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Lowering the trans-isomer content in hydrogenation of triglycerides of unsaturated fatty acids at ambient temperatures
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
2039
Lowering the t r a n s - i s o m e r content in hydrogenation of triglycerides of unsaturated fatty acids at ambient temperatures I.A.Makaryan, O.V.Matveeva, G.I.Davydova and V.I.Savchenko Institute of Problems of Chemical Physics of the Russian Academy of Sciences 142432, Chernogolovka, Moscow Region, Russia
The selective hydrogenation of the triglycerides of unsaturated fatty acids from vegetable oils has been performed using newly designed low-loaded palladium catalyst. Investigations on the effect of reaction conditions (temperature, pressure, fractional composition) on the trans-isomerization showed that the palladium catalyst unlike conventional nickel catalysts allows to decrease the trans-isomer content in final hydrogenated fat.
1. INTRODUCTION The selective catalytic hydrogenation of the triglycerides of unsaturated fatty acids from vegetable oils is the main method used to modify natural vegetable oils into a fatty base for edible margarines. In this century margarine plays an important part in the human diet and has to some extent superseded high cholesterol butters. At present its production is performed using finely-dispersed nickel catalysts at elevated temperatures from 180 to 2300C, and has some serious disadvantages which contribute to making the process inefficient. In the course of triglycerides hydrogenation a hydrogenated fat is produced, and simultaneously the trans-isomers are formed in proportion depending on hydrogenation conditions. The content of trans-isomers in final edible product could be about 60 per cent. Recent literature points to the fact that the trans-isomers of hydrogenated fats may pose a possible health hazard [1]. At the same time, edible margarines may be contaminated with very-toxic-to-humans nickel compounds from the catalyst used. Therefore, it is not clear yet either trans-isomers or nickel may
2040
cause undesirable effects to humans. In any case, the conventional technology for producing hydrogenated fat for edible margarines requires updating. The main challenge in producing pure margarines is to find replacement for the nickel-containing catalyst. One such alternative may be palladium, and several studies have been reported using palladium catalysts to hydrogenate vegetable oils [2, 3]. Thus, three palladium-containing catalysts Pd/C, Pd/A1203 and Pd/BaSot were used [3] during the investigations on the effects of pressure, temperature and catalyst concentration on the formation of trans-isomers in hydrogenation of vegetable oils at temperatures of 50 to 1100C and pressures from 3,5 to 50 atm. Our previous investigations showed [4] that newly designed low-loaded palladium catalyst which was developed to hydrogenate vegetable oils into a fatty base for edible margarines gives a highly stable process with the required product. The present work is aimed at the investigation on characteristics of newly designed low-loaded palladium catalyst in hydrogenation of triglycerides of unsaturated fatty acids under different reaction conditions and using different fractional composition of catalyst which could lead to the decrease in trans-isomer content in final hydrogenated fat.
2. EXPERIMENTAL The low-loaded palladium catalysts were prepared by deposition of palladiumcontaining compound on an active alumina support of different particle size (e.g. fractional composition). The hydrogenation was performed in reaction vessel fixed on the shaker with 10 sec -1 oscillation. Under these conditions the hydrogenation rate was not limited by mass transfer of hydrogen from the gas to the liquid phase. The hydrogenation proceeded during definite time: for soybean oil up to iodine value (I.V.) = 95 + 3, and for sunflower oil up to I.V. = 75 + 2. The initial vegetable oils were preliminarily purified. When testing the catalysts with a very low palladium content, the initial oil was purified more carefully using special technique. The fatty acid composition was measured by gas-liquid chromatography; the trans-isomer content was analyzed by IRspectroscopy (Table 1).
2041 Table 1 Fatty acid composition and iodine value for the initial vegetable oils Vegetable
I.V,
oil
Fatty acid c0mposition, %*
C14:0 C16:0 C18:0 C18:1 C18:2 C18:3 C20:0 C20:1 C22:0
Sunflower oil
137.5
0.1
6.8
4.4
16.3
71.2
-
0.2
0.4
0.6
Soybean oil
132.3
0.1
11.4
4.0
22.3
53.0
8.0
0.3
0.5
0.4
*Content of acyi radicals in triglycerides of the following fatty acids: myristic (C14:o); palmitic (C16:o); stearic (C18:o); oleic (Cls:l); linoleic (C18:e); linolenic (C18:a); eicosanic (C20:0); eicoseinic (Ce0:l); docosanoic Cee:0).
