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Catalytic deoxygenation of stearic acid over palladium supported on acid modified mesoporous silica
Zeolites and Related Materials: Trends, Targets and Challenges Proceedings of 4th International FEZA Conference A. Gédéon, P. Massiani and F. Babonneau (Editors) © 2008 Elsevier B.V. All rights reserved.
1339
Catalytic deoxygenation of stearic acid over palladium supported on acid modified mesoporous silica Siswati Lestari, Jorge Beltramini, G.Q. Max Lu ARC Centre of Excellence for Functional Nanomaterials, University of Queensland, Brisbane 4072, Australia
Abstract Catalytic deoxygenation of stearic acid were investigated over palladium supported on acid modified mesoporous silica SBA15 and MCM-41. Liquid product analysis performed by gas chromatography shows that palladium supported on acid modified SBA15 was active toward deoxygenation product of n-pentadecane with selectivity up to 67% after 5h reaction. Moreover, deoxygenation of stearic acid was also observed during the reaction that confirmed by mass spectroscopy the formation of nheptadecane. Keywords: deoxygenation, acid modified SBA15, biodiesel
1. Introduction The widely known process to convert vegetable oil is transesterification to produce fatty acid methyl ester (FAME) known as biodiesel [1] Although, it has been well reported that biodiesel has several attractive features to substitute petroleum fuel, e.g. reducing the GHG emission, high cetane number and lubricity, the amount of biodisel added is limited to 5% or less [2] Considering the fact that bio-feedstock comprised a significant amount of oxygen, this paper reported improvement of process to convert vegetable oil via deoxygenation pathway using acid modified mesoporous silica as catalyst support. Two different of mesoporous supports were use in this work, i.e MCM-41 and SBA-15 modified with acid then palladium was impregnated into the support. Mesoporous support were chosen for this application as they have large pore structure and high surface area making these materials suitable for processing large molecule of vegetable oil molecules. In order to get the balance information, we also compared the catalytic test using palladium supported on microporous zeolite. Stearic acid was used as a model compound of vegetable oil.
2. Experimental Section The catalytic deoxygenation experiments were performed in a 300 ml semi-batch reactor coupled to condenser and heating jacket with liquid phase volume of 100ml. The catalysts were reduced in-situ by hydrogen for 1h prior to the catalytic test. Then 0,1M of feed was introduced to the reactor via bubbling unit preventing re-oxygenation of catalyst. Before the reaction started, 25 ml/min of argon gas was passed through the reactor until reaching the reaction pressure. Afterward the reaction temperature was adjusted to desired reaction temperature. At this point stirring and reaction time started.
1340
S. Lestari et al.
The liquid samples were withdrawn from reactor during experiment. Typically, the samples had to be dissolved in a solution of pyridine and silylated with N,Obis(trimethyl)trifloroacetamide, BSTFA (Across Organics, 98+%) in order to conduct the gas chromatographic analysis. Typically, eicosane was used as internal standard
3. Results and Discussion The extent of thermal deoxygenation at 300oC was minor, the results showed that <5% of stearic acid was converted within 5h of reaction. The main products formed were linear C15 and C17 produced mainly from thermal cracking of stearic acid (Fig.1) In a typical heterogeneous reaction, deoxygenation experiment over 5wt% Pd supported on acid modified SBA15 catalyst, the main product was n-pentadecane with selectivity up to 67% within 5h reaction. In addition with this product, it was also found nheptadecane in minor amounts, as shown in figure 2. This result demonstrated that cracking of stearic acid occurs at 300oC in conjunction with decarboxylation reaction. This result is slightly different compare with previous work reported by [3] which revealed higher hydrocarbon of C17 as main product instead of C15, perhaps due to different active sites of support. Two different acid modified mesoporous supports were also tested in this work, i.e. SBA15 and MCM41 in order to investigate the effect of pore size. The results were compared with catalytic results obtained when using zeolite Y as support. As expected, catalytic deoxygenation of stearic acid over palladium supported on acid modified SBA15 is superior compared with the two others supports which show conversion up to 84% during 5h reaction. The main product is n-pentadecane with 67% of selectivity. Additionally, other unsaturated C15 and C17 were also formed in minor amounts.
