during TPR for each sample, calculated based on integral of the TPR peaks, is found to positively correlates with their bulk Ni content, where samples with 19 ACS Paragon Plus Environment


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higher Ni content showed higher hydrogen consumption and signal intensity in TPR analysis. However for Ni/Al2O3 sample, the hydrogen consumption is significantly higher than the ZnObearing Ni catalysts. This stems from the higher level of NiO species present in the pre-reduced Ni/Al2O3 sample compared to Ni/ZnO/Al2O3, as shown in the results of XPS fittings. XPS fittings showed that in the Ni/Al2O3 sample (Figure 1a), the amount of NiO is almost double of its Ni species, whereas in the ZnO-bearing Ni catalysts, the relative amount of both Ni and NiO species are almost the same. This finding also suggests that the presence of ZnO probably has the ability to preserve more Ni in the metallic state.

3.5

Hydrogen Pulse Chemisorption

Table 3 shows the amount of hydrogen chemisorbed as well as the calculated Ni dispersion and Ni surface area of the catalysts. In general, the catalysts exhibit consistently high Ni dispersion and Ni surface area, which can be attributed to the two-step sequential precipitation method employed in this study. Referring to Table 3, Ni dispersion and Ni surface area decrease in the order of 2_Ni/ZnO/Al2O3 > 3_Ni/ZnO/Al2O3 > Ni/Al2O3 > 1_Ni/ZnO/Al2O3. Higher bulk Ni composition generally leads to higher Ni surface area, but it is interesting to see that the ZnO-bearing catalysts such as 2_Ni/ZnO/Al2O3 and 3_Ni/ZnO/Al2O3 which exhibit lower bulk Ni composition than Ni/Al2O3, showed higher Ni dispersion than Ni/Al2O3, suggesting that the presence of ZnO enhances the Ni dispersion.


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Table 3 Amount of hydrogen adsorbed, Ni dispersion, DNi and Ni specific surface area, SANi of the catalysts obtained from hydrogen pulse chemisorption analysis as compared with bulk Ni composition, surface Zn/Ni ratio, hydrogenation rate and average pore diameter, dp H2 Catalyst

DNi a

SANi b

(%)

(m2/gNi)

624.33

19.38

1_Ni/ZnO /Al2O3

87.10

2_Ni/ZnO /Al2O3 3_Ni/ZnO /Al2O3 ZnO/Al2O3 a

Surface Hydrogenation

dp

Composition c

Zn/Ni

Rate e

(%)

ratio d

(IV/min)

129.05

32.36

-

0.60

25.92

9.84

65.53

9.83

2.31

0.15

13.68

736.74

35.13

233.99

22.17

0.85

0.38

16.39

304.10

21.17

140.98

15.73

1.27

0.74

32.93

6.45

-

-

-

-

0.06

24.59

Adsorbed (μmol/g)

Ni/Al2O3

Bulk Ni

(nm)

Calculated by dividing the amount of hydrogen adsorbed per gram of Ni (assuming 1 H2: 2 Ni

atoms) by the bulk Ni content. Adsorption of H2 on ZnO and other species is assumed to be negligible. b

Calculated based on Ni dispersion, and by assuming Ni has the proportion of low index plane fcc

(111):(100):(110) = 1:1:1, with a single Ni atom occupying 0.0649 nm2 30. c

Obtained from ICP-OES analysis.

d

Obtained from XPS analysis.

e

Calculated by the difference between final and initial iodine value divided over the total reaction

time used to reach the final iodine value.

Several studies have shown that the presence of ZnO increases the Cu surface area and dispersion by functioning as physical spacer for Cu nanoparticles

32,53–55

as well as results in

increased wetting of Cu nanoparticles under reducing conditions 56 in the Cu/ZnO/Al2O3 catalyst. In this study, it is opined that the occurrence of ZnO enrichment at the surface for Ni/ZnO/Al2O3 helps in spreading out the supported Ni nanoparticles, in which ZnO acts as physical spacer between Ni nanoparticles (as shown in Figure 3), forming more dispersed Ni clusters on the 21 ACS Paragon Plus Environment