3. R E S U L T S AND D I S C U S S I O N The thorough investigations using palladium catalyst to hydrogenate the triglycerides of unsaturated fatty acids allowed to observe the following distinctive features. The palladium is a very active catalyst for the hydrogenation of triglycerides of unsaturated fatty acids from vegetable oils, and it is difficult to find other reactions with such high hydrogenation rates. Thus, using a very pure initial oil for hydrogenation we can employ the active catalysts with a very low palladium content (down to 0.001 wt% palladium in catalyst) [4]. Such extremely high activity allows to perform hydrogenation of triglycerides event at lowered temperatures, resulting in hydrogenated fat with a low trans-isomer content. Changing the amount of palladium in the catalyst from 0.2 to 0.02 wt% has only a small effect on the characteristics of final hydrogenated fat relatively both acyl and trans-isomer selectivity (Table 2). We have ensured that the main factor affecting the trans-isomer content in hydrogenated fat is the hydrogenation temperature. The data on the hydrogenations at moderate temperatures are given in Table 3. The lower range of temperatures in these runs was limited by 300C for I.V. - 95 + 3, and by 45oc at I.V. - 75 + 2. This was caused by the fact that under these conditions the reaction mixture became very viscous and badly agitated. In order to use the range of lower temperatures we decided to perform the hydrogenation of sunflower oil in solvent medium, specifically heptane. In this case the palladium catalyst was active down to T 00C. The decrease in temperature was accompanied by decrease in trans-isomer content.
2042 Table 2 Effect of the palladium concentration in the catalyst on the characteristics of hydrogenation of sunflower oil
Characteristics
Concentration of Pd in catalyst, wt% 0.02 0.05 0.2
Hydrogenation time, min 175.0 I.V. 75.6 Fatty acid composition,%: C14:o 0.1 C16:o 6.6 Cls:O 10.8 Cls:I 75.3 C18:2 6.2 C2o:o 0.3 C2o:1 0.1 C22:o 0.6 Trans-isomer content, % 20.8 Charge of catalyst Pd/A1203 is 0.5 g per 50 g of oil. Reaction temperature is 45oc, hydrogen pressure is
70.0 75.3
29.0 74.9
0.1 6.4 9.5 79.2 4.1 0.3 0.1 0.6 23.3
0.1 6.9 11.0 75.3 5.8 0.2 0.1 0.6 23.1
4 bar.
Table 3 Characteristics of the Hydrogenation of Soybean and Sunflower Oils at Moderate Temperatures Soybean oil hydrogenation up to I . V . - 95 + 3 Temperature, 0C I.V. trans-isomer content, % 90 92.0 21.7 8O 95.O 2O.6 70 93.0 19.2 50 97.7 14.7 3O 96.6 12.5 Sunflower oil hydrogenation up to I . V . - 75 + 2 Temperature, oc I.V. trans-isomer content, % 95 73.0 33.9 70 74.6 29.4 45 76.9 21.8 Charge of Pd is 20 mg per I kg of oil.
Taking into consideration the fact that the rate determining step for the hydrogenation of vegetable oils with palladium is the transfer of hydrogen and triglycerides of unsaturated acids from the liquid phase to the catalyst surface, and that the particle size of catalyst significantly affects the process under
2043
consideration we investigated the characteristics of hydrogenation using palladium catalyst with different fractional composition. The tests were performed both when the temperature was kept constant (450C) pressures being changed from 1 to 10 bar (Table 4), and when the pressure was constant (4 bar) and temperatures being changed (Table 5). Table 4 The effect of catalyst particle size on the characteristics of hydrogenated fat produced during hydrogenation of sunflower oil at different hydrogen pressures Hydrogen pressure, bar
Characteristics
Hydrogenation time, min I.V. Trans-isomer content, % Hydrogenation time, min I.V. Trans-isomer content, % Hydrogenation time, min I.V. Trans-isomer content, % 10 Hydrogenation time, min I.V. Trans-isomer content, % Charge of catalyst 0.2 wt% Pd/A1203 is 0.5 Reaction temperature is 45oc.
Catalyst particle size, pm 63-100 100-160 160-200 72.8 91.5 153.0 76.5 75.9 76.8 28.4 27.8 27.0 29.0 51.0 99.0 74.9 76.9 75.0 23.1 21.8 20.7 19.5 34.5 66.0 81.7 71.2 74.6 20.3 19.4 18.8 15.2 25.0 65.0 77.5 74.1 77.0 17.4 16.9 17.5 g per 50 g oil.
Table 5 The effect of catalyst particle size on the characteristics of hydrogenated fat produced during hydrogenation of sunflower oil at different temperatures Hydrogenation temperature, oc 45
Characteristic
Hydrogenation time, min I.V. Trans-isomer content, % 70 Hydrogenation time, min I.V. Trans-isomer content, % 95 Hydrogenation time, min I.V. Trans-isomer content, % Charge of catalyst 0.2% PcVA1203 is 0.5 g per 50 Hydrogen pressure is 4 bar.