100
Molar fraction, %
80
n-pentadecane n-heptadecane stearic acid
60
40
20
0 0
50
100
150
200
250
300
Reaction time, min
Figure 1. Deoxygenation profile of stearic acid over thermal condition (without catalyst) at 300oC. The reaction conditions are as follow: mstearic acid = 0.1M (in mesitylene), T = 300oC, p = 17 bar, flowcarrier gas = 25 ml/min (Ar)
Catalytic deoxygenation of stearic acid over palladium supported on acid modified mesoporous silica
1341
100
n-pentadecane n-heptadecane stearic acid
Molar fraction, %
80
60
40
20
0
0
50
100
150
200
250
300
Reaction time, min
Figure 2. Deoxygenation profile of stearic acid over 5wt%Pd/SH-SBA15 catalyts. The reaction conditions are the same as Figure 1. The catalytic deoxygenation tests revealed the order of active supports as follow: SBA15 > MCM41 > zeolite Y. This result indicated that large pore size and high surface area of SBA15 are more acceptable for large molecules of stearic acid [4]. Moreover, the thicker wall of SBA15 makes this material more thermally stable at high temperature. While the catalytic test over zeolite Y support shows a low conversion of less than 10% of stearic acid after 5hours reaction, due to either poor pore diffusion and limitation of active sites [5] Table 1. Physical properties of catalysts and catalytic results Catalyts
Physical properties
Catalytic test results
SBET, m2/g
DpBJHa,
VpBJHb,
nm
5wt% Pd/SH-SBA15
996
5wt% Pd/H-MCM41 5wt% Pd/H-Y a
cc/g
Selectivityn-C15c, %
Conversiond, %
5.5
1.4
67
84
868
3.3
0.8
45
36
638
1.0
0.2
24
10
Average pore diameter Total pore volume estimated at P/P0 0.99 c Selectivity to n-heptadecane after 5hours reaction d Stearic acid conversion after 5hours reaction b
1342
CH3(CH2)15-COOH Stearic acid
S. Lestari et al.
CH3(CH2)15CH3 +CO2 n-heptadecane
1
2 CH3(CH2)13CH3 + CH3CH=CH2 n-pentadecane propene 1. decarboxylation 2. cracking 3. hydrogenation
3
H2
CH3CH2-CH3 propane
Figure 3 . Reaction pathway for deoxygenation of stearic acid From the catalytic test results it was revealed that the plausible reaction pathways for conversion of stearic acid are as follow: (i) decarboxylation of stearic acid to produce nheptadecane by releasing carbon dioxide and (ii) cracking of stearic acid into shorter hydrocarbon as shown in Figure 3.
3. Conclusion The catalytic deoxygenation of stearic acid over Pd supported on different support was successfully demonstrated with high activity and selectivity to desired product, npentadecane which achieved by SBA15 support. The plausible reaction pathways to convert stearic acid into hydrocarbon products are via decarboxylation and cracking reactions.
References [1] Knothe, G. and Steidley, K. R., Fuel, 84 (2005) 1059 [2] Kalnes T.M., Shonnard D.R. International Journal of Chemical reacto Engineering, 5 (2007) A 48 [3] Snare, M., Kubickova, I., Maki-Arvela, P., Eranen, K. and Murzin, D. Y., Ind. Eng. Chem. Res., 45 (2006), 5708 [4] Mbaraka, I. K. and Shanks, B. H., J. of the American Oil Chemists Society, 83 (2006), 79 [5] Melero, J. A., vanGrieken, R. and Morales, G., Chem. Rev., 106 (2006), 3790 .
1339
Catalytic deoxygenation of stearic acid over palladium supported on acid modified mesoporous silica Siswati Lestari, Jorge Beltramini, G.Q. Max Lu ARC Centre of Excellence for Functional Nanomaterials, University of Queensland, Brisbane 4072, Australia
Abstract Catalytic deoxygenation of stearic acid were investigated over palladium supported on acid modified mesoporous silica SBA15 and MCM-41. Liquid product analysis performed by gas chromatography shows that palladium supported on acid modified SBA15 was active toward deoxygenation product of n-pentadecane with selectivity up to 67% after 5h reaction. Moreover, deoxygenation of stearic acid was also observed during the reaction that confirmed by mass spectroscopy the formation of nheptadecane. Keywords: deoxygenation, acid modified SBA15, biodiesel
1. Introduction The widely known process to convert vegetable oil is transesterification to produce fatty acid methyl ester (FAME) known as biodiesel [1] Although, it has been well reported that biodiesel has several attractive features to substitute petroleum fuel, e.g. reducing the GHG emission, high cetane number and lubricity, the amount of biodisel added is limited to 5% or less [2] Considering the fact that bio-feedstock comprised a significant amount of oxygen, this paper reported improvement of process to convert vegetable oil via deoxygenation pathway using acid modified mesoporous silica as catalyst support. Two different of mesoporous supports were use in this work, i.e MCM-41 and SBA-15 modified with acid then palladium was impregnated into the support. Mesoporous support were chosen for this application as they have large pore structure and high surface area making these materials suitable for processing large molecule of vegetable oil molecules. In order to get the balance information, we also compared the catalytic test using palladium supported on microporous zeolite. Stearic acid was used as a model compound of vegetable oil.
2. Experimental Section The catalytic deoxygenation experiments were performed in a 300 ml semi-batch reactor coupled to condenser and heating jacket with liquid phase volume of 100ml. The catalysts were reduced in-situ by hydrogen for 1h prior to the catalytic test. Then 0,1M of feed was introduced to the reactor via bubbling unit preventing re-oxygenation of catalyst. Before the reaction started, 25 ml/min of argon gas was passed through the reactor until reaching the reaction pressure. Afterward the reaction temperature was adjusted to desired reaction temperature. At this point stirring and reaction time started.