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support, hence exposing and dispersing more Ni atoms on the surface. Conversely, in the absence of ZnO, the Ni tends to agglomerate to form larger nanoparticles on the support. This leads to more Ni atoms being enclosed than being exposed on the surface, thus having a lower Ni dispersion and Ni surface area. For the ZnO-bearing catalysts, the Ni surface area is also affected by the surface Zn/Ni ratio, where a lower ratio of surface Zn/Ni (higher Ni content) exposes more Ni atoms and leads to larger Ni surface area and higher Ni dispersion. When combined with the findings in the previous paragraph, it can be deduced that the presence of ZnO in low quantity (e.g. 2_Ni/ZnO/Al2O3) will significantly increase the Ni surface area. When ZnO content becomes higher, the benefit of ZnO as spacer for Ni nanoparticles diminish, with ZnO becoming the main constituent, which can be observed from the lower Ni surface area in 3_Ni/ZnO/Al2O3 and followed by 1_Ni/ZnO/Al2O3. Our finding is consistent with Chen et al. 57 which also shown that increasing ZnO content for Ni/ZnO/Al2O3 catalysts led to declining Ni dispersion and bigger Ni crystallites.

3.6

Hydrogenation Activity

It is widely believed that the hydrogenation of triglycerides can be described with the reaction mechanism

58

as shown in Figure 5. The nomenclature used is in the format of CX:Y where X is

the number of carbons while Y designates the number of double bonds. The fatty acids tails in the triglycerides are hydrogenated to eliminate the double bonds up to full saturation, for example linolenic acid (C18:3) would be hydrogenated to form linoleic acid (C18:2) which has one less double bond.

Figure 5 General reaction path for hydrogenation of oils from linolenic acid (C18:3) to stearic acid (C18:0) 22 ACS Paragon Plus Environment

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Figure 6 shows the change in IV with reaction time. The sunflower oil has an initial IV of 162 and the IV decreased as the sunflower oil became more saturated after hydrogenation. The gradient of the graph indicates the rate of decrement in IV, where steeper gradient indicates a greater hydrogenation activity. IV 70 was chosen as the end-point of reaction as it is a common target IV for partial hydrogenation of vegetable oils to form margarine 12. Figure 6 shows that the hydrogenation activity decreases in the order of 3_Ni/ZnO/Al2O3 > Ni/Al2O3 > 2_Ni/ZnO/Al2O3 > 1_Ni/ZnO/Al2O3. It can be seen that larger specific Ni surface area and higher Ni dispersion are generally linked to higher hydrogenation activity with the exception of 2_Ni/ZnO/Al2O3. In this instance, 2_Ni/ZnO/Al2O3 exhibits the highest Ni surface area and Ni dispersion (Table 3), but shown notably lower hydrogenation activity than 3_Ni/ZnO/Al2O3 and Ni/Al2O3. Furthermore, 3_Ni/ZnO/Al2O3 also shows higher hydrogenation activity compared to Ni/Al2O3 despite them having a similar range of Ni surface area. The above observations suggest that hydrogenation activity in this study is not directly related to Ni dispersion and Ni surface area.

Figure 6 Decrease in iodine value (IV) in the course of partial hydrogenation of sunflower oil (temperature: 180 oC, hydrogen pressure: 5 bar (g), agitation rate: 2000 rpm) over synthesised catalyst samples 23 ACS Paragon Plus Environment


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When comparing the hydrogenation activity with the average pore size information (Table 3) of each sample, it is apparent that they are well-correlated whereby the hydrogenation activity generally increases with the use of catalyst with larger pore size. Table 3 shows that sample 3_Ni/ZnO/Al2O3, which possesses the largest average pore size, had the highest hydrogenation activity among all the catalysts. On the other hand, 1_Ni/ZnO/Al2O3 which shows smallest average pore size and lowest pore volume have contributed to lower hydrogenation activity. Despite possessing the highest Ni dispersion and Ni surface area, 2_Ni/ZnO/Al2O3 catalyst did not show the highest activity as its pore size is notably smaller than 3_Ni/ZnO/Al2O3 and Ni/Al2O3, demonstrating the impact of limited accessibility for hydrogenation activity. Among the ZnO-bearing catalysts, the hydrogenation activity appears to increase with larger specific surface area and pore volume, as more surface area and space are available for occupancy of triglycerides in the pores, resulting in more reactants having higher chances of adsorbing on the active sites for reaction. Nevertheless, when compared with Ni/Al2O3, it is noted that Ni/Al2O3 has smaller specific surface area and pore volume than 2_Ni/ZnO/Al2O3 but a higher activity. This could be attributed to its larger pore diameter than 2_Ni/ZnO/Al2O3, which allows less restriction on diffusion of triglycerides into the pores. It is inferred that pore size has more obvious effect than specific surface area and pore volume on the hydrogenation activity of triglycerides. A smaller pore diameter near the pore opening particularly, restricts access of triglycerides to the pores before the specific surface area and pore volume limitation come to effect. Therefore, this further supports the widely-held view that the pore size plays an important role in contributing to hydrogenation activity, as smaller pore sizes tend to impede the diffusion activity of bulky triglyceride molecules present in the oils. Comparing between the samples, it is clear that changing the Ni:Zn molar ratio indirectly affects the overall activity, as they may either result