Catalyst particle size, pm 56-63 100-160 160-200 22.0 74.5 25.8 11.0 74.8 31.5 10.0 73.2 37.9 g oil.
51.0 76.9 21.8 14.0 74.6 29.4 12.5 73.0 33.9
99.0 75.0 20.7 34.0 74.7 29.0 27.0 74.9 34.3
2044 The data given in Table 4 and Table 5 show that: - the effect of hydrogen pressure on the hydrogenation of triglycerides of unsaturated acids is rather complicated. Although increasing the hydrogenation pressure lowers the trans-isomer content in hydrogenated fat down to a certain level, nevertheless at hydrogen pressures more than 6 bar we lose another selectivity when the content of undesirable acyls of stearic acid increases; - the effect of the particle size of the palladium based catalyst for a fixed weight ratio oil : palladium (50000:1) on the hydrogenation characteristics is also complicated. The fractional composition of catalyst has only a small effect on cistrans isomerization activity. In this case only the hydrogenation time and to some extent the fatty acid composition change.
4. CONCLUSIONS Using newly designed low-loaded palladium catalyst in selective hydrogenation of triglycerides of unsaturated fatty acids, the production of hydrogenated fat with low trans-isomer content becomes possible by performing the hydrogenation at ambient temperatures. The characteristics of catalyst used (concentration of palladium in catalyst, fractional composition, particle size) slightly affect the process. There is an optimal pressure of 4 bar when on the one hand we have the decrease in trans-isomer content, and on the other hand the selectivity of oleinic acid formation remains rather high.
REFERENCES
1. F.B.Hu, M.J.Stampfer, J.E.Manson, E.Rimm, G.A.Colditz, B.A.Rosner, C.H.Hennekens and W.C.Willet, New Engl. J. Med., 337 (1997) 1491. 2. M.Zajcew, J. Am. Oil Chem Soc., 39 (1962) 301. 3. N.Hsu, L.L.Diosady and L.J.Rubin, J. Am. Oil Chem. Soc., 65 (1988) 349. 4. V.I.Savchenko and I.A.Makaryan, Platinum Metals Rev., 43 (1999) 74.
2039
Lowering the t r a n s - i s o m e r content in hydrogenation of triglycerides of unsaturated fatty acids at ambient temperatures I.A.Makaryan, O.V.Matveeva, G.I.Davydova and V.I.Savchenko Institute of Problems of Chemical Physics of the Russian Academy of Sciences 142432, Chernogolovka, Moscow Region, Russia
The selective hydrogenation of the triglycerides of unsaturated fatty acids from vegetable oils has been performed using newly designed low-loaded palladium catalyst. Investigations on the effect of reaction conditions (temperature, pressure, fractional composition) on the trans-isomerization showed that the palladium catalyst unlike conventional nickel catalysts allows to decrease the trans-isomer content in final hydrogenated fat.
1. INTRODUCTION The selective catalytic hydrogenation of the triglycerides of unsaturated fatty acids from vegetable oils is the main method used to modify natural vegetable oils into a fatty base for edible margarines. In this century margarine plays an important part in the human diet and has to some extent superseded high cholesterol butters. At present its production is performed using finely-dispersed nickel catalysts at elevated temperatures from 180 to 2300C, and has some serious disadvantages which contribute to making the process inefficient. In the course of triglycerides hydrogenation a hydrogenated fat is produced, and simultaneously the trans-isomers are formed in proportion depending on hydrogenation conditions. The content of trans-isomers in final edible product could be about 60 per cent. Recent literature points to the fact that the trans-isomers of hydrogenated fats may pose a possible health hazard [1]. At the same time, edible margarines may be contaminated with very-toxic-to-humans nickel compounds from the catalyst used. Therefore, it is not clear yet either trans-isomers or nickel may
2040
cause undesirable effects to humans. In any case, the conventional technology for producing hydrogenated fat for edible margarines requires updating. The main challenge in producing pure margarines is to find replacement for the nickel-containing catalyst. One such alternative may be palladium, and several studies have been reported using palladium catalysts to hydrogenate vegetable oils [2, 3]. Thus, three palladium-containing catalysts Pd/C, Pd/A1203 and Pd/BaSot were used [3] during the investigations on the effects of pressure, temperature and catalyst concentration on the formation of trans-isomers in hydrogenation of vegetable oils at temperatures of 50 to 1100C and pressures from 3,5 to 50 atm. Our previous investigations showed [4] that newly designed low-loaded palladium catalyst which was developed to hydrogenate vegetable oils into a fatty base for edible margarines gives a highly stable process with the required product. The present work is aimed at the investigation on characteristics of newly designed low-loaded palladium catalyst in hydrogenation of triglycerides of unsaturated fatty acids under different reaction conditions and using different fractional composition of catalyst which could lead to the decrease in trans-isomer content in final hydrogenated fat.