1340
S. Lestari et al.
The liquid samples were withdrawn from reactor during experiment. Typically, the samples had to be dissolved in a solution of pyridine and silylated with N,Obis(trimethyl)trifloroacetamide, BSTFA (Across Organics, 98+%) in order to conduct the gas chromatographic analysis. Typically, eicosane was used as internal standard
3. Results and Discussion The extent of thermal deoxygenation at 300oC was minor, the results showed that <5% of stearic acid was converted within 5h of reaction. The main products formed were linear C15 and C17 produced mainly from thermal cracking of stearic acid (Fig.1) In a typical heterogeneous reaction, deoxygenation experiment over 5wt% Pd supported on acid modified SBA15 catalyst, the main product was n-pentadecane with selectivity up to 67% within 5h reaction. In addition with this product, it was also found nheptadecane in minor amounts, as shown in figure 2. This result demonstrated that cracking of stearic acid occurs at 300oC in conjunction with decarboxylation reaction. This result is slightly different compare with previous work reported by [3] which revealed higher hydrocarbon of C17 as main product instead of C15, perhaps due to different active sites of support. Two different acid modified mesoporous supports were also tested in this work, i.e. SBA15 and MCM41 in order to investigate the effect of pore size. The results were compared with catalytic results obtained when using zeolite Y as support. As expected, catalytic deoxygenation of stearic acid over palladium supported on acid modified SBA15 is superior compared with the two others supports which show conversion up to 84% during 5h reaction. The main product is n-pentadecane with 67% of selectivity. Additionally, other unsaturated C15 and C17 were also formed in minor amounts.
100
Molar fraction, %
80
n-pentadecane n-heptadecane stearic acid
60
40
20
0 0
50
100
150
200
250
300
Reaction time, min
Figure 1. Deoxygenation profile of stearic acid over thermal condition (without catalyst) at 300oC. The reaction conditions are as follow: mstearic acid = 0.1M (in mesitylene), T = 300oC, p = 17 bar, flowcarrier gas = 25 ml/min (Ar)
Catalytic deoxygenation of stearic acid over palladium supported on acid modified mesoporous silica
1341
100
n-pentadecane n-heptadecane stearic acid
Molar fraction, %
80
60
40
20
0
0
50
100
150
200
250
300
Reaction time, min
Figure 2. Deoxygenation profile of stearic acid over 5wt%Pd/SH-SBA15 catalyts. The reaction conditions are the same as Figure 1. The catalytic deoxygenation tests revealed the order of active supports as follow: SBA15 > MCM41 > zeolite Y. This result indicated that large pore size and high surface area of SBA15 are more acceptable for large molecules of stearic acid [4]. Moreover, the thicker wall of SBA15 makes this material more thermally stable at high temperature. While the catalytic test over zeolite Y support shows a low conversion of less than 10% of stearic acid after 5hours reaction, due to either poor pore diffusion and limitation of active sites [5] Table 1. Physical properties of catalysts and catalytic results Catalyts
Physical properties
Catalytic test results
SBET, m2/g
DpBJHa,
VpBJHb,
nm
5wt% Pd/SH-SBA15
996
5wt% Pd/H-MCM41 5wt% Pd/H-Y a
cc/g
Selectivityn-C15c, %
Conversiond, %
5.5
1.4
67
84
868
3.3
0.8
45
36
638
1.0
0.2
24
10
Average pore diameter Total pore volume estimated at P/P0 0.99 c Selectivity to n-heptadecane after 5hours reaction d Stearic acid conversion after 5hours reaction b
1342
CH3(CH2)15-COOH Stearic acid
S. Lestari et al.
CH3(CH2)15CH3 +CO2 n-heptadecane
1
2 CH3(CH2)13CH3 + CH3CH=CH2 n-pentadecane propene 1. decarboxylation 2. cracking 3. hydrogenation
3
H2
CH3CH2-CH3 propane
Figure 3 . Reaction pathway for deoxygenation of stearic acid From the catalytic test results it was revealed that the plausible reaction pathways for conversion of stearic acid are as follow: (i) decarboxylation of stearic acid to produce nheptadecane by releasing carbon dioxide and (ii) cracking of stearic acid into shorter hydrocarbon as shown in Figure 3.
3. Conclusion The catalytic deoxygenation of stearic acid over Pd supported on different support was successfully demonstrated with high activity and selectivity to desired product, npentadecane which achieved by SBA15 support. The plausible reaction pathways to convert stearic acid into hydrocarbon products are via decarboxylation and cracking reactions.
References [1] Knothe, G. and Steidley, K. R., Fuel, 84 (2005) 1059 [2] Kalnes T.M., Shonnard D.R. International Journal of Chemical reacto Engineering, 5 (2007) A 48 [3] Snare, M., Kubickova, I., Maki-Arvela, P., Eranen, K. and Murzin, D. Y., Ind. Eng. Chem. Res., 45 (2006), 5708 [4] Mbaraka, I. K. and Shanks, B. H., J. of the American Oil Chemists Society, 83 (2006), 79 [5] Melero, J. A., vanGrieken, R. and Morales, G., Chem. Rev., 106 (2006), 3790 .