in

an

enhancement

(3_Ni/ZnO/Al2O3)

or

impediment

(1_Ni/ZnO/Al2O3

and

2_Ni/ZnO/Al2O3) to the overall activity as compared to single metal Ni catalyst by changing the availability of wide pores. The hydrogenation rate (Table 3) increased by 23% with the use of 3_Ni/ZnO/Al2O3 compared to the conventional Ni/Al2O3 catalyst. Finally, the catalyst activity remained low for ZnO/Al2O3 mainly due to the low intrinsic activity of ZnO towards hydrogenation. 3.7

Hydrogenation Selectivity 24 ACS Paragon Plus Environment

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As shown in Table S1, the raw material used in this work is sunflower oil, which contains 3.3 wt% of saturated fats and no trans-fat. In order to evaluate the performance of the synthesised catalysts, hydrogenation was carried out and the catalysts’ performance was evaluated based on the formation of saturated fat (C18:0), trans-C18:1 and cis-C18:1 at specific IV values, as shown in Figure 7. Interpretation of the results in terms of yield and selectivity are shown in Table 4. With the exception of 1_Ni/ZnO/Al2O3 and ZnO/Al2O3, which had low hydrogenation activity and beyond targeted IV limit (IV 70) even after 480 minutes, the three other catalysts samples showed distinctive differences in the formation of both trans- and saturated fats at various IV intervals. It is clear from Figure 7 that the catalysts showed similar overall trend in terms of the increase in C18:0 and trans-C18:1, as well as decrease in cis-C18:1 and C18:2. The elimination of double bonds and the isomerisation process during the reaction can be tracked through the plots. For Ni/Al2O3 and 3_Ni/ZnO/Al2O3, it can be seen that the significant increase in cis-C18:1 at the beginning of hydrogenation coincided with the drastic decrease in C18:2, which implies that C18:2 were hydrogenated to form cis-C18:1. However, cis-C18:1 has shown smaller increment compared to the decrement of C18:2, and this was due to the simultaneous hydrogenation of cis-C18:1 to C18:0 or trans-C18:1. The level of cis-C18:1 reached a maximum when C18:2 is almost depleted, and then decreased thereafter to produce trans-C18:1 and C18:0, via isomerisation and hydrogenation respectively.



Page 26 of 37

Figure 7 Composition change of sunflower oil with iodine value (IV) during partial hydrogenation (temperature: 180 oC, hydrogen pressure: 5 bar (g), agitation rate: 2000 rpm) with the use of (a) Ni/Al2O3 (b) 1_Ni/ZnO/Al2O3 (c) 2_Ni/ZnO/Al2O3 (d) 3_Ni/ZnO/Al2O3 catalyst. (♦ represents C18:2; ● represents cis-C18:1; ▲ represents trans-C18:1; ■ represents C18:0).


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Table 4 Yield of trans-C18:1 and C18:0, selectivity of C18:0 over trans-C18:1 and selectivity of trans-C18:1 over cis-C18:1 of sunflower oil at specific IV upon undergoing partial hydrogenation (temperature: 180 oC, hydrogen pressure: 5 bar (g), agitation rate: 2000 rpm) over synthesised catalyst samples.