2. EXPERIMENTAL The low-loaded palladium catalysts were prepared by deposition of palladiumcontaining compound on an active alumina support of different particle size (e.g. fractional composition). The hydrogenation was performed in reaction vessel fixed on the shaker with 10 sec -1 oscillation. Under these conditions the hydrogenation rate was not limited by mass transfer of hydrogen from the gas to the liquid phase. The hydrogenation proceeded during definite time: for soybean oil up to iodine value (I.V.) = 95 + 3, and for sunflower oil up to I.V. = 75 + 2. The initial vegetable oils were preliminarily purified. When testing the catalysts with a very low palladium content, the initial oil was purified more carefully using special technique. The fatty acid composition was measured by gas-liquid chromatography; the trans-isomer content was analyzed by IRspectroscopy (Table 1).
2041 Table 1 Fatty acid composition and iodine value for the initial vegetable oils Vegetable
I.V,
oil
Fatty acid c0mposition, %*
C14:0 C16:0 C18:0 C18:1 C18:2 C18:3 C20:0 C20:1 C22:0
Sunflower oil
137.5
0.1
6.8
4.4
16.3
71.2
-
0.2
0.4
0.6
Soybean oil
132.3
0.1
11.4
4.0
22.3
53.0
8.0
0.3
0.5
0.4
*Content of acyi radicals in triglycerides of the following fatty acids: myristic (C14:o); palmitic (C16:o); stearic (C18:o); oleic (Cls:l); linoleic (C18:e); linolenic (C18:a); eicosanic (C20:0); eicoseinic (Ce0:l); docosanoic Cee:0).
3. R E S U L T S AND D I S C U S S I O N The thorough investigations using palladium catalyst to hydrogenate the triglycerides of unsaturated fatty acids allowed to observe the following distinctive features. The palladium is a very active catalyst for the hydrogenation of triglycerides of unsaturated fatty acids from vegetable oils, and it is difficult to find other reactions with such high hydrogenation rates. Thus, using a very pure initial oil for hydrogenation we can employ the active catalysts with a very low palladium content (down to 0.001 wt% palladium in catalyst) [4]. Such extremely high activity allows to perform hydrogenation of triglycerides event at lowered temperatures, resulting in hydrogenated fat with a low trans-isomer content. Changing the amount of palladium in the catalyst from 0.2 to 0.02 wt% has only a small effect on the characteristics of final hydrogenated fat relatively both acyl and trans-isomer selectivity (Table 2). We have ensured that the main factor affecting the trans-isomer content in hydrogenated fat is the hydrogenation temperature. The data on the hydrogenations at moderate temperatures are given in Table 3. The lower range of temperatures in these runs was limited by 300C for I.V. - 95 + 3, and by 45oc at I.V. - 75 + 2. This was caused by the fact that under these conditions the reaction mixture became very viscous and badly agitated. In order to use the range of lower temperatures we decided to perform the hydrogenation of sunflower oil in solvent medium, specifically heptane. In this case the palladium catalyst was active down to T 00C. The decrease in temperature was accompanied by decrease in trans-isomer content.
2042 Table 2 Effect of the palladium concentration in the catalyst on the characteristics of hydrogenation of sunflower oil
Characteristics
Concentration of Pd in catalyst, wt% 0.02 0.05 0.2
Hydrogenation time, min 175.0 I.V. 75.6 Fatty acid composition,%: C14:o 0.1 C16:o 6.6 Cls:O 10.8 Cls:I 75.3 C18:2 6.2 C2o:o 0.3 C2o:1 0.1 C22:o 0.6 Trans-isomer content, % 20.8 Charge of catalyst Pd/A1203 is 0.5 g per 50 g of oil. Reaction temperature is 45oc, hydrogen pressure is
70.0 75.3
29.0 74.9
0.1 6.4 9.5 79.2 4.1 0.3 0.1 0.6 23.3
0.1 6.9 11.0 75.3 5.8 0.2 0.1 0.6 23.1
4 bar.
Table 3 Characteristics of the Hydrogenation of Soybean and Sunflower Oils at Moderate Temperatures Soybean oil hydrogenation up to I . V . - 95 + 3 Temperature, 0C I.V. trans-isomer content, % 90 92.0 21.7 8O 95.O 2O.6 70 93.0 19.2 50 97.7 14.7 3O 96.6 12.5 Sunflower oil hydrogenation up to I . V . - 75 + 2 Temperature, oc I.V. trans-isomer content, % 95 73.0 33.9 70 74.6 29.4 45 76.9 21.8 Charge of Pd is 20 mg per I kg of oil.
Taking into consideration the fact that the rate determining step for the hydrogenation of vegetable oils with palladium is the transfer of hydrogen and triglycerides of unsaturated acids from the liquid phase to the catalyst surface, and that the particle size of catalyst significantly affects the process under
2043
consideration we investigated the characteristics of hydrogenation using palladium catalyst with different fractional composition. The tests were performed both when the temperature was kept constant (450C) pressures being changed from 1 to 10 bar (Table 4), and when the pressure was constant (4 bar) and temperatures being changed (Table 5). Table 4 The effect of catalyst particle size on the characteristics of hydrogenated fat produced during hydrogenation of sunflower oil at different hydrogen pressures Hydrogen pressure, bar
Characteristics
Hydrogenation time, min I.V. Trans-isomer content, % Hydrogenation time, min I.V. Trans-isomer content, % Hydrogenation time, min I.V. Trans-isomer content, % 10 Hydrogenation time, min I.V. Trans-isomer content, % Charge of catalyst 0.2 wt% Pd/A1203 is 0.5 Reaction temperature is 45oc.
Catalyst particle size, pm 63-100 100-160 160-200 72.8 91.5 153.0 76.5 75.9 76.8 28.4 27.8 27.0 29.0 51.0 99.0 74.9 76.9 75.0 23.1 21.8 20.7 19.5 34.5 66.0 81.7 71.2 74.6 20.3 19.4 18.8 15.2 25.0 65.0 77.5 74.1 77.0 17.4 16.9 17.5 g per 50 g oil.
Table 5 The effect of catalyst particle size on the characteristics of hydrogenated fat produced during hydrogenation of sunflower oil at different temperatures Hydrogenation temperature, oc 45
Characteristic
Hydrogenation time, min I.V. Trans-isomer content, % 70 Hydrogenation time, min I.V. Trans-isomer content, % 95 Hydrogenation time, min I.V. Trans-isomer content, % Charge of catalyst 0.2% PcVA1203 is 0.5 g per 50 Hydrogen pressure is 4 bar.
Catalyst particle size, pm 56-63 100-160 160-200 22.0 74.5 25.8 11.0 74.8 31.5 10.0 73.2 37.9 g oil.
51.0 76.9 21.8 14.0 74.6 29.4 12.5 73.0 33.9
99.0 75.0 20.7 34.0 74.7 29.0 27.0 74.9 34.3
2044 The data given in Table 4 and Table 5 show that: - the effect of hydrogen pressure on the hydrogenation of triglycerides of unsaturated acids is rather complicated. Although increasing the hydrogenation pressure lowers the trans-isomer content in hydrogenated fat down to a certain level, nevertheless at hydrogen pressures more than 6 bar we lose another selectivity when the content of undesirable acyls of stearic acid increases; - the effect of the particle size of the palladium based catalyst for a fixed weight ratio oil : palladium (50000:1) on the hydrogenation characteristics is also complicated. The fractional composition of catalyst has only a small effect on cistrans isomerization activity. In this case only the hydrogenation time and to some extent the fatty acid composition change.
4. CONCLUSIONS Using newly designed low-loaded palladium catalyst in selective hydrogenation of triglycerides of unsaturated fatty acids, the production of hydrogenated fat with low trans-isomer content becomes possible by performing the hydrogenation at ambient temperatures. The characteristics of catalyst used (concentration of palladium in catalyst, fractional composition, particle size) slightly affect the process. There is an optimal pressure of 4 bar when on the one hand we have the decrease in trans-isomer content, and on the other hand the selectivity of oleinic acid formation remains rather high.
REFERENCES
1. F.B.Hu, M.J.Stampfer, J.E.Manson, E.Rimm, G.A.Colditz, B.A.Rosner, C.H.Hennekens and W.C.Willet, New Engl. J. Med., 337 (1997) 1491. 2. M.Zajcew, J. Am. Oil Chem Soc., 39 (1962) 301. 3. N.Hsu, L.L.Diosady and L.J.Rubin, J. Am. Oil Chem. Soc., 65 (1988) 349. 4. V.I.Savchenko and I.A.Makaryan, Platinum Metals Rev., 43 (1999) 74.