Yield of

Yield of

trans-C18:1 (%)

C18:0 (%)

Selectivity of

Selectivity of

C18:0 over

trans-C18:1 over

trans-C18:1 (%)

cis-C18:1 (%)

IV values

90

70

90

70

90

70

90

70

Ni/Al2O3

24.5

31.2

18.8

25.6

43.5

45

33.2

46.2

1_Ni/ZnO/Al2O3

4.7

-

17.6

-

78.8

-

8.2

-

2_Ni/ZnO/Al2O3

28.1

47.6

14

24.1

33.2

33.6

41.4

61.0

3_Ni/ZnO/Al2O3

11.3

20.2

17.9

31.9

61.3

61.2

16.9

34.1

Furthermore, as seen from Table 4, catalyst with lower trans-C18:1/cis-C18:1 selectivity would have higher C18:0/trans-C18:1 selectivity and vice versa. This shows that when transC18:1 selectivity is low, subsequent hydrogenation of trans-C18:1 to C18:0 and direct hydrogenation of cis-C18:1 to C18:0 instead of isomerisation to trans-C18:1 tend to occur, which leads to high C18:0 formation. Conversely, when trans-C18:1 selectivity is high, isomerisation of cis-C18:1 to trans-C18:1 is favoured and subsequent hydrogenation of trans-C18:1 to C18:0 is less inclined, which results in lower C18:0 formation. Referring to the C18:0/trans-C18:1 selectivity at IV 90, it can be seen that increasing ZnO content in the Ni/ZnO/Al2O3 catalysts led to increasing selectivity for the saturated C18:0. When comparing in terms of trans-C18:1/cis-C18:1, increasing ZnO content led to decreasing selectivity for trans-C18:1. This two trend suggests that increasing ZnO content tend to suppress the formation of trans-C18:1 and promote the formation of saturated C18:0. However, Ni/Al2O3 shows higher C18:0/trans-C18:1 selectivity and lower transC18:1/cis-C18:1 selectivity than 2_Ni/ZnO/Al2O3, despite the absence of ZnO. This may be closely related to 2_Ni/ZnO/Al2O3 having high percentage of pores in the range of 10 – 20 nm, 27 ACS Paragon Plus Environment


Page 28 of 37

which is large enough for triglycerides to diffuse in, avoiding low hydrogenation activity, but also small enough to retard the outward diffusion of bulky cis-C18:1 molecules. A study by Balakos and Hernandez 13 showed that pores with intermediate width favour the formation of trans-isomers where they desorb from active sites without being hydrogenated. As the exit of the molecules from the pores is restricted due to the bent molecular structure of cis-C18:1, the molecules trapped in the pores have high tendency to proceed with isomerisation or full-hydrogenation. As isomerisation is thermodynamically more favourable, this would produce high level of linear trans-C18:1 and allow the easy escape of molecules from the intermediate-width pores. On the other hand, Ni/Al2O3 which shows significantly higher percentage of pores in the range of 20 – 50 nm would allow better access for both bulky cis-C18:1 and linear trans-C18:1, and hydrogenation with such wide pores will therefore result in higher conversion to saturated C18:0. Meanwhile, 1_Ni/ZnO/Al2O3 which has higher percentage of small pores in the range of 2 – 10 nm than 2_Ni/ZnO/Al2O3, would restrict cis-C18:1 and trans-C18:1 from leaving the pores and lead to full saturation and high C18:0 selectivity, despite the meagre pore volume and very low catalytic activity. It is interesting to see that although both Ni/Al2O3 and 3_Ni/ZnO/Al2O3 have similar range of Ni surface area (Table 3), 3_Ni/ZnO/Al2O3 exhibits lower trans-C18:1 formation and lower trans-selectivity compared to Ni/Al2O3. As shown in Table 4, there is reduction of 26.2% of the trans-selectivity at IV 70 with the use of 3_Ni/ZnO/Al2O3 compared to Ni/Al2O3. This can be attributed to the promotional effect of ZnO that functions as selectivity modifier for Ni catalyst. It is thought that the presence of ZnO change the bonding strength of adsorbates, either through increasing the energy barrier for isomerisation or lowering the energy barrier for hydrogenation and speed up the conversion from cis-C18:1 to C18:0. The shift in binding energy of Ni 2p3/2 from Ni/Al2O3 (857.63 eV) to Ni/ZnO/Al2O3 (856.09 – 856.32 eV) catalysts as shown by XPS analysis supports the view that ZnO plays a significant role in modifying the selectivity of the reaction. In order to have a better understanding on the performance of the synthesised Ni/ZnO/Al2O3 catalysts, 3_Ni/ZnO/Al2O3 and Ni/Al2O3 catalysts are chosen to be compared with other catalysts from literature as shown in Table 5. The 3_Ni/ZnO/Al2O3 catalyst appears to be the best-performing catalyst among the synthesised catalysts with consideration of its higher activity than 1_Ni/ZnO/Al2O3 catalyst and ability to achieve lower trans-C18:1 selectivity than Ni/Al2O3 28 ACS Paragon Plus Environment

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catalyst. Through comparison with results reported in the literature, it can be seen that 3_Ni/ZnO/Al2O3 has yielded comparable saturated fat percentage as the Ni-B catalyst

24

, albeit

forming more trans-fat, which may probably be resulted from operating at higher hydrogenation temperature. Overall, Ni/Al2O3 and 3_Ni/ZnO/Al2O3 have yielded lower trans-fat and higher saturated fat, as compared to Pt/Al2O3, Pd/Al2O3

14

which was performed using sunflower oil.

Comparing with other hydrogenation process using Ni based catalyst, such as Ni-Mg-Ag/D 21, NiCe 17 and Ni-B 24, this work has demonstrated comparable if not lower trans-fat formation, though it should be noted that the reaction conditions and type of oil vary slightly from one another, which may have subsequently affected the overall trans- and saturated fat contents. Therefore, this work has managed to show the potential of incorporating ZnO onto Ni/Al2O3 catalysts, seeing the positive result it may provide in enhancing the hydrogenation activity and balancing between the formation of both trans- and saturated fats, depending on individual applications and requirements.

Table 5 Comparison of trans-fats and saturated fats content with other works

Catalyst

Conditions

Pd/Al2O3

150 oC,

Pt/Al2O3

3.5 bar

Ni-Mg-Ag/D

160 oC, 1.6 bar

Reaction

Trans-

Saturated

Time (min)

Fats (%)

Fats (%)

49

44.7

16.3

102

33.0

21.8

90

255

26.3

5.8

Soybean oil 21

IV

70

Type of Oil

Sunflower oil 14

Ni-Ce

180 oC, 6 bar

70

33

26.4

16.6

Canola oil 17

Ni-B

120 oC, 5 bar

77

80

8.0

34.0

Soybean oil 24

180 oC, 5 bar

70

130

22.8

33.5

Sunflower oil,

150

30.8

24.9

present work

3_Ni/ZnO/Al2O3 Ni/Al2O3

4.0

Operating

Conclusion



Page 30 of 37

This work has shown that the presence of ZnO in co-precipitated Ni/ZnO/Al2O3 catalysts tends to migrate the Zn atoms out to the surface and varying the Ni:Zn molar ratio resulted in catalysts that have different pore characteristics. Incorporation of ZnO in 3_Ni/ZnO/Al2O3 (equimolar of Ni and Zn) has produced catalyst with wider pores and higher surface area, which is shown to improve the hydrogenation activity. Apart from modifying the pore characteristics, ZnO also serves as bifunctional promoter where it acts as: (i) structural promoter which increases Ni dispersion and stabilizes Ni crystallites; and (ii) selectivity modifier which results in lower trans-C18:1 formation. However, given that ZnO content effects a considerable change in pore characteristics and Ni surface area, it is difficult to determine the relationship between Zn/Ni ratio and trans-selectivity. Despite this limitation, it is found that the combination of wide pores and high Ni surface area is necessary to achieve high hydrogenation activity and lower trans-C18:1 formation. A balance has to be sought between trans- and saturated fats content to formulate partially hydrogenated products that will fulfil the melting profile requirement without the expense of our health and efficiency of this industrial reaction.

Acknowledgements This work was supported by the Fundamental Research Grant Scheme Ministry of Higher Education Malaysia (FRGS/1/2015/TK02/UTAR/02/1). The authors would like to thank Dr. Steven Lim and Tang Zo Ee for guidance on GC analysis as well as Loo Wai Jin, Chin Jun Hao, Wong Yen Tong, Ting Hui Ling and Tan Wen Hsiung for their assistance in catalyst synthesis and hydrogenation experiments.

Supporting Information 

Composition of sunflower oil employed in partial hydrogenation



Pore size distribution curve



Nitrogen adsorption-desorption isotherm

References


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